Highly efficient solid-state red fluorophores using ESIPT: crystal packing and fluorescence properties of alkoxy-substituted dibenzothiazolylphenols

Abstract

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.