JOURNAL OF MATERIALS SCIENCE LETTERS18 (1999 )1539– 1541

Organic-inorganic hybrid light-emitting composites: Poly(p-phenylene

vinylene) intercalated clay nanoparticles

B. WINKLER, L. DAI , A. W.-H. MAUCSIRO Molecular Science, Bag 10, Clayton South, Victoria 3169, AustraliaE-mail: Liming.Dai@molsci.csiro.au

Poly(p-phenylene vinylene) (PPV) and its derivativeshave been intensively studied since the first reporton polymeric light emitting diodes (LEDs) in 1990[1]. Owing to their relatively high photo-/electro-luminescence (PL/EL) quantum efficiencies [2] andgood color tunability through molecular engineering[3], PPV and its derivatives are among the mostattractive materials for EL applications [4–7]. Wehave recently prepared PPV derivatives substitutedwith oligo(ethylene oxide) side chains (EO-PPVs)[8] for use in both LEDs and light emitting elec-trochemical cells (LECs) [9, 10]. It was found thatthe covalent linkages between the EO and PPV con-stituents can effectively minimize the phase separationproblem often associated with conventional LEC de-vices based on mixtures of conjugate light-emittingpolymers (e.g. poly[2-methoxy-5-(2′-ethyl-hexyloxy)-p-phenylene vinylene], MEH-PPV) and polymeric ionconductors (e.g. poly(ethylene oxide), PEO) [9–11].

Organic-inorganic hybrid composites constitute anew class of materials, which could show propertiescharacteristic of both constituent components with po-tential synergetic effects [12–14]. In this context, cer-tain electroluminescent organic-inorganic hybrid ma-terials have been prepared by sol-gel chemistry [15–18] while various polymers including poly(ethyleneoxides) [19, 20], poly(olefines) [21], polyimide [22],polypyrrole [23], and polyaniline [24] have been in-corporated into clay particles through either a solutionor a melt intercalation process [19, 25]. The organic-inorganic hybrid composites thus prepared have beendemonstrated to show improved environmental stabil-ity, mechanical strength, and lower permeability forgases (e.g., O2) with respect to corresponding purepolymers [15–25]. In this letter, we report the first solu-tion intercalation of EO3-PPV into clay nanoparticlesfor light-emitting applications. The molecular structureof EO3-PPV and the intercalation process are schemat-ically shown in Scheme 1a and b, respectively.

The synthesis and characterization of poly[1,4-(2,5-bis(1,4,7,10-tetraoxaundecyl))phenylene vinylene](EO3-PPV) has been described in detail elsewhere [8].The polymer sample used in this study has a weight-average molecular weightMw= 220000 g/mol andmolecular weight distribution indexMw/Mn= 1.8. Anorganically-modified smectic clay (Bentone 34, B34,from Rheox), consisting of multilayers of negativelycharged silicate sheets (<800× 800×2.5 nm) com-pensated by organic-cations (e.g. dimethyldioctadecyl

ammonium) homogeneously distributed within theclay galleries, was used for the solution-intercalationof EO3-PPV. In a typical experiment, 160 mg B34 wasadded to a homogeneous solution of 16 mg EO3-PPVin 2 mL CHCl3 and the mixture was stirred overnightat room temperature. The resulting polymer-clay com-posites were characterized by various spectroscopictechniques after having been thoroughly rinsed withpure CHCl3 and dried in vacuum at ca. 50◦C untila constant weight was obtained. The total polymerloading for each of the samples was calculated by itsweight grain and cross-checked by thermal gravimetricanalyses (TGA, Mettler TG50).

Powder X-ray diffraction (XRD) was carried outon an X-ray spectrometer Rigaku I (CuKα, λ =0.154 nm). Ultraviolet-visible (UV-vis) measurementswere made on a Cary 5E UV/VIS/NIR spectrophoto-meter. PL spectroscopy was recorded on a luminesce-nce spectrometer SL 50 (Perkin-Elmer). EL emissionswere measured by an EG&G Model 1460 optical mul-tichannel analyzer and the corresponding diode char-acteristics were determined using a Hewlett-Packard6113A DC power supply coupled with a Fluke 45 DualDisplay Multimeter. All above measurements weremade under ambient conditions (20◦C).

Fig. 1 shows the XRD profiles for B34 before andafter the polymer intercalation. As the intercalationproceeded, a steady increase in the height of the claygalleries from ca. 2.89 nm (B34) to ca. 3.69 nm (61%

Figure 1 Powder X-ray diffraction data collected for EO3-PPV inter-calated B34 nanoparticles with different percentage contents of thepolymer.

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Figure 2 Photoluminescent emissions from the EO3-PPV intercalatedB34 nanoparticles at different intercalation levels (λEX= 475 nm), 22%(——); 8% (· · · · · ·); 0.5% (- - -). The inset shows the dependence of thePL peak position (λmax) on the polymer content.

EO3-PPV in B34) was observed, as indicated by thecorresponding shift of the (0 0 1) diffraction peak from2θ = 3.05 to 2.39◦. This was accompanied by a colorchange of the polymer-intercalated clay nanoparticlesfrom yellow (0.5% EO3-PPV) to red luminescent (>5%EO3-PPV). The PL spectra of the composites are givenin Fig. 2. As can be seen, the PL emission shows a red-shift with increasing intercalation level. Included in theinset of Fig. 2 is the dependence of the PL peak position(λmax) on the polymer content, which shows that theλmax continuously increases with increasing polymercontent to a limiting value of ca. 610 nm, characteristicof EO3-PPV [8], at ca. 20% (w/w), then, remains un-changed despite further intercalation. The above fea-tures could be attributed to an intercalation-inducedconformational transition from a “compact coil” to “ex-panded coil” (See Scheme 1b) [26–28], which shouldlead to an increase in the effective conjugation length.Luminescent color tuning by adjusting the length ofπ -conjugated segments has been previously demonstrated[29, 30]. Therefore, the intercalation provides an addi-

Scheme 1(a) Molecular structure of EO3-PPV. (b) A schematic repre-sentation of the intercalation process.

Figure 3 Electroluminescent emissions from a single layer LED devicewith the structure ITO/EO3-PPV-B34(0.95 : 0.05)/Al. The inset showsthe corresponding current-voltage (I-V) characteristics of the LED de-vice.

tional means for PL/EL color tuning associated withthe organic-inorganic hybrid composites.

Our preliminary results from a single layer LED de-vicewith thestructure ITO/EO3-PPV-B34 (0.95 : 0.05)/Al showed a similar EL spectrum (λmax= 590 nm,Fig. 3) to the PL emission. As expected, the similar-ity between the EL and PL emissions suggested thatthe same singlet excitons were generated upon both thePL- and EL-excitation [4–7]. Furthermore, the LEDshowed a typical diode characteristic with a turn-onvoltage of ca. 8 V at the forward bias (Inset of Fig. 3).These results indicate that the organic-inorganic hy-brid composites based on EO-PPVs intercalated claynanoparticles may be a class of very promising newmaterials of good color-tunability and environmentalstability for both the PL and EL applications.

AcknowledgmentsThe financial support of theAustrian Science Fund,Vienna, for B.W. is gratefully acknowledged (FWFProject J-1426-CHE). We also thank Ms. Shirley Shen,Dr. Y. B. Cheng, and Dr. G. Simon at the Departmentof Materials Engineering, Monash University, Clayton,for assistance in XRD spectroscopy.

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Received 2 Apriland accepted 26 May 1999

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