Charge-Transfer Complex Study by Chemical Force Spectroscopy: A

Charge-transfer interaction, as a reversible and rapid phenomenon, was evidenced by force microscopy. Pull-off forces were measured between a tip graf...
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Langmuir 2007, 23, 542-548

Charge-Transfer Complex Study by Chemical Force Spectroscopy: A Dynamic Force Spectroscopic Approach Richard Gil, Marie-George Guillerez, Jean-Claude Poulin,* and Emmanuelle Schulz Equipe Catalyse Mole´ culaire, Institut de Chimie Mole´ culaire et des Mate´ riaux d’Orsay, UMR 8182, UniVersite´ Paris-Sud, Baˆ t 420, 91405 Orsay Cedex, France ReceiVed July 24, 2006. In Final Form: October 6, 2006 Charge-transfer interaction, as a reversible and rapid phenomenon, was evidenced by force microscopy. Pull-off forces were measured between a tip grafted with a trinitrofluorenone derivative and a surface functionalized with an electron-rich aromatic anthracene compound in a dodecane environment. The effect of the sweep time on the measured interaction forces is described, together with an extensive study of a competitive influence of free aromatic molecules in dodecane diluted solutions. These forces depend on the nature of the competitor and its concentration as well as on the velocity of tip/sample separation.

Introduction Different aspects of force measurements by AFM (atomic force microscopy)1 were recently exhaustively reviewed. Instrumental, physical, and theoretical points of view were discussed,2 as well as force spectroscopy of molecular systems.3,4 Conventional force spectroscopy measurement was mainly used in the biology-biophysics and polymer fields. Complementary molecules as double-strands DNA5,6 and ligand-receptor (or antibody-antigen) complexes7-10 are often studied (rupture of bonds), but also phenomena such as softening of proteins,11 unfolding of filamentous proteins,12-16 elasticity of synthetic polymers,17,18 or conformational modifications of polysaccharides.19 Nevertheless, applications in chemistry are scarce, but they can be found in areas such as studies of van der Waals and hydrogen bonding interactions in different solvents.20,21 The pH * E-mail: [email protected]. (1) Binnig, G.; Quate, C. F.; Gerber, C. Phys. ReV. Lett. 1986, 56, 930. (2) Butt, H.-J.; Cappella, B.; Kappl, M. Surf. Sci. Rep. 2005, 59, 1. (3) Fisher, T. E.; Marsalek, P. E.; Oberhauser, A. F.; Carrion-Vazquez, M.; Fernandez, J. M. J. Phys. (Oxford, U. K.) 1999, 520, 5. (4) Janshoff, A.; Neinzert, M.; Oberdo¨rfer, Y.; Fuchs, H. Angew. Chem., Int. Ed. 2000, 39, 3212. (5) Lee, G. U.; Chrisley, L. A.; Colton, R. J. Science 1994, 266, 771. (6) Albrecht, C.; Blank, K.; Lalic-Multhaler, M.; Hirler, S.; Mai, T.; Gilbert, I.; Schiffmann, S.; Bayer, T.; Clausen-Schaumann, H.; Gaub, H. E. Science 2003, 301, 367. (7) Lee, G. U.; Kidwell, D. A.; Colton, R. J. Langmuir 1994, 10, 354. (8) Florin, E. L.; Moy, V. T.; Gaub, H. E. Science 1994, 264, 415. (9) Dammer, U.; Hegner, M.; Anselmetti, D.: Wagner, P.; Drier, M.; Huber, W.; Guntherodt, H. J. Biophys. J. 1996, 70, 2437. (10) Hinterdorfer, P.; Baumgartner, W.; Gruber, H. J.; Schilcher, K.; Schindler, H. Proc. Natl. Acad. Sci. 1996, 93, 3477. (11) Schliert, M.; Rief, M. J. Mol. Biol. 2005, 354, 497. (12) Kellermayer, M. S. Z.; Granzier, H. L. Biochem. Biophys. Res. Commun. 1996, 221, 491. (13) Rief, M.; Gautel, M.; Oersterhelt, F.; Fernandez, J. M.; Gaub, H. E. Science 1997, 276, 1109. (14) Tskhovrebova, L.; Trinick, J.; Sleep, J. A.; Simmons, R. M. Nature (London) 1997, 387, 308. (15) Erikson, H. P. Science 1997, 276, 1090. (16) Oberhauser, A. F.; Marsalek, P. E.; Erikson, H. P.; Fernandez, J. M. Nature (London) 1998, 393, 181. (17) Bemis, J. E.; Achremitchev, B. B.; Walker, G. C. Langmuir 1999, 15, 2799. (18) Ortiz, C.; Hadziioannou, G. Macromolecules 1999, 32, 780. (19) Li, H.; Rief, M.; Oersterhelt, F.; Gaub, H. E. AdV. Mater. 1998, 3, 316. (20) Frisbie, C. D.; Rozsnyai, L. F.; Noy, A.; Wrighton, M. S.; Lieber, C. M. Science 1994, 265, 2071. (21) Noy, A.; Frisbie, C. D.; Rozsnyai, L. F.; Wrighton, M. S.; Lieber, C. M. J. Am. Chem. Soc. 1995, 117, 7943.

dependency of carboxylic acid/amine interactions was also measured in water.22 The characterization of chirality in diastereomeric interactions such as chromatography Pirkle phase23 with mandelic acid derivatives24 or between amines and amino acids25 was reported as a work of fundamental interest. In recent publications, the chemical force spectroscopy was applied to the study of host-guest complexes.26 The surface functionalization was mainly performed by using the self-assembled monolayer technique (sulfur derivatives on gold surfaces);27 however, the rupture force of an individual sulfur link was found to be 100 pN associated to the abstraction of one Au atom from the gold surface.28 Within the domain of large biologic and synthetic molecules, adsorption29 was used to link chains without specific connection on the tip and/or the support surface. Recently, dynamic effects induced by the value of sweep duration were explored (dynamic force spectroscopy, DFS).30 Obviously, the kinetics of motion was maintained constant along the recording of a force-distance curve. With hard samples, constant mechanical speed along the sweep involved an unchanged pulling force velocity during the dissociation part of the sweep. In the biological domain, at the single-molecule level involving weak noncovalent interactions, the theory anticipates a logarithmic increase of the interaction force with the separation speed.31 This was mainly observed for DNA,32 ligand-receptor complexes,33-36 redox partners,37 lipid pull-out experiments,38 elasticity of proteins,39,40 and polysaccharides.41 (22) Vezenov, D. V.; Noy, A.; Rozsnyai, L. F.; Wrighton, M. S.; Lieber, C. M. J. Am. Chem. Soc. 1997, 119, 2006. (23) Pirkle, W. H.; Finn, J. M.; Schreiner, J. L.; Hamper, B. C. J. Am. Chem. Soc. 1981, 103, 3964. (24) McKendry, R.; Theoclitou, M. E.; Rayment, T.; Abell, C. Nature (London) 1998, 391, 566. (25) Mahapatro, M.; Gibson, C.; Abell, C.; Rayment, T. Ultramicroscopy 2003, 97, 297. (26) Kado, S.; Yamada, K.; Kimura, K. Langmuir 2004, 20, 3259. (27) Grandbois, M.; Beyer, M.; Rief, M.; Clausen-Schaumann, H.; Gaub, H. E. Science 1999, 283, 1727. (28) Skulason, H.; Frisbie, C. D. J. Am. Chem. Soc. 2000, 122, 9750. (29) Zhang, W. K.; Zou, S.; Wang, C.; Zhang, X. J. Phys. Chem. B 2000, 104, 10258. (30) Evans, E.; Ritchie, K. Biophys. J. 1997, 72, 1541. (31) Evans, E. Annu. ReV. Biophys. Biomol. Struct. 2001, 30, 105. (32) Grange, W.; Strunz, T.; Schumakovitch, I.; Guntherodt, H. J.; Hegner, M. Single Mol. 2001, 2, 75. (33) Fritz, J.; Katopodis, A. G.; Kolbinger, F.; Anselmetti, D. Proc. Natl. Acad. Sci. 1998, 95, 12283.

10.1021/la062169h CCC: $37.00 © 2007 American Chemical Society Published on Web 11/17/2006

Charge-Transfer Complex Study

In the study of β-cyclodextrin host-guest complexes42-44 and in some polymer chain elongations,45,46 the measured interaction forces were not dependent on the traction velocity. A somewhat different situation was reported for other systems. When a large number of interactions are indeed involved, an easy rebinding is in competition with the corresponding dissociation phenomenon, leading to increased measured forces with decreased speed velocity (Ni-NTA/histidine).47 In other studies on polymer chain elongations, the measured interaction forces increased with decreasing traction velocity (silica-poly(2-vinylpyrrolidone), silica-poly(2-vinylpyridine),48,49 and silicon nitride-stearic acid50 in water). As the phenomenon of interaction is highly complex, the measured interaction forces are at least also dependent on the temperature,51 contact force,52 ionic strength,53 viscosity, diffusion, and interaction time.52,54,55 Charge-transfer complexes (CTC), as reversible and weak interactions between donor and acceptor compounds, have been the subject of numerous investigations for both the understanding of their formation mechanism and their use in various fields. These complexes have been used in analytical applications (chromatographic separation)56 and have also been involved in biological systems for molecular recognition.57 Furthermore, selective organic transformations have been controlled by donoracceptor interactions,58 and CTC molecular clips have been used in supramolecular chemistry.59 These donor-acceptor complexes have also been tested for an industrial application in the purification of gas oil.60 AFM, as a direct and sensitive force measurement in a nondestructive manner, was therefore chosen to investigate these adhesion forces acting between properly (34) De Paris, R.; Strunz, T.; Oroszlan, K.; Scha¨fer, R.; Guntherodt, H. J.; Hegner, M. Single Mol. 2000, 1, 285. (35) Lo, Y. S.; Zhu, Y. J.; Beebe, T. P., Jr. Langmuir 2001, 17, 3741. (36) Neuert, G.; Albrecht, C.; Pamir, E.; Gaub, H. E. FEBS Lett. 2006, 580, 505. (37) Bonanni, B.; Kamruzzahan, A. S. M.; Bizzarri, A. R.; Rankl, C.; Gruber, H. J.; Hinterdorfer, P.; Cannistraro, S. Biophys. J. 2005, 89, 2783. (38) Wieland, J. A.; Gewirth, A. A.; Leckband, D. E. J. Phys. Chem. B 2005, 109, 5985. (39) Rief, M.; Gautel, M.; Schemmel, A.; Gaub, H. E. Biophys. J. 1998, 75, 3008. (40) Bertoncini, P.; Schoenauer, R.; Agarkova, I.; Hegner, M.; Perriard, J.-C.; Guntherodt, H. J. J. Mol. Biol. 2005, 348, 1127. (41) Stletmoen, M.; Skjåk-Bræk, G.; Stokke, B. T. Carbohydr. Res. 2005, 340, 2782. (42) Scho¨nherr, H.; Beulen, M. W. J.; Bu¨gler, J.; Huskens,. J.; van Veggel, F. C. J. M.; Reinhoudt, D. N.; Vancso, G. J. J. Am. Chem. Soc. 2000, 122, 4963. (43) Zapotocny, S.; Auletta, T.; de Jong, M. R.; Scho¨nherr, H.; Huskens, J.; van Veggel, F. C. J. M.; Reinhoudt, D. N.; Vancso, G. J. Langmuir 2002, 18, 6988. (44) Auletta, T.; de Jong, M. R.; Mulder, A.; van Veggel, F. C. J. M.; Huskens, J.; Reinhoudt, D. N.; Zou, S.; Zapotocny, S.; Scho¨nherr, H.; Vancso, G. J.; Kuipers, L. J. Am. Chem. Soc. 2004, 126, 1577. (45) Cui, S.; Liu, C.; Wang, Z.; Zhang, X. Macromolecules 2004, 37, 946. (46) Long, J.; Xu, Z.; Masliyah, J. H. Langmuir 2006, 22, 1652. (47) Le´vy, R.; Maaloum, M. Biophys. Chem. 2005, 117, 233. (48) Biggs, S. J. Chem. Soc., Faraday Trans. 1996, 92, 2783. (49) Biggs, S.; Proud, A. D. Langmuir 1997, 13, 7202. (50) Capella, B.; Baschieri, P.; Ruffa, M.; Ascoli, C.; Relini, A.; Rolandi, R. Langmuir 1999, 15, 2152. (51) Zepeda, S.; Yeh, Y.; Noy, A. Langmuir 2003, 19, 1457. (52) Vander Wal, M.; Kamper, S.; Headley, J.; Sinniah, K. Langmuir 2006, 22, 882. (53) Vengasandra, S.; Sethumadhavan, G.; Yan, F.; Wang, R. Langmuir 2003, 19, 10940. (54) Hemmerle´, J.; Altmann, S. M.; Maaloum, M.; Ho¨rber, J. K. H.; Heinrich, L.; Voegel, J.-C.; Schaaf, P. Proc. Natl. Acad. Sci. U.S.A. 1999, 96, 6705. (55) Brayshaw, D. J.; Berry, M.; McMaster, T. J. Ultramicroscopy 2004, 100, 145. (56) Pirkle, W. H.; Pochapsky, T. C.; Mahler, G. S.; Corey, D. E.; Reno, D. S.; Alessi, A. L. J. Org. Chem. 1986, 51, 4991. (57) Mulliken, R. S. J. Am. Chem. Soc. 1952, 74, 811. (58) Lemaire, M.; Guy, A.; Guette, J.-P. Bull. Soc. Chim. Fr. 1985, 3, 477. (59) Zimmerman, S. C.; Vanzyl, C. M. J. Am. Chem. Soc. 1987, 109, 7894. (60) Sevignon, M.; Macaud, M.; Favre-Re´guillon, A.; Schulz, J.; Rocault, M.; Faure, R.; Vrinat, M.; Lemaire, M. Green Chem. 2005, 7, 413.

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modified tip and surface. In this context, Skulason and Frisbie61 published the successful measurement of the chemical binding forces of discrete CTC between gold-coated tips and substrates through alkanethiol self-assembled monolayers. They were able to measure pull-off forces between modified tips and substrates in chloroform and to propose a 70 pN force assigned to the rupture of an individual CTC. We have investigated the functionalization of a tip and a probe with an electron-deficient and an electron-rich compound, respectively, using siloxane derivatives, a support modification rarely performed for AFM measurements.62 This system was used to study the charge-transfer interactions in different solvents containing other aromatic compounds as competitors. We report here the detailed AFM studies of these donor-acceptor complexes in the presence of competitors in solution at fixed velocity of the tip/sample separation. The influence of the sweep duration on the measured interaction strength, i.e., dynamic experiments, is also described. Experimental Section General. Force spectroscopy experiments were carried out using a Molecular Imaging Pico SPM equipped with an S-type AFM scanner. Molecular Imaging PicoScan 5.3.1 software was used to control force/distance spectroscopy; sweep amplitude was always 600 nm, with a duration of 0.01 to 100 s (speed: 120 µm.s-1 to 12 nm.s-1); records were done with 2000 data points with permanent sweeps (loop multiple sweeps), without limit of deflection and 10 ms of presweep delay. V-shaped, thin-legged 200-µm-long with a pyramidal unsharpened tip Nanoprobe Si3N4 cantilevers (Digital Instruments) were used (spring constant about 0.06 N.m-1). Dichloromethane and toluene were distilled over CaH2 under argon. A Bruker AM 250 spectrometer, operating at 250 MHz for 1H, and at 62.9 MHz for 13C, was used for the NMR spectra; chemical shifts are reported in parts per million (ppm) downfield from tetramethylsilane for spectra recorded in CDCl3. Infrared spectra were recorded using potassium bromide pellets on a Perkin-Elmer 1000 FT-IR spectrometer and are reported in reciprocal centimeters. High-resolution mass spectra (HRMS) were measured with a Finnigan-Mat 95 S spectrometer. Ultraviolet-visible spectra of chloroform solutions were recorded on a Bio-tek instruments Uvikon XL spectrometer. N-[3-(Trimethoxysilyl)propyl)]-2,5,7-trinitro-9-oxofluorene4-carboxamide (1). In a 100 mL round-bottomed flask, 2,5,7-trinitro9-oxofluorene-4-carboxylic acid63 (830 mg, 2.31 mmol) and dicyclohexylcarbodiimide (655 mg, 3.17 mmol) were dissolved in 50 mL of dichloromethane. To this solution, 3-(trimethoxysilyl)propylamine (480 µL, 2.54 mmol) was added. The yellow reaction mixture changed to red immediately. The solvent was removed under vacuum, and the residue was purified by using flash chromatography (ethyl acetate). To remove completely the dicyclohexylurea, a crystallization in a pentane-dichloromethane mixture was necessary to obtain 830 mg of 1 (69% yield). Mp ) 185-195 °C. 1H NMR (250 MHz, CDCl3): δ 8.90 (s, 1H), 8.79 (s, 1H), 8.72 (s, 1H), 8.66 (s, 1H), 7.35 (bs, 1H, NH), 3.63 (s, 9H, 3× -OCH3), 3.49 (m, 2H), 1.89 (m, 2H), 0.85 (t, 2H, J ) 7.3 Hz). 13C NMR (62.9 MHz, CDCl3): δ 185.41, 165.04, 149.47, 149.09, 147.21, 143.64, 139.99, 138.77, 138.01, 136.87, 127.91, 125.15, 122.03, 120.67, 50.78, 42.64, 22.07, 6.70. IR (KBr) (cm-1): ν 3088, 2942, 2842, 1738, 1630, 1591, 1531, 1452, 1345, 1195, 1081, 915, 798, 737, 705. HRMS (IE, 70 eV): calcd for C20H20N4O11Si 520.0912, found 520.0897. N-[3-(Triethoxysilyl)propyl]-anthracen-9-ylmethyl Carbamate (2) (according to a procedure described in ref 64). In a Schlenk tube, 1 mL of triethylamine was added under argon to a solution of anthracen-9-yl methanol (417 mg, 2 mmol) in 5 mL of dichlo(61) Skulason, H.; Frisbie, C. D. J. Am. Chem. Soc. 2002, 124, 15125. (62) Gil, R.; Fiaud, J.-C.; Poulin, J.-C.; Schulz, E. Chem. Commun. 2003, 2234. (63) Laboratoire de Catalyse et Synthe`se Organique, Universite´ Claude Bernard Lyon I, is gratefully acknowledged for a generous gift of this compound. (64) Hesemann, P.; Moreau, J. J. E. Tetrahedron: Asymmetry 2000, 11, 2183.

544 Langmuir, Vol. 23, No. 2, 2007 romethane. After addition of 3-(triethoxysilyl)propyl isocyanate (550 µL, 2.2 mmol), the homogeneous reaction mixture was stirred at room temperature for 18 h. The solvent was then removed under vacuum. To eliminate the excess of triethylamine, 5 mL of diethyl ether was placed in the Schlenk tube and evaporated under vacuum and heating using a heat gun. The carbamate 2 was further washed by dry hexane, and 1 g of expected product was obtained after filtration (99% yield). Mp ) 105-110 °C. 1H NMR (250 MHz, CDCl3): δ 8.48 (s, 1H), 8.39 (d, 2H, J ) 8.3 Hz), 8.01 (d, 2H, J ) 7.8 Hz), 7.4-7.6 (m, 4H), 6.10 (s, 2H), 3.75 (q, 6H, J ) 7.2 Hz), 3.11-3.20 (m, 2H), 1.58-1.54 (m, 2H), 1.19 (t, 9H, J ) 7.2 Hz), 0.55-0.64 (m, 2H). 13C NMR (62.9 MHz, CDCl3): δ 156.58, 131.24, 130.88, 128.85, 128.76, 126.78, 126.35, 124.91, 124.01, 58.83, 58.82, 43.43, 23.13, 18.10, 7.49. IR (KBr) (cm-1): ν 3450, 1713. HRMS (IE, 70 eV): calcd for C25H33NO5Si 455.2124, found 455.2126. Tip and Surface Grafting. For tip grafting, two or three cantilevers were maintained for 6 h in 1 mL of a 0.08 M solution of 1 (41.6 mg in 1 mL of dry toluene) contained in a closed, flat-bottomed vial. At the end of this time, the main part of the solution was pumped out carefully with a syringe; cantilevers were taken out one by one with small tweezers and dipped successively in three vials containing toluene and one more containing hexane before drying in air. Surface grafting was done on three or four cover glasses (16 × 16 mm2) maintained vertically by a small Teflon support (slotted half-cylinder) in a flat-bottomed vial in 5 mL of a 0.08 M solution of 2 (182 mg in 5 mL of dry toluene). After 6 h, the support with the cover glasses was taken out with tweezers, dabbed roughly onto an absorbing paper, and rinsed by three successive dippings in toluene and one in hexane before drying in air, as described previously for cantilevers. AFM Measurements. During spectroscopic experiments, the tip and the sample were maintained in organic media by a drop of dodecane solution (about 50 µL) introduced between the sample and the cantilever-mounting glass rod, all around the tip area. The low volatility of dodecane allows hours of stable working conditions. The temperature near the experiment cell was between 25 and 30 °C. For each change of the solvent composition, the sample and the cantilever were washed successively with toluene and hexane before drying. All the measurements performed at a scan rate of 100 s for 600 nm sweep amplitude were obtained in each solvent with more than 100 experiments. The dynamic force spectroscopy experiments were performed independently from the 100 s experiments. To avoid time fluctuations and allow an easy view of the phenomenon, these dynamic measurements were done according to two different complementary procedures. The limits of sweep duration are 0.01 and 100 s, and the number of buffers for a complete recording is 26; 2 measurements can be obtained for each value of sweep time onto a logarithmic scale in a recording. The first procedure is 2 successive sweeps for each duration (100, 50, 20, 10, 5, 2, 1, 0.5, 0.2, 0.1, 0.05, 0.02, and 0.01 s) and the second one (0.01, 0.02, 0.05, 0.1, 0.2, 0.5, 1, 2, 5, 10, 20, 50, 100, 50, 20, 10, 5, 2, 1, 0.5, 0.2, 0.1, 0.05, 0.02, 0.01 and 100 s). As 3 series of each procedure are performed for 1 solvent, the dfs curves are described with 12 measurements for each point. All the described experiments were done by using the same tip. Control experiments were performed in dodecane to verify the effectiveness of the tip-sample interaction along the time. The calculated forces are given as average values associated to standard deviations. After more than two months of experiments and thousands of recorded force-distance spectra, the measurements were always reproducible.

Results and Discussion We have previously evidenced CTC interactions between trinitrofluorenone (TNF) and anthracene derivatives (compounds 1 and 2 in Figure 1), both covalently bound to an AFM tip and a glass surface, respectively.62

Gil et al.

Figure 1. Representation of the modified tip and surface to measure CTC interactions.

Figure 2. Typical pull-off force curves for an AFM tip modified with 1 on a surface grafted with 2 recorded (A) in dodecane or (B) in 1-methylnaphthalene.

These interactions were measured in dodecane and in 1-methylnaphthalene, giving rise to binding forces with average values of 6.6 ( 3.5 nN and 1.7 ( 0.5 nN (Figure 2). Dodecane was chosen as a model for a nonvolatile aliphatic solvent to favor CTC interactions. Analogously, 1-methylnaphthalene is a liquid compound, to mimic aromatic hydrocarbon in which CTC interactions are strongly inhibited by competition. Since this

Charge-Transfer Complex Study

Figure 3. Pull-off forces (nN) measured with different concentrations (mM) of 1-methylnaphthalene in dodecane.

behavior was evidenced here, the force spectroscopy technique proved to be particularly suitable for the measurement of such weak reversible interactions. Control experiments were run in dodecane with the same functionalized tip and an unmodified glass sample. The corresponding force-distance curves showed only weak unspecific interactions (0.5 ( 0.5 nN) in 100 s sweep time. Experiments in 100 s Sweep Time. The tip-sample contactseparation sweep time was first performed in 100 s to allow at best quasi-reversible conditions. The influence of the competitor concentration on the average pull-off forces has been studied. Solutions with different 1-methylnaphthalene concentrations (10, 35, and 70 mmol.L-1) in dodecane have been used as solvents for measuring the CTC interactions (see Figure 3). The values of the average pull-off forces diminished with an increase in the 1-methylnaphthalene concentration. After these experiments, as expected, the initial high value of pull-off forces is recovered in dodecane. Hence, the first value recorded in dodecane is 15.2 ( 2.1 nN, and the last experiment performed after the competing tests afforded 18.0 ( 1.3 nN. It has to be noted that these values are much higher than those recorded in our preliminary studies.62 This difference seems to be due to a more important coverage of the tip surface by the electron deficient compound, coherent with a longer reaction time (6 h in place of 4 h for the grafting procedure). The addition of 1-methylnaphthalene as competitor in a 10 mmol.L-1 concentration resulted in an average pull-off force of only 12.6 ( 1.9 nN. This value decreased to 11.8 ( 2.0 nN in a dodecane solution containing 35 mmol.L-1 of 1-methylnaphthalene. Finally, a lower value of 4.8 ( 1.0 nN was obtained by increasing this concentration to 70 mmol.L-1. The competitor concentration thus played an important role by inhibiting the global CTC interactions between the modified tip and the substrate. This result is in agreement with an increase in the concentration of the free aromatic competitor thus displacing more efficiently the CTC initially formed between the linked anthracene and trinitrofluorenone derivatives. Further studies were then performed by varying the competitor structure. More electron-rich compounds, 4,6-dimethyldibenzothiophene (4,6-DMDBT) and anthracene, were tested in a 10 mmol.L-1 concentration in dodecane. An average pull-off force of 6.5 ( 1.3 nN was recorded in the presence of the anthracene solution, whereas this value was limited to 3.9 ( 1.1 nN when the same concentration of 4,6-DMDBT in dodecane was used as solvent. Both values are significantly lower than the result obtained with the 1-methylnaphthalene solution (12.6 ( 1.9 nN). It was checked that, in the presence of the strongest competitor of these series (i.e., 4,6-DMDBT), its concentration in dodecane

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also influenced the interactions measured between the modified tip and the substrate. Experiments have indeed been performed with two concentrations of 4,6-DMDBT (10 and 50 mmol.L-1 in dodecane), and the more concentrated solution led to a stronger inhibition since the average pull-off force was limited to 2.9 ( 1.0 nN. Frontier Molecular Orbital Calculations and UV-vis Spectroscopy Measurements. For the formation of chargetransfer complexes, we assumed, as a first approach, that experimental results can be correlated to the frontier molecular orbital energies of the compounds. The HOMO of the donor compound interacts with the LUMO of the acceptor molecule, and the higher the level of its HOMO, the stronger the association. Indeed, these benzo-fused aromatic compounds possess high HOMO levels that were calculated with the PM3 semiempirical method within the Hyperchem 5.11 Standard package.65 A good convergence of the orbital energies was achieved. The HOMOs of anthracene, 4,6-DMDBT, and 1-methylnaphthalene (showing different symmetries) are represented together with the LUMO of TNF in Figure 4. This classification of HOMO levels is in agreement with our experimental results for 1-methylnaphthalene with the lowest level, but not for the two other donors. Regarding the HOMO level of anthracene (-8.25 eV), slightly higher than that of 4,6DMDBT (-8.49 eV), we would expect a greater interaction between anthracene and TNF, although the best results were obtained with 4,6-DMDBT. However, the geometry of the considered molecules and the shape of their HOMO orbitals are also of major importance. The HOMO of anthracene and 1-methylnaphthalene present two nodal planes perpendicular to the molecule (one horizontal and one vertical approximated due to the presence of the methyl group of 1-methylnaphthalene). On the other hand, the HOMO of 4,6-DMDBT is symmetrical with respect to the vertical plane, as is nearly the LUMO of TNF, approximated due to the NO2 group in the fifth position. Therefore, the overlap between the frontier molecular orbitals of TNF and 4,6-DMDBT is more efficient than that of anthracene with TNF. These results are furthermore in total accordance with PM3 computations66 and total energy calculations based on density functional theory67 between analogous electron-rich compounds and tetranitrofluorenone. Complementary experiments were performed to confirm the importance of the orbital overlap in the stability of CTC. Indeed, electron donor-acceptor complexes can be detected by UV-vis spectroscopy, because their formation is generally accompanied by the appearance of a new absorption band (Benesi-Hildebrand band).68 Following the method of Foster-Hammick-Wardley,69 this phenomenon allows the calculation of the association constant characterizing the complex. The CTC between TNF and anthracene was previously studied in chloroform solution,62 giving rise to a new absorption wave (λmax 538 nm; see Table 1) corresponding to a calculated equilibrium constant of 11.5 L.mol-1. Accordingly, the association between TNF and 4,6DMDBT under analogous conditions showed a maximal absorption at 477 nm. The calculation of the corresponding constant yielded 13.9 L.mol-1 revealing a stronger association compared to the TNF/anthracene complex. (65) Hyperchem 5.11 (Standard); Hypercube, Inc.: Gainesville, FL, 1996. (66) Milenkovic, A.; Schulz, E.; Meille, V.; Loffreda, D.; Forissier, M.; Vrinat, M.; Sautet, P.; Lemaire, M. Energy Fuels 1999, 13, 881. (67) Milenkovic, A.; Loffreda, D.; Schulz, E.; Chermette, H.; Lemaire, M.; Sautet, P. Phys. Chem. Chem. Phys. 2004, 6, 1169. (68) Benesi, H. A.; Hildebrand, J. H. J. Am. Chem. Soc. 1949, 71, 2703. (69) Foster, R.; Hammick, D. L.; Wardley, A. A. J. Chem. Soc. 1953, 3817.

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Figure 4. Frontier molecular orbital diagram for interaction between TNF and 4,6-DMDBT, anthracene, and 1-methylnaphthalene. Table 1. Comparative UV-vis Spectroscopy Study of Trinitrofluorenone (TNF) Associated with Anthracene and 4,6-Dimethyldibenzothiophene (4,6-DMDBT) in Chloroform Solution CTC

λmax (nm)

K (L.mol-1)

max (L.mol-1.cm-1)

∆G (kJ.mol-1)

TNF/anthracene TNF/4,6-DMDBT

538 477

11.5 13.9

770 1030

-6 -6.4

Regarding all these results, the strongest competition measured by AFM in the presence of 4,6-DMDBT may be explained by frontier orbital considerations. To reach such an important inhibition effect in the presence of toluene as a competitor, we thought it should be present in a very high concentration, since the HOMO level was calculated to be only -9.44 eV. Indeed, when this solvent was tested at a concentration as high as 110 mmol.L-1 in dodecane, a low inhibition was observed, giving average forces of 9.2 ( 1.6 nN. Dynamic Force Spectroscopy Study. Complementary experiments were moreover carried out under dynamic conditions to study the effect of the velocity of the tip/sample contact/ separation on the observed adhesion forces. Since our experiments are performed on a large tip and highly functionalized surfaces, we may expect, according to the literature,47 that the measured interaction will depend on this velocity. These experiments have been first performed again in neat dodecane (see Figure 5). We assume that the flat region observed in dodecane at high scan rates (t < 0.2 s) with an average pull-off force at 6.75 nN represents the number of sites that are easily accessible for CTC interactions between grafted trinitrofluorenone and anthracene under those experimental conditions. Pursuing the experiments at lower velocities led in this case to an increase of the CTC interactions. They are indeed characterized by fast equilibrium, and under those conditions, the maximal number of interactions can be

Figure 5. Pull-off force measurements in different 1-methylnaphthalene concentrations depending on the sweep duration.

observed between the linked donor and acceptor compounds. In Figure 5, the curve recorded for the experiment in dodecane tends toward a maximal value of around 16.8 nN corresponding to the saturation of all accessible interacting sites. An analogous behavior related to contact time was reported recently by Logan and co-workers70 for interactions between colloids and dextran plus protein-coated surfaces. Due to the limitation of our apparatus, experiments could not be conducted at higher sweep times. In the presence of free 1-methylnaphthalene and with increasing concentrations, a similar competition phenomenon occurred, as was observed when the experiments were performed with 100 s sweep duration. This led to lower values for the average pull(70) Xu, L.-C.; Logan, B. E. Langmuir 2006, 22, 4720 and references therein.

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Figure 6. Pull-off force measurements in the presence of anthracene, 4,6-DMDBT, or 1-methylnaphthalene in dodecane at 10 mmol.L-1 concentration depending on the sweep duration.

Figure 7. Pull-off force measurements in the presence of anthracene, toluene, 4,6-DMDBT, or 1-methylnaphthalene in dodecane at high concentration depending on the sweep duration.

off forces, according to the competitor concentration. The most important inhibition was indeed observed in the presence of the highest concentration of 1-methylnaphthalene in dodecane. All solvents behaved similarly, i.e., the average pull-off forces were independent of the scan rate between 0.01 and 0.2 s for a complete contact/separation cycle. For every solvent composition, the interaction increased in a semilogarithmic scale with increasing of the sweep time (i.e., decreasing of velocity). All these flat regions were observed before a threshold located around 0.2 s (see Figure 5). This value seemed independent of the competitor concentration. Furthermore, it has to be noted that the more diluted the 1-methylnaphthalene samples, the higher the slopes of the curves. Further dynamic experiments were carried out in the presence of three competitors solubilized in dodecane at the same concentration (10 mmol.L-1). In Figure 6, 4,6-DMDBT, anthracene, and 1-methylnaphthalene were compared under those conditions, and regardless of the scan rate, 4,6-DMDBT remained a better inhibitor than anthracene, which is also more competing than 1-methylnaphthalene, as was observed in the 100 s sweep duration experiments. Hence, the flat regions corresponding to the performed experiments are located at 4.3 ( 1.0 nN with anthracene and at 1.5 ( 0.5 nN for 4,6-DMDBT. The threshold is again observed near 0.2 s, showing the independence of the competitor structure. Finally, we performed dynamic experiments in the presence of four competitors in higher concentrations (Figure 7). As expected, a 50 mmol.L-1 solution of 4,6-DMDBT inhibits strongly the interaction between the tip and the substrate, even at high sweep duration. For this compound, the flat region is located at 0.55 ( 0.25 nN, and the measured interactions do not exceed 2.5 ( 0.8 nN. As already depicted in Figure 5, a 70 mmol.L-1 solution of 1-methylnaphthalene inhibits efficiently the measured interaction at low sweep time, but this inhibition decreases for higher sweep duration, more important than in the presence of the 4,6-DMDBT solution. A concentration of 110 mmol.L-1 in the less electron-rich toluene as competitor is at least necessary for obtaining an analogous phenomenon. However, anthracene used as competitor showed a different behavior: with a saturated solution, the flat region is located at 3.4 ( 0.15 nN, and the measured interactions increased strongly with the sweep time, reaching a maximum of 13.7 ( 1.9 nN, whereas for the 10 mmol.L-1 concentration, the slope of the curve increased less significantly (see Figure 6). One may assume

that specific molecular association can occur with anthracene, depending on the concentration.71 If this was the case, the most important part of the inhibitor would no longer be available for forming competitive CTC interactions with the acceptor-modified tip. This could explain the low inhibition observed with the saturated solution of anthracene.

Conclusions Important observations concerning CTC study by DFS can be outlined from this work. Such interactions have been unambiguously evidenced by AFM spectroscopy, as inhibition phenomena in the presence of aromatic-containing solvents. All the plots of pull-off force measurements versus sweep duration exhibit comparable shapes for this system. At low duration time (high speed) and in the presence of each inhibitor (various structures and concentrations), flat regions were observed with measured forces corresponding to the expected values for the competitive tip-surface CTC interaction. A threshold was observed near 0.2 s, probably specific to the system studied here: the geometry of the tip, the functionalization of the tip and the surface (chemical structure and loading) and the solvent. As the inhibitor concentrations in dodecane are low (1% v/v or w/v as a maximum), the viscosities of all solutions are assumed to be the same. As a general observation, for a sweep time beyond the threshold, the inhibition decreases with the velocity of the tip/sample separation. Furthermore, the larger the dilution (the lower the inhibition), the larger the slope. The pitch is linked to the structure of the inhibitor, even for solutions exhibiting an equivalent inhibition before the threshold. Such a description of the curves is also valuable for the experiments performed in neat dodecane. Indeed, for every sweep duration, the contact time is modified as well as the tip/sample separation speed. At a first approximation neglecting drift during DFS experiments, the pressure time is a constant fraction of a complete sweep. Thus, when the sweep time decreases, contact time shortens, and obviously, the loading rate increases in proportion. As CTC formation is rapid and reversible, grafted molecules on the tip and the surface can rearrange for the formation of a maximum of CTC interaction. Under those conditions, the forces reached 16.8 nN, corresponding to the saturation of all accessible interacting sites. Our experiments do not allow a precise determination of the number of interacting (71) Rizkov, D.; Gun, J.; Lev., O.; Sicsic, R.; Melman, A. Langmuir 2005, 21, 12130.

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sites. However, by comparison to the values reported by Frisbie61 (70 ( 15 pN by molecular interaction between N,N,N′,N′tetramethylphenylenediamine and 7,7,8,8-tetracyanoquinodimethane), we estimate this number to reach a maximum of 400. Of course, this value fluctuates from one experiment to the other. When free aromatic molecules are introduced in solution, all the tip-accessible sites are saturated by these competing molecules in the absence of contact between the tip and the surface. At large sweep time, the free molecules displaced from the tip by those linked to the sample may have time to diffuse outside the contact area. Pull-off force measurements of CTC interaction are performed here by DFS. This technique proved to be sensitive enough to detect weak variation coming from the environment. Such a

Gil et al.

study may be highly valuable for the understanding of interactions occurring at the solid/liquid interface, as, for example, in liquid chromatographic separation. Further work is ongoing in our laboratory for analogous studies concerning chiral CTC interactions and ligand-metal coordination. Acknowledgment. We acknowledge the Ministe`re de l’Education Nationale, de l’Enseignement Supe´rieur et de la Recherche, and CNRS for financial support. We are indebted to Professor Jean-Claude Fiaud for his encouragements during this work. LA062169H