Development of bis(tridentate) ruthenium(II) complexes for various applications

This thesis describes the synthesis of several novel ruthenium(II) polypyridyl and cyclometalated complexes, some of which bear carboxylic and phosphonic acid and methyl ester groups and the study of their spectroscopic and electrochemical properties. The incorporation of carboxylic and phosphonic acid (-COOH, -PO(OH)2 moieties into the ligands allows for attachment of the complexes to the TiO2 semiconductor in DSSCs or Peptide Nucleic Acids (PNAs). Moreover, the methylester substituent also has the ability to be hydrolysed and be applied in DSSCs or bio-application. Ruthenium(II) derivatives of a variety of tridentate ligands were prepared using [Ru(L)Cl3] as the precursor. The polypyridyl and cyclometalated Ru(II) complexes prepared included, [Ru(tpyPh)(tpyCOOH)](PF6)2 (1), [Ru(tpyPh)(tpy(PO)(OH)2)](PF6)2 (2), [Ru(tpyPh)(dqpCO2Me)](PF6)2 (3), [Ru(tpyCO2Me)(dqp)](PF6)2 (4), [Ru(dqpCO2Me)(dqp)](PF6)2 (5), [Ru(tpyPh)(N^C^N)](PF6) (6), [Ru(tpyCO2Me)(N^C^N)](PF6) (7), [Ru(dqpCO2Me)(N^C^N)](PF6) (8), (tpyPh = 4'-phenyl-2,2':6',2''-terpyridine, tpyCOOH = 2,2':6',2''-terpyridine-4'-carboxylic acid, tpyCO2Me = 4'-methylcarboxy-2,2':6',2''-terpyridine, dqpCO2Me = methyl-2,6-di(quinolin-8-yl)isonicotinate and dqp = 2,6-di(quinoline-8-yl)pyridine, N^C^N = 1,3-di(2-pyridyl)benzene) were successfully synthesised in this work. For the complexes 2-8, the dehalogenation strategy was applied in which the precursor [Ru(L)Cl3] was treated with either Ag(I) in acetone, or MeCN in the case of complex (5). The UV-Vis spectroscopic studies of complexes 1-5 showed a slightly red-shifted in MLCT bands for complexes 1 and 2 compared to [Ru(tpy)2]2+, which reflected the effect of the electron withdrawing properties of the carboxylic and phosphonic acid moieties. For the other three complexes 3, 4 and 5 containing bis(quinoline)pyridine (dqp), the absorption is noticeably red shifted compared to [Ru(tpy)2]2+ and the visible region is dominated by (dπ→π*) 1MLCT bands. The emission spectroscopic studies of these polypyridyl Ru(II) complexes were in agreement with the absorption behaviour. Red shifts in the emission maxima were observed for the first two complexes 1 and 2 to 683 and 658 nm, respectively, relative to [Ru(tpy)2]2+ (629 nm), due to the effect of carboxylic and phosphonic acid substituent at the 4'-position of tpy ligand. These complexes exhibited 3MLCT excited state lifetimes of ≤ 15 ns. The dqp based complexes 3, 4 and 5 show fairly strong emission at room temperature in contrast to typical bis(tridentate) Ru(II) complexes, [Ru(tpy)2]2+. For complexes 3 and 4, the emission maximum was around 680 nm. The excited state lifetimes (τ) for these two complexes were 0.50 μs and 0.44 μs, respectively. For the final polypyridyl complex involved two dqp, the longest excited state lifetime of 3.76 μs was recorded which can be rationalized by the closer to octahedral ruthenium geometry, raising the energy level of the short-lived 3MC states. The cyclic voltammetry of these polypyridyl complexes exhibited a reversible one electron process associated with the RuII/III redox process at most of the scan rates. The anodic effect was observed for complexes 1 and 2 as a result of the electron withdrawing effect arising from the direct installation of the carboxylic or phosphonic acid groups to the 4'-tpy position with respect to [Ru(tpy)2]2+, (viz; a shift to be of 100 mV and 10 mV respectively). For the dqp complexes 3, 4, the greater electron withdrawing ability of tpy or dqp, results in a higher oxidation potential. For the final polypyridyl complex 5, the reversible one-electron RuIII/II oxidation is observed at lower potential than [Ru(tpy)2]2+ due to less efficient π back bonding in this complex compared to [Ru(tpy)2]2+. Reversible potentials associated with the RuII/III redox couple for complexes 1-5 were in the range of 837-1026 mV vs Fc0/+. Furthermore, the ligand based reduction processes were observed at negative potentials for all complexes and are sensitive to the substituents on the ligand. Extending the conjugation of the ligand shifts the LUMO to lower energy, while increasing electron density on the metal centre raises the energy of the HOMO, and to a lesser extent, the LUMO. The second series of Ru(II) complexes developed in this thesis were the cyclometalated complexes, [Ru(tpyPh)(N^C^N)](PF6) (6), [Ru(tpyCO2Me)(N^C^N)](PF6) (7), [Ru(dqpCO2Me)(N^C^N)](PF6) (8) which were also synthesised via dehalogenation method. Investigation using 1H NMR spectroscopy showed significant changes in the tpy chemical shifts as a result of cyclometalation. In the UV-Vis spectroscopic studies of 6, 7 and 8, a bathochromic shift and broadening of the MLCT transition with a slightly decrease in molar absorption coefficient compared to the polypyridyl Ru(II) complexes was observed as a result of cyclometalation. Among the three cyclometalated Ru(II) complexes, the only emission spectrum obtained was recorded for complex 6 but the emission yields were very low. The other two cyclometalated complexes 7, 8 were not luminescent at room temperature. The cyclic voltammetry of these complexes showed that the RuII/III oxidation potential can be significantly affected by the anionic cyclometalating ligand, due to the electron rich nature of this ligand. A cathodic shift of about (700-800) mV for the metal-based oxidation was observed compared to polypyridyl Ru(II) complexes. In terms of ligand based reduction, there was only one reduction wave observed for the cyclometalated complexes, which were assigned to the tpy ligand for complexes 6, 7 and dqpCO2Me for complex 8. Overall, a number of new ruthenium(II) polypyridyl and cyclometalated complexes have been synthesised and demonstrated to exhibit interesting photophysical and electrochemical properties that could make them suitable candidates for biosensor application as well as sensitizer in dye sensitized solar cells (DSSCs) and water splitting devices. Future work would involve investigation into the incorporation of these complexes via carboxylic or phosphonic acid 1, 2 or ester moieties 3, 4, 5, 7 and 8 as dyes to the TiO2 in DSSCs or introducing them at any chosen site of peptide conjugates and DNA or DNA mimics, such as Peptide Nucleic Acids (PNAs).