Structural and functional evolution of a library of constitutional dynamic polymers driven by alkali metal ion recognition

Full text of DOI:10.1002/anie.200902512

Angewandte
Chemie
DOI: 10.1002/anie.200902512
Constitutional Dynamic Chemistry
Structural and Functional Evolution of a Library of Constitutional
Dynamic Polymers Driven by Alkali Metal Ion Recognition**
Shunsuke Fujii and Jean-Marie Lehn*
Dedicated to Professor C. N. R. Rao on the occasion of his 75th birthday
Adaptive materials, which are capable of responding to
physical triggers or chemical effectors by modification of their
very constitution, represent an intriguing class of “smart”
materials that are of both basic and applied interest. Their
development rests on the implementation of constitutional
dynamic chemistry (CDC),[1] which is based on the lability of
either molecular and supramolecular entities that are derived
from the formation of reversible covalent bonds or through
noncovalent interactions between their components, respec-
tively. This lability ensures the thermodynamic control of the
dynamic libraries of constituents, which are represented by all
possible combinations of the components present, as well as
the rapid response of such libraries to changes in imposed
conditions.[2] Dynamic polymers (dynamers[3]) are constitu-
tionally dynamic polymeric entities based on monomers that
are linked through reversible connections at either the
molecular or supramolecular levels, and have the capacity
to undergo spontaneous and continuous changes in their
constitution by exchange, reshuffling, incorporation, and
decorporation of their monomeric components. A constitu-
tional dynamic polymer library (CDPL) is expected to have
the ability to respond to interacting species or to environ-
mental conditions by shifts in equilibria towards the preferred
“fittest” dynamer(s). Thus, the properties that are displayed
may evolve in response to a variety of external factors, so that
constitutional variation by component exchange in a CDPL
offers a basis for a new class of adaptive, “intelligent”
materials.
understanding of folded forms and of folding processes should
enable the design of foldamers on the nanoscale and thus
allow for control of macroscopic properties such as electrical
and optical features.[5,7]
Herein, we describe the combined adaptation and folding
behavior of a CDPL[2,3,4d,6] that is generated by condensation
polymerization through the reversible formation of imine
bonds. The bond formation occurs between a,w-diamines and
dialdehydes, which are functionalized with various groups
that potentially give rise to the supramolecular interactions
expected to induce or favor particular folded forms. We show
that these dynamic libraries can undergo driven evolution
under the double effect of donor–acceptor stacking[7] and
metal-ion binding.[8] In the presence of alkali metal ions, the
specific binding modes of these metal ions induce an
adaptation behavior, which is associated with specific con-
stitutional changes and with different optical characteristics
that reflect the presence and the positioning of donor and
acceptor units within the folded dynamer.[7,8]
Scheme 1 shows the monomer units used in this study: the
diamines AmD and AmSi, the dialdehydes AlA and AlSi, the
four different imine links that may form during the polycon-
densation reaction L1–L4, and the four possible generated
dynamers P1–P4. Key features of the design are the presence
=
of 1) C N, N, and ether O sites, which are suitable for the
binding of alkali metal ions, and 2) electron-rich 1,5-dialkoxy-
naphthalene and electron-deficient 1,4,5,8-naphthalene-tet-
racarboxylic diimide units, which can associate by charge-
transfer interactions, in the diamines and the dialdehydes,
respectively. These groups were chosen as their features are
well-documented.[4e,7a,b ] Only the dynamer P3 involves both
the donor and acceptor components, which are connected by
a heteroatom-containing chain. The chain is capable of
binding metal cations and is sufficiently flexible to allow
folding, which results in stacking and thus in CT interactions.
The dynamers P1–P4, which are formed from reversible
imine bonds of only one type, were obtained separately by
polycondensation of equimolar amounts of the relevant
diamine and dialdehyde monomers in CHCl3 in the presence
of anhydrous MgSO4. As a prelude to the study of the full
CDPL, the folding behavior of dynamer P3, which was
derived from AmD and AlA, was examined in organic
solvents. Addition of the alkali metal salts LiOTf, NaOTf, and
KOTf to 3 mm P3 in an CD2Cl2/CD3CN mixture (8:2) led, in
the first two cases, to folding that brought the donor and
acceptor units into proximity, as indicated by the correlation
of Hb, Hc, and Hd of the donor with Ha of the acceptor in the
2D NOESY 1H NMR spectrum (Figure S1 (left) in the
The understanding, design, and control of the folding of
molecular strands has attracted great interest in both
chemistry and biology.[4–8] In the area of biopolymers, such
as double-stranded DNA and proteins, folding is a crucial
aspect of the development and expression of specific func-
tions. In materials science, especially polymer science, an
[*] S. Fujii,[+] Prof. J.-M. Lehn
Institut de Science et d’Ingꢀnierie Supramolꢀculaires (ISIS)
Universitꢀ de Strasbourg
8 allꢀe Gaspard Monge, 67000 Strasbourg (France)
Fax: (+33)3-9024-5140
http://www-isis.u-strasbg.fr/supra/
E-mail: lehn@isis.u-strasbg.fr
[+] Present address: R&D Center, Mitsui Chemicals Inc.
580-32 Nagaura, Sodegaura, Chiba 299-0265 (Japan)
[**] We thank Dr. Augustin Madalan for determination of the crystal
structure in Figure 1 as well as Prof. Jack Harrowfield and Tadahito
Nobori for valuable discussions.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200902512.
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Na+ (and Li+) complexation
with respect to K+ may be
related to the size as well as to
the adaptability of the binding
cavity that is defined by the
strands of the stacked folded
form of the ligand, in analogy
to the selectivity shown by mac-
rocyclic polyethers of different
sizes. The average Na+–O and
K+–O distances are 2.6 ꢁ and
2.75 ꢁ, compared to 2.44 ꢁ in
[15]crown-5[11a] and 2.80 ꢁ[11b] in
[18]crown-6, respectively.
Addition of NaOTf to the
dynamer P3, which was obtained
from AmD and AlA, also
resulted in the formation of
Na+ complexes. Gel permeation
chromatography indicated that
the detected species showed a
mean molecular weight Mn =
45000 and dispersivity Mw/Mn =
1.2. These data indicate that
addition of NaOTf to the
dynamer P3 does not result in a
small oligomer such as a macro-
cycle, but a polymeric chain is
formed (Figure 1).
A CDPL was generated from
a mixture of equimolar amounts
of preformed[9] P1 and P2 (each
3 mm in CD2Cl2/CD3CN 8:2) in
the presence of 2 mol%
CF3COOD as the acid catalyst
for the imine equilibration. The
integrations (ꢀ1H) of the imine
and the residual terminal alde-
hyde proton signals in the
Scheme 1. Molecular structures of the donor diamines AmD and AmSi, of the acceptor dialdehydes AlA
and AlSi and of the constitutional dynamic polymers (dynamers) obtained by polycondensation: P1
(AmSi+AlA), P2 (AmD+AlSi), P3 (AmD+AlA), and P4 (AmSi+AlSi) that contain the different imine
links L1–L4.
Supporting Information), as well as by color changes. Binding
of the larger K+ ion to the polymer seems to be less
compatible with close approach of the donor and acceptor
units. An absorption band indicative of the charge-transfer
(CT) interaction that occurs in the Na+ complex appears at
517 nm (Figure S1 (right) in the Supporting Information).[7a,8]
In the experimental range of concentrations from 0.3 mm to
3.0 mm, adherence to Beerꢀs Law was observed, which
indicated that a single species was present over this range
and that the interaction must be intra- and not intermolecular.
The solid-state molecular structures of the NaOTf and
KOTf complexes of a model oligomer for P3, which was
formed by reaction of a monoamine analogue of AmD[9] with
AlA in a 2:1 molar ratio, were determined by X-ray
crystallography (Figure 1). The structures are indeed folded;
the two donor and one acceptor units form a stack, and two
equivalent ions are bound to the connecting chains. Each ion
is hexacoordinated to pyridine N, imine N, two ether O and
one carbonyl O sites of the oligomer (Figure 1), as well as to
one triflate O site (not shown in Figure 1). The preference for
1H NMR spectrum were used in Equation (1) to estimate
the degree of polymerization DP and polymer weight Mn (see
Table S1 in the Supporting Information).[12]
X
X
½DPnꢀ ¼ 1=2½
Imine 1H=
Aldehyde 1H þ 1ꢀ
ð1Þ
The mean molecular weight of the dynamers did not show
a significant change throughout the CDPL study. Thus, the
addition of the different metal triflate salts caused only a
minor effect in the overall degree of polymerization of the
dynamic library.
To ensure equilibration, the dynamer mixture was stirred
for 12 hours at room temperature. After this time, the
1H NMR spectrum of the mixture showed the four sets of
characteristic imine proton signals of the link units L1–L4 in
the P1–P4 dynamers that resulted from exchange and
recombination of the monomer constituents of P1 and P2
(Figure S2e in the Supporting Information). The ratio L1/L2/
L3/L4 was determined to be 22:22:28:28 from the peak
1
integration in the H NMR spectrum. The nearly statistical
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iently illustrated by the colors of the library mixture
in the presence of the various metal triflate salts
(Figure 3). A marked color change, from the light
brown of the initial equilibrium state to a much
deeper purple tint, was observed for LiOTf and
NaOTf, while KOTf produced a weaker effect and
the color decreased further for RbOTf and CsOTf.
The color observed for LiOTf and NaOTf corre-
sponds to the same absorption spectrum as that
observed for pure P3 in presence of NaOTf (Fig-
ure S1 in the Supporting Information). The amplifi-
cation of L3 decreased with increasing ion size
(L1/L2/L3/L4 = 12:12:38:38,
17:17:33:33,
and
20:20:30:30, for KOTf, RbOTf, and CsOTf, respec-
tively; Figure 2). This observation is consistent with
the association of these effects with the preferred
binding of the smaller metal ions in the fold provided
by the link L3 (generated in the course of the
constitutional evolution of a mixture of P1 and P2).
The behavior of this CDPL as a function of the
size of alkali metal ions demonstrates a self-sensing
property,[3i] which results from an adaptation of the
CDPL through the constitutional evolution of the
system that is driven by the recognition of each alkali
metal ion. The system displays the ability to respond
Figure 1. a) Solid-state structure of the NaOTf complex of the model compound
for dynamer P3 generated by condensation of two molecules of a monoamine
analogue of AmD with one molecule of AIA. The stacking of the donor and
acceptor units, which result from the folding of the chains and the binding of
two Na+ ions, are shown (triflate counteranions omitted). b) Solid-state
structure of the corresponding KOTf complex. c) Schematic representa-
tion of the folded dynamer P3 as its Na+ complex based on the crystal
structure of the model compound.
link distribution at equilibrium reflects the essentially iso-
energetic nature of the links under the given conditions.
Subsequent addition of alkali metal triflate salts (2 mol per
mol of imine) led to shifts in the initial equilibrium
distribution, which were dependent upon the nature of the
cation and were reflected not only in the NMR spectra but
also in color changes that arise from modification of CT
interactions. Consistent with expectations based on the
structural characterization of the NaOTf complex of the
model bis(imine) (Figure 1), and with the observation of color
development from CT interactions, addition of NaOTf (as
well as LiOTf) resulted in a strong increase in the fraction of
link L3 and therefore in the P3 constituent (or P3 blocks) of
the polymer mixture. The L1/L2/L3/L4 ratios in presence of
LiOTf and NaOTf were around 8:8:42:42 and 7:7:43:43,
respectively (Figure 2). It is noted that the enhanced forma-
tion of L3 must necessarily result in the release of the
components of L4, so that the increase of L4 and therefore of
P4 (blocks) can be regarded as an agonistic[10] consequence of
the formation of L3. The data obtained from 1H NMR
measurements and GPC analyses indicate consistently that
the overall degree of polymerization in the dynamic library is
hardly affected by the addition of the different metal triflate
salts.
Figure 2. Representation of the distribution of the imine link domains
L1–L4 generated from the library of P1, P2, P3, and P4 dynamers. The
distribution results from the addition of different alkali metal cations
after the library has reached equilibrium, and is determined by
integration of the imine proton signals in the H NMR spectra in
Figure S2 (see the Supporting Information).
1
to external factors by selective amplification of specific
constituents in an adaptive system.
In conclusion, the CDPL reported here exhibits adaptive
behavior by constitutional recombination in response to the
addition of alkali metal ions. A given metal ion induces the
formation of its binding site by synergistic 1) selection of the
optimal components, 2) chain folding, and 3) donor–acceptor
stacking interactions. Moreover, this system is capable of
recognizing alkali metal ions with generation of different
As noted previously, the constitutional evolution of the
CDPL gives rise to changing optical characteristics. The
differing abilities of the metal ions to bring donor and
acceptor units into proximity through folding are conven-
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optical signals, thus displaying a sensing function that arises
from constitutional modification. Numerous variations may
be envisaged, such as extensions to other types of metal ions
or to organic guests, incorporation of other components, and
induction of novel properties. In general terms, a CDPL may
display various features that evolve under the pressure of
external factors, thus defining a new type of smart material
based on change in chemical constitution. Such materials
should be able to generate specific responses to external
stimuli and thereby exhibit several different kinds of func-
tions through the implementation of dynamic constitutional
variation and adaptation. The development of these materials
represents a significant step in the emergence of adaptive
materials technologies.
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[9] The structure and detailed synthetic procedure for this com-
pound are given in the Supporting Information.
Received: May 12, 2009
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Published online: July 23, 2009
Keywords: adaptive materials ·
.
constitutional dynamic chemistry · combinatorial chemistry ·
polymers · supramolecular chemistry
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