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

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 ZnX2 (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 ΔEST, 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-ZnBr2 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.