Pronounced Magnetic Bistability in Highly Cooperative Mononuclear [Fe(L^npdtz)₂(NCX)₂] Complexes

Abstract

Molecular materials that exhibit stimuli-responsive bistability are promising candidates for the development of molecular switches and sensors. We herein report on the coexistence of a wide thermal hysteretic spin crossover (SCO) effect and a thermally inducible metastable high-spin state at low temperatures achieved with the two new complexes [FeII(Lnpdtz)2(NCX)2] (X = S; Se), with Lnpdtz being (2-naphthyl-5-pyridyl-1,2,4-thiadiazole) and X = S (1) and Se (2). Pronounced π–π-stacking of the aromatic side residues of the ligands enables strong intermolecular interactions, leading to abrupt SCO properties and broad magnetic hysteresis of 10 K for X = S and 58 K for X = Se. In this paper, we also present the pressure-induced spin-state switching around 0.8 GPa. A pronounced thermally induced excited spin state trapping (TIESST effect) is observed for the highly cooperative SCO compounds, which was experimentally followed by low-temperature single crystal structure analysis (20 K) and temperature-dependent Mössbauer spectroscopy.

Spin-Crossover in a New Iron(II)/Di(pyrazolyl)pyridine Complex with a Terpyridine Embrace Lattice. Thermally Induced Excited Spin State Trapping and Clarification of a Structure–Function Correlation

“…SCO in [Fe­(bpp R,Y ) 2 ]­X 2 derivatives that extends toward 100 K is often incomplete, with a residual high-spin fraction of the sample being trapped in its high-spin form below that temperature. − However, the TIESST effect shown by 1­[ClO 4 ] 2 is notably efficient, on the diffractometer and in the SQUID magnetometer. − The high-spin and low-spin forms of 1­[ClO 4 ] 2 can be produced selectively below 100 K, by cooling at >10 K min –1 or <0.5 K min –1 respectively (Figure ). While 1­[ClO 4 ] 2 itself is not an SMM, bistable multifunctional materials could still be produced from the 1­[ClO 4 ] 2 host lattice containing functional dopant molecules. , …”

“…Rewarming the sample led to a pronounced decrease in χ M T above 110 K, followed by an abrupt increase at 150 K where the sample reverted to its high-spin form. That χ M T vs T profile is characteristic of thermally induced excited spin state trapping (TIESST) of the high-spin state at low temperatures. ,− This arises when the spin-transition is kinetically slow on the time scale of the measurement, which can occur for transitions occurring below ca. 100 K. The sample, or a fraction of it, remains trapped in a metastable high-spin state if the sample is cooled too rapidly.…”

Synergetic Spin-Crossover and Luminescent Properties in a Multifunctional 2D Iron(II) Coordination Polymer

“…Inspired by the ability of polypyridine to construct coupled multifunctional coordination polymers with different dimensions, 27,28 herein we rationally designed and successfully synthesized a tetradentate pyridine, 3,6,11,12-tetra(pyridin-4yl)dibenzo[a,c]phenazine (TPDP), with strong fluorescence by grafting pyridines to fluorescent dye dibenzo[a,c]phenazine (dbpz). 29,30 The TPDP can react with the Fe(NCX) 2 (X = S and Se) unit, obtaining two new 2D square-grid complexes, {Fe (2). The X-ray single-crystal analysis, UV−vis absorption spectroscopy, variable-temperature fluorescence emission spectra, and magnetic studies all reveal the synergetic effects between SCO and fluorescence multifunctional properties in complex 2 vacuum .…”

“…Spin-crossover (SCO) complexes are the classic magnetically bistable materials, showing reversible switches between high-spin (HS) and low-spin (LS) states of transition metal octahedral complexes with 3d 4 –3d 7 electron configuration under external stimuli such as temperature, pressure, or light irradiation. − Among them, the most extensively studied species are Fe­(II)-based SCO systems with an [FeN 6 ] environment . In recent years, the design and synthesis of coupled multifunctional materials exhibiting synergy effects of two or more properties have attracted considerable interest. − Since the change in the spin state in the SCO system is often accompanied by changes in the structural, optical, vibrational, and electrical properties, , multifunctional materials combining SCO and other attractive properties can be synthesized by modifying the function of the organic ligand coordinated to the SCO center or introducing other functions into the SCO systems.…”

Cooperative spin crossover: analog to digital – and back

“…In these cases, the switching remained again incomplete, even at low temperatures, so that only a limited fraction of the potential system states could be accessed. 18, 19 A nal example was reported for a valence-tautomeric cobalt complex with a scan rate dependent hysteresis width and HS fraction, which once more suffered from very incomplete spin transitions. 20 As stated by other authors, in-depth analysis of SCO systems with thermal hysteresis is highly promising for a more profound understanding of the SCO process in the solid state.…”