Fluorescence modulation via photoinduced spin crossover switched energy transfer from fluorophores to Fe-II 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 Fe-II ions, whose electronic configuration changed from low spin to high spin upon light irradiation. The photoinduced spin crossover of Fe-II 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 Fe-II ions differed between the low spin and high spin states. The photoinduced spin crossover switched the energy transfer from the fluorophore to the Fe-II 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