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

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., λ em > 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.