Light ligand triplet excited state

Many lanthanides in the form of complexes emit fluorescence when excited with light absorbed by ligands. Lanthanide ions in crystals or in solution also exhibit fluorescence. The fluorescence observed due to energy transfer from the triplet state of the ligand to excited state of Ln3+ is known as enhanced fluorescence. The fluorescence emission spectra of Tb3+ aquo ion in the absence and presence of porcine trypsin at pH 6.3 is shown in Fig. 11.3. 

This approach of CO2 reduction in the presence of tertiary amines has been frequently used in photocatalytic systems based on metallic complexes and particularly with Ru(ll) polypyridyl complexes [28]. In these cases excitation with visible light of the Ru(n) polypyridyl complexes results in the generation of a triplet excited state with intramolecular metal-to-ligand charge transfer character that will transfer an electron to CO2 forming Ru(lll) and CO2. Subsequent Ru(lll) will undergo reduction to Ru(ll) in the presence of amine that will be the reducing species in the system (Scheme 1.8). 

A Study of the photophysical and photochemical properties of alkylidyne complexes such as W(CPh)Cl(CO)2(tmeda) was reported by Bocarsly et al. (see also Section II,B) 24). Quenching experiments with aromatic hydrocarbons indicate that a significant amount of triplet character is associated with the emissive excited state. Bimolecular oxidative and reductive quenching is observed as well. Methylviologen dichloride is reduced and iV.A, A, A, -tetramethyl-p-phenylenediamine is oxidized by W(CPh)Q-(CO)2(tmeda) on irradiation with 488 nm light. Photosubstitution of two CO ligands in W(CPh)Cl(CO)(dppe) by dppe to give W(CPh)Cl(dppe)2 was observed as well. 

Some photosensitizers, such as metal organic complexes (see also Section 6.8), can behave as either electron donors or acceptors because of their favourable redox potentials for oxidation or reduction of their excited states.670 A well-known example is tris(2,2 -bipyridine)ruthenium(II) ion ([Ru(bpy)3]2 + Scheme 6.204).10461235 When a solution of this compound is irradiated with visible light, a triplet charge-transfer excited state is formed, in which a metal-centred electron is promoted to the jt -orbital of the bipyridyl ligand [d,7t state where the metal is the electron-deficient centre (acceptor) and the ligand is the electron rich centre (donor)]. 

---