Molecular organization in self-assembled binary porphyrin nanotubes revealed by resonance Raman spectroscopy

Article abstract of DOI:10.1039/b926068d

Porphyrin nanotubes were formed by the ionic self-assembly of tetrakis(4-sulfonatophenyl) porphyrin diacid (H₄TPPS₄ ^2-) and Sn(iv) tetra(4-pyridyl) porphyrin (Sn(OH^-)(X) TPyP^4+/5+ [X = OH^- or H₂O]) at pH 2.0. As reported previously, the tubes are hollow as revealed by transmission electron microscopy, approximately 60 nm in diameter, and can be up to several micrometres long. The absorption spectrum of the porphyrin nanotubes presents monomer-like Soret bands, as well as two additional red-shifted bands characteristic of porphyrin J-aggregates (offset face-to-face stacks). To elucidate the origin of the J-aggregate bands and the internal interactions of the porphyrins, the resonance Raman spectra have been obtained for the porphyrin nanotubes with excitations near resonance with the Soret J-aggregate band and the monomer-like bands. The resonance Raman data reveal that the Sn porphyrins are not electronically coupled to the J-aggregates within the tubes, which are formed exclusively by H₄TPPS₄^2-. This suggests that the internal structure of the nanotubes has H₄TPPS ₄^2- in aggregates that are similar to the widely studied H₄TPPS₄^2- self-aggregates and that are segregated from the Sn porphyrins. Possible internal structures of the nanotubes and mechanisms for their formation are discussed.

Full text of DOI:10.1039/b926068d

View Article Online / Journal Homepage / Table of Contents for this issue
PAPER
www.rsc.org/pccp | Physical Chemistry Chemical Physics
Molecular organization in self-assembled binary porphyrin nanotubes
revealed by resonance Raman spectroscopy
a
b
c
c
Ricardo Franco,* John L. Jacobsen, Haorong Wang, Zhongchun Wang,
a
b
c
cd
Krisztina Istvan, Neil E. Schore, Yujiang Song,w Craig J. Medforth and
´
John A. Shelnutt*
ce
Received 10th December 2009, Accepted 12th February 2010
First published as an Advance Article on the web 9th March 2010
DOI: 10.1039/b926068d
Porphyrin nanotubes were formed by the ionic self-assembly of tetrakis(4-sulfonatophenyl)
2
ꢀ
ꢀ
4+/5+
porphyrin diacid (H
ꢀ
4
TPPS
4
) and Sn(IV) tetra(4-pyridyl) porphyrin (Sn(OH )(X)TPyP
[
X = OH or H
2
O]) at pH 2.0. As reported previously, the tubes are hollow as revealed by
transmission electron microscopy, approximately 60 nm in diameter, and can be up to several
micrometres long. The absorption spectrum of the porphyrin nanotubes presents monomer-like
Soret bands, as well as two additional red-shifted bands characteristic of porphyrin J-aggregates
(
offset face-to-face stacks). To elucidate the origin of the J-aggregate bands and the internal
interactions of the porphyrins, the resonance Raman spectra have been obtained for the
porphyrin nanotubes with excitations near resonance with the Soret J-aggregate band and the
monomer-like bands. The resonance Raman data reveal that the Sn porphyrins are not
electronically coupled to the J-aggregates within the tubes, which are formed exclusively by
2
ꢀ
42ꢀ
H
4
TPPS
4
. This suggests that the internal structure of the nanotubes has H
4
TPPS
ꢀ
self-aggregates and that are
in
2
aggregates that are similar to the widely studied H
TPPS
4 4
segregated from the Sn porphyrins. Possible internal structures of the nanotubes and mechanisms
for their formation are discussed.
charge are of particular interest because their hollow tubular
I. Introduction
structure lends itself to the construction of nanotube-metal
22
composites of various architectures. The PNTs are represen-
Self-assembled porphyrin nanostructures are of great interest
because of their promise as catalysts and optoelectronic
tative of a new class of porphyrin nanostructures for which the
molecular building blocks (tectons) can be altered to control
2
their structural and functional properties. However, to realize
1
–8
nanomaterials.
Porphyrin nanostructures are particularly
9
promising as light-harvesting and energy- and electron-transfer
components of solar energy utilizing devices, as they have
interesting supramolecular porphyrin structures and properties
nanodevices based on these PNTs and related porphyrin nano-
structures, the organization of the porphyrin molecules in the
tubes needs to be understood and the collective electronic
properties of the tubes determined. For nanostructures, often
direct X-ray structural methods may not be successful and
indirect techniques must be employed. Herein, we report a
resonance Raman study of the porphyrin nanotubes that
provides insights into the electronic coupling and the packing
arrangement of the porphyrin subunits.
1
0
analogous to those displayed by porphyrin hetero-dyads and
1
1
pentamers. Self-assembled porphyrin nanostructures may also
serve as illuminating models of biological light-harvesting
structures like the bacteriochlorophyll nanorods of the chloro-
1
2,13
somes of green bacteria.
Well-defined nanostructures
15,16
nanofibers,
8
such as porphyrin nanorods, nanotubes,
2,14
1
7,18
9
nanospheres, and other morphologies
19–21
nanosheets,
have recently been reported. The porphyrin nanotubes (PNTs)
formed by ionic self-assembly of two porphyrins with opposite
II. Experimental
a
REQUIMTE, Departamento de Quı´mica, Faculdade de Cieˆncias
A
Synthesis of the porphyrin nanotubes
e Tecnologia, Universidade Nova de Lisboa, 2829-516 Caparica,
Portugal. E-mail: r.franco@dq.fct.unl.pt; Tel: +251-21-294-9659
Department of Chemistry, University of California, Davis,
CA 95616, USA
Advanced Materials Laboratory, Sandia National Laboratories,
b
The nanotubes were synthesized using a procedure similar to
2
that reported previously. All solutions were prepared using
c
HPLC grade water (Aldrich). Tetrakis(4-sulfonato-phenyl)-
1
001 University Blvd SE, Albuquerque, NM 87185-1349, USA.
porphyrin (H
Aldrich and Sn(IV) tetrakis(4-pyridyl)porphyrin (SnTPyP
dichloride was purchased from Frontier Scientific. H TPPS
2 4
TPPS ) tetrasodium salt was purchased from
E-mail: jasheln@unm.edu; Fax: +1 505-272-7077;
Tel: +1 505-272-7160
Department of Chemical and Nuclear Engineering, University of
New Mexico, Albuquerque, NM 87106, USA
Department of Chemistry, University of Georgia, Athens, GA 30602,
USA
2
+
)
d
e
2
4
stock solution (98.7 mM) was prepared by dissolving 10.1 mg
of H TPPS in 100 mL of water. The stock solution was green
2
4
with a pH of 5.5. SnTPyP dichloride stock solution (42.6 mM)
was prepared by dissolving 8.6 mg of SnTPyP in 250 mL of
w Present address: Dalian Institute of Chemical Physics, Chinese
Academy of Sciences, Dalian, Liaoning, China
4
072 | Phys. Chem. Chem. Phys., 2010, 12, 4072–4077
This journal is ꢁc the Owner Societies 2010

View Article Online
water. This stock solution was magenta pink and had a pH of
.5. Small volumes (1–6 mL) of the stock solutions were
were transferred into a stoppered 3 ꢃ 3 mm cross-section
optical cell (NSG Precision Cells). Typically, the spectra were
taken at room temperature (24 1C) with data collection times
of 2–10 min using a laser beam power of 20 mW.
4
filtered immediately prior to their use in the self-assembly
reactions (Kendall monojectt Tuberculin 1 mL syringes were
s
used to pass the solutions through Pall Life Sciences Acrodisc
s
The spectra were obtained using the 413.1 and 406.7 nm lines
+
of an INNOVA 302 Kr laser (Coherent) or using the 496.5
+
and 501.7 nm lines of an INNOVA 304 Ar laser (Coherent).
1
3 mm syringe filters with 0.2 mm Supor membranes).
Self-assembly experiments were conducted in 20 mL glass
vials that had been rinsed with HPLC grade water. SnTPyP
solution (3.5 mM) was prepared by diluting 739 mL of stock
solution to a volume of 9 mL, followed by thorough mixing. A
The Raman spectrometer was a 0.75 m monochromator
(Instruments, SA) with a 512 ꢃ 2048 pixel LN
2
-cooled CCD
ꢀ1
detector. The slit width of 100 mm for the 2400 groove mm
ꢀ1
1
0.5 mM solution of H
4
TPPS
4
was prepared in a separate vial;
TPPS stock
holographic grating gives a spectral resolution of 2 cm . The
CCD array was cooled to 138 K by liquid nitrogen and
controlled by a Symphony (Horiba Jobin Yvon) CCD con-
troller unit. The columns of the CCD chip (EEV) were binned
7
.863 mL of water was added to 957 mL of H
2
4
solution, 180 mL of 1 M HCl was added, and the solution
thoroughly mixed. The diluted and acidified solutions of
ꢀ
1
H TPPS4 were green consistent with the formation of the
2
to give 2048 13.5 mm channels or 0.3 cm per channel. The
chip is back-illuminated and has visible-NIR antireflection
coatings. The spectrometer was interfaced to a personal
computer via an IEEE 488.2 PCI-GPIB interface card (National
Instruments), and LabSpec Raman Spectroscopy Software
(Horiba JobinYvon) was used to control the spectrometer.
Position mode was used for CCD detection covering about
diacid species expected at this pH.
The SnTPyP solution was then added to the H TPPS
4
4
solution and the mixture thoroughly homogenized, producing
a yellowish-green solution with H TPPS , SnTPyP and HCl
4
4
concentrations of 5.25 mM, 1.75 mM, and 10 mM. The pH
was carefully adjusted to 2.0 ꢂ 0.1 using 1 M HCl. The vial
was sealed and the solution left undisturbed in the dark for
ꢀ
1
500 cm
of the Raman spectrum. Spectra were exported
7
2 h, after which time samples were taken for TEM, UV-visible
as even-X ASCII files for plotting with SigmaPlot (SPSS).
Presented resonance Raman spectra were obtained for the
absorption, and resonance Raman experiments as described
below.
ꢀ
1
1300–1700 cm wavenumber range. Lower frequency spectra
were not useful due to the intense resonance light scattering of
ꢀ
1
.
B
Structural characterization
the nanotube samples for wavenumbers below ca. 1000 cm
To prepare the TEM samples, B5 mL of solution containing
precipitate were removed from the bottom of the vial, followed
by the removal of B5 mL of the supernatant into the same
pipette tip. This 10 mL aliquot was dispensed onto a standard
holey carbon-coated copper TEM grid resting on a Kimwipe;
the Kimwipe wicked away excess water. Transmission electron
microscopy images of the samples were obtained using a
III. Results
A
Porphyrin nanotubes
Porphyrin nanotubes were prepared by mixing aqueous solutions
of the two porphyrins shown in Fig. 1 using a procedure slightly
2
modified from that described previously (see Experimental
2
00 keV JEOL 2010 microscope.
section).
The PNTs used in the Raman study are shown in a transmission
electron microscope (TEM) image as an inset of Fig. 2. The hollow
tubes, formed by ionic self-assembly of the porphyrins, are
approximately 60 nm in diameter and can be up to several
C
UV-visible spectroscopy
UV-visible spectra of the constituent porphyrins and the
porphyrin nanotubes were obtained with a Hewlett Packard
2
ꢀ
micrometres long. The ratio of 2.0–2.5 molecules of H TPPS
4
8452A diode array spectrometer. Samples of the individual
4
0
4+/5+
0
ꢀ
[X,X = OH or H O] is
2
to 1.0 molecule of Sn(X)(X )TPyP
porphyrins were prepared by dilution of the previously prepared
stock solutions to 10 mM with 0.01 M HCl (final pH of 2).
UV-visible absorption spectra were obtained on the freshly
prepared samples.
consistent with the ratio of the charges on the two porphyrins
2
at pH 2. The fact that the composition of the PNTs is
After centrifugation, samples of the porphyrin nanotubes
were extracted from the precipitate rich layer at the bottom of
the vial in which the nanotubes were synthesized (see Section A).
Samples were transferred to a quartz cell with a beam path
length of 1 cm, and UV-visible spectra were obtained and
plotted using SigmaPlot (SPSS).
D
Resonance Raman spectroscopy
Samples of the individual porphyrins were prepared at 100 mM
concentrations in 0.01 M HCl. Samples of the porphyrin
nanotubes were prepared by centrifuging the nanotube
solution described in Section A and re-suspending the precipi-
tate in 0.01 M HCl to produce a 10 times more concentrated
suspension. The Raman samples, in a final volume of B100 mL,
Fig. 1 Porphyrins used in the assembly of the porphyrin nanotubes
depicted in the TEM image of Fig. 2. At pH 2, there is a mixture of Sn
ꢀ
0
porphyrin complexes with X = OH and X = H O (net charge +5)
2
0
ꢀ
2
and X = X = OH (net charge +4).
This journal is ꢁc the Owner Societies 2010
Phys. Chem. Chem. Phys., 2010, 12, 4072–4077 | 4073

View Article Online
Fig. 2 UV-visible spectra of the porphyrin nanotubes and the con-
42ꢀ
4+/5+
stituent monomers at pH 2: SnTPyP
(a); H
4
TPPS (b); and a
colloidal suspension of the nanotubes (c). Dashed lines correspond to
two of the laser excitation wavelengths used in the resonance Raman
experiments (near the monomeric Soret region at 413.1 nm and near
the J-aggregate band at 496.5 nm). Inset: TEM image of a PNT from
the preparation used for the Raman spectra.
determined by the ionic interactions of the porphyrins
confirms the formation of an ionic solid without additional
counter ions. Other intermolecular interactions including van
der Waals forces, hydrophobic interactions and hydrogen
bonding also contribute to the formation and stability of the
nanotubes.
B
UV-visible spectroscopy
Fig. 2 shows the position of the laser excitation wavelengths
relative to the absorption bands of the porphyrin nanotubes
(
Fig. 2, spectrum c) and the monomer-like Soret bands at
4
16 and 432 nm. These monomer-like bands of the nanotubes
+/5+
are at the same wavelengths as the Soret bands of SnTPyP
4
2ꢀ
(
Fig. 2, spectrum a) and freshly prepared H TPPS
4
(Fig. 2,
4
spectrum b). The monomer-like bands come from the nanotubes,
not from dissolved porphyrin monomers in the supernatant, as
they are also observed in the spectra of dry preparations of the
nanotubes as films on glass slides. Red-shifted from these
monomer-like bands of the tubes are two characteristic bands
1
,2
Fig. 3 Resonance Raman spectra of the porphyrin nanotubes (dark
2
for porphyrin J-aggregates at 496 nm and 714 nm (Fig. 2,
spectrum c). J-Aggregate bands also result from the coupling
of transition dipoles of adjacent porphyrin molecules in
ꢀ
magenta, blue) and the individual porphyrins, H
4+/5+
SnTPyP
4
TPPS
4
(green) and
(red) for: (a) laser excitation at the Soret region (413.1
and 406.7 nm) and (b) the J-aggregate band region (496.5 and 501.7 nm).
2
3–28
H
4
TPPS
4
self-aggregates.
Given that there are two types
Spectra of the individual porphyrins at 406.7 nm (a) and 501.7 nm
of porphyrins present in the nanotubes, the origin of the
exchange coupling emerges as a relevant question.
(
b) are not shown since they are very similar to those obtained at
13.1 nm and 496.5 nm, respectively. * indicates an emission line.
4
C
Resonance Raman spectroscopy
4
13.1 and 496.5 nm laser excitation wavelengths at resonance
with the monomer-like and J-aggregate absorption bands,
In order to elucidate the electronic coupling between the
porphyrins in the nanotubes, resonance Raman spectra of
the porphyrin nanotubes (Fig. 3) were obtained with the 496.5
respectively.
+
and 501.7 nm lines of an Ar laser for excitation near
Excitation at resonance with the monomer bands. Fig. 3a
shows the resonance Raman spectra obtained with excitation
near the monomer-like Soret bands of the nanotubes together
with Raman data for the reference porphyrins. Excitation at
406.7 and 413.1 nm is close to the 0–0 Soret peak of mono-
meric SnTPyP and the 0–1 vibrational satellite of the Soret
band of monomeric H TPPS (Fig. 2a). Thus, resonance
resonance with the J-aggregate band. In addition, spectra were
obtained near resonance with the monomer-like Soret bands
+
of the PNTs by using the 413.1 and 406.7 nm lines of a Kr
laser. For comparison, spectra of the individual constituent
porphyrins were also obtained at these wavelengths. Table 1
gives the frequencies of the Raman lines in the spectra taken at
4
4
4
074 | Phys. Chem. Chem. Phys., 2010, 12, 4072–4077
This journal is ꢁc the Owner Societies 2010

View Article Online
ꢀ1
Table 1 Frequencies (cm ) of the lines in the resonance Raman spectra of the monomeric porphyrins, the H
porphyrin nanotubes at 413.1 and 496.5 nm excitation wavelengths
4
TPPS
4
self-aggregate, and the
413.1 nm excitation
496.5 nm excitation
SnT
4
PyP
mono-H
4
TPPS
4
Nanotubes
SnT
4
PyP
agg-H TPPS
4
4
Nanotubes
Assignment
1636
—
1636
1594
—
—
n
n
10
1599
1567
1535
1498
1477
—
1593
1561
1536
—
1475
1427
—
1592
1561
1536
—
1475
1427
—
f
2
3
4
(CQC)
a
a
1546
B1530, 1535, 1547
—
—
—
1547
—
—
—
1366
n
n
n
—
—
—
1365
1374
1366
a
Shoulder.
26,29
formation of H TPPS aggregates.
enhancement of the Raman lines of both monomeric porphyrins
is expected and the lines observed for the individual porphyrins
are the monomeric forms. Note, however, that the aged
That is, agg-H TPPS
4 4
4
4
frequencies are found for the nanotubes even for resonance with
the monomer-like absorption bands of the PNTs. In contrast,
sample of H
4
TPPS
4
used in the Raman studies is a mixture
Soret excitation of the H
spectrum (mono-H TPPS
monomer-like H TPPS present in the nanotubes, but the
TPPS solution contains enough monomer so that it dominates
4
TPPS
4
solution gives the true monomer
of monomeric and aggregated forms resulting from the
4
4
). Apparently, there is very little
2ꢀ
well-known self-aggregation of H
conditions, self-aggregation of H
4
TPPS
4
. Under acidic
ꢀ
can occur by
4
4
2
4
4
TPPS
H
4
4
neutralization of its two positive charges at the porphyrin
core by the negative charges of the sulfonate groups at
the periphery. Nevertheless, only the monomeric species is
observed for Soret excitation.
the Raman spectra taken at resonance with the Soret band.
Excitation at resonance with the J-aggregate band. Raman
spectra obtained with laser wavelengths at resonance with the
J-aggregate band of the nanotubes provide insight into the
porphyrin organization and electronic coupling between
molecules within the nanostructure. Unexpectedly, resonance
Raman spectra of the nanotubes obtained at 496.5 or 501.7 nm
excitation (Fig. 3b) shows only the aggregated form of
For excitation near the monomer-like Soret bands of the
nanotubes, the spectrum is clearly dominated by the lines of
the Sn porphyrin, and the frequencies and relative intensities
of the SnTPyP lines are virtually unchanged from the monomer
spectra. The strongest lines of the nanotubes at 1366 and
ꢀ1
1
547 cm in the 413.1 nm spectrum correspond to n
4
and n
2
H
4
TPPS
4
. The PNT spectrum is almost identical to the spectra
2
6,29,30
of monomeric SnTPyP. The weak n10 line of monomeric
of H
4
TPPS
4
self-aggregates
taken at 496.5 nm, for
that is aggregated at the acidic
ꢀ1
SnTPyP also appears at 1636 cm in the porphyrin nanotube
spectrum. These SnTPyP spectral features also appear in the
spectrum of the nanotubes taken with 406.7 nm excitation.
The frequencies of these lines of SnTPyP subunits of the
nanotubes are thus very close to the frequencies obtained
from monomeric SnTPyP (see Table 1).
which the fraction of H
4
TPPS
4
pH of the experiment satisfies the resonance condition and is
preferentially enhanced. This suggests that there is some
similarity between the H
nanotubes and that of the H
Remarkably, the strong n
found in the nanotube spectra. H
TPPS
aggregation motif in the
4
4
TPPS
self-aggregates.
4
4
4
and n
2
lines of SnTPyP are not
The resonance Raman lines of the H TPPS species are also
4
4
TPPS
4
is present in only a
4
in evidence in the Soret resonance spectra, and the frequencies
are generally in good agreement with previous Raman studies
2-fold excess over the Sn porphyrin, so we would still expect to
see the strongest Raman lines of SnTPyP if they were enhanced.
2
6,29
of H
TPPS
the appearance of the phenyl line (n
shifted from the true monomer frequency of 1599 cm . Con-
tributions from other lines of H TPPS appear as shoulders on
the strong Sn-porphyrin lines (Fig. 3a). Specifically, H TPPS
4
TPPS
4
self-aggregates.
Lines attributable to the
H
4
4
subunits of the nanotubes are evident, primarily in
IV. Discussion
ꢀ
1
f
) at 1594 cm down-
ꢀ
1
Taken together, the resonance Raman results indicate that the
Sn porphyrins do not form J-aggregates within the tubes. If
the Sn porphyrin were in J-aggregates this should be evident in
the spectra obtained at resonance with the J-aggregate band,
but no Raman lines from the Sn porphyrin are observed. The
Raman data also indicate that H TPPS forms J-aggregates in
4
4
4
4
is evident in the increased intensity on the high frequency side
of n of the Sn porphyrin and the unresolved shoulder on the
4
ꢀ
1
low frequency side of n of SnTPyP at 1535 cm . Weaker lines
2
4
4
of the H
the nanotubes taken with 413.1 nm excitation. As expected,
the H TPPS contributions to the PNT spectrum subtly
4
TPPS
4
are not obviously observed in the spectrum of
the PNTs that are segregated from the SnTPyP molecules.
Thus, SnTPyP molecules do not participate in the H TPPS
4
4
4
4
J-aggregates nor do they form separate J-aggregates with a
UV-visible band near 500 nm.
decrease in intensity when the resonance condition for the
Soret bands changes with excitation at 406.7 nm (bottom
spectrum in Fig. 3a).
H TPPS is known to form J-aggregates in the form of
4
4
27,29
monolayer-thick tubes under acidic (pH 1) solution conditions.
H TPPS apparently also forms similar J-aggregates within
The frequencies of lines for H
at 413.1 nm excitation are altered from the monomer values
see Table 1). The frequencies for the PNTs are consistent with the
4 4 f
TPPS (e.g., n ) in the PNTs
4
4
the binary PNT. Segregation of the two porphyrins within the
tube structure might occur for several reasons including the
(
This journal is ꢁc the Owner Societies 2010
Phys. Chem. Chem. Phys., 2010, 12, 4072–4077 | 4075

View Article Online
disparity of charges present on the porphyrins and consequent
amounts of the two porphyrins in the tubes. Specifically, the
resonance Raman spectra indicate a segregated packing
arrangement of the two porphyrin components in the
2
tubes. The lamellar structure of the nanotubes seen in TEM
4
+/5+
higher charge on SnTPyP
and hence lower abundance
in the tubes implies it must interact with several adjacent
molecules of H TPPS to obtain local charge neutrality. For
images of the nanotubes trapped in end-on orientations
may also indicate that the nanotubes are composed of layers
of H TPPS , similar to the monolayer thick nanotubes of
4
4
this reason, the slipped stacking of the typical J-aggregate is
4
4
28
less likely to occur because the SnTPyP molecules must be
H
4
TPPS
4
observed in cryo-TEM studies of frozen suspensions,
more dispersed in the solid. In addition, the obligate axial
ꢀ
alternating with layers of SnTPyP acting as counter ions.
Resonance Raman spectroscopy provides a useful probe of
the molecular organization in the absence of detailed X-ray
structural information, which is often not attainable for
nanoscale structures. Regardless of the exact molecular organi-
zation, these nanotubes possess a unique internal structure
that may lead to novel optomechanical, photophysical, and
photochemical properties. In this regard, the delocalization of
excitons in the porphyrin nanotubes points to high exciton
mobilities, which are desirable for applications in areas such as
light-harvesting in solar energy systems, sensors, and electronics.
ligands (H O, OH ) of the Sn(IV) porphyrin is expected to
2
31–34
inhibit the porphyrin macrocycle stacking
and formation of
J-aggregates.
The formation of a binary structure with Sn porphyrin
molecules does alter the structure of the H TPPS aggregates
in the nanotubes versus H TPPS self-aggregates. Specifically,
4
4
4
4
the UV-visible absorption spectrum of the nanotubes indicates
shortening of the coherence length for the coupling of tran-
sition dipoles in the H TPPS stacks. For H TPPS self-
4
4
4
4
aggregates that form at low pH and high ionic strength, it is
estimated that transition dipoles of the molecules are exchange
4 4
coupled over 14 to 16 adjacent molecules in H TPPS slipped
2
5,29
Acknowledgements
stacks,
2
band.
giving rise to a very exchange-narrowed J-aggregate
Based on the broader J-aggregate UV-visible bands
7,28
Sandia is a multiprogram laboratory operated by Sandia
Corporation, a Lockheed Martin Company, for the United
States Department of Energy’s National Nuclear Security
Administration under Contract DEAC04-94AL85000. Research
supported by the Laboratory Directed Research and Develop-
ment program at Sandia National Laboratories and the
U.S. Department of Energy, Office of Basic Energy Sciences,
Division of Materials Sciences and Engineering. FLAD (Luso-
American Foundation, Portugal) is gratefully acknowledged
for financial support of this work.
4 4
of the nanotubes (Fig. 2) in comparison with the H TPPS
J-aggregate bands, the coupling must extend over fewer
molecules than for the self-aggregates. This could be the result
of Sn porphyrin molecules being interspersed within the
H TPPS slipped face-to-face stacks, thereby interrupting the
4
4
usual dipolar couplings that give rise to the J-aggregate bands.
In this regard, it is possible that two Sn porphyrins initially
form neutral hexamers or heptamers with 4 or 5 H
4
4
TPPS
4
+/5+
molecules, with the SnTPyP
molecules terminating the
dipole coupling. The initial formation of these neutral subunits
would largely neutralize the strong electrostatic interactions
between the porphyrin tectons, and subsequently, these puta-
tive neutral subunits could self-assemble into the nanotubes. A
similar subunit assembly mechanism has been proposed for
References
1
X. W. Li, Z. L. Zheng, M. Y. Han, Z. P. Chen and G. L. Zou,
J. Phys. Chem. B, 2007, 111, 4342.
2
9
H TPPS self-aggregates. Another attractive mechanism for
4
2 Z. C. Wang, C. J. Medforth and J. A. Shelnutt, J. Am. Chem. Soc.,
2004, 126, 15954.
4
the formation of the PNTs is the formation of a nucleating
monolayer tube of H TPPS followed by a disperse layer of
3
4
5
C. J. Medforth, Z. Wang, K. E. Martin, Y. Song, J. L. Jacobsen
and J. A. Shelnutt, Chem. Commun., 2009, 7261.
P. E. Burrows, S. R. Forrest, S. P. Sibley and M. E. Thompson,
Appl. Phys. Lett., 1996, 69, 2959.
C. Y. Liu, H. I. Pan, M. A. Fox and A. J. Bard, Science, 1993, 261,
897.
T. Malinski and Z. Taha, Nature, 1992, 358, 676.
4
4
SnTPyP molecules. Successive alternating monolayers of slip-
stacked H TPPS and more thinly spread SnTPyP would
4
4
explain the concentric fringes seen in the end-on TEM images
2
of the porphyrin nanotubes.
6
7
C. H. M. Mare
T. J. Schaafsma and F. H. P. M. Habraken, J. Appl. Phys., 1996,
0, 3381.
´
e, S. J. Roosendaal, T. J. Savenije, R. E. I. Schropp,
V. Conclusions
8
8
T. Hasobe, H. Oki, A. S. D. Sandanayaka and H. Murata, Chem.
Commun., 2008, 724.
2
4
ꢀ
homo-aggregates has
The precise structure of H TPPS
4
been the source of considerable debate. Recent structural
and spectroscopic studies strongly support the idea that
9 Z. Wang, L. E. Lybarger, W. Wang, C. J. Medforth, J. E. Miller
and J. A. Shelnutt, Nanotechnology, 2008, 19, 395604.
10 B. Ventura, F. Barigelletti, F. Lodato, D. L. Officer and
L. Flamigni, Phys. Chem. Chem. Phys., 2009, 11, 2166.
2
ꢀ
27–29
H
4
TPPS
4
forms monolayer thick nanotubes,
although
there is still no consensus on the structure of the monolayer
1
1 M. J. Ahrens, R. F. Kelley, Z. E. X. Dance and M. R. Wasielewski,
Phys. Chem. Chem. Phys., 2007, 9, 1469.
2
rolled sheets versus disk-like building blocks ). Signifi-
8
29
(
1
2 T. Jochum, C. M. Reddy, A. Eichho
H. Kalt, D. Moss and T. S. Balaban, Proc. Natl. Acad. Sci. U. S. A.,
008, 105, 12736.
¨
fer, G. Buth, J. Szmytkowski,
cantly, the internal arrangement of the porphyrin molecules
in the ionically self-assembled porphyrin nanotubes discussed
here must be more complex than for the monolayer nanotubes
2
13 S. Ganapathy, G. T. Oostergetel, P. K. Wawrzyniak, M. Reus,
A. Gomez Maqueo Chew, F. Buda, E. J. Boekema, D. A. Bryant,
A. R. Holzwarth and H. J. M. de Groot, Proc. Natl. Acad. Sci.
U. S. A., 2009, 106, 8525.
of the H
4 4 4 4
TPPS self-aggregates. Specifically, the H TPPS
component segregates and forms J-aggregates, while the other
4
+/5+
porphyrins (SnTPyP
) serve as counter ions without
1
4 T. Kojima, R. Harada, T. Nakanishi, K. Kaneko and
S. Fukuzumi, Chem. Mater., 2007, 19, 51.
forming separate J-aggregate stacks. In other words, the
4
076 | Phys. Chem. Chem. Phys., 2010, 12, 4072–4077
This journal is ꢁc the Owner Societies 2010

View Article Online
1
1
1
5 Z. Wang, K. J. Ho, C. J. Medforth and J. A. Shelnutt, Adv. Mater.,
006, 18, 2557.
6 J. H. Jang, K. S. Jeon, S. Oh, H. J. Kim, T. Asahi, H. Masuhara
and M. Yoon, Chem. Mater., 2007, 19, 1984.
7 K. Akatsuka, Y. Ebina, M. Muramatsu, T. Sato, H. Hester,
D. Kumaresan, R. H. Schmehl, T. Sasaki and M. A. Haga,
Langmuir, 2007, 23, 6730.
24 R. F. Pasternack, P. R. Huber, P. Boyd, G. Engasser,
L. Francesconi, E. Gibbs, P. Fasella, G. C. Venturo and L. de
C. Hinds, J. Am. Chem. Soc., 1972, 94, 4511.
25 A. S. R. Koti, J. Taneja and N. Periasamy, Chem. Phys. Lett.,
2003, 375, 171.
2
26 O. Ohno, Y. Kaizu and H. Kobayashi, J. Chem. Phys., 1993, 99,
4128.
1
1
2
2
8 Z. C. Wang, Z. Y. Li, C. J. Medforth and J. A. Shelnutt, J. Am.
Chem. Soc., 2007, 129, 2440.
9 Y. Kim, M. F. Mayer and S. C. Zimmerman, Angew. Chem., Int.
Ed., 2003, 42, 1121.
0 T. Yamaguchi, N. Ishii, K. Tashiro and T. Aida, J. Am. Chem.
Soc., 2003, 125, 13934.
1 K. Hosomizu, M. Oodoi, T. Umeyama, Y. Matano, K. Yoshida,
S. Isoda, M. Isosomppi, N. V. Tkachenko, H. Lemmetyinen and
H. Imahori, J. Phys. Chem. B, 2008, 112, 16517.
2 Z. C. Wang, C. J. Medforth and J. A. Shelnutt, J. Am. Chem. Soc.,
27 A. D. Schwab, D. E. Smith, C. S. Rich, E. R. Young, W. F. Smith
and J. C. de Paula, J. Phys. Chem. B, 2003, 107, 11339.
28 S. M. Vlaming, R. Augulis, M. C. A. Stuart, J. Knoester and P. H.
M. van Loosdrecht, J. Phys. Chem. B, 2009, 113, 2273.
29 B. A. Friesen, K. R. A. Nishida, J. L. McHale and U. Mazur,
J. Phys. Chem. C, 2009, 113, 1709.
30 D.-M. Chen, T. He, D.-F. Cong, Y.-H. Zhang and F.-C. Liu,
J. Phys. Chem. A, 2001, 105, 3981.
31 J. A. Shelnutt, Inorg. Chem., 1983, 22, 2535.
32 J. A. Shelnutt, M. M. Dobry and J. D. Satterlee, J. Phys. Chem.,
1984, 88, 4980.
2
2
2004, 126, 16720.
3 E. B. Fleischer, J. M. Palmer, T. S. Srivastava and A. Chatterjee,
J. Am. Chem. Soc., 1971, 93, 3162.
33 J. A. Shelnutt, J. Am. Chem. Soc., 1981, 103, 4275.
34 J. A. Shelnutt, J. Am. Chem. Soc., 1983, 105, 7179.
This journal is ꢁc the Owner Societies 2010
Phys. Chem. Chem. Phys., 2010, 12, 4072–4077 | 4077