To investigate the effect of the different pore functionality on their photocatalytic performance, the solar-driven hydrogen evolution reaction (HER) from water was performed. The PCOFs were dispersed in water in presence of a sacrificial electron donor (SED) (0.1 M ascorbic acid (AA), pH = 2.50 at room temperature) and Pt co-catalyst and the H2 production under solar-light irradiation was measured (Reaction set up, Supplementary Fig. , supporting video 1). After dispersion of the PCOFs in the water/ascorbic acid solution, an immediate darkening of the color of the pristine COF was observed, indicating the protonation of the COF backbone. This color change can also be followed by solid-state UV-vis DRS measurements after AA treatment, in which all COFs show a red shift of absorption (Supplementary Fig. ). The HER was measured using a sunlight simulator or a xenon lamp (λ>420 nm), which yielded average HERs of 7.1, 9.6, 2.4, 9.9 and 4.2 mmol g -1 h -1 ; and 35.5, 43.1, 9.9, 43.3 and 21.2 mmol g -1 h -1 for PCOF-1, 2, 3, 4, and 5, respectively (Supplementary Fig. ). In natural sunlight, PCOF-4 produces 17 mmol g -1 h -1 of hydrogen which is higher compared to sunlight simulator. When the photocatalytic HER was performed under alkaline condition using triethanolamine (TEOA) as SED at pH ~10.65, all of the MCR-COFs produced negligible H2. This result support the view that the protonation of COFs at acidic condition promotes photocatalytic HER. Also, the values are higher than for the corresponding imine COF (27 mmolg -1 h -1 when using a Xenon lamp). It can be spotted that highest HER values are achieved for the two COFs with 2-pyridine and imidazole moieties in the pores, thus with a more hydrophilic pore interior, especially when these groups are protonated. The higher hydrophilicity yields a better dispersibility of the COFs in water and also facilitate the entering of water molecules within the pores. To prove this assumption, water sorption measurements were carried out. PCOF-1-5 exhibit typical type IV sorption isotherms with hystereses and the overall uptake is in the order of PCOF-4 > PCOF-2 > PCOF-1 > PCOF-5 ~ PCOF-3 (Fig. ) thus following the trend of the photocatalytic activity. PCOF-3 showed a lower water uptake than expected, considering that a pyridine moiety is attached as for PCOF-2, which is probably due to its overall lower surface area. The amount of water molecules which are adsorbed are 512 cm 3 g -1 , 234 cm 3 g -1 , 604 cm 3 g -1 , 735 cm 3 g -1 , and 232 cm 3 g -1 at p/po ~ 0.9 for PCOF-1, 2, 3, 4 and 5, respectively. As it is assumed that the protonated forms are present under photocatalytic measurement conditions, water sorption experiments were also repeated for the ascorbic acid treated samples PCOF-4-AA and PCOF-5-AA. The overall water sorption drastically decreased for PCOF-4-A (735 to 300 cm 3 g -1 ) compared to PCOF-5 (289 to 230 cm 3 g -1 ) (Supplementary Fig. ). This is most probably due to the protonation of the quinoline and imidazole nitrogen in PCOF-4 and the presence of ascorbate counter anions which restrict the pore space further and effect which is not as strong in the already hydrophobic PCOF-5, which shows a lower amount of protonation. However, the even further increased hydrophilicity of PCOF-4 after AA treatment is seen in the improved water uptake at lower water pressures.