Effect of Solvation on Thermally Activated Delayed Fluorescence Within Electron Donor/Acceptor Chromophores

Abstract

： Thermally activated delayed fluorescence （ TADF ） materials with distinct electron donor and acceptor （ D - A ） segments can achieve theoretically 100% internal quantum efficiencies via the reverse intersystem crossing （ RISC ） process ， thus receiving tremendous attention in lighting ， display ， and biomedical fields. Generally ， D - A systems with intramolecular charge transfer （ ICT ） characteristics minimize singlet - triplet energy gap （ Δ E S - T ） by having molecular con⁃ formations twisted ， so that the RISC process takes place rapidly. Once the dihedral angle of D - A segments in the excit⁃ ed molecular conformation is further twisted ， approaching 90° ， Δ E S - T will be smaller and the TADF characteristics will be also enhanced. However ， conformational changes of excited states ， ICT process and TADF emission are often influ⁃ enced by solvent effect ， which poses a challenge for understanding luminescence mechanism of TADF molecules. This review mainly summarizes the recent progresses in the influence of solvation on the delayed fluorescence made by our groups. As a result ， it is shown that strongly polar solvents lead to an increase in non - radiative relaxation that is averse to TADF ， as well as alteration of solvent viscosities affects the excited state conformational relaxation ， resulting in the enhancement or weakening of TADF. These results are valuable for understanding the role of solvation in conformation⁃ al relaxation and TADF emission ， and provide guidance for the design and synthesis of TADF molecules