4.8 CPL-Active Carbohelicenic Derivatives
4.8.1 Pentahelicenic Structures
In 2018, Mori and coworkers reported a combined experimental and theoretical
study to elucidate the ECD and CPL behaviors of parent pentahelicene (P)-23
and of D 3 -symmetric triple pentahelicene (P,P,P)-54 [53]. They showed that
the pentahelicene unit exhibits absorption and luminescence, with dissymmetry
factors g abs and g lum that are intrinsically larger than those of higher homologues.
Thus, (P)-23 emitted strong CPL with g lum of À2.7 Â 10
À3 at low temperature
(see Table 4.9 and Fig. 4.19), which is about a half value of the g abs . Due to its
photolabile nature, 23 is not suitable to be incorporated in chiroptical materials.
However, such undesirable reactivities can be excluded by merging three
pentahelicenic units into 54, for which the g lum and g abs factors were found to be
as high as À1.3 Â 10
À3 and À1.8 Â 10
À3 , respectively, for the (P) enantiomer,
corresponding to a high g lum /g abs ratio of 0.72, indicating moderate excited-state
relaxation. Theoretical calculations provided further insights into the improved
chiroptical responses at the main band (
1 B b transition) in the triple pentahelicene
23, which was ascribed to its symmetric nature.
In 2017, Lu and coworkers prepared enantiopure tetrahydrocarbo[5]helicenic
derivatives (Fig. 4.20) with high configurational stability, thanks to the presence
of phenyl groups placed in the inner groove of the helix [54]. These compounds
displayed strong blue fluorescence with quantum yield of up to 0.59. Compounds
(P)-(+)-55a-e,56 and (M )-(À)-55a-e,56 also exhibited mirror-imaged ECD and
Table 4.9 Photophysical data of pentahelicenic derivatives
Compound
λ Abs
max a (nm)
λ Em
(nm)
Φ F
(%)
Solvent
(CPL)
10
3 g abs 10
3 g lum Ref.
(P)-(+)-23
398
406
5.7
CH 2 Cl 2
À7.6
À2.7
[53]
(M )-(À)-54 460
483
1.8
CH 2 Cl 2
À1.8
À1.3
[53]
(P)-(+)-55a 347
440
48
THF
2.63
5.63
[54]
(P)-(+)-55b 349
442
59
THF
3.55
3.66
[54]
(P)-(+)-55c 344
439
40
THF
4.34
6.32
[54]
(P)-(+)-55d 344
438
41
THF
3.82
6.41
[54]
(P)-(+)-55e 345
440
42
THF
3.21
4.58
[54]
(P)-(+)-56
338
465
3
THF
7.87
28.2
[54]
(P)-(À)-57a 305
424
32.6
CH 2 Cl 2
À0.47
À0.25
[55]
(P)-(À)-57b 310
425
41.0
CH 2 Cl 2
À0.50
À0.28
[55]
(P)-(À)-57c 301
425
39.1
CH 2 Cl 2
À0.83
À0.41
[55]
(P)-(+)-57d 330
455
18.7
CH 2 Cl 2
À0.328 À4.52
[55]
a Lowest-energy UV-vis band
4 Circularly Polarized Luminescence in Helicene and Helicenoid Derivatives
75
4.8.1 Pentahelicenic Structures
In 2018, Mori and coworkers reported a combined experimental and theoretical
study to elucidate the ECD and CPL behaviors of parent pentahelicene (P)-23
and of D 3 -symmetric triple pentahelicene (P,P,P)-54 [53]. They showed that
the pentahelicene unit exhibits absorption and luminescence, with dissymmetry
factors g abs and g lum that are intrinsically larger than those of higher homologues.
Thus, (P)-23 emitted strong CPL with g lum of À2.7 Â 10
À3 at low temperature
(see Table 4.9 and Fig. 4.19), which is about a half value of the g abs . Due to its
photolabile nature, 23 is not suitable to be incorporated in chiroptical materials.
However, such undesirable reactivities can be excluded by merging three
pentahelicenic units into 54, for which the g lum and g abs factors were found to be
as high as À1.3 Â 10
À3 and À1.8 Â 10
À3 , respectively, for the (P) enantiomer,
corresponding to a high g lum /g abs ratio of 0.72, indicating moderate excited-state
relaxation. Theoretical calculations provided further insights into the improved
chiroptical responses at the main band (
1 B b transition) in the triple pentahelicene
23, which was ascribed to its symmetric nature.
In 2017, Lu and coworkers prepared enantiopure tetrahydrocarbo[5]helicenic
derivatives (Fig. 4.20) with high configurational stability, thanks to the presence
of phenyl groups placed in the inner groove of the helix [54]. These compounds
displayed strong blue fluorescence with quantum yield of up to 0.59. Compounds
(P)-(+)-55a-e,56 and (M )-(À)-55a-e,56 also exhibited mirror-imaged ECD and
Table 4.9 Photophysical data of pentahelicenic derivatives
Compound
λ Abs
max a (nm)
λ Em
(nm)
Φ F
(%)
Solvent
(CPL)
10
3 g abs 10
3 g lum Ref.
(P)-(+)-23
398
406
5.7
CH 2 Cl 2
À7.6
À2.7
[53]
(M )-(À)-54 460
483
1.8
CH 2 Cl 2
À1.8
À1.3
[53]
(P)-(+)-55a 347
440
48
THF
2.63
5.63
[54]
(P)-(+)-55b 349
442
59
THF
3.55
3.66
[54]
(P)-(+)-55c 344
439
40
THF
4.34
6.32
[54]
(P)-(+)-55d 344
438
41
THF
3.82
6.41
[54]
(P)-(+)-55e 345
440
42
THF
3.21
4.58
[54]
(P)-(+)-56
338
465
3
THF
7.87
28.2
[54]
(P)-(À)-57a 305
424
32.6
CH 2 Cl 2
À0.47
À0.25
[55]
(P)-(À)-57b 310
425
41.0
CH 2 Cl 2
À0.50
À0.28
[55]
(P)-(À)-57c 301
425
39.1
CH 2 Cl 2
À0.83
À0.41
[55]
(P)-(+)-57d 330
455
18.7
CH 2 Cl 2
À0.328 À4.52
[55]
a Lowest-energy UV-vis band
4 Circularly Polarized Luminescence in Helicene and Helicenoid Derivatives
75