Heterotriangulenes (HTs) are promising building units since they are planar molecules with three conjugated benzene rings in one molecule , and the HOMO-LUMO gaps are in range of 3-4 eV (Figure ). The threefold symmetry and highdegree of π-conjugation of HTs allow the formation of conjugated 2D honeycombkagome lattices (Figure ) via on-surface polymerization (Figure ) . By scanning tunneling spectroscopy and DFT calculations, Steiner et al. have demonstrated that the electronic band gap of HTs decreases when going from the monomer to 1D and 2D surface-confined organic polymers comprising the same building unit . Recently, Galeotti et al. have synthesized long-range ordered 2D HTpolymers and confirmed the semiconducting properties with the presence of Dirac cone structures and flat bands by using angle-resolved photo electron spectroscopy (ARPES) . By means of first-principles calculations, we have demonstrated that 2D HTpolymers constructed from B-and N-centred HTs are semiconductors with band gaps of 1.80 -2.84 eV, thus ideally suited for water-splitting applications, and show high charge carrier mobilities . A further check of the electronic and optical properties indicated that altering the heteroatoms and bridge groups effectively adjusts the band edges and enhances light absorption (Box 1). As a result, the appropriate band edge alignment (Figure ) and good light absorption capability (Figure ) render 2D HTpolymers potential photocatalysts for HER or OER processes, respectively . We have further demonstrated that the electronic properties of 2D polymers are determined by their lattice topology (Figure ) and the honeycomb-kagome lattice contributes to single-band features of 2D HT-polymers. As a result, coupling B-and N-centered HTpolymers in a Z-scheme manner (Figure ) with spatially separate electrodes allows effective overall water splitting with suppressed electron/hole recombination. nitrogen-linked COFs as potential photocatalysts . The rational combination of the active molecular segments allows the construction of 2DCPs with appropriate band edge alignments (Figure ) and pronounced light absorption (Figure ) for overall water splitting. By constructing a Z-scheme system (Figure ), a high theoretical energy conversion efficiency of over 20.6 % can be realized. The high photocatalytic activity was further confirmed by experimental measurements, which demonstrated the reliability of theoretical calculations in predicting metal-free photocatalysts for overall water splitting.