Heteroleptic copper( i ) sensitizers with one versus two hole-transporting units in functionalized 2,9-dimethyl-1,10-phenanthroline ancillary ligands

A series of homoleptic [Cu(L) 2 ][PF 6 ] complexes in which L is a 2,9-dimethyl-1,10-phenanthroline fused at the 5,6-positions with a 2′-functionalized imidazole (ligands 1–4 ), or substituted at the 4,7-positions with electron-donating 4-(diphenylamino)phenyl groups (ligand 5 ) is described; the imidazole 2′-functionality in 1 is 4-bromophenyl, in 2 is 4-(diphenylamino)phenyl, in 3 is 4-(bis(4- n -butoxy)phenylamino)phenyl, and in 4 is 4-(carbazol-9-yl)phenyl. The copper complexes were characterized by mass spectrometry, NMR and absorption spectroscopies and cyclic voltammetry; the single crystal structure of ligand 4 has been determined. Compared to the solution absorption spectra of [Cu( 1 ) 2 ][PF 6 ], [Cu( 2 ) 2 ][PF 6 ], [Cu( 3 ) 2 ][PF 6 ] and [Cu( 4 ) 2 ][PF 6 ], that of [Cu( 5 ) 2 ][PF 6 ] shows increased absorbance at wavelengths >375 nm. An on-surface strategy was used to assemble heteroleptic [Cu( 6 )(L)] + dyes on TiO 2 electrodes where 6 is ((6,6′-dimethyl-[2,2′-bipyridine]-4,4′-diyl)bis(4,1-phenylene))bis(phosphonic acid); solid-state absorption spectra confirmed enhanced light-harvesting between 375 and 600 nm for [Cu( 6 )( 5 )] + with respect to [Cu( 6 )( 1 )] + , [Cu( 6 )( 2 )] + , [Cu( 6 )( 3 )] + and [Cu( 6 )( 4 )] + . Comparison of the performances of dye-sensitized solar cells (DSCs) containing [Cu( 6 )( 2 )] + , [Cu( 6 )( 3 )] + and [Cu( 6 )( 4 )] + with those with [Cu( 6 )( 1 )] + indicate only a marginal influence of the diphenylamine or carbazole hole-transporting domains in 5,6-substituted phenanthroline dyes. The introduction of the 4-(diphenylamino)phenyl hole-transporting units in the 4- and 7-positions of the phen unit in 5 proves to be beneficial, with DSCs containing [Cu( 6 )( 5 )] + performing better than those with the other four dyes; duplicate DSCs were tested for each dye to validate the results. While the values of the maximum external quantum efficiencies (EQE max ) for [Cu( 6 )( 1 )] + and [Cu( 6 )( 4 )] + are greater than for [Cu( 6 )( 5 )] + , the extension of the EQE spectrum for [Cu( 6 )( 5 )] + to longer wavelengths results in higher short-circuit current densities ( J SC ) compared to DSCs with [Cu( 6 )( 1 )] + , [Cu( 6 )( 2 )] + , [Cu( 6 )( 3 )] + and [Cu( 6 )( 4 )] + .