Curved Aromatics: The Fold-in Method
We have introduced the fold-in strategy for synthesizing curved aromatic molecules. In this method, a macrocyclic precursor is subjected to a strain-inducing reaction to produce a distorted conjugated structure. The fold-in method is particularly well suited for the synthesis of bowl-shaped (spherical), belt-shaped (tubular) and saddle-shaped (hyperbolic) molecules, but can be just as well applied for the preparation of planar coronoid macrocycles.
Examples of molecules synthesized in our laboratory using the fold-in method include the molecular jellyfish (named chrysaorole), a triangular carbazole belt, and fluorene-containing chrysaorenes. Octulene, the hyperbolic analogue of kekulene, was also obtained in a fold-in reaction, and showed a surprising ability to bind chloride anions.
Curved Aromatics: Other Approaches
We explore alternative approaches to highly strained aromatic systems. In particular, we showed that highly strained non-classical nanotube endcaps can be obtained from strain-free precursors by employing "synchronized homocoupling," which involves the intermediacy of multinuclear metallacycles. More recently we employed the masked phenylene strategy, originally developed for the synthesis of circular nanohoops, to synthesize a molecular lemniscate, characterized by radial conjugation.
A general discussion of the different synthetic methods available for the construction of curved aromatic molecules can be found in our review.
Heterocyclic Nanographenes: Electron-Rich Systems
Expansion of hetrocyclic motifs in two dimensions creates heteroatom-doped analogues of nanographenes, which are of high interest as dyes and materials for organic electronics. Our groups developed several types of such compounds, e.g. peripherally expanded porphyrins and azacoronenes. These electron-rich systems can be used as liquid crystals, multi-electron donors, and aromaticity switches.
Heterocyclic Nanographenes: Donor–Acceptor Systems
We reported the synthesis of donor–acceptor (DA) pyrrole building blocks that can be used to synthesize a variety of oligopyrrole chromophores, including porphyrins, snowflake-like or propeller-shaped azacoronenes, and bipyrrole boomerangs, some of which adopt persistent chiral configurations. These DA systems absorb strongly the visible and near-infrared radiation, and can be used e.g. as dyes for antimicrobial photodynamic therapy. Another notable feature is the ability of these molecules to accept multiple electrons (up to 10 in azacoronenes), which makes them of interest as charge storage materials.
Aromatic oligoradicals and oligoradicaloids have been intensely investigated for over a century because of their exceptional electronic structure, and expected practical applications. Our research on coronoids and nanographenes provides a unique opportunity to create oligoradicaloid frameworks. As part of these investigations, we reported the synthesis of fully conjugated chrysaorene, which acts as a unique macrocyclic “diiodine splitter,” combining redox switching with anion receptor chemistry. In parallel work, we established an effective strategy to obtain diindenophenanthrene, a stable derivative of Chichibabin’s hydrocarbon. In our work on DA nanographenes, we designed a 139-electron azacoronene radical which reversibly recombines into a giant sandwich-like dimer.