J. Phys. Chem. 1985,89, 1478-1485
1478
Hydrogen Bonding and Properties of Organic Conductors. 2. Electronic Properties and Structure of M2(TCNQ)3and M(TCNQ)?Systemst A. D. Bandrat&,* K. Ishii,t K. D. Truong, M. Aubin, Groupe de recherches sur les semiconducteurs et dielectriques, FacultE des sciences, Universite de Sherbrooke, Sherbrooke, Quebec, Canada
and A. W. Hanson National Research Council of Canada, Atlantic Research Laboratory, Halifax, Nova Scotia, Canada B3H 321 (Received: October 8, 1984)
Conductivity data, ESR and electronic absorption measurements, and crystal structures are presented for four semiconducting complex salts of tertiary amines with 7,7,8,8-tetracyanoquinodimethane(TCNQ). The properties of these and other known complex salts are compared and rationalized. It is concluded that the conductivity of such compounds is largely determined by (1) the connectivity of the array of charged TCNQ molecules (uniform, face-to-face stacks mediate the highest conductivity, but some conductivity occurs with other arrangements) and (2) the uniformity of the charge distribution in the TCNQ array. If the TCNQ molecules are not all equivalent the charge-transfer mechanism may inhibit conductivity by selectively “pinning” all the available charge on some fraction of the TCNQ molecules.
Introduction
Tetracyanoquinodimethane (TCNQ) is a ?r-molecular acceptor which readily forms salts with many metallic and organic cations. Some of these salts are moderate to good electrical conductors, and in order to understand this behavior it is clearly important to consider the spatial arrangement of the ions. Many of the better conductors incorporate face-to-face stacks of TCNQ anions, and it is reasonable to identify these stacks as the conducting paths. However, some salts possessing this feature are insulators while others (including one described here) are moderate conductors despite the absence of any stack. Torrance’ has considered the case of salts comprising segregated stacks of T C N Q and cations. H e has shown that the critical determinant of conductivity is the degree of charge transfer from cation to anion. This is complete when the average charge on a TCNQ molecule has the appropriate commensurate value (that is, 1 for a simple salt, ’/*,2/3, etc. for a complex salt). Electron mobiiity along the TCNQ stack is constrained by Coulomb forces, and the compgmd is an insulator or a semiconductor. However, if the average charge is less than the appropriate commensurate value (while greater than zero) the constraint is removed, and metallic conduction becomes possible. In this paper we present conductivity, ESR and electronic absorption measurements, and crystal structures for four complex salts of tertiary amines with TCNQ. These compounds all result from unsuccessful attempts to synthesize an organic metal. So far as can be determined, charge transfer is complete for all of them; none is more than moderately conductive. However, some are more conductive than others, and we would like to understand why. We shall compare the properties of these and other known salts and try to account for them. We conclude that TCNQ salts may be moderately conductive if they incorporate reasonably connected arrays (which need not be stacks) of TCNQ. However, unless the T C N Q s are all equivalent, cation-anion forces may act selectively to transfer all the charge to some of the TCNQ’s (as, for example, in C S ~ ( T C N Qwhere ) ~ 2/3 of the T C N Q s bear unit charge, and 1/3 are neutral2). This “pinning” of the available charge on specific TCNQ’s limits the charge mobility, and, therefore, the conductivity. (By pinning we understand the condition in which a unit of charge is, because of strong cationanion forces, unable to leave. Unit charge might be normally resident on a TCNQ without being pinned there.) Pinning can NRCC No. 23607. Killam (Canada Council) Research Professor. *Sabbatical Visitor, Department of Chemistry, Gakushuin University, Tokyo, Japan.
* 1982-84
0022-3654/85/2089-1478$01 SO/O
occur only in structures in which the TCNQ’s are not all equivalent, that is, in most (if not all) 1:3 and 2:3 complexes, but in only a few 1:2 complexes. Synthesis
The compounds studied were prepared by following the general procedure described by Melby et aL3 Starting with purified TCNQ (recrystallized from redistilled, dried acetonitrile) one can prepare the amine salts either by direct reaction with the purified amines or by oxidation of the amine iodides with TCNQ. For tertiary amines the latter method has been known to give an iodine complex which is generally highly conducting (quasimetallic)?,5 The reaction is 3MI
+ 3TCNQ
-
(M+)3(TCNQ)32-(13)-
where M is the counterion. Although generally written as MITCNQ, the reaction product can also be described as M+(13)(M+)2(TCNQ),z-, Le., an “alloy” of the triiodide salt and the 2:3 TCNQ salt. However, we have found that for the counterions triethylenediamine and quinuclidine (see Figure 1) this procedure yielded the 2:3 TCNQ salts exclusively, with traces (
