3.3. Optical properties
Photoluminescence excitation (PLE) and emission (PL) spectra of [Eu2(phen)2(PFbz)6] in PMMA at 5, 10 and 20% films placed on glass are reported in Fig. 4. PLE spectra show the UV light absorbed by the Eu-based PMMA films for a fixed emission at 615 nm through downconversion processes (Fig. 4a,b). Luminescent films present an UV excitation band between 250 and 360 nm (consistent with absorption by the antenna ligands). The PL spectrum displays the emission of Eu(III) ions previously excited via energy transfer from the organic antennas (Fig. 4c) [7,36,37]. Upon excitation at 290 nm, which is the maximum of the PLE spectrum, the [Eu2(phen)2(PFbz)6] complex shows the characteristic emission bands of Eu3+ ions corresponding to the electronic transitions from the excited level 5D0 to the 7FJ (J =0 − 4) ground levels. The shielding effect of electrons in the outer shells turns rare earth ions insensitive to chemical environment and causes narrow emission band from inner 4f electronic transitions [38]. The so-called hypersensitive transition,5D0 → 7F2 (λ = 615 nm) display the strongest emission among this series, being its intensity related to the coordination environment.