In the Bottlebrush Garden: The Structural Aspects ... - ACS Publications

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In the Bottlebrush Garden: The Structural Aspects of Coordination Polymer Phases formed in Lanthanide Extraction with Alkyl Phosphoric Acids Ross J. Ellis,* Thomas Demars, Guokui Liu, Jens Niklas, Oleg G. Poluektov, and Ilya A. Shkrob* Chemical Sciences and Engineering Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439, United States S Supporting Information *

ABSTRACT: Coordination polymers (CPs) of metal ions are central to a large variety of applications, such as catalysis and separations. These polymers frequently occur as amorphous solids that segregate from solution. The structural aspects of this segregation remain elusive due to the dearth of the spectroscopic techniques and computational approaches suitable for probing such systems. Therefore, there is a lacking of understanding of how the molecular building blocks give rise to the mesoscale architectures that characterize CP materials. In this study we revisit a CP phase formed in the extraction of trivalent lanthanide ions by diesters of the phosphoric acid, such as the bis(2-ethylhexyl)phosphoric acid (HDEHP). This is a well-known system with practical importance in strategic metals refining and nuclear fuel reprocessing. A CP phase, referred to as a “third phase”, has been known to form in these systems for half a century, yet the structure of the amorphous solid is still a point of contention, illustrating the difficulties faced in characterizing such materials. In this study, we follow a deductive approach to solving the molecular structure of amorphous CP phases, using semiempirical calculations to set up an array of physically plausible models and then deploying a suite of experimental techniques, including optical, magnetic resonance, and X-ray spectroscopies, to consecutively eliminate all but one model. We demonstrate that the “third phase” consists of hexagonally packed linear chains in which the lanthanide ions are connected by three O−P−O bridges, with the modifying groups protruding outward, as in a bottlebrush. The tendency to yield linear polynuclear oligomers that is apparent in this system may also be present in other systems yielding the “third phase”, demonstrating how molecular geometry directs polymeric assembly in hybrid materials. We show that the packing of bridging molecules is central to directing the structure of CP phases and that by manipulating the steric requirements of ancillary groups one can control the structure of the assembly.

1. INTRODUCTION Coordination polymers (CPs) are hybrid organometallic materials that consist of metal cation centers interconnected by organic ligands and extend indefinitely into one, two, or three dimensions. This extended structure frequently causes their insolubility, and they tend to aggregate and precipitate. The unusual properties of these CP phases have been driving a surge of interest in recent years from a number of fields including catalysis, luminescence, electronics, magnetism, and ion separations.1 Structure, with regard to both local environments around the metal ions as well as the higher-ordered architectures that define polymer material morphology, is crucial in understanding the emergent properties of these materials.2 However, the structure of CP materials can be challenging to establish, especially when they lack crystallinity. One of the longest-known and most well-documented CPs is that formed from association between lanthanide(III) ions and dialkyl phosphoric acids, most notably bis(2-ethylhexyl) phosphoric acid (HDEHP).3,4 This system was developed more than half a century ago for the liquid−liquid extractive separation of lanthanide rare earths5 but is limited by the © 2015 American Chemical Society

formation of CPs, which reports from solution as an amorphous gelatinous phase at the oil−water interface. Although extraction of lanthanides using organophosphorus acids remains the paramount technique for separating the rare earths in numerous industrial applications,6−9 the morphology of the CP phase that limits all of these systems remains underexplored, and there is no consensus as to the molecular structure in the studies that have been published. The maturity of this open problem exemplifies the challenges faced in characterizing the structure of amorphous CP phases, which have ever increasing importance in a variety of emerging applications. In this study, we seek to establish the structural motifs for the aggregates that are formed in the extraction of trivalent lanthanide (Ln3+) ions from the acidic solutions by diesters of the phosphoric acid, such as HDEHP (Scheme 1). In the following, we will use the contractions HD[R]P and D[R]P− Received: June 14, 2015 Published: August 5, 2015 11910

DOI: 10.1021/acs.jpcb.5b05679 J. Phys. Chem. B 2015, 119, 11910−11927

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The Journal of Physical Chemistry B

the stability constant for the complex to the radius of the Ln3+ ion, which in turn accounts for the rapid increase in the distribution coefficient (the ratio of the equilibrium concentrations in the organic and aqueous phases) across the lanthanide period.4,10 As the concentration of the metal ion increases, the acidity decreases, or if a polar cosolvent is added to the organic phase, CP begins to form. SANS studies of Jensen et al.13,14 on Nd3+ complex aggregation suggested that the initial step was the formation of an Nd2L6 dimer, which is the largest complex that remains in a solution (Figure 1b). Further aggregation yields a gel-like solid residue with 1:3 stoichiometry.11,15−19 As shown in previous studies, this material is partially ordered: there are several well-defined peaks in their powder X-ray diffraction pattern (XRD).15,16,20 Unfortunately, the poor quality of these X-ray data do not allow confident indexing of the unit cell, and different structural models proliferated (Figure 1). In these structures, the Ln3+ ion is believed to be octahedrally coordinated20 (as was suggested by the luminescence spectra for the Eu3+ ions; we will demonstrate below that the crystal-field symmetry is, in fact, trigonal). One of the earliest structures suggested for CPs19,21 was a linear chain with three O−P−O bridges between the Ln3+ ions, as shown in Figure 1c.20 In the following we will refer to this structure as the 3-3 chain. This CP has been identified crystallographically by Lebedev et al.22 in the triclinic neodymium(III) diethylphosphate, and other chains of this type have been found in the related systems (e.g., refs 23−25). Suglobov et al.20 suggested that these 3-3 chains for (EuL3)∞ pack into a hexagonal lattice, as illustrated in Figure 2a, and estimate the Eu−Eu distance as 5.2 Å.

(where R is the modifier group) for the acid and the corresponding base, respectively (Scheme 1). Scheme 1. Chemical Structure for the Extracting Agent HD[R]P, Where R = EH (2-Ethylhexyl)

HDEHP (with pKa 1.8) readily ionizes in mildly acidic solutions (pH 2−4). The acid (HL) extracts Ln3+ ions into a hydrocarbon solvent via a concerted proton and ion exchange that involves the complexation of the Ln3+ ion by the hydrogen bound HL···HL dimers which are preformed in the nonpolar diluent.4,10 Ln 3 +aq + 3HL ··· HLorg ⇌ Ln(L ··· HL)3aq + 3H+aq

(1)

(The indexes “aq” and “org” in reaction 1 indicate the aqueous and organic phases, respectively.) In the resulting octahedral complex the three L−···HL dimers form a tight shell around the Ln3+ ion, with the protonated HL units bound to the deprotonated L− anions through their free PO groups, as shown in Figure 1a.11,12 This causes the extreme sensitivity of

Figure 2. Packing of the (a) 3-3 chains and (b) 2-2-2 sheets in Nd(DEtP)3 crystals (MOPAC Sparkle/RM1 models). Space-filling rendering is used for the ethyl groups.

An entirely different structure was suggested for (NdL3)∞.26 The unit cell was monoclinic, and the crystal consisted of hexagonally tiled sheets (Figure 1d); each hexagon is composed of the trefoils made of Nd3+ ions bridged through two O−P−O bridges with a Nd−Nd distance of 6.5 Å, as shown in Figure 1d. This structure will be referred to as the 2-2-2 sheet. The authors speculated that extended aliphatic chains pack together in layers

Figure 1. Structural motifs for (a) Ln(L···HL)3 monomers and (b−e) {LnL3}n complexes: (b) the μ2-oxo 1-4-1 dimer (type a), (c) the 3-3 chain, (d) the 2-2-2 sheet (type a), and (e) the 2-2-2 ribbon (type b). Only the first carbon atom is shown in the R-groups. 11911


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The Journal of Physical Chemistry B between the 2-2-2 sheets, as illustrated in Figure 2b.26 This structure, too, has the single-crystal analogue in the subsequently studied 1:3 complexes of dimethylphosphate (R = Me) with La3+,27 Sm3+,28 and Eu3+.29 For the latter two Ln3+ ions the Ln−Ln distance is considerably longer than in the crystalline 3-3 polymers (ca. 5.9 Å28,29 vs 5.1 Å22). This 2-2-2 model has been accepted in most of the subsequent studies (e.g., refs 15 and 16). The 2-2-2 motif for a ribbon-like CP (Figure 1e) was also suggested by Mikulski et al.17−19 Neither one of these previously suggested structures for CP occurring in the third phase has been established experimentally or even modeled structurally. In a more recent development, Nifant’yev et al.30 isolated an Nd dimer (Nd2L6) with a short (4.02 Å) distance between the Nd3+ ions and the structure shown in Figure 1b. In this dimer, the two Nd3+ ions are bridged by four O−P−O bridges arrange to give two μ-oxo bridges, while the two remaining ligands are pointing outward (the 1-4-1 motif). The “secret” of stabilizing this 1-4-1 dimer in a crystal was in using the sterically crowded groups.30 This 1-41 structure could be related to the Nd2L6 complex identified in the initial stages of the third phase formation.13,14 There are other known examples of such μ2-oxo bridged organophosphate dimers forming hybrid materials.3132 As seen from this brief review, several structural possibilities for CP and its precursor exist, each having credible single-crystal references. In this study, we present a comprehensive deductive approach that establishes the elusive structure of amorphous lanthanide-organophosphate CP phases. With the aid of semiempirical calculations, we set up an array of possible models (including those proposed in the previous studies) and then test these models using a variety of experimental techniques including time-resolved laser-induced fluorescence measurements (TRLF), X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS), X-ray photoelectron spectroscopy (XPS), continuous wave (cw) and time-domain electron paramagnetic spectroscopy (EPR), electron spin echo envelope modulation (ESEEM) spectroscopy, and solid-state magic angle spinning NMR (MAS NMR) spectroscopy. By using this vast arsenal of experimental probes, we demonstrate that the 3-3 polymer shown in Figure 1c forms the bulk of the CP matrix. To save space, many of the supporting schemes, tables, figures, and the list of abbreviations have been placed in the Supporting Information (SI). When referenced in the text, these materials have the designator “S”, as in Figure 1S in Supporting Information.

solvent, protic or aprotic, induced gelling; methanol and 2propanol were most convenient due to rapid Ostwald ripening. After 10 min of stirring, the gel was isolated by centrifuging. The supernatant was removed, and the residue was washed with the methanol by repeated suspension of the gel in methanol followed by centrifuging. Traces of the solvent were subsequently removed in vacuum using a rotary evaporator, and the gel was dried in a vacuum oven for 12 h at 80 °C. For preparation of doped samples, the appropriate aqueous mixtures of the lanthanide ions were used. Since the treatment considerably changes the relative concentration of the lanthanide ions in the solid matrix as compared to the aqueous solution,11 the elementary analysis of the solid gel was required to determine the doping levels. To this end, weighed dry gel was placed in a glass vial, one volume of toluene-d8 containing 5 wt % cumene as an analytical standard and one volume of 3 M nitric acid were added, and the sample was sonicated and vortexed until complete digestion. The organic layer was separated and used to determine the concentration of the L− anions by 1H NMR using an Avance DMX 500 MHz spectrometer (Bruker Biospin), while the aqueous phase was analyzed using a PerkinElmer model SCIEX ELAN DRC II inductively coupled plasma mass spectrometer. Within the experimental error, the dry gel samples corresponded exactly to the LnL3 stoichiometry. For XAS, the sonicated suspension of Eu(DEHP)3 gel in 2-propanol was filtered through Whatman No. 5 paper filter with 2.5 μm pores. The filtrate was collected, the filter was replaced with a new one, and the operation was repeated several times, as shown in Figure 1S. In this fashion, samples with uniform surface covering and different particle densities were obtained. The relative coverage was determined using fluorescence spectroscopy (Figure 2S). The deposited wet gel was dried at 80 °C, and the paper strips were sandwiched between the Al foil and Kapton film and flattened with a steel roller (Figure 1S). 2.2. Time-Resolved Fluorescence Spectroscopy (TRLF). For optical measurements, solid samples were placed in sealed Suprasil glass capillaries or deposited on filter paper as described above. The 5D0 →7FJ emission (J = 0−4) of the Eu3+ ion between 570 and 720 nm was induced using 355 nm photons (1 mJ, 6 ns fwhm, 10 Hz) from a Nd:YAG laser (Quantel Brilliant or Continuum). For kinetic measurements the emitted light was passed through a color glass filter and a bandpass interference filter (20 nm fwhm) with the transmission maximum at 610 nm (corresponding to the J = 2 band). This light was sampled using a photomultiplier, and the signal was terminated into a 4 kΩ load at the digitizing oscilloscope (Tektronix model 360). The photomultiplier was operated at 0.5−1 kV, depending on the emission yield. Figure 2S gives the typical decay kinetics of the photoinduced luminescence. For spectroscopic measurements, the EuDEHP sample sealed in an evacuated glass capillary was mounted inside an Optistat bath dynamic variable cryostat (Oxford Instruments) and cooled down to 77 K. The laserinduced emission was analyzed using a Spex Model 1704 spectrometer with the spectral resolution of 0.2 Å. The photomultiplier output terminated into 1 MΩ load was sampled at a delay of 1−10 ms using a Stanford research Systems Model 250 boxcar integrator with 15 μs gate. As shown later in section 3.3, the single line of the Eu3+5D0−7F0 transition at 580 nm indicates the presence of the single emission center, and the doublet of lines in the J = 1 band suggests a trigonal or higher site symmetry for Eu3+. In

2. METHODS 2.1. Sample Preparation. All reagents were obtained from Sigma-Aldrich and used as supplied without further purification. Typically, one volume of 1 M HDRP (R = 2-ethylhexyl (EH), n-octyl (Oc), n-butyl (Bu), and phenyl (Ph)) in toluene was put in contact with one volume of 0.1−0.2 M lanthanide(III) nitrate in nitric acid solution at pH = 3. In some experiments, the aqueous solution contained 3 M NaNO3. This mixture was vortexed for 5 min, and the organic layer was separated by centrifuging. For HDOcP and HDPhP, this procedure was sufficient to induce gel formation during the extraction. For mildly polar diluents (such as 1-octanol or hydrophobic ionic liquids), in which the internal hydrogen bonding required to stabilize the monomer is weakened, the formation of CP is also prompt, for all of the alkylphosphoric acids. In other systems, a polar solvent (1:10 v/v) needs to be added to the toluene solution to initiate the third phase formation. Almost any polar 11912


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The Journal of Physical Chemistry B case of trigonal symmetry, the crystal-field (CF) Hamiltonian HCF can be expanded as33

The ZFS contribution depends on CF around the probe ion. As the 8S7/2 state cannot contribute to CF, ZFS in Gd3+ ion originates entirely from the mixing of this ground state with the excited 6P7/2 and 6D7/2 states (that account for ca. 16% and 1%, respectively), and so it is rather weak ( 2, in the principal axes of rank2 tensor this contribution is given by

HCF ≈ B02 O02 + B04 O04 + B34 O34 + B06 O06 + B34 O34 + B06 O66 (2)

Okq(J)̂

Bkq

where are real CF parameters and are the conventional Stevens operators of rank k = 2, 4, 6 and q = 0, ..., k constructed of the components of the total moment J.̂ In the superposition model,34 the CF around the Ln3+ ion can be expanded into the Wybourne normalized real spherical harmonics gk,q(θi, φi) (also called the coordination factors) using the atomic coordinates calculated in our model (section 3.1), and the coefficients Bkq are estimated from Bqk ≈

∑ B̅ k (ri)gk ,q(θi , φi) i

2 2 2 1 HZFS = D(SẐ − S(S + 1)) + E(SX̂ − SŶ ) 3

The second term in eq 6 introduces a nonaxial component that is characterized by the asymmetry parameter η = 3E/D. In matrices other than single crystals, both D and (especially) η vary for different sites (which is referred to as strain); there is no unique set of these parameters.36 In our EPR simulations, these ZFS parameters were sampled independently from the corresponding Gaussian distributions with the centroids D̅ and η̅ and the variances δD and δη, respectively. Powder EPR spectra were simulated using second-order perturbation theory4142 or the exact diagonalization,43,44 using Monte Carlo integration over the spherical angles and the strain parameters41,42,45 and the contributions from the allowed (|ΔMS| = 1 in a strong field) and forbidden transitions (|ΔMS| > 1) were calculated separately. Typically, 5 × 104 of random field directions were averaged over the spherical quadrant, and the resulting histograms were convoluted with the Gaussian line shapes with the peak-to-peak width ΔBpp of 20−50 G. 2.4. Magnetometry. The magnetometry was carried out using a Quantum Design Magnetic Property Measurement System 3. The effective magnetic moment μeff of the Ln3+ ion was found from the equation χH→0 = NLnμ2effβ/3, where χ = dM/dH is the magnetic susceptibility, M is the magnetic moment, H is the field outside of the sample, NLn is the number of Ln3+ ions, β = 1/kBT, T is the temperature, and kB is the Boltzmann constant. For the free Gd3+ ion, μ0eff/μB = g(J(J + 1))1/2 ≈ 7.94. As Lu3+ is a close shell trivalent ion, Lu-DEHP was used to determine the diamagnetic contribution (Figure 3S). After the correction for temperature-independent magnetism, the magnetic susceptibility for Gd-DEHP for T > 20 K closely followed the Curie law with μeff = 7.98 μB. The details of the analyses are given in Appendix 2S. At lower temperature, magnetic interactions between the ions become important. It can be shown (see Appendix 2S) that the sufficiently high temperature and low field

(3)

For oxygen atoms in the first coordination shell we have B̅ k (ri) = bk (r0/ri)tk

(4)

where bk are the intrinsic CF parameters at r = r0, where r0 is the standard distance and tk values are the exponential factors given in Table 1S. For all other atoms, point charge approximation was used Bk (ri) = −e 2⟨r k⟩qiri−k + 1

(6)

(5)

where ⟨r ⟩ values are the averages calculated using the 4f radial functions and shielding factors (Table 1S), and qi are the Milliken atomic charges. The summation in eq 3 was extended out to 20 nm; the electrostatic contribution given by eq 5 is significant only for B20. The estimated CF parameters can be compared to the ones obtained through the optimization of the computed energy levels for the 7FJ multiplets calculated using trial CF parameters in eq 2. Appendix 1S provides the details of these analyses. 2.3. Continuous Wave EPR Spectroscopy. For EPR measurements, the samples were placed in 3 or 4 mm outer diameter Suprasil tubes, evacuated, and flame-sealed. The firstderivative cw EPR spectra were recorded using a 9.44 GHz Bruker ESP300E spectrometer, equipped with a programmable flow helium cryostat (Oxford Instruments, model CF935). The magnetic field and the parameters of the spin-Hamiltonian are given in units of Gauss (1 G = 104 T). If not stated otherwise, the EPR spectra were obtained using 2 mW of microwave power and 5 G modulation at 100 kHz. The resonance line positions are given by the effective g-factor defined as geff = hv/ μBB0, where ν is the microwave frequency, B0 is the external magnetic field of the EPR spectrometer, and μB is the Bohr magneton. Two paramagnetic Ln3+ ions were used as a probe, Gd3+ (4f 7 , S = 7/2) and Yb3+ (4f 13, S = 1/2), while closed shell Eu3+ and Lu3+ ions served as the “inert” matrix for magnetic dilution experiments. Gd3+ is a convenient probe, as in the ground state it is an orbital singlet (8S7/2) that has the largest spin among the trivalent lanthanide ions (S = 7/2), negligible hyperfine coupling constants, and isotropic g-tensor with giso ≈ 2.35 However, there is zero-field spitting (ZFS, see below),35 which complicates the analyses of cw EPR spectra36 and even more so of ESEEM.37,38 In contrast, Yb3+ is an S = 1/2 ion, for which there is no ZFS; however, it has two magnetic isotopes, 171Yb (I = 1/2, 14.3 at. %) and 173Yb (I = 7/2, 16.1 at. %) with significant hyperfine coupling in both of them, and large g-tensor anisotropy, so the resulting EPR spectra can also be quite complicated.39 k

μeff /μeff0 ≈ 1 +

2 J(J + 1)⟨Jex ⟩β[1 + ξ(T )] 3

(7)

where ⟨Jex⟩ is the site-average isotropic component of the Heisenberg exchange coupling constants (that may also include dipole−dipole interactions of the ions) and ξ(T) are the correction in second power of β and H. 2.5. ESEEM Spectroscopy. Pulsed EPR spectra and kinetics were obtained using a Bruker ELEXSYS E580 EPR spectrometer (Bruker Biospin) equipped with a 1 kW traveling wave tube amplifier (Applied Systems Engineering). A Flexline dielectric ring resonator (Bruker EN 4118X-MD4-W1) was used. Primary and stimulated electron spin echo (pESE and sESE, respectively) were obtained using π/2−τ−π−τ and π/ 2−τ−π/2−T−π/2−τ microwave pulse sequences using the appropriate phase cycling (Figure 4S). Data processing was 11913


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Science America, Inc.). For measurement, samples were cooled to 40 K using a closed-cycle helium gas refrigerator to reduce the thermal fluctuations and to increase the signal from the more distant atoms. Many scans were summed for each sample for more than 107 photons per data point at higher k. The data were processed using the Athena interface to ifef f it.47 The edge energy E0 was determined by finding the maximum of the first derivate of the μ0(E) spectrum. Normalized data were obtained after subtracting a linear pre-edge background and three-term quadratic function for the atomic absorption background and normalized using the Lengeler−Eisenberger procedure.48 The processed data were analyzed using the Artemis suit and feff version 8.28.49−51 Pseudoradial distribution functions were obtained by FT of k3χ(k) between 2.5 and 12 Å−1 using the Hanning window. Phases and amplitudes were calculated using the test structural models as the initial input. The data were first fit using the ab initio calculation neglecting Eu−Eu scattering, and then the fit was refined in a model that included this scattering (with the exception of the 2-2-2 model, where this contribution was negligible, see below). 2.9. XRD. X-ray diffraction data were obtained over the angular range 5−150° at 295 K using Cu Kα (1.5405 and 1.5443 Å) radiation from a Scintag X1 θ−θ diffractometer. Approximately 5 mg portions of powdered samples placed on a low background flat holder were used for these measurements. The peak positions were analyzed using the CMPR suit,52 and the unit cell was indexed using TREOR program.53 2.10. Modeling. The structures were modeled at the semiempirical level using the MOPAC2012 suit (version 14.083, Stewart Computational Chemistry)54,55 with Sparkle/ RM1 Hamiltonian.56,57

carried out using Xepr software (Bruker BioSpin). The magnetic dipole interaction of unpaired electrons with magnetic nuclei in the matrix causes modulation of the envelope (ESEEM) of the primary (p-) and stimulated (s-) echo as a function of the time intervals τ and T (for fixed τ) between the microwave pulses. Modulus Fourier transform (FT) of these time-domain traces yields frequency domain ESEEM spectra that are an analogue of NMR spectra for paramagnetic ions. For an S = 1/2, I = 1/2 system, there are two NMR frequencies, να and νβ, corresponding to the up (α) and down (β) orientation of the electron spin relative to the magnetic field B0. For a distant (weakly coupled) nucleus in a matrix these frequencies are close to the Larmor frequency ν for a free nucleus. In the solid state, the decay rate of the pESE kinetics in the τ-domain is limited by the transverse relaxation time for the electron spin (T2), whereas the decay of the sESE kinetics in the T-domain is determined by the slower longitudinal relaxation (T1 ≫ T2). A longer sampling interval for the latter (due to the slower decay of the echo signal) makes the sESEEM spectra better resolved in the frequency domain as compared to the pESEEM ones. The additional advantage of using sESE is in the ability to selectively suppress or enhance contributions from different types of nuclei (with NMR frequency ν), as the amplitude changes as 1 − cos(2πντ). The standard formulas for the effective S′ = 1/2 system (the Kramers doublet corresponding to the −1/2 ↔ +1/2 transition)46 from ref 38 were used to simulate and analyze the time-domain ESEEM traces. 2.6. Solid-State Magical Angle Spinning Nuclear Magnetic Resonance (MAS NMR). The 31P and 1H MAS NMR experiments were performed at 7.02 T (300 MHz) on a Bruker Avance III HD spectrometer operating at a Larmor frequency of 121.5 MHz (for 31P) using a 1.3 mm probe. All MAS NMR spectra were acquired at 50 kHz with single pulse experiments at the room temperature. Pulse width of 1.5 μs was used with pulse recycle delays of 15 s. Chemical shifts for 31P and 1H nuclei are given with respect to 85 wt % aqueous phosphoric acid and tetramethylsilane (TMS), respectively. 2.7. XPS. The gel samples were dried in a vacuum oven for 24 h and handled inside an argon filled glovebox with the water and oxygen levels 2 ms, while at a shorter delay time the kinetics become dispersive; the same was observed at 77 K. For t > 80 μs, these room-temperature kinetics (Figure 4) can be approximated using two exponential components with the lifetimes of 7.3 and 1.4 ms (±0.02 ms) and the relative weights of 67% and 33%, respectively (Figure 4).

butyl and 2-methylbutyl groups, the 2-2-2 ribbon would be lower in energy than the 2-2-2 sheet (Table 2S), although this 2-2-2 ribbon is still >27 kcal/mol equiv higher in energy than the 3-3 chain. For the bulkier 2-ethylbutyl groups (and even more so for the 2-ethylbutyl groups), no possibility exists to place these groups on the 2-2-2 scaffolding. While the 2-2-2 motif is plausible for the n-alkyl groups and the smaller 1- and 2-methyl alkyl groups, it appears to be excluded for the branched alkyl groups having a larger cross section. The similar yet more relaxed constraint also applies to the 2-2-2 ribbons: while the phenyl, n-alkyl, and 2-methyl-n-alkyl groups can decorate this scaffolding, the 2-ethyl-n-alkyl chains cannot fit. Thus, according to our simulations, the dominant driver for the emergent structure of the amorphous CP phase is the steric requirements of the substituent R groups. In this way, only the 3-3 chain scaffolding can accommodate all of the substituting groups. Our inference, therefore, is that the 3-3 chain polymer rather than the currently favored 2-2-2 sheet is the likely structural unit of the CP. This is the assertion that we seek to verify or falsify below. As the previous researchers put much emphasis on X-ray crystallography as a means of characterizing the CP structure, we first critically examined the XRD data. 3.2. XRD. Although amorphous, the CP phase has a crystalline component that defines the inorganic core of the material. Collecting powder XRD can therefore be used to index the signature Bragg peaks. Figure 3 exhibits powder X-ray diffraction patterns obtained for Ln-DEHP gels, where Ln = Nd, Lu, and Eu. It is seen that

Figure 3. Powder diffraction patterns for Ln-DEHP at 295 K (Ln = Na, Eu, Lu). The 5° peaks are removed to facilitate the comparison between the samples. The vertical lines indicate the persistent peaks whose positions are identical in all of these samples.

the five strongest Bragg peaks are conserved, and the comparison with the data in the literature15,16,20 suggests that all of the lanthanide compounds yield essentially the same XRD pattern, although the resolution may vary considerably from sample to sample. Nothing in our data suggests that the structural motifs for Nd-DEHP and Eu-DEHP are different, as was implied by the studies of Trifonov and co-workers, who postulated the 2-2-2 sheet structure for Nd-DEHP and 3-3 chain structure for Eu-DEHP.20,26 These five conserved peaks, however, are insufficient to index the unit cell reliably. As seen from Figure 3, Lu-DEHP polymer exhibits the highest degree of crystal ordering. Taking into account the best defined 15 peaks

Figure 4. Decay kinetics of 610 nm luminescence from photoexcited Eu-DEHP matrix at 295 K (trace (i) analyzed by decomposition into one [trace (ii)] or two [trace (iii)] exponential components. 11916


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possible to fit the spectra with this model, too, unrealistic assumptions concerning the spin−orbital coupling and the Slater−Gordon parameters were needed to achieve the agreement. Furthermore, the inferred θ = 52.3° and ϕ = 32.6° significantly differ from these parameters predicted for the 2-2-2 sheet (cf. Table 2 and Table 8S in Supporting Information) and obtained in the Eu-DMeP crystals (section 3.1 and Tables 7S and 8S in Supporting Information). These results strongly imply that the 3-3 chain provides a better structural model for the Eu3+ center that is responsible for the long-lived luminescence in Eu-DEHP. 3.4. XPS and 31P MAS NMR. The difference between the proposed structural models considered in section 3.1 originates in part from the role of the organophosphate ligand that can adopt different bridging and nonbridging modes. These modes are expected to result in varying chemical environments around the O and P atoms that can be observed using X-ray photoemission spectroscopy (XPS), as the binding energies for the X-ray photoemission peaks correspond to the ligand adopting different positions. While TRLF measurements discussed in the previous section suggested that Eu3+ emission centers had trigonal symmetry and favored the 3-3 chain model, such centers may not be representative of the bulk material, given the inherent disorder and the occurrence of the energy transfer in the polymer matrix. To mitigate this shortcoming of the TRLF approach, we sought an independent verification of the structural uniformity. The XPS spectra of Lu-DEHP polymer are shown in Figure 6 (the full scan is shown in Figure 18S). For the O 1s and P 2p

In a disordered EuL3 matrix, there is apparently Förster-type energy transfer62 between the Eu3+ ions at distorted sites that leads to inhomogeneous broadening and excitation energy migration. Because energy transfer depends on Eu3+ concentration as well as on site distance, we tried to suppress this transfer using 6 at. % Eu3+-doped Gd-DEHP (Figure 14S). The resulting decay kinetics were still biexponential, with 32% of the 6.8 ms component and 68% of the 1.03 ms component. The same behavior was also observed for Ln-DPhD solids, as shown in Figure 15S. More accurate analysis of the nonexponential decay curves requires using the Inokuti−Hirayama model62 of donor−donor energy transfer. However, to serve the purpose of the present work in characterization of the Eu 3+ luminescence dynamics, we limited ourselves to using the long-range part of the fluorescence decay (>2 ms) in which energy transfer is absent (i.e., resulting from Eu3+ ions more isolated from each other). The emission lines for Eu-DEHP were poorly resolved at room temperature (Figure 16S); however, the asymmetrical J = 1 band clearly indicates splitting and excludes the octahedral CF symmetry postulated by Suglobov et al.20 The structure becomes fully resolved at 77 K (Figure 5). The J = 0 band

Figure 5. Emission spectrum of Eu-DEHP observed at 77 K (355 nm excitation). Zero energy corresponds to the 7F0 level. Normalized traces i, ii, and iii were obtained at the delay times of 1, 2, and 9 ms, respectively. The vertical bars are calculated energy levels. Note the multiplication factor of 10 for the higher energy transitions. The total moment J corresponding to these 7FJ bands is indicated in the plot.

exhibits the single peak at 579.8 nm, suggesting the existence of the single type of the emission Eu3+ center, which excludes most of the structures in Table 1 that would give more than one CF environment. The J = 1 band has two resolved lines at 591.3 and 593.4 nm, which indicates a trigonal or hexagonal type of CF (or site) symmetry. Appendix 2S, Tables 6−10S, and Figure 17S in Supporting Information give a step-by-step account as to how the emission spectrum was analyzed to obtain CF parameters and compare them with the CF parameters computed using our semiempirical models. The main conclusion is that only the 3-3 chain structure is compatible with the observed CF parameters without assuming unrealistic atomic parameters. The experimentally inferred polar angle θ of 51.5° for the S6 symmetrical crystal field (Table 10S) is close to 50.6−50.8° given by our semiempirical models (Table 2 and Table 8S). The main contender is the 2-2-2 sheet that in our model has nearly perfect C3 symmetry at Eu3+ sites (Table 7S). While it is

Figure 6. XPS of Lu-DEHP gel for (a) O 1p, (b) P 2p, and (c) Lu 4f lines (○). The solid lines are the Gaussian fits on a quadratic slope; in panel c the line is decomposed into two components each fit using 3:4 doublets of the 4f7/2 line.

lines centered at 531.2 and 133 eV (panels a and b in Figure 6), respectively, there is a single Gaussian component with the full width at half magnitude (fwhm) corresponding to 3.16 and 2.3 eV, respectively. This result suggests the existence of the single environment for these atoms. For the Lu 4f line at 9.22 eV,63 there is also a single environment (panel c in Figure 6), but there is a little shoulder centered at 5.2 eV (that integrates to 11917


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The Journal of Physical Chemistry B 3% of the main line) which may originate from a different environment, although this line would overlap with a weak line from Lu 4d. These results imply that almost all of the Ln3+ ions have an identical environment. A complementary technique to XPS is MAS NMR that is used to probe the local environments of magnetic nuclei and their interactions with the (paramagnetic) metal ion. Figure 7a

Figure 8. 31P MAS NMR spectra of Ln-DEHP for Ln = Lu, Yb, and Eu (see the legend given in the plot). The spinning side bands are indicated with the arrows.

magnetic moment μeff ≈ 4.5 μB and Eu3+ ion exhibits temperature-dependent Van Vleck paramagnetism due to the involvement of the 7F1 state, typically having μeff ≈ 3.3 μB around 300 K (vs 7.94 μB for Gd3+).64 In solid-state MAS NMR, the chemical shift Δδ induced by a paramagnetic metal ion can be separated into the Fermi contact and (pseudocontact) dipolar coupling terms.65 As demonstrated in section 3.5 using ESEEM spectroscopy, the contact term for 31P is negligible, and so (assuming fast spinning and thermal averaging),66 Δδ ∝ B0·g·g·d, where d is the tensor for magnetic dipole interaction between the Ln3+ ion and the 31P nucleus. In a first approximation, this pseudocontact shift is proportional to (3 cos2 Θ − 1)/r, where r is the distance from the Ln3+ ion to the 31P nucleus and Θ is the angle between the radius-vector r and the z-axis of the axially symmetrical g-tensor. As there is only a single strongly shifted resonance line for both of the paramagnetic ions, all of the phosphorus atoms must be at approximately the same distance from the Ln3+ ion, and the corresponding radius-vectors should make approximately the same angle with the long axis of the g-tensor. This suggests the high local symmetry of the Ln3+ ion in the CP, which is consistent with the computational models and TRLF data indicating trigonal symmetry for the Ln3+ ions. Indeed, every structure but the 3-3 and 2-2-2 polymers considered in section 3.1 would have either more than one type of 31P nuclei (in a Lu3+ compound) or more than one set of paramagnetic shifts in these 31P nuclei (in paramagnetic Ln3+ compounds). Thus, our XPS and NMR experiments indicate that the emission centers examined in section 3.3 are indeed representative of the majority of the Ln3+ ions in the CP matrix. 3.5. ESEEM and EXAFS. While the Ln−31P distances can be crudely estimated from these paramagnetic NMR shifts, ESEEM spectroscopy provides a direct method that is free from the uncertainties and simplifications inherent in such analyses. Like EXAFS and unlike TRLF, this technique does not give the directional structural information; however, the combination of EXAFS and ESEEM provides estimates for Ln−O and Ln−P

Figure 7. (a) 31P and (b) 1H MAS NMR spectra of Lu-DEHP (solid lines). In panel a, the dotted line indicates the decomposition into Lorentzian components, with the centroids and weights of these components indicated in the plot. In panel b, the chemical shifts and attributions for the resonance lines of the protons in the EH groups are given in the plot.

exhibits 31P MAS NMR spectrum of the same compound (with the chemical shifts δ in ppm given vs the aqueous phosphoric acid). We remind that Lu3+ is a closed-shell (4f14) ion; i.e., there are no paramagnetic shifts induced in the magnetic nuclei (1H, 31 P). Ca. 91% of the signal (with the spinning side bands taken into the account) is composed of a single narrow line (260 Hz fwhm) at −18.7 ppm, while three smaller resonance lines at −10.8, −16.7, and −19.5 ppm account for, respectively 1.9%, 1.1%, and 6.7% of the 31P nuclei (it was established that these ratios are undistorted by the spin relaxation). The material appears to be well-ordered, as we also obtained a fully resolved 1 H MAS NMR spectrum from the 2-ethylhexyl arms (Figure 7b). These observations (as well as XPS data examined above) indicate that in the major component of CP there is only one type of the phosphorus atom, which excludes many of the structural possibilities, such as 2-4 chains and 2-2-2 ribbons. The additional structural insight (indirectly bearing the directional information) can be gained through observation of the paramagnetic shifts induced by Ln3+ ions, as described below. For Yb-DEHP and Eu-DEHP in Figure 8 the resonance lines for 31P were considerably broader, and their positions were shifted by ∼300 ppm. We remind the reader that Yb3+ ion is a spin-1/2 ion in the ground 2F7/2 state with the effective 11918


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The Journal of Physical Chemistry B distances in the first two coordination shells around the Ln3+ ions. While EXAFS is most sensitive to the positions of the oxygens in the first coordination shell, ESEEM is sensitive only to the 31P nuclei in the second coordination shell. To avoid rapid spin relaxation and line broadening due to the magnetic dipole and Heisenberg exchange interaction of the paramagnetic ions (see sections 3.6 and 3.7), magnetically dilute samples were prepared for these ESEEM studies. Figure 9a depicts the X-band spin−echo detected EPR spectrum of

Gd3+-doped Eu-DPhP, Figure 9b shows sESEEM trace obtained for τ = 88 ns, and Figure 9c exhibits the modulus FT spectrum of the modulation pattern (section 2.5). It is seen that both 1H (>99.9 at. %; I = 1/2) and 31P (100 at. %; I = 1/2) nuclei contribute to this modulation with the frequencies that are close to the Larmor frequencies for free nuclei, suggesting that the isotropic Fermi contact hyperfine interactions aP for the 31P nuclei are weak ( 3 are not observed in the spin−echo spectrum. 11921


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The Journal of Physical Chemistry B called U-spectra of the Gd3+ ions in amorphous matrices.36 The current consensus is that these features originate through the rhombic contribution to the ZFS interaction eq 6 in the strong coupling limit (D̅ ∼ 500−600 G (0.050−0.055 cm−1)) in conjunction with the broad distribution of the asymmetry parameter |η| between 0 and 1 (section 2.3). This EPR spectrum does not change with the Gd3+ concentration and magnetic dilution of Gd3+ ions with Eu3+ ions in solution suggesting that it originates entirely from isolated Gd3+ ions. Acting together, the strong D coupling and the asymmetry result in the efficient spin-forbidden transitions in the low field36 that are observed in cw EPR experiments (when the microwave field continuously pumps such spin-forbidden transitions), but are not observed in spin−echo experiments, where this pumping does not occur. From the EPR perspective, the crystal field is predominantly rhombic (i.e., the corresponding rank-2 tensor terms in eq 6 greatly exceed the higher rank terms). Comparison of the cw EPR spectra for Gd3+ ion in Gd(L··· HL)3 complex and Gd3+-doped EuL3 (for L = DEHP) reveals the striking differences between the environment of this probe ion (Figure 14). In the polymer matrix, the forbidden

environments around the Gd3+ ion in these two LnL3 matrices. Our simulations indicate that the EPR spectra obtained at 200− 300 K can be described by an ensemble with D̅ ≈ 370 G (0.034 cm−1) and δD ∼ 30−40 G, with |η̅| ≈ δη ≈ 0.25. These transformations can be attributed to the growing axial distortion of the crystal field at a higher temperature. The same axial distortion was also observed for Yb3+ ions, as shown in Figure 29S. At 4−20 K, the EPR spectra for YbDEHP and Yb3+-doped Lu-DEHP are strikingly asymmetric, indicating the strongly axial g-tensor (note that there is no ZFS since this is a S = 1/2 ion); there is also a hint of the hyperfine structure on the magnetic Yb nuclei (the 490 G triplet indicated in the plot). As the temperature increases to 70 K, the line broadening sets in, suggesting the growing distortion. The shape of the low-temperature EPR spectrum weakly depends on the Yb3+ ion concentration, suggesting weak magnetic interaction between the adjacent spin-1/2 ions. We turn now to the EPR spectra of the Gd-D[R]P samples at low magnetic dilution. Figure 30S exhibits the EPR spectra (normalized by their double integral) obtained for a series of Ln-DEHP samples in which the mole ratio x of Eu to Gd was varied from 0 to 80. As seen from Figure 30S, at the intermediate Gd3+ concentrations, the EPR spectra can be represented as the sums of weighted traces i for x = 0 and ii for x → ∞, and the ratio of the corresponding weights changes linearly with the ratio of the ion concentrations (Figure 31S). In the caption to Figure 31S we present a simple statistical argument, suggesting that just such a behavior would be expected in a situation when there is equal probability for the Gd3+ and Eu3+ ions to occupy the sites in the 3-3 chain. It appears that the characteristic EPR spectrum of Gd-D[R]P samples emerges when a Gd3+ ion is coupled magnetically to another Gd3+ ion. The resulting EPR spectrum becomes strongly broadened. From our simulations, we estimated that the second moment M2 for this broadening is ca. 4.2 × 104 G2. Assuming that the latter originates through magnetic dipole interactions involving the adjacent Gd3+ ions, this broadening can be crudely estimated using the Van Vleck formula, M2 = 3 /5μ2eff⟨rGd⟩−6,67 which gives the estimate of ⟨rGd⟩ ≈ 6.5 Å (vs 5.5 Å in Table 2). Given the difficulty of interpreting this highly anisotropic EPR spectrum in the absence of a developed theory of magnetic resonance in high-spin chains, we turned to static magnetometry, as it averages over the anisotropic interactions. 3.7. Magnetometry. While there are several techniques that probe the first and the second coordination shells around the Ln3+ ion, obtaining an estimate for the Gd−Gd distance remained an experimental challenge. The EXAFS estimate given in Table 4 should be viewed with caution, as the fit quality improves incrementally when Eu−Eu scattering is included in the model (traces i and ii in Figure 11), which may be merely a statistical effect of increasing the number of fitting parameters. Magnetometry offers a possible solution, as the strength of the magnetic interaction between the ions is distance dependent. The isotropic Heisenberg spin exchange (− 2JexŜaŜb term in the spin-Hamiltonian) between the adjacent Gd3+ ions can result in their ferromagnetic (Jex > 0) or antiferromagnetic (Jex < 0) coupling. Due to poor overlap between the 4f orbitals, the coupling constant is rather small, but it can still be determined. In Figure 15a and Table 12S in Supporting Information, we present all of the data in the literature on this coupling constant Jex(rGd) (determined for di-, tri-, and polynuclear Gd3+ complexes) versus the crystallographic

Figure 14. Comparison of first-derivative cw X-band EPR spectra obtained from the Gd3+-doped Eu-DEHP (trace i) and the Gd3+ monomer complex extracted by HDEHP in toluene from Figure 13 (trace ii). Both EPR spectra were obtained at 50 K and 2 mW. The effective g-factors for the spin transitions are indicated in the plot. The geff = 2.85 feature is absent in trace i, while the geff = 10.9 feature is much more distinct in trace i. The geff = 2 feature is clipped from trace ii to facilitate comparison; note the magnification factor for the low field part of trace i. The forbidden transitions are more prominent in trace ii.

transitions are observed only at great magnification, suggesting weak ZFS coupling regime (i.e., weak axial distortion). These transitions can still be observed at low temperature, high gain, and high microwave power, as shown in Figure 25S. From the positions of the resonance lines at geff ≈ 10.9, 5.7, and 3.67 (Figure 14) we estimated giso ≈ 1.98, D̅ ≈ 190 G (0.018 cm−1), δD < 20 G, while the envelope of the central geff ≈ 2 component is consistent with |η|̅ ≈ δη ≈ 0.3−0.5. As the sample temperature increases, the forbidden transitions become progressively weaker (Figure 26S), and the EPR spectrum resulting from the allowed spin transitions undergoes the characteristic change shown in Figure 27S, revealing the additional features. The similar low-temperature EPR spectra (Figure 26S) and their temperature evolution (Figure 28S) were observed for Gd3+-doped Eu-DPhP suggesting very similar 11922


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