Solvent-dependent Sensitization and Photophysical Behavior of IrIII-EuIII Bimetallic Complexes.

The photophysical properties of a heterobimetallic IrIII-EuIII complex were investigated in solvents with different polarity, coordinating ability, and vibrational characteristics to elucidate the factors governing EuIII 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 IrIII → EuIII energy-transfer rate constants (kET) and donor-to-acceptor energy-transfer efficiencies (ηET) were estimated from the residual 3MLCT emission lifetimes (τq) of the IrIII-EuIII complex and the corresponding IrIII-GdIII analog (τu). 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 kET values up to 2.6 × 107 s-1 and ηET 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 EuIII 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 ηET 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.