Abstract:The complexes [Ru(bpy)2(pyESO)](PF6)2 and [Os(bpy)2(pyESO)](PF6)2, in which bpy is 2,2'-bipyridine and pyESO is 2-((isopropylsulfinyl)ethyl)pyridine, were prepared and studied by (1)H NMR, UV-visible and ultrafast transient absorption spectroscopy, as well as by electrochemical methods. Crystals suitable for X-ray structural analysis were grown for [Ru(bpy)2(pyESO)](PF6)2. Cyclic voltammograms of both complexes provide evidence for S→O and O→S isomerization as these voltammograms are described by an ECEC (elec… Show more
“…Interestingly, the related compound [Os-(bpy) 2 (pySO)] 2+ , where pySO is 2-((isopropylsulfinyl)-methyl)pyridine, does not feature phototriggered isomerization of the sulfoxide. One explanation for this difference is that the Os−S bond is weaker in the complex with a fluorinated group 41 We speculate that the longer Os−S bonds and larger bite angle associated with an expanded chelate must weaken the Os−S bond enough to permit isomerization following electrochemical oxidation. This suggestion is in accord with our observations regarding the reactivity of [Ru(bpy) 2 (OSSO)] 2+ , where one photon absorption prompts two isomerizations.…”
Section: ■ Sulfoxide Isomerization On Osmiummentioning
Molecular photochromic compounds are those that interconvert between two isomeric forms with light. The two isomeric forms display distinct electronic and molecular structures and must not be in equilibrium with one another. These light-activated molecular switch compounds have found wide application in areas of study ranging from chemical biology to materials science, where conversion from one isomeric form to another by light prompts a response in the environment (e.g., protein or polymeric material). Certain ruthenium and osmium polypyridine sulfoxide complexes are photochromic. The mode of action is a phototriggered isomerization of the sulfoxide from S- to O-bonded. The change in ligation drastically alters both the spectroscopic and electrochemical properties of the metal complex. Our laboratory has pioneered the preparation and study of these complexes. In particular, we have applied femtosecond pump-probe spectroscopy to reveal excited state details of the isomerization mechanism. The data from numerous complexes allowed us to predict that the isomerization was nonadiabatic in nature, defined as occurring from a S-bonded triplet excited state (primarily metal-to-ligand charge transfer in character) to an O-bonded singlet ground state potential energy surface. This prediction was corroborated by high-level density functional theory calculations. An intriguing aspect of this reactivity is the coupling of nuclear motion to the electronic wave function and how this coupling affects motions productive for isomerization. In an effort to learn more about this coupling, we designed a project to examine phototriggered isomerization in bis-sulfoxide complexes. The goal of these studies was to determine whether certain complexes could be designed in which a single photon excitation event would prompt two sulfoxide isomerizations. We employed chelating sulfoxides in this study and found that both the nature of the chelate ring and the R group on the sulfoxide affect the photochemical reactivity. For example, this reactivity may be tuned such that two sulfoxide ligands isomerize sequentially following two successive excitations or that two sulfoxide ligands isomerize following a single excitation. This Account explains our understanding to date of this photochemistry.
“…Interestingly, the related compound [Os-(bpy) 2 (pySO)] 2+ , where pySO is 2-((isopropylsulfinyl)-methyl)pyridine, does not feature phototriggered isomerization of the sulfoxide. One explanation for this difference is that the Os−S bond is weaker in the complex with a fluorinated group 41 We speculate that the longer Os−S bonds and larger bite angle associated with an expanded chelate must weaken the Os−S bond enough to permit isomerization following electrochemical oxidation. This suggestion is in accord with our observations regarding the reactivity of [Ru(bpy) 2 (OSSO)] 2+ , where one photon absorption prompts two isomerizations.…”
Section: ■ Sulfoxide Isomerization On Osmiummentioning
Molecular photochromic compounds are those that interconvert between two isomeric forms with light. The two isomeric forms display distinct electronic and molecular structures and must not be in equilibrium with one another. These light-activated molecular switch compounds have found wide application in areas of study ranging from chemical biology to materials science, where conversion from one isomeric form to another by light prompts a response in the environment (e.g., protein or polymeric material). Certain ruthenium and osmium polypyridine sulfoxide complexes are photochromic. The mode of action is a phototriggered isomerization of the sulfoxide from S- to O-bonded. The change in ligation drastically alters both the spectroscopic and electrochemical properties of the metal complex. Our laboratory has pioneered the preparation and study of these complexes. In particular, we have applied femtosecond pump-probe spectroscopy to reveal excited state details of the isomerization mechanism. The data from numerous complexes allowed us to predict that the isomerization was nonadiabatic in nature, defined as occurring from a S-bonded triplet excited state (primarily metal-to-ligand charge transfer in character) to an O-bonded singlet ground state potential energy surface. This prediction was corroborated by high-level density functional theory calculations. An intriguing aspect of this reactivity is the coupling of nuclear motion to the electronic wave function and how this coupling affects motions productive for isomerization. In an effort to learn more about this coupling, we designed a project to examine phototriggered isomerization in bis-sulfoxide complexes. The goal of these studies was to determine whether certain complexes could be designed in which a single photon excitation event would prompt two sulfoxide isomerizations. We employed chelating sulfoxides in this study and found that both the nature of the chelate ring and the R group on the sulfoxide affect the photochemical reactivity. For example, this reactivity may be tuned such that two sulfoxide ligands isomerize sequentially following two successive excitations or that two sulfoxide ligands isomerize following a single excitation. This Account explains our understanding to date of this photochemistry.
“…For other sulfoxide complexes (e.g., those with chelating sulfoxide ligands similar to that shown in Fig. 1a ), the 3 MLCT S state was suggested to couple non-adiabatically to the singlet ground state 20 , 21 , 23 – 25 . This was suggested as an efficient pathway to formation of the O-bonded ground state ( 1 G O ), as shown in Fig.…”
Section: Introductionmentioning
confidence: 90%
“…1a ) that can be optically triggered with high quantum efficiency 17 , 19 – 22 . Their isomerization rates are empirically tunable via ligand modification of the functional moieties, which in this study comprise a sulfoxide group attached to a chelate ring 23 . Fig.…”
Section: Introductionmentioning
confidence: 99%
“…Ultrafast optical spectroscopy studies of several sulfoxide-containing Ruthenium complexes 20 , 21 , 23 – 25 first revealed the dynamics of the isomerization process, and established that the initial excited state is predominantly metal-to-ligand charge-transfer (MLCT) in character. Moreover, the transient optical data of these complexes suggested photoisomerization mechanisms mediated by triplet MLCT ( 3 MLCT) excited states.…”
Section: Introductionmentioning
confidence: 99%
“…Thus these predictions support a model in which non-adiabatic isomerization pathways through conical intersection seams involving 3 MC states are favored over adiabatic routes, with important implications for the speed and quantum efficiency of these reactions. Other non-adiabatic isomerization pathways that are instead mediated by 3 MLCT states have been suggested for Ru sulfoxide complexes, based on optical spectroscopic studies 20 , 21 , 23 – 25 , and for a Ru nitrosyl complex, based on theoretical studies 30 – 33 . The precise role of MC states and the PESs on which isomerization occurs, therefore, remain important open questions for this class of systems.…”
Ultrafast isomerization reactions underpin many processes in (bio)chemical systems and molecular materials. Understanding the coupled evolution of atomic and molecular structure during isomerization is paramount for control and rational design in molecular science. Here we report transient X-ray absorption studies of the photo-induced linkage isomerization of a Ru-based photochromic molecule. X-ray spectra reveal the spin and valence charge of the Ru atom and provide experimental evidence that metal-centered excited states mediate isomerization. Complementary X-ray spectra of the functional ligand S atoms probe the nuclear structural rearrangements, highlighting the formation of two metal-centered states with different metal-ligand bonding. These results address an essential open question regarding the relative roles of transient charge-transfer and metal-centered states in mediating photoisomerization. Global temporal and spectral data analysis combined with time-dependent density functional theory reveals a complex mechanism for photoisomerization with atomic details of the transient molecular and electronic structure not accessible by other means.
Electrochemical and photochemical bond-activation steps are important for a variety of chemical transformations. We present here four new complexes, [Ru(L(n) )(dmso)(Cl)]PF6 (1-4), where L(n) is a tripodal amine ligand with 4-n pyridylmethyl arms and n-1 triazolylmethyl arms. Structural comparisons show that the triazoles bind closer to the Ru center than the pyridines. For L(2) , two isomers (with respect to the position of the triazole arm, equatorial or axial), trans-2sym and trans-2un , could be separated and compared. The increase in the number of the triazole arms in the ligand has almost no effect on the Ru(II) /Ru(III) oxidation potentials, but it increases the stability of the RuSdmso bond. Hence, the oxidation waves become more reversible from trans-1 to trans-4, and whereas the dmso ligand readily dissociates from trans-1 upon heating or irradiation with UV light, the RuS bond of trans-4 remains perfectly stable under the same conditions. The strength of the RuS bond is not only influenced by the number of triazole arms but also by their position, as evidenced by the difference in redox behavior and reactivity of the two isomers, trans-2sym and trans-2un . A mechanistic picture for the electrochemical, thermal, and photochemical bond activation is discussed with data from NMR spectroscopy, cyclic voltammetry, and spectroelectrochemistry.
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