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

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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 B+2u 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 V4=−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 S1 state. The exciton interaction between the coupled benzene rings in biphenyl is suggested to be large (>103 cm−1).