Spectroscopic study of conformational dynamics of 1,1′-binaphthyl in a jet

Abstract

Analysis of the fluorescence excitation spectrum of 1,1′-binaphthyl in a jet shows that in the origin region there are four transitions, in an energy range of 150 cm−1, which strongly overlap through the occurrence of strong progressions in a torsional mode. The four transitions can be described in terms of excitations to levels of an interchange-equivalent naphthalene dimer in a trans and cis configuration. The dimer splittings are 50 and 63 cm−1 for 1,1′-binaphthyl-h14 and 50 and 57 cm−1 for 1,1′-binaphthyl-d14. The Franck–Condon envelope of the librational progressions indicates that the potential energy surface for states of A and B symmetry are different, which implies that the interplanar angle in each of the four excited states is different. From the intensities in the A and B progressions it is further deduced that for the origin transitions the transition moment in each subunit is oriented in the naphthalene plane at an angle of ∼50° with respect to the long axis. From the Franck–Condon envelope in the librational progressions it is further deduced that the change in torsional angle on optical excitation to the B-potential energy surface is ∼12° to the trans and 6° to the cis side. From this a maximum (state dependent) barrier of 150 cm−1 is calculated. It is further shown that, near the intense false origin, numerous modes appear which are induced through vibrational mixing. This Fermi-resonance effect is most obvious in perdeutero binaphthyl, where the vibronically induced intensity in the region of the most intense false origin is smeared out over many modes. The emission spectrum of the molecule can be interpreted by assuming that in 1,1′-binaphthyl very efficient vibrational relaxation occurs through conversion of vibrational into librational energy with molecular rotations providing the matching energy. It is also suggested that, after vibronic excitation, vibrational relaxation occurs into the energetically most stable structure. Finally, the observed red shift of the binaphthyl fluorescence in solution must be due to solvent stabilization of the excited molecule in a structure which is more planar than the one observed in the isolated molecule.