Letter Cite This: J. Phys. Chem. Lett. 2018, 9, 6723−6730
pubs.acs.org/JPCL
Femtosecond Vibrational Sum-Frequency Generation Spectroscopy of Chiral Molecules in Isotropic Liquid Taegon Lee,† Hanju Rhee,*,† and Minhaeng Cho*,‡,§ †
Seoul Center, Korea Basic Science Institute, Seoul 02841, Republic of Korea Center for Molecular Spectroscopy and Dynamics, Institute for Basic Science (IBS), Seoul 02841, Republic of Korea § Department of Chemistry, Korea University, Seoul 02841, Republic of Korea ‡
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ABSTRACT: Vibrationally resonant optically active (VOA) sumfrequency generation (SFG) is a second-order nonlinear process sensitive to the stereospecific vibrational structure of chiral molecules. We demonstrate that a femtosecond VOA SFG signal can be measured in the isotropic bulk of a chiral liquid. The chiral, achiral, and VOA SFG spectra of R- and S-limonene and their racemic mixture in the C−H stretching frequency region are characterized. In particular, it is shown that the observed circular intensity difference (CID) signal, which can provide distinguishable stereochemical vibrational information between enantiomers, arises from interference of the electric-dipole allowed antisymmetric Raman tensor-induced and Raman optical activity (ROA) tensor-induced SFG fields. Furthermore, we show that the CID and linear polarization intensity difference (LID) SFG spectra are connected to the real and imaginary parts of the effective chiral VOA SFG susceptibility, respectively. We anticipate that the present technique will be of use in transient chiroptical spectroscopy and stereochemical vibrational imaging studies.
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VOA methods such as chiral vibrational sum-frequency generation (SFG)16−22 and CARS (coherent anti-Stokes Raman scattering)-ROA23,24 spectroscopies have attracted a great deal of attention not only as a complementary way of characterizing the VOA of chiral molecules but also because of their potential applicability to chiroptical ultrafast timeresolved and vibrational imaging studies. Vibrationally resonant optically active SFG (VOA SFG) is a second-order nonlinear chiroptical method based on a threewave mixing process.16,25−27 A resonant infrared (IR) beam (ω1, k1) with a target vibrational mode frequency of the chiral molecule and a (typically) nonresonant visible beam (ω2, k2) interact with the sample to generate an optically active SFG signal (ω3 = ω1 + ω2, k3 = k1 + k2), where ωi and ki denote the optical frequency and wave vector, respectively. In fact, IRvisible SFG spectroscopy is a widely used technique to study vibrational structure and dynamics of molecules on the surface or at the interface,28−32 where centrosymmetry is broken by anisotropic molecular arrangement. Like the VCD and ROA used to study chiral molecules in isotropic solutions, the VOA SFG can provide stereospecific vibrational information on surface or interfacial chiral molecules.18−22 However, as Giordmaine theoretically suggested, the SFG signal can be
hiral molecules lacking mirror symmetry show intriguing phenomena in diverse research areas. The chiral molecular geometry plays a critical role in various chemical and biological processes, such as asymmetric catalysis and ligand binding to biomolecules, and sometimes a small yet stereospecific structural difference between enantiomers could lead to entirely different biochemical activities.1−3 For instance, one form of enantiomeric chiral drugs has a good efficacy, but the other mirror-imaged form is ineffective or even harmful to life. To understand the underlying mechanism of those processes involving chiral molecules, one may need an incisive tool for characterizing their stereochemical structures and dynamics at the microscopic scale. Vibrational optical activity (VOA), which is the vibrational analogue of electronic optical activity such as UV−visible circular dichroism (CD), has proven to be of use in determining absolute configuration of small organic chiral molecules and secondary structure of biomolecules such as protein and DNA.4−11 Indeed, vibrational circular dichroism (VCD) and Raman optical activity (ROA) have been extensively and quite successfully used for various VOA applications.4−6 However, conventional VOA techniques often suffer from the intrinsically weak (electric-dipole forbidden) chiral signal and also a huge achiral (electric-dipole allowed) background problem, which has limited wider VOA applications. Along with advanced linear VOA techniques for enhancing the chiral-to-achiral signal ratio,12−15 nonlinear © XXXX American Chemical Society
Received: September 25, 2018 Accepted: November 7, 2018 Published: November 7, 2018 6723
DOI: 10.1021/acs.jpclett.8b02947 J. Phys. Chem. Lett. 2018, 9, 6723−6730
Letter
The Journal of Physical Chemistry Letters
Figure 1. (a) Energy-level diagram of three-wave mixing VOA SFG process. The broadband femtosecond mid-IR beam (ω1) allows the sample to be excited to a number of vibrational levels (r). Then, the narrowband NIR beam (ω2) is Raman-scattered via the virtual states (s) to generate the SFG signal (ω3 = ω1 + ω2). For the VOA SFG measurement, the polarization of the NIR beam (ω2) is switched either between LCP and RCP or between +45° LP and −45° LP, while those of the other beams (ω1, ω3) are set to be LP in the present study. (b) Experimental setup for the femtosecond VOA SFG measurement. OPA, optical parametric amplifier; LPF, long wavelength pass filter; NBF, narrow bandpass filter; BPF, broad bandpass filter; BS, Babinet−Soleil compensator; WP, half- or quarter-wave plate; P, polarizer; L, lens; C, optical chopper; EMCCD, electron-multiplying charge-coupled device detector. (c) Noncollinear three-wave mixing scheme with the crossing angle θ = 30° in the transmission geometry. The incident and detection polarizations (blue) for the SCP configuration are marked on each beam propagation. The X, Y, and Z axes denote the Cartesian coordinate axes in a space-fixed frame, where the Z axis is the propagation direction of the mid-IR beam (k1).
measured in optically active liquids,33,34 which are isotropic systems, with a mutually orthogonal polarization geometry, where the IR, visible, and SFG beam polarizations are perpendicular to one another.16 This is because the orientational average of the electric-dipole-allowed third-rank hyperpolarizability tensor does not vanish for chiral molecules because of the nonzero antisymmetric Raman tensor beyond the Born−Oppenheimer approximation.35,36 Shen and co-workers for the first time experimentally demonstrated that the VOA SFG signal from the bulk of a chiral liquid can be measured in the transmission geometry.16 They obtained the entire VOA SFG spectrum associated with the C−H stretching modes of limonene by scanning the narrowband picosecond (∼25 ps) IR laser frequency from 2800 to 3000 cm−1. By controlling the polarization direction of the linearly polarized (LP) IR, visible, and SFG signal beams, they could selectively measure not only the chiral and achiral SFG spectra but also their interference term by taking the LP intensity difference (LID) for ±45° rotated LP beams from the incident plane, which has opposite signs for the VOA SFG signals of R- and S-limonene, allowing them to distinguish the two enantiomers. However, it has not yet been clearly elucidated how the observed VOA SFG signal in a given polarization configuration is related to various field−matter interaction terms involved in the three-wave mixing VOA SFG process. Okuno and Ishibashi carried out heterodyne-detected chiral SFG spectroscopy in the reflection geometry as an alternative
approach to fully characterize the complex chiral nonlinear susceptibility in terms of its phase and amplitude.18 By introducing a relatively strong local oscillator (LO) field generated from a quartz crystal into the SFG setup, they could achieve a significantly enhanced chiral SFG sensitivity, which allowed the stereochemical structural investigation of small chiral molecules or biopolymers such as proteins in an extremely thin layer (tens of nanometers) right below the air−liquid (or solution) interface.18,19 However, as shown theoretically in ref 16, the VOA SFG for isotropic solutions containing chiral molecules, when the mutually orthogonal polarization configuration is used, can only provide information about antisymmetric Raman tensor properties of chiral molecules. Furthermore, it was theoretically predicted that the more interesting Raman optical activity (ROA) tensors of chiral molecules in isotropic solutions can be studied with circular intensity difference (CID) measurements even with nonorthogonal polarization configuration, unlike all the previous works.26 In this Letter, we demonstrate that a femtosecond VOA SFG signal can be indeed measured in the isotropic bulk of liquid limonene. By using a femtosecond (spectrally broad) mid-IR laser beam, we could obtain the entire VOA SFG spectra of R-, S-limonene and their racemic mixture in the C−H stretching frequency region without any frequency scan. In contrast to the previous VOA SFG methods employing the LP beams only, the CID SFG signal, which is defined as the difference of the chiral SFG signals for left- and right-circularly polarized 6724
DOI: 10.1021/acs.jpclett.8b02947 J. Phys. Chem. Lett. 2018, 9, 6723−6730
Letter
The Journal of Physical Chemistry Letters
Figure 2. (a) Chiral (SPP) and (b) ROA (SSP) tensor-induced LP SFG spectra of R- (red circles), S-limonene (blue squares), and their racemic mixture (black triangles). The solid lines represent the fitted curves of the complex line shape functions with the parameters in Table 1. The measured SFG intensities, (c) ISFG,ω2(L) (solid) and ISFG,ω2(R) (dashed) in the SCP configuration and (d) ISFG,ω2(+45°) (solid) and ISFG,ω2(−45°) (dashed) in the SLP configuration for R- (red, upper) and S-limonene (blue, lower). (e) CID (ΔICID,ω2 = ISFG,ω2(L) − ISFG,ω2(R)) and (f) LID (ΔILID,ω2 = ISFG,ω2(+45°) − ISFG,ω2(−45°)) SFG spectra of R- (red), S-limonene (blue), and their racemic mixture (black). The calculated (g) eff −Re[Leff VOA(ω)] and (h) Im[LVOA(ω)] spectra from the fitted complex line shape functions to the SPP and SSP spectra in panels a and b, which are , respectively. The dashed lines in panels g and h represent associated with the real and imaginary parts of the effective chiral response term, κSCP/SLP eff the zero-baseline.
Figure 1. A femtosecond mid-IR beam (
