Dimerization of conjugated radical cations: an emerging non-covalent interaction for self-assembly.

FOCUS REVIEW DOI: 10.1002/asia.201402805

Dimerization of Conjugated Radical Cations: An Emerging Non-Covalent Interaction for Self-Assembly Dan-Wei Zhang,* Jia Tian, Lan Chen, Liang Zhang, and Zhan-Ting Li*[a]

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Dan-Wei Zhang, Zhan-Ting Li et al.

Abstract: Aromatic stacking has played a significant role in the design of discrete supramolecular systems. However, for many years, the stacking of conjugated radical cations has been considered only as a novelty due to its weakness. In recent years, it has been demonstrated that the stacking of conjugated radical cations, particularly those formed by bipyridinium or tetrathiafulvalene, can be enhanced remarkably when they are incorporated into rationally designed, preorganized frameworks or entrapped into a confined space. This Focus Review highlights the recent advance in its application as an emerging non-covalent force for the construction of various supramolecular structures. Keywords: bipyridinium · dimerization · radical cation · self-assembly · tetrathiafulvalene

1. Introduction Stacking is a common phenomenon of planar conjugated molecules or segments, which can occur inter- or intramolecularly, and both in solution and in the solid state. The driving force may be electrostatic attraction between electronrich aromatic donor(s) and electron-deficient aromatic acceptor(s),[1–4] as exemplified by the widely investigated tetrathiafulvalene (TTF)-viologen (V) donor–acceptor motif.[5–16] Solvophobicity is another important driving force for the stacking of identical or different conjugated molecules or segments,[17–19] as observed in water for the base pairs of DNA and the aromatic units of phenylalanine, tyrosine, and tryptophan residues in peptides and proteins.[20] Another unconventional stacking pattern involves the p-homodimerization of conjugated radical anions or cations.[21, 22] This stacking has been demonstrated theoretically to be driven mainly by the inherent multicenter covalent p–p bonding of radical cation species,[23, 24] which dominates the repulsive electrostatic interaction of the ions, even though solvophobicity and/or attractive electrostatic interaction of the counter ions may also contribute. However, the stacking should be a typical non-covalent interaction because for all the reported examples the stacking is always reversible, with no time dependence. In the past decades, the first two non-covalent interactions have played crucially important roles in the construction of supramolecular systems from conjugated molecules. This Focus Review describes the applications of the unusual dimerization of conjugated radical cations as a noncovalent force for the assembly of supramolecular structures. In 1964, Kosower et al. reported the dimerization of the methyl viologen (MV) radical cation,[25] which might represent the first example of investigations on the p-stacking of conjugated radical cations (Figure 1). In 1979, Torrance

Figure 1. Dimeric species of representative conjugated radical cations.

et al. described the dimerization of the TTF radical cation (Figure 1).[26] In addition to these two currently widely investigated dimeric species, dimeric species of many other conjugated radical cations have also been described. Some of them, that is, the dimers of zinc porphyrin,[27] oligothiophene (OLT),[28] oligopyrrole,[29] and pleiadiene radical cations,[30] are illustrated in Figure 1. Due to their low stability, these prototypical radical cation dimers have been observed only in the solid state or in concentrated solutions at low temperatures. Even under these conditions, the equilibrium between the paramagnetic free radical species and the diamagnetic dimer still favored the dissociated, monomeric state.[22] To enhance the stability of the dimeric state, several efficient strategies have been developed: 1) using a rigid macrocycle or capsule to encapsulate the dimer,[31–33] 2) holding two radical cation units in place with a preorganized framework,[34–46] and 3) stabilizing the dimer utilizing mechanical bonding in interlocked systems.[47–49] As a result, molecular blocks, in particular those derived from MV, TTF, and OLTbased molecules, can be driven by this unique p–p interaction to produce a variety of supramolecular structures. These advances are described in this Focus Review.

[a] Prof. Dr. D.-W. Zhang, J. Tian, L. Chen, L. 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]

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2. Stabilization of Radical Cation Dimers in a Confined Environment 2.1. Viologen Radical Cation In an aqueous solution containing 1 m KCl, the monomeric radical cation of MV 1· + , which could be produced by reducing 12 + with sodium dithionite, has an absorption band centered at 600 nm, while the dimer (1· + )2 produces an absorption band around 870 nm.[25] By recording the absorption spectra at different concentrations, an association constant (Ka) of 385 m 1 was obtained for the equilibrium (Figure 2), which implies that at millimolar concentrations, the radical cation exists mainly in the dissociated monomeric state. Reducing the temperature could increase the stability of radical cation dimers. For ethyl viologen (EV, as diiodide), the Ka was determined to be 1203 m 1 in MeOH at 11 8C from electron paramagnetic resonance (EPR) experiments.[50] The monomeric radical cations are paramagnetic, while the dimers are diamagnetic and thus EPR-silent.

Figure 2. Dimerization of methylviologen 1· + in water of 1 m salt concentration.

Therefore, EPR has been a useful technique for investigating their equilibrium in solution. Tsukahara et al. reported that introducing two aliphatic amide chains favors the dimerization of viologen radical cations.[51] In sodium phosphate buffer (10 mm, pH 7.0), the Ka values at 25 8C for the dimerization of radical cations 2 a· + , 2 b· + , and 2 c· + were determined to be 9.0 105, 4.0 104, and 2.0 104 m 1, respectively. These values are remarkably higher than that of 1· + . The stacking of the appended benzene or naphthalene units and hydrogen bonding formed by the amides might account for the enhancement.

Dan-Wei Zhang received her B.Sc. and Ph.D. at the Department of Chemistry, Fudan University, under the supervision of Prof. Shihui Wu. She had been a Research Associate with Prof. Dan Yang at the Department of Chemistry, The University of Hong Kong. She is currently Associate Professor at the same department. Her research focuses on the construction of artificial secondary structures and linear and macrocyclic conjugated systems.

Liang Zhang was born in Lanzhou in 1990. He obtained his B.Sc. degree from Fudan University in 2012 and is currently second-year graduate at Fudan University Chemistry Department. His research is involved in the construction of 2D supramolecular networks and the calculation on synthetic secondary structures.

Jia Tian was born in Henan Province of central China in 1989. He received his B.Sc. degree from South-Central University for Nationalities (Wuhan) in 2011. He is currently third-year Ph.D. candidate in Lis group. His research focuses on the self-assembly of ordered supramolecular polymers and frameworks.

Zhan-Ting Li was born in 1966 in Henan Province of central China. He received his B.S. from Zhengzhou University in 1985 and his Ph.D. in Organic Chemistry with Professor Qingyun Chen at Shanghai Institute of Organic Chemistry (SIOC), Chinese Academy of Sciences in 1992. He did postdoctoral research with Professor Jan Becher at the University of South Denmark and with Professor Steven C. Zimmerman at the University of Illinois at Urbana-Champaign. From 1996 to 2010, he worked at SIOC as Associate and Full Professor. In 2010, he became Professor at Department of Chemistry, Fudan University, Shanghai. His research interests include conjugated and porous supramolecular architectures and materials, biomimetic structures, and fundamental aspects of non-covalent forces. He has co-authored 180 peer-reviewed papers and reviews and 10 book chapters, and co-edited one book.

Lan Chen was born in 1989 in Lanzhou, China. He obtained his B.Sc. degree from East China University of Science and Technology in Shanghai in 2012 and is currently second-year Ph.D. candidate with Prof. Li. His research focuses on the self-assembly of ordered supramolecular structures based on aromatic stacking.

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Park et al. further investigated the effects of g-CD on the monomer–dimer equilibria of 1,1’-dialkylviologen radical cations 1· + , 5 a· + , 5 b· + , 5 c· + , and 5 d· + in aqueous KCl (0.1 m) solution by a spectroelectrochemical technique (Figure 3).[59] They found that, different from a-CD or b-CD which suppresses the dimerization of viologen radical cations via complexing the side chains, g-CD enhanced the dimerization by including the dimers using its expanded cavity. The Ka values of the complexes between these monomeric radical cations and g-CD were 5–35 m 1, while the values of the complexes between the radical cation dimers and g-CD were 85–3500 m 1. Clearly, g-CD favors the dimers due to better fitting of the dimers to its cavity. It was also found that the complex formed by dibutyl viologen radical cation 5 c· + had the highest stability.[60] Kim et al. reported that cucurbit[8]uril (CB[8]) could complex MV 12 + in 1:1 stoichiometry in phosphate buffer. The Ka was determined to be 1.1 105 m 1 at 25 8C.[61] When the viologen was reduced to the radical cation 1· + , a 1:2 complex, that is, CB[8] (1· + )2 (Figure 4), was generated exclusively. The apparent Ka for the dimerization of the radical Figure 4. Complexation-enhanced dimerization of radical cation was as high as 2 107 m 1, cation 1· + by CB[8], giving which is about 105 times higher a highly stable three-compothan that of 1· + alone in aque- nent complex. ous media. The remarkably high stability of the dimer was attributed to the ability of CB[8] to provide the dimer not only with a hydrophobic cavity of a correct size but also with sixteen polar oxygen atoms at the portals for ion– dipole interaction with the radical cation. Kaifer et al. had utilized this complexation-enhanced dimerization motif to assemble a dumbbell-like dendrimer.[62] Stoddart et al. found that the diradical dicationic cyclobis(paraquat-p-phenylene) (CBPQT2(· +)) ring formed 1:1 inclusion complexes with dialkylviologen radical cationic guests in acetonitrile.[47, 63] Crystal structure analysis and quantum mechanical calculations suggested the existence of radical cation-pairing interactions between the stacked viologen radical cations. For 1· + , using both isothermal titration calorimetry (ITC) and UV/Vis spectroscopy, the Ka of complex CBPQT2(· +) 1· + at 25 8C was determined to be 5.0 104 and 7.9 104 m 1 in acetonitrile, respectively. This recognition motif has been successfully utilized to tune the switching of bistable and tristable [2]rotaxanes.[64] The same group also used this complexation motif to construct viologen-derived [2]rotaxanes, as shown in Scheme 1.[65] The linear viologen radical cation-incorporated diazide 6 a–i· + reacted with ditert-butyl but-2-ynedioate 7 in acetonitrile to afford [2]rotaxanes 9 a–i3(· +). The two radical cations in CBPQT2(· +) could be oxidized by oxygen in air to dications. However, the radical cation in linear components 8 a–i· + was oxidized due to the Coulombic repulsion of CBPQT4 + .

We recently reported that the N-phenyl unit could also enhance the stacking of the radical cation by extending the conjugated moiety.[46] The Ka of the dimerization of radical cation 3· + was measured to be 3.5 103 m 1 in aqueous solution of sodium dithionite (50 mm) at 25 8C. Kaifer et al. found that, in the presence of the surfactant sodium dodecyl sulfate or sodium decyl sulfate at concentrations slightly above the corresponding critical micelle concentration, 1· + underwent extensive dimerization,[52] which was attributed to the promotion for the radical cation stacking by the micelles formed by the surfactants through entrapping the radical cations in micelles. Rabani et al. showed that, in Nafion films deposited on SnO2 electrodes, the dimerization of 1· + was weakened.[53] Buttry et al. reported that in self-assembled monolayers, the stacking of the MV radical cation was enhanced remarkably.[54] Recently, Iapalucci and Zuo reported that in crystals the dimer of viologen radical cation could be stabilized by paired anions.[55, 56] In all these cases, quantitative evaluation of the dimerization of the radical cations in solution was not described. In crystals, the promotion was reflected by the closer distance between the stacking radical cations.[57] Park et al. obtained radical cations 4 a· + (R = H), 4 b· + (R = Me), and 4 c· + (R = Et) via RuACHTUNGRE(bpy)32 + -sensitized photochemical reduction of the corresponding mono-6-(1-alkyl4,4’-bipyridino)-b-cyclodextrin (b-CD) molecules (Figure 3).[58] The Ka values for their dimerization in aque-

Figure 3. a) Dimerization of b-CD-appended radical cations 4 a· + , 4 b· + , and 2 c· + . b) g-CD-enhanced dimerization of radical cations 1· + , 5 a· + , 5 b· + , 5 c· + , and 5 d· + .

ous sodium chloride (0.1 m) solution were determined to be 4.0 104, 8.9 105, and 6.8 106 m 1, respectively. These values are substantially higher than those (500, 770, and 850 m 1, respectively) of the corresponding methylalkyl viologen radical cations. Addition of b-CD or amphiphilic aliphatic molecules suppressed the dimerization, which indicates that the dimers were stabilized by inclusion of the hydrophobic alkyl chain into the cavity of the appended b-CD (Figure 3).

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2.2. TTF Radical Cation The formation of p-dimers by TTF radical cations can be monitored by the appearance of a new long-wave absorption band as well as the deviation, weakening, and disappearance of the EPR signal intensity of the monomeric radical cations. For TTF itself, in ethanol (1 mm), the absorption band for (TTF· + )2 was observed only at 48 8C,[67] and the Ka for the dimerization of TTF· + was determined to be 0.6 m 1 at 2 8C in acetone.[68] These results clearly reflected the low stability of the dimers.[69] Veciana et al. prepared chlorinated radical cations 13 a· + and 13 b· + .[70] They demonstrated that the chlorinated triphenylmethyl radical moieties could promote the dimerization of the TTF· + unit. In dichloromethane at 25 8C, the Ka values of dimerization were determined to be 8.76 and 0.5 m 1, respectively, which are markedly higher than that of TTF· + (0.07 m 1). At 93 8C, the values were increased to 8440 and 9250 m 1, respectively, which are also higher than that of TTF· + (4.79 m 1).

Scheme 1. The formation of [2]rotaxanes 9 a–i· + .

Kim et al. reported that CB[8] could enhance the stability of (TTF· + )2 in water by encapsulation.[31] Thus, in the presence of CB[8], water-soluble 14 could be oxidized by oxygen in air to 14· + at ambient temperature. The radical cation dimerized in CB[8] to form the stable three-component encapsulation complex CB[8] (14· + )2 as a red precipitate (Figure 5), even though no quantitative binding study was reported.

Scheme 2. The formation of homo[2]catenane 124(· +) driven by radical cation stacking.

Figure 5. Complexation-enhanced dimerization of TTF radical cation 14· + by CB[8], giving a stable three-component complex.

More recently, a homo[2]catenane had been assembled by the Stoddart group using this recognition motif between radical cations (Scheme 2).[66] Remarkably, the resulting [2]catenane 124(· +) could further donate or accept four electrons to generate nine redox states in total. Through careful electrochemical analysis, six of the states (0, 2 + , 4 + , 6 + , 7 + , and 8 +) could be characterized.

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Becher et al. reported that the two TTF· + units incorporated in macrocycle 152(· +) underwent strong intramolecular stacking in acetonitrile.[35] Stoddart et al. also observed a similar enhancement of dimerization of TTF· + units in macrocycle 162(· +).[49] The same group further established that catenation could also remarkably stabilize the dimer (TTF· + )2.[47, 71] As a result, the dimerization of TTF· + was observed both in

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solution and in the crystal structure of [3]catenane 176 + . The entrapping of the two TTF· + units in the extended “blue box” also supports that p-stacking between the viologen cation and the TTF· + radical cation is also favorable enthalpically.


a pentyl chain to the centrally-positioned thiophene of the 5-mer caused the value of the resulting 20 e to decrease, presumably because of the flexibility of the chain which hindered the association of the cations.

Nishinaga et al. investigated the factors that affect the dimerization of oligothiophene radical cations which are annulated with bicycloACHTUNGRE[2.2.2]octane units.[76] It was revealed that the p-dimerization energy was affected by four factors, that is, 1) the SOMO–SOMO (singly occupied molecular orbital) interaction, 2) van der Waals forces, 3) solvation, and 4) Coulomb repulsion. The overlap of the SOMOs is sensitive to the structure of the p-dimer, while the first interaction is the most important factor affecting DHdim. Janssen et al. reported that the radical cations of endcapped oligopyrroles 21 a–c and oligopyrrole/thiophene hybrids could also undergo p-dimerization in organic solvents,[29, 77] which was supported by UV/Vis, near IR, and ESR spectra. For 3-mer 21 b, the Ka for the dimerization of its radical cation was determined to be 1.8 103 and 3.5 106 m 1 at 60 and 73 8C in dichloromethane that contained tetrabutylammonium hexafluorophosphate (0.1 m). Casado et al. prepared pentathiophene-incorporated macrocycle 22.[78] The two oligothiophene units could also undergo oneelectron oxidation to afford two stable radical cations. IR and Raman spectra supported the notion that the two radical cations underwent through-space charge delocalization to yield thermally accessible triplet states.

2.3. Oligothiophene Radical Cation Miller et al. reported that oligothiophene radical cations also dimerize in solution.[28, 72–74] In acetonitrile at room temperature, the Ka values of the dimerization of 18· + and 19· + were determined to be 250 and 1.0 104 m 1, respectively.[72, 73] Dimerization was not observed in dichloromethane because a more polar solvent was needed to solvate the dicationic p-dimers.

3. Intramolecular Dimerization 3.1. Viologen Radical Cation Bucher et al. prepared bisradical cations 232(· +),[79] 242(· +),[80] and 252(· +).[81, 82] In solution, the two radical cation units dimerized intramolecularly. Upon one-electron oxidation, the resulting dicationic viologens repulsed each other. For 24, this reversible process led to redox-controlled rotary motion of the backbone due to the rigidity of the ferrocene linker. However, in all the cases, no intermolecular dimerization was observed.

Buerle et al. studied the stacking of the radical cations of end-capped oligothiophenes 20 a–d in dichloromethane.[75] It was revealed that the DH at room temperature for their dimerization increased with increasing extension of the conjugated p-system, being 42, 58, 65, and 87 kJ m 1 for the 3-, 4-, 5-, and 6-mer molecules, respectively. Introducing

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3.3. Oligothiophene Radical Cation Swager et al. prepared calix[4]arene- and xanthene-based Ustyled compounds 29 and 30.[85, 86] Single-electron oxidation of the two oligothiophene units in dichloromethane by Et3O·SbCl6 afforded two radical cation units. Their face-toface orientation, due to the confined conformation of the calix[4]arene and xanthenes linker, promoted them to stack strongly to form intramolecular p-dimers, which were confirmed by UV/Vis and EPR spectra.

3.2. TTF Radical Cation Hasegawa et al. prepared compounds 26 a–c in which the two TTF units are appended to the naphthalene ring in a face-to-face way.[39] When the two TTF units were oxidized to TTF· + in a 1:1 mixture of dichloromethane and acetonitrile, strong intra-molecular p-stacking was exhibited for the two TTF· + units, as evidenced by the appearance of a typical absorption band around 732 nm in the UV/Vis spectrum. Such intramolecularly enhanced p-stacking of the TTF· + units has also been observed for several other TTF derivatives or macromolecules.[37, 41, 83, 84] The stacking strength of the radical cations depended on their structure and the polarity of the solvent. In linear oligomers or polymers,[41, 83, 84] the TTF· + stacking could take place successfully from both sides, as observed for 274(· +).[83]

4. Intermolecular Dimerization 4.1. Viologen Radical Cation Zhao and co-workers recently reported radical cation monomers 31· + , 322(· +), and 333(· +) by reducing the corresponding viologen derivatives with Zn dust or sodium dithionate in water.[42] Different from 232(· +), 242(· +), and 252(· +), whose viologen radical cations underwent intramolecular stacking exclusively, 322(· +) and 333(· +) gave rise to stable 1 + 1 macrocyclic and capsular aggregates (Figure 6), respectively, through intermolecular p-dimerization of the radical cation units in water. The (apparent) Ka values (Table 1) for the dimerization of their viologen radical cation were determined in aqueous media by using a UV/Vis dilution method. In all the studied solvents, the Ka value increased from monotopic 31· + to ditopic 322(· +), and then to 333(· +). The fact that the values of 322(· +) and 333(· +) are more than 7600 times higher

Sessler et al. designed and prepared macrocycle 28.[40] The central Schiff-base calixpyrrole could complex two Pd2 + ions in a 4N-PdII motif, which induced the compound to form a saddle-styled conformation and to force the two TTF units to close. As a result, upon one-electron oxidation, the two resulting TTF· + units could stack to form an intramolecular dimer. Without the complexation, the compound could not undergo intramolecular TTF· + dimerization.

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Figure 6. Schematic presentation of radical cation-dimerization-induced formation of a) a 1 + 1 macrocycle by ditopic 322(· +) and b) a 1 + 1 capsule by tritopic 333(· +).

Table 1. The (apparent) dimerization equilibrium constants (KD) of the V· + unit in sodium phosphate buffer (pH 7.0) of aqueous binary media at 25 8C.[a]

31· + 322(· +) 333(· +) 434(· +)

Water

THF (15 %)

1,4-Dioxane (15 %)

EtOH (25 %)

MeOH (35 %)

1.0 103 7.6 106[b] 7.6 106[b] 7.6 106[b]

-[c] 9.5 104 7.1 105 9.7 105

-[c] 3.4 105 8.4 105 1.1 106

-[c] 2.9 105 9.6 105 1.5 106

-[c] 3.7 105 1.9 106 4.4 106

[a] In the presence of excess sodium dithionite (50 mm). [b] Estimated by assuming 95 % of the monomeric V· + to dimerize at the total [V· + ] = 2.5 10 5 m. [c] Too low to be evaluated accurately.

than that of 31· + in water suggests positive cooperation for the stacking of the radical cations of 322(· +) and 333(· +). Figure 7. TTF· + -dimerization-driven formation of clip dimer 362(· +)·362(· +).

dergo intramolecular stacking due to the rigidity of the glycoluril linker. Instead, a stable dimer 362(· +)·362(· +) was formed (Figure 7), which was stabilized by multiple intermolecular radical stacking interactions.[38] The radical cation dimer was characterized by UV/Vis spectroscopy and MS spectrometry. A systematic computational investigation was performed to determine the key interactions that govern the formation of this kind of dimers.[88] We recently prepared compound 37 in which two TTF units are appended to an aromatic amide linker.[43] The four intramolecular N H···N hydrogen bonds induced the backbone to adopt a preorganized conformation,[89] which directed the two TTF units to locate on the same side of the backbone. As a result, when the TTF units underwent one-electron oxidation, the resulting two TTF· + units dimerized intramolecularly to form a stable unimolecular non-covalent macrocycle in a less polar solvent like dichloromethane (Figure 8 a). In binary mixtures (1:1) of dichloromethane with more polar acetone, ethanol, or acetonitrile, the TTF· + units dimerized intermolecularly to give rise to bimolecular macrocycle 372(· +)·372(· +) (Figure 8 b). The apparent Ka values for the TTF· + dimerization at 25 8C were determined to be 1.0 104, 3.9 103, and 5.9 102 m 1, respectively. In contrast, in these binary media, no similar p-dimerization was observed for the simple TTF control. We also prepared compounds 38 and 39 and oxidized their TTF units to TTF· + .[44] The TTF· + units of the resulting radical species 382(· +) and 393(· +) did not stack intramolecularly. In organic solvents, they were revealed to form a bimolecular macrocyclic or capsular entity, respectively, through

Positive cooperation of the radical cation stacking was also observed for planar rigid ditopic 342(· +).[46] The apparent Ka (2.5 104 m 1) for the dimerization of its viologen radical cation units in water is also significantly higher than that of 35· + (3.5 103 m 1). Probably due to the three hydrophilic side chains, which might impose hindrance, the Ka of 342(· +) is considerably lower than that of 322(· +).

4.2. Tetrathiafulvalene Radical Cation Chiu et al. developed the rigid TTF molecular clip 36 for redox-tuned donor–acceptor binding.[87] When the TTF units were oxidized to TTF· + , the two radical cations did not un-

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5. Dendrimer 5.1. Viologen Radical Cation Crooks et al. reported that viologen radical cation units connected to the periphery of poly(amidoamine) (PAMAM) dendrimers underwent considerable intradendrimer dimerization which dominated interdendrimer dimerization within different concentration limits.[90] Wang et al. found that a similarly enhanced intradendrimer dimerization was exhibited for viologen radical cation units appended to hyperbranched polyglycerols.[91] However, due to the flexibility of the central dendrimers, no explicit stacking pattern was proposed. Trabolsi et al. prepared compound 4012 + by appending six viologen units to a phosphazene core.[92] The viologen units could be encapsulated by CB[7] in water to form [n]pseudorotaxanes.[93] When the viologen units were reduced to radical cations by sodium dithionate, the radical cation units in the resulting compound 406(· +) stacked intramolecularly to form three pairs of dimers.[92] The two viologen units connected to one P atom forms a U-shaped conformation. Thus, the above intramolecular dimerization has been ascribed to such three pairs of radical cations. This intramolecular p-dimerization was stable, even in the presence of CB[7], although CB[7] could form a 1:1 inclusion complex with the viologen radical cation. The two self-assembled systems could be switched by tuning the redox properties of the viologen units.

Figure 8. TTF· + -dimerization-driven formation of a) a unimolecular macrocycle and b) a bimolecular macrocycle by 372(· +), the formation of which depends on the polarity of the solvents.

Table 2. Apparent association constants (Ka) [M 1] of the TTF· + units of TTF derivatives in different solvents at 25 8C. Solvent PhMe/CHCl3 (1:1) CH2Cl2 CHCl3 MeCN MeCN/H2O (1:1) H2O

382(· +)

393(· +) 3

8.1 10 –[a] 6.3 104 4.5 103 2.8 104 –[b]

44 a4(· +) 4

4.5 10 –[a] 1.1 105 4.7 104 3.6 104 –[b]

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3.3 10 4.3 105 3.7 106 1.5 105 5.3 104 –[b]

44 b4(· +) 1.3 104 1.1 104 1.7 104 6.8 104 1.3 105 1.7 105

[a] The absorption band of dimer (TTF· + )2 was too weak to be measured. [b] The samples were not soluble.

intermolecular dimerization of these TTF· + units in a similar way as illustrated for 322(· +) and 333(· +) in Figure 6. The apparent Ka values of the dimerization of their TTF· + units in different solvents were determined by UV/Vis dilution experiments and are listed in Table 2. In all the solvents, the value of tritopic 393(· +) was higher than that of ditopic 382(· +). This result is in accordance with the proposed bimolecular binding motif in which they have two and three dimerization interactions, respectively. In all these solvents, the radical cation of simple dialkylthio-substituted TTF derivatives did not dimerize to an observable extent, which again supports the positive cooperativity of the radical cation dimerization of 382(· +) and 393(· +).

5.2. TTF Radical Cation It has been demonstrated that TTF· + units that are appended to the periphery of dendrimers can also undergo intradendrimer dimerization.[94, 95] For several water-soluble den-

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them stacked to form mixed-valence state (TTF)2· + , and the apparent Ka values for this dimerization at 25 8C were estimated to be 2.3 106 and 2.5 106 m 1, respectively. This strong radical cation stacking also caused both of them to form entangled fibrous materials.

6. Supramolecular Polymer drimeric species, it has been reported that the addition of CB[7] would dissociate the dimers through the formation of a more stable 1:1 inclusion complex.[95] Iyoda et al. prepared rigid planar star-styled TTF compounds 41 and 42.[96, 97] In a mixture of dichloromethane and acetonitrile (2:1 v/v), triangular 41 could be easily oxidized by FeACHTUNGRE(ClO4)3 to 413(· +).[96] This triradical species could stack intermolecularly in one-dimensional space to generate fibrous aggregates. The neutral 42 already stacks in chloroform.[97] Oxidation of 42 measured by cyclic voltammetry in CH2Cl2 shows a broad wave of the first six-electron oxidation to form 426(· +) and a second sharp six-electron oxidation wave to form 4212 + . The broadness of the first oxidation wave indicates complex aggregation of different radical cation states in solution. Analytically pure 42· + and 423(· +) could be obtained by chemical oxidation of 42 with FeACHTUNGRE(ClO4)3. The UV/Vis/NIR spectra of these two radical species in CHCl3 showed that both of

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6.1. Viologen Radical Cation Zhao and co-workers prepared the tetrahedral radical cation 434(· +) in water by reducing the corresponding octacation precursor with sodium dithionate.[42] Compared with that of ditopic 322(· +) and tritopic 333(· +), the apparent Ka value of 434(· +) for the intermolecular viologen radical cation dimerization was further increased in aqueous media (see Table 1), which reveals the largest positive cooperativity. Dynamic light scattering experiments showed that both 322(· +) and 333(· +) formed small aggregates, while 434(· +) could produce aggregates of about 34 nm diameter. This result indicates that tetratopic 434(· +) self-assembled into large threedimensional (3D) supramolecular networks driven by the strong intermolecular dimerization of the appended viologen radical cation. The formation of the 3D supramolecular polymer was also supported by EPR and CV experiments.

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did not. This result is also consistent with the formation of 3D supramolecular polymers.

6.2. TTF Radical Cation We also prepared tetrahedral TTF derivatives 44 a and 44 b for constructing 3D supramolecular polymers in both organic and aqueous media.[44] It was found that when the four TTF units were oxidized to the radical cation TTF· + , their preorganized tetrahedral feature remarkably enhanced the intermolecular dimerization of the TTF· + units, which led to the formation of new 3D spherical supramolecular polymers. The structures of the supramolecular polymers were assessed by UV/Vis absorption, EPR, CV, and DLS experiments. DLS revealed that the spherical supramolecular polymers possessed a hydrodynamic diameter of 68 nm for 44 a4(· +) (75 mm) in acetonitrile and 105 nm for 44 a4(· +) (75 mm) in water and acetonitrile (1:1). The existence of 3D spherical structures of the supramolecular polymers formed in different solvents was also supported by SEM and AFM experiments. The apparent Ka values for the dimerization of the TTF· + units of 44 a4(· +) and 44 b4(· +) in different solvents were also determined (Table 2). In all the studied solvents, the value of 44 a4(· +) was higher than that of 382(· +) and 393(· +), which again suggests the formation of a 3D network by 44 a4(· +). The fact that the TTF· + units of both 44 a4(· +) and 44 b4(· +) undergo dimerization in chloroform and dichloromethane, which do not favor aromatic stacking,[18, 98] also indicates that the TTF· + dimerization is driven mainly by the inherent multicenter covalent p–p bonding or SOMO– SOMO interaction, which dominates the electrostatic repulsion of the two cations.[23, 24]

7. Two-Dimensional Supramolecular Organic Framework A two-dimensional (2D) mono-layer supramolecular organic framework (SOF) has been assembled from a rigid triangular precursor by encapsulating an appended hydrophobic aromatic unit into the cavity of CB[8] in water.[99] We recently prepared water-soluble viologen derivatives 47 a6 + and 47 b6 + .[46] Their viologen units could be readily reduced to the radical cation by sodium dithionate. The resulting 47 b3(· +) stacked strongly in a face-to-face manner. CB[8] could encapsulate the stacked viologen radical cations to generate a 2 + 3 complex. However, the three amide chains in 47 a3(· +) prevented it from stacking in a similar manner. Instead, it stacked into a honeycomb-like 2D supramolecular organic framework, as shown in Figure 9, which was stabilized by intermolecular viologen radical cation dimeriza-

To evaluate the effect of the linker on the intermolecular stacking of the TTF· + unit, we also prepared tetrahedral TTF derivatives 45 and 46,[45] whose four TTF units are connected to the central tetraphenylmethane by the more rigid ethynyl or amide linker. After the TTF units were oxidized to TTF· + , both compounds also assembled into 3D supramolecular polymers. The apparent Ka values of their TTF· + dimerization in chloroform were determined to be lower than that of 44 a4(· +). SEM or AFM imaging showed that all the radical cation samples of 44 a, 44 b, 45, and 46 gave rise to nanometer-scale particles, but the di- or tritopic controls

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their respective property characters. The radical cation of viologen can be easily oxidized by oxygen in air. Thus, studies of its dimerization have to be performed under the protection of an inert gas or in the presence of excess reducing agent. The radical cation of tetrathiafulvalene can dimerize in solvents of both high and low polarity and is able to form a mixed-valence state of high stability with neutral tetrathiafulvalene. This unique feature may find more applications in the design of redox-tunable multi-state supramolecular devices. One valuable nature of the radical cation dimerization is the orthogonality of the binding. Thus, a combination of this binding motif with other recognition motifs may lead to the formation of many new kinds of supramolecular systems. Although the radical cation dimerizations of both viologen and tetrathiafulvalene have been demonstrated to be useful noncovalent forces, the development of new radical cation dimerizations should also be of high value in the future. In this context, the stacking of the oligothiophene radical cation is expected to be promising, considering its different linear structural feature and extensive applications in the construction of advanced organic materials. The dimerization of radical anions, such as single-electron reduced tetracyanoethylene,[100] has been recognized for many years. However, the potential of such kind of stacking patterns in self-assembly and supramolecular chemistry has not been explored. Efforts along this line may also lead to new binding motifs that are useful in the creation of new supramolecular structures.

Figure 9. Schematic presentation of the honeycomb-like 2D supramolecular organic framework formed in water by 473(· +) (top) through viologen radical cation dimerization, which is further enhanced by CB[8] through encapsulating the stacked dimer (bottom) in its cavity.

tion. The apparent Ka of this dimerization in water at 25 8C was determined to be 1.2 106 m 1, which was remarkably higher than that (3.5 103 m 1) of a monomeric control. CB[8] could further enhance the dimerization and consequently form 2D frameworks through encapsulation (Figure 9), and the corresponding Ka for the dimerization could be increased to 4.5 106 m 1. The new supramolecular networks have been characterized by UV/Vis absorption, EPR, DLS, as well as solution and solid-phase small-angle X-ray diffraction experiments. The single-layer feature of the radical cation dimerization-driven, CB[8]-enhanced SOF was confirmed by AFM imaging as the average thickness of the supramolecular framework (1.74 nm) was perfectly consistent with the diameter of CB[8] (1.75 nm), which controlled the thickness of the mono-layer aggregate.

Acknowledgements 8. Summary and Outlook We are grateful to the Ministry of Science and Technology of the China (2013CB834501) and The Ministry of Education (IRT1117) and Research Fund for the Doctoral Program of China, The Science and Technology Commission of Shanghai Municipality (13M1400200), and The National Natural Science Foundation of China (Nos. 91227108 and 21228203) for financial support.

In this Focus Review, we have summarized the typical radical cation dimerization patterns, the routes for enhancing this non-covalent bonding, and its applications in constructing new supramolecular structures. Currently, most of the advances have focused on the dimerization of the radical cation of viologen and tetrathiafulvalene. Although viologen and tetrathiafulvalene are both redox-reversible, they have

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FOCUS REVIEW With force and determination: The stacking of conjugated radical cations may be very strong, is orthogonal, and works in organic and aqueous media. This Focus Review highlights recent advances in its application as an emerging non-covalent force for the construction of various supramolecular architectures.

Radical Cations Dan-Wei Zhang,* Jia Tian, Lan Chen, Liang Zhang, &&&&—&&&& Zhan-Ting Li* Dimerization of Conjugated Radical Cations: An Emerging Non-Covalent Interaction for Self-Assembly

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Diaryldichalcogenide radical cations.

Noncovalent interaction of carbon nanostructures.

Modulation of DNA Polymerase Noncovalent Kinetic Transitions by Divalent Cations.

Biomimetic oxidative dimerization of anodically generated stilbene radical cations: effect of aromatic substitution on product distribution and reaction pathways.

ion reactions.

Peptide radical cations: gender determines dissociation chemistry.

Mechanistic aspects of hydration of guanine radical cations in DNA.

Fe65-PTB2 Dimerization Mimics Fe65-APP Interaction.

Evaluation of composite schemes for CCSDT(Q) calculations of interaction energies of noncovalent complexes.

Independent Generation and Reactivity of Thymidine Radical Cations.

Radical-cation dimerization overwhelms inclusion in [N]pseudorotaxanes.

Hydroxyl radical formation in the gas phase oxidation of distonic 2-methylphenyl radical cations.

Two phosphaalkene radical cations with inverse spin density distributions.

Distinguishing conventional and distonic radical cations by using dimethyl diselenide.

Electron transfer from plant phenolates to carotenoid radical cations. Antioxidant interaction entering the Marcus theory inverted region.

Dimerization of human immunodeficiency virus (type 1) RNA: stimulation by cations and possible mechanism.

Disruption of Rhodopsin Dimerization with Synthetic Peptides Targeting an Interaction Interface.

Interaction of two mechanisms regulating alkali cations in HeLa cells.

Catalytic olefin hydroamination with aminium radical cations: a photoredox method for direct C-N bond formation.

The interaction of organic cations with the mitochondrial membrane.

Ferrocenyl chiral bisphosphorus ligands for highly enantioselective asymmetric hydrogenation via noncovalent ion pair interaction.

Competitive interaction of monovalent cations with DNA from 3D-RISM.

Drosophotoxicology: An Emerging Research Area for Assessing Nanoparticles Interaction with Living Organisms.

Poly-arginine conjugated triarylmethyl radical as intracellular spin label.