AbstractThe cationic nature of heptamethine cyanines gives them the capacity to form aggregates with salts by electrostatic interactions. In this work, NaCl promoted J‐aggregate formation of aza‐coating heptamethine cyanines is explored. NaCl can induce theN‐benzyloxycarbonylCy‐CO2Bzto assemble into a J‐aggregate having an absorption at 890 nm. Its excellent fluorescence response to NaCl implies that it has great potential for use as a probe for tracing salt stress in plants. Moreover, NaCl also promotes formation of J‐aggregates from th...