Theory of Vibrational Spectra of Excited Quadrupolar Molecules with Broken Symmetry

Quadrupolar molecules of the structure AL–D–AR, where D is an electron donor and AL and AR are identical acceptors, are investigated. A general theory of vibrational spectra of such molecules undergoing symmetry breaking after photoexcitation is developed. The approach is based on the expansion of the electron–vibrational interaction in a series in powers of the displacements of the vibrational modes and the dissymmetry parameter. The theory provides a possibility to calculate the intensities of the IR bands and the splitting of the frequencies of local vibrations associated with vibrations of small groups of atoms symmetrically located in the left and right arms of the molecule. The theory explains the regularities of influence of symmetry breaking on the IR band intensities and the splitting of the frequencies observed recently using time-resolved infrared spectroscopy.