Self-assembly and self-metallization of porphyrin nanosheets

Article abstract of DOI:10.1021/ja068250o

Square porphyrin nanosheets with uniform thickness (7-12 nm) and aspect ratios up to 100 have been synthesized for the first time using a reprecipitation method. They retain the photocatalytic activity of the precursor molecules and can be self-metallized with Pt, Pd, and Au. Their unique morphology, photocatalytic properties, and large surface areas suggest potential applications in photo- and electrocatalysis, sensing, as components for optoelectronic nanodevices, and solar energy capture and conversion. Copyright

Full text of DOI:10.1021/ja068250o

Published on Web 02/10/2007
Self-Assembly and Self-Metallization of Porphyrin Nanosheets
†
‡
†
,†,§
Zhongchun Wang, Zhiyong Li, Craig J. Medforth, and John A. Shelnutt*
Nanostructure and Semiconductor Physics Department, Sandia National Laboratories, Albuquerque,
New Mexico 87185, Quantum Science Research, Hewlett-Packard Laboratories, 1501 Page Mill Road, Palo Alto,
California 94304, and Department of Chemistry, UniVersity of Georgia, Athens, Georgia 30602
Received November 17, 2006; E-mail: jasheln@unm.edu
The self-assembly approach has been widely adopted in the effort
1
to design and prepare functional materials. Porphyrins are par-
ticularly attractive building blocks for self-assembly because the
intimate packing of these aromatic macrocycles can result in rich
photophysical and photochemical properties that are important in
many technological applications. Covalent or noncovalent por-
phyrinic assemblies have been used to model the solar energy
capture and transfer processes seen in naturally occurring photo-
systems, as well as to create artificial photoactive molecular
2
devices. Recently, we have exploited the ionic self-assembly of
oppositely charged porphyrin tectons to synthesize porphyrin
nanotubes3 in aqueous solution and porphyrin nanofiber bundles
a,b
3c
in an aqueous-organic two-phase system, with the ultimate goal
of making novel photoactive nanodevices.
Herein, we describe the synthesis of discrete free-standing
porphyrin nanosheets using a reprecipitation method. This method
4a,b
has been widely adopted in the synthesis of organic nanoparticles
and recently by Hu et al.4c in the surfactant-assisted synthesis of
hollow hexagonal nanoprisms from Zn 5,10,15,20-tetra(4-pyridyl)-
porphyrin. While many two-dimensional (2-D) inorganic nano-
5
structures (e.g., titania nanosheets exfoliated from bulk materials )
have been synthesized and studied, there have been only a few
attempts to make well-defined 2-D organic nanostructures. One
6
example is the square platelets formed by controlled crystallization
of polystyrene-poly(ethylene oxide) (PS-PEO) diblock copoly-
mers in methylcyclohexane. Others include polymer nanosheets or
nanofilms with a 2-D hydrogen-bonding network prepared on planar
7
Figure 1. (a) SEM image of SnPyTriPP nanosheets on Si(100) substrate;
b) SEM image of porphyrin “cubes” on Si(100); (c) AFM image of
substrates using the Langmuir-Blodgett technique. However, these
(
organic structures6 do not possess the diverse photophysical and
photochemical properties possible with porphyrin-based systems.
In a typical synthesis of the porphyrin nanosheets, 200 µL of a
,7
porphyrin nanosheets adsorbed on Si(100); (d) height profiles of the lines
in panel c, showing that the nanosheets have thicknesses of 7-12 nm,
independent of their side lengths; (e) UV-visible extinction and (f) emission
spectra of aqueous suspensions of the nanosheets. The absorption (e) and
emission (f) spectra of monomeric SnPyTriPP in ethanol are also shown
for comparison. Inset in panel e shows the molecular structure of SnPyTriPP
1.0 mM solution of Sn(IV) 5-(4-pyridyl)-10,15,20-triphenyl-por-
phyrin dichloride (SnPyTriPP; Figure 1e, inset) in ethanol was
injected into 10 mL of deionized water at room temperature under
vigorous stirring. A translucent greenish-pink colloid was obtained
after stirring for approximately 10 min. Using atomic force
microscopy (AFM), transmission electron microscopy (TEM), and
scanning electron microscopy (SEM), the colloidal porphyrin
particles prepared were revealed to be a collection of nanosheets
(OH axial ligands are shown as these are expected to be present in neutral
aqueous solution).
Cubic microstructures with hollow faces (Figures 1b, S2) were
obtained by injecting the precursor ethanol solution into hot water
(
∼90 °C) and letting the mixture cool. Nanosheets like those in
Figure 1a,c but with slightly shorter average edge lengths were
obtained using chilled water (∼4 °C) (data not shown). Methanol
could also be used as the porphyrin solvent in the self-assembly
process, with little change in the product size distribution at room
temperature.
(Figure 1). The square nanosheets occur with a range of edge lengths
(0.3-1 µm), as can be best seen in Figure 1c and in the Supporting
Information Figure S1. However, they possess a fairly uniform
thickness of 7-12 nm (see Figure 1d) giving aspect ratios up to
1
00. Interestingly, a triangular sheet lies along the diagonal of the
The shape and sharp edges of the porphyrin nanosheets suggest
that they might be single crystals. A crystalline structure is also
suggested by powder X-ray diffraction (XRD) measurements of
the nanosheets cast onto a Si(100) substrate (Figure S3). Although
the nanosheets show far fewer peaks than the powder of the
square on ca. 30% of the nanosheets (Figure 1a,c and Figure S1).
The self-assembly of SnPyTriPP was also investigated using
other conditions and found to produce different nanostructures.
†
Sandia National Laboratories.
‡
2
SnPyTriPP(Cl) precursor, all the peaks in the XRD pattern of the
nanosheets are observed for the porphyrin precursor powder. The
Hewlett-Packard Laboratories.
§
University of Georgia.
2440
9
J. AM. CHEM. SOC. 2007, 129, 2440-2441
10.1021/ja068250o CCC: $37.00 © 2007 American Chemical Society

C O M M U N I C A T I O N S
Figure 2. (a) Crystal structure of Sn(OH)2TPyP showing the hydrogen-
bonding network (Sn-O‚‚‚H-O-H‚‚‚N) between molecules9 and (b) a
related hydrogen-bonding network for the Sn(OH)2PyTriPP nanosheets.
Figure 3. TEM images of (a) a SnPyTriPP nanosheet decorated with
photodeposited Pt nanoparticles and (b) a nanosheet encased within a thin
porous Pt mat of autocatalytically grown Pt dendrites. Note in Supporting
Information Figures S4 and S5 that all the triangular sheets were platinized
along with the squares.
markedly different XRD patterns may be related to the nanoscale
thickness of the sheets and their orientation on the substrate. No
long-range periodicity was observed in either the precursor powder
or the nanosheets as indicated by the absence of significant
diffraction peaks in the low-angle region (inset of Figure S3).
As expected for a multi-porphyrin system, the porphyrin
nanosheets exhibit absorption and emission spectra that are more
complicated than those of the monomers.4 Compared to mono-
meric SnPyTriPP in ethanol, which shows a Soret band at 422 nm
and Q-bands at 558 and 598 nm, the extinction spectrum of the
nanosheets is more complex (Figure 1e). The Soret band is split
into a blue-shifted band at 408 nm and two red-shifted bands at
pyrolytic graphite. Photometallization of nanosheets previously
adsorbed on the substrate (see Figures S5 and S6) suggests that
the fabrication of nanoelectrodes with ohmic contacts between the
metal and the nanosheets may be possible. This might facilitate
the integration of the porphyrin nanosheets into optoelectronic
nanodevices.
In summary, square porphyrin nanosheets with high aspect ratios
have been synthesized for the first time. Their unique morphology,
photocatalytic properties, and large surface areas suggest that they
may find a wide range of applications in electronics, photonics,
and catalytic systems. We are currently investigating the electrical
properties of the nanosheets using AFM on conducting substrates
and evaluating their potential applications in electronic and opto-
electronic nanodevices.
b,8
4
6
36 and 450 nm, while the Q-bands red shift slightly to 566 and
02 nm. The emission spectrum of the nanosheets (Figure 1f) shows
four bands at 602, 632, 656, and 693 nm, while only two bands at
602 and 656 nm are seen for monomeric SnPyTriPP.
With regards to the mechanism of formation of the nanosheets,
it is possible that hydrogen bonding between the axial hydroxyl
ligand of a Sn(OH) PyTriPP molecule and the pyridyl group of an
Acknowledgment. This work was partially supported by U.S.
DOE Grant DE-FG02-02ER15369 (J.A.S.) and DARPA (Z.L.).
Sandia is a multiprogram laboratory operated by Sandia Corpora-
tion, a Lockheed-Martin company, for the U.S. Department of
Energy’s National Nuclear Security Administration under Contract
DE-ACO4-94AL85000. We thank Dr. K. J. Ho at UNM for the
fluorescence measurements.
2
adjacent molecule (perhaps via a bridging water molecule) might
be involved in the self-assembly of the nanosheets (Figure 2). Such
a network of Sn-O‚‚‚H-O-H‚‚‚N hydrogen bonds is known to
occur in tin tetra(pyridyl)porphyrin crystal structures (Figure 2a).9
Together with hydrophobic forces and π-π interactions, such a
network might lead to formation of the nanosheets. Direct coordina-
tion between Sn(IV) and the pyridyl groups is not expected since
Sn(IV) has a weak affinity for pyridyl groups.10 Injecting the
SnPyTriPP ethanol solution into water at pH 2, where the pyridyl
groups will become fully protonated, yields no well-defined
aggregates, supporting the idea that pyridine acts as a hydrogen-
bond acceptor in such a network.
Supporting Information Available: Experimental details, XRD
data, and additional AFM, SEM, and TEM images of the nanosheets.
This material is available free of charge via the Internet at http://
pubs.acs.org.
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