Surface-Induced Changes in the Thermochromic Transformation of an Ionic Liquid Cobalt Thiocyanate Complex

Abstract

We demonstrate that a thermodynamic complex equilibrium within an ionic liquid film can be significantly influenced by the presence of the liquid–vacuum interface. Using surface-sensitive X-ray photoelectron spectroscopy, we find that the temperature-driven transition from the blue-colored tetrahedral [Co(II) (NCS)4]2– to the red-colored octahedral [Co(II) (NCS)6]4– complex already occurs within the outermost nanometers at around +4 °C as compared with −25 °C in the bulk. This thermochromic transformation in the near-surface region goes along with a loss in preferential surface orientation of free [SCN]− anions and with a pronounced decrease in the complex density; both effects are attributed to the formation of a weakly bound solvation shell around the [Co(II) (NCS)6]4– anion, leading to an effective complex dilution. Our results are not only relevant for high-surface area thin film systems, such as in sensor and catalysis applications, but also shed light on the role of ionic liquid surfaces in particular and liquid surfaces in general.

Resolving X-ray photoelectron spectra of ionic liquids with difference spectroscopy

“…CO 2 capture by amines) 39 or liquid phase (e.g. alkylations, dehydrogenation of organic compounds, or thermochromic transformations of organometallics) [40][41][42]…”

Cobalt(ii) liquid metal salts for high current density electrodeposition of cobalt

“…63 Also when cobalt(II) isothiocyanate was dissolved in [EMIM][SCN], a blue-to-pink color change was observed at 8 °C. 64,65 Thermochromism is also possible for other metal-containing ionic liquids, such as those with nickel(II) 66,67 or uranyl isothiocyanate. 68…”

Perspective: Chemical reactions in ionic liquids monitored through the gas (vacuum)/liquid interface

“…The fifth example addresses the thermochromic transition of an ionic liquid-based Co(II) complex from the tetrahedral to the octahedral configuration in the near-surface region. 108 We observed that the transformation occurs already around +4 • C, that is, well above the temperature of around 25 • C at which the bulk transition occurs. The sixth and last example concerns X-ray induced chemistry, that is, reduction or oxidation of metal complexes dissolved in ILs.…”

“…We thus extend our reaction studies to a metal-ligand complex reaction under thermodynamic equilibrium conditions. 108 For this purpose, XPS was applied to the dynamical metal complex system based on an IL with the doubly negatively charged [Co(II) (NCS) 4 ] 2 as anion; 126 for this anion, Osborne et al 127 reported a thermochromatic transformation to the doubly negatively charged [Co(II) (NCS) 6 ] 4 anion upon cooling. The scheme of the corresponding complex equilibrium is depicted in Scheme 6.…”

“…Upon moderate cooling of the mixture IL-3 to 10 • C, which is above the color transition temperature of 25 • C, a pronounced narrowing and intensity loss of the [SCN] -related N 1s and S 2p peaks (green spectra of IL-3 in Figure 6 108 anions are still present as clearly shown by the deep blue color of the liquid film covering the sample holder; at 30 • C, the IL-3 film on the sample holder has changed its color from blue to a red appearance, as witnessed by visual inspection.…”