Metallocene-containing conjugated polymers

Advances in Colloid and Interface Science 139 (2008) 97 – 149 www.elsevier.com/locate/cis

Metallocene-containing conjugated polymers Mikhail A. Vorotyntsev a,⁎, Svetlana V. Vasilyeva a,b a

b

ICMUB-UMR 5260 CNRS, University of Bourgogne, Dijon, France Inorganic Chemistry Department, A.I. Hertzen State Pedagogical University, St. Petersburg, Russia Available online 26 January 2008

Abstract The paper gives a review of publications on polymers with conjugated matrices (PPy, PTh, PAni, hydrocarbon or mixed chains…) which incorporate metallocene complexes (Fe, Ru, Co; Ni, Ti, Zr, Ta) with two cyclopentadienyl ligands (Cp) and their derivatives, in particular with methylated cyclopentadienyl rings (Cp⁎), as well as hemi-metallocene complexes (Fe, Ru, Co, Mn), as pendant groups or inside the principal chain (part B). The information on related short-chain systems, monomers and oligomers, is also included. In part A, a brief overview of various conjugated polymer materials is presented, with their classification in accordance with the conductivity mechanism (ionic, electronic or mixed conductors) or with the structural type (linear-chain organic or mixed polymers, derivatization, metallopolymers, multi-dimensional structures, alternating and block copolymers with organic or mixed units, hybrid materials with a mixture of conjugated and inert polymers, polymers inside a solid matrix, conjugated polymers with incorporated nanoelements of transition metals, carbon, semiconductors etc. © 2008 Elsevier B.V. All rights reserved. Keywords: Conducting polymer; Ferrocene; Metallocene; Hemi-metallocene; Modified electrode; Hybrid material

Contents 1. 2.

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Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Part A. Conjugated polymers: a brief overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. “Conducting” and “electroactive” polymers: mechanisms of charge transport . . . . . . . . . . . . . . . 2.1.1. Ionic conductors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.2. Electronic conductors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.3. Mixed (electron–ion) conductors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Structural aspects of conjugated polymers and related systems . . . . . . . . . . . . . . . . . . . . . . . 2.2.1. Materials composed of identical building blocks . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.2. Copolymers with a compose matrix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.3. Hybrid materials based on conjugated polymers . . . . . . . . . . . . . . . . . . . . . . . . . . Part B. Conjugated polymers containing metallocene or “hemi-metallocene” (monocyclopentadienyl) complexes 3.1. Entrapping of Fc-containing species inside polymer film/powder . . . . . . . . . . . . . . . . . . . . . . 3.2. Covalent bonding of Fc complex to pyrrole monomer/polymer . . . . . . . . . . . . . . . . . . . . . .

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Abbreviations: Cp, cyclopentadienyl anionic ligand, C5H−5 ; Cp⁎, alkyl-substituted (mostly, pentamethylated, (CH3)5C−5 ) Cp ligand; Fc, ferrocene, FeCp2 18a; Ar, aryl/arylene; Th, thiophene/thienyl/thienylene; Ph, phenyl/phenylene; EDOT, 3,4-ethylenedioxythiophene 80b; PPy, polypyrrole 2c; PTh, polythiophene 2d; PAni, polyaniline 2e; PPP, poly(p-phenylene) 2b; PEDOT, poly(3,4-ethylenedioxythiophene); PSS, polystyrene sulfonate; TTF, tetrathiafulvalene; NHS, N-hydroxy-2,5pyrrolidinedione (N-hydroxysuccinimide) 39a; NHP, N-hydroxyphthalamide; THF, tetrahydrofuran. Other abbreviations are defined in corresponding sections. ⁎ Corresponding author. ICMUB UMR CNRS 5260, Batiment Mirande, University of Bourgogne, 9 avenue A. Savary, BP 47 870, 21078 Dijon Cedex, France. Tel./ fax: +33 3 8039 6064. E-mail address: [email protected] (M.A. Vorotyntsev). 0001-8686/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.cis.2008.01.006

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3.2.1. Fc derivatives linked to Py in N-position . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.2. Fc derivatives linked to pyrrole in 3-position . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.3. Other Fc–pyrrole systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Fc bonded to/inside oligo/polythiophene chain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.1. Ester or carboxamide bridge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.2. Hydrocarbon bridge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.3. Compose Fc–thiophene chains . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4. Fc–aniline derivatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5. Fc- and hemi-ferrocene-containing polymers with conjugated hydrocarbon chains (with cyclopentadienylene, ethynylene or/and vinylene units) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5.1. Pendant Fc complex. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5.2. Fc and hemi-ferrocene complexes inside a polymer chain . . . . . . . . . . . . . . . . . . . . . . 3.6. Fc in mixed and non-carbon oligomer/polymer chains. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.6.1. Chains composed of carbon units and heterocycles . . . . . . . . . . . . . . . . . . . . . . . . . . 3.6.2. Other systems (non-carbon units, metal complexes) . . . . . . . . . . . . . . . . . . . . . . . . . 3.7. Polymers with metallocene or hemi-metallocene complexes of other metals . . . . . . . . . . . . . . . . . 3.7.1. Complexes of ruthenium and osmium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.7.2. Complexes of cobalt and nickel. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.7.3. Hemi-metallocene complexes of manganese . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.7.4. Metallocene complexes of “early transition metals” (Ti, Zr and Ta) . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction Traditionally, polymers were considered as typical organic materials with the corresponding characteristic properties, as low electric and thermal conductivities, hydrophobicity of their surfaces, chemical inertness, low mechanical resistance etc. However, a remarkable progress in the polymer science has changed gradually this image. In particular, during the last decades new types of polymeric materials have been created which possess a significant electric conductivity. It opens naturally a door to the involvement of electrochemistry into this area both for the synthesis of such systems and for their applications. These new polymers are obtained in the form of bulk materials (as powders or porous solids) or of a film at the surface of a solid support, in particular of an electrode. In view of the combination of their specific properties they opened prospects (or even used already) in various applications, e.g. as electrode materials (batteries and “supercapacitors” [1–6], polymer electrolytes [7–10], various protective films (against a static electricity or a microwave irradiation) [11], anti-corrosion protection [12–15], separating membranes for gases and liquids

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[16,17], sensors and biosensors [18–21], actuators and artificial muscles [22,23], micro- and nanoelements for the information storage and transformation [24], (electro)luminescent materials, “smart windows” and solar energy conversion [18,25,26], electrochromic media [27–30], catalysis and electrocatalysis [31–33]. The principal goal of this paper is to give an overview of conjugated polymers which contain transition metal complexes of the metallocene or hemi-metallocene type, in which the central metal is bonded to two or to one anionic cyclopentadienyl ligand(s), C5H5− (Cp), or their alkyl-substituted derivatives, Cp⁎, e.g.(CH3)5C5− (part B). These elements are either entrapped inside the polymer matrix, or linked as pendant groups, or incorporated inside the polymer matrix itself. To show the place of these functionalized materials amongst the whole area of “conducting polymers”, we'll start from a brief schematic description of various types of conjugated polymers, classified in accordance either with their electric conductivity mechanism (part A, Section 2.1), or with their structural aspects (part A, Section 2.2), without pretending for its completeness.

2. Part A. Conjugated polymers: a brief overview 2.1. “Conducting” and “electroactive” polymers: mechanisms of charge transport Polymer materials with a significant electric conductivity can be divided into three groups depending on the type of their mobile charges: ionic, electronic and mixed (electron–ion) conductors. 2.1.1. Ionic conductors Such solids with a pure ionic conduction (in normal conditions, without inducing an electronic conductivity e.g. by their irradiation) are usually called “polymer electrolytes” or “ionic polymers”. Physically, these systems represent porous membranes with a polymer matrix, with mobile ions inside their pores (with or without solvent). There are frequently functional groups, in particular charged/ionogenic ones, ensuring a very high concentration of mobile ions and correspondingly a high ion conductivity.

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These ionic polymers can be easily produced in the form of flexible thin films in contact with an electrode with a large surface area, these properties being a key advantage for various electrochemical devices, first of all in batteries [5,6,34]. 2.1.2. Electronic conductors Another group of polymeric materials possesses a significant electronic conductivity (the term “significant” depends on the particular type of application of the system and it means that the electronic transport takes place at the scale of the system's functioning), without an ionic contribution. One can distinguish two types of such electronic conductors depending on their electrontransport mechanism. 2.1.2.1. “Redox polymers”. In these systems the polymer matrix represents merely a solid or flexible support for functional groups which are attached to the matrix and which are able to change their oxidation state. Most systems of this type (see examples below) correspond to polymers with aliphatic chains (without or with other functional groups). Another possibility is to use an “inorganic polymer”, i.e. to localize a transition metal ion (or complex) inside an inorganic solid. There are various approaches to incorporate a metal complex inside such a material [35]. Complexes with ligands unstable in their free state (e.g. ferrocene (Fc) [36,37] or cobaltocene [38,39]) are covalently bonded to the matrix as integral groups via their ligands. For chemically stable ligands, like bipyridyl, pyridyl [40–44] or porphyrins [45–47], one can use the above approach or attach first this ligand to the matrix, to perform the complexation with the metal a posteriori. Examples of both types are given below. Electronic motion in these materials corresponds to the “hopping mechanism”, i.e. it is realized as an electron-exchange reaction between neighboring molecular centers [48,49]. Practically, the hopping distance is within a few nm (0.5–1 nm in most cases) which means the necessity to ensure a high concentration of these centers (mostly, above 0.1 mol/dm3 for their uniform distribution) [50] and the existence of long-distance chains of closely located centers. Even if these conditions are satisfied the transport with this mechanism is relatively slow, with the values of the effective diffusion coefficients being in the range: 10− 7–10− 11 cm2/s [51]. Systems of this type are frequently called “redox polymers” or “conventional redox polymers”. Their typical examples 1a–1d are given by transition metal complexes attached covalently to the polymer matrix, e.g. in polyvinylpyridine [52], polyvinylcarbazole [53], polyvinylferrocene [54,55], substituted polystyrenes [56].

For all these systems each molecular center can host a single mobile electron at maximum so that all centers are either in reduced (Red) or in oxidized (Ox) form. The electron exchange between the neighboring centers can only take place if one of them is Red and another is Ox. Therefore, the conductivity of the system as a function of the oxidation level (fraction of the centers in the Ox form) is correlated with the number of such Red + Ox couples and it passes through a maximum at a medium oxidation level (when about one half of all centers is in the Ox form) while it vanishes if all centers are in a single (Ox or Red) state [57–59]. It is the reason why such systems are also called “mixed-valence conductors” [60,61]. 2.1.2.2. Conjugated/conducting polymers/“synthetic metals”. In “redox polymers” the polymer matrix plays a passive role of a supporter for “electroactive centers”. The situation is opposite for another group which includes “electronically conducting polymers” (frequently called shortly “conducting polymers” or “synthetic metals”) where the polymer matrix itself may possess a sufficiently high electronic conductivity. The word “may” in the previous sentence implies that this parameter of the material depends essentially on its “doping level” (this term originated from the physics of inorganic semiconductors corresponds to the term “oxidation level' in chemistry, see also below). The most well-known group of materials of this type is given by polymers with “π-conjugated” chains. After the synthesis of the first example of this type of polymers, a well-conducting polyacetylene 2a [62,63], in early 70-th the area has been developing intensively and numerous other conjugated polymers were obtained. However, one should note that most of them were prepared with

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the use of electrochemical methods and the resulting materials corresponded to mixed conductors so that they will be discussed in the corresponding Section 2.1.3 below. In pure solid-state experiments with such polymers the change of their doping level is mostly performed by their treatment with oxidizing (like O2, I2…) or reducing (hydrazine…) agents (there may also be a “doping” owing to an acid-base equilibrium, e.g. in polyaniline) which can penetrate into the depth of the material owing to its porous structure. For example, the reaction between the neutral for of polyacetylene and oxygen leads to formation of positively charged electronic states of the polymer matrix whose charge is compensated by the counter-charge of trapped oxygen anions (the latter play the role of doping centers in usual inorganic semiconductors). These counterions do not demonstrate a significant mobility inside this material in the solid state and this feature allows one to attribute this system to a pure electronic conductor. As for the conductivity, it is low in the reduced state of the polymer matrix but it increases rapidly upon the polymer oxidation, reaching the levels comparable with the conductivities of typical metals. At high doping levels the temperature dependence and other features of the electronic conductivity testify in favor of a quasi-metallic character of the electronic transport. ESR studies of these materials reveal the presence of unpaired spins for relatively low doping levels which changes in parallel with the oxidation charge but the ESR signal vanishes at higher doping levels. These observations and their quantum-chemical interpretation represent a basis for the model attributing the ESR signal to “polarons” (electronic states of the polymer chain delocalized over several monomer units and occupied by a single electron) while mobile electronic species without spin to “bipolarons” (corresponding to the withdrawal of the second electron from the same orbital) [64–66] These terms originated from the theory of excessive electrons in polar media in which polaron meant a complex: electron + polarization of the polar medium around the electron induced by its electric field which created a well for the electron (“autolocalization”) [67–69]. Then, “bipolaron” corresponds to a structure where both electrons are trapped inside the same potential well created by the medium polarization. In solid-state conjugated polymers like polyacetylene there is no solvent or other polar medium around a polymer chain and the role of the interaction “electron-polarization” is played by the interaction between the electronic state and the deformations of the polymer molecule, see below. In chemistry, these names correspond to “cation radicals” and “dications” but the latter terms do not emphasize an essential role played by the vibrational degrees of freedom of the polymer molecule in formation of these electronic states.

2.1.3. Mixed (electron–ion) conductors 2.1.3.1. Electrochemical and chemical synthesis methods. The third type of conductivity is provided by polymer materials which possess simultaneously mobile charges of both electronic and ionic nature. The most important examples are provided chemically by the same types of polymers indicated as “electronic conductors”, in particular “redox” and “conjugated” polymers but in contact with an electrolyte solution. Owing to the porous character of the polymer matrix the electrolyte ions and solvent molecules penetrate easily into the depth of the material. Furthermore, the material may be swelled by the solvents between the polymer chains. This arrangement opens new possibilities of handling such systems, compared to the pure solid-state case. First, one can perform the synthesis of the polymer in the form of a film at the surface of any electrode (stable to oxidation up to the polymerization potential) by electrooxidation of the solute monomer. The deposition may be realized in various regimes: with cyclic voltammetry, potentiostatically [70], with imposition of a set of oxidative impulses [71]. The thickness of the film may be varied in wide limits (starting from several tens of nm to ensure a complete coverage of the electrode surface) simply by choosing a proper deposition charge. The upper limit of the film thickness depends on the conductivity of the material at the potential of the deposition: an insufficiently high conductivity leads to the increasing ohmic potential drop across the film and the wave of the monomer oxidation at its external surface is shifted progressively to higher potentials. It inhibits the polymer deposition above a

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certain range since the polymerization potential cannot be taken above a certain limit (specific for each monomer and the composition of the solution), otherwise the polymer becomes “overoxidized”, in particular it loses its conductivity and the deposition stops. Owing to these advantages this synthesis procedure is largely used for numerous conjugated polymers, in particular for PPy, PTh, PAni, PPP etc. as well as their derivatives 2b–2e, see Section 2.2.1.2 and part B. The mechanism of this electropolymerization represents a complicated process with numerous electron-transfer steps at the electrode or the film surface and chemical reactions of their solute products: monomer oxidation to a cation radical, then the coupling of two cation radicals, with a subsequent removal of two protons, a new electrooxidation step etc. The principal route among various parallel reactions depends on numerous factors, as the electrode potential/current, electrochemical regime, monomer concentration, main solvent and additions to it (e.g. water in acetonitrile), acidity of the medium etc. A detailed study of the reaction mechanism as a function of these factors as well as their effect on the properties of the deposited polymer was performed in the case of PPy and substituted PThs, see [72,73] and Refs. therein to preceding publications. The electrochemical synthesis gives the polymer in the form of a film at the electrode surface which is generally well-adherent for low thicknesses but may be easier detached for thicker layers. The procedure has got a serious disadvantage that such obtained films are generally insoluble and infusible which makes them intractable. If this aspect is of importance one should use an alternative route, via a chemical oxidation of the solute monomer by a reagent (e.g. FeCl3, CuCl2, persulfate anion, hydrogen peroxide, benzoyl peroxide…) [11,74–77]. It gives the polymer in the form of a powder which in advantageous cases, by the careful choice of the solution composition, can be dissolved in a (mostly, organic) solvent, see e.g. [78,79] for PPy). The polymer solution can be characterized by various techniques not available for solid films. Then, the polymer can be deposited at any (even non-conducting or corrosive) surface, e.g. by spin-coating. Another advantage of the chemical polymerization is the absence of limitations due to a low conductivity of the material. This point is of especial importance for “redox polymers” (Section 2.1.2.1) with their very low “hopping” conductivity. One should keep in mind that the films representing chemically the same polymer (e.g. PAni) and having the same thickness demonstrate generally rather different properties, e.g. elastic constants, redox response, color and spectral properties etc, in particular due to expected cross-linking inside the film obtained electrochemically. Properties of polymers obtained by the electrochemical or chemical method are strongly influenced by the composition of the monomer solution, e.g. the nature of solvent, pH in solution, monomer and electrolyte concentration and in particular by the nature of “background electrolyte” which in reality is not “indifferent” since its ions (mostly, counterions) are incorporated into the polymer structure. Thus, the change of the nature and the size of ions (especially, counterions) could be considered as the methodology of the polymer “structuring” on a molecular level. For electrochemically obtained films their properties may also be essentially varied by the choice of the deposition regime and its parameters. A specific variant of the chemical polymerization is to realize it with the use of an organometallic catalyst [80–82]. This method is broadly applied for the synthesis of another principal class of conjugated organic polymers which includes ethynylene units with a triple bond, as in polymer 2f, see numerous other examples in other sections below. The alternation of simple and triple bonds inside the polymer chain can also result in a significant electronic conductivity, similar to the polymers with simple and double bonds. 2.1.3.2. Chain length. One of the important parameters of polymer systems is their chain length. The information on this characteristic for conjugated polymers is very scarce. The principal reason is that these materials are mostly insoluble in common solvents and are decomposed chemically before the melting temperature. In the favorable case if the polymer may be dissolved (owing to a specially developed synthesis procedure or to side groups, e.g. long alkyl or alkoxy groups attached to the principal chain) one can apply the standard methods of polymer chemistry, in particular chromatography and MALDI-TOF MS techniques [83], to estimate the range of the chain lengths of polymer molecules or even to determine the distribution of lengths, to calculate the number and/or weight averages, the dispersion of lengths etc. One can point, as examples, to studies of the distribution of oligo(p-phenylenes) (with side alkoxy groups in 2 and 5 positions) up to about 20 monomer units obtained by polycondensation with the Suzuki coupling method [84] or of regioregular, head-to-tail coupled, poly(3-alkylthiophenes), synthesized by three organometallic-catalysis methods [85]. The latter analysis revealed a very wide dispersity of the polymer material (PDI factor being 1.74–1.94), as usual for these polymerization methods. This parameter may be strongly diminished with the use of the Soxhlet extraction with a series of organic solvents which dissolve polymer fractions with different characteristic chain lengths [86]. This treatment of poly(3alkylthiophene) materials allowed [85] to separate them into several fractional samples having much lower dispersity, their numberaverage chain lengths and PDI factors being from 12 (14.5) and 1.09 (1.09) for the fraction with the shortest molecules to 60 (160) and 1.29 (1.42) for the last fraction, according to MALDI-TOF (GPC) techniques, correspondingly. This way to segregate the polymer material into fractions with different molecular lengths was recently used [87] to apply the fraction with the longest chains (number-average length being estimated as above 50 monomer units) for fabrication of field-effect transistors.

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Conjugated polymers soluble in organic solvents are synthesized almost exclusively by organometallic catalysis (OC) or chemical oxidation (CO) methods. A very interesting exception is provided by alternate copolymer of fluorenone and dialkylbithiophene whose soluble samples were obtained with the use of both these traditional methods and the electrochemical (EC) deposition [88]. It allows the authors to characterize their mass/chain-length distributions with the use of size exclusion chromatography (SEC) and MS-ESI techniques. All methods led to materials with a very broad dispersity, polydispersity factor being 3.2 (OC), 6.3 (CO) and 2.2 (EC). The electrochemically obtained polymer, besides the lowest polydispersity, possessed the longest number-average chain length (in terms of monomer units), 4.0 (OC), 8.8 (CO) and 10.3 (EC). It means that they all represent a mixture of short-chain oligomers and high molecular weight fractions. The OC and CO materials were subject to the separation with the use of a series of solvents which gave several fractions with number-average chain lengths ranging from 1 to 3 monomer units up to 20 and even 70 (MS-ESI technique) [88]. The results on the molecular weight/chain-length distributions obtained both chromatographic and MS techniques should be treated with a care. The former is based on a styrene standard while the conformation of solute conjugated oligomers/ polymers may be different from randomly coiled styrene chains, e.g. the rod-type structure of the former will lead to a strong overestimation of their masses, with a factor up to 2.0–2.3, for relatively long chains [85,88] but for an underestimation for short chains [88]. The distribution profile with a high polydispersity measured with the MS techniques is not quite reliable, either, since the ionization efficiency generally vary with the chain length [84] (this factor should become less important with the diminution of the polydispersity factor by polymer fractionation). Another problem is a possibility of the fragmentation of molecules in the course of their ionization. A great advantage of MS technique is that they give absolute values for the masses of registered species which allows one to study not only the chain length but also the terminal atoms of the chain [85]. The whole approach is based on the assumption that the dissolution of the material leads to a molecularly distributed solutes, i.e. they do not form molecular aggregates or even colloidal solutions. An alternative approach to the estimation of the chain lengths of the conjugated polymers was proposed in Ref. [73]. It is based on the dependence of the wavenumber of the maximum in the UV–visible absorption spectrum on the chain length for a series of neutral oligopyrroles (with the number of pyrrole units from 2 to 7) [89]. Its application to the values found for two structurally different modifications of PPy, PPy I and PPy II, synthesized electrochemically in different conditions, allowed the authors [73] to estimate the chain lengths of these two polymers, 30–65 for PPy I and 8–12 for PPy II. It is interesting to note that the modification with much shorter chain lengths, PPy II, possessed a much higher electronic conductivity in the oxidized state, which was attributed [73] to its more regular structure leading to an easier electron delocalization over neighboring polymer molecules, owing to the formation of σbonds between them in the charged state. This method for estimation of the chain length has got a great advantage that it can be easily applied to numerous polymers. However, one has to keep in mind that it is based on an extrapolation of the empirical relation established for short-chain oligomers, to molecules with more extended chains, especially for PPy I. A potential danger of this procedure may be illustrated by the same dependence for substituted oligo(p-phenylene)s which was extended up to a much higher number of units, 17 [84] where one can obviously note a tendency to the flattening of the tendency, with a practical saturation above 11–12 units, the maximum position becoming identical to that for the polymer. Besides, the position of the absorption maximum for the same chain length may also be influenced by intermolecular interactions inside the solid polymer matrix, compared to the solute species. 2.1.3.3. Redox properties. Electroactive materials. If the film-coated electrode (obtained by means of any of the above procedures) is in contact with a monomer-free electrolyte solution the oxidation (doping) level of the polymer is controlled by the electrode potential. In the equilibrium state this level is uniform for the whole film and it is determined by two interfacial exchanges, with electrons at the electrode/film interface and with ions at the solution/film boundary [90]. The variation of the electrode potential is accompanied by the changes of both interfacial potential differences, in particular its change at the film/ solution boundary is generally not close to that of the overall potential. As a result, the phenomena sensitive to this characteristic of the system, e.g. kinetics of redox reactions of a solute species at this interface [91–93], demonstrate some features unusual for those at the bare electrode/solution boundary. This effect is related to the ion exchange at this boundary for the film-coated electrode and for its manifestation it is not important whether the electronic conductivity inside the film is of a “quasi-metallic” or a “hopping” type. If the electrode potential is shifted by a small value ΔE, the overall electronic charge transferred across the electrode/film boundary, ΔQ, is proportional to the potential difference: ΔQ = C ΔE, C being the (quasi-equilibrium) redox capacitance of the polymer film. The capacitance, C, is a function of the potential, E, and characterizes the redox activity of the polymer, vanishing in the potential range where the polymer is in the insulating state. On the contrary, the capacitance reaches very high values inside the “electroactivity range” where the polymer becomes electronically conducting. Even for films corresponding chemically to the same polymer and having the same number of monomer units inside the film, the dependence, C(E), and its amplitude may be quite different, depending on the synthesis procedure and the composition of the solution in which the redox activity is tested.

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The integration of the C(E) curve starting from the neutral (completely discharged) state of the film gives the dependence of the (excessive) electronic (or “redox”) charge of the polymer as a function of the electrode potential, Q(E). A typical feature of most solid conjugated polymer and oligomer films is a hysteresis of their redox properties which manifests itself in the different capacitance curves for the charging and discharging processes, Can(E) and Ccath(E), i.e. in a different behavior of the branches, Qan(E) and Qcath(E). The same (or close) capacitance curves for the charging and discharging processes may be found from the current, i, at the anodic and cathodic branches of the cyclic voltammetry (CV) curves, Cn(E) = ± in(E) / v, index n being an or cath. The measurement must be performed with a sufficiently slow scan rate, v, to allow the equilibrium for the exchange with electrons between the electrode and the film as well as with ions (compensating the variation of the electronic charge) between the solution and the film. The simple condition for the measured quantity to characterize (quasi-)equilibrium redox properties of the film which must be checked experimentally is the proportionality of the current (for each potential) to the scan rate. For higher scan rate or/and for thicker films the charging/discharging process becomes to be limited by the transport of this excessive charge across the film. For a film with homogeneous local properties both along the electrode surface and across the film, the theory expects the proportionality between the current and the square root of the scan rate, in analogy with the diffusioncontrolled faradaic process at the metal/solution interface. The potential sweep range in CV has to be limited to a certain limit, Emax (positive for p-doping and negative for n-doping) since beyond this range the film is subject to an irreversible change of its redox and related properties (called “degradation”, or “overoxidation” for p-doping) while the repetitive cycling within a lower-potential interval results in a stable voltammogram, with an approximate balance of the positive (doping) and negative (dedoping) charges, Qan(Emax) = Qcath(Emax) ≡ Qmax. This redox charge over the maximum large potential interval (without entering the range of irreversible degradation of the film), Qmax, is used to determine the “maximum charging degree/level”, x = Qmax / FNmon (Nmon, total number of monomer units inside the film), i.e. the excessive electronic charge per monomer unit at Emax. For most conjugated polymer and oligomer films the values of x are in the range of 0.25–0.3, i.e. one extra electronic charge per 4 to 3 monomer units (PPy, PTh, their substituents…). Generally, the introduction of redox-passive side substituents results in a diminution of the redox activity, C(E), for the same number of monomer units, i.e. of x. For example, even the methyl substitution at nitrogen of pyrrole leads to the loss of electroactivity of the polymer in the lower-potential range (about 400 mV wide). Nevertheless, one can reach about the same charging degree, 0.2–0.25, for poly(methylpyrrole) [70] or even for the film with a bulky metallocene substituent, poly(titanocene–propylene–pyrrole) [94], owing to a greater stability of these polymers at positive potentials, i.e. a higher Emax values. One should keep in mind that this parameter, “maximum charging level” x, is not defined quite unambiguously since the current does not vanish at Emax so that x depends essentially on the somewhat arbitrary choice of its value. Even for the same polymer this “safe potential limit” depends on experiment conditions, e.g. on the nature of the solvent and its purity. For example, the water content in organic solvents influences dramatically the overoxidation process and for sufficiently “dry conditions” one can reach a much higher level, 0.6 for PEDOT [95]. One can also observe a very stable redox response of PPy in propylene carbonate up to x = 0.5 and even to attain the charging level of one extra charge per each pyrrole ring, x = 1 [96]. The same limit, x = 1, was found for poly(4,4′-bimethoxybithiophene) [97]. In this case it was possible to register the C(E) curve even above this potential, to reveal that the capacitance vanishing, in accordance with theoretical predictions based on the assumption of a singly-occupied atomic orbital at each monomer unit contributing to the electronic states of excessive electrons. For two films treated in the identical manner but having different thicknesses the redox capacitance, C (for each potential E) is proportional to the thickness, i.e. to the number of the monomer units in the film. It is related again to the control of the oxidation level of each unit of the polymer by the electrode potential so that a certain electronic charge must be supplied to or withdrawn from the unit for the potential change between its two values. It means that the whole film is charged or discharged as a threedimensional phase. Therefore, this group of polymers represents an example of electroactive materials which include also such systems as mixed-valence inorganic solids, lithium–cation intercalation layers, “superionic conductors” with a noticeable electronic conductivity etc. In electroactive materials the bulk phase can be charged or discharged electronically. Since the total charge of any phase must rest zero the above additional electronic charge of this bulk medium, ΔQ, must be compensated by the exchange of the identical ionic charge across the film/solution boundary, this ionic charge being after it distributed across the film in the way uniform at the macroscopic scale. A combination of these properties, the ability of the constituent elements in the film to change their oxidation level, a mixed type of the conductivity inside the film and the exchange with electrons and ions at the interfaces (or at the same interface) is a necessary condition for a material to demonstrate its electroactive properties. These general features are independent of the particular type of the material with a mixed conductivity whose level may be varied by the electrode potential. The difference in properties of particular systems manifests itself in the ratio of the interfacial potential drops (which may be comparable with one another, or one of them may be dominant) or in the relation between the electronic and ionic conductivities of the material. For example, the charge transport in redox polymers (Section 2.1.2.1) well-filled with the solvent is usually limited by the slow electron hopping. The situation is more complicated in conjugated polymers since their electronic

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conductivity shows an extremely strong dependence on the charging level (i.e. on the electrode potential) so that it is frequently much greater than the ionic one, for sufficiently high (but not too high) charging levels, while the discharge of such polymers may reduce the electronic conductivity to a very low level, compared to the ionic one. 2.2. Structural aspects of conjugated polymers and related systems The number of synthesized chemically different conjugated polymers is enormous and they are based on quite different types of structures. As an illustration of this great variety some typical examples of such systems are given below, classified by their structural features. 2.2.1. Materials composed of identical building blocks 2.2.1.1. Linear-chain organic polymers. A very great number of “conducting polymers”/“synthetic metals” (with pure electronic or mixed conductivity) corresponds to polymers with a linear chain composed of a sequence of conjugated bonds, in particular the most well-known systems of this kind: given in 2a–2f, see books and reviews [31,75,98–104] for these polymers and other examples. The principal chain of the organic conjugated polymer is frequently formed by an alternating sequence of simple and double bonds 2a including often aromatic or heteroaromatic cycles 2b–2e. Polymers of another type include triple bonds inside ethynylene units 2f (other examples of this type are given in Section 2.2.2 amongst “copolymers”). An essential difference between these two classes is in their molecular geometry: the systems with triple bonds correspond to a linear “rigid rod” structure (even in their neutral state), owing to the sp1 hybridization of carbon atoms. On the contrary, in the neutral state of a polymer with simple and double bonds the π-electrons are mostly localized within a monomer unit so that the neighboring units can rotate around the sigma bond between them. The charging (“doping”) of this polymer by oxidation or reduction changes essentially the situation, by generating electronic states which are delocalized over several neighboring monomer units. This delocalization requires a parallel orientation of π-electrons in these units and correspondingly of the units themselves, e.g. to the “planification” of the polymers 2b–2e with aromatic or heteroaromatic rings. The size of the monomer units in linear organic chains may vary in very wide limits, from a few atoms as in polymers 2 to macrostructures as for electropolymerized metal-free porphyrins 3 [47,105–108].

The schematic structures 2b–2e correspond to polymers with ideally linear conjugated chains of C–C bonds. For many polymers with electron-conducting matrices of this type (in particular, those with heteroaromatic monomer units) this linear structure represents frequently an idealization since the neighboring chains may form links between them, e.g. in 3 or 4 positions of pyrrole or thiophene unites. One should keep in mind that materials with the same formal composition of the conjugated polymer (so that they are called by the same names as those obtained electrochemically) can be synthesized by “chemical polymerization” and “organometallic catalysis” (see below) and depending on their further treatment they may be a purely electronic or mixed conductors. Recently, the preparation of nanostructured conducting materials, in particular conjugated polymers, has become an important branch of material research, in view of their unusual chemical and physical properties. One of the most effective and frequently used methods to prepare nanostructured conducting polymers is the chemical oxidation of the monomer in a microemulsion medium (used as a template) which is created by surfactants. The latter represent frequently anions with an organic constituent or anionic polyelectrolyte (solute polymers with ionogenic groups), e.g. camphor

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sulfonic acid (CSA), p-toluene sulfonic acid (p-TSA), dodecyl benzene sulfonic acid (DBSA), polystyrene-sulfonic acid (PSS), which are incorporated into the polymer as its “dopants”, changing essentially the properties of the material, compared to the same polymer with small inorganic anions as countercharges. The microemulsion procedure applied to various conducting polymers, PPy, PAni, poly(3,4-ethylenedioxythiophene) (PEDOT)…, leads to these materials in the form of nanofibers, nanoparticles, nanotubes etc. [109–114]. Another route to nanostructured conjugated polymers is based on their synthesis inside pores of membrane or a solid (Section 2.2.3.3). Carbon-free conjugated chains of alternating N and P atoms have been synthesized [115,116], see examples of such systems with metallocene complexes in part B, Sections 3.6 and 3.7.1. Polymer chains can also be formed owing to interacting π-electrons of plane complexes (π-stacking), like TTF or porphyrins [117]. 2.2.1.2. Derivatized/functionalized polymers. A great number of new materials have been obtained on the basis of the above types of polymers with linear conjugated chains (or those quoted in sections below) by their derivatization. For example, longchain aliphatic substituents improve the solubility of the polymer in organic solvents and may reduce the oxidation potential of the monomer [118,119]. The latter effect may be even stronger for alkoxy derivatives [120] or owing to an additional ethylenedioxy or propylenedioxy ring added to thiophene [121,122]. The corresponding polymers, PEDOT and poly (propylenedioxythiophene), correspondingly, have been proposed for the use in actuators [123], electrochromic materials [124] and supercapacitors [125]. Conducting polymers functionalized with polyalkyl ether chains, crown ethers, and aza-crown ether moieties 4a–4c can be used in sophisticated sensor devices capable to detect a large variety of metal ions as well as neutral and charged organic species [18,126–128].

Derivatized monomers are frequently asymmetrical with respect to their terminal atoms/groups, e.g. pyrrole or thiophene functionalized in 3-position. Their polymerization (in particular electrochemical) of such monomers results generally in an irregular distribution of head or tail substitutions. The preparation of regioregular oligomers or polymers requires special synthetic procedures [129–131]. The incorporation of fullerenes into polymers can strongly modify their electronic, magnetic and optical properties, forming novel promising materials such as polymer photovoltaic and light-emitting devices. [132–136]. There are two main types of fullerene/polymer arrangements: 1) “charm bracelet” (or “cherry tree branch”) structure contains fullerene moieties covalently attached to the side chain of the polymer as pendant substituents 5ab and 2) “pearl necklace” structure contains fullerenes inside the main polymer chain, see 15 in Section 2.2.2.3 for copolymer materials with mixed chains. The materials of the former type can be prepared either via fullerene reaction with already formed polymer matrix [137–139], or by chemical or electrochemical polymerization of a monomer with attached pendant fullerene moiety [140–142].

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2.2.1.3. Conjugated metallopolymers. Incorporation of transition metal complexes into conjugated polymers represents actually a very rapidly developing area. Metal-functionalized conducting polymers are fascinating materials for various types of applications. A combination of a binding (chelate or bipyridyl) and a redox-active (e.g. Fc or Ru bipyridyl complex) groups attached to the conducting polymer may serve for sensor applications [128,143–147] and in non-linear optical devices [148,149]. Linkage of an organometallic complex to a conducting polymer matrix represents a route to immobilized molecular catalysts, see examples with metallocenes in part B below. One can distinguish several types of metal-containing conjugated polymers. First, one can tether metal complexes to the conjugated polymer matrix with an inert linker (e.g. alkyl group or chain with carboxamide bond) [102,104]. Such polymers with pendant functional groups can be synthesized applying both electrochemical and chemical/organometallic (such as condensation [150] or ring opening metathesis (ROMP) [151]) methods. Examples include various Fc and other metallocenes (see numerous examples in part B), metalloporphyrin 6a [152–157], cyclam 6b [158,159], rotaxane [160], bipyridyl 6c [161– 163] and phenanthroline [164] complexes etc. attached to the conducting polymer backbone. Since both components of such metal-containing conducting polymer systems are usually electroactive, they can be electrochemically processed and characterized.

In another group of polymers the metal is directly bonded to the principal chain of the polymer or it is attached to it via a conjugated linker. It results in an electronic interaction/communication between the polymer backbone and the metal, as well as between neighboring metal centers. As in the case of redox polymers with metal-containing substituents (Section 2.1.2.1), the incorporation of the metal may be performed before the polymerization stage (in the course of the monomer synthesis or after it) or into the polymer with a metal-free ligand. One of the ways to such systems is to incorporate a conjugated ligand into the polymer chain so that metal ion can be coordinated to this ligand before or after the polymerization, as e.g. polymers with bipyridine, mixed benzimidazole–pyridine 7 or benzimidazole–pyrazine units which are able to complex transition metal (Ru, Os…) [165,166].

In systems of another type the thiophene ring is functionalized in 3 and 4 positions with functional groups able to coordinate (in the monomer or polymer form) various metal ions, giving a M(salen)-type structure 8a [167–169]. This monomer can be electropolymerized, unlike its derivatives with methyl groups in p- and o-positions of both phenylene rings. In other structures with the metal complexes of the same type the thiophene unit is replaced by a terthiophene one 8bc [167–169], with or without substituents in p- and o-positions of Ph rings. This modification should reduce a danger of steric restrictions which may be expected for a homopolymer with the PTh chain on the basis of monomer 8a. Besides, it should

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diminish a tendency of the corresponding monomers to other routes of polymerization (see discussion below), leading to the corresponding polymer with a PTh chain 8bc, contrary to monomer 8a [167,168].

Systems 8a–8c are closely related to a class of conjugated metallopolymers 8d based on transition metal complexes with Schiff base ligands (salens) derived from salicylaldehyde and various substituted ethylenediamines, N, N'-ethylene-bis(salicylidenimine). Similar to monomers 8a–8c, these species can be oxidatively polymerized in weak donor solvents at an electrode surface. They have been the source of much interest since they exhibit high conductivity, electrochromic behavior, selective catalytic activity in various heterogeneous reactions (including electrocatalysis), that makes them suitable for numerous applications [167–177]. Despite intensive studies of several teams over the last two decades, the mechanism of polymerization, the polymer structure, the type of conductivity and their mechanism of the redox activity still remain a matter of controversy. It was suggested that the polymer chain passes through the set of conjugated bonds inside the salen ligand and the metal, the neighboring monomer units of the polymer being linked via para- or ortho-positions of their side phenyl moieties (“ligand-based polymerization process”, so that the electron transport is realized owing to the polaronic-type conductivity along the chain [178–183]. An alternative polymerization mechanism might be a metal-based polymerization process, in which the metal is first oxidized, after which the oxidized species could form a stacked deposit stabilized by the charge transfer. Then, the charge transport should take place along polymer stacks via electron hopping between adjacent metal centers assisted by the system of conjugated π-bonds in the ligand environment (redoxtype conductivity) [184–188]. At more positive potentials these polymers can be overoxidized. It leads to C–C bond formation between stacks, giving rise to a more compact (reticulated) film and resulting in a combined (both redox and polaronic) type of conductivity [184–188]. A different type of electroactive materials is presented by polymer chains 9, in which the central transition metal ions of the neighboring complexes are directly bonded [189–192].

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The principal chain in numerous conjugated “metallopolymers” is composed of an alternating sequence of transition metal complexes (in which the chain may pass through the metal itself or only via its ligand) and various organic units. Examples of this kind are given in Section 2.2.2.3 on polymers with a matrix of mixed type, as well as in part B for metallocene-containing systems. 2.2.1.4. Multi-dimensional structures: ladder polymers, dendrimers…. Structure defects have immense influence on many properties of polymers, such as electrical and photoconductivity, thermal stability, photo- and electroluminescence, etc. For example, distortion and twisting of the conjugated main chain induced by voluminous substituents (the presence of which is necessary e.g. to increase the solubility of polymers), results in a drop of the conjugative interaction and thus, leads to drastic changes in the physical properties of the material [193,194]. To overcome these problems, new polymers have been synthesized by incorporation of a π-conjugated backbone into rigid and planar structures of Ladder-type polymers containing additionally alkyl or aryl groups, to provide a high level of processability and solubility. Ladder-type polymers are a class of two-dimensional (2D) materials which are intermediate between linear systems such as rigid rod conjugated polymers and three-dimensional (3D) structures as dendrimers. Ladder-type poly(p-phenylene)s (LPPPs) [195] with different functional groups 10a–10c such as carbazole units [196–198] and polyacene ladder polymers [199,200] demonstrate excellent optoelectronic properties and a very low amount of defects.

Two-dimensional (2D) and three-dimensional (3D) defect-free charge transport could be realized in the networks containing conjugated stacked fragments with optimal overlapping of π-systems on the basis of polycyclic aromatic hydrocarbons 11ab (PAHs) [201,202] and dendrimers derived from phenylenevinylene [203], phenylacetylene [204], thiophene [205] and polyphenylene [206–209]. Terthiophenefunctionalized polyphenylene dendrimer 12 [209] can serve as a monomer unit, giving conducting polymers upon electrooxidation.

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2D PAHs 11 show promises as discotic liquid crystals with high charge carrier mobilities along the stacking. The 3D dendrimers can be successfully used as substrates for the attachment of a wide variety of functional groups, including catalytic and biologicallyactive species and thus have good perspectives in sensor, drug delivery systems, catalytic and optoelectronic devices.

2.2.2. Copolymers with a compose matrix Conjugated monomer units (Section 2.2.1) can be mixed within the polymer chain or combined with other building elements, e.g. transition metal complexes, fullerenes etc. Some systems of this kind are presented below. Numerous other examples may be found in part B on polymers with metallocene complexes incorporated into the principal chain of the polymer, or polymers with different monomer units (copolymers). There are two principal constructions of the chain from two (or a greater number of) building blocks in a regular manner: 1) a periodical alteration of their units, A and B, along the chain, as ABABAB… or ABBABBABB…, 2) a polymer composed of longchain oligomers of both types, as (A)n(B)m(A)n(B)m…with great values of n and m (“block copolymers”). Some examples are given in the next sections as well as in part B for metallocene-containing polymers. One should note that copolymers are prepared frequently by (electro- or chemical) polymerization in the mixture of solute monomers. Such a procedure can lead generally to various types of materials, from one of the above regular structures to an irregular sequence of monomer units inside the polymer chain, or to a mixture of corresponding homopolymers, with the interpenetration of their matrices or with a spatial separation of their phases. The fractions of constituent monomer units inside the polymer phase may be quite different from that in the starting mixture of monomers, up to a predominance of a single (easily polymerizable) component.

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2.2.2.1. Linear-chain organic copolymers with alternating units. Formally, such systems with a periodical sequence of different monomer units, e.g. (BAB)n, can be considered either as copolymers of A and B, or as homopolymers with a combined unit, BAB. It is probably worth to use the term “copolymer” for systems with such monomer units which are able to give separately (conjugated) homopolymers, (A)n and (B)n. In such copolymers with conjugated monomeric units the boundary electronic orbitals in the charged states are generally delocalized over several ABA or AB fragments. A route to such regular copolymers is to synthesize first a combined monomer in which the constituent units (e.g. A and B) are already present and to homopolymerize it. Generally, this combined monomer, e.g. AB, is not symmetrical so that a simultaneous activation of both terminal groups (with the possibility to form all types of bonds, A–A, A–B and B–B) may lead to an irregular coupling, giving for example sequences like ABBABA. The simplest way to overcome this problem, i.e. to ensure a regular sequence of the constituents inside the copolymer is to use a mixed monomer with identical terminal units on both sides (A), A–B–A, which leads automatically to a regular chain: –(ABA)n– 13ab, for any (chemical or electrochemical) polymerization procedure. To synthesize a copolymer with equal numbers of units A and B, –(AB)n– 13cd, one has to prevent head-to-head (A–A) and tail-to-tail (B–B) coupling by the proper choice of the terminal groups in A and B or/and of the catalyst. This goal has been achieved with the use of various organometallic polycondensation routes, leading to copolymers (and also for homopolymers with asymmetrical building units) with perfectly controlled sequence (and “head” or “tail” orientation) of different units inside the chain and its regioregularity, contrary to randomly structured copolymers obtained via electro- or usual chemical (oxidative) polymerization [210]. Numerous examples of both types, –(ABA)n– and –(AB)n–, may be found in part B.

Copolymers with a regular or chaotic sequences of monomeric units inside the chain (obtained by electrochemical or chemical methods) demonstrate frequently new physical and chemical properties, compared to the corresponding homopolymers [211–215]. For example, conjugated copolymers combining thiophene with quinoxaline, pyridine 13ac or benzimidazole unites and their derivatives demonstrate a better solubility and modified optical properties [216–218]. Copolymers containing alkylated bithiophene and electroactive azulene in the polymer backbone synthesized by chemical oxidative polymerization, have enhanced solubility in common organic solvents, possess good thermal and environmental stability and high conductivity (1–50 S cm− 1) via p-doping or protonation processes [210]. 2.2.2.2. Linear-chain block copolymers. In this kind of linear-chain systems with different building blocks, the principal chain represents an alteration of two long-chain oligomers or even polymers, Am and Bk, giving rise a “diblock copolymer”, –(Am–Bk)n– 14a or a “triblock copolymer”, –(Am–Bk–Am)n– 14b. In such copolymers with conjugated monomeric units, or if only one of the polymer components is conjugated, the boundary electronic orbitals in the charged states upon lower doping levels are confined to one of the oligomer or polymer constituents with a lower redox potential while the generation of mobile charge species in another constituent may only start upon higher doping levels. Block conjugated copolymers comprised of two or more homooligomer or homopolymer subunits linked by covalent bonds (sometimes, via intermediate non-repeating subunit or junction block) are expected to exhibit novel electronic and optoelectronic properties not found in conjugated homopolymers and linear-chain random or alternating copolymers [210,219–221]. These effects originating from the spatial restrictions of electronic states to a single polymer constituent may lead to quantum confinement/quantum well phenomena [222,223]. If the interactions between identical fragments, A with A and B with B, are energetically more favorable than the crossinteractions, A with B, the block copolymers (being uniform at a macroscopic scale) may demonstrate a micro- (or nano-) phase

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separation while the covalent bonding between different blocks prevents the system from the phases separated at the macroscale. It leads to specific electronic, absorption, luminescent, mechanical properties [224,225]. The presence of long hydrophobic side chains in monomer units results to a liquid crystal behavior at the local scale, with an interplay between mesophase formation and the microphase structure [226]. The rod-coil copolymers composed of diblock [224,225] or triblock [222] chains which combine conjugated rigid rod-like segments with inactive coil-like segments represent prospective materials as they provide novel ways of probing the influence of the distribution of π-conjugation lengths, supramolecular order, effects of spatial confinement of chromophores etc. on the optoelectronic properties of the luminescent rods.

2.2.2.3. Linear copolymers combining organic units with building blocks of another nature. Besides copolymers with conjugated organic units, a great number of such systems include both organic units and various functional groups of a different type. For example, fullerene units can be incorporated into the conjugated polymer chain 15.

Copolymer chains can also combine organic units with non-carbon conjugated elements, e.g. aza group, see examples in part B, Section 3.6. Numerous copolymers contain organometallic or coordination complexes, with the principal chain passing via the transition metal ion of the complex. In other copolymers the transition metal ion is coordinated to a conjugated ligand inside the polymer chain, which ensures an electronic exchange between the metal and the chain. These structures are realized e.g. in copolymers.with 3,4ethylenedioxythiophene (EDOT) units and Shiff base metal complexes, as Co(salen) 16a [227], or with a regular sequence of 2,2′bipyridine units (to which the Ru-bis(bipyridyl) complex can be coordinated before or after the polymerization) and thiophene tetramer blocks 16b [228,229] or diazabutadiene units [230].

Another group of “metallopolymers” 17 combines complexes of late transition metals (Pt, Au, Hg) with ethynylene-aryleneethynylene units, in which arylene is thiophene, bithiazole, fluorene, carbazole etc. or their alkyl, carbonyl, phenyl or other derivatives, R = Bu or Et. The corresponding dimeric species representing two metal complexes linked by a conjugated bridge were also studied, see review paper [231] and references therein.

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2.2.3. Hybrid materials based on conjugated polymers This section presents examples of polymer systems containing molecules of two (or more) different compositions or consisting of different phases, see also review papers [11,232–234]. These materials may generally be heterogeneous even at the macroscopic scale. There are various types of such hybrid materials in which a conjugated polymer matrix coexists with another polymer of a different nature, e.g. a non-conducting polymer which may be composed of organic or inorganic building blocks. Another type is presented by conjugated polymers with incorporated aggregates of a micro- or nanosize, e.g. inorganic clusters. Alternatively, the conjugated polymer may be grown inside pores of a non-conducting medium (porous solids, membranes etc). 2.2.3.1. Mixture of conducting and inert polymers. The components of these systems may be segregated so that the material represents a mixture of two (or more) different phases corresponding to pure polymers. An alternative type is given by materials with spatially interpenetrating matrices which may be macroscopically homogeneous if both polymer components are distributed in a uniform manner at the supramolecular scale. An example of such “macro-homogeneous” system is provided by conjugated (electron-conducting) polymers obtained by electrochemical or chemical polymerization in the presence of a polyelectrolyte, i.e. a solute inert (redox-inactive) polymer with ionogenic groups, e.g. polystyrene sulfonate (PSS) [235–238]. In such materials the conducting component may ensure a proper electronic conductivity and other functionalities (luminescent, electrochromic etc). The addition of the second polymer may be used to improve e.g. mechanical properties. It may also serve to modify the ion-exchange properties, e.g. in above example of combinations conjugated polymer–PSS: a high negative charge of sulfonate groups inside these hybrid materials leads to the alteration of the exchanged ion from anion to cation, e.g. Li+. As a further extension the polymer with an inert matrix may be functionalized, e.g. by attachment of redox-active group. Then, the presence of a conjugated matrix will be to enhance the electronic conductivity of the material, to ensure the exchange of the charge between the electrode and redox centers inside the material. An example of this kind is discussed in part B, Section 3.1, a material with the PPy matrix and poly(vinylferrocenesulfonate) or poly(styrenesulfonate-co-vinylferrocene) incorporated in the course of electropolymerization. 2.2.3.2. Conducting polymers grown inside inert matrix. Template polymerization of conjugated polymers (PPy, PEDOT, PAni…) inside pores of a matrix (zeolites, porous alumina, track-etched membranes, TiO2, CdPS3, NiPS3, MoO3 etc) allows one to produce nanostructured polymer materials [239–245]. Besides their immediate applications, e.g. in electronic and optoelectronic devices, photovoltaic solar cells, nanocomposite and hybrid solar cells, energy storage and conversion devices (batteries, fuel cells and supercapacitors, rechargeable batteries, fuel cells), electrocatalysis, sensors, membranes, biomaterials [246,247], such systems may represent a source of nanostructured conjugated polymers, e.g. nanowires for field-emission cells [248]. 2.2.3.3. Conducting polymers with incorporated micro- and nanoelements. Similar to anionic polyelectrolytes, the incorporation of heteropolyanions (in particular, polyoxometallates) inside the conjugated polymer film can be achieved by electro- or chemical polymerization in the presence of these ions in solution [249–252]. Owing to their relatively great size these species are trapped by the polymer matrix. The rate of their release to the solution in the reduced state of the polymer (if any) depends on the compactness of its matrix. Numerous publications were dealing with composite materials composed of a conjugated polymer and incorporated micro- or nanoclusters of transition metals: Pt–PPy [253–260], Pt–PAni [261–264], Pt–poly(dithiafulvene) [265], Pd–PPy [266–269], Pd–Pt–PPy [163,260,258], Pd–PEDOT [270], Pd–poly(dimethyl-aniline) [271], Pd–poly(dithiafulvene) [265], Rh–PPy [260], Ni–PPy [272], Ni– PAni [273]], Pt–Ru–PAni and Pt–Ru–Mo–PAni [262], Ru–PPy and Rh–PPy [163], Cu–PPy [274,275], Cu–PEDOT [276], Cu–PAni [277–280], Cu–PEDOT [281,282], Ag–PPy, Ag–PEDOT, Ag–poly(3-methylthiophene), Ag–polybithiophene [283–287], Ag–PAni [259,275,263,264,288–291], Au–PPy [257,258,260,275,292,293], Au–PAni [263,264,291,294,295], Au–Cu–PAni [295], Au–PEDOT, Au–poly(3-methylthiophene) [259,296], Au–poly(dithiafulvene) [265], see also review paper [297]. Carbon nanoparticles and multiwall tubes were incorporated into a PPy film owing to their negative surface charge related to a strong adsorption of polyoxometallate anions [298]. A ternary system, sulfonated PAni + multiwall carbon nanotubes + clusters of Fe, Pd or Fe–Pd alloy, were synthesized by irradiation [299]. Semiconductor and insulator nanoparticles (or matrices, Section 2.2.3.2) have also been used: silica (colloid or gel) [300–303], montmorillonite [304], TiO2 [305–308], Fe3O4 [309–311], WO3 [312,313], Prussian blue [314] or CdTe [292] in/with PPy or PPy copolymers, Cu(I) oxide/chloride in poly(3-methylthiophene) [315], TiO2, SiO2 and Al2O3 in PTh [316,317], SiO2 [318], montmorillonite [319,320], TiO2 [321,322], V2O5 [323,324], Fe oxide [325] or Ru(Mo)Se [326] in PAni. The choice of polymers in these studies, PPy, PAni or PEDOT, was determined principally by their compatibility with aqueous solutions, the use of organic solvents was rare. Various approaches to the synthesis of these systems were applied: decomposition of a solute precursor at the electrode coated by the polymer film [163,255,256,259,261,264,268,270,277–282,272,274,284,288,294,296,315], (mostly, by its electroreduction, an electroless deposition has also been applied [264,272,284]) or by the polymer powder/colloid/solution [260,265,286], “one-pot synthesis” of the composite film/powder from the solution of the monomer and the precursor [257,260,263,269,271,285,287,291,308–310], “layerby-layer deposition” [273,298,324], incorporation into the depositing polymer film, or deposition on its surface, of pre-synthesized and solubilized nanocolloids [254–257,259,266,267,290,292,293,295,300–307,311–314,316–322,325,326].

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3. Part B. Conjugated polymers containing metallocene or “hemi-metallocene” (monocyclopentadienyl) complexes Numerous publications have been published in this area, see references below as well as the corresponding chapters in review papers [80,82,102,104,210,327–332]. The term “metallocene” is used for transition metal complexes with two negatively-charged aromatic cyclopentadienyl ligands, C5H5− (Cp), so that their alternative names are e.g. bis(cyclopentadienyl)iron for ferrocene (Fc) 18a. The central metal ion in two most well-known representatives of this family, Fc and cobaltocene complexes, is bonded only with two Cp ligands which are parallel to each other. If the number of ligands required by the central metal is greater the additional ligands force the planes of the Cp ones to become non-parallel, e.g. complexes with 4 ligands form a distorted tetrahedron, like in those of “early transition metals”, see e.g. titanocene dichloride below 18b. The term “metallocene” is also used for complexes with two Cp⁎ (or one Cp and another Cp⁎) ligands where Cp⁎ is a cyclopentadienyl ligand with one or several (up to 5) substituents, mostly with methyl groups. Besides metallocenes, there is another group of relative systems called “hemi-metallocenes” containing a single Cp ligand around the metal, e.g. cyclopentadienyl-Mn-tricarbonyl, CpMn(CO)3 18c. An enormous information on the properties and applications of metallocene and hemi-metallocene complexes of various transition metals may be found e.g. in review books [333]. One of the chapters of volume 2 [334] is devoted to various polymers (mostly, with a non-conjugated chain) containing metallocenes.

The incorporation of metallocene complexes into a polymer with an electron-conducting matrix can be realized with the use of different strategies: entrapping of a metallocene-containing species inside the polymer matrix or its attachment to the matrix via covalent or non-covalent bond. The latter can be performed on the level of the monomer (“pre-functionalization”) or of the polymer (“post-functionalization”). The application of these approaches is illustrated below first for systems with a Fc group. The number of systems with other metallocenes is much smaller and in all cases the covalent bonding of the complex to the monomer was performed. 3.1. Entrapping of Fc-containing species inside polymer film/powder The principal route to incorporate a species inside the polymer without forming a bond with the matrix and to retain it there, even in contact with a new (species-free) solution, is to entrap it in the course of the polymer deposition on the electrode surface. For polymers synthesized chemically the species can be trapped by the precipitating particles or (for polymer film casting from its solution by the solvent evaporation) by the growing film. The easiest and efficient way for conjugated polymers is to charge the species negatively so that it is included into the film as counterions to compensate the positive electronic charge of the polymer matrix. This strategy has been applied successfully mostly for non-substituted polypyrrole (PPy) to incorporate the anionic species containing Fc, e.g. its mono- or disulfonate derivatives (19, R = SO3H) [335] or ferrocenic alkyl sulfonate (19, R = (CH2)nSO3H) [336]. Similar attempt to polymerize polybithiophene with the same anions was less successful: only thin films were obtained, because of their low electronic conductivity [335].

The Pt electrode coated with a PPy film obtained by electropolymerization in the presence of ferrocene sulfonate (19, R = SO3H) demonstrated catalytic properties with respect of hydrogen evolution in strong acid solutions [337]. PPy films doped with another water-soluble Fc derivative, p-ferrocenyl benzene sulfonic acid (19, R = PhSO3H), were obtained both by the chemical and electrochemical polymerization [338]. The former procedure leads to a ferromagnetic conductive PPy powder while the latter provided both surface-adjacent and free-standing conducting polymer films, with prospects for battery or membrane electrode applications. The electropolymerization of a PPy film in the presence of the ferrocene monocarboxylic acid (19, R = COOH) and the cholesterol oxidase [339] resulted in their co-entrapping inside the PPy matrix. This system was used as a cholesterol amperometric biosensor, with the Fc derivative playing the role of an electron-transfer mediator which enhanced the sensitivity and selectivity of the biosensor without modifying the dynamic parameters of the response.

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The same procedure of incorporation of negatively-charged species into the film in the course of its deposition was used for other conjugated polymers, in particular, for PAni synthesized in the presence of ferrocene phosphonic acid (19, R = SO3H) [340] or of ferrocene sulfonic acid [341], with their application for the electrocatalytic oxidation of gallic acid or catechol, respectively. Another conjugated polymer, poly-1-naphthylamine doped with ferrocene sulfonic acid (19, R = SO3H) [342] showed an excellent electrocatalytic activity for the nitrite oxidation, for its sensor application. In the case of a positively charged species (e.g. Fc with an amine group in acid media or ferrocenium cation itself) they should be entrapped in the course of the film deposition if the polymer matrix is charged negatively. It may be achieved by attaching to it negatively-charged ionogenic groups. It is interesting to note that some incorporation of the Fc+ cation has been observed [343] even for polymers without such charged groups, poly(3-methylthiophene) and poly(1-methylpyrrole). A different procedure was proposed for incorporation of the neutral species, e.g. Fc itself [344]. Conjugated polymers obtained by the monomer oxidation with an Fe(III) salt undergo shrinking when the generated polymer is washed to remove iron cations and excessive counterions. This phenomenon can be used to incorporate desirable molecules into the polymer matrix by adding them to the washing liquid. The prospects of this treatment were demonstrated for PPy, poly(terthiophene) and PEDOT by incorporation of Fc into these polymer films. The principal shortcoming of the approach based on entrapping of counterions is the electrostatic nature of their attachment to the film. Therefore, when the matrix is discharged, e.g. during the potential scan, the species can leave the film for the solution. A way to prevent this process is to increase the size of species in order to freeze them inside the matrix. The use of a negatively-charged inert polymer should ensure a reliable trapping, as it was demonstrated for Fc attached to poly(vinylferrocenesulfonate) [335] or to poly (styrenesulfonate-co-vinylferrocene) [345]. Such Fc-containing polymer films were proposed for the battery applications. Electroactive groups can also be inserted into the film by their linkage to bulky species (to avoid their subsequent leakage to solution) which are incorporated into the film in the course of polymerization. This attachment can be performed either before or after the film generation. For example, solute Fc-modified enzyme (glucose oxidase) linked together by a long and flexible chain was entrapped into an electrochemically deposited conducting polymer film (PPy) [346]. The conducting polymer backbone and Fc groups ensured an electric connection between the enzyme's reactive site and the electrode. The system was proposed for the development of non-leaking amperometric enzyme electrodes. The alternative approach (post-functionalization) was applied by realizing the PPy film deposition in the presence of sulfonated calixarenes, calix [4]-p-tetrasulfonate 20a and calix [6]-p-hexasulfonate 20b, as dopants [347]. Then, these voluminous cage molecules immobilized irreversibly inside the polymer matrix were used as complexation centers for the guest trimethyl (ferrocenylmethyl)ammonium cation 20c from solution which retained their redox activity.

3.2. Covalent bonding of Fc complex to pyrrole monomer/polymer The bonding of Fc complex was realized either on the stage of the monomer (pre-functionalization) or to the PPy matrix (postfunctionalization). The former approach allows to ensuring a very high concentration of Fc centers inside the film but only in the case if the polymerization of this pure functionalized pyrrole monomer is successful while such attempts were unsuccessful for many monomers. The post-functionalization approach makes this problem less acute owing to the possibility of a wide choice among the

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pyrrole-based monomers with an active functional group for the further attachment process. However, the structure of thus obtained polymer is adjusted to this primary functional group, without an extra space needed for a generally more bulky metallocene substituent. Besides, not all (even most of) these reactive groups are available for reactants from solution. As a result, the postfunctionalization takes only place for a fraction of the monomer units, first of all at the polymer/solution interface (including the surface of pores with sufficiently great diameters). 3.2.1. Fc derivatives linked to Py in N-position The hydrogen atom at nitrogen in the pyrrole ring is the most reactive so that it can be relatively easily substituted by a chain with attached Fc or with a reactive group (OH, COOH, Br etc) which can be used for a further functionalization. Three types of the chains between the Py and Fc groups have been mostly used: 1) a purely aliphatic chain, (CH2)n, with various numbers of methylene units, 2) a chain containing a carboxamide bond: (CH2)n–CO–NH–(CH2)m, with (or without) aliphatic subchains, 3) a chain containing a positively charged unit (e.g. substituted ammonium). 3.2.1.1. Aliphatic bond to polymer. Several monomers 21 in which the Fc complex was linked to nitrogen in pyrrole by an aliphatic chain have been synthesized with various chain lengths, n = 2 [348], 4 [349], 6 [350] and 3 [351]. The synthesis routes were different, e.g. reactions of pyrrole with Fc–(CH2)6Br and NaH [350] or of N–Py–(CH2)3Br with FcSnBu3 and BuLi [351].

The electrochemical oxidation of the Fc group inside these monomers in solution takes place near the potential of the free Fc oxidation [351]. The oxidation of the pyrrole ring occurs at a more positive potential. However, for most of these monomers [348,349,351] no polymerization was achieved, conducting polymer films were only deposited for the mixed pyrrole-functionalized monomer solutions. The only successful homopolymerization has been reported for the monomer with the longest aliphatic chain, n = 6 [350]. The reason is not clear. Structurally similar complex of titanocene dichloride (see Section 3.7.4 below) linked to N–Py via an aliphatic chain with three CH2 units can be homopolymerized, despite its even greater size than that of Fc. The electronic interaction between the Fc and Py groups along the chain is practically absent. The only visible hypothesis is an electrostatic interaction between the positively charged ferrocenium group and pyrrole which may affect both the pyrrole oxidation and the subsequent chemical steps, e.g. to induce the deprotonation of the pyrrole cation radical before the dimerization step, which should prevent the formation of conjugated polymer chains, as it was observed for the non-substituted pyrrole [352]. The explanation is in conformity with the above observation that all short-chain monomers do not polymerize. All homo- and copolymers demonstrate a well-visible response of immobilized Fc complexes whose potential is close to those for the monomers [350,351]. The response of the matrix has also been reported for the homopolymer [350], with the expected ratio (1:4) of the total oxidation charges of the matrix and of Fc centers, in conformity with the charging degree of the polymer being about 0.25. The apparent electron-exchange rate constant between the neighboring Fc centers (realized via the polymer matrix) was determined [350]. The copolymer with n = 3 [351] was used for quantitative sensing and determination of the redox-active enzyme cytochrome C in solution. Monomer (NPy–Fc-phane) 22 in which a ferrocenophane (Fc-phane) complex was attached to nitrogen in pyrrole via an aliphatic chain [353] was synthesized by reaction of 1-(3-bromopropyl)pyrrole and the anion (deprotonated Fc-phane) obtained from Fcphane and BuLi. The presence of two spatially-close Fc units results in a couple of redox peaks (both for oxidation and reduction) with the separation of about 200 mV, due to the interaction between the Fc units, similar to the free Fc-phane complex. Similar to the mono-Fc derivatives of pyrrole, the Fc-phane containing monomer 22 did not give a growing conducting polymer film while its copolymer with pyrrole was readily generated electrochemically. Elemental analysis of the copolymer deposited from the solution with equal concentrations of Py and NPy–Fc-phane showed a dominance of non-substituted units, 10 Py to 1 NPy–Fc-phane. In conformity with this ratio the redox peaks of immobilized Fc-phane (whose shape was similar to that of the free monomer in solution) are only weakly visible above the response of the PPy matrix. SEM images display a jellyfish-like morphology of its surface, with a high macroporosity.

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3.2.1.2. CO–N bond to polymer. Besides polymers in which Fc was attached by a purely aliphatic chain there are also numerous publications on the systems where the PPy matrix was functionalized with Fc centers via a chain with a CO–N bond, mostly of the carboxamide type. Both the pre- and post-functionalization approaches have been used. Historically, the first conjugated polymer 23 with attached Fc complex was based on the CO–N bond. It was obtained [354] with the use of the post-functionalization procedure in which the non-substituted PPy-BF4− film was treated with ferrocene carbonyl chloride in acetonitrile.

In accordance with expectations for this type of attachment, only pyrrole-monomer units of the polymer available for the approach of the solute reactants could be functionalized. As a result, the redox response of the immobilized Fc complexes is on the level of the current related to the redox transformation of the matrix so that the former charge is much lower than that of the PPy matrix while the ratio should be opposite in the case if all pyrrole units were functionalized. The polymers were obtained by electrooxidation of monomers, mostly in organic (acetonitrile etc) solvents, and the studies of redox properties of these monomers and polymers were also performed mostly in these organic media. However, a few attempts to use aqueous solutions, either for the polymerization or for the polymer characterization, have also been made. A single example of the study in which both the electropolymerization and the further characterization of the polymer were performed in an aqueous medium [355] was related to construction of an amperometric redox-enzyme (glucose oxidase) electrode for the determination of glucose. The enzyme was entrapped in the course of the deposition of the polymer film. This step was realized cyclic voltammetry in mixed aqueous solutions of pyrrole and one of the functionalized Fc–pyrrole monomers added as dissolved in a small amount of acetonitrile (Scheme 24, without a substituent R or with R = (CH2)5–CO–NH–(CH2)3)).

The monomers were synthesized from the corresponding acids, CpFeCp–COOH or CpFeCp–CO–NH–(CH2)5–COOH, and the amine, H2N–(CH2)3–NC4H4 (Scheme 25) These Fc–pyrrole polymers were found to be efficient oxidants of reduced glucose oxidase. In most of the other papers of this group the functionalized monomer was obtained by the reaction between ferrocene carbonyl chloride (to link each Fc complex to a single pyrrole ring) or 1,1-ferrocene dicarbonyl chloride (to link each Fc complex to two pyrrole rings) and an amine attached to nitrogen in pyrrole via an aliphatic chain.

As a result, the carbonyl group was directly linked to the Cp ligand of Fc which resulted to a significant positive shift of the potential of the redox transition of the complex, Fc+/Fc, both in the monomer and in the polymer, compared to the free Fc complex or

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to polymers with the aliphatic chain between Fc and the matrix (see above). On the contrary, the existence of an alkyl element in these chains linked to pyrrole leads to an analogy of pyrrole/PPy-related properties to those of N-alkyl pyrroles/PPy. The first synthesis (Scheme 25b, n = 3) and electropolymerization of the disubstituted Fc derivative, 1,1′-(3-N-pyrrolylpropyl) ferrocene dicarboxamide were described in Refs. [356,357]. The position of an intensive redox response of the immobilized Fc complexes in this film corresponds to the potential of the free solute monomer. The cycling of the film-coated electrode in the range of the polymer-matrix response revealed a degradation of the film, with the loss of its electroactivity properties (both of Fc and of the matrix). The electronic conductivity of the film measured in the dry state was very low (below 10− 8 S/cm). A more stable behavior was observed for the film obtained by copolymerization of this monomer with N-methylpyrrole. For the first time a successful attempt of the homopolymerization of a CO–NH bonded monomer, with a stable redox response of immobilized Fc complexes (but without a pronounced electroactivity of the PPy matrix), was achieved in Ref. [358], see also review [20]. Two types of Fc–PPy films were electropolymerized (potentiostatically or with CV) at Pt and glassy carbon surfaces from the acetonitrile solution of the corresponding monomer, N-[2-(pyrrole-1-yl)ethyl]-N-ethylferrocene-1-carboxamide or N,N′-bis-[2(pyrrole-1-yl)ethyl]-N,N′-bisethylferrocene)-1,l′-dicarboxamide. The monomers were synthesized by reaction of a mono- or dicarbonyl chloride Fc derivative and N-pyrrole diethyl amine (Scheme 26).

The influences of monomer concentration and synthesis charge on the modified electrode performances were analyzed. The redox potential of the Fc complex inside the monomers and the polymers turned out to be dependent on the presence and the concentration of solute Li+ cation, especially for the disubstituted derivatives, attributed to its selective complexation which affected both the redox response of the Fc group and the polymerization process. The redox response of Fc inside the film was also observed after the polymer-coated electrode transfer to an aqueous solution, the redox potential being dependant on the nature and the concentration of the counter-ion of the electrolyte. However, the stability of the redox response and its intensity (peak currents and the total charges) is much lower in this medium. This team synthesized later polymer films of a similar composition but without ethyl group in amine in Scheme 26a, i.e. following Scheme 25a with n = 4 [359]. The monomer was polymerized with CV or potentiostatic regime and the film demonstrated a redox response of Fc complexes similar to that for the system corresponding to Scheme 26a. This response was sensitive to H2PO4− and adenosine-5′-triphosphate anions in organic electrolytes, contrary to poor sensing properties of the monomer in solution. A similar monomer (Scheme 25a) with a different aliphatic chain length (n = 3) was deposited as a thin film on the surface of a platinum microelectrode array on silicon substrates with different geometric characteristics [360]. This modified electrode was used for detection of the dihydrogen phosphate mono-anion in non-aqueous media by means of differential pulse voltammetry. A monomer in Scheme 25a with a longer chain length, n = 6, was polymerized at the surface of a high-temperature superconductor (HTSC) [361], for the study of the electron-transfer rate between HTSC and Fc complexes immobilized near the interface. The same monomer with a shorter chain (n = 3) was electropolymerized on platinum electrode and on stainless steel meshes [362]. The deposited films demonstrated voltammetric peaks due to oxidation and reduction of immobilized Fc complexes while the response of the polymer matrix was not noticeable. The response of Fc inside the homopolymer was rapidly diminishing in the course of potential cycling. A more stable behavior was achieved by the copolymerization of the functionalized monomer and pyrrole. This copolymer deposited on the stainless steel surface was applied as a membrane with electrochemically controllable properties. In all monomers/polymers discussed above the CO–N bond was formed by the reaction of a carbonyl-containing Fc derivative (FcCOCl, Fc(COCl)2, FcCOOH or FcRCOOH) with an amine linked to nitrogen of pyrrole. It is interesting to note that an alternative way of the CO–NH attachment where the carbonyl group comes from a pyrrole-containing species while Fc is functionalized with an amine group has been realized in a single publication [363].

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Both pre- and post-functionalization approaches were applied. In the former a new Fc–pyrrole monomer was synthesized (Scheme 27), then it was electropolymerized potentiostatically at Pt surface from acetonitrile solution. This Fc–PPy film transferred in the background electrolyte solution showed the redox responses of both immobilized Fc complexes and the polymer matrix.

Several other monomers were synthesized [363] for the further polymerization and post-functionalization with the use of N-pyrrole derivatized carbonic acids with different chain lengths, N-pyrrole–(CH2)n–COOH, with n = 2, 5 or 12. It was reacted either with N-hydroxysuccinimide and dicyclohexylcarbodiimide to give monomers 28a, or with pentafluorophenol to give monomer 28c. Monomer 28b was obtained by coupling the reaction product of pyrrole potassium salt and 2-[2-(2-chloroethoxy) ethoxy]ethanol with dicyclohexylcarbodiimide.

After the synthesis these monomers 28abc were electropolymerized potentiostatically at Pt surface from acetonitrile solutions, then film-coated electrodes were transferred into the solution of β-ethyl amino ferrocene, to perform the post-functionalization of these polymers with immobilized Fc centers (Scheme 29).

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These Fc-functionalized films showed in CV tests the responses of both Fc complexes (at the same potential as free Fc species in solution) and of the PPy matrix (corresponding to its N-alkyl-substituted case). The absence of a noticeable shift of the potential for immobilized Fc centers is due to the aliphatic substitution in its Cp ligand while in other Fc–N-pyrrole systems based on CO–N bond (see above) the Cp ligand was attached directly to the carbonyl group shifting the redox potential of Fc in the positive direction. The shape of this response of Fc centers is typical for a covalently immobilized redox couple: a symmetrical shape, proportionality of the peak current to the scan rate, no loss in the intensity within a multi-cycle experiment (except for the usual first-cycle effect) or an extended holding in contact with solution. Comparison of the redox charges of the Fc centers and of the matrix for the post-functionalized polymers in Scheme 29 and the pre-functionalized one in Scheme 27 allowed the authors to estimate the efficiency of the former procedure, i.e. the fraction of pyrrole units in the polymer functionalized after the film deposition. The influence of such factors as the length of the alkyl chain in Scheme 29a (n = 2, 5 or 12) or the hydrophobic vs. hydrophilic character of this chain (comparison of polymers in Schemes 29a and 29b) as well as of the film thickness on the functionalization procedure and the response of the post-functionalized polymer was analyzed. Another application of the film with an activated ester in Scheme 29c was to functionalize it with a bio-assembly including biotin-linked amine, to immobilize after it (strept)avidin molecules owing to the high affinity of the biotin-(strept)avidin interaction. A recent review [364] outlines the developments of mixed pyrroleamide based receptors over the last decade (including bridged dipyrrole ansa-ferrocene), with an emphasis on the anion binding properties of these receptors. 3.2.1.3. Cationic bridge to polymer. Several papers reported N-substituted polypyrroles in which Fc complexes were covalently bonded via a positively charged chain (with ammonium or bipyridium groups).

The Fc–N-pyrrole monomers 30 with an ammonium bridge and different aliphatic chain lengths [365] were synthesized by stoichiometric reaction of (dimethylamino)methylferrocene and iodopropylpyrrole (for the synthesis of 2) or iodododecylpyrrole (for the synthesis of 3). They were readily electropolymerized in CV or controlled-potential regime from acetonitrile solutions. The redox response of the deposited film in the background solution was characteristic for immobilized Fc centers, with a very small difference between the peak potentials (10 mV for v = 50 mV/s) while no clear redox response of the polymer matrix was detected, probably because of its overoxidation. Owing to a high positive charge of the ammonium groups these films allowed electrochemical sensing of dihydrophosphate anions in organic solvents (acetonitrile) by means of the amperometric titration. On the contrary, this complex film-anion dissociates if the electrode is transferred into an aqueous solution. No qualitative differences were observed in the behavior of polymers with different aliphatic chain lengths, n = 3 or 12, including the anion release in the aqueous medium. The same monomer with a shorter chain length 30 (n = 3) was synthesized in a similar way (except for the use of 1-(3bromopropyl)pyrrole). Attempts of its homopolymerization were unsuccessful. Therefore, the films were obtained by its copolymerization with pyrrole in acetonitrile. The further study of the film electroactivity was performed in aqueous solutions. The transfer of the film-coated electrode between these solvents resulted in a strong change or even disappearance of the matrix electroactivity while the redox response of immobilized Fc centers retained. Owing to positive charges of the attached chains the polymer film possesses an ability to extract effectively water-soluble calix [6]arene-p-hexasulfonate from its solutions. Automated amperometric immunosensors for the quantification of transferrin were fabricated [366] with the use of transferrin antibodies immobilized owing to positive charges of the same polymer (30, n = 3). The sensor was based on the variation of the Fc response inside the film as a function of the transferrin concentration. A different type of the cationic bridge between Fc and N-pyrrole was developed in Ref. [144] which contained a doublecharged viologen group (Scheme 31). Besides, two other Fc-viologen derivatives were synthesized and characterized electrochemically in aqueous solutions of various inorganic and ATP2− anions. The combination of two components, Fc and viologen, having their redox activities in different ranges of potential allowed to use the Fc/Fc+ transformation both for sensing and for the change of the binding strength of viologen. Fc–pyrrole monomer in Scheme 31 was obtained by reaction of 4,4′-bipyridine with chloromethylferrocene, followed by action of bromopropylpyrrole. The polymer was deposited from acetonitrile solution. Both in background acetonitrile solution and in aqueous medium it demonstrated stable redox responses of both Fc and viologen groups, with selective electrochemical sensitivity to various anions, the strongest effect being observed for ATP2− anion.

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3.2.1.4. Other bridges. A different type of the chain between Fc and N-pyrrole which contains a crown ether was developed in Refs. [367,368], see also review [20]. It was obtained by reaction of ferrocene dicarbonyl chloride with 2,2′-[4-[2-(1H-pyrrol-1-yl) ethyl]-1,2-phenylene]bis(oxy-2,1-ethanediyloxy-2,1-ethanediyloxy)]bisethanol (Scheme 32).

This species in the free form demonstrates the reversible peaks due to the Fc group and irreversible responses of pyrrole and benzocrown components. Polymer films were deposited in the potential range corresponding to the onset of the pyrrole oxidation, potentiostatically or by potential cycling. In the background acetonitrile solution the response of immobilized Fc centers has got a symmetrical shape, almost without a difference of Epa and Epc. The response of the polymer matrix could only be clearly seen for thin films. The wave due to Fc was used to trace the complexation behavior of the film towards inorganic metal cations. The possibility of the amperometric recognition of Ba2+ and Ca2+ in acetonitrile was proposed. Quite a different type of Fc–pyrrole derivatives was synthesized in Ref. [369], in which both Fc (via one of its Cp ligands) and pyrrole (via nitrogen) were attached via sulfonyl groups to a macrocycle, (E,E,E)-1,6,11-tris(arenesulfonyl)-1,6,11-triazacyclopentadeca-3,8,13triene, where three aryl groups represent either two ferrocenyl and one 4-pyrrol-1-ylphenyl 33 or versa vice. Their redox properties as well as those of their complexes with Pd(0) were studied. Polymer films were obtained by the electrooxidation of both ligands and their Pd(0) complexes at glassy carbon electrode. These modified electrodes are efficient and selective heterogeneous catalysts for Suzuki crosscouplings, benefiting from simple removal of the catalyst from the reaction vessel.

3.2.2. Fc derivatives linked to pyrrole in 3-position Since the hydrogen in 3-position of pyrrole is less reactive than those in positions 1 and 2 the synthesis of 3-substituted pyrroles requires to preventing an attack to the latter positions. Two different strategies have been employed to produce Fc-3-pyrrole derivatives: 1) Substitution of 1H in pyrrole by a bulky triisopropylsilyl group protecting both 1- and 2-positions so that pyrrole ring may be substituted in 3-position by Br etc, with the deprotection of position 1 at the last step of the monomer synthesis, just

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before its polymerization. In all publications of this group the functionalized Fc-3-pyrrole monomer was obtained, with its further homo- or copolymerization, since the latter step cannot be realized before removal of i-Pr3Si group. 2) Route via pyrrole derivatives bearing “activated esters”, like pentafluorophenyl ester, N-hydroxysuccinimide (NHS) or hydroxyphtalamide (NHP) on its 3-position which may readily replaced by amine-containing species, e.g. ethyl amino ferrocene. In this case one may attach the Fc complex to the pyrrole derivative either on the stage of the monomer (pre-functionalization) or after the film deposition (post-functionalization), the latter way has mostly been used. Approach 1 was developed in the first synthesis of poly(Fc-3–Py) films [370,371] via of 3-bromo-N-(triisopropylsilyl)-pyrrole to give finally 3-(6-ferrocenyl-6-hydroxyhexyl)pyrrole 34. The attempts to deposit a homopolymer did not result in even films. Therefore, this functionalized monomer was copolymerized with pyrrole from acetonitrile solutions with various ratios of concentrations. The responses both of the PPy matrix and of Fc complexes were well visible, the potential of the latter being close to that for the monomer or for free Fc.

These Fc-functionalized species (with various aliphatic chain lengths, n = 5 or 12) were also incorporated into Langmuir–Blodgett films as mixtures with alkyl pyrrole to produce highly ordered structures [372,373]. Monomer 35 with a direct link between the Fc and 3-pyrrole groups was synthesized via ethyl 2-cyano-3-ferrocenylacrylate and 2-ferrocenylsuccinonitrile [374]. This species showed a redox response attributed to the Fc group which was shifted strongly in the negative direction (150 mV). On the contrary, no current related to oxidation of the pyrrole fragment was registered and no film deposition was observed. One can attribute these features to the conjugation between the π-electrons of Cp and pyrrole rings which may further lead to a stiff geometry of the monomer.

The first example of a monomer with a purely aliphatic chain between the Fc and 3-pyrrole fragments (36, n = 6) [350] was synthesized via the reaction between 3-bromo-N-(triisopropylsilyl)-pyrrole and a new Fc-containing derivative, (6-bromohexy1) ferrocene, with the further deprotection of 1-position of pyrrole. In acetonitrile solution this Fc-3–pyrrole derivative showed diffusion-controlled oxidation and reduction peaks at the same potential as for free Fc or its alkyl derivatives. The electrooxidation at higher potentials resulted in the deposition (on Pt surface) of the first well-conducting homopolymer with the Fc complex attached in 3-position of pyrrole. In the background electrolyte solution in acetonitrile the redox responses of both the immobilized Fc and the polymer matrix were registered. The analysis of the dependence of the film properties on the monomer concentration led to the conclusion of a specific adsorption of the monomer on the electrode surface, with an inhibiting effect, so that the deposition on the ITO surface was only possible as a copolymer with pyrrole and with addition of water (1%). The formation of self-assembled monolayers of this monomer in acetonitrile on ITO surface (with pyrrole oriented to solution and Fc group to the surface) was studied [375]. A further development of this direction [353] was to replace bromohexylferrocene as a reactant by another source of mono-anionic ferrocene, tri(n-butyl)stannylferrocene (Bu3SnFc) which gave exclusively the monolithiated salt of Fc under the action of BuLi, and then alkyl-linked Fc and 3-pyrrole (36, n= 4) from 3-(4-bromobutyl)-N-(triisopropylsilyl)pyrrole. The CV curve in the monomer solution in acetonitrile showed a couple of oxidation and reduction waves but with a very great distance between the peak potentials. Further cycling resulted in a progressive increase of their intensities (accompanied by the further displacement of both peaks for higher potentials, probably because of a high ohmic resistance), testifying in favor of the homopolymer deposition. The CV curve after the transfer of the film-coated electrode into aqueous 1 M NaNO3 solution showed only the response of immobilized Fc, again with a significant difference of the peak potentials. SEM images revealed a cloud-like morphology. Films of a copolymer with unsubstituted pyrrole were also deposited, with cauliflower morphology. The detection of cytochrome C was unsuccessful for any of these films. A similar route was used for the synthesis of a new functionalized 3-pyrrole monomer 37 and the corresponding PPy films with attached binuclear ferrocenophane complexes, which contain 2 Fc units connected by methylene bridges between their respective Cp

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ligands [353]. The monomer was obtained by reaction between 3-(4-bromobutyl)-N-(triisopropylsilyl)pyrrole and the lithium salt of ferrocenophane (produced from ferrocenophane by BuLi). Its CV response in dichloromethane solution showed a double oxidation wave typical for ferrocenophane complex. An extension to higher positive potentials in the mixed solution of the monomer and pyrrole (homopolymerization was not successful) led to a progressive increase of both anodic and cathodic waves corresponding to a film growth. SEM images demonstrated cauliflower morphology. The ferrocenophane-functionalized PPy films were tested as electrocatalysts for hydrogen generation [337]. The direction based on the use of easily removable “activated esters” employed several groups, in particular pentafluorophenyl ester. It was proposed to attach this ester via a carbonyl chain to pyrrole in 3-position (Scheme 38a), then to polymerize this monomer, to obtain a film which may be easily post-functionalized simply by soaking in a solution of an amine-containing substance, in particular biologicallyactive molecules, e.g. amino-acids and peptides, for their covalent attachment to the polymer matrix [376,377].

This possibility was used [378] to attach the Fc complex via a CO–NH bond (Scheme 38b, n =2). The treatment of PPy-ester film (the polymer should be in the reduced state) with DMSO solution of ethyl amino Fc resulted in appearance of the Fc redox couple in the CV response of the film in acetonitrile solution. The Fc attachment was confirmed by XPS and FTIR data. Later, the same films as well as copolymers of this monomer with non-substituted pyrrole were studied by means of neutron reflectivity and RAIRS techniques, to study the kinetics of the penetration of reactants, ions and solvent (water and acetonitrile) into the polymer film [379,380].

A different type of “activated esters” has also been proposed, N-hydroxysuccinimide (NHS) 39a and N-hydroxyphthalamide (NHP) 39b [381,382]. The electropolymerization of NHS or NHP derivatives of pyrrole can be realized easily in acetonitrile, giving raise to a film with a redox response typical for a substituted PPy (Scheme 39c). Then, the functionalization of the polymer is achieved simply by soaking in acetonitrile or aqueous solution of ethyl amino ferrocene (Scheme 39d), similar to the reaction in Scheme 38b. The polymer functionalization with a crown ether was also demonstrated.

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As an extension of this approach, a monomer containing pyrrole, Fc and NHP group was synthesized [383,384] (Scheme 40a). At the first stage carboxamide chains with terminal NHP groups were attached to both Cp ligands of Fc. Then, some of NHP ester groups were replaced by an amine pyrrole derivative (to avoid making this replacement in both Cp ligands of a Fc complex). Thus obtained functionalized pyrrole monomer was copolymerized (Scheme 40b) with pyrrole (to avoid steric restriction due to bulky substituents and to make the film more porous) on Pt surface from acetonitrile solution. At the final stage oligonucleotides were attached to the polymer by replacement of NHP ester groups by their terminal amines, followed by hybridization step. The system showed prospects for biosensing applications. 3.2.3. Other Fc–pyrrole systems A novel type of a conducting polymer, which was described as conjugated poly[pyrrole-2,5-diyl(ferrocenylidene)] (PPDFcE), and its precursor poly[pyrrole-2,5-diyl(ferrocenylidane)] (PPDFcA) were obtained by condensation of pyrrole with ferrocene carboxaldehyde [385]. A neutral redox-active receptor, Fc-functionalized calix[4]pyrrole 41, was used as an active component in C paste electrodes and ion-selective electrodes, for the detection of anions (dihydrophosphate, fluoride etc) in aqueous solutions [386].

3.3. Fc bonded to/inside oligo/polythiophene chain The discussion below is made separately for two principal types of systems, those with Fc complexes attached in 3-position to the polythiophene (PTh) chain and those with polymer chains composed of both Fc and thiophene groups. In the latter case the chain passed always through the whole Fc complex, i.e. its both Cp ligands are monosubstituted and linked directly to the corresponding thiophene rings. In the former type the links between PTh chain (where only 3 and 4 positions are available for functionalization) and Fc were mostly constructed with the use of ester, O–CO, or carboxamide, CO–NH, bonds. Several other systems were based on pure hydrocarbon links, saturated or conjugated. 3.3.1. Ester or carboxamide bridge The synthesis of the first Fc–thiophene polymerizable derivatives (as well as viologen-functionalized thiophenes) [387,388] was based on reaction of an ω-haloalkyl substituted thiophene derivative with ferrocene carboxylic acid (Scheme 42). Similar approach was also used to obtain mono-, bi- and terthiophenes functionalized with viologen. These monomers allowed the authors to prepare electrochemically the corresponding functionalized films with PTh matrices.

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A series of monomers based on monothiophene ring but having a similar type of the link to Fc was synthesized in Ref. [389] by reaction of [6-(trifluoromethyl)sulfonyl-oxy-alkyl-oxy-carbonyl]ferrocene with 2-(3-Thienyl)ethanol (Scheme 43) in which the length of the aliphatic chain was varied as n = 6, 8, 10, 12, 16. The functionalized monomers in acetonitrile solution showed the redox responses of both Fc and thiophene groups. Similar to other Fc-containing species, the Fc wave was quasi-reversible while the further oxidation of thiophene led to the formation of a film at the electrode surface with a monolayer thickness. A similar inhibition of the further deposition of this functionalized polymer took place also on the top of the pre-deposited conducting film of poly(3-methylthiophene) film.

A series of other thiophene-based polymers to which the Fc complex was also attached via an ester linkage was obtained by means of both pre- and post-functionalization approaches [390]. First, 2-(3-thienyl)ethanol and 3-dodecylthiophene were copolymerized and the deposited polymer was treated with ferrocene carboxylic acid (Scheme 44a). Another route was to synthesize first a thiophene-based monomer functionalized with Fc, 2-(3-thienyl)ethyl ferrocene carboxylate (Scheme 44b). Then, this monomer was copolymerized together with two previous monomers (Scheme 44b).

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A different approach to the functionalization of PTh chains with Fc complexes was proposed in Ref. [391]. Monomers were obtained by reactions of hexanoic-acid derivatives of mono-, bi- and terthiophene units with N-hydroxy-2,5-pyrrolidinedione (NHS) giving corresponding “activated esters” of NHS (Scheme 45). These monomers can be electropolymerized. Then, the NHS group can be easily replaced by a solute amine, in particular 2-aminoethoxymethylferrocene, leading to polymers functionalized with Fc complexes linked via CO–NH bond (similar approach was also used for 3- and N-substituted PPy films, see above).

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Another example of a Fc-functionalized PTh film with an ester bridge [392] was obtained by post-modification of a regioregular head-to-tail poly[3-(6-bromohexyl)thiophene] with ferrocene carboxylic acid at 100 °C for 24 h (Scheme 46). Owing to the powder state of the starting polymer the functionalization was practically complete. Both the starting and the derivatized polymers were readily soluble in some common solvents, as chloroform and dichloromethane so that NMR, UV–visible, photoluminescence spectroscopy and gel permeation chromatography could be used for its characterization. The polymer on the electrode surface showed a stable redox response in acetonitrile.

Another Fc–PTh polymer soluble in some organic solvents (chloroform, THF, acetone, xylene etc), poly[3-(ferrocenyl-ethyl formate) triethoxy] thiophene, was prepared by chemical polymerization of the monomer with iron(III) chloride as oxidant [393]. It was characterized with NMR, IR, UV–visible spectroscopies and electrochemical techniques. A stable response of Fc complexes was registered. The polymer showed a great selectivity and sensitivity towards sodium cation. A different type of the Fc–PTh polymer was synthesized [394–396] which contained a cationic group, N+Me2, inside the side chain between Fc and thiophene groups (Scheme 47). The monomer was afforded by reaction between (dimethylaminomethyl)ferrocene and 3-(2-bromoethoxy)-4-methylthiophene. Its polymerization was performed by chemical oxidation with FeCl3. This water-soluble polymer was used in a new label-free electrochemical DNA biosensor specific enough to discriminate one mismatch DNA from the perfect complementary strand. This polymer was also used in electrochemical methods for the detection of human α-thrombin.

3.3.2. Hydrocarbon bridge Surprisingly, no publications on a PTh chain with Fc complexes linked by a pure aliphatic chain are available. The found examples represent either a modified PTh polymer chain or a conjugated hydrocarbon bridge. The former variant was realized in Refs. [350,397] where the substituents were attached to a 4H-cyclopenta[2,1-b:3,4-b′] dithiophene unit (Scheme 48). These monomers were synthesized by reactions of this cyclopentadithiophene with Fc–(CH2)6Br or Fc–CH2Cl or Fc–CHO. They were homopolymerized from their acetonitrile solutions and in CV the polymer films showed stable redox responses of both immobilized Fc complexes and the polymer backbone. The redox transformation of FC centers was realized owing to the electron charge transport along the conducting polymer chains.

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A strong specific adsorption of these monomers on the electrode surface (especially, on ITO) was discovered. In particular, the hexyl-bridged derivative in Scheme 48a formed self-assembled monolayers on ITO surface [375,398,399], both in its initial (Fc) and oxidized (Fc+) state. Anodic coupling of these layers produced thin PTh films contacting directly the electrode surface. Another polymer of this type (Scheme 48c) deposited on ITO and interdigitated Au electrodes was tested as a candidate for humidity sensoring based on the variation of its electroconductivity [400]. Its gradual decomposition under the action of oxygen and water was observed. Two Fc-containing thiophene-based monomers, 1-(2,5-Dibromothiophene-3-yl)-2-ferrocenylethene (Scheme 49a) and 2(2,5-Dibromothiophene-3-yl)-3-ferrocenylacrylonitrile (Scheme 49b), with a conjugated link were synthesized via Knoevenagel base condensation or Wittig reactions [401]. The corresponding poly(thiophene)s with pendant vinylene Fc or cyanovinylene Fc units were prepared by Ni-mediated copolymerization of a functionalized monomer and 2,5-dibromo-3-butylthiophene (Schemes 49cd). The solubility of both polymers in chloroform allowed one to deposit their thin films onto ITO and evaporated Au surfaces by spin-coating, for UV–visible absorption and luminescence measurements as well as for constructing organic bilayer photocells.

Two monomers, trans-1-(3′-Thienyl)-2-(ferrocenyl)ethene 50a and trans-1-((2′,2″:5″,2‴-Terthiophen)-3″-yl)-2-(ferrocenyl) ethene 50b, with the same conjugated Fc-containing side chain, –CH = CH–Fc, linked to either thiophene or terthiophene were synthesized from ferrocene carbaldehyde and (3-thienylmethyl) triphenylphosphonium bromide or from (ferrocenylmethyl) triphenylphosphonium bromide and 3′-formyl-2,2′:5′,2″-terthiophene, respectively [402]. The homopolymerization of the former monomer 50a did not succeed because of the conjugation between the vinyl linker and thiophene ring. The monomer was copolymerized electrochemically with pyrrole, the resulting film showed in aqueous solution the redox responses of both the Fc complexes and the PPy matrix. Its morphology had a cauliflower structure. On the contrary, the second Fc-derivatized monomer 50b was homopolymerized electrochemically in acetonitrile solution, with redox responses of Fc centers and the PTh matrix. Its transfer into an aqueous solution resulted in the disappearance of the matrix response. Both polymers deposited on Pt disk electrodes facilitated access to the redox center within Cytochrome C during CV, opening prospects of their use as biosensors.

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3.3.3. Compose Fc–thiophene chains Several publications reported conjugated polymers or oligomers whose chains were composed of both thiophene and Fc units, without any other groups inside the chain. Systems with principal chains which included Fc, thiophene and other conjugated units are discussed below in Section 3.6 below. The first such system was synthesized [403] by dilithiation of Fc and subsequent conversion and copolymerization with dihalo thiophenes. Its conjugated chain contains alternating terthiophene units and Fc complexes (both Cp ligands of a complex being included into the principal chain). Magnetic properties of polymers in the oxidized state were investigated.

A series of similar monomers was synthesized [404]. Stille coupling between 1,1′-bis(tributylstannyl)ferrocene and 5-iodooligothiophenes was used to prepare. 1,1′-bis(5-oligothienyl)ferrocenes 51abc. They all showed CV peaks due to Fc redox transformation as well as the thiophene oxidation wave. The attempts of electropolymerization of the monothiophene derivative (Scheme 51a) were not successful, because of a high reactivity of the double-charged oxidation product. The two other monomers 51bc based on bi- and terthiophene groups gave conducting polymer films upon electrooxidation which contained chains of linked Fc and quater- or sexithiophene units (51d, m = 2 or 3), with substitutions in thiophene groups corresponding to 51c. No waves due to the Fc redox transformation was observed for these films in the range of the solute Fc reaction, presumably due to a low conductivity of the polymer matrix in this potential range. Complexes 1′-bis[5-(2,2′-bithienyl)]ferrocene 51b and 1,1′-bis[5-(2,2′:5′,2″-terthienyl)]ferrocene 51e were synthesized by coupling 1,1′-bis(2-thienyl)ferrocene 51a with 2-bromothiophene and 5-bromo-2,2′-bithiophene, respectively [405]. These species demonstrated the CV responses of both Fc group and oligothiophene oxidation. In dichloromethane/dichloroethane solution with a rigorous removal of water traces both monomers could be electropolymerized. The polymer films were electroactive in the potential ranges of both Fc complexes and the polymer matrix.

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A polymer with alternating Fc (with two disubstituted Cp ligands) and monothiophene units, poly(3,3′-dihexyl-4,4′-dimethyl1,1′-ferrocenylene-2,5-thienylene, was prepared via the reaction of dilithio bis(3-hexyl-4-methylcycylopentadienide)thienylene and iron(II) iodide (Scheme 52) [406].

Several molecules with Fc–oligothiophene chains were synthesized, with Fc complex at one end of the chain 53ab or at both ends 53cde, as models for a molecular wire [407,408]. The distribution of the electronic density after their electrochemical oxidation was studied.

3.4. Fc–aniline derivatives The number of aniline-type conjugated polymers with covalently bonded Fc complexes is much smaller than those for PPy or PTh systems. The first monomeric species of this kind were synthesized by reaction of [(dimethylamino)methyl]ferrocene methiodide with aniline or its derivatives, R–HN–phenyl–R′, with various substituents R and R′ (Scheme 54a) with 1) R = H, R′ = H 54b, 2) R = H, R′ =m–NH2 54c, 3) R = H, R′ =p-OCH3, 4) R = CH3, R′ = H, 5) R = H, R′ =o-NH2 in water [409]. All these molecules in acetonitrile solution showed a reversible wave of Fc redox transformation as well as an irreversible oxidation of the aniline fragment. The potential of the latter was strongly dependent on the type of the substituents R and R′, being much more positive than that of the Fc couple for species 1, 3 and 4 while overlapping with the Fc wave for aminoanilines 2 and 5. Only monomers 1 (aniline derivative, 54b) and 2 (m-aminoaniline derivative, 54c) were electropolymerizable in acetonitrile. Polymer film formation was assumed to occur by a head-to-tail coupling of the aniline moieties. Besides the above two monomers, two new Fc-containing species were studied [410], with aminophenol 54d or NH–(CH2)2– NH–C6H5 substituents 54e. All these monomers could be electropolymerized from acetonitrile solutions. Their study in a monomer-

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free acetonitrile solution revealed the redox response of pendant Fc complexes (gradually decaying with potential cycling) but no evidence for the matrix electroactivity. After the transfer of the poly(m-aminoanilinomethylferrocene) film into neutral or acidic aqueous solutions [411] a redox response of the electron-conducting polymer matrix was observed, its stability being strongly dependent on the composition of the solution. The same monomer (sparingly soluble in aqueous solutions) was used in this medium in the form of a suspension to deposit another polymer film. Layered films from this polymer and PAni were also studied.

One of these monomers, N-(ferrocenylmethyl)aniline 54b, was electropolymerized in acetonitrile on the surface of a glassy carbon electrode for cathodic determination of hydrogen peroxide and for the construction of a flavin enzyme (glucose oxidase)based biosensor [412,413]. The polymer was also used for the development of a reagentless amperometric enzyme electrode employing covalently attached horseradish peroxidase [414]. The covalent attachment of Fc to the polymer film provided a means of preventing the leaching of Fc into the surrounding organic solvent. Two Fc derivatives of a different type, p-ferrocenylaniline 55a and p-ferrocenylphenol 55b, were synthesized [415] as new watersoluble substrates for enzymes. p-ferrocenylaniline was readily electropolymerizable on a carbon electrode from acidic solutions. The deposited film mediated the electrooxidation of NADH.

Quite different systems in which both Cp ligands of a Fc complex were linked to oligoaniline chains via a carboxamide bond were synthesized [416]. First, a Fc–bianiline derivative with a CO–NH bond was afforded by condensation of 1,1′-ferrocene dicarbonyl chloride with N-phenyl-p-phenylenediamine (Scheme 56a). Then, it was subjected to oxidative coupling (by persulfate) with pphenylenediamine, leading to a polymer with Fc complexes and aniline pentamer units inside the chain (Scheme 56b). Owing to its solubility in DMF the oxidation state of the polymer could be changed by a chemical agent (persulfate) as well as deposited as a film on a solid surface. Because of non-conjugated Cp–CO–NH elements the electric conductivity is low even in the doped state. The redox response of the film is distributed in a wide potential range, with a weakly expressed structure.

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3.5. Fc- and hemi-ferrocene-containing polymers with conjugated hydrocarbon chains (with cyclopentadienylene, phenylene, ethynylene or/and vinylene units) Contrary to conjugated polymers and oligomers with PPy, PTh or PAni chains which were mostly electrodeposited at the electrode surface by the monomer oxidation, conjugated systems with linear hydrocarbon or aromatic chains are polymerized almost exclusively with the use of organometallic catalysts. In many cases the obtained polymer was soluble in some organic solvents which allowed the authors to carry out its characterization in the solute state as well as to deposit as a film on any solid substrate. Another noticeable difference between these two groups of polymers is in the type of the link between the incorporated metallocene complexes and the principal chain. In most of the synthesized PPy, PTh or PAni-based polymers these functional units are attached to this chain by an insulating (aliphatic, carboxamide etc) side chain. On the contrary, in almost all systems discussed below the metallocene complexes are either included into the principal polymer chain or linked to it with a conjugated side chain. It leads to a marked electronic interaction both between a complex and the chain (so that the change of the oxidation level of the latter influences the properties of the complex) and between two complexes via the chain (this interaction manifests itself e.g. in the redox properties of the system). A detailed discussion of the synthesis routes to these systems as well as of their properties may be found in review papers [80,82,210,332,417]. A brief outline of these systems is given below. The systems are separated below into two groups depending on whether Fc complex is outside or inside the principal polymer chain. 3.5.1. Pendant Fc complex Several publications were devoted to materials with a polyacetylene chain while Fc complexes were attached to it directly or via a side chain. In the first example one of the Cp ligand of Fc in the monomer is bonded directly to the acetylene unit 57a [418]. Its homopolymerization as well as a copolymerization with 7,8-bis(trifluoromethyl)tricyclo[4.2.2.02,5]deca-3,7,9-triene was performed by living polymerization. Then, a series of monomers with conjugated spacers between Cp in Fc and C`CH unit was synthesized [419], in particular (2-ethynylphenyl)ferrocene 57b, (Z)- and (E)-but-1-en-3-ynylferrocene 57c and 2-(4-ethynylphen-1-yl)vin-1ylferrocene 57d. The corresponding polymers were obtained by living metathesis polymerization, with predominantly head-to-tail coupling of monomer units.

The same Fc–acetylene monomer 57a was studied in Ref. [420]. First, its homopolymerization was performed by metathesis with W(CO)6 catalyst and photo-irradiation (Scheme 58a). Its copolymers with phenylacetylene or norbornene were also obtained with the same method (Scheme 58b). Besides, a homopolymer of phenylacetylene with Fc end groups was prepared. Polymers were soluble in common solvents and showed a redox activity related to interacting Fc complexes.

Several Fc-containing short-chain oligomers were synthesized [421] on the basis of (E)-1-ferrocenyl-2-(p-iodophenyl)ethene: its (E,E)-dimeric form (Scheme 59a), (E)-l-ferrocenyl-2-(p-ethynylophenyl)ethene and its (E,E)- and (Z,Z)-dimers (Scheme 59b).

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A rare example of the system in which a Fc complex was separated from the principal p-phenylene–ethynylene chain by an oligoether side chain and an ester group was synthesized in Ref. [422]. In the three Fc-containing monomers these complexes were covalently attached to diiodide phenyl ring through either a multi-ethylene oxide or a long-chain aliphatic tether 60ab. They all were copolymerized with another monomer (without Fc), also with long side chains, by a Pd-catalyzed Sonogashira coupling reaction (Scheme 60). The polymers were soluble in several solvents (THF, CHCl3, CH2Cl2 etc).The redox activity of Fc groups was characterized both in solution and as a thin deposited film.

In another publication [423] one of these polymers (60a, n = 3) was solubilized in water through complexation of Fc groups with β-cyclodextrin (CD) or β-CD modified nanoparticles. These polymers were proposed for biosensing applications. 3.5.2. Fc and hemi-ferrocene complexes inside a polymer chain Numerous studies have been devoted to purely metallocene (mostly Fc) systems which are composed only from metal cations and negatively-charged Cp ligands, as well as those where couples of Cp cycles are linked together directly or via a spacer or via two spacers (ferrocenophanes), see Refs. in review papers [80,330,332]. In the section below only the publications on the systems in which couples of Fc complexes are bonded by a conjugated hydrocarbon chain of linear or/and aromatic units connecting one Cp ligand of each Fc complex. Similar systems in which this chain is conjugated but contains also heteroatom units are discussed in the next section. A series of monomeric, dimeric and polymeric (with a low molecular weight) systems was constructed on the basis of naphthalene to which two Co rings were bonded in1 and 8 positions (Scheme 61). Most studies were performed for pure Fc system [424–426] 61ac, M_Fe). Later both Cp ligands of Fc complexes in Scheme 61 were substituted by its mono-alkyl derivative, C5H4-2-octyl, while Cp ligands in complexes with metal M (Fe or Ni) were not modified, so that a pure Fc system with all Cp ligands was replaced by a alternating sequence of complexes with two Cp or two Cp-octyl ligands [426,427] (61b, M = Fe or Ni). A mixed Fe–Co chain as well as a pure nickelocene polymer was also synthesized [428].

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Several papers reported polymers with Fc complexes inside the chain with vinylene units. Disubstituted carbaldehyde derivatives of Fc with alkyl chains of various lengths, 1,1′-Dialkylferrocene-3,3′-dicarbaldehydesn were polymerized by the titanium-induced dicarbonyl-coupling reaction resulting in chains with a single vinylene unit between substituted Cp⁎ rings (Scheme 62a) [429]. These poly(ferrocenylenevinylene) were readily soluble n chloroform, benzene, and hexane. As solid films they possessed an electronic conductivity on the level of 10 mS/cm. Poly(ferrocenylenedimethylvinylene) was prepared by the reductive polycondensation of 1,1′-diacetylferrocene with low-valent Ti component (McMurry reaction) [430]. Polymer was partially soluble in halogenated hydrocarbons and THF. Poly(ferrocenylenevinylene) without alkyl chains in Cp ligands was obtained by the ring opening metathesis polymerization of the vinylene-bridged ansa-ferrocene complex which was synthesized by the McMurry coupling of 1,1′-ferrocenedicarbaldehyde (Scheme 62b) [151]. The polymer was insoluble in usual solvents. Partially soluble materials (a block copolymer) were obtained by copolymerization of the Fc-containing monomer with n-norbornene. Several ansa–Fc complexes were synthesized with a longer (with 4 carbon atoms) chain between Cp ligands of the complex [431]. Their ring opening metathesis polymerization resulted in the corresponding polymers, poly(ferrocenylenedivinylene) and its methoxy derivative (Scheme 62c), poly(ferrocenylenebutenylene) with a non-conjugated chain (Scheme 62d). The latter polymer was soluble. Attempts to obtain a derivative of poly(ferrocenylenedivinylene) with tetramethylated Cp rings (Scheme 62e) by polymerization of octamethyl-1,4-(1,1′-ferrocenediyl)-1,3-butadiene were not successful. Poly(ferrocenylenevinylene) with an aliphatic substituent inside the vinylene chain was polymerized from the corresponding ansa–Fc derivative (Scheme 62c) [432]. The polymer was soluble in common organic solvents as benzene, dichloromethane and THF.

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The Cp ligands of Fc complexes in the next group of polymers are linked by a mixed chain composed of conjugated linear units and hydrocarbon aromatic rings. Two oligomers or low-weight polymers of this type, poly(ferrocenyl vinylene phenylene vinylene) and poly(ferrocenyl vinylene diphenylene vinylene), were synthesized by Heck-arylation of 1,1′-divinylferrocene with diiodoarenes (Scheme 63a) [433]. The products are poorly soluble. Another polymer of the same type, poly(phenylene vinylene ferrocenyl vinylene) (or poly(ferrocenyl vinylene phenylene vinylene)), but with substituent groups, was prepared by condensation polymerization of bisphosphonate reagent and ferrocene dialdehyde (Scheme 63b) [434]. Owing to long alkyl side chains the polymer is soluble in various organic solvents. The electronic conductivity in the solid state was low.

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Conjugated polymers with a different type of the chain between the Cp ligands, poly(ferrocenylene ethynylene arylene ethynylene)s, were also synthesized. In particular, the reaction of alkynyltrimethylstannanes with 1,1′-dilithioferrocene gave several ferrocenylacetylides, 1-phenylethynyl-1′-iodoferrocene, 1,1′-bis(phenylethynyl)ferrocene, 1,4-bis(1′-iodoferrocenylethynyl)benzene and poly[1,4-(1′-ferrocenylethynyl)ethynylbenzene] [435]. Several polymers of this type were prepared by the palladiumcatalyzed polycondensation between 1,1′-diiodoferrocene and various diethynyl homo- and heteroaromatic groups including phenylene (Scheme 63c) and dialkyl phenylene (Scheme 63d). Besides, a polymer with alternating biferrocenyl (from 1,6-diiodo1′,6′-biferrocene) and dialkyl phenylene units was obtained (Scheme 63e) [436,437]. The solubility of these polymers in organic solvent is correlated with the presence of long alkyl chains.

Several polymers with purely aromatic rings in the chain between Cp ligands were reported. In particular, a polymer with a single phenylene unit in the chain and all Cp ligands with two alkyl substituents, poly(3,3′-dihexyl-4,4′-dimethyl-1,1′-ferrocenylene phenylene) (Scheme 64a), was prepared via the reaction of dilithio bis(3-hexyl-4-methylcycylopentadienide)phenylene and iron(II) iodide in THF [406]. Polymers with longer chains containing from 3 to 7 p-phenylene units, poly(1,1′-ferrocenylene-alt-p-terphenyl-4,4″-ylene), poly (1,1′-ferrocenylene-alt-p-quinquephenyl-4,4⁗-ylene), and poly(1,1′-ferrocenylene-alt-p-septiphenyl-4,4‴‴-ylene), were prepared via Pd-catalyzed polycondensation (Scheme 64b) [438,439]. The central phenylene ring in each chain contained a couple of long-chain alkyl substituents, n-hexyl or n-dodecyl, to make the polymers soluble. A randomly coiled conformation was proposed for their structure. Soluble Fc-containing polyphenylenes were also synthesized by the trimerization polycondensation of 1,1′-diacetylferrocene [440]. The regulating effect of a monosubstituted acetyl derivative of Fc on the properties of the final products was studied.

A conducting polymer 65 with a pure poly(n-hexylphenylene) chain in which some rings represent ligands inside hemi-ferrocene complexes (Fe with a single Cp ligand) was synthesized by functionalization of poly(n-hexylphenylene) by Fc, Al and AlCl3 followed by an anion exchange using NH4PF6 [441]. 3.6. Fc in mixed and non-carbon oligomer/polymer chains 3.6.1. Chains composed of carbon units and heterocycles This section presents polymers with mixed conjugated chains composed by both hydrocarbon units and some other functional groups (heterocycles, azo group, non-metallocene complexes etc). In most cases the Fc complexes were inserted inside the principal

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chain. An opposite example is poly(3,4-bisphenyl-N-methylferrocene-pyrrole-2,5-dione) where the principal chain is composed by phenylene and pyrrole-2,5-dione cycles while Fc complexes were linked to nitrogen atoms by side chains (Scheme 66) [442]. It was prepared from 3,4-bis(4-bromophenyl)-N-methylferrocenylpyrrole-2,5-dione by dehalogenation polycondensation with a zero valent nickel complex. The polymer was readily soluble in organic solvents and demonstrated a redox activity in the solid state after a film deposition.

In a group of monomeric and dimeric species one of the Cp ligands of a Fc complex was linked to a conjugated chain with ethynylene and various thiophene units: one, two or three thiophene rings, (Th)n, 3,4-ethylenedioxythiophene (EDOT) or a triple unit, Th-EDOT-Th 67 [443]. They were synthesized by coupling ethynylferrocene with the appropriate mono- or dibromooligothiophene using Pd(PPh3)2Cl2 and CuI catalysts. The electrochemical oxidation of some of the mono(ferrocenylethynyl)oligothiophene complexes resulted in the deposition of a redox-active electrochromic film on the electrode surface. The films consist primarily of α,α-coupled dimers, probably with the presence of longer-chain oligomers or polymers resulting from additional α,β or β,β coupling.

The other conjugated systems included Fc into the principal chain. Mixed chains with Fc, ethynylene and pyridine or 3hexylthiophene-2,5-diyl units were synthesized in Refs. [436,437], see Scheme 63f,g and related discussion there. A polymer with mixed chains with Fc, vinylene and thiophene units is shown in Scheme 63a above [433].

Two isomeric donor-acceptor conjugated complexes 68ac containing two chains linking a Cp ligand of Fc, ethynylene unit and anthraquinone, 1,8-bis(ferrocenylethynyl)anthraquinone and 1,5-bis(ferrocenylethynyl)anthraquinone, were synthesized [332,444– 446]. Their protonation-driven structure changes and redox properties were studied. The structure of the corresponding polymer is given in 68b. 3.6.2. Other systems (non-carbon units, metal complexes) Besides monomers and polymers with purely hydrocarbon chains between Cp in Fc and C`CH unit (see structures 57 and related discussion) azo group containing monomer, 1-ferrocenyl-2-(4-ethynylphenyl)diazene 69, and the corresponding polymer were also synthesized by living metathesis polymerization [419].

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Redox and optical properties of a series of azo-bridged Fc oligomers 70a–c and a polymer 70d were studied in Refs. [332,446– 448]. A family of 3-ferrocenylazobenzene-attached dendrimers, 9-mer, 27-mer, and 81-mer, was synthesized [449] which exhibited photochromism and photoisomerization.

Quite a different type of conjugated polymers (without any carbon atom in the principal chain), phosphazenes, was used for attachment of Fc and ruthenocene complexes [115]. Several different ways to bond these complexes to P atoms in the chain are shown in 71a–c.

Fc-containing tetrayne was synthesized from HC`C–Ph–C`C–Si(i-Pr)3 via catalyzed C–C coupling (Scheme 72ab) [450]. Its desilylated product was used to obtain bimetallic oligomeric complexes of Fe (in the form of Fc) and Pd or Ni (Scheme 72c). Another copolymer contained the same Pt complex together with ethynylene and fluorene units, to which Fc was attached as a

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pendant group via a conjugated linker (Scheme 72d). Its synthesis was performed by dehydrohalogenative coupling procedure [451,452].

3.7. Polymers with metallocene or hemi-metallocene complexes of other metals 3.7.1. Complexes of ruthenium and osmium Several systems with conjugated chains have been reported for metallocene (Cp2M) or hemi-metallocene (CpM) complexes of electronic analogs of iron, Ru and Os. Ethynylruthenocene monomer 73 was synthesized and polymerized similar to its Fc analog [418], see structures 57a and related discussion.

Another series of ruthenocene-containing systems, stacked oligomeric and polymeric metallocene on the basis of naphthalene, was also prepared [424] in analogy with their Fc analogs, see Scheme 61 and related discussion. Ruthenocene complexes were also attached to a different class of conjugated polymers, phosphazenes, based on an alternating sequence of N and P atoms, see structures 71 [115].

Several studies were devoted to hemi-ruthenocene and hemi-osmocene complexes, with a single Cp (or its pentamethylated analog, Cp⁎) ligand of Ru. A series of monomeric and dimeric conjugated systems with CpRu, CpOs or Cp⁎Ru complexes

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with oligothiophene chains 74 was synthesized and studied electrochemically [453]. The oxidation and reduction of these complexes was studied. The oxidation results in conductive films on the electrode but the composition of the electrodeposited films was unclear.

Another series of analogous systems studied by the same team [454] contained one or two ruthenium complexes with Cp (or Cp⁎) and phenyl ligands attached to a terthiophene chain 75.

Several systems with a ruthenacyclopentatriene complex with a hexyl chain at its single Cp ligand 76a were synthesized including its derivative with two biphenyl branches 76b and a polymer with biphenyl spacers between the hemi-ruthenocene complexes 76c [331,455,456]. The polymer composed of identical ruthenacyclopentatriene units was prepared by metallacyclic polymerization. 3.7.2. Complexes of cobalt and nickel As in the case of Fc-modified polymers, the first publications on the incorporation of cobaltocene into the film corresponded to polymers with non-conjugated chains [38,457]. The number of publications on conjugated polymers with immobilized cobaltocene or nickelocene complexes (i.e. complexes with two Cp ligands, Cp2M) is surprisingly small, compared to that of studies dealing with a hemi-cobaltocene complex (i.e. with a single Cp ligand, CpM), see below. An approach based on a chain of face-to-face metallocene complexes kept together and parallel by naphthalene molecules (see Scheme 61 and related discussion there) was applied to generate such polymers either with an alternating sequence of ferrocene–nickelocene complexes [426,427] or of ferrocene–cobaltocene ones or with purely nickelocene units [426,428]. A different procedure based on the functionalization of a monomer with attached cobaltocene complexes for its subsequent polymerization was used by another team [458]. The synthesis of such monomers was performed following the same route as in Scheme 25 above for Fc derivatives, to form a carboxamide bond by reaction between cobaltocene carbonyl chloride (to link each complex to a single pyrrole ring, Scheme 77ac) or 1,1′-cobaltocene dicarbonyl chloride (to link each complex to two pyrrole rings, Scheme 77b) and an amine attached to nitrogen in pyrrole via an aliphatic (Scheme 77ab) or phenylene spacer (Scheme 77c). All these species demonstrated an expected behavior in electrochemical tests on two one-electron reduction steps within the cobaltocene complex. A potential scan in the positive direction resulted in the deposition of an electroactive polymer film for all these three monomers in Scheme 77. These films in acetonitrile or aqueous solutions of a background electrolyte revealed a very stable redox response of immobilized cobaltocene centers inside the film, without a noticeable response of the polymer matrix. According to SEM images, the films possessed a compact morphology at this scale, with well-visible crossed fibers on their surface. The presence of two pyrrole rings in monomer in Scheme 77b does not increase the efficiency of polymerization. Two allyl-containing monomeric cationic species, CpCo + Cp–CO–NH–CH2–CH_CH2 and CpCo + Cp–CO–O–CH2– CH_CH2, were also synthesized and characterized electrochemically. Besides, non-conjugated siloxane-based cobaltocenium polymers were prepared which demonstrated a redox response of metallocene centers inside the film.

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The monomer in Scheme 77a and the corresponding polymer were used [459] in selective amperometric sensors for anions.

Several teams developed a study of a series of systems with cobaltacyclopentadienylene complex 78a, an analog of ruthenacyclopentatriene (see structures 76 and related discussion). It included numerous oligomers and polymers in which these complexes were linked with phenylene bridges (Scheme 78b) [331,456,460,461]. The synthesis was performed by reacting (cyclopentadienyl)bis(triphenylphosphine)cobalt with aromatic diacetylenes, 1,4-diethynylbenzene, 1,4-diethynyl-2-fluorobenzene, 1,4diethynyl-2,5-difluorobenzene and 4,4′-diethynyl-1,1′-biphenyl (Scheme 78b). Soluble poly(cobaltacyclopentadienylenephenylene) was prepared by metallacycling polymerization of (hexCp)Co(PPh3)2 (hexCp_n-hexylcyclopentadienyl) with p-diethynylbenzene [462]. Metallacyclization reaction of para-bis(ferrocenylethynyl) benzene with CpCo(PPh3)2 afforded a Fc-containing cyclobutadienecobalt polymer. See also review papers [331,332,456].

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The same route via the reaction between (η5-cyclopentadienyl)bis(triphenylphosphine)cobalt complex and a diyne monomer, 4,4′-bis(phenylethynyl)biphenyl, to synthesize another polymer with cobaltacyclopentadiene and phenylene units in the principal chain (Scheme 78c) [463]. The reaction of this polymer with various isocyanates was used to prepare new polymers having pyridine moieties and aromatic units inside the main chain [464]. The reaction between cyclopentadienyl[diethynyl bis(trimethylsilyl)cyclobutadiene]cobalt and N,N,N′,N′-tetramethylethylenediamine afforded a copolymer, poly[(para-cyclobutadienylene cyclopentadienylcobalt) butadiynylene]-butadiynecyclobutadiene [465]. Novel thermotropic nematic and lyotropic smectic liquid crystal polymers were synthesized as copolymers of 1,3-diethynyl2,4-bis(trimethylsilyl)cyclobutadienyl cyclopentadienyl cobalt and 2,5-dihexyl-1,4-diiodobenzenes (hexyl, dodecyl) [466,467] or 3hexyl-2,5-diiodothiophenes [467,468] with or without alkyl substituents (Scheme 78d). 3.7.3. Hemi-metallocene complexes of manganese The only available example of conjugated polymers with this element corresponds to its hemi-metallocene complex (with a single Cp ligand). Hay's oxidative coupling of diethynylated derivatives (in 1,2 or 1,3-positions of the Cp ligand) of cymantrene, cyclopentadienyl-Mn-tricarbonyl CpMn(CO)3, resulted in polymers containing this complex (Scheme 79) [467].

3.7.4. Metallocene complexes of “early transition metals” (Ti, Zr and Ta) The previous sections contain plenty of publications on conjugated polymers with incorporated metallocene complexes of groups 8–10. The situation is quite different concerning similar systems with metals of other groups, in particular with “early transition metals”. The number of such systems is extremely limited as it may be seen from this section. On the contrary, these metallocene complexes, especially those of Ti and Zr, have been included into numerous non-conjugated polymers, frequently with the metal inside the principal chain, see recent review [469]. The only examples of titanocene-containing conjugated polymers represent films with an electron-conducting polymer matrix, to which a titanocene dichloride complex was attached with an aliphatic side chain (Schemes 80). The synthesis of the functionalized monomers passed through intermediate species in which the cyclopentadiene ring was attached to the polymerizable group, pyrrole (Scheme 80a) [94] or EDOT (Scheme 80b) [470]. The reduction of this ring resulted to its complexation (by ligand exchange) to CpTiCl3. The former titanocene-pyrrole monomer in Scheme 80a represents an analog of a Fc-functionalized monomer in Scheme 21 linked to nitrogen in pyrrole by a propyl chain (n = 3). No analogous monomers have been synthesized for other metallocenes. As for Fc–pyrrole monomers (see a discussion related to Scheme 21) the homopolymerization of those monomers with a comparable chain length, n = 2–4, was never successful so that a copolymerization with pyrrole had to be used. The latter resulted in a mixture of pyrrole and Fc–pyrrole units inside the polymer chain, i.e. to the diminution of the concentration of Fc groups inside the film. This increase of the average distance between neighboring Fc centers was not especially important for these systems since the redox potential of the Fc+/Fc transformation corresponded to the range of the electroactivity of the polymer matrix. In this potential range the principal mechanism of the electronic charge transport across the film is via the conducting PPy matrix, with the electron exchange between a Fc center and the matrix as a final or initial stage of the whole electron transport between the center and the electrode [350]. The substitutions within the pyrrole unit (especially, at nitrogen) lead to a strong diminution of the electronic conductivity of the polymer matrix (by several orders of magnitude for poly(N-alkylpyrroles) [70]) but even this reduced conductivity is quite sufficient for the change of the oxidation level of the polymer matrix following the variation of the imposed electrode potential (except for an improper choice of the polymerization conditions leading to the “overoxidation” of the film, making its conductivity too low).

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The situation is quite different for conjugated polymers with functional groups whose redox potential is away from the range of the matrix electroactivity. Within this potential range the electronic conductivity of the matrix becomes very low and it behaves as an inert insulating support for functional centers. Therefore, the change of the oxidation level of these centers (which requires an electron exchange with the electrode located at the macroscopic distance) resembles the same process in “conventional redox polymers”, i.e. it can only be realized by the hopping mechanism. Such a transport is highly critical both to an average distance between neighboring centers and to the absence of large distances inside such “hopping chains”. It implies a crucial importance for such systems to ensure both a very high concentration of the functional centers and their uniform distribution, in particular a good homogeneity of the film. Another interesting question is on the electroactivity of the polymer matrix within its own potential range. In most systems with other metallocene (Fc and cobaltocene) only the redox response due to the metallocene centers was observed, without a noticeable contribution due to the charging of the polymer matrix. Both monomers with titanocene dichloride complexes were successfully homopolymerized electrochemically (Scheme 80ab) [94,470,471], with a high current efficiency (about 70–90% for a small disk Pt electrode, the losses were higher for electrodes of larger sizes). The SEM, STM and AFM images of the deposited films as well as ellipsometry studies [472] demonstrated their very uniform morphology, with a very low surface roughness [94,470]. The redox response of the polymer matrix was rapid and stable, the shape of the voltammetric response being very similar to poly(N-methylpyrrole) in the same electrolyte, despite of the presence of a bulky metallocene group attached to each pyrrole unit. In accordance with the above reasoning the rate of the redox transformation of titanocene centers inside the film (realized by the hopping transport) was much slower. Nevertheless, a practically complete change of their redox state within the whole film is possible, owing to the film uniform structure. The mechanism of electroreduction of the complex (from Ti(IV) to Ti(III) state) in solution is influenced by the solution composition, solvent and electrolyte, because the degree of the dissociation of the reduced complex: Cp2 TiCl2 þ e− ¼ Cp2 TiCl−2 ¼ Cp2 TiCl þ Cl− depends on the coordination ability of the solvent (bond strength Ti-solvent molecule inside the dissociated complex), the solvation energy of chloride, the concentration of chloride in solution. A similar mechanism was confirmed for the attached centers [473,474]. The first studies of redox or halogen-transfer reactions were performed for these films [475,476], which supported the prospects for the use of these functionalized films for catalytic and electrocatalytic applications.

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Conjugated polymers with zirconocene complexes have also been synthesized by only one team [477–480]. These systems include zirconacyclopentadienyl units in the polymer backbone (Scheme 81), together with a spacer R. Besides non-conjugated spacers, e.g. SiMe3–Ar–SiMe3, Ar = various phenylenes or p-diphenylene [479,481], various conjugated groups were incorporated into the chain, e.g. diphenylene [478,480]. The polymers were prepared by the reaction between a diyne with “zirconocene” (produced by reaction of zirconocene dichloride and BuLi). Similar to analogous Co containing polymers, the principal chain represented a mixture of two isomeric forms of the complex, with branching in positions 1,3 or 1,4 of the butadienyl group (Scheme 81). The ratio between these forms in the synthesized polymer depends on various factors, in particular on the nature of the substituents R (Scheme 81), e.g. the fluorinated groups had a tendency to adopt preferentially 3,4-positions of zirconacyclopentadienyl ring [480]. The presence of long-chain alkyl group resulted in increase of the solubility in organic solvents. The zirconocene complex in these systems is very reactive, being easily substituted by another unit [478].

First example of a polymer with a conjugated matrix which contains a tantalocene complex has been synthesized recently (Scheme 82). The monomer synthesis was performed in a way similar to that for the titanocene monomer in Scheme 80a, in particular with the use of the same intermediate anionic species, pyrrole-cyclopentadienyl, [Cp(CH2)nNC4H4]−, Cp_C5H4. It reacted with a reduced complex of Ta(IV), Cp⁎TaCl3, which contained a pentamethylated Cp ligand, Cp⁎ = η5–C5Me5. It resulted in the first tantalocene–pyrrole complex with two chloride ligands, Cp⁎(Cp(CH2)3NC4H4)Ta(IV)Cl2. An attempt of its electropolymerization was not successful [482], probably because of the inhibitive action of chloride anions in the positive potential range [474]. Then, a cationic species combining pyrrollyl ring with a different tantalocene complex, in another oxidation state, Ta(V) and with OH ligand instead of one of the chloride ones, [Cp⁎(Cp(CH2)3NC4H4)Ta(IV)OHCl]+. This monomer was successfully electropolymerized, the deposited film demonstrated an expected redox responses of both the polypyrrole matrix and the attached tantalocene centers [483].

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