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

초록

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 F & ouml;rster 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.

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