Expanded [23]-Helicene with Exceptional Chiroptical Properties via an Iterative Ring-Fusion Strategy

Abstract

Expanded helicenes are an emerging class of helical nanocarbons composed of alternating linear and angularly fused rings, which give rise to an internal cavity and a large diameter. The latter is expected to impart exceptional chiroptical properties, but low enantiomerization free energy barriers (ΔG ‡ e ) have largely precluded experimental interrogation of this prediction. Here, we report the syntheses of expanded helicenes containing 15, 19, and 23 rings on the inner helical circuit, using two iterations of an Ircatalyzed, site-selective [2 + 2 + 2] reaction. This series of compounds displays a linear relationship between the number of rings and ΔG ‡ e . The expanded [23]-helicene, which is 7 rings longer than any known single carbohelicene and among the longest known all-carbon ladder oligomers, exhibits a ΔG ‡ e that is high enough (29.2 ± 0.1 kcal/mol at 100 °C in o-DCB) to halt enantiomerization at ambient temperature. This enabled the isolation of enantiopure samples displaying circular dichroism dissymmetry factors of ±0.056 at 428 nm, which are ≥1.7× larger than values for previously reported classical and expanded helicenes. Computational investigations suggest that this improved performance is the result of both the increased diameter and length of the [23]-helicene, providing guiding design principles for high dissymmetry molecular materials.

A π‐Extended Pentadecabenzo[9]Helicene

“…The dissymmetry factors | g abs | of ( P )‐ and ( M )‐ EP9H at 640 nm were as high as 2.73×10 −2 and 2.76×10 −2 , respectively. Both values are significantly larger than those of most helicene derivatives [14] …”

“…Both values are significantly larger than those of most helicene derivatives. [14] Although helicenes are potential CPL emitters, relevant studies remain limited. The high g abs and medium quantum yield of EP9H led us to investigate its potential application as a CPL emitter.…”

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

“…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. 23,[31][32][33][34][35][36] Although these molecules provided an opportunity for synthetic chemists to explore the limits of aromaticity and π-bonding by designing extremely twisted PAHs, [37][38][39][40][41][42] often the key challenge remained with the configurational stability and isolation of enantiopure [n]helicene derivatives as well as their stereoselective synthesis. Several stable conformers are possible for multiple [n]heliceneembedded PAHs, hence the isolation and availability of enantiopure multi[n]helicenes are cumbersome.…”

Intramolecular Alkyne Aromatization: Unexpected Synthesis of Expanded [9]Helicene and π‐Extended Double [4]Helicene, and Their Molecular Geometry Effect on Transistor Memory

“…Incorporating more benzene rings into carbohelicenes results in the extension of the π-system to form expanded helicenes or π-extended helicenes, which can lead to unique geometries and optoelectronic properties. [12][13][14][15][16][17] The helicenes with different topological geometries have shown potential application in optoelectronic devices, such as circularly polarized photodetectors, [18][19][20] light-emitting diodes, [21] spin filters, [22] biological sensors, [23] and memories. [24] The organic field-effect transistor (OFET) is an essential platform for organic electronics, especially, for the applications in data storage, sensor arrays, light-emitting devices, and flexible logic circuits.…”

“…Exploring cyclization methods to prepare molecular structures with nonplanar geometries and applying them in organic devices is of great importance for synthetic chemistry and organic materials. [46][47][48] The annulation methods for helical structures, such as photocyclization, [49] olefin metathesis cyclization, [50] Scholl reaction, [13] and alkyne benzannulation, [14,51] have been widely explored in last decade. In this study, one expanded [9] helicene (1) and one π-extended double [4]helicene (2), which are structural isomers, were obtained from the same precursor (4) through an unexpected intramolecular alkyne aromatization.…”