Design of Efficient Exciplex Emitters by Decreasing the Energy Gap Between the Local Excited Triplet (3LE) State of the Acceptor and the Charge Transfer (CT) States of the Exciplex

A series of thermally activated delayed fluorescence (TADF) exciplex based on the TX-TerPy were constructed. The electronic coupling between the triplet local excited states ((LE)-L-3) of the donors and acceptor and the charge transfer states had a great influence on the triplet exciton harvesting and Phi(PL). Herein, based on this strategy, three donor molecules TAPC, TCTA, and m-MTDATA were selected. The local triplet excited state ((LE)-L-3) of the three donors are 2.93, 2.72 and 2.52 eV in pure films. And the (LE)-L-3 of TX-TerPy is 2.69 eV in polystyrene film. The energy gap between the singlet charge transfer ((CT)-C-1) states of TAPC:TX-TerPy (7:1), TCTA:TX-TerPy (7:1) and the (LE)-L-3 of TX-TerPy are 0.30 eV and 0.20 eV. Finally, the Phi(PL) of TAPC:TX-TerPy (7:1) and TCTA:TX-TerPy (7:1) are 65.2 and 69.6%. When we changed the doping concentration of the exciplex from 15% to 50%, the ratio of the triplet decreased, and Phi(PL) decreased by half, perhaps due to the increased energy gap between (CT)-C-1 and (LE)-L-3. Therefore, optimizing the (CT)-C-1, (CT)-C-3, and (LE)-L-3 facilitated the efficient exciplex TADF molecules.