Chromophores 2, 2 -bipyridine

Meisel etal. [18-20] were the first to investigate how the addition of a polyelectrolyte affects photoinduced ET reactions. They found that charge separation was enhanced as a result of the retardation of the back ET when poly(vinyl sulfate) was added to an aqueous reaction system consisting of tris(2,2 -bipyridine)ruthenium(II) chloride (cationic photoactive chromophore) and neutral electron acceptors [21]. More recently, Sassoon and Rabani [22] observed that the addition of polybrene (a polycation) had a significant effect on separating the photoinduced ET products in an aqueous solution containing cir-dicyano-bis(2,2 -bipyridine)ruthenium(II) (photoactive donor) and potassium hexacyano-ferrate(III) (acceptor). These findings are ascribable to the electrostatic potential of the added polyelectrolytes. 

Tris(2,2 -bipyridine)ruthenium(II) complex (Ru(bpy)3+) has been most commonly employed as a chromophore in the studies of photoinduced ET. Electrostatic effects on the quenching of the emission from the Ru(II) complex covalently bound to polyeletrolytes have been studied by several groups [79-82]. 

Dendrimer 1 + is a classical example of a dendrimer containing a luminescent metal complex core. In this dendrimer the 2,2 -bipyridine (bpy) ligands of the [Ru(bpy)3] +-type core carry branches containing 1,2-dimethoxybenzene- and 2-naphthyl-type chromophoric units [15]. 

Meyer and coworkers investigated the photophysical behavior of vinyl containing Ru(II) and Os(II) complexes electropolymerized into the channels of silica sol-gel modified ITO electrodes. The monomeric complexes, [Ru(vbpy)3]2+ and [Os(vbpy)3]2+ (vbpy = 4-methyl-4/-vinyl-2,2/-bipyridine), have excited state lifetimes of approximately 900 and 60 ns, respectively. Incorporation into the sol-gel pores and polymerization (reductive polymerization initiated at the ITO electrode) results in chromophores that exhibit a remarkably small amount of self-quenching and have domains that reflect relatively isolated chromophores with excited state lifetimes longer than the solution values [125]. 

A chromophore such as the quinone, ruthenium complex, C(,o. or viologen is covalently introduced at the terminal of the heme-propionate side chain(s) (94-97). For example, Hamachi et al. (98) appended Ru2+(bpy)3 (bpy = 2,2 -bipyridine) at one of the terminals of the heme-propionate (Fig. 26) and monitored the photoinduced electron transfer from the photoexcited ruthenium complex to the heme-iron in the protein. The reduction of the heme-iron was monitored by the formation of oxyferrous species under aerobic conditions, while the Ru(III) complex was reductively quenched by EDTA as a sacrificial reagent. In addition, when [Co(NH3)5Cl]2+ was added to the system instead of EDTA, the photoexcited ruthenium complex was oxidatively quenched by the cobalt complex, and then one electron is abstracted from the heme-iron(III) to reduce the ruthenium complex (99). As a result, the oxoferryl species was detected due to the deprotonation of the hydroxyiron(III)-porphyrin cation radical species. An extension of this work was the assembly of the Ru2+(bpy)3 complex with a catenane moiety including the cyclic bis(viologen)(100). In the supramolecular system, vectorial electron transfer was achieved with a long-lived charge separation species (f > 2 ms). 

The first detailed study of the driving force dependence for photoinduced ET was reported by Elliott and co-workers [89]. The dyads used in Elliott s work consist of a tris-diimine Ru(II) chromophore covalently linked to a series of quatemized 2,2 -bipyridine (diquat) electron acceptors, 19a-c (Scheme 10, Table... 

The following sections are purposely separated into specific structural classes of square planar Pt" complexes of the general formulae Pt(NAN)(C=CR)2, [Pt(NANAN) (OCR)]+, Pt(NANAC)(C=CR), rra s-Pt(PR3)2(OCR)2, and d.v-Pt(PAP)(( =CR)2, where NAN is a bidentate 2,2 -bipyridine, NANAN and NANAC are tridentate polypyridines, PR3 is a monodentate phosphine, and PAP is a bidentate phosphine ligand. The final section of this work is dedicated to recent electronic structure calculations on these molecules with an emphasis on the successful application of DFT (density functional theory) and TD-DFT (time-dependent density functional theory) methods towards understanding the absorption and emission processes of these chromophores. 

While artificial photosynthetic mimics come in many manifestations, our efforts have focused predominantly on the class of molecules represented by the structure in Fig. 10.1. These molecules consist of a visible-light-absorbing chromophore in the form of a trisbipyridineruthenium(ll) complex (C) finked by flexible polymethylene chains to one or more electron donors (D) and an electron acceptor (A). The electron acceptor is anAl,W-dialkylated-2,2 -bipyridine (a so-called diquat ) and the electron donors are Al-aUcylated phenothiazines. The diquat type acceptor was chosen because... 

Dendrimer 1 (Fig. 5) is a classical example of a dendrimer built around a metal complex core. In this compound, the 2,2 -bipyridine ligands, that constitute the first coordination sphere of the Ru ion, carry branches containing 1,3-dimethoxybenzene and 2-naphthyl chromophoric imits separated by aliphatic connectors (10). Since the interchromophoric interactions are weak, the absorption spectrum of 1 is substantially equal to the summation of the spectra of [Ru(bpy)s], which is characterized by a broad spin-allowed Ru bpy metal-to-ligand (MLCT) band around 450 nm (11), and of the chromophoric groups contained in the branches, which show very intense bands in the near UV region. 

Veggel and coworkers [137, 138] first reported the use of ruthenium(II) tris(2,2 -bipyridine) complexes ([Ru(bpy)3] +) and ferrocene as light-harvesting chromophores for sensitization of NIR luminescence from Nd(III) and Yb(III) ions. The Ru-Ln complexes (Ln = Nd 104, Yb 105) resulted from incorporating [Ru(bpy)3] + with m-terphenyl-based lanthanide complexes. Upon excitation of the Ru(bpy)s chromophore absorption with visible light up to 500 nm, both Ru-Nd and Ru-Yb complexes exhibited typical NIR luminescence because of effective Ru Ln energy transfer with the rates of 1.1 x 10 s for Ru-Nd complex and <1.0 X 10 s for Ru—Yb species. 

Xu, H.-B., Shi, L.-X., Ma, E., et al (2006) Diplatinum alkynyl chromophores as sensitisers for lanthanide luminescence in R2Ln2 and R2Ln4 (Ln = Eu, Nd, Yb) arrays with acetylide-functionalized bipyridine/phenanthroline. Chemical Communications, 1601. 

Several kinds of supramolecular effects on the redox behavior of metal-polypyridine units were mentioned in Section 5.3.6. Besides influencing photophysical properties, incorporation of metal-bipyridine chromophores into supramolecular structures enables new electron-transfer reactions. Since these processes are dealt with in detail in other chapters, only basic principles and links between the behavior of isolated and supramolecular metal-polypyridine units will be mentioned here. 

The most widely used electron acceptors in inorganic chromophore-quencher systems have been bipyridinium dications, often called viologens (quatemarized derivatives of 4,4 -bipyridine) or diquat (cyclic quatemarized derivatives of 2,2 -bipyridine). The classical studies of Elliott, Schmehl, and Mallouk have been concentrated on dyads of types (4) and (5). For dyads (4) [168, 169], oxidative PET takes place, with forward processes in the 80 to 1700-ps time scale and very fast (<30 ps) charge recombination. The main observations are that (i) electron transfer to the diquat quencher occurs from the directly linked bipyridine ligand (ii) fast equilibration between the MLCT excited states on the three bipyridine ligands precedes electron transfer (iii) the electron transfer rates are in the normal Marcus... 

Viologens have been used as covalently linked quenchers for Cu(I) bipyridine chromophores, leading to fast (<10 ns) charge separation and remarkably slow (30 ns to 2 ps, depending on solvent) charge recombination [174]. More complex systems, similar to (5) but with two viologens on the same bypyridine of the Ru(II) chromophore, have been designed to mimic the presence of two ET in the reaction center of natural photosynthesis [175]. 

Another example of modulation of the electronic interaction between two metal units is schematically represented by compound 41 in Scheme 2(d), for which upon complexation of the vacant 2,2 -bipyridine a perturbation of the luminescence properties of the ruthenium moiety was reported [100]. Methylation caused a quenching of the emission, most likely due to photoinduced electron transfer from the terminal chromophores to the central viologen-type unit. 

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