Chiral Dibenzopentalene‐Based Conjugated Nanohoops through Stereoselective Synthesis

Abstract

Conjugated nanohoops allow to investigate the effect of radial conjugation and bending on the involved π‐systems. They can possess unexpected optoelectronic properties and their radially oriented π‐system makes them attractive for host–guest chemistry. Bending the π‐subsystems can lead to chiral hoops. Herein, we report the stereoselective synthesis of two enantiomers of chiral conjugated nanohoops by incorporating dibenzo[a,e]pentalenes (DBPs), which are generated in the last synthetic step from enantiomerically pure diketone precursors. Owing to its bent shape, this diketone unit was used as the only bent precursor and novel “corner unit” in the synthesis of the hoops. The [6]DBP[4]Ph‐hoops contain six antiaromatic DBP units and four bridging phenylene groups. The small HOMO–LUMO gap and ambipolar electrochemical character of the DBP units is reflected in the optoelectronic properties of the hoop. Electronic circular dichroism spectra and MD simulations showed that the chiral hoop did not racemize even when heated to 110 °C. Due to its large diameter, it was able to accommodate two C60 molecules, as binding studies indicate.

Conjugated Nanohoops Incorporating Donor, Acceptor, Hetero‐ or Polycyclic Aromatics

“…In 2020 Itami and co-workers reported [6]CPP derivative 87,acompound related to fluorene-containing nanohoops, where each phenylene unit is linked to its neighbors through [87] [4]40 b, [89] [4]40 c, [88] [n]40 d and e, [90] [n,n]41, [74] 87, [91] [n]42, [92] 88, [93] 43, [94] and 44. [40] The color red is used to highlight the dibenzo-[a,e]pentalene moieties due to their small band gap and red color. methylene bridges,r esulting in ab elt-shaped structure.…”

“…Figure 11. Nanohoops with five-ring-containing PAHs [n]40 a,[87] [4]40 b,[89] [4]40 c,[88] [n]40 d and e,[90] [n,n]41,[74] 87,[91] [n]42,[92] 88,[93] 43,[94] and 44 [40]. The color red is used to highlight the dibenzo-[a,e]pentalene moieties due to their small band gap and red color.…”

Conjugated Nanohoops Incorporating Donor, Acceptor, Hetero‐ or Polycyclic Aromatics

“…Lastly, our group used a bent and chiral diketone precursor to incorporate dibenzo-[a,e]pentalenes into nanohoops (Scheme 1 f), as will be discussed in Section 6.1. [40,41]…”

“…In 2020 Itami and co-workers reported [6]CPP derivative 87, a compound related to fluorene-containing nanohoops, where each phenylene unit is linked to its neighbors through [87] [4]40 b, [89] [4]40 c, [88] [n]40 d and e, [90] [n,n]41, [74] 87, [91] [n]42, [92] 88, [93] 43, [94] and 44. [40] The color red is used to highlight the dibenzo-[a,e]pentalene moieties due to their small band gap and red color. methylene bridges, resulting in a belt-shaped structure.…”

“…In 2021 we followed up with chiral DBP-based nanohoops 44 accessed in a stereoselective synthesis. [40] Both enantiomers (+)and (À)-44 were synthesized separately by using enantiomerically pure and bent diketone precursors (Scheme 1 f). The small HOMO-LUMO gap and ambipolar electrochemical character of the DBP units were reflected in the optoelectronic properties of the hoops.…”