Structural Modulation of Charge Transfer in Donor–Acceptor Systems: Effects of Donor Type and Connectivity

ABSTRACT Molecular geometry is a powerful lever for steering excited‐state dynamics in thermally activated delayed fluorescence (TADF) systems. We present a modular phthalimide‐based emitter platform that systematically decouples donor planarity and donor–acceptor (D–A) connectivity to sculpt distinct excited‐state topologies. By combining planar (2D) or non‐planar (3D) donors with direct or phenylene‐bridged linkages, we realize a tunable progression of photophysical behavior—from prompt fluorescence and phosphorescence to fast and locally excited (LE)‐mediated slow TADF. Quantum‐chemical calculations reveal how structural modulation governs singlet‐triplet energy gap (Δ E ST ) and enables vibronic coupling between 3 CT and 3 LE states. Time‐resolved spectroscopy confirms a crossover from an effective three‐state description to 3 LE‐involving, multi‐state reverse intersystem crossing (rISC) pathways. OLEDs fabricated with the most decoupled emitter reach state‐of‐the‐art efficiencies for phthalimide‐based TADF systems (EQE max ≈ 36%) and exhibit pronounced horizontal dipole orientation (anisotropy factor a ≈ 0.16). Together, these results establish direct links between molecular conformation/connectivity, excited‐state topology, and device‐level performance within an imide‐based platform, providing valuable structure‐property relations for tuning TADF kinetics.