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Article

Lanthanide-Containing Polyoxometalate Crystallized with Bolaamphiphile Surfactants as Inorganic–Organic Hybrid Phosphors

by
Rieko Ishibashi
1,
Ruka Koike
1,
Yoriko Suda
2,
Tatsuhiro Kojima
3,
Toshiyuki Sumi
1,
Toshiyuki Misawa
1,
Kotaro Kizu
1,
Yosuke Okamura
4 and
Takeru Ito
1,*
1
Department of Chemistry, School of Science, Tokai University, Hiratsuka 259-1292, Japan
2
Department of Electric and Electronic Engineering, School of Engineering, Tokyo University of Technology, Hachioji 192-0982, Japan
3
Department of Applied Chemistry, Kobe City College of Technology, Kobe 651-2194, Japan
4
Department of Applied Chemistry, School of Engineering, Tokai University, Hiratsuka 259-1292, Japan
*
Author to whom correspondence should be addressed.
Inorganics 2024, 12(6), 146; https://doi.org/10.3390/inorganics12060146
Submission received: 30 April 2024 / Revised: 20 May 2024 / Accepted: 22 May 2024 / Published: 23 May 2024
(This article belongs to the Special Issue Synthesis and Application of Luminescent Materials)
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Abstract

:
Lanthanide elements such as europium exhibit distinctive emissions due to the transitions of inner-shell 4f electrons. Inorganic materials containing lanthanide elements have been widely used as phosphors in conventional displays. The hybridization of lanthanide ions with organic components enables to control of the material’s shapes and properties and broadens the possibility of lanthanide compounds as inorganic–organic materials. Lanthanide ion-containing polyoxometalate anions (Ln-POM) are a promising category as an inorganic component to design and synthesize inorganic–organic hybrids. Several inorganic–organic Ln-POM systems have been reported by hybridizing with cationic surfactants as luminescent materials. However, single-crystalline ordering has not been achieved in most cases. Here, we report syntheses and structures of inorganic–organic hybrid crystals of lanthanide-based POM and bolaamphiphile surfactants with two hydrophilic heads in one molecule. An emissive decatungstoeuropate ([EuW10O36]9−, EuW10) anion was employed as a lanthanide source. The bolaamphiphile counterparts are 1,8-octamethylenediammonium ([H3N(CH2)8NH3]2+, C8N2) and 1,10-decamethylenediammonium ([H3N(CH2)10NH3]2+, C10N2). Both hybrid crystals of C8N2-EuW10 and C10N2-EuW10 were successfully obtained as single crystals, and their crystal structures were unambiguously determined using X-ray diffraction measurements. The photoluminescence properties of C8N2-EuW10 and C10N2-EuW10 were investigated by means of steady-state and time-resolved spectroscopy. The characteristic emission derived from the EuW10 anion was retained after the hybridization process.

1. Introduction

Lanthanide elements can attribute several functions to materials, which have been applied to ionic conductors [1], magnetic materials [2], and biological reagents [3]. One of the most distinctive characteristics are emission properties [4]. Some lanthanides such as europium and terbium exhibit distinctive emission due to the transitions of inner-shell 4f electrons. Inorganic lanthanide compounds have been widely employed as phosphors in conventional displays and imaging technologies. The combination of lanthanide ions with organic moiety enables to control of the material’s shapes and properties and broadens the application areas of lanthanide compounds as inorganic–organic hybrid luminescent materials [5,6,7,8,9].
As for the lanthanide source, lanthanide-containing polyoxometalate (Ln-POM) anions are promising inorganic components [10,11,12,13]. Various types of Ln-POM anions have been synthesized as single crystals under ambient and/or hydrothermal conditions [14,15,16,17]. Hybridizing Ln-POM with organic moieties achieves controllable inorganic-–organic hybrid materials in their luminescent properties and material shapes [18,19,20,21]. Decatungstoeuropate ([EuW10O36]9−, EuW10, Figure 1a) anion exhibits strong emission at room temperature with a decay time of millisecond order [22,23,24,25], and is often utilized to build up luminescent hybrid materials [18,19]. The EuW10 anion has two W5O186− ligands that absorb ultraviolet (UV) light by O → W ligand-to-metal charge-transfer (LMCT). The intramolecular energy transfer from the LMCT state of W5O186− ligands to the 5D0 state of Eu3+ causes distinct orange-red light emission.
Cationic surfactants and polymer matrices as well as neutral block copolymers are effectively hybridized with the EuW10 anion to obtain luminescent nanocomposites [26,27,28,29], thin films [30,31,32,33,34,35,36,37], and sensors [38,39,40,41]. These systems are functional as soft matter and compatible with living organisms. However, single-crystalline ordering has not been achieved in most cases, which can be a drawback with the use as solid-state materials. The surfactant molecules are also effective as structure-directing reagents to construct one-dimensional tunnel and two-dimensional layer structures. Additionally, the EuW10 anion has rarely been crystallized with organic cations [42] and organic moieties [43,44].
Here, we report syntheses and structures of inorganic–organic hybrid crystals of the luminescent EuW10 anion and cationic bolaamphiphile surfactants, which have two hydrophilic heads in one molecule. The bolaamphiphile counterparts employed are 1,8-octamethylenediammonium ([H3N(CH2)8NH3]2+, C8N2) and 1,10-decamethylenediammonium ([H3N(CH2)10NH3]2+, C10N2), as shown in Figure 1b. Both hybrid crystals of C8N2-EuW10 and C10N2-EuW10 were successfully obtained as single crystals, and their crystal structures were unambiguously determined using X-ray diffraction measurements. The photoluminescence properties were evaluated by means of steady-state and time-resolved spectroscopy.

2. Results

2.1. Synthesis of EuW10-Bolaamphiphile Hybrid Crystals

C8N2-EuW10 and C10N2-EuW10 hybrid crystals were synthesized via ion-exchange reactions using sodium salt of EuW10 (Na-EuW10) and bolaamphiphile cations. The as-prepared precipitate of C8N2-EuW10 was obtained in 15–20% yield, and the as-prepared precipitate of C10N2-EuW10 was obtained in 40–50% yield. In each case, single crystals were successfully isolated from the synthetic filtrate after the removal of the as-prepared precipitate of C8N2-EuW10 or C10N2-EuW10. The yields of isolated single crystals were ca. 50% for C8N2-EuW10, and ca. 30% for C10N2-EuW10. Figure 2 shows IR spectra of the as-prepared precipitates and single crystals of C8N2-EuW10 and C10N2-EuW10. The spectra of C8N2-EuW10 (Figure 2b,c) showed characteristic peaks of the EuW10 anion in the range of 400–1000 cm–1 (935–945 cm−1 [νas(W=Ot)], 820–850 cm−1 [νas(W–Ob–W)], 700–710 cm−1 [νas(W–Oc–W)]) [45]. The peaks in the range of 2800–3000 cm−1 were derived from the C8N2 cation (2920 cm−1 [νas(−CH2−)], 2850 cm−1 [νs(−CH2−)]), which indicates the successful hybridization of the EuW10 anion and C8N2 cation. The IR spectra of C10N2-EuW10 (Figure 2d,e) also verified the formation of the C10N2-EuW10 hybrid crystal.
Figure 3 demonstrates powder XRD patterns of the C8N2-EuW10 and C10N2-EuW10 hybrid crystals. The XRD patterns of the C8N2-EuW10 as-prepared precipitate (Figure 3a) were crystalline, but slightly different from those of the C8N2-EuW10 single crystal (Figure 3b), and calculated from the results using single-crystal X-ray diffraction (Figure 3c). Slight differences in the peak position and intensity of the patterns may be derived from the desolvation of water molecules of crystallization (see below). The XRD pattern of the C8N2-EuW10 single crystal was similar to that calculated from results using single-crystal X-ray diffraction (Figure 3c). The XRD patterns of the C10N2-EuW10 as-prepared precipitate (Figure 3d) and single crystals (Figure 3e) were both similar to the calculated pattern from the results using single-crystal X-ray diffraction (Figure 3f). The results of IR spectra and powder XRD patterns indicate that both C8N2-EuW10 and C10N2-EuW10 hybrid crystals were obtained in a single phase and that the as-prepared precipitate and single crystal were essentially the same in their molecular and crystal structures.

2.2. Crystal Structures of EuW10-Bolaamphiphile Hybrid Crystals

The formulae of the hybrid crystals consisting of the EuW10 anion and bolaamphiphile cations were revealed by means of single-crystal X-ray diffraction and CHN elemental analyses (Table 1). C8N2-EuW10 has a formula of [H3N(CH2)8NH3]4H[EuW10O36]·10H2O, in which four C8N2 cations (2+ charge) and one H+ (1+ charge) were connected to one EuW10 anion (9−charge) due to charge compensation (Figure 4 and Figure S1). The presence of Na+ was not detected using energy dispersive X-ray spectroscopy (EDS) analysis. Ten water molecules of crystallization were contained in the crystal lattice. As shown in the asymmetric unit (Figure S1), three crystallographically independent C8N2 cations (except for the C8N2 cation containing N1 and N2) were bent with gauche conformation. The associated H+ was not observed using X-ray diffraction, but its presence was suggested by the bond valence sum (BVS) calculations [46]. The BVS value of plausibly protonated O atom (O21) in EuW10 was 1.12, while those for other O atoms were 1.57–1.95. The Eu3+ cation held a distorted square-antiprismatic 8-fold coordination with Eu–O distances of 2.37–2.50 Å (mean value: 2.43 Å), and the shortest Eu···Eu distance was 10.49 Å, which is similar to those of Na-W10 [25].
The crystal packing of C8N2-EuW10 was a layer structure viewed along the b-axis (Figure 4a, left). The layer structure consisted of EuW10 inorganic layers and C8N2 organic layers parallel to the ab plane with a periodicity of 15.5 Å. The crystal packing viewed along the a-axis exhibited a honeycomb-like feature (Figure 4a, right). The EuW10 anions interacted with each other to form a one-dimensional chain structure (Figure 4b). The O···O distances were 2.73 Å (O12···O21) and 2.92 Å (O26···O28). As the BVS calculation suggested, the associated H+ was located onto O21, and the short contact of O12···O21 (2.73 Å) was due to O–H⋯O hydrogen bonding [47]. Some water molecules (O37, O40, O43, and O44) were located inside the inorganic EuW10 layer. These water molecules and EuW10 anions formed a two-dimensional network (EuW10-H2O layer) through O–H⋯O hydrogen bonding with O⋯O distance ranging from 2.67 to 2.94 Å (mean value: 2.81 Å) (Figure 4c). Some hydrophilic heads of C8N2 penetrated the EuW10-H2O layers with the N–H⋯O hydrogen bonding with distances of 2.71–3.04 Å (mean value: 2.85 Å) [47].
The chemical formula of C10N2-EuW10 was determined to be [H3N(CH2)10NH3]3.5H2[EuW10O36]·6.5H2O. Three and a half C10N2 cations (2+ charge) and two H+ (1+ charge) were connected to one EuW10 anion (9− charge) with four water molecules of crystallization (Figure 5). No residual Na+ was observed using EDS analysis. As shown in the asymmetric unit (Figure S2), a half C10N2 cation (containing N7) was onto the inversion center with anti-conformation. Other C10N2 cations were bent with gauche conformation, and two C10N2 cations (with N3 and N4A, N4B; with N5 and N6A, N6B) were disordered with site occupancies of 0.558 and 0.442. Four water molecules were crystallographically assigned (Figure S2), while the presence of six and a half molecules per EuW10 anion was suggested by the thermal gravimetric (TG) analyses (Figure S3). The associated H+ was not detected using X-ray diffraction. The BVS value of O22 was 1.04 and seemed to be protonated (the BVS values of other O atoms: 1.59–1.96). The second H+ was not revealed in its position but may be located in the vicinity of O18 (BVS value: 1.60) or O19 (BVS value: 1.59). The coordination environment was similar to those of C8N2-EuW10 and Na-W10 [25]: Eu–O distances of 2.43–2.47 Å (mean value: 2.45 Å) and the shortest Eu···Eu distance of 10.50 Å.
The crystal packing of C10N2-EuW10 viewed along the b-axis was a layer structure composed of EuW10 inorganic layers and C10N2 organic layers parallel to the ab plane (Figure 5a, left). The layered distance was 15.8 Å. As viewed along the a-axis (Figure 5a, right), the crystal packing was a honeycomb-like structure. The EuW10 anions formed a one-dimensional infinite chain (Figure 5b) by short contacts between O6 and O22 with an O···O distance of 2.71 Å. This short contact will be due to the O–H⋯O hydrogen bonding [47], since O22 is the plausibly protonated O atom by the BVS calculation. The crystallographically assigned water molecules were located inside the inorganic EuW10 layer to form a two-dimensional network with the EuW10 anions (EuW10-H2O layer) through the O–H⋯O hydrogen bonds (O⋯O distance: 2.72–3.04 Å; mean value: 2.88 Å) (Figure 5c). Some hydrophilic heads of C10N2 were located in the EuW10-H2O layers with N–H⋯O hydrogen bonds (N⋯O distance: 2.66–3.04 Å; mean value: 2.83 Å) [47].

2.3. Photoluminescent Properties of EuW10-Bolaamphiphile Hybrid Crystals

The hybrid crystals of C8N2-EuW10 and C10N2-EuW10 exhibited distinct photoluminescence derived from the EuW10 anion. Figure 6 shows steady-state spectra of C8N2-EuW10 and C10N2-EuW10. Diffuse reflectance spectra (Figure 6a) showed adsorptions around 395 nm and 465 nm, which were assigned as f-f transitions of Eu3+: 395 nm for 7F0 → 5L6 transition, and 465 nm for 7F0 → 5D2 transition [14,15]. Each excitation spectrum (Figure 6b) exhibited a broad peak around 200–340 nm owing to the excitation into the O → W LMCT band in the W5O186− ligands. The f-f transitions mentioned above were also observed in the excitation spectra. In the emission spectra, distinct peaks due to 5D0 → 7FJ (J = 0, 1, 2, 3, 4) transition of Eu3+ were observed around 580–710 nm (Figure 5c) [22,23,24,25].
The photoluminescent properties of the C8N2-EuW10 and C10N2-EuW10 hybrid crystals were evaluated by means of time-resolved spectroscopy. The emission spectra acquired using a single pulse excitation (Figure 7a,b) exhibited characteristic emission derived from the EuW10 [22,23,24,25]. Emission peaks at 575 nm are assigned to 5D0 → 7F0 transition, peaks at 587 and 593 nm to 5D0 → 7F1, and peaks at 611 and 618 nm to 5D0 → 7F2. The peaks around 650 nm are assignable to 5D0 → 7F3 transition, and peaks at 691 and 700 nm to 5D0 → 7F4 transition. The spectrum profiles of C8N2-EuW10 and C10N2-EuW10 were almost the same irrespective of the measurement temperatures. However, the emission decay profiles of C8N2-EuW10 and C10N2-EuW10 were different. The emission decay profiles of C8N2-EuW10 were regarded with a single exponential function (blue plots in Figure 7c,d). The emission lifetimes were 3.0 ± 0.1 ms at 15 K and 2.5 ± 0.1 ms at 300 K (Table 2). In the case of C10N2-EuW10, the emission decay profiles were approximated with two exponential functions (red plots in Figure 7c,d). The emission lifetimes at 15 K were estimated to be 0.94 ± 0.1 ms for a faster decay component and 3.1 ± 0.1 ms for a slower decay component (Table 2). The emission lifetimes at 300 K were 1.1 ± 0.1 ms and 1.8 ± 0.1 for a faster and slower component, respectively. The emission decay lifetimes of C8N2-EuW10 and C10N2-EuW10 at 15 K were comparable to that of Na-EuW10 [25,33] but became shorter at 300 K. The increase in the number of carbon atoms in the bolaamphiphile cation resulted in a shorter emission lifetime at 300 K of C10N2-EuW10 [33].
As for the preparation of inorganic–organic luminescent materials, the lasing property is a promising character to be tackled in several applications. As shown in Figure 8, the emission intensity of C8N2-EuW10 and C10N2-EuW10 depended on the excitation laser power. After the threshold value of the excitation laser power, the emission intensity increased linearly, indicating the emergence of the lasing property [48]. The threshold values at 15 K were 28.5 and 25.4 mJ cm−2 for C8N2-EuW10 and C10N2-EuW10, respectively. The threshold values at 300 K were 26.0 and 25.2 mJ cm−2 for C8N2-EuW10 and C10N2-EuW10, respectively. These threshold values will be essentially in the same order.

3. Discussion

Lanthanide-containing polyoxometalate (Ln-POM) single crystals hybridized with surfactant molecules were first obtained in this work. Using bolaamphiphile surfactants was critical for the crystallization of the Ln-POM hybrid crystals. Bolaamphiphiles have two hydrophilic heads [49,50]. Hybrid crystals of POM with bolaamphiphiles have higher solubility in conventional solvents, and it is rather easier to isolate single crystals [51,52]. The size of the hydrophilic heads of C8N2 and C10N2 are smaller than those of quaternary alkylammonium cations, which may be another reason for the successful isolation of single crystals of C8N2-EuW10 and C10N2-EuW10. The effect of surfactant length on luminescent properties will be an interesting topic; however, preparing single crystals with longer surfactants may be difficult.
The powder XRD patterns of the as-prepared precipitate (Figure 3a) and single crystal (Figure 3b) of the C8N2-EuW10 hybrid crystal were slightly different. On the other hand, the essential feature of the XRD patterns of as-prepared precipitate (Figure 3a) is similar to that calculated from the single-crystal structure of C8N2-EuW10 (Figure 3c). The as-prepared precipitate and single crystal of C8N2-EuW10 is considered to be the same phase. The differences in the peak position and intensity of the patterns will be derived from the desolvation of water molecules of crystallization, the different measurement temperatures (powder: room temperature; single crystal: 93 K), and the preferred orientation derived from the layered structure of C8N2-EuW10. In the case of C10N2-EuW10, the XRD patterns of the as-prepared precipitate (Figure 3d) and single crystal (Figure 3e) were quite similar. The water molecules in the C10N2-EuW10 hybrid crystal were located inside the inorganic layers of EuW10 with short-contact interaction, and plausibly less easily desorbed from the crystal lattice. TG analyses indicated the stability of C8N2-EuW10 and C10N2-EuW10 until 180–200 °C (Figure S3).
The structures of C8N2-EuW10 and C10N2-EuW10 hybrid crystals were unambiguously revealed by means of single-crystal X-ray diffraction measurements. In summary, the crystal structures were similar concerning the cell parameters (Table 1) and packing features (Figure 4 and Figure 5). The crystal structures of C8N2-EuW10 and C10N2-EuW10 were layer structures viewed along the b-axis, and a honeycomb-like feature viewed along the a-axis. Such structural features are observed for some POM-surfactant crystals [52,53]. The packing features of EuW10 in C8N2-EuW10 and C10N2-EuW10 were almost the same, while the number of bolaamphiphile cations and their conformations were different. In both hybrid crystals, the EuW10 anions formed one-dimensional chain structures. The residual H+ was relevant to the formation of the one-dimensional chain structure. These one-dimensional chains of EuW10 together with water molecules formed two-dimensional networks of EuW10-H2O parallel to the ab plane (Figure 4c and Figure 5c).
The photoluminescence spectroscopy of C8N2-EuW10 and C10N2-EuW10 revealed the emission properties of C8N2-EuW10 and C10N2-EuW10. The photoluminescence of the EuW10 anion was essentially retained: characteristic emission derived from Eu3+ (Figure 6 and Figure 7) and an emission lifetime of millisecond order (Table 2). The hybridization of EuW10 with organic moieties sometimes shortens the emission lifetime (<1 ms) [33,54]; however, primary ammonium cation can retain the emission lifetime of millisecond order [31]. The primary ammonium cation can form N–H⋯O hydrogen bonds between the EuW10 anion to prevent water molecules from approaching near Eu3+. The excitation energy owing to O → W LMCT can transfer to Eu3+ without nonradiative deactivation through the vibration states of the high-frequency O–H oscillators of water molecules [55,56]. The EuW10 anion has no coordinated water and therefore a long emission decay time (3.5 ms at 4.2 K) and high quantum yield (0.99) for Na-EuW10 at 4.2 K [25]. As shown in Table 2, the emission decay time of C8N2-EuW10 (3.0 ± 0.1 ms) and C10N2-EuW10 (3.1 ± 0.1 ms for a slower component) at 15 K were comparable to that of Na-EuW10 (3.5 ms) at 4.2 K. This implies that the emission behavior of C8N2-EuW10 and C10N2-EuW10 was almost identical to that of Na-EuW10 derived from the suppression of the thermal deactivation of the excitation energy at the low temperature. At the high temperature (300 K), the emission decay time of C8N2-EuW10 (2.5 ± 0.1 ms) was similar to that of Na-EuW10 (2.6 ms), but C10N2-EuW10 exhibited the faster decay time of 1.8 ± 0.1 ms. This will be due to more carbon atoms in the crystal lattice [33] and fewer N–H⋯O hydrogen bonds between EuW10 and surfactant cations. The kinetic constants of energy transfer can be estimated using the magnetic-dipole 5D0 → 7F1 transition as a standard [56], since the rate of 5D0→7F1 (1.35 × 102 s−1) is almost independent of the geometry of Eu3+. The relative intensity of 5D0→7F1 emission to the total emission (5D0→7Fn, n = 0–4) at 300 K was 0.38 for C8N2-EuW10 and 0.41 for C10N2-EuW10, respectively. Therefore, for C8N2-EuW10, the radiative rate (krad) was 3.6 × 102 (=1.35 × 102/0.36) s−1 and the experimental decay rate was (4.0 ± 0.2) × 102 (=1/((2.5 ± 0.1) × 10−3)) s−1, and then the estimated nonradiative rate (knr) was (0.4 ± 0.2) × 102 (=(4.0 ± 0.2) × 102 − 3.6 × 102) s−1. For C10N2-EuW10, the respective values of radiative rate (krad) and experimental decay rate were 3.3 × 102 (=1.35 × 102/0.41) s−1 and (5.6 ± 0.3) × 102 (=1/((1.9 ± 0.1) × 10−3)) s−1, and the estimated nonradiative rate (knr) was (2.3 ± 0.3) × 102 (=(5.6 ± 0.3) × 102 − 3.3 × 102) s−1. The reason for the presence of fast decay components in the C10N2-EuW10 emission (Table 2) was unclear but may be derived from the presence of more H+ in the crystal lattice. In addition, both C8N2-EuW10 and C10N2-EuW10 hybrid crystals exhibited lasing properties (Figure 8). The threshold values (25–28 mJ cm−2) were larger than those of recent organic lasers [57,58]. Although further improvement in materials processing will be necessary, the photoluminescence properties of C8N2-EuW10 and C10N2-EuW10 mentioned above show the possibility of a new series of inorganic–organic hybrid phosphors.

4. Materials and Methods

4.1. Materials

Chemical reagents purchased from commercial sources (FUJIFILM Wako Pure Chemical Corporation, Osaka, Japan, Tokyo Chemical Industry Co., Ltd. (TCI), Tokyo, Japan and Kanto Chemical Co., Inc., Tokyo, Japan) were utilized without further purification. Solid 1,8-octamethylenediammonium chloride ([H3N(CH2)8NH3]Cl2, C8N2-Cl) and 1,10-decamethylenediammonium chloride ([H3N(CH2)10NH3]Cl2, C10N2-Cl) were prepared by adding equimolar hydrochloric acid to 1,8-octanediamine and 1,10-decanediamine, respectively. The sodium salt of EuW10 (Na9[EuW10O36]·32H2O, Na-EuW10) was prepared according to the literature [25].

4.2. Measurements

Infrared (IR) spectra were recorded with an FT/IR-4200ST spectrometer (Jasco Corporation, Tokyo, Japan, KBr pellet method). Powder X-ray diffraction (XRD) patterns were measured on a MiniFlex300 diffractometer (Rigaku Corporation, Tokyo, Japan, Cu Kα radiation, λ = 1.54056 Å). CHN (carbon, hydrogen, and nitrogen) elemental analyses were performed with a 2400II elemental analyzer (PerkinElmer, Inc., Waltham, MA, USA). Energy dispersive X-ray (EDS) spectroscopy was performed on a JSM-6000Plus (JEOL, Tokyo, Japan). Thermal gravimetric (TG) analyses were measured with a TG/DTA-6200 (Seiko Instruments, Chiba, Japan) at a heating rate of 10 °C min−1 under a nitrogen atmosphere.
Steady-state spectra (diffuse-reflectance, excitation, and emission) were obtained at 300 K on an FP-6500 fluorescence spectrometer (Jasco Corporation, Tokyo, Japan) using Xe lamp excitation. Time-resolved emission spectra were acquired at 15 and 300 K, using an Ultra CFR 400 YAG:Nd3+ laser (Big Sky Laser Technologies, Inc., Bozeman, MT, USA, 266 nm fourth harmonics, pulse duration 10 ns with a repetition rate of 10 Hz) as an excitation source. A Spectra Pro 2300i and PI-Max intensified CCD camera (Princeton Instruments, Inc., Trenton, NJ, USA) were employed as a spectrometer and a detector, respectively. Pelletized samples of the as-prepared precipitate of C8N2-EuW10 and C10N2-EuW10 were utilized for the photoluminescence measurements.

4.3. Synthesis of C8N2-EuW10 Hybrid Crystal

A water/ethanol (20 mL, 1:1 (v/v)) solution of C8N2-Cl (0.11 g, 0.50 mmol) was added to an aqueous solution (20 mL) of Na-EuW10 (0.47 g, 0.14 mmol), and stirred for 10 min. The resultant suspension was heated until 60 °C with stirring (for 5–10 min) and quickly filtrated to obtain a colorless as-prepared precipitate of C8N2-EuW10 (0.078 g, yield 17%). Colorless plates of C8N2-EuW10 single crystal were isolated from the hot synthetic filtrate kept at 25–42 °C (0.25 g, yield 51%). No presence of Na+ in the C8N2-EuW10 single crystals was confirmed using EDS spectroscopy. Anal. Calcd for C32H105N8EuW10O44: C, 11.66; H, 3.21; N, 3.40%. Found: C, 11.45; H, 2.98; N, 3.29%. IR (KBr disk): 936 (m), 840 (s), 775 (s), 704 (s), 537 (w), 489 (w), 440 (w), 423 (m) cm−1.

4.4. Synthesis of C10N2-EuW10 Hybrid Crystal

The synthesis of C10N2-EuW10 was carried out using a similar procedure as for C8N2-EuW10. A colorless as-prepared precipitate of C10N2-EuW10 was obtained from the combined suspension of Na-EuW10 and C10N2-Cl (0.22 g, yield 49%). Colorless plate single crystals of C10N2-EuW10 were isolated from the hot synthetic filtrate (0.16 g, yield 34%). No presence of Na+ in the C10N2-EuW10 single crystals was confirmed using EDS spectroscopy. Anal. Calcd for C37H99N7EuW10O37: C, 13.65; H, 3.06; N, 3.01%. Found: C, 13.79; H, 3.15; N, 3.14%. IR (KBr disk): 946 (m), 848 (s), 824 (m), 773 (s), 703 (s), 531 (w), 494 (w), 459 (w), 418 (m) cm−1.

4.5. X-ray Crystallography

Single-crystal X-ray diffraction measurements were performed with a Rigaku XtaLAB PRO P200 diffractometer using graphite monochromated Mo Kα radiation (λ = 0.71073 Å) or Cu Kα radiation (λ = 1.54184 Å). The data collection and processing including absorption correction were performed by CrysAlisPro (Version 1.171.39.46) [59]. Crystal structures were solved by SHELXT (Version 2018/2) [60], and refined through the full-matrix least-squares using SHELXL (Version 2018/3) [61]. The diffraction data recorded at the 2D beamline in the Pohang Accelerator Laboratory (PAL) confirmed the same crystal structure. CCDC 2352151-2352152.

5. Conclusions

Lanthanide-containing polyoxometalate-surfactant hybrid crystals were first obtained as single crystals. A highly luminescent decatungstoeuropate (EuW10) anion was successfully crystallized with bolaamphiphile surfactant cations (C8N2 and C10N2). Both C8N2-EuW10 and C10N2-EuW10 hybrid crystals had a similar packing of the EuW10 anion: a layer structure viewed along the b-axis and a honeycomb-like structure viewed along the a-axis. The EuW10 anions formed a two-dimensional network parallel to the ab plane by O–H⋯O hydrogen bonding with water molecules. The luminescent properties of C8N2-EuW10 and C10N2-EuW10 were investigated by means of steady-state and time-resolved spectroscopy. The characteristic emission owing to EuW10 was essentially retained after the hybrid crystals. The emission decay time of C10N2-EuW10 became shorter than that of C10N2-EuW10, especially at a high temperature (300 K), suggesting the thermal deactivation of the excitation energy derived from the longer organic surfactant of C10N2. The C8N2-EuW10 and C10N2-EuW10 hybrid crystals exhibited preliminary lasing properties, which is promising as a new category of inorganic–organic phosphors.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/inorganics12060146/s1, Figure S1: Asymmetric unit of C8N2-EuW10; Figure S2: Asymmetric unit of C10N2-EuW10; Figure S3: TG profiles of C8N2-EuW10 and C10N2-EuW10.

Author Contributions

Conceptualization, T.I.; methodology, R.I. and T.S.; validation, Y.S., T.K. and T.I.; formal analysis, T.I., Y.S., T.K., T.M. and Y.O.; investigation, R.I., R.K., Y.S., T.S. and K.K.; resources, Y.S. and Y.O.; writing—original draft preparation, T.I.; writing—review and editing, T.I. and Y.S.; visualization, T.I., Y.S., R.I. and T.M.; funding acquisition, T.I. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded in part by JSPS KAKENHI (grant number JP21K05232), and the Research and Study Project of Tokai University Research Organization.

Data Availability Statement

Further details of the crystal structure investigation (CCDC 2352151-2352152) can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif (accessed on 30 April 2024), or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; Fax: +44-1223-336033.

Acknowledgments

This work is partially supported by the Tokai University Imaging Center for Advanced Research. X-ray diffraction measurements with synchrotron radiation were performed at the Pohang Accelerator Laboratory (Beamline 2D, proposal No. 2019-1st-2D-015), a synchrotron radiation facility in Pohang, Republic of Korea, supported by Pohang University of Science and Technology (POSTECH).

Conflicts of Interest

The authors declare no conflicts of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

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Figure 1. (a) Molecular structure of decatungstoeuropate anion, [EuW10O36]9− (EuW10). Each polyhedron represents a WO6 unit, and the red sphere represents a Eu3+ ion. (b) Molecular structure of bolaamphiphiles utilized in this work. Upper: 1,8-octamethylenediammonium, [H3N(CH2)8NH3]2+ (C8N2); bottom: 1,10-decamethylenediammonium, [H3N(CH2)10NH3]2+ (C10N2).
Figure 1. (a) Molecular structure of decatungstoeuropate anion, [EuW10O36]9− (EuW10). Each polyhedron represents a WO6 unit, and the red sphere represents a Eu3+ ion. (b) Molecular structure of bolaamphiphiles utilized in this work. Upper: 1,8-octamethylenediammonium, [H3N(CH2)8NH3]2+ (C8N2); bottom: 1,10-decamethylenediammonium, [H3N(CH2)10NH3]2+ (C10N2).
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Figure 2. IR spectra of EuW10 and bolaamphiphile hybrid crystals: (a) starting material of Na-EuW10; (b) as-prepared precipitate of C8N2-EuW10; (c) single crystal of C8N2-EuW10; (d) as-prepared precipitate of C10N2-EuW10; and (e) single crystal of C10N2-EuW10.
Figure 2. IR spectra of EuW10 and bolaamphiphile hybrid crystals: (a) starting material of Na-EuW10; (b) as-prepared precipitate of C8N2-EuW10; (c) single crystal of C8N2-EuW10; (d) as-prepared precipitate of C10N2-EuW10; and (e) single crystal of C10N2-EuW10.
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Figure 3. Powder X-ray diffraction patterns of EuW10 and bolaamphiphile hybrid crystals: (a) as-prepared precipitate of C8N2-EuW10; (b) single crystal of C8N2-EuW10; (c) calculated pattern of C8N2-EuW10 from the structure revealed using single-crystal X-ray diffraction; (d) as-prepared precipitate of C10N2-EuW10; (e) single crystal of C10N2-EuW10; and (f) calculated pattern of C10N2-EuW10 from the structure revealed using single-crystal X-ray diffraction.
Figure 3. Powder X-ray diffraction patterns of EuW10 and bolaamphiphile hybrid crystals: (a) as-prepared precipitate of C8N2-EuW10; (b) single crystal of C8N2-EuW10; (c) calculated pattern of C8N2-EuW10 from the structure revealed using single-crystal X-ray diffraction; (d) as-prepared precipitate of C10N2-EuW10; (e) single crystal of C10N2-EuW10; and (f) calculated pattern of C10N2-EuW10 from the structure revealed using single-crystal X-ray diffraction.
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Figure 4. Crystal structure of C8N2-EuW10 (Eu: pink; C: gray; N: blue; O: red). WO6 units in EuW10 are depicted in the polyhedral model. H atoms are omitted for clarity. (a) Packing diagram along b-axis (left) and a-axis (right). Solvent atoms are omitted for clarity. (b) One-dimensional arrangement of EuW10 anions. Broken lines represent short contacts between EuW10 anions. Symmetry codes: (i) 2 − x, −0.5 + y, 0.5 − z; (ii) 2 − x, 0.5 + y, 0.5 – z. (c) Molecular arrangement of the inorganic monolayer (ab plane). Broken lines represent short contacts between EuW10 anions and solvents.
Figure 4. Crystal structure of C8N2-EuW10 (Eu: pink; C: gray; N: blue; O: red). WO6 units in EuW10 are depicted in the polyhedral model. H atoms are omitted for clarity. (a) Packing diagram along b-axis (left) and a-axis (right). Solvent atoms are omitted for clarity. (b) One-dimensional arrangement of EuW10 anions. Broken lines represent short contacts between EuW10 anions. Symmetry codes: (i) 2 − x, −0.5 + y, 0.5 − z; (ii) 2 − x, 0.5 + y, 0.5 – z. (c) Molecular arrangement of the inorganic monolayer (ab plane). Broken lines represent short contacts between EuW10 anions and solvents.
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Figure 5. Crystal structure of C10N2-EuW10 (Eu: pink; C: gray; N: blue; O: red). WO6 units in EuW10 are depicted in the polyhedral model. H atoms and disordered parts are omitted for clarity. (a) Packing diagram along the b-axis (left) and a-axis (right). Solvent atoms are omitted for clarity. (b) One-dimensional arrangement of EuW10 anions. Broken lines represent short contacts between EuW10 anions. Symmetry codes: (i) 1 − x, −0.5 + y, 1.5 − z; (ii) 1 − x, 0.5 + y, 1.5 – z. (c) Molecular arrangement of the inorganic monolayer (ab plane). Broken lines represent short contacts between EuW10 anions and solvents.
Figure 5. Crystal structure of C10N2-EuW10 (Eu: pink; C: gray; N: blue; O: red). WO6 units in EuW10 are depicted in the polyhedral model. H atoms and disordered parts are omitted for clarity. (a) Packing diagram along the b-axis (left) and a-axis (right). Solvent atoms are omitted for clarity. (b) One-dimensional arrangement of EuW10 anions. Broken lines represent short contacts between EuW10 anions. Symmetry codes: (i) 1 − x, −0.5 + y, 1.5 − z; (ii) 1 − x, 0.5 + y, 1.5 – z. (c) Molecular arrangement of the inorganic monolayer (ab plane). Broken lines represent short contacts between EuW10 anions and solvents.
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Figure 6. Steady-state spectra of C8N2-EuW10 and C10N2-EuW10. The measurement temperature was 300 K: (a) diffuse reflectance spectra; (b) excitation spectra monitored on the emission at 595 nm; (c) emission spectra were measured with an excitation wavelength of 265 nm.
Figure 6. Steady-state spectra of C8N2-EuW10 and C10N2-EuW10. The measurement temperature was 300 K: (a) diffuse reflectance spectra; (b) excitation spectra monitored on the emission at 595 nm; (c) emission spectra were measured with an excitation wavelength of 265 nm.
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Figure 7. Photoluminescence properties of C8N2-EuW10 and C10N2-EuW10 investigated using time-resolved spectroscopy. Each spectrum or decay profile was obtained by a single pulse excitation with a wavelength of 266 nm. Emission spectra were acquired 50–100 μs after the excitation. Emission decays were monitored at the emission at 593 nm: (a) emission spectra measured at 15 K; (b) emission spectra measured at 300 K; (c) emission decay profiles measured at 15 K; (d) emission decay profiles measured at 300 K.
Figure 7. Photoluminescence properties of C8N2-EuW10 and C10N2-EuW10 investigated using time-resolved spectroscopy. Each spectrum or decay profile was obtained by a single pulse excitation with a wavelength of 266 nm. Emission spectra were acquired 50–100 μs after the excitation. Emission decays were monitored at the emission at 593 nm: (a) emission spectra measured at 15 K; (b) emission spectra measured at 300 K; (c) emission decay profiles measured at 15 K; (d) emission decay profiles measured at 300 K.
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Figure 8. Emission intensity–excitation laser power dependency of C8N2-EuW10 and C10N2-EuW10 at (a) 15 K and (b) 300 K. Each data point was obtained by a single pulse excitation with a wavelength of 266 nm on the emission at 593 nm. Data acquisition time: 50–100 μs after the excitation.
Figure 8. Emission intensity–excitation laser power dependency of C8N2-EuW10 and C10N2-EuW10 at (a) 15 K and (b) 300 K. Each data point was obtained by a single pulse excitation with a wavelength of 266 nm on the emission at 593 nm. Data acquisition time: 50–100 μs after the excitation.
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Table 1. Crystallographic data.
Table 1. Crystallographic data.
CompoundC8N2-EuW10C10N2-EuW10
Chemical formulaC32H88N8EuW10O46C35H91N7EuW10O42
Formula weight3311.53 3272.59
Crystal systemmonoclinicmonoclinic
Space groupP21/c (No. 14)P21/c (No. 14)
a (Å)15.5117 (6)15.2827 (2)
b (Å)15.3803 (5)16.2555 (2)
c (Å)31.3536 11) 32.0125 (6)
α (°)90.000090.0000
β (°)99.655 (4)98.8002(15)
γ (°)90.000090.0000
V (Å3)7374.2 (5)7859.2 (2)
Z44
ρcalcd (g cm−3)2.9832.766
T (K)93 (2)93 (2)
Wavelength (Å)0.710731.54184
μ (mm−1)16.47832.553
No. of reflections measured100,83865,068
No. of independent reflections19,63315,570
Rint0.09110.0596
No. of parameters852535
R1 (I > 2σ(I))0.05330.0716
wR2 (all data)0.10160.1980
Table 2. Emission lifetimes (τ/ms) of C8N2-EuW10 and C10N2-EuW10.
Table 2. Emission lifetimes (τ/ms) of C8N2-EuW10 and C10N2-EuW10.
Compound15 K300 K
C8N2-EuW103.0 ± 0.12.5 ± 0.1
C10N2-EuW100.94 ± 0.1 + 3.1 ± 0.1 11.1 ± 0.1 + 1.8 ± 0.1 1
Na-EuW103.5 22.6 3
1 Two exponential decays were applied. 2 The decay time at 4.2 K. Taken from Ref. [25] as a comparison. 3 The value at r.t. taken from Ref. [33].
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Ishibashi, R.; Koike, R.; Suda, Y.; Kojima, T.; Sumi, T.; Misawa, T.; Kizu, K.; Okamura, Y.; Ito, T. Lanthanide-Containing Polyoxometalate Crystallized with Bolaamphiphile Surfactants as Inorganic–Organic Hybrid Phosphors. Inorganics 2024, 12, 146. https://doi.org/10.3390/inorganics12060146

AMA Style

Ishibashi R, Koike R, Suda Y, Kojima T, Sumi T, Misawa T, Kizu K, Okamura Y, Ito T. Lanthanide-Containing Polyoxometalate Crystallized with Bolaamphiphile Surfactants as Inorganic–Organic Hybrid Phosphors. Inorganics. 2024; 12(6):146. https://doi.org/10.3390/inorganics12060146

Chicago/Turabian Style

Ishibashi, Rieko, Ruka Koike, Yoriko Suda, Tatsuhiro Kojima, Toshiyuki Sumi, Toshiyuki Misawa, Kotaro Kizu, Yosuke Okamura, and Takeru Ito. 2024. "Lanthanide-Containing Polyoxometalate Crystallized with Bolaamphiphile Surfactants as Inorganic–Organic Hybrid Phosphors" Inorganics 12, no. 6: 146. https://doi.org/10.3390/inorganics12060146

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