Emergence of an Antiferromagnetic Mott Insulating Phase in Hexagonal π-Conjugated Covalent Organic Frameworks

While the search for 2D organic semimetallic Dirac materials displaying, like graphene, a Dirac cone at the Fermi level remains active, attention is also being paid to the quantum phase transition from semimetal to antiferromagnet. Such a transition in graphene-like materials is predicted based on theoretical investigations of the 2D honeycomb lattice; it occurs (within a Hubbard model) when the on-site electron-electron Coulomb repulsion (U) is much larger than the nearest-neighbor inter-site electronic coupling (t). Here, monomers carrying long-lived radicals are considered and used as building blocks to design 2D hexagonal pi-conjugated covalent organic frameworks (COFs). Both the nonmagnetic semimetallic phase and magnetically ordered phases are evaluated. It is found that the electronic coupling between adjacent radical centers in these COFs is more than an order of magnitude smaller than in graphene while the on-site Coulomb repulsion is reduced to a lesser extent. The resulting large U/t ratio drives these COFs into the antiferromagnetic side of the phase diagram. This work provides a first theoretical evidence of the realization of an antiferromagnetic Mott insulating phase in 2D pi-conjugated COFs and allows a strategy to achieve quantum phase transitions from antiferromagnet to spin liquid and to semimetal to be outlined.