Published: July 19, 2011 r2011 American Chemical Society 7586 dx.doi.org/10.1021/ic200532p | Inorg. Chem. 2011, 50, 7586–7590 ARTICLE pubs.acs.org/IC Ultrafast Spectroscopy of a Photochromic Ruthenium Sulfoxide Complex Beth Anne McClure † and Jeffrey J. Rack* Department of Chemistry and Biochemistry, Nanoscale and Quantum Phenomena Institute, Ohio University, Athens, Ohio 45701, United States b S Supporting Information ’ INTRODUCTION Ultrafast visible spectroscopy reveals critical temporal infor- mation regarding the dynamics of electronic excited states of organic and inorganic dyes. These investigations have been particularly valuable for transition metal complexes related to [Ru(bpy) 3 ] 2+ , where bpy is 2,2 0 -bipyridine. 17 A number of studies have shown that the vibrationally relaxed 3 MLCT excited state is formed on the femtosecond time scale even for low- symmetry derivatives. 811 From these and other related inves- tigations, new approaches have emerged that aim to control excited state electron transfer through conformational changes of ligand structure on an ultrafast time scale, indicating the im- portance of these studies. 1215 For photochromic compounds, ultrafast spectroscopy reveals not only excited state dynamics but also information regarding excited state bond rupture and bond construction. 16 Such studies are critical for our understanding of these materials since a large nuclear reorganization immediately follows formation of the FranckCondon states. These processes are concomitant with vibrational cooling, energy redistribution, and intersystem cross- ing and highlight the complexity of excited state dynamics in this group of chromophores. 17,18 We have created a class of transition metal photochromes based on ruthenium polypyridine sulfoxide complexes, whose action is based on phototriggered SfO and OfS isomerization. 1921 The large quantum yields of isomerization (Φ SfO ) indicate that these complexes efficiently convert photonic energy to potential energy for excited state bond breaking and forming reactions. Our previous reports on [Ru(bpy) 2 (OSO)] + , where OSO is 2-methyl- sulfinylbenzoate (Scheme 1), include the photochromic behavior (Φ SfO = 0.45), isomerization kinetics, and spectral properties of both S- and O-bonded isomers (absorption spectra, Figure 1). 2224 Herein, we report our first investigation on the femtosecond time scale, which reveals the formation of multiple 3 MLCT states prior to picosecond isomerization. We conclude with a complete energy diagram that depicts the reactivity of this complex. ’ RESULTS AND DISCUSSION Shown in Figure 2 are the femtosecond transient absorp- tion data for S-bonded [Ru(bpy) 2 (OSO)] + in methanol solu- tion. These spectral traces show the appearance of a bleach centered near 400 nm, an absorption peak at 360 nm, and a broad absorption from ∼485 nm to longer wavelengths. In accordance with literature reports, 3,25 these features are ascribed to the loss of Ru II MLCT absorption following excitation, to a π* f π* transition for the reduced bipyridine and to a bpy π f Ru III LMCT transition, respectively. Global analysis of these data reveal a time constant of ∼100 fs. The spectrum at 600 fs is representative of a relaxed metal-to- ligand charge-transfer (MLCT) excited state and is reminis- cent of that observed at 300 fs in the ultrafast transient absorption spectroscopy of [Ru(bpy) 3 ] 2+ . 2,3 Thus we inter- pret these data as corresponding to complete formation of the thermally equilibrated 3 MLCT state. At longer time scales (Figure 3), the S-bonded 3 MLCT spectrum ultimately evolves to a spectrum at 502 ps that is identical to that of ground state O-bonded [Ru(bpy) 2 (OSO)] + (see Figure 1 for comparison). This spectrum represents the ground state difference spectrum of O-bondedS-bonded [Ru(bpy) 2 (OSO)] + . These data indicate that O-bonded [Ru(bpy) 2 (OSO)] + produced from excitation of S-bonded Received: March 15, 2011 ABSTRACT: Photochromic [Ru(bpy) 2 (OSO)](PF 6 ), where bpy is 2,2 0 -bipyridine and OSO is 2-methylsulfinylbenzoate, was investigated by femtosecond transient absorption spectroscopy. The results show that for both S- and O-bonded isomers, a 3 MLCT state is formed on a femtosecond time scale. Also observed is the formation of multiple metal-to-ligand charge-transfer (MLCT) states, representing different con- formers, prior to isomerization on the picosecond time scale. These results and others are compiled in an energy diagram depicting these results.