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

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'-Me 2 pqca ). Preparation of the methyl ester analogues 6'-Mepqcame , 8-Mepqcame and 8,6'-Me 2 pqcame 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 ][PF 6 ] (N^N = pqca (1), 6'-Mepqca (2), 8-Mepqca (3), 8,6'-Me 2 pqca (4), 6'-Mepqcame (6), 8-Mepqcame (7) and 8,6'-Me 2 pqcame (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·Me 2 CO·0.5H 2 O}, 4, 6·CHCl 3 ·0.13H 2 O, 2{7· C 5 H 12 }·CHCl 3 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 dm 3 mol -1 cm -1 . The photovoltaic performance of the prepared compounds was evaluated on mesoporous nanocrystalline TiO 2 dye-sensitized solar cells (DSSCs), and compared with that of the [Cu(dmdcbpy) 2 ][PF 6 ] 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.