[ Instrument Network Instrument R & D ] As an photochromic molecule, azobenzene molecules can realize the mutual conversion between cis and trans under the irradiation of ultraviolet and visible light. The use of molecular circuits to study the isomerization of azobenzene molecules at the single-molecule level can not only observe the response of a single molecule to external stimuli in real time, study its dynamics, but also hope to achieve applications such as single-molecule switches, single-molecule memory, Device miniaturization.
There are cis (Z) -trans (E) -isomers (can also be expressed as cis-trans). The trans form is orange-red prism crystal, the vapor is dark red, soluble in ethanol and acetic acid, but insoluble in water. Cis is an orange-red flake crystal, which is unstable and slowly becomes trans at normal temperature. Azobenzene is toxic and flammable; it is reduced to hydrogenated azobenzene under alkaline conditions, reduced to aniline in zinc and acetic acid, and oxidized with chromium dioxide in acetic acid to form azobenzene oxide. Industrially, it is prepared by reacting nitrobenzene with sodium hydroxide and zinc. Used in the manufacture of dyes and rubber accelerators.
Recently, Meng Linan, a PhD student in the SF10 group of the Institute of Physics of the Chinese Academy of Sciences / Beijing National Research Center for Condensed Matter Physics, under the guidance of researcher Meng Sheng and Guo Xuefeng, a professor at the School of Chemistry and Molecular Engineering, Peking University, worked with Guo Hong's group and the Institute of Physics Zhang Guangyu's team and other researchers have discovered that the light and electric fields in graphene-based single-molecule devices can effectively regulate the structure and transport properties of a single azobenzene molecule, and reveal the internal physical mechanism of azobenzene molecule isomerization. Related results were published in Nature Communications.
They and their collaborators designed and synthesized molecules with terphenyl as the main chain and azobenzene as the side chain, modified amino groups at the ends, and connected the molecules between the graphene electrodes through amide bonds. In the two conformations of the trans and cis, the molecule not only has a large difference in molecular structure, but also the projection of the dipole moment along the main chain direction is very different. In single-molecule devices, they investigated the response of azobenzene molecule transport signals to bias voltage and light. Changes in molecular structure affect the orbital energy levels of the molecules, which are then manifested by changes in conductance.
Graphene is a two-dimensional carbon nanomaterial with hexagonal honeycomb lattice composed of carbon atoms with sp² hybrid orbitals.
Graphene has excellent optical, electrical, and mechanical properties, and has important application prospects in materials science, micro / nano processing, energy, biomedicine, and drug delivery. It is considered to be a revolutionary material in the future. Physicists Andre Gem and Konstantin Novoselov of the University of Manchester, England, successfully separated graphene from graphite by micromechanical exfoliation, and thus won the 2010 Nobel Prize in Physics. The common powder production methods of graphene are mechanical peeling method, redox method, SiC epitaxial growth method, and the thin film production method is chemical vapor deposition (CVD).
Studies have found that azobenzene molecules undergo a conformational change from trans to cis under a light field or an electric field in a certain direction, that is, light / electric field induces isomerization of azobenzene molecules. Combining theoretical calculations, they found that because the dipole moments of the azobenzene molecules along the main chain direction under the trans and cis structures are different, the electric field's ability to regulate the molecular energy is different. In a certain electric field direction, as the electric field strength increases, the energy difference ΔE = Ec-Et of the cis-structure and trans-structure of the azobenzene molecule gradually decreases. When the electric field strength is increased, the energy of the cis-structure will be Below the energy of the trans structure, the cis structure is more stable. In the opposite direction of the electric field, the energy difference between the cis-structure and the trans-structure gradually increases with the increase of the electric field intensity, and the trans-structure is always a stable structure. At the same time, they also realized the conversion of azobenzene molecules between trans and cis under different wavelengths of light.
Azobenzene compounds contain a conjugated π system and have strong absorption in the ultraviolet to visible red light bands. Azobenzene compounds show a strong set of π-π * transitions in the ultraviolet region and a weak set of n-π * transitions in the visible region. For most azobenzene compounds, trans isomers can be efficiently converted to cis isomers under ultraviolet light irradiation conditions, and homeoisomers can be completely reversible under heating or visible light irradiation conditions. Conversion to the trans isomer.
This work not only combined the experimental and theoretical calculations to achieve the regulation of light / electric field on the conformation of azobenzene molecules, but also a new type of "gate voltage". At the same time, single-molecule memory and single-molecule optical switches can be realized through molecular structure regulation.
Source: Encyclopedia, Institute of Physics
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