Carbazole modification of ruthenium bipyridine-dicarboxylate oxygen evolution molecular catalysts

We synthesized new oxygen-evolving molecular Ru(II) catalysts with one or two carbazole moieties on the axial pyridyl ligands, namely [Ru(bda)(cbz-py)(py)] and [Ru(bda)(cbz-py)₂] [C1 and C2; bdaH₂ = 2,2'-bipyridyl-6,6'-dicarboxylic acid, py = pyridine, and cbz-py = 9-(pyridin-4-yl)-9H-carbazole] to investigate the effect of cbz modification on the photophysical and catalytic properties of the well-known molecular catalyst [Ru(bda)(py)₂] (C0). The initial oxygen-evolving catalytic activities of C1 and C2 were higher than that of C0 in both a chemical reaction driven by the strong oxidant (NH₄)₂[Ce(NO₃)₆] (CAN = ceric ammonium nitrate) and photochemical oxidation using a [Ru(bpy)₃]²⁺ (bpy = 2,2'-bipyridine) photosensitizer with Na₂S₂O₈ as the sacrificial oxidant. The higher activities were ascribed to the electron-withdrawing cbz groups, which promoted the radical coupling reaction to form a Ru^IV-O-O-Ru^IV species. A unique oxygen-evolution rate change behaviour was observed for both C1 and C2 in the presence of a large excess of CAN, suggesting the competitive oxidation of the cbz moiety during the chemical oxygen evolution reaction. This work suggests that the cbz modification of an oxygen evolution molecular catalyst is a promising approach for integrating the hole accumulator near the oxygen evolution catalytic centre.

Inhibiting Charge Recombination in cis-Ru(NCS)2 Diimine Sensitizers with Aromatic Substituents

A series of cis-[Ru(LL)(dcbH₂)(NCS)₂] compounds, where dcbH₂ = 2,2'-bipyridine-4,4'-dicarboxylic acid and LL = 1,10-phenanthroline (Ru(phen)), 4,7-dipyrrole-1,10-phenanthroline (Ru(pyr)), 4,7-diindole-1,10-phenanthroline (Ru(ind)), or 4,7-dicarbazole-1,10-phenanthroline (Ru(cbz)), was investigated for application as sensitizers in mesoporous TiO₂ dye-sensitized solar cells (DSSCs). A systematic increase in the number of rings of the aromatic substituents at the 4,7-positions of the 1,10-phenanthroline allowed tuning of the molecular size of the sensitizers and the energy stored in the excited state while maintaining the same ground-state Ru^3+/2+ reduction potentials. These small structural changes had a significant influence on the rates and/or efficiencies of electron injection, back-electron transfer, recombination to oxidized mediators, lateral self-exchange electron transfer, and regeneration through iodide oxidation that were reflected in distinct photoelectrochemical performance of full operating DSSCs. The global efficiencies, open-circuit voltages, and short-circuit current densities of the DSSCs consistently followed the trend Ru(pyr) < Ru(ind) < Ru(phen) < Ru(cbz), and the most optimal performance of Ru(cbz) was ascribed to dramatically slower recombination to the oxidized redox mediators. Transient photovoltage and transient absorption experiments both revealed significantly slower recombination as the size of the aromatic substituents increased with Ru(cbz) providing the most promising behavior for application in dye sensitization.