Fausto Puntoriero

Innovative oxygen evolving catalysts, taken from the pool of nanosized, water soluble, molecular metal oxides, the so-called polyoxometalates (POMs), represent an extraordinary opportunity in the field of artificial photosynthesis. These catalysts possess a highly robust, totally inorganic structure, and can provide a unique mimicry of the oxygen evolving center in photosynthetic II enzymes. As a result POMs can effect H₂O oxidation to O₂ with unprecedented efficiency. In particular, the tetra-ruthenium based POM [Ru(IV) ₄(μ-OH)₂(μ-O)₄(H₂O)₄(γ-SiW(10)O(36))₂](10-), Ru₄(POM), displays fast kinetics, electrocatalytic activity powered by carbon nanotubes and exceptionally light-driven performance. A broad perspective is presented herein by addressing the recent progress in the field of metal-oxide nano-clusters as water oxidation catalysts, including colloidal species. Moreover, the shaping of the catalyst environment plays a fundamental role by alleviating the catalyst fatigue and stabilizing competent intermediates, thus responding to what are the formidable thermodynamic and kinetic challenges of water splitting. The design of nano-interfaces with specifically tailored carbon nanostructures and/or polymeric scaffolds opens a vast scenario for tuning electron/proton transfer mechanisms. Therefore innovation is envisaged based on the molecular modification of the hybrid photocatalytic center and of its environment.

Photodynamic therapy (PDT) is a minimally invasive procedure that can provide a selective eradication of neoplastic diseases by the combined effect of a photosensitizer, light and oxygen. New amino oligo(phenylene-ethynylene)s (OPEs), bearing hydrophilic glucoside terminations, have been prepared, characterized and tested as photosensitizers in PDT. The effectiveness of these compounds in combination with UVA light has been checked on two tumor cell lines (HEp-2 and HeLa cells, derived from a larynx carcinoma and a cervical carcinoma, respectively). The compounds triggered a mitotic blockage that led to the cell death, being the effect active up to 3 μm concentration. The photophysical properties of OPEs, such as high quantum yield, stability, singlet oxygen production, biocompatibility, easy cell-internalization and very good response even at low concentration, make them promising photosensitizers in the application of PDT.

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 ...

The absorption spectra, luminescence properties, and redox behavior of Ru (II) molecular racks are reviewed, together with the same properties of closely related systems, including homo-(Fe (II)) and hetero-metallic (Fe (II)/Ru (II) and Fe (II)/Os (II)) molecular grids and ...

The increasing global energy demand has stimulated great recent efforts in investigating new solutions for artificial photosynthesis, a potential source of clean and renewable solar fuel. In particular, according to the generally accepted modular approach aimed at optimising separately the different compartments of the entire process, many studies have focused on the development of catalytic systems for water oxidation to oxygen. While in recent years there have been many reports on new catalytic systems, the mechanism and the active intermediates operating the catalysis have been less investigated. Well-defined, molecular catalysts, constituted by transition metals stabilised by a suitable ligand pool, could help in solving this aspect. However, in some cases molecular species have been shown to evolve to active metal oxides that constitute the other side of this catalysis dichotomy. In this paper, we address the evolution of tetracobalt(iii) cubanes, stabilised by a pyridine/aceta...

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.

A high-yielding synthesis of a series of polyimide dendrimers, including decacyclene- and perylene-containing dendrimer D6, in which two types of polyimide dyes are present, is reported. In these constructs, the branching unit is represented by trisphenylamine, and the solubilizing chains by N-9-heptadecanyl-substituted perylene diimides. The photophysical properties of the dendrimers have been studied by absorption, steady-state, and time-resolved emission spectroscopy and pump-probe transient absorption spectroscopy. Photoinduced charge-separated (CS) states are formed on the femtosecond timescale upon visible excitation. In particular, in D6, two different CS states can be formed, involving different subunits that decays independently with different lifetimes (ca. 10-100 ps).

ABSTRACT A detailed photophys. investigation of the photocatalytic water oxidn. system based on the tetraruthenium polyoxometalate [Ru4(μ-O)4(μ-OH)2(H2O)4(γ-SiW10O36)2]10- (1) as the catalyst, the tetranuclear ruthenium dendrimer [Ru{(μ-2,3-dpp)Ru(bpy)2}3]8+ (2) as the light-harvesting photosensitizer, and persulfate (S2O82-) as the sacrificial electron acceptor has shown that the water oxidn. mechanism proceeds through an unusual, anti-biomimetic pathway: The first photochem. event is indeed a reductive quenching of the excited photosensitizer by the catalyst, followed by electron scavenging by the sacrificial electron acceptor, both occurring on the picosecond time scale within ion-paired species. As an unprecedented photoreaction mechanism for mol. water oxidn. systems, these results suggest a new way to combine photosensitizers and catalysts, taking advantage of suitable chem. interactions and alternative photoinduced processes for the construction of efficient water-splitting devices. [on SciFinder(

The first water oxidation catalyst containing only vanadium atoms as metal centers is reported. The compound is the mixed-valence [(V(IV)5V(V)1)O7(OCH3)12](-) species, 1. Photoinduced water oxidation catalyzed by 1, in the presence of Ru(bpy)3(2+) (bpy = 2,2'-bipyridine) and Na2S2O8, in acetonitrile/aqueous phosphate buffer takes place with a quantum yield of 0.20. A hole scavenging reaction between the photochemically generated Ru(bpy)3(3+) and 1 occurs with a bimolecular rate constant of 2.5 × 10(8) M(-1) s(-1). The time-resolved formation of the oxidized molecular catalyst 1(+) in bimolecular reactions is also evidenced for the first time by transient absorption spectroscopy. This result opens the way to the use of less expensive vanadium clusters as water oxidation catalysts in artificial photosynthesis schemes.

A new tetranuclear heterometallic complex carrying six pyrenyl chromophores at the periphery has been prepared: this species is an example of a new class of light-harvesting metal dendrimer, in which multinuclear metal dendrimers are surrounded by additional purely organic chromophores.

ABSTRACT A review. New progress towards artificial photosynthetic methods and solar fuels will depend on the discovery of highly robust multi-electron catalysts and materials enabling light-activated water splitting with high quantum efficiency and low overpotential, thus mimicking the natural process. [on SciFinder(R)]

ABSTRACT Photoinduced electron transfer continues to be a key process for the design of artificial systems capable to perform an efficient solar energy conversion. In particular, linearly-arranged donor–bridge–acceptor dyads have greatly contributed to shine light on the various factors that must be taken into account when designing systems for obtaining long-lived charge separation, a useful property on the route to artificial photosynthesis. Here we summarized the results we recently obtained on the photoinduced electron transfer processes occurring in Os(II)-bis(terpyridine)-(bi)pyridinium dyads. In particular, we will focus on the role of the bridge in forward and backward electron transfer processes, and on the possibility of obtaining efficient photoinduced charge separation even when the driving force for the electron transfer process approaches zero. This latter point can be of considerable interest when several electron transfer steps are considered to ultimately yield long-range charge-separated state, with minimal energy losses from the initial, light-prepared localized excited state.

Innovative oxygen evolving catalysts, taken from the pool of nanosized, water soluble, molecular metal oxides, the so-called polyoxometalates (POMs), represent an extraordinary opportunity in the field of artificial photosynthesis. These catalysts possess a highly robust, totally inorganic structure, and can provide a unique mimicry of the oxygen evolving center in photosynthetic II enzymes. As a result POMs can effect H₂O oxidation to O₂ with unprecedented efficiency. In particular, the tetra-ruthenium based POM [Ru(IV) ₄(μ-OH)₂(μ-O)₄(H₂O)₄(γ-SiW(10)O(36))₂](10-), Ru₄(POM), displays fast kinetics, electrocatalytic activity powered by carbon nanotubes and exceptionally light-driven performance. A broad perspective is presented herein by addressing the recent progress in the field of metal-oxide nano-clusters as water oxidation catalysts, including colloidal species. Moreover, the shaping of the catalyst environment plays a fundamental role by alleviating the catalyst fatigue and stabilizing competent intermediates, thus responding to what are the formidable thermodynamic and kinetic challenges of water splitting. The design of nano-interfaces with specifically tailored carbon nanostructures and/or polymeric scaffolds opens a vast scenario for tuning electron/proton transfer mechanisms. Therefore innovation is envisaged based on the molecular modification of the hybrid photocatalytic center and of its environment.

The title compound, C(21)H(19)N(7), is a polypyridine ligand that is suitable for assembling complex metal systems capable of photoinduced electron transfer. The solid-state structure has been determined at room temperature by single-crystal X-ray diffraction. The molecule is not flat and both the bis(pyridyl)triazole and the benzylideneamine fragments show significant distortions from planarity.

The new trinuclear dendrons [Cl 2 Os {(μ-2, 3-dpp) Ru (bpy) 2} 2] 4+ 3, Cl 2 OsRu 2 (bpy= 2, 2′-bipyridine; 2, 3-dpp= 2, 3-bis (2-pyridyl) pyrazine),[Cl 2 Os {(μ-2, 3-dpp) Os (bpy) 2} 2] 4+ 4, Cl 2 OsOs 2, and [(bpy) Ru {(μ-2, 3-dpp) Os (bpy) 2} 2} 6+ 6, bpyRuOs 2 have been ...

ABSTRACT 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.

Page 76. Photophysics of transition metal complexes Fausto Puntorieroab DOI: 10.1039/ 9781849732826-00065 This report summarises the literature reported in 2010 on the photophysical properties of metal complexes and their polynuclear supramolecular assem-blies. ...

In regard to semirigid donor-spacer-acceptor (D-S-A) dyads devised for photoinduced charge separation and built from an unsaturated spacer, there exists a strategy of design referred to as "geometrical decoupling" that consists in introducing an inner-S twist angle approaching 90° to minimize adverse D/A mutual electronic influence. The present work aims at gaining further insights into the actual impact of the use of bulky substituents (R) of the alkyl type on the electronic structure of spacers (S) of the oligo-p-phenylene type, which can be critical in the functioning of derived dyads. To this end, a series of 12 novel expanded pyridiniums (EPs), regarded as model S-A assemblies, was synthesized and its structural, electronic, and photophysical properties were investigated at both experimental and theoretical levels. These EPs result from the combination of 4 types of pyridinium-based acceptor moieties with the three following types of S subunits connected at position 4 of the pyridinum core: xylyl (X), xylyl-phenyl (XP), and xylyl-tolyl (XT). From comparison of collected data with those already reported for eight other EPs based on the same A components but linked to S fragments of two other types (i.e., phenyl, P, and biphenyl, PP), the following quantitative order in regard to the pivotal S-centered HOMO energy perturbation was derived (sorted by increasing destabilization): P < X ≪ PP ≈< XP ≈< XT. This indicates that spacers (S) are primarily distinguished on the basis of their mono- or biaryl composition and secondarily by their number of methyl substituents (R). The electron-donating inductive contribution of methyl substituents (HOMO destabilization) more than counterbalances the effect of conjugation disruption (HOMO stabilization). This "compensation effect" suggests that mildly electron-withdrawing hindering groups are better suited for…

ABSTRACT A new water-soluble calix[5]arene 1, bearing 4-butylsulfonato pendant groups at the lower rim, has been synthesised. This derivative shows aggregation properties in water. Critical micellar concentration (cmc) for 1 has been calculated by means of DOSY NMR and fluorescence spectroscopy. When n-dodecylammonium chloride is added to 1 below its cmc, the resulting 1:1 endo-cavity inclusion complex behaves as a supramolecular amphiphile providing a new micellar aggregate.

ABSTRACT The first examples of photo-responsive azobenzene-containing oxacalixarenes have been synthesized via a 3+1 macrocyclization approach. Introduction of the photoresponsive unit was achieved by using 4-phenylazoresorcinol or (E)-4-(4′-nitrophenylazo)resorcinol as the nucleophilic component in the macrocyclization reaction. These novel macrocycles have been characterized by means of 1D and 2D NMR spectroscopy and DFT calculations (B3LYP/6-31G(d)). According to thermal and photoisomerization studies, tetranitro-oxacalix[4]arenes are less prone to E→Z isomerization than oxacalix[2]arene[2]triazines and, within the two series, p-nitrophenylazo derivatives are more unwilling to isomerize than their phenylazo analogues.

A new multicomponent species made of Pt(II) terpyridine and difluoroborondipyrromethene (bodipy) dyes is synthesized and its absorption, redox, and emission properties are reported; photoinduced energy transfer from the Pt-based (3)MLCT state to the bodipy triplet occurs both at room temperature and at 77 K.

By tailoring the chemistry of the 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (bodipy) core in order to change its spectroscopic properties, we linked some modified bodipy dyes to Pt(ii)-terpyridine centers thereby preparing three novel hybrid complexes. In one of such species, two pyrene subunits are also present. The absorption spectra and luminescence properties (both at room temperature in acetonitrile solution and at 77 K in butyronitrile rigid matrix) of the new hybrid complexes and of their bodipy parent species have been investigated, and compared with those of previously studied compounds. Absorption spectra showed that the different chromophoric subunits roughly keep their own features in the multicomponent systems, indicating the supramolecular nature of the compounds. Efficient photoinduced intercomponent energy transfer takes place in the various hybrid Pt(ii)-terpyridine-bodipy compounds, by taking advantage of different mechanisms: at room temperature in fluid solution very efficient energy transfer from the (3)MLCT state(s) of the Pt residue to the lowest-lying (3)bodipy is mediated by a charge transfer state via a two-step electron transfer process. Direct energy transfer from (3)MLCT state(s) to the (3)bodipy states takes place at 77 K, most likely via the Dexter mechanism. When two pyrene fragments are grafted to the boron, cascade energy transfer from the pyrene to the (1)bodipy also occurs by the Förster mechanism. For one of the new compounds, (3)bodipy emission with a maximum at 738 nm is also observed at cryogenic temperature.

Luminescence intensity changes of a Pt(II) complex which is known to bind externally to DNA at low [DNA]/[complex] ratio and to intercalate at high [DNA]/[complex] ratio are studied in the presence of calf thymus DNA. External binding is demonstrated to induce luminescence enhancement whereas intercalation leads to luminescence quenching. The reasons for this behaviour are discussed.

... Francesco Neve,* † Massimo La Deda ... It should be noted that recently Scandola and co-workers reported a bis-tridentate Ir(III) cyclometalated compound exhibiting a very small red shift of the emission band on passing from low to high temperature, still assigning the emission to ...

ABSTRACT We evidenced the conversion of an unprecedented intermolecular bridged-proton bimetallic P-metallated phosphoryl complex [{[Ru](Ph2PO)}2H][PF6]3 to its corresponding H-bonded water complex [{[Ru](Ph2PO)}2(H2O)][PF6]2, a rare motif. Their synthesis, NMR data, X-ray crystal structures, electrochemical and photophysical properties are reported.

ABSTRACT A species based on a bodipychromophore and containing bipyridine chelating sites behaves as a concealable molecular switch, featuring part of the properties of D-latch circuits by integrating two logic gates, a NOR and an INHIBIT gate, with both gates sharing the same inputs.

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.

We have studied the adducts formed by eosin (E) with a fourth generation dendrimer (D) that comprises 30 tertiary amine units in the interior and 32 naphthyl and 32 trans azobenzene units in the periphery. We have found that: (i) the all trans dendrimer D(32t) can be converted by irradiation with 365 nm light (Phi=0.12) into species containing, as an average, 4 trans and 28 cis azobenzene units, D(4t28c), that at 313 K undergoes a D(4t28c) --> D(32t) thermal back reaction (k = 7.0 x 10(-5) s(-1)); (ii) D(32t) and D(4t28c) extract 8 and, respectively, 6 eosin molecules from water at pH 7, yielding the species D(32t) subset 8E and D(4t28c) subset 6E; (iii) eosin uptake is significantly faster for D(32t) than for D(4t28c); (iv) irradiation at 365 nm of the D(32t) subset 8E species at 298 K leads to the release of two eosin molecules with formation of a photostable D(15t17c) subset 6E species (Phi = 0.15) that is also obtained from the back thermal reaction of D(4t28c) subset6E at 313 K (k = 2.7 x 10(-5) s(-1)); (v) thermal release of E from D(32t) subset 6E is much faster than from D(4t28c) subset 6E; and (vi) excitation of E in the adducts sensitizes the cis --> trans (but not the trans --> cis) isomerization. The results obtained show that the isomerization of the 32 peripheral azobenzene units controls to some extent the hosting capacity of the dendrimer and, viceversa, eosin molecules hosted in the dendrimer affect the isomerization process of its azobenzene units.