Self-assembly And Properties Of Azobenzene Films

There has been considerable interest recently in the photo-isomerization of azobenzene compounds. The structure of azobenzene compounds can transfer from its usual state, Tran state, into Cis state by UV illumination, which can return to the Tran state by visible light or heat. The isomerization of azobenzene induces the change in the dipole moment of the azobenzene molecule, UV-Vis absorption spectra, volume, surface free energy, etc. Consequently, the alternation of surface free energy results in the change of surface wettability, indicated by contact angle (CA). The observed change of CA is only about 10(?) on azobenzene film in previous reports.In this paper, ultra thin films of azobenzene are prepared through a simple electrostatic layer-by-layer self-assembly and chemical route. Wettability of thin films is influenced by molecule structure of azobenzene and surface geometrical structures. By introducing microstructures on substrate, a superhydrophobic surface is successfully obtained. Moreover, a large reversible change of wettability is realized with the change of water CA up to 66°. The investigation has important value in the microliquid transmission, and absorption-separation of biology molecule.A rough surface with special geometrical structures is prepared on silicon wafer by photolithography and the induced coupling plasma deep etching technique. Azobenzene films are fabricated on these rough substrates. The Tran-Cis change of azobenzene is recorded by UV-Vis absorption spectra, geometrical structure of films is investigated through AFM, and wettability of films is studied by measuring CA. The result of experiment:1. PPAPE and PDAC are used as polyanion and polycation in this work, respectively, monolayer and multilayer are prepared on the rough substrate by an electrostatic layer-by-layer self-assembly process. Superhydrophobicity is obtained on the rough azobenzene monolayer. Under UV light irradiation, CA decrease 66(?) (from 152.6(?) to 86.1(?)) on the rough monolayer, and analogically the large change of CA is about 66(?)(from 153.8(?)to 87.6(?)) on the rough multilayer. The CA evolves back to the original superhydrophobic state after Vis light irradiation. The reverseble change has been repeated several times, and good reversibility of the surface wettability is observed through the alteration of UV and Vis irradiation.2. A kind of azobenzene with small molecule weight is assembled on the silicon substrate by chemical route. The as-prepared film shows the superhydrophobicity, which CA is about 151.9o. After exposing on the UV light, no obvious change of CA is observed.