Claudio Chiorboli

Photoinduced electron and energy transfer in polynuclear complexes

The photochemistry and photophysics of polynuclear transition metal complexes is a new and rapidly developing area of inorganic photochemistry. The photophysical and photochemical behavior of these multi-component systems is characterized by the widespread occurrence of intramolecular, intercomponent electron and energy transfer processes. The aim of this review article is to provide a general overview of the field as it has been developing during the last ten years. The article includes a general introduction with some background material, a detailed survey of the literature, and some projections towards the future of this research field.

Triplet Pathways in Diarylethene Photochromism: Photophysical and Computational Study of Dyads Containing Ruthenium(II) Polypyridine and 1,2-Bis(2-methylbenzothiophene-3-yl)maleimide Units

A 1,2-bis(2-methylbenzothiophene-3-yl)maleimide model ( DAE) and two dyads in which this photochromic unit is coupled, via a direct nitrogen-carbon bond ( Ru-DAE) or through an intervening methylene group ( Ru-CH 2-DAE ), to a ruthenium polypyridine chromophore have been synthesized. The photochemistry and photophysics of these systems have been thoroughly characterized in acetonitrile by a combination of stationary and time-resolved (nano- and femtosecond) spectroscopic methods. The diarylethene model DAE undergoes photocyclization by excitation at 448 nm, with 35% photoconversion at stationary state. The quantum yield increases from 0.22 to 0.33 upon deaeration. Photochemical cycloreversion (quantum yield, 0.51) can be carried out to completion upon excitation at lambda > 500 nm. Photocyclization takes place both from the excited singlet state (S 1), as an ultrafast (ca. 0.5 ps) process, and from the triplet state (T 1) in the microsecond time scale. In Ru-DAE and Ru-CH 2-DAE dyads, efficient photocyclization following light absorption by the ruthenium chromophore occurs with oxygen-sensitive quantum yield (0.44 and 0.22, in deaerated and aerated solution, respectively). The photoconversion efficiency is almost unitary (90%), much higher than for the photochromic DAE alone. Efficient quenching of both Ru-based MLCT phosphorescence and DAE fluorescence is observed. A complete kinetic characterization has been obtained by ps-ns time-resolved spectroscopy. Besides prompt photocyclization (0.5 ps), fast singlet energy transfer takes place from the excited diarylethene to the Ru(II) chromophore (30 ps in Ru-DAE, 150 ps in Ru-CH 2-DAE ). In the Ru(II) chromophore, prompt intersystem crossing to the MLCT triplet state is followed by triplet energy transfer to the diarylethene (1.5 ns in Ru-DAE, 40 ns in Ru-CH 2-DAE ). The triplet state of the diarylethene moiety undergoes cyclization in a microsecond time scale. The experimental results are complemented with a combined ab initio and DFT computational study whereby the potential energy surfaces (PES) for ground state (S 0) and lowest triplet state (T 1) of the diarylethene are investigated along the reaction coordinate for photocyclization/cycloreversion. At the DFT level of theory, the transition-state structures on S 0 and T 1 are similar and lean, along the reaction coordinate, toward the closed-ring form. At the transition-state geometry, the S 0 and T 1 PES are almost degenerate. Whereas on S 0 a large barrier (ca. 45 kcal mol (-1)) separates the open- and closed-ring minima, on T 1 the barriers to isomerization are modest, cyclization barrier (ca. 8 kcal mol (-1)) being smaller than cycloreversion barrier (ca. 14 kcal mol (-1)). These features account for the efficient sensitized photocyclization and inefficient sensitized cycloreversion observed with Ru-DAE. Triplet cyclization is viewed as a nonadiabatic process originating on T 1 at open-ring geometry, proceeding via intersystem crossing at transition-state geometry, and completing on S 0 at closed-ring geometry. A computational study of the prototypical model 1,2-bis(3-thienyl)ethene is used to benchmark DFT results against ab initio CASSCF//CASPT2 results and to demonstrate the generality of the main topological features of the S 0 and T 1 PES obtained for DAE. Altogether, the results provide strong experimental evidence and theoretical rationale for the triplet pathway in the photocyclization of photochromic diarylethenes.

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.

Induced Fit Interanion Discrimination by Binding-Induced Excimer Formation

The synthesis, photophysical, and anion-binding properties of a series of di-, tri-, and tetrapodal anion-binding hosts based on aminopyridinium units with pyrenyl reporter groups are described. The ditopic mesitylene-derived calix[4]arene-based host 4 binds strongly to dicarboxylates, particularly malonate, in a 2:1 anion:host ratio but is essentially nonemissive in the presence of all anions except chloride because of intramolecular quenching by the pyridinium units. Addition of chloride results in a conformational change, giving an initial increase in emission assigned to intramolecular excimer formation. Further chloride addition also results in an increase in the intensity of the pyrenyl monomer emission as chloride binding reduces the acceptor ability of the pyridinium groups. This behavior is not exhibited by control compounds 5 and 6, which lack the ditopic geometry and calixarene spacer unit; however, tripodal 6 forms 1:2 anion:host complexes with a range of anions.

Photocatalytic Hydrogen Evolution with a Self‐Assembling Reductant–Sensitizer–Catalyst System

A noble-metal-free system for photochemical hydrogen production is described, based on ascorbic acid as sacrificial donor, aluminium pyridyl porphyrin as photosensitizer, and cobaloxime as catalyst. Although the aluminium porphyrin platform has docking sites for both the sacrificial donor and the catalyst, the resulting associated species are essentially inactive because of fast unimolecular reversible electron-transfer quenching. Rather, the photochemically active species is the fraction of sensitizer present, in the aqueous/organic solvent used for hydrogen evolution, as free species. As shown by nanosecond laser flash photolysis experiments, its long-lived triplet state reacts bimolecularly with the ascorbate donor, and the reduced sensitizer thus formed, subsequently reacts with the cobaloxime catalyst, thereby triggering the hydrogen evolution process. The performance is good, particularly in terms of turnover frequencies (TOF=10.8 or 3.6 min(-1), relative to the sensitizer or the catalyst, respectively) and the quantum yield (Φ=4.6%, that is, 9.2% of maximum possible value). At high sacrificial donor concentration, the maximum turnover number (TON=352 or 117, relative to the sensitizer or the catalyst, respectively) is eventually limited by hydrogenation of both sensitizer (chlorin formation) and catalyst.

Photochemistry and Photophysics of Coordination Compounds: Rhodium

Rhodium(III) polypyridine complexes and their cyclometalated analogues display photophysical properties of considerable interest, both from a fundamental viewpoint and in terms of the possible applications. In mononuclear polypyridine complexes, the photophysics and photochemistry are determined by the interplay between LC and MC excited states, with relative energies depending critically on the metal coordination environment. In cyclometalated complexes, the covalent character of the C–Rh bonds makes the lowest excited state classification less clear cut, with strong mixing of LC, MLCT, and LLCT character being usually observed. In redox reactions, Rh(III) polypyridine units can behave as good electron acceptors and strong photo-oxidants. These properties are exploited in polynuclear complexes and supramolecular systems containing these units. In particular, Ru(II)-Rh(III) dyads have been actively investigated for the study of photoinduced electron transfer, with specific interest in driving force, distance, and bridging ligand effects. Among systems of higher nuclearity undergoing photoinduced electron transfer, of particular interest are polynuclear complexes where rhodium dihalo polypyridine units, thanks to their Rh(III)/Rh(I) redox behavior, can act as two-electron storage components. A large amount of work has been devoted to the use of Rh(III) polypyridine complexes as intercalators for DNA. In this role, they have proven to be very versatile, being used for direct strand photocleavage marking the site of intercalation, to induce long-distance photochemical damage or dimer repair, or to act as electron acceptors in long-range electron transfer processes.

A fully self-assembled non-symmetric triad for photoinduced charge separation

A very efficient and successful metal-mediated strategy towards the formation of a non-symmetric triad is described: appropriate Lewis acid and/or base functions on the molecular components (a naphthalenediimide, an aluminium(III) porphyrin, and a ruthenium(II) porphyrin) lead to the desired product uniquely. The photophysics of the triad was investigated in detail using time-resolved spectroscopy in the pico- and nanosecond time domains. The strategy is of great potential interest as, while confining the synthetic effort to the single components, it can give access to a wide range of photoactive systems.

Photoinduced Electron/Energy Transfer Across Molecular Bridges in Binuclear Metal Complexes

Molecular bridges that efficiently move charge between remote donor and acceptor sites can be thought of as molecular wires. Insight into the properties of molecular wires can be obtained by studying photoinduced electron transfer in covalently linked donor--bridge--acceptor systems. This article summarizes some of the recent progress in the study of such systems involving transition metal complexes as donor and acceptor units. Specific classes of molecular bridges are considered, namely, polyphenylene, and polyquinoxaline bridges. Basic questions are discussed, such as the transfer mechanisms, the associated distance and bridge structure dependence, and the interplay between energy and electron transfer.

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.

Nondestructive Photoluminescence Read‐Out by Intramolecular Electron Transfer in a Perylene Bisimide‐Diarylethene Dyad

A novel, highly stable photochromic dyad 3 based on a perylene bisimide (PBI) fluorophore and a diarylethene (DAE) photochrome was synthesized and the optical and photophysical properties of this dyad were studied in detail by steady-state and time-resolved ultrafast spectroscopy. This photochromic dyad can be switched reversibly by UV-light irradiation of its ring-open form 3 o leading to the ring-closed form 3 c, and back reaction of 3 c to 3 o by irradiation with visible light. Solvent-dependent fluorescence studies revealed that the emission of ring-closed form 3 c is drastically quenched in solvents of medium (e.g., chloroform) to high (e.g., acetone) polarities, while the emission of the ring-open form 3 o is appreciably quenched only in highly polar solvents like DMF. The strong fluorescence quenching of 3 c is attributed to a photoinduced electron-transfer (PET) process from the excited PBI unit to ring-closed DAE moiety, as this process is thermodynamically highly favorable with a Gibbs free energy value of -0.34 eV in dichloromethane. The electron-transfer mechanism for the fluorescence quenching of ring-closed 3 c is substantiated by ultrafast transient measurements in dichloromethane and acetone, revealing stabilization of charge-separated states of 3 c in these solvents. Our results reported here show that the new photochromic dyad 3 has potential for nondestructive read-out in write/read/erase fluorescent memory systems.