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

• Published in: CrystEngComm Vol. 16; no. 15; pp. 3180 - 3185
• Main Authors: Sakai, Ken-ichi, Kawamura, Hiroshi, Kobayashi, Noriyuki, Ishikawa, Takaaki, Ikeda, Chiori, Kikuchi, Takemitsu, Akutagawa, Tomoyuki
• Format: Journal Article
• Language: English
• Published: 01.01.2014
• Subjects: Chemical compounds; Colors (materials); Coupling (molecular); Crystals; Derivatives; Excitation; Fluorescence; Spectra
• ISSN: 1466-8033; 1466-8033
• DOI: 10.1039/c3ce42109k

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 ( lambda sub(max)) at 619 nm. However, in the crystalline state, the lambda sub(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 ( lambda sub(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 pi -conjugation and no bulky groups.