Luminescent urea cross-linked tripodal siloxane-based hybrids

This work reports the synthesis of novel sol–gel derived urea cross-linked tripodal siloxane-based hybrids (classed as tri-ureasils) modified by the addition of phenyltriethoxysilane (PTES) and diphenyldimethoxysilane (DPDMS). The materials were produced as transparent monoliths and thin films (thickness = 3.3–17.5 ± 0.1 μm) and characterized by X-ray diffraction, mid-infrared spectroscopy, ²⁹Si and ¹³C nuclear magnetic resonance, thermogravimetric analysis, ultraviolet/visible (UV/Vis) absorption and photoluminescence spectroscopies. The role played by PTES and DPDMS as light harvesting chromophores has been discussed and quantified through emission quantum yield and UV/Vis absorption measurements. All the hybrids show efficient emission at room temperature in the blue spectral region with maximum emission quantum yield values ranging from 0.01 ± 0.001 to 0.10 ± 0.01. The highest values were found for the tri-ureasils incorporating PTES and DPDMS essentially due to an increase in the absorption coefficient (from 1.4 × 10³ cm⁻¹, for the pristine tri-ureasil, to 8.6 × 10³ cm⁻¹, for the hybrids modified by the chromophores). The existence of energy transfer between the DPDMS and PTES excited states and the hybrid host intrinsic emitting levels (NH/C=O- and siliceous-related levels) has been discussed.