Synthesis and intramolecular charge-transfer properties of new tetrathiafulvalene-σ-tetracyanoanthraquinodimethane diad (TTF-σ-TCNAQ) and triad (TTF-σ-TCNAQ-σ-TTF) molecules

We report the use of functionalised electron acceptor tetracyanoanthraquinodimethane (TCNAQ) units in the synthesis of novel diad D-sigma-A compounds 6 and 7 [D = tetrathiafulvalenyl (TTF) and ferrocenyl] and the triad TTF-sigma-TCNAQ-sigma-TTF assembly 8. Compounds 6-8 display a very weak, broad, low-energy intramolecular charge-transfer band in the UV-VIS spectra. Nanosecond laser flash photolysis of compound 6 did not lead to any new transient absorptions in the 300-800 nm region, suggesting that if a charge-separated species is formed upon excitation, then back electron transfer occurs very rapidly to regenerate the ground state. Cyclic voltammetry of compounds 6-8 shows that reversible oxidation processes occur for the TTF and ferrocene moieties, and a reversible two-electron reduction occurs for the TCNAQ moiety. Spectroelectrochemical studies on compound 6 have enabled the redox processes to be assigned to the sequential formation of the TTF radical cation and dication upon oxidation, and the TCNAQ dianion upon reduction. Simultaneous electrochemistry and EPR (SEEPR) experiments provide further evidence for intramolecular interaction between the TTF and TCNAQ moieties in compound 6. Quantum mechanical calculations on compound 6, performed by the AM1 method, predict that in its minimum energy conformation the TTF and TCNAQ moieties are approximately orthogonal to one another, with the TCNAQ unit folded into a butterfly conformation.