Photoisomerization of <i>cis</i>-1-(2-Naphthyl)-2-phenylethene in Methylcyclohexane at 77 K:  No Hula-Twist

Abstract

The cis-trans photoisomerization of cis-1-(2-naphthyl)-2-phenylethene (c-NPEA and c-NPEB) was studied in methylcyclohexane glass at 77 K. The progress of the reaction was followed by fluorescence spectroscopy. Formation of the extended trans conformer (t-NPEB) is revealed by the growth of sharp vibronic bands on the broad structureless fluorescence spectrum of c-NPE. Principal component analysis with self-modeling of a matrix consisting of spectra generated by irradiation of c-NPE at different excitation wavelengths and spectra of t-NPE measured under the same conditions as a function of excitation wavelength reveals that the diabatic photoisomerization is a completely conformer-specific one-bond twist process: c-NPEB --> t-NPEB. These observations are consistent with observations in solution at ambient temperature. They demonstrate unequivocally that free volume restrictions imposed by the amorphous glassy environment do not open a hula-twist process.