A kinetic method, analogous to those which describe completely reversible photochemical transformations, has been developed for measuring quantum yields in systems characterized by simultaneous reversible and irreversible photochemical reactions. Based on the concept of a "quasi-steady state", in which there is no net reversible reaction but a constant-composition mixture undergoing an irreversible reaction, it facilitates calculation of both reversible and irreversible quantum yields from the kinetics of a single irradiation of the interconverting specieis. The analysis has been applied to irradiation in the n,π* band of azomethane, azoisopropane, and phenylazomethane in benzene or isooctane; for the first two systems free-radical decomposition competes with reversible cis-trans isomerization, whereas for phenylazomethane only reversible isomerization occurs. For the 365-nm irradiation of azomethane (the more interesting case) the following quantum yields are found: φtrans→cis = 0.42 ± 0.01, φcis→trans = 0.45 ± 0.01, φtrans→dec = 0.089 ± 0.01, φcis→dec = 0.008 ± 0.01. These results are consistent with data from numerous other investigations of azomethane and lend support to a mechanistic interpretation of alkyldiazene photochemistry in which decomposition occurs from the unrelaxed S1 state of each isomer and isomerization takes place from an intermediate form of T1.
All Science Journal Classification (ASJC) codes
- Physical and Theoretical Chemistry