Abstract
The photophysical properties of a heterobimetallic Ir III –Eu III complex were investigated in solvents with different polarity, coordinating ability, and vibrational characteristics to elucidate the factors governing Eu III sensitization. Steady-state and time-resolved luminescence measurements revealed that the solvent profoundly influences both the donor-to-acceptor energy-transfer process and the europium-centered emission. The apparent Ir III → Eu III energy-transfer rate constants (k ET ) and donor-to-acceptor energy-transfer efficiencies (η ET ) were estimated from the residual 3 MLCT emission lifetimes (τ q ) of the Ir III –Eu III complex and the corresponding Ir III –Gd III analog (τ u ). The results showed a general tendency toward higher energy-transfer rates and efficiencies in solvents with high polarity, coordinating ability, and free of O–H oscillators, reaching k ET values up to 2.6 × 10 7 s −1 and η ET values up to 90% in DMSO. Solvent polarity was found to indirectly modulate the energy transfer (ET) process by influencing the energies of the excited states and the donor–acceptor electronic coupling. In contrast, protic solvents strongly quenched Eu III emission due to the presence of high-energy O–H oscillators, despite moderate ET efficiencies. Degassing experiments performed in dichloromethane and chloroform demonstrated that dissolved oxygen has only a minor effect on the η ET efficiency. The results reveal a complex interplay between solvent polarity, coordinating ability, donor–acceptor gap, and non-radiative deactivation pathways, providing new insights into the design of efficient d–f heterobimetallic luminescent systems.