Unlocking Spin Dynamics: Spin‐Orbit Coupling Driven Spin State Interconversion in Carbazole‐Containing TADF Emitters

Abstract The determination of transport mechanisms in organic light‐emitting diodes (OLEDs) is crucial for optimizing device performance. Magnetic field measurements enable the differentiation of spin state interconversion mechanisms, but data interpretation remains challenging. Here, experimental and theoretical investigations are combined to provide a comprehensive understanding of the underlying processes. This study systematically compares three cyanoarene‐based emitters with different singlet–triplet gaps (Δ E ST ) to explore factors influencing reverse intersystem crossing (RISC). The comparison of all‐ 1 H and all‐ 2 H 4CzIPN isotopologues confirms that RISC is governed by spin‐orbit coupling (SOC) rather than hyperfine interactions. Magnetic field‐dependent measurements reveal that charge transport in OLED devices is driven by triplet‐charge annihilation in 3CzClIPN and 4CzIPN, while triplet–triplet annihilation dominates for 5CzBN. Theoretical calculations further indicate that SOC‐mediated RISC in 3CzClIPN and 4CzIPN can additionally occur via a T 2 intermediate state with an activation energy distinct from Δ E ST . A temperature‐dependent analysis of the devices was conducted to quantify this activation energy and compare it with the computational findings. These findings establish key correlations between activation energy, spin dynamics, and magnetic field effects in TADF emitters, advancing the understanding of excitonic processes in OLEDs.