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COFLeaf Report Summary

Project ID: 639233
Funded under: H2020-EU.1.1.

Periodic Reporting for period 1 - COFLeaf (Fuel from sunlight: Covalent organic frameworks as integrated platforms for photocatalytic water splitting and CO2 reduction)

Reporting period: 2015-09-01 to 2017-02-28

Summary of the context and overall objectives of the project

The efficient conversion of solar energy into renewable chemical fuels has been identified as one of the grand challenges facing society today and one of the major driving forces of materials innovation.
Nature’s photosynthesis producing chemical fuels through the revaluation of sunlight has inspired generations of chemists to develop platforms mimicking the natural photosynthetic process, albeit at lower levels of complexity. While artificial photosynthesis remains a considerable challenge due to the intricate interplay between materials design, photochemistry and catalysis, the spotlights – light-driven water splitting into hydrogen and oxygen and carbon dioxide reduction into methane or methanol – have emerged as viable pathways into both a clean and sustainable energy future. With this proposal, we aim at introducing a new class of polymeric photocatalysts based on covalent organic frameworks, COFs, to bridge the gap between semiconductor and molecular systems and explore rational ways to design single-site heterogeneous photocatalysts offering both chemical tunability and stability.
The development of a photocatalytic model system is proposed, which will be tailored by molecular synthetic protocols and optimized by solid-state chemical procedures and crystal engineering so as to provide insights into the architectures, reactive intermediates and mechanistic steps involved in the photocatalytic process, with complementary insights from theory. We envision the integration of various molecular subsystems including photosensitizers, redox shuttles and molecular co-catalysts in a single semiconducting COF backbone. Taking advantage of the hallmarks of COFs – molecular definition and tunability, crystallinity, porosity and rigidity – we describe the design of COF systems capable of light-induced hydrogen evolution, oxygen evolution and overall water splitting, and delineate strategies to use COFs as integrated platforms for CO2 capture, activation and conversion.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

Project Goal: Kinetic Control of COF Growth by Development of Covalent Network Terminating Agents (Step 1 and 2)

COF syntheses which yield highly crystalline materials have predominantly been realized through reversible condensation reactions. The term “self-healing” is often applied to the primary mechanism operating during COF growth when reaction conditions are such that covalent bonds formed by condensation reactions are being formed, broken and re-formed simultaneously. This subproject aims to develop COF growth modulating agents which can reversibly terminate covalent network growth fronts. More controlled growth kinetics will allow “self-healing” to become more prominent, ultimately improving crystallinity in imine based COF systems which are more stable as scaffolds for water splitting catalysis.
Compounds 3C and 3D were selected as synthetic targets for growth modulating agents. Compound 3C has been prepared in its entirety in high purity. 3C possesses relatively short, solubilizing hexyloxy chains and the compound proved straightforward to synthesize and purify according to. Compound 3D possess longer polyether chains designed to solubilize the compound and forming COFs in more polar solvents. Compound 3D has not been fully prepared, and remains at the 3B intermediate stage. The 3B derivative has been prepared on a small scale but yields less high purity material and requires more tedious chromatography steps.
Going forward, compound 3B with longer polyether chains will be reacted with 4-formylphenylboronic acid to yield the final target compound 3D. Both 3C and 3D will be added in variable molar ratios with COF forming reagents 3E and hydrazine under standard COF forming reaction conditions. Resultant COFs will be characterized with regards to crystallinity and surface area as a function of modulator concentration.

Scheme 1:

Project goal: Controlling the crystallinity, layer registry and stacking mode in COFs (Step 2-iii)

The exact stacking sequence of the 2D layers in COFs is of paramount importance for the optoelectronic, catalytic and sorption properties of these polymeric materials. The weak interlayer interactions lead to a variety of stacking geometries in COFs, which are both hard to characterize and poorly understood due to the low levels of crystallinity. Therefore, detailed insights into the stacking geometry in COFs is still largely elusive. In this regard, we could show that the geometric and electronic features of the COF building blocks can be used to guide the stacking behavior of two related 2D imine COFs (TBI-COF and TTI-COF), which either adopt an averaged “eclipsed” structure with apparent zero-offset stacking or a uniformly slip-stacked structure, respectively.[1] These structural features are confirmed by XRPD and TEM measurements. Based on theoretical calculations, we were able to pinpoint the cause of the uniform slip-stacking geometry and high crystallinity of TTI-COF to the inherent self-complementarity of the building blocks and the resulting donor-acceptor-type stacking of the imine bonds in adjacent layers, which can serve as a more general design principle for the synthesis of highly crystalline COFs. Controlling the crystallinity, layer registry and stacking mode in COFs could be used to elucidate structure – activity relationships for photocatalysis.

Figure 1: A: determination of the slipping direction by XRPD. B: Different types of stacking in the TTI-COF.

Project goal: Topotactic Locking of a Covalent Organic Framework (Step 1-iii. Post-synthetic stabilization) – A novel thiazole COF

As a strategy to synthesize highly stable covalent organic framework materials – a prerequisite for robust and long-term photocatalytic action – we investigated the transformation of an imine linkage to a thiazole in a COF by the action of elemental sulfur on the COF.

Figure 2: A: formation of the imine bond and the subsequent reaction to form the thiazole. B:

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

Our work on covalent organic frameworks has not only introduced a new and highly versatile photocatalytic materials platform, but is expected to provide unique insights into the fundamental mechanisms underlying photocatalytic processes at a molecular level. So far, we have generated new understanding of the crystallization of COFs on a morphological and a molecular level that is inherently linked to their photocatalytic activity. Structural modifications developed by us, aiming at generating highly robust and long-term stable systems, as well as the introduction of functionality in the form of molecular co-catalysts drive the tunability of these molecular solids beyond the limits of conventional solid state materials. Continuous improvement of COF photocatalysts with regard to stability, light harvesting and activity could ultimately open the door to tailored commercial photocatalysts for harvesting solar energy on a large scale. The primary benefits of this research, however, will be the advancement of our understanding of what is at the heart of a “good” photocatalyst, and our ability to rationally design and tailor new photocatalytic systems with molecular precision.

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