Locking and Unlocking the Molecular Spin Crossover Transition

Abstract

The Fe(II) spin crossover complex [Fe{H B(pz) } (bipy)] (pz = pyrazol-1-yl, bipy = 2,2'-bipyridine) can be locked in a largely low-spin-state configuration over a temperature range that includes temperatures well above the thermal spin crossover temperature of 160 K. This locking of the spin state is achieved for nanometer thin films of this complex in two distinct ways: through substrate interactions with dielectric substrates such as SiO and Al O , or in powder samples by mixing with the strongly dipolar zwitterionic p-benzoquinonemonoimine C H (-⋯ NH ) (-⋯ O) . Remarkably, it is found in both cases that incident X-ray fluences then restore the [Fe{H B(pz) } (bipy)] moiety to an electronic state characteristic of the high spin state at temperatures of 200 K to above room temperature; that is, well above the spin crossover transition temperature for the pristine powder, and well above the temperatures characteristic of light- or X-ray-induced excited-spin-state trapping. Heating slightly above room temperature allows the initial locked state to be restored. These findings, supported by theory, show how the spin crossover transition can be manipulated reversibly around room temperature by appropriate design of the electrostatic and chemical environment.

Engineering On‐Surface Spin Crossover: Spin‐State Switching in a Self‐Assembled Film of Vacuum‐Sublimable Functional Molecule

“…This is akin to adding an insulating layer between the strongly interacting metallic surface and the SCO complex. Note also a recent report by Zhang et al, detailing quenching of SCO in a 5 nm film of [Fe(H 2 B(pz) 2 ) 2 (bpy)] on SiO 2 and Al 2 O 3 ; the preservation of SCO in 10 nm films observed in the present work is also important in this context and in terms of realizing large area spintronics junctions based on SCO complexes.…”

“…Relentless efforts have been made to study spin‐state switching characteristics of SCO complexes on different surfaces to harness the device utility of SCO entities. Several interesting results, e.g., the spin‐state dependence of electrical conductance, memristance behavior, electric field or electron‐induced SCO, and locking and unlocking of SCO around room temperature, have been reported. Vacuum sublimation of the complexes onto a suitable surface is the preferred method to obtain high‐quality surface layers of SCO complexes .…”

Indications of magnetic coupling effects in spin cross-over molecular thin films

“…The spin state of an ultrathin film can be deliberately manipulated: the Fe(II) spin crossover complex [Fe{H 2 B(pz) 2 } 2 (bipy), when locked in the low spin state, by the substrate, can be excited into the high spin state at room temperature by an X-ray fluence and then thermally relaxed backed to the low spin state. 4 This appears to require an insulating substrate, consistent with the observation of STM tip manipulation of the spin state of Fe(1,10-phenanthroline) 2 (NCS) 2 on a dielectric substrate like CuN. 5 Charge displacement is implicated.…”

“…T he spin crossover (SCO) phenomenon, in several classes of 3d transition metal compounds, relates to the temperature-induced transition between a low spin (LS) diamagnetic state of the metal ion to a high-spin (HS) paramagnetic state, usually stable at higher temperature. 1,2 For molecular devices or memory applications, where non-volatility is required, there is a strong need to identify and control mechanisms to lock, unlock and switch the molecular spin state at a given temperature, ideally around 300 K. 3,4 For bistability at higher temperatures exceeding the SCO transition, it is necessary to "lock" the low temperature LS state into a metastable state. Such a suppression of the spin state transition has been observed for [Fe(1,10-phenanthroline) 2 (NCS) 2 ], 5 and [Fe{H 2 B(pz) 2 } 2 (bipy)] (H 2 B(pz) 2 = bis(hydrido)bis(1H-pyrazol-1-yl)borate, bipy = 2,20-bipyridine), 4,6-10 molecules or ultra-thin films interacting with a metallic substrate, with a resulting mixed spin state dominated by the LS state at high temperatures.…”

“…Such a suppression of the spin state transition has been observed for [Fe(1,10-phenanthroline) 2 (NCS) 2 ], 5 and [Fe{H 2 B(pz) 2 } 2 (bipy)] (H 2 B(pz) 2 = bis(hydrido)bis(1H-pyrazol-1-yl)borate, bipy = 2,20-bipyridine), 4,6-10 molecules or ultra-thin films interacting with a metallic substrate, with a resulting mixed spin state dominated by the LS state at high temperatures. [4][5][6]8 This implies a substrate-induced ''locking'' of the spin state.…”