Light emission of a polyfluorene derivative containing complexed europium ions

Abstract

The photophysical properties of a new alternating copolymer containing fluorene, terpyridine, and complexed sites with trivalent europium (Eu³⁺) ions (LaPPS66Eu) were investigated, using the non-complexed backbone (LaPPS66) and a low molecular weight compound of similar chemical structure of the ligand/Eu³⁺ site (LaPPS66M) as a model compound. The analogous gadolinium complex (LaPPS66Gd) was also synthesized to determine the triplet state of the complex. ¹H and ¹³C nuclear magnetic resonance (NMR) analysis, Fourier transform infrared (FT-IR) spectroscopy, inductively coupled plasma optical emission spectroscopy (ICP-OES), elemental analyses, differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA) characterized the chemical structure and thermal properties of the synthesized materials. A level of Eu³⁺ insertion of 37% (molar basis) in the polymer backbone was achieved. The photoluminescence studies were performed in the solid state showing the occurrence of polymer-to-Eu³⁺ energy transfer brought about by the spectral overlap between the absorption spectra of the Eu³⁺ complex and the emission of the polymer backbone. A detailed theoretical photoluminescence study performed using time-dependent DFT (TD-DFT) calculations and the recently developed LUMPAC luminescence package is also presented. The high accuracy of the theoretical calculations was achieved on comparison with the experimental values. Aiming at a deeper level of understanding of the photoluminescence process, the ligand-to-Eu³⁺ intramolecular energy transfer and back-transfer rates were predicted. The complexed materials showed a dominant pathway involving the energy transfer between the triplet of the dbm (dibenzoylmethane) ligand and the ⁵D₁ and ⁵D₀ Eu³⁺ levels.