Steric Effects in Singlet and Triplet Electronic Energy Transfer to Azo Compounds.

摘要

Rate constants have been determined for the solution-phase quenching of singlet and triplet excited states of a variety of sensitizers (aromatic hydrocarbons, aryl and alkyl ketones, alpha-diketones) by a series of azo compounds with varying steric properties. Both singlet and triplet quenching are attributed to electronic energy transfer by the collisional electron-exchange mechanism. Steric hindrance to exothermic triplet energy transfer is significant, with azo-n-butane a better acceptor than azo-tert-butane by a factor of 3.6-10.7. Steric hindrance to singlet energy transfer is less pronounced (comparable steric factors are 1.7-2.9), apparently because diffusion rather than energy transfer is rate limiting in solution. For a given donor-acceptor pair, singlet energy transfer is two to three times faster than triplet energy transfer. The triplet energy level of the azofubanes is estimated to be 53 tor 1 kcal/mol. (Author)