Report

Teaser, summary, work performed and final results

Periodic Reporting for period 2 - COFLeaf (Fuel from sunlight: Covalent organic frameworks as integrated platforms for photocatalytic water splitting and CO2 reduction)

Teaser

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...

Summary

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

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 were 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 (Fig. 1). 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 – A novel thiazole COF (Step 1-iii. Post-synthetic stabilization)

Covalent organic frameworks inherently rely on the reversible bonds that are used to create a covalently connected framework. The reversible bond formation is necessary to introduce a means of error correction and defect healing, such that an ordered and crystalline material can be produced. Therefore, the reversible linkage of COFs always presents a weak link in these materials, making them inherently prone to hydrolysis. To circumvent this problem, the reactive COF linkages are further transformed into more robust functional groups that are less prone to cleavage under conditions relevant to photocatalysis, while retaining the crystallinity induced in the first step. We therefore investigated post-synthetic reaction schemes that would enable the “locking” of the crystalline state once the COF is formed under reversible reaction conditions, and thus prevent the re-opening of the COF forming linkage. To this end, we have developed an innovative strategy to transform the imine bond of a COF into an extremely stable thiazole, thus realizing COFs that are crystalline and stable at the same time [2]. This transformation was demonstrated in two different COFs – TTI-COF and a known pyrene-based COF – by reacting the COFs with elemental sulfur at elevated temperatures (Figure 2).

Figure 2: (A) Schematic representation of the transformation of the labile bonds of an imine-COF into a thiazole-moiety. This chemistry is demonstrated with TTI-COF in panel (B).

In these two COF systems we were able to unambiguously prove the transformation of the imine bond to the thiazole by 13C and 15N ssNMR in conjunction with DFT-calculated NMR chemical shifts, IR, EDX elemental mapping and elemental analysis. The structural integrity of the material was evaluated by X-ray powder diffraction (XRPD), Argon sorption and transmission electron microscopy. XRPD showed a change in the structure that can be summarized as a change in the stacking of the individual layers and a reduction in unit cell parameter that is expected from the imine-to-thiazole transformation. Rietveld analysis of the XRPD with structural models also confirmed the intensity changes in the XRPD to be due to the incorporation of sulfur into the thiazole moiety

Final results

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 developed the first integrated COF platforms active for photocatalytic hydrogen evolution, and we have generated new understanding of the crystallization and post-synthetic stabilization 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 molecular solids. 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, insights into the photocatalytic mechanism, and our ability to rationally design and tailor new photocatalytic systems with molecular precision.