Rotationally resolved electronic spectra of trans,trans-octatetraene and its derivatives

Described herein are the rotationally resolved one-photon fluorescence exci tation spectra of several vibronic bands in the S-1 <-- S-0 electronic tran sitions of three linear polyenes in the gas phase, 1,3,5,7-octatetraene (OT ), I,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 t o a and the low frequency of an in-plane bending mode (v(48)), by Coriolis coupling in the S-1 state of NT, involving v(48) and a nearby methyl torsio nal 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 eigh t 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,t rans isomers. The important role of v(48) as a promoting mode for S-1-S-2 v ibronic coupling, the source of the S-1 <-- S-0 oscillator strength, is con firmed. Finally, the measured differences in the rotational constants of th e S-0 and S-1 states (e.g., Delta A = 2532, Delta B = - 11.7, and Delta 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.