Deep eutectic silsesquioxane hybrids with quaternary ammonium/urea derivatives: synthesis and physicochemical and ion-conductive properties

We report the synthesis of deep eutectic silsesquioxane hybrids (DE-SQs) by simple mixing of quaternary-ammonium-containing SQ and urea derivatives. Cationic SQ, which was prepared by the hydrolytic condensation of a triethoxysilane precursor derived from 2-(dimethylamino)ethyl acrylate, followed by a quaternization reaction with methyl iodide, was used as a quaternary-ammonium-containing SQ component. Cationic SQ reacted with urea at a 1:2 M ratio at 80 °C for 48 h to yield a viscous DE-SQ (2Urea) liquid with a low glass transition temperature (T_g = ?11 °C). Urea derivatives—1,3-dimethylurea (DMU) and 1,3-dimethylthiourea (DMTU)—were additionally used as hydrogen bond donors to form low-T_g DE-SQs. The thermal, physical, and ion-conductive properties of the DE-SQ family of organic–inorganic hybrids were investigated and characterized, and the influences of the nature of the urea derivative and their feed ratios on DE-SQ formation were evaluated. Among the DE-SQs developed in this study, DE-SQ (2Urea) and DE-SQ (2DMTU) achieved the highest ionic conductivity, with DE-SQ (2Urea) exhibiting 2.35 × 10^?6 and 6.63 × 10^?4 S cm^?1 at 25 and 75 °C, respectively, under anhydrous conditions. This is the first report on the synthesis of DE-SQs by simple mixing of two solids, wherein the resulting compounds exhibit low T_g, thermal stability, and characteristic ionic conductivity. The ability to incorporate unique DE units into the SQ structure facilitates the development of advanced organic–inorganic hybrid materials possessing a wide range of functions and applications.