Development of Low Temperature Activatable Aryl Azide Adhesion Promoters as Versatile Surface Modifiers

An innovative approach for the immobilization of polymeric films is the use of bifunctional compounds called adhesion promoters, that create a stable chemical "bridge" between materials, allowing for versatile and permanent surface modification. To connect coating materials that lack reactive groups, the "bridge" forming adhesion promoter needs a highly reactive group that can insert in otherwise unreactive chemical bonds. Activatable perfluoro-aryl azides are commonly used to achieve this, with the limitation that their thermal activation is constrained to elevated temperatures-typically far above 100 °C-and photolytic activation is often unfeasible due to geometry or opaque materials. To overcome this limitation, we designed and synthesized three small organic molecules based on substituted aryl azides with the aim of lowering the activation temperature that restricts the use of existing aryl azides. We demonstrate both computationally via density functional theory (DFT) calculations and experimentally that the activation temperature of an aryl azide can be tuned by varying its substituents, giving access to mild activation temperatures (below 100 °C). The most reactive compound was the ο,ο-difluoro substituted <i>p</i>-phenoxy azide, which forms a nitrene and undergoes C-H insertion reactions at temperature of around 70 °C. This allows functionalization of surfaces with polymers that have no reactive groups under gentle conditions. The synthesized molecules were incorporated into a polymeric backbone to form adhesion promoters allowing covalent attachment of polymeric films to substrates by thermal activation below 100 °C. As an example, we successfully generated monomolecular films of polyvinylpyrrolidone (PVP), a polymer used and approved for medical devices due to its hydrophilic and lubricious properties. The effectiveness of attachment was assessed qualitatively and quantitatively using spectroscopic ellipsometry (ELM) and X-ray photoelectron spectroscopy (XPS).