Extended TCNQ derivatives

With the only exception of BTCNQ, which was prepared in a multistep procedure, the synthesis of the above laterally Ti-extended TCNQ derivatives has been... 

The geometry of TCAQs have also been optimized using the PM3 semiempirical method, and also at the more accurate 6-3IG ab initio level to test the reliability of the PM3 results [108]. The obtained values were almost identical, thus confirming the validity of the PM3 method to deal with extended TCNQ derivatives where the steric interactions determine the geometry and planarity or non-planarity of the molecule. 

The non-planarity of the anions justifies their low stability and explains the coalescence of the first and second reduction potentials for tetrasubstituted 1,4-TCAQs, since the first reduction potential would appear at more negative values while the second reduction potential would remain unaffected. This coalescence has also been observed for 9,10-TCAQ [21,100,101] and other 7r-extended TCNQ derivatives (see below)... 

Extended TCNQ derivatives as acceptors in composite films with photoinduced electron transfer... 

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]. 

Five aspects of the preparation of solids can be distinguished (i) preparation of a series of compounds in order to investigate a specific property, as exemplified by a series of perovskite oxides to examine their electrical properties or by a series of spinel ferrites to screen their magnetic properties (ii) preparation of unknown members of a structurally related class of solids to extend (or extrapolate) structure-property relations, as exemplified by the synthesis of layered chalcogenides and their intercalates or derivatives of TTF-TCNQ to study their superconductivity (iii) synthesis of a new class of compounds (e.g. sialons, (Si, Al)3(0, N)4, or doped polyacetylenes), with novel structural properties (iv) preparation of known solids of prescribed specifications (crystallinity, shape, purity, etc.) as in the case of crystals of Si, III-V compounds and... 

Charge-transfer complexes of variously functionalized, both electron-rich and electron-poor, [3]radialenes have attracted attention because of their magnetic properties [7] and as potential organic metals [17, 18]. Most of the [3]radialenes reported so far have been studied with respect to their oxidation and reduction chemistry (see the reviews [7, 8] and some examples mentioned in this chapter). With appropriate substitution patterns, the range of redox properties extends from the most electron-rich [3]radialene 31 to the very electron-poor radialene 24a, a strong oxidant [21]. The first one-electron reduction step of 18 (X = Y = CN) [16], 19, and 20 [17] occurs at potentials similar to TCNQ, and at potentials similar to chloranil for other derivatives of 18 (Scheme 4.3). 

Based on the TCNQ and DCNQI molecules, during the last years a variety of novel acceptors have been reported. We will present some of the most significant modifications carried out on the quinoid skeleton and will discuss separately the rr-extended systems. N,7,7-tricyanoquinomethanimines (20) (Scheme 1.8), which can be considered as hybrids of the TCNQ and DCNQI systems, were reported by Bryce and co-workers in 1989 [45], These new electron acceptors were prepared from the corresponding quinones by reaction with malononitrile in the presence of titanium tetrachloride and pyridine (Lehnert s reagent) to form the dicyano-quinomethides (18), which on reaction with BTC afforded the hybrid tricyano derivatives (20). 

The CV measurements in acetonitrile at room temperature of these TCNQ r-extended sulphur containing derivatives show only a two-electron singlewave reduction to the dianion (—0.18 eV V5. SCE) with a separation of their cathodic and anodic peaks of less than 60 mV. The number of electrons transferred was confirmed by controlled potential coulometric analysis. This result is in agreement with that previously reported for 11,1 l,12,12-tetracyano-9,10-anth aquinodimethane (TCAQ), which showed a two-electron single-wave reduction to the dianion [21,101]. Substitution on the molecule allows the modulation of the acceptor ability. Thus, the presence of two electron-withdrawing chlorine atoms (54b) significantly decreases the reduction potential (-0.10 V vs. SCE). 

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