Structure and dynamics of excited singlet states of isolated diphenylhexatriene

Abstract

The fluorescence spectrum, one photon fluorescence excitation spectrum, and single vibrational level decay kinetics have been measured for 1,6‐diphenyl‐1,3,5‐hexatriene seeded in helium in a supersonic free jet. The 2 ¹A_g doubly excited state (S₁) is located more than 3400 cm⁻¹ below the strongly absorbing 1 ¹B_u state (S₂) which is 29 157 cm⁻¹ above the ground state. The lowest energy observed vibronic band of S₁ is at 25 741.8 cm⁻¹; emission produced by excitation into this band has a lifetime of 90.7 ns. The vibronic development of the excitation spectrum shows that this band is a vibronically promoted false origin. Fluorescence excitation intensity for this band is smaller than that of the S₀ to S₂ fluorescence excitation origin by at least a factor of 10⁴. Direct excitation of S₂ in the isolated molcule is followed by relaxation, fast on the nanosecond time scale, to isoenergetic vibrationally excited levels of S₁. This is apparent both from the observed fluorescence spectrum and the fluorescence decay time (48.1 ns at the S₂ origin) which is more than an order of magnitude longer than the intrinsic fluorescence lifetime of S₂.