Fig. 6. Frontier molecular orbitals of the structural fragments of COF-HNU9 in non-water (a and b) and water environments (c and d) (C, gray; N, blue; O, red; H, white). A dimer structure fragment is built to describe the COF-HNU9 framework. All geometric structure optimizations were performed at the B3LYP/6-311+G (d, p) level. The calculated energy for HOMO in non-water and water environment was −5.62 and −5.32 eV, and that for the corresponding LUMO was −3.04 and −3.57 eV, respectively.