Photosalient and thermosalient crystalline hemithioindigo-anthracene based isomeric photoswitches

Abstract

Hemithioindigos are emerging photoswitches that have been found to undergo either [2+2] cycloaddition or reversible photoisomerization reactions. In this work we present a hemithioindigo photoswitch modified with an anthracene moiety, 2-[(anthracen-9-yl)methylidene]-2,3-dihydro-1-benzothiophen-3-one (HTI-An) and report the responses to thermal and light stimulation. In solution and in the crystalline state, HTI-An was found to selectively undergo a reversible Z–E photoisomerization reaction. Upon exposure of crystals of (Z)-HTI-An to UV light a photochromic effect is observed, while heating at 130 °C prompted a phase transition coupled to a thermosalient response. Upon visible light exposure, (E)-HTI-An crystals undergo a photosalient response, where heating to 180 °C induces thermal isomerization reversion while preserving the crystal structural integrity. Thus, HTI-An is an example of a photosalient and thermosalient responsive photoswitch adding to the library of stimuli responsive mechanical compounds.