In this communication, we report a synthesis of anisotropic colloidal superparticles from CdSe/CdS semiconductor nanorods. These anisotropic superparticles are cylindrical disks or stacked-disk arrays. We attribute the major driving forces for controlling superparticle shape to the interparticle interactions between nanorods and solvophobic interactions between a superparticle and its surrounding solvents. According to their sizes (or volumes), the superparticles adopt either single- or multi-layered structures. In addition, these SPs exhibit linearly polarized emissions, demonstrating their potential role as useful components in devices such as polarized light emitting diodes and electrooptical modulators.

Single molecule fluorescence spectroscopy has been used to probe architecturally diverse and unique model oligomers containing exactly two or four perylene tetracarboxylic diimide (PTDI) units: linear foldamers lin2 and lin4, monocyclic complement cyc2, and concatenated foldable rings cat4. Linear, cyclic, and concatenated foldamers reveal that photoabsorption and excitation induces unfolding and refolding, generating colorful spectral switching from one spectral type to another. Foldamer architectures dictate the unfolding and refolding processes, and hence the spectral dynamics. As a result, linear tetramer exhibits active frame-to-frame spectral switching accompanying dramatic changes in colors, but a concatenated tetramer displays a multicolored composite spectrum with little or no spectral switching. Excited state dynamics causes spectral switching: an electronically decoupled PTDI monomer emits green fluorescence while electronically coupled PTDI pi-stacks emit red fluorescence, with longer pi-stacks emitting redder fluorescence. A key question we address is the excited-state delocalization length, or the exciton coherence length, in the pi-stacks, which has been proven difficult to measure directly. Using foldamers having controlled sequences, structures, and well-defined length and chromophore numbers, we have mapped out the exciton coherence length in pi-stacks. Single molecule fluorescence studies on chromophoric foldamers reveal that the maximum domain length is delocalized across just four pi-stacked PTDI dyes and no new pure color can be found for oligomers beyond the tetramer. Therefore, the range of fluorescent colors in pi-stacks is a function of the number of chromophores only up to the tetramer.

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Unique perylene diastereomeric linear and cyclic dimers were synthesized from twisted perylene monomers, revealing that pi-stacking stereoisomerism imparted specific intermolecular self-assembly and intramolecular folding. Only the homochiral twisted tetrachloroperylene monomers cyclized via a cooperative reaction, forming the homochiral diastereomers. The heterochiral tetrachloroperylene monomers proceeded through a stepwise reaction and yielded a linear heterochiral dimer, which equilibrated with the linear homochiral dimers. The linear homochiral dimers cyclized to produce the same cyclic homochiral diastereomers. These results demonstrated that homochiral and heterochiral self-assemblies were two distinct molecular codes, directing two specific chemical pathways. The homochiral cyclic dimers remain isomerically pure at -20 degrees C but can be interconverted to the heterochiral cyclic dimer meso compound at room temperature. The diastereomers were readily separated by HPLC. While driven by solvophobic forces, foldable linear dimers synthesized from the same twisted monomers using phosphoramidite chemistry folded into homodimer and heterodimer, confirming the inherent molecular codes, which were dictated by the perylene chirality, ultimately gauged the weak pi-stack forces, and directed self-assembly and folding.