Halos show the path to perfection : peripheral iodo-substituents improve the efficiencies of bis(diimine)copper(I) dyes in dye-sensitized solar cells

The homoleptic copper(I) complexes [CuL2][PF6] (L = 4,4′-bis(4-halophenyl)-6,6′-dimethyl-2,2′-bipyridine with halogen = F (2) and Cl (3)) have been prepared and characterized, and their absorption spectroscopic and electrochemical properties compared to that with L = 4,4′-bis(4-bromophenyl)-6,6′-dimethyl-2,2′-bipyridine (4). The synthesis of [CuL2][PF6] (L = 4,4′-bis(4-iodophenyl)-6,6′-dimethyl-2,2′-bipyridine, 5) resulted in a mixture of [Cu(5)2][PF6] and [Cu(5)(MeCN)2][PF6]; variable temperature 1H NMR spectroscopy confirmed that the complexes are in equilibrium in CD3CN solution. The structure of [Cu(5)(MeCN)2][PF6] was determined by single crystal X-ray crystallography, and confirms a distorted tetrahedral geometry for the copper(I) centre. The heteroleptic dyes [Cu(1)(2)]+, [Cu(1)(3)]+, [Cu(1)(4)]+ and [Cu(1)(5)]+ (1 = ((6,6′-dimethyl-[2,2′-bipyridine]-4,4′-diyl)bis(4,1-phenylene))bis(phosphonic acid)) have been assembled by ligand exchange between [CuL2]+ and TiO2 functionalized with the anchoring ligand 1, and the performances of the dyes in fully masked dye-sensitized solar cells (DSCs) have been measured and compared. On the day of DSC fabrication, the trend for the global efficiencies, η, depends on the halo-substituent in the order I > F ≈ Br > Cl. Ripening of the DSCs occurs and after 7 days, the dependence of η on the halo-atom is in the order I > Cl ≈ F ≈ Br; the highest η is 3.16% for [Cu(1)(5)]+ compared to 7.63% for N719. Compared to the other halo-functionalized dyes, [Cu(1)(5)]+ shows an extended spectral response to longer wavelength, with enhanced electron injection. The results of DFT calculations suggest that the better dye performance of [Cu(1)(5)]+ may be associated with improved electron transfer over the halogen of the aryl substituent from the reduced electrolyte. The assembly of anchored dye [Cu(1)(5)]+ by treating functionalized-TiO2 with a 1 : 1 mixture of [Cu(MeCN)4]+ and 5, yields a dye which gives a DSC performance that matches that made by ligand exchange using [Cu(5)2][PF6] and [Cu(5)(MeCN)2][PF6].