Controlling the Excited State and Photosensitizing Property of a 2-(2-Pyridyl)benzo[b]thiophene-Based Cationic Iridium Complex through Simple Chemical Modification

Bis-cyclometalated cationic iridium (Ir) complexes 1-4 comprising two 2-(2-pyridyl)benzo[b]thiophene (btp) ligands and one 2,2'-bipyridyl (bpy) ancillary ligand with different substituents were prepared as new visible light-absorbing sensitizers and examined for their photophysical and electrochemical properties. Complex 1 was prepared as a parent complex without any substituents. Complexes 2-4 contained methyl-, methoxy-, and trifluoromethyl groups at 4,4'-positions on the bpy ancillary ligand. Systematic investigation of these complexes revealed that such a simple chemical modification selectively controls the excited-state lifetime, while the absorption and emission spectral features remain unchanged. Specifically, the phosphorescence lifetimes of complexes 2 and 3 with electron-donating groups (tau = 3.50 mu s, 3.90 mu s) were found to be much longer than that of complex 1 (tau = 0.273 mu s), and complex 4, possessing strong electron-withdrawing trifluoromethyl groups, did not exhibit detectable phosphorescence at room temperature. The large differences in excited-state lifetimes of complexes 1-3, as well as the nonemissive character of complex 4, are attributed to a strong influence of the substituents on the ligand field strength. The increased sigma-donating ability of the ancillary ligand in complexes 2 and 3 destabilizes a short-lived, nonemissive triplet metal-centered ((MC)-M-3) state and increases the energy separation between the (MC)-M-3 state and emissive triplet ligand-centered ((LC)-L-3) state based on the btp ligand. For complex 4, however, the (MC)-M-3 state is close in energy to the (LC)-L-3 state because of the decreased sigma-donating ability of the ancillary ligand. Additional evidence of the (MC)-M-3 state associated with the changeable excited state was also provided via low-temperature phosphorescence measurements and density functional theory calculations. Ir complexes 1-4 were tested as sensitizers in photoinduced electron-transfer reaction of triethanolamine and methylviologen chloride (MVCl2). As a result, complexes 2 and 3 exhibited much better photosensitizing property compared to complex 1 since their long-lived excited states promoted an oxidative quenching pathway. This Study has first demonstrated that simple substitution on the diimine ancillary ligand can control the (MC)-M-3 state of the biscyclometalated cationic Ir complex to finely tune the excited-state lifetime and photosensitizing property.