Descriptor-Guided Design of High-Efficiency Narrowband Phosphorescent Probes for Bioimaging.

Organic room-temperature phosphorescence (ORTP) with narrow emission bands is highly attractive for high-contrast biomedical imaging. However, most ORTP systems suffer from low efficiency due to weak spin-orbit coupling, and their emission spectra are often broadened by strong electron-vibration coupling. Herein, we propose a descriptor-guided design framework to achieve efficient and narrowband ORTP through intramolecular noncovalent conformational locks (NoCLs). Two fundamental descriptors are established to link structure to photophysical properties: the NoCL strength S, a rigidity metric that governs electron-vibration coupling and dictates spectral bandwidth, and the orbital character γ, which controls intersystem crossing and determines triplet-state abundance. Using these descriptors, we design a series of benzophenone derivatives bearing NoCL and identify the 2SOBr system, which exhibits the highest phosphorescence efficiency of 58.22% among the explored narrowband ORTP emitters, with a full width at half maximum 36 nm under ambient conditions. As a biological probe, 2SOBr enables lysosome-specific cellular imaging with high signal-to-noise ratios of 15.93, outperforming commercial green fluorescent probes (<10). This work establishes a quantitative structure-descriptor-property relationship that provides a strategy for the design of efficient narrowband organic phosphorescent materials and expands their potential for high-contrast optical imaging.