Letter pubs.acs.org/JPCL
Chirality Discriminated by Heterodyne-Detected Vibrational Sum Frequency Generation Masanari Okuno and Taka-aki Ishibashi* Department of Chemistry, Graduate School of Pure and Applied Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8571 Japan S Supporting Information *
ABSTRACT: We first demonstrated chiral vibrational sum frequency generation (VSFG) in the heterodyne detection, which enables us to uniquely determine chiral second-order nonlinear susceptibility consisting of phase and amplitude and distinguish molecular chirality with high sensitivity. Liquid limonene was measured to evaluate the heterodyne-detected chiral VSFG developed in this study. R-(+)- and S-(−)-limonene showed clearly opposite signs in the complex spectra of the second-order nonlinear susceptibility in the CH stretching region. This is the first report of the chiral distinction by VSFG without any a priori knowledge about chiral and achiral spectral response. Furthermore, from the phase of the chiral VSFG field measured in the heterodyne detection, the origin of the chiral signal was ascribed to the bulk limonene. The heterodyne detection also improves detection limits significantly, allowing us to observe weak chiral signals in reflection. The heterodyne-detected chiral VSFG can provide information on absolute molecular configuration. SECTION: Spectroscopy, Photochemistry, and Excited States molecules give the opposite signs of χ(2) chiral with the same amplitude. However, chiral VSFG spectra have been so far observed in the homodyne detection scheme, in which the (2) 2 absolute square of χ(2) chiral (|χchiral| ) is observed, meaning that the information on the phase of χ(2) chiral is missing. While we can distinguish enantiomers by using VCD and ROA spectroscopy, in which we measure Im[χ(1)] and Im[χ(3)], respectively, 2 different chirality gives the same |χ(2) chiral| spectrum in the homodyne chiral VSFG. The interference between chiral and achiral components has been used to circumvent this shortcoming.6,12,17−19 In this technique, the chiral signal is mixed with the achiral contribution and gives the total signal (2) 2 (2) proportional to |χ(2) chiral + χachiral| , where χachiral is an achiral contribution independent from the sample chirality. Thanks to the different signs of χ(2) chiral of different enantiomers, the total output depends on enantiomers. However, the phase of χ(2) chiral cannot be determined without knowing the complex spectral response χ(2) achiral in advance of the measurement with this technique. In addition, this method requires the detection of difference in a pair of measurements with two different polarization combinations for determining the chirality. Hence, so far, there has not been any direct method to address the phase of χ(2) chiral, which determines the molecular handedness, without any a priori knowledge. In this report, we have extended chiral VSFG spectroscopy to the heterodyne detection (phase-sensitive detection20−23) to
M
olecular chirality plays an important role in biological systems. Sensitive detection and discrimination of chirality is a big challenge to optical spectroscopy. Circular dichroism, vibrational circular dichroism (VCD), and Raman optical activity (ROA) are widely used and have been proven powerful tools for probing molecular chirality.1,2 These chiral spectroscopic techniques rely on the magnetic-dipole process or both the magnetic dipole and electric quadrupole processes and are forbidden in the electric-dipole approximation. The signals are thus inherently weak, which therefore results in limited detection sensitivity. Recently, optical spectroscopy of chirality has been extended to nonlinear spectroscopy including second-harmonic generation (SHG) and sum-frequency generation (SFG), which are based on electric-dipole allowed processes.3−11 Because of their high sensitivity, it has been shown that chirality from a monolayer or thin film is detectable with chiral SHG or SFG.4,8 In particular, chiral vibrational SFG (VSFG) has a large potential to provide a wealth of structural information on chiral molecules through vibrational spectra. Shen and coworkers first experimentally demonstrated chiral VSFG from chiral liquid in the transmission geometry.6 Recently, several groups have applied chiral VSFG to detect biomolecules including peptides and proteins at interfaces.12−15 The origins of chiral SFG signals have been also studied in several cases.16 Thus, chiral VSFG spectroscopy is a promising tool to investigate not only chiral molecules in bulk but also biomolecules at interfaces. In chiral VSFG spectroscopy, the phase of χ(2) chiral, which is the chiral second-order nonlinear susceptibility, directly reflects molecular chirality, in other words, left- or right-handedness. Left- and right-handed © 2014 American Chemical Society
Received: June 7, 2014 Accepted: July 31, 2014 Published: July 31, 2014 2874
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discriminate enantiomers based on the phase of second-order nonlinear susceptibilities. To our best knowledge, this is the first report of determination of both the phase and the amplitude of χ(2) chiral, which are unobtainable in the conventional detection.6,12,17−19 The heterodyne-detected VSFG has recently attracted much attention as a tool to obtain the phase information, which is associated with absolute molecular orientation at interfaces. In the heterodyne detection scheme, we can retrieve the phase of χ(2) by mixing the SFG signal with a local oscillator (LO), whose phase is well-defined. In this study, we employed a heterodyne SFG method23,24 with broad bandwidth femtosecond IR and narrow bandwidth picosecond visible probes.25−27 In our experimental setup, the LO is generated from a y-cut quartz plate prior to the sample and delayed in time. The signals from the sample and the LO interfere with each other in a spectrograph, giving rise to interference fringe pattern in the spectral domain. By analyzing the fringe, we can retrieve the phase of χ(2). We measured R(+)-, S(−)-, and the racemic mixture of limonene (Figure 1B) at the air/limonene interface in the reflection geometry (Figure 1A).
Figure 2. Complex χ(2) spectra of R-, S-, and racemic mixture of limonene at the air/limonene interface in the SSP and the PPP polarization combination. (A) Imaginary and (B) real part in the SSP and (C) imaginary and (D) real part in the PPP, respectively. The spectra are normalized by the spectrum of left-handed z-cut quartz30 plate. The coherence length of the measurement of the quartz was 30 nm.
Figure 1. (A) Schematics of the experimental setup. 532 and ∼3450 nm beams are used as the visible and infrared probes, respectively. SFG signal from samples and delayed local oscillator generated from ycut quartz interfere with each other and yield a resultant spectral fringe pattern. (B) Molecular structure of R-(+)- and S-(−)-limonene.
The heterodyne detection in chiral VSFG has the following three virtues. First, it enables us to distinguish chirality from the phase of χ(2) chiral as mentioned. Second, the phase information also reveals the origin of the signal (bulk or surface) because the phases of bulk and surface signals differ by 90° in the electric-dipole approximation.28 The sum frequency electric fields from the bulk and a surface are expressed as Ẽ SFG,bulk = (2) ̃ ̃ ̃ ̃ ̃ rbulkχ(2) bulkE1E2 and ESFG,surface = irsurfaceχsurfaceE1E2, respectively, where r are positive real constants and Ẽ 1 and Ẽ 2 are the electric fields of the visible and infrared pulses. Because we used the SFG signal of left-handed z-cut quartz, Ẽ SFG,quartz originating from the bulk, as the reference, signals from bulk and surface (2) ̃ ̃ are normalized as follows,29 χ(2) eff,bulk = (ESFG,bulk/ESFG,quartz)χquartz (2) (2) (2) and χeff,surface = (Ẽ SFG,surface/iẼ SFG,quartz)χquartz/Δkz, where χeff,bulk and χ(2) eff,surface are effective second-order nonlinear susceptibility −13 of the sample, χ(2) mV−130), quartz is that of quartz (6.0 × 10 and1/Δkz is the coherence length.31 In the spectral analysis of this study, we assumed the origin of the chiral signal to the bulk and that of the achiral signal to the surface to obtain the complex spectra shown in Figures 2 and 3. As will be shown later, these assumptions were verified by spectral features observed in our measurements. Third, the heterodyne detection can increase the detection sensitivity. The referential LO amplifies the weak signal and, as a result, improves the signal-
Figure 3. Complex χ(2) chiral spectra of R-, S-, and racemic mixture of limonene at the air/limonene interface in the PSP polarization combination. (A) Imaginary part and (B) real part of the χ(2) chiral. The spectra are normalized by the spectrum of left-handed z-cut quartz30 plate. The coherence length of the measurements of the samples was assumed to be the same as that of quartz (30 nm).
to-noise ratio, as proven not only in SFG but also in other spectroscopic techniques.23,32,33 In this study, we show that the 2875
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from the electric dipole interaction,35 while achiral SFG signals from isotropic surfaces are dominated by the electric dipole interaction, although achiral bulk SFG signals attributable to the magnetic dipole and electric quadrupole interactions might be contained in the observed achiral signals.36−38 This suggests that limonene molecules at the topmost of the air/limonene interface are oriented to some extent. Very recently, a molecular simulation study proposed that the ring structure of limonene is likely to be perpendicular to the surface.39 In the case of liquid limonene, even though the molecules may be fairly oriented at the interface, the bulk signal is dominant in the chiral spectra. In the homodyne detection, it is difficult to detect chiral signals of limonene in the reflection geometry without the electronic resonance effect. Because of its shorter coherence length, the reflection geometry gives significantly smaller signals than transmission for the detection of bulk signals and requires high sensitivity.40 Typically, the coherence length in reflection is more than 10 times smaller than that in transmission, indicating that the homodyne signal in reflection is no more than 1% of that in transmission. Indeed, the chiral signal from liquid limonene in reflection was observed by using the interference technique, but the signal-to-noise ratio was significantly lower than the transmission chiral signals.6,18 In this work, we have successfully observed the small chiral signals with the moderate signal-to-noise ratio under the electronic nonresonance condition in the reflection geometry by employing the heterodyne detection, which has a role in increasing signal sensitivity. We have estimated the coherence length for the reflected signal under the present condition to be ∼30 nm,40 implying that our technique enables us to detect chirality of a very thin sample with a thickness of tens of nanometers, which is hardly achieved by other vibrational chiral spectroscopy. In conclusion, we have successfully determined the complex χ(2) of chiral systems and distinguished enantiomers by chiral heterodyne-detected vibrational sum frequency generation spectroscopy. R- and S-limonene clearly showed different signs of complex χ(2) signals, while the racemic mixture showed no vibrational signal. We attributed the origin of the signal not to the surface but to the bulk by referring to the phase of the electric field of the chiral signal, which was observed first in this study. We believe that this technique can be easily applied to other systems including biomolecules. The phase of χ(2) that can be obtained in our method should give the information on the handedness of proteins, left or right. The experimentally determined phase information would be of crucial importance for future simulation studies to connect absolute configurations of chiral molecules with the phases of their chiral VSFG signals. Sensitive detection achieved by the heterodyne technique can be used to follow molecular dynamics of chiral molecules, such as protein folding, by combining the present method with timeresolved spectroscopy.
heterodyne detection is highly sensitive enough to detect chirality from tens of nanometers thick, even though we employ a 532 nm laser, which is far from electronic resonance, as the visible beam in the SFG process. Figure 2A,B shows the imaginary and real parts of χ(2) spectra of R-, S-, and racemic mixture of limonene at the air/limonene interface in the SSP (SFG, visible, and infrared beams set to S-, S-, and P-polarized) polarization combination, and Figure 2C,D shows those under the PPP polarization, respectively. Three spectra, both imaginary and real parts, agreed with one another almost perfectly, confirming that these originated from the achiral spectral response. The spectra showed highly congested features due to the several CH stretching (CH, CH2, CH3) resonances in the molecule. The chiral spectrum measured in the PSP combination, which is known as chiral-specific, showed completely different features. Figure 3A,B shows the imaginary and real parts of χ(2) chiral measured at the air/limonene interface in the PSP polarization combination, respectively. The spectra of R- and S-limonene had vibrational resonances at the same frequencies but with opposite signs in Figure 3, while the spectrum of the racemic mixture showed no discernible signal. Both R- and S-limonene displayed two clear vibrational peaks at 2880 and 2835 cm−1 in their Im [χ(2) chiral] spectra, which were assigned to the symmetric CH3 and CH2 stretching modes, respectively.6 Moreover, it is evident that the spectra in Figure 3 are completely different from the achiral spectra shown in Figure 2, proving that the spectra in Figure 3 were not the “leaking” spectra from either the SSP or PPP spectrum. These results ensure that we successfully distinguished one enantiomer from the other by using the heterodyne detection. It should be emphasized that left-handed z-cut quartz was used as the reference,30,34 enabling us to determine the signs of χ(2) chiral uniquely. (See the details in the Experimental Methods.) Therefore, we have not only distinguished enantiomers but also have determined their absolute χ(2) chiral values with their signs. This fact is similar to the situation of VCD and ROA, in which the signs of signals are uniquely determined. Next, we shall discuss the origin of the signals, bulk or surface. As previously mentioned, we have assumed that the chiral signal came from the bulk and the achiral signal from the interface. This is consistent with the spectral features of the complex spectra in Figures 2 and 3. We can easily see vibrational peaks in Figures 2A and 3A, while the spectra in Figures 2B and 3B look dispersive. These spectral features are characteristic to imaginary and real parts of vibrational resonant terms of molecular hyper-polarizabilities. Thus, the observed spectral shapes substantiate the assumption that the chiral signal (PSP) came from the bulk contribution but the achiral signals (SSP and PPP) mainly originated from the surface contribution. It should be noted that the units of vertical axes in Figures 2 (m2 V−1) and 3 (mV−1) are different due to the assignment of the signal origins previously discussed. The bulk signal comes from the molecules in depth within the coherence length, while the surface signal comes from the monolayer, which localizes at the surface and has no depth. This difference gives rise to the difference in the two units by a dimension of length.6,7 (See the details in the Supporting Information.) The (2) 2 | spectrum, which corresponds to a reconstructed |χchiral homodyne detected spectrum, was similar to the reported spectrum in transmission, which was ascribed to the bulk. (See Figure S1 in the Supporting Information.) This fact supports our assignment that the chiral signal originated from the bulk. It has been shown that bulk chiral SFG signals originate purely
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EXPERIMENTAL METHODS R-(+)-Limonene was purchased from Nacalai Tesque and S(−)- and the racemic mixture of liquid limonene were from Tokyo Chemical Industry. All samples were used as received. Details of our heterodyne-detected chiral VSFG setup are as follows. A Ti:sapphire regenerative amplifier (Legend Elite, Coherent) was used to generate laser pulses centered at 800 nm with pulse duration of 100 fs. The amplifier produces ∼3.5 mJ of energy/pulse with repetition rate of 1 kHz. The output was divided into two. One was used to pump a commercial 2876
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optical parametric amplifier (TOPAS-C, Coherent) to obtain a broadband infrared beam with a central wavelength of ∼3450 nm (∼2900 cm−1) with fwhm of 200 cm−1. The other part was introduced into a narrow-band second-harmonic generator (SHBC, Coherent) to generate spectrally narrow 400 nm (∼10 ps, ∼8 cm−1). The output of the second-harmonic generator was used to pump an optical parametric amplifier (TOPAS 400, Coherent) to obtain a narrowband visible beam (wavelength: 532 nm, bandwidth: ∼10 cm−1). The infrared and visible beams were overlapped in a y-cut quartz thin plate to generate a broadband sum frequency generation signal as an LO. The transmitted infrared and visible beams and the LO were refocused onto the sample. The incident angles of the IR and VIS were ∼60 and ∼70°, respectively. Among three beams, the LO passed through a fused silica plate between the sample and the concave mirror to delay the LO in time (∼2.5 ps). The LO and the SFG from the sample passed through an analyzer for selecting the probe polarization. They were introduced into a polychromator (TRIAX550, Horiba Jovin Yvon) after a prism monochromator (CT25-UV, JASCO)27 and interfered with each other in the frequency domain. The interference fringe pattern was finally detected by a LN-cooled CCD camera (Roper Scientific). The energies of the IR probe, and VIS probe at the sample were 5 and 15 μJ/pulse, respectively. The samples were placed in a homemade Teflon trough. The height of the sample surface was monitored by a displacement sensor (LT8110, Keyence) and was controlled within ±5 μm. The exposure time was 5 min for the achiral measurements and 30 min for the chiral measurements. A left-handed z-cut quartz plate was used as the reference.41 The schematics are shown in Figure 4. For SSP and PPP measurements, the x axis of the
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This study is partially supported by a Grants-in-Aid for Scientific Research from the Ministry of Education Culture, Sports, Science, and Technology of Japan (Grant No. 24350010; No. 26104504, Innovative Areas 2503) and by a Grant for Basic Science Research Projects from the Sumitomo Foundation.
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Figure 4. Experimental configuration of the reference quartz for the achiral (A) and chiral measurements (B).
quartz is set to be parallel to the plane including the visible and IR lasers (Figure 4A). For PSP measurements, the y axis is set to be parallel to that plane (Figure 4B). In this case, the SFG signal field is in effect proportional to minus the yxy component of the second-order nonlinear susceptibility, which is equivalent to its xxx component, χq(2).
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ASSOCIATED CONTENT
S Supporting Information *
2 (2) Reconstructed |χ(2) chiral| spectrum, the complex χchiral spectra when one assumes that the chiral signal comes from the surface, and calculation of surface and bulk second-order nonlinear susceptibilities and coherence length. This material is available free of charge via the Internet at http://pubs.acs.org.
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(34) Care must be taken when the susceptibilities we report in this paper are compared with those in ref 6. We employed the secondorder susceptibility of quartz of 6.0 × 10−13 mV−1 from ref 30, but ref 6 employed the value of 8.0 × 10−13 mV−1 from ref 40. (35) Fischer, P.; Beckwitt, K.; Wise, F. W.; Albrecht, A. C. The Chiral Specificity of Sum-Frequency Generation in Solutions. Chem. Phys. Lett. 2002, 352, 463−468. (36) Guyotsionnest, P.; Shen, Y. R. Bulk Contribution in Surface 2nd-Harmonic Generation. Phys. Rev. B 1988, 38, 7985−7989. (37) Maki, J. J.; Kauranen, M.; Persoons, A. Surface 2nd-Harmonic Generation from Chiral Materials. Phys. Rev. B 1995, 51, 1425−1434. (38) Kauranen, M.; Verbiest, T.; Maki, J. J.; Persoons, A. 2ndHarmonic Generation from Chiral Surfaces. J. Chem. Phys. 1994, 101, 8193−8199. (39) Zheng, R. H.; Wei, W. M.; Liu, H.; Jing, Y. Y.; Wang, B. Y.; Shi, Q. Theoretical Study of Sum-Frequency Vibrational Spectroscopy of Limonene Surface. J. Chem. Phys. 2014, 140, 104702. (40) Wei, X.; Hong, S. C.; Lvovsky, A. I.; Held, H.; Shen, Y. R. Evaluation of Surface vs Bulk Contributions in Sum-Frequency Vibrational Spectroscopy Using Reflection and Transmission Geometries. J. Phys. Chem. B 2000, 104, 3349−3354. (41) Standards on Piezoelectric Crystals. Proc. IRE 1949, 37, 1378− 1395.
Δkz = |kSFG, z − k vis, z − kIR, z| = ωSFGnSFG cos θSFG + ωvisn vis cos θvis + ωIR nIR cos θIR where kz are the surface normal components of the wave vectors, n are the refractive indices of the medium at the frequencies of the beams, ω are the frequencies of the beams, and θvis and θIR are the refraction angles of the visible and IR beams, respectively. θSFG is the reflection angle of the SFG signal in the bulk medium defined by the following equation ωSFGnSFGsin θSFG = ωvisn vissin θvis + ωIR nIR sin θIR (32) Oudar, J. L.; Shen, Y. R. Non-Linear Spectroscopy by MultiResonant 4-Wave Mixing. Phys. Rev. A 1980, 22, 1141−1158. (33) Lepetit, L.; Cheriaux, G.; Joffre, M. Linear Techniques of Phase Measurement by Femtosecond Spectral Interferometry for Applications in Spectroscopy. J. Opt. Soc. Am. B 1995, 12, 2467−2474. 2878
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