Fine tuning phosphorescent properties of platinum complexes via different N-heterocyclic-based CˆNˆN ligands

Abstract

In this article, series of platinum(II) complexes with different [CˆNˆN] cyclometalated ligand scaffolds was elaborated via density functional theory (DFT) and time-dependent density functional theory (TDDFT) methods to mainly explore how the N atomic number, the atomic number of cyclization and the position of N atom influence phosphorescent processes (radiative and nonradiative decay processes). Thereinto, the factors which determine the radiative processes, including μ(Sn) for S0–Sn transitions, ΔE(Sn–T1) and SOC matrix elements〈T1| HSOC| Sn〉were calculated. In addition, according to the energy gap law, the absorption-emission Stokes shifts as well as energy gaps between T1 and S0 states were also computed to describe the nonradiative decay processes qualitatively. As revealed, compared with six-membered N-heterocyclic compounds, five-membered N-heterocyclic compounds exhibit obvious blue-shifted behaviors, smaller nonradiative decay rate constants and similar radiative decay rate constant, especially complex 2b with the other N atom at the 4-position of five-membered N-heterocyclic ligand, which leads to a maximum hypsochromic shift of phosphorescence band and relatively larger radiative decay rate constant and smaller nonradiative decay rate constant among these complexes. These findings successfully illustrated the structure–property relationship and provide valuable information to design highly efficient phosphorescent material.