Single Atom Stabilization of Phosphinate Ester‐Containing Rhodamines Yields Cell Permeable Probes for Turn‐On Photoacoustic Imaging**

Abstract Photoacoustic imaging (PAI) is an emerging imaging technique that uses pulsed laser excitation with near‐infrared (NIR) light to elicit local temperature increases through non‐radiative relaxation events, ultimately leading to the production of ultrasound waves. The classical xanthene dye scaffold has found numerous applications in fluorescence imaging, however, xanthenes are rarely utilized for PAI since they do not typically display NIR absorbance. Herein, we report the ability of Nebraska Red ( NR ) xanthene dyes to produce photoacoustic (PA) signal and provide a rational design approach to reduce the hydrolysis rate of ester containing dyes, affording cell permeable probes. To demonstrate the utility of this approach, we construct the first cell permeable rhodamine‐based, turn‐on PAI imaging probe for hypochlorous acid (HOCl) with maximal absorbance within the range of commercial PA instrumentation. This probe, termed SNR 700 ‐HOCl , is capable of detecting exogenous HOCl in mice. This work provides a new set of rhodamine‐based PAI agents as well as a rational design approach to stabilize esterified versions of NR dyes with desirable properties for PAI. In the long term, the reagents described herein could be utilized to enable non‐invasive imaging of HOCl in disease‐relevant model systems.