Control of Conformation and Spin for Programmable Singlet‐Triplet Exciton Balance in Through‐Space Charge‐Transfer Emitters

ABSTRACT Achieving continuous and predictable control over singlet‐triplet exciton partitioning in solution remains challenging for organic molecules, partly because electronic structure, conformational dynamics, and nonradiative decay are intertwined. Here we introduce a dual‐key, coordination‐activated through‐space charge‐transfer (TSCT) platform that enables programmable modulation of spin‐dependent excited‐state pathways in solution. Flexible donor‐σ‐acceptor ligands Cz‐nC‐TPP comprising a carbazole donor (Cz) and a terpyridine acceptor (TPP) linked by alkyl spacers ( n = 2, 4, 6, 8) predominantly emit from local excited states and show negligible oxygen sensitivity before metal binding. Upon coordination with ZnX 2 (X = Cl, Br, I), a TSCT channel is activated without covalent re‐engineering, producing microsecond delayed fluorescence consistent with TSCT‐based thermally activated delayed fluorescence. Spacer length acts as a geometric key that regulates access to folded TSCT geometries and Δ E ST , whereas halide identity serves as a spin key that tunes spin‐orbit coupling and triplet involvement. Together, these inputs enable predictable modulation of delayed, triplet‐associated emission across the series. Notably, the Cz‐6C‐TPP‐ZnBr 2 complex shows the best balance between delayed‐emission prominence and overall brightness. This programmability enables a modular library of dissolved‐oxygen probes with turn‐off and ratiometric responses, visible color evolution, low detection limits, and rapid, reversible responses.