Color Changes of a Full‐Color Emissive ESIPT Fluorophore in Response to Recognition of Certain Acids and Their Conjugate Base Anions

2‐(1,3‐Benzothiazol‐2‐yl)‐4‐methoxy‐6‐(1,4,5‐triphenyl‐1H‐imidazol‐2‐yl)phenol (BTImP) is an excited‐state intramolecular proton transfer (ESIPT) fluorophore, containing an acid‐stimuli‐responsive intramolecular hydrogen bond (H‐bond) that can switch from the central phenolic proton to the imidazole (Im) or benzothiazole (BT) nitrogen atoms. Here, we demonstrate that BTImP shows full‐color (red, green, blue, and white) emission upon the addition of different concentrations of HClO4 or, with time, after the addition of HBF4. It also shows thermally dependent color changes from pink through white to blue in a narrow temperature range of 25–60 °C. 1H and 15N NMR measurements suggest that, after the green fluorescent BTImP is protonated at its Im nitrogen atom, a conjugate base anion coordinates to the imidazolium (HIm+) proton, forming two types of complexes with different coordination states. One state shows a significantly Stokes‐shifted red emission resulting from ESIPT at the BT side, whereas the other shows a typical Stokes‐shifted blue emission, probably caused by interaction of the anion with the phenolic proton, which breaks the H‐bond on the BT side. BF4− and ClO4− are effective in forming such a blue emitter, whereas Cl− and PF6− are not; this behavior depends on whether the anion can fit into the bidentate binding site consisting of HIm+ and the phenolic hydroxy group. Now in Technicolor! Stimuli‐responsive full‐color (red, green, blue, and white) emission from a single excited‐state intramolecular proton transfer (ESIPT) fluorophore of BTImP has been achieved by a unique “two‐step intramolecular H‐bond switching” mechanism. This involves acid‐induced positional switching of an H‐bond from the imidazole to the benzothiazole (BT) side, and anion‐induced on‐off switching of the H‐bond on the BT side.