Photoluminescent and Superhydrophobic [Eu(Phen)_2]~(3+)-Laponite/Polypropylene Film for Long-Term Fluorescence Stability under Conditions of High Humidity

摘要

The fascinating photophysical properties of lanthanide ions, particularly trivalent europium (Eu~(3+)) ion, play an important role in various optical applications, such as in optical amplifi ers, light-emitting devices, lasers, and luminescent thin films. In most of these applications, the Eu complexes have received much interest since the Eu~(3+) ion gives off strong red emission when the ion is complexed with suitable organic ligands. Due to the weak absorption of the Eu~(3+) ion itself based on the forbidden character of the 4f–4f transitions, ligands are employed as intense light absorbers to photosensitize the central Eu~(3+) ion through intramolecular energy transfer. Although Eu complexes have excellent luminescence characteristics, their real and long-term applications are still limited due to the gradual decrease in luminescence caused by external environment factors, such as atmospheric humidity, UV exposure, etc. In particular, water molecules have a strong suppression effect on emission due to nonradiative dissipation of high-energy O–H vibration around the Eu complex. Furthermore, it is difficult to process the Eu complex itself because of its poor thermal and mechanical properties. Thus, in order to solve these problems, Eu complexes should be introduced in stable rigid matrixes to form hybrid materials. Among the various matrixes, organic polymers offer several advantages for the broad applicability of the Eu complex, such as transparency, flexibility, and superhydrophobicity. However, the resulting hybrid product often suffers from concentration quenching by molecular aggregation of fluorescent Eu complexes after direct blending with polymers. Although attempts have been made to reduce self-aggregation by direct coordination of the Eu complex to the polymer backbone, quenching still exists at above 5% of the complex concentration by the saturation effect. In addition, most pristine polymers are permeable to gases such as water vapor and oxygen. So, the luminescent property of the Eu complex in a polymer matrix is gradually reduced. This can be overcome with an introduction of inorganic host materials, which offer not only nanospace with enhanced stability but also increased diffusion pathways for water vapor (the gas barrier property), which in turn leads to preservation of fluorescence stability.