Design of Light‐Induced Solid‐State Plasmonic Rulers via Tethering Photoswitchable Molecular Machines to Gold Nanostructures Displaying Angstrom Length Resolution

Plasmonic rulers (PRs) linking nanoscale distance dependence spectral shifts are important for studying cellular microenvironments and biomarker detection. The traditional PR design employs tethering metal nanoparticle pairs using synthetic and biopolymers that severely suffer from reproducibility issues, as well as lack reversibility. Here, the fabrication of novel PRs is reported through the formation of self‐assembled monolayers (SAMs) of photoswitchable molecular machines chemically tethered onto sharp‐tip gold nanostructures (Au NSs). This unique and highly sensitive PR utilizes localized surface plasmon resonance (LSPR) properties of Au NSs to spectroscopically evaluate dipole–dipole coupling between NSs and photoisomerizable spiropyran (SP)‐merocyanine (MC) conjugates in the solid‐state. It is observed that the SAM‐modified NSs are extremely sensitive to the photoisomerization of SP‐to‐MC, resulting in LSPR shifts as large as 5.6 nm for every 1.0 Å change in distance. The highly dipolar MC changes the NS‐SAM interfacial polarizability and alters the dipole–dipole coupling leading to the ultrasensitive PR is hypothesized. The hypothesis is supported theoretically by calculating dipole polarizability of an inorganic‐organic heterodimer model and experimentally by determining work function and interfacial dipole values. Taken together, this work represents the fabrication of next‐generation PRs, which hold great promise for advanced, plasmonic‐based sensors and optoelectronic device fabrication. This work introduces the construction of solid‐state plasmonic rulers (PR) by connecting sharp‐tip gold nanostructures and self‐assembled monolayers of alkylthiols, which contain photoswitchable molecular machines, with an Å‐level resolution. The working mechanism for this unique PR is established as dipole–dipole interactions between the highly dipolar molecular machines and the dipole of the nanostructures. Together, this work represents the fabrication of next‐generation plasmonic rulers, which hold great promise for advanced, plasmonic‐based sensors and optoelectronic device fabrication.