Circular Dichroism assisted bi-directional absorbers

Chirality, a geometric property that is of great importance in chemistry, biology, and medicine, has spurred many breakthroughs in the field of multi-dimensional metasurfaces that provide efficient ways of flexibly manipulating amplitude and phase of circular polarization (CP) waves. As one of the most important applications, chiral metamaterials can be used to implement novel absorbers. Herein, an ultra-thin wideband circular dichroic asymmetric metasurface was implemented via loading resistive film into chiral resonators. Opposite and reversible polarization conversion and circular dichroism (CD) were realized as being illuminated by CP waves from both sides meanwhile. Theoretical derivation and simulation verify that the polarization conversion and CD enhancement utilizing multi-layer CD metasurface. It is also found that the orientation angle of the meta-atom of each layer plays an important role in the CD enhancement, which paves a new way for CD enhancement. In addition, the coupling between the CD resonators was utilized to manipulate CD. On this basis, an ultra-thin polarization-insensitive absorber was achieved by employing a C4 2×2 CD resonator array, which was identical illuminating from front and back sides. Circular dichroic absorbers possess great potential in practical applications, ranging from stealth technology, antenna isolation, multi-functional microwave devices, chiral sensing, and catalysis.

Unambiguous Stereochemical Assignment of Cyclo(Phe-Pro), Cyclo(Leu-Pro), and Cyclo(Val-Pro) by Electronic Circular Dichroic Spectroscopy

2,5-diketopiperazines (DKPs) are cyclic dipeptides ubiquitously found in nature. In particular, cyclo(Phe-Pro), cyclo(Leu-Pro), and cyclo(Val-Pro) are frequently detected in many microbial cultures. Each of these DKPs has four possible stereoisomers due to the presence of two chirality centers. However, absolute configurations of natural DKPs are often ambiguous due to the lack of a simple, sensitive, and reproducible method for stereochemical assignment. This is an important problem because stereochemistry is a key determinant of biological activity. Here, we report a synthetic DKP library containing all stereoisomers of cyclo(Phe-Pro), cyclo(Leu-Pro), and cyclo(Val-Pro). The library was subjected to spectroscopic characterization using mass spectrometry, NMR, and electronic circular dichroism (ECD). It turned out that ECD can clearly differentiate DKP stereoisomers. Thus, our ECD dataset can serve as a reference for unambiguous stereochemical assignment of cyclo(Phe-Pro), cyclo(Leu-Pro), and cyclo(Val-Pro) samples from natural sources. The DKP library was also subjected to a biological screening using assays for E. coli growth and biofilm formation, which revealed distinct biological effects of cyclo(D-Phe-L-Pro).

Au Nanospirals Transferred onto PDMS Film Exhibiting Circular Dichroism at Visible Wavelengths

We propose a thin, single-layered circular dichroic filter with Au nanospiral structures on a polydimethylsiloxane (PDMS) thin film that has strong circular dichroism at visible wavelengths. Au nanospiral structures with a diameter of 70 nm were fabricated by cryogenic glancing angle deposition on a substrate with a nanodot array template patterned with the block copolymer PS-PDMS. The Au nanospiral structures were transferred onto a transparent and flexible PDMS thin film to fabricate a thin, single-layered circular dichroic filter. The filter had a very large circular dichroism peak of −830 mdeg at 630 nm. The results show that the Au nanospiral structures transferred onto PDMS thin film exhibit large circular dichroism at visible wavelengths.

Spontaneous Magnetization and Optical Activity in the Chiral Series {(L-proline)nV[Cr(CN)6]x} (0 < n < 3)

The incorporation of the natural amino acid L-proline in the synthesis to vanadium-chromium Prussian blue derivatives results in materials exhibiting magnetic ordering including chiral magnetic centers. Although the amorphous nature of these materials makes difficult to assess the structural features of these proline-containing compounds, magnetic and spectroscopic data confirms their multifunctionality. They exhibit high-temperature magnetic ordering (Tc < 255 K) and a circular dichroic signal, representing the molecule-based chiral magnets with the highest ordering temperatures reported to date. In addition, the presence of chiral L-proline (or D-proline) has additional benefits, including higher redox stability and the appearance of magnetic hysteresis. The latter was not observed in the parent compounds, the series of room temperature molecule-based magnets V[Cr(CN)6]x.