To elucidate charge separation and transfer efficiency of all COFs, electron paramagnetic resonance (EPR) (at g = 2.005) spectra were recorded in the dark and under irradiation. In the dark state all COFs showed low EPR signals which significantly increased after irradiation showing that more electrons were excited from the VB to CB (Fig. and Supplementary Figs. ). After protonation of the COFs with ascorbic acid, PCOF-1-5-AA displayed even more intense signals, showing that protonation further improved the charge separation efficiency. Notably, it can be observed that the change in intensity under light irradiation is more pronounced for PCOF-1AA, PCOF-2AA and PCOF-4AA after protonation which correspond to the observed higher H2 evolution for these three photocatalysts (Fig. ).