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 Solid State

Photochromism and photomagnetism are two important features for the generation of smart materials that are of utility for optical switches and memory devices. Very few materials that exhibit both the features at room temperature are known to date, the reported ones mostly contain metal-ions to assist the electron transfer with photo stimuli. Herein, we report a new class of organic materials that show photochromism and photomagnetism at room temperature due to the generation of free radicals in the solid-state. 1,4-bis(4,5-diphenyl-1H-imidazol-2-yl)benzene (BDPI) was found to acts as organic photomagnet without use of any traditional photochromic couplers or radical containing moieties. Solvent molecules (eg. water and DMSO) form hydrogen bonds with the imidazole rings which act as binding glue in stacking of 1,4-bis(imidazolyl)benzene moieties via π···π interactions. These interactions found to play a significant role in promoting strong spin-spin interaction in the solid-state which can be inferred from the appearance of broad EPR signal, whereas a sharp signal appeared only after irradiation. The sharp signal indicates formation of paramagnetic free radicals which influence antiferromagnetic exchange interactions with weakening of the magnetic susceptibility significantly. This work provides a new direction in the ongoing research of photochromic and photomagnetic materials, where a simple 1,4- bis(imidazolyl)benzene system changes its color, radical activity and magnetic behavior with photoirradiation.