Mechanistic Origins of Hierarchical Order in Organic Monolayers Deposited From Liquid Crystal Solvents

The orientational and chiral properties of monolayer films of the fluorescent dye 3-(2-benzothiazolyl)-7-octadecyloxy coumarin (BOC) on highly oriented pyrolytic graphite have been studied by scanning tunneling microscopy and polarized absorption spectroscopy. Films were deposited from two oriented liquid crystal solvents and one isotropic solvent using a sacrificial template method in which a solvent film applied in a first deposition step is displaced by BOC in a second deposition step. Fluid-phase orientational order parameters, monolayer order parameters, and short-range orientational and chiral autocorrelation were analyzed to determine the mechanisms by which liquid crystal solvents influence order in the deposited BOC films. By comparing BOC molecular packing characteristics and ordering statistics it is concluded that the substrate potential provides the dominant influence, with rotational and chiral symmetry among the six substrate-allowed configurations broken by much weaker liquid crystal forces. A combination of elastic and anchoring forces explains most of the observed long-range order, with templating effects making a small additional contribution at submicrometer length scales. The orientation of BOC molecules in solution appeared not to be a significant factor in affecting order in the adsorbed monolayer.