achieve strong chiroptical responses. This may have been apparent in CD, where an
effective exciton coupling should be observed, but now turned out to be also
applicable to the materials that emit strong CPL in the NIR region. Recently, a
related approach has been reported [57].
5.3 Summary
In conclusion, we successfully demonstrated the structural and photophysical control of fluorescent helicene and related zinc(II) helicates for improved CPL properties. The absolute fluorescence quantum yields (Φ FL ) of helicene derivatives are
generally extremely low because the intersystem crossing (ISC) pathways are highly
accelerated (ΦISC: $0.9). However, our synthetic strategy (e.g., “push-pull”
method) enables to significantly enhance the Φ FL values, which resulted in the
observation of CPL signals. Then, a homoleptic chiral zinc(II) helicate selectively
formed by a pair of achiral benzo[a]phenanthrene-fused dipyrromethene ligands
afforded unusually strong chiroptical responses with absorption and luminescence
dissymmetry factors such as |g abs | ¼ 0.20 (at 615 nm) and |g lum | ¼ 0.022 (at 660 nm),
respectively. More interestingly, the g lum profiles also extended up to ca. 850 nm.
They are highest values among the rare earth- and precious metal-free small
Fig. 5.9 Optimized structures of (P,P)-Zn(Phena-dpm) 2 . (a) ground state and (b) excited state.
Hydrogen atoms are omitted for clarity. Electric transition dipole moment μ and magnetic transition
dipole moment m demonstrate the S 0 -to-S 1 or S 1 -to-S 0 , respectively (Reprinted with permission
from Ref. [52] Copyright 2018 Wiley-VCH)
112
T. Hasobe
effective exciton coupling should be observed, but now turned out to be also
applicable to the materials that emit strong CPL in the NIR region. Recently, a
related approach has been reported [57].
5.3 Summary
In conclusion, we successfully demonstrated the structural and photophysical control of fluorescent helicene and related zinc(II) helicates for improved CPL properties. The absolute fluorescence quantum yields (Φ FL ) of helicene derivatives are
generally extremely low because the intersystem crossing (ISC) pathways are highly
accelerated (ΦISC: $0.9). However, our synthetic strategy (e.g., “push-pull”
method) enables to significantly enhance the Φ FL values, which resulted in the
observation of CPL signals. Then, a homoleptic chiral zinc(II) helicate selectively
formed by a pair of achiral benzo[a]phenanthrene-fused dipyrromethene ligands
afforded unusually strong chiroptical responses with absorption and luminescence
dissymmetry factors such as |g abs | ¼ 0.20 (at 615 nm) and |g lum | ¼ 0.022 (at 660 nm),
respectively. More interestingly, the g lum profiles also extended up to ca. 850 nm.
They are highest values among the rare earth- and precious metal-free small
Fig. 5.9 Optimized structures of (P,P)-Zn(Phena-dpm) 2 . (a) ground state and (b) excited state.
Hydrogen atoms are omitted for clarity. Electric transition dipole moment μ and magnetic transition
dipole moment m demonstrate the S 0 -to-S 1 or S 1 -to-S 0 , respectively (Reprinted with permission
from Ref. [52] Copyright 2018 Wiley-VCH)
112
T. Hasobe