Donor-pi-acceptor species derived from functionalised 1,3-dithiol-2-ylidene anthracene donor units exhibiting photoinduced electron transfer properties: Spectroscopic, electrochemical, X-ray crystallographic and theoretical studies

Novel single-component donor- acceptor species 7 and 8 have been synthesise d from substituted 10-(1,3-dithiol-2-ylidene)anthracen-9(10H)one 6 by react ion with Lehnert's reagent and with N,N'-bis(trimethylsilyl)carbodiimide, r espectively. The steady-state UV/visible spectra of these systems contain a solvatochromic band arising from intramolecular charge transfer between th e donor and acceptor groups Ultrafast time-resolved spectroscopy shows that the excited state formed on photolysis has a lifetime of 170-400 ps, which depends strongly on solvent polarity. Cyclic voltammetric data indicate th e presence of two active redox centres in the molecules: electrochemical ox idation and reduction form cation radicals and anion radicals, respectively . The X-ray crystal structures of compounds 6a, 7b and 11a reveal that all three molecules are severely distorted from planarity, with the central rin g of the anthracenediylidene moiety adopting a boat conformation. The molec ular structures of 6, 7 and 11 have been investigated theoretically at the ab initio 6-31G* level. Calculated geometries are in good agreement with th e X-ray data and clearly show the decrease in steric hindrance along the se ries 11>7>8>6. The calculations support the intramolecular charge-transfer nature of the lowest energy absorption band observed for 6, 7 and 8, and ex plain the origin of the redox properties for these compounds. The optimised geometries of the cation radical, anion radical and dianion of compound 7 illustrate the structural changes induced by the charging process. For the dianion, the steric interactions are alleviated by rotation of the acceptor dicyanomethylene unit, but the presence of the donor dithiole group hinder s the achievement of a fully aromatic planar structure for the central anth racene skeleton.