Optical Activity Anisotropy of Benzil

Abstract

Optical activity (OA) along the optic axis of crystalline benzil has been measured by many over the past 150 years. However, the OA anisotropy remains uncharacterized due to difficulties in sample preparation as well as competition with linear birefringence (LB). The challenges associated with measuring OA along low-symmetry directions in crystals have too often left scientists with only average values of nonresonant OA in solution, i.e., specific rotations, which continue to resist interpretation in terms of structure. Measuring OA anisotropy has been facilitated by recent advances in polarimetry and optical modeling and here we compare results from two distinct division-of-time polarimeters. The absolute structure of crystalline benzil was established for the first time. The optical rotation (OR) of (+)-crystalline benzil (space group P3121) perpendicular to the optic axis at the sodium D-line is −24.6 ± 1.1°/mm. A spectroscopic optical model in the transparent region of the crystal is provided. Electronic structure calculations of OR inform the polarimetric measurements and point to the necessity of developing linear response theory with periodic boundary conditions in order to interpret the results of chiroptical measurements in crystals.

Mueller matrix polarimetry of bianisotropic materials [Invited]

“…Some low symmetry molecular crystals resist cutting and polishing in the necessary directions. Benzil was a difficult organic crystal of this kind [74]. A tremendous savings in labor was achieved by accounting for multiply reflected incoherent waves in anisotropic, arbitrarily oriented crystals [75].…”

Mueller Polarimetry of Chiral Supramolecular Assembly

“…We show that by using appropriate polarimetric tools, namely, Mueller polarimetry, − new insights can be provided with regard to self-organization of molecules. Relevant to all fields of chirality research, the phenomenon of supramolecular spontaneous symmetry breaking has not yet been examined by Mueller polarimetry, despite the proven efficacy of the approach in nano-optics in particular. − Although seldom used in the field of molecular chirality, − extensive efforts have been made, in particular by the group of Bart Kahr, to explain the chirality and asymmetric features of crystals using this complete polarimetric tool. − One key value of Mueller polarimetry is to provide artifact-free characterizations of chiroptical properties, in particular the circular dichroism (CD). ,, Such characterizations on macroscopic supramolecular assemblies have occasionally been limited, for instance by the fact that linear dichroism (LD), arising from the orientation of the macroscopic molecular system, can be sometimes misinterpreted for a genuine CD response in the chiroptical characterization. ,− Interestingly, this difficulty can be overcome by the use of a Mueller polarimetric tool as shown here.…”

Revisiting polarimetry near the isotropic point of an optically active, non‐enantiomorphous, molecular crystal

“…Having recently analyzed a new crystal with an isotropic point in considerable detail, we set out to further characterize the structure of KTC, as well as the optical properties using complete polarimetry. The OA of KTC is small at the isotropic wavelength, and as such these crystals extend the range of the accumulated data of OA tensors that can be used to evaluate predictions of computations of the OA of crystals from first principles …”

“…small at the isotropic wavelength, and as such these crystals extend the range of the accumulated data of OA tensors 7 that can be used to evaluate predictions of computations of the OA of crystals from first principles. 8 2 | MATERIALS AND METHODS…”