A New Type of π-Electron Donors with One Dithiole Unit: Substituted 7-(1,3-Dithiol-2-ylidene)-7-hydrobenz[d,e]anthracenes

Abstract

New electron donors 11a–c bearing only one 1,3-dithiole ring conjugated with an acene backbone have been synthesized by Wittig–Horner reactions from 7H-benz[d,e]anthracen-7-one (10) and differently substituted phosphonate esters 9a–c. The donor properties of 11 have been characterized by means of both experimental techniques and quantum chemical calculations. Cyclic voltammetry measurements on the novel compounds predict donor abilities comparable to those of TTF and BEDT-TTF. The voltammograms show a multi-stage redox behaviour with oxidation up to the trication, where the first two oxidation processes take place at very close potentials. These processes have been assigned to a simultaneous oxidation of the dithiole and acene moieties on the basis of DFT/B3-P86/6-31G* theoretical calculations. Whereas the neutral compound and the monocation exhibit butterfly-shaped non-planar structures, the dication is fully aromatic and consists of a closed-shell, singly charged, planar polyacenic unit and a singly charged dithiole ring. The third redox process can be assigned to a second oxidation of the acene unit. The high aromaticity of the dication, coupled with the particular donor ability of the acene unit are shown to be the key factors in accounting for the electrochemical behaviour and the enhanced donor properties of the novel compounds. Theoretical calculations also help to rationalize the UV/Vis data in that they predict the appearance of a low-energy, intramolecular charge-transfer absorption band for the neutral compounds. Stable charge-transfer complexes with a 2:3 (D/A) stoichiometry have been prepared by reaction with the strong acceptors TCNQF4 and DDQ.