Aggregation and host-guest interactions in dansyl-substituted poly(acrylate)s in the presence of β-cyclodextrin and a β-cyclodextrin dimer in aqueous solution: A UV-vis, fluorescence, 1H NMR, and rheological study

Article abstract of DOI:10.1021/ma202055a

A UV-vis, steady-state, and time-resolved fluorescence 2D ¹H NOESY NMR spectroscopic and rheological study of new poly(acrylate)s 3% randomly substituted with either N-(2-aminoethyl)-, N-(6-aminohexyl)-, or N-(12-aminododecyl)-5-dansyl-sulfonamide to give the substituted polymers PAADSen, PAADShn, and PAADSddn, respectively, is reported. Their dansyl substituent aggregation and complexation by β-cyclodextrin, βCD, and its covalently linked dimer N,N′-bis(6^A-deoxy-6 ^A-β-cyclodextrin)urea, 66βCD₂ur, is also reported. The βCD complexation of the dansyl substituents is characterized by apparent complexation constants, K = 89, 105, and 55 dm³ mol ^-1 for PAADSen, PAADShn, and PAADSddn, respectively, and the analogous complexations by 66βCD₂ur are characterized by K = 3.04 × 10³, 3.42 × 10⁴, and 2.42 × 10⁵ dm³ mol^-1 at pH 7.0 in aqueous 0.10 mol dm^-3 NaCl at 298.2 K. Under the same conditions the dansyl substituent shows three fluorescence decay constants assigned to the aggregated (τ₁ = 2.2, 2.5, and 3.2 ns), single (τ₂ = 5.0, 5.3, and 9.5 ns), βCD (τ₃ = 13.2, 11.7 ns, and undetected), and 66βCD₂ur complexed dansyl substituent states (τ₃ = 13.9, 20.8, and 19.3 ns) where the values in each data set correspond to PAADSen, PAADShn, and PAAddn solutions, respectively. 2D ¹H NOESY NMR spectroscopy provides additional insight into dansyl substituent complexation by βCD and 66βCD₂ur, as do rheological studies. These data are interpreted in terms of the factors affecting dansyl substituent fluorescence quenching and the impact of tether length on dansyl substituent aggregation, complexation, and network formation.

Full text of DOI:10.1021/ma202055a

ARTICLE
pubs.acs.org/Macromolecules
Aggregation and HostꢀGuest Interactions in Dansyl-Substituted
Poly(acrylate)s in the Presence of β-Cyclodextrin and a β-Cyclodextrin
Dimer in Aqueous Solution: A UVꢀVis, Fluorescence, 1H NMR, and
Rheological Study
Jie Wang,† Duc-Truc Pham,‡ Tak W. Kee,‡ Scott N. Clafton,‡ Xuhong Guo,† Philip Clements,‡
Stephen F. Lincoln,*,‡ Robert K. Prud’homme,§ and Christopher J. Easton^
†State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai, 200237, China
‡School of Chemistry and Physics, University of Adelaide, Adelaide, SA 5005, Australia
§Department of Chemical Engineering, Princeton University, Princeton, New Jersey 08544, United States
^Research School of Chemistry, Australian National University, Canberra, ACT 0200, Australia
S Supporting Information
b
ABSTRACT: A UVꢀvis, steady-state, and time-resolved fluor-
1
escence 2D H NOESY NMR spectroscopic and rheological
study of new poly(acrylate)s 3% randomly substituted with
either N-(2-aminoethyl)-, N-(6-aminohexyl)-, or N-(12-
aminododecyl)-5-dansyl-sulfonamide to give the substituted
polymers PAADSen, PAADShn, and PAADSddn, respectively,
is reported. Their dansyl substituent aggregation and complexa-
tion by β-cyclodextrin, βCD, and its covalently linked dimer N,
N0-bis(6A-deoxy-6A-β-cyclodextrin)urea, 66βCD2ur, is also reported. The βCD complexation of the dansyl substituents is
characterized by apparent complexation constants, K = 89, 105, and 55 dm3 molꢀ1 for PAADSen, PAADShn, and PAADSddn,
respectively, and the analogous complexations by 66βCD2ur are characterized by K = 3.04 ꢁ 103, 3.42 ꢁ 104, and 2.42 ꢁ 105 dm3
molꢀ1 at pH 7.0 in aqueous 0.10 mol dmꢀ3 NaCl at 298.2 K. Under the same conditions the dansyl substituent shows three
fluorescence decay constants assigned to the aggregated (τ1 = 2.2, 2.5, and 3.2 ns), single (τ2 = 5.0, 5.3, and 9.5 ns), βCD (τ3 = 13.2,
11.7 ns, and undetected), and 66βCD2ur complexed dansyl substituent states (τ3 = 13.9, 20.8, and 19.3 ns) where the values in each
data set correspond to PAADSen, PAADShn, and PAAddn solutions, respectively. 2D 1H NOESY NMR spectroscopy provides
additional insight into dansyl substituent complexation by βCD and 66βCD2ur, as do rheological studies. These data are interpreted
in terms of the factors affecting dansyl substituent fluorescence quenching and the impact of tether length on dansyl substituent
aggregation, complexation, and network formation.
’ INTRODUCTION
formation in more concentrated solutions. The poly(acrylate)s
studied are 3% randomly substituted by N-(2-aminoethyl)-,
N-(6-aminohexyl)-, and N-(12-aminododecyl)-5-dansylsulfona-
mide, PAADSen, PAADShn, and PAADSddn, in which the
dansyl substituent tether length progressively increases (Scheme 1).
This facilitates studies of the effect of tether length on dansyl
substituent, aggregation and complexation by β-cyclodextrin, βCD,
and the linked dimer N,N0-bis(6A-deoxy-6A-β-cyclodextrin)-
urea, 66βCD2ur, in dilute aqueous solution where interactions
occur dominantly within individual substituted poly(acrylate)
strands.
The use of fluorescent polymers has been reported in studies
of scintillators,1 luminescent solar concentrators,2 laser-resistant
materials,3 fiber-optic sensors,4 and laser dyes.5 Such polymers
usually consist of a backbone substituted with fluorophores such
as the naphthyl,6ꢀ9 pyrenyl,7,10ꢀ14 and dansyl8,15ꢀ19 entities.
The fluorescence intensities and lifetimes of such fluorophores
are sensitive to local environment such that steady-state and
time-dependent fluorescence studies have provided considerable
insight into polymer interactions.6ꢀ19
As part of an exploration of substituent interactions of
randomly substituted poly(acrylate)s, PAA,20ꢀ25 we report a
UVꢀvis, steady-state, and time-resolved fluorescence and 2D 1H
NOESY NMR study of the variation of dansyl substituent
spectroscopic response to environmental change at the molec-
ular level in dilute aqueous solution. These observations are used
to interpret macroscopic observations gained through rheologi-
cal studies of variations in the viscosities in terms of network
In more concentrated solutions aggregation of dansyl substit-
uents between substituted poly(acrylate) strands occur to form
networks which may either be disrupted through βCD forming
Received: September 8, 2011
Revised:
November 3, 2011
Published: November 22, 2011
r
2011 American Chemical Society
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|

Macromolecules
ARTICLE
Scheme 1. 3% Randomly Substituted Poly(acrylate)s, βCD,
and the 66βCD2ur Dimer in Which Two βCD Are Each
Substituted at the C6A Carbon of One Glucopyranose Unit by
Urea and Covalently Linkeda
solutions were combined and washed successively with water (5 ꢁ
50 cm3) and brine (50 cm3) and dried over magnesium sulfate. The
solution was filtered, and the solvent was removed under reduced
pressure to give the crude product as a yellow-green waxy solid. This
was recrystallized from benzene/hexane to give the pure products as
light green needles (158 mg, 54%). 1H NMR: δH (DMSO-d6) 8.51 (d,
1H, J = 8.4 Hz, ArH); 8.35 (d, 1H, J = 8.7 Hz, ArH); 8.15 (d, 1H, J = 8.7
Hz, ArH); 7.68 (m, 2H, ArH); 7.31 (d, 1H, J = 7.5 Hz, ArH); 2.87 (s, 6H,
N(CH3)2); 2.82 (t, 2H, SO2NHCH2); 2.50 (t, 2H, CH2NH2). GCMS
calcd for C14H19N3O2S m/z 293.12; found 293.20.
Preparation of N-(6-Aminohexyl)-5-dansylsulfonamide.
A solution of dansyl chloride (270 mg, 1 mmol) in tetrahydrofuran
(25 cm3) was added dropwise over 3 h to a vigorously stirred solution
of 1,6-diaminohexane (2.0 g, 17 mmol) in tetrahydrofuran (25 cm3).
The solvent was removed under reduced pressure, and the residue was
dissolved in dichloromethane (20 cm3) and 10% hydrochloride acid
(20 cm3). The layers were separated, and the aqueous layer was washed
with dichloromethane (3 ꢁ 20 cm3). This aqueous solution was made
basic by addition of 10% w/w aqueous sodium hydroxide and extracted
with dichloromethane (5 ꢁ 20 cm3). The dichloromethane solutions
were combined and washed successively with water (5 ꢁ 50 cm3) and
brine (50 cm3) and dried over magnesium sulfate. The solvent was
removed under reduced pressure to give a yellow-green oil. This was
dissolved in dichloromethane (5 cm3) and loaded onto basic alumina
column (160 ꢁ 15 mm, Brockman activity III) which was eluted with a
gradient of dichloromethane:methanol from 100:0 to 80:20. Fractions
containing the product were combined and concentrated under reduced
pressure to give the product as a light green oil (180 mg, 52%). 1H NMR:
δH (DMSO-d6) 8.51 (d, 1H, J = 9.0 Hz, ArH); 8.35 (d, 1H, J = 8.7 Hz,
ArH); 8.14 (d, 1H, J = 6.3 Hz, ArH); 7.68 (m, 2H, ArH); 7.31 (d, 1H, J =
7.2 Hz, ArH); 2.86 (s, 6H, N(CH3)2); 2.82 (t, 2H, SO2NHCH2); 2.43
(t, 2H, CH2NH2). GCMS calcd for C18H27N3O2S m/z 349.18; found
349.20.
a The narrow and wide ends of the truncated cone representing βCD
correspond to the rings of primary and secondary hydroxyl groups,
respectively.
Preparation of N-(12-Aminododecyl)-5-dansylsulfona-
mide. A solution of dansyl chloride (270 mg, 1 mmol) in tetrahydro-
furan (25 cm3) was added dropwise over 3 h to a vigorously stirred
solution of 1,12-diaminododecane (2.0 g, 10 mmol) in tetrahydrofuran
(25 cm3). After stirring overnight the yellow-green reaction mixture was
filtered. The solvent was removed under reduced pressure, and the
residue was dissolved in dichloromethane (20 cm3). This solution was
washed successively with 10% w/w aqueous sodium hydroxide (3 ꢁ
30 cm3), water (5 ꢁ 50 cm3), and brine (50 cm3) and dried over
magnesium sulfate. The filtered solution was evaporated under reduced
pressure to give a white solid. This was suspended in dichloromethane
(10 cm3), filtered, and loaded onto a basic alumina column (160 ꢁ
15 mm, Brockman activity III). The column was eluted with a gradient of
dichloromethane:methanol from 100:0 to 90:10. Fractions containing
the product were combined and concentrated under reduced pressure to
give the product as a light green oily solid (265 mg, 61%). 1H NMR: δH
(DMSO-d6) 8.50 (d, 1H, J = 8.4 Hz, ArH); 8.34 (d, 1H, J = 8.4 Hz, ArH);
8.14 (d, 1H, J = 7.2 Hz, ArH); 7.67 (m, 2H, ArH); 7.29 (d, 1H, J = 7.5 Hz,
ArH); 2.86 (s, 6H, N(CH3)2); 2.82 (t, 2H, SO2NHCH2); 2.52 (t, 2H,
CH2NH2). GCMS calcd for C24H39N3O2S m/z 433.28; found 433.45.
Preparation of 3% Substituted Poly(acrylate)s. The new
PAADSen, PAADShn, and PAADSddn 3% randomly substituted poly-
(acrylate)s were prepared from poly(acrylic acid) and the respective
aliphatic amine substituted dansylsulfonamide in the presence of
dicyclohexylcarbodiimide in N-methy-2-pyrrolidone at 60 ꢀC in a similar
manner to that reported for other substitutions2 and according to the
general procedure shown in Scheme 2.20,21,24,25 The three substituted
poly(acrylate)s were isolated as the sodium salts. Yield: 70ꢀ80%.
hostꢀguest complexes with individual dansyl substituents or
may be strengthened through 66βCD2ur simultaneously com-
plexing dansyl substituents from adjacent substituted poly-
(acrylate) strands. This study increases understanding of the
rapidly expanding field in which cyclodextrin hostꢀguest chem-
istry is generating a wide variety of aqueous and solid polymer
systems and networks.20ꢀ32
’ EXPERIMENTAL SECTION
Materials. 1,2-Diaminoethane, 1,6-diaminohexane, and 1,12-diami-
nododecane (Strem), dansyl chloride (Fluka), tetrahydrofuran (Ajax),
N-methylpyrrolidin-2-one (Merck), dicyclohexylcarbodiimide (Aldrich),
and βCD (Nihon Shokuhin Kako Co.) were used as supplied.
Poly(acrylic acid) (Mw = 250 000, Mw/Mn ≈ 2) was purchased from
Aldrich as a 35 wt % aqueous solution and freeze-dried to a constant
weight. Basic alumina (Fluka) was supplied at Brockman activity I,
and the appropriate amount of water was added to give Brockman
activitiy III.
Preparation of N-(2-Aminoethyl)-5-dansylsulfonamide.
A solution of dansyl chloride (270 mg, 1 mmol) in tetrahydrofuran
(25 cm3) was added dropwise over 3 h to a vigorously stirred solution of
1,2-diaminoethane (1.0 g, 17 mmol) in tetrahydrofuran (10 cm3). The
solvent was removed under reduced pressure, and the residue was
dissolved in dichloromethane (20 cm3) and 10% hydrochloride acid
(20 cm3). The layers were separated, and the aqueous layer was washed
with dichloromethane (3 ꢁ 20 cm3). This aqueous solution was then
made basic by addition of 10% w/w aqueous sodium hydroxide and
extracted with dichloromethane (5 ꢁ 20 cm3). The dichloromethane
1
Characterization. Routine H NMR spectra were recorded on a
Varian Gemini ACP-300 spectrometer operating at 300.145 MHz. The
degrees of substitution of the carboxyl groups in poly(acrylate) (PAA) in
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ARTICLE
Scheme 2. General Scheme for the Preparation of 3% Ran-
domly Substituted Poly(acrylate)s
equilibrium which continually readjusts as the monomer form is
complexed by either βCD or 66βCD2ur to form the 1:1 hostꢀguest
complexes exemplified by βCD.PAADSen and 66βCD2ur.PAADSen.
Thus, βCD complexes PAADSen according to eq 1.
K
βCD þ PAADSen h βCD:PAADSen
ð1Þ
The apparent complexation constant, K, at equilibrium is given by
K ¼ ½βCD:PAADSenꢂ=½βCDꢂ½PAADSenꢂ
ð2Þ
Given that [PAADSen]total and [βCD]total are the initial concentrations
of the two complexation partners
½PAADSenꢂtotal ¼ ½βCD:PAADSenꢂ þ ½PAADSenꢂ
ð3Þ
ð4Þ
½βCDꢂtotal ¼ ½βCD:PAADSenꢂ þ ½βCDꢂ
the fluorescence at a particular wavelength is given by
F ¼ IPAADSen½PAADSenꢂ þ IβCD:PAADSen½βCD:PAADSenꢂ
ð5Þ
where F, IPAADSen, and IβCD.PAADSen represent the observed fluorescence
and apparent molar fluorescences of PAADSen and βCD.PAADSen,
respectively. Fluorescence measurements were made in 1 cm path length
cuvettes over the range 450ꢀ600 at 0.5 nm intervals as the βCD
concentration was varied. The value of K was determined by best fitting
the variation of the fluorescence spectra in the range 450ꢀ600 at 0.5 nm
intervals to an algorithm based on eqs 2ꢀ5 as the βCD concentration
was varied using the HypSpec protocol.33 The same protocol was used
for the other βCD systems and the 66βCD2ur systems. Algorithms for
complexations involving 1:2 and 2:1 host/guest stoichiometries in
addition to, or separate from, 1:1 stoichiometry could not be fitted to
the data.
Time-Resolved Fluorescence Measurements. Time-resolved
fluorescence measurements were made on 1.0 wt % substituted poly-
(acrylate) solutions where the total dansyl substituent concentration was
2.95 ꢁ 10ꢀ3 mol dmꢀ3 in aqueous 0.10 mol dmꢀ3 NaCl at pH 7.0 and
298.2 K using the time-correlated single photon counting (TCSPC)
technique.34 The laser source was a mode-locked Ti-sapphire laser
(Spectra Physics Tsunami), tunable from 720 to 1000 nm with a
repetition rate of 80 MHz. The fundamental output from the
Ti-sapphire oscillator was modulated by a Pockels cell (Model 350-160,
Conoptics Inc.) to reduce the repetition rate to about 16 MHz and was
subsequently frequency doubled by focusing tightly into a 0.5 mm BBO
crystal (Eksma Optics). The resulting blue light, which had a central
wavelength of 400 nm, provided the excitation source. A half-wave plate
before a vertical polarizer ensured the polarization of the excitation light.
The fluorescence decays were collected using a time-resolved fluores-
cence spectrometer (Ultrafast System Halcyone). Emission at 540 nm
was collected through a double monochromator fitted with a slit with a
1 nm band-pass. The instrument response function of the apparatus had
a full width at half-maximum (fwhm) of 1 ns. The acquisition time
window had a width of 62.5 ns with 0.05 ns time steps. A cuvette of
0.2 cm path length was used for the time-resolved measurements of the
samples. Multiexponential fits of the time-resolved fluorescence traces
were obtained using the Ultrafast System Surface Explorer protocol.35
Rheology. Rheological measurements were carried out with a
Physica MCR 501 (Anton Parr GmbH) stress-controlled rheometer
with a 25 mm cone and plate geometry. Temperature was controlled at
298.2 ( 0.1 K by a Peltier plate. All studies were carried out in 0.10 mol
dmꢀ3 aqueous NaCl solution at pH 7.0 adjusted with 0.10 mol dmꢀ3
aqueous NaOH solution.
PAADSen, PAADShn, and PAADSddn were determined to be 3.0% by
1H NMR spectroscopy through a comparison of the relative resonance
areas of the dansyl H2ꢀ4,6ꢀ8 and methyl protons and PAA methylene
protons. The 2D 1H NOESY NMR spectra were recorded on a Varian
Inova 600 spectrometer operating at 599.957 MHz using a standard
pulse sequence with a mixing time of 0.3 s. Solutions for hostꢀguest
complexation studies contained either PAADSen, PAADShn, or
PAADSddn (10 mg) and βCD (3.5 mg) in 1 cm3 D2O such that the
βCD and dansyl substituents of the substituted poly(acrylate)s were
equimolar. The pD was adjusted to 7.0 ( 0.2 with 0.10 mol dmꢀ3 NaOD
in D2O for each solution, which was also 0.10 mol dm3 in NaCl.
Solutions were allowed to equilibrate at the thermostated probe
temperature of 298.2 ( 0.1 K for 30 min in 5 mm NMR tubes prior
to recording their spectra.
The UVꢀvis and fluorescence spectra of solutions 0.10 mol dm3 in
NaCl whose pH was adjusted to 7.0 ( 0.2 with 0.10 mol dmꢀ3 NaOH
were recorded with a Varian-Cary 300 UVꢀvis spectrophotometer
using 1 cm path length matched quartz cells and a Varian-Cary Eclipse
fluorimeter using standard cuvettes, respectively. The fluorescence
excitation wavelength was 331 nm in all cases, and excitation and
emission slit widths were set at 5 mm. Solutions were thermostated to
within (0.1 K of the desired temperature.
The model used for determining the apparent complexation con-
stant assumes that the dansyl groups in PAADSen, PAADShn, and
PAADSddn exist in aggregated and monomeric forms in a labile
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Figure 1. UVꢀvis absorption spectra of PAADSen where the dansyl
substituent concentration is 1.5 ꢁ 10ꢀ4 mol dmꢀ3 at 283.2, 303.2, 323.2,
and 343.2 K in aqueous 0.01 mol dmꢀ3 NaCl at pH 7.0. The arrows
indicate the direction of change with increase in temperature. Isosbestic
points occur at 284 and 342 nm, and λmax and the molar absorbance,
εmax, respectively change from 327 nm and 4752 dm3 molꢀ1 cmꢀ1 at
283.2 K to 329 nm and 4392 dm3 molꢀ1 cmꢀ1 at 343.2 K. The path
length is 1 cm.
’ RESULTS AND DISCUSSION
Preparation of 3% Randomly Dansyl-Substituted Poly-
(acrylate)s. The new randomly substituted dansyl-substituted
poly(acrylate)s were prepared with differing length linear alipha-
tic tethers as shown in Scheme 2. The tethers were attached to
the dansylsulfonamide by reacting with the appropriate linear
aliphatic diamine at room temperature in tetrahydrofuran
(THF) for 3 h. These amine-substituted dansylsulfonamide
were then reacted with poly(acrylic acid) of Mw = 250 000 and
Mw/Mn ≈ 2 in N-methy-2-pyrrolidone (NMP) in the presence
of dicyclohexylcarbodiimide (DCC) at 60 ꢀC for 2ꢀ3 days in
a similar manner to that reported elsewhere for other substitu-
tions.20,21,24,25 The reacted solutions were made basic with
NaOH, and the sodium poly(acrylate)s were separated and
purified by dialysis.
Figure 2. (top) Fluorescence increase of a PAADShn solution for which
the dansyl substituent concentration is 1.0 ꢁ 10ꢀ5 mol dmꢀ3
(corresponding to 0.0034 wt % in PAADShn) with 25 sequential
additions (25 mm3 each) of βCD solution (1.41 ꢁ 10ꢀ2 mol dmꢀ3).
Both aqueous solutions are 0.10 mol dmꢀ3 in NaCl with a pH = 7.0 at
298.2 K. The arrow indicates the increase in fluorescence with each
addition of βCD solution. Excitation is at 331 nm. (bottom) Fluorescence
variation at 524 nm and the line representing the best fit of an algorithm
for a 1:1 hostꢀguest complexation by βCD of a dansyl substituent to data
at 0.5 nm intervals over the wavelength range 450ꢀ600 nm.
When the total dansyl substituent concentrations are in-
creased in the range 5.64 ꢁ 10ꢀ6ꢀ1.41 ꢁ 10ꢀ3 mol dmꢀ3 in
solutions of either PAADSen, PAADShn, or PAADSddn in
0.10 mol dmꢀ3 NaCl aqueous solution at pH 7.0 and 298.2 K
solutions, the absorbance of the dansyl substituent increases
linearly over most of the range and shows a systematic small
negative deviation from linearity at the higher end of the
concentration range consistent with intermolecular interactions
between dansyl substituents on different poly(acrylate) strands
occurring. However, these small departures from linearity are
insufficient to quantitatively characterize this interaction. The
highest concentration studied corresponds to a 0.5 wt % sub-
stituted poly(acrylate) solution where the shared solution vo-
lumes of the three substituted poly(acrylate)s are small, and
consequently any inter-poly(acrylate) strand dansylꢀdansyl
substituent interaction is likely to be minor.21 While these data
can only be interpreted qualitatively, time-dependent fluores-
cence studies allow single and aggregated dansyl substituent
excited state lifetimes to be quantified as is discussed below.
Fluorescence and UVꢀVis Studies of Dansyl Substituent
Complexation by βCD and 66βCD2ur. The variation of the
fluorescence of the dansyl substituent of PAADShn with increas-
ing βCD concentration (Figure 2) is consistent with the forma-
tion of a dominant 1:1 βCD:dansyl substituent hostꢀguest
UVꢀVis Studies of Dansyl-Substituted Poly(acrylate)s.
The dansyl substituent UVꢀvis spectra of either 3% randomly
substituted PAADSen (Figure 1), PAADShn, or PAADSddn in
0.10 mol dmꢀ3 NaCl aqueous solution at pH 7.0 exhibit
reversible temperature variations and are characterized by two
isosbestic points. This indicates that the dansyl substituents
experience two dominant environments whose populations are
temperature dependent and which are considered to be the single
and aggregated states within a single substituted poly(acrylate)
strand. (In these solutions where the poly(acrylate) is 0.05 wt %
and the dansyl substituent total concentration is 1.5 ꢁ 10ꢀ4 mol
dmꢀ3, interaction between poly(acrylate) strands is likely to be
minimal.21) In the absence of the individual single and aggre-
gated dansyl substituent spectra, this intramolecular equilibrium
cannot be quantified. However, as temperature increases, λmax
increases and εmax decreases which, on the assumption that
dansyl substituent aggregation decreases with increase in tem-
perature, is consistent with λmax and εmax for single dansyl sub-
stituents being larger and smaller, respectively, than those of the
dansyl substituent aggregate. At 283.2 K the dansyl λmax (nm)
are similar, and the molar absorbance εmax (dm3 molꢀ1 cmꢀ1
)
decreases in the sequence PAADSen (327 and 4752), PAADShn
(327 and 4198), PAADSddn (328 and 3571), as shown by the
first and second values in parentheses.
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Table 1. Fluorescence Titration and 2D 1H NOESY NMR Data
fluorescence data
2D 1H NMR NOESY data βCD or 66βCD2ur cross-peaks
system
λmax (nm)
Imax (au)
K (dm3 molꢀ1
)
with dansyl substituent
with tether to substituent
dansyl substituent
PAADSen
556
550
546
68
76
PAADShn
PAADSddn
103
complexed dansyl substituent
PAADSen/βCD
PAADShn/βCD
PAADSddn/βCD
PAADSen/66βCD2ur
PAADShn/66βCD2ur
PAADSddn/66βCD2ur
524
524
542
524
506
513
378
406
121
199
618
681
89 ( 5
105 ( 5
moderate
moderate
moderate
moderate
strong
none
weak
strong
none
weak
strong
55 ( 3
3040 ( 100
34200 ( 500
242000 ( 5000
strong
complex characterized by an apparent complexation constant,
K = 105 dm3 molꢀ1 with λmax = 524 nm and Imax = 406 au for the
βCD complexed dansyl substituent. Similar variations in fluor-
escence occur for PAADSen and PAADSddn for which K = 89
and 55 dm3 molꢀ1, respectively (Table 1). Thus, K varies within a
factor of 2, and the change in environment of the dansyl
substituent upon complexation by βCD is reflected by a decrease
in λmax and an increase in Imax of the complexed dansyl
substituents by comparison with that of the uncomplexed dansyl
substituents of PAADSen and PAADShn (Table 1). This is
consistent with a change from an aqueous environment to the
hydrophobic environment of the βCD annular interior. The
decrease in λmax and increase in Imax for the PAADSddn system
are comparatively much smaller consistent with the occurrence
of a lesser dansyl substituent environmental change occurring
upon complexation. This is probably because a substantial
proportion of the βCD passes over the dansyl substituent to
complex the dodecyl tether while the remainder of the βCD
complexes the dansyl substituent as is indicated by the 1H NMR
studies discussed below.
Complexation of the dansyl substituents of PAADSen,
PAADShn, and PAADSddn by 66βCD2ur is much stronger
than complexation by βCD (Table 1). This is exemplified by
the variation of the fluorescence of the dansyl substituent of
PAADShn with increasing concentration of 66βCD2ur (Figure 3),
consistent with the formation of a dominant 1:1 66βCD2ur.
dansyl substituent hostꢀguest complex characterized by an
apparent complexation constant, K = 34 200 dm3 molꢀ1 with
λmax = 506 nm and Imax = 619 au for the complexed dansyl
substituent. Similar variations in fluorescence occur for the
PAADSen and PAADSddn systems for which K = 3040 and
242 000 dm3 molꢀ1, respectively (Table 1). Thus, K increases
80-fold consistent with increasing cooperativity in binding of the
dansyl substituent and its tether in both cyclodextrin annuli of
66βCD2ur as tether length increases and steric hindrance caused
by the poly(acrylate) backbone lessens. This also causes sub-
stantial decreases in λmax and increases in Imax of the complexed
dansyl substituent by comparison with that of the uncomplexed
dansyl substituent. However, the change in λmax and Imax for the
PAADSen system is much less than for the PAADShn and
PAADSddn systems probably because steric hindrance from
the poly(acrylate) backbone causes the complexation of the
dansyl substituent of PAADSen to be largely limited to a single
Figure 3. (top) Fluorescence increase of a PAADShn solution for which
the dansyl substituent concentration is 1.0 ꢁ 10ꢀ5 mol dmꢀ3
(corresponding to 0.0034 wt % in PAADShn) with 25 sequential
additions (6.5 mm3 each) of 66βCD2ur solution (1.09 ꢁ 10ꢀ3 mol
dmꢀ3). Both aqueous solutions are 0.10 mol dmꢀ3 in NaCl at pH 7.0 at
298.2 K. The arrow indicates the increase in fluorescence with each
addition of βCD solution. Excitation is at 331 nm. (bottom) Fluores-
cence variation at 506 nm and the line representing the best fit of an
algorithm for a 1:1 hostꢀguest complexation by 66βCD2ur of a dansyl
substituent to data at 0.5 nm intervals over the wavelength range
450ꢀ600 nm.
cyclodextrin annulus of 66βCD2ur such that it experiences a
smaller environmental change than is the case in the analogous
PAADShn and PAADSddn systems.
The UVꢀvis spectrum of a PAADShn solution where the
dansyl substituent concentration is 1.5 ꢁ 10ꢀ4 mol dmꢀ3 also
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Scheme 3. Interactions in a PAADSddn Strand Showing Two
Possible Modes of Complexation of Dansyl Substituents by
βCD, One Double Complexation Mode, and the Single and
Aggregated (Ringed in Red and Shown for Simplicity As
Dimers Formed by Adjacent Dansyl Substituents) Forms of
the Dansyl Substituentsa
a The thick blue line represents the poly(acrylate) backbone, the shorter
green lines represent the dodecyl tethers, and the truncated cones
represent βCD.
Figure 4. 2D 1H NOESY NMR 600 MHz spectrum of a D2O solution
in which the PAADShn dansyl substituent and βCD concentrations are
2.95 ꢁ 10ꢀ3 mol dmꢀ3 (PAADShn concentration is 1.0 wt %) at pD 7.0.
The rectangles A, B, and C enclose the cross-peaks arising from
interaction of the βCD annular H3, H5, and H6 protons with dansyl
substituent H2ꢀ4,6ꢀ8, dansyl substituent methyl (NꢀCH3), and hexyl
tether (hnCH2) protons.
For protons at similar interaction distances, the strength of a cross-
peak is proportional to the number of protons involved and the
dodecyl tether is therefore expected to produce the strongest
cross-peaks when complexed in the βCD annulus. Nevertheless,
the absence of any cross-peaks associated with the ethyl tether
suggests a variation of hostꢀguest complexation with tether
length. Thus, the long dodecyl tether of PAADSddn complexes
in the βCD annulus, as do octadecyl substituents on other
substituted poly(acrylate)s,21 and competes with the dansyl
substituent for occupancy of the βCD annulus as was inferred
earlier from the fluorescence studies. For PAADSen and
PAADShn the shorter tethers render this competition less
effective.
shows a systematic increase in λmax with isosbestic points
occurring at 295 and 331 nm with increase in βCD concentration
under the same conditions as for the analogous fluorescence
studies. This is consistent with the formation of a dominant 1:1
βCD.dansyl substituent hostꢀguest complex and an increase in
λmax of the complexed dansyl substituent. However, the magni-
tude of the absorbance variation is insufficient for quantitative
characterization of the βCD.dansyl substituent hostꢀguest
complex. Analogous changes in UVꢀvis spectra occur in the
PAADSen and PAADSddn systems.
Collectively, the temperature-dependent UVꢀvis, fluores-
cence, and 2D 1H NOESY NMR spectroscopic data suggest that
the dansyl substituent and the dodecyl tether of PAADSddn are
complexed by βCD singly as shown in Scheme 3 together with
single and aggregated dodecyl substituents. The βCD competi-
tive complexation results in a lesser proportion of the dansyl
substituents occupying the βCD annulus upon complexation
such that the fluorescence decrease in λmax and the increase in
Imax for the PAADSddn system are less than for the PAADSen
and PAADShn systems. (It is possible that a dansyl substituent
and its dodecyl tether could be complexed by two βCDs as is also
shown in Scheme 3, but such complexation cannot be distin-
guished from the two singly βCD complexed forms on the basis
of the experimental data.)
1
2D H NOESY NMR Studies of Dansyl Substituent Com-
1
plexation by βCD and 66βCD2ur. The 2D NOESY H NMR
spectrum of a D2O solution of PAADShn equimolar in βCD and
dansyl substituents shows moderate cross-peaks arising from
interaction between the βCD annular H3,5,6 protons and the
dansyl H2ꢀ4,6ꢀ8 aromatic and methyl protons and the hexyl
tether protons (Figure 4). (Cross-peak intensity is assessed by
comparison with that of cross-peaks arising from interaction
between the dansyl substituent aromatic protons and methyl
protons in the same spectrum.) This is consistent with both the
hexyl tether and the dansyl substituent complexing within the
βCD annulus. While cross-peaks arise from interaction between
the βCD annular H3,5,6 protons and the dansyl H2ꢀ4,6ꢀ8 and
methyl protons for the analogous spectra of the PAADSen and
PAADSddn solutions occur, no cross-peaks occur for the ethyl
tether protons of PAADSen whereas strong cross-peaks occur
for the dodecyl tether protons of the PAADSddn (Table 1).
1
The 2D NOESY H NMR spectrum of a D2O solution of
PAADSddn equimolar in 66βCD2ur and dansyl substituents
shows strong cross-peaks arising from interaction between the
66βCD2ur annular H3,5,6 protons, the dansyl H2ꢀ4,6ꢀ8 aro-
matic and methyl protons, and the dodecyl tether protons
(Figure 5). This indicates that both the dansyl substituent and
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Figure 6. Chem3D MM2 energy-minimized models of the 1:1
66βCD2ur hostꢀguest complexes of the dansyl substituent with an
ethyl tether (top) and a dodecyl tether (bottom) with their respective
complexation constants.
Figure 5. 2D 1H NOESY NMR 600 MHz spectrum of a D2O solution
in which the dansyl substituent and 66βCD2ur concentrations are
2.95 ꢁ 10ꢀ3 mol dmꢀ3 in 3% substituted PAADSddn and 66βCD2ur
(PAADSddn concentration is 1.0 wt %) at pD 7.0. The rectangles A, B,
and C enclose the cross-peaks arising from interaction of the 66βCD2ur
annular H3, H5, H6 protons with the dansyl H2ꢀ4,6ꢀ8 protons, dansyl
(NꢀCH3), and dodecyl tether (ddn CH2) protons.
the dodecyl tether complex within the cyclodextrin annuli of
1
66βCD2ur. While the 2D NOESY H NMR spectrum of the
66βCD2ur/PAADSen solution also shows cross-peaks arising
from interaction of the 66βCD2ur annular H3, H5, H6 protons
with the dansyl H2ꢀ4,6ꢀ8 and methyl protons, no cross-peaks
are observed for the ethyl tether protons. As for the analogous
βCD solution, this is consistent with the length of the ethyl tether
being insufficient to allow its complexation by 66βCD2ur
because of steric hindrance arising from the poly(acrylate)
backbone. The 2D NOESY 1H NMR spectrum of the 66βCD2ur/
PAADShn solution shows moderate cross-peaks arising from
interaction of the 66βCD2ur annular H3, H5, H6 protons with
the dansyl H2ꢀ4,6ꢀ8 and methyl protons and also weak cross-
peaks arising from a similar interaction with the hexyl protons,
consistent with the increased tether length decreasing the effect
of steric interactions on complexation. The overall effect of tether
length on 66βCD2ur complexation is illustrated by the energy-
minimized structures in Figure 6 obtained using the Chem3D
MM2 protocol.36
Figure 7. Time-dependent fluorescence at 540 nm of a 1.0 wt %
aqueous solution of PAADShn in which the total dansyl substituent
concentration is 2.95 ꢁ 10ꢀ3 mol dmꢀ3, [NaCl] = 0.10 mol dmꢀ3, pH =
7.0 at 298.2 K. The solid line represents the best fit of the data to an
algorithm for the fluorescence biexponential decay A1 exp(ꢀt/τ1) + A2
exp(ꢀt/τ2), where A1 and A2 are the amplitudes of the decay compo-
nents and τ1 and τ2 are the corresponding decay time constants. The
resolution is 1 ns.
In addition to these 1:1 complexations, it is possible that some
66βCD2ur may also simultaneously complex two dansyl substit-
uents in a single strand of PAADSen, PAADShn, and PAADSddn,
particularly in the latter case where the longer tether confers
greater flexibility. While such simultaneous complexation cannot
be distinguished from 1:1 complexation through the fluorescence
solutions where the viscosity increases caused by the addition of
66βCD2ur are attributed to the simultaneous complexation of
two dansyl substituents on adjacent PAADSen, PAADShn, or
PAADSddn strands as is discussed below.
Time-Resolved Fluorescence Studies of Dansyl Substitu-
ent Aggregation and Complexation by βCD and 66βCD2ur.
Aqueous solutions of PAADSen, PAADShn, and PAADSddn
1
and H NMR data, the possibility of its occurrence is inferred
from the viscosity variations of more concentrated 5.0 wt %
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Table 2. Time-Dependent Fluorescence Dataa
system PAADSen A1
τ1 (ns)
A2
τ2 (ns)
A3
τ3 (ns)
ꢀ
βCD
βCD2ur
0.814 2.2 ( 0.4 0.186 5.0 ( 0.7
0.619 2.2
0.470 2.2
0.187 5.0
0.144 5.0
0.193 13.2 ( 2.0
0.386 13.9 ( 2.1
system PAADShn A 1
τ1 (ns)
A 2
τ2 (ns)
A3
τ3 (ns)
ꢀ
βCD
βCD2ur
0.792 2.5 ( 0.4 0.208 5.3 ( 0.7
0.532 2.5
0.066 2.5
0.139 5.3
0.182 5.3
0.329 11.7 ( 1.8
0.709 20.8 ( 3.0
system PAADSddn A1
τ1 (ns)
A2
τ2 (ns)
A3
τ3 (ns)
ꢀ
0.722 3.2 ( 0.5 0.278 9.5 ( 1.4
βCD
0.618 3.2
0.175 3.2
0.378 9.5
0.167 9.5
Figure 8. Viscosity shear rate dependence for 5 wt % aqueous 0.10 mol
dmꢀ3 NaCl solutions at pH 7.0 and 298.2 K of PAADSen, PAADShn,
and PAADSddn and their binary mixtures with βCD and 66βCD2ur in
which the mole ratio of the dansyl substituents to either βCD or
66βCD2ur is unity.
βCD2ur
0.627 19.3 ( 2.9
a The overall fitting error for τ1, τ2, and τ3 is (15%.
show a biexponential decay of fluorescence as exemplified by
PAADShn in Figure 7. The time-dependent fluorescence inten-
sity time dependence, I(t), is fitted to eq 6:
IðtÞ ¼ A1 expðꢀt=τ1Þ þ A2 expðꢀt=τ2Þ
ð6Þ
where A1 and A2 are the amplitudes of the decay components
assigned to the excited state aggregated and single dansyl
substituents, respectively, and τ1 and τ2 are the corresponding
time constants for the fluorescence decays for all three systems as
shown in Table 2. The assignment of τ1 is made on the basis that
it is expected that πꢀπ and other interactions where πꢀπ
stacking is stereochemically prevented between dansyl substitu-
ents in the excited state aggregates should shorten fluorescence
decay time constants, as has been extensively shown for pyrene
substituents in other polymer systems.11ꢀ13 The τ2 for the exci-
ted state single dansyl substituents of PAADSen and PAADShn
are similar, but that for PAADSddn is twice as long possibly
reflecting the greater length of the dodecyl tether, allowing
greater freedom of motion and a shorter spatial residence
time and thereby a less effective fluorescence quenching and a
longer τ2.
Figure 9. Zero-shear viscosities of 5 wt % aqueous solutions (0.10 mol
dmꢀ3 NaCl, pH 7.0, 298.2 K) of (a) PAADSen, PAADShn, and
PAADSddn and their binary mixtures with (b) native βCD and
(c) 66βCD2ur in which the mole ratio of the dansyl substituents to
either βCD or 66βCD2ur is unity. The individual zero-shear viscosities
In the presence of βCD, fluorescence time dependence for the
PAADSen and PAADShn systems becomes triexponential:
(Pa s) are PAADSen (0.015), PAADShn (0.016), PAADSddn (0.081),
3
PAADSen + βCD (0.014), PAADShn + βCD (0.015), PAADSddn +
βCD (0.031), PAADSen + 66βCD2ur (0.020), PAADShn + 66βCD2ur
(0.022), and PAADSddn 66βCD2ur (0.69).
IðtÞ ¼ A1 expðꢀt=τ1Þ þ A2 expðꢀt=τ2Þ
þ A3 expðꢀt=τ3Þ
ð7Þ
dominantly with βCD positioned over the long dodecyl tether
which has little effect on the dansyl environment or fluorescence
and a second form where βCD is positioned over the dansyl
entity and enhances fluorescence (Scheme 3). In contrast, the
relative shortness of the ethyl and hexyl tethers causes the βCD.
PAADSen and βCD.PAADShn complexes to exist dominantly in
this second form.
Triexponential dansyl substituent fluorescence time depend-
ences according to eq 7 are also observed in the presence of
66βCD2ur, and the derived τ3 appear in Table 2. The similarity of
τ3 for the 66βCD2ur/PAADSen and βCD/PAADSen systems
suggests that the excited state dansyl substituent environments are
quite similar despite the 30-fold greater stability of the 66βCD2ur.
PAADSen ground state complex by comparison with that of the
βCD.PAADSen complex (Table 1). The τ3 for the excited states
species derived from the more stable 66βCD2ur.PAADShn and
where the A3 exp(ꢀt/τ3) term in eq 7 arises from the βCD.dansyl
and 66βCD2ur.dansyl excited state hostꢀguest complexes. The
parameters in Table 2 are derived by fitting the data to eq 7, with
the previously derived τ1 and τ2 as fixed parameters, to the
fluorescence time dependencies. The overall fitting error is (15%.
The 2-fold increase in τ3 for the βCD complexed excited state
dansyl substituents of PAADSen and PAADShn complexes by
comparison with the analogous τ2 is attributed to a change for the
dansyl substituent from the aqueous environment to the hydro-
phobic βCD annular environment where quenching is less
effective. (This interpretation is similar to that made where
dansyl substituents are directly tethered to βCD and exist in an
equilibrium between the dansyl substituent inside and outside
the annulus.37ꢀ39) The amplitude of the third fluorescence decay
component for PAADSddn is too small for a reliable determina-
tion of τ3. This is consistent with βCD complexing PAADSddn
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Scheme 4. (top) Interactions within a 5 wt % PAADSddn
Aqueous Solution Showing Inter- and Intrastrand Dansyl
Substituent Interactions as Dimerizations although Higher
Aggregations May Occur; (bottom) Interactions within a
5 wt % PAADSddn Aqueous Solution after Addition of
66βCD2ur Showing Dual HostꢀGuest Interactions between
the Cyclodextrin Annuli and the Dansyl Substituents Forming
Interstand Linkages (Some Intrastrand Complexation of
Dansyl Substituents May Also Occur) and Interꢀ and
Intrastrand Dansyl Substituent Interactionsa
the aggregated and single dansyl substituent states have different
molar absorbances, ε, at 400 nm, the excitation wavelength, as
indicated in Figure 1. As the absorbance data do not allow
quantification of the dansyl substituent aggregation, as discussed
above, A1 and A2 cannot be corrected for the differing aggre-
gate and single dansyl ε values. The ε values of the βCD and
66βCD2ur dansyl complexes are also different at 400 nm (see
Supporting Information). The combination of these factors
prevents the determination of apparent complexation constants
from A1, A2, and A3.
Rheological Studies. The viscosity variation with shear rate of
aqueous solutions 5 wt % in either PAADSen, PAADShn, or
PAADSddn alone and in the presence of either βCD or
66βCD2ur where the mole ratio of the dansyl substituents to
either βCD or 66βCD2ur is unity is shown in Figure 8. Therein
the more viscous PAADSddn + 66βCD2ur and PAADSddn
solutions show small decreases in viscosity with increasing shear
rate. The zero shear viscosity variations are shown in Figure 9.
Three trends emerge, the first of which is that the PAADSddn
solution is much more viscous than the PAADShn and PAADSen
solutions consistent with the greater length of the dodecyl tether,
allowing more aggregation between substituents on adjacent
strands. A second trend is that the addition of βCD lowers the
viscosity of all three solutions through complexation of the dansyl
substituents and also their tethers in the case of PAADShn
and PAADSddn and thereby decreases substituent aggregation.
(A similar trend has been observed for octadecyl substituted poly-
(acrylate) where the high viscosity of its aqueous solution arising
from substituent aggregation is greatly diminished by addition
of βCD as a consequence of the octadecyl substituents being
complexed by βCD.25) In contrast, the addition of 66βCD2ur
increases the viscosity of all three solutions through hostꢀguest
complexation of dansyl substituents on adjacent strands to form
interstrand linkages which are either stronger or more prevalent
or both than dansyl substituent aggregation. These competing
interactions are broadly shown for the PAADSddn system in
Scheme 4.
’ CONCLUSIONS
Three new 3% randomly substituted poly(acrylate)s are
reported in which the dansyl substituent tether lengthens system-
atically as 2, 6, and 12 methylene groups are incorporated in
PAADSen, PAADShn, and PAADSddn, respectively. For dilute
aqueous solutions in which the interaction between substituted
poly(acrylate) strands is minimal, UVꢀvis and time-resolved
fluorescence studies are consistent with an equilibrium between
aggregated and single dansyl substituents characterized by fluo-
rescence lifetimes τ1 = 2.2, 2.5, and 3.2 ns and τ2 = 5.0, 5.3, and
9.5 ns, respectively, in the sequence PAADSen, PAADShn, and
PAADSddn. Steady-state and time-resolved fluorescence and 2D
1H NOESY NMR spectroscopy show the dansyl substituents to
be complexed within the annulus of βCD in 1:1 hostꢀguest
complexes characterized by apparent complexation constants,
K = 89, 105, and 55dm3 molꢀ1 and fluorescence lifetimesτ3 = 13.2
and 11.7 ns for the βCD.PAADSen and βCD.PAADShn com-
plexes. However, τ3 was not determined for βCD.PAADSddn
because its signal amplitude was insufficient for accurate deter-
mination probably because a substantial proportion exists with
βCD positioned over the dodecyl tether and has little effect on
the dansyl substituent fluorescence. This also accounts for the
smaller fluorescence change for PAADSddn on dansyl substituent
a The thick blue line represents the poly(acrylate) backbone, the shorter
green lines represent the dodecyl tethers, and the linked truncated cones
represent 66βCD2ur.
66βCD2ur.PAADSddn complexes are longer than those for the
excited state of the 66βCD2ur.PAADSen complex, consistent with
their longer hexyl and dodecyl tethers allowing greater cooperative
in complexation of the dansyl entity by 66βCD2ur and a greater
environmental change and less effective fluorescence quenching.
Overall, these time-dependentobservations areconsistent withthe
deductions made from the previously discussed steady state
UVꢀvis, fluorescence, and 1H NMR studies.
Unfortunately, the apparent formation constant for the dansyl
substituent aggregation cannot be quantified from A1 and A2 as
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complexation by βCD than is the case for PAADSen and
PAADShn. Cooperativity in dansyl substituent complexation
by the two cyclodextrin annuli of 66βCD2ur results in increases
in 1:1 complex stability in the sequence K = 3040, 34 200, and
242 000 dm3 molꢀ1 and τ3 = 13.9, 20.8, and 19.3 for the
66βCD2ur.PAADSen, 66βCD2ur.PAADShn, and 66βCD2ur.
PAADSddn complexes, respectively, coincident with the increas-
ing dansyl substituent tether length decreasing poly(acrylate)
backbone steric hindrance to complexation. These dilute solu-
tion observations aid the interpretation of rheological data for
more concentrated solutions. Thus, the increase in zero shear
viscosity in the sequence: PAADSen ∼ PAADShn < PAADSddn
for aqueous solution indicates that dansyl substituent interstrand
aggregation is favored with increase in substituent length,
whereas dansyl substituent complexation by βCD decreases such
aggregation. In contrast, viscosity increases in the sequence
PAADSen ∼ PAADShn < PAADSddn in the presence of 66βCD2ur
as complexation of dansyl substituents from adjacent poly(acrylate)
strands form interstrand linkages.
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’ ASSOCIATED CONTENT
S
Supporting Information. UVꢀvis temperature varia-
b
tions for substituted poly(acrylate)s, UVꢀvis variations for
substituted poly(acrylate)s in the presence of βCD and 66βCD2ur,
steady-state and time-dependent fluorescence variations for the
substituted poly(acrylate)s alone and in the presence of βCD and
66βCD2ur, and 2D 1H NOESY NMR spectra of the substituted
poly(acrylate)s in the presence of βCD and 66βCD2ur. This
material is available free of charge via the Internet at http://
pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: stephen.lincoln@adelaide.edu.au.
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’ ACKNOWLEDGMENT
We gratefully acknowledge Australian Research Council fund-
ing, NSFC Grant 20774028 and 20774030, 111 Project Grant
B08021, Shanghai Shuguang Plan Project 06SG35, Shanghai
Pujiang Talent Project 07PJ14022, and the China Scholarship
Council for supporting this work.
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