in the solid state (see Fig. 4.17 and Table 4.7). The CPL spectra of enantiopure
sila[7]helicene (P)- and (M )-32, which are mirror image of positive and negative
sign, respectively; dissymmetry factors of 3.5 Â 10
À4 at 470 nm were measured.
The authors concluded that the g lum derives mainly from the helical biphenanthryl
moiety, while the heterole moiety plays essential roles in the luminescent properties.
In 2015, Tanaka et al. prepared enantioenriched 1,1
0 -bis-triphenylene-based sila[7]
helicenes (P)-52 with 91% ee [48]. Compared to sila[7]helicene 51, 52 displayed
red-shifted absorption and fluorescence responses explained by the presence of
fused 1,1
0 -bistriphenylenes resulting in more extended π-conjugation. Probably
for the same reason, enantiopure 52 show high g lum values, i.e., 1.6 Â 10
À2 ,
which is uncommonly high for an organic helicene. These values appear larger
than that for the 3,3-biphenanthrene-based sila[7]helicene 51 (g lum ¼ À0.0035 at
470 nm) but smaller than that for the 1,1-bitriphenylene based carbo[7]helicene 61
(g lum ¼ À0.030 at 428 nm for the (M )-(À) enantiomer; see Sect. 4.8.3) [49].
4.7 CPL-Active Phosphahelicenes
It is now well recognized that phosphorus-containing π-conjugated small molecules,
oligomers, polymers, and supramolecular assemblies are important classes of
heteroatomic molecular materials for many applications in optoelectronics including
OLEDs [50]. P-containing building blocks can indeed lead to materials with
unique properties (emission, charge transport, coordination, (anti)aromaticity, etc.).
So far, most phosphorus derivatives having helical chirality have displayed
polyaromatic (or heteroaromatic) helical scaffolds with pendant phosphorus
functions (phosphites, trivalent phosphines and phosphine oxides, helicenephospholes derivatives) but a few classes of P-containing heterohelicenes have
appeared in the literature in the last years. Although several phospholes and
helicenes are known to exhibit efficient fluorescence properties, the only example
of CPL-active phosphane-containing helicenes has been reported recently [51]. Compounds 53a and 53b are benzooxophosphole derivatives that display a carbo[6]
helicene unit that is meta-fused with one terminal oxophosphole ring containing
a pendant phenyl ring at position 5 (Fig. 4.18). In both systems a l-menthyl
group at the P atom is directed toward the inner groove of the helix (endo isomer).
Table 4.7 Photophysical data of silahelicenes
Compound
λ Abs
max a
(nm)
λ Em
(nm)
Φ F (%)
Solvent
(CPL)
10
3
g abs
10
3
g lum
Ref.
(P)-51
412
450
23/CH 2 Cl 2
(26)
b
CH 2 Cl 2
~1.4
c
À3.5
[47]
(P)-52
400
482
6.9
CHCl 3
~4.2
c
+16
[48]
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
73
sila[7]helicene (P)- and (M )-32, which are mirror image of positive and negative
sign, respectively; dissymmetry factors of 3.5 Â 10
À4 at 470 nm were measured.
The authors concluded that the g lum derives mainly from the helical biphenanthryl
moiety, while the heterole moiety plays essential roles in the luminescent properties.
In 2015, Tanaka et al. prepared enantioenriched 1,1
0 -bis-triphenylene-based sila[7]
helicenes (P)-52 with 91% ee [48]. Compared to sila[7]helicene 51, 52 displayed
red-shifted absorption and fluorescence responses explained by the presence of
fused 1,1
0 -bistriphenylenes resulting in more extended π-conjugation. Probably
for the same reason, enantiopure 52 show high g lum values, i.e., 1.6 Â 10
À2 ,
which is uncommonly high for an organic helicene. These values appear larger
than that for the 3,3-biphenanthrene-based sila[7]helicene 51 (g lum ¼ À0.0035 at
470 nm) but smaller than that for the 1,1-bitriphenylene based carbo[7]helicene 61
(g lum ¼ À0.030 at 428 nm for the (M )-(À) enantiomer; see Sect. 4.8.3) [49].
4.7 CPL-Active Phosphahelicenes
It is now well recognized that phosphorus-containing π-conjugated small molecules,
oligomers, polymers, and supramolecular assemblies are important classes of
heteroatomic molecular materials for many applications in optoelectronics including
OLEDs [50]. P-containing building blocks can indeed lead to materials with
unique properties (emission, charge transport, coordination, (anti)aromaticity, etc.).
So far, most phosphorus derivatives having helical chirality have displayed
polyaromatic (or heteroaromatic) helical scaffolds with pendant phosphorus
functions (phosphites, trivalent phosphines and phosphine oxides, helicenephospholes derivatives) but a few classes of P-containing heterohelicenes have
appeared in the literature in the last years. Although several phospholes and
helicenes are known to exhibit efficient fluorescence properties, the only example
of CPL-active phosphane-containing helicenes has been reported recently [51]. Compounds 53a and 53b are benzooxophosphole derivatives that display a carbo[6]
helicene unit that is meta-fused with one terminal oxophosphole ring containing
a pendant phenyl ring at position 5 (Fig. 4.18). In both systems a l-menthyl
group at the P atom is directed toward the inner groove of the helix (endo isomer).
Table 4.7 Photophysical data of silahelicenes
Compound
λ Abs
max a
(nm)
λ Em
(nm)
Φ F (%)
Solvent
(CPL)
10
3
g abs
10
3
g lum
Ref.
(P)-51
412
450
23/CH 2 Cl 2
(26)
b
CH 2 Cl 2
~1.4
c
À3.5
[47]
(P)-52
400
482
6.9
CHCl 3
~4.2
c
+16
[48]
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
73