(S)-1 and (S)-2 emit CPL in CHCl 3 . Surprisingly, although (S)-1 and (S)-2 have
the same axial chirality, their CPL spectra show opposite signs; a positive (+) sign is
observed for (S)-1, while (S)-2 exhibits a negative (–) sign. To quantitatively
compared the degrees of CPL and PL, we used the dimensionless Kuhn’s anisotropy
factor in the photoexcited state, which is defined as g CPL ¼ 2(I L À I R )/(I L + I R ),
where I L and I R are the intensities of the left- and right-handed CPLs upon excitation
with unpolarized light, respectively. The values of g CPL were found to be about
+1.0 Â 10
–3 for (S)-1 and about À1.4 Â 10
–3 for (S)-2. These results suggest that the
sign of the CPL depends on the nature of the binaphthyl ring (i.e., open or closed) in
addition to axial chirality. To discuss the origin of the reversal in the sign of the CPL,
solution-state circular dichroism (CD) and unpolarized UV-Vis absorption spectra
for (S)-1 and (R)-1 were acquired, as shown in Fig. 2.3.
Several characteristic vibronic UV-Vis bands between 270 and 360 nm that arise
from the
1 B b transition moments of (S)-1 and (S)-2 were commonly observed. To
quantitatively discuss the magnitude of the CD amplitude, we used the
dimensionless Kuhn’s anisotropy factor in the ground state, which is defined as
g CD ¼ Δε/ε. Values of g CD for (S)-1 and (S)-2 at their first Cotton CD bands are
about +2.0 Â 10
–4 (345 nm) and about À9.6 Â 10
–4 (338 nm), respectively.
Evidently, the first Cotton CD band of (S)-1 has opposite sign to that of (S)-2.
This reversal in the CD sign, despite the same axial chirality, is possibly due to the
difference in their chemical structures, i.e., whether the substituents are in the open
or closed form. The opposite signs of the first Cotton CD bands are responsible for
the opposite sign of the CPL bands observed for (S)-1 and (S)-2.
To investigate the origins of the opposing CPL and CD spectral signs of (S)-1
and (S)-2, CD spectra for (S)-1 were determined computationally. Simulated
(a)
(b)
0
0.5
-40
-20
0
20
40
235
275
315
355
395
Absorbance
g
e
d
m
/
D
C
Wavelength / nm
-180
0
180
300 320 340 360 380
g
e
d
m
/
D
C
Wavelength / nm
0
0.5
-50
0
50
100
235
275
315
355
395
Absorbance
g
e
d
m
/
D
C
Wavelength / nm
-120
-80
-40
0
40
300 320 340 360 380
g
e
d
m
/
D
C
Wavelength / nm
Fig. 2.3 CD (upper) and UV-Vis absorption (lower) spectra for (a) (S)-1 (black) and (R)-1 (red)
and (b) (S)-2 (black) in CHCl 3 (1.0 Â 10
-5 and 1.0 Â 10
À3
M (insets))
2 Circularly Polarized Luminescence of Axially Chiral Binaphthyl Fluorophores
13
the same axial chirality, their CPL spectra show opposite signs; a positive (+) sign is
observed for (S)-1, while (S)-2 exhibits a negative (–) sign. To quantitatively
compared the degrees of CPL and PL, we used the dimensionless Kuhn’s anisotropy
factor in the photoexcited state, which is defined as g CPL ¼ 2(I L À I R )/(I L + I R ),
where I L and I R are the intensities of the left- and right-handed CPLs upon excitation
with unpolarized light, respectively. The values of g CPL were found to be about
+1.0 Â 10
–3 for (S)-1 and about À1.4 Â 10
–3 for (S)-2. These results suggest that the
sign of the CPL depends on the nature of the binaphthyl ring (i.e., open or closed) in
addition to axial chirality. To discuss the origin of the reversal in the sign of the CPL,
solution-state circular dichroism (CD) and unpolarized UV-Vis absorption spectra
for (S)-1 and (R)-1 were acquired, as shown in Fig. 2.3.
Several characteristic vibronic UV-Vis bands between 270 and 360 nm that arise
from the
1 B b transition moments of (S)-1 and (S)-2 were commonly observed. To
quantitatively discuss the magnitude of the CD amplitude, we used the
dimensionless Kuhn’s anisotropy factor in the ground state, which is defined as
g CD ¼ Δε/ε. Values of g CD for (S)-1 and (S)-2 at their first Cotton CD bands are
about +2.0 Â 10
–4 (345 nm) and about À9.6 Â 10
–4 (338 nm), respectively.
Evidently, the first Cotton CD band of (S)-1 has opposite sign to that of (S)-2.
This reversal in the CD sign, despite the same axial chirality, is possibly due to the
difference in their chemical structures, i.e., whether the substituents are in the open
or closed form. The opposite signs of the first Cotton CD bands are responsible for
the opposite sign of the CPL bands observed for (S)-1 and (S)-2.
To investigate the origins of the opposing CPL and CD spectral signs of (S)-1
and (S)-2, CD spectra for (S)-1 were determined computationally. Simulated
(a)
(b)
0
0.5
-40
-20
0
20
40
235
275
315
355
395
Absorbance
g
e
d
m
/
D
C
Wavelength / nm
-180
0
180
300 320 340 360 380
g
e
d
m
/
D
C
Wavelength / nm
0
0.5
-50
0
50
100
235
275
315
355
395
Absorbance
g
e
d
m
/
D
C
Wavelength / nm
-120
-80
-40
0
40
300 320 340 360 380
g
e
d
m
/
D
C
Wavelength / nm
Fig. 2.3 CD (upper) and UV-Vis absorption (lower) spectra for (a) (S)-1 (black) and (R)-1 (red)
and (b) (S)-2 (black) in CHCl 3 (1.0 Â 10
-5 and 1.0 Â 10
À3
M (insets))
2 Circularly Polarized Luminescence of Axially Chiral Binaphthyl Fluorophores
13