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Spin crossover iron(II) coordination polymer with fluorescent properties: correlation between emission properties and spin state Charles Lochenie, Konstantin Schötz, Fabian Panzer, Hannah Kurz, Bernadette Maier, Florian Puchtler, Seema Agarwal, Anna Köhler, and Birgit Weber J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b10571 • Publication Date (Web): 18 Dec 2017 Downloaded from http://pubs.acs.org on December 18, 2017

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Spin crossover iron(II) coordination polymer with fluorescent properties: correlation between emission properties and spin state. Charles Locheniea†, Konstantin Schötzb, Fabian Panzerb, Hannah Kurza, Bernadette Maiera, Florian Puchtlerc, Seema Agarwal,d Anna Köhlerb* and Birgit Webera*. a

Inorganic Chemistry II, Universität Bayreuth, Universitätsstrasse 30, NW I, 95440 Bayreuth, Germany Experimental Physics II and Bayreuth Institute of Macromolecular Research (BMBF), Universität Bayreuth, Universitätsstrasse 30, NW I, 95440 Bayreuth, Germany c Inorganic Chemistry I, Universität Bayreuth, Universitätsstrasse 30, NW I, 95440 Bayreuth, Germany d Macromolecular Chemistry II, Universität Bayreuth, Universitätsstrasse 30, NW I, 95440 Bayreuth, Germany b

ABSTRACT: A spin crossover coordination polymer [Fe(L1)(bipy)]n (where L is a N2O22- coordinating Schiff base-like ligand bearing a phenazine fluorophore and bipy = 4,4’-bipyridine) was synthesized and exhibits a 48 K wide thermal hysteresis above room temperature (T1/2↑ = 371 K and T1/2↓ = 323 K) that is stable for several cycles. The spin transition was characterized using magnetic measurements, Mössbauer spectroscopy, and DSC measurements. T-dependent X-ray powder diffraction reveals a structural phase transition coupled with the spin transition phenomenon. The dimeric excerpt {(µ-bipy)[FeL1(MeOH)]2}∙2 MeOH of the coordination polymer chain crystallizes in the triclinic space group P1� and reveals that the packing of the molecules in the crystal is dominated by hydrogen bonds. Investigation of the emission properties of the complexes with regard to temperature shows that the spin crossover can be tracked by monitoring the emission spectra, since the emission colour changes from greenish to a yellow colour upon the low spin to high spin transition.

Introduction Iron(II) spin crossover (SCO) complexes are a class of molecules where the spin state can be reversibly switched between a paramagnetic high spin (HS) and a diamagnetic low spin (LS) state upon physical stimuli such as temperature or pressure change, or light irradiation.1–9 Upon spin transition (ST), the structural, vibrational, electronic, and magnetic properties of the molecule are changing. This makes the field attractive to researchers all over the world due to possible applications in the domain of sensors or memory devices.10–16 Particularly suitable for the latter are cooperative STs presenting thermal hysteresis, if possible around room temperature.17–20 In order to achieve cooperativity between the spin centers, intermolecular interactions such as hydrogen bonds,21 van der Waals interactions,22–25 or π-interactions26–28 are needed to propagate the structural change associated with the SCO phenomenon through the crystal packing of a SCO material.29,30 The synthesis of coordination polymers with rigid bridging ligands can further increase the width of the thermal hysteresis loop.31–35 Additionally it broadens the application potential to areas, where nano-size and incorporation into composite materials or functional devices is required. Indeed, there are several examples for the combination of SCO with a second property36,37 as optical properties,38,39 conducting properties,38,40–44 magnetic properties45,46 or porosity.47–52 For the nano-structuring of SCO coordination polymers and networks, several strategies are already well established.3,53–67 SCO materials that also exhibit fluorescence would have a supplementary spin state “read out” possibility.68 This is of interest for the development of such complexes into the direction of bio-sensors for nano-

thermometry69–71 or fluorescence microscopy,72,73 provided a change of emitted wavelength and/or emission intensity upon SCO can be realized. Until now, two major strategies are adopted to build such systems: the first is to build a composite material made of a luminescent material and a SCO compound such as electroluminescent thin films doped with SCO complexes,74–76 functionalized SCO-core-luminescent-shell nanoparticles,71,77,78 or SCO complexes with fluorescent counter anion.71,79 Among those possibilities the synthesis of hybrid SiO2 nanoparticles with a SCO coordination polymer core and a luminescent shell grafted on the surface of the material allowed a successful tuning of the luminescent properties with the spin state change in several cases.80–83 This coupling is independent from size82,83 and shape (eg. nanorods)84 of the nanoparticles and is observed for different fluorophores (eg. acridine orange,82 pyrene excimers83) Time-resolved measurements show that in some cases a non-radiative energy transfer (Förster type) between the organic fluorophore and the iron(II) is responsible for this coupling, triggered by the color change of the SCO core.81 In the case of the pyrene excimers the mechanical strain induced upon spin transition influences the luminescence.83 The second strategy is to covalently bind the spin center to the fluorophore in a single (macro)molecule, however, this approach does not guarantee a coupling between the spin transition and the emission properties.77,85–88 We recently reported the synthesis of a ligand linked to a phenazine unit, which added ligand-based fluorescence to our Schiff base-like system. We have shown that the ligand-based fluorescence of the corresponding Ni(II) complex could be switched on or off upon Coordination Induced Spin State

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Switch (CISSS) between a diamagnetic square planar geometry and a paramagnetic octahedral/square pyramidal geometry.89Here we present a 1D iron(II) SCO coordination polymer built from this ligand and its detailed characterization. It exhibits a wide thermal hysteresis above room temperature and spin state-dependent emission properties. To the best of our knowledge, this is the first SCO-luminescent molecular device with thermal hysteresis above room temperature that has been reported yet. Results Synthesis The synthesis pathway of all compounds presented in this work is pictured in Figure 1. The spin crossover coordination polymer [FeL1(bipy)]n (4) was obtained in two steps from the phenazine-based ligand H2L1.89 In a first step, iron(II) acetate is used as metal source to include the iron center in the chelate cycle, using the acetate anions as base, to give the methanol complex [FeL1(MeOH)2] (2). The complexation of the iron(II) is accompanied with a typical color change of the solution from yellow to dark red, in agreement with similar complexes reported in literature.90–95 In the solid state, compound 2 is a greenish black crystalline material. The purity of compound 2 was assessed with elemental analysis (CHN) as well as mass spectrometry. Additionally, the IR spectrum shows a typical shift of the C=O bands in comparison to the free ligand, consistent with the coordination of an iron(II) metal center. From the starting compound 2, the dimeric specie {(µbipy)[FeL1(MeOH)]2}∙2 MeOH (3) and the coordination polymer [FeL1(bipy)]n (4) can be obtained by substitution of the axial MeOH ligands with 4,4’-bipyridine (bipy). The dimer 3 was firstly obtained as single crystals in an attempt to grow single crystals of polymer 4 using a liquid-liquid slow diffusion setup. 3 could also be obtained in a controlled fashion as a red fine crystalline material when the starting complex 2 is converted with 0.5 equivalents of bipy at room temperature. The exact formula of the dimer was determined on the basis of the elemental analysis, mass spectrometry, and the determined crystal structure. The formation of dimeric iron(II) complexes instead of the expected coordination polymers was recently reported to depend strongly on the size ratio of the used equatorial and axial ligands.96,97 Upon conversion of the starting material 2 with an excess of the bipy bridging ligand (10 equivalents), the black fine crystalline material [FeL1(bipy)]n (4) precipitates. The exact formula of 4 was confirmed with elemental analysis and mass spectrometry. The polymeric nature of the material was assessed with powder Xray diffraction and is discussed further. All attempts to obtain larger single crystals of 4 using slow diffusion techniques (liquid-liquid) were so far unsuccessful regardless of the equivalent of bipy. Only single crystals of the dimer 3 could be obtained. Presumably, the dimer 3 is too insoluble and precipitates right away, hindering the formation of any polymeric structures. SEM pictures of 3 and 4 confirmed the purity and assess the morphology of the samples, (see Supporting Information: Figure S1). The dimer 3 forms block-shaped crystallites with an average size of 1 µm, similar to the monocrystals grown using slow diffusion setup. The SCO coordination polymer 4

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precipitates as platelet-like crystallites, with the smaller dimension measuring 100–150 nm. The other two dimensions of the platelet measures 500 nm to several mm, the platelets presenting then a large surface. Magnetism The magnetic properties of both compounds 3 and 4 were investigated with a SQUID magnetometer in the range from 50 K to 400 K in the settle mode. The dimer 3 has a magnetic susceptibility temperature product (χMT) value of 7.40 cm3Kmol-1 at room temperature, which is in a typical range for a complex with two iron(II) HS centers. It stays HS over the whole temperature range investigated (Supporting Information: Figure S2). This result is not surprising as the FeN3O3 coordination sphere for this ligand system is too weak and leads to pure HS complexes.97–99

Figure 1: Pathway of synthesis of the compounds 3 and 4 described in this work.

Figure 2: Magnetic susceptibility temperature product (χMT) vs. temperature measurement for compound 4 displayed between 275 and 400 K. The χMT versus T plot of 4, however, shows a different temperature dependence that is displayed in Figure 2. The coordination polymer 4 is with a χMT value of 0.095 cm3Kmol-1 at room temperature in a typical range for an iron(II) center in its LS state. Upon warming, the compound undergoes a first spin transition at T½ = 380 K ending in its HS state with a χMT value of 3.11 cm3∙K∙mol-1 at 400 K. Upon cooling, the χMT product stays constant until 330 K, where the compound transits towards its LS state (T½ = 323 K), revealing a first hyste-

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resis of 57 K. If the compound is heated again, the LS to HS transition occurs again, however at a lower temperature T½ = 371 K, then upon cooling, the compound undergoes its HS to LS at the same temperature as in the first cycle, reducing the hysteresis width to 48 K. This hysteresis is then stable and can be measured several times, as illustrated in the SI, Figure S2. Leaving the sample at 400 K under vacuum in order to remove possible solvent molecules showed no further influence on the SCO behavior. X-ray Structure Analysis Single crystals of 3 suitable for X-ray structure determination were obtained from a liquid-liquid slow diffusion setup between methanolic solutions of 2 and bipy under argon atmosphere. The structure was determined at 133 K and the crystallographic data are presented in Supporting Information: Table S1. The dimer 3 crystallizes in the triclinic space group P1� and the asymmetric unit contains half a dimer molecule and one non-coordinating methanol solvent molecule. An ORTEP drawing of the dimer molecule is shown in Figure 3. The iron center has an octahedral FeN3O3 coordination sphere, with a methanol molecule and the bridging bipy as axial ligands. The iron center, whose spin state can be determined by measuring the O1–Fe–O2 angle,99,100 is with 107.34(7)° clearly in the HS state, in agreement with the magnetic data. Selected bond lengths and angles are listed in Table 1.

Figure 3: ORTEP drawing of the dimer 3. The thermal ellipsoids are shown at 50% level. Hydrogen atoms and noncoordinating solvent molecules were omitted for clarity reasons. Table 1: Selected bond lengths [Å] and angles [°]. Bond Fe1–O1 Fe1–O2 Fe1–O3 Fe1–N1 Fe1–N2 Fe1–N3

Angle 2.0249(19) 1.9953(18) 2.2839(19) 2.070(2) 2.083(2) 2.279(2)

O1–Fe1–O2 N1–Fe1–N2 O3–Fe1–N3

107.34(7) 79.78(8) 174.40(7)

The crystal packing of 3 presents a hydrogen bond network, where the axial methanol (O3–H3) of the iron center binds to the phenazine nitrogen atom (N5) of a neighboring molecule, which binds back to the first molecule in the same way. A consequence of this interaction is the formation of 1D chain of the dimers, which propagate along the vector [1 1 -1]. The chains are further connected through a non-classical hydrogen bond between the methyl group of the axial methanol (C30H30C) and one of the oxygen atoms (O6) of the ester substituents of a neighboring chain. This leads to the formation of a 2D network with base vectors [1 1 -1] and [0 0 1]. One more hydrogen bond is observed between the non-coordinating methanol solvent molecule (O51-H51) and the second phenazine nitrogen atom (N4). The crystal packing is depicted in Figure 4 and the D–H∙∙∙A distances and angles are given in Table 2.

Figure 4: Crystal packing of 3 along vectors [1 0 0] (top) and [0 0 1] (bottom). Hydrogen atoms non-participating in hydrogen bond were omitted for clarity. Hydrogen bonds are depicted as pink dashed lines. Table 2: Summary of the D–H∙∙∙A interactions in the crystal packing of 3. Distances are given in [Å] and angles in [°]. D O3 O51 C30

H H3 H51 H30C

A a

N5 N4 O6b

D–H

H∙∙∙A

D∙∙∙A

D–H∙∙∙A

0.84 0.84 0.98

2.50 2.09 2.44

2.879(3) 2.923(4) 3.411(4)

108 172 172

a = 1-x, 1-y, 1-z; b = 1-x, 1-y, 1-z.



Differential Scanning Calorimetry DSC measurements of 4 were done to determine the enthalpy and entropy changes associated with the SCO phenomenon itself, but also to investigate the possible presence of supplementary phase transitions, linked or not, with the spin transition. The DSC measurement is shown in Figure 5. Upon heating, the sample presents a sharp endothermic transition at 375 K, corresponding to the LS to HS spin transition observed in the SQUID measurement. Upon cooling, a relatively broader peak at 327 K corresponding to the HS to LS spin transition was observed. The transition temperatures are in good agreement with the magnetic measurement taken into account the different measurement modes and scanning velocities. The different broadness of the peaks reflects how abrupt the ST is, in accordance with the results of the SQUID measurement where the HS to LS transition is less abrupt than the LS to HS transition. Another feature of the SQUID measurement is the different transition temperatures between the very first LS to HS transition and the subsequent LS to HS transitions. This feature was also observed in the DSC measurement. The integrated enthalpy and entropy values are with ∆H = 16.1(3) kJ∙mol-1 and ∆S = 44.7(12) J∙mol-1∙K-1 for the LS to HS transition, and with ∆H = 15.7(2) kJ∙mol-1 and ∆S = 47.4(13) J∙mol1 ∙K-1 for the HS to LS transition, similar to the values obtained for other spin crossover complexes with abrupt ST of the same family.21,34,101,102 The enthalpy and entropy values of the very first LS to HS transition (∆H = 16.1(3) kJ∙mol-1 and ∆S = 44.7(12) J∙mol-1∙K-1) and the subsequent LS to HS transition (∆H = 16.3(3) kJ∙mol-1 and ∆S = 43.8(12) J∙mol-1∙K-1) are in the same order of magnitude. No supplementary phase transitions were observed in the investigated temperature range (300 K – 400 K).

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der patterns are presented in Figure 6. At 300 K, the powder pattern exhibit five strong reflections at 2θ = 6°, 10°, 12.5°, 17.5°, and 24°. Typically, strong reflections in the 2θ range of 8°–12° corresponds to the Fe-Fe distance within a chain, as previously reported for similar coordination polymers. 21,34,96 In this case, the peak at 10° would correspond to a Fe-Fe distance of ≈ 9 Å, which is in agreement with the length of a Fe–bipy–Fe distance.103,104 Upon heating, the pattern at 350 K is very similar to the pattern at 300 K, with minor shifts due to the temperature change. At 390 K, the spin transition took place and the diffractogram is different. Starting with five reflections in the LS state, the sample shows now six main reflections in the HS state as the peak at 24° splits into two peaks at 23.2° and 23.5°. Moreover the reflections at 10° and 17.5° are shifted to a lower diffraction angle, respectively at 9.6° and 16.7°. A shift of the reflection at 10° in the LS state to 9.6° in the HS state corresponds an elongation of the Fe-Fe distance of ≈ 1 Å, which is in agreement with the coordination sphere volume increase upon LS to HS transition. Upon cooling, the pattern at 350 K is still characteristic for the HS state. This is a further proof of the bistability of the material, as it was measured at 350 K in both spin states. When the compound is back to 300 K, a diffractogram similar to the starting diffractogram was recorded, and the sample is back to its LS state.

Figure 6: Temperature-dependent X-ray powder patterns of 4. The HS state is represented with a red line, the LS state with a blue line. Diffractograms with ↑ were measured upon heating and with ↓ upon cooling. Figure 5: DSC measurement of polymer 4 displayed between 300 and 400 K. The red line corresponds to the first heating. Temperature-dependent Powder X-ray Diffraction The X-ray diffraction powder pattern of polymer 4 was measured at 5 different temperatures in the following sequence: 300 K, 350 K, 390 K, 350 K, and 300 K. The powder patterns of both HS and LS states could then be obtained, as well as in the middle of the thermal hysteresis at the same temperature (350 K) depending on the history of the compound. The pow-

Fluorescence Spectroscopy Emission spectra of the dimer 3 and the polymer 4 in the solid state were measured with varying temperature between 300 K and 400 K, using a 337 nm nitrogen laser as excitation. For both samples, the emission is weak. Normalized spectra measured at 300 K, 350 K, and 400 K are presented in Figure 7. For reference, spectra prior to normalization as well as normalized to unit area are added as Supporting Information, Figure S3.


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The paramagnetic dimer 3 shows a dominant peak at λ2 = 686 nm. In addition, a broad shoulder with lower intensity, centered around λ1 = 550 nm is observed. The spectra do not change with temperature. Similar to the emission spectrum of the dimer 3, the photoluminescence (PL) spectra of the polymer 4 exhibits two prominent emission features at λ1 = 550 nm and at λ2 = 673 nm. However, the relative intensity PL(λ2)/PL(λ1) of these two peaks changes with temperature in a way that mirrors the spin state of the polymer. At 300 K, the peak at λ1 is more intense than at λ2. Comparison with Figure 2 shows that at 300K, sample 4 is in its LS state. Upon heating, at 350 K, the sample 4 is still in the LS state and the emission spectrum stays unchanged.

sion spectrum shows a relatively high intensity at λ1 that reduces with respect to λ2 in the HS state. We observe this behavior for all samples, though there is some variation from sample to sample, and also with sample age, regarding the value of the peak ratio. This correlation between the relative intensities of the peaks λ1 and λ2 and the spin state of the polymer is further substantiated in Figure 8, which compares the temperature evolution of the PL intensity ratio at λ1 and λ2 with the magnetic susceptibility. It is evident how their PL intensity ratio tracks the temperature dependence of the magnetic susceptibility temperature product (χMT ). In other words, the SCO phenomenon can be followed using the fluorescence spectroscopy. The CIE coordinates (CIE= Commission internationale de l’éclairage) in the LS state at 300 K for the sample shown in Figure 7 are (0.33, 0.42), corresponding to a light turquoise-greenish color, and they change to a yellow with (0.36, 0.41) in the HS state at 400 K. As detailed in the SI (Figure S4), after a full measurement cycle, the overall PL intensity at 300 K has slightly diminished, though the peak ratio is hardly affected by this (c.f. 300K (H) vs 300K (C) in Figure 7, as well as Figure 8). We attributed this overall reduction in PL to photobleaching, as rather harsh measurement conditions are used (337 nm laser, heating up to 400 K, several hours of measurements for a full cycle). We have also recorded the reflection at 337 nm (in lieu of the absorption) to probe whether the change in PL intensity with temperature is due to a change in the absorption of the sample. As shown in the SI (Figures S5), while we see a reversible decrease by 10% in the reflection upon heating, probably due to changes in sample scattering, we do not observe any correlation of significant changes in the reflection at 337 nm with the spin state. It was not possible to measure the transmission of the sample for an absorption spectrum since the polymer crystallites were scattering any incident light strongly. When we grinded the crystallites to reduce the scattering, then the SCO behavior strongly reduced. We attribute this to the increase in surface area upon grinding.

Figure 7: Temperature-dependent emission spectra taken from powders of the dimer 3 (top) and the polymer 4 (bottom) with λex = 337 nm upon heating (H) and cooling(C). The stars denote the peaks at λ1=550 nm and at λ2=686 nm (3), λ2=673 nm (4). At 400 K, after the sample underwent spin transition to HS, the ratio PL(λ2)/PL(λ1) changes insofar that the PL intensity at λ1 reduces to become closer to that at λ2. Upon cooling, at 350 K, the sample is still in the HS state and the spectrum is similar to the spectrum at 400 K. Finally, when back in the LS state at 300 K, the spectrum is similar to the starting spectrum. This temperature dependent behavior demonstrates that the spin state of the sample controls the shape of the emission bands. In the LS state of the sample, the corresponding emis-

Figure 8: Comparison of the temperature dependent photoluminescence ratio at λ2=686 nm and λ1=550 nm with the magnetic susceptibility temperature product (χMT) for compound 4 We have also measured the photoluminescence spectra of polymer 4 from 300 K down to 60 K, as detailed in the SI



(Figure S6). Even though there is no spin transition in this temperature range, we observe a reduction of the peak at λ1 relative to that at λ2, so that the ratio PL(λ2)/PL(λ1) increases by a factor of 2 from 300 K to 60 K. Discussion The SCO coordination polymer 4 presents remarkable magnetic properties, exhibiting a 48 K wide hysteresis above room temperature. Wide thermal hystereses are usually associated with a coupling between the electronic transition and structural changes. This is also true for our complex, as the DSC measurement showed that the spin transitions have rather high enthalpy and entropy values, and the powder X-ray diffraction analysis showed that the HS and the LS states present different diffractograms. All this indicates that the SCO is accompanied by a major structural change, though a single crystal X-ray structure analysis would be required to finally confirm this hypothesis. The very first LS to HS ST happens at a higher temperature as all subsequent STs, and this phenomenon was confirmed with SQUID and DSC measurements. A reason could be that minor structural changes upon the first ST, such as the movement of a substituent upon ST, happen in an irreversible way, and upon the subsequent HS to LS, the exact original structure is not recovered. As SCO is an extremely sensitive phenomenon with regard to environmental changes, such minor structural change will have an influence on the SCO properties. The emission properties of both dimer 3 and polymer 4 were measured as a function of temperature. From the fact that the peaks at λ1 and λ2 change their relative intensity depending on the spin state of the polymer we infer that these two peaks have a different electronic origin. This is further supported by the different evolution of the peaks with temperature upon cooling from room temperature to 60 K, and the sample to sample variation in the peak ratio. Since the same two peaks also occur at λ1 and λ2 for the dimer 3, albeit with different intensity, we suggest that for the high spin dimer, the two peaks can also be attributed to two electronic origins. This implies that upon photoexcitation, two different, competing radiative relaxation processes take place. In the case of the corresponding Ni(II) complex, two relaxation processes were also observed, however only the diamagnetic specie was luminescent.89 Here, we observe that the emission peak at λ1 is at lower energy (longer wavelength) than the emission of the ligand 1 in solution and its corresponding octahedral Zn(II) complex.89 The wavelength of the λ2 emission feature corresponds to an energy of 1.8 eV, which is slightly larger than the values of 1.4 to 1.6 eV typically found for the energy splitting between the t2g and eg* orbitals of an iron(II) in the HS state.2,8,105,106. It is, however, also significantly smaller than the π − π* transition of the ligand 1, which emits at 2.64 eV (470 nm).89 Thus, the λ2 emission seems to be caused by a transition that is largely iron(II)-centred, i.e. eg* to t2g, though the value of 1.8 eV suggests that there may be some admixture from ligand-based π-orbitals that increases the splitting. For an unambiguous assignment of this emission, detailed quantum chemical calculations would be required, which is beyond the scope of this paper. Nevertheless, it seems that we observe a rare example107 of an iron-based photoluminescence at room

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temperature. This can be rationalized by considering that the comparatively high energy of this transition of 1.8 eV reduces the influence of the non-radiative decay via internal conversion, which is known to decrease exponentially with increasing energy (energy gap law. e.g. 108,109). Moreover, for a largely metal-centered transition, vibrations on the rather rigid organic ligand do not contribute to the non-radiative decay as they hardly couple to the electronic transition. Regarding vibrations between the metal center and the ligand cage, these are impeded in this coordination polymer by the rigid crystal lattice in the solid state. We believe these factors contribute to rendering the radiative transition competitive with respect to the non-radiative decay. We recall that the intensity of the λ2 emission feature is weak and close to our detection limit. On the basis of these observations of two distinct emission features, we propose a competition between two relaxation processes. The first one is a metal-to-ligand-charge-transfer (MLCT) based relaxation process in which an electron of a t2g metal-centred orbital is excited into a πL* ligand-centred orbital. When it relaxes back, the emission λ1 is observed. This is illustrated at the top of Figure 9 for both spin states. The second is a relaxation process based on an entirely metalcentred transition. After the excitation of an electron into the πL* orbital, an electron transfer can take place from the ligandbased πL* orbital to the metal-based eg* orbital. The excited electron in the eg* orbital can then relax to the t2g orbital by light emission at a wavelength λ2. This processes is depicted at the bottom of Figure 9, also for both spin states. When the complex is in the HS state throughout the entire process, neither absorption nor emission require a change of spin. Both luminescence features can be attributed to emission within the quintet state manifold. When in the LS state, the emission has singlet character. The correlation of the relative peak intensities with the spin state of the iron(II) center can be understood by considering the associated changes in the crystal structure that result from the spin transition. In the HS state, the eg* orbitals are occupied and the metal-ligand bond length increases. This results in a reduction of the associated crystal field strength, and a concomitantly reduced splitting between the t2g and eg* orbitals. (Refs Chapter 7 of “Transition Metal Chemistry” by M.

Gerloch and EC Contable,110 as well as Chapter 4 in “Molecular Magnetism” by O. Khan,111 P. Gütlich, H.A. Goodwin, “Spin Crossover in Transition Metal compounds I”,2 B. Weber, “Koordinationschemie”112). The energetics in the HS and LS state are compared by the left and right panel in Figure 9. In the HS state, the crystal field strength is smaller than in the LS state, implying that the energy difference between the iron(II) centred t2g and eg* orbitals is small. As a result, electron transfer from the ligand to the metal would be favorable. However in the LS state, we estimate that the crystal field strength is larger so that the eg* orbital would come to be at higher energy than the πL* orbital, thus preventing electron transfer from photoexcited ligand to the metal. The occurrence of dual emission in the HS state is unusual. It would imply that the rates for the two processes, MLCT-based emission and metal-based emission after photoinduced electron transfer, are in competition, with the key parameter being how fast the electron exchange takes place. Unfortunately, the overall low luminescence efficiency of the samples precluded


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time-resolved measurements. However, the increase in the relative intensity of the peak at λ1 upon cooling from 300 K to 60 K is consistent with a freezing out of the electron exchange and supports our suggestion of two competing rates. The dual emission is observed in both, compound 3 and 4. For the dimer 3, the iron(II) centered emission dominates the spectrum, consistent with our suggested scenario that is summarized in Figure 9. Its photoluminescence does not show any change in the shape of the spectra with temperature between 300 K and 400 K, in agreement with the fact that it is a pure HS species. Sample 4 shows both emission peaks in both spin states, albeit with different intensities. In principle, the emission from the HS state should give a dominant contribution at λ2 analogous to dimer 3, yet we observe about equal intensities of the peaks at λ1 and λ2. The observation of a significantly higher contribution at λ1 in the HS polymer 4 than in the HS dimer 3 implies the presence of some remaining LS domains in the polymer even at elevated temperatures. In the LS state, the λ1 peak is enhanced compared to the peak at λ2, confirming that the MLCT-based relaxation process is

favored. The fact that we still observe a significant contribution from λ2 may be attributed to effects from the HS chain ends. Sample 4 is a coordination polymer, and usually the properties of such coordination polymers are analyzed and interpreted taking into account that the polymer chain is of infinite dimension, and that the contribution of the ends of the polymer chain is negligible. However, in case of nanosized material, the contribution of the end of the chains (localized on the surface) is no longer negligible, as illustrated in the SI, Figure S1.74 The SEM pictures of 4 showed that the SCO polymer consists of thin platelet-shaped crystals, with a rather high surface. The PL measurement of the crystals collects mainly emission that originates from close to the surface. Therefore, it is conceivable that the resulting spectrum is a superposition of the emission of the surface which is HS and the emission of the SCO-core. Since the dimer 3 can also be considered as a model for the two end groups of the polymer chain, we expect that the emission properties of 3 will be similar to those of the surface of 4.

MLCT-based relaxation process Fe(II) HS

E

πL*

λ1

πL*

eg*

λex

eg*

λ1

λex

t2g

πL

Fe(II) LS

E

t2g

πL

Metal-based relaxation process Fe(II) HS E

πL

*

λex πL

x

E

e- transfer

λ2

eg* t2g

πL

Fe(II) LS

eg*

*

λex πL

xx

λ'2 t2g

Figure 9: Simplified schema representing the two hypothetical relaxation processes, in the HS state (left) and LS state (right) In summary, for the polymer 4, when in the HS state, we observe similar spectra as for the dimer 3, with the two proposed relaxation processes competing with each other. However, in

the LS state, the metal-centered process is not happening in the core of the material, so that the emission is ligand-based at λ1 and the λ2 emission observed originates from the surface of



the material. Please note, that we cannot rule out that the pronounced crystallographic phase transition also triggers changes in the emission spectra. A clear assignment, which factor (electronic, phase transition or both) influences the emission properties is at this point not possible. The fact that the spinstate of the sample can be traced by monitoring the ratio of the two emission peaks is a convenient signature that may be exploited for device (sensing) purposes. It also increases the robustness with respect to possible photobleaching, as the latter tends to quench the entire emission of the molecule, thus leaving the intensity ratio of the peaks of the overall sample largely unaffected. In order to verify the proposed hypothesis regarding the origin of the luminescence features, further, more detailed measurements as well as quantum chemical calculations and the synthesis of further slightly modified examples120 are required. Experimental Section Synthesis Methanol (MeOH) was purified by distillation over Mg under argon atmosphere.113 The ligand H2L1 (1)89 and iron(II) acetate114 were synthesized as described in literature. 4,4’bipyridine (bipy) (Alfa Aesar, 99.9%) was used without further purification. All syntheses were carried out under argon using Schlenck techniques. CHN analyses were measured with a Vario El III from Elementar Analysen-Systeme. Mass spectra were recorded with a Finnigan MAT 8500 with a data system MASPEC II. IR spectra were recorded with a Perkin Elmer Spectrum 100 FT-IR spectrometer. [FeL1(MeOH)2] (2): H2L1 (0.2 g) and iron(II) acetate (0.11 g) were dissolved in 40 mL MeOH. The black solution was refluxed for 2 hours and then allowed to cool down. Upon cooling a dark green crystalline precipitate appeared that was filtered off, washed with MeOH (2×5 mL), and dried in vacuo. Yield: 0.22 g (88 %). IR: 𝝂𝝂� = 3246(b) (OH), 1625(s) (CO), 1589(s) (CO) cm-1; MS (DEI-(+), 70 eV) m/z (%): 516 (100) ([FeL1]+); elemental analysis calculated (found) for C26H28FeN4O8 (580.37 g∙mol-1): C 53.81 (53.21), H 4.86 (4.23), N 9.65 (9.62). {(µ-bipy)[FeL1(MeOH)]2}∙2 MeOH (3): 2 (0.2 g) and bipy (0.03 g) were dissolved in 10 mL MeOH. The dark red solution was left to stir at room temperature for 2 hours. After a few minutes, a red crystalline precipitate appeared in the flask, which was filtered off, washed with MeOH (2×1 mL), and dried in vacuo. Yield: 0.16 g (70 %). IR: 𝝂𝝂� = 3240(b) (OH), 1615(s) (CO), 1596(s) (CO) cm-1; MS (DEI-(+), 70 eV) m/z (%): 516 (65) ([FeL1]+), 156 (100) ([bipy]+); elemental analysis calculated (found) for C62H64Fe2N10O16 (1316.94 g∙mol-1): C 56.55 (55.98), H 4.90 (4.32), N 10.64 (10.62). [FeL1(bipy)]n (4): 2 (0.2 g) and bipy (0.54 g) were dissolved in 20 mL MeOH. The black solution was refluxed for 1 hour and then allowed to cool down to room temperature. A black crystalline material precipitated upon cooling that was filtered off, washed with MeOH (2×2 mL), and dried in vacuo. Yield: 0.19 g (82 %). IR: 𝝂𝝂� = 1630(s) (CO), 1587(s) (CO) cm-1; MS (DEI-(+), 70 eV) m/z (%): 516 (13) ([FeL1]+), 156 (84) ([bipy]+); elemental analysis calculated (found) for C34H28FeN6O6 (672.47 g∙mol-1): C 60.73 (60.53), H 4.20 (4.376), N 12.50 (12.96).

Page 8 of 12

X-ray Structure Analysis The intensity data of 3 were collected with a STOE StadiVari diffractometer using graphite-monochromated MoKα radiation. The data were corrected for Lorentz and polarization effects. The structure was solved by direct methods (SIR97)115 and refined by full-matrix least-square techniques against Fo2-Fc2 (SHELXL-97).116 All hydrogen atoms were calculated in idealized positions with fixed displacement parameters. ORTEP-III117,118 was used for the structure representation, SCHAKAL-99119 to illustrate the crystal packing. A cif file was deposited at the CCDC database (CCDC 1473427). X-ray Powder Diffraction Powder diffractograms were measured with a STOE StadiP diffractometer using CuKα1 radiation with a Ge monochromator, and a Mythen 1K Stripdetector in transmission geometry. Differential Scanning Calorimetry Calorimetric measurements were carried out with a differential scanning calorimeter DSC821e from Mettler Toledo, with a scan rate of 5 K∙min-1. Scanning Electron Microscopy Scanning electron microscopy pictures were gathered at a Zeiss LEO 1530. Samples were prepared on carbon tape. Magnetic measurements Magnetic susceptibility data were collected using a MPMSXL5 SQUID magnetometer under an applied field of 0.5 T over the temperature range 50 to 400 K in the settle mode. The samples were placed in gelatin capsule held within a plastic straw. The data were corrected for the diamagnetic contributions of the ligands by using tabulated Pascal’s constants and of the sample holder.111 Fluorescence Spectroscopy The temperature dependent emission spectra were measured in a home build setup. The samples were placed between a fused silica substrate and a coverslip, sealed with glue under argon atmosphere. A supplementary Indium wire was placed between the sample and the glue to avoid contact. The metal ring separates powder and glue thus preventing a reaction of glue and complex. The sample was then placed in an electrically heatable continuous flow cryostat (Oxford Instruments). It was excited using a nitrogen laser with 337 nm emission. The light emitted by the complex was focused into a spectrograph (Andor Technology Shamrock SR303i) and detected with a CCDcamera (Andor iDus). Each temperature was stabilized during 15 min before measurement in order to ensure homogeneous temperature of the sample. The emission spectra were corrected for the transmission through the setup. As detailed in the SI, we also confirmed that light detected at 673 nm was only PL from the sample and not from second order diffraction of the excitation laser. Conclusions We presented the synthesis of a new dimer 3 and a new SCO coordination polymer 4 based on a phenazine-derived Schiff base-like ligand. The compound 4 presents a 48 K wide thermal hysteresis which was characterized with SQUID, DSC, and X-ray powder diffraction. The large hysteresis probably


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originates from a structural phase transition, which is coupled to the electronic spin transition, as HS and LS states present different diffraction patterns. The emission properties in the solid state were measured and showed that not only the band structure is depending on the spin state, but the SCO can also be followed through measurement of the emission intensity. Future investigations as measurement of the fluorescence properties in solution/gel matrix and the synthesis of nanoparticles of this material are in progress.

ASSOCIATED CONTENT Supporting Information. SEM pictures of samples 3 and 4, crystallographic data of 3, χMT vs. T plot of 3, 4 and a finely grind sample of 4, photoluminescense spectra of 3 and 4 as function of temperature. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *Prof. Dr. Birgit Weber, Inorganic Chemistry II, Universität Bayreuth, Universitätsstrasse 30, NW I, 95440 Bayreuth, Germany [email protected] *Prof. Dr. Anna Köhler, Experimental Physics II and Bayreuth Institute of Macromolecular Research (BMBF), Universität Bayreuth, Universitätsstrasse 30, NW I, 95440 Bayreuth, Germany

[email protected] Present Addresses † Dr. C. Lochenie, Laboratoire de Chimie et des Biomatériaux Supramoléculaires, Institut de Sciences et d’Ingénierie Supramoléculaires, Université de Strasbourg

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Funding Sources German Research foundation (DFG; SFB840; project A10), German Research Foundation (DFG; GRK1640), BayNAT program of the University of Bayreuth

ACKNOWLEDGMENT Financial supports from the German Research foundation (SFB840; project A10), the BayNAT program, and the University of Bayreuth are acknowledged. Fabian Panzer acknowledges financial support by the German Research Foundation DFG through the research training group GRK1640. Ling Peng (MCII, University of Bayreuth) is thanked for the calorimetric measurement. Ottokar Klimm (ACII, University of Bayreuth) is thanked for the measurement of the SEM pictures.

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Spin crossover iron(II) - American Chemical Society

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