Ordered Blue Luminescent Ultrathin Films by the Effective

Aug 8, 2011 - Ordered Blue Luminescent Ultrathin Films by the Effective Coassembly of .... Artur J. M. Valente , Beverly Stewart , Licinia L. G. Justi...
0 downloads 0 Views 4MB Size
Download PDF
ARTICLE pubs.acs.org/Langmuir

Ordered Blue Luminescent Ultrathin Films by the Effective Coassembly of Tris(8-hydroxyquinolate-5-sulfonate)aluminum and Polyanions with Layered Double Hydroxides Shuangde Li, Jun Lu,* Hongkai Ma, Jing Xu, Dongpeng Yan, Min Wei, David G. Evans, and Xue Duan State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, PR China

bS Supporting Information ABSTRACT: This article reports a novel method to assemble a small anion with exfoliated Mg Al-layered double hydroxide (LDH) nanosheets into ordered ultrathin films (UTFs) by employing the layer-by-layer assembly technique. The premixing solution of tris(8-hydroxyquinolate-5-sulfonate)aluminum(III) (AQS3 ) with three kinds of polyanions—poly(acrylic acid), ((C3H4O2)n, PAA), poly(styrene 4-sulfonate) ([CH2CH(C6H4)SO3]m, PSS), and poly[5-methoxy-2-(3-sulfopropoxy)-1,4-phenylene vinylene] (C12H13O5S)n, PPV)—has been used as building blocks to assemble alternatively with LDH nanosheets. The UV vis absorption and fluorescence spectroscopy of (AQS-polyanion/LDH)n UTFs presents stepwise growth upon increasing deposited cycles in comparison with the (AQS/LDH)n film under the same experimental process. (AQS-PPV/LDH)n UTF displays complex fluorescence originating from AQS and PPV. The (AQS/LDH)n and (AQS-polyanion/LDH)n UTFs exhibit higher blue-polarized photoemission character with a luminescence anisotropy (r) of ca. 0.12 0.20 and a longer fluorescence lifetime than that of the Na3AQS film with r = 0.04. X-ray diffraction, scanning electron microscopy, and atomic force microscopy demonstrated that the UTFs were orderly periodically layered structures with a thickness of ca. 3.0 nm per bilayer. Therefore, this work gives a feasible method for immobilizing small anions into the gallery of LDHs.

’ INTRODUCTION Metal 8-hydroxyquinolate chelates (Mqn), which have excellent luminescence and electron-transport performance, have been widely studied for their application in organic light-emitting devices (OLEDs).1 Tris(8-hydroxyquinolate) aluminum (Alq3) and its derivatives, which feature a metal-to-ligand chargetransfer (MLCT) transition, are an important class among these materials, and they have been extensively investigated in green/ blue fluorescent2 and white light emission devices.3 Alq3 molecules exist as two geometrical isomers: meridianal and facial (mer and fac). A systematic study indicated that mer-Alq3 is the stable isomer with green luminescence2a whereas fac-Alq3 has a larger energy gap with blue fluorescence.2c As a new type of blue fluorescent material, fac-Alq3 has invigorated researchers’ interest in preparation2b,c and will advance the development of nextgeneration Alq3-based multicolor OLEDs. Recently, many studies have been devoted to the fabrication of nanorods,4a nanowires,4b d and liquid-crystalline films5 based on Alq3, which are considered to be good candidates for nano-optoelectronic devices. The layer-by-layer (LbL) assembly technique has been well established as a simple and versatile method of constructing ultrathin films (UTFs) with controlled thickness through the sequential adsorption of polycations and polyanions.6 Recent reviews highlight the fabrication of various multicharged dye ions r 2011 American Chemical Society

and polyion films7 and the postdiffusion of small organic molecules in polyion multilayer films.8 Zhang developed a method to fabricate UTFs by mixing a small molecule and polyelectrolyte to form a polymer complex by electrostatic interaction and then deposited it with a counterpolyelectrolyte.9 However, it is quite hard to introduce less-charged molecules into an LbL assembled film because they can be extracted more easily than can polyions. Therefore, it is very desirable to achieve a new approach to fabricating stable UTFs containing small ions, which could combine the merits of the LbL technique and the intrinsic advantages of the organic small ion. Layered double hydroxides (LDHs) are one family of anionic clays consisting of positively charged brucite-like layers and exchangeable interlayer anions, the general formula of which can be described as [MII1 xMIIIx(OH)2]x+An x/n 3 yH2O. MII and MIII are divalent and trivalent metals ions, respectively; An represents the guest anions that are present between hydroxide layers.10 They have attracted considerable interest for their potential applications as catalysts,11a pharmaceutically and biologically active materials,11b and electrical and optical functional Received: June 8, 2011 Revised: July 18, 2011 Published: August 08, 2011 11501

dx.doi.org/10.1021/la202139f | Langmuir 2011, 27, 11501–11507

Langmuir Scheme 1. Schematic Representation of the Process for the Fabrication of (AQS Polyanion/LDH)na

a

Structure of AQS3 and polyanions (top). (a) Pretreated quartz substrate, (b) AQS polyanion, and (c) Mg Al LDH nanosheets.

materials.11c Recently, LDHs were widely focused on the incorporation of chromophores in their interlayers, for instance, aromatic azo dyes rhodamine B and fluorescein,12 which served as new important luminous materials. The chromophore/LDHs system was identified to tune the photophysical and photochemical properties and improve the thermal stability and photostability of the guests, which were caused by the host guest and guest guest interactions within the restrained LDHs interlayer environment. Now, much attention has been paid to the delamination of LDHs to obtain positively charged nanosheets.13a,b These can serve as 2D building blocks to be alternately assembled with luminescent materials by the LbL technique in order to fabricate a new type of functional hybrid luminous UTFs, which could combine the properties of organic materials with those of inorganic parts.14 Polymer could be simply used in the fabrication of (LDH/ APPP)n (APPP, sulfonated poly(p-phenylene)) films for the prominent polymer LDH electrostatic interaction with improved luminescence properties by avoiding the π π stacking of the APPP backbones.14a Yan employed poly(vinylsulfinate) (PVS) as an intermediary to assemble a photoactive divalent cation bis(N-methylacridinium) (BNMA) film, denoted as the (BNMAPVS)/LDH film, which could enhance the adhesion of small molecules to the LDHs.15 However, to the best of our knowledge, very little attention has been focused on the assembly of LDH nanosheets with small anions.14b This has inspired us to take on the challenge to fabricate (small complex anion polyanion)/ LDH UTFs, which is expected to extend the available range of LDH-based organic inorganic hybrid UTF materials. During the LbL process, the building block of a small complex anion polyanion is anticipated to enhance the cohesion of small complex anions to form UTFs. It is advantageous to extend the LbL technique to prepare UTFs consisting of various Mqn. The luminous properties of such ordered UTFs can be modulated by changing the polyanion species. In our previous work, tris(8-hydroxyquinolate-5-sulfonate) aluminum (AQS3 ) and bis(8-hydroxyquinolate-5-sulfonate)zinc (ZQS2 ) have been cointercalated into LDH, together with dodecyl sulfonate (DDS ).16 A series of resulting products show systematically tuned luminescence for metal complexes within the interlayer of LDH. It transferred from green to blue fluorescence

ARTICLE

after AQS3 intercalate into LDH, which was proven to be the conversion of mer-AQS to fac-AQS. Furthermore, there is no report on Mqn/LDH UTFs. Herein, we demonstrate the fabrication of ordered, stacked UTFs onto pretreated quartz substrates by the LbL method, which were LDH nanosheets that were alternately assembled with complex anion tris(8-hydroxyquinolate-5-sulfonate) aluminum (AQS3 ) combined with polyanions poly(acrylic acid) ((C3H4O2)n, PAA), poly(styrene 4-sulfonate) ([CH2CH(C6H4)SO3]m, PSS), and poly[5-methoxy-2-(3-sulfopropoxy)1,4-phenylene vinylene] (C12H13O5S)n, PPV) denoted as (AQSPAA/LDH)n, (AQS-PSS/LDH)n, and (AQS-PPV/LDH)n, where n refers to the numbers of bilayers. The related molecular structures and the fabrication scheme of the UTFs are illuminated in Scheme 1. The obtained (AQS-polyanion/LDH)n UTFs show long-range-ordered structure and well-defined blue fluorescence. Moreover, they exhibit a longer fluorescence lifetime and higher polarized photoluminescence characteristics than does the pure AQS3 sample. Therefore, this work provides a feasible route for the confinement of small anion molecules into LDH interlayers with potential luminescence applications based on the organic metal complex and inorganic LDH components.

’ EXPERIMENTAL SECTION Reagents and Materials. 8-Hydroxyquinoline-5-sulfonic acid monohydrate (98%) and poly(acrylic acid) (PAA, Mw = 240 000) were purchased from Alfa Aesar Chemical. Co. Ltd., and poly(styrene sulfonic acid) (PSS, Mw = 70 000) was purchased from J&K Chemical. Co. Ltd. NaOH (AR), Mg(NO3)2 3 6H2O (AR), Al(NO3)3 3 9H2O (AR), and urea were purchased from Beijing Chemical Co. Ltd. AlCl3 (99%), poly(dimethyldiallylammonium chloride) (PDDA, Mw = 100 000 200 000), and poly[5-methoxy-2-(3-sulfopropoxy)-1,4-phenylenevinylene] potassium salt (PPV) were purchased from Sigma-Aldrich Chemical. Co. Ltd. All other chemicals were of analytical grade and used as received without further purification. Deionized and de-CO2 water were used throughout the experimental processes. Synthesis of Na3AQS and MgAl-LDH Nanosheets. Tris(8-hydroxyquinolate-5-sulfonate)aluminum was synthesized according to the method reported previously.5 8-Hydroxyquinoline-5-sulfonic acid hydrate (6 mmol, 1.35 g) was dissolved in 150 mL of a NaOH (0.72 g, 18 mmol) aqueous solution with stirring. AlCl3 powder (0.27 g, 2 mmol) was then added to the yellow solution. The pH was adjusted to 8 with 1 M NaOH solution, and the light-yellow solution was stirred at room temperature for 12 h. After the evaporation of the water, the solid was washed with hot ethanol (100 mL) and precipitated from a methanol/diethylether mixture. The process of synthesis and exfoliation of MgAl-LDH (Mg/ Al = 2:1) corresponds to our group’s previous work.14d Mg Al LDH (0.1 g) was shaken in 100 cm3 of a formamide solution for 24 h to produce a colloidal suspension of exfoliated Mg Al LDH nanosheets. Fabrication of (AQS/LDH)n and (AQS Polyanion/LDH)n UTFs. The quartz substrate was cleaned by immersion in a fresh piranha solution (3:1 v/v H2SO4 (30%)/H2O2) for 30 min and then rinsed with deionized water and dried in a flow of nitrogen gas. The cleaned substrates were dipped into a cationic PDDA solution (10 g 3 dm 3) for 30 min and then thoroughly rinsed with distilled water and dried in a flow of nitrogen gas. After deposition, the substrates have a positively charged surface. The fabrication of the (AQS/LDH)n multilayer films was carried out according to the following steps: the pretreated substrates were dipped alternately into the formamide colloidal suspension containing exfoliated MgAl LDH nanosheets (1 g 3 dm 3) for 10 min and an AQS3 aqueous solution (5 g 3 dm 1, pH 8.5 adjusted with a 0.1 M NaOH aqueous solution) for 15 min. After each assembly step, the substrates were thoroughly rinsed with distilled water without drying with nitrogen 11502

dx.doi.org/10.1021/la202139f |Langmuir 2011, 27, 11501–11507

Langmuir

ARTICLE

Figure 1. Characterization of (AQS-PPV/LDH)n (n = 2 26) UTFs: (a) UV/vis absorption spectra. The inset shows the absorbance at 202, 258, and 298 nm vs n. (b) Fluorescence spectra with 360 nm excitation. gas unless otherwise specified. (AQS/LDH)n UTFs were also fabricated and dried at each assembly for comparison. The fabrication methods of the (AQS-PAA/LDH)n, (AQS-PSS/LDH)n, and (AQS-PPV/LDH)n multilayer films were similar, except that the AQS3 (10 g 3 dm 1, 15 mL) solution was completely dissolved into a PAA (5 g 3 dm 3, 15 mL) solution, a PSS (2 g 3 dm 3, 15 mL) solution, and a PPV (0.5 g 3 dm 3, 15 mL) solution, respectively. The pH of the resulting AQS polyanion solutions was adjusted to 8.5 with a 0.1 M NaOH aqueous solution. The electric charge ratios of AQS/polyanion are 0.43:1, 2.76:1, and 18.75:1, respectively. Sample Characterization. The UV vis absorption spectra were obtained with a Shimadzu UV-2501PC spectrometer with a slit width of 1.0 nm. The fluorescence spectra were obtained with a Hitachi F-7000 FL spectrophotometer with the slit width of 5 nm, excitation at 360 nm, and a PMT voltage of 400 V. Luminescence lifetime measurements were recorded with an Edinburgh Instruments’ FL 900 fluorimeter with excitation at 360 nm and emission at 460 nm. The percentage contribution of each lifetime component to the total decay was calculated with Edinburgh F900 Instruments software. Steady-state polarized photoluminescence measurements of (AQS-polyanion/LDH) UTFs were recorded with an Edinburgh Instruments’ FLS 920 fluorospectrophotometer. A Zeiss Supra 55 scanning electron microscope (applied acceleration voltage of 10 kV) and a NanoScope IIIa atomic force microscope (Veeco Instruments Co. Ltd.) were used to investigate the surface morphology of the ultrathin films. X-ray diffraction patterns (XRD) of the films were recorded using a Rigaku 2500VB2+PC diffractometer under the following conditions: 40 kV, 50 mA, and Cu KR radiation with step scanning in steps of 0.04 (2θ) in the range from 2 to 10 using a count time of 10 s/step. The XRD of all of the films is measured under the same setting parameters.

’ RESULTS AND DISCUSSION Assembly of the (AQS/LDH)n UTFs. The (AQS/LDH)n UTFs were fabricated and the subsequent growth of the UTFs were monitored by UV vis absorption spectra immediately after two deposition cycles of AQS3 without the drying process after each assembly cycle. The nonlinear relationship observed between absorbance at 196 and 257 nm (the electron transition due to phenyl rings in AQS17a) and n indicates that the deposition process is irregular and nonreproducible from layer to layer (not shown here). The absorbance reaches a balance and then decreases when n reaches 10. The LDH nanosheet assembly solution gradually exhibits the luminescence of the AQS3 ions under 365 nm UV light when this assembly process continues, revealing the desorption of the AQS3 complex anion from the assembled film. However, when the substrates were additionally

dried in a flow of nitrogen gas for 2 min after each assembly step, the UV absorbance shows a linear relationship with n (Figure S1A) and the complex ions were not released into the LDH solution. The intensity of the maximum photoemission peaks at 472 nm also displays a monotonic increase with n (Figure S1B) indicating a stepwise, regular film-growth procedure. This can be further visualized by the gradual increase in the color intensity with the uniform bright blue light of the (AQS/LDH)n UTFs with increasing n (Figure S1B, inset) under UV irradiation. It shows the 27 nm blue shift, compared to the maximum emission at 495 nm for both the Na3AQS aqueous solution (5  10 5 mol 3 L 1) and powder with cyan luminescence in Figure S2,16a,17b indicating that there were host guest interactions between the assembled AQS3 ion and LDH nanosheets, which modified the photoemission of AQS3 . In our previous study of the AQS3 and dodecyl sulfonate cointercalating LDH system, the cyan-blue luminescence transition was also found.16a Assembly of the (AQS Polyanion/LDH)n UTFs. A traditional viewpoint is that a strong repulsion interaction between a small ion and an identically charged polyelectrolyte might be unfavorable with respect to retention in the LbL assembly after the adsorption of the next layer. On the contrary, it is easy to assemble AQS3 with the LDH nanosheet after premixing it with a polyanion, without any drying process for each assembly step. The UV absorption spectra of (AQS-polyanion/LDH)n UTFs with various bilayer numbers (n) are shown in Figures 1 and S1. The similar stepwise and regular increase in UV absorption can also be obtained in the (AQS-PAA/LDH)n, (AQS-PSS/LDH)n (Figure S1C,,E), and (AQS-PPV/LDH)n (Figure 1a) UTFs by the LbL deposition procedure. The UV absorption peaks at 202, 298 (absorption of phenylene ring14c), and 258 nm in the (AQSPPV/LDH)n film indicate that PPV and AQS3 are regularly coassembled with the LDH nanosheet (Figure 1a). A similar phenomenon occurred for the absorption of (AQS-PSS/LDH)n UTFs at 196, 225 (characteristic peaks of PSS13a), and 257 nm (Figure S1E). Figures 1b and S1D,F show the photoemission spectra of (AQS-polyanion/LDH)n UTFs with increasing n. Their luminescence peaks located at 479 and 477 nm for (AQS-PAA/LDH)n and the (AQS-PSS/LDH)n UTFs display consistent increases with n (Figure S1D,,F). For (AQS-PPV/ LDH)n, there was a broad emission band (410 575 nm) overlapped by two bands at 460 (origined from AQS3 ) and 550 nm (one phonon S1 S0 transition of PPV17c), which also increase with n (Figure 1b). The photoemission spectra of (AQS/LDH) and (AQS polyanion/LDH) UTFs with n = 18 are shown in Figure 2. They all show uniform blue luminescence, which is 11503

dx.doi.org/10.1021/la202139f |Langmuir 2011, 27, 11501–11507

Langmuir

Figure 2. Photoemission spectra of (AQS/LDH)18 and (AQS-polyanion/LDH)18 UTFs with excitation at 360 nm.

Figure 3. Morphology of (AQS-PPV/LDH)24 UTF for (a) a top-view SEM image, (b) a side-view SEM image, and (c, d) tapping-mode AFM topographical images.

remarkably different from that of the pure AQS3 solution and powder with 495 nm emission (Figure S2). The loading and emission intensity of the complex can be simply controlled by adjusting the thickness of the assembled film. The emission of the AQS polyanion solutions show a blue shift that is located at ca. 484 nm compared with that of the pristine AQS molecule (Figure S3). The blue-shifted photoemission for AQS3 dissolved in polyanions implied the existence of some interaction. The isolation effect and diverse polar environment created by the surrounding coassembled polyanions were favorable to the luminescent properties of AQS3 . The host guest interaction will prompt a further hypsochromic shift when the AQS polyanion is assembled in UTFs with LDH nanosheets. These results clearly demonstrate that the LbL assembly of AQS3 can be successfully carried out along with a wide variety of polyanions, and the detailed mechanism is worth investigating. However, it can be expected that AQS3 interacted with the polyanion chains in solution to form some supramolecular structure by the intermolecular interaction, which overwhelms the electrostatic repulsion of the negative charges, for example, the π π stacking of the benzene ring among PSS, PPV, and AQS and the hydrophobic effect from the PAA chain and AQS.18 The small AQS3 molecules are immobilized effectively by

ARTICLE

the supramolecular structure and then smoothly coassembled with LDH nanosheets by an electrostatic interaction. Therefore, the coassembly strategy solves the problem of hard assembly for small anions and paves the way for fabricating UTFs based on small functional ions. Furthermore, the coassembly method omits the fussy and time-consuming drying process of immobiling small ions, which predigests the experimental procedure. Morphology and Structural Characterization of the UTFs. The (AQS-polyanion/LDH)n UTFs show both the homogeneity and increase in surface coverage with increasing n, and (AQS-PPV/ LDH)n UTFs were selected to investigate its surface appearance by scanning electron microscopy (SEM) and atomic force microscopy (AFM). A top view of the SEM image for the (AQS-PPV/LDH)24 UTF (Figure 3a) shows that the film surface is continuous and uniform with thicknesses of ca. 72 and 3 nm per bilayer (AQS-PPV/ LDH)1 obtained from a side view (Figure 3b). Additionally, plenty of “spherical” islands that are nearly the same size (ca. 110 nm) are clearly resolved on the UTF surface, which may be attributed to the ordered stacks of (AQS-PPV/LDH) microcrystallites.14c The spherical islands are denser with increased assembly numbers, which can be further seen in Figure 4a c. The approximately linear increase in the thickness from 25 nm for 8 bilayers to 93 nm for 32 bilayers with increasing n (Figure 4 and Table S1) confirms that the UTF presents a uniform and periodic layered structure, which is in agreement with the behavior revealed by the absorption and fluorescence spectra. The AFM topographical image (10 μm  10 μm) of the (AQS-PPV/LDH)24 film indicates that the film surface is relatively smooth with a root-mean-square (rms) roughness of ca. 8 nm in Figure 3c,d. Furthermore, the roughness values for the UTFs increase slowly with the number of deposition cycles in the range of 6 10 nm (Table S1). Small-angle X-ray diffraction shows that the diffraction intensity at around 2θ = 2.75 increases gradually with the increase in the bilayer number, further testifying to the successful multilayer buildup (Figure 5). This phenomenon is usually detected in the LBL multilayer film.13a,c Besides, the small angle of the pattern varying from 2θ = 3.04 to 2.75 indicates that the d space between the LDH nanosheets apparently increased from 29.27 to 32.03 Å as the bilayers increased (Table S2). This is probably due to the enhanced adsorption of AQS and polyanion within LDH layers with increasing assembly numbers, which will enlarge the interlayer height of the LDHs. The variation in the XRD peak was also observed in ruthenium(II) complex/LDH and APPV/LDH UTFs.14b,c This indicates that the UTF is an orderly periodical structure in the normal direction with a period of ca. 3 nm in approximate accordance with the result obtained by SEM. Moreover, this is also in agreement with the ideal overlapping arrangement model of the (AQS-PPV/LDH) supramolecular structure with a thickness of about 0.48 nm for a monolayer of LDH and 2.5 nm for an interlayer of AQS3 and PPV (Figure S4). We also measured the XRD for (AQS/LDH)32 with 2θ = 3.05, (AQSPAA/LDH)18 with 2θ = 3.00, and (AQS-PSS/LDH)24 with 2θ = 2.84 UTFs (Figure S5 and Table S3), and they have an interlayer space of around 3 nm, which is expected to hold a single-layer arrangement of AQS in the interlayer region. Fluorescence Lifetime and Polarized Fluorescence of the UTFs. To understand further the influence of polyanions and LDHs on the photoluminescence properties of complex, pristine AQS3 and (AQS-polyanion/LDH)n, UTFs were studied by detecting their fluorescence decays with 360 nm excitation. The fluorescence lifetimes were obtained by fitting the decay profiles with a doubleexponential form, and the results are listed in Table 1 and Figure S6. The fluorescence lifetime analysis reveals that the lifetimes of 11504

dx.doi.org/10.1021/la202139f |Langmuir 2011, 27, 11501–11507

Langmuir

ARTICLE

Figure 4. Top view (top) and side view (bottom) of SEM images for the (AQS-PPV/LDH)n films with (a) 8, (b) 16, and (c) 32 bilayers and the plot of the thickness of (AQS-PPV/LDH)n films vs n.

Table 1. Fluorescence Lifetimes of (AQS Polyanion/ LDH)18 UTFs with Double-Exponential Fitting at 360 nm Excitation and 460 nm Detection τi (ns)a Ai (%)

UTFs AQS solution (5  10

a

2.75

1.27

3.20

1.23

1.39

1.01

5.86

32.03 67.97

5.64

50.54

0.71

49.46

5.49

20.76

0.32

79.24

(AQS-PSS/LDH)18

6.03

50.46

3.55

1.18

(AQS-PPV/LDH)18

1.02 6.59

49.54 50.23

3.97

1.22

1.32

49.77

(AQS-PAA/LDH)18

(AQS/LDH)n and (AQS-polyanion/LDH)n UTFs are all prolonged (3.2 4.0 ns) compared with that of the pristine Na3AQS solution (2.75 ns), except for (AQS-PAA/LDH)n (1.38 ns). This increase can be expected that the rigid LDH nanosheets that isolate the complex ions from one another between adjacent layers with the dispersion effect of the polyanion chains, thus eliminating the interlayer π π stacking interaction.2b,14a Meanwhile, the electrostatic interactions between AQS3 and LDH layers could offer a constrained environment for AQS3 , which will reduce the internal mobility and flexibility of AQS3 and thus enhance the fluorescence lifetime. On the contrary, PAA has a more flexible structure than do PSS and PPV, which may be the reason for the decreased lifetime in

mol 3 L 1)

χ2b

1.29 (AQS/LDH)18

Figure 5. Small-angle XRD patterns for the (AQS-PPV/LDH)n UTFs with 8, 16, 24, and 32 bilayers.

5

Æτæ (ns)

τi: the fluorescence lifetime. b χ2: the goodness of fit.

(AQS-PAA/LDH)n films. To investigate further the microenvironment of the AQS in the as-prepared UTFs, polarized luminescence was employed to measure luminescence anisotropy value r.14c,19 r = (IVV GIVH)/(IVV + 2GIVH), where G = IHV/IHH. IVH stand for the photoluminescence intensity obtained with vertical excitation and horizontal detection, and IVH, IHV, and IHH are defined in a similar way. In contrast to the Na3AQS film made by the solvent evaporation method with no luminescence polarization (r = 0.04) and bulk AQS-LDH powder with r of around 0.06, the 11505

dx.doi.org/10.1021/la202139f |Langmuir 2011, 27, 11501–11507

Langmuir

ARTICLE

Figure 6. Polarized luminescence profiles for the VV and VH modes and anisotropic value (r) for (A) (AQS/LDH)18 and (B) (AQS-PPV/LDH)18 UTFs. The bottom scheme illustrates the measurement setup for polarized fluorescence for excitation light with a 45 incident geometry.

(AQS/LDH)18 UTF displays well-defined blue photoluminescence anisotropy with r = ca. 0.20 from 450 to 550 nm, whereas r = 0.15 for the (AQS-PPV/LDH)18 UTF (Figure 6A,B). The uniform r values ranging from 450 to 575 nm were observed between 0.12 and 0.18 for (AQS-polyanion/LDH)n UTFs. They are all measured by comparing the parallel and perpendicular directions with respect to the excitation polarization (IVV vs IVH) for the polarized excitation light with 45 incidence geometry. (The measurement setup is shown at the bottom of Figure 6.) It can be speculated that the host guest interaction induces AQS3 to be arranged in an oriented manner and suppresses the π π stacking, which may be responsible for the enhanced polarized photoemission. This is also proven by experimental and theoretical studies of the Ru(dpds)3/LDH film,14b whereas the ionic self-organized film of AQS3 with synthetic ammonium surfactants exhibits no polarized emission due to the unspecific orientation of AQS3 with a pseudospherical structure.5

’ CONCLUSIONS The ordered luminous UTFs were fabricated by the effective coassembly of inorganic LDH nanosheets and building block (AQS polyanion) on quartz substrates, which combine the advantages of inorganic materials as a support matrix and the LbL method for functional complex ion immobilization. (AQS polyanion/LDH)n UTFs were more easily fabricated than (AQS/ LDH)n UTFs because AQS anions can combine to form LDH nanosheets more stably when being premixed with polyanions. In comparison with those of pure AQS3 , (AQS/LDH)n and (AQSpolyanion/LDH)n UTFs all displayed blue-shifted fluorescence, prolonged lifetimes, and improved luminescent polarization, with the exception of (AQS-PSS/LDH)n UTFs with a decreased lifetime. It may originate from the host guest interaction by suppressing the π π stacking of the AQS molecule and the restrained 2D orientation. Furthermore, (AQS-PPV/LDH)n UTFs show combined luminescence from AQS3 and PPV. It can be expected that,

by tuning and controlling the ratio of the assembled organic components, the adjustable complex photoemissive UTFs can be constructed. A structural and surface morphology study shows that the UTFs are continuous and uniform with long-range stacking order in the normal direction of the substrate. Therefore, this work provides an efficient strategy to fabricate functional films by incorporating small luminous complex ions into an inorganic layered matrix for the purpose of promising applications in optoelectronic devices.

’ ASSOCIATED CONTENT

bS

Supporting Information. UV vis absorption and photoemission spectra of (AQS/LDH)n, (AQS-PAA/LDH)n, and (AQS-PSS/LDH)n UTFs vs n. Emission spectra of AQS and PPV solutions and the AQS polyanion solution. Depth and thickness parameters for the (AQS-PPV/LDH)n UTFs. 2θ and d values for (AQS polyanion/LDH)n UTFs. Structural model of (AQS-PPV/LDH)n UTFs. Small-angle XRD for (AQSpolyanion/LDH)n UTFs. Fluorescence decay profile of the AQS solution and (AQS polyanion/LDH)n UTFs. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Fax: +86-10-64425385. Tel: +86-10-64412131.

’ ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China, the 973 Program (grant no. 2011CBA00504), the 111 Project (grant no. B07004), and Fundamental Research Funds for the Central Universities. 11506

dx.doi.org/10.1021/la202139f |Langmuir 2011, 27, 11501–11507

Langmuir

ARTICLE

’ REFERENCES (1) (a) Tang, C. W.; VanSlyke, S. A. Appl. Phys. Lett. 1987, 51, 913. (b) Chen, C. H.; Shi, J. Coord. Chem. Rev. 1998, 171, 161. (c) Sapochak, L. S.; Benincasa, F. E.; Schofield, R. S.; Baker, J. L.; Riccio, K. K. C.; Fogarty, D.; Kohlmann, H.; Ferris, K. F.; Burrows, P. E. J. Am. Chem. Soc. 2002, 124, 6119. (2) (a) Brinkmann, M.; Gadret, G.; Muccini, M.; Taliani, C.; Masciocchi, N.; Sironi, A. J. Am. Chem. Soc. 2000, 122, 5147. (b) C€olle, M.; Gmeiner, J.; Milius, W.; Hillebrecht, H.; Br€utting, W. Adv. Funct. Mater. 2003, 13, 108. (c) Kaji, H.; Kusaka, Y.; Onoyama, G.; Horii, F. J. Am. Chem. Soc. 2006, 128, 4292. (3) Chen, J.-A.; Chen, C.-H. J. Mater. Chem. 2005, 15, 1179. (4) (a) Chen, W.; Peng, Q.; Li, Y. D. Adv. Mater. 2008, 9999, 1. (b) Cho, C. P.; Wu, C. A.; Perng, T. P. Adv. Funct. Mater. 2006, 16, 81. (c) Cho, C. P.; Wu, C. A.; Perng, T. P. Nanotechnology 2006, 17, 5506. (d) Cho, C. P.; Perng, T. P. Org. Electron. 2010, 11, 115. (5) Camerel, F.; Barbera, J.; Otsuki, J.; Tokimoto, T.; Shimazaki, Y.; Chen, L.-Y.; Liu, S.-H.; Lin, M.-S.; Wu, C.-C.; Ziessel, R. Adv. Mater. 2008, 20, 3462. (6) Decher, G. Science 1997, 277, 1232. (7) (a) Ariga, K.; Lvov, Y.; Kunitake, T. J. Am. Chem. Soc. 1997, 119, 2224. (b) Linford, M. R.; Auch, M.; Mohwald, H. J. Am. Chem. Soc. 1998, 120, 178. (8) (a) Chung, A. J.; Rubner, M. F. Langmuir 2002, 18, 1176. (b) Nicol, E.; Habib-Jiwan, J. L.; Jonas, A. M. Langmuir 2003, 19, 6178. (9) Chen, H.; Zeng, G. H.; Wang, Z. Q.; Zhang, X. Macromolecules 2007, 40, 653. (10) (a) Leroux, F.; Taviot-Guehob, C. J. Mater. Chem. 2005, 15, 3628. (b) Williams., G. R.; O’Hare, D. J. Mater. Chem. 2006, 16, 3065. (c) Fogg, A. M.; Freij, A. J.; Parkinson, G. M. Chem. Mater. 2002, 14, 232. (11) (a) Sels, B.; De Vos, D.; Buntinx, M.; Pierard, F.; Kirsch-De Mesmaeker, A.; Jacobs, P. Nature 1999, 400, 855. (b) Khan, A. I.; Lei, L. X.; Norquist, A. J.; O’Hare, D. Chem. Commun. 2001, 22, 2342. (c) Gago, S.; Costa, T.; Seixas de Melo, J.; Goncalves, I. S.; Pillinger, M. J. Mater. Chem. 2008, 18, 894. (12) (a) Wang, X. R.; Lu, J.; Shi, W. Y.; Li, F.; Wei, M.; Evans, D. G.; Duan, X. Langmuir 2010, 26, 1247. (b) Yan, D. P.; Lu, J.; Wei, M.; Evans, D. G.; Duan, X. J. Phys. Chem. B 2009, 113, 1381. (c) Shi, W. Y.; He, S.; Wei, M.; Evans, D. G.; Duan, X. Adv. Funct. Mater. 2010, 20, 3856. (13) (a) Liu, Z. P.; Ma, R. Z.; Osada, M.; Iyi, N.; Ebina, Y.; Takada, K.; Sasaki, T. J. Am. Chem. Soc. 2006, 128, 4872. (b) Li, L.; Ma, R. Z.; Ebina, Y.; Fukuda, K.; Takada, K.; Sasaki, T. J. Am. Chem. Soc. 2007, 129, 8000. (c) Wang, L.; Sakai, N.; Ebina, Y.; Takada, K.; Sasaki, T. Chem. Mater. 2005, 17, 1352. (14) (a) Yan, D. P.; Lu, J.; Wei, M.; Han, J. B.; Ma, J.; Li, F.; Evans, D. G.; Duan, X. Angew. Chem., Int. Ed. 2009, 48, 3073. (b) Yan, D. P.; Lu, J.; Wei, M.; Ma, J.; Evans, D. G.; Duan, X. Chem. Commun. 2009, 42, 6358. (c) Yan, D. P.; Lu, J.; Ma, J.; Wei, M.; Evans, D. G.; Duan, X. Langmuir. 2010, 26, 7007. (d) Han, J. B.; Lu, J.; Wei, M.; Wang, Z. L.; Duan, X. Chem. Commun. 2008, 41, 5188. (15) Yan, D. P.; Lu, J.; Chen, L.; Qin, S. H.; Ma, J.; Wei, M.; Evans, D. G.; Duan, X. Chem. Commun. 2010, 46, 5912. (16) (a) Li, S. D.; Lu, J.; Wei, M.; Evans, D. G.; Duan, X. Adv. Funct. Mater. 2010, 20, 2848. (b) Li, S. D.; Lu, J.; Xu, J.; Dang, S. L.; Wei, M.; Evans, D. G.; Duan, X. J. Mater. Chem. 2010, 20, 9718. (17) (a) Ballardini, R.; Varani, G.; Indelli, M. T.; Scandola, F. Inorg. Chem. 1986, 25, 3858. (b) Muegge, B. D.; Brooks, S.; Richter, M. M. Anal. Chem. 2003, 75, 1102. (c) Kim, S. T.; Hwang, D.; Li, X. C.; Griiner, J.; Friend, R. H.; Holmes, A. B.; Shim, H. K. Adv. Mater. 1996, 8, 979. (18) (a) Crespo-Biel, O.; Dordi, B.; Reinhoudt, D. N.; Huskens, J. J. Am. Chem. Soc. 2005, 127, 7594. (b) Lojou, E.; Bianco, P. Langmuir 2004, 20, 748. (19) Valeur, B. Molecular Fluorescence: Principles and Applications; Wiley-VCH: Weinheim, Germany, 2001.

11507

dx.doi.org/10.1021/la202139f |Langmuir 2011, 27, 11501–11507