Inorganic–organic Hybrid Surfactant Crystals: Structural Aspects and Functions

Hybrid single crystals consisting of an organic surfactant and an inorganic moiety are promising functional materials. Layered crystals composed from alternate inorganic and surfactant layers are obtained by the template effect of long alkyl chain moiety. The composition, crystal packing, and molecular arrangement of the hybrid single crystals are controllable by changing the inorganic constituent and the surfactant molecular structure. The types of hybrid surfactant single crystals are twofold: (i) crystals consisting of discrete inorganic cation coordinated by ligands having amphiphilic moiety; and (ii) crystals comprising a surfactant cation and a discrete inorganic anion including polyoxometalate (POM) oxide clusters. The POM-surfactant hybrid single crystals are rather rare, and therefore promising as unprecedented functional materials. Their structural variation and functional properties are discussed.


Introduction
Construction of functional materials by a bottom-up strategy requires precise structural control of the component species.Small molecular or ionic species of the sub-nanometer order (<1 nm) are built into three-dimensional structures of the nano-or micrometer order.In the construction process, molecular or ionic component structures should be controlled, and an assembling methodology of molecular or ionic components is also quite significant.
A promising option for constructing functional materials is inorganic-organic hybrid materials [1][2][3][4][5].We can expect synergy of the merits derived from inorganic and organic components.The inorganic motif has wide options to select the consisting elements, and to bring thermal stability and various functions.On the other hand, the organic motif enables easy control of the molecular structure.We can make a flexible and precise design of the materials' structures and functions, which leads to novel functional materials.Among several assembling structures, the layered materials possess a two-dimensional arrangement of constituent ions or molecules.This structural anisotropy is beneficial to functional properties such as electronic or ionic conductivity [6][7][8][9].In addition, two-dimensional confinement of functional ions or molecules will lead to several interesting properties [10][11][12][13][14].
To construct inorganic-organic hybrid layered structures, surfactants or molecules with long alkyl chain(s) can be employed as an organic motif (Figure 1) because they self-assemble into a lamellar structure to behave as a synthetic template [15][16][17].As for inorganic motifs, metal ions such as transition metals or lanthanide ions can contribute to the emergence of physicochemical functions such as redox, magnetic, or photoluminescent properties [10][11][12][13][14].In addition, the introduction of inorganic motifs leads to an increase in melting point and thermal stability.Discrete inorganic ions contained in the hybrid surfactant materials are metal cations coordinated by organic ligands, or inorganic anions of metal halides [13], metal chalcogenides [16], and metal oxides (polyoxometalates, POMs) [18][19][20][21][22][23][24][25][26].Inorganic-organic surfactant layered materials are better for fabricating crystalline materials since the well-ordered two-dimensional arrangement of ions and/or molecules is effective for the emergence of several functions.Crystalline layered materials with higher melting point and higher thermal stability are demanded especially as solid electrolytes for motor vehicles because they work at intermediate temperatures (373-573 K) [27][28][29][30][31][32][33][34][35].Such compounds ideally should be obtained as single crystals.Single crystals have a distinct advantage in that their crystal structures can be clearly solved at an atomic level.Accurate elucidation of a material's structure enables us to construct a rational synthetic methodology.Knowing the precise structural design of molecular constituents has allowed for high performance in crystalline hybrid materials [1][2][3]8,9].
Here recent progress in inorganic-organic hybrid surfactant crystals is briefly overviewed.Organic constituents with long alkyl chain (number of C atoms ≥ 8) are referred to as "surfactants" herein.Hybrid surfactant single crystals are classified into some categories, and their structures and characteristics as functional materials are reviewed.

Hybrid Surfactant Single Crystals Constructed by Inorganic Ions
Hybrid surfactant single crystals containing inorganic ions are intrinsically ionic compounds.These crystals are at first classified into two categories: (i) crystals with discrete inorganic metal cations of M n+ ; and (ii) crystals comprising discrete inorganic anions such as [MX4] n− , [M2X6] n− , or [M4X10] n− (X = O, S, Cl, I, etc.), including POM cluster anions.

Hybrid Surfactant Single Crystals with Discrete Inorganic Cations
Hybrid surfactant single crystals containing discrete M n+ metal cations are usually metalcoordinated compounds, in which M n+ is coordinated by organic surfactant ligands.Of such compounds, the most famous examples are probably chlorophyll molecules involved in photosynthesis in plants (Figure 2) [36,37].The amphiphilic moiety of chlorophyll enables appropriate arrangements of its magnesium-containing hydroporphyrin moieties, which effectively absorb light as photo antennas.These coordination complexes composed of surfactant ligands  enable us to control the assembling manner of M n+ components in the solid state, owing to the long alkyl chains ("fastener effect") [66].Moreover, the coordination environments of metal cations can be accurately and flexibly controlled by changing the surfactant ligand structure.This leads to precise design of properties derived from electronic states of the coordinated metal ion.In addition, the surfactant moiety can add liquid crystallinity to metal coordination compounds.
Here recent progress in inorganic-organic hybrid surfactant crystals is briefly overviewed.Organic constituents with long alkyl chain (number of C atoms ě 8) are referred to as "surfactants" herein.Hybrid surfactant single crystals are classified into some categories, and their structures and characteristics as functional materials are reviewed.

Hybrid Surfactant Single Crystals Constructed by Inorganic Ions
Hybrid surfactant single crystals containing inorganic ions are intrinsically ionic compounds.These crystals are at first classified into two categories: (i) crystals with discrete inorganic metal cations of M n+ ; and (ii) crystals comprising discrete inorganic anions such as [MX 4 ] n´, [M 2 X 6 ] n´, or [M 4 X 10 ] n´( X = O, S, Cl, I, etc.), including POM cluster anions.
Crystals 2016, 6, 24 3 of 20 [58][59][60], 1,4,7-triazacyclononane [61]), and tetradentate ligands (Schiff base [62-64], 1,3,5,8,12pentaazacyclotetradecane [65]).Nitrogen atom-containing ligands have often been used, presumably due to the stability and versatility of coordination bonds.Ligands with a coordination number higher than 2 are effective due to the chelate or macrocyclic effect.Some of their coordination and packing modes are schematically extracted (Figure 4).The coordination mode depends on the number of surfactant ligands to coordinate to the metal cation, and on the number of amphiphilic chains of each ligand.Here, the coordination mode does not mean Some of their coordination and packing modes are schematically extracted (Figure 4).The coordination mode depends on the number of surfactant ligands to coordinate to the metal cation, and on the number of amphiphilic chains of each ligand.Here, the coordination mode does not mean accurate coordination geometry.There are six typical coordination and packing modes: asymmetric coordination of one single-chained ligand (Figure 4a) [38,39,55,61,65], symmetric coordination of two single-chained ligands lying on the opposite sides of the metal cation (Figure 4b) [41][42][43]49,51,53,55,59,60], symmetric coordination of one double-chained ligand with two chains at the opposite sides of the metal cation (Figure 4c) [45,47,54,55,62], asymmetric coordination of two single-chained ligands lying on the same side of the metal cation (Figure 4d) [44,46,48,56], asymmetric coordination of one double-chained ligand with narrow space between the chains (Figure 4e) [50, 54,63], and asymmetric coordination of one double-chained ligand with wide space between the chains (Figure 4f) [41].However, the essential feature is the formation of a layered structure that consists of M n+ inorganic layers and surfactant organic layers.The long amphiphilic chains are densely packed with or without interdigitation, which depends on the chain length and structure of coordinating moiety.Some ligands possess asymmetric coordination mode (Figure 4a,d-f); however, they usually form symmetric packing in the single crystals.Although it is difficult to strictly categorize these coordination and packing modes due to the variety of the ligand structures, monodentate ligands coordinate in both a symmetric and asymmetric manner (Figure 4a,b).On the other hand, bidentate and tridentate ligands seem to prefer symmetric coordination modes (Figure 4b,c) Crystals 2016, 6, 24 4 of 20 accurate coordination geometry.There are six typical coordination and packing modes: asymmetric coordination of one single-chained ligand (Figure 4a) [38,39,55,61,65], symmetric coordination of two single-chained ligands lying on the opposite sides of the metal cation (Figure 4b) [41][42][43]49,51,53,55,59,60], symmetric coordination of one double-chained ligand with two chains at the opposite sides of the metal cation (Figure 4c) [45,47,54,55,62], asymmetric coordination of two singlechained ligands lying on the same side of the metal cation (Figure 4d) [44,46,48,56], asymmetric coordination of one double-chained ligand with narrow space between the chains (Figure 4e) [50, 54,63], and asymmetric coordination of one double-chained ligand with wide space between the chains (Figure 4f) [41].However, the essential feature is the formation of a layered structure that consists of M n+ inorganic layers and surfactant organic layers.The long amphiphilic chains are densely packed with or without interdigitation, which depends on the chain length and structure of coordinating moiety.Some ligands possess asymmetric coordination mode (Figure 4a,d-f); however, they usually form symmetric packing in the single crystals.Although it is difficult to strictly categorize these coordination and packing modes due to the variety of the ligand structures, monodentate ligands coordinate in both a symmetric and asymmetric manner (Figure 4a,b).On the other hand, bidentate and tridentate ligands seem to prefer symmetric coordination modes (Figure 4b,c) Here "symmetric" means that amphiphilic chains lie in the opposite sides of metal cation, and "asymmetric" means that amphiphilic chains lie on the same side of the metal cation.
These hybrid surfactant crystals containing coordinated metal M n+ cations exhibit spin crossover [56,[58][59][60] and valence tautomerism [38] induced by changes in the coordination environment.Thermal motion in the long amphiphilic chains plausibly affects the electronic spin state of metal cations.These crystals intrinsically can exhibit the phase transition to the liquid crystalline state with an increase in the temperature as metallomesogens [67], and their mesomorphic property has been investigated [41][42][43][44][45][46]51,68,69].Ionic conductivity [70,71], photoluminescence [39,54,62,63], catalytic activity [47,48], thin-film formation ability [39,69], and behavior as preparative precursor for metal nanoparticles [43,57] have been explored recently.(e) asymmetric coordination of one double-chained ligand with narrow space between the chains; (f) asymmetric coordination of one double-chained ligand with wide space between the chains.Here "symmetric" means that amphiphilic chains lie in the opposite sides of metal cation, and "asymmetric" means that amphiphilic chains lie on the same side of the metal cation.

Hybrid Surfactant Single Crystals with Discrete Inorganic Anions
Hybrid surfactant single crystals containing discrete inorganic anions are ionic salts of amphiphilic surfactant cations and non-amphiphilic inorganic anions [72][73][74][75][76][77][78][79][80][81][82][83][84][85][86][87][88][89][90].The inorganic anions are metal halides, chalcogenides, and oxides, which vary from monomeric anions to highly polymeric anions (Figure 5).  87-90].The minus charge of the inorganic anion is compensated for by the organic surfactant cation (Figure 6).These hybrid crystals composed of discrete inorganic anions are less popular in number and variety than the hybrid crystals consisting of discrete metal cations coordinated by amphiphilic ligands, probably because a more specific combination of inorganic anion and surfactant cation is necessary.The formation of single crystals tends to require size matching between the inorganic anions and hydrophilic heads of surfactant cations, resulting in a specific combination of inorganic and surfactant constituents.Alkylammonium (Figure 6c) [72,80,81,84,86,88] and alkyldiammonium (Figure 6d) [73,89,90] often give single crystals, being different from the case of hybrid surfactant single crystals of coordinated compounds.The primary ammonium cations can form hydrogen bonds between inorganic anions by using N-H bonds to stabilize the crystal structures [87].However, these ammonium cations are not effective ligands to M n+ cations, since the stabilization by the chelate effect cannot be expected.
The hybrid surfactant crystals containing monomeric inorganic anions typically form layered packing, as shown in Figure 7a.The packing manner comprises alternate stacking of inorganic monolayers and organic surfactant layers.The single-headed surfactant forms a bilayer arrangement to realize a dense packing of long alkyl chains (Figure 7a, left), while the double-headed surfactant forms monolayer structures (Figure 7a, right).When a single-headed and single-chained surfactant is employed, the inorganic layers sometimes form a bilayer structure, probably due to the small size of monomeric ions (Figure 7b) [74,75,79,80].To compensate for the negative charge of the inorganic anion, the charged hydrophilic heads of the surfactant come close to the anion.The small monomeric anions are necessary to form a bilayer arrangement to increase the number of surfactant cations associated with the monomeric anions.The polymeric anions realize more integrated metal ions confined in the two-dimensional inorganic layer, and are expected to emerge specific electronic properties [10][11][12][13][14]16].The metal halides tend to form a perovskite-type infinite two-dimensional layered structure [13], which consists of a polymeric metal halide layer such as [PbI4] 2− (Figure 5) [87][88][89][90].On the other hand, metal chalcogenides form molecular cluster anions ( 5), and are hybridized to form layered single crystals [81][82][83][84][85].Such molecular cluster-surfactant hybrid single crystals are much more unconventional materials.
The layered structures are controllable by changing the length of surfactant alkyl chain [125].The case of hybrid crystals comprising Mo8 and CnNC3 (n = 8, 10, 12, 14, 16, 18) is shown in Figure 9. Interestingly, there are two types of homologue structure, which change owing to the surfactant The hybrid surfactant crystals containing monomeric inorganic anions typically form layered packing, as shown in Figure 8a.The packing manner comprises alternate stacking of inorganic monolayers and organic surfactant layers.The single-headed surfactant forms a bilayer arrangement to realize a dense packing of long alkyl chains (Figure 8a, left), while the double-headed surfactant forms monolayer structures (Figure 8a, right).When a single-headed and single-chained surfactant is employed, the inorganic layers sometimes form a bilayer structure, probably due to the small size of monomeric ions (Figure 8b) [74,75,79,80].To compensate for the negative charge of the inorganic anion, the charged hydrophilic heads of the surfactant come close to the anion.The small monomeric anions are necessary to form a bilayer arrangement to increase the number of surfactant cations associated with the monomeric anions.The polymeric anions realize more integrated metal ions confined in the two-dimensional inorganic layer, and are expected to emerge specific electronic properties [10][11][12][13][14]16].The metal halides tend to form a perovskite-type infinite two-dimensional layered structure [13], which consists of a polymeric metal halide layer such as [PbI4] 2− (Figure 5) [87][88][89][90].On the other hand, metal chalcogenides form molecular cluster anions ([Sn2S6] 4− , [Ge4S10] 4− , [Ge4Se10] 4− , [Re6Se8(CN)6] 4− , Figure 5), and are hybridized to form layered single crystals [81][82][83][84][85].Such molecular cluster-surfactant hybrid single crystals are much more unconventional materials.
The packing feature in the hybrid surfactant single crystals comprised of polymeric inorganic anions is depicted in Figure 8c.The inorganic layers are composed from the monolayer of large molecular cluster anions or an infinite two-dimensional anionic layer.These inorganic monolayers are sandwiched by bilayers of the single-headed surfactant (Figure 8c, left) or monolayers of the double-headed surfactant (Figure 8c, right).Due to the large size of polymeric anions, charged surfactant hydrophilic heads of the surfactant cation can easily access anions, resulting in the monolayer arrangement of the inorganic anions.
The layered POM-surfactant hybrids with two-dimensional anisotropy have been assembled as liquid crystals [105], thin films [101,103,106], and mesostructured solids [100,102,[107][108][109][110][111][112].However, most of them are materials with atomically ill-ordered structure.On the other hand, POM-surfactant single crystals having rigid structures are much rarer hybrids.This is probably because the POM-surfactant hybrids have a larger surfactant cation, which results in lower solubility to crystallize from common solvents.In addition, their anisotropic layered structures often lead to fine needle or thin plate crystals, challenging to analyze by single crystal X-ray structure analysis.
POM-surfactant hybrid layered crystals have been known since 1997 [113], and began to be successfully obtained as single crystals in this decade .The layered structure of POM-surfactant crystals comprises alternate stacking of POM monolayers and surfactant bilayers, as shown in Figure 8c.A few crystals possess the bilayer packing manner of POM anions [119,121].The inorganic POM layer can be considered as an atomically regular two-dimensional POM array, and the molecular arrangement of POM anion is controllable by changing the structures of surfactant hydrophilic head.In addition, the layered distance depends on the structures of the constituent POM and surfactant.
The layered structures are controllable by changing the length of surfactant alkyl chain [125].The case of hybrid crystals comprising Mo 8 and C n NC 3 (n = 8, 10, 12, 14, 16, 18) is shown in Figure 9. Interestingly, there are two types of homologue structure, which change owing to the surfactant length (Figure 9a,b).Shorter surfactants prefer β-Mo 8 isomer, while longer ones prefer δ-Mo 8 isomer, conceivably due to the effect of van der Waals interactions between the surfactant alkyl chains.Such a structural change in isomer molecules induced by the surfactant chain length is rather unique.The layered distances of the C n NC 3 -δ-Mo 8 crystals (n = 12, 14, 16, 18) depend linearly on the surfactant length (Figure 9c).Molecular arrangements of POMs are also controllable by changing the structure of surfactant hydrophilic heads, because POMs strongly interact with hydrophilic moieties of employed surfactants.Heterocyclic moieties of Cnpy, Cnpda, and Cnmim surfactants fully penetrate into POM inorganic layers to isolate each POM anion in many cases [118,120,123,127,128].The difference in the molecular structures of the heterocyclic moiety causes different arrangements of W10 anions [128].On the other hand, less bulky heads of CnNH3, CnNC3, and (Cn)2NC2 tend to keep POM anions close, leading to a more densely-packed arrangement of POM anions.In the case of β-Mo8 hybrid crystals, C12NH3 cations do not completely isolate the β-Mo8 anions to result in the close arrangement in the β-Mo8 layers (Figure 10a), while more bulky hydrophilic heads of C12mim penetrate into the β-Mo8 layers, resulting in the complete isolation of each β-Mo8 anion (Figure 10b) [123].CnNC3-V10 (n = 10, 12) [113,126] and (C18)2NC2-Mo6 [117] also have rather dense packing of POM anions.Such controllability in molecular arrangement and distance would affect the emergence of conductive or magnetic property.Molecular arrangements of POMs are also controllable by changing the structure of surfactant hydrophilic heads, because POMs strongly interact with hydrophilic moieties of employed surfactants.Heterocyclic moieties of C n py, C n pda, and C n mim surfactants fully penetrate into POM inorganic layers to isolate each POM anion in many cases [118,120,123,127,128].The difference in the molecular structures of the heterocyclic moiety causes different arrangements of W 10 anions [128].On the other hand, less bulky heads of C n NH 3 , C n NC 3 , and (C n ) 2 NC 2 tend to keep POM anions close, Crystals 2016, 6, 24 10 of 21 leading to a more densely-packed arrangement of POM anions.In the case of β-Mo 8 hybrid crystals, C 12 NH 3 cations do not completely isolate the β-Mo 8 anions to result in the close arrangement in the β-Mo 8 layers (Figure 10a), while more bulky hydrophilic heads of C 12 mim penetrate into the β-Mo 8 layers, resulting in the complete isolation of each β-Mo 8 anion (Figure 10b) [123].C n NC 3 -V 10 (n = 10, 12) [113,126] and (C 18 ) 2 NC 2 -Mo 6 [117] also have rather dense packing of POM anions.Such controllability in molecular arrangement and distance would affect the emergence of conductive or magnetic property.One option for the functionalized POM-surfactant crystals is to introduce other countercations such as proton or sodium into the crystal structures as well as surfactant cations.The crystals containing β-type Mo8 anions tend to crystallize with small countercations such as proton or sodium [120][121][122]124], and are promising candidates for the solid electrolyte for fuel-cell or sodium-ion batteries.The surfactant hybrid crystals containing small countercations are limited to the crystals consisting of β-Mo8, γ-Mo8, and V10 to date. Figure 11a shows the crystal structure of a protoncontaining Mo8 hybrid crystal, which has been synthesized by gradual oxidation of reduced polyoxomolybdate-surfactant hybrids [122].In this crystal, a Mo8 anion with a charge of 4− is associated with two C16py surfactant cations.The remaining minus charge is compensated for by two protons to form a diprotonated β-Mo8 anion, which has been confirmed by bond-valence-sum (BVS) calculations.Interestingly, the inorganic layers are two-dimensional anionic layers composed of Mo8 anions and ethanol molecules of crystallization (Figure 11a, right).The pyridine rings of the surfactants are not penetrated into the inorganic layer, implying the emergence of a surfactant-cation exchange.Another proton-containing hybrid crystal can be obtained by using V10 and C10NC3, and the V10 anions are diprotonated species (Figure 11b).The presence of protons has been clearly identified by the X-ray structure analysis [126].In the C10NC3-V10 crystal, the diprotonated V10 anions are hydrogen-bonded to form a one-dimensional infinite chain structure (Figure 11b, right).The hydrogen-bonded V10 chains are closely located (the distance between each chain: 3.25 Å), and not completely isolated by the C10NC3 cations as mentioned above.The inorganic layer composed of hydrogen-bonded V10 infinite chains is considered a "pseudo two-dimensional" layer of diprotonated V10 anions.The close distance between protonated V10 infinite chains would possibly contribute to the emergence of proton conductivity, as described below.
Sodium ion-containing hybrid crystals of Mo8 can be synthesized in the presence of a sodium ion by using Cnpy or Cnmim (Figure 12) [120,121,124].In cases where a C16py cation is employed, one β-Mo8 anion having a charge of 4− is associated with three C16py surfactant cations and one sodium cation [120,121].A one-dimensional chain structure composed from Mo8 and a sodium cation is formed (Figure 12b, upper).Different types of sodium ion-containing hybrid crystals have been obtained by One option for the functionalized POM-surfactant crystals is to introduce other countercations such as proton or sodium into the crystal structures as well as surfactant cations.The crystals containing β-type Mo 8 anions tend to crystallize with small countercations such as proton or sodium [120][121][122]124], and are promising candidates for the solid electrolyte for fuel-cell or sodium-ion batteries.The surfactant hybrid crystals containing small countercations are limited to the crystals consisting of β-Mo 8 , γ-Mo 8 , and V 10 to date. Figure 11a shows the crystal structure of a proton-containing Mo 8 hybrid crystal, which has been synthesized by gradual oxidation of reduced polyoxomolybdate-surfactant hybrids [122].In this crystal, a Mo 8 anion with a charge of 4´is associated with two C 16 py surfactant cations.The remaining minus charge is compensated for by two protons to form a diprotonated β-Mo 8 anion, which has been confirmed by bond-valence-sum (BVS) calculations.Interestingly, the inorganic layers are two-dimensional anionic layers composed of Mo 8 anions and ethanol molecules of crystallization (Figure 11a, right).The pyridine rings of the surfactants are not penetrated into the inorganic layer, implying the emergence of a surfactant-cation exchange.Another proton-containing hybrid crystal can be obtained by using V 10 and C 10 NC 3 , and the V 10 anions are diprotonated species (Figure 11b).The presence of protons has been clearly identified by the X-ray structure analysis [126].In the C 10 NC 3 -V 10 crystal, the diprotonated V 10 anions are hydrogen-bonded to form a one-dimensional infinite chain structure (Figure 11b, right).The hydrogen-bonded V 10 chains are closely located (the distance between each chain: 3.25 Å), and not completely isolated by the C 10 NC 3 cations as mentioned above.The inorganic layer composed of hydrogen-bonded V 10 infinite chains is considered a "pseudo two-dimensional" layer of diprotonated V 10 anions.The close distance between protonated V 10 infinite chains would possibly contribute to the emergence of proton conductivity, as described below.Conductive property has been investigated for some POM-surfactant hybrid crystals.The measurements were carried out using the alternating current (AC) impedance method for a pelletized sample under an anhydrous atmosphere at intermediate temperature.Electronic conductivities for hybrid crystals of C16py-W10 (Figure 13) [127] and C16py-Mo4 [129] are in the range of 10 −6 to 10 −5 S cm −1 order.Although these values are much lower than the radical salts of POM containing an organic donor [8,9], POM-surfactant crystals with an appropriate combination of POMs and surfactants Sodium ion-containing hybrid crystals of Mo 8 can be synthesized in the presence of a sodium ion by using C n py or C n mim (Figure 12) [120,121,124].In cases where a C 16 py cation is employed, one β-Mo 8 anion having a charge of 4´is associated with three C 16 py surfactant cations and one sodium cation [120,121].A one-dimensional chain structure composed from Mo 8 and a sodium cation is formed (Figure 12b, upper).Different types of sodium ion-containing hybrid crystals have been obtained by using ionic liquid C n mim (n = 10, 12) surfactant cations (Figure 12a) [124].Notably in these cases, two sodium ions as well as two C n mim cations are contained per one βor γ-Mo 8 having a charge of 4´, different from the C 16 py-β-Mo 8 hybrid crystals.In the C n mim-Mo 8 hybrid crystals, two sodium cations connect Mo 8 anions to form infinite one-dimensional chain structures (Figure 12b, bottom).Conductive property has been investigated for some POM-surfactant hybrid crystals.The measurements were carried out using the alternating current (AC) impedance method for a pelletized sample under an anhydrous atmosphere at intermediate temperature.Electronic conductivities for hybrid crystals of C16py-W10 (Figure 13) [127] and C16py-Mo4 [129] are in the range of 10 −6 to 10 −5 S cm −1 order.Although these values are much lower than the radical salts of POM containing an organic donor [8,9], POM-surfactant crystals with an appropriate combination of POMs and surfactants Conductive property has been investigated for some POM-surfactant hybrid crystals.The measurements were carried out using the alternating current (AC) impedance method for a pelletized sample under an anhydrous atmosphere at intermediate temperature.Electronic conductivities for hybrid crystals of C 16 py-W 10 (Figure 13) [127] and C 16 py-Mo 4 [129] are in the range of 10 ´6 to 10 ´5 S cm ´1 order.Although these values are much lower than the radical salts of POM containing an organic donor [8,9], POM-surfactant crystals with an appropriate combination of POMs and surfactants would be possible for another class of hybrid solid electrolytes.On the other hand, the proton-containing C 10 NC 3 -V 10 hybrid crystal exhibits anhydrous proton conductivity of 6.5 ˆ10 ´5 S cm ´1 at 373 K [126].Emergence of anhydrous proton conductivity at intermediate temperatures is demanded for solid electrolyte application for fuel-cell technology [29][30][31][32][33].Although the proton conductivity of C 10 NC 3 -V 10 is rather unstable and lower than other POM hybrid materials [34,35], the proton-containing POM-surfactant crystals would pave the way for an unprecedented class of proton conductors.The redox property, which characterizes POMs as specific functional materials [25,26], has also been investigated.Electrochemical property has been measured in solution state for C16NC3-SiMo12 and its reduced hybrids [130,131].A (C18)2NC2-Mo6 hybrid crystal containing a double-chained surfactant has been found to form a self-assembled helical structure in dichloromethane/propanol solution [141].This helical structure exhibits reversible transformation into spherical assemblies with accompanying photoreduction by UV light irradiation, and back to helical assemblies by H2O2 oxidation.Catalytic behavior is another characteristic of POM anions [24].Catalytic oxidation of acetaldehyde with H2O2 has been explored for bolaamphiphilic 1,ω-bis(pyridinium)alkane-Mo8 hybrid crystals (ω = 8-12) [134].

Hybrid Single Crystals Composed from Polyoxometalate Modified by Amphiphilic Moiety
A recent class of POM-surfactant hybrid crystals is single crystals composed from POMs having amphiphilic moiety [137][138][139][140].In this type of POM-surfactant crystals, amphiphilic POMs are synthesized by grafting a long alkyl chain covalently into pre-modified or lacunary POM anion by condensation reaction such as esterification [22,23].Several types of POMs with amphiphilic moiety (moieties) have been synthesized [98][99][100]112,[142][143][144][145][146][147][148][149] from Lindqvist-, Anderson-, and Keggin-type anions.However, the amphiphilic POMs analyzed from single crystals have been limited to hexavanadate and hexamolybdate derivatives so far.Anderson-type amphiphilic POMs have been reported to crystallize; however, the crystal structures have not been fully solved due to severe disorder of the amphiphilic moieties [140] 7i) [137].The hexamolybdate derivative is single-chained [Mo 6 O 18 (N-C 6 H 3 -2-(CH 3 )-4-OCOC 17 H 35 )] 2´( Figure 7j) [139].In these three cases, amphiphilic POMs having a charge of 2´crystallize with a tetrabutylammonium cation (TBA).The packing manners consist of alternate stacking of an inorganic layer of POM cluster moiety and an organic layer of amphiphilic moiety, being essentially similar to those observed in most POM-surfactant hybrid crystals.Single-chained amphiphilic POMs comprise bilayer arrangements of POMs [138,139], while a double-chained amphiphilic POM has a monolayer arrangement of POM moiety [137].The difference in the amphiphilicity due to the number of the long alkyl chain may cause the different packing manner of amphiphilic POM layers.As for these crystals, cation exchange to protons from TBA induces characteristic fluorescence [137] and catalysis [138], which demonstrates a wide potential of amphiphilic POMs for functional materials.

Summary and Outlook
Hybrid surfactant single crystals containing an inorganic moiety have been evaluated in the search for unprecedented functional materials.Layered crystals composed of inorganic layers and surfactant layers are obtained due to the template effect of a long alkyl chain moiety.These crystalline layered structures create the anisotropic arrangement of the inorganic component, which accounts for the emergence of conductivity [7][8][9], two-dimensional confinement of magnetic molecules [10], photocatalytic activity [11,12], and controlled luminescence [14,150].
The composition, crystal packing, and molecular arrangement of these hybrid single crystals are controllable by changing the inorganic constituents and surfactant moieties.There are two types of hybrid surfactant single crystals: (i) crystals consisting of a discrete inorganic cation coordinated by ligands having amphiphilic moiety; and (ii) crystals composed from discrete inorganic anion hybridized with surfactant cation.The crystals consisting of a discrete inorganic cation are suitable for controlling the coordination environment of metal cations, resulting in precisely controlled properties such as magnetism [56,58,151,152] or photoluminescence [153][154][155].On the other hand, crystals composed from discrete inorganic anions, including infinite two-dimensionally polymerized anions and molecular cluster anions, can confine the metal components in the layered structures, which leads to characteristic electronic properties [13,16] and device applications [95][96][97][156][157][158].
Some of the most promising functional inorganic anions are polyoxometalate (POM) cluster anions.POM-surfactant hybrid single crystals, which are comprised of alternate inorganic POM layers and organic surfactant layers, have been synthesized recently.POM-surfactant crystals containing a typical POM anion and a surfactant cation have been obtained via a simple cation-exchange reaction.Changing POM and surfactant molecular structures enables us to control the layered structure including two-dimensional POM arrangement, and to introduce a small countercation such as proton or sodium.Conductivity, redox property, and catalytic behavior have been explored.In addition, amphiphilic POMs have been reported to form single crystals, and to exhibit characteristic properties owing to the POM moiety.This type of POMs possesses a long alkyl chain moiety covalently grafted directly onto a POM skeleton.
There are still several difficulties to overcome for the synthesis, structural prediction, and structure analysis of hybrid surfactant single crystals.The distinct crystal and molecular structures are difficult to analyze, since their crystals are often tiny and anisotropic (thin plates or fine fibers).To obtain suitable single crystals sometimes requires slow crystal growth under mild conditions.Effective options are to adjust the appropriate concentration of the dissolved sample and keep the temperature constant during the crystallization.Synchrotron radiation is a powerful method for measurement.Powder diffraction methods are difficult to apply to solving the structures of hybrid surfactant crystal containing many heavy metals such as POM clusters, because the position of the light atoms (C and N) in the surfactant moiety is difficult to discern due to their weak reflections in the diffraction data.In addition, the emergence of characteristic properties is still developing; more intentional design of built-up hybrid crystals is needed.However, these difficulties deserve to be tackled in order to explore the synthesis and functionalization of hybrid surfactant single crystals.

Figure 1 .
Figure 1.Schematic representation of hybrid surfactant single crystals with layered structures.

Figure 1 .
Figure 1.Schematic representation of hybrid surfactant single crystals with layered structures.

Figure 4 .
Figure 4. Extracted coordination and packing modes observed in hybrid surfactant single crystals composed from discrete metal cations and surfactant ligands: (a) asymmetric coordination of one single-chained ligand; (b) symmetric coordination of two single-chained ligands; (c) symmetric coordination of one double-chained ligand; (d) asymmetric coordination of two single-chained ligands; (e) asymmetric coordination of one double-chained ligand with narrow space between the chains; (f) asymmetric coordination of one double-chained ligand with wide space between the chains.Here "symmetric" means that amphiphilic chains lie in the opposite sides of metal cation, and "asymmetric" means that amphiphilic chains lie on the same side of the metal cation.

Figure 4 .
Figure 4. Extracted coordination and packing modes observed in hybrid surfactant single crystals composed from discrete metal cations and surfactant ligands: (a) asymmetric coordination of one single-chained ligand; (b) symmetric coordination of two single-chained ligands; (c) symmetric coordination of one double-chained ligand; (d) asymmetric coordination of two single-chained ligands;(e) asymmetric coordination of one double-chained ligand with narrow space between the chains; (f) asymmetric coordination of one double-chained ligand with wide space between the chains.Here "symmetric" means that amphiphilic chains lie in the opposite sides of metal cation, and "asymmetric" means that amphiphilic chains lie on the same side of the metal cation.

Figure 5 .
Figure 5. Several discrete inorganic anions employed to construct hybrid surfactant single crystals with cationic surfactants.Oxide cluster anions are represented separately in Figure 8.

Figure 5 .
Figure 5. Several discrete inorganic anions employed to construct hybrid surfactant single crystals with cationic surfactants.Oxide cluster anions are represented separately in Figure 7.

Figure 7 .
Figure 7. Packing manners observed in the hybrid surfactant single crystals comprised of discrete inorganic anions: (a) Monomeric anions hybridized with single-headed surfactant (left) and doubleheaded surfactant (right); (b) monomeric anions to form bilayer arrangement hybridized with singleheaded and single-chained surfactant; (c) polymeric anions hybridized with single-headed surfactant (left) and double-headed surfactant (right).The inorganic layers depicted by open circles are sometimes polymerized to form infinite two-dimensional layers (see text).

Figure 7 .
Figure 7. Packing manners observed in the hybrid surfactant single crystals comprised of discrete inorganic anions: (a) Monomeric anions hybridized with single-headed surfactant (left) and doubleheaded surfactant (right); (b) monomeric anions to form bilayer arrangement hybridized with singleheaded and single-chained surfactant; (c) polymeric anions hybridized with single-headed surfactant (left) and double-headed surfactant (right).The inorganic layers depicted by open circles are sometimes polymerized to form infinite two-dimensional layers (see text).

Figure 8 .
Figure 8. Packing manners observed in the hybrid surfactant single crystals comprised of discrete inorganic anions: (a) Monomeric anions hybridized with single-headed surfactant (left) and double-headed surfactant (right); (b) monomeric anions to form bilayer arrangement hybridized with single-headed and single-chained surfactant; (c) polymeric anions hybridized with single-headed surfactant (left) and double-headed surfactant (right).The inorganic layers depicted by open circles are sometimes polymerized to form infinite two-dimensional layers (see text).

Figure 9 .
Figure 9. (a) Crystal structures of C n NC 3 -Mo 8 hybrid crystals; (b) powder X-ray diffraction patterns of as-prepared C n NC 3 -Mo 8 ; (c) variation of the interlayer spacing of as-prepared C n NC 3 -Mo 8 with n [125].Reproduced with permission from Ito, T. et al., J. Mol.Struct.; published by Elsevier, 2016.
for another class of hybrid solid electrolytes.On the other hand, the protoncontaining C10NC3-V10 hybrid crystal exhibits anhydrous proton conductivity of 6.5 × 10 −5 S cm −1 at 373 K[126].Emergence of anhydrous proton conductivity at intermediate temperatures is demanded for solid electrolyte application for fuel-cell technology[29][30][31][32][33].Although the proton conductivity of C10NC3-V10 is rather unstable and lower than other POM hybrid materials[34,35], the protoncontaining POM-surfactant crystals would pave the way for an unprecedented class of proton conductors.

Figure 13 .
Figure 13.Crystal structure and conductive property of C16py-W10 hybrid crystals.(a) Crystal packing; (b) Nyquist spectrum (open circles) at 423 K and simulated spectrum (red line) based on an equivalent electronic circuit [127].Reproduced with permission from Ito, T. et al., Crystals; published by MDPI, 2012.

Figure 13 .
Figure 13.Crystal structure and conductive property of C 16 py-W 10 hybrid crystals.(a) Crystal packing; (b) Nyquist spectrum (open circles) at 423 K and simulated spectrum (red line) based on an equivalent electronic circuit [127].Reproduced with permission from Ito, T. et al., Crystals; published by MDPI, 2012.