Circular dichroism imaging microscopy: application to enantiomorphous twinning in biaxial crystals of 1,8-dihydroxyanthraquinone.
A microscope was constructed for imaging circular dichroism of heterogeneous anisotropic media. To avoid linear biases that are common with electronic circular polarization modulation, we chose a retrogressive solution: mechanical light modulation by rotating a linear polarizer with respect to a quarter wave plate continuously tuned by tilting to the operating wavelength. Our comparatively slow technique succeeds with near-perfect circular input and signal averaging using a CCD camera. We have applied the method to anomalously birefringent crystals of 1,8-dihydroxyanthraquinone that are shown to have intergrown mirror image domains, undetected by X-ray diffraction because the twinning complexity renders differences in anomalous dispersion, already small, unreliable. The origin of the anomalous birefringence and the assignment of the absolute configuration are discussed.
Claborn
Kacey
K
Department of Chemistry, Box 351700, University of Washington, Seattle, WA 98195-1700, USA.
Puklin-Faucher
Eileen
E
Kurimoto
Miki
M
Kaminsky
Werner
W
Kahr
Bart
B