Electronic structures, bonding aspects and photophysical properties of N-heterocyclic carbene manganese complexes: DFT and TDDFT exploration

Here we explored the influence of axial ligands on the structural, electronic, and photophysical properties of Mn(II)(TPP)(1,3-Me₂Imd)(L) complexes, where L = -NH₃, -OCH₃, -SH, -Cl, and -NO₂, using density functional theory (DFT) and time-dependent DFT (TDDFT) methods. Frontier molecular orbital analysis was performed to assess the reactivity behavior of the complexes which exhibit strong nonlinear optical properties due to their high polarizability. The nature of bonding and charge distribution was further investigated through electrostatic potential mapping, quantum theory of atoms in molecules, electron localization function and localized orbital locator analyses. Natural bond orbital analysis was also conducted to identify stabilizing interactions within the species. Our calculations reveal that the complex with an NH₃ axial ligand (species 1) possesses a larger HOMO-LUMO energy gap and a more negative electrostatic potential, suggesting lower reactivity due to the electron-donating character of the ligand. Furthermore, TDDFT results indicate strong light-harvesting efficiencies in the visible region (575–735 nm), with species 4 demonstrating the highest efficiency and species 5 the lowest. These findings provide insights into the design of Mn-based complexes for optoelectronic applications.