Crystal structure of a 1,6-bis(phenylethynyl)pyrene-based cyclophane that exhibits mechanochromic luminescence

Abstract

Cyclophanes featuring luminophores are considered promising candidates as thermal or mechanical stimuli-responsive luminescent materials. The mechanism of the changes in the photophysical properties on the crystal–crystal or crystal–amorphous phase transitions can be easily clarified, owing to the crystal structure of the cyclophanes. However, in the case where flexible aliphatic chains are introduced to the target cyclophane, the procurement of the crystals suitable for single-crystal X-ray analysis is challenging. Conversely, the introduction of significantly shorter linkers between large π-conjugated groups leads to difficulties in compound purification, resulting in incorrect photophysical properties, particularly in the solid state. Herein, we report the crystal structure of a 1,6-bis(phenylethynyl)pyrene-based cyclophane and mechanoresponsive luminescence. The cyclophane has a naphthalene group as another aromatic group, and the two different π-conjugated groups are bridged by tetraethylene glycol linkers. Single-crystal X-ray analysis revealed that the pyrene and naphthalene moieties form intramolecular π-stacked structures in the crystals, and intermolecular excimer formation of the luminophores was not observed. The arrangement of the isolated luminophore results in light-blue emission with a well-resolved vibronic structure in the fluorescence spectrum. Mechanical grinding induces a phase transition from crystal to amorphous, which was confirmed by the powder X-ray diffraction measurements, and the emission colour turns green. The photoluminescence spectroscopy and emission lifetime measurements clarified that the green emission is ascribed to intermolecular excimer formation. Subsequent thermal treatment recovers the initial light-blue-emissive crystalline state. The crystalline phase is thermodynamically stable, whereas the amorphous state is thermodynamically metastable.