Fluorene‐Linked Single‐Molecule Exciplex Hosts With Acceptor‐Dependent Energy Transfer and Exciplex Formation for TADF OLEDs

ABSTRACT Single‐molecule exciplex hosts offer a promising route to overcome the solubility mismatch, morphological instability, and phase separation often encountered in conventional binary exciplex systems for solution‐processed organic light‐emitting diodes (OLEDs). Here, two cardo‐type fluorene‐linked single‐molecule exciplex hosts, FL‐PN1 and FL‐PN2, are developed by covalently connecting an identical donor unit ( p ‐type) to different acceptor units ( n ‐type) through a non‐conjugated linker. This molecular platform enables direct elucidation of acceptor‐structure effects on excited‐state dynamics and device performance. In solution, FL‐PN1 and FL‐PN2 exhibit distinct intramolecular energy‐transfer behavior governed by Förster resonance energy transfer. In the solid state, both hosts form intermolecular charge‐transfer exciplexes with small singlet‐triplet energy gaps, while displaying markedly different photophysical properties depending on the acceptor structure. Solution‐processed green OLEDs based on FL‐PN1 achieve a maximum external quantum efficiency of 23.6%, outperforming FL‐PN2 (19.7%), owing to its higher photoluminescence quantum yield (84.3% vs. 79.9%) and more balanced charge transport. These results identify acceptor molecular design as a decisive factor governing exciplex photophysics and OLED efficiency and establish a versatile strategy for developing high‐performance single‐molecule exciplex hosts.