| Phototriggered linkage S→O and O→S isomerization in ruthenium and osmium polypyridine sulfoxide complexes are of specific interest due to their potential use in photomolecular devices, information storage and for storing solar energy. Complexes of the type, [M(bpy)2(L2)] 2+ (where M is ruthenium or osmium, bpy is 2,2'-bipyridine, L2 is 2-((isopropylsulfinyl)methyl)pyridine) (pySO), 2-((trifluoroethylsulfinyl)methyl)pyridine (pySOCF3), 2-(phenylsulfinyl)methyl)pyridine (pySO-Ph), 2-((2- fluorophenylsulfinyl)methyl)pyridine (pySO-2FPh), 2-((isopropylsulfinyl)ethyl)pyridine (pyESO), dimethylbis(methylsulfinylmethyl)silane (OSSO)) were synthesized and characterized by number of techniques. Structural characterization was performed by Xray crystallography, 1H NMR, 2D NMR and IR spectroscopy. Electronic characterization of these complexes was performed by UV-visible spectroscopy, cyclic voltammetry and transient absorption spectroscopy. The ground state of these complexes features an Sbonded sulfoxide to RuII or OsII and also exhibits strong absorption in the visible region typically assigned as metal-to-ligand charge transfer (MLCT) transitions. Cyclic voltammetry often reveals irreversible one-electron oxidation behavior suggesting the Sto O-bonded isomerization to occur at the Ru3+ oxidation state. Charge-transfer excitation of these complexes also shows substantial changes in the UV-visible spectrum, consistent 4 with S→O isomerization. The reverse O→S isomerization normally occurs at room temperature or with modest heating. Femtosecond and nanosecond transient absorption spectroscopic measurements were performed to characterize the excited-state behavior and to obtain the dynamics of these molecules upon photoexcitation. Transient absorption measurements suggest a rapid non-adiabatic isomerization pathway, which involves the interplay of 3MLCT, 3LF and ground state potential energy surfaces, with S→O time constant of isomerization as fast as 47 ps in the ruthenium sulfoxide complex, [Ru(bpy) 2(pyESO)]2+. Due to the large ligand field energy gap in Os2+ and the strong Os--S bond, not all osmium sulfoxide complexes feature phototriggered isomerization behavior. Two sulfoxide isomerization with single photon excitation is also observed in ruthenium bis-sulfoxide complex, [Ru(bpy)2(OSSO)]2+, which is very rare phenomenon. These studies demonstrate the nature of the interaction between large atomic displacements and the electronic transitions in the excited-state reactions. |
