Excited States of Polypyridine Complexes

Control of the Electron-transfer Properties of MLCT Excited States of Polypyridine Complexes... 

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. 

The character of the lowest excited state depends on the nature of the metal atom, the polypyridine ligand and the ancillary ligands. MLCT excited states are the lowest excited states of those complexes which contain easily-oxidized low-valent metal atoms Cr, Mo°, W°, Mn, Re Fe Ru Os, Pt and Cu These are the polypyridine complexes whose ground-state reduction and oxidation is predominantly localized on the metal atom and the polypyridine ligand, respectively (Section 5.3.1). MLCT electronic transitions are strongly allowed and usually occur in the visible spectral region. Hence, irradiation into MLCT absorption bands of poly-... 

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. 

Polypyridine rhodium(III) complexes (RM ) may be reduced by one-electron reductants The reductants which have been successfully employed include Ru(bpy)32+, the luminescent charge-transfer excited state of Ru(bpy)32+ (J, 9)... 

In the case of tetranuclear compounds belonging to the polypyridine family, all four possible energy migration patterns, schematized in Figure 21, have been obtained. Pattern (i) is found for L = bpy, BL = 2,3-dpp, and M = Ru. In such a complex, the three peripheral units are equivalent and Aeir lowest excited state lies at lower energy than the lowest excited state of the central unit.The reverse... 

Fig. 18. Oxidation and reduction potentials in aqueous solution of the lowest excited state of some polypyridine complexes. The M+/M and M+/ M potentials of the Cr complex axe lower limiting values...

While not yet as extensive as the chemistry reported for bipy and phen complexes of ruthenium, the chemistry of rhodium polypyridine complexes (especially their excited states) has generated much recent interest. The claim of Lehn and co-workers805 that excited states of [Rh(bipy)3]3+ are involved in the photoinduced generation of H2(g) from water has sparked renewed efforts to understand the rich excited state chemistry of these complexes. 

Apart from phosphine ligands, polypyridine ligands have been incorporated into platinum(II) alkynyl moieties to give luminescent complexes. The use of polypyridine ligands is interesting because their planar structure and the square-planar coordination geometry of the d platinum(II) center could modify the excited state of these luminescent complexes by tt tt and/or Pt- Pt interactions. The complex... 

The prominent electron transfer photochemistry of polypyridine complexes stems from a unique combination of their ground state redox reactivity and excited state properties ... 

These conditions are met by excited states of many polypyridine complexes of d metals and, in part, of d Cr. Electron transfer reactivity is then retained upon electronic excitation, giving rise to important photochemistry and possible applications. 

Since the ligand-field splitting of the d-orbital manifold of low-valent d metal atoms is relatively large, LF excited states of d -metals occur at higher energy than MLCT states, with the only exception of Fe F Hence, LF states are not involved in electron transfer reactivity but they can provide a non-radiative deactivation pathway for the reactive MLCT state, shortening its lifetime. LF states do not exist for d CuF The only polypyridine complexes with a redox-active LF state are [Cr(N,N)3] +, whose T/ E LF states are strong oxidants [278, 279]. 

Excited state lifetime tq is another important parameter to be controlled, especially if the photoactive complex is intended for bimolecular photochemical electron transfer. MLCT excited states of most polypyridine complexes decay both radia-tively and non-radiatively, with the respective rate constants and k r. The inherent excited state lifetime is defined as tq = l/( r + nr)- The non-radiative decay pathway in most cases prevails k r K. Hence To = 1 /km- Non-radiative decay of MLCT excited states can be treated as intramolecular electron transfer in the Marcus inverted region ... 

Finally, it should be noted that an enormous research effort have been spent on the development of photo-redox active polypyridine complexes and optimization of their excited state redox properties. The vast literature on this topic offers discussions and theories of the relations between molecular structure, medium and excited state redox properties and lifetimes, as well as a broad choice of polypyridine complexes suitable as photosensitizers, especially those based on Ru [9, 14, 28, 74, 95, 98, 151-153, 205, 217, 222, 224, 226, 287, 289, 290, 295, 297]. 

The lowest-lying excited states of tris-polypyridine Rh " and Ir complexes have intraligand nn character [37, 269]. These states are populated by UV irradiation. They are long lived (ps range) and very strong oxidants (e.g. -1-1.42 V for [Ir(bpy)3] +/[Ir(bpy)3] ), but relatively weak reductants. In supramolecular systems, Ir and Rh usually function as excited-state energy donors or ground-state electron acceptors. 

MLCT excited states of metal-polypyridine complexes are also produced by recombination of their oxidized and reduced forms. For example, the reaction... 

Bergkamp, M. A. Gutlich, P. Netzel, T. L. Sutin, N. Lifetimes of the ligand-to-metal charge-transfer excited states of iron(III) and osmium(III) polypyridine complexes. Effects of isotopic substitution and temperature. J. Phys. Chem. 1983, 87, 3877-3883. 

Table 1. Properties of the Excited States of Transition-Metal Polypyridine Complexes ...

Like the corresponding ground states (GSs), the excited states (ESs) of polypyridine complexes can either donate or accept an electron. The intrinsic kinetic barriers to these electron transfers are reflected in the self-exchange rates of the relevant couples. We thus distinguish oxidative ... 

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