Surfactant-encapsulated europium-substituted heteropolyoxotungatate: The structural characterization and photophysical properties of its solid state, solvent-casting film, and Langmuir-Blodgett film

The surfactant-encapsulated europium-substituted heteropolyoxotungatate, (DODA)(11)H-2[Eu(SiW11O39)(2)(H2O)(4)] (SEC-2), is structurally characterized in detail and its photophysical behaviors are investigated. The DODA alkyl chains of SEC-2 are well ordered in the solid state, solvent-casting film, and Langmuir-Blodgett (LB) film. The europium-substituted heteropolyoxotungatate (POM-2a) is organized in the solvent-casting and LB films. A comparison of the intensities of the W-O-d antisymmetrical stretching band between the infrared transmission and reflection absorption spectra of the LB films suggests that the long axis direction of POM-2a is parallel to the substrates. The solid-state SEC-2 shows two different lamellar structures with interlayer distances of 3.6 and 4.9 nm, respectively. The layer structures are well defined in both the solvent-casting and LB films and have lamellar distances of 3.6 and 4.9 nm, respectively. In short periodicities, it is considered that the alkyl chains are partially interdigitated, and the interdigitated length may reach 1.3 nm. SEC-2 exhibits the characteristic D-5(0)-->F-7(j) (j = 0-4) Eu3+ emission. The bands in both the excitation and emission spectra of K13Eu(SiW11O39)(2).20H(2)O (POM-2) and SEC-2 solutions (1 mM) show different intensities because of their various chemical microenvironments. The strong oxygen-to-tungsten charge transfer (O-->W LMCT) bands of POM-2a appear in the excitation spectra of the solvent-casting and LB films, indicating an efficient intramolecular energy transfer from the O-->W LMCT states to Eu3+ ions. The luminescent lifetimes of SEC-2 are much shorter than those of the naked POM-2 because of the coordinated water molecules to Eu3+ in SEC-2. The solution, solid state, solvent-casting films, and LB films of SEC-2 display their different photophysical properties such as spectrum, lifetime, and energy transfer from the O-->W LMCT states to Eu3+ ions. The photophysical behaviors of Eu-POMs can be modified by their organizational structures, which are significant for the future realizations of functional devices.