New Dicarboxylic Acid Bipyridine Ligand for Ruthenium Polypyridyl Sensitization of TiO₂

Abstract

An ambidentate dicarboxylic acid bipyridine ligand, (4,5-diazafluoren-9-ylidene) malonic acid (dfm), was synthesized for coordination to Ru(II) and mesoporous nanocrystalline (anatase) TiO(2) thin films. The dfm ligand provides a conjugated pathway from the pyridyl rings to the carbonyl carbons of the carboxylic acid groups. X-ray crystal structures of [Ru(bpy)(2)(dfm)]Cl(2) and the corresponding diethyl ester compound, [Ru(bpy)(2)(defm)](PF(6))(2), were obtained. The compounds displayed intense metal-to-ligand charge transfer (MLCT) absorption bands in the visible region (ε > 11,000 M(-1) cm(-1) for [Ru(bpy)(2)(dfm)](PF(6))(2) in acetonitrile). Significant room temperature photoluminescence, PL, was absent in CH(3)CN but was observed at 77 K in a 4:1 EtOH:MeOH (v:v) glass. Cyclic voltammetry measurements revealed quasi-reversible Ru(III/II) electrochemistry. Ligand reductions were quasi-reversible for the diethyl ester compound [Ru(bpy)(2)(defm)](2+), but were irreversible for [Ru(bpy)(2)(dfm)](2+). Both compounds were anchored to TiO(2) thin films by overnight reactions in CH(3)CN to yield saturation surface coverages of 3 × 10(-8) mol/cm(2). Attenuated total reflection infrared measurements revealed that the [Ru(bpy)(2)(dfm)](2+) compound was present in the deprotonated carboxylate form when anchored to the TiO(2) surface. The MLCT excited states of both compounds injected electrons into TiO(2) with quantum yields of 0.70 in 0.1 M LiClO(4) CH(3)CN. Micro- to milli-second charge recombination yielded ground state products. In regenerative solar cells with 0.5 M LiI/0.05 M I(2) in CH(3)CN, the Ru(bpy)(2)(dfm)/TiO(2) displayed incident photon-to-current efficiencies of 0.7 at the absorption maximum. Under the same conditions, the diethylester compound was found to rapidly desorb from the TiO(2) surface.

Investigation of a new bis(carboxylate)triazole-based anchoring ligand for dye solar cell chromophore complexes

“…Innumerable reports have appeared in the literature on the design and modification of the ligand set for these complexes in order to enhance the optical absorption cross-section to increase light harvesting efficiency and to optimise the ground state and excited state oxidation potentials for charge-injection and dye regeneration processes. 6 A critical component of these dye complexes is therefore the ligand on which the 3 MLCT state is localised and which anchors the complexes to the elec-trode. The energy of the lowest-unoccupied molecular orbital (LUMO) of the dye, localised on this ligand then largely determines the excited state oxidation potential which must be correctly positioned relative to the Fermi level of the electrode to favour efficient charge injection, and relative to the electrolyte redox couple to minimise short-circuit reactions.…”

“…1) together with its corresponding complex [Ru(bpy) 2 (dfm)] 2+ (bpy = 2,2′-bipyridyl). 6 Mishra et al prepared the complex BCT-1, [Ru(dcthbpyH) 2 (NCS) 2 ][NBu 4 ] 2 , where the distance between the bpy core of the ligand and its anchoring carboxylate group has been extended by the introduction of a thiophene spacer (dcthbpyH 2 ). 17 This increases charge separation upon charge injection and reduces the rate of recombination but also leads to augmentation of the light absorption properties over the corresponding dcbH 2 complex.…”

Transfer hydrogenation of aryl ketones with homogeneous ruthenium catalysts containing diazafluorene ligands

“…Ruthenium(II) complexes were prepared by the reaction of corresponding ligands (1 equiv.) with [Ru(η‐ p ‐cymene)(μ‐Cl)Cl] 2 (0.5 equiv. for complexes 1 , 2 , 3 , 4 or 1 equiv.…”

Coordination chemistry and applications of versatile 4,5-diazafluorene derivatives

“…64 Another ligand design tested to anchor Ru(II) polypyridine complexes to TiO 2 was an ambidentate ligand (4,5-diazafluoren-9-ylidene)malonic acid (dfm), which has a 4,5-diazafluorenyl chelate with an olefin bridge to two carboxylic acid groups. 140 The metal-to-ligand charge transfer (MLCT) excited state lifetime of [Ru(dfm)(bpy) 2 ] 2+ was extremely short in solution, yet the interfacial electron transfer to TiO 2 was efficient (ϕ inj = 0.70 ± 0.05). 140 Some Ru photosensitizers with record high extinction coefficients possess 4,5-diazafluorenylidene ligands substituted with a 1,3-dithiole group (ε ≥ 40 000 M −1 cm −1 @ 470 nm), displaying rapid and efficient charge injection to TiO 2 .…”

“…140 The metal-to-ligand charge transfer (MLCT) excited state lifetime of [Ru(dfm)(bpy) 2 ] 2+ was extremely short in solution, yet the interfacial electron transfer to TiO 2 was efficient (ϕ inj = 0.70 ± 0.05). 140 Some Ru photosensitizers with record high extinction coefficients possess 4,5-diazafluorenylidene ligands substituted with a 1,3-dithiole group (ε ≥ 40 000 M −1 cm −1 @ 470 nm), displaying rapid and efficient charge injection to TiO 2 . 141,142 In addition, Ru(II) complexes of 4,5-diazafluorene functionalized at the 9-position with carbazole groups, 143 aryl groups, 144 or bis(thiophene)pyrrolyl groups, 139 have all been used to create DSSCs based on TiO 2 .…”