variants and easily driven to clockwise (C) or counterclockwise (CC) propeller. At
low temperatures in less polar solvents, the minor toroidal conformer is almost
negligible and the enantiomeric propeller, i.e., C-CC equilibrium becomes practically dominant in chiral HABs, while the propeller conformation is less significant in
heptaaryl-BODIPY where the radial aromatic rings are mutually more separated in
space. The enhanced chiroptical responses and the highly susceptive and manipulatable nature of propeller molecules will contribute to amplified chirality sensing as
well as advanced chiroptical materials. The contribution of whizzing toroids
becomes substantial at temperatures higher than critical temperature T c to affect
the observed (chir)optical properties. Likewise, the propeller chirality is highly
sensitive to the solvent employed. The smaller solvent may intervene between the
radial blades in different degrees of penetration, depending on the size and shape of
the solvent molecule. This will cause the increase of effective tilt angles of blades,
which in turn reduces the observed chiroptical responses. A conventional solvation,
feasible at the polar peripheries with polar solvent, also induces the increase of the
blade angles to impact the observed CD. Recently, the propeller chirality and the
corresponding CD responses have been demonstrated to be controlled also by
applied pressure [86]. Such propeller inversion process and dynamics have to be
taken into account in fine-tuning the structure and properties of propeller-shaped
molecules and related materials. Remarkably, heptaaryl-BODIPY, lacking one blade
in the propeller, was found to form supramolecular dimer at low temperatures in
non-polar solvent. Thus, at temperatures lower than T d , the bisignate Cotton effects
become apparent, representing the strong excitonic coupling between the two
BODIPY chromophores. Consequently, this supramolecular dimer affords intense
CPL (g lum ¼ 2.0 Â 10
À3 ) with a good fluorescent quantum yield (Φ lum ¼ 0.45),
which is opposite in sign to those observed for the corresponding HAB propellers
due to the different origins.
In conclusion, the propeller chirality of HABs and related molecules is not simply
multiple combination of axial chirality. A domino-like corporation effect between
the radial aromatic blades is crucial to harvest the highly enhanced chiroptical
responses. More significantly, such chirality is dynamic in nature, which can be
easily controlled by subtle environmental factors such as temperature, solvent, etc.
We encourage further research on the propeller chirality in the relevant systems to
boost the development of new design principle for advanced and expectantly
superior chiroptical materials, especially those that can emit efficient circularly
polarized light at desired wavelength and is easily switched on-off or modulated
at will.
References
1. von Delius M, Leigh DA (2011) Walking molecules. Chem Soc Rev 40:3656–3676. https://doi.
org/10.1039/c1cs15005g
2. Michl J, Sykes ECH (2009) Molecular rotors and motors: recent advances and future challenges. ACS Nano 3:1042–1048. https://doi.org/10.1021/nn900411n
170
T. Mori
low temperatures in less polar solvents, the minor toroidal conformer is almost
negligible and the enantiomeric propeller, i.e., C-CC equilibrium becomes practically dominant in chiral HABs, while the propeller conformation is less significant in
heptaaryl-BODIPY where the radial aromatic rings are mutually more separated in
space. The enhanced chiroptical responses and the highly susceptive and manipulatable nature of propeller molecules will contribute to amplified chirality sensing as
well as advanced chiroptical materials. The contribution of whizzing toroids
becomes substantial at temperatures higher than critical temperature T c to affect
the observed (chir)optical properties. Likewise, the propeller chirality is highly
sensitive to the solvent employed. The smaller solvent may intervene between the
radial blades in different degrees of penetration, depending on the size and shape of
the solvent molecule. This will cause the increase of effective tilt angles of blades,
which in turn reduces the observed chiroptical responses. A conventional solvation,
feasible at the polar peripheries with polar solvent, also induces the increase of the
blade angles to impact the observed CD. Recently, the propeller chirality and the
corresponding CD responses have been demonstrated to be controlled also by
applied pressure [86]. Such propeller inversion process and dynamics have to be
taken into account in fine-tuning the structure and properties of propeller-shaped
molecules and related materials. Remarkably, heptaaryl-BODIPY, lacking one blade
in the propeller, was found to form supramolecular dimer at low temperatures in
non-polar solvent. Thus, at temperatures lower than T d , the bisignate Cotton effects
become apparent, representing the strong excitonic coupling between the two
BODIPY chromophores. Consequently, this supramolecular dimer affords intense
CPL (g lum ¼ 2.0 Â 10
À3 ) with a good fluorescent quantum yield (Φ lum ¼ 0.45),
which is opposite in sign to those observed for the corresponding HAB propellers
due to the different origins.
In conclusion, the propeller chirality of HABs and related molecules is not simply
multiple combination of axial chirality. A domino-like corporation effect between
the radial aromatic blades is crucial to harvest the highly enhanced chiroptical
responses. More significantly, such chirality is dynamic in nature, which can be
easily controlled by subtle environmental factors such as temperature, solvent, etc.
We encourage further research on the propeller chirality in the relevant systems to
boost the development of new design principle for advanced and expectantly
superior chiroptical materials, especially those that can emit efficient circularly
polarized light at desired wavelength and is easily switched on-off or modulated
at will.
References
1. von Delius M, Leigh DA (2011) Walking molecules. Chem Soc Rev 40:3656–3676. https://doi.
org/10.1039/c1cs15005g
2. Michl J, Sykes ECH (2009) Molecular rotors and motors: recent advances and future challenges. ACS Nano 3:1042–1048. https://doi.org/10.1021/nn900411n
170
T. Mori