Fausto Puntoriero

Photochemistry and Photophysics of Coordination Compounds: Ruthenium

Ruthenium compounds, particularly Ru(II) polypyridine complexes, are the class of transition metal complexes which has been most deeply investigated from a photochemical viewpoint. The reason for such great interest stems from a unique combination of chemical stability, redox properties, excited-state reactivity, luminescence emission, and excited-state lifetime. Ruthenium polypyridine complexes are indeed good visible light absorbers, feature relatively intense and long-lived luminescence, and can undergo reversible redox processes in both the ground and excited states. This chapter presents some general concepts on the photochemical properties of Ru(II) polypyridine complexes and gives an overview of various research topics involving ruthenium photochemistry which have emerged in the last 15 years. In particular, aspects connected to supramolecular photochemistry and photophysics are discussed, such as multicomponent systems for light harvesting and photoinduced charge separation, systems for photoinduced multielectron/hole storage, and photocatalytic processes based on supramolecular Ru(II) polypyridine species. Interaction with biological systems and dye-sensitized photoelectrochemical cells are also briefly discussed.

Dendrimers based on photoactive metal complexes. Recent advances

Abstract Recent advances in the field of photoactive dendrimers containing metal complexes are reviewed. Dendrimers with [Ru(bpy)3]2+ as a core exhibit the characteristic [Ru(bpy)3]2+-type luminescence that can be (i) protected from external quenchers by the dendrimer branches and (ii) sensitized by chromophoric groups contained in the periphery of the dendrimer (antenna effect). Several examples of dendrimers fully based on transition metal complexes (i.e., containing a metal at each branching point of the dendrimer structure) have been investigated with the purpose of light harvesting. Dendrimers containing one or more free base and metal porphyrin units have been investigated for light harvesting and for a variety of other purposes. Scattered examples of other types of photoactive dendrimers are also reviewed.

Dendrimers based on ruthenium(II) and osmium(II) polypyridine complexes and the approach of using complexes as ligands and complexes as metals

The use of the “complexes as ligands and complexes as metals” synthetic strategy for the preparation of luminescent and redox-active Ru(II) and Os(II) dendrimers, the dominant synthetic approach for this novel class of compounds, is reviewed. A few comments on the photophysical and redox properties of the metal dendrimers are provided and an overview of alternative synthetic approaches is also presented.

Photocatalytic water oxidation: tuning light-induced electron transfer by molecular Co4O4 cores.

Isostructural cubane-shaped catalysts [Co(III)(4)(μ-O)(4)(μ-CH(3)COO)(4)(p-NC(5)H(4)X)(4)], 1-X (X = H, Me, t-Bu, OMe, Br, COOMe, CN), enable water oxidation under dark and illuminated conditions, where the primary step of photoinduced electron transfer obeys to Hammett linear free energy relationship behavior. Ligand design and catalyst optimization are instrumental for sustained O(2) productivity with quantum efficiency up to 80% at λ > 400 nm, thus opening a new perspective for in vitro molecular photosynthesis.

Photochemistry and Photophysics of Coordination Compounds: Overview and General Concepts

Investigations in the field of the photochemistry and photophysics of coordination compounds have proceeded along several steps of increasing complexity in the last 50 years. Early studies on ligand photosubstitution and photoredox decomposition reactions of metal complexes of simple inorganic ligands (e.g., NH3, CN – ) were followed by accurate investigations on the photophysical behavior (luminescence quantum yields and lifetimes) and use of metal complexes in bimolecular processes (energy and electron transfer). The most significant differences between Jablonski diagrams for organic molecules and coordination compounds are illustrated. A large number of complexes stable toward photodecomposition, but capable of undergoing excited-state redox processes, have been used for interconverting light and chemical energy. The rate constants of a great number of photoinduced energy- and electron-transfer processes involving coordination compounds have been measured in order to prove the validity and/or extend the scope of modern kinetic theories. More recently, the combination of supramolecular chemistry and photochemistry has led to the design and construction of supramolecular systems capable of performing light- induced functions. In this field, luminescent and/or photoredox reactive metal complexes are presently used as essential components for a bottom-up approach to the construction of molecular devices and machines. A few examples of molecular devices for processing light signals and of molecular machines powered by light energy, based on coordination compounds, are briefly illustrated.

Star-shaped multichromophoric arrays from Bodipy dyes grafted on truxene core.

Efficient photoinduced energy migration is obtained in a new star-shaped multichromophoric species made of three different Bodipy dyes and a truxene core.

Recent advances in luminescent polymetallic dendrimers containing the 2,3-bis(2′-pyridyl)pyrazine bridging ligand

Abstract Some new developments in the area of metal-based light-harvesting dendrimers based on the 2,3-bis(2′-pyridyl)pyrazine bridging ligand are presented, with particular regard to unidirectional energy transfer, enhanced light absorption, coupling dendritic structures with electron donors, and measurement of some of the ultra-fast processes occurring in this class of compounds.

Synthetic, Structural, and Photophysical Exploration of meso‐Pyrimidinyl‐Substituted AB2‐Corroles

meso-Pyrimidinyl-substituted AB(2)-corroles were efficiently synthesized starting from 5-mesityldipyrromethane and various 2-substituted 4,6-dichloropyrimidine-5-carbaldehydes. The corrole yield was significantly enhanced by optimization of the amount of Lewis acid catalyst (BF(3).OEt(2)). The main advantage of pyrimidinylcorroles over other meso-triarylcorroles is their wide range of functionalization possibilities, which has been explored by nucleophilic and electrophilic aromatic substitution, and Pd-catalyzed cross-coupling reactions. Stepwise substitution of the chlorine functions afforded asymmetrically substituted pyrimidinylcorroles. Due to the lability of the free-base corrole macrocycles, functionalization of the corrole periphery was preferentially performed on the Cu-metalated counterparts. Functionalized free-base AB(2)-pyrimidinylcorroles were, however, readily accessible by the reversible sequence Cu insertion and subsequent reductive demetalation. AB(2)-pyrimidinylcorroles can hence be regarded as highly versatile platforms towards more sophisticated corrole systems. X-ray analysis of a bis(4-tert-butylphenoxy)-substituted Cu-pyrimidinylcorrole showed the typical features of a Cu-corrole: short N-Cu distances and a saddled corrole plane. The absorption spectra and photophysical properties of some representative free-base AB(2)-pyrimidinylcorroles were examined in depth. The absorption spectra displayed typical corrole features: intense spin-allowed pi-pi* bands, which can be classified as Soret- and Q-type bands. The photophysical properties, investigated both in fluid solution at room temperature and in rigid matrix at 77 K, were governed by the lowest-lying pi-pi* singlet state; however, in most cases, a state with partial charge-transfer character (from the corrole ring to the pyrimidinyl group) was proposed to contribute to the dynamic properties of the emissive level.

Synthesis, characterization, absorption spectra, and luminescence properties of multinuclear species made of Ru(II) and Ir(III) chromophores.

A series of new mixed-metal Ru(II)-Ir(III) trinuclear complexes have been prepared and characterized, together with their mononuclear parents and a series of closely related dinuclear and trinuclear homometallic Ru(II) and Ir(III) species, and their absorption spectra and luminescence properties (both at 77 K in rigid matrix and at room temperature in fluid solution) have been studied. The absorption spectra and luminescence properties of the Ru(II) species and subunits are dominated by metal-to-ligand charge-transfer (MLCT) transitions and excited states, whereas ligand centered (LC) transitions and excited states govern the spectroscopic and photophysical properties of most of the Ir(III) species here studied, with MLCT states playing an important role when cyclometalated Ir(III) subunits are present. Each metal-based subunit retains in the multinuclear arrays its own spectroscopic properties, but in the case of the mixed Ru-Ir species an efficient, additional decay channel is opened for the excited states involving the Ir-centered subunits, that is, photoinduced energy transfer to the lower-lying MLCT state(s) involving the Ru centers.

meso-Pyrimidinyl-Substituted A2B- and A3-Corroles

A variety of meso-pyrimidinyl-substituted A(2)B- and A(3)-corroles (A = 4,6-dichloropyrimidin-5-yl) have been synthesized by careful optimization of the macrocyclization conditions. meso-Pyrimidinylcorroles offer the distinct advantage of an unprecedented broad scope of functionalization options. Highly sterically encumbered triarylcorroles were readily prepared via efficient nucleophilic aromatic substitution and Suzuki cross-coupling procedures.