Sulfur atom substitution strategy achieves deep-red emission from a compact organic emitter

Achieving deep-red (DR) emission from compact organic molecules remains challenging, as traditional strategies require extended π-conjugation or strong donor–acceptor (D–A) interactions, leading to broad spectra and high molecular weights. Here, we demonstrate a simple one-step sulfur substitution that converts a deep-blue emitter (CzCO) into a thionated derivative (CzCS), which exhibits activated nπ* emission in the DR region when embedded in a rigid host matrix. The replacement of the oxygen of carbonyl with sulfur switches the lowest excited state from ππ* to nπ*, with the HOMO localized on the sulfur lone pair and the LUMO delocalized over the π framework. The rigid matrix suppresses nonradiative decay, yielding a photoluminescence peak at 674 nm with a full width at half-maximum (FWHM) of 70 nm, smaller than that of conventional D–A-type DR emitters (>100 nm). The corresponding OLED emits at 672 nm with an FWHM of 84 nm and a maximum external quantum efficiency approaching 1%, while its molar mass is only 397 g mol –1 . Notably, simple sulfur substitution shifts the electroluminescence from 432 nm (CzCO) to 672 nm (CzCS), a redshift of 240 nm. This work provides a cost-effective and straightforward molecular design to access nπ*-based DR emission through sulfur atom substitution.