and blue fluorescence (λ Em ~ 420–450 nm) with rather strong quantum yields
(0.21–0.49) for azaborahexahelicenes 46a,c and more modest ones (~0.07) for
the octa- and decahelicenes 46b,d. The introduction of one additional boron
atom on 46c strongly increased the emission efficiency compared to 46a, but at
the same time strongly decreased the configuration stability (enantiomerization
barrier ΔG
6 ¼ of 27.5 kcal mol
À1 at 78
C, in ethanol) due to the presence of two
azaborapentacycles. From the UV-vis spectra, the longer the helicene, the stronger
were the absorption coefficients and the more red-shifted the absorption wavelengths. Similarly, the ECD spectra were more red-shifted and more intense for
azaboraoctahelicene 46b and azaboradecahelicene 46d as compared to
azaborahexahelicenes 46a,c. Note that, except for 46c, the overall ECD signature
appeared typical of helicene derivatives and that the (P)-enantiomers display positive optical rotation values. Regarding the CPL responses, g lum values were found
negative for (P)-46a-c and positive for (P)-46d (see Table 4.6). As mentioned above,
the sign of CPL greatly varies with the substituents grafted onto the helicenic core
and generally follows the sign of the lower energy ECD-active band. The absolute
values of g lum (between 7 Â 10
À4 and 10
À3 ) for 46a-d are typical of enantiopure
organic helicenes.
Enantiopure azabora[5]helicenes 47–49 were also prepared; they displayed
different charge transfer characters and fluorescence quantum yields ranging from
0.13 to 0.30 in toluene, governed by the electron-donor substitution ( p-MeO-phenyl,
p-Me 2 N-phenyl) at the helicene [43]. The dimethylamino-substituted derivative
emitted at the most red-shifted wavelength and showed the highest Stokes shift in
toluene. These helicenes also showed CPL activity with dissymmetry factors g lum
between 2.5 Â 10
À4 and 3.5 Â 10
À3 . Their ECD spectra and optical rotation values
of 47–49 were very different from azaborahelicenes 46a–d, and it was shown that
the sign of the ECD band corresponding to the first transition and the CPL spectrum
depended on the electron-donor substitution.
Table 4.6 Photophysical data of borahelicenes
Compound
λ Abs
max a
(nm)
λ Em
(nm)
Φ F (%)
Solvent
(CPL)
10
3
g abs
10
3
g lum
Ref.
(P)-46a
398
404
21
CH 2 Cl 2
~À1.9
b
À0.9
[42]
(P)-46b
429
435
6.9
CH 2 Cl 2
~À2.7
b
À0.7
[42]
(P)-46c
391
427
49
CH 2 Cl 2
~À0.8
b
À2.3
[42]
(P)-46d
440
471
7.4
CH 2 Cl 2
~+2.6
b
+1
[42]
(P)-47
414
495
29/toluene
CHCl 3
À0.7
À0.25 [43]
(P)-48
420
502
30/toluene
CHCl 3
+1.1
+0.95
[43]
(P)-49
433
586
13/toluene
CHCl 3
+2
+3.5
[43]
(P,P)-50
411
436
65/CH 2 Cl 2
(26)
b
CH 2 Cl 2
~À1.1
c
À1.7
[44]
a Lowest-energy absorption band
b
Fluorescence quantum yield measured in the solid state
c Taken from [12]
4 Circularly Polarized Luminescence in Helicene and Helicenoid Derivatives
71
(0.21–0.49) for azaborahexahelicenes 46a,c and more modest ones (~0.07) for
the octa- and decahelicenes 46b,d. The introduction of one additional boron
atom on 46c strongly increased the emission efficiency compared to 46a, but at
the same time strongly decreased the configuration stability (enantiomerization
barrier ΔG
6 ¼ of 27.5 kcal mol
À1 at 78
C, in ethanol) due to the presence of two
azaborapentacycles. From the UV-vis spectra, the longer the helicene, the stronger
were the absorption coefficients and the more red-shifted the absorption wavelengths. Similarly, the ECD spectra were more red-shifted and more intense for
azaboraoctahelicene 46b and azaboradecahelicene 46d as compared to
azaborahexahelicenes 46a,c. Note that, except for 46c, the overall ECD signature
appeared typical of helicene derivatives and that the (P)-enantiomers display positive optical rotation values. Regarding the CPL responses, g lum values were found
negative for (P)-46a-c and positive for (P)-46d (see Table 4.6). As mentioned above,
the sign of CPL greatly varies with the substituents grafted onto the helicenic core
and generally follows the sign of the lower energy ECD-active band. The absolute
values of g lum (between 7 Â 10
À4 and 10
À3 ) for 46a-d are typical of enantiopure
organic helicenes.
Enantiopure azabora[5]helicenes 47–49 were also prepared; they displayed
different charge transfer characters and fluorescence quantum yields ranging from
0.13 to 0.30 in toluene, governed by the electron-donor substitution ( p-MeO-phenyl,
p-Me 2 N-phenyl) at the helicene [43]. The dimethylamino-substituted derivative
emitted at the most red-shifted wavelength and showed the highest Stokes shift in
toluene. These helicenes also showed CPL activity with dissymmetry factors g lum
between 2.5 Â 10
À4 and 3.5 Â 10
À3 . Their ECD spectra and optical rotation values
of 47–49 were very different from azaborahelicenes 46a–d, and it was shown that
the sign of the ECD band corresponding to the first transition and the CPL spectrum
depended on the electron-donor substitution.
Table 4.6 Photophysical data of borahelicenes
Compound
λ Abs
max a
(nm)
λ Em
(nm)
Φ F (%)
Solvent
(CPL)
10
3
g abs
10
3
g lum
Ref.
(P)-46a
398
404
21
CH 2 Cl 2
~À1.9
b
À0.9
[42]
(P)-46b
429
435
6.9
CH 2 Cl 2
~À2.7
b
À0.7
[42]
(P)-46c
391
427
49
CH 2 Cl 2
~À0.8
b
À2.3
[42]
(P)-46d
440
471
7.4
CH 2 Cl 2
~+2.6
b
+1
[42]
(P)-47
414
495
29/toluene
CHCl 3
À0.7
À0.25 [43]
(P)-48
420
502
30/toluene
CHCl 3
+1.1
+0.95
[43]
(P)-49
433
586
13/toluene
CHCl 3
+2
+3.5
[43]
(P,P)-50
411
436
65/CH 2 Cl 2
(26)
b
CH 2 Cl 2
~À1.1
c
À1.7
[44]
a Lowest-energy absorption band
b
Fluorescence quantum yield measured in the solid state
c Taken from [12]
4 Circularly Polarized Luminescence in Helicene and Helicenoid Derivatives
71