dihedral-angle-dependent (θ ¼ (O)C-C-C-C(O)) CD and UV-Vis absorption spectra
of (S)-1 are presented in Fig. 2.4. These simulations reveal that the sign of the first
Cotton CD band of (S)-1 is negative (À) for dihedral angles (θ) between about +50
and +85
, whereas the sign is positive (+) for θ between +85
and +120
.
These results suggest that the opposite first CD and CPL bands observed for (S)-1
and (S)-2 are attributable to differences in the θ values of the binaphthyl units in the
ground and photoexcited states. In fact, the calculated equilibrium structures of (S)-1
and (S)-2 were observed to have θ values of +89.6
and +73.8
respectively, which
suggests that the former has a positive first Cotton CD band (~340 nm) and the latter
has a negative first Cotton CD band (~340 nm); these simulated results are consistent
with experimental observations.
In conclusion, the CPL properties of an axially chiral binaphthyl fluorophore can
be controlled by adjusting the dihedral angle of the binaphthyl unit in addition to its
chirality. In general, the enantiomeric organic fluorophore is usually required to
invert the sign of the CPL of a chiral fluorophore; however, the enantiomeric organic
molecule is sometimes difficult to obtain. Therefore, controlling the sign of the CPL
of axially chiral fluorophores through dihedral angle, without the need for the
enantiomer, is a very useful technique.
2.3 Controlling the Sign of the Circularly Polarized
Luminescence (CPL) from an Axially Chiral
Binaphthyl Fluorophore by Solvent [22]
In Sect. 2.2, we reported that the sign of the CPL from a chiral binaphthyl
fluorophore can be controlled by tuning the dihedral angle of the binaphthyl unit.
In this section, solvent polarity control of the CPL sign of an axially chiral
0
10000
20000
30000
40000
50000
-100
-80
-60
-40
-20
0
20
250
300
350
400
Molar absorptivity (ε) / mol -1 dm -1 cm -1
CD intensity (Δε) / mol -1
dm -1
cm -1
Wavelength / nm
Fig. 2.4 CD and UV-Vis
absorption spectra of (S)-1
calculated as functions of
the dihedral angle θ [+50
( ), +60
( ), +70
( ),
+80
( ), +90
( ), +100
( ), +110
( ), +120
( )]
in the binaphthyl unit
14
Y. Imai
of (S)-1 are presented in Fig. 2.4. These simulations reveal that the sign of the first
Cotton CD band of (S)-1 is negative (À) for dihedral angles (θ) between about +50
and +85
, whereas the sign is positive (+) for θ between +85
and +120
.
These results suggest that the opposite first CD and CPL bands observed for (S)-1
and (S)-2 are attributable to differences in the θ values of the binaphthyl units in the
ground and photoexcited states. In fact, the calculated equilibrium structures of (S)-1
and (S)-2 were observed to have θ values of +89.6
and +73.8
respectively, which
suggests that the former has a positive first Cotton CD band (~340 nm) and the latter
has a negative first Cotton CD band (~340 nm); these simulated results are consistent
with experimental observations.
In conclusion, the CPL properties of an axially chiral binaphthyl fluorophore can
be controlled by adjusting the dihedral angle of the binaphthyl unit in addition to its
chirality. In general, the enantiomeric organic fluorophore is usually required to
invert the sign of the CPL of a chiral fluorophore; however, the enantiomeric organic
molecule is sometimes difficult to obtain. Therefore, controlling the sign of the CPL
of axially chiral fluorophores through dihedral angle, without the need for the
enantiomer, is a very useful technique.
2.3 Controlling the Sign of the Circularly Polarized
Luminescence (CPL) from an Axially Chiral
Binaphthyl Fluorophore by Solvent [22]
In Sect. 2.2, we reported that the sign of the CPL from a chiral binaphthyl
fluorophore can be controlled by tuning the dihedral angle of the binaphthyl unit.
In this section, solvent polarity control of the CPL sign of an axially chiral
0
10000
20000
30000
40000
50000
-100
-80
-60
-40
-20
0
20
250
300
350
400
Molar absorptivity (ε) / mol -1 dm -1 cm -1
CD intensity (Δε) / mol -1
dm -1
cm -1
Wavelength / nm
Fig. 2.4 CD and UV-Vis
absorption spectra of (S)-1
calculated as functions of
the dihedral angle θ [+50
( ), +60
( ), +70
( ),
+80
( ), +90
( ), +100
( ), +110
( ), +120
( )]
in the binaphthyl unit
14
Y. Imai