Arylazopyrazole Photoconversion Enables Tunable Morphology and Mechanical Properties

A red-light switchable fast, near-quantitative two-way photoswitching of an azoheteroarene, arylazo-N-phenyl-3,5-diphenyl pyrazole (AAP1), with a high thermal half-life of the cis-isomer is reported here. The photochromic molecule switches in the solution phase and even in thin films. The design of AAP1 incorporates structural features that enable cis → trans reversal under red light quantitatively. Solid-state photoswitching in pristine thin films has been demonstrated using AAP1. Various photo patterns on paper with AAP1 as the photochromic ink are generated and their switching displaying the change in color is investigated. Single-crystal XRD structures of the trans-isomer as well as, the more exciting and rarely observed form, the cis-isomer of AAP1 is obtained. The SCXRD analysis reveals that the phenyl groups in the pyrazole ring engage in an intramolecular π-stacking interaction only in the cis-isomer which significantly enhances its thermal stability. The needle-shaped crystals of the thermodynamically stable trans-AAP1 display exceptional elastic mechanical flexibility, whilst the plate-like cis-isomers are brittle. The quantitative difference of the mechanical properties of both the isomers is exhibited by nanoindentation experiments. The trans crystals are significantly softer than the cis ones and a roughly three-fold difference in mechanical characteristics is identified.