Redox-active polyaromatic hydrocarbons (PAHs) have been widely pursued as a means of investigating fundamental aromaticity and developing stimuli-responsive π-conjugated materials for advanced organic device applications. Unlike conventional redox-active systems based on rigid 2D-planar PAH topologies, however, in which electron delocalisation is generally confined to conjugated through-bond pathways, helically strained PAHs known as helicenes offer a dynamic non-planar scaffold to investigate the effect of intrannular through-space conjugation on functional properties and assembly under a external stimulus. We have found that the precise installation of atomic electrophores (Figure 1a) in the fjord region of redox-active helicenes encourages the formation of a new intraannular molecular orbital that spans through space (Figure 1b) and acts as an attractive electron reservoir. Indeed, detailed spectroscopic investigations show that subsequent electron population of this 'super' MO can activate strong NIR absorption indicative of extensive π-conjugation, while theoretical aromaticity indicies (NICS, AICD (Figure 1c), IMS) all imply the generation of a new aromatic ring current across the fjord — despite the absence of a formal sigma scaffold! We note that such non-bonded aromatic ring currents are Mobius-like on account of an orbital node present between opposing fjord sites, and are fundamentally different from other forms of through-space cyclic conjugation leading to typically Hückel homoaromaticity. Redox-active helicenes thus lend themselves to thermally-, electrochemically- and chemically-responsive degrees of fjord aromaticity and spin–spin interactions in doubly reduced samples, as detected by advanced EPR analyses. Depending on the nature of aryl-extension employed, racemic helicene mixtures assemble to form heterochiral or self-sorted homochiral arrays of helical enantiomers (Figure 1d), giving rise to solid-state fluorescence. The Avestro Group at York is currently elaborating upon this new helicene chemistry to deepen our understanding of emergent fjord-based electronic phenomena with an eye towards future chiral spintronic and circularly polarised light technologies.

Intra-annular fjord aromaticity within helically chiral aromatic electrophores

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