Photoelectron conductivity was evaluated through impedance spectroscopy, revealing a reduction in resistivity under light exposure across all COFs (Fig. ). PCOF-1, PCOF-2 and PCOF-4 display the lowest resistivity thus higher photoelectron conductivity under light exposure. These trends in photoconductivity and charge carrier separation efficiency and lifetime are all consistent with the observed photocatalytic hydrogen evolution. It can be concluded that even though the backbone of the PCOFs is constructed from identical donor-acceptor (triphenylamine-triazine) building units, the alteration in the pendent aromatic group distinctly influences their polarity and optoelectronic properties. The apparent quantum efficiency (AQE) of PCOF-1-5 was determined using a sunlight simulator and a xenon lamp at wavelength of 420 and 500 nm. Using the xenon lamp, the AQE values are observed to be 2.55, 3.31, 0.61, 3.43 and 1.60% at 420 nm (violet light), whereas at 500 nm (green light) they are determined as 3.84, 2.89, 0.52, 2.96 and 1.08% for PCOF-1-5, respectively (Fig. ). These values are comparable with other COF-based materials at 420 nm. Using a sunlight simulator the AQE values were found to be 2.25, 3.56, 0.96, 4.15 and 1.85% at 420 nm (violet light), whereas at longer wavelength 500 nm (green light) they were determined as 3.43, 2.06, 0.56, 2.03 and 1.02% for PCOF-1-5, respectively (Supplementary Fig. ). The overall performance of PCOF-4, with its higher surface area and wettability, is notable. The solar-to-hydrogen efficiency was found to be 0.21%, which is higher and comparable to that of other semiconducting materials. To determine the origin of the evolved H2, hydrogen isotope labeling experiments were conducted in a pure D2O environment for PCOF-4. The acquired mass spectra provided unequivocal confirmation that the predominant gas produced was D2 (Supplementary Fig. ), proving that the source of the generated H2 is water exclusively. When we performed the same experiment in equimolar mixtures of H2O and D2O, the formation of H2, HD and D2 was observed. Notably, the amount of H2 is higher compared to HD and D2 (H2 > HD > D2), which can be attributed to the higher bond dissociation energy of D2O compared to H2O which further confirmed the H2 evolution source.