Energy transfer from the excited 3*MLCT states to organic acceptors – Temperature effect studies

• Energy transfer processes involving 3*Ir(ppy)3 or 3*Ru(bpy)32+ have been studied.
• Solvent and temperature effects have been investigated.
• The overall energy transfer rates have been found to be slower than diffusion.
• Temperature affects the energy transfer rates through medium viscosity changes.
• Magnetic quantum number conservation rule explains the observed deviations.

Energy transfer from the excited (3*D) metal-to-ligand-charge-transfer triplet state (3*MLCT) to selected organic energy acceptor quenchers (A) has been investigated for Ir(ppy)3–chrysene and Ru(bpy)32+–pyrene systems at different temperatures in several organic solvents. For both systems studied, reversible, nearly iso-energetic energy transfer processes 3*D + A ⇌ D + 3*A have been observed. In the presence of an organic quencher, the 3*D emission decays change from the mono-exponential (characteristic for the excited 3*D state of the investigated complexes) to the bi-exponential ones that enables the determination of the kinetic parameters characterizing the investigated forward 3*D + A → D + 3*A and backward 3*D + A ← D + 3*A energy transfer processes. Analysis of the obtained kinetic (rate constants) and thermodynamic (activation energies) data have been performed in the framework of different kinetic models, with the main conclusion that the “classical” Sandros and Balzani models are inadequate for proper description of the experimental results. A much better agreement between theoretical predictions and experimentally found results can be found if the magnetic number conservation rule is implemented in the 3*D + A ⇌ D + 3*A reaction scheme. The obtained results suggest that during the energy transfer processes occurring according to the Dexter mechanism, not only spin but also magnetic quantum number within 3*D…A and D…3*A activated complexes are conserved.

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