Chromium complexes doublet excited states

Gnmwald-Winstein analysis, 505 Chromium(III) complexes doublet excited states reactions, 400 emission rules, 395 ligand field states reactivity, 397 magnetic behavior, 272 mixed-ligand reactivity, 398 nitrito... 

Manipulation of Doublet Excited State Lifetimes in Chromium (III) Complexes... 

The relaxation rates observed for the lowest energy doublet excited states ( E) of chromium Ill) complexes can be represented, kre k°... 

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. 

Quenching of the ( CT)[Ru(bipy)3] by [Cr(bipy)]3 has been studied. This is via electron transfer to the Cr complex and a rapid back reaction. The ruthenium complex will also quench the 727 nm emission of the metal-centred doublet excited state of the chromium species, by a similar mechanism. Evidently both ligand- and metal-centred excited states can be quenched by bimolecular redox processes. A number of Ru complexes, e.g. [Ru(bipy)3] and [Ru(phen)3] also have their luminescence quenched by electron transfer to Fe or paraquat. Both the initial quenching reactions and back reactions are close to the diffusion-controlled limit. These mechanisms involve initial oxidation of Ru to Ru [equation (1)]. However, the triplet excited state is more active than the ground state towards reductants as well as... 

Two mechanisms have been put forward to explain the photochemistry of chromium(in) complexes. Their photoreactivity has been ascribed to excitation either to the lowest spin-forbidden excited state, Eg, or to the lowest quartet excited states, and Mxg. Several years ago some quenching experiments on the photoaquation of the [Cr(NH3)2(NCS)4]- anion indicated that a quartet state was at least partially involved. Recently two papers concerning the photochemistry of the [Cr(CN)6] anion, chosen for the known large energy difference between the doublet and quartet states, have provided strong evidence for a quartet state as the photointermediale. Thus the observations that pyrazine and xanthone sensitize the photoaquation of [Cr(CN)e] , but that Michler s ketone and [Ru(bipy)3] + do not, rule out photoaquation via a doublet state. Likewise a comparison of phosphorescence and photolysis of solutions of [Cr(CN)6] in dimethylformamide showed that the photolysis, to [Cr(CN)5(DMF)] , could not proceed via the same excited state, Eg, as phosphorescence. Both of these investigations led to the implication of the... 

---