Vibronic structure in the optical absorption spectra of phenylene vinylene oligomers: a joint experimental and theoretical study

We present a joint experimental and theoretical investigation of the optical absorption spectra of phenylene vinylene oligomers. The vibronic structure, which is clearly observed in the experimental spectra, is well described by a simple model based on the Frank-Condon approximation but requires the use of at least two effective modes coupled to the electronic states. Using a fitting procedure, we deduce the displacement of the 1Bu excited state potential energy curve relative to that of the ground state and compare it to the results of recent Hartree-Fock semi-empirical calculations. Such information is of prime importance in the description of the frequency-dependent linear as well as nonlinear optical responses.