Metal luminescent states

TABLE 6. Decay Rate Constants of the Metal Luminescent States and Number of Solvent Molecules Coordinated to the Metal Ion11... 

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... 

It has been proposed (5) that in the case of the Ln + ions an excited state involving a metal d state is responsible for the absence of luminescence. It is then necessary that this state has a large Franck-Condon shift and is situated at energies not very much higher than those of the 4/-c.t. state. Whatever the solution of this problem may be, it is clear that either c.t. or 4f 5d states play an important role in the quenching process of the luminescence. The conclusion of all the material presented in this section is that this is true for all types of lanthanide luminescence. 

In complexes of the quinoline based ligands we saw room-temperature emission which was weak or absent and could be related to unfavorable steric factors. Most likely [Ru(dpt)2]2+ is non-luminescent for the same reason. Kirchhoff et al.258) have argued that steric repulsions may cause a 3MC state to lie at lower energy than the 3CT state so that no CT emission occurs. A metal centered state should be more photoactive and [Ru(dpt)2]2+ does indeed undergo photolysis in the presence of nucleophiles. The photolysis product has been formulated as containing a bidentate dpt ligand. 

In photochemical reduction of CO2 by metal complexes, [Ru(bpy)3] is widely used as a photosensitizer. The luminescent state of [Ru(bpy)3] is reductively quenched by various sacrificial electron donors to produce [Ru(bpy)3] . Metal complexes used as catalyst in the photochemical reduction of CO2 using [Ru(bpy)3] are prerequisites which are reduced at potentials more positive than that of the [Ru(bpy)3] " redox couple (-1.33 V vs SCE) (72). Irradiation with visible light of an aqueous solution containing [Co (Me4(14)-4,ll-dieneNJ], [Ru(bpy)3], and ascorbic acid at pH 4.0 produces CO and H2 with a mole ratio of 0.27 1 (73). Similarly, photochemical reduction of CO2 is catalyzed by the [Ru(bpy)3] /[Ni(cyclam)] system at pH 5.0 and also gives H2 and CO. However, the quantum efficiency of the latter is quite low (0.06% at X = 400 nm), and the catalytic activity for the CO2 reduction decreases to 25% after 4 h irradiation (64, 74, 75). This contrasts with the high activity for the electrochemical reduction of CO2 by [Ni(cyclam)] adsorbed on Hg. 

Arfsten DP, Davenport R, and Schaffer DJ, Reversion of bioluminescent bacteria (Mutatox ) to their luminescent state upon exposure to organic compounds, munitions, and metal salts, Biomed. Environ. Sci., 7, 144, 1994. 

If d>M cannot be measured, it may be substituted, as in Eq. 2, by the efficiency of the metal luminescence (i/M), which is derivable from Eq. 3. Since in this study the radiative (kr), and nonradiative (kar) rate constants are calculated from the experimentally obtained lifetimes, Eq. 3 may be replaced by Eq. 4 on the assumption that the decay of the metal emitting state in D20 at 77 K is purely radiative. 

In order to discuss the efficiency of the metal luminescence in the complex, the radiative and nonradiative decay rate constants of the Eu3+ 5D0 and Tb3+ 5D4 emitting states (Table 6) have been calculated using Eqs. 10-12. 

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