Experimental investigations on the viscosity effects on photoinduced electron transfer reactions in solution

The viscosity dependence of the photoinduced electron transfer reaction bet ween perylene and two different aromatic amines, N,N-dimethylaniline and 2, 4,6,N,N-pentamethylaniline, is studied. In order to increase the viscosity, mixtures of methanol and cyclohexanol, and of methanol and glycerol are us ed as solvents. The advantage of this particular mixtures is that the diele ctric constant does not change significantly in the range of viscosities mo nitored and that there is only one microscopic environment, so the value of the macroscopic viscosity is really a sensible parameter. Perylene is chos en because of its well known photochemical properties, like high fluorescen ce quantum yield. Quenching rate constants are obtained in two ways, by flu orescence lifetimes and by fluorescence intensities measurements of perylen e with increasing quenchers concentrations, according to the Stern-Volmer p rocedure. Results show that there is always a positive deviation from the d iffusion limit (Einstein-Smoluchowski law), that can be explained employing the recently published finite lifetime theory. Although at very high visco sities the intensity data show even higher values than predicted. That may indicate a second quenching mechanism. But no pseudostatic quenching can be postulated, since there are no changes in the absorption spectra within th e whole concentration range of the quenchers.