Comprehensive Review on the Structural Diversity and Versatility of Multi-Resonance Fluorescence Emitters: Advance, Challenges, and Prospects toward OLEDs.

Fluorescence emitters with a multiple-resonant (MR) effect have become a research hotspot. These MR emitters mainly consist of polycyclic aromatic hydrocarbons with boron/nitrogen, nitrogen/carbonyl, and indolocarbazole frameworks. The staggered arrangement of the highest occupied molecular orbital and the lowest unoccupied molecular orbital facilitates MR, resulting in smaller internal reorganization energy and a narrower emission bandwidth. Optimal charge separation suppresses the energy gap between singlet and triplet excited states, favoring thermally activated delayed fluorescence (TADF). These MR-TADF materials, due to color purity and high emission efficiency, are excellent candidates for organic light-emitting diodes. Nevertheless, significant challenges remain; in particular, the limitation imposed by the alternated core configuration hinders their diversity and versatility. Most existing MR-TADF materials are concentrated in the blue-green range, with only a few in red and near-infrared spectra. This review provides a timely and comprehensive screening of MR emitters from their pioneering work to the present. Our goal is to gain understandings of the MR-TADF structure-performance relationship from both basic and advanced perspectives. Special emphasis is placed on exploring the correlations between chemical structure, photophysical properties and electroluminescent performance in both depth and breadth with an aim to promote the future development of MR emitters.