Triplet ligand-localized

A schematic diagram of the ligand localized and Eu3+ excited energy levels are shown in fig. 16. The ligand levels are denoted with S and T for the singlet and triplet states, respec-... 

The photochemistry of Re-bound L occurs from a ligand-localized triplet excited state (3nn or im ). This contrasts the photochemistry of free species L, which react from the corresponding singlet states. Binding L to Re in [Re(L) (CO)3(N,N)]+ or [Re(Cl)(CO)3(L)2] thus presents an excellent opportunity to generate the lowest triplet state of L in high yields and study its spectroscopic properties and reactivity. This is very difficult to accomplish for free L. 

It has been very difficult to unambiguously prove which state is responsible for the energy transfer processes because of the lack of information regarding the emission from the excited states of the coordinated ligand and the difficulties in determining ligand-localized triplet-triplet absorption spectra of lanthanide complexes. All the experimental work conducted seemed to support case (a) in Figure 1.8. 

The lowest absorption (and the emission) transition is ligand localized on the right-hand side and charge transfer on the left-hand side of Fig. 22. In between is an intermediate region. Since the excited states arc open shell, spin-singlets and spin-triplets appear. This simple approximation can be extended by including more excited configurations. 

The experimentally determined values for the zero-field splitting parameters, D = 3445 MHz and E = 865 MHz, are typical for the thpy ligand localized triplet state. Note that these values differ appreciably from those found above for the bpy ligand. Thus the values of the fine structure parameters also reflect that the triplet state is localized on the thpy ligand and not on the bpy ligand. Assuming that dipolar spin-spin interactions predominantly contribute to the... 

With reference to the molecular axis frame of Fig. 16, it is recalled from the ODMR results that the ligand-localized triplet state excitation in Rh(TTB)+ is on the fragment (of Cj symmetry) marked with the single asterisk. We consider now, as in Sect. 6.2, that the transition moment for radiative decay of the triplet state is determined by (Hsqc) of Eq. (19), with a major contribution from the SOC... 

In conclusion, the emissive properties of the lowest excited triplet state of the Rh + complexes are mainly determined by the heavy atom effect. The SOC mechanism mixes ddjn (in the case of Rh(bpy) + and Rh(phen)3 +) and Ha-i-(5dj7T (in the case of Rh(TTB)+, Rh(TPB)+, and Rh(PTB)+) configurations. Of course, the admixed d-orbital character implies charge transfer (CT) character of the ligand-localized excited state. Assuming comparable... 

Similar examples for energy transfer from ligand localized levels to highly localized 4f levels are represented by the rare-earth chelates. Voloshin and Savutskii (1976) studied europium benzoylacetonate imder pressures up to 6 GPa. Exciting the triplet level they could observe the luminescence from the Eu " " ion. It was possible to describe the observed initial increase in the quantum yield of the Eu " " luminescence up to 2.5 GPa and the following decrease by the exchange resonance theory (Dexter, 1953). A more detailed study on different Tris chelates of Sm +, Eu +, Gd " ", and Tb " with -diketonates was performed by Hayes and Drickamer (1982), where the most dramatic effects of pressure on energy transfer phenomena were found for the Eu + chelates. 

Photocyclization also takes place upon illumination into the MLCT band of the rhenium(i) complex 88 [232]. Here, the quantum yield for photocyclization reaches about 0.62. The closed form, complex 89, can be converted back to the original open form by illumination at about 580 nm. The closed form also undergoes slow thermal opening but this occurs on a time scale far removed from that related to the photochemical processes. It seems that the ligand-localized triplet state lies at lower energy than the MLCT triplet in the closed form but the opposite is true in the open form. 

Spectroscopic properties of [Ru(bpy)3] " ", and the effects of varying the diimine ligands in [Ru(bpy)3 L ] + (L = diimine) on the electronic spectra and redox properties of these complexes have been reviewed. The properties of the optical emission and excitation spectra of [Ru(bpy)3] +, [Ru(bpy)2(bpy-d )] + and [Ru(bpy-d )3] " " and of related Os, Rh , and Pt and Os species have been analyzed and trends arising from changes in the metal d or MLCT character in the lowest triplet states have been discussed. A study of the interligand electron transfer and transition state dynamics in [Ru(bpy)3] " " has been carried out. The results of X-ray excited optical luminescence and XANES studies on a fine powder film of [Ru(bpy)3][C104]2 show that C and Ru localized excitation enhances the photoluminescence yield, but that of N does not. 

---