The polymorphism of porphyrin 2D assemblies at the liquid-graphite interface: The effect of a polar solvent additive and a flexible spacer on the face-on and edge-on type molecular arrangements

Article abstract of DOI:10.1039/c9cc02579k

Self-assembly structures of 5,10,15,20-tetrakis(4-substituted phenyl)porphyrins at the liquid-graphite interface were investigated by scanning tunneling microscopy. We found that the presence of a small amount of a polar solvent, i.e., only 0.5 vol% of octanoic acid in phenyloctane, significantly affected the selective formation of the face-on polymorph over the edge-on one likely due to the solvent-molecule interactions at the 2D interface.

Full text of DOI:10.1039/c9cc02579k

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Polymorphism of Porphyrin 2D Assemblies at the Liquid–Graphite
Interface: Effect of Polar Solvent Additive and Flexible Spacer on
the Face-On and Edge-On Type Molecular Arrangements
Received 00th January 20xx,
Accepted 00th January 20xx
a
b
a
DOI: 10.1039/x0xx00000x
Kenya Adachi, Takashi Hirose* and Kenji Matsuda*
www.rsc.org/
Self-assembly structures of 5,10,15,20-tetrakis(4-substituted "face-on" arrangement where the molecular plane of porphyrin
phenyl)porphyrins at the liquid–graphite interface was arranges parallel to the substrate,23,25 and the other is "edge-on"
investigated by scanning tunneling microscopy. We found that the arrangement in which porphyrins are perpendicularly oriented
presence of a small amount of polar solvent, i.e., only 0.5 vol % of to the substrate and the porphyrin cores stack on top of each
octanoic acid in phenyloctane, significantly affected the selective other on the substrate.26–30 It is noted that for porphyrins the
formation of the face-on polymorph over the edge-on one likely face-on ordering is much more common than the other
due to the solvent–molecule interactions at the 2D interface.
regardless of the type of substrate, i.e., graphite or metal,23,26,27
suggesting a significant contribution of van der Waals
interactions between the planar porphyrin molecules and
substrate to the formation of 2D self-assemblies.
The edge-on arrangement should be formed by stabilizing
the perpendicular - stacking structure of porphyrin cores. We
have recently reported that a zinc-coordinated 5,10,15,20-
tetraphenylporphyrin (Zn-TPP) with four alkoxy groups
showed a polymorphic transition from a face-on arrangement to
Precise control over intermolecular arrangement in ordered
assemblies is of primary importance for the development of
functional materials in the solid state.1–3 When multiple crystal
structures are obtained, i.e., so-called polymorphism,4–7
selecting a specific molecular arrangement is a straightforward
approach to control the solid-state properties. By designing a
specific crystal structure out of possible polymorphs, physical
and chemical properties in the solid state could be greatly
controlled as desired, which is a major concern in materials
chemistry and pharmaceutical industry.
In contrast to the three-dimensional (3D) ordered
assemblies that can be unambiguously determined by X-ray
single crystal analyses, two-dimensional (2D) self-assemblies
have been investigated in detail using scanning tunnelling
microscopy (STM) by which each molecule in 2D ordered
assembly is well visualized with the single-molecule resolution
in real time.8–11 Recently, polymorphism in 2D assemblies has
been receiving increasing attention because of their potential
applications for sophisticated surface functionalization working
at the single-molecule level.12–16
an edge-on one at the phenyloctane (PO)–highly oriented
pyrolytic graphite (HOPG) interface.31 Interestingly, this
polymorphic transition proceeded on a slow timescale of
several hours. On the other hand, a Zn-TPP with four
alkylcarbamoyl groups, i.e., 1-Zn in Fig. 1, formed an edge-on
ordering in a cast film prepared from chloroform solution,
which was stable during the STM measurements at the PO–
HOPG interface. These results suggested that the introduction
of amide group stabilizing the - stacking structure of TPPs
would be a key molecular design for stabilizing the edge-on
arrangement of porphyrins in 2D self-assemblies.
The face-on type porphyrin assembly can be used as an
ideal molecular template of multi-component 2D self-
assemblies, by which molecular conductance of functional
Molecular rigidity, planarity, and high symmetry are desired
requirements to assemble molecules into 2D ordered
nanostructures.17–19 Porphyrins are widely-used organic
molecules for constructing 2D assemblies because of their rigid
square-planar molecular geometry with D4h symmetry.20–24 Two
major polymorphs have been reported for porphyrins; one is
R
R
R
R
N
NH HN
N
N
N Zn N
N
a
R
R
R
R
Department of Synthetic Chemistry and Biological Chemistry, Graduate School of
Engineering, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan. E-
mail: kmatsuda@sbchem.kyoto-u.ac.jp
Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan.
1
-Fb, R = -CONH-C16H33
-Fb, R = -(CH2)2-CONH-C16H33
Chemical structures of meso-5,10,15,20-substituted free-base porphyrin
derivatives (1-Fb and 2-Fb), and a zinc complexes (1-Zn).
1-Zn, R = -CONH-C16H33
2
b
E-mail: hirose@scl.kyoto-u.ac.jp
Fig. 1
†
Electronic Supplementary Information (ESI) available: Experimental details and
additional STM images. See DOI: 10.1039/x0xx00000x
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molecules can be quantitatively investigated using STM.32,33 On
the other hand, the edge-on assembly can be applicable to
photovoltaic cells and molecular wires in molecular electronics
because of the high carrier mobility and good conductance of
the porphyrin aggregates with - stacking structure.34–37 In
this context, systematic investigation on molecular design for
the control over the 2D polymorphs is of particular importance
for the development of porphyrin-based functional 2D systems.
In this work, we conducted careful investigations on the
stability of the edge-on type 2D molecular orderings composed
of alkylcarbamoyl substituted TPPs, i.e., 1-Fb and 2-Fb with
and without flexible ethylene spacer (Fig. 1), in terms of the
effects of (1) concentration, (2) polar solvent additives, and (3)
the position of amide group in alkyl side chains. We revealed
that 1-Fb formed an edge-on ordering at the PO–graphite
interface only at high concentration more than 100 M,
whereas a loosely packed face-on arrangement became
dominant in dilute conditions. Interestingly, a small amount of
polar additive, i.e., only 0.5 vol % of octanoic acid (OA) in PO,
significantly stabilized the face-on ordering likely due to the
solvent–adsorbate interactions at the liquid–graphite interface.
Introduction of flexible ethylene spacer significantly
contributed to the stabilization of the edge-on arrangement, and
the edge-on ordering of 2-Fb was exclusively formed even at
low concentrations (~20 M) at the PO–graphite interface.
Porphyrin 1-Fb with four hexadecylcarbamoyl groups was
synthesized according to the literature procedure.31 To
investigate the effect of flexible ethylene spacer that links
carbamoyl group with porphyrin core on the stability of the
edge-on type 2D polymorph, 2-Fb was synthesized according
to the route shown in Scheme 1. Synthetic details and
characterization of 2-Fb are shown in the ESI†.
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DOI: 10.1039/C9CC02579K
Fig. 2
(a) Chemical structures of octanoic acid (OA) and phenyloctane (PO) (b,c)
High-resolution STM images of 1-Fb at the (a) octanoic acid– and (b) phenyloctane–
graphite interfaces (c = 100 M, Iset = 20 pA, Vbias = −0.80 V).
Table 1. Lattice parameters of the 2D orderings at the liquid–graphite interfacea
b
unit cell parameters
a (nm) b (nm)
c
crit
compd solvent
OA
polymorph
 (°)
(M)
3.47 ± 0.01 1.81 ± 0.02 79 ± 2
3.63 ± 0.05 1.82 ± 0.03 82 ± 2
face-on
face-on
edge-on
face-on
face-onc
edge-on
–
1
-Fb
PO
OA
PO
100
–
4
.6 ± 0.2
3.57 ± 0.05 2.05 ± 0.01 77 ± 1
–c –c –c
4.76 ± 0.12 0.47 ± 0.03 88 ± 1
0.50 ± 0.03 89 ± 1
2-Fb
< 60
aDetermined by STM measurements. OA, octanoic acid; PO, phenyloctane.
bCritical concentration at which surface coverage of edge-on ordering saturates.
cFace-on ordering was not detected for 2-Fb even at low concentration (20 M).
COOH
R =
R
R
C16H33NH2
EDC, HOBt
interdigitated between the adjacent rows of porphyrins. Lattice
parameters of the face-on ordering of 1-Fb were a = 3.47 ±0.01
nm, b = 1.81 ± 0.02 nm, and  = 79 ± 2° (Table 1).
CHO
N
NH HN
N
N
H
2-Fb
propionic acid
dry CHCl3
27%
HOOC
3
2%
Absorption spectra of 1-Fb in PO, recorded at
concentrations of 6.25–100 M, was broadened compared to
that in OA, suggesting some aggregation of 1-Fb in PO
solution (Figs. S1 and S2, ESI†). When a solution of 1-Fb in
PO (100 M, 10 L) was deposited on HOPG, a continuous
stripe-patterned ordering with a bright line in the middle of
stripes was observed, which was reproduced by MM/MD
calculations assuming the edge-on arrangement (Fig. 2c). The
model simulation suggested that the porphyrin cores stacked on
top of each other to form the middle bright line with a 4-fold
rows of intermolecular hydrogen bonding comprising of amide
group. Only the two out of four alkyl chains adsorbed on the
graphite substrate and remaining the other half of alkyl chains
are considered to dissolve into supernatant solution phase.
Lattice parameters of the edge-on ordering of 1-Fb were a =
R
R
3
4
Scheme 1
Synthesis of porphyrin with ethylene spacer 2-Fb. EDC, 1-ethyl-3-(3-
dimethylaminopropyl)carbodiimide hydrochloride; HOBt, 1-hydroxybenzotriazole.
The formation of 2D polymorphs of 1-Fb was investigated
by STM using two different solvents, i.e., OA and PO (Fig. 2).
Due to a low solubility of 1-Fb, especially in PO, concentrated
chloroform solution of 1-Fb (~10 L) was injected in an excess
amount of each solvent (i.e., OA and PO, ~190 L) to prepare
solution samples. When a solution of 1-Fb in OA (100 M, 10
L) was deposited on a freshly cleaved HOPG surface, isolated
bright squares with a butterfly-like contrast were observed with
the sub-molecule resolution (Fig. 2b). The ordering pattern was
successfully reproduced by a molecular mechanics/molecular
dynamics (MM/MD) calculations assuming the face-on
arrangement, suggesting that the porphyrin core and four
alkylcarbamoyl groups were fully adsorbed on graphite
substrate in cofacial manner and alkyl chains were
4.6 ± 0.2 nm, b = 0.50 ± 0.03 nm, and  = 89 ± 1° determined
by high-resolution STM images (Table 1). It is noted that the
length of the a-axis for the edge-on ordering (4.6 nm) was
significantly larger than that for the face-on one (3.47 nm),
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Fig. 3
(a) Representative STM images showing the polymorphic transition from the face-on (highlighted by blue in the bottom images) to the edge-on (highlighted by red)
orderings composed of 1-Fb with increasing sample concentration at the phenyloctane–graphite interfaces (c = 25–100 M, Iset = 20 pA, Vbias = −0.80 V) (b) Surface coverage plot
depending on concentration (blue square and red circle, the face-on and edge-on orderings, respectively; filled symbols, phenyloctane was used as a liquid phase; open symbols, a
mixed solvent of octanoic acid/phenyloctane = 0.5/99.5 (v/v) was used as a liquid phase). For STM images and surface coverage data at each concentration, see Figs. S6 and S7 and
Tables S6 and S7 in the ESI†.
which is reasonable considering the lack of interdigitation of face-on ordering increased up to 95% at 25 M and the critical
alkyl side chains in the edge-on arrangement. The difference in concentration (ccrit) increased by 2 times from 100 to 200 M
the length of the a-axis was used as a decisive parameter to (Fig. 3b). It was further demonstrated that a complete
distinguish the two polymorphs especially in low-resolution polymorphic transition from the edge-on to face-on orderings
STM images as described below.
was achieved by addition of 5 vol % of OA at the PO–HOPG
Polymorphic transition depending on concentration interface when concentration of 1-Fb was set at 100 M (Fig.
provides a clear overview on the emergence of each 2D S8, ESI†). Thus, the concentration region where two different
assemblies. The edge-on ordering of 1-Fb was dominantly polymorphs switches was found to be very sensitive to the
observed at the PO–HOPG interface at high concentrations presence of polar solvent additives.
more than 100 M, whereas the two polymorphs, i.e., the edge-
To investigate a dynamic nature of the polymorphic
on and face-on orderings, was competitively formed at low transition, time course change of STM images were recorded at
concentration conditions less than the critical concentration at the same place at the PO–HOPG interface in the presence of 0.5
which the surface coverage of the edge-on ordering saturates vol % of OA. The real-time STM measurements suggested that
(ccrit = 100 M for 1-Fb) (Fig. 3). The fractional surface one minor polymorph tended to be replaced by the other
coverage of the face-on domains of 1-Fb has maximum of 58% polymorph that is major in a concentration condition; the
at concentration of 25 M. It is noted that the observation of polymorphic transition from a small edge-on domain to major
densely-packed polymorph at higher concentration was face-on orderings was observed at 25 M, whereas a transition
supported by adsorption energies calculated by MM/MD model with the opposite direction at 150 M (Fig. S9, ESI†), which is
(
Fig. S3 and Tables S2–S5, ESI†) model, and is consistent with likely explained by Ostwald ripening in which small crystals
the literature16,38,39 including our previous works.40,41
dissolve with concomitant growth of large crystals because of a
In contrast to the polymorphic transition depending on high surface energy of the small particles.42–45
concentration observed at the PO–HOPG interface, the face-on
Slow dynamics of 2D ordering formation was also
ordering was predominantly observed when OA was used as supported by annealing experiments. The face-on ordering, that
liquid phase. The face-on ordering was constantly observed at is co-existed with the edge-on ordering at concentrations of 50
OA–HOPG interface even at high concentrations up to 600 M and 125 M before annealing (Fig 3b), was almost completely
(
Figs. S4 and S5, ESI†), suggesting that the face-on ordering of replaced with the edge-on ordering after annealing at 60 °C
-Fb is kinetically or thermodynamically stabilized by the (Fig. S10, ESI†), suggesting that the face-on ordering of 1-Fb
presence of polar solvent. is likely kinetically formed at the OA/PO–HOPG interface
1
To investigate the effect of polar solvent on the emergence (0.5/99.5, v/v) and the edge-on type arrangement is
of the 2D polymorphs of porphyrins more carefully, a small thermodynamically stable for 1-Fb.
volume of OA was added to an excess amount of PO solution
Next, 2D assembly structures of 2-Fb with flexible ethylene
of 1-Fb before the deposition on HOPG, and concentration group was investigated by STM. When concentration of 2-Fb
dependence of surface coverage was then investigated by STM. was set at 100 M, a selective formation of the face-on
When 0.5 vol % of OA was added in the solvent, the ordering at the OA–HOPG interface and the edge-on at the PO–
concentration region at which the face-on ordering was formed HOPG interface was observed (Fig. S11, ESI†), which is the
has significantly widen; the fractional surface coverage of the same polymorphic behavior to 1-Fb in the same condition (Fig
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). Lattice parameters of the two polymorphs for 2-Fb were
Journal Name
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1
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4 R. Gutzler, L. Cardenas and F. Rosei, Chem. Sci., 2011, 2,
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290.
DOI: 10.1039/C9CC02579K
summarized in Table 1. The length of the a-axis for each
polymorph of 2-Fb was longer than that of 1-Fb by 1–2 Å,
which is consistent with the introduction of ethylene spacer. An
important finding was that the edge-on ordering was
exclusively observed for 2-Fb at the PO–HOPG interface even
when surface coverage was not saturated at low concentration
of 20 M (Fig. S12, ESI†), which is marked contrast to the
predominant formation of the face-on ordering of 1-Fb at low
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This work was supported by a Grant-in-Aid for Scientific
Research on Innovative Areas “Photosynergetics” (JSPS
KAKENHI Grant Number JP26107008) from MEXT, Japan. T.
H. acknowledges JSPS for a Grant-in-Aid for Scientific
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Graphical Abstract
Communication
Polymorphism of Porphyrin 2D Assemblies at the Liquid–Graphite Interface: Effect of
Polar Solvent Additive and Flexible Spacer on the Face-On and Edge-On Type
Molecular Arrangements
Kenya Adachi, Takashi Hirose and Kenji Matsuda
Edge-on type 2D self-assembly can be selectively formed by managing hydrogen bonding
interactions at the liquid–solid interface.