Supramolecular aspects of organic conductors

Chapter 7

^^___

SUPRAMOLECULAR ASPECTS OF ORGANIC CONDUCTORS Tomoyuki Akutagawa, Tatsuo Hasegawa, Takayoshi Nakamura* Research Institute for Electronic Science, Hokkaido University, N12W6, Kita-ku, Sapporo 060-0812, Japan Contents 1. 2.

3.

4. 5. 6.

Introduction TTF-based Supramolecular System 2.1. Multi-TTF Annulated Molecules 2.2. TTF Cyclophanes 2.3. TTF Annulated Crown Ethers 2.4. TTF Catenanes and Rotaxanes Supramolecular Entities in the Molecular Conductor Crystals 3.1. Supramolecular Cations in TCNQ and DCNQI-based Molecular Conductors . . . 3.2. Supramolecular Cations in Ni(dmit)2-based Molecular Conductors 3.3. Supramolecular Anion Structures in TTF-based Molecular Conductors Molecular Conductors in Ordered Thin Films 4.1. Langmuir-Blodgett Films 4.2. Self-assembly Films Supramolecular Aspects of Electronic Phenomena in Organic Charge-Transfer SoUds . . 5.1. Collective Electronic Phenomena in Charge-Transfer Complexes 5.2. Supramolecular Aspects of Electronic Phenomena in BEDT-TTF Complexes . . . Concluding Remarks References

1. INTRODUCTION The field of supramolecular chemistry maintains an important position in science. There are several definitions for the term "supramolecular chemistry" or "supramolecules" and quite a few books and articles are already pubHshed on the subject [1-4]. Here we define the supramolecular chemistry in a usual way as a field concerning the supramolecular entities composed of molecules (m addition to atoms and ions) without covalent , , ^ \. ^. , , ./. ^ ^ J bond formation. Since supramolecular entities are constructed r 1 / 1 \ u j: 1 1 ^u 1 1 from several (or large) number of molecules, the supramolecules often have beautiful shapes, which is one of the motivations for research in the supramolecular-chemistry field. The more important point, however, is that a supramolecular entity shows functions which cannot be attained by a single molecule. The functions of supramolecular entities arise from the intermolecular interactions, which are determined by the relative position

267 268 268 269 270 271 272 273 274 279 280 280 286 288 288 291 297 297

of each molecule in a three-dimensional assembly and its temporal change. The spatial-temporal control over the molecular positions and interactions to realize highly desirable and sophisticated functions is the main interest of the supramolecular chemistry. Organic conductors are divided into two categories, i.e., ^^olecular conductors and conducing polymers. In this chapter, ^ ^ ^^.^j ^^^^^^ ^^^ ^^^^^^ ^^^j^ j^ ^^^ j ^ ^ ^ ^ ^ .^ ^^^^ .^^^^^^^_ . r ^^ • - . r .-i i-.T. . J J mg from the viewpoint of practical application. Interested readr . . . r .ers are referred to the volume of proceedings on the progress in , . ^ ,, r i i , n ^^'' ^^^^ t^^ According to the definition m the previous para^^^P^' ^^^ ^he molecular crystals including molecular conductors ^^^ supramolecular entities. Of course, we do not discuss the molecular conductors in general in the following sections. However, before discussing our subject, we will briefly discuss the common structural feature of the molecular conductor [6].

* Corresponding author

ISBN 0-12-513753-2/$35.00

Handbook of Advanced Electronic and Photonic Materials and Devices, edited by H.S. Nalwa Volume 3: High T^ Superconductors and Organic Conductors Copyright © 2001 by Academic Press All rights of reproduction in any form reserved. 267

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AKUTAGAWA ET AL.

Molecular conductors are composed of redox active molecules. If a molecule of small ionization potential (donor, D) is combined with a molecule of large electron affinity (acceptor, A), a part of the charge is transferred from D to A to form a charge-transfer complex D+^A~^(0 < 6 < 1). The D or A may be closed-shell ions. In these cases, the cation radical salts or anion radical salts are formed. The D and A molecules should have an extended 7r-electron system. The 7r-molecular orbitals overlap each other through the stacking of D or A molecules in the crystal to form 7r-electron bands of highest occupied molecular orbital and/or lowest unoccupied molecular orbital (HOMO and/or LUMO bands). If the charge on the molecule is less than unity, the band is partially filled and high electrical conductivity is expected (a more expHcit picture is presented in Section 5). The common structural feature of the molecular conductors is the overlap of the 7r-orbitals between adjacent molecules, and only through this overlap, the electrical conduction, which cannot be attained by a single molecule, emerges. Consequently, the molecular conductors are the typical example of the supramolecular entity. The purpose of this chapter, as previously mentioned, is not to review the general feature of molecular conductors as supramolecular entities. On the contrary, we rather select the molecular conductors which have supramolecular structures and functions other than those common to molecular conductors. In the next section, we start with individual molecules designed for constructing supramolecular entities. These molecules already form interesting supramolecular systems in solution. The crystals, which have intriguing supramolecular structures such as ion cavities and channels, are also reviewed in Section 3 in connection with the structure-function relationship. We deal with the thin films of molecular conductors in Section 4. These films are promising candidates for the apphcation of molecular conductors in future molecular electronic devices. In the films, we can control the molecular assembhes and thus we can control the functions of molecular conductors to a certain extent. The Langmuir-Blodgett technique [7] is mainly reviewed, which was apphed to the molecular conductors. The self-assembly film technique [8, 9], which shows rapid progress, is also addressed. In Section 5, we review the electron and spin systems as supramolecular entities. Since the molecular conductors have highly correlated electronic systems, conduction electrons mainly locate on each molecule in addition. The main subject of this section is not the molecular assembly in crystals but the structure of exotic electronic systems that emerge from molecular assemblies. The dynamics of electrons and spins in terms of supramolecular chemistry is the subject of this section. In these sections, we describe the supramolecular entity as a self-assembly system. Science in complexity drew attention to the self-organizing system such as a dissipative structure far from the equilibrium [10]. The supramolecular chemistry should set foot on this field in the near future. The future molecular electronic devices may also be self-organizing systems composed of those self-assembly systems interconnected to each other. In the concluding remarks, we go back to this point. 2. TTFBASED SUPRAMOLECULAR SYSTEM The supramolecular entities with active 7r-electron systems were extensively prepared, which included redox active donor and

^S

S^^S

ON

"S

S" ^s

ON

CKX

S

S-^sCs*

Scheme I. acceptor molecules in connection with constructing novel molecular conductors. The redox active 7r-molecules typically employed in molecular conductors are /?-benzoquinones, tetracyano;7-quinodimethane (TCNQ), DCNQI, M(dmit)2, and fuUerene for electron acceptors, and tetra thiafulvalene (TTF), aromatic amines, and phthalocyanines for electron donors [6, 11]. In the case of electron acceptors, the synthetic approach is limited and only the molecular frameworks of /?-benzoquinone or nitrobenzene having macrocyclic moiety were utilized for ion-sensing [12]. On the other hand, the synthesis of TTF-based supramolecular systems advanced considerably since the discovery of the powerful protection-deprotection protocol of the cyanoethylene groups of the outer substituted sulfur atoms in the TTF framework, which enabled us to isolate complex TTF-based molecules (Scheme I) [13]. The protected cyanoethylene groups can be easily deprotected using cesium hydroxide, then the various functional units can be incorporated into the TTF molecular framework, which are the useful building blocks for supramolecular systems. The TTF-based complex 7r-systems were applied to construct supramolecular systems in which the ion-electron, molecular conformation-electron, and molecular motion-electron couplings are realized. The ion-electron interaction described here is an interconversion of redox properties through the ion-recognition process in solution. The molecular conformation (or motion)— electron coupling is the redox induced conformational transformations of the molecule. These transformations are accompanied by the drastic change in optical properties and are proposed as possible candidates for the molecular switching or memory devices. In this section, we overview multi-TTF annulated molecules, TTF cyclophanes, TTF annulated crown ethers, and TTF catenanes and rotaxanes (Fig. 1) as examples of the TTFbased supramolecular system. 2.1. Multi-TTF Annulated Molecules When the multihalogenated reagents are used to react with the dithiolate TTF intermediate, macrocycles which contain more than two TTF units (Scheme II) are obtained [14]. The yield of these multicomponent products depends on the condensation reaction, which is usually prepared in high-dilution conditions. The TTF dimers are used as the 7r-electron donors in the molecular conductors for constructing two- or three-dimensional electronic 7r-band structures (Fig. la) [15]. Among them, highly electrical conducting cation radical salts and CT complexes were obtained. For tris-TTF annulated analogues, the cation radical salts of tris(dimethylthioTTF), X = CH2CH2CH2 in Scheme II, were reported [16]. Since the Unker groups of each TTF unit have a structuralflexibihtywithin the molecule, the TT-TT overlap mode within the crystal depends on its molecular conformation. The ideal C3 symmetry of the tris(dimethylthio-TTF) molecule was broken in the cation radical salts with Ij and IBr2 (Cj symmetry). Two TTF molecules within the molecule formed the intramolecular dimer structure, while the molecular plane of the other TTF

SUPRAMOLECULAR ASPECTS OF ORGANIC CONDUCTORS

a.

269

b.

CK r^o

^ 0 ^

s-'^s

-o

S

s

e.

A "Y >k T

- T -

Fig. 1. TTF-based supramolecular systems, (a) multi-TTF system, (b) macrocyclic TTF system, (c) TTF cyclophane, (d) TTF catenane, and (e) TTF rotaxane.

MeS...^S

T

>=< T

Br~X-Br

S^-^S-X—S^

MeS

SMe

energy conversion systems, and electron transport systems at the surface and within the restricted area etc. [17].

SMe

2.2. TTF Cyclophanes

rCs'S-^ - ^^ ^ :SMe SMe etc.

Scheme II.

unit was normal to that of the TTF dimer. Cydic multi-TTF derivatives with a larger number of TTF units, tetrakis-TTF and pentakis-TTF, were already prepared by the stepwise protectiondeprotection protocol. The crystal structures and molecular conformations of these donors, however, are not reported yet. The information on molecular conformation of these multi-TTFs with an open-shell electronic state is desired for designing threedimensional TT-electronic systems in the molecular conductors. The TTF dendorimers with controlled molecular weights were also extensively studied [17-20]. The redox activity of TTF units is retained in these dendroimers, in which all TTF units undergo two-step single electron oxidations producing highly charged species in solution. These extended TTF-based molecular systems are the promising candidates to develop single-molecular conducting systems, multi-redox active electron-transfer catalysis.

The other molecules we are currently interested in are the TTF cyclophanes in which two or more TTF molecules are covalently linked through two or more linkers (Fig. lb) [21-29]. The TTF cyclophane with crisscross-overlapped arrangement of TTF units is synthesized by two separated groups [30-32]. Since the molecular arrangement of each TTF unit within the molecule is restricted by the covalent bonds and has steric constraints of 77-77 interactions, the redox properties of TTF cyclophanes are influenced from the other TTF unit within the molecule [27, 32]. In general, the first oxidation potential shifted to lower potential compared to the corresponding monomer TTF due to delocalization of the positive charge over both TTF units, while the second oxidation potential shifted to higher potential due to the Coulombic repulsion between the oxidized TTF units. The open-shell species of TTF cyclophanes were isolated as CT complexes with TCNQ and cation radical salts with CIO4, PF^, and Br~ as counteranions [31, 32]. The electrical conductivities of these salts were in the range of the insulator (p^j = 10~^ ^ 10~^n cm) due to insufficient intermolecular interactions within the crystals. More complicated cage-type novel macrobicyclic trisTTF cyclophanes were also prepared using the stepwise selective protection-deprotection of TTF-thiolate under high dilution conditions [33]. The neutral macrobicyclic tris-TTF cyclophane had the Cj symmetry rather than the C3 in the crystal. The crystals of the open-shell electronic species were not obtained. The cagetype donors in the open-shell electronic state should be good candidates for constructing a three-dimensional 77-electron system when the counteranions are included in their cage.

270

AKUTAGAWA E T AL.

Co

(^^

s.

3 3

"

C

sX>-
SMe

4

Fig. 2. Typical molecular structures of macrocyclic TTF 1 ~ 4.

2.3. TTF Annulated Crown Ethers Molecules for ion-electron interaction systems can be synthesized by hybridizing redox active 7r-molecules with ion-sensing moieties such as crown ethers. The crown ethers can include ions in their cavity. The coordination abiUty depends on the cavity structure [34, 35]. For instance, typical oxygen-containing crown ethers have high ion selectivity for alkali and alkaline earth metal ions, while the sulfur-containing crown ethers (thiacrown ethers) can recognize transition metal ions [36, 37]. In addition to crown ethers, a large number of host molecules such as cryptands, macrocyclic polyamines, cyclodextrines, cyclophanes, calixarenes etc., were synthesized for ion- or molecular-recognition [34, 35]. Figure 2 shows the examples of macrocyclic TTF analogues for the ion-electron interaction system. Molecule 1 was first synthesized by Otsubo and co-workers in 1985 [38]. After that, similar TTF annulated crown-ether derivatives were extensively synthesized by Becher et al. [39-42] and Dicing et al. [43]. Until now, various types of ion-sensing macrocycles such as azacrown ether, thiacrown ether, and calix[4]arene were introduced into the TTF-based molecular framework [44-48]. The crown ether part usually has an ion-sensing abihty. Donor 1, however, did not show such properties in solution [38] and formed CT complexes with strong electron acceptors such as 2,3,5,6-tetraf[urotetracyano-/?-quinodimethane (F4TCNQ), TCNQ, dichlorodicyano-;?-benzoquinone (DDQ), and HCBD by the direct mixing method [49]. Since the crystal structure of the (1)(TCNQ) was composed of mixed stacks, the electrical conductivity at room temperature was quite low (-1.5 X 10-^ S cm-^. The simple salts of (l)(AgN03), (l)(CuCl2), and (l)(PdCl2) were also prepared, however, the evidence of ion inclusion within the 15-crown-5 cavity was not obtained yet. Donor 2 first showed ion-sensing redox properties (Fig. 2) [42], and later, donors 3 and 4 also proved to have such properties. When a molecule has one TTF and one crown ether, it may have six identical states in solution according to its chargetransfer (CT) and ion coordinated (ICO) states (Fig. 3). In Figure 3, vertical and horizontal directions correspond to the ICO

crownTTF"^

TiTTF^^

MVownTTF)^^

Fig. 3. Charge-transfer (CT) and ion-coordination (ICO) (IR) diagram of monocrown ether-annulated TTF (crown TTF). Vertical and horizontal processes correspond to one-step ICO and two-step CT processes, respectively. Six kinds of chemical species exist in this diagram.

and CT processes, respectively. The two-step oxidation processes can be easily evaluated by the cyclic voltammetry (CV) method in solution. The difference of E^^ and El^ (E^hift = ^ox " ^ox) means the ion-sensitivity of the redox active 7r-system. Figure 4a shows the CV chart of donor 2. Since donor 2 has two ion-sensing 18-crown-6 units per one TTF, nine kinds of species according to CT and ICO processes should be considered (Fig. 4b). The CV curves of free 2 (line b) and 2 + 250 folds of Na+ (line a) in CH3CN (0.1 M TEA PFg vs. Saturated calomel electrode (SCE)) show a clear difference in the redox properties [42]. The two-step reversible oxidation peaks are observed at 0.48 and 0.64 V (route i) in the case of the free 2 (soUd line in Fig. 4a). Under the excess Na+ condition (dashed Une in Fig. 4a), the positive peak shift of the first oxidation potential (E^hift = +80 mV) was observed, which corresponds to the oxidation process of Na"^(2) -^ Na^(2)+ (route ii). Since the Coulomb repulsion energies of ion-including species such as the Na+(2)+ and Na+(2)2+ are larger than those of free 2+ and 2^+ species, the first oxidation potential is expected to shift to the positive direction. On the other hand, the second oxidation potential

SUPRAMOLECULAR ASPECTS OF ORGANIC CONDUCTORS

< 1.5

MeS

4PR

271

MeSv-S

S-^SMe k.0

•^::r b. E „ ' = -K).48V

(Na-')2 .iL

Eo/ = -H).56V '' ..:^ - . . ^ (Na"*)(2)^

1 l0gKe2

(NaV

(2f

"• I logK,5

logK,3

logKe, = 2.5

E, ^ —

^

(Na-')2(2f

(Na^)(2)2^ l0gKe6

I l0gKc4

-^

b.

Eo/ = -K).64V

- (2f -

2

4PF,

<

^==

(Na^)2(2)'^ 400

Fig. 4. CT and ICO properties of the TTF macrocycle 2. (a) Cyclic voltammetry (CV) diagram of free 2 (solid line) and 2 + excess Na+ under 250 equivalent Na+ ion (dashed line) in CH3CN solution (0.1 M TBAPFg, vs. SCE). (b) CT and ICO diagram of TTF macrocycle (2). Vertical and horizontal processes correspond to two-step ICO and two-step CT processes, respectively. Nine kinds of chemical species exist in this system. Source: M. B. Nielsen et al., Chem. Commun. 475 (1998).

under the excess Na+ condition was consistent with that of free 2. Thus, the second redox process under the excess Na^ ions is the (2) -^ (2)2+ rather than the Na+(2)+ -^ Na+(2)2+. The improvement of the ion-capturing ability through chemical modifications of the macrocyclic TTF will provide a stronger ion-electron coupling redox system. 2.4. TTF Catenanes and Rotaxanes TTF catenanes were prepared from a macrocyclic compound by using the self-assembly strategy through charge-transfer interaction by Stoddart et al. [50-52]. The topologically complex macrocyclic TTF catenanes containing multi-TTF units were also prepared by Becher et al. [23, 27, 53-55]. These catenanes are obtained by treating the macrocyclic TTF with 1, r-[l, 4-phenylenebis(methylene)]bis-4,4'-bipyridinium bis(hexafluorophosphate) and l,4-bis(bromomethyl)benzene in dimethylformamide (DMF) and subjecting the mixture to high pressure (~ 10 kbar) for 4 days. Some interesting progress in the macrocyclic, catenane, and rotaxane TTF was reported from the viewpoints of the molecular electronic devices. The TTF macrocycle 5 shows the molecular conformation change accompanied by color changes [56]. The intramolecular CT complex formation in the self-assembly process is the origin of the molecular conformation change.

500

500

700 800 XI nm

900

1000 1100

Fig. 5. Conformational equilibrium of the TTF macrocycle. (a) Equilibrium between the CT (left-side) and non-CT (right-side) conformations, (b) Time-dependent UV-vis spectra in CH3CN. The inset is the intensity of the CT transition (~800 nm) vs. time (t/h). Source: J. Lau and J. Becher, Synthesis 1015 (1997).

Figure 5a shows a conformational equilibrium of molecule 5 between the intramolecular CT (left-side) and the elongated non-CT (right-side) forms. In the CT conformation, the TTF unit is surrounded by the electron acceptor of macrocyclic cyclobis(paraquate-/?-phenylene) moiety, and the TT-TT overlap causes the CT interaction. The crystalline state has a CT conformation with green color, and an adequate linker length is necessary to occur in the CT arrangement [56]. In the CH3CN solution, compound 5 has a non-CT conformation with yellow color. However, in several hours the solution changes its color from yellow to green. Figure 5b shows the time dependence of the UV-visible (UV-Vis) absorption spectra. The intensity of the CT transition at around 800 nm increases with time, and the equilibrium is achieved after 10 h. The electron-transfer assisted molecular conformation change was also reported in the TTF catenane (Fig. 6a) [57]. Both of the NMR and X-ray crystal structural analysis show the CT state of the TTF unit surrounded by cyclobis(paraquate/?-phenylene) both in solution and in solid. The deep bluishgreen color of the crystal originates from the CT interaction of the electron donating TTF unit to the electron accepting cyclobis(paraquate-/?-phenylene) moiety (left conformation in Fig. 6). The chemical oxidation of the TTF unit by Fe^+(C104)3 or o-chloranil in the CH3CN solution changes the color of the solution from dark green to maroon. Since the cyclobis(paraquate-j!?-phenylene) molecule has four positive charges on the molecule, the oxidized TTF cation feels

272

AKUTAGAWA ET AL.

CT interaction

.4/)?|oN|^^ \

^

»

Oxidation of TTF unit or Reduction of paraquat

HO

300

500

700 A/nm

900

1100

\ 0 /

OHzCv.^^

^s-

tf^Jlc.\^A)/"

'

—

Fig. 6. Conformational equilibrium of the TTF catenane. (a) Redox induced conformational changes between the green CT (left-side) and yellow non-CT (right-side) conformations, (b) The UV-vis absorption spectra in CH3CN solution (curve a) and after addition of Fe(C104)3 (curves b-d). Addition of two equivalent ascorbic acids (curve c). Source: M. Asakawa et al., Angew. Chem. Int. Ed. Engl. 37 (1998). Coulomb repulsion from the cyclobis(paraquate-;?-phenylene) moiety. As a result, the cyclobis(paraquate-;?-phenylene) moiety moves to the neutral naphthalene side (CT2 conformation in Fig. 6). This molecular motion is reversible and is recovered from the initial state by reduction with vitamin-C, Na2S205, and/or electrochemical reduction. Figure 6b shows the change in the UV-vis spectra during the redox process. The initial greencolor CT transition (X^^ax = 835 nm) disappears upon chemical oxidation, then new bands at 450 and 600 nm are observed, which are attributed to the intramolecular excitations of the TTF cation radical. In addition, weak absorption at 515 nm is observed due to the CT transition from the weak electrondonor naphthalene to the cyclobis(paraquate-/?-phenylene). The completely reversible transformation between two intramoleculer CT isomers is realized through the electron-transfer assisted intramolecular-conformation change in the TTF catenane. The TTF rotaxanes are also prepared by using the selfassembly method (Fig. 7) [23, 58-60]. The CT interaction between the electron donating and the electron accepting moieties is a driving force to create the rotaxane structure, in which the TTF and the cyclobis(paraquate-/?-phenylene) are the electron-donor and the electron-acceptor moieties, respectively. The applications to electrochemically induced molecular devices were proposed in the TTF rotaxane [60, 61]. These TTFbased supramolecular systems from the viewpoints of molecular conductors, molecular sensing systems, or molecular electronic devices are now extensively studied. The electrochemically induced molecular motion was reported in the TTF rotaxane [57, 61]. The TTF-based linear chain

or HO

OH2C.

^%r Fig. 7. Redox induced molecular motion of TTF rotaxane. The CT interaction between the TTF and bis(paraquate-/?-phenylene) moiety supports the initial rotaxan structure, and the oxidation of TTF or the reduction of bis(paraquate-p-phenylene) dethread the rotaxan structure. compound and the cyclobis(paraquate-/>-phenylene) form a selfcomplexing rotaxane structure (Fig. 7). This complex formation process also originates from the CT interaction between the TTF and the cyclobis(paraquate-p-phenylene). The reduction of cyclobis(paraquate-^-phenylene) or oxidation of the TTF unit increases the Coulomb repulsion, which dethreads the rotaxan structure of each component molecule. The reversible threading and dethreading processes in the solution are confirmed by the CV and UV-vis spectra. The electrochemically driven dethreading-rethreading molecular motion may be appHed to the molecular electronic devices.

3. SUPRAMOLECULAR ENTITIES IN THE MOLECULAR CONDUCTOR CRYSTALS Although the complex TTF-based supramolecules described earlier show interesting functions associated with redox reactions in solution, functions arising from bulk systems such as electrical conduction have not been obtained yet. In this section, we overview the molecular conductors based on rather simple molecules in which supramolecular entities play an important role. The molecular conductors can be conventionally classified into two types: the donor-acceptor type CT complexes such as

273

SUPRAMOLECULAR ASPECTS OF ORGANIC CONDUCTORS (TTF)(TCNQ), and the cation radical or anion radical salts [11]. The latter salts are composed of open-shell donor or acceptor molecules and closed-shell counteranions or cations. The TT-TT stacks of the open-shell electronic species provide an electrically conducting path along the stacking direction, while the closed-shell counterions are necessary to compensate the total charge of the crystal. A simple approach to introduce the supramolecular structure within molecular conductors is the chemical modifications of the counteranion or cation structures. Usually, simple counteranions (Cl~, Br~, I", I3, BF4, PF^, Cu(SCN~)2, etc.) or cations (Li+, Na+, alkylammonium, etc.) were employed within molecular conductors [11]. These simple counterions are replaced with the supramolecular cation or anion structures in the TCNQ, DCNQI, Ni(dmit)2, and TTF-based molecular conductors.

.
12-crown-4

18-crown-6

Dicyclohexyl-18-crown-6

O § ^bS> Dibenzo-18-crown-6

Diaza-18-crown-6

Cyclam

Di(8-quinolyl)pentaoxatridecane

xcc -^xKcy

3.1. Supramolecular Cations in TCNQ and DCNQI-based Molecular Conductors A large number of host molecules were synthesized, which form a supramolecular structure by including metal cations. The cation including host macrocycles were incorporated into anion radical salts. Figure 8 shows the examples of macrocycHc host molecules introduced into TCNQ, DCNQI, Ni(mnt)2, and Ni(dmit)2-based molecular conductors. The typical crown ethers such as 12-crown-4, 15-crown-5, and 18-crown-6 have high coordination abilities to alkali (M+) and alkahne earth metal ions (M^+) [34, 35]. The resulting M+(crown ether) or the M^+(crown ether) supramolecular cation structures can coexist with anion radicals in the crystal. The M+(crown ether) or the M^+(crown ether) structures were first incorporated into the TCNQ-based anion radical salt by Morinaga et al. [62]. A large number of M+(crown ether) type cation structures of the monovalent ions (Li"^, Na+, K+, Rb+, Cs+, and NH^) and crown ethers (12-crown-4, 15-crown-5, 18-crown-6, dibenzo-18-crown-6, dicyclohexyl-18-crown-6, and dibenzo-24-crown-8) were tried to be incorporated into the TCNQ salts [62]. The partial CT state of TCNQ was observed for M+(crown ether)(TCNQ)2 and M+(crown ether)2(TCNQ)2 salts. Some of them show the electrical conductivity as high as 0.01 S cm-^ By using divalent ions (Mg^^, Ca^^, Sr^^, Ba^^, and Pb^+), partial CT salts with the stoichiometries of M^+(crown ether)(TCNQ)3, M2+(crown ether)2(TCNQ)3 and M2+(crown ether)(TCNQ)4 were obtained with the room-temperature conductivities ranging from 0.001 to 0.25 S cm~^ [63]. They also attempted the crystal preparation with the other macrocycles such as azacrown ethers, cryptand, and l,13-di(8-quinolyl)1,4,7,10,13-pentaoxatridecane (Fig. 8a) [64]. The crystal structures of completely ionized K+(18-crown6)TCNQ, Rb+(18-crown-6)TCNQ, Tl+(18-crown-6)TCNQ, and K+(15-crown-5)2TCNQ salts were reported [65]. In the case of the M+(18-crown-6)(TCNQ) salts (M+ = K+, Rb+, and T1+), the M+ ions are completely included in the 18-crown-6 cavity through six M+ -- O coordination, which forms a typical diskshaped planar supramolecular cation structure. Figure 9 shows the unit cell of the K+(18-crown-6)TCNQ salt viewed along the b'Sods. The monovalent TCNQ molecules form an isolated dimer structure within the crystal. The K+ ion found in the K+(18crown-6)2(TCNQ) salt is coordinated by 12 oxygen atoms of two 18-crown-6 molecules, which forms a sandwich-type supramolecular structure.

15-crown-5

TCNQ

2,5-dimethyl-DCNQI

Ni(innt)2

Ni(dmit)2

Fig. 8. Molecular structures of (a) macrocycHc host molecules and (b) electron-acceptor molecules used in preparations of anion radical salts.

f-l -

J:

if

v^ 7

c^JLi •b^\f4 fp^r^ cr^jD •9,1 \ 4 TT^'

0 ^ cr^—'*i-^

'^t'

•'pv s^

•c- g-j.-

^^ "^.^ "^c^^b d**"^i"~^~T

^ ,!:* ""^^^9

\ ^^O^

Icri

Fig. 9. Crystal structures of the K+(18-crown-6)TCNQ salt. Unit cell viewed along the £>-axis. Source: M. Morinaga et al., Bull Chem. Soc. Jpn. 53 (1980). The nitrogen-based macrocycHc host molecules have a high ion-capturing abihty to transition metal ions compared with that of the oxygen-based crown ethers [36, 37]. The magnetic Cu^+ and Ni^+ ions included with the nitrogen-based cyclam provide the Cu^+(cyclam) and Ni^+(cyclam) supramolecular cation structures, which were utilized to introduce the magnetic spins within the molecular conductors of TCNQ and DCNQI [66, 67]. The magnetic spins of transition metal cations incorporated into molecular conductors influence the behavior of conduction electrons [68]. The crystal structure, electrical conductivity, and magnetic property of Cu^^(cyclam)(TCNQ)3 salt was reported [66]. The Cu^+ ion was completely included in the cavity of cyclam through

274

AKUTAGAWA ET AL.

four Cu^+ ~ NH coordination, which supported the planar Cu^+(cyclam) structure. The cyano groups in the TCNQ were coordinated with the Cu^+ ion at an axial position to the cyclam plane. The segregated stacking structure of TCNQ molecules was observed, in which the dimers of TCNQ anions (TCNQ~ ^ TCNQ-) were sandwiched by the neutral TCNQ. The TCNQ molecules are in a charge separated electronic state, accordingly. The semiconducting behavior with room-temperature conductivity of 1.4 XIQ-"^ S cm"^ was consistent with this electronic state of this salt. Each Cu^+(cyclam) unit was isolated from each other, and magnetic behavior was explained by the sum of contributions from the Curie spins of the Cu^+ ion and a thermally activated triplet spin of the dimerized TCNQ anions. The Cu^+(cyclam) supramolecular cation structure was also incorporated into the [Cu^+(cyclam)]2(2,5-dimethyl-DCNQI)5 salt [67]. The coordination feature of Cu^+(cyclam) was the same as that of the TCNQ salt. The cyano group of the 2,5-dimethylDCNQI was coordinated axially to the Cu^+(cyclam). The trimer and dimer units of 2,5-dimethyl-DCNQI molecules formed a nonuniform segregated stacking structure. Although the partial CT state of the 2,5-dimethyl-DCNQI molecule satisfied the criteria for the high electrical conduction, no conductivity data were reported. The magnetic data indicate independent magnetic behaviors of Cu^+(cyclam) and 2,5-dimethyl-DCNQI units to each other. Spins on 2,5-dimethyl-DCNQI molecules disappeared through the strong antiferromagnetic interaction, while the S = 1/2 spin of Cu^+(cyclam) unit showed the Curie type magnetic behavior. In these TCNQ and DCNQI salts, the cyano groups of the acceptor molecules coordinate with the magnetic Cu^+ ions axially. The magnetic spin ordering of the Cu^+ ions, however, was not observed through the interaction with 7r-electrons on acceptor molecules. Further design of magnetic supramolecular cation structures is necessary to realize the magnetic spinconduction electron interaction within the molecular conductors. For this purpose, the magnetic ions should exist near enough to the conduction electrons to have large magnetic exchange energy (J). The coupling with the magnetic spin and delocalized conduction electron causes the interesting phenomena such as magnetic field induced metal-insulator transition, colossal magnetoresistance, and coexistence of superconductivity and ferromagnetism [68]. 3.2. Supramolecular Cations in Ni(dniit)2-based Molecular Conductors The planar metal-coordinated 7r-molecule of Ni(dmit)2, dmit = 2-thioxo-l,3-dithiole-4,5-dithiolate, provides highly conducting molecular crystals. Since the atomic coefficient of sulfur atoms in the LUMO is large compared to that of the d-orbital in the central Ni metal, the conduction electrons on Ni(dmit)2 are mainly delocalized on the 7r-ligand [11]. The Ni(dmit)2-based conducting salts can be easily prepared by the electrochemical oxidation of the monovalent Ni(dmit)2 salt. A large number of Ni(dmit)2-based molecular conductors were already prepared, and the metallic conducting behavior and superconducting transition were reported [69]. One of the characteristic electronic features in the Ni(dmit)2 system is the highly one-dimensional band structure. The dispersion in the LUMO band is only observed along the TT-TT stacking direction. Such one-dimensional electronic structure is unstable and causes metal-insulator transition at the low temperature (see also Section 5) [11]. High electronic

LUMO

HOMO

xK5; HOMO

Fig. 10. LUMO and HOMO symmetry of the Ni(dmit)2 molecule (left side) and HOMO symmetry of TTF (right side). dimensionality is important for obtaining a highly conducting system. The Ni(dmit)2 molecule has b^^^ symmetry of the LUMO (Fig. 10), through which the side-by-side S ~ S intermolecular interactions are not effective. On the other hand, the side-byside interactions between TTF molecules are effective enough to increase the electronic dimensionahty due to ^2M symmetry of the HOMO (Fig. 10). The two-dimensional electronic band structures in the TTFbased molecular conductors stabilize the metaUic conducting state. The HOMO in the Ni(dmit)2 molecule has the same symmetry as that of TTF. Simple M+(crown ether) structures can be incorporated into the Ni(dmit)2-based molecular conductors as in the case of TCNQ salts. Here we overview the M+(crown ether) type supramolecular cation structures in the Ni(dmit)2 salts in addition to the Ni(mnt)2 salt. The Na+(15-crown-5) unit was incorporated into the monovalent Na+(15-crown-5)[Ni(mnt)2]H20 salt [70]. The room-temperature conductivity (^10"^ S cm~^) was consistent with the monovalent electronic state of the Ni(mnt)2 molecule. Since the ion radius of Na+ is larger than the cavity radius of the 15-crown-5 molecule, the pyramidal Na+(15-crown-5) structure was observed in this salt (Fig. 11). The dimerized Ni(mnt)2 molecules formed a segregated stacking structure, and the Na+(15-crown-5)H20 units were isolated from each other within the crystal. The magnetic behavior was explained by the thermally activated triplet spins of the Ni(mnt)2 dimer. The monovalent Ni(dmit)2 salt, NHj(15-crown-5)2 [Ni(dmit)2], was reported [71]. Since the ion radius of NH^ is larger than that of Na+, the NHj ion was largely displaced from the 15-crown-5 plane (Fig. 11), these formed a sandwich-type NHj(15-crown-5)2 structure. The NHj(15-crown5)2 units were further dimerized to form barrel type large [NH|(15-crown-5)2]2 supramolecular cation structures. Since the monovalent Ni(dmit)2 molecule carries the magnetic spin of S = 1/2, the control of Ni(dmit)2 arrangements within the crystal by using the supramolecular cation structures is interesting from the viewpoint of constructing molecular magnets. The magnetic supramolecular cations of Cu^^ (cyclam) (CH3CN)2 and Ni2+(cyclam)(CH3CN)2 were introduced into the isostructural Cu2+(cyclam)[Ni(dmit)2]2(CH3CN)2 and Ni2+(cyclam)[Ni(dmit)2]2(CH3CN)2 salts, respectively, [68]. The Cu^+ and Ni^+ ions were completely included in the cyclam cavity as in the case of TCNQ salts previously described, and the solvent molecules (CH3CN) further coordinated axially. The Ni(dmit)2 molecules formed a dimer structure, and the onedimensional Ni(dmit)2 dimer chain was elongated within the

275

SUPRAMOLECULAR ASPECTS OF ORGANIC CONDUCTORS crystal. The total magnetic susceptibilities were fitted by the sum of the Curie spins of Cu^+ or Ni^+ ions and the thermally activated triplet spins of the Ni(dmit)2 dimer. Highly conducting Ni(dmit)2 salts with supramolecular cation structures were reported. The LiJ(12-crown-4)3 [Ni(dmit)2]7(acetone)2 had a supramolecular cation unit of dimeric pentacoordinated LiJ(12-crown-4)3 (Fig. 12b). Since the Li+ ion is larger than the 12-crown-4 cavity, a pyramidal Li+(12crown-4) cavity was a fundamental unit [72]. The two pyramidal Li+(12-crown-4) units were further connected by oxygen atoms of a free 12-crown-4 molecule. The segregated nonuniform stacking of Ni(dmit)2 was observed as the A ~ B ~ C ^ D tetramer arrangement, which was consistent with the semiconducting temperature dependence (O-RJ = 30 S cm~^) (Fig. 12c). Two kinds of semiconducting phases were observed at a higher (a-phase: T > 230 K) and a lower temperature region ()3-phase: T < 230 K). This phase transition can be suppressed by the application of pressure up to 5.8 kbar. Various types of supramolecular cation structures will be expected from the M+(crown ether) units depending on the alkali metal ions and crown ethers. The size and shape of the supramolecular cation structures influence the composition of the crystal and the stacking manner of the Ni(dmit)2 molecules. From the combination of NH^ and the 18-crown-6 partial CT salt, NH^(18-crown-6)[Ni(dmit)2]3 was obtained [73]. Figure 13a

•oV"

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Fig. 11. Supramolecular cation structures of M+(15-crown-5). (a) The pyramidal Na+(15-crown-5)(H20) unit in Na+(15-crown-5)[Ni(mnt)2] (H2O) and (b) the barrel type [NHj(15-crown-5)]2 unit in NHj(15-crown-5)2[Ni(dmit)2]. Sources: N. Robertson et al, /. Chem. Res. 54 (1999); T Akutagawa et al., Thin Solid Films 331, 264 (1998).

Fig. 12. Crystal structures and electrical conductivity of LiJ(12-crown-4)3[Ni(dmit)2]7(CH3CN)2. (a) Unit cell viewed along the 6-axis. (b) Supramolecular cation structure of the LiJ(12-crown-4)3 unit, (c) Temperature-dependent logarithmic resistivity (P/PRT) VS. inverse of temperature {T~^) at 1 bar, 5.2, and 10.2 kbar. Inset shows the pressure (/?) dependence of conductivity at room temperature (cr^j). Source: T. Akatagawa et al, /. Mater. Chem. 7, 135 (1996).

276

AKUTAGAWA ET AL.

Na-'

Fig. 13. Crystal structure of NHj(18-crown-6)[Ni(dmit)2]3. (a) Unit cell viewed along the «-axis. (b) Supramolecular cation structure of typically disk-shaped NHj(18-crown-6). Source: T. Akutagawa et al., Synth. Met 86, 1961 (1997).

shows the unit cell of this salt viewed along the a-axis. A typical planar NHj(18-crown-6) supramolecular cation structure (Fig. 13b) was found as in the case of the K+(18-crown-6)TCNQ salt. Since the ion radius of the N H j ion fits the cavity size of the 18-crown-6 molecule, a planar NH^(18-crown-6) structure was obtained through six N H j - O interactions. The A -^ B - A stack of Ni(dmit)2 molecules formed a nonuniform Ni(dmit)2 column in the crystal, which was consistent with the semiconducting behavior of the salt with the room-temperature conductivity of 0.4 S cm"^ and activation energy of 0.15 eV. The size of the supramolecular cation structure also influences the stoichiometry of the Ni(dmit)2 salt. The supramolecular cation of Na+(cw-d5-dicyclohexyl-18-crown-6) was incorporated into the Ni(dmit)2 salt to form Na+(d5-cw-dicyclohexyl-18-crown6)[Ni(dmit)2]4(acetone)2 [74]. The smaller (supramolecular cation)/[Ni(dmit)2] ratio of 1/4 compared to NH|(18-crown-6) should be due to the larger size of the countercation through the substitutions of cyclohexyl rings to the parent 18-crown6 molecule. Figure 14a shows the unit cell of Na+(cw-cwdicyclohexyl-18-crown-6)[Ni(dmit)2]4(acetone)2 viewed along the c-axis. The Ni(dmit)2 stack was composed of A - B dimers, and a two-dimensional conducting layer was constructed by the sideby-side S - S contacts. The Na+ ion was completely included in the crown-ether cavity (Fig. 14b). Since the ion radius of Na+ is smaller than that of N H | , the Na+ ion was not tightly fixed by the six oxygen atoms of cw-cw-dicyclohexyl-18-crown-6 molecule. The salt was semiconducting with the room-temperature conductivity of 0.1 S cm-i (Fig. 14c). The transition in the activation energy (EJ was observed at 250 K with E^ (< 250 K) = 0.56 eV and E^ (> 250 K) = 0.13 eV. AcycUc polyethers have coordination abihty to M+ ions, and the M+(acycHc polyether) type supramolecular cation

C=>

Na (cis-cis-dicyclohexyl-18-crown-6)

/ ^

c

^^ Oi~ E E

0 0 0 0

-r

^^

^ -^..^. 3.2

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y

q H

— \ :

-z q

I . 1 . I 1 . < ! < i . t L i L.I< I,I I I i l 3.8 4.0 4.2 3.6

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Fig. 14. Crystal structure and electrical conductivity of Na+(cw-cw -dicyclohexyl-18-crown-6)[Ni(dmit)2]4(acetone)2. (a) Unit cell viewed along the c-axis. (b) Supramolecular cation structure of Na+(d5-cwdicyclohexyl-18-crown-6). (c) Temperature dependence log resistivity (log p / a cm) vs. inverse of temperature {J~^). Source: N. Robertson et al., /. Mater. Chem. 9, 1233 (1999).

Structure can also be incorporated into the electrically conducting Ni(dmit)2 salts. The removal of one ethylene unit from the 18-crown-6 molecule gives an acycUc pentaethyleneglycol (PEG) structure. Since the K+ ion and the PEG molecule have a similar ion radius and molecular size to NH^ and 18-crown-6, respectively, the K+(PEG)[Ni(dmit)2]3 salt was obtained with the same stoichiometry as NH^(18-crown-6)[Ni(dmit)2]3 [75]. Figure 15a shows the unit cell of the K+(PEG)[Ni(dmit)2]3 salt viewed along the c- and the a-axis. The non-uniform Ni(dmit)2 dimer structure was consistent with the semiconducting properties with the

SUPRAMOLECULAR ASPECTS OF ORGANIC CONDUCTORS

277

room-temperature conductivity of 0.001 S cm"^ The supramolec- in the crystal was closely related to the electrical conducting ular cation structure was the [K+(PEG)]2 dimer, in which the K+ property [80]. The segregated uniform stacks of Ni(dmit)2 and ion was completely included in the PEG cavity with six K+ ^ 15-crown-5 molecules were observed in this crystal (Fig. 16a). O coordination (Fig. 15b). Further axial K+ -- O coordination The uniform 15-crown-5 stack provides the ion-channel structure resulted in the dimerized [K+(PEG)]2 structure. The coordina- along the stacking direction, which offers the ion-moving field tion ability of the PEG molecule to the K+ ion was weaker within the crystal. The Lij6(15-crown-5) unit has two kinds of disorder, one is than that of the cyclic 18-crown-6. Weak cation-binding (cationcoordination) increases the motional freedom of cations within the orientational disorder of the 15-crown-5 molecule and the the soUd state. The polyethyleneoxide (PEO) derivatives have other is the positional disorder of the Li+ ion. Since the crystal been extensively examined from the viewpoint of organic non- structure was determined using the centrosymmetric space group crystalHne ion-conducting materials [76-78]. These polymer elec- of P2i/c, the 15-crown-5 molecule was located on the inversion trolytes progressed the technology of the lithium-ion battery by center with a half crystallographically asymmetrical unit. It is replacing the Hquid electrolyte. The ion conductivity of the Li+- impossible to determine whether the orientational disorder of the PEO film reaches at about 10""^ S cm~^ in the temperature range 15-crown-5 molecule is intrinsic or not, because the 15-crown-5 from 60 to 80 °C. The structural determinations of these Li+- molecule is noncentrosymmetrical. Similarly, the occupational PEO films were tried to clarify the coordinated environment of sites of the Li+ ion were found at the upper and lower positions of the 15-crown-5 plane. Due to the smaller ion radius of Li"^ mobile Li+ ions [79]. than to the cavity radius of 15-crown-5, the translational Li+ In the LiJ;6(15-crown-5)[Ni(dmit)2]2(H20) salt, the ionmotion through the cavity was possible. The microscopic aspect electron hybrid conduction was realized, and the ionic motion of Li+ motion was evaluated by the solid-state ^Li-NMR measurement (Fig. 16d). The motional narrowing of the Hnewidth above 200 K was observed, which suggested the short-range Li+ motion between the two upper and lower sites of the 15-crowna. 5 plane. The direct measurement of Li+ ion conductivity clariA-B fies the long-range Li+ motion within the ion-channel. The Li+ ion conductivity was measured by using the electron-blocking method. The Li+ ion conductivity was determined as 3 x 10"^ S B-A cm~^ at 333 K. However, the magnitude of ion conductivity was quite significantly lower than that of electrical conductivity. A-B The ion-channel is one of the intriguing supramolecular assembUes because it often plays an important role in biological systems such as the signal transmission of the nervous system [81, 82]. Moreover, the construction of artificial ionchannel structures in the crystalline solid attracted much attention from the viewpoint of the applications in molecular ionic devices having a biomimetic signal processing ability [83-85]. The Lij6(15-crown-5)[Ni(dmit)2]2H20 salt may be a model system for constructing such biomimetic systems. The metallic conductivity is expected from the partial CT state and the regular stack of Ni(dmit)2 molecules. Two Ni(dmit)2 columns were arranged as a two-leg ladder through the side-byb. side S '^ S contacts. Since the magnitude of the transfer integral along the stacking direction was quite significantly larger than those of the side-by-side interactions, the salt has a highly onedimensional electronic structure. The temperature dependence of electrical conductivity showed a weak metaUic character above 250 K (Fig. 16b). By lowering the temperature below 250 K, a semiconducting behavior appeared. The magnitude of roomOH temperature conductivity (240 S cm~^) suggests the metallic state + r c=!> above 250 K. On the other hand, the low temperature semiconOH ducting state was consistent with the temperature dependence of magnetic susceptibihty (Fig. 16c), in which a one-dimensional Heisenberg antiferromagnetic interaction was observed below 200 K with the exchange energy of IJ//:^! = 90 K and 0.45 spin per Ni(dmit)2. The deviation from the localized Heisenberg antiferromagnetic state above 200 K was attributed to the Pauli paramagnetism of the delocalized metaUic state. It is noteworthy that this metal-semiconductor transition should be due to the freeze Fig. 15. Crystal structure of K+(PEG)[Ni(dmit)2]3. Unit cell viewed of Li+ motion within the ion-channel. along the c-axis. (b) Supramolecular cation structure of the dimerized The Li+ ions produce Coulomb potential in the Ni(dmit)2 [K+(PEG)]2 unit. Source: T. Akutagawa et al, Chem. Commun. 2599 crystal, since the motion in Li+ ions diffuse the Coulomb poten(1998). tial, which does not effectively influence the conduction electrons

c:

278

AKUTAGAWA ET AL.

J

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200

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Fig. 16. Crystal structure and physical properties of Lij6(15-crown-5) [Ni(dmit)2]2 H2O. (a) Unit cell viewed along the a- (upper figure) and the c-axis (lower figure), (b) Temperature dependence of electrical resistivity, (c) Molar magnetic susceptibility (XM) VS. temperature. The solid line is the fitting curve of the one-dimensional Heisenberg model, (d) Temperature dependence of the linewidth of cross-polarization magnetic angle spinning ^Li-NMR spectra. Source: T Nakamura et al, Nature 394, 159 (1998).

SUPRAMOLECULAR ASPECTS OF ORGANIC CONDUCTORS on the Ni(dmit)2 stack. On the other hand, Li+ ions are randomly frozen at low temperatures, which produce incoherent Coulomb potentials. The conduction electrons are trapped by the pinning potential, which results in the metal-semiconductor transition. The ionic motion strongly influences the conduction electrons especially in a one-dimensional system through the electron-ion coupling. We described the M(crown ethers) type supramolecular structures within the Ni(dmit)2-based molecular conductors. The design of the crystal structures is not straightforward even in such a simple system. However, the prediction of the coordination feature in the M(crown ethers) type supramolecules should be possible by the use of the crystal-structure library so far obtained. When the ion radius fits well into the cavity radius of crown ethers such as in K+(18-crown-6) and Na+(15-crown-5), the ions are completely included in the cavity of crown ethers. The ions larger than that of the crown ether cavity are located outside of the crown ethers, which form the pyramidal or sandwich-type supramolecules. On the other hand, the ions smaller than that of the crown-ether cavity such as Li"^ vs. 15-crown-5 may have a motional freedom within the supramolecular structure and, in some cases, may show an ionic conduction through the translational motion within the ion-channel structure. For the formation of the supramolecular assemblies such as ion-channel structure, several kinds of interactions should be taken into account to connect each supramolecular unit. In the case of the Li+(15-crown-5)(H20) ion-channel, the Li+ ions were connected with water molecules through the Li"^ ^ O interaction, which results in an infinite Li+ ~ O chain. In this sense, the pyramidal supramolecular units of Li+(15-crown-5)(H20) compose the ion-channel structure. Since the coordination of the M+ ions to the oxygen atoms of crown ethers also determine the structure of supramolecular assembly, the complex formation constant (Kc) between the ions and crown ethers in solution can be one of the parameters for designing supramolecular structure [86]. However, the crystal data of these supramolecular systems within the molecular conductors are limited for the prediction of the supramolecular structure, and the structures of supramolecular assemblies also depend on the 7r-electron system. In fact, the crystal packing of the 7r-system and the supramolecules in the Ni(dmit)2 molecular conductors and the TCNQ-based molecular solids are quite different from each other. 3.3. Supramolecular Anion Structures in TTF-based Molecular Conductors A large number of molecular metals and superconductors of TTF-based CT complexes or cation radical salts were prepared [11]. The outer substituted calcogen atoms in bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) and BEDOTTF are effective to stabilize the metallic conducting state through the side-by-side S ~ S contacts to form a twodimensional electronic band structure. The TTF-based molecular conductors should have a potential to keep the highly electrical conducting state even if the supramolecular structures coexist in the crystal. One approach to introduce a supramolecular structure into TTF-based molecular conductors is the replacement of a simple counteranion to the supramolecular anion structure in the cation radical salts. Various types of counteranions were already utilized in the BEDT-TTF salts. In the crystal, partially oxidized BEDT-TTF molecules form a two-dimensional electrical conducting layer, and counteranions are usually sandwiched

279

by the BEDT-TTF layers. The isolated anions (CI", Br", I", I3 etc.) or anion networks (CuN(CN)2X, X = CI", Br", I", CN" etc.) were employed as the counteranion of BEDT-TTF radical salts. The metal-coordination, halogen-halogen interaction and/or hydrogen-bonding interaction construct the network-type anion structures. Yamaomoto et al. designed the structure of the anion network from the viewpoint of the supramolecular assembly within molecular conductors [87]. The Lewis acid-base type interaction is utilized to construct the anion networks. The electrocrystallization of BEDT-TTF or EDT-TTF (ethylenedithio-TTF) in the presence of iodine-containing compound, diiodoacetylene (DIA), ;7-bis(iodoethylnyl)benzene (;7-BIB), or tetraiodoethylene (TIE), and counteranion X" (Cl~, Br", I", or AuBr2) gave conducting crystals with the anion network structures constructed through the X~ '^ I interactions. One-, two-, and three-dimensional anion network structures are incorporated into the electrical conducting cation radical salts of BEDT-TTF or EDT-TTF The one-dimensional ^ Cl~ ~ (DIA) ^ C\~ ^ infinite chain structure through the CI" ^ I contacts was observed in the crystals of (BEDT-TTF)2C1-(DIA) (Fig. 17a). The highly electrical conducting character is expected from the packing of BEDT-TTF molecules. In fact, the metallic electrical conductivity was observed in the temperature range from room temperature to 1.6 K. The salt has a two-dimensional Fermi surface. Similar metallic electrical conductivity and one-dimensional ^ C\~ ~ ;7-BIB ^ C r ^ i?~BIB ~ anion network structures were observed in the (BEDT-TTF)3Cl-(;?-BIB) salt. The two-dimensional anion network structure in the (BEDTTTF)6(AuBr2)6Br-(TIE)3 salt was obtained by the electrocrystallization of BEDT-TTF with AuBr2, Br", and TIE molecules. The Lewis acid-base interaction between the AuBr~ and iodine atoms of TIE molecules forms a planar anion network structure (dashed fines in Fig. 17b), while the BEDT-TTF layer has a novel r-type donor arrangement with a semimetalHc temperature dependence down to 50 K. Another two-dimensional nonplanar anion network structure was observed in the isostructural (BEDT-TTF)2Cl2(DIA)(TIE) and (BEDT-TTF)2Br2 (diiodoethylene (DIE))(TIE) salts (Fig. 17c). The DIE - X" -TIE X" — anion network penetrated into the conducting layer, in which the BEDT-TTF molecules formed a dimer unit. These salts showed semiconducting temperature dependence of conductivity with activation energy of 0.08 and 0.15 eV, respectively. The (EDT-TTF)4Br"l2 (TIE)5 salt has a three-dimensional anion network structure (Fig. 17d). The Br" - I (TIE) and I" - I (TIE) contacts form the three-dimensional channel structure, in which a one-dimensional EDT-TTF stack runs through the channel. This salt showed semiconducting temperature dependence with activation energy of 0.027 eV. The TTF-based molecular conductors have an advantage for constructing a highly electrical conducting system because of the two-dimensional character of its Fermi surface. The dimensionafity and contact mode of the anion network structure influence the packing of donor molecules and the electrical band structures. For instance, a large number of molecular superconductors were found in the K-type donor packing of BEDT-TTF molecules, which structures are composed of the BEDT-TTF dimer pairs [11]. The design of the K-type structure was proposed through the design of anion structures [88]. The anion network structures should be predicted by considering the intermolecular interaction scheme from the viewpoint of supramolecular chemistry.

280

AKUTAGAWA ET AL. C.

a.

Conduction layer

AuBr," (type B)

\

/

Anion sheets

0-' I ft' 0 BEDT-TTF (lype 1)

\9' V

^0

0

\

/ AuBr," (type A)

. TIE

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Fig. 17. Network anion structures of BEDT-TTF-and EDT-TTF-based organic conductors, (a) One-dimensional Cl~ ~ DIA chain in the (BEDT-TTF)2 CI" (DIA) salt, (b) Two-dimensional planar network anion structure in the (BEDT-TTF)6(AuBr2 )6 Br-(TIE)3 salt, (c) Two-dimensional nonplanar anion network in the (BEDT-TTF)2 Br2 (DIA)(TIE) salt, (d) Three-dimensional channel network in the (EDT-TTF)4 Brl2 (TIE)5 salt. Source: H. M. Yamamoto et al, /. Am. Chem. Soc. 120, 5905 (1998). See also Plate 1 (Fig. 17d).

4. MOLECULAR CONDUCTORS IN ORDERED THIN FILMS 4.1. Langmuir-Blodgett Films In this section, we describe thin ordered films of molecular conductors and related materials as examples of supramolecular assemblies. Langmuir-Blodgett (LB) films are formed by the deposition of the monolayers at the air-water interface to the solid substrates. The comprehensive reviews on this technique are published and interested readers are referred to one of these books [7]. The prominent feature of the LB films is as follows.

(a) Since the films are deposited layer-by-layer, the film has an ordered structure (at least) in the surface-normal direction, which is controlled through the deposition procedure. (b) Films are formed under ambient conditions, which is a keen difference to the dry processes such as molecular beam epitaxy. Thus, the process is suitable for thermally unstable materials like molecular conductors. (c) Heterostructures with different film forming materials are easily realized. In other words, the positional control in the film normal direction is readily attainable.

281

SUPRAMOLECULAR ASPECTS OF ORGANIC CONDUCTORS A large variety of conducting LB films is so far reported. The interested readers are referred to review articles [89]. Here we review only selected LB systems, which are the possible candidates for future molecular electronic devices. 4.1.1. Metallic Langmuir-Blodgett

conductivity (a) curve decreases with a decrease in temperature above 250 K. The conductivity curve in the whole measuring range, however, was fitted by a single formula for a semiconductor, taking into account the mobiUty change, which is often applicable to the TCNQ-based semiconducting salts.

Films

The ultrathin organic films of metallic properties should be essential for the construction of future molecular electronic devices. Especially, the films of molecular conductors are desirable considering the connectivity with the active parts composed of organic molecules. Such films may be conducting polymer films. However, the polymer films are essentially semiconducting due to the disorder in the polymer assemblies. The same problem, however, arises for the LB films of molecular conductors, because the introduction of the defects and disorders is inevitable (at least with the present LB technique) during the film forming process, even if using the molecular conductors shows a metallic conductivity in the crystalline state. Most of the conducting LB films show semiconducting behavior in the temperature-dependent conductivity measurements, that is the conductivity decreased with the decrease in temperature. There are two possibilities for the cause of the semiconducting behavior. One is, of course, that the film is made of an intrinsic semiconductor, which is the case for most of the LB films so far reported and is out of scope of this section. Another possibility is that the LB film is composed of metal and the semiconducting behavior is due to the defects and disorders in the film. In this case, the metallic nature of the films may be evaluated by the optical measurements, or some other noncontact or zero-current electrical measurements such as microwave conductivity and thermoelectric power measurements. In some cases, degree of the defects or disorders is low enough to observe the metallic properties even in dc conductivity measurements. The metallic conductivity (that is the negative slope in the conductivity vs. temperature curve) was first observed in the LB films of the BEDO-TTF-tetradecyl-TCNQ charge-transfer complex [90]. The conductivity behavior is shown in Figure 18. The

100

a = AT-"" exp

(1)

In this sense, the "metallic" behavior in the conductivity curve could not be positively regarded as evidence for the metallic nature of the film. However, the thermoelectric power measurements clearly indicated the metaUic properties of the film as shown in Figure 19. This is because the thermoelectric power is not affected strongly from defects and disorders which have only a small area fraction in the film. On the other hand, the resistance of the film is determined by the series of metallic parts and semiconducting (defects and disorder) parts, and only the high resistance region reflects to the conductivity measurement [90]. The dc conductivity originating from metaUic parts was observed for the LB film of tridecylmethylammonium[Au(dmit)2] after electrochemical oxidation [91]. The temperature dependence of the conductivity is shown in Figure 20. The conductivity was also measured under hydrostatic pressure up to 17 kbar and the conductivity attained was as high as 140 S cm~^ [92]. The semiconducting behavior suppressed by applying pressure and the conductivity at 4.2 K were comparable to those at room temperature under high pressure. These conductivity behaviors apparently cannot be explained by regarding the film as a usual semiconductor. The thermoelectric power measurements also showed the metaUic nature of the film. The conductivity was fitted by a hnear combination of that of a usual metal (the conductivity is proportional to the inverse of temperature) and that caused by the thermal-fluctuation induced

150

200

300

Temperature / K

Fig. 18. Temperature dependence of the conductivity of an LB film of the BEDO-TTFtetradecyl-TCNQ charge-transfer complex. Source: T Nakamura et al., /. Phys. Chem. 98, 1882 (1994).

282

AKUTAGAWA ET AL.

tunneling (TFIT) between the metallic domains [93]. The latter mechanism was applied for the conducting behavior of polyacety-

lene and expressed as (2)

(7 = 00 exp

\T, + T)

where fitting parameters Tg and T^ can be expressed as a function of height {VQ), width (it;), and cross-section {A) of the barrier between the metallic domains as (3) TQ =

sV2hAVQ^^eo/{7re^kBy/m;w^)

(4)

The LB films of tridecylmethylammonium-[Au(dmit)2] in the oxidized form are composed of metalhc domains. Between the domains and the domain boundaries, there should be semiconducting parts due to the defects and disorders. The electron conducts between domains by the tunneling mechanism. As mentioned before, the total resistance in dc measurements is determined by the linear combination of the semiconducting part (in this case the resistance due to the tunneling) and the metallic part. Usually the resistance in the metallic part is very low, and we can no longer observe the resistance due to the metalhc part, especially in the polyacetylene having a conductivity as high as 10^ S cm~^ On the other hand, the molecular conductors exhibit the conductivity at most 10^-10^ S cm~^ because of the highly correlated electronic systems they have. Consequently, the conductivity behavior of the metallic domains was observable in the LB films of tridecylmethylammonium-[Au(dmit)2] and the conductivity behavior was well fitted by the formula. 1 /IPTFIT + 100

150

200

Temperature / K

Fig. 19. Temperature dependence of the thermoelectric power of an LB film of the BEDOTTFtetradecylTCNQ charge-transfer complex. Source: T Nakamura et al., /. Phys. Chem. 98, 1882 (1994).

40

-

^

^

—

-

(5) /2PM

where p^pix ^^^ PM ^^^ the resistivity of TFIT and metallic domains, respectively. The resistivity of the metalhc island was assumed to be p^ = PM ^ ^• An LB film showing a metalhc conductivity down to 14 K was reported using an unsubstituted BEDO-TTF molecule (Fig. 21). Ohnuki et al. formed mixed LB films of neutral BEDO-TTF and behenic acid [94]. A certain oxidation occurred during the

^

/

"^E 30 0

ff 8

e? >% •5

0 0

0 20

3 •a

0 — 00 0 0

c 0 0

0 Q

10

1

0 •h

1

50

1 1 1 100 150 200 Temperature / K

1 250

1 300

Fig. 20. Temperature dependence of the conductivity of an LB film of tridecylmethylammonium-Au(dmit)2 after electrochemical oxidation. Source: T Nakamura et al., Synth. Met. 71, 1993 (1995).

50

100 150 200 250 Temperature (K)

300

Fig. 21. Temperature dependence of the conductivity of a mixed LB film of BEDO-TTF and behenic acid. Source: H. Ohnuki et al. Phys. Rev. B 55, RIO, 225 (1997).

SUPRAMOLECULAR ASPECTS OF ORGANIC CONDUCTORS film forming process and a conducting film was obtained without postoxidation treatments. The countercation incorporated is not clear at present, although the layered structure of the film was confirmed by the X-ray diffraction measurements. The size of the metalUc domains is usually in the submicron range or smaller. The metallic domains of a larger area or the defect free LB films may be favorable. However, considering the application in molecular devices, the small size of the domains should not be a fatal limitation. Rather, the development of a novel technique for dealing with these small domains and for the interconnection between these domains and with other supramolecular entities is important. More favorably, these connections are realized through the bottom-up technique. That is, the connecting ability should be programmed from the beginning and the higher level architecture of molecular devices should be developed through the self-assembly of the agents such as metallic domains and other supramolecular entities. The metallic state in the LB films was estabUshed through extensive research. The next step should be to realize a superconducting transition in the LBfilms.Two separate groups confirmed the superconducting transition in C^o LB films [95, 96]. Wang et al. doped the C6o LB film with potassium vapor and observed the superconducting critical temperature of 12.9 K by low magnetic field microwave absorption (LPS) (Fig. 22). Ikegami et al. used the decomposition of RbN3 for rubidium doping to C^Q LB films. They observed superconducting onset temperature at 23 K by LPS. In both cases, the superconductivity was not confirmed by the conductivity measurements and the doping of the LB film was inhomogeneous. The superconducting transition is quite sensitive to the defects and disorders in the system. However, if we can construct clean metallic domains interconnected with each other comparable to the superconducting coherent length, we should obtain a superconducting supramolecular entity which is a promising agent for constructing a supramolecular device system. 4.1.2, Molecular Rectifiers One of the most important functions in the present silicon device technology is rectification. Although the molecular electronic devices in the future are not necessarily based on the present logic architecture, the rectification should be inevitable for the construction of any future device system. The idea of the single molecule rectification is already presented by Aviram and Ratner in 1974 [97]. They proposed a unimolecular rectification through the molecular orbitals of a single D-cr-A molecule. Since the energy for the electron transfer from acceptor to donor is much higher (several electronvolts) than that from donor to acceptor, the unidirectional electrical conduction will be realized. In the field of LBfilms,the electron transfer and energy transfer between the donor and acceptor layers fabricated in an LB multilayer were observed in early 1980's [98]. An electrochemical LB film photodiode was reported by Pujihira et al. by using pyrene derivatives [99]. Metzger prepared several D-a-A type molecules and employed them for LB film formation, but evidence of rectification was not found [100]. However, they finally overcame the difficulties and demonstrated the rectification in the monolayer film using a highly polarized molecule, y(rt-hexadecylquinolinium-tetracyanoquinodimethanide, C16H33Q3CNQ (Pig. 23) [101]. The orientation of the monolayer device is shown in Figure 23. To avoid the effect from the asymmetries of electrodes, Al was

283

12.9 K

12.5 K

Fig. 22. LFS spectra for a Cgo LBfilmafter potassium doping. Source: P. Wang et al, Langmuir 12, 3932 (1996).

used as the sandwiching electrodes in both sides. Using this device, rectification was observed as shown in Figure 24. Unimolecular electrical rectification by the monolayer of C16H33Q3CNQ was unequivocally confirmed. 4.1.3. Molecular Switches Together with molecular rectifiers, molecular switches were considered to be the basic parts of future molecular electronic devices. So far, the soliton switch is the most famous example theoretically proposed. In the field of LB films, quite a few works were reported on cw-^ran^photoisomerization phenomena of the molecules having double bond(s) [102]. Carotenoid are typical examples of such cw-^fl/i^isomerization and the retinal molecule is utilized for the visual system of human eyes. The artificial molecules extensively used in this field, however, are azobenzene derivatives. Since the shape of the molecule, and thus, the cross-sectional area in the film changes largely through the (photo)isomerization of azobenzene derivatives, quite a few systems are reported so far utilizing this photochemical isomerization to the photo-irradiation triggered switch of the system.

284

AKUTAGAWA ET AL.

\ . _/

c 16^33"- N ^ J

=\

transmission! unit

C

\ _ J~

CH3(CH2)7-<^N=N-0-O(CH2)n-N^(TCNQ)^ A P T ( 8 - n ) ( n = 6, 12, 14)

H

Fig. 25. Azobenzene derivatives for the photochemical switching LB films. Source: H. Tachibana et al, Chem. Lett. 173 (1992).

C,6H33Q-3CNQ

Fig. 23. Molecular structure of C16H33Q-3CNQ and orientation of the LB monolayer device. Source: R. M. Metzger et al., /. Am. Chem. Soc. 119, 10, 455 (1997).

ing LB films with the room-temperature conductivity around 10-2 S cm-i [104]. The azobenzene unit showed cw-^ra«5-phtoisomerization in the LB multilayer and the conductivity of the film simultaneously changed with the photoisomerization [105]. The results are shown in Figure 26. The photochemical switching phenomena are clearly observed. It should be noted that the photochemical switching scheme largely depends on the structure of the side chain. For example, the conductivity increased on trans- to cw-isomerization for the APT(8-12) LB film and the opposite phenomena were observed for the LB film of APT(8-14). Some accumulation effect was observed in APT(8-6). By using a naphthalene derivative, the light frequency for cw-^an5-isomerization can be changed and by hybridizing the azobenzene and naphthalene derivatives in an LB heterostructure, multiwavelength switching was also possible [106].

UV ,

For example, the orientation of the liquid crystals can be changed by the photo-isomerization of an azobenzene attached to the surface of a liquid crystal cell [103]. Tachibana et al. utilized azobenzene derivatives for the construction of a photochemical switching system in LB films. They prepared pyridinium salts which have an azobenzene unit in the side chain attached to the pyridinium nitrogen. The salts then converted to the TCNQ complex salts. The chemical structures and their abbreviations are shown in Figure 25. The TCNQ salts are fabricated into LB multilayers to form semiconduct-

(a)

< ^

r—

r—

T

1—

T—

1.0 0.9 0.8 0.7

LV/

1.0

L(b)

Cfi"^

rP^\

'^V/

VOJ

^^

1

0.9 h 0.8 C4

—1

VIS ^

^ark

I

1.5

L-(c)

^ ^

0.0004

1 1

0.0003

r \

H o o

(d)

•\

0.0002

0.0001

0 I

-

2

1

-

I

1

I

..... 1

0

J

J_

1

\

r

1.0

A

^ 0.9 H

11

1 2

1 10

1 20

30

Time / min

Vollage/V

Fig. 24. Rectification through a single monolayer of C16H33Q-3CNQ sandwiched between Al electrodes. Source: R. M. Metzger et al, /. Am. Chem. Soc. 119, 10, 455 (1997).

Fig. 26. The change in fraction of rra«5-azobenzene of APT(8-12) in the LB film (a) and the change in the conductivity of the LB films of APT(8-12) (b), APT(8-14) (c) and APT(8-6) (d) upon alternate irradiation of UV and visible light.

SUPRAMOLECULAR ASPECTS OF ORGANIC CONDUCTORS 4,1.4, Magnetic Langmuir-Blodgett

Films

Magnetic devices are also indispensable for the present silicon device technology especially for data storage. The colossal magnetoresistance is now extensively studied, which is one of the most promising candidates for data storage devices in the next generation [107]. The switching phenomena in magnetism are also attracting considerable attention in the field of solidstate physics and chemistry, and the bistability between ferroand para-magnetism is already reported for a Prussian blue analog [108]. Pure organic materials are composed of covalent bonds in which electrons form singlet pairs, and as a result, are diamagnetic. Only radicals and carbenes (nitrenes) have isolated spins, which are important components of organic magnets [109]. However, these molecules are usually unstable (except for anion and cation radical compounds, which are rather used for conducting LB films) and the spin density is relatively low. Consequently, transition metal complexes are suitable for constructing magnetic LB films. An early report on a magnetic LB film of a transition metal complex is on the phenanthroline complex LB film aiming at low-spin-high-spin transition in the LB film [110]. The Fe(phen)2(NCS)2 (phen = phenanthroline) is known as a spin transition material [HI], which shows an abrupt spin transition with T^ = 176 K. The ligand field changes on cooling the crystal and the high-spin state of Fe(II) at higher temperatures becomes the low-spin state. The complex used for constructing the LB film is shown in Figure 27. The LB films were prepared by accumulating 200 monolayers. Due to the limited amount of film forming materials of each LB film, the magnetism arising from these LB films is extremely small. A powerful tool for the investigation of such magnetism is ESR, and magnetic properties of molecular conductors and semiconductors are extensively studied [112]. The reports on magnetic properties of the LB films other than conducting LB films, however, are limited so far. The authors examined the spin transition by the temperature dependence of IR spectra. At room temperature, the LB film exhibited a doublet corresponding to the high-spin species (2065 and 2075 cm~^). Upon cooling, the intensity of the doublet from the high-spin species became weak and a doublet at a higher frequency (2108 and 2116 cm~^) corresponding to the

Fig. 27. Chemical structure of the Fe^"^ complex of the phenanthroline derivative used for constructing a magnetic LB film.

285

low-spin species became predominant below 250 K. The temperature dependence of the molar fraction of the high-spin species calculated from IR intensity is shown in Figure 28. Although the transition was far from being abrupt, the transition was less smooth than that expected from the Boltzmann distribution law between a spin singlet and a spin quintet separated by an energy gap. Moreover, a weak hysterisis ca. 4 K was present. The authors concluded that the spin transition was observed in an LB film and the cooperativity was retained even in the two-dimensional network constructed in the LB system. The magnetic properties of the LB film measured directly by a magnetometer were reported. Clemente-Leon et al. constructed an organic-inorganic hybrid LB film using Keggin polyoxymetalate and dioctadecyldimethylammonium bromide (DODA) [113]. The ;^r-plot of the DODA-C0W12O40 LB film is shown in Figure 29. The magnetic susceptibility of the LB film follows the Curie law down to ca. 20 K. The spin susceptibility of the polyanion decreases at low temperature because of the zero-field splitting of Co(II) in a tetrahedral environment. The appropriate choice of polyoxymetalate and a lipid counterion should allow LB films having particular magnetic properties to be constructed. A weak ferromagnetic state at low temperature was realized in the LB film of manganese octadecylphosphonate [114]. The magnetic order occurred at 13.5 K and a spin-flop transition was observed at 2.5 T in the ordered state. The ferromagnetic behavior in the LB film was first reported for a hybrid organic-inorganic system containing ferri-ferrocyanide [115]. The well-defined multilayer architecture was obtained by accumulating a DODA monolayer on a dilute Prussian blue solution. A schematic view of the LB film is shown in Figure 30. The LB films presented a ferromagnetic behavior below 5.7 K. The hysteresis loops of a 300-layer LB film at 2 K is shown in Figure 31. The LB film obtained from the monolayer spread on a subphase containing copper hexacyanoferrate also showed a spin ordering below 25 K [116].

Fig. 28. Temperature dependence of the molar fraction of a high-spin species calculated from IR intensity for the LB film in the cooling (full circles) and warming (open circles) measurements. Source: P. Coronel et al., /. Chem. Soc. Chem. Commun. 193 (1989).

286

AKUTAGAWA ET AL.

molecules was employed ranging from normal alkyl thiols to complex biomolecules. The molecules mainly used are alkane thiols ••and their derivatives, which are readily attached to gold surfaces *•«•• 1 0^rjX^C^p^^^^CCCCPOCl XXXXXXIC 0 0 0 Q o n fl and form ordered monolayer films. On the other hand, aromatic ^ ¥ • • " ^ j thiols, which offer advantages over alkane thiol derivatives in I 4x10*1 1.2x10* stability due to their rigid shape and conjugated nature, were 0.9, 1.0x10* not employed extensively as film forming materials so far [118]. 8.0x10* However, by using aromatic thiols we can attach the functional 0.86.0x10* group, or extended 7r-system, directly to the gold (or some other "'4.0x10* / metal) surface. Consequently, the functions based on the direct • 2,0x10* interaction between the jr-system and the metal-surface elec0.700x10**' trons, in addition to those associated with electrochemical conj c 20 40 60 80 100 trol through the metal surface are expected. The limited number T\K\ —,—,—0.6of reports of aromatic thiol-based self-assembly monolayers are 40 60 80 100 on porphyrin [119], phthalimides [120], in addition to the phenyl, 20 T[K] ;?-biphenyl, and /?-terphenylderivatives [121]. From the viewpoint of electronic and photonic materials as Fig. 29. Plot of xT vs. temperature for the K5HC0W12O40 polyanion in the powder (open circles) and for the DODA/C0W12O40 LB film (full well as for the application to molecular conductors, more eleccircles). The inset is a plot of reciprocal magnetization M~^ [emu] of the troactive functional groups are desirable. As for alkylthiol derivaLBfilmvs. temperature. Source: M. Clemente-Leon et al., Angew. Chem. tives, electroactive functional groups such as ferrocenes [122], viologenes [123], and Ru-complexes [124] were examined aiming Int. Ed. Engl. 36, 1114 (1997). at sensors, electron-transfer mediators, etc. Kondo et al. reported the electrochemical control of the second harmonic generation using ferrocene derivatives shown in Figure 32 [125]. The ferro4.2. Self-Assembly Films cenyl group was attached through the alkyl thiolate to the gold surface, and the second harmonic generation from the surface 4.2.1. Self-Assembly Films on Metal Surface was monitored during the redox reaction of the ferrocene unit. Self-assembled monolayers on the solid substrates were known The results are shown in Figure 32. The second harmonic intenfrom the middle of the twentieth century [117]. An explo- sity increased when the ferrocene moiety was oxidized electrosive growth in the field was witnessed and a large diversity of chemically and it returned to the original value upon reduction of the ferrocene group. The second harmonic intensity change was mainly due to the difference of the molecular hyperpolarizability between neutral and oxidized states, in addition to the molecular orientation change upon oxidation.

f

4.2.2. Layered

44 A

Multicomposites

An alternative approach for constructing ordered thin films was reported, that is, fabrication of multilayers by consecutive adsorption of poly anions and polycations [126]. Since the method is essentially applicable to polymers, the resulting superlattice (alternate layers of cationic and anionic polymers) has fussy structures due to the flexibiUty of polymer backbones. A schematic view of the film deposition process is shown in Figure 33. The

2H

B

Fig. 30. Schematic view of the DODA-Prussian blue LBfilm.The exact structure and formula of the inorganic part are unknown. However, it contains Fe"^-CN-Fe" units and two types of CN groups (only the bridging CN molecules are presented): Fe^"^\ large open circles, Fe^"\ large full circles, C and N, small full circles. Source: C. Mingotaud et al., Langmuir 15, 289 (1999).

1

I • ' ' I '

-600 -400 -200

0

200

I".

400

' 1

600

H(G) Fig. 31. Hysteresis loops of an LB film (300 layers) at 2 K with the magnetic field parallel (open circles) or perpendicular (full circles) to the LB layers. Source: C. Mingotaud et al. Langmuir 15, 289 (1999).

SUPRAMOLECULAR ASPECTS OF ORGANIC CONDUCTORS

(i)

287

to^to^to

CS>-{CH2)6 • SH NOj

(a) NPEFcCeSH i^^3r-C-(CH2)5-SH Pe

-C C-

(b) NPEFcCOCcSH

(ii)

0

5

10 15 20 25 Charge / ^C cm-2

30

Fig. 33. Schematic view of the layer-by-layer film deposition method, (a) Deposition process using slides and beakers, (b) Molecular picture of the first two adsorption steps, (c) Chemical structures of two typical polyions. Source: G. Decher, Science 211, 1232 (1997). See also Plate 2.

10 15 20 25 Charge / jxC cm-2

Fig. 32. (i) Ferrocene derivatives used for constructing SAM films, (ii) Charge dependence of the second harmonic intensity from the (a) NPEFcCgSH and (b) NPEFCCOC5SH SAM modified gold electrodes. Source: T Kondo et al, /. Am. Chem. Soc. Ill, 391 (1999).

method provides a convenient way to construct a polymer superlattice, which is promising in the application of polymers in electronics and photonics. For example, using polyion-based multicomposites, organic light-emitting diodes are already reported. Rubner et al. applied the method to fabricate high-efficiency light-emitting devices based on the Ru complex [127]. First, they used a polypridyl Ru(II) complex dispersed in a matrix of poly(ethylene oxide) and prepared a thin film by the spin coating technique [128]. The film was sandwiched by ITO and Al electrodes. The device thus obtained produced a luminescence level of 300 cd m"^, but operated with low external device efficiencies (0.02%). Polypyridyl Ru(II) complexes produce Hght in a process known as electrogenerated chemiluminescence (ECL). Critical issues for light emission are (1) the successful creation of the Ru(III) and Ru(I) species at the anode and cathode, respectively, and (2) the balanced redox transport of these species away from the electrodes to annihilate to produce the light-emitting Ru(II)* excited state. For the high efficiency of the chemiluminescence, consequently, it is necessary to design thin films that support some level of ionic conductivity and to gain control over the site-to-site distance of the Ru(II) complexes to reduce selfquenching effects.

They used a water soluble Ru(II)-containing polyester as a polycation in conjunction with the polyanion, poly(acrylic acid) (Fig. 34). By the pH adjustment of the solution, they controlled the charge density of poly(acrylic acid), both as an adsorbing layer and as a previously adsorbed layer. It was possible to use this approach to systematically gain control over the composition of the film. Moreover, the polyanion and polycation layers of sequentially adsorbed films were highly interpenetrated. The resultant thin films were considered as an essentially homogeneous blend of the two polyelectrolytes. The best result was obtained for the thin film containing 46% of Ru(II) containing polyester, which exhibited an external quantum efficiency in the 1-3% range with a maximum light output of 40-50 cd m"^. It was also possible to create compositionally graded heterostructures, which emit light either only in the forward bias or only in the reverse bias [127]. The film deposition is possible with polymer pairs which can form hydrogen bonds [129], and more interestingly, with those that can form charge-transfer complexes [130]. The covalent bond formation is also possible between the polymer layers [131]. By using this technique, fullerene molecules were incorporated into polyamine layers where fullerene molecules form covalent bonds with amine moiety [131]. The layered multicomposite of molecular conductors has not been realized so far to our knowledge. However, molecular conductors which have a strong chargetransfer interaction between donor and acceptor moiety should be appropriate building blocks for constructing electronic and photonic devices by multicomposite method. Such multicomposite methods even with appropriate polymers as one of the composite components should open up new possibilities for the application of molecular conductors.

288

AKUTAGAWA ET AL.

OCO(CHj),„OCO-f—

^

2+

^

I Ru(bpy)3'**2 polyester (X=Cl- or PF^")

-CHj-CH^ 1 COOH

-CH,-CH-

2 poly(allylamine hydrochloride) PAH

3 polyCaciylic acid) PAA

Fig. 34. Structure of the polycations and polyanions used for ECL device fabrication.

5. SUPRAMOLECULAR ASPECTS OF ELECTRONIC PHENOMENA IN ORGANIC CHARGE-TRANSFER SOLIDS In organic charge-transfer (CT) solids, valence electrons of constituent molecules can move from molecule to molecule with CT interactions, which is the foundation for various electronic phenomena, including electronic phase transitions, observed in this kind of material. In this section, we describe the supramolecular aspects of collective electronic phenomena in organic CT complexes, with reviewing the mechanisms of Peierls transitions and of neutral-ionic phase transitions. Then, we will proceed to describe the relationship between the supramolecular structures and the electronic properties of organic CT solids, referring to the development of BEDT-TTF-based cation radical salts and BEDT-TTF-based donor-acceptor type CT complexes. 5.1. Collective Electronic Phenomena in Charge-Transfer Complexes 5.1.1. Charge-Transfer Interaction in Organic Conductors Various types of intermolecular force are known to construct supramolecular assemblies [132]. Van der Waals force, chargetransfer (CT) force [133], hydrogen bond, and Coulombic force are well known for molecular materials. Among these, the CT force is essential for organic conductors, not only as it mainly contributes to form the supramolecular assembly, but also as it arises from the CT interaction which governs the electronic properties of organic conductors. The CT mechanism can be regarded as a kind of coordinate bond between electron-donor (D) and electron-acceptor (A) molecules, where the electrons on the highest-occupied molecular orbital (HOMO) of the D molecule intrude into the lowest unoccupied molecular orbital (LUMO) of the A molecule. As a result, part of the valence electrons is transferred from D to A, or is delocalized over the DA pair. This mechanism lowers the total energy, which becomes the origin of the CT force. A similar CT mechanism is also effective for a pair of the same molecules

DOC

D0D2+

D+D^

A=t

JA

Aoc

A-

D+A-

D^AO

poAO

1A

D+A- JA

A=2t2/U

Fig. 35. Dimer components constructing charge-transfer complexes.

like the DD pair. In this case, the constituent molecules should be ionized either partially or completely, and the CT interaction is sometimes called the charge resonance. As components for CT complexes, the formation of the following four kinds of dimers are the starting point; (A) D+D^, (B) D+D+, (C) D^A^, and (D) D+A", shown in Figure 35. Here, dimer (A) is constituted of donors with 1/2 average charge, dimer (B) is constituted of completely ionized donors, dimer (C) is constituted of the neutral donor-acceptor pair, and dimer (D) is constituted of the ionized donor-acceptor pair. The ground state of the respective dimers is stabilized due to the CT mechanism, which can be understood as a hybridization between ground states and the CT excited states. The gains of the CT mechanism are (A) t, (B) If/U,

(C) f/Ed,

and (D) ^V^CT.

respectively, as shown in Figure 35 [134]. Here, "^" is the CT energy between molecules, "t/" denotes the Coulomb repulsion on a single molecule, which corresponds to the energy difference between D+D"^ and D^+D°, and EQJ denotes the energy difference between D^A^ and D+A". It is noted that the formation of neutral dimers, denoted as D^D^, is difficult, since the energy of the CT excited state, shown as D"^D~, is extremely high as compared with those of the dimers (A)~(D). In organic conductors, 7r-conjugated aromatic planar molecules, such as TTF or TCNQ, come face-to-face with each other so as to maximize the intermolecular 7r-orbital overlap.

289

SUPRAMOLECULAR ASPECTS OF ORGANIC CONDUCTORS

(b) TTF

CA

Xr
TCNQ

TMTSF

_J

CXHI) substituted

BEDT-TTF

CN-^CN

TCNQ

Fig. 36. Electron-donor and electron-acceptor molecules for organic charge-transfer complexes.

(a)

(b)

H

i f M^

-Acceptor

Donor

Fig. 37. Segregated molecular stacks and mixed stacks observed in organic charge-transfer complexes.

which affords large CT interaction. The representative organic donor and acceptor molecules are shown in Figure 36. The molecules form stacks or layer structures in the solid state mainly with face-to-face overlap of the planar molecules. The organic metals and superconductors are constituted of component (A) having partial charge transfer, while component (B) affords Mott-Hubbard insulators. These are called the segregated stack compounds shown in Figure 37a, while the components (C) and (D) give alternate stacks of donor and acceptor molecules, which are called the mixed-stack compounds, shown in Figure 37b. In addition to the CT force, various factors contribute to determine the supramolecular structure of organic conductors. They include the following factors; van der Waals contacts, Coulombic force, and hydrogen bond, etc. However, the electronic properties of organic conductors are primarily associated with the CT interaction. In the experimental and theoretical research, the extended Htickel monoelectronic (MO) calculation of the intermolecular overlap is conventionally utilized for the numerical estimation of the CT interaction to understand electronic structures of the various CT complexes [135, 136]. 5,1,2. Peierls Instability in Conductors

Quasi-one-dimensional

The CT interaction is usually much stronger along the stacks than between the stacks. Due to such an anisotropic nature of the CT interaction, most of the organic conductors afford quasi-one-dimensional metals or semiconductors, in which the TTelectrons are delocalized along the stack [137, 138]. On the basis of the strong anisotropy in organic CT complexes, the following important concepts in solid-state physics were established for the quasi-one-dimensional electron system; charge-density waves (CDW) responsible for metal-insulator transitions observed in plenty of organic metals including (TTF)(TCNQ) [139], spindensity waves (SDW) responsible for metal-insulator transitions

Fig. 38. Schematic for supramolecular configuration of Peierls and spin-Peierls transitions.

in (TMTSF)2PF6 and related complexes [140, 141], and spinPeierls transitions (SPT) responsible for the anomaly in the magnetic susceptibility of (TTF)CuBDT, etc. [142]. The CDW is understood as a periodic distortion of the lattice with which the energy gain for the CT interaction is to be maximized in a supramolecular assembly. Molecules slightly shift the position in the CDW state at low temperature, to form the alternating "dense-and-rare" packing pattern of molecules, from the uniform-stack configuration at high temperature, as schematically shown in Figure 38. Here, "wave" does not mean the dynamic but the static patterns with a periodicity, which afford the periodical electrostatic potential for delocalized valence electrons. If every two valence electrons are accommodated in respective dense packing regions, large energy gain is obtained, and the system exhibits insulating behavior. This periodicity of CDW is corresponding to the wave vector of Ik^, where /^p is the Fermi wave numbers [143]. This is a kind of CT mechanism, in which molecular configuration is distorted spontaneously, by way of the CT force, to maximize the energy gain. Such a kind of metal-insulator transition is called a Peierls transition, which occurs at a specific temperature, depending on the energy balance between the gain due to the CT mechanism and the loss due to the lattice transformation. As an example, (TTF)(TCNQ) exhibits Peierls transition at 53 K [144-146]. The insulating phase is called a Peierls insulator, which has a nonmagnetic ground state because of the singlet pair formation of valence electrons. For this reason, the transition accompanies the abrupt decrease of the magnetic susceptibility at low temperature [147, 148]. The Peierls transition was observed in plenty of organic CT complexes [148]. The charge-density wave is confirmed by the satelHte reflection observed between the conventional Bragg spot in the X-ray scattering measurement [149, 150]. The alternating dense-andrare molecular packing causes the satellite reflection at low temperature. In addition, it is reported that the scanning tunneling microscope (STM) directly imaged the alternating dense-andrare molecular packing [151]. Figure 39 shows the results for the STM image of the crystal surface of (TTF)(TCNQ) (a) at room temperature and (b) at 42 K, where the segregated molecular stacks are observed along the Z?-axis. As shown in the figure, the high electron density due to the alternating slightly shifted dense-and-rare packing along the a-axis is observed in the image at 42 K, while uniform stacks are observed at room temperature. In (TTF)(TCNQ), one period of the alternating dense-and-rare packing almost amounts

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Fig. 40. Images of the sample surface under a microscope at 290 K: (a) before the electric field was applied, and (b) the low-resistive state with an electric current flow of 0.69 mA. Source: R. Kumai et al., Science 284, 1645 (1999).

Fig. 39. STM image of the crystal surface of (TTF)(TCNQ) (a) at room temperature and (b) at 42 K. Arrowheads in the lower right of the image (b) indicate the noticeable peak position of CDW in TCNQ chains. Source: T Nishiguchi et al., Phys. Rev. Lett 81, 3187 (1988).

to 3.2 molecules, as obtained in the X-ray scattering measurement. The STM result is in good agreement with this periodicity. In contrast to the CDW, the SDW is the density wave for the spin-direction of valence electrons. It caused the metal-insulator transition of (TMTSF)2PF6 and related complexes at low temperature [152]. The spin-density wave is distinguished from the CDW in terms of the spin magnetic order, or magnetic anisotropy [153]. In the complexes composed of component (B) (Fig. 1), charges are localized on respective molecules due to the electron correlation energy U on each molecule (see Fig. 1). It is noted that charges are also localized on dimers in the case of strong dimerization, if the degree of charge transfer is 0.5 [154]. This type of insulator is called a Mott-Hubbard insulator [155]. Such insulators exhibit paramagnetic behavior in magnetic susceptibility that is ascribed to the unpaired electrons on a respective molecule [156]. Spin-Peierls transition is another kind of Peierls transition, where the previously mentioned Mott-Hubbard insulator turns to the nonmagnetic Peierls insulator with periodic lattice distortions (Fig. 38b) [157, 158]. In the case of p = 1, the molecules form dimers, and in the case of p = 0.5, the molecules form tetramers, respectively, to form electron pairs [159]. This transition is also associated with the CT mechanism, where the energy gain is obtained magnetically by CT interaction due to the singlet pair formation of unpaired valence electrons. As described earUer, there are various types of ground states exhibited by the supramolecular assembly with a quasi-onedimensional electronic nature. For these types of ground states, current researches is devoted to the coexistence or competition

between the ground states in the field of solid-state physics. It is reported that SDW transition is highly unconventional and slightly coexists with the CDW-type periodic lattice distortion with 2k^ periodicity in (TMTSF)2PF6 at low temperature [160, 161]. The competition of the ground states is also reported between spin-Peierls and antiferromagnetic states in (TMTTF)2PF6 at high pressure [162]. Peierls insulators are known to show nonlinear electric conduction, which is associated with the collective sliding motion of charge density waves [163]. The spin-Peierls insulators as well as the Mott-Hubbard insulators also show nonlinear conduction upon appUcation of a quite high electric field [164]. Experiments show that the current-driven low-resistive state realized by the high electric field in K(TCNQ) is stabilized down to 2 K. In addition, the current path in the low-resistive state was visualized at room temperature with a microscope, the result of which is shown in Figure 40 [165]. It is extremely interesting in this figure that the current flow causes a stripelike periodic phase-segregation in the carrier-rich and carrier-poor regions along the current path. 5.1,3, Valence Instability in Donor-Acceptor CT Complexes

'type

In the mixed-stack compounds shown in Figure 37b, Coulombic force takes important roles in an interesting phenomenon, associated with valence instabihty, called neutral (N)-ionic (I) phase transition. This section briefly reviews the mechanism and the research progress for this phenomenon in this type of supramolecular assembly. Mixed-stack CT complexes have either neutral or ionic ground states, depending on the balance between the energy gain for the electrostatic potential called Mardelung energy (Ey^) in an ionic crystal lattice, and the energy cost for ionization of the DA pair in a neutral crystal lattice [166]. The latter amounts to / Q - E^, where /p is the ionization potential of a donor and E^ is the electron affinity of an acceptor. On the other hand, in the mixedstack compounds, the CT mechanism as shown in Figure 35 (C) or (D) affords the ground states with a partial charge transfer [D+^ A~^(0 < p < 1)], due to the hybridization between the neutral [D°A^] and the ionic [D+A"] states. Nevertheless, the mixed-stack compounds are classified into the nominal "neutral" and nominal "ionic" compounds, in terms of the magnetic properties, due to the existence of spins of cation and anion radicals in an ionic crystal lattice.

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291

Both of the neutral and ionic complexes show semiconduct(b) ing and/or insulating behaviors. However, in some specific combinations of D and A, the complexes exhibit valence instability between neutral and ionic states, accompanied by an anomaly of various solid-state properties in the vicinity of phase transition. The transition is first discovered in the complex of (TTF)(CA) having strong one dimensionahty [167]. The transition is also observed in the bis(ethylenedithio)tetrathiafulvalene Neutml (BEDT-TTF)(TCNQ) analogues in which the side-by-side interIonic molecular CT interaction of BEDT-TTF could suppress the one (c) dimensionality as shown in the later section [168]. iyAoiyA<>DgA^[D^'D*A'rrAJ ly^A^WA^xy^A^ The neutral-ionic phase transition is, either directly or indirectly, triggered by the variation of Coulombic force, that is Mardelung energy, in the ionic crystal lattice with application of Fig. 41. Schematic for neutral-ionic phase transition in quasipressure or temperature change. The fundamental features of one-dimensional CT complexes, (a) Neutral phase, (b) ionic phase, and (c) the domain excitation in the vicinity of the phase transition. The tranthis transition are; sition accompanies the spin-Peierls type periodic lattice distorsions in the The intramolecular absorption spectra associated with ionic phase in quasi-one-dimensional CT complexes. either electronic or vibronic excitation undergo appreciable changes due to the discontinuous variation of the CT degree accompanied by the transition. transition is also observed in the (BEDT-TTF)(ClMeTCNQ) Both of the cation and anion radical spins appear in the complex [168], where the component donor, BEDT-TTF, is ionic states. known for the characteristic side-by-side intermolecular CT interThe notable increase of the conductivity is observed in the action, as discussed in the next section. The valence instabihty vicinity of the phase transition [169]. observed in the quasi-two-dimensional complexes is related with In addition to these, the following important aspect is the magnetic and electrical properties in the vicinity of the NI observed in the complexes with strong one transition. dimensionahty, which are mainly investigated so far. The existence of radical spins in the ionic states induces the spin-Peierls-type lattice dimerization accompanied by the neutralto-ionic phase transition. This feature indicates that the ionic complexes are also nonmagnetic Peierls insulators in such quasi-one-dimensional complexes. Meaning, the transition occurred between the nonmagnetic neutral and nonmagnetic ionic ground states, as schematically shown in Figure 41. Due to the preceding features, the neutral-ionic phase transition exhibits the characteristic structural, magnetic, optical, and electrical properties. The symmetry breaking due to the structural dimerization observed in (TTF)(CA) was investigated by infrared spectra [170], X-ray scattering [171], neutron scattering [172], and chlorine-nuclear quadrupole resonance (NQR) [173]. Since the symmetry breaking is associated with the appearance of ferroelectricity in the ionic states, the Curie-like behavior is observed in the dielectric constants of the neutral states just above the phase transition [174]. In the magnetic properties related with the radical spins of ionic states, the paramagnetic component is observed in the ESR measurement that is ascribed to the local defect for dimerized lattices [175]. In such compounds, the energy of the collective domain excitation, as shown in Figure 41c, is predicted to be much smaller than the local CT excitation of DA pairs, due to the long-range part of the Coulomb interaction [176]. The notable increase of the conductivity in the vicinity of the phase transition is discussed to be associated with the domain excitation [169]. Chemically substituted crystals of (TTF)i_^(TSF)^(CA) are synthesized successfully to tune the NI transition temperature. In the compounds having very low transition temperature, it is discussed that the observed critical behavior of the dielectric constants is associated with the quantum para-electric effect [177, 178]. Another topic is that the neutral-ionic phase

5.2. Supramolecular Aspects of Electronic Phenomena in BEDT-TTF Complexes 5.2.1. Development of BEDT-TTF-based Cation Radical Salts The Peierls-type ground state that appeared in quasi-onedimensional conductors at low temperature is disadvantageous to obtain organic superconductors. Among various constituent molecules for organic CT complexes, an extended-TTF donor molecule named BEDT-TTF and their analogues, shown in Figure 36, are known to afford quasi-two-dimensional conductors that can suppress the Peierls instability. Among these, nearly half of the organic superconductors, reached more than 50, so far found are radical cation salts of BEDT-TTF, and the organic superconductor with the highest Tc to date is also based on the BEDT-TTF molecule [136]. Up to now, synthesis of more than 200 CT complexes were explored in the combination of BEDTTTF with a variety of inorganic counteranions (Xs), denoted as (BEDT-TTF)^X„. Most of the BEDT-TTF-based complexes have twodimensional layered structures whose supramolecular structure is partly stabilized due to the characteristic "side-by-side'' and 'Jace-to-face" intermolecular CT interaction [169]. It is characteristic of BEDT-TTF that this molecule forms a rich variety of structural phases depending on the combination with inorganic counteranions. The various crystal structures are customarily classified as a, j8, ^, K, and so on. Although the relationship between the structures and electronic properties of (BEDT-TTF)^X„ salts was intensively reviewed in the literature [135, 136, 179-181], we briefly describe the fundamental aspect of the supramolecular structure in these salts. The HOMO of BEDT-TTF molecule has a large coefficient on the p7r orbital of the sulfur atoms, which has a large extension over the side of the molecules. The intermolecular CT interaction is mainly contributed by the sulfur-sulfur

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^hs

100

^

^

Fig. 43. Schematic crystal structure of BEDT-TTF donor sheets in a-type salts viewed along the molecular long axis. Source: T Mori et al, Bull Chem. Soc. Jpn. 72, 179 (1999).

Figure 44 shows the BEDT-TTF donor arrangement of K-type salts [190]. In these types of complexes, the two-dimensional layered structure is composed of dimer units, which are located on both corners and centers of the lattice. The intermolecular interaction inside of the dimer is much larger than that between the dimers. (BEDT-TTF)2Cu(NCS)2 [191], (BEDTTTF)2Cu[N(CN)2]Cl, and (BEDT-TTF)2Cu[N(CN)2]Br [192] are the representative K-type salts, which afford a group of organic superconductors exhibiting the highest critical temperature among CT complexes. On the basis of (BEDT-TTF)^X„ salts with a variety of crystal structures, superconductivity, metal-insulator transition, Fig. 42. (a) Overlap integrals between BEDT-TTF molecules calculated Fermiology in quasi-two-dimensional metal, and various magas a function of the relative configuration of the molecules, (b) Definition netic properties associated with electron-electron repulsion were of the relative configuration of the molecules used for the overlap calcu- investigated. Meaning, it is epoch-making for a donor molecule lation. The displacement along the molecular long axis is designated as BEDT-TTF in supramolecular chemistry that the various strucD, and the angle between the molecular planes is defined as cf). Source: tural types are realized and are classified with enormous experiT. Mori et al., Bull. Chem. Soc. Jpn. 71, 2509 (1999). mental data of their electronic properties. According to plenty of experimental and theoretical studies, the electronic structure of these complexes can be explained on contact between BEDT-TTFs. The side-by-side intermolecular CT the basis of the following two principal factors; one is the interinteraction is as large as the face-to-face interaction in BEDT- molecular CT interaction which can be calculated based on the TTF, as is shown by the intermolecular overlap calculation based extended Hiickel method [179-181]. Magnetotransport experion the extended Hiickel method. As an example, the calcu- ments for various organic metals at low temperature are well lated results for the dependence of intermolecular overlap inte- explained by the calculated band structure, based on the intergral on the relative configuration of the molecules are shown in molecular CT interaction [193, 194]. The other is the electronFigure 42. In the (BEDT-TTF)^X„ system, the formation of the electron repulsion on the BEDT-TTF dimer. It is believed to regular stack of planar molecules is also prevented by the exis- afford a consistent picture to explain the relationship between tence of the end ethylene group, which is out of the molecular plane. Due to such a characteristic feature of the BEDT-TTF molecules, the two-dimensional layered structures are easily realized. Two examples are shown for the molecular arrangements of layered structure in the following. Figure 43 shows the BEDT-TTF donor arrangement of a-type salts [182]. In these types of complexes, the stacked column is seen along the c-axis. However, the side-by-side intermolecular interaction is larger than the face-to-face interaction. As representative compounds for these types of complexes, (BEDT-TTF)2l3 [183], and (BEDT-TTF)2KHg(SCN)4 [184] are well known. The former salt exhibits a metal-insulator transition at 135 K, above what the semimetallic behavior is investigated at [185]. The latter salt shows metallic behavior down to low temperature, while a part of the Fermi surface disappears due to the SDW mechanism at 8 K [186]. An interesting feature is observed in angle-dependent magnetoresistance oscillation measurements, Fig. 44. Schematic crystal structure of BEDT-TTF donor sheets in which is associated with the reconstruction of the Fermi surface K-type salts viewed along the molecular long axis. Source: T. Mori et al., Bull. Chem. Soc. Jpn. 72, 179 (1999). with the SDW mechanism [187-189].

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tbi' 'tb2' Fig. 45. A schematic view of the electronic phases of a- and K-type (BEDT-TTF)2X compounds derived from the results of Hartree-Fock calculations. AF„ and AF^ are the antiferromagnetic ordered phase for a- and K-type compounds, respectively. (I) and (M) imply insulating and metallic phases, respectively. Source: H. Kino and H. Fukuyama, / Fhys. Soc. Jpn. 67, 4523 (1995).

superconductivity and the Mott-Hubbard insulator phase in K-type BEDT-TTF complexes at high pressure [195]. Based on the CT interaction and electron-electron repulsion on dimers, a universal phase diagram, shown in Figure 45, was proposed to understand the electronic properties of a-type and K-type salts depending on the dimerization and the bandwidth [196]. 5.2.2. BEDT-TTF-based Donor-Acceptor Type CT Complexes Since the discovery of high electrical conductivity in (TTF)(TCNQ), a large variety of "donor-acceptor type" CT complexes were synthesized [197]. Both of two "organic" components can contribute to the electronic properties in these types of complexes, which are in contrast to the radical cation (or anion) salts composed of organic donor (acceptor) and "inorganic" counteranions (cations), like (BEDT-TTF)^X„ discussed earher. The combination of planar donor and acceptor molecules is known to afford the following two kinds of molecular stacks; one is the segregated stacks in which the donors and the acceptors form stacks independently, as shown previously in Figure 37. They are composed of the dimer component (A) or (B) shown in the former section. The other is the mixed stacks composed of the dimer component (C) or (D). In addition to the two forms mentioned in the foregoing text, investigation reveals that the BEDT-TTF-based donor-acceptor type CT complexes give novel crystal structures. We review here the structural aspects of BEDT-TTF-based donor-acceptor type CT complexes, mainly focusing on those of a family of (BEDTTTF)(TCNQ) analogues. The combination of BEDT-TTF, having characteristic face-to-face and side-by-side intermolecular interaction, with TCNQ, which has face-to-face highly anisotropic interaction, is shown to afford novel crystal structures. As for BEDT-TTF analogues, BEDT-TTF, BEDT-TSeF, BEDO-TTF, BMDT-TTF, and BEDO-TTF are the candidates for the constituent donor molecules. In contrast, various acceptors of TCNQ derivatives can be prepared by chemical modification to have various electron affinity and molecular polarizabilities, by substituting with halogen atoms.

Fig. 46. Typical four types of molecular arrangements of BEDT-TTF-based donor-acceptor type charge-transfer complexes, (a) Complexes with two-dimensional conducting layers, (b) complexes with double columnar structures, (c) complexes composed of side-by-side BEDT-TTF chains, and (d) complexes with mixed-stack structures.

The crystal structures of the obtained (BEDT-TTF)(TCNQ) family can be classified into the following four categories; (i) (ii) (iii) (iv)

Complexes Complexes Complexes Complexes

with two-dimensional conducting layer, with double columnar structure, composed of side-by-side BEDT-TTF chain, with mixed-stack structure.

These four types of crystal structures are schematically shown in Figure 46. In the following, each of the structural types are briefly reviewed. 5.2.2.1. Complexes with Two-dimensional Conducting Layer The complexes with two-dimensional conducting layer were obtained for more than 20 compounds including (BEDTTTF)(TCNQ) [198, 199]. Among these compounds, the crystal structures of (BEDT-TTF)(Fj TCNQ) [200, 201] and (BEDTTSeF)(Cl2TCNQ) [202] are described in the following text. The former compound preserves metallic properties down to 2 K, while the latter is a first donor-acceptor type superconductor. Figure 47 shows the crystal structure of (BEDTTTF)(FiTCNQ). As shown in Figure 47, the donors and the acceptors form sheets independently along the (010) plane. The BEDT-TTFs are stacked face-to-face along the c-axis, and arranged side-by-side along the [100] and the [101] directions (Tig. 47b). The intermolecular S-S distance is found to be 3.48 A between ETs aligned along [101], and 3.71 A along [100], indicating the two-dimensional nature of the conducting donor sheet. Molecular packing of BEDT-TTFs is uniform in the sheets, which is in contrast to most of the other BEDT-TTF-based conductors. It is noted that the hydrogen bonds between FjTCNQs contribute to stabilize the present structure as discussed in the later section. In (BEDT-TTF)(FiTCNQ), metaUic properties are preserved down to 2 K. The BEDT-TTF two-dimensional donor layer mainly contributes to the conducting properties of this compound. In contrast, acceptor molecules of FjTCNQ are strongly localized, as they exhibit Curie-Weiss behavior in the spin magnetic susceptibility.

AKUTAGAWA ET AL.

(a)

Fig. 47. Crystal structure of (BEDT-TTF)(FiTCNO). (b) View of the molecular packing of BEDT-TTFs along the long molecular axis, (c) Molecular arrangement of FiTCNQs. Source: T. Hasegawa et al.,/. Chem. Soc. Chem. Commun. 1377 (1997). Figure 48 shows the crystal structure of (BETDTSeF)2(Cl2TCNQ), (a) viewed along the stacking axis (c-axis), and (b) viewed along the molecular long axis of BEDT-TSeF. As seen in these figures, donors form two-dimensional conducting layers, while acceptor columns are strongly isolated from each other. The structure of the donor layer is characterized by a umioim face-to-face stack along the c-axis and double periodicity in the side-by-side direction along the a-axis. It is noted that the uniform stack is in sharp contrast to the quadratic periodicity of the A-type BETS-based superconductors [203-205]. (BEDT-TSeF)(Cl2TCNQ) is a superconductor, whose superconducting transition temperature is 1.3 K at pressure of 3.5 kbar. The interesting feature of this compound is that the insulating behavior is observed below 20 K at higher pressure, which is followed by a superconducting transition at lower temperature. Investigations are now under way to elucidate the roles of acceptor molecules on the insulating phase appearing at high pressure. 5.2.2.2. Complexes with Double Columnar Structure The double columnar structure is characteristic of the BEDTTTF molecule, side-by-side contact of which contributes to the pair formation of the stacked columns. The 2:1 isomorphous compounds with double columnar structure are obtained in (BMDT-TTF)2(ClMeTCNQ), (BMDT-TTF)2(BrMeTCNQ), (BMDT-TTF)2(C1TCNQ), and (BMDT-TTF)2(BrTCNQ) [206]. In contrast, the 1:1 compounds are obtained in (BEDT-TTF)(2,3F2TCNQ). Figure 49 shows the crystal structure of (BMDTTTF)2(ClMeTCNQ) viewed along the stacking axis. The two segregated stack columns of BMDT-TTF donors form pairs each, where the short side-by-side S-S contacts are observed between the two columns in pairs, realizing a unique "double columnar" structure. The pairs are strongly isolated from each other, by adjacent acceptor columns. The intermolecular overlap integrals were calculated by the extended Hiickel method for the donor

(b)

Fig. 48. Crystal structure of (BETD-TSeF)2(Cl2TCNQ), (a) viewed along the stacking axis (c-axis), and (b) viewed along the molecular long axis of BEDT-TSeF Source: T Mochida et al., Synth. Met. 102, 1680 (1999).

Fig. 49. Crystal structure of (BMDT-TTF)2(ClMeTCN0), viewed along the stacking axis. Source: T. Mochida et al., Synth. Met. 102, 1680 (1999).

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to thefl-axis,[100], and (c) parallel to the c-axis, [001]. As seen in the figure, no prominent peak is observed in the reflectivity spectra polarized parallel to the Z?-axis (Fig. 51a). The absence of the CT band along the columnar direction, which is usually observed in the mixed-stack compounds, suggests that the electronic structure of (ET)(F2TCNQ) significantly differs from the conventional mixed-stack compounds. In contrast, an intense peak is observed at around 5,500 cm~^ for the direction parallel to the a-axis. Considering the characteristic side-by-side contact of ET and the 5.2.2.3. Complexes with One-dimensional strongly localized nature of F2TCNQ along the «-axis as observed Side-by-Side ET Arrangement in the crystal structure, this band is attributed to the CT excitaThe complexes composed of the side-by-side one-dimensional tion from ET to the neighboring ET, as schematically shown on BEDT-TTF chain are novel crystal structures, which are observed the right side of Figure 51. in (BEDT-TTF)(F2TCNQ) [207]. While the crystal structure Based on the earlier observations, it can be concluded that seems to be a kind of mixed-stack compound at first glance, the the intermolecular CT interaction along the a-axis is fairly large intermolecular CT interaction in the crystal is completely differ- and predominant, while the hybridization is negligible between ent from that of the mixed-stack complexes. Many other com- donors and acceptors stacking face-to-face along the b-axis. It plexes in the combination of BEDT-TTF analogues with F4TCNQ is noted that the acceptors in this crystal are strongly localcan be categorized as this type. ized, as is evidenced from the Curie-Weiss behavior in the Figure 50 shows the crystal structure of (BEDT- spin susceptibility of F2TCNQ, while the one-dimensional MottTTF)(F2TCNQ). Molecular arrangements on the (001) plane are Hubbard insulating chain is constructed by BEDT-TTF side-byshown in Figure 50b; molecular planes of donors and acceptors side arrangements. stack on top of each other along the Z^-axis while donors are arranged side-by-side along the a-axis. In this view, the crystal can be regarded as a mixed-stack CT crystal with the column 5.2.2.4. Complexes with Mixed-stack Structure aligned parallel to the ^-axis. However, there are several unusual The mixed-stack structure is one of the most typical crysaspects in the crystal structure in contrast to the conventional tal structures observed in donor-acceptor type CT complexes. mixed-stack CT complexes: Among a variety of (BEDT-TTF)(TCNQ) analogues, (BEDTIntermolecular distance between ET and F2TCNQ is 4.13 A, TTF)(Me2TCNQ), (BEDT-TTF)(ClMeTCNQ), and (BEDOwhich is significantly larger than that of usual mixed-stack type TTFXClsTCNQ) are categorized into these types of CT complexes, 3.3 ~ 3.7 A. structures [168, 208]. Intermolecular S-S contact between the ET molecules along Figure 52 shows the crystal structure of (BEDT-TTF) the a-axis is 3.59 A which is shorter than the sum of the van der (Me2TCNQ), viewed along the stacking axis. In the crystal, Waals radius (3.7 A). donors and acceptors stack alternately with each other along the The unique intermolecular CT interaction in the crystal is c-axis. The intermolecular distance between donor and acceptor understood with use of the polarized reflectivity spectra, which molecules is 3.79 A inside the columns, which is comparable to sensitively detect the direction of the intermolecular CT excithat of usual mixed-stack CT complexes. The adjacent stacked tation. Figure 51 shows the polarized reflectivity spectra, for columns are arranged next to each other with a slippage of a half the electric vector (a) parallel to the b-axis, [010], (b) parallel length of molecule, along the molecular long axis, which make arrangements of (BMDT-TTF)2(ClMeTCNQ). Inside the double columns, tht face-to-face interaction of donors is calculated to be dominant (t = 19.7 x 10"^ eV), and the intercolumn side-by-side interaction is obtained as about 1/3 of it (t = 6.7 x 10"^ eV). The (BMDT-TTF)2(ClMeTCNQ) exhibits metalHc properties at room temperature, and undergoes metal-insulator transition at around 80 K.

(a)

Charge Tnnsfer (0101 (100)

(b) ET inlra-molecuUr excitation LUMO" HOMO. -8000 .I2000cni'

10000 20000 30000 Wivc number (cm*')

Fig. 50. (a) Crystal structure of (BEDT-TTF)(F2TCNQ) composed of the side-by-side BEDT-TTF chain, viewed along the a-axis, and (b) molecular arrangements on the (001) plane. Molecular planes of donors and acceptors stack on top of each other along the b-sods while donors are arranged side-by-side along the fl-axis. Source: T Hasegawa et al., Solid State Commun. 103, 489 (1997).

Fig. 51. Polarized reflectivity spectra of (BEDT-TTF)(F2TCNQ), for the electric vector (a) parallel to the 6-axis, [010], (b) parallel to the fl-axis, [100], and (c) parallel to the c-axis, [001]. The CT band observed at around 5500 cm~^ is strongly polarized parallel to the a-axis. Source: T. Hasegawa et al, Solid State Commun. 103, 489 (1997).

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Fig. 52. Crystal structure of (BEDT-TTF)(Me2TCNQ) with mixed-stack crystal structure. Source: T. Mochida et al., Synth. Met. 86, 1791 (1996). the columns more isolated. The CT exciton is polarized parallel to the stacking axis as conventional mixed-stack compounds, indicating the predominant face-to-face CT interaction between donors and acceptors inside the stack. Among complexes with mixed-stack structure, (BEDTTTF)(Me2TCNQ) and (BEDT-TTF)(ClMeTCNQ) have a neutral ground state which is constructed from the dimer component (C), while (BEDO-TTF)(Cl2TCNQ) has a ionic ground state, constructed from dimer component (D). It is noted that (BEDT-TTF)(ClMeTCNQ) undergoes neutral-ionic phase transition under the pressure, corresponding that the ground state transforms from (C) to (D), which is discussed in the later section. As a short summary for the structural aspects of (BEDTTTF)(TCNQ) analogues, the combination of BEDT-TTF with TCNQ, having different types of intermolecular CT interaction affords some novel crystal structures. The variety of crystal structures, the two dimensionality of the electronic structures, and the chemical tunability of organic acceptor molecules are promising to afford exotic electronic properties of CT complexes in future development of this family of compounds. 5,2,3. Effect of Intermolecular Hydrogen Bonds in CT Complexes In addition to the CT force stated previously, the hydrogen bond is also important to construct the supramolecular assembly in organic conductors. The hydrogen-bond is partly understood as another kind of CT force, in which the bond electrons on X—H (X is electronegative atom) are delocalized over the weak connection between H and Y (Y is electronegative atom) as X—H Y [209]. It contributes to the local connection between molecules. The local nature of the hydrogen bond is in sharp contrast to the CT force originating from thQ face-to-face or sideby-side intermolecular ir-orbital overlap. As an example of a hydrogen bond contributing the supramolecular structure of highly conductive compounds, the crystal structures of (BEDT-TTF) (Fi TCNQ) and its isomeric compound (BEDT-TTF)i(F2TCNQ)o.5(TCNQ)o.5 are described later. Figure 53 shows the molecular arrangement of FiTCNQ viewed along the axis perpendicular to the molecular plane.

Fig. 53. Molecular arrangement of FITCNQ viewed along the axis perpendicular to the molecular plane. Source: T. Hasegawa et al., /. Chem. Soc. Chem. Commun. 1377 (1997). The orientation of FiTCNQ is obtained as disordered, on the average for the whole crystal of (BEDT-TTF)(FiTCNQ). However, the intermolecular F C distance of 3.70 A (Fig. 53), which is shorter than the sum of the van der Waals radii of the F H—C arrangement, suggests a local structure represented as (TCNQ)F (TCNQ)F (TCNQ)F , based on the hydrogen bondlike interaction between CH and F. As evidence to support the foregoing hypothesis, the organic ternary compound (BEDT-TTF)i(F2TCNQ)^(TCNQ)i_^(0 < X < 1) gave an isostructural compound with (BEDTTTF)(FiTCNQ) only at x - 0.5. The result indicates that the present crystal structure is only realized in the composition, where the similar local structure for a hydrogen bond chain can be expected. (BEDT-TTF)i(F2TCNQ)^(TCNQ)i_^ were obtained from a concentrated solution in chlorobenzene. The composition shown here is based on the ingredients in the solution, and is almost identical to the chemical composition of the sample. X-ray powder diffraction patterns were measured to investigate crystal structures of (BEDT-TTF) i(F2TCNQ)^ (TCNQ) i_^(0 < X < 1). The results are presented in Figure 54 for (BEDTTTF)! (F2TCNQ);,(TCNQ)i_^ at X ~ 0.5, together with those of [A] (BEDT-TrF)(F2TCNQ), [B] (BEDT-TTF)(FiTCNQ), and [C] (BEDT-TTF)(TCNQ). The structures of [A]-[C] are clearly different from each other. In the ternary system, on the other hand, the compound of [B'] (BEDT-TTF)i(F2TCNQ)^(TCNQ)i_^ at x - 0.5 is considered to be isostructural with [B]. In this composition, crystalline flake samples are obtained as (BEDT-TTF) j (F2TCNQ)^(TCNQ)i_Jx - 0.5], which exhibits metallic properties down to 2 K just like (BEDT-TTF)(FITCNQ). In the composition other than x ~ 0.5, the diffractograms show that they are composed of two phases, like [A] (BEDT-TTF)(F2TCNQ) + [B'] (BEDT-TTF)(FiTCNQ). The previous result indicates that the introduction of one fluorine atom into each TCNQ, on the average, commonly gives isostructural compounds. Considering the molecular arrangements of FjTCNQ molecules in (BEDT-TTF)(FiTCNQ) denoted before, we can notice that the similar local structure can also be expected in (BEDT-TTF) i(F2TCNQ);, (TCNQ) i_^ with forming a hydrogen bond chain of alternating F2TCNQ

SUPRAMOLECULAR ASPECTS OF ORGANIC CONDUCTORS

10

20

30

40

50

2or Fig. 54. X-ray powder diffraction pattern of (BEDT-TTF)i (F2TCNQ)^(TCNQ)i_^ [jc~0.5], in comparison with those of (BEDT-TTF)(F„TCNQ) {n = 0, 1, 2). [A] (BEDT-TTF)(F2TCNQ), [B] (BEDT-TTF)(FiTCNQ), and [C] (BEDT-TTF)(TCNQ). The structure of [B'] (BEDT-TTF)i(F2TCNQ)^(TCNQ)i_, [x ~ 0.5] is similar to that of [B] (BEDT-TTF)(FiTCNQ). Source: T Hasegawa et al.,/. Chem. Soc. Chem. Commun. 1377 (1997).

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and are the agents for composing supramolecular entity. The chemionics device should be constructed from these supramolecular entities as agents for the next step assembly. Alternatively further steps may be needed to obtain more complicated chemionics devices. The chemionics system in the future should be (hopefully) the self-organizing system. The terms "self-assembly" and "self-organization" are sometimes mixed up especially among chemists. However, the latter should be defined as the dissipative system far from the equilibrium. For example, every living system is a self-organizing system. There is a field that utilizes the living organs, e.g., neural cells, for the construction of the electronic devices (biochips). If we have artificial molecular systems, which have a similar or a higher ability compared to the living systems, it is no longer necessary to utilize such biosystems. The top-down strategy of the present silicon-based devices succeeded excellently and our life changed largely from the development of silicon technology. To find a niche of the present silicon world, and even to overcome it in part, the chemionics system should have novel logic as well as novel molecular architecture. In this context, the chemionics system should be a self-organizing system and the logic to operate the system should more or less resemble those of the living systems. At present, we do not have a sufficient answer about how to construct both the logic and the molecular architecture. However, we hope that the studies on the molecular conductors from the supramolecular aspects will provide a new paradigm to answer these questions in the future.

REFERENCES and TCNQ arrangement as F(TCNQ)F (TCNQ) F(TCNQ)F (TCNQ) It accounts for the exclusive formation of the (BEDTTTF)(FiTCNQ) structure when one fluorine is introduced per TCNQ (at X ~ 0.5). As a short summary of this section, it is highly probable that the hydrogen bond for the F H-C arrangement, which locally connects the FiTCNQs or between TCNQ and F2TCNQ, can contribute to stabilize the structure of highly conductive compounds.

6. CONCLUDING REMARKS The supramolecular systems of molecular conductors described here have intriguing electronic systems and the possible extension of these systems is the appUcation to molecular electronics. There are several definitions of molecular electronics [102]. The electronics using organic molecules is one definition; the liquid crystal displays and polymer-based photolithography may set up this category. However, here we consider the molecular electronics as molecular-size electronics, or molecular- and supramolecularbased electronics. The more general term "chemionics" is proposed by Lehn in his book Supramolecular Chemistry [1]. The chemionics is composed of molecular electronics, molecular ionics, and molecular photonics, where electron, ion, and photon, respectively, play a central role. The chemionics device may be constructed from "programmed molecules" through the bottomup strategy. As described earlier, the complicated supramolecular assemblies are formed by the appropriate molecular design. Such designed molecules may be called programmed molecules

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(d)

Plate 1. Network anion structures of BEDT-TTF-and EDT-TTF-based organic conductors, (d) Three-dimensional channel network in the (EDT-TTF)4 Brl2 (TIE)5 salt. Source: H. M. Yamamoto et al., /. Am. Chem. Soc. 120, 5905 (1998). See also p. 280.

g*g*g*g

Plate 2. Schematic view of the layer-by-layer film deposition method, (a) Deposition process using slides and beakers, (b) Molecular picture of the first two adsorption steps, (c) Chemical structures of two typical polyions. Source: G. Decher, Science 277, 1232 (1997). See also p. 287.