Conformational equilibria in 1,2-diarylethylenes manifested in their emission spectra and lifetimes

The emission spectra of trans-1-(2-naphthyl)-2-arylethylenes (where aryl = phenyl, 1-naphthyl, or 2-naphthyl) vary with the wavelength of excitation, indicating the presence of two species of each compound, with slightly shifted absorption and emission peaks. The emission decay curves of two of these compounds can be expressed only as the sum of at least two exponentials, the contribution of each varying with the wavelength of excitation. Both phenomena are restricted to those trans diarylethylenes capable of existing as a mixture of two or three almost isoenergetic conformers, and it is therefore suggested that such conformers should be assigned to the two species postulated above. These differ in their emission spectra, decay rates, and quantum yields. In stilbene and in 1-phenyl-2-(1-naphthyl)- and 1,2-di-(1-naphthyl)ethylene the same emission spectra were observed even when exciting all through the range of the first emission band.