A Flexible Chelate Leads to Phototriggered Isomerization in an Osmium Sulfoxide Complex

We report the preparation and spectroscopic properties of [Os(bpy)2(pySO)]2+ and [Os(bpy)2(pySOCF3)]2+, where bpy is 2,2′-bipyridine, pySO is 2-(propane-2-sulfinylmethyl)pyridine, and pySOCF3 is 2-[(trifluoroethylsulfinyl)methyl]pyridine. The charge-transfer absorption spectra of [Os(bpy)2(pySO)]2+ and [Os(bpy)2(pySOCF3)]2+ are 378 and 367 nm, respectively. Charge-transfer excitation of [Os(bpy)2(pySOCF3)]2+ shows substantial changes in the UV/Vis spectrum with a shift in the lowest energy absorption maximum from 367 to 415 nm, which is consistent with phototriggered SO isomerization. Irradiation of [Os(bpy)2(pySO)]2+ shows no evidence of isomerization. Cyclic voltammograms of [Os(bpy)2(pySO)]2+ and [Os(bpy)2(pySOCF3)]2+ feature reversible one-electron couples that are ascribed to Os3+/2+ reduction potentials (E°′) at 1.01 V (vs Ag/AgCl; = 0.1 V/s; CH3CN; 0.1 M TBAPF6) and 1.17 V, respectively. Femtosecond to nanosecond transient absorption spectra for [Os(bpy)2(pySOCF3)]2+ are reported. For [Os(bpy)2(pySOCF3)]2+, fits of data obtained at 370 nm yield lifetimes of 0.20 (±0.02) and 19.7 (±8) ps, whereas for [Os(bpy)2(pySO)]2+ lifetimes of 0.47 (±0.2) and 12.7 (±11) ps are obtained at 378 nm. These lifetimes are ascribed to the formation of a 3MLCT state and solvent reorganization, respectively. Excited state lifetimes of 331(±4) and 348 (±25) ns are found for [Os(bpy)2(pySO)]2+ and [Os(bpy)2(pySOCF3)]2+, respectively. The time constant for isomerization is 213 μs for [Os(bpy)2(pySOCF3)]2+. Transient absorption spectra reveal an excited state sulfoxide to Os3+ transition. The electrochemical and spectroscopic data can be combined to yield electronic state diagrams for [Os(bpy)2(pySO)]2+ and [Os(bpy)2(pySOCF3)]2+.