Vertical two-dimensional layered conjugated porous organic network structures of poly-benzimidazobenzophenanthroline (BBL): A first-principles study

Very recently, the 2D form of poly-benzimidazobenzophenanthroline (BBL) structures has been successfully fabricated [Noh et al., Nat. Commun. 369, 670 (2020)]. Motivated by these exciting experimental results on 2D layered BBL structures, herein we perform density functional theory-based first-principles calculations in order to gain insight into the structural, electronic, and optical properties of the BBL monolayer and bilayer honeycomb crystal structures (planar and vertical). Our computational structural optimization reveals that the BBL monolayer crystallizes in a puckered, anisotropic hexagonal structure, while the BBL bilayer is composed of covalently bonded shifted one with respect to the other BBL layers. Two terminations with hydrogen and fluorine atoms are considered for the BBL bilayer, namely, H-BBL and F-BBL, respectively. The direct bandgaps of H-BBLs and F-BBLs are similar to 1eV and similar to 1.2eV. The top of the valence band and the bottom of the conduction band are flat due to the localized carbon states. The BBL monolayer and bilayer can absorb a wide range of visible light. The calculated refractive index of the BBL monolayer is similar to 1, i.e., it is smaller than the refractive index of the common natural or synthetic polymers.