CL-140163
Received: February 27, 2014 | Accepted: March 25, 2014 | Web Released: March 29, 2014
Electrochiroptical Response in Aqueous Media: 9,10-Dihydrophenanthrene-9,10-diyl Dications
with Michlar’s Hydrol Blue Chromophores Attached with Oligoethylene Glycol Units
Takanori Suzuki,*1 Keisuke Hanada,1 Ryo Katoono,1 Yusuke Ishigaki,1 Sho Higasa,2 Hiroki Higuchi,3
Hirotsugu Kikuchi,3 Kenshu Fujiwara,1 Hidetoshi Yamada,2 and Takanori Fukushima4
1Department of Chemistry, Faculty of Science, Hokkaido University, Sapporo, Hokkaido 060-0810
2School of Science and Technology, Kwansei Gakuin University, Gakuen, Sanda, Hyogo 669-1337
3Institute for Materials Chemistry and Engineering, Kyushu University, Kasuga, Fukuoka 816-8580
4Chemical Resources Laboratory, Tokyo Institute of Technology, Yokohama, Kanagawa 226-8503
(E-mail: tak@mail.sci.hokudai.ac.jp)
Upon electrochemical oxidation of configurationally stable
biphenyl-based electron donors (Rax)-1a and -1b to butane-1,4-
diyl dications (M)-2a2+ and -2b2+, drastic changes were induced
not only in UV-vis but also in CD spectra. Because of the
hydrophilic oligoethylene glycol units on the amino nitrogens,
(Rax)-1b and (M)-2b2+ are soluble in water, thus realizing the
electrochiroptical behavior not only in organic solvents but also
in aqueous media.
C-C bond formation-breaking is accompanied by electron
transfer (“dynamic redox behavior”).5
Another interesting feature of this redox pair is the drastic
change in helical geometry (e.g., the twisting angle of the
biphenyl unit) upon electron transfer. Hence, the pair might be
used as a redox-switchable dopant for the chiral nematic phase
of lyotropic liquid crystals, which form mesophases by mixing
amphipathic molecules with water. To employ the rac-3a/rac-
4a2+ pair for this purpose, there are two major issues to be
concerned: 1) good solubility of the redox pair in aqueous media
and 2) configurational stability of the redox pair for intercon-
version, without loss of enantiomeric purity.
Butane-1,4-diyl dications belong to an interesting class of
carbocations with high reactivity. In general, they are short-lived
due to the rapid deprotonation to give the corresponding 1,3-
dienes.1 During the course of our continuing studies2 on novel
electrochromic systems3 based on organic cationic dyes, attach-
ment of strong electron-donating aryl groups at the 1,4-positions
was shown to stabilize these intriguing dications by electronic
effects.4 For example, we previously found that 9,10-dihydro-
phenanthrene-type dication rac-4a2+ with two MHB (Michler’s
hydrol blue) chromophores was generated upon the two-electron
oxidation of 2,2¤-bis[2,2-bis(4-dimethylaminophenyl)ethenyl]bi-
phenyl (rac-3a) (Scheme 1a). The dication was isolated as a
stable salt, from which rac-3a was regenerated upon reduction.4b
The high-yield interconversion indicates that rac-3a and rac-
4a2+ can be considered as a “reversible” redox pair, although
Regarding the first point, we envisaged that attachment of
hydrophilic substituents (e.g., 2-{2-[2-(2-methoxyethoxy)eth-
oxy]ethoxy}ethyl group, TEG) to the amino nitrogens would
make it possible to conduct an electrochromic response not only
in organic solvents but also in aqueous media. Thus, we
designed the rac-3b/rac-4b2+ pair as a promising candidate for
less well-developed electrochromic systems working in aqueous
media (Scheme 1a). The MHB-type chromophore in rac-4b2+
is expected to be less electrophilic and would remain intact in
neutral water (pKa+ of MHB: 5.61).6 The second point concerns
the common requirements in developing electrochiroptical
response systems,4b,4c,7 which exhibit dual electrochromic
behavior of UV-vis and circular dichroism (CD). Thus, we
designed a redox pair attached with benzyloxy groups on the
biphenyl skeleton by assuming high configurational stability
of the axial chirality in (Rax)-1a and helicity in (M)-2a2+
(Scheme 1b). By combining the two strategies for modification
shown above, we then designed the configurationally stable
redox pair with TEG units, (Rax)-1b/(M)-2b2+. By using this
pair, we successfully demonstrated an electrochiroptical re-
sponse in aqueous media for the first time, as detailed below.
Diarylketone 5b8 with four TEG units was prepared from
the benzophenone derivative with two diethanolamine moieties9
in 96% yield. This ketone was converted to the corresponding
Horner-Emmons reagent 6b8 in 61% yield. By using 6b,
optically pure dial (Rax)-710 was converted to chiral donor (Rax)-
1b8 in 75% yield (Scheme 2a). When (4-Me2NC6H4)2CHPO-
(OEt)2 6a11 was used, (Rax)-1a8 was similarly obtained in 41%
yield. All these hexabenzyloxy derivatives were configura-
tionally stable, and there was no sign of racemization upon
transformation or standing. Configurationally unstable rac-3b8
was more conveniently prepared, as in the case of rac-3a,4b by
the reaction of ketone 5b with 2,2¤-bis(lithiomethyl)biphenyl
generated from 2,2¤-bitolyl and BuLi/TMEDA, followed by
dehydration under acidic conditions, in two-step yield of 52%
a)
Ar Ar
Ar Ar
2e–
2e–
Ar
H
Ar
H
Ar
Ar
rac-4a,b2+
3a,b
rac-
b)
Ar Ar
Ar Ar
2e–
2e–
Ar
Ar
Ar
Ar
H
H
BnO
OBn
BnO
OBn
BnO BnO OBn OBn
BnO BnO OBn OBn
2a,b2+
1a,b
(Rax)-
(M)-
NMe2
b: Ar =
a: Ar =
N[(C2H4O)4CH3]2
Scheme 1. Butane-1,4-diyl dication-based dynamic redox
pairs.
© 2014 The Chemical Society of Japan