Solvent-dependent fluorescence and circular dichroism properties of poly(quinoxaline-2,3-diyl)s bearing pyrene pendants

Article abstract of DOI:10.1039/c2cc36275a

Poly(quinoxaline-2,3-diyl)s bearing pyrene pendants exhibited solvent-dependent changes of the fluorescence color (blue and green) and the screw-sense selectivity (left-handed and racemic) in chloroform and 1,1,1-trichloroethane, respectively.

Full text of DOI:10.1039/c2cc36275a

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Solvent-dependent fluorescence and circular dichroism properties of
poly(quinoxaline-2,3-diyl)s bearing pyrene pendants
Yuuya Nagata,a Tsuyoshi Nishikawaa and Michinori Suginome*ab
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
preference leads to the induction of a single screw sense, which
can be detected by fluorescence spectroscopy. We have also
established a quick and simple method for screening solvents for
efficient induction of singleꢀhanded helices.
Poly(quinoxaline-2,3-diyl)s
bearing
pyrene
pendants
exhibited solvent-dependent changes of the fluorescence color
(blue and green) and the screw-sense selectivity (left-handed
and racemic) in chloroform and 1,1,1-trichloroethane,
10 respectively.
Reversible and efficient control of the screw sense of helical
polymers has attracted considerable interest, because it leads to
the exploration of new chiral functional materials.1ꢀ6 We have
recently reported that poly(quinoxalineꢀ2,3ꢀdiyl)s bearing chiral
15 side chains adopt pure rightꢀ and leftꢀhanded screw senses in
chloroform and 1,1,2ꢀtrichloroethane, respectively.7 We also
reported a new class of polymerꢀbased chiral ligand PQXphos;8
its enantioselectivity could be perfectly controlled by solventꢀ
dependent helix inversion of its backbone of poly(quinoxalineꢀ
20 2,3ꢀdiyl).9, 10 Although we suggested the conformational change
of chiral side chains as the mechanism of the helix inversion,7 the
details were not clear. We have expanded much effort to find
evidence for the proposal, concluding that any conformational
probes should be introduced to the polymer side chains.
25
Pyrene has been widely used as a probe to determine
conformations and conformational changes of biomolecules,11, 12
molecular switches,13ꢀ15 and macromolecules,16ꢀ18 because an
excimer emission of pyrene reflects the presence of another
pyrene unit in spatial proximity. In general, when two pyrene
30 rings are located within a few angstroms of each other, they
exhibit a broad, unstructured band at longer wavelengths around
500 nm (green color) as an excimer emission.19 In this
Communication, poly(quinoxalineꢀ2,3ꢀdiyl)s bearing pyrene
pendants were prepared to obtain information about the
35 conformation of the side chain. We observed a fluorescent color
change between chloroform and 1,1,1ꢀtrichloroethane (1,1,1ꢀ
TCE), which may originate from adjacent conformation of two
pyrene rings on the same monomer unit. This conformational
55
Scheme
1 Syntheses, number average molecular weights, and
polydispersity indexes of polymers 1–3 and their model compounds 4–6:
(i) oꢀTolNiCl(PMe3)2, THF, RT, 3 h; (ii) NaBH4, RT, 1 h; (iii) Bu4NF,
THF, RT, 10 h; (iv) pyreneꢀ1ꢀcarbonyl chloride, Et3N, CH2Cl2, RT, 17 h.
60
Poly(quinoxalineꢀ2,3ꢀdiyl)s bearing pyrene pendants 1–3 were
synthesized by postꢀpolymerization functionalization of the
corresponding silylꢀprotected polymer (Scheme 1). All
conversions proceeded quantitatively, as confirmed by H NMR.
All the polymers yielded in this experiments showed good
40 a Department of Synthetic Chemistry and Biological Chemistry, Graduate
School of Engineering, Kyoto University, Katsura, Nishikyo-ku, Kyoto
615-8510, Japan.
1
E-mail: suginome@sbchem.kyoto-u.ac.jp
65 solubility in organic solvents such as toluene, THF, CHCl3, and
1,1,1ꢀTCE. The numberꢀaverage molecular weights (Mn) and
polydispersity indexes (Mw/Mn) determined by size exclusion
chromatography using polystyrene standards suggested that this
polymerization proceeded in a living fashion. The corresponding
70 model compounds 4–6 were also prepared for comparison.
Fax: +81-75-383-2723; Tel: +81-75-383-2722;
45 b JST, CREST (Creation of Next-Generation Nanosystems through
Process Integration), Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-
8510, Japan.
†Electronic Supplementary Information (ESI) available: Experimental
procedures and characterization data for new compounds. See DOI:
50 10.1039/b000000x/
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centers, no CD signals were observed. However, the observed
35 solventꢀdependent fluorescent color change was the same as for
polymers 1 and 2.
Two side chains on the same quinoxaline ring have opposed,
‘upꢀandꢀdown’ arrangements because of steric hindrance. These
enantiomeric conformations are defined as
40 shown in Fig. 3. Considering these conformers, possible
conformations of polymer are shown in Fig. using
heptaquinoxaline bearing two pyrene groups on the central
quinoxaline rings. Note that the ꢀform and Mꢀhelix state is
identical to the ꢀform and Pꢀhelix state, because polymer 3 has
δꢀ and λ
ꢀforms20 as
3
4
λ
δ
45 no chiral centers. However, A′ and B′ could be distinguished
because of their diastereomeric relationship. Polymer 3 might
adopt conformer A in CHCl3 and conformer B in 1,1,1ꢀTCE.
Fig. 1 CD and FL spectra of polymer 1 (a: CD, b: FL) and model 6 (c: CD,
d: FL) in CHCl3 and in 1,1,1ꢀTCE.
50 Fig. 3 Conformations of two side chains on a same quinoxaline ring (
and ꢀform). aꢁꢁꢁa indicates the plane of the quinoxaline ring, and bꢀb
represents the line passing through the two side chains, R.
δꢀ
λ
5
Fig. 2 CD and FL spectra of polymer 2 (a: CD, b: FL) and polymer 3 (c:
CD, d: FL) in CHCl3 and in 1,1,1ꢀTCE.
To evaluate the relationship between fluorescence properties
and the screwꢀsense selectivities, circular dichroism (CD) spectra
and fluorescence (FL) spectra were measured in CHCl3 and 1,1,1ꢀ
Fig. 4 Possible conformations of polymer 3. Only seven quinoxaline units,
55 of which side chains except for those on the central quinoxaline ring are
omitted for clarity, are shown.
10 TCE. Polymer 1 showed a clear negative Cotton effect in CHCl3
(Fig. 1a), suggesting that polymer 1 adopted a leftꢀhanded helical
backbone. As none of model compounds 4–6 showed CD signals
in the observed region, pyrene units may have little influence on
the CD spectra of polymer 1. Polymer 1 exhibited an intense
15 green emission at 514 nm in CHCl3, which originated from the
pyrene excimer. On the other hand, CD signals and the excimer
emission of polymer 1 were dramatically suppressed in 1,1,1ꢀ
TCE. FL measurements of model 6 gave almost the same spectra
in CHCl3 and in 1,1,1ꢀTCE as shown in Fig. 1d, suggesting the
20 changes in the fluorescence spectra of polymer 1 rely on the
existence of the polymer main chain.
CD and FL measurements of polymer 2 bearing additional
chiral side chains were also performed. As shown in Fig. 2a,
polymer 2 adopted leftꢀ and rightꢀhanded screw senses in
25 chloroform and 1,1,1ꢀTCE, respectively. This helix inversion was
caused by the chiral side chains on the pyreneꢀfree monomer
units. The FL spectra of polymer 2 were almost identical to those
of polymer 1, indicating that the conformation of the two pyrene
units on the quinoxaline ring was not affected by the screwꢀsense
30 of the polymer backbone. This means that the solventꢀdependent
fluorescence color change reflected a local conformation of the
pyrene pendants. In Figs. 2c and 2d, the CD and FL spectra of
polymer 3 are shown. Because polymer 3 possessed no chiral
According to the results of the CD and FL measurements,
possible conformations of polymers 1 and 2 can be suggested.
Figure 5 shows representative conformers A–D, which have a
60 diastereomeric relationship because of the chiral centers on the
side chains. Polymer 1 showed an intense excimer emission and
Mꢀhelical structure in CHCl3, and adopted conformer A in CHCl3.
In contrast, in 1,1,1ꢀTCE, polymer 1 did not show good
selectivity among conformers A–D because the CD signals and
65 the excimer emission were suppressed. Polymer 2 showed a leftꢀ
handed screw sense with intense excimer emission in chloroform
and rightꢀhanded screw sense with weak excimer emission in
1,1,1ꢀTCE. Thus, polymer 2 adopted conformer A in CHCl3 and
conformer D in 1,1,1ꢀTCE. These observations of the solventꢀ
70 dependent fluorescent color changes conformed that the
conformational change of the side chain certainly occurred.
Although this color change was not directly dependent on the
screwꢀsense selectivity of the polymer main chain, it could reflect
a local conformation of the chiral side chain modified by the
75 pyrene unit. Therefore, as far as the chiral pyrene units were the
sole source of chirality, the screwꢀsense selectivity should be
related to the intensity of the excimer emission. We performed a
fluorescence screening of solvents for efficient induction of a
singleꢀhanded helical backbone. Polymer 1 was dissolved in
2
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intense excimer emission in CHCl3 and adopted the Mꢀhelical
form. However, the CD signals and the excimer emission of
30 polymer 1 were suppressed in 1,1,1ꢀtrichloroethane. Polymers 2
(pyreneꢀmodified chiral side chains and chiral side chains) and 3
(pyreneꢀmodified achiral side chains and achiral side chains)
showed similar solventꢀdependent fluorescence changes in CHCl3
and 1,1,1ꢀtrichloroethane. Based on these results, we showed that
35 the helix and fluorescence switches depend on solventꢀinduced
conformational changes of the chiral side chains. Finally, we
have established a quick and simple method for screening
solvents for the efficient induction of a singleꢀhanded helical
backbone. In addition, 1,1,2,2ꢀtetrachloroethane was found to be
40 an efficient solvent for the asymmetric helix induction. Studies
for establishing the polymer design, which can report the change
of the screw sense of the polymer backbone as fluorescence
changes, are currently being undertaken in this laboratory.
Fig. 5 Possible conformations of polymers 1 and 2. Only seven
quinoxaline units, of which side chains except for those on the central
quinoxaline ring are omitted for clarity, are shown.
This work is supported by a GrantꢀinꢀAid for Scientific
45 Research from the Ministry of Education, Culture, Sports,
Science and Technology, Japan.
Notes and references
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in Fig. 6a, polymer 1 showed intense excimer emissions in CHCl3
and 1,1,2,2ꢀtetrachloroethane. The relative intensities of the
excimer emissions in various solvents are shown in Fig. 6b. The
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recognizable by the naked eye. Selected CD spectra are shown in
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an almost racemic helical backbone, only CHCl3 and 1,1,2,2ꢀ
tetrachloroethane could induce singleꢀhanded helical backbones.
20 Interestingly, polymer 1 adopted the Pꢀhelical form in toluene,
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Conclusions
In summary, we have synthesized a series of poly(quinoxalineꢀ
25 2,3ꢀdiyl)s bearing pyrene pendants to reveal the conformational
changes of their side chains. Polymer 1, which has pyreneꢀ
modified chiral side chains and achiral side chains, exhibited
80
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