One-step synthesis of spirobi[fluorene] and spiro[fluorene-9,9'-xanthene] derivatives and their applications in organic light-emitting devices: Performance enhancement and related optical phenomena

Employing fluorene as substrate, we synthesized a new spirobifluorene derivative, 2'-(9-phenylfluoren-9-yl)-9,9'-spirobi[fluorene] (PF-SBF), through a one-step palladium-catalyzed cross-coupling reaction. Utilizing PF-SBF as an emitter and as a host of the blue phosphor bis(3,5-difluoro-2-(2-pyridyl)phenyl-(2-carboxypyri-dyl))iridium(III)(FIrpic) in organic light-emitting devices (OLEDs), we observed a red light band different from the intrinsic blue emission of PF-SBF and FIrpic. This was attributed to the intermolecular aggregation of PF-SBF and to exciplexes generated at the interfaces of the emitting layer and the electron transport layer. The exciplex emission was then restrained through a suitable selection of hole and electron transport layer. Employing PF-SBF with di-[4-(N,N-ditolyl-amino)-phenyl]cyclohexane (TAPC) as a cohost, we obtained high-performance blue and green emissions from FIrpic and tris(2-phenylpyridine)iridium(III)(Ir(ppy)₃). The maximum current efficiencies and luminances of the blue and green OLEDs were as high as 16.7 and 50.5 cd·A^-1 and 7857 (at 11 V) and 23390 cd·m-2 (at 8 V), respectively. As an alternative to PF-SBF, we also synthesized a new xanthene derivative, 2-(9-phenyl-fluoren-9-yl)spiro[fluorene-9,9'-xanthene] (PF-SFX), with a large triplet energy level of 2.8 eV. Using PF-SFX similarly as a host of FIrpic, the current efficiency and luminance were significantly improved to 22.6 cd·A^-1 and 6421 cd·m^-2 (at 10 V). These results demonstrate the potential of PF-SBF and PF-SFX as new building blocks for high-efficiency green/blue phosphorescent host materials.