Theoretical study of the force field of the lowest singlet electronic states of long polyenes

The force field of all-trans decapentaene, dodecahexaene, tetradecaheptaene, and hexadecaoctaene have been analyzed in the diabatic and adiabatic representations for the 1A−g and 2A−g states by an extended Pariser–Parr–Pople Hamiltonian. The adiabatic frequencies have been computed and found to agree with the existing experimental frequencies. In particular, the calculations reproduce well the observed increase of the frequency of the Franck–Condon active ag C=C stretch upon excitation to the 2A−g state. This mode has been found to be increasingly localized on the CC oscillators of the central moiety as the polyenic chain gets longer. This, together with the alternate signs of the adjacent oscillators, is the reason of its Franck–Condon activity. The effects of electronic excitation and vibronic interactions on the force constant matrix are discussed.