PHOTOISOMERIZATION KINETICS OF STILBENE-TYPE FLUORESCENT WHITENING AGENTS

Photoisomerization of stilbene-type fluorescent whitening agents (FWAs ) is an important process that governs their isomer distributions and photodegradation rates in the aquatic environment. Kinetic expressions relating parameters for singlet excited-state dynamics, photoisomeriz ation, and photodegradation were derived. Picosecond time-correlated s ingle photon counting and stationary monochromatic irradiation were us ed to study the fluorescence lifetimes and steady-state isomer ratios of three widely used FWAs in aqueous solution. Fluorescence lifetimes decrease and Z/E-isomer ratios increase with increasing temperature, w hich can be interpreted in terms of an activated rate process in the f irst execited singlet state of the E-isomer having an activation energ y of 20-26 kJ mol(-1). The data obtained in the laboratory enable pred iction of the FWAs isomer distributions as a function of temperature a nd under various irradiation conditions in sunlit freshwaters.