Dynamic rearrangement of bilayers of porphyrin hetero-dimers at a solid/liquid interface

Article abstract of DOI:10.1039/c4cc02963a

STM reveals that self-assembled bilayers of covalently linked Cu-Mn porphyrin dimers can dynamically rearrange at a solid/liquid interface by rapid exchange of molecules between the bilayer and the supernatant, or via a slower two-dimensional translation of the whole top layer.

Full text of DOI:10.1039/c4cc02963a

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Dynamic rearrangement of bilayers of porphyrin
hetero-dimers at a solid/liquid interface†
Cite this: Chem. Commun., 2014,
50, 7291
Dennis Lensen, Thomas Habets and Johannes A. A. W. Elemans*
Received 21st April 2014,
Accepted 18th May 2014
DOI: 10.1039/c4cc02963a
www.rsc.org/chemcomm
STM reveals that self-assembled bilayers of covalently linked Cu–Mn oriented pyrolytic graphite (HOPG) surface via p–p and van der
porphyrin dimers can dynamically rearrange at a solid/liquid inter- Waals interactions.
face by rapid exchange of molecules between the bilayer and the
We first investigated the self-assembly behaviour of the
supernatant, or via a slower two-dimensional translation of the Cu–Cu porphyrin dimer 1. After the addition of a droplet of a
whole top layer.
solution of 1 in 1-phenyloctane ([1] = 4.6 Â 10À5 M) onto a
freshly cleaved HOPG surface, STM revealed the formation of
In nature metal–porphyrins are involved in processes such as an extended monolayer structure of bright zig-zag like arrays
light harvesting in plants and oxygen binding in blood. Synthetic (Fig. 1B). In the magnification in Fig. 1C, the porphyrin dimers
metal–porphyrins have been widely used as catalysts, e.g. for the are submolecularly resolved, revealing the individual porphyrin
oxidation of alkanes to alcohols and of alkenes to epoxides.1 planes, which appear as cross-like features. The alkyl tails are
We have recently investigated manganese porphyrin-mediated less resolved and reside in the dark regions between the bright
oxidation reactions at the single molecule level in real-time at a signatures. The proposed orientation of one dimer molecule is
solid/liquid interface, employing Scanning Tunnelling Micro- indicated by a molecular model superimposed on the STM
scopy (STM) on gold2 and graphite3 surfaces. The superior image in Fig. 1C. The unit cell contains one molecule of 1 and
resolution offered by STM revealed that when O2 was used as
the oxidant, pairs of manganese porphyrins at adjacent locations
in the monolayer were preferentially oxidised, yielding Mn(IV)QO
species. This correlation suggested the occurrence of homolytic
dissociation of O2. To investigate such correlative effects in
reactive porphyrin monolayers further, we decided to synthesise
covalently linked metal–porphyrin dimers and investigate their
physical and chemical properties with STM. Here we present
porphyrin dimers with two identical and with two different
metals in their cores, and analyse their self-assembly behaviour
at the solid–liquid interface with the help of STM.
Two porphyrin dimers were synthesised, one with two Cu(II)
centres in its cores (1) and the other with one Cu(II) centre and
one Mn(III)Cl centre (2) (Fig. 1A) (see ESI†). The acetylene spacer
between the two porphyrin moieties ensures a rigid and well-
defined connection, while it still allows their rotation along the
phenyl–acetylene bonds. When the porphyrins of a dimer are
coplanar they have an ideal geometry to adsorb at a highly
Fig. 1 (A) Molecular structures of Cu–Cu porphyrin dimer 1 and Cu–Mn
porphyrin dimer 2. (B) STM image of a self-assembled monolayer of 1 at
the graphite/1-phenyloctane interface. The white asterisk represents the
symmetry directions of the underlying graphite lattice. Vbias = À450 mV and
Iset = 3 pA. (C) Magnification, with a molecular model of the conjugated
parts of 1 superimposed and the unit cell drawn in.
Radboud University Nijmegen, Institute for Molecules and Materials,
Heyendaalseweg 135, 6525 AJ, Nijmegen, The Netherlands.
E-mail: J.Elemans@science.ru.nl; Tel: +31 24 3653099
† Electronic supplementary information (ESI) available: Synthesis and characteri-
sation of 1 and 2, statistical data, and additional STM images. See DOI: 10.1039/
c4cc02963a
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was determined to have unit cell vectors a = 3.53 Æ 0.1 and that the Mn–Cu dimers do not have a significant preferred
b = 4.07 Æ 0.1 nm2, at an angle of 68 Æ 21 (Fig. 1C).
orientation with respect to their neighbours.
While covalently linked porphyrin dimers have been imaged
Occasionally the Cu–Mn porphyrin dimers appeared to be
by STM under ultrahigh vacuum conditions,4 so far no STM arranged in a double layer6 (Fig. 2C and D), in which one of the
studies of covalently linked porphyrin hetero-dimers have been porphyrins of a dimer in the top layer is stacked in an H-type
reported. When a droplet of a solution of 2 in 1-phenyloctane geometry on top of a porphyrin of a dimer in the bottom layer,
([2] = 4.6 Â 105 M) was placed on HOPG, immediately large and the other porphyrin in an offset, J-type geometry with
domains of the porphyrin dimers were observed by STM respect to a porphyrin of another dimer in the bottom layer.
(Fig. 2A and B). The two halves of each of the dimers clearly This arrangement is schematically depicted by the red and blue
appear as different signatures, which is attributed to the squares in Fig. 2D. Similar double layers were never observed
presence of different metals in the porphyrin cores. Similar for Cu–Cu dimer 1, which suggests that the Mn porphyrin
as in the case of 1 the Cu–porphyrin moiety of 2 appears moiety of 2 plays an essential role in bilayer formation, possibly
relatively dark, while the MnCl–porphyrin has a bright feature because of specific, favourable p–p stacking interactions between
in its core. Previous STM studies of mixed monolayers of porphyrins in the top and bottom layer. So far we were not able to
monomeric porphyrins with different metal centres also control or induce bilayer formation, e.g. by varying the concen-
revealed that the difference in their electronic signature can tration of 2 in the supernatant. An interesting aspect of the
allow easy distinction of the different porphyrin species.5 The bilayer structure is the significant difference in unit cell para-
unit cell parameters of the monolayer of 2 are equal to those of meters when compared to those of a monolayer of 2: the unit
the monolayer of 1, which is not surprising since only one cells of both the bottom and the top layer of the Cu–Mn dimers
metal centre was replaced. The rotational symmetry of the have vectors a = 4.02 Æ 0.4 and b = 4.30 Æ 0.1 nm2, at an angle of
dimers along the centre of the acetylene bond in the plane of 76 Æ 21. This means that the presence of the top layer is
the interface changes though, from C2 for 1 to C1 for 2. To accompanied by changes in the structural properties of the whole
investigate whether this symmetry change led to a preference system, i.e. also in the organisation of the bottom layer of
for either Cu porphyrin or Mn porphyrin nearest neighbour porphyrin dimers with respect to the graphite surface. This is
moieties in the case of 2, 635 molecules in the monolayer were probably the result of specific stacking interactions between the
analysed. In case of 338 of these dimers (53%), a Mn porphyrin porphyrins in the layers.
moiety was located next to a Cu porphyrin moiety in an
Statistical analysis of 1223 dimer molecules in the top layer
adjacent dimer in the same lamellar array, which indicates revealed that 60 Æ 2% of the top porphyrins that are arranged
in an H-type geometry with respect to the porphyrin in the
bottom layer are MnCl–porphyrins (ESI,† Fig. S1). As a conse-
quence, 60% of the Cu–porphyrins in the top layer are oriented
in a J-type geometry with respect to a porphyrin in the bottom
layer. The identity and orientation of the porphyrin moieties in
the dimers in the bottom layer could not be derived directly
from the STM images. It would, however, be realistic to assume
that the porphyrins in the bottom layer that exist in an H-type
geometry contain a Cu centre, since it lacks an axial ligand, in
contrast to a Mn–porphyrin. The latter has a chloride axial
ligand that introduces a steric bulk, which does not favour
stacking of a porphyrin directly on top of it. In case the bottom
porphyrin is a Cu one, the Mn–porphyrin can stack on top of
it in a face-to-face geometry, with its chloride ligand facing
upward.
The top layer turned out to be capable of reorganising itself
by means of two different dynamic processes: (i) by desorption
and readsorption of single porphyrin dimers in the top layer,
mediated by the supernatant, or (ii) by a 2-dimensional transla-
tion of the complete top layer. The first process was evident from
the observation of switching of the orientation of Cu and MnCl
centres of molecules of 2 in the top layer between consecutive
STM scans (Dt = 5 min, contrast-enhanced STM images in Fig. 3).
When a 50 Â 50 nm2 area of the bilayer was monitored for 10
Fig. 2 (A) STM image of a monolayer of Cu–Mn porphyrin dimer 2 at the
graphite/1-phenyloctane interface. Vbias
=
À450 mV and Iset
= 5 pA.
consecutive scans, a minimum of 22 porphyrin dimers (about
20%) per scan changed orientation in such a way, while the
other dimers in the top layer seemed to retain their position.
The dynamic behaviour can be explained by assuming that
(B) Magnification of the STM image in (A). (C) STM image of a bilayer of 2,
recorded under the same conditions as the monolayer in (A). (D) Magnification
of the STM image in (C). The red squares represent porphyrin dimers in the
top layer and the blue squares porphyrins in the bottom layer.
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twice as low as that of porphyrin moiety II, which indicates that
the former belongs to a porphyrin dimer situated in the bottom
layer. It has an apparent height of 0.8–1.0 Å, which is typically
observed for a monolayer of Cu–Mn–porphyrin dimers physi-
sorbed on HOPG (ESI,† Fig. S2).
Porphyrin moiety II belongs to a dimer in the top layer and
interacts in a J-type geometry with porphyrin moiety I. An
H-type assembly of two porphyrins on top of each other is
observed at the location labelled III. Its apparent height is twice
that of porphyrin moiety I and only slightly higher than the
offset-stacked porphyrins at location II. This difference might
be the result of a somewhat larger steric hindrance between two
porphyrins in an H-type assembly, or of a higher conductivity
through the coplanar p-stacked aromatic systems. In the
consecutive scan, the geometry between the porphyrins in the
top and bottom layers has changed completely to an H-type
structure (Fig. 4B). This geometry change was accompanied and
confirmed by changes in the apparent height profiles, and is
apparently the result a lateral movement of porphyrin dimers in
the top layer, along the lamella (Fig. 4A - B). Although a fully
H-type geometry is not expected to be the energetically most
favourable one, it seems that when a transformation to such
type assembly occurs somewhere in the top layer, it is subse-
quently transferred throughout the entire domain (ESI,† Fig. S3
and S4). We have no evidence that the translation of the top
layer is a direct result of scanning by the tip, since STM images
show the two different aggregation types at different locations
in a single scan line (ESI,† Fig. S3A, B and E). However, the tip
might have induced the movement of the first porphyrin
dimers into their new geometry, after which the packing
gradually changed over the entire domain. After its formation,
the full H-type packing of the porphyrins was observed during
only four consecutive scans (15 min), which indicates that it is
metastable. The porphyrin dimers in the top layer subsequently
moved again along the lamella into a new double layer geometry,
(Fig. 4B - C), resembling the first one. This packing remained
stable for hours. In this final packing the offset-stacked
porphyrin moieties were positioned at the other side of the
porphyrins in the bottom layer when compared to the packing
in Fig. 4A. This difference was also confirmed by the apparent
height profiles. The observed movement of apparently the whole
top layer domain is another indication of the relatively weak
interactions between the two molecular layers. The coherence of
the top layer, which behaves like a two-dimensional sheet, is
attributed to sufficiently strong intermolecular van der Waals
interactions between the alkyl chains.
Fig. 3 Three consecutive STM images (contrast-enhanced, Vb = À450 mV,
Is = 5 pA, Dt = 5 min, scale bar 3 nm) of a bilayer of Cu–Mn porphyrin dimer
2 at the same location at the graphite/1-phenyloctane interface, revealing
the dynamics between the two layers. The arrows point from the Cu to the
MnCl centre within a dimer. Dimers that reoriented with respect to the
previous scan are indicated by a red arrow.
the interactions between the top and bottom layer are relatively
weak, allowing porphyrin dimers from the top layer to desorb and
dissolve into the supernatant solution, while the same or another
molecule from that solution can subsequently fill the vacancy.
This incoming dimer can either orient itself in the same direction
as the previously present one, or adopt an opposite orientation.
We never observed vacancies in the top layer, which indicates
that it is energetically unfavourable to leave the vacancies that are
created by desorption of a porphyrin dimer unoccupied, and that
the dynamic exchange process of desorption and adsorption is
fast on the STM scanning scale. It is also possible that a vacancy
is filled by lateral movement of neighbouring porphyrin dimers,
but in that case we would have expected to observe vacancies at
other locations in the top layer.
Intriguingly, the top layer was also able to reorient itself as a
whole, over rather large domains of 50 Â 50 nm2 (Fig. 4 and
ESI,† Fig. S3). The STM images in Fig. 4 show magnifications of
such a moving domain, revealing a dynamic process which we
attribute to a rearrangement of the top layer. Consider the
porphyrin moieties labelled I, II and III along the cross-section
in Fig. 4A. The apparent height of porphyrin moiety I is almost
Future work will be directed towards investigating the
reactivity of Cu–Mn porphyrin dimer 2 in oxidation reactions,
and in particular whether this reactivity is different in a
monolayer or a bilayer arrangement of the molecules at the
Fig. 4 Top: three consecutive STM images (Vb = À450 mV, Is = 5 pA,
Dt = 5 min, scale bar 3.4 nm) of a bilayer of Cu–Mn porphyrin dimer 2 at the solid/liquid interface.
graphite/1-phenyloctane interface, showing a repositioning of the top layer with
This research was supported by the Council for the Chemical
respect to the bottom layer, first to an H-type (A - B) and then to a J-type
assembly (B - C). Red squares represent porphyrin dimers in the top layer, blue
squares those in the bottom layer. The red arrows indicate the direction in which
porphyrin dimers in the top layer (red squares) move along a lamellar array.
Sciences of the Netherlands Organisation for Scientific Research
(CW-NWO) (VIDI grant 700.58.423 and ECHO grant 700.57.023),
by the European Research Council (ERC Starting grant NANO-
Bottom: cross sections corresponding to the yellow dashed traces in the images. CAT – 259064), and by the Ministry of Education, Culture and
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