Sterically demanding pyridine-quinoline anchoring ligands as building blocks for copper(i)-based dye-sensitized solar cell (DSSC) complexes

Abstract

The Pfitzinger condensation reaction was employed to synthesise N^N sterically demanding ligands bearing carboxylic acid anchoring groups, namely 2,2′-pyridyl-quinoline-4-carboxylic acid (pqca), 6′-methyl-2,2′-pyridyl-quinoline-4-carboxylic acid (6′-Mepqca), 8-methyl-2,2′-pyridyl-quinoline-4-carboxylic acid (8-Mepqca) and 8,6′-dimethyl-2,2′-pyridyl-quinoline-4-carboxylic acid (8,6′-Me₂pqca). Preparation of the methyl ester analogues 6′-Mepqcame, 8-Mepqcame and 8,6′-Me₂pqcame is also described. All ligands were fully characterised including the X-ray structures of pqca, 6′-Mepqca and 8-Mepqca. We also describe the synthesis and characterisation of seven homoleptic copper(I) complexes of the formula [Cu(N^N)₂][PF₆] (N^N = pqca (1), 6′-Mepqca (2), 8-Mepqca (3), 8,6′-Me₂pqca (4), 6′-Mepqcame (6), 8-Mepqcame (7) and 8,6′-Me₂pqcame (8)). Characterisation of the copper(I) complexes includes FT-IR, elemental analyses, multinuclear NMR spectroscopy, UV-vis spectroscopy, cyclic voltammetry, and a single-crystal X-ray diffraction study. The molecular structures of 1·DMSO, 2{2·Me₂CO·0.5H₂O}, 4, 6·CHCl₃·0.13H₂O, 2{7·C₅H₁₂}·CHCl₃ and 8 have been determined, revealing that these complexes adopt a distorted tetrahedral geometry. These are the first crystallographically characterised examples of copper(I)-based coordination compounds incorporating the above mentioned N^N pyridyl-quinoline ligands. In solution, the new complexes are purple to red colored, while 2 displayed excellent stability in acetone at ambient temperature over a month. The absorption spectra of 1–8 display a main broad MLCT band with values of λ_max at ∼530 nm and ε values ranging from 1800 to approximately 10 000 dm³ mol⁻¹ cm⁻¹. The photovoltaic performance of the prepared compounds was evaluated on mesoporous nanocrystalline TiO₂ dye-sensitized solar cells (DSSCs), and compared with that of the [Cu(dmdcbpy)₂][PF₆] dye (dmdcbpy = 6,6′-dimethyl-2,2′-bipyridine-4,4′-dicarboxylic acid) (5), that has been used as standard, under the same experimental conditions. From a combination of electrochemical and absorption spectroscopy experiments, the MLCT energy levels of 2 are appropriate for electron injection onto the titania conduction band. Upon optimisation of the semiconductor's architecture, 2 proved to be the most efficient dye, reaching a conversion efficiency of η = 1.20%, which is slightly higher than that of 5 (η = 1.05%), mainly attributed to higher V_oc values.