Light-Induced Antiferromagnetic to Ferromagnetic Transition in Halogen Substituted 1,4-Bis(imidazolyl)benzene Systems: An Effect of Spin-Orbit Coupling and π-Stacking in Enhanced Photomagnetism.

Employing the spin-orbit coupling effect by introducing halogen substituents is an excellent strategy to tune the magnetic behavior of organic or metal-organic materials. Light is an alternative tool to modulate the magnetic behavior of a material through a photoinduced electron transfer process, without changing its chemical identity. In this work, three halogen containing 1,4-bis(4,5-diphenyl-1H-imidazol-2-yl)benzene (F-BDPI, Cl-BDPI and Br-BDPI) systems have been chosen to exploit the role of halogen substituents on solid-state photoinduced phenomena. Through a comprehensive analysis involving various characterization techniques, including UV/vis diffuse reflectance, solid-state photoluminescence, and EPR measurements, it was found that the as-synthesized forms Cl-BDPI-IA and Br-BDPI-IA (IA denotes the hexahydrate form of Cl/Br-BDPI) exhibited fast photochromic response through the generation of photoinduced free radicals in the solid state. Moreover, the SQUID analysis revealed an antiferromagnetic to ferromagnetic transition in Cl-BDPI-IA through photoirradiation, which led to an increase in the magnetic moment value up to 38% at room temperature. This signifies the first occurrence of such a significant level of magnetization amplitude compared with previously reported metal-organic photomagnets. This investigation underscores the significance of halogen substitution in tailoring the magnetic properties of organic photomagnets, where strong halogen-π and π-π interactions facilitate the spin-orbit coupling effect in the solid state.