In Situ Luminescence of Self-Assembled Eu(III)-Naphthoic Acid Complex in Langmuir and LB Films

Highly luminescent nanostructured films with controlled molecular organization may find applications in various areas, ranging from new drug delivery systems to solid-state lighting. In this work, we synthesized and characterized amphiphilic complexes [Ln(dion)₃(H₂O)(DMSO)] where Ln(III) = Eu or Gd and dion is 6-dodecyloxy-2-naphthoic acid. The ligand dion was prepared in high yield through a sequence of esterification, alkylation, and hydrolysis reactions, and its structure was confirmed by ¹H NMR analysis. The synthesis of the complexes was confirmed by CHN elemental analysis and FT-IR spectral data. The triplet level (T = 26,332 cm^-1) obtained from an isostructural Gd(III) complex confirms that the dion ligand acts as an antenna in the energy absorption and transfer process. The photoemission spectra exhibit intraconfigurational transitions from the Eu(III) ion, with the hypersensitive transition ⁵D₀ → ⁷F₂ at 623 nm being the most intense line. An intrinsic quantum yield $\left(Q_{Eu}^{Eu}\right)$ of 40.45% indicates that the long chain of dion (C₁₂) causes losses by nonradiative processes, in addition to the low value of the Ω₂ parameter, characterizing a lower covalence of the chemical coordination environment. Luminescent monolayer films were obtained through the complexation that occurred at the interface, and the photoemission was monitored in situ using a Langmuir trough. The results indicate that the dion carboxylic acid-Eu(III) interfacial film exhibits f-f intraconfigurational transitions. Therefore, the behavior of the molecularly organized films demonstrates that dion plays the roles of both surfactant and photoantenna, enabling the deposition of stable and luminescent Langmuir-Blodgett (LB) films.