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Fluorescent imaging of single helicene molecules is applied to study the optical activity of chiral fluorophores. In contrast to the previous report by Hassey et al. (Science 2006, 314, 1437), the dissymmetry factors of single chiral fluorophores are found not to differ significantly from the bulk value of |g| < 10^{−4} at 457 nm. Linear dichroism and birefringence of the dichroic mirror inside the fluorescence microscope change the polarization state of the incoming laser beam significantly; i.e., circular polarized light sent into the microscope becomes highly elliptically polarized after reflection from the dichroic mirror. Compensation for this effect should be made to avoid artifacts brought by linear dichroism in single immobilized molecules., Chemistry and Chemical Biology, Physics

We report enhanced optical Faraday rotation in gold-coated maghemite (γ-Fe2O3) nanoparticles. The Faraday rotation spectrum measured from 480-690 nm shows a peak at about 530 nm, not present in either uncoated maghemite nanoparticles or solid gold nanoparticles. This peak corresponds to an intrinsic electronic transition in the maghemite nanoparticles and is consistent with a near-field enhancement of Faraday rotation resulting from the spectral overlap of the surface plasmon resonance in the gold with the electronic transition in maghemite. This demonstration of surface plasmon resonance-enhanced magneto-optics (SuPREMO) in a composite magnetic/plasmonic nanosystem may enable design of nanostructures for remote sensing and imaging of magnetic fields and for miniaturized magneto-optical devices., Chemistry and Chemical Biology, Physics, Other Research Unit

This paper describes the formation and electrical properties of a new Hg-based metal−molecules−metal junction that incorporates charged redox sites into the space between the electrodes. The junction is formed by bringing into contact two mercury-drop electrodes whose surfaces are covered by COO^{−}-terminated self-assembled monolayers (SAMs) and immersed in a basic aqueous solution of Ru(NH_3)_6Cl_3. The electrical behavior of the junction, which is contacted at its edges by aqueous electrolyte solution, has been characterized electrochemically. This characterization shows that current flowing through the junction on the initial potential cycles is dominated by a redox-cycling mechanism and that the rates of electron transport can be controlled by controlling the potentials of the mercury electrodes with respect to the redox potential of the Ru(NH_3)_6^{3+/2+} couple. On repeated cycling of the potential across the junction, the current across it increases by as much as a factor of 40, and this increase is accompanied by a large (>300 mV) negative shift in the formal potential for the reduction of Ru(NH_3)_6^{3+}. The most plausible rationalization of this behavior postulates a decrease in the size of the gap between the electrodes with cycling and a mechanism of conduction dominated by physical diffusion of Ru(NH_3)_6^{3+/2+} ions (at larger interelectrode spacing), with a possible contribution of electron hopping to charge transport (at smaller interelectrode spacing). In this rationalization, the negative shift in the formal potential plausibly reflects extrusion of the solution of electrolyte from the junction and an increase in the effective concentration of negatively charged species (surface-immobilized COO^{−} groups) in the volume bounded by the electrodes. This junction has the characteristics required for use in screening and in exploratory work, involving nanogap electrochemical systems, and in mechanistic studies involving these systems. It does not have the stability needed for long-term technological applications., Chemistry and Chemical Biology, Physics

A theory is presented for how magnetic nanostructures can catalyze intersystem crossing in molecular radical pairs. Magnetic field gradients near physically realistic nanostructures are strong enough to induce a relative reorientation of two electronic spins in <1 ns, overwhelming nuclear hyperfine coupling as a driver of intersystem crossing. Nanomagnetic control of intersystem crossing represents a form of heterogeneous catalysis that does not require molecular contact, but only short-range magnetic coupling., Chemistry and Chemical Biology, Physics