Toroidal Interaction and Propeller Chirality of Hexaarylbenzenes. Dynamic Domino Inversion Revealed by Combined Experimental and Theoretical Circular Dichroism Studies

Abstract

Hexaarylbenzenes (HABs) have greatly attracted much attention due to their unique propeller-shaped structure and potential application in materials science, such as liquid crystals, molecular capsules/rotors, redox materials, nonlinear optical materials, as well as molecular wires. Less attention has however been paid to their propeller chirality. By introducing small point-chiral group(s) at the periphery of HABs, propeller chirality was effectively induced, provoking strong Cotton effects in the circular dichroism (CD) spectrum. Temperature and solvent polarity manipulate the dynamics of propeller inversion in solution. As such, whizzing toroids become more substantial in polar solvents and at an elevated temperature, where radial aromatic rings (propeller blades) prefer orthogonal alignment against the central benzene ring (C6 core), maximizing toroidal interactions.

Hydrostatic Pressure on Toroidal Interaction and Propeller Chirality of Hexaarylbenzenes: Explicit Solvent Effects on Differential Volumes in Methylcyclohexane and Hexane

“…Note that although C and CC conformers are diastereomers of each other, the point chirality at the periphery (on the order of <1 m −1 cm −1 ) is negligible compared with the CD derived from the propeller shape (which is typically on the order of >100 m −1 cm −1 , owing to the strong excitonic coupling). The value of ±0.003 was obtained by extrapolation of the observed g factor values to infinite pressure, which agrees well with the experimental g value at very low temperature for H6 . (3) Experimentally, we observed two distinct pressure‐dependent regimes for the g factors, both of which provided good linear correlations within an experimental error.…”

“…(2) We assumed the g factors for these conformers as 0, + g 0 (=+0.003), and − g 0 (=−0.003), respectively. These conformation‐dependent changes in the CD spectra were also supported by quantum chemical calculations . Note that although C and CC conformers are diastereomers of each other, the point chirality at the periphery (on the order of <1 m −1 cm −1 ) is negligible compared with the CD derived from the propeller shape (which is typically on the order of >100 m −1 cm −1 , owing to the strong excitonic coupling).…”

“…This inconsistency may well be explained as the latter is an averaged structure; a rotation of the peripheral ring against the central benzene ring in HAB is quite feasible even at ambient temperature . As such, although clockwise ( C ) and counterclockwise ( CC ) propeller conformations are at local minima, intermediate geometries between the C and CC isomers populated on the tilt‐angle surface, or whizzing toroids ( T ), play significant roles on the observed (chir)optical properties of HABs and related molecules (Figure ) …”

“…In previous studies, we investigated the dynamic nature of the propeller chirality of HABs in various solvents at atmospheric pressure. By introducing small ( R )‐point‐chiral group(s) at the periphery of HABs, as in H6 or H2 (Figure ), preference for the C over CC conformer was induced through the equilibrium shift to express strong Cotton effects (CEs) in their circular dichroism (CD) spectra.…”

Multiple Acetylation of Pentaphenylferrocene – Synthesis and Asymmetric Reduction of 1‐Acetyl‐1′,2′,3′,4′,5′‐penta(para‐acetylphenyl)ferrocene

“…Comparison of stepwise produced 22* to ( R , R , R , R , R , R )‐ 22 , by both optical rotation and circular dichroism (Figure ), revealed that the R configured stereogenic centres created on ( S )‐CBS catalysed reduction do not control the selectivity of reduction with NaBH 4 . In addition, the similarity of the CD spectrum of ( R )‐ 16 to that of ( R , R , R , R , R , R )‐ 22 suggests that, under the conditions used for the recording of these spectra, the para‐ substituted stereogenic centres of the latter do not induce a preference for M or P propeller chirality in the pentaarylcyclopentadienyl moiety …”