Effect of secondary substituent on the physical properties, crystal structures, and nanoparticle morphologies of (porphyrin)Sn(OH)2: Diversity enabled via synthetic manipulations

Article abstract of DOI:10.1039/b804629h

A highly efficient porphyrin synthesis facilitates a systematic investigation of the effects that secondary substituents have on the physical properties, crystal structures, and nanoparticle morphologies of amphiphilic (porphyrin)Sn(OH)₂. The Royal Society of Chemistry.

Full text of DOI:10.1039/b804629h

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Effect of secondary substituent on the physical properties, crystal structures,
and nanoparticle morphologies of (porphyrin)Sn(OH)2: diversity enabled via
synthetic manipulations†
*
Suk Joong Lee, Rebecca A. Jensen, Christos D. Malliakas, Mercouri G. Kanatzidis, Joseph T. Hupp
*
and SonBinh T. Nguyen
Received 18th March 2008, Accepted 6th June 2008
First published as an Advance Article on the web 8th July 2008
DOI: 10.1039/b804629h
assistance of surfactants, into macroscopic objects. In this commu-
nication, we focus on a strategy for quickly generating porphyrin
nanocrystals displaying diverse morphologies from easily accessible
(porphyrin)Sn(OH)2 molecular precursors. Our premise is that simple
changes in the van der Waals surfaces of the (porphyrin)Sn(OH)2
building blocks, tuned via secondary substituents, can significantly
affect the morphologies of the crystals and nanocrystals grown from
these molecules.4a
A highly efficient porphyrin synthesis facilitates a systematic
investigation of the effects that secondary substituents have on the
physical properties, crystal structures, and nanoparticle morphol-
ogies of amphiphilic (porphyrin)Sn(OH)2.
The enormous potential that metalloporphyrins possess in catalysis,
photochemistry, sensing, and as optical devices has made them
favorite building blocks in the emerging field of molecular materials.1
With attractive structural features such as large bulk, rigid planarity,
and a highly conjugated framework that can be readily modified with
a variety of functional groups, porphyrins can be assembled into
supra- and super-structures in a relatively facile manner.2 Indeed,
porphyrin building blocks have been used extensively in the
construction of nano-, micro-, and macroscopic structures,3 including
nanomaterials4 using p–p interactions, electrostatic interactions, and
metal coordination with appropriate choice of primary substituents.
Largely overlooked has been the manipulation of secondary
substituents (i.e., substituents that are not directly bonded to the
porphyrin ring) to engender van der Waals interactions beyond the
immediate porphyrin ring. Herein, we demonstrate the potential for
using such substituents to influence the physical properties, crystal
structures, and nanocrystal morphologies of materials based on
amphiphilic (porphyrin)Sn(OH)2. This endeavor was facilitated by
applying the Lindsey dipyrromethane methodology5 to quickly
generate secondary substituent diversity on a porphyrin framework.
The barriers most often encountered in systematic investigations of
porphyrin-based materials are synthetic in nature. Such studies,
especially those concerning crystalline phases, often require macro-
scopic quantities of materials; yet porphyrins are typically obtained
only in low yield and large crystals of porphyrins are difficult to grow.
The latter challenge can be addressed by growing porphyrin nano-
crystals4 and assembling them into macroscopic objects. In this spirit,
we have recently demonstrated the facile hierarchical assembly of
crystalline porphyrin nanorods into micron-size prisms.4a Under
highly agitated conditions, [5,15-bis(pyridyl)-10,20-diarylporphyrin]
Sn(OH)2, possessing both hydrogen-bond-acceptor and -donor
functionalities, proved to be an ideal precursor for growing amphi-
philic nanocrystals that could subsequently be assembled, with the
The condensation reaction between 4-pyridylaldehyde and the
corresponding meso-aryldipyrrolemethane affords 5,15-bis(pyridyl)-
10,20-diarylporphyrin in excellent yield (26–35%) (Scheme 1).
Subsequent metallation with SnCl2 and hydrolysis with K2CO3
readily affords the dihydroxytin derivatives 1–3. In contrast to
traditional multistep methods for dipyridyl porphyrin synthesis
where the dipyridyl moieties are elaborated through cross-coupling
reactions,6 the overall yield of the sequence shown in Scheme 1 is high
(19–30%) by porphyrin standards, making large quantities of 1–3
readily available in two steps. As the meso-aryldipyrrolemethane can
be accessed readily in one step from the corresponding aldehyde and
pyrrole,5 a wide range of secondary aryl substituents can be deployed
to facilitate subsequent investigations into nanocrystal growth.
Using the Shelnutt strategy,4c we have recently prepared several
porphyrin nanorods and plates from (porphyrin)Sn(OH)2 with
different main-ring hydrophobicity and pyridine–pyridine distances.4a
Following this protocol, when an ethanolic solution of 1 or 2 is
injected into stirring water at room temperature (rt), a light brown
suspension is obtained. Analysis by scanning electron microscopy
(SEM) reveals these porphyrin suspensions to be collections of fairly
uniform diamond plates that are 400–600 nm long for 1 and 750–900
nm long truncated bipyramidal particles for 2 (Fig. 1a and 1c,
respectively). However, for the more hydrophobic (porphyr-
in)Sn(OH)2 3, only an amorphous solid is observed (Fig. 1e).
Interestingly, when a solution of 1 in ethanol is injected in water
and agitated via microwaves at 45 ꢀC for 1 min, hexagonal plates ꢁ2
mm long are obtained. For 2, well-defined ꢁ2 mm long herring bone
double blades result. Both materials are remarkably homogeneous in
Northwestern University, Department of Chemistry, 2145 Sheridan Road,
Evanston, IL 60208, USA. E-mail: j-hupp@northwestern.edu; stn@
northwestern.edu; Fax: +847-491-7713; Tel: +847-467-3347
† Electronic supplementary information (ESI) available: Details of
synthesis and characterization, including solution-phase X-ray
scattering. See DOI: 10.1039/b804629h
Scheme 1
3640 | J. Mater. Chem., 2008, 18, 3640–3642
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secondary substituents in 1 uniquely enables one pendant pyridyl
group to approach closely enough to the axial OH ligand of an
adjacent porphyrin to achieve direct interaction. In contrast, 2, which
features slightly larger dibutoxy secondary substituents, packs with
longer distances between its pyridyl groups and the OH ligands of the
adjacent porphyrin cores. This packing arrangement necessitates
mediation by water to form a network of Sn–OH–H2O–N hydrogen-
bonding bridges (Fig. 3, third column) that are less dense than that in
1, with two crystallographically independent porphyrin molecules
alternating in a pseudo-herringbone fashion. One of the two
porphyrins enlists both of its OH groups for H-bonding while the
other only employs its pyridyl groups, thereby allowing a water
molecule to be accommodated between the two porphyrins.
The (2,6-dialkoxy)phenyl substituents in 1 and 2 pack so as to
render two faces of the crystals hydrophobic while the orthogonal
HO–Sn–OH moiety makes the remaining face more hydrophilic
(Fig. 3, looking down the diagonal direction of the abc-axes),
suggesting a natural tendency for crystal growth in water to occur
faster along the hydrophilic direction than the hydrophobic one,
resulting in nanocrystals that are flat and long. In 2, where water
molecules must be recruited as structural components and the
secondary substituents on the phenyl ring are more hydrophobic,
crystal growth along the hydrophilic direction presumably becomes
more retarded relative to that in 1, resulting in thicker nanoplates.
In the case of 3, the bulky and more hydrophobic dioctoxyphenyl
groups force the porphyrins apart and prevent intermolecular
H-bonding (Fig. 3, third column). These differences are manifested
most clearly in the contact angle experiments shown in the fourth
column of Fig. 3 where thin films8 of porphyrins 1–3 repel water at
different contact angles. Together with the structural data presented
above, these results illustrate how seemingly small changes to
porphyrin building blocks can lead to significant changes, not only in
the physical properties of the thin films derived from them, but also in
the growth and morphology of the corresponding nanocrystalline
materials.
Fig. 1 SEM images of the different nanocrystal morphologies obtained
from porphyrin building blocks 1–3 under various conditions; a) and b)
from 1, c) and d) from 2, e) and f) from 3. Conditions: a), c), and e): rt,
stirring; b), d), and f): 45 ꢀC, microwaves.
size (Fig. 1b and 1d). Again, 3 gives only undefined aggregates
(Fig. 1f) under these conditions.
The two nanocrystal samples from 1 exhibit powder X-ray
diffraction (PXRD) patterns (Fig. 2) that closely resemble the
diffraction pattern for a single crystal sample grown under non-
agitated conditions. Such similarities imply that the corresponding
unit cells for the three samples are identical. The PXRD patterns for
nanocrystals and single crystals obtained from 2 are likewise similar
(ESI†‡), suggesting a conservation of the molecular packing
arrangement.
For amphiphilic (porphyrin)Sn(OH)2, the pyridyl substituents and
Sn(OH)2 core of one porphyrin can hydrogen-bond to adjacent
molecules in the solid state to afford ordered porphyrin arrays.4c,7
Because direct coordination of the pyridyl nitrogen to the
hexacoordinate Sn(IV) center is not expected,7 variously weighted
combinations of hydrogen bonding, hydrophobic interactions, and
p–p stacking can account for the formation of well-ordered arrays of
(porphyrin)Sn(OH)2—with the different weightings from secondary
substituents resulting in different nanocrystals (Fig. 3).
Interestingly, the single crystal structure of 1 indicates that 1 uses
only one of its two pyridine/OH pairs for hydrogen bonding. The
result is the relatively symmetrical head-to-tail arrangement shown in
Fig. 3 (right column). The comparatively small size of the methoxy
As multi-porphyrin materials, the crystalline nanoparticles
obtained from 1 and 2 exhibit absorption and emission spectra that
differ considerably from those of the monomers (ESI,†Fig. S1 and
S2).4e,9 For example, compared to monomer 2 in ethanol, which show
a B-band at 425 nm and Q-bands at 557 and 593 nm, the extinction
spectrum of the nanoparticle colloidal dispersions in water are more
complicated. The B-band is split, with sub-peaks at 426, 428 and 452
nm, while the Q-bands red-shift slightly to 567 and 603 nm. The
emission spectrum of nanoparticles of 2 shows multiple bands at 611,
633, 661, and 693 nm, while only two bands at 604 and 659 nm are
seen for a solution of 2.
In summary, crystalline nanoparticles of [5,15-bis(pyridyl)-10,20-
diarylporphyrin]Sn(OH)2 with a variety of morphologies have been
grown in a facile fashion from readily accessible molecular precur-
sors. Simple synthetic modification of the secondary substituents on
the diaryl substituents of the porphyrin rings led to widely different,
but understandable, crystal packing arrangements, as well as differing
degrees of hydrophobicity. Under conditions of agitated growth,
these molecular properties are expressed as diverse macroscopic
nanocrystal morphologies. Together with the unique electronic and
catalytic properties of the porphyrin building blocks, the ability to
assemble these molecular materials into well-defined nano- and
macrostructures may enhance the status of porphyrins as candidates
for applications in electronics, photonics, and/or catalysis.
Fig. 2 The PXRD patterns for the two nanoparticle samples of 1 as well
as that obtained for a single-crystal sample of 1. a) Nanoplates grown
under stirring at rt. b) Nanoplates grown under brief exposure to
microwaves at 45 ꢀC. c) Single crystal grown under non-agitated condi-
tions. d) Simulated PXRD pattern obtained from the X-ray crystal
structure of 1.
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Fig. 3 Structural, physical, and morphological data for metalloporphyrins 1, 2 and 3 (for a color version of this figure with clear depiction of the atomic
labels, see Fig. S6 in ESI†). From the left: First column: Stick representations of the X-ray structures of the (porphyrin)Sn building blocks. Second
column: Photographs of single crystals of 1, 2 and 3, and corresponding SEM images of the nanocrystals prepared under agitation. Third column:
Packing-diagram representations of the X-ray structures, illustrating the intermolecular H-bonding pattern between adjacent porphyrin layers in 1, water-
mediated H-bonding pattern between adjacent porphyrin layers in 2, and the absence of such a pattern for 3. These differences lead to completely different
nanoscale morphologies for 1, 2, and 3 under agitated growth conditions. Those that can form H-bonds (1 and 2) are more likely to afford well-defined
nanocrystals while the porphyrin that does not (3) tends to yield amorphous nanoparticles. Fourth column: The relative increase in hydrophobicity of thin
films of 1, 2 and 3, as shown via contact-angle measurements (69ꢀ for 1, 74ꢀ for 2, and 83ꢀ for 3) when drops of colored water are placed on top of glass-
supported (porphyrin)Sn(OH)2 films. Insets show the side views of the water drops: the most hydrophobic film of 3 cause water to bead up the most.
A. J. Goshe, P. J. Wesson, S. T. Nguyen, J. T. Hupp and
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Notes and references
‡ Synthesis of porphyrin nanoparticles (see also ESI†). Method A: An
ethanol solution of porphyrin (200 mL of a 1 mM solution) was injected
into a vial containing stirring deionized water (10 mL) at room temper-
ature. Stirring was maintained for approximately 2 to 20 min whereupon
suspensions of nanoparticles were obtained. Method B: An ethanol
solution of porphyrin (200 mL of a 1 mM solution) was injected into a vial
containing stirring deionized water (5 mL) at room temperature.
Suspensions of nanoparticles were obtained after 1 to 2 min of microwave
irradiation at 45 ꢀC. Isolation of all porphyrin nanoparticles was easily
carried out via centrifugation and decantation of the mother liquor.
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3642 | J. Mater. Chem., 2008, 18, 3640–3642
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