Strong chiroptical effects recently reported result from the interaction of light with chiral plasmonic nanostructures. Such nanostructures can be used to enhance the chiroptical response of chiral molecules and could also significantly increase the enantiomeric excess of direct asymmetric synthesis and catalysis. Moreover, in optical metamaterials, chirality leads to negative refractive index and all the promising applications thereof. In this Progress Report, we highlight four different strategies which have been used to achieve giant chiroptical effects in chiral nanostructures. These strategies consecutively highlight the importance of chirality in the nanostructures (for linear and nonlinear chiroptical effects), in the experimental setup and in the light itself. Because, in the future, manipulating chirality will play an important role, we present two examples of chiral switches. Whereas in the first one, switching the chirality of incoming light causes a reversal of the handedness in the nanostructures, in the second one, switching the handedness of the nanostructures causes a reversal in the chirality of outgoing light.
A new approach to second-order nonlinear optical (NLO) materials is reported, in which chirality and supramolecular organization play key roles. Langmuir-Blodgett films of a chiral helicene are composed of supramolecular arrays of the molecules. The chiral supramolecular organization makes the second-order NLO susceptibility about 30 times larger for the nonracemic material than for the racemic material with the same chemical structure. The susceptibility of the nonracemic films is a respectable 50 picometers per volt, even though the helicene structure lacks features commonly associated with high nonlinearity. Susceptibility components that are allowed only by chirality dominate the second-order NLO response.
Employing high-throughput methods, the synthesis conditions for a series of six new MOFs based on aluminium ions and the V-shaped linker molecule 1,3-benzene dicarboxylic acid, denoted as CAU-10-X (CAU = Christian-Albrechts-University) with the sum formula [Al(OH)(C 8 H 3 O 4 X)]•solvent, were established (X = functional group in 5-position of the aromatic ring; X = H (1), CH 3 (2), OCH 3 (3) NO 2 (4), NH 2 (5), or OH (6)). Due to the absence of macroscopic crystals, the obtained compounds were structurally characterized employing XRPD-methods. The crystal structures of 1, 2 and 3 were refined using Rietveld methods. Although the described MOFs are isoreticular, they crystallize in several, sometimes non-centrosymmetric space groups (1, 4, 6), due to slight structural changes induced by the functionalization. These space groups were confirmed with second-harmonic generation measurements. All compounds are highly stable as confirmed by temperature-dependent XRPD-and IR-experiments and decompose at temperatures above 350 °C. The stabilities of all compounds in aqueous solutions of varying pH were confirmed by XRPD-measurements and their sorption properties towards nitrogen, hydrogen, carbon dioxide and water vapor at low pressures are reported. A drastic influence of the functional group on affinity, capacity and accessibility of the pores for these gases is observed. These properties depend on the polarity and size of the functional group as well as on subtle structural differences between the CAU-10-X compounds.
We demonstrate circular dichroism (CD) in the second harmonic generation (SHG) signal from chiral assemblies of G-shaped nanostructures made of gold. The arrangement of the G shapes is crucial since upon reordering them the SHG-CD effect disappears. Microscopy reveals SHG "hotspots" assemblies, which originate in enantiomerically sensitive plasmon modes, having the novel property of exhibiting a chiral geometry themselves in relation with the handedness of the material. These results open new frontiers in studying chirality.
We present a theory of second-harmonic generation from chiral surfaces including contributions of electric and magnetic dipole transitions to the surface nonlinearity. The nonlinear polarization and magnetization of the surface as well as the second-harmonic fields that are radiated in the reflected and transmitted directions are expressed in terms of the six possible bilinear combinations of the components of the electric field of the fundamental beam. For the case in which the polarization of the fundamental beam is controlled by means of a quarter-wave plate between p-polarized linear and left- and right-hand circular, the second-harmonic fields can be expanded in terms of only three different functions of the rotation angle of the wave plate. The process exhibits nonlinear optical activity, i.e., it responds differently to the two circular polarizations of the fundamental beam if the phases of certain expansion coefficients are different. The theory is used to explain the results of a recent experiment and excellent agreement is found. The results suggest that in the experiment the largest components of the susceptibility tensors that include magnetic contributions were of the order of ∼10% of those of the electric dipole-allowed susceptibility tensor.
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