One- and two-photon fluorescence excitation spectra of the 2 1A g states of linear tetraenes in free jet expansions

Abstract

One- and two-photon fluorescence excitation spectra of the S1←S0 transitions of the all-trans isomers of 1,3,5,7-octatetraene, 1,3,5,7-nonatetraene, and 2,4,6,8-decatetraene have been obtained in free jet expansions. Comparison of the one- and two-photon spectra allows the unambiguous identification of electronic and vibronic origins and, for octatetraene and decatetraene, provides clear evidence for molecular inversion symmetry. One-photon spectra show ag progressions built on Herzberg–Teller, bu promoting modes, while two-photon spectra are built on progressions of ag modes starting from the 2 1Ag←1 1Ag electronic origins. In nonatetraene, the absence of inversion symmetry results in an allowed electronic origin in both the one- and two-photon spectra. Nevertheless, bands built on vibronic origins dominate the one-photon spectrum. The S1←S0 spectra of nonatetraene and decatetraene exhibit characteristic splittings of vibronic bands that can be quantitatively explained by the tunneling of the methyl groups through low energy, torsional barriers in the S1 states. Couplings between methyl torsions and low frequency skeletal modes further complicate the optical spectra. Fluorescence lifetimes indicate abrupt onsets of nonradiative decay processes (tentatively attributed to trans→cis isomerization) at ∼2100 cm−1 excess energy. Systematic differences in the energy dependencies of S1 nonradiative decays in the three polyenes can be explained by the higher densities of vibronic states in the methyl-substituted compounds.