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Ru or Pt nanoparticles have been prepared following the organometallic approach and deposited onto the surface of mesoporous graphitic carbon nitride (mpg-CN). Three different Ru-based samples have also been compared to investigate the effect of 4-phenylpyridine as a stabilizing agent. The photocatalytic performance towards the hydrogen evolution reaction (HER) has been tested showing that all hybrid systems clearly outperform the photocatalytic activity of bare mpg-CN. In particular, Pt-decorated mpg-CN yields the largest H

A collection of cobalt polypyridine complexes employed for the light-driven HER in aqueous solution is presented with the aim of highlighting the relevant chemical motifs that play a role in supporting efficient catalytic activity.

The conjugation of photoactive benzophenone with diphenylalanine yielded a self-assembling photocatalyst that was probed in the E → Z photoisomerisation of stilbene derivatives.

International audience; Cobalt(II) complexes featuring hexadentate amino-pyridyl ligands have been recently discovered as highly active catalysts for the Hydrogen Evolution Reaction (HER), whose high performance arises from the possibility to assist proton transfer processes via intramolecular routes involving detached pyridine units. With the aim of gaining insights in such catalytic routine, three new proton reduction catalysts based on amino-polypyridyl ligands are reported, focusing on substitution of the pyridine ortho- position. Especially, a carboxylate (C2) and two hydroxyl substituted pyridyl moieties (C3, C4) are introduced with the aim of promoting intramolecular proton transfer possibly enhancing the efficiency of the catalysts. Foot-of-the-wave and catalytic Tafel plot analyses have been utilized to benchmark the catalytic performances under electrochemical conditions in acetonitrile using trifluoroacetic acid as the proton source. In this respect, the cobalt complex C3 results as the fastest catalyst in the series, maximum turnover frequency (TOF) of 1.6 (± 0.5)x105 s-1, but at the expenses of large overpotentials. Mechanistic investigation by means of Density Functional Theory (DFT) suggest a typical ECEC mechanism (i.e. a sequence of reductions -E- and protonations -C- events) for all the catalysts, as previously envisioned for the parent unsubstituted complex C1. Interestingly, in the case of complexes C2, the catalytic route is triggered by initial protonation of the carboxylate group resulting in a less common (C)ECEC mechanism. The pivotal role of the hexadentate chelating ligand in providing internal proton relays to assist hydrogen elimination is further confirmed within this novel class of molecular catalysts, thus highlighting the relevance of a flexible polypyridine ligand in the design of efficient cobalt complexes for the HER. Photochemical studies in aqueous solution using [Ru(bpy)3]2+ (where bpy = 2,2’-bipyridine) as the sensitizer and ascorbate as the sacrificial electron donor support the superior performance of C3.

The luminophore Ru(bpy)2 (dcbpy)2+ (bpy=2,2'-bipyridine; dcbpy=4,4'-dicarboxy-2,2'-bipyridine) is covalently linked to a chitosan polymer; crosslinking by tripolyphosphate produced Ru-decorated chitosan fibers (NS-RuCh), with a 20 : 1 ratio between chitosan repeating units and RuII chromophores. The properties of the RuII compound are unperturbed by the chitosan structure, with NS-RuCh exhibiting the typical metal-to-ligand charge-transfer (MLCT) absorption and emission bands of RuII complexes. When crosslinks are made in the presence of IrO2 nanoparticles, such species are encapsulated within the nanofibers, thus generating the IrO2 ⊂NS-RuCh system, in which both RuII photosensitizers and IrO2 water oxidation catalysts are within the nanofiber structures. NS-RuCh and IrO2 ⊂NS-RuCh have been characterized by dynamic light scattering, scanning electronic microscopy, and energy-dispersive X-ray analysis, which indicated a 2 : 1 ratio between RuII chromophores and IrO2 species. Photochemical water oxidation has been investigated by using IrO2 ⊂NS-RuCh as the chromophore/catalyst assembly and persulfate anions as the sacrificial species: photochemical water oxidation yields O2 with a quantum yield (Φ) of 0.21, definitely higher than the Φ obtained with a similar solution containing separated Ru(bpy)3 2+ and IrO2 nanoparticles (0.05) or with respect to that obtained when using NS-RuCh and "free" IrO2 nanoparticles (0.10). A fast hole-scavenging process (rate constant, 7×104 s-1 ) involving the oxidized photosensitizer and the IrO2 catalyst within the IrO2 ⊂NS-RuCh system is behind the improved photochemical quantum yield of IrO2 ⊂NS-RuCh.

The cobalt substituted polyoxotungstate [Co6 (H2 O)2 (α-B-PW9 O34 )2 (PW6 O26 )]17- (Co6) displays fast electron transfer (ET) kinetics to photogenerated RuIII (bpy)33+ , 4 to 5 orders of magnitude faster than the corresponding ET observed for cobalt oxide nanoparticles. Mechanistic evidence has been acquired indicating that: (i) the one-electron oxidation of Co6 involves Co(II) aquo or Co(II) hydroxo groups (abbreviated as Co6(II)-OH2 and Co6(II)-OH, respectively, whose speciation in aqueous solution is associated to a pKa of 7.6), and generates a Co(III)-OH moiety (Co6(III)-OH), as proven by transient absorption spectroscopy; (ii) at pH>pKa , the Co6(II)-OH→RuIII (bpy)33+ ET occurs via bimolecular kinetics, with a rate constant k close to the diffusion limit and dependent on the ionic strength of the medium, consistent with reaction between charged species; (iii) at pH

By conducting in-depth mechanistic investigations to rationalize reaction manifolds, chemists can expand the generality of synthetic processes and discover novel reactivities. The challenge is to use the resulting information to control the selectivity of a given synthetic process. A particular challenge for organic chemists is to use an external stimulus to switch a reactivity on or off. Here, we mechanistically investigated light-driven [2+2] heterocycloadditions (Paternò-Büchi reactions) between indoles and ketones. We used ground-state UV-Vis absorption and transient absorption spectroscopy (TAS) at ns and fs timescale, together with DFT and TD-DFT calculations. We found that the reaction can proceed via an exciplex or electron-donor-acceptor (EDA) complex manifold, which are key intermediates in determining the reaction’s stereoselectivity. We used this discovery to control the reaction’s diastereoselectivity, gaining access to previously inaccessible diastereoisomeric variants. When moving from 370 to 456 nm irradiation, the EDA complex is increasingly favored, and the diastereomeric ratio dr (en-do:exo) moves from >99

Synthesis of a novel bulky diimine ligand and its corresponding heteroleptic Cu(i). Unusual fluorescence behavior of a novel Cu(i) complex due to a strong electron-donor diimine ligand.

The utilization of carbon dioxide as a raw material represents nowadays an appealing strategy in the renewable energy, organic synthesis, and green chemistry fields. Besides reduction strategies, carbon dioxide can be exploited as a single-carbon-atom building block through its fixation into organic scaffolds with the formation of new C-C bonds (carboxylation processes). In this case, activation of the organic substrate is commonly required, upon formation of a carbanion C−, being sufficiently reactive toward the addition of CO2. However, the prediction of the reactivity of C− with CO2 is often problematic with the process being possibly associated with unfavorable thermodynamics. In this contribution, we present a thermodynamic analysis combined with density functional theory calculations on 50 organic molecules enabling the achievement of a linear correlation of the standard free energy (ΔG0) of the carboxylation reaction with the basicity of the carbanion C−, expressed as the pKa of the CH/C− couple. The analysis identifies a threshold pKa of ca 36 (in CH3CN) for the CH/C− couple, above which the ΔG0 of the carboxylation reaction is negative and indicative of a favorable process. We then apply the model to a real case involving electrochemical carboxylation of flavone and chalcone as model compounds of α,β-unsaturated ketones. Carboxylation occurs in the β-position from the doubly reduced dianion intermediates of flavone and chalcone (calculated ΔG0 of carboxylation in β = −12.8 and −20.0 Kcalmol-1 for flavone and chalcone, respectively, associated with pKa values for the conjugate acids of 50.6 and 51.8, respectively). Conversely, the one-electron reduced radical anions are not reactive toward carboxylation (ΔG0 > +20 Kcalmol-1 for both substrates, in either α or β position, consistent with pKa of the conjugate acids < 18.5). For all the possible intermediates, the plot of calculated ΔG0 of carboxylation vs. pKa is consistent with the linear correlation model developed. The application of the ΔG0 vs. pKa correlation is finally discussed for alternative reaction mechanisms and for carboxylation of other C=C and C=O double bonds. These results offer a new mechanistic tool for the interpretation of the reactivity of CO2 with organic intermediates.

Light-driven water oxidation is achieved with the Ru(bpy)32+/S2O82– cycle employing the highly active Ir-blue water oxidation catalyst, namely, an IrIV,IV2(pyalc)2 μ-oxo-dimer [pyalc = 2-(2′-pyridy...

Twelve naphthochromenone photocatalysts (PCs) were synthesized on gram scale. They absorb across the UV/Vis range and feature an extremely wide redox window (up to 3.22 eV) that is accessible using simple visible light irradiation sources (CFL or LED). Their excited-state redox potentials, PC*/PC.- (up to 1.65 V) and PC.+ /PC* (up to -1.77 V vs. SCE), are such that these novel PCs can engage in both oxidative and reductive quenching mechanisms with strong thermodynamic requirements. The potential of these bimodal PCs was benchmarked in synthetically relevant photocatalytic processes with extreme thermodynamic requirements. Their ability to efficiently catalyze mechanistically opposite oxidative/reductive photoreactions is a unique feature of these organic photocatalysts, thus representing a decisive advance towards generality, sustainability, and cost efficiency in photocatalysis.

The luminophore Ru(bpy)

Four novel polypyridine cobalt(II) complexes were developed based on a hexadentate ligand scaffold bearing either electron-withdrawing (-CF3 ) or electron-donating (-OCH3 ) groups in different positions of the ligand. Experiments and theoretical calculations were combined to perform a systematic investigation of the effect of the ligand modification on the hydrogen evolution reaction. The results indicated that the position, rather than the type of substituent, was the dominating factor in promoting catalysis. The best performances were observed upon introduction of substituents on the pyridine moiety of the hexadentate ligand, which promoted the formation of the Co(II)H intermediate via intramolecular proton transfer reactions with low activation energy. Quantum yields of 11.3 and 10.1 %, maximum turnover frequencies of 86.1 and 76.6 min-1 , and maximum turnover numbers of 5520 and 4043 were obtained, respectively, with a -OCH3 and a -CF3 substituent.

The cobalt substituted polyoxotungstate [Co6(H2O)2(α‐B‐PW9O34)2(PW6O26)]17− (Co6) displays fast electron transfer (ET) kinetics to photogenerated RuIII(bpy)3 3+, 4 to 5 orders of magnitude faster than the corresponding ET observed for cobalt oxide nanoparticles. Mechanistic evidence has been acquired indicating that: (i) the one‐electron oxidation of Co6 involves Co(II) aquo or Co(II) hydroxo groups (abbreviated as Co6(II)−OH2 and Co6(II)−OH, respectively, whose speciation in aqueous solution is associated to a pKa of 7.6), and generates a Co(III)−OH moiety (Co6(III)−OH), as proven by transient absorption spectroscopy; (ii) at pH>pKa, the Co6(II)−OH→RuIII(bpy)3 3+ ET occurs via bimolecular kinetics, with a rate constant k close to the diffusion limit and dependent on the ionic strength of the medium, consistent with reaction between charged species; (iii) at pH, Kinetic investigation by laser flash photolysis supports the occurrence of a concerted proton electron transfer (CPET) from a CoII−OH2 unit embedded in a polyoxotungstate to photogenerated RuIII(bpy)3 3+, with water acting as the base within a hydrogen bonding network.

Aromatic amino acids such as l-tyrosine and l-tryptophan are deployed in natural systems to mediate electron transfer (ET) reactions. While tyrosine oxidation is always coupled to deprotonation (proton-coupled electron-transfer, PCET), both ET-only and PCET pathways can occur in the case of the tryptophan residue. In the present work, two novel conjugates 1 and 2, based on a Sn-IV tetraphenylporphyrin and Sn-IV octaethylporphyrin, respectively, as the chromophore/electron acceptor and l-tryptophan as electron/proton donor, have been prepared and thoroughly characterized by a combination of different techniques including single crystal X-ray analysis. The photophysical investigation of 1 and 2 in CH2Cl2 in the presence of pyrrolidine as a base shows that different quenching mechanisms are operating upon visible-light excitation of the porphyrin component, namely photoinduced electron transfer and concerted proton electron transfer (CPET), depending on the chromophore identity and spin multiplicity of the excited state. The results are compared with those previously described for metal-mediated analogues featuring Sn-IV porphyrin chromophores and l-tyrosine as the redox active amino acid and well illustrate the peculiar role of l-tryptophan with respect to PCET.

Photocatalytic water oxidation is a promising process for the production of solar fuels and the elucidation of factors that influence this process is of high significance. Thus, we have studied in detail light‐driven water oxidation with a trinuclear Ru(bda) (bda: 2,2’‐bipyridine‐6,6’‐dicarboxylate) macrocycle MC3 and its highly water soluble derivative m‐CH\(_2\)NMe\(_2\)‐MC3 using a series of ruthenium tris(bipyridine) complexes as photosensitizers under varied reaction conditions. Our investigations showed that the catalytic activities of these Ru macrocycles are significantly affected by the choice of photosensitizer (PS) and reaction media, in addition to buffer concentration, light intensity and concentration of the sensitizer. Our steady‐state and transient spectroscopic studies revealed that the photocatalytic performance of trinuclear Ru(bda) macrocycles is not limited by their intrinsic catalytic activities but rather by the efficiency of photogeneration of oxidant PS\(^+\) and its ability to act as an oxidizing agent to the catalysts as both are strongly dependent on the choice of photosensitizer and the amount of employed organic co‐solvent.

The generation of hydrogen from water represents an important task towards a carbon neutral economy. Within this context, the preparation of hybrid electrodes merging the versatility of solid-state porous substrates and the catalytic ability and tunability of molecular complexes represents a great challenge. In the present work, we report on the preparation of hybrid cathodes for the hydrogen evolution reaction (HER) through an unprecedented combination of single-walled carbon nanohorns (SWCNHs) and two novel cobalt(II) polypyridine complexes based on the tris(2-pyridylmethyl)amine (TPMA) ligand scaffold. Suitable pyrene groups are introduced in the ligand framework in different positions to provide a way for direct anchoring onto the carbonaceous substrate by exploiting non-covalent π–π interactions. The present systems behave as competent cathodes for the HER in neutral aqueous solution with overpotentials of η ∼ 0.5 V and stable current densities (within 1 h electrolysis) up to −0.50 mA cm−2, whose exact values depend on the catalyst used and are mainly related to the respective loading on the electrode surface. In both cases, hydrogen evolution is detected under continuous electrolysis for up to ca. 12 h leading to maximum turnover numbers (TONs) of 4700 and 9180 molH2 molCo−1 for the two different complexes. The progressive deactivation under electrolytic conditions is mainly ascribed to leaching of the metal centre from the polydentate ligand, likely occurring from the competent catalytic intermediates involved in the HER.

The design of active photocathodes for the hydrogen evolution reaction (HER) is a crucial step in the development of dye-sensitized photoelectrochemical cells (DS-PECs) aimed at solar-assisted water splitting. In the present work, we report on the use of orange CdTexS1-x quantum dots (QDs) with an average diameter of ca. 3.5 nm, featuring different capping agents (MAA, MPA, and MSA) for the sensitization of electrodes based on nanostructured NiO. Photoelectrochemical characterization of the resulting NiO|QDs electrodes in the presence of [CoIII(NH3)5Cl]Cl2 as an irreversible electron acceptor elects MAA-capped QDs as the most active sample to achieve substantial photocurrent densities thanks to both improved surface coverage and injection ability. Functionalization of the NiO|QDs electrodes with either heterogeneous Pt or the molecular nickel bis(diphosphine) complex (1) as the hydrogen evolving catalysts (HECs) yields active photocathodes capable of promoting hydrogen evolution upon photoirradiation (maximum photocurrent densities of -16(±2) and -20(±1) μA·cm-2 for Pt and 1 HECs, respectively, at 0 V vs. NHE, 70-80% faradaic efficiency, maximum IPCE of ca. 0.2%). The photoelectrochemical activity is limited by the small surface concentration of the QD sensitizers on the NiO surface and the competitive light absorption by the NiO material which suggests that the match between dye adsorption and the available surface area is critical to achieving efficient hydrogen evolution by thiol-capped QDs.

Aromatic amino acids such as l‐tyrosine and l‐tryptophan are deployed in natural systems to mediate electron transfer (ET) reactions. While tyrosine oxidation is always coupled to deprotonation (proton‐coupled electron‐transfer, PCET), both ET‐only and PCET pathways can occur in the case of the tryptophan residue. In the present work, two novel conjugates 1 and 2, based on a SnIV tetraphenylporphyrin and SnIV octaethylporphyrin, respectively, as the chromophore/electron acceptor and l‐tryptophan as electron/proton donor, have been prepared and thoroughly characterized by a combination of different techniques including single crystal X‐ray analysis. The photophysical investigation of 1 and 2 in CH2Cl2 in the presence of pyrrolidine as a base shows that different quenching mechanisms are operating upon visible‐light excitation of the porphyrin component, namely photoinduced electron transfer and concerted proton electron transfer (CPET), depending on the chromophore identity and spin multiplicity of the excited state. The results are compared with those previously described for metal‐mediated analogues featuring SnIV porphyrin chromophores and l‐tyrosine as the redox active amino acid and well illustrate the peculiar role of l‐tryptophan with respect to PCET., Photoinduced electron transfer (ET) or concerted proton‐electron transfer (CPET) occur in tin(IV) porphyrin‐tryptophan conjugates in dichloromethane in the presence of an organic base depending on the chromophore identity and spin multiplicity of the excited state which point out how peculiar the l‐tryptophan amino acid is with respect to charge transfer reactions.

The design of active photocathodes for the hydrogen evolution reaction (HER) is a crucial step in the development of dye-sensitized photoelectrochemical cells (DS-PECs) aimed at solar-assisted water splitting. In the present work, we report on the use of orange CdTexS1-x quantum dots (QDs) with an average diameter of ca. 3.5 nm, featuring different capping agents (MAA, MPA, and MSA) for the sensitization of electrodes based on nanostructured NiO. Photoelectrochemical characterization of the resulting NiO|QDs electrodes in the presence of [CoIII(NH3)5Cl]Cl2 as an irreversible electron acceptor elects MAA-capped QDs as the most active sample to achieve substantial photocurrent densities thanks to both improved surface coverage and injection ability. Functionalization of the NiO|QDs electrodes with either heterogeneous Pt or the molecular nickel bis(diphosphine) complex (1) as the hydrogen evolving catalysts (HECs) yields active photocathodes capable of promoting hydrogen evolution upon photoirradiation (maximum photocurrent densities of -16(±2) and -20(±1) μA·cm-2 for Pt and 1 HECs, respectively, at 0 V vs. NHE, 70-80% faradaic efficiency, maximum IPCE of ca. 0.2%). The photoelectrochemical activity is limited by the small surface concentration of the QD sensitizers on the NiO surface and the competitive light absorption by the NiO material which suggests that the match between dye adsorption and the available surface area is critical to achieving efficient hydrogen evolution by thiol-capped QDs.

International audience; Ru nanomaterials have recently emerged as potential substitutes for classical Pt-based cathodes for the hydrogen evolution reaction (HER). In this regard, nanoparticle surface-functionalization through the so-called organometallic approach is a promising strategy towards synthesizing tailored highly active and durable HER (photo)electrocatalysts with limitless tunability. Herein, efficient (turnover numbers over 480 molH2 molRu−1 and a turnover frequency of 21.5 molH2 h−1 molRu−1; apparent quantum yield of 1.3%) and durable (>100 h) visible-light-driven hydrogen evolution has been achieved at neutral pH with a ternary hybrid nanomaterial combining 4-phenylpyridine-capped Ru nanoparticles (RuPP), TiO2 nanocrystals and [Ru(bpy)2(4,4′-(PO3H2)2(bpy))]Cl2 (RuP) using triethanolamine (TEOA) as a sacrificial electron-donor. Photophysical analysis by means of transient absorption spectroscopy has been performed in order to shed light on the kinetics of the electron transfer events and to identify the rate-determining step of the overall photocatalytic process. TiO2 is shown to have a key role as (1) the support aiding the dispersion of the photocatalyst and limiting its agglomeration under turnover conditions and (2) the electron-transfer mediator enabling efficient electron communication between the catalyst and the anchored molecular photoabsorber. Finally, the evolution and fate of the photocatalyst in long-term HER photocatalysis are thoroughly analyzed.

Solar-to-chemical (STC) energy conversion is the fundamental process that nurtures Earth's ecosystem, fixing the inexhaustible solar resource into chemical bonds. Photochemical synthesis endows plants with the primary substances for their development; likewise, an artificial mimic of natural systems has long sought to support human civilization in a sustainable way. Intensive efforts have demonstrated light-triggered production of different solar fuels, such as H2 , CO, CH4 and NH3 , while research on oxidative half-reactions has built up from O2 generation to organic synthesis, waste degradation and photo-reforming. Nevertheless, while extensive utilization of the radiant chemical potential to promote a manifold of endergonic processes is the common thread of such research, exploration of the chemical space is fragmented by the lack of a common language across different scientific disciplines. Focusing on colloidal semiconductor materials, this Viewpoint discusses an inclusive protocol for the discovery and assessment of STC redox reactions, aiming to establish photon-to-molecule conversion as the ultimate paradigm beyond fossil energy exploitation.

Nickel(ii) bis(diphosphine) complex 1 bearing carboxylic acid functional groups at the periphery has been prepared and characterized. Its catalytic ability towards the hydrogen evolution reaction (HER) has been evaluated under different experimental conditions: (i) under homogeneous electrocatalysis in acetonitrile using trifluoroacetic acid as the proton source, (ii) under homogeneous photocatalysis in aqueous solution using Ru(bpy)32+ (where bpy = 2,2'-bipyridine) as the sensitizer and ascorbic acid as the sacrificial electron donor, and (iii) under heterogeneous electrochemical conditions upon grafting onto mesoporous TiO2. The results show that complex 1 is a competent and efficient catalyst for the HER under both homogeneous electro- and photochemical conditions, while undergoes rapid deactivation once attached onto the TiO2 surface. This is attributed to both catalyst binding mode and structural rigidity imparted by covalent grafting, which highlights how catalyst design has to be considered for the direct transposition of molecular catalysis on (photo)electrode surfaces.

Iridium complexes [IrClCp*diNHC]PF6, with N-heterocyclic dicarbene (diNHC) and pentamethylcyclopentadienyl (Cp*) ligands, have been investigated in light driven water oxidation catalysis within the Ru(bpy)32+/S2O82− cycle (bpy = 2,2′-bipyridine). In particular, the effect of different diNHC ligands was evaluated by employing the complex 1a (diNHC = 1,1′-dimethyl-3,3′-ethylenediimidazol-2,2′-diylidene) and the novel and structurally characterised 2 (diNHC = 1,1'-dimethyl-3,3′-ethylene-5,5′-dibromodiimidazol-2,2′-diylidene) and 3 (diNHC = 1,1′-dimethyl-3,3'-ethylene-dibenzimidazol-2,2′-diylidene). The presented results include: (i) a photon management analysis of the 1a/Ru(bpy)32+/S2O82− system, revealing two regimes of O2 evolution rate, being dependent on the light intensity at low photon flux, where the system reaches an overall quantum yield up to 0.17 ± 0.01 (quantum efficiency 34 ± 2%), while being independent of light intensity at high photon flux thus indicating a change of limiting step; (ii) a trend of O2 evolution activity that follows the order 1a > 2 > 3 both under low and high photon flux conditions, with the reactivity that is favoured by the electron donating nature of the diNHC ligand, quantified on the basis of the carbene carbon chemical shift; (iii) an analogous trend also in the bimolecular rate constants of electron transfer kET from the iridium species to photogenerated Ru(bpy)33+, with kET values in the range 4.2–6.1 × 104 M−1 s−1, thus implying a significant reorganisation energy to the iridium sphere; (iv) the evolution of 1a, as the most active Ir species in the series, to mononuclear iridium species with lower molecular weight and originating from oxidative transformation of the organic ligand scaffold, as proven by converging UV-Vis, MALDI-MS and 1H-NMR evidences. These results can be used for the further design and engineering of novel catalysts.

In this contribution we report on an in-depth electrochemical impedance spectroscopy (EIS) study of the interfaces and processes involved in the photoelectrochemical hydrogen production by efficient p-type silicon (pSi) photocathodes, designedly functionalized with protective and catalytic layers. In particular, we have optimized the thermal evaporation of compact and conformal ultra-thin (5 nm) Ti films on pSi, with the double aim of (i) protecting the pSi surface from passivation and photocorrosion, as well as of (ii) introducing platforms with improved adhesion properties for the further functionalization with co-catalysts (in the specific, Pt nanoparticles). The best performing electrodes (labelled as pSi-5Ti-Pt) display an improved photocurrent onset ( ca. 200 mV positive shift in 1 M H 2 SO 4 ), as well as a more than doubled saturation photocurrent (up to −27 mA/cm 2 ) when compared to the unmodified pSi photocathodes. As evidenced by the EIS analysis, the proposed modifications of pSi surface led to an enhancement of the charge extraction from the semiconductor, most likely due to surface dipole effects able to reduce the resistance associated to the transport through the space charge layer. With respect to unmodified silicon, the presence of the Ti overlayer, partially oxidized to TiO 2 , also allows for a denser surface coverage of the electrodeposited Pt nanoclusters, resulting in decreased interfacial charge transfer resistance for hydrogen evolution. The proposed functionalization strategy relies on common fabrication methods (already applied at industrial level) and can be easily extended to other photoelectrode materials prone to passivation/photocorrosion and/or difficult to functionalize, thus introducing more flexibility in the choice of materials for photoelectrochemical cells.

Four new mixed ligand Cu(I) complexes bearing methoxycarbonyl imidoyl-indazole and bis[2-(diphenylphosphino)-phenyl]ether (POP) ligands were synthesized and characterized by variable-temperature NMR, FT-IR, EA and HRMS. For three of them, the molecular structures were obtained by X-ray diffraction analysis. The electrochemical and absorption–emission properties of all the complexes were investigated by using cyclic voltammetry, UV-Vis spectroscopy, and spectrofluorometric measurements in a CH2Cl2 solution at room temperature and in different solid-state matrices. In addition, quantum chemical computations were performed to gain insight into their electronic and photophysical properties. The complexes showed an MLCT band, which is more influenced by the position of the electron-withdrawing methoxycarbonyl substituent in the indazole ring rather than by the π-extension introduced by the alkene moiety. Besides, all the complexes were found to be weak emitters in the CH2Cl2 solution while they were brighter emitters in the solid-state.

International audience; H2 evolution catalyzed by a heptacoordinate complex under both electro- (1) and photo-chemical (2) conditions is analyzed in detail in order to gain insights into the mechanism of the processes. The complex shows high catalytic activity for hydrogen production in acetonitrile, in the presence of trifluoroacetic acid and triethylammonium tetrafluoroborate as proton sources. Foot-of-the-wave analysis (FOWA) and Tafel plot analysis have been applied to elucidate the kinetics of the hydrogen evolution mechanism and to benchmark the catalytic performances, respectively, with remarkably high rates being obtained at the expense of high overpotentials. Transient absorption spectroscopy measurements have been performed to characterize the kinetics and relevant intermediates formed under photocatalytic conditions. A computational investigation, based on density functional theory (DFT) and time-dependent DFT (TD-DFT), has also been carried out to characterize the intermediate species and support the experimental results. A combination of both experimental and theoretical data suggests the formation of catalytic intermediates displaying dangling pyridine groups in both the one- and two-electron reduced species, possibly acting as proton-transfer relays to enable efficient H–H bond formation.

Proton-coupled electron transfer (PCET) plays a key role in many biological processes, and a thorough comprehension of its subtle mechanistic complexity requires the synthesis and characterization of suitable artificial systems capable of mimicking this fundamental, elementary step. Herein, we report on a detailed photophysical investigation of conjugate 1, based on a tin(IV) tetraphenylporphyrin (SnTPP) chromophore bound to two l-tyrosinato amino acids, in CH2Cl2 in combination with organic bases of different strength and the preparation of a novel conjugate 3, based on a tin(IV) octaethylporphyrin (SnOEP) in place of the tetraphenyl analogue, and its photophysical characterization in CH2Cl2 in the presence of pyrrolidine. In the case of compound 1 with all bases examined, quenching of both the singlet and triplet excited states is observed and attributed to the occurrence of concerted proton-electron transfer (CPET). Rates and quenching yields decrease with the strength of the base used, consistent with the decrease of the driving force for the CPET process. Conjugate 3 with pyrrolidine is quenched only at the triplet level by CPET, albeit with slower rates than its parent compound 1, ascribable to the smaller driving force as a result of SnOEP being more difficult to reduce than SnTPP. For both systems, the quenching mechanism is confirmed by suitable blank experiments, specific kinetic treatments, and the observation of kinetic isotope effects (KIEs). Differently from what has been previously proposed, a detailed reinvestigation of the triplet quenching of 1 with pyrrolidine shows that no long-lived radical pair state is formed, as diradical recombination is always faster than formation. This is true for both 1 and 3 and for all bases examined. The kinetics of the CPET pathways can be well described according to Marcus theory and point toward the involvement of substantial reorganization energy as typically observed for PCET processes of concerted nature.

Two new complexes [Ir(ppy)2ppdh](PF6) (C1) and [Ir(F2ppy)2ppdh](PF6) (C2), where ppy is 2-phenylpyridine, F2ppy is 2-(2,4-difluorophenyl)pyridine, and ppdh is pteridino[7,6–f][1,10]phenanthroline-1,13(10H,12H)-dihydroxy have been synthesized. Their photophysical properties have been characterized by means of steady-state and time-resolved emission techniques and supported by TD-DFT calculations. Both complexes C1 and C2 are luminescent in fluid solutions at room temperature with quantum yields around ca 1%, and the nature of the emitting excited state has a predominant 3MLCT/3LLCT character with some mixed contributions from a 3LC state on the ancillary ligand. However, in C2 the contribution from the 3LC state becomes dominating, as was identified from a dual emission observed in CH2Cl2 arising from a slow equilibration between the 3MLCT/3LLCT and 3LC states. Therefore, the corresponding luminescence is indeed composed of two different patterns (respective maxima at 533 and 580 nm) and two different lifetimes, whose contributions in the whole emission spectrum is strongly dependent on the excitation wavelength. Overall, the results here presented add further fundamental knowledge on the photophysical properties of iridium(III) complexes of potential interest for applications in diverse research fields such as optoelectronics, bioimaging, and theranostics.

Hydrogen production via light-driven water splitting is a key process in the context of solar energy conversion. In this respect, the choice of suitable light-harvesting units appears as a major challenge, particularly as far as stability issues are concerned. In this work, we report on the use of CdTe/CdS QDs as photosensitizers for light-assisted hydrogen evolution in combination with a nickel bis(diphosphine) catalyst (1) and ascorbate as the sacrificial electron donor. QDs of different sizes (1.7–3.4 nm) and with different capping agents (MPA, MAA, and MSA) have been prepared and their performance assessed in the above-mentioned photocatalytic reaction. Detailed photophysical studies have been also accomplished to highlight the charge transfer processes relevant to the photocatalytic reaction. Hydrogen evolution is observed with remarkable efficiencies when compared to common coordination compounds like Ru(bpy)32+ (where bpy = 2,2′-bipyridine) as light-harvesting units. Furthermore, the hydrogen evolution performance under irradiation is strongly determined by the nature of the capping agent and the QD size and can be related to the concentration of the surface defects within the semiconducting nanocrystal. Overall, the present results outline how QDs featuring large quantum yields and long lifetimes are desirable to achieve sustained hydrogen evolution upon irradiation and that a precise control of the structural and photophysical properties thus appears as a major requirement towards profitable photocatalytic applications.

Three perfluorinated Zn

A side-to-face array DPy-gPBI[Ru(4-tBuTPP)(CO)]

Overall water splitting represents one of the most promising approaches toward solar energy conversion and storage, which is, however, severely challenged by the four-electron/four-proton nature of the oxygen evolution reaction (OER). One option to overcome this issue is to replace OER with a more useful reaction, for simultaneous production of both hydrogen and chemicals of interest. For the purpose, in this paper a cheap, hydrothermally prepared Ti-doped nanostructured hematite photoanode was employed for the first time as highly stable, heterogeneous catalyst for the low bias, efficient and highly selective photoinduced oxidation of benzylamine to N-benzylidenebenzylamine, and for the simultaneous production of hydrogen in a double solvent/environment cell. A preliminary estimate indicates the possibility to obtain a similar to 150 mu mol h(-1) H-2 production, with the contemporary production of stoichiometric benzylidene N-benzylamine in a 5 x 5 cm(2) area electrode. This study contributes to overcome the 40-year lasting issues limiting the use of hematite in industrial photo-electrochemical sunlight conversion and storage, due to poor performance of hematite and lack of economic value of oxygen production, providing solid evidence for the use of hematite in alternative oxidation reactions of industrial importance.

A series of four arrays made of a central Sn(IV) porphyrin as scaffold axially connected, via carboxylate functions, to two free-base porphyrins has been prepared and fully characterized. Three arrays in the series feature the same free-base unit and alternative tin-porphyrin macrocycles, and one consists of a second type of free-base and one chosen metallo-porphyrin. A thorough photophysical investigation has been performed on all arrays by means of time-resolved emission and absorption techniques. Specific focus has been given at identifying how structural modifications of the free-base and tin-porphyrin partners and/or variation of the solvent polarity can effectively translate into distinct photophysical behaviors. In particular, for systems SnTPP(Fb) 2 (1) and SnOEP(Fb) 2 (2), an ultrafast energy transfer process from the excited Sn(IV) porphyrin to the free-base unit occurs with unitary efficiency. For derivative SnTPP(FbR) 2 (3), the change of solvent from dichloromethane to toluene is accompanied by a neat change in the intercomponent quenching mechanism, from photoinduced electron transfer to energy transfer, upon excitation of the Sn(IV) porphyrin unit. Finally, for array SnTpFP(Fb) 2 (4), an ultrafast electron transfer quenching of both chromophores is detected in all solvents. This work provides a general outline, accompanied by clear experimental support, on possible ways to achieve a systematic fine-tuning of the quenching mechanism (from energy to electron transfer) of Sn(IV) multiporphyrin arrays.

Three perfluorinated ZnII porphyrins were evaluated as n-type sensitizers in photoelectrosynthetic cells for HBr and water splitting. All the dyes are featured by the presence of pentafluorophenyl electron-withdrawing groups to increase the ground-state oxidation potential and differ for the nature and position of the ?-conjugate linker between the core and anchoring group tasked to bind the metal oxide, in order to assess the best way of coupling with the semiconductor. A phenyl-triazole moiety was used to link the carboxylic anchoring group onto the meso position, while an ethynyl-phenyl linker was chosen to bridge carboxylic and cyanoacrylic groups onto the ?-pyrrolic position. A combination of electrochemical, computational, and spectroscopic investigations confirmed the strong electron-withdrawing effect of the perfluorinated porphyrin core, which assures all the investigated dyes of the high oxidation potential required to the coupling with water oxidation catalysts (WOC). Such an electron-poor core, however, affects the charge separation character of the dyes, as demonstrated by the spatial distribution of the excited states, leading to a nonquantitative charge injection, although tilting of the molecules on the semiconductor surface could bring the porphyrin ring closer to the semiconductor, offering additional charge-transfer pathways. Indeed, all the dyes demonstrated successful in the splitting of both aqueous HBr and water, with the best results found for the SnO2/TiO2 photoanode sensitized with the ?-substituted porphyrin equipped with a cyanoacrylic terminal group, achieving 0.4 and 0.1 mA/cm2 photoanodic currents in HBr and water under visible light, respectively. The faradaic yield for oxygen evolution in the presence of an IrIV catalyst was over 95%, and the photoanode operation was stable for more than 1000 s. Thus, the perfluorinated porphyrins with a cyanoacrylic anchoring group at the ?-position should be considered for further development to improve the charge-transfer character.

Four mononuclear Cu(I) complexes were prepared, described as [Cu(N,N)2]PF6 (1) and [Cu(N,N)(P,P)]PF6 (2–4), where N,N is N-(1-(2H-indazol-2-yl)ethylidene)-2,6-diisopropylaniline and P,P are phosphine derived ancillary ligands (bis[2-(diphenylphosphino)phenyl]ether (POP), bis(diphenylphosphino)ethane (dppe) or 2 PPh3). These new species were characterized by NMR, FT-IR, elemental analyses, cyclic voltammetry, UV-Vis – emission spectroscopy, transient absorption spectroscopy and DFT calculations. In addition, complexes 1 and 2 were characterized by X-ray diffraction. The four complexes showed an MLCT absorption band between 400 and 450 nm, in addition to a weakly structured phosphorescence in a 4:1 ethanol:methanol glassy matrix at 77 K. Complexes 2–4 have emission profiles that resemble the phosphorescence of the protonated N,N ligand, suggesting a triplet LC character of the lowest lying excited state at 77 K. By contrast, a mixed MLCT/LC triplet emission is most likely responsible for the phosphorescence in complex 1. Weak ligand-centered emission is also detected in the solid state at room temperature but only in the case of complexes 2 and 4, suggesting thermally activated deactivation processes in the case of 1 and 3. Notably, the transient absorption spectroscopy of complexes 2–4 in CH2Cl2 solution confirms a strong contribution from a ligand-centered (LC) triplet excited state, pointing towards a mixed 3MLCT/3LC character of the transient species in solution at room temperature, undergoing a non-radiative deactivation in the μs time-scale. This behavior markedly differs from that observed for complex 1, whose short-lived 3MLCT excited state is followed by ultrafast transient absorption spectroscopy.