Parametric method in the theory of vibronic spectra of complex molecules. Excited state structure and fluorescence spectrum of stilbene

The parameters of the adiabatic model of the stilbene molecule in the excited state and the vibrational structure of the fluorescence spectrum are calculated by the parametric method of the theory of vibronic spectra of complex molecules. The first and second orders of theory are used. The molecular models are constructed by the fragment method. Satisfactory agreement with experiment is obtained. The parameters of molecular fragments are shown to be highly transferable. The model and the spectrum may be refined in the second order of theory using only one additional parameter for CCC angles (optimal values were obtained). The parametric method under discussion makes it possible to predict variations of structural parameters due to excitation and to calculate the vibrational structure of the electronic spectra of complex molecules, including characteristic variations of the spectra in series of related molecules (stilbene-di-phenylpolyenes). This method surpasses all previous procedures (in particular, the correlation method) in completeness and accuracy of results