On the Nanoaggregated Emitter of All sp²-Hybridized Bistriphenylenyl in the Device Layout of Organic Light-Emitting Diodes

Atomic force microscopy (AFM) is employed to unveil the film morphology of 2,2‘-bistriphenylenyl (BTP), an all sp²-hybridized polyaromatic blue emitter derived from two triphenylenes (TP) in the OLED layout (organic light-emitting diode). It is generally believed that molecules with such planar cores require derivatization or steric hindrance to hamper intermolecular π−π stacking and reduce self-absorption. X-ray crystallographic results of BTP crystals show that the two triphenylenyls are coplanar, indicative of a likely tendency toward self-quenching. Surprisingly, AFM reveals grainy nanoaggregates of the BTP layer in the device, yet BTP films exhibit excellent performance for OLED applications. To correlate luminescent spectra with the molecular structure, BTP films of crystalline, nanoaggregated, and amorphous features are prepared and subjected to spectroscopic studies. These AFM and spectroscopic results in conjunction with simulation results of a small torsion barrier to rotate the TP rings suggest that in the OLED device BTP molecules adopt a range of dihedral angles which confer nonself-quenched BTP even without being derivatized with steric side chains.