the CPL activities of 23 and 24 drastically changed when the molecules formed
films. They made spin-coated thin films, drop-casted thin films, and drop-casted
thick films of 23 and 24. The films were annealed at 65
C (for 23) and 90
C (for 24)
for 3 h. The CPL properties of the films are summarized in Table 9.1. The g lum values
of 24 were increased after annealing, whereas those of 23 were not increased
as much. For example, the g lum value of the drop-casted thick film of (R)-24
was À3.0 Â 10
À2 before annealing and À2.5 Â 10
À1 after annealing. The value
after annealing is quite large for organic compounds. The authors proposed that
the self-assembly of (R)-24 in the annealed film results in the increase of the g lum .
Thus far, the CPL properties of chiral organic luminophores and chirally arranged
achiral luminophores through covalently linked spacers have been summarized.
Three important findings are obtained from the results: (1) the distortion of the
π-plane is effective for CPL activity, but it hinders the fluorescence efficiency
of the planar π-conjugated luminophore; (2) a chiral arrangement of an achiral
luminophore is effective for CPL activity; and (3) the aggregation of CPL-active
organic compounds often increases their CPL activity. These findings encourage
the use of the supramolecular assembly of achiral luminophores as CPL-active
materials.
9.4 CPL Produced by Helical Supramolecular Assemblies
A supramolecular assembly is a well-defined molecular assembly held by
noncovalent bonds such as hydrogen-bonding, π–π stacking, dipole–dipole, and
hydrophilic/hydrophobic interactions. A finely designed supramolecular system
provides a highly ordered structure of the assembly like the double helix of DNA.
Recently, various highly ordered supramolecular assemblies have been developed
[32–34]. Among them, supramolecular assemblies equipped with chirality have
attracted attention due to their potential applications in the fields of asymmetric
catalysts, chiral sensors, and chiroptical materials. Here, it is important to remember
that the CPL properties of some chiral organic compounds increase in the condensed
conditions compared to in the diluted solutions as mentioned above [12, 28,
35]. The increase would result from the formation of aggregates in the condensed
conditions. Then, the idea to use supramolecular methods for constructing
CPL-active organic materials is attractive since the luminophores form highly
Table 9.1 CPL properties of the films of 23 and 24
Film
state
Spin-coated film
Drop-casted thin film
Drop-casted thick film
Before
annealing
After
annealing
Before
annealing
After
annealing
Before
annealing
After
annealing
(R)-23 À0.0061
À0.0087
À0.012
À0.026
À0.075
À0.0043
(S)-23
+0.0056
+0.010
+0.0096
+0.034
+0.056
+0.0015
(R)-24
+0.021
À0.12
À0.0086
À0.17
À0.030
À0.25
(S)-24 À0.014
+0.13
+0.016
+0.13
+0.011
+0.27
9 Circularly Polarized Luminescence of Chirally Arranged Achiral Organic. . .
205
films. They made spin-coated thin films, drop-casted thin films, and drop-casted
thick films of 23 and 24. The films were annealed at 65
C (for 23) and 90
C (for 24)
for 3 h. The CPL properties of the films are summarized in Table 9.1. The g lum values
of 24 were increased after annealing, whereas those of 23 were not increased
as much. For example, the g lum value of the drop-casted thick film of (R)-24
was À3.0 Â 10
À2 before annealing and À2.5 Â 10
À1 after annealing. The value
after annealing is quite large for organic compounds. The authors proposed that
the self-assembly of (R)-24 in the annealed film results in the increase of the g lum .
Thus far, the CPL properties of chiral organic luminophores and chirally arranged
achiral luminophores through covalently linked spacers have been summarized.
Three important findings are obtained from the results: (1) the distortion of the
π-plane is effective for CPL activity, but it hinders the fluorescence efficiency
of the planar π-conjugated luminophore; (2) a chiral arrangement of an achiral
luminophore is effective for CPL activity; and (3) the aggregation of CPL-active
organic compounds often increases their CPL activity. These findings encourage
the use of the supramolecular assembly of achiral luminophores as CPL-active
materials.
9.4 CPL Produced by Helical Supramolecular Assemblies
A supramolecular assembly is a well-defined molecular assembly held by
noncovalent bonds such as hydrogen-bonding, π–π stacking, dipole–dipole, and
hydrophilic/hydrophobic interactions. A finely designed supramolecular system
provides a highly ordered structure of the assembly like the double helix of DNA.
Recently, various highly ordered supramolecular assemblies have been developed
[32–34]. Among them, supramolecular assemblies equipped with chirality have
attracted attention due to their potential applications in the fields of asymmetric
catalysts, chiral sensors, and chiroptical materials. Here, it is important to remember
that the CPL properties of some chiral organic compounds increase in the condensed
conditions compared to in the diluted solutions as mentioned above [12, 28,
35]. The increase would result from the formation of aggregates in the condensed
conditions. Then, the idea to use supramolecular methods for constructing
CPL-active organic materials is attractive since the luminophores form highly
Table 9.1 CPL properties of the films of 23 and 24
Film
state
Spin-coated film
Drop-casted thin film
Drop-casted thick film
Before
annealing
After
annealing
Before
annealing
After
annealing
Before
annealing
After
annealing
(R)-23 À0.0061
À0.0087
À0.012
À0.026
À0.075
À0.0043
(S)-23
+0.0056
+0.010
+0.0096
+0.034
+0.056
+0.0015
(R)-24
+0.021
À0.12
À0.0086
À0.17
À0.030
À0.25
(S)-24 À0.014
+0.13
+0.016
+0.13
+0.011
+0.27
9 Circularly Polarized Luminescence of Chirally Arranged Achiral Organic. . .
205