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Core level photoemission of rotaxanes: A summary on binding energies
Journal of Electron Spectroscopy and Related Phenomena 165 (2008) 42–45
Contents lists available at ScienceDirect
Journal of Electron Spectroscopy and Related Phenomena journal homepage: www.elsevier.com/locate/elspec
Core level photoemission of rotaxanes: A summary on binding energies b ´ S.M. Mendoza a , J. Berna´ b , E.M. Perez , E.R. Kay b , A. Mateo-Alonso c , C. De Nada¨ı d , S. Zhang b , J. Baggerman e , P.G. Wiering e , D.A. Leigh b , M. Prato c , A.M. Brouwer e , P. Rudolf a,∗ a
Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands School of Chemistry, University of Edinburgh, The King’s Buildings, West Mains Road, Edinburgh EH9 3JJ, United Kingdom c Dipartimento di Scienze Farmaceutiche, Universit` a di Trieste, Piazzale Europa 1, 34127 Trieste, Italy d Laboratoire Interdisciplinaire de Spectroscopie Electronique, Facult´es Universitaires Notre Dame de la Paix, 61 Rue de Bruxelles, B-5000 Namur, Belgium e Van’t Hoff Institute for Molecular Sciences, University of Amsterdam, Nieuwe Achtergracht 129, NL-1018 WS Amsterdam, The Netherlands b
a r t i c l e
i n f o
Article history: Received 2 November 2007 Received in revised form 7 May 2008 Accepted 14 May 2008 Available online 21 May 2008
a b s t r a c t Several rotaxanes were studied by XPS in the form of thin films or monolayers on gold substrates. Here we report a database of photoemission spectra of the C 1s, N 1s and F 1s core levels. Binding energy ranges are summarized, classifying the core levels according to the chemical groups that form part of the rotaxanes. © 2008 Elsevier B.V. All rights reserved.
Keywords: X-ray photoelectron spectroscopy Rotaxane Nitrogen Carbon Fluorine
1. Introduction The manipulation of single atoms and molecules for the construction of nanoscale devices is a fascinating challenge in nanotechnology. Although nanotechnological applications seem to be in their infancy, they already have a great impact in areas such as electronics, material science, medicine, biotechnology, and information storage [1]. Synthetic molecular devices are inspired by nature and by macroscopic objects that we know from every day life. Nature uses molecular machines to transport components within cells for many functions such as DNA replication and ATP synthesis [2]; the proteins kinesin and dynein move along microtubules to induce directional motility of membranous vesicles, organelles, chromosomes, and RNA [3]; the F1F0-ATP synthase is a rotary biological motor located in the mitochondrial, bacterial and chloroplast membranes, which is responsible for ATP generation [4]. Performing macroscopic mechanical tasks with the rudimentary early generations of synthetic molecular machines has proved to be quite elusive. However, molecules with mechanically inter-
∗ Corresponding author. Tel.: +31 50 3634736. E-mail address: [email protected] (P. Rudolf). 0368-2048/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.elspec.2008.05.004
locked architectures are particularly suited for these types of applications because they permit the controlled, large amplitude movement and positioning of one mechanically interlocked component with respect to another. Rotaxanes (Fig. 1) are molecules composed of one or more macrocyclic structures locked onto a linear thread by bulky stoppers placed at both ends. Through the pioneering work of Stoddart and Sauvage [5–7], it has become clear that rotaxanes have the ability to change the relative positions of their interlocked components – macrocycle and thread – through an external trigger (e.g. light, electrons, temperature, pH, nature of the environment, reversible covalent bond formation, etc.). Thus, rotaxanes seem to have appropriate structure to build molecular machines. Most studies have focused on the behaviour of rotaxanes in solution [5,7–10], while it is by controlling their properties in the solid state that they could offer the greatest potential as versatile building blocks for the design of nanodevices [11]. In particular, immobilization on a substrate is a key step in nanoengineering [12–15]. Indeed, placing the molecules on a surface (i) allows to have them assume a desired configuration, (ii) gives the possibility of keeping them far apart from each other to avoid intermolecular interactions if needed, (iii) can prevent the collisions with undesired surrounding molecules, and (iv) will limit the Brownian motion and the freedom of movement to which every nanoscale structure is subject.
S.M. Mendoza et al. / Journal of Electron Spectroscopy and Related Phenomena 165 (2008) 42–45
43
Fig. 1. Chemical structure of the six measured rotaxanes.
This report represents a small step in the understanding of novel synthetic molecular devices on surfaces, summarizing XPS spectra obtained from rotaxane films for the development of advanced materials [16]. 2. Experimental The XPS measurements were performed using an X-PROBE Surface Science Laboratories photoelectron spectrometer with the monochromatic Al K␣ X-ray source (h = 1486.6 eV). The energy resolution (FWHM measured on the substrate Au 4f7/2 core level) was set to 1.2 or 1.5 eV to minimize data acquisition time and maximize the signal-to-noise ratio. The photoelectron take-off angle was 90◦ for rotaxane 1 and 37◦ for all the other samples. The binding energies were referenced to the Au 4f7/2 core level [17]. The base pressure in the spectrometer was 1 × 10−10 Torr. A minimum number of scans were accumulated to avoid any X-ray damage [18–21]. Spectral analysis included a background subtraction and peak separation using mixed Gaussian–Lorentzian functions in a least squares curve-fitting program (Winspec) developed in the ´ Universitaires Notre-Dame de la LISE laboratory of the Facultes Paix, Namur, Belgium. Since in practice, XPS may not distinguish between the various types of atoms of the same element present in a molecule, the fitting procedure consists in reconstructing the XPS spectrum with a minimum number of peaks consistent with the raw data, the experimental resolution and the molecular structure, Table 1 Binding energy ranges found for N 1s, and F 1s in rotaxane molecules Core level
Chemical group
Binding energy (eV)
N 1s F 1s
–CONH–; pyridine –CF2 –
399.8–401.4 688.2
in which atoms of the same element under very similar chemical environments, i.e. very close in binding energy, are considered equivalent and represented by one peak. When more than one component was needed to reproduce the raw data, the error in peak position was ±0.1 eV. All the measurements were accomplished on freshly prepared samples in order to guarantee the reproducibility of the results. Films of six different rotaxane molecules on gold substrates were analyzed (see chemical structures in Fig. 1). They are (1) [2]-(1,7,14,20-tetraaza-2,6,15,19-tetraoxo-3,5,9,12,16,18,22,25-tetrabenzocyclohexacosane)-[N,N -bis(2,2-diphenylethylamine)-1,2ethanediylidene]-rotaxane; (2) [2]-(1,7,14,20-tetraaza-2,6,15,19tetraoxo-3,5,9,12,16,18,22,25-tetrabenzocyclohexacosane)-[(E)-N, N -bis(2,2-diphenylethyl)butendiamide]-rotaxane; (3) [2]-(1,7,14, 20-tetraaza-2,6,15,19-tetraoxo-3,5,9,12,16,18,22,25-tetrabenzocyclohexacosane)-(N-[[2-[[2-[2-(2 H-[5,6]fullereno-C60-Ih-[1,9-c] pyrrol-1 (5 H)-yl)ethoxy]ethoxy]ethyl]amino]-2-oxoethyl]-␣,␣ , diphenylacetamide)-rotaxane; (4) [2]-(1,7,14,20-tetraaza-2,6,15, 19-tetraoxo-9,12,22,25-dibenzo-3,5,16,18-di[5-pyridyl]cyclohexacosane)-[N,N -bis(12-([4-(2,2-diphenylethylamino)-4-oxobutanoyl]amino)dodecyl)-1,6,7,12-tetra(4-tert-butylphenoxy)perylene3,4,9,10-tetracarboxylic diimide]-rotaxane; (5) [2]-(1,7,14,20-tetrTable 2 Binding energy ranges found for C 1s core level in rotaxanes for several chemical groups Chemical group
Binding energy (eV)
–Ph Shake-up –CH2 –; –CH3 C–O–C, C–N, etc. –CONH– –CF2 –
284.4–284.9 290.5–292.3 284.9–285.4 285.7–286.9 287.9–288.8 293.1
44
S.M. Mendoza et al. / Journal of Electron Spectroscopy and Related Phenomena 165 (2008) 42–45
Fig. 2. C 1s core levels and fits for the six rotaxane films on gold substrates (1–6) and rotaxanes 5 and 6 on SAM (5SAM and 6SAM). X-ray spectra of 5SAM and 6SAM were acquired with resolution 1.5 eV, while the other films were measured with a resolution of 1.2 eV.
aaza-2,6,15,19-tetraoxo-9,12,22,25-dibenzo-3,5,16,18-di[5-pyridyl] cyclohexacosane)-[N-(2,2-dipehnylethyl)-N -(12-([4-(2,2-diphenylethylamino)-4-oxobutanoyl]amino)dodecyl)naphthalene-1,4, 5,8-tetracarboxylic diimide]-rotaxane; (6) [2]-(1,7,14,20-tetraaza-2,6,15,19-tetraoxo-9,12,22,25-dibenzo-3,5,16,18-di[5-pyridyl] cyclohexacosane)-[(E)-N1 -(12-(2,2,3,3-tetrafluoro-N1 -(2,2-diphenylethyl)succinamido)dodecyl)-N4 -(2,2-diphenylethyl)fumaramide]-rotaxane. Additionally, two of them (rotaxanes 5 and 6) were grafted to a self-assembled monolayer (SAM) of alkanethiol as described below. Some rotaxane molecules can be sublimed in vacuum without being damaged. This advantage was used to prepare films of rotaxanes 1 and 2, which were deposited onto Au(1 1 1) kept at room temperature, using a custom built cell which consisted of a Pyrex crucible topped with a 2-mm stainless steel collimator. The crucible was heated resistively to 470 K with the temperature being measured by a chromel–alumel junction fixed at the tube exit. Exposures were monitored using an uncalibrated Bayard–Alpert ionization gauge. The organic film thickness was such that any interfacial interaction with the substrate was obscured in the XPS spectra, to the extent that the Au 4f photoemission signal was totally
Fig. 3. N 1s core levels and fits for the six rotaxane films (1–6) and rotaxanes 5 and 6 on SAM (5SAM and 6SAM). X-ray spectra of 5SAM and 6SAM were acquired with resolution 1.5 eV, while the other films were measured with a resolution of 1.2 eV.
Fig. 4. F 1s core level and fit for rotaxane 6 on gold substrate, acquired with a resolution of 1.2 eV.
S.M. Mendoza et al. / Journal of Electron Spectroscopy and Related Phenomena 165 (2008) 42–45
attenuated. This method has been used elsewhere [22,23]. Films of rotaxanes 3–6 were prepared on gold on mica substrates from a dichloromethane solution: few drops of the solution containing the rotaxane were placed on the substrate; then the solvent evaporated within a few seconds resulting in a thin film of molecules. The samples were immediately introduced in the spectrometer to be analyzed. No chlorine was detected by XPS indicating that no traces of solvent were left on the sample. The thicknesses of the films were such that the photoemission signal of the Au 4f core level was not fully attenuated (∼70% of attenuation) and there was no evidence of charging of the surface. Rotaxanes 5 and 6 contain pyridine functions in the macrocyclic component that were used to graft them on an acid-terminated SAM of 11-mercaptoundecanoic acid (11-MUA), giving rise to submonolayers of rotaxanes [24]. The procedure consists of immersing an 11-MUA SAM prepared on gold on mica [25] in the rotaxane solution for 5 days. The rotaxane solutions were prepared with dichloromethane ([CAS 75-09-2] of purity better than 99.8%, purchased from Acros Organics). The concentration of the solution can vary for every rotaxane molecule, depending on its availability and solubility, in a range of 0.05–0.1 mM. We will refer to these samples as 5SAM and 6SAM. 3. Experimental results The photoemission spectra of carbon nitrogen and fluorine and their fits are shown in Figs. 2–4. The chemical shift of the elements in these molecules depends mainly on the functional groups in which they are involved. The XPS data grouped in binding energy ranges for every element are summarized in Tables 1 and 2. The core levels appear in narrow ranges of energy. In particular, there is no overlapping in binding energy for carbon atoms of different chemical groups. Thus, the various functionalities in every molecule can be very well identified. Of course, similar information can be found in the literature, but the expected ranges of energy for every functional group are larger because they refer to a larger variety of molecules than just rotaxanes. Acknowledgements This work has been supported by the European Commission (contracts HPRN-CT2002-00178, HPRN-CT-2002-00168, and NMP4-CT-2004-013525). Additional support was provided by the Dutch Foundation for fundamental research on matter (FOM), the
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Breedtestrategie Program of the University of Groningen and the Italian Ministry of Education, University and Research (MIUR). References [1] B. Bhushan (Ed.), Handbook of Nanotechnology, Springer-Verlag, Berlin Heidelberg, Germany, 2004. [2] M. Schliwa (Ed.), Molecular Motors, Wiley–VCH, Weinheim, 2003. [3] D. Grigoriev, D. Moll, J. Hall, P. Guo, in: Hari Singh Nalwa (Ed.), Encyclopedia of Nanoscience and Nanotechnology, vol. 1, American Scientific Publishers, Indiana, USA, 2004. [4] H. Noji, M. Yoshida, J. Biol. Chem. 276 (2001) 1665. [5] J.-P. Sauvage, C. Dietrich-Buchecker (Eds.), Molecular Catenanes, Rotaxanes and Knots, Wiley–VCH, Weinheim, 1999. [6] C.P. Collier, G. Mattersteig, E.W. Wong, Y. Luo, K. Beverley, J. Sampaio, F.M. Raymo, J.F. Stoddart, J.R. Heath, Science 289 (2000) 1172. [7] J.P. Sauvage, Acc. Chem. Res. 31 (1998) 611. [8] F.G. Gatti, D.A. Leigh, S.A. Nepogodiev, A.M.Z. Slawin, S.J. Teat, J.K.Y. Wong, J. Am. Chem. Soc. 123 (2001) 5983. [9] A. Altieri, G. Bottari, F. Dehez, D.A. Leigh, J.K.Y. Wong, F. Zerbetto, Angew. Chem. Int. Ed. 42 (2003) 2296. [10] A. Mateo-Alonso, G. Fioravanti, M. Marcaccio, F. Paolucci, G.M. Aminur Rahman, C. Ehli, D.M. Guldi, M. Prato, Chem. Commun. (2007) 1945. [11] J. Areephong, W.R. Browne, N. Katsonis, B.L. Feringa, Chem. Commun. 37 (2006) 3930. [12] G.S. Kottas, L.I. Clarke, D. Horinek, J. Michl, Chem. Rev. 105 (2005) 1281. [13] S. Fournier-Bidoz, A.C. Arsenault, I. Manners, G.A. Ozin, Chem. Commun. (2005) 441. [14] K. Nørgaard, B.W. Laursen, S. Nygaard, K. Kjaer, H.-R. Tseng, A.H. Flood, J.F. Stoddart, T. Bjørnholm, Angew. Chem. Int. Ed. 117 (2005) 7197. [15] A.L. Vance, T.M. Willey, T. van Buuren, A.J. Nelson, C. Bostedt, G.A. Fox, L.J. Terminello, Nanoletters 3 (2003) 81. ´ E.M. Perez, ´ [16] S.M. Mendoza, M. Lubomska, P. Rudolf, J. Berna, D.A. Leigh, Phys. Mag. 28 (2006) 217. [17] J.K. Moulder, W.F. Stickle, P.E. Sobol, K.D. Bomben, Handbook of X-ray photoelectron Spectroscopy, Physical Electronics Inc., Eden Prairie, MN, 1995. [18] C.D. Bain, E.B. Troughton, Y.-T. Tao, J. Evall, G.M. Whitesides, R.G. Nuzzo, J. Am. Chem. Soc. 111 (1989) 321. [19] K. Heister, M. Zharnikov, M. Grunze, L.S.O. Johansson, A. Ulman, Langmuir 17 (2001) 8. [20] G. Gonella, O. Cavalleri, S. Terreni, D. Cvetko, L. Floreano, A. Morgante, M. Canepa, R. Rolandi, Surf. Sci. 638 (2004) 566. [21] T. Laiho, J.A. Leiro, M.H. Heinonen, S.S. Mattila, J. Lukkari, J. Electron. Spectrosc. 142 (2005) 105. [22] C.M. Whelan, F. Cecchet, R. Baxter, F. Zerbetto, G.J. Clarkson, D.A. Leigh, P. Rudolf, J. Phys. Chem. B 106 (2002) 8739. [23] C.-A. Fustin, R. Gouttebaron, C. de Nada¨ı, R. Caudano, P. Rudolf, F. Zerbetto, D.A. Leigh, Surf. Sci. 474 (2001) 37. [24] F. Cecchet, P. Rudolf, S. Rapino, M. Margotti, F. Paolucci, J. Baggerman, A.M. Brouwer, E.R. Kay, J.K.Y. Wong, D.A. Leigh, J. Phys. Chem. B 108 (2004) 15192. [25] S.M. Mendoza, I.E. Arfaoui, S. Zanarini, F. Paolucci, P. Rudolf, Langmuir 23 (2007) 582.
Contents lists available at ScienceDirect
Journal of Electron Spectroscopy and Related Phenomena journal homepage: www.elsevier.com/locate/elspec
Core level photoemission of rotaxanes: A summary on binding energies b ´ S.M. Mendoza a , J. Berna´ b , E.M. Perez , E.R. Kay b , A. Mateo-Alonso c , C. De Nada¨ı d , S. Zhang b , J. Baggerman e , P.G. Wiering e , D.A. Leigh b , M. Prato c , A.M. Brouwer e , P. Rudolf a,∗ a
Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands School of Chemistry, University of Edinburgh, The King’s Buildings, West Mains Road, Edinburgh EH9 3JJ, United Kingdom c Dipartimento di Scienze Farmaceutiche, Universit` a di Trieste, Piazzale Europa 1, 34127 Trieste, Italy d Laboratoire Interdisciplinaire de Spectroscopie Electronique, Facult´es Universitaires Notre Dame de la Paix, 61 Rue de Bruxelles, B-5000 Namur, Belgium e Van’t Hoff Institute for Molecular Sciences, University of Amsterdam, Nieuwe Achtergracht 129, NL-1018 WS Amsterdam, The Netherlands b
a r t i c l e
i n f o
Article history: Received 2 November 2007 Received in revised form 7 May 2008 Accepted 14 May 2008 Available online 21 May 2008
a b s t r a c t Several rotaxanes were studied by XPS in the form of thin films or monolayers on gold substrates. Here we report a database of photoemission spectra of the C 1s, N 1s and F 1s core levels. Binding energy ranges are summarized, classifying the core levels according to the chemical groups that form part of the rotaxanes. © 2008 Elsevier B.V. All rights reserved.
Keywords: X-ray photoelectron spectroscopy Rotaxane Nitrogen Carbon Fluorine
1. Introduction The manipulation of single atoms and molecules for the construction of nanoscale devices is a fascinating challenge in nanotechnology. Although nanotechnological applications seem to be in their infancy, they already have a great impact in areas such as electronics, material science, medicine, biotechnology, and information storage [1]. Synthetic molecular devices are inspired by nature and by macroscopic objects that we know from every day life. Nature uses molecular machines to transport components within cells for many functions such as DNA replication and ATP synthesis [2]; the proteins kinesin and dynein move along microtubules to induce directional motility of membranous vesicles, organelles, chromosomes, and RNA [3]; the F1F0-ATP synthase is a rotary biological motor located in the mitochondrial, bacterial and chloroplast membranes, which is responsible for ATP generation [4]. Performing macroscopic mechanical tasks with the rudimentary early generations of synthetic molecular machines has proved to be quite elusive. However, molecules with mechanically inter-
∗ Corresponding author. Tel.: +31 50 3634736. E-mail address: [email protected] (P. Rudolf). 0368-2048/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.elspec.2008.05.004
locked architectures are particularly suited for these types of applications because they permit the controlled, large amplitude movement and positioning of one mechanically interlocked component with respect to another. Rotaxanes (Fig. 1) are molecules composed of one or more macrocyclic structures locked onto a linear thread by bulky stoppers placed at both ends. Through the pioneering work of Stoddart and Sauvage [5–7], it has become clear that rotaxanes have the ability to change the relative positions of their interlocked components – macrocycle and thread – through an external trigger (e.g. light, electrons, temperature, pH, nature of the environment, reversible covalent bond formation, etc.). Thus, rotaxanes seem to have appropriate structure to build molecular machines. Most studies have focused on the behaviour of rotaxanes in solution [5,7–10], while it is by controlling their properties in the solid state that they could offer the greatest potential as versatile building blocks for the design of nanodevices [11]. In particular, immobilization on a substrate is a key step in nanoengineering [12–15]. Indeed, placing the molecules on a surface (i) allows to have them assume a desired configuration, (ii) gives the possibility of keeping them far apart from each other to avoid intermolecular interactions if needed, (iii) can prevent the collisions with undesired surrounding molecules, and (iv) will limit the Brownian motion and the freedom of movement to which every nanoscale structure is subject.
S.M. Mendoza et al. / Journal of Electron Spectroscopy and Related Phenomena 165 (2008) 42–45
43
Fig. 1. Chemical structure of the six measured rotaxanes.
This report represents a small step in the understanding of novel synthetic molecular devices on surfaces, summarizing XPS spectra obtained from rotaxane films for the development of advanced materials [16]. 2. Experimental The XPS measurements were performed using an X-PROBE Surface Science Laboratories photoelectron spectrometer with the monochromatic Al K␣ X-ray source (h = 1486.6 eV). The energy resolution (FWHM measured on the substrate Au 4f7/2 core level) was set to 1.2 or 1.5 eV to minimize data acquisition time and maximize the signal-to-noise ratio. The photoelectron take-off angle was 90◦ for rotaxane 1 and 37◦ for all the other samples. The binding energies were referenced to the Au 4f7/2 core level [17]. The base pressure in the spectrometer was 1 × 10−10 Torr. A minimum number of scans were accumulated to avoid any X-ray damage [18–21]. Spectral analysis included a background subtraction and peak separation using mixed Gaussian–Lorentzian functions in a least squares curve-fitting program (Winspec) developed in the ´ Universitaires Notre-Dame de la LISE laboratory of the Facultes Paix, Namur, Belgium. Since in practice, XPS may not distinguish between the various types of atoms of the same element present in a molecule, the fitting procedure consists in reconstructing the XPS spectrum with a minimum number of peaks consistent with the raw data, the experimental resolution and the molecular structure, Table 1 Binding energy ranges found for N 1s, and F 1s in rotaxane molecules Core level
Chemical group
Binding energy (eV)
N 1s F 1s
–CONH–; pyridine –CF2 –
399.8–401.4 688.2
in which atoms of the same element under very similar chemical environments, i.e. very close in binding energy, are considered equivalent and represented by one peak. When more than one component was needed to reproduce the raw data, the error in peak position was ±0.1 eV. All the measurements were accomplished on freshly prepared samples in order to guarantee the reproducibility of the results. Films of six different rotaxane molecules on gold substrates were analyzed (see chemical structures in Fig. 1). They are (1) [2]-(1,7,14,20-tetraaza-2,6,15,19-tetraoxo-3,5,9,12,16,18,22,25-tetrabenzocyclohexacosane)-[N,N -bis(2,2-diphenylethylamine)-1,2ethanediylidene]-rotaxane; (2) [2]-(1,7,14,20-tetraaza-2,6,15,19tetraoxo-3,5,9,12,16,18,22,25-tetrabenzocyclohexacosane)-[(E)-N, N -bis(2,2-diphenylethyl)butendiamide]-rotaxane; (3) [2]-(1,7,14, 20-tetraaza-2,6,15,19-tetraoxo-3,5,9,12,16,18,22,25-tetrabenzocyclohexacosane)-(N-[[2-[[2-[2-(2 H-[5,6]fullereno-C60-Ih-[1,9-c] pyrrol-1 (5 H)-yl)ethoxy]ethoxy]ethyl]amino]-2-oxoethyl]-␣,␣ , diphenylacetamide)-rotaxane; (4) [2]-(1,7,14,20-tetraaza-2,6,15, 19-tetraoxo-9,12,22,25-dibenzo-3,5,16,18-di[5-pyridyl]cyclohexacosane)-[N,N -bis(12-([4-(2,2-diphenylethylamino)-4-oxobutanoyl]amino)dodecyl)-1,6,7,12-tetra(4-tert-butylphenoxy)perylene3,4,9,10-tetracarboxylic diimide]-rotaxane; (5) [2]-(1,7,14,20-tetrTable 2 Binding energy ranges found for C 1s core level in rotaxanes for several chemical groups Chemical group
Binding energy (eV)
–Ph Shake-up –CH2 –; –CH3 C–O–C, C–N, etc. –CONH– –CF2 –
284.4–284.9 290.5–292.3 284.9–285.4 285.7–286.9 287.9–288.8 293.1
44
S.M. Mendoza et al. / Journal of Electron Spectroscopy and Related Phenomena 165 (2008) 42–45
Fig. 2. C 1s core levels and fits for the six rotaxane films on gold substrates (1–6) and rotaxanes 5 and 6 on SAM (5SAM and 6SAM). X-ray spectra of 5SAM and 6SAM were acquired with resolution 1.5 eV, while the other films were measured with a resolution of 1.2 eV.
aaza-2,6,15,19-tetraoxo-9,12,22,25-dibenzo-3,5,16,18-di[5-pyridyl] cyclohexacosane)-[N-(2,2-dipehnylethyl)-N -(12-([4-(2,2-diphenylethylamino)-4-oxobutanoyl]amino)dodecyl)naphthalene-1,4, 5,8-tetracarboxylic diimide]-rotaxane; (6) [2]-(1,7,14,20-tetraaza-2,6,15,19-tetraoxo-9,12,22,25-dibenzo-3,5,16,18-di[5-pyridyl] cyclohexacosane)-[(E)-N1 -(12-(2,2,3,3-tetrafluoro-N1 -(2,2-diphenylethyl)succinamido)dodecyl)-N4 -(2,2-diphenylethyl)fumaramide]-rotaxane. Additionally, two of them (rotaxanes 5 and 6) were grafted to a self-assembled monolayer (SAM) of alkanethiol as described below. Some rotaxane molecules can be sublimed in vacuum without being damaged. This advantage was used to prepare films of rotaxanes 1 and 2, which were deposited onto Au(1 1 1) kept at room temperature, using a custom built cell which consisted of a Pyrex crucible topped with a 2-mm stainless steel collimator. The crucible was heated resistively to 470 K with the temperature being measured by a chromel–alumel junction fixed at the tube exit. Exposures were monitored using an uncalibrated Bayard–Alpert ionization gauge. The organic film thickness was such that any interfacial interaction with the substrate was obscured in the XPS spectra, to the extent that the Au 4f photoemission signal was totally
Fig. 3. N 1s core levels and fits for the six rotaxane films (1–6) and rotaxanes 5 and 6 on SAM (5SAM and 6SAM). X-ray spectra of 5SAM and 6SAM were acquired with resolution 1.5 eV, while the other films were measured with a resolution of 1.2 eV.
Fig. 4. F 1s core level and fit for rotaxane 6 on gold substrate, acquired with a resolution of 1.2 eV.
S.M. Mendoza et al. / Journal of Electron Spectroscopy and Related Phenomena 165 (2008) 42–45
attenuated. This method has been used elsewhere [22,23]. Films of rotaxanes 3–6 were prepared on gold on mica substrates from a dichloromethane solution: few drops of the solution containing the rotaxane were placed on the substrate; then the solvent evaporated within a few seconds resulting in a thin film of molecules. The samples were immediately introduced in the spectrometer to be analyzed. No chlorine was detected by XPS indicating that no traces of solvent were left on the sample. The thicknesses of the films were such that the photoemission signal of the Au 4f core level was not fully attenuated (∼70% of attenuation) and there was no evidence of charging of the surface. Rotaxanes 5 and 6 contain pyridine functions in the macrocyclic component that were used to graft them on an acid-terminated SAM of 11-mercaptoundecanoic acid (11-MUA), giving rise to submonolayers of rotaxanes [24]. The procedure consists of immersing an 11-MUA SAM prepared on gold on mica [25] in the rotaxane solution for 5 days. The rotaxane solutions were prepared with dichloromethane ([CAS 75-09-2] of purity better than 99.8%, purchased from Acros Organics). The concentration of the solution can vary for every rotaxane molecule, depending on its availability and solubility, in a range of 0.05–0.1 mM. We will refer to these samples as 5SAM and 6SAM. 3. Experimental results The photoemission spectra of carbon nitrogen and fluorine and their fits are shown in Figs. 2–4. The chemical shift of the elements in these molecules depends mainly on the functional groups in which they are involved. The XPS data grouped in binding energy ranges for every element are summarized in Tables 1 and 2. The core levels appear in narrow ranges of energy. In particular, there is no overlapping in binding energy for carbon atoms of different chemical groups. Thus, the various functionalities in every molecule can be very well identified. Of course, similar information can be found in the literature, but the expected ranges of energy for every functional group are larger because they refer to a larger variety of molecules than just rotaxanes. Acknowledgements This work has been supported by the European Commission (contracts HPRN-CT2002-00178, HPRN-CT-2002-00168, and NMP4-CT-2004-013525). Additional support was provided by the Dutch Foundation for fundamental research on matter (FOM), the
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