Photoregeneration of Biomimetic Nicotinamide Adenine Dinucleotide Analogues via a Dye-Sensitized Approach

Two-step photochemical reduction of an acridinium-based cation 2O+ to the corresponding anion 2O– was investigated using a dye-sensitized approach involving 2O+–COOH attached to the surface of a wide-bandgap semiconductor, p-NiO. The cation 2O+ and corresponding one-electron reduced radical form, 2O• were synthesized and characterized using steady-state UV/vis and electron paramagnectic resonance spectroscopy. The thermodynamics for the photoinduced hole injection from 2O+ and 2O• were evaluated and found to be favorable. Subsequent femtosecond transient absorption spectroscopy was utilized to evaluate the photoinduced hole injection into NiO, starting from 2O+–COOH/NiO and 2O•–COOH/NiO samples. The excitation of 2O+–COOH at 620 nm initiated fast (2.8 ps) hole injection into NiO. However, 90% of the charge-separated population recombined within ∼40 ps, while ∼10% of the charge-separated population exhibited lifetimes longer than the time scale of our instrument (1.6 ns). In the case of 2O•–COOH/NiO, the light absorption occurs predominantly by NiO (2O•–COOH absorbs at 310 nm) and is associated with the electron transfer from the conduction band of NiO to the radical. The charge-separated state in this case appears to be long-lived, based on the slow (ns) growth of the trapped carriers formed on the NiO surface. The results of this work indicate that the photochemical reduction of 2O+ to the corresponding hydride form (2OH) can be achieved, opening the possibility of using such a dye-sensitized approach for regeneration of nicotinamide adenine dinucleotide analogues in enzymatic and chemical catalysis.