Revealing the marked differences of phosphorescence efficiencies on C˄N˄N‐coordinated Pt(II) complexes: A theoretical study

Abstract

In this study, green phosphorescent Pt(II) complexes with N,N‐diphenyl‐6‐(1H‐pyrazol‐1‐yl)pyridin‐2‐amine (Ndpp) coordinated ligands, [Pt (Ndpp)Cl] 2a, [Pt (Ndpp)Pb, Pb = (prop‐1‐ynyl)benzene] 2b, and [Pt (Ndpp)CN] 2aCN were theoretically investigated by means of density functional theory and time‐dependent density functional theory calculations to reveal their marked distinct phosphorescence quantum yields. These complexes exhibit evident absorption bands in the 200–450 nm region but emit strong green light with marked differences of phosphorescence quantum yields. Compared with the complex 2a, the complex 2b possesses large oscillator strengths of absorption spectra, strong spin‐orbit coupling, and transition electric dipole moment, as well as small singlet‐triplet splitting energies, which conduces to enhancing its radiative decay. To illustrate the nonradiative decay process, the transition state (TS) between the triplet metal‐centered (3MC) state and the excited state (T1) was optimized. The 3MC state is found to be the minimum energy crossing point (MECP) between the T1 state and the S0 state. Compared with the complex 2a, the complex 2b possesses a much larger energy barrier to the MECP state from the T1 state, so it is strongly emissive in the green region. Besides, the introduction of CN substitutions on 2a is useful for enhancing the energy barrier to the thermal deactivation pathway of 3MLCT → TS → MECP. These results demonstrate that the modification of metal–ligand conjugation is an effective way to develop high‐performance phosphorescent materials.