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 (ΔE_ST), 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 ΔE_ST of 0.001 eV and the highest k_TADF 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.