Geometry and torsional motion of biphenyl in the ground and first excited singlet state

Abstract

The lowest excited singlet state of biphenyl (BP) and a number of its isotopically and chemically substituted analogs have been studied by supersonic jet laser spectroscopy. The symmetry species of this excited state in BP can be classified as B2u+ in the G16 extended molecular symmetry group G16 (EM). The symmetry-allowed origin of the biphenyl-h10 S1←S0 electronic transition occurs at 35 268 cm-1. The frequency of the torsional motion in S1 is determined to be ∼65 cm-1. The potential parameters for this motion in S1 are V2 = 1195 cm -1, V4 = -190 cm-1, and V6 = 18 cm-1. The torsional motion for the ground state (∼50 cm -1) can be described by V2 = 50 cm-1 and V 4 = -148 cm-1. The change in the dihedral angle between the two benzene rings in BP upon S0 to S1 excitation is determined to be ∼44° based on a Franck-Condon factor calculation. Several fundamentals of the molecular vibrations are assigned in the S 1 state. The exciton interaction between the coupled benzene rings in biphenyl is suggested to be large (> 103 cm-1). © 1988 American Institute of Physics.