Highly Efficient Luminescence from a Red Thermally Activated Delayed Fluorescence Emitter with Flexible Conformation of Ancillary Groups.

Robust scaffolds were typically applied in thermally activated delayed fluorescence (TADF) molecules to suppress the non-radiative decay, trigger the fast spin-flipping, and enhance the light out-coupling efficiency. Herein, we disclosed for the first time the positive effect of flexible conformation of ancillary groups on the photophysical properties of TADF emitter. The red TADF emitter Ph-TPA with flexible conformation demonstrated small excited-state structural distortion and low reorganization energy compared to the counterpart Mc-TPA with a rigid macrocycle. Consequently, Ph-TPA showed an excellent photoluminescent quantum yield (PLQY) of 92 % and a state-of-the-art external quantum efficiency (EQE) of 30.6 % at 630 nm. This work could deepen our understanding of structure-property relationships of organic luminophores and help us to rationalize the design of efficient TADF materials.

Using azaacene as an acceptor unit to construct an ultraefficient red fluorophore with an EQE over 40.

Azaacenes, which have been known for a long time, are of scientific and practical importance in organic electronics. Azaacenes once shone as the luminophore in organic light-emitting diodes (OLEDs). However, due to the low exciton utilization efficiency and/or the aggregation induced quenching (ACQ) effect, N-heteroacene based OLEDs generally showed inferior device performance. In this work, azaacene has been revisited and applied as an acceptor for a red fluorophore (AZA-TPA), where the judicious connection pattern between donor and acceptor maximized the harvest of singlet and triplet excitons, resulting in a high photoluminescence efficiency of 94.6% in doped films (3 wt%). In addition, the linearly-fused polycyclic structure contributed to a high horizontal emitting dipole ratio (Θ‖ = 90%). As a result, an AZA-TPA-based OLED achieved an unprecedented external quantum efficiency of 41.30% at 610 nm. This work will pave a new path for the development of efficient N-heteroacene-based fluorophores.

Intramolecular charge transfer effect for highly efficient deep red and near infrared thermally activated delayed fluorescence.

Thermally activated delayed fluorescence (TADF) materials with emission in the deep red and near infrared (DR/NIR) region are underresearched due to the limited choice of strong donor/acceptor units. The current mainstream strategy for the design of DR/NIR TADFs is to increase the acceptor strength via the introduction of multiple sub-acceptor units, thereby narrowing the bandgap. In this work, the intramolecular charge transfer (ICT) effect was applied for the development of acceptor units to achieve efficient DR/NIR TADFs. The ICT effect within the acceptor unit enhanced the π-electron delocalization, lowered the LUMO and redshifted the emission wavelength. In addition, the fusion of the donor unit into the planar acceptor skeleton rigidified the molecular structure and reduced the non-radiative decay. This proof-of-concept study demonstrated that ICT is an undoubtedly effective strategy for the rational design of efficient DR/NIR TADFs.