Cyclophanes that Change the Fluorescence Properties on the Phase Transitions

Abstract

Cyclophanes comprise aromatic groups and aliphatic linkers bridging the aromatic moieties. Photoluminescent cyclophanes have been mainly investigated in solution because formation of inclusion complexes with guest ions or molecules changes the photophysical properties. In contrast, stimuli-responsive luminescent properties of luminescent cyclophane in the crystalline or liquid-crystalline states have not been explored well, though cyclophanes would be promising candidates of thermo- and/or mechanoresponsive luminescent materials. In this contribution, recent research progress concerning luminescent cyclophanes prepared in my group is introduced. A 1,6-bis(phenylethynyl)pyrene-based cyclophane featuring hexaethyleneglycol linkers shows a nematic liquid-crystalline phase showing blue-green emission due to the excimer formation. Rapid cooling from the nematic phase leads to appearance of a supercooled nematic phase and both the blue-green emission and the nematic molecular order are well retained. Subsequent annealing procedure induces a phase transition to a crystalline phase showing blue emission. A 9,10-bis(phenylethynyl)anthracene-based cyclophane having tetraetylene glycol linkers have been found to form two different quasipolymorphs. One crystalline state exhibits two-step mechanochromic luminescence. Another crystal contains chloroform as the guest molecules and mechanical or thermal stimuli induces release of the guest molecules. Differential scanning calorimetry and thermogravimetric analysis clarified the phase transition behavior of the luminescent cyclophanes.