Magnetic Field Effects on Photochemical Reactions in Ionic Liquids

ARTICLE pubs.acs.org/JPCC

Magnetic Field Effects on Photochemical Reactions in Ionic Liquids with Short Alkyl Chains Tomoaki Yago,† Atom Hamasaki,†,§ Miyuki Tanaka,† Tadashi Takamasu,‡ and Masanobu Wakasa*,† †

Department of Chemistry, Graduate School of Science and Engineering, Saitama University, 255 Shimo-ohkubo, Sakura-ku, Saitama, 338-8570, Japan ‡ National Institute for Material Science (NIMS), 3-13 Sakura, Tsukuba, 305-003, Japan ABSTRACT: Magnetic field effects (MFEs) on the photoinduced hydrogen abstraction reactions of benzophenone and thiobenzophenone from thiophenol were investigated in several ionic liquids with short alkyl chains by a nanosecond laser flash photolysis technique. In each ionic liquid, escaped radical yields of benzophenone and thiobenzophenone ketyl radicals gradually decreased with increasing magnetic field strength. The observed results were analyzed by using the stochastic Liouville equation (SLE), employing a solvent separated radical pair (SSRP) model where the SSRP with a specific radical
radical distance is stable and has a long lifetime. The SLE analysis revealed that the lifetime of the SSRP and the radical rotation in the SSRP states are strongly correlated with the macroviscosity of the ionic liquids.

’ INTRODUCTION Ionic liquids (ILs) are currently a promising new class of solvent in green chemistry, electrochemistry, and nanochemistry because of their exceptional combination of properties: nonvolatility, noncorrosiveness, nonflammability, stability to air and moisture, and designability provide new environments for chemical reactions.1
7 Although a number of studies on their structures and functions have been reported,8
11 the mechanistic insight into chemical reactions in ILs is still unclear because of the complex solvent structures in ILs. In particular, it has been reported that the ILs have nanoscale ordering structures as a result of the strong columbic interactions and the aggregation of the nonpolar parts of ionic molecules.9
11 One of the biggest controversies regarding the nanoscale ordering structures in ILs is the solute molecule diffusion, which is important to understand the mechanisms of the chemical reactions in liquid phase. Several time-resolved optical studies suggested that the microviscosities in the vicinity of the solute molecules are completely different from the macrovicosities of the ILs.12
15 Recently, we have applied a strategy of the magnetic field effect probe (MFE probe) to study the molecule diffusion in ILs.16
19 The magnetic field interacts with the electron spin of radical pairs (RPs) generated by the photochemical reactions, and thus the spin conversion between singlet and triplet RPs is influenced by the magnetic fields. The lifetime of the RPs and the yield of escaped radical show appreciable magnetic field effects (MFEs).20
22 Thus, we demonstrated that one can probe the microenvironment around the specific solute molecules by observing MFEs on photochemical reactions through radical pairs (RPs).16
19,23 In an IL of N,N,N,trimethyl-N-propylammonium bis(trifluoromethanesulfonyl)amide r 2011 American Chemical Society

(TMPA TFSA), we have observed the large MFEs for the photoinduced hydrogen abstraction reaction of benzophenone (BP) with thiophenol (PhSH) in the range of 0 T < B e 28 T.16,17 The observed MFEs have the following characteristics: (1) The escape yield of benzophenone ketyl radical (BPH•) generated from the hydrogen abstraction rapidly decreased with increasing magnetic field strength (B) in the range of 0 T < B e 2 T. (2) At much higher fields (2 T < B e 28 T), the yield gradually deceased, resulting in a 25% decrease at 28 T. We have preliminarily analyzed the observed MFEs using the stochastic Liouville equation (SLE) with the cage18 and the solvent separated radical pair (SSRP)19 models. The observed MFEs were explained by the transverse spin relaxations in the RPs caused by the large anisotropy of the g-value and the slow rotation of the phenylthiyl radical (PhS•). The calculated MFE was dependent on a rotational correlation time of the radical as well as the mutual diffusion coefficients for the translational diffusion of the radicals. The SLE analysis revealed that the IL has at least two different viscosity regions associated with the solvation dynamics of the IL. We proposed that such inhomogeneity causes the nanoscale cage effects on the translational diffusion of the RPs. Our MFE probe study on IL is, however, limited with BP in an IL of TMPA TFSA. Here, we extended the study by using several ILs having short alkyl chains: 1-ethyl-3-methyl-imidazolium tetrafluoroborate (Emim BF4), N-methyl-N-propyl pyrrolidinium bis(trifluoromethanesulfonyl) amide (P13 TFSA), and N-methyl-N-propylpiperidinium bis(trifluoromethanesulfonyl) amide (PP13 TFSA) and TMPA TFSA. Received: August 15, 2011 Revised: September 13, 2011 Published: September 20, 2011 21063

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The Journal of Physical Chemistry C Chart 1. Structures and Abbreviations of Ionic Liquids Used in the Present Work

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Table 1. Viscosities (η) of Ionic Liquids Used in the Present Study and Magnetic Field Effects on the Yield of the Escaped Radical Observed at 1.6 T (R(1.6 T)) ionic liquids

temperature/K

η/cPa

R (1.6 T)

296

45

0.84

343

13

0.89

296

56

0.81

308

35

0.85

343 296

16 73

0.89 0.81

Emim BF4 P13 TFSA

TMPA TFSA PP13 TFSA

a

Moreover, a new probe reaction of thiobenzophenone and PhSH was also studied in TMPA TFSA. In this study, the clear MFEs originated from the long-lived SSRPs were observed in each IL. The lifetime and rotational correlation time in the SSRP states are strongly correlated with the macroviscosity of the ILs. These results suggest that the coulomb interactions between cation and anion molecules of ILs provide not only the high viscosities but also rigid solvation structure in the vicinity of the radicals causing the nanoscale cage effects on photochemical reactions.

’ EXPERIMENTAL SECTION Materials. Thiobenzophenone (TBP) was synthesized as described in the literature.24,25 Purity of synthesized TBP which was determined by gas chromatography (SHIMADZU GC-18A) was >95%. Benzophenone (BP) was recrystallized twice from methanol. Thiophenol (PhSH) was purified by vacuum distillation. 1-Ethyl-3-methyl-imidazolium tetrafluoroborate (Emim BF4), N-methyl-N-propyl- pyrrolidinium bis(trifluoromethanesulfonyl) amide (P13 TFSA), N,N,N-trimethyl-N-propylammonium bis(trifluoromethanesulfonyl) amide (TMPA TFSA), and N-methyl-N-propylpiperidinium bis(trifluoromethanesulfonyl) amide (PP13 TFSA) were used as received. All chemicals except for TBP were purchased from Kanto Chemical Co., Inc. (Cica). The concentrations of BP, TBP, and PhSH in employed ILs were 2.0  10
2, 2.0  10
3, and 1.2
2.0  10
1 mol dm
3, respectively. The kinetic viscosity and density of each IL were measured by a viscometer (Yamauchi VM-10A-L) and a density meter (Anton Paar DMA 5000), respectively. The viscosities (η) obtained from the kinetic viscosity and density are summarized in Table 1. Nanosecond Laser Flash Photolysis. Laser flash photolysis experiments were carried out with an apparatus that was essentially the same as the apparatus described elsewhere.26,27 The third harmonic (355 nm) of a Nd:YAG laser (Quanta-Ray INDI) with a pulse width of 7 ns was used as an exciting light source. The probe light from a Xe short arc lamp (Perkin-Elmer Optoelectronics Lx-300 or Ushio UXL-500D) with a custom-built pulse current generator was divided into two beams, creating a doublebeam probe system. One beam was guided to a quartz sample cell by a quartz optical fiber (3M FT1.0-UMT) and passed through the cell. The other was detected directly. Both beams were guided by

343

19

0.88

288

190

0.76

296

137

0.77

303

100

0.79

323

50

0.82

Obtained from the kinetic viscosities and densities.

optical fibers to photomultipliers (Hamamatsu R636-10) through a monochromator (Oriel MS257 and Shimadzu SPG-120S, respectively). This double-beam probe system was constructed to accurately observe transient absorption by maintaining a flat baseline signal. In the present study, the baseline contained somewhat large noise contributions, which were generated from Q-switching of the laser and the pulsed trigger of the Xe lamp. Signal voltage from the photomultiplier was terminated by a 50 Ω resistor and was recorded by a digitizing oscilloscope (LeCroy Wave Pro 960, 2 GHz). A personal computer was used to control the apparatus and record data. Magnetic fields of up to 1.6 T were provided by an electromagnet (Tokin SEE-10W). The pulsed magnet having a roomtemperature bore with a diameter of 20 mm and a length of 160 mm was used to provide the magnetic field up to 28 T. Pulsed magnetic fields were generated by supplying intense pulsed currents from a 10 mF capacitor bank of 125 kJ at 5 kV. The maximum field was 32.2 T at 3.5 kV. Applied magnetic field was measured with a gauss meter (Lake-Shore model 421, for the electromagnet) and a search coil (for the pulsed magnet) placed right next to the quartz cell. Argonbubbled solutions in quartz cell were placed at the center of the electromagnet or pulsed magnet. Laser flash photolysis measurements were carried out at 288
343 K.

’ RESULTS AND DISCUSSION Reaction Scheme. The following reactions occur by the photoexcitation of BP in the presence of PhSH:27
29

BP þ hν ð355nmÞ f 1 BP f 3 BP 3

BP þ PhSH f 3 ðBPH••SPhÞ

3

ðBPH••SPhÞ T 1 ðBPH••SPhÞ

1

ðBPH••SPhÞ f recombination products

1;3

ð1Þ ð2Þ

B

ð3Þ

krec

ð4Þ

kesc

ðBPH••SPhÞ f escaped radicals

ð5Þ

Here, 1BP* and 3BP* represent the singlet and triplet excited states of BP, respectively. BPH• and •SPh represent the benzophenone ketyl and phenylthiyl radicals, respectively. 1(BPH• •SPh) and 21064

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Figure 1. A(t) curves observed at 380 nm in the absence and presence of a magnetic field of 1.6 T in (a) Emim BF4 (η = 45 cP), (b) P13 TFSA (η = 56 cP), (c) TMPA TFSA (η = 73 cP), and (d) PP13 TFSA (η = 137 cP) at 296 K.

(BPH• •SPh) denote singlet and triplet RPs, respectively, that are composed of BPH• and •SPh. Upon laser excitation of BP, 3 BP* is immediately generated via the fast intersystem crossing of 1 BP* (eq 1). The triplet RP of BPH• and •SPh is formed by a hydrogen abstraction reaction of 3BP* with PhSH (eq 2). In this photochemical reaction system, the MFEs originated from the magnetic field (B) dependent transverse spin relaxation in RPs caused by the large anisotropy of the g-value and the slow rotation of phenylthiyl radical (PhS•) in the IL.18,19 The singlet close RPs recombine to form the recombination products with a rate constant krec (eq 4), whereas radicals escape from the singlet and triplet RPs with a rate constant kesc (eq 5). In the presence of spin
orbit coupling (SOC) induced by a heavy atom, triplet close RPs can also recombine with an associated rate constant of kSOC, 3

3

kSOC

ðBPH••SPhÞ f recombination products

ð6Þ

In this reaction system, the MFEs are quenched by the recombination reactions from the triplet RPs (eq 6) in the highly viscous solvents where the lifetimes of the close RPs and 1/kSOC are comparable, leading to the loss of the spin selectivity in the recombination reactions from RPs.23 Similar hydrogen abstraction reaction and MFEs are believed to occur with thiobenzophenone (TBP) and PhSH in ILs.30,31 Magnetic Field Effects on the Reaction of BP and PhSH up to 1.6 T. The MFEs on the yield of escaped BPH• were measured in each IL under magnetic fields of 0
1.6 T. Typically, Figure 1 shows time profiles of the transient absorption, A(t), observed at

380 nm in the absence and presence of a magnetic field of 1.6 T in (a) Emim BF4 (η = 45 cP), (b) P13 TFSA (η = 56 cP), (c) TMPA TFSA (η = 73 cP), and (d) PP13 TFSA (η = 137 cP) at 296 K. Each A(t) curve has two decay components. The first component (0 < t < 0.3 μs) is ascribable to the decay of the T
T absorption (3BP*), and the second component (0.3 μs e t) is ascribable to the decay of RPs and escaped BPH•.16 As shown in Figure 1a
d, the second decay component was clearly affected by the presence of the magnetic field. As reported previously, these observed MFEs are caused by the B-dependent transverse spin relaxations.18,19 In each IL, MFEs were generated within 500 ns after laser excitation of BP. After the delay time of 500 ns, the magnitude of the MFEs remained nearly constant as the RPs and the escaped BPH• decayed. These results indicate that MFEs were generated in geminate RPs, but the free RPs did not induce the MFEs observed in the present photochemical reaction systems. Since the generation of MFEs occurs within 500 ns and the MFEs are caused by the geminate RPs, the A (0.75 μs) value is proportional to the escaped radical yield for a given magnetic field (Y(B)). Thus, the relative yield, R(B) = Y(B)/Y(0 T) = A (0.75 μs, B T)/A (0.75 μs, 0 T), gives the MFE on the yield of the escaped BPH•. Figure 2 shows the magnetic field dependence of R(B) observed at 296 K in (a) Emim BF4, (b) P13 TFSA, (c) TMPA TFSA, and (d) PP13 TFSA. In each IL, the R(B) value decreased with increasing B at 0 T < B e 1 T and the decrease was almost saturated at 1 T < B e 1.6 T. The R(B) value observed at the almost saturated field of 1.6 T is 21065

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Figure 2. Magnetic field dependence of R(B) observed at 380 nm for escaped BPH• in (a) Emim BF4 (η = 45 cP), (b) P13 TFSA (η = 56 cP), (c) TMPA TFSA (η = 73 cP), and (d) PP13 TFSA (η = 137 cP) at 296 K. Red line shows simulated curves obtained from the SLE analysis.

Figure 3. Relationship between R(1.6 T) (= Y(1.6 T)/Y(0 T) = A (0.75 μs, 1.6 T)/A (0.75 μs, 0 T)) observed at 380 nm for escaped BPH•, and solvent viscosity (η). Red line shows simulated curves obtained from the SLE analysis. Bottom axis shows η for the experimental R(1.6 T) values while the top axis shows the ηSSRP values used in the simulations.

dependent on η of the IL. In Figure 3, the R(1.6 T) values observed in the various viscosities of ILs are plotted against η. The R(1.6 T) values decreased smoothly with increasing η in the range of η e 100 cP, and then exhibited the saturation in the range of 100 cP < η < 200 cP. These results indicate that the magnitudes of the MFEs observed are well correlated with the macroviscosity of η in the present ILs. In the previous our paper, MFEs on the same reaction were observed in alcoholic solutions of varying viscosities (η = 0.55
59.2 cP).17 For comparison, the R(1.6 T) values are also plotted against η in Figure 3. In the case of alcoholic solutions, R(1.6 T) decreased with increasing η in the range of 0.55 cP e η e 5 cP, and then increased with increasing η in the range of 5 cP < η e 55.3 cP. When η was higher than 55.3 cP, the R(1.6 T) value became 1.0 and the MFEs were completely quenched. Such quenching of MFE can be explained by the SOC recombination of triplet close RPs associated with high viscosity. From the η-dependence observed in alcoholic solutions, the microviscosity in the ILs is expected to be much smaller than macroviscosity. Moreover, the magnitudes of the MFEs observed in ILs were much larger than those in alcoholic solutions and depended on macroviscosity of the ILs. These results can be explained by the cage effects from the nanoscale heterogeneous structure of the 21066

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The MFEs on the yield of escaped radicals (Rcalc) are then calculated as

Scheme 1

Rcalc ðBÞ ¼

Ycalc ðB TÞ Ycalc ð0 TÞ

ð11Þ

The spin Hamiltonian used here is composed of the Zeeman interactions for the radicals, the hyperfine interactions between electron and nuclear spins with a hyperfine coupling constant (A), and the r-dependent exchange interaction (J): ILs. To clarify this point, theoretical analyses were carried out using the stochastic Liouville equation (SLE). Analysis of the MFEs with the Stochastic Liouville Equation. In the present study, we performed the SLE analysis with the solvent separated radical pair (SSRP) model where the SSRP with a specific radical
radical distance (R) are stable and have the longer lifetime.19 Scheme 1 represents the possible solvation scheme after the bimolecular photochemical reaction. First the photochemical reaction occurs at the contact pairs. After the photochemical reaction, the anion and cation molecules of ILs around the radicals are reoriented, responding to the change of the charge distribution on the solute molecules. Because the solvent structures of ILs have the ordered structure with the range of 1
2 nm around the solute molecules, the most stable RP is an SSRP with R = 1
2 nm. To account for such a stable SSRP, we employed two different viscosities for the RP diffusions in the SLE analysis. One is the viscosity (ηSSRP) for the SSRP diffusion with R = 1
2 nm and the other is the viscosity (ηRP) for the other RP. High ηSSRP values indicate that the two radicals are rigidly solvated and SSRP is stable and has a long lifetime. The SLE includes the effects of spin
spin interactions, Zeeman interactions, molecular diffusion, recombination reactions at the contact RP, and spin relaxations as follows32
35 ∂Fðr, tÞ ¼
iHðrÞ Fðr, tÞ þ DΓr Fðr, tÞ þ Kr Fðr, tÞ þ Rr Fðr, tÞ ∂t

ð7Þ

In this equation, H(r) is the commutator associated with the spin Hamiltonian H(r) at distance r. F(r, t) is density matrix of the RP at time t and distance r. D and Γr are the diffusion coefficient and the diffusion operator, respectively. Kr and Rr are the superoperators for the recombination reactions and the spin relaxations, respectively. The Laplace transform of eq 7 is ^ðr, sÞ þ D s^ Fðr, sÞ
Fðr, t ¼ 0Þ ¼
iHðrÞ F

∂2 ^ðr, sÞ F ∂r 2

þ K^Fðr, sÞ þ R^ Fðr, sÞ

ð8Þ

HðrÞ ¼ μB p
1 Bðga Saz þ gb Sbz Þ þ ASa Ia
JðrÞ

^ðr, sÞ  F

0

e

rFðr, tÞ dt 

Z ∞
st 0

e

r^ Fðr, tÞ dt

Ycalc ¼ lim sTr½ sf0

Z ∞ d

r^ Fðr, sÞ dr

ð10Þ



Here, S and I represent the electron and the nuclear spin operators, respectively. Subscripts a and b denote the individual radicals. Here radical a represents benzophenone ketyl radical (BPH•) or thiobenzophenone ketyl radical (TBPH•) and radical b is phenylthiyl radical (PhS•). ga and gb are the isotropic g-factor for radical a and b, respectively. In the present reaction, the MFEs due to the hyperfine coupling mechanism have been hardly observed.16,17 One magnetic nucleus in radical a was included. The J gives the energy gap between the S and T states and is exponentially decayed with radical
radical distance (r) as follows J ¼ J0 exp½βðr
dÞ

ð13Þ

where J0 is a magnitude of the exchange interaction (J) at the closest distance d and β is an exponential falloff parameter. In the contact RP, the large J value inhibits the S-T conversion. In the separate RPs, on the other hand, the S and T states are nearly degenerate, permitting the S-T conversions by the spin interactions and the relaxations in RPs. The diffusion of the radicals was assumed to proceed by a simple Brownian motion and was treated by the finite difference technique with mutual diffusion coefficient D. The Kr matrix includes recombination reactions from the singlet RP with a rate constant krec and that from the triplet RP with a rate constant kSOC at the closest distance d.23 In confined systems where the lifetimes of RPs are comparable with the spin relaxation times, the spin relaxations play an important role on the spin conversion in RPs.21,22 The rate constants for the spin relaxations depend on the magnitude of the fluctuating local magnetic field and the frequency for the fluctuation represented with the rotational correlation time (τ). In the Rr matrix, spin relaxations by the anisotropy (δA) of the hyperfine interaction, the anisotropies (δg) of the g-factors, the dipole
dipole interactions and the spin rotational interactions were taken into account. The longitudinal relaxation rate constant (1/T1) is represented as22,36 1 3ðga 0 : ga 0 ÞμB 2 B2 þ ðA0 : A0 Þ τa ¼  T1 1 þ ω2 τ a 2 30p2

ð9Þ

The magnitudes of the MFEs were nearly constant after a delay time of 0.5 μs in the transient absorption measurements.16,17 The experimental data used for the SLE analysis were obtained at a delay time of 0.75 μs. Therefore, only the values calculated with the limiting condition of t f ∞ were used in the SLE. The yield of escaped radicals (Ycalc) is given by

1 þ 2Sa Sb 2

ð12Þ

where Z ∞
st



þ

3ðgb 0 : gb 0 ÞμB 2 B2 τb 1  þ TSR 1 þ ω2 τb 2 30p2

ð14Þ

where (g0 :g0 ) and (A0 :A0 ) are the magnitudes of the anisotropies for the g-factor and the hyperfine coupling constant, respectively. ω is the Larmor frequency for the unpaired electron spin and is dependent on B; ω = gμBp
1B. τa and τb are rotational correlation time for radical a and b, respectively. TSR is the relaxation time due to the spin-rotational interaction. In the 21067

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presence of the large J value in the closed RP, the transverse relaxation rate constant (1/T2) for the S-T0 relaxation is represented as, for 2J(r) > {[3(ga0 :ga0 ) μB2B2 + (A0 :A0 )]/30p2}1/2, 1 3ðga 0 : ga 0 ÞμB 2 B2 þ ðA0 : A0 Þ ¼ T2 30p2 

τa 3ðgb 0 : gb 0 ÞμB 2 B2 2 2 þ 30p2 1 þ ½2JðrÞ τa



τb 1 2 2 þ T 1 þ ½2JðrÞ τb SR

ð15Þ

When the S and T0 states are nearly degenerated, on the other hand, 1/T2 is represented as, for 2J(r) < {[3(ga0 :ga0 ) μB2B2 + (A0 :A0 )]/30p2}1/2, 1 1 1 ¼ 0 þ T2 T2 2T1

ð16aÞ

1 3ðga 0 : ga 0 ÞμB 2 B2 þ ðA0 : A0 Þ ¼  4τa T2 0 30p2 þ

3ðgb 0 : gb 0 ÞμB 2 B2 1  4τb þ 2 TSR 30p

ð16bÞ

For the T-T relaxation, the following longitudinal relaxation rate constant due to the dipole interactions is added:22 1 T1

DDI

¼

μB 4 ga 2 gb 2 3τab  10p2 r 6 1 þ ðω þ ωDDI Þ2 τab 2

3 μ 2 ga gb ωDDI ¼ pffiffiffi  B 3 pr 2 5 τab ¼

4πηr 3 3kB T

ð17aÞ

ð17bÞ ð17cÞ

ωDDI corresponds to the averaged energy splitting due to the dipole
dipole interactions for the spin-sublevel states in the triplet state.37 τab is the rotational correlation time of the vector directing radical a to radical b. Equation 17 cannot account for the spin relaxation rate at the magnetic field lower than 0.01 T.38 We have estimated the spin relaxation rate at the zero-field with the comparison of the data obtained at the magnetic field of up to 30 T. The estimated spin relaxation rate at the zero-field was on the order of 107 s
1.26 At the magnetic field lower than 0.01 T, we used a spin relaxation rate of 1  107 s
1 for the spin relaxation due to the dipole
dipole interactions. In the SLE analysis, we assumed that the RP was initially populated with the three triplet state (T+, T0, T
) equally at the closest distance d. In the model, we assumed that the Stokes
Einstein and the Stokes
Einstein
Debye relationships are always held for the translational diffusion and the rotational motion of solute radicals, respectively. The diffusion coefficients for radical a and b (Da and Db, D = Da + Db), τa and τb are represented with the viscosity (η) as follows Da ðηÞ ¼ τa ¼

kB T kB T , Db ðηÞ ¼ 6πηda 6πηdb

4πηda 3 4πηdb 3 , τb ¼ 3kB T 3kB T

ð18Þ ð19Þ

where da and db are the radius of each radical, respectively. Nishiyama et al. reported that the diffusion of radicals in ILs can be described by the Stokes
Einstein relationships with the stick boundary condition from the transient grating measurement.39 For the radius of solute molecules, we used the value of da = 0.4 nm for BPH• and db = 0.2 nm for PhS•. We assumed that, once the radicals escape from the SSRP, the RP does not form again in the calculations. The most of the parameters needed for the SLE analysis including J0 and β have been fixed by fitting the MFEs data obtained for the same photochemical reaction system in the various alcoholic solvents.23 These parameters are listed in Table 3. The fitting parameters here are associated with the SSRP model: R, ηRP, and ηSSRP. The simulated results for the MFEs under the magnetic field up to 1.6 T are depicted in Figure 2 by red lines with corresponding experimental data. The observed MFEs are mainly explained by the transverse spin relaxation caused by the large δg for •SPh. As was described in the previous paper, the long-lived SSRPs with R = ∼2 nm are generated in the ILs due to the nano-ordered solvent structure created by the solute
solvent and the solvent
solvent interactions.19 The SLE analysis indicates that the η dependence of the MFE observed in the present ILs is related with the parameters of ηSSRP rather than ηRP and R. As can be seen in Figure 2, the MFEs observed in the several ILs are well reproduced by the SLE by varying the value of ηSSRP. There are possibilities that the parameters of R depend on η or ILs. In the SLE analysis, we can fit the experimental data with a wide range of R; 1.4 nm < R < 2.4 nm. In the present study, therefore, we cannot discuss the η dependence of R in detail. In the SSRP model, ηRP is related with the solvation dynamics for the RPs. The SLE analysis indicated that the ηRP does not depend on the η or ILs. This fact implies that the solvation dynamics of the ILs are not largely dependent on η. The η dependence of R (1.6 T) depicted in Figure 3 was also reproduced by the SLE analysis with the assumption that the ηSSRP value is linearly correlated with the macroviscosity of η. The lifetime (tSSRP) of the SSRPs are estimated to be a few nanoseconds from the relation of tSSRP = Δr2/D. When the coulomb interactions between cation and anion molecules become strong, giving the high viscosity, solute
solvent interactions also get stronger. As a result, the high η value causes the stronger solvation, resulting in a long lifetime of the SSRP. Time resolved fluorescence study reported by Iwata et al. shows that the solute molecule forms the π
π aromatic complexes in the aromatic ILs but not in the nonaromatic ILs.40 The results suggested that there are specific interactions between solute molecules and the aromatic ILs, which may affect the radical diffusion in ILs. In the present study, the observed MFEs are simply correlated with the macroviscosity of ILs, and we could not find the specific interactions between the radicals and Emim BF4 where the cation has the aromatic ring. Thus, in the present study, the π
π interactions between the solute molecules and Emim are not strong enough to change the radical diffusion in Emim BF4 where the coulomb interactions play an important role. Magnetic Field Effects up to 28 T. Figure 4 shows the magnetic field dependence of R(B) observed in TMPA TFSA (η = 73 cP) and PP13 TFSA (η = 137 cP) at 296 K under magnetic fields of 0
28 T. In both ILs, the gradual decreases of R(B) with the increase of B were observed in the magnetic field range of 2 T < B e 28 T together with rapid decreases of R(B) with the increase of B in the range of B < 2 T. The MFEs observed 21068

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in 2 T < B e 28 T are due to the B-dependent longitudinal spin relaxation between T(1-T0 and T(1-S states assisted by the large anisotropy of g-factor for •SPh. The observations of the MFEs due to the longitudinal spin relaxation also support the large cage effect of the ILs since the MFEs due to the longitudinal spin relaxation have been limited to a confined system such as micellar solutions. The R(B) values observed at 2 T < B e 28 T in PP13 TFSA are larger than those observed in TMPA TFSA at 296 K, indicating that the cage effects in PP13 TFSA are much larger than that in TMPA TFSA at 296 K. In TMPA TFSA and PP13 TFSA, the MFEs on R(B) was simulated under the magnetic field up to 28 T and the results are depicted in Figure 4. The observed MFEs were well reproduced by the SLE analyses with the parameters listed in Tables 2 and 3. As was discussed in the previous section, the η-dependence of the MFE observed in the present ILs is explained by the variation of the ηSSRP value and the higher η value gives the higher ηSSRP value, resulting in the larger cage effect in the present ILs. MFEs on the Reaction of TBP and PhSH. In TMPA TFSA, laser flash photolysis of TBP (2.0  10
3 mol dm
3) was performed in the presence of PhSH (2.0  10
1 mol dm
3). The transient absorption spectra observed at delay times of 0.1, 0.3, 0.5, and 1.0 μs after laser excitation are typically shown in Figure 5. The spectra were essentially same as the spectra reported in the previous study in alcohols.30,31 Transient peaks observed at 480 and 510 nm can safely be assigned to the triplet
triplet absorption of TBP (3TBP*).41 The decay of the T
T absorption accelerated with increasing concentration of PhSH. The PhSH concentration dependence of the 3TBP* decay rate constant (k) observed at 510 nm is shown in Figure 6. For

the comparison, the dependence observed for the reaction of BP is also plotted in Figure 6. In both reactions, good linear relationships were observed between k and PhSH concentration. From the slope of the plots, the rate constant for the reaction of 3 TBP* and 3BP* with PhSH in TMPA TFSA was determined to be 1.4  107 and 2.7  108 s
1 mol
1 dm3, respectively. The rate constant for the reaction between 3TBP* is 20 times smaller than that observed for the reaction of 3BP*. Such a small rate constant of 3TBP* may be due to the lower triplet energy of 3TBP* (166 kJ mol
1) than that of benzophenone (290 kJ mol
1). From our previous paper,30,31,42 the transient absorption observed at 380 and 450 nm can be assigned to thiobenzophenone ketyl radical (TBPH•) and •SPh, respectively. From these results, the reaction scheme of TBP and PhSH is described as follows: TBP þ hν ð355nmÞ f 1 TBP f 3 TBP 3

ð20Þ

TBP þ PhSH f 3 ðTBPH••SPhÞ

ð21Þ

Here, 1TBP* and 3TBP* represent the singlet and triplet excited states of TBP, respectively. 3(TBPH• •SPh) denotes triplet RPs of TBPH• and •SPh. The MFEs on the yield of escaped TBPH• were measured at 380 nm in TMPA TFSA under magnetic fields of 0
1.6 T. The obtained R(B) values are plotted against B in Figure 7. The R(B) Table 3. Magnetic and Kinetic Parameters for Radical Pairs Used for the SLE Analysis used values parameters for calculationa J0, rad s
1
1


1  1012

20

20

0.4

0.4

0.2

0.2

5  1010 4  109

5  1010 9  109

A, mT


0.4


0.4

δA, mT

0.2

0.2

ga

2.003d

2.005e

gb

2.0082f

2.0082f

d

0.006e

δga

0.002

δgb

f

0.02f

0.02

2  10

rate for the spin rotational relaxation, s
1 e


1  1012

β, nm

krec, s
1 kSOC, s
1

a

TBPH• •SPhc

da, nm db, nm

Figure 4. Magnetic field dependence of R(B) observed at 380 nm for escaped BPH• in TMPA TFSA (O) and PP13 TFSA (b) at 296 K. Blue and red lines show simulated curves in TMPA TFSA and PP13 TFSA, respectively, obtained from the SLE analysis.

BPH• •SPhb

6

6  106

Abbreviations are described in ref 23. b Ref 19. c Ref 31. d For BPH•. For TBPH•. f For •SPh.

Table 2. Parameters for Radical Pair Diffusion Used in the SLE Analysis hydrogen acceptors BP

TBP a

ionic liquids

T/K

η/cPa

R/nm

ηRP/cP

ηSSRP/cP

tSSRP/nsb

Emim BF4

296

45

1.9 ( 0.5

3

4

1800

P13 TFSA

56

4

2600

4

TMPA TFSA

73

4c

2800c

4

3 6

5000 1700

8 3

PP13 TFSA TMPA TFSA

296

137 73

1.9 ( 0.5

Obtained from the kinetic viscosities and densities. b Calculated from Δr2/DSSRP. c Ref 19. 21069

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The Journal of Physical Chemistry C

Figure 5. Transient absorption spectra observed for the reaction of TBP (2.0  10
3 mol cm
3) and PhSH (2.0  10
1 mol cm
3) in TMPA TFSA solution at delay times of 0.1 (b), 0.3 (O), 0.5 (2), and 1.0 μs (4) after laser irradiation.

Figure 6. PhSH concentration dependence of the decay rate constant (k) observed for 3TBP* at 510 nm (O) and 3BP* (b) at 525 nm in TMPA TFSA.

value decreased with increasing B at 0 T < B e 1 T and the decrease was almost saturated at 1 T < B e 1.6 T. Finally, R(1.6 T) became 0.91. Thus, the magnitude of the MFEs observed for the reaction of TBP with PhSH is much smaller than that for the reaction of BP (R(1.6 T) = 0.81). In the reaction of TBP with PhSH, TBPH• and •SPh are generated and two heavy sulfur atoms induce the strong SOC. This strong SOC causes the following spin dynamics in the RPs: (1) acceleration of spin relaxation by increasing anisotropy of the g-tensor; (2) acceleration of spin relaxation by enhancing spin rotational coupling; and (3) mediation of the recombination of triplet RPs.43
45 Since these three effects have been known to quench the MFEs on RPs containing heavy atoms,43 smaller MFEs observed for the present reaction of TBP with PhSH can be explained by the strong SOC. To clarify this point, theoretical analyses were carried out using the SLE. The parameters used in the SLE analysis for the reaction of TBP with PhSH are also listed in Tables 2 and 3. The simulated results for the MFEs under the magnetic fields up to 1.6 T are depicted in Figure 7. The experimental results were well reproduced by the SLE

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Figure 7. Magnetic field dependence of R(B) observed at 380 nm for escaped TBPH• in TMPA TFSA at 296 K. Red line shows simulated curves obtained from the SLE analysis.

analysis. As shown in Table 3, the rate constants of SOC induced recombination (kSOC) and the spin rotational relaxation used for the SLE analysis of TBPH• •SPh are much larger than that of BPH• •SPh. Thus, it is concluded that the strong SOC by two heavy sulfur atoms causes quenching of the MFE in the present photochemical reaction. Moreover, as shown in Table 2, the radical diffusion parameters used for TBPH• •SPh were somewhat different from that used for BPH• •SPh. The ηSSRP value for TBPH• •SPh was smaller than that for BPH• •SPh, while the ηRP values for TBPH• •SPh was slightly larger than that for BPH• •SPh. Since the electronegativity of sulfur atom is smaller than that of oxygen atom, the dipole moment of TBPH• may be smaller than that of BPH•. The solute molecule with the small dipole moment may be weakly solvated in ILs with short alkyl chains causing the decrease of ηSSRP. In our SSRP model, the ηRP value is associated with the solvation dynamics after the birth of RP by the photochemical reaction.19 The higher ηRP value used for TBPH• •SPh suggests that the time needed for the solvation of TBPH• is longer than that for BPH•. This is consistent with the fact that ηSSRP for TBPH• •SPh is smaller than that for BPH• •SPh because of the weaker solvation of TBPH• than BPH•. These results indicate that the stability of the nanoscale solvation structures in ILs is dependent on both ILs and solute molecules, causing the variation of the cage effects for RP diffusions. In the present SLE analysis, the ratio ηSSRP/ηRP indicates the magnitude of the cage effect.35 As shown in Table 2, the cage effect of TMPA TFSA for the reaction of TBP with PhSH may be 2.5 times smaller than that for BP with PhSH. In addition, the cage effect of PP13 TFSA is roughly 3 times larger than those of Emim BF4, P13 TFSA, and TMPA TFSA.

’ CONCLUSION MFEs on the photoinduced hydrogen abstract reactions of benzophenone and thiobenzophenone with thiophenol in the ILs having the short alkyl chains were investigated by the nanosecond laser flash photolysis. The results were analyzed by the stochastic Liouville equation (SLE) with the solvent separated radical pair (SSRP) model. The observed MFEs were explained by the transverse spin relaxation caused by the long lifetime and the slow radical rotation in the SSRP states with 21070

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The Journal of Physical Chemistry C radical
radical distance of about 2 nm. The SLE analysis revealed that the lifetime of the SSRP and the radical rotation in the SSRP states are strongly correlated with the macroviscosity of the ILs. We suggest that the increases in the macroviscosity of the present ILs cause the stronger solvation for radicals, resulting in the longer lifetime of the SSRP and the slow radical rotation in the SSRP state. These results indicate that the stability of the nanoscale solvation structures in ILs is dependent on both ILs and solute molecules, causing the variation of the cage effects for RP diffusions.

’ AUTHOR INFORMATION Corresponding Author

*Tel: +81-48-858-3909. Fax: +81-48-858-3909. E-mail: mwakasa@ chem.saitama-u.ac.jp. Present Addresses §

Department of Chemistry, Faculty of Science, Shinshu University, 3-1-1 Asahi, Matsumoto, Nagano 390-8621, Japan.

’ ACKNOWLEDGMENT This work was partially supported by a Grant-in-Aid for Scientific Research (Nos. 20081005 and 20031002) in Priority Area “Science of Ionic Liquids” (Area No. 452) and “High Field Spin Science in 100 T” (No. 451) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan, and by a Grant-in-Aid for Scientific Research (B) (No. 22350003) from Japan Society for the Promotion of Science. ’ REFERENCES (1) Seddon, K. R. J. Chem. Technol. Biotechnol. 1997, 68, 351–356. (2) Welton, T. Chem. Rev. 1999, 99, 2071–2083. (3) Wasserscheid, P.; Kein, W. Angew. Chem., Int. Ed. 2000, 39, 3772–3789. (4) Sheldon, R. Chem. Commun. 2001, 2399–2407. (5) Dupont, J.; Souza, R. F.; Saurez, P. A. Z. Chem. Rev. 2002, 102, 3667–3691. (6) Ohno, H. Electrochemical Aspects of Ionic Liquids; John Wiley & Sons, Inc.: Hobeken, NJ, 2005. (7) Antonieei, M.; Kuang, D.; Smarsly, B.; Zhou, Y. Angew. Chem., Int. Ed. 2004, 43, 4988–4992. (8) Iwata, K.; Okajima, H.; Saha, S.; Hamaguchi, H. Acc. Chem. Res. 2007, 40, 1174–1181. (9) Wang, Y.; Jiang, W.; Yan, T.; Voth, G. A. Acc. Chem. Res. 2007, 40, 1193–1199. (10) Hardacre, C.; Holbrey, J. D.; Nieuwenhuyzen, M.; Youngs, T. G. A. Acc. Chem. Res. 2007, 40, 1146–1155. (11) Padua, A. A.; Gomes, M. F. C.; Lopes, J. N. A. C. Acc. Chem. Res. 2007, 40, 1087–1096. (12) McLean, A. J.; Muldoon, M. J.; Gordon, C. M.; Dunkin, I. R. Chem. Commun. 2002, 1880–1881. (13) Neta, P.; Skrzypczak, A. J. Phys. Chem. A 2003, 107, 7800–7803. (14) Paul, A.; Samanta, A. J. Phys. Chem. B 2007, 111, 1957–1962. (15) Takahashi, K.; Tezuka, H.; Kitamura, S.; Satoh, T.; Katoh, R. Phys. Chem. Chem. Phys. 2010, 12, 1963–1970. (16) Wakasa, M. J. Phys. Chem. B 2007, 111, 9434–9436. (17) Hamasaki, A.; Yago, T.; Takamasu, T.; Kido, G.; Wakasa, M. J. Phys. Chem. B 2008, 112, 3375–3379. (18) Wakasa, M.; Yago, T.; Hamasaki, A. J. Phys. Chem. B 2009, 113, 10559–10561. (19) Yago, T.; Wakasa, M. J. Phys. Chem. C 2011, 115, 2673–2678. (20) Steiner, U. E.; Ulrich, T. Chem. Rev. 1989, 89, 51–147. (21) Nagakura, S; Hayashi, H.; Azumi, T. Dynamic Spin Chemistry; Kodansha-Wiley: Tokyo, NY, 1998.

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