Dynamic control over catalytic function using responsive bisthiourea catalysts.

Two novel bisthiourea molecular motor-based catalysts were designed and their dynamic control of activity and stereoselectivity in the Henry reaction using heat and light as external stimuli was investigated. Enantioselective organocatalysis studies established that a basic functional group is not required within the catalyst core to effect efficient dual stereocontrol, illustrating novel stimuli responsive organocatalysts.

The Art of Building Small: From Molecular Switches to Motors (Nobel Lecture).

A journey into the nano-world: The ability to design, use and control motor-like functions at the molecular level sets the stage for numerous dynamic molecular systems. In his Nobel Lecture, B. L.

Supramolecular Low-Molecular-Weight Hydrogelator Stabilization of SERS-Active Aggregated Nanoparticles for Solution and Gas Sensing.

The potential of surface-enhanced Raman scattering (SERS) spectroscopy in both laboratory and field analyses depends on the reliable formation of so-called SERS hot spots, such as those formed during gold or silver nanoparticle aggregation. Unfortunately such aggregates are not stable in solution because they typically grow until they precipitate. Here we describe the use of low-molecular-weight hydrogels formed through pH-triggered self-assembly that occurs at a rate that well matches the rates of aggregation of Au or Ag colloids, allowing them to be trapped at the SERS-active point in the aggregation process.

Surface Inclusion of Unidirectional Molecular Motors in Hexagonal Tris(o-phenylene)cyclotriphosphazene.

A new unidirectional light-driven molecular motor suitable for host-guest surface inclusion complexes with tris(o-phenylene)cyclotriphosphazene (TPP) was synthesized. The motor molecules formed regular two-dimensional trigonal arrays covering the large facets of disc-shaped TPP nanocrystals. Photochemical and thermal isomerization studies demonstrated that the light-driven rotation of the anchored motors is similar to that observed in solution and is not compromised neither by either the surface confinement or the density of surface coverage (50 vs 100%).

Catalytic Asymmetric Synthesis of Butenolides and Butyrolactones.

γ-Butenolides, γ-butyrolactones, and derivatives, especially in enantiomerically pure form, constitute the structural core of numerous natural products which display an impressive range of biological activities which are important for the development of novel physiological and therapeutic agents. Furthermore, optically active γ-butenolides and γ-butyrolactones serve also as a prominent class of chiral building blocks for the synthesis of diverse biological active compounds and complex molecules. Taking into account the varying biological activity profiles and wide-ranging structural diversity of the optically active γ-butenolide or γ-butyrolactone structure, the development of asymmetric synthetic strategies for assembling such challenging scaffolds has attracted major attention from synthetic chemists in the past decade.

Third-Generation Light-Driven Symmetric Molecular Motors.

Symmetric molecular motors based on two overcrowded alkenes with a notable absence of a stereogenic center show potential to function as novel mechanical systems in the development of more advanced nanomachines offering controlled motion over surfaces. Elucidation of the key parameters and limitations of these third-generation motors is essential for the design of optimized molecular machines based on light-driven rotary motion. Herein we demonstrate the thermal and photochemical rotational behavior of a series of third-generation light-driven molecular motors.

Locked synchronous rotor motion in a molecular motor.

Biological molecular motors translate their local directional motion into ordered movement of other parts of the system to empower controlled mechanical functions. The design of analogous geared systems that couple motion in a directional manner, which is pivotal for molecular machinery operating at the nanoscale, remains highly challenging. Here, we report a molecular rotary motor that translates light-driven unidirectional rotary motion to controlled movement of a connected biaryl rotor.

Cold Snapshot of a Molecular Rotary Motor Captured by High-Resolution Rotational Spectroscopy.

We present the first high-resolution rotational spectrum of an artificial molecular rotary motor. By combining chirped-pulse Fourier transform microwave spectroscopy and supersonic expansions, we captured the vibronic ground-state conformation of a second-generation motor based on chiral, overcrowded alkenes. The rotational constants were accurately determined by fitting more than 200 rotational transitions in the 2-4 GHz frequency range.

In situ control of polymer helicity with a non-covalently bound photoresponsive molecular motor dopant.

The transfer of chirality from a molecular motor to a dynamic helical polymer via ionic interactions was investigated. A dopant with photoswitchable chirality was able to induce a preferred helicity in a poly(phenylacetylene) polymer and the helicity is inverted upon irradiation. The findings described herein will advance the development of functional and responsive polymeric systems.

Defocused Imaging of UV-Driven Surface-Bound Molecular Motors.

Synthetic molecular motors continue to attract great interest due to their ability to transduce energy into nanomechanical motion, the potential to do work and drive systems out-of-equilibrium. Of particular interest are unidirectional rotary molecular motors driven by chemical fuel or light. Probing the mechanistic details of their operation at the single-molecule level is hampered by the diffraction limit, which prevents the collection of dynamic positional information by traditional optical methods.

Ultrafast Dynamics in Light-Driven Molecular Rotary Motors Probed by Femtosecond Stimulated Raman Spectroscopy.

Photochemical isomerization in sterically crowded chiral alkenes is the driving force for molecular rotary motors in nanoscale machines. Here the excited-state dynamics and structural evolution of the prototypical light-driven rotary motor are followed on the ultrafast time scale by femtosecond stimulated Raman spectroscopy (FSRS) and transient absorption (TA). TA reveals a sub-100-fs blue shift and decay of the Franck-Condon bright state arising from relaxation along the reactive potential energy surface.

Asymmetric Synthesis of Second-Generation Light-Driven Molecular Motors.

The enantiomeric homogeneity of light-driven molecular motors based on overcrowded alkenes is crucial in their application as either unidirectional rotors or as chiral multistate switches. It was challenging to obtain these compounds as single enantiomers via the established synthetic procedures due to loss of optical purity in the key step, i.e.

Chirality controlled responsive self-assembled nanotubes in water.

The concept of using chirality to dictate dimensions and to store chiral information in self-assembled nanotubes in a fully controlled manner is presented. We report a photoresponsive amphiphile that co-assembles with its chiral counterpart to form nanotubes and demonstrate how chirality can be used to effect the formation of either micrometer long, achiral nanotubes or shorter (∼300 nm) chiral nanotubes that are bundled. The nature of these assemblies is studied using a variety of spectroscopic and microscopic techniques and it is shown that the tubes can be disassembled with light, thereby allowing the chiral information to be erased.

Artificial molecular motors.

Motor proteins are nature's solution for directing movement at the molecular level. The field of artificial molecular motors takes inspiration from these tiny but powerful machines. Although directional motion on the nanoscale performed by synthetic molecular machines is a relatively new development, significant advances have been made.

Fluorine-Substituted Molecular Motors with a Quaternary Stereogenic Center.

A series of unprecedented second generation molecular motors featuring a quaternary stereogenic center substituted with a fluorine atom has been synthesized. It is demonstrated that a seemingly benign replacement of the stereogenic hydrogen for a fluorine atom, regarded as a common substituent in pharmacology, resulted in a dramatic change in the energetic profile of thermal helix inversion. The barrier for the thermal helix inversion was found to increase considerably (by 20-30 kJ mol ), presumably due to destabilization of the transition state by increased steric hindrance when the fluorine atom is forced to pass over the lower half of the motor.

Visible-Light Excitation of a Molecular Motor with an Extended Aromatic Core.

Exploring routes to visible-light-driven rotary motors, the possibility of red-shifting the excitation wavelength of molecular motors by extension of the aromatic core is studied. Introducing a dibenzofluorenyl moiety in a standard molecular motor resulted in red-shifting of the absorption spectrum. UV/vis and H NMR spectroscopy showed that these motors could be isomerized with light of wavelengths up to 490 nm and that the structural modification did not impair the anticipated rotary behavior.

Dynamic control of chirality and self-assembly of double-stranded helicates with light.

Helicity switching in biological and artificial systems is a fundamental process that allows for the dynamic control of structures and their functions. In contrast to chemical approaches to responsive behaviour in helicates, the use of light as an external stimulus offers unique opportunities to invert the chirality of helical structures in a non-invasive manner with high spatiotemporal precision. Here, we report that unidirectional rotary motors with connecting oligobipyridyl ligands, which can dynamically change their chirality upon irradiation, assemble into metal helicates that are responsive to light.

Ultrafast Excited State Dynamics in Molecular Motors: Coupling of Motor Length to Medium Viscosity.

Photochemically driven molecular motors convert the energy of incident radiation to intramolecular rotational motion. The motor molecules considered here execute four step unidirectional rotational motion. This comprises a pair of successive light induced isomerizations to a metastable state followed by thermal helix inversions.

Oxygen Activated, Palladium Nanoparticle Catalyzed, Ultrafast Cross-Coupling of Organolithium Reagents.

The discovery of an ultrafast cross-coupling of alkyl- and aryllithium reagents with a range of aryl bromides is presented. The essential role of molecular oxygen to form the active palladium catalyst was established; palladium nanoparticles that are highly active in cross-coupling reactions with reaction times ranging from 5 s to 5 min are thus generated in situ. High selectivities were observed for a range of heterocycles and functional groups as well as for an expanded scope of organolithium reagents.

Cu-catalyzed enantioselective allylic alkylation with organolithium reagents.

This protocol describes a method for the catalytic enantioselective synthesis of tertiary and quaternary carbon stereogenic centers, which are widely present in pharmaceutical and natural products. The method is based on the direct reaction between organolithium compounds, which are cheap, readily available and broadly used in chemical synthesis, and allylic electrophiles, using chiral copper catalysts. The methodology involves the asymmetric allylic alkylation (AAA) of allyl bromides, chlorides and ethers with organolithium compounds using catalyst systems based on Cu-Taniaphos and Cu-phosphoramidites.

Recent developments in reversible photoregulation of oligonucleotide structure and function.

There is a growing interest in the photoregulation of biological functions, due to the high level of spatiotemporal precision achievable with light. Additionally, light is non-invasive and waste-free. In particular, the photoregulation of oligonucleotide structure and function is a rapidly developing study field with relevance to biological, physical and material sciences.

Remarkable solvent isotope dependence on gelation strength in low molecular weight hydro-gelators.

A delicate interplay of anisotropic hydrophobic/hydrophilic, π-π stacking, ionic and hydrogen bond interactions determine the strength of hydrogelators and are considered key factors in efforts to design potent small molecule hydrogelators. Here we show that solvent deuteration and electrolytic strength affect the strength of hydrogels formed from amino acid modified C-symmetric cyclohexane trisamides profoundly. Gels formed by self-assembly through heating/cooling of solutions or by pH switching show up to a 30 °C increase in their melting temperatures in DO compared to HO.

Photocontrol of Anion Binding Affinity to a Bis-urea Receptor Derived from Stiff-Stilbene.

Toward the development of photoresponsive anion receptors, a stiff-stilbene photoswitch has been equipped with two urea anion-binding motifs. Photoinduced E/Z isomerization has been studied in detail by UV-vis and NMR spectroscopy. Titration experiments (H NMR) reveal strong binding of acetate and phosphate to the (Z)-isomer, in which the urea groups are closely together.

Bidirectional Photomodulation of Surface Tension in Langmuir Films.

Switching systems operating in a cooperative manner capable of converting light energy into mechanical motion are of great interest for optical devices, data storage, nanoscale energy converters and molecular sensing. Herein, photoswitchable monolayers were formed at the air-water interface from either a pure bis(thiaxanthylidene)-based photoswitchable amphiphile or from a mixture of the photoswitchable amphiphile with a conventional lipid dipalmitoylphosphatidylcholine (DPPC). Efficient photoisomerization of the anti-folded to syn-folded geometry of the amphiphile's central core induces changes in the surface pressure in either direction, depending on the initial molecular density.

Bifunctional Molecular Photoswitches Based on Overcrowded Alkenes for Dynamic Control of Catalytic Activity in Michael Addition Reactions.

The emerging field of artificial photoswitchable catalysis has recently shown striking examples of functional light-responsive systems allowing for dynamic control of activity and selectivity in organocatalysis and metal-catalysed transformations. While our group has already disclosed systems featuring first generation molecular motors as the switchable central core, a design based on second generation molecular motors is lacking. Here, the syntheses of two bifunctionalised molecular switches based on a photoresponsive tetrasubstituted alkene core are reported.