Excited states metal-ligand charge transfer MLCT

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...

In order to illustrate the approach suggested above, it is of value to consider a specific case. Visible or near-UV excitation of the complex RuCbpy results in excitation and formation of the well-characterized metal to ligand charge transfer (MLCT) excited state Ru(bpy)32+. The consequences of optical excitation in the Ru-bpy system in terms of energetics are well established, and are summarized in eq. 1 in a Latimer type diagram where the potentials are versus the normal hydrogen electrode (NHE) and are... 

We have reported the first direct observation of the vibrational spectrum of an electronically excited state of a metal complex in solution (40). The excited state observed was the emissive and photochemically active metal-to-ligand charge transfer (MLCT) state of Ru(bpy)g+, the vibrational spectrum of which was acquired by time-resolved resonance Raman (TR ) spectroscopy. This study and others (19,41,42) demonstrates the enormous, virtually unique utility of TR in structural elucidation of electronically excited states in solution. 2+... 

Charge transfer excited states internal charge transfer (ICT) states, metal-to-ligand charge transfer (MLCT) and twisted internal charge transfer (TICT) states... 

The versatility of the structural types possible for 1,10-phenanthroline ligands is exemplified in a review article on metal-to-ligand charge-transfer (MLCT) excited states of copper(ll) bis-phenanthroline coordination compounds, where 14 different 1,10-phenanthroline-based ligands were discussed (Figure 2) <2000CCR243>. 

Fluorescent redox switches based on compounds with electron acceptors and fluorophores have been also reported. For instance, by making use of the quinone/ hydroquinone redox couple a redox-responsive fluorescence switch can be established with molecule 19 containing a ruthenium tris(bpy) (bpy = 2,2 -bipyridine) complex.29 Within molecule 19, the excited state of the ruthenium center, that is, the triplet metal-to-ligand charge transfer (MLCT) state, is effectively quenched by electron transfer to the quinone group. When the quinone is reduced to the hydroquinone either chemically or electrochemically, luminescence is emitted from the ruthenium center in molecule 19. Similarly, molecule 20, a ruthenium (II) complex withhydroquinone-functionalized 2,2 6, 2"-terpyridine (tpy) and (4 -phenylethynyl-2,2 6, 2"- terpyridine) as ligands, also works as a redox fluorescence switch.30... 

Aqueous [Ru"(bpy)3]2+ is a model system for Metal-to-Ligand Charge Transfer (MLCT) reactions. Its excited state properties have been readily studied with optical spectroscopies [15,16]. However, little is known about its excited state structure, which we investigated via time-resolved x-ray absorption spectroscopy. The reaction cycle is described by Fig. 3 (where the superscripts on the left hand side of the ground and excited state compounds denote the... 

The low-energy metal to ligand charge-transfer (MLCT) excited state has been extensively studied and involves low-lying ft -acceptor orbitals of the pyridine ligands ( ). ... 

Almost in all cases (Table 3) photosubstitution reactive excited states are longer-lived spin-forbidden states involving an antibonding d-orbital of the central atom (usually dzi). The ability of complexes to undergo a photoejection of one axial ligand stems from the dissociative character of such a state. A special attention should be paid to complexes in metal-to-ligand charge-transfer (MLCT) reactive excited states (d, n ). On one side, the increase of positive... 

In dichloromethane solution, the [Ru(bpy)2(l)]2+ complex (Scheme 1) exhibits an absorption band at 455 nm (emax = 10400 M em Figure 5) and an emission band at 619 nm (x = 733 ns, cf> = 0.05, Figure 5, Table 1). These bands can straightforwardly be assigned to spin-allowed and, respectively, spin-forbidden metal-to-ligand-charge-transfer (MLCT) excited states, characteristic of Ru(II) polypyridine complexes[6a,c,e]. 

It can be seen that the coupling of the formation and decay processes increases with the width of the flash. In an intermediate case, the time dependence of the absorbance change will have the functional form of a double exponential, A exp(—f/i ) + B exp (—tlx"). One lifetime will be close to the lifetime of the transient species and the other to the lifetime of the pump. In the most unfavorable conditions, the functional form will be a single exponential with nearly the lifetime of the pump. The determination of the lifetime of a transient species formed by the decay of transformation of an excited state offers a similar difficulty. The reduction of methylviologen, MV2 +, by the metal to ligand charge transfer (MLCT) state19 of the Re(I) complex and the reoxidation of the produced radical, MV +, are illustrated in Equations 6.57-6.59. 

Quantum yields have been measured for the photoaquation of a large range of substituted pyridine complexes of the type [Ru(NH3)5(pyX)]2+.53 The marked dependence of quantum yields on the nature of X indicates that metal-to-ligand-charge-transfer (MLCT) excited states are not involved in the photosubstitution. Presumably, a ligand-field excited state is responsible. Evidence has been reported for a simple outer-sphere reduction of cytochrome c by [Ru(NH3)6]2+.54 Such a... 

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