Carbazole-linked through-space TADF emitters for OLEDs: tuning photophysics via molecular architecture and exciton dynamics

Abstract

Thermally activated delayed fluorescence (TADF) offers a promising route to highly efficient organic light-emitting diodes (OLEDs), yet conventional D–A–D and A–D–A architectures often suffer from conformational flexibility, leading to multiple singlet excited states and enhanced non-radiative decay. These effects compromise both emission efficiency and color purity. While multi-resonant TADF (MR-TADF) systems provide improved rigidity, their planar structures favor π–π stacking, causing aggregation-induced quenching (ACQ). This study presents a molecular design strategy integrating a carbazole unit as a rigid, non-planar bridge to mitigate intramolecular rotation and suppress ACQ by disrupting parallel stacking. A set of 21 such D–A–D and A–D–A type molecules was computationally designed and analyzed. The optimized structures exhibit spatially separated frontier orbitals, resulting in small singlet–triplet energy gaps (ΔEST), fast radiative and reverse intersystem crossing rates, and near-unity photoluminescence quantum yields (PLQYs). Exciton dynamics simulations further confirm efficient TADF behavior, while molecular dynamics trajectories reveal conformational stability and through-space charge transfer characteristics. Notably, A1–D3–A1 achieves an exceptionally small ΔEST of 0.001 eV and the highest kTADF of 1.34 × 106 s−1, enabling rapid triplet harvesting, while D1–A2–D3 combines high oscillator strength with efficient TADF dynamics. These results demonstrate that subtle architectural tuning can yield substantial performance improvements, highlighting carbazole-bridged TADF emitters as a pathway toward stable, high-efficiency OLED materials.