Time-resolved ESR in a spin-correlated radical pair with large hyperfine coupling constant at 31P. Micellar size effects and the role of flip-flop transitions Original Research Article

Time-resolved ESR spectra from spin-correlated pairs of diphenyl phosphonyl and 2,4,6-trimethylbenzoyl radicals were observed in 308 nm laser photolysis of (2,4,6-trimethylbenzoyl)-diphenylphosphine oxide in sodium alkylsulfate micelles (C8–C12). These ESR spectra exhibit unusual polarization patterns in which the M=+1/2 resonance line of diphenyl phosphonyl is split into two inversely polarized components while the M=−1/2 line is in the net absorption. This pattern is shown to result from flip-flop electron–nuclear spin transitions in the diphenyl phosphonyl radical (A[31P]=38.5 mT). The corresponding mechanism is an extension of the familiar ST− radical pair mechanism to spin-correlated pairs. Our analysis indicates that in micelles, flip-flop transitions, in concert with the exchange relaxation, equilibrate all three spin states in the Fz=−1/2 manifold of the pair (where Fz is the projection of the total spin of the pair). In the studied system, this equilibration occurs in ∼100 ns. The equilibration of the Fz=+1/2 states, due to less efficient non-adiabatic ST+ transitions, is much slower and takes microseconds.