Dynamics of the Energy Transfer Process in Eu(III) Complexes Containing Polydentate Ligands Based on Pyridine, Quinoline, and Isoquinoline as Chromophoric Antennae


In this work, we investigated from a theoretical point of view the dynamics of the energy transfer process from the ligand to Eu(III) ion for 12 isomeric species originating from six different complexes differing by nature of the ligand and the total charge. The cationic complexes present the general formula [Eu(L)(H2O)2]+ (where L = bpcd2- = N,N '-bis(2-pyridylmethyl)-trans-1,2-diaminocyclohexane N,N '-diacetate; bQcd2- = N,N '- bis(2-quinolinmethyl)-trans-1,2-diaminocyclohexane N,N '-diace-tate; and bisoQcd2- = N,N '-bis(2-isoquinolinmethyl)-trans-1,2-diaminocyclohexane N,N '-diacetate), while the neutral complexes present the Eu(L)(H2O)2 formula (where L = PyC3A3- = N-picolyl-N,N ',N '-trans-1,2-cyclohexylenediaminetriacetate; QC3A3- = N-quinolyl-N,N ',N '-trans-1,2-cyclohexylenediaminetriacetate; and isoQC3A3- = N-isoquinolyl-N,N ',N '-trans-1,2-cyclohexylenediaminetriacetate). Time-dependent density functional theory (TD-DFT) calculations provided the energy of the ligand excited donor states, distances between donor and acceptor orbitals involved in the energy transfer mechanism (RL), spin-orbit coupling matrix elements, and excited-state reorganization energies. The intramolecular energy transfer (IET) rates for both singlet-triplet intersystem crossing and ligand-to-metal (and vice versa) involving a multitude of ligand and Eu(III) levels and the theoretical overall quantum yields (phi ovl) were calculated (the latter for the first time without the introduction of experimental parameters). This was achieved using a blend of DFT, Judd-Ofelt theory, IET theory, and rate equation modeling. Thanks to this study, for each isomeric species, the most efficient IET process feeding the Eu(III) excited state, its related physical mechanism (exchange interaction), and the reasons for a better or worse overall energy transfer efficiency (eta sens) in the different complexes were determined. The spectroscopically measured phi ovl values are in good agreement with the ones obtained theoretically in this work.



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Neto, ANC; Moura, RT; Carlos, LD; Malta, OL; Sanadar, M; Melchior, A; Kraka, E; Ruggieri, S; Bettinelli, M; Piccinelli, F

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This work was developed within the scope of the projects CICECO-Aveiro Institute of Materials, UIDB/50011/2020, UIDP/50011/2020, and LA/P/0006/2020, and The Shape of Water (PTDC/NAN-PRO/3881/2020) financed by national funds through the FCT/MCTES (PIDDAC). F.P., A.M., M.S., S.R., and M.B. thank the Italian Ministry of University and Research for the received funds in the frame of PRIN (Progetti di Ricerca di Rilevante Interesse Nazionale) project CHIRALAB, Grant No. 20172M3K5N). Also, COST Action CA18202, NECTAR-Network for Equilibria and Chemical Thermodynamics Advanced Research, supported by COST (European Cooperation in Science and Technology), is acknowledged. R.T.M., Jr. and E.K. thank SMU's Center for Scientific Computing for providing generous computational resources. This work was financially supported by the National Science Foundation (Grant CHE 2102461).

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