Exploiting trifluoromethyl substituents for tuning orbital character of singlet and triplet states to increase the rate of thermally activated delayed fluorescence

This work shows that trifluoromethyl (CF3) substituents can be used to increase the rate of thermally activated delayed fluorescence (TADF) in conjugated organic molecules by tuning the excitonic character of the singlet and triplet excited states. The synthesis and detailed photophysical characterization of four new functionalized donor-acceptor-donor (D-A-D) compounds using CF3-substituted-thioxanthene-S,S-dioxide and dimethylacridine units are presented. Several compounds are reported to exhibit rapid blue/green thermally activated delayed fluorescence with major delayed fluorescence lifetime components of approximate to 1-2 mu s. Changes in the orbital character of singlet and triplet states (local vs. charge transfer) result in significant changes in rates of delayed fluorescence, despite similar Delta E-ST values. The change in orbital character is well supported by TD-DFT calculations. Electrochemical measurements reveal strong shifts in redox potentials that can be induced by sigma-electron withdrawing CF3 substituents. Trifluoromethyl substituted compounds with a wider Delta E-ST also perform more efficiently than might be expected due to the demonstrated changes in excited state character. This study reveals important photophysical and molecular design implications for the future development of TADF emitters.