Metal-ligand charge-transfer MLCT

To substitute the strongly bound axial CO ligand of the ruthenium or osmium center, it is necessary to employ more drastic conditions than simple stirring at room temperature. Imamura (11,20) used photolysis to synthesize porphyrin trimers on the basis of simultaneous coordination of two 4-pyridyl porphyrins to the same ruthenium porphyrin (12, Fig. 3). Some interesting photophysical behavior was observed for these systems. The trimers have an extra UV-Vis absorption band at about 450 nm which is ascribed to metal-ligand charge transfer (MLCT), a d7r(Ru(II))-7r (OEP) transition. This band shows a batho-chromic shift in more polar solvents, and decreased in intensity when... 

Fig. 1 Qualitative MO diagram for Cr(CO)g. There are two forbidden ligand-field (LF) excited states a Tu and a T2g) arising from (2t2gf (2f2g) (6e ) and a manifold of metal-ligand charge-transfer (MLCT) states (A - D), which give rise to two one-photon dipole allowed...

Fig. 2 Qualitative MO diagram for Fe(CO)j. There are many allowed ligand-field (LF) and metal-ligand charge-transfer (MLCT) transitions that can occur. Selection rules mean that one-photon population of A 2 states is allowed, two-photon population of and states is allowed, while both one- and two- photon population of states is allowed and the spectra may overlap...

In the parallel investigation of systems where metal-ligand charge-transfer (MLCT) occurs, studies into transition metal complexes of the redox-active quinone ligand have unravelled crossover behaviour that accompanies electron transfer. This reversible process, known as valence tautomerism, has been observed with bistability in the solid state. ° Exotic photomechanical behaviours, such as the bending of crystals of ID chain materials with IR irradiation,have been attributed to the unique structural consequences of electron transfer within the solid. 

Bands due to metal-ligand charge transfer (MLCT). 

In this review, we restrict ourselves to metal-mediated assemblies of functional 7T-systems, which absorb light in the visible region (i.e. A. > 400 nm). Thus, materials whose color arises from metal ions or metal-ligand charge transfer (MLCT) are not included here. Self-assembhes derived from free base porphyrins, metalloporphyrins and perylene bisimide dyes have shown promising applications in artificial LH systems. Therefore, some fundamental properties of a few representative chromophores, namely meso-tetraphenylporphyrin (TPP), zinc tetraphenylporphyrin (ZnTPP), tetra-phenoxy-substituted perylene bisimide (PPBI) and bay area imsubstituted perylene bisimide (PBI), are briefly summarized herein. These properties are essential for the understanding of the photophysical and photochemical behavior of assemblies derived from such dyes. 

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