Morpholine-Substituted Rhodamine Analog with Multi-Configurational Switches for Optical Sensing of pH Gradient under Extreme Acidic Environments.

Superior pH-responsive molecules are required for the development of functional materials applicable to advanced molecular technologies. Despite having been widely developed, many rhodamine-based pH-responsive molecules exhibit a single configurational switch for "turn-on." Herein, we report a new type of rhodamine-based pH-responsive molecule with multi-configurational switches displaying stable two-step structural and color conversion in response to pH. This rhodamine analog could be successfully applied to optical sensing of pH gradient under extreme acidic environments both in solution and on hydrogel through high-contrast color change. We demonstrated that this multi-responsive character enabled optical memory of different pH information.