Charge carrier transport on molecular wire controlled by dipolar species: towards light-driven molecular switch

Thin Solid Films 438 – 439 (2003) 268–278

Charge carrier transport on molecular wire controlled by dipolar species: towards light-driven molecular switch ˇ ˚ a,b,*, Petr Tomana, Juliusz Sworakowskic Stanislav Nespurek a

Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, Heyrovsky Sq. 2, 162 06 Prague, Czech Republic b Technical University of Brno, Purkynova 118, 612 00 Brno, Czech Republic c Institute of Physical and Theoretical Chemistry, Wroclaw University of Technology, Wyb. Wyspianskiego 27, 50-370 Wroclaw, Poland

Abstract Charge carrier transport on molecular wire (conjugated polymer chain) is limited by polaron formation and by Coulomb interactions between a charge localized on a chain and another charge moving on the same or a neighboring chain. The ‘Coulomb limitation’ results from a feature characteristic of one-dimensional systems in which carriers cannot be scattered. Polar side groups attached to the molecular wire locally modify the ionization energy of the polymer segments and energies of electrostatic interactions of charge carriers on the wire and the dipoles. These changes result in the formation of a potential well structure and in the creation of local states for charge carriers. If the side groups are photochromic species changing their dipole moments upon illumination, then a light-driven molecular switch can be constructed. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: Molecular electronics; Polymers; Quantum effects; Conductivity; Polysilylene

1. Introduction The architecture of molecular-scale electronic devices can be designed starting from molecular segments whose properties have been known from experiment andyor suitable theoretical models. Such an approach can also be employed in designing a molecular switch—one of the basic components for the construction of memories and logic elements. Some examples of molecules or nanostructures that might be used as switching units have recently been demonstrated (e.g. Refs. w1–4x). A pioneering construction of a molecular switch was based on the electron tunneling principle w1x. An electron travels along a ‘molecular wire’ (e.g. a conjugated polymer chain) containing a finite series of periodic potential walls. The tunnel switch is ‘on’ if the transmission coefficient of the electron is close to unity, i.e. if the electron energy matches pseudostationary energy levels of the walls, and can be turned off by either changing a barrier height or the depth of a potential *Corresponding author. Tel.: q420-222-514-610; fax: q420-222516-969. ˇ ˚ E-mail addresses: [email protected] (S. Nespurek), [email protected] (P. Toman), [email protected] (J. Sworakowski).

well, which can be controlled by the dipole moment of polymer side groups. The Carter’s model w1x describes only the basic idea of the switch, no real and chemically available architecture has been put forward. Recently, Chen et al. w5x proposed a new type of electrical switch based on a two-step reduction process occurring on a molecular wire containing a donor– acceptor substituent. As the voltage is increased, the molecular wire initially undergoes a one-electron reduction on the place of donor–acceptor unit, thereby supplying a charge carrier for electron flow through the system. A further increase in voltage causes a second electron reduction with subsequent blocking of the current. The negative resistance behaviour of this type was demonstrated on a molecule containing a nitroamine redox center 29-amino-4,49-di(ethynylphenyl)-59-nitro-1benzenethiolate. However, detailed studies of Donhauser et al. w4x showed that the switching is due to conformational changes in the molecule or bundles, rather than to the electrostatic effects of charge transfer. Thus, the basic philosophy, physical backgrounds and material engineering of molecular switches are still open problems. In this article, we present an alternative approach to the construction of the molecular switch, and discuss its

0040-6090/03/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0040-6090(03)00799-5

ˇ ˚ S. Nespurek et al. / Thin Solid Films 438 – 439 (2003) 268–278

molecular architecture and basic electrical properties. The problem considered in this article is, to some extent, a microscopic (one-dimensional, 1D) equivalent of the case of ‘dipolar traps’ considered in our earlier articles w6–9x. The calculations w6–9x demonstrate, in agreement with the results of experiments performed by several groups w10–16x, that parameters of such traps (their depths, cross-sections, etc.) depend on the magnitude of the dipole moment as well as on the concentration and mutual orientation of the dipoles. It follows from model calculations that in 3D molecular solids local states are formed on the adjacent molecules in the vicinity of the dipole. On the other hand, the polar molecules themselves can act as chemical traps w17,18x. As will be discussed in this article, dipolar species of side groups of 1D polymer chains are expected to create local states on the chain (‘molecular wire’) and, having fulfilled certain requirements, to act as chemical traps, competing with the local states created on the chains. The discussion is supplemented with Appendix A describing results of the measurements of ‘on-chain’ charge carrier mobility and conditions of quantum-chemical calculations of quantities relevant to the functioning of the switch. 2. The architecture of the switch The proposed molecular switch consists of a molecular wire (a conjugated polymer chain) with photochromic side units chemically attached via a spacer w19,20x. In principle, both p- or s-conjugated polymer chains (see Scheme 1, structures (I)–(III)) can be taken into account. ‘On-chain’ mobilities in p-conjugated carbon polymers are usually higher (up to 7=10y5 m2 Vy1 sy1) than those in s-conjugated silicon ones (up to 2=10y5 m2 Vy1 sy1) w21x. In the latter polymers, the mobilities of mobile charge carriers (holes) depend on the chemical nature and on the size of the side groups. In alkyl-substituted polysilylenes, the hole mobilities seem to increase with the increasing length of the alkyl substituent: for example, in polywhexyl(phenyl)silylenex the ‘on-chain’ mobility equal to 2=10y5 m2 Vy1 sy1 was reported w21x, whereas m-10y6 m2 Vy1 sy1 was found for polywethyl(phenyl)silylenex by time-resolved microwave conductivity technique w21x. The on-chain hole mobility in polywmethyl(phenyl)silylenex, which in the present contribution will be considered a model for a molecular wire, was found to amount to m;2=10y6 m2 Vy1 sy1 using time-resolved microwave photoconductivity (TRMP) w22x, as will be described in Appendix A. In this article, the functioning of the switch will be discussed on the example of a suitably modified sconjugated polywmethyl(phenyl)silylenex (PMPSi, see Scheme 1, (III), R1sphenyl, R2smethyl). This polymer is a good hole-transporting material w23x. The HOMO level (‘the edge of the valence band’ in the

269

solid state terminology) consists of mixed s and p orbitals, as follows from Fig. 1. The polymer chain (molecular wire) is modified by chemically attached photochromic side groups, the magnitude andyor orientation of their dipole moments differing as much as possible between the stable and the metastable form. There are many photochromic materials which can be used for this purpose, see e.g. Ref. w24x and references therein. Two photochromic systems will be mentioned in this context: 6-nitro-19,39,39-trimethylspirow2H-1-benzopyran-2,29-indolinex (see Scheme 1, structure (IV)), whose dipole moment is higher in the metastable form, and a substituted azobenzene (see Scheme 1, structure (V)), whose photochromism is based on the trans–cis isomerization, and in which the orientations of the dipole moments of the stable and the metastable forms are mutually nearly orthogonal. In this article, we shall focus our attention on the former system. The dipole moments of stable forms of spiro-molecules (S) amount typically to 2–7 D. Depending on the substituents attached to their backbones, the dipole moments of the respective metastable forms (merocyanines, M) may exceed 15 D. The overall mechanism may be described as a heterocyclic bond cleavage and ring opening, but there are also cis–trans bond conversion and triplet– triplet contribution to the mechanism. The photochromic group can be attached to the molecular wire directly, as was proposed by Carter w1x, or via a spacer. Apart from a complicated chemical synthesis of the Carter’s structures, any distribution of geometries of the chain segments of the molecular wire should lead to a dispersion of the positions of energy levels throughout the polymer chain and, consequently, to a sharp decrease of the tunnel transmission coefficient of the charge carrier. Thus, the idea of the Carter’s switch is likely to fail in confrontation with the situation in real polymer chain. Therefore, we propose the switch with a spacer between the molecular wire and photochromic side group moieties, its functioning being based on electrostatic interactions between charges travelling on the molecular wire and dipolar species of the photochromic side groups. The spacer can be realized by several chemical structures, e.g. as those given in Scheme 1, (VI)–(VIII). Two important features should be considered, determining the action of the spacer: its chemical structure and its length. There are two limiting factors for the chemical structure: the spacer can be ‘quasi-neutral’ from the point of view of the charge transfer, like the spacer (VI) presented in Scheme 1, or it can freely transfer the charge during the formation of the dipole moment in the side group (e.g. spacer (VII) in Scheme 1). In the former case, the electrostatic contribution of the polar side groups, based on the charge–dipole interactions, is the most important factor influencing the behaviour of the charges on the molecular wire. In the

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Scheme 1. Chemical structures of some materials suitable for the construction of a molecular switch. The s-conjugated polymer (III) in which R 1sphenyl, R 2smethyl, will be referred to as polywmethyl-(phenyl)silylenex (PMPSi).

latter case, the situation is more complex: the change of the chemical structure and a charge redistribution on the substituent, and a charge redistribution between the molecular wire and the photochromic group during the photochemical transformation may influence the ionization energy of the entire polymer segment containing the polar side group. Such a modification may result in the creation of a chemical trap on the segment w17,18x. Another problem, which should be solved, is that of the length of the spacer. From the point of view of chemical synthesis it is difficult to attach chemically the photochromic group directly to the polymer chain. The

energy of electrostatic (charge–dipole) interactions depends on the distance between the interacting species. Therefore, the question arises what is the maximum length of the spacer at which the charge–dipole interactions are still on a suitable level of the operation. The question can be answered on the basis of numerical calculations w6–9x of the polarization energy, performed on a model molecular lattice. The results showed that the number of molecules whose energies are perturbed by the presence of a polar dopant strongly depends on the dipole moment. Hence, one can calculate the average cross-section of the local states created due to the

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271

Fig. 1. Shape of the HOMO-Si orbital of decawmethyl(phenyl)silylenex. Positive and negative values of the wavefunction are indicated with grey and black colours, respectively.

presence of the dipoles. For the purpose of this article, however, the critical distance (rcrit), defined as a dipolar equivalent of the Coulombic radius, will be estimated from the basic equation describing the ion–dipole interaction in a continuous dielectric medium w25x. Limiting oneself for the most favourable geometry of the system, one may write B

™™

D

4p´´okT

E1y2

ep cos(r,p) F rcritsC G

.

(1)

Here e is the unit charge, p is the dipole moment, ´ is the relative electric permittivity (assumed equal to 3.4), ´o is the permittivity of free space, k is the Boltzmann constant, and T is the temperature. Limiting oneself to ™ ™ and the most favourable geometry of the system (rIp), taking ps10 D, one may estimate rcrit to amount to approximately 1.8 nm at room temperature, equivalent to 12 C–C bonds. In fact, the spacer should be significantly shorter for the dipolar trap to create a sufficiently deep trap on the chain. It should be mentioned here that the estimate has been made neglecting the effect of the non-zero polarizability of the polar side group and any field-induced change of its orientation. One may demonstrate, however, that both effects may be neglected in semi-quantitative calculations presented in this article. For typical values of the molecular polarizability, the contribution of the induced dipole moment to the energy of ion–dipole interactions at the distance of 1 nm amounts to approximately 6% of the total energy and decreases with the distance. Similarly, a reorientation of the dipole by 208 would result in some 3–5% change.

Fig. 2. Changes of Si–Si bond lengths (a) and Si–Si–Si bond angles (b) during the positive polaron formation in decawmethyl(phenyl)silylenex.

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In the above discussion, we took into account the electrostatic component only, i.e. we assumed a ‘totally neutral’ spacer linking the polar moiety to the chain.

One should expect, however, that the presence of any conjugated spacer, allowing for a redistribution of charge between the chain and the side group, would result in

Fig. 3. HOMO orbitals of the PMPSi molecular wire with spiropyran side groups in closed S and open M form. HOMO-LOC ‘M’ means the orbital where charge is localized after the phototransformation.

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additional effects which will be discussed in the next section. 3. Factors influencing functioning of the switch

Table 1 Ionization energies Ic and dipole moments p of 7MPSi, the heptamer with chemically attached closed and open forms of spiropyran (7MPSi-S and 7MPSi-M, respectively), and dipole moments of isolated closed and open form of the photochromic system (S and M, respectively)

3.1. Charge carrier transport in 1D molecular wire Because the chain of the molecular wire is conjugated, one can expect quite effective charge carrier transport. The question then arises why the values of on-chain mobilities are so low (cf. the data in the preceding section and in Appendix A). One possible reason is the formation of polarons. The strong electron–phonon coupling causes carrier self-trapping and creates a quasiparticle, a polaron, which can move only by carrying along the associated molecular deformation. The motion of such a charge carrier, dressed into a cloud of local deformation of the nuclear subsystem, can be phenomenologically described by introducing a temperaturedependent effective mass which is higher than the mass of the electron. The formation of polarons in poly(silylene) samples has been confirmed using the technique of thermally stimulated luminescence w26x. The presence of polarons results in a deformation of the Si–Si bond lengths and of the inter-bond angles. The change can be visualized for the limiting case of a charge fully localized on a site of the Si–Si chain. The results of quantum-chemical calculations (Appendix A), shown in Fig. 2, demonstrate that the presence of the charge on the molecule indeed deforms the chain. Thus, one may expect that even a partial localization of the charge results in a substantial deformation of the chain parameters extended over several Si–Si bonds. Another important feature influencing the charge movement on a molecular wire is the presence of traps. An isolated molecular wire is a 1D system in which the presence of any defect results in a localization of charge carriers w27,28x. Consider a system containing traps which are neutral when empty. In a 3D system, once such traps are filled, they act as scattering centres for other carriers approaching their sites, without otherwise hindering their motion. The situation is, however, qualitatively different in 1D systems: (i) Any localized carrier becomes an obstacle rendering it impossible for another carrier to follow the same path. Because in strictly 1D systems the probability of lateral jumps is none, any filled ‘primary’ trap produces a ‘secondary’ localization centre at a distance of the order of the Coulombic radius rc. In other words, a single primary trap may localize several carriers. This phenomenon was referred to as ‘degenerate trapping’ w27,28x. (ii) Both the trapping and release rates from primary traps are electric field dependent w29x. Consequently, the kinetics of filling and emptying the secondary traps should also depend on electric field.

273

Ic (eV)

7MPSi 7MPSi-S 7MPSi-M

7.4 7.6 6.9

Resulting dipole moment of the segment of the molecule wire p (D) – 9.3 20.1

Dipole moment of the isolated unit (S or M) p (D) S M

– 6.6 14.6

In real 1D systems, the situation is more complex but in any case the effect of traps should be more pronounced than in isotropic 3D solids w28x. 3.2. Dipolar traps vs. chemical traps Apart from changes of the dipole moment of the side group, the formation of the ‘open’ merocyanine form results in changes of the ionization energy of the polymer segment containing the dipolar side group. The situation is demonstrated in Fig. 3. The result of quantum mechanical ab initio Hartree–Fock calculations is given in Table 1. The polymer chain (molecular wire) was represented in the calculations by an oligomer of PMPSi, heptawmethyl(phenyl)silylenex ((IX), Scheme 1), the side group R was that of spiropyran (Scheme 1, (IV)), either in its closed form (hereafter denoted S) or a strongly polar open form (merocyanine, referred to as M), linked by a spacer (VIII) in Scheme 1. As polysilylene is a hole-transporting material, chemical traps affecting the charge transport are expected to occur on sites with locally modified ionization energies. Considering the behaviour of 1D molecular-scale systems, one should, however, introduce an important distinction. As was already mentioned above, isolated molecular wires should behave as 1D systems, where there is no scattering. In this case, both the increase and decrease of the ionization energy should result in the formation of local states on the molecular wire. It becomes evident from a comparison of ionization energies given in Table 1 that holes should be nearly unaffected by the vicinity of a spiropyran unit in the closed form S, but they will be effectively localized when spiropyran unit will be in the open form (metastable form, merocyanine, M). The influence of the side group on the modification of the ionization energy of the molecular wire extends several monomeric units, as follows from the calculation of the parameters of methyl(phenyl)silylene oligomers of different lengths w30x. Thus, the general picture for the charge carrier switching is fulfilled—the open form generates local centers capa-

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ble of trapping holes. This contribution is mainly related to the change of the ionization energy given by charge redistribution, orbital occupation, and geometry of the molecular segments during the photochromic conversion. The charge redistribution is demonstrated in Table 2. The merocyanine (M) form is more polar as follows from the differences of the atomic charges on atoms 39S and 21C. Whilst the dipole moment of free standing side group changes from 6.6 (S) to 14.6 D (M) during

the phototransformation, the change in the case that the photochromic unit is chemically attached to the polysilylene chain is from 9.3 to 20.1 D. This feature demonstrates the participation of electrons of the main chain in the photochromic process of the side group and points to the importance of the chemical structure of the spacer. The formation of local states for holes during the photochromic transformation can also be inferred from the characters of the orbitals as follows from Fig. 3.

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275

Fig. 4. Values of the ionization energies for PMPSi molecular wire with spiropyran (S) and merocyanine (M) photochromic groups.

The character of the HOMO‘S’ orbital of 7-MPSi-S is nearly the same as that of 7-MPSi (cf. Fig. 1). During the phototransformation, the orbital HOMO‘M’ maintains nearly the same energy position, but orbital HOMO-1‘S’ before the phototransformation is shifted to lower energy and represent actually localized HOMOLOC‘M’ level after the phototransformation. Thus, after the phototransformation the highest HOMO level is the localized one, the charge is localized on the M-side group. To be re-activated to the main chain, this charge should overcome the potential barrier wE(HOMO-Si)– E(HOMO-LOC‘M’)x ;0.5 eV and move through the main chain backbone. Alternatively, it tunnels between the side groups of the molecular wire provided the intersite distances are sufficiently short. Finally, a possible total effect of the chemical and dipolar traps on the on-chain charge mobility should be considered. The case will be illustrated with PMPSi containing spiropyran side groups. The energy diagram summarizing results of quantum-chemical calculations is shown in Fig. 4. The ionization energy of PMPSi

molecular wire, representing the energy of holes transported on the molecular chain measured with respect to the vacuum level, is Ics7.4 eV. If a spiropyran (S) unit is chemically bound on an MPSi segment, the ionization energy is higher, Ics7.6 eV, and after the phototransformation of S to merocyanine (M) lower, Ics6.9 eV. In the latter case, the local states can be formed. These changes are of chemical origin, given by charge redistribution in the PMPSi—spiropyranymerocyanine system. If a charge is injected into the wire, an additional modification of Ic—an electrostatic contribution given by charge–dipole interactions—must be taken into account. Here, it is y0.3 eV for the PMPSi-S system and y0.6 eV for PMPSi-M. For the final effect, we must take into account both contributions. The result is that MPSi-S units in PMPSi wire only slightly influence the on-chain mobility (cf. Ic of PMPSi 7.4 eV and of PMPSi-S 7.3 eV). The situation is different after the phototransformation. Here, the differences of the ionization energies are quite high, 7.4 eV for PMPSi vs. 6.3 eV for PMPSi-M. Thus, the localized states for holes

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Fig. 5. Schematic experimental arrangement for TRMP.

are created with energies in the interval 0–1.1 eV, if the effect of chemical and dipolar traps is additive. However, in reality, the situation can be more complex and strongly dependent on the chemical structures of the molecular wire and photochromic side groups. 4. Conclusion The estimates put forward in this article point to a possibility of a controlled modulation of charge carrier mobilities by a light-induced reversible photochemical reaction. Moreover, one may envisage the construction of a molecular-scale switch acting on the same principle. Discussing the architecture and function of the molecular switch, it is important to mention the role of the spacer. With a neutral spacer fixing the geometry of the photoactive side group, the electrostatic contribution of the dipole to the function of the switch is dominant. With the spacer electronically connecting the side group with the main chain, i.e. allowing for a charge redistribution, the trapping of a charge carrier on the side group becomes more probable if only its ionization energy is lower than that of the main chain. The electrostatic contribution is then superimposed on the main term. In the former case, one deal with a dipolar trap created on the chain due to electrostatic interactions of the carrier with the dipole of the side group, whereas the latter case is equivalent to creation of a chemical trap mainly on the side group.

Acknowledgments The research was supported by the Grant Agency of the Academy of Sciences of the Czech Republic (Grant No. AV0Z4050913), Grant Agency of the Czech Republic (Grant No. 202y01y0518), and by the Polish State Committee for Scientific Research (Grant No. 4 T09A 132 22). The quantum-chemical calculations were supported by the Grant Agency of the Czech Republic (Grant No. 203y02yD074). The computer time in the Joint Supercomputing Center at the Czech Technical University (Research plan MSM 216200031) is gratefully appreciated. The authors thank Prof. J. Lipinski for numerous discussions and helpful comments. Appendix A: (a) Hole mobility in PMPSi Measurements of the mobility of holes in polywmethyl(phenyl)silylenex were carried out using the time-resolved microwave photoconductivity (TRMP). The basic experimental arrangement is given in Fig. 5. TRMP measurements were performed with the aid of Ka-band equipment (30.4 GHz). Microwaves generated by a tunable Gunn oscillator were directed via a broadband isolator, an attenuator and a circulator to the sample. The reflected microwaves separated by the circulator passed, via a microwave switch, either to a power meter (Hewlett Packard, 432B) or to a fast detection diode (Hughes, 47321H-2111). The latter was

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277

Fig. 6. TRMP trace for PMPSi lexcs355 nm, bandwidth of the amplifier 500 MHz.

connected to the vertical plug-in of a storage oscilloscope (Tektronix, 7834). The polymer powder, positioned at the open end of the cell branch of the waveguide system, was irradiated with 12 ns light flashes emitted from a Nd–YAG laser (1064 nm), using the third (355 nm) harmonics. Generally, three types of charged species can be detected by the microwave detection method: free and localized charge carriers, and correlated charges (i.e. dipoles). In organic semiconductors, in initial stages of the excitation, mainly electron– hole pairs, in which the separation between the charged species is smaller than the Onsager ‘escape distance’ rc (i.e. a distance at which the Coulomb energy is equal to kT)

fast chain geminate recombination to a significant extent. The remaining portion of ion pairs escape the fast geminate recombination presumably due to the fact that s-conjugation extents only over a limited number of silicons with the consequence that only electron–hole pairs formed in the same conformational chain segment can recombine fast. The remainder of electron–hole pairs relaxes to the ion-pair (s, p*) charge-transfer state. This implies a transfer of the electron from the silicon backbone to the pendant phenyl group. For the homogeneous diffusion-controlled recombination of electrons and holes, the recombination half-time tR is given by the relation

e2 rcs , 4p´´okT

tRs (A1)

are detected by the microwave technique. A decay curve of the microwave signal then reflects the recombination kinetics of the pairs generated on the main chain of the molecular wire. The TRMP trace w22x measured on PMPSi powder is shown in Fig. 6. The signal shows a dispersive behaviour: at least two components of the decay can be distinguished. The initial rapid decay of the microwave signal with a half-life t1y2s(80"20) ns, ascribed to the fast geminate recombination, is followed by a slow process. The kinetics of the latter process can be fitted with a stretched exponential function up to the milliseconds range. Excitation at 355 nm results in the fast formation of electron–hole pairs in the Si backbone that undergo a

´´o emNp

(A2)

where m is the charge mobility and Np is the pair concentration. Taking tR (st1y2) from the fast part of the experimental kinetics and Np equal to 1.2=1021 my3 (Np was determined by the calibration of the microwave signal using TiO2), one can estimate the value of the on-chain mobility as 2=10y6 m2 Vy1 sy1, in a good agreement with the data presented in Ref. w31x. This value is, very probably, still limited by conformational disorder and structural defects in the backbone. In the case of polysilylene chain, a perfect all-trans arrangement of the s-bonds has been shown to be the optimal configuration of intrachain electronic coupling w32,33x. Lower on-chain mobilities in s-conjugated polymers in comparison with p-conjugated ones can be attributed

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to a greater conformational disorder and a decrease in the overall electronic coupling due to lower barriers to conformations other than all-trans. The long-time process can be related to the intrachain recombination (Si chain cation-radicals with phenyl anion-radicals) andy or to the interchain electron–hole recombination. The latter value must be close to the zero-field drift mobility measured on 3D samples as was indeed observed (at room temperature m(F™0)f10y9 m2 Vy1 sy1 w23x, whereas from the measurements of microwave conductivity one obtains 2=10y9 m2 Vy1 sy1 w31x). (b) Quantum-chemical calculations The conformations of the neutral oligomers under studies were optimised by means of the Hartree–Fock (HF) methods and Becke’s three parameter hybrid method using the LYP correlation functional (B3LYP) at 3-21G(*) level. Both methods yield almost the same equilibrium conformations. The conformation of the positive polaron was calculated by B3LYP method. The oligomers were capped by methyls. Population analysis (atomic charges, dipole moments, ionization energies and molecular orbitals) was done using the HF method and 6-31G* basis set. The computer program GAUSSIAN 98 was used for the calculations. References w1x F.L. Carter (Ed.), Molecular Electronic Devices, M. Dekker, New York, 1982. w2x S. Nespurek, ˇ ˚ J. Sworakowski, IEEE Eng. Med. Biol. 13 (1994) 45. w3x D.I. Gittins, D. Bethell, D.J. Schiffrin, R.J. Nichols, Nature 408 (2000) 67. w4x Z.J. Donhauser, B.A. Mantooth, K.F. Kelly, L.A. Bumm, J.D. Monnell, J.J. Stapleton, D.W. Price Jr., A.M. Rawlett, D.L. Allara, J.M. Tour, P.S. Weiss, Science 292 (2001) 2303. w5x J. Chen, M.A. Reed, A.M. Rawlett, J.M. Tour, Science 286 (1999) 1550. w6x J. Sworakowski, Proc. SPIE 37DP (1999) 83. w7x J. Sworakowski, S. Nespurek, ˇ ˚ Polish J. Chem. 72 (1998) 163. w8x J. Sworakowski, IEEE Trans. Diel. Electr. Insul. 7 (2000) 531. w9x J. Sworakowski, S. Nespurek, ˇ ˚ in: A. Graja, B.R. Bulka, F. Kajzar (Eds.), Molecular Low Dimensional and Nanostructured Materials for Advanced Applications, Kluwer Acad. Publishers, Dordrecht, 2002, p. 25.

w10x S. Nespurek, ˇ ˚ ´ J. Pfleger, E. Brynda, I. Kmınek, A. Kadashchuk, A. Vakhnin, J. Sworakowski, Mol. Cryst. Liq. Cryst. 355 (2001) 191. w11x H. Valerian, ´ E. Brynda, S. Nespurek, ˇ ˚ W. Schnabel, J. Appl. Phys. 78 (1995) 6071. w12x S. Nespurek, ˇ ˚ ´ A. Eckhardt, V. Herden, W. Schnabel, H. Valerian, Polym. Adv. Technol. 12 (2001) 306. w13x A. Dieckmann, H. Baessler, P.M. Borsenberger, J. Chem. Phys. 99 (1994) 8136. w14x R.H. Young, J.J. Fitzgerald, J. Chem. Phys. 102 (1995) 9380. w15x S. Nespurek, ˇ ˚ J. Sworakowski, A. Kadashchuk, IEEE Trans. Diel. Electr. Insul. 8 (2001) 432. w16x A. Kadashchuk, N.I. Ostapenko, Yu.A. Skryshevskii, V.I. Sugakov, M.T. Shpak, Pisma v Zh.E.T.F. (USSR) 46 (1987) 165, English transl. JETP Lett. 46 (1987) 207. w17x J. Sworakowski, Mol. Cryst. Liq. Cryst. 11 (1970) 1. w18x E.A. Silinsh, Organic Molecular Crystals. Their Electronic States, Springer, Berlin, 1980, Ch. 2 and 3. w19x S. Nespurek, ˇ ˚ J. Sworakowski, Thin Solid Films 393 (2001) 168. w20x S. Nespurek, ˇ P. Toman, J. Sworakowski, J. Lipinski, in: A. Graja, B.R. Bulka, F. Kajzar (Eds.), Molecular Low Dimensional and Nanostructured Materials for Advanced Applications, Kluwer Acad. Publishers, Dordrecht, 2002, p. 37. w21x F.C. Grozena, L.D.A. Siebbeles, J.M. Warman, S. Seki, S. Tagawa, U. Scherf, Adv. Mater. 14 (2002) 228. w22x S. Nespurek, ˇ V. Herden, M. Kunst, W. Schnabel, Synt. Met. 109 (2000) 309. w23x S. Nespurek, ˇ ˚ A. Eckhardt, Polym. Adv. Technol. 12 (2001) 427. w24x H. Durr, ¨ H. Bouas-Laurent (Eds.), Photochromism, Molecules and Systems, Elsevier, Amsterdam, 1990. w25x C.J.F. Bottcher, ¨ O.C. van Belle, P. Bordewijk, A. Rip, Theory of Electric Polarization, vol. 1, second ed., Elsevier, Amsterdam, 1973. w26x V.I. Arkhipov, E.V. Emelianova, A. Kadashchuk, I. Blowsky, S. Nespurek, D.S. Weiss, H. Baessler, Phys. Rev. B 65 (2002) 165218. w27x J. Sworakowski, G.F. Leal Ferreira, J. Phys. D: Appl. Phys. 17 (1984) 135. w28x J. Sworakowski, M. Samoc, IEEE Trans. Electr. Insul. EI-22 (1987) 13. w29x E.G. Wilson, J. Phys. C: Solid State Phys. 13 (1980) 2885. w30x P. Toman, J. Lipinski, unpublished results. w31x H. Frey, M. Moller, ¨ M.P. de Haas, N.J.P. Zenden, P.G. Schouten, G.P. van der Laan, J.M. Warman, Macromolecules 26 (1993) 89. w32x R.D. Miller, J. Michl, Chem. Rev. 89 (1989) 1359. w33x J.W. Mintmire, Phys. Rev. B 39 (1989) 13350.