| The photophysical and photochemical behavior of Cr(NN)(,3)('3+) complexes in aqueous solution has been investigated using techniques of continuous and flash photolyses and luminescence intensity and emission measurements. These investigations have allowed for an intensive characterization of the lowest excited state(s) of these complexes, including the identity, the lifetime and the factors affecting the lifetime, and the chemical reactivity in aqueous and mixed solvent solutions.;Continuous photolysis (313 - 450 nm) of Cr(NN)(,3)('3+) in aqueous alkaline solution reveals that (('2)T(,1)/('2)E)Cr(NN)(,3)('3+) is produced with near-unitary efficiency from the initially-produced Franck-Condon quartet excited states. The photolysis results in the release of one polypyridine ligand:(UNFORMATTED TABLE FOLLOWS); h(nu); Cr(NN)(,3)('3+) *Cr(NN)(,3)('3+) Cr(NN)(,2)(OH)(,2)('+) + NN;The transient absorption produced on flash excitation of Cr(NN)(,3)('3+) has been identified as the lowest excited doublet state, ('2)E, through comparison with the 727-nm phosphorescence emission lifetime and quenching studies. The (('2)E)Cr(NN)(,3)('3+) lifetimes were found to be sensitive to the nature and pH of the solvent, the nature of (NN), the presence of added anions and temperature. Furthermore, (('2)E)Cr(NN)(,3)('3+) has been shown to be a strong oxidizing agent, capable of undergoing excited-state bimolecular reductive electron transfer reactions. The product of such a reaction, Cr(NN)(,3)('2+), has been characterized with respect to absorption spectra. Also, (('2)E)Cr(NN)(,3)('3+) undergoes excited-state bimolecular energy transfer reactions with molecular oxygen.; H(,2)O/OH('-)(TABLE ENDS);Photolysis experiments have shown that the quenchable reaction component exhibits a pH dependence identical to that of the thermal reaction.('1) From the pH dependences, and the activation parameters obtained from temperature dependence studies, an associative interchange mechanism is proposed involving nucleophilic attack by H(,2)O on the Cr(III) center to yield a seven-coordinate intermediate species. The excited state responsible for this reaction component is the lowest excited doublet manifold, (('2)T(,1)/('2)E). The unquenchable photoaquation reaction component has been ascribed to direct reaction from the ('4)T(,2) excited manifold, via a similar associative interchange mechanism; this assignment was based on pH- and temperature-dependence studies of the quantum yields of reaction under conditions where > 99.9% of the (('2)T(,1)/('2)E) reaction is quenched. . . . (Author's abstract exceeds stipulated maximum length. Discontinued here with permission of author.) UMI... |
