Determination of the Molecular Arrangement Inside Cyanine Dye Aggregates by Magnetic Orientation

The average magnetic anisotropy of polystyrene in polymersomes self-assembled from poly(ethylene glycol)-b-polystyrene

Using the diamagnetic anisotropy of polymers for the characterization of polymers and polymer aggregates is a relatively new approach in the field of soft-matter and polymer research. So far, a good and thorough quantitative description of these diamagnetic properties has been lacking. Using a simple equation that links the magnetic properties of an average polymer repeating unit to those of the polymer vesicle of any shape, we measured, using magnetic birefringence, the average diamagnetic anisotropy of a polystyrene (PS) repeating unit, ΔχPS, inside a poly(ethylene glycol)-polystyrene (PEG-PS) polymersome membrane as a function of the PS-length and as a function of the preparation method. All obtained values of ΔχPS have a negative sign which results in polymers tending to align perpendicular to an applied magnetic field. Combined, the same order of magnitude of ΔχPS (10-12 m3 mol-1) for all polymersome shapes proves that the individual polymers are organized similarly regardless of the PS length and polymersome shape. Furthermore, the value found is only a fraction (∼1%) of what it can maximally be due to the random coiling of the polymers. We, therefore, predict that further ordering of the polymers within the membrane could lead to similar responses at much lower magnetic fields, possibly obtainable with permanent magnets, which would be highly advantageous for practical applications.

Cyanine dyes

Cyanine dyes are characterized by an odd number 2n + 3 of π-centers and 2n + 4 π-electrons (where n is the number of vinyl groups –CH = CH–). This special feature has a marked impact on their electronic structure and thus their equilibrium structure in the electronic ground state as well their color and electronic spectrum, respectively. Their first technical application was the use as spectral sensitizers in silver halide photography. Today they have numerous of applications in digital optical data storage, Computer-to-Plate lithographic printing plates, bio-analysis and medical diagnostics.

Harnessing the interparticle J-aggregate induced plasmonic coupling for surface-enhanced Raman scattering

The kinetic linkage between surface plasmon resonance absorption and surface-enhanced Raman scattering data highlights an effective pathway for harnessing the plasmonic coupling of gold nanoparticlesviaJ-aggregation in solutions.

Forcing aggregation of cyanine dyes with salts: a fine line between dimers and higher ordered aggregates

It is uncommon to read about cyanine dyes in the literature and not have their aggregation discussed. They are of high interest considering their propensity to undergo self-organization in aqueous solution, leading to interesting photophysical properties resulting from the formation of their dimers and higher ordered aggregates. Currently, the study of their aggregation is in high demand due to their diverse application range including dye-sensitized solar cells. However, their aggregation in high salt solutions is under studied, and the effect on aggregation in congruence with high ionic strength is often overlooked. In a previous study, our group established the role of specific ion effects and in particular the necessity of matching water affinity to induce aggregation of a cationic cyanine dye, thiazole orange. In order to advance the understanding of this topic, we present in this article the diverse aggregation of cyanine dyes, as a single monovalent salt can cause different aggregation responses in a variety of these dyes. We established via absorption spectroscopy combined with chemometric analyses that the inherent monomer-dimer equilibrium of a dye depends on its geometry. More interestingly, experimental data coupled with DFT calculations reveal that not only the geometry of a dye but also its charge location plays a role in the aggregate morphology formed by the interaction of a cationic cyanine dye and an anion. It is thought that contact ion pair formation and effective charge screening generated within that ion pair are responsible for aggregates with a greater order.

Giant magnetic susceptibility of gold nanorods detected by magnetic alignment

We have determined the magnetic properties of single-crystalline Au nanorods in solution using an optically detected magnetic alignment technique. The rods exhibit a large anisotropy in the magnetic volume susceptibility (Δχ(V)). Δχ(V) increases with decreasing rod size and increasing aspect ratio and corresponds to an average volume susceptibility (χ(V)), which is drastically enhanced relative to bulk Au. This high value of χ(V) is confirmed by SQUID magnetometry and is temperature independent (between 5 and 300 K). Given this peculiar size, shape, and temperature dependence, we speculate that the enhanced χ(V) is the result of orbital magnetism due to mesoscopic electron trajectories within the nanorods.

Selection of supramolecular chirality by application of rotational and magnetic forces

Many essential biological molecules exist only in one of two possible mirror-image structures, either because they possess a chiral unit or through their structure (helices, for example, are intrinsically chiral), but so far the origin of this homochirality has not been unraveled. Here we demonstrate that the handedness of helical supramolecular aggregates formed by achiral molecules can be directed by applying rotational, gravitational and orienting forces during the self-assembly process. In this system, supramolecular chirality is determined by the relative directions of rotation and magnetically tuned effective gravity, but the magnetic orientation of the aggregates is also essential. Applying these external forces only during the nucleation step of the aggregation is sufficient to achieve chiral selection. This result shows that an almost instantaneous chiral perturbation can be transferred and amplified in growing supramolecular self-assemblies, and provides evidence that a falsely chiral influence is able to induce absolute enantioselection.

Molecular organization of cylindrical sexithiophene aggregates measured by X-ray scattering and magnetic alignment

We have determined the internal organization of elongated sexithiophene aggregates in solution by combining small-angle X-ray scattering and magnetic birefringence experiments. The different aggregate axes can be probed independently by performing the experiments on magnetically aligned aggregates. We have found multiwalled cylindrical aggregates consisting of radially oriented sexithiophene molecules with pi-pi-stacking in the tangential direction, a structure that is considerably different from those previously found in other solvents. The aggregate morphology of this semiconducting material can thus be tuned by using different solvents, which offers the attractive perspective to steer chemical self-assembly toward nanostructures with desired functionalities, especially in combination with the alignment in a magnetic field.

Study of internal structure of meso-tetrakis (4-sulfonatophenyl) porphine J-aggregates in solution by fluorescence microscope imaging in a magnetic field

To determine the internal molecular arrangement of organic dye aggregates, a technique for observing the fluorescence microscope image of a solution consisting of dye aggregates in a magnetic field was developed. Using this technique, the fluorescence image of meso-tetrakis (4-sulfonatophenyl) porphine (TPPS) J-aggregates in a solution in a magnetic field of 10 T was observed. It was observed that individual rod-shaped TPPS aggregates (4-20 microm in length) were aligned parallel to the applied field. The polarized absorption spectra of the sample solution were also measured in the fields of up to 10 T. The spectra show the magnetic field dependence of the J-band intensity, reflecting the magnetic alignment of the aggregates. On the basis of the magnetic and optical properties obtained by the experiments, it was proposed that TPPS J-aggregates have a tube-like structure and are constructed from one-dimensional molecular arrays that are stacked parallel to the long axis of the tube.