Xiao-Jun
Wang
ab,
Ling-Bao
Xing
ac,
Bin
Chen
a,
Ying
Quan
b,
Chen-Ho
Tung
a and
Li-Zhu
Wu
*a
aKey Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, The Chinese Academy of Sciences, Beijing 100190, P. R. China. E-mail: lzwu@mail.ipc.ac.cn; Fax: +86 108254 3580; Tel: +86 108254 3580
bJiangsu Key Laboratory of Green Synthetic Chemistry for Functional Materials, School of Chemistry and Chemical Engineering, Jiangsu Normal University, Xuzhou 221116, P. R. China
cSchool of Chemical Engineering, Shandong University of Technology, Zibo 255049, P. R. China
First published on 19th November 2015
The first example of tetrathiafulvalene (TTF)-based vesicle fabricated in water solution with 1 vol.% tetrahydrofuran that could be prevented by chemical oxidant Fe(ClO4)3 or electron-deficient cyclobis(paraquat-p-phenylene) tetracation cyclophane (CBPQT4+) is described.
Tetrathiafulvalene and its derivatives (TTFs) have attracted extensive research interest as strong electron donors for the development of organic electrical conductors and superconductors over the past decades.35–37 In addition, they are also used as building blocks in macromolecular and supramolecular systems for various applications.38,39 Zhao and Li reported the first self-assembling vesicles from TTF derivatives in organic solvents.40 However, their self-assembling behaviors and applications in aqueous environments have been far less explored41 owing to the pronounced hydrophobicity of TTFs. Recently, we reported that a water-soluble small molecular weight amphiphile with one TTF unit can self-assemble into micelles in water.42 The encouraging result prompted us to design a new different amphiphile 1 containing two TTF units and to explore further its assembly behaviors in aqueous environment (Scheme 1). In comparison to the reported amphiphilic molecule,42 the introduction of one more TTF units in 1 results in the whole molecular structural shape changing from wedge and dumbbell (Fig. S1†).43,44 More importantly, the self-assembly of dumbbell-shaped 1 in aqueous solution results in TTF-based vesicles and shows different self-assembly aggregates as a result of the new balance and orientation of hydrophobic moiety and hydrophilic chain.
The synthesis of amphiphile 1 was carried out as described in Scheme 2. The starting N-Boc protected compound 2 with hydrophilic group was deprotected under excess trifluoroacetic acid (TFA). Subsequently, an excess of triethylamine (TEA) was slowly added to neutralize TFA. The generated amine was directly coupled with 3,5-diiodobenzoic acid with the aid of PyBOP to give compound 3. The Sonogashira reaction of 3 with an excess (trimethylsilyl)acetylene (TMSA) yielded 4, which was then deprotected by excess K2CO3 to afford the terminal alkyne compound 5. Then, 5 was reacted with 2-iodotetrahiafulvalene to produce target amphiphilic compound 1. The target compound was identified by 1H NMR and 13C NMR spectroscopy, matrix-assisted laser desorption/ionization mass spectrometry (MALDI-TOF MS).
Due to the very limited solubility of 1 in water, it cannot directly dissolve into water for self-assembly. Therefore, a little amount of organic solvent tetrahydrofuran (THF) that may have some solvation or even templation effect was used to aid the solvation.6 Specifically, to initiate the self-assembly of 1 in aqueous solution, a schematic representation of the preparation procedure is shown in Fig. S2.† In a typical experiment, a concentrated THF solution of 1 (2 × 10−3 M, 20 μL) was injected into a rigoursly stirred water (2 mL) to induce aggregate formation.
The aggregated species of 1 in aqueous solution was corroborated by dynamic light scattering (DLS) experiments. CONTIN analysis of the DLS autocorrelation function of 1 shows a broad distribution of hydrodynamic diameter (Dh) of the aggregates centered at ∼70 nm in aqueous solution (Fig. 1a). The observation suggested the presence of large aggregates. Cryogenic transmission electron microscopy (cryo-TEM) was performed to gain a direct visualization of the size and morphology (Fig. 1b and S3†). As shown in Fig. 1b, these aggregate species have a typical vesicular structure by the distinct contrast between the dark periphery and the light central part with a diameter of 50–90 nm, which are consistent with the results of DLS measurement. The wall thickness (dark part) is estimated to be about 4–7 nm, which indicated the hydrophobic segments consisting of bilayer of amphiphilic molecules (Fig. 2). The driving force for the formation of vesicles of 1 lies with the balance of π–π and S–S interactions45–51 and solvophobic interactions between TTF cores and interior hydrophilic interaction. Moreover, the absorption spectra of 1 in water exhibited a weaker absorbance with a red shift than that in CH3CN (Fig. S4†), confirming the π–π interactions between TTF cores. We postulated, based on the evidence to be shown, that 1 self-assembled in a bilayer fashion with the hydrophilic ethoxy chains exposed both internally and externally to the polar solvent with the hydrophobic TTF groups interacting in the middle of the bilayer.
Then, we explore the possibility of prevention of such vesicular aggregates by oxidation or complexation with TTF unit. Firstly, we found it was difficult to fully oxidize the TTF units of 1 in aqueous solution to its cation TTF2+, as shown in Fig. S5.† Whereas only 4 equiv. of Fe(ClO4)3 was needed to produce dication 12+ (TTF2+) in CH3CN. Such differences could be interpreted by the fact that the TTF moiety is buried in the hydrophobic core of vesicular aggregate that is not easily accessible for Fe(ClO4)3. Due to the low solubility of 1 in water, we first oxidized the TTF unit to its dication TTF2+ in CH3CN, and then added it to deionized water for cryo-TEM measurement (Fig. 3, left). As shown in Fig. 3, no vesicular structures could be observed, indicating the prevention of such vesicular aggregates by the oxidation of TTF units to more hydrophilic TTF2+, together with the electrostatic repulsion between the oxidized species. However, the trial for regeneration of vesicles by addition of reductant, such as Vitamin C and NaBH4, was unsuccessful mainly due to the limited solubility of 1 in water.
Fig. 3 The representative cryo-TEM images of 1 with TTF units being fully oxidized by Fe3+ (left) or complexation with CBPQT4+ (right). |
Due to the high π-electron donating ability, TTFs can form stable charge-transfer complexes with electron-deficient species, tetracationic cyclobis(paraquat-p-phenylene) (CBPQT4+). Therefore, we took advantage of the complexation of 1 and CBPQT4+ in solution to disassemble the vesicular architectures. When amphiphile 1 and CBPQT4+ were mixed in water and CH3CN, respectively (Fig S6†), a green solution appeared immediately accompanied with a very broad charge-transfer band around 800 nm in the absorption spectra. Additional insight into the complexation of 1 and CBPQT4+ was obtained by means of 1H NMR spectroscopy studies in CD3CN (Fig. S7†). A mixture of 1 and CBPQT4+ (1:2) exhibited a quite different spectrum from that of amphiphile 1 itself in CD3CN. Upon addition of CBPQT4+ to solution, the proton H (●, ■) resonance of TTF unit shifted to upfield position, whereas the aromatic protons H (◆, ◊) neighboring the TTF unit shifted to downfield. These results suggested the formation of charge-transfer complex between 1 and CBPQT4+, and thereby leading to the strong shielding and deshielding effect of the aromatic rings in CBPQT4+. The disassembling vesicles of 1 upon complexation with CBPQT4+ were further studied using cryo-TEM techniques (Fig. 3, right). Fig. 3 (right) showed the typical cryo-TEM images of 1 after complexation with CBPQT4+. Also, no vesicular structures were observed, indicating the damage of vesicles. This prevention may be explained by considering the two factors: (1) the original hydrophobic part of 1 became more hydrophilic upon complexation with CBPQT4+; and (2) the strong electrostatic repulsion interactions between these terminus tetracationic species caused the separations of charge-transfer complexes.
Footnote |
† Electronic supplementary information (ESI) available: Detailed experimental procedures, and other supporting data. See DOI: 10.1039/c5ob02214b |
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