The electronic structures of the two COFs were systematically investigated by ultraviolet/visible diffuse reflectance spectroscopy (UV/vis DRS), photoluminescence (PL) spectroscopy and ultraviolet photoelectron spectroscopy (UPS). As shown in Figure , COF-DFB shows an absorption maxima at ca. 475 nm, and COF-BPDA exhibits a relatively broad absorption band with a tail covering 450 ~ 700 nm. Accordingly, the optical band gaps of the COFs were determined from Kubelka-Munk function as 2.5 eV and 2.3 eV for COF-DFB and COF-BPDA, respectively (Figure inset). The energy of valence band maximum (EVB) of the three COFs relative to vacuum level were evaluated by UPS analyses (Figure ). The EVB values of COF-DFB and COF-BPDA were calculated by subtracting the UPS width from excitation energy (He I, 21.2 eV), which were -6.4 and -6.3 eV, respectively. Coupled with their optical band gaps, the energy of conduction band minimum (ECB) of COF-DFB and COF-BPDA were determined as -3.9 and -4.0 eV, respectively. The ECB of both COFs are positioned higher than the reduction potential of proton (EredH + / 𝟏 𝟐 H2 = -4.44 eV) (Figure ), allowing for reducing proton to hydrogen once the COFs were excited by light with photon energy larger than their band gaps. The steady-state photoluminescence (PL) spectra of the two COFs were measured. Upon excitation at 365 nm, the emission maxima of COF-BPDA appeared at λ em = 605 nm, distinctly red-shifted by 30 nm over that of λem = 575 nm for COF-DFB (Figure ), attributable to the larger π-extended structure of COF-BPDA. Additionally, the PL decay curves of two COFs are well fitted with the lifetimes with two exponential components (Figure ). The average lifetimes of COF-DFB and COF-BPDA were estimated to be 7.08 and 7.56 ns, respectively. The longer lifetime of COF-BPDA indicates the suppressed radiative recombination of photogenerated excitons, associated with its extended π-conjugated backbone. As a proof of concept, we studied the visible light photocatalytic H2 evolution activities of COF-DFB and COF-BPDA from water reduction. Both COF samples can be easily dispersed in water, which is related to their pyridine-rich backbones that strongly interacted with water. The photocatalysis experiments were performed by dispersing 20.0 mg of COF powder into 0.1 M aqueous ascorbic acid solution, in which ascorbic acid was applied as a sacrificial electron donor, and 3 wt % platinum was in situ photo-deposited on the COF. Upon visible light illumination, COF-BPDA showed an average hydrogen evolution rate (HER) of 3229.01 μmol h -1 g -1 , nearly 1.54 times faster than that of COF-DFB (2099.60 μmol h -1 g -1 ) (Figure ). This indicated that the photocatalytic H2 generation performance of COF-DFB and COF-BPDA were mainly influenced by their intrinsic semiconducting properties. COF-BPDA with longer conjugation length and narrower band gap delivered higher activity as compared to COF-DFB.