C. A. Bignozzi

Electronic coupling between remote metal centers in cyanobridged polynuclear complexes

Abstract Several cyanide-bridged polynuclear complexes have been synthesized in the context of intramolecular energy and electron transfer studies. A valencelocalized description is generally appropriate for such complexes. Within such a localized picture, however, the cyanide bridge is found to provide a remarkable degree of metal-metal electronic coupling. This conclusion can be drawn from a variety of experimental results. Specific attention is devoted here to the spectroscopic observation of appreciable second-order interactions between remote (i.e., non directly bridged) sites in a polynuclear complex.

Intramolecular energy transfer in Ru(II)-Ru(II) and Ru(II)-Cr(III) polynuclear complexes

Abstract Ligand-bridged polynuclear complexes made up of metal-containing subunits with long-lived excited states are suited for the study of intramolecular electronic energy transfer. Using Ru(II) and Cr(III) complex units as building blocks and cyanide as bridging ligand, a number of Ru(II)-Ru(II), Ru(II)-Cr(III) polynuclear complexes have been synthesized. Efficient intramolecular energy transfer between adjacent metal centers occurs in these systems, as shown by quenching and photosensitization of the appropriate emissions. When the behavior of the polynuclear complex is compared with that of the component subunits, a number of interesting effect, luminescence shift, enhanced population of the emitting state, photoprotection, excited-state intervalance transfer, chemiluminescent charge recombination. Possible developments of the results towards the design of photonic molecular devices are discussed.

Intramolecular energy transfer in Cr(III)-Cr(III) and Ru(II )-Cr(III)-Cr(III) polynuclear complexes

Abstract The binuclear NC- trans -Cr(cyclam)-CN-Cr(CN) 5 complex has been synthesized and characterized. Efficient energy transfer from the NC- trans -Cr(cyclam)-CN + to the -CN-Cr(CN) 5 3− component was observed to occur. The reaction of cis -Ru(bpy) 2 [NC- trans -Cr(cyclam)-CN] 2 4+ with Cr(CN) 5 (DMF) 2- in DMF was studied. In the pentanuclear product cis -Ru(bpy) 2 [-NC- trans -Cr(cyclam)-CN-Cr(CN) 5 ] 2 , which was detected in the reacting solution but not isolated, stepwise energy transfer occurs along the Ru(II)-Cr(III)-Cr(III) cyano bridged chain.

Hydrogen production with nanostructured and sensitized metal oxides.

Recent advances in the field of photoelectrochemical cells (PECs) applied to solar water and H₂S splitting and hydrogen production are reviewed with meaningful examples and case studies. At the molecular level, significant recent efforts have been directed towards the development of stable dye sensitizers/water oxidation catalyst assemblies. In the field of photoactive nanostructured materials and interfaces, novel highly ordered semiconductors nanostructures (i.e., anodically grown titania nanotubes) are drawing an increasing interest, under both the fundamental and applicative points of view, due to improved charge transfer kinetics with respect to more conventional sintered nanoparticle substrates. These features, coupled with low cost and ease of fabrication, stand as a good promise for the realization of solar devices capable of solar hydrogen production at a useful rate.

Intramolecular Energy and Electron Transfer in Polynuclear Metal Complexes

As indicated by their traditional name of “complexes”, coordination compounds have a composite nature. Their relevant molecular orbitals are often predominantly localized either on the metal or on the ligands, which can thus be considered as electronically independent fragments. Because of this composite nature, coordination compounds of transition metals exhibit a remarkable variety of electronically excited state types. The common classification includes metal-centered (MC), ligand-centered (LC), or charge transfer (CT) states, with the possibility of metal-to-ligand (MLCT) or ligand-to-metal (LMCT) subtypes. A given type of excited state usually gives rise to a typical spectroscopic transition and (when it occurs as the lowest excited state of the system) to a specific type of photophysical and photochemical behavior [1–5]. The possibility to play with metal and ligands in a large number of combinations provides a remarkable degree of synthetic control on excited state properties [6–13]. This flexibility can be useful, e.g., in the design of redox photosensitizers [14] for various applications, including chemical conversion of light energy [15].

Intercomponent and interfacial electron transfer processes in polynuclear metal complexes anchored on transparent TiO2 films

A series of polynuclear complexes based on RuII, OsII, ReI and RhIII polypyridine moieties have been prepared in the context of intramolecular energy and electron transfer studies and of interfacial electron transfer with nanocrystalline TiO2. The polynuclear complexes allow for the occurrence of vectorial intramolecular energy and electron transfer and have been proven to be efficient sensitizers of the wide band-gap semiconductor. The performance of photoregenerative cells based on these systems and the dynamics of the excited state intramolecular processes and of the interfacial electron transfer processes are discussed.

Antenna Effects and Photoinduced Electron Transfer in Polynuclear Metal Complexes

Interesting light-induced functions, such as antenna effect and charge separation, can be performed by suitably designed supramolecular systems. Metal polypyridine complexes are good building blocks for the design and synthesis of such supramolecular systems. Photoinduced electron transfer and charge shift processes have been observed in Ru(II)-Rh(III) and Rh(III)-DQ dyads (DQ = Ν,Ν’-cycloalkyl bpy derivative). The possibility to obtain related triads for photoinduced charge separation is discussed. Trinuclear cyano-bridged Ru(II) complexes feature very efficient antenna effects and can be profitably used, following appropriate functionalization, in spectral sensitization of semiconductors. An extension towards larger one-dimensional antenna systems is discussed.

Photophysics of Polynuclear Complexes. Intercomponent Energy and Electron Transfer Processes

Polynuclear complexes add a new dimension to the photochemistry of coordination compounds. Their photophysics is characterized by intercomponent transfer processes: optical electron transfer, photoinduced electron transfer, electronic energy transfer. Examples of such type of processes are discussed in some detail for polynuclear complexes containing Ru(II)-Ru(III), Ru(II)-Ru(II), Ru(II)Rh( III), and Ru(II)-Cr(III) centers.