Molecular Design of Non-doped OLEDs Based on a Twisted Heptagonal Acceptor: A Delicate Balance between Rigidity and Rotatability

The development of efficient non-doped organic light-emitting diodes (OLEDs) is highly desired but very challenging because of a severe aggregation-caused quenching effect. Herein, we present a heptagonal diimide acceptor (BPI), which can restrict excessive intramolecular rotation and inhibit close intermolecular π–π stacking due to well-balanced rigidity and rotatability of heptagonal structure. The BPI-based luminogen ( DMAC-BPI ) shows significant aggregation-induced delayed florescence with an extremely high photoluminescence quantum yield (95.8 %) of the neat film, and the corresponding non-doped OLEDs exhibit outstanding electroluminescence performance with maximum external quantum efficiency as high as 24.7 % and remarkably low efficiency roll-off as low as 1.0 % at 1000 cd m⁻², which represents the state-of-the-art performance for non-doped OLEDs. In addition, the synthetic route to DMAC-BPI is greatly streamlined and simplified through oxidative Ar−H/Ar−H homo-coupling reaction.     

Highly Efficient Nondoped OLEDs with Negligible Efficiency Roll‐Off Fabricated from Aggregation‐Induced Delayed Fluorescence Luminogens

Purely organic emitters that can efficiently utilize triplet excitons are highly desired to cut the cost of organic light‐emitting diodes (OLEDs), but most of them require complicated doping techniques for their fabrication and suffer from severe efficiency roll‐off. Herein, we developed novel luminogens with weak emission and negligible delayed fluorescence in solution but strong emission with prominent delayed components upon aggregate formation, giving rise to aggregation‐induced delayed fluorescence (AIDF). The concentration‐caused emission quenching and exciton annihilation are well‐suppressed, which leads to high emission efficiencies and efficient exciton utilization in neat films. Their nondoped OLEDs provide excellent electroluminescence efficiencies of 59.1 cd A⁻¹, 65.7 lm W⁻¹, and 18.4 %, and a negligible current efficiency roll‐off of 1.2 % at 1000 cd m⁻². Exploring AIDF luminogens for the construction of nondoped OLEDs could be a promising strategy to advance device efficiency and stability.     

Restriction of Access to the Dark State: A New Mechanistic Model for Heteroatom‐Containing AIE Systems

Restriction of intramolecular motion (RIM), as the working mechanism of aggregation‐induced emission (AIE), cannot fully explain some heteroatom‐containing systems. Now, two excited states are taken into account and a mechanism, restriction of access to dark state (RADS), is specified to elaborate RIM and complete the picture of AIE mechanism. A nitrogen‐containing molecule named APA is chosen as a model compound; its weak fluorescence in solution is ascribed to the easy access from the bright (π,π*) state to the close‐lying dark (n,π*) state. By either metal complexation or aggregation, the dark state is less accessible due to restriction of the molecular motion leading to the dark state and elevation of the dark state energy, thus the bright state emission is restored. RADS is powerful in elucidating the AIE effect of molecules with excited states favoring non‐radiative decay, including overlap‐forbidden states such as (n,π*) and CT states, spin‐forbidden triplet states, and so on.