1. Introduction

As an infrared (IR)-to-visible light conversion process, the upconversion luminescence (UCL) of rare-earth (RE) ions have been known for decades. This process is highly demanded for implementation of important photonic technologies in display systems [1], photovoltaics [2], multiplexed biosensing [3], diagnosis [4] and therapeutics [5]. The Energy Transfer Upconversion (ETU) involves the stepwise excitation among the discrete energy levels of RE ions by absorbing multiple infrared photons. The ETU mechanism is accepted as the most important in terms of relative efficiency among several two-photon upconversion processes, such as two-step absorption, cooperative sensitization, cooperative luminescence, second-harmonic generation, and two-photon absorption excitation [6]. The investigation on the basic mechanism for UCL was an active research field in the 1960’s. The fluorescence excited by two consecutive photons was first reported by Porter in 1961, who used two independent infrared sources to excite Pr³⁺ doped in a host lattice of LaCl₃ [7]. Auzel in 1966 reported the upconverted emission of Er³⁺ ions in germanate oxide glasses [8,9]. In late 1980s, there was a boom of the UCL by the appearance of various fluoride glasses [10]. The second boom was leaded by the miniaturization of UCL phosphors to submicron-sized crystals, which opened the possibilities for applications as bioprobes as an alternative to conventional dye-based ones [11,12]. In 2004, first proposals of synthetic methods for nanoparticles were reported to prepare NaYF₄ with reduced size (< 50 nm) [13–15]. The reduced phonon energy of fluoride with respect to other hosts host improved the brightness [14]. All of those conventional materials are solid state inorganic crystals. Eventually, an organic system showing the UCL was reported for MOF (metal organic framework) in 2007 [16]. Even at this state, the UCL phenomenon was reported for “solid state” crystals.

Along with the development of the materials, various theories for understanding the UCL phenomenon had been proposed. Most important matter is the suppression of thermal relaxation of the intermediate energy levels during the multistep excitation. Since the solid-state system was ionic one, most of the theories are developed for ionic solid-state substances. In ionic solid materials, thermal vibrations of atoms surrounding optically active ions provide a field of phonons leading to luminescence quenching. The thermal relaxation, as a non-radiative relaxation process, is understood as multi phonon relaxation (MPR) based on the weak coupling between electrons and phonons for RE ions, approximated as:

Molecular based systems for UC luminescence, from organic ligands [20–25] to the supramolecular level in metal–organic frameworks (MOF) [26,27] is an emerging research topic with few examples in the literature to date. The use of UC molecular composites for designing hybrid materials exhibit advantages over conventional solid-state compounds in terms of optical transparency and structural versatility [28]. On the other hand, improvements are necessary compensate the more pronounced influence of surrounding phonons in molecule-based system. The stabilization of the obtained complexes in a polymeric host with high hydrophobicity (water free) can overcome the thermal quenching of UC luminescence caused by OH oscillators present in aqueous environment [29]. In the case of UC-MOF, luminescent active RE ions are assembled in a porous organic material with well-organized structure, which is finally a solid-state crystalline host. The molecular systems face limitations to absorb more than a single photon and further populate higher excited state levels, because of their inherent high phonon energy caused by molecular vibrations, thus high probability of non-radiative processes [30–32]. Therefore, the lifetime of RE excited-states when coordinated to organic ligands is expected to be short, reaching up to few microseconds [33]. For this reason, early examples of UC in molecular systems used double femtosecond laser beams for simultaneously pumping with extremely high-power density [34,35]. The formation of a shielding layer (e.g. fluorination or deuteration of ligands) around RE, aiming at protecting the ions from vibrating quenchers, have been proposed as an efficient strategy to extend the lifetime of intermediate excited levels [30]. In view of these facts, the rational design of UC luminescent materials implies shielding the RE ions from the effect of high-vibrational energy of OH oscillators, including water molecule. In this contribution, we describe the design of molecular complexes capable to emit UCL, which was guided by a systematic spectroscopic investigation for each type of complex prepared to unravel the grafted species. The obtained materials are intended for designing flexible and transparent luminescent materials, as an alternative to conventional solid-based ones. The strategy to minimize the emission intensity quenching was constructing a special layer around the emitter ions based on tetrafluoroborate as a coordinating ligand capable to replace the OH oscillators in the system, often coordinated to RE ions. We selected N,N-dimethylformamide as the system. The solubility parameter for the DMF is around 12 [36]. If the value is close to 8, as the case of cyclohexane, the system is too hydrophobic for not dissolving any ions. We dissolved erbium chloride and tetrafluoroborate salts (NH₄BF₄ or NaBF₄) into DMF. The BF₄⁻ was a Cl⁻ replacing ligand for Er³⁺ in the system. By this process, we could observe green bright UCL in transparent liquid materials, which is useful for a further development of optoelectronic and photonic materials.

2. Experimental methods

2.1 Materials

Ammonium tetrafluoroborate, sodium tetrafluoroborate and dehydrated N,N-dimethylformamide were purchased from Wako Pure Chemical Industries (Osaka, Japan). Erbium (III) chloride hexahydrate and europium (III) chloride hexahydrate were purchased from Sigma-Aldrich (St Louis, MO, USA). Syringe filters (Nylon, 13 mm, 0.45 μm) were purchased from RephiQuik (Rephile Bioscience Ltd., Shanghai city, China).

2.2 Synthesis

The complexes were prepared by mixing rare-earth chlorides with tetrafluoroborate salts in DMF. The variations were performed in terms of: (i) the RE ion (Er³⁺ or Eu³⁺); (ii) the tetrafluoroborate salt (NH₄BF₄ or NaBF₄); (iii) the initial molar ratio between the RE chloride and the tetrafluoroborate salt, RECl₃:(NH₄/Na)BF₄ = 1:n (considering a fixed RECl₃ concentration); and (iv) aqueous phase, according to the molar relation H₂O:DMF. Table 1 summarizes these variables among samples.

Table 1. Summary of set of complex samples and corresponding variables in terms of RE ion, tetrafluoroborate salt, molar ratio between RE chloride and tetrafluoroborate salt, and molar ratio between water and DMF.

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In the first step of the synthesis, erbium(III) chloride hexahydrate solution in water (0.4 M, 0.2125 mL) was added to DMF (4 mL). Then, NH₄BF₄ powder (n eq) was added to the previous mixture containing ErCl₃ (1 eq, 0.085 mmol) in order to obtain a molar ratio ErCl₃:NH₄BF₄ = 1:n (n = 0, 5, 7, 10; i.e. 0, 44.6, 62.4 or 89.1 mg, respectively). The mixtures were sonicated for 40 min, followed by stirring at 300 rpm for 1h. The formed precipitate of NH₄Cl was removed by using centrifugation at 2600 g for 5 min. The supernatants were filtered through a 0.45 μm membrane and allowed to react for further 4 days under gentle shaking motion (50 rpm).

In a similar way, the complexes were prepared using NaBF₄ as the source of tetrafluoroborate anions, instead of the ammonium salt. First, erbium(III) chloride hexahydrate solution in water (1 eq, 0.085 mmol) was added to DMF (4 mL, 52 mmol). The initial concentration of ErCl₃ in stock aqueous solution (i.e., before addition to DMF) varied as follows: 2.1, 0.7, 0.4 and 0.2 M. Therefore, in order to add 0.085 mmol of ErCl₃ to DMF, the transferred volumes of such stock solutions were 0.040, 0.121, 0.213 and 0.425, respectively. In this sense, the water phase in DMF varied as follows: molar ratio H₂O/DMF = 0.04, 0.13, 0.23 and 0.48, respectively. NaBF₄ powder was then added (10 eq, 0.85 mmol, 93.3 mg) to obtain a molar ratio ErCl₃:NaBF₄ = 1:10. The mixtures were sonicated for 40 min followed by stirring at 300 rpm for 1h. The non-dissolved NaCl was removed by centrifugation (2600 g for 5 min) and filtration (0.45 μm membrane). The samples were maintained under gentle shaking motion (50 rpm) for 4 days.

The Eu³⁺-complexes were prepared in a similar way starting from europium(III) chloride hexahydrate solution in water (1 eq, 0.085 mmol, 0.4 M, 0.2125 mL) and DMF (4 mL, 52 mmol). Accordingly to this, the molar ratio H₂O:DMF in the mixture was 0.23. Then, different amounts of NH₄BF₄ or NaBF₄ were added in order to obtain the molar ratios EuCl₃:(NH₄/Na)BF₄ 1:n (n = 0, 5, 7, 10). As described above, samples undergone sonication (40 min) followed by stirring (300 rpm for 1h). The precipitated chloride salts were removed by centrifugation (2600 g for 5 min) and filtration (0.45 μm membrane). The samples were maintained under gentle shaking motion (50 rpm) for 4 days.

2.3 Characterization

UV-visible-NIR absorption spectra were measured using a spectrophotometer (V-770; JASCO, Tokyo, Japan). Luminescence spectra were obtained using a fluorescence spectrophotometer (RF-5300PC; Shimadzu Co., Kyoto, Japan). For measuring NIR-to-visible upconversion emission spectra, an external 980-nm excitation source, fibre-coupled laser diode (SP-976-5-1015-7 Laser Components, Ltd., Olching, Germany), was attached to the spectrophotometer. The impurity powder removed by centrifugation during synthesis was identified by X-ray powder diffraction (XRPD) using CuK_α radiation (RINT-TTR III, RIGAKU, Japan). Fourier-transformed infrared (FTIR) spectra were measured on a FTIR spectrometer (FT/IR-6500; JASCO, Tokyo, Japan). Complex samples, NH₄BF₄ and NaBF₄ were analyzed in KBr powder. The liquid complex samples were dropped and into KBr powder, the homogenized mixtures were oven-dried at 60 °C, and finally made into pellets. Quantifications of rare-earth elements, sodium and boron were performed by using inductively coupled plasma (ICP) emission spectrometry (ICPE-9000, Shimadzu Co., Kyoto, Japan). For the ICP determination, samples (100 μL) were digested in aqua regia (1 mL), a mixture of hydrochloric acid and nitric acids in a 3:1 molar ratio. The mixture was kept under stirring overnight and finally dissolved in water to a final volume of 10 mL. Serial dilutions of standard with concentrations of 0, 0.001, 0.01, 0.1 and 1 ppm were prepared from a multi-element calibration solutions with 5% aqueous HNO₃ containing the elements: (i) Ce, Dy, Er, Eu, Gd, Ho, La, Lu, Nd, Pr, Sc, Sm, Tb, Th, Tm, Y, Yb (Multi-Element Calibration Standard 2, Perkin Elmer, Shelton, CT, USA); (ii) Al, B, Ba, Be, Bi, Ca, Cd, Co, Cr, Cu, Fe, Ga, K, Li, Mg, Mn, Na, Ni, Pb, Se, Sr, Te, Tl, Zn (PerkinElmer Pure VIII, Perkin Elmer, Shelton, CT, USA).

3. Results and discussion

3.1 Effect of BF₄⁻ coordinating ligand on UC luminescence

Luminescent complexes in liquid solution were obtained through the reaction of rare-earth chlorides and tetrafluoroborate precursors in N,N-dimethylformamide. The effect of BF₄⁻ coordinating ligand was investigated by analyzing complexes prepared at different molar ratios of ErCl₃ and NH₄BF₄. The addition of NH₄BF₄ in excess has led to the precipitation of NH₄Cl, as evidenced by XRD pattern shown in Fig. 8 in the Appendix. Absorption spectra from 400 to 1650 nm of Er³⁺-complexes prepared at different molar ratios of ErCl₃:NH₄BF₄ (Fig.1a) show the Er³⁺ absorption band at 978 nm (⁴I_15/2 → ⁴I_11/2) suggesting the potential of the complex to upconvert 980 nm NIR light into visible emission. The peaks at 406, 442, 450, 488, 522, 542, 652 and 804 nm are assigned to Er³⁺ ion transitions from ⁴I_15/2 to the levels: ²G_9/2, ⁴F_3/2, ⁴F_5/2, ⁴F_7/2, ²H_11/2, ⁴S_3/2, ⁴F_9/2 and ⁴I_9/2, respectively. The 1556 nm band, ascribed to the Er³⁺ transition ⁴I_15/2 →⁴I_13/2, is overlapped with large water absorption band at around 1430 nm. A weak baseline rise was observed at shorter wavelengths as the concentration of BF₄⁻ increases, probably related to the scattering from coordination compounds. The photograph of samples (Fig. 1(b)) shows their transparency in the visible range.

 

Fig. 1. (a) UV–vis–NIR absorption spectra of complexes at different molar ratio of ErCl₃:NH₄BF₄ and (b) photograph of the corresponding samples.

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The UC emission spectra were recorded for complexes prepared at different molar ratio of ErCl₃:NH₄BF₄ 1:n (n = 0, 5, 7, 10) under 980-nm excitation (Fig. 2(a)). The photograph of a cuvette containing the Er³⁺-complex (ErCl₃:NH₄BF₄ = 1:5) under 980 nm laser excitation is shown in Fig. 2(b). The energy-level diagram of Er³⁺ shown in Fig. 2(c) illustrates the mechanism of UC emission under 980 nm laser excitation. Upon NIR irradiation, a ground state absorption (GSA) promotes the Er³⁺ ion from the ground state ⁴I_15/2 to the intermediate state ⁴I_11/2. Then, an excited state absorption (ESA) occurs, in which another photon at 980 nm is absorbed promoting the ion to an even higher excited level ⁴F_7/2. From this level, non-radiative relaxations occur to the levels ⁴S_3/2, ²H_11/2 and ⁴F_9/2. The radiative decays to the ⁴I_15/2 ground state result in the visible emissions at 525 nm, 550 nm and 660 nm. Alternatively, after absorbing the first photon, Er³⁺ may undergo a non-radiative relaxation from ⁴I_11/2 level to the lower lying ⁴I_13/2 level. The absorption of a second photon (ESA) promotes the ion to ⁴F_9/2 level, from which the red upconverted emission is observed. The ESA is a mechanism for the UC luminescence involving an individual ion, which we consider to be the main process for the obtained complexes. Jia et al. [37] reported that at high concentration of Er³⁺, the energy transfer between two Er³⁺ ions occur as a consequence of their stronger interactions. In the case of a dominant ETU mechanism, the green emission is not observed, being the red one the only visible emission upon excitation at 980 nm.

 

Fig. 2. (a) UC emission spectra under 980-nm laser excitation (192 W/cm²) of Er³⁺-complexes at different molar ratio of ErCl₃:NH₄BF₄. (b) Photograph of Er³⁺-complex (ErCl₃:NH₄BF₄ = 1:5) in a cuvette under 980-nm irradiation. (b) Schematic energy-level diagram of Er³⁺ and mechanism of UC emissions under 980 nm laser excitation. (d) Red-to-green emission intensity ratio for different molar amounts of NH₄BF₄ (ErCl₃:NH₄BF₄ 1:n; n = 0, 5, 7, 10). Red and green emission intensities correspond to integrated areas in the ranges 630–680 nm and 509–576 nm, respectively.

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The changes in the spectral shape in the red region (630–680 nm) between Er³⁺-complexes and ErCl₃ (control sample ErCl₃:NH₄BF₄ = 1:0) suggests a variation of the anion coordinated to Er³⁺ and allow us to propose that the BF₄⁻ ions were successfully grafted to Er³⁺ ions. Figure 2(d) depicts how intensity ratio between red and green emissions changes with the amount of NH₄BF₄ used for complex preparation. The red-to-green ratio values are higher in the presence of BF₄⁻ ions, and such value decreased by adding the NH₄BF₄ at a molar ratio ErCl₃:NH₄BF₄=1:7, indicating the optimal condition to obtain the most efficient upconverting complex. This parameter is related to the multiphonon relaxation rate, since the red emission intensity increases to the detriment of the green one because of an increase of non-radiative decays from the levels ⁴S_3/2 and ⁴I_11/2 to the next lower ones ⁴F_9/2 and ⁴I_13/2, respectively. As described above in Eq. (1), W_MPR is controlled by both electron-phonon coupling and phonon energy. After absorbing a NIR photon, the relaxation of excited electrons is influenced by the dynamic change of the electric dipole moment upon vibrational motions. Such effect is associated with the covalency of the metal-ligand bonding. The electric dipole moment is quite smaller in the case of covalent bonding, since the distance between the cation positive charges and covalent bonding electron is much closer than in the case of ionic bonding, in which the electrons are localized on the anions. Therefore, the weaker electrostatic interaction between RE and ligands reflects in a lower value of B parameter of Eq. (1), thus lower W_MPR, and finally lower red-to-green ration is expected. In other words, the more surrounding bonding covalency, the higher radiative transition is expected. The replacement of Cl⁻ coordination by BF₄⁻ leads to a decrease in covalency character of the bonding with the ligands, because of the tetrahedral molecular geometry structure, and thus an increase in W_MPR, as evidenced by the higher red-to-green ratio values for BF₄⁻-containing complexes [38]. Indeed, the W_MPR for RE ions in a chloride host is expected to be many orders of magnitude lower than other hosts, as demonstrated by Soga et al. [39]. The shifted center of gravity for the red emission to longer wavelength (lower energy) after replacement of Cl⁻ by BF₄⁻, suggests an increase in ΔE of non-radiative transitions (⁴S_3/2 → ⁴F_9/2 and ⁴I_11/2 → ⁴I_13/2) and also reflects on the decreased covalency for BF₄⁻ ligands.

The higher emission intensity observed in presence of BF₄⁻ was unexpected considering the increased multiphonon relaxation rate (MPR) caused by such ligand. On the other hand, a relevant factor here is that Cl⁻ ligands were replaced by larger BF₄⁻ ligands. The degree of interaction of RE ions with ligands is modulated by their separation distance. The results indicate that in the absence of BF₄⁻, the OH oscillators would induce a more pronounced quenching effect, leading to lower intensity in the full wavelength range. The introduction of a larger ligand (BF₄⁻) can displace water from coordination in the inner sphere, and result in higher emission intensity in the full range. Therefore, suggesting the BF₄⁻ capacity to shield Er³⁺ from OH oscillators. In other words, BF₄⁻ located at inner coordination sphere, while water is possibly in a second coordination sphere.

3.2 Effect of water on the coordination environment and UC luminescence

In order to better control the coordination environment, Er³⁺-complexes were prepared from the reaction of erbium chloride with an excess of sodium tetrafluoroborate (ErCl₃:NaBF₄ = 1:10) at different hydration levels. Such reaction leads to the precipitation of sodium chloride. The removal of NaCl by centrifugation was confirmed by the X-ray diffraction pattern of the precipitate dry powder (Fig. 3(a)). The control over the amount of sodium and chlorine ions in the coordination site was performed by simply adjusting the initial water amount, thus the NaCl precipitation, as displayed in Fig. 3(b) of samples before purification by centrifugation. A transparent solution is observed at the highest water phase (H₂O:DMF = 0.48), thus not being possible to remove Na⁺ and Cl^– ions by centrifugation. The samples became transparent after purification by centrifugation and filtration, as shown in Fig. 3(c). The concentrations of the elements Er, Na and B in purified samples were estimated by ICP (Fig. 3(d)). The results confirm that Na concentration increases (from 118 to 143 mM) at higher hydration levels. Concentrations of Er (20.2 ± 1.1 mM) and B (218.8 ± 6.3 mM) are similar among the samples. The concentration of B is around 10 times higher than Er, as expected.

 

Fig. 3. (a) X-ray powder diffraction (XRPD) pattern of precipitate collected by centrifugation and reference pattern of NaCl [40]. Photograph of samples prepared at different molar ratios of water (b) before and (c) after NaCl removal by centrifugation and filtration. Erbium, sodium and boron concentrations estimated by ICP as a function of molar ratio H₂O:DMF for complexes prepared with ErCl₃ and NaBF₄. at different hydration levels. Error bars correspond to standard error of mean (n = 2).

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The absorption spectra from 400 to 1650 nm of purified complexes prepared with ErCl₃:NaBF₄ = 1:10 at different water amounts (Fig. 4(a)) show the characteristic Er³⁺ absorption bands as previously mentioned. The water absorption at around 1430 nm increased linearly with the hydration level (Fig. 4(b)). The UC emission spectra of Er³⁺-complexes under 980-nm excitation (Fig. 4(c)) show the typical erbium emission bands centered around 525, 540 and 660 nm, corresponding to transitions from ²H_11/2, ⁴S_3/2 and ⁴F_9/2 levels. The changes in the spectral shape among samples suggest a variation of the ligands coordinated to Er³⁺. Therefore, the different spectral shape of the purified samples (H₂O:DMF = 0.04, 0.13, and 0.23) allow us to propose that the BF₄⁻ ions were successfully grafted to Er³⁺ ions after replacement of Cl⁻. The obvious shift of the emission peaks at highest water amount (H₂O:DMF = 0.48) indicated a change in the crystal field surrounding the erbium ions. An additional red emission at 630 nm is observed, which is overlapped with the emission at 660 nm, both attributed to ⁴F_9/2→⁴I_15/2 transitions [37,41]. UC emission broadening at highest hydration level indicates an inhomogeneous distribution of Er³⁺ site [41]. At a hydration level corresponding to H₂O:DMF = 0.23 the maximum UC emission intensity is observed. Figure 4(d) shows the lowest red-to-green intensity ratio values of such sample reinforcing the idea of reaching at this hydration level the best ionic condition for charge stabilizing the upconverting complex, together with brighter luminescence and narrow emission lines. The increase of UCL intensity with water concentration, up to a certain hydration level, indicates that OH oscillators does not contribute linearly with the non-radiative deactivation of Er³⁺ excited states. This increase may be attributed to a more favorable ionic environment related, which hampers the direct coordination of water to RE ions. Na⁺ and Cl⁻ concentrations increase with the ratio H₂O:DMF. As mentioned above, the Cl⁻ ions have lower contribution for MPR rate when compared with other hosts [39], and with respect to BF₄⁻ (Fig. 2). Therefore, the increase in emission intensity may be related to less Cl⁻ ligand substitution by BF₄⁻. The increased water amount also increases the presence of positive ions (Na⁺ in this case, or NH₄⁺), which are possibly in a second coordination sphere and have an important role for keeping OH oscillators far enough from RE. The coordination structure seems to be stable up to a limit, indicated by broadening of emission lines at the highest hydration level investigated (0.48), which may be related to an ionic disorder, since purification step was not performed for this sample due to absence of NaCl precipitation. The quenching effect caused by water molecules was not so relevant as expected possibly because of a shielding shell of BF₄^−.

 

Fig. 4. (a) UV–vis–NIR absorption spectra of Er³⁺-complexes obtained from ErCl₃ and NaBF₄ reaction at different molar ratios between water and DMF. (b) Water absorption at 1440 nm as a function of hydration level. (c) UC emission spectra under 980-nm laser excitation (192 W/cm²) and their respective (d) red-to-green emission intensity ratios. Emission intensities correspond to integrated areas in the ranges 615–700 nm (red) and 500–575 nm (green).

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3.3 Upconversion mechanism

The upconverting property of the Er³⁺-complex prepared with ErCl₃ and NaBF₄ at a molar ratio H₂O:DMF = 0.23 was investigated by recording the UC emission spectra at different laser power (Fig. 5(a)). As discussed above, the main mechanism for the erbium UC luminescence in these single ion coordination complexes is ESA. In the UC process, the emission intensity (I_em) increases non-linearly with the pumping intensity (I_ex), i.e. I_em α (I_ex)ⁿ, where n is the number of NIR photons that are absorbed for the emission of a single visible photon. The value of n is obtained as the slope from the plot of the logarithm of I_em (which is the integrated area of the emissions) vs. the logarithm I_ex (i.e. the laser power density). The n value is necessarily integer, however, the slope may decrease as a consequence of non-radiative processes, such as thermal effects. The values of n estimated for the Er³⁺ transitions ²H_11/2 / ⁴S_3/2 / ⁴F_9/2 → ⁴I_15/2 (Fig. 5(b)) suggests that two-photons are involved in such emission processes. Similar results for Er³⁺ UCL upon 980 nm excitation were reported in literature for ESA mechanism involving two IR photons in classical inorganic UC materials [42,43]. The non-linear UC processes via ESA mechanism involving the sequential absorption of two photons was reported for Tm salt in solution [35]. The observation of UCL via successive absorption of multiple photons is limited in molecular systems by their high phonon energy. The approach used to overcome short lifetime of RE intermediate excited stated in molecular systems is based shielding the RE ion from vibrational molecules, by fluorination or deuteration of local site [22]. The UC luminescence efficiency (i.e. the number of emitted visible photons divided by the number of the absorbed NIR photons) strongly depends on the excitation density due to the nonlinear nature of UC phenomenon [44]. The low absorption cross sections of f−f transitions in RE (approximately 10⁻²⁰ cm²) drastically limit the UCL efficiency [45]. To overcome the small absorption cross-section, Hyppänen et al. [46] used an organic sensitizer IR-806 for sensitizing a nonlinear UC luminescence. The antenna effect under 806 nm irradiation has led to UCL via absorption of two photons at quite low power excitation density (5–6 W/cm²).

 

Fig. 5. (a) UC emission spectra of Er³⁺-complex recorded by pumping at 980 nm at different powers. (b) Emission intensity evolution as a function of laser power for different Er³⁺ transitions. Emission intensities correspond to integrated areas in the ranges 500–534 nm (²H_11/2 → ⁴I_15/2), 534–575 nm (⁴S_3/2 → ⁴I_15/2), and 625–700 nm (⁴F_9/2 → ⁴I_15/2).

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3.4 Downshifting Eu³⁺-complexes

In order to better understand the coordination environment, complexes containing europium ions were prepared. Luminescence of Eu³⁺ gives important information about grafted species in the coordination sphere by analyzing transitions that are governed by the local ligand field [47]. The emission spectra of Eu³⁺-complexes prepared at different molar ratios EuCl₃: EuCl₃:(NH₄/Na)BF₄ = 1:n (n = 0, 5, 7, 10) at a fixed concentration of Eu³⁺ are shown in Fig. 6(a),b. The spectra were recorded under excitation at 394 nm, according to the maximum intensity recorded in the excitation spectra (Fig. 6(c) and Fig. 9(a,b) in the Appendix) and maximum absorbance in the UV–vis–NIR absorption spectra (Fig. 10 in Appendix), which corresponds to the Eu³⁺ transition ⁷F₀ → ⁵L₆. The Eu³⁺ energy level diagram showing the mechanism of downshifting emission under UV excitation is presented in Fig. 6(d). Upon 394-nm irradiation, the Eu³⁺ ion is excited to the ⁵L₆ energy level. Non-radiative relaxations lead to the population of ⁵D_J manifold (J = 1 and 0) and then radiative transitions to ⁷F_J (J = 0–4) manifold result in the Eu³⁺ luminescence. The pure magnetic-dipole (MD) character of ⁵D₀ →⁷F₁ transition allows its use as a reference, since it does not depend on the local ligand field [48]. Thus, the emission spectra were plotted by normalizing the emission intensity at 592 nm so that the same area is observed for MD transition (582–603 nm). Absence of change in shape of emission spectra among the samples allows us to consider the ligands chlorine and tetrafluoroborate as points of charge. The intensity of the hypersensitive electric dipolar transition ⁵D₀ →⁷F₂ clearly changed with the amount of BF₄^–. This result evidences the effect of BF₄^– in the in the local ligand field environment, since the ⁵D₀ →⁷F₂ transition is related to partly covalent bonding and local polarization [48]. Figure 6(e) displays the integrated intensity ratio between ⁵D₀→⁷F_0–4 and MD transitions (I_TOT / I_MD) with variation in NH₄BF₄ or NaBF₄ amounts. The relative intensity showed a dependence on BF₄^– concentration. The decrease in the relative intensity I_TOT / I_MD with temperature increase was demonstrated by Labrador-Páez et al. for Eu³⁺ aqueous complexes [49], which can be explained by an increase in W_MPR. Therefore, as well as observed above for Er³⁺-complexes, the introduction of BF₄^– increases the probability of multiphonon relaxation. Moreover, the introduction of tetrafluoroborate anion have led to higher emission intensities (Fig. 6(f), non-normalized emission spectra presented in Fig. S2.c,d) with respect to the chloride complexes (control samples: EuCl₃:(NH₄/Na)BF₄ = 1:0). Similarly as discussed above, this may be explained by the fact that Cl^– is smaller enough to allow the coordination of water molecules. The coordination of OH groups leads to non-radiative deexcitations due to their high vibrational mode. In the case of BF₄^–, the larger ligand may displace water from the coordination sphere, leading to brighter complexes [49]. The complexes prepared with NaBF₄ display higher emission intensity in comparison with those obtained from NH₄BF₄. The use of sodium salt allows to higher amount of precipitate (Fig. 11 in the Appendix), which means more efficient chloride removal. The more efficient ionic exchange of Cl^– by BF₄^– resulted in brighter complexes.

 

Fig. 6. Emission spectra (excitation at 394 nm) of Eu³⁺-complexes prepared with different amounts of (a) NH₄BF₄ or (b) NaBF₄. Relative molar concentration EuCl₃:(NH₄/Na)BF₄ 1:n (n = 0, 5, 7, 10). (c) Photoexcitation spectrum of a Eu³⁺-complex (EuCl₃:NaBF₄ = 1:10) by monitoring the emission at 612 nm. (d) Energy-level diagram of Eu³⁺ and mechanism for downshifting emission under 394 nm excitation. (e) Integrated intensity ratio between total emission from ⁵D₀ level (570–725 nm) with respect to MD emission (582–603 nm), and (f) integrated intensity in the full range (500–750 nm).

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3.5 FTIR spectroscopy

The FTIR spectra of Er³⁺-complexes prepared with sodium and ammonium tetrafluoroborate salts are presented in Fig. 7 together with spectra of NaBF₄ and NH₄BF₄ for comparison purposes. The characteristic vibrations of BF₄⁻ are present in all spectra at 1300, 1084, 772, 522 and 533 cm⁻¹ [50,51]. The spectra of the ammonium-containing complex and NH₄BF₄ salt display a large band at around 3122 cm^-1 and a peak at 1400 cm^-1, which are assigned to NH₄⁺ ions [52]. The large absorption band at around 3417 cm⁻¹ is related to O-H stretching of water molecules [53]. The broad water absorption band observed in the spectra of liquid complexes (Fig. 1(a), Fig. 4(a) and Fig. 10) at around 1430 nm (ca. 7000 cm⁻¹) corresponds to the first harmonic of such O-H absorption observed in FTIR spectra at ca. 3400 cm⁻¹. Additional weak peaks in the spectra of complexes that do not appear in the spectra of tetrafluoroborate salts, located at 2992, 2780 and 1652 cm^–1, are related to DMF. The main features in FTIR spectrum of neat DMF is the maximum absorption at 1683 cm^–1, which is related to the high dipole moment of the carbonyl group, and highest frequency bands at 2930 cm^-1 and 2858 cm^{-1 ­­}due to C-H bonds of methyl and formyl groups, respectively [54]. The shift of carbonyl and formyl bands to lower wavenumbers may be due to lower strength of such bonds after coordination. The poor coordinating ability of BF₄⁻ allows for vacant coordination sites available for DMF involvement as coordinating solvent through these groups [53].

 

Fig. 7. FTIR spectra of Er³⁺-complexes prepared with tetrafluoroborate salts of sodium (ErCl₃.NaBF₄) and ammonium (ErCl₃.NH₄BF₄), and spectra of salts NaBF₄ and NH₄BF₄.

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4. Conclusion

We concluded that upconversion luminescent coordination complex can be fabricated in organic solvent by adjusting the ionic environment. The effect of ligands chlorine and tetrafluoroborate on luminescence properties were discussed based on how their covalency affected the multiphonon relaxation rate. By analyzing the emission properties of both Er³⁺- and Eu³⁺-complexes, we have demonstrated that introduction of tetrafluoroborate induced a change in the local polarization, decreasing the covalency character of the coordination bonding. The larger BF₄^– ion may also contribute to shield RE ions from OH coordination. Counter cations of tetrafluoroborate salts, NH₄⁺ and Na⁺_, play an important role to remove Cl^– as a precipitate of NH₄Cl or NaCl, thus improving the efficiency of BF₄^– grafting on RE ions. The use of sodium salt rendered brighter Eu³⁺-complexes. Moreover, control over the water phase was essential to obtain the optimal ionic environment for higher emission intensity, lower red-to-green ratio, and narrower emission lines. Complexation of RE with BF₄^– in DMF was confirmed by FTIR spectroscopy. The Er³⁺-complex obtained in this work showed excellent luminescent properties for NIR to visible upconversion luminescence. This work brings a novel approach to upconverting materials design, extending the possibilities to organic hosts and liquid systems. The promising results stimulates the development of new luminescent materials for optics and photonic applications. This finding is innovative since by adding soluble polymer to the DMF, one can possibly fabricate transparent polymer as a material for luminescent devices such as flexible displays.

Appendix 1

 

Fig. 8. X-ray powder diffraction pattern of precipitate collected by centrifugation and reference pattern of NH4Cl [55].

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Fig. 9. Photoexcitation spectra by monitoring the emission at 612 nm (a,b) and emission spectra under excitation at 394 nm (c,d) of Eu³⁺-complexes prepared at different molar ratios of EuCl₃:(NH₄/Na)BF₄ 1:n (n = 0, 5, 7, 10).

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Fig. 10. UV–vis–NIR absorption spectra in the ranges (a,c) from 350 to 1650 nm and (b,d) from 350 to 600 nm of Eu³⁺-complexes prepared with NH₄BF₄ (a,b) or NaBF₄ (c,d) at different molar ratios EuCl₃:(NH₄/Na)BF₄ = 1:n (n = 0, 5, 7, 10).

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Fig. 11. Photograph of Eu³⁺-complexes prepared at different molar ratios EuCl₃:(NH₄/Na)BF₄ = 1:n (n = 0, 5, 7, 10) before removal of (a) NH₄Cl or (b) NaCl by centrifugation and filtration.

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Funding

Ministry of Education, Culture, Sports, Science and Technology (15H05950, S1511012); Japan Science and Technology Agency.

Disclosures

The authors declare no conflicts of interest.

References

1. R. Deng, F. Qin, R. Chen, W. Huang, M. Hong, and X. Liu, “Temporal full-colour tuning through non-steady-state upconversion,” Nat. Nanotechnol. 10(3), 237–242 (2015). [CrossRef]  

2. D. Y. Wang, P. C. Ma, J. C. Zhang, and Y. H. Wang, “Efficient down-and up-conversion luminescence in Er³⁺–Yb³⁺ co-doped Y₇O₆F₉ for photovoltaics,” ACS Appl. Energy Mater. 1(2), 447–454 (2018). [CrossRef]  

3. S. Wu, N. Duan, Z. Shi, C. C. Fang, and Z. Wang, “Simultaneous aptasensor for multiplex pathogenic bacteria detection based on multicolor upconversion nanoparticles labels,” Anal. Chem. 86(6), 3100–3107 (2014). [CrossRef]  

4. D. Ni, W. Bu, S. Zhang, X. Zheng, M. Li, H. Xing, Q. Xiao, Y. Liu, Y. Hua, L. Zhou, W. Peng, K. Zhao, and J. Shi, “Single Ho³⁺-doped upconversion nanoparticles for high-performance T2-weighted brain tumor diagnosis and MR/UCL/CT multimodal imaging,” Adv. Funct. Mater. 24(42), 6613–6620 (2014). [CrossRef]  

5. L. Cheng, K. Yang, Y. Li, J. Chen, C. Wang, M. Shao, S. T. Lee, and Z. Liu, “Facile preparation of multifunctional upconversion nanoprobes for multimodal imaging and dual-targeted photothermal therapy,” Angew. Chem., Int. Ed. 50(32), 7385–7390 (2011). [CrossRef]  

6. F. Auzel, “Upconversion and anti-stokes processes with f and d ions in solids,” Chem. Rev. 104(1), 139–174 (2004). [CrossRef]  

7. J. F. Porter Jr, “Fluorescence excitation by the absorption of two consecutive photons,” Phys. Rev. Lett. 7(11), 414–415 (1961). [CrossRef]  

8. F. Auzel, “Compteur quantique par transfert d'energie entre deux ions de terres rares dans un tungstate mixte et dans un verre,” C. R. Acad. Sci. Paris 262, 1016–1019 (1966).

9. F. Auzel, “Compteur quantique par transfert d'energie de Yb3+ a Tm3+ dans un tungstate mixte et dans verre germanate,” C. R. Acad. Sci. Paris 263, 819–821 (1966).

10. R. S. Quimby, M. G. Drexhage, and M. J. Suscavage, “Efficient frequency up-conversion via energy transfer in fluoride glasses,” Electron. Lett. 23(1), 32–34 (1987). [CrossRef]  

11. H. J. M. A. A. Zijlmans, J. Bonnet, J. Burton, K. Kardos, T. Vail, R. S. Niedbala, and H. J. Tanke, “Detection of cell and tissue surface antigens using up-converting phosphors: a new reporter technology,” Anal. Biochem. 267(1), 30–36 (1999). [CrossRef]  

12. J. Hampl, M. Hall, N. A. Mufti, M. Y. Yung-Mae, D. B. MacQueen, W. H. Wright, and D. E. Cooper, “Upconverting phosphor reporters in immunochromatographic assays,” Anal. Biochem. 288(2), 176–187 (2001). [CrossRef]  

13. G. Yi, H. Lu, S. Zhao, Y. Ge, W. Yang, D. Chen, and L. H. Guo, “Synthesis, characterization, and biological application of size-controlled nanocrystalline NaYF₄:Yb,Er infrared-to-visible up-conversion phosphors,” Nano Lett. 4(11), 2191–2196 (2004). [CrossRef]  

14. S. Heer, K. Kömpe, H.-U. Güdel, and M. Haase, “Highly efficient multicolour upconversion emission in transparent colloids of lanthanide-doped NaYF₄ nanocrystals,” Adv. Mater. 16(23-24), 2102–2105 (2004). [CrossRef]  

15. H. Lu, G. Yi, S. Zhao, D. Chen, L.-H. Guo, and J. Cheng, “Synthesis and characterization of multi-functional nanoparticles possessing magnetic, up-conversion fluorescence and bio-affinity properties,” J. Mater. Chem. 14(8), 1336–1341 (2004). [CrossRef]  

16. D. Weng, X. Zheng, X. Chen, L. Li, and L. Jin, “Synthesis, upconversion luminescence and magnetic properties of new lanthanide–organic frameworks with (43)2(46, 66, 83) topology,” Eur. J. Inorg. Chem. 2007(21), 3410–3415 (2007). [CrossRef]  

17. R. C. Powell, “Physics of Solid-State Laser Materials,” (Springer, 1998).

18. B. Stuart, “Infrared Spectroscopy: Fundamentals and Applications,” (John Wiley & Sons, 2004).

19. A. Beeby, I. M. Clarkson, R. S. Dickins, S. Faulkner, D. Parker, L. Royle, A. S. de Sousa, J. A. G. Williams, and M. Woods, “Non-radiative deactivation of the excited states of europium, terbium and ytterbium complexes by proximate energy-matched OH, NH and CH oscillators: an improved luminescence method for establishing solution hydration states,” J. Chem. Soc., Perkin Trans. 2(3), 493–504 (1999). [CrossRef]  

20. A. Hauser and C. Piguet, (2016). U.S. Patent No. 9,291,561. Washington, DC: U.S. Patent and Trademark Office.

21. L. Aboshyan-Sorgho, C. Besnard, P. Pattison, K. R. Kittilstved, A. Aebischer, J. C. G. Bünzli, A. Hauser, and C. Piguet, “Near-infrared→visible light upconversion in a molecular trinuclear d–f–d complex,” Angew. Chem., Int. Ed. 50(18), 4108–4112 (2011). [CrossRef]  

22. A. Nonat, C. F. Chan, T. Liu, C. Platas-Iglesias, Z. Liu, W. T. Wong, W. K. Wong, K. L. Wong, and L. J. Charbonnière, “Room temperature molecular up conversion in solution,” Nat. Commun. 7(1), 11978–8 (2016). [CrossRef]  

23. B. Golesorkhi, A. Fürstenberg, H. Nozary, and C. Piguet, “Deciphering and quantifying linear light upconversion in molecular erbium complexes,” Chem. Sci. 10(28), 6876–6885 (2019). [CrossRef]  

24. A. Nonat, S. Bahamyirou, A. Lecointre, F. Przybilla, Y. Mély, C. Platas-Iglesias, F. Camerel, O. Jeannin, and L. J. Charbonnière, “Molecular upconversion in water in heteropolynuclear supramolecular Tb/Yb assemblies,” J. Am. Chem. Soc. 141(4), 1568–1576 (2019). [CrossRef]  

25. B. Golesorkhi, H. Nozary, A. Fürstenberg, and C. Piguet, “Erbium complexes as pioneers for implementing linear light-upconversion in molecules,” Mater. Horiz. 7, 1279–1296 (2020). [CrossRef]  .

26. X. Zhang, B. Li, H. Ma, L. Zhang, and H. Zhao, “Metal–organic frameworks modulated by doping Er³⁺ for up-conversion luminescence,” ACS Appl. Mater. Interfaces 8, 17389–17394 (2016). [CrossRef]  

27. M. Li, S. Gul, D. Tian, E. Zhou, Y. Wang, Y. Han, L. Yin, and L. Huang, “Erbium(III)-based metal–organic frameworks with tunable upconversion emissions,” Dalton Trans. 47(37), 12868–12872 (2018). [CrossRef]  

28. D. W. Bruce, D. O’Hare, and R. I. Walton, Eds., “Molecular Materials” John Wiley & Sons14 (2011).

29. D. Manzani, K. Nigoghossian, M. F. Iastrensk, G. R. Coelho, M. V. Dos Santos, L. J. Q. Maia, S. J. L. Ribeiro, and M. G. Segatelli, “Luminescent silicone materials containing Eu³⁺-complexes for photonic applications,” J. Mater. Chem. C 6(30), 8258–8265 (2018). [CrossRef]  

30. Y. Suffren, B. Golesorkhi, D. Zare, L. Guénée, H. Nozary, S. V. Eliseeva, S. Petoud, A. Hauser, and C. Piguet, “Taming lanthanide-centered upconversion at the molecular level,” Inorg. Chem. 55(20), 9964–9972 (2016). [CrossRef]  

31. L. J. Charbonnière, “Bringing upconversion down to the molecular scale,” Dalton Trans. 47(26), 8566–8570 (2018). [CrossRef]  

32. A. M. Nonat and L. J. Charbonnière, “Upconversion of light with molecular and supramolecular lanthanide complexes,” Coord. Chem. Rev. 409, 213192 (2020). [CrossRef]  

33. C. Reinhard and H. U. Güdel, “High-resolution optical spectroscopy of Na₃[Ln(dpa)₃]·13H₂O with Ln = Er³⁺, Tm³⁺, Yb³⁺,” Inorg. Chem. 41(5), 1048–1055 (2002). [CrossRef]  

34. X. Xiao, J. P. Haushalter, and G. W. Faris, “Upconversion from aqueous phase lanthanide chelates,” Opt. Lett. 30(13), 1674–1676 (2005). [CrossRef]  

35. O. A. Blackburn, M. Tropiano, T. J. Sorensen, J. Thom, A. Beeby, L. M. Bushby, D. Parker, L. S. Natrajan, and S. Faulkner, “Luminescence and upconversion from thulium(III) species in solution,” Phys. Chem. Chem. Phys. 14(38), 13378–13384 (2012). [CrossRef]  

36. C. M. Hansen, “Solubility Parameters: A User’s Handbook,” (CRC Press, 2007).

37. R. Jia, W. Yang, Y. Bai, and T. Li, “Upconversion photoluminescence of ZrO₂:Er³⁺ nanocrystals synthesized by using butadinol as high boiling point solvent,” Opt. Mater. 28(3), 246–249 (2006). [CrossRef]  

38. R. Reisfeld and N. Lieblich, “Optical spectra and relaxation of Eu⁺³ in germanate glasses,” J. Phys. Chem. Solids 34(9), 1467–1476 (1973). [CrossRef]  

39. K. Soga, W. Wang, R. E. Riman, J. B. Brown, and K. R. Mikeska, “Luminescent properties of nanostructured Dy³⁺-and Tm³⁺-doped lanthanum chloride prepared by reactive atmosphere processing of sol-gel derived lanthanum hydroxide,” J. Appl. Phys. 93(5), 2946–2951 (2003). [CrossRef]  

40. Y. I. Vesnin and S. N. Zakovrjashin, “About the decay of solid-solutions KCl-NaCl,” Solid State Commun. 31(9), 635–639 (1979). [CrossRef]  

41. L. V. Maneeshya, P. V. Thomas, and K. Joy, “Visible PL Emission from erbium doped BaTiO₃ thin films deposited by RF magnetron sputtering,” Mater. Today: Proc. 2(3), 992–996 (2015). [CrossRef]  

42. J. Zhang, S. Wang, T. Rong, and L. Chen, “Upconversion luminescence in Er³⁺ doped and Yb³⁺/Er³⁺ codoped yttria nanocrystalline powders,” J. Am. Ceram. Soc. 87(6), 1072–1075 (2004). [CrossRef]  

43. T. Jiang, M. Xing, Y. Tian, Y. Fu, X. Yin, H. Wang, X. Feng, and X. Luo, “Up-conversion luminescence of Er₂Mo₄O₁₅ under 980 and 1550 nm excitation,” RSC Adv. 6(111), 109278 (2016). [CrossRef]  

44. G. Chen, H. Qiu, P. N. Prasad, and X. Chen, “Upconversion nanoparticles: Design, nanochemistry, and applications in theranostic,” Chem. Rev. 114(10), 5161–5214 (2014). [CrossRef]  

45. C. Strohhöfer and A. Polman, “Absorption and emission spectroscopy in Er³⁺–Yb³⁺ doped aluminum oxide waveguides,” Opt. Mater. 21(4), 705–712 (2003). [CrossRef]  

46. I. Hyppänen, S. Lahtinen, T. Ääritalo, J. Mäkelä, J. Kankare, and T. Soukka, “Photon upconversion in a molecular lanthanide complex in anhydrous solution at room temperature,” ACS Photonics 1(5), 394–397 (2014). [CrossRef]  

47. S. Chaussedent, A. Monteil, M. Ferrari, and L. Del Longo, “Molecular dynamics study of Eu³⁺ in an aqueous solution: Luminescence spectrum from simulated environments,” Philos. Mag. B 77(2), 681–688 (1998). [CrossRef]  

48. R. Reisfeld, E. Greenberg, R. N. Brown, M. G. Drexhage, and C. K. Jørgensen, “Fluorescence of europium (III) in a flouride glass containing zirconium,” Chem. Phys. Lett. 95(2), 91–94 (1983). [CrossRef]  

49. L. Labrador-Páez, E. Montes, M. Pedroni, P. Haro-González, M. Bettinelli, D. Jaque, J. García-Solé, and F. Jaque, “Effect of H₂O and D₂O thermal anomalies on the luminescence of Eu³⁺ aqueous complexes,” J. Phys. Chem. C 122(26), 14838–14845 (2018). [CrossRef]  

50. M. Owczarek, R. Jakubas, G. Bator, A. Pawlukojc, J. Baran, J. Przesławski, and W. Medycki, “Vibrational and thermodynamic properties and molecular motions in the incommensurate crystal of morpholinium tetrafluoroborate studied by ¹H NMR,” Chem. Phys. 381(1-3), 11–20 (2011). [CrossRef]  

51. H. D. Lutz, J. Himmrich, and M. J. Schmidt, “Lattice vibration spectra. Part LXXXVI. Infrared and Raman spectra of baryte-type TICIO₄, TIBF₄, and NH₄BF₄ single crystals and of ¹¹B-enriched NH₄BF₄,” J. Alloys Compd. 241(1-2), 1–9 (1996). [CrossRef]  

52. E. L. Wagner and D. F. Hornig, “The vibrational spectra of molecules and complex ions in crystals IV. Ammonium bromide and deutero-ammonium bromide,” J. Chem. Phys. 18(3), 305–312 (1950). [CrossRef]  

53. K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds, Part B, Applications in Coordination, Organometallic and Bioinorganic Chemistry, 4th Edn. (John Wiley & Sons, Inc.).

54. N. Biliškov, “Infrared optical constants, molar absorption coefficients, dielectric constants, molar polarisabilities, transition moments and dipole moment derivatives of liquid N,N-dimethylformamide–carbon tetrachloride mixtures,” Spectrochim. Acta, Part A 79(2), 302–307 (2011). [CrossRef]  

55. B. K. Vainshtein, “Refinement of the structure of the group NH₄ in the structure of ammonium chloride,” Tr. Inst. Kristallogr. Akad. Nauk SSSR. 12, 18–24 (1956).