Molecular Configuration Fixation with C–H···F Hydrogen Bonding for Thermally Activated Delayed Fluorescence Acceleration

Suppressing dimer formation by increasing conformational freedom in multi-carbazole thermally activated delayed fluorescence emitters

“…In recent work by Woo et al 53 DFT calculations have shown that carbazole units around a highly congested central ring instead have dihedral angles of >60°, similar to those in 4CzCNPy and 4CzTPN. High torsion angles, >60°, of carbazole units have also been observed in the single crystal structure of a highly crowded TADF emitter in work by Yuan et al 34 The relaxed (flatter) dihedral angles of the carbazole units in 2Cz2CNPyz, 3CzCNPyz, and 4CzPyz mean that some C-H bonds will protrude at shallower angles from the compound preventing regular stacking of neighbouring molecules. For 4CzCNPy, 4CzTPN and 4CzIPN the carbazole units are more perpendicular meaning the C-H bonds protrude in such a way as to leave the π systems exposed to dimerization.…”

“…1), are prone to dimerization, which may also be responsible for their mechanochromism [24][25][26][27][28] and thermochromism. 23 The investigation by Etherington et al has been followed by further detailed investigation into the stimuliresponsive behaviour of TADF emitters 13,[29][30][31][32][33][34] and a recent minireview. 35 This previous work established that dimerization causes significant shifts in the photoluminescence and electroluminescence colour ( 1 CT emission energy) of the TADF materials, beyond that expected by packing and polarizability effects alone.…”

Intramolecular‐Locked High Efficiency Ultrapure Violet‐Blue (CIE‐y <0.046) Thermally Activated Delayed Fluorescence Emitters Exhibiting Amplified Spontaneous Emission

“…[ 17–25 ] To effectively minimize singlet‐triplet splitting energy (Δ E ST ), so far, most efficient TADF emitters are constructed by twisted donor–acceptor (D–A) structures to separate frontier molecular orbitals. [ 26–33 ] However, such architecture often accompanies a large Stokes‐shift and a broadened spectrum (>70 nm) because of the structural relaxation at the singlet excited state ( S 1 ) state and vibronic coupling between S 1 and the ground state ( S 0 ), which brings about detrimental effects on the luminescence purity. [ 34–39 ]…”

Exploring the possibility of using fluorine-involved non-conjugated electron-withdrawing groups for thermally activated delayed fluorescence emitters by TD-DFT calculation

“…The first family consists of D–A–D (Type I) molecules containing two carbazole donors disposed each ortho to the acceptor group, while the second family consists of five carbazole donors substituted about a central benzene ring and the sixth position occupied by the acceptor moiety (Type II). Adachi et al have shown that compounds that fall within the Type I family can simultaneously show high singlet and triplet energies and small Δ E ST while compounds that are a part of Type II family possess a more dense number of low-lying excited states [ 22 ], the presence of which has been shown to assist in the rISC process through spin-vibronic coupling [ 23 – 24 27 ]. The energy levels and electronic configurations of S 1 and T 1 in these molecules were analysed and we found that compounds possessing either SCF 3 and SF 5 groups as acceptors ( 2CzSCF 3 / 2CzSF 5 in Type I, 5CzSCF 3 / 5CzSF 5 in Type II), possessed LUMOs that are mainly located on the central benzene ring and the acceptor group while the HOMOs are mainly localized on the carbazoles, thereby leading to small Δ E ST s. The calculated Δ E ST s for 2CzSCF 3 / 2CzSF 5 are 0.22 eV and 0.07 eV, respectively, which are comparable to the calculated results for 2CzBN (0.18 eV) and 2CzTRZ (0.08 eV); likewise, the calculated Δ E ST s for 5CzSCF 3 / 5CzSF 5 are 0.27 eV and 0.12 eV, respectively, which are close to the calculated results of 5CzBN (0.20 eV) and 5CzTRZ (0.17 eV).…”

“…The promising performance of emitters possessing a CF 3 acceptor group prompted us to investigate other fluorinated weakly-conjugated acceptor units in order to assess their potential within TADF emitter design (Figure 1) [25][26][27]. In the present study, we report on the impact of incorporating other fluorine-containing electron-withdrawing groups beyond trifluoromethyl (CF 3 ), including trifluoromethoxy (OCF 3 ), trifluoromethylthio (SCF 3 ), and pentafluorosulfanyl (SF 5 ) groups, and explore their potential computationally within TADF emitter design.…”