Polypyridine complexes

Dendrimers built around a metal complex as a core. These compounds can be considered metal complexes of ligands carrying dendritic substituents (Fig. 1 a). The most commonly used metal complex cores are porphyrin complexes, polypyridine complexes, and ferrocene-type compounds. 

The electroactive units in the dendrimers that we are going to discuss are the metal-based moieties. An important requirement for any kind of application is the chemical redox reversibility of such moieties. The most common metal complexes able to exhibit a chemically reversible redox behavior are ferrocene and its derivatives and the iron, ruthenium and osmium complexes of polypyridine ligands. Therefore it is not surprising that most of the investigated dendrimers contain such metal-based moieties. In the electrochemical window accessible in the usual solvents (around +2/-2V) ferrocene-type complexes undergo only one redox process, whereas iron, ruthenium and osmium polypyridine complexes undergo a metal-based oxidation process and at least three ligand-based reduction processes. 

Denti, G., Campagna, S., Serroni, S., Ciano, N., and Balzani, V. (1992) Decanuclear homo- and hetero-metallic polypyridine complexes Synthesis, absorption spectra, luminescence, electrochemical oxidation, and intercomponent energy transfer./. Am. Chem. Soc. 114, 2944-2950. 

The historical development and elementary operating principles of lasers are briefly summarized. An overview of the characteristics and capabilities of various lasers is provided. Selected applications of lasers to spectroscopic and dynamical problems in chemistry, as well as the role of lasers as effectors of chemical reactivity, are discussed. Studies from these laboratories concerning time-resolved resonance Raman spectroscopy of electronically excited states of metal polypyridine complexes are presented, exemplifying applications of modern laser techniques to problems in inorganic chemistry. 

The photochemical and photophysical properties of Ru(bpy)3 and related d° polypyridine complexes have been the subject of intense recent interest (43-49). This is due to their potential in photochemical energy conversion, their intrinsically significant excited state behavior, and their attractive chemical properties. The structures of the excited states of these complexes are clearly of great interest. 

Laser Studies of Radiationless Decay Mechanisms in Os2+/3+ Polypyridine Complexes... 

P. R. Ashton, R. Ballardini, V. Balzani, E. C. Constable, A. Credi, O. Kocian, S. J. Langford, J. A. Preece, L. Prodi, E. R. Schofield, N. Spencer, J. E Stoddart, S. Wenger, Ru(II)-Polypyridine Complexes Covalently Linked to Electron Acceptors as Wires for Light-Driven Pseudorotaxane-Type Molecular Machines , Chem. Eur. J. 1998, 4, 2411-2422. 

Dixon IM, Lebon E, Sutra P, Igau A (2009) Luminescent ruthenium-polypyridine complexes phosphorus ligands anything but a simple story. Chem Soc Rev 38 1621-1634... 

Table 6 Formal electrode potentials (V vs. SCE) for the redox processes exhibited by polypyridine complexes of chromium...

All the three polypyridyl complexes display the reversible reduction sequence 2 + / + /0. The relative potential values are reported in Table 7. As far as the nature of such redox changes is concerned, it is important to recall the ambiguity that exists in attributing metal-centred or ligand-centred redox processes for metal-polypyridine complexes. 

Table 13 summarizes the electrode potentials of the redox processes for the complete set of polypyridine complexes. 

Passing to the polypyridine complexes, Figure 111 shows the octahedral geometry of the [Ni(bipy)3]2+ ion.163... 

The redox potentials of the nickel(II) polypyridine complexes are reported in Table 16.167 168... 

As usual, we conclude with the CuN6 coordination of polypyridine complexes. As an example, Figure 132 shows the solid state structure of [Cu(terpy)2]2 +. 185 The X-ray structure of [Cu(bipy)2]2+ and [Cu(phen)3]2+ are also available.186,187... 

In these cases, there is the expected tetragonal distortion due to the Jahn-Teller effect in addition to the often mentioned octahedral distortion associated with polypyridine complexes. For the terpyridine complex there is the usual difference between the distances separating the metal and nitrogen atom of the central pyridine and that of the metal and the nitrogen atom of the peripheral pyridines. 

A. Juris, V. Balzani, F. Barigellitti, S. Campagna, P. Belser, andA. vonZelewsky, Ru(II) Polypyridine complexes Photophysics, photochemistry, electrochemistry, and chemiluminescence, Coord. Chem. Rev. 84, 85-277 (1988). 

Since the propensity to form adducts in chemistry is high and these adducts undergo a variety of reactions, the rate law (1.98) is quite common. This is particularly true in enzyme kinetics. In reality, these reaction schemes give biphasic first-order plots but because the first step is usually more rapid, for example between A and B in (1.101) we do not normally, nor do we need to, examine this step in the first instance. The value of A", in (1.107) obtained kinetically can sometimes be checked directly by examining the rapid preequilibrium before reaction to produce D occurs. In the reactions of Cu(I) proteins with excited Cr and Ru polypyridine complexes, it is considered that (a) and (b) schemes may be operating concurrently. 

The polymers derived from ruthenium(II)-polypyridine complexes have demonstrated promising potential for application in solar energy conversion, sensors, polymer-supported electrodes, nonlinear optics, photorefraction, and electroluminescence [27-32]. 

Thus, the synthetic control translates into a high degree of control on the direction of energy flow within these molecules. For the well-known correlation between electrochemical and photophysical properties in these polypyridine complexes, the above series is the same as that reported in Section IV.B. 

Probably the larger class of light-harvesting dendrimers investigated up to now is constituted of dendrimers based on Ru(II) and Os(II) polypyridine complexes, and the more extensive studies within these species involve the systems containing... 

Rotaxane 316+ was specifically designed36 to achieve photoinduced ring shuttling in solution,37 but it also behaves as an electrochemically driven molecular shuttle. This compound has a modular structure its ring component is the electron donor macrocycle 2, whereas its dumbbell component is made of several covalently linked units. They are a Ru(II) polypyridine complex (P2+), ap-terpheny 1-type rigid spacer... 

The electrochemical properties of another series of Cu(I) complexes, based on substituted bipyridine and quinoline derivatives, have been also investigated68 (Fig. 17.31). To stabilize the Cu(I) oxidation state of Cu(I) polypyridine complexes, electron-withdrawing substitutents like esters have been considered. The same effect was also obtained with pyridyl-quinoline and biquinoline complexes, thanks to the increased 7i-accepting properties of the quinoline condensed aromatic ring. 

Room temperature emission has been observed for a number of transition metal complexes. Examples include Rh111 ammines,53 [Pt(CN)4]2-,54 and some Cu1 phosphine complexes.55 An important class is that of the polypyridine complexes of Ru11 and related species.56 This last emission, probably from a 3CT state, is quite strong and its occurrence has made possible a number of detailed studies of electron transfer quenching reactions. 

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