Symmetric, Unsymmetrical, and Asymmetric [7]‐, [10]‐, and [13]Helicenes

Abstract

Fully aromatic helicenes with more than one pitch‐length are illustrious synthetic targets with potential applications in advanced optical devices and nano‐electronics. The task of extending the length of fully conjugated helicenes past one pitch length is challenging. Now, the synthesis of a series of azaoxa[7]‐, [10]‐, and [13]helicenes is described. The synthesis is based on iterative oxidative furan formation between 3,6‐dihydroxycarbazoles and/or 2‐naphthols. The flexibility of the presented method allows the convenient and scalable synthesis of symmetric, unsymmetrical, and asymmetric homo‐chiral structures. The [13]helicenes can be synthetically functionalized both at the termini and the periphery. The full range of helicenes were characterized using NMR and optical spectroscopy (UV/Vis, fluorescence, and CD) along with single‐crystal X‐ray crystallography. The enantiomers of the [13]helicenes are the longest optically pure helicenes isolated to date.

Bowl‐Shaped Pentagon‐ and Heptagon‐Embedded Nanographene Containing a Central Pyrrolo[3,2‐b]pyrrole Core

“…Inducing a curvature in π‐extended systems, usually requires harsh conditions which rarely tolerate electron‐rich heterocycles. Incorporation of heteroatoms into polycyclic frameworks, however, is an elegant method of tuning the intrinsic physicochemical properties of polycyclic aromatic hydrocarbons (PAHs) [30–32] . Furthermore, azabuckybowls have been envisaged as precursors for the bottom‐up syntheses of azafullerenes [33] and nitrogen‐doped CNTs [34] .…”

Pyrene bridged double[7]helicene embedded with a heptagonal ring

“…Overcoming the shortcomings of pristine [n]helicenes, helicene fused polycyclic aromatic hydrocarbons (PAHs) have shown a significant increase in the overall functional properties. [11][12][13][14][15][16][17][18][19][20][21][22][23][24] Moreover during the last few years multiple [n]helicene-embedded nanographenes with numerous aromatic cores have been developed. 23,[25][26][27][28][29][30] Preserving their core properties, multi[n]helicene showed unique characteristics due to an increased π-conjugation and molecular packing.…”

“…13 C NMR (151 MHz, CD 2 Cl 2 , 25 °C): δ [ ppm] = 149.57, 149.47, 143.91, 143.39, 140.40, 137.08, 136.59, 134.62, 134.35, 133.96, 133.28, 133.24, 133.07, 132.76, 132.67, 132.51, 132.49, 131.95, 131.52, 131.39, 131.31, 130.80, 130.55, 130.50, 130.49, 130.27, 129.77, 129.75, 129.73, 129.65, 129.11, 129.06, 128.98, 128…”