Self-Assembled Monolayers and Solvent Composition Control the Orientation and Work Function of Pyrene-Based [7]Helicenes

Abstract

Self-assembled monolayers (SAMs) and solvent composition critically influence the orientation and electronic properties of functional organic molecules, paving the way for energy conversion and optoelectronic devices. PeakForce quantitative nanomechanics atomic force microscopy (AFM) was used to investigate pyrene-based 2H-pyran[7]helicene (P,S,S)-(+) and its mirror image, 2H-pyran[7]helicene (M,R,R)-(−), deposited on self-assembled monolayers (SAMs) with one of the following headgroups: adamantyl, phenyl, and trityl. By varying toluene content and dilution during spin-coating, we achieve changes to the topography, mechanical properties, and work function of our surfaces and are able to link these changes to the achievement of limited control over the modulation of the helicene orientation and gain insight into the self-assembly process on said SAMs. At high dilutions, we were able to detect distinct nucleation sites for helicene structures. These orientational changes and associated disorder correlate directly with work function modulation, suggesting a way and a mechanism for tuning electronic properties via these interfaces. Our findings hint at a simple processing strategy to engineer the molecular orientation and work function, advancing the design of helicene-based optoelectronic materials.