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∧NtBu donor motif, constructed from either imidazole-2-ylidene or benzimidazole-2-ylidene-based N-heterocyclic carbenes (NHCs) linked to p-tbutylpyridine. The C∧NtBu 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 Ru2+/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.