In situ Second-Harmonic Generation Circular Dichroism with Submonolayer Sensitivity

Negative Nonlinear CD-ee Dependence in Polycrystalline BINOL Thin Films

Generally, the relationship between the observed circular dichroism and the enantiomeric excess in chiral systems (CD-ee dependence) is linear. While positive nonlinear behavior has often been reported in the past, examples of negative nonlinear (NN) behavior in CD-ee dependence are rare and not well understood. Here, we present a strong NN CD-ee dependence within polycrystalline thin films of BINOL by using second-harmonic-generation circular dichroism (SHG-CD) and commercial CD spectroscopy studies. Theoretical calculations, microscopy, and FTIR studies are employed to further clarify the underlying cause of this observation. This behavior is attributed to the changing supramolecular chirality of the system. Systems exhibiting NN CD-ee dependence hold promise for highly accurate enantiomeric excess characterization, which is essential for the refinement of enantio-separating and -purifying processes in pharmaceuticals, asymmetric catalysis, and chiral sensing. Our findings suggest that a whole class of single-species systems, i.e., racemate crystals, might possess NN CD-ee dependence and thus provide us a vast playground to better understand and exploit this phenomenon.

One-Fold Anisotropy of Silver Chiral Nanoparticles Studied by Second-Harmonic Generation

Second-harmonic generation (SHG) integrated with diverse nonlinear optical activity characterization has high sensitivity to detect the symmetry of materials at an interface, but the study is in its infancy. Here, we employ SHG with linear dichroism (or SHG-LD) to study the chiroptical origin of silver (Ag) chiral nanoparticles (CNPs) deposited by glancing angle deposition (GLAD). It is found that Ag CNPs show the chiroptical activity ascribed to not only the structural chirality (i.e., atomically chiral lattices) but also one-fold anisotropy at an interface due to the substrate rotation during GLAD. Therefore, the SHG-LD shows great potential to provide valuable complementary information to study the chiroptical properties of chiral metamaterials.

Understanding laser desorption with circularly polarized light

We present aspects of emerging optical activity in thin racemic 1,1'-Bi-2-naphthol films upon irradiation with circularly polarized light and subsequent resonant two-photon absorption in the sample. Thorough analysis of the sample morphology is conducted by means of (polarization-resolved) optical microscopy and scanning electron microscopy (SEM). The influence of crystallization on the nonlinear probing technique (second harmonic generation circular dichroism [SHG-CD]) is investigated. Optical activity and crystallization are brought together by a systematic investigation in different crystallization regimes. We find crystallization to be responsible for two counter-acting effects, which arise for different states of crystallization. Measuring crystallized samples offers the best signal-to-noise ratio, but it limits generation of optical activity due to self-assembly effects. For suppression of crystallization on the other hand, there is a clear indication that enantiomeric selective desorption is responsible for the generation of optical activity in the sample. We reach the current resolution limit of probing with SHG-CD, as we suppress the crystallization in the racemic sample during desorption. In addition, intensity-dependent measurements on the induced optical activity reveal an onset threshold (≈0.7 TW cm-2), above which higher order nonlinear processes impair the generation of optical activity by desorption with CPL.

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.

Enantiospecific Desorption Triggered by Circularly Polarized Light

The interest in enantioseparation and enantiopurification of chiral molecules has been drastically increasing over the past decades, since these are important steps in various disciplines such as pharmaceutical industry, asymmetric catalysis, and chiral sensing. By exposing racemic samples of BINOL (1,1′‐bi‐2‐naphthol) coated onto achiral glass substrates to circularly polarized light, we unambiguously demonstrate that by controlling the handedness of circularly polarized light, preferential desorption of enantiomers can be achieved. There are currently no mechanisms known that would describe this phenomenon. Our observation together with a simplified phenomenological model suggests that the process of laser desorption needs to be further developed and the contribution of quantum mechanical processes should be revisited to account for these data. Asymmetric laser desorption provides us with a contamination‐free technique for the enantioenrichment of chiral compounds.

Spectroscopy for model heterogeneous asymmetric catalysis

Heterogeneous catalysis has vastly benefited from investigations performed on model systems under well-controlled conditions. The application of most of the techniques utilized for such studies is not feasible for asymmetric reactions as enantiomers possess identical physical and chemical properties unless while interacting with polarized light and other chiral entities. A thorough investigation of a heterogeneous asymmetric catalytic process should include probing the catalyst prior to, during, and after the reaction as well as the analysis of reaction products to evaluate the achieved enantiomeric excess. I present recent studies that demonstrate the strength of chiroptical spectroscopic methods to tackle the challenges in investigating model heterogeneous asymmetric catalysis covering all the abovementioned aspects.