Molecular organic crystals have highly ordered structures that can be prepared in a modular way using solution-processable units. This makes molecular crystals attractive candidates for studying the effect of secondary structure on photocatalytic activity, since it is possible to compare materials that are chemically identical and that differ only in terms of their solid-state packing. By contrast, such structural comparisons are challenging for amorphous polymers and extended organic networks, such as covalent-organic frameworks (COFs), which often have only moderate crystallinity: for example, there are only a handful of single crystal structures reported in the literature for COFs , and none of those materials have been shown to have photocatalytic activity. On the other hand it is relatively straightforward to grow high-quality single crystals of organic molecules. Until now, however, there are no examples of appreciable photocatalytic hydrogen evolution from molecular organic crystals.