suggests that the aggregates adopt the same chiral geometry in the ground and
the excited states. However, the increased ordering of these aggregates also
resulted in a large degree of linear polarization (P ¼ 0.39) which can greatly affect
the CPL measurement [38].
In 2016, Yamamoto et al. reported the synthesis of tetrasulfone[9]helicene 43 via
the oxidation of tetrathia[9]helicene 42 [39]. Remarkably, it was found that the
quantum yield of fluorescence for sulfone[9]helicene 43 (Φ F ¼ 0.27) was ten times
higher than tetrathia[9]helicene precursor 42 (Φ F ¼ 0.03). The author explain this
strong enhancement by a significant increase of the energy gap between the lowest
singlet (S 1 ) and the triplet (T 1 ) excited states ΔE ST in the case of 43 (ΔE ST ¼ 1.02 eV)
compared to (ΔE ST ¼ 0.60 eV) for 42, which may efficiently decrease the intersystem
crossing (ISC) rate. Additionally, ECD spectra for 43 were recorded and an anisotropy
factor g abs ¼ À4.7 Â 10
À3 was measured for the (P) enantiomer. Likewise, plotting the
fluorescence CPL mirror-image spectra 43 enantiomers gave an estimated anisotropy
factor value of g lum ¼ À8.3 Â 10
À4 for the (P)-43 (Fig. 4.14).
A series of fluorescent “push-pull” tetrathia[9]helicenes based on quinoxaline
(acceptor) fused with tetrathia[9]helicene (donor) derivatives was synthesized
for control of the excited-state dynamics and circularly polarized luminescence
(CPL) properties [40]. Introduction of a quinoxaline onto the tetrathia[9]helicene
skeleton induced a “push-pull” character, which was enhanced by further introduction
of electron-releasing or electron-withdrawing groups onto the quinoxaline unit
(Fig. 4.15). Significant enhancement in the fluorescence quantum yields (Φ F ) was,
for instance, obtained for 44: (Φ F ¼ 0.30, Table 4.5), which is more than 20 times
S
S
S
S
n Pr
n Pr
42
(P)-43
F = 0.03
1) Sulfonation
F = 0.27
|g lum | = 8.3 x 10 -4
2) HPLC
separation
S
S
S
S
n Pr
n Pr
O
O
O
O
O O
O
O
Fig. 4.14 Chemical
structures of 42 and 43 and
their emission data
(P)-45
(P)-44
S
S
Pt
N
N
S
S
S
S
N
N
NC
NC
Fig. 4.15 Chemical
structures of push-pull
systems with improved
luminescence and CPL
emission and of a [6]
helicene Pt(diimine)
(dithiolene) complex
4 Circularly Polarized Luminescence in Helicene and Helicenoid Derivatives
69
the excited states. However, the increased ordering of these aggregates also
resulted in a large degree of linear polarization (P ¼ 0.39) which can greatly affect
the CPL measurement [38].
In 2016, Yamamoto et al. reported the synthesis of tetrasulfone[9]helicene 43 via
the oxidation of tetrathia[9]helicene 42 [39]. Remarkably, it was found that the
quantum yield of fluorescence for sulfone[9]helicene 43 (Φ F ¼ 0.27) was ten times
higher than tetrathia[9]helicene precursor 42 (Φ F ¼ 0.03). The author explain this
strong enhancement by a significant increase of the energy gap between the lowest
singlet (S 1 ) and the triplet (T 1 ) excited states ΔE ST in the case of 43 (ΔE ST ¼ 1.02 eV)
compared to (ΔE ST ¼ 0.60 eV) for 42, which may efficiently decrease the intersystem
crossing (ISC) rate. Additionally, ECD spectra for 43 were recorded and an anisotropy
factor g abs ¼ À4.7 Â 10
À3 was measured for the (P) enantiomer. Likewise, plotting the
fluorescence CPL mirror-image spectra 43 enantiomers gave an estimated anisotropy
factor value of g lum ¼ À8.3 Â 10
À4 for the (P)-43 (Fig. 4.14).
A series of fluorescent “push-pull” tetrathia[9]helicenes based on quinoxaline
(acceptor) fused with tetrathia[9]helicene (donor) derivatives was synthesized
for control of the excited-state dynamics and circularly polarized luminescence
(CPL) properties [40]. Introduction of a quinoxaline onto the tetrathia[9]helicene
skeleton induced a “push-pull” character, which was enhanced by further introduction
of electron-releasing or electron-withdrawing groups onto the quinoxaline unit
(Fig. 4.15). Significant enhancement in the fluorescence quantum yields (Φ F ) was,
for instance, obtained for 44: (Φ F ¼ 0.30, Table 4.5), which is more than 20 times
S
S
S
S
n Pr
n Pr
42
(P)-43
F = 0.03
1) Sulfonation
F = 0.27
|g lum | = 8.3 x 10 -4
2) HPLC
separation
S
S
S
S
n Pr
n Pr
O
O
O
O
O O
O
O
Fig. 4.14 Chemical
structures of 42 and 43 and
their emission data
(P)-45
(P)-44
S
S
Pt
N
N
S
S
S
S
N
N
NC
NC
Fig. 4.15 Chemical
structures of push-pull
systems with improved
luminescence and CPL
emission and of a [6]
helicene Pt(diimine)
(dithiolene) complex
4 Circularly Polarized Luminescence in Helicene and Helicenoid Derivatives
69