Intermolecular interaction on excited-state properties of fluoro-substituted thermally activated delayed fluorescence molecules with aggregation-induced emission: a theoretical perspective

Abstract

ABSTRACT Thermally activated delayed fluorescence (TADF) molecules have shown promising applications in organic light- emitting diodes. Theoretical investigations to reveal the fluoro-substitution and intermolecular interaction on their photophysical properties especially for aggregation-induced emission (AIE) and TADF mechanisms are highly desired. In this work, two efficient TADF molecules SFDBQPXZ and DFDBQPXZ with fluoro-substitution are selected and studied. The solvent and solid environments are considered by polarisable continuum model and the combined quantum mechanics and molecular mechanics (QM/MM) method, respectively. The frontier molecular orbital properties, heat maps, adiabatic singlet–triplet energy gap (ΔEst ) and intermolecular interactions as well as excited-state dynamic process are analysed in both toluene and solid phase. Results show that the small ΔEst and large spin-orbital coupling constant facilitate the up-conversion process. Moreover, small reorganisation energies and large intermolecular interactions are determined in the solid phase, the non-radiative energy consumption processes of SFDBQPXZ and DFDBQPXZ are significantly suppressed in the aggregation state due to the restricted rotations in low-frequency region, the AIE feature is illustrated, and inner mechanisms for higher luminescence efficiency of SFDBQPXZ than that of DFDBQPXZ ae determined. The experimental measurements are reasonably elucidated and the effect of fluoro-substitution on excited-state dynamics is revealed.