Two-photon spectroscopy of diphenylbutadiene. The nature of the lowest-lying 1A g*−ππ* state

A two-photon excitation spectrum of all-trans 1,4-diphenyl-1,3-butadiene is taken in EPA solvent glass at 77 K. The system origin of the lowest-lying 1Ag*− ππ* state is observed at 27 900±20 cm−1, 130 cm−1 below the system origin of the strongly allowed 1Bu*+ state. The vibronic analysis and the Franck–Condon intensity distribution indicate considerable bond order reversal in the 1Ag*− state localized in the polyene portion of the molecule. Configurational analysis using PPP–SCF–MO–CISD procedures predicts that the bond order reversal is primarily associated with a LUMO←HOMO double excitation. Although the bond order reversal is polyene localized, the electronic transition is calculated to be highly delocalized with 55% of the ’’optical’’ electron density on the phenyl groups. A single intermediate state approximation based on the strongly allowed 1Bu*+ state is demonstrated to adequately describe the two-photon absorptivity and polarization properties of the lowest-lying 1Ag*− state. The remarkably short intrinsic lifetime of fluorescence is shown to be associated with strong vibronic coupling of the 1Ag*− state with the nearby 1Bu*+ state.