Three-dimensional supramolecular polymers driven by rigid tetrahedral building blocks through tetrathiafulvalene radical cation dimerization

Tetrahedron 70 (2014) 4778e4783

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Three-dimensional supramolecular polymers driven by rigid tetrahedral building blocks through tetrathiafulvalene radical cation dimerization Lan Chen, Shao-Chen Zhang, Hui Wang, Ya-Ming Zhou *, Zhan-Ting Li, Dan-Wei Zhang * Department of Chemistry, Fudan University, 220 Handan Road, Shanghai 200433, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 2 April 2014 Received in revised form 2 May 2014 Accepted 8 May 2014 Available online 21 May 2014

Rigid tetrahedral compounds T1 and T2 that bear four tetrathiafulvalene (TTF) units, which are connected to a tetraphenylmethane core by the ethynylene or amide linker were designed and prepared.
Upon one-electron oxidation of the TTF units by Fe(ClO4)3, the resulting TTF þ radical cations stacked intermolecularly to give rise to three-dimensional supramolecular polymers, which were supported by UVevis spectroscopy, cyclic voltammetry, dynamic light scattering, and scanning electron microscopy. Ó 2014 Elsevier Ltd. All rights reserved.

Keywords: Supramolecular polymer Tetrathiafulvalene Hydrophobic effect Aromatic stacking Cooperativity

1. Introduction In recent years, there has been considerable interest in developing supramolecular polymers by holding monomers with discrete non-covalent forces, such as intermolecular hydrogen bondings, hosteguest molecular recognitions, and so on.1,2 Most of the reported supramolecular polymers are constructed from simple ditopic monomers, which lead to the formation of linear backbones. Many examples of cross-linked supramolecular polymers have also been reported, among which are respectable cases with the functions of self-healing, stimuli responsiveness, etc.3 Given the extensive investigations on the constructions and functions of threedimensional (3D) metal-organic and covalent-organic frameworks,4 it would be of importance to develop 3D soft networks through self-assembly strategy. Tetraphenyl-methane has been demonstrated as a useful subunit for the design of preorganized tetratopic building blocks for the generation of macromolecular systems.5 As a versatile electron donor, tetrathiafulvalene (TTF) has been widely applied in researches in materials and supramolecular sciences.6 Wudl and Torrance reported that, upon one-electron
oxidation, the resulting TTF þ radical cation can dimerize in solid 7 state. However, the stability of the dimer is very low. In the past

* Corresponding authors. Tel.: þ86 21 65643576; fax: þ86 21 65641740; e-mail address: [email protected] (D.-W. Zhang). 0040-4020/$ e see front matter Ó 2014 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.tet.2014.05.035

decade, several supramolecular methods have been developed to enhance the stability of this dimeric motif.8e10 We recently reported that tetraphenylmethane-based tetratopic building blocks that connect four TTF units with flexible linkers could self-assemble into stable 3D supramolecular polymers after
the TTF units were oxidized to TTF þ.11 To exploit the factors that affect the formation of this kind of supramolecular polymers, we have synthesized two new tetrahedral building blocks T1 and T2, whose four TTF units are connected to the central tetraphenylmethane unit with rigid ethynylene or amide linkers (Fig. 1). We herein describe their self-assembly into 3D supramolecular poly
mers driven by the dimerization of the TTF þ units. 2. Results and discussion 2.1. Synthesis Synthetic routes for compounds T1, T2, M1, and M2 are shown in Scheme 1. Compound 1 was first synthesized according to reported literature.12a Treatment of 1 with bromine afforded compound 2 in 80% yield. The tetrabromide was then reacted with trimethylsilylacetylene, which was followed by removal of the trimethylsilyl group with sodium hydroxide, to give tetraalkyne 3 in 40% yield.12b Compound 1 was further nitrated to give 4, which was then treated with stannous chloride to produce 5.12a Compounds 7 and 8 were prepared from commercially available tetrathiafulvalene 6

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Fig. 1. Rigid tetrahedral compounds T1 and T2 bearing four TTF units and model compounds M1 and M2.

according to reported methods.13a Then Sonogashira crosscoupling reaction of 7 with 3 or ethynylbenzene afforded T1 in 20% yield and M113b in 50% yield, respectively. Hydrolysis of compound 8 in aqueous sodium hydroxide solution produced the corresponding acid, which was further reacted with 5 or aniline to afford T2 in 48% yield and M213c in 58% yield, respectively. 2.2. UVevis spectroscopy The self-assembly of compounds T1, M1, T2, and M2 at different oxidation states for the TTF units, which was realized by adding different equiv of Fe(ClO4)3, were first studied by UVevis spectroscopy. For T1 and M1, the experiments were conducted in chloroform, and for T2 and M2, the chloroform/acetonitrile mixture (2:1 v/v) was used.14 Adding 0.25, 0.5, 0.75, and 1 equiv of Fe(ClO4)3, which were relative to the concentration of the TTF unit, to the solution of T1 (0.05 mM) in chloroform caused the TTF unit to be
oxidized to its radical cation TTF þ gradually.14 In the recorded absorption spectra (Fig. 2), in addition to the absorption bands cen
tered at 410 and 600 nm, which could be assigned to the TTF þ 14 monomer, two absorption bands, centered around 845 and 1600 nm, were also displayed. These two bands are typical for the
radical cation dimer (TTF þ)2 and the mixed-valence dimer
þ 8,15 (TTF)2 , respectively. The intensity of the absorption peak around 1600 nm reached maximum when 2 equiv of Fe(ClO4)3 relative to T1 was added (Fig. 2b,c), which allowed for the oxidation of half of the
TTF units to the TTF þ unit. With the addition of more Fe(ClO4)3, the absorption band became weakened because more neutral TTF units
were oxidized to TTF þ and when 4 equiv of Fe(ClO4)3 relative to T1 was added, the absorption band around 845 nm became the
strongest as all the TTF units were oxidized to TTF þ and thus the
þ concentration of (TTF )2 reached to its maximum (Fig. 2a). For control compound M1, with the addition of Fe(ClO4)3, only the ab
sorption of the radical cation TTF þ was observed (Fig. 2d), indicating that its dimerization tendency was very weak. In chloroform, when the TTF units of compound T2 (0.025 mM)
were oxidized to radical cation TTF þ, green precipitation was
formed due to the strong dimerization of TTF þ. In the chloroform/ acetonitrile (2:1) mixture containing 1% 2-phenoxyethanol, no precipitation occurred. Thus, the UVevis spectra were recorded in this solvent system. The spectra, together with those of M2, are provided in Fig. 3. Again, the spectra of T2 displayed the absorption
bands of radical cation dimer (TTF þ)2, which was centered around
800 nm (Fig. 3a), and mixed-valence dimer (TTF)2þ, which was centered around 1450 nm (Fig. 3b,c). In contrast, compound M2 did

Scheme 1. The synthesis of compounds T1, T2, M1, and M2.

not exhibit similar absorption bands (Fig. 3d). On the basis of UVevis dilution experiments, we could determine the apparent
association constants (Ka) of the dimer of the TTF þ unit of T1 in chloroform and T2 in chloroform and acetonitrile (2:1) were 2.2104 and 5.9104 M1, respectively, by fitting the absorption of
(TTF þ)2 to the 1:1 nonlinear equation.10b,11 Given the rigidity of tetrahedral backbone of compounds T1 and
T2, it was not possible for their peripheral TTF þ units to undergo intramolecular dimerization. Thus, their stacking should occur

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Fig. 2. UVevis absorption spectra of (aec) T1 and (d) M1, after different equiv of Fe(ClO4)3 was added, in chloroform at 25  C ([TTF]¼0.2 mM, 1.0 cm path length).

intermolecularly. The fact that no absorption of (TTF þ)2 or (TTF)2þ was observed for M1 and M2 suggested that the preorganized tetrahedral backbone of T1 and T2 provided cooperativity for the
homodimerization of the TTF þ unit and its stacking with the neutral





Fig. 3. UVevis absorption spectra of (aec) T2 and (d) M2, when different equiv of Fe(ClO4)3 was added, in chloroform and acetonitrile (2:1, v/v) at 25  C ([TTF]¼0.1 mM, 1.0 cm path length).

L. Chen et al. / Tetrahedron 70 (2014) 4778e4783

TTF unit through the formation of 3D supramolecular polymers,10b,11 as shown in Fig. 4. For both T1 and T2 (0.05 mM), when the UVevis spectra were recorded in more polar chloroform and acetonitrile
mixture (1:1, v/v), neither the absorption of the (TTF þ)2 dimer nor
þ that of the (TTF)2 mixed-valence dimer was observed. This observation indicates that the increase of the polarity of the medium
weakened the stacking of the TTF þ via better solvation probably.

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oxidation and reduction peaks of compounds T1 and T2 appeared with a lower potential. This observation indicated that, for both
compounds, the two oxidation processes, that is, TTF/TTF þ and
TTF þ/TTF2þ, were easier to occur, whereas the two reduction

processes, that is, TTF2þ/TTF þ and TTF þ/TTF, were more difficult to occur. The shifting of the first oxidation and the second reduction to the low-potential direction may be attributed to the

formation of (TTF þ)2 by TTF þ, which caused TTF to lose the first electron more easily and led to the formation of the 3D supramolecular networks. The fact that the second oxidation appeared with
a lower potential implied that the TTF þ units in the dimeric state lost another electron more easily. It is intriguing that the first reduction also shifted to the low-potential direction. One possible reason for this result is that their large tetrahedral backbone made the octacationic species to diffuse much more slowly, which
delayed the reduction of TTF2þ to TTF þ. 2.4. Dynamic light scattering (DLS)

2.3. Cyclic voltammetry (CV) CV experiments were also carried out to investigate the electrochemical properties of T1 and T2 and the voltammograms, together with those of M1 and M2, are shown in Fig. 5. All the potentials were referenced against an Ag/AgCl electrode. All the four compounds gave rise to two oxidation and reduction peaks. However, compared with that of the controls M1 and M2, both the

(a)

1.0

0.98 0.61

0.54

25

84.1 nm

20

Number (%)

Fig. 4. Schematic representation of the 3D supramolecular polymers formed by tet
rahedral compounds T1 and T2 via intermolecular dimerization of their TTF þ units.

It was envisioned that the 3D supramolecular networks formed
by T1 and T2 in the TTF þ state would produced aggregates of large size. We thus performed DLS experiments for T1 in chloroform and T2 in chloroform and acetonitrile (2:1) after the TTF units were
oxidized to TTF þ. The apparent hydrodynamic diameter (DH) of the formed aggregates was determined to be 116.5 and 84.1 nm (Fig. 6), respectively. Under the identical measurement conditions, both the solution of M1 and M2 did not produce any large aggregates of >3 nm in size. Thus, the above large DH values could be regarded as strong evidence for the formation of 3D supramolecular networks for T1 and T2 through the intermolecular dimerization of their
appended TTF þ units.

116.5 nm

15 10 5 0

0.48

10

0.38

0.00

0.25

T1 M1

0.77

0.50

0.75

1.00

1.25

E/V

(b)

0.97 T2 M2

0.53

100

1000

DH (nm)

0.91

Fig. 6. DLS results of T14( þ) (e,e) in CHCl3 ([TTF]¼0.2 mM), and T24( þ) (eBe) in CHCl3/MeCN 2:1 ([TTF]¼0.08 mM) at 25  C.





2.5. Scanning electron microscopy (SEM) In order to investigate the morphological character of the supramolecular aggregates formed by T1 and T2, SEM images were also recorded after evaporating their respective solution (Fig. 7).

0.90

0.48

0.37

0.82

0.90

0.46 0.00

0.25

0.50

0.75

1.00

1.25

E/V Fig. 5. Cyclic voltammograms of (a) T1 and M1 in chloroform ([TTF]¼0.2 mM), and (b) T2 and M2 in chloroform and acetonitrile (2:1) ([TTF]¼0.1 mM) at 25  C. Electrolyte: Bu4NPF6 (0.1 M), the scan rate: 100 mV/s.

Fig. 7. SEM images of the samples of (a) T1 (0.1 mM) in chloroform, and (b) T2 (0.1 mM) in chloroform and acetonitrile (2:1) on mica after their TTF units were oxi
dized to TTF þ and the solvent was evaporated.

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4.3. Characterization of compounds M1 and M2

The SEM images of both samples showed the formation of spherical particles with the size ranging from 1 mm to 10 mm. Again, similar spherical particles were not observed from the images of M1 or M2. Thus, the large spherical particles observed for the samples of T1 and T2 should reflect that they formed 3D supramolecular networks in solution after their TTF units were oxi
dized to TTF þ.

Compound M1:13b 1H NMR (400 MHz, CDCl3) d 7.45e7.44 (m, 2H), 7.35e7.33 (m, 3H), 6.55 (s, 1H), 6.33 (s, 2H). Compound M2:13c 1H NMR (400 MHz, DMSO-d6) d 10.23 (s, 1H), 7.80 (s, 1H), 7.60 (d, J¼8.2 Hz, 2H), 7.34 (t, J¼7.9 Hz, 2H), 7.12e7.09 (m, 1H), 6.76 (s, 2H).

3. Conclusion

Acknowledgements

We have demonstrated that rigid tetrahedral TTF molecules could self-assemble into 3D supramolecular networks in solution after their peripheral TTF units were oxidized to radical cations, which is driven by preorganization-promoted dimerization of the radical cations. This work, together with our previous studies on the self-assembly of viologen or TTF-based tetrahedral radical cation building blocks, illustrates the potential of the stacking of conjugated radical cations in driving the formation of advanced supramolecular architectures. In the future, we will design building blocks that bear groups for the formation of reversible covalent bonds, which can be tuned by the directed stacking of radical cations.

The authors thank the National Natural Science Foundation of China (Nos. 21172042 and J1103304), Science and Technology Commission of Shanghai Municipality (13NM1400200), and the Ministry of Science and Technology of the People’s Republic of China (2013CB834501) for financial support of this work.

4. Experimental section 4.1. Synthesis and characterization of compound T1 2-Iodo-tetrathiafulvalene (7, 33 mg, 0.1 mmol), Pd(PPh3)2Cl2 (2 mg, 0.0034 mmol), CuI (1 mg, 0.0057 mmol), and tetrakis(4ethynylphenyl)methane (3, 14 mg, 0.017 mmol) were dissolved in the mixture of 4 mL of NEt3 and 4 mL of THF. Then the mixture was degassed with N2. After stirring at 40  C over 24 h under N2 atmosphere, the mixture was cooled to room temperature and the solvent was evaporated, the residue was dissolved in dichloromethane and filtered. Purification by column chromatography on silica gel (hexane/dichloromethane 5:1 to 2:1, v/v, as eluent) followed by recrystallization from ethanol gave T1 (5 mg) in 20% yield as a yellow solid. FT-IR (n) 3070, 2964, 2923, 2198, 1629, 1492, 1384 cm1; 1H NMR (400 MHz, CDCl3) d 7.35 (d, J¼8.0 Hz, 8H), 7.12 (d, J¼8.0 Hz, 8H), 6.54 (s, 4H), 6.33 (s, 8H); 13C NMR (100 MHz, CDCl3) d 146.4, 131.3, 131.0, 124.9, 120.4, 119.3, 119.0, 116.1, 113.4, 93.1, 81.2, 58.9; HRMS (ESI): calcd for C57H28S16, 1223.7722 [M]þ, found: 1223.7725. 4.2. Synthesis and characterization of compound T2 Dissolve 2-(Ethoxycarbonyl)-tetrathiafulvalene (8, 110 mg, 0.4 mmol) in 15 mL of THF, to which 5 mL of 0.1 M NaOH aqueous solution was added. The mixture was stirred at room temperature until all of the compound 8 was consumed. Then 0.5 M of HCl aqueous solution was added dropwisely until pH 7. The mixture was extracted by dichloromethane and washed by 250 mL of water. The organic layer was dried over anhydrous MgSO4 and the solvent was removed under vacuum. To the residue dissolved in 10 mL of DMF were added EDCI (87 mg, 0.45 mmol), DMAP (55 mg, 0.45 mmol), and tetrakis(4-amino-phenyl)methane (6, 19 mg, 0.05 mmol) subsequently. The mixture was stirred at room temperature for 5 days and the solvent was removed under vacuum, the residue was recrystallized from dichloromethane to give T2 (31 mg) in 48% yield as a red solid. FT-IR (n) 3070, 2975, 2923, 2360, 1637, 1596, 1508, 1405, 1322 cm1; 1H NMR (400 MHz, DMSO-d6) d 10.33 (s, 4H), 7.83 (s, 4H), 7.55 (d, J¼8.0 Hz, 8H), 7.07 (d, J¼8.0 Hz, 8H), 6.75 (s, 8H); 13C NMR (100 MHz, DMSO-d6) d 157.4, 142.0, 139.1, 136.1, 133.8, 130.6, 126.9, 120.3, 120.0, 119.4, 112.5, 107.0, 105.8, 42.0; HRMS (ESI): calcd for 1/2 (C53H32N4O4S16), 649.8977 [M]2þ, found: 649.8986.

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