Achieving highly efficient near-ultraviolet emitters via optimizing molecular configuration by the intramolecular-locked donor and acceptor

In this work, three D-A-locked near-ultraviolet (NUV) emitters (DPhPO-Oz, DPhPO-Ad and DPhPO-Tz) with atypical planar structures were synthesized by an intramolecular-locked of donor and acceptor (D and A) units. They adopt triphenylphosphine oxide (TPhPO) moiety as accepter, which can improve the electron injection and transportation, reduce the LUMO energy level to widen the bandgap and limit the emission red-shift. By locking the D and A units got D-A-locked emitters with atypical planar structure. This atypical planar structure can increase molecular rigidity and reduce the non-radiative decay of the D-A-locked emitters. It also can increase the PLQY and decrease the intermolecular π–π stacking. The highest PLQY of the D-A-locked emitters is 0.64. Due to their atypical planar structure, all of the three emitters show blue-shifted emission and narrower FHWM with the Commission Internationale de L'Eclairage (CIE_y) ≤ 0.03 in toluene solution. In order to further investigate the effect of the atypical planar configurations of the molecules on their emissions. Three additional D-locked emitters (TPhPO-Oz, TPhPO-Ad and TPhPO-Tz) with distinct D-A structures were synthesized. Although the D-locked emitters can also show near-ultraviolet emission with narrow FHWM in toluene, they still exhibit red-shift and wider FHWM in their emission spectra with respect to the D-A-locked emitters. These results indicate that the shorter emission wavelength in the D-A-locked emitters with narrow FHWM should be due to their atypical planar structure. As the above consequence, the NUV organic light-emitting diodes based on DPhPO-Ad can achieve forward-viewing maximum external quantum efficiency (η_ext) of 4.58 % with electroluminescence (EL) at 404 nm, CIE_y coordinate at 0.05 and narrow FHWM (54 nm).