Non-centrosymmetric homochiral supramolecular polymers of tetrahedral subphthalocyanine molecules

Article abstract of DOI:10.1002/anie.201411272

A combination of spectroscopy (UV/Vis absorption, emission, and circular dichroism), microscopy (AFM and TEM), and computational studies reveal the formation of non-centrosymmetric homochiral columnar subphthalocyanine assemblies. These assemblies form th

Full text of DOI:10.1002/anie.201411272

Angewandte
Chemie
DOI: 10.1002/anie.201411272
Supramolecular Chemistry
Non-Centrosymmetric Homochiral Supramolecular Polymers of
Tetrahedral Subphthalocyanine Molecules**
Julia Guilleme, Marꢀa J. Mayoral, Joaquꢀn Calbo, Juan Aragꢁ, Pedro M. Viruela, Enrique Ortꢀ,*
Tomꢂs Torres,* and David Gonzꢂlez-Rodrꢀguez*
Abstract: A combination of spectroscopy (UV/Vis absorption,
emission, and circular dichroism), microscopy (AFM and
TEM), and computational studies reveal the formation of non-
centrosymmetric homochiral columnar subphthalocyanine
assemblies. These assemblies form through a cooperative
supramolecular polymerization process driven
molecular convex–concave polymerization of related bowl- or
cone-shaped monomers is in itself a challenging task.[7] As
a result of their 3D structure, most bowl- or cone-shaped
monomers are obviously reluctant to aggregate in solution.
Moreover, many bowl-shaped aromatic molecules, such as
by hydrogen-bonding between amide groups,
p–p stacking, and dipolar interactions between
ꢀ
axial B F bonds.
A
s a result of their singular structure, the
columnar stacking of tetrahedral-shaped mol-
ecules can provide materials with exclusive
properties that cannot be obtained using
common planar p-conjugated discotics.[1]
First, these molecules may exhibit intrinsic
chirality,[2] which may potentially lead to the
formation of non-centrosymmetric stacks in
which helical chirality evolves from the mol-
ecule itself[3] and not from chiral centers
present in peripheral side chains.[4] Second,
these molecules are often endowed with axial
dipoles that may add up along the stacks
resulting in the formation of polar nanostruc-
tures, an effect which may be exploited to
uniaxially align the columns in an electric
field[5] or for the development of polarized
semiconducting films.[6] However, the supra-
Figure 1. a) Top and side views of the two enantiomers of a model trisubstituted
SubPcBF macrocycle and the corresponding homochiral head-to-tail columnar stacks.
b) Molecular structures of the two enantiomers of compound 1 (1a and 1b),[14,15] and
compounds 2, 3 and 4. Atom colors in (a): N=blue, C=turquoise, H=white, and the
yellow sphere represents a generalized R functional group.
[*] J. Guilleme, Dr. M. J. Mayoral, Prof. T. Torres,
Dr. D. Gonzꢀlez-Rodrꢁguez
corannulane, sumanene, calixarene, or cyclotriveratrylene,
undergo cone inversion,[8] which leads to depolarization and
stack racemization.
Departamento de Quꢁmica Orgꢀnica, Facultad de Ciencias
Universidad Autꢂnoma de Madrid, 28049 Madrid (Spain)
E-mail: tomas.torres@uam.es
Subphthalocyanines (SubPcs; Figure 1a)[9] are a rare
example of p-conjugated aromatic molecules with a rigid
tetrahedral structure.[10] They are composed of three isoindole
units condensed around a boron atom which also bears an
axial ligand perpendicular to the macrocyclic core. In contrast
with their higher analogues the phthalocyanines (Pcs), whose
supramolecular chemistry is dominated by p–p interactions
between planar surfaces, SubPcs do not exhibit a strong
tendency to aggregate in solution. This property, which stems
from their nonplanar geometry, has proven beneficial for
many applications in which dye aggregation needs to be
prevented.[11] However, because of their intense absorption
and emission in the visible region of the electromagnetic
spectrum and their tunable electronic properties,[12] SubPcs
have recently attracted significant technological interest in
the fields of organic semiconductors and optoelectronics.[13]
david.gonzalez.rodriguez@uam.es
J. Calbo, Dr. J. Aragꢂ, Dr. P. M. Viruela, Prof. E. Ortꢁ
Instituto de Ciencia Molecular, Universidad de Valencia
46980 Paterna (Valencia) (Spain)
E-mail: enrique.orti@uv.es
Prof. T. Torres
IMDEA Nanociencia, Facultad de Ciencias
Cantoblanco, 28049 Madrid (Spain)
[**] Funding from MINECO (CTQ2011-24187, CTQ2011-23659,
MAT2012-38538-CO3-01 and 02, FEDER funds CTQ2012-31914 and
CTQ2012-35513-C02-01), CAM (S2013/MIT-2841 FOTO-CARBON),
GVA (PROMETEO/2012/053 the European Research Council (StG-
279548), and MECD (F.P.U. fellowships to J.G. and J.C.) is gratefully
acknowledged.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201411272.
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Such applications often demand suitable columnar organiza-
tions in which close p–p contacts contribute to efficient
exciton and charge transport.[1]
Herein, we show for the first time that tetrahedral-shaped
molecules such as SubPcs can be polymerized in solution to
form self-assembled non-centrosymmetric homochiral col-
umnar nanostructures. The key to overcome the challenging
head-to-tail (convex–concave) stacking is the combination of
hydrogen-bonding interactions between three peripheral
amide groups and the use of an axial fluorine atom which
provides minimal steric hindrance upon stacking[7c] and
a strong axial dipole moment.
C3-symmetric SubPcs 1, 2, 3, and 4 (Figure 1b) were
synthesized as racemic mixtures and characterized as de-
scribed in the Supporting Information. The two enantiomers
of 1, hereafter called 1a and 1b, were then separated by
analytical chiral HPLC (see Figure S1 in the Supporting
Information) on a milligram scale.[15]
The first insights into the aggregation of enantiomers 1a
or 1b in solution came from UV/Vis absorption and emission
spectroscopy. In 1,4-dioxane, a competing solvent for hydro-
gen bonding, compounds 1a/b display the characteristic
narrow Q band features of monomeric SubPcs, that is,
a maximum at l = 576 nm (e = 86000mꢀ1 cmꢀ1) with shoulders
at l = 530 and 560 nm. In sharp contrast, 1a/b solutions in
methylcyclohexane (MCH) or dodecane exhibit a broader,
blue-shifted Q band with a maximum at l = 520 nm (e =
38.000mꢀ1 cmꢀ1; Figure 2a,b and Figure S2).[16] Similarly,
molecules 1a and 1b in dioxane show a strong emission
band, typical of SubPc monomers, with emission maxima at
l = 590 nm. In contrast, MCH solutions of 1a/b are virtually
non-emissive (Figure 2c,d and S2). Both the presence of the
small axial fluorine atom and the peripheral amide substitu-
ents are required for these spectroscopic changes in apolar
solvents, as none of compounds 2, 3, and 4 display such
features. Thus, we attribute the spectral features measured in
apolar solvents to the formation of head-to-tail columnar
supramolecular polymers of 1a/b, as depicted in Figure 1a.
CD spectroscopy was consistent with this hypothesis (Fig-
ure 2e,f). Compounds 1a or 1b in the monomer state exhibit
a mirror-imaged CD signal centered at l = 578 nm, negative
for 1a and positive for 1b.[14] In MCH, in contrast, M- and P-
helical homochiral stacks are formed that show a Cotton
effect with a zero-crossing at the l = 520 nm aggregate
absorption maximum wavelength. Hydrogen bonding
between the exocyclic amide groups was confirmed by
1H NMR and FTIR spectroscopy.
The dissociation of 1a/b stacks can be monitored by
spectroscopy as a function of sample concentration, temper-
ature, or solvent composition. Figures 2a, c, and e show the
absorption, emission, and CD changes, respectively, occurring
in the transition from the supramolecular polymer to the
monomer as the volume fraction of dioxane (cd) in
MCH:dioxane mixtures is increased. Increasing the sample
temperature of MCH solutions results in similar changes
(Figure 2b, d, and f). Consistent with a supramolecular poly-
merization process, the degree of aggregation (a) decreases at
high temperatures and at low concentrations (Figure S3). The
self-assembly mechanism has been analyzed by fitting the
Figure 2. Q-band a,b) UV/Vis absorption, c,d) emission, and e, f) cir-
cular dichroism spectral changes of SubPc 1a and 1b. Changes are
recorded as a function of a,c,e) the volume fraction of dioxane (cd) in
MCH:dioxane mixtures (from cd =0–0.35; [1a]=[1b]=5.0ꢃ10ꢀ5 m),
or b,d,f) the temperature (T=368–268 K; [1a]=[1b]=3.9ꢃ10ꢀ6 m) in
MCH solutions. Arrows indicate the trends with increasing cd or
T values, respectively.
non-sigmoidal cooling curves, calculated from the UV/
Vis absorption extinction coefficient at l = 576 nm, to the
cooperative nucleation–elongation model developed by
Meijer and ten Eikelder and co-workers (Figure S4).[17] This
model assumes that that after an unfavourable nucleation
process, the system can abruptly elongate yielding extended
supramolecular polymeric species. The data fitted reasonably
well to a dimeric nucleus that then grows by successive
SubPc stacking.[18] Applying the model to 1a/b in MCH, we
obtained the main thermodynamic parameters, both for the
nucleation: DH8nucl = ꢀ11.7 ꢁ 1.3 kJmolꢀ1, and Knucl = 7.1 ꢀ
103 mꢀ1, as well as for the elongation process: DH8e =
ꢀ56.1 ꢁ 3.4 kJmolꢀ1, DS8 = ꢀ55.0 ꢁ 2.8 Jmolꢀ1 K, Ke = 4.2 ꢀ
105 mꢀ1, and Te = 344.9 ꢁ 1.1 K (where Knucl and Ke refer to
the equilibrium constants for nucleation and elongation,
respectively). The degree of cooperativity in the self-assembly
process was calculated at s = 0.017.[15,16] These values agree
reasonably well with those reported for other hydrogen-
bonded polymers obtained from C3-symmetric monomers.
Our entropic term is, however, relatively small, which may
reflect the high structural rigidity of our SubPc monomers.[18b]
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The formation of supramolecular polymeric stacks of
SubPcs 1a/b was confirmed by scanning probe and electron
microscopies. MCH solutions were dropcast onto highly
oriented pyrolitic graphite (HOPG) and the surface was
imaged by AFM after solvent evaporation. As shown in
Figure 3a, long fibrillar objects were observed on the surface
Figure 4. a) Amide conformations that lead to triple intermolecular
hydrogen bonding in the 1b helical stacks. Syn and anti orientations
are defined as depending on whether the amide carbonyl dipole is
ꢀ
aligned parallel or antiparallel, respectively, to the axial B F bond.
In and out orientations describe the position of the carbonyl oxygen
(red atom) with respect to the isoindole ring. b) Side and c) top views
of P-helical all-anti-in head-to-tail arrangement of 1b stacks calculated
by DFT. Dodecyl chains have been replaced with hydrogen for
calculations. d) Stabilization energy per monomer unit (DEmon,n) calcu-
lated at the at wB97X-D/cc-pVTZ level as the number of monomers
(n) in the all-anti-in aggregate increases. Energy values are fitted to
a biexponential decay function (solid line). Atom colors in (a, b, c):
O=red; N=dark blue; H=white; simplified R group in (a)=yellow;
C=green (in (b) and (c); C=turquoise in (a)).
Figure 3. a) AFM height image of 1 dropcast onto HOPG
([1]=3.2ꢃ10ꢀ6 m in MCH). b) TEM image of a negatively stained
solution of 1 in MCH on a carbon-coated copper grid. c) Dodecane
gels under no (left) or l=365 nm (right) UV irradiation. d) SEM image
of the corresponding xerogel.
whose average height match those of single SubPc fibers
(2.5 nm in diameter) and fiber bundles (5–6 nm), likely
formed by bundling of two or three individual fibers. The
high propensity of these fibers to bundle may be the reason
why we could not observe helical structures. TEM analysis
(Figure 3b) also showed the formation of bundled fibers in
MCH or dodecane. Additionally, SubPc 1 formed stable red–
magenta-colored gels in dodecane at a concentration of
4.2 mgmLꢀ1, whose fluorescence emission is quenched with
respect to non-aggregated solutions in dioxane (Figure 3c).
SEM images of the xerogel (Figure 3d), prepared by vacuum-
drying the dodecane gels, revealed an extended and inter-
connected fibrous network.
Intrigued by these unique non-centrosymmetric homo-
chiral assemblies, quantum-chemical calculations were car-
ried out to gain further insight into the structural features of
the head-to-tail stacks of 1a/b at the molecular scale. We took
1b as the model enantiomer, but the analysis performed
hereafter can be equally applied to 1a. Prior to optimizing
a columnar structure, we examined the diverse conforma-
tional possibilities for the H-bonding association between
amides. In 1a/b there are four types of amide conformations,
depending on the C-C-N-H dihedral angle formed between
this group and the isoindole ring (Figure 4a). Syn-in and anti-
out conformations are complementary and lead to left-
handed (M) helices, whereas syn-out and anti-in conforma-
tions result in right-handed (P) helices (Figure S5). To
identify which conformation is energetically favored, we
considered three different issues: intermolecular interactions,
the net dipole moment, and chiral helicity.
First, head-to-tail 1b dimers were built and fully opti-
mized at the B97-D/6-31G** level (Figure S6–S8). Right-
handed syn-out and anti-in conformations lead to P-helical
dimers stabilized by both p–p and H-bonding interactions
between the three amide groups (see Figure S8). A signifi-
cantly different situation is noted however for M-helical syn-
in and anti-out conformations, for which theoretical models
reveal that the amide groups cannot adopt an optimal
orientation for H-bond formation and the dimers are, as
a result, considerably destabilized.[15] Further comparison
between syn-out and anti-in dimers (Figure S9) showed that
although both dimers present similar structural parameters,
the anti-in arrangement is computed 7.8 kcalmolꢀ1 more
stable than the syn-out configuration.
To explain this energy difference we then considered
discrete interactions between dipoles. As the stack grows,
a global dipole is generated along the z axis that feeds from
ꢀ
two main contributions: a) the axial B F bond, which leads to
a rigid dipole that increases with stack length,[7h] and b) the z-
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component of the amide carbonyl dipole, which can change
Figure 4d shows the stabilization energy per monomer
unit (DEmon,n) calculated for (1b)n all-anti-in columnar
aggregates with increasing numbers of monomeric units
(n = 1–6, 8, 10, 12). As the columnar stack grows, the H-
bonding network strengthens because of the larger polar-
ization, and the aggregate becomes more stable. It is worth
noting that the asymptotic limit (n = 1) is rapidly approached
upon addition of 10–12 monomer units, as the increase in
stabilization is relatively small with subsequent monomer
additions from this point. The stabilization per monomer unit
predicted for the dodecamer is ꢀ232.9 kJmolꢀ1, very close
to that obtained from the extrapolation to n = 1
(ꢀ238.4 kJmolꢀ1). The enhancement in DEmon,n with increas-
ing numbers of monomer units (n) suggests a large cooper-
ative character for the self-association of SubPc 1, in agree-
ment with the experiments.
ꢀ
orientation in order to add (syn) or subtract (anti) to the B F
dipole.[5] A vacuum environment (in which theoretical
calculations were carried out) or solvents with low dielectric
constants (such as MCH or dodecane) are not suited to
stabilize polar structures. Thus we reasoned that the non-
centrosymmetric stacks of 1a/b must grow with a minimum
global dipole moment, which may be achieved by participa-
tion of anti conformations. Calculations show that columnar
aggregates having the three amides in the anti-in conforma-
ꢀ
tion, that is, their z-component dipoles opposing the B F
dipole, have minimum net dipole moments (Figure S12).
Finally, the theoretical CD spectrum was computed for
both the monomer and trimer species of 1b (Figure S10). For
the monomer, only one positive CD signal is predicted, in
agreement with the experimental results (Figure S1). How-
ever, a more intense, blue-shifted, negative-to-positive CD
signal is predicted for the anti-in right-handed trimer of 1b
(Figure S10a), which also corresponds to the experimental
CD spectra (Figure 2e,f) and confirms the P helicity of the
columnar stacks formed by 1b. The theoretical CD spectrum
computed for the less stable P-helical syn-out conformation of
1b (Figure S10b) significantly differs from the experimental
data, so this conformation was again ruled out.[15] A similar
analysis was made for 1a that corroborates the formation of
all-anti-in M-helical stacks.
We have demonstrated for the first time the formation of
non-centrosymmetric homochiral columnar SubPc assemblies
through a cooperative supramolecular polymerization pro-
cess driven by a combination of noncovalent interactions: H-
bonding, p–p stacking, and dipolar interactions between axial
ꢀ
dipolar B F bonds. Future work will focus on exploiting the
axial dipole moments generated in these assemblies to
produce electric-field-responsive polar materials that may
exhibit ferroelectricity.
Received: November 20, 2014
Published online: && &&, &&&&
In summary, theoretical calculations select the anti-in as
the most stable configuration for the triple array of hydrogen
bonds in our homochiral SubPc assemblies. This amide
conformation maximizes intermolecular interactions, leads
to a minimum net dipole moment in the stacking direction,
and corresponds to the experimental CD spectra of the
assemblies.[19] With this information in hand, 1b all-anti-in
columnar aggregates of increasing size (up to the dodecamer)
were built up and theoretically investigated by using the long-
range corrected wB97X-D functional and the more extended
triple-z cc-pVTZ basis set.[20] As observed in Figures 4b and c,
a right-handed anti-in helical stacking favors the coexistence
of intermolecular H-bonding, p–p stacking, and dipolar
interactions. Neighboring molecules are separated by 4.12 ꢁ
and rotated by 23.08. This torsional angle, which establishes
that 16 molecules are necessary to complete one helical pitch,
is dictated by the triple array of hydrogen bonds. The amide
groups are twisted out the plane of the isoindole units by 36.58
to maximize the intermolecular H-bonding interactions, and
give rise to NH···O intermolecular contacts of 1.93 ꢁ.
Intermolecular C···C contacts between the isoindole phenyl
rings of neighboring molecules are found in the range 3.50–
4.10 ꢁ indicating that, despite the steric effect of the axial
ligand, stabilizing p–p interactions are also present in these
columnar arrangements. The Fdꢀ···Bd+ contacts between
adjacent molecules are calculated to measure 2.78 ꢁ, which
Keywords: homochiral aggregates · porphyrinoids ·
.
self-assembly · subphthalocyanine ·
supramolecular polymerization
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8676; c) C. Rest, M. J. Mayoral, K. Fucke, J. Schellheimer, V.
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705; Angew. Chem. 2014, 126, 716 – 722.
[19] N. Kobayashi, A. Muranaka, J. Mack, Circular Dichroism and
Magnetic Circular Dichroism Spectroscopy for Organic Chem-
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[20] The monomer used to build up the oligomers and the relative
disposition of the adjacent molecules along the stack were
obtained from the wB97X-D/6-31G** fully optimized structure
of the heptamer (see the Supporting information for full
computational details).
[12] a) D. Gonzꢄlez-Rodrꢃguez, T. Torres, D. M. Guldi, J. Rivera,
M. A. Herranz, L. Echegoyen, J. Am. Chem. Soc. 2004, 126,
Angew. Chem. Int. Ed. 2015, 54, 1 – 6
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

.
Angewandte
Communications
Communications
Supramolecular Chemistry
Piling up rigid tetrahedral-shaped sub-
phthalocyanine dye molecules in
J. Guilleme, M. J. Mayoral, J. Calbo,
J. Aragꢁ, P. M. Viruela, E. Ortꢂ,*
T. Torres,*
a convex-to-concave fashion results in the
formation of unconventional homochiral
non-centrosymmetric columnar assem-
blies (see picture). Assembly occurs
through a cooperative supramolecular
polymerization process driven by a com-
bination of noncovalent interactions
(C green, N blue, O red, H white).
D. Gonzꢃlez-Rodrꢂguez*
&&& — &&&
Non-Centrosymmetric Homochiral
Supramolecular Polymers of Tetrahedral
Subphthalocyanine Molecules
6
www.angewandte.org
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2015, 54, 1 – 6
These are not the final page numbers!