Rational construction of self-assembly azobenzene derivative monolayers with photoswitchable surface properties

Photo-responsive azobenzene (ABZ) derivatives with different end groups (R) as photoswitchable molecules were employed to construct self-assembled monolayers (SAMs) on silicon substrate by using 3-glycidoxypropyltrimethoxysilane (GPTS) as the bridging molecules. The assembly process was optimized by changing various parameters, including the type and concentration of ABZ derivatives, reaction time, etc. The obtained SAMs were fully characterized and evaluated using UV spectroscopy, atomic force microscope (AFM), elllipsometer, static contact angle and X-ray photoelectron spectroscopy (XPS). It is found that the end group property of azobenzene derivatives is critical to the obtained SAMs’ photoresponsive properties. Compared with hydrophobic compounds (4-(4′-aminophenylazo) benzoic acid, ABZ-CF₃), the hydrophilic compounds (4-(4′-aminophenylazo) benzoic acid, ABZ-COOH) show excellent reversible photoswitching performance with a large contact angle change of 35° under optimized process, and the SAMs are removable by thermal treatment at 240 °C in air for only 5 min.