hydrogen bonds are formed to stabilize the stacked assembly of the π-conjugated
molecules.
Various helical stacked assemblies via hydrogen-bonding interactions have been
reported, and some of them exhibited CPL activity. Ajayaghosh and coworkers
reported the self-assembly behavior as well as optical and chiroptical properties of
25, an oligo-p-phenylene-ethynylene derivative possessing an amide moiety and
chiral side-chains (Fig. 9.10) [37]. The molecule mostly existed as a monomer in
tetrahydrofuran (THF) solution (1 Â 10
À5 mol L
À1 ), whereas 25 formed stacked
assemblies in methylcyclohexane (MCH) solution (1 Â 10
À5 mol L
À1 ). 25 is
equipped with an azobenzene moiety that exhibits photo-induced isomerization. In
fact, photoisomerization from the (E,E)-isomer to (Z,E)- and (Z,Z )-isomers was
observed with the irradiation of ultraviolet (UV) light. The formation of a helical
assembly was confirmed by using CD spectroscopy. The assembly of the (E,E)isomer of (S)-25 exhibited a positive CD signal at 464 nm with two negative CD
signals at 407 nm and 322 nm, corresponding to the π ! πà transition of the oligo-pphenylene-ethynylene moiety. Interestingly, the CD signals inverted in the CD
spectrum of the assembly of the (Z,Z )-isomer of (S)-25. This suggests that the helical
sense of the assembly inverted during the photoisomerization of 25. The helical
assembly of the (E,E)-isomer of (S)-25 exhibited CPL on its emission at 503 nm with
a g lum of 8 Â 10
À3 . The photoisomerization of (S)-25 inverted the CPL as in the case
of the CD. The helical assembly of the (Z,Z)-isomer displayed a negative CPL signal
with a g lum of À2 Â 10
À3 . (R)-25 gives a mirror-image CD and CPL of (S)-25.
Recently, Takeda, Akutagawa, and coworkers reported a CPL-active assembly of
a pyrene derivative 26 (Fig. 9.11) [38]. They synthesized a pyrene possessing four
amide moieties, and a chiral side-chain was introduced onto the amides. The simple
molecule formed a helical assembly in chloroform, THF, and MCH through the
one-dimensional hydrogen-bonding interactions of the amide moieties, which
was confirmed by UV-vis and CD spectroscopy techniques. The helical assembly
exhibited CPL on its excimer emission band at 500 nm in chloroform, THF, and
MCH (1.0 Â 10
À4 mol L
À1 ). The g lum value of the assembly in MCH (3.0 Â 10
À2 )
was one order of magnitude higher than those in chloroform and THF (ca. 2 Â 10
À3 ).
Furthermore, the CPL signal of the helical assembly in chloroform was inverted
HN
NH
O
O
N
N
N
N
OR
RO
OC 12 H 25
C 12 H 25 O
OC 12 H 25
C 12 H 25 O
Vis
UV
(E,E)-25
(S): R=
(R): R=
HN
NH
O
O
N
N
N
N
OR
RO
OC 12 H 25
C 12 H 25 O
OC 12 H 25
C 12 H 25 O
(Z,Z)-25
Fig. 9.10 CPL-active helical assembly of a photoresponsive oligo-p-phenylene-ethynylene
derivative
9 Circularly Polarized Luminescence of Chirally Arranged Achiral Organic. . .
207
molecules.
Various helical stacked assemblies via hydrogen-bonding interactions have been
reported, and some of them exhibited CPL activity. Ajayaghosh and coworkers
reported the self-assembly behavior as well as optical and chiroptical properties of
25, an oligo-p-phenylene-ethynylene derivative possessing an amide moiety and
chiral side-chains (Fig. 9.10) [37]. The molecule mostly existed as a monomer in
tetrahydrofuran (THF) solution (1 Â 10
À5 mol L
À1 ), whereas 25 formed stacked
assemblies in methylcyclohexane (MCH) solution (1 Â 10
À5 mol L
À1 ). 25 is
equipped with an azobenzene moiety that exhibits photo-induced isomerization. In
fact, photoisomerization from the (E,E)-isomer to (Z,E)- and (Z,Z )-isomers was
observed with the irradiation of ultraviolet (UV) light. The formation of a helical
assembly was confirmed by using CD spectroscopy. The assembly of the (E,E)isomer of (S)-25 exhibited a positive CD signal at 464 nm with two negative CD
signals at 407 nm and 322 nm, corresponding to the π ! πà transition of the oligo-pphenylene-ethynylene moiety. Interestingly, the CD signals inverted in the CD
spectrum of the assembly of the (Z,Z )-isomer of (S)-25. This suggests that the helical
sense of the assembly inverted during the photoisomerization of 25. The helical
assembly of the (E,E)-isomer of (S)-25 exhibited CPL on its emission at 503 nm with
a g lum of 8 Â 10
À3 . The photoisomerization of (S)-25 inverted the CPL as in the case
of the CD. The helical assembly of the (Z,Z)-isomer displayed a negative CPL signal
with a g lum of À2 Â 10
À3 . (R)-25 gives a mirror-image CD and CPL of (S)-25.
Recently, Takeda, Akutagawa, and coworkers reported a CPL-active assembly of
a pyrene derivative 26 (Fig. 9.11) [38]. They synthesized a pyrene possessing four
amide moieties, and a chiral side-chain was introduced onto the amides. The simple
molecule formed a helical assembly in chloroform, THF, and MCH through the
one-dimensional hydrogen-bonding interactions of the amide moieties, which
was confirmed by UV-vis and CD spectroscopy techniques. The helical assembly
exhibited CPL on its excimer emission band at 500 nm in chloroform, THF, and
MCH (1.0 Â 10
À4 mol L
À1 ). The g lum value of the assembly in MCH (3.0 Â 10
À2 )
was one order of magnitude higher than those in chloroform and THF (ca. 2 Â 10
À3 ).
Furthermore, the CPL signal of the helical assembly in chloroform was inverted
HN
NH
O
O
N
N
N
N
OR
RO
OC 12 H 25
C 12 H 25 O
OC 12 H 25
C 12 H 25 O
Vis
UV
(E,E)-25
(S): R=
(R): R=
HN
NH
O
O
N
N
N
N
OR
RO
OC 12 H 25
C 12 H 25 O
OC 12 H 25
C 12 H 25 O
(Z,Z)-25
Fig. 9.10 CPL-active helical assembly of a photoresponsive oligo-p-phenylene-ethynylene
derivative
9 Circularly Polarized Luminescence of Chirally Arranged Achiral Organic. . .
207