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

## TL;DR

This paper describes the creation of luminescent films using a europium complex that could be useful in lighting and drug delivery.

## Contribution

The novel contribution is the synthesis of a self-assembled Eu(III)-naphthoic acid complex that acts as both a surfactant and a photoantenna in Langmuir and LB films.

## Key findings

- The dion ligand functions as an antenna for energy transfer in the Eu(III) complex.
- Luminescent monolayer films were successfully obtained and monitored in situ.
- The complex exhibits f–f intraconfigurational transitions with a quantum yield of 40.45%.

## Abstract

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
(QEuEu)
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.