Aggregation-assisted energy gap modulation controls delayed emission in hybrid charge-transfer emitters

Abstract

Exploring the impact of molecular aggregation on excited-state energy landscapes is key to tailoring long-lived delayed emission in organic materials. Here, we demonstrate aggregation-assisted energy gap modulation governing the balance between delayed fluorescence (DF) and room-temperature phosphorescence (RTP) in the indole-based hybrid charge-transfer emitters – HD and its brominated analogue BrD. In HD, J-aggregation, facilitated by a collinear molecular arrangement, significantly reduces the singlet–triplet energy gap (ΔE_ST), enhances reverse intersystem crossing (RISC), and increases the efficiency of radiative decay from the regenerated singlet state, thereby favouring DF. In contrast, BrD forms H-aggregates through strong π–π interactions, which increase ΔE_ST, suppress RISC and stabilize triplet excitons, resulting in prominent RTP. Time-resolved spectroscopy and theoretical calculations reveal that the hybridization of local-excited and charge-transfer states mediates excited-state evolution, with J-aggregation promoting DF and H-aggregation favouring RTP. These results establish a direct link between aggregation and distinct photoluminescence pathways, offering a strategy to tune delayed emission in organic materials.