Crystal structure and thermoresponsive luminescence of a 9,10-bis(phenylethynyl)anthracene-based cyclophane

Abstract

Introducing a cyclic structure to luminophores is a promising approach for achieving external stimuli-responsive luminescence. This is because luminescent cyclophanes containing flexible linkers tend to form several molecular assembled states. However, previously reported cyclophanes exhibiting thermoresponsive and/or mechanoresponsive luminescence have not given crystals suitable for single crystal X-ray structure analysis because of the flexible cyclic molecular structures. Such analysis is important because solved crystal structures can show unambiguous correlation between the arrangement of luminophores and photophysical properties. Here, we report the crystal structure of a cyclophane featuring a 9,10-bis(phenylethynyl)anthracene group and the thermoresponsive luminescence. The cyclophane was designed with shorter flexible oligo(ethyleneglycol) chains used as linkers bridging the luminophore and another aromatic group. The two different π-conjugated groups were orthogonally arranged in the individual molecule, and the luminophores partially overlapped between adjacent molecules. The cyclophane showed a supercooled nematic phase at room temperature upon cooling. Thermal treatment for the kinetically trapped state led to a transition to another crystalline state and, consequently, a change in photoluminescence colour. Emission spectroscopic studies and emission lifetime measurements revealed that the luminophores formed excimers in the supercooled nematic phase, whereas no excimer formation was observed for the crystalline phases.