Understanding of vibrational spectra of polyconjugated molecules by means of the “effective conjugation coordinate”

Abstract

All the features of vibrational spectra of pristine and doped polyenes can be explained in a simple way in terms of the effective conjugation of the chain. The effective conjugation is affected both by the length of the molecule or of the linear segment of polymer and by conjugational perturbations introduced in the chain by charge transfer and/or photoexcitation. A uniquely defined vibration, described by the я coordinate, is affected by the variations of effective conjugation much more than any other vibration. The diagonal force constant associated to я gives a measure of the state of conjugation of the chain. This may be of great help when using the vibrational spectra for characterization of the sample.