Different mechanisms for the protection of polymers against photodegradation are discussed. (1) The u.v.-absorbers of the 2-(2-hydroxyphenyl)-benzotriazolc and the o-hydroxybcnzophenone type havc rates of internal conversion much higher than the rate of intersystem crossing and of fluorescence dccay. Evidence is presented that this rapid non-radiativc deactivation cannot be explaincd very satisfactorily by a simple 'cnol-keto'-cquilibrium in the first excited singlct state only.
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Heller et al. (1973) studied this question.