Monitoring the Spin States of Ferrous Ions by Fluorescence Spectroscopy in Spin-Crossover-Fluorescent Hybrid Materials

The grafts of fluorophores 9-anthraldehyde (AD) and 9-phenanthrenecarboxaldehyde (PD), respectively, on the one-dimensional spin-crossover compound [Fe(L)₃ ](ClO₄ )₂ (FeL, L=4-amino-1,2,4-triazole) by post-synthetic aldimine condensation reactions produced two spin-crossover (SCO)-fluorescent hybrid materials, that is, FeL-AD and FeL-PD. The spin-crossover critical temperatures of the two materials both centered at T_c ↓=254 and T_c ↑=256 K, whereas the fluorescence intensities of the two materials featured functions of the temperature that strictly synchronized with the spin-crossover processes, which showed that the ligand-centered fluorescence was dominated by the spin states of the ferrous ions. The bifunctional entities (spin-crossover centers and fluorophores) in FeL-AD or FeL-PD showed spectral band overlap that purported the Förster resonance energy transfer mechanism of such spin-crossover-fluorescence correlation. The post-synthetic modification of SCO materials and the relationship between the fluorescence and the SCO may be helpful in the development of multifunctional materials that can be sensitive to multiple stimuli.