In situ elucidation of the formation mechanism of donor-acceptor complexes responsible for exciplex generation

Exciplex-forming systems offer tunable emission and dual emitter/host functionality in TADF OLEDs, yet their inherently amorphous nature obscures molecular-level structure-property relationships, hindering further application development. Herein, in situ techniques were applied for the first time to investigate the intermolecular interactions driving exciplex formation. NMR analyses revealed that the ubiquitous exciplex acceptor PO-T2T functions as a supramolecular host (H), encapsulating the carbazole-based donor ICzMe (G) in solution. Photophysical measurements confirmed that the PO-T2T/ICzMe complexes exhibit prominent thermally activated delayed fluorescence (TADF) characteristic of exciplex emission. Fluorescence titration and theoretical calculations indicated a stepwise complexation process: initial 1:1 (H:G) binding, followed by a 1:1:1 (H:G:H) sandwich-like arrangement. This growth mechanism was further corroborated by the cocrystal structure of PO-T2T/ICzMe, which revealed that PO-T2T adopts a claw-like geometry to form the sandwiched supramolecular complex. Moreover, NMR investigations across multiple PO-T2T-based exciplexes demonstrated that supramolecular interactions are a general driving force for exciplex formation.