that exhibits its fluorescence at approximately 550 nm with a high emission quantum
yield. They synthesized (R)- and (S)-19, with two PBI moieties attached to the
2 and 2
0 positions of the binaphthyl. (R)- and (S)-19 exhibited characteristic π–πÃ
absorption and emission for the pair of PBI units. Chiroptical dissymmetry was
observed in the circular dichroism (CD) spectrum, indicating that the two PBI
moieties take a chiral orientation. (S)-19 displayed CPL with a g lum of 2 Â 10
À3
in their diluted solution (1 Â 10
À3 mol L
À1 ). The g lum value increased as the
concentration of (S)-19 increased and reached 6 Â 10
À3 at the concentration of
1 Â 10
À3 mol L
À1 , where (S)-19 formed an opaque colloidal solution. The authors
suggest that the formation of the aggregate in the condensed solution results in an
increase in the g lum value.
Pieters and coworkers developed a CPL-active delayed fluorescence system [29].
In thermally activated delayed fluorescence (TADF) emitters, the energy gap
between their singlet and triplet states is so small that reverse intersystem crossing
processes easily occur. Thus, both singlet and triplet excitons can be harvested for
their fluorescence from the singlet excited state. This property has an advantage
in developing motivated organic light emitting diodes (OLEDs) because of the
possibility to overcome the theoretical maximum efficiency. They synthesized
TADF emitter 20 possessing two achiral carbazoles as luminophores and a
binaphthyl (Fig. 9.6). The TADF character of 20 was demonstrated using timeresolved fluorescence analysis. 20 exhibited CPL with a g lum of 1.3 Â 10
À3 . The
combination of CPL and TADF may provide a remarkable CPL-OLED material.
The use of a chiral cyclophane is also effective for creating CPL-active organic
materials. A cyclophane is a cyclic compound that includes aromatic moieties
as an integral part of its structure. [2.2]Paracyclophane has two benzene rings
linked by two ethylenes at the 1,1
0 and 4,4
0 positions. The orientations of two
benzene rings of a [2.2]paracyclophane are fixed since the benzene rings cannot
invert. Thus, chirally substituted [2.2]paracyclophanes provide a chiral arrangement
of luminophores that exhibit chiroptical properties. Recently, CPL-active [2.2]
paracyclophanes have been developed by Morisaki, Chujo, and coworkers
(Fig. 9.7) [30]. They synthesized propeller-shaped [2,2]paracyclophane derivatives
(R)- and (S)-21 and their precursors (R)- and (S)-22. (S)-21 exhibited CPL with a
g lum of 1.1 Â 10
À2 and fluorescence quantum yield of 45%. These values are quite
good compared to ordinary chiral organic luminophores. Interestingly, the g lum value
of (S)-21 is approximately ten-fold higher than that of (S)-22 (g lum ¼ 1.1 Â 10
À3 ).
The good CPL activity of (S)-21 comes from its highly extended and crisscrossed
delocalized structure. The distortion of the π-conjugated plane in (S)-21 may hinder
its fluorescence efficiency, but it is clear that the distortion causes the high g lum
value of (S)-21 compared to that of (S)-22. Morisaki, Chujo, and coworkers also
synthesized planar chiral tetra-substituted [2.2]paracyclophanes (R)- and (S)-23
and 24 [31]. 23 and 24 have a chirally orientated two para-phenylene-ethynylene
luminophore that is not distorted. 24 has a more extended π-conjugated structure
compared to 23. 23 and 24 displayed optical dissymmetry in their emissions
with good fluorescence quantum yields (65% for 23 and 87% for 24) in diluted
chloroform solution (1 Â 10
À6 mol L
À1 ). The g lum values of (R)-23 and (R)-24 in the
diluted solution were À1.7 Â 10
À3 and À1.2 Â 10
À3 , respectively. Interestingly,
9 Circularly Polarized Luminescence of Chirally Arranged Achiral Organic. . .
203
yield. They synthesized (R)- and (S)-19, with two PBI moieties attached to the
2 and 2
0 positions of the binaphthyl. (R)- and (S)-19 exhibited characteristic π–πÃ
absorption and emission for the pair of PBI units. Chiroptical dissymmetry was
observed in the circular dichroism (CD) spectrum, indicating that the two PBI
moieties take a chiral orientation. (S)-19 displayed CPL with a g lum of 2 Â 10
À3
in their diluted solution (1 Â 10
À3 mol L
À1 ). The g lum value increased as the
concentration of (S)-19 increased and reached 6 Â 10
À3 at the concentration of
1 Â 10
À3 mol L
À1 , where (S)-19 formed an opaque colloidal solution. The authors
suggest that the formation of the aggregate in the condensed solution results in an
increase in the g lum value.
Pieters and coworkers developed a CPL-active delayed fluorescence system [29].
In thermally activated delayed fluorescence (TADF) emitters, the energy gap
between their singlet and triplet states is so small that reverse intersystem crossing
processes easily occur. Thus, both singlet and triplet excitons can be harvested for
their fluorescence from the singlet excited state. This property has an advantage
in developing motivated organic light emitting diodes (OLEDs) because of the
possibility to overcome the theoretical maximum efficiency. They synthesized
TADF emitter 20 possessing two achiral carbazoles as luminophores and a
binaphthyl (Fig. 9.6). The TADF character of 20 was demonstrated using timeresolved fluorescence analysis. 20 exhibited CPL with a g lum of 1.3 Â 10
À3 . The
combination of CPL and TADF may provide a remarkable CPL-OLED material.
The use of a chiral cyclophane is also effective for creating CPL-active organic
materials. A cyclophane is a cyclic compound that includes aromatic moieties
as an integral part of its structure. [2.2]Paracyclophane has two benzene rings
linked by two ethylenes at the 1,1
0 and 4,4
0 positions. The orientations of two
benzene rings of a [2.2]paracyclophane are fixed since the benzene rings cannot
invert. Thus, chirally substituted [2.2]paracyclophanes provide a chiral arrangement
of luminophores that exhibit chiroptical properties. Recently, CPL-active [2.2]
paracyclophanes have been developed by Morisaki, Chujo, and coworkers
(Fig. 9.7) [30]. They synthesized propeller-shaped [2,2]paracyclophane derivatives
(R)- and (S)-21 and their precursors (R)- and (S)-22. (S)-21 exhibited CPL with a
g lum of 1.1 Â 10
À2 and fluorescence quantum yield of 45%. These values are quite
good compared to ordinary chiral organic luminophores. Interestingly, the g lum value
of (S)-21 is approximately ten-fold higher than that of (S)-22 (g lum ¼ 1.1 Â 10
À3 ).
The good CPL activity of (S)-21 comes from its highly extended and crisscrossed
delocalized structure. The distortion of the π-conjugated plane in (S)-21 may hinder
its fluorescence efficiency, but it is clear that the distortion causes the high g lum
value of (S)-21 compared to that of (S)-22. Morisaki, Chujo, and coworkers also
synthesized planar chiral tetra-substituted [2.2]paracyclophanes (R)- and (S)-23
and 24 [31]. 23 and 24 have a chirally orientated two para-phenylene-ethynylene
luminophore that is not distorted. 24 has a more extended π-conjugated structure
compared to 23. 23 and 24 displayed optical dissymmetry in their emissions
with good fluorescence quantum yields (65% for 23 and 87% for 24) in diluted
chloroform solution (1 Â 10
À6 mol L
À1 ). The g lum values of (R)-23 and (R)-24 in the
diluted solution were À1.7 Â 10
À3 and À1.2 Â 10
À3 , respectively. Interestingly,
9 Circularly Polarized Luminescence of Chirally Arranged Achiral Organic. . .
203