Toward the design of inorganic–organic hybrid Ir(III) complexes containing borazine and benzene ligands with excellent second-order NLO responses: An appropriate substitution and π-conjugated extension

The high nonlinear optical (NLO) efficiency and good chemical stability render inorganic–organic hybrid materials widely applied in NLO filed. In this paper, we theoretically design a sequence of new cationic Ir(III) complexes based on parent complex Ir[(C^N)2(N^N)]+ (C^N = 2-phenylpyridine, N^N = 2,2′-bipyridine) through substituting pyridine by borazine ligands or extending borazine/benzene ligands on C^N/N^N position. The geometric and electronic structures, second-order NLO properties and absorption spectrum are evaluated by employing density functional theory (DFT) and time-dependent DFT methods. All designed complexes have smaller HOMO-LUMO energy gaps (0.74–1.82 eV) than parent complex (2.78 eV). N-5 (with borazine-benzene-borazine π-conjugated long chain on N^N ligands) has an unusually small Egap of 0.74 eV which may facilitate an easier electronic transition than other complexes. Interestingly, introducing borazine ligands on parent complex can increase first hyperpolarizability (βtot), but further addition of borazine not lead to positive effect on increasing βtot values. Hybridizing borazine and benzene ligands significantly improve the second-order NLO properties. Thereinto, the maximum βtot of N-5 is found to be 27,648 a.u. which is about 22 times larger than parent complex. The origin of largest βtot values can be reasonably explained by the two-level model due to low transition energy and high transition dipole moment. Further, the UV–Vis spectrum analysis is performed for all complexes, indicating that substituting or hybridizing inorganic and organic ligands are feasible way to make the maximum absorption wavelength red-shifted toward longer wavelength. Our proposed complexes may be used as efficient infrared (IR) NLO materials because of there is no obvious absorption detected in the IR region. This work may offer valuable guidelines for designing high-performance second-order NLO materials.