The syntheses of five homoleptic copper(I) complexes [CuL2][PF6] are described in which L is a 4,4'-di(4-bromophenyl)-6,6'-dialkyl-2,2'-bipyridine ligand (compounds 1-4 with methyl, (n)butyl, (iso)butyl and hexyl substituents, respectively) or 4,4'-di(4-bromophenyl)-6,6'-diphenyl-2,2'-bipyridine (5). The new ligands 2-5 and copper(I) complexes [CuL2][PF6] (L = 1-5) have been fully characterized. The single crystal structures of 2{[Cu(1)2][PF6]}·3Me2CO, [Cu(2)2][PF6], 2{[Cu(3)2][PF6]}·Et2O and [Cu(5)2][PF6]·CH2Cl2 have been determined. The first three structures show similar distorted tetrahedral environments for the Cu(+) ions with angles between the least squares planes of the bpy domains of 85.6, 86.4 and 82.9°, respectively; in contrast, the Cu(+) ion in [Cu(5)2][PF6]·CH2Cl2 is in a flattened coordinate environment due to intra-cation face-to-face π-interactions. The solution absorption spectra of the complexes with ligands 1-4 are virtually identical with an MLCT band with values of λmax = 481-488 nm. In contrast, the absorption spectrum of [Cu(5)2][PF6] shows two broad bands in the visible region. Cyclic voltammetric data show that oxidation of the copper(I) centre occurs at a more positive potential in [Cu(2)2][PF6], [Cu(3)2][PF6] and [Cu(4)2][PF6] than in [Cu(1)2][PF6] or [Cu(5)2][PF6] with the latter being oxidized at the lowest potential. The complexes have been used to prepare dye-sensitized solar cells (DSCs) incorporating heteroleptic dyes of type [Cu(L)(Lanchor)](+) where L is 1-5 and Lanchor is a 6,6'-dimethyl-2,2'-bipyridine functionalized in the 4- and 4'-positions with phosphonic acid groups with (Lanchor = 7) and without (Lanchor = 6) a spacer between the metal-binding and anchoring domains. The presence of the spacer results in enhanced performances of the dyes, and the highest energy conversion efficiencies are observed for the dyes [Cu(3)(7)](+) (η = 2.43% compared to 5.96% for standard dye N719) and [Cu(5)(7)](+) (η = 2.89% compared to 5.96% for N719). Measurements taken periodically over the course of a week indicate that the cells undergo a ripening process (most clearly seen for [Cu(5)(6)](+) and [Cu(5)(7)](+)) before their optimum performances are achieved. IPCE (EQE) data are presented and confirm that, although the photo-to-current conversions are promising (37-49% for λmax≈ 480 nm), the copper(I) dyes do not realize the broad spectral response exhibited by N719.

DSCs containing simple copper(i) dyes with peripheral halo (X) ligands give global efficiencies >3% for X = I; a new strategy for stepwise surface dye assembly is demonstrated.

The synthesis and characterization of ((6,6 0 -diphenyl-[2,2 0 -bipyridine]-4,4 0 -diyl)bis(4,1-phenylene)) bis(phosphonic acid), 2, are described. Compound 2 has been incorporated as an anchoring ligand in copper(I)-containing dyes in n-type dye-sensitized solar cells (DSCs), combined with 2,2 0 -bipyridine (bpy), 6-methyl-2,2 0 -bipyridine (6-Mebpy), 6,6 0 -dimethyl-2,2 0 -bipyridine (6,6 0 -Me 2 bpy), 4,4 0 -di(4-bromophenyl)-6,6 0 -dimethyl-2,2 0 -bipyridine (3) or 4,4 0 -di(4-bromophenyl)-6,6 0 -diphenyl-2,2 0 -bipyridine (4) as ancillary ligands (L ancillary ). Dyes were assembled on mesoporous TiO 2 using an on-surface assembly strategy which relies on ligand exchange between surface-anchored L anchor and [Cu(L ancillary ) 2 ] + ; 1 H NMR spectroscopy was used to confirm that the bulky phenyl substituents did not hinder ligand exchange. Comparison of values of the open-circuit voltages (V OC ), short-circuit current densities (J SC ) and external quantum efficiency (EQE) spectra for DSCs with model dyes [Cu(2)(bpy)] + , [Cu(2)(6-Mebpy)] + and [Cu(2)(6,6 0 -Me 2 bpy)] + confirm that methyl-substituents in L ancillary are beneficial. Performance data for DSCs with dyes [Cu(1)(3)] + , [Cu(1)(4)] + , [Cu(2)(3)] + and [Cu(2)(4)] + where 1 is the anchor ((6,6 0 -dimethyl-[2,2 0 -bipyridine]-4,4 0 -diyl)bis(4,1-phenylene))bis(phosphonic acid) show that dyes with anchor 2 (phenyl substituents in the 6-and 6 0 -positions) give relative conversion efficiencies #10% with respect to standard dye N719 set at h ¼ 100%; this compares with relative efficiencies of up to 34.5% for the dyes [Cu(1)(3)] + and [Cu(1)(4)] + . The performance of [Cu(2)(3)] + can be improved by the addition of the co-adsorbant chenodeoxycholic acid. Although the phenyl (versus methyl) substituentslead to enhanced light absorption to lower energies, dyes with anchor 2 quickly bleach when exposed to the I À /I 3 À electrolyte; bleaching also occurs after soaking in solutions of LiI. The dye can be regenerated by treatment of a bleached electrode with L ancillary , or with [Cu(NCMe) 4 ] + followed by L ancillary .

Photoconversion efficiencies reaching 40% that of the ruthenium dye N719 are achieved in n-type DSCs using structurally simple heteroleptic copper(i) dyes; the effects on DSC performance of using mixed copper(i) dyes are presented.

Surface-bound heteroleptic copper(I) dyes [Cu(L anchor )(L ancillary )] + are assembled using the "surfaces-as-ligands, surfaces as complexes" (SALSAC) approach by three different procedures. The anchoring and ancillary ligands chosen are ((6,6 -dimethyl-[2,2 -bipyridine]-4,4 -diyl)-bis(4,1-phenylene))bis(phosphonic acid) (3) and 4,4 -bis(4-iodophenyl)-6,6 -diphenyl-2,2 -bipyridine (4), respectively. In the first SALSAC procedure, the FTO/TiO 2 electrode is functionalized with 3 in the first dye bath, and then undergoes ligand exchange with the homoleptic complex [Cu (4)