GOALI: Chiroptical Anisotropy

New York University, New York NY

Investigators

Abstract

TECHNICAL The two-century struggle to measure chiroptical properties of organized, anisotropic media leaves an enormous hole in the science of molecular and materials chirality. What little has been learned during the past three decades has come haltingly. In order to unravel averaged, pseudo-scalar solution chiroptical effects into quantities that can be compared with electronic structures of real molecules, a database of molecular crystal tensors is required. They must be measured using a methodology that is robust, can be easily adopted, and can generate the quantity of data that enables comparison, analysis, and understanding. Mueller matrix imaging polarimetry is the solution because all linear optical properties can be determined simultaneously, it can assay depolarization in imperfect samples, and is suited to the treatment of non-normal incidence analytically. A fast device without moving optical components will require the synchronous operation of four photoelastic modulators. This has never been achieved but will be carried out with Hinds Instruments of Hillsboro, Oregon, a leading developer of photoelastic modulators and polarimeters, through collaboration of postdoctoral researchers in Oregon and New York. The need for a commercial, turnkey polarimeter for oriented materials is essential because chiroptical anisotropy is a chasm too large for one group to bridge. With this device, the optical rotation of isomorphous molecular crystals that lend themselves to the interpretation of small structural perturbations will be measured. To avoid complications associated with excitonic interactions in resonance, the anisotropy of circular dichroism of "oriented gases" of dyes in host crystals will be studied. Mueller matrix microscopes and polarimeters are also applicable to meso-structured materials such as cholesteric liquid crystals and chiral sculpted meta-materials. NON TECHNICAL Exactly two hundred years ago (1811) François Arago first observed the rotation of the plane of light polarization passing through a crystal of quartz along the direction of highest symmetry. It is been said that no phenomenon "has had so profound an effect on chemical thought as that of natural optical rotatory power" (Liehr, 1954. Unfortunately, since that time it has been almost impossible to measure optical rotation in organized media like crystals along general directions because the electromagnetic field of light suffers greater perturbations in low-symmetry environments that mask the phenomenon of interest. Thus, we remain ignorant about the orientation dependence of optical rotation in molecules, a fundamental light-matter interaction. Our project is aimed at developing an instrument for measuring the polarization state of light in any medium quickly and accurately enough so that we can derive the essential quantities. The device is based on photoelastic modulators that can change the polarization state of light at a rate of ~50,000 times per second. Using four such modulators, built by our GOALI partner, Hinds instruments, we can generate a complex signal that can be treated by the mathematical techniques of one of Arago's colleagues', Fourier. We have established a relationship with the Bronx Academy of Science and Technology, an underserved public high school with 98% native Spanish speakers. We provide, in addition to research opportunities in our lab during the summer and academic year, SAT tutoring, an advantage commonly exercised in wealthy school districts. We are convinced that creating scientists, especially those from underrepresented groups, requires first developing within students, one-by-one, scientific identities, and ensuring that the basics are attended.