Spectroscopic and Kinetic Characterization of Photochromic Ruthenium Chelating Sulfoxide Complexes

Complexes of the type [Ru(bpy)2(OSOR)]+, where OSOR is a sulfinyl benzoate chelate with various substituents, R, attached to the sulfur, and [Ru(bpy)2(pySO)]2+, (pySO = 2-(isopropylsulfinylmethyl)pyridine) were synthesized and characterized by a number of spectroscopic techniques. Structural characterization was done by 1H NMR and IR spectroscopy as well as X-ray crystallography for some complexes. Analysis of the structural features such as S-O bond lengths and nu(S=O) vibrational stretches are suggestive of a pi-back bonding interaction between the ruthenium metal center and the sulfoxide group in the S-bonded isomer. Electrochemical measurements also support such an interaction by large positive Ru2+/3+ reduction potentials for the S-bonded isomer. Cyclic voltammetry reveals irreversible oxidation behavior that agrees with previous studies finding an S-to-O isomerization to occur after oxidation to the Ru3+ state. However, compared to similar non-chelate ruthenium sulfoxides, the rate of isomerization is significantly slower indicating that the chelate hinders the degrees of freedom utilized in this reaction. Electronic absorption spectra are also consistent with a stabilization of the dpi orbitals as a result of pi-back bonding, especially as compared to similar non-chelate sulfoxide complexes. Irradiation of the complexes was found to promote an S-to-O isomerization. The complex [Ru(bpy)2(pySO)]2+ was found to undergo reversible O-to-S phototriggered isomerization as well which is very rare for transition metal based photochromes. Analysis of the UV-Vis absorption and low temperature emission spectra of these complexes reveal significant distortion of nuclear coordinates between the ground state potential energy surface and the Franck-Condon state and lowest energy excited state potential energy surfaces. Emission spectral fitting indicates strong vibronic coupling between a vibrational mode similar in energy to the nu(S=O) mode with relaxation to the ground state. Transient absorption measurements indicate rapid non-adiabatic isomerization from a MLCT state of S-bonded or eta2-SO character to the O-bonded ground state, with time constants of isomerization as fast as 84 ps. The complex O-[Ru(bpy)2(pySO)]2+ shows a similar reverse isomerization with relaxation to the S-bonded ground state as well. The non-adiabatic mechanism is consistent with the large distortion indicated by spectral analysis, further supporting an eta2-SO geometry for the excited state minimum geometry.