Octagon‐Embedded Carbohelicene as a Chiral Motif for Circularly Polarized Luminescence Emission of Saddle‐Helix Nanographenes

Abstract

We report a new family of hexa‐peri‐hexabenzocoronene (HBC)‐based helical nanographenes incorporating π‐extended carbo[5]helicenes bearing an octagonal carbocycle. This family represents a new kind of highly distorted saddle‐helix hybrid nanographenes. For the first time, the eight‐membered ring becomes a constituent of both a carbo[5]helicene and a HBC and thus, the negative curvature is responsible for twisting both units. This novel chiral motif, namely, oct‐[5]helicene results in the largest torsion angle recorded so far for a carbo[5]helicene (θ=79.5°), as it has been suggested by DFT‐calculations and confirmed by X‐ray crystallography. Consequently, the barriers of isomerization become exceptionally high for a [5]helicene unsubstituted in the fjord region since neither racemization nor decomposition were observed at 200 °C for 1 or 3 during 5 h. Therefore, racemic resolutions allowed subsequent chiroptical studies showing the ECD and CPL responses of this novel family of chiral nanographenes.

Chiral Distorted Hexa‐peri‐hexabenzocoronenes Bearing a Nonagon‐Embedded Carbohelicene

“…Both terms also depend on the number and type of rings that make up the helicene skeleton. In this sense, we demonstrate that the inclusion of nonagon ring has a tremendous effect on the average torsion‐angle since it is also much bigger (45.2–46.0°) than those previously reported: hept ‐[5]helicene (35.6°), [15] HBC‐ oct ‐[5]helicene (35.0°), [13] carbo[5]helicene (22.1°) [18] or the multiple hexapole[5]helicene (22.1–22.2°) [9c–d] . In fact, the large ring ( n =9) is responsible for the maximal dihedral angle in the rim ( φ 3 =106.9°–108.3°, see Figure 2 d).…”

Octagon‐Embedded Carbohelicene as a Chiral Motif for Circularly Polarized Luminescence Emission of Saddle‐Helix Nanographenes

“…X-Ray crystal structure of 2:[42] a) Structure of P-2 with the hexabenzo-oct-[5]helicene moiety highlighted in color;b )ORTEP drawing of the asymmetric unit showing thermal ellipsoids set at 50 % probability;c)Dimensions of the saddle shape induced by the octagonal ring []: a, 6.35;b,2.63 (see Supporting Information); d) eight-membered ring bond lengths []: c, 1.49;d,1.50;e,1 .47;f, 1.46;g,1 .49;h,1.40;i ,1.50;j ,1.37;e )section of the crystal packing. Color code:C(gray), O(red), H( white), Cl (green), N(blue).…”

Chiral Distorted Hexa‐peri‐hexabenzocoronenes Bearing a Nonagon‐Embedded Carbohelicene

“…Very recently, our group has developed an unprecedented strategy for the insertion of saddle curvature into both the carbohelicene and the HBC unit, leading to rigid chiral helical nanographenes. [13] Typically, the replacement of one or more six‐membered rings by four‐, five‐ or seven‐membered rings in a carbo[ n ]helicene leads to a significant decrease of the configurational stability. [14] However, Müllen and Narita proved that the simultaneous inclusion of two heptagonal carbocycles into both the π‐extended structure and the carbo[5]helicene (Figure 1 ) gives rise to more rigid structures (Δ G ≠ =25.41 kcal mol −1 at 298 K) [15] than either carbo[5]helicenes or nanographenes containing heptagonal rings alone.…”