Chirality induction in metal-induced achiral polythiophene aggregates assisted by optically active amines and polythiophene

Article abstract of DOI:10.1039/c2cc17940g

A regioregular achiral polythiophene bearing oxazoline pendant groups formed a unique optically active metal-induced supramolecular aggregate upon complexation with chiral amines or in the presence of a chiral polythiophene in a good solvent for the polym

Full text of DOI:10.1039/c2cc17940g

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COMMUNICATION
Chirality induction in metal-induced achiral polythiophene aggregates
assisted by optically active amines and polythiophenewz
Hidetoshi Goto,y Yuko Yokochi and Eiji Yashima*
Received 20th December 2011, Accepted 30th January 2012
DOI: 10.1039/c2cc17940g
A regioregular achiral polythiophene bearing oxazoline pendant
groups formed a unique optically active metal-induced supra-
molecular aggregate upon complexation with chiral amines or in
the presence of a chiral polythiophene in a good solvent for the
polymers, thus showing an induced circular dichroism.
Polythiophenes (PTs) are a class of popular p-conjugated
polymers that have been extensively studied with much interest
because of their possible applications in electric and opto-
electric devices1 and actuators.2 On the other hand, chiral PTs
have also attracted significant interest in recent years due to
their unique chiroptical properties showing an optical activity
when they are aggregated to form supramolecular p-stacked
assemblies in poor solvents or in a film,3 whereas they exhibit
no optical activity in good solvents, except for water-soluble
chiral PTs that show an optical activity solely due to a single-
handed helical conformation in the non-aggregate state.4
In general, chiral PTs have been prepared by the homo-
polymerization of chiral thiophenes or the copolymerization
with achiral ones.3 Recently, Shinkai and coworkers reported
that the chirality, probably a helical chirality with an excess of
one-handedness, could be induced in an achiral cationic PT
when complexed with DNA5a or adenosine triphosphate5b or
included in a helical cavity of polysaccharides5c,d in an aqueous
solution, thus showing an induced circular dichroism (ICD) in
the p-conjugated chromophore regions of the achiral PT.
Chart 1 Structures of polythiophenes and chiral amines.
We previously found that a chiral, regioregular (head-to-
tail) PT (poly[3-{4-((R)-4-ethyl-2-oxazolin-2-yl)phenyl}thio-
phene]) (PEOPT, Chart 1) bearing an optically active
oxazoline residue self-assembled into supramolecular chiral
aggregates in poor solvents9 or in the presence of metal salts
capable of coordinating to the oxazoline residues even in
CHCl3, a good solvent,10 which showed a characteristic ICD
in the p–p* transition region. Such a supramolecular chirality
of the PEOPT aggregates could be reversibly controlled (ON and
OFF) by the addition or removal of an electron from the polymer
main chain.11 Here we report that an analogous regioregular,
achiral PT bearing an achiral oxazoline residue, poly[3-{4-(4,4-
dimethyl-2-oxazolin-2-yl)phenyl}thiophene] (P1, Chart 1), aggre-
gated in the presence of metal salts in a good solvent, and a
unique supramolecular chirality was induced upon complexation
with chiral amines or in the presence of PEOPT, thus showing an
ICD in the achiral P1 chromophore region.
Inganas et al. also found a preferred-handed helicity induction
¨
on achiral PTs in the presence of polypeptides in water.6 Meijer
et al.7 and Koeckelberghs et al.8 also reported an interesting
supramolecular chirality induction in achiral PTs derived from
the chiral information transfer from chiral PT chains to achiral
ones in poor solvents via intermolecular p-stacking when both
homopolymers were mixed in poor solvents or both segments
covalently linked via block-copolymerization.
The P1 was prepared according to Scheme S1 (ESIz) in a
way similar to that previously reported for the synthesis of
PEOPT.9 The number-average molecular weight (Mn) was
estimated to be 5.8 ꢀ 103 as determined by size exclusion
chromatography with polystyrene standards using CHCl3 as
1
the eluent. The H NMR spectrum of P1 in CDCl3 showed a
sharp singlet centered at 6.85 ppm due to the thiophene ring
proton at the 4-position, indicating that the polymer possesses
a head-to-tail structure.9,10
Department of Molecular Design and Engineering, Graduate School of
Engineering, Nagoya University, Chikusa-ku, Nagoya 464-8603,
Japan. E-mail: yashima@apchem.nagoya-u.ac.jp;
Metal-induced aggregations of P1 were first investigated using
silver trifluoromethanesulfonate (AgOTf) because it lacks absorp-
tion in the polythiophene absorption region above 250 nm. The
UV/vis titrations indicated a 2 : 1 stoichiometry between the
oxazoline residues of P1 and Ag(I) ions as previously observed
for the PEOPT–metal complexations (Fig. S1, ESIz).10 With the
aim of inducing chirality in the achiral P1–metal salt aggregates,
commercially available chiral amino alcohols (2–6) and amine (7)
Fax: +81-52-789-3185
w This article is part of the ChemComm ‘Chirality’ web themed issue.
z Electronic supplementary information (ESI) available: Experimental
details. See DOI: 10.1039/c2cc17940g
y Present address: Department of Industrial Chemistry, Faculty of
Engineering, Tokyo University of Science, 12-1 Ichigaya-funagawara-
machi, Shinjuku-ku, Tokyo 162-0826, Japan.
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 3291–3293 3291

Fig. 1 CD and absorption spectra of P1 in the presence of
AgOTf–(1S,2R)- and –(1R,2S)-2 complexes (a) and AgOTf–chiral
amine complexes (b) in CHCl3/CH3CN (97.5/2.5 vol/vol) at 25 1C;
[P1] = 1 mM unitꢁ1, [P1] : [AgOTf] : [chiral amine] = 1 : 1 : 1. These
spectra were measured after the mixtures had been allowed to stand
for 30 min at rt.
Fig. 2 Possible models for chiral supramolecular aggregates (a–d) of
P1 in the presence of Ag(I) and chiral amines. P1 takes a random-coil
conformation in a good solvent (a), but can possess a helically twisted
s-cis or s-trans conformation (b) with dihedral angles of 201 and 1201
between the adjacent thiophene rings, respectively. Upon complexation
with Ag(I) ions, an equal mixture of right- and left-handed helical
aggregates is formed. Further addition of chiral amines produces either
a right- or left-handed helical aggregate.
(Chart 1), which can also coordinate to the metal ions, were added
to the P1–metal salt aggregates and then their CD spectra were
measured.
Fig. 1a and b shows the typical CD and absorption spectra of
P1 in the presence of AgOTf and (1S,2R)- or (1R,2S)-2 in CHCl3/
CH3CN (97.5/2.5 vol/vol). The ternary mixtures exhibited split-
type ICDs of the mirror images of each other in the p–p* transition
region of P1 and the ICD intensity gradually increased with time,
showing a constant value after 30 min (Fig. S2, ESIz).12 Similar
time-dependent ICD changes were observed for the PEOPT–metal
complexes.10 The appearance of the ICDs was accompanied by a
blue-shift in lmax (18 nm) in the absorption, and the ICDs are
completely different from the previously reported ICDs based on
the chiral PT aggregates due to the intermolecular p-stacked
interactions in poor solvents.9 As we previously reported, the chiral
PEOPT exhibited an ICD upon complexation with various metal
ions in the absence of the chiral amines, and we proposed a
predominantly one-handed helical conformation of PEOPT
induced by the intermolecular coordination of the oxazoline group
to metal ions, resulting in the formation of supramolecular chiral
aggregates without any p-stacking.10
For the present achiral P1, the metal coordination most
likely produces an equal mixture of right- and left-handed
helical aggregates of P1 and the chiral amines may contribute
to inducing either a right- or left-handed helical aggregate via
coordination to the metal ions (Fig. 2). The CD and UV/vis
titration experiments of P1 with a 1 : 1 complex of AgOTf and
(1S,2R)-2 were then performed (Fig. S3, ESIz). The ICD
intensity gradually increased in a sigmoidal fashion by the
addition of an increasing amount of the AgOTf–(1S,2R)-2
complex and reached an almost constant value at around
[AgOTf–(1S,2R)-2]/[P1] = 1.0. These ICD changes were
accompanied by clear-cut isosbestic points in the absorption
spectra, suggesting that the polymer forms a slight excess of a
one-handed helical aggregate at [AgOTf–(1S,2R)-2]/[P1] =
0–0.5 upon binding of P1 to Ag(I) ions as supported by the
1H NMR spectroscopy (Fig. S4, ESIz); the proton resonances
derived from the oxazoline residues significantly broadened in
the presence of Ag(I) ions. Further change in the population
of the right- and left-handed helical aggregates into a one-
handedness may cooperatively take place by the further addition
of the AgOTf–(1S,2R)-2 complex because the UV-visible spectra
negligibly changed at [AgOTf–(1S,2R)-2]/[P1] = 0.5–2.0.
The diffusion-ordered 1H NMR spectroscopy (DOSY) measure-
ments showed that the diffusion constant (D) of P1 significantly
ꢁ1
decreased from 5.25 ꢀ 10ꢁ10 to 2.09 ꢀ 10ꢁ10 m2
s
upon
complexation with AgOTf, but increased to 2.50 ꢀ 10ꢁ10 m2 sꢁ1
in the presence of the chiral amine 2, while the D value of 2
decreased from 13.8 ꢀ 10ꢁ10 to 7.43 ꢀ 10ꢁ10 m2 sꢁ1 in the presence
of Ag(I) and P1 (Table S1, ESIz). These data support the fact that
the chiral amine 2 coordinates to the Ag(I) ions to dissociate the
metal-induced P1 aggregates, resulting in a decrease in the size of
the aggregates.
P1 also formed chiral aggregates with Ag(I) ions in the
presence of other chiral primary amines, thus showing ICDs
with similar spectral patterns, but different intensities (Fig. 2b
and Table S2, ESIz); mono-substituted chiral amino alcohols
(5 and 6) and amine (7) tended to exhibit weaker ICDs. In
addition, structurally similar chiral amino alcohols of the same
configurations at the stereogenic center with an amino group
(2–4) afforded the same Cotton effect signs, suggesting that the
P1–Ag aggregation system may be utilized as a novel chirality
sensor for determining the absolute configuration of chiral
amino alcohols.
On the other hand, the types of metals were also anticipated
to affect the P1-aggregate formation as observed for chiral
PEOPT-aggregate formation.10 Among the metal salts tested
(Cu(I), Cu(II), Fe(II), Fe(III), Co(II), Ni(II), and Zn(II)),
Cu(OTf)2 and Cu(ClO4)2 as well as AgOTf induced a chiral
aggregate formation of P1 with chiral amines (Fig. S6 and
Table S3, ESIz), although the CD spectral patterns were
different from each other, while the other metal salts with
chiral amines showed no ICD at all.
Finally, we investigated the chiral information transfer from the
chiral PEOPT to the achiral P1 in a good solvent through metal-
induced aggregate formation derived from the intermolecular
coordination of metals to the oxazoline residues between PEOPT
and P1. Such a chiral information transfer (‘‘sergeant and soldiers
effect’’)13 was achieved when the chiral and achiral PTs were
c
3292 Chem. Commun., 2012, 48, 3291–3293
This journal is The Royal Society of Chemistry 2012

PT with chiral amines may be used as supramolecular chiral
catalysts for asymmetric synthesis and also as chiral materials for
chiral recognition.14
Notes and references
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Fig. 3 (a) Plots of the molar circular dichroism (2nd Cotton effect)
(De2nd) of P1–PEOPT mixtures with Cu(OTf)2 (red) and AgOTf (blue)
in CHCl3/CH3CN (99/1 vol/vol) versus PEOPT content; [P1] +
[PEOPT] = 0.2 mM unitꢁ1, ([P1] + [PEOPT]) : [Cu(OTf)2 or AgOTf] =
1 : 1. These spectra were measured after the mixtures had been allowed
to stand for 3 h at rt. (b) CD and absorption spectra of P1 (black),
P1/PEOPT (1/1) mixture (red), and PEOPT (blue) with Cu(OTf)2 in
CHCl3/CH3CN (99/1 vol/vol) at 25 1C; [P1] + [PEOPT] = 0.2 mM
unitꢁ1, ([P1] + [PEOPT]) :[Cu(OTf)2] = 1 : 1. These spectra were
measured after the mixtures had been allowed to stand for 3 h at rt.
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p–p stacking took place,7,8 thus showing an ICD in the achiral PT
chromophore region. Fig. S7 (ESIz) shows the CD and absorption
spectra of mixtures of PEOPT and P1 with different molar ratios
in the presence of Cu(II) (a) and Ag(I) ions (d). The ICD intensities
of the mixed chiral and achiral PT–metal complexes gradually
increased with time, but they did not reach constant values even
after 3 h. The plots of the second Cotton effect intensity (De2nd) of
the PT mixtures with Ag(I) ions versus the amounts of PEOPT in
the mixtures showed an almost linear relationship between them,
indicating that almost no chiral information transfer took place
for the mixtures in the presence of Ag(I) (Fig. 3a). In sharp
contrast, the same plots with Cu(II) ions displayed a remarkable
nonlinear relationship between the content of PEOPT and
the De2nd values. Most importantly, the Cotton effect signs of
the PEOPT and P1 mixtures were reversed from those of the
PEOPT–Cu(II) complex, and their ICD patterns were significantly
different from that of the PEOPT–Cu(II) complex10 (Fig. 3b and
Fig. S7a, ESIz). These dramatic changes in the ICD patterns and
signs clearly suggest that the chirality of PEOPT is definitely
transferred to the achiral P1 chains, which are incorporated into
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In conclusion, we found a unique chirality induction in a
metal-induced achiral PT aggregate bearing an achiral oxazoline
residue by chiral amines. In addition, the supramolecular
chirality is also transferred from the chiral PT to achiral PT
chains when specific metal salts are used as an intermolecular
cross-linking agent via coordination to the functional pendant
residues. The present metal-induced chiral aggregates derived
from the PEOPT and P1 mixtures assisted by chiral PT in a
good solvent were completely different from their chiral aggre-
gates induced in poor solvents via well-accepted p-stacked,
interchain interactions (Fig. S8, ESIz). We expect that the
metal-induced chiral supra-molecular aggregates of the achiral
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12 The ICD intensity and sign of the ternary mixtures were independent
of the mixing orders of P1, Ag, and chiral amines.
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c
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Chem. Commun., 2012, 48, 3291–3293 3293