Page - 39 - in Photovoltaic Materials and Electronic Devices

Materials 2016, 9, 137 20 of 37 Figure 28. Crystal structure of complex 37 in form of its dimer [132] (Adapted from Ref 131 with permission of The Royal Society of Chemistry). N^N’^C cyclometalated compounds showed better sensitization properties respect to the bis-tpy complexes; while the lower efficiencies of the N^C^N' 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 absorption of the corresponding ruthenium complex. Functionalization on the N^C^N’ ligand with another tpy resulted in the synthesis of dinuclear Ru(II)-complexes [134]. Kisserwan et al. [135] further engineered the 6-phenyl-2,2’-bipyridyl (C^N^N’) ligand with a thiophene and carboxylic acid moieties in the 4- and 4’- positions of the bipyridine moiety (39, Figure 29). The thienyl group was chosen with the purpose of increasing the molar extinction coefficient, while COOH had the aim to further strengthen the coupling with TiO2. With respect to Wadman’s works, tctpy was used instead of tpy. The work focused more on electrolyte composition than on sensitizer design, providing better performances when CuI was used as an additive. The same group in 2012 [57] extendend the investigation on the 6-phenyl-2,2’-bipyridyl (C^N^N’) ligand, studying the influence of either donor or acceptor substituents on the phenyl and the presence of COOH on the bipyridine. When the thienyl group was replaced by COOH, lower efficiencies were observed, attributed to a less efficient electron injection. The best sensitizer was also studied for its long-term stability, showing better results when compared to N719. Figure 29. Bis-tpy-based Ru(II) complex proposed by Kisserwan et al. [135]. In 2011, Robson et al. [136] published an extensive study in which a series of asymmetric bis-tridentated ruthenium complexes was synthesized, whose ligands ranged from terpyridine (N^N’^N’’) to phenyl-bipyridine (C^N^N’) and di-(2-pyridyl)-benzene (N^C^N’), bearing anchoring Figure28. Crystal structureofcomplex37 in formof itsdimer [132] (Adaptedfrom Ref131withpermissionofTheRoyalSocietyofChemistry). Kisserwan et al. [135] furtherengineeredthe6-phenyl-2,2’-bipyridyl (CˆNˆN’) ligandwithathiopheneandcarboxylicacidmoieties in the4-and4’-positionsof the bipyridinemoiety (39, Figure29). Thethienylgroupwaschosenwiththepurpose of increasing the molar extinction coefficient, while COOH had the aim to further strengthen the coupling with TiO2. With respect to Wadman’s works, tctpy was used instead of tpy. The work focused more on electrolyte composition than on sensitizer design, providing better performances when CuI was used as an additive. The samegroupin2012[57]extendendthe investigationonthe6-phenyl-2,2’-bipyridyl (CˆNˆN’) ligand,studyingthe influenceofeitherdonororacceptorsubstituentson the phenyl and the presence of COOH on the bipyridine. When the thienyl group wasreplacedbyCOOH,lowerefficiencieswereobserved,attributedtoalessefficient electron injection. The best sensitizer was also studied for its long-term stability, showingbetter resultswhencomparedtoN719. Materials 2016, 9, 137 20 of 37 Figure 28. Crystal structure of complex 37 in form of its dimer [132] (Adapted from Ref 131 with permission of The Royal Society of Chemistry). N^N’^C cyclometalated compounds showed better sensitization properties respect to the bis-tpy complexes; while the lower efficiencies of the N^C^N' co plexes 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 absorption of the corresponding ruthenium complex. Functionalization on t e N^C^N’ ligand with another tpy resulted in the synthesis of dinuclear Ru(II)-com lexes [134]. Kisserwan et al. [135] further e gineered the 6-phenyl-2,2’-bipyri yl (C^N^N’) ligand with a thiophene and carboxylic acid moieties in the 4- and 4’- p sitions of the bipyridine moiety (39, Figure 29). The thienyl group was chosen with the purpose of increasing the molar extinction coefficient, while COOH had the aim to further strengthen the coupling with TiO2. With respect to Wadman’s works, tctpy was sed instead of tpy. The work focused more on electrol te composition than on sensitizer design, provi ing better performances when CuI was used as an additive. The same group in 2012 [57] exte dend the investigation on the 6-p enyl-2,2’-bipyridyl (C^N^N’) ligand, studying the influence of either donor or acceptor substituents on the phenyl and the presence of COOH on the bipyridine. When the thienyl group was replaced by COOH, lower efficiencies were observed, attributed to a less efficient electron injection. The best sensitizer was also studied for its long-term stability, showing better results when compared to N719. Figure 29. Bis-tpy-based Ru(II) complex proposed by Kisserwan et al. [135]. In 2011, Robson et al. [136] published an extensive study in which a series of asymmetric bis-tridentated ruthenium complexes was synthesized, whose ligands ranged from terpyridine (N^N’^N’’) to phenyl-bipyridine (C^N^N’) and di-(2-pyridyl)-benzene (N^C^N’), bearing anchoring electron-withdrawing groups on one ligand and, on the other, a thienyl-triphenylamino group as donor counterpart (40, Figure 30). A thorough investigation of the photophysical and electrochemical properties was pursued in order to understand the role of the organometallic bond Figure29. Bis-tpy-basedRu(II) complexproposedbyKisserwan et al. [135]. 39