Time-resolved luminescence investigations of the reversible energy transfer from the excited 3*MLCT states to organic acceptors-An alternative method for the determination of triplet state energies and lifetimes

Decays of emission from the excited metal-to-ligand-charge-transfer triplet state (3*MLCT) have been investigated for several luminescent Ir(III) complexes in acetonitrile solutions. Experiments have been performed in the absence as well as in the presence of organic acceptor quenchers (A) having triplet state (3*A) energies close to that characteristic of the 3*MLCT state. This similarity in energies between the 3*MLCT and 3*A states allows observing reversible energy transfer (REN) processes that manifest as changes in the emission decay profiles. In the presence of an organic quencher, the 3*MLCT emission decays change from mono-exponential (characteristic of the excited 3*MLCT state of the investigated complexes) to bi-exponential. The analysis of these observed bi-exponential decays (performed using the Birks' formalism) enables the determination of the kinetic parameters characterizing the investigated forward and backward REN processes. The applied approach is proposed as an alternative method for determining molecular triplet state energies and lifetimes that are sometimes difficult to measure in liquid media.