Abstract
Abstract— The mechanism for photodegradation of the ultraviolet photostabilizer 2‐(2'‐hydroxy‐5'‐methylphenyl)benzotriazole (TIN P) upon direct and dye‐sensitized (singlet molecular oxygen [O2(1Δg)]‐mediated) irradiation was studied. From the experimental TIN P photodegradation rate data, and low temperature (77 K) fluorescence and phosphorescence quantum yields, one can conclude that the photodegradative process involves phosphorescent states of TIN P. The open conformer of TIN P quenches O2(1Δg) by physical scavenging with a rate constant (kq) in dimethylsulfoxide of 2.8 times 106M‐1 s‐1. The intramolecular hydrogen‐bonded conformer does not appreciably interact with O2(1Δg). In the presence of a relatively high concentration of OH‐ (either 5 times 10‐2M KOH in ethanol or water at pH 13), the ionic form of TIN P (with an ionized phenol group) physically and chemically quenches O2(1Δg). The reaction rate constant (kr) is 1 times 108M‐1 s‐1, and the ratio kq/kr is approximately three in alkaline aqueous media.
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