The photoyellowing of stilbene-derived fluorescent whitening agents--mass spectrometric characterization of yellow photoproducts.

The application of fluorescent whitening agents (FWAs) significantly accelerates the photoyellowing of wool and silk under exposure to the ultraviolet and visible components of sunlight <500 nm. The photochemistry involved in this process is poorly understood, particularly the role of photoproducts derived directly from the FWA itself. Hydroxylation was identified as the key initial mechanism of photodegradation leading to coloration of the solution in the irradiation of the stilbene-derived FWA 4,4'-bis(2-sulfostyryl)biphenyl (DSBP) in the presence of hydrogen peroxide (H2O2). Polyhydroxylated DSBP derivatives were implicated as critical intermediates in the formation of yellow photoproducts under these conditions. The formation of trace quantities of DSBP quinone derivatives subsequent to hydroxylation was identified as the key cause of DSBP photoyellowing. These results are the first successful characterization of yellow photoproducts resulting directly from irradiation of a stilbene-based FWA. Formation of these yellow stilbene-based FWA-derived photoproducts may occur on the surface of FWA-treated wool exposed to simulated sunlight, as previous work has shown that H2O2 is photogenerated when wet FWA-treated wool is exposed to light. These results therefore suggest that yellow FWA-derived photoproducts contribute to the accelerated photoyellowing of FWA-treated wool.