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

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 or orthogonal donors with direct or phenylene-bridged linkages, we realize a tuneable progression of photophysical behaviour - 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 (ΔEST) and enables vibronic coupling between 3CT and 3LE states. Time-resolved spectroscopy confirms a switch from conventional three-level spin flipping to dual channel four-state reverse intersystem crossing (rISC) pathways. OLEDs fabricated with the most decoupled emitter exhibit high external quantum efficiency (~30%) and favourable horizontal alignment (anisotropy factor a ≈ 0.16), validating the design concept at the device level. This unified framework demonstrates how molecular conformation and connectivity together dictate spin dynamics, establishing a blueprint for kinetic control in next-generation TADF materials.