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Light effected autonomous molecular pumps: Towards active transporters and actuating materials

Periodic Reporting for period 2 - LEAPS (Light effected autonomous molecular pumps: Towards active transporters and actuating materials)

Reporting period: 2018-04-01 to 2019-09-30

Movement is one of the central attributes of life, and a key feature in many technological processes. While artificial motion is typically provided by macroscopic engines powered by internal combustion or electrical energy, movement in living organisms is produced by machines and motors of molecular size that typically exploit the energy of chemical fuels at ambient temperature to generate forces and ultimately execute functions.
In the past two decades the progress in several areas of chemistry, together with an improved understanding of biomolecular machines, has led to the development of a large variety of wholly synthetic molecular machines. Although these systems have the potential to bring about radical innovations in several areas of technology and medicine, a significant research effort still needs to be done to make molecular devices that can effectively and continuously convert energy from a source into directed motion, and use such motion to carry out a useful task.
The goal of the LEAPS project is to make new prototypes of molecular motors powered by light energy, and to use them to perform functions that include controlled transport of molecules at the nanometer scale, and mechanical actuation at the macroscopic scale. These systems are rationally designed on the evolution of previously reported working prototypes, and are based on synthetically affordable compounds.
The demonstration of such functionalities would not only be a landmark accomplishment in chemistry and nanoscience, but also open up stimulating scenarios in nanotechnology, materials science, and nanomedicine. On the one hand, the development of artificial nanomotors that can use light energy to displace molecules can lead to radically new routes for solar energy conversion and medical phototherapies. On the other hand, devising novel photosensitive materials capable of macroscopic mechanical actuation has evident technological significance in fields such as robotics and prosthetics. Reaching these goals will constitute significant steps forward towards the real world exploitation of chemical systems that, until just a few years ago, were considered no more than laboratory curiosities.
"The main idea of the LEAPS project is to combine a photochromic reaction (a chemical transformation induced by light) with a self-assembling rotaxane architecture – consisting of a molecular axle threaded into a molecular ring - to achieve the light-powered unidirectional translation of the ring relatively to the axle. The compounds are designed following a minimalistic approach, that is, to incorporate all the functional elements necessary for the motor operation while keeping the structural complexity (and hence the synthetic and characterization effort) to a minimum. In the first three years of the project (months 1-36) we have designed and synthesized new molecular axles that form the basis for second generation light-driven molecular pumps. We have also prepared molecular rings tagged with fluorescent units to facilitate the observation of the molecular movements. Progress was made towards the construction of molecular machines, based on the new pump modules, capable of transforming light energy into continuous unidirectional rotary motion, or storing light energy in the form of unstable supramolecular intermediates. Unexpected outcomes of this research were the electrochemical modulation of the acid/base properties of a remote site in a rotaxane (published in the prestigious PNAS journal), the photoswitching of single molecules that diffuse on a surface, and the chemical switching of mechanically planar chirality and enantioselective anion recognition in a molecular shuttle.
The LEAPS team has published seventeen peer-reviewed publications and has presented the project results in more than sixty-five contributions to national and international conferences. Renowned experts in the field have been invited to give seminars in our institute, and an international scientific workshop on “Light-activated nanostructures” (free registration) has been organized. Moreover, the ""Molecular Machines Days"" - a series of public events with the participation of the three Nobel Laureates in Chemistry 2016 - have been held in Bologna in November 2018. All the details are available in the LEAPS web site (site.unibo.it/leaps/en).
Another significant achievement of the project is its contribution to the setup of a joint research laboratory between the University of Bologna and the National Research Council of Italy. This lab, called Center for Light Activated Nanostructures (CLAN), aims at becoming a center of excellence for research in photochemical nanosciences in Europe."
Unlike the molecular axle of the first generation of pumps (Nat. Nanotechnol. 2015, 10, 70), the axles developed in the first period of the LEAPS project can be functionalized while maintaining their photoinduced pumping functionality. The construction of autonomous light driven rotary motors based on catenanes, and of rotaxane-type systems in which macrocyclic rings can be transferred and stored in a molecular reservoir under light irradiation are underway. The new molecular pumps are also being modified for incorporation in membranes or in polymers.
Another significant result of the project is the demonstration that the ring shuttling motion in a rotaxane can lead to new and potentially useful functions, such as the transfer of chemical information between two distant sites, and the control of mechanical chirality and chiral anion recognition. Possibilities emerge not only for the rational design of species with tailor-made functional and structural properties, but also for the development of model systems to understand some of Nature’s most effective regulatory mechanisms, namely, allostery, proton-coupled electron transfer and enantioselective molecular recognition.
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