Abstract
It is generally believed conjugated and planar molecules that are appropriate for light-emitting diodes (OLEDs) application due to strong intermolecular □□□ stacking and induce self-absorption. The triphenylene (TP)-derivatives, 2,2’-bistriphenylenyl (BTP), 1,4-di(triphenylen-2-yl)benzene (T1) and 4,4’-di(triphenylen-2-yl)biphenyl (T2), are all sp2-hybridized polyaromatic compounds without derivatization. Also, X-ray crystallographic results of BTP and T1 crystals show the chemical structure are coplanar, indicative of a likely tendency toward self-quenching. Surprisingly, The device layouts prove the three compounds are excellent blue emitters. To correlate, AFM, theoretical calculation, and luminescence spectra are used to study the correlation between optical properties and the film morphology. AFM reveals grainy nanoaggregates of the emitting layer in the device based on the three TP-derivatives as emitters. The molecular aggregation makes ensembles of the emitters luminesce deep blue light. The small torsion barrier to rotate the TP rings suggest that these compounds adopt a range of dihedral angles which confer steric hinderance on molecular packing. Therefore, self-quenching and self-absorption are obstructed in the OLED device. Comparing BTP, T1, and T2-nased devices, the EQE increases with the number of phenylene inserted between two TP-rings. The wavelengths of the EL for T1 and T2 appear no red-shift from that of BTP, suggesting that phenylene and biphenylene effectively reduce the intermolecular □□□ stacking and do not increase the conjugation length of the biaryls in the devices. The effect of the inserted phenylene-moiety on the enhancement of film amorphism and the luminescence efficiency for small molecules is verified.