Quenching metal complex excited state

It is also necessary to take account of outer sphere contributions to the quenching of the metal-centered excited state. The nature of the outer sphere will vary from one complex to another, depending on the size, charge, and shape of the complex. However, it is possible to correct for a generalized outer sphere contribution for each lanthanide giving a new equation for q which can be expressed in several ways 74... 

Flowever, there is a trade-off in using near-IR emissive lanthanides, in that luminescence lifetimes are shorter, and quantum yields lower, compared to complexes of Tb and Eu. This arises because the near-IR emissive lanthanides are quenched by lower harmonics of the O-H oscillator, increasing the Franck-Condon overlap with the metal excited state. For neodymium, matters are further complicated by the manifold of available metal-centered excited states, which leads to particularly effective quenching by C-H oscillators. Thus, complexes in which there are few C-H oscillators close to the metal are desirable if the luminescence lifetime is to be optimized (e.g. 44).76 97-101... 

BCPDA is a tetradentate ligand, and forms 1 1 complexes in these conditions. The metal ion is thus coordinatively unsaturated, and also binds a number of water molecules, which can lead to non-radiative quenching of the metal-centered excited state (vide supra). These must be removed by drying to achieve optimal luminescence intensity. This form of assay is, in fact, slightly less sensitive than DELFIA. 

Bimolecular quenching of the excited states of metal complexes generally involves electron transfer or energy transfer processes ( 1). Recently, however, Pt2(pop)4 " has been found to undergo a photochemical reaction involving atom abstraction as a primary photoprocess (.26). The reaction involves the catalytic conversion of isopropanol to acetone ... 

The quenching of an excited state of a transition metal complex by chemical reaction can occur, in principle, by means of any of the intermolecular reactions which transition metal complexes are able to undergo. It should be noted, however, that intermolecular excited state reactions can only occur if they are fast enough to compete with the intramolecular deactivation modes of the excited state and with the other quenching processes (Fig. 2). 

Rate constants for quenching of the excited states of metal complexes are available through the Notre Dame Radiation Laboratory DataBase http //allen.rad.nd.edu/... 

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... 

The development of metal complexes, particularly those based on nickel, resulted in compounds which exhibited relatively low extinction coefficients in the near ultraviolet region and yet in many instances were found to be superior in performance to the currently available ortho-hydroxyaromatic compounds [3,5]. This finding resulted in a search for some other feasible mechanistic function. As carbonyl photolysis was considered to be an important initiation process which gives rise to both free radicals and backbone cleavage, then deactivation or quenching of the excited-state precursor should prove an effective means of photoprotection. Effective structures in this respect were tris(dibenzoylmethanato) chelates of Fe and Cr (Structure 5), nickel oxime chelates (Structure 6), and the nickel complex of Structure 7, nick-el(ii) 2,2 -thiobis(4-f-octylphenolato)- -butylamine. 

Bimolecular quenching reactions of excited states can involve atom transfer from the quencher to the metal complex. Such reactions are shown diagrammati-cally ... 

Relaxation of Thermally Quenched or Optically Excited High-Spin States Resulting from the Spin-State Transition in Solid Metal Complexes... 

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