Optically Tunable Properties of Small Organic Molecules via Photomediated Aza Nazarov Cyclizations

We unveil the spectral characteristics exhibited by a group of benzoyl methyl pyrazinium molecules undergoing aza-Nazarov photocyclization to luminescent pyrazino-indoles upon ultraviolet (UV) irradiation between 280 and 400 nm. The reaction, driven by photogenerated radical formation, induces structural transformations that manifest as a striking red-shift of up to 100 nm in the photoluminescent spectra, a highly unusual and visually discernible spectral change. Chemical substitutions further allow fine-tuning of these spectral properties before and after cyclization, offering an additional degree of control over the optical characteristics. Further, when deposited as thin films, the progression of photocyclization can be precisely regulated, enabling the molecules to remain highly fluorescent while arresting the cyclization at desired stages. This ability to modulate the spectral emission by controlling the extent of photocyclization presents unprecedented opportunities for designing advanced functional materials with tailored light-matter interactions for biological and environmental applications. The byproduct-free nature of the reaction, coupled with the tunable optical properties achieved through variations in concentration, solvent, and excitation wavelength, further accentuates the uniqueness of these molecules in the development of materials with customized spectral responses.