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

Here, a comprehensive photophysical investigation of a the emitter molecule , showing thermally activated delayed fluorescence (TADF), with near-orthogonal electron donor (D) and acceptor (A) units is reported. It is shown that has minimal singlet-triplet energy splitting due to its near-rigid molecular geometry. However, the electronic coupling between the local triplet (LE) and the charge transfer states, singlet and triplet, (CT, CT), 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 CT state couples directly to the near-degenerate LE, 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 shifts below (the static) LE, leading to decreased TADF efficiencies. The relatively large energy difference between the CT and LE states and the extremely low efficiency of the CT to CT hyperfine coupling is responsible for the reduction in TADF efficiency. Both the electronic coupling between CT and LE, 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.