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, photoisomerization, and photodegradation were derived. Picosecond time-correlated single photon counting and stationary monochromatic irradiation were used 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, which can be interpreted in terms of an activated rate process in the first execited singlet state of the E-isomer having an activation energy of 20-26 kJ mol(-1). The data obtained in the laboratory enable prediction of the FWAs isomer distributions as a function of temperature and under various irradiation conditions in sunlit freshwaters.