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We have constructed an experimental setup for Faraday rotation dispersion imaging and demonstrated the performance of a novel imaging principle. By us...
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Faraday Rotation Dispersion Microscopy Imaging of Diamagnetic and Chiral Liquids with Pulsed Magnetic Field Masayori Suwa,*,† Yusuke Nakano,† Satoshi Tsukahara,† and Hitoshi Watarai*,‡ †

Department of Chemistry, Graduate School of Science, Osaka University, 1-1 Machikaneyama, Toyonaka, Osaka 560-0043, Japan Institute for Nanoscience Design, Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka 560-8531, Japan

‡

S Supporting Information *

ABSTRACT: We have constructed an experimental setup for Faraday rotation dispersion imaging and demonstrated the performance of a novel imaging principle. By using a pulsed magnetic field and a polarized light synchronized to the magnetic field, quantitative Faraday rotation images of diamagnetic organic liquids in glass capillaries were observed. Nonaromatic hydrocarbons, benzene derivatives, and naphthalene derivatives were clearly distinguished by the Faraday rotation images due to the difference in Verdet constants. From the wavelength dispersion of the Faraday rotation images in the visible region, it was found that the resonance wavelength in the UV region, which was estimated based on the Faraday B-term, could be used as characteristic parameters for the imaging of the liquids. Furthermore, simultaneous acquisition of Faraday rotation image and natural optical rotation image was demonstrated for chiral organic liquids.

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In the case of weak magnetic sample, the FR angle, θF, is directly proportional to the external magnetic field, B, and the optical path length, l, as the following expression

INTRODUCTION lot of effort to invent a novel imaging technique has been paid, because direct observation of any events supplies strong evidence in every field of science. For example, a fluorescence microscope,1−5 which can visualize not only organelles but also small molecules and ions in a cell, is one of the most powerful tools in bioscience and biotechnology. Raman scattering imaging6−9 can observe the distribution of molecules in a sample, for example living cells, 10,11 pharmaceutical tablet,12 minerals,13 semiconductors,14 based on its characteristic vibrational frequency without labeling. Scanning probe microscope (SPM)15,16 has been utilized in many regions of science to observe nanometer sized structure. The image of property provided by SPM depends on the nature of a probe. For example, magnetization of specimen can be visualized by a scanning superconducting quantum interference device,17−19 and a scanning electrochemical microscope20−22 can spatially characterize surface electron transfer reactivity and mass transfer fluxes at interface. The Faraday rotation (FR) is an optical gyration phenomenon in a magnetized material.23 Since the FR is a magneto-optical effect using light as a probe, the minimum observation area has to be limited up to the dimension of its wavelength even using an optical microscope. In solid state physics and material science, the magneto-optical effects, such as the FR and Kerr effect, have been widely applied to observe the magnetic domain of ferromagnetic materials because those effects directly relate to the direction of magnetization. Therefore, several magneto-optical microscopic imaging techniques have been demonstrated so far.24−29

A

© 2013 American Chemical Society

θF = VBl

(1)

where V is the Verdet constant, which is an inherent property of the sample. Since the 1960s, the FR of weak magnetic compounds has been observed, and the origin of the Verdet constant has been discussed.30−36 We have also investigated the Verdet constants of paramagnetic rare earth ion solutions and diamagnetic organic liquids in a transparent spectral region, systematically. In the former case, the Faraday C-term plays a dominant role,37 and the magnetic moment and the electric configuration in 4f orbital of rare earth ion governed by the magnitude of the molar Verdet constant.38 In the latter case, the Faraday B-terms are larger than the other terms because these molecules have no magnetic moment, and most molecules have no degeneracy both in the ground state and in the excited states. We have observed the wavelength dispersion of the FR in organic liquids and have demonstrated that the resonance wavelength lying in the UV region can be determined from the FR dispersion curve observed in the transparent visible region.39 Furthermore, it has also been demonstrated that the natural optical rotatory (NOR) dispersion in chiral liquids can simultaneously be measured. Received: February 20, 2013 Accepted: April 13, 2013 Published: April 14, 2013 5176

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Figure 1. The schematic drawing of the experimental setup for acquiring FR image (a), the timing diagram of the pulsed magnetic field, the Xe flash lamp, and the exposure of CCD camera (b), and the device to keep the capillary cells in the solenoidal coil (c).

was established from the FR measurements under parallel and antiparallel magnetic fields.

The FR takes place even in weak magnetic materials, though the magnitude of rotation is much smaller than that in ferromagnetic materials and a longer optical path is required for precise measurement. Therefore, there have been some attempts to minimize the sample volume required for FR measurement. Fischer and his colleagues utilized the difference of beam deflection between left and right circular polarized light at the surface of FR medium.40 This approach is very unique because the consideration of an optical path length is principally not needed. As expected by eq 1, the FR in a weak magnetic sample increases with the strength of a magnetic field. A pulsed magnetic field is suitable for FR measurements41 because (a) a high magnetic field up to 15 T can be easily generated by a small homemade coil and a condenser bank, (b) the pulse duration with the order of ms is sufficiently long for the magneto-optical measurement, and (c) a difference between the light intensities in the absence and the presence of pulsed magnetic field is attained in each measurement. In this paper, first, we describe an improved experimental setup for acquiring a quantitative FR image by using a pulsed magnetic field. We had demonstrated the FR imaging previously,42 but a clear image could not be obtained because the exposure time of the charge coupled device (CCD) camera (33 ms) was much longer than the duration time of the pulsed magnetic field (0.7 ms). In order to overcome this problem, the timing of the irradiation of the incident light, the CCD exposure time, and the generation of the pulsed magnetic field were controlled to optimize the sensitivity of the FR image in the present study. Second, we obtained the characteristic resonance wavelength in the UV region from the measurement of FR dispersion in the nonabsorbing visible wavelength region of organic liquids, and then used it as a new parameter for the imaging. Finally, simultaneous acquisition of the FR image and the NOR image

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EXPERIMENTAL SECTION Faraday Rotation Imaging Microscope. Figure 1(a) illustrates the apparatus for the measurement of microscopic FR images. Pulsed magnetic field was provided by a homemade solenoidal coil (see the SI), the bore of which was 1 cm in diameter and 1 cm in length, and a capacitor bank (MFC-404, Magnet Force Co., Ltd.) having 4000 μF as capacitance. The strength of the magnetic field was controllable with charging voltage on the capacitor (shown in Figure S2). The duration of the pulsed magnetic field was 0.6 ms. A Xe flash lamp (L7684, Hamamatsu Photonics) was used as a light source. The incident light was collimated to a parallel light and was monochromated by a band-pass filter of 420 nm, 450 nm, 480 nm, or 510 nm in wavelength. The bandwidth of all filters was ±2 nm. The light passed through a calcite polarizer (extinction ratio of 5 × 10−6) and a sample cell. The transmitted light was collected with an objective of 4× magnification and passed through an analyzer (extinction ratio of 4 × 10−5), which was set to 45° clockwise with respect to the polarizer as viewed from the light source. The image of the sample was focused onto a CCD camera (Pixelfly qe, pco. Imaging). Figure 1 (b) shows the timesequence diagram of the pulsed magnetic field, the flash lamp, and the exposure of CCD camera. The flash irradiation of 3 μs in full width and the exposure time of 1 ms were synchronized with the pulsed magnetic field by using an oscilloscope (DPO4032, Tektronics) and a pulse generator (WF1974, NF Corporation). The delay time of the flash from the initial rise of the pulsed magnetic field was set to 200 μs so that the time of flushing corresponds to the peak of the magnetic field (see Figure S3). 5177

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Under the condition that the angle between a polarizer and an analyzer is 45°, the FR angle can be calculated by the following expression:43 ⎛ I π θF = − cos−1⎜⎜ B 4 ⎝ 2I0

⎞ ⎟⎟ ⎠

to modulate the polarized state of light by the large FR of 1methylnaphthalene.

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RESULTS AND DISCUSSION The Faraday Rotation Images of Diamagnetic Organic Liquids. Figure 2 shows the normal optical microphotograph

(2)

Here, IB and I0 are the intensities of the transmitted light under the magnetic field and no magnetic field, respectively. Five I0 images were taken before and after one magnetic pulse, respectively. These ten I0 images were averaged, and then the IB/I0 image was obtained. Fifty to 100 IB/I0 images were averaged for enough reduction of noise. The FR image was acquired by carrying out the operation of eq 2 for every element of 1392 × 1024 pixels of the CCD image. The minimum resolution of the observed FR angle was estimated as 1.8 mrad in the case of 100 times averaging, taking into account the standard deviation of FR angle (0.6 mrad). The magnetic field in coil was measured by the FR of a Pyrex glass plate of 1 mm thickness. For simultaneous acquisition of FR and NOR images, the measurement was done in the normal and inverted directions of the magnetic field. This was performed by switching the direction of the electric current to the coil. The temperature of the coil was monitored in order to avoid destruction, and the subsequent measurement was started after waiting the coil to be cooled, when the temperature was raised over 30 °C. The whole measurement system was controlled by a program built with LabVIEW and Vision developing module (National Instruments). All measurements were carried out in a thermostatted room at 25 ± 1.5 °C. Faraday Rotation Dispersion Measurement. To confirm the wavelength dependence of the FR images, separately from the imaging experiment mentioned above, FR dispersion, so-called magneto-optical rotatory dispersion (MORD), of organic liquids was measured by the nonmicroscopic system in the wavelength region from 390 to 650 nm. The details of the setup for the dispersion measurement were described in the previous paper.39 A glass cylindrical cuvette with the optical path length of 7 mm was used. A pulsed magnetic field was generated by the homemade coil mentioned above and a capacitor bank (MAG-2520-3A, Magnet Force Co., LTD) having 2000 μF capacitor. The magnetic field can also be controlled by adjusting the charging voltage on the capacitor. The FR dispersion was measured under the magnetic field of 2.5 T. Sample Preparation. All organic liquids, n-decane, ndodecane, o-xylene, 1-methylnaphthalene, 1,4-dimethylnaphthalene, R-(+)-limonene, L-(−)-limonene, R-(+)-1-phenylethanol, RS-(±)-1-phenylethanol, and S-(−)-1-phenylethanol were used as received. RS-(±)-limonene was prepared by mixing the same volume of R- and S-isomer. Figure 1(c) illustrates the sample cell and its holder. Square capillary, whose inner section was 200 μm × 200 μm (VitroCom, Inc.), was used as an optical cell for liquid samples. The sample was introduced by capillary action. In order to keep the cells horizontally in the bore of the coil, the capillary was adhered onto the outside surface of a glass cuvette with vaseline. The cuvette was made by assembling a glass tube, a glass plate, and a thin cover glass, whose thickness was 0.2 mm, as shown in Figure 1(c). The diameter and the length of the cuvette were 8 mm and 25 mm, respectively. In simultaneous measurement of FR and NOR, the glass cuvette was filled with 1-methylnaphthalene in order

Figure 2. The dependence of the FR angle on the magnetic field: 1,4dimethylnaphthalene and n-dodecane were included in the capillaries numbered (II) and (III), respectively. The capillaries (I) and (IV) were empty. The FR images acquired under 0.7 T (b), 1.2 T (c), and 1.7 T (d). The average values of the FR angle of 1,4dimethylnaphthalene (red) and n-dodecane (black) were plotted against the magnetic field (e). The dashed lines were obtained by the least-squares method. The image of the Verdet constant (f) was estimated from the FR image taken under 1.2 T, taking into account the path length of the capillary cell (200 μm). The scale bar in (a) indicates 200 μm. The color in (b), (c), and (d) shows the FR angle as scaled by the color bar below the images.

and the FR image of n-dodecane and 1,4-dimethylnaphthalene, which were included in capillaries of 200 μm × 200 μm inner section. Two empty capillaries were also set in order to compensate the FR in the capillary wall and the cover glass. The wavelength of the probe light was 420 nm. In the normal optical image shown in Figure 2(a), these two liquids were not able to be distinguished. However, in the FR image (Figure 2(b)−(d)), a clear contrast was observed, and the FR angle increased with the magnetic field. Even in a weaker magnetic field of 0.7 T, n-dodecane and 1,4-dimethylnaphthalene can be discriminated. The FR angle of the liquid, which was obtained 5178

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by subtracting that of empty capillary and by averaging over the whole region of the image of sample, was plotted against the magnetic field as shown in Figure 2(e). The linear correlation between the observed FR angle and the magnetic field confirmed the relationship expressed by eq 1. From the slopes in Figure 2(e), the Verdet constants of n-dodecane and 1,4dimethylnaphthalene were determined as 8.6 rad m−1 T−1 and 36.5 rad m−1 T−1, respectively. Figure 2(f) shows the image of Verdet constant estimated from the FR image taken under the magnetic field of 1.2 T (Figure 2(c)), considering the FR angle of glass and the optical path length of 200 μm. It was confirmed that the Verdet constant image did not depend on the magnetic field strength (see Figure S4). The Verdet constants of several liquids were also measured and summarized in Table 1. All Table 1. Determined Verdet Constants of Organic Liquids at 420 nm compounds n-decane n-dodecane toluene o-xylene mesitylene 1,4-dimethylnaphthalene 1-methylnaphthalene

Va (rad m−1 T−1) 7.8 ± 0.7 8.6 ± 0.7 17.3 ± 1.5 17.5 ± 1.0 15.2 ± 0.9 36.5 ± 1.9 35 ± 2

(8.04) (8.50) (17.9) (17.8) (14.9) (37.9) (34.7)

Figure 3. The dependence on the wavelength: n-dodecane (I), oxylene (II), and 1-methylnaphthalene (III) and empty cell (IV) were examined. The normal optical image (a), the FR images observed by the light of 420 nm (b), 480 nm (c), and 510 nm (d).

a

The values in the parentheses were measured by the apparatus for the MORD. The optical path length was 7 mm.

observed values were in good agreement with those measured in the previous study.39 Therefore, it was concluded that quantitative FR images could be obtained using a rotation angle and Verdet constant measured by the present microscopic FR system. The observed FR angle was equivalent over a sample region (Figure S5). This means that the magnetic field in the observed area was homogeneous, and the turbulence of the polarization state of light due to objective could be neglected. The lower limit volume of 1,4-dimethylnaphthalene for the FR measurement by the present system was estimated as ∼90 fL, taking into account the dimension of the pixel of image (1.5 μm × 1.5 μm), the maximum magnetic field of 2 T, the standard deviation of measurement (1 mrad), and the optical path length of 200 μm. It was confirmed that the Verdet constant image did not depend on the magnetic field strength (see Figure S3). The Verdet constants of several liquids were also measured and summarized in Table 1. All observed values were in good agreement with those measured in the previous study.39 Therefore, it was concluded that quantitative FR images could be obtained using a rotation angle and a Verdet constant measured by the present microscopic FR system. The observed FR angle was equivalent over a sample region (Figure S4). This means that the magnetic field in the observed area was homogeneous, and the turbulence of the polarization state of light due to objective could be neglected. The Wavelength Dispersion of the Faraday Rotation and the Resonance Wavelength Imaging Based on the Faraday B-Term. We observed the wavelength dispersion of the FR image by the present setup with the four wavelengths of 420 nm, 450 nm, 480 nm, and 510 nm. The examined samples were n-dodecane, o-xylene, and 1-methylnaphthalene, and the magnetic field was set to 1.2 T. The FR angle in every image increased with a decrease of the wavelength of light as shown in Figure 3. Figure 4(a) is the plots of the average FR angle

Figure 4. The wavelength dispersion of the FR angle of 1methylnaphthalene (black), o-xylene (red), and n-dodecane (blue) measured with the present setup (a): The curves are drawn from the results obtained in the previous study.39 The reciprocal FR angle vs λ2 (b): the regression lines, corresponding to eq 4, were estimated by the least-squares method.

against the wavelength. The solid lines indicate the calculated value from the Verdet constants measured in the previous study,39 taking into account the present experimental condition (l = 200 μm and B = 1.2 T). For every sample, the results were in good agreement with those measured with the nonmicroscopic apparatus for FR dispersion. In the previous study, it was revealed that the Faraday B-term took a dominant role in the FR of most organic liquids, and it was possible to determine the resonance wavelength from the ground state to the jth excited state, λjn, in the UV region from the FR dispersion curve in the visible wavelength region and the coefficient of the B-term, B(j ← n), which was attributed to the transition probability of FR including the mixing of the molecular orbital due to the magnetic field. The Faraday B-term far from the resonance wavelength can be expressed as37 5179

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θF = −K K=

λjn 2 − λ

Article

benzene (, mixes into the ground state, |n >, and into the other excited state, |j>, the B(j ← n) can be written as B(j ← n) ⎡ ⟨k|mz |n⟩ ∝ Im⎢∑ (⟨n|μx |j⟩⟨j|μy |k⟩ − ⟨n|μy |j⟩⟨j|μx |k⟩) ⎢⎣ k ≠ n Wkn +

∑ k≠j

⎤ ⟨j|mz |k⟩ (⟨n|μx |j⟩⟨k|μy |n⟩−⟨n|μy |j⟩⟨k|μx |n⟩)⎥ ⎥⎦ Wkj (4)

Here, Im denotes the imaginary part, mz is the magnetic dipole operator parallel to the magnetic field, μx and μy are the electric dipole operators perpendicular to the magnetic field, and Wkn is the energy difference between |k > and |n >. Equation 4 shows that B(j ← n) contains the magnetic and electric transition dipole moments not only from a ground state to an excited state but also from one excited state to another excited state. The first and second terms in eq 4 represent the contributions of mixing |k > into |n > and into |j>, respectively. Inversing eq 3 yields contributions of mixing |k > into |n > and into |j>, respectively. Inversing eq 3 yields θF−1 = −

1 1 λ2 + KB(j ← n) Kλjn 2B(j ← n)

(5) −1

Hence, from the slope and the intercept of θF vs λ plot, the resonance wavelength and KB(j ← n) can be determined. It is noteworthy that the λjn can be obtained without the information of the thickness and the concentration of sample from the FR dispersion. Figure 4(b) shows the results of this plot, and the slope and the intercept were acquired by the leastsquares method. The estimated values for λjn and B(j←n) were summarized in Table 2. In the case of the 1-methylnaphthalene and o-xylene, the resonance wavelengths were slightly smaller than those of La ← Ag transition of naphthalene (284 nm) and benzene groups (217 nm) and larger than that of Bb ← Ag transition of naphthalene (227 nm) and Ba, b ← Ag transition of 2

Figure 5. The resonance wavelength images of n-dodecane (I), oxylene (II), and 1-methylnaphthalene (III) and empty cell (IV) (a) and its histogram (b): the pixels, where the correlation coefficient, R2 (see Figure S6), was smaller than 0.95, set to zero (white). The scale bar indicates 200 μm.

Table 2. Estimated Resonance Wavelength and B(j ← n) Values of Organic Liquids

shows the resonance wavelength, λjn, image, which was obtained for the first time in the present study. The correlation coefficient, R2, for every pixel could also be calculated (see Figure S6), and the λin values were set to zero at the pixel where R2 was lower than 0.95 (white pixels in Figure 5(a)). Figure 5(b) shows the histogram of λjn values read from the image for each sample. For 1-methylnaphthalene and o-xylene, the images of the resonance UV wavelength gave clearly different intensities, suggesting that the λjn value could be used as new imaging parameters to identify the compound from the transition energy. Note that these images were obtained from

B(j ← n) (10−3 D2 β [cm−1]−1)b a

compounds n-dodecane o-xylene 1-methylnaphthalene

λjna (nm) 111 195 266

(105) (199) (264)

−5.09 −2.07 −2.01

(−7.60) (−1.97) (−1.96)

a

The values in the parentheses were the results obtained in the previous study. bTraditionally used unit of B(j ← n); D is debye (3.336 × 10−30 C m), β is Bohr magneton (9.274 × 10−24 J T−1), and cm−1 is used as a unit of energy (1 cm−1 = 1.985 × 10−23 J). 5180

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polarization. Larger FR is preferable for precise measurement of NOR. In the present experiment, the FR of the glass cuvette filled with 1-methylnaphthalene was used as a reference for the rotation of polarized light. Figure 6 shows the normal optical microphotograph, the [IB/ I0]p image, the FR image, and the NOR image of R-(+)-, S-

the FR measurements in the visible wavelength region. However, for n-dodecane, the image was very noisy. The FR angle of n-dodecane was very small, and the relative error of the measurement was 20% when the light of 510 nm in wavelength was used. Furthermore, the inverse of small θF caused a large error in the slope and the intercept as appeared in Figure 4(b). The error of the measurement should arise from noise in the CCD camera and the fluctuation of the light source. This problem would be solved by increasing the magnetic field and the accumulation time. In the present setup, it took much time to wait cooling of the coil. Any cooling devices would allow us to take more a precise λjn image. Alternatively, a shorter magnetic pulse, which is available by using a smaller capacitor, would satisfy this requirement because this suppresses Joule heating of the coil by current. Supposing the flash lamp was used, the pulsed magnetic field of 50 μs in width would be sufficient to acquire the FR image. Simultaneous Acquisition of the Faraday Rotation Image and the Natural Optical Rotation Image. A chiral liquid exhibits not only the FR but also the NOR under magnetic field. Hence, the observed FR angle is affected by NOR in such cases. However, both effects of FR and NOR should be distinguishable, since NOR is independent of the magnetic field. The ratio of the intensities of the detected light under the presence of magnetic field, IB, to that under the absence of magnetic field, I0, can be expressed as ⎡I ⎤ cos2(π /4 − α − θF) ⎢ B⎥ = ⎣ I0 ⎦ p cos2(π /4 − α)

(6)

where α is the angles of the natural optical rotation, and the subscript “p” was added to explicit that the direction of the magnetic field is the same with that of the light propagation (parallel mode). The obvious difference between the FR and the NOR is the dependency on the magnetic field. The direction of the FR inverts by applying the magnetic field of the opposite direction (antiparallel mode) is indicated by eq 1, while the natural optical rotation does not change. Hence, the measurements of IB/I0 under the parallel and antiparallel modes allow us to obtain θF and α simultaneously by using the following equations ⎧ ⎛ ⎪1 ⎡I ⎤ θF = cos ⎨ ⎜⎜ ⎢ B ⎥ + ⎪ 2 ⎝ ⎣ I0 ⎦ p ⎩ −1

⎞⎫ ⎡ I ⎤ ⎟⎪ B ⎢ ⎥ ⎟⎬ ⎣ I0 ⎦ap ⎪ ⎠⎭

⎧ ⎛ ⎪ 1 ⎜ ⎡ IB ⎤ π −1 α = − tan ⎨ ⎢ ⎥ − 4 ⎪ 2sin θF ⎜⎝ ⎣ I0 ⎦ p ⎩

Figure 6. Simultaneous acquisition of the FR and NOR images: the normal optical (a), the [IB/I0]p (b), the FR (c), and the NOR (d) images of an empty cell (I), R-(+)- (II), RS-(±)- (III), and S-(-)-1phenylethanol (IV). The scale bar in (a) shows 200 μm. The line profiles of the images of FR(e) and NOR(f) were acquired along the dashed lines in (c) and (d), respectively.

(7)

⎞⎫ ⎡ I ⎤ ⎟⎪ B ⎢ ⎥ ⎟⎬ ⎣ I0 ⎦ap ⎪ ⎠⎭

(−)-1-phenylethanol and their racemic mixture filled in the square capillaries. These isomers could not be discriminated from the normal optical image, but clear contrast was observed in the [IB/I0]p image (Figure 6(b)). However, it was not clear whether this difference ascribed to the FR or the NOR. The contributions of the FR and the NOR in IB/I0 were divided by carrying out the operation expressed in eqs 7 and 8 for each pixel, and the FR and the NOR images were obtained separately. In the FR image (Figure 6(c)), the intensity of the FR angle for each isomer was almost equal. Note that all of the FR images shown in Figure 6(c) included that of the cuvette filled with 1-methylnaphthalene. Meanwhile, the NOR images (Figure 6(d)) of the three different racemic 1phenylethanols were clearly distinguishable. It was confirmed that the FR signal of 1-methylnaphthalene depending on the magnetic field direction enhanced the NOR signal of the

(8)

which can be derived from eq 6 and the corresponding equation for the antiparallel mode. The separation of FR and NOR has been demonstrated by several groups.44,45 Since the reported methods by those groups were based on a lock-in technique under weak ac magnetic field, a sophisticated instrumentation was required to attain a very precise measurement of FR angles. On the other hand, the advantage of the use of the pulse magnet is that a strong magnetic field can be generated and a reliable FR image can be obtained easily by using CCD camera. Furthermore, dynamic measurements of magnetization are also possible.46 In the calculation of NOR expressed in eq 8, the FR is recognized to be the modulation of 5181

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superimposed samples of R-(+)- and S-(−)-1-phenylethanols. We also examined other chiral samples, R-(+)-, S-(−)-1limonene and their racemic mixture (see Figure S7). The average value of the Verdet constant and the NOR angle in respective liquids were summarized in Table 3. The observed

excitation states in UV region, its magnitude reflects the FR resonance wavelength, λjn, and the transition probability, B(j ← n). Therefore, one of the advantages of FR imaging is that the excited state lying in higher energy, which cannot be observed by nonresonant visible light, can be used for imaging. Taking into account the wavelength dispersion of Faraday B-term, the resonance wavelength imaging could be attained. Usually, the resonance wavelength is observed directly by using a vacuum UV spectroscopy, but there are severe requirements for optical components to be used for UV light and to avoid interference gases. The imaging of λjn will be useful to discriminate compounds due to the transition energy of π- and σ-electrons. Furthermore, the simultaneous acquisition of FR and NOR images was established in the present study. The present technique is suitable for optional equipment for optical microscope, because the FR image can be observed by simply attaching a coil generating pulsed magnetic field on a sample stage and a pair of polarizer and analyzer as well as a CCD camera. This novel magneto-optical microscope imaging method can visualize not only magnetization in ferromagnetic materials but also heterogeneous distribution of paramagnetic ions, aromatic molecules, and chiral molecules in diamagnetic materials. For example, the distribution of proteins or iron metabolite in a biological cell can be probably measured by this method without fluorescence labeling.

Table 3. Verdet Constants and the Normal Optical Rotation Angles of Chiral Liquids at 420 nm compounds R-(+)-limonene RS-(±)-limonene S-(−)-limonene R-(+)-1-phenylethanol RS-(±)-1-phenylethanol S-(−)-1-phenylethanol

Va (rad m−1 T−1) 7±3 8±3 12 ± 2 17 ± 2 18 ± 2 18 ± 3

(10.2) (10.5) (10.6) (16.6) (16.6) (17.1)

αb (mrad) −4.2 ± 1.7 2.0 ± 1.1 7.7 ± 1.0 −1.8 ± 1.3 2.2 ± 0.9 6.1 ± 1.9

(−7.67) (0.00) (7.58) (−3.36) (0.00) (3.34)

a

The values in the parentheses were measured by the apparatus for the MORD. The optical pass length was 7 mm. bSign of the NOR is opposite to that in common use in chemistry (ref 43). The values in the parentheses were estimated from the NOR angle measured with a spectropolarimeter, taking into account the optical path length.

Verdet constant values corresponded with those measured by the MORD apparatus, although the error was larger in the microscope system. On the other hand, the values of NOR measured by the microscope system were different between Rand S-isomers and that of racemic mixture was not zero. As seen in Figure 6(f) and Figure S7(f), which show the intensity profile on a crossing line of the NOR image, even in the empty capillary, the NOR of 1.8 mrad was observed. It is likely to ascribe this to misalignment of the polarizer and the analyzer or the slight difference of the magnetic field strength between parallel and antiparallel mode. However, the precision of the adjustment of the polarizer and analyzer was ±0.06 mrad, so it is unreasonable to attribute to the misalignment. To evaluate the difference of the magnetic field, the FR angles of the empty cell in parallel antiparallel mode were estimated by using eq 2, and the values were 424.1 ± 1.0 mrad and −425.4 ± 0.8 mrad, respectively. Therefore, the magnetic field strength was slightly different due to the contact resistance of the switch to change the polarity, but the error of the FR angle, which might be ascribed to the noise of the CCD camera and the fluctuation of the intensity of incident light as mentioned above, was comparable to this difference. To overcome this problem, more averaging time should be needed. Alternatively, the use of an achiral reference material would allow us to analyze the image, quantitatively. If the NOR angle of the cuvette with 1methylnaphthalene (1.8 mrad) was set as a base, the NOR angles of R- (−3.6 mrad), S-isomers (3.9 mrad) of 1phenylethanol, and their racemic mixture (0.4 mrad) came close to the results measured by spectropolarimeter shown in Table 3. Similar results were obtained in the case of limonene isomers (R: −6.0 mrad, S: 5.9 mrad, RS: 0.2 mrad). Thus, the present method is promising as a simple solution for acquiring quantitative a NOR image.

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ASSOCIATED CONTENT

S Supporting Information *

Description of the magnetic coil and supplementary figures. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (M.S.), watarai@chem. sci.osaka-u.ac.jp (H.W.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by a Grant for Basic Science Research Projects (No. 090811) from The Sumitomo Foundation and in part by a Grant-in-Aid for Young Scientists (B) (No. 21750076) and a Grant-in-Aid for Scientific Research (A) (No. 21245022) from the Japan Society for the Promotion of Science (JSPS).

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CONCLUSION We have demonstrated the promising potential of the novel imaging principle employing FR measurements. By using a pulsed magnetic field and a synchronized flash light, quantitative FR images due to θF values or Verdet constants were obtained. Since the FR of transparent materials in nonabsorbing visible wavelength region is attributed to 5182

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Analytical Chemistry

Article

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