Steric Effects in Singlet and Triplet Electronic Energy Transfer to Azo Compounds,2

Abstract: Rate constants have been determined for the solution-phase quenching of singlet and triplet excited states of a vari-ety of sensitizers (aromatic hydrocarbons, aryl and alkyl ketones, a-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 accep-tor 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 azobutanes is estimated to be 53 f 1 kcal/mol. Bimolecular electronic energy transfer reactions may D * + A- D + A * proceed by a t least three different r n e ~ h a n i s m s: ~. ~ (1) long-range resonance energy transfer, which acts over distances up to about 50 8, by dipole-dipole interactions; (2) short-range collisional energy transfer, which requires electron-exchange interactions between the donor and acceptor mo