Tailoring the Weight of Surface and Intralayer Edge States to Control Lumo Energies.

The energies of the frontier molecular orbitals determine the electronic and optical properties in organic films, which is crucial for their application as organic semiconductor materials, e.g., in organic solar cells, and strongly depend on the morphology and supramolecular structure. The impact of the latter two properties on the electronic energy levels relies primarily on nearest neighbor interactions, which are difficult to study in thin films due to their nanoscale nature and heterogeneity. Here  we present an automated method for fabricating layered thin films with a tailored ratio of surface to bulk sites and a controlled extent of domain edges within the layers, both of which  we use to control nearest neighbor interactions. This method uses a Langmuir-Schaefer-type rolling transfer of Langmuir layers (rtLL) to minimize flow during deposition of rigid Langmuir layers composed of π-conjugated molecules. Using UV-vis absorption spectroscopy, atomic force and transmission electron microscopy  we show that the rtLL method significantly advanced the deposition of multi-Langmuir layers and enables the production of films with highly defined morphology. The variation in nearest neighbor interactions thus achieved and the resulting systematically tuned lowest unoccupied molecular orbital (LUMO) energies (determined via square wave voltammetry) enabled us to establish a handy model that functionally relates the LUMO energies to a morphological descriptor, allowing for prediction of the range of experimentally accessible LUMO energies. This article is protected by copyright. All rights reserved.