Intermolecular pi-pi-packing-induced thermally activated delayed fluorescence: a novel pathway toward luminescence efficiency of nearly 100%

Efficient utilization of triplet excitons is vital in organic optoelectronics. One important strategy for harvesting triplets is thermally activated delayed fluorescence (TADF). In most TADF materials, the first singlet (S1) and triplet (T1) excited states both show strong charge-transfer (CT) character to reduce their energetic gap (EST); however, the negative consequence is small spin-orbit coupling and broad fluorescence emission width. To overcome this trade-off, our present work developed a novel strategy, i.e., intermolecular pi-pi-packing-induced (pi3) TADF, named pi3TADF. Distinct from the previously reported TADF systems, the excited states of our intermolecular pi3TADF systems show weak CT-excitation character. Our designed coplanar molecules based on a 1,5,9-trioxo-13-azatriangulene core show low photoluminescent quantum yields (PLQYs) in dilute solutions while their PLQYs in solid films reach ~ 100%. Their face-to-face pipi packings lead to the hybridization of intermolecular CT and localized pi-pi* excitations as well as electronic delocalization in the S1 states, while their T1 states show little changes. Consequently, with a dense manifold of the triplet states, the EST is significantly reduced while the large spin-orbit couplings are induced, thus leading to efficient TADF and significantly enhanced PLQYs in films. Organic light-emitting diodes exploiting the intermolecular pi3TADF systems as emitters show simultaneously high maximum external quantum efficiencies (e.g., over 30%) and narrow emission spectral widths (e.g., 44 nm). Our present work not only develops a new strategy, i.e., pi3TADF, for efficient TADF but also provides an in-depth understanding of its photophysical mechanism, thus opening a new approach for designing novel and efficient TADF materials.