Jf. Pfanstiel et al., Rotationally resolved electronic spectra of trans,trans-octatetraene and its derivatives, J PHYS CH A, 103(14), 1999, pp. 2337-2347
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.