8084 Inorg. Chem. 2009, 48, 8084–8091 DOI: 10.1021/ic900421v
Photochromic Ruthenium Sulfoxide Complexes: Evidence for Isomerization Through a Conical Intersection Beth Anne McClure,† Nicholas V. Mockus,† Dennis P. Butcher, Jr.,† Daniel A. Lutterman,†,‡ Claudia Turro,*,†,‡ Jeffrey L. Petersen,†,§ and Jeffrey J. Rack*,† †
Department of Chemistry and Biochemistry, Ohio University, Athens, Ohio 45701, ‡Department of Chemistry, The Ohio State University, Columbus, Ohio 43210, and §C. Eugene Bennett Department of Chemistry, West Virginia University, Morgantown, West Virginia 26506-6045
Received March 4, 2009
The complexes [Ru(bpy)2(OS)](PF6) and [Ru(bpy)2(OSO)](PF6), where bpy is 2,20 -bipyridine, OS is 2-methylthiobenzoate, and OSO is 2-methylsulfinylbenzoate, have been studied. The electrochemical and photochemical reactivity of [Ru(bpy)2(OSO)]+ is consistent with an isomerization of the bound sulfoxide from S-bonded (S-) to O-bonded (O-) following irradiation or electrochemical oxidation. Charge transfer excitation of [Ru(bpy)2(OSO)]+ in MeOH results in the appearance of two new metal-to-ligand charge transfer (MLCT) maxima at 355 and 496 nm, while the peak at 396 nm diminishes in intensity. The isomerization is reversible at room temperature in alcohol or propylene carbonate solution. In the absence of light, solutions of O-[Ru(bpy)2(OSO)]+ revert to S-[Ru(bpy)2(OSO)]+. Kinetic analysis reveals a biexponential decay with rate constants of 5.66(3) 10-4 s-1 and 3.1(1) 10-5 s-1. Cyclic voltammograms of S-[Ru(bpy)2(OSO)]+ are consistent with electron-transfer-triggered isomerization of the sulfoxide. Analysis of these voltammograms reveal ES°0 = 0.86 V and EO°0 = 0.49 V versus Ag/Ag+ for the S- and O-bonded Ru3+/2+ couples, respectively, in propylene carbonate. We found kSfO = 0.090(15) s-1 in propylene carbonate and kSfO = 0.11(3) s-1 in acetonitrile on RuIII, which is considerably slower than has been reported for other sulfoxide isomerizations on ruthenium polypyridyl complexes following oxidation. The photoisomerization quantum yield (ΦSfO = 0.45, methanol) is quite large, indicating a rapid excited state isomerization rate constant. The kinetic trace at 500 nm is monoexponential with τ = 150 ps, which is assigned to the excited SfO isomerization rate. There is no spectroscopic or kinetic evidence for an O-bonded 3MLCT excited state in the spectral evolution of S-[Ru(bpy)2(OSO)]+ to O-[Ru(bpy)2(OSO)]+. Thus, isomerization occurs nonadiabatically from an S-bonded (or η2-sulfoxide) 3MLCT excited state to an O-bonded ground state. Density functional theory calculations support the assigned spectroscopy and provide insight into ruthenium ligand bonding.
Introduction In 1970, Forster classified photochemical reactions as either adiabatic or nonadiabatic, depending upon whether the chemical reaction occurred on a potential energy surface of identical multiplicity or one of different multiplicity.1 These concepts have since been expanded to focus not solely on spin, such that adiabatic reactions are those that separate atomic and electronic movement (Born-Oppenheimer approximation), while nonadiabatic reactions are those that involve nuclear dynamics on two separate potential energy curves.2,3 The latter may indeed be much more common than *To whom correspondence should be addressed. E-mail:
[email protected] (J.J.R.). (1) Forster, T. Pure Appl. Chem. 1970, 24, 443–449. (2) Domcke, W.; Yarkony, D. R.; Koppel, H. Conical Intersections: Electronic Structure, Dynamics and Spectroscopy; World Scientific: River Edge, NJ, 2004. (3) Worth, G. A.; Cederbaum, L. S. Annu. Rev. Phys. Chem. 2004, 55, 127–158.
pubs.acs.org/IC
Published on Web 05/12/2009
originally thought. In mapping potential energy surfaces, recent computational studies have revealed conical intersections at which the excited-state molecule may rapidly undergo nonradiative decay to the ground-state potential energy surface.4-6 Such regions occur at the intersection of two or more orthogonal reaction coordinates in the evolution of an excited state. These pathways provide a facile mechanism for the formation of a photoproduct with little to no activation barrier. This issue is especially important in photochemical isomerizations where substantial changes in atomic connectivity and electronic structure occur on a femtosecond or picosecond time scale. Efficient conversion of photonic energy to potential energy in these reactions requires minimal energy pathways for the formation of photoproducts. (4) Dick, B.; Haas, Y.; Zilberg, S. Chem. Phys. 2008, 347, 65–77. (5) Piryatinski, A.; Tretiak, S.; Chernyak, V. Y. Chem. Phys. 2008, 347, 25–38. (6) Quenneville, J.; Martinez, T. J. J. Phys. Chem. A 2003, 107, 829–837.
r 2009 American Chemical Society
Article
Photochromic molecules are a special case of photochemical isomerization.7-11 The resultant photochemical product, or metastable state, exhibits a substantially different atomic and electronic structure than the initial ground state. Mechanistic studies that reveal the nature of coupling between atomic structure and electronic state relaxation in these complexes are of fundamental importance. For example, photochromic molecules have featured prominently in the construction of logic gates,12-14 photoswitches,15-19 and other photoresponsive materials.20-26 For our part, we have developed a class of synchronously photochromic and electrochromic ruthenium and osmium polypyridine sulfoxide compounds that operate at room temperature, in the solid state, and switch on the picosecond to nanosecond time scale.27-36 The photochromic action is due to a phototriggered isomerization of the bound sulfoxide from S- to O-bonded. Reversion of the metastable O-bonded product to
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Inorganic Chemistry, Vol. 48, No. 17, 2009
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the S-bonded starting material occurs either photochemically or thermally, depending upon the nature of the metal atom and the ancillary ligands. In our studies of [Ru(bpy)2(OSO)]+,37 where bpy is 2,20 -bipyridine and OSO is 2-methylsulfinylbenzoate, we have found evidence for a nonadiabatic isomerization on the picosecond time scale from S- to O-bonded sulfoxide. Experimental Section The compound cis-Ru(bpy)2Cl2 3 xH2O was either synthesized using published methods38 or purchased from Strem and used as received. The reagents bpy, 2-(methylthio)benzoic acid (OS), and 3-chloroperbenzoic acid (m-cpba) were purchased from Aldrich and used as received. Solvents such as methanol (MeOH), ethanol (EtOH), anhydrous propylene carbonate, 1,2-dichloroethane, and diethyl ether (Et2O) were purchased from Aldrich and used without further purification. Acetonitrile for electrochemical measurements was HPLC-grade purchased from Burdick and Jackson and used without further purification. Tetrabutylammonium hexafluorophosphate (TBAPF6) was purchased from Aldrich and recrystallized three times from ethanol before use. [Ru(bpy)2(OS)](PF6). Dark purple cis-Ru(bpy)2Cl2 (200 mg, 0.384 mmol), OS (71.1 mg, 0.422 mmol), 120 μL of triethylamine, and 2 equiv of AgPF6 (194 mg, 0.768 mmol) were dissolved in 125 mL of 1,2-dichloroethane. The reaction was refluxed for 4 h under argon. The solution changed from purple to deep red as the reaction progressed, during which time solid AgCl precipitated. The solution was cooled to -30 °C overnight to ensure full precipitation of AgCl and then filtered to collect 2 equiv of AgCl. The filtrate was subsequently removed by rotary evaporation. The resulting solid was dissolved in 25 mL of dichloromethane and extracted with a 10 mL of an aqueous solution of 30 mg of LiOH 3 H2O to remove NEt3HPF6 formed during reaction. The dichloromethane layer was dried with magnesium sulfate, and the solvent was removed by rotary evaporation. The solid was dissolved in about 5 mL of ethanol. Ether was added to precipitate a red solid. The product was isolated via vacuum filtration, washed with ether (3 15 mL), and air-dried. Yield: 210 mg (75%). UV-vis (MeOH): λmax= 467 nm (5510 M-1cm-1). E°0 Ru3+/2+ vs Ag/AgCl = 1.0 V. 1H NMR (CD3CN, 300 MHz): δ 9.15 (d, bpy, 1 H), 8.84 (d, bpy, 1 H), 8.67 (d, bpy, 1 H), 8.53 (d, 3 H), 8.27 (t, bpy, 1 H), 8.00 (m, 3 H), 7.90 (t, bpy, 1 H), 7.80 (m, 2 H), 7.70 (d, S-O, 1 H), 7.50 (m, 3 H), 7.33 (m, 2 H), 7.28 (t, bpy, 1 H), 1.86(s, SCH3, 3 H). Elem anal. calcd for [Ru(C10H8N2)2(C8H7O2S)]PF6 3 H2O 3 0.2Et2O: C, 45.64%; H, 3.60%; O, 6.76%; N, 7.39%; S, 4.23%. Found: C, 45.79%; H, 3.50%; O, 6.54%; N, 7.46%; S, 4.11%. [Ru(bpy)2(OSO)](PF6). Red [Ru(bpy)2(OS)](PF6) (50.00 mg, 0.0689 mmol) and m-CPBA (26.16 mg, 0.1516 mmol) were dissolved in 50 mL of methanol. The reaction was stirred at room temperature in the dark for 4 h. The progress of the reaction was monitored by the 3MLCT transition via UV-vis spectroscopy (MLCT = metal-to-ligand charge transfer). The solution volume was reduced to
