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. 1 H and 15 N 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.