Three hypothetical complexes were designed using diimides (PMDI, NTCDI, and PTCDI) as the acceptor unit and B(III)‐submonoazaporphyrin (1) as the donor unit. These complexes have smaller HOMO‐LUMO energy gaps (3.39–3.96 eV) than pristine 1 (6.61 eV). Further, the energy gap can be tuned by changing the number of benzene rings of these diimides. Remarkably, these proposed complexes possess considerable first hyperpolarizabilities (β0) (4865–6921 a.u.), and the regularity of the β0 values remained the same in the gas phase and toluene solvent conditions. There is an inverse relationship between the energy gap and the polarizability/first hyperpolarizability. In addition, absorption spectra, frontier molecular orbitals, and hole electron distributions were obtained using time‐dependent density functional theory calculations to emphasize the relationship between structure and properties. Ultraviolet‐Visible absorption spectra reveals that all complexes show satisfying IR working regions. Further analysis of the first hyperpolarizability density reveals the nature of the excellent NLO properties of the studied systems. This study can provide valuable insights for the development of potential high‐performance NLO molecules. Three hypothetical complexes were theoretically designed using diimides (PMDI, NTCDI, and PTCDI) as the acceptor unit and B(III)‐submonoazaporphyrin (1) as the donor unit. Theoretical calculations show that the first hyperpolarizabilities of these complexes can increase significantly with increasing size of the diimide unit.