Quantum Chemical Insights Into Tunable Charge Transfer and Singlet–Triplet Splitting in TADF Emitters

Thermally activated delayed fluorescence (TADF) materials hold the key to next-generation optoelectronics by converting both singlet and triplet excitons into light via rapid reverse intersystem crossing (RISC). However, designing chromophores that simultaneously feature a small singlet-triplet gap (ΔE_ST), strong charge-transfer (CT) character, and high photoluminescence efficiency remains elusive, as conventional donor-acceptor scaffolds suffer from nonradiative losses. Here, we harness through-space charge transfer (TSCT) architectures, which spatially separate donor and acceptor units to suppress vibrational quenching and boost RISC, to overcome this trade-off. We pair an N-(4-methylphenyl)-1,8-naphthalimide acceptor with a suite of heteroatom-modified 9,9-dimethyl-9,10-dihydroacridine donors (D-A), including a dual-donor (D-A-D) analogue, and systematically tune electronic coupling and molecular rigidity. To evaluate the photophysical implications of these structural modifications, we performed quantum-chemical analyses of ground and excited-state parameters (S₁, T₁, ΔE_ST, ΔE_HL), along with natural transition orbital and energy decomposition studies to probe the nature of electronic transitions. Spin-orbit coupling constants and ISC/RISC rate estimates were calculated to assess the excited-state mechanism, while electron-hole correlation metrics quantified the extent of charge separation. Together, these descriptors offer a comprehensive understanding of how donor identity, spatial arrangement, and conjugation control TADF behavior.