light technologies in the development of OLEDs in which the electroluminescence
is directly circularly polarized thus giving CP-OLEDs. Indeed, antiglare filters
commonly used for OLED displays exploit the physics of CP light to eliminate
glare from external light sources (e.g., the sunlight). Unfortunately, this technology
removes approximately 50% of the non-polarized light emitted from the OLED
pixels. If the non-polarized OLEDs are replaced with CP-OLEDs (with a comparable
device performance), an improved amount of CP light component of the correct
handedness would pass through the antiglare filter with less loss, thus increasing
the energy efficiency of the display in proportion to the increasing dissymmetry
of the light. In addition, the use of CP-OLED will enable to simplify the architecture
of the device by avoiding the use of extra filter components, which will directly
impact the overall cost of the device.
In 2013, Fuchter and coworkers, reported the use of 1-aza[6]helicene 9 as a
chiral dopant in light-emitting polymer, i.e., poly[9,9-dioctylfluorene-cobenzothiadiazole] 80 (Fig. 4.31a) [21]. It was found that blends consisting
of a small amount (7%) of enantiopure 1-aza[6]helicene dopant gave a strong
CP-photoluminescence response of the 80 films. Increasing the 1-aza[6]helicene
blending ratio resulted in improvements of the g PL factor, up to a significantly
high value of 0.5 for the 53% helicene blend (while the starting azahelicene
displayed only modest g lum ~10
À4 to 10
À3 ). To explain this behavior, the authors
suggested the formation of a chiroptical co-crystalline phase. The authors were
then able to fabricate a single-layer polymer LED (PLED) device emitting circularly
polarized light from the 80 blends containing 7% of either (À)-1-aza[6]helicene or
(+)-1-aza[6]helicene with a dissymmetry factor of electroluminescence (g EL ) factor
as high as 0.2. In 2016, Fuchter and Campbell succeeded in preparing a single layer
CP-phosphorescent OLEDs (CP-PHOLEDs), using 66a as a chiral emissive dopant;
these PHOLEDs displayed strong circularly polarized electrophosphorescence
(CPEL), with g EL reaching À0.38 and + 0.22 at 615 nm for (À)- and (+)-66a,
respectively (see Fig. 4.31b) [75]. Although not yet clearly demonstrated, the
increase of g EL as compared to the molecular g lum value (10
À2 ) may be explained
by a supramolecular organization of 66a in the solid state. Recently, by decorating
the pyridyl-helicene ligands with –CF 3 and –F groups [66], the platinahelicene
enantiomers 66e featured good configurational stability as well as high sublimation
Table 4.13 Photophysical data of helicene-bipy and terpy ligands together with heir Zn and proton
complexes
Compound
λ Abs
max a
(nm)
λ Em
(nm)
Φ
(%)
Solvent
(CPL)
10
3
g abs
10
3
g lum
Ref.
(P,P)-(+)-72
416
421
8.4
CH 2 Cl 2
~6.5
b
+8.6
[69]
(P,P)-(+)-73
430
480
19
CH 2 Cl 2
~4.3
b
+1.2
[69]
(P,P)-(+)-74
420
421
22
CH 2 Cl 2
–
+4.8
[70]
(P,P)-(+)-75
453
520
44
CH 2 Cl 2
–
+1.8
[70]
(P,P)-(+)74.2H
+
507
600
28
CH 2 Cl 2
–
+2.5
[70]
a Lowest-energy UV-vis band
b
Taken from 12
90
J. Crassous
is directly circularly polarized thus giving CP-OLEDs. Indeed, antiglare filters
commonly used for OLED displays exploit the physics of CP light to eliminate
glare from external light sources (e.g., the sunlight). Unfortunately, this technology
removes approximately 50% of the non-polarized light emitted from the OLED
pixels. If the non-polarized OLEDs are replaced with CP-OLEDs (with a comparable
device performance), an improved amount of CP light component of the correct
handedness would pass through the antiglare filter with less loss, thus increasing
the energy efficiency of the display in proportion to the increasing dissymmetry
of the light. In addition, the use of CP-OLED will enable to simplify the architecture
of the device by avoiding the use of extra filter components, which will directly
impact the overall cost of the device.
In 2013, Fuchter and coworkers, reported the use of 1-aza[6]helicene 9 as a
chiral dopant in light-emitting polymer, i.e., poly[9,9-dioctylfluorene-cobenzothiadiazole] 80 (Fig. 4.31a) [21]. It was found that blends consisting
of a small amount (7%) of enantiopure 1-aza[6]helicene dopant gave a strong
CP-photoluminescence response of the 80 films. Increasing the 1-aza[6]helicene
blending ratio resulted in improvements of the g PL factor, up to a significantly
high value of 0.5 for the 53% helicene blend (while the starting azahelicene
displayed only modest g lum ~10
À4 to 10
À3 ). To explain this behavior, the authors
suggested the formation of a chiroptical co-crystalline phase. The authors were
then able to fabricate a single-layer polymer LED (PLED) device emitting circularly
polarized light from the 80 blends containing 7% of either (À)-1-aza[6]helicene or
(+)-1-aza[6]helicene with a dissymmetry factor of electroluminescence (g EL ) factor
as high as 0.2. In 2016, Fuchter and Campbell succeeded in preparing a single layer
CP-phosphorescent OLEDs (CP-PHOLEDs), using 66a as a chiral emissive dopant;
these PHOLEDs displayed strong circularly polarized electrophosphorescence
(CPEL), with g EL reaching À0.38 and + 0.22 at 615 nm for (À)- and (+)-66a,
respectively (see Fig. 4.31b) [75]. Although not yet clearly demonstrated, the
increase of g EL as compared to the molecular g lum value (10
À2 ) may be explained
by a supramolecular organization of 66a in the solid state. Recently, by decorating
the pyridyl-helicene ligands with –CF 3 and –F groups [66], the platinahelicene
enantiomers 66e featured good configurational stability as well as high sublimation
Table 4.13 Photophysical data of helicene-bipy and terpy ligands together with heir Zn and proton
complexes
Compound
λ Abs
max a
(nm)
λ Em
(nm)
Φ
(%)
Solvent
(CPL)
10
3
g abs
10
3
g lum
Ref.
(P,P)-(+)-72
416
421
8.4
CH 2 Cl 2
~6.5
b
+8.6
[69]
(P,P)-(+)-73
430
480
19
CH 2 Cl 2
~4.3
b
+1.2
[69]
(P,P)-(+)-74
420
421
22
CH 2 Cl 2
–
+4.8
[70]
(P,P)-(+)-75
453
520
44
CH 2 Cl 2
–
+1.8
[70]
(P,P)-(+)74.2H
+
507
600
28
CH 2 Cl 2
–
+2.5
[70]
a Lowest-energy UV-vis band
b
Taken from 12
90
J. Crassous