Self-assembly of an oligo(p-phenylenevinylene)-based molecule on an HOPG surface: insights from multi-scale simulation and STM observation

To gain knowledge of the most important weak interactions for supramolecular self-assembly and observe molecular structure for self-assembled architectures, the two-dimensional self-assembly of an oligo(p-phenylenevinylene)-based molecule (AS-OPV) on highly oriented pyrolytic graphite has been investigated. Accurate atomic configuration for the AS-OPV self-assembled pattern has been identified by means of multi-scale simulation combined with scanning tunneling microscopy (STM) experiments. The weak interactions which contribute to the formation of AS-OPV self-assembly are studied by analysis of non-covalent interactions existing in the system and theoretical calculation of their energy values. Investigation of the molecular structure of self-assembly and STM images at a certain temperature range is performed by molecular dynamics and density functional theory simulations. This work paves the way to explore the contribution of weak interactions for the self-assembly system, as well as providing a reference to observe the possible self-assembled structure at temperatures not convenient for direct experimental observation.

Unmasking bulk exciton traps and interchain electronic interactions with single conjugated polymer aggregates

For conjugated polymer materials, there is currently a major gap in understanding between the fundamental properties observed in single molecule measurements and the bulk electronic properties extracted from measurements of highly heterogeneous thin films. New materials and methodologies are needed to follow the evolution from single chain to bulk film properties as multiple chains begin to interact. In this work, we used a controlled solvent vapor annealing process to assemble single chains of phenylene-vinylene conjugated polymers into aggregates that can be individually spectroscopically interrogated. This approach allowed us to probe the effects of interchain coupling in isolated conjugated polymer nanodomains of controlled size. By assembling these aggregates from building blocks of both pristine MEH-PPV and MEH-PPV derivatives containing structure-directing ortho- or para-terphenyl inclusions, we were able to control the ordering of these nanodomains as measured by single aggregate polarization anisotropy measurments. Depending on the individual chain constituents, these aggregates varied from highly anisotropic to nearly isotropic, respectively facilitating or inhibiting interchain coupling. From the single chain fluorescence lifetimes, we demonstrated that these structure directing inclusions effectively break the phenylene-vinylene conjugation, allowing us to differentiate interchain electronic effects from those due to hyper-extended conjugation. We observed well-defined bathochromic shifts in the fluorescence spectra of the aggregates containing extensive interchain interactions, indicating that low-energy exciton traps in MEH-PPV are the result of coupling interactions between neighboring chain segments. These results demonstrate the power of the synthetic inclusion approach to control properties at not just the single chain level, but as a comprehensive approach toward ground-up design of bulk electronic properties.

Interactions of zeatin with gold ions and biomimetic formation of gold complexes and nanoparticles

We report here a simple one-pot synthesis for the preparation of gold nanoparticles biomimetically using zeatin nanostructures. Zeatin, a plant phytohormone was self-assembled into nanospheres. Those nanospheres transformed into nanoribbons over a period of time upon formation of zeatin-gold (III) complexes in the presence of hydrogen tetrachloroaurate. Further, upon heating, gold nanoparticles were formed due to mineralization in the presence of zeatin nanofibers. The effect of pH on the self-assembly of zeatin and the formation of gold nanoparticles, was investigated. We also compared the preparation of gold nanoparticles in the presence of zeatin nanoribbons, using a known reducing agent such as hydrazine, which resulted in loss of morphology control and alignment of the gold nanoparticles. Thus zeatin nanoribbons act as templates which allow for size as well as alignment control for the gold nanoparticles. The materials obtained were analyzed using FTIR, absorbance spectroscopy as well as by transmission electron microscopy, EDX, SEM and AFM. The method involved here is a mild, green-synthetic process, which could be used for facile preparation of morphology controlled gold nanoparticles and may open up new avenues for device fabrications for a wide range of applications, particularly in optoelectronics and sensors.

Structure and structural transition of chiral domains in oligo(p-phenylenevinylene) assembly investigated by scanning tunneling microscopy

OPV3-CHO molecules are employed to prepare assembly on highly oriented pyrolytic graphite, and the so-prepared assembly is investigated by scanning tunneling microscopy. In the assembly chiral domains are observed with various structures such as linear and windmill. The chiral structural formation, stability, transition, and possible unification are intensively studied. After thermal annealing, linear structure was the only structure. To achieve a unified assembly with a single structure, an efficient method is proposed by coadsorption of OPV3-CHO with selected molecules. For example, an assembly with side-by-side helix structure is formed by a simple coadsorption of OPV3-CHO with alkyl bromide (C(n)H(2n+1)Br, n = 15-18). The experiments by cocrystallization of OPV3-CHO/C(n)H(2n+1)X (X = Cl, Br, and I) show the important role of halogen bonding in formation of the uniform structure. The results are significant in understanding the intermolecular noncovalent interactions that dominate the surface structure and chirality.