Chem. Mater. 1993,5, 1199-1203
1199
Articles Structural and Electronic Study of Donors Composed of Two TTF Moieties Linked by Tellurium Bridges James D. Martin and Enric Canadell* Laboratoire de Chimie ThSorique, UniversitO de Paris-Sud, 91405 Orsay Cedex, France
James Y. Becker and Joel Bernstein' Department of Chemistry, Ben-Gurion University of the Negev, Beer Sheva 84105, Israel Received February 2, 1993. Revised Manuscript Received June 14, 1993 The details of the crystal and electronic structure of two compounds composed of two TTF (tetrathiafulvalene) moieties linked by one and two tellurium atoms have been studied. In each case the two TTF units in a given molecule differ: in one the difference is manifested in the intramolecular bond lengths, while in the second case there is a difference in the degree of molecular planarity. Because of the difference in conformational degrees of freedom, the mutual disposition of the TTF units also differs in the two structures, as do the type and number of the intermolecular interactions. It is suggested that the inequivalence between the two chemically identical TTF units in donors of this type, can lead to a quite large localization of the HOMO in one of the two TTF moieties. The implications of such a localization for the transport properties of this type of donors are discussed.
Introduction When there is only one kind of interacting species in a molecular crystal, the orbital interactions which determine the structure must derive from the same molecule as those which determine the properties. However,the interactions which play a dominant role in determining the crystal structure can implicate orbitals which are different from those determining the transport properties. For instance, it has been suggested that intermolecular hydrogen bonding plays a significant role in determining the crystal structure of molecular organic conductors.l However, for donor molecules like those discussed here, the resulting interactions between the HOMO (highest occupied molecular orbital) of nearby molecules, generally of *-type character and without any significant contribution from the hydrogens,will determine the band structure and hence to a large extent the resulting properties of electrical conductivity. Recently there has been increasing interest and activity in the preparation and study of candidate molecules for organic conductors which are based on a molecular motif linking two (or more) classic donor moieties by a variety of molecular connectors.2" If the two donor moieties are (1) Whangbo, M.-H.;Jung, D.; Ren, J.; Evain, M.; Novoa, J. J.; Mota, F.; Alvarez, S.;Williams, J. M.; Beno, M. A.; Kini, A. M.; Wang, H. H.; Ferraro, J. R. The Physics and Chemistry of Organic Superconductors; Saito, G., Kagoshima, S.,Eds.; Springer-Verlag: Berlin, 1990; p 262. (2) Mizutani, M.; Tanaka, K.; Ikeda, K.; Kawabata, K. Synth. Met. 1992,46,201. Bechgaard, K.; Lerstrup, K.; Jorgensen, M.; Johannaen, I.; Christensen, J. The Physics and Chemistry of Organic Superconductors; Saito, G., Kagoshima, S., Eds.; Springer-Verlag: Berlin, 1990; p 349. (3) Becker, J. Y.; Bernstein, J.;Bittner, S.;Sarma, J. A. R. P.;Shahal, L. Tetrahedron Lett. 1988,29, 6177. (4) Becker, J. Y.;Bemstein, J.; Sarma, J. A. R. P.;Shahal, L. J.Chem. SOC.,Chem. Conmun. 1992, 1048. ( 5 ) Bryce, M.R.; Cooke, G.; Dhindsa, A. S.; Ando, D. J.; Hurthouse, M. B. Tetrahedron Lett. 1992,33,1783. Khodorkovsky, V. Y.;Becker, J. Y.; Bernstein, J. Synthesis 1992, 1071.
not symmetricallyequivalent in the crystal, intermolecular interactions for one of the two chemically identical units may be structure determining, whereas interactions between the second of the two units may play the key role in determining the properties. Compounds based on the two linked donors motif may also exhibit structural features which are not found in the parent donor compounds. In some cases the intramolecular atoms connecting the donor moieties participate in the intermolecular network which characterizesthe three-dimensional structure. Such contacts can lead to electronic intermolecular interactions which will generate electronic structures and solid-state electronic properties also differing from those of the parent donors. Understanding the relationship between the structure-determining and property-determining features of the structure can thus lead to new design strategies for organic conductors. Two examples of such a linked multidonor motif are the molecules Ia and Ib, in which two TTF units are linked respectively by one or two tellurium atoms. The use of
fHS] S S
[)==-(s7jA S
I, a: A
Te, b: A = Te-Te
a one-atom connector prescribes certain geometric constraints on the molecular conformation and presumably intermolecular interactions as well. Many of these are relaxed when the connector is composed of two atoms mutually linked by a single bond. In both cases the incorporation of tellurium into the molecule provides an (6) Fourmigu6, M.;Batail, P. Chem. Commun. 1991,1370.
0897-4756/93/2805-ll99$04.00/00 1993 American Chemical Society
Martin et al.
1200 Chem. Mater., Vol. 5, No. 9, 1993 a
C
Figure 1. Ball-and-stickdrawingsof the (TTF)2Temolecule (a) giving a view perpendicular to the C(l)-Te(l)-C(7) plane and the atom numbering scheme and (b) showing the crystal packing arrangement.
Figure 2. Ball-and-stick drawings of the (TTF)2Te2 molecule (a) giving a view nearly down the Te-Te vector and the atom numbering scheme, (b) emphasizing the planarity and degree of bending of the two TTF fragments, and (c) showing the crystal packing arrangement.
additional potential degree of intermolecular interaction, involving Te-Te and T e . 4 interactions, as well as the expected S-S interactions between TTF units. We have recently reported the synthesis and crystal determination of both of these compound^^?^ which, as will be shown below, exhibit chemically equivalent but structurally inequivalentTTFunits. We here use the crystal structures of Ia and Ib to explore the possible consequences for the electronic structure (and transport properties) of the structural inequivalence of the two moieties in this class of organic donors.
infinite stacks fulfills one of the necessary structural conditions for electronic conductivity. Being a TTF derivative, such a mechanism should arise from short intermolecular contacts between chalcogen at0ms.I All intra- and interstack chalcogen-chalcogen short contacts are summarized in Table I. The different molecule-. molecule interaction types are depicted graphically in Figure lb. Each of these interaction types exhibit chalcogem-chalcogen distances which are less than the s u m of the van der Waals radii, suggesting that there are both intra- and interstack interactions. Thus, additional requirements for conductivity seem to be qualitatively met. (TTF)zTe2(Ib). The additional Te atom in the bridge of Ib (Figure 21, adds considerable conformational flexibility to the molecule resulting in an entirely different intramolecular relationshipbetween the "TFunits. While in this case the central C = C distances are more equivalent (C(3)-C(4) = 1.324(18)A and C(9)-C(lO) =: 1.342(19) A), the two TTF moieties are quite distinct with respect to their planarity. Whereas the terminal C = C bonds of the TTF fragment bound to Te(1) are nearly coplanar with the central SZCCSZ unit (bent 3.4' and 4.1°, respectively, out of the SzCCS2 least-squares plane), the terminal C-C bonds of the TTFfragment bound to the Te(2) are notably bent with respect to the central SZCCSZ plane ( 1 8 . 8 O and 12.9', respectively), as shown in Figure 2b. The two TTF units are twisted about the Te-Te bond such that the C(1)Te(l)-Te(2)-C(7) torsion angle is 99.0'. Furthermore, the least-squares planes of the two TTF moieties are not parallel, but are twisted with an angle between the two planes of 36.6O. The crystal structure of Ib (space group P21/n) also exhibits a variety of stackings of the TTF moieties (see Figure 2c). Translation along c leads to face-to-facestacks
Molecular and Crystal Structures (TTF)zTe(Ia). The crystal structure of Ia (spacegroup P21/a),Figure 1,exhibits two crystallographically distinct TTFmoieties linked by a T e atom. The primary difference between the two TTF fragments lies in the different bond length of the central C = C bond, C(3)-C(4)= 1.367(17)A and C(9)-C(lO) = 1.336(16) A, respectively. The leastsquares planes of the two TTF fragments are twisted from the C(l)-Te(l)-C(7) plane by 76.7' and 100.Oo, respectively, withan angleof 97.7O between the two planes (Figure la). These molecules stack with the TTF fragments faceto-face by a translation along the c direction. Parallel stacks are related by an a-glide and by centers of inversion. Those related by inversion give rise to a herringbone pattern. Stacks with edge-to-edge contacts are found between molecules related by the inversion centers or the screw axis along the b direction. In the stacks along c, the tellurium lies only at 3.69 and 3.68 A, respectively, from the least-squares planes of the two TTF fragments. This leads to two Te.4 contacts shorter than the sum of their van der Waals radii within the stacks. There are also two interatomic S . 4 distances along these stacks which are less than 3.8 A (see Table I, interaction type VI. Provided that there is an overlap mechanism which would allow for the flow of charge, the presence of parallel
(7) Wudl, F. Acc. Chem. Res. 1984, 17,227. Williams,J. M.; Beno, M. A.; Wang, H. M.; Leung, P. C. W.; Emge, T. J.; Geiser, U.; Carbon, K. D. Acc. Chem. Res. 1985,18,261.
Chem. Mater., Vol. 5, No. 9, 1993 1201
Donors with Two TTF Moieties Linked By Te Bridges
Table 1. Different Types of Intermolecular Interactions in the Crystal Structure of TTFITe (la) Associated with 9 . 4 and S-Te Contacts Smaller than 3.8 and 4.0 A, Respectively interaction
I
sy"etW
I11 IV V
(1 --*,1 - y , 1 - 2 ) (1 - x , Y ,-2) (1-z,?,l--I) ('I*+x,'/z-y.1+2) (X,Y, 1 + 2)
VI
(1 --I, 1 - y ,
11
--I)
S...Sb
types of ?TFs involvedc
S ( 2 ) 4 5 ) = 3.525d S ( 4 ) 4 4 ) = 3.763 S(l)-S(4) = 3.65od S(3).-S(7) = 3.685 S(3)-S(1) = 3.784 S(7)...S(5) = 3.794 S(3)-S(8) = 3.57Zd
longahort long-long long-long longahort long-long shortshort longahort
TeSb
Te.-S(l) = 3.925 Te-S(5) = 3.864
Key to the symmetry operation relating the second sulfur atom to the first one at ( x y , z ) . Angstroms. Shortllong refer to the ?TF moieties with the shortllang central carban-carbon double bond. Because this contact is across an inversion center it should be counted twice in its interaction energy. Table 11. Different Types of Intermolecular Interactions in the Crystal Structure of TTFsTer (Ih) Associated with S-S and 9.-Te Contacts Smaller than 3.8 and 4.0 A. Respectively interaction
sy"etW
+ x , 'I2 - Y, ' 1 2 + 2 ) + x , 'I2 - Y . -'I2 + 2 )
I I1
('12 ('12
111
(X,Y,1 + I )
IV
e x , y , -1
V
~'12-*,-'1~+Y,-'/2-2~
S...Sb
types of lTFs involvedc
S(Z)-S(8) = 3.708 S(4)-S(5) = 3.720 S(4)-S(7) = 3.635 S ( 5 ) 4 ( 6 ) = 3.757
planarhent planarbent planarbent bent-bent
-2)
Te-Sb
Te(l)-S(3) = 3.530 Te(l)-S(6) = 3.851 Te(Z)-.S(6) = 3.740 Te(Z)-S(6) = 3.487d T e ( 2 ) 4 ( 4 ) = 3.739
Key to the symmetry operation relating the second sulfur atom to the first one at ( x y . 2 ) . Angstroms. Bentllinear refer to the nature of the "'ITSinvolved in the desienated S-S interaction. Because this contact is a c r w an inversion center it should be counted twice in ita interaction energy.
of planar TTF moieties as well as to edge-to-edge stacks of the bent TTF moieties. These two types of contacts will give different orbital overlaps along that direction. In fact, onlythe edge-to-edge stacks exhibitshortchalcogenchalcogen distances. Molecules related by an n-glide form edge-to-face stacks and molecules related by inversion centers form stacks along b. The short chalcogen-. chalcogen contacts are summarized in Table 11, and the different molecule-molecule interaction types are graphically depicted in Figure 2c. Electronic S t r u c t u r e
General Considerations. The crystal structures of TTFzTe (In) and TTFzTez (Ib) are quite complex, and in spite of the qualitative remarks above it is not immediately apparent whether the necessary structural conditions for achieving conductivity are realized. Though there are no Te-Te short contacts in either structure, short Te-S and S-S contacts suggest the possibility of intermolecular electronic delocalization. However, some of these contacts may be ineffective because (a) the orientation of the corresponding p orbitals is not appropriate for orbital overlap and/or (b) the p orbital of one of the two chalcogen atomsdoes notparticipatein ther-typemolecularorbital( 8 ) leading to the band(s) responsible for conductivity. In the following we briefly explore the latter aspect for the general case where two TTF units are coupled through a bridging group A (see I). This analysis is not only useful in the study of compounds In and Ib but will provide some guidelines by which to consider the electronic structure of possible charge transfer salts of these and other donors of type I. Indeed charge-transfer salts of I b have recently been prepared and exhibit good conductivity? Our qualitative arguments are supported by extended Hiickel typegmolecular calculations for isolated molecules and dimers and tight binding band structure calculations1° for the crystal structures of In and Ib. Electronic S t r u c t u r e a n d Intermolecular Orbital Interactions. Since molecules I are donors, we consider only the two orbitals which originate from the HOMO of
each TTF. These are the orbitals which can lead to partially filled bands in the metallic charge transfer salts. In the neutral semiconducting In and I b the bands built from both the HOMO and LUMO of I should be considered. However, whereas the TTF HOMO contains strong sulfur contributions, the TTF LUMO, which will lead to the LUMO and Ia and Ib, is localized on the outer carbon-carbon double bonds and contains only small contributions from the sulfur orbitals. Thus the corresponding bands will be quite flat and need not be considered. A. Nature of the HOMO ofMolecules I. As discussed elsewhere" the coupling between the two TTF HOMOS in molecules of type I is usually quite weak because of the relatively small contribution of the outer carbon atoms in the TTF HOMO 11. However, even if the interaction
_..
HOMO-
,." '.
"
I T F -A-TTF
T F .....TrF
I1
-HOMO.
111
through the bridging group A is weak, the splitting between the HOMO+ and HOMO. orbitals (see 111) can be important if the two TTF units are inequivalent (the case where the two coupled TTFs are identical has been discussed elsewhere)." This inequivalencecan result from theexistenceofdifferentbondlengths withintbetwolTFs or because one of the two TTFs is planar, whereas the other is bent. As discussed above, examples of the two situationsareprovided by Ia and Ib, respectively. Though both In and Ib are closed-shell molecular species, the arguments put forward here must be considered in order to understand the electronic structure and transport (8) Becker, J.; Bernstein, J.: Shshal, L.; Dayan, M., manuecript in preparation.
1202 Chem. Mater., Vol. 5, No. 9, 1993
Martin et al.
B. Molecular Interactions in the Crystal Structure of TTFzTe (la). Listed in Table I are the different types of interaction between Ia molecules associated with So-S and Te-S contacts smaller than 3.8 and 4.0 A,13 respectively. As shown in Figure 1 there are TTF stacks (associated with interaction type V) along the c axis. A look at Table I reveals that there are several S.-S contacts considerably shorter than those of interaction V along the b direction (particularly those of interaction types I and VI). Interaction IV connects molecules of different stacks along a . Thus, there is a 3D network of T e . 4 and S.-S interactions in this material. Our calculations for the Ia molecule show that the tellurium contribution to the HOMO+ and HOMOorbitals is about 0.1%. This result is consistent with cyclic voltammetry results for Ia3 which indicate that the two TTF units are very nearly electronically independent and has the important implication that the Te.4 contacts in the crystal structure (i.e., those of interaction V) are completely ineffective in leading to good conductivity. In addition, the HOMO+ and HOMO- orbitals are very strongly localized (approximately 98 % ) in one of the two IV TTF moieties of the molecule. The HOMO- is localized on the TTF containing the longer central C=C bond sometimes observed in both neutral and charge-transfer (C(3)=C(4) 1.37 A) whereas the HOMO+ is localized on molecular solids.lJ2 According to our calculations, the the TTF containing the shorter central C=C bond energy of the TTF HOMO is lowered while the total energy (C(9)=C(lO) 1.34 A). These results are in complete of both TTF and TTF+ is slightly raised for reasonable agreement with our previous qualitative analysis. Acvalues of 8 (i.e., between 0 and 15’). Consequently, if one cording to the single-rand double-lcalculations,the energy of the two TTFs in I is bent, the HOMO+ and HOMOsplitting between the HOMO+ and HOMO- orbitals is orbitals will be strongly localized on the bent and planar 0.08 and 0.10 eV, respectively. According to our qualitative TTFs, respectively. Since the partially filled band in analysis,the highest occupied band should mainly originate metallic charge-transfer salts of I and the highest occupied from the TTF with the longer central C-C bond, thus band in semiconducting Ib is built from the HOMO-, only the S 4 contacts between pairs of this type of TTF localized on the the planar TTF, the X-X (X= S or Te) need to be considered. These include interactions 11,I11 contacts associated with this TTF play the major role in and V. Interactions I1 and I11lead to delocalization along determining the conducting properties of the system. the b and c directions and V along the c direction. Hence As it will be shown later, careful use of these results can the qualitative analysis leads to the prediction of a 2D greatly simplify the analysis of the electronic structure of type conductivity. solids built from molecules of type I. If, however, the To have a more quantitative estimate we have evaluated difference in bond lengths of B angles is small, the energy j3ij interaction energies14( i ,j = HOMO+ and HOMO-) the splitting between the HOMO+ and HOMO- orbitals will for the six different interaction types of Table I. These also be small and all X-X contacts should then be interaction energies reflect the strength of interaction considered. The above analysis may also break down when between a pair of molecular orbitals in adjacent sites of the X-X interactions of the TTF leading to the upper the crystal. As shown by Whangbo et al.14 they give band are weak while those of the TTF leading to the lower important clues relating the electronic and crystal strucband are strong. In that case the lower band will be more tures of molecular conductors. The main result of this dispersive than the upper one. If the dispersion of the study is that except for the HOMO--HOMO- interaction lower band is greater than the initial energy splitting associated with I1and I11(0.137 and 0.088 eV, respectively, between HOMO+ and HOMO-, the two bands will try to according to our double-{ calculations), all other intercross leading to an upper band with mixed character. Here actions are negligible. The fact that all the HOMO-too all X-X contacts must be considered. When one of HOMO+ interactions associated with long-short TTF pairs these two situations apply actual calculation of the band and the HOMO+-HOMO+ interactions associated with structure is indispensable for an understanding of the short-short TTF pairs are weak provides support for our conductivity behavior. qualitative analysis. The fact that interaction 11, associated with an S-S distance longer than that of interaction (9)Hoffmann, R. J. Chem. Phys. 1963,39,1397.A modified Wolfsberg-Helmholz formula was used to calculate the off-diagonal H,,,” I11and very similar to that of interaction V, is the stronger elements: Ammeter, J. H.; Biirgi, H.-B.; Thibeault, J.; Hoffmann, R. J. one shows the importance of the relative orientation of Am. Chem.SOC.1978,100,3686.Both single-{and double-{type orbitals the two sulfur p orbitals. Since interaction V is about 4 for C, S, and Te were used. The parameters and single-{exponents were times smaller than 11, we can conclude that only intertaken from previous work (Canadell, E.; Rachidi, I. E.-I.; Ravy, S.;Pouget, properties of any charge transfer compounds exhibiting the described structural distortions. The energy of the TTF HOMO (11) is very sensitive to the central carbon-carbon double-bond length. This orbital is destabilized when the bond length increases. Thus, in compounds like I, the HOMO- will originate from the TTF with the longer central C=C bond. Since the interaction through the bridging group is small, the HOMO+ and HOMO- orbitals will be strongly localized on the TTFs with short and long central C=C bonds, respectively. Thus, the partially filled band in metallic charge-transfer salts of I and the highest occupied band in semiconducting Ia will be built primarily from the HOMO of the TTF with the longer C=C central bond and consequently, the X-X (X = S or Te) contacts associated with this TTF largely determine the conducting properties of the system. Bending of the external ends of TTF (or BEDT-TTF) while keeping planar the central SzC=CS2 core (IV) is
J. P.; Brossard, L.; Legros, J. P. J. Phys. Fr. 1989,50,2967. Canadell, E.; Mathey, Y.; Whangbo,M.-H., J. Am. Chem. SOC.1988,110,104).The double-{type orbitals were taken from: Clementi, E.; Roetti, C. At. Nucl. Data Tables 1974,14, 177. (10) Whangbo, M.-H.; Hoffmann, R. J. Am. Chem. SOC.1978,100, 6093. (11) Dolbecq, A.; Batail, P.; Lerstrup, K.; Bechaaard, K.: Canadell, E., to be published; (12) FourmiguB, M.; Batail, P. Bull. SOC. Chim. Fr. 1992,129, 29.
(13) Based on the van der Waals radii of 1.85A for S and 2.00A for Te with tolerance of 0.10 and 0.15A respectively for the two types of interactions. Pauling, L. The Nature of the Chemical Bond, 3rd ed.; Cornell University Press: Ithaca, NY, 1990;p 260. (14) Williams, J. M.; Wang, H. H.; Emge, T. J.; Geiser, U.; Beno, M. A.;Leung,P. C. W.;Carlson,K. D.;Thorn,R. J.;Schultz,A. J.; Whangbo, M.-H. Prog. Inorg. Chem. 1987,35, 51.
Donors with Two TTF Moieties Linked By Te Bridges actions I1 and I11play a significant role in determining the conductivity of Ia. Because both I1 and I11 lead to delocalization along the b and c directions, we conclude that the system should be a 2D semiconductor. This analysis is substantiated by our band structure calculations which show that the highest occupied band of Ia is dispersive along both b and c, with a total width of 0.25 eV while the lowest empty band in very flat. Unfortunately, we were not able to obtain information concerning the conductivity of this material. C . Molecular Interactions in the Crystal Structure of TTFzTez (Ib). The crystal structure of Ib contains four different types of intermolecular interactions with S - 4 and Te.4 contacts smaller than 3.8 and 4.0 A, respectively (see Table I1 and Figure 2). Interactions I and I1 are associated with S.43 contacts, interaction I11 with Te-S and S - 4 contacts and interactions IV and V involve only Te.43 contacts. The T e S contacts are notably shorter than in the Ia crystal structure. However, our extended Huckel calculations for Ib show that again both the HOMO+ and HOMO- orbitals contain a negligible contribution from tellurium (lessthan 0.5 % ). In consequence, only the four different S-S contacts of interactions I, 11, and I11 need to be considered. Interactions I and I1 lead to delocalization along the a and c directions and interaction I11 along the c direction. Again, although there is a 3D network of Te-S and S . 4 interactions, the system can be a 2D conductor at most. As mentioned, one of the two TTF moieties of the unique Ib molecule is noticeably bent. Thus, according to our qualitative analysis the highest occupied band should originate from the HOMOSof the planar TTFs and hence the most significant S-S interactions are those associated with pairs of planar TTFs. Inspection of Table I1 reveals that there are no short Sa-S contacts of this type. This would lead to the conclusion that the highest occupied band of Ib should be completely flat. As noted above, one has to be careful in such situations. Here, the splitting between the HOMO+ and HOMO- orbitals is smaller than in the case of TTFzTe (0.04 eV instead of 0.08 according to the single-t calculations and 0.08 instead of 0.10 according to the double-t calculations). Accordingly, the localization of the HOMO+ and HOMO- in one of the two TTF fragments is not as strong as in TTFzTe although localization in the predicted sense is observed (i.e., approximately 67 % contribution from the planadbent T T F fragments in the HOMO-/HOMO+). This suggests that the interactions between sulfurs of bent and planar TTFs can confer some dispersion to the highest occupied band. In addition we note that interaction I11is associated with S 4 interactions between pairs of bent TTFs which, if they are strong enough, can lead to mixing of the two HOMO bands. Actual computation of the Pi,j interaction energies14 (i,j = HOMO+ and HOMO-) for I, 11, and I11 show that except for the HOMO+-HOMO+ interaction associated with bent-bent TTF pairs of I11 (0.097 eV according to our double-t calculations) all other interactions are small. The S-S contacts of interaction I11 lead only to delocalization along the c direction and consequently Ib should exhibit 1D conductivity. In addition, model calculations for independent chains of the bent andplanar T T F molecules in the crystal structure of Ib show that the HOMO band of the chain of planar molecules is nondispersive, while that of the chain of bent molecules has a width of 0.23 eV. The top part of the latter is found to overlap in energy the flat band of the planar molecules.
Chem. Mater., Vol. 5, No. 9, 1993 1203 From the above results we expect that the highest occupied band for the crystal structure of Ib will result from the mixing of these two types of bands and thus have a modest dispersion along the c direction. In agreement with this analysis, our calculations for the 3D structure of Ib show that the highest occupied band has mixed bent-planar TTF character and is only dispersive along the c direction with a bandwidth of 0.13 eV, smaller than in the case of Ia. In summary, our study suggests that Ib should be a semiconductor with a greater anisotropy but lower conductivity than Ia. Indeed, the four-probe conductivity measurement on a singlecrystal of Ib affords a conductivity of 5 X Q-l cm-l. We also examined the flexibility of a model Ib molecule with planar TTFs as a function of 8 (see V). The lowest -3Te $1
-3Te
I
T
$2
V
energy was found for values of 8 between 90° and 120°, which is in good agreement with the X-ray results and previous descriptions of the gauche effect.15 Although we did not cany out a complete study, when the motion around this angle is coupled with those around 41 and 42, a quite flat surface results except for small values of 0. Hence Ib seems to be structurally quite adaptable to variations in the crystal packing. Thus the study of charge-transfer salts of this donor seems to be quite promising.
Concluding Remarks We have studied the electronic structure of two molecules where two classical donor moieties (i.e., TTF) are linked through a connector. Charge-transfer salts of this type of molecules are likely to have crystal structures with two chemically equivalent but structurally inequivalent donor components. Our study suggeststhat this structural inequivalence can lead to an important localization of the HOMO in one of the two moieties. Although this is not expected to have dramatic consequences for neutral molecular solids where the structural inequivalence can only be relatively small, it can have very important consequences for charge transfer salts of these molecules. In these salts, some of the chalcogen-chalcogen contacts in the crystal structure will play a role in determining the structure but not the transport properties. The qualitative ideas reported here should help providing guidelines for structure-property correlations in charge-transfer salts of this new class of donors.
Acknowledgment. The stay of James D. Martin at Orsay was supported by a postdoctoral grant from the National Science Foundation of USA (INT-9007963).Part of this work was supported by the Israel-Germany Cooperative Science program administered by the Israel Council for Research and Development, and the United States Army Research, Development and Standarization Group (UK). (15) Wolfe,
S.Acc. Chem. Res. 1972, 5, 102.
