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

: 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 ([EuW 10 O 36 ] 9 − , EuW 10 ) anion was employed as a lanthanide source. The bolaamphiphile counterparts are 1,8-octamethylenediammonium ([H 3 N(CH 2 ) 8 NH 3 ] 2+ , C 8 N 2 ) and 1,10-decamethylenediammonium ([H 3 N(CH 2 ) 10 NH 3 ] 2+ , C 10 N 2 ). Both hybrid crystals of C 8 N 2 -EuW 10 and C 10 N 2 -EuW 10 were successfully obtained as single crystals, and their crystal structures were unambiguously determined using X-ray diffraction measurements. The photoluminescence properties of C 8 N 2 -EuW 10 and C 10 N 2 -EuW 10 were investigated by means of steady-state and time-resolved spectroscopy. The characteristic emission derived from the EuW 10 anion was retained after the hybridization process.


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].

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 Cationic surfactants and polymer matrices as well as neutral block copolymers are effectively hybridized with the EuW 10 anion to obtain luminescent nanocomposites [26-29], 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 EuW 10 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 EuW 10 anion and cationic bolaamphiphile surfactants, which have two hydrophilic heads in one molecule.The bolaamphiphile counterparts employed are 1,8-octamethylenediammonium ([H 3 N(CH 2 ) 8 NH 3 ] 2+ , C 8 N 2 ) and 1,10-decamethylenediam monium ([H 3 N(CH 2 ) 10 NH 3 ] 2+ , C 10 N 2 ), as shown in Figure 1b.Both hybrid crystals of C 8 N 2 -EuW 10 and C 10 N 2 -EuW 10 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 timeresolved spectroscopy.

Synthesis of EuW 10 -Bolaamphiphile Hybrid Crystals
C 8 N 2 -EuW 10 and C 10 N 2 -EuW 10 hybrid crystals were synthesized via ion-exchange reactions using sodium salt of EuW 10 (Na-EuW 10 ) and bolaamphiphile cations.The asprepared precipitate of C 8 N 2 -EuW 10 was obtained in 15-20% yield, and the as-prepared precipitate of C 10 N 2 -EuW 10 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 C 8 N 2 -EuW 10 or C 10 N 2 -EuW 10 .The yields of isolated single crystals were ca.50% for C 8 N 2 -EuW 10 , and ca.30% for C 10 N 2 -EuW 10 .Figure 2 shows IR spectra of the as-prepared precipitates and single crystals of C 8 N 2 -EuW 10 and C 10 N 2 -EuW 10 .The spectra of C 8 N 2 -EuW 10 (Figure 2b,c) showed characteristic peaks of the EuW 10 anion in the range of 400-1000 cm -1 (935-945 cm −1 [ν as (W=O t )], 820-850 cm −1 [ν as (W-O b -W)], 700-710 cm −1 [ν as (W-O c -W)]) [45].The peaks in the range of 2800-3000 cm −1 were derived from the C 8 N 2 cation (2920 cm −1 [ν as (−CH  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.3a) were crystalline, but slightly different from those of the C 8 N 2 -EuW 10 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 C 8 N 2 -EuW 10 single crystal was similar to that calculated from results using single-crystal X-ray diffraction (Figure 3c).The XRD patterns of the C 10 N 2 -EuW 10 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 C 8 N 2 -EuW 10 and C 10 N 2 -EuW 10 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.

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 (Figures 4 and 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 Eu 3+ cation held a distorted square-antiprismatic 8-fold coordination with Eu-O distances of 2.37-2.50Å (mean value: 2.43 Å), and the shortest Eu•••Eu distance was 10.49 Å, which is similar to those of Na-W10 [25].

Crystal Structures of EuW 10 -Bolaamphiphile Hybrid Crystals
The formulae of the hybrid crystals consisting of the EuW 10 anion and bolaamphiphile cations were revealed by means of single-crystal X-ray diffraction and CHN elemental analyses (Table 1).C 8 N 2 -EuW 10 has a formula of [H 3 N(CH 2 ) 8 NH 3 ] 4 H[EuW 10 O 36 ]•10H 2 O, in which four C 8 N 2 cations (2+ charge) and one H + (1+ charge) were connected to one EuW 10 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 C 8 N 2 cations (except for the C 8 N 2 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 EuW 10 was 1.12, while those for other O atoms were 1. (Figure 4c).Some hydrophilic heads of C 8 N 2 penetrated the EuW 10 -H 2 O layers with the N-H• • • O hydrogen bonding with distances of 2.71-3.04Å (mean value: 2.85 Å) [47].were connected to one EuW 10 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 C 10 N 2 cation (containing N7) was onto the inversion center with anticonformation.Other C 10 N 2 cations were bent with gauche conformation, and two C 10 N 2 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 EuW 10 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.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 The crystal packing of C 10 N 2 -EuW 10 viewed along the b-axis was a layer structure composed of EuW 10 inorganic layers and C 10 N 2 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 EuW 10 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 EuW 10 layer to form a two-dimensional network with the EuW 10 anions (EuW
Inorganics 2024, 12, x FOR PEER REVIEW 8 of 15 observed in the excitation spectra.In the emission spectra, distinct peaks due to 5 D0 → 7 FJ (J = 0, 1, 2, 3, 4) transition of Eu 3+ 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 5 D0 → 7 F0 transition, peaks at 587 and 593 nm to 5 D0 → 7 F1, and peaks at 611 and 618 nm to 5 D0 → 7 F2.The peaks around 650 nm are assignable to 5 D0 → 7 F3 transition, and peaks at 691 and 700 nm to 5 D0 → 7 F4 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 es- The photoluminescent properties of the C 8 N 2 -EuW 10 and C 10 N 2 -EuW 10 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 EuW 10 [22][23][24][25].Emission peaks at 575 nm are assigned to 5 D 0 → 7 F 0 transition, peaks at 587 and 593 nm to 5 D 0 → 7 F 1 , and peaks at 611 and 618 nm to 5 D 0 → 7 F 2 .The peaks around 650 nm are assignable to 5 D 0 → 7 F 3 transition, and peaks at 691 and 700 nm to 5 D 0 → 7 F 4 transition.The spectrum profiles of C 8 N 2 -EuW 10 and C 10 N 2 -EuW 10 were almost the same irrespective of the measurement temperatures.However, the emission decay profiles of C 8 N 2 -EuW 10 and C 10 N 2 -EuW 10 were different.The emission decay profiles of C 8 N 2 -EuW 10 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 C 10 N 2 -EuW 10 , 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 C 8 N 2 -EuW 10 and C 10 N 2 -EuW 10 at 15 K were comparable to that of Na-EuW 10 [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 C 10 N 2 -EuW 10 [33].

Compound 15 K 300 K
1 Two exponential decays were applied. 2 The decay time at 4.2 K. Taken from Ref. [25] as a comparison. 3The value at r.t.taken from Ref. [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.  Two exponential decays were applied. 2 The decay time at 4.2 K. Taken from Ref. [25] as a comparison. 3The value at r.t.taken from Ref. [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 C 8 N 2 -EuW 10 and C 10 N 2 -EuW 10 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 C 8 N 2 -EuW 10 and C 10 N 2 -EuW 10 , respectively.The threshold values at 300 K were 26.0 and 25.2 mJ cm −2 for C 8 N 2 -EuW 10 and C 10 N 2 -EuW 10 , respectively.These threshold values will be essentially in the same order.

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 (Figures 4 and 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 (Figures 4c and 5c).

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 C 8 N 2 and C 10 N 2 are smaller than those of quaternary alkylammonium cations, which may be another reason for the successful isolation of single crystals of C 8 N 2 -EuW 10 and C 10 N 2 -EuW 10 .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 C 8 N 2 -EuW 10 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 C 8 N 2 -EuW 10 (Figure 3c).The as-prepared precipitate and single crystal of C 8 N 2 -EuW 10 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 C 8 N 2 -EuW 10 .In the case of C 10 N 2 -EuW 10 , the XRD patterns of the as-prepared precipitate (Figure 3d) and single crystal (Figure 3e) were quite similar.The water molecules in the C 10 N 2 -EuW 10 hybrid crystal were located inside the inorganic layers of EuW 10 with short-contact interaction, and plausibly less easily desorbed from the crystal lattice.TG analyses indicated the stability of C 8 N 2 -EuW 10 and C 10 N 2 -EuW 10 until 180-200 • C (Figure S3).
The structures of C 8 N 2 -EuW 10 and C 10 N 2 -EuW 10 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 (Figures 4 and 5).The crystal structures of C 8 N 2 -EuW 10 and C 10 N 2 -EuW 10 were layer structures viewed along the b-axis, and a honeycomb-like feature viewed along the aaxis.Such structural features are observed for some POM-surfactant crystals [52,53].The packing features of EuW 10 in C 8 N 2 -EuW 10 and C 10 N 2 -EuW 10 were almost the same, while the number of bolaamphiphile cations and their conformations were different.In both hybrid crystals, the EuW 10 anions formed one-dimensional chain structures.The residual H + was relevant to the formation of the one-dimensional chain structure.These onedimensional chains of EuW 10 together with water molecules formed two-dimensional networks of EuW 10 -H 2 O parallel to the ab plane (Figures 4c and 5c).
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:Nd 3+ 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 C 8 N 2 -EuW 10 and C 10 N 2 -EuW 10 were utilized for the photoluminescence measurements.

Conclusions
Lanthanide-containing polyoxometalate-surfactant hybrid crystals were first obtained as single crystals.A highly luminescent decatungstoeuropate (EuW 10 ) anion was successfully crystallized with bolaamphiphile surfactant cations (C 8 N 2 and C 10 N 2 ).Both C 8 N 2 -EuW 10 and C 10 N 2 -EuW 10 hybrid crystals had a similar packing of the EuW 10 anion: a layer structure viewed along the b-axis and a honeycomb-like structure viewed along the a-axis.The EuW 10 anions formed a two-dimensional network parallel to the ab plane by O-H• • • O hydrogen bonding with water molecules.The luminescent properties of C 8 N 2 -EuW 10 and C 10 N 2 -EuW 10 were investigated by means of steady-state and time-resolved spectroscopy.The characteristic emission owing to EuW 10 was essentially retained after the hybrid crystals.The emission decay time of C 10 N 2 -EuW 10 became shorter than that of C 10 N 2 -EuW 10 , especially at a high temperature (300 K), suggesting the thermal deactivation of the excitation energy derived from the longer organic surfactant of C 10 N 2 .The C 8 N 2 -EuW 10 and C 10 N 2 -EuW 10 hybrid crystals exhibited preliminary lasing properties, which is promising as a new category of inorganic-organic phosphors.

Figure 3
Figure 3 demonstrates powder XRD patterns of the C 8 N 2 -EuW 10 and C 10 N 2 -EuW 10 hybrid crystals.The XRD patterns of the C 8 N 2 -EuW 10 as-prepared precipitate (Figure3a) were crystalline, but slightly different from those of the C 8 N 2 -EuW 10 single crystal (Figure3b), and calculated from the results using single-crystal X-ray diffraction (Figure3c).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 C 8 N 2 -EuW 10 single crystal was similar to that calculated from results using single-crystal X-ray diffraction (Figure3c).The XRD patterns of the C 10 N 2 -EuW 10 as-prepared precipitate (Figure3d) and single crystals (Figure3e) were both similar to the calculated pattern from the results using single-crystal X-ray diffraction (Figure3f).The results of IR spectra and powder XRD patterns indicate that both C 8 N 2 -EuW 10 and C 10 N 2 -EuW 10 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.

Figure 4 .
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

Figure 4 .
Figure 4. Crystal structure of C 8 N 2 -EuW 10 (Eu: pink; C: gray; N: blue; O: red).WO 6 units in EuW 10 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

Figure 6 .
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 .
Figure 6.Steady-state spectra of C 8 N 2 -EuW 10 and C 10 N 2 -EuW 10 .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.

Inorganics 2024 , 15 Figure 7 .
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 .
Figure 7. Photoluminescence properties of C 8 N 2 -EuW 10 and C 10 N 2 -EuW 10 investigated using timeresolved 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 8 .
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 .
Figure 8. Emission intensity-excitation laser power dependency of C 8 N 2 -EuW 10 and C 10 N 2 -EuW 10 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.