Synthesis, photophysical and electrophosphorescent properties of fluorene-based platinum(II) complexes

Abstract

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 Î 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 3IL/ 3MLCT 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. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.