Photochemistry and Photophysics of Coordination Compounds

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62 N.A.P. Kane-Maguire nal fate of the nitrene intermediate was not established, Sriram and Endicott noted that quasi-thermodynamic calculations suggested the lowest energy product ground state for this system would be a Cr(V) species [133]. More recent photochemical investigations on azido complexes of Cr(III) have focused on systems containing tetradentate ligands such as N2O4 Schiffbases [135] and N3 or N4 macrocyclic ligands [136, 137] as stable non-leaving groups. For many of these systems, air-stable, solid products have been isolated, and have been fully characterized by a variety of spectroscopic probes (including X-ray crystallography). These studies provide convincing evidence for the formation of stable Cr(V) complexes containing the nitrido ligand, N3– , formed via the generic photoreaction shown in Eq. 5: [CrIII – N3] 2+ + hν → [CrV ≡ N] 2+ +N2 (5) Evidence for a Cr(V) product is based on the diagnostic EPR signature displayed by this d1 metal ion. The presence of a Cr ≡ Ntriplebondinthe product is also in accord with the short Cr – N bond distances observed in Xray crystallographic studies, and the presence of a strong infrared absorption in the 1020–1150 cm –1 region [135–138]. During the present review period, the charge transfer photochemistry of several azido-Cr(III) complexes containing new Schiff-base ligands as the non-leaving groups were examined [139, 140]. In the first of these papers, no crystallographic evidence was presented for Cr(V)-nitrido product formation, but this product assignment was strongly supported by EPR and infrared spectral results [139]. In the second contribution, a potentially valuable biochemical application of azido-Cr(III) photochemistry is reported by Shrivastava and Nair [140]. An azido-Cr(III) Schiff-base complex was irradiated in the presence of bovine serum albumin (BSA), and the photolyte examined by sodium dodecyl sulfate-polyacrylamide disc electrophoresis (SDS-PAGE). The SDS-PAGE results revealed that the BSA protein was cleaved at multiple sites, non-specifically into smaller peptide fragments. The protein cleavage was attributed to the azido-Cr(III) complex binding at multiple sites, and being subsequently converted to the reactive nitrido-Cr(V) species upon light activation. This light-promoted protease activity bears some analogies with the photonuclease activity discussed earlier for [Cr(diimine)3] 3+ interactions with DNA (Sect. 7.1). A non-selective photonuclease could be utilized in a variety of applications, including protein sequencing. 9 Final Comments In this chapter, the author has attempted to provide an overview of recent progress in the field of Cr(III) photochemistry and photophysics, with a more detailed focus on certain topics of interest. For cohesiveness, it was not pos-
Photochemistry and Photophysics of Coordination Compounds: Chromium 63 sible to cover some aspects of the field that did not naturally fit within the scope of those focus areas. Included in the material not covered, is the recent contribution by Ronco and coworkers [141], where advantage is taken of the sometimes exquisite dependence on environmental factors of the emission intensity and lifetime of Cr(III) polypyridyls. This sensitivity was used to probe hydrophobic sites in anionic polyelectrolytes, where such information may provide useful guidance in attempts to enhance the rates of photoactivated electron transfer processes and/or retard recombination events. More recently [142], in an effort to more finely tune 2 Eg excited state properties, their group synthesized a range of mixed ligand polypyridyls of Cr(III), using a procedure we had developed earlier [143]. Another area not addressed is the increasing use of Cr(III) complexes in spectral hole-burning experiments to overcome spectral broadening in condensed phases [144, 145]. Finally, one of the more intriguing topics omitted is a report in Nature in 2000 describing an experimental confirmation [146] of a theoretical prediction, termed magnetochiral dichroism, that a chiral medium would absorb light traveling parallel to a magnetic field differently from light traveling antiparallel [147]. The compound investigated was [Cr(oxalate)3] 3– , and a very small, strongly excitation wavelengthdependent, induction of optical activity was observed on laser irradiation in a very powerful magnetic field (up to 15 Tesla). In conclusion, it is noted that although the subject of Cr(III) photochemistry and photophysics is unlikely to reassume the degree of prominence it held up until the early 1970s, the present condition of the field is good and the long-term prognosis is excellent. Who is to say that the sign on my laboratory door which boldly states: “Chromium – The Final Frontier”, will not one day be more than just a catchy phrase? Acknowledgements The author gratefully acknowledges stimulating discussions with Paul Wagenknecht and John Wheeler during the preparation of this chapter. In early 2006, the field of Cr(III) photochemistry and photophysics lost one of its young luminaries, Marc Perkovic. This chapter is dedicated to his memory. References 1. Balzani V, Carassiti V (1970) Photochemistry of coordination compounds. Academic, London 2. Zinato E (1975) Substitutional photochemistry of first-row transition metal elements. In: Adamson AW, Fleischauer PD (eds) Concepts in inorganic photochemistry. Wiley, New York (Chap 4) 3. Roundhill DM (1994) Photochemistry and photophysics of metal complexes. Plenum, New York 4. Kirk AD (1999) Chem Rev 99:1607 5. Zinato E, Riccieri P (2001) Coord Chem Rev 211:5 6. Irwin G, Kirk AD (2001) Coord Chem Rev 211:25
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280 Topics in Current Chemistry Edi
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Subject Index Alkyne bridges 148 An
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Subject Index 273 Rh(III) cyclometa
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