Surface-tuned assembly of porphyrin coordination oligomers

Article abstract of DOI:10.1021/ja800039a

Two self-complementary phenanthroline-strapped porphyrins bearing imidazole arms and C₁₂ or C₁₈ alkyl chains were synthesized, and their surface self-assembly was investigated by atomic force microscopy (AFM) on mica and highly ordered pyrrolitic graphite (HOPG). Upon zinc(II) complexation, stable porphyrin dimers formed, as confirmed by DOSY ¹H NMR and UV-visible spectroscopy. In solution, the dimers formed J-aggregates. AFM studies of the solutions dip-coated onto mica or drop-casted onto HOPG revealed that the morphologies of the assemblies formed were surface-tuned. On mica, fiber-like assemblies of short stacks of J-aggregates were observed. The strong influence of the mica's epitaxy on the orientation of the fibers suggested a surface-assisted assembly process. On HOPG, interactions between the alkyl chains and the graphite surface resulted in the stabilization and trapping of monomer species followed by their subsequent association into coordination polymers on the surface. Interdigitation of the alkyl chains of separate polymer strands induced lateral association of wires to form islands that grew preferentially upon drop-casting and slow evaporation. Clusters of laterally assembled wires were observed for the more mobile functionalized porphyrins bearing C₁₂ chains.

Full text of DOI:10.1021/ja800039a

Published on Web 07/09/2008
Surface-Tuned Assembly of Porphyrin Coordination
Oligomers
Matthieu Koepf,† Jennifer A. Wytko,*,† Jean-Pierre Bucher,‡ and Jean Weiss*,†
CLAC, Institut de Chimie de Strasbourg, UMR 7177, CNRS-ULP, 4 rue Blaise Pascal,
BP 1032, 67070 Strasbourg, France, and IPCMS, UMR 7504 CNRS-ULP, 23 rue du Loess,
BP 43, 67034 Strasbourg, France
Received January 3, 2008; E-mail: jwytko@chimie.u-strasbg.fr, jweiss@chimie.u-strasbg.fr
Abstract: Two self-complementary phenanthroline-strapped porphyrins bearing imidazole arms and C12
or C18 alkyl chains were synthesized, and their surface self-assembly was investigated by atomic force
microscopy (AFM) on mica and highly ordered pyrrolitic graphite (HOPG). Upon zinc(II) complexation, stable
porphyrin dimers formed, as confirmed by DOSY 1H NMR and UV-visible spectroscopy. In solution, the
dimers formed J-aggregates. AFM studies of the solutions dip-coated onto mica or drop-casted onto HOPG
revealed that the morphologies of the assemblies formed were surface-tuned. On mica, fiber-like assemblies
of short stacks of J-aggregates were observed. The strong influence of the mica’s epitaxy on the orientation
of the fibers suggested a surface-assisted assembly process. On HOPG, interactions between the alkyl
chains and the graphite surface resulted in the stabilization and trapping of monomer species followed by
their subsequent association into coordination polymers on the surface. Interdigitation of the alkyl chains
of separate polymer strands induced lateral association of wires to form islands that grew preferentially
upon drop-casting and slow evaporation. Clusters of laterally assembled wires were observed for the more
mobile functionalized porphyrins bearing C12 chains.
Introduction
the influence of the surface on the assembly of supramolecular
nanoobjects needs to be documented as it has been for other
In the past decade, several examples of photoinduced energy
or electron transfer reactions have been reported in geometrically
well defined porphyrin dyads, triads, and higher oligomers,1,2
and much effort has been devoted to the production of large
multiporphyrinic arrays for molecular electronics,3 for electronic2,4
or photonic conduction.5 On the basis of synthetic consider-
ations, self-assembled multiporphyrin scaffolds appear to have
a bright future; however, the controlled formation of quasiinfinite
linear arrangements is difficult to achieve, mostly because
assembly in solution is entropy controlled.6 In addition, before
incorporating porphyrin-based materials in operating devices,
self-assembled species.7 The results hereafter suggest that
surface/object interactions may be used advantageously to direct
the self-assembly process.
We have previously reported a phenanthroline-strapped
porphyrin that selectively binds imidazole within the phenan-
throline strap.8 The high association constants (>106 M-1
)
observed in chlorinated solvents originate from a combination
of weak interactions (metal-ligand coordination, H-bond
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† CLAC.
‡ IPCMS.
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Vol 18,Chapter 113. (c) Wytko, J. A.; Weiss, J. In N4-Macrocyclic
metal complexes; Zagal, J., ; Bedioui, F., ; Dodelet, J. P., Eds.;
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4291, and references cited.
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10.1021/ja800039a CCC: $40.75
2008 American Chemical Society

Surface-Tuned Porphyrin Assemblies
A R T I C L E S
Scheme 1. Comparison of Assembly Modes Observed on Mica
(left) and on HOPG (right).
Figure 1. Building blocks for multiporphyrin wires.
formation, and π-π stacking).9 The association of imidazole
within the zinc(II) porphyrin is entropy favored due to the loss
of two water molecules bound within the strap of the free
zinc(II) porphyrin.10 As a first step toward the preparation of
molecular wires, this imidazole recognition was used to design
geometrically well defined, self-assembled dyads11 and triads.12
A second step has now been taken by preparing porphyrins
1Zn-3Zn that self-assemble into oligomers under the proper
conditions. The assembly properties and AFM studies of two
types of nanoobjects (fibers or wires) obtained from the same
molecular species on two different substrates (mica and HOPG)
are reported. The results show that the nature of the deposition
substrate has a significant impact on the morphology of the
resulting objects. Hereafter, wires refer to assemblies with
coordination bond connectivity between consecutive monomer
units, whereas fibers refer to linear objects formed by aggrega-
tion of smaller assemblies that result from weak interactions.
The monomeric units 1H2-3H2 (Figure 1) were designed
such that the formation of long oligomeric or polymeric species
was expected to be initiated by insertion of Zn(II) within the
porphyrins. Preliminary attempts to metalate 1H2 with Zn(II)
led to insoluble material. This problem was circumvented by
anchoring long alkyl chains on the strapped porphyrin frame-
work13 to obtain 2H2 and 3H2 in good yields (see Supporting
Information for synthetic details). Zinc was inserted by treating
the free bases 2H2 and 3H2 with excess zinc acetate in refluxing
THF. The resulting zinc(II) derivatives 2Zn or 3Zn were highly
soluble in most organic solvents. Initially, linear assembly
(Scheme 1, right) was expected because of the unique arrange-
ment previously observed in photochemical triads built on
1-Zn.12
(end to end distance between two opposite, fully extended alkyl
chains) could be calculated. By fixing the smallest diameter (l)
(top of phenanthroline strap to top of phenanthroline strap) at
l ) 2 nm, L was found to be 4.0 nm for 2Zn and 6.0 nm for
3Zn, in agreement with respective theoretical values of L )
3.9 and 5.5 nm for head-to-tail dimer arrangements, obtained
from standard MM2 calculations. An association constant of
Kassoc) 109 M-1 in CHCl3 (see SI Figure S2 for a UV-visible
titration) was determined for this dimer formation and contrasts
with the association constants in the range of 107 M-1 usually
observed in self-assembled dyads and triads.11,12
UV-visible studies showed that the position of the Soret band
was concentration and solvent dependent. In chloroform at
concentrations of 5 × 10-5 M and higher, 2Zn exists primarily
as dimers which was also supported by DOSY experiments at
similar concentrations (SI Figure S1). The Soret band was
observed at 439 nm with a small shoulder at 430 nm, indicating
the presence of residual, unbound monomers in solution. Upon
dilution to 5 × 10-7 M, the Soret band, with no shoulder, was
shifted back to the maximum expected for the four coordinate
zinc(II) species at 432 nm (Figure 2). This disappearance of
the red-shift is a significant indicator of dissociation and also
indicates that only monomers were present in dilute chloroform
solutions. In less polar CH2Cl2 solutions, 2Zn and 3Zn existed
as dimers at concentrations of 10-7 to 10-4 M. Neither the blue
shift of the Soret band nor the shoulder on its higher energy
side was observed upon dilution. When a 10-4 M dichlo-
romethane stock solution of 2Zn was diluted to 1 × 10-6 M in
acetone, the Soret band displayed a broad shoulder at 425 nm
and a maximum at 435 nm (SI Figure S3). Therefore, it is
assumed that both monomers and dimers of 2Zn were present
in acetone-diluted solution later used for drop-casting deposition
on HOPG.
Solution Studies
Solutions of 2Zn or 3Zn in halogenated solvents were stable.
1H NMR DOSY experiments, provided in the Supporting
Information (SI Figure S1), of 2Zn or 3Zn in deuterated
chloroform (5 × 10-4 M CDCl3) indicated in each case the
presence of only one species with relatively large diffusion
coefficients of 300 and 250 µm/s, respectively. Using an
ellipsoidal model and presuming a head-to-tail dimer arrange-
ment as shown on the left in Scheme 1, the large diameter L
(9) Paul, D.; Melin, F.; Hirtz, C.; Wytko, J.; Ochsenbein, P.; Bonin, M.;
Schenk, K.; Maltese, P.; Weiss, J. Inorg. Chem. 2003, 42, 3779.
(10) Brandel, J.; Trabolsi, A.; Melin, F.; Elhabiri, M.; Weiss, J.; Albrecht-
Gary, A.-M. Inorg. Chem. 2007, 46, 9534.
(11) Leray, I.; Valeur, B.; Paul, D.; Regnier, E.; Koepf, M.; Wytko, J. A.;
Boudon, C.; Weiss, J. Photochem. Photobiol. Sci. 2005, 4, 280.
(12) Koepf, M.; Trabolsi, A.; Elhabiri, M.; Wytko, J. A.; Paul, D.; Albrecht-
Gary, A.-M.; Weiss, J. Org. Lett. 2005, 7, 1279.
Figure 2. UV-visible spectra of 2Zn in CHCl3 at 25 °C: (---) 5 × 10-5
M; (s) 5 × 10-7 M.
(13) Koepf, M.; Melin, F.; Jaillard, J.; Weiss, J. Tetrahedron Lett. 2005,
46, 139.
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A R T I C L E S
Koepf et al.
the case of dip coating, the excess solvent was blotted off prior
to air drying. For drop casting, the sample was air-dried or
exposed to a vapor saturated atmosphere.
Both dip-coating and drop-casting methods of deposition were
preliminarily tested to determine the best method for each
substrate (detailed procedures in SI, page S10). In the case of
mica, the following solvents were tested: acetone, dichlo-
romethane, heptane, and THF. Dip-coating procedures generated
suitable samples and organized assemblies only when heptane
solutions were used. By drop-casting, assemblies similar to those
observed by dip-coating were observed only when the drop of
solution was allowed to evaporate slowly in a heptane-saturated
atmosphere. Thus, for mica deposition, only results obtained
by dip-coating will be presented. The same solvents as well as
dioxane were tested for deposition onto HOPG, but samples
generated by dip-coating were poorly reproducible and hardly
exploitable. Drop-casting gave more interesting preliminary
results and was therefore pursued in more detail.
AFM Studies on Mica. Using tips with a typical curvature
radius of 10-12 nm on mica, we observed regular arrangements
of linear assemblies of 2Zn, as shown in Figure 4. Although
other types of assemblies were occasionally observed (SI Figure
S4), the images in Figure 4 were highly reproducible and best
represent structures that were observed over most of the
substrate. A strong influence of mica’s hexagonal epitaxy was
visible in the orientation of the assemblies (Figures 4b-d), as
well as in the 2-D fast Fourier transform (FFT) (middle column
Figures 4b-d). As a result of the dip-coating deposition method,
the size distribution of the objects varied depending on the area
of the substrate explored (Figure 4).
The length of the objects increased going from the top of the
mica support to the bottom. Within the same horizontal zone
(a, b, c, or d in Figure 4), the assemblies were relatively
homogeneous. Objects near the top of the support (Figure 4a)
had an average length of 30 nm, whereas those closer to the
bottom (Figure 4d) averaged 204 nm in length. The size of self-
assembled structures on surfaces is known to be concentration
dependent,17 which, because of the deposition method, is
consistent with the observation of longer fibers near the bottom
of the mica support.
At the top of the support (Figure 4a), 70% of the small
aggregates observed were shorter than 25 nm long. Several
millimeters lower on the support (Figure 4b), short fibers
covered the surface. In this zone, 70% of the objects were >25
nm long in at least one direction. The influence of mica’s
hexagonal epitaxy was already visible in this second zone. When
an area in the bottom half of the support was explored (Figure
4c), fibers that measured 43-209 nm in length were observed.
The length of 70% of the objects was >70 nm. The longest
strands were present at the bottom of the mica support (Figure
4d), where 70% of the fibers were longer than 110 nm. In this
area, the orientation of longest fibers was more random.
To investigate whether or not the length of the alkyl chains
played a role in the type of objects formed on mica, 3Zn with
C18 chains was studied under the same condition as for 2Zn.
Compound 3Zn was also deposited on mica supports by dip-
coating the support in a 1 µM heptane solution of 3Zn for 15 s.
As shown in Figure 5, deposits of nonorganized material and
small films were observed near the top of the mica support
(Figures 5a,b), whereas complex networks of fiber-like struc-
Figure 3. Normalized absorption spectra of 2Zn at 25 °C: (---) 5 × 10-5
M chloroform solution; ( · · · · ) initial chloroform solution diluted to 5 ×
10-6 M in heptane; (s) initial chloroform solution diluted to 5 × 10-7 in
heptane. The broadening of the Soret band at 440 nm is attributed to the
presence of greater π-stacking interaction between dimers induced by the
nonpolar heptane. At 5 × 10-7 M, most species are isolated (noninteracting)
dimers.
Dilution of chlorinated solutions to micromolar concentrations
(10-6 M) in heptane afforded slightly red-shifted absorption
bands, as shown in the UV-visible spectra in Figure 3. The
Soret band was broader with a larger low energy component
compared to the Soret in CHCl3 at 10-5 M. Such a combination
of broadening and red-shifting suggests a slipped lateral
interporphyrin association via π-π stacking that is favored in
aliphatic solvents. Furthermore, previous observations of such
interactions were found in solid-state structures of strapped
porphyrins.14 Thus, dimers of 2Zn and 3Zn must assemble by
additional noncovalent interactions into J-aggregates. Further
dilution to 10-7 M in heptane resulted in a Soret band that was
the same as that of the initial chloroform solution. This latter
observation is consistent with the loss of dimer-dimer interac-
tion due to dilution and is supported by previous observations
that the formation of dimers or higher aggregates not only
depends on the intrinsic properties of the molecules but also
on external conditions.15
Solution studies supported the presence of only monomers,
dimers, or dimer aggregates, depending on the concentration.
The absence of coordination oligomers in solution can be
explained by solution thermodynamics in which entropy favors
the presence of a larger number of smaller species. These results
contrast with those observed in the surface studies presented
below.
AFM Studies
To investigate the aggregation tendency observed in solution,
AFM studies were undertaken based on previous reports of the
surface deposition of J-aggregates and their ability to form
higher assemblies on substrates.16 Noncontact mode AFM
studies showed that identical species can exhibit markedly
different and reproducible behaviors regarding their self-
assembly during a surface-tuned process. Deposition of 2Zn or
3Zn was carried out in various solvents (∼1 µM) by vertical
dipping of freshly cleaved mica or drop-casting on HOPG. In
(14) Melin, F.; Boudon, B.; Lo, M.; Schenk, K. J.; Bonin, M.; Ochsenbein,
P.; Gross, M.; Weiss, J. J. Porphyrins Phthalocyanines 2007, 11, 212.
(15) For examples of distinct monomer-dimer equilibria of dye aggregates
see Wurthner, F.; Yao, S.; Debaerdemaeker, T.; Wortmann, R. J. Am.
Chem. Soc. 2002, 124, 9431.
(17) Guo, Q.; Yin, J.; Palmer, R. E.; Bampos, N.; Sanders, J. K. M. J.
Phys.: Condens. Matter 2003, 15, S3127.
(16) Balaban, T. S. Acc. Chem. Res. 2005, 38, 612.
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Surface-Tuned Porphyrin Assemblies
A R T I C L E S
Figure 4. Influence of the position explored on mica on the morphology of species formed by dipping a support in a ∼1 µM. heptane solution of 2Zn for 15 s. Left
column: topographic images; middle column: 2-D FFT; right column: length distribution histograms. (a) Top edge of support; (b and c) intermediate positions of mica
support; (d) bottom edge of support. The histogram of each image represents the contour length distribution of species observed on the mica surface. For positions c and d,
the histograms and the average length measurements were based on several images, taken on the same horizontal line. Scale bar: 1 µm; vertical scale: 6 nm.
tures, similar in height to the fibers observed for 2Zn (see the
(Figures 5c,d). Mica’s epitaxy was found to be less pronounced
following section), tended to form at the bottom of the support
on the orientation of the fibers formed from 3Zn than from 2Zn,
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A R T I C L E S
Koepf et al.
from 2Zn could be seen on detailed AFM images such as those
in Figure 6. Better resolved images were obtained using AFM
super sharp tips with a much lower curvature radius (2-3 nm).
The high resolution images showed that the fibers were
fragmented rather than continuous, suggesting that the fibers
were formed by lateral association of smaller, linear segments
that were spaced 4.5 ( 0.5 nm apart for the most compact
arrangements (Figures 6b and 6c). The most homogeneous fibers
were observed when each linear segment adopted an orientation
parallel to its neighboring segments and perpendicular to the
axis of the fiber.
The assemblies in Figure 6 measured between 8-21 nm in
width and 1.63 ( 0.43 nm in height. The dimensions of a 2Zn
dimer (Figure 7) calculated with ChemDraw 3D are 3.85 nm
× 1.62 nm (Figure 7). The width of the dimer (1.6 nm) is the
only calculated dimension that matches the height of the objects
(1.63 ( 0.43 nm) observed on mica. This observation suggests
the edge-on deposition of objects that are primarily formed in
solution (Figure 7). The width of the linear objects did not match
any of the calculated dimensions of the dimer; therefore,
additional structuring interactions were considered.
We propose that the formation of the objects thus starts with
the J-type aggregation of dimers (Figure 7c) present in the initial
1 µM heptane stock solution used for dip coating. Small stacks
of four to five dimers are then deposited edge-on onto the
surface, thus allowing association of their lipophilic contours
and minimization of unfavorable interactions with the hydro-
philic surface. The progressive increase in the size of the objects
toward the bottom of the plate and their orientation with respect
to a 6-fold symmetry suggest that growth into larger assemblies
occurs on the surface and is mostly directed by the crystal-
lographic symmetry of mica surface (epitaxial growth). The
dependency of assembly orientation on the epitaxy may be due
to interactions between the phenolic oxygens and the potassium
cations present on the mica surface. Contact between the solution
and substrate was the longest at the bottom of the plate, where
solution was still visible for 20-30 s after the residual drop
was absorbed with paper. Therefore, the small assemblies
already present on the substrate maintain their mobility for a
longer period, allowing extended assemblies to form. Interest-
ingly, the number of small aggregates observed diminished as
longer fibers were observed on the plate. Such a seeding effect
of porphyrin stacks has been proposed to explain the growth of
large lamellar porphyrin assemblies on HOPG.27
AFM of 2Zn and 3Zn on HOPG. Based on the established
stabilizing effect of alkyl chains on HOPG,28 AFM studies of
2Zn and 3Zn deposited on lipophilic HOPG surface were
undertaken. Acetone and dioxane were chosen for in-depth AFM
studies on HOPG because these solvents initially demonstrated
the best wetting ability and produced the highest degree of
homogeneity of assemblies on the graphite surface. Both the
behavior and the growth process of the objects obtained from
2Zn or 3Zn on HOPG (Figure 8) are strikingly different from
those observed on mica. Drop-casting deposition and air
evaporation of 0.5 µM acetone solutions of 2Zn on HOPG
produced short linear objects (Figure 8a) that were 13-18 nm
wide, 20-40 nm long, and 1.26 ( 0.09 nm high. These objects
were consistently shorter than the fibers observed on mica.
Under similar deposition conditions, 3Zn led to slightly longer
Figure 5. AFM images of 3Zn dip-coated onto mica. Mica supports were
dipped for 15 s into a 1 µM heptane solution of 3Zn. (a and b) Top area
of mica support. (c and d) Bottom area of the support. Scale bar: 1 µm;
vertical scale: 5 nm.
as exemplified by the more uniform dispersion of frequencies
in 2D-FFT spectra (insets in Figures 5c,d). These observations
are consistent with an erratic contribution of a spinodal
dewetting phenomenon that is widely used to explain self-
organization and molecular ordering on various surfaces,18–22
including the case of porphyrin species.17,23,24
It should be noted that spinodal dewetting patterns are less
frequently observed for the C12-appended 2Zn structure than
for the C18-appended 3Zn. In the case of 2Zn, dewetting patterns
were observed on very small, localized sections of the substrates
(SI Figure S4), but the corresponding images were not repre-
sentative of the assemblies observed over the major portion of
the substrates. The low occurrence of random distribution (as
opposed to aligned patterns) of the assemblies demonstrates that
in our case, in heptane, spinodal dewetting does not play an
important role in the formation of assemblies.25 Furthermore,
the fast Fourier transforms shown in Figures 4b-d are atypical
of spinodal dewetting26 and show a pronounced influence of
mica’s epitaxy.
Fine Structure and Organization of 2Zn Assemblies on
Mica. Because of their greater homogeneity over a larger surface
of deposition, assemblies produced from 2Zn were selected for
more detailed investigation. The fine structure of fibers formed
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(25) An insignificant role of spinodal dewetting was suggested in the case
of n-hexane solutions on mica in ref 24.
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(26) FFT of spinodal dewetting processes are spherical. See ref 21.
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A R T I C L E S
Figure 6. (a and b) Details of 2Zn fibers. (c) Cross-section taken from image b (white line). Sample obtained by dip-coating a mica sample in a ∼1 µM
heptane solution of 2Zn for 15 s. AFM tips with a typical radius of 2-3 nm were employed to increase the resolution. Scale bar: 100 nm; vertical scale:
5 nm.
Figure 7. Representation and dimensions of dimer 2Zn and its proposed organization on mica. (a) Profile and theoretical dimensions (in bold) of 2Zn
dimer. The measured height of the assemblies on the mica surface is given in italic. The large diameter of the dimer determined by DOSY NMR calculations
is given in italic. (b) Proposed assembly of 2Zn dimers. (c) Proposed orientation and assembly mode of strands of dimers on the mica surface (top view)
and indication of the theoretical periodicity for an assembly without intercalation of the lipophilic chains. Theoretical values are given in bold, and values
measured from the AFM images are given in italic.
linear objects and more aggregation (Figure 8d) than for 2Zn.
The linear objects formed from 3Zn were 15-19 nm wide and
1.25 ( 0.22 nm high.
interactions are always maximized between the lateral side
chains and the graphite surface.
In larger aggregates in which individual linear wires were
distinct, such as the circled assemblies of 3Zn in Figure 8e, the
lowest measurable center-to-center distance between two wires
was 9 nm (SI Figure S5). This distance characterizes an
assembly assigned to an intermediate situation between isolated
linear wires and the islands of films formed by lateral inter-
digitation of alky side chains. The interdigitation of alkyl chains
would be characterized by distances shorter than 6 nm for 3Zn,
and less than 5 nm for 2Zn. Such short spacing was not observed
in the islands of films, suggesting that a very compact arrange-
ment is reached (SI Figure S6). In this higher degree of
organization the height of the islands was highly uniform, with
no distinguishable individual segments, and no corrugation. Such
a uniform aspect confirmed that individual segments (wires)
were in close proximity.
Thus, contrary to the situation with heptane and mica in which
the hydrophilic surface forces a solution self-assembly process,
the affinity of the side chains and of the porphyrin’s flat
lipophilic surface for HOPG leads to a complete surface-assisted
self-assembly process. At submicromolar concentrations in
acetone, dimers are mostly dissociated, as described above.
Thus, the formation of coordination polymers takes place a
posteriori to the deposition of mobile isolated monomers onto
When acetone solutions were evaporated more slowly in a
solvent-saturated atmosphere, a higher degree of association was
observed. For 3Zn, the lateral assembly of wires was observed,
as shown in Figures 8e and 8f (circled objects). Under the same
deposition conditions, 2Zn formed small islands of film as seen
in Figure 8b via progressive lateral association of objects
depicted in Figure 8c. These islands are reminiscent of as-
semblies observed for zinc porphyrins bearing four methyl ester
side chains and two (trimethylsilylethynyl)phenyl groups.17
Organization of 2Zn and 3Zn on HOPG. The heights of
assemblies of 2Zn and 3Zn on HOPG were respectively 1.26
( 0.09 and 1.25 ( 0.22 nm. These values measured on isolated
linear objects are 0.4-0.5 nm smaller than those observed for
the same assemblies on mica, suggesting a different mode of
organization. The theoretical distance from the phenolic oxygen
to the top of the phenanthroline strap was calculated to be 1.25
nm using Chem 3D standard modeling. The correspondence
between calculated height and the observed AFM heights of
the objects indicates that the isolated linear objects and films
were a single molecule thick. Thus, it is proposed that the
porphyrins lie flat on the graphite surface with C12 or C18 alky
chains to either side, as depicted in Figure 9. In this way,
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A R T I C L E S
Koepf et al.
Figure 8. AFM images of 2Zn and 3Zn drop-casted (0.5 µM in acetone) onto HOPG: (a) 2Zn evaporation in air; (b) 2Zn slow evaporation in an acetone
saturated atmosphere; (c) 2Zn slow evaporation in an acetone saturated atmosphere: detail of the lateral aggregation process; (d) 3Zn evaporation in air; (e
and f) 3Zn slow evaporation in an acetone saturated atmosphere. Amplitude image (deflection scale ) 0.02 V) that illustrates the side by side aggregation
of 3Zn. Scale bar: 500 nm; vertical scale: 5 nm.
Figure 9. Calculated height (in bold) and proposed mode of assembly of 2Zn or 3Zn on HOPG. Typical height range observed by AFM is given in italic.
the HOPG surface. Thus, the assembly process begins when
the imidazole arm of one monomer is bound within the
phenanthroline strap of a second monomer that is sufficiently
close to the first monomer. The assembly process continues in
this manner until complete evaporation of the solvent. Accord-
ingly, it would be expected that monomers with a moderate
affinity for HOPG would preserve some mobility over a longer
period of time than monomers with a higher affinity, especially
when slow evaporation proceeds under a solvent saturated
atmosphere.
observed for 2Zn (Figure 8b), only small groups of laterally
assembled wires were visible for 3Zn (Figure 8e). The relatively
small size of the islands of film in Figure 8b was probably
caused by fast evaporation of acetone (bp 56 °C). The
concomitant lower occurrence of small objects on the surround-
ing HOPG steps confirms that the short C12 functionalized linear
segments tend to be mobile on the surface. Assemblies of 3Zn
with longer C18 side chains should intuitively have a greater
affinity than 2Zn for the graphite surface. The absence of island
formation for 3Zn that is evaporated slowly in acetone-saturated
atmosphere is probably due to the reduced mobility.
This hypothesis is supported by comparison of the aggregates
of 2Zn and 3Zn formed by evaporation in an acetone-saturated
atmosphere. Whereas small islands of oriented films were
Further confirmation that aggregate size is mobility dependent
comes from the observation of films of 2Zn formed by slow
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Surface-Tuned Porphyrin Assemblies
A R T I C L E S
will raise problems related to the perturbation of these interac-
tions by molecule/surface interactions. These results show how
interactions between a surface and molecules can influence the
self-assembly process. In solution, porphyrins 2Zn and 3Zn both
exist as dimers that form J-aggregates. When deposited on mica,
the dimer and J-aggregate structures remain intact and assemble
into larger fibers by weak intermolecular interactions. On HOPG,
favorable CH-π interactions between the porphyrins’ long alkyl
side chains and the graphite surface prevent dimer formation
and favor the association of monomers into linear coordination
polymers. In the prospect of incorporating self-assembled
molecular materials on patterned surfaces, recent results em-
phasize the surface-molecule interactions as a crucial parameter
to control.31 This series of self-assembling strapped porphyrins
represent ideal candidates for a systematic exploration of side
chain effects on the growth of monomolecular films which is
under progress.
Figure 10. AFM images of 2Zn in dioxane drop-casted on HOPG: (a)
evaporation in air; (b) evaporation in a dioxane saturated atmosphere. Scale
bar: 500 nm. Vertical scale: 6 nm.
evaporation of drop-casted dioxane solutions on HOPG (Figure
10b). By using dioxane, a less polar (0.45 D) and less volatile
(bp 102 °C) solvent, the mobility of the coordination polymer
segments was maintained over a longer period, allowing larger
films to form (thickness ) 1.25 ( 0.22 nm). In this case, almost
no isolated short segments were seen on the surface. The
evolution of organization as a function of the species’ mobility
is reminiscent of that reported by Otsuki for tripod molecules
bearing long alkyl side chains.29
Acknowledgment. M.K. is grateful to the Re´gion Alsace and
the CNRS for a Ph.D. fellowship. We are grateful to Dr. M. Elhabiri
for calculation of the association constant for 2Zn dimer. This work
was supported by the CNRS (ACI-NX001) and Research Council
of the Universite´ Louis Pasteur.
Supporting Information Available: Synthetic and surface
deposition procedures, DOSY NMR, Ka determination by
UV-visible spectroscopy, UV-vis of 2Zn in CH2Cl2, acetone,
and CHCl3, and occasionally observed AFM images of 2Zn on
mica are provided free of charge via the Internet at http://
pubs.acs.
Conclusion
Molecular recognition by combined weak interactions is often
used to obtain self-corrected assembly of materials.30 The
incorporation of self-assembled materials onto patterned surfaces
(29) Otsuki, J.; Shimizu, S.; Fumino, M. Langmuir 2006, 22, 6056.
(30) During the course of this work, a similar iterative approach using axial
base coordination in mutated hemoprotein was reported. Kitagishi,
H.; Oohora, K.; Yamaguchi, H.; Sato, H.; Matsuo, T.; Harada, A.;
Hayashi, T. J. Am. Chem. Soc. 2007, 129, 10326.
JA800039A
(31) Wei, Y.; Tong, W.; Zimmt, M. B. J. Am. Chem. Soc. 2008, 130, 3399.
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