Exploring the possibility of using fluorine-involved non-conjugated electron-withdrawing groups for thermally...

The trifluoromethyl group has been previously explored as a non-conjugated electron-withdrawing group in donor-acceptor thermally activated delayed fluorescence (TADF) emitters. In the present study, we investigate computationally the potential of other fluorine-containing acceptors, trifluoromethoxy (OCF₃), trifluoromethylthio (SCF₃), and pentafluorosulfanyl (SF₅), within two families of donor-acceptor TADF emitters. Time-dependent density functional theory calculations indicate that when only two ortho-disposed carbazole donors are used (Type I molecules), the lowest-lying triplet state possesses locally excited (LE) character while the lowest-lying singlet state possesses charge-transfer character. When five carbazole donors are present in the emitter design (Type II molecules), now both S₁ and T₁ states possess CT character. For molecules 2CzOCF and 5CzOCF , the singlet energies are predicted to be 3.92 eV and 3.45 eV; however, the singlet-triplet energy gaps, ΔE _STs, are predicted to be large at 0.46 eV and 0.37 eV, respectively. The compounds 2CzCF , 2CzSCF , and 2CzSF , from Type I molecules, show significant promise as deep blue TADF emitters, possessing high calculated singlet energies in the gas phase (3.62 eV, 3.66 eV, and 3.51 eV, respectively) and small, ΔE _STs, of 0.17 eV, 0.22 eV, and 0.07 eV, respectively. For compounds 5CzSCF and 5CzSF , from Type II molecules, the singlet energies are stabilized to 3.24 eV and 3.00 eV, respectively, while ΔE _STs are 0.27 eV and 0.12 eV, respectively, thus both show promise as blue or sky-blue TADF emitters. All these six molecules possess a dense number of intermediate excited states between S₁ and T₁, thus likely leading to a very efficient reverse intersystem crossing in these compounds.