Rotationally Resolved Electronic Spectra of trans,trans-Octatetraene and Its Derivatives

Described herein are the rotationally resolved one-photon fluorescence excitation spectra of several vibronic bands in the S 1 ← S 0 electronic transitions of three linear polyenes in the gas phase, 1,3,5,7-octatetraene (OT), 1,3,5,7-nonatetraene (NT), and 2,4,6,8-decatetraene (DT). Several of the spectra are significantly perturbed by an apparent centrifugal distortion in the S 1 state of OT, owing to the high frequency of rotations parallel to a and the low frequency of an in-plane bending mode (ν 48 ), by Coriolis coupling in the S 1 state of NT, involving ν 48 and a nearby methyl torsional level, and by torsion−rotation coupling in the S 1 states of NT and DT, owing to a significant reduction in the excited-state torsional barrier(s) compared to the ground state. Nonetheless, the inertial parameters of eight different S 0 and S 1 vibronic levels have been determined, from which it is concluded that the carriers of the spectra are in all cases the trans, trans isomers. The important role of ν 48 as a promoting mode for S 1 −S 2 vibronic coupling, the source of the S 1 ← S 0 oscillator strength, is confirmed. Finally, the measured differences in the rotational constants of the S 0 and S 1 states (e.g., Δ A = 2532, Δ B = −11.7, and Δ C = −11.0 MHz for the vibronic origin of OT) provide new information about the changes in geometry that occur when the photon is absorbed.