Fluorescent Molecular Scaffold-Building Mononuclear Iron(II) Complexes Display Synergistic Spin Crossover and Fluorescence Behaviors

Abstract Magnetic fluorescent bifunctional spin crossover (SCO) materials show switchable magnetic and optical properties and have attracted continuous research interest due to their potential application in magneto-optical switches and multi-sensors. Herein, we report two octahedral Fe(II) SCO complexes, namely, [Fe2(1-isoTQA)2(NCS)4]·CH2Cl2 (1·CH2Cl2) and its desolvated sample, Fe2(1-isoTQA)2(NCS)4 (1d), based on the fluorescent molecular scaffold ligand of tris(1-isoquinolylmethyl)amine (1-isoTQA). Magnetic studies revealed that 1·CH2Cl2 exhibits two-step SCO behavior, while 1d displays one-step, incomplete SCO behavior. Temperature-dependent fluorescence measurements indicated that the emission intensity of 1d can be observed to increase synchronously with the spin transition. Moreover, the temperature-dependent structural analyses and theoretical calculations confirmed that the emission intensity can be modulated by the spin state through the energy-transfer mechanism.