Upconverting erbium(III) complexes in N,N-dimethylformamide (DMF) were prepared via chloride ligand replacement by tetrafluoroborate, as suggested by changes in the spectral profile. Cl− removal as precipitated salts was evidenced by X-ray diffraction (XRD) analysis. The systematic spectroscopic work indicated optimal condition for complex preparation. Ions in the coordination site were controlled by adjusting the water phase, thus the amount of removed chloride salts. Maximum emission intensity, lower red-to-green ratio and narrower emission lines were achieved at molar ratios Er3+:BF4− = 1:7 and H2O:DMF = 0.23. Studies extended to downshifted luminescence of Eu3+-complex provided more evidences of effective BF4− coordination, through dependence of relative intensities between 5D0 electric dipole and magnetic dipole (5D0→7F1) transitions. Infrared spectra suggest BF4− coordination to RE.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
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 , photovoltaics , multiplexed biosensing , diagnosis  and therapeutics . 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 . 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 . 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 . 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 . 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 . 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 . 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 . 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 . 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 . 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 . 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
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).
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.
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.
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 CuKα 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 (4I15/2 → 4I11/2) 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 4I15/2 to the levels: 2G9/2, 4F3/2, 4F5/2, 4F7/2, 2H11/2, 4S3/2, 4F9/2 and 4I9/2, respectively. The 1556 nm band, ascribed to the Er3+ transition 4I15/2 →4I13/2, 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.
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 4I15/2 to the intermediate state 4I11/2. 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 4F7/2. From this level, non-radiative relaxations occur to the levels 4S3/2, 2H11/2 and 4F9/2. The radiative decays to the 4I15/2 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 4I11/2 level to the lower lying 4I13/2 level. The absorption of a second photon (ESA) promotes the ion to 4F9/2 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.  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.
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 4S3/2 and 4I11/2 to the next lower ones 4F9/2 and 4I13/2, respectively. As described above in Eq. (1), WMPR 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 WMPR, 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 WMPR, as evidenced by the higher red-to-green ratio values for BF4−-containing complexes . Indeed, the WMPR 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. . 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 (4S3/2 → 4F9/2 and 4I11/2 → 4I13/2) 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.
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 2H11/2, 4S3/2 and 4F9/2 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 4F9/2→4I15/2 transitions [37,41]. UC emission broadening at highest hydration level indicates an inhomogeneous distribution of Er3+ site . 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 , 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−.
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 (Iem) increases non-linearly with the pumping intensity (Iex), i.e. Iem α (Iex)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 Iem (which is the integrated area of the emissions) vs. the logarithm Iex (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 2H11/2 / 4S3/2 / 4F9/2 → 4I15/2 (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 . 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 . 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 . The low absorption cross sections of f−f transitions in RE (approximately 10−20 cm2) drastically limit the UCL efficiency . To overcome the small absorption cross-section, Hyppänen et al.  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).
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 . 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 5DJ manifold (J = 1 and 0) and then radiative transitions to 7FJ (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 . 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 . Figure 6(e) displays the integrated intensity ratio between 5D0→7F0–4 and MD transitions (ITOT / IMD) with variation in NH4BF4 or NaBF4 amounts. The relative intensity showed a dependence on BF4– concentration. The decrease in the relative intensity ITOT / IMD with temperature increase was demonstrated by Labrador-Páez et al. for Eu3+ aqueous complexes , which can be explained by an increase in WMPR. 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 . 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.
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 . The large absorption band at around 3417 cm−1 is related to O-H stretching of water molecules . 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 . 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 .
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.
Ministry of Education, Culture, Sports, Science and Technology (15H05950, S1511012); Japan Science and Technology Agency.
The authors declare no conflicts of interest.
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