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La fotosintesi artificiale, ovvero la conversione diretta di energia solare in energia chimica attraverso la sintesi fotoindotta di specie chimiche ad alta energia, quali idrogeno e altri combustibili, a partire da specie a bassa energia come acqua e anidride carbonica, un processo ispirato alla fotosintesi naturale, è stato per lungo tempo un sogno inseguito dalla comunità scientifica internazionale. Gli sviluppi degli ultimi anni permettono adesso di affrontare il problema con buone speranze di successo. L’articolo presenta una breve panoramica delle problematiche relative alla fotosintesi artificiale, delle strutture e dei progetti di ricerca nel settore attualmente presenti nei vari Paesi, con particolare attenzione per gli studi che vengono portati avanti dai ricercatori del SOLAR-CHEM, centro interuniversitario italiano per la conversione chimica dell’energia solare.

A series of rodlike ruthenium(II)-rhodium(III) polypyridine dyads based on modular oligo-p-phenylene bridges, of the general formula [(Me2phen)2Ru-bpy-(ph)n-bpy-Rh(Me2bpy)]5+ (Me2phen = 4,7-dimethyl-1,10-phenanthroline; bpy = 2,2'-bipyridine; ph = 1,4-phenylene; n = 1-3), have been synthesized and their photophysical properties investigated. The dyad [(Me2bpy)2Ru-bpy-(ph)3'-bpy-Rh(Me2bpy)]5+ with the central phenylene unit bearing two hexyl chains has also been studied. The metal-to-metal distance reaches 24 Å for the longest (n = 3) spacer in the series. For all of the dyads in a room-temperature CH3CN solution, quenching of the typical metal-to-ligand charge-transfer luminescence of the Ru-based chromophoric unit is observed, indicating that an efficient intramolecular photoinduced electron transfer from the excited Ru moiety to the Rh-based unit takes place. The rate constants for the electron-transfer process have been determined by time-resolved emission and absorption spectroscopy in the nanosecond and picosecond time scale. An exponential dependence of experimental transfer rates on the bridge length is observed, consistent with a superexchange mechanism. An attenuation factor of 0.65 Å-1 is determined, in line with the behavior of other systems containing oligo-p-phenylene spacers. Interestingly, for n = 3, the presence/absence of hexyl substituents in the central p-phenylene ring causes a 10-fold difference in the rates between otherwise identical dyads. This comparison highlights the importance of the twist angle between adjacent spacers on the overall through-bond donor-acceptor coupling.

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 twoelectron 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.

The primary photoinduced charge separation and recombination processes occurring in a dinuclear Ru(II) complex capable to perform multielectron storage upon light excitation have been studied in different solvents: the charge-separated state obtained in dichloromethane is, to the best of our knowledge, the longest-lived ever reported for a ruthenium polypyridine complex.

The supramolecular systems [Ru(Pyrnbpy)(CN)4]2- (n ) 1, 2), where one and two pyrenyl units are linked via two-methylene bridges to the [Ru(bpy)(CN)4]2- chromophore, have been synthesized. The photophysical properties of these systems, which contain a highly solvatochromic metal complex moiety, have been investigated in water, methanol, and acetonitrile. In all solvents, prompt and efficient singlet-singlet energy transfer takes places from the pyrene to the inorganic moiety. Energy transfer at the triplet level, on the other hand, is dramatically solvent dependent. In water, the metal-to-ligand charge transfer (MLCT) emission of the Ru-based chromophore is completely quenched, and rapid (200 ps for n ) 1) irreversible triplet energy transfer to the pyrene units is detected in ultrafast spectroscopy. In acetonitrile, the MLCT emission is practically unaffected by the presence of the pyrenyl chromophore, implying the absence of any intercomponent triplet energy transfer. In methanol, triplet energy transfer leads to an equilibrium between the excited chromophores, with considerable elongation of the MLCT lifetime. The investigation of the [Ru(Pyrnbpy)(CN)4]2- systems in methanol provided a very detailed and self-consistent picture: (i) The initially formed MLCT state relaxes toward equilibrium in 0.5-1.3 ns (n ) 1, 2), as monitored both by ultrafast transient absorption and by time-correlated single photon counting. (ii) The two excited chromophores decay with a common lifetime of 260-450 ns (n ) 1, 2), as determined from the decay of MLCT emission (slow component) and of the pyrene triplet absorption. (iii) These equilibrium lifetimes are fully consistent with the excitedstate partition of 12-6% MLCT (n ) 1-2), independently measured from preexponential factors of the emission decay. Altogether, the results demonstrate how site-specific solvent effects can be used to control the direction of intercomponent energy flow in bichromophoric systems.

The energy and electron transfer processes taking place in binuclear polypyridine complexes of ruthenium and osmium based on the tetrapyrido[3,2- a:2¢,3¢- c:3¢¢,2¢¢- h:2¢¢¢-3¢¢¢- j]phenazine bridging ligand (tpphz) have been investigated by ultrafast absorption spectroscopy. In the binuclear complexes, each chromophore is characterized by two spectrally distinguishable metal-to-ligand charge transfer (MLCT) excited states: MLCT1 (with promoted electron mainly localized on the bpy-like portion of tpphz, higher energy) and MLCT0 (with promoted electron mainly localized on the pyrazine-like portion of tpphz, lower energy). In the homodinuclear complexes Ru(II)-Ru(II) and Os(II)-Os(II), MLCT1 f MLCT0 relaxation (intraligand electron transfer) is observed, with strongly solvent-dependent kinetics (ca. 10-10 s in CH2Cl2, ca. 10-12 s in CH3CN). In the heterodinuclear Ru(II)-Os(II) complex, *Ru(II)-Os(II) f Ru(II)-*Os(II) energy transfer takes place by two different sequences of time-resolved processes, depending on the solvent: (a) in CH2Cl2, ruthenium-to-osmium energy transfer at the MLCT1 level followed by MLCT1 f MLCT0 relaxation in the osmium chromophore, (b) in CH3CN, MLCT1 f MLCT0 relaxation in the ruthenium chromophore followed by osmium-to-ruthenium metal-to-metal electron transfer. In the mixed-valence Ru(II)-Os(III) species, the *Ru(II)-Os(III) f Ru(III)-Os(II) electron transfer quenching is found to proceed by two consecutive steps in CH3CN: intraligand electron transfer followed by ligand-to-metal electron transfer. On a longer time scale, charge recombination leads back to the ground state. Altogether, the results show that the tpphz bridge plays an active mechanistic role in these systems, efficiently mediating the transfer processes with its electronic levels.