Morphology-controlled self-assembled nanostructures of 5,15-di[4-(5- acetylsulfanylpentyloxy)phenyl]porphyrin derivatives. Effect of metal-ligand coordination bonding on tuning the intermolecular interaction

Article abstract of DOI:10.1021/ja8067337

Novel metal-free 5,15-di[4-(5-acetylsulfanylpentyloxy)phenyl]porphyrin H₂[DP(CH₃COSC₅H₁₀O)₂P] (1) and its zinc congener Zn[DP(CH₃COSC₅H ₁₀O)₂P] (2) were designed and synthesized. Single-crystal X-ray diffraction (XRD) analysis confirmed the tetrapyrrole nature of these two compounds, revealing the existence of metal-ligand coordination bond between the carbonyl oxygen in the aryloxy side chain of meso-attached phenyl group in the porphyrin molecule with the zinc center of neighboring porphyrin molecule in the crystal structure of 2. This intermolecular Zn-O coordination bond induces the formation of a supramolecular chain structure in which the porphyrinato zinc moieties are arranged in a "head-to-tail" mode (J-aggregate), which is in contrast to a "face-to-face" stacking mode (H-aggregate) in the supramolecular structure formed depending on the C-H?π interaction in the crystal of 1. Their self-assembling properties in MeOH and n-hexane were comparatively investigated by scanning electronic microscopy and XRD technique. Intermolecular π-π interaction of metal-free porphyrin 1 leads to the formation of hollow nanospheres and nanoribbons in MeOH and n-hexane, respectively. In contrast, introduction of additional Zn-O coordination bond for porphyrinato zinc complex 2 induces competition with intermolecular π-π interaction, resulting in nanostructures with nanorod and hollow nanosphere morphology in MeOH and n-hexane. The IR and XRD results clearly reveal the presence and absence of such metal-ligand coordination bond in the nanostructures formed from porphyrinato zinc complex 2 and metal-free porphyrin 1, respectively, which is further unambiguously confirmed by the single-crystal XRD analysis result for both compounds. Electronic absorption spectroscopic data on the self-assembled nanostructures reveal the H-aggregate nature in the hollow nanospheres and nanoribbons formed from metal-free porphyrin 1 due to the π-π intermolecular interaction between porphyrin molecules and J-aggregate nature in the nanorods and hollow nanospheres of 2 depending on the dominant metal-ligand coordination bonding interaction among the porphyrinato zinc molecules. The present result appears to represent the first effort toward controlling and tuning the morphology of self-assembled nanostructures of porphyrin derivatives via molecular design and synthesis through introduction of metal-ligand coordination bonding interaction. Nevertheless, availability of single crystal and molecular structure revealed by XRD analysis for both porphyrin derivatives renders it possible to investigate the formation mechanism as well as the molecular packing conformation of self-assembled nanostructures of these typical organic building blocks with large conjugated system in a more confirmed manner.

Full text of DOI:10.1021/ja8067337

Published on Web 11/13/2008
Morphology-Controlled Self-Assembled Nanostructures of
5,15-Di[4-(5-acetylsulfanylpentyloxy)phenyl]porphyrin
Derivatives. Effect of Metal-Ligand Coordination Bonding on
Tuning the Intermolecular Interaction
Yingning Gao, Xiaomei Zhang, Changqin Ma, Xiyou Li, and Jianzhuang Jiang*
Department of Chemistry, Shandong UniVersity, Jinan 250100, China
Received August 26, 2008; E-mail: jzjiang@sdu.edu.cn
Abstract: Novel metal-free 5,15-di[4-(5-acetylsulfanylpentyloxy)phenyl]porphyrin H2[DP(CH3COSC5H10O)2P]
(1) and its zinc congener Zn[DP(CH3COSC5H10O)2P] (2) were designed and synthesized. Single-crystal
X-ray diffraction (XRD) analysis confirmed the tetrapyrrole nature of these two compounds, revealing the
existence of metal-ligand coordination bond between the carbonyl oxygen in the aryloxy side chain of
meso-attached phenyl group in the porphyrin molecule with the zinc center of neighboring porphyrin molecule
in the crystal structure of 2. This intermolecular Zn-O coordination bond induces the formation of a
supramolecular chain structure in which the porphyrinato zinc moieties are arranged in a “head-to-tail”
mode (J-aggregate), which is in contrast to a “face-to-face” stacking mode (H-aggregate) in the
supramolecular structure formed depending on the C-H · · · π interaction in the crystal of 1. Their self-
assembling properties in MeOH and n-hexane were comparatively investigated by scanning electronic
microscopy and XRD technique. Intermolecular π-π interaction of metal-free porphyrin 1 leads to the
formation of hollow nanospheres and nanoribbons in MeOH and n-hexane, respectively. In contrast,
introduction of additional Zn-O coordination bond for porphyrinato zinc complex 2 induces competition
with intermolecular π-π interaction, resulting in nanostructures with nanorod and hollow nanosphere
morphology in MeOH and n-hexane. The IR and XRD results clearly reveal the presence and absence of
such metal-ligand coordination bond in the nanostructures formed from porphyrinato zinc complex 2 and
metal-free porphyrin 1, respectively, which is further unambiguously confirmed by the single-crystal XRD
analysis result for both compounds. Electronic absorption spectroscopic data on the self-assembled
nanostructures reveal the H-aggregate nature in the hollow nanospheres and nanoribbons formed from
metal-free porphyrin 1 due to the π-π intermolecular interaction between porphyrin molecules and
J-aggregate nature in the nanorods and hollow nanospheres of 2 depending on the dominant metal-ligand
coordination bonding interaction among the porphyrinato zinc molecules. The present result appears to
represent the first effort toward controlling and tuning the morphology of self-assembled nanostructures of
porphyrin derivatives via molecular design and synthesis through introduction of metal-ligand coordination
bonding interaction. Nevertheless, availability of single crystal and molecular structure revealed by XRD
analysis for both porphyrin derivatives renders it possible to investigate the formation mechanism as well
as the molecular packing conformation of self-assembled nanostructures of these typical organic building
blocks with large conjugated system in a more confirmed manner.
Introduction
groups onto the peripheral positions of conjugated molecules
with rigid molecular structure induces additional intermolecular
Self-assembled nanostructures have created a wide range of
ever more sophisticated molecular materials associated with their
diverse applications. In particular, self-assembly of functional
molecular materials with large conjugated electronic structure
into well-defined nanostructures depending on various nonco-
valent interactions has attracted increasing research interests in
both material and chemistry fields.1-8 On the basis of different
noncovalent intermolecular interactions, a wide variety of
nanostructures such as particles, fibers, rods, sheets, cubes, and
tubes have been fabricated with potential application in the fields
of sensors,9-12 field-effect transistors,13-15 and photovoltaics.16-21
Extensive investigations have revealed that self-assembly of
conjugated molecular systems depends mainly on the intermo-
lecular π-π interaction. Introduction of different functional
interactions including van der Waals, hydrogen bonding, hy-
drophilic/hydrophobic, electrostatic, and metal-ligand coordina-
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10.1021/ja8067337 CCC: $40.75
2008 American Chemical Society

Self-Assembled Nanostructures of Porphyrin Derivatives
A R T I C L E S
tion. Cooperation and/or competition between the original π-π
interaction and additionally introduced intermolecular interaction
lead to self-assembly of functional molecular materials into
novel nanostructures with different morphology. As a conse-
quence, incorporation of different functional groups onto the
peripheral positions of conjugated molecules has been widely
utilized to tune the morphology of self-assembled nanostructures.
For example, cooperation between the π-π interaction and
hydrogen bonding among the molecules of phthalocyanine
copper complexes decorated with optically active diol units was
revealed to lead to the formation of fibrous assemblies.22
Additional sulfur-sulfur interaction between the thioether side
chains at the peripheral positions of phthalocyanine ring
promotes the intermolecular π-π interaction, which in turn
induces the self-assembly of corresponding phthalocyanine
molecules into rodlike aggregates.23 Incorporation of different
side chains onto different positions of the perylene diimide ring
has been revealed to induce different effects on the intermo-
lecular interactions, which in turn results in formation of
nanostructures with different morphology according to Zang and
co-workers.24,25 Very recently, this group incorporated different
numbers of hydroxyl groups onto the meso-substituted phenyl
groups of porphyrin ligand in mixed (phthalocyaninato)(por-
phyrinato) europium triple-decker complexes to tune the inter-
molecular interaction.26 Cooperation and/or competition between
the intermolecular π-π interaction and hydrogen bonding for
different sandwich mixed (phthalocyaninato)(porphyrinato) eu-
ropium triple-decker compounds result in the formation of
nanostructures with different morphologies. However, self-
assembly of functional molecular materials into a prerequisite
nanostructure with desirable dimension and morphology through
controlling intermolecular interaction still remains a great
challenge.
As the typical representative of functional molecular materials
with large conjugated electronic molecular structure, porphyrins
have been extensively studied over the past century because of
their wide range of biological relevance and industrial applica-
tions.27 In recent years, the self-assembly behavior and nano-
structures of these tetrapyrrole derivatives have also started to
attract extensive research interests. For example, hollow capsules
with potential application as drug delivery agents were produced
using a phase-transfer method depending on the strong π-π
interaction between porphyrin molecules.28 Nanotubes with
photocatalytic activity were synthesized by electrostatic force
between two oppositely charged porphyrins.29 Investigation over
the organogel formation properties of a series of porphyrins with
amide groups as peripheral hydrogen bonding sites indicates
that the aggregation mode of porphyrin stacks can be tuned by
the hydrogen-bonding interaction.30 However, as mentioned
above, great efforts have been continuously paid toward self-
assembling porphyrin molecules into prerequisite nanostructures
with desirable dimension and morphology by optimizing
intermolecular interaction.
In the present article, we describe the design, synthesis, single
crystal structure, and self-assembly properties of two novel 5,15-
di[4-(5-acetylsulfanylpentyloxy)phenyl]porphyrin derivatives,
namely H2[DP(CH3COSC5H10O)2P] (1) and Zn[DP(CH3COS-
C5H10O)2P] (2) (Scheme 1). Introduction of the two carbonyl
side chains onto the meso-attached phenyl groups in the
tetrapyrrole ligand of porphyrin ring and in particular the central
zinc ion induces additional metal-ligand coordination bonding
interaction between neighboring molecules of porphyrinato zinc
complex 2 in addition to intermolecular π-π interaction, leading
to the formation of nanostructures with different morphology
in both MeOH and n-hexane in comparison with those of metal-
free porphyrin 1 and clearly revealing the effect of coordination
bonding interaction in tuning the intermolecular interaction
between porphyrin molecules in the self-assembly process. This
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A R T I C L E S
Gao et al.
Scheme 1. Synthesis of 5,15-Di[4-(5-acetylsulfanylpentyloxy)phenyl]porphyrin Derivatives H2[DP(CH3COSC5H10O)2P] (1) and
Zn[DP(CH3COSC5H10O)2P] (2)
represents part of our continuous efforts toward understanding
the relationship between the molecular structure and corre-
sponding nanostructures of tetrapyrrole derivatives with large
conjugated π system. Nevertheless, it is also worth noting that
investigation of the formation mechanism of organic aggregates
has stimulated great research interest associated with the
important role toward preparing monodispersed organic nano-
structures with controllable morphology and dimension for
potential practical applications. However, because of the lack
of single-crystal molecular structure for most organic frame-
works used to construct organic nanostructures, the formation
mechanism as well as the molecular packing conformation of
corresponding self-assembled aggregates were usually proposed
on the basis of aggregate X-ray diffraction (XRD) result (which
is absent for a lot of cases) and the theoretical simulated
molecular structure. This results in relatively high uncertainty
in the proposed formation mechanism as well as proposed
molecular packing conformation for organic self-assembled
nanostructures. In the present work, the availability of single
crystal and molecular structures revealed by XRD analysis for
both porphyrin derivatives renders it possible to study the
formation mechanism and molecular packing conformation of
self-assembled nanostructures of these typical organic building
blocks in a more confirmed manner.
Zn(OAc)2 ·2H2O in chloroform and methanol. Satisfactory
elemental analysis results were obtained for both the two newly
prepared porphyrin derivatives after repeated column chromato-
graphic purification and recrystallization. The MALDI-TOF
mass spectra of both compounds clearly showed intense signals
for the molecular ion (M)+/(HM)+. The isotopic pattern closely
resembles the simulated one as exemplified by the spectrum of
compound 1 given in Figure S1 in the Supporting Information.
These two new porphyrin compounds were also characterized
1
with a range of spectroscopic methods. The H NMR spectra
of compounds 1 and 2 were recorded in CDCl3 at room
temperature. All the signals can be readily assigned (Figure S2
and Table S1 in the Supporting Information).
X-ray Single-Crystal Structures. The crystal and molecular
structures of porphyrin derivatives 1 and 2 were determined by
XRD analyses. Single crystals of metal-free porphyrin 1 suitable
for XRD analysis were obtained by slow diffusion of MeOH
into the corresponding CHCl3 solution. However, single crystals
of porphyrianto zinc complex 2 were obtained from natural
solvent evaporation of the chloroform solution of this compound
in air. Compound 1 crystallizes in the monoclinic system with
a P2(1)/c space group with two molecules in a unit cell. In
contrast, compound 2 crystallizes in the triclinic system with a
j
P1 space group with only one molecule per unit cell. It is worth
noting that the crystal structure of the metal-free porphyrin
contains four solvated CHCl3 molecules. The crystal data are
summarized in Table 1. Figure S3 in the Supporting Information
shows the molecular structures of both metal-free porphyrin 1
and porhyrinato zinc complex 2 viewed along the C2 axis, from
which their tetrapyrrole nature and in particular molecular
dimension are clearly revealed.
Figure 1 shows the crystal packing diagrams for both
porphyrin derivatives. As revealed in the crystal-packing
diagram of metal-free porphyrin 1 (Figure 1A), the solvated
chloroform molecules connect neighboring metal-free porphyrin
molecules into a face-to-face stacking supramolecular structure
(H-aggregate) depending on the C-H · · · π interaction in the
crystal of 1, leading to a relatively large separation between
the neighboring metal-free porphyrin rings, 5.19 Å. This in turn
Results and Discussion
Molecular Design, Synthesis, and Characterization. Porphy-
rins are of typical large conjugated molecular electronic
structure. The dominant intermolecular interaction among the
porphyrin molecules is therefore π-π interaction. Tuning the
intermolecular interaction of such a tetrapyrrole derivative can
therefore be easily reached by incorporating functional groups
(actually additional noncovalent interactions) onto the peripheral
positions of porphyrin ring. On the basis of consideration of
introducing additional metal-ligand coordination bonding
interaction, two acetylsulfanylpentyloxy side chains were in-
corporated onto the meso-attached phenyl groups of diphe-
nylporphyrin ring and zinc introduced into the center of the
porphyrin ligand (Scheme 1).
The target metal-free porphyrin H2[DP(CH3COSC5H10O)2P]
(1) was synthesized in good yield from the reaction between
thioacetic acid S-[5-(4-formylphenoxy)-pentyl] ester and dipyr-
romethane in the presence of BF3 ·Et2O in dichloromethane.31,32
Its zinc complex Zn[DP(CH3COSC5H10O)2P] (2) was obtained
from the reaction between metal-free porphyrin 1 and
(31) (a) Nagarkatti, J. P.; Ashley, K. R. Synthesis 1974, 186–187. (b) Treibs,
A.; Haberle, N. Liebigs Ann. Chem. 1968, 718, 183–191. (c) Wallace,
D. M.; Leung, S. H.; Senge, M. O.; Smith, K. M. J. Org. Chem. 1993,
58, 7245–7257.
(32) Laha, J. K.; Dhanalekshmi, S.; Taniguchi, M.; Ambroise, A.; Lindsey,
J. S. Org. Process Res. DeV. 2003, 7, 799–812.
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Self-Assembled Nanostructures of Porphyrin Derivatives
A R T I C L E S
Table 1. Crystallographic Data for Compounds 1 and 2
C2 axis direction). In contrast, π-π interaction between
neighboring porphyrin ligands in the direction along the
supramolecular chain direction in the crystal of 2 is possible
because of the close intermolecular distance in this direction.
This is indeed demonstrated by the electronic absorption
spectroscopic result of aggregates formed from this complex
in both methanol and n-hexane as detailed below.
1
2
formula
Mr
C46H46O4N4S2 ·2CHCl3 C46H44O4N4S2Zn
1021.77
0.21 × 0.17 × 0.15
monoclinic
P21/c
846.41
crystal size (mm3)
crystal system
space group
a (Å)
0.32 × 0.23 × 0.18
triclinic
j
P1
10.8849(3)
22.7573(6)
11.3839(3)
90.00
116.2250(10)
90.00
10.6174(6)
10.7550(8)
11.2109(12)
114.014(6)
101.658(6)
110.184(4)
1006.18(14)
1
Electronic Absorption Spectra. The electronic absorption
spectra of the two porphyrin derivatives 1 and 2 in CHCl3 were
recorded, and the data are compiled in Table 2. As expected,
compound H2[DP(CH3COSC5H10O)2P] (1) and Zn[DP(CH3-
COSC5H10O)2P] (2) showed typical features of metal-free and
zinc porphyrin compounds, respectively, in their electronic
absorption spectra, revealing the nonaggregated molecular
spectroscopic nature of both compounds in CHCl3. As shown
in Figure S4 in the Supporting Information, in line with that
for H2TPP (H2TPP ) 5,10,15,20-tetraphenylporphyrin),35 the
absorption around 410 nm for 1 can be attributed to the
porphyrin Soret band, while the four typical weak absorptions
at 504, 541, 578, and 633 nm can be attributed to the metal-
free porphyrin Q bands. Upon complexation with zinc metal
ion, the increase in the molecular symmetry from C2h for 1 to
D2h for 2 induces change in the electronic absorption spectrum
from typical feature for metal-free porphyrin to that for typical
porphyrinato metal species (Figure S5 in the Supporting
Information). With the porphyrin Soret band remaining un-
changed at 410 nm, three weak Q absorptions appear at 501,
537, and 573 nm, respectively, for 2 in CHCl3.
b (Å)
c (Å)
R (deg)
ꢀ (deg)
γ (deg)
V (Å3)
2529.65(12)
2
Z
F(000)
1060
1.341
0.468
2.09-25.00
21326
442
1.397
0.764
2.17-25.05
11732
Fcalcd (mg m-3
)
µ (mm-1
θ range (deg)
total no. of reflns
)
no. of independent reflns 4409 (Rint ) 0.0320)
3460 (Rint ) 0.0588)
260
parameters
290
R1 [I > 2σ(I)]
wR2 [I > 2σ(I)]
goodness of fit
0.0778
0.2177
1.042
0.0743
0.1965
1.031
will result in a relatively weak π-π interaction between
neighboring stacking porphyrin ligands in the direction perpen-
dicular to the porphyrin rings because of the relatively large
intermolecular distance.
The porphyrinato zinc analogue 2 is packed differently in
the crystal structure. As shown in Figure 1B, each porphyrinato
zinc molecule is bound with two neighboring molecules, forming
a head-to-tail porphyrin supramolecular chain (J-aggregate) via
the Zn-O coordination bond between the central metal zinc of
porphyrin molecule with the carbonyl oxygen in the aryloxy
side chain of meso-attached phenyl groups in the neighboring
porphyrin molecules. In the supramolecular chain of 2, each
Zn(II) metal ion complexed by four pyrrole nitrogen atoms of
the porphyrin ligand is sandwiched between two pairs of
antiparallel carbonyl arms of adjacent molecules because of the
Zn-O coordination bonding, forming a slightly distorted
octahedral geometry of zinc ion with the angle of Zn-O bond
with respect to the N(pyrrole)4 mean plane of porphyrin ring
ranging from 84.48 to 95.52°. The distance between the
coordinated oxygen and zinc is 2.60 Å, as expected, which is
longer than that reported in pentacoordinated zinc porphyrin
compound ZnTrTP (H2TrTP ) 5,10,15-triarylyl-20-quinone
porphyrin), 2.28 Å.33 The ring-to-ring separation of the
two virtually parallel N(pyrrole)4 mean planes of
DP(CH3COSC5H10O)2P along the C2 axis is 3.16 Å. This value
is only a bit longer than that found in the bis(porphyrinato)tho-
rium double-decker complex of Th(TPP)4 reported earlier, 2.94
Å, in which the two porphyrin ligands are associated closely in
a face-to-face manner by a thorium metal atom.34 As a
consequence, this result suggests feasible intense π-π interac-
tion between porphyrin rings along this C2 axis direction.
However, the separation between the two C2 axes of neighboring
Zn[DP(CH3COSC5H10O)2P] unit, 16.63 Å, is too far to induce
effective π-π interaction between porphyrin ligands in the
direction perpendicular to the porphyrin ring (actually along the
The aggregate electronic absorption spectra of these two
porphyrin compounds in methanol and n-hexane are also
recorded and given in Figures S4 and S5 in the Supporting
Information, respectively, which are different from the spectra
of corresponding compounds 1 and 2 in CHCl3. Nevertheless,
significant difference also exists in the electronic absorption
spectrum between the two compounds in both methanol and
n-hexane, indicating different aggregation type formed from
metal-free porphyrin and porphyrinato zinc compounds in these
two solvents. As shown in Figure S4 in the Supporting
Information, the porphyrin Soret band at 410 nm in CHCl3 for
metal-free porphyrin 1 takes a slight blue shift to 404 and 406
nm, respectively, when dispersed in methanol and n-hexane,
and the four porphyrin Q absorptions in chloroform also take a
more or less blue shift (Table 2). However, introduction of zinc
ion into the center of porphyrin ligand induces a significant red
shift in all three porphyrin Q absorption bands for 2 from 501,
537, and 573 nm in CHCl3 to 505, 544, and 581 nm in methanol
and 508, 547, and 583 nm in n-hexane (Table 2) with the
porphyrin Soret band remaining unchanged or slightly blue-
shifted, respectively, indicating the effect of central zinc ion
on the molecular packing conformation in aggregates due to
the introduction of metal-ligand (Zn-O) coordination bonding.
On the basis of Kasha’s exciton theory,36 blue shifts in the main
absorption bands of the metal-free porphyrin 1 upon aggregation
are typically a sign of the effective π-π interaction between
metal-free porphyrin molecules, indicating the formation of
H-aggregate from the compound in both methanol and n-hexane.
In contrast, the red-shifted absorption bands in the main
electronic absorption spectra of porphyrinato zinc complex 2
upon aggregation in methanol and n-hexane imply the porphy-
(33) Senge, M. O.; Speck, M.; Wiehe, A.; Dieks, H.; Aguirre, S.; Kurreck,
H. Photochem. Photobiol. 1999, 70, 206–216.
(35) Ulman, A.; Manassen, J. J. Am. Chem. Soc. 1975, 97, 6540–6544.
(36) Kasha, M.; Rawls, H. R.; EI-Bayoumi, M. A. Pure Appl. Chem. 1965,
11, 371–392.
(34) Girolami, G. S.; Milam, S. N.; Suslick, K. S. J. Am. Chem. Soc. 1988,
110, 2011–2012.
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A R T I C L E S
Gao et al.
Figure 1. Crystal-packing diagrams for H2[DP(CH3COSC5H10O)2P] (1) (A) and Zn[DP(CH3COSC5H10O)2P] (2) (B) in side view. Hydrogen atoms are
omitted for clarity.
cm-1 is clearly due to the carbonyl vibration.37 This is also true
Table 2. Electronic Absorption Spectral Data for Compounds 1
and 2 Dissolved in CHCl3 and the Self-Assemblies Dispersed in
for the IR spectra of its aggregates formed in both methanol
Methanol and n-Hexane
and n-hexane (Figure S6 in the Supporting Information).
However, because of the formation of Zn-O coordination bond
λ (max/nm)
compound
CHCl3
methanol
n-hexane
of the carbonyl oxygen in the aryloxy side chains in porphyrin
molecule with zinc center of neighboring porphyrin molecule
in the crystal of porphyrinato zinc analogue 2, the typical
carbonyl vibration splits into a shoulder band at 1688 and a
main band at 1657 cm-1 in the IR spectrum of 2 (Figure S7 in
the Supporting Information), with the main absorption taking
an obvious red shift in comparison with that for metal-free
porphyrin 1. As expected, split absorptions with the shoulder
at 1691 or 1690 cm-1 and main absorption at 1646 or 1661
cm-1 were also observed in the IR spectrum of nanorods or
hollow nanospheres formed from 2 in methanol or n-hexane
(Figure S7 in the Supporting Information), indicating the
formation of Zn-O coordination bond of the carbonyl oxygen
in the aryloxy side chains in porphyrin molecule with zinc center
of neighboring porphyrin molecules in these nanostructures.
Morphology of the Aggregates. The morphology of the
aggregates formed was examined by scanning electron micros-
copy (SEM) and transmission electronic microscopy (TEM).
Samples were prepared by casting a drop of sample solution
onto a carbon-coated grid. The SEM images of aggregates of 1
and 2 were shown in Figure 2. It is well known that, according
to the model set up by Srinivasarao and co-workers,38 molecular
materials dissolved in a solvent with density higher than that
of water usually yield two-dimensional air bubbles and a three-
dimensional network, respectively, after the natural evaporation
of solvent in air in the cases of lack of and existence of
significant intermolecular interaction. As shown in Figure 2A,
a two-dimensionally ordered array of air bubbles with relatively
high monodispersed pores with the diameter of ca. 500 nm was
1
410, 504, 541,
578, 633
410, 501, 537,
573
404, 501, 538,
576, 632
410, 505, 544,
581
406, 501, 535,
578, 633
406, 508, 547,
583
2
rinato zinc molecules of this compound are enforced to adopt
the J-aggregation mode. These results indicate the dominant
role of π-π interaction between the molecules of metal-free
porphyrin 1 but of intermolecular metal-ligand coordination
bonding due to the Zn-O coordination bond formed between
central zinc ion of porphyrinato zinc molecule and a carbonyl
oxygen atom in the aryloxy side chain of neighboring molecules
for compound 2, revealing the effect of metal-ligand coordina-
tion bonding interaction on tuning the intermolecular interaction
of porphyrin molecules and in turn the aggregation mode. This
is in good accordance with the single-crystal XRD analysis,
XRD, and IR spectroscopic results as detailed above and below,
respectively. In addition, comparison in the aggregate electronic
absorption spectra for either metal-free porphyrin 1 or porphy-
rinato zinc complex 2 in methanol and n-hexane reveals the
effect of the solvents on tuning the intermolecular interaction
and in turn the molecular packing mode in aggregates due to
different interaction between porphyrin and solvent molecules
for both porphyrin derivatives.
IR Spectra. The IR spectra of these two porphyrin derivatives
and their self-assembled nanostructures obtained are shown in
Figures S6 and S7 in the Supporting Information. The similar
feature in the IR spectra of the nanostructures to that of
corresponding compounds for both H2[DP(CH3COSC5H10O)2P]
(1) and Zn[DP(CH3COSC5H10O)2P] (2) unambiguously confirms
the composition of nanostructures from corresponding porphyrin
compounds. In the IR spectrum of 1, the absorption at 1684
(37) Orita, A.; Fukudome, M.; Ohe, K.; Murai, S. J. Org. Chem. 1994, 59,
477–481.
(38) Srinivasarao, M.; Collings, D.; Philips, A.; Patel, S. Science 2001,
292, 79–83.
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Self-Assembled Nanostructures of Porphyrin Derivatives
A R T I C L E S
true for injecting a small volume of solution of these two
compounds dissolved in CHCl3 into n-hexane. Depending
mainly on the intermolecular π-π interaction in cooperation
with the van der Waals interaction, molecules of metal-free
porphyrin 1 self-assemble into nanostructures with hollow sphere
morphology with a diameter of ca. 700 nm (Figure 2C). The
hollow interior nature is confirmed by the two broken spheres
with hollow interior displayed at the top of Figure 2C. In the
TEM image of Figure S8A in the Supporting Information,
convincing hollow-sphere structures can be seen with the
presence of circular rings of sectioned spheres and a cavity in
the interior. As suggested by the single-crystal XRD analysis,
Zn-O coordination bonding interaction between a carbonyl
oxygen in the aryloxy side chain of meso-attached phenyl group
in porphyrinato zinc molecule and the zinc center of neighboring
molecules of 2 dominates the intermolecular interaction of the
porphyrinato zinc compound. As a result, self-assembly of 2
induces the formation of nanostructures with nanorod morphol-
ogy with ca. 50-nm width and 800-nm length (Figure 2D).
When the solvent is changed from methanol to n-hexane,
further change in the morphology of self-assembled nanoscale
aggregates was revealed, showing the solvent effect on the
formation of self-assembled nanostructures. As can be seen from
Figure 2E, molecules of metal-free porphyrin 1 self-assemble
into nanostructures with nanoribbon morphology with uniform
size and orientation. A zoom-in image of a piece of nanoribbon
(Figure 2e) shows their average width of ca. 18 nm. The
nanoribbons were ordered over several micrometers, in line with
the long-range periodicity along the (010) direction as revealed
by the XRD result due to the intermolecular π-π stacking
interaction. For porphyrinato zinc complex 2, self-assembly
process in n-hexane leads to the formation of nanostructures
with hollow sphere morphology with diameter of ca. 40 nm
(Figure 2F). Figure 2f is a zoom-in image of some broken
hollow spheres, revealing the hollow interior nature of nano-
spheres formed. Further evidence was given in Figure S8B in
the Supporting Information, from which we can see that the
inner compartments of the spheres are colorless.
At the end of this section, it is worth noting that, in line with
the previous results of Hupp and co-workers,41 the morphology
of aggregates formed from the two porphyrin compounds 1 and
2 clearly shows dependence on the solvent it precipitated from
since different solvent molecules with different mixing rates
and viscosities can compete for specific intermolecular interac-
tion moieties and therefore alter both the thermodynamics of
the intermolecular interactions and the kinetics of the assembly
process during nanomaterial formation.
X-ray Diffraction Patterns of the Aggregates. The nanostruc-
tures of the two compounds were fabricated by injecting a small
volume of solution of 1 and 2 in chloroform (1 mM) into a
large volume of methanol and n-hexane, respectively. The
internal structure of self-assembled nanostructures of these two
compounds was investigated by XRD analysis (Figures 3 and
S9 in the Supporting Information). As shown in Figure 3A, in
the low angle range, the XRD diagram of the nanoscale hollow
spheres formed from compound 1 in methanol shows three
refraction peaks at 2θ ) 1.76° (corresponding to 5.02 nm), 2.38°
(3.71 nm), and 6.09° (1.45 nm), respectively, which are ascribed
to the refractions from the (010), (100), and (001) planes.23
Figure 2. SEM images of nanostructures of compounds 1 and 2. (A) Air
bubbles formed from compound 1 in CHCl3. (B) Three-dimensional network
nanostructures formed from compound 2 in CHCl3. (C) Nanoscale hollow
spheres formed from compound 1 in methanol. (D) Nanorods formed from
compound 2 in methanol. (E) Nanoribbons formed from compound 1 in
n-hexane. (e) Zoom-in image of the rectangle part in E. (F) Nanoscale hollow
spheres formed from compound 2 in n-hexane. (f) Zoom-in image of the
rectangle part in F.
obtained by natural evaporation of the chloroform solution of
metal-free porphyrin 1,38,39 indicating the less intermolecular
interaction for this metal-free porphyrin compound 1 dissolved
in chloroform. In good contrast, natural evaporation of the
chloroform solution of porphyrianto zinc complex 2 led to the
formation of three-dimensional network nanostructures (Figure
2B), revealing the significant intermolecular interaction due to
the Zn-O coordination bond between the carbonyl oxygen in
the aryloxy side chain of meso-attached phenyl group in
porphyrin molecule with the zinc center of neighboring por-
phyrin molecules. This is in line with a previous result reported
on another hexa-coordinated Zn(II) porphyrin complex, for
which a three-dimensional coordination polymer was obtained
through the same process.40 When injecting a small volume of
solution of these two compounds dissolved in CHCl3 into
methanol, different nanostructures were obtained. This is also
(39) (a) Schenning, A. P. H. J.; Benneker, F. B. G.; Geurts, H. P. M.; Liu,
X. Y.; Nolte, R. J. M. J. Am. Chem. Soc. 1996, 118, 8549–8552. (b)
Beysens, D.; Knobler, C. M. Phys. ReV. Lett. 1986, 57, 1433–1436.
(c) Family, F.; Meakin, P. Phys. ReV. Lett. 1988, 61, 428–431. (d)
Limaye, A. V.; Narhe, R. D.; Dhote, A. M.; Ogale, S. B. Phys. ReV.
Lett. 1996, 76, 3762–3765.
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4210.
(41) Lee, S. J.; Hupp, J. T.; Nguyen, S. T. J. Am. Chem. Soc. 2008, 130,
9632–9633.
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A R T I C L E S
Gao et al.
distance between tetrapyrrole cores of neighboring stacking
porphyrin molecules, respectively. It is worth noting that the
lack of the (001) refraction in the XRD diagram of the
nanoribbons formed from metal-free porphyrin compound 1 in
n-hexane indicates the lack of long-range periodicity along this
direction in the self-assembled nanostructures of this metal-free
porphyrin compound. As a result, growing in this direction
during the self-assembly process is limited or prohibited. In
contrast, observation of the higher-order refraction peak of the
(010) plane suggests the advantage of crystal growth along this
direction, resulting in the nanoribbons of the metal-free por-
phyrin.42
According to the XRD diffraction analysis result for the
nanorods of porphyrinato zinc complex 2 formed in methanol
(Figure S9A in the Supporting Information) and the single-
crystal molecular structure size of this compound revealed by
single-crystal XRD analysis (Figure S3B in the Supporting
Information), the unit cell consisting of four porphyrinato zinc
molecules from a parallelepipedal lattice (including triclinic
system) with cell parameters of a ) 2.36, b ) 8.74, and c )
0.91 nm is given in Figure S10 in the Supporting Information
for the nanorods of 2 formed in methanol. The sole sharp
refraction at 0.31 nm for the nanorods of 2 formed in methanol
in the wide-angle region can be assigned to the stacking distance
between tetrapyrrole cores of neighboring porphyrin molecules
(Figure S9 in the Supporting Information). This is in good
accordance with that revealed by single-crystal X-ray analysis
for this compound as detailed above. As revealed by the single-
crystal XRD analysis of this compound, existence of the Zn-O
coordination bond between the carbonyl oxygen in the aryloxy
side chain of meso-attached phenyl group in porphyrinato zinc
molecule with the zinc center of neighboring molecules of 2
induces the packing of porphyrinato zinc molecules in a one-
dimensional direction. As a consequence, needlelike single
crystals were obtained from this complex under suitable
experimental condition in a macroscopic view. This is also
responsible for the formation of aggregates with nanorod
morphology during the self-assembly process of the same
complex in methanol.30
The XRD pattern of self-assembled nanoscale hollow spheres
of complex 2 in n-hexane shows two peaks at 5.56 and 4.96
nm in the low-angle region and six peaks at 0.63, 0.62, 0.52,
0.43, 0.31, and 0.26 nm in the wide-angle region (Figure S9B
in the Supporting Information). As shown in Figure S11B in
the Supporting Information, a unit cell consisting of two
porphyrinato zinc molecules of 2 from a parallelepipedal lattice
(including triclinic system) with cell parameters of a ) 4.96, b
) 5.56, and c ) 0.63 nm can be concluded on the basis of this
XRD result as well as the single-crystal molecular structure size
of this compound. On the basis of the single-crystal XRD
analysis result of 2, the two diffraction peaks at 0.31 and 0.62
nm for the nanorods of 2 correspond to the stacking distance
between porphyrin-porphyrin and porphyrin-porphyrin-
porphyrin cores along the direction perpendicular to the por-
phyrin ring, respectively, whereas the two diffraction peaks at
0.26 and 0.52 nm correspond to the metal-ligand coordination
bond length between zinc and oxygen and the two oxygen atoms
coordinated with the central zinc atom below and above the
porphyrin ring. The remaining peak at 0.43 nm is assigned to
Figure 3. XRD profiles of the aggregates of compound 1 formed in
methanol (A) and n-hexane (B).
These diffraction results could be assigned to the refractions
from a parallelepipedal lattice (including monoclinic system)
with cell parameters of a ) 3.71, b ) 5.02, and c ) 1.45 nm
(Figure 4). As can be seen from Figure S3A in the Supporting
Information, the molecular dimensional size for 1 is 3.41 nm
(length) × 1.06 nm (width) on the basis of single-crystal XRD
analysis. According to the XRD result and the single-crystal
molecular structure, the unit cell consisting of four molecules
is given for metal-free porphyrin 1 (Figure 4B). Comparison
between this unit cell of the nanoscale hollow spheres for 1
and that revealed for single crystals of the same compound
formed also in methanol reveals their different molecular
packing diagram, clearly indicating the effect of crystal growth
speed on the system and dimension size of crystal unit cell.
This is also true for the porphyrinato zinc complex as detailed
below. It is worth noting that, in the wide-angle region, the XRD
pattern presents two sharp refractions at 2θ ) 29.46 and 30.96°
(corresponding to 0.30 and 0.28 nm), respectively, which might
be attributed to the distance between tetrapyrrole cores of
neighboring stacking porphyrin molecules (Figure 4A). Because
of the different experimental condition from the single crystals
of the same compound, the solvent molecules of chloroform
appear not to get into the aggregates, resulting in close
intermolecular distance between neighboring stacking porphyrin
molecules in the self-assembled nanostructures of 1 in com-
parison with that in single crystals.
As shown in Figure 3B, in the low-angle range, the XRD
diagram of the nanoribbons formed from metal-free porphyrin
compound 1 in n-hexane shows two narrow refraction peaks at
2.88 and 1.44 nm, which are ascribed to the refractions from
the (010) and (020) planes, respectively. In addition, the XRD
pattern also displays three well-defined peaks at 0.72, 0.48, and
0.27 nm. The former originates from the refraction of the (100)
plane, whereas the latter two are assigned to the van der Waals
distance between the pentyloxy side chains attached at the meso-
phenyl groups of two neighboring porphyrin molecules and the
(42) (a) Hudson, S. D.; Jung, H.-T.; Percec, V.; Cho, W.-D.; Johansson,
G.; Ungar, G.; Balagurusamy, V. S. K. Science 1997, 278, 449–452.
(b) Pan, Z. W.; Dai, Z. R.; Wang, Z. L. Science 2001, 291, 1947–
1949.
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Self-Assembled Nanostructures of Porphyrin Derivatives
A R T I C L E S
Figure 4. Schematic representation of the unit cell in the aggregates of compound 1 formed in methanol.
the van der Waals distance between the pentyloxy side chains
attached at the meso-phenyl groups of two neighboring por-
phyrin molecules (Figure S9B in the Supporting Information).
At the end of this section, it is worth noting that the molecular
packing as represented by the dimension of unit cell in the
nanorods for 2 formed in methanol is different from that revealed
for single crystals of the same complex formed also in methanol,
which is also different from that calculated on the basis of
experimental XRD data for the nanoscale hollow spheres of
this complex formed in n-hexane. This is also true for the metal-
free porphyrin 1. These results are in good accordance with that
for other tetrapyrrole derivatives as exemplified by 2,3,7,8,12,
13,17,18-tetracyclopentyl-5,10,15,20-n-pentylporphyrinato nickel
complex and bis(phthalocyaninato)neodymium, for which dif-
ferent single crystals with different dimensions were obtained
from different or even the same solvent,43 confirming the effect
of both solvent and growth speed on the system and size of
crystal unit cell.
nanostructures for metal-free porphyrin compound 1, while the
dominant Zn-O coordination bonding interaction between the
carbonyl oxygen in the aryloxy side chain of meso-attached
phenyl group in porphyrinato zinc molecule with the zinc center
of neighboring molecules of 2 leads to nanostructures such as
nanorods and hollow spheres for this complex in both methanol
and n-hexane. In particular, availability of single crystal and
molecular structure revealed by XRD analysis for both metal-
free porphyrin and porphyrinato zinc compounds renders it
possible to investigate the formation mechanism and the
molecular packing conformation of self-assembled nanostruc-
tures for these typical organic building blocks with a large
conjugated system in a more confirmed manner. The present
result appears to represent the first effort toward realization of
controlling and tuning the morphology of self-assembled
nanostructures of porphyrin derivatives via molecular design
and synthesis through the competition between intermolecular
π-π stacking interaction and metal-ligand coordination bond-
ing interaction and, more importantly, is believed to be helpful
toward preparation of monodispersed organic nanostructures
with controllable morphology and dimension for potential
practical applications.
Conclusions
In summary, we designed, synthesized, and investigated the
self-assembly properties of two novel porphyrin derivatives with
two 5-acetylsulfanylpentyloxy side chains at the opposite meso-
attached phenyl groups. Comparative investigation results reveal
that competition of intermolecular metal-ligand coordination
bonding with the π-π interaction between tetrapyrrole rings
in tetrapyrrole molecules leads to different molecular packing
conformation and in turn different self-assembled nanostructure
morphology. The π-π interaction dominates the formation of
Experimental Section
Measurements. The nanostructures of the two compounds 1 and
2 were fabricated by the phase-transfer method according to the
following procedure.23,24,27,44,45 A minimum volume (30-50 µL)
of concentrated chloroform solution of compounds (1 and 2) (1
mM) was injected rapidly into a large volume of methanol or
n-hexane (1 mL), respectively, and subsequently mixed with a
microinjector. The results were reproducible under the experimental
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Sorbier, J.-P.; Fournel, A.; Benlian, D. C. R. Seances Acad. Sci., Ser.
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Darovsky, A.; Keserashvili, V.; Harlow, R.; Mutikainen, I. Acta
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A R T I C L E S
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1
conditions described above. H NMR spectra were recorded on a
Bruker DPX 300 spectrometer (300 MHz) in CDCl3 using the
residual solvent resonance of CHCl3 at 7.26 ppm relative to that of
SiMe4 as internal reference. Electronic absorption spectra were
recorded on a Hitachi U-4100 spectrophotometer. XRD experiment
was carried out on a Rigaku D/max-γB X-ray diffractometer.
MALDI-TOF mass spectra were taken on a Bruker BIFLEX III
ultrahigh resolution Fourier transform ion cyclotron resonance mass
spectrometer with R-cyano-4-hydroxycinnamic acid as matrix.
Elemental analyses were performed by the Institute of Chemistry,
Chinese Academy of Sciences. TEM images were measured on a
JEOL-100CX II electron microscope operated at 100 kV. SEM
images were obtained using a JEOL JSM-6700F field-emission
scanning electron microscopy. For TEM imaging, a drop of sample
solution was cast onto a carbon copper grid. For SEM imaging,
gold (1-2 nm) was sputtered onto the grids to prevent charging
effects and to improve the image clarity.
Acknowledgment. Financial support from the Natural Science
Foundation of China, Ministry of Education of China, and Shandong
University is gratefully acknowledged.
Supporting Information Available: Preparation of 4-(5-
bromopentyloxy)benzaldehyde; preparation of thioacetic acid
S-[5-(4-formylphenoxy)pentyl]ester; preparation of metal-free
5,15-di[4-(5-acetylsulfanylpentyloxy)phenyl]porphyrin (1); prepa-
ration of 5,15-di[4-(5-acetylsulfanylpentyloxy)phenyl]porphyrinato
zinc complex (2); experimental and simulated isotopic patterns
for the molecular ion of the metal-free porphyrin (1); 1H NMR
spectra of H2[DP(CH3COSC5H10O)2P] (1) and Zn[DP(CH3-
COSC5H10O)2P] (2) in CDCl3; molecular structures of metal-
free 1 and porphyrinato zinc complex 2; electronic absorption
spectra of 1 and 2 in CHCl3 and their self-assembled aggregates
in methanol and n-hexane; IR spectra of compound 1, aggregates
of compound 1 with nanoscale hollow sphere morphology
formed in methanol, and aggregates of compound 1 with
nanoribbon morphology formed in n-hexane in the region
400-1800 cm-1 with 2 cm-1 resolution; IR spectra of compound
2, aggregates of compound 2 with nanorod morphology formed
in methanol, and aggregates of compound 2 with nanoscale
hollow sphere morphology formed in n-hexane in the region
400-1800 cm-1 with 2 cm-1 resolution; TEM images of
nanoscale hollow spheres formed from compounds 1 and 2 in
methanol; XRD profiles of the aggregates of compound 1
formed in methanol and n-hexane; XRD profiles of the
aggregates of compound 2 formed in methanol and n-hexane;
schematic representation of the unit cell in the aggregates of
compound 1 formed in methanol; schematic representation of
the unit cell in the aggregates of compound 2 formed in
methanol; schematic representation of the unit cell in the
Chemicals. Column chromatography was carried out on silica
gel (Merck, Kieselgel 60, 70-230 mesh) with the indicated eluents.
All other reagents and solvents were used as received. The
compound of dipyrromethane was prepared according to the
published procedures.
X-ray Crystallographic Analyses of 1 and 2. Crystal data and
details of data collection and structure refinement are given in Table
1. Data were collected on a Bruker SMART CCD diffractometer
with an Mo KR sealed tube (λ ) 0.71073 Å) at 293 K, using a ω
scan mode with an increment of 0.3°. Preliminary unit cell
parameters were obtained from 45 frames. Final unit cell parameters
were obtained by global refinements of reflections obtained from
integration of all the frame data. The collected frames were
integrated using the preliminary cell-orientation matrix. The
SMART software was used for collecting frames of data, indexing
reflections, and determining lattice constants, SAINT-PLUS was
used for integration of intensity of reflections and scaling,46
SADABS was used for absorption correction,47 and SHELXL was
used for space group and structure determination, refinements,
graphics, and structure reporting.46 CCDC-697437 and 697438
contain the supplementary crystallographic data for this article.
These data can be obtained free of charge from the Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/
cif.
1
aggregates of compound 2 formed in n-hexane; and H NMR
spectroscopic data for the porphyrin derivatives 1 and 2. This
material is available free of charge via the Internet at http://
pubs.acs.org.
(46) Sheldrick, G. M. SADABS, A Software for Empirical Absorption
Correction; University of Go¨ttingen: Go¨ttingen, Germany, 1997.
(47) SHELXL Reference Manual, version 5.1; Bruker Analytical X-Ray
Systems: Madison, WI, 1997.
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17052 J. AM. CHEM. SOC. VOL. 130, NO. 50, 2008