and À3.40 Â 10
À3 to À2.73 Â 10
À3 for the (M) enantiomers, respectively.
These compounds also showed intense CPL signals, matching with the region of
emission spectra and corresponding ECD signals at the longest wavelength
(305–400 nm). The pure enantiomers all exhibited relatively high g lum with
values between +3.66 Â 10
À3 and + 6.41 Â 10
À3 for the (P) configurations
and À3.40 Â 10
À3 to À6.59 Â 10
À3 for the (M) configurations (Table 4.9) thus
showing that chirality existed in both the ground and excited states and could
be attributed to their rigid helical structures. Interestingly, absorption dissymmetry
factors (g abs ) of (P)-(+)-56 and (M )-(À)-56 were found stronger (+7.9 Â 10
À3 and
À7.7 Â 10
À3 , respectively), as well as the emission dissymmetry factors (g lum ) as
large as 2.8 Â 10
À2 and À3.1 Â 10
À2 at 466 nm, respectively, considerably higher
than values reported for other CPL-active single organic molecules. Similar
tetrahydropentahelicenic structures, i.e., (P)-(À)-57a-c and (M)-(+)-57a-c, were
prepared in 2018 by Chen et al. [55]. They exhibited mirror-image ECD and CPL
spectra in dichloromethane, with g abs of À4.7 Â 10
À4 to À8.3 Â 10
À4 for the (P)
configuration, and +4.9 Â 10
À4 to +8.7 Â 10
À4 for the (M) configuration (Table 4.10
and [55]). As for fully conjugated derivative pentahelicene (P)-(+) and (M)-(À)-57d,
they display g abs values of similar magnitude (+3.28 Â 10
À4 and À3.57 Â 10
À4 ,
respectively). Furthermore, the enantiomers of 57a–c showed CPL signals with g lum
values between À2.5 Â 10
À4 and À4.1 Â 10
À4 for the (P) enantiomers and
+2.6 Â 10
À4 and +4.2 Â 10
À4 for the (M) ones. (P)-(+) and (M )-(À)-57d
in dichloromethane exhibited stronger CPL activity, with g lum values of
À4.52 Â 10
À3 and +4.43 Â 10
À3 , respectively. In 2018, Chen et al. also reported
the one-pot oxidative aromatization and dearomatization (OADA) reactions of
similar tetrahydro[5]helicene diols, with DDQ as the oxidant, which provided
a new method for the synthesis of novel CPL-active chiral π-extended diones [58].
Table 4.10 Photophysical data of carbon-based helicenes
Compound
λ Abs
max a (nm)
λ Em
(nm)
Φ (%)
Solvent
(CPL)
10
3
g abs
10
3
g lum
Ref.
(P)-58a
582
610
39
CH 2 Cl 2
+0.15
+0.1
[56]
(P)-58b
582
610
41
CH 2 Cl 2
+0.9
+0.6
[56]
(P,P)58c
622
650
35
CH 2 Cl 2
+1.3
+0.9
[56]
(P)-(+)-59a
326
442
14
CHCl 3
À0.56
~0
[57]
(P)-(+)-59b
327
452
29
CHCl 3
À3.1
~0
[57]
(P,P)-(+)59c
291
464
11
CHCl 3
À0.68
À2.7
[57]
(P,P)-(+)59d
292
466
7.6
CHCl 3
À4.5
À1.5
[57]
(P)-10
410
413
14
CHCl 3
À1.2
b
À0.9
[23]
(P)-11
410
416
5
CHCl 3
À2.5
b
À0.02
b
[20]
(P,P)-60
471
494
1.8
CH 2 Cl 2
À2.6
À2.5
[23]
(P,P)-61
430
434
4.1
CH 2 Cl 2
À2.8
À2.1
[23]
a Lowest energy UV-vis band
b
Taken from [12]
4 Circularly Polarized Luminescence in Helicene and Helicenoid Derivatives
77
À3 to À2.73 Â 10
À3 for the (M) enantiomers, respectively.
These compounds also showed intense CPL signals, matching with the region of
emission spectra and corresponding ECD signals at the longest wavelength
(305–400 nm). The pure enantiomers all exhibited relatively high g lum with
values between +3.66 Â 10
À3 and + 6.41 Â 10
À3 for the (P) configurations
and À3.40 Â 10
À3 to À6.59 Â 10
À3 for the (M) configurations (Table 4.9) thus
showing that chirality existed in both the ground and excited states and could
be attributed to their rigid helical structures. Interestingly, absorption dissymmetry
factors (g abs ) of (P)-(+)-56 and (M )-(À)-56 were found stronger (+7.9 Â 10
À3 and
À7.7 Â 10
À3 , respectively), as well as the emission dissymmetry factors (g lum ) as
large as 2.8 Â 10
À2 and À3.1 Â 10
À2 at 466 nm, respectively, considerably higher
than values reported for other CPL-active single organic molecules. Similar
tetrahydropentahelicenic structures, i.e., (P)-(À)-57a-c and (M)-(+)-57a-c, were
prepared in 2018 by Chen et al. [55]. They exhibited mirror-image ECD and CPL
spectra in dichloromethane, with g abs of À4.7 Â 10
À4 to À8.3 Â 10
À4 for the (P)
configuration, and +4.9 Â 10
À4 to +8.7 Â 10
À4 for the (M) configuration (Table 4.10
and [55]). As for fully conjugated derivative pentahelicene (P)-(+) and (M)-(À)-57d,
they display g abs values of similar magnitude (+3.28 Â 10
À4 and À3.57 Â 10
À4 ,
respectively). Furthermore, the enantiomers of 57a–c showed CPL signals with g lum
values between À2.5 Â 10
À4 and À4.1 Â 10
À4 for the (P) enantiomers and
+2.6 Â 10
À4 and +4.2 Â 10
À4 for the (M) ones. (P)-(+) and (M )-(À)-57d
in dichloromethane exhibited stronger CPL activity, with g lum values of
À4.52 Â 10
À3 and +4.43 Â 10
À3 , respectively. In 2018, Chen et al. also reported
the one-pot oxidative aromatization and dearomatization (OADA) reactions of
similar tetrahydro[5]helicene diols, with DDQ as the oxidant, which provided
a new method for the synthesis of novel CPL-active chiral π-extended diones [58].
Table 4.10 Photophysical data of carbon-based helicenes
Compound
λ Abs
max a (nm)
λ Em
(nm)
Φ (%)
Solvent
(CPL)
10
3
g abs
10
3
g lum
Ref.
(P)-58a
582
610
39
CH 2 Cl 2
+0.15
+0.1
[56]
(P)-58b
582
610
41
CH 2 Cl 2
+0.9
+0.6
[56]
(P,P)58c
622
650
35
CH 2 Cl 2
+1.3
+0.9
[56]
(P)-(+)-59a
326
442
14
CHCl 3
À0.56
~0
[57]
(P)-(+)-59b
327
452
29
CHCl 3
À3.1
~0
[57]
(P,P)-(+)59c
291
464
11
CHCl 3
À0.68
À2.7
[57]
(P,P)-(+)59d
292
466
7.6
CHCl 3
À4.5
À1.5
[57]
(P)-10
410
413
14
CHCl 3
À1.2
b
À0.9
[23]
(P)-11
410
416
5
CHCl 3
À2.5
b
À0.02
b
[20]
(P,P)-60
471
494
1.8
CH 2 Cl 2
À2.6
À2.5
[23]
(P,P)-61
430
434
4.1
CH 2 Cl 2
À2.8
À2.1
[23]
a Lowest energy UV-vis band
b
Taken from [12]
4 Circularly Polarized Luminescence in Helicene and Helicenoid Derivatives
77