Acidity-driven bidirectional room-temperature spin-state switch and fluorescence modulation of a mononuclear Fe(II) complex

Abstract

Room-temperature switchable materials showing multiple responses toward external stimuli are highly desired. Herein, we report bidirectional spin-state switch and fluorescence modulation of an Fe(II) complex (1) based on a rhodamine B 2-pyridinecarbaldehyde hydrazone ligand in both the solid state and solution. The complex is predominantly stabilized in the low-spin (LS) state at room temperature due to the strong ligand-field strength imposed by acylhydrazone pyridine. Heating to 400 K results in changes of the spin state to predominantly high spin (HS) due to solvent loss, and the dehydrated sample shows reversible incomplete spin crossover (SCO) in the following thermal cycles. Besides thermally-induced spin-state change, acidification treatment of complex 1 results in iminol–amide tautomerization and dissociation of the complex, and, hence, a switch of the spin state to HS at room temperature. Concurrent fluorescence enhancement was observed and attributed to the intrinsic response of the rhodamine luminophore toward acid perturbation. Reverse switching was achieved upon further alkalization treatment.