Light-Induced Bistability in the 2D Coordination Network {[Fe(bbtr)3][BF4]2}∞: Wavelength-Selective Addressing of Molecular Spin States

Whereas the neat polymeric Fe-II compound {[Fe(bbtr)(3)][ClO4](2)}(infinity) (bbtr=1,4-di(1,2,3-triazol-1-yl)butane) shows an abrupt spin transition centered at 107K facilitated by a crystallographic symmetry breaking, in the covalently linked 2D coordination network of {[Fe(bbtr)(3)][BF4](2)}(infinity), Fe-II stays in the high-spin state down to 10K. However, strong cooperative effects of elastic origin result in reversible, persistent, and wavelength-selective photoswitching between the low-spin and high-spin manifolds. This compound thus shows true light-induced bistability below 100K. The persistent bidirectional optical switching behavior is discussed as a function of temperature, irradiation time, and intensity. Crystallographic studies reveal a photoinduced symmetry breaking and serve to establish the correlation between structure and cooperative effects. The static and kinetic behavior is explicated within the framework of the mean-field approximation.