The Chemical Structure, Dynamics and Mechanisms Program in the Chemistry Division at the National Science Foundation supports Professor Nicholas Turro of Columbia University, who will study electronic and nuclear spin pair recognition in supramolecular systems, specifically hydrogen and water molecules encapsulated inside fullerenes ("buckyballs"). The synthetic and spectroscopic investigations in this work will determine how, on the atomic level, two or more small molecules recognize and communicate with each other. The novelty of the approach in this work is to create strong spin polarization in the encapsulated molecules, to enhance the sensitivity of the magnetic resonance signals. Professor Turro's group also plans to label mRNA in living cells in a unique way, with fluorescent quantum dots.
Professor Turro has a keen interest in attracting young people to science, and to chemistry in particular. He and his research group have developed websites, instructional videos and books to enhance chemical knowledge across all age groups. He has multiple long-standing collaborations with scientists from all corners of the globe, and he has established a multi-user-friendly shared instrumentation laboratory for spin chemistry and physical organic chemistry studies at Columbia.
In our research program we view photons (light) both as a reagent for initiating chemical reactions and as a tool to image molecules in space and time. Light absorption generates electronic excited states, which can undergo a variety of reactions. For example, we have developed photo-initiator systems to initiate free radical polymerizations that are useful in making polymer coatings on surfaces. The imaging of a single molecule is an indispensable tool for advancing our understanding of both chemical and biological systems. Because fluorescence imaging is one of the most sensitive detection techniques, it is ideally suited for single molecule spectroscopy. The main problem in fluorescence imaging at the single-molecule level is the poor photo-stability of organic fluorophores. We synthesized and tested stabilized fluorophore systems based on the popular fluorescent molecule, Cy5, which was covalently linked to stabilizers, such as the triplet state quencher cyclooctatetraene. Image 1 shows an example. We found that these stabilized fluorophores performed significantly better than conventional Cy5 fluorophores, and they are less prone to photo-degradation. Most cellular components of living organisms are held together by supramolecular forces (non-covalent bonds). Interaction and communication between molecules and molecular assemblies separated by supramolecular structures is critical for the cell to function. Studying supramolecular host-guest systems is ideally suited to enhance our understanding of these interactions. For example, we have observed, using time-resolved Electron Paramagnetic Resonance (EPR), that a ketone (thioxanthone), when excited with light pulses, generates spin-polarized triplet states, which transfer their polarization to a stable radical (nitroxide). This spin-polarization transfer is faster when the molecules are brought together by supramolecular interactions, even when separated by the molecular wall of a supramolecular container (octa acid). Image 2 illustrates how two molecules communicate through this molecular wall. Small molecules, such as H2 and H2O, have two nuclear spin isomers, which interconvert rapidly under normal conditions. However, if isolated by encapsulation inside fullerenes, such as C60 or C70, inter-conversion is slower. This allows for a detailed quantum dynamic study of the rotational transitions using inelastic neutron scattering, far-infrared spectroscopy, and cryogenic nuclear magnetic resonance. As an example, image 3 shows an encapsulated water molecule inside C60 and the H2O quantized rotational energy levels. To achieve these diverse research results, we have built an interdisciplinary network of over 40 national and international senior collaborators from academic institutions and industry, who have participated in the work described here. The results of this NSF funded research are documented in 60 publications in scientific journals.
