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

Abstract

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.

Rare-Earth Metal Complexes of the Antibacterial Drug Oxolinic Acid: Synthesis, Characterization, DNA/Protein Binding and Cytotoxicity Studies

“…The coordination sphere around the metal center in the complexes involve the following atoms: O3, O25 (belonging to the deprotonated carboxylic group), O2, O24 (belonging to the carbonyl groups) and O46, O48 (belonging to a hydroxyl group and a water molecule, respectively). The M-O bond lengths are comparable with those previously reported for other complexes with quinolones[91][92][93][94][95]: the bond distances between Sm-O2 and Sm-O3, Sm-O24 and Sm-O25 are similar to values of 2.517 and 2.506 Å, 2.582 and 2.519 Å, respectively; still, it can be observed how the bond between Sm 3+ and the oxygen atoms from the carboxyl moieties are shorter than the ones Sm 3+ forms with the pyridonic oxygens. The bonds length for Sm-O46 is 2.557 Å, while the one for Sm-O48 is 2.664 Å.…”

“…Since all attempts to isolate single crystals of the complexes have failed, the geometry optimization for Sm oxo was carried out via density functional theory (DFT) studies. Basis set 6-31G(d,p) was used for all H, C and O atoms and the small core potential ECP52MWB was used for Sm, as previously reported [91]. The fully optimized geometry of the complex is shown in Figure 4.…”

“…The fully optimized geometry of the complex is shown in Figure 4. The selected bond lengths, angles and charge densities are presented in Table 3 31G(d,p) was used for all H, C and O atoms and the small core potential ECP52MWB was used for Sm, as previously reported [91]. The fully optimized geometry of the complex is shown in Figure 4.…”

First-principles method for calculating the rate constants of internal-conversion and intersystem-crossing transitions

“…Since the obtained k IC and k ISC rate constants depend strongly on the accuracy of the calculated values for the singlet and triplet excitation energies, we calculated the excitation energies at the XMC-QDPT2 level of theory, which is computationally expensive but a feasible level for molecules of this size. 69,70 The present approach makes it possible to estimate rate constants for nonradiative transitions (k IC and k ISC ) for organic and organometallic compounds within the FC and adiabatic approximation by using calculated values for the matrix elements of the spin-orbit coupling interaction and nonadiabatic coupling interaction. Table 5 Calculated wavenumbers (o j in cm À1 ) of the significant vibrational modes for the nonradiative transitions and the corresponding Huang-Rhys factors (y j ) for the S 1 to S 0 nonradiative transition of the polyacenes…”

Solution Structure of a Europium-Nicotianamine Complex Supports That Phytosiderophores Bind Lanthanides

“…TDDFT or multi-reference calculations are necessary for more robust quantitative validation of Ln 3+ complexes stabilities; 32,[89][90][91][92] however, in this work we focus on describing the structure of the complexes, and we will quantify stabilities in the near future. Thus, in the present paper, we provide a qualitative description of the electronic structure of the previously unresolved Eu 3+nicotianamine complexes to complement our molecular structure findings (Section III.B).…”