VIBRATIONAL POTENTIALS OF THE LOW-FREQUENCY OUT-OF-PLANE MOTION IN THE GROUND AND EXCITED SINGLET ELECTRONIC STATES OF 9,10-DIHYDROPHENANTHRENE

The absorption and single vibronic level fluorescence spectra of jet cooled 9, 10-dihydrophenanthrene possesses long progressions of a low-frequency vibration identified as the out-of-plane nontotally symmetric CH2-CH2 bridge deformation. The intensity distribution in the observed progressions indicate that the molecule is nonplanar in the ground and the first two excited singlet electronic states described as the plus and minus combinations of the L(b) states of the two perturbed benzene rings. The barrier height between the two equivalent nonplanar configurations, with the planar configuration being the transition state, is 2640 cm-1 in S0 and 1430 cm-1 in S1 . These values are much larger than those in 9,10-dihydroanthracene. The observed intensity distributions are reproduced well by model calculations with double minimum potentials whose parameters are in good agreement with those obtained by geometry optimization and force field and vibrational frequency calculations by the quantum chemical force field pi-electron method. The analysis yields the following values for the angle between the average planes of the two benzene rings: 18.4-degrees in S0 and 12.1-degrees in S1. It is shown that the origin of the S0 --> S2 transition lies ca. 240 cm-1 above the origin of the S0 --> S1 transition and that its doublet structure results from a near resonance vibronic interaction between the S1 and S2 states via a b mode of the calculated frequency of 221 cm-1.