N-Heterocyclic Carbene-Based Tetradentate Pd(II) Complexes for Deep-Blue Phosphorescent Materials

A series of N-heterocyclic carbene (NHC)-based tetradentate Pd(II) complexes employing phenylcarbene (Ph-NHC)-, benzocarbene (Ph/NHC)-, and pyridinocarbene (Py/NHC)-containing ligands were designed and synthesized. The NHC-based Pd(II) complexes could be prepared by a metalation of the corresponding hexafluorophosphate ligand with Pd(OAc)2 using K2CO3 as base in dioxane at 110 °C in 36-66% yields. All the Pd(II) complexes are air-stable and not sensitive to moisture, and PdON5N-tt exhibits a thermal decomposition temperature (Td) of up to 416 °C. The electrochemical and photophysical properties of the Pd(II) complexes are systematically investigated through experimental research and theoretical calculation. Their reduction potentials, frontier orbitals, and excited-state properties can be efficiently tuned through the ligand modification of the NHC moieties and also perturbed by the alkyl substituents on the pyridine and phenyl rings. Differential pulse voltammetry curves show two obvious reduction peaks for all the Pd(II) complexes involvement with the electron-deficient alkyl pyridine and NHC moieties. Time-dependent density functional theory and natural transition orbital calculations reveal that the T1 excited-state properties are strong admixed metal-to-ligand charge-transfer (3MLCT)/ligand-centered (3LC) with some intraligand charge-transfer (3ILCT) characters for PdON5 analogues, admixed 3MLCT/3LC characters for PdON7p analogues, and admixed 3MLCT/3ILCT for PdON5N-tt. The Pd(II) complexes emit deep-blue light in various matrixes and exhibit narrow emission spectra with a dominant peak at 437-439 nm and FWHM of 34-48 nm in dichloromethane solution and PMMA film. The Pd(II) complexes show a PLQY and excited-state lifetime of 1-11% and 1.1-40 μs in dichloromethane, and 5-25% and 2.6-51 μs in PMMA film.