Water-soluble nanorods self-assembled via pristine C60 and porphyrin moieties

Article abstract of DOI:10.1039/b908050c

A novel water-soluble nanorod is discussed, which is prepared via the self-assembly of pristine C₆₀ and a double-sided porphyrin projecting four β-cyclodextrins from each face.

Full text of DOI:10.1039/b908050c

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COMMUNICATION
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Water-soluble nanorods self-assembled via pristine C60 and porphyrin
moietiesw
Maher Fathalla,a Shao-Chun Li,b Ulrike Diebold,b Alina Albb
and Janarthanan Jayawickramarajah*a
Received (in Austin, TX, USA) 22nd April 2009, Accepted 15th May 2009
First published as an Advance Article on the web 12th June 2009
DOI: 10.1039/b908050c
A novel water-soluble nanorod is discussed, which is prepared via
the self-assembly of pristine C60 and a double-sided porphyrin
projecting four b-cyclodextrins from each face.
The design of molecules imbued with specific structural
information that encodes their spontaneous assembly into
organized nanoscale structures is a salient goal in supra-
molecular chemistry1 and is critical for further advancements
in the bottom-up approach to nanofabrication.2 From this
point of view, supramolecular structures consisting of fullerene
C60 and porphyrin subunits are particularly attractive for the
fabrication of functional nanomaterials, given the potential
light-harvesting and photo-induced electron transfer properties
of their constituents.3
While elegant nanostructures composed of a C60–porphyrin
self-assembly have been studied in low dielectric solvents,4
the development of such architectures in water—an attractive
Scheme 1 The synthesis of porphyrin 4. Conditions: (a) propionic
solvent from environmental, economic and biocompatibility
points of view—remains a formidable challenge.5 The reason
for this difficulty is due to the low solubility of both components
in water and their tendency to form ill-defined homomeric
aggregates with diminished photophysical properties.6
Furthermore, the few water-soluble C60–porphyrin nano-
structures developed thus far are largely based on either
favorable van der Waals interactions between C60 and the
porphyrin plane, or via axial coordination to metallated
porphyrins. Hence, in order to enhance the repertoire of
potential nanostructures that can be prepared, alternative
modes of self-assembling porphyrin and C60 modules are
required.
acid, reflux; (b) Zn(OAc)2 in CHCl3–methanol (10 : 1); (c) 6-azido-
6-deoxy-PMb-CD (5), sodium ascorbate, CuSO4 in THF–H2O.
b-CDs from each of its two faces. Such functionalization not
only enhances aqueous solubility and precludes porphyrin-
based stacking interactions, but importantly also facilitates
directional host–guest inclusion complexation with pristine
C60. In particular, the resultant self-assembly, while generated
in DMF–toluene, is water soluble and forms unidirectional
C60–porphyrin nanorod 6 (Fig. 1) by exploiting repeats of four
2 : 1 b-CD–C60 interactions. The formation of nanorod 6 and
its higher-order assembly has been extensively characterized
via analytical and microscopic techniques, including dynamic
light scattering (DLS), scanning tunneling microscopy (STM),
transmission electron microscopy (TEM), and scanning electron
microscopy (SEM).
Our strategy to answer this problem exploits the unique
ability of b-cyclodextrin (b-CD) to encapsulate pristine C60 in
a 2 : 1 stoichiometry.7a While this ternary supramolecular
motif has been used to assemble higher-order structures,7b,c to
the best of our knowledge, there has been no exploration
of using this motif to assemble porphyrin–C60 nanoscale
architectures. In this communication, we detail the synthesis
and assembly studies of ‘‘double-sided’’ zinc porphyrin deri-
vative 4 (Scheme 1), judiciously designed to project four
A central feature of our assembly design is octadentate
tetraphenylporphyrin derivative 4, with permethylated
b-cyclodextrin (PMb-CD) substituents on each meta-position
of the phenyl rings. It was expected that torsional strain would
place the plane of the meso-phenyl rings close to perpendicular
to the plane of the porphyrin macrocycle, thereby resulting in
a double-sided porphyrin that projects four PMb-CD units
from each face. Furthermore, PMb-CD was specifically
chosen because methylation has been reported8 to increase
the effective diameter of the hydrophobic cavity of b-CDs,
thereby better facilitating fullerene inclusion.
a Department of Chemistry, Tulane University, 2015 Percival Stern
Hall, Louisiana 70118, USA. E-mail: jananj@tulane.edu;
Fax: +1 504 865 5596; Tel: +1 504 862 3580
b Department of Physics, Tulane University, 2001 Percival Stern Hall,
Louisiana 70118, USA
w Electronic supplementary information (ESI) available: Synthetic
details, spectroscopic characterization, IR and UV-vis spectra,
molecular models, DLS measurements, and TGA curves. See DOI:
10.1039/b908050c
The synthesis of porphyrin 4 is outlined in Scheme 1 (supra)
and features the highly efficient Huisgen [3 + 2] cycloaddition,
which allows for the facile attachment of all eight PMb-CD
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Fig. 1 The preparation of assembly 6 via inclusion complexation of
pristine C60 by porphyrin 4.
Fig. 2 UV-vis spectra of assembly 6 (red) and porphyrin 4 (black)
upon extraction into toluene (from an aqueous solution).
arms in a single step. Briefly, aldehyde 1 was prepared by
the reaction of 3,5-dihydroxybenzaldehyde with propargyl
bromide in excellent yield. Formyl derivative 1 was subsequently
refluxed in the presence of pyrrole in propionic acid, followed
by purification via column chromatography (silica gel column
using methylene chloride as the eluent) to afford porphyrin 2
in 13% yield. Metallation of porphyrin 2 with zinc acetate
afforded Zn porphyrin derivative 3 and the requisite double-
sided porphyrin 4 was prepared by ‘‘clicking’’9 Zn porphyrin 3
with 6-azido-6-deoxy-PMb-CD.10
Prior to investigating the potential of 4 to assemble in the
presence of C60, water-solubility studies were performed, with
porphyrin 4 alone, using UV-vis spectroscopy. These experi-
ments indicated that monomer 4 is indeed readily soluble in
water. Furthermore, no porphyrin–porphyrin stacking inter-
actions were observed (i.e., the Soret band of 4 shows no
broadening, even at 0.015 mM concentrations, and no
absorption peaks characteristic for porphyrin aggregates were
observed).6 Encouraged by these preliminary studies, the
self-assembly of porphyrin 4 in the presence of C60 was
undertaken. Briefly, porphyrin 4 was incubated with 6 equiv.
of fullerene C60 in a solution of toluene–DMF (a mixture that
maximizes C60 solubility) for three weeks. Upon evaporation
of the solvents, the resulting assembly was found to be soluble
in water, and thus was dissolved in this same solvent and
purified by filtration to remove any insoluble fullerene mono-
mers and aggregates.
indicate, even at sub-micromolar concentrations of 6 (0.3 Â
10À6 M), the presence of large aggregates with an average
diameter of 263 nm.
In addition to the above mentioned solution phase studies,
solid-state microscopies were undertaken to better character-
ize the nature of assembly 6. The most telling information was
gleaned from STM experiments. A dilute aqueous solution of
assembly 6 (1 Â 10À6 M) was deposited on a highly ordered
pyrolytic graphite (HOPG) surface. Subsequent to evapora-
tion of the solvent, STM investigations (Fig. 3a) displayed
hundreds of nm long, one-dimensional nanorods, a finding not
observed for porphyrin 4 alone. For the most part, these
nanorods are located at steps of the HOPG substrate,
probably because step edges provide more favorable binding
sites than the flat terraces. As shown in the small-scale image
in Fig. 3b, assembly 6 consists of a chain of bright protrusions.
These protrusions have a periodicity of B2 nm, and an
apparent height of 2.5 nm above the lower terrace next to
the step edge (see line profiles in Fig. 3d and e).
The contrast in STM is not only a function of height but
also of electronic structure, and tip convolution effects prevent
accurate measurements of the dimensions of nanoscopic
objects. Nevertheless, the periodicity as well as the measured
Preliminary evidence for the formation of a complex invol-
ving porphyrin 4 and C60 came from absorption studies. For
instance, the FT-IR spectrum of assembly 6 (ESI, Fig. S1w)
shows a characteristic peak at 527 cmÀ1 assigned to C60.4e In
addition, the electronic absorption spectrum of assembly 6 in
water (ESI, Fig. S2w) displays broad bands (at 275 and 350 nm),
albeit at low intensity, ascribed to C60. In order to verify that
these observed bands indeed correspond to C60-based
absorption, an aqueous solution of assembly 6 was extracted
into toluene. The result of this experiment is illustrated in
Fig. 2, clearly indicating the presence of C60 (via sharp
absorptions bands at 278 and 332 nm) in the toluene layer.4e
In contrast, an analogous extraction experiment with only
porphyrin 4 does not show these dual absorption bands.
Although the aforementioned experiments demonstrate the
presence of fullerene within assembly 6, they do not, however,
shed light on the size of the resulting supramolecular species.
Hence, DLS studies were employed (ESI, Fig. S3w) that
Fig. 3 STM images of assembly 6 on HOPG. (a) Image size 100 nm Â
85 nm, sample bias voltage V = +0.5 V; tunneling current I = 0.1 nA);
(b) 20 nm  22 nm, V = +0.5 V; I = 0.1 nA. (c) Pictorial
representation of assembly 6 and (d and e) line profiles of assembly
6 taken at the positions indicated by the arrows in (b).
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height of the bright protrusions compare well with a suggested
model that assumes a four-fold bundle structure (see Fig. 1
and 3c), where each bright spot11 represents the four
head-to-head PMb-CD dimers (with each dimer encasing one
C60 molecule). For example, the height of the nanorods
(ca. 2.5 nm) is larger than the outer diameter of one
PMb-CD (ca. 1.6 nm) and thus supports a possible model
that is two PMb-CD units high. Likewise, the periodicity of
B2 nm is consistent with repeats of head-to-head PMb-CD
dimers interdigitated by a porphyrin core with flexible linkers
(see ESI, Fig. S4w).
Further evidence for a self-assembly consisting of four C60
molecules per unit of porphyrin 4 came from thermo-
gravimetric analysis (TGA). Specifically, a sample containing
only 4 loses 80% of its weight below 700 1C. On the other
hand, a sample of assembly 6 loses only 63% of its weight
below 700 1C (ESI, Fig. S5 and S6w). Thus, the remaining 17%
that does not decompose under these experimental conditions
is assigned to C60 (which is known to only decompose over
850 1C)12. This percentage is close to the theoretical value
(18%) for four C60 molecules per unit of 4.
In conclusion, we have successfully designed and constructed
a novel water soluble porphyrin–fullerene C60 nanorod via the
inclusion complexation of a porphyrin-bridged octa-(b-CD)
module 4 and pristine C60. The nanorod structure is thought to
be composed of sequential repeats of four 2 : 1 b-CD–C60
interactions. The development of other nanostructures based
on water soluble porphyrins is currently under way in our
laboratory.
This work was partly supported by the Tulane University
Start Up Funds (to J. J). M. F. would like to thank the
Egyptian government for a graduate fellowship to undertake
research at Tulane. The authors are grateful for the
assistance given by Dr Jibao He in acquiring TEM and
SEM measurements.
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With compelling evidence that assembly 6 forms a nanorod-
type morphology at low concentrations, we were keen to
discern the higher-order self-assembly of 6 in the solid-state.
Thus, TEM studies were also performed. The sample for the
TEM experiment was prepared by placing a drop of aqueous
solution of assembly 6 (1 Â 10À4 M) onto a carbon-coated
copper grid, followed by evaporation of the aqueous solvent.
As can be gathered from inspection of the TEM image of
assembly 6 (Fig. 4a), several unidirectional structures with
various lengths ranging from 300 to 500 nm are present. Based
on the dimensions of assembly 6 (determined by STM studies)
it is deduced that the long axes of these nanorods are formed
by the inclusion complexation of approximately 150–250 units
of porphyrin 4 with flanking fullerene C60 molecules.
In addition, SEM experiments were performed to determine
whether porphyrin 4 (alone) and assembly 6 differed in their
overall surface topology in terms of size and shape in the
micron regime. As shown in Fig. 4b and c, both assembly
6 and porphyrin 4 display three-dimensional morphologies;
however, the former forms significantly smaller macrostructures.
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Fig. 4 TEM micrograph of assembly 6 (a), and SEM images of
assembly 6 (b) and porphyrin 4 (c).
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Chem. Commun., 2009, 4209–4211 | 4211