Phosphorescent platinum(II) complexes based on spiro linkage-containing ligands
Two highly phosphorescent platinum(II) complexes (Pt1–Pt2) supported by spiro linkage-containing tetradentate ligands (L1–L2) were designed and prepared. The bulky spiro linkage was applied in these compounds to minimize the intermolecular Pt⋯Pt interaction and to suppress the formation of an excimer, thus leading to efficient electroluminescence. The picolinate coordination group was introduced into both L1 and L2 to bind Pt(II) ions. In L1, a pyridyl group serving as a coordination unit was attached to the spiro linkage; while in L2, an electron-rich functional group, N-methyl imidazole, was used. The molecular structures and packing patterns of these two Pt(II) complexes in the solid state were revealed by single-crystal X-ray diffraction measurements. The effect associated with the variation of coordination groups in Pt1–Pt2 on their photophysical and electroluminescence properties was studied. Organic light-emitting diodes (OLEDs) based on Pt1 and Pt2 were fabricated by the vacuum thermal evaporation approach. Both Pt1 and Pt2 showed yellowish green electro-phosphorescence with high efficiency. The device based on Pt1 at a doping ratio of 15 wt% achieved a maximum efficiency of 83.0 cd A−1, 63.8 lm W−1 and 22.9% with Commission Internationale de L'Eclairage (CIE) coordinates of (0.36, 0.60) without any light extraction technologies. Remarkably, the EQE roll-off ratio of Pt2 at the doping level of 10 wt% from the peak value (20.2%) to that (19.5%) at a brightness of 1000 cd m−2 was 3.5%.