A Configurationally Stable Helical Indenofluorene

Abstract

We report the synthesis and study of the optoelectronic, magnetic, and chiroptical properties of a helically chiral diradicaloid based on dibenzoindeno[2,1-c]fluorene. The molecule shows a small HOMO-LUMO gap and a moderate singlet-triplet gap, which agrees with the results of DFT calculations. The helical structure of the compound, confirmed by X-ray diffraction, is configurationally stable, which allows the isolation of both enantiomers and the evaluation of the chiroptical properties (ECD).

Figure 1

Five regioisomeric indenofluorenes.

Figure 2

Structure of indeno[2,1-c]fluorene IF5H (left) and our synthesized extended compound IF7H (right).

Scheme 1. Synthesis of Dibenzoindeno[2,1-c]fluorene IF7H

Conditions: (i) 2,4,6-trimethylphenylmagnesium bromide, tetrahydrofuran (THF), 0 °C, 68%; (ii) SnCl₂, toluene, 40 °C, 84%.

Figure 3

(a) Aromatic region of the ¹H NMR spectrum of IF7H. (b) Bond distances (in Å) in the as-indacene core: (a) 1.37–1.38; (b) 1.43; (c) 1.35–1.36; (d) 1.43; (e) 1.48; (f) 1.36–1.38; (g) 1.49; h, 1.37. (c) Top view of the structure showing the dihedral angles (in deg) in the helicene moiety: ϕ₁, 1.6–6.0; ϕ₂, 23.8–31.5; ϕ₃, 4.6–5.6; ϕ₄, 24.3–27.4; ϕ₅, 2.5–3.1. (d) Front view showing the distance between the outer helicene rings: (i) 4.61–4.63 Å. H atoms have been omitted for the sake of clarity. Two values are given for some distances and angles, as there are two molecules of IF7H in the asymmetric unit.

Figure 4

UV–vis spectrum of IF7H in CH₂Cl₂. Inset shows a magnified view of the 600–1100 nm region.

Figure 5

(a) SQUID measurement for IF7H. (b) Spin density distribution calculated of the singlet open-shell (top) and triplet (bottom) states of IF7H ((BS)-U-LC-BLYP/6-311G(d), isoval = 0.004).

Figure 6

(a) ECD spectra of (M)-IF7H and (P)-IF7H in CH₂Cl₂. (b) Energy diagram for the isomerization from (M)-IF7H to (P)-IF7H; values in kcal mol^–1.