Donor-acceptor biomorphs from the ionic self-assembly of porphyrins

Article abstract of DOI:10.1021/ja102194x

Microscale four-leaf clover-shaped structures are formed by self-assembly of anionic and cationic porphyrins. Depending on the metal complexed in the porphyrin macrocycle (Zn or Sn), the porphyrin cores are either electron donors or electron acceptors. Al

Full text of DOI:10.1021/ja102194x

Published on Web 05/14/2010
Donor-Acceptor Biomorphs from the Ionic Self-Assembly of
Porphyrins
Kathleen E. Martin,† Zhongchun Wang,† Tito Busani,‡ Robert M. Garcia,†
Zhu Chen,† Yingbing Jiang,†,§ Yujiang Song,† John L. Jacobsen,| Tony T. Vu,|
Neil E. Schore,| Brian S. Swartzentruber,† Craig J. Medforth,†,§,| and
John A. Shelnutt*,†,⊥
AdVanced Materials Laboratory and Center for Integrated Nano Technologies, Sandia National
Laboratories, Albuquerque, New Mexico 87185-1349, UniVersidade NoVa de Lisboa at
CENIMAT/I3N, Departamento de Ciência dos Materials, Faculdade de Ciências e Tecnologia,
FCT, UniVersidade NoVa de Lisboa and CEMOP-UNINOVA, 2829-516 Caparica, Portugal,
Department of Chemical & Nuclear Engineering, UniVersity of New Mexico, Albuquerque, New
Mexico 87106, Department of Chemistry, UniVersity of California, DaVis, California 95616, and
Department of Chemistry, UniVersity of Georgia, Athens, Georgia 30602
Received March 15, 2010; E-mail: jasheln@unm.edu
Abstract: Microscale four-leaf clover-shaped structures are formed by self-assembly of anionic and cationic
porphyrins. Depending on the metal complexed in the porphyrin macrocycle (Zn or Sn), the porphyrin cores
are either electron donors or electron acceptors. All four combinations of these two metals in cationic tetra(N-
ethanol-4-pyridinium)porphyrin and anionic tetra(sulfonatophenyl)porphyrin result in related cloverlike
structures with similar crystalline packing indicated by X-ray diffraction patterns. The clover morphology
transforms as the ionic strength and temperature of the self-assembly reaction are increased, but the
structures maintain 4-fold symmetry. The ability to alter the electronic and photophysical properties of these
solids (e.g., by altering the metals in the porphyrins) and to vary cooperative interactions between the
porphyrin subunits raises the possibility of producing binary solids with tunable functionality. For example,
we show that the clovers derived from anionic Zn porphyrins (electron donors) and cationic Sn porphyrins
(electron acceptors) are photoconductors, but when the metals are reversed in the two porphyrins, the
resulting clovers are insulators.
Introduction
true when the subunits of these nanostructures are the chlorophyll-
related porphyrins.9-23 In fact, molecules of bacteriochlorophyllsa
Nanostructures self-assembled from organic molecules are
of great interest because of their potential applications in areas
such as organic solar cells and electronics, sensors, and
catalysis.1-8 In particular, these organic nanostructures offer new
opportunities for mimicking the processes that occur in biologi-
cal photosynthesis to produce fuels or to produce electrical
energy in organic solar cells, and this possibility is especially
porphyrin derivativesassemble to form the nanorods in the
chlorosomes of green bacteria, and these chlorosomal rods are
(9) Wang, Z.; Ho, K. J.; Medforth, C. J.; Shelnutt, J. A. AdV. Mater. 2006,
18, 2557.
(10) Wang, Z.; Li, Z.; Medforth, C. J.; Shelnutt, J. A. J. Am. Chem. Soc.
2007, 129, 2440.
(11) Wang, Z.; Lybarger, L. E.; Wang, W.; Medforth, C. J.; Miller, J. E.;
Shelnutt, J. A. Nanotechnology 2008, 19, 395604.
(12) Wang, Z.; Medforth, C. J.; Shelnutt, J. A. J. Am. Chem. Soc. 2004,
126, 16720.
† Sandia National Laboratories.
‡ Universidade Nova de Lisboa at CENIMAT/I3N.
§ University of New Mexico.
| University of California.
(13) Wang, Z.; Medforth, C. J.; Shelnutt, J. A. J. Am. Chem. Soc. 2004,
126, 15954.
⊥ University of Georgia.
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8194 J. AM. CHEM. SOC. 2010, 132, 8194–8201
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Porphyrin Biomorphs
A R T I C L E S
the largest and most efficient harvesters of light known.24-29 It
is of interest to examine the possibility of producing similar
self-assembled porphyrin-based structures from synthetic por-
phyrins for the purpose of generating efficient light-harvesting
components of artificial photosynthetic systems, dye-sensitized
solar cells, and organic photovoltaics.
We and others have been exploring the self-assembly of
porphyrin nanostructures by various methods including repre-
cipitation, coordination polymerization, surfactant-induced self-
assembly, and ionic self-assembly.9-23,30-32 Ionic self-assem-
bly33 is particularly interesting; while the properties of these
binary porphyrin nanomaterials are relatively unknown, they
potentially offer unique and tunable characteristics that result
from the cooperative interactions between the two types of
porphyrin cores in the solid. Herein, we describe some
extraordinary microscale-sized porphyrin biomorphs (nonbio-
logical structures that are shaped like living organisms) obtained
by the self-assembly of two oppositely charged porphyrin ions
(tectons). For the Zn(II) and Sn(IV) complexes investigated,
the overall shape, size, and crystalline structures of these
biomorphs are largely independent of which metals are in the
two porphyrins. However, the metal-centered interactions play
a dominant role in determining the electronic characteristics of
the porphyrin macrocycles (e.g., electron donor versus electron
acceptor). This combination of factors potentially allows a high
degree of control over the cooperative interactions between the
porphyrin cores (e.g., charge transfer) and consequently the
functionality (e.g., charge separation and migration) of the or-
ganic solid. We expect that these new materials will have
applications in solar energy technologies and organic electronics
and optoelectronics.
Figure 1. Structures of (a) tin(IV) tetrakis(4-sulfonatophenyl)porphyrin
(SnTPPS4-) and (b) zinc(II) tetrakis(N-ethanol-4-pyridinium)porphyrin
[ZnT(N-EtOH-4-Py)P4+].
Clover Characterization. The composition of the clovers was
determined by calculating the amount of each porphyrin removed
from solution and incorporated into the product. Optical absorption
spectra of the supernatants were measured after centrifugation to
remove the precipitated product, and porphyrin concentrations were
calculated from extinction coefficients measured using diluted stock
solutions of the porphyrin monomers.
Samples for imaging were prepared by pipetting 50 µL of the
precipitate layer onto Si wafers (scanning electron microscopy,
SEM) or p-type Si wafers (atomic force microscopy, AFM). Excess
solvent was wicked away after 10 min using a Kimwipe tissue,
and the wafer was air-dried. SEM imaging was performed on a
Hitachi S-5200 Nano Scanning Electron Microscope operating at
1-2 keV. AFM measurements were carried out on a Nanoscope
III Multimode AFM (Digital Instruments, United States) in contact
mode using Si cantilevers.
Samples for X-ray diffraction (XRD) measurements were
prepared either by depositing the powder (dried by mild heating)
onto glass slides (VWR) or Si wafers or by depositing a drop of a
suspension of the clovers onto the glass slide and allowing it to
dry in air. XRD spectra were recorded on a Siemens D500
diffractometer using Ni-filtered Cu KR radiation with λ ) 1.5418
Å in θ-2θ scan mode using a step size of 0.05° and a 90 s step
time.
Salt-, Temperature-, and Time-Dependent Studies. The self-
assembly reaction of ZnT(N-EtOH-4-Py)P4+ and SnTPPS4- was
repeated using modified versions of the procedure described above.
All reactions were carried out by mixing 1 mL aliquots in a 4 mL
glass vial. For the salt-dependent studies, sodium chloride (99+%,
Aldrich) was added to the 210 µM porphyrin stock solutions to
produce salt concentrations of 1, 2, 5, 10, 15, or 20 mM. The saline
stock solutions were then added to a 4 mL glass vial, mixed by
shaking for 30 s, and left undisturbed and shielded from light for
24 h. For the temperature-dependent studies, 1 mL aliquots were
equilibrated at the required temperature (10, 23, 60, or 80 °C) for
1 h, rapidly mixed, and then returned to the temperature-controlled
environment for 24 (10 or 23 °C) or 4 h (60 or 80 °C). In the
time-dependent study, aliquots of the stock solutions were mixed
by shaking the vial for 5 s, and 50 µL portions were removed and
placed onto Si wafers after 0.5, 5, 10, 20, 30, 60, and 120 min.
The excess liquid was immediately wicked away, the wafer was
washed with 2 drops of NANOpure water, excess liquid was again
removed, and the wafer was allowed to air dry.
Materials and Methods
Synthesis of Clovers. ZnIITPPS4- and SnIVTPPS4- were obtained
from Frontier Scientific. SnIVT(N-EtOH-4-Py)P4+ was purchased
from Ambinter. ZnIIT(N-EtOH-4-Py)P4+ was prepared by dissolving
H2T(N-EtOH-4-Py)P4+ (58 mg) (Frontier Scientific) in methanol
(5 mL), adding Zn(OAc)2 (29 mg), and stirring the solution for
1 h. Chloroform (90 mL) was then added, and a stream of air was
passed over the solution until a green film developed on the surface.
The film was removed using a pipet and dried under vacuum for
24 h. The purity of the porphyrins was confirmed by proton NMR
spectroscopy of D2O solutions of the materials. Stock solutions of
the porphyrins (210 µM) were prepared in NANOpure water and
used in the self-assembly reactions. In a typical self-assembly
reaction, 10 mL aliquots of stock solutions were added to a 20 mL
glass vial, mixed by shaking for 30 s, and left undisturbed and
shielded from light for 2 days. The clovers were obtained as a dark
green precipitate at the bottom of the glass vial.
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Results and Discussion
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J. Bacteriol. 1981, 147, 1021.
Zinc(II) or tin(IV) was chosen as the metal for the porphyrins,
as these give electron donor or acceptor porphyrin macrocycles,
respectively (see below). Self-assembly of SnTPPS4- and
ZnT(N-EtOH-4-Py)P4+ (Figure 1) produced a dark green
precipitate. SEM images of the material (Figure 2) reveal
remarkable “four-leaf clover”-like structures. The clovers are
approximately square with average edge lengths ranging from
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Shelnutt, J. A. Chem. Commun. 2009, 7261.
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Figure 2. Low magnification SEM image of the Sn/Zn porphyrin clovers. Inset: High magnification image of a clover in an edge-on orientation.
2.0 to 10.0 µm (as determined by measuring randomly selected
clovers from several images and preparations). The clovers are
a few hundred nanometers thick, as determined from SEM
images of clovers trapped edge on to the electron beam (see
Figure 2, inset). Four leaves with well-defined “veins” (espe-
cially the midrib vein) are readily apparent, as well as a central
“stem” or “stalk”. Complex nanoscale features are visible in
high magnification images shown in the inset of Figure 2 and
in Figure S1 of the Supporting Information.
The formation of inorganic biomorphic structures during the
bottom-up synthesis of nanomaterials is well-known in the
literature.34 However, this is probably the most striking example
of a porphyrin-related biomorph. The most closely related
biomorphic structure previously observed is the nanoflower
obtained by vapor deposition of a phthalocyanine.20 There have
been no previously reported biomorphs composed of a binary
porphyrin solid like those described here.
Analysis of the UV-visible spectra of the supernatant
remaining after the synthesis of the clovers using equimolar
porphyrin concentrations shows that greater than 95% of each
porphyrin present initially has reacted. This indicates a 1:1 ratio
of the porphyrin ions in the product and is expected9,13 for an
ionic solid obtained from the self-assembly of porphyrin ions
that have equal but opposite charges. There is also no evidence
from EDX that significant amounts of small counterions (e.g.,
Na+ or Cl-) are present in the clovers.
Figure S2 of the Supporting Information shows SEM images
of the clovers after different reaction times. The images show
that the cloverlike morphology is already established after only
30 s, with nascent square clovers as large as 500 nm on an
edge. In agreement with this result, cloudiness of the solution
is observed immediately after mixing the porphyrin solutions.
Mature clovers are observed after only 5 min. These findings
suggest rapid growth by diffusion-limited aggregation from an
initially supersaturated binary porphyrin solution (100 µM).
Consistent with this interpretation, dissolution of the cloverlike
dendrites in water produces very low (<1 µM) concentrations
of SnTPPS4- or ZnT(N-EtOH-4-Py)P4+ monomers.
SEM images in Figure 3 show that ionic strength (i.e., NaCl
added to the reactant solutions) influences the morphology of
the clovers. Specifically, a smooth four-pointed star shape is
seen at the highest salt concentration (20 mM) in contrast to
the complex four-leaf clover motif shown in Figure 2. The salt
dependence of the morphology might be explained by a variation
in the diffusion-limited rate of growth. Increasing the ionic
strength shields the charged groups on the anionic and cationic
porphyrins, likely slowing the ionic self-assembly process. We
also note an increase in the diversity of morphologies obtained
as the salt concentration increases (Figure S3 of the Supporting
Information).
The temperature at which self-assembly takes place has an
even more pronounced influence on morphology than the
changes in ionic strength. Figure 4 shows SEM images of the
structures obtained for growth at four temperatures between 10
and 80 °C. Notice that the structures retain the basic 4-fold
symmetry of the clovers but have less pronounced nanoscale
features as the growth temperature is increased, until at 80 °C
the structures have transformed into a smooth and podlike shape.
Just as for high ionic strength, high temperature might influence
the diffusion-limited self-assembly of these dendritic structures,
changing the morphology. Varying the growth temperature is
also expected to change the solubility of the ionic solid, and
this might also influence the diffusion-limited growth rate.
Greater diversity is also seen in the structures obtained at high
temperatures (Figure S4 of the Supporting Information), as was
noted for structures obtained at increased ionic strength (see
Figure S3 of the Supporting Information).
Microclovers of the same general shape and size are obtained
by switching the metals in the two porphyrins (Figure 5b), that
is, using ZnTPPS4- and SnT(N-EtOH-4-Py)P4+ to prepare Zn/
Sn clovers. Moreover, cloverlike structures are even obtained
when either both tectons are tin porphyrins (Sn/Sn clovers,
Figure 5c) or both are zinc porphyrins (Zn/Zn clovers, Figure
5d). This is consistent with the interactions between the
porphyrin ions being the dominant factor determining the struc-
ture. That is, intermolecular interactions arising from the metals
(Zn, Sn) in the porphyrin play only a minor role, so that
cloverlike structures are produced regardless of the metals
contained in the porphyrins. Other six-coordinate metallopor-
(34) Ruiz, J. M. G.; Carnerup, A.; Christy, A. G.; Welham, N. J.; Hyde,
S. T. Astrobiology 2002, 2, 353.
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Figure 3. Growth of SnTPPS4- and ZnT(N-EtOH-4-Py)P4+ clovers at 23 °C with differing concentrations of added NaCl. SEM images obtained for (a) 5,
(b) 10, (c) 15, and (d) 20 mM added salt. Excess salt has been washed away for these images.
Figure 4. Growth temperature dependence of SnTPPS4- and ZnT(N-EtOH-4-Py)P4+ structures. SEM images obtained for growth at 10 (blue), 23 (green),
60 (gold), and 80 °C (pink).
phyrins can also be used to generate cloverlike structures. Ionic
self-assembly of ZnTPPS4- with Co(III)T(N-EtOH-4-Py)P4+ or
Mn(III)TPPS4- with ZnT(N-EtOH-4-Py)P4+ (Figure S5 of the
Supporting Information) both produce cloverlike structures. In
contrast, ionic self-assembly reactions where one of the met-
alloporphyrins is four-coordinate [e.g., Ni(II)TPPS4- with
ZnT(N-EtOH-4-Py)P4+ or Cu(II)TPPS4- with SnT(N-EtOH-4-
Py)P4+] have not yielded clovers or other discrete nanostructures.
Further studies are underway to determine the range of porphyrin
metal complexes and axial ligands that produces clovers. In
addition, we are investigating what other morphologies can be
obtained with different combinations of porphyrin ions and
coordinated metals.
spite of these minor differences, the strong similarities in the
dendritic features and basic 4-fold symmetry suggest that they
might all share a common molecular packing structure.
Single crystals suitable for X-ray structure determination of
the solids that form these cloverlike structures have not yet been
obtained. However, XRD measurements of the clovers supported
on glass slides or Si wafers (Figure 6) indicate that all of the
clovers shown in Figure 5 have similar crystalline structures.
Specifically, the reflections from the clovers occur at almost
the same angles and have roughly similar line widths. The
largest distance (smallest angle) common to all of the clovers
is 1.1 nm. On the other hand, the intensities of the reflections
vary greatly. It remains to be determined whether each clover
is a single crystal despite its complex shape, as is often the
case for snowflakes, which also form by a diffusion-limited
growth process.
The most obvious difference among the structures obtained
with the Zn and Sn complexes of TPPS and T(N-EtOH-4-Py)P
is that only clovers obtained with SnTPPS4- (Figure 5a,c) have
pronounced central stems. In addition, the clovers obtained with
ZnTPPS4- (Figure 5b,d) have more angular leaf structures. In
The UV-visible absorption spectrum of the Zn/Sn clovers
is shown in Figure 7. The spectrum shows evidence of one or
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Martin et al.
Figure 5. Cloverlike structures obtained from (a) SnTPPS4- and ZnT(N-EtOH-4-Py)P4+, (b) ZnTPPS4- and SnT(N-EtOH-4-Py)P4+, (c) SnTPPS4- and
SnT(N-EtOH-4-Py)P4+, and (d) ZnTPPS4- and ZnT(N-EtOH-4-Py)P4+
.
Figure 7. UV-visible absorption spectra of a suspension of the Zn/Sn
clovers (green) and the constituent porphyrins ZnTPPS4- (blue) and SnT(N-
EtOH-4-Py)P4+ (red). The J-aggregate band seen near 500 nm for the Zn/
Sn clovers is indicative of exciton delocalization over multiple porphyrins.
monomer-like Q bands. Porphyrin J-aggregates typically have
molecules in a slipped stacked configuration, and several
porphyrins of the same type in the stacks are coupled electroni-
cally,35 that is, excitons are delocalized over at least several
molecules. To date, attempts to obtain UV-visible spectra of
the Sn/Zn clovers have not produced reliable results, probably
because their greater thickness relative to the Zn/Sn clovers
insures almost complete extinction of visible light. Nevertheless,
the similarity of the XRD patterns would suggest that a similar
stacking of the porphyrins and exciton delocalization occurs for
the Sn/Zn clovers.
Conductive AFM studies were undertaken for the clovers
composed of equal proportions of electron donor molecules (Zn
porphyrins) and electron acceptor molecules (Sn porphyrins).
Figure 8a and Figure S6a of the Supporting Information show
AFM images of the Zn/Sn clovers on p-doped conductive Si
substrates. The tunneling currents from the surface through the
Zn/Sn clovers to a point on the top surface, in the absence and
presence of illumination by visible light, are shown in Figure
8b,c. A more than 5-fold increase in current (at -12 V) between
the substrate and the tip is observed upon illumination of the
Figure 6. (a) XRD data for Sn/Zn clovers [SnTPPS4- and ZnT(N-EtOH-
4-Py)P4+] (green) and for the Zn/Sn clovers (dark blue) on glass slides.
Inset: Sn/Zn (green) and Zn/Sn (dark blue) clovers on the Si wafer and the
Si substrate (gray); the stars indicate reflections from the clovers. (b) XRD
data for the Zn/Zn clovers (blue) and Sn/Sn clovers (pink) on glass (inset).
All samples are dried as follows: Sn/Zn, Sn/Sn, and Zn/Zn by mild heating
and Zn/Sn at room temperature.
two J-aggregate bands near 490 nm originating from the Soret
transitions, and the long wavelength tail above 620 nm is
suggestive of further J-aggregate bands associated with the
(35) McRae, E. G.; Kasha, M. J. Chem. Phys. 1958, 28, 721.
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A R T I C L E S
Figure 8. AFM image (5 µm × 5 µm) of a ZnTPPS4- and SnT(N-EtOH-4-Py)P4+ clover (a) and TUNA current-voltage curve for conduction to the AFM
Ti/Pt tip at a point on the clover surface through the clover from a p-doped conductive Si substrate with 20 ohm cm resistivity in the dark (b) and when
illuminated with the cold incandescent lamp of the AFM instrument (c).
Figure 9. Schematic showing the approximate valence and conduction band energies of Sn(IV) and Zn(II) porphyrins, based on the redox potentials of the
metallooctaethylporphyrin ground and lowest excited states.37,38 An extended version showing the porphyrin potentials in relation to common inorganic
semiconductors is provided in Figure S7 of the Supporting Information.
Zn/Sn clover. Conducting AFM measurements on other Zn/Sn
clovers shows similar changes in current upon illumination (see
Figure S6b of the Supporting Information) but do not show the
increase in dark current seen in Figure 8b that is probably due
to stray light from an alignment laser. Remarkably, photocon-
ductivity has not been observed for the Sn/Zn clovers (Figure
S6c in the Supporting Information), despite the similarity in
the crystal structures indicated by the XRD data.
transfer excitons)36 occurring in the illuminated solid. Sn(IV)
porphyrins are considered acceptors, and Zn(II) porphyrins are
considered donors because of their respective redox potentials
(Figure 9 and Figure S7 of the Supporting Information). For
both the ground state and the (triplet) excited states, the
potentials for related redox processes are almost 1.0 V more
negative for Zn(II) than for Sn(IV) porphyrins (based on
(36) Knoester, J.; Agranovich, V. M. In Electronic Excitations in Organic
Based Nanostructures; Agranovich, V. M., Bassani, G. F., Eds.;
Elsevier Academic Press: Amsterdam, 2003; Chapter 1.
The photoconductivity of the Zn/Sn clovers can be understood
in terms of the energetics of the molecular excitations (charge-
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Martin et al.
published potentials SnOEP and ZnOEP complexes).37 The
states connected by solid black lines in Figure 9 indicate those
relevant for photoexcitation of the clovers containing Zn and
Sn porphyrins. The excited triplet state of the Zn porphyrin has
a more negative potential than, and can easily reduce, the Sn
porphyrin in either its ground or its excited state. Reduction
moves an electron to an adjacent Sn porphyrin and leaves a
hole at the Zn porphyrin. The electron (anion) and hole (cation)
can then stay bound, but having the hole and electron on
different porphyrin molecules increases their separation distance
and the probability of free charge-carrier formation. The
reduction of the Sn porphyrin in its excited state by the ground-
state Zn porphyrin can also occur and leads to a similar charge-
separated species.
Another consideration for electron transfer in the clovers is
the shift in the energy levels in Figure 9 (shown for SnOEP
and ZnOEP)37,38 that are present for the porphyrin having the
extremely electron-withdrawing pyridinium substituents. [There
is a +500 mV shift in the ring reduction potential for T(N-Me-
4-Py)P4+, the methyl analogue of T(N-EtOH-4-Py)P4+, relative
to TPPS4-.38] For the photoconductive Zn/Sn clovers, the levels
shown in Figure 9 should be shifted down an additional 500
mV for SnT(N-EtOH-4-Py)P4+ because of the effect of the
pyridinium substituents. This further enhances the electron
acceptor ability of the Sn porphyrin and favors electron transfer.
In contrast, for the Sn/Zn clovers, it is the energy levels of the
donor ZnT(N-EtOH-4-Py)P4+ that shift by +500 mV, decreasing
the potential difference between the donor and the acceptor and
the driving force for electron transfer.
Finally, models of conductivity in classical donor-acceptor
solids such as TTF-TCNQ39-41 typically invoke the stacking
pattern of the molecules:36 A segregated structure (Figure 10a)
generally results in conductivity or photoconductivity, whereas
interleaved stacking (Figure 10b) usually leads to electrical
insulators. Recent resonance Raman studies of binary porphyrin
nanotubes13 have indicated an internal structure where the two
types of porphyrins (H4TPPS2- and SnTPyP4+/5+) are segregated
in cylindrical layers.42 While further studies are needed to
definitively determine the internal stacking arrangement in the
clovers, we note that the slipped stacks of π-π interacting
porphyrins, suggested by the optical spectrum in Figure 7, would
form four columns of the ionic substituents of the porphyrins,
leading to electrostatic π-system channels in the clovers. (These
electrostatic channels can be visualized as the pink or blue
columns in Figure 10a having either positive or negative charges
at the corners of the square prisms representing the porphyrins.)
These columns of ions may also affect the photoconductivity.
The photoexcitation of the Zn/Sn clovers results in electrons
on SnT(N-EtOH-4-Py)P4+ in columns of positive charges of
the pyridinium groups. Conversely, the holes remaining on
ZnTPPS4- see channels formed by the negative charges of the
sulfonate groups. In contrast, the Sn/Zn clovers, which are not
Figure 10. Idealized packing for donor-acceptor solids: (a) segregated
stacking and (b) interleaved stacking.36 As drawn, the blocks in panel a
would represent H-aggregates, so the stacks must actually be tilted to
represent the slipped stacks of J-aggregates. Slipped stacks would still
produce the columns of charges at the corners as discussed in the text.
photoconductive (see Figure S6c of the Supporting Information),
have the holes on the positively charged porphyrins and the
electrons on the negatively charged porphyrinssa condition that
may be energetically unfavorable and conducive to electron-hole
recombination.
Conclusions
The self-assembled porphyrin structures reported here are
unusual not only because of their elaborate dendritic morphol-
ogies but also because they provide examples of self-assembled
organic materials where the structure is largely independent of
the electron donor or acceptor nature of the constituent
molecules. This raises the possibility of making binary solids
with tunable functional characteristics. To reflect the potential
for cooperative interactions between the component porphyrins,
we refer to this class of organic materials as cooperatiVe binary
ionic (CBI) solids. Such materials may be of interest for solar
energy and other applications, as they provide new possibilities
for varying important photophysical and electronic properties,
as well as the possibility of emergent properties. Other CBI
materials self-assembled from donor and acceptor porphyrins
or other combinations of cooperative functionality (e.g., light-
harvesting porphyrins and catalytic porphyrins) have recently
been prepared in our laboratories. An example to be reported
shortly is the cloverlike structure shown in Figure S5a of the
Supporting Information, produced from ZnTPPS4- and CoIIIT(N-
EtOH-4-Py)P4+. This combines a Zn-porphyrin electron donor
and a Co-porphyrin catalyst for CO2 reduction43-45 to produce
a nanostructure potentially capable of photoassisted CO2
conversion to CO.
(37) Davis, D. G. In The Porphyrins; Dolphin, D., Ed.; Academic Press:
New York, 1978; Vol. V, Chapter 4.
(38) Kadish, K. M.; Royal, G.; Van Caemelbecke, E.; Gueletti, L. In The
Porphyrin Handbook; Kadish, K. M., Smith, K. M., Guilard, R., Eds.;
Academic Press: San Diego, CA, 2000; Vol. 9, Chapter 59.
(39) Cavalione, F.; Clementi, E. J. Chem. Phys. 1975, 63, 4304.
(40) Alves, H.; Molinari, A. S.; Xie, H.; Morpurgo, A. F. Nat. Mater. 2008,
7, 574.
Acknowledgment. Sandia is a multiprogram laboratory operated
by Sandia Corporation, a Lockheed Martin Company, for the United
(41) Fraxedas, J. Molecular Organic Materials: from Molecules to Crystal-
line Solids; Cambridge University Press: Cambridge, 2006.
(42) Franco, R.; Jacobsen, J. L.; Wang, H.; Wang, Z.; Istvan, K.; Schore,
N. E.; Medforth, C. J.; Shelnutt, J. A. Phys. Chem. Chem. Phys. 2010,
12, 4072.
(43) Furuya, N.; Koide, S. Electrochim. Acta 1991, 36, 1309.
(44) Furuya, N.; Matsui, K. J. Electroanal. Chem. 1989, 271, 181.
(45) Magdesieva, T. V.; Yamamoto, T.; Tryk, D. A.; Fujishima, A. J.
Electrochem. Soc. 2002, 149, D89.
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8200 J. AM. CHEM. SOC. VOL. 132, NO. 23, 2010

Porphyrin Biomorphs
A R T I C L E S
States Department of Energy’s National Nuclear Security Admin-
istration under Contract DEAC04-94AL85000.
biomorphs, TEM image and MnTPPS/ZnT(N-EtOH-4-Py)P
biomorphs, AFM image of Sn/Zn clover and photoconductivity
data for Sn/Zn and Zn/Sn clovers, and an extended illustration
of the energy levels of Zn and Sn porphyrins and common
semiconductors. This material is available free of charge via
the Internet at http://pubs.acs.org.
Supporting Information Available: High magnification SEM
images of Sn/Zn clovers, SEM images showing the development
of the clovers during growth, SEM images showing morpho-
logical diversity for clovers grown at high ionic strengths and
high temperatures, SEM image of ZnTPPS/CoT(N-EtOH-4-Py)P
JA102194X
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