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The Journal of Physical Chemistry, Vol. 83, No. 3, 1979
(64) A recent calculation of azomethane itsel?’ shows only one significant discrepancy with the diimide values-an improbably low minimum in Tl(w) for the former. (65) (a) T. Mill and R. S. Stringham, Tetrahedron Lett., 1853 (1969); (b) V. I. Pergushov, Vesfn. Mosk. Univ., Khim., 16, 740 (1975); (c) E. M. Chen, Diss. Abstr. Int. 8 ,33, 3017 (1972). (66) Engel and Bartlett’s result” has been questioned by Steel,” who
T. Werner
obtained a lower triplet-sensitized isomerization quantum yield using acetophenone. Since acetophenone is a higher energy donor than benzophenone, it seems likely that the lower amount of isomerization was accompanied by a greater amount of decompositionundetected in Steel’s analysis for C, and CBhydrocarbon dissociation products. Further measurementsof sensitized quantum yields for azomethane would help resolve the matter.
Triplet Deactivation in Benzotriazole-Type Ultraviolet Stabilizers T. Werner Institut fur Physikalische Chemie der Universitat Stuttgarf, Pfaffenwaldring 55,0-7000 Stuttgarf 80, Vaihingen, West Germany (Received May 23, 1978; Revised Manuscript Received September 19, 1978)
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 Forster cycle. The equilibrium constants between the tautomeric derivatives protonated at nitrogen or oxygen are estimated to be pK,(So) 5 12, pK,(S1) 5 -3 (*3) and pK,(Tl) I O (h3)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
