Photocurrent generation based on a light-driven proton pump in an artificial liquid membrane

Design and Synthesis of Nonequilibrium Systems

The active transport of ions and molecules across cell membranes is essential to creating the concentration gradients that sustain life in all living organisms, be they bacteria, fungi, plants, animals or Homo sapiens. Nature uses active transport everywhere for everything. Molecular biologists have long been attracted to the study of active transport and continue to this day to investigate and elucidate the tertiary structures of the complex motor proteins that sustain it, while physicists, interested in nonequilibrium statistical mechanics, have developed theoretical models to describe the driven ratcheting motions that are crucial to its function. The increasingly detailed understanding that contemporary science has acquired relating to active transport, however, has yet to lead to the design and construction of artificial molecular motors capable of employing ratchet-driven motions that can also perform work against concentration gradients. Mechanically interlocked molecules (MIMs) in the form of pseudo- and semirotaxanes are showing some encouraging signs in meeting these goals. This review summarizes recent progress in making artificial molecular motors that can perform work by "pumping" tetracationic rings into high-energy states. The launching pad is a bistable [2]rotaxane whose dumbbell component contains two electron-donating recognition sites, one, a tetrathiafulvalene (TTF) unit, which interacts more strongly with the ring component, cyclobis(paraquat-p-phenylene) (CBPQT(4+)), containing two electron-accepting bipyridinium units, than does the other 1,5-dioxynaphthalene (DNP) unit. Switching can be induced electrochemically by oxidizing the TTF unit to a TTF(•+) radical cation, whereupon Coulombic repulsion takes care of moving the ring to the DNP unit. Reduction of the radical cation resets the switch. Molecular switches operate at, or close to, equilibrium. Any work done during one switching event is undone during the reset. Molecular motors, on the other hand, rely on a flux of energy, and a ratchet mechanism to make periodic changes to the potential energy surface of a system in order to move molecules uphill to higher energy states. Forging a path from molecular switches to motors involved designing a molecular pump prototype. An asymmetric dumbbell with a 2-isopropylphenyl (neutral) end and a 3,5-dimethylpyridinium (charged) end with a DNP recognition site to entice CBPQT(4+) rings out of solution exhibits relative unidirectional movement of the rings with respect to the dumbbell. Redox chemistry does the trick. During the oxidative cycle, the rings enter the dumbbell by passing over the neutral end onto the recognition site; in the reduction cycle, much of the recognition is lost and the rings find their way back into solution by leaving the dumbbell from the charged end. This on-one-end, off-the-other process can be repeated over and over again using light as the energy source in the presence of a photosensitizer and a compound that shuttles electrons back and forth. Although this prototype demonstrates ratchet-driven translational motion, no work is done. A ring enters the dumbbell from one end and leaves from the other end. Another deficiency of the prototype is the fact that, although the recognition site is muted on reduction, it retains some attraction for the ring. What if the recognition site was attractive initially and then became repulsive? This question was answered by turning to radical chemistry and employing the known stabilization behavior of a bipyridinium radical cation and the bisradical dication, generated on reduction of the CBPQT(4+) ring, to pluck rings out of solution and thread them over the charged end of the pump portion of a semidumbbell. On subsequent oxidation, the pump is primed and the rings pass through a one-way door, given a little thermal energy, onto a collecting-chain where they find themselves accumulating where they would rather not be present. In this manner, an artificial molecular pump mimics the pumping machinery commonplace in biological systems. Looking beyond this state-of-the-art artificial molecular pump, we discuss, from a theoretical standpoint, the measures that would need to be taken in order to render its operation autonomous.

An artificial molecular pump

Carrier proteins consume fuel in order to pump ions or molecules across cell membranes, creating concentration gradients. Their control over diffusion pathways, effected entirely through noncovalent bonding interactions, has inspired chemists to devise artificial systems that mimic their function. Here, we report a wholly artificial compound that acts on small molecules to create a gradient in their local concentration. It does so by using redox energy and precisely organized noncovalent bonding interactions to pump positively charged rings from solution and ensnare them around an oligomethylene chain, as part of a kinetically trapped entanglement. A redox-active viologen unit at the heart of a dumbbell-shaped molecular pump plays a dual role, first attracting and then repelling the rings during redox cycling, thereby enacting a flashing energy ratchet mechanism with a minimalistic design. Our artificial molecular pump performs work repetitively for two cycles of operation and drives rings away from equilibrium toward a higher local concentration.

Photo-control of the thermal radical recombination reaction: photochromism of an azobenzene-bridged imidazole dimer

Among various kinds of photochromic compounds, bridged imidazole dimers have been known as fast photo-switch molecules. Bridged imidazole dimers have opened up various potential applications to photochromic lenses and real-time holographic displays. The optical properties of bridged imidazole dimers strongly depend on the bridging moiety to tether two imidazole rings. Therefore, the control of the bridging structure by introducing another photochromic moiety would increase the versatility of bridged imidazole dimers. In this study, we designed and synthesized a new type of the bridged imidazole dimer 1 which has the azobenzene moiety as the photo-responsive linker. The cis–trans isomerization of the azobenzene moiety enables to change the distance between the photogenerated radical pairs. The two structural isomers, cis–1 and trans–1, are observed and both compounds undergo the photochromism to produce the imidazolyl radicals. We found that the two imidazolyl radicals generated from cis–1 are close enough to form the intramolecular C–N bond, whereas the imidazolyl radicals of trans–1 undergo the intermolecular recombination reaction due to the long distance between the radicals. Our results demonstrate the control of intra-/intermolecular radical recombination reactions by the combination of the two photochromic compounds.

Light-powered autonomous and directional molecular motion of a dissipative self-assembling system

Biomolecular motors convert energy into directed motion and operate away from thermal equilibrium. The development of dynamic chemical systems that exploit dissipative (non-equilibrium) processes is a challenge in supramolecular chemistry and a premise for the realization of artificial nanoscale motors. Here, we report the relative unidirectional transit of a non-symmetric molecular axle through a macrocycle powered solely by light. The molecular machine rectifies Brownian fluctuations by energy and information ratchet mechanisms and can repeat its working cycle under photostationary conditions. The system epitomizes the conceptual and practical elements forming the basis of autonomous light-powered directed motion with a minimalist molecular design.

Photocurrent generation in a light-harvesting system with multifunctional artificial nanochannels

Inspired by the biological light-driven proton pump, a light-harvesting system based on multifunctional artificial nanochannels is developed for photocurrent generation.

Multi-responsive supramolecular hydrogels based on merocyanine–peptide conjugates

Stimuli-responsive hydrogels are “smart” materials with diverse applications.

Pseudo crystalline state thermochromic and reverse-photochromic reactivity of spiroindolinobenzopyran upon encapsulation into Zn-MOF-74

Drastic changes observed in chromic properties of an unsubstituted spiroindolinobenzopyran encapsulated into a MOF lattice are reported.

Light-driven directed proton transport across the liposomal membrane

We have developed a simple artificial photoresponsive ion-gating device by inserting molecular switches in the membrane of liposomes. A controlled and directed proton transport across the bilayer membrane can lower the internal pH of the liposomes from neutral to around 4 under combined light and chemical stimulation.

Light-controlled reversible release and uptake of potassium ions from ion-exchanging nanospheres

Here, we report for the first time on photoswitchable nanospheres containing spiropyran (Sp) for reversible release and uptake of metal ions. K(+) is used as a model ion to demonstrate the chemical principle of this approach. Valinomycin is incorporated in the nanospheres to stabilize K(+). Upon UV illumination, Sp transforms to the more basic ring-opened merocyanine form, which takes up H(+) from the surrounding aqueous solution and expels K(+) from the nanospheres. The process can be reversed by irradiation with visible light to reduce the surrounding K(+) concentration.

Creating electrochemical gradients by light: from bio-inspired concepts to photoelectric conversion

Artificial light harvesting systems can be used to convert light into electrochemical gradients and photocurrents.