Aromatic versus polyenic linkers in push–pull chromophores: electron–phonon coupling effects

The linear absorption spectra and absolute resonance Raman excitation profiles of [(2E,4E,6E,8E)-9-[4-(dibutylamino)phenyl]-2,4,6,8-nonatetraenylidene]propanedinitrile, an electron donor–acceptor push–pull chromophore with a polyenic inker group, have been measured in cyclohexane and acetonitrile solution. Numerical simulation of the spectra yields solvent reorganization energies and the excited-state geometry changes along the strongly Raman-active vibrations. The ground-state vibrational frequencies and excited-state reorganization energies are considered within the context of two-state valence bond models. Comparison of the results for this chromophore with p-nitroaniline, which has a purely aromatic linker group, suggests that the two-state model is more applicable to molecules with polyenic rather than aromatic linkers.