Spin-Crossover in Supramolecular Iron(II)–2,6-bis(1H-Pyrazol-1-yl)pyridine Complexes: Toward Spin-State Switchable Single-Molecule Junctions

Abstract

Spin-crossover (SCO) active iron(II) complexes are an integral class of switchable and bistable molecular materials. Spin-state switching properties of the SCO complexes have been studied in the bulk and single-molecule levels to progress toward fabricating molecule-based switching and memory elements. Supramolecular SCO complexes featuring anchoring groups for metallic electrodes, for example, gold (Au), are ideal candidates to study spin-state switching at the single-molecule level. In this study, we report on the spin-state switching characteristics of supramolecular iron(II) complexes 1 and 2 composed of functional 4-([2,2′-bithiophen]-5-ylethynyl)-2,6-di(1 H -pyrazol-1-yl)pyridine ( L1 ) and 4-(2-(5-(5-hexylthiophen-2-yl)thiophen-2-yl)ethynyl)-2,6-di(1 H -pyrazol-1-yl)pyridine ( L2 ) ligands, respectively. Density functional theory (DFT) studies revealed stretching-induced spin-state switching in a molecular junction composed of complex 1 , taken as a representative example, and gold electrodes. Single-molecule conductance traces revealed the unfavorable orientation of the complexes in the junctions to demonstrate the spin-state dependence of the conductance.

Computational demonstration of isomer- and spin-state-dependent charge transport in molecular junctions composed of charge-neutral iron(ii) spin-crossover complexes

“…This displacement could take the aforementioned molecular states into energy regions where the electrodes DOS drastically drops, decreasing the coupling with the electrodes and, thus, reducing the transmission. Also, both complexes exhibit a high SFE, in line with the values obtained for other iron(II)-based single-molecule junctions, 22,23,60,73 in contrast to devices based on different transition metal-Co(II), Cr(II)-complexes that display much lower SFE values. 74,75 Finally, we report the results obtained for the ligands L1 and L2, and their respective complexes in the LS configuration.…”

“…59 Apart from the photo-induced SCO discussed above, spinstate switching is also induced at the single-molecule level in molecular junctions by applying mechanisms such as stretching and electric field. 2,60 Examples include reports by Mayor and co-workers. 16,61 Ideally speaking, the complexes studied by Mayor and co-workers are not SCO active in the bulk state.…”

Bistable spin-state switching characteristic of a charge-neutral iron(II) complex

“…25 Our recent report dealing with a mononuclear iron(II) complex showing markedly different SCO characteristics despite showing comparable SWAXS patterns is another example. 26 Establishment of a relation between molecular structure and SCO property-for example, what contributes to reverse SCO in [Fe(H2Bpz2)2(4,4'-Br2-bpy)]? -in the bulk-state requires single-crystal X-ray structure determination of a molecule under investigation in the LS and HS states.…”

“…A similar observation was made in the case of a binuclear iron(II) complex, whose polymorphs showed markedly different spin-state switching characteristics, despite featuring comparable spectral characteristics 25. Our recent report dealing with a mononuclear iron(II) complex showing markedly different SCO characteristics despite showing comparable SWAXS patterns is another example 26.…”

Lattice solvent- and substituent-dependent spin-crossover in isomeric iron(ii) complexes

“…3a, the coordinating β-nitrogens of the pyrazole rings are in the trans position with respect to the pyridyl nitrogen, in line with the previously reported structures of bpp-based ligands. [37][38][39] The trans orientation of the nitrogen atoms indicates the need for the rotation of the pyrazole rings to coordinate with iron(II) centres. Structure determination of wine-red crystals at 173 K (Fig.…”