SUPRAMOLECULAR SYSTEMS FOR ARTIFICIAL PHOTOSYNTESIS

Artificial photosynthesis, i.e., conversion of solar energy into fuels, represents one of the most promising research fields which could potentially provide clean and renewable sources of energy for a sustainable development of the future generations. Among several possibilities, water splitting into molecular hydrogen and oxygen is one of the most challenging and appealing reaction schemes. Taking inspiration from nature several functional units are required to this aim: (i) an antenna system, responsible for light harvesting, (ii) a charge separating system, where the absorbed energy is converted into an electrochemical potential (electron/hole pair), and (iii) appropriate catalyst units, capable of stepwise storing the photogenerated electrons and holes in order to drive multi-electron transfer processes at low activation energy. In the present thesis several points regarding both the photoinduced oxidation and reduction of water as well as charge separation are studied. In more detail, three different classes of water oxidation catalysts are examined, namely tetrametallic polyoxometalate, tetracobalt cubanes, and single-site cobalt salophen complexes, within light-activated catalytic cycles involving tris(bipyridine) ruthenium and persulfate as photosensitizer and sacrificial electron acceptor, respectively. Particular attention is paid to the evaluation of the interactions between the catalyst and the sensitizer and to the kinetics of both photochemical and thermal electron transfer steps. Concerning water reduction, the following systems are investigated: a self-assembling reductant/sensitizer/catalyst triad based on an aluminum pyridylporphyrin central unit, a cobaloxime catalyst, and an ascorbate electron donor, a cationic cobalt porphyrin catalyst in the presence of tris(bipyridine) ruthenium as sensitizer and ascorbic acid as electron donor, and a PAMAM dendrimer decorated with ruthenium polypyridine dyes at the periphery and inside of which platinum nanoparticles have been grown. Beside the optimization of the photocatalytic performance, detailed insights into the photoinduced hydrogen evolving mechanism are carefully provided. Finally, a triad system for photoinduced charge separation, based on a naphthalene bisimide electron acceptor, a zinc porphyrin electron donor, and a ferrocene secondary electron donor, connected via 1,2,3- triazole bridges, is also described. Detailed photophysical investigation of the system will show an unusual behavior with respect to photoinduced electron transfer. Results obtained from a side-project in the field of molecular electronics will be also discussed, where photoinduced electron transfer processes are used to different aims

Elucidating the Key Role of pH on Light-Driven Hydrogen Evolution by a Molecular Cobalt Catalyst

Photochemical hydrogen generation from aqueous solutions can be accomplished with a combination of at least three molecular components: namely, a photosensitizer, a hydrogen-evolving catalyst, and an electron donor. A parameter that plays a key role in the light to hydrogen efficiency of such three-component systems is the solution pH. While this evidence has been usually observed in several works aiming at identifying catalysts and optimizing their performances, detailed studies capable of shining light on this issue have been extremely rare. Hence, the pH dependence of a reference three-component system based on Ru(bpy)32+ (where bpy = 2,2′-bipyridine) as the sensitizer, a cobaloxime HEC, and ascorbic acid as the sacrificial donor has been studied with care by merging photocatalytic hydrogen evolution kinetic data and detailed time-resolved spectroscopy results. The photocatalytic activity shows a bell-shaped profile as a function of pH which peaks at around pH 5. While at acidic pH (pH 5) the production of hydrogen is hampered by the disfavored protonation of the reduced Co(I) species. In this latter instance, however, hydrogen evolution is mainly slowed down rather than inhibited, as it is instead in the former case. This evidence affects the time scale of the photocatalysis and gives the opportunity to rationalize and correlate different results obtained with the same cobaloxime catalyst but under rather diverse experimental conditions

Photocatalytic Hydrogen Evolution with Ruthenium Polypyridine Sensitizers: Unveiling the Key Factors to Improve Efficiencies

Photochemical hydrogen evolution studies aimed at evaluating new molecular catalysts have usually exploited Ru(bpy)32+ (where bpy = 2,2’-bipyridine) as the photosensitizer of reference, thanks to its suitable optical and redox properties. In principle, an additional improvement of the photocatalytic performances can be achieved also by a careful adjustement of the photophysical and/or electrochemical characteristics of the ruthenium-based sensitizer. Herein we describe homogenous molecular systems for photocatalytic hydrogen evolution composed of a series of ruthenium polypyridine complexes as the photosensitizers (Ru1-4), a cobaloxime catalyst, and ascorbic acid as the sacrificial electron donor. Suitable functionalizations of the 4,4’ positions of the bipyridine ligands have been addressed in order to modify the redox properties of the chromophores rather than their optical ones. A careful and detailed kinetic characterization of the relevant processes at the basis of the hydrogen evolving photocatalysis have been addressed to rationalize the observed behavior. The results show that the ruthenium complex involving two 2,2’-bipyridines and one 4,4’-dimethyl-2,2’-bipyridine (Ru2), may outperform the standard Ru(bpy)32+ (Ru1), combining the right balance of structural and redox properties, thus posing as an alternative benchmark photosensitizer for the study of new hydrogen evolving catalysts

Photoinduced Charge Separation in Porphyrin Ion Pairs

Ion pairs between porphyrin-type compounds have been successfully employed for spectral sensitization of semiconductor surfaces and for the preparation of collective binary ionic materials for photonic and (photo)catalytic applications. The understanding of the photophysical processes occurring within ion-paired porphyrin dimers is thus of remarkable importance for the optimization and improvement of such systems. Herein the ion-pair species formed between ZnTMePyP4+ (Zn1) or H2TMePyP4+ (H21) and ZnTPPS4- (Zn2) or H2TPPS4- (H22) in a variety of solvent mixtures are characterized and their photophysics thoroughly investigated by time-resolved techniques. In all the systems studied, very fast and efficient photoinduced charge separation is observed, with the cationic porphyrin being reduced and the anionic one oxidized. Interestingly, despite the very short charge separation distance, the lifetime for charge recombination, depending on the energy gap, can extend into the nanosecond time domain, showing great potential for the utilization of this molecular design within energy conversion schemes.Ion pairs between porphyrin-type compounds have been successfully employed for spectral sensitization of semiconductor surfaces and for the preparation of collective binary ionic materials for photonic and (photo)catalytic applications. The understanding of the photophysical processes occurring within ion-paired porphyrin dimers is thus of remarkable importance for the optimization and improvement of such systems. Herein the ion-pair species formed between ZnTMePyP4+ (Zn1) or H2TMePyP4+ (H(2)1) and ZnTPPS4- (Zn2) or H2TPPS4-(H(2)2) in a variety of solvent mixtures are characterized and their photophysics thoroughly investigated by time-resolved techniques. In all the systems studied, very fast and efficient photoinduced charge separation is observed, with the cationic porphyrin being reduced and the anionic one oxidized. Interestingly, despite the very short charge separation distance, the lifetime for charge recombination, depending on the energy gap, can extend into the nanosecond time domain, showing great potential for the utilization of this molecular design within energy conversion schemes

Bioinspired motifs in proton and CO2 reduction with 3d-metal polypyridine complexes

The synthesis of active and efficient catalysts for solar fuel generation is nowadays of high relevance for the scientific community, but at the same time poses great challenges. Critical requirements are mainly associated with the kinetic barriers due to the multi-proton and multi-electron nature of the hydrogen evolution reaction (HER) and the CO2 reduction reaction (CO2RR) as well as to selectivity issues. In this regard, natural enzymes can be a source of inspiration for the design of effective and selective catalysts to target such fundamental reactions. In this Feature Article we review some recent works on molecular catalysts for both the HER and the CO2RR performed in our labs and other research teams which mainly address (i) the role of redox noninnocent ligands, to lower the overpotential for catalysis and control the selectivity, and (ii) the role of internal relays, to assist formation of catalytic intermediates via intramolecular routes. The selected exemplars have been chosen to emphasize that, although the molecular structures and the synthetic motifs are different from those of the active sites of natural enzymes, many affinities in terms of catalytic mechanism and functionality are instead present, which account for the observed remarkable performances under operative conditions. The data discussed herein thus demonstrate the great potential and the privileged role of molecular catalysts towards the design and construction of hybrid photochemical systems for solar energy conversion into fuels

Porphyrins as Versatile Molecular Components for Photoinduced Hydrogen Production

Artificial photosynthesis, i.e. conversion of solar energy into fuels, is considered as an attractive potential solution to the global energy issue.1 Among various possibilities, water splitting represents one of the most challenging reaction schemes, involving generation of hydrogen as a clean and renewable fuel.2 Herein we report on the use of porphyrins as molecular platforms for the construction of photoactive systems capable of hydrogen production. In particular, porphyrins either free-base or involving closed-shell metal centers such as zinc(II) and aluminium(III) can be coupled to a cobaloxime catalyst and a sacrificial electron donor and used for the preparation of photocatalytic hydrogen evolving dyad or triad systems.3,4 On the other hand, upon introduction of a redox-active metal center, such as cobalt(II), the porphyrin macrocycle can be used as a suitable catalyst in light-activated water reduction experiments, in the presence of tris(bipyridine) ruthenium(II) as photosensitizer and ascorbic acid as sacrificial electron donor.5 In all these cases, particular attention has been dedicated to the investigation of the photoinduced electron transfer dynamics in order to shine light into the overall photoreaction mechanism

Formation of a long-lived radical pair in a Sn(IV) porphyrin-di(L-tyrosinato) conjugate driven by proton-coupled electron-transfer

The novel conjugate 1, featuring two L-tyrosinato residues axially coordinated to the tin centre of a Sn(IV)-tetraphenylporphyrin, is reported as the first example of a supramolecular dyad for photochemical PCET. It is noteworthy that the excitation of 1 in the presence of a suitable base is followed by photoinduced PCET leading to a radical pair state with a surprisingly long lifetime

Photoinduced Electron vs. Concerted Proton Electron Transfer Pathways in SnIV (l-Tryptophanato)2 Porphyrin Conjugates

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

Evaluation of Pt Deposition onto Dye‐Sensitized NiO Photocathodes for Light‐Driven Hydrogen Production

The design of photocathodes for the hydrogen evolution reaction (HER), which suitably couple dye‐sensitized p‐type semiconductors and a hydrogen evolving catalyst (HEC), currently represents an important target in the quest for artificial photosynthesis. In the present manuscript, we report on a systematic evaluation of simple methods for the deposition of Pt metal onto dyesensitized NiO electrodes. The standard P1 dye was taken as the chromophore of choice and two different NiO substrates were considered. Both potentiostatic and potentiodynamic procedures were evaluated either with or without the inclusion of an additional light bias. Photoelectrochemical characterization of the resulting electrodes in an aqueous solution at pH 4 showed that all the methods tested are effective to attain photocathodes for hydrogen production. The best performances (maximum photocurrent densities of −40 μA∙cm−2, IPCE of 0.18%, and ⁓60% Faradaic yield) were achieved using appreciably fast, light‐assisted deposition routes, which are associated with the growth of small Pt islands homogenously distributed on the sensitized NiO

Mechanistic Insights into Light-Activated Catalysis for Water Oxidation

The development of catalysts for water oxidation to oxygen has been the subject of intense investigation in the last decade. In parallel to the search for high catalytic performance, many works have focused on the mechanistic analysis of the process. In this perspective, the oxidation of water through light-assisted cycles composed of an electron acceptor (EA), a photosensitizer (PS), and a water oxidation catalyst (WOC) can provide insightful and complementary information with respect to the use of chemical oxidants or to electrochemical techniques. In this minireview, we discuss the mechanistic aspects of the EA/PS/WOC photoactivated cycles, and in particular: (i) the general elementary steps; (ii) the required features and the nature of the PS employed; (iii) the electron transfer processes and kinetics from the WOC to PS+ (hole scavenging); (iv) the detrimental quenching of the PS by the WOC and the alternative mechanistic routes; (v) the identification of WOC intermediates and, finally, (vi) the transposition of the above processes into a dye-sensitized photoanode embedding a WOC