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Color Changes of a Full‐Color Emissive ESIPT Fluorophore in Response to Recognition of Certain Acids and Their Conjugate Base Anions
Author(s) -
Tsuchiya Saki,
Sakai Kenichi,
Kawano Keiichi,
Nakane Yuta,
Kikuchi Takemitsu,
Akutagawa Tomoyuki
Publication year - 2018
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201705622
Subject(s) - chemistry , intramolecular force , photochemistry , protonation , hydrogen bond , stokes shift , fluorescence , fluorophore , proton , molecule , ion , stereochemistry , organic chemistry , physics , quantum mechanics
2‐(1,3‐Benzothiazol‐2‐yl)‐4‐methoxy‐6‐(1,4,5‐triphenyl‐1 H ‐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 HClO 4 or, with time, after the addition of HBF 4 . 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. BF 4 − and ClO 4 − are effective in forming such a blue emitter, whereas Cl − and PF 6 − are not; this behavior depends on whether the anion can fit into the bidentate binding site consisting of HIm + and the phenolic hydroxy group.