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

Abstract

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 ¹A_g*⁻ ππ* state is observed at 27 900±20 cm⁻¹, 130 cm⁻¹ below the system origin of the strongly allowed ¹B_u*⁺ state. The vibronic analysis and the Franck–Condon intensity distribution indicate considerable bond order reversal in the ¹A_g*⁻ 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 ¹B_u*⁺ state is demonstrated to adequately describe the two‐photon absorptivity and polarization properties of the lowest‐lying ¹A_g*⁻ state. The remarkably short intrinsic lifetime of fluorescence is shown to be associated with strong vibronic coupling of the ¹A_g*⁻ state with the nearby ¹B_u*⁺ state.