Ligand Engineering toward Deep Blue Emission in Nonplanar Terdentate Platinum(II) Complexes

Eight nonplanar terdentate cyclometalated Pt(II) complexes ( Pt1 – Pt8 ) have been designed and synthesized with the general formula [Pt(N ∧ C ∧ N)Cl], where N ∧ C ∧ N represents 1,3-bis( N -methyl- N ′-(pyridin-2-yl)amino)benzene (H- L ) derivatives with various substituents (−CH 3, −OCH 3, −Cl, and −CF 3 ). These complexes were fully characterized via a wide array of spectroscopic and X-ray single-crystal diffraction techniques. The electrochemical, photophysical and electroluminescence properties of these compounds have been investigated. As determined by the X-ray single-crystal structures of Pt1 with −CF 3 substituents and [Pt( L )Cl] ( Pt3 ), both of them possess a nonplanar configuration but show different intermolecular interactions. In dilute solution, all of these complexes show deep blue emission. However, the emission of the corresponding doped PMMA films can be varied from greenish yellow to deep blue by introducing electron-withdrawing (−Cl or −CF 3 ) to electron-donating (−CH 3 or −OCH 3 ) substituents. Accordingly, Pt1 with −CF 3 substituents displays concentration-dependent emission properties with an additional excimeric emission band centered at 550 nm, resulting in a greenish yellow emission in the doped PMMA film. Pt7 and Pt8 with electron-donating groups (−CH 3 for Pt7, −OCH 3 for Pt8 ) show deep blue emission in both solution and the doped thin films. Sky blue emission was observed in PMMA films doped with Pt2 – Pt6 . A bluish green organic light-emitting diode with a maximum EQE of 3.7% was fabricated using Pt7 as the dopant emitter with CIE coordinates of (0.2822, 0.3602). In addition, density functional theory (DFT) and time-dependent DFT (TDDFT) calculations were carried out to investigate the theoretical molecular orbitals and energies of the studied complexes. This work provides an effective molecular-structure-design approach to develop deep blue phosphorescent emitters.