FULL PAPER DOI: 10.1002/asia.201301729
Tetrathiafulvalene-Based Macrocycles Formed by Radical Cation Dimerization: The Role of Intramolecular Hydrogen Bonding and Solvent Wei-Kun Wang,[a] Yuan-Yuan Chen,[a] Hui Wang,*[a] Dan-Wei Zhang,[a] Yi Liu,*[b] and Zhan-Ting Li*[a] Abstract: Compounds 1 a and 1 b were prepared by appending two tetrathiafulvalene (TTF) units to an aromatic amide segment that is driven by six or two intramolecular NH···O hydrogen bonds to adopt a folded conformation. UV/Vis absorption experiments revealed that if the TTF units were oxidized to TTFC + radical cations, the two compounds could form a stable single molecular noncovalent macrocycle in less polar dichloromethane or dichloro-
ethane or a bimolecular noncovalent macrocycle in a binary mixture of dichloromethane with a more polar solvent owing to remarkably enhanced dimerization of the TTFC + units. The stability of the (TTFC + )2 dimer was evaluated through UV/Vis absorption, elecKeywords: foldamers · hydrogen bonds · macrocyclization · radical cation dimerization · solvent effects
Introduction
tron paramagnetic resonance, and cyclic voltammetry experiments and also by comparing the results with those of control compound 2. The results showed that introduction of the intramolecular hydrogen bonds played a crucial role in promoting the stability of the (TTFC + )2 dimer and thus the noncovalent macrocyclization of the two backbones in both uni- and bimolecular manners.
drive the framework to form a new kind of 3D supramolecular network,[13] which demonstrates the utility of this inherently weak (TTFC + )2 dimer as a basic binding motif for the assembly of new supramolecular architectures. Hydrogen-bonding-induced aromatic amide oligomers may adopt folded conformations.[14–18] In the past decade, this structural preorganization has been widely applied to construct a variety of macrocyclic systems by facilitating the formation of amide bonds,[19] coordination bonds,[20] imine and hydrazone bonds,[21] and triazole rings.[22] Examples of making use of the backbones to promote intramolecular donor–acceptor interactions have also been reported.[23] We herein describe that preorganized, folded arylamide segments can remarkably enhance the dimerization of two appended TTFC + units, which leads to the formation of noncovalent uni- and bimolecular macrocycles that is tunable by solvents.
In the past decades, tetrathiafulvalene (TTF) has received great attention as a prototypical electron donor in supramolecular and materials chemistry because of its reversible one- and two-electron redox character.[1–7] Although it was long ago reported that upon its one-electron oxidation, the resulting TTFC + radical cation unit could dimerize in the solid state,[8] the low stability of the dimer limited its application as a useful binding motif for molecular recognition and self-assembly.[9] In the last decade, several strategies have been developed to stabilize this radical cation dimer, including holding two TTF units together with a preorganized framework,[10] using a rigid macrocycle to encapsulate the dimer,[11] and entrapping two TTFC + units in an interlocked system through mechanical bonds.[12] We recently reported that TTFC + units appended to a rigid tetrahedral framework underwent remarkably enhanced dimerization to
Results and Discussion [a] W.-K. Wang, Y.-Y. Chen, Dr. H. Wang, Prof. Dr. D.-W. Zhang, Prof. Dr. Z.-T. Li Department of Chemistry, Fudan University 220 Handan Road, Shanghai 200433 (China) Fax: (+ 86) 21-6564-1740 E-mail: [email protected] [email protected]
Compounds 1 a and 1 b, which form six and two intramolecular hydrogen bonds, respectively, were designed and prepared. Owing to intramolecular hydrogen bonding, both compounds were expected to adopt a V-shaped conformation, which should facilitate the stacking of the TTF units, once oxidized to TTFC + , from one side. Comparison of their stacking tendency would reveal the effect of the intramolecular hydrogen bonds. Compound 2 was used as a control. The synthesis routes to 1 a and 1 b are provided in Scheme 1. For the synthesis of 1 a, 6 was first prepared in 72 % yield from the coupling of 4[19j] and 5.[24] Then, 7[13] was treated
[b] Prof. Dr. Y. Liu The Molecular Foundry Lawrence Berkeley National Laboratory Berkeley, CA 94720 (USA) E-mail: [email protected] Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/asia.201301729.
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Scheme 1. The synthesis of 1 a, 1 b, and 2. DIEA = N,N-diisopropylethylamine, DMAP = 4-(N,N-dimethylamino)pyridine.
dized to the TTFC + radical cation (Figure S1, Supporting Information), no absorption of the (TTFC + )2 dimer was observed. Instead, its spectrum only gave rise to the absorption of the TTFC + radical cation at approximately 717 nm. The above observations suggest that the dimerization of the TTFC + radical cation formed by ditopic compounds 1 a and 1 b was substantially strengthened relative to the dimerization of a simple TTF derivative. The dimerization of the two TTFC + units occurred cooperatively, and thus, the possibility of forming linear oligomers or polymers can be excluded. Because the appended TTF units are quite far away from the amide units, it is reasonable to assume that the formation of the radical cation would not impose an important effect on the intramolecular NH···OR hydrogen bonding. Thus, we propose that the dimerization of the TTFC + units took place through the formation of a cyclic structure, which was favored by their hydrogen-bonding-driven folded conformation. It is expected that in such a cyclic structure, the TTFC + radical cations can be stabilized by the formation of the (TTFC + )2 dimer. To investigate the effect of the solvent on this cyclization process, we also recorded the UV/Vis spectra of 1 a and 1 b in a 1:1 v/v mixture of dichloromethane with other solvents of varying polarity (Figure 2). It can be seen that, for both samples, in a mixture of dichloromethane with another solvent of comparable polarity, the strong absorption at approximately 865 nm for the (TTFC + )2 dimer was also exhibited, together with the absorption of the TTFC + unit at approximately 670 nm. In all other systems involving more polar solvents, the strong absorption of the TTFC + unit at approximately 715 nm was observed. In mixtures with ace-
with bromoacetic acid in the presence of cesium hydroxide to produce 8 in 80 % yield. Further coupling of 6 and 8 in the presence of 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDCI) and hydroxybenzotriazole (HOBt) afforded 1 a in 40 % yield. For the preparation of 1 b, 10 was first synthesized in 56 % yield from the reaction of 4 and 9 and then treated with 8 to give 1 b in 42 % yield. Compound 2 was prepared from 8 and n-octylamine in 70 % yield. In the 1H NMR spectrum of 1 a (2.0 mm) in CDCl3, the signals of H-a and H-b appear at d = 9.70 and 8.27 ppm, respectively, whereas in the spectrum of 1 b (2.0 mm), these signals appear at d = 9.65 and 6.53 ppm, respectively. These results showed that the NH protons of 1 a and the Ha proton of 1 b were all engaged in intramolecular N H···OR hydrogen bonding. The UV/Vis spectra of 1 a, 1 b, and 2 in dichloromethane were first recorded for different oxidation states of TTF, which were obtained by adding different equivalents of FeACHTUNGRE(ClO4)3 with respect to the concentration of TTF.[13, 25] For 1 a, the addition of 2.0 equivalents of FeACHTUNGRE(ClO4)3 caused the TTF units to be oxidized to the TTFC + radical cation, which exhibited two strong absorption bands at approximately 650 and 890 nm (Figure 1 a). The first band is typical for the single TTFC + radical cation,[11, 12] whereas the second is the typical absorption of the (TTFC + )2 dimer.[12] The TTFC + radical cation could be further oxidized to TTF2 + by the addition of another 2.0 equivalents of FeACHTUNGRE(ClO4)3, which was evidenced by the formation of the typical absorption band at approximately 569 nm and the disappearance of the absorptions of both TTFC + and (TTFC + )2.[11, 12] Similar results were obtained for 1 b (Figure 1 b). In contrast, after 2 was oxi-
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Figure 2. UV/Vis absorption spectra of the TTF· + radical cation of a) 1 a in 1:1 mixtures of dichloromethane with dichloroethane (I), acetone (II), ethanol (III), THF (IV, 0.05 mm), acetonitrile (V), and methanol (VI) and of b) 1 b in 1:1 mixtures of dichloromethane with dichloroethane (I), acetone (II), THF (III, 0.035 mm), ethanol (IV), and acetonitrile (V) at 25 8C. The concentration was 0.1 mm unless otherwise indicated.
Figure 1. UV/Vis absorption spectra of a) 1 a (0.08 mm) with the addition of I) 0, II) 0.5, III) 1.0, IV) 1.5, V) 2.0, VI) 2.5, VII) 3.0, VIII) 3.5, and IX) 4.0 equivalents of FeACHTUNGRE(ClO4)3 and of b) 1 b (0.08 mm) with the addition of I) 0, II) 0.50, III) 1.0, IV) 1.5, V) 2.0, VI) 3.0, and VII) 4.0 equivalents of FeACHTUNGRE(ClO4)3 in dichloromethane at 25 8C.
tone, ethanol, and acetonitrile, 1 a produced a shoulder band at approximately 930 nm; this suggests the formation of a weaker (TTFC + )2 dimer. A similar shoulder band was also observed for 1 b in a mixture with acetone or ethanol but not with MeCN. In more polar methanol, neither compound gave rise to the absorption of the (TTFC + )2 dimer. Owing to solubility limits, the spectra of 1 a and 1 b in the mixture with THF were recorded at lower concentrations of 0.05 and 0.035 mm. Both solutions displayed a quite strong absorption band of the (TTFC + )2 dimer. Again, in all these binary solvents, 2 in the TTFC + state did not exhibit the absorption of the (TTFC + )2 dimer, but only the strong absorption band of the monomer at approximately 714 nm (Figure S2). These results indicated that increasing the polarity of the medium weakened or suppressed the dimerization of the TTFC + state of 1 a and 1 b. Another reason is that their conformational flexibility was also increased as a result of the weakening of the intramolecular hydrogen bonding. UV/Vis dilution experiments were then performed for 1 a and 1 b in the TTFC + state in dichloromethane or in a 1:1 v/v mixture with dichloroethane (Figures S3–S5). It was found that within the studied concentration range, the absorbance of the TTFC + and (TTFC + )2 states was linearly correlated
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Figure 3. Plots of (TTF· + )2 absorption versus concentration, highlighting a linear correlation for 1 a in dichloromethane (&) and in dichloromethane/dichloroethane (1:1, ~) and for 1 b in dichloromethane (*) and in dichloromethane/dichloroethane ( ! ) at 25 8C.
with the concentration (Figure 3) and thus obeyed Beers Law, which supported the fact that the dimerization of the TTFC + unit occurred intramolecularly (Figure 4 a). For 1 a, the molar extinction coefficients (e) of the maximum absorption of the (TTFC + )2 dimer in dichloromethane and in
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with acetone, ethanol, or acetonitrile, the (TTFC + )2 absorption fitted well to a 1:1 binding mode,[13, 26] and the apparent association constants (Ka) were determined to be 1.0 104, 3.9 103, and 5.9 102 m1 (Figures S8–S10), respectively. Using the same method, the apparent Ka of the dimer of the TTFC + unit of 1 b in a mixture with ethanol was evaluated to be 1.8 103 m1. In mixtures of more polar acetonitrile, the absorption was too weak to be used for quantitative evaluation of the Ka values. Because the TTFC + unit of control 2 did not undergo dimerization in any of these solvent systems, the enhanced dimerization of the TTFC + unit of 1 a and 1 b should be attributed to the formation of a 1+1 macrocyclic structure (Figure 4 b). The fact that the (TTFC + )2 dimer of 1 a was more stable than that of 1 b showed that the extra four intramolecular hydrogen bonds of 1 a could further promote the formation of the macrocycle. Electron paramagnetic resonance (EPR) experiments were further performed to investigate the dimerization of the TTFC + unit of 1 a, 1 b, and 2 in dichloromethane and in a 1:1 v/v mixture with ethanol. For comparison, the total concentration of the TTF unit was kept at 0.2 mm for all of the studied samples, and TTF was quantitatively oxidized to TTFC + by FeACHTUNGRE(ClO4)3. All the samples exhibited the typical signal of the paramagnetic TTFC + radical cation (Figure 5). However, the intensity of this signal was substantially different. In dichloromethane, the signals of both 1 a and 1 b were weak, but for the latter, the signal was approximately two times stronger than that for the former. In contrast, the signal of 2 was very strong: it was approximately 35 times stronger than that of 1 a. These results are consistent with the TTFC + radical cation of 1 a and 1 b undergoing spin–spin coupling to form the diamagnetic EPR-silent (TTFC + )2 dimer, and this process was even stronger for 1 a, whereas the dimerization of TTFC + of 2, if any, was quite weak. Compared with that in dichloromethane, the signal intensity of 1 a, 1 b, and 2 in the more polar 1:1 mixture of dichloromethane/ethanol increased by 12, 11, and 0.09 times, respectively. The fact that the signals of 2 in both media were quite
Figure 4. Proposed a) intramolecular and b) intermolecular dimerization of the TTF· + radical cation units of ditopic 1 a and 1 b.
a 1:1 mixture with dichloroethane were 1.0 104 and 1.1 104 m1 cm1, respectively, and for 1 b, the values were 9.7 103 and 7.9 103 m1 cm1, respectively. These data are quite close, and this suggests that the two intramolecular N H···OR hydrogen bonds in 1 b are able to efficiently induce a folded conformation of the two appended TTFC + units so that they can stack intramolecularly. The results in the two solvent systems also reflect the comparable polarity of the two solvents. The UV/Vis spectra of 1 a and 1 b in the TTFC + state were further recorded at varying concentrations in other binary solvents (1:1 v/v), and the spectra exhibited the absorption of the (TTFC + )2 dimer. For both compounds in a mixture of dichloromethane/tetrahydrofuran (1:1), a nonlinear decrease in the absorption of the (TTFC + )2 dimer was observed (Figures S6 and S7), which, however, did not fit the 1:1 binding stoichiometry. This result suggests that the dimerization might take place both intra- and intermo- Figure 5. EPR spectra of a) 1 a, b) 1 b, and c) 2 in dichloromethane (I) and dichloromethane/ethanol (II, 1:1 v/ lecularly. In contrast, for 1 a in v), which were recorded at 25 8C after the addition of 1.0 equivalent of FeACHTUNGRE(ClO4)3 relative to the amount of mixtures of dichloromethane [TTF] (0.2 mm).
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comparable further supported the fact that the TTFC + unit did not dimerize noticeably, whereas the remarkable signal enhancement observed for 1 a and 1 b in the polar binary solvent indicated a substantial weakening of the dimerization of the TTFC + unit and an increase in the concentration of the monomeric TTFC + unit. All these observations were in accordance with the above UV/Vis experiments, which support the formation of single molecular macrocycles by 1 a and 1 b in dichloromethane. The electrochemical properties of 1 a, 1 b, and 2 were also systematically studied by recording their cyclic voltammograms in different solvents, including dichloromethane and its 1:1 mixtures with dichloroethane, acetone, acetonitrile, tetrahydrofuran, and ethanol. All the compounds exhibited two typical one-electron oxidation and reduction processes (Figure 6 and Figures S12–S17). In all the mixed polar solvents, the first and second oxidation and reduction potentials of the three compounds were close, and the difference was up to 0.03 V. In less polar dichloromethane and dichloroethane, the first oxidation and second reduction potentials were also comparable. However, the second oxidation and first reduction potentials, which correspond to the TTFC + !TTF2 + and TTF2 + !TTFC + processes, were notably different. For example, in dichloromethane, relative to the values of 2 (1.01 and 0.94 V), the second oxidation and first reduction potentials of 1 a (0.90 and 0.82 V) and 1 b (0.89
Hui Wang, Yi Liu, Zhan-Ting Li et al.
and 0.83 V) were notably lower. This shifting of 1 a and 1 b to lower potentials may be rationalized by considering that their TTFC + units, formed through the one-electron oxidation of neutral TTF or one-electron reduction of dication TTF2 + , underwent intramolecular stacking to give (TTFC + )2 and to the formation of a noncovalent unimolecular macrocycle. The macrocyclization might generate strain, which would facilitate the oxidation of the TTFC + unit to TTF2 + , or it may have delayed the reduction of the TTF2 + unit to TTFC + . In more polar binary media, the dimerization of the TTFC + unit, if any, was weak, and thus the related redox potentials of the TTF units of the three compounds were all comparable. Reducing the scan rate from 200 to 100 mV s1 did not cause a clear change in the redox potentials of the three compounds (Figures S12–S17). Generally, the reduction waves of 1 a and 1 b were stronger than those of control 2. This result may be rationalized by considering that the increased ability of the TTFC + units to undergo dimerization made it easier for the TTF2 + unit to accept electrons to produce TTFC + , and it was also easier for the (TTFC + )2 dimer to accept electrons than for the single TTFC + unit. As a result, both processes exhibited stronger waves.
Conclusions We have demonstrated that the intramolecular hydrogenbonding-driven folded conformation of aromatic amide segments can be utilized to direct the formation of macrocyclic structures by enhancing the dimerization of the appended TTF· + radical cations, which are inherently weak. Depending on the polarity of the solvent, the macrocyclization may take place intramolecularly or in a 1+1 binding pattern. Considering the easy access to TTF derivatives, the strong dimerization of the TTF· + radical cation in solution bodes well for further application of this stacking pattern for the design of new supramolecular polymers or duplexes by appending multiple TTF units to a preorganized linear backbone. Because TTF is a typical electron donor with reversible redox properties, this increased stacking pattern of the radical cations may also be utilized to create new redoxtuned interlocked supramolecular architectures.
Acknowledgements We are grateful to the Ministry of Science and Technology (2013CB834501), the Ministry of Education (IRT1117), the Science and Technology Commission of Shanghai Municipality (13M1400200), and the Postdoctoral Science Foundation and National Natural Science Foundation (91227108, 21228203, and J1103304) of China for financial support. Y.L. thanks the support of the Molecular Foundry, Lawrence Berkeley National Laboratory, supported by the Office of Science, Office of Basic Energy Sciences, Scientific User Facilities Division, the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. Figure 6. Cyclic voltammograms of 1 a (I), 1 b (II), and 2 (III) in a) dichloromethane and b) dichloromethane/ethanol (1:1 v/v, [TTF] = 0.2 mm). Supporting electrolyte = Bu4NPF6 (0.2 m), scan rate = 200 mV s1.
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Tetrathiafulvalene-Based Macrocycles Formed by Radical Cation Dimerization: The Role of Intramolecular Hydrogen Bonding and Solvent Wei-Kun Wang,[a] Yuan-Yuan Chen,[a] Hui Wang,*[a] Dan-Wei Zhang,[a] Yi Liu,*[b] and Zhan-Ting Li*[a] Abstract: Compounds 1 a and 1 b were prepared by appending two tetrathiafulvalene (TTF) units to an aromatic amide segment that is driven by six or two intramolecular NH···O hydrogen bonds to adopt a folded conformation. UV/Vis absorption experiments revealed that if the TTF units were oxidized to TTFC + radical cations, the two compounds could form a stable single molecular noncovalent macrocycle in less polar dichloromethane or dichloro-
ethane or a bimolecular noncovalent macrocycle in a binary mixture of dichloromethane with a more polar solvent owing to remarkably enhanced dimerization of the TTFC + units. The stability of the (TTFC + )2 dimer was evaluated through UV/Vis absorption, elecKeywords: foldamers · hydrogen bonds · macrocyclization · radical cation dimerization · solvent effects
Introduction
tron paramagnetic resonance, and cyclic voltammetry experiments and also by comparing the results with those of control compound 2. The results showed that introduction of the intramolecular hydrogen bonds played a crucial role in promoting the stability of the (TTFC + )2 dimer and thus the noncovalent macrocyclization of the two backbones in both uni- and bimolecular manners.
drive the framework to form a new kind of 3D supramolecular network,[13] which demonstrates the utility of this inherently weak (TTFC + )2 dimer as a basic binding motif for the assembly of new supramolecular architectures. Hydrogen-bonding-induced aromatic amide oligomers may adopt folded conformations.[14–18] In the past decade, this structural preorganization has been widely applied to construct a variety of macrocyclic systems by facilitating the formation of amide bonds,[19] coordination bonds,[20] imine and hydrazone bonds,[21] and triazole rings.[22] Examples of making use of the backbones to promote intramolecular donor–acceptor interactions have also been reported.[23] We herein describe that preorganized, folded arylamide segments can remarkably enhance the dimerization of two appended TTFC + units, which leads to the formation of noncovalent uni- and bimolecular macrocycles that is tunable by solvents.
In the past decades, tetrathiafulvalene (TTF) has received great attention as a prototypical electron donor in supramolecular and materials chemistry because of its reversible one- and two-electron redox character.[1–7] Although it was long ago reported that upon its one-electron oxidation, the resulting TTFC + radical cation unit could dimerize in the solid state,[8] the low stability of the dimer limited its application as a useful binding motif for molecular recognition and self-assembly.[9] In the last decade, several strategies have been developed to stabilize this radical cation dimer, including holding two TTF units together with a preorganized framework,[10] using a rigid macrocycle to encapsulate the dimer,[11] and entrapping two TTFC + units in an interlocked system through mechanical bonds.[12] We recently reported that TTFC + units appended to a rigid tetrahedral framework underwent remarkably enhanced dimerization to
Results and Discussion [a] W.-K. Wang, Y.-Y. Chen, Dr. H. Wang, Prof. Dr. D.-W. Zhang, Prof. Dr. Z.-T. Li Department of Chemistry, Fudan University 220 Handan Road, Shanghai 200433 (China) Fax: (+ 86) 21-6564-1740 E-mail: [email protected] [email protected]
Compounds 1 a and 1 b, which form six and two intramolecular hydrogen bonds, respectively, were designed and prepared. Owing to intramolecular hydrogen bonding, both compounds were expected to adopt a V-shaped conformation, which should facilitate the stacking of the TTF units, once oxidized to TTFC + , from one side. Comparison of their stacking tendency would reveal the effect of the intramolecular hydrogen bonds. Compound 2 was used as a control. The synthesis routes to 1 a and 1 b are provided in Scheme 1. For the synthesis of 1 a, 6 was first prepared in 72 % yield from the coupling of 4[19j] and 5.[24] Then, 7[13] was treated
[b] Prof. Dr. Y. Liu The Molecular Foundry Lawrence Berkeley National Laboratory Berkeley, CA 94720 (USA) E-mail: [email protected] Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/asia.201301729.
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Scheme 1. The synthesis of 1 a, 1 b, and 2. DIEA = N,N-diisopropylethylamine, DMAP = 4-(N,N-dimethylamino)pyridine.
dized to the TTFC + radical cation (Figure S1, Supporting Information), no absorption of the (TTFC + )2 dimer was observed. Instead, its spectrum only gave rise to the absorption of the TTFC + radical cation at approximately 717 nm. The above observations suggest that the dimerization of the TTFC + radical cation formed by ditopic compounds 1 a and 1 b was substantially strengthened relative to the dimerization of a simple TTF derivative. The dimerization of the two TTFC + units occurred cooperatively, and thus, the possibility of forming linear oligomers or polymers can be excluded. Because the appended TTF units are quite far away from the amide units, it is reasonable to assume that the formation of the radical cation would not impose an important effect on the intramolecular NH···OR hydrogen bonding. Thus, we propose that the dimerization of the TTFC + units took place through the formation of a cyclic structure, which was favored by their hydrogen-bonding-driven folded conformation. It is expected that in such a cyclic structure, the TTFC + radical cations can be stabilized by the formation of the (TTFC + )2 dimer. To investigate the effect of the solvent on this cyclization process, we also recorded the UV/Vis spectra of 1 a and 1 b in a 1:1 v/v mixture of dichloromethane with other solvents of varying polarity (Figure 2). It can be seen that, for both samples, in a mixture of dichloromethane with another solvent of comparable polarity, the strong absorption at approximately 865 nm for the (TTFC + )2 dimer was also exhibited, together with the absorption of the TTFC + unit at approximately 670 nm. In all other systems involving more polar solvents, the strong absorption of the TTFC + unit at approximately 715 nm was observed. In mixtures with ace-
with bromoacetic acid in the presence of cesium hydroxide to produce 8 in 80 % yield. Further coupling of 6 and 8 in the presence of 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDCI) and hydroxybenzotriazole (HOBt) afforded 1 a in 40 % yield. For the preparation of 1 b, 10 was first synthesized in 56 % yield from the reaction of 4 and 9 and then treated with 8 to give 1 b in 42 % yield. Compound 2 was prepared from 8 and n-octylamine in 70 % yield. In the 1H NMR spectrum of 1 a (2.0 mm) in CDCl3, the signals of H-a and H-b appear at d = 9.70 and 8.27 ppm, respectively, whereas in the spectrum of 1 b (2.0 mm), these signals appear at d = 9.65 and 6.53 ppm, respectively. These results showed that the NH protons of 1 a and the Ha proton of 1 b were all engaged in intramolecular N H···OR hydrogen bonding. The UV/Vis spectra of 1 a, 1 b, and 2 in dichloromethane were first recorded for different oxidation states of TTF, which were obtained by adding different equivalents of FeACHTUNGRE(ClO4)3 with respect to the concentration of TTF.[13, 25] For 1 a, the addition of 2.0 equivalents of FeACHTUNGRE(ClO4)3 caused the TTF units to be oxidized to the TTFC + radical cation, which exhibited two strong absorption bands at approximately 650 and 890 nm (Figure 1 a). The first band is typical for the single TTFC + radical cation,[11, 12] whereas the second is the typical absorption of the (TTFC + )2 dimer.[12] The TTFC + radical cation could be further oxidized to TTF2 + by the addition of another 2.0 equivalents of FeACHTUNGRE(ClO4)3, which was evidenced by the formation of the typical absorption band at approximately 569 nm and the disappearance of the absorptions of both TTFC + and (TTFC + )2.[11, 12] Similar results were obtained for 1 b (Figure 1 b). In contrast, after 2 was oxi-
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Figure 2. UV/Vis absorption spectra of the TTF· + radical cation of a) 1 a in 1:1 mixtures of dichloromethane with dichloroethane (I), acetone (II), ethanol (III), THF (IV, 0.05 mm), acetonitrile (V), and methanol (VI) and of b) 1 b in 1:1 mixtures of dichloromethane with dichloroethane (I), acetone (II), THF (III, 0.035 mm), ethanol (IV), and acetonitrile (V) at 25 8C. The concentration was 0.1 mm unless otherwise indicated.
Figure 1. UV/Vis absorption spectra of a) 1 a (0.08 mm) with the addition of I) 0, II) 0.5, III) 1.0, IV) 1.5, V) 2.0, VI) 2.5, VII) 3.0, VIII) 3.5, and IX) 4.0 equivalents of FeACHTUNGRE(ClO4)3 and of b) 1 b (0.08 mm) with the addition of I) 0, II) 0.50, III) 1.0, IV) 1.5, V) 2.0, VI) 3.0, and VII) 4.0 equivalents of FeACHTUNGRE(ClO4)3 in dichloromethane at 25 8C.
tone, ethanol, and acetonitrile, 1 a produced a shoulder band at approximately 930 nm; this suggests the formation of a weaker (TTFC + )2 dimer. A similar shoulder band was also observed for 1 b in a mixture with acetone or ethanol but not with MeCN. In more polar methanol, neither compound gave rise to the absorption of the (TTFC + )2 dimer. Owing to solubility limits, the spectra of 1 a and 1 b in the mixture with THF were recorded at lower concentrations of 0.05 and 0.035 mm. Both solutions displayed a quite strong absorption band of the (TTFC + )2 dimer. Again, in all these binary solvents, 2 in the TTFC + state did not exhibit the absorption of the (TTFC + )2 dimer, but only the strong absorption band of the monomer at approximately 714 nm (Figure S2). These results indicated that increasing the polarity of the medium weakened or suppressed the dimerization of the TTFC + state of 1 a and 1 b. Another reason is that their conformational flexibility was also increased as a result of the weakening of the intramolecular hydrogen bonding. UV/Vis dilution experiments were then performed for 1 a and 1 b in the TTFC + state in dichloromethane or in a 1:1 v/v mixture with dichloroethane (Figures S3–S5). It was found that within the studied concentration range, the absorbance of the TTFC + and (TTFC + )2 states was linearly correlated
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Figure 3. Plots of (TTF· + )2 absorption versus concentration, highlighting a linear correlation for 1 a in dichloromethane (&) and in dichloromethane/dichloroethane (1:1, ~) and for 1 b in dichloromethane (*) and in dichloromethane/dichloroethane ( ! ) at 25 8C.
with the concentration (Figure 3) and thus obeyed Beers Law, which supported the fact that the dimerization of the TTFC + unit occurred intramolecularly (Figure 4 a). For 1 a, the molar extinction coefficients (e) of the maximum absorption of the (TTFC + )2 dimer in dichloromethane and in
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with acetone, ethanol, or acetonitrile, the (TTFC + )2 absorption fitted well to a 1:1 binding mode,[13, 26] and the apparent association constants (Ka) were determined to be 1.0 104, 3.9 103, and 5.9 102 m1 (Figures S8–S10), respectively. Using the same method, the apparent Ka of the dimer of the TTFC + unit of 1 b in a mixture with ethanol was evaluated to be 1.8 103 m1. In mixtures of more polar acetonitrile, the absorption was too weak to be used for quantitative evaluation of the Ka values. Because the TTFC + unit of control 2 did not undergo dimerization in any of these solvent systems, the enhanced dimerization of the TTFC + unit of 1 a and 1 b should be attributed to the formation of a 1+1 macrocyclic structure (Figure 4 b). The fact that the (TTFC + )2 dimer of 1 a was more stable than that of 1 b showed that the extra four intramolecular hydrogen bonds of 1 a could further promote the formation of the macrocycle. Electron paramagnetic resonance (EPR) experiments were further performed to investigate the dimerization of the TTFC + unit of 1 a, 1 b, and 2 in dichloromethane and in a 1:1 v/v mixture with ethanol. For comparison, the total concentration of the TTF unit was kept at 0.2 mm for all of the studied samples, and TTF was quantitatively oxidized to TTFC + by FeACHTUNGRE(ClO4)3. All the samples exhibited the typical signal of the paramagnetic TTFC + radical cation (Figure 5). However, the intensity of this signal was substantially different. In dichloromethane, the signals of both 1 a and 1 b were weak, but for the latter, the signal was approximately two times stronger than that for the former. In contrast, the signal of 2 was very strong: it was approximately 35 times stronger than that of 1 a. These results are consistent with the TTFC + radical cation of 1 a and 1 b undergoing spin–spin coupling to form the diamagnetic EPR-silent (TTFC + )2 dimer, and this process was even stronger for 1 a, whereas the dimerization of TTFC + of 2, if any, was quite weak. Compared with that in dichloromethane, the signal intensity of 1 a, 1 b, and 2 in the more polar 1:1 mixture of dichloromethane/ethanol increased by 12, 11, and 0.09 times, respectively. The fact that the signals of 2 in both media were quite
Figure 4. Proposed a) intramolecular and b) intermolecular dimerization of the TTF· + radical cation units of ditopic 1 a and 1 b.
a 1:1 mixture with dichloroethane were 1.0 104 and 1.1 104 m1 cm1, respectively, and for 1 b, the values were 9.7 103 and 7.9 103 m1 cm1, respectively. These data are quite close, and this suggests that the two intramolecular N H···OR hydrogen bonds in 1 b are able to efficiently induce a folded conformation of the two appended TTFC + units so that they can stack intramolecularly. The results in the two solvent systems also reflect the comparable polarity of the two solvents. The UV/Vis spectra of 1 a and 1 b in the TTFC + state were further recorded at varying concentrations in other binary solvents (1:1 v/v), and the spectra exhibited the absorption of the (TTFC + )2 dimer. For both compounds in a mixture of dichloromethane/tetrahydrofuran (1:1), a nonlinear decrease in the absorption of the (TTFC + )2 dimer was observed (Figures S6 and S7), which, however, did not fit the 1:1 binding stoichiometry. This result suggests that the dimerization might take place both intra- and intermo- Figure 5. EPR spectra of a) 1 a, b) 1 b, and c) 2 in dichloromethane (I) and dichloromethane/ethanol (II, 1:1 v/ lecularly. In contrast, for 1 a in v), which were recorded at 25 8C after the addition of 1.0 equivalent of FeACHTUNGRE(ClO4)3 relative to the amount of mixtures of dichloromethane [TTF] (0.2 mm).
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comparable further supported the fact that the TTFC + unit did not dimerize noticeably, whereas the remarkable signal enhancement observed for 1 a and 1 b in the polar binary solvent indicated a substantial weakening of the dimerization of the TTFC + unit and an increase in the concentration of the monomeric TTFC + unit. All these observations were in accordance with the above UV/Vis experiments, which support the formation of single molecular macrocycles by 1 a and 1 b in dichloromethane. The electrochemical properties of 1 a, 1 b, and 2 were also systematically studied by recording their cyclic voltammograms in different solvents, including dichloromethane and its 1:1 mixtures with dichloroethane, acetone, acetonitrile, tetrahydrofuran, and ethanol. All the compounds exhibited two typical one-electron oxidation and reduction processes (Figure 6 and Figures S12–S17). In all the mixed polar solvents, the first and second oxidation and reduction potentials of the three compounds were close, and the difference was up to 0.03 V. In less polar dichloromethane and dichloroethane, the first oxidation and second reduction potentials were also comparable. However, the second oxidation and first reduction potentials, which correspond to the TTFC + !TTF2 + and TTF2 + !TTFC + processes, were notably different. For example, in dichloromethane, relative to the values of 2 (1.01 and 0.94 V), the second oxidation and first reduction potentials of 1 a (0.90 and 0.82 V) and 1 b (0.89
Hui Wang, Yi Liu, Zhan-Ting Li et al.
and 0.83 V) were notably lower. This shifting of 1 a and 1 b to lower potentials may be rationalized by considering that their TTFC + units, formed through the one-electron oxidation of neutral TTF or one-electron reduction of dication TTF2 + , underwent intramolecular stacking to give (TTFC + )2 and to the formation of a noncovalent unimolecular macrocycle. The macrocyclization might generate strain, which would facilitate the oxidation of the TTFC + unit to TTF2 + , or it may have delayed the reduction of the TTF2 + unit to TTFC + . In more polar binary media, the dimerization of the TTFC + unit, if any, was weak, and thus the related redox potentials of the TTF units of the three compounds were all comparable. Reducing the scan rate from 200 to 100 mV s1 did not cause a clear change in the redox potentials of the three compounds (Figures S12–S17). Generally, the reduction waves of 1 a and 1 b were stronger than those of control 2. This result may be rationalized by considering that the increased ability of the TTFC + units to undergo dimerization made it easier for the TTF2 + unit to accept electrons to produce TTFC + , and it was also easier for the (TTFC + )2 dimer to accept electrons than for the single TTFC + unit. As a result, both processes exhibited stronger waves.
Conclusions We have demonstrated that the intramolecular hydrogenbonding-driven folded conformation of aromatic amide segments can be utilized to direct the formation of macrocyclic structures by enhancing the dimerization of the appended TTF· + radical cations, which are inherently weak. Depending on the polarity of the solvent, the macrocyclization may take place intramolecularly or in a 1+1 binding pattern. Considering the easy access to TTF derivatives, the strong dimerization of the TTF· + radical cation in solution bodes well for further application of this stacking pattern for the design of new supramolecular polymers or duplexes by appending multiple TTF units to a preorganized linear backbone. Because TTF is a typical electron donor with reversible redox properties, this increased stacking pattern of the radical cations may also be utilized to create new redoxtuned interlocked supramolecular architectures.
Acknowledgements We are grateful to the Ministry of Science and Technology (2013CB834501), the Ministry of Education (IRT1117), the Science and Technology Commission of Shanghai Municipality (13M1400200), and the Postdoctoral Science Foundation and National Natural Science Foundation (91227108, 21228203, and J1103304) of China for financial support. Y.L. thanks the support of the Molecular Foundry, Lawrence Berkeley National Laboratory, supported by the Office of Science, Office of Basic Energy Sciences, Scientific User Facilities Division, the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. Figure 6. Cyclic voltammograms of 1 a (I), 1 b (II), and 2 (III) in a) dichloromethane and b) dichloromethane/ethanol (1:1 v/v, [TTF] = 0.2 mm). Supporting electrolyte = Bu4NPF6 (0.2 m), scan rate = 200 mV s1.
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