Ligand-Driven Light-Induced Spin Change Activity and Bidirectional Photomagnetism of Styrylpyridine Iron(II) Complexes in Polymeric Media

Two pseudo-octahedral iron(II) complexes, Fe(stpy)(4)(NCSe)(2), containing photoresponsive ligands (cis <-> trans isomerization of -CH=CH-) were prepared with trans- or cis-styrylpyridine (stpy) isomers. The magnetic behavior of the polycrystalline solids was previously shown to depend on the configuration of the stpy ligand. The crystal X-ray structures were determined at 293 and 104 K for both isomers. The all-traps and all-cis compounds crystallize in the orthorhombic (Pna2(1)) and the monoclinic space groups (C2/c), respectively. No symmetry change occurs upon cooling to 104 K. The Fe(II) centers lie in axially compressed octahedra with NCSe anions in the apical position and the four pyridinic nitrogens in the meridional plane. The variation of metal-ligand bond lengths as a function of temperature reflects the thermal S=0 <-> S=2 crossover of all-traps complexes and the S=2 ground state of all-cis complexes. The unit-cell volumes per metal ion also change accordingly, and the relative variation due to the spin-crossover compares those associated with the formal change of configuration of the four stpy isomers. The photomagnetic responses were investigated at 130 K with doped polymer thin films containing all-cis (high-spin) or all-traps species (partly low-spin). The 130 K illumination of these doped poly(methyl methacrylate) (PMMA) films leads to the UV-vis absorption features typical for the cis <-> trans photo isomerization of the stilbenoid moiety. The direct magnetic measurements have unambiguously established the photomagnetic effect named ligand-driven light-induced spin change (LD-LISC). The 355 nm excitation of doped thin films produces very long lifetime states that are manifested by high-spin to low-spin (all-cis complex) and low-spin to high-spin (all-traps complex) changes of the Fe(II) magnetic behavior; the process is bidirectional. A structural analysis based on the single-crystal X-ray diffraction data has been proposed to rationalize the LD-LISC activity detected here for doped PMMA thin films.