About Supramolecular Assemblies of π-Conjugated Systems

Self-assembly of -bond assisted supramolecular architectures

We report on the self-assembly of a functionalized hexaazatriphenylene into supramolecular architectures where the single hexaazatriphenylene molecules are held together primarily through intermolecular hydrogen bonds between amide units. Wide and small angle X-ray scattering, polarized light microscopy, and differential scanning calorimetry revealed bulk self-organization into columnar structures. At the surfaces, scanning force microscopy experiments showed that it is possible to drive the self-organization from solutions of N-(2-ethylhexyl)-hexacarboxamidohexaazatriphenylene, towards either layers on a conductive surface like graphite or supramolecular anisotropic assemblies on an electrically insulating substrate such as muscovite mica. The growth of this latter type of architecture is primarily driven by the physical dewetting of the solution cast on the surface combined with intermolecular hydrogen bonds between the amide moieties exposed in the peripheral positions that lead to the formation of the columnar stack. Therefore, the anisotropic supramolecular azatriphenylene assemblies observed in the bulk have been also observed in thin films on a substrate poorly interacting with the adsorbate. In view of the interesting electronic properties of hexaazatriphenylene based architectures as n-type semiconductors, these results might be of interest for applications in the field of organic electronics.

Supramolecular Helical Columns from the Self-Assembly of Chiral Rods

Chiral-bridged rod molecules (CBRs) that consisted of bis(penta-p-phenylene) conjugated to an opened or closed chiral bridging group as a rigid segment and oligoether dendrons as flexible segments were synthesized and characterized. In the bulk state, both molecules self-assemble into a hexagonal columnar structure, as confirmed by X-ray scatterings and transmission electron microscopy (TEM) observations. Interestingly, these structures display opposite Cotton effects in the chromophore of the aromatic unit in spite of the same chirality (R,R) of the chiral bridging groups. The molecules were observed to self-assemble into cylindrical micellar aggregates in aqueous solution, as confirmed by light scattering and TEM investigations, and exhibit intense signals in the circular dichroism (CD) spectra, which are indicative of one-handed helical conformations. The CD spectra of each molecule showed opposite signals to each other, which were similar to those in the bulk. Notably, when the opened CBR was added to a solution of closed CBRs up to a certain concentration, the CD signal of the closed CBR was amplified. This implies that both molecules co-assemble into a one-handed helical structure because the opened chiral bridge is conformationally flexible, which is inverted to co-assemble with the closed CBR. These results demonstrate that small structural modifications of the chiral moiety can transfer the chiral information to a supramolecular assembly in the opposite way.

Diastereochemically controlled porphyrin dimer formation on a DNA duplex scaffold

DNA-porphyrin conjugates were designed and synthesized for the preparation of the conformationally controlled porphyrin dimer structures constructed on a d(GCGTATACGC)2. Porphyrin derivatives were introduced to the central TATpA sequence where p represents the phosphoramidate for the attachment of the free-base porphyrin (FbP) and zinc-coordinated porphyrin (ZnP), which allows contact of the two porphyrins in the minor groove. The porphyrin dimers were characterized using CD, UV-vis, steady-state, and time-resolved fluorescence spectroscopies, indicating that the porphyrins form face-to-face conformations. Also the co-facial conformation was confirmed by comparison with spectra of the non-self-complementary duplex containing one porphyrin moiety. Introduction of zinc into porphyrin moiety destabilized the duplex formation. Two diastereomers showed different thermal stabilities and affected the conformations of porphyrin dimers. The temperature-dependent assembly and the conformational change of the porphyrin dimer on the duplex DNA were observed in the UV-vis spectra, indicating that the dynamic movement of the porphyrin dimer occurs on the duplex. The results indicate that the porphyrin dimers of DNA-FbP conjugates are overlapped clockwise and are located in the minor groove of the usual B-form DNA backbone. The interaction and conformation of two porphyrin moieties are controlled by the following three factors: (1) temperature change during and after formation of the duplex porphyrins at lower temperature; (2) diastereochemistry of the phosphoramidates where porphyrins are connected via a linker; and (3) zinc ion coordination that destabilizes the interaction of porphyrins as well duplex formation.

Light harvesting and energy transfer in multiporphyrin-modified CdSe nanoparticles

Multiporphyrin-modified CdSe nanoparticles (CdSe-H2P) were prepared to elucidate the interaction between chromophores and luminescent semiconducting nanoparticles in the excited and ground states. The CdSe-H2P nanoparticles were obtained by place-exchange reactions of hexadecylamine-thiophenol-modified CdSe nanoparticles with porphyrin alkanethiols in toluene. The number of porphyrin molecules on the surface of a single CdSe nanoparticle increased with increasing reaction time to reach a saturated maximum of 21. The porphyrins as well as the core in CdSe-H2P can absorb UV/Vis radiation. Steady-state emission and emission-lifetime measurements reveal efficient energy transfer from the CdSe excited state to the porphyrins in the CdSe-H2P nanoparticles. The resulting porphyrin excited singlet state is not quenched by the CdSe core. These unique properties are in sharp contrast with those of multiporphyrin-modified metal and silica nanoparticles. Thus, semiconducting nanoparticle-multiporphyrin composites are highly promising as novel artificial photosynthetic materials.

Synthesis of substituted conjugated polymers: Tuning properties by functionalization

The review addresses features of special interest concerning two classes of functionalized semiconducting polymers: poly(aryleneethynylene)s (PAEs) bearing biomolecules as chiral nonracemic pendant groups and poly(phenylenevinylene)s (PPVs), which are fluorinated in various positions of the repeating units. Molecular design and choice of specific substituents, synthetic protocols, and the effect of functionalization on properties of the polymers both in solution and in the solid state are discussed.

Fluorescent DNA base modifications and substitutes: multiple fluorophore labeling and the DETEQ concept

There is an increasing need for fluorescent nucleic acid probes that are able to sense genetic variations without the application of enzymes. The incorporation of organic fluorophores either as DNA base modifications or as DNA base substitutions represents a powerful and versatile method for such new fluorescent DNA assays. Multiple labeling of oligonucleotides using several adjacent chromophore-modified DNA bases yields fluorescence enhancement and modulation that are sensitive to single-base mismatches in the complementary oligonucleotide. Charge transfer processes that cause fluorescence quenching are DNA-base mediated and occur over several base pairs distance. Our "DETEQ" setup, consisting of a fluorescence DNA base substitution and the charge acceptor as a second modification two base pairs away, allows the homogeneous detection of single-base mutations simply by fluorescence readout. This could lead to new DNA microarrays which are based on charge transfer processes and can be analyzed by commonly used fluorescence readout techniques.

Divergent approach for synthesis and terminal modifications of dendritic polyphenylazomethines

In the past, dendritic polyphenylazomethines (DPA) have been synthesized via the convergent method. However, the convergent method has problems such as difficult terminal-group modifications and an increased number of steps. Therefore, we synthesized the terminal-protected DPA to construct the dendritic structure via the divergent method. The combination of the convergent method and the divergent method easily achieved the rapid synthesis of a higher generation dendrimer. Furthermore, we succeeded in the synthesis of the water-soluble PEG-DPA via the divergent method using the ester-DPA.

X-ray microscopy study of chromonic liquid crystal dry film texture

Soft x-ray spectromicroscopy has been used to investigate the degree of the molecular alignment of sulfonated benzo[de]benzo[4.5]imidazo[2,1-a]isoquinoline[7,1], a lyotropic chromonic liquid crystal (LCLC). LCLC thin films cast from concentrated aqua solution (20%wt.) , aligned by shear flow and dried, show strong linear dichroism in their C-, N-, O-, S- K edge near edge x-ray spectra (NEXAFS). The carbon K edge has been used for quantitative evaluation of the orientational texture of the films at a submicron spatial scale. This has verified there is predominantly in-plane alignment of the LC director. To highlight the role of hydrophobic-hydrophilic interactions, two stereoisomers of the same dye has been synthesized with different positioning of terminal sulfonate groups, in the form of a mixture of isomers with sulfonate groups in 2,10 and 2,11 positions (Y104 compound) and in a 5,10-disulfo arrangement (Y105). Both compounds develop characteristic herringbone-type texture with similar domain sizes. Polarized optical microscopy and higher resolution x-ray microscopy show sinusoidal-like undulations of the molecular director, with occasional crisscross appearance. Such behavior is found to be consistent with earlier observation of striations, characteristic of the columnar phase. The drastic difference in the degree of undulation ( +/-15 degrees in Y104 and +/-7 degrees in Y105 films) and long period of undulation (approaching the film thickness) requires further analysis. It was also found that the degree of in-plane order within domains changes from 0.8 for Y104 to >0.9 in Y105 films.

Temperature-induced chiroptical changes in a helical poly(phenylacetylene) bearing N,N-diisopropylaminomethyl groups with chiral acids in water

A stereoregular poly(phenylacetylene) bearing an N,N-diisopropylaminomethyl group as the pendant (poly-1) changed its structure into the prevailing one-handed helical conformation upon complexation with optically active acids in water. The complexes exhibited induced circular dichroism (ICD) in the UV/Vis region of the polymer backbone. Poly-1 is highly sensitive to the chirality of chiral acids and can detect a small enantiomeric imbalance in these acids, in particular, phenyl lactic acid in water. For example, a 0.005% enantiomeric excess of phenyl lactic acid can be detected by CD spectroscopy. The observed ICD intensity and pattern of poly-1 were dependent on the temperature and concentration of poly-1, probably due to aggregations of the polymer at high temperature as revealed by dynamic light scattering and AFM. On the basis of the temperature-dependent ICD changes, the preferred chiral helical sense of poly-1 was found to be controlled by noncovalent bonding interactions by using structurally different enantiomeric acids.