Boosting Self-Assembly Diversity in the Solid-State by Chiral/Non-Chiral ZnII-Porphyrin Crystallization

Article abstract of DOI:10.1002/chem.201802031

A chiral Zn^II porphyrin derivative 1 and its achiral analogue 2 were studied in the solid state. Considering the rich molecular recognition of designed metalloporphyrins 1 and 2 and their tendency to crystallize, they were recrystallized from t

Full text of DOI:10.1002/chem.201802031

A Journal of
Accepted Article
Title: Boosting Self-Assembly Diversity in the Solid-State by Chiral/non-
Chiral ZnII-Porphyrin Crystallization
Authors: Wenjie Qian, Arántzazu González-Campo, Ana Pérez-
Rodríguez, Sabina Rodríguez-Hermida, Inhaz Imaz, Klaus
Wurst, Daniel Maspoch, Eliseo Ruiz, Carmen Ocal, Esther
Barrena, David B. Amabilino, and Núria Aliaga-Alcalde
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To be cited as: Chem. Eur. J. 10.1002/chem.201802031
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Boosting Self-Assembly Diversity in the Solid-State by
Chiral/non-Chiral ZnII-Porphyrin Crystallization
Wenjie Qian[a], Arántzazu González-Campo[a], Ana Pérez-Rodríguez[a], Sabina Rodríguez-Hermida[b],
Inhaz Imaz[b], Klaus Wurst[c], Daniel Maspoch[b,d], Eliseo Ruiz[e], Carmen Ocal[a], Esther Barrena[a], David
B. Amabilino[f]* and Núria Aliaga-Alcalde[a,d]
*
Abstract: This work bases on the solid-state study of a chiral ZnII-
porphyrin derivative (5,10,15,20-tetra[(4-R,R,R,R)-methyl-2-
Owing to the premature stage of the subject basic studies are
required, where a key-point is the use of molecular units that can
be easily tailored.
phenoxy-propanoate, 1) building block and its achiral analogous (2).
Here, foreseen the rich molecular recognition of the designed
metallo-porphyrins (1 and 2) and tendency to crystallize, we
recrystallized both using two sets of solvents (CH2Cl2/CH3OH and
CH2Cl2/hexane). As a result, four different crystalline arrangements
(1a-b, 2a-b, from 0D to 2D) were successfully achieved. We
performed solid state studies for all the species, analysing the role
played by chirality, solvent mixtures and surfaces (mica and HOPG),
on the supramolecular arrangements. As for the combination of
solvents and substrates we obtained a variety of micro-sized species,
from vesicles to flower-shaped arrays, including geometrical
microcrystals. Overall, our results emphasize the environmental
susceptibility of metallo-porphyrins and how this feature must be
taken into account in their design.
Regarding this, porphyrin derivatives are excellent molecular
prototypes due to the possibility of direct information transfer
from bench-experiments into biological facts.[6] We can ensure
self-assembly because of the sum of its manifold features: (i) the
conjugated core that has given copious studies with plenty of
data regarding the formation of H/J aggregates;[7] (ii) the use of a
metal centre that provides additional interactive site in the final
metallo-porphyrins;[8] and (iii) the addition of organic groups in
the meso- positions of the tetrapyrrolic units that afford extra
interactions depending on their number and nature.[1, 9]
Self-assembled aggregates of meso-substituted metallo-
porphyrins are therefore quite difficult to analyse and specially to
anticipate. An approach, adopted by us[10-11] and others[12-13]
relies on chirality as the driving force in the achievement of
highly ordered structures and the analyses in solution of their
noncovalent interactions (hydrogen bonds, - stacking or
coordination of through the metal centre) and self-assembly
mechanisms. However, much less it is known about the genuine
effect of chirality on the final architectures, with no strict
comparison of chiral/non-chiral analogous porphyrin types, as
well as there is scare information on the projection of the
knowledge gathered in solution to the solid state. Here, we unify
these two ideas toward the analysis of two novel Zn-porphyrins
containing phenoxy propanoate and methyl 2-phenoxy acetate
groups (1 and 2, respectively) in the four meso-positions of
porphyrins (Scheme 1). The choice of such final entities relates
to previous experiences with related systems[10, 14-17] foreseen
their rich molecular recognition capacity and tendency to
crystallize.
Introduction
Structural diversity and advanced activity based on
supramolecular self-assembly macrocycles are challenging
research topics in the field of materials science.[1-2] In the design
of non-covalent intermolecular interactions that resembles
nature,[3-5] the synthetic methodologies existing are mostly
limited by the difficulties of anticipating the final arrangements
and the lack of reversibility, where the thermodynamic products
are the ending tract, hampering both, control and functioning.
[a]
[b]
Ms. W. Qian, Dr. A. González-Campo, Dr. A. Pérez-Rodríguez, Dr.
C. Ocal, Dr. E. Barrena, ICREA Prof. N. Aliaga-Alcalde.
Institut de Ciència de Materials de Barcelona (ICMAB–CSIC),
Campus Universitari, 08193 Bellaterra, Spain
E-mail:nuria.aliaga@icrea.cat
Dr. S. Rodríguez-Hermida, Dr. I. Imaz, ICREA Prof. D. Maspoch
Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC
and The Barcelona Institute of Science and Technology, Campus
UAB, Bellaterra, 08193 Barcelona, Spain
Restricting our recrystallization methods, identical otherwise, for
our chiral and non-chiral Zn-porphyrin systems allow new
findings in the solid state primarily by the use of X-ray diffraction.
In addition, our study integrates the effect of solvent polarity
and highlights the polyvalent coordination of the ZnII ions
together with the application of solid-state techniques to
[c]
[d]
[e]
Dr. K. Wurst
describe in
architectures.
a
great manner the final self-assembled
Institut für Allgemeine Anorganische und Theoretische Chemie,
Universität Innsbruck, A-6020, Innrain 52a, Austria
ICREA Profs. D. Maspoch, N. Aliaga-Alcalde
ICREA (Institució Catalana de Recerca i Estudis Avançats), Passeig
Lluís Companys 23, 08010 Barcelona, Spain.
Prof. Eliseo Ruiz
Departament de Química Inorgànica i Orgànica, Universitat de
Barcelona, Barcelona 08007, Spain.
Institut de Química Teórica i Computacional de la Universitat de
Barcelona (IQTCUB), Barcelona 08007, Spain.
Prof. D. A. Amabilino
[f]
School of Chemistry, The University of Nottingham, University Park,
Nottingham, NG7 2RD, UK.
Scheme 1. General scheme of the disposition of the arms in
the meso-porphyrins, chiral (1) and non-chiral versions (2).
Supporting information for this article is given via a link at the end of the
document.
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Here, we do not focus on the dynamics of the supramolecular
arrangements but on the outcomes, portraying how small
changes can make a great difference in the organization of the
porphyrin entities.
Results and Discussion
Metallo-porphyrins recrystallization method. The free
porphyrins 4R-H2PPP and H2PPP (Scheme S1 top) were
reacted with ZnII salts and the resulting coordination compounds
(Zn(4R-PPP), 1, and Zn(PPP), 2) were re-crystallized by slow
liquid-liquid diffusion method using CH2Cl2/CH3OH and
CH2Cl2/hexane mixtures, respectively (Scheme S1 bottom),
when in a final step the samples were left open to air and most
of the solvent evaporated. As a result, we found out crystals for
all the combinations. 1a and 2a for the former and 1b and 2b for
the second, respectively. Attempts to re-crystallize the free
porphyrins (4R-H2PPP and H2PPP) using the procedure
described above did not provide crystals limiting their study to
the characterization of 1a-b and 2a-b. (Figures S1-S2)
It has been postulated that solvent mixtures containing water
rich solvents can guide self-aggregation processes.[18-19]
However, here additional factors require our attention as (i) the
non-innocent action of the side groups of the porphyrin cores
(noncovalent coordination), (ii) their chiral and non-chiral nature,
and (iii) the different coordination possibilities of the ZnII ions;
altogether they trigger the final structures (1a-b, 2a-b).
Structural Descriptions. Table S1 and Figure S3 show general
crystal data information of the four ZnII-porphyrin species (1a-b,
2a-b). Selected bond lengths and angles for each system are
listed in Tables S2-S5, respectively, as well as additional
Figures in S4-S7 displaying different projections. Overall, the
core, for all the porphyrin units shown in 1a-b and 2a-b is
identical, described as a ZnII ion coordinated to the N atoms of a
porphyrin ring. On the other hand, the main difference between
single molecules resides in the chiral centre on the peripheral
groups in the formers, as Scheme 1 shows. Here we focus on
the major differences between the four species and provide
basic molecular descriptions but concentrate in the
supramolecular arrangements of all the systems.
[Zn(OH2)(4R-PPP)]. 1a crystallizes in the orthorhombic P212121
space group. The mononuclear species contain one penta-
coordinated ZnII centre bonded to four N atoms from the
porphyrin core and one O from a H2O molecule (Figure 1). The
ZnII site is above the porphyrin plane (by 0.207 Å) toward the
axially bonded H2O. Such binding is nearly perpendicular to the
plane of the Zn-porphyrin core (O-Zn-N angles between 89.85 –
101.43°) with a Zn-O bond length of 2.247 Å. This meso-ZnII-
porphyrin displays four identical substituents containing chiral
ester groups (C45, C49, C53, C57 in Figure 1), all of them with
an absolute R configuration. These peripheral moieties, from the
4R-PPP2- system, spread in different orientations from the
porphyrin plane with a variety of dihedral angles (-122.04, -
109.56, 77.88 and-62.42°).
Figure 1. (Top) View of 1a with thermal ellipsoids fixed at 50 %.
Protons are omitted for the sake of simplification. (Bottom) View
of the supramolecular arrangement of molecules of 1a,
hydrogen bonding between coordinated H2O molecules and one
specific branch of the porphyrin moieties from neighbour
molecules. Hydrogen bonds (except for the ones of the H2O
molecule) and side branches not involved in the intermolecular
interactions are omitted for clarity. Color legend: Zn in purple, O
in red, N in blue, C in gray, and H in yellow.
The supramolecular arrangement of 1a molecules consists on
linear arrays of the porphyrin species, held together by O-H···O
hydrogen bonds (Figure 1 bottom, O···O 2.901 Å) between the
O from the H2O molecule coordinated to the ZnII centre and the
O from the C=O of one of the peripheral arms of a neighbouring
porphyrin unit. Thus, every porphyrin is hydrogen bonded to one
adjacent moiety. 1D arrays are formed by the linear coordination
of one of the four branches of a porphyrin with the nearest
neighbour. The Zn-porphyrin molecules in the chain are facing
opposite to each other in an alternating fashion, where the
hydrogen bonding makes the porphyrin moieties displaced. The
chains are aligned with closes distances superiors to 3.3 Å.
Further interactions of 1a molecules between chains lead to the
2D and 3D organizations shown in the SI (Figures S8-S9); in
particular, the lattice parameters of the 3D orthorhombic unit cell
being a = 9.579Å, b = 17.304 Å and c = 32.657 Å.
The large size of the crystals obtained for 1a (see inset in Figure
2 and Figure S3) permitted positioning them in the stage of an
AFM equipment (see physical measurements) to check the
quality of the crystal surface. The surface of this particular 3D
crystal as measured by AFM is shown in Figure 2a. It consists of
large and atomically flat terraces (several micrometers long and
hundreds of nm wide in average) separated by well-defined
steps. As it can be extracted from the line profile in Figure 2b,
the steps heights are  1.8 nm, which fully agrees with the
crystallographic parameter b and multiples of it (see details of
the 3D structure of 1a in Table S1 in the SI).
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always the coordination of two branches of the same monomeric
porphyrin unit with two others.[22-23]
Figure 3. (Top) View of 1b with thermal ellipsoids fixed at 50 %.
Hydrogen atoms are omitted for clarity. Color legend as the
previous. (Bottom) Arrangement of the coordination polymer (1D
system). Side branches not involved in the intermolecular
interactions and hydrogen atoms are omitted for clarity.
Figure 2. (a) Topographic AFM image of the surface of the
crystal of 1a. (b) Line profile corresponding to the segment
signalled in (a). A cartoon with molecules at scale and with the
appropriate orientation has been included. Inset: optical image
of the particular crystal analysed. Note the sharp angles defining
the macroscopic shape of the crystal.
[Zn(4R-PPP)n]. 1b crystallizes in the orthorhombic P212121 space
group. 1b is a coordination polymer made by the self-assembly
of the same porphyrin molecule described in 1a (same
porphyrinic ring and chiral branches, Figure 1). The difference
now resides in the fact that the ZnII centre of each porphyrin unit
is not coordinated to a molecule of H2O but to the C=O group of
an ester branch from the adjacent molecule. Such coordinative
bond provides the final polymeric structure depicted in Figure 3.
Therefore, each monomeric unit shows a penta-coordinated ZnII
centre, with the metal slightly shifted up from the plane of the
porphyrinic core (by 0.188 Å). Here, the organic moiety, 4R-
PPP2-, presents sprains, where one of the phenyl rings is
bended up with respect to the others. The Zn-O bond length
distance is now of 2.210 Å, shorter than the Zn-O distance
shown in 1a (Zn-OH2), with a noticeable deviation from previous
perpendicular disposition (O-Zn-N angles of 88.60 °, 88.72 ° and
reciprocal). As it happened in 1a, only one C=O from a chiral
ester group interacts with the neighbour molecule, leaving free
Figure 4. (Top) Side-view of the 1D supramolecular structure of
1a (left) and the polymeric 1b system (right) emphasizing the
distances among porphyrin units. Side branches not involved in
the intermolecular interactions and hydrogen atoms are omitted
for clarity. Figures enlarged in SI. (Bottom) Scheme of 1a and 1b
emphasizing the general orientation of the interactions.
[Zn(PPP)]. Compound 2a crystallizes in the C2/c monoclinic
space group and the discrete unit consists exclusively in one
molecule of the ZnII-porphyrin moiety. Here, the ZnII centre is
tetra-coordinated and adopts a square planar geometry, forming
a perfect plane with the four porphyrin nitrogen atoms of the
core. The planarity of the PPP2- ligand does not present any
deformation, in contrast with what it happens in the units of 1b.
As Figure 5 (Top) shows, the structure has some resemblance
to that describe in 1a, due to the similarities of the peripheral
groups, but now the replacement of the CH3 group in all four
branches by a proton lets to achiral molecules. Although the
compacted packing of the molecules shows proximity among
the other three. The final arrangement of
a single 1D
coordination system is shown in Figures 3 bottom and 4.
From our best knowledge, system 1b is the first chiral
coordination polymer of porphyrin nature that is formed by the
coordination of one branch of the porphyrin system and one
metal of the nearby molecule and the first one displaying chiral
properties. Other two coordination polymers have been
described in the literature where the metallo-porphyrin units are
connected by means of coordinative bounding of the metallic
centre and groups from the neighbours, providing also 1D
systems.[20-21] However, the ones described in the past, present
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neighbours, the ZnII centre in 2a is further away from the C=O
moiety (3.333 and 3.908 Å) and no molecules of solvent/H2O
appear in the final supramolecular architecture (Figure 5,
Bottom).
Figure 6. (Top) View of 2b with thermal ellipsoids fixed at 50 %.
Hydrogen atoms are omitted for clarity. Color legend: Color
legend: Zn in purple, O in red, N in blue and C in gray. (Bottom)
General view of nine molecules of 2b forming the 2D structure.
ZnII···ZnII distances are in all cases 13.663 Å.
Figure 5. (Top) View of 2a with thermal ellipsoids fixed at 50 %.
Hydrogen atoms are omitted for clarity. Color legend: Zn in
purple, O in red, N in blue and C in gray. (Bottom) Side views of
the disposition of 2a molecules and intermolecular interactions
among them. Some of the side branches not involved in the
intermolecular interactions and hydrogen atoms are omitted for
clarity.
Having a ZnII centre and four terminal benzylic ester moieties at
the meso- positions, both electron-acceptor and donor parts in
that order, we expected self-assemble through coordinative
bonds, fact that we see in two of the systems under study, 1b
and 2b. However, the final picture is more complex, where the
coordination number of the ZnII metal centre varies from four to
six, unlikely to anticipate. Single crystal X-ray diffraction shows
that in the case of the chiral systems, 1a and 1b, the Zn is
always pentacoordinated, choosing as the fifth ligand a molecule
of H2O or the terminal benzylic branch of one neighbouring
species, respectively. The same methodology applied to the
non-chiral versions provides tetra- and hexacoordinated ZnII
centres, 2a and 2b, respectively. In all cases, the size and
geometry of the crystals for all the four samples differ (Figure
S10). We could argue that the changes in solubility regarding
the existence of a –CH3 vs. a –H (1 vs. 2) in the winds of the
porphyrins, makes unfeasible a rigorous comparison but the two
methodologies used work in a similar manner in the four
systems, having crystals in all the cases after few days.
Therefore, chirality may be involved in the results; where the –
CH3 of the coordinated branches are always outside, improving
the disposition of the C=O group toward the ZnII centres (Figure
S11). Here, it is worth mentioning the formation of
(supramolecular or covalent) 1D structures, that display helical
shapes along the direction of the chains (Figure 4 bottom), only
in the case of the chiral systems (1a and 1b). This is a key factor
for the finding of chiral response in the solid state for such
species, absent in their achiral versions.
[Zn(PPP)n]. 2b crystallizes, as the previous, in the monoclinic
space group C2/c. The asymmetric unit is described by a
molecule of “Zn(PPP)O2”, where the oxygen atoms relate to the
C=O groups of two neighbouring molecules (Figure 6, top). The
ZnII centre is therefore hexa-coordinated, with
a pseudo-
octahedral symmetry due to its coordination to four nitrogen
atoms from the PPP2- organic moiety and to the two oxygen
atoms already mentioned. As it happens in 2a, the ZnII centre
forms a perfect plane within the chromophore core. In the
rearrangement, the oxygen atoms are tilted from 90° (84.27°,
85.64° and reciprocal). Each ZnPPP is attached to four other
units creating 2D networks with sql topology, where the ZnII-
porphyrin molecules present alternating orientations and one
single Zn···Zn distance of 13.663 Å (Figure 6, bottom). The
structure grows layer by layer, where the adjacent 2D networks
are parallel to each other with small interactions between them
through the branches that are not involved in coordination
(Figure S10).
Altogether, we found two Zn-porphyrin setups (1 and 2) with
great ability toward crystallization, and confined the combination
of solvents for such task (CH2Cl2/CH3OH and CH2Cl2/hexane) to
investigate chirality and polarity. By doing so, we created a map
with a rich variety of supramolecular arrangements.
On the other hand, the polarity of the solvent provided us with
additional features, where the use of CH3OH as a precipitating
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agent gave always isolated molecules (0D). Here, the
S20). The former area shows the appearance of a broad peak in
the case of 1a missing in 1b that may correspond to the inserted
H2O molecule as the structure shows. On the other hand, the
former area relates to the C=O stretching vibrational modes from
the terminal ester groups of the four branches. In the case of 1a,
appear two peaks at 1732 and 1750 cm-1 but 1b shows three
peaks, at 1719, 1739 and 1757 cm-1, agreeing well with the fact
of having one ester group coordinated, different from the rest.
Regarding the same area, compound 2a displays a single peak
with a shoulder at 1752 cm-1 (Figure S21) and in agreement with
the absence of different coordination on the branches with ZnII
centres. Thermogravimetric analyses (TG-DSC) of 1a-b and 2a
were also carried out. 2a and 1b show critical weight loss above
200 °C and 300 °C, respectively; TG data of 1a displayed the
release of water molecules starting at 85 °C (Figure S22).
Our studies show that the existence of different supramolecular
formations has scarce effect on the solid-state UV-Vis
absorbance of the final systems, remaining all, chiral and achiral,
very similar as it happens in solution (Figure S23). Additional
fluorescent experiments in solution or solid state were not further
pursued.
intermolecular
associations
within
discrete
porphyrin
coordination compounds rely on H-bonds involving coordinated
water molecules (1a), or C-H···π interactions (2a). Instead,
hexane always provided compact supramolecular moiety,
although compounds 1b and 2b present different arrangements.
This seems to indicate that the most apolar solvent forces
aggregation of the molecules that re-organize through the
metallic centre and moieties on the arms. The differences in the
self-assemble however, must be related to the affinity among the
molecules and therefore to the nature of such branches, chiral
and achiral.
In addition, studies in solution by 1H NMR and UV-Vis absorption
spectroscopy in CD2Cl2 (Figures S12-S14) of all ZnII-porphyrins
showed isolated porphyrin units, chiral (ZnII (4R-PPP), 1) and
achiral (ZnII(PPP), 2), with no additional info regarding
aggregation even at different concentrations (Figures S12, S13).
In the case of 1, the absence of CD signals using the same
solvent was expected, due to the distance between the
chromophore core and the chiral centres and the freedom of
rotation of the latest.[24] MALDI experiments of 1a and 1b in
CH2Cl2 provided almost identical spectra, showing one m/z
signal with maximum at 1084.29 and isotropic patterns that
matched well with the existence of [ZnII(4R-PPP)] units.
Additional experiments were performed with samples 1a and 1b
to contemplate the origin of the coordinated H2O in the case of
1a, missed in 1b. For that, we used dehydrated Zn acetate salts
and play with the H2O content of the solvents following the same
methodology explained above. Our analyses showed that the
the H2O molecules from crystal processing, therefore contained
in the solvents used to recrystallize the samples (Figure S15).
In a further step to characterize in detail the systems and chiral
nature in the case of 1a and 1b, we gathered information with
the use of current and specialized solid-state techniques, having
the advantage of correlating the changes to the structures
described above. The following paragraphs describe the
outcome encountered and our impressions regarding their
practical use.
On the other hand, high-resolution solid-state 13C NMR
spectroscopy was one of the most sensitive techniques for our
studies; Figure 7 shows the spectra of 1a, 1b and 2a (4R-H2PPP
is shown in Figure S24). The general assignments of the
chemical shifts were made by comparison with the free achiral
porphyrin and the contrast among the coordination compounds
(1a-b and 2a) and reported literature.[25] From the latest,
similarities of our porphyrins to the 5,10,15,20-tetraphenyl
porphyrin ZnII compound, studied by Grant et al.[26] allowed us to
interpret the nature of most of the chemical shifts by their range
of appearance although specifics about individual shift
assignations were not possible to elucidate.
Solid state studies. In a first step, we performed XRD analyses
on different crystalline batches for each compound (1a-b, 2a-b)
for phase identification and consistency of the final structures.
Figures S16-S19 depict the diagrams found experimentally for
the four species and the comparison with their simulated spectra,
respectively. 1a-b and 2a presented reproducible patterns that
allow us to use crystalline samples toward additional
characterizations in the bulk shown below. However, crystals of
2b were scarce. Attempts to recrystallize such solid showed
PXRD patterns that differed from the expected from the powder
pattern of 2b; this was a drawback for the rest of the analyses in
the solid-state where high amounts of samples were required.
Hence, the comparison of 2b with the rest of systems was
limited to the crystallographic information.
Figure 7. Comparison between the solid-state 13C NMR spectra
of 1a (top), 1b (central) and 2a (bottom) between 180 – 0 ppm.
We found out that multiple signals appeared in the range of 180
to 0 ppm for compounds 1a and 1b and from 180 to 30 ppm for
2a. The shifts could be grouped in four areas: signals appearing
between 180-165 ppm (A), 165-125 ppm (B), 125-100 ppm (C)
and from 80-20/0 (D), from down fields to high ones (Figure 7).
“A” encloses C3 (Figure 7), related to the C=O part of the ester.
“B” is the sum of Cpara, C, Cortho, Ci and probably part of the C
shifts. “C” would include C, Cmeso and Cmeta and finally “D” will
Following with our analysis, ATR-FTIR provides us clear
evidences of the changes shown in the crystal structures. In the
case of the chiral systems, 1a and 1b, the two molecules
present almost identical ATR-FTIR spectra but differing in two
areas between 3450-3470 cm-1 and 1720-1760 cm-1 (Figures
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rely to the non-conjugated part of the molecule (branches)
therefore to C1, C4 and C2, from down to high fields, in that
order. The absence of C2 in 2a (Figure 7) corroborates our
assignation in the other two systems (1a and 1b).
Taking advantage of the collected X-ray diffraction data, we now
could match structural differences between the molecules with
changes in the CD spectra. This way, the R-nature of the ligand,
for 1a shows a bisignate signal displaying a negative CE at the
lowest wavelength and a positive Cotton effect of similar
intensity at higher wavelengths. This matches well with the
position of the Soret absorption band of the porphyrinic
chromophor in solid and solution (Figure S28), where the
presence of bisignate CD may also relate to the orientation of
the porphyrin cores in a sliding face-to-face creating a 1D
supramolecular structure through the H2O interactions (Figure 1
bottom and 8 bottom).
In the solid state, compound 1a displays two intense and
proximate CD bands of opposite sign (positive-negative; positive
chirality)[31] between 375 and 500 nm (black solid line) centred at
431 and 444 nm, respectively. In the case of 1b (dotted line), CE
are observed in the same range, although now three CD signals
appear (positive-negative-positive) being the one in the middle
the most intense.
The variation in the number and shift of the chemical signals
differ, from one sample to another, in a complex manner. It is
already established that such variations can relate to overlap of
the signals, small structural differences inside the molecules
(e.g.: bendings provide different number and shifts for C, C,
etc.) and to the proximity of neighbouring molecules (e.g.: 
electron cloud of the core) providing different environments to
most of the groups.[27] Comparing 1a and 1b, the multiple
chemical shift corresponding to C3 of 1b suffers (at least one
over four per molecule) such alterations in a strong manner than
the others. The final shape of these signals is different in both
and shifted to higher fields for 1b (172.5 ppm (1a) and 170.7
ppm (1b)). The shift and shape of C1 also clearly differs in all
three systems showing the dramatic effect that different
environment has on the chiral/achiral carbons.
Chirality. Taking advantage of the chiral nature of some of the
samples, we performed solid-state circular dichroism studies of
1a and 1b using a KBr matrix due to the absence of chiral
response in solution. The key aspects for finding optimum
conditions in the achievement of the spectra were described
elsewhere[28-29] and detailed information regarding our procedure
here is described in the SI. In the past, some of us studied the
correlation of experimental Cotton effects (CE) with the
conformational stereoisomerism of chiral systems in the solid
state.[30] Our objective here was to establish the relationship
between the CD spectra of the compounds and their
supramolecular conformations, comparing systems with identical
chiral species. Figure 8 shows the comparison between the two
species (1a-b) under such conditions and Figure 4 bottom
emphasizes the crystallographic disposition of the two
compounds. Here again, it is important to underline the spatial
disposition observed in both systems (missed in the achiral
structures) that give rise to helical architectures in the chains
(supramolecular and coordinative 1D arrangements, for 1a and
1b, respectively).
The above features are present in the CD spectrum of 1b too,
however, this system differs from the previous by displaying a
third positive CD signal, absent in 1a, headed by the already
mentioned strong positive-negative bisignate CD sign. The
negative Cotton effect is here more pronounced than for 1a.
Such results could be associated to the expanded 1D
coordination network of 1b, by the coordination of the ZnII
centres with CH3CO2-R moieties (instead of molecules of H2O
(1a)) and differences in the tridimensional packing between the
two systems (Figure 8 bottom).[32] The first positive CD signals
(highest wavelengths) for the two systems, 1a and 1b, with
maxima at 444 nm and 448 nm, respectively, follow similar
trends, and intensity, having again close relation to the Soret
band and therefore intrinsic structure of the otherwise identical
porphyrinic units. These results highlight the high sensitivity of
the technique and how smooth supramolecular arrangements
can tune the appearing signals. They also show the advantage
of using chiral sources to study changes in self-assembly.
Finally, we analysed the chiral systems using solid-state
vibrational circular dichroism (VCD). VCD has great advantages
toward the detailed analysis of molecular conformations
because the number of molecular vibrations, in the IR region,
and the sensitivity of the technique are larger than the electronic
transitions in the UV-Vis region.[33]
Nevertheless, working in the solid state one must be extremely
careful minimizing the spectral artefacts. For that, we took into
account the noise in each experiment, the signals found in the
non-chiral system 2a (Figure S29) and performed theoretical
calculations on 1a (Figure S30). The workout of the material and
procedure is described in the SI. Figure 9 shows the comparison
between the IR and the VCD spectra of both systems. As
expected, each absorbance band in the IR spectrum has a
correspondence with a VCD band. DFT geometry optimization
and frequency calculation were performed (all-electron 6-311G*
[35]
B3LYP[34] calculations using Gaussian09 code,
see Fig. S31)
Figure 8. Solid state CD spectra of 1a and 1b.
taking into account the single molecule 1a (the length of the
chain of 1b difficult the study) displayed a number of motifs in
the window under study in a similar manner although few shifted
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from the experimental values displaying, at times, opposite sign
from the observed. Albeit contradiction, such results point out
that the chiral response of the system involves the surroundings,
where the supramolecular network is the responsible for the
chiral phenomenon.[36]
aggregates and microplates bundles shapes, depending on the
conditions used (Figures 10a-d and S31-S40).
Hollow microvesicles appeared by dissolving 1 in CH2Cl2/CH3OH
and depositing it on mica (Figures S31) and graphite (Figure 10a
and S32). On mica, the size and shapes of the aggregates were
the most
irregular, presenting sometimes sectioned
microvesicles; instead, on graphene the microballs presented an
average size of 600 nm with hole diameters of ~300 nm. Overall,
the vesicles in the latest were distinguishable and presented
homogenous shapes, where some of them connected to others
through small cords. In CH2Cl2/CH3OH, hollow vesicles made of
Zn-porphyrin systems have been related to the  stacking of
the porphyrin rings and formation of J-type aggregations.[37]
Guided by the crystallographic data of 1a, we can relate our
porphyrin microvesicles to the ligand-assisted long-distance J-
type formation[38] In our case, the supramolecular 1D structure of
1a, with up-down distributions of the porphyrins cores (Figures
1-bottom and S33), differs from the regular staircase situation
described in previous works.[39] The general explanation for the
holes formation, of encapsulation of the volatile solvents inside
the vesicles and later evaporation, is also applicable here.
Figure 9. Solid state VCD spectra (top) and solid-state
absorbance IR spectra (bottom) of 1a and 1b.
The measured and calculated VCD spectra of each system
match well with those found in the IR experiment corresponding
to 4R-systems (intensities are not comparable). The bands
described between 1800-1600 cm-1 vary between the two
species, as anticipated due to the difference upon coordination
of the ZnII centres of the porphyrinic moieties with H2O (1a) or
some C=O groups (1b). In this region, the number of VCD bands
found in this region for 1b is higher than 1a, with some of them
shifted to lower energies, probably due to the difference
between the C=O groups bounded with respect to the non-
bounded ones and additional intermolecular interactions in the
tridimensional map of each system.
Figure 10. SEM images on HOPG substrates of Zn(4R-PPP)
prepared in CH2Cl2/CH3OH (a), Zn(PPP) in CH2Cl2/CH3OH (b),
Zn(4R-PPP) in CH2Cl2/hexane (c) and Zn(PPP) in
CH2Cl2/hexane (d).
Overall, all the techniques used in the solid-state studies agree
with the structures but emphasize their restricted outcome
individually.
Repeating the same exact experiment but using 2, the
architectures found on mica and graphene were totally different
than before. Now, on mica well-shaped microcrystals of different
sizes presented parallelogram and prismatic geometries
(Figures S34). On graphite, four-pointed star shapes were the
most common morphology (Figures 10b) although bigger and
amorphous aggregates were seen too (Figure S35); all shapes
intuitively relate to the layer-by-layer aggregation present in the
structure of 2a (Figure 4-bottom) and to the formation of short-
distance J-type aggregates (staircase disposition). Overall, the
quantity of material deposited on the different substrates limited
the use of additional techniques, as for example XRD, toward
the possible identification of 1a/2a or related species.
Nevertheless, it is important to stress the differences in
Surface studies. We performed SEM images of the samples
prepared by drop casting solutions (10-3 M) of the chiral (Zn(4R-
PPP), 1) and achiral (Zn(PPP), 2) porphyrins in the same
solvents mixture used to achieve 1a-b and 2a-b: CH2Cl2/CH3OH
(1:1) or CH2Cl2/hexane (1:1), respectively, onto mica or highly
oriented pyrolytic graphite (HOPG) substrates. Here, the effects
of solvents and surfaces were studied by the comparison with
the morphologies found in the single crystal X-ray diffraction
experiments.
A
variety of Zn(4R-PPP) and Zn(PPP)
nanoarchitectures were obtained by an evaporation-driven self-
assembly process (Figure 10). The chiral and achiral porphyrins
generate reproducible microvesicles, microrods flower-shaped
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10.1002/chem.201802031
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morphology between the chiral and achiral units, providing the
first microvesicles and the second prismatic shapes. Taking into
account that the comparison was performed in an identical
manner (hence, mixed of solvents and surfaces), the difference
between the two obvious architectures may reside in their
intrinsic nature, chiral and achiral.
future applications and face the present challenges, as the lack
of anticipation of the final architectures. The addition of a metal
centre increases the range of diversity in the coordinative way
meanwhile blocks strong stacking interactions among the
porphyrin cores giving priority to the nature of the arms, now key
parts in the creation of the supramolecular arrays. The four arms
are flexible enough and the carbonyl moieties, from the ester
groups at the edges, present the freedom to attach the ZnII
centres in different manners where the final coordination number
of the metallic centre can go from four to six, depending on the
overall conditions. The comparison between chiral metallo-
porphyrin (1) and non-chiral (2) one, repeating exact conditions,
provides different crystallographic species, pointing out the
complexity of adding chirality among the other factors already
mentioned (coordination and solubility). The nature of the
precipitant solvents, from polar (CH3OH) to apolar (C6H14), can
promote the stabilization of mononuclear entities, like in the case
of former, in both, chiral and non-chiral metallo-porphyrins (1a
and 2a, respectively). In addition, 1a shows a quite distinctive
1D supramolecular structure. Hexane promotes aggregation in
the chiral and non-chiral systems, having at the end coordination
1D structures (1b) or 2D systems (2b), respectively. Regarding
the effect of chirality, 1a and 1b, exhibit supramolecular and
coordinative 1D architectures that differ, but share certain
similarities like the disposition of the chromophore units, giving
as a result helical chains and the possibility of analyzing the
systems by additional techniques (as CD) in the solid state. The
achiral versions lack for this type of rearrangements.
On the topic of chirality, CD and VCD solid-state experiments
presented differences between 1a and 1b, stressing their
sensitivity as well, although the variations in the former were not
intuitive and difficult to understand without the assistance of the
crystallographic data. In the case of the second, it presented
relevant changes in the area related to the C=O stretching, in
agreement with the coordination to Zn atoms. Yet, our results
emphasize that the proper analysis of the structures by the
exclusive use of such techniques requires the creation of
extended libraries and corroboration from theoretical
calculations.
Finally, the studies of our systems on two different substrates
show the enormous effect of the surfaces on the final structures,
providing a variety of vesicles, flower-shapes and well defined
geometrical architectures, increasing at times the capacity of
achieving microcrystals (e.g.: 2b in graphite). Here again,
chirality should be underlined due to the differences found
between the chiral and achiral systems under the same
conditions.
Moving to CH2Cl2/C6H14, with 1, on mica, we observed again
microballs, some of them with holes, together with slide
microvesicles (Figure S36). The difference with the previous
resides in that most of the microball edges smear, connecting
each other, probably because of solvent flow and the longer
sintering state during the evaporation process. The shape of the
nanostructures agrees well with 1b, where the 1D coordination
structure provides J-type aggregates (up-down structure, Figure
3-bottom). On graphite, two distinctive morphologies were
observed depending on the area of analysis. This way, some of
the parts presented microvesicles with sizes between 300-400
nm (Figures S37). The topographic information in such cases
was very similar to that found in CH2Cl2/CH3OH; however,
trigonal prisms were observed mostly at the surface edges
(Figure 10c). They presented homogeneous sizes, with a length
of 8 m, and spherical defects in some of the prismatic faces
(Figures 10c and S38). Such prismatic microcrystals show
similarities with those at the macroscopic scale used for the X-
ray diffraction analysis (Figure S3). They were in areas where
the evaporation process is slower, providing more time for the
ensemble and organization of the matter. This was established
by the fact that few of them appeared cracked exposing the
aggregation of microvesicles like expanded polystyrene bowls
(Figure 10c).
At last, the same mixture using 2 on mica showed microplates,
with diverse shapes and sizes, together with flower-shaped
topologies (Figure S39). In former works, the formation of the
latest relates usually to H2O.[40] Here this association cannot be
discriminated but neither defended. However, in both cases, in
microplates and micropetals, it is clear the existence of an
extended face versus the others, connecting with the 2D layers
observed in the 2b system typical in other layered materials as
well.[41] Regarding the experiment in graphite, now multiple
microblocks were present again with a variety of sizes and
prismatic shapes (Figure 10d and S40). The size may depend
on the deposition and evaporation processes, where the
concentration of the material can also rely on the different
terraces of the graphite surface. Either way, this result differs
with the macroscopic scale in solution, where the crystals were
scarce and difficult to achieve, pointing out the tendency of
graphite to provide crystalline material under these conditions.
Our studies show all the factors that lead supramolecular
design: metallo-coordination, chirality, nature of solvents and
surfaces; play relevant roles; however, they also provide
Conclusions
fingerprints
that
repeat
(e.g.:
coordinative-bindings,
supramolecular chirality, shapes on surfaces,…) being of great
relevance the creation of libraries with extended studies as the
present here. The realistic application of molecular materials will
undergoes through their disposal on substrates in the solid state.
The understanding of the supramolecular nature is vital for
Using metallo-porphyrin derivatives as unit models, we
described how through a unique molecular design, we created
well-defined and isolable nanostructures; where, the tuning of
solvent conditions directly affected the self-assembly process,
and therefore the morphologies of aggregates. Our results
emphasize the relevance of porphyrin crystallization toward their
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by vacuum distillation. The dark viscous material that remained was
washed with saturated sodium carbonate solution. The free-base
porphyrin (H2PPP) was isolated as a purple solid (723 mg, 26 %) after
purification by column chromatography (SiO2, CH2Cl2/ CH3OH 100:0.5).
Anal. calcd for C56H46N4O12 (966.98 g·mol−1): C 69.56; H 4.79; N 5.79.
Found: C 69.45; H 4.67; N 5.64. 1H NMR (300 MHz, CDCl3, 25 ℃): δ
8.85 (s, 8H), 8.13 (d, J = 8.5 Hz, 8H), 7.29 (d, J = 8.5 Hz, 8H), 4.94 (s,
8H), 3.95 (s, 12H), -2.79 (s, 2H). ATR-FTIR date (cm−1): 3650(w),
2954(m), 1753(s), 1604(m), 1507(m), 1219(s), 1170(s), 1084(m), 997(w),
966(w), 802(m) Maldi-TOF/MS m/z (%): 966.22 ([H2PPP]+).
future uses in several fields, as for example photovoltaics, that
will benefit from such knowledge.
Experimental Section
Materials and Methods. Experiments were carried out in aerobic
conditions or under N2 atmosphere when required, using commercial
grade solvents for the synthesis of the four crystallographic species.
Solvents were dried and distilled for some of the synthetic steps and for
the absorption UV-Vis studies. Methyl (4-formylphenoxy) acetate was
purchased in Activate Scientific. (R)-methyl-2-(4-formyl phenoxy)-
propionate was achieved by modifying prior procedure improving the
Synthesis of [Zn(PPP)] (2a). The procedure was identical to that
described in 1a but using H2PPP. Yield: 249 mg (78 %). Crystals of 2a
were achieved as for 1a. Anal. calcd for C56H44N4O12Zn·1.85H2O: C
63.23; H 4.52; N 5.27. Found: C 63.01; H 4.29; N 5.07. ATR-FTIR date
(cm−1): 3347(w), 2851(w), 1752(s), 1606(m), 1509(s), 1434(m), 1284(w),
1217(s), 1204(s), 1171(s), 1084(s), 997(s), 799(s), 717(m). Maldi-
TOF/MS m/z (%): 1028.15 ([Zn(PPP)]+).
yield.[14]
5,10,15,20-tetra[(4-R,R,R,R)-methyl-2-phenoxy-propanoate]-
porphyrin, here described as 4R-H2PPP, was synthesized according to
the procedure described elsewhere.[10]
Synthesis. Synthesis of 5,10,15,20-tetra[(4-R,R,R,R)-methyl-2-phenoxy-
propanoate]-porphyrin, (4R-H2PPP). Freshly distilled pyrrole (810 μL,
11.53 mmol) was mixed with 4-formylphenoxy propanoate (2.4 g, 11.53
mmol) and refluxed during 2 h using propionic acid as solvent (42 mL).
After the vacuum distillation of propionic acid, the remaining dark viscous
solid was washed with a saturated sodium carbonate solution to remove
residual acid. The free-base porphyrin (4R-H2PPP) was then isolated as
Synthesis of [Zn(PPP)]n (2b). The procedure was as described for 2a but
using H2PPP. Crystals of 2b were achieved by following the procedure
described in 1b. Crystallographic data was pursued and shown below.
Further analyses were unfeasible due to the scarce amount of the
crystalline samples.
a
purple solid after purification by column chromatography (SiO2,
Physical Measurements. C, H and N analyses were performed with a
Perkin-Elmer 2400 series II analyser in London Metropolitan University
and Thermo Fisher Scientific Flash EA 2000 CHNS with accessory
Microbalança MX5 Mettler Toledo in Servei d’Anàlisi Química in UAB.
Correlation of the experimental data was performed with the use of
http://www.chem.yorku.ca/profs/potvin/Jasper/jasper2.htm. Simultaneous
thermogravimetric analysis (TG)-differential scanning calorimetry/
differential thermal analysis (heat flow DSC/DTA) system NETZSCH -
STA 449 F1 Jupiter. MALDI-TOF/TOF MS: Mass spectra were recorded
using matrix assisted laser desorption ionization with time of flight
(MALDI-TOF) mass spectrometer ULTRAFLEXTREME (Bruker) at
Servei de Proteòmica i Biologia Estructural (SePBioEs) from UAB. FTIR:
Infrared spectra (4000-450 cm-1) were recorded from Spectrometer
Perkin-Elmer Spectrum One. The measurement was performed in
universal attenuated NMR spectra: 1H-NMR spectra were obtained on
Bruker Advanced at 300 MHz and 298 K. The 13C-NMR spectra were
recorded using frequency 100 MHz in the Servei de Ressonància
Magnètica of the Universitat Autònoma de Barcelona, performed on a
Bruker AVANCE-III 400 MHz spectrometer. UV-Visible absorption
measurements: All UV-visible spectra for liquid samples were obtained
on a Varian Cary 5000 using quartz cells. For solid samples it is available
a Diffuse Reflectance Sphere DRA-2500 accessory in the UV-Vis-NIR
Varian Cary 5000 spectrophotometer, with operational range of 190-3300
nm, and be measured mainly in reflectance or transmittance mode.
Powder X-ray diffraction (PXRD): Dates were collected on Panalytical
X'PERT PRO MPD diffractometer using Cu K α radiation at 295K. CD:
KBr discs were used as the solid solution for the study of circular
dichroism spectra. The solid-state circular dichroism spectra were
recorded on a JASCO-715 spectrometer fitted with a sample holder. For
vibrational circular dichroism (VCD), the equipment is a PMA50 module
coupled to a Bruker Tensor 27 FT-IR spectrometer. Investigation of
morphology and surface self-organization phenomena were performed
on Luminescence spectrometer Perkin Elmer LS4 and Scanning Electron
Microscope (SEM) QUANTA FEI 200 FEG-ESEM. Atomic force
microscopy (AFM) contact mode measurements were carried out at room
temperature using a commercial head and control unit from anotec
Electr nica and Si tips mounted in soft (k  0.01-0.5 Nm-1) Veeco
cantilevers. All data were analysed with the WSxM freeware.[42]
CH2Cl2/CH3OH 100:0.5)[15]
.
Yield: 790 mg (27 %). Anal. calcd for
C60H54N4O12 (1023.09 g·mol−1): C 70.44; H 5.32; N 5.48. Found: C 70.57;
H 5.27; N 5.39. 1H NMR (300 MHz, CDCl3, 25 ℃): 1H NMR (300 MHz,
CDCl3, 25 ℃): δ 8.84 (s, 8H), 8.10 (d, J = 8.7 Hz, 8H), 7.24 (d, J = 8.7 Hz,
8H), 5.23 – 4.95 (m, 4H), 3.93 (s, 12H), 1.83 (d, J = 6.8 Hz, 12H), -2.80
(s, 2H). ATR-FTIR date (cm−1): 3319(w), 2925(w), 1755(s), 1739(s),
1605(m), 1504(m), 1133(s), 967(m), 736(w). Maldi-TOF/MS m/z (%):
1022.68 ([4R-H2PPP]+).
Synthesis of [Zn(OH2)(4R-PPP)] (1a). 4R-H2PPP (300 mg, 293 μmol)
was dissolved and refluxed in 40 mL of CH2Cl2 under N2 atmosphere. A
solution of Zn(CH3COO)2·2H2O (220 mg, 1 mmol) in a mixture of
CH3OH/CH2Cl2 (10 ml:10 ml) was added drop wise. The reaction was
monitored by absorption UV-Vis spectroscopy (~ 3 h). Afterward, the final
solution was washed with NaHCO3 saturated aqueous solution, brine and
distilled water. The organic phase was extracted using CH2Cl2. The
removal of the solvent gave the desired product as a shining purple
solid.[17] Yield: 278 mg (86 %). Suitable crystals for X-ray diffraction
analyses of 1a were achieved after few days by dissolving the solid in a
1:1 mixture of CH2Cl2 and CH3OH, leaving the final solution open to air.
Anal. calcd for C60H54N4O13Zn·0.45H2O: C 64.77; H 4.97; N 5.04. Found:
C 64.64; H 4.80; N 4.94. ATR-FTIR date (cm−1): 3642(w), 3478(w),
2989(w), 1751(s), 1733(s), 1605(m), 1505(s), 1488(s), 1445(m), 1338(m),
1277(m), 1203(s), 1172(s), 1129(s), 995(s), 846(m), 795(s). Maldi-
TOF/MS m/z (%): 1084.62 ([Zn(4R-PPP)]+).
Synthesis of [Zn(4R-PPP)]n (1b). 1b was achieved by following previous
procedure but using dehydrated Zn(CH3COO)2. Yield: 277 mg (86 %).
Crystals of 1b were achieved by dissolving the final solid in CH2Cl2 and
layering the solution with C6H14. Anal. calcd for C60H52ZnN4O12 (1086.45
g·mol−1): C 66.33; H 4.82; N 5.16. Found: C 66.61; H 5.07; N 5.02. ATR-
FTIR date cm−1): 2988(w), 1757(s), 1739(s), 1721(s), 1605(m), 1506(s),
1445(m), 1340(m), 1201(s), 1176(s), 1131(s), 996(s), 799(s). Maldi-
TOF/MS m/z (%): 1084.63 ([Zn(4R-PPP)]+).
Synthesis of H2PPP. Freshly distilled pyrrole (800 μL, 11.53 mmol) was
mixed with methyl (4-formylphenoxy) acetate (2.239 g, 11.53 mmol) in
refluxing propionic acid (42 mL). After 2 h, propionic acid was removed
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Acknowledgements
A This work has been supported by the Spanish Government
under projects MAT2013-47869-C4-1-P, MAT2015-65354-C2-1-
R, MAT2016-77852-C2-1-R and CTQ2015-64579-C3-1-P
(AEI/FEDER, UE) and by the Generalitat de Catalunya, FI-DGR
grant 2014 and ICREA academia. We also acknowledge the
MINECO project MAT2015-68994-REDC and the ‘‘Severo
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Keywords: Zinc-porphyrin • chirality • supramolecular
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This article is protected by copyright. All rights reserved.

10.1002/chem.201802031
Chemistry - A European Journal
FULL PAPER
Entry for the Table of Contents
FULL PAPER
Perspectives on metallo-porphyrin
aggregates are revised by means of
two Zn-porphyrin derivatives, chiral
and achiral analogue, using slow
liquid-liquid diffusion methods with
two sets of solvents, CH2Cl2/MeOH
and CH2Cl2/Hexanes, finding
Wenjie Qian, Arántzazu González-
Campo, Ana Pérez-Rodríguez, Sabina
Rodríguez-Hermida, Inhaz Imaz, Klaus
Wurst, Daniel Maspoch, Eliseo Ruiz,
Carmen Ocal, Esther Barrena, David B.
Amabilino* and Núria Aliaga-Alcalde*
different crystalline architectures for
each combination. This paper
Page No. – Page No.
shows the analysis performed in the
solid state and the relevance that
chirality, solvent mixtures and
surfaces (mica and HOPG) have on
the supramolecular arrangement.
Boosting Self-Assembly Diversity in
the Solid-State by Chiral/non-Chiral
ZnII-Porphyrin Crystallization
This article is protected by copyright. All rights reserved.