Energy-donor phosphorescence quenching study of triplet-triplet energy transfer between UV absorbers

The intermolecular triplet-triplet energy transfer from a photounstable UV-A absorber, 4-tert-butyl-4′-methoxydibenzoylmethane (BMDBM), to UV-B absorbers, 2-ethylhexyl 4-methoxycinnamate (octyl methoxycinnamate, OMC), octocrylene (OCR) and dioctyl 4-methoxybenzylidenemalonate (DOMBM) has been observed using a 355 nm laser excitation in rigid solutions at 77 K. The decay curves of the energy-donor phosphorescence in the presence of the UV-B absorbers deviate from the exponential decay at the initial stage of the decay. The Stern-Volmer formulation is not valid in rigid solutions because molecular diffusion is impossible. The experimental results indicate that the rate constant of triplet-triplet energy transfer from BMDBM to the UV-B absorbers, kT-T, decreases in the following order: kT-T (BMDBM-DOMBM)>kT-T (BMDBM-OMC)≥kT-T (BMDBM-OCR). The presence of DOMBM enhances the photostability of the widely used combination of UV-A and UV-B absorbers, BMDBM and OCR. The effects of the triplet-triplet energy transfer on the photostability of BMDBM are discussed.