Optical properties and time-resolved photoluminescence of conjugated polymers with europium complex side chain as an emitter

We investigated four conjugated polymers of the same backbone with different lengths of side chains attached with europium (Eu³⁺) complex (herein labeled P2, P4, P6 and P8). All samples exhibited strong luminescence belonging to f-f* electronic transitions of Eu³⁺. Sharp emission peaks of ⁵D₀-⁷F_J (J=0, 1, 2, 3) transitions were observed with the strongest peak at ∼612 nm (i.e. ⁵D₀-⁷F₂ transition). Ligand field splitting was seen for the ⁵D₀-⁷F_J (J=1, 2, 3) transitions. Weaker and very broad emission due to singlet S₀→S₁ transition of the polymer was also observed. The temperature dependent fluorescence measurement indicating the energy transfer from polymer S₁ state to the Eu excited D levels is not a barrierless process; some kind of activation energy may be involved. The fluorescence rise-time indicates that the energy transfer process may take several hundred nanoseconds to be fully completed. Emission lifetimes at 612 nm were measured at 0.72, 0.37, 0.39 and 0.61 ms for P2, P4, P6 and P8, respectively. However, there was a fast decay component for the 612 nm emission, indicating some initial rapid relaxation process.