Reaction yield detected magnetic resonance in exciplex systems

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Abstract
Reaction yield detected magnetic rescnance (RYDMR) experiments are reported from separate solutions of pyrene in the presence of different electron acceptors, the three isomers of dicyanobenzene, and the electron donors, N,N-dimethyl- and N,N-diethyl-aniline. For the first time a complete set of RYDMR B 0 and B 1 spectra are provided from radical pairs in low viscosity organic solvents. The data are of high quality, due to apparatus development, and provide detailed insight into the processes occurring within the radical pair. The results have been interpreted using a stochastic Liouville equation to account for the simultaneous action of spin and molecular dynamics within the spin correlated radical pair. Two different models of the latter have been used, a simple exponential one, which allowed the influence of the electron spin exchange interaction to be investigated, and a full diffusional one, with and without a potential between the radicals. In the dicyanobenzene systems, the RYDMR behaviour is shown to be dominated by spin mixing under the influence of the hyperfine interactions, although the fit to experimental data is improved if the Coulombic interaction between the radical ions, and the exchange interaction, are taken into account. In the dimethylaniline case, the effect of the exchange interaction seems considerable, and the behaviour is also affected by degenerate electron hopping between the cation and the parent molecule. Analysis gives the rate of this process, and allows an estimate of the lifetime of the geminate radical pair in this system. This appears to be in the region of 60 to 300 ns, and is remarkably long. Theory also indicates that the effect of relaxation due to electron spin exchange processes may be significant in RYDMR measurements, particularly those performed in the higher microwave fields, or in viscous media. A method for calibrating the microwave field in the resonant cavity from a comparison of RYDNR and magnetic effect on reaction yield (MARY) results from the same experimental system is described.