Research - Chiroptics
Measurement
Computation
New Chiroptical Effects
In conjunction with chiroptical imaging, we must gain a deeper understanding of the chiroptical properties of simple, organized media - single crystals. We set out to influence the chemists understanding of chiroptics by measuring the optical rotation tensor of a crystal made from a tetrahedral carbon compound with four chemically equivalent ligands that are related by a symmetry operation of the second kind both in the gas phase and in the crystalline state. In other words, we set out to measure the optical rotation of a strictly achiral compound of the form CL4 such as S4 symmetric pentaerythritol (C(CH2OH)4). Kacey Claborn has recently succeeded in determining its optical rotation tensor (Figure 1).
Figure 1. (a) Measured rotations, (b) optical rotation tensor, and (c) most rotatory direction in pentaerythritol.
All quantum mechanical calculations of the optical rotation of molecules have reduced second rank tensors to traces that can be conveniently compared with average rotations from solution. Interpreting the orientational dependence of chiro-optical properties is an enormous hole in the science of molecular chirality, even for a molecule as small as pentaerythritol. In order to interpret our measurements it is essential that we have computational tensor elements for comparison. Christine Isborn and Kacey have undertaken a quantum computational investigation of the optical rotatory power of the water molecule, the simplest optically rotatory compound. By applying classical models of optical rotation to the limited numbers of excitation in H2O (Figure 2), we were able to develop an intuitive understanding of the optical rotatory power of H2O from the computed transition moments.
Figure 2. Classical Fresnel interpretation of optical rotation for the B1 and B2 transitions of water in wave vector directions that are related to one another by a mirror plane that bisects the polar axis of the electric dipole moment. The signs of the induced moments are all interchanged to maintain the fixed phase relationships.
Kacey has applied our circular extinction imaging microscope (patent pending) to dyed crystals and in this way discovered new chiroptical effects that are neither optical rotation or circular dichroism. Chicago sky blue (CSB) dyed LiKSO4 crystals have mirror image related faces that potentially incorporate the conformationally chiral guest molecules enantioselectively during growth (Figure 3). Although mirror image related domains in the twinned LiKSO4 crystals displayed marked contrast, the signal changed sign with inversion of the sample with respect to the light path, a transformation inconsistent with circular dichroism. We proposed a model whereby the anisotropy of the host contributes a linear component to the polarization state of light, attenuating right and left circularly polarized light (CPL) equal amounts in opposite directions. A bias therefore becomes manifest in the interaction of rod-like absorbing dipoles and the two elliptical light forms. Unlike intrinsic CD, anomalous circular extinction (ACE effect) does not convey absolute configuration but may be used instead to determine the absolute orientation of adsorbed dye molecules and thereby resolve an ambiguity present in polarized absorption measurements.
Figure 3. Chicago sky blue dyed LiKSO4 crystals. (top) Photograph of Chicago sky blue dyed LiKSO4 in polarized light along the direction of the arrow. (bottom) Circular extinction image of dotted region above. Signal is apparent in the {011} growth sectors of the host. Arrows are the direction of the transition dipole moments of the dyes.