Planar-chiral through-space conjugated oligomers: Synthesis and characterization of chiroptical properties

Article abstract of DOI:10.1002/chem.201402930

Optically active through-space conjugated oligomers, namely, a dimer, trimer, tetramer, and cyclic trimer, consisting of a planar-chiral [2.2]paracyclophane skeleton were synthesized. In the ground state, observed similarities in the chiroptical properties of the oligomers were attributed to the equivalent orientations of two adjacent chromophores. In the excited state, the oligomers were folded into a form analogous to a one-handed helix by photoexcitation. All the compounds in dispersed solution exhibited intense circularly polarized luminescence with relatively large anisotropy factors on the order of 10^-3.

Full text of DOI:10.1002/chem.201402930

DOI: 10.1002/chem.201402930
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&
Chirality
Planar-Chiral Through-Space Conjugated Oligomers:
Synthesis and Characterization of Chiroptical Properties
Yasuhiro Morisaki,* Kenichi Inoshita, and Yoshiki Chujo*[a]
Abstract: Optically active through-space conjugated oligo-
mers, namely, a dimer, trimer, tetramer, and cyclic trimer,
consisting of a planar-chiral [2.2]paracyclophane skeleton
were synthesized. In the ground state, observed similarities
in the chiroptical properties of the oligomers were attributed
to the equivalent orientations of two adjacent chromo-
phores. In the excited state, the oligomers were folded into
a form analogous to a one-handed helix by photoexcitation.
All the compounds in dispersed solution exhibited intense
circularly polarized luminescence with relatively large aniso-
tropy factors on the order of 10À3
.
Introduction
systems are partly stacked (Figure 1). Important findings on
the mechanism for the intense CPL were obtained in this
study.
Planar chirality is one of the distinguishing physical characteris-
tics of [2.2]paracyclophane derivatives and arises from the
proximally fixed face-to-face phenylene moieties.[1] Unlike the
planar chirality of metal complexes such as ferrocene and (h6-
arene)chromium complexes, the planar chirality of [2.2]paracy-
clophane compounds is conformationally stable due to the
suppression of axial rotation. Various methods for the optical
resolution of planar chiral [2.2]paracyclophanes have been de-
veloped so far to afford the corresponding enantiopure deriva-
tives,[2–4] thus leading to their primary use as chiral auxiliaries
in the field of organic chemistry. Considering their conforma-
tional stability, planar-chiral [2.2]paracyclophanes can be used
as chiral scaffolds to construct a variety of optically active
structures. From this viewpoint, we have recently developed
a practical method for the optical resolution of disubstituted
[2.2]paracyclophane compounds[3i] and synthesized optically
active through-space conjugated polymers[3j] in contribution to
the fields of polymer and materials chemistry.[1c] The resulting
optically active polymers exhibited circularly polarized lumines-
cence (CPL) with large dissymmetry factors (glum), even in
dilute solutions. To discuss the chiroptical properties in detail,
it is desirable to understand the properties of well-defined op-
tically active through-space conjugated oligomers rather than
polymers. Thus, we report herein detailed studies on the syn-
theses and properties of optically active through-space conju-
gated [2.2]paracyclophane oligomers, in which 2–4 p-electron
[a] Dr. Y. Morisaki, K. Inoshita, Prof. Dr. Y. Chujo
Department of Polymer Chemistry
Graduate School of Engineering, Kyoto University
Katsura, Nishikyo-ku, Kyoto 615-8510 (Japan)
Fax: (+81)75-383-2607
E-mail: ymo@chujo.synchem.kyoto-u.ac.jp
chujo@chujo.synchem.kyoto-u.ac.jp
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201402930.
Figure 1. Compounds prepared in this study. The Sp isomers are shown.
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sults of the UV spectroscopic analysis (see Figure S17 in the
Supporting Information). The uniformity of the phases and
overlap of the orbitals between the face-to-face aromatic rings
of the [2.2]paracyclophane moiety decreased the energy band
gap of the pseudo-para-dimer relative to the pseudo-ortho-
dimer.
Results and Discussion
Optical and chiroptical properties in the ground state
Figure 1 shows monomeric compound 1 and Sp enantiomers
(Sp)-2–4 prepared by using the Sonogashira–Hagihara coupling
(see Schemes S1–S3 in the Supporting Information).[5] Cyclic
compound (Sp)-C3 was obtained as a byproduct of polymeriza-
tion[3j] (see the Supporting Information for the synthetic proce-
dures and spectral data).
The CD spectra of (Sp)-2–4 and (Rp)-2–4 exhibited mirror
image Cotton effects in the p–p* transition region of the p-
electron systems with relatively large molar ellipticity ([q])
values on the order of 105 (Figure 2A). Bisignate exciton-cou-
pled CD signals[8] were observed owing to the rigidly stacked
p-electron systems at an angle of 608. The [q] values became
larger with the increase in the number of the stacked p-elec-
tron systems.
The absorption (UV) spectra and circular dichroism (CD)
spectra of (Sp)-2–4 and (Rp)-2–4 were measured in dilute CHCl3
(1.0ꢁ10À5 m) and are shown in Figure 2A with the UV spec-
In contrast, their absorption anisotropy factors (gabs, defined
as gabs =De/e) were almost identical (Figure 2B). Orientations
of the electric transition moments of two adjacent interacting
p-electron systems were fixed in a V-shape by the planar chiral
[2.2]paracyclophane scaffold,[8,9] whereas higher-ordered struc-
tures of each compound were not formed through molecular
motions (discussed later in Figure 4); therefore, the chirality of
the whole molecule was intrinsically equivalent in the ground
state and resulted in the almost constant gabs values.
Figure 3 shows the UV spectra, CD spectra, and gabs plots of
the linear trimers (Sp)-3 and (Rp)-3 and the cyclic trimers (Sp)-C3
Figure 2. A) UV and CD spectra of 1, (Sp)-2–4, and (Rp)-2–4 in CHCl3
(1.0ꢁ10À5 m) and B) their gabs plots.
trum of 1. The obtained UV spectra were bathochromically
shifted as the number of the stacked p-electron systems in-
creased as a result of the through-space conjugation.[6,7] The
effect of the through-space conjugation of the pseudo-ortho-
oligomer was slightly smaller than that of the corresponding
pseudo-para-oligomer; for example, the absorption maxima of
2 and 3 were lmax =374.5 and 375.5 nm, respectively, whereas
those of the corresponding pseudo-para-dimer and trimer
were lmax =378 and 381 nm, respectively.[7] DFT calculations for
the pseudo-ortho- and pseudo-para-dimers supported the re-
Figure 3. A) UV and CD spectra of (Sp)-3, (Rp)-3, (Sp)-C3, and (Rp)-C3 in CHCl3
(1.0ꢁ10À5 m) and B) their gabs plots.
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and (Rp)-C3. The UV spectra of (Sp)-C3 and (Rp)-C3 were hypso-
chromically and hypochromically shifted relative to (Sp)-3 and
(Rp)-3 (Figure 3A). A twist in the didodecyloxyphenylene unit
of the stacked p-electron system in (Sp)-C3 and (Rp)-C3 sup-
pressed the extension of p conjugation due to steric hin-
drance. The Cotton effects exhibited by (Sp)-C3 and (Rp)-C3
were slightly larger than those of (Sp)-3 and (Rp)-3 (Figure 3A);
in addition, the gabs values of (Sp)-C3 and (Rp)-C3 were larger
than those of (Sp)-3 and (Rp)-3 (Figure 3B). In the cyclic trimer,
the orientation of the electric transition moments of two adja-
cent p-electron systems was the same as that of the linear
trimer (Figure 4). The chain ends of each stacked p-electron
system were completely fixed by the [2.2]paracyclophane to
construct an optically active higher-ordered structure. Thus,
noticeable changes in the [q] and gabs values were observed
between 3 and C3; particularly, a clear enhancement in the
gabs value was observed (Figure 3A,B).
Optical and chiroptical properties in the excited state
Photoluminescence (PL) spectra of 1, (Sp)-2–4, and (Rp)-2–4 in
dilute CHCl3 (1.0ꢁ10À6 m) are shown in Figure 5A. The PL spec-
tra of oligomers were also gradually red-shifted in accordance
with the number of stacked p-electron systems. Vibronic struc-
tures were observed in all the PL spectra, and their decay
curves were fitted with
a
single exponential relation.[10]
Pseudo-ortho-linked through-space conjugated oligomers
emitted from the chromophore state rather than from the
phane state,[6] a behavior similar to that of the pseudo-para-
linked compounds.[6,7]
Figure 4. Plausible structures of (Sp)-3 and (Sp)-C3 in the ground state.
CPL measurements for (Sp)-2–4 and (Rp)-2–4 in dilute CHCl3
(1.0ꢁ10À5 m) were carried out. Intense CPL signals of ILÀIR,
Figure 5. A) PL and CPL spectra of 1, (Sp)-2–4, and (Rp)-2–4 in CHCl3
(1.0ꢁ10À6 m, excitation at UV lmax for PL; 1.0ꢁ10À5 m, lex =320 nm for CPL).
FPL =PL absolute quantum efficiency (average of the Sp and Rp isomers).
B) The glum plots of (Sp)-2–4 and (Rp)-2–4.
Figure 6. A) PL and CPL spectra of (Sp)-3, (Rp)-3, (Sp)-C3, and (Rp)-C3 in CHCl3
(1.0ꢁ10À6 m, excitation at UV lmax for PL; 1.0ꢁ10À5 m, lex =320 nm for CPL).
FPL =absolute PL quantum efficiency. B) The glum plots of (Sp)-3, (Rp)-3, (Sp)-
C3, and (Rp)-C3.
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It should be noted that, as
discussed above, all the optically
active compounds (Sp)-2–4, (Rp)-
2–4, (Sp)-C3, and (Rp)-C3 exhibit-
ed intense CPL with relatively
high PL quantum efficiencies
and large
glum values on the
order of 10À3. Monodispersed
CPL-active organic molecules re-
ported to date consist of helical
or axial chirality.[12–15] To achieve
the intense CPL, a chiral orienta-
tion of the emitting species in
the excited state is required. The
present study suggests that
planar chirality of [2.2]paracyclo-
phane is a promising option for
chiral scaffolds to produce CPL
materials.
Conclusion
We have investigated the chi-
roptical properties of planar-
chiral [2.2]paracyclophane-based
through-space conjugated olig-
omers. In the ground state, ob-
served similarities in the chirop-
tical properties of these oligo-
mers were attributed to the
equivalent orientations of two
adjacent chromophores. In the
excited state, the chiroptical
properties of the oligomers
Figure 7. Plausible structures of the oligomers: The linear trimer versus the cyclic trimer in the ground and excited
states.
where IL and IR are intensities of the left- and right-handed lu-
minescence, respectively, appeared with mirror images in the
PL peak ranges (Figure 5A). Intensities of the CPL peak tops
and CPL dissymmetry factors glum =2(ILÀIR)/(IL+IR) increased
gradually as the number of p-electron systems increased, in
contrast to the constant gabs values.
were gradually enhanced as the number of the stacked p-elec-
tron systems increased, thus implying that the oligomers were
folded into a form analogous to a one-handed helix by photo-
excitation. Moreover, an intense CPL response was observed
from the oligomers; this property, attributed to the planar chir-
ality of the [2.2]paracyclophane skeleton, holds potential for
their use in practical CPL materials.
Figure 6 illustrates the PL spectra, CPL spectra, and glum plots
of the linear trimers (Sp)-3 and (Rp)-3 and the cyclic trimers (Sp)-
C3 and (Rp)-C3. Interestingly, there were negligible spectral dif-
ferences between the linear and cyclic compounds. In particu-
lar, the almost identical CPL spectra and glum values suggest
that the linear and cyclic compounds form similar optically
active higher-ordered structures in the excited state.
Acknowledgements
This work was supported by Grant-in-Aid for Young Scientists
(A) (No. 24685018) from the MEXT. Financial support from the
Foundation for Interaction in Science & Technology is also
gratefully acknowledged. The authors are grateful to Professor
Kazuo Akagi and Mr. Kazuyoshi Watanabe (Department of Poly-
mer Chemistry, Graduate School of Engineering, Kyoto Univer-
sity) for CPL data analysis.
It is expected that the para-phenylene ethynylenes form
a planar quinoid-like structure in the excited state (Figure 7);[11]
consequently, trimer 3 and tetramer 4 adopt optically active
higher-ordered structures, that is, zigzag and/or helical struc-
tures in the excited state (Figure 7). The change in glum values
arises from this higher-ordered structural contribution. Consid-
ering the completely fixed cyclic structure of (Sp)-C3 and (Rp)-
C3, it is speculated that the linear trimers (Sp)-3 and (Rp)-3 are
folded to form a one-handed helix rather than a zigzag struc-
ture in the case of excitation (Figure 7).
Keywords: circular dichroism · cyclophanes · luminescence ·
higher-ordered structure · planar chirality
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Received: April 3, 2014
Published online on May 30, 2014
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