Horst Kunkely

Photoreactivity of gold complexes

Abstract The light sensitivity of gold compounds has been known since 1737. More recent observations have led to a deeper understanding of the photochemistry of gold complexes, which are discussed on the basis of selected examples. Light sensitive Au(I) and Au(III) compounds are classified according to the nature of reactive excited states: metal-centered (MC), ligand-to-metal charge transfer (LMCT), metal-to-ligand charge transfer (MLCT), metal-to-metal charge transfer (MMCT) and intraligand (IL). Accordingly, some basic information on the electronic spectra of gold complexes including luminescence spectra is provided. The photochemistry is described in more detail. Essentially, our own observations are reported but some relevant observations of other groups are also included.

Luminescent Metal Complexes: Diversity of Excited States

The photoluminescence of metal complexes has attracted much recent interest since it can be utilized for a variety of applications such as optical sensors and LEDs. Moreover, the emission behavior provides a probe for the investigation of photoreactions including artificial photosynthesis. In this review, emitting compounds are classified according to the nature of their excited states: metal-centered, ligand-to-metal charge transfer, metal-to-ligand charge transfer, ligand-to-ligand charge transfer, metal-to-metal charge transfer, ligand-centered (or intraligand), and intraligand charge transfer excited states. Complexes of transition metals (dn with n=0 – 10), main group metals (s2), lanthanides and actinides (fn) are included in our discussion. However, this review does not cover the photoluminescence of metal complexes comprehensively, but illustrates this subject by selected examples. The viewpoint is that of a coordination chemist and not of a spectroscopist. Accordingly, molecular complexes which emit under ambient conditions are preferably chosen.

Absorption and emission spectra of tetrameric gold(I) complexes

The lowest-energy absorption and emisson bands of the complexes [Au(dithioacetate)]4 and [Au(piperidine)Cl]4 are assigned to a metal-centered 5d-6s (A2g↔1g) transition which is modified by the metal-metal interaction in the square-planar Au(I)4 moiety.

Photoredox Reactions of Mixed-Valence Compounds Induced by Intervalence Excitation

Abstract A variety of mixed-valence systems were shown to be photoactive in aqueous solution upon light absorption by the intervalence (IT) band. IT excitation leads to an electron transfer from a reducing to an oxidizing metal center. Co(III), Ru(III), Cr(III), and Os(III) complexes were used as electron acceptors while [Fe(CN) 6 ] 4- , [Ru(CN) 6 ] 4- , and [Os(CN) 6 ] 4- servede as electron donors. Inner- as well as outer-sphere systems were studied. The inner-sphere IT interaction was mediated by bridging cyanide ligands in binuclear complexes. Outer-sphere IT systems were formed by ion pairs of the oxidizing complexes as cations and the reducing hexacyanide anions. Light-induced metal to metal electron transfer was followed by secondary processes which yields stable photoproducts.

Excited state properties of organometallic compounds of rhenium in high and low oxidation states

Abstract Organometallic compounds of rhenium occur in high and low oxidation states. These complexes are characterized by a variety of excited states including ligand field (LF), ligand-to-metal charge transfer (LMCT), metal-to-ligand charge transfer (MLCT), ligand-to-ligand charge transfer (LLCT), metal-to-metal charge transfer (MMCT), and intraligand (IL) excited states. The nature of the lowest-energy excited state can be tuned by the choice of appropriate ligands and metal oxidation states. This diversity is illustrated by selected examples of organometallics of Re(VII) and Re(I). Our account emphasizes spectral (absorption and emission) as well as photochemical properties.

Excited state properties of transition metal phosphine complexes

Phosphine ligands determine the excited state properties of a variety of coordination compounds. Phosphines not only influence metal-centered excited states, but participate directly in charge transfer transitions owing to their electron donating and accepting ability. Moreover, intraligand excited states are accessible if the phosphine carries suitable substituents. This diversity is illustrated by selected examples. The excited state behavior is discussed on the basis of spectral (absorption and emission) and photochemical properties of appropriate phosphine complexes.

Photoreactivity of metal-to-ligand charge transfer excited states

Abstract Generally, transition metal complexes in metal-to-ligand charge transfer (MLCT) excited states are considered to be less reactive than in other states (e.g. ligand field, ligand-to-metal charge transfer) because the orbitals which participate in MLCT transitions are frequently of the π type and, thus, less involved in strong bonding interactions. However, contrary to these expectations, numerous complexes, in particular the organometallic compounds, are characterized by intrinsically reactive MLCT states. The photoreactivity may be correlated with the electron distribution in the excited state which roughly corresponds to an oxidized metal and reduced ligand when compared with the ground states. An attempt is made to classify reactive MLCT states according to the reactive center. The reactivity of a MLCT state may be based predominantly on the oxidation of the metal or reduction of the ligand. Finally, charge-transfer-to-solvent (CTTS) transitions which are closely related to MLCT transitions are included in this discussion.

PHOTOCHEMISTRY OF COORDINATION COMPOUNDS OF THE MAIN GROUP METALS

A general concept is developed which relates characteristics excited states of main group metal complexes to typical photoreactions. With regard to their electronic spectra and photochemistry the main group metals are classified according to their ground state electron configuration nsxnpy. The photochemistry is generally dominated by the reactivity of metal-centered sp and ligand to metal charge transfer excited states which in most cases initiate inter- and intramolecular photoredox processes.

Photochemistry of transition metal complexes induced by outer-sphere charge transfer excitation

The intermolecular (outer sphere, OS) interaction of a reducing and an oxidizing metal complex generates a new optical transition involving charge transfer (CT) from the electron donor to the acceptor. OS CT transitions are classified according to the redox site (metal or ligand). Generally, the interaction between donor and acceptor is facilitated by ion pairs consisting of an oxidizing complex cation and a reducing complex anion. There are also ion pairs which are composed of a metal complex and an organic counter ion as electron donor or acceptor. In addition, the review includes examples of OS CT interaction which do not involve ion pairs at all. — A short introduction into the theory is followed by the discussion of the spectroscopy of OS CT of transition metal complexes. Finally, photoreactions induced by OS CT transitions are reviewed. The optical transfer is frequently followed by a rapid back electron transfer which regenerates the starting complexes. In many cases the primary products are kinetically labile and substitution reactions compete successfully with back electron transfer. As a result stable redox products may be formed. As an alternative, the substitution can be followed by back electron transfer. Product formation appears then as a substitution of the starting complexes. The various possibilities are illustrated by appropriate examples.

Optical Ligand to Ligand Charge Transfer of Metal Complexes Including Ligand-Based Mixed-Valence Systems

Abstract The electronic spectra of metal complexes which contain a reducing and an oxidizing ligand (Lred–M–Lox) are characterized by ligand to ligand charge transfer (LLCT) absorptions. A specific form of Lred–M–Lox complexes are ligand-based mixed-valence (LBMV) compounds which contain the same ligand in two different redox states. In this case the ligand-ligand interaction may lead to a partial or even complete electronic delocalization between the reduced and oxidized form of the ligands. As a consequence, the “LLCT” transition loses its CT character since it takes place between delocalized orbitals.