structure tuning and environmental setting. In comparison to the conventional
covalently bonded systems, chiral supramolecular systems are more facile and
hence advantageous in controlling their chiroptical responses by various internal
and external factors.
In this chapter, we will mainly describe the results of our recent experimental and
theoretical studies on the propeller chirality of hexaarylbenzenes (HABs) and related
molecules [7, 8, 29]. Some of these propeller-shaped molecules, when properly
arranged, form supramolecular assemblies (e.g., dimer) to exhibit divergent
chiroptical responses. The propeller chirality is fundamentally different in origin
and dynamism from mechanical planar chirality in rotaxanes [30–33], although the
correlation between propeller and thrust is conceptually similar to the wheel and axle
relationship. In the former, the molecular structure is inherently chiral in conformation and the chiral sense is dynamically switchable, while the latter is
supramolecularly induced by threading a wheel composed of three or more different
segments with an unsymmetrical axle and the chiral sense is not manipulable or
switched upon shuttling of the wheel. Indeed, the propeller chirality of HABs is not a
result of simple or random combination of the six axially chiral aryls connected to
the central benzene ring but is emerged by their synchronized unidirectional twists to
the equilibrium angle to attain strong chiroptical responses. More crucially, the twist
angle of synchronized propeller blades is dynamic in nature and hence susceptible to
various environmental factors such as temperature, solvent polarity, viscosity, and
pressure, eventually allowing us to manipulate the chiroptical properties. In the
toroidal form, all the aromatic blades become orthogonal to the central benzene
core and the chiroptical responses are practically diminished. In contrast to chiral
supramolecular systems, the structures of propeller-chiral molecules presented in the
following sections are precisely defined, but the extensive inter-blade interactions
may dynamically alter the shape (and therefore the properties) by various environmental factors. As such, the combined experimental and theoretical studies to
elucidate how and to what extent the chiroptical responses are affected by the
intramolecular (inter-blade), intermolecular, and/or solvent interactions provide us
with the substantial insights bridging the gap between molecular and supramolecular, as well as rigid and flexible, chirality.
7.2 Propeller Chirality in Hexaarylbenzenes
In designing propeller-shaped molecules, two- or three-bladed propeller may seem
simpler, but firmly anchoring each propeller blade to a hub at a fixed twist angle is
synthetically highly challenging. We chose a different strategy for contracting a
flexible propeller molecule, in which multiple blades are anchored to a hub through a
single bond to allow the blade to automatically adjust the twist angle by inter-blade
interactions. In such an approach for studying propeller chirality, the HAB is one of
the most ideal motifs [34]. In the HAB, all the six hydrogens of benzene are replaced
by identical or divergent aromatic groups to endow the propeller structure, in which
7 Propeller Chirality: Circular Dichroism and Circularly Polarized Luminescence
153
covalently bonded systems, chiral supramolecular systems are more facile and
hence advantageous in controlling their chiroptical responses by various internal
and external factors.
In this chapter, we will mainly describe the results of our recent experimental and
theoretical studies on the propeller chirality of hexaarylbenzenes (HABs) and related
molecules [7, 8, 29]. Some of these propeller-shaped molecules, when properly
arranged, form supramolecular assemblies (e.g., dimer) to exhibit divergent
chiroptical responses. The propeller chirality is fundamentally different in origin
and dynamism from mechanical planar chirality in rotaxanes [30–33], although the
correlation between propeller and thrust is conceptually similar to the wheel and axle
relationship. In the former, the molecular structure is inherently chiral in conformation and the chiral sense is dynamically switchable, while the latter is
supramolecularly induced by threading a wheel composed of three or more different
segments with an unsymmetrical axle and the chiral sense is not manipulable or
switched upon shuttling of the wheel. Indeed, the propeller chirality of HABs is not a
result of simple or random combination of the six axially chiral aryls connected to
the central benzene ring but is emerged by their synchronized unidirectional twists to
the equilibrium angle to attain strong chiroptical responses. More crucially, the twist
angle of synchronized propeller blades is dynamic in nature and hence susceptible to
various environmental factors such as temperature, solvent polarity, viscosity, and
pressure, eventually allowing us to manipulate the chiroptical properties. In the
toroidal form, all the aromatic blades become orthogonal to the central benzene
core and the chiroptical responses are practically diminished. In contrast to chiral
supramolecular systems, the structures of propeller-chiral molecules presented in the
following sections are precisely defined, but the extensive inter-blade interactions
may dynamically alter the shape (and therefore the properties) by various environmental factors. As such, the combined experimental and theoretical studies to
elucidate how and to what extent the chiroptical responses are affected by the
intramolecular (inter-blade), intermolecular, and/or solvent interactions provide us
with the substantial insights bridging the gap between molecular and supramolecular, as well as rigid and flexible, chirality.
7.2 Propeller Chirality in Hexaarylbenzenes
In designing propeller-shaped molecules, two- or three-bladed propeller may seem
simpler, but firmly anchoring each propeller blade to a hub at a fixed twist angle is
synthetically highly challenging. We chose a different strategy for contracting a
flexible propeller molecule, in which multiple blades are anchored to a hub through a
single bond to allow the blade to automatically adjust the twist angle by inter-blade
interactions. In such an approach for studying propeller chirality, the HAB is one of
the most ideal motifs [34]. In the HAB, all the six hydrogens of benzene are replaced
by identical or divergent aromatic groups to endow the propeller structure, in which
7 Propeller Chirality: Circular Dichroism and Circularly Polarized Luminescence
153