Magnetic field effects on the pyrene—dicyanobenzene system: determination of electron self-exchange rates by MARY spectroscopy

Abstract

An experimental determination has been made of electron self-exchange rates between the radical anions of 1,2-, 1,3- and 1,4-dicyanobenzene (DCB) and the respective neutral molecules applying steady-state field-modulated MARY (magnetic field effect on reaction yield) spectroscopy. For the first time this has been achieved successfully for compounds whose self-exchange rate constants can be obtained independently by alternative methods such as EPR linebroadening. In this study, pyrene was used as an electron donor to generate the spin-correlated radical ion pair (pyrene·+ DCB·−) essential for MARY spectroscopy. The radical ion pair is in equilibrium with an exciplex whose magnetic field affected fluorescence was recorded as a function of the magnetic field to yield the MARY spectrum. Due to lifetime uncertainty energy broadening of spin levels caused by electron self-exchange, the characteristic B 1/2 value increases with the concentration of DCB in the sample. The rate constant of self-exchange was obtained from the slope of the linear part in the plot of B1/2 versus DCB concentration. The values range between 6 x 108 M−1 s−1 and 1.4 x 1010 M−1 s−1, depending on the DCB isomer and solvent. Comparison with literature data from EPR linebroadening measurements shows good agreement.