NEW RED-EDGE EFFECT IN AROMATIC-MOLECULES - ANOMALY OF APPARENT ROTATION REVEALED BY FLUORESCENCE POLARIZATION

Measurements of stationary fluorescence polarization at wavelengths of excitation that yield limiting polarizations close to 1/2 and 1/7, respectively, permit the characterization of the rates of in-plane and out-of-plane rotation in aromatic compounds. Propylene glycol solutions of 1- and 2-naphthylamine, some of their derivatives, anthracene, and indole show the expected dependence of the rate of rotation upon limiting polarization, but all of them additionally display on excitation at the red edge of the absorption, a decrease in the rotational rate, which then approaches the rate observed for pure out-of-plane rotations. Differential phase measurements of the polarized components of the fluorescence of 1-naphthylamine confirm this effect. After careful check of the effects of temperature and excitation wavelength upon the emission spectrum and the fluorescence lifetimes, it is concluded that the red-edge rotational anomaly arises from the existence of out-of-plane transition moments in absorption and emission in this spectral region. The physical origin of the effects and their relation to other observed red-edge phenomena are discussed. © 1978 American Institute of Physics.