Improving the Singlet Oxygen Photosensitization Activity of Fluorenone-Based Materials

Abstract

The rational design of three coordination polymers based on copper(I) iodide, UDS-7 (1D-Cu₂I₂L7₂)_n, UDS-8 (2D-Cu₂I₂L8₂)_n, and UDS-9 (1D-Cu₂I₂L9₂)_n, using new thioether fluorenones, respectively, 2,7-bis(propylthio)-9-fluorenone (L7), 2,7-bis(4'-methylthiophenyl)-9-fluorenone (L8), and 2,7-bis(methylthio)-9-fluorenone (L9), is reported. The identity of the coordination materials is elucidated by using X-ray diffraction techniques. An in-depth photophysical analysis of L7, L8, L9, UDS-7, UDS-8, and UDS-9 enabled a rational analysis of the properties of the newly synthesized compounds, which are corroborated by density functional theory (DFT) and time-dependent DFT (TD-DFT) computations. The coordination species exhibit ligand centered ^1,3LC emissions, radically contrasting the largely expected metal halide-to-ligand charge-transfer (M/XLCT) behavior. Time-resolved spectroscopic techniques show a relationship between the ligand design and the multiplicity of the ^1,3LC emissive state. Excitation power-dependent photoluminescence analyses indicate the presence of singlet-singlet and triplet-triplet annihilation phenomena in UDS-8 and UDS-9, which is associated with efficient exciton migration within both structures. Near-infrared photoluminescence spectroscopy of the ¹Δ_g → ³∑_g^- emission at 1275 nm provides evidence of singlet oxygen (¹O₂) photosensitization at the solid-gas interface. The photosensitization phenomenon is found to be improved by the incorporation of the L7 and L9 ligands into coordination polymers using CuI (UDS-7 and UDS-9), while L8, due to differing ligand design, remains silent.