Chirality, or handedness, associated with molecular systems plays important roles in chemistry, perhaps most notably in biomolecular recognition processes and pharmacological function. Most chemistry students are introduced to the concept of chirality through demonstrations of the interaction of chiral molecules with polarized light manifested as an "optical rotation" (with a well-defined direction and magnitude) which gives rise to the familiar "R" and "S" designations of chiral centers in molecular systems. While the intrinsic molecular parameters that control the dissymmetric light-matter interaction in chiral systems are well understood, the extrinsic factors (coupling to solvent or matrix) remain largely unexplored. The fundamental issue is that much of the detailed information on the influence of local environment on the molecular chiroptical response is obscured through necessary ensemble averaging inherent in conventional (bulk) absorption (circular dichroism) or non-resonant scattering (optical rotatory dispersion) measurements. The principal aim of this project is to use single-molecule chiroptical spectroscopy as a tool for probing the "inhomogeneous broadening" of chiroptical dissymmetry in the condensed phase. This proposed cross-disciplinary research has high fundamental scientific impact and will have a technological impact in the development of advanced display technologies, optical storage devices, and in asymmetric synthesis. The proposed project also serves as an attractive vehicle for broadening the participation and retention of students. Minority and underrepresented students will be recruited for this research project. During summer, two middle or high school students from local area schools will be recruited to participate in this research, thus creating advanced opportunities for pursuing advanced work in science and mathematics. THe group will also increase the interaction between UMass Amherst Chemistry and the middle/high students and science teachers in the Five College area school districts and to provide opportunities for advanced work in science. Such opportunities will help enhance interest in science and mathematics at the middle and high school levels.
Over the past 20 years, single-molecule spectroscopy - the study of the interaction of light with isolated molecular species - has yielded fantastic new insights into dynamics and mechanisms of a wide variety of chemical processes in materials and life sciences. Our goal was to apply this same methodology to the study of chirality - the peculiar and fundamental aspect of molecular systems to show a "handed-ness" (right OR left) at the single-molecule level. The fundament question we sought to address was whether a collection of single chiral molecules, whose ensemble (bulk-average) chiroptical signature is known, might individually show deviations or flucutations from the ensemble average. Such information could provide new insights into the emergence of, for example, preferred left handed-ness for amino acids, as well as guide development for novel optical-electronic devices using circular polarized light. Moreover, we anticipated that this research would result in new insights into how to control or tune molecular chiroptical response by specific (fixed) molecular orientation. Finally, as a potential analytical technique, the instrumentation and methodology developed in this research could provide new routes to fast chiral screening of reaction products requiring only picomoles of sample. We faced several significant technical challenges in this interdisciplinary work. First, as nearly all single-molecule studies use visible molecular fluorescence as the information-carrying signal, we needed to synthesize chirally pure organic molecules with robust fluorescence. Second, we had to design and implement an experimental strategy for detecting the (weak) differential absorption of left- and right-circularly polarized light of a single chiral molecule, while also taking great care to avoid artifactual results from residual linear polarization in the excitation beam. Finally, and perhaps most difficult, is the fact that the chiroptical response (characterized by preference for absorbing left- or right-circularly polarized light) is intimately tied with molecular orientation. Thus any meaningful connection of our experimental results with theoretical calculations requires knowledge of the orientation of the chiral axis in the laboratory frame. Using a generalized extension of coupled antenna theory, we showed how this determination could be made by analysis of the "antenna patterns" generated from a slightly defocused image of the single molecule fluorescence. What did we learn? We showed how single-molecule spectroscopy techniques could be applied to the study of chiroptical phenomena. With this technique, we were able to learn about some of the dynamical behavior resulting in fluctuations in chiroptical dissymmetry that derive from changes in local environment and/or molecular orientation. We demonstrated further some of the interesting optical phenomena associated with coupled chiral chromophores showing in some cases what appear to be non-additive coupling of optical rotatory strength between the two chromophores. Finally, this research program provided invaluable support and training for both graduate students and undergraduates, resulting in two Ph.D. dissertations awarded for work supported by this grant.
