Electronic excitation in the benzonitrile dimer: The intermolecular structure in the S0 and S1 state determined by rotationally resolved electronic spectroscopy

The rotationally resolved UV spectrum of the electronic origin of the benzonitrile dimer has been measured and analyzed using a genetic algorithm based fitting strategy. For the electronic ground state, a C2h symmetric structure is found in which the permanent dipole moments of the benzonitrile monomers are aligned anti-parallel. The orientation of the transition dipole moment could be shown to be parallel to the orientation in the monomer moiety. The distance between the two monomer moieties decreases slightly upon electronic excitation and the symmetry of the benzonitrile dimer changes from C2h in the electronic ground state to Cs in the electronically excited state. This break of symmetry is probably caused by the local excitation of only one benzonitrile moiety in the cluster due to the weak electronic coupling between the cluster moieties.