EXCITED-STATE DYNAMICS AND VISCOSITY-DEPENDENT INTERNAL-CONVERSION IN 3,3-DIMETHYL-2-PHENYL-3H-INDOLE

The fluorescence quantum yields and lifetimes (tau(F)) of 3,3-dimethyl-2-phenyl-3H-indole (1), in a series of eighteen nonpolar, polar aprotic and/or protic solvents, range from 2.4 x 10(-4) to 0.12 and from 1 to 520 ps, respectively, as a result of viscosity dependent fast internal conversion. Rates of internal conversion are shown to be dependent upon the bulk viscosity of the solvent as opposed to the microviscosity (free volume effect of the solvent). Both phi(F) and tau(F) were found to vary with solvent viscosity (eta) according to a Forster-Hoffmann mechanism: phi(F) (or tau(F)) = Ceta2/3 with an identical value of C whatever the nature of the solvent is. This molecule may then be regarded as an excellent viscosity probe for heterogeneous systems and polymers over a large range of viscosities. Various solvatochromic methods have been used to evaluate the ground and excited state dipole moments of 1. The Bilot-Kawski method gives optimized values of 3.8 and 7.8 D, respectively, with a negligible polarizability of the molecule.