Curtis P. Berlinguette

A series of heteroleptic bis(tridentate) Ru^II complexes featuring N^C^N‐cyclometalating ligands is presented. The 1,2,3‐triazole‐containing tridentate ligands are readily functionalized with hydrophobic side chains by means of click chemistry and the corresponding cyclometalated Ru^II complexes are easily synthesized. The performance of these thiocyanate‐free complexes in a dye‐sensitized solar cell was tested and a power conversion efficiency (PCE) of up to 4.0 % (J_sc=8.1 mA cm⁻², V_oc=0.66 V, FF=0.70) was achieved, while the black dye ((NBu₄)₃[Ru(Htctpy)(NCS)₃]; Htctpy=2,2′:6′,2′′‐terpyridine‐4′‐carboxylic acid‐4,4′′‐dicarboxylate) showed 5.2 % (J_sc=10.7 mA cm⁻², V_oc=0.69 V, FF=0.69) under comparable conditions. When co‐adsorbed with chenodeoxycholic acid, the PCE of the best cyclometalated dye could be improved to 4.5 % (J_sc=9.4 mA cm⁻², V_oc=0.65 V, FF=0.70). The PCEs correlate well with the light‐harvesting capabilities of the dyes, while a comparable incident photon‐to‐current efficiency was achieved with the cyclometalated dye and the black dye. Regeneration appeared to be efficient in the parent dye, despite the high energy of the highest occupied molecular orbital. The device performance was investigated in more detail by electrochemical impedance spectroscopy. Ultimately, a promising Ru^II sensitizer platform is presented that features a highly functionalizable “click”‐derived cyclometalating ligand.

Amorphous and More Active: The electrochemical generation of hydrogen from water could help in the storage of energy generated by renewable resources at off-peak times. However, catalysts for the slow step of this reaction, the oxygen evolution reaction (OER), are based on oxides of noble metals (iridium and ruthenium) that have limited abundance. A strategy for improving the performance of earth-abundant elements is to explore mixed-metal oxides and to prepare these as amorphous phases. Smith et al. (p. 60 , published online 28 March) developed a general method for preparing amorphous oxides, based on photodecomposition of organometallic precursors. Amorphous mixed-metal oxides of iron, nickel, and cobalt were more active than comparable crystalline materials and provided OER performance comparable to noble metal oxides.

Cycloruthenated complexes of the type [Ru^II(N^N)₂(C^N)]⁺ (N^N = substituted 2,2′-bipyridine; C^N = substituted 3-(2′-pyridyl)-1,8-naphthalimide ligand) are shown to generate high power conversion efficiencies (PCEs) in the dye-sensitized solar cell (DSSC). It is shown that substitution of the pyridine ring of the C^N ligand with conjugated groups can enhance molar absorption extinction coefficients, while the electron density imparted on the metal center is alleviated by the 1,8-naphthalimide fragment. This latter feature maintains a Ru(III)/Ru(II) redox couple more positive than 0.8 V versus NHE, thereby accommodating regeneration of the oxidized dye by an iodide-based redox mediator. This dye platform can consequently be modulated at various sites to enhance light absorption and suppress recombination between the redox mediator and the TiO₂ surface without compromising dye regeneration, thereby maintaining device PCEs as high as 7%. We also introduce a new phosphine-based coadsorbent, bis(2-ethylhexyl)phosphinic acid (BEPA), which is significantly easier to synthesize than the widely used bis(3,3-dimethylbutyl)phosphinic acid (DINHOP) while also facilitating high dye loading.

The proton-coupled electron transfer (PCET) chemistry associated with the [Co-OH]²⁺/[Co-OH₂]²⁺ redox couple for [Co(PY5)(OH₂)]²⁺ (1; PY5 = 2,6-(bis(bis-2-pyridyl)-methoxymethane)-pyridine) and [Co^II(pz₄depy)(OH₂)]²⁺ (2; pz₄depy = 2,6-bis(1,1-di(1H-pyrazol-1-yl)ethyl)pyridine) is reported. It is found that the couple is acutely sensitive to the geometry of the axially ligated group in addition to the electronic-donating/-withdrawing character of the ligand. Interrogation of the electron-transfer kinetics by electrochemical methods also shows for the first time that the interconversion of [Co^III-OH]²⁺ and [Co^II-OH₂]²⁺ proceeds through a concerted pathway in favour of energetically unfavourable stepwise electron-transfer or proton-transfer reaction steps.

The recent recognition that a single metal site is capable of mediating the multiple electron and proton transfer events associated with water oxidation represents a pivotal discovery for the field. This finding has led to a remarkable expansion of known synthetic water oxidation catalysts, and has provided the means to gain unprecedented insight into the reaction steps involved with O–O bond formation. This perspective reflects on the key studies that have advanced our understanding of water oxidation catalysis while summarizing molecular features that are integral to negotiating this complicated reaction pathway with the goal of helping identify new frontiers of discovery for the field.

A series of heteroleptic bis(tridentate) ruthenium(II) complexes, each bearing a substituted 2,2′:6′,2″-terpyridine (terpy) ligand, is characterized by room temperature microsecond excited-state lifetimes. This observation is a consequence of the strongly σ-donating and weakly π-accepting tridentate carbene ligand, 2′,6′-bis(1-mesityl-3-methyl-1,2,3-triazol-4-yl-5-idene)pyridine (C^∧N^∧C), adjacent to the terpy maintaining a large separation between the ligand field and metal-to-ligand charge transfer (MLCT) states while also preserving a large ³MLCT energy. The observed lifetimes are the highest documented lifetimes for unimolecular ruthenium(II) complexes and are four orders in magnitude higher than that associated with [Ru(terpy)₂]²⁺.

We report the first case of Ru(II) dipyrrinates employed as dyes in dye-sensitized solar cells. These complexes exhibit panchromatic light harvesting that results in significant DSSC current densities, rendering them promising for photovoltaic applications. Adjustment of the lowest excited state energy is required to boost the power conversion efficiency.

A divergence from the conventional approach to chromophore design has led to the establishment of many exciting new benchmarks for the dye-sensitized solar cell (DSSC), including the first documented power conversion efficiency in excess of 12% at 1 sun illumination [Yella et al., Science 2011, 334, 629]. Paramount to these advances is the deviation from polypyridyl ruthenium dyes bearing NCS⁻ ligands, such as [Ru(dcbpy)₂(NCS)₂] (N3; dcbpy = 4,4′-dicarboxy-2,2′-bipyridine). While metal-free and porphyrin dyes have demonstrated much promise, the discovery that the NCS⁻ ligands of N3 can be replaced by anionic, chelating cyclometalating ligands without compromising device efficiencies has ushered in a new era of ruthenium dye development. A particularly appealing feature of this class of dyestuff is that they offer acute control of the frontier molecular orbitals to enable the precise attenuation of both the ground and excited state redox potentials through judicious chemical modification of the aryl ring. This Perspective summarizes very recent developments in the field, and demonstrates how the new and rapidly expanding class of Ru-based sensitizers provides a conduit for enhancing the performance (and potentially the stability) of the DSSC.

A cyclometalated Ru dye bearing two triphenylamine groups to augment light absorption exhibits a power conversion efficiency (PCE) in excess of 7% in the dye-sensitized solar cell despite having a large molecular footprint on TiO₂.