Vibrational spectra of oligothienyl-vinylenes with donor-π-donor and donor-π-acceptor substitution patterns

The Raman spectra of a vinylene-bridged quaterthiophene with their thienyl beta-positions occupied by hexyl groups and with the terminal alpha,alpha' sites substituted with ferrocene and dicyano-vinylene groups in symmetric and asymmetric fashions have been reported on. The Raman lines have been assigned to vibrational normal modes with the help of theoretical calculations in the framework of the density functional theory. There is spectroscopic evidence that substitution with two ferrocene groups leads to a central conjugated backbone with increased conjugation. Donor/acceptor substitution however provokes the appearance of a molecular subunit around the ferrocene group characterized by the aromatic character of the thiophene rings, while in the proximity of the dicyano-vinyl group the thienyl rings acquire a pseudo-quinoidal structure. (c) 2006 Elsevier B.V. All rights reserved.