Photoalignment of poly(di-n-hexylsilane) by azobenzene monolayer.: 2.: Structural optimization of the surface azobenzene monolayer

For the optimization of the surface-mediated photoalignment process of a poly(di-n-hexylsilane) (PDHS) film, a systematic investigation was conducted with respect to the design of the surface azobenzene (Az) monolayer. The preparative conditions and synthetic design of the Az monolayer were investigated in terms of exposure energy, lateral packing density, and length of n-alkyl tail attached to the Az units. The in-plane dichroic ratio of the Az monolayer was enhanced monotonically with increased exposure energy of the linearly polarized visible light (LPL) from 0.15 to 3.0 J cm(-2). A good correlation was obtained for the orientational order of PDHS with the dichroic ratio of the Az monolayer. The lateral packing density was varied by changing the level of compression in the LB deposition process. The highest orientational order of PDHS was obtained for the Az monolayer at ca. 0.4 nm(2) per Az unit. The alignment behavior of PDHS was further dependent on the tail length of Az unit. The most efficient in-plane control was achieved for the Az monolayer having an octyl (C8) tail. As a whole, the Az monolayer having the octyl tail prepared at 0.4 nm(2) per Az after exposure to 3.0 J cm(-2) of LPL yielded the highest anisotropy, leading to the highest order parameter of the PDHS film reaching ca. 0.7. Micropatterned irradiation through a photomask performed in the above-optimized conditions provided a locally addressed uniaxial alignment at a resolution of 6 mum.