Effect of ring torsion on intramolecular vibrational redistribution dynamics of 1,1′-binaphthyl and 2,2′-binaphthyl

Abstract

The role of ring torsion in the enhancement of intramolecular vibrational energy redistribution (IVR) in aromatic molecules was investigated by conducting excitation and dispersed fluorescence spectroscopy of 1,1′-binaphthyl (1,1′-BN) and 2,2′-BN. The dispersed fluorescence spectra of 1,1′-BN in the origin region of S₁–S₀ were well resolved, which presented 25–27 cm⁻¹ gaps of torsional mode in the ground state. The overall profile of the dispersed spectra of 1,1′-BN is similar to that of naphthalene. In contrast, the spectra of 2,2′-BN were not resolved due to the multitude of the active torsional modes. In both cases, dissipative IVR was observed to take place with a relatively small excess vibrational energy: 237.5 cm⁻¹ for 1,1′-BN and 658 cm⁻¹ for 2,2′-BN, which clearly shows that ring torsion efficiently enhances the IVR rate. Ab initio and density functional theory calculations with medium-sized basis sets showed that the torsional potential of 1,1′-BN has a very flat minimum over the range of torsional angles from ca. 60° to 120°, whereas that of 2,2′-BN showed two well-defined potential minima at ca. 40° and 140°, in resemblance to the case of biphenyl. In this work, we propose that aromatic molecules be classified into “strong” and “weak” torsional hindrance cases: molecules with strong hindrance case show shorter torsional progressions and more effective IVR dynamics than do those with weak hindrance.