Mechanoresponsive luminescence and liquid-crystalline behaviour of a cyclophane featuring two 1,6-bis(phenylethynyl)pyrene groups

Abstract

Cyclophanes have been attractive targets over recent decades because of their fascinating molecular structures and inherent ability to act as supramolecular hosts. However, cyclophanes that exhibit stimuli-responsive luminescence in their condensed states are still very rare, although luminescent cyclophanes could be suitable platforms for sophisticated photofunctional molecular assembled materials. We here report that a pyrenophane featuring two π-extended pyrene groups exhibits mechanoresponsive luminescence in the solid states and shows a nematic liquid-crystalline phase at elevated temperature. Two 1,6-bis(phenylethynyl)pyrene moieties form intramolecular excimers in a diluted chloroform solution, whereas no clear intra- and/or intermolecular excimer formation was observed for the solid state accessed through slow cooling from the nematic phase. Photoluminescence spectroscopic measurements, emission lifetime measurements, and X-ray diffraction patterns confirmed that mechanical stimuli result in conversion to much less ordered molecular assembled states in which some luminophores form excimers, leading to changes in photoluminescence colours.