Thermoreversible covalent self-assembly of oligo(p-phenylenevinylene) bridged gold nanoparticles

Article abstract of DOI:10.1021/la903838w

Organic-inorganic hybrids have been fabricated through mild Diels-Alder cross-linking between maleimide bearing oligo(p-phenylenevinylene) (OPV) and furan functionalized gold nanoparticles with, diameter smaller than 2 nm. The OPV ligands afford strong reaction, ability toward furan group due to their maleimide moieties. These small gold nanoparticles form close-packed homogeneous hybrids with well-defined interfaces by incorporating OPV ligands in solutions. Covalent assembly and disassembly of gold nanoparticles can be achieved by repeated thermal stimuli on as-obtained hybrids, which can be monitored by fluorescence changes of OPVs and surface plasmon resonance absorption. Moreover, the dramatic photophysical properties and assembly behavior of these hybrids allow this procedure to be performed as a smart assay for monitoring the process of the Diels-Alder reaction.

Full text of DOI:10.1021/la903838w

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© 2009 American Chemical Society
Thermoreversible Covalent Self-Assembly of Oligo( p-phenylenevinylene)
Bridged Gold Nanoparticles
Xiaofeng Liu,†,§ Huibiao Liu,*,† Weidong Zhou,†,§ Haiyan Zheng,†,§ Xiaodong Yin,†,§ Yuliang Li,*,†
Yanbing Guo,†,§ Mei Zhu,†,§ Canbin Ouyang,†,§ Daoben Zhu,† and Andong Xia‡
†Beijing National Laboratory for Molecular Sciences (BNLMS), CAS Key Laboratory of Organic Solids,
Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China, ‡State Key Laboratory of
Molecular Reaction Dynamics, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190,
P. R. China, and §Graduate University of Chinese Academy of Sciences, Beijing 100190, P. R. China
Received August 21, 2009. Revised Manuscript Received December 11, 2009
Organic-inorganic hybrids have been fabricated through mild Diels-Alder cross-linking between maleimide bearing
oligo(p-phenylenevinylene) (OPV) and furan functionalized gold nanoparticles with diameter smaller than 2 nm. The
OPV ligands afford strong reaction ability toward furan group due to their maleimide moieties. These small gold
nanoparticles form close-packed homogeneous hybrids with well-defined interfaces by incorporating OPV ligands in
solutions. Covalent assembly and disassembly of gold nanoparticles can be achieved by repeated thermal stimuli on
as-obtained hybrids, which can be monitored by fluorescence changes of OPVs and surface plasmon resonance
absorption. Moreover, the dramatic photophysical properties and assembly behavior of these hybrids allow this
procedure to be performed as a smart assay for monitoring the process of the Diels-Alder reaction.
1. Introduction
their characteristic optical and electronic properties.6,7 The
assembly of metal nanoparticles into defined macromolecular
structures provides potential access to nanocomposites featuring
useful chemical and optoelectronic applications.8 The internal
organization of such composite materials on the nanoscale is crucial
for determining the desired properties.9 Hybrid nanomaterials
consisting of both inorganic nanoparticles and organic counterparts
have been exploited extensively due to their unique properties
that combining the best properties in both materials into one.10
Unfortunately, favorable properties are often lost arising from poor
dispersion of each component within the hybrid materials.
During the past decade, scientists have developed techniques
for synthesizing new materials on the nanoscale, especially
nanoparticles.1-4 Still, the design and synthesis of nanoparticle
materials with controlled properties is a significant and ongoing
challenge. Understanding the structure-property relationships
that relate specifically to nanomaterials could lead to new strategy
for producing benign, high-performance nanoscale materials.
Metal nanoparticles exhibit intense size- and shape-dependent
properties due to the surface plasmon resonance (SPR),5 which
have attracted considerable recent interest as materials due to
The Diels-Alder reaction is one of the most important ways in
which chemists make new carbon-carbon bonds and which has
been extendedly used for the preparation of thermally responsive
polymers, dendrimers, and biocompatible materials.11 It has also
been found useful as a means to modify self-assembled mono-
layers (SAMs).12 Recently, new methodologies have been deve-
loped to functionalize monolayer-protected gold cluster (MPC)
surfaces using Diels-Alder reaction, facilitating further chemical
modifications.13 Gates et al.14 reported that the release of a
*Corresponding authors. E-mail: ylli@iccas.ac.cn (Y.L.), liuhb@iccas.ac.
cn (H.L.).
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Langmuir 2010, 26(5), 3179–3185
Published on Web 12/22/2009
DOI: 10.1021/la903838w 3179

Article
Liu et al.
Scheme 1. Chemical Structures of OPVs and F-Au
fluorescent dye from core-shell nanoparticles using retro-
Diels-Alder reaction. Gold nanoparticle systems possess rever-
sible behaviors which have been investigated extensively in recent
researches;15 for example, Workentin et al.15a demonstrated the
reversible formation of 3D MPC assemblies utilizing the rever-
sible Diels-Alder reaction. On the basis of researches in the past
decades, Diels-Alder reaction exhibits reversible behavior on the
carbon-carbon bond formation, which has been used widely to
build thermoreversible polymeric materials with diverse function-
alities.11,16 However, there is rare report on the thermoreversi-
bility of gold nanoparticle hybrid composites directed by
Diels-Alder reaction.12,15
Considering the facile and thermally reversible formation
process, the Diels-Alder reaction is expected to lead to develop-
ment of new methodologies for fabricating novel organic-
inorganic hybrid nanomaterials with thermoresponsive behavior.
Recently, we have reported a developed chemical approach that
fabricating organic-inorganic composites by utilizing mild
Diels-Alder reaction.17 The as-prepared hybrid materials exhibit
well-defined interfaces and well dispersity in both inorganic
nanoparticles and organic conjugated part. Although the orga-
nically shelled MPCs have been explored intensively in recent
researches, there have been no reports or practical applications
of organic fluorophore-nanoparticles concerning covalent self-
assembly and thermoswitchable PL properties. Considering the
advantages of both covalently assembly and thermal responsive-
ness, we aim on fabricating such smart materials capable of
modulating both MPCs assembly and optical properties upon
thermal stimuli. Herein, we demonstrated that maleimide bearing
oligo(p-phenylenevinylene) (OPVs) reacts with furan-modified
MPCs (F-Au) to form covalently 3D cross-linked structures
(Scheme 1). These materials represent PL reversibility and assem-
bly behavior with thermal responsiveness.
carbon nuclear magnetic resonance spectra (1H NMR and 13C
NMR) were carried out on Bruker ARX300 and ARX400
spectrometers using tetramethylsilane (TMS) as internal stan-
dard; chemical shifts (δ) are given in ppm relative to TMS. Mass
spectra were measured with a Bruker Biflex III MALDI-TOF
spectrometer and an APEX II FT-ICRMS spectrometer. UV-vis
absorption spectra were obtained from sample solutions on a
JASCO V-570 spectrophotometer. Fluorescence spectra were
obtained on a JASCO FP-6600 fluorimeter. Quartz cuvettes of
1 cm path length were employed. Transmission electron micro-
scopy (TEM) samples were prepared by applying a droplet of
sample solution on carbon-coated copper grids (400 mesh) and
evaporated. Images of representative areas were recorded on a
JEOL JEM-2011 transmission electron microscope operating at
200 kV. Fluorescence lifetimes were measured by a standard time-
correlated single-photon counting (TCSPC) from Ortac at room
temperature using 400 nm laser pulses at 4.7 MHz for excitations
of both samples. Fluorescence was detected at 450 nm by using a
monochromator (EI-121, Edinburgh Instruments) and fast
photomultiplier tube (XP2020). Dynamic light scattering experi-
ments were performed on a nano-ZS zen3600 purchased from
Malvern Instruments, Ltd., with 600 nm laser. Data were aver-
aged by three times of measurements for each sample.
Synthesis and Characterization of Diethyl 4-Nitrobenzyl-
phosphonate (1). A mixture of 1-(chloromethyl)-4-nitrobenzene
(3.0 g, 17.5 mmol) and triethyl phosphite (3.5 g, 21.1 mmol) was
heated at 140 °C for 5 h before being cooled to room temperature.
The excess triethyl phosphite was removed in vacuo. The residue
was purified by silica gel column (400 mesh, petroleum ether/ethyl
acetate = 3:1 as eluent) to give pure 1 as brown oil (4.1 g, 85.9%).
1H NMR (CDCl3, 400 MHz, ppm): δ = 1.26-1.29 (t, 6H),
4.09-4.11 (q, 4H), 3.16-3.21 (d, 2H), 7.47-7.49 (d, 2H),
8.13-8.15 (d, 2H). 13C NMR (CDCl3, 100 MHz, ppm): δ =
16.3, 39.5, 62.2, 123.8, 131.6, 141.5, 144.9. EI MS (MW = 273.22):
m/z: 273.08.
Synthesis and Characterization of (E)-1,2-Bis(4-nitro-
phenyl)ethane (2). A clear solution of 1 (570 mg, 2.1 mmol)
and 4-nitrobenzaldehyde (315 mg, 2.1 mmol) in dry THF (30 mL)
was treated with NaH (151 mg, 6.3 mmol) under a nitrogen
atmosphere at 50 °C, followed by stirring for 30 min, after which
the reaction was quenched with water carefully. Washed the
mixture with saturated aqueous NaCl and dried over MgSO4
anhydrous. Column chromatography (silica gel, 400 mesh, pet-
roleum ether/ethyl acetate = 6:1 as eluent) afforded pure 2
(235 mg, 41.7%) as a pale yellow solid. 1H NMR (CDCl3,
400 MHz, ppm): δ = 7.34 (s, 2H), 7.68-7.71 (d, 4H),
8.26-8.28 (d, 4H). 13C NMR (CDCl3, 100 MHz, ppm): δ =
123.8, 127.4, 129.1, 143.6, 147.3. EI MS (MW = 270.24): m/z:
270.03.
2. Experimental Section
General Information. Chemicals and solvents were reagent
grades and all commercially available. Compound 4 was synthe-
sized according to previous report.18 Solvents were redistilled
prior to use. The water used throughout all experiments was
purified with a Milli-Q equipment. Detailed synthesis routes for
M2 and M3 are outlined as shown in Scheme 1. Proton and
(14) Bakhtiari, A. B. S.; Hsiao, D.; Jin, G.; Gates, B. D.; Branda, N. R. Angew.
Chem., Int. Ed. 2009, 48, 4166.
(15) (a) Kim, K.-H.; Kim, J.-U.; Cha, S.-H.; Lee, J.-C. J. Am. Chem. Soc. 2009,
131, 7482. (b) Wang, Z.; Li, Z.; Liu, Z. J. Phys. Chem. C 2009, 113, 3899.
(16) (a) Adzima, B. J.; Aguirre, H. A.; Kloxin, C. J.; Scott, T. F.; Bowman, C. N.
Macromolecules 2008, 41, 9112.
(17) Liu, X.; Zhu, M.; Chen., S.; Yuan, M.; Guo, Y.; Song, Y.; Liu, H.; Li, Y.
Langmuir 2008, 24, 11967.
(18) Eldo, J.; Arunkumar, E.; Ajayaghosh, A . Tetrahedron Lett. 2000, 41, 6241–
6244.
Synthesis and Characterization of (E)-4,40-(Ethene-1,2-diyl)-
dianiline (3). To a suspension of 2 (150 mg, 0.55 mmol) in mixed
solvent of ethanol (10 mL) and ethyl acetate (10 mL) was added
concentrated HCl(aq) (5 mL) under stirring at room temperature.
Subsequently, SnCl2 2H2O (1.25 g, 5.5 mmol) was added, and the
3
3180 DOI: 10.1021/la903838w
Langmuir 2010, 26(5), 3179–3185

Liu et al.
Article
mixture was heated to 80 °C for 5 h. After cooling to room
temperature, the reaction mixture was filtered and the precipitate
was discarded. The filtrate was concentrated and washed with
saturated aqueous NaCl and dried over MgSO4 anhydrous. After
removal of the solvent, it affords pure 3 (112 mg, 96.0%) as yellow
solid. 1H NMR (CDCl3, 400 MHz, ppm): δ = 6.66-6.67 (d, 4H),
6.84 (s, 2H), 7.29-7.30 (d, 4H). 13C NMR (CDCl3, 100 MHz,
ppm): δ = 114.1, 122.5, 127.4, 129.6, 147.6. EI MS (MW =
210.27): m/z: 210.12.
was heated at 100 °C for another 2 h. The resulting clear solution
was poured into ice water after cooling to room temperature. A
precipitate was formed after 10 min, and the mixture was vigo-
rously stirred for an additional hour. The precipitate was filtered
and washed with cold distilled water, which was further purified
by silica gel column (400 mesh, petroleum ether/ethyl acetate =
4:1 as eluent) to afford pure M3 (145 mg, 47.1%) as a yellow solid.
1H NMR (CDCl3, 400 MHz, ppm): δ = 1.01-1.05 (t, 6H),
1.57-1.60 (m, 4H), 1.85-1.89 (m, 4H), 4.06-4.09 (t, 4H), 6.87 (s,
4H), 7.13 (s, 2H), 7.14-7.17 (d, 2H), 7.34-7.37 (d, 4H),
7.41-7.44 (d, 2H), 7.47-7.51 (d, 4H). 13C NMR (CDCl3,
100 MHz, ppm): δ = 14.0, 18.8, 31.7, 68.7, 114.6, 115.9, 123.6,
124.3, 125.3, 128.1, 128.8, 131.9, 135.8, 145.7, 161.5. MALDI
TOF MS (MW = 616.70): m/z: 616.40. The UV-vis absorption
spectrum of M3 is shown in Figure S1.
Synthesis and Characterization of (E)-1,10-(4,40-(Ethene-
1,2-diyl)bis(4,1-phenylene))bis(1H-pyrrole-2,5-dione) (M2).
A suspension of 3 (90 mg, 0.43 mmol) and maleic anhydride
(126 mg, 1.29 mmol) in chloroform (30 mL) was heated to reflux
for 8 h. The resulting dark yellow precipitate was filtered, washed
with chloroform, and dried. Subsequently, acetic anhydride
(5 mL) and sodium acetate (14 mg, 0.17 mmol) were added. The
mixture was heated at 100 C for another 1.5 h. The resulting clear
solution was poured into ice water after cooling to room tem-
perature. A precipitate was formed after 10 min, and the mixture
was vigorously stirred for an additional hour. The precipitate was
filtered and washed with cold distilled water, which was further
purified by silica gel column (400 mesh, petroleum ether/ethyl
acetate = 5:1 as eluent) to afford pure M2 (120 mg, 75.7%) as a
beige solid. 1H NMR (CDCl3, 400 MHz, ppm): δ = 6.94 (s, 4H),
6.93-6.95 (d, 2H), 7.14-7.16 (d, 4H), 7.89-7.91 (d, 4H). 13C
NMR (CDCl3, 100 MHz, ppm): δ = 124.3, 127.4, 128.1, 129.0,
131.9, 135.8, 161.8. EI MS (MW = 370.36): m/z: 370.08. The
UV-vis absorption spectrum of M2 is shown in Figure S1.
Synthesis and Characterization of 4,40-(1E,10E)-2,20-(2,5-
Dibutoxy-1,4-phenylene)bis(ethene-2,1-diyl)bis(nitrobenzene)
(5). A clear solution of 4 (1.2 g, 2.3 mmol) and 4-nitrobenz-
aldehyde(867mg, 5.7 mmol) indry THF(50mL) wastreated with
NaH (276 mg, 11.5 mmol) under a nitrogen atmosphere at 50 °C,
followed by stirring for 30 min, after which the reaction was
quenched with water carefully. The mixture was washed with
saturated aqueous NaCl and dried over MgSO4 anhydrous.
Column chromatography (silica gel, 400 mesh, petroleum ether/
ethyl acetate = 6:1 as eluent) afforded pure 5 (680 mg, 57.3%) as
dark red solid. 1H NMR (CDCl3, 400 MHz, ppm): δ = 1.03-1.06
(t, 6H), 1.58-1.62 (m, 4H), 1.86-1.93 (m, 4H), 4.08-4.11 (t, 4H),
7.14 (m, 2H), 7.20-7.24 (m, 2H), 7.61 (m, 2H), 7.63-7.65 (d, 4H),
8.16-8.19 (d, 4H). 13C NMR (CDCl3, 100 MHz, ppm): δ = 14.1,
19.1, 31.8, 68.8, 114.8, 116.0, 123.8, 125.6, 128.5, 129.0, 143.6,
145.9, 147.1. EI MS (MW = 516.58): m/z: 516.31.
Synthesis and Characterization of Furan Ligand. To a
solution of furfuryl alcohol (981 mg, 10 mmol) and 11-mercap-
toundecanoic acid (2.18 g, 10 mmol) in dry dichloromethane
(85 mL) was added 1-ethyl-3-(3-dimethyllaminopropyl)carbo-
diimide hydrochloride (EDC HCl, 1.92 g, 10 mmol) and
3
4-(dimethylamino)pyridine (DMAP, 122 mg, 1 mmol). The flask
was purged under a N2 atmosphere. The mixture was stirred at
room temperature overnight. After evaporating off the solvent,
the residue was redissolved in a small amount of ethyl acetate
(30mL), washed with water, and dried. Afterpurificationthrough
silica gel (400 mesh, petroleum ether/ethyl acetate = 7:1 as
eluent), it affords pure furan ligand as yellow oil (1.35 g,
45.2%). 1H NMR (CDCl3, 400 MHz, ppm): δ = 1.26-1.38 (m,
12H), 1.55-1.66 (m, 4H), 2.30-2.34 (t, 2H), 2.49-2.54 (q, 2H),
5.06 (s, 2H), 6.35-6.36 (d, 1H), 6.39-6.40 (d, 1H), 7.42 (s, 1H)
13C NMR (CDCl3, 100 MHz, ppm): δ = 24.6, 25.0, 28.2, 28.9,
29.0, 29.3, 29.5, 29.6, 33.3, 34.2, 60.4, 107.6, 110.7, 143.6, 152.1,
173.1. MALDI TOF MS (MW = 298.44): m/z 298.16.
Synthesis and Characterization of Furan-Modified
MPCs (F-Au) via Direct Method (Scheme 2). Following a
modified Brust-Schiffrin method,14 to a clear solution of THF
(40 mL) containing HAuCl4 (aqueous solution, 0.1 M ꢀ 1 mL)
was added dodecanethiol (MD, 20.2 mg, 0.1 mmol) and furan
ligand (29.8 mg, 0.1 mmol) under vigorous stirring at room
temperature. After 20 min of reaction, a freshly prepared aqueous
solution of NaBH4 (19.0 mg, 0.5 mmol) was added dropwise with
a syringe in 10 min. Subsequently, the mixture was stirred for
another 1 h to form a clear brown solution without further change
in color. After removal of the solvent under reduced pressure, the
residue was redispersed in ethanol. Upon repeated centrifugation
and washing with ethanol, the obtained brown powder was dried
Synthesis and Characterization of 4,40-(1E,10E)-2,20-(2,5-
Dibutoxy-1,4-phenylene)bis(ethene-2,1-diyl)dianiline (6). To
a suspension of 5 (517 mg, 1.0 mmol) in mixed solvent of ethanol
(15 mL) and ethyl acetate (15 mL) was added concentrated
HCl(aq) (7 mL) under stirring at room temperature. Subse-
1
in vacuo overnight to afford F-Au (28 mg). H NMR (CDCl3,
400 MHz, ppm): δ = 0.88 (br, 6H), 1.26 (br, 50H), 1.54-1.76 (br,
8H), 2.17 (br, 2H), 2.51 (br, 1H), 2.84 (br, 1H), 5.05 (br, 2H), 6.35
(br, 1H), 6.39 (br, 1H), 7.41 (br, 1H). The F-Au stock solution was
prepared by dissolving F-Au (10 mg) in degassed THF (10 mL),
which was examined by UV-vis absorption spectrum and TEM
observation (Figure S3). The average size of F-Au was determined
as 1.8 ( 0.4 nm in diameter.
quently, SnCl2 2H2O (2.27 g, 5.5 mmol) was added, and the
3
mixture was heated to 80 °C for 6 h. After cooling to room
temperature, the reaction mixture was filtered and the precipitate
was discarded. The filtrate was concentrated and washed with
saturated aqueous NaCl and dried over MgSO4 anhydrous. After
removal of the solvent, it affords pure 6 (445 mg, 97.5%) as yellow
solid. 1H NMR (CDCl3, 400 MHz, ppm): δ = 1.00-1.04 (t, 6H),
1.53-1.60 (m, 4H), 1.80-1.87 (m, 4H), 3.74 (br, 4H), 4.05-4.07
(d, 4H), 6.67-6.70 (d, 4H), 6.95 (s, 2H), 7.00-7.08 (d, 4H),
7.34-7.36 (d, 4H). 13C NMR (CDCl3, 100 MHz, ppm): δ =
14.3, 19.2, 32.0, 69.1, 114.1, 114.9, 116.3, 122.5, 123.9, 125.8,
129.6, 146.0, 147.6. EI MS (MW = 456.62): m/z: 456.21.
3. Results and Discussion
The OPVs (M2, M3) were obtained in several steps (Scheme 2).
Initially, nitro-substituted OPVs were obtained through several
Wittig-Horner reactions. After reduction with SnCl2, the amino-
OPVs undergo cyclocondensation with maleic anhydride to
afford M2 and M3 in good yields. UV-vis absorption spectra
show typical peaks located at 320 nm for M2 and 340, 395 nm for
M3, which mainly ascribe to π-π* transitions along the OPV
backbones. Furan-modified MPCs (F-Au) were synthesized
following a modified Brust-Schiffrin method (Scheme 3).19
Synthesis and Characterization of 1,10-(4,40-(1E,10E)-
2,20-(2,5-Dibutoxy-1,4-phenylene)bis(ethene-2,1-diyl)bis-
(4,1-phenylene))bis(1H-pyrrole-2,5-dione) (M3). A sus-
pension of 6 (228 mg, 0.5 mmol) and maleic anhydride (147 mg,
1.5 mmol) in chloroform (30 mL) was heated to reflux for 12 h.
The resulting dark yellow precipitate was filtered, washed with
chloroform, and dried. Subsequently, acetic anhydride (6 mL)
and sodium acetate (16.4 mg, 0.2 mmol) were added. The mixture
(19) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem.
Soc., Chem. Commun. 1994, 801–802.
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Liu et al.
Scheme 2. Reaction Sequence Leading to OPV Ligands M2 and M3
Scheme 3. Synthesis Procedure for Furan-Modified MPCs (F-Au)
The furan ligand was facile prepared before subjected to one-pot
synthesis of small furan-modified MPCs. As determined by TEM,
the as-obtained MPC was well-dispersed and ∼1.8 ( 0.4 nm in
diameter (Figure S3 in Supporting Information). The
Diels-Alder reaction between OPVs and F-Au leads to formation
of hybrid networks, while the PL from OPV backbones recovered
efficiently along reaction time. It is noted that molar ratio of
OPVs and F-Au plays a key role in both reaction efficiency and
physical properties of the resulting hybrid complexes. By per-
forming a concentration-dependent experiment, we found that
molar ratio at 12 exhibits best in PL recovery (Figure S4 in
Supporting Information). The investigation of photophysical
properties and morphologies was carried out in situ to maintain
the intrinsic properties from as-obtained OPV-MPCs using as-
determined molar ratio throughout all experiments.
The thermal-driven covalent assembly of M3-Au particles with
dAu = 1.8 nm was investigated in detail. UV-vis absorption
studies in toluene indicate an isolated species featuring sum
absorption of M3 (395 nm) and F-Au. TEM on samples cast
from toluene confirm the presence of isolated particles. After 12 h
of Diels-Alder reaction, however, we found that M3-Au self-
assemble into large structures in a reversible manner. Evidence for
this comes from the observation of a typical SPR absorption band
with a maximum at λ = 530 nm (Figure 1A). While heating the
M3-Au solution at 110 °C, the SP band disappears with a spectral
position similar to that found before Diels-Alder reaction. Upon
cooling at room temperature for 12 h, the SP band reappears at
530 nm. We ascribe the emergence of SP band at 530 nm to a
thermoreversible transition from dispersed to aggregated hybrid
complexes. The SP band can be interpreted in terms of the Mie
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Liu et al.
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exhibits completely quenched PL, which mainly due to conjuga-
tion with maleimide moieties.20 The conjugated CdC bond of
maleimide is also known to undergo Diels-Alder reaction with
furan.21 The resulting succinimide group no longer exhibits a
strong quenching effect, and the PL of chromophore is recovered.
As shown in Figure 1B, the relative PL intensities (I - I0)/I0
gradually increased as the time prolonged, while I denotes the
observed PL intensity of M3-Au at various stage of Diels-Alder
reaction, and I0 is the original PL intensity of M3. Upon repeated
thermal treatments, the PL intensities show well-reversible transi-
tion states (Figure 1C). These results indicate that M3 and F-Au
were progressively cross-linked to form hybrid superstructures
through Diels-Alder reaction.
TEM investigation ondifferent reactionstages provides further
insight into the formation of M3-Au superstructures. The MPCs
remain dispersed within first 1 h without obvious size and shape
changes (Figure 2A). After 12 h of reaction, MPCs cross-link to
form large-scale self-assemblies consisting of densely packed M3-
Au (Figure 2B, also see Figure S6 in Supporting Information).
While zooming in, TEM reveals internal structure that the MPCs
remain discrete and hold similar void space (Figure 2C and
Figure S7 in Supporting Information). This illustrates ordered
self-assembly driven by the bridge effect of peripheral OPV. The
rigid backbones with covalently bonding on MPC surfaces
prevent them from agglomeration, which enable thermally driven
disassembly upon retro-Diels-Alder reaction. The reversibility of
Diels-Alder reaction has been explored extensively for fabrica-
tion of thermally “remendable” polymeric materials without
sacrificing stability and specific functionalities.4a,22 In our system,
the reversibility of M3-Au was examined by monitoring the PL
and superstructures changes over repeated thermal treatments.
After Diels-Alder reaction, the PL in M3-Au is recovered
efficiently as compared to that of M3 (Figure 1C). However,
the M3-Au exhibits quenched PL upon treatment at elevated
temperature (110 °C) for 2 h. The thermochromic effect reveals
that the PL of these hybrids can be switched “on” and “off” with
specific thermal stimulus. In other words, the MPCs may undergo
a disassembly process to recover dispersed clusters. Upon thermal
treatment at 110 °C after five heating cycles, the MPCs remain
well-dispersed with little increase in particle diameter and size
distribution (Figure 2D). Dynamic light scattering (DLS) experi-
ments were performed on thermal reversible formation of M3-Au
hybrids (Figure S8 in Supporting Information). The results show
that MPCs remain dispersed with diameter around 2 nm at initial
stage. After sufficient cross-linking, the M3-Au hybrid exhibits
large-scale aggregation with average diameter of ca. 600 nm,
which is in good accordance with TEM observation. We have
performed further measurement on samples after retro-
Diels-Alder reaction. DLS size distribution data show dramatic
decrease in average diameter (ca. 3 nm) of nanoparticles as
compared to highly cross-linked hybrids. However, it shows a
little increase in diameter in comparison to initial state, which
is mainly due to slight agglomeration of nanoparticles under
repeated thermal treatments. Figure 3 shows typical Diels-Alder
reaction and retro-Diels-Alder between M3 and F-Au, which
leads to order self-assembly of MPCs as well as capability of PL
modulation in such hybrid systems. At initiate stage, M3 exhibits
weak PL emission due to photoinduced electron transfer (PET)
Figure 1. (A) UV-vis absorption of M3-Au (dAu = 1.8 ( 0.4 nm,
optical path length 1 cm, [M3-Au] = 4.2 μM) in toluene. (B) Time-
dependent PL enhancement on M3-Au (open square, dAu =
1.8 nm, [M3-Au] = 4.2 μM) and data obtained in mixture of M3
and MD-Au (close square). (C) PL spectra of M3 (5 ꢀ 10-5 M,
black curve), M3-Au ([M3-Au] = 4.2 μM, red curve), and M3-Au
after being treated at 110 °C for 2 h (blue curve). All the PL spectra
were recorded under 395 nm light illumination.
scattering theory results from transition dipoles coupling while
metal particles were restricted at close distance. Our assignment is
also supported by PL measurements that reveal progressively PL
increase and well reversibility upon thermal treatments. M3
(21) (a) Shi, M.; Wosnick, J. H.; Ho, K.; Keating, A.; Shoichet, M. S. Angew.
Chem., Int. Ed. 2007, 46, 6126. (b) Szalai, M. L.; McGrath, D. V.; Wheeler, D. R.; Zifer,
T.; McElhanon, J. R. Macromolecules 2007, 40, 818.
(22) (a) Chen, X.; Wudl, F.; Mal, A. K.; Shen, H.; Nutt, S. R. Macromolecules
2003, 36, 1802. (b) Murphy, E. B.; Bolanos, E.; Schaffner-Hamann, C.; Wudl, F.; Nutt,
S. R.; Auad, M. L. Macromolecules 2008, 41, 5203.
(20) Guy, J.; Caron, K.; Dufresne, S.; Michnick, S. W.; Skene, W. G.; Keillor, J.
W. J. Am. Chem. Soc. 2007, 129, 11969.
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Liu et al.
Figure 2. TEM images obtained at different time of D-A reaction between M3 and F-Au: (A) 15 min, (B) 12 h, (C) magnification of (B), (D)
treated at110 °C afterfiveheating cycles. Redcirclein(C) indicatestypicalordered aggregation ofM3-Au. Arrows in(D) showpartlyincrease
in diameter of MPCs after thermal treatment. Scale bar in (B) represents 50 nm; others are 5 nm.
from OPV backbone to maleimide. After efficient Diels-Alder
reaction, MPCs are restricted to form close packed assemblies,
and the PL emission enhances upon depression of the PET
process. Moreover, the cross-linked superstructures can also
decompose to release M3 and F-Au (i.e., thermal reversible
formation of OPV-MPCs hybrids). It should be pointed out here
that thermal treatment on MPCs at high temperature may cause
partly detachment of ligands. The ligand-deficient MPCs tend to
form large, insoluble aggregates. During each heating cycle,
ligand detachment from gold cluster surfaces is regarded as main
obstacle for reversibility of covalent assemblies between OPVs
and MPCs. Nevertheless, the forward and backward Diels-Alder
reaction has very good reversibility even after 20 cycles.
On the basis of the results obtained in both optical and size
distribution observations, we suggest that surface-initiated cross-
linking progressively occurs between furan group and pedant
maleimide at ambient temperature. The sufficient Diels-Alder
reactionreveals immobilization of nanoparticles within polymeric
network, which exhibit large-scale self-assembled MPC super-
structures. Thermally active OPV-MPCs are capable to self-
assemble in a reversible manner. After treated under elevated
temperature, retro-Diels-Alder reaction leads to decomposition
of each component. Thus, the F-Au gets redispersed, which can
undergo forward and backward Diels-Alder reaction repeatedly.
Specifically, Diels-Alder reaction between M3 and F-Au was
Figure 3. Representative illustration for the thermal reversible
Diels-Alder reaction leading to ordered MPC self-assemblies.
further investigated by time-resolved photoluminescence spec-
troscopy (Figure 4, see Supporting Information for details). The
PL decay curve obtained from initial stage of Diels-Alder
reaction exhibits dual exponential fitting model. PL lifetime was
obtained as τ1 = 328 ps and τ2 = 2.72 ns. It is considered that
there are mainly physical blend of M3 and F-Au at very early stage
of reaction. Thus, Brownian motion of free F-Au leads to
collisions between ligands and MPCs. Obviously, both collision
state and free state between M3 and F-Au are almost equally
proportional. τ1 mainly arises from M3 and F-Au in collision state
because of PET between them, and τ2 is from intrinsic OPV
backbones of free M3. The proportion is calculated as 46% and
54% for τ1 and τ2, respectively, which prove further evidence on
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Liu et al.
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present study on the OPV linked MPCs suggests a dramatic PL
enhancement (several hundred times) while performing Diels-
Alder reaction. We suppose that the reversibility for modulating
OPV PL as well as gold clusters assembly/disassembly mainly arises
from thermal reversible Diels-Alder reaction. The reasons can be
summarized as follows: First, OPVs show totally quenched PL due
to PET from OPV backbone to maleimide. After covalently
bonding to furan on the MPC surfaces, the intrinsic PL from
OPV recovered due to depression of PET process. Second, the size of
as-prepared F-Au is too small to support SP band. Thus, the less
spectral overlap with emission of OPVs decrease the excited state
energy transfer between OPV and F-Au.24 On the basis of present
experiment data, we note that the PET process shows less effect on
PL recovery during Diels-Alder reaction. This is mainly because
that the quenching efficiency induced by PET is less comparable
with that of PL enhancement arising from Diels-Alder adducts,
and the electron transfer resulted in feeble PL quenching and can be
neglected in such systems herein. Third, rigid OPV structures
provide interparticle underprop during the formation of OPV-
MPCs superstructure. These enable the intrinsic properties of ligand
and nanoparticles to be maintained, which facilitating construction
and reversible modulation of such well-defined and large-scale
ordered superstructures.
In conclusion, we have fabricated organic-inorganic hybrid
materials consisting of conjugated oligomer and gold nano-
particles by performing mild Diels-Alder reaction on both sides
of the counterparts. The as-prepared hybrid materials exhibit
well dispersity and well-defined interfaces between organic com-
ponent and inorganic nanoparticles. Furthermore, we have
demonstrated that thermal modulation of PL and MPCs assem-
bly/disassembly is possible with smart reversibility over repeated
heating cycles. With dramatic PL enhancement and well-defined
superstructures resulting from hybrid OPV-MPCs, these environ-
mentally stimuli-responsive hybrid materials can be considered as
homogeneous and intelligent materials with a variety of novel
technological uses.
Figure 4. Time-resolved photoluminescence for M3 and F-Au
before and after Diels-Alder cross-linking.
the transient state of M3 and F-Au blend. After sufficient
Diels-Alder cross-linking, the M3-Au hybrids exhibit PL decay
curve as single-exponential fitting model with PL lifetime of τ =
1.05 ns. In comparison with τ1 in the physical blend, the longer
lifetime owes to depressed PET from OPV backbone to MPC.
Additionally, cross-linked M3-Au no longer shows random colli-
sion due to covalent restriction between organic ligands and
MPCs. It should be noted that PL decay curve of M3-Au shows
an initial increase followed by decay as compared to that of
physical mixture. This behavior probably dues to energy transfer
from SPR absorption of MPCs to bonded OPV ligand. Further-
more, electron transfer in physical mixture of M3 and F-Au leads
to sharp rising processes because of the random collisions effects,
which is more close to instrument response function (IRF) curve.
However, PL investigations from physical mixture of OPVs
and furan-deficient MPCs (dodecanethiolate-modified MPCs,
MD-Au, same concentration as F-Au) reveal that no PL recovery
occurs (Figure 2A), and the MPCs remain dispersed (data not
shown). This suggests that there is no interaction between furan-
deficient MPCs and OPVs, which provides further evidence for
the Diels-Alder reaction between OPVs and F-Au.
Acknowledgment. This work was supported by the National
Nature Science Foundation of China (20531060 and 20721061)
and the National Basic Research 973 Program of China.
Supporting Information Available: Additional large-scale
and detailed TEM images of OPV-MPCs, fluorescence
lifetime measurements, DLS size distribution data, and
Figures S1-S8. This material is available free of charge via
the Internet at http://pubs.acs.org.
It is known that either electron transfer or energy transfer between
chromophore and gold cluster is considered to be a major pathway
for quenching excited state of chromophore.23 Interestingly, the
(23) (a) Thomas, K. G.; Kamat, P. V. Acc. Chem. Res. 2003, 36, 888. (b) Sen, T.;
Patra, A. J. Phys. Chem. C 2008, 112, 3216. (c) Kotiaho, A.; Lahtinen, R.; Lehtivuori,
H.; Tkachenko, N. V.; Lemmetyinen, H. J. Phys. Chem. C 2008, 112, 10316.
(24) 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.
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