Chiral sensing for induced circularly polarized luminescence using an Eu(iii)-containing polymer and d- or l-proline

Article abstract of DOI:10.1039/c3cc42323a

An Eu(iii)-containing polymer can exhibit intense induced circularly polarized luminescence (CPL) in the presence of proline. In addition, the optical anisotropy factor (g_lum) of the polymer for ⁵D₀ → ⁷F₁/⁷F ₂ transition was much higher than that of a single model molecule, which reveals the amplification effect of CPL arising from the conjugated polymer structure.

Full text of DOI:10.1039/c3cc42323a

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Chiral sensing for induced circularly polarized
luminescence using an Eu(III)-containing polymer
and D- or L-proline†
Cite this: Chem. Commun., 2013,
49, 5772
Received 30th March 2013,
Accepted 7th May 2013
Fengyan Song, Guo Wei, Xiaoxiang Jiang, Fei Li, Chengjian Zhu* and
Yixiang Cheng*
DOI: 10.1039/c3cc42323a
www.rsc.org/chemcomm
An Eu(III)-containing polymer can exhibit intense induced circularly Another question is whether the conjugated polymer can lead to the
polarized luminescence (CPL) in the presence of proline. In addition, chiral sensing enhancement or the chiroptical signal amplification
the optical anisotropy factor (glum) of the polymer for 5D0 - 7F1/7F2 effect. These questions may be resolved by enhancing the purity of
transition was much higher than that of a single model molecule, the chiroptical response from the intrinsic interaction between
which reveals the amplification effect of CPL arising from the chiral molecules and the conjugated polymer structures, which
conjugated polymer structure.
can afford an opportunity to develop more sensitive CPL materials.
It is well known that chiral lanthanide(III) complexes are
Enantioselective recognition of chiral molecules is one of the of great practical value because optical anisotropy factors
most important fundamental processes in nature and plays vital (glum values) of f–f transitions are much higher than those of chiral
roles in asymmetric synthesis and biological systems.1 More organic molecules.5c,6 In our previous work, we reported the energy
sophisticated chiroptical probes have recently brought fresh transfer from the conjugated polymer to the Eu(TTA)3 moiety by
insights to the role of solvation and the local dielectric environ- choosing different excitation wavelengths for tunable chromaticity.7
ment in the problem of light–matter interactions with chiral In this work, we have further designed and synthesized an achiral
materials.2 Circularly polarized luminescence (CPL) is now a Eu(III)-containing conjugated polymer P3 in which the Eu(III) metal
ubiquitous tool for characterizing and quantifying natural and ion is tightly coordinated with the b-diketone unit in the polymer
synthetic chiral systems.3 In the past several years, there has been main chain. The higher optical anisotropy factor (glum) of the
growing interest in the CPL-active materials4 and chemical- polymer for 5D0 - 7F1/7F2 transition could be detected by induced
stimuli-responsive CPL properties.5 Maeda and co-workers first CPL in the presence of D- or L-proline, which reveals the amplifica-
observed anion-driven CPL enhancement response behaviour tion effect of CPL arising from the conjugated polymer structure.
of chiral BINOL-based boron hybrid complexes.5a,b Recently,
The neutral polymer P1, water-soluble polymer P2, Eu(III)-
Iwamura et al. have also found that the chiral sensing effect containing polymer P3 and the corresponding model molecule
could be detected by using achiral europium(III) complexes in are shown in Fig. 1. The neutral polymer P1 could be obtained
the presence of chiral 2-pyrrolidone-5-carboxylic acid.5c
by the polymerization of 2,20-((2,5-diiodo-1,4-phenylene)bis-
Although there have been some reports on CPL-induced (oxy))bis(N,N-diethylethanamine) (M1) with 1,3-bis(4-ethynylphenyl)-
chemical sensors, most of them are based on organic small 3-hydroxyprop-2-en-1-one (M2) via a typical Pd-catalyzed Sonogashira
molecules. So far there has been no report on polymer-based cross-coupling reaction (Scheme S1, ESI†). Conversion of the neutral
CPL sensors. Compared with organic small molecules, one of
the most important questions is whether the achiral polymer
sensor can offer a significantly higher luminescence dissymmetry
factor glum by the induction of chiral molecules. Herein, glum
=
2(IL ꢀ IR)/(IL + IR), where IL and IR are the emission intensities of
left and right circularly polarized luminescence, respectively.2a
Key Lab of Mesoscopic Chemistry of MOE, School of Chemistry and
Chemical Engineering, Nanjing University, Nanjing 210093, China.
E-mail: yxcheng@nju.edu.cn, cjzhu@nju.edu.cn; Fax: +86-25-88317761;
Tel: +86-25-83686508
† Electronic supplementary information (ESI) available: Synthesis, experimental
details, NMR spectra and MS spectra of important compounds, and competition
experiment data. See DOI: 10.1039/c3cc42323a
Fig. 1 Structures of the neutral polymer P1, water-soluble polymer P2,
Eu(III)-containing polymer P3 and the corresponding model molecule.
c
5772 Chem. Commun., 2013, 49, 5772--5774
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polymer P1 to the cationic polymer P2 was achieved by treating
P1 with bromoethane in THF–DMSO at 50 1C for 3 days. Mw, Mn
and PDI of polymer P1 were determined by gel permeation
chromatography using polystyrene standards in THF and their
values are 34 720, 32 800 and 1.05, respectively. The GPC result
of polymer P1 shows a high molecular weight. P1 is soluble in
THF and DMF solvent, but insoluble in methanol. P2 exhibits
good solubility in water and DMF. The water soluble polymer
P2 does not form the completely quaternized cationic polymer.
By comparing the 1H NMR spectra of the subunits M2, S2, and
S4, the quaternization degree (QD) of P2 could be determined
1
using H NMR spectroscopy as shown in Fig. S1 (ESI†). Based
on the relative integrals of each pair of split peaks (–CH2N–, low
field: the quaternized component, upfield: unquaternized com-
ponent8), the QD of P2 is 42%. In addition, these spectral
features support the view that a well-defined polymer P2
structure was obtained as we expected. P2 incorporating a
b-diketone unit in the polymer backbone could directly coordinate
with EuCl3 and the TTA anion ligand (4,4,4-trifluoro-1-(thiophen-
2-yl)butane-1,3-dione) to afford P3 as a deep red solid in a
solution of pH = 6.5. The b-diketone unit in the main chain
backbone of P2 does not completely coordinate with Eu ions
according to their elemental analysis data. Furthermore, the
molar ratio of sulfur and nitrogen could be obtained from
elemental analysis results. The molar ratios of S and N obtained
from elemental analysis is 0.51 [3.28/(32.0 ꢁ 2) : 2.85/(14.01 ꢁ 2)]
indicating that the content of Eu(TTA)2 in P3 is 51%. Herein, the
content of Eu in P3 is 7.74%, similar to the elemental analysis
data obtained using ICP determination.7
The fluorescence spectra of the model molecule, P2 and P3
were obtained in a mixed solution of DMSO and H2O (v/v, 1.0%,
1 ꢁ 10ꢀ5 mol Lꢀ1 corresponding to the b-diketone unit). As
evident from Fig. 2a, the water soluble polymer P2 appears to
exhibit a strong fluorescence emission centered at 410 nm
upon excitation at 350 nm. The Eu(III)-containing polymer P3
exhibits characteristic red fluorescence situated at 615 nm
while excited at 350 nm, which can be assigned to the Eu(III)
Fig. 2 (a) Fluorescence spectra of P2, P3 and the model molecule in a mixed
solution of DMSO and H2O (v/v, 1.0%, 1 ꢁ 10ꢀ5 mol Lꢀ1 corresponding to
b-diketone unit) lex = 350 nm. (b) Fluorescence excitation (J) and absorption (n)
spectra of P3: [P3] = 1 ꢁ 10ꢀ5 mol Lꢀ1, lmoni = 615 nm. (c) Time-resolved
fluorescence decays (t) of P2 and P3 in DMSO and H2O (v/v, 1.0%, 1 ꢁ 10ꢀ5 mol Lꢀ1
corresponding to the b-diketone unit) obtained by excitation at 345 nm and
detection at 410 nm.
7
(5D0 - F2) transition. The weak emission peaks of Eu(III) at
577, 596 and 653 nm can be fingerprinted to Eu(III)’s 5D0 - 7F0,
5D0
- -
7F1 and 5D0 7F3 transitions, respectively. However,
compared with P2, P3 exhibits a weak fluorescence emission at
410 nm arising from the conjugated polymer backbone,
demonstrating that the energy transfer process can partially emission peak at 410 nm with several weak emission peaks
take place from the polymer main chain conjugated backbone from 577 to 653 nm.
to the Eu(TTA)2 complex moiety. As evident from Fig. 2b,
Fig. 3 shows the CPL and total luminescence spectra for an
a perfect overlap between the excitation (lmoni = 615 nm) and aqueous solution of P3 and a model molecule in the presence
absorption spectra of P3 can also support the hypothesis that and absence of proline (0.1 mol Lꢀ1 in DMSO and H2O v/v,
the fluorescence emission of polymer P3 at 410 nm shows 1.0%). In the absence of proline, no CPL spectrum was
obvious quenching due to efficient intramolecular energy observed. However, upon addition of D- or L-proline to the
transfer.7,9 In addition, the fluorescence decay profiles of P2 solution of P3 and the model molecule, intense CPL signals
and P3 were obtained with excitation at 345 nm and detection situated at 596 nm of mirror image 5D0 - 7F1 transition could
at 410 nm (Fig. 2c). The weighted average lifetimes ht0i10 of P2 be clearly observed as shown in Fig. 3. More interestingly, we
and P3 were 2.08 ns and 1.53 ns, respectively, which provides found that P3 when used as a CPL-active sensor for enantio-
further evidence of efficient energy transfer.11 And the mean selective recognition of proline can exhibit better sensitivity and
square deviation w2 (CHISQ) values of P2 and P3 were 0.94866 selectivity than the model molecule. With the model molecule,
and 0.85040 (Fig. S2, ESI†). Similar to P3, the model molecule a glum value of +0.028 was observed for L-proline versus glum
=
also shows a sharp emission peak at 615 nm and a weak ꢀ0.036 for D-proline (Fig. 3a). As for P3, the 5D0
-
7F1
c
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More importantly, the optical anisotropy factor (glum) of the
polymer for 5D0
-
7F1/7F2 transition was much higher than
that of the single model molecule demonstrating that the
amplification effect of CPL could be produced by the conju-
gated polymer by the induction of chiral organic small mole-
cules. This research work could provide a new strategy to obtain
higher glum values for CPL-active polymer materials.
This work was supported by the National Natural Science
Foundation of China (No. 21074054, 51173078, 21172106) and
the National Basic Research Program of China (2010CB923303).
Notes and references
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5
7
in the presence of L-proline in aqueous solution for D0 - F1
transition, while right-CPL was observed in the presence of
L-proline for 5D0 - 7F1 transition. In addition, the CPL spectra
5
7
of the D0 - F2 transition of P3 also gave a low glum value of
ꢀ0.0055 for L-proline versus glum = +0.0056 for D-proline. How-
5
7
ever, the glum value of the D0 - F2 transition for the model
molecule was measured to be less than 0.001. We thought that a
well-defined structure of the conjugated polymer main chain
can lead to a specific guest-molecule orientation, which can be
beneficial to obtain a higher luminescence dissymmetry factor.2a
Herein, this kind of specific guest-molecule orientation can be
induced by the interaction between the Eu-containing polymer
and L- or D-proline so that distinct contributions to the average
g value can be observed. As for the single molecule, the lower
g value can be attributed to cancellation effects in ensemble
measurements of a randomly oriented bulk sample. Our
research suggests that the amplification effect of chiral sensing
for induced CPL could be realized by using an achiral Eu(III)-
containing conjugated polymer and D- or L-proline.
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An achiral Eu(III)-containing polymer can exhibit intense left 10 Weighted average: ht0i = A1t1 + A2t2, for P2: A1 = 0.035 t1 = 0.1713
A2 = 0.965 t2 = 2.15, for P3: A1 = 0.11 t1 = 0.6852 A2 = 0.89 t2 = 1.64.
circularly polarized luminescence (CPL) in the presence of L-proline
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7
observed in the presence of D-proline for 5D0 - F1 transition.
c
5774 Chem. Commun., 2013, 49, 5772--5774
This journal is The Royal Society of Chemistry 2013