To create a new series of mechanofluorochromic materials and to elucidate the mechanism of the phenomenon of mechanofluorochromism, 1-alkanoylaminopyrenes including 1-acetylaminopyrene (AAPy), 1-octanoylaminopyrene (OAPy), and 1-stearoylaminopyren (SAPy) were prepared. It was found that these materials exhibited mechanofluorochromism with emission colors in the crystalline samples changing reversibly from bluish purple to yellowish green, which could be induced by mechanical grinding. X-ray crystal structure analysis, electronic absorption, and fluorescence spectroscopies, as well as fluorescence lifetime analysis and powder X-ray diffraction analysis of AAPy suggested that the present mechanofluorochromism was caused by developing crystal defects through grinding. Intermolecular hydrogen bonds were suggested to play an important role in the occurrence of mechanofluorochromism, suppressing the face-to-face overlapping of pyrene moieties to form excimers in the pristine crystal.
Binary films composed of azobenzene-based amorphous molecular materials and p-toluene sulfonic acid were found to exhibit drastic and reversible colour change when we breathed onto their films.
We have previously reported that crystalline samples of 1-acetylaminopyrene (AAPy), 1-octanoylaminopyrene (OAPy), and 1-stearoylaminopyrene (SAPy) exhibited mechanochromic luminescence. In the present study, we have found that these 1alkanoylaminopyrenes adsorbed onto cellulose papers also exhibited mechanochromic luminescence; however, their behaviors were essentially different from those observed for their crystalline samples. In addition, these cellulose papers were found to exhibit reversible emission color change upon alternate exposure to ethanol and water vapors.
We have found that fluorescence emission of binary films composed of fluorescent amorphous molecular materials, 4-[bis(4methylphenylaryl)amino]benzaldehyde (BMABA) and 4-[bis(4methylphenylaryl)amino]acetophenone (BMAAP), with p-toluenesulfonic acid (TsOH) was quenched at ambient dry atmosphere but enhanced when we breathed onto their films. When pausing the breath, the emission was again quenched. Moisture in the exhaled breath was suggested to play a role for such phenomena. It was suggested that some BMABA and BMAAP molecules were protonated in their films as prepared at ambient dry atmosphere and that protonation and deprotonation took place corresponding to change in surrounding humidity, resulting in modulation of emission intensity.
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