Electron acceptors of the fluorene series. Part 8. Electrochemical and intramolecular charge transfer studies of thiophene functionalised fluorenes

The degree of intramolecular charge transfer (ICT) in 9-(1,3-dithiol-2-ylidene)fluorenes bearing fused thiophene and 2,5-dihydrothiophene units (2 and 16, respectively), has been investigated by UV-VIS spectroscopy. Both series of compounds show a good correlation between the ICT energies and Sigma sigma(p)(-) values of the corresponding molecules. The sensitivity parameters rho(ICT)(-) are 0.18 eV for 2 and 0.16 eV for 16. Cyclic voltammetry of both series, 2 and 16, reveals a single irreversible oxidation process in all compounds. However, electropolymerisation under oxidative conditions does not take place, since the radical cation intermediate is most probably derived from the electroactivity of the 1,3-dithiole-fluorene fragment. In parallel to ICT, the values of E-1red(1/2) and E-2red(1/2) in relevant compounds also show a linear relationship with sigma(p)(-) constants in the fluorene ring. The sensitivity parameters for series 2 are 0.20 V (rho(1red)(-)) and 0.17 V (rho(2red)(-)), whilst those for 16 are 0.26 V (rho(1red)(-)) and 0.18 V (rho(2red)(-)).