Soft-landing Isolation of Gas-phase-synthesized Transition Metal

Sep 13, 2008 - This orientational preference is measurably different from that of complexes isolated in alkanethiol SAM matrices, a discrepancy that m...
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J. Phys. Chem. C 2008, 112, 15824–15831

Soft-landing Isolation of Gas-phase-synthesized Transition Metal-Benzene Complexes into a Fluorinated Self-assembled Monolayer Matrix Shuhei Nagaoka,† Kaori Ikemoto,† Takeshi Matsumoto,† Masaaki Mitsui,† and Atsushi Nakajima*,†,‡ Department of Chemistry, Faculty of Science and Technology, Keio UniVersity, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan and CREST, Japan Science and Technology Agency (JST), c/o Department of Chemistry, Keio UniVersity, Yokohama 223-8522, Japan ReceiVed: June 24, 2008; ReVised Manuscript ReceiVed: July 30, 2008

Gas-phase-synthesized chromium-benzene 1:2 sandwich cation complexes [Cr+(benzene)2] were soft-landed on a self-assembled monolayer (SAM) of fluorinated alkanethiol (C10F-SAM) at a hyperthermal collision energy of ∼20 eV. The adsorption properties and thermal stability of the soft-landed complexes were studied with infrared reflection absorption spectroscopy (IRAS) and temperature-programmed desorption (TPD). The landed complexes were neutralized due to charge transfer from the SAM substrate, but their native sandwich structure remained intact. The hyperthermal collision event resulted in the penetration of the incoming complexes into the C10F-SAM matrix. The embedded complexes then tended to orient their molecular axes approximately along the surface normal. This orientational preference is measurably different from that of complexes isolated in alkanethiol SAM matrices, a discrepancy that might be caused by a repulsive interaction between the π cloud of the capping benzene rings of the complex and the side-chain CF2 groups of the fluorocarbon chains in the C10F-SAM matrix. The thermal desorption study showed that the complexes supported inside the C10F-SAM could resist thermal desorption until the high-temperature region of ∼320 K, a persistence revealing a large desorption activation energy (∼190 kJ/mol). 1. Introduction Gas-phase synthesis provides an advantage for producing novel chemical compounds that are unsuccessful in a solutionphase chemical reaction. Over the past decade, indeed, the combination of laser-vaporization and molecular beam methods has yielded various kinds of metal clusters, as well as organometallic complexes, in the gas phase.1-6 These gas-phasesynthesized clusters have attracted a great deal of attention due to their size-specific characteristics that are often rather different from those of their corresponding bulk materials. Since the discovery of multiple-decker vanadium (V)-benzene sandwich complexes,7 in particular, transition metal-benzene:, Mn(benzene)n+1 sandwich complexes have been actively studied both experimentally and theoretically. The source of interest lies in their unique size-dependent electronic8-10 and magnetic11-15 properties, originating from their one-dimensional (1D) anisotropic structure. The Mn(benzene)n+1 sandwich complexes are therefore expected to be promising candidates for application in new types of nanometer-scale devices. The advent of a soft-landing technique,16,17 that is, nondissociative deposition of size-selected gas-phase-synthesized compounds onto an appropriate substrate, opens up the possibility of utilizing them as building blocks in nanostructured functional materials. Recently, surface modification via the softlanding of gas-phase ionic compounds, such as metal clusters and biomolecules onto a solid surface, increasingly enables the creation of new types of nanoscale materials, for example, nanocatalysts,18-20 nanomagnetics,21-23 and biological micro* To whom correspondence should be addressed. E-mail: nakajima@ chem.keio.ac.jp; Fax: +81-45-566-1697. † Keio University. ‡ CREST, Japan Science and Technology Agency.

arrays.24-30 In particular, research on the catalytic properties of metal clusters soft-landed on metal-oxide surfaces has currently become one of the most active scientific fields since the discovery of the catalytic activity of gold clusters supported on a magnesia surface by Heiz and co-workers.20,31 They discovered the size-dependent catalytic activity of Aun/MgO for CO combustion, where the Au8 cluster is the smallest catalytically active size, and further demonstrated that the catalytic activity could be changed by doping impurities into the supported gold clusters. These findings provide evidence that the clusters possess novel size-specific and compositiondependent properties, even when supported on a solid surface via soft-landing, and that these supported clusters have the potential for use in nanoscale functional materials. The use of a self-assembled monolayer (SAM) of organic molecules as a supporting substrate for the soft-landed species is an effective way to provide a wide variety of adsorption regimes and/or supporting systems.17,27-30,32,33 Cooks and coworkers first examined the soft-landing of gas-phase polyatomic ions onto a SAM consisting of fluorocarbon chains (F-SAM). They also demonstrated that the incoming ions penetrate into the F-SAM matrix and are sterically trapped inside the matrix while retaining their charge.17,32 On the other hand, Laskin and co-workers recently showed that the reactive landing of massselected biomolecules on the surface provides a terminally modified SAM.27-30 They have achieved immobilization of the landed molecules on the SAM surface via the covalent linking of the molecules to the terminal groups of the SAM matrices. In our previous work, we soft-landed gas-phase-synthesized Mn(benzene)n+1 sandwich complexes onto an n-alkanethiol SAM (CnH-SAM, n: carbon number in alkyl chain) with a hyperthermal collision energy (∼20 eV).34-38 The matrix-isolation (softlanding isolation) of the neutralized Mn(benzene)n+1 complexes

10.1021/jp8055784 CCC: $40.75  2008 American Chemical Society Published on Web 09/13/2008

Soft-landing Isolation of Metal-Benzene Complexes succeeded because they penetrated the SAM while keeping their native sandwich structure intact. Furthermore, In the CnH-SAM matrices the sandwich complexes were oriented with their molecular axes largely canted from the surface normal and were trapped so that their thermal desorption was suppressed up to around room temperature. For the most advantageous use of the Mn(benzene)n+1 complexes’ optical and magnetic functionalities, development of advanced techniques that could control the adsorption regimesthat is, the orientational preference and thermal stability of the complexes at the surfacessis highly desired. In this study, therefore, we examined the effect of fluorination of the molecular chains in the SAM matrices on the soft-landing isolation of transition metal-benzene complexes. A fluorinated SAM (C10F-SAM), namely CF3(CF2)7(CH2)2SH, provided a supporting substrate for the soft-landing of gas-phase-synthesized Cr(benzene)2 complexes with a hyperthermal collision energy of 20 eV, as well as thermal deposition (physical vapor deposition)38-41 of Cr(benzene)2 with an extremely low deposition energy of ∼25 meV. The adsorption properties and thermal stability of the landed complexes were investigated by means of infrared reflection absorption spectroscopy (IRAS) and temperature-programmed desorption (TPD) measurements. Compared to the soft-landing isolation in the CnH-SAM,34-38 the fluorination of the SAM provides a measureable change in orientational preference as well as a drastic increment in the thermal stability of the matrix-isolated Cr(benzene)2 complexes. The Cr(benzene)2 isolated inside the C10F-SAM exhibits a large desorption activation energy (∼190 kJ/mol), sufficient to suppress its thermal desorption until it reaches a high-temperature region of ∼320 K. 2. Experimental Section The details of the experimental setup have been described elsewhere.36,44 Briefly, a commercially available 10 × 10 mm2 gold substrate, Au (100 nm thickness)/Ti/Silica, was immersed in a piranha solution (3:1 concentrated H2SO4/H2O2) for about 20 min to remove organic contaminants from the surface.42,43 Dipping the gold substrate into a 0.5 mM ethanolic solution of heptadecafluoro-1-decanethiol (>99%, Aldrich) at ambient temperature for 20 h formed a C10F-SAM on the surface. The formation of the C10F-SAM/gold was confirmed by infrared reflection absorption spectroscopy (IRAS) and contact angle measurements (Drop Master 300, Kyowa Interface Science) of a water droplet (10 µL) at room temperature. The substrates were mounted on a sample holder capable of being heated by a ceramic heater and cooled with a liquid nitrogen reservoir. Chromium (Cr)-benzene complexes were produced in a molecular beam by laser vaporization, and only the Cr(benzene)2 complexes were size-selected by a quadrupole mass spectrometer (QMS). The cations were subsequently deposited onto the C10F-SAM substrate, which was cooled to 220 K by a liquid nitrogen cryostat, with an incident energy of ∼20 eV. In contrast, the thermal deposition of Cr(benzene)2 vapor onto the C10FSAM was conducted via the admission of the sublimed complexes at 300 K to the chamber trough through a stainless steel doser line (1/4 in. diameter), which provides an extremely low-energy deposition (∼25 meV) of the sandwich complexes onto the SAMs.38 The exposures were recorded in Langmuirs (1 L ) 10-6 Torr · s), and during the exposure time, the substrate temperature was kept at 180 K. The complex deposited on the substrate was analyzed by IRAS and TPD. The IRAS was carried out by an FT-IR spectrometer at a grazing incident angle of ∼80°; the IR optics

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Figure 1. IRAS spectra in the 600-1550 cm-1 region for heptadecafluoro-undecanethiol (C10F16H4SH) adsorbed on the gold substrate, measured at 160 K.

TABLE 1: Spectral Mode Assignments for Heptadecafluoro-undecanethiol (C10F15H4SH) Self-assembled on a Gold Surfacea observed frequency/cm-1

mode assignment

direction of momentb

667 693 709 962 1003 1036 1076 1119 1156 1207 1220 1245 1284 1335

w(CF2), r(CF2) w(CF2), r(CF2) V(CF3) end group δ(CCC), r(CH2) r(CH2) V(CC), trans-planar V(CC), gauche V(CC), trans-planar Va(CF2), δ(CF2) Va(CF2), Va(CF3) δ(CCC), V(CC) Va(CF2), r (CF2) w(CH2), r(CF2) V(CF2) progression: axial CF2 stretching V(CF2) progression: axial CF2 stretching

parallel parallel parallel parallel parallel, perpendicular perpendicular parallel perpendicular perpendicular perpendicular perpendicular perpendicular parallel parallel

1372

parallel

a Vibraional modes: V, stretching; δ, bending; r, rocking; w, wagging. b Directions of transition dipole moments with respect to the fluorocarbon axis.

and detector were mounted in a vacuum chamber pumped to a pressure of about 0.1 Torr to remove spectral background contributions due to atmospheric gases. Five hundred scans with a spectral resolution of 2 cm-1 were accumulated for the background and sample spectra, which were recorded, respectively, before and after the cluster deposition. The TPD measurements were made with the substrate placed at a location of ∼1 mm in front of the entrance aperture of a mass spectrometer ionizer. A heating rate of 1 K/s was used in the TPD measurements. 3. Results 3.1. Characterization of C10F-SAM substrate. The IRAS spectrum for the C10F-SAM on the gold surface in the 600-1550 cm-1 region, measured at surface temperature of 160 K, is shown in Figure 1. These data are in good accordance with previously reported IRAS spectra of the monolayers of perfluoroalkanethiols on a gold surface.45,46 The observed frequencies and corresponding vibrational modes for the C10FSAM are listed in Table 1, together with the directions of the transition dipole moment toward the fluorocarbon helical axis. Contact angle measurements, using a water droplet on the C10FSAM, yielded large contact angles (θc) of ∼112°. The hydrophobic nature of the C10F-SAM was thus established, while the bare gold surface was completely wetted (θc ∼ 0°). The contact

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Nagaoka et al.

Figure 2. IRAS spectra in the 650-1550 cm-1 region with thermal deposition of Cr(benzene)2 complexes onto C10F-SAM at several different coverages.

Figure 3. IRAS spectra in the 650-1550 cm-1 region with the softlanding of Cr+(benzene)2 complexes onto C10F-SAM at several different deposition numbers.

angles obtained are in good agreement with literature values for the corresponding SAMs anchored on gold surfaces.47,48 During the deposition of the gas-phase-synthesized Cr(benzene)2 cation complexes onto the C10F-SAM substrate, we could measure the ion current through the substrate, caused by the neutralization of the deposited complexes. It has been reported that fluorinated SAMs reduce the neutralization of projectile ions during the deposition process and also after trapping the ions.17,32 In this study, however, a similar ion current value (∼12 nA) was obtained for the deposition of Cr(benzene)2+ onto all kinds of substrates: a fluorinated SAM, an alkanethiolate SAM, and bare gold, under the same experimental conditions. Thus, Cr(benzene)2+ seems to be immediately neutralized after the deposition onto the C10F-SAM matrix, just as it was onto the alkanethiolate SAM matrices or a metal gold surface. 3.2. Infrared Spectroscopic Study. The IRAS spectra in the 650-1550 cm-1 region of the thermally deposited Cr(benzene)2 complexes on the C10F-SAM substrate at several coverage levels are shown in Figure 2. After an exposure of 4 L, two absorption bands with E1u symmetric vibrational modes at 996 and 1429 cm-1 appear, and an additional band with the A2u symmetric mode appears at 972 cm-1 when the exposure reaches 8 L. The peak positions of these IR bands are in good agreement with the IR fundamentals for the condensed-phase Cr(benzene)2 measured in a KBr pellet. Thus, the thermally deposited Cr(benzene)2 molecules are molecularly adsorbed while keeping their native sandwich structure, even on the C10F-SAM. In the IRAS spectra, however, the relative intensities of the IR bands for the thermally deposited Cr(benzene)2 are rather different from those for the condensed-phase ones. For instance, the IR bands at 996 cm-1 (E1u symmetric mode) are prominent, compared to those at 972 cm-1 (A2u symmetric mode) for the thermally deposited Cr(benzene)2 supported on the C10F-SAM. The KBr-isolated Cr(benzene)2 exhibits a comparable peak intensity between the two modes. Owing to the IRAS surface selection rule,49 the relative IR intensity of the A2u and E1u modes reflects the orientational preference of the ideally D6h-symmetric Cr(benzene)2 complexes on the substrate,. The predominant observation of E1u modes implies that the Cr(benzene)2 tilts its molecular axis (D6h-symentric axis) far from the surface normal on the SAM.35,50

In addition, as shown in Figure 2, IR peaks with negative intensity are observed with the thermal deposition of the complexes. The peak positions of these negative peaks are assignable to the vibrational frequencies for the CF3(CF2)7(CH2)2SH molecules making up the C10F-SAM listed in Table 1.51,52 The negative peaks at 667 and 693 cm-1 originate from CF2 wagging and rocking modes, and the peaks at 1341 and 1378 cm-1 are attributed to axial CF2 stretching modes. Because the background IR spectra were recorded before the depositions of the complexes, the negative peaks in the measured IRAS spectra represent a thermal deposition-induced decrease in the IR absorption intensity of the vibrational modes for CF3(CF2)7(CH2)2SH molecules. Furthermore, it should be noted that the transition moments of all the negative peaks lie along the fluorocarbon axis. Displayed in Figure 3 is the IRAS spectra for the Cr(benzene)2 complexes soft-landed on the C10F-SAM at 220 K with hyperthermal collision energy of ∼20 eV, as a function of the deposition-number of the Cr(benzene)2 cations. Two bands at 972 and 1429 cm-1 first appeared at the deposition number of 1.0 × 1014 cations, and an additional mode was clearly observed at 996 cm-1 at 3.0 × 1014 ions. As mentioned above, the peak positions of these IR bands are in excellent agreement with the IR fundamentals for the complexes in a KBr pellet and the thermally deposited complexes on the C10F-SAM substrate. Although it has been reported that fluorinated SAMs have the ability to trap the penetrated projectile ions while retaining their charge after the soft-landing,17,32 the IRAS spectra obtained herein represent the IR signals originating only from neutral complexes. Thus, the results reveal that the soft-landed Cr(benzene)2 cation complexes lost most of their charge, and the resulting neutral complexes adsorbed on the C10F-SAM substrate with their native sandwich structure intact. Furthermore, compared to the IR spectrum for condensedphase Cr(benzene)2 in a KBr pellet, for the complexes softlanded on the C10F-SAM, the IR peak at 972 cm-1 with A2u symmetry was strongly observed with respect to that at 996 cm-1 with E1u mode. According to the IRAS selection rule, the results suggest the orientational preference of the Cr(benzene)2 complexes soft-landed on the C10F-SAM. The preponderant observation of the A2u mode reflects the fact that the Cr(ben-

Soft-landing Isolation of Metal-Benzene Complexes

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Figure 4. IRAS spectra showing the temperature-dependent variation of Cr(benzene)2/C10F-SAM with (a) thermal deposition of 8 L and (b) soft-landing of 3.0 × 1014 complexes.

zene)2 complexes tend to prefer the orientation with their molecular axes approximately perpendicular to the surface. The orientational preference of the complexes is strikingly different between the soft-landed and thermally deposited ones. As seen in Figure 3, negative peaks were also observed in the IRAS measurements of the soft-landing of the Cr(benzene)2 complexes onto the C10F-SAM. However, the peak positions of the negative IR bands are markedly different from those observed after the thermal deposition experiment. The prominent peaks at 1156 and 1252 cm-1 are assigned to the CF2 stretching, bending, and rocking modes, and the peak at 1220 cm-1 is assigned to several bending and stretching modes of the carbon chain within the CF3(CF2)7(CH2)2SH molecules. These pronounced vibrational modes possess a transition moment perpendicular to the fluorocarbon axis. Although the thermal deposition generates the predominantly negative peaks of the axial CF2 stretching modes (transition moments along the fluorocarbon axis) at 1341 and 1378 cm-1, the corresponding negative peaks of 1339 and 1377 cm-1 with the soft-landing of Cr(benzene)2 appeared very weak, even at for a deposition of 3.0 × 1014 ions. 3.3. Thermal Desorption Study. As shown in Figure 4, to evaluate the thermal stability of thermally deposited and softlanded Cr(benzene)2 on the C10F-SAM, we examined temperature-induced variations of the IRAS spectra. The top traces in panels a and b of Figure 4 represent the IR spectra of the Cr(benzene)2 on the C10F-SAM with, respectively, the thermal deposition of 8 L and the soft-landing of 3.0 × 1014 cations. For the thermally deposited complexes, the IR intensity reduction began around 240 K, and the bands completely disappeared at 270 K. These IR bands never returned to their initial intensity, even if the substrate was cooled down again to the initial temperature of 220 K. Evidently, the decrease in IR intensity reflects thermal desorption from the SAM substrates. For the soft-landed complexes, in contrast, all the IR peaks started to decrease at the much higher temperature of 320 K and completely disappeared at 330 K. Specifically, on the C10FSAM, the desorption temperature of the soft-landed Cr(benzene)2 was approximately 60 K higher than that of the thermally deposited compound. Raising the surface temperature caused an increase in the intensity of negative peaks, originating from the vibrations of fluorocarbon chains. The growth of the negative peaks reflects the decrease in the vibrational absorption due to the change in the tilt angle and/or a surface-temperature-induced conforma-

Figure 5. TPD spectra obtained for (a) Cr(benzene)2 with thermal deposition of 1 L and (b) Cr(benzene)2 with soft-landing of 4.0 × 1013 complexes onto C10F-SAM substrate at 220 K.

tional disordering of the molecular chains within the SAM matrices.53-55 The thermal desorption kinetics of Cr(benzene)2 supported on the C10F-SAM was evaluated in detail by the TPD study. To reduce the aggregations of and interactions between the adsorbed complexes themselves on the substrate, we set the deposition number of the landed complexes at a relatively low coverage level; the TPD measurements were performed after an exposure of 2 L for thermal deposition or after the softlanding of 1.0 × 1014 ions/cm2. These deposition numbers were estimated to provide similar coverage (adsorbed amounts) by comparing the IR absorption strength of the thermally deposited and soft-landed complexes. The TPD spectra for the parent-mass ion signals of the Cr+(benzene)2 desorbed from the C10F-SAM are shown in Figure 5. In the experiment, the fragment ionssfor example, Cr+ (benzene)1, benzene+, and chromium atoms (Cr+)swere also monitored. The peak profiles of these fragment ions were identical to those of the corresponding parent ions, a result indicating that the fragments were produced not on the substrate but in the electron impact ionization in the mass spectrometer. It is noted that no desorption ions were observed without an electron ionization event in the mass spectrometer until a higher temperature range (over 420 K) where thiol molecules forming the C10F-SAM are thermally desorped. The result strongly supports the neutralization of Cr(benzene)2+ after its soft-landing onto the C10F-SAM. For the thermally deposited complexes, as shown in Figure 5a, the thermal desorption starts at ∼210 K, and the desorption rate reaches a maximum at ∼240 K. The TPD curve displays a nearly symmetric shape in which the ion intensity slowly decreases after the peak maximum, forming a tail on their hightemperature side. Because an identical feature was also observed in the TPD spectra of physisorbed M(benzene)2 complexes on a gold surface and on the methyl-surface of the CnH-SAM matrices,38 thermally deposited Cr(benzene)2 complexes would thus be physisorbed on the surface of the C10F-SAM. In addition, the indication of tailing in the high-temperatures in the TPD spectrum could be brought about by surface diffusion of the complexes on the SAM surface.36,38,56,57 For the soft-landed complexes, however, the TPD spectral feature is pronouncedly different from that for the thermally

15828 J. Phys. Chem. C, Vol. 112, No. 40, 2008 deposited ones. The thermal desorption of the soft-landed complexes starts at a much higher temperature of ∼320 K, and the desorption rate reaches a maximum at ∼350 K. The drastic increase in the desorption temperature indicates that the softlanded Cr(benzene)2 are more strongly bonded to the C10F-SAM than are the thermally deposited complexes. In addition, the TPD curves for the soft-landed complexes show asymmetric peak profiles in which the desorption rate rapidly decreases after the peak maximum. This result demonstrates that the soft-landed complexes desorb from the C10F-SAM substrate via a first order desorption kinetics: in particular, thermal diffusion (s) of the complexes in the desorption process are effectively suppressed. The increase in the desorption temperature and first-order desorption kinetics most likely suggest the incorporation of the M(benzene)2 complexes inside the SAM matrix (i.e., matrixisolation regime); the soft-landing with the hyperthermal deposition allows the complexes to penetrate inside the SAM. As a consequence, the Cr(benzene)2 complexes, trapped firmly by fluorocarbon chains inside the C10F-SAM, acquire a high thermal stability. The activation energy for the desorption of the complexes can be quantitatively determined by taking Arrhenius plots of the measured TPD spectra.58,59 For the thermally deposited Cr(benzene)2, the desorption activation energy was determined to be 68 ( 18 kJ/mol. This value is comparable to the desorption activation energy for other M(benzene)2 complexes physisorbed on a gold surface or on a methyl-surface of CnH-SAM.36,38 In contrast, for the soft-landed Cr(benzene)2 the much larger desorption activation energy of 195 ( 31 kJ/mol was obtained. This considerable enhancement of the desorption activation energy clearly demonstrates that the soft-landed Cr(benzene)2 complexes are not physisorbed on the surface but instead are firmly trapped inside the C10F-SAM matrix. 4. Discussion 4.1. The Orienrtation of Cr(benzene)2. The hyperthermal collision of the incoming Cr(benzene)2 with the C10F-SAM surface produces a penetration of the complexes into the SAM matrix, whereas the thermally deposited Cr(benzene)2 are certainly adsorbed on the matrix surface. The drastically increased thermal stability of the soft-landed complexes indicates that the penetrating complexes are more strongly trapped within the C10F-SAM than by its surface. Indeed, the penetration of landed complexes into SAM matrices during a hyperthermal collision event has also been demonstrated for the soft-landing onto CnH-SAM substrates.34-38 Both the thermal deposition and soft-landing studies provide the orientational preference of the Cr(benzene)2 complexes on/ inside the C10F-SAM. The orientational preference of the complex inside the SAM is likely due to intermolecular interactions between the complex and the organic chains forming the SAM. Inside the CnH-SAM, an attractive CH-π interaction60-63 operates between the complex’s capping benzene rings and the hydrogen atoms of methylene groups of the surrounding alkyl chains, and the complex strongly tilts its molecular axis to the surface plane.35,37,38 The complexes isolated inside the C10F-SAM, however, favor a different orientation, such that the complexes have their molecular axes approximately perpendicular to the surface. This orientation would result from repulsive interactions between the capping benzene rings of the complex and the fluorine atoms of the side-chain CF2 groups of fluorocarbon molecules. Because of the high electronegativity of fluorine atoms, most of the electric charge of the C-F bond is distributed on the fluorine atom. Hence, electrostatic repul-

Nagaoka et al. sions between the highly charged fluorine atoms and the π-electron clouds of the benzene rings probably govern the orientation of the complex inside the C10F-SAM. The results of the IRAS measurements also support the possibility that such electrostatic repulsion plays an important role in the interactions between the benzene π rings and the C-F group. Our previous study demonstrated that when the Cr(benzene)2 complexes are adsorbed on the methyl surface via their thermal deposition onto the CnH-SAM that they are randomly oriented.38 However, the IRAS spectra obtained herein suggest that, with thermal deposition onto the C10F-SAM, the Cr(benzene)2 complexes adsorbed on the CF3-terminated surface are oriented with their benzene planes approximately perpendicular to the surface. The electrostatic repulsion between the π cloud of the benzene ring and the terminal C-F group of the SAM seemingly results in such an orientation. Indeed, electrostatic attractive interactions between the hydrogen atoms of the benzene ring and the fluorine atoms of the terminal C-F group might orient the complexes on the SAM surface. However, the electrostatic H-F interaction is negligibly weak in this adsorption system, because the desorption activation energy for Cr(benzene)2 on the CF3-terminated surface (68 ( 18 kJ/mol) is almost the same as that for complexes physisorbed on the CH3-terminated surface (71 ( 12 kJ/mol).38 Thus, the orientational preference of the Cr(benzene)2 would be dominated by the repulsion between the benzene π rings and the C-F group. 4.2. Negative IR Peaks of the SAM. With both the thermal and hyperthermal depositions, the IRAS measurements exhibited a series of negative peaks originating from the vibrational modes of the fluorocarbon chains in the C10F-SAM. As shown in Figure 4, however, the characteristics of the negative IR peaks are completely different between the thermal deposition and the softlanding. For the thermal deposition, only the vibrational modes whose transition dipole moment arises along the fluorocarbon axis exhibited negative peaks; in contrast, for the soft-landing, vibrational modes with a transition dipole moment perpendicular to the fluorocarbon axis did generate negative peaks. This dissimilar mode-selective decrement in IR intensity might result from an induced dipole field produced by the adsorption or penetration event of the complexes on/in the SAM matrices. Indeed, such an induced dipole field usually influences the IR absorption intensity of the vibrations of molecules at surfaces.49 Although a fluorocarbon chain and benzene monomer have few or no permanent dipole moments, the coordination of the benzene monomer to the fluorocarbon chain can induce a local dipole moment from the benzene monomer to the fluorocarbon chain. To roughly evaluate the magnitude of these induced dipole moments, we used the Gaussian 03 program to perform density functional theory (DFT) calculations.64 Because Cr(benzene)2 also has no permanent dipole moment, its orientation is most likely caused by an interaction between the capping benzene and the organic-chains of the SAMs. We focused on the local interaction between a fluorocarbon chain and benzene monomer. Thus, the optimized geometries and dipole moments were obtained at the B3LYP/6-31G* level with two initial geometries, as shown in Figure 6.65 Displayed in Figure 6a is a model complex in which a benzene monomer attaches to the terminal CF3 group of a fluorocarbon chain, with the benzene plane parallel to the chain axis. This model simply represents the adsorption state of Cr(benzene)2 on the surface of the C10F-SAM after thermal deposition. In this model, an induced dipole moment of 0.1897 Debye arises along the molecular axis of the fluorocarbon chain. On the other hand, the other model, shown in Figure 6b, demonstrates that a

Soft-landing Isolation of Metal-Benzene Complexes

Figure 6. Induced dipole moments in complexes of a C6F14 chain and benzene monomer calculated using DFT at the B3LYP/6-31G* level.

benzene monomer adheres to the side CF2 groups of a fluorocarbon chain, with the benzene plane perpendicular to the chain axis; this configuration represents the Cr(benzene)2 inside the C10F-SAM after the soft-landing. The latter model provides an induced dipole moment of 0.4072 Debye perpendicular to the molecular axis of the fluorocarbon chain. The theoretical calculations suggest that the Cr(benzene)2 thermally deposited onto the C10F-SAM induces a dipole field parallel to the fluorocarbon axis. In contrast, the Cr(benzene)2 soft-landed onto the C10F-SAM produces a dipole field perpendicular to the fluorocarbon axis. The contrasting differences between the induced dipole field consistently explains the characteristics of the negative peaks of the vibrational modes; the direction of the induced dipole field for complex deposition causes the change in the oscillator strength of the IR modes. Such induced dipole moments might have an effect on both the enhancement and diminishment of the IR absorption intensity of the fluorocarbon chains; however, only the negative IR peaks were clearly observed in this study. The results imply that the induced dipole moments act to decrease the polarization between the fluorine and carbon atoms in the fluorocarbon chains. The polarization of chemical bonds strongly influences their IR intensities; due to the high-polarization of the C-F bond, the IR intensity of C-F vibrations of a fluorocarbon chain is enormously increased relative to that of C-H vibrations of a hydrocarbon chain. Hence, the growth of the negative IR peaks with the deposition number of the complexes can be explained by the partial decrease in the polarization of C-F bonds of the C10F-SAM, a lessening that is caused by the induced dipole formed after the landing of the Cr(benzene)2. In addition, it should be noted that the effect of an induced dipole field on the IR absorption intensity can be readily detected with a regime in which the Cr(benzene)2 is deposited onto the highly polarized C10F-SAM rather than onto the CnH-SAM. Indeed, negative peaks could not be observed in our previous IRAS measurements for the thermal deposition of Cr(benzene)2 onto the CnHSAM substrate.38 4.3. Desorption Triggered by Phase Transition of the SAM. The thermal desorption study disclosed an unusually large desorption activation energy for the Cr(benzene)2 complexes trapped after the soft-landing inside the C10F-SAM matrix. Interestingly, the activation energy for the C10F-SAM is about 60 kJ/mol larger than that for the C18H-SAM. One of the most plausible explanations for the pronounced increase in the desorption temperature could involve the enthalpy of the phase transition of the monolayer matrix. As previously demonstrated, the thermal desorption of the matrix-isolated complexes is most likely associated with a crystal-rotator phase transition of the monolayer matrices;38 thus, the enthalpy increase at the crystal-rotator phase transition should increase the desorption

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Figure 7. Temperature dependence of the integrated intensity of the C-F stretching mode at 1156 cm-1. The dashed-line represents the slope due to the decrease in the tilt angle of the fluorocarbon chains, and a distinct slope transition is brought about by the rotator phase in the C10F-SAM.

activation energy of the complexes from the SAM. In contrast to the flexible n-alkyl chain, the fluorocarbon axis possesses a large intramolecular rotational barrier providing a conformational rigidity.45,46 Because the rotator phase is brought about by the rotational excitation of molecular chains, the enthalpy at the phase transition of the C10F-SAM would be larger than that of CnH-SAM. For the SAM matrices, the advent of the rotator phase can be nicely followed by probing the temperature dependence of the IRAS spectra.53,54 Indeed, the temperature of crystal-rotator phase transition (marked by the appearance of gauche conformational defects) of the n-alkanethiol SAM have been demonstrated by plotting the IR intensity for CH2 stretching.38,54 In this study, therefore, we estimated the temperature for the crystal-rotator phase transition of the C10F-SAM by plotting the IR intensity for CF2 stretching at 1156 cm-1 versus surface temperature. This plot is shown in Figure 7. The IR intensity decreases at a constant rate up to ∼320 K, induced by a decrease in the tilt angle of the molecular chains in the SAM.55,66-68Above 320 K, however, the intensity undergoes a further decrease, which is attributed to the crystal-rotator phase transition54,69,70 in the C10F-SAM matrices. Therefore, we can confirm that the phase transition of the highly ordered phase (crystal phase) to the rotator phase takes place around 320 K for the C10F-SAM substrate used in this study, and the value is indeed about 30 K higher than the temperature of the crystal-rotator phase transition for the C18H-SAM38 estimated in our previous work. Additionally, the phase-transition temperature accords well with the threshold desorption temperature of the Cr(benzene)2 complexes supported inside the C10F-SAM matrix (see Figure 5b). The results strongly suggest that (1) the crystal-rotator phase transition most likely assists the release of the embedded complexes from the SAM matrix and that (2) the increment in temperature (or enthalpy) at the crystal-rotator phase transition of the organic monolayer matrix causes the increase in the desorption temperature (or desorption activation energy) of the matrix-isolated complexes. Because the enthalpy for the crystal-rotator phase transition should increase with the length of fluorocarbon chains, it is expected that the use of the longerchain CnF-SAM substrate could achieve a much higher temperature isolation of gas-phase-synthesized complexes inside the SAM matrices. 5. Conclusions To investigate the effect of fluorination of SAM matrices on the soft-landing isolation regime, we size-selectively deposited

15830 J. Phys. Chem. C, Vol. 112, No. 40, 2008 gas-phase-synthesized Cr+(benzene)2 complexes onto a C10FSAM matrix with a hyperthermal collision energy of 20 eV. The complexes were neutralized, even on the fluorinated SAM substrate, while keeping their native sandwich structure intact. The hyperthermal collision event in the soft-landing process results in a penetration of the incoming complexes into the C10FSAM matrix. In contrast, the extremely low energy (thermal) deposition (∼25 meV) of Cr(benzene)2 complexes creates a physisorption state of the complexes on the SAM surface. The Cr(benzene)2 complexes orient with their molecular axes largely canted to the surface plane on the surface of the C10F-SAM, whereas the complexes embedded in the C10F-SAM matrix are oriented with their molecular axes approximately perpendicular to the surface plane. The orientational preference is probably due to a repulsive interaction between the π cloud of capping benzene rings of the complex and the outmost CF3 group and side-chain C-F groups of the fluorocarbon axes of the C10FSAM matrix. In addition, the embedded complexes exhibit a large desorption activation energy of over 190 kJ/mol, with the result that the thermal desorption of the complexes can be suppressed to above room temperature, ∼320 K. This significant increment in the thermal stability of the matrix-isolated complexes evidently results from an increase in enthalpy at the crystal-rotator phase transition of the supporting SAM matrix, as a consequence of the fluorination of its molecular chains. Chemical fluorination modification of the organic chains making up the SAM matrices highly influences the soft-landing isolation regime of the transition metal-benzene complexes in the SAM matrices. Hence, the soft-landing isolation technique using the SAM matrices has an advantage not only for trapping the gas-phase-synthesized complexes and clusters as they are, but also for facilitating fine control of their orientation and thermal stability that optimizes their optical and magnetic functionality on the substrate. Acknowledgment. This work is partly supported by a grantin-aid for scientific research (A) (No. 19205004) from the Ministry of Education, Culture, Sports, Science, and Technology. S.N. expresses his gratitude for a research fellowship from Japan Society for the Promotion of Science for Young Scientist. References and Notes (1) Leuchtner, R. E.; Harms, A. C.; Castleman, A. W., Jr. J. Chem. Phys. 1989, 91, 2753. (2) Jena, P.; Castleman, A. W., Jr. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 10560. (3) Li, J.; Li, X.; Zhai, H. J.; Wang, L. S. Science 2003, 299, 864. (4) Nakajima, A.; Kaya, K. J. Phys. Chem. A 2000, 104, 176. (5) Duncan, M. A. Int. J. Mass Spectrom. 2008, 272, 99. (6) Oomens, J.; Moore, D. T.; vonHelden, G.; Meijer, G.; Dunbar, R. C. J. Am. Chem. Soc. 2004, 126, 724. (7) Hoshino, K.; Kurikawa, T.; Takeda, H.; Nakajima, A.; Kaya, K. J. Phys. Chem. 1995, 99, 3035. (8) Yasuike, T.; Yabushita, S. J. Phys. Chem. A 1999, 103, 4533– 4542. (9) Pendy, R.; Rao, B. K.; Jena, P.; Blanco, M. A. J. Am. Chem. Soc. 2001, 123, 3799–3808. (10) Kandalam, A. K.; Rao, B. K.; Jena, P.; Pandey, R. J. Chem. Phys. 2004, 120, 10414–10422. (11) Wang, J.; Acioli, P. H.; Jellinek, J. J. Am. Chem. Soc. 2005, 127, 2812. (12) Miyajima, K.; Nakajima, A.; Yabushita, S.; Knickelbein, M. B.; Kaya, K. J. Am. Chem. Soc. 2004, 126, 13202. (13) Miyajima, K.; Yabushita, S.; Knickelbein, M. B.; Nakajima, A. J. Am. Chem. Soc. 2007, 129, 8473. (14) Kua, J.; Tomlin, K. M. J. Phys. Chem. A 2006, 110, 11988. (15) Goto, A.; Yabushita, S. Chem. Phys. Lett. 2008, 454, 382. (16) Heiz, U.; Vanolli, F.; Trento, L.; Schneider, W. D. ReV. Sci. Instrum. 1997, 68, 1986. (17) Miller, S. A.; Luo, H.; Pachuta, S. J.; Cooks, R. G. Science 1997, 275, 1447.

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