NUS Chemists Synthesize Novel Non-Alternant Carbon Nanobelts

Researchers at the National University of Singapore, led by Associate Professor Chi Chunyan, have synthesised a new class of fully-conjugated, pentagon-embedded non-alternant carbon nanobelts (CNBs). Published in Nature Synthesis, the work details a multi-step chemical process, initiated by a Diels-Alder reaction and culminating in oxygen removal, to create stable ring structures. These CNBs exhibit small energy gaps and emit bright red light under ultraviolet illumination, suggesting potential applications in organic light-emitting diodes and solar cells. Notably, one CNB can be chemically oxidised into a charged form with an open-shell singlet ground state, displaying characteristics of loosely coupled annulenes with Baird-type aromaticity.

Novel Carbon Nanobelts Synthesised

Researchers at the National University of Singapore have successfully synthesised a new class of carbon nanostructures termed fully-conjugated, pentagon-embedded non-alternant carbon nanobelts (CNBs). These CNBs feature non-alternant carbon frameworks, a structural design intended to enhance electron delocalisation and impart unique electronic properties to the molecules. The research, led by Associate Professor Chi Chunyan, addresses a long-standing challenge in molecular design and has been published in the journal Nature Synthesis.

The newly synthesised CNBs differ from previously reported structures, which typically consist of benzene rings that restrict electron flow to localised regions. To overcome this limitation, the NUS team incorporated five-membered cyclopentadienyl units, introducing curvature and moderate strain while facilitating greater electron mobility across the molecule. This approach resulted in the creation of CNBs that are both structurally novel and electronically functional, according to Associate Professor Chi.

The researchers employed a multi-step chemical process to create these nanobelts, beginning with the assembly of molecular building blocks via a Diels-Alder reaction. A key step involved the removal of oxygen atoms, leading to the formation of stable, fully conjugated ring structures. This method successfully yielded two carbon nanobelts exhibiting bright red light emission under ultraviolet illumination and possessing small energy gaps.

One of the resulting CNBs can be chemically oxidised into a charged form with an open-shell singlet ground state, an unusual electronic configuration linked to global aromaticity. Theoretical studies suggest this state behaves as two loosely coupled [32]annulenes with Baird-type aromaticity along its edges, offering new insights into charge and spin behaviour in curved carbon systems.

The synthetic strategy developed in this work enables the gradual buildup of strain while maintaining effective conjugation, ultimately allowing access to the target carbon nanobelts (CNBs). This research provides a new platform for exploring correlated electron behaviour in carbon-based systems, according to Associate Professor Chi.

Enhanced Electron Delocalisation and Unique Properties

The incorporation of five-membered cyclopentadienyl units into the non-alternant carbon nanobelts (CNBs) enables enhanced electron delocalisation. These non-alternant structures introduce curvature and moderate strain, allowing electrons to move more freely across the molecule, a departure from previously reported CNBs consisting of benzene rings that restrict electron flow to localised regions.

Theoretical studies suggest that one of the resulting CNBs, when chemically oxidised into a charged form, exhibits an open-shell singlet ground state, an unusual electronic configuration linked to global aromaticity. This state behaves like two loosely coupled [32]annulenes with Baird-type aromaticity along its edges, offering new insights into charge and spin behaviour in curved carbon systems.

Implications for Future Technologies

The successful synthesis of these fully-conjugated non-alternant carbon nanobelts (CNBs) provides a new platform for exploring correlated electron behaviour in carbon-based systems. This research represents a significant step forward in reimagining how carbon nanostructures are designed and utilised in future technologies, according to Associate Professor Chi.

The synthetic strategy developed in this work enables the gradual buildup of strain while maintaining effective conjugation, ultimately allowing access to the target carbon nanobelts (CNBs). The resulting CNBs exhibit bright red light emission under ultraviolet illumination and possess small energy gaps, making them highly suitable for use in organic light-emitting diodes and solar cells.

Theoretical studies suggest that the charged form of one of the resulting CNBs, achieved through chemical oxidation, behaves like two loosely coupled [32]annulenes with Baird-type aromaticity along its edges. This behaviour offers new insights into charge and spin behaviour in curved carbon systems, linked to its unusual open-shell singlet ground state and global aromaticity.