Page - 41 - in Photovoltaic Materials and Electronic Devices

comparison to homolog complexes with terpyridine, dipyrazinyl-pyridine led to higher oxidation potential. Exchanging SCN improved HOMO and LUMO while substitutingtpywithdipyrazinyl-pyridine loweredthesevalues. 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. Figure 31. Example of dipyrazinyl-pyridine ligand [135]. Sepehrifard et al. [138,139] investigated a series of homoleptic bis-tridentate ruthenium complexes, employing both tpy and dipyrazinyl-pyridine ligands. The poorer performances of the latter ones were attributed to lower LUMO levels and weaker bonding to TiO2. The best results were obtained with terpyridine ligands bearing COOH grafting groups (1.53% efficiency) while the use of dipyrazinyl-pyridine ligands, ester groups or the introduction of a phenylene spacer between the pyridine and the anchoring group all resulted in lower efficiencies. 3.2.8. Triazolate Schulze et al. investigated triazolate as chelating moiety in a series of N^C^N’ cyclometalated ligands [140] and N^N’^N’’ ligands [141]. 1,3-Di(4-triazolyl)benzene and 2,5-di(4-triazolyl)pyridine Figure31. Exampleofdipyrazinyl-pyridine ligand[135]. Sepehrifard et al. [138,139] investigated a series of homoleptic bis-tridentate rutheniumcomplexes,employingbothtpyanddipyrazinyl-pyridine ligands. The poorer performances of the latter ones were attributed to lower LUMO levels and weaker bonding to TiO2. The best results were obtained with terpyridine ligands bearing COOH grafting groups (1.53% efficiency) while the use of dipyrazinyl-pyridine ligands, ester groups or the introduction of a phenylene spacer betweenthepyridineandtheanchoringgroupall resulted in lowerefficiencies. 3.2.8. Triazolate Schulze et al. investigated triazolate as chelating moiety in a series of NˆCˆN’ cyclometalated ligands [140]andNˆN’ˆN”ligands [141]. 1,3-Di(4-triazolyl)benzene and 2,5-di(4-triazolyl)pyridine were used in association with tctpy as the grafting moiety (42, Figure 32). In the case of the NˆCˆN’ ligand, the substitution with electron-withdrawing groups suchasF orNO2 stabilizes the HOMOenergy level providingblueshiftand loss incharge injection,whilehydrophobicalkyl chainsare expected to be beneficial for the long-term stability. The relatively low efficiency obtained as the best result (η = 4.9%; TiO2: 12 + 3µm, 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 NˆN’ˆN” ligand with respect to N749 (6.1%inthesameconditions,dippingsolution inethanol)wasexplainedbyloss in panchromaticabsorption. 41