Rational Molecular Design for Balanced Locally Excited and Charge Transfer Character to Achieve Dual Function of Two-Photon Absorption and TADF

The pursuit of molecular design of highly efficient thermally activated delayed fluorescence (TADF) emitters with two-photon absorption (2PA) character is hampered by the concurrent achievement of a small singlet-triplet energy gap (Δ<i>E</i>_ST) and a large oscillator strength (<i>f</i>). Here, by introducing a terephthalonitrile unit into a sterically crowded D-π-D structure, we designed TADF emitters with a 2PA phenomenon bearing hybrid electronic excitation character. This rational molecular design was achieved through a main π-conjugated donor-acceptor-donor (π-DAD) in line with locally excited features and an auxiliary N-donor-acceptor-donor (N-DAD) with charge transfer (CT) character, highly balancing the TADF phenomenon by a small Δ<i>E</i>_ST and maintaining high 2PA cross-section with a large <i>f </i>value. Moreover, the incorporation of naphthyl groups was found to manipulate the emission properties without surrendering the Φ_PLvalues. Through this approach, the emission modulation from yellow to orange-red was successfully realized by compromising the trade-off between a small ΔE_ST and a large <i>f </i>towards the goal of efficient OLEDs while retaining the TADF and 2PA character with rigid molecular framework. A near-unity Φ_PL value with a large radiative decay rate over an order of magnitude higher than the intersystem crossing (ISC) rate and a high horizontal orientation ratio (Θ<i>_//</i>) of 0.95 were simultaneously obtained for the emitter of <b>TPCz2NP</b>. The organic light-emitting diode (OLED) fabricated with this material exhibits a record-high maximum external quantum efficiency (EQE) of 25.4% and EL peak at 573 nm with CIE coordinates of (0.50, 0.49). Notably, the <b>TPCzTPA</b> and <b>TPCz1NP</b>-based OLEDs displayed improved performance of a high EQE of 24.2% and 23.1% with EL peak at 564 nm and 550 nm, respectively, when compared with <b>2PhCzTPN </b>suggesting the merit of the design approach. Moreover, it is envisioned that the balanced RISC with high Φ_PL and radiative decay also render these compounds an ideal emitter for 2PA applications. Consequently, the elevated <i>σ</i><i>²</i>_max value of 143 GM at 850nm was realized for <b>TPCz2NP </b>which is highest among the reported TADF emitters with EQE_max values exceeding 25%. The successful integration of asymmetric electron-donating segments into the acceptor core underscores the full potential of this approach for advanced molecular systems with TADF and 2PA capabilities and would shed light on the futuristic realization of bio-applications. These findings offer a venue for designing high-performance TADF emitters with exceptional 2PA properties, expanding future OLEDs.