The first observation of CPL of a nonaggregated helicene was reported by
Venkataraman and coworkers in 2003. They synthesized a racemic pair of
a helicene, and then, the helicene was reacted with (1S)-camphanate to give a
corresponding diastereomeric pair of the helicene ((1S,M )- and (1S,P)-2, Fig. 9.3)
[13]. Both of the diastereomers exhibited CPL on the monomeric fluorescence
with a g lum of ca. 1 Â 10
À3 . Interestingly, the CPL spectrum of (1S,M )-2 was
a mirror-image of that of (1S,P)-2 in spite of a diastereomer, indicating that
the (1S)-camphanate attached on the helicene core does not perturb the CPL of
the helicene. They also synthesized the more extended [5]helicene-like compounds
(1S,M)- and (1S,P)-3 that display CPL too.
After the pioneering work by Venkataraman and coworkers, various chiral
helicenes exhibiting CPL have been investigated. Tanaka and coworkers have
developed several [7]helicene-like compounds displaying CPL since 2012 (Fig. 9.3)
[14, 15]. For example, (M)-4 and (M)-5 exhibited CPL on its monomeric fluorescence
in CHCl 3 with a g lum of ca. 3 Â 10
À2
, which is a high value for an organic compound
[15]. Tanaka and coworkers have also developed the CPL of aza[6]helicene-like
compounds [14]. They synthesized aza[6]helicene (M)-6 and S-shaped double
aza[6]helicene-like compound (M,M)-7. Interestingly, the CPL of (M,M)-7
(g lum ¼ 1.1 Â 10
À2
) was enhanced compared to that of (M)-6 (g lum < 1 Â 10
À3
).
Shinokubo and coworkers reported the CPL of aza[7]helicenes (M)- and (P)-8 in
2012 (Fig. 9.3) [16]. They exhibited CPL with a g lum of 3 Â 10
À3 in CH 2 Cl 2 .
Nozaki and coworkers have developed CPL-active sila[7]helicenes (M)- and (P)-9
that exhibited CPL with a g lum of 3.5 Â 10
À3 in CH 2 Cl 2 (Fig. 9.3) [17].
Takeuchi and coworkers have reported an interesting CPL behavior of a
phthalhydrazide-functionalized [7]helicene-like compound 10 (Fig. 9.4) [18]. 10
formed a trimeric disk in chloroform via the hydrogen-bonding interaction of
a phthalhydrazide moiety. Further aggregation of the trimeric disk resulted in
the formation of a one-dimensional fiber-like assembly. The CPL property of
10 was found in chloroform, but not in methanol, which suggests that the
trimeric disk was dissociated by breaking the hydrogen bonds. The g lum values of
(M)-10 were estimated to be À3.5 Â 10
–2 in chloroform and À2.1 Â 10
–2 in
methanol (4 Â 10
–4 mol L
–1 ), respectively. The formation of the trimeric disk
and one-dimensional fiber-like assembly may increase the g lum value of (M)-10
in chloroform compared to that in methanol.
Thus, various CPL-active helicene-like compounds have been reported.
However, the fluorescence quantum yields of CPL-active helicenes are moderate
(32% for 4, 36% for 8, and 23% for 9, respectively). The distortion of the π-plane
may decrease the effective fluorescence of planar π-conjugated luminophores.
200
T. Ikeda and T. Haino
Venkataraman and coworkers in 2003. They synthesized a racemic pair of
a helicene, and then, the helicene was reacted with (1S)-camphanate to give a
corresponding diastereomeric pair of the helicene ((1S,M )- and (1S,P)-2, Fig. 9.3)
[13]. Both of the diastereomers exhibited CPL on the monomeric fluorescence
with a g lum of ca. 1 Â 10
À3 . Interestingly, the CPL spectrum of (1S,M )-2 was
a mirror-image of that of (1S,P)-2 in spite of a diastereomer, indicating that
the (1S)-camphanate attached on the helicene core does not perturb the CPL of
the helicene. They also synthesized the more extended [5]helicene-like compounds
(1S,M)- and (1S,P)-3 that display CPL too.
After the pioneering work by Venkataraman and coworkers, various chiral
helicenes exhibiting CPL have been investigated. Tanaka and coworkers have
developed several [7]helicene-like compounds displaying CPL since 2012 (Fig. 9.3)
[14, 15]. For example, (M)-4 and (M)-5 exhibited CPL on its monomeric fluorescence
in CHCl 3 with a g lum of ca. 3 Â 10
À2
, which is a high value for an organic compound
[15]. Tanaka and coworkers have also developed the CPL of aza[6]helicene-like
compounds [14]. They synthesized aza[6]helicene (M)-6 and S-shaped double
aza[6]helicene-like compound (M,M)-7. Interestingly, the CPL of (M,M)-7
(g lum ¼ 1.1 Â 10
À2
) was enhanced compared to that of (M)-6 (g lum < 1 Â 10
À3
).
Shinokubo and coworkers reported the CPL of aza[7]helicenes (M)- and (P)-8 in
2012 (Fig. 9.3) [16]. They exhibited CPL with a g lum of 3 Â 10
À3 in CH 2 Cl 2 .
Nozaki and coworkers have developed CPL-active sila[7]helicenes (M)- and (P)-9
that exhibited CPL with a g lum of 3.5 Â 10
À3 in CH 2 Cl 2 (Fig. 9.3) [17].
Takeuchi and coworkers have reported an interesting CPL behavior of a
phthalhydrazide-functionalized [7]helicene-like compound 10 (Fig. 9.4) [18]. 10
formed a trimeric disk in chloroform via the hydrogen-bonding interaction of
a phthalhydrazide moiety. Further aggregation of the trimeric disk resulted in
the formation of a one-dimensional fiber-like assembly. The CPL property of
10 was found in chloroform, but not in methanol, which suggests that the
trimeric disk was dissociated by breaking the hydrogen bonds. The g lum values of
(M)-10 were estimated to be À3.5 Â 10
–2 in chloroform and À2.1 Â 10
–2 in
methanol (4 Â 10
–4 mol L
–1 ), respectively. The formation of the trimeric disk
and one-dimensional fiber-like assembly may increase the g lum value of (M)-10
in chloroform compared to that in methanol.
Thus, various CPL-active helicene-like compounds have been reported.
However, the fluorescence quantum yields of CPL-active helicenes are moderate
(32% for 4, 36% for 8, and 23% for 9, respectively). The distortion of the π-plane
may decrease the effective fluorescence of planar π-conjugated luminophores.
200
T. Ikeda and T. Haino