Self-assembling triazolophanes: From croissants through donuts to spherical vesicles

Article abstract of DOI:10.1039/c4cc09587a

Macrocyclic compounds M1-M3 with different ring sizes containing amide and triazole units were synthesized. These triazolophanes displayed a variety of self-assembled structures such as hemi-toroids, toroids, and vesicles in a concentration dependent manner. Detailed ultramicroscopic and crystallographic investigations delineated a hierarchical mechanism of self-assembly. This journal is

Full text of DOI:10.1039/c4cc09587a

Volume 51 Number 32 25 April 2015 Pages 6869–7074
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V. Haridas, Jerry P. Jasinski et al.
Self-assembling triazolophanes: from croissants through donuts to
spherical vesicles

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Self-assembling triazolophanes: from croissants
through donuts to spherical vesicles†
V. Haridas,*a Appa Rao Sapalaa and Jerry P. Jasinski*b
Cite this: Chem. Commun., 2015,
51, 6905
Received 1st December 2014,
Accepted 11th February 2015
DOI: 10.1039/c4cc09587a
www.rsc.org/chemcomm
Macrocyclic compounds M1–M3 with different ring sizes containing groups may impart rigidity and self-assemble to form a variety of
amide and triazole units were synthesized. These triazolophanes displayed 3D architectures based on conventional and non-conventional
a variety of self-assembled structures such as hemi-toroids, toroids, and hydrogen bonds.10 The triazole unit is considered as a mimic of
vesicles in a concentration dependent manner. Detailed ultramicroscopic an amide bond, apart from the fact that it adds rigidity to the
and crystallographic investigations delineated a hierarchical mechanism structure and provides a non-classical H-bond donor site.11 We
of self-assembly.
set out to synthesize a variety of triazolophanes making use of a
Cu-catalyzed azide–alkyne cycloaddition (CuAAC) reaction. The
The physicochemical basis of morphogenesis has continued to first series of macrocycles based on an isophthaloyl unit resulted
intrigue scientists for over half a century.1 The formations and in insoluble compounds. The high insolubility of amide-linked
transformations of biomolecular assemblies are intricately linked isophthaloyl-based triazolophanes is believed to be a result of
with the molecular structure. The link between functional bio- intermolecular interactions due to their flat structure, leading to
molecular assemblies and molecular structure is an abiding theme strong p–p stacking and hydrogen bonding interactions. This
of biochemical research.2 The fabrication of specific 2D and 3D prompted us to place a t-butyl substituent on the phenyl ring
assemblies from small molecules by the process of self-assembly (Fig. 1) to reduce the intermolecular stacking interactions in order to
is a powerful strategy for the generation of functional materials improve the solubility. The cyclophanes M1–M3 with amble rigidity
and is also useful for unravelling the mechanism of the chemical and hydrogen bond donor and acceptor moieties are designed for
basis of morphogenesis.3 The design and synthesis of molecules investigating the self-assembly behavior.12 Macrocycles M1–M3 were
with the ability to self-assemble to specific 3D shapes such as synthesized by a CuAAC reaction between dialkynes and the corres-
toroids and vesicles have attracted attention recently.4 Despite the ponding diazides in B40% yield (ESI†).
diverse applications of vesicles, the mechanism of their formation
The relatively rigid molecular frameworks of M1–M3 and
is not well understood.5 Toroidal structures are observed in the the amide linkages give them self-assembling properties. The
self-assembly of proteins,6 block copolymers,4c,d surfactants7 and acidic triazole CHs are additional structural attributes of these
other organic molecules.4f,g Fabrication of toroidal nanostructures molecules that can facilitate self-assembly by non-classical
with uniform size and shape is a challenging endeavor, strategies CHÁ Á ÁX (X = O, N) interactions.
such as flow-induced synthesis8 and solidification of polymer
droplets in a microfluidic device are used for this purpose.9
Here we present a class of triazolophanes with a hierarchical
mechanism of assembly to toroids and vesicles. We envisaged
that macrocyclic compounds incorporating aromatic and amide
a Department of Chemistry, Indian Institute of Technology Delhi (IITD), HauzKhas,
New Delhi-110016, India. E-mail: haridasv@iitd.ac.in, h_haridas@hotmail.com;
Tel: +91 01126591380
b Department of Chemistry, Keene State College, 229 Main Street, Keene,
NH 03435-2001, USA. E-mail: jpjasinski@keene.edu
† Electronic supplementary information (ESI) available: Details on synthesis, and
data of all compounds. SEM, TEM, AFM images and crystallographic data. CCDC
1015636. For ESI and crystallographic data in CIF or other electronic format see
Fig. 1 Structures of triazolophanes M1, M2 and M3.
DOI: 10.1039/c4cc09587a
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The self-assembly of M1–M3 was studied by electron micro- albeit with some flattening of the toroid due to adsorption on the
scopy, atomic force microscopy (AFM) and X-ray crystal structure surface. The vertical height of the toroids is 20–30 nm, the
analysis. In order to probe the self-assembly of macrocycles, horizontal diameter is 100–450 nm and the thickness 45–80 nm
isomeric triazolophanes M1 and M2 with slight differences in (Fig. S2, ESI†). At 1 mM concentration, only vesicles were
the linker regions were synthesized. Macrocycle M3 with leucine observed (Fig. 2c, f and Fig. S1–S4, ESI†), and vesicular evolu-
in the framework was synthesized in order to see the effects of tion is schematically presented in Fig. 2.
chirality and macrocyclic size on self-assembly.
Concentration dependent SEM and HR-TEM studies clearly
Solutions of these compounds (M1–M3) with different concen- show the same trend as observed from AFM, and thus indicated
trations in 1 : 1 chloroform : methanol were placed on freshly cleaned that triazolophanes M1–M3 form a hierarchical self-assembly
mica and allowed to evaporate in air and analyzed by AFM. Mica pattern (Fig. 2, and Fig. S1–S7, ESI†). SEM analysis of M1–M2
provides a macroscale atomically flat surface by simply removing showed that the majority of the toroids and vesicles are in
the top layers. This removes the influence of surface roughness the range of 200–250 nm diameter (Fig. S3 and S4, ESI†). SEM
on the self-assembly process. Careful analysis using AFM of a and TEM data showed good correlation for the dimensions of the
range of concentrations (0.1 mM to 1 mM) of triazolophanes toroids and vesicles (Fig. S1, S3 and S4, ESI†). AFM also supported
revealed a variety of structures, such as hemi-toroids, toroids and the same observation, but the dimension ranges are bit higher
vesicles. At 0.1 mM, the macrocycles show mostly hemi-toroidal compared to SEM and TEM.13
structures as is evident from AFM and transmission electron
The hemi-toroid is an intermediate structure formed en
microscopy (TEM) images (Fig. 2a and Fig. S1, ESI†). At about route to toroids, and toroids are in turn an intermediate self-
0.25 mM concentration, mostly toroidal structures were observed assembled structure formed en route to vesicles. Increasing
(Fig. 2b, d and Fig. S1–S4, ESI†).
concentration resulted in gradual changes from hemi-toroids
Scanning electron microscopy (SEM) analysis of M2 showed to vesicles (Fig. 2). Similarly, dilution also resulted in vesicles
that the diameters of the toroids are in the range of 100–350 nm, changing to hemi-toroids, supporting that the transformations are
matching the diameters of the toroids of M1 (Fig. S3 and S4, ESI†). bidirectional in nature. The definite concentration dependence
Increasing the concentration resulted in the shrinkage of the on the topology indicates that the desired shape could be attained
internal cavity of the toroids (Fig. 2d and e). At 0.5 mM, toroids by changing the concentration.
and vesicles were found to co-exist, supporting the concentration
The macrocycle design can incorporate chiral units such as
effect on the self-assembly process. Analysis using AFM, TEM and amino acids in its framework. This has been demonstrated by
SEM revealed the toroidal assembly at 0.25 mM (Fig. 2 and synthesizing macrocycle M3 containing leucine in its frame-
Fig. S1–S4, ESI†). The thickness and diameter of the toroids are in work. Macrocycle M3 shows self-assembly behavior similar to
the range of 45–80 nm and 100–450 nm, respectively (Fig. S2, ESI†). that of M1–M2 (Fig. 2, and Fig. S5–S7, ESI†). This opens a wider
AFM cross-sectional analysis further supports the toroidal shape, opportunity for the design and synthesis of self-assembling
Fig. 2 Ultramicroscopic analysis of the self-assembly of triazolophanes. Only selected images are presented. (I) (a) AFM image of M2 at 0.1 mM showing
hemi-toroids. The inset shows a single magnified hemi-toroid. (b) AFM image of M2 at 0.25 mM showing toroids. (c) AFM image of M2 at 1 mM showing
vesicles. (d) SEM image of M2 at 0.25 mM showing mostly toroids. (e) FE-SEM image of M2 at 0.5 mM showing toroids with decreasing internal cavities.
(f) FE-SEM and AFM image of M2 at 1 mM showing vesicles. (II) Schematic representation showing the evolution of vesicles. The increase in concentration
results in the transformation of hemi-toroids (left) to vesicles (right).
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from M1 and M3 were sliced off, revealing hollow interiors
(Fig. S9, ESI†).16 Interestingly, in the TEM images, the thickness
of the membrane of the vesicles could not be seen, and hence
appeared as dark spheres (Fig. S1c, f and S5c–e, ESI†). This has
also been observed by other researchers and is attributed to the
soft and rubbery-like nature of the vesicles.17
To form closed 3D surfaces such as toroids and vesicles, a
significant curvature in the self-assembly is required, suggesting a
possible non-planar assembly adopted by triazolophanes. In order to
obtain insight into their self-assembly, we attempted crystallization
of the triazolophanes. Macrocycle M2 was crystallized from a
mixture of methanol, chloroform and acetonitrile. The X-ray crystal
structure of macrocycle M2 revealed an asymmetric unit comprised
of two molecules of M2 (Fig. 4a, Fig. S10 and Table S1, ESI†) which
differ in the arrangement of the triazole units. In one conformer,
both triazole CHs are pointing to the same side (syn), while in the
other, the CHs of the triazoles are pointing in opposite directions
(anti). The amide carbonyls adopt an anti-conformation in both
forms. The phenyl ring carrying the t-butyl group is almost
perpendicular to the plane of the 1,4-phenyl unit. The macrocycle
M2 has both a concave and convex surface. In the syn conformer,
the nitrogens are on the concave face. The packing diagram
reveals a very unique self-assembly pattern, in which different
conformers of M2 (syn and anti) arrange alternately to form a
Fig. 3 FE-SEM images of M2 before (a and c) and after FIB milling (b and d).
The red circles indicate the portions where FIB milling was performed. The
observed surface softening of the toroids after milling is due to the high FIB
energy employed in the experiment.14
amino acid-based triazolophanes. Interestingly, the CD spec- cyclic tetrad resembling a toroid (Fig. 4, Fig. S11 and S12, ESI†).18
trum of M3 showed no characteristic CD signal indicating that Symmetry expansion reveals a circular tetrameric framework
the macrocycle has no specific folded conformation in solution. in the unit cell (maximum width 27 Å), in which each of the syn
The formation of vesicles for all macrocycles (M1–M3) follows the and anti molecules of M2 are arranged in an alternate ‘‘up(syn)–
same path as revealed from their detailed ultramicroscopic down(anti)–down(syn)–up(anti)’’ fashion (Fig. S12, ESI†). The
studies (Fig. 2, Fig. S1–S7, ESI†).
alternate ‘‘syn–anti’’ motif coupled with an up–down packing
Dynamic light scattering (DLS) of M2 and M3 at different arrangement of the macrocycles results in the formation of a
concentrations in 1 : 1 MeOH/CHCl3, revealed the presence of circular assembly with a hole positioned at the centre.
particles with sizes in the range of B198–478 nm, matching
well with the microscopic studies (Fig. S8, ESI†), demonstrating together by four CHÁÁÁN and two CHÁÁÁO hydrogen bonds (Table S2,
the presence of aggregate structures in solution. ESI†). The methylene attached to the triazole of the anti conformer
The tetrameric supramolecular assembly in the unit cell is held
SEM equipped with focused ion beam (FIB) was used to obtain bonds to the triazole nitrogen of the syn conformer (2.69 Å), and one
finer details of the toroids and vesicles (Fig. 3 and Fig. S9, ESI†).14 of the benzylic hydrogens of the syn conformer makes a non-
The well-formed toroid from triazolophane M2 was subjected to covalent contact with the amide carbonyl of the anti-conformer
FIB milling using a gallium ion beam. The selected portion of the (2.56 Å). Each molecule in the tetrad forms three hydrogen bonds
toroid was excised out (Fig. 3b, d) which revealed that the internal with its two neighbors (C25B–H25CÁÁÁN6A; C15A–H15AÁÁÁO2B and
cavity of the toroid was empty.15 Similarly, a portion of the vesicles C25B–H25DÁÁÁN3A). Both triazole moieties of the syn conformer
Fig. 4 X-ray crystal structure of M2: (a) capped stick representation of syn and anti conformers, (b) capped stick representation showing tetrad assembly
in the unit cell through hydrogen bonding, viewed along the b-axis, (c) the packing diagram of M2, viewed along the b-axis. A small portion of the
assembly is shown.
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take part in hydrogen bonding. The anti conformer provides HRMS and single crystal XRD facilities at IITD. We thank
both the methylene protons attached to the triazole and the Prof. Narayanan Kurur and Prof. Nalin Pant, Department of
amide carbonyl. A noteworthy point is that the molecules in the Chemistry, IITD for discussions. We thank Prof. S. Aravindan
tetrad are held only by weak intermolecular non-classical and Mr Amit Gupta, Department of Mechanical engineering,
hydrogen bonds. The tetrad assembly generates a hole at the IITD for help in FIB experiments. JPJ acknowledges the NSF-MRI
centre that has a diameter of approximately B10 Å.
program (grant No. CHE-1039027) for funds to purchase the
Careful analysis of the packing revealed that the syn con- X-ray diffractometer.
formers are assembled by two N–HÁ Á ÁO (N1A–H1AÁ Á ÁO2A; N8A–
H8AÁ Á ÁO1A) and two C–HÁ Á ÁO interactions (C1A–H1AAÁ Á ÁO1A;
C23A–H23AÁ Á ÁO1A) to form stacks (Fig. S11, ESI†). In a similar
way, the anti conformers form stacks, wherein two intermole-
cular N–HÁ Á ÁO (N1B–H1BÁ Á ÁO2B; N8B–H8BÁ Á ÁO1B) H-bonds are
present. The separate syn and anti stacks indicate a clear case of
self-sorting. The syn conformers in the tetrad make interactions
with the upper layer of anti conformers by (C15A–H15BÁ Á ÁN6B;
C15A–H15BÁ Á ÁN7B) hydrogen bonds (Fig. S13, ESI†). Taken
together the results from the microscopic analysis and X-ray
structure analysis imply an array of M2, with self-sorted stacks
of syn and anti molecules and their inter-stack interactions
leading to a continuous surface (Fig. 4c), which may eventually
fold into hemi-toroids, toroids and vesicles (Fig. S14, ESI†).
Bensimon and coworkers reported the self-assembly of phos-
pholipids to various topological genuses.19 Their experimental and
theoretical studies indicated that the shape of the vesicle is a result
of the minimization of its elastic curvature under various physical
constraints. Our experimental results suggest that the intrinsic
tendency of triazolophanes to form toroids may be due to their
bent molecular architecture. The bent molecular shape causes
spontaneous curvature of the assembly leading to the minimum
energy toroidal architecture. An increase in concentration leads to
the assembly of more molecules in the poloidal direction leading to
vesicles (Fig. S14, ESI†).
Notes and references
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In conclusion, we have designed, synthesized and studied the
self-assembly behavior of a series of triazolophanes by ultramicro-
scopy and X-ray crystallography. The studies presented here serve
to support a new hypothesis regarding the mechanism of vesicle
formation. Furthermore, the 3D assemblies have many applica-
tions in chemistry and biology. The physicochemical basis of
vesicle morphogenesis from simple molecular building blocks
has profound significance from an evolutionary point of view.
The concentration dependent self-assembly from hemi-toroid to
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6908 | Chem. Commun., 2015, 51, 6905--6908
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