Contact electrification of polymers due to electron transfer among mechano anions, mechano cations and mechano radicals

Journal of Electrostatics 72 (2014) 412e416

Contents lists available at ScienceDirect

Journal of Electrostatics journal homepage: www.elsevier.com/locate/elstat

Short communication

Contact electrification of polymers due to electron transfer among mechano anions, mechano cations and mechano radicals Masato Sakaguchi a, *, Masakazu Makino a, Takeshi Ohura b, Tadahisa Iwata c a

University of Shizuoka, Institute for Environmental Sciences, 52-1, Yada, Suruga-ku, Shizuoka 422-8526, Japan Meijo University, Faculty of Agriculture, 1-501, Shiogamaguchi, Tempaku-ku, Nagoya 468-8502, Japan c The University of Tokyo, Department of Biomaterial Sciences, Graduate School of Agricultural and Life Sciences, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 133-8657, Japan b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 21 October 2013 Received in revised form 1 June 2014 Accepted 27 June 2014 Available online 18 July 2014

Many studies have been reported for contact electrification based on the electron transfer from donors to acceptors. However, the chemical structures of donors and acceptors have not been identified. Here we calculated the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels of model structures of mechano anions, mechano cations and mechano radicals which were produced by the heterogeneous and homogeneous scissions of covalent bonds comprising polymer main chain in vacuum at 77 K. We identified the donors are mechano anions(HOMO) and mechano radicals(HOMO), and the acceptors are mechano cations(LUMO) and mechano radicals(LUMO). The contact electrification is due to the electron transfer from the donors to the acceptors during contacting on the friction surface, and produces mosaic nano-scopic domains with opposite sign. The sign of the net charge of polymer was deduced from the number of paths of electron acceptance reaction. The relative sign of charge and position on the triboelectric series were deduced from their chemical structure. © 2014 Elsevier B.V. All rights reserved.

Keywords: Contact electrification Electron transfer Mechano anion Mechano cation Mechano radical

1. Introduction The contact electrification is still incompletely understood for polymers. The contact electrification mechanisms based on the electron transfer from donors to acceptors have been reported [1e6]. However, the chemical structures of donors and acceptors have not been identified. Recently, Baytekin, et al. [4] have reported that the random mosaic of oppositely charged nano-scopic domains was formed by contacting between two polymers. The phenomenon was due to electron transfer from donors to acceptors. Unfortunately, they did not identify the donors and the acceptors based on their chemical structure. Until now, there is no theory to reveal the phenomenon of the random mosaic of oppositely charged nano-scopic domains and the relative sign of contacting polymer from their chemical structures. The frictional contact between two polymeric materials induces a macroscopic fracture of polymers and should simultaneously induce microscopic fractures, i.e., scissions of covalent bonds

* Corresponding author. Tel.: þ81 54 264 5786. E-mail address: [email protected] (M. Sakaguchi). http://dx.doi.org/10.1016/j.elstat.2014.06.006 0304-3886/© 2014 Elsevier B.V. All rights reserved.

comprising the main chain of polymers, on the friction surface. This concept was supported by the reports of Baytekin et al. [4,7] who demonstrated that the contact charging between two polymers was accompanied by a mass transfer. We think the mass transfer should accompany with mechanical scission of polymer main chain and produce mechano radicals [8]. However, on a conventional experiment, the mechano radicals react with oxygen molecules, and result in peroxide radicals [9]. The mechano radicals almost decay out at room temperature [10]. We previously reported that mechanical fracture of polymers in vacuum at 77 K produced anionic products (mechano anions) [11e13], cationic products (mechano cation) [14] and free radicals (mechano radicals) [8,10,13e15] by the heterogeneous and homogeneous scissions of covalent bonds comprising polymer main chain. Therefore, it is reasonable to set an experimental condition that the mechanical fracture of polymers is executed in vacuum at 77 K. This experimental system is predominant to the conventional experiment because the real contact area and surface geometry do not need to be known, oxygen and water molecules are free, and the decay of primary products is prevented. The important terms are chemical species produced by mechanical fracture of polymers. Here we propose novel contact electrification of polymers due to electron transfer among mechano anions, mechano cations and

M. Sakaguchi et al. / Journal of Electrostatics 72 (2014) 412e416

mechano radicals which are produced by heterogeneous and homogeneous scissions of covalent bond comprising the polymer main chain, and are trapped on the surface of the polymer. 2. Experimental section: calculations of HOMO and LUMO energy levels All calculations reported in the present study were carried out using the density functional theory (DFT), as implemented in the Gaussian R 09W ver.7.0 program suite [16]. Becke's 3-parameter hybrid functional combined with the LeeeYangeParr correlation functional, abbreviated as B3LYP level of DFT, was used [17]. The input geometry-data of model molecules for submission to the Gaussian were prepared by using a GaussView ver.5.0, and to adjust the geometry ‘Clean’ option in the viewer program was used [18]. The geometries of all molecules were fully optimized to ground states in vacuum using 6-311G** basis set and the Berny algorithm with the geometry optimization using direct inversion in the iterative subspace (GEDIIS) in redundant internal coordinates [19]. We have found optimized the ground-state geometries for (i) mechano cation, (ii) mechano anion, and (iii) mechano radical, and calculated the HOMO and the LUMO energy levels to estimate the ease of electron transfer among these species. For radicals, since we used unrestricted method, a single occupied molecular orbital (SOMO) was divided into occupied and unoccupied one. In this paper, the former will be simply expressed as HOMO and the latter be expressed as LUMO. The calculations were performed using an HP Compaq 8100 Elete SF/CT AY032AV-A and Windows 7.

413

Table 1-1 HOMO and LUMO energy levels for model structures of mechano anions of polymers and BC. Anion

Model structure of mechano anion

HOMO energy level (au)

LUMO energy level (au)

PP1 PP2 PE PVF1 PVF2 PVDF1 PVDF2 PVC1 PVC2 PTFE BCIa BCIIa BCIb BCIIb

CH3CH(CH3)eCH 2 (CH3)2CH(CH3)  CH3CH2eCH2 CH2FCH2eCHF CH3CHFeCH 2 CF3CH2eCF 2 CH3CF2eCH 2 CH2ClCH2eCHCl CH3CHCleCH 2 CF3CF2eCF 2  BC Iaem BCIIaem BCIbem BCIIbem

0.11274 0.12199 0.11694 0.12627 0.10857 0.13282 0.09889 0.06700 0.09314 0.09858 0.08354 0.01528 0.03694 0.06160

0.20957 0.19260 0.22878 0.23334 0.22613 0.23542 0.22324 0.19426 0.21606 0.26371 0.17159 0.15572 0.14786 0.17031

The production of mechano cations was confirmed in the case of PVDF [14]. In the case of contact electrification, for example, between PVC and PTFE, when PVC contacts with PTFE, the mechanical scission of covalent bond comprising polymer main chain occurs, and produces mechano anions (Table 1-1), mechano radicals (Table 1-2)

Tables 1-2 HOMO and LUMO energy levels for model structures of mechano radicals of polymers and BC.

3. Results and discussion

Radical

We systematically calculated the HOMO and LUMO energy levels of model structures of polypropylene (PP), polyethylene (PE), poly(vinyl fluoride) (PVF), poly(vinylidene fluoride) (PVDF), poly(vinyl chloride) (PVC), poly(tetrafluoroethylene) (PTFE) and bacterial cellulose (BC), in which, particularly, the model structures of BC are shown in Fig. 1. The model structures and their HOMO and LUMO energy levels are shown in Table 1-1 for mechano anions, Table 1-2 for mechano radicals, and Table 1-3 for mechano cations. We reported [8e15,20e22] that the mechano radicals and the mechano anions of all these polymers were produced by mechanical fracture in vacuum at 77 K, and were trapped on the surface.

PP 1  PP 2  PE  PVF 1  PVF 2  PVDF 1  PVDF 2  PVC 1  PVC 2  PTFE  BC Ia  BC IIa  BC Ib  BC IIb



Model structure of mechano radical 

CH3CH(CH3)eCH2  (CH3)2CH (CH3)  CH3CH2eCH2  CH2FCH2eCHF  CH3CHFeCH2  CF3CH2eCF2  CH3CF2eCH2  CH2ClCH2eCHCl  CH3CHCleCH2  CF3CF2eCF2  BC Iaem  BC IIaem  BC Ibem  BC IIbem

Fig. 1. Model structures of mechano radicals, mechano anions and mechano cations of BC.

HOMO energy level (au)

LUMO energy level (au)

0.20529 0.18202 0.20503 0.20496 0.22194 0.20757 0.24339 0.22376 0.22663 0.24455 0.20652 0.27562 0.25294 0.21540

0.05185 0.03379 0.05110 0.05452 0.06668 0.05375 0.08421 0.08577 0.07229 0.09374 0.06393 0.16872 0.13320 0.08491

414

M. Sakaguchi et al. / Journal of Electrostatics 72 (2014) 412e416

Table 1-3 HOMO and LUMO energy levels for model structures of mechano cations of polymers and BC. Cation

Model structures of mechano cation

HOMO energy level (au)

LUMO energy level (au)

PPþ1 PPþ2 PEþ PVFþ1 PVFþ2 PVDFþ1 PVDFþ2 PVCþ1 PVCþ2 PTFEþ BCþIa BCþIIa BCþIIb BCþIIb

CH3CH(CH3)eCHþ 2 (CH3)2CHþ(CH3) þ CH3CH2eCH2 CH2FCH2eCHFþ CH3CHFeCHþ 2 CF3CH2eCFþ 2 CH3CF2eCHþ 2 CH2ClCH2eCHClþ CH3CHCleCHþ 2 CF3CF2eCFþ 2 þ BC Iaem BCþIIaem BCþIbem BCþIIbem

0.54815 0.51923 0.57603 0.54443 0.56894 0.61115 0.57450 0.47085 0.50460 0.61472 0.43408 0.45204 0.45178 0.45303

0.39472 0.35509 0.40480 0.40864 0.42138 0.41936 0.44921 0.39067 0.41251 0.45944 0.35976 0.42313 0.39079 0.36067

and mechano cations (Table 1-3) on each surface. Next, these chemical species contact each other by physical mixing, and then electron transfers execute from the donors; mechano anion with HOMO energy level (mechano anions(HOMO)) and mechano radicals(HOMO) to the acceptors; mechano radicals(LUMO) and mechano cations(LUMO). The number of all possible electron transfer paths are 14, except the paths involving PTFE(LUMO), PVC1(LUMO) and PVC2(LUMO), because we think they cannot receive more electrons, and also except the paths involving PTFEþ(HOMO), PVCþ1(HOMO) and PVCþ2(HOMO), because they cannot also release more electrons. More detail electron transfer, for example, the forward electron  transfer path from PVC1 to PTFE is shown in Eq. (1) as follows; PVC1 þ PTFE / PVC 1 þ PTFE 



(1)

The energy difference DEf is defined and calculated as; jDEfj ¼ jPVC1(HOMO)  PTFE (LUMO)j ¼ 0.16074 au 

On the backward electron transfer path from PTFE to PVC 1; The energy difference DEb is defined and calculated as; 

Therefore, the electron transfer executes from PTFE(HOMO) to   PVC 1(LUMO), and then the PVC 1 becomes the anion PVC1, and   PTFE simultaneously becomes the neutral radical PTFE . Thus, the electron donor is identified as PTFE(HOMO), and the electron  acceptor is identified as PVC 1(LUMO). After separation, the contact  point located at PVC 1 on the surface of PVC becomes the negative in charge with nano-scopic domain, and the contact point located at PTFE on the surface of PTFE becomes neutral in charge with nano-scopic domain. All electron transfer paths (1)e(14) and related values of jDEfj, jDEbj, jDEfjjDEbj and direction of electron transfer on the paths are shown in Table 2. According to this mechanism, the contact charging should form a random mosaic pattern with oppositely charged nano-scopic domain on each surface (Fig. 2.). This concept is supported by the report [4] that Baytekin et al. observed a random mosaic of oppositely charged regions produced by contact between two polymers. Each sign of the net charge on the surface of PVC or PTFE can be deduced as follows: The number of all possible electron transfer paths is 14; Eqs. (1)e(14). The number of electron transfer paths from PVC to PTFE is 8; Eqs. (2)e(4),(7),(8),(10),(12) and (14), and the number of inverse paths from PTFE to PVC is 6; Eqs. (1),(5),(6),(9),(11) and (13). Therefore, although a random mosaic pattern with oppositely charged nano-scopic domain forms on the surfaces (Fig. 2), the net charge of PTFE should be negative in charge; PTFE(), and the net charge of PVC should be positive in charge; PVC(þ). To elucidate the relative sign of net charge, we analysed PEePP, PEePVC, PVCePVDF, PVFePVDF, PVFeBC and BCePTFE based on the proposed mechanism. The electron donors, electron acceptors, number of electron paths with direction of electron streaming on the paths and net charge of each contacting polymer are shown in Table 3. These results indicate that a sign of net charge of the polymer depends on the chemical structure of counter polymer, and the sign is relatively determined based on each chemical structure. We elucidated the relative signs of net charge for PP, PE, PVC, PVF, PVDF, BC and PTFE based on the proposed mechanism (shown in Table 3). The order of polymers from positive to negative sign was obtained as follows;

jDEbj ¼ jPTFE(HOMO)  PVC 1(LUMO)j ¼ 0.18435 au.

Positive, PP < PE < PVC < PVF y PVDF < BC < PTFE, Negative

The jDEfj and jDEjb roughly reflect the degree of progress for the forward and the backward paths.

The position on the order of PE, PVC, and PTFE is identical to the triboelectric series reported by Heinniker [23]. This result strongly suggests that the relative sign of net charge can be deduced from its chemical structure.



jDEfj  jDEbj ¼ 0.02361 au < 0,

Table 2 jDEfj, jDEbj, jDEfjjDEbj values on the electron transfer paths and the direction of electron transfer on the path. Electron transfer path PVC1 þ PTFE / PVC 1 þ PTFE (1)   PVC1 þ PTFEþ / PVC 1 þ PTFE (2)   PVC2 þ PTFE / PVC 2 þ PTFE (3)   PVC2 þ PTFEþ / PVC 2 þ PTFE (4)   PVC 1 þ PTFE / PVC1 þ PTFE (5)   PVC 1 þ PTFE / PVCþ1 þ PTFE (6)   PVC 1 þ PTFEþ / PVCþ1 þ PTFE (7)   PVC 2 þ PTFE / PVC2 þ PTFE (8)   PVC 2 þ PTFE / PVCþ2 þ PTFE (9)   PVC 2 þ PTFEþ / PVCþ2 þ PTFE (10)   PVCþ1 þ PTFE / PVC 1 þ PTFE (11)   PVCþ1 þ PTFE / PVC 1 þ PTFEþ(12)   PVCþ2 þ PTFE / PVC 2 þ PTFE (13)   PVCþ2 þ PTFE / PVC 2 þ PTFEþ(14) 



jDEfj (au)

jDEbj (au)

jDEfjjDEbj (au)

Direction of electron transfer on the path

0.16074 0.52644 0.18688 0.55258 0.18435 0.13002 0.23568 0.17087 0.13289 0.23281 0.48925 0.14612 0.51996 0.16796

0.18435 0.00797 0.17087 0.17226 0.16074 0.48925 0.14612 0.18688 0.51109 0.16796 0.02079 0.23568 0.12820 0.23281

0.02361 0.51847 0.01601 0.38032 0.02361 0.35923 0.08956 0.0161 0.37820 0.06485 0.46846 0.08956 0.39176 0.06485

PTFE(HOMO) / PVC 1(LUMO) PVC1(HOMO) / PTFEþ(LUMO)  PVC2(HOMO) / PTFE (LUMO) PVC2(HOMO) / PTFEþ(LUMO)  PTFE(HOMO) / PVC 1(LUMO), PTFE(HOMO) / PVCþ1(LUMO)  PVC 1(HOMO) / PTFEþ(LUMO)  PVC2(HOMO) / PTFE (LUMO) PTFE(HOMO) / PVCþ2(LUMO)  PVC 2(HOMO) / PTFEþ(LUMO) PTFE(HOMO) / PVCþ1(LUMO)  PVC 1(HOMO) / PTFEþ(LUMO) PTFE(HOMO) / PVCþ2(LUMO)  PVC 2(HOMO) / PTFEþ(LUMO) 

M. Sakaguchi et al. / Journal of Electrostatics 72 (2014) 412e416

415

Fig. 2. Possible scenarios of electrification due to electron transfer among mechano anions, mechano cations and mechano radicals. The random mosaic pattern with oppositely charged nano-scopic domain on the surface of PVC and that of PTFE are produced by mechanical fracture of covalent bonds comprising main chain of polymers on the surface, where the symbols; , þ and  are mechano radical, mechano cation and mechano anion, respectively (shown in left hand side). The electron transfer paths from mechano anions(HOMO) or mechano radicals(HOMO) to mechano radicals(LUMO) or mechano cations(LUMO) execute under contact on the surface (shown in the center). After separation a new different random mosaic pattern is produced, and the net charge of PVC is positive in charge; PVC(þ) and the net charge of PTFE is negative in charge; PTFE() (shown in right hand side).

We elucidated the sign of net charge between identical PE (PE1 and PE2). The number of all possible electron transfer paths is 8. The number of electron transfer paths from PE1 to PE2 is 2, and that of inverse paths is 2. Since the 4 electron transfer paths have jDEfj  jDEbj ¼ 0.0, the electron transfer has equal probability between PE1 and PE2. Thus, although a random mosaic pattern with oppositely charged nano-scopic domain forms on the surfaces, the net charges of PE1 and PE2 should be neutral in charge. This concept is supported by the reports [4,24]. In conclusion, we identified that the electron donors are mechano anions(HOMO) and mechano radicals(HOMO), and the

electron acceptors are mechano radicals(LUMO) and mechano cations(LUMO). The mosaic charging with nanoscopic domain on the polymer surface is revealed by electron transfer from the donors to the acceptors which are produced by mechanical fracture of covalent bond comprising polymer main chain and are trapped on the surface. The sign of net charge was deduced that the polymer having large number of paths of electron acceptance becomes negative in charge, in which the direction of electron transfer depends on the difference of each HOMO or LUMO energy level. The position on the triboelectric series was deduced from its chemical structure.

Table 3 Electron donors, acceptors, number of electron paths with direction of electron streaming and net charge of each contacting polymer. Contacting polymers

Electron donors

Electron accepters

PP and PE

PE(HOMO), PP1(HOMO),  PP2(HOMO), PP 1(HOMO),  PP 2(HOMO PE(HOMO), PVC1(HOMO),  PVC2(HOMO), PE (HOMO)

PE (LUMO), PP 1(LUMO), PEþ(LUMO), PPþ1(LUMO), PPþ2(LUMO)   PVC 1(LUMO), PVC 2(LUMO), PEþ(LUMO), PVCþ1(LUMO), PVCþ2(LUMO)   PVC 1(LUMO), PVC 2(LUMO),  PVDF 2(LUMO), PVCþ1(LUMO), PVCþ2(LUMO), PVDFþ1(LUMO), PVDFþ2(LUMO)   PVF 1(LUMO), PVF 2(LUMO),  PVDF 2(LUMO), PVFþ1(LUMO), PVFþ2(LUMO), PVDFþ1(LUMO), PVDFþ2(LUMO)   BC Ia(LUMO), BC IIa(LUMO),   BC Ib(LUMO), BC IIb(LUMO), BCþIa(LUMO), BCþIIa(LUMO), BCþIb(LUMO), BCþIIb(LUMO), PVFþ1(LUMO), PVFþ2(LUMO)   PTFE (LUMO), BC IIa(LUMO),   BC Ib(LUMO), BC IIb(LUMO), BCþIa(LUMO), BCþIIa(LUMO), BCþIb(LUMO), BCþIIb(LUMO), PTFEþ(LUMO)

PE and PVC

PVC and PVDF

PVF and PVDF

PVF and BC

BC and PTFE

PVC1(HOMO), PVC2(HOMO), PVDF1(HOMO), PVDF2(HOMO),   PVC 1(HOMO), PVC 2(HOMO),  PVDF 1(HOMO) PVF1(HOMO), PVF2(HOMO), PVDF1(HOMO), PVDF2(HOMO),   PVF 1(HOMO), PVF 2(HOMO),  PVDF 1(HOMO) PVF1(HOMO), PVF2(HOMO), BCIa(HOMO), BCIIa(HOMO), BCIb(HOMO), BCIIb(HOMO),   PVF 1(HOMO), PVF 2(HOMO),   BC Ia(HOMO), BC IIb(HOMO) PTFE(HOMO), BCIa(HOMO), BCIIa(HOMO), BCIb(HOMO),  BCIIb(HOMO), BC Ia(HOMO),   BC IIa(HOMO), BC Ib(HOMO),  BC IIb(HOMO)





Number of electron paths with direction of electron streaming

Net charge of each polymer

8: PP / PE 6: PE / PP

PP (þ) PE ()

10: PE / PVC 4: PVC / PE

PE (þ) PVC ()

16: PVC / PVDF 12: PVDF / PVC

PVC (þ) PVDF ()

14: PVF / PVDF 14: PVDF / PVF

PVF (neutral) PVDF (neutral)

32: PVF / BC 24: BC / PVF

PVF (þ) BC ()

18: BC / PTFE 10: PTFE / BC

BC (þ) PTFE()

416

M. Sakaguchi et al. / Journal of Electrostatics 72 (2014) 412e416

References [1] A.F. Diaz, D. Fenzel-Alexander, Langmuir 9 (1993) 1009e1015. [2] L.S. McCarty, A. Winkleman, G.M. Whitesides, J. Am. Chem. Soc. 129 (2007) 4075e7088. [3] L. Chong-yang, A.J. Bard, Chem. Phys. Lett. 480 (2009) 145e156. [4] H.T. Baytekin, A.Z. Patashinski, M. Branicki, S. Soh, B.A. Grzybowski, Science 333 (2011) 308e312. [5] T.A.L. Brugo, T.R.D. Ducati, K.R. Francisco, K.J. Clinckspoor, F. Galembeck, S.E. Galembeck, Langmuir 28 (2012) 7407e7416. [6] S. Martsusaka, H. Maruyama, T. Matsuyama, M. Ghadiri, Chem. Eng. Sci. 65 (2010) 5781e5807. [7] H.T. Baytekin, B. Baytekin, J.T. Incovati, B.A. Grzybowski, Angew. Chem. Int. Ed. 51 (2012) 4843e4847. [8] M. Sakaguchi, J. Sohma, J. Polym. Sci. Polym. Phys. 13 (1975) 1233e1245. [9] M. Sakaguchi, J. Sohma, Polym. J. 7 (1975) 490e497. [10] M. Sakaguchi, T. Ohura, T. Iwata, S. Takahashi, S. Akai, T. Kan, H. Murai, M. Fujiwara, O. Watanabe, M. Narita, Biomacromolecules 11 (2010) 3059e3066. [11] M. Sakaguchi, K. Kinpara, H. Hori, S. Shimada, H. Kashiwabara, Polym. Commun. 26 (1985) 142e144.

[12] M. Sakaguchi, K. Kinpara, H. Hori, S. Shimada, H. Kashiwabara, J. Polym. Sci. Polym. Phys. 25 (1987) 1431e1437. [13] M. Sakaguchi, K. Kinpara, H. Hori, S. Shimada, H. Kashiwabara, J. Polym. Sci. Polym. Phys. 26 (1988) 1307e1312. [14] M. Sakaguchi, K. Kinpara, H. Hori, S. Shimada, H. Kashiwabara, Macromolecules 22 (1989) 1277e1280. [15] J. Sohma, M. Sakaguchi, Adv. Polym. Sci. 20 (1976) 109e158. [16] M. J.Frisch, et al., Gaussian 09, Revision A.1, Gaussian, Inc., Wallingford, CT, 2009. [17] A.D. Becke, J. Chem. Phys. 98 (1993) 5648e5652. [18] R. Dennington, T.D. Keith, J. Millam, GaussView, Version 5, Semichem Inc., Shawnee Mission, KS, 2009. [19] X. Li, M.J. Frisch, J. Chem. Theory Comput. 2 (2006) 835e839. [20] M. Sakaguchi, S. Shimada, H. Kashiwabara, Macromolecules 23 (1990) 5038e5040. [21] M. Sakaguchi, Y. Miwa, S. Hara, Y. Sugino, K. Yamamoto, S. Shimada, J. Electrostat. 62 (2004) 35e49. [22] M. Sakaguchi, M. Makino, T. Ohura, T. Iwata, J. Phys. Chem. A 116 (2012) 9872e9877. [23] J. Henniker, Nature 196 (1962) 474. [24] M.M. Apodica, P.J. Wesson, K.J. M Bishop, M.A. Ratner, B. Grzybowski, Angew. Chem. Int. Ed. 49 (2010) 946e949.