Luminous Ultrathin Films by the Ordered Micellar Assembly of Neutral

May 14, 2012 - Luminous Ultrathin Films by the Ordered Micellar Assembly of. Neutral Bis(8-hydroxyquinolate)zinc with Layered Double. Hydroxides...
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Luminous Ultrathin Films by the Ordered Micellar Assembly of Neutral Bis(8-hydroxyquinolate)zinc with Layered Double Hydroxides Shuangde Li, Jun Lu,* Hongkai Ma, Dongpeng Yan, Zhen Li, Shenghui Qin, David G. Evans, and Xue Duan State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, P. R. China S Supporting Information *

ABSTRACT: This article describes a novel method to effectively assemble a neutral complex molecule, bis(8hydroxyquinolate)zinc (Znq2) with the exfoliated Mg−Allayered double hydroxide (LDH) nanosheets, to obtain the ordered ultrathin films (UTFs) by employing a layer-by-layer assembly technique. Anionic block copolymer micelles, poly(tert-butyl acrylate-co-ethyl acrylate-co-methacrylic acid) (PTBEM), were chosen as a molecular carrier for the incorporation of Znq2 molecules and then alternatively electrostatic assembly with the cationic LDH nanosheets. (Znq2@PTBEM/LDH)n UTFs present a stepwise growth upon the increasing deposited cycles monitored by the UV−vis absorption and fluorescence spectroscopy. The UTFs exhibit the blue-shifted luminescence (λem = 488 nm) of 57 nm by comparison to the Znq2 solution and enhanced cyan polarized photoemission character with the luminescence anisotropy (r) of ca. 0.14 in comparison with Znq2@PTBEM micelle film (r = 0.07) and the Znq2 solution (r = 0.02), due to the orientation arrangements induced by the LDH layers. The orderly periodical layered structure with a thickness of ca. 13 nm per UTF bilayer can be predicted from the small-angle X-ray diffraction pattern, being in approximate accordance with the height of a single layer of Znq2@PTBEM micelle in the interlayer region. Scanning electron microscopy and atomic force microscopy indicate that the film surface is continuous and relatively smooth. This work gives a feasible method for immobilizing functional neutral molecules into the gallery of LDHs for designing and achieving novel organic−inorganic ultrathin films. illustrated by Thomsen III et al.9 Hopkins prepared the substituted Znq2 with blue-shifted luminescence and then immobilized it in a poly(vinylcarbazole) thin film.10 Ogawa et al. reported the in situ formation of Znq2 in the interlayer space of smectites, demonstrating that the host can influence its photoemission.11 The above resolutions usually need a complicated synthesis and can resolve only some problems. It will be quite useful to find a feasible approach to solve all the problems at the same time. Nowadays, the fabrication of inorganic−organic hybrid materials with ordered nanostructure are attracting much attention because they have novel functionalities affected by the host−guest interaction, which generally differ from those of the pure consisting components.12 Layered double hydroxides (LDHs, [M2+1−xM3+x(OH)2]x+An− x/n·mH2O, where M2+ and M3+ are metal cations and An− is an anion) are one family of anionic clay consisting of positively charged brucite-like layers and exchangeable interlayer anions.13−18 They are broadly surveyed in catalyst,19 absorption,20 gene delivery system,21

1. INTRODUCTION Metal 8-hydroxyquinolate chelates (Mqn) have been broadly studied due to their excellent luminescence and electron transport performance for their application in organic lightemitting devices (OLEDs) since the first report of tris(8hydroxyquinolate) aluminum (Alq3) in 1987.1,2 There are many detailed studies about the structure−property relationships of Mqn, aimed to design molecular structures for improved and controllable optical properties.3−6 The vacuum-deposited thin film of bis(8-hydroxyquinolate) zinc (Znq2) has a lower operating voltage and higher quantum yields than the widely used Alq3.5 However, as in the case with Alq3, Znq2 would inevitably crystallize partially during the vacuum deposition process,7 and degrade during the prolonged thermal treatments, which would ultimately compromise the device operation. Furthermore, the performance of OLEDs is particularly sensitive to the morphology variation of the film, which needs a technique to easily control the homogeneity of the film.1 To design Znq2-based compounds with a tunable emission color is also the aim of researchers. Weck et al. synthesized a Znq2-functionalized polymer system, which can overcome the processable limitation.8 A uniform Znq2 film grown on Si substrate by the coordination self-assembly was © 2012 American Chemical Society

Received: January 16, 2012 Revised: May 9, 2012 Published: May 14, 2012 12836

dx.doi.org/10.1021/jp3005025 | J. Phys. Chem. C 2012, 116, 12836−12843

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Scheme 1. Schematic Representation of the PTBEM, Znq2, and Mg−Al/LDH Nanosheet and the Process for the Fabrication of Znq2@PTBEM Micelle and (Znq2@PTBEM/LDH)n Film

optical functional materials,22−24 and so on based on their chemical composition, layer charge, capability of ion exchange, and good thermal stability. The chromophore/LDHs system fabricated by the incorporation of chromophores, like metal complexs22,23 or fluorescein,24 into their interlayers was identified with a kind of new important luminous composite material. The system can tune the photophysical and photochemical properties, and improve the thermal- and photostability of the guests, which were caused by the host− guest and guest−guest interaction within the restrained LDH interlayer gallery. Recently, the exfoliated LDH nanosheets25,26 were employed to fabricate functional ultrathin films (UTFs) by a layer by layer (LbL) electrostatic assembly with many species of anions, like π-conjugated polymers,27 or thermoresponsive small molecules,28 which will enhance the property of the guests or display span-new performances. However, because of the unique positively charged LDH monolayer, only anions could be intercalated into the interlayer gallery, except neutral C60 and cucurbituril,29,30 which will greatly restrict the development of LDH-based functional materials. Very recently, Yan et al. employed a polyanion as an intermediary to assemble (small cation−polyanion)/LDH film.31 However, the LbL assembly of LDH nanosheets with neutral molecules has never been reported due to the absence of electrostatic interaction between them. Herein, we describe a facile method to fabricate novel neutral molecule/LDH UTFs with an amphiphilic block copolymer as a carrier, which is easy to replicate and expand into the fabrication of practical devices. The UTFs are fabricated by the following sequential procedures: first, as is known, the amphiphilic block copolymers composed of hydrophilic and hydrophobic segments are easy to produce numerous core−shell micellar structures by self-organizing, thus can spontaneously encapsulate the water-insoluble small molecules into the hydrophobic core of micelles.32,33 Then, these micelles could be assembled with conventional polyelectrolytes using the LbL method. Applications of such micelle assemblies include the fabrication of core−shell film (pyrene@PS-b-PAA/PDDA) for the controlled release of water-insoluble dyes34 and the antireflective micelle/micelle films.33 Therefore, the highly desirable approach would apply for the assembly of LDH layers with the various functional neutral molecule, which will greatly extend

the scope of layered composite materials with prospective applications. Poly(tert-butyl acrylate-co-ethyl acrylate-co-methacrylic acid) (PTBEM), a block copolymer for drug delivery,35 was selected to form an anionic micelle matrix for the incorporation of the water-insoluble molecule Znq2, yielding the neutral molecule@ block copolymer micelles, denoted as Znq2@PTBEM, based on the hydrophobic interaction. PTBEM forms anionic micelles due to the conversion from uncharged carboxylic acid groups (−COOH) to charged carboxylate groups (−COO−) at pH > 3,36 and the Znq2@PTBEM anionic micelles exist in the neutral solutions (pH = 7). The negatively charged micelles were assembled with the exfoliated Mg−Al/LDH nanosheets on the quartz substrate by the LbL method. Two components were alternatively deposited for the fabrication of (Znq2@PTBEM/ LDH)n UTFs (Scheme 1). Some Na+ ions exist in the (Znq2@ PTBEM/LDH)n UTFs inferred from SEM-EDX for the LDH as outer layer (shown in Figure S1 of the Supporting Information) and partially balance the anionic micelles in the LDH interlayers, which originated from the Znq2@PTBEM micelle solution. In addition, there have been some reports of the existence of an ordered interlayer containing SO42− anions and Na+ cations in minerals of [MnII6Al3(OH)18][(SO4)2{Na(OH 2 ) 6 }(H 2 O) 6 ] or [Mg 5.6 Al 3.4 (OH) 18 ][(SO 4 ) 1.3 (CO 3 )Na0.6(H2O)12].37 Compared with the Znq2 film with spincoating or coordinate assembly method, the technique used in (Znq2@PTBEM/LDH)n UTFs preparation is extremely simple and low-cost. The (Znq2@PTBEM/LDH)n UTFs possess a precise tuning of its UV−visible absorption and photoemission, combined with the blue-shifted fluorescence and improved fluorescence polarization compared with pristine Znq2 and Znq2@PTBEM. The resulting UTFs combined the advantage of the luminescence properties of Znq2 and the character of LDH monolayers, and its fluorescence, composition, and film thickness can be modified.27,28

2. EXPERIMENTAL SECTION 2.1. Reagents and Materials. Bis(8-hydroxyquinolate)zinc (C18H12N2O2Zn, Znq2), poly(tert-butyl acrylate-co-ethyl acrylate-co-methacrylic acid) [CH2CH[CO2C(CH 3 ) 3 ] 3 ] x [CH 2 CH(CO 2 C 2 H 5 )] y [CH 2 C(CH 3 )(CO 2 H)] z (PTBEM), poly(thiophene-3-[2-(2-methoxyethoxy)ethoxy]12837

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PTBEM micelle solution and Pulsed diode laser PDL 800-B at 375 nm for the films. The percentage contribution of each lifetime component to the total decay curve was calculated by the Edinburgh F900 instrument software. Photoluminescence quantum yield (PLQY) was measured using a HORIBA JobinYvon Nanolog FL3-2 iHR spectrofluorimeter. Steady-state polarized photoluminescence measurements were recorded with an Edinburgh Instruments’ FLS 920 fluorimeter with the 385 nm excitation, and polarizers were used in the excited and detected directions. The r value was observed by comparing the parallel and perpendicular directions to the excitation polarization (IVV vs IVH) in which the polarized excitation light keeps at 45° incidence geometry (as shown in Scheme 2). A Zeiss

2,5-diyl) sulfonated solution (C9H12O6S2)x(C9H11O3S)y (denoted as PT), and poly dimethyldiallylammonium chloride (PDDA, Mw = 100 000−200 000) were purchased from Sigmaaldrich development Co. Ltd. Analytical grade Mg(NO3)2·6H2O, Al(NO3)3·9H2O, NaOH, and ethylene glycol monobutyl ether [CH3(CH2)3OCH2CH2OH] were purchased from Beijing Chemical Co. Ltd. All other chemicals were analytical grade and used as received without further purification. Deionized water was used throughout the experimental process. 2.2. Preparation of Znq2@PTBEM, Znq2@PT Micelle, and (Znq2@PTBEM)n and (Znq2@PT)n UTFs. The Znq2 solution was prepared by dissolving Znq2 in ethylene glycol monobutyl ether solvent (GME, 1 × 10−4 mol·L−1). The Znq2@PTBEM micelle solution was obtained as follows. The 2 mL Znq2 solution (1 mg/mL in GME, under ultrasonic) was added dropwise into the 38 mL PTBEM micelle solution (0.368 mg/mL, pH = 7.0, adjusted by 0.1 M NaOH solution) under ultrasonic, keeping the pH value of the final micellar solution. The resulting micelle solution was filtered by a filter of 200 nm. Then, the pH value of Znq2@PTBEM micelle was kept at 7.0 for the layer by layer (LbL) deposition. The Znq2@ PT micelle solution was obtained with the same method except that the 1 mL Znq2 GME solution (1 mg/mL, under ultrasonic) dissolved in 19 mL of PT solution (0.5 mL of PT diluted with GME and water (vol/vol = 9:10)). Then, the pH value of Znq2@PT micelle was also adjusted to 7.0. The Znq2 film and Znq2@PTBEM micelle film were prepared by the solvent evaporation method: the suspension of Znq2 GME solution and Znq2@PTBEM micelle solution were thoroughly ultrasonically dispersed and spread on a precleaned quartz substrate, and then dried in vacuum at ambient temperature. The process of synthesis and exfoliation of Mg−Al/LDH was similar to that described in the previous work;18 0.1 g Mg−Al/ LDH was ultrasonically shaken in 100 mL of formamide solution for 30 h to produce a colloidal suspension of exfoliated Mg−Al/LDH nanosheets. The quartz substrate was first cleaned in concentrated H2SO4/H2O2(30%) (vol/vol = 7:3) for 40 min and then thoroughly washed with pure water. The cleaned substrates were dipped into a cationic PDDA solution (10 g·L−1) for 30 min, then thoroughly rinsed with distilled water and dried in a nitrogen gas flow, which resulted in a positively charged surface on the substrates. The pretreated substrate was dipped into the Znq2@PTBEM micelles for 10 min followed by washing thoroughly and then treated with a colloidal suspension (1 g·L−1) of LDH nanosheets for 10 min and washed thoroughly. The (Znq2@PTBEM/LDH)n multilayer UTFs were fabricated by the alternately depositing Znq2@ PTBEM micelle and LDH nanosheets suspension for n cycles, and after every deposition, the UTF was dried under a nitrogen gas flow. The fabrication of (Znq2@PT/LDH)n UTFs were employed with the same procedure. 2.3. Characterization. The UV−vis absorption spectrum was performed on a Shimadzu UV-2501PC spectrometer with the slit width of 1.0 nm. The fluorescence spectra were performed on a RF-5301PC fluorospectrophotometer in the identical condition with an excitation wavelength of 385 nm. The fluorescence lifetime was fitted by recording the fluorescence decay curve with an Edinburgh Instruments’ FL 900 fluorimeter with the 385 nm excitation and the detection wavelengths at their own maximal photoemissions with hydrogen flash lamp for Znq2 GME solution and Znq2@

Scheme 2. Scheme Illustrates the Measurement Setup of the Polarized Fluorescence for the Excitation Light from the 45° Incidence Geometry

Supra 55 scanning electron microscope (the accelerating voltage applied was 20 kV) was used to investigate the surface morphology of ultrathin films. NanoScope III Atomic Force Microscope (Veeco Instruments Co. Ltd.) was used to investigate the surface morphology and surface smooth with the UTFs. X-ray diffraction (XRD) patterns of the films were recorded using a Rigaku 2500VB2+PC diffractometer using the Cu Kα radiation (λ = 1.541844 Å) at 40 kV and 50 mA with the step-scanned mode in 0.04° (2θ) per step and count time of 10 s/step in the range from 0.5 to 5°.

3. RESULTS AND DISCUSSION 3.1. Characterization of the (Znq2@PTBEM/LDH)n Assembly UTFs. The Znq2@PTBEM micelle solution with pale yellow showed the Tyndall effect and green fluorescence at 365 nm UV irradiation, while Znq2 GME solution presented yellow-green color (Figure 1A). This will be directly observed from the photoemission spectra in Figure 1B. The Znq2 solid and GME solution (1 × 10−4 mol·L−1) have a photoemission band at 536 and 545 nm,38 respectively, while that of the Znq2@PTBEM micelle solution blue-shifted to 494 nm with largely enhanced fluorescent intensity than Znq2 GME solution (Figure 1B), which manifested that there existed a prominent interaction between Znq2 and PTBEM. The photoemission band of Znq2 at 536 nm is assigned to the π*−π transition from the quinoliate ligand, which is also the reason for the photoemission of Mq2 (M = metal) complex. Furthermore, this photoemission is very sensitive to the change of the ligand environment.2 It was reported that Znq2 and Alq3 showed the blue-shifted and enhanced fluorescence in the surfactant micellar solutions, which could be related to the changes in the localization of the chelates.39,40 Hopkins also found the emission maxima blue shift for Znq2 and Alq3 dissolved in poly(N-vinylcarbazole) (PVK) polymer compared with those 12838

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The growth of the (Znq2@PTBEM/LDH)n UTFs was monitored by the UV−vis absorption and photoemission spectra measured immediately after several deposition cycles (n). The linear relationship was observed between the absorbance at 257 nm (the transition of phenyl rings in Znq2) and n indicates that the deposition process is stepwise and regular from layer to layer (Figure 2a). The intensity of the maximum photoemission peaks with 488 nm also displays a monotonic increase in n (Figure 2b). This can be further visualized by the gradual increase in color with uniformly bright cyan light of the (Znq 2 @PTBEM/LDH)n UTFs with increasing n (Figure 2b, inset). The as-prepared (Znq2@ PTBEM/LDH)n film shows a little further blue-shift of 6 nm compared to that of the Znq2@PTBEM micelle solution, indicating that the interaction between LDH and Znq2@ PTBEM micelle in the UTFs will affect the Znq2 surroundings through the influence on the Znq2@PTBEM micelle. The phenomenon of the photoemission shift is frequently observed for the system of fluorescent molecules intercalated into LDH interlayer. Our previous study of the sulfonated-Znq2 and dodecyl sulfonate cointercalating LDH system presents the notably blue-shifted transition.23 Shukla also reported the conversion of green light emitting Znq2 thin film deposited by thermal evaporation on quartz substrates to a stable blue luminescence film because of the transformation after aging and degradation behavior of the Znq2 film.36 The results clearly demonstrate the successful assembly of Znq2 into the UTFs with the PTBEM micelles as carriers. The micelles assembly strategy solves the problem of impossible assembly for neutral molecules and paves the way for fabricating UTFs based on many functional neutral molecules. The small-angle X-ray diffraction pattern displays that the peak at (2θ, d nm) = (0.74, 11.9) and (0.65, 13.6) for n = 22 and 30, respectively, as shown in Figure 3 and Table S1, Supporting Information. Although there is a little variation of the basal spacing from 11.9 to 13.6 nm for different n, it indicates that the UTFs present an orderly periodical layered structure in the normal direction with a period of ca. 13 nm. Moreover, the gallery height slightly dwindled due to the existence of the electrostatic interaction between micelles and LDH nanosheets, but it is still in approximate accordance with the height of the ideal overlapping arrangement model of the (Znq2@PTBEM/LDH)1 UTF structure constituted with one LDH monolayer with 0.48 nm thick and Znq2@PTBEM micelle of ca. 15 nm thick.

Figure 1. (A) Photograph of (a) Znq2 GME solution at the 365 nm UV illumination and Znq2@PTBEM micelles (b) at daylight and (c) at the 365 nm UV illumination. The light beam in panel A-b was an incident from the side to demonstrate the Tyndall effect. (B) Photoemission spectra of Znq2 (1 × 10−4 mol·L−1) GME solution and powder and Znq2@PTBEM micelle solution with the 385 nm excitation.

of pure complex films, which can be attributed to the reduced interactions between lumophores in the PVK matrix corresponding to the reduced excimer formation.10 It has previously been described for Alq3 that the emission was dependent on the packing of the quinolate ligands. The larger interligand spacing leads to more blue-shift of the emission.6 Weck studied the emission of the copolymer containing quinolato−zinc and found that the system with low complex content showed blueshifted at 548−505 nm in comparison to that with high quinolato−zinc content.8 The possible explanation for the hypsochromic shift phenomenon of Znq2 in our system may be the isolation effect and the hydrophobic environment created by the surrounding block copolymer.

Figure 2. (a) UV−vis absorption and (b) photoemission spectra of (Znq2@PTBEM/LDH)n UTFs (n = 6−26). The insets in panels a and b show the plot of the absorbance at 257 nm vs n and the photograph of UTFs under the 365 nm UV illumination, respectively. 12839

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PTBEM/LDH)26 film is 4.76%, which declined a little to that of the Znq2@PTBEM micelle film around 7.73%. The PLQY is reported to be 0.03 and 0.45 for Znq2 in the solution and powder, respectively.41 3.3. Steady-State Fluorescence Polarization. To understand the influence of the interlayer microenvironment on the orientation of Znq2 molecules, the Znq2 molecules in the micelle solution and as-prepared UTFs were further investigated by measuring their fluorescence anisotropy. One of the most common methods for evaluating fluorescence polarization is the measurement of the anisotropic value r, as fully described by Valeur.42 r can be expressed as 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. Theoretically, the value of r is in the range of −0.2 (absorption and emission transition dipoles perpendicular) to 0.4 (two transition dipoles parallel), and the deviation from this value indicates the reorientation of the emission dipole moment. Although the Znq2 solution displays no luminescence polarization (r = 0.02) shown in Figure 5a, Znq2@PTBEM micelle solution and Znq2@PTBEM micelle film reveal a little increased r with 0.05 and 0.07, respectively (Figure 5b,d). (Znq2@PTBEM/LDH)16 and (Znq2@PTBEM/LDH)26 UTF show a uniform r = 0.10 and 0.14 ranging in 455−550 nm for the cyan photoluminescence (Figure 5c,d) indicating an improvement in the orientation of the Znq2 molecules between the LDH nanolayers. It can be speculated that the host−guest interaction induces the Znq2 molecule orientable arranged by affecting the alignment of the interlayer micelles, which may be responsible for the enhanced polarized photoemission. This phenomenon has turned up frequently in the films, which are fabricated by the alternate assembly of organic molecules or polymers with LDH nanosheets.31,43 3.4. Structural and Morphological Characterization. The surface structure of the Znq2@PTBEM micelle in multilayer films and the (Znq2@PTBEM/LDH)n UTFs were characterized by the scanning electron microscope (SEM) and atomic force microscope (AFM), and the results are shown in Figures 6 and 7. The micelles show individual spheric morphology with the uniform diameter around 15 nm (Figure 6a). (Znq2@PTBEM/LDH)22 UTF with the LDH nanosheets as the terminal layer shows that the film surface is smooth and uniform (Figure 6b), while Figure 6c takes on the subspherical morphology around 20 nm in diameter with the Znq2@ PTBEM as the terminal layer. We can clearly observe lots of subspherical surface appearance from the zoomed-in image (Figure 6d). This further indicated the successful periodic assembly of the UTFs by the alternate deposition of Znq2@ PTBEM micelles and LDH nanosheets, which is in agreement with that revealed by absorption and fluorescence spectra. In fact, when mixing the formamide suspension of exfoliated LDH nanosheets with the Znq2@PTBEM micelles, the floccule precipitated due to the electrostatic interaction between the Znq2@PTBEM micelles and LDH nanosheets. This floccule was deposited on silicon substrate to characterize their structure. The AFM image (Figure 7A,B) shows that the Znq2@PTBEM micelles, when binding on the surface of LDH nanosheets, hold near-spherical with the uniform diameter around 15 nm. The structure of Znq2@PTBEM micelles in the assembled UTFs was also detected. The AFM topographical image (480 nm × 480 nm, Figure 7C,D) of (Znq2@PTBEM/ LDH)16 UTF with the Znq2@PTBEM micelle as the terminal

Figure 3. Small angle XRD of (Znq2@PTBEM/LDH)n UTFs with n = 22 and 30.

3.2. Fluorescence Lifetime. The fluorescence decays of the pristine Znq2, Znq2@PTBEM micelle solution, and (Znq2@PTBEM/LDH)n UTFs with different assembly numbers were detected in 385 nm excitation to further understand the influence of micelle and LDH on the photoluminescence property of the complex. The precise nature of the fluorescence decay can reveal the details about the interaction of the fluorophores with their environment. The fluorescence lifetime analysis obtained by fitting the decay profile with a doubleexponential form (Table 1; Figures 4 and S2, Supporting Table 1. Fluorescence Lifetimes of (Znq2@PTBEM/LDH)n UTFs with Double-Exponential Fitting under the 385 nm Excitation and Detection at Their Own Maxima Emission UTFs −4

Znq2 GME solution (1 × 10 mol·L−1) Znq2 solid film Znq2@PTBEM micelle (Znq2@PTBEM/LDH)16 (Znq2@PTBEM/LDH)26

τi (ns)a

Ai (%)

1.41 5.45 7.51 21.07 4.22 26.34 3.77 16.98 3.42 16.69

3.33 96.67 10.49 89.51 7.35 92.65 16.79 83.21 12.75 87.25

⟨τ⟩ (ns)

χ2b

5.32

1.04

19.65

1.03

24.71

1.12

14.76

1.10

15.00

1.09

τi (i = 1, 2) is the fitted fluorescence lifetime. Ai is the percentage of τi. In the biexponential case, ⟨τ⟩ = A1τ1 + A2τ2; A1 + A2 = 1.). bThe goodness-of-fit is indicated by the value of χ2; a fluorescence decay curve and residual plots of fits with biexponential for Znq2 GME solution and Znq2@PTBEM micelle solution are shown in Figure S2 of the Supporting Information. a

Information) reveals that the lifetime of Znq2@PTBEM micelle with 24.71 ns enlarged 4 times compared with that of the pristine Znq2 GME solution of 5.32 ns, which evidence that the micellization of Znq2 molecules enhanced the stability of the excited state of the Znq2. Tsuboi reported that lifetimes of Znq2 powder and acetonitrile solution are 25 and 5.42 ns, respectively.41 The (Znq2@PTBEM/LDH)n UTFs show a little decreased lifetime of around 15 ns, (Table 1) in comparison with that of the Znq2 film of 19.65 ns made by the solvent evaporation method, which manifested that the assemble film can keep the essence of the complex. The lifetime values for n = (16, 14.76 ns) and (26, 15.00 ns) are almost the same, which is independent of the assembled numbers n. The photoluminescence quantum yield (PLQY) of the (Znq2@ 12840

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Figure 4. Fluorescence decay curves and residual plots of the double-exponential fitting for (a) the Znq2 film and (b) (Znq2@PTBEM/LDH)16.

Figure 5. Polarized luminescence profiles and anisotropic value (r) for the (a) Znq2 GME solution, (b) Znq2@PTBEM micelle solution, and (c) (Znq2@PTBEM)26 film and (d) r values for the (Znq2@PTBEM)16 film and Znq2@PTBEM micelle film.

layer shows the morphology and roughness information for the UTF, which indicates that the film surface looks like relatively subspherical appearance with the root-mean-square (rms) roughness of ca. 36 nm. Furthermore, the roughness values for the UTFs increase slowly from ca. 28 to 57 nm with increasing deposition cycles n in the range of 14−32. The sizes of the respective micelles change to ca. 30−50 nm in diameter, which indicates there exist some aggregation of micelles. The micelles in the LDH interlayer maybe have certain robustness and can keep the spherical shape to some extent. In many other micelle/polymer or micelle/micelle assembly systems, the polymer micelles are proved quite stable and of engineering robustness, allowing for layer-by-layer fabrication.33,34 Therefore, on the basis of the existence of interlayer Na+ ions and the above AFM results, it can be concluded temporarily that the Znq2@PTBEM micelle within the LDH interlayers prefer to the sphrical shape rather than other structures. Of course, it is also possible for other structures, which will be the research focus for future work. 3.5. Micellar Assembly of (Znq2@PT/LDH)n UTFs. To extend the methodology, we provide another block copolymer

to prove the generality. Poly(thiophene-3-[2-(2methoxyethoxy)ethoxy]-2,5-diyl) sulfonated (PT) was chosen as the block copolymer micelle to incorporate Znq2 molecules and then to alternatively assemble with LDH nanosheets. The stepwise increase of (Znq2@PT/LDH)n UTFs with the increasing assembly layer n from 2 to 18 was also observed from the UV−vis absorption spectra of (Znq2@PT/LDH)n UTFs in Figure S3 of the Supporting Information. The linear augment of the UV absorption peaks at 194, 225 (absorption of the phenylene ring), and 268 nm indicate that PT and Znq2 are coassembled with the LDH nanosheet regularly (Figure S3 inset in the Supporting Information). Moreover, the diffraction peak of the (Znq2@PT/LDH)27 film from the small-angle XRD indicated that the UTFs show a periodical layered structure with a period of ca. 14.4 nm (Figure S4 of the Supporting Information), which is in accordance with the height of the (Znq2@PT/LDH)1 UTF structure composed of one monolayer of LDH and Znq2@PT micelle of ca. 18 nm in thickness. 12841

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Figure 6. Top view SEM images of (a) Znq2@PTBEM micelle; (b,c) (Znq2@PTBEM/LDH)22 UTFs with LDH nanosheet as the terminal layer and micelle as the terminal layer, respectively; (d) zoomed-in image for the red square in panel c.

Figure 7. Tapping-mode AFM topographical 2D images for (A) a film composed of LDH nanosheets and Znq2@PTBEM micelles and (C) (Znq2@ PTBEM/LDH)18 UTF with Znq2@PTBEM micelle as the terminal layer. (BmD) Three-dimensional surface plot of panels A and C, respectively.

shifted fluorescence and improved polarization value compared with Znq2 molecules and Znq2@PTBEM micelles. This may be originated from the dispersion effect from the micelles and the host−guest electrostatic interaction, which could suppress the π−π stacking and induce the orientable arrangement of the complex. The UTFs are uniform with long-range stacking order in the normal direction of the substrate inferred from structural and surface morphology studies. The method is also extended to the other block copolymer systems, like PT. Therefore, this work solves the problem of the difficult assembly of neutral molecules and provides an efficient strategy for fabricating

4. CONCLUSIONS In conclusion, we employed a micellar assembly route for the incorporation of neutral molecules (Znq2) into the inorganic LDHs to avoid the absence of interaction between them. Znq2 was first encapsulated into the block copolymer PTBEM micelles, and then the luminous cyan UTFs were fabricated by the effective assembly of LDH nanosheets and Znq2@PTBEM micelles on the quartz substrates, which show continuous and uniform surface with long-range stacking order and well-defined photoemission. The (Znq2@PTBEM/LDH)n UTFs show blue12842

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The Journal of Physical Chemistry C

Article

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functional UTFs based on small neutral molecules for the purpose of promising applications in optoelectronic devices.

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ASSOCIATED CONTENT

S Supporting Information *

SEM-EDX date of (Znq2@PTBEM/LDH)22 film; fluorescence decay profile for Znq2 GME solution and Znq2@PTBEM micelle solution; XRD diffraction data for (Znq2@PTBEM/ LDH)n film; UV absorption spectra and XRD profile of (Znq2@PT/LDH)n UTFs. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China, the 111 Project (Grant No. B07004), the 973 Program (Grant No. 2011CBA00504), Program for New Century Excellent Talents in University (NCET-11-0560), and the Fundamental Research Funds for the Central Universities.

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REFERENCES

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dx.doi.org/10.1021/jp3005025 | J. Phys. Chem. C 2012, 116, 12836−12843