Vanishing Electronic Band Gap in Two-Dimensional Hydrogen-Bonded Organic Frameworks

We introduce the concept of two-dimensional donor–acceptor (DA) hydrogen-bonded organic frameworks (HOFs) as a general design strategy to realize low-band-gap flat-band materials. We report the electronic structure calculations for a series of hexagonal and tetragonal DA-HOFs from aromatic quinones and cyclic fused oligopyrroles. Density functional theory calculations show that these HOFs possess reasonable binding energies (∼10 kcal/mol). The HOFs show very strong charge transfer interactions that enhance the DA abilities while maintaining localized orbital distributions. This leads to vanishingly low band gaps (Eg ∼ 0.02–0.07 eV) but with extremely flat bands (bandwidth <0.06 eV). The partial filling of these bands via p- or n-doping leads to Stoner ferromagnetism, with stabilization energies up to 100 meV. Stacking the layers results in the metallization of the porous networks with varying energy dispersions. The symmetry of the band structures can be easily modified by the chemical constituent to represent, for example, Kagome and Lieb lattices.