Rational Molecular Design for Balanced Locally Excited and Charge‐ Transfer Nature for Two‐Photon Absorption Phenomenon and Highly Efficient TADF‐Based OLEDs

Abstract

The pursuit 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 (ΔEST) and high photoluminescence quantum yield (ФPL). Here, by introducing a terephthalonitrile unit into a sterically crowded donor‐π‐donor structure, inducing a hybrid electronic excitation character, we designed unique TADF emitters possessing 2PA ability. This rational molecular design was achieved through a main π‐conjugated donor‐acceptor‐donor backbone in line with locally excited feature renders a large oscillator strength and transition dipole moment, maintaining a high 2PA cross‐section value. The ancillary N‐donor‐acceptor‐donor with charge transfer character highly balances the TADF phenomenon by minimizing ΔEST. A near‐unity ФPL value with a large radiative decay rate over an order of magnitude higher than the intersystem crossing rate and a high horizontal orientation ratio of 0.95 were simultaneously attained for TPCz2NP. The organic light‐emitting diodes fabricated with this material exhibit a high maximum external quantum efficiency of 25.4% with an elevated 2PA cross‐section (σ2) value up to 143 GM at 850 nm. These findings offer a venue for designing high‐performance TADF emitters with exceptional performance inclusive of 2PA properties, expanding for future functional material design.