Variations in molecular electronic structures related to conformational change are exceedingly attractive because of their key role in the understanding and development of functional processes in molecular electronics and biology. We observed, for the first time, the novel phase switching of a photoactive isomeric molecule, N-(2-mercaptoethyl)-4-phenylazobenzamide (Azo molecule) at a single-molecule level, which exhibits a distinctive change in the conductive characteristic under scanning tunneling microscope (STM) measurement. In comparison with the results obtained by the measurement of photoactive isomerization of the isolated Azo molecule, which was performed also for the first time, the observed characteristics are attributed to the results of the trans and cis phase transformation of the Azo molecule, under the condition of an external electric field and current flow. A specific point is that the potential landscape of the system is controllable by the electric field and provides a conformational stability with asymmetric bias dependence resulting in rectification.
The Langmuir-Blodgett technique has been applied to build optically homogeneous thin films of chemically solubilized single-wall carbon nanotubes (s-SWNTs) which possess good surface spreading properties at the air/water interface. Deposition can be performed in a layer-by-layer fashion up to 100 or more layers either by horizontal lifting or vertical dipping, allowing to readily control the film thickness. Their visible to near-infrared absorption spectra showing the characteristic features of semiconducting and metallic SWNTs prove the intactness of their one-dimensional electronic states during the preparation process. Polarized absorption spectroscopy and atomic force microscope (AFM) observation demonstrate that the tubes are oriented in the direction of the trough barrier (horizontal lifting) or in the dipping direction (vertical dipping). These are attributed to compression-induced or flow-induced orientation, respectively, the latter found to be much stronger than the former. The realization of homogeneous thin films of SWNTs with a controllable thickness and tube orientation should be an important basis for the future development of their scientific understanding and technological applications.
An amphiphilic anionic azobenzene derivative, soluble in water,
formed a monolayer on an aqueous
subphase containing a water-soluble polycation. The monolayers
were transferred onto solid substrate by the
Langmuir−Blodgett (LB) technique. XPS measurements showed that
ion exchange reaction proceeded almost
completely at the air−water interface and that the ratio of the
monomer unit of the polycation to the azobenzene
was almost unity. UV/vis absorption and IR measurements indicated
that the azobenzene photoisomerized
reversibly in the LB films on alternate illumination with UV and vis
light. Furthermore, a reversible
morphological change induced by light was observed in the LB films with
AFM. Before illumination, the
surface of the single-layer LB film was very smooth with a surface
undulation of less than 1 nm. On illumination
with UV light, however, a number of hills, with the height of ca. 5 nm
and the diameter of the base of ca. 100
nm, appeared on the film surface. These structures almost
disappeared on illumination with vis light. This
indicates that the widely accepted assumption that photoisomerization
should not change the two-dimensional
structures significantly does not hold in the present
case.
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