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.