Charge Transfer Excitation of Coordination Compounds. Generation of Reactive Intermediates

Abstract

Reactive compounds exhibit frequently catalytic properties. Particularly, many reactive metal complexes are characterized by their catalytic activity. Photochemistry presents one of the most important and versatile methods for the generation of reactive compounds. The design of photochemically generated catalysts requires a deeper insight in the photochemistry in general. The choice of appropriate light-sensitive precursors can be guided by several considerations. A straight-forward approach involves the selection of compounds according to the type of electronic excitation which often predetermines the nature of the photoproducts.In a metal complex the metal-ligand bonds are generally polar. The molecular orbitals are then not equally delocalized between metal and ligands but predominantly located at the metal or the ligands. The electronic transitions are classified according to this localization [1]. In simple homoleptic mononuclear complexes three types of electronic excitations can be distinguished. Electronic transitions are localized at the metal (metalcentered, MC), at the ligand (ligand-centered, LC or intraligand, IL) or they take place between MOs at the metal and the ligand. The latter transitions are associated with a spatial redistribution of electrons and therefore are termed charge transfer (CT) transitions. While this review deals with CT excited states let us first have a short look at MC and LC states and their significance for the generation of reactive species which might be catalytically active.Since the valence orbitals of metals may be of the s, p, d, and f type all these orbitals can be involved in MC transitions. Ligand field (LF) or dd excited states have been investigated in great detail [2][3][4][5][6]. In many cases LF (or dd) excitation leads to a weakening of metal-ligand bonds. As a result a ligand may be released without any change of the oxidation state of the metal. This process is very important for the generation of catalysts because the product is a coordinatively unsaturated complex which provides an

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“…Ligand-to-metal charge transfer (LMCT) photocatalysis has emerged as a powerful tool to easily access reactive electrophilic radicals from abundant and inexpensive metal complexes enabling diverse reactivity . Light absorption by an LMCT band induces a homolytic cleavage of a metal-ligand bond resulting in a reduced metal complex and an oxidized ligand radical . Even though these photophysical principles were never restricted to certain metals, modern synthetic applications have been limited mostly to cerium and its alkoxy and carboxyl radical chemistry.…”

Highly regenerative, fast colorimetric response for organo-toxin and oxo-anions in an aqueous medium using a discrete luminescent Cd(ii) complex in a heterogeneous manner with theoretical revelation

“…The higher the oxidation state of the metal, the larger will be the electron deficiency and hence there will be a facile charge transfer from electron-rich species to electron-deficient metal. 48 The oxidation states of Mn and Cr in MnO 4 − , Cr 2 O 7 2− and CrO 4 2− are +7, +6 and +6, respectively. Hence, the more electronic charge will be transferred from complex 1 to the MnO 4 − .…”

Coordination Polymers Based on Carbazole-Derived Chromophore Linkers for Optimized Multiphoton Absorption: A Structural and Photophysical Study

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