requisite [8]. Nevertheless, the clockwise propeller chirality induced by the peripheral point-chiral (R)-alkoxy auxiliaries and the average twist angle of 62 Æ 2
, as
well as the quasi-C 6 symmetric overall structure, well accord with the theoretical
predictions (vide supra). Possessing smaller numbers of chiral auxiliaries, H2P and
H0 become less symmetrical (less uniform in tilt angle), as exemplified by a wider
range (Δ ¼ 23
) of the tilt angles, i.e., 66, 57, 77, 89, 74, and 66
(average: 72 Æ 5
),
found in the crystal of H0. The crystal of H2P contains two independent but similar
structures. The range of the tilt angles becomes much broader (Δ ¼ 41
and 32
) and
one of the radial aromatic rings is twisted counterclockwise. The actual tilt angles
widely vary, i.e., 111, 80, 70, 73, 71, and 77
and 99, 80, 67, 67, 69, and 79
(grand
average: 79 Æ 6
), probably reflecting the high susceptibility to small perturbations
(e.g., packing forces) as a consequence of the shallow potentials for the inversion
process (vide infra) [7].
7.5 Solvent and Temperature Effects on Propeller Chirality
Figure 7.6 demonstrates very strong chiroptical responses of the propeller chirality in
HABs, where the point chirality is accumulated in the propeller geometry through
the cooperative or domino effect. However, such strong Cotton effects are observed
only in non-polar hydrocarbon solvents. In fact, the use of more polar solvents, such
as diethyl ether and acetonitrile, greatly reduces the CD intensity of H6, as shown in
Fig. 7.10. Obviously, a single solvent parameter, such as polarity, viscosity, or size
alone, cannot well rationalize such behaviors, for which the complex conformational
H0
H1
H2
H2P
H4
H6
Fig. 7.8 Optimized structures of HABs calculated by the dispersion-corrected DFT method
7 Propeller Chirality: Circular Dichroism and Circularly Polarized Luminescence
159
, as
well as the quasi-C 6 symmetric overall structure, well accord with the theoretical
predictions (vide supra). Possessing smaller numbers of chiral auxiliaries, H2P and
H0 become less symmetrical (less uniform in tilt angle), as exemplified by a wider
range (Δ ¼ 23
) of the tilt angles, i.e., 66, 57, 77, 89, 74, and 66
(average: 72 Æ 5
),
found in the crystal of H0. The crystal of H2P contains two independent but similar
structures. The range of the tilt angles becomes much broader (Δ ¼ 41
and 32
) and
one of the radial aromatic rings is twisted counterclockwise. The actual tilt angles
widely vary, i.e., 111, 80, 70, 73, 71, and 77
and 99, 80, 67, 67, 69, and 79
(grand
average: 79 Æ 6
), probably reflecting the high susceptibility to small perturbations
(e.g., packing forces) as a consequence of the shallow potentials for the inversion
process (vide infra) [7].
7.5 Solvent and Temperature Effects on Propeller Chirality
Figure 7.6 demonstrates very strong chiroptical responses of the propeller chirality in
HABs, where the point chirality is accumulated in the propeller geometry through
the cooperative or domino effect. However, such strong Cotton effects are observed
only in non-polar hydrocarbon solvents. In fact, the use of more polar solvents, such
as diethyl ether and acetonitrile, greatly reduces the CD intensity of H6, as shown in
Fig. 7.10. Obviously, a single solvent parameter, such as polarity, viscosity, or size
alone, cannot well rationalize such behaviors, for which the complex conformational
H0
H1
H2
H2P
H4
H6
Fig. 7.8 Optimized structures of HABs calculated by the dispersion-corrected DFT method
7 Propeller Chirality: Circular Dichroism and Circularly Polarized Luminescence
159