In summary, we report here the first example of a molecular crystalline HOF with high photocatalytic activity. The porous, hydrogen-bonded structure of TBAP-α was anticipated using CSP methods and ESF maps. TBAP-α has high photocatalytic activity for proton reduction under sacrificial conditions, while the amorphous TBAP material has an activity that is up to 200 times lower over the first 5 hours of photolysis. To our knowledge, this is the first example where two molecular organic materials have been studied for photocatalysis where the solids are chemically identical and differ only in their solid-state packings, and it allows us to deconvolute the effects of molecular structure and crystal structure. The huge difference in catalytic activity for crystalline and amorphous forms of the same molecule shows unambiguously that crystal packing can have a large effect on photocatalytic activity. Detailed comparisons of the activities and crystal structures of TBAP, TPyP and TPhP suggest that πstacked columns with strong π-π overlap are a desirable packing motif. The specific role of porosity in the photocatalytic activity of TBAP-α is less clear, but porosity is, in general, a desirable feature for heterogeneous catalysis and it also opens up other opportunities, such as doping or dye sensitization. While it is not yet possible to predict photocatalytic activity directly from crystal structure alone, CSP has strong potential to guide the discovery of new molecular building blocks by searching for crystal structure landscapes that contain stable structures with useful features, such as extended columns of π -stacked molecules with good overlap of the molecular faces. This is also useful for other energy materials, such as organic electronics.