Three-in-One: Dye-Volatile Cocrystals Exhibiting Intensity-Dependent Photochromic, Photomechanical, and Photocarving Response

Abstract

Cocrystallization of a cis-azobenzene dye with volatile molecules, such as pyrazine and dioxane, leads to materials that exhibit at least three different light-intensity-dependent responses upon irradiation with low-power visible light. The halogen-bond-driven assembly of the dye cis-(p-iodoperfluorophenyl)­azobenzene with volatile halogen bond acceptors produces cocrystals whose light-induced behavior varies significantly depending on the intensity of the light applied. Low-intensity (<1 mW·cm–2) light irradiation leads to a color change associated with low levels of cis → trans isomerization. Irradiation at higher intensities (150 mW·mm–2) produces photomechanical bending, caused by more extensive isomerization of the dye. At still higher irradiation intensities (2.25 W·mm–2) the cocrystals undergo cold photocarving; i.e., they can be cut and written on with micrometer precision using laser light without a major thermal effect. Real-time Raman spectroscopy shows that this novel photochemical behavior differs from what would be expected from thermal energy input alone. Overall, this work introduces a rational blueprint, based on supramolecular chemistry in the solid state, for new types of crystalline light-responsive materials, which not only respond to being exposed to light but also change their response based on the light intensity.

The Halogen Bond to Ethers - Prototypic Molecules and Experimental Electron Density

“…The earliest example of a crystallographically investigated halogen bond with an ether acceptor molecule is the study of the structure of 1,4-dioxane and elemental bromine by Hassel et al from 1954 . Since these early analyses, further systems with etheric oxygen acceptors have been found, but many of them are characterized by additional noncovalent intermolecular interactions, like hydrogen bonds, π–π-stacking interactions or N···X-halogen bonds. , Also bivalent etheric halogen bond acceptors like dioxane have been presented …”