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Copyright © 2006 American Scientific Publishers All rights reserved Printed in the United States of America
Journal of Nanoscience and Nanotechnology Vol. 6, 1–3, 2006
Synthesis of Fluorescent Composite Macromolecules by Using Organic/Inorganic Assemblies as Structural Units Ze-Hua Liu† , Yan Wang‡ , Guo-Ping Ge, and Hai-Qing Guo∗ State Key Laboratory of Rare Earth Materials Chemistry and Applications, College of Chemistry and Molecular Engineering, Peking University, 100871 Beijing, P. R. China A synthetic pathway is introduced to construct fluorescent composite macromolecules with supramolecular assemblies as structural units. The supramolecular assembly that contains polymerizable groups is used as a starting “monomer.” The supramolecular assembly is composed of nanoparticle core of II ∼ IV group semiconductor and organic ammonium shell. Polymerization of the assemblies yields soluble composite macromolecules. Light scattering data show that the macromolecule has an average size of about 310 nm in diameter in chloroform; AFM image illustrates that the macromolecule has an average diameter of 120 nm and an average height of 35 nm on a mica surface and photoluminescent spectra reveal that the macromolecule performs an extraordinary enhancement in fluorescence intensity of the semiconductor nanoparticles. These observations suggest that construction of macromolecules with supramolecular assembly as starting monomer may produce generations of materials with novel properties.
Keywords: Supramolecular Assembly, Nano-Materials, Fluorescence, Energy Transfer, CdSe.
2. EXPERIMENTAL SECTION
It is well known that covalently combined polymers are generally obtained from small molecules “monomer” by chemical polymerization reactions.1 It is also well known that many functions come from “supramolecular assemblies.”2 It is anticipated that application of the supramolecular assembly to the covalently combined polymer formation can produce new types of polymers with novel functions. One of the strategies is to use the supramolecular assembly as “monomer” (Scheme 1). We got succeeded in preparation of fluorescent composite macromolecules by using inorganic/organic assemblies as structural units.3 4 The inorganic/organic composite macromolecule showed high fluorescence and good solubility in general organic solvents. These properties make it potential candidate in many photoelectric applications.5 In this paper, we will report the preparation strategy, characterization, and properties of the polymer.
2.1. Chemicals and Instruments
∗
Author to whom correspondence should be addressed. Present address: College of Material Science and Chemical Engineering, Tianjin University of Science and Technology, Tianjin 300222, P. R. China. ‡ Visiting student from College of Material Science and Technology, Beijing Forestry University, Beijing, P. R. China. †
J. Nanosci. Nanotechnol. 2006, Vol. 6, No. 12
Selenium (AR grade, Beijing Zhongliante Chemical Company Ltd.), anhydrous Na2 SO3 (AR grade, Beijing Chemical Factory), CdCl2 (AR grade, Beijing Shuanghuan Chemical Reagent Factory), Mercaptoacetic acid (AR grade, Beijing Chemical Reagent Factory), Hexadecyltrimethylammonium bromide (CH3 (CH2 )15 N(CH3 )3 Br (HTAB), AR grade, Beijing Xizhong Chemical Factory), 4-methoxy-stilbenyldimethylethylammonium bromide (MODAB) and 4-methacyloyloxy-trans-stilbenyldimethylammonium bromide (ASDAB) (prepared in our laboratory). Photoluminescence (PL) experiments were conducted on a Hitachi F-4500 fluorescent spectrometer. Dynamic Light scattering (DLS) data were obtained using an ALV/DLS/SLS-5022F laser light scattering instrument (ALS/Laser Vertrisebsgesellschaft m.b.H company). A 22 mW helium neon laser with wavelength of 632.8 nm was focused on a precision scattering cell containing a sample solution. Dynamic light scattering measurements were conducted at angular of 90 . All light scattering measurements were performed at 25 C. Atomic force microscopy (AFM) image were conducted on a
1533-4880/2006/6/001/003
doi:10.1166/jnn.2006.670
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RESEARCH ARTICLE
1. INTRODUCTION
Synthesis of Fluorescent Composite Macromolecules by Using Organic/Inorganic Assemblies as Structural Units Table I. Supramolecular Chemistry Small Molecules and/or Particles
Chemistry
No. Assembly
Covalent Polymer
Scheme 1.
SPI3800N-SPA 400 (Seiko Instruments Inc.) scan probe microscope determined in tapping mode. 2.2. Assemble of CdSe Nanoparticles with Organic Molecules CdSe nanoparticles were prepared according to the reported procedure.3 4 A mixed suspension of HTAB, MODAB, and MSDAB in water was added dropwisely to the CdSe nanoparticle aqueous solution under stirring at room temperature to give a yellow precipitate. The precipitate was separated by centrifugation followed by washing with water and dried at 50 C under vacuum for 24 hrs.
RESEARCH ARTICLE
2.3. Polymerization of the Assembly A solution of the assembly and a solution of AIBN (a radical polymerization initiator) in chloroform were added to a polymerization ampoule. After degassing in a vacuum line, the ampoule was sealed off and placed in a water bath at 80 C for 4 hrs. The precipitate (insoluble parts) in the ampoule was centrifuged off and the supernatant was decanted and poured into methanol to give a yellow precipitate. The precipitate was dried at 50 C under vacuum for 24 hrs. The polymers precipitated from methanol are soluble in chloroform and many other organic solvents.
Liu et al.
Radical polymerization of the assembly∗ . HTAB:MODAB:MSDAB (molar ratio)
Polymer yield (%)
Soluble part (%)
1:5.00:1.00 1:5.50:0.50 1:5.90:0.10 1:5.98:0.02
63 58 62 58
62 78 90 97
1 2 3 4
Polymerization temperature: 80 C, polymerization time: 4 h, solvent: CHCl3 , concentration of the assembly: 20.0 (g · L−1 ), initiator: AIBN ([AIBN] = 5.0 × 10−4 mol · L−1 ).
kinds of organic molecules were used to adjust the fluorescence, solubility, and polymerizability, respectively. As shown in Figure 1, the alkyl flexible chain of HTAB give the assembly solubility, the luminescent stilbene group can transfer energy to CdSe nanoparticles to increase the fluorescence intensity of the particles, and the double bonds in MSTAB make the assembly polymerizable to macromolecules. The relationship between the solubility and the fluorescence properties of the assembly and the HTAB/MODAB ratio in charging in the preparation of the assembly was investigated.4 It was clear that there is a good balance of the two properties when the HTAB/MODAB ratio is 1:6. There is also a good balance between the fluorescence, solubility, and polymerizability when the molar ratios of HTAB:MODAB:MSDAB in
155.2 1.0
0.5
3. RESULTS AND DISCUSSION 3.1. Composition of the Assembly
0.0
A schematic representation of the assembly of organic molecules with CdSe nanoparticles and the polymer thereof is shown in Figure 1. To obtain a polymer with high fluorescence and solubility in organic solvents, three
1
10
100
1000
10000
Radius (nm) Fig. 2. Dynamic light scattering data of the polymer.
R
R
CdSe
R
R
:
CH3(CH2)15N(CH3)3OOCCH2S (HTAB)
:
R
CH
CH
NR3OOCCH2S
R: CH3O, MODAB R: CH2=C(CH3)CO2 , MSDAB R Fig. 1.
2
Schematic representation of the assembly.
J. Nanosci. Nanotechnol. 6, 1–3, 2006
Liu et al.
Fig. 3.
Synthesis of Fluorescent Composite Macromolecules by Using Organic/Inorganic Assemblies as Structural Units
AFM image of the polymer.
charging are from 1:5.90:0.10 to 1:5.98:0.02 as shown in Table I. 3.2. Characterization of the Polymer
The assembly of a long alkyl chain ammonium of HTAB with CdSe nanoparticle showed a weak bandedge emission at 490 nm and a broad emission at about 600 nm due to the surface state. However, the polymeric sample with the unit structure shown in Figure 1 showed a great enhancement of the emission at 490 nm that results from the energy transfer from the stilbene luminescent groups to CdSe nanoparticles (Fig. 4).
4. CONCLUSION A new method for preparation of functional polymeric materials was introduced. A new type of organic/inorganic composite macromolecule was successfully prepared by using supramolecular assemblies of organic molecules and CdSe nanoparticles as structural units. The macromolecule shows good solubility in organic solvents and high fluorescence. It is anticipated that the method can be expended to prepare many kinds of functional materials by designing the assembly with different molecules combinations.
8
PL intensity (a.u.)
6 CdSe Polymer Organic Molecule (MODAB)
4
2
Acknowledgments: The project supported by the National Nature Science Foundation of China (No. 20374001 and No. 90401028). The AFM determination was conducted in the laboratory of Prof. Xinsheng Zhao, College of Chemistry and Molecular Engineering, Peking University).
References and Notes
0 400
500
600
700
Wavelength (nm) Fig. 4. Photoluminescence: Assembly of HTAB and CdSe nanoparticle; Organic molecule, MODAB; ( the polymer. In chloroform (5 × 10−4 mol/L), excitation at 370 nm.
1. W. Fred and Billmeyer, Jr., Textbook of Polymer Science, 3rd edn., John Wiley and Sons, Singapore (2000). 2. J. M. Lehn, Makromol. Chem. Macromol. Symp. 69, 1 (1993). 3. H. H. Song, Z. H. Liu, and H. Q. Guo, Acta Polym. Sinica 5, 609 (2003). 4. Z. H. Liu, R. F. Lai, and H. Q. Guo, Acta Phys.-Chim. Sinica 9, 1067 (2004). 5. N. Tessle, V. Medvedev, M. Kazes, S. Kan, and U. Banin, Science 295, 1506 (2002).
Received: XX xxxx xxxx. Revised/Accepted: 24 March 2006. J. Nanosci. Nanotechnol. 6, 1–3, 2006
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The polymer (the polymer sample obtained by the condition of No. 3 in Table I) was characterized by dynamic light scattering and AFM experiments. Dynamic light scattering data showed the polymer with an apparent mean diameter of about 310 nm (the hydrodynamic radius (Rh was 155.2 nm) at room temperature in chloroform (Fig. 2). AFM image of the polymer molecules showed an average diameter of 120 nm and an average height of 35 nm (Fig. 3). The dimension of a polymer chain in dry state given by AFM is, generally, smaller than the one in solution given by DLS. Whether the polymer chain is linear or branched need further investigation. However, the polymer is soluble in many organic solvents that make the polymer attractive in device manufacture and other applications.
3.3. Photoluminescence of the Polymer
