Controlling Anisotropic Properties Through Manipulation of Chiral Small Molecule Orientation

Chiral π-conjugated molecules offer new functionality to thin film technologies and represent a highly exciting and rapidly expanding area of scientific research. Due to the anisotropic functional properties of such materials such as the absorption and emission of circularly polarised light or the transport of spin-polarised electrons, the functionality, performance, and efficiency of chiral devices is critically dependent on the orientation of chiral molecules in thin films. Despite this, broadly applicable, simple methods to control molecular orientation of chiral materials in the bulk are yet to be developed. Here a novel approach is presented to control the orientation of a chiral small molecule (2,2’-dicyano[6]helicene, <b>CN6H</b>) in thin films: the use of organic (3,4,9,10-perylenetetracarboxylic dianhydride, PTCDA) and inorganic (copper(I) iodide, CuI) templating layers. All <b>CN6H</b> and templating layers were grown via organic molecular beam deposition (OMBD), ensuring good film quality and reproducibility. Using grazing incidence wide angle X-ray scattering (GIWAXS) measurements, it is shown that using 3,4,9,10-PTCDA templating layers, <b>CN6H</b> molecules adopt a ‘<i>face-on</i>’ orientation and self-assemble into upright supramolecular columns oriented with their helical axis perpendicular to the substrate. Alternatively, copper(I) iodide (CuI) templating induces the formation of supramolecular columns in an ‘<i>edge-on</i>’ orientation with their helical axis lying parallel to the substrate. Structural templating is further verified by 2D GIWAXS simulations of the <b>CN6H</b> crystal structure in face-on and edge-on orientations, demonstrating excellent agreement with experimental results. Through such control of orientation, it is shown that low- and high-energy chiroptical responses can be independently ‘turned on’ or ‘turned off’ as a function of molecular orientation. As such, the templating methodologies described here provide a simple way to engineer orientational control, and by association, anisotropic functional properties of chiral molecular systems for a range of emerging technologies.