Turn on of sky-blue thermally activated delayed fluorescence and circularly polarized luminescence (CPL) via increased torsion by a bulky carbazolophane donor

Abstract

The carbazolophane (Czp) donor unit (indolo[2.2]paracyclophane) is introduced to the design pool of donors in thermally activated delayed fluorescence emitters. The increased steric bulk of the annelated donor unit forces an increased torsion between the carbazole and the aryl bridge resulting in a decreased ΔE_ST and an enhancement of the thermally activated delayed fluorescence in the triazine-containing emitter CzpPhTrz. Further, the closely stacked carbazole and benzene units of the paracyclophane show through-space π–π interactions, effectively increasing the spatial occupation for the HOMO orbital. The chiroptical properties of enantiomers of [2.2]paracyclophane reveal mirror image circular dichroism (CD) and circularly polarized luminescence (CPL) with g_lum of 1.3 × 10⁻³. rac-CzpPhTrz is a sky-blue emitter with λ_PL of 480 nm, a very small ΔE_ST of 0.16 eV and high Φ_PL of 70% in 10 wt% doped DPEPO films. Sky blue-emitting OLEDs were fabricated with this new TADF emitter showing a high maximum EQE of 17% with CIE coordinates of (0.17, 0.25). A moderate EQE roll-off was also observed with an EQE of 12% at a display relevant luminance of 100 cd m⁻². Our results show that the Czp donor contributes to both a decreased ΔE_ST and an increased photoluminescence quantum yield, both advantageous in the molecular design of TADF emitters.