Other polycyano derivatives as acceptors

These A, 7,7-tricyanoquinomethanimines (20) are strong acceptors which readily form radical anions and dianions in solution as shown by cyclic voltammetry studies. The acceptor strength of the parent compound (R = = R = H) turned out to be 

Most of the novel acceptors prepared have kept the dicyanomethylene or cyanoimino terminal groups in the molecule, with only the quinoid skeleton modified. A very successful, more radical, variation leading to good conducting CT complexes was carried out with the thieno[3,2-Z7]thiophene system. Thus, the novel sulphur containing heteroquinoid electron acceptors 2,5 - bis(dicyanomethylene) -2,5- dihydrothieno[3,2 - Z ]thio-phenes(21) [46] and, more recently, 2,5-bis(cyanoimi-no)-2,5-dihydrothieno[3,2-b]thiophenes (22) (DCNTT) 

In this case the standard conditions for the preparation of DCNQIs were not satisfactory and the reaction had to be carried out in ort/io-dichlorobenzene at 60°C. These compounds showed good acceptor ability which can be tuned to within wide limits depending upon the substituents Ri and R2. 

Attempts to increase the dimensionality by means of interstack sulphur-sulphur contacts led to the synthesis of other TCNQ analogues of thiophene. Thus, the thiophene-TCNQ (23) [46] and its vinilogue (24) [48] were prepared and, more recently, the benzothiophene-TCNQ (25) and its vinilogue (26) as the first electron acceptors derived from isothianaphthene were synthesized [49]. 

The half-wave redox potentials of the novel acceptors are summarized in Table 1.2 and, in general, possess a considerably weak accepting ability compared with that of TCNQ. However, the presence of bromine atoms on the central thienoquinoid ring allows the tuning of the acceptor strength. The single-crystal X-ray analysis of benzothiophene (25) showed the interesting feature of a planar molecule. 

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