Exclusion of Aromatic Radical Cations from Cyclodextrin Nanocavity

constants (Kapp) for the formation and decay processes of S•+. ... determined Kapp values, it was found that the binding ability of S•+ with HP-β...
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17460

J. Phys. Chem. B 2005, 109, 17460-17466

Exclusion of Aromatic Radical Cations from Cyclodextrin Nanocavity Studied by Pulse Radiolysis Takashi Tachikawa, Sachiko Tojo, Mamoru Fujitsuka, and Tetsuro Majima* The Institute of Scientific and Industrial Research (SANKEN), Osaka UniVersity, Mihogaoka 8-1, Ibaraki, Osaka 567-0047, Japan ReceiVed: May 31, 2005

The influence of hydroxypropyl-β-cyclodextrin (HP-β-CD) on the one-electron oxidation reaction of aromatic sulfides (S) with Br2•- and the decay process of the S radical cation (S•+) was investigated by pulse radiolysis. The dissociation kinetics of S•+ from the CD cavity was examined in terms of the apparent equilibrium constants (Kapp) for the formation and decay processes of S•+. Inhibition of the one-electron oxidation reaction of S by Br2•- was clearly observed in the presence of HP-β-CD. On the basis of a comparison between the determined Kapp values, it was found that the binding ability of S•+ with HP-β-CD is much lower than that of S, because of the hydrophobic nature of the cavity. The formation process of the dimer radical cation of 4-(methylthio)phenylmethanol ((MTPM)2•+), which is generated between MTPM•+ and neutral MTPM in solution, was also inhibited by the addition of HP-β-CD.

1. Introduction Cyclodextrins (CDs) are doughnut-shaped molecules consisting of D-glucose units with a fairly hydrophobic interior.1 The number of D-glucose units (6, 7, and 8 for R-, β-, and γ-CDs) determines the diameter (ca. 5-8 Å) and internal volume (ca. 170-430 Å3) of the cavity. CDs are versatile host molecules that can include a variety of organic and inorganic compounds. Therefore, they have been employed as biomimetic cavities for analytical applications and for industrial applications, such as solubilization agents and drug carriers.2,3 The ability of CDs to engage in molecular and chiral recognitions through host-guest inclusion complexes has attracted renewed interest in the construction of various supramolecular architectures.4-6 For example, β-CD, when linked to semiconductor nanoparticles, could act as multiple charge relays between photogenerated holes and guest molecules.6 The field of supramolecular chemistry has been driven by the synthesis of elaborate and sophisticated structures. The structures of supramolecular systems are routinely established by NMR or X-ray crystallography, and the thermodynamics is frequently reported. On the other hand, kinetic measurements have been less prevalent, because many of the processes involved occur on fast time scales (nano- to microseconds) and require fast kinetic techniques. In addition, it is an important issue to clarify the effects of CDs on the bimolecular reactions such as energy and electrontransfer reactions between guests and quenchers. Generally, the bimolecular reaction processes are inhibited and the selectivity of the reactions increases due to the shielding effects of CDs.7 Such CD effects have been studied by the product analyses or fluorescence measurements, while the transient absorption measurements have rarely been performed.8-12 Much less information is also available on the kinetics of association and dissociation of guests, such as radical ions, with CDs. Such information is very useful for the controlled release of guest molecules, such as drugs, by the redox reactions. * Address correspondence to this author. Phone: +81-6-6879-8495. Fax: +81-6-6879-8499. E-mail: [email protected].

The pulse radiolysis technique is a powerful tool for the investigation of mechanisms involving photochemical reactions. Hapiot and co-workers found that the produced radical cations of small oligothiophenes and oligopyrroles are not associated with β-CD and are rapidly expelled from the host after its formation.11a On the contrary, if the radical cation is deprotonated to form a neutral radical, the neutral radical is associated with β-CD. Recently, our research group found that the oxidation of 1-pyrenesulfonic acid (PySA-) by SO4•- was completely inhibited in β- and γ-CDs.12a Herein, we investigated the formation and decay kinetics of radical cations (S•+) of aromatic sulfides (S) included inside β-CD using a pulse radiolysis technique. Aromatic sulfides were chosen because of (a) a number of studies on CD complexation with benzene derivatives,13-16 (b) their importance as intermediates in many chemical processes including those of organic synthesis,17 environmental,18 and biological significance,19,20 and (c) specific spectroscopic properties of their intermediates, such as S•+ and the dimer radical cation (S2•+), which have absorption inthevisible(Vis)andnear-infrared(NIR)regions,respectively.21-25 We discussed the effects of β-CD on the one-electron oxidation reaction of S by Br2•-, which is generated by pulse radiolysis, and the kinetics of association and dissociation of S•+ with the CD cavity. The influence of β-CD on the formation of S2•+ was also examined. 2. Experimental Section 2.1. Materials. Hydroxypropyl-β-CD (HP-β-CD) (Aldrich) and carboxymethyl-γ-CD (CM-γ-CD) (Aldrich) were used without further purification. 4-(Methylthio)toluene (MTT) (Tokyo Kasei) was used without further purification. 4-(Methylthio)phenol (MTP) (Tokyo Kasei) and 4-(methylthio)phenylmethanol (MTPM) (Aldrich) were purified by vacuum sublimation before use. 4-(Methylthio)phenylacetic acid (MTPA) was recrystallized from ethanol. 2.2. Instrumentation. The steady-state UV-Vis absorption spectra were measured by an UV-Vis-NIR spectrophotometer (Shimadzu, UV-3100) at room temperature.

10.1021/jp0528663 CCC: $30.25 © 2005 American Chemical Society Published on Web 08/30/2005

Pulse Radiolysis Study of the Cyclodextrin Nanocavity

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Pulse radiolysis experiments were performed using an electron pulse (28 MeV, 8 ns, 0.87 kGy per pulse) from a linear accelerator at Osaka University. All experiments were performed with aqueous acidic solutions (pH 2) that had been saturated with purified N2O gas for a minimum of 20 min by using a capillary technique. Saturation of the solution with N2O was done to quantitatively convert all solvated electrons (eaq) to the hydroxyl radical (OH•) in the radiolysis experiments. A 4-mL sol solution was placed in a quartz cell (10 × 10 × 40 mm3) that was sealed with a silicon rubber stopper. The kinetic measurements were performed using a nanosecond photoreaction analyzer system (Unisoku, TSP-1000). The monitor light was obtained from a pulsed 450-W Xe arc lamp (Ushio, UXL451-0), which was operated by a large current pulsed-power supply that was synchronized with the electron pulse. The monitor light was passed through an iris with a diameter of 0.2 cm and sent into the sample solution at a perpendicular intersection to the electron pulse. The monitor light passing through the sample was focused on the entrance slit of a monochromator (Unisoku, MD200) and detected with a photomultiplier tube (Hamamatsu Photonics, R2949). The transient absorption spectra were measured using a photodiode array (Hamamatsu Photonics, S3904-1024F) with a gated image intensifier (Hamamatsu Photonics, C2925-01) as a detector. Cyclic voltammograms were obtained using a conventional three-electrode system (BAS, CV-50W) in acetonitrile at room temperature. A platinum electrode was used as the working electrode and a Ag/AgNO3 electrode was used as the reference electrode. 3. Results and Discussion 3.1. Complex Formation between S and CDs. First, we determined the binding constant (K) of S with HP-β-CD in acidic water (pH 2) at room temperature. In the present work, we used HP-β-CD, which is more soluble in water than simple β-CD. The S binds with both one and two molecules of CD as defined by the equilibria in eqs 1 and 2, in which K11 and K12 are stepwise K values as follows,15b

S + CD {\ } S/CD K

(1)

} S/CD2 S/CD + CD {\ K

(2)

11

12

This system is described by eq 3 in which A(λi) is the absorbance at i nm and 0(λi), 11(λi), and 12(λi) are the molar absorbance coefficients () for free S, S/CD, and S/CD2, respectively.

0(λi) + 11(λi)K11[CD]0 + 12(λi)K11K12[CD]02 A(λi) ) [S]0 1 + K11[CD]0 + K11K12[CD]02 (3) Figure 1A is typical of the spectral changes observed for 4-(methylthio)phenylacetic acid (MTPA) in the absence and presence of HP-β-CD ([HP-β-CD] ) 0-10 mM). The binding of one molecule of HP-β-CD results in a red shift together with a decrease in absorbance at the maximum wavelength (λmax), and the subsequent binding of a second HP-β-CD results in a red shift and an increase in the absorbance at λmax. Similar spectral changes were observed for other S. The determined  and K values are summarized in Table 1. The K11 values decreased in the order 4-(methylthio)phenol (MTP) > MTPA > 4-(methylthio)phenylmethanol (MTPM) > 4-(methylthio)toluene (MTT). It is noted that the differences in

Figure 1. (A) Steady-state UV absorption spectra observed for MTPA ([MTPA] ) 0.5 mM) in the absence and presence of HP-β-CD ([HPβ-CD] ) 0-10 mM) in acidic water (pH 2). (B) The HP-β-CD concentration dependences of the absorption values at 245 (a), 254 (b), and 280 nm (c). Solid lines indicate the best fits to eq 3.

the relative K11 values are much smaller than those among the reported K11 values for R-CD (K11R) (see Table 1). Such a difference in K11 should be mainly due to the differences in mechanisms for the formation of CD inclusion complexes.15 In view of the wide variety of guest types, which can form complexes with CDs, there are likely to be several different types of driving forces acting in various combinations. Inoue lists the main factors influencing complexation, such as van der Waals interactions between CD and guest, hydrophobic interactions, release of high energy water or ring strain energy upon binding, hydrogen bonding between guest and hydroxyl groups of the CD, and the effects of solvent surface tension.2d Davies and co-workers have proposed a model of the factors influencing the stability constants of the host-guest complexes of R-CD and 1,4-disubstituted benzenes.15 They found that the logarithm of the stability constant for R-CD shows a fairly good correlation with an expression including the Hammett constant (σ) and the molar refractivity (Rm) of the substituent located at the narrower end of the CD cavity, and specific substituent solvation assisted resonance.26 However, the same method failed when applied to β-CD complexation. Guo et al. calculated the K11 value for the inclusion complexation of R- and β-CDs with a number of 1,4-disubstituted benzenes by the model from Rm, hydrophobic constant (π), and σ, which respectively reflect the volume, hydrophobicity, and electronic property of the substituents in the guest molecules.16 They concluded that van der Waals force plays a dominant role in the R-CD complexation; on the other hand, van der Waals force and hydrophobic interactions play the major roles in the β-CD complexation. At the probable position proposed for the phenolic compounds in the β-CD cavity, there is strong steric interaction between the ortho- and meta-aromatic protons and H-3 and H-5, which are located inside the cavity from rings with radii of 4.1 and 3.5 Å for β-CD, respectively.13 Molecular models indicate the restriction of the motion of the included phenolic compound imposed by this steric interaction to be larger in the β-CD cavity than in that of R-CD, as also found by 13C spin-lattice relaxation times.27 Therefore, we can assume that the guests in the β-CD complexes are more deeply inserted into the cavity than is the case for R-CD (see images in Figure 2), although there are slight differences in the proposed positions of the respective phenolic compounds included in the β-CD cavity, which reflect the nature of the para substituents.

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TABLE 1: Molar Absorbance Coefficients (E) for Free S (E0), S/CD (E11), and S/CD2 (E12), and Binding Constants (K) for S/CD (K11) and S/CD2 (K12) Sa

,b M-1 cm-1

11,b M-1 cm-1

K11, M-1 (rel)

12,b M-1 cm-1

K12, M-1

K11R,c M-1 (rel)

K12R,c M-1

MTP MTT MTPM MTPA

9 400 9 100 11 600 12 000

6 800 7 100 9 500 10 500

360 ( 50 (2.0) 180 ( 30 (1.0) 200 ( 30 (1.1) 320 ( 40 (1.8)

10 700 11 000 13 000 12 600

150 ( 40 160 ( 50 130 ( 40 120 ( 40

d 41 ( 25 (1.0) 85 ( 11 (2.1) 520 ( 260 (13)

d 1000 ( 700 32 ( 9 90 ( 70

a

[S] ) 0.05 mM. b At λmax. c Reference 15b. d Not available.

TABLE 2: Oxidation Potentials (Eox) of Substrates (S) and the Molar Absorbance Coefficients of S•+ (E•+)

a

S

Eox, V vs NHE

•+,a M-1 cm-1 (λ, nm)

MTP MTT MTPM MTPA

1.42 1.57 1.59 1.67

4000 (530)b 6700 (530)c 5500 (545)d 5000 (550)b

At λmax. b This work. c Reference 21. d Reference 22.

Figure 2. Top (A) and side (B) views of the structure of the MTPM/ β-CD complex obtained by AM1 calculation, where a model for the inclusion complexation of β-CD with MTPM by taking the possibility of different inclusion orientation into consideration was used. The calculated K values are 240 and 60 M-1 for KX and KY, where X and Y are -SCH3 and -CH2OH groups, respectively. The structures of two isomeric complexes of β-CD with 1,4-disubstituted benzenes and the numerical treatment are given in ref 16. The images were drawn with the use of MOLEKEL software.28

3.2. Formation and Decay Processes of S•+. The pulse radiolysis of water solution leads to the production of eaq-, H•, OH•, and so on (eq 4). The eaq- and OH• can be respectively converted into OH• and Br• in N2O-saturated aqueous solutions containing NaBr (eqs 5 and 6). Homogeneous one-electron oxidation of S then occurs via a bimolecular reaction with Br2•(E1/2 ) 1.69 V vs NHE),8 which can oxidize all S as listed in Table. 2, by the following reactions:

H2O ' eaq-, H•, OH•, H+, H2, H2O2

(4)

eaq + N2O + H2O f N2 + OH- + OH•

(5)

OH• + Br- f OH- + Br•

(6)

Br• + Br- f Br2•-

(7)

Br2•- + S f 2Br- + S•+

(8)

In experiments containing HP-β-CD, OH• should react with the CDs. For example, the rate constant for the bimolecular reaction of OH• with β-CD has been reported to be 4.2 × 109 M-1 s-1,29 while the bimolecular reaction rates of OH• with the precursor Br- and Br• with Br- are 1.1 × 1010 and ∼1010 M-1 s-1, respectively.30,31 To minimize the reaction of OH• with HP-β-CD, in most of the experiments, we used HP-β-CD concentrations 10 or 20 times lower than that of NaBr. Under these experimental conditions, it was also observed that most of the starting S were included inside HP-β-CD.Figure 3 shows the transient absorption spectra during the pulse radiolysis of N2O-saturated water (pH 2) in the presence of MTPM (0.5 mM) and NaBr (100 mM) with an HP-β-CD concentration of 10 mM. Transient absorption spectra with two peaks at about 360 and 550 nm attributable to Br2•- and MTPM•+, respectively, were clearly observed.21 These spectra are the same as those obtained

Figure 3. (A) Transient absorption spectra obtained at 0 (black), 0.5 (red), 1 (green), and 2 µs (blue) after an electron pulse during the pulse radiolysis of N2O-saturated water (pH 2) in the presence of MTPM (0.5 mM) and NaBr (100 mM) with the HP-β-CD concentration of 10 mM. (B) Time profiles of ∆OD550 at HP-β-CD concentrations of 0 (gray), 5 (red), and 10 mM (green). The black lines represent nonlinear least squares curve fits based on eq 8. (C) Second-order plots for the decay kinetics of MTPM•+ at HP-β-CD concentrations of 0 (gray), 5 (red), and 10 mM (green).

in water without HP-β-CD at the same pH, indicating that almost the same MTPM•+ was produced. Similar experiments were performed with MTP, MTT, and MTPA. 3.3. Formation and Decay Kinetics of S•+. Here, we have focused our attention on the influence of HP-β-CD on the formation and decay kinetics of S•+. It was found that kox decreased with the increasing HP-β-CD concentration as shown in Figure 3B. The one-electron oxidation reaction rate constants (kox) were determined by the single-exponential fitting of the kinetic traces to the expression for a quasi-first-order reaction. The determined kox values are summarized in Table 3. A decrease in reactivity is expected when S is included inside HPβ-CD due to the shielding from Br2•-. Such effects have been observed in several instances in the prevention of excited states from reacting with quenchers.7 By using the absorption coefficient values of S•+ (•+), which are summarized in Table 2, the second-order rate constants (kd) for the decay kinetics of S•+ were calculated to be 109-1010

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TABLE 3: One-Electron Oxidation Reaction Rate Constants (kox) and Decay Rate Constants of S•+ kox, 109 M-1 cm-1 [HP-β-CD]/mM

kd, 109 M-1 cm-1 [HP-β-CD]/mM

S

0

5

10

0

5

10

MTP MTT MTPM MTPA

5.1 ( 0.5 3.5 ( 0.5 6.0 ( 0.4 4.6 ( 0.4

2.1 ( 0.3 2.9 ( 0.3 3.3 ( 0.4 2.9 ( 0.3

1.5 ( 0.2 2.3 ( 0.3 2.5 ( 0.3 2.1 ( 0.3

0.7 ( 0.2 4.1 ( 0.4 4.1 ( 0.4 7.3 ( 0.9

0.7 ( 0.2 3.9 ( 0.4 3.9 ( 0.4 5.6 ( 0.6

0.5 ( 0.1 3.8 ( 0.4 3.4 ( 0.3 4.7 ( 0.6

TABLE 4: Apparent Equilibrium Constants for One-Electron Oxidation (Koxapp) and Decay Processes (Kdapp) S

Koxapp, M-1

Kdapp, M-1

MTP MTT MTPM MTPA

250 ( 50 90 ( 20 160 ( 40 190 ( 40

32 ( 7 6(1 20 ( 4 55 ( 10

M-1 s-1, as summarized in Table 3. The measured values were found to be almost independent of the dose, that is, independent of the initial concentration of S•+, confirming the second-order nature of the process. The second-order decay of S•+ would lead to products,32 because first-order reactions should be observed if S•+ reacted with Br- or with the neutral S. Our results show that first-order processes do not occur in our systems or have only minor contributions. Interestingly, the kd values were not sensitive to the concentration of HP-β-CD, compared with the kox values. If one assumes that the complexation equilibrium (eq 1) is quickly reached, the apparent reaction rates for CD (kCD) are related to that in water without CD (kwater) by the following relation,11a

kwater/kCD ) 1 + Kapp[HP-β-CD]

(9)

where Kapp is the apparent equilibrium constant. Figure 4 shows the relationship between the kwater/kCD values for kox (solid symbols) and kd (open symbols) and the HP-β-CD concentration for MTP (black), MTT (red), MTPM (green), and MTPA (blue). From eq 9, we obtained the Kapp values for the one-electron oxidation (Koxapp) and decay processes (Kdapp). It should be noted that the limiting kinetic case will give the largest value of Kapp for a given decrease in the kCD values, as mentioned elsewhere.11a In a situation where the dissociation kinetics becomes involved in the reaction processes, a larger decrease in Kapp would be expected. As shown in the inset of Figure 4, relatively small Koxapp values were obtained compared to K11, suggesting that the one-electron oxidation pathways of free S in solution and S

Figure 4. HP-β-CD concentration dependences of kox (solid symbols) and kd (open symbols) observed during the pulse radiolysis of N2Osaturated water (pH 2) in the presence of NaBr (100 mM) containing MTP (black), MTT (red), MTPM (green), and MTPA (blue) (0.5 mM).

included inside CD by Br2•-, which are characterized by different rate constants, exist. On the other hand, considerably smaller Kdapp values were obtained compared with the K11 and Koxapp values. For example, we derived Koxapp and Kdapp values of 150 and 20 M-1 for MTPM, respectively, suggesting that the binding ability of MTPM•+ with HP-β-CD is much lower than that of MTPM because of the hydrophobic nature of the cavity. The difference in behavior observed for the decay of S•+ strongly suggests that S•+ is not included in the CD host and that the subsequent reaction occurs outside the cavity. From the good second-order fits found for the decay process of S•+ in the presence of HPβ-CD, we can expect that the dissociation or association reactions take place during several tens of microseconds. The complexation equilibrium between CDs and a series of electroactive molecules, such as ferrocenes,14,33 viologens,34 and cobaltocenes,35 has been investigated in detail and found to involve dissociation rate constants of the order of 104 s-1. Recently, Dimitrijevic and co-workers studied the photoinduced charge transfer between guest molecules and hybrid TiO2/ carboxyethyl-β-CD nanoparticles using low-temperature EPR and cyclic voltammetry.6 They used ferrocenemethanol (K11 ) 2.1 × 103 M-1) as the guest molecule and observed the formation of ferrocenium cation, which is generated by an electron transfer from ferrocenemethanol to the TiO2 nanoparticles, by UV irradiation. It was also mentioned that the dissociation of the charged ion from the hydrophobic cavity of carboxyethyl-β-CD into the bulk of the aqueous solution leads to efficient charge separation. To date, we have studied the TiO2 photocatalytic one-oxidation processes of aromatic compounds such as aromatic sulfides using laser flash photolysis and pulse radiolysis techniques.25 It is worthwhile to clarify the influences of CDs on the one-electron oxidation processes of aromatic compounds on the TiO2 surface and the subsequent dissociation of their radical cations from the TiO2/CD nanoparticles. Such studies are now underway. 3.4. Formation of the Dimer Radical Cation. The S•+ and •+ S2 can provide information on the dynamics in the time range of 0.1-100 µs.12b,36 Recently, our research group clarified the formation and decay processes of the radical cation (Py•+SA-) and dimer radical cation ((PySA-)2•+) of PySA- included in β- and γ-CDs during the 355-nm resonant two-photon ionization of PySA- using transient absorption measurements.12b To clarify the dissociation and subsequent reactions of S•+, we examined the influence of HP-β-CD and CM-γ-CD on the formation and decay processes of S2•+. Figure 5A shows the transient absorption spectra during the pulse radiolysis of N2Osaturated water in the presence of NaBr (100 mM) with MTPM concentrations of 1 (black), 2.5 (red), 5 (green), and 10 mM (blue). The spectral intensities are normalized at 580 nm for comparison. The absorption band around 550 nm was blueshifted by about 20 nm with the increasing MTPM concentration, while a new absorption band clearly appeared around 800 nm. Recently, Sawaki et al. investigated the one-electron oxidation processes of aromatic sulfides in acetonitrile by the laser flash

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Figure 5. (A) Transient absorption spectra obtained after an electron pulse during the pulse radiolysis of N2O-saturated water (pH 2) in the presence of NaBr (100 mM) with MTPM concentrations of 1 (black), 2.5 (red), 5 (green), and 10 mM (blue). (B) Transient absorption spectra obtained at 0.5 µs after an electron pulse during the pulse radiolysis of N2O-saturated water (pH 2) in the presence of MTPM (10 mM) and NaBr (100 mM) with HP-β-CD concentrations of 0 (black), 10 (red), and 25 mM (green).

photolysis method and concluded that S2•+ compounds absorbing at around 460-500 nm involve the σ-type complex of the sulfur-sulfur three-electron bond and that S2•+ compounds absorbing at around 800 nm involve the π-type complex associated with two phenylthio groups.24 As shown in Figure 5A, the absorption bands at around 520 and 800 nm observed for the MTPM system are similar to those for S2•+ of thioanisoles such as the p-chloro- and p-methylthioanisoles. Therefore, we conclude that the formation processes of σ- and π-types of S2•+ are involved in the reaction dynamics of MTPM•+ at high MTPM concentrations. It is noteworthy that this is a rare example of an S2•+ formation of an aromatic sulfide in water. Almost the same kd values were also obtained for MTPM at 520, 550, and 780 nm, which are mainly attributable to σ-type (MTPM)2•+, MTPM•+, and π-type (MTPM)2•+, respectively, suggesting that MTPM•+ is in rapid equilibrium with (MTPM)2•+ as indicated by eq 10,

MTPM•+ + MTPM a (MTPM)2•+

(10)

This result is consistent with that reported for S2•+ of other aromatic sulfides in acetonitrile.24 In the case of MTP, the transient absorption spectra attributed to (MTP)2•+ were not observed in the absence and presence of HP-β-CD, although MTP•+ was clearly observed. It is wellknown that the formation of S2•+ is very sensitive to the steric and electronic factors of the substituents.37,38 MTP has a strong electron-donating -OH group, and hence the delocalization of the positive charge over the aromatic rings is more effective than that in MTPM. Therefore, the formation of S2•+ is unfavorable and could not be clearly observed for MTP. Unfortunately, we cannot observe the transient absorption spectra of S2•+ for MTT and MTPA because of their poor solubility in acidic water.

Figure 6. (A) Time profiles of ∆OD obtained at 780 nm after an electron pulse during the pulse radiolysis of N2O-saturated water (pH 2) in the presence of MTPM (10 mM) and NaBr (100 mM) with HPβ-CD concentrations of 0 (black), 10 (red), 25 (green), and 50 mM (blue). (B) The HP-β-CD concentration dependences of kox (black circles) and the apparent (MTPM)2•+ formation rate (kdimerapp) (red triangles). The inset shows the relationships between the kwater/kCD values and the HP-β-CD concentration. The Kox11 and Kox12 indicate Koxapp for the MTPM•+/HP-β-CD and MTPM•+/HP-β-CD2 complexes, respectively, and Kdimer indicates the Kapp value for the formation process of (MTPM)2•+.

In the presence of HP-β-CD, a significant change in the spectral shape was observed as shown in Figure 5B. The absorption band around 520 nm was red-shifted with the increasing HP-β-CD concentration, while the absorption band around 800 nm significantly decreased. These results clearly indicate that the formation of (MTPM)2•+ is inhibited by the addition of HP-β-CD. It is interesting to discuss the influence of HP-β-CD on the formation kinetics of (MTPM)2•+. Figure 6A shows the time profiles of ∆OD obtained at 780 nm after an electron pulse during the pulse radiolysis of N2O-saturated water in the presence of MTPM (10 mM) and NaBr (100 mM) with the HPβ-CD concentrations of 0 (black), 10 (red), 25 (green), and 50 mM (blue). The apparent formation rates (kdimer) of (MTPM)2•+ clearly decreased with the increasing HP-β-CD concentration. We then estimated the Kapp values for the formation of MTPM•+ (Koxapp, which is already defined by eq 9) and (MTPM)2•+ (Kdimer) from the kox and kdimer values in the absence and presence of HP-β-CD. As shown in the inset of Figure 6B, the relationship between the kwater/kCD values for kox and the HP-β-CD concentration clearly suggests two distinguishable slopes of 160 and 370 M-1 at low and high HP-β-CD concentrations, respectively, while the kwater/kCD values for kdimer indicate a good linear correlation with the HP-β-CD concentration (Kdimer ) 110 M-1). Because it is expected that MTPM can form a 1:2 complex, that is, MTPM/β-CD2, at high HP-βCD concentration (>10 mM), the inhibition effect of HP-βCD on the one-electron oxidation process should be enhanced. One or two CD molecules can contain one or more entrapped “guest” molecules.1 The host:guest ratio of 1:1 is most frequently seen. However, 2:1, 1:2, 2:2, or even more complicated associations, and higher order equilibria exist, almost always simultaneously. For example, The K11 value of PySA- and γ-CD giving the 1:1-inclusion compound (PySA-/γ-CD) has been

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Figure 8. Reaction schemes for (A) the association and dissociation of the starting S with β-CD, (B) the one-electron oxidation of S in water and included inside β-CD by Br2•-, (C) the association and dissociation of S•+ with β-CD, where K11•+ is K11 for the complexation of S•+ with β-CD, and (D) the formation process of S2•+.

Figure 7. (A) Transient absorption spectra obtained 1 µs after an electron pulse during the pulse radiolysis of N2O-saturated water (pH 2) in the presence of MTPM (10 mM) and NaBr (100 mM) with CMγ-CD concentrations of 0 (black), 10 (red), and 25 mM (green). (B) Time profiles of ∆OD at 780 nm during the pulse radiolysis of N2Osaturated water (pH 2) in the presence of MTPM (10 mM) and NaBr (100 mM) with CM-γ-CD concentrations of 0 (black), 10 (red), and 25 mM (green). (C) Second-order plots for the decay kinetics of (MTPM)2•+ at CM-γ-CD concentrations of 0 (black), 5 (red), and 10 mM (green).

found to be 1 M-1.39 Those of PySA-/γ-CD and PySA- giving the 2:1-inclusion compound ((PySA-)2/γ-CD), and of (PySA-)2/ γ-CD and γ-CD giving the 2:2-inclusion compound ((PySA-)2/ γ-CD2) have been reported to be 106 and 2.0 × 104 M-1, respectively.39 It was found that (Py•+SA-)2/β-CD2 is generated from the dimerization of Py•+SA-/β-CD and PySA-/β-CD, while (Py•+SA-)2/(γ-CD)2 is directly generated from the resonant two-photon ionization of (PySA-)2 in (PySA-)2/γCD2.12b This is explained by the existence of PySA-/β-CD and (PySA-)2/γ-CD2 in the ground state. Here, we confirmed whether the generated (MTPM)2•+ is stabilized by the inclusion inside CM-γ-CD. Figure 7A shows the transient absorption spectra obtained at 1 µs after an electron pulse during the pulse radiolysis of N2Osaturated water in the presence of MTPM (10 mM) and NaBr (100 mM) with CM-γ-CD concentrations of 0 (black), 10 (red), and 25 mM (green). The absorption bands of σ- and π-types (MTPM)2•+ were clearly observed in the presence of CM-γCD. As shown in Figure 7B, both the formation and decay rates of (MTPM)2•+ slightly decreased with the increasing CM-γCD concentration. From the steady-state UV absorption measurements, both the K11 and K12 values for the complexation of MTPM with CM-γ-CD were determined to be 20 ( 10 M-1. Therefore, the dimerization to give (MTPM)2•+ mainly occurred in the bulk solution, and the formation and decay processes of (MTPM)2•+ were slightly inhibited by the addition of CM-γCD because of the shielding effects of CM-γ-CD. 3.5. Reaction Mechanisms. We summarize the reaction schemes as follows. As a first step, S associates with HP-β-CD as shown in reaction A of Figure 8. The inhibition of the oneelectron oxidation of S included inside HP-β-CD by Br2•occurred to give S•+ (reaction B). From the second-order fits found for the decay process of S•+ in the presence of HP-βCD, we can conclude that the dissociation or association reactions take place during several tens of microseconds,

indicating that the dissociation rate constant should be higher than 104-105 s-1 (reaction C). It was found that the binding ability of S•+ with HP-β-CD is much lower than that of S, because of the hydrophobic nature of the cavity. The desorbed S•+ from β-CD can react with the free S to give S2•+ (reaction D). 4. Conclusions We have investigated the one-electron oxidation reactions of aromatic sulfides (S) included inside HP-β-CD by Br2•- using the pulse radiolysis technique. Inhibition of the one-electron oxidation reaction of S with Br2•- was clearly observed in the presence of HP-β-CD. From a comparison between the determined K values, it was found that the binding ability of S•+ with HP-β-CD is much lower than that of S because of the hydrophobic nature of the cavity. The formation process of (MTPM)2•+, which is generated between MTPM•+ and neutral MTPM in solution, was also inhibited by the addition of HPβ-CD. It was found that the generated (MTPM)2•+ is not stabilized by the addition of HP-β- and CM-γ-CDs. Acknowledgment. The authors wish to thank the people in the Radiation Laboratory, The Institute of Scientific and Industrial Research, Osaka University, for running the pulse radiolysis. This work has been partly supported by a Grant-inAid for Scientific Research (Project 17105005, Priority Area (417), 21st Century COE Research, and others) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of the Japanese Government. References and Notes (1) For example: (a) Szejtli, J. Chem. ReV. 1998, 98, 1743. (b) ComprehensiVe Supermolecular Chemistry; Szejtli, J., Osa, T. Eds.; Pergamon: New York, 1996; Vol. 3. (2) (a) Breslow, R.; Dong, S. D. Chem. ReV. 1998, 98, 1997. (b) Hedges, A. R. Chem. ReV. 1998, 98, 2035. (c) Connors, K. A. Chem. ReV. 1997, 97, 1325. (d) Rekharsky, M. V.; Inoue, Y. Chem. ReV. 1998, 98, 1875. (3) (a) Davis, M. E.; Brewster, M. E. Nat. ReV. Drug DeliV. 2004, 3, 1023. (b) Uekama, K.; Hirayama, F.; Irie, T. Chem. ReV. 1998, 98, 2045. (4) Harada, A. Acc. Chem. Res. 2001, 34, 456. (5) (a) Willner, I.; Eichen, Y. J. Am. Chem. Soc. 1987, 109, 6862. (b) Willner, I.; Eichen, Y.; Frank, A. J. J. Am. Chem. Soc. 1989, 111, 1884. (6) (a) Dimitrijevic, N. M.; Saponjic, Z. V.; Bartels, D. M.; Thurnauer, M. C.; Tiede, D. M.; Rajh, T. J. Phys. Chem. B 2003, 107, 7368. (b) Dimitrijevic, N. M.; Rajh, T.; Saponjic, Z. V.; de la Garza, L. Tiede, D. M. J. Phys. Chem. B 2004, 108, 9105. (7) Evans, C. H.; De Feyter, S. D.; Viaene, L.; Van Stam, J.; De Schryver, F. C. J. Phys. Chem. 1996, 100, 2129 and references therein.

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