Hexa-peri-hexabenzo[7]helicene: Homogeneously π-Extended Helicene as a Primary Substructure of Helically Twisted Chiral Graphenes

Abstract

Helically twisted graphenes can be considered as a promising candidate for the nanometer-sized molecular inductors in molecular electronics and molecular spring materials in nanomechanics. Here, we report the synthesis of hexa- peri-hexabenzo[7]helicene, which represents a primary substructure of the helical graphenes. The helically twisted polycyclic aromatic hydrocarbon was synthesized by a tetrasubstituted alkene formation using McMurry coupling followed by stepwise photocyclodehydrogenation and aromatization reactions. The π-extended helicoid structure with a noticeable intramolecular π-π interaction was unambiguously determined by X-ray crystallography. The primary helical nanographene molecule has a small HOMO-LUMO band gap evidenced by the absorption edge that appeared at ca. 800 nm, which exhibits an excellent chiroptical property with a dissymmetry factor of circular dichroism of | g| = 0.016 at 680 nm. The femtosecond transient absorption spectroscopy revealed the ultrafast excited-state dynamics of the helical nanographene molecule, with a lifetime of only few picoseconds in the lowest-energy excited (S) state.

Control of Circularly Polarized Luminescence by Orientation of Stacked π‐Electron Systems

“…The sign of a g lum value is decided by the orientation of the dipole moments between μ and m . As shown in Figure S9, the angle θ between μ and m of ( S p )‐ 8 was estimated to be 144°, and that of ( S p )‐ 9 was 87° . Theoretical results suggested their negative and positive CPL signs for ( S p )‐ 8 and ( S p )‐ 9 , respectively, and supported the experimental results.…”

Negative Charge as a Lens for Concentrating Antiaromaticity: Using a Pentagonal “Defect” and Helicene Strain for Cyclizations

“…Helicenes are helically chiral aromatic compounds, [1] where the helical distortion is imposed by intramolecular steric demands.The inherent twist in such extended p-systems leads to special optical and electronic properties. [2] Recently, considerable progress has been achieved in the synthesis and applications of various helicenes, [3] including heterohelicenes [4] and fused "multipole" helicenes. [5] Theh elical twist depends on the length of the helicenes,t he nature of the substituents,a nd the presence of defects in the backbone.…”

Stereocontrolled Synthesis of Chiral Heteroaromatic Propellers with Small Optical Bandgaps

“…Embedding helical motifs into polyaromatic structures [1] provides access to inherently curved and multilayered aromatic molecules with exceptional chiroptical properties and the potential to be used in organic electronics. [2][3][4][5][6] Recent activity in this field has produced adiverse range of new chiral aromatics,i ncluding helicenes fused to large polycyclic structures, [7][8][9][10][11] helically hindered PA Hs, [12,13] helical nanoribbons, [2][3][4] and multi-helicene architectures. [14][15][16][17][18][19][20][21][22][23][24][25][26][27] Inclusion of heteroatoms or nonbenzenoid rings in such systems can have ap rofound effect on their electronic and chiroptical characteristics.H owever,s uch modifications are synthetically challenging and thus rarely attempted.…”