Narrowband emission: organic thermally-activated delayed fluorescence materials and underlying mechanisms

Organic narrowband emitters inherit the advantages of conventional emitters while combining the characteristics of narrowband emission and high luminous efficiency, which are particularly important for color purity and energy saving. Currently, narrowband emissions with thermally activated delayed fluorescence (TADF) features, i.e., multiresonant TADF (MR-TADF) emitters, are attracting growing interest in cutting-edge research from both academic and industrial perspectives. The underlying mechanisms behind the structure-property relationship for narrowband emission, high photoluminescence quantum yield (phi(PL)), and emission color tunability are worth exploring, which will pave the way for further refinements and contribute to real-world applications. In this review, representative experimental studies in purely organic MR-TADF emitters will be comprehensively summarized from the molecular level, with a focus on rational molecular design strategies for varying emission color, correlations between molecular geometries and photoluminescence (PL) properties, and underlying mechanisms for achieving narrowband emissions with tunable colors. Furthermore, the computational explorations of MR-TADF materials are also investigated. Finally, the challenges in realizing practical applications for MR-TADF emitters are discussed. Our review hopefully paves the way for designing novel MR-TADF emitters with narrowband emission and high color-tuning.