Nature's highly efficient light-harvesting antennae, such as those found in green sulfur bacteria, consist of supramolecular building blocks that self-assemble into a hierarchy of close-packed structures. In an effort to mimic the fundamental processes that govern nature's efficient systems, it is important to elucidate the role of each level of hierarchy: from molecule, to supramolecular building block, to close-packed building blocks. Here, we study the impact of hierarchical structure. We present a model system that mirrors nature's complexity: cylinders self-assembled from cyanine-dye molecules. Our work reveals that even though close-packing may alter the cylinders' soft mesoscopic structure, robust delocalized excitons are retained: Internal order and strong excitation-transfer interactions-prerequisites for efficient energy transport-are both maintained. Our results suggest that the cylindrical geometry strongly favors robust excitons; it presents a rational design that is potentially key to nature's high efficiency, allowing construction of efficient lightharvesting devices even from soft, supramolecular materials. supramolecular assembly | self-assembled excitonic nanoscale systems | photosynthesis | exciton theory | light-harvesting antennae systems T he most remarkable materials that demonstrate the ability to capture solar energy are natural photosynthetic systems such as those found in primitive marine algae and bacteria (1-10). Their light-harvesting (LH) antennae are crucial components, because they absorb the light and direct the resulting excitation energy efficiently to a reaction center, which then converts these excitations (excitons) into charge-separated states (1,4,11,12). Although the noncovalent interactions that link the individual molecules within the LH antennae are weak, the excitation transfer interactions between the molecules are relatively strong; new excited states, so-called Frenkel excitons (13), are generated that are delocalized over a number of molecules (1). These delocalized excitons are key to nature's efficiency and are therefore of high interest (14)(15)(16)(17)(18)(19)(20).To create such efficient LH systems, nature assembles molecular subunits into individual supramolecular structures, which are then further assembled into close-packed superstructures (1, 4, 7-10, 12, 21). This hierarchical assembly is a generic motif of nature's photosynthetic systems. As with natural systems, assembling artificial LH devices from supramolecular structures will require close packing into hierarchical assemblies to maximize the amount of absorbed light (19). Therefore, key to our ability to tune materials properties for efficient LH applications is a basic understanding of the role of each level of the hierarchy: from the individual molecule, to the individual supramolecular building block, to the close-packed assembly. Whereas the role of the individual molecules in the excitonic properties of the building blocks is well-studied (1, 7-10, 20, 22-37), the effect of structural hierar...

We demonstrate bowtie apertures that were designed and fabricated by a lift-off process to optically trap individual, 30 nm, silica-coated quantum dots (scQD). Simulations and experiments confirm the trapping capability of the system with a relatively low continuous wave trapping flux of 1.56 MW/cm2 at 1064 nm. Additionally, the scQD emits upon trapping via two-photon excitation from the trapping laser due to strong field enhancement inside the aperture. This system is an exciting platform for studying light–matter interactions and mulitphoton processes in single emitters.

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Internal water molecules are important to protein structure and function, but positional disorder and low occupancies can obscure their detection by x-ray crystallography. Here we show that water can be detected within the distal cavities of myoglobin mutants by subtle changes in the absorbance spectrum of pentacoordinate heme, even when the presence of solvent is not readily observed in the corresponding crystal structures. A well defined, non-coordinated water molecule hydrogen bonded to the distal histidine (His64) is seen within the distal heme pocket in the crystal structure of wild type (wt) deoxymyoglobin. Displacement of this water decreases the rate of ligand entry into wt Mb, and we have shown previously that the entry of this water is readily detected optically after laser photolysis of MbCO complexes. However, for L29F and V68L Mb no discrete positions for solvent molecules are seen in the electron density maps of the crystal structures even though His64 is still present and slow rates of ligand binding indicative of internal water are observed. In contrast, time-resolved perturbations of the visible absorption bands of L29F and V68L deoxyMb generated after laser photolysis detect the entry and significant occupancy of water within the distal pockets of these variants. Thus, the spectral perturbation of pentacoordinate heme offers a potentially robust system for measuring non-specific hydration of the active sites of heme proteins.

We monitored the occupancy of a functionally important non-coordinated water molecule in the distal heme pocket of sperm whale myoglobin over the pH range 4.3-9.4. Water occupancy was assessed by using time-resolved spectroscopy to detect the perturbation of the heme visible band absorption spectrum caused by water entry after CO photodissociation (Goldbeck, R. A., Bhaskaran, S., Ortega, C., Mendoza, J. L., Olson, J. S., Soman, J., Kliger, D. S., and Esquerra, R. M. (2006) Proc. Natl. Acad. Sci. U. S. A. 103, 1254 -1259). We found that the water occupancy observed during the time interval between ligand photolysis and diffusive recombination decreased by nearly 20% as the pH was lowered below 6. This decrease accounted for most of the concomitant increase in the observed CO bimolecular recombination rate constant, as the lower water occupancy presented a smaller kinetic barrier to CO entry into the pocket at lower pH. These results were consistent with a model in which the distal histidine, which stabilizes the water molecule within the distal pocket by accepting a hydrogen bond, tends to swing out of the pocket upon protonation and destabilize the water occupancy at low pH. Extrapolation of this model to lower pH suggests that the additional increase in ligand association rate constant observed previously in stopped-flow studies at pH 3 may also be due in part to reduced distal water occupancy concomitant with further His 64 protonation and coupled protein conformational change.

Fabrication of large area plasmonic nanoparticle grating structure on silver halide based transmission electron microscope film and its application as a surface enhanced Raman spectroscopy substrate

Colloidal CdSe quantum dot (QD) core ensembles were photodimmed and allowed to recover in the dark using ambient thermal energy at a range of temperatures. Nonlinear thermal recovery is well described by a stretched exponential function, and further analysis yields an underlying probability distribution of rate constants. Casting the rate constants as a collection of first-order activated processes provides an activation barrier probability distribution with significant density at room-temperature thermal energy that peaks at 200 meV before decaying to zero. This treatment for the recovery transition intuitively describes the distributed kinetics observed and complements commonly proposed blinking mechanisms.