Page - 40 - in Photovoltaic Materials and Electronic Devices

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 (NˆN’ˆN”) to phenyl-bipyridine (CˆNˆN’) and di-(2-pyridyl)-benzene(NˆCˆN’),bearinganchoringelectron-withdrawinggroupson one ligandand,ontheother,a thienyl-triphenylaminogroupasdonorcounterpart (40, Figure30). Athoroughinvestigationof thephotophysicalandelectrochemical propertieswaspursuedinorder tounderstandtheroleof theorganometallicbond andterminal substituentsand to tunetheenergetic levels. Broad absorptionspectra were generated in Ru(II) complexes containing an organometallic bond because of the electronic dissymmetry about the octahedral Ru(II) center. The intensity of thespectra in thevisibleregionwasenhancedwhentheorganometallicbondwas orthogonal to the principal axis (i.e., CˆNˆN’ ligand). When the anchoring ligand is represented by a NˆCˆN’ tridentate combination, the LUMO is placed remotely fromTiO2, andthispreventsanefficientcharge injection. Ontheotherhand, if the organometallic bond is placed on the donor ligand, HOMO level can be localized eitheronthetriphenylaminomoietyoronRu(II),maximizinglightharvesting inthe visibleregion;while,at thesametime, theLUMOontheanchoringligandensuresan efficientelectrontransfer towards thesemiconductorsurface. Thehighest recorded efficiency reached 8.02% (TiO2: 15 + 4.5µm, dye: 0.3 mM ethanol, Z1137 electrolyte: 1.0M 1,3-dimethylimidazoliumiodide, 60mM I2, 0.5M t-bupy, 0.05 MNaI, 0.1M GuNCSinCH3CN/valeronitrile85:15). Materials 2016, 9, 137 21 of 37 organometallic bond was orthogonal t the principal axis (i.e. C^N^N’ ligand). When the anchoring ligand is represented by a N^C^N’ triden ate combination, the LUMO is placed remotely from TiO2, and this prevents an efficient charge injection. On the other hand, if the organometallic bond is placed on the donor ligand, HOMO level can be localized either on the triphenyl amino moiety or on Ru(II), maximizing light harvesting in the visible region; while, at the same time, the LUMO on the anchoring ligand ensures an efficient electron transfer towards the semiconductor surface. The highest recorded efficiency reached 8.02% (TiO2: 15 + 4.5 μm, dye: 0.3 mM ethanol, Z1137 electrolyte: 1.0 M 1,3-dimethylimidazolium iod de, 60 mM I2, 0.5 M t-bupy, 0.05 M NaI, 0.1 M GuNCS in CH3CN / valeronitrile 85:15). Figure 30. Robson et al. [136] series of bis-tridentated ruthenium complexes bearing triphenyl amino groups. 3.2.7. Dipyrazinyl-Pyridine Another series of bis-tridentate complexes was reported in 2007 by Al-mutlaq et al. [137] using dipyrazinyl-pyridine ligands with different substituents on 4’- position, and cathecol moieties as grafting groups (41, Figure 31). In comparison to homolog complexes with terpyridine, dipyrazinyl-pyridine led to higher oxidation potential. Exchanging SCN improved HOMO and LUMO while substituting tpy with dipyrazinyl-pyridine lowered these values. Figure30. Robsonetal. [136]seriesofbis-tridentatedrutheniumcomplexesbearing triphenylaminogroups. 3.2.7. Dipyrazinyl-Pyridine Another series of bis-tridentate complexes was reported in 2007 by Al-mutlaq et al. [137]usingdipyrazinyl-pyridine ligandswithdifferentsubstituents on 4’- position, and cathecol moieties as grafting groups (41, Figure 31). In 40