The nature of chirality induced by molecular aggregation and self-assembly

Physical Mechanism of Nonlinear Spectra in Triangene

In this work, we theoretically investigate the linear and nonlinear optical absorption properties of open triangulene spin chains and cyclic triangulene spin chains in relation to their lengths and shapes. The physical mechanism of local excitation within the triangular alkene unit and the weak charge transfer between the units are discussed. The uniformly distributed electrostatic potential allows the system to have a small permanent dipole moment that blocks the electronic transition in the light excitation such that the electronic transition can only be carried out between adjacent carbon atoms. The one-photon absorption (OPA) spectra and two-photon absorption (TPA) spectra are red-shifted with the addition of triangulene units compared to N = 3TSCs (triangulene spin chains, TSCs). Here, TPA is mainly caused by the first step of the transition. The length of the spin chain has a significant adjustment effect on the photon cross-section. TSCs of different lengths and shapes can control chirality by adjusting the distribution of the electric dipole moment and transition magnetic dipole moment. These analyses reveal the photophysical properties of triangulene and provide a theoretical basis for studying the photophysical properties of triangulene and its derivatives.

A fluorescent sensing platform based on collagen peptides-protected Au/Ag nanoclusters and WS2 for determining collagen triple helix integrity

The unique triple helix structure of collagen plays an important role in its biological properties, and the triple helix integrity is closely correlated with its molecular behavior and biological functions. Nevertheless, there is still a lack of convenient, accurate and practical methods for quantitatively determining collagen triple helix integrity. Herein, we first prepared bovine skin collagen peptide (BSCP)-protected Au/Ag nanoclusters (Au/AgNCs@BSCP) with excellent optical properties, high stability and good biocompatibility, which could adsorb on WS₂ surface leading to fluorescence quenching. Upon the addition of collagen, the interaction of collagen and Au/AgNCs@BSCP led to the detachment of Au/AgNCs@BSCP from the WS₂ surface, causing an increase in the fluorescence signal. Using the difference in the fluorescence recovery of the different samples, we achieved the quantitative determination of collagen triple helix integrity. This developed strategy exhibited excellent accuracy, selectivity, and practicality, thus showing promising potentials in biomedical applications.

Chiral inorganic nanomaterials for biological applications

Chiral nanomaterials in biology play indispensable roles in maintaining numerous physiological processes, such as signaling, site-specific catalysis, transport, protection, and synthesis. Like natural chiral nanomaterials, chiral inorganic nanomaterials can also be established with similar size, charge, surface properties, and morphology. However, chiral inorganic nanomaterials usually exhibit extraordinary properties that are different from those of organic materials, such as high g-factor values, broad distribution range, and symmetrical mirror configurations. Because of these unique characteristics, there is great potential for application in the fields of biosensing, drug delivery, early diagnosis, bio-imaging, and disease therapy. Related research is summarized and discussed in this review to showcase the bio-functions and bio-applications of chiral inorganic nanomaterials, including the construction methods, classification and properties, and biological applications of chiral inorganic nanomaterials. Moreover, the deficiencies in existing studies are noted, and future development is prospected. This review will provide helpful guidance for constructing chiral inorganic nanomaterials with specific bio-functions for problem solving in living systems.

Physical mechanism on linear spectrum and nonlinear spectrum in double helical carbon nanomolecule-infinitene

In this work, based on density functional theory (DFT) and wave function analysis, the properties of absorption spectrum, electronic circular dichroism (ECD) spectrum and Raman spectrum of infinitene (monomer and dimer) with double helical structure are theoretically studied. The electronic excitation properties of infinitene were investigated based on the visualization method charge density difference (CDD) and transition density matrix (TDM). It is found that there is obvious intermolecular charge transfer behavior in the dimer. The electromagnetic interaction mechanism of the chirality of infinitene is explained by decomposing transition electric\magnetic dipole moments (TEDMs\TMDMs). The response of Raman spectra to excitation light of different wavelengths was calculated. Then, the electron delocalization degree and magnetic response intensity of infinitene were studied based on the magnetically induced current under external magnetic field. The interaction of infinitene with the external environment was studied by electrostatic and van der Waals potentials, and it was shown that non-polar or low-polar molecules are more inclined to be adsorbed at the groove position of infinitene. Finally, the mechanism of intermolecular interactions in dimer was investigated based on independent gradient model based on Hirshfeld partition (IMGH), Atoms-In-Molecules (AIM), and energy decomposition analysis based on forcefield (EDA-FF). And revealed that the stacking in the dimer is dominated by dispersive interactions.

Anti-Symmetric Electromagnetic Interactions' Response in Electron Circular Dichroism and Chiral Origin of Periodic, Complementary Twisted Angle in Twisted Bilayer Graphene

Many novel physical properties of twisted bilayer graphene have been discovered and studied successively, but the physical mechanism of the chiral modulation of BLG by a twisted angle lacks theoretical research. In this work, the density functional theory, the wavefunction analysis of the excited state, and the quantum theory of atoms in molecules are used to calculate and analyze the anti-symmetric chiral characteristics of zigzag-edge twisted bilayer graphene quantum dots based on periodic complementary twisted angles. The analysis of the partial density of states shows that Moiré superlattices can effectively adjust the contribution of the atomic basis function of the fragment to the transition dipole moment. The topological analysis of electron density indicates that the Moiré superlattices structure can enhance the localization of the system, increasing the electron density of the Moiré central ring, reducing the electron surge capacity in general and inducing the reversed helical properties of the top and underlying graphene, which can be used as the origin of the chiral discrimination; it also reveals the mole in the superlattice chiral physical mechanism. On this basis, we will also study the nonlinear optical properties of twisted bilayer graphene based on a twisted angle.

Correlation of electronic and vibrational properties with the chiro-optical activity of polyfluorene copolymers

The rationale of this paper is to shed some light on the origin of the optical response of two similar chiral fluorene copolymers in correlation with their vibrational modes, to understand how a chiral center placed in a ramification affects the optical properties of the main chain. Various spectroscopic ellipsometric techniques, in the scope of the Stokes theory were used to characterize the optical-vibrational behavior of the polyfluorenes: ellipsometry in emission (EE), transmission (TE), and Raman (ERS). The results showed that the optical activity and the emission of the circularly polarized light depends substantially on the interaction of the chiral carbon in the ramification and the main chain through specific optically active vibrational modes, for each sample. One interesting achievement was to find the absolute dextrorotatory configuration of the studied molecules, that could induce a helicoidal structure to the entire material.

Broadband Optical Activity Spectroscopy with Interferometric Fourier-Transform Balanced Detection

Spectrally resolved measurements of optical activity, such as circular dichroism (CD) and optical rotatory dispersion (ORD), are powerful tools to study chiroptical properties of (bio)molecular and nanoplasmonic systems. The wider utilization of these techniques, however, has been impeded by the bulky and slow design of conventional spectropolarimeters, which have been limited to a narrowband scanning approach for more than 50 years. In this work, we demonstrate broadband measurements of optical activity by combining a balanced detection scheme with interferometric Fourier-transform spectroscopy. The setup utilizes a linearly polarized light field that creates an orthogonally polarized weak chiral free-induction-decay field, along with a phase-locked achiral transmitted signal, which serves as the local oscillator for heterodyne amplification. By scanning the delay between the two fields with a birefringent common-path interferometer and recording their interferogram with a balanced detector that measures polarization rotation, broadband CD and ORD spectra are retrieved simultaneously with a Fourier transform. Using an incoherent thermal light source, we achieve state-of-the-art sensitivity for CD and ORD across a broad wavelength range in a remarkably simple setup. We further demonstrate the potential of our technique for highly sensitive measurements of glucose concentration and the real-time monitoring of ground-state chemical reactions. The setup also accepts broadband pulses and will be suitable for broadband transient optical activity spectroscopy and broadband optical activity imaging.

Excited States Symmetry Breaking and In-Plane Polarization Cause Chiral Reversal in Diastereomers

In this work, we investigate the electronic transitions and chirality of three isomers of huge conjugated systems: asymmetric diastereomers (MMMM) and two symmetrical diastereomers (PMPM and PPMM). The physical mechanism of flipping has been studied theoretically. The new ribbon-shaped polycyclic aromatic hydrocarbons (PAHs) molecule is formed by connecting three graphene-like systems with large conjugated π orbitals. By calculating and analyzing electromagnetic interaction decomposition over distance, it can be found that the chirality reversal of different energies is caused by the symmetrical fracture of TMDM in the Z direction. The chirality reversal at the same energy is caused by the in-plane polarization of the TMDM along the Y direction.

Chiral surface plasmon-enhanced chiral spectroscopy: principles and applications

In this review, the development context and scientific research results of chiral surface plasmons (SPs) in recent years are classified and described in detail. First, the principle of chiral SPs is introduced through classical and quantum theory. Following this, the classification and properties of different chiral structures, as well as the superchiral near-field, are introduced in detail. Second, we describe the excitation and propagation properties of chiral SPs, which lays a good foundation for the application of chiral SPs and their chiral spectra in various fields. After that, we have summarized the recent research results of chiral SPs and their applications in the areas of biology, two-dimensional materials, topological materials, analytical chemistry, chiral sensing, chiral optical force, and chiral light detection. Chiral SPs are a new type of optical phenomenon that have useful application potential in many fields and are worth exploring.

Optical Properties of Artemisinin and Its Derivatives

Artemisinin and its derivatives are of great research value in biology. In this work, we study their chiral and optical properties. The multidimensional multifunction analysis method is used to analyze the linear and nonlinear optical processes (one-photon and two-photon absorption: OPA and TPA), electronic circular dichroism (ECD), and Raman optical activity (ROA) mechanisms under light excitation. Transition dipole moments (TDMs) and charge difference density (CDD) are used to describe the electromagnetic interaction between ECD and ROA when a substance is excited by light. The theoretical research results of the study show that the dioxygen atoms provide an intermediary for the transfer between charges and also enhance the role of the TDMs. This generalized chiral theory can not only explain the traditional sources of chirality but also distinguish whether the molecule has chirality when the chiral center is not obvious. By analyzing ROA and different vibration modes, we can clearly observe that each part of the molecule responds differently when excited.

Chiroptical Properties in Thin Films of π-Conjugated Systems

Chiral π-conjugated molecules provide new materials with outstanding features for current and perspective applications, especially in the field of optoelectronic devices. In thin films, processes such as charge conduction, light absorption, and emission are governed not only by the structure of the individual molecules but also by their supramolecular structures and intermolecular interactions to a large extent. Electronic circular dichroism, ECD, and its emission counterpart, circularly polarized luminescence, CPL, provide tools for studying aggregated states and the key properties to be sought for designing innovative devices. In this review, we shall present a comprehensive coverage of chiroptical properties measured on thin films of organic π-conjugated molecules. In the first part, we shall discuss some general concepts of ECD, CPL, and other chiroptical spectroscopies, with a focus on their applications to thin film samples. In the following, we will overview the existing literature on chiral π-conjugated systems whose thin films have been characterized by ECD and/or CPL, as well other chiroptical spectroscopies. Special emphasis will be put on systems with large dissymmetry factors (g_abs and g_lum) and on the application of ECD and CPL to derive structural information on aggregated states.

Visible Light Electromagnetic Interaction of PM567 Chiral Dye for Asymmetric Photocatalysis, a First-Principles Investigation

In asymmetric photocatalytic reactions, it is necessary to study the mechanism of the asymmetric electromagnetic interaction between molecules and light. In this work, we theoretically studied the electromagnetic interactions between the light-induced charge transfer reaction and the chiral reaction of PM567 dye. We found that the chiral responses of molecules in different wavelength ranges were partially due to pyrromethene and binaphthalene. Therefore, the catalytic sites with different chirality also corresponds to the two-part groups. Through quantitative analysis, we found the entire analysis process to be complete and self-consistent.

Controlling the nonadiabatic electron-transfer reaction rate through molecular-vibration polaritons in the ultrastrong coupling regime

Recent experiments showed that the chemical reaction rate is modified, either increased or decreased, by strongly coupling a nuclear vibration mode to the single mode of an optical cavity. Herein we investigate how the rate of an electron-transfer reaction depends on the molecule-cavity coupling in the ultrastrong coupling regime, where the coupling strength is comparable in magnitude with both the vibrational and the cavity frequencies. We found two main factors that determine the modification of the reaction rate: the relative shifts of the energy levels induced by the coupling and the mixing of the ground and excited states of molecular vibration in the ground state of the hybrid molecule-plus-cavity system through which the Franck-Condon factor between the initial and final states of the transition is altered. The former is the dominant factor if the molecule-cavity coupling strengths for the reactant and product states differ significantly from each other and gives rise to an increase in the reaction rate over a wide range of system's parameters. The latter dominates if the coupling strengths and energy levels of the reactant and product states are close to each other and it leads to a decrease in the reaction rate. The effect of the mixing of molecular vibrational states on the reaction rate is, however, suppressed in a system containing a large number of molecules due to the collective nature of the resulting polariton, and thus should be observed in a system containing a small number of molecules. In contrast, the effect of the relative shifts of the energy levels should be essentially independent of the number of molecules coupled to the cavity.

Physical mechanism of concentration-dependent fluorescence resonance energy transfer

We experimentally report fluorescence resonance energy transfer (FRET), using a novel visualization method of excitation-emission mapping. Firstly, both the absorption and fluorescent spectra of donor and acceptor are measured, respectively, under different molecular concentrations for verifying that these two molecules are suitable for exploring FRET. And then, the excitation-emission mappings of FRET are investigated to reveal the internal regular pattern of FRET. Our theoretical calculations strongly support experimental results of FRET. Our experimental results provided a new visualization method to clearly understand the mechanism of FRET.

Donor-length dependent photoninduced charge transfer in two-photon absorption

In this paper, we study the donor-length dependent photoinduced charge transfer in two-photon absorption (TPA). In the donor-acceptor (D-A) system of oligo-thiophene-fullerene, two kind of lengths of oligo-thiophene are chosen to study the donor-length dependent photoinduced charge transfer in TPA. It is found that donor length can significantly influence the rate of charge transfer. When the donor length is quarter-thiophene, the rate of charge transfer is less than 50%; while with the increase of donor-length to octant-thiophene, the rate of charge transfer can be reach up to 100%. The transition channels of photoinduced charge transfer in TPA are visualized with transition density matrix and charge difference density. Our results promote deeper understanding and rational design for donor-acceptor system with large rate of photoinduced charge transfer in TPA.

Visualization of vibrational-resolution charge transfer enhanced resonance Raman scattering spectroscopy

In this paper, we report visualization of vibrational-resolution charge transfer enhanced resonance Raman scattering Spectroscopy. Based on the first-principles calculation method, we calculated and analyzed the electronic excitation characteristics of the molecules as well as the spontaneous Raman and resonance Raman spectra. Through the visualization of the electronic excitation characteristics, it is found that the Raman signal of the atomic group vibration mode occupied by the charge transfer excitons is significantly enhanced. Super-exchange charge transfer excitons enhance the polarizability by enhancing the dipole moment, ultimately enhancing the Raman optical signal.

Optical properties of S2 and S3 excited states of protonated schiff-base retinal chromophores in TPA, ECD and ROA

The electronic transitions of the protonated Schiff base of 1l-cis-retinal (PSB11) and protonated Schiff bases of all-trans retinal (PSBT) for the second or higher electronic excited states are hard to be observed experimentally, due to weak intensities of electronic state excitations. In this paper, we propose visualizations method to investigate these weak electronic state transitions of PSB11 and PSBT, using two-photon absorption (TPA), electronic circular dichroism (ECD) and Raman optical activity (ROA) spectra. Because of the resonance excitations of PSB11 and PSB11 in TPA, the transition intensity of the third electronic state is significantly enhanced, which are much larger than that of S₁ and S₂ electronic transitions. The charge transfer and electron-hole coherence of these electronic transitions in each step in TPA are visualized with charge difference density and transition density matrix. Also, the strong absorptions of S₁ and S₂ electronic excited states are observed with ECD spectra, and the physical mechanism of electric and magnetic interactions for these electronic transitions are revealed by visualization method. The large intensity of ROA at S₃ excited state results from transition electric and magnetic dipole interactions, not from transition electric dipole and transition electric quadrupole interactions. Our results provide a new visualization method to study the optical properties of biological system using TPA and ECD spectra.

Visualizations of Electric and Magnetic Interactions in Electronic Circular Dichroism and Raman Optical Activity

The chiral source and its mechanism in the molecular system are of great significance in many fields. In this work, we proposed visualized methods to investigate the physical mechanism of a chiral molecule, where the electric and magnetic interactions are visualized with the transitional electric dipole moment, the transitional magnetic dipole moment, and the transitional electric quadrupole moment, and their tensor product. This will also serve as an effective means of visualizing the interaction of light with matter. The relationship between the molecular Raman optical activity (ROA) response and molecular structure was analyzed in an intuitive way. The relationship between chromophore chirality and molecular vibration mode are revealed via interaction between the transition electric dipole moment and the transition magnetic dipole moment. The molecular chirality is derived from the anisotropy of the molecular transition electric dipole moment and the transition magnetic dipole moment. The anisotropic dipole moment localized molecular chromophore is the source of the vibration mode in which the ROA responds to the reverse.