Engineering Spiropyran-Based Ratiometric ¹⁹F MRI Probes with Ultralarge Chemical Shift Variations

Fluorine magnetic resonance imaging (¹⁹F MRI) is a rapidly growing field with significant potential for clinical diagnostics, but it is hindered by inherently low sensitivity. Developing stimuli-responsive ¹⁹F MRI probes with dual-signal capabilities enhances diagnostic sensitivity but remains highly challenging. Herein, we designed pH-activatable ¹⁹F MRI probes based on fluorinated spiropyrans, engineered through spiro-isomerism (pH-triggered merocyanine-spiropyran interconversion) and positional-isomerism (strategically tuning F-substitution positions on the spiropyran core) strategies to achieve unprecedented ratiometric ¹⁹F NMR chemical shift variations (Δδ_F) of up to 13 ppm. The pH-triggered interconversion between fluorinated merocyanine (open-form) and fluorinated spiropyran (closed-form) alters the π-electron conjugation of these two isomers, thereby modulating the fluorine atomic charge density and eliciting measurable Δδ_F. Critically, we demonstrate that positional-isomerism amplifies Δδ_F from 1 to 13 ppm, enabling precise control over the response magnitude. Notably, the probe with the largest Δδ_F (4F-MCH) exhibited pH-dependent proportionality in ¹⁹F NMR signals across the physiologically relevant pH range (5.0-7.0), establishing a robust ratiometric readout. With the unique feature of ultralarge ratiometric Δδ_F, the probe 4F-MCH achieves direct visualization of pH gradients in ¹⁹F MRI, yielding sharp, interference-free, and pH-dose-dependent images. By integration of spiro-isomerism and positional-isomerism strategies, this work establishes a generalizable platform for engineering ¹⁹F MRI probes with tunable, high-contrast ratiometric chemical shift variations, enabling the rational design of ratiometric ¹⁹F MRI probes for biological imaging.