Abstract
The photo‐induced oxidation of NADH and its model compound, 1‐benzyl‐1,4–dihydronicotinamide (BNAH), by N,N′‐dimethyl‐4,4′‐bipyridinium dichloride (methyl viologen: MV^+^ ^+^) was studied in homogeneous and in micellar solution. In the overall, slightly endoergonic, reaction: NADH + 2 MV^+ +^ + hv → NAD^+^ + 2 MV ^+^• + H^+^ the dihydropyridines lose a hydride equivalent via a stepwise e,H^+^,e sequence in which only the initial one‐electron transfer requires photostimulation. It is shown that the singlet‐excited state of the dihydropyridine is the photoreactive intermediate in this initial electron transfer process and that the overall quantum yield depends critically on the ratio of the rates for ionic dissociation and back‐electron transfer in the encounter complex between this excited state and MV^+ +^ molecules. This ratio can be influenced dramatically by changing the solvent medium. In aqueous solution (for NADH) and especially in anionic micellar solution (for BNAH) strong ground state association between MV^+^ ^+^ and the dihydropyridines occurs. Evidence for this association is provided by the appearance of a long‐wavelength charge transfer absorption, as well as by the large deviations from a Stern‐Volmer behaviour observed in the quenching of the dihydropyridine fluorescence by MV^+^ ^+^. The ground state complexation appears to hamper ionic dissociation of the excited complex. This is evident from the low quantum yield for photooxidation of such systems, upon excitation of the dihydropyridine chromophore (365 nm), and the complete absence of such oxidation upon their excitation within the charge transfer absorption (546 nm). The data obtained are shown to allow for a complete thermodynamic analysis of the Gibbs free‐energy changes involved in loss of a hydride equivalent from a 1,4‐dihydropyridine via a stepwise e,H^+^,e mechanism. The consequences of this analysis for the mechanism of reactions in which 1,4‐dihydropyridines transfer a hydride equivalent to various substrates under thermal conditions are discussed.