Electronic Structure and Energy Transfer in Europium(III)–Ciprofloxacin Complexes: A Theoretical Study

The structure and ligand-localized excited states of [Eu(cfqH) (cfq)(H2O)4]Cl2 (cfqH is ciprofloxacin) are studied by XMCQDPT2/CASSCF with full geometry optimization. The complex includes one anionic and one zwitterionic ligand. Two low-lying triplet states, both localized on the anionic ligand, are found. One of them has sufficient energy to transfer to the (5)D1 sublevel of Eu(3+), because its T-S0 vertical transition energy is equal (or very close) to the (7)F0-(5)D1 Eu(3+) excitation energy. The other triplet state has a very small S0-T1 gap, which favors fast nonradiative relaxation. Two other triplet states are localized on the zwitterionic ligand. One low-lying excited singlet state (S1) is localized on the anionic ligand; the other excited singlet is localized on the zwitterionic one. Spin-orbit coupling constants were calculated for the relaxed geometry of each state (ground state, two low-lying triplets, and one low-lying excited singlet) by spin-orbit configuration interaction (CI) with Pauli-Breit Hamiltonian. Large spin-orbit coupling constants between S1 and both triplets together with small energy gaps are indicative of fast intersystem crossing (ISC) from the excited singlet state to the triplet manifold. This ISC process is followed by energy transfer from the ligand-localized triplet states to the (5)D1 sublevel of Eu(3+). However, relatively large spin-orbit coupling constants between S0 and one of the triplet states together with the small T-S0 energy gap shows that this state can decay without transferring its energy to Eu(3+). This mechanism is expected to be common for other Ln(3+)-fluoroquinolone complexes.