Chiroptical anisotropy of crystals and molecules

Abstract

Optical activity, a foundational part of chemistry, is not restricted to chiral molecules although generations have been instructed otherwise. A more inclusive view of optical activity is valuable because it...

Extracting Pure Circular Dichroism from Hierarchically Structured CdS Magic Cluster Films

“…In general, there are five major mechanisms that produce a signal in a CD spectrometer, recording the differential absorption between left- and right-handed circularly polarized light. These sources of CD are (1) chiral atomic structure, (2) chiral surface structure or chiral ligands on an inorganic core, (3) helical assemblies of nanoparticles, (4) LDLB contributions as outlined in section 2, and/or (5) anisotropic alignment (ordered orientation) of the transition dipoles. ,, The previous sections with derivations and studies now enable us to eliminate many of these effects as the source for the CD signals in our MSC films.…”

“…Circular dichroism (CD) spectroscopy is widely used for the detection and quantification of chiroptic activity by measuring the differential absorption of left- and right-handed circularly polarized light. The main shortcoming with all CD measurements is that materials with macroscopic anisotropies can have undesired contributions to the signal from the linear dichroism (LD) and linear birefringence (LB) and the residual static birefringence (imperfection) of photoelastic modulators that are used to modulate between left and right circular polarization states. , These collective contributions have been termed “apparent CD” or “pseudo CD”. , The intensity of the “chiroptic-CD” signal (also known as “true CD”) that is generated from the coupling between the transition electronic dipole and transition magnetic dipole can be orders of magnitude smaller than linear anisotropy-induced CD signals (which are generated solely from electronic dipole-allowed transitions). , It is important to note that the necessary condition for CD in an isotropic solution is chirality, but this is not the case for oriented materials: the chiroptic-CD signal (true CD) can be nonzero in an achiral system that is highly oriented . In these cases, the coupling between the electronic transition dipoles and magnetic transition dipoles or electric quadrupoles still produces the CD signal, but the chromophore is achiral.…”

Twisted tetrathiafulvalene crystals

“…LR and LR′ are the linear retardances for the same sets of axes (Scheme 1c). Elements along the anti-diagonal represent circular retardance, CR, and circular extinction, CE, (Scheme 1c) 30,31 while elements along the diagonal capture the isotropic absorption, A.…”

Chirality determination in crystals

“…Furthermore, adulteration by linear birefringence and dichroism limits this technique to cubic crystals, when using unoriented samples. 87 Thus, highly colored or anisotropic crystals are not readily suitable, and for this reason, most deracemization studies using POM have been undertaken on sodium chlorate or bromate. [88][89][90][91][92][93] There exist other microscopic techniques, which are also limited to certain materials.…”