Temperature and Viscosity Effects on an Excited State Equilibrium as Revealed from the Dual Fluorescence of Very Dilute Solutions of 1‐Dimethylamino‐4‐cyanonaphthalene

It is shown that polar solutions of 1‐dimethylamino‐4‐cyanonaphthalene display dual fluorescence, closely related to that of p‐dimethylaminobenzonitrile. — The kinetic model introduced by Grabowski et al. has been used. This involves a temperature dependent excited state equilibrium, B → A, which is responsible for the characteristic maximum observed for the fluorescence quantum yield ratio ϕ A /ϕ B as a function of temperature. — With the help of the degree of polarization of the short wavelength fluorescence band, F B , as a function of temperature, an inversion of the 1 L a and 1 L b fluorescent states is shown to occur in propylene glycol at room temperature. This fluorescence level inversion is assumed to arise from reorientation effects of the solvent molecules. — The long wavelength fluorescence band, F A , is emitted from a third state which is formed via an adiabatic photoreaction along a reaction coordinate involving solvent reorientation as well as solute geometrical relaxation (intramolecular twisting).