Photosensitized Generation of Singlet Oxygen from Re(I) Complexes: A Photophysical Study Using LIOAS and Luminescence Techniques

Abstract

Quantum yields and efficiencies of 1O2 (1Δg) production along with photophysical properties for a number of Re(I) complexes in acetonitrile solutions are reported. Two different classes of Re(I) complexes, LS-CO2-Re(CO)3(bpy) (LS = 2-pyrazine, 2-naphthalene, 9-anthracene, 1-pyrene, 2-anthraquinone) and XRe(CO)3L (X =CF3SO3, py; L = bpy, phen), were probed as photosensitizers for 1O2 (1Δg) production in air-saturated acetonitrile solutions. Depending on the nature of the Re(I) complex, the excited state responsible for the generation of 1O2 (1Δg) is either a metal-to-ligand charge transfer (3MLCT) or a ligand centered (3LC) state. With LS-CO2-Re(CO)3(bpy)complexes, 1O2 (1Δg) is produced by oxygen quenching of 3LC states of anthracene and pyrene with high quantum yields (ΦΔ between 0.8 and 1.0), while the complexes bearing the ligands LS = 2-anthraquinone, 2-pyrazine, and 2-naphthalene did not yield 1O2. XRe(CO)3L complexes generate 1O2 (1Δg) mainly by oxygen quenching of their 3MLCT luminescence with an efficiency of 1O2 (1Δg) formation close to unity. Bimolecular rate constants for the quenching of the XRe(CO)3L complexes? emission by molecular oxygen range between 1 × 109 and 2 × 109 M−1 s−1, and they are all ≤ 1/9kd,in good agreement with the predominance of the singlet channel in the mechanism of 1O2 (1Δg) generation using these Re(I)complexes as photosensitizers. All the experimental singlet oxygen efficiencies are consistent with calorimetric and luminescence data for the studied complexes. With LS-CO2-Re(CO)3(bpy) complexes, calorimetric experiments were utilized in the calculation of the quantum yields of triplet formation; namely ϕT = 0.76 and 0.83 for the triplet states of anthracene and pyrene, respectively.