Two-dimensional (2D) porous graphitic carbon nitrides (PGCNs) with semiconducting features have attracted wide attention because of built-in pores with various active sites, large surface area, and high physicochemical stability. However, only a few PGCNs have been synthesized, covering a 1.23-3.18 eV band gap. We systematically investigate two new 2D PGCN monolayers, T-C3N2 and H-C3N2, including possible pathways for their experimental synthesis. Based on first-principles calculations, the mechanical, electronic, and optical properties of T-C3N2 and H-C3N2 have been systematically investigated. These two architectural frameworks exhibit contrasting mechanical characteristics owing to their structural differences. Both T-C3N2 and H-C3N2 monolayers are predicted to be intrinsic semiconductors. Exceptionally, the stacking bilayers of T-C3N2 can transform into a rare 2D nodal-line semimetal structure. The narrow bandgap (0.35 eV) of the T-C3N2 monolayer and its extraordinary transformation in the bilayer electronic structure fill the vacancy of PGCNs as electronic devices in the middle/long wave infrared region. C3N2 structures possess ultrahigh anisotropic carrier mobilities (×104 cm2 V-1 s-1) and exceptional absorption coefficients (×105 cm-1) in the near-infrared and visible light regions, suggesting its possible optoelectronic applications. The findings expand the scope of 2D PGCNs and offer guides for their experimental realization.