Determination of the Intersystem Crossing Probabilities of Acridine in Various Alcohols

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Abstract
Intersystem crossing probabilities φST of acridine in methanol, ethanol, and 2-propanol were determined by two newly developed methods, relative (r) and absolute (a). The results are as follows. Methanol; 0.27 (r), 0.31 (a). Ethanol; 0.29 (r), 0.33 (a). 2-Propanol; 0.43 (r), 0.45 (a). The above φST values added to the quantum yield values already obtained of photoreduction occurring at T(n-π*), viz., 2φRT (radical mechanism via T(n-π*), disproportionation of semiquinone being taken into account by a factor of 2) + φMT (molecular mechanism via T(n-π*)), become about the same for the three alcohols. Thus the present data are consistent with our previous data on photoreduction if the intersystem crossing is assumed to occur mainly via S*(π-π*)→T(n-π*)→T(π-π*) with the same total φST for the three alcohols but with T(n-π*) participating in the reaction in different extent according to the alcohol.

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