Influence of the Length of the Donor-Acceptor Bridge on Thermally Activated Delayed Fluorescence.

Small singlet-triplet energy splitting is a dominant condition for efficient thermally activated delayed fluorescence (TADF), which could be obtained by increasing the length of the connecting bridge between the donor (D) and the acceptor (A) units in molecules. However, the long connecting bridge could also give rise to a small radiative decay rate, which is harmful for the overall luminescent efficiency of TADF. Herein, we calculate the singlet-triplet energy splitting and the radiative decay rate and discuss the bridge length effect on the TADF efficiency. The results indicate that there is an optimal value of the D-A bridge length at which the delayed fluorescence efficiency and internal quantum efficiency reach their maxima. The optimal value depends on the size of the donor or acceptor unit. Furthermore, molecules with larger unit size have larger optimal TADF efficiency. Our findings shed new light on the design strategies of high-efficiency TADF molecules.