(Φ F ¼ 0.80). The ECD spectrum of (+)-17 showed several positive Cotton bands in
the longer wavelength region with very similar shapes for both neutral and acidic
forms. These observations imply that the electronic transitions have similar features
in the ground state. Under both conditions, helicene 17 also shows strong CPL
activity with a g lum value under neutral conditions of 0.009 at 473 nm which is quite
large for a heptahelicene derivative. Upon addition of 200 equivalent amounts of
TFA, the g lum value remained high (0.008 at 514 nm). Overall, these systems
combine both high g lum value with high quantum yields.
4.2.5 Azahelicenes with Fused Carbazole Cycles
(Pyrrolohelicenes)
Pyrrole-incorporating PHAs have been shown to possess remarkable physical
properties such as effective hole-transporting ability and bright emission. In
2012, Hiroto et al. reported pyrrole-fused system 18 displaying an aza[5]helicenic
structure with a stable helical conformation, thanks to the presence of bulky
ethynylsilylated groups (Fig. 4.7) [24]. Good fluorescence quantum yields were
obtained for 18 (Φ F ¼ 0.36) with a Stokes shift of 2220 cm
À1 reflecting the distorted
conformation of 18; a CPL anisotropy factor |g lum | of 3 Â 10
À3 was measured
for both enantiomers. The corresponding bis-butadiyne bridged azahelicene dimer
(M,M)- and (P,P)-20 with a figure-eight shape was prepared and exhibited
red-shifted absorption and emission, with a higher fluorescence quantum yield
(Φ F ¼ 0.55) and higher g abs and g lum values as compared to (M )- and (P)-18
(with opposite signs, see Table 4.2). This enhancement was attributed to the rigid
conformation of the dimer [25]. See Sect. 4.3 for the corresponding CPL-active
oxygen-containing helicene derivative 19.
Fig. 4.6 Chemical structure, X-ray structure, ECD, and CPL spectra of neutral and acidic form of
17 enantiomers. Adapted with permission [19]. Copyright 2017, Wiley
60
J. Crassous
the longer wavelength region with very similar shapes for both neutral and acidic
forms. These observations imply that the electronic transitions have similar features
in the ground state. Under both conditions, helicene 17 also shows strong CPL
activity with a g lum value under neutral conditions of 0.009 at 473 nm which is quite
large for a heptahelicene derivative. Upon addition of 200 equivalent amounts of
TFA, the g lum value remained high (0.008 at 514 nm). Overall, these systems
combine both high g lum value with high quantum yields.
4.2.5 Azahelicenes with Fused Carbazole Cycles
(Pyrrolohelicenes)
Pyrrole-incorporating PHAs have been shown to possess remarkable physical
properties such as effective hole-transporting ability and bright emission. In
2012, Hiroto et al. reported pyrrole-fused system 18 displaying an aza[5]helicenic
structure with a stable helical conformation, thanks to the presence of bulky
ethynylsilylated groups (Fig. 4.7) [24]. Good fluorescence quantum yields were
obtained for 18 (Φ F ¼ 0.36) with a Stokes shift of 2220 cm
À1 reflecting the distorted
conformation of 18; a CPL anisotropy factor |g lum | of 3 Â 10
À3 was measured
for both enantiomers. The corresponding bis-butadiyne bridged azahelicene dimer
(M,M)- and (P,P)-20 with a figure-eight shape was prepared and exhibited
red-shifted absorption and emission, with a higher fluorescence quantum yield
(Φ F ¼ 0.55) and higher g abs and g lum values as compared to (M )- and (P)-18
(with opposite signs, see Table 4.2). This enhancement was attributed to the rigid
conformation of the dimer [25]. See Sect. 4.3 for the corresponding CPL-active
oxygen-containing helicene derivative 19.
Fig. 4.6 Chemical structure, X-ray structure, ECD, and CPL spectra of neutral and acidic form of
17 enantiomers. Adapted with permission [19]. Copyright 2017, Wiley
60
J. Crassous