The 2 1A g state of trans,trans-1,3,5,7-octatetraene in free jet expansions

Abstract

One- and two-photon fluorescence excitation and emission spectra of the S1↔S0 transition of trans,trans-1,3,5,7-octatetraene have been measured for the first time in free jet expansions. The one-photon excitation spectrum is the same, with the exception of significant differences in the intensities of a few lines, as the two-color, resonance-enhanced, two-photon ionization spectrum, previously assigned to the 2 1A′←1 1A′ transition of cis,trans-1,3,5,7-octatetraene. However, comparison of the one- and two-photon fluorescence excitation spectra shows clearly that the carrier of the spectrum has inversion symmetry, as expected for trans,trans-1,3,5,7-octatetraene. The one-photon spectrum is built on bu Herzberg–Teller promoting modes, which are origins of progressions in ag modes, while the two-photon spectrum is due to a single progression in ag modes starting from the 2 1Ag←1 1Ag electronic origin. The appearance of out-of-plane vibrations, possibly including torsions of the polyene framework, suggests large differences in force constants and perhaps in the geometries of the 2 1Ag and 1 1Ag potential surfaces. For 2 1Ag vibronic levels with energies ≤1000 cm−1, the fluorescence lifetimes vary between 170 and 450 ns due to the dependence of radiative and nonradiative decay rates on the vibronic state. An abrupt increase in the nonradiative decay rates at ∼2100 cm−1 excess energy is tentatively ascribed to trans→cis isomerization. This work demonstrates that the one- and two-photon cross sections of the 2 1Ag←1 1Ag transitions of all-trans linear polyenes are sufficiently large to allow the study of 2 1Ag states under isolated, unperturbed conditions.

Vibronic activity in trans,trans-1,3,5,7 octatetraene: The S0→S1 spectrum

“…The onset of the spectrum is therefore a false-origin band due to the lowest frequency b u mode. 9 This band is the most prominent one in the spectrum and is followed, at higher energies, by other bands of b u symmetry. Upon this set of false origins, progressions of totally symmetric modes are built.…”

“…With this assumption, the calculated relative intensities are shown in Table II where they are compared with the experimental results reported in Ref. 9. In this table both the experimental and the calculated data are given as relative intensities with respect to the 48 false origin.…”

“…By inspection of Table I in Ref. 9 whose results are summarized in the second last column of Table II of the present work, one can identify three regimes of lifetimes. To the first regime, which extends up to slightly above 500 cm Ϫ1 from the true origin ͑observed in two-photon excitation experiments 9 ͒, belong the first three false origins, i.e., 48 successful calculations are sets 4 and 8: a result that, in a sense, might have been expected since these calculations are the largest.…”

“…If one accounts for the 76.2 cm Ϫ1 shift of the true origin with respect to the 48 band, the fundamentals of 42 and 41 should fall between 1100 and 1200 cm Ϫ1 . 9 In this region, no bands were assigned in Table I of Ref. 9, but inspection of Fig.…”

“…Spectroscopic measurements in supersonic jet expansions have sufficient resolution to allow detection and accurate measurement of these false origins. 9 They also enable one to disentangle part of the effects that usually operate in the excited state dynamics by exciting directly into the 2 1 A g manifold and thereby avoiding excitation to the 1 1 B u state.…”