Luminescence excited states

The luminescent excited state of MogClii reacts rapidly with electron acceptors(24). The powerfully oxidizing MogCli is produced in these reactions. Experiments with BSEP as acceptor in... 

Rehm-Weller method (21) for estimating the reduction potentials of excited molecules As our homologous series of reductants, we have used the RuL3 +/ RuL32+ couples (22,23), where RuL32+ is the luminescent excited state of Rul 7 The electron exchange rate constants for these couples are very large... 

Keywords Photochemistry Photophysics Rhenium Luminescence Excited-state Abbreviations... 

An interesting facet of the photochemistry of [Re2Cl8]2- involves the electron-transfer chemistry of the luminescent excited state [Re2Clg]2-, which is an 88 singlet30,31. ... 

P-Cyclodextrines, appended to a ruthenium complex, have been employed as hosts for iridium and osmium complexes bearing adamantyl or biphenyl moieties, which form strong host-guest complexes with P-cyclodextrines (see Fig. 3). In such systems, photoinduced energy transfer can occur from the periphery, upon complexation of the iridium units, toward the central ruthenium acceptor, or switched in the other direction, from the ruthenium to the periphery when the osmium moieties are assembled (see Fig. 3) 42). The lowest excited state is in fact localized on the osmium center, while the highest luminescent excited state belongs to the iridium complex (see Fig. 3 right). 

A paper has appeared which provides a procedure for determining quantum yields of non-luminescent excited states and photoproducts having sub-nanosecond lifetimes. The method has been applied to inorganic complexes and photosynthetic models. 

The CT excited states usually are not emissive because the low energy gap and the strong distortion with respect to the ground state favor the occurrence of radiationless deactivation. Furthermore, the presence of the low-energy CT excited states causes rapid radiationless decay of the upper lying, potentially luminescent excited states localized on the molecular components. Therefore, rotaxanes and catenanes based on CT interactions usually do not exhibit any luminescence. For example, the intense emission band with maximum at 320 nm (t = 2.5 ns) exhibited by macrocycle 7 [1 la] is no longer present in catenane [23]. 

Circularly polarized luminescence spectroscopy (CPLS) is a measure of the chirality of a luminescent excited state. The excitation source can be either a laser or an arc lamp, but it is important that the source of excitation be unpolarized to avoid possible photoselection artifacts. The CPLS experiment produces two... 

A more detailed description of the nature and decay properties of the luminescent excited states of these compounds has come from investigations of the effects of temperature (2-77°K)... 

FIGURE 10. Calculated splittings and component decay constants of the luminescent excited state of ruthenocene (53). 

Other OTM compounds that are indicated by preliminary studies to be efficient triplet sensitizers include ClRe(C0)2L (Table 12). In dichloromethane at room temperature these complexes are strongly luminescent and photo-inert (41). Under the same conditions, the emissions are quenched by oxygen, anthracene, and trans-stilbene. The trans cis isomerization of trans-stilbene is efficiently sensitized by several of these species. Quantitative sensitization experiments indicated that the luminescing excited states of ClRe(C0)3L are produced with 100 percent... 

The [Ru(bipy)3] ion reacts with [e ]" to yield the luminescent excited state of [Ru(bipy)3] ", owing to the great exoergicity of the reaction. ... 

Intercomponent CT interactions introduce low energy excited states which are responsible for the presence of broad and weak absorption bands in the visible region (and therefore, for the color) exhibited by pseudorotaxanes, rotaxanes, and catenanes based on this kind of interactions. Furthermore, the low energy (non-emissive) CT excited states offer favorable deactivation paths to the potentially luminescent excited states localized on the molecular components, so that this kind of pseudorotaxanes, rotaxanes, and catenanes usually does not exhibit luminescence [11, 18]. 

Hydrated electrons react with certain water-soluble metalloporphyrin complexes, reducing the porphyrin ligands to pi-radical species. When the metal centers are Zn(II), Pd(II), Ag(II), Cd(II), Cu(II), Sn(IV), and Pb(II), the radical complexes are produced at diffusion-controlled rates and decay with second-order kinetics.188 Fe(III) porphyrins, on the other hand, yield Fe(II) porphyrins.189 Rather different behavior is seen in the reaction of e (aq) with [Ru(bpy)3]3 + here, parallel paths generate the well-known luminescent excited-state [ Ru(bpy)3]2 + and another reduced intermediate, both of which decay to the ground-state [Ru(bpy)3]2+, 190 In a direct demonstration of the chemical mechanism of inner-sphere electron transfer, [Coni(NH3)5L]2+ complexes where L = nitrobenzoate and dinitrobenzoate react with e (aq) to form Co(III)-ligand radical intermediates, which then undergo intramolecular electron transfer to yield Co(II) and L.191... 

For Pd(ppy)2, the red shift from the spectrum of the C-protonated ligand is larger ( 1400 cm-1), the relative intensities of the vibrational features are different, and the radiative lifetime is estimated to be >0.05 s. Therefore, the luminescent excited state is mainly 3LC in nature, but some MLCT contribution is present. 

The number of cyclometallated complexes that have been examined so far is not very large. Nevertheless, it is interesting to compare the available results to draw some preliminary conclusions on the possibility of tuning their excited state properties. We now focus our attention on a series of closely related cyclometallated complexes and we examine the trends shown by energies and lifetimes of the luminescent excited states. 

Information on the orbital nature (ligand centered, LC, or metal-to-ligand charge-transfer, MLCT) of the luminescent excited state can be drawn from the data reported in Table 2 and from their trends shown in Figure 13. For the Pt(IV) complexes the emission energy is very close to (less than 1000cm-1 red-shifted from) that of the free ligand and the luminescence lifetime is... 

Figure 13. Energy (a) and lifetime (b) of the luminescent excited states of the free protonated cyclometallating ligands and their complexes. From Ref. 91 with permission of Springer-Verlag.

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