Visible light driven hydrogen (H2) production from water is a promising strategy to convert and store solar energy as chemical energy. Covalent organic frameworks (COFs) are front runners among different classes of organic photocatalyst, owing to their tunable porosity, crystallinity, optical and electronic properties.Photocatalytic activity of COFs depends on numerous factors such as band gap, crystallinity, porosity, exciton migration, charge separation and transport, stability etc. However, it is challenging to fine tune all these factors simultaneously to enhance the photocatalytic activity. Hence, in this report, we have prioritized the key factors for efficient photocatalytic H2 production through structure-property-activity relationship combined with microwave spectroscopy and first-principles calculations. Careful molecular engineering allowed us to tune the light absorption (i.e. band gap), crystallinity, porosity, layer stacking and charge carrier generation and transport of a series of isoreticular COFs. We have assessed how these properties and the interplay between them impact photocatalytic activity of studied COFs. From the structure-property-activity relationship, we found that light absorption and charge carrier generation and transport are the prime factors, which influence the photocatalytic H2 production of COFs in much greater extent than other factors. File list (2)download file view on ChemRxiv Ghosh_etal_Manuscript.pdf (5.34 MiB) download file view on ChemRxiv Ghosh_etal_SI.pdf (4.80 MiB) Scheme 1. Chemical structures of the building blocks along with corresponding molecular lengths and torsional angles. Reaction scheme for the synthesis of COFs under different solvothermal conditions, condition A and condition B.

The structural topology of a 2D network defines its electronic structure.

Metal-organic frameworks (MOFs) and covalent organic frameworks (COFs) consist of molecular building blocks being stitched together by strong bonds. They are well known for their porosity, large surface area, and related properties. The electronic properties of most MOFs and COFs are the superposition of those of their constituting building blocks. If crystalline, however, solid-state phenomena can be observed, such as electrical conductivity, substantial dispersion of electronic bands, broadened absorption bands, formation of excimer states, mobile charge carriers, and indirect band gaps. These effects emerge often by the proximity effect caused by van der Waals interactions between stacked aromatic building blocks. Herein, it is shown how functionality is imposed by this proximity effect, that is, by stacking aromatic molecules in such a way that extraordinary properties emerge in MOFs and COFs. After discussing the proximity effect in graphene-related materials, its importance for layered COFs and MOFs is shown. For MOFs with well-defined structure, the stacks of aromatic building blocks can be controlled via varying MOF topology, lattice constant, and by attaching steric control units. Finally, an overview of theoretical methods to predict and analyze these effects is given, before the layer-by-layer growth technique for well-ordered surface-mounted MOFs is summarized.