Kinetic solvent isotope effect of the photoinduced thermal cis-trans isomerization of an azo dye

Rate constants k(iso) of the photoinduced thermal cis-trans isomerization o f azo dyes are known to be dependent on polarity and viscosity of the mediu m. In poly(ethylene oxide) 600/water mixtures with varying water content, k (iso)-values of 4,4'-nitro-anilinoazobenzene show an appearently stronger d ependence on bulk viscosity which depends on the water content, than in wat er-free aprotic solvents. Nearly constant UV/Vis absorption maxima lambda ( max) of the dye (trans) however indicated almost constant micropolarity and hydrogen bonding effects on a time average. This effect of water on k(iso) and the stronger kinetic solvent isotope effect on exchange of H2O by D2O compared to polarity and viscosity isotope effects, is assumed to be due to the dynamics of hydrogen bonding on cis-trans isomerization.