Theoretical study and design of cyclometalated platinum complexes bearing innovatively a highly-rigid terdentate ligand with carboranyl as a chelating unit

摘要

Density functional theory (DFT) and time-dependent density functional theory (TD-DFT) were employed to explore the electronic structures and phosphorescence properties of synthesized terdentate Pt( II ) complexes bearing highly-rigid 3,6-bis( p -anizolyl)-2-carboranyl-pyridine as a cyclometalated ligand and triphenylphosphine ( 1 ) or t -butylisonitrile ( 2 ) as ancillary ligand. To understand the marked difference in phosphorescence quantum efficiency between 1 and 2 , the relaxation dynamics of excited states were elucidated in detail. Aiming to formulate the radiative relaxation, the zero-field splitting (ZFS) and the radiative decay rate constant ( k _(r) ) were calculated by SOC-perturbed TDDFT (pSOC-TDDFT). Meanwhile, the temperature-independent non-radiative relaxation was analyzed by calculating the Huang–Rhys factor ( S ), the SOC interaction between the emitting state and the ground state. While the temperature-dependent non-radiative decay mechanism was studied by depicting the thermal deactivation process via a metal-centered excited ~(3) MC state. Based on the results, 1 and 2 show a few differences in their temperature-independent non-radiative rates. However, the activation barrier for the population of non-emissive ~(3) MC is greatly raised for complex 2 . Therefore, the temperature-dependent non-radiative decay behavior of 2 is considerably suppressed, which ultimately leads to a substantially enhanced phosphorescence quantum efficiency for 2 . To further tune the emission wavelength towards blue, four new complexes 3–6 were theoretically designed by modifying the terdentate ligand with azole groups based on the parent complex 2 . As a result, pyrazole modified complex 4 stands out with enhanced deep-blue phosphorescence located at 434 nm.