Fullerodendrimers with a tris-isothiocyanate core allowing their anchoring onto gold electrodes

Article abstract of DOI:10.1039/b706319a

Dendrimers with peripheral fullerene subunits and a tris-isothiocyanate core have been prepared and self-assembled onto a gold surface; detailed electrochemical studies revealed that electron transfer from the electrode to the fullerene subunits occurs th

Full text of DOI:10.1039/b706319a

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LETTER
www.rsc.org/njc | New Journal of Chemistry
Fullerodendrimers with a tris-isothiocyanate core allowing their
anchoring onto gold electrodesw
Jose Antonio Camerano,a Miguel Angel Casado,*a Uwe Hahn,b
Jean-Franc¸
ois Nierengarten,*b Emmanuel Maisonhaute*c and Christian Amatore*c
Received (in Montpellier, France) 25th April 2007, Accepted 30th May 2007
First published as an Advance Article on the web 12th June 2007
DOI: 10.1039/b706319a
tive, however cleavage of the benzyl unit failed to give targeted
Dendrimers with peripheral fullerene subunits and a tris-isothio-
cyanate core have been prepared and self-assembled onto a gold
surface; detailed electrochemical studies revealed that electron
transfer from the electrode to the fullerene subunits occurs
through space at a short distance from the electrode.
alcohol 4. Thus, the protecting group in 2 was exchanged for a
tert-butyldimethylsilyl (TBDMS) ether. Cleavage of the benzyl
group was achieved by treatment of 2 with Pd/C in THF under
H2 atmosphere (1 bar). Reaction of the resulting alcohol with
TBDMSCl in the presence of imidazole gave 3 in 99% yield.
Treatment of 3 with an excess of KSCN in DMF at 140 1C
afforded directly compound 4, the TBDMS being cleaved
during the work-up procedure. Dendrimers G1–3 were then
obtained by reaction of 4 with the corresponding fullerene-
containing dendrons bearing a carboxylic acid group at the
focal point (G1–3CO2H)10 under esterification conditions
using N,N0-dicyclohexylcarbodiimide (DCC), 1-hydroxybenzo-
triazole (HOBt) and 4-dimethylaminopyridine (DMAP).
We let gold ball ultramicroelectrodes,11 obtained by melting
a 25 mm gold wire in a blue flame, bathe during 72 h in THF
solutions of compounds G1–3. After a quick rinse with neat
THF, these electrodes were transferred into the electrochemi-
cal cell where no electroactive compound was present. All
compounds give several reduction waves, but only the first one
is chemically reversible, as was previously observed for similar
products.6 We focus below on the first reduction wave of the
C60 entities. Cyclic voltammograms are presented in Fig. 1.
We observed that the current scales proportionally with the
scan rate v (with a maximum error of 10%), i.e. is typical for
species anchored onto electrodes.12 It is difficult to measure
precisely the dendrimer coverage since the area of the gold ball
electrode can only be roughly measured. However, considering
that the gold ball is a hemisphere with a 25 mm radius, and that
the molecular area of G2 is 310 A2,13 the charge integration of
the faradaic peak reveals that there is less than a monolayer
adsorbed onto the electrode. However, the data acquisition
needs to be performed within a short time (15 min) to avoid
desorption of the compound. Indeed, since these compounds
are soluble in THF, their desorption is slow but thermodyna-
mically favored. Moreover, a fast decrease of the signal is also
observed if slow scan rates (less than 30 V sꢀ1) are used,
attesting that the negatively charged radicals desorb faster
than their neutral parent. In that case, cyclic voltammetry is a
powerful technique since it allows one to position the starting
potential at a potential where the monolayer is kinetically
stable, thus minimizing the dynamic instability. This is an
advantage compared to other techniques where the potential
has to remain in the faradaic region during a longer time.14
In order to minimize the impact of the slight variation in
charge during the experiment, we decided to extract the
In recent years, fullerene-rich dendrimers have generated
significant research activities.1 In particular, the peculiar elec-
tronic properties of fullerene derivatives2 make such dendri-
mers attractive candidates for a variety of interesting features
in materials science.3 As part of this research, we have recently
shown that dendrimers bearing multiple C60 units are inter-
esting candidates for solar energy conversion.4 This prompted
us to study in detail their electrochemical properties as the
accepting ability of the fullerene subunits as well as the charge
mobility in thin films are key parameters in photovoltaic
devices.5 In this respect, ultrafast cyclic voltammetry has
proven to be a powerful tool to understand intra- and inter-
molecular interactions between C60 units within thin films of
fullerene-rich dendritic materials deposited onto the electrodes
during their electrochemical analysis.6 To gain further insight
in the electrochemical behavior of such molecules, we have
decided to develop dendrimers with peripheral fullerene sub-
units and a head-group allowing the preparation of self-
assembled monolayers onto gold surfaces. With this idea in
mind, we have designed compounds G1–3 with a tris-isothio-
cyanate core. Indeed, Tour revealed that isothiocyanate deri-
vatives may chemically adsorb onto gold substrates7 in an
analogous manner with what is very commonly observed with
thiols.8
The synthesis of G1–3 is depicted in Scheme 1. Reaction of
compound 19 with tosyl chloride (TsCl) in the presence of
pyridine gave 2 in 80% yield. Subsequent treatment with
KSCN afforded the corresponding tris-isothiocyanate deriva-
a
´
Dpto. de Quımica Inorganica-Instituto de Ciencia de Materiales de
Aragon, Universidad de Zaragoza-C.S.I.C., 50009 Zaragoza, Spain.
´
´
E-mail: mcasado@posta.unizar.es; Fax: +34-976-761187
b
`
`
´
Groupe de Chimie des Fullerenes et des Systemes Conjugues,
Laboratoire de Chimie de Coordination du CNRS (UPR 8241), 205
route de Narbonne, 31077 Toulouse Cedex 4, France. E-mail:
jfnierengarten@lcc-toulouse.fr; Fax: +33 (0) 561 55 30 03
c Departement de Chimie, UMR CNRS 8640 ‘‘PASTEUR’’, Ecole
´
Normale Superieure, Universite Pierre et Marie Curie-Paris, 624 rue
Lhomond, 75231 Paris Cedex 05, France. E-mail:
´
christian.amatore@ens.fr; Fax: +33 (0) 144 32 38 63
w Electronic supplementary information (ESI) available: Experimental
details. See DOI: 10.1039/b706319a
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electron transfer rate constant from the peak potential evolu-
tion with the scan rate since this quantity is independent of the
precise number of molecules adsorbed (as long as the electro-
active entities from different adsorbed molecules do not inter-
act as is the case in this study).12 Moreover, the peak potential
is only very slightly sensitive to the rather flat capacitive
background underlying below the faradaic current. At slow
scan rates, there is a very slight (about 10 mV) peak to peak
potential difference which we ascribe to a structural reorgani-
sation of the monolayer after electron transfer (this phenom-
enon is commonly observed). When the scan rate increases,
there is only a very slight evolution of the anodic and cathodic
peak potentials between 40 and 3200 V sꢀ1. From comparison
with theory, we could extract a rate constant k0ads = 1 ꢂ
105 sꢀ1 for G2 and G3, and ka0ds = 2 ꢂ 105 sꢀ1 for G1. It is well
known that the electron transfer rate through molecular
bridges decreases exponentially with the distance to the elec-
trode. For single C–C bonds, the attenuation factor b is
estimated at about 1.2 per methylene unit. It has also been
observed that an ester linkage behaves essentially as two
methylene units.15 We have no experimental data about
electron transfer through the chains constituting the dendritic
branches of our compounds. However, we can notice that
there are 16 non-conjugated (C–C or C–O) bonds for G1, 22
for G2 and 28 for G3. Therefore, the electron transfer effi-
ciency should decrease by several orders of magnitude within
our series if it occurred along the dendritic branches. More-
over, a comparison with the literature indicates that for G1 a
rate constant in the range of 1–100 sꢀ1 should be obtained for
electron transfer through the chain linkers.15 Our experimental
values are several orders of magnitude above this estimate,
which evidences that when the charge transfer occurs the C60
entities are close enough to the electrode surface to give rise to
direct efficient electron transfer through the solvent. There-
fore, the dendritic branches necessarily have the possibility of
folding to reach the electrode. A similar situation was pre-
viously examined experimentally and theoretically by Moiroux
and co-workers16 in the case of ferrocene moieties attached by
polyethyleneglycol linkers. Two limiting situations are possi-
ble. If the flexibility of the chains is large, or if the electroactive
entities lie very close to the electrode, adsorption behaviour is
observed. Conversely, if the chains are stiff and the electro-
active centers are far enough from the electrode, their diffusive
movement should limit the current and a voltammogram
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< x >< =x >
for 5 may occur at varying distances from the electrode, so
that in order to compare both quantities we should compare
k0ads with:18
R
k0diff
/
Nexp(ꢀbSx)dx = bSkd0iff
(1)
0
where bS is the attenuation factor for the solvent. We are not
aware of a calculated or experimental value for bS. However,
taking 1 o bS o 2 Aꢀ1 seems reasonable. This leads to 1.4 ꢂ
107 sꢀ1 o bSk0diff o 2.8 ꢂ 107 sꢀ1. ka0ds is then two orders of
magnitude smaller than what could be expected if electron
transfer occurred at the same distance from the electrode.
Taking into account that xdiff, the closest distance for 5, and
xads, the distance for G1, G2 or G3, may be different, eqn (1)
should be corrected to yield:17
Scheme 1 Reagents and conditions: (a) TsCl, pyridine, 0 1C to room
temp., 24 h (80%); (b) H2, Pd/C, THF, room temp., 12 h (78%); (c)
TBDMSCl, imidazole, CH2Cl2, 0 1C to room temp., 24 h (80%); (d)
KSCN, DMF, 140 1C, 6 h (54%); (e) GnCO2H, DCC, DMAP, HOBt,
CH2Cl2, 0 1C to room temp. (G1: 64%; G2: 58%; G3: 85%).
kd0iff = ka0ds ꢂ exp(bS(xads – xdiff))/bS
(2)
having the characteristics of diffusion is observed. We pre-
sently do not have structural data about the formed mono-
layers, such as the effective thickness and the position of the
C60 entities. Since the current peak is proportional to v within
10% in the whole scan range of this study, we deduce that the
eventual diffusional movement of the C60 moieties is transpar-
ent in the registered data.
We thus obtain 2.8 o xads ꢀ xdiff o 5 A. This analysis then
suggests that the local organisation of the film allows the redox
centers to approach the electrode, but not to touch it. Never-
theless, we want to stress that this analysis remains subordi-
nate to other factors such as double layer effects or a different
local dielectric constant which may induce slight variation in
the reorganisation energy of C60 or in the electronic coupling
with the electrode. Indeed, eqn (2) is valid only provided all
parameters are the same for both homogeneous and linked
systems except for the electronic coupling.
Following this assumption, we compared our experimental
value (k0ads = 1 ꢂ 105 sꢀ1) to the one obtained for fullerene
derivative 5. In this case, 5 is well soluble in THF and there is
no anchoring group so that the signal is purely diffusive. From
the peak current intensity of the voltammogram (Fig. 2a), we
could extract an approximate diffusion coefficient of 1.6 ꢂ
In conclusion, we have developed fullerene-substituted den-
drimers with a tris-isothiocyanate core moiety enabling us to
self-assemble them onto gold electrodes. Detailed cyclic vol-
tammetry studies revealed an efficient through space electron
transfer from the electrode to the fullerene moieties revealing
that these entities are or can get very close to the electrode.
10ꢀ6 cm2 sꢀ1
.
The peak potential evolution gives a heterogeneous rate
constant of k0diff = 0.14 cm sꢀ1, in qualitative agreement with
previous measurements performed by Bard and co-workers17
onto pristine C60 under different conditions. Electron transfer
Fig. 1 Cyclic voltammograms of G2 adsorbed onto a gold ball ultramicroelectrode at 60 V sꢀ1 (a) or 2400 V sꢀ1 (b). (c) Peak potential evolution
with the scan rate. Filled circles: experimental values. Solid line: theoretical evolution for k0ads = 1 ꢂ 105 sꢀ1
.
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Fig. 2 Cyclic voltammograms of a 4.8 mM solution of 5 at 0.1 V sꢀ1 onto a 0.5 mm diameter electrode (a) or at 3200 V sꢀ1 onto a 25 mm diameter
electrode (b). (c) Peak potential evolution with the scan rate. Filled circles: experimental values. Solid line: theoretical evolution for kd0iff
=
0.14 cm sꢀ1
.
This work was supported by the CNRS (UPR 8241 and
UMR 8640), the Ecole Normale Superieure, the Universite
Compound G2. Prepared from G2CO2H and purified by
column chromatography on silica gel (SiO2, CH2Cl2 to CH2Cl2
+ 0.2% MeOH) followed by gel permeation chromato-
graphy (Biorad, Biobeads SX-1, CH2Cl2) to yield G2
(184 mg, 58%) as a dark orange glassy product. IR: n/cmꢀ1
´
´
Pierre et Marie Curie, fellowships from the Deutscher Akade-
mischer Austausch Dienst (DAAD) to U.H. and from the
Minesterio de Educacion y Ciencia (Project CTQ2005-06807/
´
BQU) to J.A.C.
= 1747 (CQO); UV/Vis (CH2Cl2): l/nm (e/dm3 molꢀ1 cmꢀ1
)
1
= 260 (305 990), 315 (93 400), 378 (32 475), 438 (10 100); H
NMR (300 MHz, CDCl3): d = 0.88 (t, J = 7 Hz, 24 H),
1.23–1.45 (m, 80 H), 1.71 (m, 16 H), 3.18 (s, 6 H), 3.56 (s, 2H),
3.69 (m, 2H), 3.82 (t, J = 7 Hz, 16 H), 4.38 (m, 2 H), 4.68 (s, 2
H), 4.70 (s, 4 H), 5.03 (d, J = 13 Hz, 4 H), 5.22–5.30 (m, 12 H),
5.71 (d, J = 13 Hz, 4 H), 6.32 (s, 4 H), 6.45 (s, 8 H), 6.78 (s, 4
H), 6.89 (m, 2 H), 6.98 (s, 1 H), 7.12 (s, 2 H); 13C NMR (75
MHz, CDCl3): d = 14.3, 22.8, 26.2, 29.4, 29.6, 32.0, 36.9, 46.1,
49.2, 64.0, 65.4, 65.6, 66.5, 67.0, 67.2, 68.2, 68.9, 69.3, 70.1,
70.7, 101.7, 107.2, 111.7, 112.7, 114.7, 116.2, 121.4, 134.5,
135.9, 136.2, 136.6, 137.5, 137.9, 138.6, 140.1, 141.2, 141.3,
142.3, 142.8, 143.3, 143.7, 143.9, 144.1, 144.3, 144.4, 144.7,
145.0, 145.1, 145.3, 145.4, 145.7, 145.8, 146.2, 147.4, 147.6,
148.7, 157.9, 158.4, 160.5, 162.6, 162.7, 168.5, 168.6; MALDI-
TOF-MS: m/z = 4050.86 [M + H]+; Anal. calc. for
Experimental
General procedure for the preparation of G1–3
DCC (1 equiv.) was added to a stirred solution of 4
(1.1 equiv.), the appropriate fullerene derivative (GnCO2H,
1 equiv.), and DMAP (0.4 equiv.) in CH2Cl2 at 0 1C. After 1 h,
a catalytic amount of HOBt was added, the mixture allowed to
slowly warm to room temperature and stirred at this tempera-
ture for several days (G1: 1 d, G2: 2 d, and G3: 4 d). The
resulting solid was filtered off, the solvent evaporated and the
crude product purified as outlined in the following text.
C
264H195O34N3S3 (4049.63): C 78.30, H 4.85, N 1.04; found:
Compound G1. Prepared from G1CO2H and purified by
column chromatography on silica gel (SiO2, CH2Cl2 to CH2Cl2
+ 0.2% MeOH) followed by gel permeation chromato-
graphy (Biorad, Biobeads SX-1, CH2Cl2) to yield G1
(290 mg, 64%) as a dark orange glassy product. IR: n/cmꢀ1
C 78.51, H 4.86, N 0.86%.
Compound G3. Prepared from G1CO2H and purified by
column chromatography on silica gel (SiO2, CH2Cl2 to
CH2Cl2 + 0.2% MeOH) followed by gel permeation chroma-
tography (Biorad, Biobeads SX-1, CH2Cl2) to yield G3
(185 mg, 85%) as a dark orange glassy product. IR: n/cmꢀ1
= 1746 (CQO); UV/Vis (CH2Cl2): l/nm (e/dm3 molꢀ1 cmꢀ1
)
= 260 (174 100), 315 (54 250), 378 (19 920), 438 (4790); 1H
NMR (300 MHz, CDCl3): d = 0.89 (t, J = 7 Hz, 12 H),
1.20–1.43 (m, 40 H), 1.71 (m, 8 H), 3.20 (s, 6 H), 3.58 (s, 2 H),
3.76 (m, 2 H), 3.84 (t, J = 7 Hz, 8 H), 4.41 (m, 2 H), 4.73 (s, 2
H), 5.05 (d, J = 13 Hz, 2 H), 5.29 (s, 4 H), 5.81 (d, J = 13 Hz,
2 H), 6.35 (t, J = 2 Hz, 2 H), 6.46 (d, J = 2 Hz, 4 H), 6.78 (s, 2
H), 7.16 (s, 1 H); 13C NMR (75 MHz, CDCl3): d = 14.2, 22.8,
26.2, 29.4, 29.5, 31.9, 36.8, 46.1, 49.1, 64.0, 65.7, 67.0, 67.2,
68.2, 68.8, 69.2, 70.0, 70.7, 101.7, 107.2, 111.6, 112.8, 116.3,
134.5, 135.9, 136.1, 136.6, 138.0, 138.6, 140.1, 141.2, 141.3,
142.3, 142.8, 143.3, 143.7, 143.9, 144.1, 144.26, 144.30, 144.36,
144.7, 145.0, 145.1, 145.3, 145.5, 145.7, 145.81, 145.84, 145.88,
146.2, 147.4, 147.6, 148.7, 158.1, 160.5, 162.6, 162.7, 168.8;
MALDI-TOF-MS: m/z = 2080.13 [M + H]+; Anal. calc. for
= 1748 (CQO); UV/Vis (CH2Cl2): l/nm (e/dm3 molꢀ1 cmꢀ1
)
1
= 260 (551 830), 315 (177 290), 378 (63 580), 438 (18 000); H
NMR (300 MHz, CDCl3): d = 0.87 (t, J = 7 Hz, 48 H),
1.20–1.43 (m, 160 H), 1.71 (m, 32 H), 3.20 (s, 6 H), 3.57 (s, 2
H), 3.72 (m, 2 H), 3.83 (t, J = 7 Hz, 32 H), 4.34 (m, 2 H), 4.64
(s, 2 H), 4.71 (m, 12 H), 5.05 (d, J = 13 Hz, 8 H), 5.15–5.28 (m,
28 H), 5.71 (d, J = 13 Hz, 8 H), 6.32 (s, 8 H), 6.45 (s, 16 H),
6.78 (s, 8 H), 6.89 (m, 7 H), 6.98 (s, 2 H), 7.12 (s, 4 H); 13C
NMR (75 MHz, CDCl3): d = 14.2, 22.8, 26.2, 29.4, 29.5, 31.9,
37.0, 45.9, 49.1, 63.9, 65.4, 65.5, 66.4, 66.9, 67.2, 68.2, 68.8,
69.3, 70.2, 70.7, 101.7, 107.2, 111.8, 112.6, 114.5, 114.6, 116.1,
121.3, 121.4, 134.5, 135.8, 136.1, 136.6, 137.4, 137.5, 137.9,
138.6, 140.0, 141.1, 141.2, 142.3, 142.8, 143.2, 143.6, 143.8,
144.0, 144.2, 144.3, 144.6, 145.0, 145.1, 145.2, 145.4, 145.6,
C
132H99O16N3S3 (2079.43): C 76.24, H 4.80, N 2.02; found: C
76.45, H 4.82, N 1.88%.
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145.8, 146.1, 147.4, 147.5, 148.7, 157.9, 158.2, 158.3, 160.5,
5 (a) H. Imahori and S. Fukuzumi, Adv. Funct. Mater., 2004, 14, 525;
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´
6 U. Hahn, E. Maisonhaute, C. Amatore and J.-F. Nierengarten,
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162.6, 162.7, 168.3, 168.4, 168.5; MALDI-TOF-MS: m/z =
7992.47 [M
+
H]+; Anal. calc. for C528H387O70N3S3
(7990.04): C 79.37, H 4.88, N 0.53; found: C 79.37, H 4.89,
N 0.48%.
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Electrochemistry
All electrochemical experiments were conducted in freshly
distilled THF + 0.3 M tetrabutylammonium hexafluorophos-
phate (Aldrich) as supporting electrolyte. We used a conven-
tional three electrode cell with a platinum counter electrode
and a saturated calomel electrode as a reference. The working
electrodes were gold balls obtained by melting a 25 mm gold
wire in a blue flame.19 The home-made potentiostat allows full
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