RESONANCE RAMAN AND ABSORPTION-SPECTROSCOPY OF THE LOWEST TRIPLET-STATE OF 1,3,5-HEXATRIENE AND DEUTERIATED DERIVATIVES AT 183-K - MOLECULAR-STRUCTURE IN THE T1 STATE

Time-resolved resonance Raman (RR) spectra of the lowest excited triplet state T1 of (E)-1,3,5-hexatriene, (E)-3-deuterio-1,3,5-hexatriene, and (E)-1,1-dideuterio-1,3,5-hexatriene are obtained at 183 K. The T1 potential energy surface (PES), determining energy minima, equilibrium geometries, frequencies, and normal modes of vibration, is calculated, and RR intensities are obtained on the basis of the Franck-Condon mechanism. By comparison with the calculated spectra, the observed RR spectra are assigned to an equilibrium mixture of planar E and Z species in the T1 state. The relative intensity of the E and Z bands at temperatures between 293 and 183 K gives an upper limit of 0.22 kcal/mol for the energy difference between the planar E and Z forms. Time-resolved triplet-triplet absorption measurements as a function of temperature between 296 and 183 K yield a frequency factor A of 1.6 x 10(7) s-1 and an apparent activation energy E(a) of 0.7 kcal/mol for the decay of the T1 state. Consequently, an upper limit of 0.7 kcal/mol above the planar E and Z geometries is established for the possible local energy minimum at the twisted geometry. On the basis of these data, a PES is proposed for the T1 state of 1,3,5-hexatriene with respect to torsion around the central CC bond and the C2C3 and C4C5 bonds.