Steering the luminescence of donor–acceptor materials by regioisomerism of triazole linkers

Abstract

We examine the impact of triazole connectivity on the photophysical properties of donor–acceptor emitters. In these systems, the electron donor is connected either to the triazole's nitrogen atom (N1, “N-connected”) or carbon atom (C4, “C-connected”), giving rise to regioisomeric pairs with distinct excited-state behaviours. Donor strength and triazole orientation together govern energy levels, charge-transfer character, and delayed emission pathways. N-connected derivatives exhibit higher singlet energies and lower triplet energies compared to their C-connected counterparts, resulting in consistently larger ΔE_ST values. These trends are attributed to disrupted donor–acceptor conjugation through the nitrogen linkage, thereby rendering the acceptor less electron-deficient in the N-connected series. Most compounds favour room-temperature phosphorescence (RTP) due to large singlet–triplet energy gaps (ΔE_ST > 0.4 eV), while phenoxazine C-connected (PXZ-C) uniquely exhibits thermally activated delayed fluorescence (TADF), attributed to its small ΔE_ST of 0.1 eV. Computational studies support these experimental trends, showing that the lowest triplet state is predominantly localised on the quinoxaline acceptor. These findings highlight the critical role of regioisomeric control in tuning excited-state dynamics in TADF materials.