Abstract

We report a homologous series of [Re(CO)s(N-N)Cl] complexes, Re-nap, Re-ant and Re-pyr, based on 1H-imidazo[4,5-f][1,10]phenanthroline ligands substituted with naphthalen-1-yl (nap), anthracen-9-yl (ant), and pyren-1-yl (pyr) fragments, respectively. The complexes were characterized by 1H NMR, FT-IR, and HRMS, while the solid-state structure of Re-nap was established using single-crystal X-ray diffraction. Coordination induces a highly coplanar arrangement of the naphthyl-imidazo-phenanthroline framework, with a dihedral angle of only 2.56(10)degrees between the phenanthroline and naphthyl fragments, supported by parallel-displaced pi-stacking interactions with centroid-centroid distances of 3.800-3.905 & Aring;. Absorption and photoluminescence studies revealed that increasing the aromaticity modulates the interplay between the intraligand (IL), intraligand charge-transfer (ILCT), and metal-to-ligand charge-transfer (MLCT) states. Re-nap displayed a distinct MLCT absorption band at 425 nm in CH2Cl2 and exhibited the highest MLCT photoluminescence quantum yield of the series, Phi PL = 1.90%, along with a remarkably long 3MLCT lifetime of 9354 ns, compared with 158 ns for Re-ant in CH2Cl2. All Re(I) complexes efficiently photosensitized singlet oxygen formation upon 405 nm excitation, with Phi Delta values increasing from 0.708, 0.844, and 0.887 for Re-nap, Re-ant and Re-pyr, respectively. These values exceeded those of the free ligands. Under CO2, both ligands and complexes displayed enhanced cathodic currents; however, the Re complexes were markedly more responsive, with apparent icat/ip ratios of 8.94, 8.80, and 12.0 for Re-nap, Re-ant, and Re-pyr, respectively. These results demonstrate that systematic aromatic extension, particularly through pyrene incorporation, is an effective ligand design strategy for enhancing singlet oxygen photosensitization and CO2 reduction activity in Re(I) imidazo-phenanthroline complexes.