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

Abstract

Here, a comprehensive photophysical investigation of a the emitter molecule <b>DPTZ-DBTO2</b>, showing thermally activated delayed fluorescence (TADF), with near-orthogonal electron donor (D) and acceptor (A) units is reported. It is shown that <b>DPTZ-DBTO2</b> 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 <b>DPTZ-DBTO2</b> 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.