Orientational Transitions of Liquid Crystals Driven by Binding of Organoamines to Carboxylic Acids Presented at Surfaces with Nanometer-Scale Topography

We recently reported that the orientations of nematic phases of 4-cyano-4‘-pentylbiphenyl (5CB), when supported on surfaces with nanometer-scale topography and oriented carboxylic acid groups, are strongly influenced by n -H 2 N(CH 2 ) 5 CH 3 bound to these surfaces. The changes in orientation of the liquid crystals were found to be triggered by amounts of bound n -H 2 N(CH 2 ) 5 CH 3 that were not measurable by optical methods such as ellipsometry (ellipsometric thicknesses <1 Å). Here we describe a detailed experimental investigation of this phenomenon. We report that the transition in orientation of the 5CB corresponds to an azimuthal rotation of 90° with no measurable change in the tilt of the liquid crystal out of the plane of the surface. The magnitude of the azimuthal anchoring energy of 5CB is determined to be substantially weaker when n -H 2 N(CH 2 ) 5 CH 3 is bound to the surface. We also report that it is possible to manipulate the orientational response of 5CB to bound n -H 2 N(CH 2 ) 5 CH 3 by controlling the areal density of carboxylic acid groups presented on the surface. This is accomplished by forming mixed monolayers from H 3 C(CH 2 ) 10 SH and HOOC(CH 2 ) 10 SH on the surface of the gold films. Finally, the orientational response of 5CB to vapor-phase concentration of n -H 2 N(CH 2 ) 5 CH 3 is found to be strongly influenced by mass transport of n -H 2 N(CH 2 ) 5 CH 3 to the surface, an effect that is exploited to create spatial gradients in the amount of n -H 2 N(CH 2 ) 5 CH 3 bound to a given surface. These spatial gradients can be imaged by using nematic 5CB.