Organic−Inorganic Nanohybrids via Directly Grafting Gold

Langmuir 2008, 24, 11967-11974

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Organic-Inorganic Nanohybrids via Directly Grafting Gold Nanoparticles onto Conjugated Copolymers through the Diels-Alder Reaction Xiaofeng Liu,†,‡ Mei Zhu,†,‡ Songhua Chen,†,‡ Mingjian Yuan,†,‡ Yanbing Guo,†,‡ Yinglin Song,§,| Huibiao Liu,† and Yuliang Li*,† Beijing National Laboratory for Molecular Sciences (BNLMS), CAS Key Laboratory of Organic Solids, Center for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China, Graduate UniVersity of Chinese Academy of Sciences, Beijing 100190, P. R. China, Suzhou UniVersity, Suzhou 215006, P. R. China, and Department of Physics, Harbin Institute of Technology, Harbin 150001, P. R. China ReceiVed July 1, 2008. ReVised Manuscript ReceiVed July 24, 2008 Nanocomposites of poly-p-phenyleneethynylene gold nanoparticles (PPE-Au) were synthesized via directly grafting maleimide functionalized gold nanoparticles (MA-Au) onto PPE chains by a mild Diels-Alder reaction. The Diels-Alder reaction between copolymers and MA-Au leads to self-assembly of the MA-Au as well as enhances electronic communication between the copolymers and inorganic particles. The as-prepared hybrid nanoassemblies show homogeneous status and well-defined interfaces, which facilitate the electronic interaction between conjugated polymers and gold nanoparticles. Moreover, dramatic photophysical properties and an influence on the assembly behavior of gold nanoparticles are also exhibited, which allows this procedure to be performed as a smart assay for monitoring the process of the Diels-Alder reaction.

Introduction The design and synthesis of organic-inorganic hybrid nanostructures is of great interest and holds a significant promise in applications of catalysis, optoelectronics, and biomedical diagnostics.1-4 Metal nanoparticles exhibit intense size- and shape-dependent properties due to the surface plasmon resonance (SPR);5 thus, they are an increasing important colorimetric reporter for signifying events associated with transferring metal nanoparticles from dispersion to aggregation.6 Significant development has been made recently for functional materials by designing monolayer protected gold clusters (MPCs) and exploiting them as building blocks for supramolecular structures and sensory applications.7 The internal organization of such composite materials on the nanoscale is crucial for determining the desired properties. Hybrid nanomaterials consisting of gold nanoparticles (GNPs) and low molecular weight * To whom correspondence should be addressed. E-mail: [email protected]. † Chinese Academy of Sciences. ‡ Graduate University of Chinese Academy of Sciences. § Suzhou University. | Harbin Institute of Technology. (1) (a) Tang, Z.; Kotov, N. A. AdV. Mater. 2005, 17, 951–962. (b) Dai, Q.; Worden, J. G.; Trullinger, J.; Huo, Q. J. Am. Chem. Soc. 2005, 127, 8008–8009. (c) Wang, C.-W.; Moffitt, M. G. Chem. Mater. 2005, 17, 3871–3878. (2) (a) Boal, A. K.; Ilhan, F.; DeRouchey, J. E.; Thurn-Albrecht, T.; Russell, T. P.; Rotello, V. M. Nature 2000, 404, 746–748. (b) Alivisatos, A. P.; Johnsson, K. P.; Peng, X. G.; Wilson, T. E.; Loweth, C. J.; Bruchez, M. P.; Schultz, P. G. Nature 1996, 382, 609–611. (3) Zhou, Y.; Antonietti, M. J. Am. Chem. Soc. 2003, 125, 14960–14961. (4) (a) Lin, Y.; Emrick, T.; Dinsmore, A. D.; Russell, T. P. Science 2003, 299, 226–229. (b) Pyun, J.; Matyjaszewski, K. Chem. Mater. 2001, 13, 3436–3448. (5) (a) Daniel, M.-C.; Astruc, D. Chem. ReV. 2004, 104, 293–346. (b) Burda, C.; Chen, X.; Narayanan, R.; El-Sayed, M. A. Chem. ReV. 2005, 105, 1025–1102. (6) (a) Klajn, R.; Bishop, K. J. M.; Grzybowski, B. A. Proc. Natl. Acad. Sci. U.S.A. 2007, 104, 10305–10309. (b) Shen, Z.; Yamada, M.; Miyake, M. J. Am. Chem. Soc. 2007, 129, 14271–14280. (c) Drechsler, U.; Erdogan, B.; Rotello, V. M. Chem.sEur. J. 2004, 10, 5570–5579. (7) (a) Ipe, B. I.; Yoosaf, K.; Thomas, K. G. J. Am. Chem. Soc. 2006, 128, 1907–1913. (b) Oh, E.; Hong, M.-Y.; Lee, D.; Nam, S.-H.; Yoon, H. C.; Kim, H.-S. J. Am. Chem. Soc. 2005, 127, 3270–3271. (c) Ray, P. C.; Fortner, A.; Darbha, G. K. J. Phys. Chem. B 2006, 110, 20745–20748.

ligands5a,8 or nonconjugated polymers2a,9 have been explored intensively. However, only a few examples on conjugated polymer (CP) coated GNPs are known due to difficult control over the self-organization properties and morphologies.10 CPs have been investigated intensively in recent decades due to their remarkable electronic and optical properties.11 Therefore, the combination of gold particles and conjugated polymers for control in producing new structured GNPs may include either new or improved chemical and physical properties that can be exploited for fabricating novel nanoscale devices. Developing a methodology capable of efficiently and reproducibly fabricating CPs-GNP hybrid materials remains an important challenge in nanotechnology. Current trends for preparation of CPs-GNP composites mainly include mixing these two components or constructing them either physically or chemically. Results show that it is hard to control the detailed morphologies as well as the dispersion of GNPs within CP networks. The interface between CPs and GNPs, formed during film processing, is not well-defined, thereby depressing the electronic interaction efficiency between these two components. This implies that CP-GNP composites with homogeneous phases and well-controlled interfaces are favorable (8) (a) Templeton, A. C.; Wuelfing, W. P.; Murray, R. W. Acc. Chem. Res. 2000, 33, 27–36. (b) El-Sayed, M. A. Acc. Chem. Res. 2001, 34, 257–264. (9) (a) Chiu, J. J.; Kim, B. J.; Kramer, E. J.; Pine, D. J. J. Am. Chem. Soc. 2005, 127, 5036–5037. (b) Lee, H.; Choi, S. H.; Park, T. G. Macromolecules 2006, 39, 23–25. (c) Mandal, T. K.; Fleming, M. S.; Walt, D. R. Nano Lett. 2002, 2, 3–7. (d) Frankamp, B. L.; Uzun, O.; Ilhan, F.; Boal, A. K.; Rotello, V. M. J. Am. Chem. Soc. 2002, 124, 892–893. (10) (a) Sih, B. C.; Wolf, M. O. Chem. Commun. 2005, 3375–3384. (b) Ozawa, H.; Kawao, M.; Tanaka, H.; Ogawa, T. Langmuir 2007, 23, 6365–6371. (11) Skotheim, T. A.; Elsenbaumer, R. L.; Reynolds, J. R. Handbook of Conducting Polymers; Marcel Dekker: New York, 1998. (12) (a) Chan, E. W. L.; Lee, D.-C.; Ng, M.-K.; Wu, G.; Lee, K. Y. C.; Yu, L. J. Am. Chem. Soc. 2002, 124, 12238–12243. (b) Cho, S. H.; Park, S.-M. J. Phys. Chem. B 2006, 110, 25656–25664. (c) Nakashima, H.; Furukawa, K.; Ajito, K.; Kashimura, Y.; Torimitsu, K. Langmuir 2005, 21, 511–515. (d) Brinkmann, M.; Pratontep, S.; Chaumont, C.; Wittmann, J.-C. Macromolecules 2007, 40, 9420–9426.

10.1021/la8020639 CCC: $40.75  2008 American Chemical Society Published on Web 08/30/2008

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Scheme 1. Schematic Illustration for the Fabrication of PPE-GNP Composites via the Diels-Alder Reaction

for electronic communication and charge transportation, which is now difficult to achieve by utilizing conventional blending approaches.12 The Diels-Alder reaction is one of the most important reactions for carbon-carbon bond formation, which has been extendedly used for the preparation of thermally responsive polymers, dendrimers, and biocompatible materials.13-15 It has also been found useful as a means to modify self-assembled monolayers (SAMs) and silicon surfaces.16,17 Considering the facile and thermally reversible formation process, the Diels-Alder reaction is expected to lead to development of new methodologies for fabricating novel organic-inorganic hybrid nanomaterials. More recently, Workentin and co-workers have provided an efficient route for the preparation of maleimide modified GNPs via a thermally activated retro-Diels-Alder reaction, which enabled further chemical modifications.18 Taken together, we are interested in the physical properties of assemblies in which GNPs are linked covalently to conjugated copolymers via (13) (a) Chen, X.; Dam, M. A.; Ono, K.; Mal, A.; Shen, H.; Nutt, S. R.; Sheran, K.; Wudl, F. Science 2002, 295, 1698–1702. (b) Gotsmann, B.; Duerig, U.; Frommer, J.; Hawker, C. J. AdV. Funct. Mater. 2006, 16, 1499–1505. (c) Bailey, G. C.; Swager, T. M. Macromolecules 2006, 39, 2815–2818. (14) (a) Shi, Z.; Hau, S.; Luo, J.; Kim, T.-D.; Tucker, N. M.; Ka, J.-W.; Sun, H.; Pyajt, A.; Dalton, L.; Chen, A.; Jen, A. K.-Y. AdV. Funct. Mater. 2007, 17, 2557–2563. (b) Szalai, M. L.; McGrath, D. V.; Wheeler, D. R.; Zifer, T.; McElhanon, J. R. Macromolecules 2007, 40, 818–823. (15) Shi, M.; Wosnick, J. H.; Ho, K.; Keating, A.; Shoichet, M. S. Angew. Chem., Int. Ed. 2007, 46, 6126–6131. (16) (a) Gawalt, E. S.; Mrksich, M. J. Am. Chem. Soc. 2004, 126, 15613– 15617. (b) Dillmore, W. S.; Yousaf, M. N.; Mrksich, M. Langmuir 2004, 20, 7223–7231. (17) Wang, Y.; Cai, J.; Rauscher, H.; Behm, R. J.; Goedel, W. A. Chem.sEur. J. 2005, 11, 3968–3978. (18) (a) Zhu, J.; Kell, A. J.; Workentin, M. S. Org. Lett. 2006, 8, 4993–4996. (b) Zhu, J.; Ganton, M. D.; Kerr, M. A.; Workentin, M. S. J. Am. Chem. Soc. 2007, 129, 4904–4905. (c) Zhu, J.; Lines, B. M.; Ganton, M. D.; Kerr, M. A.; Workentin, M. S. J. Org. Chem. 2008, 73, 1099–1105.

thermally reversible bonding processes. Herein, we present a facile pathway for the fabrication of CP-GNP nanohybrids through a mild Diels-Alder reaction as outlined in Scheme 1. The as-prepared hybrid nanoassemblies show homogeneous status and well-defined interfaces, which facilitate the electronic interaction between CPs and GNPs. Moreover, dramatic photophysical properties and an influence on the assembly/disassembly behavior of GNPs are also exhibited, which allows this procedure to be performed as a smart assay for monitoring the process of the Diels-Alder reaction.

Results and Discussion Synthesis and Characterization of CPs. Conjugated copolymers PPE-F1-3 with different contents of the furan pendanthavebeensynthesizedviaastandardSonogashira-Hagihara polycondensation reaction (see the Supporting Information). The furan ratios of copolymers PPE-F1, PPE-F2, and PPE-F3 have been calculated to be 10%, 40%, and 50%, respectively. The as-obtained copolymers are soluble in most common organic solvents with a polydispersity index (PDI) ranging from 1.44 to 1.94 (see the Supporting Information for details). The photophysical properties of copolymers PPE-F1-3 have been determined (Figure 1) and summarized as shown in Table 1. The maximum absorption peaks are located at 438, 432, and 437 nm for PPE-F1, PPE-F2, and PPE-F3, respectively, which are mainly due to π-π* transitions of the conjugated main chains. Meanwhile, the maximum emission peaks are almost the same at 474 nm, indicating that the pendant furan group has nearly no affect on the photophysical properties of conjugated main chains. Synthesis and Characterization of Maleimide Modified GNPs. The maleimide modified GNPs (MA-Au) were synthesized following a modified literature method.18a Mercaptododecane (MD)-Au underwent a ligand-exchange

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Figure 1. Chemical structure of PPE-F1-3 (left) and corresponding UV-vis absorption spectra and fluorescence emission spectra (right) with an excitation wavelength of 430 nm.

Figure 2. (A) UV-vis absorption spectra, TEM images of (B) MD-Au and (C) MA-Au, and corresponding size distribution histograms of (b) MD-Au and (c) MA-Au. Both scale bars represent 10 nm.

process followed by thermal retro-Diels-Alder reaction to afford MA-Au. A detailed description of the synthesis procedure of MA-Au is outlined in the Supporting Information. The as-prepared MA-Au was confirmed by 1H NMR, UV-vis spectroscopy, and transmission electron microscopy

(TEM). After the ligand-exchange reaction, the broadened peaks from surface-attached masked maleimide ligand can be clearly observed in the 1H NMR spectrum. The chemical shift at 5.25 and 2.83 disappeared after thermal treatment, indicating a loss of the furan group (see the Supporting Information). UV-vis

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Figure 3. UV-vis spectra (A, B, and C for PPE1, PPE2, and PPE3, respectively) of copolymers PPE-F1-3 (black curve) and of mixtures of PPE-F1-3 and modified GNPs after reaction for 2 h (red curve) and 5 days (blue curve). TEM images (a-c) of corresponding samples obtained after reaction for 5 days. Scale bar represents 20 nm.

spectra of GNPs before and after ligand replacement are shown in Figure 2A; the surface plasmon (SP) band red-shifts from 519 to 523 nm, indicating a slight agglomeration due to ligand exchange, which was clearly confirmed by TEM observation. Figure 2B and C shows typical high resolution TEM (HRTEM) images of MD-Au and MA-Au, respectively. MD-Au exhibits randomly dispersed behavior with 5.5 ( 1.5 nm in diameter. However, replacement of MD partly with MA on GNPs leads to nearly hexagonal arrangement with narrower size distribution at 5.5 ( 1.0 nm (Figure 2b and c). The reason might be the enhanced driving force arising from additional π-π interactions between maleimide ligand, which become dominant over van der Waals interactions of alkyl chains after ligand replacement. This is supposed to be a further reason for the slight red-shift of the SP band after ligand exchange. It is also noted that the as-prepared MA-Au is stable either in solution or in the solid state for months, which allows additional chemical modifications and characterizations. Fabrication of PPE-Au Nanohybrids. Chemically attached functionalized conjugated oligomers or polymers onto functionalized nanoparticles have been a common approach to modify nanoparticles (NPs), such as gold NPs5a,19 and semiconductor NPs.20,21 The key is to first synthesize both NPs and conjugated ligands with compatible functional groups, which allow them to react with each other under relatively mild conditions without sacrificing the stability and photophysical properties of each component.21b In our study, furan pendant (19) Zubarev, E. R.; Xu, J.; Sayyad, A.; Gibson, J. D. J. Am. Chem. Soc. 2006, 128, 15098–15099. (20) (a) Lin, Z. Chem.sEur. J. 2008, 14, 6294–6301. (21) (a) Skaff, H.; Sill, K.; Emrick, T. J. Am. Chem. Soc. 2004, 126, 11322– 11325. (b) Xu, J.; Wang, J.; Mitchell, M.; Mukherjee, P.; Jeffries-EL, M.; Petrich, J. W.; Lin, Z. J. Am. Chem. Soc. 2007, 129, 12828–12833. (c) Zhang, Q.; Russell, T. P.; Emrick, T. Chem. Mater. 2007, 19, 3712–3716. (d) Girolamo, J. D.; Reiss, P.; Pron, A. J. Phys. Chem. C 2007, 111, 14681–14688. (e) Querner, C.; Reiss, P.; Bleuse, J.; Pron, A. J. Am. Chem. Soc. 2004, 126, 11574–11582. (f) Locklin, J.; Patton, D.; Deng, S.; Baba, A.; Millan, M.; Advincula, R. C. Chem. Mater. 2004, 16, 5187–5193.

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CPs were synthesized via typical Sonogashira-Hagihara polycondensation, while maleimide modified GNPs were obtained by ligand replacement and thermal treatment. Subsequently, PPE-Au nanocomposites were prepared via a mild Diels-Alder reaction of furan-pendant PPE side chains with MA-Au. Briefly, solutions of series of copolymers PPE-F (5 × 10-5 M) are mixed with MA-Au (2.3 × 10-9 M) in chloroform (see the Supporting Information for a detailed synthesis procedure). The mixtures are led to react at ambient conditions, avoiding light, while the changes over time are monitored. In such a system, the conjugated copolymers act as electron donors as well as soft templates for MA-Au self-assembly, while MA-Au acts as an electron acceptor or fluorescence quencher with maleimide as the active site for performing the Diels-Alder reaction. Unfortunately, the formed PPE-F-Au nanocomposites cannot be directly characterized by 1H NMR spectroscopy because only a small fraction of the maleimide and furan end groups on the nanoparticles and PPE-F need to undergo a Diels-Alder reaction before the formation of PPE-F-Au, which appear to aggregate and begin to precipitate out of the solution. However, UV-vis absorption spectroscopy and TEM provide more direct insight for the Diels-Alder reaction induced covalent assembly between PPE-F and MA-Au, as illustrated in Figure 3. At the early stage (e.g., 2 h) of the Diels-Alder reaction, the surface plasmon (SP) bands of all samples remain unchanged located at 526 nm, while the SP bands of different samples change time-dependently (Figure 3A-C) as the reaction proceeds. For PPE-F1-Au with less furan content, the SP band remained unchanged even after 5 days of reaction. For PPE-F2-Au and PPE-F3-Au with more furan content, the SPs band red-shifted from 526 to 564 and 614 nm, respectively. TEM observation provides further evidence for different samples of PPE-Au nanocomposites as shown in Figure 3a-c. Different types of self-assemblies occurred along with the Diels-Alder reaction. With lower furan content PPE, GNPs remained discrete, while with higher furan content PPE GNPs appeared to form large-scale aggregation. Thus, they exhibit furan-content-dependent behaviors after efficient reaction times, which further confirms the Diels-Alder reaction between furanpendant PPEs and maleimide functionalized GNPs. It is clear that there is a covalent reaction between PPE-F and MA-Au, because PPE-F is mixed with MD-Au without maleimide which is unable to result in molecular aggregation. (data not shown). Moreover, the distinct changes in absorption can be observed under ambient conditions, which provide a more easy access manner for monitoring the process of the Diels-Alder reaction. We have examined the effect of the molar ratio (r) between MA-Au and conjugated copolymers PPE-F. As shown in Figure 4A, by mixing different r of MA-Au and PPE-F in chloroform after 5 days of reaction, the fluorescence intensity of each sample changes dramatically. In accordance with that observed in the time-dependent experiment, higher furan content results in enhanced fluorescence quenching efficiencies. For high r values of MA-Au and PPE-F, part of the fluorescence quenching arises from long-range energy transfer as compared with that of low r, which mainly resulted from the electron transfer process. On the basis of previous research, it is known that both charge transfer and energy transfer play a role in quenching in fluorophore-gold particle nanoassemblies.22 For GNPs lager than 2 nm, the intense SP absorption band located at >500 nm, which overlaps significantly with polymer emission, guarantees efficient energy transfer. Conversely, for GNPs smaller than 2 nm, electron transfer may also contribute to the fluorescence (22) Fan, C.; Wang, S.; Hong, J. W.; Bazan, G. C.; Plaxco, K. W.; Heeger, A. J. Proc. Natl. Acad. Sci. U.S.A. 2003, 100, 6297–6301. (23) Thomas, K. G.; Kamat, P. V. Acc. Chem. Res. 2003, 36, 888–898.

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Figure 4. (A) Fluorescence quenching efficiency (QE) of conjugated copolymer PPE-F1 (blue curve), PPE-F2 (red curve), and PPE-F3 (black curve) versus r of MA-Au and conjugated copolymers PPE-F after 5 days of reaction. (B) Fluorescence emission spectra that compare the difference between chemical bound complexes PPE-F3-Au (dotted curve) and the physical mixture PPE-F3/Au (dashed curve) after 3 days of reaction. All samples in B were prepared using the same r ) 1.2 × 10-3 of PPE-F3 and GNPs.

Figure 5. TEM images of PPE-F3-Au nanocomposites at increasing r (A, 1.0 × 10-4; B, 5 × 10-4; C, 1.2 × 10-3; D, 2.4 × 10-3) of PPE-F3 and MA-Au. Scale bar is 40 nm for A and B, and 100 nm for C and D.

quenching. In our experiment, we suggest that, with the formation of PPE-F-Au nanocomposites, energy transfer as well as electron transfer result in fluorescence quenching. At low r of MA-Au and PPE-F, the Diels-Alder reaction can proceed efficiently. Decreased distance between the polymer and gold particles enables excited-state electron transfer between the two components. Increasing r of MA-Au and PPE-F results in deficient cycloaddition between maleimide and the furan pendant. Thus, long-range energy transfer dominates the fluorescence quenching.

TEM images cast from nanocomposites of different r of MA-Au and PPE-F3 provide additional information on the effect of the molar ratio. For relatively low r value (1.0 × 10-4), the GNPs remain dispersed after 5 days of reaction, with a slight effect on the polydispersity (Figure 5A). At r ) 5.0 × 10-4, the functionalized GNPs start to form loose aggregates (Figure 5B). On increasing r value up to 1.2 × 10-3, large-scale GNP self-assembly can be observed (Figure 5C). Furthermore, at higher r value (2.4 × 10-3), bulklike aggregation partly occurs

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Figure 6. Fluorescence quenching efficiency (QE) of conjugated copolymers PPE-F1 (blue curve), PPE-F2 (red curve), and PPE-F3 (black curve) in the presence of MA-Au versus the reaction time of the Diels-Alder reaction in chloroform.

(Figure 5D). By varying the molar ratio of MA-Au and PPEF3, we can conclude that both higher and lower molar ratios result in deficient interaction between PPE-F3 and MA-Au. For relatively low r values, excess MA-Au was well-encapsulated by PPE-F3, which prevents GNPs from aggregation. However, the electronic interaction between the two components is relatively low with little quenching of the fluorescence. While, for higher r value, MA-Au was cross-linked through PPE-F3 to form a large bulk of aggregation. The electronic interaction is more efficient, but the composite is not homogeneous, which precludes them from being applied in optoelectronic applications.

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To probe the impact of constraining the NPs along the CPs main chain as well as difference of photophysical properties between PPE-F-Au nanocomposites (chemically attaching) and PPE-F/Au composite (physical mixing), a composite of PPE-F3/Au with same r as PPE-F3-Au was prepared to serve as a control, which containing MD-Au that lacking of maleimide moiety. Figure 4B shows the emission spectra of the PPE-F3/Au composite and PPE-F3-Au nanocomposite in chloroform after 3 days of reaction. For the PPE-F3/Au composite in chloroform solution, the emission intensity of PPE-F3 maintained because of deficient interaction between two subunits due to phase separation of CPs and NPs. It should be pointed out here that physical mixture of fluorophore and GNPs also can resulted in feeble fluorescence quenching, which mainly due to excited-state energy transfer from organic molecules to GNP core.23 To elucidate the formation process of PPE-Au nanocomposites, time-dependent fluorescence changes and TEM were carried out at the molar ratio (r) of 1.2 × 10-3 between MA-Au and PPE-F. Figure 6 shows fluorescence changes of PPE-F as a function of reaction time. The conjugated copolymers PPE-F exhibit intense fluorescence emission before complexation with MA-Au. At the early stage after the copolymers mixed with MA-Au, the fluorescence of the copolymers is relatively high and MA-Au remains dispersed. As the Diels-Alder reaction proceeds between the furan pendant copolymers and MA-Au, the fluorescence of the conjugated copolymers PPE-F is progressively quenched by MA-Au due to covalent linkage of GNP pendants on copolymers and the self-assembly of MA-Au occurs concomitantly. The fluorescence quenching efficiency (QE) from the electron donor PPE-F to the electron acceptor

Figure 7. TEM images of PPE-F3-Au nanocomposites at different reaction times (A, 2 h; B, 1 day; C, 5 days) of PPE-F3 and MA-Au. (D) HRTEM image of sample C. Scale bar is 50 nm for A-C and 10 nm for D.

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Figure 8. Normalized transmission curves of Z-scan data without (A) and with (B) an aperture in the far-field (linear transmission of s ) 0.13) for PPE-F1-Au (blue dots), PPE-F2-Au (red dots), and PPE-F3-Au (black dots) in chloroform using 4 ns laser pulses at 532 nm. The corresponding solid curves are the best theoretical fitting lines. Table 1. Photophysics for Copolymers PPE-F1-3 (ε, Extinction Coefficient; QY, Quantum Yield) copolymer

UV λmax [nm]

FL λmax [nm]

ε × 104 [M-1 · cm-1]

QY [%]

438 432 437

474 473 474

3.41 2.57 3.31

26.4% 28.0% 27.2%

PPE-F1 PPE-F2 PPE-F3

Table 2. Third-Order Optical Nonlinear Parameters of the Samples in Chloroform at 532 nma samples

E (µJ)

T0

PPE-F1-Au PPE-F2-Au PPE-F3-Au

9.91 9.85 9.85

0.59 0.54 0.52

γ (×10-18 m2/W) 5.20 10.7 12.0

β (×10-12 m/W) 105 150 250

a E ) input energy, T0 ) linear transmission index, γ ) effective thirdorder refractive index, and β ) nonlinear absorption coefficient.

MA-Au is calculated according to a previous report.24 For PPEF2 and PPE-F3 with higher furan content, the QE value increases more intensely, by about 28% and 40%, respectively, after 5 days of reaction. For PPE-F1 with less furan pendants, the QE increases slightly with time, reaching a plateau of 17% at most. These results clearly indicate that more furan content in the conjugated copolymers leads to a more efficient Diels-Alder reaction, thus leading to more efficient fluorescence quenching (i.e., electronic interaction). Moreover, the fluorescence changes of such a system can act as a platform for monitoring the process of the Diels-Alder reaction with different furan content systems. Figure 7 shows TEM images obtained at different stages of the Diels-Alder reaction between furan-rich PPE-F3 and MA-Au. The PPE-F3-Au nanocomposite presents slight aggregation after 2 h of reaction. With the reaction proceeding for 1 day, particle aggregation is enhanced, while after 5 days of reaction the nanocomposite forms densely packed aggregation. However, both time scale samples exhibit homogeneous and well-defined interfaces and well dispersed PPE-F-Au nanocomposites. The HRTEM image in Figure 7D exhibits homogeneous void spaces and GNP arrangement. It mainly arises from covalent bonding between PPE-F and MA-Au, in which the encapsulation of GNPs with PPE-F prevents them from bulky aggregation. Such a well-defined interface and homogeneous nanocomposite may find potential use in solid-film photoelectronic devices.

Significantly, the as-synthesized PPE-Au nanohybrids show pronounced third-order nonlinear optical properties. Both the nonlinear absorption and refractive effects of three sample solutions were measured in chloroform at the same r value (1.2 × 10-3, after 5 days of reaction) by the normalized energy transmission Z-scan technique (detailed information is outlined in the Supporting Information). The open and close aperture Z-scan curves of polymers are shown in Figure 8. It can be easily seen that all samples exhibit reverse-saturable nonlinear absorption and self-focusing behavior as revealed in the valley-peak shaped curves. The third-order optical nonlinear parameters obtained through the theoretical fitting curves are summerized in Table 2. These data reveal that the third-order nonlinear optical properties were enhanced with the increase of furan content (i.e., contents of grafted GNPs) of the polymers in the nanohybrids, while there is no similar nonlinear effect observed from intrinsic PPE-F. On the basis of previous research, the nonlinear scattering can be induced by the light-excited SPR in the GNPs, which interacted with the designed ligands.25,26 In addition, the frequency of the incident pulses is within the SPR spectral range of the hybrids. Thus, it is believed that the nonlinear optical response of the sample is ascribed to the light-excited SPR in the PPE-Au hybrids in which PPE main chains interacted with covalently grafted GNPs. Therefore, the optical nonlinearities could be highly enhanced with efficient interactions between the polymer and metal nanoparticles.

Conclusion In summary, we have developed a new strategy for the fabrication of hybrid nanomaterials of conjugated copolymers and grafted functionalized GNPs via the Diels-Alder reaction. The conjugated copolymer directed the self-assembly of functionalized GNPs to form large superstructures after covalently linking to copolymers pendantly. The grafting method is easily accessed, and there is no need for ligandexchange chemistry. The resulting PPE-Au nanocomposites (24) He, F.; Tang, Y.; Yu, M.; Wang, S.; Li, Y.; Zhu, D. AdV. Funct. Mater. 2007, 17, 996–1002. (25) Link, S.; El-Sayed, M. A. J. Phys. Chem. B 1999, 103, 8410–8426. (26) (a) Qu, S.; Song, Y.; Du, C.; Wang, Y.; Gao, Y.; Liu, S.; Li, Y.; Zhu, D. Opt. Commun. 2001, 196, 317–323. (b) Galletto, P.; Brevet, P. F.; Girault, H. H.; Antoine, R.; Broyer, M. Chem. Commun. 1999, 581–582. (c) Vance, F. W.; Lemon, B. I.; Hupp, J. T. J. Phys. Chem. B 1998, 102, 10091–10093. (d) Fang, H.; Du, C.; Qu, S.; Li, Y.; Song, Y.; Li, H.; Liu, H.; Zhu, D. Chem. Phys. Lett. 2002, 364, 290–296.

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exhibit homogeneous and well-defined interfaces, which provides a novel pathway to achieve uniform composites, thereby promoting the efficient electronic interfacial interaction between the two constituents. Meanwhile, pronounced optical nonlinearities can be observed with nanohybrid formation. Such organic-inorganic nanohybrids may hold promise for investigating electron interaction and fabricating novel nanoscale device applications. Acknowledgment. This work was supported by the National Nature Science Foundation of China (20531060 and 20571078,

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20721061) and the National Basic Research 973 Program of China (Grant No. 2006CB932100, 2007CB936401 and 2005CB623602). Supporting Information Available: Detailed synthetic procedure for the preparation of PPE-F1, PPE-F2, PPE-F3, MD-Au, and MA-Au; additional TEM images of MD-Au, MA-Au, and PPE-F-Au nanocomposites. This material is available free of charge via the Internet at http://pubs.acs.org. LA8020639