Aggregation-Enhanced Emission in Gold Nanoparticles Protected by

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Aggregation-Enhanced Emission in Gold Nanoparticles Protected by Tetradentate Perylene Derivative Jing Lv,†,‡ Yingjie Zhao,†,‡ Guoxing Li,†,‡ Yongjun Li,† Huibiao Liu,† Yuliang Li,*,† Daoben Zhu,† and Shu Wang† † CAS Key Laboratory of Organic Solids, Beijing National Laboratory for Molecular Sciences (BNLMS), Institute of Chemistry Chinese Academy of Sciences, Beijing 100190, P.R. China, and ‡Graduate School of Chinese Academy of Sciences, Chinese Academy of Sciences, Beijing 100190, P.R. China

Received April 3, 2009. Revised Manuscript Received August 12, 2009 Three novel gold nanoparticle (AuNPs) composites protected by perylene bisimide derivatives have been designed and synthesized. Thanks to the rational molecular design, AuNPs capped by N,N0 -2,6-bis(4-aminomethylpyridine)1,6,7,13-tert-[4-(hydroxymethyl)phenoxy-5-(1,2-dithiolan-3-yl)pentanoate]-3,4,9,10- tetracarboxylic acid bisimide (8SP-AuNPs) exhibited unusual enhancement in fluorescence intensity, while the other two nanoparticle composites showed slight enhancement or quench in emission spectra. The structural effect of ligands on aggregation enhanced emission had been studied, and temperature dependence experiments had been conducted. Deductive explanation had been made to elucidate the special spectral behavior of 8SP-AuNPs led by restricted motion of perylene chromophores in the packed nanoculsters.

Introduction Gold nanoparticles coated by an alkanethiol monolayer have attracted a great deal of attention due to their unique optical and electronic properties that can be controlled by the particle size as well as the nature of the protecting molecules.1-5 Many ligandfunctionalized metal nanoparticles have been reported based on ligation using chemical affinity of organic functional groups toward the nanoparticle surface to stabilize the nanoparticles, and many nanoscale devices such as memories, sensors, and catalysts have been created by this powerful surface modification method.6-11 Combination of conjugated organic chromophores and AuNPs has emerged as one of the most exciting areas of scientific endeavor in this decade, which is able to produce new class organic/inorganic hybride nanomaterials having distinct structure, photophysical, and photochemical properties that were not observed in the individual component. This may include either new or improved chemical and physical properties that can *Corresponding author. E-mail: [email protected]. (1) Daniel, M. C.; Astruc, D. Chem. Rev. 2004, 104, 293–346. (2) En€ust€un, B. V.; Turkevich, J. J. Am. Chem. Soc. 1963, 85, 3317–3328. (3) He, X. R.; Liu, H. B.; Li, Y. L.; Wang, S.; Li, Y. J.; Wang, N.; Xiao, J. C.; Xu, X. H.; Zhu, D. B. Adv. Mater. 2005, 17, 2811–2815. (4) Schmid, G. Chem. Rev. 1992, 92, 1709–1727. (5) Gross, S. Colloidal Dispersion of Gold Nanoparticles in Material Synthese; Springer-Verlag: Wien, 2008. (6) (a) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Chem. Rev. 2005, 105, 1103–1170. (7) Teranishi, T.; Kiyokawa, I.; Miyake, M. Adv. Mater. 1998, 10, 596–599. (8) Li, H. K.; Huang, J. H.; Lv, J. H.; An, H. J.; Zhang, X. D.; Zhang, Z. Z.; Fan, C. H.; Hu, J. Angew. Chem., Int. Ed. 2005, 44, 5100–5103. (9) Bertolotti, S. G.; Zimmerman, O. E.; Cosa, J. J.; Previtali, C. M. J. Lumin. 1993, 55, 105–113. (10) Ghosh, S. K.; Pal, A.; Kundu, S.; Mandal, M.; Nath, S.; Pal, T. Langmuir 2004, 20, 5209–5213. (11) Li, G. T.; Fuhrhop, J. H. Langmuir 2002, 18, 7740–7747. (12) Mirkin, C. A.; Letsinger, R. L.; Mucic, R. C.; Storhoff, J. J. Nature 1996, 382, 607–609. (13) Hainfeld, J. F.; Powell, R. D. J. Histochem. Cytochem. 2000, 48, 471–480. (14) Nam, J. M.; Park, S. J.; Mirkin, C. A. J. Am. Chem. Soc. 2002, 124, 3820– 3821. (15) Hens, Z.; Tallapin, D. V.; Weller, H.; Vanmaekelbergh, D. Appl. Phys. Lett. 2002, 81, 4245–4227.

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be exploited for optical-based devices.12-16 However, almost all the investigated AuNPs played as a photoluminescent quencher to quench the molecular excitation energy in chromophoreAuNPs composites.17,18 Total quenching of the singlet excited states of the chromophores limits the applications of chromophore-labeled metal nanoparticles in optoelectronic devices and photonic materials. Thus, as expected, ongoing research in choosing the extending π-conjugated system molecules to protect AuNPs aims to bring obvious advances on the materials optical properties. In recent years, aggregation-induced emission (AIE) and aggregation-enhanced emission (AEE) phenomena in silole systems and other conjugated system attract more and more attention due to their extensive application in the field of photoelectronic device.19-24 In these molecular systems, the energy relaxation processes through nonradiant channels via intramolecular vibration/torsion are blocked in aggregate state; thus, the energy relaxes through the radiant state of the exciplex and turns on the emission of this kind of compounds. In AuNPs composites, the addition of ligands with multidentate usually induces the aggregation of AuNPs, which may present the possibility to restrict the motions of the ligands in the packed nanoclusters. It has been reported that aggregation could induce enhancement of phosphorescence by surface plasmon resonances in colloidal (16) Otsuka, H.; Akiyama, Y.; Nagasaki, Y.; Kataoka, K. J. Am. Chem. Soc. 2001, 123, 8226–8230. (17) Jenekhe, S. A.; Osaheni, J. A. Science 1994, 265, 765–768. (18) Ishida, A.; Sakata, Y.; Majima, T. J. Chem. Soc., Chem. Commun. 1998, 57–58. (19) Luo, J.; Xie, Z.; Lam, J. W. Y.; Cheng, L.; Chen, H.; Qiu, C.; Kwok, H. S.; Zhan, X.; Liu, Y.; Zhu, D.; Tang, B. Z. Chem. Commun. 2001, 1740–1741. (20) Chen, J.; Xie, Z.; Lam, J. W. Y.; Law, C. C. W.; Tang, B. Z. Macromolecules 2003, 36, 1108–1117. (21) An, B. K.; Kwon, S. K.; Jung, S. D.; Park, S. Y. J. Am. Chem. Soc. 2002, 124, 14410–14415. (22) Chen, J.; Law, C. C. W.; Lam, J. W. Y.; Dong, Y.; Lo, S. M. F.; Williams, I. D.; Zhu, D.; Tang, B. Z. Chem. Mater. 2003, 15, 1535–1546. (23) Chen, J.; Peng, H.; Law, C. C. W.; Dong, Y.; Lam, J. W. Y.; Williams, I. D.; Tang, B. Z. Macromolecules 2003, 36, 4319–4327. (24) Lim, S. J.; An, B. K.; Jung, S. D.; Chung, M. A.; Park, S. Y. Angew. Chem., Int. Ed. 2004, 43, 6346–6350.

Published on Web 09/02/2009

DOI: 10.1021/la901173s

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metal nanoparticles.25 On the basis of these facts, we have devoted much effort to exploit emissive AuNPs composites with AIE or AEE effects. In our previous work, a series of AuNPs protected by carbazolyldiacetylenes with unusual fluorescence enhancements has been designed and the mechanism has been preliminarily discussed.26,27 When carbazolyldiacetylenes derivatives were bound to the surface of gold nanoparticles, the nanoparticle composites underwent a substantial enhancement in fluorescence intensity and the emission enhancement behavior is mainly due to restricted intramolecular rotation/torsion in the packed nanoclusters. In this work, great efforts have been made to exploit novel gold nanoparticle composites with fluorescence enhancements leading by the restriction of intramolecular motions. The red emitting material perylene bisimide28-31 has been taken as investigative object, and three kinds of AuNPs labeled by perylene derivatives had been designed and synthesized. Their optical properties have been studied, and the mechanism of the fluorescence enhancements has been discussed. This work might support an efficient method for the study and exploitation of novel gold nanoparticle composites with fluorescence emission.

Experimental Section General Information. Reagents were purchased and utilized as received unless indicated otherwise. All solvents were purified using standard procedures. Evaporation and concentration in vacuum were carried out at water aspirator pressure. Column chromatography: SiO2 (200-300 mesh). 1H and 13C NMR spectra were obtained with a Bruker ARX400 spectrometer using tetramethylsilane (TMS) as internal standard. MALDI-TOF mass spectrometric measurements were performed on a Bruker Biflex MALDI-TOF spectrometer. HRMS (ESI) spectra were obtained at FT_MS_Bruke APEX IV (7.0 T). UV-vis spectra were measured on a Hitachi U-3010 spectrophotometer. The fluorescence spectra were measured on a Hitachi F-4500 fluorimeter. N,N0 -2,6-Bis(4-aminomethylpyridine)-1,7-bis(4-iodophenoxy)3,4,9,10-tetracarboxylic acid bisimide (1), 9-(prop-2-ynyl)-9Hfluorene-3-carbaldehyde (2), N,N0 -2,6-bis(4-aminomethylpyridine)-1,7-bis[4-(hydroxymethyl)phenoxy]-3,4,9,10-tetracarboxylic acid bisimide (4), and N,N0 -2,6-bis(4-aminomethylpyridine)-1, 6,7,13-tert-[4-(hydroxymethyl)phenoxy]-3,4,9,10-tetracarboxylic acid bisimide (5) were synthesized according to refs 32, 33, and 34, respectively. Tetra-n-octylammonium bromide-capped gold nanoparticles (TOAB-AuNPs) were synthesized according to the standard procedure in the literature.35 The 8SP-AuNPs, AuNPs protected by N,N0 -2,6-bis(4-aminomethylpyridine)-1,7-bis[4-(hydroxymethyl)phenoxy-5-(1,2-dithiolan-3-yl)pentanoate]3,4,9,10-tetracarboxylic acid bisimide (4SP-AuNPs), and N,N0 (25) Ostrowski, J. C.; Mikhailovsky, A.; Bussian, D. A.; Summers, M. A.; Buratto, S. K.; Bazan, G. C. Adv. Funct. Mater. 2006, 16, 1221–1227. (26) Li, C. H.; Liu, X. F.; Yuan, M. J.; Li, J. B.; Guo, Y. B.; Xu, J. L.; Zhu, M.; Lv, J.; Liu, H. B.; Li, Y. L. Langmuir 2007, 23, 6754–6760. (27) Lv, J.; Jiang, l.; Li., C. H.; Liu, X. F.; Yuan, M. J.; Xu, J. L.; Zhou, W. D.; Song, Y. L.; Liu, H. B.; Li, Y. L.; Zhu, D. B. Langmuir 2008, 24, 8297–8302. (28) Schmidt-Mende, L.; Fechtenkotter, A.; M€ullen, K.; Moons, E.; Friend, R. H.; MacKenzie, J. D. Science 2001, 293, 1119–1122. (29) Toda, Y.; Yanagi, H. Appl. Phys. Lett. 1996, 69, 2315–2317. (30) Liu, Y.; Xiao, S. Q.; Li, H. M.; Li, Y. L.; Liu, H. B.; Lu, F. S.; Zhuang, J. P.; Zhu, D. B. J. Phys. Chem. B 2004, 108, 6256–6260. (31) Kraft, A.; Grimsdale, A. C.; Holmes, A. B. Angew. Chem. 1998, 110, 416– 443. Angew. Chem., Int. Ed. 1998, 37, 402-428. (32) Liu, Y.; Wang, N.; Li, Y. J.; Liu, H. B.; Li, Y. L.; Xiao, J. C.; Xu, X. H.; Huang, C. S.; Cui, S.; Zhu, D. B. Macromolecules 2005, 38, 4880–4887. (33) He, X. R.; Liu, H. B.; Li, Y. L.; Liu, Y.; Lu, F. S.; Li, Y. J.; Zhu, D. B. Macromol. Chem. Phys. 2005, 206, 2199–2205. (34) Liu, Y.; Zhuang, J. P.; Lu, H. B.; Li, Y. L.; Lu, F. S.; Gan, H. Y.; Jiu, T. G.; Wang, N.; He, X. R.; Zhu, D. B. ChemPhysChem 2004, 5, 1210–1215. (35) Brust, M.; Fink, J.; Bethell, D.; Schiffrin, D. J.; Kiely, C. J. J. Chem. Soc., Chem. Commun. 1995, 117, 1655–1656.

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2,6-bis(4-aminomethylpyridine)-1,7-bis[9-(prop-2-ynyl)9H-carbazole-3-yl)methyl-5-(1,2-dithiolan-3-yl)pentanoate]-3,4,9,10-tetracarboxylic acid bisimide binding AuNPs (4SCP-AuNPs) colloid solution was prepared according to the ligand exchange method36 by mixing TOAB-AuNPs and ligands (8SP, 4SCP, or 4SP) at a specific concentration in methanol. The synthetic route for compounds is shown in Scheme 1.

Synthesis of N,N0 -2,6-Bis(4-aminomethylpyridine)-1,7bis[9-(prop-2-ynyl)-9H-carbazole-3-carbaldehyde]-3,4,9, 10-tetracarboxylic Acid Bisimide (3). To a solution of 1

(115 mg, 0.1 mmol) and 2 (595 mg, 0.25 mmol) in tetrahydrofuran (THF)/triethylamine (1:1, v/v) was added the catalysts CuI and PdCl2(PPh3)2 under a nitrogen atmosphere at room temperature (rt). After stirring for 20 h, the resulting mixture was concentrated under reduced pressure. Purification of the crude product by column chromatography (eluent: petroleum/chloroform, 1:4) yielded pure 5 (82 mg, 60%) as a dark red solid. 1H NMR (400 MHz, CDCl3): δ (ppm) 10.12 (s, 2H), 9.49 (d, 2H), 8.6 (d, 2H), 8.35 (d, 2H), 8.16 (m, 4H), 7.6-7.3 (m, 18H), 7 (m, 6H), 5.3 (s, 4H), 4.2 (s, 4H), 2.7 (m, 4H), 1.1 (d, 24H). MS (TOF) m/z: C94H28N2O8, M = 1355; found 1356. Synthesis of 4SCP. To a solution of 3 (137 mg, 0.1 mmol) in THF/methanol (1:1, v/v) was added NaBH4 (150 mg). After stirring for 4 h, the resulting mixture was concentrated under reduced pressure, washed with water, and extracted with dichloromethane (3  30 mL). The combined organic layer was dried over anhydrous Na2SO4 and concentrated under reduced pressure. The yielded dark red solid together with 5-(1,2-dithiolan-4-yl)pentanoic acid (52 mg, 0. 25 mmol) was dissolved in 25 mL of chloroform. To that solution was added 1-[3-(dimethylamino)propyl]-3-ethylcarbodiimide hydrochloride (EDCI) (56 mg, 0.25 mmol) and 4-(dimethylamino)pyridine (DMAP) (32 mg, 0.25 mmol) under a nitrogen atmosphere at 0 °C. After stirring for 1 h, it was allowed to be warmed up to rt, and the mixture was further stirred for another 28 h. The reaction mixture was washed successively with saturated citric acid aqueous solution (3  20 mL) and water (2  30 mL). Then the aqueous layer was extracted with CH2Cl2 three times. The combined organic phase was dried over anhydrous Na2SO4 and was removed under vacuum. Purification of the crude product by column chromatography (eluent: chloroform) yielded pure 4SCP (118 mg, 68%) as dark red solid. 1H NMR (400 MHz, CDCl3): δ (ppm) 9.5 (m, 2H), 8.67 (m, 2H), 8.36 (m, 2H), 8.1 (m, 4H), 7.54-7.27 (m, 18H), 6.9 (m, 6H), 5.18 (s, 4H), 5.12 (s, 4H), 3 (m, 4H), 2.7 (m, 4H), 2.3 (m, 6H), 1.8-1.26 (m, 16H), 1.16 (d, 24H). 13C NMR ((CDCl3, δ, ppm): 173, 163, 162, 155, 153, 145, 140.3, 139.9, 134, 133, 130, 129, 127, 126, 125.2, 124.3, 123.4, 123, 122.6, 121.2, 120.6, 119.8, 118, 109, 83.6, 83, 67, 56, 40, 38, 34, 29.8, 29, 24, 23. MS (MALDI-TOF) m/z: calcd for C108H96N2O10S4 1738; found 1737.5 Synthesis of 4SP. The synthetic route of 4SP was similar to synthetic process of 4SCP, and the starting materials are listed as follows: 4 (191 mg, 0.2 mmol), 5-(1,2-dithiolan-4-yl)pentanoic acid (124 mg, 0.6 mmol), EDCI (136 mg, 0.6 mmol), and DMAP (76 mg, 0.6 mmol). Purification of the crude product by column chromatography (eluent: chloroform) yielded pure 4SP (186 mg, 70%) as a dark red solid. 1H NMR (400 MHz, CDCl3): δ (ppm) 9.6 (d, 2H), 8.7 (d, 2H), 8.4 (s, 2H), 7.5 (m, 6H), 7.38 (m, 4H), 7.2 (m, 4H), 5.11 (s, 4H), 3.2 (m, 4H), 3 (m, 4H), 2.6 (m, 6H), 1.7-1.38 (m, 16H), 1.15 (d, 24H). 13C NMR ((CDCl3, δ, ppm): 173, 163, 162, 155, 154, 145, 133, 132, 131, 130, 129, 126, 125, 124, 122, 119, 66, 56, 40, 38, 34, 29, 28, 24, 23. MS (MALDI-TOF) m/z: calcd for C78H78N2O10S4 1331; found 1330.4. HRMS (ESI) m/z calcd for C78H78N2O10S4 [M þ Na]þ: 1353.4431, found 1353.4367; calcd for [M þ K]þ: 1369.4171; found 1369.4133.

(36) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 116, 801–802.

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Article Scheme 1. Synthetic Route of 4SCP, 4SP, and 8SP

Synthesis of 8SP. The synthetic route of 8SP was similar to synthetic process of 4SCP and 4SP, and the starting materials are listed as follows: 5 (120 mg, 0.1 mmol), added 5-(1,2-dithiolan4-yl)pentanoic acid (124 mg, 0.6 mmol), EDCI (136 mg, 0.6 mmol), and DMAP (76 mg, 0.6 mmol). The stirring time of the reactant mixture at rt is about 48 h, which is longer than 4SP and 4SCP. Disposal of the reaction mixture was similar to 4SCP. Purification of the crude product by column chromatography (eluent: chloroform) yielded pure 8SP (126 mg, 65%) as dark red solid. 1H NMR (400 MHz, CDCl3): δ (ppm) 9.6 (d, 2H), 9.6 (d, 2H), 7.46 (m, 4H), 7.3 (m, 10H), 6.96 (m, 8H), 5.07 (s, 8H), 3.14 (m, 8H), 2.72 (m, 4H), 2.45 (m, 12H), 1.9-1.43 (m, 32H), 1.13 (d, 24H). 13C NMR ((CDCl3, δ, ppm): 173, 163, 155.6, 155.3, 145.6, 133.2, 132.5, 130.5, 130.3, 129.6, 124, 123.1, 121, 120.6, 119.9, 66.5, 56.4, 40.3, 38.5, 34.6, 34.1, 29.2, 28.8, 24.7, 24.1. MS Langmuir 2009, 25(19), 11351–11357

(MALDI-TOF) m/z: calcd for C108H114N2O16S8 1952.5; found 1952. HRMS (ESI) m/z calcd for C108H114N2O16S8 [M þ Na]þ: 1973.5826; found 1973.5707; calcd for [M þ K]þ: 1989.5560; found 1989.5678.

Results and Discussion As most of the investigations in chromophore-AuNPs composites shown, AuNPs usually played as photoluminescent quencher to quench the molecular excitation energy in chromophore-AuNPs composites.17,18 In our previous study, carbazolyldiacetylene-capped AuNPs were found to show unusual fluorescence enhancement according to the restriction of the intramolecular motion in packed nanoclusters.26,27 In order to understand the mechanism for the unusual emission enhancement DOI: 10.1021/la901173s

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Figure 1. (A) UV-vis absorption spectra of AuNPs in toluene solution with the titration of 8SP. TEM image of TOAB-AuNPs (B) and TEM image of 8SP-AuNPs in toluene suspension (C).

of the gold nanoparticle composites, the structure of the ligands and the temperature influence should be considered. In this work, perylene bisimide derivatives with large rigid conjugation system were used to develop ligands. The modification of the 1,6,7, 13-position of the perylene derivatives (bay region)37 is crucial for developing chromophore-AuNPs composites with emission enhancements. 8SP, 4SCP, and 4SP were designed upon the different degree of rotation/torsion of the perylene chromophores in the packed chromophore-AuNPs composites. The temperature dependence of the emission intensity disclosed the mechanism of the unusual emission behaviors of 8SP-AuNPs composites. Spectral Study of the 8SP-AuNPs. The position and the shape of the plasmon absorption of metal nanoclusters are strongly dependent on the particle size, dielectric medium, and surface adsorbed species.38-40 The absorption spectra of TOAB-AuNPs (3  10-4 mol/L in toluene) exhibited a strong surface plasmon absorption band centered at 528 nm in a toluene solution. As shown in Figure 1A, upon continuous addition of 8SP (0-5  10-5 mol/L in toluene) into the gold nanoparticle suspension (3  10-4 mol/L in toluene), UV absorption spectra exhibited dampening, and broadening of the gold surface plasmon absorption band. The plasmon absorption band red-shifted from 528 to 575 nm, which indicated the tetradentate ligand complexed with the gold surface and induced the aggregation of the AuNPs.41 The red shift can be interpreted in terms of the Mie scattering theory and results from the coupling of the transition from dipoles associated with the surface plasmon band of metal particles at close distance.42 TEM provides direct information about the shape, size, and size distribution of ligand-capped gold nanoparticles. As shown in Figure 1B,C, TEM image showed numerous discrete TOAB-AuNPs with an average diameter of 3-5 nm, while 8SP-AuNPs aggregated obviously. Unusual Fluorescence Enhancements of 8SP-AuNPs. It was shown that the fluorescence of the most investigated fluorescent chromophores, labeling on the surface of AuNPs, was

(37) Osswald, P.; Reichert, M.; Bringmann, G.; W€urthner, F. J. Org. Chem. 2007, 72, 3403–3411. (38) Kreibig, U.; Vollmer, M. Optical Properties of Metal Clusters; Springer: Berlin, 1995. (39) Mulvaney, P. In Semiconductor Nanoclusters: Physical, Chemical and Catalytic Aspects; Kamat, P. V., Meisel, D., Eds.; Elsevier Science: Amsterdam, 1997; pp 99-123. (40) Kreibig, U.; Gartz, M.; Hilger, A.; Hovel, H. In Fine Particles Science and Technology; Pelizzatti, E., Ed.; Kulwer Academic Publishers: Boston, MA, 1996; p 499. (41) Herrikhuyzen, J. V.; Janssen, R. A. J.; Meijer, E. W.; Meskers, S. C. J.; Schenning, A. P. H. J. J. Am. Chem. Soc. 2006, 128, 686. (42) Cuniberti, C.; Dellepiane, G.; Piaggio, P.; Franco, R.; Musso, G. F. Chem. Mater. 1996, 8, 708–713.

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totally quenched.17,18,32 8SP-AuNPs colloid solution was found to exhibit distinctive optical properties compared with most of the investigated gold nanoparticle composites. Emission enhancement was observed in the solution of 8SP-AuNPs in methanol. When AuNPs (5  10-5 mol/L) was added into 8SP in a methanol solution (2.5  10-5 mol/L), the fluorescence intensity of 8SP decreased at first; however, enhancement in fluorescence intensity was observed by time aging (Figure 2A, traces were measured at every 5 min; all the emission spectra of 8SP and 8SP-AuNPs were excited by 576 nm). As shown in Figure 2B, the addition of AuNPs suspension into 8SP in a methanol solution induced fluorescence intensity decrease initially. When the solution was kept for 12 h, the fluorescence intensity of 8SP-AuNPs solution increased more than 10 times compared with the fluorescence intensity of 8SP in a methanol solution. The quantum yield of 8SP in a methanol solution is 0.09%; N,N0 -2,6-bis(4-aminomethylpyridine)-1,7-bisbromo-3,4,9,10-tetracarboxylic acid bisimide was used as standard material.43 When AuNPs suspension was added into 8SP solution, the energy transfer between the ligands and AuNPs led the decrease in fluorescence intensity.17,18,32 As the time aging, AuNPs aggregated together which probably induce the increase in fluorescence intensity.26,27 Minor differences were observed between the TEM images of the sample soon after the mixture of 8SP in a methanol solution and AuNPs suspension and the TEM images of sample incubated for 12 h (Figure S1) because the drying process of the samples led to aggregation of AuNPs inevitably. TEM images of the nanoparticle composites gave a clear-cut evidence for that 8SP could induce the aggregation of AuNPs (Figure 2C). In the packed nanocluster, the AuNPs provided scaffolding to hold the rigid π-conjugated perylene chromophores in an optimal position where the intramolecular rotation and torsion of the perylene chromophores were significantly restricted. Because of the restricted motion of chromophores, the energy relaxation channels of nanoclusters through intramolecular motions were hampered. Hence, the energy was probably relaxed through radiant ways. On the basis of our previous experiments and study, the emission enhancements in 8SP-AuNPs composites were supposed to be the results of the aggregation of the AuNPs. Besides, there was no obvious wavelength change in spectra, which reflected the absence of the electron-transfer process in 8SP-AuNPs composites44-46 (43) Chao, C.; Leung, M.; Su, Y. O.; Chiu, K. Y.; Lin, T. H.; Shieh, S. J.; Lin, S. C. J. Org. Chem. 2005, 70, 4323. (44) Beny, J. P.; Dhawan, S. N.; Kagan, J.; Sundlass, S. J. Org. Chem. 1982, 47, 2201–2204. (45) George Thomas, K.; Zajicek, J.; Kamat, P. V. Langmuir 2002, 18, 3722– 3727. (46) George Thomas, K.; Kamat, P. V. J. Am. Chem. Soc. 2000, 122, 2655–2656.

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Figure 2. (A) Time-dependent fluorescence spectra of 8SP in a methanol solution (2.5  10-5 mol/L) with the addition of AuNPs (5  10-5

mol/L); every trace was measured at every 5 min. (B) Fluorescence spectra of 8SP in a methanol solution (2.5  10-5 mol/L), soon after the addition of AuNPs (5  10-5 mol/L) into the solution and incubated for 12 h. (C) TEM image of 8SP-AuNPs with fluorescent enhancements.

Figure 3. (A) Molecular structure of 4SP, 4SCP, and 8SP. The general model for ligand-AuNPs composites (B) and the local magnification

of connection ways in the cases of 4SP-AuNPs (C), 4SCP-AuNPs (D), and 8SP-AuNPs (E). (F-H) Fluorescence spectra of 4SP (2.5  10-5 mol/L), 4SCP solution (2.5  10-5 mol/L), and 8SP solution (2.5  10-5 mol/L) in methanol solution before and after the addition of AuNPs (5  10-5 mol/L) (keeping for 12 h).

and indicated no overlap between the perylene chromophores. The enhancement in fluorescence intensity of 8SP-AuNPs was probably attributed to the effect of restricted motion of perylene chromophores in the packed nanoclusters. Structural Effect of Ligands on the Fluorescence Enhancement of Gold Nanoparticle Composites. In order to investigate the possible mechanism for the emission enhancement of the 8SP-AuNPs composites, the structural effect of the ligands were taken into consideration. Other two kinds of nanoparticle composites labeled by 4SP and 4SCP were prepared and studied. As shown in Figure 3A, molecules of 4SP, 4SCP, and 8SP shared the identical chromophore, perylene bisimide. The structural differences among them lie in the different modification approaches of the perylene chromophore. In the molecular of 8SP, the Langmuir 2009, 25(19), 11351–11357

four positions of bay region were linked with dithiolan which could interact with the gold surface of the AuNPs. In 4SP, only the 1,7-positions of bay region were connected with dithionlan. While in 4SCP two rigid carbazole were used to connected the perylene bisimide and dithiolan, which might make the rotation/ torsion of the perylene chromophore difficult. Because of the multidentates, all the three ligands could induce the aggregation of the AuNPs. The gold plasmon resonance band of the AuNPs colloid solution red-shifted with the addition of 4SP, 4SCP, and 8SP, respectively. TEM images of the 4SP-AuNPs, 4SCP-AuNPs, and 8SP-AuNPs showed aggregates and networks due to the multidentates of the ligands (Figure S2). Thanks to the different modification of the bay region of 8SP, 4SP, and 4SCP, the nanoparticle composites of 8SP-AuNPs, DOI: 10.1021/la901173s

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Figure 4. (A) Temperature-dependent fluorescence spectra of 8SP-AuNPs in a methanol solution. (B) Fluorescence spectra of the initiative cooling process of 8SP-AuNPs solution after it was heated to 50 °C. (C) Fluorescence spectra of the spontaneous warming process of 8SP-AuNPs colloid solution after it was cooled to -10 °C.

4SP-AuNPs, and 4SCP-AuNPs exhibit different optical behaviors. As indicated previously, 8SP-AuNPs showed fluorescence enhancement in a methanol solution (Figure 3H). The fluorescence intensity of 4SP in a methanol solution (2.5  10-5 mol/L) decreased upon the addition of AuNPs (5  10-5 mol/L), and the intensity showed no obvious change by keeping the solution for 12 h (Figure 3F). Under the same conditions, fluorescence intensity of 4SCP solution decreased right after the addition of AuNPs; however, faint enhancement in fluorescence intensity was observed in the emission spectra of the 4SCP-AuNPs solution after 12 h (Figure 3G). All the spectra measurements were conducted under the same conditions, and identical AuNPs colloid solution was applied in our experiments. As a matter of fact, decrease or enhancement in fluorescence intensity of the nanoparticle composites disclosed the competition between energy transfer process between ligands and AuNPs and radiant energy relaxation induced by the restricted motion of chromophores in nanoclusters. The energy transfer process is responsible for the initial quenching,17,18,32 while the restricted motion of the perylene attributed to the fluorescence enhancement.26,27 This competition is bound up with the ratios of ligand/gold particles and the structure of the ligands. The degree of rotation/torsion of the perylene chromophores in the packed nanolusters depended on the different modification approaches of the bay region. As exhibited by the local magnified model of 4SP-AuNPs in Figure 3C, 4SP could induce the aggregation of the AuNPs due to the didentates stretching from inclined para-positions of bay region. Nevertheless, the rotation/torsion of the perylene chromophore was almost not influenced in the nanoclusters, resulting in no enhancement in emission spectra. The motions of perylene bisimide were restrained to certain extent by linking the rigid carbazole to the perylene bisimide, which led to the faint emission enhancement of 4SCP-AuNPs composites (Figure 3D). While in the case of 8SP-AuNPs, the perylene chromophores were firmly fixed among the particles by tetradentates binding on the surface of AuNPs, which made the motions of chromophores quite difficult in the packed nanoclusters (Figure 3E). That means in 8SP-AuNPs composites the energy relaxation processes through nonradiant channels via intramolecular rotation/torsion are hampered, and the radiant way of energy relaxation was in activation which led to emission enhancement of 8SP-AuNPs.19-24 Aggregation of the AuNPs is a basic requirement for the emission enhancement of the chromophore-AuNPs composites, while the structure of the ligand is one of the determinant factors in these cases. These results supported the supposed aggregation enhanced emission mechanism for 8SP-AuNPs. 11356 DOI: 10.1021/la901173s

Effect of Temperature on the Fluorescence Enhancement of Gold Nanoparticle Composites. The temperature experiments were carried out to further demonstrate the supposed mechanism for emission enhancement of the 8SP-AuNPs composites. It has been proposed that the restricted motion of AIE or AEE dyes may block their nonradiative channels, resulting in high emission. In the packed nanoclusters, the AuNPs could provide scaffolding to hold the ligands which presents a possibility to exploit novel emissive AuNPs according to the AEE mechanism. The emission of 8SP-AuNPs should get stronger with the decreasing of temperature, when the AEE mechanism played a major role in the emission process.20,23-25 As shown in Figure 4A, the emission spectra of 8SP-AuNPs in a methanol solution showed enhancement in fluorescence intensity as temperature get lower. The fluorescence intensity increased gradually when the heated solution was cooled down (Figure 4B); correspondingly, the fluorescence intensity decreased as the cooled solution got warmer (Figure 4C). Both the processes could be repeated. In the temperature experiments, all the used chromophore-AuNPs solutions were been kept for 12 h. The thermal motion of the perylene chromophores in 8SP-AuNPs became vigorous when the temperature increased to 50 °C; thus, the energy could partially relax through the nonradiant vibration/ torsion channels which was responsible for the decrease of the fluorescence intensity. As the temperature decreased, the activity of thermal motion of the choromphores was reduced and fluorescence signal appeared to be intensified. When temperature got even lower, the fluorescence intensity became stronger, which effectively supported the aggregation-enhanced fluorescence in 8SP-AuNPs. In contrast, there is no temperature dependence of the fluorescence intensity of 4SP--AuNPs and 4SCP-AuNPs composites. The motion of the perylene chromophores was relatively free in composites of 4SP-AuNPs and 4SCP-AuNPs, which determined the weak influence of the temperature on the motion of chromophores in the tested range of temperature. The structural differences among 8SP, 4SP, and 4SCP determine their different emission behaviors.

Conclusion We have demonstrated a novel organic/inorganic hybrid material, 8SP-AuNPs composites, which showed the unusual fluorescence enhancement. With the perylene chromophore firmly fixed among the nanoparticles in the nanoclusters, 8SP-AuNPs underwent a substantial enhancement of fluorescence. This fluorescence enhancement behavior is mainly due to restricted intramolecular rotation/torsion of the perylene chromophores in the packed nanoclusters. Our present work will provide a novel concept and Langmuir 2009, 25(19), 11351–11357

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an efficient method for fabricating chromophore labeled metal nanoparticles with unusual fluorescence enhancement. Acknowledgment. This work was supported by the National Nature Science Foundation of China (20531060, 10874187, and 20721061) and the National Basic Research 973 Program of China.

Langmuir 2009, 25(19), 11351–11357

Article

Supporting Information Available: TEM images of the samples (8SP-AuNPs) right after preparation and incubation for 12 h, 4SP-AuNPs, 4SCP-AuNPs, and 8SP-AuNPs in toluene suspension. This material is available free of charge via the Internet at http:// pubs.acs.org.

DOI: 10.1021/la901173s

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