Tunable Induced Circular Dichroism in Thin Organic Films

In this work, we study the induced optical activity (OA) of spin-coated molecular films made of a pristine optically inactive laser dye (Rhodamine 110) in the visible range. The OA is brought about by a "transparent" chiral modifier (1,1′-Bi-2-naphthol). For the first time, it was experimentally shown how two separate mechanisms yield induced circular dichroism (ICD) in such a film and how they are describable through a model. In one, ICD occurs via a direct interaction between dye and modifier molecules. In the other, ICD takes place in optical transitions of present dye aggregates. It is proposed that this is caused by a chiral bias through an enantiomeric excess in inherently chiral and optically active dye aggregates promoted by the chiral modifier in a sergeant-soldier manner. Further, the strength of ICD strongly depends on the ratio between the modifier and the dye as well as on the preparation conditions. The resulting high tunability of the OA paired with a straightforward film preparation opens up a route for fabrication of homogeneous thin films with customized OA.