Multiple Fused Anthracenes as Helical Polycyclic Aromatic Hydrocarbon Motif for Chiroptical Performance Enhancement

Abstract

Polycyclic aromatic hydrocarbons consisting of three fused anthracene units were designed as new π‐conjugated compounds having helical structures. These expanded helicenes were synthesized by Pt‐catalyzed cycloisomerization of the corresponding ethynyl‐substituted precursors. The nonplanar and helical structure was confirmed by X‐ray analysis and DFT calculations, and the barrier to helical inversion was estimated to be 34 kJ mol−1. The enantiomers of the diphenyl derivative were successfully resolved by chiral HPLC. Enantiopure samples showed good chiroptical performance in the CD (|Δϵ| 1380 L mol−1cm−1) and CPL (|glum| 0.013) spectra, and these values were considerably large for simple organic molecules. The unique chiroptical properties are discussed on the basis of the molecular structure and the electronic state with the aid of time‐dependent DFT calculations.

Construction of Helical Structures with Multiple Fused Anthracenes: Structures and Properties of Long Expanded Helicenes

“…The target compounds were synthesized by cycloaromatization of terminal alkyne precursors prepared by the Suzuki–Miyaura coupling of suitable arene building units, as was adopted for the synthesis of [3]HA [4] . The synthetic routes are shown in Scheme 1.…”

“…The signals due to the inner hydrogen atoms in the nonterminal anthracene units were observed at 12.6 ppm for [4]HA and 12.5 and 13.3 (central) ppm for [5]HA (cf. 12.11 ppm for [3]HA ) [4] . These hydrogen atoms were extraordinarily deshielded as ordinary neutral aromatic compounds because of the enhanced ring current effect of the aromatic moieties.…”

“…Because of this high barrier, the unsuccessful resolution was attributable to poor separation as well as low stability. The barriers to helical inversion increased in the order of [2]HA (21.8 kJ mol −1 )< [3]HA (25.2 kJ mol −1 ) [4] ≪ [4]HA < [5]HA . The introduction of the fourth unit considerably enhanced the barrier because of the destabilization of the transition state by steric interactions between the terminal moieties.…”

“…The helical structure of the longest helicene, [16]helicene, has more than 2.5 turns, and some benzene units are triply layered [3] . Recently, we reported new aromatic compound [3]HA (hereinafter, [ n ]HA refers to a helical anthracene with n fused anthracene units) as an expanded helicene, [4] which is defined as a helicene with extra linearly fused benzene rings into angularly fused units (Figure 1). [5, 6] Even though the racemization via helical inversion takes place rapidly for [3]HA , the barrier is sufficiently enhanced by the introduction of diphenyl substituents enough to enable resolution of its enantiomers at room temperature.…”

Synthesis, Structures, and Properties of Helically Fused Anthraquinones with Unusually Close Carbonyl‐Carbonyl Contacts

“…We have recently reported multiply fused anthracene [3]ANa as an example of expanded helicene (Figure 1), [1] which has been defined as a helicene with additional linearly fused benzo moieties into angularly fused units. [2,3,4] Compound [3]ANa possesses a helical structure to avoid steric interactions between the terminal aromatic moieties, and the enantiomers of its diphenyl derivative show very high circular dichroism and circularly polarized luminescence activities.…”