Self-assembly of supramolecular fractals from generation 1 to 5

Article abstract of DOI:10.1021/jacs.8b05530

In the seeking of molecular expression of fractal geometry, chemists have endeavored in the construction of molecules and supramolecules during the past few years, while only a few examples were reported, especially for the discrete architectures. We herein designed and constructed five generations of supramolecular fractals (G1-G5) based on the coordination-driven self-assembly of terpyridine ligands. All the ligands were synthesized from triphenylamine motif, which played a central role in geometry control. Different approaches based on direct Sonogashira coupling and/or (tpy-Ru(II)- tpy) connectivity were employed to prepare complex Ru(II)-organic building blocks. Fractals G1-G5 were obtained in high yields by precise coordination of organic or Ru(II)-organic building blocks with Zn(II) ions. Characterization of those architectures were accomplished by 1D and 2D NMR spectroscopy, electrospray ionization mass spectrometry (ESI-MS), traveling-wave ion mobility mass spectrometry (TWIM-MS), and transmission electron microscopy (TEM). Furthermore, the two largest fractals also hierarchically self-assemble into ordered supramolecular nanostructures either at solid/liquid interface or in solution on the basis of their well-defined scaffolds.

Full text of DOI:10.1021/jacs.8b05530

Article
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Cite This: J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
Self-Assembly of Supramolecular Fractals from Generation 1 to 5
Lei Wang,†,¶ Ran Liu,‡,§,¶ Jiali Gu,∥ Bo Song,† Heng Wang,† Xin Jiang,⊥ Keren Zhang,§ Xin Han,#
Xin-Qi Hao,# Shi Bai,∇ Ming Wang,*,⊥ Xiaohong Li,*,∥ Bingqian Xu,*,§ and Xiaopeng Li*,†
†Department of Chemistry, University of South Florida, Tampa, Florida 33620, United States
‡Shandong Province Key Laboratory of Medical Physics and Image Processing Technology, School of Physics and Electronics,
Shandong Normal University, Jinan 250358, China
§Single Molecule Study Laboratory, College of Engineering and Nanoscale Science and Engineering Center, University of Georgia,
Athens, Georgia 30602, United States
∥College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, China
⊥State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun, Jilin 130012,
China
#College of Chemistry and Molecular Engineering, Zhengzhou University, Zhengzhou, Henan 450001, China
∇Department of Chemistry and Biochemistry, University of Delaware, Newark, Delaware 19716, United States
S
* Supporting Information
ABSTRACT: In the seeking of molecular expression of fractal
geometry, chemists have endeavored in the construction of molecules
and supramolecules during the past few years, while only a few
examples were reported, especially for the discrete architectures. We
herein designed and constructed five generations of supramolecular
fractals (G1−G5) based on the coordination-driven self-assembly of
terpyridine ligands. All the ligands were synthesized from triphenyl-
amine motif, which played a central role in geometry control. Different
approaches based on direct Sonogashira coupling and/or ⟨tpy-Ru(II)-
tpy⟩ connectivity were employed to prepare complex Ru(II)-organic
building blocks. Fractals G1−G5 were obtained in high yields by
precise coordination of organic or Ru(II)-organic building blocks with
Zn(II) ions. Characterization of those architectures were accom-
plished by 1D and 2D NMR spectroscopy, electrospray ionization
mass spectrometry (ESI-MS), traveling-wave ion mobility mass spectrometry (TWIM-MS), and transmission electron
microscopy (TEM). Furthermore, the two largest fractals also hierarchically self-assemble into ordered supramolecular
nanostructures either at solid/liquid interface or in solution on the basis of their well-defined scaffolds.
well-known fractal geometry, viz., Sierpinski triangle,11 through
́
INTRODUCTION
■
theoretical prediction,12 halogen bonding,13 hydrogen bond-
The concept of “fractal geometry” is coined by Mandelbrot to
describe the natural geometries, which cannot be described
ing14 and coordination15−17 in ultrahigh vacuum. However,
different generations of fractals were assembled as a mixture on
the surface. With the goal of exerting more elegant
precisely using ideal constructions of Euclidean geometry, such
as lines, triangles, squares, circles, spheres, etc.1 As an
manipulation over structural features of fractals, coordina-
important characteristic, fractal geometry displays self-
tion-driven supramolecular chemistry has stepped into the
similarity at different levels of magnification. Inspired by
́
construction of discrete metallo-supramolecular Sierpinski
nature and mathematics, the fractal geometry was introduced
into several different scientific fields from physics to geo-
morphology, from chemistry to materials science, and from
economics to biology.2 In particular, the fractal geometry has
been behind an enormous change in the way chemists
perceive, and subsequently rebuild, the chemistry field in
which we explore.3,4 For instance, dendrimers have been
extensively demonstrated to be iconic molecular fractals in
conventional synthetic chemistry.5−10
triangles18,19 and gaskets20 through the self-assembly in
solution with high yields. Although metallo-supramolecular
chemistry has witnessed a spectacular explosion in constructing
a great variety of polygons and polyhedrons in the field of
Euclidean geometry,21−37 it still remains a formidable
challenge to assemble nontrivial architectures, particularly
high generation of fractals. Up to date, most of these metallo-
In the seeking of alternatives to avoid multistep synthesis,
surface self-assembly recently was employed to construct a
Received: May 26, 2018
© XXXX American Chemical Society
A
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supramolecular fractals are limited to low generation of (≤3)
RESULTS AND DISCUSSION
■
́
Sierpinski geometry (Figure 1a) due to the challenge of design,
In an effort to elaborate the concept of fractal by coordination-
driven self-assembly, a series of ligands was designed with the
goal of assembling a set of discrete structures with Zn(II) ions
rather than infinite framework. Those structures possess the
same repeating shape unit, namely G1, but display at different
scales, leading to different molecular geometry and symmetry.
G2 contains three repeating units with a triangle geometry,
while G3 is comprised of five repeating units with a dumbbell-
shaped geometry. The higher generations G4 and G5 consist
of 6 and 12 repeating units, respectively. Moreover, all the
geometry of G1 to G4 can be found as a proportion in G5,
which exhibits the extended supramolecular fractal with high
complexity. According to molecular modeling, the size is
enlarged more than four times from the smallest supramolecule
G1 (∼2.5 nm) to the largest one G5 (∼11 nm) (Figure 2).
In our synthetic design, triphenylamine motifs played a
central role in controlling the geometry of assemblies due to
their specific internal bond angles with appropriate molecular
rigidity.52 Tpy units, which were widely used in coordination-
driven self-assembly,43−52 were introduced to provide coordi-
nation sites for metal ions. Considering the accessibility, three
approaches were utilized to construct building blocks.
Approach 1 was using Sonogashira coupling reaction to
connect the triphenylamine motifs and tpy directly, for
instance, organic ligands LA and LD (Scheme 1). Approach
2 was bridging two organic tpy ligands using Ru(II) chemistry
to form ROBB, such as LE with increasing complexity and
avoiding multiple self-assembly products.49a,50 Approach 3 was
a combination of the first two methods through performing
Sonogashira coupling on the Ru(II)-tpy complexes to precisely
control the structures of ligands and the self-assembly
supramolecules as well as increasing the diversity and
complexity, such as LB and LC. Note that the ⟨tpy−
Ru(II)−tpy⟩ connectivity was able to sustain the basic
condition and high temperature in the cross-coupling reaction
due to its high stability.49a
́
Figure 1. (a) Sierpinski triangle fractals and (b) supramolecular
fractals designed in this study.
synthesis and separation. An important question was raised:
can one assemble other high generations of supramolecular
fractals rather than Sierpinski fractals?
́
In this study, we used well-documented 2,2′:6′,2″-
terpyridine (tpy)-based coordination chemistry38−48 to con-
struct discrete supramolecular fractals ranging from generation
1 (G1) to 5 (G5) in high yields. Except for G1 and G4, all
other fractals were assembled using two combinations of
connectivity, i.e., ⟨tpy-Ru(II)-tpy⟩ and ⟨tpy-Zn(II)-tpy⟩.
Briefly, a series of Ru(II)-organic building blocks (ROBBs)
was synthesized on the basis of the strong binding ability
between tpy and Ru(II).49−51 In the following self-assembly,
such ROBBs can assemble with weak coordination metal, i.e.,
Zn(II) to construct G2, G3 and G5 fractals with increasing size
and complexity. More importantly, these architectures exhibit
fractal characteristics through repeating the basic shape unit
(G1) to form branched frameworks. It is worth noting that
supramolecular fractals we reported here display only statistical
self-similarity, which are not identical at all levels of
magnification (Figure 1b). By contrast, computer generated
(i.e., mathematical) fractals can repeat their self-similarity at all
scales.
Synthesis of Ligands and Fractals. We initiated the
ligand synthesis from organic ligands L and LA, which were
obtained easily by approach 1 through connecting different
triphenylamine motifs and tpy units using Sonogashira
Figure 2. Energy-minimized structures of fractal complexes G1[Zn2LA2], G2[Zn3LB], G3[Zn6LC2], G4[Zn12LD6], G5[Zn12LE6] from molecular
modeling (the alkyl chains are omitted for clarity).
B
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Scheme 1. Different Approaches to Prepare the Ligands and Fractal Structures
Figure 4. (a) ESI-MS and (b) TWIM-MS plot (m/z vs drift time) of
fractal G3.
Figure 3. (a) ESI-MS and (b) TWIM-MS plot (m/z vs drift time) of
fractal G2.
coupling (Schemes S2 and S5). L was designed to provide
scaffolds of the higher generations of fractals, while LA was
able to end-cap the outer binding sites to prevent the
formation of infinite structure. LA itself could self-assemble
into complex [Zn2LA2] in a stoichiometric ratio (1:1) with
Zn(NO3)2·6H2O, resulting in the formation of the smallest
C
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a
Scheme 2. Synthesis of ROBB LE
Figure 5. (a) ESI-MS spectra and (b) TWIM-MS plot (m/z vs drift
time) of fractal G4.
a
(i) Pd(PPh3)2Cl2, CuI, THF/Et3N, 60 °C. (ii) Pd(PPh3)4, CuI,
TBAF, THF/Et3N, 70 °C. (iii) Ru(DMSO)2Cl2, 1,2-dichloroethane,
80 °C. (iv) 1,2-dichloroethane/EtOH, 80 °C.
In the assembly of G2, we initially designed LB′ through
end-capping L with three equivalents of LA (Figure S1).
However, no desired product was achieved after vigorous
efforts due to the inert coordination of Ru(II). Therefore, we
redesigned and synthesized LB using approach 3 to overcome
the inertness of coordination with the combination of
Sonogashira coupling reaction and Ru(II) chemistry. Ironi-
cally, LB with three Ru(II) ions and six free tpy groups is the
most challenging one among all the building blocks for the
fractals, although G2 is a relatively small structure among those
five fractals.
1
Figure 6. H NMR spectra (500 MHz, 300 K) of LD in CDCl3 and
G4 in CD3CN.
Two steps of Sonogashira coupling were performed on the
Ru(II)-tpy complex S-18 to control the structure of the target
ligand and assembled supramolecule (Scheme S3). Complex
[Zn3LB] or fractal G2 with three repeating units was then
obtained by assembling LB with Zn(II) in a 1:3 ratio, followed
by the counterions exchange from NO3 to PF6 for mass
spectrometry characterization. Note that G2 was not accessible
through direct mixing L and LA in 1:3 ratio with
corresponding Zn(II) because of self-sorting of individual
building block.
structure G1, which represented the basic repeating shape unit
in the complex fractal set. Electrospray ionization−mass
spectrometry (ESI-MS) and traveling wave ion mobility mass
spectrometry (TWIM-MS)53−56 provided strong evidence for
the formation of G1 with a molecular weight of 2722 Da
−
−
1
(Figure S8). H NMR, 2D COSY (correlation spectroscopy),
and 2D NOESY (nuclear Overhauser effect spectroscopy)
spectra supported the formation of the symmetrical structure
(Figures S76−S81).
D
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and TWIM-MS spectra provided strong evidence for the
formation of G4 with molecular weight of 14865 Da. Only one
prominent set of signals for multicharged signals (from 10+ to
19+) was observed in ESI-MS spectrum (Figure 5a).
Moreover, a narrow distribution of drift time for different
charge states in TWIM-MS spectrum (Figure 5b) indicated
there were no isomers or structural conformers generated.
Furthermore, the isotope pattern of the multicharged ions
agreed well to the simulated one, suggesting the proposed
1
structure (Figure S12). H NMR, 2D COSY and 2D NOESY
spectra (Figures S96−S99) supported the formation of
symmetrical structures. For instance, in the 1H NMR spectrum
of G4, two types of tpy units were observed and the resonance
signals had distinguishable shift compared to LD (Figure 6).
The synthesis of LE was achieved by approach 2 through
end-capping hexatopic tpy ligand L with one LA using Ru(II)
chemistry in two steps (Scheme 2). Such end-capping
approach left four free tpy moieties for further coordination.
Complex [Zn12LE6] (i.e., G5), the largest supramolecule in the
fractal set was obtained in high yield (96%) by treatment LE
with Zn(NO3)2·6H2O in a stoichiometric ratio (1:2) in
CHCl3/MeOH (v/v 1:3) at 50 °C for 10 h, followed by
counterion exchange. ESI-MS and TWIM-MS spectra
identified the formation of desired fractal G5 with molecular
weight of 28 384 Da. Also, one prominent set of signals was
observed for multicharged signals (from 14+ to 28+) in ESI-
MS spectrum (Figure 7a) and TWIM-MS spectrum (Figure
7b) as other fractals did. 1H NMR spectra of LE and G5 were
consistent with the assigned structure. Four sets of tpy signals
were observed in both ligand and complex, indicating a highly
symmetrical structure was formed after complexation (Figure
8). All the protons of 3′, 5′ positions on the metal-free tpy
units of LE, i.e., A3′,5′ and B3′,5′ were shifted downfield (∼0.4
ppm) due to electron deficiency upon coordination with metal
ions, while those protons of 6, 6″ positions were significantly
shifted upfield (∼0.7 ppm) due to electron shielding effects
according to the previous reports.45a,50 In addition, the
characteristic protons on alkoxy chain (−OCH2−) with a
1:2:1 integration ratio also suggested the formation of discrete
structure rather than polymers. All the assignments were
confirmed by detailed 2D COSY and 2D NOESY spectra,
respectively (Figures S102−S105).
Diffusion-ordered NMR spectroscopy (DOSY) experiments
were conducted to provide dimensional information on those
supramolecular assemblies.48,57 All the five spectra showed
narrow bands of signal that suggested the self-assemblies
formed discrete supramolecular architectures (Figure 9).
Moreover, the diffusion coefficient (D, m2/s) decreased from
4.99 × 10−10 to 1.46 × 10−10 as the size gradually increased
from G1 to G5.57,58 And the experimental hydrodynamic
radius (rH) of the fractals from the D value agreed well to the
molecular modeling. For instance, the obtained radii for G4
and G5 were 4.2 and 5.7 nm, respectively, which were
comparable to the sizes obtained from the modeling. And the
results were summarized in Table S1.
Figure 7. (a) ESI-MS spectra and (b) TWIM-MS plot (m/z vs drift
time) of fractal G5.
In the design of G3, the preparation of LC′ was unsuccessful
using approach 2 because of the inert coordination of Ru(II)
(Figure S1). As such LC was synthesized using the same
strategy as LB but underwent only one step of Sonogashira
coupling on ROBB as shown in Scheme S4. Briefly, LC was
prepared through coupling of organic precursor S-13 and
Ru(II)-tpy complex S-21, which was obtained via double end-
capping of precursor S-8 with LA and Ru(II). Such
combination strategy allowed us to prepare LC in a moderate
isolation yield after a column chromatography separation.
Complex [Zn6LC2] or fractal G3 was obtained in a yield of
94% by mixing LC and Zn(NO3)2·6H2O in a 1:3 ratio,
followed by the same treatment as the other fractals. G3 is
comprised of 5 repeating units with 2 organic precursor
compound L and 4 LA in each structure; similarly, the direct
self-assembly of L and LA in 1:2 ratio with corresponding
Zn(II) was unable to form G3-like fractal because of the self-
sorting of individual building block.
ESI-MS and TWIM-MS spectra provided structural
information on G2 and G3 with one set of multiple charged
signals observed from 1D ESI-MS and 2D TWIM-MS spectra.
Corresponding isotope pattern of each charged state agreed
1
well with theoretical one (Figures 3 and 4). H NMR spectra,
2D COSY and 2D NOESY spectra of ligand LB and fractal G2
1
showed four types of tpy (Figure S106). H NMR spectra
Transmission electron microscopy (TEM) was also utilized
to provide further structural information on the shape and size
of the complexes, especially the large fractals G4 and G5. The
measured dimension of individual supramolecule deposited on
the surface of copper grid showed the diameter around 7 and
12 nm (the average distance between the farthest corners) for
G4 and G5, respectively from TEM images (Figures 10a−d),
which were also consistent with the sizes obtained from
showed four types of tpy for Ligand LC but five types of tpy for
fractal G3 (Figure S107). The assigned structures were
supported by detailed 2D COSY and 2D NOESY spectra
(Figures S82−S93).
Organic ligand LD was obtained in a straightforward manner
by approach 1 through direct Sonogashira coupling. The
organic ligand could form the complex [Zn12LD6], or G4 in
the fractal set, with Zn(NO3)2·6H2O in a 1:2 ratio. ESI-MS
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1
Figure 8. H NMR spectra (500 MHz, 300 K) of LE in DMSO-d6 and fractal G5 in CD3CN.
Physical Properties. Given that triphenylamine motifs and
tpy-metal complexes were extensively studied for optoelec-
tronic application,52,59 a series of experiments was performed
to study photophysical and electrochemical properties of the
complexes. Absorption spectra for complexes are shown in
Figure S112. All the complexes exhibited the typical π → π*
bands localized on the tpy-Ph subunits of ligands at around
285 and 325 nm because of intraligand charge transfer
(1ILCT). The band formed at around 485 nm in G2, G3 and
G5 is assigned to the Ru(II)-tpy moiety because of the metal-
to-ligand charge transfer (MLCT) transitions.60 All the five
fractals also show a characteristic absorption band centered at
390 nm, which corresponds to the intramolecular charge
transfer transition from the triphenylamine motif to the tpy-
metal component.61,62
Figure 9. 2D DOSY NMR spectra (600 MHz, 300 K) of G1−G5 in
CD3CN.
molecular dynamic modeling as well as DOSY NMR
experiments.
Oxidation and reduction properties of all the five fractals
were studied using cyclic voltammetry (CV), and the results
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from energy-minimized structure by molecular modeling
(Figure 11b). The formation of those ordered nanostructures
were probably ascribed to the multiple intermolecular
interactions such as π−π stacking, CH−π hydrophobic/
hydrophilic interactions.66 The proposed stacking models
were displayed in Figures 11c and 11d. We speculated that
the diameter and the orientation of the repeating units might
result in the different hierarchical self-assembly behaviors of
G4 and G5 in solution.
The hierarchical self-assembly behaviors of complexes G4
and G5 at liquid/solid interface were further investigated using
scanning tunneling microscopy (STM). It is well-known that
molecules could self-assemble into ordered 2D materials at
liquid/solid interface through noncovalent interactions such as
intermolecular, molecule−substrate, molecule−solvent, sol-
vent−substrate interactions.67,68 However, giant metallo-supra-
molecules were less explored because of the challenges in
construction of large supramolecules with well-defined
geometry. Furthermore, many previous studies reported that
alkylated compounds tended to form ordered supramolecular
assemblies on the highly oriented pyrolytic graphite (HOPG)
surface due to their high affinities.69,70 We herein reasoned that
the giant complexes G4 and G5 with many alkyl chains on the
rigid architectures could directly self-assemble into ordered 2D
nanopattern on HOPG surface. Ambient STM imaging
showed the formation of the anticipated honeycomb 2D
supramolecular networks for both G4 (Figures 12a, 12b, and
12d) and G5 (Figures 12f, 12g, and 12i) after dropcasting of
the fresh prepared solution onto HOPG surface. Obviously,
the networks formed by G4 had higher resolution, which might
result from its higher rigidity compared to G5.
Figure 10. TEM images of (a,c) G4 and (b,d) G5.
More interestingly, with short incubation time, 1D supra-
molecular metal−organic nanoribbons (SMON)58,71 with
single-molecule width were also observed on HOPG surfaces
(Figures 12e, 12h, and 12k). The SMON might grow along
specific lattice direction of HOPG according to the reported
gold(I) cyanide nanowires formed on graphene;72 however,
the current resolution by ambient STM was unable to provide
more conclusive evidence. The achievements of those 2D
networks and 1D orientated nanostructures on surfaces from
giant supramolecular fractals might advance the study of
surface chemistry for molecular-scale applications.73,74
Figure 11. TEM images of nanostructures formed by (a) G4 and (b)
G5 in solution and (c,d) proposed packing models (c for G4, d for
G5).
CONCLUSIONS
were summarized in Figure S113. Two irreversible couples
observed between −1.8 and −0.5 V are ascribed to the tpy-
ligand-centered redox process, while overlapped by the broad
peak from reduction process of triphenylamine motifs.52 The
Ru(II)/Ru(III) oxidation process of tpy-based complex
occurred at 0.8−1.0 V according to the literature,63 and the
oxidation of the organic amine unit also occurred at around 1.0
and 1.2 V with broad peak corresponding to successive one
electron removal.64,65 That leads to an overlap in G2, G3 and
G5 in the positive potential region.
Hierarchical Self-Assembly Behaviors. We investigated
the hierarchical self-assembly behaviors of fractals G4 and G5
under different conditions. TEM imaging showed that G4
could further self-assemble into columnar nanostructures,
which were bound together to form twisted fiber-like
aggregates (Figure 11a) by diffusing THF into the solution
of the complex (5 mg/mL in DMF) slowly. G5 also
hierarchically assembled into a well-organized tubular structure
under the same condition and the diameter (∼12 nm) of
nanostructures was close to that of the individual fractal G5
■
In summary, five supramolecular architectures using triphenyl-
amine and tpy moieties were successfully designed to
demonstrate the concept of “supramolecular fractal”. In
preparation of the ligands, different approaches were applied
to precisely control the geometry as well as increasing the
complexity. Particularly, the extensive utilization of Sonoga-
shira coupling reaction on the Ru(II)-tpy complexes was able
to significantly increase the complexity of building blocks, and
thus substantially increased the diversity and complexity of
metallo-supramolecules. With such strategy, we were able to
construct nontrivial architectures, particularly high generation
of fractals, which were not achievable using conventional direct
self-assembly approach. Their photophysical and electro-
chemical properties were investigated for further application
study. Besides the self-assembly of the discrete structures, the
hierarchical self-assembly of the largest fractals, G4 and G5
were also studied under different conditions. They were found
to form tubular aggregates in solution while 2D networks and
1D orientated SMON on HOPG surface. Those unique
G
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Figure 12. STM images of 2D networks formed by (a, b, d) G4 and (f, g, j) G5; 1D nanoribbons formed by (e) G4 and (h, k) G5 on HOPG (note
that d, e, j are 3D STM images); energy-minimized structures for (c) G4 and (i) G5.
hierarchical self-assembly behaviors of the giant supra-
molecular architectures might broaden the avenue of novel
materials such as semiconducting nanowires and 2D supra-
molecular networks for specific applications at molecular level.
Ming Wang: 0000-0002-5332-0804
Xiaohong Li: 0000-0003-3190-7214
Xiaopeng Li: 0000-0001-9655-9551
Author Contributions
¶L.W. and R.L. contributed equally.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/jacs.8b05530.
■
Notes
S
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Synthetic details, ligands and complexes characterization
The authors gratefully acknowledge the support from NSF
(CHE-1506722 to X.L.; ECCS-1609788 to B.X.), NIH
(1R01GM128037 to X.L.), and National Natural Science
Foundation of China (No. 21528201 and 21305098), and the
Program for Science & Technology Innovation Talents in
Universities of Henan Province (17HASTIT004 to X.-Q.H.),
Natural Science Foundation of Jilin Province (20180101297JC
to M. W.).
1
including H NMR, 13C NMR, 2D COSY, 2D NOESY,
ESI-MS, TWIM-MS, UV−vis, CV, TEM and STM
images (PDF)
AUTHOR INFORMATION
■
Corresponding Authors
*mingwang358@jlu.edu.cn
*lxh83@suda.edu.cn
*bxu@engr.uga.edu
*xiaopengli1@usf.edu
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