The replacement of fossil fuels by a clean and renewable energy source is one of the most urgent and challenging issues our society is facing today, which is why intense research has been devoted to this topic recently. Nature has been using sunlight as the primary energy input to oxidize water and generate carbohydrates (a solar fuel) for over a billion years. Inspired, but not constrained by nature, artificial systems^[1] can be designed to capture light and oxidize water and reduce protons, CO₂ or other compounds to generate useful chemical fuels and feedstocks. In this context, this contribution will describe the preparation of efficient molecular water oxidation catalysts both in homogeneous phase and confined into solid conductive or semiconductive supports. Further the nature of the anchoring strategy on the performance of these molecular (photo)anode will be further discussed as well their implications for the generation of solar fuels.^[2]

[1] (a) Berardi, S.; Drouet, S.; Francàs, L.; Gimbert-Suriñach, C.; Guttentag, M.; Richmond, C.; Stoll, T.; Llobet, A. Chem. Soc. Rev., 2014, 43, 7501-7519. (b) Matheu, R.; Ertem, M. Z.; Gimbert-Suriñach, C; Sala, X.; Llobet, A. Chem. Rev. 2019, 119, 3453–3471. (c) Matheu, R.; Garrido Barros, P.; Gil Sepulcre, M.; Ertem, M. Z.; Sala, X; Gimbert-Suriñach, C; Llobet, A. Nat. Rev. Chem. 2019, 3, 331–341.

[2] (a) Matheu, R.; Gray, H. B.; Brunschwig, B. S.; Llobet, A; Lewis, N. S. et al., J. Am. Chem. Soc. 2017, 139, 11345-11348. (b) Garrido-Barros, P.; Gimbert-Suriñach, C.; Llobet, A. et al., J. Am. Chem. Soc. 2017, 139, 12907–12910. (c) Hoque, Md. A.; Gil-Sepulcre, M.; Llobet, A. et al. Nat. Chem. 2020, 12, 1060–1066. (d) Schindler, D.; Gil‐Sepulcre, M.; Llobet, A.; Würthner, F. Adv. Energy Mater. 2020, 2002329. (e) Gil-Sepulcre, M.; Lindner, J. O.; Schindler, D.; Velasco, L.; Moonshiram, D.; Rüdiger, O.; DeBeer, S.; Stepanenko, V.; Solano, E.; Würthner, F.; Llobet, A. J. Am. Chem. Soc. 2021, 143, 11651–11661.

In a world that is running out of natural resources, there is a growing need to design and develop sustainable and green energy resources. In that respect, photo-electrocatalytically driven reactions for the production of alternative fuels (such as water splitting or CO₂ reduction) hold the potential to provide a route for future carbon neutral energy economy. Nevertheless, the slow kinetics of those catalytic reactions demands the development of efficient catalysts in order to drive it at lower overpotentials. Indeed, a variety of molecular catalysts based on metal complexes are capable of electrochemically reducing CO₂ and/or protons. Yet, despite the significant progress in this field, practical realization of molecular catalysts will have to involve a simple and robust way to assemble high concentration of these catalysts in an ordered, reactant-accessible fashion onto a conductive electrode. 

Our group utilizes Metal-Organic Frameworks (MOFs) based materials as a platform for heterogenizing molecular electrocatalysts. Their unique properties (porosity and flexible chemical functionality), enables us to use MOFs for integrating all the different functional elements needed for efficient catalysts: 1) immobilization of molecular catalysts, 2) electron transport elements, 3) mass transport channels, and 4) modulation of catalyst secondary environment. Thus, in essence, MOFs could possess all of the functional ingredients of a catalytic enzyme.  

In this talk, I will present our recent study on (photo)-electrocatalytically active MOFs incorporating molecular catalysts for solar fuel reactions.

A long-standing challenge is the development of artificial photosynthetic approaches for capturing and storing the energy of sunlight in the chemical bonds of a fuel, e.g. molecular hydrogen, offering a clean, sustainable and abundant source of energy. In this respect, colloidal nanostructured semiconductor materials are explored extensively as photosensitizers and photocatalysts. To push forward the development of functional materials based on semiconductor nanocrystals, function immanent exciton and charge-separation/recombination dynamics in relation to structural parameters need to be understood. Factors like composition, structure and dimensions of the semiconductor particles, the nature of cocatalysts and the type of surface ligands are severely influencing the efficiencies for solar to hydrogen conversion. By applying time-resolved transient absorption and photoluminescence spectroscopy we strive to reveal the connections between structure, charge carrier dynamics and the targeted function. To illustrate this approach, I will give a brief overview on the results of our studies on CdSe@CdS seeded nanorods functionalized with metal particles of varying composition and morphology as cocatalyst and the insights gained on the connection between charge separation at the nanosized semiconductor/metal interface and activity for hydrogen evolution.[1, 2] Besides the transfer of electrons to the cocatalyst, a key step in these heterostructures is hole localization in the CdSe seed which enables the formation of long-lived charge separation supporting charge accumulation at the catalytic reaction centers necessary to drive multi-electron redox reactions. Latest results revealed the influence of the nature of the surface ligands on the efficiency of the hole localization process.[3] Finally, interfacing semiconductor nanocrystals with molecular reaction centers presents an attractive alternative to metal particles and equally high charge transfer rates as at semiconductor/metal interfaces can be reached in principle.[4] Nevertheless, anchoring strategies with sufficient stability and supporting efficient transfer of multiple charge carriers and reducing unwanted charge recombination, e.g. by anchoring units serving as electron relay, still need to be established. First results indicate that bioinspired redox-active polymer matrices could play an important role in the future design of photocatalytically active inorganic/organic hybrid materials.[5]

Transparent conductive oxides are well-known inorganic materials that have found wide use in various fields of both research and industry. In particular, doped metal oxides have aroused great interest in power generation applications thanks to a very convenient combination of optical and electronic properties. To the effective absorption of light usually in the ultraviolet frequency range and to the typical optical transparency window in the visible, a tunable feature is added that enriches the versatility of these materials. It is the plasmon resonance in the infrared. This property is the result of the aliovalent doping of the oxide, which induces the typical conductive character thanks to the excess of extrinsic charges [1]. Further advantages in terms of preparation, costs and applications derive from processing in the nanoscale and, among the nanocrystalline forms of these oxides, one of the most widely used species is indium tin oxide (ITO) [2]. More recently, the possibility of a further increase in charge density through a zero environmental impact post-synthetic treatment has renewed the attraction for these materials and intensified their study. Many metal oxides nanocrystals have in fact demonstrated an exponential increase in charges through photo-doping induced by multiple events of light absorption [3]. This phenomenon encourages even more the use of these semiconductors in the field of solar energy conversion, but also in related fields such as photoelectrochemistry and photocatalysis. In each of these applications, one of the most current research lines in fact concerns the analysis of the multiple charges accumulated in the light-driven process and the possibility of accessing processes involving multiple charge transfers [4]. The consequences would concern both the increase in efficiency and the widening of the application window of these materials. In this work,[5] we report a first evidence, in the case of photodoped ITO nanocrystals, of the possible transfer process of more than one electron by means of a gradual oxidative treatment using an electron-acceptor molecule, the F4TCNQ. The spectroscopic analysis of the titration allows shedding light on the reactivity of the nanocrystals induced by light and on the potential transfer of multiple charges.

Nanocarbon in Photochemistry and Electrochemistry

Artificial photosynthesis, a process in which the light energy is converted into the chemical bond, represents an important area of research aimed toward generation of Sun-derived fuels and value-added chemicals. To successfully utilize photons from Sun, such solar reactors need to contain light-absorbing chromophores that efficiently and rapidly channel the absorbed energy toward desired catalytic sites where useful chemistry can takes place with high selectivity and low kinetic barriers. The Glusac group investigates molecular chromophores and catalysts for artificial photosynthesis. We utilize advanced time-resolved laser spectroscopy techniques to investigate mechanisms of energy and charge migration in molecular excited states and evaluate the parameters that control undesired energy losses through fast charge recombination. We also explore molecular electrocatalysts that are able to receive electrons and holes from excited chromophores and covert then into desired products.

Three projects are currently under investigation in our labs: (i) Bio-inspired CO2 reduction using metal-free NAD+/NADH analogs, where we look for strong hydride donors that can selectively reduce CO2 to methanol and that can be recycled photochemically; (iii) Light-harvesting by graphene quantum dot assemblies, where we explore excited-state energy and charge redistributions in chromophore-catalyst assemblies using advanced time-resolved laser spectroscopy methods; (iii) Carbon-based platforms for electrocatalysis. In this project, we investigate methods to decorate carbon electrodes with molecular catalytic motifs that can perform useful chemistry.

Syngas, an industrially relevant mixture of CO and H₂, is a high-priority intermediate resource for producing several commodity chemicals, e.g., ammonia, methanol, and synthetic hydrocarbon fuels. Accordingly, parallel photocatalytic conversion of CO2 and protons to syngas is a key step in fostering a sustainable energy cycle. State-of-the-art catalytic systems often fall short as concurrent CO and H₂ evolution requires challenging reaction conditions which can hamper stability, selectivity, and efficiency under application-oriented conditions. Here a light-harvesting metal-organic framework and two molecular catalysts are associated to design colloidal, water-stable, versatile nanoreactors for photocatalytic syngas generation with highly controllable product ratios. The host provides efficient directional light-harvested energy transport to the active sites, yielding sustained CO₂ reduction and H₂ evolution with incident photon conversions up to 36% and turnover numbers setting a benchmark, thus paving the way for application in solar energy-driven syngas generation. System bottlenecks, such as limited light absorption and kinetics, were identified, and proven addressable upon modularly co-hosting molecular additives owing to the system’s intrinsic versatility. Based on our findings, we highlight the uniqueness of MOF materials’ coordination space for rational designing and directing of such multicomponent supramolecular assemblies including antenna chromophores, auxiliary photosensitizers, and several catalysts of orthogonal photo-physical and chemical properties to minimize energy loss. This opens perspectives to tackle scalability, synthesize value-added products from the full redox cycle, harness the full solar spectrum energy, and precisely implement active sites with co-factors to mimic natural enzyme functionalities.[1-3]

Metal–organic frameworks (MOFs) represent a new class of materials, and one of their striking features lies in the tunable, designable, and functionalizable nanospace. The nanospace within MOFs allows the designed incorporation of different functionalities for targeted applications, such as gas storage, gas separation, sensing, drug delivery, environmental remediation, heterogeneous catalysis; and it has also provided plenty of opportunities for photocatalysis application. We will discuss a series of strategies to tailor MOFs as photocatalysts for photocatalytic small molecule activation under visible light irradiation including hydrogen production from water and water oxidation coupled carbon dixoide reduction reminiscent of photosynthesis in nature.

Carbon dioxide reduction by porphrynic metal-organic frameworks has been widely studied. Changes in porphyrin moiety (non-metallated, different central metal atoms) and three-dimensional structures result in putative changes in mechanism and resulting products, namely carbon monoxide and formate. There are two prevailing mechanisms that are debated in the literature (1) the formation of a porphyrin-node charge-separated state, where the node acts as the catalytic site, (2) porphyrin driven chemistry where the linker acts as both the photo-active species and catalytic site with the nodes simply serving as structural supports. Our collaborators have systematically explored the reduction rates and products for different non-metallated and metallated MOF-525. To support the mechanistic implications, we conducted photophysical studies, including transient absorption spectroscopy. Interestingly, the results do not support either of the prevailing mechanisms. Transient absorption spectroscopy eliminates the possibility for the formation of a charge-separated state. EPR spectroscopy confirms that the photogenerated Fe(II) species is not capable of carbon dioxide activation, consistent with previously published molecular mechanisms. Therefore, we turned to theory to provide insight. The computational results support a mechanism where the sacrificial donor plays a key role in the catalytic mechanism. Other recent mechanistic studies on molecular carbon dioxide reduction catalysts have put forth similar stories with sacrificial donors playing multiple roles in catalysis. The implications of sacrificial reducing agents in the field will be mentioned, as well as potential pathways for future research.