Excited State Dynamics Govern Emission Properties of Unique Silsesquioxane-Salphen-Based Zinc Compounds

This study aims to develop a synthetic protocol for preparing salphen-based hybrid compounds with silsesquioxane T₈ cages anchored at the molecule's periphery. Three types of coordination compounds featuring κ⁴-NO-donating atoms were obtained via a sequence of reactions. These compounds differ in the arene linker between the salphen and silsesquioxane fragments. An individual synthetic pathway was developed for the preparation of aldehydes, followed by a tailored strategy for the synthesis of the final complexes employing both solution-based and mechanochemical methods in the solid state. The latter represents a novel technique in silsesquioxane chemistry. The newly designed ligands were used for the coordination of Zn²⁺ ions to evaluate their ligation properties and to determine the photophysical properties of the resulting complexes in comparison to their bare ligand molecules. Using absorption and emission spectroscopy, combined with advanced time-resolved spectroscopic methods, we demonstrated that the photochemical efficiency of these compounds is influenced by their tendency to aggregate in solution, which positively affects their photophysical properties and enhances their potential for photodynamic therapy (PDT). Additionally, we explored the ability of these complexes to generate singlet oxygen (¹O₂) depending on the architecture of the designed ligands. The results indicate that the excited state dynamics plays a crucial role in determining the emission properties of the studied compounds, which may have significant implications for their applications in medicine and materials science.