Mechanochromic luminescent liquid crystals based on a bianthryl moiety

Abstract

Smectic liquid crystals consisting of an aromatic moiety based on 10,10′-bis(phenylethynyl)-9,9′-bianthryl at the central part and one and two mesogenic moieties on each side connected through oxyethylene spacers have been prepared to develop stimuli-responsive materials. The compound having two mesogenic moieties on each side (i.e. four total) forms an ordered smectic phase. Mechanochromic luminescence is observed for the smectic liquid crystal. A sample of this molecule formed by thermal annealing shows green photoluminescence at room temperature. The photoluminescent color changes from green to blue-green with mechanical stimulation at room temperature. The change of the photoluminescent color by mechanical shearing is ascribed to the disturbance of the π–π interactions between the adjacent molecules. Moreover, the original green photoluminescent color is recovered by heating followed by cooling to room temperature. The effects of the number, and thus volume of mesogenic moieties were also examined. Smaller mesogenic moieties lead to stronger ground state interactions between adjacent luminescent molecules and red-shifted emission. In contrast, the compound having three mesogenic moieties on each side shows no mechanochromic luminescence. The large mesogenic moieties disturb intermolecular interactions, resulting in no detectable change in the assembled structure of luminescent moieties upon mechanical stimulation. These results suggest that the present molecular design is useful for fabricating new stimuli-responsive materials with liquid-crystalline behavior.