Electron Self-Exchange Kinetics Determined by MARY Spectroscopy: Theory and Experiment

The electron self-exchange between a neutral molecule and its charged radical, which is part of
a spin-correlated radical ion pair, gives rise to line width effects in the fluorescence-detected MARY (magnetic
field effect on reaction yield) spectrum similar to those observed in EPR spectroscopy. An increasing self-exchange rate (i.e., a higher concentration of the neutral molecule) leads to broadening and subsequent
narrowing of the spectrum. Along with a series of MARY spectra recorded for several systems (the
fluorophores pyrene, pyrene-d10 and N-methylcarbazole in combination with 1,2- and 1,4-dicyanobenzene)
in various solvents, a theoretical model is developed that describes the spin evolution and the diffusive
recombination of the radical pair under the influence of the external magnetic field and electron
self-exchange, thereby allowing the simulation of MARY spectra of the systems investigated experimentally.
The spin evolution of the radicals in the pair is calculated separately using spin correlation tensors, thereby
allowing rigorous quantum mechanical calculations for real spin systems. It is shown that the combination
of these simulations with high resolution, low noise experimental spectra makes the MARY technique a
novel, quantitative method for the determination of self-exchange rate constants. In comparison to a simple
analytical formula which estimates the self-exchange rate constant from the slope of the linear part of a
line width vs concentration plot, the simulation method yields more reliable and accurate results. The
correctness of the results obtained by the MARY method is proved by a comparison with corresponding
data from the well-established EPR line broadening technique. With its less stringent restrictions on radical
lifetime and stability, the MARY technique provides an alternative to the classical EPR method, in particular
for systems involving short-lived and unstable radicals.