CHCl 3 and DMF (Fig. 2.7; upper). The negative (À) sign (λ CD ¼ 356 nm) for (R)-4
in CHCl 3 became positive (+) (λ CD ¼ 337 nm) in DMF. The g CD value at the first
Cotton CD band of (R)-4 in CHCl 3 was about À0.33 Â 10
–3 , while a value of about
+0.38 Â 10
–4 was observed in DMF.
The observed reversals of CPL and CD signs are possibly ascribable changes in
the dihedral angles of the binaphthyl units. In DMF, the dihedral angle in the
binaphthyl unit is smaller since the carboxylic acid groups in 4 interact efficiently
with DMF molecules. Dipole-dipole and/or hydrogen-bonding interactions between
DMF and the carboxylic acid groups on the binaphthyl units may also be responsible
for the observed CD and CPL inversions in the ground and photoexcited states.
In conclusion, an axially chiral binaphthyl fluorophore can selectively emit
positive or negative CPL through control of solvent type.
2.4 Controlling Circularly Polarized Luminescence (CPL)
Through Neighboring Group Effects Involving
Binaphthyl Units [23]
In Sect. 2.2, the signs of the CPL from chiral binaphthyl fluorophores were shown to
be controllable by tuning the dihedral angle of the binaphthyl unit. As a more facile
method for controlling the sign of the CPL from a chiral binaphthyl moiety while
maintaining the same axial chirality, CPL sign control through neighboring-group
effects between binaphthyl units has been reported. To enable this form of control,
two binaphthyl derivatives with different numbers of chiral binaphthyl units that
(a)
(b)
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
-200
-150
-100
-50
0
50
100
250
300
350
400
Absorbance
g
e
d
m
/
D
C
Wavelength / nm
-1
-0.5
0
0.5
1
300
350
400
g
e
d
m
/
D
C
Wavelength / nm
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
-200
-150
-100
-50
0
50
100
250
300
350
400
Absorbance
g
e
d
m
/
D
C
Wavelength / nm
-3
-2
-1
0
1
2
3
300
350
400
g
e
d
m
/
D
C
Wavelength / nm
Fig. 2.7 CD (upper) and UV-Vis absorption (lower) spectra of (R)-4 (red) and (S)-4 (black) in (a)
CHCl 3 and (b) DMF (1.0 Â 10
À4 and 1.0 Â 10
À3 M (insets))
16
Y. Imai
in CHCl 3 became positive (+) (λ CD ¼ 337 nm) in DMF. The g CD value at the first
Cotton CD band of (R)-4 in CHCl 3 was about À0.33 Â 10
–3 , while a value of about
+0.38 Â 10
–4 was observed in DMF.
The observed reversals of CPL and CD signs are possibly ascribable changes in
the dihedral angles of the binaphthyl units. In DMF, the dihedral angle in the
binaphthyl unit is smaller since the carboxylic acid groups in 4 interact efficiently
with DMF molecules. Dipole-dipole and/or hydrogen-bonding interactions between
DMF and the carboxylic acid groups on the binaphthyl units may also be responsible
for the observed CD and CPL inversions in the ground and photoexcited states.
In conclusion, an axially chiral binaphthyl fluorophore can selectively emit
positive or negative CPL through control of solvent type.
2.4 Controlling Circularly Polarized Luminescence (CPL)
Through Neighboring Group Effects Involving
Binaphthyl Units [23]
In Sect. 2.2, the signs of the CPL from chiral binaphthyl fluorophores were shown to
be controllable by tuning the dihedral angle of the binaphthyl unit. As a more facile
method for controlling the sign of the CPL from a chiral binaphthyl moiety while
maintaining the same axial chirality, CPL sign control through neighboring-group
effects between binaphthyl units has been reported. To enable this form of control,
two binaphthyl derivatives with different numbers of chiral binaphthyl units that
(a)
(b)
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
-200
-150
-100
-50
0
50
100
250
300
350
400
Absorbance
g
e
d
m
/
D
C
Wavelength / nm
-1
-0.5
0
0.5
1
300
350
400
g
e
d
m
/
D
C
Wavelength / nm
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
-200
-150
-100
-50
0
50
100
250
300
350
400
Absorbance
g
e
d
m
/
D
C
Wavelength / nm
-3
-2
-1
0
1
2
3
300
350
400
g
e
d
m
/
D
C
Wavelength / nm
Fig. 2.7 CD (upper) and UV-Vis absorption (lower) spectra of (R)-4 (red) and (S)-4 (black) in (a)
CHCl 3 and (b) DMF (1.0 Â 10
À4 and 1.0 Â 10
À3 M (insets))
16
Y. Imai