The Electronic Spectroscopy of 2,2‘-Binaphthyl in Solution, Cryogenic Matrix and Supersonic Jet

Abstract

The electronic spectroscopy of 2,2‘-binaphthyl near 330 nm has been investigated by absorption spectroscopy in room-temperature solution and Shpolskii matrix at 4 K and by fluorescence excitation spectroscopy in a supersonic free-jet expansion. In the solution and gas phase, where the molecules are free to form the minimum-energy conformation, the spectra are quite different from those of 1,1‘-binaphthyl and the parent naphthalene. However, when frozen in a matrix, aspects of the electronic spectrum resembled features of the spectra of these molecules. The fluorescence excitation spectrum in the free jet showed several long progressions in the torsional vibration with a characteristic, but anharmonic, frequency of about 30 cm-1. The spectrum was assigned as the 11B ← 11A transition with an origin transition, which, although it could not be observed, was estimated to be at 30 060 ± 30 cm-1. The frequency of 29 members of this progression allowed an accurate torsional potential to be calculated, which was flattened at low energy and approached harmonic at higher energy. The change in equilibrium torsional angle was estimated to be 32° upon electronic excitation, with the excited state being planar (either cis or trans). Several low-frequency vibrations were assigned in the gas phase that probably involve motions of the naphthalene structures as a whole about the central C−C bond. In the matrix, the torsional vibrations were frozen out, which allowed for the observation and tentative assignment of several higher-frequency vibrations resembling those of the bare naphthalene. The origin frequency was measured at 28 460 cm-1. The difference between the gas-phase and solid-phase frequencies was attributed to the constrained structure of the molecule in the matrix.