Two-color reversible switching in a photochromic ruthenium sulfoxide complex.

Reversible, external triggering of molecules between two ground states is central to the operation of modern molecular machines. A desirable trigger is light, as it provides an ample amount of energy in a short burst of time. Although light is commonly employed for the formation of many different types of metastable states, in practice it is difficult to employ light to regenerate the initial ground state. The enthalpic and entropic factors that favor the photochemical generation of the metastable state from the ground state must necessarily be disfavored for the opposing photochemical reaction. Moreover, the photoproduct often reverts to the ground state thermally. Irradiation of photochromic compounds yields metastable states that exhibit distinct electronic structures compared to their ground states. Whilst a number of organic photochromic compounds feature reversible or two-color photoswitching between metastable and ground states, such reactivity is rare in transition metal complexes. Herein, we report our findings on a ruthenium sulfoxide complex that features two-color photochromism. The complex [Ru(bpy)2(pySO)](PF6)2 (bpy = 2,2’-bipyridine, pySO = 2-(isopropylsulfinylmethyl)pyridine) was prepared from reaction of cis-[Ru(bpy)2Cl2] with pySO. The structure and formulation of the complex was verified by oneand-two-dimensional H NMR, IR spectroscopy, and elemental analysis. The H NMR spectrum is consistent with the presence of two isomers, which are most likely diastereomers, as both the ruthenium and sulfur centers are chiral. Elemental analysis of the isomeric mixture supports this assignment. The lowest-energy transition in the electronic spectrum occurs at 370 nm (e = 7250 cm m ) and is assigned to a Ru dp! bpy p* charge-transfer transition based on its energy and intensity (Figure 1). This transition is similar to that observed for other chelating S-bonded sulfoxides. Furthermore, the structurally characterized complex [Ru(bpy)2(py)(dmso)] 2+ (dmso = dimethylsulfoxide), which has no chelation, features a maximum absorbance at 400 nm for the S-bonded isomer. Therefore, the chelate complex exhibits an additional 2000 cm 1 of stabilization relative to the non-chelate form. White light irradiation of [Ru(bpy)2(pySO)] 2+ in propylene carbonate solution results in a decrease in the absorbance at 370 nm concomitant with an increase at 472 nm (Figure 1, inset). In accord with other ruthenium sulfoxide complexes, the spectral changes indicate an intramolecular S! O isomerization, with an isosbestic point at 408 nm. However, the observed spectral changes are muted in comparison to these other complexes. We questioned whether or not this spectrum represented a photostationary state. Accordingly, irradiation of this solution with 355 nm light resulted in a more pronounced absorbance at 472 nm (quantum efficiency FS! O = 0.11(2)). This value is similar to that of the Obonded isomer of [Ru(bpy)2(py)(dmso)] 2+ (476 nm) and [Ru(bpy)2(pic)] + (483 nm; pic = 2-pyridinecarboxylate), which feature N and O donors in the Ru(bpy)2 coordination sphere. The S-bonded and extracted O-bonded spectra (e472nm = 6400 cm m ) are shown in Figure 1. The IR spectrum provides structural evidence for isomerization; the ground-state S-bonded isomer has a band at ñ(S=O) = 1090 cm 1 which is replaced by a band at ñ(S=O) = 1060 cm 1 upon irradiation. This new feature is ascribed to the O-bonded isomer. The presence of a photostationary state with white light irradiation indicates an excited-state photochemical pathway for O! S isomerization from longer wavelengths. Accordingly, 470 nm irradiation of a solution that contains predominately the O-isomer resulted in a decrease of the absorbance at 472 nm concomitant with an increase at 370 nm (FO! S = Figure 1. Absorption of S-bonded (c) and O-bonded (b) isomers. The spectrum of the O-bonded isomer was extrapolated from the photostationary state spectrum. Inset: Spectrum depicting the photostationary state obtained following white-light irradiation of the S-bonded isomer.