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

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(Sn) for S0–Sn excitations, the energy gap between the triplet and singlet states ΔE(Sn–T1) and the spin–orbital coupling 〈T1|ĤSOC|Sn〉, 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.