Synthesis and Comprehensive Characterizations of New cis-RuL2X2 (X = Cl, CN, and NCS) Sensitizers for Nanocrystalline TiO2 Solar Cell Using Bis-Phosphonated Bipyridine Ligands (L)

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Abstract
The preparation and the properties of several ruthenium complexes of the general formula cis-RuL2X2 with L = 2,2‘-bipyridine-4,4‘-bisphosphonic acid, L‘ = 2,2‘-bipyridine-5,5‘-bisphosphonic acid, and X = Cl, CN, or NCS are reported. The synthesis of these complexes relies on the preparation of the key intermediates cis-Ru(bipyridinebis(diethyl ester phosphonate))Cl2. The ground-state second pKa values of the thiocyanato complexes were determined and are 6.0 and 6.1 for cis-RuL2(NCS)2 and for cis-RuL‘2(NCS)2, respectively. For these species, 13C NMR and IR demonstrate that the thiocyanato ligands are bound to Ru via the N atom. The new complexes exhibit a blue-shifted electronic absorption spectrum with respect to the analogous complexes containing carboxylic acid groups. Density functional theory molecular orbital calculations show that the LUMO of the bipyridine phosphonated ligands is at higher energy than the corresponding dicarboxylate complexes and that the thiocyanato ligands are not simple spectator ligands, whose role is to enrich electron density on the ruthenium, but are also involved in transitions from Π*Ru−NCS to Π*bpy that extend the absorbance of the dye in the low energy part of the absorption spectrum. The photoaction spectra recorded in a sandwich regenerative photovoltaic cell indicate that the cyano and thiocyanato complexes containing the bipyridine substituted in 4,4‘ positions exhibit a 90−95% photoconversion efficiency on the MLCT band, whereas those containing the bipyridine substituted in 5,5‘ positions display lower efficiency (60−65%). The most efficient complex in the series is cis-RuL2(NCS)2; however, its overall efficiency is about 30% lower than the analogue cis-Ru(H2dcb)2(NCS)2 (H2dcb = 2,2‘-bipyridine-4,4‘-dicarboxylic acid) due to a lower absorbance in the red part of the visible spectrum.

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