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Novel Cyclocarbon-Porphyrin Systems: Synthesis and Properties

Periodic Reporting for period 1 - CarboPorph (Novel Cyclocarbon-Porphyrin Systems: Synthesis and Properties)

Reporting period: 2020-01-06 to 2022-01-05

The structural diversity of organic compounds is accompanied by a broad range of chemical and physical properties, making them the building blocks of life, as well as suitable for many technological applications, from synthetic materials to drug design. This is a consequence of carbon’s ability to bind with itself and nearly all elements in almost limitless variety. Among the many different classes of compounds, porphyrins - aromatic macrocycles composed of four nitrogen-containing, smaller rings connected by carbon bridges, are of great interest for modern chemists. Apart from their key role in biological functions (active sites in heme for oxygen transport by blood cells, in chlorophyll for photosynthesis performed by plants) they are studied as elements of novel, molecular-scale materials. They can be easily functionalized by different methods used in organic synthesis (changing the side groups, incorporating different metals in the center, connecting many porphyrins together) to fine tune their magnetic, electronic and optical properties in order to create nanoscale structures with precisely tailored characteristics. Porphyrin arrangement in larger, cyclic structures of different ring size and connectivity of the subunits is of special interest, as it enables studying charge and energy transport and unlocking new phenomena not seen in linear structures. Some of the properties can be compared to those observed in very long, and thus harder to study, organic polymers - materials of interest as "molecular wires". Cyclic arrays of porphyrins are also known to form in biological, photosynthetic systems, making such structures relevant in the search for novel light-harvesting materials for energy production. Many of such nanorings of different size and structure have been reported in recent years, in particular by the Anderson group in Oxford, opening a whole field for future investigations. The goal of this research project was to use existing synthetic methodologies - in particular the use of internal templates, acting as "moulds" to facilitate ring formation, in order to create and study new porphyrin architectures. Main focus was placed on making compounds with porphyrin subunits directly fused together, potentially improving electronic communication along the large rings. The project was based in Oxford to make use of the extensive hands-on experience in making and investigating of similar compounds in the group.
At the beginning of the project a critical overview of existing subject literature and unpublished results was made with cooperation of the group leader: prof. H.L. Anderson. This lead to slight refinement of the proposed structures in light of recent progress made by other group members. The target molecules were then chosen and synthetic routes were theoretically established and modified with project progress to circumvent any difficulties discovered along the way. Some initial theoretical modelling was also done to visualize the geometry of targets and their feasibility in terms of practical considerations. Then a large series of different, mostly non-literature, porphyrin monomers (“building blocks”) were made, separated and purified and their structure characterized by means of state-of-the-art methods (Nuclear Magnetic Resonance and Ultraviolet-visible spectroscopy, mass spectrometry). They incorporated varying solubilizing groups made with long hydrocarbon chains to facilitate the processability of larger assemblies made further on, as well as different metal centers (zinc, nickel or magnesium) to modify their reactivity. Then the possibility of direct porphyrin-porphyrin fusion was explored and a series of different porphyrin dimers and trimers were created and studied. A range of templates of different size and reaction sites were made in parallel, intended to pre-form the porphyrin building blocks in macrocyclization steps. Some of these templates, similar to known ones, were made to act in a reversible manner by forming coordination bonds with metal centers of the porphyrins. A completely novel set of templates was also explored, intended to pre-arrange the porphyrins by their peripheries via formation of covalent, reversible or non-reversible bonds with the templates. The final part involved using these newly-created porphyrin assemblies in combination with specific templates in order to create the target, three-dimensional porphyrin nanorings. While initial experiments showed that the designed porphyrin substrates tend to degrade or oligomerize into linear structures instead of cyclic ones, lengthily modification of the designed building blocks and optimization of reaction procedures lead to very encouraging initial results of final assembly steps. Unfortunately, due to COVID pandemic restrictions (lockdowns, shift work and reduced on-site presence) lasting for the majority of the projects timeline, this part could not be finalized as intended. The project is, however, continued by current group members and results are expected to be published in the coming months in high-impact chemical journals. Initial results were shown on several online outreach events for the general public, as well as targeted groups of school student of all ages, as part of presentations aimed to visualize and highlight the importance of science and research. These talks showcased not only the created data, but promoted scientific and academic careers in chemistry and natural sciences in general.
During the course of work two major and two minor research projects relating to novel porphyrin nanostructures and cyclocarbons were planned and executed. This involved initial synthesis optimization, up-scaling and characterization of 50+ non-literature porphyrin and phenylene compounds, explored further toward novel cyclic structures. Obtained results give new insight into the structure-property relationships in such molecular architectures and the established synthetic methodologies can be used for the development of similar compounds in the future. This research contributes toward creating and understanding the phenomena observed in functional materials in the field of molecular electronics, such as flexible and tunable organic wires, solar cells and magnets. On the other hand, multiple outreach activities focused on showcasing not only the results, but also the scientific process and career development, communicate the importance of chemical research, as well as inspire future generations to pursue interest in natural sciences in general. Showing the MSCA Fellows path, from early interests to the current position and achievements, hopefully will encourage some to take on science studies and become a scholar and/or a scientist in the future.
simplified synthetic scheme toward target molecules