Synthesis, structure, electronic state, and luminescent properties of novel blue-light-emitting aryl-substituted 9,9-di(4-(di-p-tolyl)aminophenyl)fluorenes

Novel blue-light-emitting fluorene derivatives 5a-c and 7a-c containing bulky and highly emissive groups, namely pyrene, 10-phenylanthracene-9-yl and 10-(4'-diphenylaminophenyl)anthracene-9-yl groups, as well as hole-injecting/transporting triarylamines were synthesized. Single crystals of compounds 5a, 5c, 7a, and 7c were grown and their crystal structures were determined by X-ray diffraction. The four fluorene derivatives have nonplanar molecular structures, which reduce the intermolecular interaction and the likelihood of molecular aggregation or excimer formation. No unwanted long-wavelength emission was observed in the photoluminescence (PL) spectra of the 5a-c and 7a-c thin films. Their PL spectra reveal excellent thermal stability after annealing treatment under air and ambient light. All of the six compounds show high fluorescence quantum yields and outstanding thermal stabilities. The 2-aryl and 2,7-diaryl substituents at the fluorene molecule have a significant effect on the photophysical properties and the thermal characteristics. The six compounds show almost the same energy levels for the highest occupied molecular orbitals (HOMOs) of about -5.20 eV, which allows effective hole injection. The C2- and C7-aryl substituents play a relatively less-important role in the HOMO energy levels, which depend mainly on the triphenylamino groups at the C9 position. The molecular orbitals, excitation energy, and emission energy were calculated to explain the real origin of their photophysical characteristics. The HOMOs are mainly localized on the triphenylamino groups at the C9 position, while the lowest unoccupied molecular orbitals (LUMOs) have a significant orbital density at the C2- and/or C7-aryl substituents. Pure-blue-light-emitting diodes based on 2,7-diaryl-9,9-di(triarylamino)fluorenes were fabricated.