Quenching of Collision between the Solute Molecules in the

Jun 2, 2007 - This sharp line width indicates that collision between the solute molecules is quenched in the nanochannel. The effect is more ... Cover...
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9122

J. Phys. Chem. C 2007, 111, 9122-9129

Quenching of Collision between the Solute Molecules in the Nanochannel of MCM-41: A Spin-Probe ESR Study on the Alcoholic Solutions Masaharu Okazaki* and Kazumi Toriyama Research Institute of Instrumentation Frontier, National Institute of AdVanced Industrial Science and Technology (AIST), 266-98, Shimoshidami, Moriyama-ku, Nagoya, 463-8560 Japan ReceiVed: January 24, 2007; In Final Form: March 30, 2007

ESR spectra were observed for the alcoholic solutions of spin probes at a very high concentration, 30 mM, both in the clear solution and encapsulated in the nanochannel of MCM-41. At a high temperature, the clear solution shows a very broad ESR spectrum, whose line width is mainly due to the spin exchange induced by collision between the solute spin probes. On the other hand, the solution encapsulated in the nanochannel of MCM-41 shows a much sharper spectrum. This sharp line width indicates that collision between the solute molecules is quenched in the nanochannel. The effect is more prominent for the MCM-41 system with smaller channel diameter. In the suspension of MCM-41, the ESR spectrum is composed of the two parts, the sharp one from the solution in the nanochannel and the broad one from the bulk phase. Thus, the quenching of collision in the nanochannel is independent of the coexistence of the solution out of the nanochannel. When the system is cooled down at a temperature 10-20 degree above the melting point, the ESR spectrum of the solution in the nanochannel changes to the glassy signal. Upon warming the system, the spectrum is sharpened gradually and the glassy pattern disappears at around 293 K. These observations have been explained with a model that the solvent molecules form a liquid-crystalline like structure in the nanochannel with hydrogen bonding which prevents the solute from making translational diffusion in the nanochannel. The unexpected rapid flow of the alcoholic solution through the nanochannel and a large cage effect in a photoreaction of the solute in it, both of which were reported by the present authors, are consistent with this model.

Introduction Many interesting findings have been accumulated on the physical1-5 as well as the chemical processes1,2,6,7 for the solution systems in the nanochannel of MCM-41,8 a mesoporous silica9 that is one of the members of nanomaterials10 and expected as the base material of new catalysts.6 For example: the melting point of water becomes as low as 230 K in the MCM-41 nanochannel;3 benzene and diethylether diffuse very rapidly through the nanochannel at ambient temperatures,4 but water diffuses slower compared with that in the bulk.5 Among these, a large “cage effect”11 observed for the photoreaction of xanthone in 2-propanol that flows in a column packed with MCM-417,12 is very important, since the experimental setup can be applied to many systems as a new type of technique to control chemical reactions. A rapid solution flow in the nanochannel without a translational diffusion of individual molecules is definitely necessary7,12 to interpret the above results and a new concept, “collective molecular flow”, has been proposed.12,13 In the present work, a spin probe-ESR14-16 study has been made for the alcoholic solutions of spin probes at the concentration of 30 mM encapsulated in the nanochannel of MCM-41. At this concentration, the translational diffusion rate of the solute spin probe is directly related to the additional line width caused by collision between the spin probe molecules. As a result, a new model has been presented that the alcohol molecules exist as liquid-crystalline like clusters when included in the nanochannel.12,13 It is discussed that the rapid solution flow in the nanochannel13,17 at a rate much higher than that predicted by * To whom correspondence should be addressed. E-mail: masa-okazaki@ aist.go.jp.

Poiseulle’s law is consistent with this solution model. DTBN (di-t-butylnitroxide) and TEMPOL (2,2,5,5-tetramethylpiperidine-1-oxyl-4-ol) were employed as the spin probes (Chart 1).14 Experimental Section MCM-41 with different pore sizes was synthesized following the methods given in the references from tetraethylorthosilicate (TEOS) and cetyltrimethylammoniumbromide (MCM-41(16))18 or fumed silica (MCM-41(10)) and decyltrimethylammoniumbromide.19 Template molecules were removed by calcination at 820 K for 5 h. MCM-41(16) prepared in the present study has the granular size of about 5 µm on average, and the poresize was estimated as 3.76 nm from the lattice constants obtained by the XRD method and the surface area of 980 m2/g obtained by the BET method (Flowsorb II 2300, of Micrometrics Inc.). On the other hand, the corresponding values for MCM-41(10) employed here are ca. 0.3 µm, 2.88 nm, and 870 m2/g, respectively. Here, the density of silica was assumed as 2.25.20 Specific capacities of the nanochannels have been estimated as 0.92 mL/g (MCM-41(16)) and 0.63 mL/g (MCM-41(10)) by the simple calculation based on the model in which the area of outer surface is neglected [Supporting Information (I)]. A suprasil-made ESR cell with the pyrex joint purchased from Eiko-sha (Osaka, Japan) is fused to glassware that has been designed and made for the present study [Supporting Information (II)]. Samples were prepared as follows. (1) Spin probe solution encapsulated in the nanochannel: MCM-41 was dried for 1 h at 403 K and a 100 mg portion was introduced in the glassware and dried another 1.0 h in vacuum at the same temperature. After cooling, an appropriate amount of an alcoholic solution of a spin probe, 90 µL for MCM-41(16) and 50 µL for MCM-

10.1021/jp070605i CCC: $37.00 © 2007 American Chemical Society Published on Web 06/02/2007

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CHART 1: Molecular Structure of the Spin Probes Employed in the Present Study

41(10), respectively, was injected through the septum attached on the glassware, degassed twice by the freeze and thaw cycles, and sealed off. The MCM-41 powder involving the solution was vigorously shaken to disperse the coagulated MCM-41 powder. After waiting for a while, the argon gas was injected through the same septum to the pressure a little less than 1.0 atm. The MCM-41 powder holding the solution in the nanochannel was moved to the tip of the ESR cell, which was then sealed off for observation. (2) MCM-41 suspension: 40 mg of MCM41 was dried in vacuum as described above. A 400 µL portion of the alcoholic solution was added through the septum, and the system was degassed by the pump and thaw cycles. After sealing off the apparatus, the container was shaken vigorously, and then the suspension was introduced into the ESR cell. The sample cell was set upright still until the sedimentation was completed.21 ESR measurements were made with a JEOL RX-1 spectrometer (9.1 GHz) with a temperature control unit. The analog data was acquired to a computer (NEC PC9801US) via AD converter. The microwave power was set at 1.0 mW and the field modulation amplitude was 0.01 mT. The integrated spectra have been obtained from the usual first derivative spectra by integrating numerically. DTBN and TEMPOL were purchased from Aldrich Japan (Tokyo), and other chemicals were from Wako Pure Chemicals (Tokyo, Japan). Results and Discussion (i) Overview of the Phenomena at High Temperature for the 2-Propanol System. Figure 1 shows the ESR spectra observed at 313 K for the 2-propanol solutions of spin probes under various conditions. Spectrum a for the DTBN solution at 0.3 mM in the absence of MCM-41 is a sharp triplet with equal heights. Thus, the rapid rotational diffusion averages the most part of the intramolecular anisotropic interactions, and the spin exchange between the radicals does not affect the line width very much. The three lines in spectrum b for the same solution in the nanochannel of MCM-41(16) are a little broader than those in spectrum a and have heights different from each other: h(0) > h(-1) > h(+1), where h(MN) indicates the line height for the nuclear quantum number MN. As has been analyzed in detail22 with the line width theory,23 this ESR pattern indicates that the rotational diffusion of the DTBN molecule becomes restricted considerably along both the x and z axes, but not so much along the y axis (Chart 1). We refer to this kind of rotational diffusion as y-axis rotation hereafter. Since the rotation along the y axis prevents the NO group from making interaction with the nanochannel surface and the DTBN molecule does not have other functional groups for special interactions, the molecule must be free from the surface. The diameter of the nanochannel is about five times larger than the molecular diameter of DTBN; thus, the anisotropic rotation may be due to both the elliptic shape of DTBN and the anisotropy in the solvent 2-propanol induced in the nanochannel.24,25 The very large line width of spectrum c for the clear solution at 30 mM is due to spin exchange induced by collisions between the DTBN

Figure 1. ESR spectra of DTBN (a-e) and TEMPOL (f) in 2-propanol at 313 K with or without MCM-41(16): (a) DTBN at 0.3 mM in the bulk, (b) in the nanochannel; (c) DTBN at 30 mM in the bulk; (d) in the nanochannel; (e) in the MCM-41(16) layer of its suspension; (e) TEMPOL at 30 mM in the nanochannel. The gray spectrum attached below (e) is a reconstructed one by mixing (c) and (d) at the ratio of 0.79 and 0.21.

molecules at a high rate. This is usually observed for a spin probe in a low molecular weight alcohol, where the translational diffusion of the spin probe molecule is very rapid. Once the solution is encapsulated in the nanochannel, the spectrum changes to that in (d). The great reduction in the line width must be due to inhibition of the translational diffusion of DTBN molecules. The relative heights of the three lines are almost the same with those of spectrum b.26 When the translational diffusion is suppressed, the line width induced by the dipolar interaction between the two electron spins becomes substantial. Thus, the spin exchange frequency for (d) may be much smaller than the value calculated from the difference between the linewidths of the two spectra b and d. Thus, the viscosity of 2-propanol27 in the nanochannel must be larger than that simply estimated from only the decrease in the spin exchange rate. Spectrum f for TEMPOL is also sharpened as in the DTBN case for spectrum d; thus, the translational diffusion is suppressed also for the TEMPOL molecule at a very low level. The relation h(0) ) h(-1) > h(1) indicates that the rotational diffusion of TEMPOL is retarded along all of the directions in the nanochannel.25 The great decrease in the collision frequency between the radicals, the retarded and anisotropic rotational diffusion of the elliptic DTBN molecule, and the retarded isotropic rotation for the TEMPOL molecule suggests strongly that 2-propanol carries liquid-crystalline character, i.e., becomes anisotropic and viscous, when existing in the nanochannel of MCM-41. Spectrum e is observed for the sediment layer of MCM-41 in the 2-propanol solution of DTBN at 30 mM. This spectrum can be reconstructed from the two spectra c and d at the areal ratio of 0.79 vs 0.21 as shown with a gray line just below the observed spectrum. The good coincidence between the two spectra indicates that the dynamics of the solution in the nanochannel is not very much dependent on the solution outside the nanochannel. The mixing ratio of the two signals is not so far from that between the space volumes,21 so the adsorption of the solute DTBN in the nanochannel is not extensive at this temperature, i.e., the DTBN molecule works as a good probe. (ii) Temperature Dependence for the DTBN/2-PrOH System in MCM-41(16). The temperature dependence of the ESR spectrum may give useful information on the mechanism of the nanospace effect in the present system. Shown in Figure 2 are the ESR spectra for the 2-propanol solution of DTBN at 30 mM at various temperatures, (A) the clear solution, (B) the solution encapsulated in the nanochannel of MCM-41(16), and (C) the solution in the MCM-41(16) sediment layer of its

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Figure 2. ESR spectra (integrated) of the 2-propanol solution of DTBN at 30 mM at various temperatures: (A) in the bulk, (B) in the nanochannel of MCM-41(16), and (C) in the MCM-41(16) layer of its suspension.

suspension. The spectra are shown in the integrated form here, since it is easy to compare by eye the relative contributions of two or more components having different linewidths. In the clear solution at 193 K, which is a little higher than the melting point of 183.5 K, the spectrum appears as broad but resolved three lines. The line width decreases upon warming the system, becomes minimum at around 233 K, and increases again at still higher temperatures. The line broadening at high temperatures above 233 K is clearly due to spin exchange induced by collisions between the radicals.28 As has been pointed out by Berner and Kivelson,28 however, the relative contributions between the two intermolecular mechanisms, the intermolecular electron-electron dipolar interaction and the exchange interaction, are not well understood for the spin probes in the lower temperature region. For the solution encapsulated in the nanochannel (B), on the other hand, the shapes and the temperature dependence of the ESR spectrum are completely different from those in (A). A spectrum in column (C) is approximately the linear combination between the two spectra in (A) and (B) at the corresponding temperature, suggesting that the DTBN radical behaves almost independently in the two spaces as already mentioned in the above section. However, a mild adsorption of DTBN occurs in the nanochannel of MCM41 especially at low temperatures, since the component from (B) increases gradually upon lowering the temperature. The exact analysis, however, was difficult in contrast to the cases in the former report for the benzene solution,21 since the spectrum is constituted of many lines with different broadenings. The ESR spectrum at 193 K in column (B) is that of completely immobilized DTBN, indicating that 2-propanol in the nanochannel is in the glassy state in the ESR time domain.29 However the DTBN molecules are not immobilized by adhesion on the nanochannel surface by some chemical forces, since (a) almost identical spectrum is observed for the other spin probe, TEMPOL, that is more soluble in alcohol (see Figure 4A at 193 K); (b) the same type of spectrum is observed with other solvents, methanol and ethanol (Figure 7 at 183 K and Supporting Information III), as will be discussed later; (c) if some chemical force acts between the surface and the spin probe as the essential cause of this immobilization, the spin probe should have been highly condensed into the nanochannel in the suspension of MCM-41(16) as has been observed in the previous reports.21,30 In the present case, however, the three-line spectrum is observed also as a major component for the suspension of MCM-41(16) at 193 K (Figures 2C and 4B). The wide rigid ESR pattern gradually “softened” and became almost homogeneous three-line at around 293 K. For these spectra the relative

Okazaki and Toriyama

Figure 3. Peak to peak line width of the first derivative ESR line (MN ) 0) for the 2-propanol solution of DTBN: (a) clear solution at 30 mM, (b) clear solution at 3.0 mM, (c) solution at 30 mM encapsulated in the nanochannel of MCM-41(16), and (d) solution at 30 mM encapsulated in the nanochannel of MCM-41(10). The line width data at low temperatures are given only for the bulk system (see the text).

Figure 4. ESR spectra (integrated) of the 2-propanol solution of TEMPOL at 30 mM at various temperatures: (A) in the nanochannel of MCM-41(16) and (B) in the MCM-41(16) sediment layer of its suspension.

Figure 5. ESR spectra for the spin probe solutions of 2-propanol at 30 mM in the nanochannel of MCM-41(16) at the temperature of 313 K: (a) DTBN solution observed, (b) simulation of (a), (c) TEMPOL solution observed, and (d) simulation of (c). Hyperfine coupling of the distant protons are considered in the simulation.25

height of the high field line to that of the central line, h(+1)/ h(0), which is sometimes employed to discuss the rotational correlation time of the spin probe,31 is in the range similar to that for the clear solution at temperatures in the range of 193233 K. Roughly speaking, therefore, the rotational mobility of the spin probe in the nanochannel at temperatures in the range 293-333 K is similar to that in the clear solution in the range of 193-233 K. Therefore, the effective viscosity for the

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Figure 6. ESR spectra for the DTBN solution of 2-propanol at 30 mM encapsulated in the nanochannel of (A) MCM-41(16) and (B) MCM-41(10).

rotational diffusion of a solute might be more than 30 times larger in the nanochannel than that in the bulk. (Supporting Information IV) In conclusion, the quenching of collision between the solute molecules is due to the liquid crystalline structure of 2-propanol in the nanochannel, which also reduce the rotational mobility of the solute. A small sharp signal in the spectrum observed at 213 K for the 2-propanol solution encapsulated in the nanochannel (Figure 2B) may be assigned to the DTBN radical trapped in the central portion of the wide channels or in an imperfect channel structure, where the 2-propanol solution remains mobile. This small signal increases with the temperature and finally fused into the main signal at the highest temperature. Since MCM-41 is inherently inhomogeneous in its structure, it is quite difficult to know whether this signal is from the normal channel structure of MCM-41. In any case, we must not exaggerate this signal at a temperature below 273 K, even though it appears intensely in the first derivative ESR spectrum.25 (iii) Additional Line Broadening by Spin Exchange. The spin exchange frequency in solution, ωex, is usually evaluated by using eq 1.16

4 T ωex ) nfλk ) FCollfλ 3 η

()

(1)

Here, the collision radius is assumed as being equal to the diameter of the radical, n represents the concentration (number density) of radical, fλ is the statistical factor giving the ratio of the effective collision for spin exchange to the total collision frequency of FColl (fλ ) 2/3; thus FColl is equal to 1.5ωex), η is the viscosity of the solvent, k is the Boltzman’s constant, and T is the absolute temperature. The additional peak-to-peak line broadening for the first derivative ESR spectrum caused by spin exchange is related with ωex as

∆HEX pp )

2 ωex x3 γe

(2)

Here, γe is the gyro-magnetic ratio of the electron.15,16 Equations 1 and 2 indicate that the collision frequency FColl is related with the additional line broadening of the ESR spectrum. The important relation is that the collision frequency thus the exchange broadening is proportional to both n and T/η. In stead of ∆Hpp, the peak-to-peak total width, the half width at halfheight of the integrated ESR spectrum, ∆H1/2,1/2 ()x3∆Hpp/ 2), is also employed frequently as the line width parameter.

Figure 7. ESR spectra for the DTBN solution of methanol at 30 mM encapsulated in the nanochannel of (A) MCM-41(16) and (B) MCM41(10).

Figure 3 shows the observed peak-to-peak linewidths of the first derivative ESR spectra (∆Hpp) for the 2-propanol solution of DTBN in the bulk (a) at 30 mM and (b) at 3 mM, or for the DTBN solution at 30 mM encapsulated in the nanochannel of (c) MCM-41(16) and (d) MCM-41(10). Since the sharp minor components appear intensely in the first derivative ESR spectra, the line width data at temperatures T < 293 K are omitted from curves c and d. The line width for the DTBN solution at 30 mM in the bulk increases steeply as the temperature increases in the range of 273-333 K. This is a normal feature of the exchange broadening of a solution sample, since it depends on T/η. The slope of curve b in the range of 273-333K is around 1/15 of that in curve a in partial agreement with eq 1.32 On the other hand, the apparent line width for the DTBN solution at 30 mM in the nanochannels c and d decreases gradually with an increase of the temperature in the range of 293-333 K. The negative slopes of these indicate that the spin exchange mechanism is not the main one for the line broadening mechanism in these systems. Thus, the collision frequency in the nanochannel of MCM-41(16) is actually much less than that estimated from the difference of curves a and d. The difference between the two curves c and d will be discussed later. (iv) Solute Dependence of Exchange Broadening in the Nanochannel of MCM-41(16). Figure 4 shows the ESR spectra at various temperatures for the TEMPOL solution of 2-propanol existing in the nanochannel (A) or in the MCM-41 sediment layer of its suspension (B). In both cases, the temperature dependences are qualitatively the same with those for the corresponding systems with DTBN. One difference is that the linewidths of the triplet components in Figure 4 are considerably larger than those in the corresponding spectrum of DTBN in Figure 2. This is partly due to a higher concentration of TEMPOL in the region of high fluidity, in addition to the larger hyperfine coupling constants for the γ-protons of TEMPOL. In fact, the relative area of the three-line signal of Figure 4B in the low-temperature region is considerably larger than that for the DTBN system in Figure 2C. This is because TEMPOL is not condensed distinctly into the nanochannel from the bulk in contrast to DTBN. In general, the physical character of a polar solvent is modified by the silica surface of the nanochannel, which may trap a rather hydrophobic molecule, DTBN, more than the hydrophilic molecule, TEMPOL, in the nanochannel. In the case of a nonpolar solvent, on the other hand, the reverse relation holds: a hydrophilic solute is adsorbed more than a hydrophobic one in the nanochannel. The mechanism of the solute condensation in the MCM-41 nanochannel has been

9126 J. Phys. Chem. C, Vol. 111, No. 26, 2007 discussed.21,30 The spectrum of TEMPOL at 193 K in Figure 4A also strongly suggests that 2-propanol becomes in the glassy state in the ESR time scale when encapsulated in the nanochannel of MCM-41 at this temperature. At the temperature above 293 K, the solution holds the liquid crystalline state in the nanochannel, which prevents the free radicals from exchanging their spins rapidly. The fact that qualitatively the same results have been obtained with two spin probes, whose molecular structures and interactions with the solvent alcohol are considerably different from each other, assures its success. Figure 5 shows the ESR spectra at 313 K for the DTBN solution (a) and for the TEMPOL solution (c) at 30 mM encapsulated in the nanochannel of MCM-41. Their simulation spectra, b and d, respectively, are calculated by the method given in the literature taking into account the anisotropy in rotational diffusion.23 In this method, the hyperfine interaction with nitrogen nucleus, g-anisotropy, and the isotropic hyperfine coupling with γ-protons are considered.22 The rotational correlation times τll and τ⊥ for the calculation of spectrum b are, 0.095 and 1.43 ns (τ⊥ /τll ) 15), respectively. On the other hand, spectrum d has been obtained by the single correlation time of 0.42 ns. The linewidths 0.04 and 0.08 mT, respectively, are employed as those that are caused by other mechanisms and work equally to all three lines. For the DTBN system, we obtain 0.02 mT () 3.5 × 106 s-1) as the contribution of the intermolecular spin interactions by reducing the intrinsic or residual line width of about 0.02 mT from the component line width of 0.04 mT employed for the simulation. The residual line width of 0.02 mT has been estimated in a previous report,25 which is due to, e.g., the spin-rotation interaction, the magnetic field modulation, and the inhomogenuity of the static magnetic field. If the rest of the line width of 0.02 mT by the intermolecular interactions is totally due to the spin exchange induced upon collision between the radicals, the viscosity of 2-propanol is calculated as 21 cp in the nanochannel at 313 K. Since a large part of the line width may be due to the electronelectron dipolar interaction, the viscosity must be much larger than the above value. In this case, however, it is quite questionable whether we can define the viscosity of a solvent in the nanochannel in the same way for the bulk solution. The shape of DTBN molecule, which is approximated as an ellipsoid with the diameters of 0.65 nm for directions x and z and 0.85 nm for direction y (Chart 1), must be the cause of the very anisotropic rotational diffusion mentioned above. Since the shape of TEMPOL cannot be approximated as an ellipsoid, on the other hand, the rotational diffusion is not anisotropic. A little larger contribution of the intermolecular spin interactions in the line width, 0.06 mT, for TEMPOL might be due to a little larger diffusion rate of this molecule in the nanochannel. Although the detail for this cause is not known, a possible mechanism is that TEMPOL works to soften 2-propanol in the nanochannel, since the TEMPOL molecule has an OH group and the concentration of 30 mM is rather high. It is important to point out that the small difference in the molecular shapes of the spin probes (Chart 1) results in a large difference in the rotational motion, though the present nanospace is wide enough to make rotation in every direction. In summary, the spin exchange frequency decreases to a very small level and the rotational diffusion becomes very anisotropic for DTBN, whose molecular shape is oval, in 2-propanol when included in the nanochannel of MCM-41. (v) Effect of Diameter of the Nanochannel. It is important to observe the effect of space diameter in this kind of discussion to derive the essential feature of the space effect. Figure 6 shows

Okazaki and Toriyama the ESR spectra for the 2-propanol solution of DTBN encapsulated in the nanochannel of mesoporous silica with different (averaged) channel diameters, (A) MCM-41(16), d ) 3.76 nm (identical to Figure 2B), and (B) MCM-41(10), d ) 2.88 nm.33 ESR spectra of B are considerably narrower at higher temperatures (T > 293 K) than those of A. This is clearly due to the intensified space effect in the narrower channel: i.e., a narrower channel causes a strict quenching in the translational diffusion of the spin probes. The broad ESR pattern at 193 K for the MCM-41(10) system is almost the same as that for the MCM41(16) system, but the low field edge appears dull. The sharpening of spectra proceeds at higher temperatures in the same way for both systems, except that the very small sharp component appears only in the MCM-41(16) system. This sharp component may be neglected for the time being, since the integrated area is very small. If the alcohol molecules are packed in the nanochannel of MCM-41(10), only four or five molecules can make an array in the radial direction. So, it is rather difficult for the solvent alcohol to make a transient network structure, and the DTBN molecule may be a little more mobile than that in MCM-41(16) in the lowest temperature region (T < 233 K). If the DTBN molecule avoids making contact with the nanochannel, it can take a nearly unique position at the central part of the nanochannel. Therefore, the sharpening of the ESR pattern is rather simple. Meanwhile, the MCM-41(16) nanochannel may accommodate seven or more molecular layers radially, and the transient network structure is made rather easily. This may be the reason why the guest molecule is held rather tightly at a lower temperature. This wider space allows the spin probe to find several different positions in the nanochannel of MCM41(16); thus, the ESR spectrum is composed of several components from the very sharp signal to the very broad one even at the lowest temperature. When the temperature becomes high, redistribution of the spin probes occurs among these positions. Since the sharp component appears prominently in the first derivative ESR, the ∆Hpp value may not be the most suitable to assess the total feature of the spin-probe mobility. (vi) Solvent Effect in the Solute Mobility in the Nanochannel. Figure 7 shows the ESR spectra for the methanol solution of DTBN at 30 mM in the nanochannel of (A) MCM-41(16) and (B) MCM-41(10) at various temperatures. Both the temperature dependence and the pore-diameter dependence are essentially the same as those for the 2-propanol solution system (Figure 6). Although the viscosity of liquid methanol is lower than that of 2-propanol, the three lines observed at a temperature higher than 293 K are narrower than the corresponding spectra for the 2-propanol system in Figure 6. If the spin exchange worked as the main mechanism in line broadening, the system with methanol should have given wider spectra. The reverse observation, therefore, indicates that no appreciable spin exchange is expected in these nanopore systems even at the highest temperature, i.e., the spin exchange; thus, the translational molecular diffusion of a solute is inhibited strictly in 2-propanol when the MCM-41with a narrower channel is employed, since 2-propanol has a higher viscosity and a larger molecular volume. Figure 8 shows the ∆Ηpp of the first derivative ESR line as functions of temperature for the DTBN systems in methanol. The line width for the bulk system changes in the same way as that in the 2-propanol system, but the line width is considerably larger in the high-temperature region. This is simply due to the lower viscosity of methanol than that of 2-propanol. At 293 K, for example, the ESR line width in methanol is 1.8 times larger

Collision Quenching of Solutes in the Nanospace

Figure 8. Peak to peak line width of the first derivative ESR line (MN ) 0) for the methanol solution of DTBN at 30 mM: (a) bulk solution (circle), (b) encapsulated in the nanochannel of MCM-41(16) (square), and (c) encapsulated in the nanochannel of MCM-41(10) (diamond). The line width data at low temperatures are given only for the bulk system.

than that in 2-propanol, reflecting the difference in the bulk viscosities of the two solvents.34 As for the system encapsulated in the nanochannel, the temperature dependences are not large in the region of 273 K < T < 333 K for both systems. Since the nanopore effect must be reduced in the methanol system, the small temperature dependences may result from the cancellation of the two effects: narrowing of the line width due to the anisotropic intramolecular interactions and broadening of it due to the spin exchange. We would point out again here that the ∆Ηpp does not reflect the total appearance well for spectra in Figure 7, since (1) the sharp component is more conspicuous in the first derivative ESR spectrum of an inhomogeneous system and (2) the ESR spectrum of the MCM-41(16) system contains several components with different line width as mentioned above (section v). All of these observations indicate that the nanospace effect is observed more clearly in the MCM-41(10) system than that in the MCM-41(16) system and also more extensive in the 2-propanol system compared with that in the methanol system. This agrees well with the fact that the cage effect in a photochemical reaction appears larger in the system with narrower MCM-41 nanochannel for the 2-propanol solution.7,12,13 At this stage, we can safely assign the broader line width in the high-temperature region T > 293 K in the 2-propanol system to the higher viscosity of 2-propanol which restrict the rotational diffusion more and not to spin exchange due to collision between the solutes. We do not refer to the ethanol system with experimental results since the ethanol system behaves in a manner just between those in the 2-propanol system and the methanol system. The observed spectra for the ethanol system in MCM-41(10) is given as Supporting Information (Supporting Information III). (vii) Model for the Alcoholic Solution of DTBN in the Nanochannel of MCM-41. A model for the alcoholic solution of DTBN in the nanochannel is visualized in Figure 9. The alcohol molecules take an ordered structure like a liquid crystal in the nanochannel of MCM-41, and the DTBN molecule takes a position as if it is in a cage of alcohol. This model is similar to that presented in a previous study for the aqueous solution of DTBN.30 The DTBN molecule tends to be in parallel with the line of alignment of the solvent clusters and the translational diffusion of the solute DTBN molecule is quenched by this cage. Since the longest molecular axis is y (Chart 1), the DTBN molecule makes y axis rotation at a considerable rate. The interaction between the 2-propanol molecules that cause clustering must be hydrogen bonding; thus, a zigzag chain may be formed temporally as depicted in the figure. The alcohol

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Figure 9. Model of DTBN solution in the alcohol solution encapsulated in the nanochannel of MCM-41: The alcohol molecules acquire a liquid crystal structure with hydrogen bonding in the nanochannel and the DTBN molecule is dissolved among the solvent cluster and making preferential rotation along the y axis.

Figure 10. ESR spectrum for the methanol solution of DTBN at 30 mM in the presence of MCM-41(10): (A) first-derivative spectra and (B) integrated spectra. The ratio r indicates the solution volume relative to the capacity of nanochannel, which is defined as 1.0 when 50 µL of the solution is added to 100 mg of MCM-41(10). All spectra except the bottom one are normalized to have the same intensity.

molecules in the nanochannel are on the surface or close to the surface; thus, the molecular configuration may be arranged to lower the surface enthalpy by making H-bonding between them. This molecular configuration is characteristic of the crystalline alcohol.35 The OH groups in the chain may not be directed toward the silica surface on average, since the surface of pure silica MCM-41 is hydrophobic.30,36 In fact, the population of the Si-OH groups is low and only one in a few square nm.37 In this model, the MCM-41 nanochannel works as a nonpolar hollow space, and the DTBN molecule is condensed into the nanochannel in the MCM-41 suspension of alcohols at a low temperature.21 If the nanochannel surface is more polar than the alcohol, the nonpolar DTBN must not be condensed in the nanochannel in the MCM-41 suspension.30,38 It may be difficult to understand this model from the conventional point of view, since the interaction between the Si-OH group on the surface of the nanochannel and the OH group of alcohol does not appear as an important factor. However, the interaction between the Si-OH group and the R-OH group should be important as the basic factor for the nanochannel to encapsulate the solution smoothly. We consider that the solution molecules are incessantly changing the network structure, and sometimes they have interaction with the Si-OH group on the surface. When the solution is not enough to fill the nanochannel, on the other hand, the contribution of the hydrogen bond network among the neighboring alcohols decreases and that of the interaction with the surface Si-OH group increases. Therefore, the assumed H-bond network structure is broken in the region where the solution does not fill the space radially. To check the above interpretation on the model we would like to refer to another experiment. Figure 10 shows the ESR spectrum at 293 K for the various volume of methanol solutions of DTBN at the concentration of 30 mM having contact with MCM-41(10) power of a constant weight. The first derivative ESR spectrum in A is convenient to measure the ∆Ηpp for the

9128 J. Phys. Chem. C, Vol. 111, No. 26, 2007

Okazaki and Toriyama

sharp component, and the integrated spectrum in B is convenient to detect the existence of the broad compoent. The ratio between the solution volume and the volume of nanochannel is defined as r

Acknowledgment. We thank Dr. Sindhu Seelan for sample preparation. The internal grant from the Research Institute of Instrumentation Frontier is also acknowledged.

V(solution) r) V(nanochannel)

Supporting Information Available: (I) Surface area and the volume and the diameter of the nanochannel. (II) Glassware for the sample preparation. (III) ESR spectrum of DTBN in ethanol. (IV) Viscosity of 2-propanol. This material is available free of charge via the Internet at http://pubs.acs.org.

(3)

When the solution volume is much larger than the volume of nanochannel, or r . 1.0, the very broad component from the bulk solution filling the space between the MCM-41 granules overlaps with the sharp three lines (top spectrum of B). At r ) 1, all of the solution is in the nanochannel, and thus, the very broad component disappears. The line width starts broadening gradually with a further decrease in the solution volume, and at r ) 0.4, the line width becomes very broad abruptly. We consider that this large spectral change corresponds to the breaking of the structure drawn in Figure 9. When the solution volume is not enough to fill the nanochannel, some solvent alcohols directly make H-bonding with the Si-OH groups, and the network is broken. The random molecular distribution of the solvent molecules does not allow the DTBN molecule to rotate smoothly and causes a serious line broadening as in the bottom spectrum of Figure 10. Above all, this observation may indicate that the network-structure formation is a cooperative one, and it collapse at the volume ratio of around 0.4. A more detailed study is now in progress. The model presented here and the dynamic characters clarified for the solute molecules in the nanochannel accords with our mechanism for the photoreaction of xanthone in 2-propanol flowing in the column packed with MCM-41. The dull and large magnetic field effect on this reaction observed previously7,12,13 implies that the two radicals generated as the intermediates keep pairing for a long time,39,40 e.g., more than several microseconds in the nanochannel. This long lifetime of the radical pair accords with the present result that the mutual collision of the solute molecules is quenched;41 that is, the separation of the two paired radicals is not expanded; thus, the two radicals keep pairing during the flow through the nanochannel, since the translational diffusion of the solutes are quenched. The re-encounter is possible both in the nanochannel and at the end of the nanochannel. In this experiment, the reactant solution should flow through the nanochannel. This is the definitely a necessary condition for the system to show a large cage effect. If adhesion of the solvent molecules occurs on the surface, the friction force between the molecules must cause clogging of the solution as Poiseulle’s law predicts. Therefore, if molecules make clusters and there is no strong interaction between the solvent molecules and the nanochannel surface, the solution may flow in a single file by slipping on the nanochannel surface. In this case, the lifetime of the “geminate” pair can be as long as the value of the channel length divided by the flow speed. Conclusion Translational diffusion of the solute spin probes in alcoholic solutions is quenched in the nanochannel of MCM-41, and the rotational diffusion of an elliptic spin probe is a little retarded and highly anisotropic at an ambient temperature. A model for these dynamic behaviors of the solute is presented, in which the hydrogen-bonded network of solvent molecules is assumed in the nanochannel. It was confirmed that the network structure is broken when the relative volume of the solution is reduced at a level. This model is compatible with the rapid solution flow through the nanochannel of MCM-41 under a moderate pressure.13,15 The large cage effect in a photochemical reaction7,12 is self-explanatory in this model.

References and Notes (1) Zhao, X. S.; Lu, G. Q.; Millar, G. J. Ind. Eng. Chem. Res. 1996, 35, 2075. (2) Coma, A. Chem. ReV. 1997, 97, 2373. (3) Morishige, K.; Nobuoka, K. J. Chem. Phys. 1997, 107, 6965. (4) Stallmach, F.; Graeser, A.; Kaerger, J.; Krause, C.: Jeschke, M.; Oberhagemann, U.; Spange, S. Micopor. Mesopor. Mater. 2001, 44-45, 745. (5) Takahara, S.; Sumiyama, N.; Kittaka, S.; Yamaguchi, T.; BellissentFunel, M.-C. J. Phys. Chem. B 2005, 109, 11231. (6) Sayari, A. Chem. Mater. 1996, 8, 1840. (7) Okazaki, M.; Konishi, Y.; Toriyama, K. Chem. Phys. Lett. 2000, 328, 251. (8) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710. (9) Yanagisawa, T.; Shimizu, T.; Kuroda, K.; Kato, C. Bull. Chem. Soc. Jpn. 1990, 63, 988-992. (10) Ozin, G. A.; Arsenault, A.C. In Nanochemistry A Chemical Approach to Nano Materials; RSC Publishing: Cambridge, U.K., 2005; Chapter 8. (11) Turro, N. J. Proc. Natl. Acad. Sci. U. S. A. 1983, 80, 609. (12) Konishi, Y.; Okazaki, M.; Toriyama, K.; Kasai, T. J. Phys. Chem. B 2001, 105, 9101. (13) Okazaki, M.; Toriyama, K.; Oda, K.; Kasai, T. Phys. Chem. Chem. Phys. 2002, 4, 1201. (14) Keana, J. F. K. Chem. ReV. 1978, 78, 15. (15) Marsh, D.; Horvath, L. In AdVanced EPR-Application in Biology and Biochemistry; Hoff, A. J., Ed.; Elsevier: Amsterdam, 1989; Chapter 20. (16) Berliner, L. J., Ed.; Spin Labeling; Academic Press: New York, 1976. (17) Okazaki, M.; Toriyama, K.; Sawaguchi, N.; Oda, K. Appl. Magn. Reson. 2003, 23, 435. (18) Burkett, S. L.; Sims, S. D.; Mann, S. Chem. Commun. 1996, 1367. (19) Sayari, A.; Yang, Y. J. Phys. Chem. B 2000, 104, 4835. (20) Handbook of Chemistry, Volume for Applied Chemistry; Chemical Society of Japan: Japan; p 257. (21) Okazaki, M.; Anandan, S.; Seelan, S.; Nishida, M.; Toriyama, K. Langmuir 2007, 23, 1215. (22) Okazaki, M.; Kuwata, K. J. Phys. Chem. 1984, 88, 4181. (23) Freed, J. H.; Fraenkel, G. K. J. Chem. Phys. 1963, 39, 326. (24) Okazaki, M.; Toriyama, K. J. Phys. Chem. B 2003, 107, 7654. (25) Okazaki, M.; Toriyama, K. J. Phys. Chem. B 2005, 109, 13180. (26) It is difficult to understand with the usual strong collision model for spin exchange that the relative heights of the three lines are almost invariant with the concentration. (27) The word “viscosity” is also used to characterize the solution in the nanochannel, where the macroscopic concept cannot be used in a straightforward way. (28) Berner, B.; Kivelson, D. J. Phys. Chem. 1979, 83, 1406. (29) The melting point of 2-propanol may be lowered in the nanochannel of MCM-41; thus, the 2-propanol in the nanochannel may not be solid but in a liquid-crystalline like state. The ESR spectrum for the DTBN molecule whose rotational correlation time is much larger than 10-7 s appears almost the same as that for the rigid sample. The dynamic characterization of the system in the present paper is valid only in the ESR time domain. (30) Okazaki, M.; Toriyama, K. J. Phys. Chem. B 2005, 109, 20068. (31) See, for example: Likhtenshtein, G. I. Spin Labelling Methods in Molecular Biology; John wiley; New York, 1976; Chapter 1. (32) The linear dependence of ∆HEX pp to the radical concentration is limited in the cases where the lineshape is approximated with the Lonrentzian function. Since the lineshapes of the spectra in Figure 1A are distorted considerably, we cannot expect the linear dependence. (33) The diameter of the nanochannel calculated for MCM-41(10) is a little larger than those in the literature. In fact, we employed 50 µL to fill up the nanochannel of a 100 mg portion of MCM-41(10). If this volume

Collision Quenching of Solutes in the Nanospace setting is correct, the averaged nanochannel diameter must be reduced to 2.56 nm. (34) The viscosities of 2-propanol and methanol at 293 K are 0.239 and 0.0611 (Kg/m.s), respectively. The much larger linewidth for the methanol system is due to this large difference in the viscosities. (35) Tauer, K. J.; Lipscomb, W. N. Acta. Crystallogr. 1952, 5, 606. (36) Zhao, X. S.; Lu, G. O. J. Phys. Chem. B 1998, 102, 1556. (37) Zhao, X. S.; Lu, G. O.; Whittaker, A. K.; Millar, G. J.: Zhu, H. Y. J. Phys. Chem. B 1997, 101, 6525. (38) When the nanochannel of MCM-41works as a polar space in a nonpolar solvent, the nanochannel collects polar solute molecules in the suspension of MCM-41. This type of adsorption has been observed in, e.g., benzene (ref 21). On the other hand, when the nanochannel works as a nonpolar space in a polar solvent, the nanochannel collects nonpolar solute

J. Phys. Chem. C, Vol. 111, No. 26, 2007 9129 molecules in the MCM-41 suspension. This type of adsorption is observed in water (see section iv). (39) Okazaki, M. In Dynamic Spin Chemistry; Nagakura, S., Hayashi, H., Azumi, T., Eds.; Kodansha-Wiley: New York, 1998; Chapter 8. Other chapters are also informative on the chemistry of the radical pair. (40) Okazaki, M.; Tai, Y.; Nunome, K.; Toriyama, K.; Nagakura, S. Chem. Phys. 1992, 161, 177. (41) It should be emphasized that only an increase in the solvent viscosity cannot explain the long pairing time of the two radicals. The “geminate pair” radicals should be in a cage, e.g., a micelle, to make a reencounter after a certain period, e.g., several µs. Since the radical pair decomposes at a time when they start to diffuse independently and lose the probability to make a reencounter, the lifetime of the pair will not be as long as 1 µs even in glycerol.