A potential strategy used for controlling the phosphorescence quantum yield of cyclometalated (CˆC*) platinum(II) NHC complexes: The theoretical insight

Abstract

Effectively adjusting the phosphorescent quantum efficiency of transition metal complexes is of great significance for the development of new energy-saving organic light-emitting diodes devices. In this article, density functional theory (DFT) and time-dependent density functional theory (TDDFT) methods are employed to investigate the effects of electronegativity caused by introduction of azole moieties on the phosphorescence properties, radiative and nonradiative decay rate constants and photodeactivation mechanism of cyclometalated (CˆC*) platinum(II) NHC complexes. According to calculated results, by decreasing the electronegativity of NHC ligand, the maximum wavelength of the complex appears blue shift; the electron transition density of the lowest triplet state is concentrated to the Pt(II) atom, the singlet-triplet splitting energy and the spin-orbital coupling (SOC) matrix elements are obviously increased, leading to a significant increase of radiative decay rate constant. However, the decrease of ligand electronegativity results in a slight decrease in molecular rigidity and the SOC matrix elements between the ground state and the lowest triplet excited state increase, the energy barriers in photodeactivation pathway decrease, which eventually leads to the increase of nonradiative decay rate constant. Considering its prominent effect on the enhancement of the radiative decay rate constant, this method may still effectively improve the phosphorescence quantum yield of the complexes. This work has a certain theoretical reference value for the design of highly efficient phosphorescent complexes.