ABSTRACT A p-toluenesulfonate salt of benzothiazonium (merocyanine) dye substituted with diethylamine was synthesized to act as an optical probe. The probe was characterized using FT-IR, NMR, HR-MS and examined for a response towards polar molecular and ionic species using UV-Visible, fluorescence spectroscopic techniques. The probe displayed remarkable formic acid-responsive optical and aggregation properties in distilled water and at pH 7.6 (1.0 mM HEPES). The change in color was also exploited for smartphone assisted digital colorimetric analysis of formic acid and formate ions. The formic acid response can be switched off thermally. The emission response of the probe appeared in the visible region. The receptor can also be used to detect the vapors of formic acid reversibly in air. The computational calculations based on M06-2X/6-31+G(d), M06-2X/6-31+G(d,p) and CAM-B3LYP/6-31+G(d,p) methods were performed to investigate the complex formation at a molecular level. The time-dependent CAM-B3LYP/6-31+G(d,p) method-based calculations were also implemented to study the observed absorption spectra of the complex.