Self-assembly of three-dimensional supramolecular polymers through cooperative tetrathiafulvalene radical cation dimerization

Article abstract of DOI:10.1002/chem.201302951

The self-assembly of a new type of three-dimensional (3D) supramolecular polymers from tetrahedral monomers in both organic and aqueous media is described. We have designed and synthesized two tetraphenylmethane derivatives T1 and T2, both of which bear four tetrathiafulvalene (TTF) units. When the TTF units were oxidized to the radical cation TTFC+, their pre-organized tetrahedral arrangement remarkably enhanced their intermolecular dimerization, leading to the formation of new 3D spherical supramolecular polymers. The structure of the supramolecular polymers has been inferred on the basis of UV/Vis absorption, electron paramagnetic resonance, cyclic voltammetry, and dynamic light scattering (DLS) analysis, as well as by comparing these properties with those of the self-assembled structures of mono-, di-, and tritopic control compounds. DLS experiments revealed that the spherical supramolecular polymers had hydrodynamic diameters of 68 nm for T1 (75 mm) in acetonitrile and 105 nm for T2 (75 mm) in water/ acetonitrile (1:1). The 3D spherical structures of the supramolecular polymers formed in different solvents were also supported by SEM and AFM experiments.

Full text of DOI:10.1002/chem.201302951

DOI: 10.1002/chem.201302951
Full Paper
&
Supramolecular Polymers
Self-Assembly of Three-Dimensional Supramolecular Polymers
through Cooperative Tetrathiafulvalene Radical Cation
Dimerization
Jia Tian,[a] Yu-Di Ding,[a] Tian-You Zhou,[c] Kang-Da Zhang,[c] Xin Zhao,[c] Hui Wang,*[a]
Dan-Wei Zhang,[a] Yi Liu,*[b] and Zhan-Ting Li*[a]
Abstract: The self-assembly of a new type of three-dimen-
sional (3D) supramolecular polymers from tetrahedral mono-
mers in both organic and aqueous media is described. We
have designed and synthesized two tetraphenylmethane de-
rivatives T1 and T2, both of which bear four tetrathiafulva-
lene (TTF) units. When the TTF units were oxidized to the
tion, electron paramagnetic resonance, cyclic voltammetry,
and dynamic light scattering (DLS) analysis, as well as by
comparing these properties with those of the self-assembled
structures of mono-, di-, and tritopic control compounds.
DLS experiments revealed that the spherical supramolecular
polymers had hydrodynamic diameters of 68 nm for T1
(75 mm) in acetonitrile and 105 nm for T2 (75 mm) in water/
acetonitrile (1:1). The 3D spherical structures of the supra-
molecular polymers formed in different solvents were also
supported by SEM and AFM experiments.
radical cation TTF +, their pre-organized tetrahedral arrange-
C
ment remarkably enhanced their intermolecular dimeriza-
tion, leading to the formation of new 3D spherical supra-
molecular polymers. The structure of the supramolecular
polymers has been inferred on the basis of UV/Vis absorp-
ricated materials.[4] 3D self-assembled soft architectures are not
only structurally unique as a new kind of supramolecular poly-
mers,[5–20] but also are promising for applications in absorption,
separation, sensing, and drug delivery. In principle, pre-organ-
ized tetrahedral monomers should be ideal for generating
such kinds of supramolecular polymers, if the intermolecular
interaction between the appended binding sites is robust
enough to hold a large number of monomers together. How-
ever, to the best of our knowledge, this possibility has not
been addressed to date.
Introduction
In the last decade, chemists have created a large number of
metal–organic and covalent organic frameworks (MOFs and
COFs),[1] the diamond topology of which endows such frame-
works with high rigidity and large surface areas.[2] To date,
these kinds of crystalline materials have found extensive appli-
cations in gas storage, catalysis, luminescence, and biotech-
nology.[1–3] However, the modification or functionalization of
such three-dimensional (3D) MOFs and COFs has often proved
challenging due to their heterogeneity or insolubility, although
post-synthetic strategies have been developed that allow the
introduction of several kinds of functional groups into pre-fab-
As a prototypical electron donor, tetrathiafulvalene (TTF) has
found tremendous applications in supramolecular and materi-
als chemistry over the past decades.[21–27] In 1979, it was report-
ed that, upon one-electron oxidation of neutral TTF com-
+
[a] J. Tian, Y.-D. Ding, Dr. H. Wang, Prof. D.-W. Zhang, Prof. Z.-T. Li
Department of Chemistry, Fudan University
220 Handan Road, Shanghai 200433 (China)
Fax: (+86)21-65641740
C
pounds, the resulting radical cation TTF could dimerize in
the solid state.[28] However, due to its low stability, the radical
cation dimer (TTF +)2 was for a long time considered merely as
C
a novelty.[29] In the last decade, several strategies have been
developed to stabilize this radical cation dimer, which involve:
1) holding two TTF units in place with a pre-organized frame-
work,[30] 2) using a rigid macrocycle or capsule to complex the
dimer,[31] or 3) stabilizing the dimer by virtue of mechanical
bonds in an interlocked system.[32] Given the excellent reversi-
ble redox nature of TTF,[33] it should be of value if new ap-
proaches could be developed that allow the direct utility of
the radical cation dimer as a basic binding motif for assem-
E-mail: wanghui@fudan.edu.cn
ztli@fudan.edu.cn
[b] Prof. Y. Liu
The Molecular Foundry
Lawrence Berkeley National Laboratory
Berkeley, CA 94720 (USA)
E-mail: yliu@lbl.gov
[c] T.-Y. Zhou, Dr. K.-D. Zhang, Prof. X. Zhao
Shanghai Institute of Organic Chemistry
Chinese Academy of Sciences
345 Lingling Lu
bling functional supramolecular architectures. Herein, we dem-
Shanghai 200032 (China)
+
C
onstrate that TTF units appended to a rigid tetrahedral
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201302951.
framework can undergo remarkably strengthened intermolecu-
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lar dimerization in both organic and aqueous media, which
leads to the formation of a new kind of 3D spherical supra-
molecular polymers.
The synthetic routes leading to the TTF compounds are
shown in Scheme 1. Thus, compound 4[34] was first deprotect-
ed with cesium hydroxide and then treated with iodomethane
to give compounds 1a[35] and 5. With compound 5 available,
tetrabromide 7 was prepared by the reaction of 6[36] with me-
thoxymethyl bromide (MOMBr) in the presence of titanium(IV)
bromide. Treating 7 with 5 in the presence of cesium hydrox-
ide afforded T1 in 80% yield. Compound 4 was then depro-
Results and Discussion
Design and synthesis
The tetraphenylmethane unit has been widely exploited as
a rigid tetrahedral building block for the formation of 3D MOFs
and COFs.[2] We conjectured that, for the formation of soluble
supramolecular networks, a certain degree of flexibility would
be important for tetraphenylmethane-derived building blocks
to self-assemble into 3D “soft” networks. Compounds T1 and
T2 were thus designed and prepared, in which four TTF units
are attached to the rigid tetraphenylmethane framework
through flexible ꢀSCH2ꢀ linkers. Moreover, compounds 1a and
b, dipodal 2, and tripodal 3 were also prepared as controls.
Molecular dynamics modeling showed that the appended TTF
units of compounds T1 and T2 could not undergo intramolec-
ular stacking due to the rigidity of the central tetraphenylme-
thane framework.
Scheme 1. Synthesis of compounds T1, T2, 1a, 1b, 2, and 3.
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tected and reacted with bromide 8 to give compounds 9 and
1b. Further reaction of 9 with 7 produced T2 in 42% yield. For
the synthesis of 2, compound 12 was first prepared by the re-
action of 10 and 11[37] and then treated with N-bromosuccini-
mide (NBS) to give dibromide 13. The latter was then reacted
with 5 to give 2 in 44% yield. For the preparation of 3, com-
pounds 10 and 14[38] were first refluxed in acetic acid and then
treated with isoamyl nitrite to produce 16. This intermediate
was then treated with NBS to afford tribromide 16. The latter
was further reacted with 5 to produce 3 in 35% yield. All of
the TTF compounds were found to be soluble in common or-
ganic solvents, including chloroform, dichloromethane, and
acetonitrile, while T2 and 1b were rendered moderately solu-
ble in water after their TTF units had been oxidized to mono-
or dications.
UV/Vis absorption spectroscopy
UV/Vis spectroscopic experiments were first carried out for T1,
1a, 2, and 3 in chloroform and acetonitrile at different TTF oxi-
dation states, which were attained by adding different num-
bers of equivalents of Fe(ClO4)3 with respect to the total con-
centration of TTF, as reported by Amabilino and co-workers.[39]
Typical spectra are presented in Figure 1. As expected, upon
addition of 0.5 or 1.0 equiv of Fe(ClO4)3, half or all of the TTF
+
C
units were oxidized to the TTF radical cation, as evidenced
by the emergence of absorption bands at around 660–
738 nm.[31,32] The spectra of compounds T1 and 3 exhibited
the characteristic absorption of the dimer (TTF +)2 centered at
C
850 nm,[32] which was notably stronger for the former than for
the latter. In contrast, compound 2 showed only a weak
shoulder around this wavelength, indicative of the formation
of a smaller amount of the dimer, while compound 1a did not
exhibit any noticeable absorption of the dimer (TTF +)2. The
C
colors of the solutions matched well with the above UV/Vis
spectra (Figure S25 in the SI). In chloroform, the colors of the
solutions changed gradually from brown for 1a and 2 to
brownish-green for 3 and green for T1, reflecting the strongest
+
C
propensity of the TTF unit of 1a to undergo dimerization. In
acetonitrile, the four compounds gave rise to similar spectra,
but the absorption of the dimer (TTF +)2 was generally weaker,
C
with only T1 displaying the relevant absorption band as
+
+
C
C
a strong feature. The relative strengths of the TTF and (TTF
)2 absorptions in all of the spectra were concentration-depen-
+
C
dent. Because the dimerization of the TTF unit of tetratopic
T1 was substantially stronger than that of ditopic 2, it is rea-
+
C
sonable to infer that the dimerization of their TTF units oc-
curred intermolecularly. Further addition of 1.0 equiv of
Fe(ClO4)3 to the solutions caused the exclusive formation of
the dication TTF2+, as reflected by the characteristic absorption
Figure 1. UV/Vis absorption spectra of the neutral (TTF, I), mixed-valence
state (1/2TTF , II), radical cation (TTF +, III), and dication state (TTF2+, IV) of
a) 1a, b) 2, c) 3, and d) T1 in chloroform at 258C ([TTF]=60 mm, 1.0 cm path
length). Fe(ClO4)3 was used as the oxidant for all samples. The molar concen-
tration represents the total [TTF].
+
C
C
at around 557–580 nm and the disappearance of the absorp-
+
tions of both TTF and (TTF +)2.[31,32]
C
C
The UV/Vis absorption spectra of T2 at different oxidation
states in chloroform, acetonitrile, and water are shown in
830 nm. This absorption band was also observed for T1 in the
+
Figure 2. In contrast to that of T1, the absorption of the (TTF
same solvent. In water, the absorption of (TTF +)2 of T2, cen-
C
C
)2 of T2 in chloroform was quite weak. In acetonitrile, T2 exhib-
tered at 870 nm, became even stronger. Consistent with this,
the solution took on a green color, while a solution of control
ited an increased absorption band due to (TTF +)2 centered at
C
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T2 tend to self-coil, resulting in a steric repulsion that signifi-
cantly weakens the dimerization. Similarly to 1a, control 1b
did not exhibit any absorption of (TTF +)2 regardless of the sol-
C
vent, which is consistent with the hypothesis that strong dime-
+
C
rization of the TTF unit of T2 is enabled by its tetrahedral
framework.
The apparent equilibrium constants (Ka) for the dimerization
+
C
of the TTF units of the TTF compounds were then evaluated
in different solvents on the basis of UV/Vis dilution experi-
+
C
ments, fitting the maximum absorption of the dimer (TTF
)
2
to a nonlinear self-binding equation. The data are summarized
in Table 1. The Ka values matched well with those derived from
+
C
the absorptions of (TTF
) at even longer absorption wave-
2
lengths, which indicated that, at these wavelengths, the ab-
+
C
sorptions of TTF and other possible TTF species were negligi-
bly weak, confirming the reliability of the data in Table 1. For
both 1a and 1b, no absorption of (TTF +)2 was observed, indi-
C
cating very low, if any, propensity for dimerization of their
+
C
TTF units. In all cases, the Ka values were in the order T1 > 3
+
C
> 2, clearly reflecting the increased cooperativity of the TTF
units of these compounds in dimerization. Comparison of the
data for T1 and T2 in the same solvent reveals that in less
+
C
polar media, the TTF dimerization of the former was always
stronger than that of the latter. However, in highly polar aque-
ous media, the latter displayed a greater propensity for dimeri-
+
C
zation. The fact that strong dimerization of the TTF unit
could occur in solvents of both low and high polarity may be
rationalized by considering the solvophobicity differences in
+
C
the respective processes. In solvents of low polarity, the TTF
unit displays solvophobicity by acting as a polar aromatic spe-
cies, while in highly polar aqueous media, it might exhibit hy-
drophobicity as a less polar aromatic unit. As a result, the di-
merization could take place in both organic and aqueous
media.
Progressive addition of Fe(ClO4)3 to a solution of compound
T1 in chloroform also led to the formation of a weak but no-
ticeable absorption band at around 1800 nm (Figure 3). The in-
tensity of this absorption was maximized when 2.0 equiv of
Fe(ClO4)3 had been added. Further addition of Fe(ClO4)3 caused
the absorption to weaken. Thus, this absorption may be attrib-
+ [32,39,40]
C
uted to the mixed-valence dimer (TTF)2
.
Under identi-
cal conditions, no similar absorption was observed for any of
the other samples, reflecting the fact that the tetrahedral
framework of T1 not only maximized the homo-stacking of its
+
C
radical cation TTF in chloroform, as reflected by the highest
Figure 2. UV/Vis spectra of the neutral (TTF, I), mixed-valence state (1/
+
2TTF , II), radical cation (TTF +, III), and dication state (TTF2+, IV) of T2 a) in
chloroform, b) in acetonitrile, and c) in water, and d) of 1b in water at 258C
([TTF]=60 mm, 1.0 cm path length, Fe(ClO4)3 as oxidant).
C
C
Ka value (Table 1), but also its stacking with the neutral TTF
unit.
Self-assembled patterns
+
C
The differences in the dimerization propensities of the TTF
1b was light-brown (Figure S25 in the SI). These observations
+
C
suggested that the TTF unit of T2 preferentially dimerized in
units of the new TTF derivatives could be rationalized by con-
polar media. We postulate that in polar solvents, such as ace-
tonitrile or water, the long polar triglycol chains are better sol-
vated and can adopt an extended form, and thus have minimal
sidering different binding patterns. For ditopic 2, a 1+1 macro-
cyclic dimer could account for the remarkably increased dime-
+
C
rization propensity of its TTF units relative to those of 1a or
+
impact on the intermolecular stacking of TTF +. In less polar
1b. Further enhanced dimerization of the TTF units of tritop-
C
C
solvents, such as chloroform, the long polar triglycol chains of
ic 3 could be ascribed to the formation of a 1+1 capsular
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shape because the tetrahedral
structure of the two building
blocks should favor growth of
the network in 3D space.
Table 1. Apparent equilibrium constants (Ka) [mꢀ1] of the TTF + units of various TTF derivatives in different sol-
vents.
C
Solvent
2
3
T1
T2
PhMe/CHCl3 (1:1, v/v)
CH2Cl2
CHCl3
MeCN
MeCN/H2O (1:1, v/v)
H2O
8.1(ꢁ0.3)ꢁ103
4.5(ꢁ0.8)ꢁ104
3.3(ꢁ0.2)ꢁ105
4.3(ꢁ0.8)ꢁ105
3.7(ꢁ0.2)ꢁ106
1.5(ꢁ0.1)ꢁ105
5.3(ꢁ0.7)ꢁ104
1.3(ꢁ0.1)ꢁ104
1.1(ꢁ0.06)ꢁ104
1.7(ꢁ0.4)ꢁ104
6.8(ꢁ0.2)ꢁ104
1.3(ꢁ0.1)ꢁ105
1.7(ꢁ0.1)ꢁ105
[a]
[a]
–
–
6.3(ꢁ0.2)ꢁ104
4.5(ꢁ0.1)ꢁ103
2.8(ꢁ0.4)ꢁ104
1.1(ꢁ0.2)ꢁ105
4.7(ꢁ0.2)ꢁ104
3.6(ꢁ0.5)ꢁ104
Electron paramagnetic reso-
nance (EPR) spectroscopy
[b]
[b]
[b]
–
–
–
The 1H NMR spectra of the TTF
derivatives did not display re-
+
C
[a] The absorption band of dimer (TTF
) was too weak to be measured. [b] The samples were not soluble.
2
solved signals when the TTF unit
+
C
was oxidized to TTF due to the
paramagnetic character of the radical cation (Figures S45–S47
in the SI). EPR experiments were further performed to investi-
+
C
gate the dimerization of the generated TTF units in each of
the TTF derivatives. To allow for comparison, the total concen-
tration of TTF units was kept at 0.6 mm for all of the studied
+
C
samples, and they were quantitatively oxidized to TTF with
Fe(ClO4)3. Upon incremental addition of Fe(ClO4)3, all of the
samples exhibited the typical signal of the paramagnetic radi-
cal cation TTF +. The signal intensity was maximized after
C
about 0.5 equiv of Fe(ClO4)3, relative to the total concentration
Figure 3. Near-infrared UV/Vis absorption spectra of compound T1 (0.15mm)
in chloroform with the addition of Fe(ClO4)3, indicating that the absorption
at around 1800 nm first increased (left) and then decreased (right) with the
incremental addition of Fe(ClO4)3.
+
C
dimer, which could be stabilized by forming three (TTF
)
2
dimers. For tetratopic T1 and T2, the tetrahedral structural
framework should favor the formation of 3D supramolecular
polymers. In a “perfect” supramolecular structure, a spherical
3D diamondoid network may be generated, with the mono-
mers being connected through intermolecular dimerization of
+
the appended TTF units (Figure 4).[41] Considering the dy-
C
namic nature of the intermolecular noncovalent interactions
and the consequent movement and rotation of the monomers,
the 3D supramolecular networks should be polydisperse and
their ordering may be between this ideal self-assembled struc-
ture and a randomly arranged 3D polymer. To some extent,
however, the networks should feature the ordered diamondoid
topology, because only in this 3D self-assembled pattern can
+
C
the intermolecular dimerization of their TTF moieties be
Figure 4. Schematic representation of the 3D supramolecular polymers
maximized, as reflected by the highest Ka values shown in
formed by tetrahedral compounds T1 and T2 through intermolecular dimeri-
+
C
Table 1. The supramolecular network should adopt a spherical
zation of their TTF units.
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of the TTF units, had been added. The signal then weakened
and broadened upon addition of further Fe(ClO4)3. These ob-
to a smaller extent, but not for 1a or 2. These differences
should also reflect the incrementally reduced spin–spin cou-
+
C
servations indicated that, at a relatively high concentration of
pling of their TTF units and, more importantly, the “stickiest”
+
C
0.6 mm, even the TTF units of 1a and 1b could form the
environment within the supramolecular polymer formed only
+
+
C
C
spin-coupled diamagnetic dimer (TTF
)
to some extent, al-
C
by T1. In acetonitrile, the signal intensities of the TTF moiet-
2
+
though the mixed-valence dimer TTF/TTF may not have
been generated. The EPR spectra recorded after the addition
of 1.0 equiv of Fe(ClO4)3, which oxidized all of the TTF units to
ies of the four compounds followed the same order, but the
signal of T1 was considerably stronger than that observed in
chloroform, indicating a lower dimerization capacity of its
TTF +, are shown in Figure 5a. In chloroform, the spectra of
TTF units. All of these results are in accordance with those of
+
C
C
the above UV/Vis absorption experiments.
The EPR spectra of T2 in chloroform, acetonitrile, and water
are presented in Figure 5b. As expected, the signal intensities
+
C
of its TTF units in the three solvents were consistently
weaker than those of the control compound 1b in the respec-
tive solvents, again supporting the formation of a 3D supra-
molecular network. For comparison, the spectrum of 1b re-
corded in water is also included in Figure 5b. The order of the
signal intensities of T2 in the three solvents was also in agree-
+
C
ment with the relative dimerization capacities of the TTF
units as revealed by the UV/Vis experiments. In water, the EPR
signal nearly disappeared, with the intensity being only about
+
C
0.5% of that of 1b, suggesting that the TTF units of T2 were
almost completely dimerized in this most polar medium.
Cyclic voltammetry (CV)
CV experiments were also carried out to investigate the elec-
trochemical properties of the TTF compounds. First, we record-
ed their voltammograms in chloroform (Figure 6). Compounds
1a, 2, and 3 each exhibited two common one-electron oxida-
tion and reduction processes. However, T1 gave rise to three
oxidation and reduction peaks. The first oxidation potentials of
the four compounds, corresponding to oxidation of the neutral
TTF unit to radical cation TTF +, gradually shifted to lower
C
Figure 5. EPR spectra of a) 1a, 2, 3, and T1 in chloroform and b) T2 in
chloroform, acetonitrile, and water, and 1b in water, recorded at 258C after
the addition of 1.0 equiv of Fe(ClO4)3 relative to [TTF] (0.6 mm).
values, from 0.67 V for 1a to 0.50 V for T1. The signals of the
1a, 2, 3, and T1 all featured the signal of the radical cation
TTF +. The signal intensities of 2, 3, and T1 were about 80%,
C
3.1%, and 2.4% relative to that of 1a. Assuming that all of the
+
C
TTF units in the various TTF derivatives had identical signal
+
C
intensity and that no TTF dimerization occurred in the case
+
C
of 1a, we may deduce that at least 97.6% of the TTF units of
T1 underwent dimerization,[32c] corresponding to an apparent
Ka of 1.4ꢁ106 mꢀ1. This low-limit value is reasonably compara-
ble to the value evaluated on the basis of UV/Vis dilution ex-
periments (Table 1). The strong propensity for dimerization of
+
C
the TTF units of 3 implies generation of the aforementioned
1+1 dimerization pattern, and the even stronger dimerization
+
C
of the TTF moieties of T1 supports the formation of a 3D
supramolecular network.
Compared with the signal recorded after 0.5 equiv of
Fe(ClO4)3 had been added, the EPR signal of T1 obtained in the
presence of 1.0 equiv of Fe(ClO4)3 was noticeably broadened.
Similar broadening was also observed for compound 3, albeit
Figure 6. Cyclic voltammograms of a) 1a, b) 2, c) 3, and d) T1 recorded in
chloroform and electrolyte Bu4NPF6 (0.1m) at 258C ([TTF]=0.6 mm). The
scan rates were 100 mVsꢀ1 for 1a, 2, and 3 and 1–200 mVsꢀ1 for T1.
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1
TTF protons in the H NMR spectra of the four compounds in
CDCl3 appeared at nearly the same chemical shifts, indicating
that the neutral units were not engaged in any significant
intra- or intermolecular interactions. Thus, the above shifting
of the first oxidation potentials should reflect the incrementally
+
C
increasing propensity of their TTF units to undergo spin–spin
coupling to form (TTF +)2. The second oxidation peaks of 1a,
C
2, and 3 clearly corresponded to the same one-electron oxida-
+
tion of TTF to TTF2+. For T1, the second and third oxidation
C
peaks may be ascribed to two one-electron oxidations of
+
+
+
(TTF )2, that is, the processes (TTF )2!TTF +/TTF2+ and TTF
C
C
C
C
/TTF2+!2TTF2+. The separate second and third oxidation
+
C
steps suggest that the oxidation of TTF to TTF greatly facili-
tated the formation of radical cation dimer (TTF +)2 in the 3D
C
supramolecular networks, this being the dominant species that
might undergo two consecutive one-electron oxidation pro-
cesses towards the formation of TTF2+ via a less stable mixed-
Figure 7. DLS results for solutions of 1a, 2, 3, and T1 in acetonitrile and 1b
and T2 in water/acetonitrile (1:1, v/v) at 258C ([TTF]=0.3 mm) when the TTF
+
C
units were oxidized to the radical cation TTF with Fe(ClO4)3.
valence state TTF +/TTF2+
.
C
The re-reduction of the TTF2+ species of compound T1 also
supported the formation of the 3D supramolecular network.
ular polymer. In principle, the strong propensity for dimeriza-
+
C
The first re-reduction at 0.62 V should involve the one-electron
tion of TTF observed in tritopic 3 could lead to the formation
+
reduction of TTF2+ to TTF and was similarly observed for 1a,
of a 2D honeycomb-like supramolecular network. However, the
DLS results excluded this possibility and pointed to the forma-
tion of a smaller supramolecular entity, most likely in the form
of a dimeric capsule. In aqueous acetonitrile, the DH of 1b was
comparable to the size of a single molecule, indicating that, as
in the case of 1a, the one-electron oxidation state of this com-
pound did not undergo appreciable dimerization. However, T2
exhibited a remarkably large DH, corresponding to about 20
times the size of the single molecule in the extended confor-
mation (~5.2 nm), which again supported the formation of
a supramolecular network. As expected, under the same condi-
tions, the samples of T1 and T2 in the neutral and mixed-va-
lence states did not exhibit any large entities.
C
2, and 3. The radical cations then underwent spin–spin cou-
pling to produce (TTF +)2 before being further reduced with
C
the formation of the 3D supramolecular network. The second
peak at 0.54 V is ascribed to the one-electron reduction of
+
(TTF )2 to (TTF)2 +, which underwent another one-electron re-
duction giving rise to the third peak at 0.27 V with the genera-
tion of neutral TTF. Within the studied scan rate range of 1–
200 mVsꢀ1, these second oxidation and reduction peaks were
always observed for T1, but not for any of the other com-
pounds. In acetonitrile, the TTF units of the four compounds
C
C
all exhibited the same two one-electron oxidation and reduc-
+
C
tion processes, suggesting that the (TTF
) formed by T1 in
2
chloroform was most stable entrapped within the 3D supra-
molecular polymer by virtue of the tetrahedral structure of the
monomer.
DLS measurements were also conducted at different concen-
trations of T1 in acetonitrile and T2 in aqueous acetonitrile.
The results are presented in Figure 8. It can be seen that, in
the studied concentration range, the DH values of both com-
Dynamic light scattering (DLS)
The above investigations indicated that T1 and T2 formed 3D
supramolecular networks due to the strong dimerization of
+
C
their TTF moieties. In order to evaluate the size of these
supramolecular networks, DLS experiments were performed for
all of the compounds in acetonitrile at the one-electron oxida-
tion state of their TTF units. Chloroform was not chosen as the
solvent to avoid interference from Fe(ClO4)2, the reduction
product of Fe(ClO4)3, which is insoluble in this solvent. The re-
sults are presented in Figure 7. The apparent hydrodynamic di-
ameter (DH) of compound 1a was the smallest, which support-
ed its single molecular state. The values for 2 and 3 reasonably
matched with the calculated sizes of their respective dimeric
macrocycle (ca. 2.1 nm) and capsular dimer (ca. 3.2 nm). In
sharp contrast, tetratopic T1 exhibited a substantially larger DH
value, which corresponded to more than 19 times the size of
the tetrahedral molecule in its extended conformation. This
result strongly supported the formation of the 3D supramolec-
*
Figure 8. The concentration dependences of DH of T1 ( ) in acetonitrile and
&
T2 ( ) in water/acetonitrile (1:1, v/v) at 258C, when their TTF units were oxi-
+
C
dized to the radical cation TTF with Fe(ClO4)3.
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pounds increased markedly with the concentration of the
monomers, but tended to reach maximum values in the high
concentration range. At even higher concentrations, the solu-
tions became turbid and dark-green precipitates were eventu-
ally precipitated. This result implied that even larger solid ag-
gregates might have been produced. In no case was a supra-
molecular gel observed. Upon dilution of the mixtures, the
solid aggregates were redissolved and the solutions showed
the formation of the homogeneous 3D supramolecular poly-
mers, as evidenced by DLS experiments. The results showed
the self-assembly of the new 3D supramolecular networks to
be reversible, a key feature of such supramolecular polymers.
Scanning electron microscopy (SEM) and atomic force mi-
croscopy (AFM)
+
C
SEM images were first obtained for T1 and T2 in the TTF
state after evaporating the solvents from solutions in chloro-
form or acetonitrile. For all of the studied samples, the SEM
images showed the formation of isolated particles (Fig-
ure 9a,b). The sizes of these particles roughly ranged from 100
to 500 nm. A sample of T2 obtained by concentration to dry-
ness of a solution in water exhibited a similar result (Figure S39
in the SI). Generally, the particles formed by T2 were larger
than those formed by T1, which is consistent with the above
DLS results in solution. Notably, reducing or increasing the
concentrations of the solutions did not substantially change
the size of the particles. In addition, no linear fibrous aggre-
gates were observed in any of the images. No spherical parti-
cles were observed for any of the other TTF samples in chloro-
form or acetonitrile at different concentrations. Thus, these ob-
servations support the formation of 3D spherical supramolec-
ular polymers in solution by T1 and T2, which spread on the
surface upon evaporation of the solvent. The fact that the size
of the particles was independent of the concentration of the
samples implies that the formation of the 3D supramolecular
polymers was rapid. Thus, during evaporation of the solvent
from the dilute solutions, the monomers or small supramolec-
ular entities could further form large species prior to deposi-
tion on the surface. On average, the sizes of the aggregates
observed in the SEM images were larger than those evaluated
in solution by DLS. This is not unexpected considering that the
3D spherical supramolecular entities would collapse on the
surface upon evaporation of the solvent, producing disk-like
larger aggregates. X-ray diffraction experiments were per-
formed on the samples both in solution and in the solid state.
Unfortunately, however, we did not obtain diffraction signals
to support the ordered arrangement of the monomers.
Figure 9. SEM images of samples of a) T1 (0.1 mm) in CHCl3 and b) T2
(0.1 mm) in MeCN. Tapping-mode AFM images of samples of c) T1 and d) T2
in MeCN (0.1 mm). The samples were prepared on mica wafers by evaporat-
+
C
ing the solvent. The TTF units were oxidized to the radical cation TTF with
Fe(ClO4)3.
supramolecular polymers present in solution on the surface
upon removal of the solvent. The samples of both compounds
prepared from chloroform and the sample of T2 prepared
from water all showed similar results (Figures S40–S44 in the
SI), suggesting that the formation of spherical supramolecular
polymers by the two compounds constitutes general behavior
in solution.
AFM images of T1 and T2 prepared by concentration to dry-
ness of solutions in acetonitrile are presented in Figure 9c,d.
Similar to the SEM images, the AFM images also showed the
formation of separate particles, and the particles formed by T2
were generally larger than those formed by T1. Typically, the
heights of the particles were in the range 50–100 nm, which
was substantially smaller than their diameters. This height
range ruled out the possibility of formation of hollow supra-
molecular entities and suggested collapse of the spherical
Conclusion
In conclusion, we have demonstrated that 3D spherical supra-
molecular polymers can be self-assembled from tetrahedral
molecular building blocks by making use of cooperatively en-
hanced homo-stacking of TTF radical cations, which is remarka-
Chem. Eur. J. 2014, 20, 575 – 584
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We thank the Ministry of Science and Technology
(2013CB834501), the Ministry of Education (IRT1117), the Sci-
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Keywords: dimerization
·
radical cations
·
stacking
interactions · supramolecular polymers · tetrathiafulvalene
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Received: July 26, 2013
Published online on December 2, 2013
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