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Time-resolved resonance Raman spectra are reported for the lowest excited triplet state of stilbene and three of its isotopomers. The spectra were obtained using a two-laser pump-and-probe arrangement under various experimental conditions. The spectrum of trans-stilbene after direct excitation in a glassy medium at low temperature (glycerol at 203 K) is compared with that of cis-stilbene under sensitized excitation in solution at room and low temperature. The dependence of resonance Raman spectra on excitation wavelength in both cases is investigated. The observed spectra and isotopic shifts are discussed and interpreted on the basis of quantum chemical molecular orbital calculations. Optimized geometries and vibrational frequencies in the T1 state are calculated by a semiempirical QCFF/PI Hamiltonian and by means of ROHF ab initio methods using the 6-31G basis set. TI - T, transition energies and moments are calculated using QCFF/PI and CNDO/S methods, and the triplet-triplet transition responsible for the observed T1 - T, absorption and resonance Raman spectra is identified as theT1 -Tlo transition. Corresponding resonance Raman intensities arecalculated by QCFF/PI. It is concluded that trans- and cis-stilbene adopt a common equilibrium geometry in the TI state, with the ethylenic C=C bond of the ground state being weakened to a bond with essentially single-bond character in the T1 state. The observed spectra in the glass are assigned to a planar geometry, implying that a relative minimum is found at a planar trans geometry (C2h point group) on the potential energy surface of the T1 state. A number of observed non-totally symmetric vibrational modes are tentatively assigned to combinations of either a,, or b, modes, deriving their intensities from relatively large frequency changes upon T1 - Tlo excitation.

Molecular Structure of Stilbene in the T1 State. Transient Resonance Raman Spectra of Stilbene Isotopomers and Quantum Chemical Calculations