Light-Induced Spin Crossover in an Fe(II) Low-Spin Complex Enabled by Surface Adsorption

Abstract

Understanding and controlling the spin-crossover properties of molecular complexes can be of particular interest for potential applications in molecular spintronics. Using near-edge X-ray absorption fine structure spectroscopy, we investigated these properties for a new vacuum-evaporable Fe(II) complex, namely [Fe(pypyr(CF))(phen)] (pypyr = 2-(2'-pyridyl)pyrrolide, phen = 1,10-phenanthroline). We find that the spin-transition temperature, well above room temperature for the bulk compound, is drastically lowered for molecules arranged in thin films. Furthermore, while within the experimentally accessible temperature range (2 K < T < 410 K) the bulk material shows indication of neither light-induced excited spin-state trapping nor soft X-ray-induced excited spin-state trapping, these effects are observed for molecules within thin films up to temperatures around 100 K. Thus, by arranging the molecules into thin films, a nominal low-spin complex is effectively transformed into a spin-crossover complex.

Influence of Substrate Electronic Properties on the Integrity and Functionality of an Adsorbed Fe(II) Spin-Crossover Compound

“…Thin films of this complex on 1T TiTe 2 undergo light-induced spinstate trapping (LIESST) and soft x-ray-induced excited spin-state trapping (SOXIESST) for temperatures below 100 K (Ref. 20). Because the spin-transition temperature of Fe-pypyr is relatively high (T 1/2 = 390 K for powder samples), mostly LS-to-HS transitions induced by 532 nm laser light for temperatures below 100 K are considered throughout the manuscript.…”

“…Because the spin-transition temperature of Fe-pypyr is relatively high (T 1/2 = 390 K for powder samples), mostly LS-to-HS transitions induced by 532 nm laser light for temperatures below 100 K are considered throughout the manuscript. XAS at the Fe L 3,2 absorption edges is particularly suitable to probe the spin state averaged over an ensemble of molecules 12,16,17,19,20,[22][23][24][25]30,[32][33][34][40][41][42][43][44] . Indeed, the fine structure and energy of the Fe L 3,2 edges sensitively depend on the occupation and the ligand field of the Fe d orbitals.…”

“…This leads to characteristic and distinct x-ray absorption (XA) spectra for the HS and LS states. The fraction of HS molecules in a given sample can be extracted from the corresponding XA spectrum at the Fe L 3,2 edges using a linear combination of the HS and LS reference spectra (SI section 3) as previously reported 12,16,17,19,20,[22][23][24][25]30,[32][33][34]42,44 . Figure 1c shows XA spectra measured at 3 K under illumination and at 120 K on 5 ML Fe-pypyr on Au(111).…”

“…Fe-pypyr was synthesized as described in Ref. 20. The Fe-pypyr complexes were sublimed in ultrahigh vacuum (base pressure of 10 9 mbar) from a Knudsen cell heated to 170 C onto samples held at room temperature.…”

“…The conformation and spin-state bistability of spincrossover (SCO) compounds make them attractive for applications [1][2][3][4][5][6] . In that respect, considerable efforts were made to realize thin films of SCO complexes using physical vapor deposition [2][3][4][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24] . These films usually exhibit decent SCO properties.…”