Optically Functional Di-Urethanesil Nanohybrids Containing Eu3+ Ions

Abstract

Sol−gel derived poly(oxyethylene)/siloxane hybrids (di-urethanesils) doped with europium triflate, Eu(CF3SO3)3, were examined by XRD, 13C and 29Si MAS NMR and FT-IR, FT-Raman, and photoluminescence spectroscopies. The host framework of these materials consists of a siliceous network grafted through urethane linkages to both ends of polymer chains with 6 oxyethylene repeat units. Xerogels with ∞ ≥ n ≥ 1 (where n is the molar ratio (OCH2CH2)/Eu3+) were analyzed. The compounds with n ≥ 10 are amorphous. In samples with n ≤ 5 crystalline Eu(CF3SO3)3 was detected. In the di-urethanesils with n ≥ 10 the cations interact with the urethane carbonyl oxygen atoms. The complexation of the polyether chains to the cations is initiated at approximately n = 10. At n ≤ 10 both types of cation bonding situations occur. “Free” and weakly coordinated CF3SO3- ions exist in the whole range of salt concentrations analyzed. Ionic aggregates are formed in samples with n = 5 and 1. For 200 ≥ n ≥ 20 the emission quantum yields range from 0.7 to 8.1%. The decrease in the quantum yield with the increase of the Eu3+ concentration markedly depends on the activation of the energy transfer between the hybrid host's emitting centers and the cations, permitting, therefore, a fine-tuning of the emission chromaticity across the CIE (Commission Internationale d'Éclairage) diagram (e.g., (x, y) color coordinates from (0.19, 0.18) to (0.50, 0.49), for n = 200 and 5, respectively).