Benedict Jason B, Freudenthal John H, Hollis Eva, Kahr Bart
Department of Chemistry, Box 351700, University of Washington, Seattle Washington 98195-1700, USA.
J Am Chem Soc. 2008 Aug 13;130(32):10714-9. doi: 10.1021/ja802322t. Epub 2008 Jul 18.
D-sorbitol forms so-called spherulites from under-cooled melts. These polycrystalline formations have optically uniaxial radii. Melts pressed between glasses crystallize as plane sections of spheres. Dyes that are soluble in molten sorbitol become oriented as the crystallization front passes through the melt so as to form disks with large linear dichroism in the absorption bands of the dyes. The dyeing of spherulites is thus a general method of solute alignment. The linear optical properties of sorbitol spherulites containing the azo dye amaranth were analyzed in detail so as to correct a persistent confusion in the literature regarding the orientational dependence of linear dichroism. In cases involving thin film dichroism of multilayered samples requiring many corrections of intensity data in non-normal incidence, some authors have taken transmittance and others absorbance as having a cosine-squared angular dependence on the plane of the electric vector of linearly polarized light. Plane sections of doped spherulites present all orientations of an electric dipole oscillator in spatially localized region in normal incidence. As such, the samples described herein are ideally suited to resolving this confusion. Images of transmittance of dyed spherulites in polarized light were recorded with a CCD camera and simulated under the assumption that both absorbance and transmittance show a cosine-squared angular dependence but with respect to different angles. Transmittance with a cosine-squared dependence follows azimuthal rotations of the spherulite radii around the wave vector, while absorbance with a cosine-squared dependence follows rotations about axes perpendicular to the wave vector, natural consequences of the properties of the optical indicatrix that are often overlooked. Spherulites obviate the substantial experimental complexities that are engendered in non-normal incidence by sample reorientation. Thus, the principles of anisotropic absorption are given in a complete and intuitive fashion.
D -山梨醇从过冷熔体中形成所谓的球晶。这些多晶结构具有光学单轴半径。夹在玻璃之间压制的熔体结晶为球体的平面切片。可溶于熔融山梨醇的染料在结晶前沿穿过熔体时会发生取向，从而形成在染料吸收带中具有大线性二色性的圆盘。因此，球晶染色是溶质取向的一种通用方法。详细分析了含有偶氮染料苋菜红的山梨醇球晶的线性光学性质，以纠正文献中关于线性二色性取向依赖性的长期混淆。在涉及多层样品薄膜二色性且需要对非正入射强度数据进行多次校正的情况下，一些作者将透射率视为与线偏振光电矢量平面呈余弦平方角依赖关系，而另一些作者则将吸光度视为如此。掺杂球晶的平面切片在正入射时在空间局部区域呈现电偶极子振荡器的所有取向。因此，本文所述的样品非常适合解决这种混淆。用电荷耦合器件（CCD）相机记录了偏振光下染色球晶的透射图像，并在假设吸光度和透射率均呈余弦平方角依赖关系但相对于不同角度的情况下进行了模拟。具有余弦平方依赖关系的透射率跟随球晶半径围绕波矢的方位旋转，而具有余弦平方依赖关系的吸光度跟随围绕垂直于波矢的轴的旋转，这是光学指标性质的自然结果，却常常被忽视。球晶避免了非正入射时因样品重新取向而产生的大量实验复杂性。因此，以完整且直观的方式给出了各向异性吸收的原理。
