Improving the Singlet Oxygen Photosensitization Activity of Fluorenone-Based Materials

The rational design of three coordination polymers based on copper(I) iodide, <b>UDS-7</b> (1D-Cu₂I₂<b>L7</b>₂)_<i>n</i>, <b>UDS-8</b> (2D-Cu₂I₂<b>L8</b>₂)_<i>n</i>, and <b>UDS-9</b> (1D-Cu₂I₂<b>L9</b>₂)_<i>n</i>, using new thioether fluorenones, respectively, 2,7-bis(propylthio)-9-fluorenone (<b>L7</b>), 2,7-bis(4'-methylthiophenyl)-9-fluorenone (<b>L8</b>), and 2,7-bis(methylthio)-9-fluorenone (<b>L9</b>), is reported. The identity of the coordination materials is elucidated by using X-ray diffraction techniques. An in-depth photophysical analysis of <b>L7</b>, <b>L8</b>, <b>L9</b>, <b>UDS-7</b>, <b>UDS-8</b>, and <b>UDS-9</b> 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 <b>UDS-8</b> and <b>UDS-9</b>, which is associated with efficient exciton migration within both structures. Near-infrared photoluminescence spectroscopy of the ¹Δ_<i>g</i> → ³<i>∑</i>_<i>g</i>^- 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 <b>L7</b> and <b>L9</b> ligands into coordination polymers using CuI (<b>UDS-7</b> and <b>UDS-9</b>), while <b>L8</b>, due to differing ligand design, remains silent.