MOLECULAR-STRUCTURE OF STILBENE IN THE T-1 STATE - TRANSIENT RESONANCE RAMAN-SPECTRA OF STILBENE ISOTOPOMERS AND QUANTUM-CHEMICAL CALCULATIONS

Time-resolved resonance Raman spectra are reported for the lowest exci ted triplet state of stilbene and three of its isotopomers. The-spectr a were obtained using a two-laser pump-and-probe arrangement under var ious experimental conditions. The spectrum of trans-stilbene after dir ect excitation in a glassy medium at low temperature (glycerol at 203 K) is compared with that of cis-stilbene under sensitized excitation i n solution at room and low temperature. The dependence of resonance Ra man spectra on excitation wavelength in both cases is investigated. Th e observed spectra and isotopic shifts are discussed and interpreted o n the basis of quantum chemical molecular orbital calculations. Optimi zed geometries and vibrational frequencies in the T-1 state are calcul ated by a semiempirical QCFF/PI Hamiltonian and by means of ROHF ab in itio methods using the 6-31G basis set. T-1 -> T-n transition energies and moments are calculated using QCFF/PI and CNDO/S methods, and the triplet-triplet transition responsible for the observed T-1 -> T-n abs orption and resonance Raman spectra is identified as the T-1 -> T-10 t ransition. Corresponding resonance Raman intensities are calculated by QCFF/PI. It is concluded that trans- and cis-stilbene adopt a common equilibrium geometry in the T-1 state, with the ethylenic C=C bond of the ground state being weakened to a bond with essentially single-bond character in the T-1 state. The observed spectra in the glass are ass igned to a planar geometry, implying that a relative minimum is found at a planar trans geometry (C-2h point group) on the potential energy surface of the T-1 state. A number of observed non-totally symmetric v ibrational modes are tentatively assigned to combinations of either a( u) or b(g) modes, deriving their intensities from relatively large fre quency changes upon T-1 -> T-10 excitation.