Photochromism and Photomagnetism in 1,4-bis(4,5-diphenyl-imidazolyl)benzene Chromophores: Water-Assisted π-Stacks for the Generation of Stable Free Radicals in the Solid State
Subhajit Saha, Sudipta Khamrui, Rajib Moi, Anakuthil Anoop, Владимир В. Чернышев, Debamalya Banerjee, Kumar Biradha
Abstract
Photochromism and photomagnetism are two important properties for the generation of smart materials that are useful for optical switches and memory devices. Very few materials, mostly containing metal ions, are known to date to exhibit both properties at room temperature. Herein, we report a class of bis-imidazole-based organic molecules that show reversible photochromism and photomagnetism at room temperature due to the generation of free radicals in the solid state. 1,4-Bis(4,5-diphenyl-1 H -imidazol-2-yl)benzene ( BDPI ) was found to act as an organic photomagnet without the use of any traditional photochromic couplers or radical containing moieties. Solvent molecules (e.g., water and DMSO) form hydrogen bonds with the imidazole rings that act as a binding glue in the stacking of 1,4-bis(imidazolyl)benzene moieties via π···π interactions. These interactions were found to play a significant role in promoting strong spin–spin interaction in the solid state that can be inferred from the appearance of a broad EPR signal, whereas a sharp signal appeared only after irradiation. The sharp signal indicates the formation of paramagnetic free radicals that influence antiferromagnetic exchange interactions with significant weakening of the magnetic susceptibility. This work provides a unique direction in the generation of metal-free photochromic and photomagnetic materials, where a simple 1,4- bis(imidazolyl)benzene system changes its color, radical activity, and magnetic behavior with photoirradiation.