Aza‐Coating Heptamethine Cyanines for Sensing pH Dynamics of Plants

ABSTRACT As a key parameter in organismal growth and development, pH is essential for maintaining internal environmental homeostasis, ensuring normal metabolic processes, and coordinating the functions of various organ systems. In this study, we developed a series of aza‐IR780‐based heptamethine cyanine ketones that exhibit remarkable pH‐responsive behavior, governed by keto–enol tautomerism occurring along the conjugated polyene backbone of the cyanine scaffold. Under neutral conditions, these cyanines predominantly adopt the keto form, displaying a fluorescence emission maximum at approximately 640 nm. In acidic environments, however, they shift to the enol form, resulting in a distinct fluorescence redshift to around 740 nm. This pronounced pH‐dependent spectral response enables real‐time visualization and monitoring of pH fluctuations within plant tissues. Our work thus offers a promising molecular tool for noninvasively imaging the microenvironment in plants.