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Langmuir 2008, 24, 6640-6648
Grafting of Monocarboxylic Substituted Polychlorotriphenylmethyl Radicals onto a COOH-Functionalized Self-Assembled Monolayer through Copper (II) Metal Ions O. Shekhah,† N. Roques,‡ V. Mugnaini,‡ C. Munuera,‡ C. Ocal,‡ J. Veciana,*,‡ and C. Wo¨ll*,† Ruhr-UniVersita¨t Bochum, Institut fu¨r Physikalische Chemie 1, UniVersita¨tsstrasse 50, D-44801 Bochum, Germany, and Institut de Cie`ncia de Materials de Barcelona (CSIC)-CIBER/BBN, Campus UniVersitari de Bellaterra, E-08193 Cerdanyola, Spain ReceiVed March 11, 2008 A monocarboxylic substituted polychlorotriphenylmethyl radical (PTMCOOH) has been grafted onto a COOHfunctionalized SAM (mercaptohexadecanoic acid, MHDA SAM), using copper (II) metal ions as linkers between the carboxyl groups of the SAM and the ligand. The metal-radical adlayer has been characterized thoroughly using different surface analysis techniques, such as contact angle, IRRAS, XPS, SPR, ToF-SIMS, SFM, and NEXAFS. The magnetic character was confirmed by EPR. The density of unoccupied states was investigated using X-ray absorption spectroscopy. A low-energy peak in the NEXAFS spectrum directly revealed the presence of partially occupied electronic levels, thus proving the open-shell character of the grafted ligands. SEM measurements on a laterally patterned sample prepared by µCP of MHDA in a matrix of hexadecane thiolate (a CH3-terminated SAM) was performed to demonstrate that the metal-assisted anchoring of the open-shell ligand occurs selectively on the COOH terminated SAM. These results represent an easy and new approach to anchor organic radicals on surfaces and constitute a first step toward the growth of magnetic metal-organic radical-based frameworks on solid substrates.
Introduction To control the organization of nano-objects on surfaces represents one of the main challenges in the field of nanotechnology. To date, one of the most widely used approaches to manipulate a surface coverage is the use of self-assembled monolayers (SAMs)1 composed of laterally organized molecules whose ending group confers relevant physical properties to the substrate.2 These materials hold considerable potential in the field of molecular electronics and optoelectronics, as well as sensors. Nevertheless, the preparation of functional molecules substituted with the appropriate reactive group to bind the substrate often requires fine synthetic chemistry work, which constitutes, together with the unpredictable self-assembly behavior of these functionalized molecules, the main drawback of this approach. Another particularly appealing strategy to reach the same objective deals with the use of well-known SAMs that are formed with basic molecules. In this approach, the ending groups of the homogeneous and well-packed monolayer are used to graft the functional molecule to the surface thanks to covalent bonds or electrostatic interactions. To date, SAMs functionalized with suitable ending groups have been widely used to graft single
† Ruhr-Universita¨t Bochum, Institut fu¨r Physikalische Chemie 1, Universita¨tsstrasse 50, D-44801 Bochum, Germany. ‡ Institut de Cie`ncia de Materials de Barcelona (CSIC), Campus Universitari de Bellaterra, E-08193 Cerdanyola, Spain.
(1) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Chem. ReV. 2005, 105, 1103. (2) (a) Matsushita, M. M.; Ozaki, N.; Sugawara, T.; Nakamura, F.; Hara, M. Chem. Lett. 2002, 596. (b) Areephong, J.; Browne, W. R.; Katsonis, N.; Feringa, B. L. Chem. Commun. 2006, 3930. (c) Namiki, K.; Sakamoto, A.; Murata, M.; Kume, S.; Nishihara, H. Chem. Commun. 2007, 4650. (d) Mannini, M.; Sorace, L.; Gorini, L.; Piras, F. M.; Caneschi, A.; Magnani, A.; Menichetti, S.; Gatteschi, D. Langmuir 2007, 23, 2389. (e) Cornia, A.; Fabretti, A. C.; Pacchioni, M.; Zobbi, L.; Bonacchi, D.; Caneschi, A.; Biagi, R.; Del Pennino, U.; De Renzi, V.; Gurevich, L.; Van der Zant, H. S. J. Angew. Chem., Int. Ed. 2003, 42, 1645.
Figure 1. Chemical structure for the PTMCOOH radical.
molecule magnets (SMMs),3 to construct molecular metal-organic nanowires on top of electrodes,4 or even to build porous coordination polymers.5,6 Recently, a feasible approach was demonstrated to form one of the few examples of molecular magnetically active surfaces,7 a fundamental starting point toward advanced nanodevices for magnetic data storage. In that work, a carboxylic-substituted polychlorotriphenylmethyl radical (PTMCOOH, Figure 1) was grafted to a SAM functionalized with suitable ending groups, thanks to covalent and electrostatic interactions. These radical molecules not only allowed conferring a magnetic nature to the surface, but also a multifunctional character, thanks to their relevant electrochemical and optical properties. (3) (a) Zobbi, L.; Mannini, M.; Pacchioni, M.; Chastanet, G.; Bonacchi, D.; Zanardi, C.; Biagi, R.; Del Pennino, U.; Gatteschi, D.; Cornia, A.; Sessoli, R. Chem. Commun. 2005, 1640. (b) Gomez-Segura, J.; Ruiz-Molina, D.; Veciana, J. Chem. Commun. 2007, 3699. (4) Lin, C.; Kagan, C. R. J. Am. Chem. Soc. 2003, 125, 336. (5) (a) Hermes, S.; Schro¨der, F.; Chelmowski, R.; Wo¨ll, C.; Fischer, R. A. J. Am. Chem. Soc. 2005, 127, 13744. (b) Biemmi, E.; Scherb, C.; Bein, T. J. Am. Chem. Soc. 2007, 129, 8054. (6) (a) Shekhah, O.; Wang, H.; Strunskus, T.; Cyganik, P.; Zacher, D.; Fischer, R.; Wo¨ll, C. Langmuir 2007, 23, 7440. (b) Shekhah, O.; Wang, H.; Kowarik, S.; Schreiber, F.; Paulus, M.; Tolan, M.; Sternemann, C.; Evers, F.; Zacher, D.; Fischer, R. A.; Wo¨ll, C. J. Am. Chem. Soc. 2007, 129, 15118. (7) (a) Crivillers, N.; Mas-Torrent, M.; Perruchas, S.; Roques, N.; VidalGancedo, J.; Veciana, J.; Rovira, C.; Basabe-Desmonts, L.; Ravoo, B. J.; GergoCalama, M.; Reinhoudt, D. N. Angew. Chem., Int. Ed. 2007, 46, 2215. (b) Crivillers, N.; Mas-Torrent, M.; Vidal-Gancedo, J.; Veciana, J.; Rovira, C. J. Am. Chem. Soc. 2008, 130, 5499.
10.1021/la800771q CCC: $40.75 2008 American Chemical Society Published on Web 06/04/2008
Grafting of Monocarboxylic PTMCOOH Radicals onto a SAM
Despite the fact that coordination chemistry provides a flexible method for creating organic-inorganic assemblies, the use of metal ions as linkers to functionalize SAMs with radical molecules has not been reported to date. We propose the carboxylicsubstituted PTM radicals as suitable ligands to explore this approach. In the past few years, these open-shell molecules have been proven to act as particularly efficient chemical and magnetic linkers to connect transition metal ions in magnetic and porous coordination polymers.8 The possibility to modulate the ligand shape and topicity, modifying both the number and the relative position of the carboxylic groups introduced on the PTM skeleton has been successfully exploited to prepare crystalline coordination polymers associating the expected topologies and dimensionalities (from 1D-chains to 3D-coordination polymers) with relevant magnetic properties.9 Thus, the monocarboxylic PTMCOOH radical,10 which was reported to form mononuclear complexes with transition metal ions,11 was chosen as a model ligand for our first experiments. We report herein, for the first time, the grafting of this open-shell molecule on top of carboxylicterminated SAMs through copper (II) linkers. The surface functionalization has been investigated thanks to different surface techniques such as contact angle, infrared reflection-absorption spectroscopy (IRRAS), surface plasmon resonance (SPR), time of flight- secondary ion mass spectrometry (ToF-SIMS), X-ray photoelectron spectroscopy (XPS), near edge X-ray absorption fine structure (NEXAFS), scanning force microscopy (SFM), and scanning electron microscopy (SEM), while its magnetic character has been evidenced by electron paramagnetic resonance (EPR). In addition to the interest for the organization of magnetically active species on surfaces in an easy way, this approach also represents a first step toward the construction of extended PTM-based coordination polymers following a “surfacedriven” metal-radical approach.12
Experimental Section a. General Considerations and Synthesis of the PTMCOOH Radical. All solvents were reagent grade and were used as received and distillated, unless otherwise indicated. Absolute ethanol was used in the different reactions and to clean the surfaces. Mercaptohexadecanoic acid (abbreviated as MHDA) and copper (II) acetate (Cu2(OAc)4 · 2H2O, abbreviated as Cu(OAc)2) were of high purity grade and were obtained from Aldrich. The PTMCOOH radical was prepared in rather good yields following a well-known seven-step procedure.10 The radical purity was controlled by elemental analysis, IR, UV-vis, and EPR before its use as a ligand. Since PTM radicals are highly sensitive to light when they are in solution, the synthesis and grafting experiments were performed under red light. b. Preparation of Au Substrates. For infrared reflectionabsorption spectroscopy (IRRAS), NEXAFS, and XPS measure(8) (a) Maspoch, D.; Ruiz-Molina, D.; Wurst, K.; Domingo, N.; Cavallini, M.; Biscarini, F.; Tejada, J.; Rovira, C.; Veciana, J. Nat. Mater. 2003, 2, 190. (b) Maspoch, D.; Ruiz-Molina, D.; Wurst, K.; Rovira, C.; Veciana, J. Chem. Commun. 2004, 1164. (c) Maspoch, D.; Domingo, N.; Ruiz-Molina, D.; Wurst, K.; Herna´ndez, J. M.; Vaughan, G.; Rovira, C.; Lloret, F.; Tejada, J.; Veciana, J. Chem. Commun. 2005, 5035. (9) Roques, N.; Maspoch, D.; Luis, F.; Camon, A.; Wurst, K.; Datcu, A.; Rovira, C.; Ruiz-Molina, D.; Veciana, J. J. Mater. Chem. 2008, 18, 98. (10) Maspoch, D.; Catala, L.; Gerbier, P.; Ruiz-Molina, D.; Wurst, K.; Tejada, J.; Rovira, C.; Veciana, J. Chem. Eur. J. 2002, 8, 3635, and references cited therein. (11) (a) Maspoch, D.; Ruiz-Molina, D.; Wurst, K.; Vidal-Gancedo, J.; Rovira, C.; Veciana, J. Chem. Commun. 2002, 2958. (b) Maspoch, D.; Ruiz-Molina, D.; Wurst, K.; Vidal-Gancedo, J.; Rovira, C.; Veciana, J. J. Chem. Soc., Dalton Trans. 2004, 1073. (c) Maspoch, D.; Domingo, N.; Ruiz-Molina, D.; Wurst, K.; Hernandez, J. M.; Lloret, F.; Tejada, J.; Rovira, C.; Veciana, J. Inorg. Chem. 2007, 46, 1627. (12) (a) Caneschi, A.; Gatteschi, D.; Sessoli, R.; Rey, P. Acc. Chem. Res. 1989, 22, 392. (b) Caneschi, A.; Gatteschi, D.; Rey, P. Prog. Inorg. Chem. 1991, 39, 331. (c) Iwamura, H.; Inoue, K.; Hayamizu, T. Pure Appl. Chem. 1996, 68, 43. (d) Lemaire, M. T. Pure Appl. Chem. 2004, 76, 277.
Langmuir, Vol. 24, No. 13, 2008 6641 ments, polycrystalline Au substrates were prepared by evaporating a 5-nm buffer layer of titanium (99.8%, Chempur) and subsequently 100 nm of gold (99.995%, Chempur) onto polished silicon-wafers (Wacker silicone) at room temperature in an evaporation chamber operating at a base pressure of about 10-7 mbar. For SPR, D263 thin glass (Schott) were rinsed with absolute ethanol, dried in a nitrogen stream, and then installed in a Leybold Inficon XTC/2 metal evaporator. Gold was evaporated onto a 12 Å buffer layer of titanium up to reach a final thickness close to 485 Å. Evaporation was performed at room temperature under a pressure of approximately 10-7 mbar. For SFM measurements, gold substrates were purchased from Arrandee (200-300 nm of gold over 1-4 nm of chromium on glass). After a 10-min. sonication in n-hexane, dichloromethane, and ethanol, the gold substrates were prepared by flame-annealing in air after cleaning by immersion in a piranha solution (1:3; H2O2:H2SO4). After this procedure, the resulting surface consisted of large grains with flat terraces of (111) orientation (sizes up to 400 nm) separated by monatomic steps. c. Preparation of MHDA SAMs (S1). The mercaptohexadecanoic acid (MHDA) solution was prepared as described by Arnold et al. by dissolving the MHDA in a 5% (by volume) solution of acetic acid in ethanol to reach the desired concentration of 20 µM.13 A clean gold substrate was placed in this solution for 24 h and then rinsed with the pure solvent and gently dried under nitrogen flux. d. Preparation of Laterally Patterned Surfaces (p-S1). The gold substrates were cut into pieces of 10 × 10 mm, and a lateral pattern of organothiols was fabricated by employing the microcontact printing method (µCP).14 A PDMS stamp was loaded by incubation with a little pile (80 µL applied from a pipet) of 1 mM ethanolic solution of MHDA for 60 s. The stamp was then dried in a stream of nitrogen and laid gently toward the gold surface for 90 s. After removing the stamp, the samples were first cleaned with ethanol, then dried under a nitrogen stream and finally placed into a 1 mM ethanolic solution of hexadecanethiol for 24 h. After removal from the solvent, the sample was rinsed and dried using the same procedures applied to the nonpatterned SAM samples. e. Grafting of Copper-PTM Units on MHDA SAMs. Method 1: Preparation of S2. The MHDA SAM sample S1 was placed in a 1/1 mixture of Cu(OAc)2 and PTMCOOH in absolute ethanol (1 mM) for 90 min. The sample was then removed, rinsed with absolute ethanol, and dried under a nitrogen stream to afford S2. A similar method was used to prepare the patterned p-S2 from p-S1. Method 2: Step-by-Step Preparation of S4 Via S3. The MHDA SAM S1 sample was placed in a 1 mM ethanol solution of Cu(OAc)2 for 30 min. The sample was then removed, rinsed with absolute ethanol, and dried under a nitrogen stream to afford SAM S3. Subsequently, S3 was placed in a 1-mM PTMCOOH ligand solution in ethanol for 60 min. After this time, the sample was removed from the solution, carefully washed with absolute ethanol, and finally dried with a nitrogen stream to afford S4. A similar method was used to prepare the patterned p-S3 and p-S4 from p-S1. f. Characterizations. Sessile water-drop contact angle (CA) measurements were obtained by using a video camera-based commercial apparatus (Surface Electron Optics Co., Ltd., Korea; Phoenix 150). The reported value is the average of three measurements. IRRAS data were recorded using a Biorad Excalibur FTIR spectrometer (FTS 3000) equipped with a grazing incidence reflection unit (Biorad Uniflex) and a narrow band MCT detector. All spectra were recorded with 2 cm-1 resolution at an angle of incidence of 80° relative to the surface normal and further processed by using boxcar apodization. A commercial surface plasmon resonance system (Reichert SR7000DC) was used to record the real-time kinetics of the Cu(OAc)2 and PTMCOOH ligand adsorption to the organic surface. A ToF-SIMS IV mass spectrometer (Ion-Tof GmbH, Mu¨nster, Germany) equipped with a bismuth cluster (Bi3) ion source and (13) Arnold, R.; Azzam, W.; Terfort, A.; Wo¨ll, C. Langmuir 2002, 18, 3980. (14) Xia, Y.; Whitesides, G. M. Annu. ReV. Mater. Sci. 1998, 28, 84.
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Scheme 1. Schematic Representation of (a) Mononuclear and (b) Dinuclear Copper (II) Complexes Obtained Reacting PTMCOOH or its Carboxylate PTMCOO- with Copper (II) Acetate, and (c) Idealized Representation for the Grafting of the PTMCOO- Ligand on Top of a COOH-Terminated SAMa
a
The choice of binding modes to the copper (II) ions for both the surface and the COO- groups is arbitrary.
operated at a pressure of 5 × 10-9 mbar was used for these experiments. Eighteen ns pulses of 25 keV Bi3++ (primary ions) were bunched to form ion packets with a nominal temporal extent of 10 000, which was more than enough to resolve all relevant mass spectral peaks. The ion abundances were normalized relative to the total ion abundance to compensate for differences in the numbers of Bi shots that each sample received together with other instrumental factors.15 SFM measurements and particularly scratching experiments were carried out to determine the thickness of all the adlayers under study. These measurements were performed under ambient conditions with a microscope head and control unit from Nanotec.16 Rectangular, Si cantilevers with a nominal force constant of k ) 2.8 Nm-1 (Nanosensors) were used. Force versus distance curves were systematically obtained to check tip and film conditions by measurement of the adhesion force. Since accurate film thickness measurements require the correct choice of substrate reference, we resorted to scratching the film with the SFM tip by imaging at relatively high loads (150-200 nN) to leave visible the bare gold underneath. Immediately after, an area including the scratched region was imaged while keeping the applied load at the lowest value (the pull-off force). This guarantees no deformation of the grown film and permits using the gold substrate surface as an excellent in situ reference for height determination. The film thickness was obtained in all cases by both height histograms and line profiles averaged within a magnified image of the modified region. XPS measurements were done in an UHV apparatus based on a modified Leybold XPS system with a double-anode X-ray source. For the measurements reported here, the Al KR X-ray source was used. The base pressure of the apparatus was below 3 × 10-10 mbar. Additional XPS and NEXAFS measurements were performed at the HE-SGM beam line of the synchrotron BESSY II in Berlin, Germany. The NEXAFS spectra17 were recorded at the C K-edge (15) For a detailed description of ToF-SIMS experiments, refer to the following: ToF-SIMS: Surface Analysis by Mass Spectrometry; Vickerman J. C., Briggs, D., Eds.; IM Publications and SurfaceSpectra, Ltd. : West Sussex, 2001. (16) Horcas, I.; Fernandez, R.; Gomez-Rodriguez, J. M.; Colchero, J.; GomezHerrero, J.; Baro, A. M. ReV. Sci. Instrum. 2007, 78, 013705. (17) For a detailed description of the NEXAFS data, see: Stepanow, S.; Strunskus, T.; Lingenfelder, M.; Dmitriev, A.; Spillmann, H.; Lin, N.; Barth, J. V.; Wo¨ll, C.; Kern, K. J. Phys. Chem. B 2004, 108, 19392.
in the partial electron yield mode with a retarding voltage of -150 V. Linear polarized synchrotron light with a polarization factor P of 82% was used. The energy resolution amounted to 0.40 eV. The incidence angle of the light was varied from 90° (E-vector in surface plane) to 30° (E-vector near surface normal). The raw NEXAFS spectra were normalized to the incident photon flux by division through a spectrum of a clean, freshly sputtered gold sample. The energy scale was referenced to the pronounced π* resonance of highly oriented pyrolytic graphite at 285.38 eV. The EPR spectra were recorded on a Bruker Elexsys E500 using a standard cavity. For these measurements, 1.0 × 1.1 cm goldcoated mica substrates (1500 Å of Au(111), Scientec) were used after being mechanically cut into pieces of 0.2-0.3 × 1.0 cm to fit the quartz 5 mm i.d. of the EPR tube. To enhance sensitivity, two or three pieces of gold/mica were introduced inside one single tube.
Results and Discussion As highlighted in the Introduction, substituted PTM radicals have been widely used as ligands to construct extended porous and magnetic coordination polymers.8,9 A great deal of work has also been dedicated to the preparation of low dimensional complexes to better understand the monocarboxylic-PTM reactivity toward metal ions as well as to explore, in simple systems, the magnetic properties resulting from its coordination to metal centers.11 PTMCOOH complexes have been prepared with copper,11a,b nickel, cobalt, and zinc (II) salts.11c Among all the explored coordination complexes, the copper (II) systems are of particular interest because the copper coordination sphere can be easily modulated depending on the reaction conditions. Thus, starting from copper (II) acetate as a metal source, it is possible to form mononuclear complexes where two PTMCOO- units bind the central copper ions through monodentate coordination bonds. To complete the copper coordination sphere of such a complex, auxiliary ligands (L1 to L3), ranging from water molecules to pyrimidine and ethanol molecules or to the sole pyridine, were added (Scheme 1a).11a,b Also particularly appealing is the possibility of forming paddlewheel supramolecular structures with these bulky ligands. In such a case, two PTMCOO- radicals and two acetate groups are involved in the formation of the dinuclear paddle-wheel, while the two copper (II) axial positions are occupied by auxiliary water molecules (Scheme 1b).11a These different complexes are easily prepared either by reaction of the copper acetate salt with the PTM carboxylate following a slow diffusion technique, or by mixing ethanolic solutions of
Grafting of Monocarboxylic PTMCOOH Radicals onto a SAM
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Scheme 2. Schematic Representation of the Two Methodologies Used to Graft the PTMCOOH Radical on Top of MHDA SAMs Using Copper (II) Ionsa
a Left: immersion of the starting SAM S1 in a copper acetate/PTMCOOH mixture to form S2. Right: “step-by-step” procedure involving the formation of an intermediate copper-functionalized SAM S3 previous to its final functionalization with the radical (S4). For the sake of simplicity, copper acetate is represented as a monomeric species and its binding to the surface as well as the binding of the radical carboxylate to the metal ion are arbitrarily represented to occur in a monodendate fashion.
Figure 2. Negative mode ToF-SIMS mass spectra of (a) S2 and (b) bare gold. In the inset, the region of the highest m/z fragment (m/z ) 718), corresponding to the loss of the carboxylate (MW ) 44) from the PTMCOO- molecule (MW ) 762). The isotopic distribution due to the presence of the 14 chlorine atoms is clearly visible.
the PTMCOOH radical and of the metal source.11 We considered this latter approach to be the most appropriate to graft the PTMCOOH radical thanks to coordination bond on suitable COOH-terminated SAMs (Scheme 1c). As a first attempt, a freshly prepared MHDA SAM (S1) was immersed in a PTMCOOH/copper acetate mixture in ethanol for 90 min (Method 1, see Experimental Section and Scheme 2 left). A preliminary analysis to check the anchoring of the metal coordination complex was given by water contact angle measurements. While a contact angle of 18° is observed for S1, the surface S2 shows a contact angle close to 80°. This result clearly indicates an increase in surface hydrophobicity after immersion in the PTMCOOH/copper acetate mixture as a result of the hydrophobic perchlorinated PTM molecules grafted on top of the surface. To discard possible interactions of the (18) Hydrogen-bond mediated bilayer formation has been previously observed during the preparation of mercaptomethyl-p-terphenylcarboxylic acid (MMTP) SAMs. See: Himmel, H. J.; Terfort, A.; Wo¨ll, C. J. Am. Chem. Soc. 1998, 120, 12069.
carboxylic-substituted radicals with the COOH-terminated surface through hydrogen-bonds18 instead of through the metal linker bonds as desired, S1 was immersed for the same time in an ethanol solution solely containing the radical, and then carefully washed with absolute ethanol. The contact angle obtained for the resulting SAM is very close to the one obtained for the starting S1 (around 18°), clearly demonstrating that the hydrogen bonds, which could possibly be formed between the COOH groups of S1 and the ones of PTMCOOH units, are not strong enough to resist the cleaning step. A further proof of the anchoring of PTM units to the surface in S2 was given by ToF-SIMS measurements (Figure 2). For sample S2, the highest m/z fragment detected was the anion corresponding to the loss of the CO2 group for PTMCOO-, i.e. [M-CO2], m/z ) 718. The other stable anions correspond to the subsequent losses of one and two chlorine atoms, with m/z ) 683 and m/z ) 648, respectively.19 All of these peaks show isotopic (19) Fox, M. A.; Gaillard, E.; Chen, C-C. J. Am. Chem. Soc. 1987, 109, 7088.
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Figure 3. Top: topographic SFM images of S1 and S2 before ((a) and (c)) and after ((b) and (d)) the scratching experiments at high loads (150-200 nN). Bottom: the height histograms of magnified images (insets) of areas including the scratched region are used to determine the corresponding adlayer thickness.
Figure 5. CW EPR spectrum of S2 after subtraction of the spurious signal from the substrate. EPR parameters: frequency, 9.40543 GHz; 2048 points; modulation amplitude, 3G; number of scans, 20; power, 7.86 mW. The position of this line corresponds to g ) 2.0037. S4 surface shows an EPR signal very similar to that of S2 (See Supporting Information Figure ESI 6).
substrate gold grain covered by each particular adlayer (Figure 3, parts a and c), the load is increased up to about 150-200 nN to scratch the film by sweeping over a smaller area (lateral size ≈ 200-300 nm in these cases). The effect of the scratching process can be observed, after releasing the load, in Figure 3, parts b and d, respectively. Height histograms performed on selected regions containing the scratch, are used to calculate the film thickness. As it can be clearly observed, the S2 adlayer is considerably thicker (by about ∼1.5nm) than the MHDA monolayer. This value is in good agreement with the expected one for additional vertically oriented PTMCOO-Cu units on top of the starting MHDA SAM.21 S1 and S2 surfaces were also characterized by IRRAS (Figure 4). As anticipated from the contact angle measurements, the IR-spectrum recorded for S1 presents the typical set of bands expected for a fully protonated COOH SAM, as evidenced by the presence of IR bands at 1717 and 1739 cm-1.22 In contrast, the spectrum for S2 presents several bands in agreement with the presence of the PTM units on top of the surface with weak intensity bands at 827 cm-1, typical of halogenated phenyl groups and at 1330 and 1257 cm-1, already observed in the starting ligand spectrum. It is also important to notice the strong decrease of the bands at 1717 and 1739 cm-1 as well as the presence of two additional bands at 1430 and 1544 cm-1 which are characteristic wavenumbers for PTMCOO- groups bound to metal ions.23 Altogether, these modifications demonstrate the following: (i) most part of the carboxylic groups from S1 are now involved in coordination bonds with copper ions; and (ii) the copper ions are bound with PTM radicals or with remaining acetate groups. To further characterize the S2 surface and hence verify the formation of the paramagnetic metal complex, room temperature CW X-band EPR experiments were also performed. We first recorded the EPR spectrum of S1 and use it as blank, to get rid
patterns corresponding to the 14, 13, and 12 chlorine atoms present in these ions, respectively.20 Substrates S1 and S2 have also been characterized by SFM scratching experiments. As illustrated for both samples in Figure 3, the thicknesses of the adlayers are determined following the procedure described in the Experimental Section f. After imaging at the lowest practical load, a relatively large area containing a
(20) In Mass Spectrometry and its Applications to Organic Chemistry; Beynon, J. H., Ed.; Elsevier: Amsterdam, 1960, p 298. (21) Expected adlayer thickness could be estimated to be around 1.5 nm analyzing the crystallographic data reported for the (PTMCOO)2-Cu mononuclear complex. See ref 11. (22) Smith, E. L.; Alves, C. A.; Anderegg, J. W.; Porter, M. D.; Siperko, L. M. Langmuir 1992, 8, 2707. (23) Ballester, M.; Riera, J.; Castaner, J.; Badía, C.; Monso, J. J. Am. Chem. Soc. 1971, 93, 2215.
Figure 4. IRRAS spectra of (a) S1, (b) S1 after immersion in a PTMCCOH ethanol solution and careful cleaning with absolute ethanol, (c) S2 and (d) PTMCOOH ligand (registered in KBr pellets), as a reference.
Grafting of Monocarboxylic PTMCOOH Radicals onto a SAM
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Figure 6. C1s, O1s, Cl2p, Au4f, and Cu2p XP spectra of S1 (filled dots) and S2 (void dots).
Figure 7. C K-edge NEXAFS spectra of SAMs (a) S1 and (b) S2.
of any spurious signal that could arise from the Au/mica substrate. As expected, no EPR signal could be recorded for the diamagnetic S1 (Supporting Information Figure 1). For the freshly prepared S2, instead, we recorded a signal of rather low intensity but characteristic of a PTM derivative (Figure 5).7 These experiments confirm that the S2 surface is magnetically active. XPS and NEXAFS experiments provided deeper information on the sample composition. XPS data obtained for S2 and S1 (as a reference) are presented in Figure 6. The relative intensities for the C1s, O1s, and Au4f lines obtained for S1 are in perfect agreement with the data previously reported for MHDA SAMs.13 The intense peak (around 285 eV) in the C1s spectrum (Figure 6a) is due to the C15-alkyl chains of the MHDA molecules, while the signal from the remaining Csp2 carbon atoms (from COOH) is much weaker and appears at around 289.5 eV. The single peak observed in the O1s spectrum (Figure 6b) is assigned to oxygen atoms in the COOH groups. Strong modifications are observed in the case of S2. The intensity decrease observed for the Au4f peaks clearly indicates, in agreement with the SFM experiments, an increase of the layer thickness when moving from S1 to S2 (Figure 6d). Modifications are also observed in the S2 C1s and O1s spectra. The slight increase in the C1s peak
at 284.5 eV is in agreement with the presence of acetate groups bonded to Cu2+ ions (CH3 groups) while the appearance of a shoulder at 286.4 eV fully agrees with the grafting of molecules containing sp2 carbon atoms on top of the surface (chlorinated phenyl groups of the PTM ligand). The shift observed for the peak characteristic of the COO–carbon atoms from either the SAM, the acetate, or the PTM molecules may indicate their bonding to copper ions. This hypothesis is confirmed by the S2 O1s XP spectrum: the intensity of the O1s peak at 533 eV increases indicating an increase in the amount of oxygen atoms and the peak becomes more symmetrical clearly evidencing the deprotonation of the COOH groups which interact with copper ions as carboxylates. However, the most important results are provided by the presence of the typical signature for chlorine atoms and copper (II) ions observed in the corresponding Cl2p and Cu2p spectra (Figure 6, parts c and e, respectively). These observations clearly confirm the presence of both copper ions and chlorinated molecules in S2. Figure 7 shows the C K-edge polarization dependence NEXAFS of S1 and S2. Figure 7a shows the expected features for the MHDA SAM, namely the C(C-H) f R*, the C(CdO) f π*, and the C-C f σ* resonances.22,24 Resonance values and signal assignments are listed in Table 1. Interestingly, the spectrum registered for S2 exhibits 4 additional resonance peaks at 282.9, 285.4, 285.5, and 286.6 eV, respectively (Figure 7b). These last two resonance peaks could be assigned in a straightforward fashion to the C(Ph) f π* transition in the perchlorinated phenyl rings of the PTM molecule.25 The peak at 285.4 eV could be attributed, in agreement with the IRRAS and XPS results, to the presence of residual acetate groups coordinated to copper ions. The presence of the lower energy peak is surprising at first sight, since in previous work26 it has been demonstrated that the Fermi edge states of measured elements are located above the corresponding XPS C1s binding energy, which is equal to 284.9 eV in the present case. Thus, the (24) Sto¨hr, J. Springer Ser. Surf. Sci. 1996, 25. (25) Orlov, A.; Watson, D. J.; Williams, F. J.; Tikhov, M.; Lambert, R. M. Langmuir 2007, 23, 9551. (26) Bruhwiler, P. A.; Karis, O.; Martensson, N. ReV. Mod. Phys. 2002, 74, 703.
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Table 1. Assignments of NEXAFS Resonances resonance
energy (eV)
assignment
1 2 3 4 5 6 7 8
282.9 285.4 285.5, 285.6 287.9 288.7 291.5 293.6 299.5
C 1s f “radical” (HOMO of ligand) C(CdO) f π* (CH3COO-Cu2+) C(Ph) f π* (perchlorinated ligand) Rydberg of alkyl chain C(COOH) f π*(COOH) σ* σ* σ*
small peak located at an energy of 282.9 eV (which is actually more prominent in the case of S4, see below) indicates the presence of empty states in the ligand with energies below the Fermi level, as expected for a radical grafted to the substrate (see also discussion below). In order to assign all of the resonance peaks observed in NEXAFS, but also to explore the possibility of functionalizing the MHDA SAM following a step-by-step procedure, S1 substrate was first immersed in a copper acetate solution in ethanol during 30 min. The resulting SAM S3 was carefully washed with ethanol, gently dried under nitrogen flux, and subsequently immersed in a PTMCOOH solution during one hour (Method 2, see Experimental Section and Scheme 2-right,), to afford the functionalized surface S4. The legitimacy of this step-by-step procedure was first controlled by means of SPR experiments, since this technique allows monitoring the deposition of molecular species on Au surfaces with submonolayer resolution.27 Starting from the S1 surface, Figure 8 shows a representative kinetic curve obtained by monitoring the change in percentage of reflectivity caused by the sequential adsorption of copper (II) acetate (to form S3) and of the PTMCOOH ligand (to form S4). As expected, upon the sequential introduction of both Cu(OAc)2 (A) and PTMCOOH ligand (C), an increase of the signal intensity is observed due to the adsorption of both species on the initial surface. This result clearly reflects the metal-organic adlayer formation on the SAM surface. Also noteworthy is the fact that the cleaning procedure with ethanol (B) allows to eliminate both nonreacted metal ions and PTM ligands after each step, as attested by the signal intensity decrease observed after each ethanol injection. As in the case of SAM S2, surfaces S3 and S4 were fully characterized by contact angle, IRRAS, SPR, ToF-SIMS, SFM, XPS, EPR, and NEXAFS measurements. As shown in Supporting Information Figure ESI 2, contact angle measurements reflect an increase of the surface hydrophobicity moving from the starting surface S1 to the surface treated by copper acetate S3 (43°). Values found for S4 are in perfect agreement with the ones encountered for S2, (80°) transducing a comparable surface coverage whatever the methodology used to functionalize S1. The IRRAS spectrum of S4 (Supporting Information Figure ESI 3) is similar to that of S2 with bands at 827 cm-1, 1257 and 1330 cm-1, and 1430 and 1544 cm-1. Nevertheless, the last two bands are also observed in the case of S3, in agreement with the presence of remaining acetate groups in the final surfaces S4 (and by extension, in the S2 surface resulting from Method 1). The ToFSIMS mass spectrum recorded for S4 (Supporting Information ESI Figure 4) shows the same fragments as S2, further confirming that both preparation procedures lead to the anchoring of the radical to the surface mediated by the metal linker. XPS measurements clearly show the presence of copper (II) ions on (27) (a) Mrksich, M.; Sigal, G. B.; Whitesides, G. M. Langmuir 1995, 11, 4383. (b) Spinke, J.; Yang, J.; Wolf, H.; Liley, M.; Ringsdorf, H.; Knoll, W. Biophys. J. 1992, 63, 1667. (c) Spinke, J.; Liley, M.; Guder, H. J.; Angermaier, L.; Knoll, W. Langmuir 1993, 9, 1821.
Figure 8. SPR-signal as a function of time recorded in situ during sequential injections of an ethanolic solution of Cu(OAc)2 (A), absolute ethanol (B), and an ethanolic solution of PTMCOOH (C) in the SPRcell containing S1.
Figure 9. Left: C-K edge NEXAFS spectra of (a) S1 (as a reference), (b) S3, and (c) S4. Right: Schematic electronic structure of radical ligand. Depending on the value of d (distance between radical center and metal surface), the partially filled HOMO will be occupied by an electron tunneling from the substrate (see text).
the S3 surface, while both copper (II) ions and perchlorinated molecules are evidenced on S4 surface (Supporting Information Figure ESI 5, parts d and e, respectively). EPR spectra of each step of the preparation of S4 have been recorded. As already said, no signal could be detected for S1 (Supporting Information Figure ESI 6c). Also, the freshly prepared S3 turned out to be EPR-silent (Supporting Information Figure ESI 6b). First, we checked whether the absence of signal was related to a lack in sensitivity. To discard this possibility, we deposited a few drops of a concentrated solution of copper acetate onto the MHDA functionalized gold in order to form a thin film rather than a single layer of Cu (II) ions. None EPR signal (Supporting Information Figure ESI 7) was either detected in this case. On the basis of the XPS measurements, we know that Cu (II) ions have been grafted onto S1; so this result is explained by the fact that the copper acetate maintains its EPR silent paddlewheel structure when bound to the carboxylic group of the MHDA.28 This result can be considered as the first experimental evidence of the specific geometry of the copper when bound to the MHDA SAM. After immersion of S3 into the radical solution to get S4, we recorded a signal attributable to the PTM radical
Grafting of Monocarboxylic PTMCOOH Radicals onto a SAM
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Figure 10. SEM images of 3-µm stamped samples for (a) p-S1, (b) p-S3 and (c) p-S4.
grafted on the surface (Supporting Information Figure ESI 6a). This rather low signal is very similar to the one recorded for S2 shown in Figure 5. The absence of two distinct signals for the Cu(II) and the PTMCOO- species and instead the presence of only one asymmetric line attributable to the grafted radical7 for both S2 and S4 might be ascribed either to the EPR silent paddlewheel structure of the Cu(II) complex anchored to the surface, or to the presence of an exchange coupling interaction between the Cu(II) and the PTM radical.11b Unfortunately, the signalto-noise ratio of the recorded spectra is too low to discriminate between these two hypotheses. Striking results are supplied by NEXAFS measurements performed on the two different SAMs S3 and S4 (Figure 9). From the S3 spectrum (Figure 9b), the resonance peak at 285.4 eV can be clearly assigned to acetate groups coordinated to copper ions (C(CdO) f π* of CH3COO-Cu2+). In the reference case of the S1 surface (Figure 9a), where the carboxylic groups are associated through hydrogen bonds, the peak at 288.7 eV does not significantly change with the incidence angle. On the contrary, in the case of S4 (Figure 9c), the intensity of the peak at 288.7 eV varies with the incidence angle, evidencing a clear change in the COOH groups orientation22 and demonstrating that the SAM COO–groups are now involved in coordination bonds. In the S4 spectrum the peak corresponding to the acetate group is clearly visible as well. The observation of the acetate group signature in the spectra of both S2 and S4 surfaces (where it overlaps or shoulders with one of the resonances corresponding the C(Ph) f π* transition in the perchlorinated ligand at 285.5 eV), can be analyzed taking into account the EPR results as follows: (i) the surface is exclusively functionalized by copper paddle-wheel units involving PTM and acetate groups (as in the case of the PTMCOO-copper dinuclear complex, Scheme 1b), (ii) these latter PTM-acetate paddle wheel units coexist together with copper paddle-wheel units exclusively based on acetate ligands (as in the case of the starting copper acetate). The results supplied by the S4 spectrum give even more interesting information: since the resonance peak at 282.9 eV only appears in S4, it corresponds to a signature of the PTM radical. This peak, which is actually more intense than in the case of S2 discussed above, is located below the corresponding C1s binding energy of 284.9 eV, and thus unequivocally reveals the presence of empty states with energies below the substrate Fermi level. Accordingly, this peak has to be assigned to an excitation of C1s core-electrons in the half-filled highest occupied molecular orbital (HOMO) of the radical ligand. The fact that we can observe this peak demonstrates that the distance between the metal substrate and the surface is so large that electrons cannot tunnel from the Au-metal into these empty states. In fact, from the thickness of the SAM (19 Å) and the distance between the radical center of the PTMMC-unit (13 Å), we estimate this distance to be 32 Å, which is consistent with a very low tunneling probability. This peak, already observed for S2, clearly demonstrates that the ligand is still an open-shell molecule after grafting it to the surface
following both Method 1 and Method 2, thus confirming the EPR results. SFM images analysis for each step of the S4 preparation and the comparison with S2 have been collected in the Supporting Information (Supporting Information Figure ESI 8). Surprisingly, in the case of S4 a less important film thickness difference (∼1 nm) with respect to S2 was observed. While in Method 1, the starting SAM is immersed in a mixture containing both the PTM radical and the metal source (where also ethanol insoluble copperPTM complexes could be formed), in the step-by-step procedure used in Method 2, the nonreacted material after each step is removed thanks to the cleaning process with ethanol, as evidenced by the SPR experiments. Consequently, the thickness obtained by this second approach really corresponds to the thickness of the PTMCOO-Cu adlayer. The discrepancy between the measured adlayer thickness and the expected one can be either ascribed to a tilt of the PTMCOO-Cu units with respect to the surface or to a lower than 100% surface coverage.7 Both of these hypotheses are in good agreement with a surface functionalized by both copper-acetate and copper-acetate-PTM metal-organic units, as evoked above. Therefore, the larger adlayer thickness (by ∼1.5 nm) observed for S2 could be attributed to the presence of copper-PTM mononuclear complexes interacting with the PTMCOO-Cu units grafted to the surface thanks to chlorinechlorine short contacts.29,30 With the aim of verifying if the copper complex is anchored to the surface through the carboxylic groups of the SAM, Methods 1 and 2 were used on MHDA SAM surfaces patterned by microcontact printing (µCP). If, as expected, selective grafting takes place, patterned complex formation would be achieved. The SEM measurements for the laterally structured surfaces p-S3 and p-S4 resulting from Method 2 are shown in Figure 10. The contrast difference before and after interaction of the p-S1 surface with copper acetate (to form p-S3) and then with the PTMCOOH solutions (to obtain p-S4) clearly demonstrate the selective grafting of the PTMCOO- ligand on the COOH terminated areas of the surface thanks to coordination bonds with copper (II) ions.
Conclusions In summary, the PTMCOOH radical was successfully grafted on a COOH-functionalized SAM using copper (II) ion as linkers. The Contact Angle, IRRAS, SPR, ToF-SIMS, SFM, EPR, XPS, and NEXAFS data clearly demonstrate that the grafting of this ligand can be easily carried out either immersing the MHDA SAM in copper-radical mixture or by a step-by-step procedure, (28) (a) Carlin, R. L. Magnetochemistry; Springer-Verlag: Berlin, Heidelberg; 1986, Chapter 5. (b) By drop-casting onto the gold surface of CuCl2, a Cu (II) salt not presenting the paddle-wheel structure, a weak EPR signal corresponding to Cu(II) could be recorded. (29) Chlorine-chlorine short contacts are encountered in all of the PTMbased coordination polymers. See refs 8-11. (30) (a) Sarma, J. A. R. P.; Desiraju, G. R. Acc. Chem. Res. 1986, 19, 222. (b) Desiraju, G. R.; Parthasarathy, R. J. Am. Chem. Soc. 1989, 111, 8725.
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introducing first the starting surface in a metal source and then the resulting copper functionalized surface into a radical solution. The EPR-data indicate the presence of a magnetic surface, whereas the NEXAFS-results clearly reveal that the highest occupied molecular orbital (HOMO) is only partially filled, evidencing the radical nature of the grafted ligand. The SEM measurements on a laterally structured SAM clearly show that the grafting of the PTMCOOH radical is selective for the COOH functionalized SAM. This approach represents a new and easy way to organize organic radicals on surfaces. This work also constitutes a first step toward the construction of extended, porous, and magnetic PTM-based coordination polymers on top of surfaces if polyfunctionalized PTM radicals are used in the
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grafting process. We are currently working on the realization of such “surface-driven” 3D structures based on PTM radicals, starting from colloidal suspensions5 or even following a “stepby-step” procedure.6 Acknowledgment. This project has been funded by the European Union under FP 6 (Contract No. STRP032109 “SURMOF”) and by DGI, Spain (Project CTQ2006-06333/BQU; “EMOCIONA”). Supporting Information Available: This material is available free of charge via the Internet at http://pubs.acs.org. LA800771Q
