Thin Solid Films 517 (2009) 6497–6501

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Multilayer films consisted of azulene-based dye molecules and polyelectrolyte:Preparation, characterization and photoluminescent property

Xiuli Wang ⁎, Jiani Fang, Daolin Wang, Wenyan ZhengFaculty of Chemistry and Chemical Engineering, Liaoning Key Laboratory of Applied Chemistry, Bohai University, Jinzhou, 121000, PR China

⁎ Corresponding author. Tel./fax: +86 416 3400158.E-mail address: wangxiuli@bhu.edu.cn (X. Wang).

0040-6090/$ – see front matter © 2009 Elsevier B.V. Adoi:10.1016/j.tsf.2009.03.215

a b s t r a c t

a r t i c l e i n f o

Article history:Received 3 July 2008Received in revised form 26 March 2009Accepted 27 March 2009Available online 7 April 2009

Keywords:Azulene-based dye moleculesLayer-by-layer self-assemblyMultilayersThin filmsPhotoluminescence

Ultrathinmultilayerfilms of two azulene-based (Az-based) dyemolecules (Az), 3-methylazulene-1-carboxylicacid hydrazide (Az-1) and 5-(4-phenylamine)-2-(3-methylazulene-1-yl)-1,3,4-oxadiazoles (Az-2), and poly(sodium 4-styrenesulfonate) (PSS) have been prepared by layer-by-layer self-assembly and characterized byUV–vis spectroscopy,fluorescence spectroscopy, small-angle X-ray reflectivitymeasurements and atomic forcemicroscopy (AFM) imaging. UV–vis spectra show that the characteristic absorbance values of the multilayerfilms increase almost linearly with the number of PSS/Az bilayers, suggesting that the deposition process isregular andhighly reproducible from layer to layer. Average thicknesses for the PSS/Az-1 and PSS/Az-2 bilayersof the multilayer films are ca. 0.9 and 1.4 nm, respectively. AFM images provide the surface morphology of thePSS/Az films, indicating that the film surface is relatively uniform and smooth. The occurrence ofphotoluminescent activity conforms the potential for creating luminescent multilayer films with Az-baseddye molecules.

© 2009 Elsevier B.V. All rights reserved.

1. Introduction

The design and preparation of functional organic thin-filmmaterials with tailored architecture and properties have receivedconsiderable interest [1]. Various methods for preparation of thinfilms have been developed such as physical deposition [2], spincoating [3], Langmuir–Blodgett (LB) technique [4] and layer-by-layer(LBL) self-assembly method [5–7]. The LBL deposition developed byDecher [8,9] is a facile method of obtaining such thin-film materials,which is desirable because it is a simple procedure, easy to automateand friendly to the environment [10]. In this technique, two oppositelycharged polyelectrolytes dissolved in aqueous solution are alternatelydeposited on a support surface by means of electrostatic attraction.The electrostatic nature of the interaction allows a variety of chargedspecies to be deposited, thereby forming multifarious thin-filmmaterials. Many different multilayers with nanocrystallites, colloidalgold, titanium dioxide, magnetic nanoparticles, and other functionalcomponents incorporated into polymer matrices have been fabricated[7,11,12].

Azulene is a nonalternant aromatic hydrocarbon featuring quiteunique characteristics of blue color and a permanent dipole momentof ca. 1 D [13]. In recent years, considerable efforts have been madetoward the exploration of a variety of functional dyes and opticalmaterials based on such π-system [14,15]. On the other hand, azuleniccompounds have been the subject of many areas of materials sciencedue to their favorable characteristics inphysical and chemical properties

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and biological activities and their potential applications as colorant,laser-printing, xerography, liquid crystal display, optical filter, opticsrecorder, photoreceptor and organic light-emitting devices [15–17].Despite unique characteristics and high potential applications ofazulenes, there is no example of Azulene-based (Az-based) dyemolecules multilayer films by LBL self-assembly. Therefore, it will beof critical importance to incorporate the Az-based dye molecules intomultilayer films by this method. As a simple yet effective technique toprepare uniformmultilayerfilms, LBL self-assembly provides a powerfultool for the formation and development of Az-based dye moleculesfilms.

Herein, two photoluminescent azulenic compounds (Az), 3-methylazulene-1-carboxylic acid hydrazide (Az-1) and 4-[5-(3-methylazulene-1-yl)-[1,3,4]oxadiazol-2-yl]-phenylamine (Az-2)were purposely synthesized and subsequently incorporated intomultilayer films by LBL methods. The growth of {P/(PSS/Az)n} filmswasmonitored by UV–vis spectroscopy and the surfacemorphology ofthe films was observed by atomic force microscopy (AFM). The filmstructure was characterized by small-angle X-ray reflectivity (XRR)measurements, and the photoluminescent properties of these multi-layer films were also investigated by fluorescence spectroscopy.

2. Experimental details

2.1. Materials

3-methylazulene-1-carboxylic acid methyl ester (Az-1a) was pre-pared according to the method described in the literature [18]. Poly(ethyleneimine) (PEI), MW 50,000, Poly(sodium 4-styrenesulfonate)

Scheme 1. Synthesis routes to Az-1, Az-2a, and Az-2.

Scheme 2. Fabrication of multilayer assemblies by consecutive absorption of PSS andAz-based dye molecules on precursor (P) films.

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(PSS), MW 70,000, and poly(allylamine hydrochloride) (PAH), MW70,000, were purchased fromAldrich. Thewater used in all experimentswas deionized to a resistivity of 18 MΩ cm−1. All other chemicalspurchasedwere of reagent grade and usedwithout further purification.

2.2. Synthesis of azulene-based dye molecules

2.2.1. 3-methylazulene-1-carboxylic acid hydrazide (Az-1)The 3-methylazulene-1-carboxylic acid hydrazide (Az-1) was

obtained by refluxing the starting material Az-1a with hydrazinehydrate in the presence of ethanol (Scheme 1) for 24 h. The crudeproduct was concentrated with rotary evaporator and extracted withethyl acetate, then purified by column chromatography, to give Az-1.Violet needles (ethanol), mp 151–152 °C; IR (KBr) ν: 3311, 3058, 1618,1578, 1532 cm−1; 1H NMR (CDCl3) δ: 2.61 (s, 3H, CH3), 4.09 (brs, 2H,NH2), 7.28 (dd, 1H, J=9.9, 9.6 Hz, 5-H), 7.36 (dd, 1H, J=10.0, 10.8 Hz,7-H), 7.52 (brs,1H, NH), 7.70 (t,1H, J=9.6, 9.6Hz, 6-H), 7.84 (s,1H, 2-H),8.29 (d,1H, J=9.6 Hz, 4-H), 9.52 (d,1H, J=9.6 Hz, 8-H). Anal. Calcd. forC12H12N2O: C, 71.98; H, 6.04; N, 13.99%. Found: C, 71.72; H, 6.21; N,14.14%.

2.2.2. 5-(4-nitro-phenyl)-2-(3-methylazulene-1-yl)-1,3,4-oxadiazoles(Az-2a)

The 5-(4-nitro-phenyl)-2-(3-methylazulene-1-yl)-1,3,4-oxadia-zoles (Az-2a) was prepared by the microwave irradiation of Az-1 with4-nitro-benzoic acid in the presence of phosphorous oxychloride(Scheme 1). The product was poured into crushed ice, and neutralizedby 5% sodium bicarbonate. The solid obtained finally purified by columnchromatography, to give Az-2a. Green needles (benzene), mp 221–222 °C; IR (KBr) ν: 3064, 1647, 1579, 1525 cm−1; 1H NMR (CDCl3,300MHz) δ: 2.67 (s, 3H, CH3), 7.27 (d, 2H, J=8.6 Hz, 2′, 6′-H, ArH), 7.34(dd, 1H, J=9.6, 8.0 Hz, 5- or 7-H), 7.45 (t, 1H, J=9.6 Hz, 6-H), 7.73 (dd,1H, J=9.6, 9.6 Hz, 7- or 5-H), 8.01 (d, 2H, J=8.6 Hz, 3′, 5′-H, ArH), 8.26(s, 1H, 2-H), 8.34 (d, 1H, J=9.6 Hz, 4-H), 9.56 (d, J=9.6 Hz, 1H, 8-H).Anal. calcd for C19H13N3O3: C, 68.87; H, 3.96; N,12.68%. Found: C, 68.84;H, 4.03; N, 12.63%.

2.2.3. 5-(4-phenylamine)-2-(3-methylazulene-1-yl)-1,3,4-oxadiazoles(Az-2)

The 5-(4-phenylamine)-2-(3-methylazulene-1-yl)-1,3,4-oxadia-zoles (Az-2) was prepared by refluxing Az-2a with iron flour in thepresence of glacial acetic acid (Scheme 1). The crude product wasconcentrated with a rotary evaporator and neutralized by 5% sodiumbicarbonate, then extracted with ethyl acetate, to give Az-2. Greenishprisms (benzene), mp 234–235 °C; IR (KBr) ν: 3328, 3056, 1643, 1569,1528 cm−1; 1H NMR (CDCl3, 300 MHz)δ: 2.70 (s, 3H, CH3), 6.91 (d, 2H,J=8.8 Hz, 3′, 5′-H, ArH), 7.29 (dd, 1H, J=9.6, 8.0 Hz, 5- or 7-H), 7.43 (t,1H, J=7.6 Hz, 6-H), 7.52 (dd, 1H, J=9.6, 9.6 Hz, 7- or 5-H), 7.91 (d, 2H,J=7.6 Hz, 2′, 6′-H, ArH), 8.30 (s, 1H, 2-H), 8.33 (d, 1H, J=9.6 Hz, 4-H),

9.57 (d,1H, J=9.6 Hz, 8-H). Anal. calcd for C19H15N3O: C, 75.73; H, 5.02;N, 13.95%. Found: C, 75.68; H, 5.13; N, 13.84%.

2.3. Assembly of azulene-based dye molecules multilayer films

The multilayer films were grown on quartz surfaces for UV–visabsorption, photoluminescence, XRR, and single crystal silicon wafersfor AFM. All solid substrates were cleaned with “piranha” solution(H2O2:H2SO4=3:7 v/v) at 80 °C for 1 h, followed by thorough rinsingwith deionized water. Further purification was carried out byimmersion of the substrates in NH4OH:H2O2:H2O (1:1:5 v/v) solutionat 70 °C for 30 min and then extensive washing with water and dryingwith N2. The cleaned substrates were hydrophilic and stored indeionized water prior to use [19].

The following solutions were used to prepare PSS/Az multilayerfilms: Aqueous PEI solution (5 mg·mL−1), aqueous PSS solution(2mg·mL−1 and containing 0.5mol·L−1 NaCl), aqueous PAH solution(2 mg·mL−1 and containing 0.5 mol·L−1 NaCl), and aqueous Azsolution (2×10−3 M, pH≈2.5).

The fabrication of the multilayer films (n=1–8) was carried outaccording to the following steps (Scheme 2). At first, a PEI/PSS/PAHprecursor (P) film was deposited onto a cleaned substrate (quartz,silicon) by immersing the substrate alternately in PEI, PSS, and PAHsolutions for 20 min each, followed by rinsing with deionized waterand drying in nitrogen after each immersion. Then the substrate-supported precursor (P) film was alternately dipped into the PSS andAz solutions for 20 min, rinsed with deionized water and dried in anitrogen stream after each dipping, respectively. The procedureresults in the build-up of the multilayer films containing Az-baseddye molecules (Az-1 and Az-2), which can be expressed as P/(PSS/Az)n, where n is the number of bilayers.

2.4. Characterization

The reactions were carried out under microwave irradiation at360 W. Thin liquid chromatography was used to monitor the progressof the reaction. All melting points were determined with a digitalWRS-1B apparatus and uncorrected. Fourier transform infraredspectroscopy (FT-IR) spectra (KBr pellets) were taken on a MagnaFT-IR 560 spectrometer and Nuclear magnetic resonance (NMR)analyses were performed on a JEOL JNM-EX 300 spectrometer. Massspectra were recorded on a HP 1100. Elemental analyses wereperformed at the Center for Instrumental Analysis, Beijing University.UV–vis absorption spectrawere recorded on a quartz slide using a UV-2550 spectrophotometer. Fluorescence spectra were performed on aHitachi F-4500 fluorescence spectrophotometer at room temperature.XRR experiments were performed with a Philips X'Pert instrumentusing Cu-Kα radiation (λ=1.5405 Å). AFM images were obtained in a

Fig. 2. UV–vis adsorption spectra of multilayer assemblies: (a) P/(PSS/Az-1)n, and(b) P/(PSS/Az-2)nwith n=0–8 (from bottom to top) grown on PEI/PSS/PAH-modifiedquartz substrates. The lowest curve corresponds to the precursor (P) film (n=0).Insets: relation between absorbance and number of cycles measured in the absorbancespectra at (a) 226, 303, 375 nm, and (b) 226, 310, 406 nm.

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commercial microscope (AJ-III Scanning Probe Microscope, ShanghaiAJ Nano-Science Development Co., Ltd.) by tapping mode withpyramidal tips and cantilevers made from etched silicon probes.

3. Results and discussion

3.1. Synthesis of Az-1 and Az-2

The functional azulene hydrazide Az-1 was prepared by thereaction of startingmaterial Az-1a and hydrazine hydrate as describedin Scheme 1. Az-1 was reacted with 4-nitro-benzoic acid inphosphorous oxychloride to yield Az-2a. Then Az-2a was deoxidizedwith iron flour in glacial acetic acid, to give Az-2.

The Az-1 and Az-2 dye molecules were further positively chargedby adjusting the pH value to ca. 2.5 with 1 M HCl for the following LBLassembling.

3.2. Assembly of Az-1 and Az-2 multilayer films

Scheme 2 shows a schematic illustration of the multilayer filmfabricated with alternating layers of PSS and Az-based dye molecules.The substrate was precoated with a (PEI/PSS/PAH) precursor film inorder to form an anionic and homogeneous surface, which leads touniform and regular adsorption of PSS polyanions and Az dye cations,and thus the reproducible layer-by-layer growth. After each dipping,the substrate must be washed with copious water to maintain thesurface hydrophilic and homogeneous.

3.3. UV–vis spectra

Fig.1 showed theUV–vis spectra of 10−6MAz-1 (blue line) andAz-2(red line) aqueous solution (pH=2.5). The absorption peaks around303, 375nmare the characteristic bands ofAz-1, and 310, 406nmare thecharacteristic bands of Az-2 dye molecules. The growth of PSS/Az thinmultilayer films formed by the sequential adsorption of PSS and Azwasexamined by UV–vis spectroscopy. Fig. 2a and b showed the UV–visspectra of (PSS/Az-1)n and (PSS/Az-2)n multilayer films (n=0–8)deposited on precursor (PEI/PSS/PAH) film-modified quartz substrate,respectively.

Since PEI and PAH does not absorb above 200 nm, the absorptionband at 226 nm is due to the benzene ring of PSS molecules in theprecursor (P) film with n=0 [20]. The characteristic absorption bandsof Az-1 around 303, 375 nm and Az-2 around 310, 406 nm appeared inthe multilayer films, which is consistent with those of their solutions. It

Fig. 1. UV–vis spectra of 10−6 M pH=2.5 Az-1 aqueous solution (blue line) and Az-2aqueous solution (red line), respectively. (For interpretation of the references to color inthis figure legend, the reader is referred to the web version of this article.)

can be clearly seen that the characteristic absorption peak intensities ofAz-1andAz-2 increasewith thenumberof PSS/Azbilayers. The insets inFig. 2a andbpresent226, 303, 375nmand226, 310, 406nmasa function

Fig. 3. Emission spectra of 10−6 M pH=2.5 Az-1 aqueous solution (blue line) excited at245 nm, and Az-2 aqueous solution (red line) excited at 262 nm, respectively. (Forinterpretation of the references to color in this figure legend, the reader is referred tothe web version of this article.)

Fig. 5. Small-angle X-ray reflectivity curve of 6-bilayer PSS/Az-1 (a), and PSS/Az-2(b) film deposited on PEI/PSS/PAH-modified quartz substrates, and PEI/PSS/PAH (c)film deposited on quartz substrates, respectively.

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of the number of deposition cycles, respectively. This linear nature of theplots also indicates that an equivalent amount of Az-based functionalmaterials are adsorbed after each deposition cycle, and further confirmsthat the deposition process is very consistent from layer to layer andhighly reproducible.

3.4. Fluorescence properties

Photoluminescence measurements were performed to study thefluorescent properties of Az aqueous solutions. The emission spectraof 10−6 M Az-1 (blue line) and Az-2 (red line) aqueous solution wereshown in Fig. 3. The S2 fluorescence emission spectra of Az-1 solutionexhibit the characteristic emission peak at ca. 381 nm upon excitationat 245 nm. The S2→S0 emission peak at ca. 374 nm upon excitation at262 nm is the characteristic emission of Az-2, with a great increase inintensity comparing with that of Az-1, which could be assigned to thecontribution of oxadiazoles ring of Az-2 molecule [21].

Photoluminescence measurements were also performed to moni-tor the assembly process of the Az-based dye molecule multilayerfilms. The fluorescence spectra of (PSS/Az-1)n and (PSS/Az-2)n filmswith the bilayer numbers from 1 to 8 on quartz slide are shown inFig. 4a and b, respectively. Characteristic emission peaks are observedat the same spectra position as the corresponding aqueous solutions,indicating that the characteristic emission of Az-based dye molecules

Fig. 4. Fluorescence spectra of P/(PSS/Az-1)n (a), and P/(PSS/Az-2)n (b) multilayerfilms with n=1–8 on PEI/PSS/PAH-modified quartz substrates, respectively. The insetsshow the intensity growth at 381 nm (λex=245 nm) (a), and 374 nm (λex=262 nm)(b) as a function of the number of bilayers, respectively.

remained after being incorporated in the LBL films. The stable growthof LBL assembly yields a linear increase of fluorescence intensity(Fig. 4 inset), which correlates to the amount of Az assembled in the

Fig. 6. Tappingmode AFM images of P/(PSS/Az-1)8 (a), andP/(PSS/Az-2)8 (b)multilayerfilms, respectively.

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PSS/Az bilayer films. Fluorescence data provide further supportingevidence of the stepwise and uniform assembly process. Theoccurrence of photoluminescent activity confirms the potential forcreating luminescent multilayer films with Az-based dye moleculesand the corresponding functional materials may be achieved byadjusting the thickness, composition, and structure of the multilayerfilms.

3.5. X-ray reflectivity measurements

XRRwas used to investigate the film structure. Fig. 5 shows the XRRcurves of a 6-bilayer film of PSS/Az-1(curve a) and PSS/Az-2(curve b)prepared onP film, respectively. The XRR curves of themultilayer filmsare considered to be a series of Kiessig fringes,which results fromX-rayinterferences from the substrate-film and film–air interfaces, suggest-ing that the film surfaces are moderately uniform and smooth. Theabsence of Bragg peaks in XRR curves indicates the absence of a lattice-like layer structure and points to entanglement between the layers.The thickness of PSS/Az bilayer in multilayer films was obtained fromXRR data. The total thickness of the films were calculated to be about10.6 and 13.2 nm for (PSS/Az-1)6 and (PSS/Az-2)6 films, respectively,from the spacing of the Kiessig fringes [22]. The thickness of theprecursor PEI/PSS/PAH film was calculated to be about 5 nm formcurve c in Fig. 5. Since the UV–vis results demonstrated that thethickness of film increased uniformly with the number of layers, theaverage thickness of one bilayer were calculated to be about 0.9 and1.4 nm for (PSS/Az-1)6 and (PSS/Az-2)6 films, respectively.

3.6. AFM images

The AFM images of the P/(PSS/Az)n (n=8) multilayer films (Az-based dye molecules as the outermost layer) were taken to providedetailed information about the surface morphology and the homo-geneity of the deposited films. Fig. 6a and b displayed an AFM image ofthe surface of the P/(PSS/Az-1)8 and P/(PSS/Az-2)8 multilayer filmson a single crystal silicon substrate, respectively. The surfaces of thesetwo LBL films are relatively uniform and smooth. The diameters of thegrains are approximately 10.6 and 11.9 nm along the horizontal axis,respectively. The rms roughness estimated by AFM are approximately1.65 and 1.85 nm for the P/(PSS/Az-1)8 and P/(PSS/Az-2)8 films,respectively. In addition, a vertical grain topography in three-dimensional AFM image present a lot of protuberant structures,which are probably composed of Az ion pairs formed by the PSSpolyanion chains and Az-based dye cations via electrostatic interac-tions [23], and closely packed with each other and uniformlydistributed in the whole films.

4. Conclusions

In this work, two Az-based dye molecules Az-1 and Az-2 weresynthesized and used to fabricate ordered thin multilayer films by LBL

method, respectively. UV–vis spectra suggest that deposition processof the multilayer films is regular and highly reproducible from layer tolayer. Average thicknesses of ca. 0.9 and 1.4 nm were determined forthe PSS/Az-1 and PSS/Az-2 bilayers, respectively. AFM imagesindicate that the film surface is relatively uniform and smooth. Thephotoluminescence properties of the P/(PSS/Az)nmultilayer films aresimilar to those of theAz solutions, which is of potential importance inthe fabrication of photoluminescent thin film incorporated Az-baseddye molecules. The preliminary results in the present paper clearlyindicate that the LBL method can be extended to the construction ofmultilayer thin films exhibiting excellent optical properties byincorporating appropriate Az-based dye molecules.

Acknowledgements

The support of the Natural Science Foundation of LiaoningProvince (Grant 20061073) and the National Natural Science Founda-tion (Grant 20871022) are gratefully acknowledged.

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