Visualization of temperature distribution and flow behavior in water evaporation using water-soluble spiropyran

Understanding the convection flows driven by the uneven temperature distribution in heated aqueous solutions is fascinating and meaningful both in fundamental and industrial aspects. Therefore, it is desirable to develop a simple and convenient method for real-time tracking of surface temperature distribution and the solvent flow state. Herein, we designed and screened a water-soluble spiropyran derivative (BS-SP₁) that contains benzothiazole and sulfonic acid groups. The BS-SP₁ molecule was found to exceptionally maintain the reversible photochromic characteristics in water, and then served as a photo-responsive indicator for visualizing the temperature distribution and flow behavior on water surface with high resolution. This study provides feasible guidance on the dynamic self-assembly and thermodynamics occurring in the water evaporation system and offers a practical tool for investigating these behaviors.