and ECD spectra seemed hardly sensitive to the formation of such aggregates, CPL
measurements of 34 afforded g lum of À0.035 for the (M) enantiomer in chloroform
solutions, which was larger than in methanol solutions (À0.021). Similar systems,
namely, [7]oxahelicene 35 (71% ee) and [9]oxahelicene derivative 36 (88% ee) were
reported in 2017 [33]. Comparison of the photophysical data of these 9-oxahelicene
36 compared to 35 shows a redshift of both the absorption and luminescence spectra
by ~20 to 50 nm, along with a decrease in fluorescence quantum yield (0.23–0.18).
ECD and CPL spectra followed the same trend as for unpolarized UV-vis and
fluorescence measurements, but with more intense ECD and CPL signals for 36.
Here the g lum values around 10
À3 in chloroform are classical for helicenic solutions
HCl (1M)
MeO
N
N
nPr
nPr
+
OMe BF 4
-
(P)-29
CO 2
-
MeO
N
N
nPr
nPr
+
OMe BF 4
-
(P)-30
CO 2 H
NaOH (1M)
O
N
nPr
+
BF 4
-
(P)-32
O
O
+
BF 4
-
(P)-31
N
N
nPr
nPr
+
BF 4
-
(P)-33
Fig. 4.10 Chemical structures of carbocationic diaza[4]helicenes 29–30 (pH-triggered switch) and
O,N-containing carbocationic [6]helicenes [30, 31]
Table 4.3 Photophysical data of carbocationic azahelicenes
Compound
λ Abs
max a
(nm)
λ Em
(nm)
Φ F (%)
Solvent
(CPL)
10
3
g abs
10
3
g lum
Ref.
(P)-(+)-25 616
667
13
(CH 3 CN)
CH 2 Cl 2
+0.15
+1.3
[29]
(P)-(+)-26 575
624
35
(CH 3 CN)
CH 2 Cl 2
+0.45
+1.6
[29]
(P)-(+)-27 582
640
37
(CH 3 CN)
CH 2 Cl 2
+0.36
+0.9
[29]
(P)-(+)-29 626
709
1
CH 3 CN
~0.4
0
[30]
(P)-(+)-30 590
654
29
CH 3 CN
0.4
0.5
[30]
(P)-(+)-31 562
595
44.5
CH 2 Cl 2
À0.86
b
À0.32
[31]
(P)-(+)-32 562
614
28
CH 2 Cl 2
À1.3
b
À2.1
[31]
(P)-(+)-33 614
658
28.4
CH 2 Cl 2
À0.57
b
À1.2
[31]
a Lowest-energy absorption band
b
Taken from [12]
4 Circularly Polarized Luminescence in Helicene and Helicenoid Derivatives
65