Fluorescence modulation via photoinduced spin crossover switched energy transfer from fluorophores to FeII ions.

Molecular materials possessing phototunable fluorescence properties have attracted great interest owing to their potential applications in optical switches and storage. However, most fluorescence modulation is realized through light-responsive structural isomerization in solution. It is a formidable challenge to achieve phototunable fluorescence emission with high fatigue resistance and a fast response rate in the solid state for the development of devices. Here, a mononuclear compound was constructed via the coordination of fluorophores with FeII ions, whose electronic configuration changed from low spin to high spin upon light irradiation. The photoinduced spin crossover of FeII ions was accompanied by a 20% increase in the fluorescence emission intensity. A temperature-dependent spectroscopic study together with time-dependent density functional theory calculations revealed that the effective spectral overlap between the emission of the fluorophores and the absorption band of the FeII ions differed between the low spin and high spin states. The photoinduced spin crossover switched the energy transfer from the fluorophore to the FeII ion, resulting in fluorescence modulation. The presented results provide a novel approach for developing optical memory and sensors via electron rearrangement of photoinduced spin crossover.