Excited state intersystem crossing and the relaxation dynamics of phosphorescent Ir(III) complexes bearing bipyridine-based C^N ligand

Abstract

Abstract The photo-deactivation mechanism of heteroleptic Ir(III)(C^N)2 (LX) complexes [(dfpypy)2Ir(pic)](1) and [(dfppy)2Ir(pic)](2) [where dfpypy = 2′,6′-difluoro-2,3′-bipyridine, dfppy = 2-(2,4-difluorophenyl)pyridine, pic = 2-picolinate] were elucidated by density functional theory/time-dependent density functional theory (DFT/TD-DFT) to rationalize the nature of the significant differences in luminescence efficiency between 1 and 2. Specifically, the radiative deactivation was formulated through the calculation of zero-field splitting (ZFS) and the radiative decay rate constant (kr) based on spin-orbit coupling (SOC). Meanwhile, the non-radiative deactivation was estimated by considering the structural distortion, the SOC between the emitting state and the ground state, as well as the thermal deactivation process via metal-centered excited 3MC state. Based on the results, 1 has much smaller SOC matrix elements between T1 and S0 than 2, which determines its small non-radiative decay rate constant, thus one may understand why 1 has higher phosphorescence quantum efficiency than 2. To further explore the structure-property relationship of Ir(III) complexes, four other new complexes 3–6 were designed by incorporating trimethylphenyl(R1), phenyl(R2), ter-butyl (R3), diphenylamine(R4) to the pyridine rings of dfpypy ligand of 1, respectively. Through analyzing, complex 4 with larger radiative decay rate and smaller non-radiative decay rate may be considered as a potential candidate as robust blue-emitting material applied in organic light emitting diodes (OLEDs).