Surface induces different crystal structures in a room temperature switchable spin crossover compound

Abstract

We investigated the influence of surfaces in the formation of different crystal structures of a spin crossover compound, namely [Fe(L) 2 ] (LH: (2-( pyrazol-1-yl)-6-(1H-tetrazol-5-yl)pyridine), which is a neutral compound thermally switchable around room temperature. We observed that the surface induces the formation of two different crystal structures, which exhibit opposite spin transitions, i.e. on heating them up to the transition temperature, one polymorph switches from high spin to low spin and the second polymorph switches irreversibly from low spin to high spin. We attributed this inversion to the presence of water molecules H-bonded to the complex tetrazolyl moieties in the crystals. Thin deposits were investigated by means of polarized optical microscopy, atomic force microscopy, X-ray diffraction, X-ray absorption spectroscopy and micro Raman spectroscopy; moreover the analysis of the Raman spectra and the interpretation of spin inversion were supported by DFT calculations.

Spin-Crossover in Iron(II) Complexes of N,N′-Disubstituted 2,6-Bis(Pyrazol-3-yl)Pyridines: An Effect of a Distal Substituent in the 2,6-Dibromophenyl Group

“…The former allows for measuring the magnetic susceptibility of a solution as an alternative to a more cumbersome magnetometry [12], while the latter allows for screening of paramagnetic complexes that may exist in mixtures of unknown concentrations [40]. The two approaches rooted in the widely available NMR spectroscopy are helpful tools in molecular design of SCO compounds [12,40] exploiting the spin state behaviour decoupled from crystal packing [23,24] or substrate [25,26] effects that sometimes block a SCO [23,24].…”

“…The required SCO behavior, which is most often observed in N6-coordinated complexes of iron(II) [10,11], is tailored for the listed applications [12] by chemical modifications to the heterocyclic N-donor ligands [13][14][15][16][17][18][19][20][21][22]. Recognizing them for the 'truly molecular' design [12] of SCO compounds (free from crystal packing [23,24] or substate [25,26] effects) relies on structure-function relations [12][13][14][15][16]18] identified for selected series of metal complexes in their systematic studies by an available solution-state technique such as NMR spectroscopy [12]. The results of these studies agree on a rather [27] general tendency of bulky groups close to the donor nitrogen atoms to stabilize the HS state of the metal ion through the steric demand that makes its LS state more unlikely, with a less straightforward role for other, more remote substituents [12].…”

In Situ NMR Search for Spin-Crossover in Heteroleptic Cobalt(II) Complexes

“…Transition metal complexes that undergo a spin-crossover (SCO) are targeted by scientists for their ability to switch between low-spin (LS) and high-spin (HS) states in response to an external stimulus (such as temperature, pressure, light irradiation, electric or magnetic field1) for use in high density data storage, switching, sensing, and other molecular devices. , For these purposes, an abrupt SCO with a thermal hysteresis that is sometimes observed in a solid state as a result of cooperative intermolecular interactions is preferred; , however, even gradual and nonhysteretic SCO occurring in solutions is useful for some applications, such as thermometry or pH and chemical sensing. − With no interference from crystal packing , or substrate , effects, a solution-state SCO behavior , is more amenable to “truly molecular” design through structure–function relationships − identified for complexes with bi- − and tridentate ,− or higher denticity − ligands to control the spin state of the metal ion.…”

Vacuum-Evaporable Spin-Crossover Complexes in Direct Contact with a Solid Surface: Bismuth versus Gold

“…The transition from the paramagnetic ( S = 2) high-spin (HS) state to the diamagnetic ( S = 0) low-spin (LS) state can be induced by temperature, pressure, or light. Thermal- and light-induced spin-state switching in films of SCO complexes has been monitored by ultraviolet photoelectron spectroscopy, inverse photoemission, and X-ray absorption spectroscopy (XAS) as well as scanning tunneling microscopy (STM). ,− In the course of these investigations it became evident that in direct contact with a Au(111) surface the complex [Fe­(H 2 B­(pz) 2 ) 2 (phen)] ( 1 ) undergoes fragmentation to [Fe­(H 2 B­(pz) 2 ) 2 ] and 1,10-phenanthroline . Keeping the gold substrate at elevated temperatures, however, it was observed that ordered monolayers of intact molecules of 1 are formed which can be spin-switched by electron-induced excited spin-state trapping (ELIESST) .…”