Molecular orientation of single and two-armed monodendron semifluorinated chains on "soft" and "hard" surfaces studied using NEXAFS

Near-edge absorption fine structure (NEXAFS) measurements are used to probe the molecular orientation of semifluorinated (SF) mesogens, -(CH2)(x)(CF2)(y)F, which are attached to (i) the isoprene backbone of polyisoprene or a styrene-isoprene diblock copolymer ("soft" substrate), and (ii) a Au-covered solid substrate via a thiol link ("hard" substrate). The SF groups on both surfaces are oriented and on average are tilted from the sample normal. The tilt angle, [tau(F)-helix], of the fluorinated part of the SF group on each substrate is determined exclusively by the combination of x and y, increasing with increasing x and with decreasing y. Moreover, [tau(F)-helix] is found to be independent of the surface topology (flat surfaces vs surfaces covered with holes or islands of the copolymer), casting solvent, and the architecture of the SF group (single vs 2-armed monodendron). Comparing the orientation of the SF groups on both substrates reveals that [tau(F)-helix] is approximately 14 degrees higher on the "soft" substrate.