emissions were recorded between 650 and 700 nm and were characterized by a g lum
of ~10
À4 to 10
À3 . Such cationic chiral dyes were also used as pH-triggered ECD
and CPL chiroptical switches when they possessed pH-sensitive group such as
carboxylic acid. For instance, zwitterionic [4]helicene 29 was reported in 2016 as
a reversible pH-triggered ECD/CPL chiroptical switch (Fig. 4.10) [30]. Protonated
30 displayed g lum of around 5 Â 10
À4 and of similar order as g abs (4 Â 10
À4 ), while
the carboxylate derivative 29 displayed no CPL, probably due to very low emission.
Overall it represented an on-off CPL switch. Similarly, longer O-containing and
N-containing [6]helicenium derivatives 31–33 displayed CPL signals (with opposite
signs compared to their optical rotation values) with |g lum | between 0.32 Â 10
À3
and 2.1 Â 10
À3 in the infrared region [31].
4.3 CPL-Active Oxygen-Containing Helicene Derivatives
In 2011, Tanaka et al. reported a phthalhydrazide-functionalized [7]oxahelicene
derivative 34 (Fig. 4.11), displaying a strong increase of g lum , i.e., one order of
magnitude, as compared to other helicenic derivatives [32]. This strong CPL
enhancement was attributed to the presence of multiple-hydrogen-bonding sites
enabling the formation of a trimeric structure which further organizes into chiral
fibers (Fig. 4.12). These chiral fibers were 200 nm wide and 3–4 μm long in
chloroform solutions, as characterized by SEM and AFM images. While UV-vis
Fig. 4.8 (a) Chemical structure of 24a-e ((P)-(À) enantiomers); (b) emission color panel of 24a-e.
(c) ECD spectra in THF of pure enantiomers. (d) CPL spectra in THF of pure enantiomers. Adapted
with permission [28]. Copyright 2016, Royal Society of Chemistry
4 Circularly Polarized Luminescence in Helicene and Helicenoid Derivatives
63
of ~10
À4 to 10
À3 . Such cationic chiral dyes were also used as pH-triggered ECD
and CPL chiroptical switches when they possessed pH-sensitive group such as
carboxylic acid. For instance, zwitterionic [4]helicene 29 was reported in 2016 as
a reversible pH-triggered ECD/CPL chiroptical switch (Fig. 4.10) [30]. Protonated
30 displayed g lum of around 5 Â 10
À4 and of similar order as g abs (4 Â 10
À4 ), while
the carboxylate derivative 29 displayed no CPL, probably due to very low emission.
Overall it represented an on-off CPL switch. Similarly, longer O-containing and
N-containing [6]helicenium derivatives 31–33 displayed CPL signals (with opposite
signs compared to their optical rotation values) with |g lum | between 0.32 Â 10
À3
and 2.1 Â 10
À3 in the infrared region [31].
4.3 CPL-Active Oxygen-Containing Helicene Derivatives
In 2011, Tanaka et al. reported a phthalhydrazide-functionalized [7]oxahelicene
derivative 34 (Fig. 4.11), displaying a strong increase of g lum , i.e., one order of
magnitude, as compared to other helicenic derivatives [32]. This strong CPL
enhancement was attributed to the presence of multiple-hydrogen-bonding sites
enabling the formation of a trimeric structure which further organizes into chiral
fibers (Fig. 4.12). These chiral fibers were 200 nm wide and 3–4 μm long in
chloroform solutions, as characterized by SEM and AFM images. While UV-vis
Fig. 4.8 (a) Chemical structure of 24a-e ((P)-(À) enantiomers); (b) emission color panel of 24a-e.
(c) ECD spectra in THF of pure enantiomers. (d) CPL spectra in THF of pure enantiomers. Adapted
with permission [28]. Copyright 2016, Royal Society of Chemistry
4 Circularly Polarized Luminescence in Helicene and Helicenoid Derivatives
63