Page - 30 - in Photovoltaic Materials and Electronic Devices

favourable oxidationand reduction potentials, the overall performanceswere quite low,mainlydueto thepoorgeneratedphotocurrent thatwaspossiblyrelatedtoan unfavorable localizationofLUMO,far fromtheanchoringsitesontitania. (bottom). Very recently, Koyyada et al. [100] reported other bipyridines 4,4’- substituted with fluoren-2-yl (21a in Figure 16) or carbazol-3-yl (21b) groups, as ancillary ligands. Even if the proposed structures reported good molar extinction coefficients and favourable oxidation and reduction potentials, the overall performances were quite low, mainly due to the poor generated photocurrent that was possibly related to an unfavorable localization of LUMO, far from the anchoring sites on titania. Figure 16. Bipyridine ancillary ligands with fluoren-2-yl or carbazol-3-yl substitutions [100]. In 2015 Pavan Kumar et al. [101] modified complex 6 [77] by substituting two thiocyanates with an asymmetrical bipyridine ligand bearing hexylthiophene and mesityl subtituents on each pyridine ring (22, Figure 17). Figure 16. Bipyridine ancillary ligands with fluoren-2-yl or carbazol-3-yl substitutions [100]. In2015PavanKumar et al. [101]modifiedcomplex 6 [77]bysubstitutingtwo thiocyanateswithanasymmetricalbipyridine ligandbearinghexylthiopheneand mesityl subtituentsoneachpyridinering(22, Figure17). Materials 2016, 9, 137 14 of 37 Figure 17. Ancillary ligands modifications of complex 6 [101]. In the same paper, a Ru complex was reported, in which the bipyridine bears two carboxyl substituents. While having four anchoring groups, this complex led to lower efficiencies (23, Figure 18). With similar purposes, Kanniyambatti [76] modified complex 5, achieving a three-anchored sensitizer (24, Figure 18) with higher molar extinction coefficient and higher efficiency with respect to both complex 5 and BD tested in the same conditions ( = 7.5 vs 6.1%; TiO2: 10 + 4 μm, dye: 0.5 mM t-butanol / acetonitrile 1:1 with with CDCA 0.5 mM, electrolyte: 0.6 M [bmim][I], 0.03 M I2, 0.1 M GuSCN and 0.5 M t-bupy in CH3CN / valeronitrile 85:15). Figure17. Ancillary ligandsmodificationsofcomplex 6 [101]. In thesamepaper,aRucomplexwasreported, inwhichthebipyridinebears two carboxyl substituents. While having four anchoring groups, this complex led to lower efficiencies (23, Figure 18). With similar purposes, Kanniyambatti [76] modifiedcomplex5, achievingathree-anchoredsensitizer (24,Figure18)withhigher molar extinction coefficient and higher efficiency with respect to both complex 5 and 30