Effect of Main Versus Ancillary Ligand Substitution on the Photophysical Properties of a Series of Ir(III) Complexes: A Detailed Theoretical Investigation.

Abstract

Herein, we report the effects of different electron-withdrawing groups (EWG) (-F) and electron-donating groups (EDG) (-OMe and -NH2) on main ligands (ppy) and ancillary (acac) of [Ir(ppy)2(acac)] [ppy = 2-phenylpyridine; acac = acetylacetonato] using seven complexes by DFT and TDDFT calculations. We find that irrespective of the substituents, absorption of ppy-substituted complexes is blue-shifted, while for the acac-substituted complexes, it is red-shifted. The calculations also show that the substitution of EWGs causes an overall drop in the frontier molecular orbital energy levels; however, we observed a reverse effect for EDGs. To calculate the radiative rate kr, we considered the spin-orbit coupling matrix element (SOCME) (⟨T1|HSOC|Sn⟩) between Sn (n = 1, 2, etc.) excited state and T1, transition dipole moment (μ(Sn)), and the energy difference between excited singlet states Sn and T1 state (ΔE(Sn - T1)). To compare the temperature-independent nonradiative process, we considered SOC between T1 and S0 (⟨T1|HSOC|S0⟩) and the energy gap between optimized T1 and S0 states. Furthermore, to formulate the temperature-dependent nonradiative rate, we computed the activation barrier (E1) for the metal-to-ligand state (3MLCT) to a metal-centered state (3MC) conversion. The emission peaks show that the changes of triplet state T1 from 3MLCT → 3MC via transition states (3TS) and 3MLCT → 1GS (GS = ground state) via the 3MC/1GS minimum energy crossing point are not much affected by the nature of substituents in the ancillary and the main ligand. The order of E1 for the investigated complexes indicates that electron-donating substituents -OMe at both ppy and acac ligands can cause a decrease in nonradiative rate constants. Natural transition orbitals of the complexes show that they are mainly localized on the main ligand ppy and the Ir atoms and hardly on the ancillary ligand acac.