Vibrational fingerprint of the structural tuning in push-pull organic chromophores with quinoid or proaromatic spacers

The Raman spectra of a series of push-pull molecules containing probenzenoid or quinoid spacers which are substituted with 1,3-dithiol-2-ylidene as donor and dicyano-methylene or barbituric acid as acceptors have been analyzed. The experimental spectra have been assigned and interpreted according to density functional theory calculations. Correlations between the Raman spectra of the isolated spacers and of the substituted molecules have been done. Raman bands in the 1620-1560 cm(-1) interval provide vibrational markers of the quinoid <-> aromatic structural evolution. This finding is supported by a careful inspection of geometrical parameters, namely, bond length alteration data and particular bond distances. As a result, the peak positions and relative intensities of these Raman features can be used to evaluate the benzenoid character of the spacer as a function of the donor/acceptor substitution pattern. This paper shows that Raman spectroscopy is a powerful spectroscopic tool for the analysis of the conjugational properties (i.e., intramolecular donor -> acceptor charge transfer) of new organic materials. (c) 2007 American Institute of Physics.