Marching Toward Long‐Wavelength Narrowband Emissive Multi‐Resonance Delayed Fluorescence Emitters for Organic Light Emitting Diodes

Abstract

Over the past decade, thermally activated delayed fluorescence (TADF) emitters have garnered tremendous impetus because of their ability to harvest 100% excitons for the light emission in organic light emitting diodes (OLEDs). However, despite their superior external quantum efficiencies (> 35%), the broad emission spectra with associated full‐width‐at‐half maximum (FWHM > 70 nm) present a limiting factor that must be solved. Recently, multiple‐resonance TADF (MR‐TADF) materials based on the heteroatom doped polyaromatic hydrocarbons have gained astonishing attention owing to their remarkable narrowband emission (FWHM < 30 nm). However, emission of the majority of reported MR‐TADF emitters falls in the blue/green region, which inevitably jeopardizes their application in full‐color OLEDs. Therefore, there is an urgent need to develop the new molecular designs for expanding the color‐gamut of MR‐TADF emitters, i.e., λem > 550 nm without compromising the narrowband emission. To the best of current knowledge, no detailed reviews focusing on the different design strategies for producing long‐wavelength (> 550 nm) MR‐TADF emitters have been reported to date. To this end, a review highlighting the recent design advances for constructing long‐wavelength MR‐TADF emitters is presented, and their photophysics and OLED performance is discussed. Finally, the current status and future prospects of long‐wavelength MR‐TADF materials are discussed.

“Core–Shell” Wave Function Modulation in Organic Narrowband Emitters

“…34,35 In contrast, (iii) enables a sufficient redshift because of the substantially enhanced electron-withdrawing and -donating strength achieved by delocalizing the HOMO and LUMO wave functions in the rigid backbone. 26,36 However, the highly extended heterocyclic aromatic skeleton leads to a laborious synthesis process and considerably large molecular weight, resulting in degradation during the long deposition process (at the sublimation temperature) in display manufacturing. 37 Facing challenges in achieving the wide-range color tuning of narrowband emitters, we recognized the primary obstacle in enhancing the ICT character without compromising the narrow FWHMs in MR emitters.…”

“…Ensuring high color purity and primary red–green–blue (RGB) emission with wide-range wavelength maximum (630, 532, and 467 nm for RGB) for each individual pixel is mandatory. Notably, a wavelength shift exceeding 163 nm (∼0.69 eV) between blue and red emissions is required. − However, achieving wide-range color tuning while preserving a narrow FWHM for MR-TADF emitters is challenging owing to the atomically separated highest occupied molecular orbital (HOMO)/lowest unoccupied molecular orbital (LUMO) distribution in the rigid heterocyclic skeleton, resulting in weak and short-range intramolecular charge transfer (ICT). − Figure a shows three mainstream strategies to redshift the MR emission, including (i) peripheral decoration of an MR unit by substituting bulky donor/acceptor (D/A) moieties, , (ii) π-conjugation extension in an MR-unit by fusing D/A fragments, , and (iii) para -(B−π–B)/(N−π–N) modulation by merging MR units. ,− Regarding (i) and (ii), the bathochromic shift of emission is constrained by the prerequisite of small FWHMs, as the MR effect should dominate over the induced twisted-ICT. , In contrast, (iii) enables a sufficient redshift because of the substantially enhanced electron-withdrawing and -donating strength achieved by delocalizing the HOMO and LUMO wave functions in the rigid backbone. , However, the highly extended heterocyclic aromatic skeleton leads to a laborious synthesis process and considerably large molecular weight, resulting in degradation during the long deposition process (at the sublimation temperature) in display manufacturing …”