Photochemical reduction of 1-methyl-3-carbamoyl-pyridinium by hexacyanoferrate(II) induced by outer-sphere charge transfer excitation

The NAD+ subunit 1-methyl-3-carbamoyl pyridinium cation (or 1-methyl nicotinamide cation, MNA+) forms an ion pair with [Fe(CN)6]4− which shows a new absorption (λmax=392 nm) in the electronic spectrum. This band is assigned to an outer-sphere charge transfer (OSCT) transition from [Fe(CN)6]4− to MNA+. In aqueous solution OSCT excitation leads to a photoredox reaction with the formation of [Fe(CN)6]3− and the radical MNAradical dot which undergoes a dimerization or, more likely, a further reduction to MNAH.