Anion-dependent selective formation of intermolecular non-covalent bonds

Journal of Molecular Structure 829 (2007) 168–175 www.elsevier.com/locate/molstruc

Anion-dependent selective formation of intermolecular non-covalent bonds Kanako Tsutsui a, Take-aki Koizumi a, Koji Tanaka

a,*

, Hideki Hayashi

b

a

b

Institute for Molecular Science, 5-1 Higashiyama, Myodaiji, Okazaki, Aichi 444-8787, Japan, and CREST, Japan Science and Technology Agency (JST), Japan Nagoya Municipal Industrial Research Institute, 3-4-41, Rokuban, Atsuta-ku, Nagoya, Aichi 456-0058, Japan Received 28 January 2006; received in revised form 18 June 2006; accepted 21 June 2006 Available online 14 August 2006

Abstract The crystal structures and packings of bis(pyridiniopropyl)benzene derivatives, [1,4-(4-R-C5H4N+CH2CH2CH2)2C6H4](PF6)2 (1, R = H; 2, R = tert-Bu), were investigated by X-ray diffraction studies. In compound 1, two pyridiniopropyl chains attached at the para positions of the phenylene group are in a linear conformation, and the central phenylene ring forms an intermolecular acceptor–donor– acceptor p–p stacking with neighboring two pyridinium rings. Compound 2 also has a linear conformation similar to 1, but intermolecular p–p stackings were not observed in the crystal stackings of 2. Instead, there is found a C–H    p interaction between the central phenylene ring and one of the hydrogen atoms in the tert-Bu group linked at the 4 position of pyridinium ring. The difference of the packing modes between 1 and 2 apparently results from steric hindrance caused by bulkiness of the substituents and counter anions. Ó 2006 Elsevier B.V. All rights reserved. Keywords: p-interactions; Donor-acceptor systems; Supramolecular chemistry; X-ray crystallography; Infrared spectroscopy

1. Introduction Non-covalent inter- and intramolecular forces, such as hydrogen bonds, p–p stackings, and C–H    p interactions have attracted much interest in the past years, since those interactions often play a key role in the crystal and molecular structures of various compounds [1], organic synthesis [2], biochemistry [3], macromolecular chemistry [4], and so on. When electron-rich aromatic rings (i.e., p-electron donor) and electron-deficient ones (i.e., p-electron acceptor) are linked with flexible chains such as poly(methylene), a donor–acceptor (D–A) type p–p stacking is formed in their crystal structures [5–7]. Recently, we have reported that dicationic bis(pyridiniopropyl)benzene and bis(p-tertbutylpyridiniopropyl)benzene selectively form linear and S-shaped configurations, respectively, in the crystal structure depending on inter- and intramolecular A–D–A type *

Corresponding author. Tel.: +81 564 59 5580; fax: +81 564 59 5582. E-mail address: [email protected] (K. Tanaka).

0022-2860/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2006.06.020

p–p stackings as well as an intermolecular C–H    p interactions. The observation that introduction of t-butyl group into the terminal pyridinium rings resulted in a remarkable change in the p–p stacking modes in the solid-state has driven us to regulate the molecular conformations and packing structures of bis(pyridiniopropyl)benzene analogs through the change of substituents and counter anions. The crystal structures and solid-state spectra of bis(pyridiniopropyl)benzene derivatives 1 and 2 having a bulky PF 6 counter anion are investigated in this work, 2. Experimental 2.1. Measurements and materials 1

H, 13C{1H}, and 1H–1H COSY NMR spectra were recorded on a JEOL GX 500 FT NMR spectrometer. ESI-MS spectra were obtained on a Shimadzu LCMS2010 spectrometer. Electronic spectra were recorded on a Shimadzu UV-Vis-NIR scanning spectrophotometer

K. Tsutsui et al. / Journal of Molecular Structure 829 (2007) 168–175

UV-3100PC and a JASCO V-570 spectrophotometer. Elemental analyses were conducted at the Molecular Scale Nano-Science Center of IMS. NH4PF6 was purchased from Wako Pure Chemical Industries. The syntheses of 1,1 0 -(1,4-Phenylenedi-3,1-propanediyl)bispyridinium diiodide (3a) [6], 1,1 0 -(1,4-Phenylenedi-3,1-propanediyl)bispyridinium dibromide (3b) [10], and 4,4 0 -Di-tert-butyl-1, 1 0 -(1,4-phenylenedi-3,1-propanediyl)bispyridinium dibromide (4) [6] have been previously reported. The numbering of protons is shown in Scheme 1.

169

tion and dried in vacuo, and colorless powder of 2 was obtained in a yield of 71% (0.31 g). 1H NMR (500 MHz in CD3CN): d 8.49 (d, 4H, J = 6.5 Hz, Ha), 7.94(d, 4H, J = 6.5 Hz, Hb), 7.10 (s, 4H, H(aromatic)), 4.46 (t, 4H, J = 7.5 Hz, Hc), 2.64 (t, 4H, J = 8.0 Hz, Ha), 2.32 (tt, 4H, J = 7.5 and 8.0 Hz, Hb), 1.37 (s, 9H, tert-Bu). MS [ESI (MeOH)]: m/z = 215 [M–2PF6]2+. C30H42F12N2 P2(720.30): Calcd C 50.00, H 5.87, N 3.89; Found C 50.05, H 5.92, N 3.83%. 2.4. X-ray crystallographic studies

2.2. Preparation of 1,1 0 -(1,4-Phenylenedi-3,1propanediyl)bispyridinium dihexafluorophosphate (1) To a methanol solution (2 mL) of 3a (0.15 g, 0.26 mmol) was added an NH4PF6 aqueous solution (10 mL). White precipitate was collected by filtration and dried in vacuo. Colorless powder of 1 was obtained in a 66% yield (0.11 g). 1H NMR (500 MHz in CD3CN): d 8.64 (d, 4H, J = 5.5 Hz, Ha), 8.47 (t, 2H, J = 8.0 Hz, Hc), 7.99 (t, 4H, J = 7.5 Hz, Hb), 7.12 (s, 4H, H(aromatic)), 4.53 (t, 4H, J = 7.5 Hz, Hc), 2.65 (t, 4H, J = 8.0 Hz, Ha), 2.45 (tt, 4H, J = 7.5 and 8.0 Hz, Hb). MS [ESI (MeCN)]: m/z = 159 [M–2PF6]2+. C22H26F12N2 P2 (608.39): Calc. for C, 43.43; H, 4.31; N, 4.60; Found C, 43.33; H, 4.33; N, 4.55%. 2.3. Preparation of 4,4 0 -Di-tert-butyl-1,1 0 -(1,4-phenylenedi3,1-propanediyl)bispyridinium dihexafluorophosphate (2) To a methanol solution (2 mL) of 4 (0.36 g, 0.60 mmol) was added an NH4PF6 aqueous solution (10 mL). White solid precipitated out of the solution was collected by filtra-

R

R

N+

N+

NH4 PF6 aq.

2 X-

N+

N+ R

R

3a : X = I , R = H 3b : X = Br , R = H 4 : X = Br, R = tert-Bu

Hp

2 PF6-

Ha

Hc N+ H

1 : X = PF6 , R = H 2 : X = PF6 , R = tert-Bu

Hα Hβ

b

R

Scheme 1.

(1)

Suitable crystals were mounted on a glass fiber. Data collections for 1 were performed at 100 °C on a Rigaku/MSC Mercury CCD diffractometer with graphite ˚ ), and2 monochromated Mo-Ka radiation (k = 0.71070 A was performed at 100 °C on a Rigaku/MSC Saturn CCD area detector with graphite monochromated Mo˚ ). The structure of 1 was Ka radiation (k = 0.71070 A solved and refined by using the teXsan software package [11], and 2 was solved by using the CrystalStructure package [12]. Atomic scattering factors were obtained from the literature [13]. Refinements were performed anisotropically for all non-hydrogen atoms by the full-matrix least-squares method. In compound 1, hydrogen atoms were placed at calculated positions and were included in the structure calculation without further refinement of the parameters. In compound 2 [14], hydrogen atoms were located from the difference map and refined isotropically. H(8), H(29), and H(37) were also located, however, further refinement was Table 1 Crystal data and details of the structure refinement of 1 and 2 Compound

1

2

Formula Mol wt Cryst syst Space group ˚) a (A ˚ b (A) ˚) c (A b (deg) ˚ 3) V (A Z l (cm1) F (000) Dcalcd (g cm1) No. of reflns measured No. of unique reflns No. of reflns used No. of variables Rint GOF R1a R Rw

C22H26F12N2P2 608.39 monoclinic P21/c (No. 14) 6.138(4) 13.077(8) 15.717(10) 96.375(7) 1253(1) 2 2.78 620.00 1.612 9944 5440 5308 172 0.051 1.449 0.063 0.083 0.141b

C30H42 F12N2P2 720.60 monoclinic P21/c (No. 14) 15.882(9) 19.251(10) 17.444(9) 109.658(3) 5022(5) 6 2.21 2244.00 1.429 36713 11142 11142 861 0.093 0.571 0.056 0.177 0.073c

a R1 = RiFojjFci/RjFoj for I > 2.0r(I) data, Rw = R [wðF 2o  F 2c Þ2 = RwðF 2o Þ2 1=2 . b w ¼ 1=½r2 ðF 2o Þ þ ð0:07800ðMaxðF 2o ; 0Þ þ 2F 2c Þ=3Þ2 . c w ¼ 1=½1:0000rðF 2o Þ.

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not achieved. Two hydrogen atoms (in C(30) and C(44)) could not be located. The residual electron densities were of no chemical significance. Crystal data and processing parameters are summarized in Table 1. CCDC-294973 (1), and CCDC-294974 (2) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge at www.ccdc.cam.ac.uk/conts/retrieving.html [or from the Cambridge Crystallographic Data Centre, 12,

Union Road, Cambridge CB2 1EZ, UK; Fax: (internet.) +44-1223/336-033; E-mail:[email protected]]. 2.5. DFT calculation The DFT method employed in this study is Beck’s threeparameter hybrid model using the Lee–Yang–Parr correlation functional (B3LYP) with 6-31G basis set (denoted B3LYP/6-31G(d)).

Table 2 ˚ ) and angles (deg) of 1 Selected bond lengths (A C1–C2 C2–C3* C4–C5 N1–C6 N1–C11 C8–C9 C10–C11 C1    C9a C3    C10a C1–C4–C5 N1–C6–C5 a b

1.396(2) 1.386(3) 1.518(2) 1.480(2) 1.347(2) 1.370(3) 1.369(3) 3.568(3) 3.584(3) 116.1(1) 111.4(1)

C1–C3 C1–C4 C5–C6 N1–C7 C7–C8 C9–C10 C3    C11

394(3) 1.511(2) 1.518(2) 1.346(2) 1.370(3) 1.382(3) b

3.581(3)

C4–C5–C6

110.2(1)

Symmetry operations: X, Y+1/2, Z1/2. X+1, Y1/2, Z+1/2.

Table 3 ˚ ) and angles (deg) of 2 Selected bond lengths (A

Fig. 1. ORTEP drawing of 1 with 50% thermal ellipsoids. Hydrogen atoms and PF 6 are omitted for clarity.

A

B

C13 C14 C12 C9 C8

C15 C10

C7

C11 N1 C5 C2

C23 C24 C28 C29

394(3)

C4–C5–C6 C16–C19–C20 C20–C21–N2 C34–C35–C36

113.4(5) 112.7(5) 111.2(4) 113.2(5)

C10    C45a C3    C43b C17    C43b C17    H54b

3.728(12) 3.581(3) 3.723(12) 3.21(6)

Symmetry operations: X, Y+1/2+1, Z+1/21. X1, Y+1, Z+1.

C41

C35

C34

C32

C3

C33 C31*

C32*

C17

C34*

N3* C37*

C41*

C26 C25

C40* C39* C42*

C27

b

C35*

C36*

N2

C22

N2–C21

N3

C31 C33*

C20 C21

C37

a b

C4 C1

C18 C19

C45 C38

2.396(2) 1.386(3) 110.9(5) 110.8(3) 112.9(5) 110.8(5) 112.1(4) 3.522(10) 3.406(10) 3.584(3) 2.97(4)

C36 C6

C16

C44 C42 C39 C40

C43

N1–C6 N3–C36 C1–C4–C5 C5–C6–N1 C19–C20–C21 C31–C34–C35 C35–C36–N3 N1    C45 a C11    C45a C16    C43b C11    H59a

C38* C45*

a

C43*

C30

C44*

Fig. 2. ORTEP drawings of 2 with 50% thermal ellipsoids. There are two independent molecules in the unit cell. Hydrogen atoms and PF 6 are omitted for clarity.

c

0

Fig. 3. Packing diagram of 1. Hydrogen atoms and counter anions are omitted for clarity.

K. Tsutsui et al. / Journal of Molecular Structure 829 (2007) 168–175

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zene ring, and the other (A) does not have any symmetrical elements in the molecule. In each structure, two pyridiniopropyl groups attached at the para position of phenylene are in a linear fashion, and two pyridine rings are situated at mutually an opposite side from the phenylene ring-containing plane. Recently, we have reported the crystal structures of 3a and 4 [6]. A non-substituted pyridinium compound 1 has a similar conformation with corresponding iodide 3a. In contrast, the conformations of tert-Bu substituted pyridinium derivatives 2 and 4 having PF 6 and Br, respectively, as an counter ion are very different from each other. Thus, not only substituents of the pyridium but also counter ions cause crucial conformational changes in the solid-state structures (vide infra). The dihedral angle of the central phenylene ring and one terminal pyridinium ring in 1 is 94.17(5)°, which is more acute than that of 3a (104.0(1)°). The crystal packing of 1 is shown in Fig. 3. The central phenylene ring is stacked by two pyridinium rings of neighboring molecules from upper and lower sides, and acceptor–donor–acceptor (A–D–A) triplets are formed. The overall crystal structure is also almost similar to that of 3a, and PF 6 anions are situated at the corresponding position with I in 3a (see supporting

3. Results and discussion 3.1. Preparation of compounds 1 and 2 Compounds 1 and 2 were prepared by the anion exchange reaction of corresponding halide compounds (3 and 4 [6]) with excess amount of NH4PF6 in water (eq 1). Each compound was obtained in good yield (1, 66%; 2, 71%). Results of the elemental analysis of 1 and 2 indicated that the anion exchange had progressed completely. 3.2. Crystal structures of compounds 1 and 2 Single crystals of 1 and 2 suitable for X-ray diffraction study were grown from CH3CN and CH3COCH3, respectively, into which diethyl ether vapor was allowed to slowly diffuse. The molecular structures of 1 and 2 are shown in Figs. 1 and 2 respectively. The selected bond lengths and angles are given in Tables 2 and 3, respectively. The Z value of 2 is 6 although its space group is P21/c. This means one and half crystallographically independent molecules are contained in the asymmetric unit. As shown in Fig. 2, the molecule B has a C2 axis through the center of the ben-

A

C1 C3 C11*

C9*

B C11* C9* 3.581(3) Å (C3-C11*) 4.032(3) Å (centroid-centroid) 3.568(3) Å (C1-C9*) C3 C1

Fig. 4. Partial packing diagram for 2. (A) Top view; (B) side view.

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information; S1). However, a bulkiness of PF 6 anion compared with the size of I widens the distances of each cation. The centroid–centroid p–p stacking distance of phenylene– ˚ , which is ca. 0.34 A ˚ larger than that pyridinium is 4.032(3) A ˚ of 3a (3.695(7) A). The shortest atomic distance between the

a c

0

b

Fig. 5. Packing diagram of 2. Hydrogen atoms and counter anions are omitted for clarity.

phenylene ring and the neighboring pyridinium one is ˚ (C(1) and C(9)*), and those two aromatic rings 3.568(3) A are almost parallel (dihedral angle : 8.9(5)°), indicating the formation of the p–p stacking among them (Fig. 4) [8,9]. The crystal structures of tert-Bu-substituted derivatives 2 and 4 are largely dependent on the counter anion. The conformation of the cationic part of 2 having a PF 6 salt shows a ‘‘linear’’ structure (Fig. 2). On the other hand, the cationic part of 4 that has a Br counter ion is an Sshaped conformation, and exhibits not only the intramolecular A–D–A p–p stackings among the central phenylene ring and two terminal pyridinium ones but also the intermolecular C–H    p interaction between the central phenylene hydrogen and the neighboring terminal pyridinium ˚ ) [6]. The dihedral angle between the central ring (3.04 A benzene ring and the terminal pyridinium one is 122.4(1)°, which is slightly larger than that of 1. In contrast with 1, the compound 2 does not show any intermolecular p–p stackings, as shown in Fig. 5. On the other hand, one of the methyl groups of tert-butyl in the pyridinium unit is located at a near position of the central phenylene ring of the neighboring molecule (distances of the methyl car˚ ; C(43)– bon–benzene carbons: C(43)–C(3)*, 3.581(3) A ˚ ; C(43)–C(17)*, 3.723(12) A ˚ , the nearest C(16)*, 3.584(3) A atomic distance between hydrogen in tert-butyl and carbon ˚ (H(54)–C(17)) [15]), and in the benzene ring is 3.21(6) A other methyl group in the same tert-butyl is positioned close to the pyridinium ring in the other neighboring molecule the distances of the methyl carbon – the atoms in the ˚ ; C(45)–C(10)*, pyridinium ring: C(45)–N(1)*, 3.522(10) A ˚ ; C(45)–C(11), 3.406(10) A ˚ , the nearest hydro3.728(12) A gen atom – the carbon atom in the pyridinium ˚ (H(59)–C(11)). These structural features ring = 2.97(4) A indicate intermolecular network formation through the

C10 N1 C11 H60 H61

C45 H56

H59 H54

C43 C3

H55 C17 C16

Fig. 6. A partial packing diagram for 2. The intermolecular C–H    p interactions are shown as dashed lines. PF 6 ions are omitted for clarity.

K. Tsutsui et al. / Journal of Molecular Structure 829 (2007) 168–175

C–H    p interactions (Fig. 6). Previously, Tsuzuki et al. evaluated the strengths of C–H    p interactions between benzene and several model hydrocarbons [16]. They calculated six kinds of benzene–methane complexes, and showed a potential minimum value at a distance between an aromatic ring and the carbon atom in methane in a range of ˚. 3.6 to 4.0 A As stated above, substituents in the terminal pyridinium ring and counter anions give strong influences on the crystal structures. In the crystal packing of 4, the S-shape-folded dication shows not only intramolecular p–p interactions among the central phenylene ring and two terminal pyridinium ones but also an intermolecular C–H    p interaction between one hydrogen of the central phenylene ring and a neighboring pyridinium ring, in which the two bromide ions are situated at a center of the space surrounded by four cations (See supporting information; S2). Taking  into account that a size of PF 6 is larger than Br , the space formed in the framework of 4 is not enough to accommodate two PF 6 ions. In addition, the bulkiness of the tert-Bu group of 2 probably hinders the formation of the intermo-

173

lecular p–p stacking such as 1 and 3 by the steric repulsion. As a result, the crystal packing of 2 is dramatically changed from that of 4. That is to say, the size and shape of PF 6 would obstruct the formation of the intramolecular p–p stacking of the dication, and construct a new intermolecular C–H    p interaction network to stabilize the crystal packing as shown in Fig. 5. The optimized structure of dicationic 1 obtained by DFT calculations is a linear and trans conformation where two pyridinium units are located in an opposite side to the central benzene ring (Fig. 7). Irrespective of the result, the S-shaped packing of 4 in the crystal structure indicates that the modes of p–p and C–H    p interactions of the present dicationic compounds are regulated by the selection of the substituent of the terminal pyridinium and the counter ions Fig. 8. 3.3. Solid-state UV-Vis and IR spectra The crystal-state molecular structures of compounds 1–4 are expected to reflect their solid-state spectra. Table 4

Linear conformation

A

the most stable structure

ΔE = 1.80 kcal mol-1

Initial structures

After optimization

B S-shape conformation

Initial structures

After optimization

Fig. 7. Initial and optimized molecular structures of 2,6-bis(pyridiniopropyl)benzene by using the DFT calculation. The DFT method employed in this study is Beck’s three-parameter hybrid model using the Lee–Yang–Parr correlation functional (B3LYP) with 6-31G basis set (denoted B3LYP/6-31G(d)). (A) Linear conformation; (B) S-shape conformation.

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K. Tsutsui et al. / Journal of Molecular Structure 829 (2007) 168–175

A

1170 1221 1180

T r a n s m i tt a n c e

1492

B 1230 1457 1190 1117

1459

C

1212

1160

1180 1486

D

1218 1178 1490

1800

1700 1600

1500 1400

1300 1300 ~ ν / cm-1

1200

1100

1000

900

Fig. 8. Infrared spectra of 1 (A), 2 (B), 3a (C), and 4 (D).

Table 4 UV-Vis spectral data for 1 and 2 kmax (nm) (e (L mol1 cm1))

Species Solid 1 2 3a 3b 4 a

Solution (MeCN)

260 257

194 227

195 222 263

258

194

192

259 256 260 259 256

(11746) (11907) (sh) (10240) (13412)

211 223 242 214 220

(23329) (29269) (50012)a (39905) (41908)

based on the I absorption.

summarizes the data for electronic spectra of the present compounds in the solid-state and in a CH3CN solution. In the solid-state UV-Vis absorption spectrum, the p–p* transition based on the pyridinium of 1 is observed at kmax = 260 nm, which is similar to that of 1 measured in a CH3CN solution (kmax = 259 nm). Similarly, each of the kmax of 2–4 in the solid-state and in CH3CN solutions

shows almost same values (257–263 nm in the solid-states and 256–260 nm in solutions) although the crystal structures are so different. Fig. 8 shows the solid-state infrared spectra of 1–4 in KBr. As expected from the similarity of the crystal structure and packing of 1 and 3, the IR spectra of Figs. 7A and C resemble each other. On the other hand, 2 forms neither intermolecular nor intramolecular p–p stacking in the crystal state, but contains intermolecular C–H    p interaction. On the other hand, 4 is an S-shaped conformation and forms intramolecular p–p stackings. It is worthy of note that 1, 3, and 4, which are involving a p–p stacking structure, show characteristic absorption bands at ca. 1180 and 1490 cm1 in the area of the stretching of aromatic rings and C–H bending vibration modes. However, these peaks were not observed in 2, instead, two remarkable peaks appeared at 1117 and 1459 cm1. Based on these assignments, the p–p stacking in the present compounds

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causes the shift of the peaks assigned to the stretching of aromatic rings and/or C–H deformation vibrations toward higher frequency by ca. 30–70 cm1. 4. Conclusions We have demonstrated the crystal structures of bis(pyridiniopropyl)benzenes 1 and 2, and characterized their spectral properties. Compound 1 has a linear conformation and forms an intermolecular p–p stacking similar to 3. On the other hand, 2 builds neither intra- nor intermolecular p–p stackings probably due to the steric repulsion of the tert-Bu group and PF 6 , but it involves an intermolecular C–H    p interaction between the methyl groups in the tert-butyl and aromatic rings (phenylene and pyridinium) in neighboring molecules. The compounds 2 and 4 have the same cation. The former is a linear configuration and shows an intermolecular C–H    p interaction between one of the methyl of tert-butyl group attached at the terminal pyridium and the central phenylene ring. On the other hand, the latter forms an S-shaped configuration and has not only intramolecular p–p interactions among the central phenylne and two terminal pyridinium rings but also an intermolecular C–H    p interaction between the central phenylene hydrogen and the neighboring pyridiunum ring. In other words, a large influence of counter anions on the crystal packing is explained by a small difference in the stabilizing energy generated by inter- and intramolecular p–p and C–H    p interactions. To our knowledge, this is the first example that not only molecular configurations but also modes of non-covalent interactions in the same framework are completely changed by the selection of counter ions. These features may hold significant implications for the design of crystal packages of the compounds involving non-covalent interaction – shapeable units. Acknowledgement

[2] [3]

[4]

[5]

[6] [7]

[8]

We thank Dr. Tatsuya Imase, Tokyo Institute of Technology, for the DFT calculation and helpful discussions. Appendix A. Supplementary data

[9] [10] [11]

The packing diagrams of 3a and 4 (3 page). Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.molstruc. 2006.06.020.

[12] [13] [14]

References [15] [1] (a) For recent reviews, see; M.C.T. Fyfe, J.F. Stoddart, Coord. Chem. Rev. 183 (1999) 139; (b) X. Huang, K. Nakanishi, N. Berova, Chirality 12 (2000) 237; (c) J.D. Hartgerink, E.R. Zubarev, S.I. Stupp, Curr. Opin. Solid State Mater. Sci. 5 (2001) 355; (d) J. Cornil, D. Beljonne, J.–P. Calbert, J.L. Bre´das, Adv. Mater. 13

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