The influence of molecular geometry on the fluorescence spectra of biphenyl and the polyphenyls

The shapes of the fluorescence spectra of rigid solutions (in ethanol at 77 K) of biphenyl,p-terphenyl, andp-quaterphenyl are shown to depend on the wavelength of the exciting light: the spectral resolution enhances with increasing wavelength of excitation. These spectra are interpreted in terms of a model wherein the equilibrium configuration of the fluorescent state is planar, but that of the ground state deviates from planarity.Some other results reported in the literature are also discussed and it is concluded that the ground states of the singly and doubly charged ions of these molecules, as well as the lowest triplet states of the neutral molecules, are nearly planar.