Terahertz Circular Dichroism Spectroscopy of Molecular Assemblies and Nanostructures

Abstract

Chemical, physical, biological and materials engineering disciplines use a variety of chiroptical spectroscopies to probe geometrical and optical asymmetry in molecules and particles. Electronic (ECD) and vibrational (VCD) circular dichroism are the most common of these techniques and collectively enable the studies of electronic and vibronic transitions with energies between 0.1 and 5.0 eV. The vibrational states with characteristic energies in the range of 0.001−0.01 eV carry valuable information about concerted intermolecular motions in molecules and crystals involving multiple atoms. These vibronic transitions located in the terahertz (THz) part of the spectrum become increasingly more important for the chemistry, physics, and biology of complex molecules and materials However, the methodology and hardware of THz circular dichroism (TCD) are much less developed than the chiroptical spectroscopies for ultraviolet, visible, near-and mid infrared photons. Here we provide theoretical foundations, practical implementations, comparative assessments, and exemplary applications of TCD spectroscopy. We show that the sign, intensity, and position of TCD peaks are highly sensitive to the three-dimensional structure and long-range organization of molecular crystals, which offer unique capabilities to study (bio) molecules, their crystals, and nanoscale assemblies and apply the novel data processing methodologies. TCD also offers a convenient toolbox to identify new physical phenomena, such as chiral phonons and their propagation in nanostructured matter. We also discuss the major challenges, emerging opportunities and promising research directions, including broad investigation of chiral phonons observed in chiral (nano) crystals and emerging machine learning methodologies for TCD in biological and nanoscale structures. Ubiquity of low-frequency vibrations with rotational components in biomolecular structures, combined with sharpness of peaks in TCD spectra, enables a variety of technological translations.

Lithography‐Free Fabrication of Terahertz Chiral Metamaterials and Their Chirality Enhancement for Enantiomer Sensing

“…The distinct characteristics of the CHAM are beneficial for broadband circular polarizers and biosensing applications for which a broad range of chiroptical responses, homogeneous polarity required for accessibility of the surface to the target analyte, and a detectable response upon a polarization change is necessary. [30,58] Our approach will widen the applicability of chiral metamaterials for optical, biomedical, and pharmaceutical applications.…”

Phonon Polaritonics in Broad Terahertz Frequency Range with Quantum Paraelectric SrTiO3

“…[ 3–9 ] Vibrational resonance‐enhanced THz polarimetry and imaging, can reveal the long‐range order of biomaterials, including chirality, intramolecular coupling, and interaction with water, as well as detect and manipulate cell membrane dynamics. [ 10–15 ] Many of these applications require a high electric field, widely tunable frequency range and polarization, and proper spatial resolution, which are more commonly available toward the high‐frequency end (≳15 THz, ≲20 µm, or ≳62 meV) and low‐frequency end (≲5 THz, ≳60 µm, or ≲21 meV), but less accessible for the range of 5–15 THz (20–60 µm) in between, which is sometimes called “the new terahertz gap”. [ 1,2 ] This intermediate frequency range is important for resonant phonon‐driven phenomena in quantum materials such as enhanced superconductivity in K 3 C 60 and magnetization in CeF 3 , [ 16,17 ] as well as the characterization of nucleobases in biological studies.…”

Spatially Correlated Chirality in Chiral Two-Dimensional Perovskites Revealed by Second-Harmonic-Generation Circular Dichroism Microscopy

“…However, a major drawback of linear CD spectroscopy is the low signal-to-noise ratio that results from the small difference in absorbance (fractions of a percent) for LCP and RCP light. 19,20 It is also worth noting that chiral samples may exhibit chirality that is strongly dependent on position due to spontaneous crystallographic heterogeneity and the formation of localized supramolecular orientation domains. 21−24 However, the conventional linear CD measurement of chiral samples is based on the macroscopic ensemble averaging effect and thus cannot provide a microscopic spatial distribution of CD signals.…”

“…However, a major drawback of linear CD spectroscopy is the low signal-to-noise ratio that results from the small difference in absorbance (fractions of a percent) for LCP and RCP light. , It is also worth noting that chiral samples may exhibit chirality that is strongly dependent on position due to spontaneous crystallographic heterogeneity and the formation of localized supramolecular orientation domains. − However, the conventional linear CD measurement of chiral samples is based on the macroscopic ensemble averaging effect and thus cannot provide a microscopic spatial distribution of CD signals. To date, various microscopic techniques have been developed to characterize spatially correlated chirality, such as wide-field CD imaging, confocal CD mapping, two-photon fluorescence CD mapping, terahertz CD spectroscopy, and second-harmonic-generation CD (SHG-CD) imaging. , As a nonlinear counterpart of linear CD, SHG-CD is defined as the normalized difference in SHG intensity upon excitation with LCP versus RCP light. , Because SHG is highly sensitive to symmetry and the inherent noncentrosymmetry of chiral perovskites provides adaptability, it is feasible to investigate their chirality using SHG-CD spectroscopy. In comparison to linear CD, which is based on the asymmetry of the electronic ground states at thermal equilibrium, SHG-CD employs longer excitation wavelengths and can provide complementary information about the spatially correlated chirality of chiral samples.…”

“…21−24 However, the conventional linear CD measurement of chiral samples is based on the macroscopic ensemble averaging effect and thus cannot provide a microscopic spatial distribution of CD signals. To date, various microscopic techniques have been developed to characterize spatially correlated chirality, such as wide-field CD imaging, 25 confocal CD mapping, 26 two-photon fluorescence CD mapping, 24 terahertz CD spectroscopy, 19 and second-harmonic-generation CD (SHG-CD) imaging. 23,27 As a nonlinear counterpart of linear CD, SHG-CD is defined as the normalized difference in SHG intensity upon excitation with LCP versus RCP light.…”