Characterization of micelle: Magnetic field effects on photoexcited reactions of 2-methyl-1, 4-naphthoquinone

Colloids and Surfaces A: Physicochem. Eng. Aspects 287 (2006) 126–131

Characterization of micelle: Magnetic field effects on photoexcited reactions of 2-methyl-1, 4-naphthoquinone Yuhe Gao a , Jiafu Chen a,∗ , Yang Pan b , Xiujuan Zhuang b , Shuqin Yu b a

b

Hefei National Laboratory for Physical Sciences at Microscale, China Department of Chemical Physics, University of Science and Technology of China, China

Received 6 January 2006; received in revised form 12 February 2006; accepted 14 March 2006 Available online 11 May 2006

Abstract Magnetic field effects (MFEs) of the radical pairs on the reaction of the photoexcited 2-methyl-1, 4-naphthoquinone in two sodium alkyl sulfate micelles (SDS and SDeS) were investigated under magnetic fields below 1.7 T by a nanosecond laser flash photolysis technique. The result showed that MFEs on the decay of radical pairs and yields of escaped radicals in SDeS micellar solution were saturated at lower magnetic field than in SDS micellar solution. Such saturation could be qualitatively interpreted by relaxation mechanism. Peculiar MFEs on the second chemical and physical process were found for the first time in SDeS micellar solution. © 2006 Elsevier B.V. All rights reserved. Keywords: MFEs; MNQ; Radical pairs; Micellar solution; Relaxation mechanism

1. Introduction Magnetic filed effects (MFEs) on chemical reactions of radical pairs have been extensively studied for a long period [1–5]. Related studies can provide a possibility to control reaction rate or product selectivity by external magnetic field. This is a promising branch, which called “Dynamic Spin Chemistry”, encompassing chemistry, physics and biology [1]. Reactions taking place in micellar solution generally have larger MFEs than in homogeneous solution due to the micelles act as semi-reflective barrier (if correct) to the diffusive motion of the radical pairs. In the micelle cage, the spatial separation as well as the relative orientation of the spins of the two correlated radicals leads to evolving with time in an interdependent way. The yields of the cage products and escaping products, as well as the radical pairs decay rate, depended on the spin-evolution process of the radical pairs, and are also affected by the diffusion path and rate. MFEs of naphthoquinone derivatives in sodium dodecyl sulfate (SDS) micellar

∗

Corresponding author. Tel.: +86 551 3602807; fax: +86 551 3602803. E-mail address: [email protected] (J. Chen).

0927-7757/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2006.03.038

solutions have been deeply investigated [6–9]. Hayashi’ group [10–12] even studied MFEs of radical pairs under ultrahigh magnetic fields up to 30 T, and MFEs observed for the decay of radical pairs had been interpreted by relaxation mechanism. The effects of micelle size and microviscosity on addition of small dioxane were discussed by A. Misra [13]. Later, J. Chen investigated the H transfer to 2-methyl-1, 4-naphoquinone (MNQ) in SDS in the presence of radical, 4-(lauroyl-amino)TEMPO, and referred that the spin relaxation process was induced through the dipole–dipole interaction and spin exchange between one of the component radical and additional radical [14,15]. In the present paper, we reported the comparison studies of MFEs on decay of radical pairs and the escaped radical yields upon photoexcited reactions of MNQ in SDS and sodium decyl sulfate (SDeS) micellar solutions, and also discussed the effect of micellar microenvironment by the analysis of kinetic parameters in two micellar solutions. It is found that the MNQ is located at water–oil interface of micelle through studying the steady-state absorption spectra of MNQ in micellar solutions. In this case, the micelle barrier might not greatly influence the MFEs and this is different as it was regarded before. Furthermore, Peculiar MFEs observed in SDeS micellar solution were reported for the first time.

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2. Experimental

Table 1 Critical micelle concentrations (cmc) [16,17]

2.1. Materials MNQ (Kanto chemical Co.) was repeatedly recrystallized from benzene and heptane (1:1, v/v). SDS was purified by repeated recrystallization from ethanol and methanol mixture (1:1, v/v). SDeS (Acros) was used as received. Acetonitrile (MeCN), hexane and ethanol (EtOH) used as the solvents were spectroscopic grade. Triply distilled deionized water was used for preparation of solutions and carefully deoxygenated by bubbling with nitrogen for at least 30 min. All micellar solutions were made by sonication and stored under a nitrogen atmosphere during experiments. For two micellar solutions, [S] substitutes for the surfactant concentration of SDS, SDeS, of 8 × 10−2 , 7 × 10−2 M, respectively, to give a micelle concentration [M], about 1 ×10−3 M. The micelle concentrations of two surfactants are calculated by [M] =

127

[S] − cmc Nagg

where cmc denotes the critical micelle concentration and Nagg denotes the aggregation number. Values of these parameters for each micelle are given in Table 1. 2.2. Absorption spectra and fluorescence Absorption spectra were obtained at room temperature using a Shimadu UV-2401PC spectrophotometer controlled by a personal computer. As for the fluorescence measurements, a PerkinElmer LS55 instrument was used. Pyrene was used as a fluorescence probe to investigate the formation of hydrophobic microdomains by surfactants. The ratio I1 /I3 of the intensities of the first and third vibronic peak is a sensitive indicator for the polarity of microenvironment [19]. As shown in Table 1, the results indicate that the polarity of the microenvironment in SDS is smaller than that of in SDeS. Conversely, the microviscosity in SDS is larger than that in SDeS and much larger fluorescence for MNQ in SDS solution can further testify this. 2.3. Laser flash photolysis Laser flash photolysis experiments were performed at 298 K. The third harmonic (355 nm) of a Quanta-Ray GCR-103 Nd: YAG laser was used as the exciting light source. The transient

SDS SDeS

cmc (M)

Nagg

˚ R (A)

η (cP)

I1 /I3

8.6 × 10−3

71 35

∼15.4 ∼13.1

>30 28

1.05 1.12

3.4 × 10−2

Aggregation numbers (Nagg ) [17,18], micelle radii (R) [17,18], microviscosity (η) [19], I1 /I3 values (this work).

absorption was recorded by a Hewlett-Packard HP54522A digitizing oscilloscope (2 GHz) with a photomultiplier. Magnetic fields (B) of 0–1.7 T were provided by a Tokin SEE-10W electromagnet. Other experimental conditions are similar to those described elsewhere [6,20,21]. 3. Results 3.1. Photochemical reaction scheme of MNQ in micellar solutions The reaction processes of the photoreduction of naphthoquinone derivatives in micellar solution are well known [6,14]. In the case of MNQ in SDS/or SDeS micellar solution, the reaction pathways are shown in Scheme 1. 1 MNQ* and 3 MNQ* represent the singlet and triplet excited states of MNQ. RH indicates a micelle molecule and S, T (MNQH··R) are the singlet and triplet radical pairs. T± , T0 represent the specific sublevels of triplet under external magnetic field. The rate constants of hydrogen abstraction (kH ), relaxation between spin states (krlx ), escaping radicals in radical pairs (kesc ), S–T0 state conversion (ks-T0 ), and the recombination rate (krex ) of singlet radical pairs are shown near the corresponding arrows in Scheme 1. 3.2. Magnetic field effects on the A(t) curves in SDS and SDeS systems Time profiles of transient absorption (A(t)) were measured in SDS and SDeS micellar solutions at 380 nm under magnetic fields of 0–1.7 T. As shown in Figs. 1 and 2, the A(t) curves for two systems are composed of several decay components under magnetic fields. In order to eliminate the contribution to the decay of triplet and the growth of radical pair, we analyzed the A(t) curves at t > 200 ns in SDS System and t > 120 ns in SDeS System. The initial decaying part of each of the A(t) curves represents the absorption of both radical pairs and the escaped

Scheme 1.

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radicals, while the constant part t > 2 ␮s corresponds to only that of the escaped ones. According to relaxation mechanism theory [22], the decay curves of radical pairs in the absence and presence of a magnetic filed can be given as follows: B = 0;

A(t) = A0 exp(−k0 t) + C

B > B1/2 ;

(1)

A(t) = Af exp(−kf t) + As exp(−ks t) + C

(2)

Here, k0 is the decay rate constant of the radical pairs in zero field, kf and ks are the fast and slow ones in presence of magnetic fields, respectively. A0 , Af and As are the constants corresponding to the initial populations of sublevels T0 + T± , T0 , and T± , respectively. Then, the k0 , kf and ks values are given as follows:

Fig. 1. A(t) curves observed at 380 nm for MNQ/SDS system under various magnetic fields.

k0 =

krec + kesc 4

(3)

kf =

krec + ks 2

(4)

ks = krlx + kesc

Fig. 2. A(t) curves observed at 380 nm for MNQ/SDeS under various magnetic fields.

(5)

Eqs. (1) and (3) indicate that the decay of the radical pair is governed by the recombination process and the escape process, not by the S–T conversion. However, with increasing magnetic fields, the dynamic is governed by the conversion of the radical pairs between singlet and triplet states, which divided into three specific sublevels (T+ , T− and T0 ). Under different magnetic field, S–T0 mixing (ks-T0 ) is rapid whereas relaxation from T+ , T− (krlx ) is impeded by the energy separation due to the Zeeman interaction. The recombination process is reduced but the escape process is enhanced. The ks values for two systems are listed in Table 2. From Eqs. (1)–(5) and the method given by Igarshi [21], we had determined the values of krec , kesc in two systems (Here, it is found that krec and kesc for MNQ/SDS system are in good agreement with the reported ones, the rate constants kf could not be obtained precisely, due to the separation of 3 MNQ* and the fast radical pairs component is too difficult to get reliable values).

Table 2 Rate constants for the processes of radical pairs obtained for MNQ/SDS and MNQ/SDeS system at 298 K MNQ/SDeS

MNQ/SDS (this work)

MNQ/SDS (literature)

kesc krec /107 s−1

2.45 1.72

0.64 1.37

0.58a 1.4 ∼ 1.5a

Magnetic field (B) (T) 0 0.02 0.04 0.10 0.33

k0 or ks (krlx )/106 s−1 6.74 4.58 (2.13)f 3.95 (1.50)f 3.25 (0.80)f –d

4.1 2.81 (2.17)f 2.24 (1.60)f 1.42 (0.78)f 0.95 (0.31)f

4.1b –c –c 1.4b 0.9 ∼ 0.7e

/106 s−1

a b c d e f

[14]. [6]. ks = 1.9 × 106 s−1 was reported for B = 0.05 T [6]. ks could not be obtained precisely due to the effect of second chemical process under high magnetic filed. ks = 0.9 × 106 s−1 and ks = 0.7 × 106 s−1 were reported for B = 0.2 and 0.5 T, respectively. krlx , obtained as ks –kesc in two systems.

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radicals. R(B) =

Fig. 3. Absorption spectra of MNQ in various solvents, [MNQ]: 3 × 10−4 M.

A380 (7.0 ␮s, B) Yesc (B) = Yesc (0) A380 (7.0 ␮s, 0)

Here, A380 (7.0 ␮s, B) and A380 (7.0 ␮s, 0) are the absorbance values at 380 nm measured at 7.0 ␮s in the presence and absence of a magnetic field B, respectively. Fig. 4 shows that the R(B) increases with increasing B from 0 to 1.70 T for both systems. It is found that the magnitude of the MFEs on R(B) observed for MNQ/SDeS system is smaller than that observed for MNQ/SDS system, although the values of Yesc (0) with zero field and Yesc (B) in presence of magnetic fields for MNQ/SDeS system are higher than those for MNQ/SDS system. This feature can be understood as follows. The Yesc (0) and Yesc (B) can be expressed by Eqs. (7) and (8), respectively. kesc kesc = k0 (krec /4 + kesc )

 2 kesc 2 kesc Yesc (B) ≈ = 3 ks 3 (krlx + kesc )

Yesc (0) ≈ 3.3. Absorption spectra of MNQ in various solvents For the sake of better understanding MFEs in micellar solutions, it is provided here some other experimental results related to the microenvironment changes of MNQ for two micellar solutions. Fig. 3 presents the steady-state absorption spectra of MNQ in several solvents wilh different polarities. The absorption peaks of MNQ in SDS and SDeS micellar solutions are red shifted from the absorption peaks of MNQ in hexane or ethanol. This suggested that the MNQ in two micellar systems was in a water-richer condition, such as that at the water–oil interface of a micelle or in bulk water rather than inside the hydrophobic micelle. The maximum absorption peaks are similar and shifted from 339 nm in SDS and SDeS micelle solution to 340 nm in water and acetonitrile mixture (19:1, v/v). This bathochromic shift further indicates that MNQ is solubilized at water–oil interface of micelle. It is noticed that the width of the bands and the maximum intensities are considerably increased in SDS, SDeS and aqueous solutions. The larger bandwidth observed in water probably indicates a strong association of the solvent molecules with MNQ molecules in the ground state via formation of intermolecular hydrogen bonds and the energy level of ππ* is stabilized. Surely, there are more H2 O molecules penetrating into microenvironment where MNQ is located in SDeS than that in SDS due to the shorter length of the hydrocarbon chain and lower micelle aggregation [23].

(6)

(7) (8)

Therefore, in a certain extent, Fig. 4 is consistent with Fig. 5, which was obtained by using the data in Table 2. As clearly seen in these plots, the MFEs for MNQ/SDeS system are relatively easier to approach saturation (Fig. 5). This saturation of MFEs observed for two micellar solution can be explained by the spin relaxation due to the relaxation mechanism [24,25]. It suggested that the saturation originated from the fact that kesc of the system is larger than krlx . In the present case of MNQ/SDeS system, it is clearly seen that the escape process is much faster than S–T conversion process (kesc > krlx ). The triplet radical pair lifetimes at high filed are mainly determined by filed independent term, kesc , meanwhile krlx decreases with increasing magnetic field (as case a in [24]). However, spin conversion process competes with the escape process (kesc ≈ krlx , as case b in [24]) in MNQ/SDS system. As a result, MFEs depend on the filed dependent term, krlx . Thus, the MFEs on the decay of radical pairs and yields of escaped

4. Discussion 4.1. MFEs on the yield of the escaped radical and the rate values for two micelles MFEs were observed on the yields of the escaped radicals (Yesc ). Judging from A(t) curves shown in Figs. 1 and 2, it was supposed the absorbance at 380 nm at t > 7.0 ␮s was mainly due to the escaped MNQ−• and MNQH• [6]. R(B) values were used to represent the magnitude of MFEs on the yields of the escaped

Fig. 4. Magnetic field dependence of the escaped radical yields, R(B) obtained for MNQ/SDS and SDeS systems.

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Fig. 5. Magnetic field dependent of the ks (B)/k0 for two systems.

radicals in SDeS micellar solution should be saturated at lower magnetic field than in SDS micellar solution. The absolute escape rate of radicals (kesc ) is mainly dominated by the escape of ketyl radicals (MNQH• and MNQ−• ) through the interface of the micelle and into bulk water. There is a very significant decrease for the escape rate from SDeS to SDS. So, described as above, the escape process of MNQ ketyl radicals is possibly controlled by the microviscosity and the location of MNQ in micelles. On the other hand, the ratio of kesc for two systems is lager than that expected. It is assumed that part of kesc is dominated by the escape of the micelle derived radical in SDeS micelle due to the low aggregation number [26]. However, the distinction of krlx values is small due to uniform anisotropic g values and hyperfine coupling constants for two systems. The recombination rate krec is subject to affect by the rate of the S–T0 mixing and the re-encounter frequency. There is no difference on krlx for two systems, which implies that recombination is mainly determined by the re-encounter frequency. MFEs have been expected to increase on reduction of the micellar size or the increase of microviscosity, because of an increase in frequency of the repeated collision [27]. The present results indicate that the micelle barrier might not greatly influence the MFEs as it was regarded before. The fact that MNQ solubilized at the micelle surface implies that the MNQ ketyl radicals are easily diffusing out the micelles and recombination processes are significant as well as escape process. Therefore, the most important factor for the MFEs in two systems is the diffusion paths wilh different microviscosity. 4.2. Peculiar MFEs for the second reaction process in MNQ/SDeS As it is found, the yields of the escaped radicals in MNQ/SDS are changing appreciably with time delaying under different magnetic fields, and we attributed that the formation of MNQ−• comes from MNQH• (Ref. reaction 9). However, as clearly seen in Fig. 2, there is obvious enhanced absorption after laser pulse 1.0 ␮s in the absence of magnetic field. Then, the absorption at

Fig. 6. Magnetic field effects for MNQ/SDeS after different delayed time.

2.4 ␮s after laser pulse is increasing with increasing magnetic field. There are obvious MFEs but different from MFEs for the final yields of escaped radicals (Fig. 6). Reaction (9) should have no magnetic field effects due to no S–T0 spin relaxation conversion existing, although the yield of MNQ−• may increase due to the high value of pH (10.23) in SDeS micelle solution. The observed MFEs at 2.4 ␮s should be dependent on the separation of radical pairs. Considering that the micelle is alkaline, it is reasonably believed that the re-encounter of the escaped radicals and reverse hydrogen abstraction from MNQH• to SDeS derived radical should play an important role (reaction 10). pH>4.6

MNQH• −→ MNQ−• + H + MNQH• + R• separation

re–encounter

−→

(9)

[MNQ−• · · ·H+ · · ·R• ]

−→ MNQ−• + R• H+

(10)

This peculiar nature of SDeS and radicals derived should be attributed to varying extents of the water penetration into the head group regions of SDeS micelle. Also, the re-encounter frequency increases in this microenvironment with low microviscosity. 5. Conclusion We had studied and compared magnetic field effects on the reaction of photoexcited triplet of MNQ in two sodium alkyl sulfate micelles. The singlet state recombination rate and escaped rate in MNQ/SDeS system were found to be larger than that in MNQ/SDS system, and the spin relaxation rates were shown to be similar as expected with ketyl and homothetic alkyl interaction. MFEs on yield of two systems can be explained by relaxation mechanism. It is also observed that peculiar MFEs on the second process in MNQ/SDeS system. In some extent, peculiar MFEs come from the characterization of SDeS micelle itself, e.g. its high pH value, low microviscosity, the escaped radicals easily re-encountering and so on. It is expected that an extra-

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