TRIPLET DEACTIVATION IN BENZOTRIAZOLE-TYPE ULTRAVIOLET STABILIZERS

The triplet-state properties of 2-(2′-hydroxy-5′-methylphenyl)benzotriazole (TIN) and related compounds are investigated. It is shown by excitation spectra that molecules with an intramolecular hydrogen bond emit no slowly decaying phosphorescence. For the example of TIN, excited-state proton transfer is considered by application of the Förster cycle. The equilibrium constants between the tautomeric derivatives protonated at nitrogen or oxygen are estimated to be pKz(S0) ≤ 12, pKz(S1) ≤ -3 (±3) and pKz(T1) ≤ 0 (±3) in the ground state, first excited singlet state, and triplet state, respectively. By a three-level energy-transfer experiment with nanosecond resolving time it is found that the triplet state shows no photosensitizing activity. A triplet decay time <20 ns is concluded. © 1979 American Chemical Society.