The Role of Local Triplet Excited States and D‐A Relative Orientation in Thermally Activated Delayed Fluorescence: Photophysics and Devices

摘要

Here, a comprehensive photophysical investigation of a the emitter molecule >DPTZ‐DBTO2, showing thermally activated delayed fluorescence (TADF), with near‐orthogonal electron donor (D) and acceptor (A) units is reported. It is shown that >DPTZ‐DBTO2 has minimal singlet–triplet energy splitting due to its near‐rigid molecular geometry. However, the electronic coupling between the local triplet (³LE) and the charge transfer states, singlet and triplet, (¹CT, ³CT), and the effect of dynamic rocking of the D–A units about the orthogonal geometry are crucial for efficient TADF to be achieved. In solvents with low polarity, the guest emissive singlet ¹CT state couples directly to the near‐degenerate ³LE, efficiently harvesting the triplet states by a spin orbit coupling charge transfer mechanism (SOCT). However, in solvents with higher polarity the emissive CT state in >DPTZ‐DBTO2 shifts below (the static) ³LE, leading to decreased TADF efficiencies. The relatively large energy difference between the ¹CT and ³LE states and the extremely low efficiency of the ¹CT to ³CT hyperfine coupling is responsible for the reduction in TADF efficiency. Both the electronic coupling between ¹CT and ³LE, and the (dynamic) orientation of the D–A units are thus critical elements that dictate reverse intersystem crossing processes and thus high efficiency in TADF.