Locally Excited States Guided Enhancement in Reverse Intersystem Crossing Rate in Unconventional Acceptor-free Thermally Activated Delayed Emitters

Contrary to the conventional molecular design strategy for obtaining efficient thermally activated delayed fluorescent emitters, we report a couple of acceptor-free molecules ( CZ 2 Bn, PhO 2 Bn, and PhS 2 Bn) that might provide a new design paradigm for the TADF molecules. The presence of locally available triplet states on the secondary donor units is found to be the most critical factor that controls the reverse intersystem crossing rate ( k risc ) and is explained in detail. Our approach to structural design was substituting the central carbazole unit with various secondary electron-rich aromatic units. The selection of the peripheral secondary donors contemplates their triplet-state energies, and in turn well-known aromatic units such as carbazole ( CZ 2 Bn ), phenoxazine ( PhO 2 Bn ), and phenothiazine ( PhS 2 Bn ) were chosen. The proof of concept of locally excited states facilitating the reverse intersystem crossing rate ( k risc ) is elaborated in the article. Despite having a significantly larger singlet–triplet energy gap (Δ E ST ) in the first derivative CZ 2 Bn, the large radiative rate of the singlet excited state and the availability of the isoenergetic local triplet states ( 3 LE) led to the highest TADF efficiency. Detailed experimental outcomes and analysis are discussed in the present manuscript.