Porphyrin Nanorings

Lead Research Organisation: University of Oxford
Department Name: Oxford Chemistry

Abstract

Conjugated organic materials are important because of their unique optical and electronic properties. For example organic semiconductors are finding applications in displays, flexible transistors and photovoltaic materials, while two-photon dyes are used in microfabrication, biological imaging and optical signal processing. This field has advanced tremendously during the past 20 years and provides excellent examples of the translation of fundamental science into practical applications that impact everyday life. However there are many open questions concerning the engineering and control of organic pi-systems. Fundamental studies are required to underpin further long-term technological innovations. In this proposal, we build on a new synthetic route to cyclic conjugated polymers, based on supramolecular organisation of precursor molecules. 'Vernier templating' is a new strategy for creating molecules with dimensions comparable to those of structures made by top-down engineering techniques, such as electron-beam lithography. The chemical synthesis of mesoscopic structures provides benefits through the introduction of functionality with atomic precision. We will exploit this methodology to study a previously unexplored region of structure-space and to generate completely new functional materials.

Conjugated macrocycles or 'annulenes' have been a focus of research ever since the classical studies by Sondheimer in the 1960s. Large annulenes exhibit remarkable optoelectronic properties, and this has stimulated a resurgence of activity in the field. Anderson's group has contributed to the area by developing the template-directed synthesis of belt-like nanorings of 6, 8 or 12 porphyrin units, with diameters of 2-5 nm. This project will investigate the chemistry and physics of these nanorings, and extend the synthesis to even larger structures.

The porphyrin nanorings targeted here are of great interest in their own right, owing to their similarity to natural light harvesting systems, and because they are expected to support the quantum coherent transport of charge and excitation. They also provide a new model system in which to explore the synthesis of mesoscopic molecules with well defined shapes. The templating procedures pioneered in Oxford are likely to stimulate developments in related fields such as molecular machines and biomimetic chemistry, where the controlled synthesis of large molecules with complex functionality remains a bottleneck for future developments. We focus on alkyne-linked metalloporphyrin oligomers as test systems which allow access to new functional materials. The fascinating interplay between synthesis, structure and function for these materials motivates the collaborative approach proposed here.

Understanding the flow of energy and electrons through molecules is fundamental to many areas of science. We will investigating the delocalisation of energy and charge in nano-scale molecular wires with well defined tertiary structures. Conventional ring currents have only been observed in molecules with diameters of less than 2 nm, however semiconductor rings with diameters of about 10 nm exhibit persistent ring currents, in the absence of an applied voltage. These quantum-interference phenomena are analogous to the ring currents of aromatic molecules, except that they vary with the applied magnetic field (i.e. they exhibit Aharonov-Bohm oscillations). We will investigate whether molecular nanorings exhibit behaviour intermediate between those of small aromatic molecules and large quantum rings. Porphyrin nanorings resemble the light harvesting chlorophyll arrays which funnel energy into the reaction centre during photosynthesis. We will explore whether they mimic the excitonic behaviour of natural light harvesting arrays. Ultimately this work may lead to improved materials for solar power generation, or to molecular solenoids and split-ring resonators which function as optical metamaterials.

Planned Impact

(A) Generation of Knowledge.
This project cuts across the Chemistry/Physics/Materials interface. It will lead to advances in fundamental understanding in diverse areas: (a) Understanding light-harvesting in natural photosynthetic systems and synthetic photovoltaic devices; (b) Understanding charge transport in molecular wires; (c) Understanding aromaticity; (d) Understanding cooperativity and multivalency; (e) Understanding template-directed synthesis; (f) Understanding interactions between molecules on surfaces; (g) Understanding how to characterise synthetic nanostructures in the 10 - 100 kDa mass range; (h) Understanding the nonlinear optical behaviour of cyclic pi-systems.

The scientific impact of the project will be maximised by presenting the results at international meetings and by publishing in top journals. The seven investigators have outstanding publication records: during the last 5 years (since January 2006) they have published 189 journal articles, including 20 in J. Am. Chem. Soc., 18 in Angew. Chem. Int. Ed., 1 in Science and 1 in Nature.

(B) Training.
This project will deliver high-level scientific training for three PDRAs in topical interdisciplinary areas:
PDRA-1: synthesis, self-assembly, SAXS, NMR and EPR spectroscopy.
PDRA-2: surface science, STM, XPS, XAS and REPS.
PDRA-3: time-resolved photoluminescence up-conversion spectroscopy.

The PDRAs will gain a broad understanding of the whole project, and learn from other on-going projects in the investigators' laboratories. They will benefit by participating in weekly group meetings (including problem classes and literature surveys). Drafting manuscripts, delivering research seminars, presenting results at conferences, interacting with collaborators in different fields and helping to supervise research students are all important aspects of the educational experience.

The quality of the training will be optimised by careful mentoring of each PDRA, by close attention to their career development and training needs and through participation in well selected international meetings. Mentoring will be delivered effectively through weekly meetings (with Anderson, Beton or Herz) and by two-monthly project reviews.

(C) Economic and Societal Impacts.
Organic electronic materials are making an increasing economic impact. The UK is strong in this area. Organic semiconductors are being applied in displays, flexible transistors and photovoltaic devices, while two-photon dyes are used in microfabrication, biological imaging and optical signal processing. However the engineering of pi-systems is still poorly understood. We urgently need better fundamental understanding of these materials. Basic research of the type proposed here will generate completely new types of functional materials.

This research has the potential to yield a wide range of long term economic and social benefits. It may lead ultimately to improved materials for harvesting solar power, or to molecular solenoids and split-ring resonators which function as optical metamaterials. The nonlinear optical materials created in this project could eventually have applications in all-optical signal processing, in telecommunications and other related areas. Results with potential for commercial exploitation will be treated confidentially, patented and exploited in conjunction with the Oxford University intellectual property company and the corresponding organisations at Nottingham and Diamond. A consortium agreement between the participating institutions will be agreed and will cover issues related to intellectual property ownership, dissemination and related management procedures.

The impact of the project will be maximised by striving to carry out research of the highest possible quality which addresses important fundamental scientific questions; by thinking unconventionally and by pursing the most original experiments. Maximising impact is an integral part of the whole project.

Publications

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Bols PS (2018) Template-Directed Synthesis of Molecular Nanorings and Cages. in Accounts of chemical research

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Bols PS (2018) Shadow Mask Templates for Site-Selective Metal Exchange in Magnesium Porphyrin Nanorings. in Angewandte Chemie (International ed. in English)

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Cremers J (2018) Template-Directed Synthesis of a Conjugated Zinc Porphyrin Nanoball. in Journal of the American Chemical Society

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Favereau L (2015) Six-Coordinate Zinc Porphyrins for Template-Directed Synthesis of Spiro-Fused Nanorings. in Journal of the American Chemical Society

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Gong JQ (2016) Breaking the Symmetry in Molecular Nanorings. in The journal of physical chemistry letters

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Gong JQ (2015) Structure-Directed Exciton Dynamics in Templated Molecular Nanorings. in The journal of physical chemistry. C, Nanomaterials and interfaces

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Judd CJ (2019) On-Surface Synthesis within a Porphyrin Nanoring Template. in Scientific reports

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Karunanithy G (2016) Harnessing NMR relaxation interference effects to characterise supramolecular assemblies. in Chemical communications (Cambridge, England)

 
Description We have developed the template-directed synthesis of new porphyrin arrays and shown that they mimic many aspects of natural light-harvesting chlorophyll systems. We have shown that these large ring-shaped molecules can be imaged by STM.
Exploitation Route The insights that we have gained will be used to design advanced materials for a variety of applications in light-harvesting.
Sectors Chemicals,Energy

 
Description Supramolecular Nanorings for Exploring Quantum Interference
Amount £362,292 (GBP)
Funding ID EP/M016110/1 
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Public
Country United Kingdom
Start 06/2015 
End 10/2018