Redox Chemistry of Benzimidazoline (BIH) and Benzimidazolium (BI<sup>+</sup>)-Initiation, Exploration and Expansion-

Reduction and oxidation (redox) reactions are fundamental chemical processes and frequently involve single electron transfer (SET). Nicotinamide adenine dinucleotide is an eminent redox coenzyme, and the oxidized form (NAD+) is converted to the reduced form (NADH) for its use in biological redox processes. Benzimidazoline (BIH) and benzimidazolium (BI+) are recognized as artificial analogues of NADH and NAD+. Redox chemistry of BIH has started in the mid 80s in which 1,3-dimethyl-2-phenylbenzimidazoline (BIH-Ph) was used as a hydride donor. Subsequently, electron and hydrogen atom donating property of BIH-Ph was explored. Following thermodynamic as well as kinetic investigations of 2-substituted-1,3-dimethylbenzimidazolines (BIH-R) revealed that SET from BIH-R produces their radical cations (BIH-R•+) which release hydrogen atom to give the oxidized forms (BI+-R). Deprotonation of BIH-R•+ by appropriate bases produces radicals BI•-R which act as strong electron donors. We began to investigate photoinduced electron transfer (PET) reactions of BIH-R in the mid 90s. Breakthroughs of BIH-R redox chemistry were made in the areas of photocatalytic CO2 reduction as well as n-type doping to organic semiconductors around 15 years ago. Thus far, BIH-R have been utilized in various chemical processes and applications such as hydride reduction, photoredox catalysis, artificial photosynthesis, O2 reduction, and organic semiconductor devices. Redox chemistry of the oxidized form BI+-R which is much less explored than that of BIH-R has recently begun. In this review article, we described the brief history and the representative investigations of BIH-R. In addition, recent studies of BI+-R which are utilized as redox catalysts were presented.