Photoinduced racemization of an optically active helical polymer formed by the asymmetric polymerization of 2,7-bis(4-tertbutylphenyl)fluoren-9-yl acrylate

Article abstract of DOI:10.1002/anie.200904259

(Figure Presented) A trick of light: An optically active preferred-handed helical polymer synthesized by asymmetric anionic polymerization underwent Stereomutation upon photoirradiation, whereas the helix remained stable upon heating (see picture). This p

Full text of DOI:10.1002/anie.200904259

Communications
DOI: 10.1002/anie.200904259
Photoinduced Stereomutation
Photoinduced Racemization of an Optically Active Helical Polymer
Formed by the Asymmetric Polymerization of 2,7-Bis(4-tert-
butylphenyl)fluoren-9-yl Acrylate**
Takeshi Sakamoto, Yasuyuki Fukuda, Shin-ichiro Sato, and Tamaki Nakano*
The helix is a fundamental macromolecular structure. The
construction and mutation of helical structures have been
studied for a variety of synthetic polymer systems.[1] Macro-
molecular stereomutation is triggered by chemical or thermal
stimuli in most cases, and fewer examples are known of
stereomutation triggered by light. Existing examples of
photoinduced stereomutation involve photochromic groups
incorporated into the side chains or main chain of polymers.
These groups cause the isomerization of a double bond[2–4] or
bond cleavage and formation.[5]
Herein we report the synthesis of optically active pre-
ferred-handed helical poly(2,7-bis(4-tert-butylphenyl)fluo-
ren-9-yl acrylate) (poly(BBPFA)), a novel polyacrylate, and
its effective racemization induced by photoirradiation. In
contrast to the previous examples,[2–5] the stereomutation
observed for this polymer is governed by simple rotation
about the single bonds between the side-chain fluorenyl
moieties and the tert-butylphenyl substituents without any
changes in chemical bonding.
Scheme 1. Asymmetric polymerization of BBPFA.
Polymerization was carried out with the complex formed
between 9-fluorenyllithium (FlLi) and (+)-1-(2-pyrrolidinyl-
methyl)pyrrolidine (PMP) as the initiator in toluene at ꢀ788C
(Scheme 1, Table 1). This initiator system has been used to
synthesize preferred-handed helical polymers from bulky
(meth)acrylates.[1a,b,6] The polymerization of BBPFA occurred
with almost quantitative monomer conversion. The observed
increase in Mn with an increase in the [BBPFA]/[FlLi] ratio
(from 10:1 to 40:1) implies that the polymerization has
Table 1: Asymmetric anionic polymerization of BBPFA with FlLi–PMP in
toluene at ꢀ788C.[a]
Conv.[b] Yield[c] Mn
Mw/Mn
[a]435
[8]
[a]D
[8]
[d]
[d]
[e]
[e]
½BBPFAꢁ
½FlLiꢁ
Entry
[%]
[%]
1
2
3
10
20
40
>99
>99
>99
93
92
90
2830 1.09
8010 1.23
12730 1.48
+6
+2
+162 +28
+213 +35
[a] Reaction conditions: [monomer]0 =0.108 molLꢀ1, [PMP]/[FlLi]=1.2,
24 h (entries 1 and 2) or 48 h (entry 3). [b] Conversion was determined by
1H NMR spectroscopic analysis of the reaction mixture. [c] Yield of the
methanol-insoluble portion of the product mixture. [d] The number-
average molecular weight and the polydispersity index were determined
by SEC in THF with polystyrene as the standard. [e] The values of optical
rotation were determined in CHCl3 at room temperature (concentration:
0.50 gdLꢀ1, cell length: 1 dm).
[*] Dr. T. Sakamoto, Dr. S.-i. Sato, Prof. Dr. T. Nakano
Division of Biotechnology and Macromolecular Chemistry
Graduate School of Engineering, Hokkaido University
Sapporo 060-8628 (Japan)
Fax: (+81)11-706-6869
E-mail: nakanot@eng.hokudai.ac.jp
Homepage: http://www.eng.hokudai.ac.jp/
“living” characteristics to a certain extent. Polymerization
was also attempted at a [BBPFA]/[FlLi] ratio of 60; however,
the reaction system gelled owing to poor solubility of the
products, and relatively low monomer conversion (37%)
resulted.
Y. Fukuda
Graduate School of Materials Science
Nara Institute of Science Technology
Takayama-cho 8916-5, Ikoma, Nara 630-0101 (Japan)
[**] We thank Dr. Motoo Shiro (Rigaku Co.) for crystal-structure analysis
of BBPFA and Dr. Hideyuki Higashimura (Sumitomo Chemical) for
helpful discussions. This study was supported in part by the
Ministry of Education, Culture, Sports, Science, and Technology
(Japan) through Grant-in-Aid No. 21655037 and the Global COE
Program (Project No. B01: Catalysis as the Basis for Innovation in
Materials Science), and in part by the Asahi Glass Foundation.
The obtained polymers were optically active and exhib-
ited intense circular dichroism (CD) spectra (Figure 1).
Optical activity and CD intensity increased with an increase
in the Mn value of the polymer. These results, combined with
the fact that BBPFA monomer does not have any configura-
tional chirality, strongly suggest that the observed chiroptical
properties can be ascribed to a preferred-handed helical
polymer conformation. This conclusion is supported by the
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200904259.
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standing at room temperature in ambient light. Since no clear
decomposition of the polymers was detected, we concluded
that this mutarotation was due to conformational transition
(stereomutation) of the polymer chain.
To identify the cause of the stereomutation, we examined
heat and light as possible stimuli. When we heated a solution
in THF of the polymer from entry 2 in Table 1 (at a
concentration of 1.16 ꢀ 10ꢀ4 molLꢀ1 with respect to the
monomeric residues) at 608C for 6 h in the dark, no change
was evident in the CD spectrum. In contrast, the peaks in the
CD spectrum of the same polymer in THF (2.26 ꢀ
10ꢀ4 molLꢀ1 with respect to the monomeric residues) nearly
completely disappeared within 4 min when the solution was
irradiated with a 500 W Xe–Hg lamp monochromatized at
320 nm (Figure 3). These results clearly indicated that light
had induced the stereomutation. Interestingly, however,
irradiation at 254 nm for 12 h had no effect on the CD
spectrum of the polymer.
Figure 1. CD and UV spectra of poly(BBPFA) with different Mn values
in tetrahydrofuran: blue Mn =12730, red Mn =8010, black Mn =2830.
1
very broad signals observed in the H NMR spectrum of the
polymer; the broadness of the signals indicates a rigid
conformation of the polymer chain (see the Supporting
Information). Only acrylate monomers with bulky tertiary
ester groups[6c,d] have previously been reported to afford
preferred-handed helical polyacrylates. Hence, poly(BBPFA)
may be the first polyacrylate with a secondary side-chain
group to have a helical conformation. The helix-sense excess
of poly(BBPFA) is not yet known: it may potentially be
determined by the method used for the chromatographic
resolution of poly(triphenylmethyl methacrylate).[6e]
Although BBPFA monomer does not have configura-
tional chirality, axial chirality may result from a twisted
conformation of the two biphenyl moieties in the 2,7-bis(4-
tert-butylphenyl)fluorenyl group. X-ray crystal-structure anal-
ysis of the monomer revealed that the biphenyl moieties are
twisted, with a dihedral angle of 278[7] (Figure 2). Although
Figure 3. CD and UV spectra of poly(BBPFA) (Mn =8010) in tetrahy-
drofuran after photoirradiation at 320 nm for 0 (blue), 2 (green), 3
(orange), and 4 min (red). (Concentration: 2.26ꢀ10ꢀ4 molLꢀ1 with
respect to the monomeric residues; cell length: 1 mm).
Upon irradiation at 320 nm, the UV spectrum of the
polymer showed little change; thus, it appeared that the
chemical structure of the polymer was not affected. This
conclusion was supported by analysis of the polymer by size-
exclusion chromatography (SEC) before and after photo-
irradiation: virtually no change in peak shape and position
was observed after irradiation (see the Supporting Informa-
tion).
Figure 2. Crystal structure of BBPFA monomer.
The mutarotation tended to be faster at a lower concen-
tration; thus, chain aggregation appears to retard the muta-
tion. In fact, no clear CD absorptions were observed for the
polymer from entry 2 in Table 1 in THF at a concentration of
2.26 ꢀ 10ꢀ5 molLꢀ1 (with respect to the monomeric residues)
even immediately after dissolution (see the Supporting
Information). Ambient light probably caused rapid stereo-
mutation in this case.
We assume that the stereomutation is triggered by
photoexcitation of the side-chain chromophores of poly-
(BBPFA), which leads to a conformational transition of the
two biphenyl moieties in the side-chain groups from a twisted
form to the coplanar form. A twist–coplanar transition upon
photoexcitation has been reported for the biphenyl com-
the monomer has a meso twist conformation in the crystal, a
chiral, preferred-handed twist conformation may be adopted
during the process of asymmetric polymerization. This
behavior is suggested by the split-type CD pattern in the
longer-wavelength range. This type of pattern is often
observed for chromophores with a chiral, twisted spatial
arrangement. Hence, we propose that optically active poly-
(BBPFA) not only has a preferred-handed helical main-chain
conformation, but also that the side-chain biphenyl moieties
have a preferred-handed twist conformation.
During the course of analytical studies of the chiroptical
properties of the polymers, the CD intensity of solutions of
poly(BBPFA) in THF was found to decrease gradually on
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Communications
pound.[8] This type of transition would lead to racemization of
late)[9] was subjected to HPLC under the same conditions
for comparison (Figure 4). If the stereomutation took place
only in the side chain, and the main-chain helix remained
preferred-handed, HPLC resolution of the stereomutation
products would not be possible. Furthermore, an optically
inactive poly(BBPFA) (Mn = 6000, Mw/Mn = 2.50) prepared
as an authentic racemic mixture by radical polymerization
with a,a’-azobisisobutyronitrile in chloroform at 608C was
also resolved by using the same HPLC setup.
Molecular-mechanics simulations on poly(BBPFA) 20-
mer models indicated that combinations of a right- or left-
handed main chain with a randomly twisted (racemized) side
chain had similar steric energies and were more stable than
other combinations involving the preferred-handed or meso
twisted side chain (see the Supporting Information). There-
fore, the proposed mechanism, according to which side-chain
racemization leads to main-chain racemization, is energeti-
cally rational.
Although the racemization of optically active 1,1’-biphe-
nanthrenes by photoirradiation[10] and the racemization of
preferred-handed helical polymers by thermal and chemical
stimuli[11] have been reported previously, this study, to the best
of our knowledge, provides the first example of the photo-
induced racemization of a preferred-handed helical polymer
without any rearrangement of chemical bonds, such as bond
formation, bond cleavage, or the isomerization of a double
bond. Studies are in progress towards the elucidation of the
complete mechanism of the stereomutation, the development
of a reversible stereomutation with circularly polarized light,
and the application of poly(BBPFA) for the construction of
photoswitching materials.
the proposed preferred-handed twist of the side-chain
biphenyl moieties of poly(BBPFA). This assumption is
supported by the fact that mutarotation was effected by
irradiation not at 254 nm but at 320 nm, that is, within the
longer-wavelength band of the UV spectrum. HOMO–
LUMO interactions should contribute to transitions in this
region of the UV spectrum. As HOMO and LUMO orbitals
are delocalized over the biphenyl moieties (see the Support-
ing Information), it is very likely that light with a wavelength
of 320 nm induces the conformational transition of the
biphenyl moieties.
Upon the twist–coplanar transition of the side-chain
biphenyl moieties, this information would be transferred to
the helical backbone of the polymer chain to cause total
racemization of the preferred-handed helix into an equimolar
mixture of right- and left-handed helices. This hypothesis was
supported by the resolution of a polymer that had been
subjected to photoirradiation and showed no CD absorptions
into dextrorotary and levorotatory fractions by HPLC on a
chiral stationary phase. The known preferred-handed helical
polymer poly(1-(p-vinylphenyl)dibenzosuberyl methacry-
Received: July 31, 2009
Revised: September 24, 2009
Published online: October 30, 2009
Keywords: chiral resolution · conformation analysis ·
.
helical structures · polymerization · stereomutation
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¯
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,
T= 93.1 K, 38963 collected reflections, 10293 unique reflections,
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R
wR2 = 0.2130 (all data). CCDC 742495 contains the supplemen-
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