Effect of ligand methylation on the spin-switching properties of surface-supported spin-crossover molecules

X-ray absorption spectroscopy investigations of the spin-state switching of spin-crossover (SCO) complexes adsorbed on a highly-oriented pyrolytic graphite (HOPG) surface have shown so far that HOPG is a promising candidate to realize applications such as spintronic devices because of the stability of SCO complexes on HOPG and the possibility of highly efficient thermal and light-induced spin-state switching. Herein, we present the spin switching of several Fe(II) SCO complexes adsorbed on an HOPG surface with particular emphasis on the thermally induced spin transition behaviour with respect to different structural modifications. The complexes of the type [Fe(bpz)2(L)] (bpz  =  dihydrobis(pyrazolyl)borate, L  =  1,10-phenanthroline, 2,2′-bipyridine) and their methylated derivatives exhibit SCO in the solid state with some differences regarding cooperative effects. However, in the vacuum-deposited thick films on quartz, complete and more gradual spin transition behavior is observable via UV/vis spectroscopy. In contrast to that, all complexes show large differences upon direct contact with HOPG. Whereas the unmodified complexes show thermal and light-induced SCO, the addition of e.g. two or four methyl groups leads to a partial or a complete loss of the SCO on the surface. The angle-dependent measurement of the N K-edge compared to calculations indicates that the complete SCO and HS-locked molecules on the surface exhibit a similar preferential orientation, whereas complexes undergoing an incomplete SCO exhibit a random orientation on the surface. These results are discussed in the light of molecule-substrate interactions.

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Introduction
The two closely related spin-crossover [1][2][3] complexes [Fe(bpz) 2 (phen)] (bpz = dihydrobis(pyrazolyl)borate, phen = 1,10-phenanthroline) and [Fe(bpz) 2 (bipy)] (bipy = 2,2′-bipyridine) are (arguably) the most widely investigated SCO complexes on a surface. Over the years, only a few SCO complexes were reported to be vacuum-evaporable [4][5][6][7][8][9][10][11][12]. On highlyoriented pyrolytic graphite (HOPG), both molecules undergo a complete thermal-and light-induced spin transition [13][14][15]. On a Au(1 1 1) surface, [Fe(bpz) 2 (phen)] decomposes into HS [Fe(bpz) 2 ] and phenanthroline, while remaining intact and hence preserving its spin-crossover (SCO) behaviour in the second layer [4,5]. In fact, the first observation of electric current-induced spin switching of SCO complexes-termed electron-induced excited spin-state trapping (ELIESST), in analogy to light-induced excited spin-state trapping (LIESST) [17,18]-was made in a bilayer of [Fe(bpz) 2 (phen)] deposited on a Au(1 1 1) surface [19]. On a Bi(1 1 1) surface, about 50% of the molecules in direct contact with the surface lose their SCO property, without any indication of molecular fragmentation [20]. This underscores the influence of the underlying substrate on the integrity and functionality of surface-adsorbed SCO complexes. On the other hand, it is a wellknown phenomenon that in the solid state (bulk), the nature of spin transition of SCO complexes is highly dependent upon the chemical structure of the ligands [21]. What is of interest is to see if the SCO can similarly be tuned for SCO complexes deposited on surfaces via ligand modification. Naggert et al. [21] reported the tuning/modification of both the thermaland light-induced SCO behaviour of [Fe(bpz) 2 (phen)] by chloro-and methyl-substitutions on the phen ligand, in the bulk and in thin films. In particular, [Fe(bpz) 2 (me 2 -phen)], i.e. [Fe(bpz) 2 (phen)] functionalized with two methyl groups at the phenanthroline leading to 4,7-dimethyl-1,10-phenanthroline, exhibits contrasting SCO in the bulk and in the thin film: while in the former, the molecule is trapped in the HS state from 300-20 K, in the latter, the molecule undergoes both temper ature-and light-induced SCO, in a manner similar to that of the parent molecule in the thin film, albeit with a slightly higher transition temperature [21].
As will be shown in the following sections, methyl substitutions have a profound effect on the SCO behaviour of derived Fe(II) complexes upon direct contact with an HOPG surface: addition of two methyl groups to the phen or bipy ligands leads to partial SCO; addition of four methyl groups leads to complete loss of SCO. However, addition of four methyl groups to bpz ligands-[Fe(4-me 2 -bpz) 2 (phen)]-results in a partial SCO. [Fe(bpz) 2 (tbu 2 -bipy)]-investigated both in the submonolayer and bilayer coverages, is also observed to undergo a partial SCO, while [Fe(bpz) 2 (dpq)] is locked in the HS state.
For probing the molecular spin states of submonolayer or bilayer coverages on HOPG, near-edge x-ray absorption finestructure (or simply referred to as x-ray absorption spectroscopy, XAS) is used. The high sensitivity and the element selectivity of XAS enables one to probe the spin states of the central Fe(II) ion, whereby the spectral pattern recorded at the Fe L 2,3 -edge (or just at the L 3 -edge) has a unique pattern for the HS and LS states [10,23]. Further, XAS recorded at the N K-edge is employed to estimate the molecular orientation on HOPG surface, making use of the measured XA-intensity dependence upon the angles between the surface and the direction of the x-ray beam. For the bulk-and thin film-sample investigation of the spin states, magnetic measurements, Mössbauer and UV/vis spectroscopy are used. The spin crossover from HS to LS is accompanied by a dramatic decrease of paramagnetism down to diamagnetism (for Fe(II) compounds) due to a decrease of the number of unpaired electrons. Therefore, the magnetic susceptibility as a function of temperature (χ M T ) changes dramatically when the thermal spin transition temperature (T 1/2 ) is reached [2,24]. Temperaturedependent 57 Fe Mössbauer spectroscopy can also be used to probe the spin states of iron in coordination compounds as two of the parameters derived from the Mössbauer spectrum, i.e. the isomer shift δ and the quadrupole splitting ΔE Q , vary significantly between the HS and LS state of Fe(II) [2,24]. Finally, the spin transition of SCO complexes with low-lying π * orbitals of ligands with an extended π-system, as present in [Fe(bpz) 2 (phen)], can be monitored via MLCT transitions observable via UV/vis spectroscopy. Due to SCO the bands dramatically increase in the intensity from HS to LS because of the larger overlap of the metal-and ligand-orbitals which results from the shorter metal-ligand bond lengths in the LS state [2,24].

X-ray absorption spectroscopy
All compounds have evaporation temperatures ranging from 150 °C to 190 °C. The samples with sub-monolayer (and multilayer) coverages on the HOPG substrate were prepared by evaporating the molecular powder from a home-made tantalum Knudsen cell onto the substrate, at a chamber pres sure of ~2 × 10 −8 mbar. The evaporation rate is monitored from the frequency change of a quartz crystal attached to the Knudsen cell. The molecular coverages were estimated from the integrated Fe L 3 -edge XA intensity, using the previously reported values as [13,14]. The HOPG substrate is purchased from Structure Probe, USA; a clean surface is obtained by cleaving away the top-layers using carbon tape in an auxiliary chamber maintained at a pressure of ~10 −7 mbar, and valveconnected to the deposition chamber.
The XAS measurements were carried out in-situ at the high-field diffractometer (5 × 10 −11 mbar) of the beamline UE46-PGM1, the VEKMAG end-station (2 × 10 −10 mbar) of the beamline PM2 of BESSY II, and the group-owned endstation (5 × 10 −10 mbar) of the beamline UE56/2-PGM-2, of BESSY II. The photon fluxes of the beamlines are ~1 × 10 11 photons s −1 cm −2 , ~1.6 × 10 11 photons s −1 cm −2 , and ~1 × 10 13 photons s −1 cm −2 , respectively. The data shown in figures 4(c) and 6 originates from the high-field diffractometer end-station; figure 4(d) was recorded at the VEKMAG endstation, while figures 4(b) and 7 were recorded at the groupowned end-station. All XAS spectra are recorded by means of total electron yield-the sample drain current is recorded as a function of the x-ray photon energy and normalized to the photocurrent of a Au grid (a Pt grid in the case of PM2-VEKMAG) upstream to the experiment; the background signal from a clean HOPG substrate is also subtracted. The Fe L 3 -edge spectra are recorded at the so-called magic angle of 54.7° between the surface and the k-vector of the linearly p -polarized x rays; at this angle, the XA resonance intensities are independent of the orientations of the molecular orbitals on the surface. For the N K-edge spectra, the indicated angles (in figures 6 and 7) are between the k vector and the surface. The light-induced spin-state switching was performed with a green LED of wavelength 520 nm with a spectral width (fwhm)  Figure 2 shows the magnetic susceptibility of the bulk sample of [Fe(4-me 2 -bpz) 2 (phen)] as a function of temperature. The new complex shows thermal SCO with χ M T values ranging from 3.15 cm 3 K mol -1 at 300 K to 0.09 cm 3 K mol -1 at 5 K. At T 1/2 = 112 K (temperature where the high-spin fraction γ HS is 50%) a fairly abrupt spin transition occurs. This behaviour is similar to other complexes of the formula [Fe(bpz) 2 (L)] with L = diimine [4,[20][21][22].

Bulk properties of [Fe(4-me 2 -bpz) 2 (phen)]
The slight increase of χ M T with decreasing temperature prior to the spin transition can be attributed to a distinct anisotropy of the g-factor as found for another example of [Fe(bpz) 2 (L)] compounds [22].
To obtain further information on the spin state of the iron center, Mössbauer spectra were recorded, indicating also HS at 300 K and LS at 80 K (see SI, figure S6).
Importantly, vacuum-deposited films of [Fe(4-me 2bpz) 2 (phen)] exhibit LIESST and reverse-LIESST [17,18]; i.e. at 5 K the low-spin state can be converted to the high-spin state by irradiation at 519 nm for 5 min and then-to a small extent -back to the low-spin state by irradiation at 810 nm for 15 min (figure 3(a)). As the intensity of the MLCT band differs in the HS and the LS state (see figure 3(a)), it can be taken to monitor the spin transition. In figure 3(b), the high-spin fraction γ HS derived from the intensity of the MLCT band is displayed versus the temperature [4,20,21]. The thermal transition is significantly more gradual than in the solid state (see figure 2), presumably due to weaker cooperative effects in the vacuum-deposited film. Moreover, the thermal spin trans ition temperature T 1/2 of 159 K is significantly higher than in the bulk material (see above). Finally, the critical temper ature for  the HS-to-LS relaxation is T LIESST = 59 K. The other vacuumdeposited films exhibit similar properties (see SI; figure S13).

XA spectroscopy in submonolayers on graphite surfaces
The x-ray absorption spectroscopy recorded at the Fe L 3 -edge has been reported-and is well-documented by now-to be a good indicator of the spin states of SCO molecules [13,14,23]. As an example, figure 4(a) shows the Fe L 3 -edge-reproduced from [13]-of 0.4 monolayer (ML) of [Fe(bpz) 2 (phen)] on an HOPG surface, undergoing a complete spin-state switching induced by temperature and light.
The molecule in the HS state at 300 K is characterized by two main peaks at 708.4 and 709.1 eV, while the LS state at 6 K is characterized by a single main peak at 709.7 eV (solid and dashed lines, respectively). On illumination with green light at 6 K, the molecule switches to the HS state, as indicated by the resultant spectral pattern (dotted line, figure 4(a)) similar to that recorded at 300 K.
The spectral patterns of the HS and LS states can be explained within the framework of a one-electron model as: the presence of holes at both the e g and t 2g levels in the HS configuration-depicted in the inset of figure 4(a)-accounts for the double-peak feature of the HS spectrum; while the presence of holes only at the e g level in the LS configuration-as depicted in the inset of figure 4(b)-accounts for the single-peak feature of the LS spectrum. A realistic understanding of the spectral patterns is possible within the atomic multiplet calculations, which take into account the multi-electron nature of the initial and final states [13,31]. Figure 4(b) shows the Fe L 3 -edge XA spectra of about 0.4 ML of [Fe(bpz) 2 (me 2 -phen)] on an HOPG surface, recorded at 300 K (solid line), at 20 K and also at 20 K after illumination with a green LED for about 10 min (dashed and dotted lines). While the spectra at 300 K and at 20 K after illumination indicate the HS state of the molecule-similar to that of the parent molecule of figure 4(a)-the spectrum at 20 K before illumination with light shows both the HS and LS peaks at 708.4 and 709.7 eV, respectively. The spin-state composition at 20 K of the sample can be estimated by a linear combination of the HS and LS spectra of the parent molecule [Fe(bpz) 2 (phen)] given in figure 4(a); by this procedure, the HS fraction is estimated to 65(5)% at 20 K. In contrast, the complex [Fe(bpz) 2 (me 2 -phen)] in the solid state (bulk) is reported to be trapped in the HS state from 300 K down to 20 K [21].
On adding four methyl groups to the phen ligand, i.e. in [Fe(bpz) 2 (me 4 -phen)], the molecule loses its SCO property and is trapped in the HS state from 300 K down to low temperatures on an HOPG surface; this is shown in figure 4(c) for a coverage of 0.3 ML. This contrasts sharply with the SCO behaviour of the bulk molecule, which is reported to undergo  [13]. The inset of (a) shows the high-spin configuration where electrons occupy both the e g and t 2g levels, while the inset of (b) shows the low-spin configuration where the lower t 2g level is fully occupied by electrons.
both temperature-and light-induced SCO, in a manner quite similar to the parent, non-methylated molecule [20].
The implementation of methyl groups to the bpz ligand in the complex [Fe(4-me 2 -bpz) 2 (phen)] changes the SCO behaviour on a HOPG surface less than methylation of the phen ligand. Nevertheless, the complex undergoes only a partial spin switching upon contact with an HOPG surface, unlike the thin film and bulk samples that exhibit a complete thermaland light-induced SCO. This is shown in figure 4(d)  Similar SCO behaviour is also observed for the closely related complex, [Fe(bpz) 2 (bipy)], on ligand-methylation and upon contact with an HOPG surface; this is shown in figure 6 for the complexes [Fe(bpz) 2 (me 2 -bipy)] and [Fe(bpz) 2 (tbu 2 -bipy)], each deposited in coverage of about 0.5 ML. In figure 6, the SCO behaviour of a bilayer sample of [Fe(bpz) 2 (tbu 2 -bipy)] on HOPG is also shown. It is obvious that the thermal-induced spin-state transition for the bilayer sample is more complete as compared to the submonolayer sample-minimum HS fraction of about 44(5)% and 61(5)% at 60 K, respectively-indicating the role of the surface in constraining/impeding the SCO behaviour of SCO complexes.
The utility of the N K-edge in probing the stability of [Fe(bpz) 2 (phen)] upon contact with a gold surface has been reported earlier [16,20]. Notably, the XA intensity depends upon the relative orientation between the k-vector and the N-π * -orbitals of the complex ( figure 7). This will be used to probe the preferential orientation (and also the integrity) on the HOPG surface of the complex [Fe(bpz) 2  and [Fe(bpz) 2 (bipy)] recorded at 25° (solid lines) and 90° angles (dotted lines) between the k-vector of the x-ray beam and the HOPG surface. Importantly, all the samples showed identical spectral patterns-with similar XA intensities at the peaks #1 at 399.1 eV, #2 at 400.8 eV, #3 at 401.3 eV. The peaks marked #1, #2 and #3 in figure 8 correspond to transitions to the π * -antibonding orbitals of the three respective classes of N atoms of the SCO molecules. The broad peaks at the higher energies (>405 eV) are due to transitions to the σ * -antibonding orbitals. The difference in the XA intensity of the spectra recorded at 25° and 90° angles reflect the preferential orientation-as opposed to a random one-of the abovementioned complexes on the HOPG surface. In particular, the higher intensity for the peak marked #1 at 25° is due to the  greater overlap of the π * -antibonding orbitals of #1 N-atoms to the k-vector of the x-ray beam than at 90°. But in the case of the peaks #2 and #3, the reverse is true.
For the orientation and geometry of the molecules in direct contact with the graphite surface we assumed a non-distorted geometry similar to the discrete complex in the crystal structure of [Fe(bpz) 2 (phen)] and [Fe(bpz) 2 (bipy)]. The complexes should stand with at least three H atoms from one pyrazole ring and the co-ligand phen or bipy on the surface ( figure  7(a)). This orientation is similar to that of [Fe(bpz) 2 (phen)] in the second layer on Au(1 1 1) proposed by Gopakumar et al [19]. The assumed orientation was used for the calculation of the theoretical transition dipole moment orientations and expected intensity ratios (R = I grazing /I normal ). Comparison with the measured values given in figure 8 should confirm or disprove the assumed orientation. The angle of the p-orbital on each N atom was determined from the undistorted molecular structure. It was thereby assumed that due to rotational distribution of the molecules all corresponding angles are arranged on a cone around the surface normal ( figure 7(b) and see SI).
The values of the experimental and calculated intensity ratios R match well (Table 1) (table 1) indicates that these three complexes probably stand in the proposed, almost undistorted orientation on a graphite surface.
As already mentioned, [Fe(bpz) 2 (phen)] and [Fe(bpz) 2 (bipy)] exhibit complete SCO with temperature and light on an HOPG surface [13,14]. Therefore, it is not surprising that both retain their molecular geometry (octahedral symmetry) on the surface-a basic requirement for the observance of the SCO phenom enon. What is surprising, however, is the complete loss of SCO properties of [Fe(bpz) 2 (me 4phen)], although the N K-edge spectra of figure 8(b) indicate that these molecules retain both, the molecular integrity and the octahedral symmetry on the HOPG surface.
For the molecule [Fe(bpz) 2 (me 2 -phen)], which exhibits a partial thermal-induced spin transition on HOPG, the N Kedge spectra recorded at two different angles (25° and 90°) between the k-vector of the x-ray beam and the HOPG surface are shown in figure 9 for two different coverages: 0.4 ML (a) and 4 ML (b). The XA intensities for both angles in all the three peaks are approximately the same. The 4-ML sample is expected to be amorphous, and as such, its N K-edge spectra shown in figure 9(b) would indicate a random orientation of the molecules, irrespective of the molecular geometry. The N K-edge XA spectral pattern of the 0.5-ML sample, as shown in figure 9(a), is also independent of the angle, indicating a random distribution of the molecules on the surface-much like the multi-layered amorphous sample of figure 9(b). Notably, the N K-edge XA spectra of [Fe(bpz) 2 (me 2 -bipy)] (0.5 ML) and [Fe(bpz) 2 (tbu 2 -bipy)] (0.5 and 2 ML)-both exhibiting an incomplete thermal-induced SCO, figure 6also show similar pattern to that shown in figure 9 (figure S16 of SI).
The partial spin crossover or the coexistence of both the spin states for SCO molecules in direct contact with a solid surface is quite a common phenomenon [7,16,[32][33][34], attributed to the molecule-substrate interactions.   In particular, in an STM-based investigation of [Fe(1,10phenanthroline) 2 (NCS) 2 ] on the Cu(1 0 0) surface with coverage ranging from single molecules to bilayers [35], the specific absorption sites are found to determine the spin state (HS or LS) of the molecule-some sites favouring the HS state, and yet others favouring the LS state. In a recent investigation of a monolayer of an SCO molecule [Fe II ((3,5-(CH 3 ) 2 pz) 3 BH) 2 ] deposited on a Au(1 1 1) surface-and apart from establishing the epitaxial relationship between the molecule and the surface using x-ray diffraction-the authors established that, indeed, mixed spin states are favoured for SCO complex-substrate systems, and their interaction energy (epitaxial strain energy) as being responsible for stabilizing a certain proportion of HS molecules at low temperaturesusing DFT calculation and a mechanoelastic model [36].

Conclusions
The combined Fe L 3 -edge and N K-edge NEXAFS studies revealed that the SCO complexes undergoing a complete temperature-and light-induced SCO on an HOPG surface, namely [Fe(bpz) 2 (phen)] and [Fe(bpz) 2 (bipy)], exhibit a preferred orientation on the surface in which the molecules stand with one pyrazole ring and phen/bipy on the surface. Importantly, the addition of methyl groups to the parent SCO complexes always results in a partial or a complete loss of the SCO (see SI, table S3). N K-edge XA spectra of the methylated complexes indicate that the addition of methyl groups leads to two different classes of molecules. In the case of [Fe(bpz) 2 (me 4phen)] on the HOPG surface, the molecule shows an orientation and a geometry similar to the parent molecule, but the complex is HS-locked, probably due to CH-π interactions between the methyl groups and HOPG [37]. In contrast to that, the complexes [Fe(bpz) 2 (me 2 -phen)], [Fe(bpz) 2 (me 2bipy)] and [Fe(bpz) 2 (tbu 2 -bipy)]-all undergoing incomplete SCO on HOPG-show a random orientation on the surface. The most simple rationalization of these findings would be that these molecules exhibit at least two different orientations, one that shows SCO and another which is HS-lockedprobably in a similar orientation such as the parent systems and [Fe(bpz) 2 (me 4 -phen)]-leading in total to an incomplete SCO on HOPG. While the introduction of methyl groups thus may enhance the SCO complexes-HOPG interactions for this class of molecules as well, the N K-edge NEXAFS spectra clearly show that this also occurs without causing any observable molecular distortions.