Near-infrared emissive Er(III) and Yb(III) molecular nanomagnets in metal–organic chains functionalized by octacyanidometallates(IV)

Robert Jankowskia, Jakub J. Zakrzewskia, Olga Surmaa, Shin-ichi Ohkoshib, Szymon Chorazy*ab and Barbara Sieklucka*a
aFaculty of Chemistry, Jagiellonian University, Gronostajowa 2, 30-387 Kraków, Poland. E-mail: chorazy@chemia.uj.edu.pl; barbara.sieklucka@uj.edu.pl
bDepartment of Chemistry, School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan

Received 21st May 2019 , Accepted 19th July 2019

First published on 22nd July 2019


The self-assembly of lanthanide(3+) ions with pyrazine N,N′-dioxide (pzdo) and bis(triphenylphosphine)iminium (PPN+) salts of diamagnetic octacyanidometallates of MoIV and WIV results in one-dimensional (PPN)[LnIII(pzdo)2(MeOH)0.3(H2O)3.7][MIV(CN)8]·7.7H2O·2MeOH (Ln = Er and Yb; M = Mo and W) coordination networks. They are constructed of zigzag metal–organic {LnIII(μ-pzdo)}n chains with terminal [MIV(CN)8]4− metalloligands attached to the LnIII centres. Both Er- and Yb-containing frameworks exhibit field-induced slow magnetic relaxation originating from the field-dependent equilibrium between quantum tunnelling of magnetization and the direct process along with the temperature-dependent Raman and Orbach relaxation processes. Transition metal substitution on [MIV(CN)8]4− sites modifies the intrinsic magnetic anisotropy of lanthanides, as depicted by the subtle change of thermal energy barriers of Orbach relaxation in Er-based systems, while it significantly affects Raman relaxation due to the modulated phonon mode scheme. All reported compounds exhibit strong visible light absorption due to a series of electronic transitions of pzdo ligands and [M(CN)8]4− ions and the appearance of low energy anion–π charge transfer band involving pzdo and octacyanidometallates. These charge transfer states were utilized in achieving the sensitized near-infrared photoluminescence of ErIII and YbIII centers; thus, the energy transfer process is strongly dependent on the nature of the metal centre in the [MIV(CN)8]4− ion. The W(IV)-pzdo system is a good sensitizer for ErIII, while the Mo(IV)-pzdo unit is better for YbIII emission which can be rationalized in terms of the positions of energy levels of their respective donor states. Therefore, we report NIR emissive ErIII and YbIII single-molecule magnets with tunable magnetic and optical properties, uncovering the crucial role of octacyanidometallates and their anion–π interactions with pzdo ligands.


Introduction

Emissive lanthanide-based materials enthral researchers due to their applications in display devices and light emitting diodes,1 optical storage,2 optical communication,3 chemical sensing,4 molecular thermometers,5 and bioimaging.6 The last decade brought also an increasing attention to near-infrared (NIR) emissive lanthanides, such as Er(III) and Yb(III), exhibiting luminescence above 700 nm.7 It opened a new route for development of the next generation of energy conversion systems,8 optical devices,9 sensors,10 and bioimaging tools.11

High performance emissive lanthanide-based materials need efficient sensitization of their luminescence to overcome the forbidden character of f–f electronic transitions.12 This issue is broadly addressed in the literature where various inorganic matrices, organic ligands and metal complexes with good light-harvesting abilities were shown to efficiently sensitize the 4f-centered emission due to the energy transfer effect.12,13

A different approach consisting of the functionalization of emissive materials towards multi-functional multi-responsive optical devices was reported.14–17 Following this idea, luminescent ferroelectrics,14 conductive luminophors,15 emissive magnetically ordered systems,16 and enantiopure luminescent materials17 were reported. In this context, emissive lanthanide molecular magnets, named Single-Molecule Magnets (SMMs),18 appear as a promising possibility.19 SMMs are constructed of mono- or polynuclear d- or f-block metal complexes and exhibit strong magnetic anisotropy, leading to the slow relaxation of magnetization and the resulting magnetic hysteresis loop.20 They are considered for applications in information storage,21 quantum computing,22 and spintronics.23 Using 4f metal ions inserted in specific coordination environments, high performance SMMs were obtained.24 Moreover, lanthanides offer emission properties; thus, they are a rich source of bifunctional luminescent molecular nanomagnets, showing a correlation between optical and magnetic characteristics.19,25

Emissive lanthanide SMMs are usually obtained by attachment of selected organic ligands to 4f metal centres, such as yellow emissive Dy(III) or NIR-emissive Yb(III).19,26 The organic ligands serve as sensitizers for 4f-based luminescence, constraining also the coordination geometry towards improved magnetic anisotropy. The alternative strategy may utilize d-block metal complexes which can efficiently transfer the energy to lanthanides, enhancing their luminescence.27 In this regard, we found great potential in polycyanidometallates, which are anionic coordination complexes bearing transition metals and potentially bridging cyanide ligands.28 Among them, square planar [MII(CN)4]2− (M = Pd and Pt) and linear [MI(CN)2] (M = Ag and Au) anions sensitize the visible emission of lanthanides,29 while octahedral [MII(CN)4(L)]2− (M = Ru and Os; L = aromatic chelates) improves the NIR emission of Yb(III) and Nd(III).30 In these cases, the metal-to-ligand charge transfer (MLCT) states of cyanide metal complexes are of crucial importance, being responsible for efficient energy transfer to the 4f metal centre. We proved that an analogous role can be played by low energy d–d transitions of [MIII(CN)6]3− (M = Co, Rh, and Ir) ions which results not only in sensitized 4f-centred emission but also in a unique family of emissive SMMs.31 In these studies, we explored the idea of emissive molecular nanomagnets achieved by insertion of lanthanides into the coordination networks to control the surrounding of a magnetic ion.32 As a next step, we decided to examine the diamagnetic and photoactive octacyanidometallates of Mo(IV) and W(IV).33 They are analogues of paramagnetic [MV(CN)8]3− (M = Mo and W) ions which were investigated in the luminescent and magnetically ordered d–f frameworks.34 Diamagnetic [MIV(CN)8]4− (M = Mo and W) ions are more suitable for lanthanide SMMs but they do not possess appropriate CT states for the energy transfer process. However, it was lately recognized that redox active cyanide metal complexes can be involved in anion–π interactions with electron deficient aromatic systems, giving rise to novel anion–π CT states.35 This can lead to strongly coloured materials suitable for sensitized NIR emission.36 To test this idea, we combined NIR emissive Er(III) and Yb(III) with redox active [MIV(CN)8]4− (M = Mo and W) and π-acidic pyrazine N,N′-dioxide (pzdo).35b Here, we present four (PPN)[Ln(pzdo)2(MeOH)0.3(H2O)3.7][MIV(CN)8]·7.7H2O·2MeOH (LnM = ErMo, ErW, YbMo, and YbW) coordination systems built of Ln(pzdo) metal–organic chains functionalized by [MIV(CN)8]4−. They exhibit slow magnetic relaxation tuned by both 4f and 4d/5d metals and Er/Yb NIR emission sensitized by the anion–π CT states related to the supramolecular interaction of cyanide complexes with pzdo.

Results and discussion

Structural studies

The red block crystals of ErMo and YbMo were obtained from H2O–MeOH solutions of lanthanide(III) chloride, (PPN)4 [Mo(CN)8nH2O salt and pyrazine N,N′-dioxide. An analogous synthetic method was applied for ErW and YbW, resulting in dark grey crystals (see the Experimental Section). All samples were characterized by IR spectra, TGA and CHN elemental analysis (Fig. S1 and S2, ESI), followed by single-crystal X-ray diffraction analysis (Fig. 1 and Fig. S3–S5 and Tables S1–S7, ESI).
image file: c9qi00583h-f1.tif
Fig. 1 Crystal structure of all reported compounds (ErMo, ErW, YbMo, and YbW) presented using the structural model for ErMo: the view of a single metal–organic chain decorated by octacyanidometallates (a), the details of lanthanide(III) coordination geometry and the view of the molecular building unit (b) and the supramolecular arrangement of the coordination chains and organic counterions (c). Solvent molecules are omitted everywhere for clarity.

The crystal data indicate that ErMo, YbMo, ErW, and YbW are isostructural, crystallizing in the identical P21/c space group (Table S1 and Fig. S3–S4, ESI). They consist of zigzag metal–organic chains based on lanthanide(III) ions bridged by organic pzdo linkers. The chains are branched by octacyanido-metallate anions bonded to each lanthanide(III) centre by a cyanide bridge (Fig. 1a). The coordination sphere of 4f metal ions is completed by terminal pzdo ligands and solvent molecules, methanol and water, forming a geometry of a strongly distorted dodecahedron (Fig. 1b and Table S6, ESI). The metal–organic chains are stabilized by intramolecular anion–π interactions involving the electron deficient pzdo aromatic system embedded between two negatively charged [MIV(CN)8]4− ions of a square antiprism geometry (Fig. 1a and Table S7, ESI). The representative distances between the pzdo rings and the closest lying nitrogen atoms are within the range of 3.0–3.2 Å, only dependent on the d–f metal pairs. Due to the presence of octacyanidometallate(4-) anions arranged within the coordination chains, these 1-D coordination polymers are negatively charged. Thus, they crystallize together with the expanded bis(triphenylphosphine) iminium (PPN+) cations (Fig. 1c and Fig. S5, ESI). Negatively charged metal–organic chains form supramolecular layers within the ab plane due to the hydrogen bonding network involving terminal cyanides, methanol and water molecules, as well as the π–π interactions between neighbouring pyrazine N,N′-dioxide ligands. These supramolecular metal–organic layers are separated by the cationic layers of PPN+ cations which are stabilized mainly due to the steric adjustment of their expanded aromatic systems (Fig. 1c). As a result of such coordination and supramolecular arrangement, lanthanide(III) complexes are well separated. The shortest Ln–Ln distances within the chains are 9.19(3), 9.18(3), 9.16(3) and 9.13(3) Å for ErMo, ErW, YbMo and YbW, respectively, while the shortest Ln–Ln distances between neighbouring chains are 8.10(3), 8.10(3), 8.09(3) and 8.06(3) Å for ErMo, ErW, YbMo and YbW, respectively. This shows that the Ln–Ln distances are very similar for all compounds. The validity of the structural models and the isostructurality of the reported compounds were confirmed using the powder X-ray diffraction method (Fig. S6, ESI).

Magnetic properties

The results of magnetic characterization including static direct- (dc) and dynamic alternate-current (ac) measurements for ErMo, ErW, YbMo and YbW are gathered in Fig. 2–3, Table 1, and Fig. S7–S16, ESI. For all reported materials, the values of the room temperature χT product are typical for the uncoupled trivalent lanthanide ions (Fig. S7, ESI). They reach 11.1–11.2 cm3 mol−1 K for Er-containing derivatives and 2.3 cm3 mol−1 K for Yb-based analogues which are close to the expected respective limits of 11.4 cm3 mol−1 K and 2.6 cm3 mol−1 K. Upon decreasing the temperature, a decrease of the χT product is observed as an effect of the thermal depopulation of the mJ sublevels within the respective 4I15/2 (Er) and 2F7/2 (Yb) ground multiplets. DC magnetic curves do not indicate noticeable magnetic interactions down to 1.8 K thanks to the sufficient separation of lanthanide(III) centres by the bridging organic ligand and diamagnetic octacyanidometallates. The field dependence of the magnetization at 1.8 K shows a typical increase of the signal without a hysteresis loop in all compounds. For ErMo and ErW, no saturation of the magnetization is reached even after the application of 70 kOe external dc field. The maximal value of 4.7μB is reached both in ErMo and ErW. For YbMo and YbW, an external dc magnetic field results in a much slower increase of magnetization and the saturation observed with the 1.1μB value at 70 kOe. The similarities in the dc magnetic properties of Mo(IV)- and W(IV)-based compounds indicate similar crystal field splitting of the mJ sublevels which agrees with the small changes in the structural parameters occurring on Mo-to-W metal substitution (Tables S2–S5, ESI).
image file: c9qi00583h-f2.tif
Fig. 2 Dynamic (ac) magnetic properties of ErMo and ErW: field dependences of the relaxation time at T = 1.8 K with the indicated ranges of dominant direct and QTM processes (a), frequency dependences of the out-of-phase magnetic susceptibility χ′′ gathered in the 1.8–2.8 K range at Hdc = 600 Oe for ErMo (b) and Hdc = 1000 Oe for ErW (c), for both Hac = 1 Oe, together with the fitting according to the generalized Debye model. The related temperature dependences of the relaxation time together with the fitting taking into account the combined contributions from Orbach, Raman, direct and QTM relaxation processes are shown in (d).

image file: c9qi00583h-f3.tif
Fig. 3 Dynamic (ac) magnetic properties of YbMo and YbW: field dependences of the relaxation time at T = 1.8 K with the indicated ranges of dominant direct and QTM processes (a), frequency dependences of the out-of-phase magnetic susceptibility χ′′ gathered in the 1.8–2.8 K range at Hac = 1 Oe and Hdc = 1000 Oe for YbMo (b) and YbW (c) together with the fitting according to the generalized Debye model. The related temperature dependences of the relaxation time together with the fitting taking into account the combined contributions from Raman, direct and QTM relaxation processes are shown in (d).
Table 1 Summary of the slow magnetic relaxation effects in ErMo, ErW, YbMo and YbW
Compound ErMo ErW YbMo YbW
Field dependence of relaxation time (Fig. 2a and 3a)
T [K] 1.8 1.8 1.8 1.8
A [s−1 K−1 Oe−4] 1.41(4) × 10−10 3.33(4) × 10−11 4.9(4) × 10−13 2.4(2) × 10−13
a [s−1] 1539(21) 2261(10) 512(13) 638(12)
b [Oe−2] 1.5(4) × 10−5 4.0(2) × 10−6 4.3(8) × 10−6 4.1(4) × 10−6
c [Oe−1] 3.6(4) × 10−3 1.38(3) × 10−3 1.5(2) × 10−3 1.03(7) × 10−3
Temperature dependence of relaxation time (Fig. 2d and 3d)
Hdc [Oe] 600 1000 1000 1000
BRaman [s−1 Kn] 0.26(3) 0.57(9) 1.3(3) 0.56(9)
n 9 (fixed) 9 (fixed) 5.1(2) 6.0(1)
τ0 [s] 2.1(7) × 10−9 5.3(8) × 10−8
Ueff/kB [K] 32(2) 26(3)
Temperature dependence of relaxation time – Arrhenius plot
τ0 [s] 2.3(4) × 10−8 2.4(7) × 10−8 8.8(6) × 10−7 4.6(9) × 10−7
Ueff/kB [K] 24(1) 24(1) 26(1) 27(2)


To study the possible SMM behaviour originating from Er3+ and Yb3+ metal ions, dynamic (ac) magnetic measurements were performed (Fig. 2–3, Table 1 and Fig. S8–S15, ESI). Under zero dc field, slow magnetic relaxation was not observed for all reported compounds. However, the application of even a small dc external field of 200 Oe resulted in the appearance of the maxima in the frequency dependences of the imaginary part of the magnetic susceptibility, χ′′(ν). These plots were analysed using the generalized Debye model (Fig. S8–S15, ESI).37 The optimal dc fields were found from the field dependences of the resulting relaxation times at T = 1.8 K (Fig. 2a and 3a and Fig. S8, S10, S12 and S14, ESI). At this temperature, the combined contributions from the quantum tunnelling of magnetization (QTM) and the field-induced direct process were assumed, as depicted in eqn (1):

 
image file: c9qi00583h-t1.tif(1)

The first term represents the direct process characterized by the constant A, while the second originates from the QTM effect described by the values of a, b, and c.37 The best fit parameters are gathered in Table 1. Using the optimal dc field values, the temperature dependent ac characteristics were collected (Fig. 2b–d and 3b–d and Fig. S9, S11, S13, S14 and S15, ESI). In the high temperature regime, a classical Arrhenius law, given by eqn (2), could be applied:

 
ln(τ) = Ueff/kBT + ln(τ0). (2)

τ0 represents the relaxation attempt time for spin reversal at infinite temperature, whereas Ueff is an effective thermal energy barrier. Alternatively, we fitted the full temperature dependence of the relaxation times using the more complex eqn (3) taking into account multiple relaxation pathways:

 
image file: c9qi00583h-t2.tif(3)

Here, the respective terms show thermally activated Orbach relaxation, two-phonon Raman relaxation, the QTM effect and the direct process.37 To avoid over-parameterization, the contributions from QTM and direct processes were extracted from the field dependences of χ′′(ν) plots. Then, the Raman and Orbach contributions could be considered. In ErMo and ErW, we applied the Raman process with the power (n) fixed at 9 as expected for the Kramers ions.38 Then, to reproduce the experimental data, the Orbach process had to be taken into account (Fig. 3d). We checked the possibility to exclude the Orbach process leaving free Raman power (n). The reasonable, yet noticeably worse, fit was achieved but the power (n) was found to exceed the expected range of 2–9. Moreover, the B parameters differ by an order of magnitude between ErMo and ErW which is rather not realistic for such closely related structures (Fig. S16, ESI). Therefore, we postulate that the Orbach process can be valid for Er analogues. In YbMo and YbW, the Raman relaxation, along with QTM and direct processes, was sufficient to reproduce the ac data. The power of Tn was lowered to the 5.1–6 range, indicating the role of both optical and acoustic phonons in spin–lattice relaxation of Yb derivatives.38 Thus, the Orbach process was not necessary for the fitting procedure. The summary of the fitting parameters of magnetic relaxation in all compounds is shown in Table 1.

A simple Arrhenius law approximation gave similar results for all derivatives, which are effective thermal energy barriers varying from 24(1) K for ErMo and ErW to 26(1) and 27(2) for YbMo and YbW, respectively. The related pre-exponential factors are within the limits of 10−7–10−8 s. All these parameters are in the ranges characteristic of single-molecule magnets with moderate magnetic anisotropy. However, there are non-negligible differences in the scheme of magnetic relaxation processes between Mo- and W-containing compounds. There is a significant difference in the field dependence of the relaxation times between ErMo and ErW (Fig. 2a and Fig S8 and S10, ESI). For ErMo, at low dc fields, the magnetic relaxation is longer, suggesting a weaker QTM effect.37 In addition, a very small impact of the external dc field upon quantum tunnelling is observed in the 200–600 Oe range, and this stage is quickly followed by a strong direct process; thus, the optimal field was identified as 600 Oe. For ErW, we found a stronger QTM effect which further causes the direct process to start to dominate above a higher optimal field of 1000 Oe. For the temperature dependence, the ac magnetic signal was detected in the 1.8–2.8 K range, indicating relatively weak magnetic anisotropy in both ErMo and ErW. While fitting full temperature dependences, it was necessary to include both Raman and Orbach relaxation routes as no satisfactory fit could have been obtained without the Orbach relaxation. With this more precise fitting, the extracted values of the thermal energy barrier are 32(2) K for ErMo and 26(3) K for ErW. This suggests that the single-ion anisotropy of Er(III) is only slightly dependent on the polycyanidometallate ion. This observation is supported by the rather subtle changes in the erbium(III) first coordination sphere between ErMo and ErW (Tables S2 and S6, ESI). This small structural variation of bond lengths and angles within Er(III) complexes can be non-negligible, especially when compared with our latest report on the Co-to-Rh transition metal substitution effect on magnetic anisotropy in two isostructural cyanido-bridged {DyIII[MIII(CN)6]} (M = Co and Rh) dinuclear molecules with a tunable thermal energy barrier from 187(6) to 214(4) K.39 The noticeable differences in the temperature dependences of the relaxation times are also strongly related to the Raman relaxation. In both ErMo and ErW, this contribution is of a typical character for the Kramers ions but the related BRaman parameter is more than two times higher in ErW than in ErMo. Such a difference in the strength of two-phonon assisted Raman relaxation can be ascribed to the modification of the phonon mode scheme on going from lighter [Mo(CN)8]4− to heavier [W(CN)8]4− complexes.40 It is also expected to affect the field dependences of the QTM and direct processes in ErMo and ErW. In addition, a non-negligible role can be played by weak magnetic interactions between neighbouring Er(III) magnetic centres. They are very weak because of long Er–Er distances above at least 8 Å but even such weak exchange interactions can contribute to a better suppression of the QTM effect in the Mo-based analogue.41

An analogous behaviour is observed for YbMo and YbW (Fig. 3, and Fig S9, S11, ESI). The field dependences of the relaxation times reveal stronger QTM for YbW, which is a similar effect to that described above for Er-based compounds. As a result, the dc field required to induce the direct process is higher for YbW. In general, Yb-based compounds exhibit slower overall relaxation times than Er-based derivatives, allowing the extension of the maxima in the χ′′(ν) plots to 5.0 and 4.6 K in YbMo and YbW, respectively. The fitting performed by using the Arrhenius law resulted in the effective energy barriers of 26(1) K for YbMo and 27(2) K for YbW. However, these values incorrectly describe the systems with Yb(III) complexes as further detailed analysis of the full temperature dependences of the relaxation times suggested the lack of significant contribution from Orbach relaxation.42 Therefore, the Raman relaxation effects govern their thermally dependent magnetic relaxation, and they are clearly dependent on the transition metal ion. There is a higher power (n) of the Tn Raman-type contribution for YbW which is again assignable to the modulated phonon modes affecting the efficiency of the relaxation process.43 In particular, in the presented compounds, the heavier W center promotes stronger Raman relaxation than the lighter Mo center which could be correlated with the higher energies of the related vibrational states within cyanide complexes.40 This observation was found for both Er- and Yb-based analogues.

Optical properties

The obtained compounds exhibit intense colours, deep red for ErMo and YbMo or dark grey for ErW and YbW (Fig. 4), and they contain Er(III) or Yb(III) centres which are able to show near-infrared photoluminescence.7 Therefore, we precisely examined their optical properties, including both absorption and emission spectra (Fig. 4–6 and Fig. S17–S22 and Table S8, ESI). ErMo and YbMo compounds show strong absorption ranging from the UV region to the visible range up to ca. 650 nm, independently of the 4f metal ion (Fig. 4). Even more broadened absorption is observed in ErW and YbW where the absorption tail to ca. 750 nm was found. The absorption spectra of all compounds could be deconvoluted into six components, named 1–6 for ErMo and YbMo or I–VI for ErW and YbW (Fig. 4). Four absorption components from the UV range (1–3 and I–III) and at the edge of the UV-Vis region (4 and IV) are related to the combined contributions from the metal-to-ligand charge transfer (MLCT) and ligand-field (LF) electronic transitions of the respective octacyanidometallates as well as the singlet-to-singlet n–π* and π–π* electronic transitions of pzdo ligands (Fig. 4 and Table S8).44 The arrangement of these broad absorption peaks is essentially identical for all compounds, as both Mo(IV) and W(IV) cyanide complexes show similarly strong absorption in these ranges as proved by the comparison with their inorganic salts (Fig. S17). Band 5 in ErMo and YbMo and the analogous band V in ErW and YbW with the maxima at ca. 430 nm mainly correspond to the low energy LF transitions of the respective [MIV(CN)8]4− ions. They are similar for all compounds; however, the relative intensity is higher for the Mo(IV)-based systems which usually exhibit stronger and slightly red-shifted visible absorption than the W(IV)-based analogues.44b There is no important contribution from pzdo or lanthanide(3+) ions to the visible region absorption due to the very weak absorption coefficients of the related forbidden electronic transitions (Fig. S17).16b,44a Therefore, there is a different origin of the additional low energy absorption bands which appeared with the maximum at 515 nm in ErMo and YbMo or 575 nm in ErW and YbW. With the support of structural data showing close supramolecular interaction between pzdo and anionic cyanide metal complexes, we assign these bands to the anion–π charge transfer (CT) transitions within the [MIV(CN)8]4−–(pzdo) moieties. Such anion–π CT states have been reported in some supramolecular systems including those exhibiting anion–π interactions between polycyanidometallates and π-acidic aromatic systems.35 To prove that this absorption does not originate from the charge transfer states between 4f and 4d/5d metal centres, we prepared analogous compounds with innocent Lu(III) centres with a completely filled 4f shell (Fig. S18 and S19). The resulting LuMo and LuW derivatives exhibit identical absorption spectra to the Er- and Yb-based analogues, so the additional low energy absorption band could be convincingly assigned to the anion–π CT originating from supramolecular interactions of pzdo and cyanide complexes. It is important to mention that this absorption is not observed in the solution which shows only yellow colour. Therefore, it appears exclusively in the solid state after the self-assembly of the metal–organic chains with octacyanidometallates.
image file: c9qi00583h-f4.tif
Fig. 4 Room temperature solid state UV-Vis-NIR absorption spectra of ErMo (a), ErW (b), YbMo (c) and YbW (d) together with the deconvoluted absorption components 1–6 for Mo-containing compounds and I–VI for W-containing derivatives. The coloured solid lines represent the experimental data, and the black solid lines show the calculated sums while the black dotted lines represent the absorption components. The interpretation of the bands 1–6 and I–VI is discussed in the text.

image file: c9qi00583h-f5.tif
Fig. 5 Low temperature (T = 77 K) solid state photoluminescence properties of ErW: NIR emission spectrum of ErW under the excitation light of 430 nm (a), the excitation spectrum for the monitored emission at 1480 nm (b) together with the indicated assignment of the main contributions to the observed bands and the related schematic energy level diagram showing the main electronic states in ErMo (pzdo, Mo, and Er) and ErW (pzdo, W, and Er) (c). Abbreviations: A = absorption, ET = energy transfer, L = luminescence, CT = charge transfer, LF = ligand field states, and MLCT = metal-to-ligand charge transfer. No detectable emission signal of ErMo was observed.

image file: c9qi00583h-f6.tif
Fig. 6 Room temperature solid state photoluminescence properties of YbMo and YbW: NIR emission spectra under the excitation light of 400 nm (a), the excitation spectra for the monitored emission at 980 nm (b) together with the indicated assignment of the main contributions to the observed bands and the related schematic energy level diagram showing the main electronic states in YbMo (pzdo, Mo, and Yb) and YbW (pzdo, W, and Yb) (c). Abbreviations: A = absorption, ET = energy transfer, L = luminescence, CT = charge transfer, LF = ligand field states, and MLCT = metal-to-ligand charge transfer.

Thanks to the strong absorption in the UV-Vis range, the reported chain materials were checked as the source of sensitized NIR emission related to the f–f electronic transitions of the respective lanthanide ions (Fig. 5–6 and Fig. S20–S22, ESI). Among Er-containing derivatives the characteristic NIR emission band at 1480 nm was detected at 77 K for ErW, while no emission was observed even at low temperature for ErMo (Fig. 5). This emission in ErW is realized by the energy transfer (ET) from octacyanidometallate ions and their anion–π charge transfer states toward Er(III) as proved by the broad excitation band with the maxima around 430 and 590 nm (Fig. 5b). Using both these excitation wavelengths identical patterns of NIR Er(III) emission were observed (Fig. 5a and S20). The analogous energy transfer is not efficient in ErMo; thus, the luminescence of Er(III) is not enhanced enough to exist in the presence of quenching O–H oscillators related to water molecules in the first coordination sphere of Er(III). The lack of efficient ET in ErMo can be rationalized on the basis of the schematic energy level shown in Fig. 5c. The UV-to-Vis excitation obviously populates the excited electronic states of pzdo and [Mo(CN)8]4− ions, and the energy is presumably relaxed to the low lying anion–π CT state. It can further transfer the energy toward the excited states of Er(III); however, it shows energy positions very close to the 4S3/2 acceptor state so the energy back transfer process can easily occur. In contrast, the lower energy position of the respective donor state in ErW favours the better sensitization of the final NIR Er(III) emission.

Near-infrared emission properties were also found for YbMo and YbW, exhibiting characteristic Yb(III)-centered emission with the maxima around 980 nm. This emission is easily detectable not only at a low temperature of 77 K as for ErW but also at room temperature (Fig. 6 and S21). This intuitively suggests that Yb-based analogues exhibit stronger emission than Er-based derivatives, and the related excitation spectra contain a broad band ranging from 250 nm up to 700 nm (Fig. 6b). Thus, various excitation wavelengths from the UV range can be utilized to induce Yb(III) emission patterns (Fig. 6a and S22). The broad excitation band can be assigned to both pzdo ligands and octacyanidometallates and their common anion–π charge transfer states. The excitation and emission spectra were normalized to the direct f–f excitation represented by the sharp excitation peaks around 900 nm. Using this comparison, we observed a much stronger excitation band in the UV-Vis range for YbMo than for YbW. This indicated that the energy transfer excitation pathway in YbMo is better than in YbW when compared with the direct f–f excitation which should be similarly efficient for both compounds due to the identical coordination sphere for Yb(III). Interestingly, the observation of presumably better sensitization in YbMo than in YbW is in contrast to the trend detected in the case of Er(III) NIR emission. This effect can be rationalized by a precise analysis of the schematic energy level diagrams of YbMo and YbW (Fig. 6c). The UV or Vis excitation leads to the population of the excited states of the pzdo ligands and octacyanidometallates as Yb(III) has no significant absorption in this range. Therefore, the final Yb(III)-centred emission has to be realized by the energy transfer process. It typically occurs through lowest lying excited states acting as a donor electronic state. Therefore, the presence of the anion–π CT states in the 550–750 nm range is of crucial importance. They are situated slightly above the emissive state of Yb(III) which results in an efficient energy transfer process giving the sensitized NIR Yb(III) emission. The difference between YbMo and YbW may be correlated with the overall stronger absorption of Mo(IV) analogues. In addition, a close distance between the anion–π CT state of the W(IV)-based compound and the acceptor state of Yb(III) favours a partial energy back transfer process.16a,31

It is important to note that all optical measurements were performed on air-dried powder samples. We checked that during drying in air, all investigated compounds undergo the exchange of the uncoordinated solvent content from mixed methanol/water composition (7.7H2O and 2MeOH per Ln centre) to only water content (9H2O per Ln centre). This was shown by CHN elemental analysis (see the Experimental Section) and additionally proved by a single-crystal X-ray diffraction analysis performed for the air-dried crystals of YbMo and YbW (Fig. S23 and Table S9, ESI). The structural analysis revealed also that the first coordination sphere of lanthanide ions remains essentially unchanged upon exposure to air; thus, the general interpretation of the optical spectra is independent of the solvent content. It hampers the precise discussion on the emission intensity and lifetime but it is beyond the scope of this work. It is important to note that luminescence measurements could not be performed in the mother solution because it contains a high concentration of emissive pzdo molecules. In addition, we tried to perform optical studies in a pure solvent of water/methanol but it leads to gradual decomposition of the sample. Therefore, air-dried samples with a stable composition in air were used for optical characterization.

Conclusions

We report a unique family of metal–organic coordination chains based on lanthanide(III) complexes with bridging pyrazine N,N′-dioxide (pzdo) ligands which are further branched by octacyanidometallate(IV) anions, [MIV(CN)8]4− (MIV = MoIV and WIV). The use of electron deficient pzdo ligands along with negatively charged 4d/5d metal cyanide complexes allowed the formation of anion–π interactions in the solid state which became the source of a strong optical absorption in the visible range. We found that the resulting anion–π charge-transfer band is suitable for sensitization of NIR emission of Er(III) and Yb(III) metal centres. Therefore, we present a novel strategy to apply cyanide metal complexes as sensitizers of the lanthanide emission by utilizing their supramolecular interactions with organic ligands.28b Moreover, the application of diamagnetic octacyanidometallate(IV) molecular building blocks allowed the observation of slow relaxation of magnetization for all presented compounds which originates from paramagnetic lanthanide ions embedded in the complexes with an anisotropic distribution of the electron density. Furthermore, we found that the d-block metal substitution within the polycyanidometallate unit has a non-negligible impact upon both optical and magnetic properties. The Mo-to-W substitution leads to a change in the energy of the charge transfer band and the overall absorption intensity which affects the Ln-centred photoluminescence in the NIR region. In this context the MoIV-based complex interacting with pzdo ligands was identified as better for the Yb(III)-based derivative, while the [WIV(CN)8]4− anion was found to be suitable for the NIR Er(III) emission. On the other hand, metal cyanide anions affect magnetic relaxation processes, e.g. the balance between the QTM effect and the direct process and the strength of Raman relaxation processes through the modification of their phonon mode schemes. Raman relaxation seems to be increased on going from lighter Mo-based to heavier W-containing analogues for both applied lanthanides. This gives a practical guide for future design of SMMs with improved characteristics. However, more examples of the related materials are needed to clarify general conclusions. Therefore, it will be worthwhile to expand the family of lanthanide–[MIV(CN)8]4− systems also towards taking control over their ability to sensitize the 4f-metal-centered emission through their charge transfer states. Octacyanidometallates(IV) were also lately presented as photoactive molecular species undergoing photoinduced spin transition, opening a route towards luminescent photomagnets.33b This will be the subject of a future work.

Experimental section

Materials

Ytterbium(III) chloride hexahydrate, erbium(III) chloride hexahydrate, pyrazine and bis(triphenylphosphine)iminium (PPN) chloride were purchased from Sigma-Aldrich and used without further purification. K4[W(CN)8]·2H2O, K4[Mo(CN)8]·2H2O and pyrazine N,N′-dioxide (pzdo) were prepared according to published methods.31c,45 PPN4[Mo(CN)8]·10H2O and PPN4[W(CN)8]·10H2O salts were prepared by adaptation of a published procedure.46 All other reagents and solvents were purchased from commercial sources and used as received.

Synthetic procedures

Synthesis of ErMo. An aqueous solution (2 mL) of 19.1 mg (0.05 mmol) of ErCl3·6H2O and 28 mg (0.25 mmol) of pzdo was added to 131.9 mg (0.05 mmol) of PPN4[Mo(CN)8]·10H2O in a 2 mL portion of methanol. The resulting solution was stirred for 5 min and kept at 4 °C. After three days, deep red crystals of ErMo appeared. The composition of PPN{[Er (MeOH)0.3(H2O)3.7][Mo(CN)8]·7.7H2O·2MeOH was determined by using single crystal X-ray diffraction analysis. The obtained crystals were found to be stable in the mother solution or in paraffin oil. They were also fairly stable in air, but underwent the gradual exchange of crystallization methanol to water molecules. This results in the air-stable composition of PPN{[Er(pzdo)2(MeOH)0.3(H2O)3.7][Mo(CN)8]·9H2O (ErMo-hyd) as found in CHN elemental analysis and TGA measurements. Yield: 22.8 mg, 31% (based on Er). Anal. Calcd C, 42.7%; H, 4.5%; N, 12.4%. Found: C, 42.5%, H, 4.4%; N, 11.9%. IR spectrum (Fig. S1). Cyanide stretching vibrations at 2149, 2132, 2124 and 2111 cm−1 indicate the presence of both bridging and terminal cyanides within the [Mo(CN)8]4− building unit.33
Synthesis of YbMo. The compound YbMo was synthesised in the same manner as ErMo but using 19.4 mg (0.05 mmol) of YbCl3·6H2O instead of ErCl3·6H2O. Analogously to ErMo, the crystals of YbMo maintain their structure in the mother solution or in paraffin oil. The composition of PPN{[Yb(MeOH)0.3(H2O)3.7][Mo(CN)8]·7.7H2O·2MeOH was determined by single crystal X-ray diffraction analysis. The air-stable composition was estimated as PPN{[Yb(pzdo)2(MeOH)0.3 (H2O)3.7][Mo(CN)8]·9H2O (YbMo-hyd) by CHN elemental analysis and TGA. Yield: 20.7 mg, 28% (based on Yb). Anal. Calcd C, 42.5%; H, 4.4%; N, 12.3%. Found: C, 43.0%, H, 4.1%; N, 12.0%. IR spectrum (Fig. S1). Cyanide stretching vibrations at 2148, 2132, 2124 and 2117 cm−1 are related to both bridging and terminal cyanides within the [Mo(CN)8]4− building unit.33
Synthesis of ErW. Synthesis of ErW was performed by mixing 2 ml of an aqueous solution containing 19.1 mg (0.05 mmol) of ErCl3·6H2O and 28 mg (0.25 mmol) of pzdo with 2 ml of the methanolic solution of 136.3 mg (0.05 mmol) of PPN4[W(CN)8]·10H2O. The resulting yellowish solution was stirred for 5 min and kept at 4 °C. Dark grey crystals of ErW crystalized after three days. The composition of PPN{[Er (MeOH)0.3(H2O)3.7][W(CN)8]·7.7H2O·2MeOH was determined using single crystal X-ray diffraction structural analysis. Moderate stability in air of the aforementioned crystals is similar to that of ErMo. The air-stable composition of PPN{[Er(pzdo)2(MeOH)0.3(H2O)3.7][W(CN)8]·9H2O (ErW-hyd) was determined by CHN elemental analysis and TGA. Yield: 19.5 mg, 25% (based on Er). Anal. Calcd C, 40.3%; H, 4.2%; N, 11.7%. Found: C, 40.3%, H, 4.0%; N, 11.5%. IR spectrum (Fig. S1). Cyanide stretching vibrations at 2148, 2130, 2121 and 2109 cm−1 can be assigned to both bridging and terminal cyanides.
Synthesis of YbW. The analogous compound YbW was synthesised in a similar way to ErW but using 19.4 mg (0.05 mmol) of YbCl3·6H2O instead of ErCl3·6H2O. The composition of PPN{[Yb(MeOH)0.3(H2O)3.7][W(CN)8]·7.7H2O·2MeOH was found by single crystal X-ray diffraction analysis. The air-stable composition was determined as PPN{[Yb(pzdo)2(MeOH)0.3(H2O)3.7][W(CN)8]·9H2O (YbW-hyd) (CHN elemental analysis and TGA). Yield: 25.9 mg, 33% (based on Yb). Anal. Calcd C, 40.1%; H, 4.2%; N, 11.6%. Found: C, 40.0%, H, 3.9%; N, 11.7%. IR spectrum (Fig. S1). Cyanide stretching vibrations at 2148, 2130, 2121 and 2109 cm−1 are related to both bridging and terminal cyanides.

Crystallography

The crystalline samples of ErMo, YbMo, ErW and YbW were taken directly from the respective mother solutions, dispersed in Apiezon® N grease, mounted on a Micro Mounts™ holder, and measured at a low temperature of 100 K, using a Bruker D8 Quest Eco Photon50 CMOS diffractometer equipped with graphite monochromated Mo Kα radiation. Data reduction and cell refinement were performed by using the SAINT and SADABS programs. Absorption correction was executed using a multi-scan procedure. The crystal structures were solved by an intrinsic phasing method using SHELXT within Apex3 software. Crystal structure refinement was conducted using WinGX (ver. 2014.1) integrated software following a weighted full-matrix least-squares method on F2 on SHELX-2014/7.47 All non-hydrogen atoms were refined anisotropically. Hydrogen atoms of the pzdo ligands, PPN+ cations, and methyl groups of methanol ligands were calculated in the idealized positions and refined using a riding model. Hydrogen atoms of coordinated water and partially of crystallization solvent molecules were found from the residual electron density map and refined isotropically. In all compounds, it was necessary to ensure the proper geometry of part of the structures by using the DFIX command. Moreover the ISOR restraints were applied for the selected atoms of organic units, solvent molecules and cyanide ligands to stabilize the refinement process. Details of crystal data and structure refinement are gathered in Table S1 (ESI) and the detailed structure parameters are depicted in Tables S2–S5 (ESI). CCDC reference numbers are 1911096, 1911094, 1911091 and 1911095, for ErMo, YbMo, ErW and YbW, respectively. Single-crystal X-ray structural analysis was also performed for the air-dried crystals of YbMo and YbW, named YbMo-hyd and YbW-hyd, respectively. The crystals were covered by Apiezon® grease and measured at 100(2) K. All details of the X-ray diffraction experiment, structure solution and refinement are analogous to those shown above for the as-synthesized samples. Details of the related crystal data and structure refinement are given in Table S9 (ESI). CCDC reference numbers are 1938580 and 1938579 for YbMo-hyd and YbW-hyd, respectively. Powder X-ray diffraction patterns for all compounds were obtained using a Bruker D8 Advance Eco powder X-ray diffractometer equipped with a Cu Kα radiation source and with a capillary spinning add-on.

Physical techniques

Infrared absorption spectra were gathered for selected single crystals using a Nicolet iN10 MX FTIR microscope. The UV-Vis-NIR absorption spectra were collected on a PerkinElmer Lambda 35 spectrophotometer using a transmission mode for the thin films of powder samples dispersed in paraffin oil between two quartz plates. The thermogravimetric (TGA) curves were gathered for the polycrystalline samples using Rigaku Thermo Plus TG8120 apparatus with aluminium oxide as a reference material while CHN elemental analysis was performed on an Elementar Vario Micro Cube CHNS analyzer. Solid state photoluminescence spectra, including emission and excitation spectra, were measured on a Horiba Jobin–Yvon Fluorolog-3 (FL3-211) spectrofluorimeter equipped with a 450 W Xe lamp. The measurements were conducted using an InGaAs photodiode detector DSS-IGA020L cooled using liquid nitrogen. The emission and excitation data were collected and analysed using FluorEssence® software. Part of the data was gathered at 77 K in an optical cryostat filled with liquid nitrogen. Magnetic properties were investigated by using a Quantum Design MPMS-3 Evercool magnetometer. The powder samples were measured in sealed glass tubes covered with a small amount of the respective mother solution. Magnetic data were corrected for the diamagnetic contributions from the sample and the sample holder. Continuous shape measure analysis for the coordination sphere of eight-coordinated complexes was performed using SHAPE software ver. 2.1.48

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financed by the National Science Centre, Poland, within the OPUS-15 project, grant no. 2018/29/B/ST5/00337. Part of the work was supported by the Japan Society for the Promotion of Science, the Grant-in-Aid for Specially Promoted Research, grant no. 15H05697.

Notes and references

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Footnote

Electronic supplementary information (ESI) available: IR spectra, TGA curves, detailed structure parameters and views, PXRD patterns, complete dc and ac magnetic properties, additional optical spectra. CCDC 1911091, 1911094–1911096, 1938579 and 1938580. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c9qi00583h

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