Surfactant-encapsulated europium-substituted heteropolyoxotungstates: Structural characterizations and photophysical properties

The surfactant-encapsulated europium-substituted heteropolyoxotungstates (Eu-POMs), (DODA)(10)H[Eu(H2O)(PW11O39)(2)].17H(2)O (SEC-2, DODA: dimethyl dioctadecylammonium), (DODA)(12)H-3[Eu(BW11O39)(2)].13H(2)O (SEC-3), and (DODA)(9)[EuW10O36].9H(2)O (SEC-4), are structurally characterized in detail and their photophysical behavior is investigated. The DODA alkyl chains of SEC-2, SEC-3, and SEC-4 are well ordered. The solid-state SEC-2 and SEC-3 show two different lamellar structures with interlayer distances of 3.6 and 4.9 nm, respectively, but SEC-4 only offers a much shorter lamellar distance of 3.2 nm. In the case of short periodicities, it is considered that the alkyl chains are partially interdigitated, and the interdigitated length may reach 1.3 nm in SEC-2 and SEC-3. Of difference is that DODA alkyl chains are almost fully interdigitated in SEC-4. On the basis of a series of SECs with 15, 13, 11, and 9 negative charges, we suggest that the DODA molecule can occupy a minimum area of 0.57 +/- 0.04 nm(2) on the surface of polyoxometalate. SEC-2, SEC-3, and SEC-4 exhibit the characteristic D-5(0) --> F-7(j) (j = 0, 1, 2, 3, 4) Eu3+ emission. The photophysical properties of Eu-POMs have been modified such as spectra, lifetimes, and intramolecular energy transfer from heteropolytungstate ligands to Eu3+ ion after encapsulating them with DODA. Therefore, the nanostructures of Eu-POMs can influence their photophysical behavior. X-ray photoelectron spectroscopy shows that the amphiphiles strongly impact the electronic properties of Eu-POMs, which rationally explains the various photophysical properties before and after the encapsulation with cationic surfactants. The controllable relationship between the nanostructures and the photophysical properties of Eu-POMs is significant for the future realization of POM-based functional devices.