the S-shaped double azahelicenes was significantly higher than that of the single
azahelicenes. Indeed, CPL measurements showed intensities for azahelicenes 12 and
13 were below their measurable limit (g lum < 0.001), whereas double azahelicenes
15 and 16 exhibited strong CPL activities, with g lum ¼ 0.028 at 492 nm for (+)-15
and g lum ¼ +0.011 at 454 nm for (+)-16 in CHCl 3 [17]. In 2016, (À) and (+)-aza[10]
helicenes 14 were found to display |g abs | ¼ 4.5 Â 10
À3 at 303 nm which correspond
to a smaller value than for S-shaped 16 (|g abs | ¼ 6.5 Â 10
À3 at 331 nm) but no CPL
activity could be measured for 14 [18].
To explain the enhancement of CPL in S-shaped double helicenes, Mori et al.
proposed in 2018 a protocol for rationally aligning multiple chiral units to boost the
chiroptical responses, using hexahelicene 10 as a prototype [23]. To do so, they
aligned two hexahelicenes in various orientations and examined by theoretical
calculations which orientation resulted in the highest chiroptical performance from
X-shaped or S-shaped double hexahelicenes (see Sect. 4.8.2).
4.2.4 Polyaza[7]helicenes
In 2017, Shibata et al. reported the synthesis of enantiopure polyaza[7]helicenes
such as 17 (Fig. 4.6) possessing a 6-5-6-6-6-5-6 skeleton [19] which showed high
fluorescence quantum yields under both neutral (Φ F ¼ 0.39) and acidic conditions
N
N
N
Cl
Cl
(P)-14
(92% ee)
OMe
MeO
N
O
MeO
OMe
N
Cl
MeO
(M)-12
(73% ee)
(M)-13
(74% ee)
N
Cl
MeO
N
Cl
OMe
(P,P )-16
(>99% ee)
N
O
MeO
N
O
OMe
(P,P )-15
(>99% ee)
R 1
R 1
R 1 = CH 2 C 6 H 4 -4-OC 10 H 21
Fig. 4.5 Chemical structures of enantioenriched single aza[6]helicenes 12–14 and S-shaped double
aza[6]helicenes 15,16 [17, 18]
4 Circularly Polarized Luminescence in Helicene and Helicenoid Derivatives
59