ALL-TRANS-1,4-DIPHENYL-1,3-BUTADIENE. A ROTATIONALLY RESOLVED FLUORESCENCE EXCITATION SPECTRUM OF THE ONE-PHOTON $S_{1}\leftarrow S_{0}$ OPTICAL $TRANSITION.^{1}$

$^{1}$ Work supported by NSF. $^{2}$ L. A. Heimbrook, B. E. Kohler, and T. A. Spiglanin. Proc. Natl. Acad. Sci. USA 80. 4580 (1983).Author Institution: Department of Chemistry, University of Pittsburgh; Division of Physics., NRC. Canada.We have obtained rotationally resolved spectra of several bands in the $S_{1}-S_{0}$ one-photon transitions of all-trans-1,4-diphenyl-1,$3-butadiene^{2}$ using the new high resolution spectrometer Band 1 is an a-type transition and can be fit to an asymmetric top rigid rotor Hamiltonian. The change in rotational constants observed on electronic excitation are consistent with the bond order reversal expected for a HOMO-LUMO excitation in which the orbital exhibits both polyene and phenyl character. The bands to the blue of band 1 exhibit a dependence of the rotational constants upon vibrational mode and, of greater interest, show a marked departure from rigid rotor behavior. K-dependent perturbations are observed as early as band 3: their magnitude appears to increase with K and with vibrational excitation. The perturbation appears to shift entire K subbands which may be individually fit to a rigid rotor Hamiltonian and which yield a dependence of the rotational constants and inertial defects on K. The possibility that these changes are associated with `motion' along a reaction coordinate associated with isomerization of the excited molecule will be discussed