Solution Processable Deep-Red Phosphorescent Pt(II) Complex: Direct Conversion from Its Pt(IV) Species via a Base-Promoted Reduction

Abstract

Color purity is a critical prerequisite for full color displays. Creation of deep-red phosphorescent materials with high PLQYs is particularly challenging because of “energy gap law”. Simultaneously achieving high yielding solution-processable Pt(II) complexes further complicates this challenge. In this report, we developed a high-yielding synthetic route to a solution processable/deep-red Pt(II) complex with a rigid tetradentate structure, in which we identified octahedral Pt(IV) as a major side product formed under the standard complexation conditions. We managed to effectively transform the octahedral Pt(IV) into a luminescent deep-red square planar Pt(II) complex through a base promoted reduction. We found the Pt(II) complex has high solution and blend film PLQYs. X-ray crystal structure and DFT calculations of the Pt(II) complex showed that perpendicular orientation of molecular dipoles enhanced the luminescence properties. In neat films, there was no luminescence enhancement due to interdigitation of the attached hexyloxy tails, preventing strong Pt∙∙∙Pt interactions in the solid state. Solution processed OLEDs based on the Pt(II) complex showed a low turn-on voltage of 3.3 V (at 1 cd/m2) with a maximum brightness of 2,000 cd/m2 and a maximum EQE of ≈6% (4% at 100 cd/m2). A narrow electroluminescence with a full-width-at-half-maximum of ≈50 nm was observed with a peak at 623 nm, and provided a deep-red emission with 1931 CIE co-ordinates of (0.65, 0.35). Transient electroluminescence measurements were used to investigate the EQE roll-off of the OLEDs.