An isomeric strategy for enhancing phosphorescence efficiency of iridium(III) complexes with N-heterocyclic naphthyridine ligands: A theoretical study

Abstract

The electronic structures and photophysical properties of six isomeric Ir(III) complexes with different N-heterocyclic naphthyridine ligands were investigated by density functional theory (DFT) and time dependent DFT (TD-DFT) approach. The radiative transition rates (kr) were determined through calculated the spin–orbital coupling (SOC) matrix elements TmHSOCSn and the energy levels (ESn and ETm). The non-radiative transition rates (knr) were estimated through analysis of the structural distortions, the d-orbital splittings and the energy differences between the S0 and T1 states ΔE(T1−S0). As the results, the ESn, the ETm and the energy splittings ΔES1-TmandΔETm-Tm-1 can be regulated by the position of two nitrogen atoms in naphthyridine ring for studied complexes. Moreover, Ir(III) complex inclusive of quinoxaline heterocyclic ring presents large kr and knr, so its phosphorescence quantum efficiency is difficult up to be 100%. While two Ir(III) complexes bound to quinazoline heterocyclic ring show weakly emissive because of large knr. Notably, the presence of the cinnoline heterocyclic ring in the Ir(III) complex makes singlet–triplet intersystem (ISC) rate and kr fast but knr slow, then leads to its high phosphorescence quantum efficiency.