Naphthalimide-Tagged Iron(II) Spin Crossover Complex with Synergy of Ratiometric Fluorescence for Thermosensing

Spin crossover (SCO) materials that possess switchable and cooperative fluorescence have long focused interest in photonic sensor devices to monitor the variations in the physicochemical parameters of the external environment. However, the lack of quantified cooperativity for the SCO transition operating in isolated molecules is detrimental to short-term technological applications. In this study, a pretwisted energy D–A system combining the deep-blue naphthalimide fluorophore (donor) and the FeN6 SCO chromophore (switchable acceptor) has been developed with the formula of Fe(naph-abpt)2(NCS)2·2DMF (1), where naph-abpt is N-[3,5-di(pyridin-2-yl)-4H-1,2,4-triazol-4-yl]-1,8-naphthalimide. Dual emission from the naphthalimide function based on its vibronic structure exhibits a different synergy effect with SCO, providing a new platform for ratiometric fluorescence thermosensing. Theoretical calculations and optical experimental results demonstrate an excellent correlation between luminescence intensity ratio signals and magnetic data of spin transition, promising a high sensitivity of the optical activity of the ligand to the spin state of the active iron(II) ions, with the maximum relative sensitivity as 0.7% K–1 around T1/2.