Luminescence quenching in supramolecular assemblies of quantum dots and bipyridinium dications

Article abstract of DOI:10.1039/b720038b

We have investigated the ability of two bipyridinium dications, with either octyl or decyl groups on their nitrogen atoms, to quench the luminescence of CdSe-ZnS core-shell quantum dots coated by either tri-n-octylphosphine oxide or a tris(phenylureido)calix[6]arene. Our studies demonstrate that both bipyridinium dications adsorb on the surface of the quantum dots with association constants ranging from 10⁴ to 10⁷ M ^-1 and quench the luminescence of the inorganic nanoparticles with rate constants ranging from 10⁸ to 10⁹ s^-1. The association constants of these supramolecular assemblies vary significantly with the counterions of the bipyridinium dications and the ligands on the nanoparticle surface. Their quenching rate constants vary with the length of the alkyl chains appended to the bipyridinium core and, once again, the ligands on the nanoparticle surface. Furthermore, our studies show that the addition of a calix[6]arene able to compete with the quantum dots for the bipyridinium quenchers restores the original luminescence intensity of the nanoparticles. Indeed, the supramolecular association of the calix[6]arene with the bipyridinium dication removes the quencher from the nanoparticle surface and, hence, is transduced into a luminescent enhancement.

Full text of DOI:10.1039/b720038b

View Article Online / Journal Homepage / Table of Contents for this issue
PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Luminescence quenching in supramolecular assemblies of quantum dots
and bipyridinium dications†
a
b
a
c
Benoıt Gadenne, Ibrahim Yildiz, Matteo Amelia, Flavio Ciesa, Andrea Secchi, Arturo Arduini,
c
c
ˆ
Alberto Credi and Franc¸isco M. Raymo
a
b
*
*
Received 2nd January 2008, Accepted 18th February 2008
First published as an Advance Article on the web 6th March 2008
DOI: 10.1039/b720038b
We have investigated the ability of two bipyridinium dications, with either octyl or decyl groups on
their nitrogen atoms, to quench the luminescence of CdSe–ZnS core–shell quantum dots coated by
either tri-n-octylphosphine oxide or a tris(phenylureido)calix[6]arene. Our studies demonstrate that
both bipyridinium dications adsorb on the surface of the quantum dots with association constants
ranging from 104 to 107 Mꢀ1 and quench the luminescence of the inorganic nanoparticles with rate
constants ranging from 108 to 109 sꢀ1. The association constants of these supramolecular assemblies
vary significantly with the counterions of the bipyridinium dications and the ligands on the
nanoparticle surface. Their quenching rate constants vary with the length of the alkyl chains appended
to the bipyridinium core and, once again, the ligands on the nanoparticle surface. Furthermore, our
studies show that the addition of a calix[6]arene able to compete with the quantum dots for the
bipyridinium quenchers restores the original luminescence intensity of the nanoparticles. Indeed, the
supramolecular association of the calix[6]arene with the bipyridinium dication removes the quencher
from the nanoparticle surface and, hence, is transduced into a luminescent enhancement.
adsorption of appropriate bipyridinium dications on the surface
of the inorganic nanoparticles in the ground state and on the
exchange of electrons between them in the excited state. In order
to elucidate further the fundamental factors governing these
processes and broaden the array of basic building blocks avail-
able for the implementation of these strategies, we have prepared
QDs coated with either tri-n-octylphosphine oxide (TOPO) or
a tris(phenylureido)calix[6]arene (3 in Fig. 1) and investigated
Introduction
Semiconductor quantum dots (QDs) are inorganic particles with
nanoscaled diameters and outstanding photophysical proper-
ties.1–5 Their spectroscopic signature can be manipulated with
fine adjustments in elemental composition and physical
dimensions to afford luminescent probes with tunable emissive
behavior. Indeed, the modularity of these inorganic nanostruc-
tures coupled to intrinsically high absorption cross sections,
outstanding photobleaching resistances and long luminescence
lifetimes are encouraging their use in biomedical imaging and
sensing applications as an alternative to conventional organic
fluorophores.6–12 Specifically, protocols to signal target analytes
with luminescent QDs are starting to be developed on the basis
of electron and energy transfer processes.13,14 In fact, funda-
mental investigations have demonstrated that these inorganic
nanostructures can exchange electrons with complementary
organic partners upon excitation with a concomitant lumine-
scence quenching.15–20 In particular, we have exploited the ability
of CdSe–ZnS core–shell QDs to inject electrons into bipyridi-
nium dications to develop photochromic materials18a and probe
protein–ligand interactions.18d These strategies are based on the
aDipartimento di Chimica ‘‘G. Ciamician’’, Universita` di Bologna, via Selmi
2, 40126 Bologna, Italy. E-mail: alberto.credi@unibo.it
bCenter for Supramolecular Science, Department of Chemistry, University
of Miami, 1301 Memorial Drive, Coral Gables, FL 33146-0431, USA.
E-mail: fraymo@miami.edu
c
`
Dipartimento di Chimica Organica e Industriale, Universita di Parma, via
G.P. Usberti 17/a, 43100 Parma, Italy
† Electronic supplementary information (ESI) available: Calculation of
1
the size of the inorganic nanocrystals; DLS experiments; H NMR and
Fig. 1 The bipyridinium dications 12+ and 22+, the calixarenes 3 and 4
and schematic representations of QD–TOPO and QD–3 (the schematic
representations are not to scale).
ESIMS spectra of 3; analysis of the Stern–Volmer plots. See DOI:
10.1039/b720038b
2022 | J. Mater. Chem., 2008, 18, 2022–2027
This journal is ª The Royal Society of Chemistry 2008

View Article Online
Fig. 2 Synthesis of the calixarene 3.
the interactions of the resulting assemblies QD–TOPO and
QD–3 with the bipyridinium dications 12+ and 22+ (Fig. 1).
calculated size of the inorganic nanocrystal. Calixarene-coated
QD–3 have been found to exhibit a hydrodynamic diameter of
16 ꢁ 3 nm, i.e., considerably larger than that observed for QD–
TOPO. This difference can hardly be accounted for by the larger
size of 3 compared to TOPO; in fact, the high polydispersion index
(>0.7) suggests that nanoparticle aggregates may be present.
Results and discussion
Synthesis and characterization
QDs. We have synthesized QD–TOPO according to estab-
lished literature procedures.21,22 Specifically, we have reacted
CdO and Se at high temperature in the presence of TOPO and
treated the resulting nanoparticles with Et2Zn and hexamethyl-
disilathiane to generate QD–TOPO. We have then heated
a dispersion of QD–TOPO and 3 in CHCl3 under reflux for
one day and isolated QD–3 after reiterative precipitation,
centrifugation and filtration steps.
Bipyridinium derivatives. We have synthesized compounds 12+
and 22+ according to established literature procedures.24,25 In
particular, we have reacted 4,40-bipyridine with an excess of
either decylbromide or octylbromide and then exchanged the
bromide counterions of the resulting dications with either
hexafluorophosphate or chloride anions to generate 1$2Cl,
1$2PF6 and 2$2PF6.
The diameter of the CdSe core and that of the CdSe–ZnS
core–shell assembly have been estimated to be 2.8 and 5.3 nm,
respectively, from absorption data and stoichiometric consider-
ations.‡ The hydrodynamic diameters of QD–TOPO and
QD–3 have been determined by dynamic light scattering (DLS)
experiments. QD–TOPO particles exhibit a diameter of 9 ꢁ 1 nm,
i.e., much larger than the 5.3 nm calculated for the inorganic
core–shell assembly. However, both the contribution of the ligand
shell and solvation sphere effects need to be taken into account.
Recent experiments have shown that the apparent thickness of
the TOPO shell ranges from 1.3 to 2 nm.23 Therefore, the value
of the diameter measured by DLS is in full agreement with the
Calixarene derivative. We have prepared 3 in six steps (Fig. 2)
starting from known precursors.26,27 In particular, we have alkyl-
ated the calixarene 5 with toluene-4-sulfonic acid undec-10-enyl
ester, under the assistance of K2CO3, and reacted the resulting
compound 6 with thioacetic acid, in the presence of AIBN. After
the acid hydrolysis of the thioester groups of 7, we have alkylated
the thiol groups of 8 with trityl chloride, under the assistance of
Et3N. Then, we have reduced the nitro groups of 9 with hydra-
zine, in the presence of catalytic amounts Pd(C), and reacted
the amino groups of 10 with phenylisocyanate. Finally, we
have removed the trityl groups of 11 with NaBH3CN and TFA
to afford the target molecule 3.x
‡ The determination of the core and core–shell diameters is discussed in
the ESI.†
1
x The H NMR and ESIMS spectra of 3 are reported in the ESI.†
This journal is ª The Royal Society of Chemistry 2008
J. Mater. Chem., 2008, 18, 2022–2027 | 2023

View Article Online
Fig. 3 Emission spectra of QD–TOPO (0.08 mM, CHCl3, lEx ¼ 400 nm)
before (thick line) and after (thin lines) the addition of increasing
amounts (0–1.5 mM) of 1$2Cl.
Fig. 5 Plots of the average luminescence lifetime (a, right scale) and
luminescence intensity (b, left scale) of QD–TOPO (0.08 mM, CHCl3,
lEx ¼ 400 nm) against the concentration of 1$2Cl.
Luminescence quenching
Table 1 Association constants (K) for the complexes formed between
QD–TOPO and the dicationic quenchers in CHCl3 and the correspond-
ing quenching rate constants (kq)
The emission spectrum (Fig. 3, thick line) of a dispersion of QD–
TOPO in CHCl3 shows an intense band centered at 547 nm with
a luminescence quantum yield of 0.38 and an average lumines-
cence lifetime of 16 ns. This band decreases in intensity (Fig. 3,
thin lines) with the addition of increasing amounts of either
1$2Cl or 1$2PF6 or even 2$2PF6. However, the presence of either
12+ or 22+ has negligible influence on the absorption spectrum of
QD–TOPO (Fig. 4).
K/Mꢀ1
kq/sꢀ1
1$2Cl
1$2PF6
2$2PF6
(1.1 ꢁ 0.1) ꢂ 107
(3.2 ꢁ 0.3) ꢂ 105
(1.7 ꢁ 0.2) ꢂ 105
2.2 ꢂ 109
1.7 ꢂ 109
2.7 ꢂ 108
In all instances, the concentration of the bipyridinium dication
has negligible influence on the average luminescence lifetime (a in
Fig. 5), but affects drastically the luminescence intensity (b in
Fig. 5). These observations are consistent with static quenching
and demonstrate that the luminescent nanoparticles and the
dicationic quencher associate in the ground state. Interestingly,
DLS measurements showed that the addition of a stoichiometric
amount of 1$2Cl to QD–TOPO causes an increase of the mean
hydrodynamic diameter from 9 ꢁ 1 to 12 ꢁ 2 nm. This result is
in agreement with the ground-state association of QD–TOPO
with 12+ and can be ascribed to an increase in the thickness of
the organic shell, originating from the presence of the positively
charged guest, its counteranions and, probably, additional
solvent molecules. The association constants (K) and quenching
rate constants (kq) are listed in Table 1 and have been determined
from the analysis of the Stern–Volmer plots.{
A comparison of the data in Table 1 indicates that K increases
by two orders of magnitude with the transition from hexafluoro-
phosphate to chloride counterions and by ca. 50% with the
elongation of the alkyl substituents from octyl to decyl chains.
The former observation can be ascribed to a stronger solvo-
phobic effect experienced by 1$2Cl compared to 1$2PF6, as
reflected by the solubilities of these salts in chloroform. In
contrast, the type of counterion of the quencher has little effect
on the quenching rate constant, which however drops by one
order of magnitude upon replacing the decyl side chains with
octyl ones. In agreement with literature data,28 electron transfer
from the excited nanoparticles to either 12+ or 22+ is, presumably,
responsible for quenching. Nonetheless, the characteristic
spectroscopic signature of the radical cation of either 12+ or 22+
cannot be observed by nanosecond flash photolysis measure-
ments, suggesting that back electron transfer occurs on a
subnanosecond timescale. Since the redox properties of bipyridi-
nium dications are nearly unaffected29 by the type of alkyl
substituent present on the nitrogen atoms, one can infer that
the difference in the quenching rate constant between 12+ and
22+ is related to the fact that the former can approach closer to
the QD surface compared to the latter.
The treatment of QD–TOPO with 3 results in the adsorption
of the macrocyclic receptor on the surface of the nanoparticles.k
Fig. 4 Absorption spectra of QD–TOPO (0.08 mM, CHCl3) before
(thick line) and after (thin lines) the addition of increasing amounts
(0–1.5 mM) of 1$2Cl. The small absorbance decrease can be ascribed to
dilution effects.
{ The analysis of the Stern–Volmer plots is discussed in the ESI.†
k The number of macrocyclic ligands adorbed on each QD was not
determined.
2024 | J. Mater. Chem., 2008, 18, 2022–2027
This journal is ª The Royal Society of Chemistry 2008

View Article Online
The kq values are one order of magnitude smaller than those
measured for QD–TOPO. These observations can be ratio-
nalized considering that the inclusion of 12+ or 22+ into 3 is
expected to have two main consequences. First, their bipyridi-
nium units are kept separated from the QD surface by the
monolayer formed by the undecanethiol-derivatized calixarene;
second, they become more difficult to reduce, thereby decreasing
the driving force for the electron-transfer process.29
Competitive binding
The calixarene 4 (Fig. 1) binds bipyridinium dications in
nonpolar solvents with association constants of ca. 106 Mꢀ1 29
.
As a result, this macrocyclic receptor can compete with QD–
TOPO and capture any of the three bipyridinium quenchers
1$2Cl, 1$2PF6 or 2$2PF6. For example, the luminescence inten-
sity of QD–TOPO (a in Fig. 7) is almost completely suppressed
in the presence of ca. 19 equivalents of 1$2Cl (b in Fig. 7).
However, the original luminescence intensity is fully restored
after the addition of an excess of 4 (c in Fig. 7). Indeed, the
supramolecular association of 4 and 12+ prevents the interaction
of the latter with QD–TOPO and suppresses the photoinduced
electron-transfer process. Consistently, this supramolecular
event is transduced into a pronounced enhancement in lumines-
cence. Thus, these operating principles can, in principle, be
exploited to probe receptor–substrate interactions with lumines-
cence measurements.
Fig. 6 Absorption spectra of QD–3 (0.12 mM, CHCl3) before (thick line)
and after (thin lines) the addition of increasing amounts (0–50 mM) of
1$2Cl. The inset shows a magnification of the region between 350 and
650 nm.
As a result, the luminescence quantum yield decreases by 30%
with a bathochromic shift of 5 nm and the average luminescence
lifetime drops to 9 ns. After the addition of increasing amounts
of either 1$2Cl or 1$2PF6 or even 2$2PF6, a broad and weak
band extending up to 600 nm appears in the absorption spectrum
(Fig. 6). Presumably, this absorption arises from charge-transfer
interactions between the electron rich calixarene and the electron
deficient dications.29
Once again, the luminescence intensity of the QDs decreases
with the concentration of the bipyridinium dications, while the
average luminescence lifetime remains essentially unaffected.
This behavior is consistent with static quenching and confirms
the formation of ground-state complexes between the lumines-
cent nanoparticles and the dication quenchers. The correspond-
ing K and kq are listed in Table 2 and have been determined from
the analysis of the Stern–Volmer plots.{
Conclusions
We have prepared CdSe–ZnS core–shell quantum dots coated
with either TOPO or calixarene 3, and investigated the inter-
action of these nanoparticles with two bipyridinium derivatives.
Our results demonstrate that bipyridinium dications associate
with either QD–TOPO or QD–3 in the ground state with K
ranging from 104 to 107 Mꢀ1 in CHCl3. The counterions of the
bipyridinium dications as well as the ligands on the surface of
the QDs control the stability of the resulting complexes. The
length of the aliphatic chains appended to the bipyridinium
core has a minor influence on the binding event. In these supra-
molecular assemblies, the bipyridinium dications quench the
luminescence of the nanoparticles with kq ranging from 108 to
A comparison of the data in Table 2 indicates that K increases
by one order of magnitude with the transition from hexafluoro-
phosphate to chloride counterions and by ca. 50% with the
elongation of the alkyl substituents from octyl to decyl chains.
Interestingly, the nature of the ligands on the surface of the
nanoparticles has a pronounced influence on K. Specifically,
the bipyridinium complexes of QD–TOPO are significantly
more stable than those of QD–3. This trend is in apparent
contradiction with the known ability of calixarenes to encapsu-
late bipyridinium dications in nonpolar solvents with high
association constants.29 Presumably, the confinement of 3 on
the surface of the inorganic nanoparticles hinders the insertion
of either 12+ or 22+ into its cavity. Chloride anions may favor
the association between QD–3 and 12+ because they can form
hydrogen bonds with the ureidic units of 3.29
Table 2 Association constants (K) for the complexes formed between
QD–3 and the dicationic quenchers in CHCl3 and the corresponding
quenching rate constants (kq)
K/Mꢀ1
kq/sꢀ1
1$2Cl
1$2PF6
2$2PF6
(1.0 ꢁ 0.1) ꢂ 105
(2.4 ꢁ 0.4) ꢂ 104
(1.0 ꢁ 0.1) ꢂ 104
5.4 ꢂ 108
1.2 ꢂ 108
2.0 ꢂ 108
Fig. 7 Emission spectra of QD–TOPO (0.43 mM, CHCl3, lEx ¼ 350 nm)
before (a) and after the sequential addition of 1$2Cl (b, 8.0 mM) and 4
(c, 160 mM).
This journal is ª The Royal Society of Chemistry 2008
J. Mater. Chem., 2008, 18, 2022–2027 | 2025

View Article Online
109 sꢀ1, presumably by photoinduced electron transfer from the
inorganic core. Interestingly, the transition from TOPO to 3
ligands has a depressive effect on the stability of the bipyridinium
complexes and delays the quenching process significantly. In the
presence of 4, the bipyridinium quenchers desorb from the surface
of QD–TOPO and associate with the calixarene instead. As
a result, the original luminescence intensity of the nanoparticles
is restored. Thus, the supramolecular association of the quencher
with a complementary receptor can be transduced into a lumines-
cence enhancement with these operating principles.
Synthesis of 6
Calix[6]arene 5 (1.5 g, 1.5 mmol) and toluene-4-sulfonic acid
undec-10-enyl ester (1.7 g, 5.4 mmol) were dissolved in 500 ml
of acetonitrile. K2CO3 (0.8 g, 5.4 mmol) was added and the
resulting heterogeneous mixture was refluxed for 7 days. After
this period HCl (300 ml 10% in water) and ethyl acetate
(100 ml) were added. The organic phase was separated, dried
on Na2SO4 and concentrated under reduced pressure. The crude
product was purified by column chromatography (eluent:
hexane–ethyl acetate ¼ 95 : 5) obtaining calix[6]arene 6 as
a yellowish solid (yield 69%). 1H NMR (300 MHz, CDCl3)
d 7.67 (bs, 6 H), 7.21 (bs, 6 H), 5.9–5.7 (m, 3 H), 5.0–4.9 (m, 6
H), 4.6–4.1 (m, 6 H), 3.83 (bs, 6 H), 3.7–3.4 (m, 6 H), 2.86 (s,
9 H), 2.1–2.0 (m, 6 H), 1.85 (bs, 6 H), 1.6–1.0 (m, 53 H). 13C
NMR (75 MHz, CDCl3): 146.8, 143.6, 139.1, 127.8, 127.3,
123.1, 114.1, 77.4, 73.9, 59.9, 34.2, 33.7, 31.4, 31.2, 30.9, 30.5,
30.2, 29.4, 29.2, 29.0, 28.8, 26.0. Mp 138–140 ꢃC. ESIMS:
1461.68 [M + Na+].
Experimental
Materials and methods
Chemicals. QD–TOPO, 12+, 22+, 4, 5 and toluene-4-sulfonic
acid undec-10-enyl ester were synthesized according to literature
procedures.21,22,24–26 All reactions were carried out under N2 and
all solvents were freshly distilled under N2 prior to use. All other
chemicals were reagent grade quality, obtained from commercial
supplies and used without further purification.
Synthesis of 7
Calix[6]arene 6 (1.3 g, 0.9 mmol) and thioacetic acid (0.2 g,
2.8 mmol) were dissolved in 100 ml of dry toluene. AIBN was
added (catalytic amount) and the mixture refluxed for 12 h. After
this period distilled water (300 ml) was added. The organic phase
collected and concentrated under reduced pressure. Pure 7
was obtained by column chromatography (eluent: hexane–
Purification and characterization. Thin-layer chromatography
was performed on aluminium sheets coated with silica gel 60F
(Merck 5554). Column chromatography was carried out by using
silica gel (ICN 4663, 63–200 mesh). 1H NMR spectra were
recorded at 300 MHz. 13C NMR spectra were recorded at
75 MHz. Mass spectra were recorded in the ESI mode.
1
THF ¼ 9 : 1) as a yellowish solid (yield 83%). H NMR (300
MHz, CDCl3) d 7.68 (bs, 6 H), 7.23 (bs, 6 H), 4.6–4.1 (m,
6 H), 3.83 (bs, 6 H), 3.7–3.4 (m, 6 H), 2.9–2.8 (m, 15 H), 2.3
(s, 9 H), 1.85 (bs, 6 H), 1.7–1.0 (m, 53 H). 13C NMR (75 MHz,
CDCl3): 195.8, 159.8; 154.2; 146.7; 143.5, 135.7, 132.1, 129.7,
127.8, 127.3, 123.1, 73.9, 59.8, 34.2, 31.4, 30.9, 30.5, 30.2, 29.4,
29.0, 28.7, 26.8, 26.0, 23.3. Mp 146–148 ꢃC. ESIMS: 1686.66
[M + Na+].
Particle size distribution. The nanoparticle size distribution was
determined by DLS with a Malvern Nano ZS instrument
equippedꢃwith a 633 nm laser diode. Measurements were carried
out at 0 C in air-equilibrated CHCl3 solutions placed in 1 cm
quartz cuvettes. The samples were filtered twice through 0.1 mm
PTFE membrane filters prior to the measurements. The resulting
size is the average of at least 20 independent experiments, and the
standard deviation is taken as the experimental error.
Synthesis of 8
Photophysical experiments. Absorption spectra were recorded
with a Perkin Elmer l40 spectrophotometer in air equilibrated
CHCl3 (Merck Uvasolꢀ) solutions at room temperature (ca.
25 ꢃC), contained in 1 cm spectrofluorimetric quartz cells.
Luminescence spectra were recorded with either an Edinburgh
Instruments FLS920 or a Varian Cary Eclipse, exciting the
sample at either 400 or 350 nm respectively. Luminescence
lifetimes were determined with the Edinburgh Instruments
FLS920, exciting the sample at 400 nm. The experimental errors
are ꢁ1 nm for the wavelengths and ꢁ10% for the luminescence
intensities and lifetimes.
Calix[6]arene 7 (1 g, 0.6 mmol) was dissolved in THF (50 ml) and
HCl (50 ml 10% in water) and the resulting homogeneous
mixture refluxed for 3 days. After this period ethyl acetate
(20 ml) was added, the organic phase separated, dried on
Na2SO4 and concentrated under reduced pressure to obtain
compound 8, which was used for the next reaction without any
other purification.
Synthesis of 9
Calix[6]arene 8 (0.6 g, 0.4 mmol), trityl chloride (0.5 g, 1.8 mmol)
and triethylamine (0.2 g, 2 mmol) were dissolved in acetonitrile
(50 ml) and refluxed for 12 h. After this period the solvent was
removed and the residue dissolved in ethyl acetate (100 ml).
The organic phase was washed twice with HCl (100 ml 10% in
water), dried on Na2SO4 and concentrated under reduced
pressure. Compound 9 was purified by column chromatography
(eluent: hexane–ethyl acetate ¼ 9 : 1) as a yellowish solid (yield
The luminescence decay of the QDs, both alone and in the
presence of the quencher, is not monoexponential and could be
fitted by a double exponential function. Mean lifetime values,
obtained from a weighted average of the two individual lifetimes
on the basis of their relative contribution to the decay, were
taken as an indication of the QDs excited-state lifetime.30 The
titration of the QDs with the quencher was performed by adding
small aliquots (typically 5 ml) of a 10 mM solution of the
quencher to 2.5 mL of a dilute (<0.2 mM) solution of the QDs
by using a microsyringe.
1
77%). H NMR (300 MHz, CDCl3) d 7.67 (bs, 6 H), 7.6–7.1
(m, 51 H), 4.6–4.1 (m, 6 H), 3.81 (bs, 6 H), 3.7–3.4 (bs, 6 H),
2.85 (s, 9 H), 2.12 (t, 6 H, J ¼ 7.3 Hz), 1.83 (bs, 6 H), 1.8–1.0
2026 | J. Mater. Chem., 2008, 18, 2022–2027
This journal is ª The Royal Society of Chemistry 2008

View Article Online
(m, 53 H). 13C NMR (75 MHz, CDCl3): 146.8, 145.1, 132.1,
129.9, 129.6, 129.5, 129.3, 128.9, 128.7, 128.3, 128.2, 127.9,
127.7, 127.6, 127.4, 127.3, 127.2, 126.6, 126.4, 126.3, 123.1,
115.3, 73.9, 66.33, 34.2, 32.0,ꢃ31.4, 30.2, 29.7, 29.5, 29.4, 29.2,
29.0, 28.6, 25.0. Mp 100–103 C. ESIMS: 2288.66 [M + Na+].
ambient temperature, the solvent was distilled off under reduced
pressure. The residue was dissolved in CHCl3 (2 ml) and diluted
with MeOH (30 ml). The resulting precipitate was filtered off
after centrifugation. This procedure was repeated three more
times to afford QD–3 (8 mg) as a reddish powder.
Synthesis of 10
References
In a 250 ml two neck round bottom flask calix[6]arene 9 (0.58 g,
0.26 mmol) was suspended in n-butanol (150 ml). Pd on activated
charcoal (catalytic amount) was added under nitrogen. The
mixture was stirred for a few minutes and then hydrazine
monohydrate (0.64 g, 13 mmol) was added. The resulting hetero-
geneous mixture was refluxed for 12 h. The crude of the reaction
was cooled to room temperature and the Pd/C removed by filtra-
tion on Celite maintaining the equipment under N2 atmosphere.
The solvent was removed under reduced pressure and the
triamine calix[6]arene 10 was used for the next reaction without
any other purification.
1 M. G. Bawendi, M. L. Steigerwald and L. E. Brus, Annu. Rev. Phys.
Chem., 1990, 41, 477–496.
2 A. P. Alivisatos, Science, 1996, 271, 933–937.
3 Al. L. Efros and L. Rosen, Annu. Rev. Mater. Sci., 2000, 30, 475–521.
4 A. D. Yoffe, Adv. Phys., 2001, 50, 1–208.
5 C. Burda, X. B. Chen, R. Narayana and M. A. El-Sayed, Chem. Rev.,
2005, 105, 1025–1102.
6 C. M. Niemeyer, Angew. Chem., Int. Ed., 2003, 42, 5796–5800.
7 I. Willner and E. Katz, Angew. Chem., Int. Ed., 2004, 43, 6042–6108.
8 A. P. Alivisatos, Nat. Biotechnol., 2004, 22, 47–52.
9 N. L. Rosi and C. A. Mirkin, Chem. Rev., 2005, 105, 1547–1562.
10 X. Gao, L. Yang, J. A. Petros, F. F. Marshall, J. W. Simons and
S. Nie, Curr. Opin. Biotechnol., 2005, 16, 63–72.
11 I. G. Medintz, H. T. Uyeda, E. R. Goldam and H. Mattoussi,
Nat. Mater., 2005, 4, 435–446.
Synthesis of 11
12 X. Michalet, F. F. Pinaud, L. A. Bentolila, J. M. Tsay, S. Doose,
J. J. Li, G. Sundaresan, A. M. Wu, S. S. Gambhir and S. Weiss,
Science, 2005, 307, 538–544.
13 R. C. Somers, M. G. Bawendi and D. G. Nocera, Chem. Soc. Rev.,
2007, 36, 579–591.
14 F. M. Raymo and I. Yildiz, Phys. Chem. Chem. Phys., 2007, 9, 2036–
2043.
15 (a) M. G. Sandros, D. Gao and D. E. Benson, J. Am. Chem. Soc.,
2005, 127, 12198–12199; (b) B. P. Aryal and D. E. Benson, J. Am.
Chem. Soc., 2006, 128, 15986–15987.
Calix[6]arene 10 (0.55 g, 0.25 mmol) and phenylisocyanate
(0.15 g, 1.26 mmol) were dissolved in dry CH2Cl2 (100 ml) and
stirred at room temperature for 4 h. The organic solvent was
removed under reduced pressure and the crude compound 11
was triturated in methanol to obtain calix[6]arene as a pale
1
yellow solid (yield 84%). H NMR (300 MHz, CDCl3) d 7.7–
6.8 (m, 72 H), 6.30 (s, 6 H), 4.38 (d, 6 H, J ¼ 15.1 Hz), 3.89
(bt, 6 H), 3.54 (d, 6 H, J ¼ 15.4 Hz), 2.82 (bs, 9 H), 2.13 (t, 6
H, J ¼ 7.2 Hz), 1.87 (bs, 6 H), 1.55 (bs, 6 H), 1.5–1.0 (m, 47
H). 13C NMR (75 MHz, CDCl3): 154.9, 154.4, 152.0, 146.8,
145.0, 138.0, 135.5, 132.9, 132.3, 129.5, 128.8, 128.2, 127.8,
127.7, 127.2, 126.4, 123.4, 122.5, 120.5, 73.1, 66.3, 60.2, 34.2,
32.0, 31.ꢃ5, 30.9, 30.5, 29.5, 29.3, 29.1, 28.9, 28.5, 26.2. Mp
128–130 C. ESIMS: 2557.23 [M + Na+].
16 K. Palaniappan, C. Xue, G. Arumugam, S. A. Hackney and J. Liu,
Chem. Mater., 2006, 18, 1275–1280.
17 R. Gill, R. Freeman, J.-P. Xu, I. Willner, S. Winograd, I. Shweky and
U. Banin, J. Am. Chem. Soc., 2006, 128, 15376–15377.
18 (a) I. Yildiz and F. M. Raymo, J. Mater. Chem., 2006, 16, 1118–1120;
(b) M. Tomasulo, I. Yildiz and F. M. Raymo, J. Phys. Chem. B, 2006,
110, 3853–3855; (c) M. Tomasulo, I. Yildiz, S. L. Kaanumalle and
F. M. Raymo, Langmuir, 2006, 22, 10284–10290; (d) I. Yildiz,
M. Tomasulo and F. M. Raymo, Proc. Natl. Acad. Sci. U. S. A.,
2006, 103, 11457–11460.
`
19 V. Maruel, M. Laferriere, P. Billone, R. Godin and J. C. Scaiano,
J. Phys. Chem. B, 2006, 110, 16353–16358.
Synthesis of 3
20 S. J. Clarke, C. A. Hollmann, Z. Zhang, D. Suffern, S. E. Bradforth,
N. M. Dimitrijevic, W. G. Minarik and J. L. Nadeau, Nat. Mater.,
2006, 5, 409–417.
21 B. O. Dabbousi, J. Rodriguez-Viejo, F. V. Mikulec, J. R. Heine,
H. Mattoussi, R. Ober, K. F. Jensen and M. G. Bawendi, J. Phys.
Chem. B, 1997, 101, 9463–9475.
22 Z. A. Peng and X. Peng, J. Am. Chem. Soc., 2001, 123, 183–184.
23 J. M. Tsay, S. Doose and S. Weiss, J. Am. Chem. Soc., 2006, 128,
1639–1647.
Calix[6]arene 11 (0.05 g, 0.02 mmol) was dissolved in CH2Cl2
(20 ml) and TFA (10 ml). NaBH3CN (0.006 g, 0.1 mmol) was
added and the mixture stirred at room temperature for 3 h.
Water (30 ml) was added and the organic phase collected, dried
on Na2SO4 and concentrated under reduced pressure. The crude
product was triturated in methanol to obtain calix[6]arene 3 as
1
a pale yellow solid (yield 86%). H NMR (300 MHz, CDCl3)
24 A. Arduini, R. Ferdani, A. Pochini, A. Secchi and F. Ugozzoli,
Angew. Chem., Int. Ed., 2000, 39, 3453–3456.
d 7.4–6.8 (m, 27 H), 6.27 (s, 6 H), 4.38 (d, 6 H, J ¼ 15.4 Hz),
3.90 (t, 6 H, J ¼ 6.0 Hz), 3.55 (d, 6 H, J ¼ 15.6 Hz), 2.82 (bs,
9 H), 2.51 (q, 6 H, J ¼ 7.2 Hz), 1.88 (t, 6 H, J ¼ 6.8 Hz), 1.8–
1.0 (m, 53 H). 13C NMR (75 MHz, CDCl3):154.9, 154.5, 152.3,
146.7, 138.2, 135.7, 133.0, 132.3, 129.4, 129.3, 128.8, 128.7,
128.20, 127.6, 126.2, 123.4, 123.1, 120.5, 73.0, 60.2, 34.1, 34.0,
31.8, 31.5, 31.0, 30.5, 29.6, 29.5, 29.4, 29.3, 29.2, 29.0, 28.8,
25 R. J. Alvarado, J. Mukherjee, E. J. Pacsial, D. Alexander and
F. M. Raymo, J. Phys. Chem. B, 2005, 109, 6164–6173.
26 J. J. Gonzales, R. Ferdani, E. Albertini, J. M. Blasco, A. Arduini,
A. Pochini, P. Prados and J. De Mendoza, Chem.–Eur. J., 2000, 6,
73–80.
´
27 R. Metivier, I. Leray, B. Lebeau and B. Valeur, J. Mater. Chem.,
2005, 15, 2965–2973.
28.3, 26.2, 24.6. Mp 137–142 C. ESIMS: 1830.6 [M + Na+].
ꢃ
28 (a) S. Logunov, T. Green, S. Marguet and M. A. El- Sayed, J. Phys.
Chem. A, 1998, 102, 5652–5658; (b) C. Burda, T. C. Green, S. Link
and M. A. El-Sayed, J. Phys. Chem. B, 1999, 103, 1783–1788.
29 A. Credi, S. Dumas, S. Silvi, M. Venturi, A. Arduini, A. Pochini and
A. Secchi, J. Org. Chem., 2004, 69, 5881–5887.
Synthesis of QD–3
A dispersion of QD–TOPO (10 mg) and 3 (40 mg) in CHCl3
30 J. R. Lakowicz, Principles of Fluorescence Spectroscopy, Springer,
New York, 2006.
(20 ml) was heated under reflux for 24 h. After cooling to
This journal is ª The Royal Society of Chemistry 2008
J. Mater. Chem., 2008, 18, 2022–2027 | 2027