  While graphene oxide (GO) is representative of a disordered phase of
oxocarbons with lackluster electronic properties, the coexistence of ordered,
stoichiometric solid-state carbon oxides with graphene brings renewed momentum
to the exploration of two-dimensional crystalline oxocarbons. This enduring
subject, spanning decades, has recently witnessed significant advancements. In
this context, our study delves into a novel material class, COF-66, notable for
its meticulously ordered two-dimensional crystalline structure and intrinsic
porosity. Employing a global optimization algorithm alongside
density-functional calculations, our investigation highlights a standout member
within the COF-66 family exceptional quasi-flat oxocarbon (C6O6)exhibiting an
unconventional oxygen-decorated pore configuration. This pioneering study
introduces C6O6 as an innovative entrant into the crystalline carbon oxide
arena, augmenting the established understanding alongside the well-recognized
graphene oxide and two graphene monoxide, i.e. {\alpha}-GMO and \b{eta}-GMO.
Expanding the exploration, the COF-66 series encompasses 2D-porous carbon
nitride (C6N6) and the recently synthesized 2D-porous boroxine (B6O6), adhering
to a generalized stoichiometry of X6Y6, where X = B, C, and Y = B, N, O, with X
6= Y. Remarkably, the entire COF-66 ensemble adopts a 2D-crystalline framework,
with the exception of C6B6, which assumes a distinct 3D-crystalline
arrangement. Employing the PBE (HSE06) level of theory, our electronic
structure calculations yield band gap values of 0.01 (0.05) eV, 3.68 (5.29) eV,
0.00 (0.23) eV, and 1.53 (3.09) eV for B6N6, B6O6, C6B6, and C6N6,
respectively, reinforcing and aligning with prior investigations.
