A General Strategy to Develop Intramolecular Spirocyclic Boron Dipyrromethene Fluorophores for Self-Blinking...

Intramolecular spirocyclic fluorophores, with their controllable on-off switching, have enabled diverse stimuli-responsive probes and serve as cornerstones of self-blinking super-resolution imaging. However, their design has been largely restricted to Rhodamine frameworks, while boron dipyrromethene (BODIPY) dyes, renowned for their superior photostability, brightness, and near-infrared tunability, remain underutilized due to the inherent rigidity of the indacene core. To overcome this limitation, we developed a general molecular engineering strategy to confer BODIPY with spirocyclic functionality: Introducing 2-(hydroxymethyl)phenyl or 2-carboxyphenyl groups at the meso-position as intramolecular nucleophiles, coupled with electron-withdrawing groups at α/β-positions to modulate core electron density and enhance meso-carbon's electrophilicity. We present the first single-crystal X-ray structure of a spirocyclized BODIPY, unambiguously confirming the closed form and its blinking mechanism. By tuning this system, we established a clear pK_a-structure-activity relationship, enabling precise prediction and customization of the open-closed equilibrium. The resulting spirocyclization-capable BODIPYs exhibited exceptional brightness up to ∼87,122 M^-1 cm^-1, and enable self-blinking super-resolution imaging of mitochondria and lysosomes in living cells under low-power laser density (≥32 W cm^-2) without requiring imaging-enhancing buffers. This work pioneers the extension of intramolecular spirocyclization to the BODIPY scaffold, establishing a general design strategy for a new class of high-performance, stimuli-responsive probes.