Donor Relocation Enables Simultaneous Strong Absorption and Small Singlet‐Triplet Gap in 1,8‐Naphthalimide Derivatives

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ABSTRACT Photoabsorption in emissive organic molecules requires extensive overlap between orbitals involved in the transition, while charge transfer‐based thermally activated delayed fluorescence (CT‐TADF) demands minimal overlap for a small ∆E ST (singlet‐triplet gap), creating a fundamental CT absorption‐∆E ST trade‐off. Here, we present a molecular example that decouples this effect in 1,8‐naphthalimide (NI) derivatives by changing the position of donor attachment. We synthesized three NI‐derivatives (SD1‐SD3) and demonstrated that the CT absorption is quasi‐dark in SD1 but increases dramatically as the substituent position is changed (SD2 and SD3). In these derivatives, 35‐fs ultrafast spectroscopy reveals vibronic couplings with short‐lived vibronic coherences (∼3 ps, SD1) versus persistent coherences (∼6 ps, SD2 and SD3). While SD1 exhibits dual S 1 /S 2 emission due to competitive kinetics, SD2 and SD3 obey Kasha's rule via slow S 1 decay. Both SD2 and SD3 deliver TADF driven by strong spin‐orbit coupling (4.5–10 cm −1 ) and small ∆E ST . SD2 organic light‐emitting diodes (OLEDs) confirm feasibility (EQE max = 3.2%, V on = 2.8 V, L max = 6900 cd m −2 ). This donor‐relocation principle, which we propose modulates TDM interference, offers a plausible qualitative framework for decoupling the absorption‐TADF trade‐off.

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