Two-Color Reversible Switching in a Photochromic Ruthenium Sulfoxide Complex

Photocrystallography of [Ru(bpy)2(dmso)2]2+ reveals an O-bonded metastable state

We report the first instance of observing the phototriggered isomerization of dmso ligands on a bis sulfoxide complex, [Ru(bpy)₂(dmso)₂], in the crystalline solid state. The solid-state UV-vis spectrum of the crystal demonstrates an increase in optical density around 550 nm after irradiation, which is consistent with the solution isomerization results. Digital images of the crystal before and after irradiation display a notable color change (pale orange to red) and cleavage occurs along planes, (1̄01) and (100), during irradiation. Single crystal X-ray diffraction data also confirms that isomerization is occurring throughout the lattice and a structure that contains a mix of the S,S and O,O/S,O isomer was attained from a crystal irradiated ex situ. In situ irradiation XRD studies reveal that the percentage of the O-bonded isomer increases as a function of 405 nm exposure time.

Main Group Molecular Switches with Swivel Bifurcated to Trifurcated Hydrogen Bond Mode of Action

Artificial molecular machines have captured the full attention of the scientific community since Jean-Pierre Sauvage, Fraser Stoddart, and Ben Feringa were awarded the 2016 Nobel Prize in Chemistry. The past and current developments in molecular machinery (rotaxanes, rotors, and switches) primarily rely on organic-based compounds as molecular building blocks for their assembly and future development. In contrast, the main group chemical space has not been traditionally part of the molecular machine domain. The oxidation states and valency ranges within the p-block provide a tremendous wealth of structures with various chemical properties. Such chemical diversity─when implemented in molecular machines─could become a transformative force in the field. Within this context, we have rationally designed a series of NH-bridged acyclic dimeric cyclodiphosphazane species, [(μ-NH){PE(μ-N^tBu)₂PE(NH^tBu)}₂] (E = O and S), bis-P^V₂N₂, displaying bimodal bifurcated R²₁(8) and trifurcated R³₁(8,8) hydrogen bonding motifs. The reported species reversibly switch their topological arrangement in the presence and absence of anions. Our results underscore these species as versatile building blocks for molecular machines and switches, as well as supramolecular chemistry and crystal engineering based on cyclophosphazane frameworks.

A Future Perspective on Phototriggered Isomerizations of Transition Metal Sulfoxides and Related Complexes

Photochromic molecules are examples of light-activated bistable molecules. We highlight the design criteria for a class of ruthenium and osmium sulfoxide complexes that undergo phototriggered isomerization of the bound sulfoxide. The mode of action in these complexes is an excited-state isomerization of the sulfoxide from S-bonded to O-bonded. We discuss the basic mechanism for this transformation and highlight specific examples that demonstrate the effectiveness and efficiency of the isomerization. We subsequently discuss future research directions within the field of phototriggered sulfoxide isomerizations on transition metal polypyridine complexes. These efforts involve new synthetic directions, including the choice of metal as well as new ambidentate ligands for isomerization.

An excited state dynamics driven reaction: wavelength-dependent photoisomerization quantum yields in [Ru(bpy)2(dmso)2]2

We report the excited-state behavior of a structurally simple bis-sulfoxide complex, cis-S,S-[Ru(bpy)2(dmso)2]2+, as investigated by femtosecond pump-probe spectroscopy. The results reveal that a single photon prompts phototriggered isomerization of one or both dmso ligands to yield a mixture of cis-S,O-[Ru(bpy)2(dmso)2]2+ and cis-O,O-[Ru(bpy)2(dmso)2]2+. The quantum yields of isomerization of each product and relative product distribution are dependent upon the excitation wavelength, with longer wavelengths favoring the double isomerization product, cis-O,O-[Ru(bpy)2(dmso)2]2+. Transient absorption measurements on cis-O,O-[Ru(bpy)2(dmso)2]2+ do not reveal an excited-state isomerization pathway to produce either the S,O or S,S isomers. Femtosecond pulse shaping experiments reveal no change in the product distribution. Pump-repump-probe transient absorption spectroscopy of cis-S,S-[Ru(bpy)2(dmso)2]2+ shows that a pump-repump time delay of 3 ps dramatically alters the S,O : O,O product ratio; pump-repump-probe transient absorption spectroscopy of cis-O,O-[Ru(bpy)2(dmso)2]2+ with a time delay of 3 ps uncovers an excited-state isomerization pathway to produce the S,O isomer. In conjunction with low-temperature steady-state emission spectroscopy, these results are interpreted in the context of an excited-state bifurcating pathway, in which the isomerization product distribution is determined not by thermodynamics, but rather as a dynamics driven reaction.

Photoisomerization and thermal isomerization of ruthenium aqua complexes with chloro-substituted asymmetric bidentate ligands

A series of ruthenium complexes with chloro-substituted bidentate ligands, proximal-[Ru(tpy)(Cl-pyqu)L] n+ [n = 1 for L = Cl, and n = 2 for L = OH2, tpy = 2,2';6',2''-terpyridine, pyqu = 2-(2'-pyridyl)quinoline] were synthesized and their reversible photoisomerizations and thermal isomerizations were investigated experimentally. The crystal structures of the complexes indicated that introduction of a chloro substituent at the 4- or 4'-position of the pyqu ligand did not change the structure around the metal center from that of the non-substituted complex, proximal-[Ru(tpy)(pyqu)L] n+. In contrast, the 6'-substituted complexes had sterically hindered environments around the metal center. The ruthenium aqua complexes showed reversible photoisomerization between the proximal and distal isomers. The quantum yield for photoisomerization of the 6'-substituted ruthenium aqua complex was almost twice as large as those of the other derivatives. This is explained by weakening of the ligand field on the ruthenium center by introduction of a chloro substituent at the 6'-position. Thermal back isomerization from the distal isomer to the proximal one was observed for the 6'-substituted complex, but such reactions were not observed for the other derivatives. The steric hindrance in the 6'-substituted aqua complex enhanced both thermal isomerization and photoisomerization.

Transient metal-centered states mediate isomerization of a photochromic ruthenium-sulfoxide complex

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.

An essential open question in functional transition metal complexes is the relative roles of charge-transfer and metal-centered excited states. Here the authors identify the important role of metal-centered excited states in the linkage photoisomerization of a photochromic Ru-sulfoxide complex.

Photochromism into nanosystems: towards lighting up the future nanoworld

The ability to manipulate the structure and function of promising nanosystems via energy input and external stimuli is emerging as an attractive paradigm for developing reconfigurable and programmable nanomaterials and multifunctional devices. Light stimulus manifestly represents a preferred external physical and chemical tool for in situ remote command of the functional attributes of nanomaterials and nanosystems due to its unique advantages of high spatial and temporal resolution and digital controllability. Photochromic moieties are known to undergo reversible photochemical transformations between different states with distinct properties, which have been extensively introduced into various functional nanosystems such as nanomachines, nanoparticles, nanoelectronics, supramolecular nanoassemblies, and biological nanosystems. The integration of photochromism into these nanosystems has endowed the resultant nanostructures or advanced materials with intriguing photoresponsive behaviors and more sophisticated functions. In this Review, we provide an account of the recent advancements in reversible photocontrol of the structures and functions of photochromic nanosystems and their applications. The important design concepts of such truly advanced materials are discussed, their fabrication methods are emphasized, and their applications are highlighted. The Review is concluded by briefly outlining the challenges that need to be addressed and the opportunities that can be tapped into. We hope that the review of the flourishing and vibrant topic with myriad possibilities would shine light on exploring the future nanoworld by encouraging and opening the windows to meaningful multidisciplinary cooperation of engineers from different backgrounds and scientists from the fields such as chemistry, physics, engineering, biology, nanotechnology and materials science.

Theoretical studies on the redox-stimulated isomerization in electrochromic osmium sulfoxide complexes

Redox-stimulated intramolecular isomerization of the DMSO ligand in [Os(bpy)2(DMSO)2](2+) (bpy = 2,2'-bipyridine; DMSO = dimethyl sulfoxide) was explored theoretically for better understanding the electrochromic properties of osmium sulfoxide complexes. It is found that the HOMO-LUMO gap is decreased because the electron transfer amount from DMSO1 ligand to Os center using Os-S1 linkage is larger than that using Os-O1 linkage, which makes the absorption of such electrochromic Os(II) sulfoxide complexes red-shifted. Moreover, it is observed that Os-O linkage is preferred by the "hard" Os(III) metal and the "soft" Os(I) metal prefers Os-S linkage, compared with Os(II). Intrinsic reaction pathway calculation results demonstrate that Os-S1 → Os-O1 isomerization is favored by Os(II) oxidation, while Os-O1 → Os-S1 isomerization is much easier to be triggered by reduction of Os(III) or Os(II). In addition, DMSO2 linkage isomerization becomes much harder to proceed attributed to the increased bond-strength between DMSO2 and Os center upon Os(II)-O1 → Os(II)-S1 rearrangement, which makes only one DMSO ligand isomerized observed experimentally.

Linkage Isomerization via Geminate Cage or Bimolecular Mechanisms: Time-Resolved Investigations of an Organometallic Photochrome

The extent of the photoinitiated linkage isomerization of dicarbonyl(3-cyanomethylpyridine-κN)(η(5)-methylcyclopentadienyl)manganese (4) to dicarbonyl(3-cyano-κN-methylpyridine)(η(5)-methylcyclopentadienyl)manganese (5) was examined by time-resolved infrared spectroscopy on picosecond to microsecond time scales in room temperature isooctane to determine the extent the isomerization occurs as a geminate cage rearrangement. We previously reported that a substantial part of the conversion between 4 and 5 must be a bimolecular reaction between a solvent coordinated dicarbonyl(η(5)-methylcyclopentadienyl)manganese (3) and uncoordinated 3-cyanomethylpyridine. For the purpose of designing a molecular device, it would be desirable for the photoisomerization to occur in a geminate cage reaction, because the faster the isomerization, the less opportunity for side reactions to occur. In this study, assignments of transients are identified by comparison with transients observed for model reactions. Within 100 μs after photolysis of 4 in isooctane, no 5 is observed. Instead, the solvent coordinated 3 is observed within 25 ps after irradiation. The formation of 5 is observed only in the presence of 9 mM 3-cyanomethylpyridine but not until 10-50 μs after irradiation of 4. Within the limits of detection, these results indicate the conversion of 4 to 5 occurs exclusively via a bimolecular reaction of 3-cyanomethylpyridine with solvent coordinated 3 and not a geminate cage reaction between 3-cyanomethylpyridine and the dicarbonyl(η(5)-methylcyclopentadienyl)manganese fragment.

Theoretical studies on the photoisomerization mechanism of osmium(ii) sulfoxide complexes

Role of ³LF state in the photoisomerization mechanism of photochromic osmium sulfoxide complex, [Os(bpy)₂(DMSO)₂]²⁺, is examined theoretically.

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.

One photon yields two isomerizations: large atomic displacements during electronic excited-state dynamics in ruthenium sulfoxide complexes

Photochromic compounds efficiently transduce photonic energy to potential energy for excited-state bond-breaking and bond-forming reactions. A critical feature of this reaction is the nature of the electronic excited-state potential energy surface and how this surface facilitates large nuclear displacements and rearrangements. We have prepared two photochromic ruthenium sulfoxide complexes that feature two isomerization reactions following absorption of a single photon. We show by femtosecond transient absorption spectroscopy that this reaction is complete within a few hundred picoseconds and suggest that isomerization occurs along a conical intersection seam formed by the ground-state and excited-state potential energy surfaces.

Stimulating changes in the elastic modulus of polymer materials by molecular photochromism

Photonastic effects are observed in an amorphous polymer via irradiation of a pendant photoreversible photochrome.

Tuning excited state isomerization dynamics through ground state structural changes in analogous ruthenium and osmium sulfoxide complexes

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 (electrochemical-chemical electrochemical-chemical) mechanism in which isomerization follows Ru(2+) oxidation and Ru(3+) reduction. The S- and O-bonded Ru(3+/2+) couples appear at 1.30 and 0.76 V versus Ag/AgCl in propylene carbonate. For [Os(bpy)2(pyESO)](PF6)2, these couples appear at 0.97 and 0.32 V versus Ag/AgCl in acetonitrile, respectively. Charge-transfer excitation of [Ru(bpy)2(pyESO)](PF6)2 results in a significant change in the absorption spectrum. The S-bonded isomer of [Ru(bpy)2(pyESO)](2+) features a lowest energy absorption maximum at 390 nm and the O-bonded isomer absorbs at 480 nm. The quantum yield of isomerization in [Ru(bpy)2(pyESO)](2+) was found to be 0.58 in propylene carbonate and 0.86 in dichloroethane solution. Femtosecond transient absorption spectroscopic measurements were collected for both complexes, revealing time constants of isomerizations of 81 ps (propylene carbonate) and 47 ps (dichloroethane) in [Ru(bpy)2(pyESO)](2+). These data and a model for the isomerizing complex are presented. A striking conclusion from this analysis is that expansion of the chelate ring by a single methylene leads to an increase in the isomerization time constant by nearly two orders of magnitude.

Transition Metal Compounds Towards Holography

We have successfully proposed the application of transition metal compounds in holographic recording media. Such compounds feature an ultra-fast light-induced linkage isomerization of the transition-metal–ligand bond with switching times in the sub-picosecond regime and lifetimes from microseconds up to hours at room temperature. This article highlights the photofunctionality of two of the most promising transition metal compounds and the photophysical mechanisms that are underlying the hologram recording. We present the latest progress with respect to the key measures of holographic media assembled from transition metal compounds, the molecular embedding in a dielectric matrix and their impressive potential for modern holographic applications.

Sulfoxide stretching mode as a structural reporter via dual-frequency two-dimensional infrared spectroscopy

The S=O stretching mode in sulfoxides, having a frequency in the 950-1150 cm(-1) range, is tested as a structural label via dual-frequency two-dimensional infrared (2DIR) spectroscopy. The properties of this structural reporter are studied in several compounds, including (4,4(')-dimethyl-2,2(')-bipyridyl)(o-methylsulfinylbenzoate) ruthenium II, [Ru(dmb)(2)(BzSO)](+), (RuBzSO), octylsulfinylpropionic acid (OSPA), and o- and p-methylsulfinyl-benzoic acid (oMSBA and pMSBA). The mode assignment in the fingerprint region for these compounds is made using a combination of density functional theory calculations and 2DIR and relaxation-assisted 2DIR (RA 2DIR) spectroscopies. The SO stretching mode frequency and IR intensity demonstrate substantial sensitivity to the molecular structure. Multiple cross peaks of the C=O and S=O stretching modes with modes in the fingerprint region (930-1450 cm(-1)) were recorded. The 2DIR and RA 2DIR spectra focusing at interactions of a high-frequency mode of a ligand with the modes in the fingerprint region provide a spectral fingerprint of a compound and help mode assignment in the often congested fingerprint region. The cross-peak amplitudes in oMSBA, pMSBA, and OSPA were compared with the theoretical predictions based on the computed values for the off-diagonal anharmonicities and a reasonable match is found. The SO stretching mode provides means for assigning other modes in the fingerprint region and constitutes a promising structural reporter for the 2DIR and RA 2DIR spectroscopy measurements.

Thermal stability, photochromic sensitivity and optical properties of [Ru(bpy)(2)(OSOR)]+ compounds with R = Bn, BnCl, BnMe

The influence of ligand substitution on the photochromic properties of [Ru(bpy)(2)(OSOR)]∙PF(6) compounds (bpy = 2,2'-bipyridine, OSO = 2-methylsulfinylbenzoate) dissolved in propylene carbonate is studied by UV/VIS laser-spectroscopy as a function of temperature and exposure. The group R is either Bn (CH(2)C(6)H(5)), BnCl or BnMe. The photochromic properties originate from a phototriggered linkage isomerization located at the SO ligand. It is shown that the thermal stability of the studied compounds can be varied by ligand substitution in the range of 1.6 × 10(3) s to 3.9 × 10(4) s. In contrast, absorption spectra of ground and metastable states as well as the characteristic exposure of the photochromic response remain unchanged. The results are explained in the frame of photoinduced linkage isomerization located at the SO ligand.