60 K wide hysteresis embracing room temperature in a fluorescent FeII spin transition complex

Abstract

This work presents the synthesis and characterization of a new neutral mononuclear Fe(II) spin transition (ST) complex, [FeII(L)2]0 (4Cl), where L is an asymmetrically substituted tridentate ligand 2-(3-(4-chlorophenyl)-(1H-1,2,4-triazol-5-yl)-6-(1H-pyrazol-1-yl)pyridine. 4Cl exhibits a remarkably wide stable ST hysteresis loop ca. 60 K wide embracing room temperature (T1/2↑ = 308 K and T1/2↓ = 248 K). Detailed structural, calorimetric and spectroscopic investigations, including X-ray diffraction with Rietveld analysis, Raman spectroscopy, and photoluminescence, reveal the crucial role of subtle structural changes within the crystal lattice in driving the cooperative spin transition. The DFT energy framework analysis highlights the interplay of balanced stabilizing and destabilizing intermolecular interactions that contribute to the observed hysteresis. Furthermore, the fluorescent properties of 4Cl exhibit distinct changes in emission intensity and wavelength upon spin state switching with emission color change from sky blue in the high spin state to violet in the low spin state. The compound is the first example of the spin transition compound with bistable fluorescence response at room temperature.