Bottom-Up Hierarchical Self-Assembly of Chiral Porphyrins through Coordination and Hydrogen Bonds

Article abstract of DOI:10.1021/jacs.5b08081

A series of chiral synthetic compounds is reported that shows intricate but specific hierarchical assembly because of varying positions of coordination and hydrogen bonds. The evolution of the aggregates (followed by absorption spectroscopy and temperature-dependent circular dichroism studies in solution) reveal the influence of the proportion of stereogenic centers in the side groups connected to the chromophore ring in their optical activity and the important role of pyridyl groups in the self-assembly of these chiral macrocycles. The optical activity spans 2 orders of magnitude depending on composition and constitution. Two of the aggregates show very high optical activity even though the isolated chromophores barely give a circular dichroism signal. Molecular modeling of the aggregates, starting from the pyridine-zinc(II) porphyrin interaction and working up, and calculation of the circular dichroism signal confirm the origin of this optical activity as the chiral supramolecular organization of the molecules. The aggregates show a broad absorption range, between approximately 390 and 475 nm for the transitions associated with the Soret region alone, that spans wavelengths far more than the isolated chromophore. The supramolecular assemblies of the metalloporphyrins in solution were deposited onto highly oriented pyrolitic graphite in order to study their hierarchy in assembly by atomic force microscopy. Zero and one-dimensional aggregates were observed, and a clear dependence on deposition temperature was shown, indicating that the hierarchical assembly took place largely in solution. Moreover, scanning electron microscopy images of porphyrins and metalloporphyrins precipitated under out-of-equilibrium conditions showed the dependence of the number and position of chiral amide groups in the formation of a fibrillar nanomaterial. The combination of coordination and hydrogen bonding in the complicated assembly of these molecules-where there is a clear hierarchy for zinc(II)-pyridyl interaction followed by hydrogen-bonding between amide groups, and then van der Waals interactions-paves the way for the preparation of molecular materials with multiple chromophore environments.

Full text of DOI:10.1021/jacs.5b08081

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Article
Bottom-up hierarchical self-assembly of chiral
porphyrins through coordination and hydrogen bonds
Cristina Oliveras-González, Florent Di Meo, Arántzazu González-Campo, David
Beljonne, Patrick Norman, Maite Simon-Sorbed, Mathieu Linares, and David B. Amabilino
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.5b08081 • Publication Date (Web): 23 Nov 2015
Downloaded from http://pubs.acs.org on November 24, 2015
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Bottom-up hierarchical self-assembly of chiral porphyrins through
coordination and hydrogen bonds
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Cristina Oliveras-González,a Florent Di Meo,b Arántzazu González-Campo,a David Beljonne,c
Patrick Norman,b Maite Simón-Sorbed,a Mathieu Linaresb and David B. Amabilino*a,d
aInstitut de Ciència de Materials de Barcelona (ICMAB-CSIC), Campus Universitari de Bellaterra, 08193 Cerdanyola
del Vallès, Catalonia, Spain.
bDepartment of Physics, Chemistry and Biology (IFM), Linköping University, SE-58 583 Linköping, Sweden.
cLaboratory of Chemistry for Novel Materials, Mons University, Place du Parc, Mons B-9000, Belgium.
dSchool of Chemistry, The University of Nottingham, University Park, Nottingham NG7 2RD, United Kingdom.
KEYWORDS (Word Style “BG_Keywords”). If you are submitting your paper to a journal that requires keywords,
provide significant keywords to aid the reader in literature retrieval.
ABSTRACT: A series of chiral synthetic compounds is reported that show intricate but specific hierarchical assembly
because of varying positions of coordination and hydrogen bonds. The evolution of the aggregates (followed by
absorption spectroscopy and temperature-dependent circular dichroism studies in solution) reveal the influence of the
proportion of stereogenic centers in the side groups connected to the chromophore ring in their optical activity and the
important role of pyridyl groups in the self-assembly of these chiral macrocycles. The optical activity spans two orders of
magnitude depending on composition and constitution. Two of the aggregates show very high optical activity even
though the isolated chromophores barely give a circular dichroism signal. Molecular modeling of the aggregates, starting
from the pyridine-zinc(II) porphyrin interaction and working up, and calculation of the circular dichroism signal confirm
the origin of this optical activity as the chiral supramolecular organization of the molecules. The aggregates show a broad
absorption range, between approximately 390 and 475 nm for the transitions associated with the Soret region alone, that
spans wavelengths far more than the isolated chromophore. The supramolecular assemblies of the metalloporphyrins in
solution were deposited onto highly oriented pyrolitic graphite in order to study their hierarchy in assembly by atomic
force microscopy. Zero and one-dimensional aggregates were observed, and a clear dependence on deposition
temperature was shown, indicating that the hierarchical assembly took place largely in solution. Moreover, scanning
electron microscopy images of porphyrins and metalloporphyrins precipitated under out-of-equilibrium conditions
showed the dependence of the number and position of chiral amide groups in the formation of a fibrillar nanomaterial.
The combination of coordination and hydrogen bonding in the complicated assembly of these molecules - where there is
a clear hierarchy for zinc(II)-pyridyl interaction followed by hydrogen-bonding between amide groups, and then van der
Waals interactions - paves the way for the preparation of molecular materials with multiple chromophore environments.
systems mimicking the natural pigments have been
synthesized and studied, confirming the importance of
the supramolecular structure on the light harvesting
ability.8-12
Introduction
Porphyrins and their derivatives based on a tetrapyrrolic
macrocycles linked by methine bridges are natural
chromophores1 that are essential for the photosynthetic
pathway2 in electron transport and light harvesting
processes.3 This property is shown exemplarily by
chlorophyll a4,5 or the chlorosomes of green-sulfur
Self-assembled chiral systems based on synthetic
porphyrins present a broad range of potential applications
in areas such as photovoltaic cells,13 sensors,14
optoelectronics15 and nanoelectronic devices16 and also
non-linear optical materials.17 The number and position of
the stereogenic centers in molecules united by covalent or
non-covalent bonds directly affect the chiral transfer from
the molecular level up to the supramolecular aggregates
that they form in solution and consequently their optical
activity can be modified.18-20
bacteria
which
mainly
comprises
aggregated
bacteriochlorophyll.6,7 These macrocycles have chiral
environments arising from self-organization through non-
covalent interactions. In all of these systems, a metal ion
is coordinated in the core of the chromophore and this
feature plays an important role in their self-assembly and
function. To understand the role of metal ions and self-
organization of porphyrins, enzymes and light-harvesting
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The self-assembly of porphyrins in supramolecular
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1
stacked systems can be controlled by a combination of
non-covalent interactions such as hydrogen-bonding, van
der Waals interactions and π-π-interactions between the
porphyrin cores. The presence of exo-facing coordinating
ligands and any metal ion coordinated at the center of the
porphyrin ring also influence their self-assembly.21-23 The
ability of metalloporphyrins to coordinate with axial
ligands containing nitrogen, oxygen or sulfur atoms gives
a great variety of self-assembled superstructures, with
facility to tune their optical and electronic properties, as
is the case of many zinc (II) metalloporphyrins that form
stable supramolecules in non-polar media when they self-
assemble through coordinating ligands.24,25 The
coordination of zinc porphyrins with axial nitrogen
derivative ligands, such as imidazolyl or pyridyl groups
afford the formation of well-organized macromolecules in
the form of cyclic structures,26-29 or linear polymers30,31
and oligomers.32-34 The coordination between zinc(II) and
pyridyl ligands has a relatively large association constant
(~103 M-1)35,36 and usually does not disturb the photo-
excited state of the metalloporphyrin.37,38
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Zinc (II) porphyrins have a strong tendency to form
pentacoordinate aggregates with a square pyramidal
geometry, although the hexacoordination of this class of
metalloporphyrins have also been reported in solid state
structures.39-42 The control of the fifth coordination site of
metalloporphyrins with nitrogen ligands is an important
parameter in order to study the kinetic and
thermodynamic effects in the self-assembly process of
these compounds.43,44
Figure 1. Target functionalized metalloporphyrins.
In this work the influence of the proportion and position
of the 4-pyridyl and 4-phenyl rings bearing the
Bearing in mind this wealth of former knowledge,
compounds Zn-(R,R,R)-1, Zn-(R,R)-2, Zn-(R,R)-3 and Zn-
(R)-4 have been designed and synthesized to probe
hierarchical assembly in solution using different non-
covalent interactions at each stage. Each compound
differs from the other in the number and position of
stereogenic centers - and associated hydrogen bond-
forming unit - and pyridyl groups (Figure 1). Alkyl chains
provide solubility for the systems in solution as well as
adhesion to each other and non-polar surfaces through
van der Waals interactions,45 and pyridyl groups allow
self-assembly through coordination with the zinc (II) ion
in the core of porphyrinic rings.
stereogenic center in
a lactamide moiety on the
aggregation of the molecules in solution and upon
precipitation was studied. In solution, absorption and
circular dichroism (CD) spectroscopy - a chiroptical tool
that allows precise monitoring of aggregation - gave
information about the complex assembly and relative
orientations of molecules in aggregates.27,31,46-48 In
conjunction, simulations provided insight into the nature
of the hierarchical assembly that led to the very high
optical activity. The morphology of the aggregates was
investigated by atomic force microscopy (AFM) when
they were transferred from solution to surface and by
scanning electron microscopy (SEM) to analyze their
morphology in the solid state when precipitated far from
equilibrium conditions.
For these compounds, two different coordinating atoms
can in principle bind the metal ion, the pyridyl nitrogen
atom or the carbonyl oxygen atom. Furthermore
hydrogen-bonding between amide groups can be the
main non-covalent interactions to form an aggregate.18,21
However, more than one interaction can be participating
in the self-assembly of these chromophores: Figure 2
shows the stronger potential interactions that need to be
considered.
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The insertion of zinc (II) into the porphyrin core was
done by reaction of the acetate metal salt with free-base
porphyrin derivative in refluxing mixture of
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a
CH2CH2/CH3OH to give the desired metallocompounds
Zn-(R,R,R)-1, Zn-(R,R)-2, Zn-(R,R)-3 and Zn-(R)-4. The 1H-
NMR spectra of all the metalloporphyrins in CDCl3
showed broad resonances at room temperature because of
equilibria between aggregated states that occur at a rate
comparable to the timescale of the spectroscopy. When
the NMR was run at lower temperature for
metalloporphyrin Zn-(R,R,R)-1 clear peaks were observed,
and will be discussed in detail in the next section. IR and
mass spectroscopy allow the identification of the desired
compounds. The UV-visible absorption spectra of
metalloporphyrins in CHCl3 were the typical ones for
these compounds, showing a Soret band at around 420
nm and two Q-bands at approximately 550 and 650 nm.
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Self-assembly of porphyrins and metalloporphyrins
in solution. The porphyrins and metalloporphyrins
synthesized here were prepared to be able to self-
assemble in solution by different non-covalent
interactions such as hydrogen bonding, π-π stacking
interactions directly related with the π-conjugated
skeleton or by coordination with the metal ion in the core
of porphyrin.50 UV-visible absorption spectroscopy is a
good technique to study the aggregation of the
chromophores because the electronic π-π* transitions
typical for these compounds are extremely sensitive to the
aggregation state. The electronic absorption spectra
depend on the exocyclic functionalization and the
coordinated metal ion; they are well explained by the
Gouterman four-orbital model.51
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Figure 2. Non-covalent interactions that can in principle
drive the self-assembly of the porphyrins presented here.
Results and Discussion
Synthesis and characterization. The metalloporphyrin
derivatives Zn-(R,R,R)-1, Zn-(R,R)-2, Zn-(R,R)-3 and Zn-
(R)-4 were prepared following the synthetic route detailed
in the supporting information (Scheme S1). Briefly, the
free base chiral porphyrin intermediates were prepared in
one step - that required their separation by column
chromatography - from the condensation of (R)-methyl 2-
(4-formylphenoxy)propanoate)49 and 4-formylpyridine-
with pyrrole in refluxing propionic acid in air. Habitual
analytical and spectroscopic techniques were used to
characterize each one. The constitutional isomers
containing two stereogenic centers were distinguished by
the multiplicity of the resonances of the hydrogen atoms
of the pyrrole group in their nuclear magnetic resonance
(NMR) spectra. In the case of -(R,R)-2 a double doublet
resonance (AB system) was observed. The NMR of the
other isomer ((R,R)-3) showed an AB system with two
singlets located between the doublets. The amidation of
the ester groups in these porphyrins at reflux in toluene
with the presence of an excess of octadecylamine afforded
the free base porphyrin derivatives (R,R,R)-1, (R,R)-2,
(R,R)-3 and (R)-4.
In general, free-base porphyrins and metalloporphyrins
present an intense band (the Soret band) at around 420
nm with an extinction coefficient over 104 mol/L cm-1 was
the focus of our studies. For some metalloporphyrins the
Soret band can be shifted to lower wavelength because of
the metal in the relative energy transitions of the
chromophores ring.27 Closed-shell ions (d0-d10) like zinc
(II) have a weak effect on the π-π* gap, meaning that
Soret band does not move significantly with respect to the
free-base porphyrin.52-54 This band provides us useful
information for the study of aggregation of our
compounds. Two general types of stacked aggregates
exist: H-aggregates, where porphyrins overlap face-to face
and present a blue (hypsochromic) shift respect to the
Soret band of the monomer, and J-aggregates, where a red
(bathochromic) shift of the Soret band is observed
because of staircase-type packing with partial overlapping
of the porphyrins.55
A decrease in solubility with increase number of pyridyl
groups attached to the porphyrin made us consider a new
route to obtain the porphyrin derivatives. The strategy to
prepare the free base porphyrin derivatives (R,R,R)-1,
(R,R)-2, (R,R)-3 and (R)-4 resolved the problem of low
solubility of the intermediate reactant porphyrins was via
an imine derived from the lactate. Condensation of the
imine and 4-formylpyridine with pyrrole in refluxing
propionic acid in air gave the desired free-base porphyrin
derivatives directly, although yields were similar to the
more conventional route.
The absorption measurements of free-base porphyrins
and the corresponding metalloporphyrins were initially
carried out in chloroform at room temperature. The
spectra of the free-base porphyrins showed a Soret band
centered at 419 nm and for metalloporphyrins at 421 nm,
indicating that in this solvent molecules behave as
isolated chromophores surrounded by solvent.56-58 The
absorption
spectra
were
also
recorded
in
methylcyclohexane (with 3% chloroform to ensure
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solubility) for the same compounds and it was observed
porphyrin complex, suggesting
disposition of the two metalloporphyrins and
a
perpendicular
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that the free-base porphyrins do not show any significant
change in the position of the bands (Figure S1) indicating
no aggregation of the chromophores and therefore
insignificant hydrogen bonding under these conditions.18
The absorption spectra of the metalloporphyrins show
both slight blue-shifted (hypsochromic effect) Soret
bands and new bands at longer wavelengths that might
correspond to J-aggregates which are caused by the
exciton coupling between porphyrins (Figure 3).59 The
spectral patterns are different for each metalloporphyrin
indicating a distinct assembly for each, leading to unique
structures. The spectra for compounds Zn-(R,R,R)-1 and
Zn-(R,R)-3 show a shoulder at around 435 nm that has
been assigned previously to the coordination of pyridyl
groups with the zinc (II) ion of the neighboring
metalloporphyrin.60-61 The other two metalloporphyrins
Zn-(R,R)-2 and Zn-(R)-4 show more red shifted bands
with a maxima at 450 nm, suggesting a higher number of
electronic transitions due to formation of a larger
aggregate or even another kind of J-aggregate because of
the different position of the peripheral groups.
a
thermodynamically stable aggregate in the range of
temperatures from 300.5 K to 338 K (Figure 4, red lines).
The isodichroic points at 415 nm and 443 nm in the CD
spectra suggest equilibria between two species, either
monomer and cyclic tetramer or monomer and linear
aggregate. Furthermore in this range of temperature, a
clear isosbestic point at 425 nm appears in the absorption
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spectra, indicating
a
co-existence of monomer
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metalloporphyrin with an absorption maximum at 419.5
nm and the cyclic tetramer (the most stable aggregate in
dilute solutions).35,63 The evolution of the aggregate to a
larger superstructure was observed when the temperature
was decreased.64,65 A high intense positive Cotton effect
with maximum at 419.5 nm and a huge negative CD signal
at 450 nm indicate extension of the inter-chromophore
interactions (Figure 4, black lines). In the corresponding
absorption spectra a new isosbestic point at 441 nm
appears in the evolution of the aggregate suggesting
equilibria between the discrete aggregate and the larger
superstructure.
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Zn-(R,R)-3
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Zn-(R)-4
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 (nm)
Figure 3. UV-visible absorption spectrum of compounds Zn-
(R,R,R)-1, Zn-(R,R)-2, Zn-(R,R)-3 and Zn-(R)-4 at room
temperature in methylcyclohexane/3% CHCl3 (5 μM).
440
400
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The development of the equilibria in the self-assembly of
each metalloporphyrin was studied at different
temperatures by UV-visible absorption and CD
spectroscopies, which gave information about the
orientation that the chromophores adopt with respect to
one another as the intensity of the optical signals is
directly proportional to the interactions between
electronic transitions.62
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425
 (nm)
450
475
Figure 4. Variable temperature CD spectra (top) and
corresponding absorption signals from the CD spectrometer
Solutions
of
compounds
Zn-(1-4)
in
a
methylcyclohexane/3% CHCl3 were prepared (at
(bottom)
for
compound
Zn-(R,R,R)-1
in
methylcyclohexane/3% (5 μM). The red lines correspond to
the temperature range from 300 K upwards and the black
lines below this temperature.
concentration 5 μM), and CD spectra were recorded in
the range of temperatures from 338 K to 263 K after a
heating and cooling cycle to ensure that equilibrium
conditions applied.
The temperature dependence of the CD spectra of
compound Zn-(R,R,R)-1 shows two clearly distinct states
of self-assembly. A weak Cotton effect arises at the
position of the band associated with the pyridyl-Zn(II)
The evolution of the tetramer aggregate with temperature
is shown in the plots of band intensity versus temperature
(Figure 5). The band at 438 nm in the CD spectra and
431.5 nm in the corresponding absorption reach a
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maximum at around 300 K and then an abrupt decay of Three isodichoric points are evident in the CD spectra, at
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these bands is apparent, while the 451 nm band starts to
increase below this critical temperature. This behavior
suggests that the cyclic tetramer aggregates to a larger
superstructure at lower temperatures, as corroborated by
the presence of an isosbestic point at 441 nm.66
approximately 415 nm, 435 nm and 453 nm, meaning that
two different species are in equilibrium in the
temperature range studied. The absence of a band at 435
nm in the absorption spectra could indicate that the
species involved in the equilibrium are the monomer
metallocompound and a larger aggregate. An isosbestic
point at 437 nm is present in the absorption spectra
corroborating the existence of an equilibrium between
monomer and aggregate. The remarkable point of this
spectrum is the presence of two bands at 448 and 453 nm,
clearly observable at and above room temperature. As the
temperature is decreased these bands collapse into one. A
possible explanation of this behavior is the formation of a
J-aggregate whose excited state energies became closer as
it gets longer resulting in a band with a shoulder. Even at
the higher temperatures studied (up to 338 K), a
supramolecular aggregate is present in solution as
indicated by the high intensity in the CD spectra. This
feature shows that metalloporphyrin Zn-(R,R)-2 presents
a much greater tendency to aggregate than Zn-(R,R,R)-1,
although the CD spectra infer a somewhat similar
arrangement of the chromophores within the aggregates.
Therefore, it is believed that the tetramour formed by the
pyridyl coordination giving pentacoordinate zinc(II) ions
is present in the aggregates formed by Zn-(R,R)-2.
For compound Zn-(R,R)-2 the evolution from monomer
to large aggregate does not pass through a specific
intermediate. The formation of a discrete structure is not
evident when the aggregation is followed by CD or
absorption spectroscopy (Figure 6).67 Two intense non-
symmetrical bisignate CD signals were observed, one with
positive maximum at 421 nm and the other one
corresponding to a supramolecular arrangement at longer
wavelength with maximum at 449 nm. The CD spectra
and corresponding absorption spectra of Zn-(R,R)-2 were
simpler than those of metalloporphyrin Zn-(R,R,R)-1
presumably because of the higher symmetry in the former
compound. The presence of two pyridyl groups in Zn-
(R,R)-2 is apparently an important factor for linear growth
in this compound, which must involve cooperative
assembly in which both pyridyl-zinc(II) coordination and
hydrogen bonding, as the CD pattern for the aggregate is
remarkably similar to the larger aggregates observed for
Zn-(R,R,R)-1, pointing to a similar chiral arrangement of
the porphyrin moieties in the aggregate.
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The behavior in solution of compound Zn-(R,R)-3 was
completely different to either compound Zn-(R,R,R)-1 or
Zn-(R,R)-2. The difference with compound Zn-(R,R)-2 is
particularly striking, because both have two chiral amide
groups and two pyridyl substituents in the periphery of
the metalloporphyrin core. This contrasting behavior
indicates the importance of the position of the groups on
the porphyrin ring for self-assembly. The CD spectra of
Zn-(R,R)-3 (Figure 7) show two bisignate signals of low
intensity. This pattern is similar to that of compound Zn-
(R,R,R)-1 above 300 K when it is present as a cyclic
tetramer. Therefore, this feature indicates that for Zn-
(R,R)-3 the main aggregate form in the whole range of
temperature studied is the cyclic tetramer and there is no
subsequent aggregation through secondary interactions
between tetramers. In the corresponding absorption
spectra only a broad band at 435 nm is seen, similar to
that of Zn-(R,R,R)-1 and attributed to a discrete aggregate.
Moreover, the broadness of the band might corroborate
the formation of a cyclic aggregate.63
1.0
0.8
451.0nm
438.0nm
419.5nm
413.0nm
0.6
0.4
0.2
0.0
260 270 280 290 300 310 320 330 340
T (K)
1.0
0.8
0.6
451.0nm
431.5nm
419.0nm
0.4
0.2
0.0
260 270 280 290 300 310 320 330 340
T (K)
Figure 5. Evolution of the aggregate versus temperature in
CD (top) and UV-Visible absorption (bottom) bands
(normalized intensity to maxima) for Zn-(R,R,R)-1. The lines
are merely to guide the eye.
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300
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-4
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-8
-100
-200
-300
-400
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268 K
273 K
278 K
283 K
288 K
293 K
298 K
303 K
308 K
310.5 K
313 K
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318 K
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323 K
328 K
330,5 K
333 K
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338 K
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425
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475
263 K
268 K
273 K
278 K
283 K
288 K
293 K
298 K
303 K
308 K
310.5 K
313 K
315.5 K
318 K
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323 K
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328 K
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333 K
338 K
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 (nm)
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Figure 7. Variable temperature CD spectra (top) and
corresponding absorption signal (bottom) from CD
spectrometer
methylcyclohexane/3% CHCl3 (5 μM).
Figure 6. Variable temperature CD spectra (top) and
corresponding absorption signal (bottom) from CD
spectrometer
methylcyclohexane/3% (5 μM).
of
compound
Zn-(R,R)-3
in
of
compound
Zn-(R,R)-2
in
Mathematical models for the aggregation curves from CD
intensity at distinct wavelengths versus temperature were
applied in order to determine precisely the self-assembly
processes taking place in each Zn (II) porphyrin. Two
main models are associated with the self-assembly
processes in this kind of aggregation. The cooperative
mechanism has two regimes, the (costly) formation of
nuclei with critical size which acts as an activation step
followed by a favorable elongation process. On the other
hand, the isodesmic model is governed by a gradual
increase in the number of aggregate species with the same
energy increment at each addition of monomer.71,72 To
analyze the temperature-dependent data from CD spectra
of each metalloporphyrin, both models were applied and
the best fits identified the most suitable model in each
case (Figure S3-S4).
Compound Zn-(R)-4 with only one stereogenic center in
its structure and three pyridyl groups showed contrasting
spectra compared to the other compounds, but
principally because the solubility of the compound is so
low in the solvent used. Nonetheless, the appearance of
the CD spectrum reflects an environment of the
molecules (Figure S2) that is different to the homologues.
The results of the temperature-dependent studies of the
metalloporphyrins clearly demonstrate the importance of
the position and number of pyridyl groups and chiral
amide in the periphery of the macrocycle. In the case of
metalloporphyrin Zn-(R,R,R)-1 two distinct assembly
regimes arise. Above 300 K a tetramer structure is formed
and an elongation process takes place at lower
temperatures.68-70 A linear growth for metalloporphyrins
Zn-(R,R)-2 and Zn-(R)-4 appears likely according to the
changes in the CD and absorption spectra with
temperature. On the other hand, the results for Zn-(R,R)-
3 suggest the cyclization of the aggregate , but the lack of
effective hydrogen bonding negates further aggregation.
The self-assembly of Zn-(R,R,R)-1 clearly shows two
distinct states in its assembly in the CD and absorption
spectra, suggesting that a nucleation-growth process take
place (Figure 8). Fitting reproduced very well the shape of
the observed curves. The formation of a tetramer
aggregate follows a sigmoidal shape as a function of
temperature which is indicative of isodesmic self-
assembly and acts as nucleation structure. The following
elongation regime appears as the temperature decreases
below 300 K; the nuclei evolve to a larger aggregate
following a cooperative process.
Given the restrictions in solvent and concentration a
direct comparison between CD and NMR data was not
possible. However, a concentrated solution (20mM) in a
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more competitive solvent was used. The comparison of
free-base porphyrin (R,R,R)-1 H-NMR spectrum and at
correlation between protons extracted from the ROESY
studies.
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that of Zn-(R,R,R)-1 in d-chloroform at-45°C (where it was
possible to distinguish clearly all the peaks of the
molecule) shows a clear shift of the pyridyl protons to
high field. In the Zn(II) compound they resonate at 6.32
and 6.07 ppm for the Hβ-pyridyl and 2.58 and 1.97 ppm for
the Hα-pyridyl, while for the free-base porphyrin they
appear as a doublets at 8.88 and 8.17 ppm hidden by the
pyrrole peak and the aromatic peak, respectively. A
splitting of the signals is observed in the Zn(II)
compound, mostly the aromatic protons and pyridyl
protons close to the porphyrin ring because the two faces
of porphyrin are not equivalent due to the pyridyl-zinc
(II) ion coordination (Figure 9).73-76
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1
Figure 9. H-NMR in d-chloroform of metalloporphyrin Zn-
451.0 nm band in CD spectra
Fitting
(R,R,R)-1 (at -45 °C) and free-base porphyrin (R,R,R)-1.
1.0
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0.2
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T (K)
451.0 nm band in CD spectra
fitting
1.0
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0.6
0.4
0.2
0.0
Figure 10. Tetramer structure of compound Zn-(R,R,R)-1
with arrows showing the cross-correlations observed in the
ROESY NMR experiment. The arrows between hydrogen
atoms with bonded correlations are not indicated for major
clarity of the drawing.
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T (K)
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300
Figure 8. Evolution of the CD intensity (black squares)for
Zn-(R,R,R)-1 versus temperature and the data fitted using the
equations detailed in the text (red line). The top graph shows
the regime where a cyclic tetramer is formed, and the lower
graph the extension to larger aggregates.
Modeling of the metalloporphyrin and pyridine-
metalloporphyrin systems. Theoretical models of a Zn-
(R,R,R)-1 and its pyridine complex were constructed by
replacement of octadecyl with ethyl chains since alkyl
side chains are not relevant to the Soret band of ECD
spectra. First-principles quantum mechanical calculations
on the models were carried out at the ωB97X-D/6-
31+G(d,p)/LANL2DZ//ωB97X-D/6-31G(d)/LANL2DZ level
of theory. The free and complexed metalloporphyrin have
The complexity of the 1H-NMR spectrum of
metalloporphyrin Zn-(R,R,R)-1 required 2D 1H-NMR
studies in order to identify correctly all the protons of the
molecule. A ROESY (Rotating Frame Overhauser effect
spectroscopy) experiment allowed full assignment of the
signals and indicated those protons in close proximity
(Figure S8).77 Figure 10 shows the proposed tetramer
structure for Zn-(R,R,R)-1 with arrows indicating the
planar
tetracoordinate
and
square-pyramidal
pentacoordinate geometry, respectively. Both singlet and
triplet states were considered for both systems. Singlet
states are significantly more stable than triplets, with
singlet-triplet splittings being -30.9 and -26.1 kcal.mol-1 for
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the free metalloporphyrin and the pyridine complex, mechanical model, although this very slight bond length
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respectively. Pyridine complexation is highly favorable,
difference is not expected to influence dramatically the
superstructure, especially given the flexibility in the
bonding scenario. The NPy-Zn bond distances are shorter
than Npyrrole-Zn ones leading to a certain flexibility
between metalloporphyrins (Ni-Zni+1 bond distances and
NPy-Zn-Npyrrol angles ranges are respectively [2.1; 2.6]Å and
[75, 100]°, Figure 11a and b). The meso-substituted pyridyl
moieties are twisted to the tetrapyrrole core with either
right or left chirality (Figure 11c). No preference between
right- and left-handed deviations of the pyridyl dihedral
angle is observed. The deformations of zinc atoms out of
the tetrapyrrole cores obtained from MD simulations are
slightly smaller than those observed in the QM
calculations, possibly because of the deformation of the
tetrapyrrole core in the tetramer. Indeed, tetrapyrrole
cores tend to lose their planarity in the tetramer. For
example, improper dihedral angles formed by each N-
atom of tetrapyrrole cores exhibits distribution from -10°
to 10°.
showing ∆Eassociation= -20.1 kcal.mol-1.
Pyridine complexation to the metalloporphyrin leads to
slight conformational changes of the tetrapyrrole core.
Bond distances between the metal ion center and the
tetrapyrrole nitrogen-atoms are slightly increased owing
to the presence of the new electron-donor pyridine
ligand. For instance, typical Ntetrapyrrole-Zn bond distances
are respectively 2.06 and 2.09 Å in the metalloporphyrin
and its complex (Table S1), and the zinc atom is moved
out of the tetrapyrrole core plane as seen experimentally
in this type of complex.78,79 Passing from planar
tetracoordinate to square pyramidal pentacoordinate Zn-
(II) also leads to a redistribution of charges. The pyridine
ligand plays a significant electron-donor role, decreasing
the positive charge of the zinc atom from 1.185e for the
free metalloporphyrin to 1.038e for the pyridine complex,
as well as increasing charges of pyridine and tetrapyrrole
N-atoms (Table S2). An important electron exchange
feature is observed in the pyridine-zinc bond suggesting a
strong dative nature. The π-back-donation between 3d
orbitals of Zn-atom and π-orbital of pyridine is not
important (verified by analyzing bond distances of the
pyridine ligand that are not affected with respect to the
standalone pyridine). The π-back-donation is thus
restricted to the Zn-tetrapyrrole core. Thereby,
electrostatic interactions between pyridine and zinc are
expected to play an important role in such association.
The nitrogen-atom charge of standalone pyridine is very
similar to that of the moiety in Zn-(R,R,R)-1, confirming
the relevance of this model for the Zn-pentacoordinate
complexation.
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The global tetramer structure can be defined by the
square formed by each zinc-atom, with edge Zn-Zn
distances of around 9.7Å. A square shape is confirmed by
the observation of diagonal Zn-Zn distances to be 13.7Å,
which is in perfect agreement with the square diagonal
equation. The dot products between vectors defined from
Zn to Npy atoms were also calculated (Table S3), and they
show that adjacent metalloporphyrins remain orthogonal
with respect to each other along the MD trajectory
whereas diagonal metalloporphyrins keep an antiparallel
orientation.
The highest occupied and the lowest unoccupied
molecular orbitals (HOMO and LUMO) are destabilized
by approximately 0.3 and 0.2 eV upon pyridine
complexation leading to a smaller HOMO-LUMO gap by
around 0.1 eV. HOMO and LUMO distributions are rather
similar between the free metalloporphyrin and the
pyridine complex confirming the experimentally
suggested π-π* gap (Figure S15). Only a very weak change
is expected in energy transitions of the tetrapyrrole
chromophore while bound to pyridine (i.e., in the Soret
band). Such complexation would lead to a red-shift of the
high energy band as observed experimentally.
Self-assembly leading to proposed tetramer
structure. NPT molecular dynamics (MD) simulations
were performed at room temperature to establish the self-
assembly of the ethyl chain equivalent of Zn-(R,R,R)-1,
starting with the proposed tetramer structure to test its
existence and stability. For sake of clarity, each sequential
metalloporphyrin (i.e., monomer) of a tetramer square are
numbered 1, 2, 3 and 4 and 12, 23, 34 and 41 are
metalloporphyrin pairs connected by NPy-Zn bonds. The
tetramer is actually stable throughout the simulation and
pentacoordinate mode is maintained with an average NPy-
Zn bond distance of 2.35 Å as well as an average Npyrrol-Zn-
NPy angle of approximately 91°. The zinc(II) atom is
slightly more in the plane than in the quantum
Figure 11. Geometrical parameters of the porphyrin tetramer
from 20 ns MD simulation.
In spite of the importance of the Zn-NPy bonds in the
tetramer, other non-covalent interactions also play a
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crucial role in these aggregates. The total non-covalent respectively coplanar and orthogonal to the drawing
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interaction energy in a tetramer was calculated to be -94.7
kcal.mol-1. This energy can be decomposed into non-
covalent dimer energy between each metalloporphyrin.
The main contribution to non-covalent interactions in the
tetramer arises from adjacent metalloporphyrin dimers
(i.e., 12, 23, 34 and41). There is an equal
contribution of van der Waals and Coulombic
interactions (average Coulombic and van der Waal dimer
energy interactions are 11.7 and 11.9 kcal.mol-1
respectively). It is worth noting that electrostatic
interactions are less dominant probably due to the
solvation in the very non-polar solvent. The perpendicular
T-shape arrangement of adjacent metalloporphyrins
underlines the importance of dispersive σ-π interactions
between the pyridyl groups and the tetrapyrrole cores.
Cofacial antiparallel metalloporphyrins (i.e., 1-3 and 2-4)
are weakly interacting since the respective intermolecular
distances are at the upper limit of such interactions.
Average Coulombic and van der Waals interaction
energies are -0.3 and -1.0 kcal.mol-1, respectively.
plane.
Mode A may be defined by an almost face-to-face
arrangement of tetramers that have hydrogen-bonds
between lateral side chains. Mode B and C slide one
tetramer upon another allowing hydrogen-bonding
between lateral and top-side chains. It must be stressed
that mode B allows two hydrogen-bonds while mode C
allows four hydrogen-bonds to be made. In the latter, one
side chain of one tetramer is between two top side chains
of another allowing the central amide group to be bonded
to one electron-acceptor and one electron-donor
respectively, thanks to NH and C=O moieties, thus
satisfying all its hydrogen-bonding potential.
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Particular attention has been paid to mode A since several
sub-modes of this arrangement are possible according to
the position of the side chains involved in hydrogen-bond
networks (Figure 13). Ladder and pyramidal shape are
defined when both side chains of one tetramer are both
on the top (bottom) of the side chains of the other
tetramer square. They differ only when at least three
tetramers are self-assembled. In the latter, the middle
tetramer side chains are below both neighbor ones while
the former exhibits a kind of staircase shape. On the other
hand, helical structures may be obtained when one side
chain is on the top of the other tetramer side chain while
the other side chain of the former tetramer is under. Both
P- and M-helices were considered according to the
tetrapyrrole planes rotation along helical axis (Figure 13).
Self-assembly of the tetramer structures. The key
parameter in the calculation of the elongation step is the
elucidation of the association mode between each
tetramer. Since, the process takes place at temperatures
below 300
K
experimentally, weak non-covalent
interactions are the driving force. Amide groups in the
metalloporphyrin side chains are expected to play a
crucial role as hydrogen-bonding between tetramer
nuclei. Likewise, π-delocalization over the tetrapyrrole
cores could lead to π-π interactions in tetramer
aggregates. Any association mode between tetramer
aggregates must balance these interactions. In theory,
three binding modes can be proposed (Figure 12).
Dodecamer structures (trimers of tetramer squares) were
built following the several modes of association and were
minimized at the molecular mechanics (MM) level of
theory . Again, the ethyl chain equivalent of Zn-(R,R,R)-1
was used to optimise computational time in these very
large and complex superstructures, and therefore allow a
reasonable exploration of the supramolecular possibilities
in the structure. The binding modes considered as the
Pyridine N-atom
strongest here
– through metal coordination and
Side chains
Eclipsed chains
Hydrogen bond
Zn-(R,R,R)-1
hydrogen bonding - are not apparently influenced
significantly by the weak interactions between alkyl side
chains, which are located at the periphery of the self-
assembled structure. However, this simplification should
not be taken as a general result since in other systems
lateral alkyl chains might play a crucial role in the self-
assembly process,80,81 and might influence in the
subsequent hierarchical levels of assembly. The use of
energies to select the most stable conformation is
challenging because of (i) the small difference in relative
energies per molecule ranging from 0.0 to 13.0 kcal.mol-1
and (ii) the considered level of theory. However, such
results associated to theoretical and experimental CD
spectra permit trends about self-assembly modes to be
gleaned. Mode C seems to be unlikely to occur owing to
steric hindrance of alkyl side chains leading to the least
stable structure. Interestingly, the pyramidal mode A may
also be discarded while ladder mode A is one of the most
plausible at first sight. Finally, both P- and M-helices also
appear possible, the latter being more stable in vacuum.
Mode A
Mode B
Mode C
Figure 12. Proposed self-assembly modes of the tetramer
metalloporphyrin complex. Tetramer square nuclei are
depicted from the top view (porphyrins perpendicular to
the plane of the drawing).
In the top view, two types of side chain can be defined in
each tetramer, lateral and top/under side chains are
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Ladder
Pyramidal
P-Helix
M-Helix
Figure 13. Proposed variants of mode A tetramer self-assembly from molecular dynamics simulations. XYZ coordinates
are available in supporting information. One monomer per tetramer is colored in red to highlight the different sub-
modes.
Towards the elucidation of second-order self-
assembly using semi-empirical CD calculations.
Experimental CD in the Soret region below 300 K is
mainly characterized by very high optical activity with
respect to higher temperature spectra. We stress that the
CD signals arises from induced chirality of tetrapyrrole
cores by self-assembly (and not from intrinsic molecular
Ladder and helix arrangements appear to be the only
relevant modes for the tetramer aggregation because they
exhibit cofacial arrangements of the tetrapyrrole cores
and short inter-core distances that would lead to efficient
exciton coupling. Moreover, these association modes are
the only ones comprising both hydrogen-bonding and π-π
interactions between tetramers. As mode B and C have
been discarded, semi-empirical excitonic calculations of
CD spectra were carried out only on the four assemblies
of mode A (Figure 14). Despite the limited potential
energy surface exploration available, the high optical
activity is reproduced only with helical shapes. Inter-
tetramer cofacial tetrapyrrole cores are twisted with
respect to each other leading to a strong chiral exciton
coupling. According to clockwise or anticlockwise twist,
P- and M-helix CD spectra are mirror images. It must be
stressed that the high optical activity is not observed in
the ladder arrangement since excitonic metalloporphyrins
are not twisted. That should thus discard ladder shape as
a relevant association mode.
chirality).
As
suggested
experimentally,
higher
temperature CD signals should be considered as
tetramers. The low intensity CD signals within a range
from -5 to 5 mdeg suggests that the chiral deformation of
the tetrapyrrole core is significant but small.
Furthermore, the theoretical CD spectra of the monomer
corroborate that even though the metalloporphyrin is
chiral, it does not present a significant optical activity
(Figure 14). This low signal can be attributed to the
pyridyl moiety binding to the zinc (II) metal ion. It must
be stressed that a similar environment-induced chirality
has already been observed in metalloporphyrins with, e.g.,
DNA G-quadruplexes.82 However, the CD signal at low
temperature shows that the tetrapyrrole core chirality is
strongly affected by hierarchical assembly. This might be
attributed to large chiral excitonic coupling between two
tetrapyrrole cores that belong to two different tetramer
structures. To be effective, exciton coupling requires a
relatively short distance between tetrapyrrole cores.
Therefore, mode B and mode C should be discarded as in
this self-assembly mode the tetrapyrrole cores are neither
facing each other nor close (average distance being ca.
18Å).
Self-assembly studies of porphyrins and their
metalloporphyrins on a surface. So as to probe the
hierarchically grown structure of the aggregates of the
porphyrins more microscopically, solutions used for
spectroscopic studies were deposited on highly oriented
pyrolytic graphite (HOPG) by casting and the solvent was
allowed to evaporate. The material was investigated with
intermittent mode atomic force microscopy (AFM). As
was observed in solution, the self-assembly was
dependent on temperature, so the organization of the
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supramolecular aggregate of metallocompounds on the Figure 15 shows a statistic study about the dimensions of
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surface can also be affected by this external factor.
Molecule-surface interactions are an important factor to
take in account to successfully adsorb the aggregates from
solution on to a solid support in a controlled way.83-85 The
metallocompounds under observation here have a long
alkyl chains in the side positions that can help the
adsorption of the aggregates due to the van der Waals
interaction with the hydrophobic surface.86
the globular objects from a solution of metalloporphyrin
Zn-(R,R,R)-1 where it can be seen that most of the
particles present diameter between 20 to 30 nm (from 100
particles). This behavior could be attributed to different
numbers of tetramers close to one another because of the
hydrogen-bonding between amide groups of different
tetramer aggregates that lead to larger structures at lower
temperature.
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Figure 14. Theoretical CD spectra of Zn-(R,R,R)-1 monomer
(i.e., single molecule), tetramer and dodecamers (i.e., three
tetramers) following A-mode self-assembly.
Figure 15. Tapping mode AFM image (top) of Zn-(R,R,R)-1
deposited on freshly cleaved HOPG at room temperature, a
profile of the line on this image and statistical graph of object
width and breadth (measured at the center parallel to the
edges of the image).
Metalloporphyrin solutions (used in CD) were drop cast
onto highly oriented pyrolitic graphite (HOPG) surface at
room temperature and let the solvent evaporate.
Topographic images of Zn-(R,R,R)-1 showed disc-like
objects distributed evenly
over the surface after
In order to corroborate that the aggregates formed in
solution were transferred to the surface suffering a
minimum effect of the adsorption, the metalloporphyrin
solution was cooled down to 0°C and then drop-cast onto
HOPG. Two different regions of the surface were
observed by AFM with different behavior of the
aggregates (Figure 16). The image in Figure 16a shows big
planar blocks with a height around 12 nm (see profile) and
small rods of 3 nm height around them. The blocks have
well defined edges characteristic of very small crystals
(the biggest blocks are around 700 nm long). Figure 16b
shows a region with a fibrillar organization with vertical
dimensions between 1.5 to 2 nm, that could correspond to
the porphyrin size width. This behavior coincides with
the evolution of the aggregates in solution because, even
though the fibers are not well defined, they are clearly
evaporation of the solvent. The measured heights of the
discs were around 2.5 nm, that must correspond to a
standing up porphyrins (with their planes perpendicular
to the surface), otherwise a coordinating pyridyl group of
aporphyrin flat on the surface could not interact with
another to produce the cycle for steric reasons (Figure
S16). The ill-defined shape is presumably because the
alkyl chains atop the structure are free to slump in no
preferred direction on or beside the aggregate.
The globular stacks presents on surface presumably come
directly from the solution, because the molecule-molecule
interaction is apparently stronger than the molecule-
surface forces. These results were in agreement with the
observation in solution where tetrameric structures were
formed from 300 K upward for this compound.
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distinct from the discs seen for the same sample when that physisorbs to the HOPG on evaporation and aligns
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deposited at room temperature.
with its axes (Figure S17). A close up image implied
helicity in the fiber formation, clearly observable in the
corresponding phase image (Figure 17). This helicity
indicates a clear chiral transfer from monomer to
aggregate that was observed in the high intensity CD
signal.
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b
Figure 17. Tapping mode AFM images (topography top and
phase bottom) of Zn-(R,R)-2 deposited at 0°C on freshly
cleaved HOPG.
While the AFM images of assemblies of Zn-(R,R)-2
deposited solutions at low temperature matched well with
the observation of long aggregates in circular dichroism,
the room temperature solution transferred onto HOPG
did not show significant large aggregates. This difference
between solution and surface studies could be noted for
the lability of the tetramer that evolves towards the
fibrillar structure. As mentioned before, the transfer of
aggregates from solution to surface can be affected by
different factors such as molecule-surface interaction and
the solvent used. The faster evaporation of the solvent at
room temperature might favor the observation of the
discrete aggregate that was not possible to detected by
spectroscopic techniques.
Figure 16. Tapping mode AFM images of Zn-(R,R,R)-1 cast at
0°C on freshly cleaved HOPG.
Topographic images of a solution of Zn-(R,R)-2 deposited
at room temperature onto HOPG showed a mixture of
spherical particles with heights around 10 nm and a
fibrillar network with heights around 3 nm. When the
same solution was cooled down at 0°C and cast onto
graphite (HOPG) a fiber network was observed as the
dominant self-assembled structure. The profile showed a
3 nm height for the fibers and a 1.5 nm height for an
The statistical study of the room temperature deposited
Zn-(R,R)-2 in one area where round particles predominate
as aggregate showed a narrow dimensions distribution of
the particles presenting diameters between 14 to 26 nm as
a result of interactions between different tetramers
aggregates (Figure S18). The objects are smaller than
underlying structure possibly arising from
aggregated metalloporphyrin, always present in solution,
a non-
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CHCl3/EtOH and deposited on a carbon tape : (R,R,R)-1 and
Zn-(R,R,R)-1 (a), (R,R)-2 and Zn-(R,R)-2 (b), (R,R)-3 and Zn-
(R,R)-3 (c), (R)-4 and Zn-(R)-4 (d).
At a yet larger scale, the solid morphology of free-base
porphyrins and the corresponding metalloporphyrins
were analyzed by scanning electron microscopy (SEM)
after precipitation in far from equilibrium conditions
from CHCl3/EtOH solution (Figure 18).
those formed by the mono-pyridyl homologue, possibly
because of mass depletion to the fibers that are also
present on the surface.
1
2
3
4
5
6
7
8
Topographic images of Zn-(R,R)-3 showed a globular
structure as a main morphology on graphite, with heights
between 3 and 6 nm when the solution were adsorb at
room temperature, while a bigger globular structures
where observed when it was deposited at 0°C (Figure S19).
Statistical analyzes of the dimensions of the small
particles in the AFM images at room temperature for
metalloporphyrin Zn-(R,R)-3 showed that the major
number of the small globular stacks have a diameter
between 14 to 20 nm, although they have a great tendency
to form a bigger aggregates as the topographic images
showed (Figure S20). Therefore, little specificity in
assembly is observed, and no significant fibrillar
morphology was seen.
As observed in the solution studies, free-base porphyrins
(R,R,R)-1, (R,R)-2, (R,R)-3, (R)-4 did not show any kind of
aggregate, but under precipitation the same porphyrins
showed a kind of fibrillar organization.
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Porphyrins (R,R,R)-1 and (R,R)-2 revealed a much clearer
fibrillar organization in their SEM images (Figure 18 a and
b) than porphyrins (R,R)-3 and (R)-4 (Figure 18 c and d)
where a more compact solids were observed. Especially
for porphyrin (R)-4 the lack of alkyl chains might be
responsible for this effect. The main force for the
arrangement of free-base porphyrins is the hydrogen-
bonding between amide groups. However, porphyrins
(R,R)-2 and (R,R)-3, both with two chiral amide groups
and two pyridyl groups connected in the meso position of
the ring, showed a different morphology that is an
indicative that the constitution of the chromophore ring
related with the position of the groups has a direct effect
in the way of precipitation, and this is not only attributed
to the aggregation in solution.
Metalloporphyrin Zn-(R)-4 showed an apparent
monolayer organization on the surface aligned with the
substrate symmetry axes and with a height lower than 0.5
nm in their topographic images, corresponding to flat-
adsorbed molecules. Moreover at room temperature
spheres were appreciated that presented height of 2.5 and
1 nm (Figure S21). The dimensions for the spherical
structure of Zn-(R)-4 when the solution was drop-cast
onto graphite surface at room temperature were around
14 to 20 nm in diameter (Figure S22).
The solid hierarchy organization for metalloporphyrins
observed in the SEM images reveled how Zn-(R,R,R)-1 and
Zn-(R,R)-2 precipitated in a fibrillar manner (Figures 18a
and b), whereas Zn-(R,R)-3 and Zn-(R)-4 a more chaotic
solid was observed after precipitation (Figures 18c and d).
Both Zn-(R,R,R)-1 and Zn-(R,R)-2 show a thicker fibrillar
structure compared with the corresponding free base
porphyrins, showing how the hierarchy of organization is
propagated from the smallest unit and strongest
interaction (the zinc(II)-pyridyl) up to the solid where
van der Waals forces must dominate.
a
30μm
30μm
30μm
30μm
30μm
20μm
b
Conclusions
It has been shown that a family of chiral zinc(II)
porphyrins can exhibit hierarchical self-assembly with a
c
complicated
combination
of
supramolecular
environments that give rise to high optical activity over a
wide range of wavelength - from 390 to 475 nm - from a
single chromophoric unit. The zinc(II) interaction with
pyridyl moieties is the dominant interaction, but
hydrogen bonding and van der Waals interactions are
responsible for the massive increase in optical activity
that is observed.
Molecular modeling has confirmed the hierarchy in the
self-assembly of the compound bearing one pyridyl group
and that weaker interactions between side-chains is a
viable way of inducing high optical activity in the second
step of the self-assembly of this compound. The second
step is well differentiated from the second because of the
difference in binding energy of the hydrogen bonds
compared with the zinc(II) to pyridyl coordination bond.
The optical activity of this compound was important for
d
20μm
30μm
Figure 18. SEM images of free-base porphyrins (left) and
metalloporphyrins (right) obtained from precipitation in
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the interpretation of the other self-assembled structures,
Page 14 of 17
ACKNOWLEDGMENT
1
2
3
4
5
6
7
8
particularly for Zn-(R,R)-2.
This work was supported by MINECO (Spain, Project
CTQ2010-16339 and MAT2013-47869-C4). M.L. acknowledges
SeRC (Swedish e-Science Research Center) for funding. P.N.
and F.D.M. thank the Swedish Research Council (Grant No.
621-2014-4646). Authors P.N., F.D.M. and M.L. acknowledge
SNIC (Swedish National Infrastructure for Computing) for
providing computer resources (snic001-12-192). D.B. is a
FNRS Research Director.
The compound Zn-(R,R)-2 incorporating two pyridyl
groups at the 12 and 6 o'clock positions of the porphyrin
core is the system that aggregates most strongly. The
optical activity of the aggregate indicates that inter-
porphyrin stacking through weak interactions is very
important as it is in the compound with one pyridyl
group. However, the concerted nature of the assembly
involving coordination and hydrogen bonds in this case
suggests an arrangement where the pyridyl group not
bonded to zinc(II) can interact with the amide function of
a neighboring tetramer. Therefore, the assembly of these
compounds is extremely sensitive to molecular
constitution, as demonstrated by the other zinc porphyrin
compounds with either two pyridyl groups at 12 o'clock
and 3 o'clock or three pyridyl groups, where only small
aggregates are observed with no obvious specific assembly
pattern.
9
ABBREVIATIONS
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AFM atomic force microscope; CD circular dichroism;
HOPG highly oriented pyrolitic graphite; NMR nuclear
magnetic resonance;
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ASSOCIATED CONTENT
Supporting Information. Contains full synthetic
experimental
procedures
and
characterization
of
compounds, additional absorption and CD spectra and
titration curves, 2D NMR spectrum, IR spectra of all the
porphyrins, computational details, additional AFM images
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AUTHOR INFORMATION
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Corresponding Author
* david.amabilino@nottingham.ac.uk
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Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version
of the manuscript.
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