Beth Anne McClure

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.

Light Induced Carbon Dioxide Reduction by Water at Binuclear ZrOCoII Unit Coupled to Ir Oxide Nanocluster Catalyst

An all-inorganic polynuclear unit consisting of an oxo-bridged binuclear ZrOCo(II) group coupled to an iridium oxide nanocluster (IrO(x)) was assembled on an SBA-15 silica mesopore surface. A photodeposition method was developed that affords coupling of the IrO(x) water oxidation catalyst with the Co donor center. The approach consists of excitation of the ZrOCo(II) metal-to-metal charge-transfer (MMCT) chromophore with visible light in the presence of [Ir(acac)3] (acac: acetylacetonate) precursor followed by calcination under mild conditions, with each step monitored by optical and infrared spectroscopy. Illumination of the MMCT chromophore of the resulting ZrOCo(II)-IrO(x) units in the SBA-15 pores loaded with a mixture of (13)CO2 and H2O vapor resulted in the formation of (13)CO and O2 monitored by FT-IR and mass spectroscopy, respectively. Use of (18)O labeled water resulted in the formation of (18)O2 product. This is the first example of a closed photosynthetic cycle of carbon dioxide reduction by water using an all-inorganic polynuclear cluster featuring a molecularly defined light absorber. The observed activity implies successful competition of electron transfer between the IrO(x) catalyst cluster and the transient oxidized Co donor center with back electron transfer of the ZrOCo light absorber, and is further aided by the instant desorption of the CO and O2 product from the silica pores.

Investigating the effects of solvent on the ultrafast dynamics of a photoreversible ruthenium sulfoxide complex.

The photochromic complex [Ru(bpy)2(pySO)](2+) [pySO is 2-(isopropylsulfinylmethyl)pyridine] undergoes wavelength specific, photoreversible S → O and O → S linkage isomerizations. Irradiation of the ground state S-bonded complex with blue light produces the O-bonded isomer, while irradiation of the O-bonded isomer with green light produces the S-bonded isomer. Furthermore, isomerization time constants are solvent-dependent. Ultrafast transient absorption spectroscopy has been employed to investigate the relaxation processes that lead to S → O isomerization in 1,2-dichloroethane, propylene carbonate, and ethylene glycol. The isomerization is most rapid in 1,2-dichloroethane and slowest in ethylene glycol. Photochemical reversion of the O-bonded isomer in propylene carbonate has further been investigated and indicates similar relaxation or isomerization kinetics, though the excited states that lead to isomerization are distinct between the S- and O-bonded isomers.

Relaxation-Assisted Dual-Frequency Two-Dimensional Infrared Spectroscopy: Measuring Distances and Bond Connectivity

Potential of a novel relaxation-assisted 2DIR spectroscopy method is demonstrated on several molecular systems, including model compounds, peptides, and transition metal complexes. The cross-peaks for modes separated by distances greater than 11A can be easily detected using RA 2DIR. A correlation of the energy transport time (arrival time) with the intermode distance is shown in several molecular systems for a number of mode pairs.