RADICAL PAIR PHENOMENA IN EXCIPLEX AND EXCITED RADICAL SYSTEMS

The results from continuous wave, time resolved and pulsed RYDMR exper iments are presented from two different experimental systems to demons trate the detailed and complementary information available from them. One system is the familiar exciplex one in which the radical pairs ori ginate from reaction of singlet excited pyrene with 1,3-dicyanobenzene on the one hand, and dimethyl aniline on the other. In these it is sh own that the results are affected by rapid electron hopping. This is c onfirmed by a parallel study of the dicyanobenzene system using Chemic ally Induced Dynamic Electron Polarization (CIDEP) observations. Here the exchange leads to a spectrum exhibiting excess absorption very soo n after radical creation, which does not originate in the Triplet Mech anism of spin polarization. Coupled with the RYDMR results, the CIDEP ones demonstrate a positive value for the electron exchange interactio n in this system. The second system involves radical pairs consisting of diphenylmethyl radicals, one of which is excited in the same light flash that creates the radical pairs, and it is the spin evolution of the pair containing one excited radical that is studied. The RYDMR res ults differ according to the environment, viscous solution or micelle, as do related magnetic field effect (MARY) studies. Spin mixing has a lesser effect on this system, and the magnitude of the field effect c orrelates neither with the magnetic properties of the radicals nor wit h the molecular diffusion. It is suggested that the reaction exit chan nel plays a prominent role. Different behaviour of both RYDMR and MARY results from this system as compared with the first leads to the sugg estion that the spin dynamics in this case ale affected by energy tran sfer processes.