Synthesis, characterization and photoelectrochemical performance of a tris-heteroleptic ruthenium(II) complex having 4,7-dimethyl-1,10-phenanthroline

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.

Mechanistic Insights into the Stepwise Assembly of Ruthenium(II) Tris-heteroleptic Compounds

The ruthenium(II) tris-heteroleptic compounds cis-[Ru(NN)(dcbH₂)(NCS)₂], NN = polypyridyl ligand and dcbH₂ = 2,2'-bipyridine-4,4'-dicarboxylic acid, can be synthesized by a one-pot route starting from [Ru( p-cymene)Cl₂]₂, followed by the sequential addition of ligands. In this work, each synthetic step for the cis-[Ru(R-phen)(dcbH₂)(NCS)₂] (R = H, Me, Ph, MeO, or Cl) preparation was individually investigated, aiming to identify reaction intermediates and to establish correlations among temperature, reaction time, reactant concentration, and the identity of the substituent of the polypyridyl ligand with the kinetics of the reactions and distribution of the products. The first step is the cleavage of [Ru( p-cymene)Cl₂]₂, followed by the coordination of R-phen via an associative mechanism and establishment of a direct correlation between the electron-donating or electron-withdrawing character of R and the reaction rates. The second step is the conversion of [Ru(R-phen)( p-cymene)Cl]Cl to cis-[Ru(R-phen)(dcbH₂)Cl₂], and the rate-determining step is the formation of the intermediate [Ru(R-phen)Cl₂], which exhibits a low dependence on R. The last step is the substitution of Cl^- by NCS^-, and the N-bound isomer is the major product. The reaction temperature, time, and identity of R influence the relative distribution of the linkage isomers. The comprehension of each of these processes is a key factor to develop new strategies to optimize the one-pot synthetic route.

Correlation Between Charge Recombination and Lateral Hole-Hopping Kinetics in a Series of cis-Ru(phen')(dcb)(NCS)2 Dye-Sensitized Solar Cells

Four complexes of the general form cis-Ru(phen')(dcb)(NCS)₂, where dcb is 4,4'-(CO₂H)₂-2,2'-bipyridine and phen' is 1,10-phenanthroline (phen), 4,7-(C₆H₅)₂-phen (Ph₂-phen), 4,7-(CH₃)₂-phen (Me₂-phen), or 3,4,7,8-(CH₃)₄-phen (Me₄-phen), were anchored to mesoporous TiO₂ thin films for applications as sensitizers in dye-sensitized solar cells (DSSCs). The compounds displayed metal based reductions E^o(Ru^III/II) = 1.01 ± 0.05 V vs NHE and were potent reductants competent of excited-state electron transfer to TiO₂ with yields ϕ_inj ≥ 0.75 in acetonitrile electrolytes. Average charge recombination rate constants, k_cr, abstracted from nanosecond transient absorption measurements, and the apparent diffusion coefficients for lateral hole-hopping, abstracted from chronabsorptometry measurements, showed the same sensitizer dependency: Ru(Me₄-phen) > Ru(Ph₂-phen) > Ru(Me₂-phen) ≈ Ru(phen). When used in operational solar cells, Ru(Ph₂-phen) was most optimal with an efficiency of (6.6 ± 0.5)% in ionic liquids under 1 sun illumination. The superior performance of Ru(Ph₂-phen) was traced to a higher injection yield and more efficient regeneration due to an unusually small sensitivity of k_cr to the number of injected electrons.

Influence of Ancillary Ligands in Dye-Sensitized Solar Cells

Dye-sensitized solar cells (DSSCs) have motivated many researchers to develop various sensitizers with tailored properties involving anchoring and ancillary ligands. Ancillary ligands carry favorable light-harvesting abilities and are therefore crucial in determining the overall power conversion efficiencies. The use of ancillary ligands having aliphatic chains and/or π-extended aromatic units decreases charge recombination and permits the collection of a large fraction of sunlight. This review aims to provide insight into the relationship between ancillary ligand structure and DSSC properties, which can further guide the function-oriented design and synthesis of different sensitizers for DSSCs. This review outlines how the new and rapidly expanding class of chelating ancillary ligands bearing 2,2'-bipyridyl, 1,10-phenanthroline, carbene, dipyridylamine, pyridyl-benzimidazole, pyridyl-azolate, and other aromatic ligands provides a conduit for potentially enhancing the performance and stability of DSSCs. Finally, these classes of Ru polypyridyl complexes have gained increasing interest for feasible large-scale commercialization of DSSCs due to their more favorable light-harvesting abilities and long-term thermal and chemical stabilities compared with other conventional sensitizers. Therefore, the main idea is to inspire readers to explore new avenues in the design of new sensitizers for DSSCs based on different ancillary ligands.

A high efficiency ruthenium(ii) tris-heteroleptic dye containing 4,7-dicarbazole-1,10-phenanthroline for nanocrystalline solar cells

cis-[Ru(cbz₂-phen)(dcbH₂)(NCS)₂] sensitized cells exhibited higher performance than those by N3. The efficiency is discussed in terms of its thermodynamic and structure.