Exciplex emission from the mixed dimer of naphthalene and 2-cyanonaphthalene in a supersonic jet

The photophysical behavior for the S₁←S₀ transition of the van der Waals mixed dimer of naphthalene and 2-cyanonaphthalene has been investigated by use of laser-induced fluorescence spectroscopy in a supersonic free jet expansion. The structure of the dimer in the ground state has been computed by electrostatic and ab initio quantum chemical calculations. Our experimental results show that unlike the behavior of the molecules in nonpolar solutions, the jet-cooled dimer emits exciplex fluorescence upon excitation to the S₁ state. Charge transfer interactions between the naphthalene donor and the cyanonaphthalene acceptor and orbital overlap between the nearly identical π-systems of the two molecules are considered to be the main factors for the stability of the exciplex in the jet.