Spectroscopic and dynamical studies of the S, and S, states of decatetraene in supersonic molecular beams

Fluorescence and fluorescence excitation spectra of all-trans-2,4,6,8-decatetraene have been obtained in free jets and in inert-gas clusters. In isolated decatetraene, excitation into 1 ‘B, (S, ) results in emission from both S, ( 1 ‘B, + 1 ‘A, ) and S, (2 ‘A, --) 1 ‘A, ) on time scales that are faster than the 10 ns experimental resolution. In clusters, rapid electronic and vibrational relaxation leads to long-lived (360 ns) emission from thermally relaxed levels of S, . Direct excitation of low-lying, S, vibronic levels in cold, isolated molecules also results in longlived S’ -S, fluorescence, as expected for this symmetry-forbidden transition. The detection of S, emission in free decatetraene has permitted the first detailed study of the vibronic structure and kinetics of the 2 ‘A, state of an isolated, all-trans linear polyene. The S, +S, fluorescence excitation spectrum is rich in low-frequency vibronic progressions. Analysis of this spectrum suggests that the transition not only is made allowed by vibronic coupling involving lowfrequency 6, skeletal modes ( Herzberg-Teller coupling), as for polyenes in condensed phases, but also gains intensity from interactions between the electronic motion and the hindered rotations (torsions) of the terminal methyl groups. Preliminary analysis suggests that the barriers to internal rotation of the methyl groups must be substantially reduced in the 2 ‘A, (S, ) state. For isolated decatetraene, the 2 ‘A, fluorescence lifetimes show a monotonic decrease with increasing vibrational energy, presumably due to increased mixing with the 1 ‘B, state.