Stepwise Spin-State Switching with Symmetry Breaking and ON/OFF Photo-switching in Iron(II) Complex

In recent world advancement and the quest for smart multi-functional material, stimuli-responsive molecular bistable systems offer unique opportunities to explore their applicability in molecular switches, data storage, and sensing technologies. Multi-stimuli responsive stepwise Spin Crossover (SCO) systems stand tall in this area. While the effects of external stimuli, particularly thermal variations and photoirradiation on the magneto-structural properties of SCO systems have been extensively studied, the area of pressure-modulated stepwise spin crossover and its associated magneto-structural changes remains lesser explored. Herein, we report a mononuclear iron(II) complex containing tetradentate macrocyclic ligand with -diimine-based bidentate coligand, [Fe(L)(bik*)](BPh4)2 (1) (L = N,N'-di-iso-propyl-2,11-diaza[3,3](2,6)pyridinophane and bik* = bis(1-ethyl-1H-imidazol-2-yl)ketone)) undergoing a reversible stepwise thermo-induced spin-state switching with the presence of three spin-states HS, LS, and an ordered HS-LS with the exciting re-entrant symmetry breaking during the spin-state switching process. The influence of external pressure on the structure and magnetic response is thoroughly studied, where the pressure-induced modification in the intermolecular interactions leads to enhanced cooperativity and a hysteretic stepwise spin state switching. The versatility of the systems is further explored where 1 displays a reversible ON/OFF photo-switching between a photo-induced paramagnetic metastable HS and diamagnetic LS states under light irradiations at low temperatures along with light-induced excited spin state trapping (LIESST).