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 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 ((LE)-L-3) and the charge transfer states, singlet and triplet, ((CT)-C-1, (CT)-C-3), 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)-C-1 state couples directly to the near-degenerate (LE)-L-3, 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) (LE)-L-3, leading to decreased TADF efficiencies. The relatively large energy difference between the (CT)-C-1 and (LE)-L-3 states and the extremely low efficiency of the (CT)-C-1 to (CT)-C-3 hyperfine coupling is responsible for the reduction in TADF efficiency. Both the electronic coupling between (CT)-C-1 and (LE)-L-3, 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.