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., lambda(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.