We have studied, in some detail, the relationship between the spectroscopic and photochemical properties of substituted benzophenones. A rather complete interpretation of these properties can now be given in terms of the electron distribution in the lowest triplet level. Three types of excited state are important, which may be classified, in decreasing order of reactivity, as n-n*, v-n* and charge-transfer (C-T) states. The relative position of these states is changed by substitution in the ring and by change of solvent polarity. For example, p-amino-and p-hydroxy-benzophenones abstract hydrogen from paraffins with high quantum yield but not from alcohols since in the latter solvents, the C-T triplet is of lower energy than the n-T* triplet.The different reactivity of hydroxy and methoxy compounds is a result of rapid deprotonation of the former in the excited state so that the equilibrium species is the base, which has a lowest triplet of C-T type. Phenyl substituted benzophenones have lowest triplet levels of T-v* type and, therefore, have intermediate reactivity.
A study of photo-induced electron-transfer reactions in MeCN with 9,IO-dicyanoanthracene as acceptor and 21 electron donors with transient photoconductivity measurements is reported. The free-ion yield and the rate constant of back electron transfer are determined. For exergonic reactions, the 'Marcus-inverted' region is observed. The fit with the theory is best, when a nearly solvent-independent Coulomb term is used in the calculation of the energy balance.
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