Theoretical study of the substituent effect controlling the radiative and non-radiative decay processes of platinum(ii) complexes

Abstract

Six platinum complexes bearing different electron-withdrawing groups (–CN, –NO₂, –o-carborane, –SF₅ and –CF₂CF₂CF₃) have been designed to explore the electron-withdrawing capability and the conjugative effect of the substituents, and density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations have been performed to determine their electronic structures and phosphorescent properties. Three factors, including the oscillator strength μ(S_n) for S₀–S_n excitations, the energy gap between the triplet and singlet states ΔE(S_n–T₁) and the spin–orbital coupling 〈T₁|Ĥ_SOC|S_n〉, have been calculated to analyze the radiative processes. In addition, temperature-independent, temperature-dependent and triplet–triplet annihilation (TTA) have been analyzed to determine the non-radiative decay processes. Introducing strong electron-withdrawing groups into phosphorescent transition-metal complexes has a significant impact on the phosphorescent properties and some regularity besides the inductive effect (the electron-withdrawing capability) and the conjugative effect of the substituents. The stronger electron-withdrawing capability and smaller conjugative effect can give rise to blue-shifted emission behavior and give larger radiative decay rate constants. The results demonstrate that complex 4 (–NO₂ substituted) and complex 2 (–o-carborane) are possible candidates for blue-emitting materials.