Abstract
It is shown that unusual frequency differences of the totally symmetric C=C bond stretching vibration in the S1(2Ag) state between 2,12-dimethyltridecahexaene and two of its derivatives N-(2,4,6,8,10-dodecapentaenylidine) butylamine (12SB) and dodecapentaenal (12R), result from vibronic coupling between the S1(2Ag) and S2(1Bu) electronic states via totally symmetric vibrations in 12SB and 12R. The vibronic coupling matrix elements between the lower ππ* electronic states of the three molecules are evaluated by CNDO/S+CISD calculations in a floating atomic orbital basis set. The resulting adiabatic frequencies of the Franck–Condon active totally symmetric vibrations in the S0(1Ag) and S1(2Ag) electronic states are in good agreement with experiment. High-resolution fluorescence and fluorescence excitation spectra of 12SB are successfully interpreted in terms of the vibronic coupling between the S1(2Ag) and S2(1Bu) electronic states via the totally symmetric C=C and C–C bond stretching vibrations. The analysis of these spectra provides values of the displacement parameters for these vibrations in the S1(2Ag) and S2(1Bu) states.
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