Rate constants of the thermal cis-trans isomerization of azobenzene dyes in solvents, acetone/water mixtures, and in microheterogeneous surfactant solutions

Rate constants k(iso) of the thermal cis-trans isomerization of four 4,4'-n itro-aminoazobenzenes with different amino groups have been determined in h omogeneous aprotic solvents and polyglykol oligomers, primarily by means of conventional flash photolysis. The rate constants have been correlated wit h polarity (according to lambda(max) from UV/Vis absorption spectra of the trans isomers) and bulk viscosity of the solvents. Qualitative conclusions about the influence of varying concentrations of water with respect to pola rity and hydrogen banding on k(iso)- and lambda(max)-values in acetone/wate r mixtures were derived. Based on these results the data from microheteroge neous solutions have been interpreted. In microheterogeneous water/surfactant solutions k(iso)-values of selected azo dyes were strongly dependent on the concentrations of SDS, Triton(R) X- 100, C12EO8 in water, and varied with the composition of bicontinuous micro emulsions of Igepal(R) CA-520/ heptane/water. The large spread of isomeriza tion rate constants is in part due to varying microviscosity. Replacement of H2O by D2O in aqueous surfactant solutions produced surprisi ngly large kinetic solvent isotope effects. (C) 1999 John Wiley & Sons, Inc .