A planar rigid steric group-modified spiropyran derivative with photochromism and mechanochromism for optical printing applications

Abstract

Spiropyran (SP) experiences isomerization from a colorless SP form to a colored merocyanine (MC) form and has received great attention due to its widespread use in optical printing materials and anti-counterfeiting. However, the isomerization mostly occurs in the solution state because of requiring free volume for the change in conformation of the SP molecule, which is hindered in the solid state. Here, we propose a molecular design strategy to link planar rigid carbazole (CZ) and naphthalene (NA) molecules with an SP molecule. Detailed photophysical property analysis and theoretical calculations suggest that the planar molecular conformation of the CZ and NA moieties contributes to a large free volume, promoting isomerization of SP under ultraviolet (UV) irradiation and mechanical grinding. The powder X-ray diffraction (XRD) pattern suggests that NA-SP has more loose intermolecular stacking than CZ-SP, leading to more efficient photo-isomerization. These results are beneficial to understand SP structures and the packing mode-to-stimuli responsive characteristic relationship. Consequently, by utilizing the efficient photochromic properties, we successfully demonstrate the application of CZ-SP and NA-SP as optical printing materials and anti-counterfeiting inks.