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

Abstract

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 Ru3+/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.

Dye-sensitized solar cells strike back

“…Studies of a homologous series of four sensitizers that maintain the cis-Ru(NCS) 2 coordination environment with one surface anchoring group show that they undergo rapid holehopping. 113,114 The hole-hopping rate constants -k hh -measured electrochemically spanned about a factor of seven and followed the same trend as did the charge recombination kinetic data. 114 Subsequent temperature and surface coverage-dependent kinetic studies with sensitizers that displayed very different hole-hopping rates also supported the conclusion that rapid hole-hopping promotes charge recombination.…”

A novel triruthenium nitrosyl bearing a quinolinic ligand: a comparison of its spectroscopic behavior with its pyridine analogues

“…[42][43][44][45] In such examples, different approaches have been employed to rationalize the mutual dependence of the metal center and ligand properties. The most common are the ordering of a property such as the redox potential or band maximum with different ligands, 45,46,[50][51][52][53][54][55][56][57] the construction of correlation plots between rate constant parameters of a given reaction and the electrochemical potentials of the metal center redox processes, 44,53,[58][59][60] and the construction of correlation plots of different data with ligand parameters like the pK a , 39,51 the Hammett constant for ligand substituents (s), 61,62 or E L , 63 a ligand electrochemical parameter established in the classic work authored by Lever. Such analysis was first performed for triruthenium compounds in the 1980s in an attempt to correlate redox potentials with the pK a values of the ancillary N-heterocyclic ligands. 64 The pK a is a good parameter to rationalize the properties of triruthenium compounds since the basicity of the ligands modulates the electronic density of the metallic core via s-donation.…”

Perspectives on Dye Sensitization of Nanocrystalline Mesoporous Thin Films

“…Sensitizers that undergo fast hole hopping recombine more rapidly than those that hop more slowly. This correlation was evident in a collaboration with the Polo research group through a study of cis- Ru­(dcb)­(phen′)­(NCS) 2 sensitizers, where phen′ is a 4,7-disubstituted 1,10-phenantholine, Figure a. , Chronoabsorptometry data, where the color change was monitored after a potential step sufficient to oxidize the sensitizers was applied, were recast as an Anson plot from the apparent diffusion constant, D app , and the hole-hopping rate constant, k hh , were extracted, Figure b. The k hh values spanned about a factor of 7 and followed the same trend as did charge recombination: Ru­(Me 4 -phen) ≪ Ru­(Ph 2 -phen) < Ru­(Me 2 -phen) ≈ Ru­(phen), Figure c.…”