Regulation of internal reorganization energy to change the non-radiative channel in the Ir(iii) complex: the role of N atoms

Abstract

Iridium-based metallization complexes constitute the preferred phosphor materials for organic light-emitting diodes (OLEDs). In this study, we examine the influence of ligands with different N-substitution modes and heavy atom effects on transition metal complexes. Our findings offer valuable insights into the properties of these complexes, facilitating the design and optimization of materials with desired characteristics. By employing density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations, we investigate the thermal vibration correlation function rate theory for Ir(III) tri-dentate ligand complexes that possess exceptional phosphorescence efficiency and broad application prospects. We draw three main conclusions from our investigations. Firstly, substituting 9-N-phenyl and 10-2N-phenyl in the iridium complex Ir(ppy)₃ reduces spin–orbit coupling (SOC), resulting in decreased non-radiative rate constants compared to Ir(ppy)₃. Conversely, substituting 10-N-phenyl and 11-N-phenyl enhances SOC, leading to varying degrees of increased non-radiative rate constants. Among these, substituting 11-N-phenyl exhibits the highest non-radiative rate constant. Secondly, for the ligand pyridine ring of Ir(ppy)₃, 1,3-2N-substitution reduces the reorganization energy while slightly increasing the SOC. Moreover, the energy barrier through the non-radiative path of the metal-centered (MC) state significantly increases. Consequently, the non-radiative decay rate decreases by a factor of 3.1, outperforming other schemes. Notably, 1,2-2N-substitution substantially raises the reorganization energy (by 2.1 times) and SOC, resulting in a five-order increase in the non-radiative decay rate. Therefore, adjacent N-substitutions should be avoided. Thirdly, the crucial activation barriers for the thermal non-radiative deactivation process in compounds 3 and 4 were determined to be 12.61 and 26.04 kcal mol⁻¹, respectively. Compound 4 exhibits the highest non-radiative rate constant (9.68 E + 12), demonstrating that Br substitution on the pyridine of Ir(ppy)₃ significantly enhances the non-radiative rate constant. Overall, this study provides valuable insights into the rational design of efficient fluorescent materials based on transition organometallic complexes.