2,2 -Bipyridine, 4,4 -dicarboxy

Scheme 3 shows the details of the synthetic strategy adopted for the preparation of heteroleptic cis- and trans-complexes. Reaction of dichloro(p-cymene)ruthenium(II) dimer in ethanol solution at reflux temperature with 4,4,-dicarboxy-2.2 -bipyridine (L) resulted the pure mononuclear complex [Ru(cymene)ClL]Cl. In this step, the coordination of substituted bipyridine ligand to the ruthenium center takes place with cleavage of the doubly chloride-bridged structure of the dimeric starting material. The presence of three pyridine proton environments in the NMR spectrum is consistent with the symmetry seen in the solid-state crystal structure (Figure 24). 

Figure 24 A view of the molecular structure of [Ru(4,4 dicarboxy-2,2 -bipyridine)Cl(cymene)]Cl showing the symmetrical orientation of 4,4 dicarboxy-2,2 -bipyridine ligand in relation to the cymene...

As described in Section II.A.3, dx-bis(4,4 -dicarboxy-2,2 -bipyridine)dithiocya-nato ruthenium(II) (N3 dye) and trithiocyanato 4,4 4"-tricarboxy-2,2 6, 2"-terpyr-idine ruthenium(II) (black dye) exhibit quite good performance therefore, the systems using these two Ru dyes have been intensively investigated. However, other Ru dye photosensitizers have also been synthesized and characterized, and their performances as photosensitizers in DSSCs have been reported by many researchers [98-116]. Several kinds of electron withdrawing ligand for Ru dye photosensitizers are shown in Fig. 13. Figure 14 shows the structures of newly synthesized Ru dye photosensitizers and their absorption properties. The axis shows the molar absorption coefficient, e (i.e., absorption coefficient per M unit M 1 cm-1). 

New metal complexes other than Ru have also been investigated. These include Fe complexes [117,118], Os complexes [119-122], Re complexes [123], and Pt complexes [124]. A nanocrystalline 2 solar cell sensitized by a square-planar platinum(II) complex containing 4,4 -dicarboxy-2,2 -bipyridine and quin-oxaline-2,3-dithiolate ligands showed an efficiency of 2.6% (Jsc = 6.14 mA/cm2... 

Other recent work on the temperature dependence of the luminescence behavior of Os(II) diimine complexes was reported by Ogawa and coworkers in the investigation of [Os(bpy)2(4,4/-dcbpy)] and [Os(bpy)2(3,5-dcbpy)], where 4,4 -dcbpy = 4,4/-dicarboxy-2,2/-bipyridine and 3,5-dcbpy = 3,5-dicarboxy-2,2/-bipyridine. The 4,4 -dcbpy complex exhibited a decrease in the luminescence lifetime with increasing temperature and had an activation barrier of approximately 350 cm x. The luminescence lifetime of the 3,5-dcbpy complex, however, increased with increasing temperature. This surprising observation remains unexplained [14]. 

The lowest energy MLCT transition of Ru polypyridyl complexes of the type tris-[Ru(4,4/-dicarboxy-2,2/-bipyridine)3] (1), can be lowered so that it absorbs more in the red region of the visible spectrum by replacing one 4,4/-dicarboxy-2,2/-bipyridine (dcbpy) with two thiocyanate donor ligands [Ru(dcbpy)2(NCS)2] (2). In complex 2, the two 4,4/-dicarboxylic acid 2,2 -bipyridine ligands pull while the two thiocyanate donor ligands push electrons. The oxidation potential of the complex 2 is 0.85 V vs. SCE, which is cathodically shifted significantly (0.65 V vs. SCE) compared to the homoleptic type of complex 1, which shows Ru(III/II) couple at 1.5 V vs. SCE. Thus, the... 

The absorption spectra of complexes 18-20 are dominated by the MLCT transitions in the visible region, and the lowest allowed MLCT bands appearing at 400 and 545 nm. The molar extinction coefficients of these bands are close to 35 000 and 19 000 M 1cm 1, respectively, which are significantly higher than those of the standard sensitizer ris-dithiocyanatobis(4,4 -dicarboxy-2,2/-bipyridine)Ru(II), (2) (Fig. 9). 

The X-ray and UV-photoemission spectroscopy study has revealed that ruthenium (II) phthalocyanine 20 has similar HOMO and LUMO energy with that of cis-bis(4,4-dicarboxy-2,2-bipyridine)-bis-(isothiocyanato)-ruthenium(II), [Ru(dcbpy)2 (NCS)2], the most efficient sensitizer so far. The oxidation potentials for both... 

Ellington RJ, Asbury JB, Ferrene S, Ghosh HN, Sprague JR, Lian T, Nozik AJ. Dynamics of electron injection in nanocrystalline titanium dioxide films sensitized with [Ru(4,4,-dicarboxy-2,2 -bipyridine)2(NCS)2] by infrared transient absorption. J Phys Chem B 1998 102 6455-6458. 

Mallouk and co-workers [6, 26, 206] reported the photodecomposition of acidic KI to H2 and iodine under visible light irradiation. For this purpose, IQNbeOn was internally loaded with Pt, ion exchanged partially with acid to obtain K4 jcHj Nb60i7 /JH2O (x 2.5) and then sensitized with RuLs (L = 4,4 -dicarboxy-2,2 -bipyridine). It was found that KI is the best reactant among the alkali metal iodides (Figure 16). 

Kim et al. [6] reported hydrogen evolution from aqueous Kl/Csl solutions under visible light irradiation k > 400 nm) employing several KTiNbOs-type oxides and KNb30g (see Table 7). In these experiments, internally Pt-loaded oxides were proton exchanged and then sensitized using a monolayer of Ru-L3 (L = 4,4 -dicarboxy-2,2 -bipyridine). All the sensitized oxides exhibited photocatalysis. There was no... 

The photoactivity of the Pt/Ti02 system in the visible region was improved [208] by the addition of the sensitizer ([Ru(dcbpy)2(dpq)]2+) [where dcbpy = 4,4 -dicarboxy 2,2 -bipyridine and dpq = 2,3-bis-(2 -pyridyl)-quinoxaline] leading to an efficient water reduction. 

Kruger J, Plass R, Gratzel M, Matthieu H-J (2002) Improvement of the photovoltaic performance of solid-state dye-sensitized device by silver complexation of the sensitizer cis-bis(4,4-dicarboxy-2,2-bipyridine)-bis(isothiocyanato)ruthenium(II). Appl Phys Lett 81 367... 

In another research, fullerene (Ceo) was attached to N3 dye (cis-bis(4,4-dicarboxy-2,2-bipyridine)dithiocyanato ruthenium(II)) via diaminohydrocarbon linkers (L) with different carbon chain lengths. The results of this study indicated that in the case of the linker 1,6-diaminohexane, the current density, applied potential and conversion efficiency of the pertaining cell were 11.75 mA/cm, 0.70 V and 4.5%, respectively, as compared with the values of 10.55 mA/cm, 0.68V and 4.0%, respectively, for a DSSC with an ordinary N3 dye [198]. 

Other studies complemented the previous results and have also shown that a homochiral ion-pairing is favoured for Ru(DEAS-bpy)3 + complex (4.9) DEAS-bpy = 4,4 -bis(diethylamino-styryl)-[2,2 ]-bipyridine " and for the compounds [Ru(bpy)2(L-L)] [PFgli L-L = cmbpy = 4-carboxy-4 -methyl-2,2 -bipyridine (4.10) L-L = dcbpy = 4, 4 -dicarboxy-2,2 bipyridine (4.11). Interestingly, in the previous two examples the octahedral metal complexes have D3 symmetry as A-TRISPHAT (4.7) while in the last example complex 4.10 possesses a C symmetry while 4.11 displays a C2-symmetry (Figure 4.6). These results suggest that a homochiral association between A-TRISPHAT and the octahedral ruthenium complexes is not dependent on a prerequisite D3-symmetry of the metal complex. 

Acid-Base Equilibria of the 4,4-Dicarboxy-2,2/-Bipyridine and its Complexes... 

In a related complex of the type [Ru(bpy)2(dcbpy)], two inflection points at pH 2.7 and <0.5 were found suggesting that the dissociation of the carboxylic acid groups of 4,4-dicarboxy-2,2 -bipyridine ligand is a sequential process. The pKai of the free 4,4-dicarboxy-2,2 -bipyridine ligand is 4.2 and the second one is below 2. The difference between the free ligand and... 

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