Chromium-polypyridyl complexes

In this report we describe some of the studies which have been initiated to investigate the factors contributing to the behavior of the lowest doublet excited state in chromium(III). The principle goal of our work has been to explore those steric constraints, introduced by the ligands, which greatly alter the photophysical properties of the 2E excited state. In pursuit of this goal, we have re-investigated some features of well known amine and polypyridyl complexes in order to obtain internally consistent reference systems. 

Fig. 24. Proposed mechanism for flash photolysis of polypyridyl complexes of chromium(UI) hound to macromolecules. Reprinted with permission from K. Viswanathan and P. Natarajan,/. Pofynt. ScL, Potym. Chem. 29,1734 (1991). Copyright 1991, Wiley, Chichester.

Photoreduction of chromium(III) polypyridyl complexes by Fe " allows examination of the thermal reverse reaction. For a series of complexes, reactions with Fe show very similar rate constants reflecting internal activation free energies which do not differ markedly. 

Two useful reviews have appeared. One general review deals with the photochemistry and photophysics of chromium(III)/ and the second specifically with the advances in the photochemistry and photophysics of polypyridyl complexes of chromium(III). ... 

Photolysis of [Cr(bipy)3] in basic or alcoholic media results in formation of the corresponding chromium(II) species. Selective photolysis of one polypyridyl complex in the presence of another allows determination of the Cr(III)/(II) self-exchange rate which, in methanolic solution, is close to the diffusion limit. A two-term rate law in the reduction of [Co(NH3)6] by [Cr(bipy)3] is the result of hydrolysis of the chromium(II) complex to give the more reactive [Cr(bipy)2(OH2)2] - However, addition of excess bipy has no effect on the reaction rate. 

Chromium(III) polypyridyl complexes undergo quenching by both electron transfer and energy transfer processes. The ground and excited state potentials (Table show that the excited states are better oxidants than are their... 

Harriman, A. Photochemistry of Manganese Complexes, Coord. Chem. Revs. 1979,28,147. Jamieson, M. A. Seipone, N. Hoffinan, M. Z. Advances in the Photochemistry and Photophysics of Chromium(ni) Polypyridyl Complexes in Fluid Solution, Coord. Chem. Revs. 1981,39,121. Kirk, A. D. Chromium(ni) Photochemistry and Photophysics, Coord. Chem. Revs. 1981,39,225-293. Kutal, C. Spectroscopic and Photochemical Properties of d Metal Complexes, Coord. Chem. Revs. 1990,99,213. 

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