Acid/base-switched solid-state fluorochromism and excitation-wavelength-dependent fluorescence color tuning have been demonstrated using an ESIPT fluorophore with a switchable intramolecular hydrogen bond.

An excited-state intramolecular proton transfer (ESIPT) fluorophore, 2,6-bis(benzothiazol-2-yl)phenol, was modified with alkoxy groups at the 4-position to obtain the methoxy (OMe), ethoxy (OEt), propoxy (OPr), and butoxy (OBt) derivatives. The derivatives exhibit bright red fluorescence in chloroform, giving the same fluorescence spectra with a maximum (λ max ) at 619 nm. However, in the crystalline state, the λ max values of OMe and OEt are bathochromically shifted, producing a deeper red color, whereas those of OPr and OBt are hypsochromically shifted producing an orange color. X-ray analysis of the OMe and OPr crystals shows that OMe molecules interact strongly with each other through sulfur-sulfur contacts, whereas the OPr molecules are stacked in an eclipsed arrangement. Assuming that the OMe and OPr crystals are J-and H-aggregates, respectively, the difference in solid-state fluorescence could be explained by the Davydov exciton coupling theory. The OEt derivative was the best solid-state red fluorophore (λ max = 633 nm) with a fluorescence quantum yield of 0.32. Therefore, ESIPT fluorophores are promising for developing a highly efficient solid-state red-emitting material with relatively small π-conjugation and no bulky groups.