Aligned 1-D nanorods of a π-gelator exhibit molecular orientation and excitation energy transport different from entangled fiber networks.

Linear π-gelators self-assemble into entangled fibers in which the molecules are arranged perpendicular to the fiber long axis. However, orientation of gelator molecules in a direction parallel to the long axes of the one-dimensional (1-D) structures remains challenging. Herein we demonstrate that, at the air-water interface, an oligo(p-phenylenevinylene)-derived π-gelator forms aligned nanorods of 340 ± 120 nm length and 34 ± 5 nm width, in which the gelator molecules are reoriented parallel to the long axis of the rods. The orientation change of the molecules results in distinct excited-state properties upon local photoexcitation, as evidenced by near-field scanning optical microscopy. A detailed understanding of the mechanism by which excitation energy migrates through these 1-D molecular assemblies might help in the design of supramolecular structures with improved charge-transport properties.

Self-assembly of optical molecules with supramolecular concepts.

Fabrication of nano-sized objects is one of the most important issues in nanoscience and nanotechnology. Soft nanomaterials with flexible properties have been given much attention and can be obtained through bottom-up processing from functional molecules, where self-assembly based on supramolecular chemistry and designed assembly have become crucial processes and techniques. Among the various functional molecules, dyes have become important materials in certain areas of nanotechnology and their self-assembling behaviors have been actively researched. In this short review, we briefly introduce recent progress in self-assembly of optical molecules and dyes, based mainly on supramolecular concepts. The introduced examples are classified into four categories: self-assembly of (i) low-molecular-weight dyes and (ii) polymeric dyes and dye self-assembly (iii) in nanoscale architectures and (iv) at surfaces.

Solvent-induced aggregation and cation-controlled self-assembly of tripodal squaraine dyes: optical, chiroptical and morphological properties.

The synthesis, characterisation, optical, chiroptical, aggregation, and alkaline earth metal cation assisted self-assembly properties of tripodal squaraine dyes have been described. In the tripodal geometry, these dyes exhibit three absorption bands around 650, 620 and 580 nm in contrast to the single, sharp absorption of a simple dye (SQ) at 640 nm. The fluorescence quantum yield of the squaraine dyes are 25-30 times lower when compared to that of SQ, which indicates intramolecular exciton interaction as a result of the confinement of the dyes. The evaporation of an acetonitrile solution of the dye resulted in the formation of vesicular objects as confirmed by AFM and TEM analyses. However, evaporation of an acetonitrile solution containing 10-12 % water gave short fibrous aggregates. In the presence of Ca(2+) or Mg(2+) ions, the dyes exhibit an intense and sharp absorption band at 547 nm with a concomitant decrease of the native absorption. Interestingly, the dye with chiral groups failed to give a circular dichroic signal during aggregation in solvent mixtures, whereas strong signals were observed in the presence of Ca(2+) and Mg(2+) ions. AFM and TEM analyses of the corresponding cation complexes revealed the formation of worm like nanohelices. However, addition of EDTA to the Ca(2+) or Mg(2+) complex exhibited a reversal of the absorption, emission, and circular dichroic spectra to that of the native dye, indicating decomplexation. AFM analyses revealed the transformation of the helices to particles. These observations reveal the difference in the nature and properties of the simple aggregates formed in solvent mixtures and those formed in the presence of cations. In the present study, we were able to establish the importance of specific cation binding in controlling the size, shape, and properties of the hierarchical assemblies of tripodal squaraines.

Controlled self-assembly of squaraines to 1D supramolecular architectures with high molar absorptivity.

A tailor made squaraine dye upon binding with Ca2+ self-assembles to form a spherical micellar assembly that reorganises to thermodynamically stable 1D cylindrical rods with high molar absorptivity.