A Family of Superhelicenes: Easily Tunable, Chiral Nanographenes by Merging Helicity with Planar π Systems

Abstract

We designed a straightforward synthetic route towards a full-fledged family of π-extended helicenes: superhelicenes. They have two hexa-peri-hexabenzocoronenes (HBCs) in common that are connected via a central five-membered ring. By means of structurally altering this 5-membered ring, we realized a versatile library of molecular building blocks. Not only the superhelicene structure, but also their features are tuned with ease. In-depth physico-chemical characterizations served as a proof of concept thereof. The superhelicene enantiomers were separated, their circular dichroism was measured in preliminary studies and concluded with an enantiomeric assignment. Our work was rounded-off by crystal structure analyses. Mixed stacks of M- and P-isomers led to twisted molecular wires. Using such stacks, charge-carrier mobilities were calculated, giving reason to expect outstanding hole transporting properties.

Keywords: circular dichroism; helicenes; luminescence; nanographene; polycyclic aromatic hydrocarbons.

Figure 1

Synthesis of superhelicene derivatives. 16–20 (blue box) were synthesized directly from precursors 1–5 via a reaction sequence including a double Sonogashira coupling to obtain 6–10 followed by a Diels–Alder reaction with tetraphenylcyclopentadienone 24 to obtain 11–15. Those double HAB derivatives were finally closed to the helicenes 16–20 under oxidative cyclodehydrogenation conditions. 21–23 (green box) were synthesized via post‐functionalization of 16 and 19. Conditions: a) Pd(PPh₃)₄, (5 mol %), CuI (5–10 mol %), 4‐tert‐butylphenylacetylene (2.2–3.0 equiv.), DMF/NEt₃, N₂, 16–19.5 h, 90–100 °C; 29–82 %; b) 24 (2.5 equiv.), toluene, N₂, 26–131 h, 220 °C (pressure flask), 67–93 %; c) FeCl₃ (30 equiv.) in MeNO₂ (ca. 300 mg(FeCl₃) mL⁻¹), dichloromethane, N₂, 2–4.5 h, 0 °C to rt., 64–96 % (for 16–19), DDQ (12 equiv.), triflic acid (28 equiv.), dichloromethane, N₂, 3 h, 0 °C to rt., 79 % (for 20); conditions for 21: 16, malononitrile (15 equiv.), pyridine, 1.5 h, 80 °C, 96 %; conditions for 22: 16, CBr₄ (2 equiv.), PPh₃ (4 equiv.), toluene, N₂, 24 h, reflux, 58 %; conditions for 23: 19, m‐CPBA (5 equiv.), dichloromethane, 68 h, 0 °C to rt., 52 %.

Figure 2

Absorption spectra of 19, 23, 17, 16, and 21 in toluene (turquoise), THF (purple), PhCN (orange). The charge transfer (CT) absorption bands of 16 and 21 are highlighted by the insets.

Figure 3

Femtosecond transient absorption spectra of 21 in PhCN with different time delays between 0 and 200 ps.

Figure 4

Evolution associated spectrum of 21 in PhCN with time delays from 0–5500 ps upon excitation at 550 nm.

Figure 5

Crystal structure analysis of 16. a) Dimensions of the π‐system; b) interplanar angle; c) torsion angles; d) single molecular wire formed by a mixed stack of M‐ and P‐enantiomers; e) side view of a bundle of those π‐aggregated wire; f) top view of the bundle of π‐aggregated wires.

Figure 6

Hole (blue) and electron (green) charge carrier mobility (cm² V⁻¹ s⁻¹) for the crystal structure of 16 in the ab, ac and bc plane.

Figure 7

Determined g_Abs for P‐enantiomers of a) 16, 17 and b) 18, 19.