Page - 42 - in Photovoltaic Materials and Electronic Devices

result (η = 4.9%; TiO2: 12 + 3 μm, dye: 0.25 mM methanol, electrolyte: 0.6 M 1,3-dimethylimidazolium iodide, 0.06 M I2, 0.1 M LiI, 0.5 M t-bupy, 0.1 M GuSCN in CH3CN) in the case of the N^N’^N’’ ligand with respect to N749 (6.1% in the same conditions, dipping solution in ethanol) was explained by loss in panchromatic absorption. Figure 32. Triazolate ligand studied by Schulze et al. [140,141]. 3.2.9. Other Ligands C^N^C’ ligands have been tested by Park et al. [142] in a series of bis-tridentate ruthenium complexes, exploiting N-heterocyclic carbenes such as 2,6-bis-(3-methylimidazolium-1-yl)pyridine (43a-c, Figure 33). Figure 33. Ru(II) complexes proposed by Park et al. [142]. X-ray crystal structure of 43b shows a typical geometry with both ligands coordinated in a meridional fashion; bond distances between Ru and the coordinated N or C are similar and the carboxyl function is deprotonated (Figure 34). Overall efficiencies were far from N719 tested in the same conditions, a result that was mainly attributed to low charge injection. Figure 34. ORTEP drawing of complex 43b [142] (Reprinted with permission from Park, H.-J.; Kim, K. H.; Choi, S. Y.; Kim, H.-M.; Lee, W. I.; Kang, Y. K.; Chung, Y. K. Unsymmetric Ru(II) Complexes Figure32. Triazolate ligandstudiedbySchulze et al. [140,141]. 3.2.9. OtherLigands CˆNˆC’ ligands have been tested by Park et al. [142] in a series of bis-tridentate ruthenium complexes, exploiting N-heterocyclic carbenes such as 2,6-bis-(3-methylimidazolium-1-yl)pyridine (43a-c, Figure33). were used in association with tctpy as the grafting moiety (42, Figure 32). In the case of the N^C^N’ ligand, the substitution with electron-withdrawing groups such as F or NO2 stabilizes the HOMO energy level providing blueshift and loss in charge injection, while hydrophobic alkyl chains are expe ted to be beneficial for the long-term stability. The relatively low efficiency obtained as the best result (η = 4.9%; TiO2: 12 + 3 μm, dye: 0.25 mM methanol, electrolyte: 0.6 M 1,3-dimethylimidazolium iodide, 0.06 M I2, 0.1 M LiI, 0.5 M t-bupy, 0.1 M GuSCN in CH3CN) in the case of the N^N’^N’’ ligand with respect to N749 (6.1% in the same conditions, dipping solution in ethanol) was explained by loss in panchromatic absorption. Figure 32. Triazolate ligand studied by Schulze et al. [140,141]. 3.2.9. Other Ligands C^N^C’ ligands have been tested by Park et al. [142] in a series of bis-tridentate ruthenium complexes, exploiting N-heterocyclic carbenes such as 2,6-bis-(3-methylimidazolium-1-yl)pyridine (43a-c, Figure 33). Figure 33. Ru(II) complexes proposed by Park et al. [142]. X-ray crystal structure of 43b shows a typical geometry with both ligands coordinated in a meridional fashion; bond distances between Ru and the coordinated N r C are similar an the carboxyl function is eprotonat d (Figur 34). Overall efficiencies were far from N719 tested in the same conditions, a result that was mainly attributed to low charge injection. Figure 34. ORTEP drawing of complex 43b [142] (Reprinted with permission from Park, H.-J.; Kim, K. H.; Choi, S. Y.; Kim, H.-M.; Lee, W. I.; Kang, Y. K.; Chung, Y. K. Unsymmetric Ru(II) Complexes X-ray crystal structure of 43b shows a typical geometry with both ligands coordinatedinameridional fashion;bonddistancesbetweenRuandthecoordinated N or C are similar and the carboxyl function is deprotonated (Figure 34). Overall efficiencieswerefarfromN719testedinthesameconditions,aresult thatwasmainly attributedto lowcharge injection. Bonacin et al. [143] proposed a complex of Ru(II) with carboxyphenyl tpy, thiocyanate, and 8-hydroxy quinoline in order to host a carboxymethyl cyclodextrin anchored to TiO2. Even if poor results were reported (ascribed to high HOMO potential and low regeneration), the host-guest interaction of the dye with the cyclodextrin increased the performances by preventing dye aggregation and limiting thedarkcurrent. 42