Melt, mix, and glow: emulsion-based fabrication of polyphenylsilsesquioxane microspheres with embedded hydrophobic fluorophores

Abstract

A novel melt-emulsion strategy is presented for synthesizing polyphenylsilsesquioxane (PPSQ) microspheres embedded with hydrophobic perylene-based fluorescent dyes. The approach utilizes a low-crosslinked, thermally softenable PPSQ precursor – referred to as a “melting gel” – which incorporates polycyclic aromatic dyes such as Lumogen® F Red 305 (LG305) taking advantage of both hydrophobic compatibility and π–π interactions within the phenyl-rich matrix. Upon heating above 70 °C, the precursor forms an emulsion in boiling water containing Triton™ X-405, followed by sodium hydroxide-induced condensation to yield solid microspheres (∼3 μm diameter). A subsequent thermal treatment at 200 °C enhances crosslinking, forming a condensed PhSiO_1.5 network and removing surfactant residues, thereby shifting surface polarity from hydrophilic to hydrophobic. Comprehensive characterization using NMR, FTIR, XRD, TGA, fluorescence spectroscopy, fluorescence lifetime and quantum yield analysis confirms the formation of a ladder-type silsesquioxane structure and retention of dye fluorescence. This two-step process enables efficient encapsulation of various hydrophobic dyes across a wide concentration range, offering a versatile platform for developing stable, processable luminescent materials for applications such as LED encapsulants and luminescent solar concentrators.