Three-in-one: dye-volatile cocrystals exhibiting intensity-dependent photo-chromic, photo-mechanical and photo-carving response

Abstract

Cocrystallization of a cis-azobenzene dye with volatile cocrystal former molecules, such as pyrazine and dioxane, leads to materials that exhibit at least three different light intensity-dependent responses upon irradiation with low-energy visible light. Specifically, the halogen bond-driven assembly of cis-(p-iodoperfluorophenyl)azobenzene with volatile halogen bond acceptors produces cocrystals whose light-induced behaviour varies significantly depending on the intensity of the light applied. Low-intensity (<1 mW∙cm-2) light irradiation leads to a colour change due to low levels of cis-trans isomerization. Irradiation at higher intensities (150 mW∙mm-2) produces photo-mechanical bending, caused by more extensive azo dye isomerization. At still higher irradiation intensities (2.25 W∙mm-2) the cocrystals undergo photo-carving, i.e. they are readily shaped, punctured, and cut with micrometer precision using laser light. This work demonstrates how the recently reported photo-carving behaviour can be combined with different types of photo-responses, providing a design for multi-responsive materials that can respond to different levels of irradiation with optical colour change, photo-mechanical bending, or photo-carving, as laser power is increased