Charge-Transfer Excited States of Transition Metal Complexes

Charge-Transfer Excited States of Transition Metal Complexes... 

Excited State Characteiizatioii. Charge-transfer excited states of transition-metal complexes fulfill many important photochemical roles, and an... 

Endicott, J. F. Charge-transfer Excited States of Transition Metal Complexes. In Electron Transfer in Chemistry Balzani, V., Ed. Wiley-VCH Weinheim, 2001 Vol. 1, Chapter 7, pp 238-270. 

A novel ECL assay for the determination of 2,4- and 3,4- diaminotoluene (DAT) isomers is based on reaction of these molecules with Au" and Cu +, respectively, in aqueous solution under oxidizing conditions in buffer-containing tripropylamine [72]. Luminescence is observed upon potential ramping from 0 to -1-2.8 V. The nature of the emitting species was not specified, but could involve a charge-transfer excited state of the metal complex with DAT or an oxidized form of DAT. DAT isomers were screened for ECL enhancement against 32 metals the apparent specificity of Au+ for 2,4-DAT and Cu + for 3,4-DAT is believed to be linked to the radii of each ion. This ECL approach could lead to applications in the determination of some aminoaromatics from degradation of explosives (e.g., TNT) as well as detection and quantifiation of various transition metals in water supplies. 

The excited states of transition metal complex having organic ligands are classified into three classes 1) metal-localized dd states, 2) ligand-localized titi states, and 3) metal-to-ligand charge-transfer dn states. The features of the spin sublevels are vastly different depending on the class of the states, and therefore, we will discuss spin sublevels for these classes of excited states separately. 

Figure 4.75 Schematic representation of the charge transfer in various excited states of a metal complex. M is the metal centre and L stands for a ligand. LF is a ligand field transition, CTs are the charge transfer transitions, LL is an intraligand transition, and CTTS is a charge transfer to solvent...

An alternative route to long-lived excited states in transition metal complexes is to populate an excited state localised on a pendant arm. For example, a switch from a very short lived MLCT state in [Pt(tpy)Cl] to a long-lived intra-pyrene excited state in [(4 -pyrene)-tpy-Pt-Cl] , has been demonstrated an electron-rich aryl substituent at the 4 position of the tpy ligand promotes the low-lying excited state with intra-ligand charge-transfer (ILCT) character, enhances the emission intensity, and extends the excited-state lifetime up to 64 ps in r.t. dichloromethane solution [35]. 

Casper, J.V., Westmoreland, T.D., Allen, G.H., Bradley, P.G., Meyer, T.J., et al. Molecular and electronic structure in the metal-to-ligand charge-transfer excited states of d6 transition-metal complexes in solution. J. Am. Chem. Soc. 106, 3492-3500 (1984)... 

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