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 C2 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.