Evidence for the marcus inverted region of back-electron-transfer in solution by means of chemically induced dynamic nuclear polarization

Abstract

In order to detect the Marcus inverted region in electron-transfer reactions in solution it is more appropriate to follow not the forward but the back reaction. This can be done by means of chemically induced dynamic nuclear polarization, measuring the polarization of re-formed educts. The ¹³C-CIDNP enhancement factors V(¹³C) of re-formed arene diazonium salts in the photochemical electron transfer from singlet-excited rubrene to a series of para-susbstituted benene diazonium tetrafluoroborates in chloroform have been determined. On plotting V(¹³C) against the standard free reaction enthalpies of the back-electron-transfer, bell-shaped behaviour of the Marcus type is obtained. Taking the reorganizational energy (λ= 0.81eV) from the maximum of this curve the Franck–Condon factors F_FC= exp (–ΔG^‡/RT) were calculated according to the Marcus quadratic equation, which, on plotting against V(¹³C), give a linear correlation. The Franck–Condon factors (0, 2,…,1) show that the back-electron-transfer proceeds with nearly the theoretical maximum value k₀.