The effect of torsion-inducing methyl substituents on the delayed luminescence of star-shaped emitters

Abstract

We probed the impact of metaagainst para-conjugation in donor-acceptor emitters containing phenylene-linked carbazole and 1,3,5-triazine moieties and their effect on thermally activated delayed fluorescence (TADF). The influence of torsionenhancing methyl substitution of the phenylene linkers was also investigated. We found that the delayed emission properties change dramatically upon these structural modifications: Due to deteriorated oscillator strength, the quantum efficiency drops strongly when torsion comes very close to orthogonality upon double methyl substitution; only para bridging allows coexistence of delayed fluorescence with room temperature phosphorescence in the solid; the smallest singlet-triplet energy gap (ΔES-T) and the fastest TADF result from methyl-substituted meta-bridges, whereas by far the greatest emission quantum yields are observed upon methyl-substituted para-bridging. The optical characterizations in solid film as well as in oxygen-containing and oxygen-free solution were supported by theoretical calculations to determine the torsion angles and the spatial distribution of the frontier orbitals, quantify the excited state energies, rationalize the experimental spectra and identify the locally excited (LE) or charge transfer (CT) nature of the observed excitations. This work is of fundamental interest for the understanding of delayed emission based on a simple set of structural ingredients.