Acetylene Functionalization of Multiple‐Resonant Materials Enabling Strong Two‐Photon Absorption and Delayed Luminescence With Highly Efficient Exciplex‐Sensitized Electroluminescence

ABSTRACT Two‐photon absorbing (2PA) luminophores with narrowband emission are rare despite growing demand for color‐pure optoelectronic materials. Multiple‐resonance thermally activated delayed fluorescence (MR‐TADF) emitters offer intrinsically narrow emission, but their nonlinear optical response remains largely unexplored. We report a dipolar MR‐TADF emitter, DtCzBCCH, where installation of a terminal acetylene moiety in carbazole‐fused 1,4‐azaborine DtCzB‐core enhanced 2PA, two‐photon delayed luminescence and enabled energy‐efficient exciplex‐sensitized electroluminescence. Installation of a terminal acetylene moiety extends π‐conjugation and delocalizes the LUMO, introducing partial intramolecular CT (ICT) character to the second singlet excited state (S 2 ). Such modification of DtCzB with an acetylene group enhances the two‐photon absorption cross‐section from 36 GM for DtCzB to 156 GM for DtCzBCCH, while preserving unity photoluminescence quantum yield and narrowband emission. Theoretical calculations revealed the excited‐state decoupling mechanism, where the S 2 state mainly contributes to two‐photon absorption, whereas the S 1 state is responsible for narrowband emission. Exciplex‐host sensitized solution‐processed and vacuum‐deposited OLEDs deliver green electroluminescence with a peak external quantum efficiency of 25.2%, retaining 94.2% efficiency at 1000 cd m −2 and operational stability up to LT 50 = 30 h. This acetylene‐mediated multiresonance CT design provides a general route to integrate strong 2PA with color‐pure, and stable electroluminescence for integrated optoelectronic‐photonic applications.