Synthesis, Air Stability, Photobleaching, and DFT Modeling of Blue Light Emitting Platinum CCC-N-Heterocyclic Carbene Pincer Complexes

The recently reported metalation/transmetalation route for the synthesis of CCC-bis(NHC) pincer ligand supported transition-metal complexes was extended to Pt. 2-(1,3-Bis(N-butylimidazol-2-ylidene)phenyiene)(chloro)platinum(II) (1) and its bromo analogue 2 were synthesized and characterized. X-ray crystal structure determinations revealed complexes 1 and 2 to have distorted-square-planar configurations around the metal. Photophysical and thermal properties of these complexes are reported, and their extended photostability in air is discussed and contrasted. Density functional theory (DFT) and time-dependent density functional theory (TD-DFT) computations of the ground state and various low-lying excited states have revealed admixing of Pt Sd orbitals and the ligand pi* orbitals for both the ground state and the low-lying excited states of complex 1, indicating the low-lying states to be a mixture of metal-to-ligand charge-transfer and ligand-centered transition (MLCT-LC). Somewhat surprisingly, the computed gas-phase geometry of the excited state of complex I had a significant distortion, mostly about the C-aryl-Pt-Cl angle. These complexes were congeners of materials for organic light emitting diodes (OLEDs).