Excited-state species, transition metal complexes

A novel ECL assay for the determination of 2,4- and 3,4- diaminotoluene (DAT) isomers is based on reaction of these molecules with Au" and Cu +, respectively, in aqueous solution under oxidizing conditions in buffer-containing tripropylamine [72]. Luminescence is observed upon potential ramping from 0 to -1-2.8 V. The nature of the emitting species was not specified, but could involve a charge-transfer excited state of the metal complex with DAT or an oxidized form of DAT. DAT isomers were screened for ECL enhancement against 32 metals the apparent specificity of Au+ for 2,4-DAT and Cu + for 3,4-DAT is believed to be linked to the radii of each ion. This ECL approach could lead to applications in the determination of some aminoaromatics from degradation of explosives (e.g., TNT) as well as detection and quantifiation of various transition metals in water supplies. 

The prototypical photochemical system for CO2 reduction contains a photosensitizer (or photocatalyst) to capture the photon energy, an electron relay catalyst (that might be the same species as the photosensitizer) to couple the photon energy to the chemical reduction, an oxidizable species to complete the redox cycle and CO2 as the substrate. Figure 1 shows a cartoon of the photochemical CO2 reduction system. An effective photocatalyst must absorb a significant part of the solar spectrum, have a long-lived excited state and promote the activation of small molecules. Both organic dyes and transition metal complexes have been used as photocatalysts for CO2 reduction. In this chapter, CO2 reduction systems mediated by cobalt and nickel macrocycles and rhenium complexes will be discussed. 

Although the models have proved to be useful tools for rationalizing some aspects of the photosubstitutional behavior of simple transition metal complexes, they are not without deficiencies. For example, the predictions of reactivity made with the models are only qualitative. Thus a reaction that is predicted for a particular complex may not occur at all. Another important deficiency of the models was recently discussed by Ford (50). In the series of analogous rhodium(III) j omplexes, Rh(NH3)5X"" (X = NH3, H2O, 0H , Cl, Br, and I"), relative quantum yields of ligand substitution are strongly dependent on the rates of physical radiationless decay of the excited complexes to the ground state species. According to the Zink model, however, relative quantum yields within such a series should reflect "reactivities" of the excited state complexes (44,47). 

In contrast to the typical behavior of organic compounds discussed above, many photoreactions of transition metal complexes have wavelength-dependent quantum yields (7). Generally, these wavelength effects have been interpreted in terms of more than one reactive excited state of the photolyzed species. The photoreactivity of V(CO) L (L = amine), for example, has been interpreted in this manner with the previously mentioned model of substitutional photoreactivity proposed by Wrighton et al. (42, 49,73). Assuming ligand dissociation to be the only primary photochemical process (Section III-B-1), photolysis of W(C0)5L could produce three primary products ... 

The persistent radical effect must always play a role when transient and persistent radicals are formed with equal or nearly equal rates. It leads to the formation of the mutual reaction products in high yields and to the virtual absence of the self-termination reactions. In the few examples given earlier, the persistent species were radicals and transition metal complexes, but other reaction partners such as molecular ions and even normal molecules may take their place. Furthermore, the phenomenon can also work with other transient species, such as carbenes, nitrenes, and molecules in electronically excited states. A literature search would probably reveal a large variety of diverse reactions that exhibit the effect to some degree, although this went unnoticed, so far. Here, we restrict the survey to evident cases. A few of the reactions have even been designed to exploit the persistent radical effect in synthesis. 

The reaction dynamics of few excited complexes are known however, the opportunities provided by pulsed lasers promise to make this research area one of major emphasis of mechanistic studies. Such methods are necessary because few transition-metal complexes exist as electronically excited states in RT solutions with lifetimes exceeding 1 fjis, and many are shorter lived. Several competing processes lead to ES decay nonra-diative deactivation to the ground state (GS), radiative deactivation (i.e., emission) to the GS, unimolecular reaction to products (such as ligand substitutions or redox decomposition) or bimolecular electron transfer or energy transfer with another species, Q, in solution. These processes are indicated in Eqs. (a)-(e) for a hypothetical complex [MLJ" + ... 

The electronically excited state of a molecule is a new species with chemical properties that can differ from those of the corresponding ground state. Many of the properties of excited states can be predicted from those of ground-state species with comparable electronic configurations, especially the electron-transfer properties of excited states of metal complexes. In this section the relations between ground- and excited-state electron-transfer reactions of transition-metal complexes are discussed. 

---