2806
Langmuir 2006, 22, 2806-2810
Regular Two-Dimensional Molecular Array of Photoluminescent Anderson-type Polyoxometalate Constructed by Langmuir-Blodgett Technique Takeru Ito,† Hisashi Yashiro,‡ and Toshihiro Yamase*,†,§ Chemical Resources Laboratory, Tokyo Institute of Technology, R1-21, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan, Application Laboratory, Rigaku Corporation, 3-9-12 Matsubara-cho, Akishima-shi, Tokyo 196-8666, Japan, and Core Research for EVolutional Science and Technology, Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi 332-0012, Japan ReceiVed NoVember 4, 2005. In Final Form: January 14, 2006 A regular two-dimensional photoluminescent array of Anderson-type polyoxometalates (POMs) was constructed as built-up Langmuir-Blodgett (LB) films. LB films of hexatungstoantimonate (SbW6) and -manganate (MnW6) were successfully fabricated by using dimethyldioctadecylammonium (DODA) as cationic partner, while hexamolybdochromate (CrMo6) was unsuccessful. Specular X-ray reflectivity (SXR) and grazing incidence X-ray diffraction (GIXD) measurements revealed that both SbW6/DODA and MnW6/DODA LB films exhibited well-ordered layers consisting of periodic arrangement of the planar-structured Anderson-type molecules. Surprising periodicity was observed in SbW6/DODA LB film, in which the distance between SbW6 and DODA layers was 4.40 nm, and SbW6 anions would form a two-dimensional square lattice with a length of 1.4 nm. SbW6/DODA LB films exhibited photoluminescence at 77 K, while emission spectra were observed at room temperature for SbW6 solid.
Introduction To fabricate films composed of functional molecules is crucial to both fundamental research and application. For example, thin films of functional dye such as cyanine or phthalocyanine have been industrialized for photonic recordable media.1 Among several kinds of molecules, inorganic functional clusters such as polyoxometalates (POMs) are superior to organic molecules due to their stability against oxidation and variety of physicochemical properties.2-8 Therefore, precise construction of a thin film or two-dimensional array of POMs is desired for realizing POMbased molecular devices. To prepare well-defined two-dimensional POM arrays with smooth surfaces and homogeneous thickness, the LangmuirBlodgett (LB) technique9,10 has been selected as an effective method. Several built-up LB films have been constructed with POM anions11-21 and surfactant-encapsulated POMs.22-26 As for LB films fabricated by using naked POM anions, alternate * To whom correspondence should be addressed: tel +81-45-924-5260; fax +81-45-924-5260; e-mail
[email protected]. † Tokyo Institute of Technology. ‡ Rigaku Corporation. § Japan Science and Technology Agency. (1) Verhoeven, J. A. Th.; Mischke, W. S. Proc. SPIE 2001, 4085, 298. (2) Chem. ReV. 1998, 98, 1 (thematic issue on polyoxometalates). (3) Polyoxometalate Chemistry for Nano-Composite Design; Yamase, T., Pope, M. T., Eds.; Kluwer Academic/Plenum Publishers: New York, 2002. (4) Pope, M. T. Heteropoly and Isopoly Oxometalates; Springer: Berlin, 1983. (5) Polyoxometalate Chemistry: From Topology Via Self-Assembly to Applications; Pope, M. T., Mu¨ller, A., Eds.; Kluwer: Dordrecht, The Netherlands, 2001. (6) Polyoxometalate Molecular Science; Borra´s-Almenar, J. J., Coronado, E., Mu¨ller, A., Pope, M. T., Eds.; Kluwer: Dordrecht, The Netherlands, 2003. (7) Day, V. W.; Klemperer, W. G. Science 1985, 228, 533. (8) Okuhara, T.; Mizuno, N.; Misono, M. AdV. Catal. 1996, 41, 113. (9) Langmuir-Blodgett Films; Roberts, G., Ed.; Plenum: New York, 1990. (10) Ulman, A. An Introduction to Ultrathin Organic Films from LangmuirBlodgett to Self-Assembly; Academic Press: San Diego, CA, 1991. (11) Clemente-Leo´n, M.; Mingotaud, C.; Agricole, B.; Go´mez-Garcı´a, C. J.; Coronado, E.; Delhae`s, P. Angew. Chem., Int. Ed. Engl. 1997, 36, 1114. (12) Clemente-Leo´n, M.; Agricole, B.; Mingotaud, C.; Go´mez-Garcı´a, C. J.; Coronado, E.; Delhaes, P. Langmuir 1997, 13, 2340. (13) Clemente-Leo´n, M.; Coronado, E.; Go´mez-Garcı´a, C. J.; Mingotaud, C.; Ravaine, S.; Romualdo-Torres, G.; Delhae`s, P. Chem.sEur. J. 2005, 11, 3979.
inorganic and organic layers consisting of POMs and surfactants usually had ordered structure in the growth direction (“out-ofplane direction”),11-18 and even in-plane periodicity of the surfactant molecules (ca. 0.4 nm) was sometimes observed.14 Regular arrays of POM molecules were observed directly by transmission electron microscopy (TEM) for thin films with surfactant-encapsulated POMs,22,23 but microscopic characterization has an intrinsic limitation on the observation range. Regular two-dimensional arrangement of POM molecules has never been observed in terms of X-ray measurement as periodic structure with long-range order. A reasonable way to construct a periodic array of POMs within LB films is to aim for a single-crystal-like structure,27-30 which (14) Giannini, C.; Tapfer, L.; Sauvage-Simkin, M.; Garreau, Y.; Jedrecy, N.; Ve´ron, M. B.; Pinchaux, R.; Burghard, M.; Roth, S. Thin Solid Films 1996, 288, 272. (15) Giannini, C.; Tapfer, L.; Burghard, M.; Roth, S. Mater. Sci. Eng. C 1998, 5, 179. (16) Wang, J.; Wang, H. S.; Fu, L. S.; Liu, F. Y.; Zhang, H. J. Thin Solid Films 2002, 414, 256. (17) Wang, J.; Wang, H.; Fu, L.; Liu, F.; Zhang, H. Thin Solid Films 2002 415, 242. (18) Liu, S.; Tang, Z.; Wang, E.; Dong, S. Thin Solid Films 1999, 339, 277. (19) Kim, H. S.; Lee, B.-J.; Huh, H. C.; Park, D. H. Mol. Cryst. Liq. Cryst., Sect. A 2002, 377, 157. (20) Chambers, R. C.; Osburn Atkinson, E. J.; McAdams, D.; Hayden, E. J.; Ankeny Brown, D. J. Chem. Commun. 2003, 2456. (21) Qian, D.-J.; Huang, H.-X.; Huang, W.; Wakayama, T.; Nakamura, C.; Miyake, J. Colloids Surf. A: Physicochem. Eng. Aspects 2004, 248, 85. (22) Kurth, D. G.; Lehmann, P.; Volkmer, D.; Co¨lfen, H.; Koop, M. J.; Mu¨ller, A.; Du Chesne, A. Chem.sEur. J. 2000, 6, 385. (23) Volkmer, D.; Du Chesne, A.; Kurth, D. G.; Schnablegger, H.; Lehmann, P.; Koop, M. J.; Mu¨ller, A. J. Am. Chem. Soc. 2000, 122, 1995. (24) Kurth, D. G.; Lehmann, P.; Volkmer, D.; Mu¨ller, A.; Schwahn, D. J. Chem. Soc., Dalton Trans. 2000, 3989. (25) Bu, W.; Fan, H.; Wu, L.; Hou, X.; Hu, C.; Zhang, G.; Zhang, X. Langmuir 2002, 18, 6398. (26) Sousa, F. L.; Ferreira, A. S.; Sa´ Ferreira, R. A.; Cavaleiro, A. M. V.; Carlos, L. D.; Nogueira, H. I. S.; Trindade, T. J. Alloys Compd. 2004, 374, 371. (27) Ito, T.; Sawada, K.; Yamase, T. Chem. Lett. 2003, 32, 938. (28) Fosse, N.; Brohan, L. J. Solid State Chem. 1999, 145, 655. (29) Janauer, G. G.; Dobley, A. D.; Zavalij, P. Y.; Whittingham, M. S. Chem. Mater. 1997, 9, 647. (30) Nyman, M.; Ingersoll, D.; Singh, S.; Bonhomme, F.; Alam, T. M.; Brinker, C. J.; Rodriguez, M. A. Chem. Mater. 2005, 17, 2885.
10.1021/la052972w CCC: $33.50 © 2006 American Chemical Society Published on Web 02/16/2006
Photoluminescent Anderson-type Polyoxometalate
requires appropriate balance between size and charge of POM molecules.12,13,27 Anderson-type POMs (XM6O24n-) are promising candidates due to their planar shape and the tendency to form layered structures in the single crystals.31-36 In addition, they exhibit characteristic photoluminescence by excitation of O f W ligand-to-metal charge-transfer (LMCT) band,37-39 and they have functional potential as a component of luminescent film devices. Here, two-dimensional arrays of Anderson-type POMs, hexatungstoantimonate (SbW6O24,7- denoted as SbW6) and -manganate (MnW6O24,8- denoted as MnW6), were successfully fabricated as LB films by using dimethyldioctadecylammonium (denoted as DODA) as cationic partner. The films showed the well-defined layer consisting of periodic arrangement of the planar-structured Anderson-type molecules. The photoluminescence of SbW6/DODA LB films was also investigated. Experimental Section Materials. Anderson-type polyoxometalates (Anderson POMs), K5.5H1.5[SbW6O24]‚6H2O (denoted as K‚SbW6),36,38 K6Na2[MnW6O24]‚ 12H2O (K‚Na‚MnW6),33,39 and Na3H6[CrMo6O24]‚8H2O (Na‚ CrMo6),32,38,40 were synthesized according to the procedures previously reported. Dimethyldioctadecylammonium bromide (DODA‚Br, Kanto) and chloroform (Kanto) were used without further purification. Subphase solutions were prepared with ultrapure water (∼18 MΩ‚cm) delivered from Toraypure LV-08 (Toray) system. LB Film Fabrication. LB film fabrication was carried out by spreading chloroform solution of DODA‚Br (1 mmol dm-3) onto Anderson POM aqueous solutions (1 µmol dm-3) on a KSV3000 apparatus (KSV Instruments). The compression rate and subphase temperature were 15 cm2 min-1 and 20 ( 1 °C, respectively. After the solvent was evaporated, the floating layer was compressed up to 30 mN m-1 and then stabilized for 50-90 min. In the case where a stable Langmuir monolayer of DODA‚Br was formed on the subphase, the monolayer was subsequently deposited onto ZnSe (for infrared spectroscopy) or quartz substrates (for other measurements) by the vertical dipping method with a rate of 10 mm min-1 under N2-purged atmosphere. The number of layers of LB film prepared here is equal to the number of dipping or lifting processes, on each of which a floating Langmuir monolayer was transferred onto the substrate. Spectroscopy. Ultraviolet-visible (UV-vis) and infrared (IR) spectra were measured on a Jasco V-570 spectrometer and a Jasco FT/IR-40 spectrometer, respectively. X-ray photoelectron spectroscopy (XPS) measurements were carried out on a Shimadzu ESCA3200 spectrometer with Mg KR X-ray line (1250 eV). Photoluminescence spectra were recorded at room temperature (∼295 K) or 77 K on a Hitachi F-4500 fluorescence spectrometer equipped with a Xe lamp. Specular X-ray Reflectivity and Grazing Incidence X-ray Diffraction. Specular X-ray reflectivity (SXR) and grazing incidence X-ray diffraction (GIXD) measurements were done by using a RINTUltima III diffractometer (at Rigaku Corporation) with a combination of an X-ray source (Cu KR 0.154 nm, 40 kV/30 mA) and a parabolic multilayered mirror. Rigaku GXRR and Phillips WinGixa analysis software was used for curve-fitting of SXR profiles. Experimental (31) Evans, H. T., Jr. J. Am. Chem. Soc. 1948, 70, 1291. (32) Perloff, A. Inorg. Chem. 1970, 9, 2228. (33) Sergienko, V. S.; Molchanov, V. N.; Porai-Koshits, M. A.; Torchenkova, E. A. SoV. J. Coord. Chem.(Engl. Transl.) 1979, 5, 740. (34) Lee, U.; Kobayashi, A.; Sasaki, Y. Acta Crystallogr., Sect. C 1983, 39, 817. (35) Lee, U.; Ichida, H.; Kobayashi, A.; Sasaki, Y. Acta Crystallogr., Sect. C 1984, 40, 5. (36) Naruke, H.; Yamase, T. Acta Crystallogr., Sect. C 1992, 48, 597. (37) Yamase, T. Chem. ReV. 1998, 98, 307. (38) Yamase, T.; Sugeta, M. J. Chem. Soc., Dalton Trans. 1993, 759. (39) Yamase, T.; Kobayashi, T.; Kettle, S. F. A. J. Electrochem. Soc. 1996, 143, 1678. (40) Nomiya, K.; Takahashi, T.; Shirai, T.; Miwa, M. Polyhedron 1987, 6, 213.
Langmuir, Vol. 22, No. 6, 2006 2807
Figure 1. Surface pressure-area (π-A) isotherms of DODA‚Br on (a) K‚SbW6, (b) K‚Na‚MnW6, and (c) Na‚CrMo6 aqueous solutions. The π-A isotherm of DODA‚Br on pure water is presented as a broken curve. The hypothetical areas of DODA were estimated by extrapolating the isotherms in the condensed region to zero pressure (dotted lines). Polyhedral representation of Anderson-type POM structure is also demonstrated. data over 2θ ) 5° of SXR profiles were not used for the fitting due to insufficient diffraction intensity. Fitting parameters calculated by the Rigaku GXRR are listed in Table 1.
Results and Discussion Surface Pressure-Area Isotherms. Figure 1 shows surface pressure-area (π-A) isotherms of DODA‚Br on Anderson POM solutions. Isotherms on SbW6 and MnW6 aqueous solutions (Figure 1a,b) exhibited a decrease in the molecular area and an increase in the surface pressure compared with the case of pure water (shown by a broken curve in Figure 1), where the dense molecular layers of DODA‚Br were formed at the liquid-air interface. The anion charge of Anderson-type polyoxotungstates is considered to decrease the repulsion between DODA.11-13,16-18 The hypothetical molecular area of DODA on the SbW6 solution was estimated to be 0.58 nm2 by extrapolating the isotherm in the condensed region to zero pressure, and 0.61 nm2 in the case of the MnW6 solution (dotted lines in Figure 1). These values were close to the cross-section of hydrophilic head of DODA (0.57 nm2) estimated from the crystal structure,41 suggesting close packing of DODA on SbW6 and MnW6 solutions. After the surface pressure was kept at an appropriate value (30-35 mN m-1), stable Langmuir monolayers were formed. In the case of Anderson-type polyoxomolybdate, CrMo6, the isotherm of DODA‚Br (Figure 1c) was more expanded than in the case of Anderson-type polyoxotungstates, and no stable Langmuir monolayer was formed at the liquid-air interface. LB Film Fabrication. SbW6/DODA and MnW6/DODA LB films were successfully deposited by the vertical method. The transfer ratio was almost unity in both dipping and lifting processes, indicating the formation of Y-type LB films. As shown in Figure 2a, UV-vis spectrum of SbW6/DODA LB film deposited on quartz (13 layers) exhibited intense O f W ligandto-metal-charge-transfer (LMCT) bands in the region below 300 nm as well as a broad shoulder around 230 nm. This spectroscopic feature was the same as that of K‚SbW6 aqueous solution (Figure 2b), demonstrating that SbW6 anions were successfully organized onto the quartz substrate. The deposition of MnW6 anion was also confirmed by UV-vis spectra (Figure S1a, Supporting Information). IR spectra of SbW6/DODA LB film and K‚SbW6 solid are shown in Figure 3. The IR spectrum of K‚SbW6 had characteristic peaks around 940, 880, and 689 cm-1 (Figure 3b). These peaks (41) Okuyama, K.; Soboi, Y.; Iijima, N.; Hirabayashi, K.; Kunitake, T.; Kajiyama, T. Bull. Chem. Soc. Jpn. 1988, 61, 1485.
2808 Langmuir, Vol. 22, No. 6, 2006
Ito et al. Table 1. Fitting Parameters for the SXR Profile of SbW6/ DODA LB Film (25 Layers)a stack repeat no. of stack
layer consisting of stack
density thickness roughness (g cm-1) (nm) (nm)
3 3
1 1
DODA (monolayer) SbW6 (monolayer)
0.79 3.94
1.81 0.42
0.63 0.57
2 2
12 12
DODA (bilayer) SbW6 (monolayer)
0.81 4.44
4.10 0.36
0.59 1.12
quartz (substrate)
2.20
1
0.50
Experimental data in the range of 2θ ) 0.26-5° were used. Fitting procedure was performed with Rigaku GXRR software. a
Figure 2. UV-vis spectra of (a) SbW6/DODA LB film (13 layers) and (b) K‚SbW6 aqueous solution (16 µmol dm-3).
Figure 5. SXR profiles of MnW6/DODA LB film (29 layers). Indices for Bragg reflections are added within the figure.
Figure 3. IR spectra of (a) SbW6/DODA LB film (51 layers) and (b) K‚SbW6 solid (KBr pellet). The peak indicated by an asterisk was derived from DODA cation.
Figure 6. GIXD profiles of (a) SbW6/DODA LB film (25 layers) and (b) MnW6/DODA LB film (29 layers).
Figure 4. SXR profiles of SbW6/DODA LB film (25 layers): (a) experimental and (b) fitted profile. Experimental data in the range of 2θ ) 0.26-5° were used for the fitting. Indices for Bragg reflections are in the figure.
were also observed in the spectrum of SbW6/DODA LB film (Figure 3a), which also indicates the successful deposition of SbW6 anions onto the solid substrate. In SbW6/DODA LB film, the position and relative intensity of the peaks slightly changed, and a new peak emerged around 815 cm-1. Successful deposition was also observed for MnW6/DODA LB film (Figure S1b, Supporting Information). Characterization of LB Film Structure. X-ray diffraction measurements revealed high periodicity of built-up LB films containing Anderson-type POMs. SXR profiles of both SbW6/ DODA (Figure 4a) and MnW6/DODA LB (Figure 5) films exhibited well-defined Bragg peaks as well as Kiessig fringes as reported in the literature.14,15,22,24 These Bragg peaks can be assigned from 001 to 004 reflections, demonstrating highly ordered stacking of POM and DODA layers along the growth direction (“out-of-plane direction”). The periodicity of layered
structure was 4.40 and 4.28 nm for SbW6/DODA and MnW6/ DODA LB films, respectively. Since these LB films were Y-type, DODA cations are considered to form bilayer arrangement between the inorganic POM monolayers, as verified for other POM LB films11-16 and LB-film-like crystals.27,28 This structure model composed of DODA bilayer and POM monolayer was used for the SXR profile fitting, in which the layer density, layer thickness, and surface roughness were optimized on the basis of X-ray reflection theory.42,43 A successful fit for the SXR profiles was obtained for SbW6/DODA LB films (Figure 4b) with internal thickness of 4.10 and 0.36 nm for the DODA bilayer and SbW6 monolayer, respectively (Table 1). Assuming a bilayer arrangement without interdigitation, the alkyl chains of DODA (ca. 2.3 nm in length)44 would tilt by 27° () cos-1 [4.10/(2.3 × 2)]) from the direction normal to the substrate. When the thickness of SbW6 and MnW6 (ca. 0.5 nm)45 is taken into account, the plane composed of six W atoms was probably parallel to the quartz (42) Parratt, L. G. Phys. ReV. 1954, 95, 359. (43) Croce, P.; Ne´vot, L. ReV. Phys. Appl. 1976, 11, 113. (44) Average length of bended and nonbended octadecyl chains of DODA estimated from (DODA)2Mo6O19 crystal structure27 by using Diamond 3 (Crystal Impact).
Photoluminescent Anderson-type Polyoxometalate
Langmuir, Vol. 22, No. 6, 2006 2809
Figure 7. XPS spectra of SbW6/DODA LB film (21 layers): (a) W 4d, (b) W 4f, (c) K 2s, and (d) Br 3d regions.
substrate. The close packing of DODA monolayer on the SbW6 and MnW6 solutions discussed above (Figure 1) would lead to the highly ordered layered structures. In the SXR profile of MnW6/ DODA LB film (Figure 5), the intensity of the 004 reflection was weaker and the depth of Kiessig fringes smaller than in SbW6/DODA LB film (Figure 4a), and good fitting was not obtained. Since the deep Kiessig fringes reflect the difference in the electronic density of POM and DODA layers, SbW6/ DODA LB film had a more well-defined structure with distinct layer interface than MnW6/DODA LB film. As can be anticipated from such well-defined layered structures, regular in-plane periodicity was observed in both SbW6/DODA and MnW6/DODA LB films (Figure 6). In Figure 6a, GIXD profile of SbW6/DODA LB film exhibited a clear peak corresponding to periodicity of 1.4 nm inside the LB film plane, which was much larger than the conceivable periodicity of surfactant aliphatic chains (∼0.4 nm).14 Therefore, the diffraction peak in Figure 6a was derived from regular arrangement of SbW6 anions in the inorganic layers. This is the first observation of in-plane periodicity of POM molecules in built-up LB films, indicating the formation of regular two-dimensional POM arrays with long-range order. The peak position did not shift even when the angle between the direction of incident X-ray and the LB film substrate was changed by 90° (Figure S2, Supporting Information),46 suggesting that the in-plane arrangement of SbW6 anions was a square lattice with C4 axis. In the case of MnW6/ DODA LB film (Figure 6b), the peak intensity corresponding to the periodicity of 1.1 nm was lower than in the case of SbW6/ DODA LB film. This demonstrates the less-ordered structure of MnW6/DODA LB film, consistent with the results of SXR measurements (Figure 5). As for SbW6/DODA LB film, XPS analysis was performed in order to detect the elements contained in the LB film. XPS spectra exhibited peaks assignable to C 1s (binding energy (BE) ) 285.1 eV), N 1s (BE ) 402.7 eV), and O 1s (BE ) 533.3 eV) (Figure S3, Supporting Information). Peaks assignable to W 4d3/2 (BE ) 260.4 eV) and W 4d5/2 (BE ) 247.7 eV) were also observed (45) Estimated value from space-filling model of K5.5H1.5[SbW6O24]‚6H2O crystal structure36 by using Diamond 3. (46) Kojio, K.; Takahara, A.; Omote, K.; Kajiyama, T. Langmuir 2000, 16, 3932. (47) Instruction manual for Shimadzu ESCA-3200.
Figure 8. Emission spectra of (a) SbW6/DODA LB film (11 layers) measured at 77 K and (b) K‚SbW6 solid measured at room temperature. Excitation wavelengths were 250 and 288 nm for LB film and solid, respectively. The intensity of the spectra is normalized by the maximum intensity.
(Figure 7a), while peaks attributable to Sb 4d (BE ) 32.0 and 33.1 eV)47 were hardly detected due to the overlap of the peaks derived from W 4f5/2 (BE ) 37.4 eV) and W 4f7/2 (BE ) 35.5 eV) (Figure 7b). A shoulder around 530.7 eV observed in the O 1s peak (Figure S3) may correspond to Sb 3d (BE ) 528.2 and 537.5 eV).47 There is no observation of the signal of K 2s (BE ) 377.8 eV)46 or Br 3d (BE ) 68.6 eV)46 in the corresponding region at 365-390 eV (for K 2s, Figure 7c) or 55-80 eV (for Br 3d, Figure 7d). Even if the peak around 73 eV in Figure 7d was assigned to Br 3d, the atomic ratio of Br to W was 0.06. Therefore, it can be concluded that counterions (K+ for SbW6 and Br- for DODA) hardly remained in the SbW6/DODA LB film and that the cation exchange of K‚SbW6 completely proceeded at the air-liquid interface when the stable Langmuir monolayer of DODA formed. Photoluminescence. Photoluminescence property was investigated for SbW6 samples. As shown in Figure 8, broad-band emissions induced by 3T1u f 1A1g transition derived from the excitation into O f W LMCT band37,38 were observed at 505 and 490 nm for SbW6/DODA LB film and K‚SbW6 solid, respectively. The position of the peak top slightly shifted to higher wavelength region for LB film, which may be affected by the two-dimensional organization of SbW6 anions. Since the shapes of the emission spectra were almost the same, the
2810 Langmuir, Vol. 22, No. 6, 2006
Ito et al.
Figure 9. Schematic representations of SbW6/DODA LB film: (a) out-of-plane structure, (b) in-plane structure, and (c) interface between SbW6 and DODA layers.
conservation of SbW6 structure in the LB film was confirmed. However, the temperature for measurement was quite different: 77 K for SbW6 LB film and room temperature (∼295 K) for SbW6 solid. Therefore, the drastic reduction in the emission efficiency for the SbW6 LB film seems to reflect nonradiative deactivation of the O f W LMCT band excitation energy through the vibration states of the high-frequency C-H oscillators of DODA. Model of SbW6/DODA LB Film. Here, the structure of the highly ordered SbW6/DODA LB film will be discussed as a representative of Anderson-type POM LB films. As revealed from the SXR profile in Figure 4, the distance between SbW6 and DODA layers was 4.40 nm in the out-of-plane structure (Figure 6a). Since XPS measurements demonstrated that counterions such as K+ or Br- hardly remained in SbW6/DODA LB film, each SbW6 layer is considered to be sandwiched by DODA layers with a SbW6:DODA ratio of 1:7 in order to balance electronic charges. The DODA cations showed the bilayer arrangement in the organic layers, and the inorganic layers consist of SbW6 monolayers. As shown in Figure 9b, the in-plane structure of the SbW6 anion layer is proposed to form a two-dimensional square lattice of 1.4 nm in length (Z ) 1), consistent with experimental results (Figures 6a and S2). This presumed structure is different from the packing of SbW6 anions in the solid state.36 In the inorganic layer of SbW6/DODA LB film, the area occupied by a SbW6 anion can be calculated to be 1.96 nm2 () 1.4 nm × 1.4 nm, Figure 9b), and each SbW6 anion would be sandwiched by seven DODA cations (Figure 9a,c). The cross section of hydrophilic head for each DODA has been reported as 0.57 nm2.41 On the basis of these values, the matching of cross section at the interface between SbW6 and DODA layers will be investigated: for one side of the interface, seven DODA associated two SbW6 due to the charge compensation. The cross section of the DODA layer, estimated to be 3.99 nm2 () 0.57 nm2 × 7), is almost equal to that of the SbW6 layer, calculated to be 3.92 nm2 () 1.96 nm2 × 2). Such a good match implies the high periodicity in both
out-of-plane and in-plane structures, and the schematic representation depicted in Figure 9 can be reasonable. SbW6 anion has an exquisite balance between size and charge in order to form the well-arranged layer with DODA. MnW6/DODA LB film was less periodic, probably because the higher negative charge of MnW6 (8-) was inappropriate for good matching of cross sections of MnW6 and DODA at the layer interface.
Conclusions Built-up LB films containing photoluminescent Andersontype POMs (SbW6 and MnW6) were successfully fabricated to exhibit high periodicity in both out-of-plane and in-plane structures, revealed by SXR and GIXD measurements. SbW6 anions within the LB film are considered to arrange in the twodimensional square lattice, which has potential for POM-based two-dimensional molecular devices. The planar structure of Anderson-type POMs seems effective for the formation of highly ordered LB films. Selection of POM molecules with appropriate size, structure, and charge will lead to realizing a variety of regular two-dimensional array with functional properties. Acknowledgment. This work was supported by the CREST program from JST and by a Grant-in-Aid for Scientific Research from Japan Society for the Promotion of Science (JSPS). We thank Professor Michikazu Hara and Mr. Kazuya Hasegawa (Tokyo Institute of Technology) for the measurements of XPS spectra. Professor Kei Inumaru (Hiroshima University) is acknowledged for kind permission and help on using the WinGixa analysis software. Supporting Information Available: Figure S1, UV-vis and IR spectra of MnW6/DODA LB film and K‚Na‚MnW6 aqueous solution; Figure S2, GIXD profiles of SbW6/DODA LB film measured when the angle between the direction of incident X-ray and LB film substrate is changed by 90°; and Figure S3, XPS spectra of SbW6/DODA LB film for the regions C 1s, N 1s, and O 1s. This material is available free of charge via the Internet at http://pubs.acs.org. LA052972W
