Theoretical investigation of electronic structures, second-order NLO responses of cyclometalated Ir(III) and Rh(III) counterpart complexes: effect of metal centers

In this paper, eight complexes M-1, M-2, M-3 and M-4 (M = Ir or Rh) in the general formula [M(C<^>N)(2)(N<^>N)] (C<^>N = cyclometalated ligands and N<^>N = ancillary ligands) are systematically investigated by density functional theory (DFT) and time-dependent DFT methods. With the aim to study the influence of metal centers for nonlinear optical (NLO) responses, the geometrical and electronic structures, first hyperpolarizabilities (beta(tot)) and UV-Vis absorption spectra are investigated in detail. It is found that the beta(tot) values increase 1.8-3.7 times as the metal atom changes from Rh to Ir. Among all the studied complexes, Ir-3 (C<^>N = 4-chloro-2-phenylquinoline with N<^>N = 2,9-dimethyl-1,10-phenanthroline) shows the highest beta(tot) value with an amplitude of 2415.4 a.u. computed at the omega B97XD/6-31G+(d)/SDD level, which can be seen as a promising candidate for second-order NLO materials. In addition, the HOMO energy levels increase upon increasing the size of the metal atom, which contributes to a significant reduction in HOMO-LUMO gaps. The beta(tot) values and HOMO-LUMO gaps are in an inverse relationship. Furthermore, tuning the center metals Ir/Rh can be seen as an effective method to modulate the charge transfer degree. We anticipate that this study may serve as a new strategy for the rational design and synthesis of second-order NLO materials.