Solvent and Temperature Effects on Dynamics and Chiroptical Properties of Propeller Chirality and Toroidal Interaction of Hexaarylbenzenes

Abstract

Because of the unique interaction of radial aromatic blades, propeller-shaped hexaarylbenzenes (HABs) attract much research interest and find various practical applications. By introducing a small point-chiral group at the tip of aromatic blade(s), HAB becomes propeller-chiral to exhibit strong Cotton effects. Because of the dynamic nature of propeller chirality, the chiroptical properties of HAB critically responded to minute changes in the environment. Using a series of chiral HABs with one to six ( R)-1-methylpropyloxy substituent(s) introduced at the blade tip, we elucidated how the smallest chiral auxiliary at the HAB periphery progressively and cooperatively boosts the overall chiroptical properties and also how subtle changes in temperature and solvent structure affect the propeller dynamics and thus the chiroptical responses. The unique features of propeller-chiral HABs further enabled us to switch on/off their circularly polarized luminescence.

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

“…The pressure dependence of H6 was also examined in hexane. In our previous investigation, we demonstrated that the shape of solvent molecule definitely matters in the observed (chir)optical responses of the HAB derivatives . As expected, the pressure dependence of H6 in hexane was somewhat different from those observed in methylcyclohexane.…”

“…This is in accord with our explanation where the contribution of the propeller against the toroidal conformation becomes dominant at higher pressure. Such significant spectral changes are characteristic for flexible HABs, owing to the synchronized unidirectional rotation of peripheral blades …”

“…Apparently, the HABs are more sensitive to the applied pressure through the domino‐like interaction between the peripheral aromatic groups. Indeed, the theoretical calculation for more stable C conformers of H2 and H6 revealed that the blades’ tilt angle in the energy‐minimized geometry is approximately 60° in the ground state . Consequently, the blades in HABs become more flattened in the excited state, the tile angle being 30–40° at atmospheric pressure.…”

“…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.…”