Sequential Picosecond Isomerizations in a Photochromic Ruthenium Sulfoxide Complex Triggered by Pump-Repump-Probe Spectroscopy

The complex [Ru(bpy)2(bpSO)](PF6)2, where bpy is 2,2′-bipydine and bpSO is 1,2-bis(phenylsulfinyl)ethane, exhibits three distinct isomers which are accessible upon metal-to-ligand charge-transfer (MLCT) irradiation. This complex and its parent, [Ru(bpy)2(bpte)](PF6)2, where bpte is 1,2-bis(phenylthio)ethane, have been synthesized and characterized by UV–visible spectroscopy, NMR, X-ray crystallography, and femtosecond transient absorption spectroscopy. A novel method of 2-color Pump-Repump-Probe spectroscopy has been employed to investigate all three isomers of the bis-sulfoxide complex. This method allows for observation of the isomerization dynamics of sequential isomerizations of each sulfoxide from MLCT irradiation of the S,S-bonded complex to ultimately form the O,O-bonded metastable complex. One-dimensional (1-D) and two-dimensional (2-D) (COSY, NOESY, and TOCSY) 1H NMR data show the thioether and ground state S,S-bonded sulfoxide complexes to be rigorously C 2 symmetric and are consistent with the crystal structures. Transient absorption spectroscopy reveals that the S,S to S,O isomerization occurs with an observed time constant of 56.8 (±7.4) ps. The S,O to O,O isomerization time constant was found to be 59 (±4) ps by pump-repump-probe spectroscopy. The composite S,S- to O,O-isomer quantum yield is 0.42.