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Fuel from sunlight: Covalent organic frameworks as integrated platforms for photocatalytic water splitting and CO2 reduction

Periodic Reporting for period 4 - COFLeaf (Fuel from sunlight: Covalent organic frameworks as integrated platforms for photocatalytic water splitting and CO2 reduction)

Reporting period: 2020-03-01 to 2020-08-31

The goal of the COFLeaf project is to develop an artificial photosynthetic platform based on a class of crystalline porous polymers known as covalent organic frameworks, COFs.
One of the main challenges of efficiently converting sunlight into solar fuels is to orchestrate a complex suite of physico-chemical processes, within an earth-abundant and stable materials platform. COFLeaf takes advantage of macromolecular photocatalysts – COFs – which are heterogenous yet molecularly precise.
As demonstrated by the results of this project, the exceptional blend of solid-state character together with modularity, porosity, and crystallinity make COFs a highly promising photocatalytic platform for solar fuel research.
1) Synthesis of robust photocatalytically active COFs:
A primary goal of this project was the development of COFs that can withstand the often harsh photocatalytic conditions. Our research has led to the development of several such platforms. These include the azine-linked Nx-COFs (Nat. Commun. 6, 8508 (2015), Faraday Discuss. 201, 247 (2017)), the A-TEXPY-COFs containing pyrene building blocks for enhanced light harvesting and photo(electro)catalysis (Adv. Energy Mater. 8, 1703278 (2018)), and thiazolothiazole containing imine-linked TpTDz-COF based on the inert keto-enamine linkage (J. Am. Chem. Soc. 141, 11082 (2019)). We have further developed a post-synthetic stabilization strategy for “locking” imine-linked COFs, where the labile imine linkage is converted to a robust and conjugated thiazole linkage (Nat. Commun. 9, 2600 (2018)). We have also developed an ionothermal methodology for the synthesis of stable imide-linked COFs that are not easily accessible using standard solvothermal conditions (Angew. Chem. Int. Ed. 59, 15750 (2020)). In our search for new types of COF platforms amenable to post-synthetic functionalization, we have discovered sub-stoichiometric COFs which contain periodic uncondensed functional groups (Nat. Commun. 10, 2689 (2019)).
2) Establishing structure-property relationships in photocatalytic activity:
We have demonstrated that the photocatalytic activity of a COF can be tuned via an activity determining descriptor at the molecular level. For example, we have shown that the gradual substitution of the aromatic C–H units with N atoms in the central aryl ring of a series of azine-linked Nx-COFs (Nat. Commun. 6, 8508 (2015)) decreases the dihedral angle of the triaryl building block, which increases the planarity of the individual 2D COF layers, leading to higher crystallinity and surface area with increasing N content. This gradual change in structural properties leads to a 4-fold increase in the photocatalytic hydrogen evolution rate with each isolobal N substitution. High-level quantum chemical calculations of large COF model systems turned out to be instrumental in carving out and interpreting mechanistic details in these systems.
3) Development of all-single-site heterogeneous photocatalysts:
COFLeaf has laid the foundations to macromolecular heterogenous catalysts with atomically distributed and spatially separated catalytic centers integrated with the COF backbone. This achievement paves the way to the vision of ‘COF leafs’ which orchestrate different subunits in a single platform to drive light-induced water splitting and CO2 reduction.
We developed the first all-single-site COF photocatalyst based on the COF—cobaloxime system for proton reduction (J. Am. Chem. Soc. 139, 16228 (2017)) and further refined this concept by devising a photocatalytic system that works under aqueous conditions with high stability (J. Am. Chem. Soc. 141, 11082 (2019)). In order to further improve the efficiency of charge transfer, we have developed a covalently linked COF—molecular co-catalyst assembly. This system is photocatalytically more active and shows extended long-term stability over the corresponding physisorbed mixture, suggesting improved charge transfer and confinement effects to be at play (J. Am. Chem. Soc. 142, 12146 (2020)).
4) Understanding the “real structure” of COFs:
Post-synthetic stabilization of thiazole linked TTT-COF has enabled HRTEM analysis of real structure effects, including grain boundaries and edge dislocations (Nat. Commun. 9, 2600 (2018)). Defects and disorder can have multiple, intertwined functions affecting light harvesting, charge transport and catalysis. Thus, understanding their role is crucial to the rational design of an earth-abundant, artificial photosynthetic platform.
We have been able to show that not only the geometric and electronic features of the COF building blocks, but also the reaction temperature can be used to guide the stacking behavior of COFs (Mater. Chem. Front. 1, 1354 (2017), Chem. Sci. DOI - 10.1039/D0SC03048A (2020)). Our analysis suggests that kinetically trapped local structures that give rise to higher apparent symmetry may be prevalent in 2D COFs, with profound influence on their optoelectronic properties.
5) COFs as a platform for CO2 capture and storage:
We have studied the interactions of CO2 and water based on hydrazone-linked COFs functionalized by tertiary-amine-bearing linkers (Chem. Mater. 31, 1946 (2019)). Our study revealed that the specific CO2 sorption capacity can be enhanced through amine-functionalization, while bicarbonate species are formed concomitantly, which suggests an important role of water for CO2 uptake.
6) Carbon nitride photocatalysts for “dark” photocatalysis and beyond:
We have developed 2D carbon nitrides known as poly(heptazine imides), PHI (Chem. Mater. 31, 747 (2019), Adv. Energy Mater. 7, 1602251 (2017)), which are closely related to COFs through their reversible bond formation and crystallinity. We have shown that in PHI ultra-long-lived electrons can be trapped after photoreduction (Angew. Chem. Int. Ed. 56, 510 (2016), J. Am. Chem. Soc. 138, 9183 (2016)). The trapped electrons can be released on demand for the generation of H2 by addition of a Pt colloid in the dark.
The unusual charge-storage capability of PHI has been used to create a solar battery photoanode (Adv. Mater. 30, 1705477 (2018)). Besides, we have developed photocapacitive PHI microswimmers, which can be charged by solar energy enabling light-induced propulsion even in the dark (PNAS 117, 24748 (2020)). This mechanism significantly extends the capabilities of micromachines in targeted drug delivery and environmental remediation.
COFLeaf has introduced a new materials platform – COFs – into the photocatalysis arena (Chem. Mater. 28, 5191 (2015); ACS Energy Lett. 3, 400 (2018); Nat. Rev. Mater., The impact of the results obtained transcend the fields of COFs and photocatalysis, and will be equally relevant to the scientific communities working on porous materials, solar energy conversion, sensing, autonomous systems, and many others. COFLeaf thus serves as a cross-fertilizer and accelerator for the many interdisciplinary challenges that lie ahead by taking advantage of the essentially limitless tunability of COFs. With the hallmarks of COFs – crystallinity, porosity, tunability and earth-abundance – it is to be expected that COF photocatalysts will continue to have a firm place in our quest for renewable chemical fuels from sunlight.