DFT Study on the Second-Order NLO Responses of 2‑Phenyl\nBenzoquinoline Ir(III) Complexes by Substituents and Redox Effects

Metal\ncomplexes have received extensive attention in nonlinear\noptical (NLO) materials because of their advantages, such as shorter\nresponse times and more flexible structural properties. Density functional\ntheory is used in investigating the geometric structures, electronic\nstructures, charge centroid, and first hyperpolarizability (β<sub>tot</sub>) of a series of selected 15 coordinated Ir(III) complexes.\nThe substitute effect and one-electron redox process effects on the\nstructures and properties of 15 coordinated Ir(III) complexes are\nconsidered. When the electron-withdrawing group is introduced into\nthe ligand, the HOMO–LUMO energy gap decreases and the β<sub>tot</sub> value increases, positively correlating with the electron-withdrawing\nability. The single electron redox process can also improve the NLO\nresponses of complexes, especially the reduction process. The β<sub>tot</sub> value of complex 4<sup>–</sup> is the largest, 2078\ntimes higher than that of complex 4. The analysis shows that the variation\nof NLO responses of complexes is ascribed to the change of electronic\nstructures and the charge transfer modes induced by the ligand modification\nand redox process, which are considered to be two effective methods\nin enhancing the NLO responses of 2-phenyl benzoquinoline Ir(III)\ncomplexes. This study aims to offer design insights into high-performance\nnonlinear optical materials.