Organic metals

Synthetic Metals, 4 (1981) 1 - 34

1

© Elsevier Sequoia S.A., Lausanne --Printed in The Netherlands

ORGANIC METALS

M. L. KHIDEKEL and E. I. ZHILYAEVA

Institute of Chemical Physics, USSR Academy of Sciences, 142 432 Chernogolovka, Moscow Region (USSR) (Received November 12, 1979)

Summary This paper is concerned with the problem of the preparation of "organic metals". Methods of synthesis and purification of organic metals are described. The general features of donors and acceptors which are basic to compounds of identified metallic state, specific peculiarities of conducting chains, and the main factors capable of stabilizing the metallic state in organic compounds are discussed.

Introduction One of the most important events of the last few years in the field of electroconductive substances is the synthesis of organic compounds possessing metallic type electrical conductivity [1 - 17] *. At present the number of workers studying "organic" or "molecular" metals and the number of such compounds are rapidly increasing. This paper is concerned with the main structures and factors stabilizing the metallic state, and with problems of purification and synthesis. The capacity for metallic behavior is most common among inorganic compounds: three-quarters of the total number of simple substances formed by elements, some of their compounds, and a large number of alloys are so constructed that the metallic state is the main stable state [19]. By contrast, in organic compounds the metallic state is very rarely encountered. Graphite (in the plane of aromatic rings) is perhaps the only instance of metallic or semimetallic conductivity [20]. The terms "electrical conductivity" and "metallicity" are sometimes applied to conjugated and aromatic systems referring to cooperation of n-electrons within a molecule and to specific *A metal may be defined as a substance characterized by high thermal and electrical conductivity, lustre, dense packing of the atoms, and which is opaque in the entire spectrum. The electrical conductivity of metals increases with reduction of temperature [18, 19].

magnetic, optical and other physical and chemical properties associated with this phenomenon [21]. In the study of redox processes the formal analogy between systems involving a metal (M -- e ~ M ÷) and systems involving a free radical [(C6H5)3C" - - e -~ (C6H5)3 C÷] is sometimes used [22, 23]. In some respects the problem of preparation of "organic" metals is traditional for synthetic chemistry. In the 19th century, during the rapid development of organic synthesis, the idea of possible similarities between alkali metals and organic radicals was quite popular. Research in this direction led to the discovery of several important reactions (Wurts, Frankland, Kolbe) and at the same time revealed one of the main difficulties in the preparation of organic metals, namely, the pairing tendency of valence electrons. The synthesis of stable free radicals produced paramagnetic insulators rather than metals: unpaired electrons were mainly localized on separate molecules. Numerous attempts to synthesize metals on the basis of polymers with conjugated bonds failed, usually yielding substances with semiconductor properties [1, 24, 25]. Attempts to stabilize the metallic state with organic amalgams were also unsuccessful [ 2 6 - 29]. The absence of organic compounds with metallic properties and the failure to synthesize such compounds was first of all due to the stringent requirements which must be met in order to obtain organic substances with a stabilized metallic state: (1) unpaired electrons; (2) a uniform crystal struc. ture so that, in the absence of electron-electron interactions, unpaired electrons would be delocalized in the same metallic energetic zone; (3) relatively weak electron-electron repulsive interactions [3]. In the sixties work began in the USA and the USSR to determine whether organic compounds with metallic properties might exist, concentrating mainly on ion-radical salts (IRS} and charge transfer complexes (CTC}. Several highly conductive compounds were already known among these compounds [30, 31], and it was thought that these types of compounds might satisfy the conditions which, following the analogy with " c o m m o n " metals, are necessary for the realization and stabilization of the metallic state. At the same time certain difficulties connected with selection of IRS and CTC were recognized. An important feature of such compounds is the presence in most of them of linear chains (stacks) consisting of donors (D) [7, 15] and acceptors (A) (Fig. 1). The spacings between components in these chains, as a rule, are shorter than van der Waals distances and considerably shorter than the distances between the chains. Hence, the compounds referred to are characterized by a quasi-one-dimensional type of packing of donor and acceptor molecules. These stacks may be arranged in various ways, for instance, structures of the t y p e . . . DADADA ... [7], ... DAADAA . . . [ 3 2 ] , . . . DDAADDAA . . . [33] and of the type . . . AAAAA . . . or . . . DDDDD ... [7, 15] have been described.

A ~

A

A ~ (a)

A

A~ (b)

(c)

Fig. 1. Structure of linear chains in CTC and IRS of the t y p e . . . A D A D A D . . . (a), •.. D D D D . . . o r . . . A A A A . . . (b, c): a, non-conducting chain; b, conducting regular chain; c, alternating chain.

For the latter type, regular chains with one molecule as a repeating unit (Fig. l(b)) and alternating with a period of two, three or four molecules (Fig. l(c)) are known. The second component is usually arranged in stacks also. Only structures with regular chains form metallic states (Fig. l(b)). However, on account of specific properties of one-dimensional systems it is very difficult to obtain compounds with regular quasi-one-dimensional structures possessing metallic properties. It is known from theory that such systems cannot exist in the metallic state [8]. In a hypothetical one-dimensional metal a decrease of temperature should lead to restructuring of the crystal lattice which involves conversion of regular chains (Fig. l(a)) into alternating chains (Fig. l(e)). As a result, a one-dimensional metal is invariably converted into an insulator on reduction of temperature (the so-called Peierls transition, similar to the Jahn-Teller effect in coordination chemistry) At lower temperatures the Coulomb interaction of electrons destabilizes the metallic state and this also leads to conversion into a dielectric (the socalled Mott-Hubbard transition). Finally, theory states that at low temperatures the conductivity of a one-dimensional system of electrons in a lattice with defects cannot be metallic [ 8]. The question of the applicability of these theoretical conclusions to real compounds with quasi-one-dimensional structures remained open for a long time. At present the existence of the metallic state in quasi-one-dimensional systems has been proved [7, 9] and satisfactorily explained [8]. The main experimental result obtained over this period is that the instability of the metallic state, characteristic of one-dimensional struetures, has been overcome. Achievements in the synthesis of highly conductive organic compounds based on ion-radical salts and charge transfer complexes are apparent from Fig. 2, which shows the values and temperature dependence of electrical conductivity for several good and medium conductors [ 7].

4

i

~

a

2

-2

(a) (b) (c) Fig. 2. Temperature dependence of electrical conductivity in the direction of the conducting chain for some ion-radical salts and charge transfer complexes: (a): 1, (Quin) (TCNQ)2; 2, (NMF)(TCNQ); 3, (ACr)(TCNQ)2 ; (b): 4, (TTF)(TCNQ); 5, (TTT)213; (c): 6, (HMTSeF)(TCNQ); 7, (TSeT)2Br; 8, (TSeT)2 Cl.

Data for (Quin)(TCNQ)2*, (NMF)(TCNQ) and (Acr)(TCNQ)2 salts are shown in Fig. 2(a). The o300K value is very high in this group (~102 ohm -1 cm -1 ), and in the 200 < T < 300 K range the conductivity exhibits a metallic dependence (increases with temperature decrease). At lower temperatures the conductivity decreases, the compound becoming a dielectric. It was on these compounds that the existence of the metallic state was first convincingly established [2]. The next group of curves (Fig. 2(b)) shows results obtained after 1972: the synthesis of compounds with even higher o values (1000 - 2000 ohm-a cm-a ). The metallic state is stable in these compounds down to very low temperatures (40- 60 K) (complexes (TTF)(TCNQ) and (TTT)213). Note the temperature dependence of electrical conductivity: high o values at low temperatures, the presence of an inflection, and a decrease of o below the temperature of the maximum. In this case the metallic state is not stabilized over the entire temperature range. This has been achieved recently in compounds of the third group: (HMTSeF)(TCNQ) [36, 37], (TSeT)2Br and (TSeT)2C1 [38, 39], the first organic metals that retain high electrical conductivity down to the very lowest temperatures. Maxima are also observed on the conductivity curves of these compounds ( o ~ 104 ohm -1 cm-1), but they correspond to a transition into a semimetallic state [40]. The number of organic compounds reported with high conductivity has grown rapidly. Together with the papers on the analog compounds of the well-known classes TCNQ, TNAP, TTF and TTT [41 - 46], there have appeared communications on highly conductive compounds of new types.

*Designations of the substances are given in Table 1.

In 1977 Sandman et al. [47] synthesized a number of charge transfer complexes of TCNQ with A4,4,-bithiopyran (Table 1) with the same conductivity as polycrystalline samples of (TTF)(TCNQ). Pearstein reported a higher conductivity for the CTC of the phenyl derivative, A4,4'-bithiopyran with TCNQ, than for (TTF)(TCNQ) [9]. Partially oxidized tetrabenzoporphyrins and phthalocyanines [ 17, 48, 49] form another class of "molecular metals". The monocrystals NiPc(I3)0.33 and Ni(OMTBP)(I3)0.35 studied by e.p.r, showed the ligand rather than the metal [48, 49] to be subjected to oxidation. The fact that in oxidizing phthalocyanine, which does not contain metal, a highly conductive c o m p o u n d [17] is also formed, shows that the conductivity in partially oxidized phthalocyanines is realized by the ligand part rather than by the metallic chain, unlike the one-dimensional conductor of lead phthalocyanine [50]. Recently, Heeger and co-workers have obtained complexes with a charge transfer of polyacetylene, (CH)x (PA), with halogens of metallic properties [ 51]. Conductor PAI2 can be considered as a partially oxidized chain of PA with counter-ions 13- and likely with 15- [52 - 54]. The increase of initial PA conductivity is also observed by treatment with Lewis acids (for example, conversion of cis-(CH)x into cis-[(CH)(AsFs)o.14] ~ producing a conductivity rise of 1011 times [55] ), or with metallic sodium [ 5 2 ] , or strong oxidant FSO2OOSO2F [56]. The conductivities resulting in the last case are: o30o K 700 ohm -1 cm-1; O150K 800 and ~4.2K = 640 ohm -1 cm -1. The main factors which facilitate stabilization of the metallic state in CTC and IRS are discussed below. =

=

Specific features of donors and acceptors Donors and acceptors used to prepare c o m p o u n d s with identified metallic states are listed in Table 1. Let us discuss some c o m m o n features of these compounds. (1) They all form redox systems capable of bielectron oxidation-reduction. The donors have a low ionization potential, I, and the acceptors a high electron affinity E, e.g., for TTF (a donor) I = 6.95 _+ 0.1 eV and for TCNQ (acceptor) E = 2.88 eV [7]. The radical cations or anions formed as a result of electron transfer are very stable, and, in conductive complexes, even in the ground state, electron transfer is considerable from the donor to the acceptor. It may be assumed that the principle of "forced monovalent oxidation", true, in general, for many oxidation reactions of organic substances, may be applied to processes of electron transfer in the solid phase of these compounds. In accordance with this principle, the reaction rate significantly increases when "bivalent" oxidation may proceed in two steps of consecutive "monovalent oxidation" [22, 57]. This is also true of the reduction process:

6 TABLE 1 Chemical formula and designations of donors and acceptors included in conducting chains Donor 1

Structure 2

Designation 3

Tetrathiofulvalene

(sk)=(/s ~

TTF

\S / cis- Dimethyltetra-

",S/ cis- DMTTF

thiofulvalene

H~cN/s\ /s\/cn'

trans-Dimethyl-

\ s / \S / H~C\./S\ / S \ It > ~ ( ]1 \S / \S/\cH ~

trans-DMTTF

~ : i ) ( : >~<:~(cH'

TMTTF

tetrathiofulvalene Tetramethyltetrathio-fulvalene

fl )=(

II

CH, Dimethyl-diethyltetrathiofulvalene

n~c .s. .s. .CH~ ~ ~=( ~( H~C~ S S C~H~

DMDETTF

Tetraethyltetrathiofulvalene

H~c~ s, .s. /C~H= n~c~(s ~ < s ~ c ~ H °

TETTF

Bicyclopentenyltetrathiofulvalene (hexamethylenetetrathiofulvalene)

s s (~(S>=(S~

Bicyclohexenyltetrathiofulvalene (oetamethylenetetrathiofulvalene)

(~s~ s

=

(s~(~

HMTTF

OMTTF

s

Dibenzotetrathiofulvalene

s s ~ \/ \F~/~] ~\S)%S/~

DBTTF

thiofulvaleneTetrapr°pyltetra"

HH;C~(S>~c~ S =
TPTTF

Benzotetrathiofulvalene

]~ / sl[ ~=(~ s."~l

BTTF

Tetraselenofulvalene

%/\s / \s / /se Jr ~=< Se."~

TSeF

\Se

Se/ (continued on facing page)

TABLE 1 (continued) 1

2

3

A4'4"bithi°pyran

~ / - - ~ ~/~Xs - \__/=~____/~

BTP

Nickel phthalocyanine

A

~

NiPc

h I II

N

I II N%/£ N %, / N N / N I \ N --~//N

Nickel octamethyltetrabenzoporphyrin

CHs

Ni(OMTBP)

CH3

cH" - . ~ V/ \ II

II / / W cu, I

I

I

11

cu~

IIC < ~ NN,N,,/ N --~CH N-/ N II:,C | . II [ CHs \/%./~ CH~

Diselenodithiofulvalene Tetrathiotetracene

/Se

SeN

CH3

/se

s X]

DSeDTF

TTT

S--S

L I S--S

Tetraselenotetracene

TSeT

Se Se I

i

i

L

Se-- Se

Hexamethylenetetraselenofulvalene

HMTSeF /Se

Se\

0%>( Jr7 (continued overleaf)

8 TABLE 1 (continued) 1

2

3

Tetramethyltetraselenofulvalene

H,C~ Se ie..CH~ H,{'" (Sc'~-~Se)~CH~

TMTSeF

Hexamethylenedithiodiselenofulvalene

~)(S

So

se

Se " S 5=~Se)(5

HMDTDSeF

! ()(75~(S2,)C.~

OMDTDSeF

5=~S )(5 ~-~ ) ( S

Octamethylenedithiodiselenofulvalene

~(75==~T'~()

Acceptor

Structure

Designation

Tetracyanoethylene

NO.

TCE

.CN

N(:5 :~cx 7,7,8,8 -Tetracyano -p-quinodimethane

xeLracyanonapnmo-

quinodimethane

TCN Q ~]5__~5=~i ~

TNAP

NC _ NC5- (--~=

= ~¢N CN

Cation Triethylammonium N-Methylphenazinium

Structure

Designation TEA

(C~H~hNH+ A/

I

N \A

I

I

NMF

N 1 CHj Quinolinium

~x.A

UU

Quin

NH

Acridinium

Acr

NH

D

--e

>

D -+

(a)

--e > (b)

D 2+

and

A

+e (a')

A=

+e (b')

)

A 2-

"

The contributions of steps (b) and (b') (formation of dications D 2÷ and dianions A 2-) during electron transfer in the solid state may be different. When these steps are important, the stable ion radicals D'*and A- act as catalysts for electron transfer in the overall processes D ° -+ D 2÷ and A ° -+ A 2-. The best, or "optimal" catalyst should, by analogy with homogeneous processes [ 58, 59], carry out electron transfer at each step with the highest rate and at maximally close rate constants for each step. This conclusion is also confirmed by the fact that, with decrease of the difference between the first and second half-wave potentials of the polarographic reduction of some acceptors (tetracyanoethylene, tetrafluorotetracyanoquinodimethane, TCNQ, tetracyanonaphthoquinodimethane), the conductivity of these compounds increases [3]. It should be noted that the formation of diions (for instance, according to schemes (1) and (2)) requires that the Coulomb repulsion of the two electrons in the same molecule be overcome [60]. A~

A~

Ao

A 2-

D ÷

D +

-+ D+

D .+

D*.

D 2÷

D°

A-

A-

A-

A-.

D÷ I

(I)

(2)

According to the authors of ref. 3, in Order to weaken the Coulomb interaction of electrons and enhance the delocalization of the charges in the diion, the electron-withdrawing and electron-releasing substituents should be arranged as far as possible from each other. Comparison of the conductivities of TCNQ compounds and tetracyanobenzene agree with this statement. In the TCNQ molecule the cyano groups are arranged at opposite sites of the molecule, weakening the Coulomb repulsion of the two "surplus" electrons in the dianion and consequently facilitating electron transfer in the conducting chain. In the tetracyanobenzene molecule the substituents are arranged in the ring close to each other, and the energy of anion-radical conversion into a dianion is much greater than for TCNQ. This explains the dielectric properties of tetracyanobenzene [7]. Therefore it may be assumed that the most favourable systems, generally speaking, are those that function only with the formation of free radicals or radical ions and do not form diions, for instance following schemes 3 and 4:

10

A;

A°

>

A°

D +

A;

(3)

D +

D°

D~

> D~

A-

D° A-

(4)

The absence of diions in the conducting chain leads to substantial weakening of the Coulomb repulsion, reduces the activation energy of electron transfer, and enhances conductivity. (2) The components forming the conducting stack usually form a planar (or near-planar) structure. This provides a high degree of spatial conformity between components in the stack and creates conditions for the closest packing. The planarity requirement however is not very stringent. The intermolecular interaction of components in the conducting stack of CTC is realized through interaction of molecular n-orbitals formed by wave functions of unpaired electrons. Despite the directional character of the norbitals, they are delocalized and belong, to a certain extent, to the entire (20 - 30 A) organic molecule. This is one of the reasons why deviations from planarity are permissible. Thus, fulvalene rings in the TTF molecule are slightly distorted, deviating from planarity by 2 ° [61, 62] ; the cyano groups in TCNQ are also arranged slightly out of the ring plane [63]. The longitudinal shift of components in relation to each other is also possible. Small substituents (methyl, ethyl) in TTF and TCNQ do not destroy the metallic state in the corresponding complexes [63]. The planarity of compounds is retained during formation of cation and anion radicals while the degree of electron delocalization increases. Consequently, in accordance with the principle of "forced one-electron oxidation" [22, 57], various types of interactions which facilitate stabilization of radicals (hydrogen bonds, complexation) may be useful. An interesting suggestion was made by Perlstein [ 9]. He suggested that the appearance of an additional (new) aromatic sextet in the radical ion formed during one-electron oxidation is a necessary condition for a conducting stack component. For example, TTF and TTT form radical cations with one and three aromatic sextets, respectively, when oxidized, whereas the original TTF molecule has no sextet and TTT only one: IS

S\

s--s I

S--S

I

-e / S

S.

s--s I~1

11 According to Perlstein [9], the same should be true of reduction, e.g., an aromatic sextet of electrons appears as a result of TCNQ reduction: NC

CN

NC

CN

NC

CN

NC

CN

In order to evaluate the generality of this suggestion, additional studies must be performed since several problems still require solution. For example, a highly conductive organic metal has been recently obtained by oxidation of DBTTF, (DBTTF)s(SnCle) 3 [64, 65], although Perlstein lists DBTTF among donors forming "donor-acceptor salts without a new aromatic sextet":

)=\

IOl

t

V\s / \s/V

o I ~

I o I

}~-~<

I,~;/

V\s / \s/~/

It should also be noted that experimental data on bond lengths in TCNQsuggest only the increase of electron delocalization and retention of the quinoid character of the bonds: on conversion of neutral TCNQ into the anion radical the length difference between two alternating bonds in the ring decreases from 0.10 to 0.05 A [7]. The fact that the TCNQ molecule retains its quinoid character when accepting an electron led Andre et al. [7] to assume the presence of structures (a) and (b) rather than (c).

NC

CN

NC

II

NC

~

NC

CN

II

II

(a)

CN

I

II

CN

NC

.,,2",..

CN

NC

(b)

~

I. CN

(c)

Properties of conducting chains A major feature of highly conductive quasi-one-dimensional systems is strong interaction between components and equal spacing in the stack. (i) Spacings in the stack Unfortunately the nature of bonds in the conducting stack is still unclear [ 9]. The spacings between components are rather large, but are still substantially smaller than van der Waals distances. Thus, the shortest interplanar distance in a TCNQ stack is 3.17 A [7], in a TTT stack 3.32 A [34,

12

35], in a TSeT stack 3.37 A [39] and in a TTF stack 3.47 A [7], whereas the distance between molecules in the crystals of neutral TCNQ is 3.74 A [7], of neutral TTF 3.62 A [62, 66], and the double van der Waals radius of sulfur is 3.7 A [7]. The role of the distances between components of a regular stack in organic metals is easily seen by comparison of the electroconductive iodide properties .of nickel octamethyltetrabenzoporphyrin, Ni(OMTBP)(I3)0.35 [48, 49], with those of nickel phthalocyanine, Ni(Pc) (I3)o.33. The distance between macrocycles of Ni(OMTBP) in the conducting stack (3.778 A) is substantially larger than those between Ni(Pc)(3.24 A). Accordingly, the conductivity of Ni(OMTBP)(I3)o.35 (16 - 40 ohm -1 cm -1 ) is much less than the conductivity of NiPc(I3)0.33 (250 - 650 ohm-1 cm-1 ). The temperature of maximum conductivity of the first of these compounds (260 - 330 K) is also indicative of the instability of its metallic state at room temperatures. The metallic state of the second compound is stable up to 90 K [48, 49].

(ii) Variation of component geometry Let us examine the effect of substituents that change the geometry of each component of the TTF-TCNQ system on electrical conductivity. A specific feature of this system is the presence of both donor and acceptor conductive chains. The effect of substituents in TTF molecules on electrical conductivity of charge transfer complexes with TCNQ is shown in Table 2. TCNQ compounds with TTF and dimethyltetrathiofulvalene (DMTTF) (cis, trans and mixed) exhibit metallic conductivity at room temperature although they are not isostructural [63]. Introduction of two more methyl groups also does not destabilize the metallic state although it alters the crystal structure of the TCNQ complex. It should be noted that the introduction of two and four methyl groups into TTF affects the temperature of the metalinsulator transition differently: Tmax of (DMTTF)(TCNQ) is lower and Tmax of (TMTTF)(TCNQ) is higher than Tm~x of (TTF)(TCNQ) [63]. TABLE 2 Electrical c o n d u c t i v i t y o f charge t r a n s f e r c o m p l e x e s o f T C N Q w i t h T T F analogs

Donor

a (ohm cm) -1 polycryst, sample

Ref.

TTF DMTTF TMTTF DMDETTF TETTF TPTTF HMTTF OMTTF BTTF DBTTF

30 60 5 10 7 x 7 x 15 5 × 3 x 10 - 7

63 63 63 67 63 63 68 68 69 70

*Single crystal

conductivity.

10 - 5 10 - 3 . 10 - 5 10 - 6

13

Replacement of t w o symmetrically arranged methyl groups with ethyl radicals in TMTTF does not destroy the high conductivity of the TCNQ c o m p o u n d [67]. The introduction of four ethyl or propyl radicals into TTF, on the other hand, yields a TCNQ c o m p o u n d with semiconductor or dielectric properties [ 6 3 ] . It has been shown recently that the introduction of four npropyl substituents into a TTF molecule destroys the regularity of the chain and produces a structure typical of charge transfer c o m p l e x e s , . . . D A D A . . . [71. The introduction of cyclic substituents into TTF has also been studied (HMTTF and OMTTF, Table 1). Expansion of each side HMTTF ring by one methylene group sharply changes the conduction of the TCNQ comp o u n d [68] (Table 2). Similar changes in conducting properties are observed when passing from HMDTDSeF (G = 900 ohm -1 cm -1 [ 7 2 ] , T m a x = 76 K) to OMDTDSeF (o = 2 X 10 -6 ohm -1 cm -1) [71, 72]. Let us now consider the effect of substituents in the acceptor component of (TTF)(TCNQ) on electrical conductivity. In ref. 73 it was shown that introduction of alkyl groups of different sizes at positions 2 and 5 in TCNQ has a rather weak effect up to a certain critical size of the substituent, after which a sharp change in conductivity is observed, e.g., when passing from (TTF)(TCNQ(C2Hs)2):to (TTF)(TCNQ(iso-C3Hv)2), o changes from 10 ohm -1 cm -1 to 5 X 10 -6 ohm -1 cm -1 [73]. Similarly, introduction of two methyl groups into positions 2 and 5 of TCNQ in the (TMTSeF)(TCNQ) system does n o t distort the metallic state and even slightly stabilizes it (the transition temperature decreases by 20 K) [74, 75]. In the (HMTSeF)(TCNQ) system, the introduction of methyl groups leads to reduction of electrical conductivity by eleven orders of magnitude (!) [36, 75].

(iii) Chain geometry Structural elements of the conducting chains should be arranged in a quite definite manner in order to ensure stabilization of the metallic state. In the crystals of highly conductive TCNQ salts, TCNQ molecules overlap each other with a longitudinal shift of 1A of the molecule length. The center of one molecule is situated over the center of the double quinodimethane bond of another molecule (Fig. 3) [4, 7, 15]. In highly conductive salts of TTF and its analogues [37, 76 - 85] two modes of molecule arrangement in stacks were established (Fig. 4): (a) one

Fig. 3. Overlap of TCNQ molecules in chains of highly conductive TCNQ compounds.

(a) (b) Fig. 4. Overlap of T T F molecules in conducting chain.

14

precisely over another with the plane of each molecule normal to the direction of the conducting chain; (b) with a longitudinal shift equal to the length of the central carbon-carbon bond and the plane of the molecules forming a certain angle with the chain direction. The interplanar spacings, overlap modes, and electrical conductivity of conductive TTF salts are listed in Table 3. TABLE 3 Interplanar spacings, methods of molecule overlap and electrical conductivity of TTF salts Compound

Ref.

Overlap, Fig. 4

Interplanar spacing in TTF chain

Room temperature conductivity (ohm cm) - ~

(h) (TTF)Br0.76 (TTF)I0.71 (TTF)C10.92 - 0.68 (TTF)(SCN)o.58 (TTF)(SeCN)0.58 (TTF)(TCNQ) (TSeF)(TCNQ) (HMTSeF)(TCNQ) (TMTTF)I.3(TCNQ) 2 (DSeDTF)(TCNQ)

76 77, 78 79 80 80 63 83, 84 37 82 83

a a a a a b b b b b

3.572 3.56 3.588 3.60 3.61 3.47

200 - 500 350 550 750 350 - 500 800 1800 10 550

3.61 3.59

The structures of conducting chains of TTT and TSeT have been studied relatively little. It follows from the two established structures of (TTT)(TCNQ)2 and (TTT)I1.5 [34, 35, 87] that there are at least two versions of TTT arrangement in the conducting chain (Figs. 5 and 6, respectively) and, as in the case of TTF, the intermolecular distances in the stack are smaller the larger the shift of overlapping molecules (Table 4). The salts (TTT)o.5(TSeT)0.5(TCNQ)2 and (TSeT)(TCNQ)2 were found to be isostructural with (TTT)(TCNQ)2, consequently the overlap of the donor molecules corresponds to Fig. 5 [88].

J

x\ ("

'

^,

",¢"

~,

i,

)

i "~" A "'(

\.__] Fig. 5. Overlap of TTF molecules in conducting chain of (TTT)(TCNQ) 2.

Fig. 6. Overlap of TTT molecules in conducting chain of (TTT)2I 3.

15 TABLE 4 Interplanar distances and overlap modes in highly conductive TTT and TSeT compounds Compound

Interplanar distance

Overlap mode

o (ohm 1 cm-1)

Ref.

(TTT)(TCNQ) 2 (TTT)I1. 5 (TSeT)2C1 (TSeT)2Br (TSeT)(TCNQ) 2

3.52 3.32 3.37 3.37

Fig. Fig. Fig. Fig. Fig.

2 0 - 160 103 2.1 x 103 1.3 × 103

86, 87 34, 35 89 89 88

5 6 6 6 5

It has been established recently that in highly conductive TSeT salts of composition (TSeT)Clo.~ and (TSeT)Br0.5 the conducting chains are arranged like the stacks in TTT2I 3 [84] (Fig. 6). Comparison of interplanar TTT-TTT and TSeT-TSeT separations in such chains (Table 4} shows that the increase of interplanar distances in TSeT chloride by 0.05 A is much less than could have been expected from the van der Waals radii of sulfur and selenium (1.85 and 2.0 A). This indicates stronger interaction in (TSeT) C10.5 stacks than in TTT213 stacks and explains the higher (by 2.5 times} room temperature conductivity of TSeT chloride as compared with TTT iodide [ 89, 90].

(iv) Incomplete charge transfer A typical feature of conducting stacks in quasi-one-dimensional organic metals is effective intermolecular delocalization of electrons over the stack. One of the major factors that determines preparation of appropriate systems is incomplete electron transfer in the formed charge transfer complex or ion-radical salt [6]. Owing to this effect, formally neutral complexes and radical ions coexist in the conducting chain and mixed-valence sta~es are formed. All the compounds of TTF, TTT and TCNQ with metallic properties, including (NMP)(TCNQ), were shown to have incomplete electron transfer from a donor to an acceptor [6, 89, 91 - 93]. The salts of TTF, TTT and TCNQ with full charge transfer possess dielectric properties. Also, incomplete charge transfer in the salts of TCNQ does not necessarily lead to metallic properties of compounds. Of great importance here is the effect of the components which are not involved in the conductive chains and are not directly responsible for conductivity. Incomplete charge transfer stabilizes the crystal structure by reducing the energy of electrostatic repulsion of the stack components [6, 92, 97, 98] and leads to a decrease of Coulomb repulsion during electron transport [6]. In the latter case the energy of Coulomb repulsion between electrons is reduced and the process of electron transfer requires much lower activation energies as compared with electron transfer in stacks consisting solely of radical ions and ions.

16

Calculation of the degree of electron transfer (p) from donor to acceptor in Rb(TCNQ) and (NMF)(TCNQ) crystals carried out recently demonstrated that in the highly conductive compound p = 1/2 (i.e., transfer is incomplete), whereas in poorly conductive Rb(TCNQ) p = 1 [97]. It was also calculated [92] that electrostatic repulsion of like charges in the TTF stack of the T T F - B r system is minimized at an average charge on the TTF of +0.74 e, i.e., at incomplete transfer from TTF. Such incomplete transfer may be realized in compounds with nonstoichiometric compositions. Indeed, oxidation of neutral TTF with bromine yields a highly conductive salt of nonstoichiometric composition (TTF)Br0.71.0.76 (o = 200 ohm -1 cm -1) in which TTF is in a mixed-valence state [94] *. Whereas, the ion-radical 1:1 salt, obtained by electrochemical oxidation of TTF and the addition of bromine anion, is a poor conductor. This is connected with the fact that increase of the electrostatic repulsion energy of components in a stack precludes formation of regular chains in (TTF)Br and leads to alternation of TTF ~ dimers and bromide anion pairs in the crystal structure of this salt [ 76]. A similar distortion of the regular stack structure seems to occur in the DBTTF-FeIiIc14 system with full oxidation of DBTTF. The conductivity of the salt of composition (DBTTF)FeC14 is lower by 8 orders than the conductivity of the (DBTTF)(FeC14)0.4 salt with incomplete electron transfer from DBTTF [101]. The general character of the effect of incomplete electron transfer should be emphasized. Inorganic and coordination compounds in the presence of mixed-valence metal ions, as a rule, exhibit unusual physical properties (including high electrical conductivity) [102].

Effect on the conducting chain

The components which are not directly responsible for conductance and are not included in the quasi-one-dimensional chains may exert a strong influence on the stability of the metallic state and properties of the conducting chain. It is evident from the above-mentioned that the effect on the chain should ensure its regularity, the closest arrangement, effective inter-

*The unusual composition of T T F halides and of other similar compounds (e.g., (DBTTF)s(SnCI6)3 [64, 65] ) indicates the presence o f neutral molecules in the conducting chain along with radical ions. For example, (TTF)I0.71(TTF715) may be represented as TTF2 ° (TTF+)5(I-)5. It should be noted that X-ray analysis did not reveal the presence of neutral and charged components in the stack (bond lengths in all molecules were the same). This apparently indicates the high rate of electron transitions between components. The characteristic charge life on the molecule was estimated in ref. 99 to be ;~ 10 -13 s. It should also be noted that the surface of T T F - T C N Q films contains 20 - 50% of neutral TCNQ molecules [ 100].

17 action of the participating compounds, and the presence of mixed-valence states. TCNQ salts containing metal ions, aliphatic bases, and some other nonplanar systems as cations usually exhibit low conductivity; in these compounds the distances between TCNQ molecules in the stack alternate. On the other hand, TCNQ salts containing planar heterocyclic compounds (phenazinium, acridinium, quinolinium) as cations are characterized by high electrical conductivity. The regular structure of conducting chains and effective overlap of TCNQ molecules is observed in the crystals of the latter compounds. The presence of the metallic state was first convincingly established for the following compounds: (NMF)(TCNQ), (Quin){TCNQ)2 and (Acr)(TCNQ)2. Up to temperatures of ~200 K their electrical conductivity increases, and then drops above 200 K, the compounds becoming dielectrics (Fig. 2} [8]. Paramagnetic susceptibility remains constant up to 30 K, a feature that is typical for metals. One explanation of the relative stability of the metallic state in these TCNQ salts is based on the assumption that electronic polarizability of the cation is an important factor affecting the conductivity of TCNQ salts [ 103]. According to LeBlanc [103], Coulomb repulsion may be compensated by high cation polarizability. If cation polarizability is high, as is the case for cations of heterocyclic aromatic compounds, the activation energy of electron transfer should be strongly reduced and, under favourable steric conditions, metallic-type conductivity may be expected in TCNQ compounds. Indeed, replacement of sulfur with selenium in TTF, by increasing the polarizability of the molecule, Stabilizes the metallic state (the temperature of the Peierls transition is 45 K for (DSeDTF)(TCNQ) and 29 K for (TSeF)(TCNQ) [ 104, 105] ). Of considerable interest are the data on synthesis and properties of TCNQ compounds containing easily polarizable cation dyes of the cyanine type [106, 107]. Although the physical features of these compounds are, to a considerable extent, determined by the steric features of the cation (size, substituents), its polarizability undoubtedly has a strong effect also [1,106 - 108]. It should be noted, however, that it is impossible to stabilize the metallic state exclusively by these factors (Fig. 2); corresponding compounds containing a symmetrical, easily polarizable cation undergo a metal-diamagnetic dielectric transition on reduction of temperature, presumably of the Peierls type [8, 104, 109- 111]. It is important to remember that the notions presented above are only of a relative nature since there are other factors which affect the stability of the metallic state. The positive charge in cations forming nonconducting chains in metallic TCNQ salts (N-methylphenazinium, quinolinium, acridinium) is asymmetrically localized. Arbitrary orientation of such cations causes random variation of the potential along the conducting TCNQ chain [8]. The resulting slight

18 disordering of the conducting chain may lead to stabilization of the metallic state [ 8]. Introduction of weak disorder into the conducting chain (which, according to theoretical conceptions, should enhance stabilization of the metallic state in the case of possible Peierls transitions [8] ) may be carried out by the method of "alloying" [104, 105, 112- 114]. It was shown, for instance, that the introduction of three mole percent, of methyltetracyano-pquinodimethane into the acceptor chain of TTF-TCNQ salt shifts the temperature of phase transitions in TCNQ stacks towards lower temperatures (from 53 to 47 K) and broadens the corresponding transition maximum [ 113]. The paramagnetic susceptibility of the acceptor stacks decreases more slowly with reduction of temperature than in the non-alloyed systems. Introduction of three mole percent, of TSeF into the donor stack of the TTF-TCNQ system broadens the anomalies on the conductivity and magnetic susceptibility curves at 38 K, which are attributed to a quasi-onedimensional transition in the donor stacks, almost to the point of complete disappearance (the transition is "blurred") [104, 113]. At the same time, the weak disorder of the cation chain shifts the Peierls transition in the TCNQ chain by 2 K in the direction of lower temperatures. The addition of methyltetracyano-p-quinodimethane (10%)into (TSeF)(TCNQ) "blurs" the phase transition at 28 K and increases conductivity at 4 K by four orders of magnitude [105]. The conductivity maximum is shifted from 40 to 75 K and a sharp reduction of conductivity at phase transition (28 K) is no longer observed. Data on the effect of the degree of disorder in TTT213 on the phase transition temperature and metallic state stability are very indicative [ 90]. The periods of TTT and I sublattices coincide in (TTT)I1.5 and are 9.78 A. With increase of iodine content to (TTT)I1.55, the incommensurability of the sublattices increases (9.92 and 9.66 A for TTT and iodine sublattices, respectively), disorder increases, and the temperature of the metal-dielectric transition drops from 45 to 30 K. The dimensions of a counter-ion greatly affect the geometry and properties of the conductive chain. Thus, when substituting a cation of Nethylphenazinium for NMF, the conductivity of the salt with TCNQ decreases sharply [7]. In the case of the ion-radical salts (DBTTF)8(SnC16)3 and (DBTTF)s(PtCl6)3 (G ~ 350 ohm -1 cm -1), substitution of bromine for chlorine while increasing the volume of a counter-ion makes it impossible to retain the original crystal lattice with regular stacks of DBTTF and results in an initiation of alternating stacks with spacings of 3.44; 4.47; 3.58 A between DBTTF molecules. The compounds formed are of composition (DBTTF)3(SnBr 6) and (DBTTF)3(PtBr6) and possess semiconductor properties [65]. The size of the counter-ion may affect not only the geometry and properties of the conducting chain but also the separation of conductive stacks. Thus, in the series of isostructural compounds TTFTI5, TTF~I(SCN)6, and TTFll(SeCN)6 the spacing between stacks may be increased (7.92, 7.99, 8.03 A, respectively,) by increasing the "width" of the anion (I- > Se > S)

19 [ 81]. The isolation of stacks should be considered to be rather relative. In some cases their interaction {both direct and through the counter-ion) may be used to stabilize the metallic state (increasing the system dimension, alloying, etc.). The distance between chains usually may be increased by using large organic molecules instead of smaller inorganic counter-ions. Thus, the amount of TCNQ chains per unit area in (NMF)(TCNQ) constitutes only 60% of the number of chains in TCNQ salts with alkali metals and (NH4)(TCNQ) [92], indicating an increase of chain separation. This separation is usually realised only in one direction. Even in the case of large organic cations (quinolinium, acridinium, methylphenazinium) there is, generally, a direction along which the conduction stacks are not separated by counter-ion columns. Small, inorganic counter-ions (metal cations, anions CI-, Br-, I-, I3-, SCN-) may easily arrange themselves between linear conducting chains. The distance between ions in channels usually exceeds their ion radii. Thus, the cylindrical anions SCN- and SeCN-, arranged parallel to the conducting TTF stack, occupy only 40 - 50% of the anion volume in the channels [81]. This provides interesting possibilities for "alloying" organic metals. For instance, increase of iodine content in tetraselenotetracene iodides from (TSeT)I0.7.0.s to (TSeT)I2.2.2.a practically does not change [115] the character of TSeT conducting stacks. The further increase of iodine content in the channels leads to stabilization of the metallic state at lower temperatures without distorting the conducting chain structure in (TTT)I1. 5 [90]. The formation of intermolecular bonds (including hydrogen bonds) between functional groups of the conducting chain and counter-ions may play an important role in metallic state stabilization [9, 116]. The possible hydrogen bond interaction between the cyano group of TCNQ and the (NH) ÷ of the cation in (Quin)(TCNQ) 2 and (Acr)(TCNQ)2 may affect the physical properties of these compounds [9, 117]. The use of additional interactions may be very productive, in particular for increasing the dimensions of the system from quasi-one-dimensional towards two-dimensional in order to control the instability of quasi-one-dimensional compounds in relation to various transitions. For the spin density distribution in the TTF and TSeF cation radicals, it has been recently shown [ 118] that in the TSeF molecule, the selenium atoms possess the highest spin density, while in the molecule of TTF the highest spin density is focused on the central carbon atoms. The high spin density of the selenium atoms suggests a stronger interaction between donor and acceptor stacks and, hence, a higher dimensionality [ 118]. In (TTF)(TCNQ) there are two separate transitions in cationic and anionic stacks (T1 = 53, T2 = 38 K), whereas in the isostructural (TSeF)(TCNQ) there is only one simultaneous transition (T = 29 K) [104]. This is explained by interchain interaction in (TSeF)(TCNQ), which results in a slight deviation from the one-dimensional state. Even stronger effects caused by increase of order are observed in recently synthesized hexamethylenetetraselenofulvalenecomplex with tetra-

20 cyanoquinodimethane and tetraselenotetracene chloride and bromide [36, 37]. These c o m p o u n d s are the first organic metals which do n o t undergo transition into the insulator state down to the lowest temperatures. In the sphere of low temperatures these two compounds are characterized by the presence of a metallic state transition into that of the halfmetal [36, 4 0 ] . In the case of (HMTSeF)(TCNQ), the conductivity along the direction parallel to the conducting stack axis increases by three times as the temperature is decreased from room temperature to 30 K. Below this temperature o decreases slightly reaching the values of conductivity at room temperature at 4 K [36]. Anomalies in heat capacity and magnetic receptivity confirm the presence of the transition at 32 K [ 119]. It should be noted that the anisotropy of conductivity of (HMTSeF)(TCNQ) is low: a II/°l ~- 30 [ 120, 121]. The structure of this c o m p o u n d is characterized by a shortened distance between atoms of selenium in the donor stack and atoms of nitrogen in the acceptor stack (3.10 A) [ 3 7 ] , which allows us to speak of the increasing dimension of the system. It is also possible to stabilize the metallic state in the chloride and bromide of tetraselenotetracene (TSeT)2C1 and (TSeT)2Br [39, 40]. The magnitude of the conductivity in these compounds at low temperatures is approximately the same value as it is at room temperature. In the region of 20 K the complex (TSeT)2C1 undergoes a transition from the metallic to the semimetallic state [ 4 0 ] . As a result of this, in t h e interval from 25 to 10 K its o drops, but at T -~ 0 it tends to the finite value of the order of the room temperature magnitude, i.e., equal to 2.1 × 1 0 3 ohm -1 cm -1. The distances between chlorine atoms and the four selenium atoms of neighboring stacks are shortened in (TSeT)2C1 and are equal to the sum of the ion radius of chlorine and the covalent radius of selenium (1.87 + 1.17 = 3.02 h), pointing to interstack interaction. Possibly this two-dimensional factor may be the reason why the isostructural tetraselenotetracene chloride and bromide do not go through a metal-insulator transition and remain metallic at the lowest temperatures [ 39]. It should be noted that at elevated pressures (HMTSeF)(TCNQ) and (TSeT)2C1 become more metallic [ 1 2 2 , 1 2 3 ] . To achieve metallic levels in the conductivity of (HMTSeF)(TCNQ) in the entire temperature range it is necessary to use a pressure of 4 kbar or more [ 122]. At pressures more than 5 - 7 kbar, the ion-radical salt (TSeT)2C1 undergoes the phase transition into the new metallic state stable in the whole temperature range. The resistivity of the high pressure phase falls when the temperature decreases and becomes equal to 10 -5 ohm cm below 10 - 15 K [123]. In contrast to these compounds of increased dimensionality, the quasione-dimensional compounds (TTF)(TCNQ) and (TSeF)(TCNQ) transform, with pressure, into the dielectric state at higher temperature than without pressure, i.e., stabilization of the phase distorted by the Peierls transition is observed [121, 124 - 126]. When the pressure increases to 10 kbar, the transition temperature of (TSeF)(TCNQ) increases at 6%/kbar [ 121] and at 1.4%/kbar for (TTF)(TCNQ) [ 1 2 4 - 126].

21

Specific features of organic metals Organic metals based on CTC and IRS differ substantially from the usual three-dimensional metals. The peculiarity of their behavior has led to them being called the "fourth state of solids" [127]. The main feature of organic metals is the lowered order of the electron systems. This feature determines the character of phase transitions, the strong anisotropy of various physical properties, and the specificity of many properties (thermodynamic, optical, magnetic} governed by electron interaction [7 - 9]. Such compounds exhibit, for instance, high anisotropy of conductivity, optical properties, and dielectric permeability. In (TTF)(TCNQ) the conductivity along the chains (along axis B) is higher by several orders of magnitude than in the transverse direction (OB/O,~ = 500 at 300 K and oB/~a = 104 near the transition point [ 8] ). The anisotropic ratio of room temperature conductivity (oil/oi) for (CH3)aNH (I)(TCNQ) is 400 [128]. At wavelengths corresponding to electron transfer in the conducting chain, only light polarized parallel to the stack axis is absorbed [ 129]. At wavelengths corresponding to intramolecular transitions, light polarized in the plane of TCNQ molecules is absorbed. The mirror reflection spectra of single crystals of TCNQ ionradical salts are also anisotropic [130]. The anisotropy of dielectric permeability at 4.2 K for (TTF)(TCNQ) is ell/el = 500 [8]. The lability of the metallic state in compounds with a quasi-one-dimensional electron system (as compared with "common" three-dimensional metals} should be emphasized once again. In metal-insulator transitions a dielectric state is formed which is characterized by exceptionally high polarizability of the electron system. For example, at 4.2 K the dielectric constant along the conducting chains is TABLE 5 Dependence of the composition of TTF, TTT and TSeF halides on the ratio of initial components Halogen/donor Br/TTF Br/TTF I/TT F I/TTF I/TTF I/TMTTF I/TMTTF I/TMTTF I/TMTTF I/TSeT I/TSeT I/TSeT I/TTT I/TTT

Initial ratio 0.7 2 0.7 2.3 3.0 < 0.5 1.2 2.4 3 ~<1 1.8 2.3 0.5 1.7

Product

Ref.

(TTF)Bro.71 -o.76 (TTF)Br 2 (TTF)Io.71 (TTF)I2. 3 (TTF)I 3 (TMTTF)Io. 5 (TMTTF)I1. 3 (TMTTF)I2.5.2.6 (TMTTF)I 3 (TSeT)Io.7-o.s (TSeT)I 1.8 (TSeT)I2. 2_ 2.4 (TTT)I (TTT)2I 3

76 76 85 85 85 132 132 132 132 115 115 115 90 90

22 3200 for (TTF)(TCNQ) [5], 4500 for (HMTTF)(TCNQ), 1 4 0 0 0 for (TTT)213, 2 2 6 0 0 for (TSeT)2Br [ 1 3 1 ] . Depending on the structure of the organic metal the dielectric may be diamagnetic ((TTF)(TCNQ), (TTT)(TCNQ)2) or paramagnetic [8]. Transitions of the metal-semimetal type have also been observed [40]. In the presence of two conducting chains, simultaneous phase transitions of both chains and separate transitions in each chain are possible. According to some theoretical estimates, a one-dimensional metal may undergo transition n o t only into a dielectric state but also into other states, including superconductor states [ 8]. This problem requires further experimental study. Owing to their unusual properties, organic metals may find interesting applications in various fields of technology. For instance, the combination of exceptionally high optical anisotropy with metallic properties may be used in the manufacture of polarization-sensitive pyroelectrical detectors [6].

Synthesis of organic metals As mentioned above, one of the main conditions for preparing highly conductive organic c o m p o u n d s is the formation of conducting chains with components in mixed-valence states. Let us now consider the main methods of synthesis of organic metals.

(i) Donor-acceptor interaction The majority of ion-radical salts and charge transfer complexes with equally spaced infinite chains was obtained by direct interaction of the donor and acceptor in solution. The preparation of mixed-valence compounds may be achieved by different methods: by variation of the donor/ acceptor ratio, by variation of the redox potentials of the donor and acceptor, and by variation of the cooling rate of the reaction mixture.

(a) Variation of the donor/accep tor ratio This method may be illustrated on systems TTF-halogen, TTT-iodine, and TSeT-iodine (Table 5). As can be seen from the Table, variation of the quantity of iodine leads to different product compositions. As a rule, nonstoichiometric ratios of components (halogen: donor < 1 for bromine and < 3 for iodine) give mixed-valence compounds.

(b) Selection of appropriate oxidizing and reducing agents The redox potentials of the reagents should presumably be such as to ensure after interaction the presence of both neutral and radical-ion components in solution. Thus, the reaction of DBTTF with TCNQ does not produce radical species and the CTC so formed is an insulator [70] (the first redox potential of DBTTF (E~ = 0.72 V) is much higher than the corresponding potential of TCNQ (E ° = 0.17 V) [73] ). Oxidation of TTF and

23 some of its derivatives to ion-radical salts with complete charge transfer (the absence of neutral species and the presence of only cation radicals) also leads to nonconductive compounds, e.g., (TTF)Br and (SeF)C1. The use of an oxidizing agent that established the required equilibrium between the cation radical and neutral species is very important, if not crucial, and yields highly conductive nonstoichiometric compounds. Thus, oxidation of DBTTF with tin tetrachloride gives an ion-radical salt (DBTTF)s(SnC16)3 with metallic properties, o = 350- 500 ohm -1 cm -1 [65]. (c) Variation o f the cooling rate o f the reaction mixture This method is also used to prepare compounds of various compositions, including nonstoichiometric. Rapid cooling of the system I2 :TTT = 0.5 produces only a mixture of iodides (TTT)I and (TTT)2Is, whereas gradual cooling of the same system yields a conductor with metallic properties, (TTT)213 [90]. This method (interaction of donor with acceptor in solution) was used to prepare (TTT)(TCNQ)2 [86], (TTT)0.5(TSeT)0.5(TCNQ)2, (TSeT)(TCNQ)2 [88], (TTF)(TCNQ) and its analogs [36, 63, 68, 69, 71, 72, 74, 75, 83, 84, 133 - 135], bromides of TTF [76], TMTTF [132] and DBTTF [60], iodides of TTT [34, 35], TSeT [115], TTF [85] and TMTTF [132], TTF chloride [ 136] and a series of TCNQ complex salts with aliphatic amines [137]. Donor-acceptor interaction is sometimes carried out in the gaseous phase by mutual sublimation of components under vacuum. This method was used to obtain solvent-free (TTT)(TCNQ)2 [ 138] and for the synthesis of (TTT)213 [35]. Interaction of TMTTF with TCNQ in solution results in a mixture of crystals of at least three compounds, whereas in the gaseous phase (TMTTF)I.3(TCNQ)2 was isolated [82]. (ii) Exchange reactions between donor and acceptor salts Another general method for preparing highly conductive ion-radical salts involves an exchange reaction between donor and acceptor salts already possessing mixed-valence chains. This method was used to obtain (N-methylphenazinium)(TCNQ) [137], thiocyanates and selenocyanates of TTF [80, 139] and of TMTTF [140, 141] and several TTF iodides [139] :

/v

q CH~

F LI(TCNQ)= ~

[I

II

'

(TCNQ) =

CH~

(TTF)3(BF4) 2 + KCNS -+ (TTF)15(CNS)s (TTF)3(BF4)2 + [(CaH9)4N] +(I3)- -~ (TTF)sI15. Direct interaction of TTF with TCNQ and its derivatives, as a rule, leads to complexes of 1:1 composition in a wide range of component ratios [142],

24 whereas the exchange reaction carried out at low temperatures made it possible to obtain 2:1 complexes [142] : -

(TTFJs(BF4)5+[(n--C,I[~).,

NC V

CN -

II / ~ / ///~v

N'l,

NC

~'CvCN Jl

~ hou,-s --~ (TTF), - :,5-(-4o~'c, ('IhCN

H~C / ~ , _

-

I

CN

_

II

-

I II

/ V II~C ,~ _

NC

CN

_

(iii) Oxidation (reduction) o f donors (acceptors) in the presence o f anions or cations Organic metals may be prepared by oxidation of donors in the presence of anions or reduction of acceptors in the presence of cations. This m e t h o d allows an increase in the number of counter-ions which may be used in the synthesis of conductive ion-radical salts. Synthesis of several salts by donor oxidation is presented below [38, 3 9 , 1 3 9 , 1 4 1 ] : 2TSeT + TCE + Br- = (TSeT)2Br + TCE ~ TTF + 2HBF4 + H202 = TTF3(BF4)~ + 2H20 5TSeT + 3TCNQ + 3KCNS = 5TSeT(CNS)o.6 + 3K(TCNQ) T. In certain cases [64, 89] the oxidant may also provide the anion: 2TSeT + FeC13 = (TSeT)2C1 + FeC12 8(DBTTF) + 6SnCl4 = (DBTTF)8(SnC16)3 + 3SnC12. The m e t h o d of acceptor reduction in the presence of cations is used to obtain TCNQ complex salts [31], with the iodide ion playing the role of reducing agent: :2

+ [I

II

~ (1~}-

Ternary compounds containing another acceptor in addition to TCNQ are also prepared by this m e t h o d [128, 143] : [(CH3)3NH] I + TCNQ = [(CH3)3NH] (I)(TCNQ). When using H2TCNQ-amine or durohydroquinone-quinoline systems as reducing agents the amine provides the cation [137] : A

'2

+ H2TCNQ + 3TCNQ ~ 2 [1 N OH [

113C

f-.../% 21

[ (TCNQ)~

NH

II

I tI~C

II I OH

.~,v/% -L 4 T C N Q = 2 1

CI13

0 ]l

II~C

\.j,..,/ I ÷

N

CH3 II N

I

H

C1t3

\/-%./ I (TCNQ)2 &

II H~C

II

II O

CH3

25

(iv) Photo- and electrochemical methods Synthesis of highly conductive organic salts by photooxidation of tetrathiofulvalene and its analogs in carbon tetrabromide or tetrachloride has been described by Scott et al. [ 1 4 4 ] . The authors assumed that the complex of the donor with tetrahalogenomethane forming in solution dissociates under the effect of light with wavelength corresponding to the charge transfer band: TTF + CC14 (TTF)(CC14)

K

> (TTF)(CC14)

hPct

> (TTF+')(CC14 ~) -~ (TTF') + C1- + CC13

TTF~xcess + TTF +" + C1- -* (TTF)C10.ss (precipitate). This method is used to obtain TTF halides with a minimal a m o u n t of halogen: (TTF)Br0.59 + 0.03 and (TTF)C10.es ± 0.03, whereas direct oxidation of TTF with halogen yields (TTF)Br0.74.0.79 [76] and (TTF)CI0.v9 [92]. It was recently proposed to prepare halide ion-radical salts b y the interaction of donor cations created electrochemically with appropriate halide anions [ 1 4 5 ] , e.g., TTF TSeF

electrochem. oxidation electrochem.

oxidation

Br-

> TTF +"

> (TTF)Br

Cl-

> TSeF +"

> (TSeF)C1.

This method is suitable for the preparation of both salts with a 1:1 composition, possessing dielectric properties and nonstoichiometric salts which are highly conductive [ 1 4 5 ] .

(v) Interaction o f stoichiometric salts with neutral donor (acceptor) An example of this method is the preparation of highly conductive TCNQ salts with ditoluenechromium [108] by interaction of the 1:1 salt with neutral TCNQ acceptor:

I(TCNQ)~]=

The m e t h o d may be used to obtain ternary c o m p o u n d s [143] : NO2

I\,/\

~ \//\

/ I [(TCNQ):I +

~,HsCH CH ~,N s] ~lHs

i

II

II

I

/ V ~ \ NOa N NO2 C NC/ Xcs

26

--

~

;I /

S --

"1 CH l X

CH

Ct[

C~l-l~

N

NO2

(

I~ /

"

C211s

\

)

' [

II

NO~

1

I

l! C

NO~

/\ NC

CN

(vi) Reduction of the salts containing organic dications Ueno et al. [146] proposed another m e t h o d of preparation of nonstoichiometric ion-radical salts by reduction of the salts containing organic dications with lithium iodide: ÷

+

S

S

S

S

/

,x / -

s\ )-.

\S /

\--/ X s /

'

Although the resulting p r o d u c t is dielectric, it is t h o u g h t that this m e t h o d may be used for synthesizing substances with metallic properties.

(vii) Recrystallization of the stoichiometric salts 1:1 Some radical-anion salts of 1:1 composition may be transformed into 1:2 salts by simple recrystallization in pure CH3CN [147]. The authors assumed that the diion TCNQ 2-, formed as a result of disproportionation of TCNQ-, is oxidized irreversibly by oxygen in air: 2 {Jl I~(TCNQ)~~ 2 t' \ ' NH ~] \ ,NH F

2TCNQ~

I 0' CN

CN I ~ ~ 1 (OCN)- ~ xx~

) C---( ~ O

TCNQ:-+ T(:NQ

=-c \/ CN

\II / . I I(TCNQM = . NH

Admixtures and m e t h o d s of purification Many properties of crystals are strongly affected by the presence of admixtures and lattice defects. This effect may be particularly strong in the case of quasi-one-dimensional compounds. Admixtures may be introduced through the initial solutions or by inclusion of solvent molecules in the crystal [7]. Therefore, attention should be paid to purification of starting materials and to perfection of crystal growth techniques [ 163]. Soon after the first synthesis of (TTF)(TCNQ) [148] a huge increase of conductivity at 58 K [149] (O5s/O298 ~ 100) in three of the seventy crystals studied was reported, which was assigned to superconductivity fluctuations

27 [149, 150]. The absence of a similar effect in other crystals was explained by the possible influence of impurities and defects. Although these record increases were never reproduced by other researchers [150], this work stimulated study of various methods of purification of the initial components (TTF and TCNQ). A comparison of the three most popular methods (recrystallization in inert atmosphere, sublimation in Teflon sublimators and gradient sublimation) is given in ref. 151. It was found that gradient sublimation has no advantages over the usual methods of sublimation and is even less effective than the more simple methods of purification [ 151]. After studying the conductance properties of crystals of different purity, the authors came to the conclusion that, for the purity level usually obtained, the conductivity of (TTF)(TCNQ) crystals is affected more by lattice defects than by chemical admixtures [ 151]. The requirement of high chemical purity, however, is important when studying e.s.r, spectra, thermopower, magnetic susceptibility, and the heat capacity of (TTF)(TCNQ): these properties are highly sensitive to the presence of admixtures [ 152]. Thus, it was shown in ref. 152 that impurities strongly affect the heat capacity of (TTF)(TCNQ). Similar effects apparently take place in (quinolinium)(TCNQ)2 and (acridinium)(TCNQ) [153]. The deviation of magnetic suceptibility from the Curie law at low temperatures is also attributed to the effect of impurities [ 7] ;this fact was used to determine admixture concentrations [ 7, 86]. Recrystallization of the end product (IRS or CTC) cannot usually be used for purification of organic metals because the thermal instability of IRS and CTC in solution results in contaminated "defect" crystals. The method of direct interaction of donor and acceptor should be used when the reaction mixture contains a minimal amount of substances which do not form a part of the end product. Oxidation of donors in the presence of anions and exchangereactions usually results in less pure crystals. The effect of various oxidants used in the preparation of (TSeT)2CI on the temperature dependence was studied in ref. 89. The conductivity behavior differs only below 25 K, presumably on account of the change of the conductivity mechanism [89]. Therefore, the use of various oxidants gives reproducibleresults up to the temperature of transition from one metallic state into another. However, the use of 3,5-dimethyl-l,2-dithioletetrachloroferrate (CsHTS2)(FeC14) as an oxidant results in less pure or "defective" crystals. They exhibit much lower conductivity values, broadening of the conductivity maximum, and displacement of the latter towards higher temperatures. The effect of defects on the temperature dependence of (TTF)(TCNQ) was investigated recently [153]. Crystals obtained after irradiation with deuteron nuclei, like "defective" (TSeF)2C1 crystals, are characterized by reduced maximal conductivity, broadening of the maximum and its shift in the direction of higher temperatures. Special attention should be paid to crystal growth techniques. The most extensively used are the methods of slow crystallization of the reaction

28

mixture (with a given rate) and diffusion in U-shaped tubes. Comparison of these methods for (TSeT)2Br crystals was carried out in ref. 89. Diffusion carried out for one month at room temperature gave crystals with very high values, whereas diffusion at 40 °C for three days produced crystals of the same quality as by slow crystallization. Prolonged diffusion at room temperature gives the most perfect crystals. The method of crystal growth in the gas phase is used less frequently. It was used for preparing single crystals of (c/s- and trans.DMTTF)(TCNQ) from mixed DMTTF* and for (TTT)213 crystals [35, 63] (rapid cooling of solution gives a mixture with (TTT)I). This method apparently yields the thermodynamically most stable phase. It has been proposed quite recently [154] to grow single crystals of conductive nonstoichiometric salts of TTF of composition (TTF)Xn by electrocrystallisation on a platinum anode in solutions of TTF in CH3CN containing (NR4)X salts. This method of growing crystals is used together with the method of diffusion in the U-shaped tube when obtaining ionradical salts electrochemically [94, 154].

Conclusions

The considerable scientific interest in organic metals and the prospect of technical applications has stimulated a search for new types of these compounds. It has been suggested that conducting chains be created from neutral components, planar n-delocalized free radicals [ 155]. Neutral odd alternating hydrocarbons (OAH) proposed for this purpose are free radicals and therefore possess one unpaired electron on the non-degenerate molecular orbital (like TTF +.and TCNQ~). In the case of odd alternating hydrocarbons, electron transport may result in disproportionation: OAH'...OAH'...OAH"

-~ O A H + . . . O A H - . . . O A H ' .

The energy of such disproportionation, as was shown for sym-tribenzophenalenyl, is of the same order as the energy of an anion radical TCNQ disproportionation into a dianion: TCNQ~... TNCQ~... TCNQ ~ -. TCNQ~-... TCNQ°... TCNQ 2-.

Sym-tribenzophenalenyl. *A mixed (DMTTF)(TCNQ) salt is o b t a i n e d in s o l u t i o n [63].

29 Introduction of substituents or the use of OAH of higher molecular weight should reduce the disproportionation energy due to higher delocalizat/on of charge [155]. A higher degree of overlap of molecular orbitals of neighboring molecules seems possible in chains consisting of odd alternating hydrocarbons. Another advantage of neutral free radicals as compared with radical ions, according to Haddon, is the absence of Coulomb repulsion in stacks of neutral molecules, and therefore shorter interplanar distances in the conducting chain. The possibility of creating conducting chains on the basis of neutral TTF and TCNQ molecules and stable free radicals is discussed in ref. 156. It has recently been suggested that complexes with charge transfer be obtained by treating two-dimensional layer structures [ 157] of type A with compounds which are planar and suitable in size for filling the rings of the molecular core. In particular, one of the compounds [ 157] is a planar cation of guanidinium [C(NH2)a] ÷ which fits well to the hole of the structure A and is capable of remaining inside the ring by hydrogen bonds of three aminogroups.

N

N

\ / ' V / N / C .g C I N N J \

N/~ C I N J S I[ N

N

s Jl N N i \ / S C f N %, S If N

",,I C I N X

N N

N C

N S II N

N

N

N/N/N/ C

S II N

S

C

t

S II N

N

\I~#\I S

N N \ / \ / N / C S C I I N N # \ S N N

\

C I N

C I N

X

S li N

N

N

NIXfN/ C

/ S iJ N

g

C

i

Structure

A.

It has also been suggested that the possibility of forming a new aromatic sextet during electron transfer in various (4n + 2) Hiickel aromatic systems be investigated [9], to pay attention to free iminyl radicals [158], to form (SCH)x-type systems similar to (SN)x polymer which exhibits metallic properties [159] and so on [14, 164]. The number of compounds possessing high electrical conductivity is continuously increasing. Several new compounds with conduct/v/ties of (10 - 102) ohm -1 cm -1 have been described only recently [17, 47, 160 162]. The physical properties of many of these compounds are absent at the

30

present time, but it is apparent now that organic molecular metals are not an isolated phenomenon but an important class of compounds, the study of which will undoubtedly contribute to the advancement of the physics and chemistry of solids. References 1 H. Meier, in H. F. Ebel (ed.), Organic Semiconductors, Vol. 2, Weinheim, New York, 1974. 2 I. F. Shchegolev, Phys. Status Solidi A, 12 (1972) 9. 3 A. F. Garito and A. J. Heeger, Acc. Chem. Res., 7 (1974) 232. 4 E. B. Yagubsky and M. L. Khidekel, Usp. Khim., 41 (1972) 2133. 5 M. L. Khidekel, R. P. Shibaeva, I. F. Shchegolev and E. B. Yagubsky, Vestn. Akad. Nauk SSSR, 11 (1975) 41. 6 E. M. Engler, Chem. Technol., 6 (1976) 274. 7 J . J . Andre, A. Bieber and F. Gantier, Ann. Phys., 1 (1975) 145. 8 V. L. Ginzburg and D. A. Kirzhnits (eds.), Problema vysokotemperaturnoy sverkhprovodimosti (The problem o f high-temperature superconductivity), Nauka, Moscow, 1977, p. 271. 9 J. H. Perlstein, Angew. Chem., Int. Ed. Engl., 16 (1977) 519. 10 Z. G. Soos, Annu. Rev. Phys. Chem., 25 (1974) 121. 11 L o w Dimensional Cooperative Phenomena, Nato Advanced Study Institutes Series, Plenum Press, 1975. 12 H. G. Schuster (ed.), One-Dimensional Conductors, New York, Berlin, Heidelberg, 1975. 13 K. Masuda and M. Silver (eds.), Energy and Charge Transfer in Organic Semiconductors, New York, 1974. 14 I. V. Krivoshei, Zh. Strukt. Khim., 17 (1976) 110. 15 R. P. Shibaeva and L. O. Atovmyan, Zh. Strukt. Khim., 13 (1972) 514. 16 F. H. Herbstein, Perspectives in Structural Chemistry, Wiley, New York, 1971. 17 J. L. Peterson, C. S. Schramm, D. R. Stojakovic, B. M. Hoffman and T. J. Marks, J. Am. Chem. Soc., 99 (1977) 286. 18 A. Bernard, Teoretischeskie osnovy neorganicheskoy khimii (Theoretical Concepts o f Inorganic Chemistry), Mir, Moscow, 1968, p. 114. 19 L. Polling, Obshchaya khimiya (General Chemistry), Mir, Moscow, 1964. 20 M. V. Volkenshtyein, Stroenie i fizicheskie svoistva molekul, Publ. Akad. Nauk SSSR, Moscow, Leningrad, 1955. 21 J. Armit and R. Robinson, J. Chem. Soc., 127 (1925) 1604. 22 A. Remik, Elektronnye predstavleniya v organicheskoi khimii (Electronic Concepts in Organic Chemistry), Foreign Lit. Publ., Moscow, 1950, p. 268. 23 J. B. Conant, L. F. Small and B, S. Taylor, J. Am. Chem. Soc., 47 (1925) 1959. 24 F. Gutman and L. Layons, Organic Semiconductors (in Russian), Mir, Moscow, 1970. 25 E. P. Goodings, Chem. Soc. Rev., 5 (1976) 95. 26 H. N. McCoy and W. C. Moore, J. Am. Chem. Soc., 33 (1911) 273. 27 H. Kraus, J. Am. Chem. Soc., 34 (1913) 1732. 28 B.H.M. Billinge and B. G. Gowenlock, J. Chem. Soc., (1962) 1201; B. G. Gowenlock, P. P. Jones and D. W. Ovenall, J. Chem. Soc., (1958) 535; B. G. Gowenlock and J. Trotman, J. Chem. Soc., (1957) 2114. 29 J. L. Maynard and H. C. Howard, J. Chem. Soc., (1923) 960. 30 R. G. Kepler, J. Chem. Phys., 39 (1963) 3528. 31 L. R. Melby, R. J. Harder, W. R. Hertler, W. Mahler, R. E. Benson and W. E. Mochel, J. Am. Chem. Soc., 84 (1962) 3374. 32 J. L. Caline, E. M. Fabre and L. Giral, Acta Crystallogr., Sect. B, 33 (1977) 3827.

31 33 A. Singhabhandhu, P. D. Robinson, J. H. Fang and W. E. Geiger, Inorg. Chem., 14 (1975) 318. 34 L. I. Buravov, G. I. Zvereva, V. F. Kaminskii, L. P. Rosenberg, M. L. Khidekel, R. P. Shibaeva, I. F. Shchegolev and E. B. Yagubskii, J. Chem. Soc., Chem. Commun., (1976) 720. 35 L. C. Izett and E. A. Perez-Albuerne, Solid State Commun., 21 (1977) 433. 36 A. N. Bloch, D. O. Cowan, K. Bechgaard, K. E. Pyle, R. H. Banks and T. O. Pochler, Phys. Rev. Lett., 34 (1975) 1561. 37 T. E. Phillips, T. J. Kistenmacher, A. N. Bloch and D. O. Cowan, J. Chem. Soc., Chem. Commun., (1976) 344. 38 L. I. Buravov, A. I. Kotov, M. L. Khidekel, I. F. Shchegolev and E. B. Yagubskii, Izv. Akad. Nauk SSSR, Set. Khim., (1976) 475. 39 S. P. Zolotukhin, V. F. Kaminskii, A. I. Kotov, R. B. Lubovskii, M. L. Khidekel, R. P. Shibaeva, I. F. Shchegolev and E. B. Yagubskii, Zh. ETPH Lett., 25 (1977) 480. 40 S. P. Zolotukhin, U. S. Karimov and I. F. Shchegolev, Zh. ETPh, 75 (1978) 12. 41 G. J. Ashweil, Phys. Status Solidi B, 85 (1978) k7 ; P. Dupuis, S. Flandrois, P. Delhaes and C. Conlon, J. Chem. Soc., Chem. Commun., (1978) 337. 42 D. J. Sandman, A. P. Fisher, T. J. Holmes and A. J. Epstein, J. Chem. Soc., Chem. Commun., {1977) 687. 43 J. P. Ferraris and G. Saito, J. Chem. Soc., Chem. Commun., (1978) 992. 44 G. R. Johnson, M. G. Miles and J. D. Wilson, Mol. Cryst. Liq. Cryst., 33 (1976) 67. 45 K. Bechgaard, C. S. Jacobsen and N. H. Andersen, Solid State Commun., 25 (1978) 875. 46 M. Mizuno, A. F. Carito and M. P. Cava, J. Chem. Soc., Chem. Commun., (1978) 18. 47 D.J. Sandman, A. J. Epstein, T. J. Holmes and A. P. Fisher, J. Chem. Soc., Chem. Commun., (1977) 177. 48 C. J. Schramm, D. R. Stojakovic, B. M. Hoffman and T. J. Marks, Science, 200 (1978) 47. 49 T. E. Phillips and B. M. Hoffman, J. Am. Chem. Soc., 99 (1977) 7734. 50 K. Ukei, J. Phys. Soc. Jpn., 40 (1976) 140. 51 H. Shirakawa, E. J. Lowis, A. G. MacDiarmid, C. K. Chiang and A. J. Heeger, J. Chem. Soc., Chem. Commun., (1977) 578. 52 C. K. Chiang, M. A. Druy, S. C. Gau, A. J. Heeger, E. J. Louis, A. G. MacDiarmid, Y. W. Park and H. Shirakawa, J. Am. Chem. Soc., 100 (1978) 1013. 53 S. L. Hsu, A. J. Signorelli, G. P. Pez and R. H. Baughman, J. Chem. Phys., 69 (1978) 106. 54 R. H. Baughman, S. L. Hsu, G. P. Pez and A. J. Signorelli, J. Chem. Phys., 68 (1978) 5405. 55 C. K. Chiang, C. R. Fincher, Y. W. Park, A. G. Heeger, H. Shirakawa, E. J. Louis, S. C. Gau and A. G. MacDiarmid, Phys. Rev. Lett., 39 (1977) 1098. 56 L. R. Anderson, G. P. Pez and S. L. Hsu, J. Chem. Soc., Chem. Commun., (1978) 1066. 57 L. Michaelis, Trans. Electrochem. Soc., 71 (1937) 107; L. Michaelis, Annu. Rev. Biochem., 7 (1938) 1. 58 S. N. Zelenin and M. L. Khidekel, Usp. Khim., 39 (1970) 209. 59 E. G. Chepaikin and M. L. Khidekel, Zh. Koord. Khim., 4 (1978) 643. 60 W. A. Barlow, G. R. Davies, E. P. Goodings, R. L. Hand, G. Owen and M. Rhodes, Mol. Cryst. Liq. Cryst., 33 (1976) 123. 61 V. K. Belsky and D. Voet, Acta CrystaUogr., Sect. B, 32 (1976) 272. 62 W. F. Cooper, N. C. Kenny, J. W. Edmonds, A. Nadel, F. Wudl and P. Coppens, J. Chem. Soc., Chem. Commun., (1971) 889. 63 L.B. Coleman, Dissertation in Physics, University of Pennsylvania, Pennsylvania, 1975. 64 M. Z. Aldoshina, L. S. Veretennikova, R. N. Luboskaya and M. L. Khidekel, Izv. Akad. Nauk SSSR, Ser. Khim., (1978) 940. 65 L. S. Veretennikova, R. N. Lubovskaya, R. B. Lubovskii, L. P. Rozenberg, M. A. Simonov, R. P. Shibaeva and M. L. Khidekel, Dokl. Akad. Nauk SSSR, 241 (1978) 862.

32 66 W. F. Cooper, J. W. Edmonds, F. Wudl and P. Coppens, Cryst. Struct. Commun., 3 (1974) 23. 67 P. Calas, J. M. Fabre, M. K. EI-Saleh, A. Mass, E. Torrieles and L. Giral, C. R. Acad. Sci., Set. C, 281 (1975) 1037. 68 H. K. Spencer, M. P. Cava, F. G. Yamaguchi and A. F. Garito, J. Org. Chem., 41 (1976) 730. 69 H. K. Spencer, M. P. Cava and A. F. Garito, J. Chem. Soc., Chem. Commun., (1976) 966. 70 R. N. Lubovskaya, M. Z. Aldoshina, V. Ya. Rodionov, T. A. Chibisova and M. L. Khidekel, Izv. Akad. Nauk SSSR, (1975) 177. 71 H. K. Spencer, M. V. Lakshimikanatham, M. P. Cava and A. F. Garito, J. Chem. Soc., Chem. Commun., (1975)867. 72 S. L. Hsu, A. N. Bloch, T. E. Carruthers and D. O. Cowan, J. Chem. Soc., Chem. Commun., (1977) 50. 73 R. C. Wheland, J. Am. Chem. Soc., 98 (1976) 3296. 74 K. Bechgaard, D. O. Cowan and A. N. Bloch, J. Chem. Soc., Chem. Commun., (1974) 937. 75 J. R. Anderson, C. S. Yacobson, G. Rindorf, H. Soling and K. Bechgaard, J. Chem. Soc., Chem. Commun., (1975)883. 76 S.J. La Placa, P. W. R. Corfield, R. Thomas and B. A. Scott, Solid State Commun., 17 (1975) 635. 77 J. J. Daly and F. Sanz, Acta Crystallogr., Sect. B, 31 (1975) 620. 78 C. K. Johnson and C. R. Watson, J. Chem. Phys., 64 (1976) 2271. 79 D.J. Dahm, G. R. Johnson, F. L. May, M. G. Miles and J. D. Wilson, Cryst. Struct. Commun., 4 (1975) 673. 80 R. B. Somoano, A. Gupta, V. Hadek, M. Movotny, M. Jones, T. Datta, R. Deck and A. M. Hermann, Phys. Rev. B, 15 (1977) 595. 81 F. Wudl, D. E. Schafer, W. M. Walsh, L. W. Rupp, F. J. Duslvo, J. V. Waszcak and G. A. Thomas, J. Chem. Phys., 66 (1977) 377. 82 T. J. Kistenmacher, T. E. Phillips. D. O. Cowan, J. P. Ferraris, A. N. BIoch and T. O. Pohler, Acta Crystallogr., Sect. B, 32 (1976) 539. 83 S. Etemad, T. Penney, E. M. Engler, B. A. Scott and P. E. Selden, Phys. Rev. Lett., 34 (1975) 741. 84 E. M. Engler and V. V. Patel, J. Am. Chem. Soc., 96 (1974) 7376. 85 R.B. Samoano, A. Gupta, V. Hadek, T. Datta, M. Jones, R. Deck and A. M. Hermann, J. Chem. Phys., 63 (1975) 4970. 86 L. I. Buravov, O. N. Eremenko, R. B. Lubovskii, L. P. Rozenberg, M. L. Khidekel, R. P. Shibaeva, I. F. Shchegolev and E. B. Yagubskii, Zh. ETF, Pis'ma Red., 20 (1974) 457. 87 R. P. Shibaeva and L. P. Rozenberg, Kristallografiya, 20 (1975) 943. 88 E. B. Yagubskii, M. L. Khidekel, lzv. Akad. Nauk SSSR, Set. Khim., (1975) 1213. 89 O. N. Eremenko, S. P. Zolotukhin, A. I. Kotov, M. L. Khidekel and E. B. Yagubskii, Izv. Akad. Nauk SSSR, Ser. Khim., (1978) 2117. 90 V. F. Kaminskii, M. L. Khidekel, R. B. Ljubovskii. T. F. Shchegolev, R. P. Shibaeva, E. B. Yagubskii, A. V. Zvarykina and G. L. Zvereva, Phys. Status Solidi A, 44 (1977) 77. 91 J. B. Torrance, B. A. Scott and F. B. Kaufman, Solid State Commun., 17 (1975) 1369. 92 J. B. Torrance and B. D. Silverman, Phys. Rev. B, 15 (1977) 788. 93 F. Devrenx, M. Guglielmi and M. Nechtschein, J. Phys. (Paris), 39 (1978) 541. 94 B. A. Scott, S. J. La Placa, J. B. Torrance, B. D. Silverman and B. Welber, J. Am. Chem. Soc., 99 (1977) 6631. 95 A. R. Siedle, G. A. Candela, T. F. Finnegan. R. P. Van Duyne, T. Cape, G. F. Kokoszka and P. M. Woyciesjes, J. Chem. Soc., Chem. Commun., (1978) 69. 96 E. A. Peres-Albuerne, H. Johnson and D. J. Trevov, J. Chem. Phys., 55 (1971) 1547. 97 I. I. Ukrainskii, V. E. Klymenko and A. A. Ovchinnikov, J. Phys. Chem. Solids, 39 (1978) 359.

33 98 V. E. Klimenko, V. Ya. Krivlov, A, A. Ovchinnikov, I. I. Ukrainskii and A. F. Shvets, Zh. ETF, 69 (1975) 240. 99 L. M. Kachapina, M. G. Kaplunov, A. I. Kotov, E. B. Yagubskii and U. G. Borod'ko, Opt. Spectrosk., 45 (1978) 82. 100 J. J. Ritsko, L. J. Brillson and D. J. Sandman, Solid State Commun., 24 (1977) 109. 101 M. Z. Aldoshina, R. N. Lubovskaya, T. I. Larkina and M. L. Khidekel, Izv. Akad. Nauk SSSR, Ser. Khim., (1979) 2377. 102 M. B. Robin and P. Day, Adv. Inorg. Chem. Radiochem., 10 (1967) 247. 103 H. LeBlanc, J. Chem. Phys., 42 (1965) 4307. 104 S. Etemad, Phys. Rev. B, 13 (1976) 2254. 105 E. M. Engler, R. A. Graven, Y. Tomkiewicz, B. A. Scott, K. Bechgaard and J. R. Andersen, J. Chem. Soc., Chem. Commun., (1976) 337. 106 M. G. Kaplunov, D. N. Fedutin, M. L. Khidekel, I. F. Shchegolev and E. B. Yagubskii, Zh. Obshch. Khim., 42 {1972) 2295. 107 J. H. Lupinsky, K. R. Walter and R. H. Vogt, Mol. CrystaUogr., 3 (1967) 241. 108 E. B. Yagubskii, M. L. Khidekel, I. F. Shchegolev, L. I. Buravov, V. G. Gribov and M. K. Makova, Izv. Akad. Nauk SSSR, Ser. Khim., 9 (1968) 2124. 109 F. Denoyer, R. Comes, A. F. Garito and A. J. Heeger, Phys. Rev. Lett., 35 (1975) 445. 110 Y. Tomkievicz, A. R. Taranko and J. B. Torrance, Phys. Rev. Lett., 36 (1976) 751. 111 C. Weyl, Bull. Am. Phys. Soc., 21 (1976) 287. 112 Y. Tomkiewicz, B. A. Scott, L. J. Tao and R. S. Title, Phys. Rev. Lett., 32 (1974) 1363. 113 Y. Tomkiewicz, R. A. Graven, T. D. Schultz, E. M. Engler and R. A. Taranko, Phys. Rev. B, 15 (1977) 3643. 114 E. M. Engler, B. A. Scott, S. Etemad, T. Penney and V. V. Pate, J. Am. Chem. Soc., 99 (1977) 5909. 115 S. P. Zolotukhin, V. F. Kaminskii, A. I. Kotov, M. L. Khidekel, R. P. Shibaeva and E. B. Yagubskii, Izv. Akad. Nauk SSSR, Ser. Khim., (1978) 1816. 116 M. L. Khidekel, N. I. Martem'yanova, M. V. Noriysina and A. P. Kriven'ko, Izv. Akad. Nauk SSSR, Ser. Khim., {1972) 450. 117 H. Kobayashi, Bull. Chem. Soc. Jpn., 47 (1974) 1346. 118 F. B. Bramwell, R. C. Haddon, F. Wudl, M. L. Kaplan and J. H. Marshall, J. Am. Chem. Soc., 100 (1978) 4612. 119 A. N. Bloch, Lecture Notes in Physics, Vol. 65, Springer Verlag, Berlin, 1977, p. 317. 120 J. R. Cooper, M. Weger, G. Delphanque, D. Jerome and R. Bechgaard, J. Phys. Lett., 37 (1976) 349. 121 J. R. Cooper, D. Jerome, S. Etemad and E. M. Engler, Solid State Commun., 22 (1977) 257. 122 J. R. Cooper, M. Weger, D. Jerome, D. Lefur, K. Bechgaard, A. N. Bloch and D. O. Cowan, Solid State Commun., 19 (1976) 749. 123 V. N. Laukhin, A. I. Kotov, M. L. Khidekel, I. F. Shchegolev and E. B. Yagubskii, Zh. ETF, Pis'ma Red., 28 (1978) 284. 124 C. W. Chu, J. M. E. Harper, T. H. Geballe and R. L. Green, Phys. Rev. Lett., 31 (1973) 1491. 125 D. Jerome, W. Mfiller and M. Weger, J. Phys. Left., 35 (1974) L77. 126 J. R. Cooper, D. Jerome, M. Weger and S. Etemad, J. Phys. Lett., 36 (1975) L219. 127 A. I. Larkin, Znanie-Sila, 10 (1977) 46. 128 M. A. Abkowitz, A. J. Epstein, C. H. Griffiths, J. S. Miller and M. J. Slade, J. Am. Chem. Soc., 99 (1977) 5304. 129 J. B. Torrance, B. A. Scott and F. B. Kaufman, Solid State Commun., 1 7 (1975) 1369. 130 R. M. Vlasova, A. I. Gutman, N. F. Karpenko, L. D. Rozenshtein, L. S. Agroskin, G. V. Papayan, L. P. Rautian and A. I. Sherle, Fizika Tverdogo Tela, 1 7 (1975) 3529. 131 W. J. Gunning, A. J. Heeger, I. F. Shchegolev and S. P. Zolotukhin, Solid State Commun., 25 (1978) 981.

34 132 E. I. Zhilyaeva, L. P. Rozenberg, R. N. Lubovskaya, R. B. Lubovskii, O. Ya. Neiland, D. V. Bite and M. L. Khidekel, Izv. Akad. Nauk SSSR, Ser. Khim., (1978) 695. 133 R. C. Wheland and J. L. Gillson, J. Am. Chem. Soc., 98 (1976) 3916. 134 E. M. Engler and V. V. Patel, J. Chem. Soc., Chem. Commun., (1975) 671. 135 M. V..Lakshmikantham, M. P. Cava and A. F. Garito, J. Chem. Soc., Chem. Commun., (1975) 383. 136 F. Wud], S. M. Smith and E. J. Hufnagel, J. Chem. Soc., Chem. Commun., (1970) 1453. 137 B. P. Bespalov and V. V. Titov, Usp. Khim., 44 (1975) 2249. 138 V. V. Aleksandrov, S. B. Aleksandrov, D. R. Balode, A. I. Belkind, A. A. Murashov, M. L. Khidekel and E. B. Yagubskii, Dokl. Akad. Nauk SSSR, 232 (1977) 825. 139 F. Wudl, J. Am. Chem. Soc., 97 (1975) 1962. 140 G. Brun, S. Peytavin, B. Liantard, M. Maurin, E. Toreilles, J. -M. Fabre and L. Giral, C. R. Acad. Sci., Set. C, 284 (1976) 211. 141 P. Delhaes, C. Condon, J. Amiell, S. Flandrois, E. Toreilles, J. M. Fabre and L. Giral, Mol. Cryst. Liq. Cryst., 50 (1979) 43. 142 W. Wheland, J. Am. Chem. Soc., 99 (1977) 291. 143 B. F. Kaminskii, R. P. Shibaeva and L. O. Atovmyan, Zh. Strukt. Khim., 15 (1974) 509. 144 B. A. Scott, F. B. Kaufman and E. M. Engler, J. Am. Chem. Soc., 98 (1976) 4342. 145 F. B. Kaufman, E. M. Engler, D. C. Green and J. Q. Chambers, J. Am. Chem. Soc., 98 (1976) 1595. 146 Y. Ueno, M. Bahry and M. Okawara, Tetrahedron Lett., (1977) 4607. 147 A. Lombardo and T. R. Fico, J. Org. Chem., 44 (1979) 209. 148 A. N. Bloch, J. P. Ferraris, D. O. Covan and T. O. Pochler, Solid State Commun., 13 (1973) 753. 149 C. B. Coleman, M. J. Cohen, D. J. Sandman, F. G. Yamagashi, A. F. Garito and A. J. Heeger, Solid State Commun., 12 (1973) 1125. 150 Y. Tomkiewicz, J. B. Torrance, B. A. Scott and D. C. Green, J. Chem. Phys., 60 (1974) 5111. 151 R. V. Gemmer, D. O. Cowan, T. O. Pochler, A. N. Bloch, R. E. Pyle and R. H. Blanks, J. Org. Chem., 40 (1975) 3544. 152 R. Viswanathan and D. C. Johnston, J. Phys. Chem. Solids, 36 (1975) 1093. 153 C. K. Chiang, M. J. Cohen, P. R. Newman and A. J. Heeger, Phys. Rev. B, 16 (1977) 5163. 154 P. Kathirgamanathan, S. A. Mucklejohn and D, R. Rosseinsky, J. Chem. Soc., Chem. Commun., (1979)86. 155 R. C. Haddon, Nature (London), 256 (1975) 394. 156 F. Wudl, in Proc. Conf. Organic Conductors and Semiconductors, Siofok, Hungary, 1976. 157 M.H. Whangbo, R. Hoffman and R. B. Woodward, Conjugated One- and Two-Dimensional Polymers, Preprint. 158 W. C. Dahen and F. A. Neugebauer, Angew. Chem., Int. Ed. Engl., 14 (1975) 783. 159 T. Yamabe, K. Tanaka, A. Imamura, H, Kato and K. Fukui, Bull. Chem. Soc. Jpn., 50 (1977) 798. 160 F. Wudl, D. E. Schafer and B. Miler, J. Am. Chem. Soc., 98 (1976) 252. 161 N.M. Rivera and E. M. Engler, J. Chem. Soc., Chem. Commun., (1979) 184. 162 J. Meinwald, D. Dauplais, F. Wudi and J. J. Hanser, J. Am. Chem. Soc., 99 (1977) 255. 163 (a) J. R. Andersen, Organic Metals: A Study o f Interstack Interactions, Ris~ Rep. No. 397, Ris~" National Laboratory, Roskilde, 1978, 87 pp.; (b) Ann. N. Y. Acad. Sci., 313 (1978). 164 R. M. Metzger, J. B. Torrance and J. J. Mayerle, Bull. Am. Phys. Soc., 23 (1978) 356, GI17.