Magnetic field induced alignment of cyanine dye J-aggregates

Fabrication of Dipole-Aligned Thin Films of Porphyrin J-Aggregates over Large Surfaces

We report a new approach for large-scale alignment of micron-sized J-aggregates of a derivative of porphyrin onto planar glass substrates. We applied a unidirectional nitrogen flow to an aqueous dye drop deposited onto a glass substrate to form an about 5 nm thick film of aligned J-aggregates over macroscopic surface areas up to several millimeters. The inter-aggregate distance is ∼500 nm, and it scales with the nitrogen pressure. We verified the film thickness and J-aggregate alignment using multimodal microscopy and spectroscopy techniques. Our technique is fast, simple, and cost-effective for producing large two-dimensional (2-D) arrays of aligned emitters.

Highly Oriented J-Aggregates of Nitroazo Dye and Its Surface-Induced Chromism

Highly oriented J-aggregates of a nitroazo dye were obtained in solid thin films on aligned poly(tetrafluoroethylene) surfaces. During film deposition on a friction-transferred poly(tetrafluoroethylene) layer, a sharp peak grew in the polarized absorption spectra around 613 nm, which was red-shifted 117 nm from the peak in dilute dichloromethane solution. The peak showed remarkable optical anisotropy: dichroic ratios D of up to 22 were observed, and the intrinsic D value should substantially exceed this value. These results indicate that the peak is attributable to highly oriented J-aggregates. On glass, however, H-like aggregates grew, exhibiting an absorption peak at 410 nm. Hence, the substrate surface induced the remarkable chromism observed as a 203 nm red shift.

Self-assembled coronene nanofibers: optical waveguide effect and magnetic alignment

To fabricate organic nanofibers that function as active optical waveguides with semiconductor properties, a facile procedure was developed to grow single crystalline nanofibers via π-π stacking of the polycyclic aromatic molecule, coronene, through solution evaporation on a substrate. The fabricated nanofibers with millimeter-scale lengths have well-defined shapes, smooth surfaces, and low-defect structures. The nanofibers are demonstrated to function as efficient active waveguides that propagate their fluorescence (FL) along the fiber axis over their entire length. We further demonstrate that the nanofibers can be highly aligned on the substrate when solution evaporation is conducted in a magnetic field of 12 T. The mechanism of the magnetic alignment can be elucidated by considering the anisotropy of the diamagnetic susceptibility of a single coronene molecule and the crystal structure of a nanofiber. Owing to the high degree of alignment, the nanofibers rarely cross each other, allowing for measurement of the waveguiding properties of single isolated nanofibers. The nanofibers propagate their FL of λ > 500 nm with a low propagation loss of 0-3 dB per 100 μm, indicating that the nanofibers function as sub-wavelength scale, low-loss waveguides. Thus, they are promising building blocks for miniaturized optoelectronic circuits.

Manipulation of micro- and nanostructure motion with magnetic fields

In this review we will focus on how magnetic fields can be used to manipulate the motion of various micro- and nanostructures in solution. We will distinguish between ferromagnetic, paramagnetic and diamagnetic materials. Furthermore, the use of various kinds of magnetic fields, such as homogeneous, inhomogeneous and rotating magnetic fields, is discussed. To date most research has focused on the use of ferro- and paramagnetic materials, but here we also describe the possibilities of magnetic manipulation of diamagnetic materials. Since the vast majority of soft matter is diamagnetic, this paves the way for many new applications to manipulate the motion of micro- and nanostructures.

Imaging capability of patterned liquid crystals

We demonstrate the ability to make high resolution arbitrary patterned optical retarders using liquid crystal polymer (LCP). Contact lithography is used to define unique LCP alignment domains. Patterned LCP retarders are imaged between crossed polarizers to determine pattern visibility as a function of feature size. It was determined that patterned retarders for wavelengths between 250 nm and 2500 nm can be constructed with feature sizes as small as 4 microm. We also showed that multiple patterns can be created on the same substrate using a combination of patterned LCP and opaque features. Our process has applications in displays, double-patterning lithography, and imaging polarimetry.

Guiding the self-assembly of a second-generation polyphenylene dendrimer into well-defined patterns

A second-generation polyphenylene dendrimer 1 is shown to self-assemble into nanofibers. To guide the formation of the dendrimer fibers into well-defined patterns, 1H,1H,2H,2H-perfluorodecyltrichlorosilane is grafted in the gas phase onto a silicon substrate. De-wetting of the solution on the nanopatterned surface results in the formation of a nanostructured template, into which fiber growth subsequently occurs under the constraints set by the de-wetted morphology.

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.

Magnetic Deformation of Self-Assembled Sexithiophene Spherical Nanocapsules

We report the experimental observation of magnetic field deformation of spherical nanocapsules, self-assembled from sexithiophene molecules, into oblate spheroids, confirming a long-standing theoretical prediction. The magnetically deformed objects can be trapped in a compatible organogel to make them suitable for further investigations and applications. Our results show that strong magnetic forces can be effectively used, in a contact-free manner, as a tool to control the self-organization of a whole class of functional organic molecules.

Spectroscopic studies of the interaction of bichromophoric cyanine dyes with DNA. Effect of ionic strength

Spectroscopic characteristics of a cyanine dye with two chromophores (biscyanine dye, BCD) in aqueous solutions and effects of NaCl and DNA upon these characteristics have been studied by optical absorption, circular dichroism (CD) and fluorescence spectroscopies. In homogeneous solutions, BCD is characterized by intense optical absorption (epsilon =1.33 x 10(5) M(-1) x cm(-1)) and weak fluorescence (phi(fl)=0.018) in the wavelength region greater than 600 nm. The dye forms H-aggregates at low concentrations (10(-6) M). NaCl stimulates the formation of both H- and J-aggregates of the dye at much lower dye concentrations, while DNA in low concentrations (<10(-6) M) stimulated the formation of just J-aggregates on the surface of the DNA molecule. Higher DNA concentrations induce the dye to disaggregate, and there exists an equilibrium between three dye forms: free monomers, J-aggregates and bound monomers, the maximum content of J-aggregates was observed at [DNA]/[BCD]=0.6+/-0.2 and total disaggregation at [DNA]/[BCD]=190+/-20. J-aggregates are characterized by phi(fl)=0.05 and bound monomers by phi(fl)=0.44. In the presence of NaCl, total disaggregation was observed at [DNA]/[BCD]=570+/-10 due to competition between Na(+) and the dye molecules for DNA electronegative binding sites.