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Asymmetric Photochemical Reactions in confined Chiral space

Periodic Reporting for period 1 - A-PREACH (Asymmetric Photochemical Reactions in confined Chiral space)

Reporting period: 2019-06-01 to 2021-05-31

Light-driven molecular machines offer tremendous opportunities in the development of systems that can be operated with spatiotemporal precision at the nanoscale. Among these so called photoactuators, two main classes of compounds emerge. The former are the molecular photoswitches, systems able to interconvert between two (or more) states by the action of light (or thermal) stimuli. Light allows these molecules to escape microscopic reversibility and populate a metastable state, whose steric and electronic properties differ from the starting, stable, form. The main characteristic of photoswitches is the stochastic nature of their motion in the interconversion between one state and the other. On the other hand, photochemically-driven molecular motors based on overcrowded alkenes introduce point and helical chirality in their core structure and can perform a fully unidirectional 360° rotation about their axle, which is normally composed by a C=C double bond.
Understanding and applying the same concepts that drive the unidirectionality of molecular photoactuators at the ground and at the excited state is a fundamental challenge in order to design novel, more efficient chemical structures that can be addressed by light, that can be applied in the context of smart materials, soft robotics and photopharmacology.
Consequently, the goal of the action was to study the principles and processes that are at the basis of the chiral information transfer in supramolecular structures containing photoswitches and molecular motors, both to impart unidirectionality to otherwise achiral photochemical switches by means of external interactions with the chiral environment, and to exploit the chiral nature of the molecular motors to affect an (a)chiral environment.
Over the past two years we have developed, synthesized and studied (both spectroscopically and computationally) photoswitches able to interact with (a)chiral guests. We could develop an achiral stiff-stilbene that upon irradiation could interact with a chiral guest, unequivocally proving that unidirectionality of motion can be achieved in the thermal process following the photochemical conversion. A second stiff-stilbene that we designed was able to form chiral supramolecular metal-organic complexes in a 2:2 host-guest system, whose enantiomers could be interconverted thermally at room temperature. We could prove the supramolecular chirality transfer from a double stranded DNA to an achiral azobenzene decorated with DNA intercalators. We started studying the enchlatration of different iminothioindoxyl (ITI) photoswitches inside a supramolecular metal-organic cage, to probe the effects at the excited and ground state of the inclusion in the container. In order to allow better addressability of the photoswitch, we designed a novel class of C=N based, green-light triggered, scaffolds starting from ITI. Due to the unpredicted event of the COVID-19 pandemic, more results on the interaction between these achiral switches and chiral supramolecular flasks was not completed during the runtime of this action and will be the content of ongoing and future investigation.
In order to exploit more effectively the molecular motors, we studied the excited state dynamics of new oxindole based systems, to obtain more insight on the excited state properties of these structures. Moreover, we studied the effects of a motor on a macrocycle and bis-macrocycle structures, transmitting the chiral information contained in the action of the molecular motor itself to the whole system.
The studies that were performed in the framework of the action allowed the production of nine scientific papers published in peer-reviewed journals and a preprint. Several other publications are in phase of preparation or submitted for peer-review.
The development of methodologies to study supramolecular structures attached to artificial rotary molecular motors which can be reversibly modulated and bias their interaction with their environment is a significant step beyond the state of the art prior to the onset of this action. Our research aiming to the gain more insights on the motion of molecular motors and the chiral induction in supramolecular structures containing photoswitches will spur more work at the interface between molecular motors, host–guest and macrocyclic chemistry. Moreover, the development of a novel class of green-light addressable photoswitches and the full study of their rotational mechanism will be the foundation of more research into molecular machines. Society will profit from the multidisciplinary nature of the fields touched by the course of this action, which will affect future applications in smart materials, molecular machinery and photopharmacology.
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