Self-ordering electron donor-acceptor nanohybrids based on single-walled carbon nanotubes across different scales

Article abstract of DOI:10.1002/anie.201207006

From nano- to macroscale: The immobilization of photo- and redox-active 9,10-di(1,3-dithiol-2-ylidene)-9,10-dihydroanthracene-based dipeptide 1 onto SWCNTs resulted in a high order of self-alignment among 1/SWCNTs (see picture). The assembly of the 1/SWCNTs is the key for stabilizing long-lived charge-separated states that are formed upon photoexcitation of the SWCNTs. Copyright

Full text of DOI:10.1002/anie.201207006

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DOI: 10.1002/anie.201207006
Donor–Acceptor Nanohybrids
Self-Ordering Electron Donor–Acceptor Nanohybrids
Based on Single-Walled Carbon Nanotubes Across
Different Scales**
Fulvio G. Brunetti, Carlos Romero-Nieto, Javier Lꢀpez-Andarias,
Carmen Atienza, Juan Luis Lꢀpez, Dirk M. Guldi,* and Nazario Martꢁn*
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Control over self-assembling nano-
structures constitutes a major challenge
in contemporary research. As a reflec-
tion of the latter, a great deal of time
and effort has gone into understanding
the factors and parameters that govern
the shape, size, and composition of
nanostructures.[1]
An intriguing class of materials is
carbon nanotubes (CNTs). CNTs have
attracted considerable attention owing
to their outstanding features, especially
in electronics, mechanics, and optics. In
fact, they are found in applications that
range from drug delivery to organic
electronics.[2] Soon after the discovery
of multi-walled carbon nanotubes
(MWCNTs) in 1991 by Iijima, single-
walled carbon nanotubes (SWCNTs)
were independently reported by Iijima
and Bethune in 1993.[3] Conceptionally,
SWCNTs are thought of as one-dimensionally extended
fullerenes, in which the sidewalls are composed of rolled up
graphene sheets and the caps are made out of hemispherical
fullerenes.[4]
When dealing with SWCNTs, limited control over their
growth and homogeneous production imposes, however,
major drawbacks for emerging areas of nanotechnology.
Equally problematic is their rather poor solubility in common
organic solvents. Large spaghetti-like bundles, that originate
from attractive interactions such as p–p stacking and London
dispersion forces, are the cause of insolubility.[5] In fact, the
latter renders purification, separation, and manipulation of
SWCNTs a sheer impossible task.[6]
Scheme 1. Retrosynthetic scheme of the p-extended tetrathiafulvalene 1.
and mature field.[7] For example, functional groups have
covalently been added to the sidewalls and caps of SWCNTs
by means of versatile synthetic protocols. In such cases, the
electronic structure of the SWCNTs is irreversibly altered
and, in turn, the overall p-systems reveal notable perturba-
tion. Contrarily, noncovalent functionalization of SWCNTs
has mostly been based on fairly weak p–p and/or hydrogen-
bonding interactions. In the resulting nanohybrids, SWCNTs
are coated/wrapped, and, thus, intertube interactions are
minimized, whereas the intrinsic electronic properties are
preserved.[8] To date, the noncovalent strategy en route
towards multifunctional systems is the method of choice to
fully harvest the unique features of SWCNTs in, for example,
p-/n-type electron donor–acceptor nanohybrids.[9]
Usually, p-/n-type electron donor–acceptor nanohybrids
based on SWCNTs have been obtained by employing
conjugated materials that lack the means to control the
morphology.[10] On the contrary, polypeptides,[11] polysaccha-
rides,[12] DNA strains,[13] and foldable oligomers[14] have
extensively been used to form ordered SWCNT nanohybrids,
albeit they fail to feature the electron-donating ability and, in
turn, appreciable electronic communication.
To exploit the full potential of SWCNTs and to overcome
any of the aforementioned limitations, their covalent and
noncovalent chemistry has developed into a rather valuable
[*] Dr. F. G. Brunetti, J. Lꢀpez-Andarias, Dr. C. Atienza, Dr. J. L. Lꢀpez,
Prof. N. Martꢁn
Departamento de Quꢁmica Orgꢂnica, Facultad de C.C. Quꢁmicas
Ciudad Universitaria sn (Spain)
and
IMDEA-nanociencia, 28049, Madrid (Spain)
E-mail: nazmar@quim.ucm.es
Homepage: http://www.ucm.es/info/fullerene
Here, we have designed a photo- and redox-active 9,10-
di(1,3-dithiol-2-ylidene)-9,10-dihydroanthracene
(exTTF)
covalently linked to an alanyl-glycine dipeptide sequence
bearing a polyethylene glycol dendrimer (Scheme 1). Our
straightforward design enables supramolecular organization
across scales, that is, from the nano- to the macroscale, and
probing the photophysical properties of the resulting electron
donor–acceptor nanohybrids. In fact, we will demonstrate
that exTTF-based dipeptide 1 guarantees the noncovalent
functionalization of SWCNTs affording unique p-/n-type
nanohybrids, the generation of long-lived radical ion pairs,
and the introduction of SWCNT nanostructuring at the
nanoscale amalgamating into macroscale domains of aligned
SWCNTs, especially in polar and aqueous media.
Dr. C. Romero-Nieto, Prof. D. M. Guldi
Department of Chemistry and Pharmacy & Interdisciplinary Center
for Molecular Materials
University of Erlangen-Nuremberg
Egerlandstr. 3, 91058 Erlangen (Germany)
[**] Financial support by the Ministerio de Ciencia e Innovaciꢀn
(MINECO) of Spain (projects CTQ2011-24652 and Consolider-
Ingenio CSD2007-00010), the EU (FUNMOLS FP7-212942-1) the
CAM (MADRISOLAR-2 project S2009/PPQ-1533) is acknowledged.
We also thank to the Deutsche Forschungsgemeinschaft (SFB583),
the Office of Basic Energy Sciences of the U.S., and Solar
Technologies Go Hybrid. The MINECO of Spain is thanked by C.A.
for a Ramꢀn y Cajal contract.
The retrosynthetic analysis for 1 is illustrated in Scheme 1.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201207006.
1 was obtained by linking building blocks 2,[15] 3, and 4 (see
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the Supporting Information). Initially, 2 and 3 were coupled
by amidation in the presence of activating agents. Depro-
tection of the methyl esthers and subsequent reaction with
deprotected 4 led to the formation of target 1 in excellent
yields.
1/SWCNT nanohybrids were obtained following an
already established protocol without, however, the needs of
employing any surfactants (see the Supporting Informa-
tion).[16] Upon their formation, 1/SWCNTs were fully char-
acterized by means of TEM, SEM, AFM, small angle X-ray
scattering (SAXS), absorption, and Raman spectroscopy.
For 1/SWCNTs, representative TEM images are displayed
in Figure 1a,b and Figure S1. At first glance, a high degree of
nanohybrid organization is discernable. A closer look reveals
the formation of fairly well-aligned SWCNT structures (Fig-
ure 1b,c). The latter agrees well with a report by Dieckman
et al., who have reported that with the help of a polypeptide
(i.e., 29 amino acids lacking an electron donor) aligned water-
soluble SWCNTs were obtained.[11] In stark contrast to their
case, we have focused on aligning SWCNTs with a water-
soluble dipeptide sequence bearing dendrimeric and electron-
donating termini. When compared to pristine SWCNTs, 1/
SWCNTs lack any randomly aggregated bundles. SWCNTs
reveal, for instance, bundles with diameters that range from
12 to 50 nm (Figure S1b).
In control experiments, dichloromethane, which was used
instead of water, gave an organization that is best described as
the coexistence of 1, unfunctionalized, pristine SWCNTs, and
1/SWCNTs (Figure 1d). The different organization relates to
the rather strong change in solvent polarity, that is, water
versus dichloromethane. Well-aligned SWCNTs have only
been seen in experiments with chemical vapor deposition
techniques.[17] In such a specific protocol SWCNTs are forced
to align in parallel.
Figure 2. SEM images of coated 1/SWCNTs (left) and an amplification
of the self-organization of SWCNT hybrids (right).
Likewise, our AFM assays prompt to the formation of 2D
organized domains that are exclusively composed of 1/
SWCNTs. On highly ordered pyrolytic graphite (HOPG)
wafers, AFM images reveal upon correcting for the tip-
broadening, a lateral d-spacing for the 2D nanocomposite of
around 10 nm. (See the Materials and Methods Section in the
Supporting Information.) FFT analyses of the AFM images
point unmistakably towards the preferred SWCNT alignment
in the resulting 2D order (Figure 3a). Height profiles under-
score that 1/SWCNTs posses heights in excess of 2.5 nm
(Figure 3c).
Histograms, in which the AFM diameter distributions of
1/SWCNTs are gathered, are displayed in Figure 3d. These
imply the homogeneous immobilization of 1 onto individu-
alized SWCNTs. Accordingly, we derive from AFM that more
than 80% of the nanostructures are formed by individual 1/
SWCNTs laterally assembled into two dimensions (Fig-
ure 3b).
In solution, the presence of well-ordered 1/SWCNTs was
confirmed by means of SAXS measurements. X-ray scattering
of 1/SWCNTs in aqueous media at 258C gives rise to a qÀ4
dependence within the low “q” range, which is indicative for
the presence of large aggregates. In addition, a scattering peak
in the intermediate “q” range with a spacing of approximately
8.97 nm is discernible (Figure 3e). Please note that this
spacing is in excellent agreement with the lateral d-spacing
observed after the tip-broadening correction in AFM. On one
hand, such spacing is in the range of 1/SWCNT diameters
(Figure 3 f). On the other hand, such spacing suggests that an
effective shielding/coating of 1 leads to the individualization
of SWCNTs and results from a uniform and homogeneous
SWCNT functionalization. Important are also the hydrophilic
dendrimer termini, which favor engagement with the sur-
rounding water (Figure 3 f). SAXS measurements with just
1 as control lack any ordered supramolecular ensemble in this
media (Figure S3).
SEM analysis further confirmed self-aligning 1/SWCNTs
and a clear higher organization of these nanocomposites is
discerned in Figure 2. Particularly, Figure 2b shows a magnifi-
cation of the self-oriented SWCNTs and their ordered
structure.
The results of our noncovalent functionalization strategy
were further investigated by Raman spectroscopy, that is, the
radial breathing mode (RBM) and D, G, and G’ modes. It
should be mentioned upfront that the D band intensities in
the Raman spectra of pristine SWCNTs and of 1/SWCNTs
reveal no difference. Such a similarity corroborates the
absence of oxidative/damaging processes during the workup
procedure. As a matter of fact, we hypothesize that the
original SWCNT properties must be preserved in 1/SWCNTs.
What did change, however, were the RBM and D’ modes.
Figure 1. a–c) TEM images of 1/SWCNTs drop-coated from water at
different magnifications—scale bars of 100, 20, and 20 nm, respec-
tively. d) TEM image of 1/SWCNTs drop-coated from DCM—scale bar
of 200 nm.
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Figure 3. a) AFM images of 1/SWCNTs on a HOPG surfaces (1 mmꢃ1 mm). Inset: FFT analysis of the highlighted area in the square. b) Schematic
drawing of 1/SWCNTs on a HOPG surface. c) Height profile of the section highlighted in (a). d) Histogram of the height distribution. e) SAXS
scattering of 1/SWCNTs in aqueous solution at 258C. f) Schematic drawing of 1/SWCNTs in solution. g and h) Differential absorption spectra
obtained upon femtosecond pump–probe experiments (387 nm) of 1/SWCNT in D2O with several time delays between 1.0 and 13.0 ps at room
temperature and in the visible/near-infrared and extended near-infrared regions, respectively. Inset h) Time absorption profiles at 1130, 1225, and
1450 nm monitoring the electron transfer.
They changed in intensity and frequency with an overall
decrease and a 3.5 cmÀ1 shift to higher frequencies (Figure S2)
revealing the needs of higher activation energies. RBMs are
strictly related to SWCNT diameters, chiralities, and the
presence of bundles.[18] Thus, modified RBMs are in agree-
ment with data reported for wrapped SWCNTs.[19] Only in
cases of high molecular weight macromolecules, which
thoroughly wrap around SWCNTs, Raman shifts emerged
that are as high as 3–5 cmÀ1.[13] Commonly, no appreciable
changes in the RBMs are detected for noncovalent inter-
actions based on p–p interacting building blocks.[20]
Owing to the electron-donating and chromophoric fea-
tures of the exTTFs, spectroscopic measurements were
performed. To this end, we monitored the absorption of 1 in
water and in the presence of increasing amounts of SWCNTs
(Figure S4). Interestingly, the addition of SWCNTs resulted
in an intensity decrease of the exTTFs centered features in the
visible (i.e., 453 and 382 nm), the simultaneous increase of
SWCNTabsorptions in the near-infrared (i.e., vide infra), and
the formation of pseudo-isosbestic and isobestic points at 363
and 489 nm, respectively. Further insights into the formation
of 1/SWCNTs came from variable temperature absorption
analyses (Figure S5). Here, the absorption intensity of
1 increased as the temperature was raised up from RT to
908C without the formation of SWCNT precipitates (Fig-
ure S6). More importantly, the initial spectrum was recovered
when the temperature was adjusted to the initial value. The
aforementioned is in sound agreement with SWCNT debun-
dling in water, especially at higher temperatures.[21] In
addition, we monitored the absorption features of SWCNTs
in the absence and presence of 1. The addition of 1 to aqueous
suspensions of SWCNTs evoked an appreciable red-shift of
the absorption maxima from 570, 590, 653, 727, 985, 1023, and
1123 nm for sodium dodecylbenzene sulfonate (SDBS)/
SWCNT to 576, 601, 661, 737, 1011, 1052, and 1157 nm for
1/SWCNT (Figure S7). But it is not only the absorption
features that are impacted by 1, also the SWCNT fluorescence
maxima shift bathochromically from 965, 1032, 1127, and
1256 nm for SDBS/SWCNT to 995, 1059, 1158, and 1296 nm
for 1/SWCNT. Interestingly, an overall fluorescence quench-
ing of 91% goes hand in hand with the formation of 1/
SWCNTs and, as such, prompts to a fast deactivation of
photoexcited SWCNTs in the presence of 1 (Figures S8 and
S9).
To shed light onto the fast excited-state deactivation in 1/
SWCNT, transient absorption measurements were per-
formed.[22] Following laser excitation at 387 nm, SDBS/
SWCNT gives rise to the instantaneous formation of a tran-
sient spectrum that features minima at 512, 597, 647, 1022, and
1131 nm, as well as maxima at 485 and 1430 nm. The SWCNT
centered transient is metastable and its decay is dominated by
two major processes. In fact, lifetimes of 2.8 and 67 ps result
from a multiexponential decay of the aforementioned fea-
tures. Owing to the shifts seen in the absorption spectra,
photoexcitation of 1/SWCNT at 387 nm leads to transient
features that are red-shifted with respect to SDBS/SWCNT.
For 1/SWCNT, minima emerge at 462, 515, 601, 651, 1062, and
1155 nm, together with maxima at 485, 853 and 1492 nm.
Figure 3g,h shows that these features transform with a lifetime
of 6 ps into a new transient. Importantly, the newly develop-
ing transient is composed in the visible range of a broad
maximum at 685 nm. The latter resemble the one-electron
oxidized radical cation of 1. In the near-infrared, minima at
1014 and 1131 nm as well as maxima at 485, 535, 616, 883, and
1415 nm imply new conduction band electrons in SWCNTs. In
other words, upon photoexcitation of 1/SWCNT the singlet
excited state gives rise to an electron transfer product, in
which 1 is oxidized and SWCNTs are reduced. From multi-
wavelength analyses of the charge-separated state, we deduce
a lifetime of 280 Æ 20 ps, which is considerably longer than
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dithiol-2-ylidene)-9,10-dihydroanthracene (p-extended tetra-
thiafulvalene)/SWCNT.[20]
To probe the electron-transfer impact of 1 on different
SWCNTs, we turned to (6,5) and (7,6) enriched SWCNTs
(Figures S11 and S12, respectively). Because of the SWCNT
enrichment fewer minima, that is, at 475, 515, 665, 1055, and
1150 nm, evolve for 1/(7,6) than for the mixture of 1/SWCNT.
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differs between 1/(7,6) and 1/(6,5) with minima at 1025/1140
and 1000 nm, respectively. Quite interesting is the trend seen
in the charge recombination dynamics with 260 Æ 10 ps for
1/(6,5) and 105 Æ 20 ps for 1/(7,6). In other words, the lower
charge-separated state energy in (6,5) SWCNTs is beneficial
to stabilize the charge-separated state. From a comparison of
the spectral changes we conclude that 1/SWCNTs contain
predominantly 1/(7,6). On the contrary, comparing the charge
recombination dynamics it appears that the dynamics in 1/
SWCNTs are governed predominantly by 1/(6,5).
In summary, we have described a versatile noncovalent
functionalization of SWCNTs in water. Our protocol enables
the spontaneous self-assembly of SWCNTs into well-ordered
structures of different scales. As such 1/SWCNTs represent
a unique example of short- and long-range organization
induced by a short peptide sequence linked to electron-
donating exTTF. Importantly, photoexcitation of 1/SWCNTs
affords SWCNT-centered excited states that transform into
long-lived charge-separated states in water. Their stability is
enhanced by long-range order. Likewise, self-ordering p-/n-
type composites on the nano- and mesoscale are a rational
and practical approach to the control of the morphology in
a variety of electronic devices, the efficiency of which is
decisively impacted by the molecular order and/or phase
segregation.
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Received: August 29, 2012
Revised: December 21, 2012
Published online: January 22, 2013
Keywords: carbon nanotubes · charge separation · donor–
.
acceptor nanohybrids · self-assembly · spectroscopy
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[22] 1 reveals upon 387 nm excitation a short lived transient—
Figure S10.
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