Heterocyclic fused TCNQ acceptors

The other approach used to form more planar n-extended acceptor molecules involves the use of heterocyclic systems. Despite the steric crowding introduced by ring fusion, it has been shown experimentally that planar molecules are formed when pentagonal heterocyclic systems bearing no peri hydrogen atoms are fused to the TCNQ ring. 

We will briefly show some relevant examples from the literature together with other TCNQ derivatives containing fused heterocyclic rings prepared by our group. 

Although the synthesis of thiophene-fused TCNQs was first reported in 1986 by Kobayashi and Gajurel [113], later in 1992, in a series of four interesting papers, the preparation and physico-chemical properties of an isomeric series of thiophene-fused TCNQs [114] was reported together with their crystal and molecular structures [115], the crystal structures of CT complexes with the strong donor TTF [116], and also the crystal structure of a 2 3 CT salt with tetraethylammonium [117]. 

The syntheses were carried out from the corresponding heterocyclic quinones by treatment with the Lehnert s reagent by following Hunig s procedure reported for the carbocyclic TCAQ [21]. The isomeric dithiophene analogues of TCAQ were thus obtained in moderate yields (15-37%) [114], 

The cyclic voltammetry data of this series of thiophene-fused TCNQs are collected in Table 1.7 and show the presence of two single-electron reduction waves to the radical anion and dianion. This behaviour is in sharp contrast with that of the dibenzo-fused TCNQ (TCAQ) which displayed only one wave involving two electrons (see Table 1.5). This finding suggests that the steric interactions caused by the peri 

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Another interesting type of heterocyclic-fused TCNQ acceptors were the thiadiazole-fused TCNQ derivatives 49 and 50 reported by Yamashita and co-workers who described TCNQ acceptors bearing two thiadiazole units fused to the central TCNQ ring (49) [119] or only one (50) [120]. 

These results can be generalized in the sense that they allow a correlation of the acceptor ability of fused TCNQ derivatives with Ti-deficient or Ti-excessive character of the heterocyclic rings. Thus, the reduction potentials for a number of heterocyclic fused TCNQ derivatives [128] reveals a pattern in which the higher reduction potentials belong to systems with Ti-deficient heterocycles (see Table 1.9). 

In agreement with previous studies [107,108], recently the use of ab initio molecular orbital (mo) calculations have been used to account for the distortion from planarity of some derivatives of TCNQ and, particularly, those containing heterocycles fused to the TCNQ ring. Additionally, MO methods have also been used to predict the reduction potential of novel TCNQ derivatives and the results suggest that it is possible to extend the 7r-system of TCNQ without weakening the electron acceptor ability [129]. 

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