Periodic Reporting for period 2 - ARO-MAT (Nanoscale Aromaticity and Supramolecular Electronic Materials)
Reporting period: 2022-04-01 to 2023-09-30
The ARO-MAT project seeks to understand emergent cooperative electronic and magnetic phenomena in large synthetic molecules with dimensions of 5–25 nm (i.e. as big as many proteins). The project is developing supramolecular architectures with large pi-systems and well-defined geometries, in which the frontier orbitals coherently delocalize charge over the whole nanostructure. Aromaticity is one example of the emergent phenomenon targeted in this project; it can be defined as the ability of a cyclic molecule to sustain a ring current when placed in a magnetic field. Until recently, it was thought that aromaticity is restricted to small molecules with circuits of less than about 22 pi-electrons. Anderson has shown that circuits of more than 160 pi-electrons (circumference > 15 nm) can exhibit strong aromatic ring currents. Testing even larger rings will elucidate the link between aromaticity and the persistent currents that arise in non-molecular mesoscopic rings (diameter 50–500 nm). ARO-MAT will explore the effects of molecular size and topology on nanoscale aromaticity. Other emergent phenomena to be addressed include the formation of open-shell singlet polyradical ground states, magnetic bistability in systems with many paramagnetic metal centers, and the control of charge transport through single-molecule devices by quantum interference. This multidisciplinary project combines organic synthesis, supramolecular chemistry, theory, electronic structure calculations, NMR and EPR spectroscopy, magnetochemistry, molecular electronics and low-temperature charge transport experiments. The core objective is to create low band gap materials with unprecedented electronic and magnetic properties, and to understand the structure-property relationships governing the behavior of these new materials.
One of the goals of ARO-MAT is to synthesize edge-fused porphyrin nanobelts, which are expected to exhibit strong global ring currents. This objective has not yet been achieved, but substantial progress led to three publications: “Bending a photonic wire into a ring” (Nat. Chem. 2022, 14, 1436–1442), “Covalent template-directed synthesis of a spoked 18-porphyrin nanoring” (Angew. Chem. Int. Ed. 2023, 62, e202302114) and “b,b-Directly linked porphyrin rings: Synthesis, photophysical properties and fullerene binding” (J. Am. Chem. Soc. 2023, doi: 10.1021/jacs.3c03549). Work is in progress towards using related structures to access porphyrin nanobelts.
Another goal of ARO-MAT is to investigate porphyrin nanoribbons with multiple paramagnetic metal centers, as an approach to spintronic materials. Substantial progress has been made in this area. We investigated poly-lanthanide nanoribbons and this work was published in “Singly and triply linked magnetic porphyrin lanthanide arrays” (J. Am. Chem. Soc. 2022, 144, 8693–8706). Low temperature SQUID magnetometry measurements reveal intramolecular antiferromagnetic exchange coupling between GdIII centers and the phase memory times are long enough to test quantum computational schemes using microwave pulses.
During the second reporting period (01/04/2022 to 30/09/2023), we made substantial advances in understanding the electronic and magnetic properties of porphyrin-based nanoribbons (Work Packages 1 and 5), leading to several high-profile publications. This includes investigation of anthracene-porphyrin nanoribbons (Angew. Chem. Int. Ed. 2023, 62, e202307035), demonstrating phase-coherent charge transport through a porphyrin nanoribbon-graphene junctions (J. Am. Chem. Soc. 2023, 145, 15265–15274) and exploring the enhanced coherence generated by coupling spins through a delocalized π-system in vanadyl porphyrin oligomers (Chem 2023, DOI: 10.1016/j.chempr.2023.09.013). We also demonstrated key steps towards the synthesis of edge-fused porphyrin nanobelts. We also devised a synthetic route to porphyrin nanorings with para-phenylene bridges (Org. Lett. 2023, 25, 378–383). Our theoretical work on understanding aromaticity in large π-conjugated macrocycles generated two publications (Angew. Chem. Int. Ed. 2022, 61, e202201231 and Chem. Sci. 2023, 14, 1762–1768), and has contributed towards a change in the way the community thinks about molecular nanostructures. We are also comparing the electronic structure of large porphyrin-based nanorings with smaller analogous rings constructed purely from carbon atoms, i.e. cyclo[N]carbons. As part of this work, we reported the first structural characterisation of a doubly anti-aromatic cyclocarbon, C16 (Nature 2023, DOI: 10.1038/s41586-023-06566-8) and we published a strategy for creating cyclocarbons that are stabilised by catenane formation (Nat. Chem. 2023, DOI: 10.1038/s41557-023-01374-z). Another strand of this work has been the investigation of polaron delocalisation on linear and cyclic molecular wires using EPR spectroscopy. We made substantial progress in this area during this reporting period and several publications on these results are in preparation.
Another goal of ARO-MAT is to investigate porphyrin nanoribbons with multiple paramagnetic metal centers, as an approach to spintronic materials. Substantial progress has been made in this area. We investigated poly-lanthanide nanoribbons and this work was published in “Singly and triply linked magnetic porphyrin lanthanide arrays” (J. Am. Chem. Soc. 2022, 144, 8693–8706). Low temperature SQUID magnetometry measurements reveal intramolecular antiferromagnetic exchange coupling between GdIII centers and the phase memory times are long enough to test quantum computational schemes using microwave pulses.
During the second reporting period (01/04/2022 to 30/09/2023), we made substantial advances in understanding the electronic and magnetic properties of porphyrin-based nanoribbons (Work Packages 1 and 5), leading to several high-profile publications. This includes investigation of anthracene-porphyrin nanoribbons (Angew. Chem. Int. Ed. 2023, 62, e202307035), demonstrating phase-coherent charge transport through a porphyrin nanoribbon-graphene junctions (J. Am. Chem. Soc. 2023, 145, 15265–15274) and exploring the enhanced coherence generated by coupling spins through a delocalized π-system in vanadyl porphyrin oligomers (Chem 2023, DOI: 10.1016/j.chempr.2023.09.013). We also demonstrated key steps towards the synthesis of edge-fused porphyrin nanobelts. We also devised a synthetic route to porphyrin nanorings with para-phenylene bridges (Org. Lett. 2023, 25, 378–383). Our theoretical work on understanding aromaticity in large π-conjugated macrocycles generated two publications (Angew. Chem. Int. Ed. 2022, 61, e202201231 and Chem. Sci. 2023, 14, 1762–1768), and has contributed towards a change in the way the community thinks about molecular nanostructures. We are also comparing the electronic structure of large porphyrin-based nanorings with smaller analogous rings constructed purely from carbon atoms, i.e. cyclo[N]carbons. As part of this work, we reported the first structural characterisation of a doubly anti-aromatic cyclocarbon, C16 (Nature 2023, DOI: 10.1038/s41586-023-06566-8) and we published a strategy for creating cyclocarbons that are stabilised by catenane formation (Nat. Chem. 2023, DOI: 10.1038/s41557-023-01374-z). Another strand of this work has been the investigation of polaron delocalisation on linear and cyclic molecular wires using EPR spectroscopy. We made substantial progress in this area during this reporting period and several publications on these results are in preparation.
The synthesis of edge-fused porphyrin nanobelts, and investigation of their magnetic and electronic properties, is still a top objective of this project. We are exploring several strategies for achieving this objective, using both template-directed and non-templated strategies. It is challenging because smaller nanobelts are highly strained, whereas larger nanobelts require the spatial organization of many porphyrin units, and the controlled formation of many carbon-carbon bonds.
We also aim to test the limits of aromaticity by investigating large butadiyne-linked porphyrin nanorings. We hope soon to have results on the 18-porphyrin ring and to discover whether the 10+ oxidation state of this nanoring with a circuit of 242 pi-electrons exhibits a global aromatic ring current. The next step will be to extend these studies to a 36-porphyrin nanoring. Macrocycles in this size regime are interesting because they are large enough that their aromatic ring currents could be reversed by accessible magnetic fields.
A more challenging objective, which we also hope to achieve during this project, is to connect molecular nanorings between electrodes and demonstrate that they can behave as sensitive single-molecule transistors, by virtue of coherent transport through both pathways around the ring.
In the context of linear molecular wires, we are working on the synthesis of highly transmissive nanoribbons that have single-molecule conductances near the theoretical limit (G0) and independent of length. The design of wires of this type has been a long-standing goal in the field of molecular electronics. We are seeking to understand the electronic behavior of these systems in detail by using EPR spectroscopy to probe charge delocalization. We also plan to deliver functional single-molecule spintronic devices such as spin-valves, in which the different magnetic behavior of two paramagnetic metal centers enables the conductance of the device to be controlled with an external magnetic field. Another objective in this area is to combine metal-centered and pi-centered spins by oxidizing or reducing the pi-system of a porphyrin-based molecular wire with several paramagnetic metal centers. We anticipate that the presence of a spin in the pi-systems may mediate efficient coupling between the metal centers, providing a strategy for creating single-molecule magnets.
We also aim to test the limits of aromaticity by investigating large butadiyne-linked porphyrin nanorings. We hope soon to have results on the 18-porphyrin ring and to discover whether the 10+ oxidation state of this nanoring with a circuit of 242 pi-electrons exhibits a global aromatic ring current. The next step will be to extend these studies to a 36-porphyrin nanoring. Macrocycles in this size regime are interesting because they are large enough that their aromatic ring currents could be reversed by accessible magnetic fields.
A more challenging objective, which we also hope to achieve during this project, is to connect molecular nanorings between electrodes and demonstrate that they can behave as sensitive single-molecule transistors, by virtue of coherent transport through both pathways around the ring.
In the context of linear molecular wires, we are working on the synthesis of highly transmissive nanoribbons that have single-molecule conductances near the theoretical limit (G0) and independent of length. The design of wires of this type has been a long-standing goal in the field of molecular electronics. We are seeking to understand the electronic behavior of these systems in detail by using EPR spectroscopy to probe charge delocalization. We also plan to deliver functional single-molecule spintronic devices such as spin-valves, in which the different magnetic behavior of two paramagnetic metal centers enables the conductance of the device to be controlled with an external magnetic field. Another objective in this area is to combine metal-centered and pi-centered spins by oxidizing or reducing the pi-system of a porphyrin-based molecular wire with several paramagnetic metal centers. We anticipate that the presence of a spin in the pi-systems may mediate efficient coupling between the metal centers, providing a strategy for creating single-molecule magnets.