equilibrium operative in hexaarylbenzenes and/or the dynamic nature of propeller
chirality should be responsible. The varied CD intensities are explained by the
variation of effective tilt angles of radial aromatic blades against the central benzene
ring, which is caused by several mechanisms. Under the condition without any
perturbation by solvent, the angles are kept ideal at %60
(although either in
clockwise or counterclockwise manner), where strong CE intensity is expected
[8]. The reduced g factors in smaller alkanes are explained by the (partial) penetration of solvent molecule(s) into the space between the radial aromatic blades, which
increases the tilt angles near to perpendicular. The degree of penetration probably
depends on the size of solvent molecule, which justifies the correlation among the
linear alkanes. In larger cyclic alkanes (i.e., methylcyclohexane and decalin),
g factors are greatly enhanced as the penetration of solvent is not feasible. The
reduced g factors in polar solvents can be easily understood by conventional
solvation at the periphery, which in turn increase the effective tilt angles of the
radial aromatic blades. In addition, solvent viscosity also affects the dynamics
between C-CC propeller equilibrium to reduce the contribution of whizzing toroids,
circumventing the undesired reduction in CD intensity.
The CD intensity of flexible molecules generally increases at lower temperatures,
due to the conformational freezing [12, 59]. The
1 L b band is split to two transitions in
HABs, but not in biaryls or teraryls [62], and shows bisignate Cotton effects in CD
spectra, the magnitudes of which significantly increase with decreasing temperature
(Fig. 7.11). Thus, the molar CD (Δε) at the lowest-energy transition of H6 is
augmented to À200 M
À1 cm
À1 in isopentane at À150
C, which is roughly six
times larger than that observed at 25
C in methylcyclohexane and equivalent to a
400 to 500-fold enhancement from the corresponding value for the parent chiral
alkyl phenyl ether. Such an extraordinary CD augmentation by reducing temperature
does not appear to arise from the conformational freezing alone. It is also to note that
decreasing temperature does not cause peak sharpening but induces considerable red
shifts of the CD extrema with appreciable band-broadening, suggesting existence of
complex conformational equilibria independently affected by the temperature
variation.
Fig. 7.11 Variabletemperature CD spectra of
H6. The spectrum at
À150
C is obtained in
isopentane, while those at
other temperatures (+85 to
À120
C) are in
methylcyclohexane
7 Propeller Chirality: Circular Dichroism and Circularly Polarized Luminescence
161
chirality should be responsible. The varied CD intensities are explained by the
variation of effective tilt angles of radial aromatic blades against the central benzene
ring, which is caused by several mechanisms. Under the condition without any
perturbation by solvent, the angles are kept ideal at %60
(although either in
clockwise or counterclockwise manner), where strong CE intensity is expected
[8]. The reduced g factors in smaller alkanes are explained by the (partial) penetration of solvent molecule(s) into the space between the radial aromatic blades, which
increases the tilt angles near to perpendicular. The degree of penetration probably
depends on the size of solvent molecule, which justifies the correlation among the
linear alkanes. In larger cyclic alkanes (i.e., methylcyclohexane and decalin),
g factors are greatly enhanced as the penetration of solvent is not feasible. The
reduced g factors in polar solvents can be easily understood by conventional
solvation at the periphery, which in turn increase the effective tilt angles of the
radial aromatic blades. In addition, solvent viscosity also affects the dynamics
between C-CC propeller equilibrium to reduce the contribution of whizzing toroids,
circumventing the undesired reduction in CD intensity.
The CD intensity of flexible molecules generally increases at lower temperatures,
due to the conformational freezing [12, 59]. The
1 L b band is split to two transitions in
HABs, but not in biaryls or teraryls [62], and shows bisignate Cotton effects in CD
spectra, the magnitudes of which significantly increase with decreasing temperature
(Fig. 7.11). Thus, the molar CD (Δε) at the lowest-energy transition of H6 is
augmented to À200 M
À1 cm
À1 in isopentane at À150
C, which is roughly six
times larger than that observed at 25
C in methylcyclohexane and equivalent to a
400 to 500-fold enhancement from the corresponding value for the parent chiral
alkyl phenyl ether. Such an extraordinary CD augmentation by reducing temperature
does not appear to arise from the conformational freezing alone. It is also to note that
decreasing temperature does not cause peak sharpening but induces considerable red
shifts of the CD extrema with appreciable band-broadening, suggesting existence of
complex conformational equilibria independently affected by the temperature
variation.
Fig. 7.11 Variabletemperature CD spectra of
H6. The spectrum at
À150
C is obtained in
isopentane, while those at
other temperatures (+85 to
À120
C) are in
methylcyclohexane
7 Propeller Chirality: Circular Dichroism and Circularly Polarized Luminescence
161