Chelating ligands for nanocrystals' surface functionalization

Article abstract of DOI:10.1021/ja047882c

A new family of ligands for the surface functionalization of CdSe nanocrystals is proposed, namely alkyl or aryl derivatives of carbodithioic acids (R-C(S)SH). The main advantages of these new ligands are as follows: they nearly quantitatively exchange th

Full text of DOI:10.1021/ja047882c

Published on Web 08/24/2004
Chelating Ligands for Nanocrystals’ Surface Functionalization
Claudia Querner,† Peter Reiss,*,† Joe¨l Bleuse,‡ and Adam Pron†
Contribution from SerVice des Interfaces et des Mate´riaux Mole´culaires et Macromole´culaires,
Laboratoire Physique des Me´taux Synthe´tiques, and SerVice de Physique des Mate´riaux et des
Microstructures, Laboratoire Physique des Semi-Conducteurs, De´partement de Recherche
Fondamentale sur la Matie`re Condense´e, CEA Grenoble, 17 rue des Martyrs,
38054 Grenoble Cedex 9, France
Received March 30, 2004; E-mail: preiss@cea.fr
Abstract: A new family of ligands for the surface functionalization of CdSe nanocrystals is proposed, namely
alkyl or aryl derivatives of carbodithioic acids (R-C(S)SH). The main advantages of these new ligands are
as follows: they nearly quantitatively exchange the initial surface ligands (TOPO) in very mild conditions;
they significantly improve the resistance of nanocrystals against photooxidation because of their ability of
strong chelate-type binding to metal atoms; their relatively simple preparation via Grignard intermediates
facilitates the development of new bifunctional ligands containing, in addition to the anchoring carbodithioate
group, a second function, which enables the grafting of molecules or macromolecules of interest on the
nanocrystal surface. To give an example of this approach, we report, for the first time, the grafting of an
electroactive oligomer from the polyaniline familysaniline tetramerson CdSe nanocrystals after their
functionalization with 4-formyldithiobenzoic acid. The grafting proceeds via a condensation reaction between
the aldehyde group of the ligand and the terminal primary amine group of the tetramer. The resulting organic/
inorganic hybrid exhibits complete extinction of the fluorescence of its constituents, indicating efficient charge
or energy transfer between the organic and the inorganic semiconductors.
shell of a second, larger band-gap semiconductor such as ZnS,4
ZnSe,5 or CdS6 on the CdSe surface. On the other hand, the
Introduction
Research on semiconductor nanocrystals is a field of steadily
growing interest due to their unique photophysical properties1
and to their high potential for substituting traditional compounds
in various domains such as optoelectronics2 or biological
labeling.3 A key issue for many of these applications is the
control of the nanocrystals’ surface chemistry. As the number
of surface atoms depends on the inverse of the diameter, for
sizes below ca. 2 nm more than half of the atoms are located
on the nanocrystal surface, dominating a large number of its
properties. In contrast to core atoms, the coordination sphere
of surface atoms is not complete. This gives rise to the formation
of dangling bonds, which, among others, can act as traps for
photogenerated charge carriers and decrease the nanocrystals’
emission efficiency. By proper passivation of these surface trap
states, the photoluminescence quantum yield (QY) of CdSe
nanocrystals can be improved by more than 1 order of mag-
nitude. This has been achieved by growing an epitaxial-type
question of surface chemistry remains essentially unchanged,
being now associated with the shell surface.
From a chemist’s standpoint, the nature of the ligand being
coordinated to the nanocrystal surface, and in particular the type
of bonds which it forms with the nanocrystal surface atoms, is
of crucial importance. Via ligand design, several important
properties of the nanocrystals can be tuned, such as their
processibility, reactivity, and stability, with direct consequences
on their spectroscopic properties. Nevertheless, relatively few
literature examples exist concerning the conception of new types
of nanocrystal surface ligands. Among them, recently developed
oligomeric phosphines7 and thiol dendrimers8 should be men-
tioned.
Three principal functions of the surface ligands can be briefly
described as follows: (1) They prevent individual colloidal
nanocrystals from aggregation. (2) They facilitate nanocrystals’
dispersion in a large variety of solvents. In the presence of
surface ligands, the ability to disperse nanocrystals is governed
by the difference between the ligand and the solvent solubility
† Service des Interfaces et des Mate´riaux Mole´culaires et Macromole´cu-
laires, Laboratoire Physique des Me´taux Synthe´tiques.
‡ Service de Physique des Mate´riaux et des Microstructures, Laboratoire
Physique des Semi-Conducteurs.
(1) Woggon, U. Optical Properties of Semiconductor Quantum Dots;
Springer: New York, 1997.
(2) (a) Coe, S.; Woo, W.-K.; Bawendi, M. G.; Bulovic, V. Nature 2002, 420,
800-803. (b) Huynh, W. U.; Dittmer, J. J.; Alivisatos, A. P. Science 2002,
295, 2425-2427.
(3) (a) Bruchez, M.; Moronne, M.; Gin, P.; Weiss, S.; Alivisatos, A. P. Science
1998, 281, 2013-2016. (b) Chan, W. C. W.; Nie, S. M. Science 1998,
281, 2016-2018. (c) Mattoussi, H.; Mauro, J. M.; Goldman, E. R.;
Anderson, G. P.; Sundar, V. C.; Mikulec, F. V.; Bawendi, M. G. J. Am.
Chem. Soc. 2000, 122, 12142-12150.
(4) (a) Guyot-Sionnest, P.; Hines, M. A. J. Phys. Chem. 1996, 100, 468-471.
(b) Daboussi, O.; Rodriguez-Viejo, J.; Mikulec, F. V.; Heine, J. R.;
Mattoussi, H.; Ober, R.; Jensen, K. F.; Bawendi, M. G. J. Phys. Chem. B
1997, 101, 9463-9475.
(5) Reiss, P.; Bleuse, J.; Pron, A. Nano Lett. 2002, 2, 781-784.
(6) Peng, X.; Schlamp, M. C.; Kadavanich, A. V.; Alivisatos, A. P. J. Am.
Chem. Soc. 1997, 119, 7019-7029.
(7) Kim, S.; Bawendi, M. G. J. Am. Chem. Soc. 2003, 125, 14652-14653.
(8) (a) Wang, Y. A.; Li, J. J.; Chen, H. Y.; Peng, X. J. Am. Chem. Soc. 2002,
124, 2293-2298. (b) Guo, W. H.; Li, J. J.; Wang, Y. A.; Peng, X. J. Am.
Chem. Soc. 2003, 125, 3901-3909.
9
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J. AM. CHEM. SOC. 2004, 126, 11574-11582
10.1021/ja047882c CCC: $27.50 © 2004 American Chemical Society

Ligands for Surface Functionalization of Nanocrystals
A R T I C L E S
parameters, which can be precisely tuned. In view of nano-
crystals’ applications in biological labeling, ligands enabling
their dispersion in aqueous solutions are of special interest. (3)
Ligands containing appropriate functional groups may serve as
“bridging” units for the coupling of molecules or macromol-
ecules to nanocrystals or their grafting on substrates. Thus,
appropriate ligand design opens up the possibility of fabrication
of new nanocrystal-based organic/inorganic hybrid materials.
In a typical nanocrystals’ synthesis, ligands preventing their
aggregation are present in the reaction medium. Depending on
the intended application, these “original” ligands must be
replaced by new ones, which introduce the desired solubility
and/or functionality. For this purpose, bifunctional ligands
X-Y-Z are frequently used, in which X is a chemical function
with a high affinity for the nanocrystal surface, Y is a spacer
of alkyl or aryl type, and Z is the group transferring the desired
property to the nanocrystal. The main advantages of this
approach are the well-defined surface chemistry of the nano-
crystals after functionalization and the possibility to maintain
essentially their original size. In most literature examples, the
function X stands for one or two thiol groups.3,9 This preference
can be partially attributed to the large commercial availability
of thiols with numerous spacers Y and functional groups Z.
Although convenient in use, thiol-based ligands show however
some weaker points. For example, photodegradation studies
reveal that thiols coordinated on CdSe nanocrystals readily
undergo photooxidation accompanied by the formation of
disulfides, which leads to the precipitation of the crystals.10 This
photochemical instability can be ascribed to the relatively weak
interaction between the nanocrystal surface and the thiol ligand,
which is not of a covalent nature. Furthermore, quasi-quantitative
substitution (>95%) of the original surface ligands (e.g. TOPO)
by thiols requires usually extended exchange reaction times and
can be achieved only in relatively drastic reaction conditions11
which, in turn, might affect other properties of the nanocrystals,
such as their QY.
These shortcomings motivated us to develop a new type
of bifunctional ligands, which contain a carbodithioate
(-C(S)S-) function serving as the anchor group for the
nanocrystal surface. Several features make them attractive
candidates for nanocrystal functionalization. Their high affinity
for metal atoms, which arises from the bidentate chelating
binding of the carbodithioate group, facilitates the replacement
of the original surface ligands. Quasi-quantitative exchange is
achieved in a room-temperature reaction within 1-3 h. In
addition, nanocrystals capped with the carbodithioate ligands
exhibit a significantly enhanced stability against photodegra-
dation with respect to the corresponding thiol ligands. Finally,
the new ligands can be easily synthesized via a Grignard
reaction. The simplicity and generality of the proposed synthetic
approach open up the possibility of the preparation of “tailor-
made” molecules. This is illustrated here by the synthesis of a
ligand which has been designed for the grafting of electroactive
molecules (aniline tetramers) on the nanocrystal surface, result-
ing in a new organic/inorganic hybrid compound with potential
interest for use in photovoltaic devices.
Experimental Section
Chemicals. Anhydrous solvents and all reagents (Aldrich) were of
the highest purity available. Trioctylphosphine (TOP, 90%) and
hexadecylamine (HDA, 90%) were additionally purified by distillation.
Characterization Techniques. All synthesized precursors and
ligands were identified by 1H and 13C NMR spectroscopy on a Bruker
AC 200-MHz spectrometer; acetone-d6, chloroform-d, and DMSO-d6
containing TMS as internal standard were used as solvents according
to the solubility of the product. The purity of the synthesized molecules
was verified by elemental analysis (Analytical Service of CNRS
Vernaison, France). FT-IR spectra were recorded on a Perkin-Elmer
Paragon500 spectrometer (wavenumber range 4000-400 cm-1; resolu-
tion 2 cm-1) using the ATR technique. Solution UV-vis spectra were
recorded on a Hewlett Packard 8452A spectrometer (wavelength range
180-820 nm). Photoluminescence spectroscopy investigations were
carried out on a Jobin-Yvon HR 550 monochromator equipped with a
CCD silicon detector cooled at 140 K. Excitation was performed with
an argon laser at 365 nm or with a blue, 400-nm LED.
Synthesis of CdSe Nanocrystals. The synthetic procedure used in
this research for the preparation of CdSe core nanocrystals has been
described in ref 5, using a molar ratio of 70% HDA in the TOPO/
HDA solvent mixture. The obtained TOPO-capped CdSe nanocrystals
had an average diameter of 4.2 nm (determined by TEM).
1H NMR (CDCl3, 200 MHz): δ 1.7-1.4 (s, 6H), 1.19 (s, 36H),
0.81 (s, 9H). UV-vis (CHCl3): λ ) 561 nm (excitonic peak), 460 nm
(higher excited state). PL (CHCl3): 575 nm.
General Procedure for the Synthesis of Carbodithioic Acids. The
preparation consists of a Grignard reaction of alkyl- or arylmagnesium
bromides with carbon disulfide (CS2), based on procedures described
in the literature.12 Caution: Carbon disulfide is highly toxic and has
to be handled under a fume hood or in a drybox. Dried magnesium
turnings (5 equiv) were covered with anhydrous THF in a round-bottom
flask under argon flux. A solution of alkyl- or arylbromine (1 equiv)
dissolved in anhydrous THF was added dropwise. The resulting mixture
was vigorously stirred under reflux for 2 h. The colored solution was
then cooled and transferred via cannula to a flask containing carbon
disulfide (3 equiv) in anhydrous THF, previously cooled to -5 to 0
°C. After addition, the reaction mixture was allowed to warm to room
temperature and left overnight with constant stirring. In the next step,
the product was hydrolyzed with a 1:1 diethyl ether:water mixture. The
aqueous layer was then acidified using 0.2 M HCl and extracted with
diethyl ether. The organic layer was repeatedly washed with water,
dried over MgSO4, and concentrated using a rotary evaporator.
Tridecanedithioic Acid (1). A 3.46-g yield of a yellow, partially
crystallized product was obtained from 5 g of n-bromododecane (70%
yield), using the procedure described above.
1H NMR (CDCl3, 200 MHz): δ 2.99 (t, 2H, J ) 6.7 Hz), 1.78 (q,
2H, J ) 6.7 Hz), 1.4-1.0 (m, 18H), 0.81 (t, 3H, J ) 6.7 Hz); S-H is
not visible. 13C NMR (CDCl3, 200 MHz): δ 14.02, 22.64, 28.57, 29.17,
29.31, 29.40, 29.57 (2C), 29.64 (2C), 31.87, 52.89, 238.76. Elemental
analysis calculated for C13H26S2: C, 63.35; H, 10.63; S, 26.02. Found:
C, 62.88; H, 10.52; S, 25.17. UV-vis (CHCl3): λmax ) 300 nm.
4-Methyldithiobenzoic Acid (2). A 0.72-g yield of a violet solid
was obtained from 1.7 g of 4-bromotoluene (43% yield), using the
procedure described above. This crude product was then purified by
recrystallization in MeOH.
1H NMR (CDCl3, 200 MHz): δ 7.89 (d, 2H, J ) 8.3 Hz), 7.10 (d,
(9) (a) Pathak, S.; Choi, S. K.; Arnheim, N.; Thompson, M. E. J. Am. Chem.
Soc. 2001, 123, 4103-4106. (b) Wuister, S. F.; Swart, I.; van Driel, F.;
Hickey, S. G.; de Mello Donega, C. Nano Lett. 2003, 3, 503-507.
(10) Aldana, J.; Wang, Y. A.; Peng, X. J. Am. Chem. Soc. 2001, 123, 8844-
8850.
(11) (a) Kuno, M.; Lee, J. K.; Dabbousi, B. O.; Mikulec, F. V.; Bawendi, M.
G. J. Chem. Phys. 1997, 106, 9869-9882. (b) Peng, X.; Wilson, T. E.;
Alivisatos, A. P.; Schultz, P. G. Angew. Chem., Int. Ed. Engl. 1997, 36,
145-147.
2H, J ) 8.3 Hz), 6.20 (S-H, s, 1H), 2.30 (s, 3H). 13C NMR (CDCl3,
(12) (a) Houben, J.; Kesselkaul, L. Ber. Dtsch. Chem. Ges. 1902, 35, 3695-
3699. (b) Houben, J.; Pohl, H. Ber. Dtsch. Chem. Ges. 1907, 40, 1725-
1730. (c) Ramadas, S. R.; Srinivasan, P. S.; Ramachandran, J.; Sastry, V.
V. S. K. Synthesis 1983, 605-622. (d) Colorado, R., Jr.; Villazana, R. J.;
Lee, T. R. Langmuir 1998, 14, 6337-6340.
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A R T I C L E S
Querner et al.
200 MHz): δ 21.52, 126.91 (2C), 129.00 (2C), 140.96, 144.46, 224.55.
Elemental analysis calculated for C8H8S2: C, 57.10; H, 4.79; S,
analysis calculated for C24H20N4‚1H2O: C, 75.20; H, 5.78; N, 14.61;
O, 4.41. Found: C, 75.86; H, 5.25; N, 14.54. MS-ESI (H+ mode): m/z
calcd ) 364.17, mH+/z found ) 365.30.
38.11. Found: C, 57.00; H, 4.87; S, 37.70. UV-vis (CHCl3): λmax
)
315 nm.
Model Compound 7. A solution of 6 (18 mg, 0.05 mmol) in 10
mL of anhydrous ethanol was added dropwise to the solution of
p-bromobenzaldehyde (9 mg, 0.05 mmol) in 5 mL of the same solvent.
The mixture was heated overnight under reflux. The precipitate was
filtered under an inert atmosphere and washed with a large excess of
methanol, then with chloroform, and finally dried in a vacuum to give
a black solid (20 mg, 80% yield). 1H NMR (DMSO-d6, 200 MHz): δ
7.99 (s, 6H), 7.2-6.6 (m, 17H). UV-vis (DMSO): λmax ) 325 nm, λ
) 618 nm.
4-Formyldithiobenzoic Acid (5). 4-Bromobenzaldehyde (1.93 g,
10 mmol) and 2,2-dimethyl-1,3-propanediol (10.88 g, 100 mmol) were
dissolved in 60 mL of toluene in a round-bottom flask. Trifluoroacetic
acid (0.25 mL, 3 × 10-3 mmol) was then added, and the mixture was
heated at 90 °C for 15 h. In the next step, the reaction mixture was
washed with a saturated solution of K2CO3 and then with water. The
organic layer was concentrated, and the product was recrystallized in
water to give 2.63 g (94% yield) of 2-(4-bromophenyl)-5,5-dimethyl-
1,3-dioxane (3) (white solid).
1H NMR (CDCl3, 200 MHz): δ 7.43 (dd, 2H, J ) 8.8 Hz, 2.2 Hz),
7.31 (dd, 2H, J ) 8.8 Hz, 2.2 Hz), 5.27 (s, 1H), 3.63 (dd, 4H, J ) 26
Hz, 11 Hz), 1.20 (s, 3H), 0.73 (s, 3H). 13C NMR (CDCl3, 200 MHz):
δ 21.83, 23.00, 30.18, 77.62 (2C), 100.92, 122.84, 127.93 (2C), 131.37
(2C), 137.58. Elemental analysis calculated for C12H15BrO2: C, 53.16;
H, 5.58; Br, 29.47; O, 11.80. Found: C, 52.93; H, 5.57.
From 4.12 g of 3, 2.12 g of 4-(5,5-dimethyl-1,3-dioxan-2-yl)-
dithiobenzoic acid (4) was obtained (62% yield) using the general
procedure described above. The product in the form of a violet solid
was recrystallized in methanol.
1H NMR (CDCl3, 200 MHz): δ 8.04 (dd, 2H, J ) 8.4 Hz, 2.2 Hz),
7.53 (dd, 2H, J ) 8.4 Hz), 6.38 (S-H, s, 1H), 5.41 (s, 1H), 3.72 (dd,
4H, J ) 26 Hz, 11 Hz), 1.28 (s, 3H), 0.81 (s, 3H). 13C NMR (CDCl3,
200 MHz): δ 21.87, 23.01, 30.29, 77.66 (2C), 100.67, 126.23 (2C),
126.80 (2C), 143.38, 143.65, 224.95. Elemental analysis calculated for
C13H16O2S2: C, 58.17; H, 6.01; S, 23.89; O, 11.92. Found: C, 58.10;
H, 6.06; S, 23.86. UV-vis (CHCl3): λmax ) 305 nm.
General Procedure of Ligand Exchange. A solution of the ligand
(thiol or carbodithioic acid, 20 mg in 1 mL of CHCl3) was added to
the colloidal solution of CdSe nanocrystals (10 mg in 1 mL of CHCl3).
The obtained mixture was then stirred at constant temperature. In the
case of carbodithioic acids, the ligand exchange carried out at room
temperature for 1-3 h led to a nearly quantitative replacement of the
initial surface ligands, as confirmed by 1H NMR. For thiols, the same
procedure required strirring at 40 °C for periods up to 3 days. The
nanocrystals coated with the new ligands were purified by two cycles
of precipitation with methanol and dispersion in chloroform. After being
dried under vacuum, they could readily be redispersed in chloroform.
Using the above-outlined procedures, CdSe nanocrystals functionalized
with dodecanethiol (Alk-SH), tridecanedithioic acid (1), 4-methoxy-
benzenethiol (Ar-SH), 4-methyldithiobenzoic acid (2), and 4-(5,5-
dimethyl-1,3-dioxan-2-yl)dithiobenzoic acid (4) were prepared.
1
CdSe-Alk-SH (40 °C, 3 days). H NMR (CDCl3, 200 MHz): δ
1.6-1.4 (s, 2H), 1.4-1.0 (s, 18.7H), 1.0-0.6 (s, 3H) (15% of the initial
TOPO content remained unexchanged). UV-vis (CHCl3): λ ) 561
nm (excitonic peak), λ ) 460 nm (higher excited state).
One milliliter of H2O and 10 mL of concentrated CF3COOH were
added to a solution of 4 (205 mg, 0.76 mmol) in 2 mL of CHCl3. The
solution was stirred at room temperature for 4 h. The reaction mixture
was then diluted with 10 mL of CHCl3 and neutralized with a saturated
aqueous solution of K2CO3. 4-Formyldithiobenzoic acid (5) was
obtained either in its potassium salt form (5′) (yellow-orange solid after
extraction from the aqueous layer) or as the free acid 5 (dark-red solid,
85 mg, 61%) by acidifying with 0.2 M HCl.
1
CdSe-1 (25 °C, 1 h). H NMR (CDCl3, 200 MHz): δ 3.0-2.8 (s,
2H), 1.6-1.4 (s, 2H), 1.4-1.0 (s, 17.4H), 1.0-0.6 (s, 3H) (10% of the
initial TOPO content remained unexchanged). UV-vis (CHCl3): λ )
578 nm (excitonic peak), λ ) 470 nm (higher excited state).
CdSe-Ar-SH (25 °C, 3 days). 1H NMR (CDCl3, 200 MHz): δ 7.33
(d, 2H, J ) 8.8 Hz), 6.77 (d, 2H, J ) 8.8 Hz), 3.71 (s, 3H), 1.6-1.4
(s, 25H), 1.4-1.0 (s, 120H), 1.0-0.6 (s, 38H) (70% of the initial TOPO
content remained unexchanged). UV-vis (CHCl3): λ ) 557 nm
(excitonic peak), λ ) 457 nm (higher excited state).
1
(a) Characterization of the Free Acid 5. H NMR (CDCl3, 200
MHz): δ 10.01 (s, 1H), 7.92 (d, 2H, J ) 8.6 Hz), 7.85 (d, 2H, J )
8.6 Hz), 6.35 (S-H, s, 1H). 13C NMR (CDCl3, 200 MHz): δ 129.67
(2C), 130.04 (2C), 191.19, 228.86. Elemental analysis calculated for
C8H6OS2: C, 52.71; H, 3.32; O, 8.78; S, 35.17. Found: C, 52.91; H,
3.39; S, 33.74.
(b) Characterization of the Potassium Salt 5′. 1H NMR (acetone-
d6, 200 MHz): δ 10.03 (s, 1H), 8.32 (d, 2H, J ) 8.4 Hz), 7.72 (d, 2H,
J ) 8.6 Hz). 13C NMR (acetone-d6, 200 MHz): δ 128.76 (2C), 129.71
(2C), 137.34, 193.75, 252.80. Elemental analysis calculated for
C8H5OS2K: C, 43.60; H, 2.29; K, 17.74; O, 7.26; S, 29.10. Found: C,
43.15; H, 2.66; S, 28.67.
CdSe-2 (25 °C, 2 h). 1H NMR (CDCl3, 200 MHz): δ 7.96 (d, 2H,
J ) 8.3 Hz), 7.17 (d, 2H, J ) 8.3 Hz), 2.34 (s, 3H), 1.6-1.4 (s, 0.2H),
1.4-1.0 (s, 1.2H), 1.0-0.6 (s, 0.3H) (<5% of the initial TOPO content
remained unexchanged). UV-vis (CHCl3): λ ) 323 nm (ligand π-π*
transition).
Complexation of Cd2+ Ions with 2. A 19-mg sample of CdCl2 (0.1
mmol) was suspended in a solution of 2 (34 mg, 0.2 mmol) in 2 mL
of CHCl3 and stirred at room temperature. Within 2 h a homogeneous
solution formed. UV-vis (CHCl3): λmax ) 317 nm.
Grafting of Aniline Tetramer on CdSe Nanocrystals. The
procedure consisted of two steps: first, original TOPO ligands were
exchanged with potassium 4-formyldithiobenzoate (5′) to give aldehyde-
capped nanocrystals on which, in a second step, aniline tetramer was
grafted (Scheme 1).
Aniline tetramer in the oxidation state of emeraldine (6) was
prepared using a modification of the method described in ref 13. The
exact preparation procedure used in this research can be found
elsewhere.14
1H NMR (DMSO-d6, 200 MHz): δ 8.37 (N-H, s, 1H), 7.23 (d, 2H,
J ) 7 Hz), 7.11 (d, 4H, J ) 5 Hz), 7.0-6.7 (m, 9H), 6.62 (d, 2H, J )
8 Hz), 5.51 (N-H2, s, 2H). UV-vis (DMSO): λmax ) 306 nm, λ )
591 nm. IR: ν (cm-1) 1597 (s), 1497 (s), 1320 (m), 1166 (w), 839
(m), 745 (m), 692 (w).
(a) Ligand Exchange. Ligand 5′ (12 mg, 0.055 mmol) was placed
under an inert atmosphere in a round-bottom flask and dissolved in 10
mL of ethanol. CdSe nanocrystals (10 mg) (0.2 mL of a 50 mg/mL
CHCl3 solution) in 10 mL of chloroform were then added, and the
mixture was stirred for 2 h at room temperature to give CdSe-5′. No
intermediate purification was carried out.
(b) Aniline Tetramer Grafting. A solution of 6 (50 mg, 0.136
mmol) in 10 mL of ethanol was added dropwise to the solution of
aldehyde-functionalized nanocrystals CdSe-5′. The mixture was re-
fluxed overnight under an inert atmosphere. The formed black
precipitate was then filtered and washed repeatedly first with methanol,
to extract nonreacted tetramer, and then with chloroform, with the goal
Spectroscopic studies as well as elemental analysis are consistent
with the presence of one water molecule per tetramer unit. Elemental
(13) Feng, J.; Zhang, W.; MacDiarmid, A. G.; Epstein, A. P. Proc. Soc. Plast.
Eng., Annu. Technol. Conf. (ANTEC97) 1997, 2, 1373-1377.
(14) Dufour, B.; Rannou, P.; Travers, J. P.; Pron, A.; Zagorska, M.; Korc, G.;
Kulszewicz-Bajer, I.; Quillard, S.; Lefrant, S. Macromolecules 2002, 35,
6112-6120.
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Ligands for Surface Functionalization of Nanocrystals
A R T I C L E S
Scheme 1. Chemical Structures of the Bifunctional Ligand 5, Grafted and Nongrafted Aniline Tetramer Derivatives, and Their Synthetic
Pathwaysa
a Reagents and conditions: (i) neopentyl glycol, CF3COOH, toluene, 90 °C; (ii) Mg, THF, 60 °C; CS2, THF, 0 °C; 2 M HCl; (iii) CHCl3:H2O:CF3COOH
(2:1:10), 25 °C; (iv) K2CO3, 25 °C; (v) 5′, CHCl3:EtOH (1:1), 25 °C; (vi) 6, EtOH, 80 °C.
to remove nongrafted aldehyde ligand. The final black solid, CdSe-8
(7 mg, 64% yield), which consists of aniline-tetramer-functionalized
CdSe nanocrystals, is soluble in DMSO or NMP.
1H NMR (DMSO-d6, 200 MHz): δ 7.99 (s, 6H), 7.2-6.6 (m, 17H).
UV-vis (DMSO): λmax ) 325 nm, λ ) 618 nm. IR: ν (cm-1) 1697
(w), 1597 (s), 1504 (vs), 1297 (s), 1166 (m), 1012 (w) (Ar-CS2-),
819 (m), 750 (m), 696 (w).
Photostability Investigations. Colloidal solutions of tridecanedithio-
ate-capped CdSe (CdSe-1) and dodecanethiol-capped CdSe nanocrystals
(CdSe-Alk-SH) in chloroform were placed in quartz cells of 1-mm
path length (sample volume 0.3 mL) and purged with air. Their
concentrations were adjusted by dilution to be identical for comparative
studies, with the absorbance being in the range 0.1-0.3 for the first
exciton absorption peak. Photooxidation experiments were carried out
by continuous irradiation with an UV lamp (254 or 365 nm, 25 W),
placed at a distance of 5 cm from the sample, and recording the
UV-vis spectra of the samples in regular intervals.
prevents them from decomposition and their acidic forms are
stable under an inert atmosphere.
Ligand Exchange. Different exchange procedures can be
found in the literature. In the simplest way, nanocrystals
containing initial surface ligands are brought into contact with
a large excess of the new ligand, which serves as a solvent
for nanocrystals’ dispersion.3b Alternatively, the ligand can be
dissolved in an organic cosolvent9b or nonsolvent for the
nanocrystals.9a,11b If used as the solvent, the new ligand must
be a liquid at the temperature of the exchange reaction; more-
over, a relatively high quantity of the ligand molecules is
required. This is a significant drawback when expensive or
tailor-made compounds are to be used, since the excess of the
new ligand is usually discarded during purification steps. For
this reason, here the second procedure was applied. The
synthesized ligands were dissolved in chloroform, which is also
a cosolvent for nanocrystals containing TOPO surface ligandss
and, importantly, also for free TOPO. An approximately 10-
fold excess of the carbodithioate or thiol ligand with respect to
the calculated molar quantity of surface TOPO molecules was
used.16 This homogeneous reaction leads to nearly quantitative
exchange with TOPO for the alkyl- or arylcarbodithioate ligands
at room temperature and for the corresponding thiols at 40 °C.
The functionalized nanocrystals are then purified by precipitation
and washing with MeOH.
Results and Discussion
Ligands containing the carbodithioic acid group and their salts
have been known for more than one century for their ability to
strongly complex metal ions. Their most important literature
examples are dithiocarbamates (RR′N-C(S)S-) and xanthates
(RO-C(S)S-).15 A common disadvantage of these molecules,
in view of a homogeneous exchange reaction with nanocrystals’
surface ligands, is the fact that they aresin their salt forms
only soluble in water and not in the same solvent as TOPO-
capped nanocrystals, used in this research. On the other hand,
they generally decompose in acidic conditions, making it
difficult to prepare their free acid form, which is usually soluble
in organic solvents. As a consequence, we were not able to
perform efficient ligand exchange with dithiocarbamates or
xanthates. We found, however, that in the case of alkyl- and
arylcarbodithioic acids, which are much less studied to date,
the lack of a heteroatom in the vicinity of the -C(S)SH group
The degree of surface ligand exchange can be controlled by
1H NMR spectroscopy if appropriate diagnostic peaks are
selected. TOPO-capped CdSe nanocrystals exhibit charac-
teristic peaks of alkyl chains with chemicals shifts in the
range of 0.5-1.5 ppm. In 4-methoxybenzenethiol-capped CdSe
(CdSe-Ar-SH), the methoxy group gives rise to a peak at
3.7 ppm, and finally 4-methyldithiobenzoate-capped CdSe
(CdSe-2) can be identified by its methyl protons at 2.3 ppm.
(16) Striolo, A.; Ward, J.; Prausnitz, J. M.; Parak, W. J.; Zanchet, D.; Gerion,
D.; Milliron, D.; Alivisatos, A. P. J. Phys. Chem. B 2002, 106, 5500-
5505.
(15) For reviews, see: (a) Coucouvanis, D. Prog. Inorg. Chem. 1970, 11, 233-
371. (b) Eisenberg, R. Prog. Inorg. Chem. 1971, 12, 295-369.
9
J. AM. CHEM. SOC. VOL. 126, NO. 37, 2004 11577

A R T I C L E S
Querner et al.
Figure 1. (a) 1H NMR spectra of 4-methoxybenzenethiol (top) and of TOPO-capped CdSe nanocrystals after surface ligand exchange with this compound
(bottom, reaction time 72 h at room temperature). (b) 1H NMR spectra of 4-methyldithiobenzoic acid (2) (top) and of TOPO-capped nanocrystals after
exchange with 2 (bottom) (reaction time 2 h at room temperature).
This separation of the diagnostic peaks facilitates quantitative
determination of the degree of ligand exchange.
presented in Figure 1. With the goal of easier line identification,
the spectra of the thiol and the carbodithioate-capped nano-
crystals are compared with those registered for the free ligands.
At room temperature, the exchange of TOPO by 4-methoxy-
benzenethiol (Ar-SH) is very inefficient, and even after 3 days
exchange time approximately 70% of TOPO, as determined by
the integration, still remains on the nanocrystal surface (Figure
1a). To the contrary, if 4-methyldithiobenzoic acid (2) is used
in the exchange reaction, within 2 h almost no TOPO remains
on the nanocrystal surface. Integration exhibits that the TOPO
content is less than 5% (Figure 1b), which is at the same time
the limit of reliable determination by this method. Moreover, it
has to be noted that the signal at 6.2 ppm, originating from the
proton in the carbodithioic acid group (C(S)SH), is absent in
the spectrum of CdSe-2, revealing that the ligand binds to the
nanocrystal surface in its deprotonated form as carbodithioate.
Finally we must notice that the ligand coordination on the
nanocrystal surface induces a significant downfield shift of the
lines corresponding to all aromatic protons (Ha, Hb, Hc). As
expected, this shift, caused by the deshielding effect, is the most
pronounced for Ha protons, which are in the closest vicinity of
the anchor groups and equal to 0.16 and 0.08 ppm for the thiol
and carbodithioate ligands, respectively. With increasing dis-
tance from the anchoring group this downfield shift decreases,
being the least pronounced for Hc protons. This behavior can
be taken as a spectroscopic manifestation of the ligand coor-
dination on the nanocrystal surface.
Such quantitative analysis is, more difficult in the case of
aliphatic ligands because chemical shifts ascribed to their protons
are very similar to those recorded for the protons of the TOPO
alkyl chains. In both cases, however, in the aliphatic part of
the spectrum, three relatively well separated groups of lines can
be distinguished: the up-field triplet (0.8 ppm) corresponding
to the methyl group, the low-field poorly resolved line (1.5 ppm)
corresponding to the methylene group adjacent to phosphorus
or sulfur, and a poorly resolved multiplet in the mid-field (1.2
ppm), corresponding to the remaining methylene groups. From
the ratio of the integrated intensities of the mid-field peak and
the up-field peak, the degree of ligand exchange can be
estimated because this ratio depends on the mutual TOPO and
Alk-SH content. This method was used for the calculation of
the exchange degree reported in the Experimental Section (vide
supra).
1
It is clear from H NMR data that the use of the newly
developed carbodithioate chelating ligands results in a faster
and more complete ligand exchange as compared to the corre-
sponding thiol ligands, at least in the mild reaction conditions
applied in this research. This indicates a much higher affinity
of the anchoring carbodithioate group toward the surface of
CdSe nanocrystals. An instructive example of this significant
difference in the complexing ability between the carbodithioate
and thiol ligands can be provided by analyzing the NMR spectra
9
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Ligands for Surface Functionalization of Nanocrystals
A R T I C L E S
The NMR lines recorded for the nanocrystal surface ligands
are relatively narrow, especially in view of the fact that the
ligands’ coordination should result in their restricted motion and,
as a consequence, in a NMR line broadening. It has been
however demonstrated that the coordination of thiophenol
ligands on the nanocrystal surface does not result in a significant
line broadening because of the possibility of a rotation around
the Cd-S bond.17 This motion is less hindered as the nanocrystal
size increases due to a lower coverage of ligands on the surface
resulting in a decreasing steric hindrance. Similar or even more
pronounced effects are expected for 4-methoxybenzenethiol
ligands used in our study, taking into account their chemical
similarity to the thiophenol ligand and the relatively large size
of the nanocrystals used (diameter 4.2 nm). In the case of
carbodithioate ligands, the rotation about the Cd-S bond is
hindered because of the chelate-type bonding of these molecules
to the nanocrystal surface. Though broader than in the case of
the thiol-capped nanocrystals, the observed NMR lines are still
relatively narrow, and we are assuming that the carbodithioate
ligand molecules are able to rotate around the C-C bond
between the anchor group and the phenyl ring.
Figure 2. PL spectra of CdSe nanocrystals before and after exchange of
the surface TOPO ligands with dodecanethiol (CdSe-Alk-SH) and with
tridecanedithioate (CdSe-1), excited at 400 nm.
Aryl carbodithioate ligands, containing an aromatic ring adjacent
to the anchor group, efficiently quench the photoluminescence
of nanocrystals. This phenomenon is discussed in detail in the
last section of the paper, devoted to CdSe-aniline tetramer
organic/inorganic hybrids.
In the case of carbodithioate ligands containing aryl subsitu-
ents, UV-vis spectroscopy is an additional tool to monitor the
complexation process since the chelating effect induces spectral
changes in the absorption spectrum of the ligand. In particular,
it is known that the bidentate coordination of carbodithioates
to metal ions (formation of four-membered-ring chelates) results
in a bathochromic shift of the band ascribed to the π-π*
transition in the aromatic ring as compared to the same band in
the corresponding carbodithioic acid.18 The expected, complex-
ation-induced, bathochromic shift is also observed for ligand
2. Its free acid form gives rise to an absorption band with a
maximum at 304 nm. Upon coordination to Cd2+ ions, the band
shifts to 317 nm. An even more pronounced bathochromic shift
is observed after the exchange of TOPO on the nanocrystal
surface with ligand 2, since for CdSe-2 the π-π* transition
band is located at 321 nm. This can be considered as an
indication of a strong coordination of the ligand in its bi-
dentate chelating form, similar to that observed in mono-
nuclear metal complexes. It should be noted here that, for
CdSe nanocrystals capped with carbodithioate ligands containing
alkyl or aryl substituents, the ligand proper bands and the
nanocrystal excitonic bands are spectrally well separated and
do not interfere.
We have also investigated the effect of the ligand exchange
process on the photoluminescence (PL) properties of the nano-
crystals. For comparative reasons, in Figure 2 the PL spectrum
of CdSe capped with the initial surface ligands (CdSe-TOPO)
is shown together with the spectra of the same nanocrystals after
the exchange with tridecanedithioate and dodecanethiol ligands,
termed CdSe-1 and CdSe-Alk-SH, respectively. Upon ligand
exchange, we first notice a decrease of the PL, which is slightly
more pronounced for CdSe-Alk-SH (90%) than for CdSe-1
(85%). In the latter case, this is accompanied by a small
bathochromic shift of the PL peak maximum from 577 to 583
nm, while, in the former case, a hypsochromic shift by 6 nm is
observed. For both systems studied, the full width at half-
maximum (fwhm) of the PL peak remains the same (ca. 35 nm).
Photostability. Strong coordination of the newly developed
ligands on the surface of CdSe nanocrystals may increase
considerably their photochemical stability, since appropriate
surface passivation should prevent photooxidation or at least
significantly slow this process. Poor resistance against photo-
oxidation is frequently one of the weakest points of nanocrystal-
based materials, which severely limits their technological
applications. Therefore, any improvement in the photochemical
stability of CdSe nanocrystals is not only of fundamental but
also of practical interest. For this reason we have undertaken
the task of comparative investigations of tridecanedithioate-
capped CdSe nanocrystals (CdSe-1) and dodecanethiol-capped
ones (CdSe-Alk-SH). We have chosen this set of ligands
because thiols containing long alkyl chains have been reported
to be the most resistant systems among other thiols in a
photooxidation study carried out in water.10 As the two ligands
differ only in the chemical nature of the anchor group, its
influence on the photochemical stability of nanocrystals can be
selectively investigated. Figure 3 shows the results of photo-
degradation experiments carried out under continuous mono-
chromatic irradiation (λ ) 365 nm). For λ ) 254 nm similar
behavior is observed, although, as expected, the photodegra-
dation is accelerated by the irradiation at higher energy. For
comparative reasons the spectra of nanocrystals capped with
initial surface ligand (TOPO) are shown in Figure 3a. These
nanocrystals are the least UV stable and start precipitating after
9 h of constant irradiation. Simultaneously, an increasing blue
shift of the excitonic peak is observed with increasing irradiation
time. As can be clearly seen in Figure 3b, for short exposure
times (up to 16 h), the thiol-capped nanocrystals show no signs
of photodegradation, such as precipitation or blue shift of the
excitonic peak. They then start to oxidize, and after ca. 30 h
they are totally degraded, which results in their quantitative
precipitation. The carbodithioate-capped nanocrystals behave
differently (Figure 3c). In this case, a small blue shift of the
excitonic peak is observed for irradiation times shorter than 3
h, and then its position stabilizes. The nanocrystals do not
precipitate even after 67 h of constant exposure to the UV
irradiation. Moreover, their spectral features remain essentially
(17) Sachleben, J. R.; Colvin, V.; Emsley, L.; Wooten, E. W.; Alivisatos, A. P.
J. Phys. Chem. B 1998, 102, 10117-10128.
(18) Furlani, C.; Luciani, M. L. Inorg. Chem. 1968, 7, 1586-1592.
9
J. AM. CHEM. SOC. VOL. 126, NO. 37, 2004 11579

A R T I C L E S
Querner et al.
Figure 3. Photostability investigations by UV-vis spectroscopy of (a) TOPO-capped CdSe nanocrystals, (b) after exchange of TOPO with dodecanethiol
(CdSe-Alk-SH), and (c) after exchange with tridecanecarbodithioic acid (CdSe-1). The spectra are taken after the indicated periods of continuous UV
irradiation (365 nm); they are vertically shifted for clarity.
unchanged apart from some broadening of the first excitonic
peak observed for exposure times exceeding 40 h. The small
blue shift of the excitonic peak observed for short exposure times
could in principle be considered as a spectroscopic manifestation
of the onset of nanocrystals’ oxidation. However, taking into
account the quick stabilization of the nanocrystals’ optical
parameters with further irradiation, we are tempted to explain
the observed phenomenon by changes in the ligand-nanocrystal
interactions rather than as a result of photooxidation. Such
interactions can, for example, be modified by irradiation-induced
structural rearrangement. Furthermore, it should be noted here
that the position of the excitonic peak of CdSe-1 is red-shifted
by 17 nm with respect to that of CdSe-TOPO. The registered
irradiation-induced blue shift reverses this behavior by only 7
Figure 4. 1H NMR spectra of (a) the bifunctional ligand 5′, (b) CdSe
nanocrystals after capping with ligand 5′ and grafting of aniline tetramer
(6), resulting in CdSe-8, and (c) aniline tetramer (emeraldine base
nm, indicating that it does not originate from the photooxidation
of the nanocrystal surface, which should result in a more
pronounced and continuous evolution.
form) (6).
Organic/Inorganic Hybrids via Grafting of Aniline Tet-
ramer on Nanocrystals Capped with an Aldehyde-Func-
tionalized Carbodithioate Ligand. The facility of quantitative
exchange with TOPO and their good anchoring properties make
carbodithioic acids excellent candidates for the preparation of
special bifunctional ligands, which could be used for the grafting
of organic molecules of interest on semiconductor nanocrystals.
The elaboration of such hybrid systems consisting of conjugated
oligomers or polymers and nanocrystals is especially interesting
in view of the recent developments in organic and molecular
electronics.19 A conjugated oligomer (polymer)/nanocrystal
hybrid can be considered as a molecular junction of two semi-
conductors of different electron-donating and electron-accepting
properties orsif the conjugated oligomer (polymer) is dopeds
as a junction between a semiconductor and a metal. Such a
molecular interface facilitates exciton dissociation and separation
in organic solar cells, which as a consequence improves the
device efficiency.20 In this research we demonstrate that aniline
tetramer can be grafted on CdSe nanocrystals using a car-
bodithioate-based bifunctional ligand. We have chosen aniline
tetramer because it can be considered as a good model com-
pound for polyaniline. Polyaniline and its oligomers show in-
teresting nonlinear electrical transport properties in their undoped
(semiconducting) state21 and high electrical conductivity after
doping.22 These properties, combined with good stability, make
the polyaniline family of organic conductors very important for
future organic electronics development.
To facilitate possible charge transfer between the nanocrystal
and the conjugated oligomer, we designed a bifunctional ligand
with a conjugated spacer, 4-formyldithiobenzoic acid (5). The
aldehyde group allows for the direct grafting of aniline tetramer
via a condensation reaction with the amine group of the tetramer,
during which an azomethine linkage is formed (see Scheme 1).
During the ligand synthesis, the aldehyde group requires
appropriate protection when introducing the carbodithioic acid
anchoring group by Grignard reaction. Furthermore, deprotection
in acidic conditions would be advantageous. Taking into account
these restrictions, acetale formation by means of a diol turned
out to be the most efficient protection method for the synthesis
of 5. After transformation into its salt form 5′, the bifunctional
ligand rapidly (within 1 h) and quasi-quantitatively (>95%)
exchanges the initial TOPO ligands on the surface of CdSe
nanocrystals. It should be emphasized that the use of ligand 5′
does not only permit the conjugation of aniline tetramer; in
principle, any kind of primary amine can be grafted on the
nanocrystal surface by this means.
The completeness of the grafting reaction is fully confirmed
1
by the analysis of the H NMR spectra of both reagents and
the product (Figure 4). First, we notice that the peak at 10.0
ppm, characteristic of the aldehyde group in potassium
(19) (a) Katz, H. E.; Bao, Z.; Gilat, S. L. Acc. Chem. Res. 2001, 34, 359-369.
(b) Dimitrakopoulos, C. D.; Malenfant, P. R. L. AdV. Mater. 2002, 14,
99-117.
(20) Brabec, C. J.; Sariciftci, N. S.; Hummelen, J. C. AdV. Funct. Mater. 2001,
11, 15-26.
(22) (a) Cao, Y.; Smith, P.; Heeger, A. J. Synth. Met. 1992, 48, 91-97. (b)
Adams, P. N.; Devasagayam, P.; Pomfret, S. J.; Abell, L.; Monkman, A.
J. Phys. Condens. Matter. 1998, 10, 8293-8303.
(21) Kieffel, Y.; Travers, J.-P.; Ermolieff, A.; Rouchon, D. J. Appl. Polym. Sci.
2002, 86, 395-404.
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Ligands for Surface Functionalization of Nanocrystals
A R T I C L E S
Figure 5. FT-IR spectra of aniline tetramer (6) and of CdSe nanocrystals after grafting of 6, resulting in CdSe-8.
4-formyldithiobenzoate (5′), completely disappears. The same
applies to the line at 5.5 ppm ascribed to the protons of the
terminal amine group of “free”, nongrafted, aniline tetramer.
The spectrum of the condensation reaction product consists of
two sets of broad, poorly resolved lines. This broadening is a
simple consequence of the restricted rotation of the grafted
organic molecules on the nanocrystal surface.11a The set of lines
at ca. 8 ppm comprises the contribution from the aromatic
protons of the phenyl group in ligand 5′ and the contribution
from the amine protons of the aniline tetramer, as well as the
contribution from the proton of the azomethine group. The
second set of broad peaks, between 6.6 and 7.2 ppm, corre-
sponds to the aromatic protons of the aniline tetramer.
its UV-vis spectrum with the emphasis on grafting-induced
spectral changes. The UV-vis spectrum of the “free” tetramer
6, recorded in DMSO (see Figure 6), consists of two bands at
306 and 591 nm, which are ascribed to the π-π* transition in
the benzene ring and to an excitonic-type transition between
the HOMO level of the quinone ring and the LUMO level of
the benzene ring, respectively.25 The presence of the latter band
confirms that the tetramer is in the oxidation state of emeraldine
base. The positions of both bands are very sensitive to the extent
of conjugation, being bathochromically shifted for oligomers
with increasing chain length.26 If we compare this spectrum with
the one recorded for CdSe-8, we first notice that it is fully
dominated by the absorption bands originating from the tetramer
part. This is not unexpected, taking into consideration the rather
high value of the molar absorption coefficient of the oligomer.
Second, both absorption bands are bathochromically shifted by
19 and 27 nm, respectively. The band originating from the
excitonic transition decreases in intensity with respect to the
π-π* transition band and slightly broadens. These changes
manifest a decreasing contribution of the quinoid structure and
an increasing conjugation length in the grafted tetramer, which
now extends to the phenyl ring, originating from ligand 5′, via
the azomethine link (see Scheme 1). To prove this hypothesis,
we have synthesized model compound 7 by a condensation
reaction between aniline tetramer and p-bromobenzaldehyde.
Apart from the carbodithioate group, this molecule is identical
to the organic moiety grafted on the nanocrystal surface in
CdSe-8 (see Scheme 1). Its spectrum is very similar to that
recorded for CdSe-8, which unequivocally indicates the ex-
tended conjugation (Figure 6). As already discussed above, the
carbodithioate ligand has an absorption band in the spectral
region of the band characteristic of the π-π* transition in the
grafted tetramer. This band, being bathochromically shifted upon
the grafting reaction to CdSe nanocrystals, strongly overlaps
with the π-π* transition band and causes an apparent increase
of the π-π* transition band intensity in CdSe-8 as compared
to 7.
Additional insight into the grafting reaction can be extracted
from the IR spectra of the “free” aniline tetramer (6) and the
nanocrystal-tetramer hybrid, CdSe-8 (Figure 5). Consistent
with the 1H NMR data, the band at 1697 cm-1 in the spectrum
of the hybrid, which originates from the CdO stretchings in
the aldehyde group, is very weak, indicating that aniline tetramer
is grafted on almost all aldehyde functions on the surface, i.e.,
that the grafting process is essentially quantitative. Additionally,
a band at 1012 cm-1, ascribed to the aromatic carbodithioate
group,23 appears upon grafting. All bands attributed to phe-
nylene/phenyl rings vibrations24 increase in intensity upon
grafting. This is partially a consequence of the fact that the ratio
of benzoid to quinoid rings increases in the nanocrystal-
tetramer hybrid as compared to the “free” tetramer, due to the
phenyl ring in ligand 5′. Consistent with the grafting via the
primary amine group, its characteristic band at 1380 cm-1
,
present in the “free” tetramer spectrum, is absent in the spectrum
of the hybrid. The band characteristic of the CdN stretchings
in the azomethine group, usually located in the vicinity of
1600-1620 cm-1, is obscured by the strong band at 1597 cm-1
originating from the CdC stretchings in the quinoid ring,23 and
is present as a shoulder. Note also small grafting-induced shifts
of bands attributed to phenylene/phenyl C-H out-of-plane
deformations in the 840-690 cm-1 spectral region.
Finally, we have carried out PL measurements in order to
identify possible charge or excitation transfer between the
nanocrystal and the electroactive capping molecules. Significant
Since CdSe-8 can be considered as an adduct of two
chromophores of different nature, it is worthwhile to analyze
(23) (a) Gotthardt, H.; Pflaumbaum, W.; Gutowski, P. Chem. Ber. 1988, 121,
313-322. (b) Gade, T.; Streek, M.; Voss, J. Chem. Ber. 1988, 121, 2245-
(25) Zheng, W.; Angelopulos, M.; Epstein, A. J.; MacDiarmid, A. J. Macro-
molecules 1997, 30, 7634-7637.
(26) Boyer, M.; Quillard, S.; Cochet, M.; Lourn, G.; Lefrant, S. Electrochim.
Acta 1999, 44, 1981-1987.
2249.
(24) Boyer, M. I.; Quillard, S.; Louarn, G.; Lefrant, S. Synth. Met. 1999, 101,
782-783.
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A R T I C L E S
Querner et al.
Figure 8. PL spectra of TOPO-capped CdSe nanocrystals, of model
compound 7, as well as of 4-formyldithiobenzoate-capped nanocrystals
before (CdSe-5′) and after (CdSe-8) grafting of aniline tetramer (excitation
wavelength 365 nm). The TOPO-capped CdSe nanocrystals’ spectrum has
been downscaled in order to fit to the same graph.
Figure 6. UV-vis absorption spectra of aniline tetramer (6), of CdSe
nanocrystals after grafting of aniline tetramer (CdSe-8), and of the model
compound 7.
conjugated model compound 7, which, apart from its terminal
group, can be considered as an adduct of the ligand 5′ and the
aniline tetramer 6, gives rise to a large emission band between
350 and 600 nm. This is essentially the same as the PL of the
aniline tetramer, which can be expected due to the structural
similarity between the two molecules. Finally, CdSe-8 shows
a nearly complete extinction of the PL of both the nanocrystal
and organic moieties. The arguments based on the relative
positions of the electronic energy levels apply as above:
excitation of the nanocrystal can lead to the transfer of holes
toward the oligomeric ligand, while excitation of the latter can
result in the transfer of electrons onto the nanocrystal. Further
experiments, namely PL excitation spectroscopy, are underway
in order to identify the location of the absorbing levels in the
newly synthesized hybrid compound.
Figure 7. Electronic energy level alignment for bulk CdSe (dashed lines),
4.2-nm-diameter CdSe nanocrystals, and aniline tetramer (full lines).27 The
energy shift due to the quantum confinement in the nanocrystals is shared
between the electron (conduction band) and hole (valence band) states with
a 3:1 ratio, taking the electron affinity of bulk CdSe as 4.5 eV.29
changes in the fluorescence properties of the newly synthesized
hybrid are expected because of the relative positions of the
nanocrystal and ligand electronic levels, depicted in Figure 7.
This scheme is constructed assuming the electron affinity of
the nanocrystals to be 4.2 eV (diameter 4.2 nm, gap 2.2 eV),27
and the HOMO level of aniline tetramer in the oxidation state
of emeraldine base to be 4.7 eV.28 Once an electron-hole pair
is excited in the system, the electron should relax to the lower
energy state located on the nanocrystal, while the hole should
relax toward states located on the aniline tetramer. The hybrid
compound should thus provide a very efficient separation of
charges, resulting in PL quenching.
The hypothesis outlined above is corroborated by the PL
spectra presented in Figure 8. The spectrum of TOPO-capped
CdSe nanocrystals is characterized by a narrow emission peak
centered at 575 nm. The ligand exchange leading to CdSe-5′
results in a total extinction of this PL. We explain that by a
charge transfer from the nanocrystal to the ligand aromatic ring,
which inhibits the radiative recombination of the excitons
photocreated on the nanocrystal. On the other hand, the
Conclusion
To summarize, we have demonstrated that carbodithioic acids
and their salts, never previously used for the functionalization
of CdSe nanocrystals, provide a facile and quasi-quantitative
exchange with original nanocrystal surface ligands (TOPO).
Moreover, because of the formation of strong chelate-type
bonding with the metal atoms at the nanocrystal surface, carbo-
dithioates significantly improve nanocrystals’ stability against
photooxidation. Finally, a simple synthesis method allows for
the integration of functional groups, offers high flexibility in
the design of new ligands, and facilitates the grafting of other
molecules and macromolecules, such as conjugated oligomers
or polymers. We strongly believe that the use of carbodithioic
acids and their derivatives is by no way restricted to the
functionalization of CdSe nanocrystals but can be extended to
other types of semiconductor and metal nanocrystals as well as
to core/shell systems. The synthesis of a new carbodithioate
ligand permitting the hydro-solubilization of highly luminescent
core/shell nanocrystals is underway.
Acknowledgment. The authors thank Dr. B. Dufour and Dr.
P. Rannou for providing a sample of aniline tetramer, as well
as Dr. N. Charvet for helpful discussions.
(27) Greenham, N. C.; Peng, X.; Alivisatos, A. P. Phys. ReV. B 1996, 54, 17628-
17637.
(28) Libert, J.; Cornil, J.; dos Santos, D. A.; Bre´das, J. L. Phys. ReV. B 1997,
56, 8638-8650.
(29) Chiang, T. C.; Himpsel, F. J. Electronic Structure of Solids; Landolt-
Bo¨rnstein, New Series, Group III, Vol. 23, Pt. 2; Springer: Berlin, 1989.
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