Imaging Block-Selective Copolymer Solvation.

Understanding individual-block solvation in self-assembled block copolymer systems is experimentally difficult, but this solvation underpins the assembly and disassembly observed at the bulk scale. Here, covalently attached viscosity-sensitive molecular rotors for fluorescence lifetime imaging microscopy uncover and quantitatively elucidate previously undisclosed differential block-selective responses toward solvation changes upon addition of DMSO and THF to self-assembled ROMP-based amphiphilic block copolymers. The sensitivity of this method provides unique information on block-selective solvent-triggered assembly and disassembly mechanisms, revealing behaviors invisible to or with superior sensitivity to traditional 1H NMR spectroscopy. These experiments demonstrate an analytical method and provide a granular mechanistic understanding, both suitable for fine tuning block copolymer assembly and disassembly processes.

Real-Time Polymer Viscosity-Catalytic Activity Relationships on the Microscale.

Polymer growth induces physical changes to catalyst microenvironments. Here, these physical changes are quantified in real time and are found to influence microscale chemical catalysis and the polymerization rate. By developing a method to "peer into" optically transparent living-polymer particles, simultaneous imaging of both viscosity changes and chemical activity was achieved for the first time with high spatiotemporal resolution through a combination of fluorescence intensity microscopy and fluorescence lifetime imaging microscopy techniques. Specifically, an increase in microenvironment viscosity led to a corresponding local decrease in the catalytic molecular ruthenium ring-opening metathesis polymerization rate, plausibly by restricting diffusional access to active catalytic centers. Consistent with this diffusional-access model, these viscosity changes were found to be monomer-dependent, showing larger changes in microenvironment viscosity in cross-linked polydicyclopentadiene compared to non-crosslinked polynorbornene. The sensitivity and high spatial resolution of the imaging technique revealed significant variations in microviscosities between different particles and subparticle regions. These revealed spatial heterogeneities would not be observable through alternative ensemble analytical techniques that provide sample-averaged measurements. The observed spatial heterogeneities provide a physical mechanism for variation in catalytic chemical activity on the microscale that may accumulate and lead to nonhomogeneous polymer properties on the bulk scale.

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.

Reactivity and Selectivity in Ruthenium Sulfur-Chelated Diiodo Catalysts.

A trifluoromethyl sulfur-chelated ruthenium benzylidene, Ru-S-CF3 -I, was synthesized and characterized. This latent precatalyst provides a distinct activity and selectivity profiles for olefin metathesis reactions depending on the substrate. For example, 1,3-divinyl-hexahydropentalene derivatives were efficiently obtained by ring-opening metathesis (ROM) of dicyclopentadiene (DCPD). Ru-S-CF3 -I also presented a much more effective photoisomerization process from the inactive cis-diiodo to the active trans-diiodo configuration after exposure to 510 nm (green light), allowing for a wide scope of photoinduced olefin metathesis reactions. DFT calculations suggest a faster formation and enhanced stability of the active trans-diiodo species of Ru-S-CF3 -I compared with Ru-S-Ph-I, explaining its higher reactivity. In addition, the photochemical release of chloride anions by irradiation of Cl-BODIPY in the presence of DCPD derivatives with diiodo Ru benzylidenes, led to in situ generation of chloride complexes, which quickly produced the corresponding cross-linked polymers. Thus, novel selective pathways that use visible light to guide olefin metathesis based synthetic sequences is presented.

Sunscreen-Assisted Selective Photochemical Transformations.

In this review, we describe a simple and general procedure to accomplish selective photochemical reaction sequences for two chromophores that are responsive to similar light frequencies. The essence of the method is based on the exploitation of differences in the molar absorptivity at certain wavelengths of the photosensitive groups, which is enhanced by utilizing light-absorbing auxiliary filter molecules, or "sunscreens". Thus, the filter molecule hinders the reaction pathway of the least absorbing molecule or group, allowing for the selective reaction of the other. The method was applied to various photochemical reactions, from photolabile protecting group removal to catalytic photoinduced olefin metathesis in different wavelengths and using different sunscreen molecules. Additionally, the sunscreens were shown to be effective also when applied externally to the reaction mixture, avoiding any potential chemical interactions between sunscreen and substrates and circumventing the need to remove the light-filtering molecules from the reaction mixture, adding to the simplicity and generality of the method.

Regioselective chromatic orthogonality with light-activated metathesis catalysts.

The ability to selectively guide consecutive chemical processes towards a preferred pathway by using light of different frequencies is an appealing concept. Herein we describe the coupling of two photochemical reactions, one the photoisomerization and consequent activation of a sulfur-chelated latent olefin-metathesis catalyst at 350 nm, and the other the photocleavage of a silyl protecting group at 254 nm. Depending on the steric stress exerted by a photoremovable neighboring chemical substituent, we demonstrate the selective formation of either five- or six-membered-ring frameworks by light-triggered ring-closing metathesis. The orthogonality of these light-induced reactions allows the initiation of these processes independently and in interchangeable order, according to the wavelength of light used to promote them.

Plasmonic visible-near infrared photothermal activation of olefin metathesis enabling photoresponsive materials.

Light-induced catalysis and thermoplasmonics are promising fields creating many opportunities for innovative research. Recent advances in light-induced olefin metathesis have led to new applications in polymer and material science, but further improvements to reaction scope and efficiency are desired. Herein, we present the activation of latent ruthenium-based olefin metathesis catalysts via the photothermal response of plasmonic gold nanobipyramids. Simple synthetic control over gold nanobipyramid size results in tunable localized surface plasmon resonance bands enabling catalyst initiation with low-energy visible and infrared light. This approach was applied to the ROMP of dicyclopentadiene, affording plasmonic polymer composites with exceptional photoresponsive and mechanical properties. Moreover, this method of catalyst activation was proven to be remarkably more efficient than activation through conventional heating in all the metathesis processes tested. This study paves the way for providing a wide range of photoinduced olefin metathesis processes in particular and photoinduced latent organic reactions in general by direct photothermal activation of thermally latent catalysts.

Guiding a divergent reaction by photochemical control: bichromatic selective access to levulinates and butenolides.

Allylic and acrylic substrates may be efficiently transformed by a sequential bichromatic photochemical process into derivatives of levulinates or butenolides with high selectivity when phenanthrene is used as a regulator. Thus, UV-A photoinduced cross-metathesis (CM) couples the acrylic and allylic counterparts and subsequent UV-C irradiation initiates E-Z isomerization of the carbon-carbon double bond, followed by one of two competing processes; namely, cyclization by transesterification or a 1,5-H shift and tautomerization. Quantum chemical calculations demonstrate that intermediates are strongly blue-shifted for the cyclization while red-shifted for the 1,5-H shift reaction. Hence, delaying the double bond migration by employing UV-C absorbing phenanthrene, results in a selective novel divergent all-photochemical pathway for the synthesis of fundamental structural motifs of ubiquitous natural products.

Modulation of photodeprotection by the sunscreen protocol.

A protocol for the selective photoremoval of alcohol protecting groups modulated by the presence of auxiliary light absorbing molecules is presented. Thus, by this method, a single light source was used to selectively remove a specific protecting group in the presence of another chromophore with a lower molar absorption coefficient. The use of a molecular sunscreen, either internal or external, was found to be crucial to achieve high selectivities.