Seite - 38 - in Photovoltaic Materials and Electronic Devices

Recently, Chang et al. [131] reported pyrazolyl-pyridine ancillary ligands bearing a series of donor groups in which the simplest substituents (such as t-butyl group) leads to better efficiency with respect to the triphenylamino andbenzothiadiazolylgroups. 3.2.6. PhenylBipyridines In2007Wadmanetal. [132]comparedabis-tpyRucomplexbearingonecarboxyl groupinthe4-position,with twostructurallyhomologcomplexes inwhichthe tpy was replaced by 6-phenylbipyridines, with one or two carboxyl groups (36 and 37, Figure 27). The NˆN’ˆC Ru(II)-complex with two anchoring groups showed performancessimilar toN719. Thus, in2010[133], thesamegroupfurtherextendend the series, including NˆCˆN’ ligands based on 3,5-bis(2-pyridyl)benzoic acid (38, Figure27). Figure 26. Pyridyl-pyrazolate ligand and quinolyl-bipyridine ligand for Ru(II)-complexes [22,127-129]. 3.2.6. Phenyl Bipyridines In 2007 Wadman et al. [132] compared a bis-tpy Ru complex be ring ne carboxyl group in th 4- position, with two structurally homolog complexes in which the tpy was replaced by 6-phenylbipyridines, with one or two carboxyl groups (36 and 37, Figure 27). The N^N’^C Ru(II)-complex with two anchoring groups showed performances similar to N719. Thus, in 2010 [133], the same group further extendend the series, including N^C^N’ ligands based on 3,5-bis(2-pyridyl)benzoic acid (38, Figure 27). Figure 27. Tpy and phenyl-bipyridines complexes investigated by Wadman et al. [132]. X-ray structural determination on the mono carboxyl complex (37, R1 = H, R2 = COOH) showed a distorted octahedral coordination, with the cyclometalated ligand perpendicular to the terpyridine and elongation of the nitrogen to Ru bond opposite to the C-Ru bond. In the solid state, the complex forms dimers via hydrogen bonds between the carboxyl functions (Figure 28). Figure 27. Tpy and phenyl-bipyridines complexes investigated by Wadman e al. [132]. X-ray structural determination on the mono carboxyl complex (37, R1 = H, R2 =COOH)showedadistortedoctahedralcoordination,with thecyclometalated ligandperpendicular to the terpyridineandelongationof thenitrogentoRubond opposite totheC-Rubond. Inthesolidstate, thecomplexformsdimersviahydrogen bondsbetweenthecarboxyl functions (Figure28). NˆN’ˆC cyclometalated compounds showed better sensitization properties respect tothebis-tpycomplexes;whilethelowerefficienciesoftheNˆCˆN'complexes were ascribed to a LUMO localization which prevented an efficient electron injection into the TiO2 conduction band. The replacement of a coordinative Ru-N bond with a covalent carbon-ruthenium bond led to a redshift and to a broadening in the optical absorptionofthecorrespondingrutheniumcomplex. FunctionalizationontheNˆCˆN’ ligand with another tpy resulted in the synthesis of dinuclear Ru(II)-complexes [134]. 38