Synthesis, Photophysical and Electrophosphorescent Properties of Fluorene‐Based Platinum(II) Complexes

A series of platinum(II) complexes bearing tridentate cyclometalated C^N^N (C^N^N=6‐phenyl‐2,2'‐bipyridine and π‐extended R‐C^N^N=3‐[6'‐(naphthalen‐2''‐yl)pyridin‐2'‐yl]isoquinoline) ligands with fluorene units have been synthesised and their photophysical properties have been studied. The fluorene units are incorporated into the cyclometalated ligands by a Suzuki coupling reaction. An increase in the π‐conjugation of the cyclometalated ligands confers favourable photophysical properties compared to the 6‐phenyl‐2,2'‐bipyridine analogues. The fluorene‐based platinum(II) complexes display vibronic‐structured emission bands with λ max =558–601 nm, and high emission quantum yields up to 0.76 in degassed dichloromethane. Their emissions are tentatively assigned to excited states with mixed 3 IL/ 3 MLCT parentage (IL=intraligand, MLCT=metal‐to‐ligand charge transfer). The crystal structures of these platinum(II) complexes reveal extensive Pt II ⋅⋅⋅π and/or π–π interactions. The fluorene‐based platinum(II) complexes are soluble in organic solvents, have high thermal stability with decomposition temperature >350 °C, and can be thermally vacuum‐sublimed or solution‐processed as phosphorescent dopants for the fabrication of organic light‐emitting diodes (OLEDs). A monochromic OLED with 3 d as dopant (2 wt %) fabricated by vacuum deposition gave a current efficiency of 14.7 cd A −1 and maximum brightness of 27000 cd m −2 . A high current efficiency (9.2 cd A −1 ) has been achieved in a solution‐processed OLED using complex 3 f (5 wt %) doped in a PVK (poly(9‐vinylcarbazole)) host.