Field emission scanning electron microscopy (FESEM) images showed irregular sometimes rod-shaped morphologies for PCOF-1-5 (Supplementary Figs. ). The periodic hexagonal structure can be visualized by high-resolution transmission electron microscopy (HRTEM). Using low-dose HRTEM, a uniformly ordered honeycomb-like porous hexagonal arrangement is seen for PCOF-1-5 (Fig. , and Supplementary Figs. ). Porosity, Stability, Wettability and Optical Properties of MCR-COFs. PCOF-1-5 exhibit characteristic microporous type-I Ar sorption isotherms measured at 87 K with BET surface areas of 1720, 1750, 1050, 2200 and 1410 m 2 g -1 , respectively, which roughly follows the change in size and weight of the groups (benzene, pyridine, imidazole, fluorobenzene) pointing into the pore space (beside for PCOF-3) (Fig. ). For PCOF-3, we were unable to improve the surface area despite several synthetic modifications, including changing the temperature and solvent (details are in the SI). This is likely because 4-vinyl pyridine is less reactive and tends to undergo selfpolymerization. NLDFT pore size distribution results in two to three types of pores in PCOF-1-5 with pore sizes between 1-1.1, 1.4-1.6 and 2.0-2.1 nm (Supplementary Fig. ). This might be explained by the trefoil-like pore shape which arise due to the additional aromatic moieties dangling into the pores. Motivated by the high surface areas and unusual pore shapes we performed H2 sorption experiments. All PCOFs show hydrogen absorption abilities in the range of 1 and 0.6 wt% at 1 bar and 77 and 87 K, respectively. Overall, the uptake of H2 at 1 bar and 77 K was found to be 100, 110, 67, 120 and 90 cm 3 /g, with a heat of absorption (Qst) of 7.2, 7.5, 6.4, 7.3 and 8.6 kJ/mol for PCOF-1, 2, 3, 4 and 5, respectively, based on temperature of 77 and 87 K (Supplementary Fig. ). Notably, PCOF-5 show a relatively high Qst reflected by the high uptake at low pressures. This can be probably explained by the fluoro-benzene groups pointing into the pores yielding the smallest pore spacings, yielding multiple interactions with incorporated hydrogen molecules. These results depict that MCR COFs are promising materials for hydrogen storage applications. The thermal stability of PCOF-1-5 was tested by thermogravimetric analysis (TGA) and the COFs show just minor weight loss until a temperature of 500 o C under N2 atmosphere (Supplementary Fig. ). Further the chemical stability of PCOF-1-5 was tested by immersing the COFs in 6M HCl (aq) (7 days), 6 M NaOH (7 days), boiling water with stirring for 1 day and heating to 350 o C in high vacuum (12 h). All COFs showed an impressing stability and no loss in crystallinity as seen from the retained PXRD patterns (Supplementary Fig. ). In case of PCOF-4, we additionally performed Ar sorption measurements after all treatments, showing that also the porosity and surface area is mainly preserved. (Supplementary Fig. ). Although all COFs are composed out of similar organic building block, the change of the aromatic moiety in the 4-position of the quinoline linkage causes noticeable alterations in their optoelectronic characteristics. The solidstate ultraviolet-visible diffuse reflection spectra (UV-vis DRS) showed different absorption onsets reflected in the color of the PCOFs (Fig. and). The most pronounced red shift was observed for PCOF-1 (dark red brown color) compared to other COFs. Using the Tauc plot, the corresponding optical band gaps were calculated to be 2.38, 2.44, 2.37, 2.39 and 2.42 eV for PCOF-1, 2, 3, 4, and 5, respectively (Detailed calculation in the SI, Supplementary Figs. ). The band structure was calculated from this band gap and valence band (VB) XPS measurements (Fig. ). The VB and conduction band (CB) of all PCOFs are suitable to produce H2 from water (Fig. ).