Linkage Photoisomerization of an Isolated Ruthenium Sulfoxide Complex: Sequential versus Concerted Rearrangement

Unravelling the kinetics of electro- and photochemical S → O linkage isomerization in Ru(II)-NHC-DMSO complexes utilised for photoinduced substitution reactions

Our recently reported Ru(III)-NHC complexes 1a and 1b were utilized as suitable precursors to prepare new Ru(II)-NHC-(DMSO)2 complexes 2a and 2b. Complexes 2a and 2b reacted with 2,2'-bipyridine to give complexes 3a and 3b, respectively, with substitution of only one of the DMSO ligands. All new complexes were characterized using various spectroscopic techniques and the molecular structures of 2a and 3a were determined using single-crystal X-ray diffraction technique. Complexes 2a, 2b, 3a, and 3b showed the S → O linkage isomerization of the DMSO ligand upon oxidation of the Ru centre from +II to +III, as confirmed by the thermodynamic and kinetic data obtained from cyclic voltammetry experiments. It was observed that in the bisdimethylsulfoxide complexes 2a and 2b, only one DMSO ligand isomerized, which was further corroborated by the computational studies performed to optimize the geometry of the possible linkage isomers of complexes 2a and 3a in +2 and +3 oxidation states, whereas complexes 3a and 3b showed a high preference for the O-bound isomer in the Ru(III) redox state. The role of NHC in stabilizing the mixed isomer in complexes 2a and 2b and preventing the isomerization of both DMSO ligands coordinated to the Ru centre was studied; moreover, NHC provided good solvent compatibility for photochemical S → O isomerization in all the complexes. Taking advantages of the photoinduced linkage isomerization in 2a and 2b, the synthesis of 3a and 3b was revisited and performed using 2a and 2b, respectively, following a photoinduced substitution reaction in the presence of 2,2'-bipyridine. The kinetics of the reversion from the O-bound to S-bound isomer was found to follow the DMSO-assisted intermolecular S → O isomerization pathway.

Photo-induced 6π-electrocyclisation and cycloreversion of isolated dithienylethene anions

The diarylethene chromophore is commonly used in light-triggered molecular switches. The chromophore undergoes reversible 6π-electrocyclisation (ring closing) and cycloreversion (ring opening) reactions upon exposure to UV and visible light, respectively, providing bidirectional photoswitching. Here, we investigate the gas-phase photoisomerisation of meta- (m) and para- (p) substituted dithienylethene carboxylate anions (DTE^-) using tandem ion mobility mass spectrometry coupled with laser excitation. The ring-closed forms of p-DTE^- and m-DTE^- are found to undergo cycloreversion in the gas phase with maximum responses associated with bands in the visible (λ_max ≈ 600 nm) and the ultraviolet (λ_max ≈ 360 nm). The ring-open p-DTE^- isomer undergoes 6π-electrocyclisation in the ultraviolet region at wavelengths shorter than 350 nm, whereas no evidence is found for the corresponding electrocyclisation of ring-open m-DTE^-, a situation attributed to the fact that the antiparallel geometry required for electrocyclisation of m-DTE^- is energetically disfavoured. This highlights the influence of the carboxylate substitution position on the photochemical properties of DTE molecules. We find no evidence for the formation in the gas phase of the undesirable cyclic byproduct, which causes fatigue of DTE photoswitches in solution.

Photo-control of bimolecular reactions: reactivity of the long-lived Rhodamine 6G triplet excited state with ˙NO

Photo-chemistry provides a non-intuitive but very powerful way to probe kinetically limited, sometimes thermodynamically non-favored reactions and, thus, access highly specific products. However, reactivity in the excited state is difficult to characterize directly, due to short lifetimes and challenges in controlling the reaction medium. Among photo-activatable reagents, rhodamine dyes find widespread uses due to a number of favorable properties including their high absorption coefficient. Their readily adaptable synthesis allows development of tailor-made dyes for specific applications. Remarkably, few studies have directly probed the chemical reactivity of their triplet excited state. Here we present a new conceptual approach to examine the specific chemistry of the triplet excited state. We have developed a pump (488 nm) - probe (600 nm) strategy to examine the gas-phase lifetime and reactivity of the triplet cation of Rhodamine 6G (³Rh6G⁺) in an ion trap mass spectrometer. The confounding effects of solvent, aggregation and formation of other reactive intermediates is thus avoided allowing fundamental reactivity to be explored. In the presence, in the ion trap, of helium seeded with 1% of nitric oxide (˙NO) (∼ 60 ion/˙NO collisions per second), the triplet lifetime is shortened from 1.9 s to 0.7 s. Simultaneously, the reaction products [Rh6G-H]˙⁺ and [Rh6G-H + NO]⁺ are observed. Reaction of ³Rh6G⁺ with ˙NO₂ yields [Rh6G-H]˙⁺, [Rh6G-H + NO₂]⁺ and [Rh6G-2H]⁺. None of these products are observed for the singlet, ¹Rh6G⁺. DFT calculations suggest a stepwise mechanism only allowed from ³Rh6G⁺, in which H atom abstraction by ˙NO_x (x = 1 or 2) yields [Rh6G-H]˙⁺ which, then, reacts with another ˙NO_x molecule. This illustrates the power of light to initiate specific chemical reactions, and the relevance of gas-phase ion-molecule reaction approaches to understand stepwise reaction mechanism from specific excited states.

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.

Light-Activated Olefin Metathesis: Catalyst Development, Synthesis, and Applications

The most important means for tuning and improving a catalyst's properties is the delicate exchange of the ligand shell around the central metal atom. Perhaps for no other organometallic-catalyzed reaction is this statement more valid than for ruthenium-based olefin metathesis. Indeed, even the simple exchange of an oxygen atom for a sulfur atom in a chelated ruthenium benzylidene about a decade ago resulted in the development of extremely stable, photoactive catalysts. This Account presents our perspective on the development of dormant olefin metathesis catalysts that can be activated by external stimuli and, more specifically, the use of light as an attractive inducing agent.The insight gained from a deeper understanding of the properties of cis-dichlororuthenium benzylidenes opened the doorway for the systematic development of new and efficient light-activated olefin metathesis catalysts and catalytic chromatic-orthogonal synthetic schemes. Following this, ways to disrupt the ligand-to-metal bond to accelerate the isomerization process that produced the active precatalyst were actively pursued. Thus, we summarize herein the original thermal activation experiments and how they brought about the discoveries of photoactivation in the sulfur-chelated benzylidene family of catalysts. The specific wavelengths of light that were used to dissociate the sulfur-ruthenium bond allowed us to develop noncommutative catalytic chromatic-orthogonal processes and to combine other photochemical reactions with photoinduced olefin metathesis, including using external light-absorbing molecules as "sunscreens" to achieve novel selectivities. Alteration of the ligand sphere, including modifications of the N-heterocyclic carbene (NHC) ligand and the introduction of cyclic alkyl amino carbene (CAAC) ligands, produced more efficient light-induced activity and special chemical selectivity. The use of electron-rich sulfoxides and, more prominently, phosphites as the agents that induce latency widened the spectrum of light-induced olefin metathesis reactions even further by expanding the colors of light that may now be used to activate the catalysts, which can be used in applications such as stereolithography and 3D printing of tough metathesis-derived polymers.

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.

Reversible Photoswitching of Isolated Ionic Hemiindigos with Visible Light

Indigoid chromophores have emerged as versatile molecular photoswitches, offering efficient reversible photoisomerization upon exposure to visible light. Here we report synthesis of a new class of permanently charged hemiindigos (HIs) and characterization of photochemical properties in gas phase and solution. Gas-phase studies, which involve exposing mobility-selected ions in a tandem ion mobility mass spectrometer to tunable wavelength laser radiation, demonstrate that the isolated HI ions are photochromic and can be reversibly photoswitched between Z and E isomers. The Z and E isomers have distinct photoisomerization response spectra with maxima separated by 40-80 nm, consistent with theoretical predictions for their absorption spectra. Solvation of the HI molecules in acetonitrile displaces the absorption bands to lower energy. Together, gas-phase action spectroscopy and solution NMR and UV/Vis absorption spectroscopy represent a powerful approach for studying the intrinsic photochemical properties of HI molecular switches.