FACTORS WHICH DETERMINE THE EFFICIENCY OF SENSITIZED SINGLET OXYGEN PRODUCTION

Nanosecond laser photolysis measurements of sensitized phosphorescence from oxygen have been used to obtain values for singlet oxygen format ion efficiencies during oxygen quenching of excited singlet and triple t states of anthracene and naphthalene derivatives. Oxygen quenching o f excited singlet states of anthracene and dicyanoanthracene in cycloh exane has been shown to lead to catalysed production of triplet states with unit efficiency in both cases, but concurrent production of sing let oxygen only occurs in the case of 9,10-dicyanoanthracene with effi ciency close to unity whereas the efficiency for singlet oxygen produc tion due to direct oxygen quenching of excited singlet anthracene is c lose to zero. In contrast to these results, oxygen quenching of the tr iplet states of anthracene and dicyanoanthracene in cyclohexane yields singlet oxygen with unit efficiency whereas the singlet oxygen format ion efficiency during oxygen quenching of triplet 1-ethylnaphthalene i s only 0-86 in cyclohexane and drops to 0.51 in acetonitrile. This sol vent dependence demonstrates the role which charge transfer interactio ns play in determining singlet oxygen yields. Further information conc erning the decay of excited oxygen-aromatic hydrocarbon charge-transfe r complexes have been obtained from picosecond laser pump-probe studie s where direct excitation is into the charge-transfer bands of oxygena ted 1-ethylnaphthalene. Following the excitation of the charge-transfe r complex, the triplet state of 1-ethylnaphthalene is rapidly produced with an efficiency which shows a marked solvent dependency, being 0.4 and 0.8 in acetonitrile and cyclohexane, respectively. The measured y ields of singlet oxygen formation following excitation into 1-ethylnap hthalene-oxygen charge-transfer complexes are 0.36 and 0.78 in these t wo solvents which is greater than that expected on the basis of the me asured triplet yields. Mechanisms of quenching of excited states by ox ygen which explain these results are discussed.