Ruthenium Dyes with N-Heterocyclic Carbene Ligands for Solar Energy Conversion

We present a novel series of ruthenium dyes tailored for solar energy conversion. Each dye features a C^∧N^tBu donor motif, constructed from either imidazole-2-ylidene or benzimidazole-2-ylidene-based N-heterocyclic carbenes (NHCs) linked to p-^tbutylpyridine. The C^∧N^tBu donor set, along with a solitary thiocyanate ligand, augments donor strength, while mono- or tricarboxylic acid-functionalized terpyridyl acceptors complete the coordination sphere and facilitate robust anchoring of the dyes onto TiO₂ surfaces. These complexes demonstrate extensive absorption of visible light up to 790 nm and exhibit notable molar absorptivity of 84,200 M^-1 cm^-1 at 500 nm. Dyes based on benzimidazole-2-ylidene display protracted excited-state lifetimes (212-210 ns) and possess energy levels that are ideal for electron injection into the TiO₂ conduction band. Upon integration into dye-sensitized solar cells, this dye yields photocurrent densities of 25 mA cm^-2. The synergistic effect of NHC and p-^tbutylpyridine donors serves to lower the Ru^2+/3+ oxidation potential to 0.66 V below the I^-/I₃^- redox couple, thereby promoting efficient dye regeneration. Devices employing these dyes achieve power conversion efficiencies up to 7.09%, exceeding the N3 and N749 benchmarks under identical conditions. This paper aims to elucidate the principles guiding the design of NHC-based ruthenium photosensitizers for optimal solar energy conversion.