2.2 Controlling Circularly Polarized Luminescence (CPL)
Through the Dihedral Angle of the Axially Chiral
Binaphthyl [21]
In this section, we report the chiroptical properties of axially chiral binaphthyl
compounds in relation to the dihedral angle in the binaphthyl unit. For this purpose,
two almost identical binaphthyl derivatives with the same (S)-axial chirality,
namely, (S)-2,2
0 -diethoxy-1,1
0 -binaphthyl [(S)-1], as an open-type binaphthyl, and
(S)-2,2
0 -(1,4-butylenedioxy)-1,1
0 -binaphthyl [(S)-2], as a closed-type, were chosen
(Fig. 2.1).
The solution-state circularly polarized luminescence (CPL) and unpolarized
photoluminescence (PL) spectra of the open-type (S)-1 and the closed-type (S)-2
in chloroform (CHCl 3 ) are shown in Fig. 2.2. (S)-1 and (S)-2 exhibit similar PL in
CHCl 3 ; PL maxima (λ PL ) for (S)-1 and (S)-2 were observed at 365 and 368 nm,
respectively, and the absolute values of their PL quantum yields (Ф F s) were 19% and
25%, respectively. The Ф F value of (S)-2 is greater than that of (S)-1 because the
ro-vibrational modes of (S)-2 are limited compared to those of (S)-1, and the
binaphthyl unit of (S)-2 is more planar than that of (S)-1.
0.0
1.0
2.0
340
380
420
460
-40
-30
-20
-10
0
10
20
30
DC / V
g
e
d
m
/
L
P
C
Wavelength / nm
Fig. 2.2 CPL (upper) and
PL (lower) spectra of (S)-1
(black) and (S)-2 (red) in
CHCl 3 (1.0 Â 10
À3
M)
O
O
(S)-2
(S)-1
O
O
Fig. 2.1 Chiral binaphthyl
fluorophores (S)-1 and (S)-2
12
Y. Imai