L-2o, respectively, suggest the preferential formation of right- and left-handed
helices, respectively. In the helical molecular geometry of 2o, the pyrene units
were stacked in a chiral manner. Owing to this chiral arrangement, compounds 2o
are CPL active showing a positive and negative CPL signal in the region of excimerlike emission band for D- and L-isomers, respectively (Fig. 8.7b). The signs of CPL
signal coincide with those of the first Cotton effect observed in the CD study. The
anisotropy factor, which is given by the equation g lum 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 circularly polarized
emission, respectively, was estimated to be 0.01. This relatively high value for the
small chiral organic molecule [38] is consistent with previously reported values for
chiral pyrene excimers [39, 40].
UV irradiation to 2o solution resulted in the emergence of an absorption band in
the visible region extended to 800 nm due to the formation of 2c (Fig. 8.8a).
Meanwhile, a drastic change was recorded in the CD spectra, in which broad CD
signals at the wavelength of the visible absorption band of photo-products emerged
(Fig. 8.8c). The emergence of CD band in the visible region suggested the diastereoselective photoreaction in the 6π-system. A combination of
1 H NMR and chiral
HPLC studies for the photo-products indicated an absolute stereo-selective ringcyclization reaction for 2. The intensity of both monomer and excimer-like emission
bands decreased with the progress of ring-cyclization reaction by the UV irradiation
(Fig. 8.7a). Along with the decrease in the excimer-like emission intensity by the
photo-isomerization, the CPL intensity decreased (Fig. 8.7b). Since the CPL measurement was performed at a relatively high concentration (1.7 Â 10
À4 M) in
comparison to that for absorption spectra, the conversion ratio was not enough to
achieve the completely off-state (Fig. 8.7b). In principle, the isolated 2c gave a
negligible emission quantum yield far less than 0.1%.
The emission from both the monomer and the excimer decreased in a liner fashion
as a function of the conversion ratio to the colored form 2c (Fig. 8.9). The quenching
Monomer at 390 nm
Excimer at 500 nm
Relative intensity (I/I
0 )
0
1.0
0.8
0.6
0.4
0.2
Conversion ratio
([c-form]/[o-form + c-form])
0
0.2
0.4
0.6
0.8
1.0
Fig. 8.9 Plots of relative
emission intensity in
monomer (circle, at 390 nm)
and excimer (square, at
500 nm) emission. I 0
corresponds to the intensity
at a conversion of 0
8 Photo-Switching of Circularly Polarized Luminescence
183
helices, respectively. In the helical molecular geometry of 2o, the pyrene units
were stacked in a chiral manner. Owing to this chiral arrangement, compounds 2o
are CPL active showing a positive and negative CPL signal in the region of excimerlike emission band for D- and L-isomers, respectively (Fig. 8.7b). The signs of CPL
signal coincide with those of the first Cotton effect observed in the CD study. The
anisotropy factor, which is given by the equation g lum 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 circularly polarized
emission, respectively, was estimated to be 0.01. This relatively high value for the
small chiral organic molecule [38] is consistent with previously reported values for
chiral pyrene excimers [39, 40].
UV irradiation to 2o solution resulted in the emergence of an absorption band in
the visible region extended to 800 nm due to the formation of 2c (Fig. 8.8a).
Meanwhile, a drastic change was recorded in the CD spectra, in which broad CD
signals at the wavelength of the visible absorption band of photo-products emerged
(Fig. 8.8c). The emergence of CD band in the visible region suggested the diastereoselective photoreaction in the 6π-system. A combination of
1 H NMR and chiral
HPLC studies for the photo-products indicated an absolute stereo-selective ringcyclization reaction for 2. The intensity of both monomer and excimer-like emission
bands decreased with the progress of ring-cyclization reaction by the UV irradiation
(Fig. 8.7a). Along with the decrease in the excimer-like emission intensity by the
photo-isomerization, the CPL intensity decreased (Fig. 8.7b). Since the CPL measurement was performed at a relatively high concentration (1.7 Â 10
À4 M) in
comparison to that for absorption spectra, the conversion ratio was not enough to
achieve the completely off-state (Fig. 8.7b). In principle, the isolated 2c gave a
negligible emission quantum yield far less than 0.1%.
The emission from both the monomer and the excimer decreased in a liner fashion
as a function of the conversion ratio to the colored form 2c (Fig. 8.9). The quenching
Monomer at 390 nm
Excimer at 500 nm
Relative intensity (I/I
0 )
0
1.0
0.8
0.6
0.4
0.2
Conversion ratio
([c-form]/[o-form + c-form])
0
0.2
0.4
0.6
0.8
1.0
Fig. 8.9 Plots of relative
emission intensity in
monomer (circle, at 390 nm)
and excimer (square, at
500 nm) emission. I 0
corresponds to the intensity
at a conversion of 0
8 Photo-Switching of Circularly Polarized Luminescence
183