New light on the ring-chain equilibrium of a hydrogen-bonded supramolecular polymer based on a photochromic dithienylethene unit and its energy-transfer properties as a storage material

Article abstract of DOI:10.1002/chem.201100691

A novel, bifunctional, quadruple hydrogen-bonding ureido-pyrimidinone (UPy) unit bridged by photochromic dithienylethene (1) has been synthesized, which affords linear assemblies in solution and undergoes concentration-dependent ring-opening polymerizatio

Full text of DOI:10.1002/chem.201100691

DOI: 10.1002/chem.201100691
New Light on the Ring–Chain Equilibrium of a Hydrogen-Bonded
Supramolecular Polymer Based on a Photochromic Dithienylethene Unit and
its Energy-Transfer Properties as a Storage Material
Shao-Lu Li,[a] Tangxin Xiao,[a] Wei Xia,[a] Xia Ding,[b] Yihua Yu,[b] Juli Jiang,[a] and
Leyong Wang*[a]
Abstract: A novel, bifunctional, quad-
ruple hydrogen-bonding ureido-pyrimi-
dinone (UPy) unit bridged by photo-
chromic dithienylethene (1) has been
synthesized, which affords linear as-
semblies in solution and undergoes
concentration-dependent ring-opening
polymerization. The two UPy function-
al groups of 1 can dimerize intramolec-
ularly to form a cyclic monomer with
the two thienyl rings fixed in a parallel
conformation, which prohibits its pho-
tocyclization. We exploited the photo-
chemical reactivity and resonance dif-
ference of the linker of the bis-UPy de-
rivative as well as using the more typi-
cal 1H NMR, DOSY, and Ubbelohde
viscometry methods to investigate for
the first time the ring–chain polymeri-
zation mechanism. Moreover, we fabri-
cated a mixed polymer film with a fluo-
rescent dye noncovalently endcapping
the linear photochromic assemblies
through quadruple hydrogen bonds,
which showed nondestructive fluores-
cent read-out ability for data storage
by fluorescence resonance energy
transfer (FRET) from the fluorescent
dye to the closed form of the diaryl-
Keywords: energy transfer · photo-
chromism · polymers · ring-opening
polymerization
chemistry
·
supramolecular
AHCTUNGTRENGNeUN thene.
Introduction
units in supramolecular polymer construction due to its high
association constant (Kass >107 mÀ1 in chloroform) and syn-
thetic accessibility.[6] In general, a monomer containing two
of these self-complementary units can self-assemble into
polymers. To date, many of the bifunctional UPy derivatives
reported have 1) a short alkyl chain or long flexible polymer
chain spacer to form linear supramolecular polymers,[5,7] 2) a
rigid highly preorganized monomeric subunit to form cyclic
dimers,[8] tetramers,[9] pentamers, or hexamers,[10] 3) 2- and 6-
substituted bifunctional UPys to form the preferred cyclic
heterodimers in solution,[11] and 4) several substituted alkyl
spacers to form cyclic dimers in equilibrium with linear
polymers in dilute solutions.[12] However, to the best of our
knowledge, an AA-type bifunctional UPy derivative that ex-
hibits a distinct concentration-dependent cyclic monomer–
linear polymer equilibrium of the AB (UPy-Napy) type[13]
has not yet been reported.
Supramolecular polymers[1,2] have attracted increasing atten-
tion in recent years as they not only possess many of the
properties of traditional polymers, but also unprecedented
distinct new material properties due to their dynamic rever-
sible characteristics, which ensures their potential applica-
tion in a broad range of fields. The noncovalent interac-
tions[3] that are present in supramolecular polymers include
hydrogen bonding, hydrophobic and hydrophilic forces, p–p
stacking, ion-pairing, metal–ligand binding, and other host–
guest interactions of which multiple hydrogen bonding is
strong, directional, and with a high-fidelity that has unsur-
passed potential for specificity, as shown by nucleic acids.[4]
The ureido-pyrimidinone (UPy) moiety, discovered by
Meijer and co-workers in 1997,[5] is one of the most used
The ring–chain equilibrium is an important phenomenon
in both covalent and supramolecular polymerization pro-
cesses. It is vital for the construction of useful polymer ma-
terials to understand well the parameters related to the
supramolecular polymerization mechanism. In the produc-
tion of covalent polymers, ring formation is prevented as
much as possible because rings usually form as byproducts.
However, optimization of the yield of single rings in self-as-
sembly supramolecular polymer systems is one of the goals
of synthetic research. In all cases, understanding the param-
eters relating to the ring–chain equilibrium is useful for con-
trolling the product distribution.[1a] Ring formation is inevi-
table in stepwise growth polymerization and the percentage
[a] S.-L. Li, T. Xiao, W. Xia, Dr. J. Jiang, Prof. L. Wang
Key Laboratory of Mesoscopic Chemistry of the
Ministry of Education
School of Chemistry and Chemical Engineering
Nanjing University
Nanjing 210093 (P.R. China)
Fax : (+86)25-83317761
E-mail: lywang@nju.edu.cn
[b] X. Ding, Prof. Dr. Y. Yu
Shanghai Key Laboratory of Magnetic Resonance
Department of Physics
East China Normal University, Shanghai 200062 (P. R. China)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201100691.
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FULL PAPER
of ring formation increases with dilution of solutions be-
cause at low concentration the system favors intramolecular
ring formation and not intermolecular chain extension. In
ing unit and two flexible alkyl C3 chains (Figure 1). Molecu-
lar modeling shows that the two UPy units of derivative 1
can dimerize intramolecularly to yield a cyclic monomer
with two thienyl rings fixed in mirror symmetry. In 2005,
Takeshita et al.[16] reported a similar structure in which the
UPy group directly connected to the dithienylethene unit
achieves a photoreversible, assembled system that can
change its size, but the two UPy units could not dimerize in-
tramolecularly and the chemical shifts of these protons in
1
general, viscometry, H NMR, and DOSY NMR[12] methods
are usually used to determine the critical concentration of
the ring–chain equilibrium in supramolecular polymer sys-
tems. However, as far as we know, there has been no report
of the investigation of ring–chain equilibria by photochromic
methods. Moreover, the incorporation of certain functional
subunits might enrich the methods used to investigate the
supramolecular polymerization process and broaden the ap-
plication of supramolecular materials.
1
the H NMR spectrum did not change upon changing their
Organic photochromic materials have attracted considera-
ble interest due to their potential applications in photo-
ACHTUNGTRENNUNGmemory and -switching devices. In particular, diarylethene,
as an excellent photochromic compound, has proved to be
the most promising switchable unit due to its excellent fa-
ACHTUNGTRENNUNG
tigue resistance and thermal stability.[14] Furthermore, the
photochemical reactivity of diarylethene can be blocked
when two thienyl rings are fixed in mirror symmetry in the
molecule (Scheme 1),[15] and thus this property can be used
to study the ring–chain equilibrium in supramolecular poly-
merization. In this work we have designed and synthesized a
bifunctional UPy derivative 1 with a dithienylethene bridg-
Scheme 1. Photochromic reaction of diarylethenes.
Figure 1. Proposed ring-opening polymerization of 1 and its photoreaction under UV or visible irradiation.
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L. Wang et al.
concentration in solution. In this work we sought 1) to use
the photochemical reactivity of the diarylethene block,
along with 1H and DOSY NMR spectroscopy and Ubbe-
lohde viscometry, to investigate the ring–chain equilibrium
in the supramolecular polymerization process in solution,
2) to study the UPy dimer dissociation in DMSO/CHCl3 by
photochemical methods, and 3) to form a mixed polymer
film with a fluorescent dye noncovalently endcapping the
photochromic linear assemblies through quadruple hydrogen
bonds. The film fluorescence could be switched by UV/Vis
light and thus represents a fluorescent switch with nonde-
F2 was then treated with imidazolide 3 to achieve F1 in a
yield of 55% based on F3.
Self-assembly of the photochromic monomer 1: It is very
convenient to “read” photoreaction processes with just the
naked eye; color change is a well-known characteristic of
photochromic molecules. A colorless 5 mm solution of 1 in
DMSO turned pale purple immediately (<5 s) and gradual-
ly changed to a deep red-purple (2 min) under 313 nm light
irradiation, which indicates the formation of the closed-ring
isomer (the cyclohexadiene type) in the strongly polar sol-
vent DMSO. However, the colors of 5 and 8 mm solutions of
1 in less polar CHCl3 irradiated for 20 s did not change at
all, which implies that the two thienyl rings of 1 are fixed in
a parallel orientation in CHCl3 by intramolecular hydrogen
bonding, as shown by the CPK model. However, a 20 mm
solution of 1 in CHCl3 turned to a light red-purple under
313 nm irradiation (<5 s) and then gradually to a deep red-
purple, which indicates that a concentration-dependent ring-
opening polymerization process occurs to form linear assem-
blies that contain the anti-parallel-orientated diarylethene,
as depicted in Figure 1. This is in accordance with the
1H NMR experiments reported below.
ACHTUNGTRENNUNG
Results and Discussion
Synthesis of monomer 1 and mono-UPy fluorescent dye F1:
The procedures for the synthesis of monomer 1 and mono-
UPy fluorescent dye F1 are depicted in Scheme 2. Bis-amine
2
[17] was treated with imidazolide 3[18] in chloroform to readi-
ly give monomer 1 in a yield of 75%. The mono-UPy fluo-
rescent dye F1 was synthesized in three steps from naphthal-
A
The photoreactivity of diarylethene-based bifunctional
UPy derivatives 1 in CHCl3 (2ꢁ10À5 m) was also explored by
the addition of the mono-UPy derivative and oxoanion salt
as strong competitors for the urea hydrogen donors[8a]
(Scheme 3). The addition of 200 equiv of mono-UPy 4[18] led
tained by nucleophilic substitution of the nitro group of
compound F4 by mono-N-Boc-protected ethylenediamine
F5[20] in a yield of 47% and then N-deprotection of F3 with
CF3COOH in CH2Cl2 afforded the free amine derivative F2.
Scheme 3. a) Structure of mono-UPy derivative 4 and b) the complex
formed by a triflate ion and UPy unit.
to no obvious change in the color of the solution. When the
proportion of 4 was increased to 500 equiv, the solution
turned slightly red under 313 nm irradiation in the photosta-
tionary state (PSS). There was just a slight absorbance in
the visible-light region (Figure S5), which shows that most
of the bifunctional UPy derivatives 1 form cyclic monomers,
even at very “high” relative concentrations of 4 in solution.
It is notable that upon the addition of 500 equiv of tetrabu-
tylammonium trifluoromethanesulfonate, bifunctional UPy
derivatives 1 could not undergo photocyclization. These re-
sults all demonstrate that the two UPy units of 1 dimerize
intramolecularly in CHCl3 (2ꢁ10À5 m), and the cyclic mono-
mer is much harder to destroy than the linear assemblies re-
sulting from intermolecular dimerization (Scheme 3).
The photochemical reaction studied in this work was used
to determine the dissociation ratio of the intramolecular
UPy dimer at a lower concentration (2ꢁ10À5 m), although
Scheme 2. Synthesis of a) monomer 1 and b) mono-UPy fluorescent dye
F1.
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Ring–Chain Equilibrium of a Supramolecular Polymer
FULL PAPER
1H NMR spectroscopy is the universal tool for calculating
the dissociation ratio of the UPy dimer in the mixed CHCl3
and DMSO.[6b,8a] In this study, the unrestrained monomer
should be the exclusive species in pure DMSO (lmax
=
528 nm, e=15.2ꢁ103 mÀ1 cmÀ1 at the PSS) because DMSO is
a strong hydrogen-bonding acceptor solvent for the UPy
unit. Note, only the molecules in the antiparallel conforma-
tion can undergo the photocyclization reaction to give the
closed form of 1. In other words, the extent of dissociation
of cyclic 1 is proportional to the amount of unrestricted mol-
ecules in DMSO/CHCl3 mixtures at the absorbance of
528 nm (lmax of the closed form of 1) at the PSS. The appar-
ent association constant at DMSO=0.80 (K*ass =8150mÀ1
(Æ20%)) and at DMSO=0.20 (K*ass =1.64ꢁ105 mÀ1
(Æ20%)) were determined from Figure 2.[8a] To the best of
our knowledge, the photochemical reaction was firstly ap-
plied to determine the extent of dissociation of the UPy
dimer at a very low concentration (2ꢁ10À5 m).
Figure 3. Partial 1H NMR spectra (300 MHz, CDCl3, 248C) of 1 at differ-
ent concentrations: a) 5, b) 10, c) 15, d) 20,e) 30, f) 40, g) 60, h) 80, and
i) 100 mm. Signals originating from cyclic aggregates are labeled “c”,
those from polymeric aggregates are labeled “p”.
creased progressively, whereas the intensity of the former
peak decreased. The assignment of H1 in the cyclic oligomer
and the linear assemblies was based on published re-
sults,[15b,17,21] the following DOSY NMR results, and the pho-
toreaction measurement. Meanwhile, we found that the
thienyl and phenyl protons H2, H3, and H4 are also concen-
tration-dependent. The 6.67 ppm signal of the thienyl pro-
tons H2 of the cyclic oligomer decreased gradually with in-
creasing concentration (>15 mmol) and another signal ap-
peared at 6.91 ppm, which gradually increased. There were
similar changes observed for the phenyl protons (Figure 3).
The higher field signals of the cyclic oligomer relative to the
linear compound may be attributed to the shielding effect of
the aromatic rings in the cyclic monomeric form in which
the two thienyl rings and the two phenyl rings are close
enough to influence the chemical environment of each
other.
The ureido-pyrimidinone functionality has three tauto-
meric structures and exists as dimers of the pyrimidin-
4(1H)-one and pyrimidinol tautomeric forms in solution
(see Figure S6). The pyrimidinone tautomer with the alkyl
substituent at the 6-position is highly favored over the pyri-
midin-4-ol tautomer in CHCl3. Although the presence of
substantial amounts of the enol tautomer was confirmed for
the bifunctional UPy derivatives bearing alkyl substituents
at the 6-position based on a m-xylene linker or with several
substituted linear alkyl spacers in dilute solutions,[8b,12a] the
ureido-pyrimidin-4(1H)-one tautomer is the only one ob-
served by FTIR (KBr, typical peak: 1701, 1656, 1583 cmÀ1,
absence of a 2500 cmÀ1 band for the enol tautomer) and
1H NMR spectroscopy (only one signal of the pyrimidinone
Figure 2. Plot of the extent of dissociation versus solvent composition for
the intramolecular dimerization of 1 in CHCl3/DMSO mixtures. The line
connecting the data points serves to guide the eye.
NMR spectroscopy is a powerful tool for investigating the
ring–chain transition. It is well known that a diarylethene
with heterocyclic aryl groups has two conformations: The
two rings with mirror or C2 symmetry. The photocyclization
reaction is prohibited when the compound is fixed in the
conformation with mirror symmetry (Scheme 1). Moreover,
the methyl protons at the 2-position of the thienyl rings give
1
signals at different fields in the H NMR spectrum depend-
ing on the conformation.[15b,21] In our system, the two thienyl
rings are fixed in the mirror symmetry in the cyclic mono-
mer but are free in linear assemblies, so there should be dif-
ferent resonance signals for the methyl protons in the cyclic
and linear assemblies in solution. As a result, photochromic
bifunctional UPy derivative 1 might be an ideal model com-
pound for monitoring the ring–chain equilibrium in solution.
1H NMR spectroscopy (300 MHz, CDCl3, 248C) was per-
formed at concentrations in the range of 3–150 mm
(Figure 3). At low concentration (<15 mm), the methyl pro-
tons H1 appeared at 2.17 ppm and correspond to the cyclic
oligomer. As the concentration was increased, a new peak
appeared clearly at 1.95 ppm, which corresponds to the
linear assemblies at 20 mm, and its relative intensity in-
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L. Wang et al.
proton H8 from 3 to 150 mm in CDCl3) for the photochro-
mic bifunctional UPy derivative 1.
the peak at 10.12 ppm were not less than the peralkylated b-
CD, except at 5 and 12 mm (see Figures S7–S12). We de-
duced that this should be the reason that the linear polymer
is the majority form at “higher concentration” and that the
larger molecules can affect the small cyclic monomer by
packing around it, and hence the reason for the apparent
magnitude of the diffusion coefficient of the cyclic monomer
in the DOSY spectra.
À
The hydrogen bonds typical of the UPy N H protons
have two sets of well-defined signals with a concentration-
dependent relative intensity (Figure 2). Having excluded the
presence of the enol tautomer, the resonances correspond to
the different assemblies of the pyrimidinone tautomer. At
low concentration, there was only one set of signals for the
À
UPy N H protons, whereas at higher concentrations, for in-
For ring–chain supramolecular polymerization mechanism
systems, there exists a critical concentration below which
the system is composed of cyclic products only and above
which the concentration of cyclic species remains constant
and excess monomer mainly produces linear species.[12a] The
cyclic monomers and linear assemblies of our system can be
stance, above 15 mm, an additional set of peaks showed up.
Moreover, the relative intensities of the new peaks in-
creased progressively and the original peaks decreased as
the concentration was increased.
Diffusion-ordered 1H NMR spectroscopy (DOSY) mea-
surements provide useful information about the size of the
1
G
distinguished by H NMR spectroscopy. Therefore the par-
assemblies in supramolecular aggregate systems.[9,10,12b,22] We
performed the DOSY experiments on monomer 1 in CDCl3
at different concentrations using heptakis(2,3,6-tri-O-
methyl)-b-cyclodextrin (MW=1429 gmolÀ1) as an internal
reference. We found out that the assembly diffusion con-
stant of 1 decreased from 1.07 at 5 mm to 0.032 at 120 mm,
which indicates the formation of much larger aggregates at
“higher” concentrations (Figure 4). At 5 mm, the diffusion
constant is slightly larger than that of the cyclodextrin,
which implies that the assembly is smaller than that of the
cyclodextrin; thus the monomer (MW=1028 gmolÀ1) of 1 is
the dominant species at this concentration. There was also
tial monomer concentrations of cyclic monomers, calculated
from the signals of H5, were plotted against the total mono-
mer concentration (Figure 5a). There was a steady increase
in the concentration of the cyclic monomer (slope of nearly
1) as the concentration increased and then a plateau; the re-
sults indicate a critical concentration of 13 mm (deduced
from the graph).
À
only one set of signals for the UPy N H protons; thus the
d=10.12 ppm (H5) peak should correspond to the cyclic
monomer. This is consistent with the photochemical mea-
ACHTUNGTRENNUNGsurements. A small new peak appeared at d=10.33 ppm at
12 mm, which has a smaller diffusion coefficient than the
peralkylated b-CD and should correspond to the larger ag-
gregates. As the concentration increased, the intensity of the
peak at d=10.12 ppm decreased, whereas that at d=
10.33 ppm increased progressively, which suggests the con-
centration dependence of the ring-opening linear supra-
molecular polymerization of monomer 1. At higher concen-
trations, the diffusion constant of the assemblies correspond-
ing to the peak at d=10.33 ppm decreased progressively,
but the diffusion constants of assemblies corresponding to
Figure 5. a) Partial concentration of cyclic monomers in CDCl3 solutions
of increasing concentration of monomer 1, calculated from the 1H NMR
spectra (9.5–10.5 ppm range of H5) and b) specific viscosity (hsp) of 1
(CHCl3, 258C) versus monomer concentration.
To further test the validity of the ring–chain transition,
viscosity measurements were taken at 258C in CHCl3 by
using a micro-Ubbelohde viscometer. A double-logarithmic
plot of the specific viscosity versus concentration is shown in
Figure 5b. The slope tended to 1 in the low-concentration
Figure 4. Concentration dependence of the diffusion coefficient D of 1 in
CDCl3 at 208C using heptakis(2,3,6-tri-O-methyl)-b-cyclodextrin as an in-
ternal standard (when there were different assemblies in the system, we
chose the larger one).
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Ring–Chain Equilibrium of a Supramolecular Polymer
FULL PAPER
region, which is a characteristic of noninteracting assemblies
of constant size and indicates the predominance of cyclic
oligomers in dilute solutions. Upon increasing the concen-
tration above 14 mm, a sharp rise in the slope to 4.20 was
observed, which is consistent with the reported ureido-pyri-
midone-based self-assembling systems (slopes of 2.8–
6.1).[12a,13] This indicates the formation of linear polymers at
high concentration, assemblies with increasing size concomi-
tant with increasing concentration. The critical concentra-
tion is 14 mm, which is approximately in agreement with the
1H NMR results (13 mm). Such a low critical concentration
might be due to the ring strain in the cyclic monomer form,
which is supported by the shift of 0.21 ppm of the H5 reso-
nance signals for the cyclic and linear assemblies in the
1H NMR spectra. The degree of polymerization (DP) of the
polymeric fractions at 100 mm of 1 was estimated to be 3.7ꢁ
103 at 298 K on the basis of the Kdim value of 5.7ꢁ107 for
the UPy groups.[12b]
The viscosities of the 20, 25, and 40 mm solutions of 1 in
CHCl3 at 258C under 313 nm light irradiation were investi-
gated. Although the solutions all turned a deep purple-red
under 313 nm irradiation, there was little change in the vis-
cosity of the solutions, which indicates that the assemblies
do not change much. From the open form to the closed
form, the increased rigidity of the monomer in the linear as-
semblies did not affect the ring–chain equilibrium. The
quantity of the open form in the cyclic monomer remained
unchanged, which is indicative of the innate character of the
critical concentration. The closed form, to some extent, is
also flexible and the assemblies are linear, so the solution
viscosity remained almost unchanged.
is known that bis-UPy derivatives can be easily spin-coated
to yield a smooth film and that energy transfer occurs effec-
tively both in solution and in the solid state[26] because the
donor and acceptor are hydrogen-bonded. Inspired by the
above idea and to explore the potential applications of a
prepared polymer, a small amount of mono-UPy-terminated
fluorescent dye F1 was added to a solution of 1 to endcap
the linear polymer 1 and then the mixture was spin-coated
to fabricate a smooth mixed film.
We chose F1 as the fluorescent dye because the emission
spectrum of the 4-aminonaphthalimide overlaps well the ab-
sorption of the closed form but not the open form of 1,
which can efficiently quench the fluorescent emission.[25a,27]
In addition, the selected excitation wavelength (400 nm)
causes little structural change in either the open or closed
isomer of diarylethene, and thus it can act as a fluorescent
switch with nondestructive readout ability. Monomer 1
(25 mm) mixed with 3% of F1 in CHCl3 was easily spin-
coated on to silicon or quartz slides to give smooth films. In
the film, no phase separation took place in the linear poly-
mers of 1 endcapped with mono-UPy-functionalized F1 be-
cause of the quadruple hydrogen bonds. The UV/Vis absorb-
ance and energy-transfer properties of the thin films before
and after irradiation with appropriate light are shown in
Figure 6. The closed form of the diarylethene can absorb 4-
aminonaphthalimide fluorescence emission, whereas the
open form cannot. Nearly 90% of the fluorescence could be
quenched by UV irradiation and regenerated by visible light
irradiation of the thin film, concomitant with the color
switching between yellow and purple-red.
Energy-transfer experiment of
mixed film: Modulating lumines-
cence intensity is one of the most
attractive features of photome-
mory and -switching devices be-
cause of the high sensitivity, reso-
lution, contrast, and fast response
times of luminescence technolo-
gy.[23] In addition, fluorescence
on-off or fluorescence color
switching can be applied in new
high-resolution imaging tech-
niques such as stochastic optical
reconstruction
microscopy
(STORM).[24] Diarylethene is
usually used to covalently con-
nect a fluorescent dye or disperse
the dye and diarylethene in a
polymer matrix.[25] The lumines-
cence of such systems can be
quenched from the excited lumi-
nescent center to the colored
state of the diarylethene through
intra- or intermolecular reso-
Figure 6. a) Absorption (a, b) and fluorescence (c, d) spectra, b) the fluorescence intensity at 490 nm of the
film upon alternating irradiation at 313 nm for 3 min and at 530 nm for 15 min, and c) depiction of the energy-
nance energy transfer (FRET). It transfer (ET) process of the thin film spin-coated from solutions of 97:3 1–F1 under UV/Vis irradiation.
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L. Wang et al.
6.93 (d, J=8.7 Hz, 4H), 5.73 (s, 2H), 4.02 (t, J=6.5 Hz, 4H), 3.30 (t, 4H,
J=6.5 Hz, overlapped with water signals), 2.82 (t, J=7.5 Hz, 4H), 2.19
(m, 2H), 2.03 (m, 2H), 1.92 (s, 6H), 1.89 (m, 4H), 1.50–1.36 (m, 8H),
1.25–1.07 (m, 8H), 0.82–0.70 (m, 12H) ppm; 13C NMR (75 MHz,
[D6]DMSO, 248C): d =157.9, 154.7, 151.7, 138.9, 136.5, 134.1, 132.3,
126.6, 126.1, 123.0, 115.0, 105.0, 65.2, 47.8, 38.0, 36.2, 33.0, 29.1, 28.8, 26.6,
22.3, 22.2, 14.0, 13.9, 11.75 ppm; FTIR (KBr): n˜ =3216, 2958, 2929, 2871,
1701, 1656, 1583, 1509, 1466, 1245, 1176, 1110, 1023, 823 cmÀ1; MS
(MALDI-TOF): m/z: 1029.98 [M+H]+; HRMS (ESI): m/z: calcd for
C57H72N8O6S2Na [M+Na]+: 1051.49139; found: 1051.49037, error
0.97 ppm.
Conclusion
We have prepared a novel, bifunctional, quadruple hydro-
gen-bonding ureido-pyrimidinone (UPy) unit bridged by
photochromic dithienylethene 1. It exists as a cyclic mono-
mer at low concentrations and undergoes a concentration-
induced ring-opening polymerization process. We exploited
the photochemical reactivity and resonance difference of
the linker of the bis-UPy derivative for the first time to in-
vestigate the ring–chain transition as well as using the more
Compound F3: A solution of F4 (2.17 g, 7.3 mmol) and F5 (1.52 g,
9.4 mmol) in DMF (50 mL) was stirred for 2 days. The resulting mixture
was poured into water (400 mL) and the resulting solution extracted with
CHCl3 (4ꢁ100 mL). The combined organic layers were dried over
Na2SO4 and the solvent was evaporated. Purification of the crude product
by column chromatography (silica, CH2Cl2) gave F3 as a yellow solid
(1.42 g, 47%). 1H NMR (300 MHz, CDCl3, 248C): d=8.57 (d, J=7.3 Hz,
1H), 8.43 (d, J=8.3 Hz, 1H), 8.25 (d, J=8.3 Hz, 1H), 7.61 (dd, J=8.2,
7.5 Hz, 1H), 6.58 (d, J=8.4 Hz, 1H), 5.13 (brs, 1H), 4.16 (t, J=7.5 Hz,
2H), 3.81 (brs, 1H), 3.66–3.62 (m, 2H), 3.47–3.44 (m, 2H), 1.76–1.67 (m,
2H), 1.48 (s, 9H), 1.47–1.40 (m, 2H), 0.97 (t, J=7.3 Hz, 3H) ppm;
13C NMR (75 MHz, CDCl3, 248C): d=164.9, 164.3, 158.7 , 150.2, 134.6,
131.1, 129.8, 127.1, 124.7, 122.9, 120.4, 109.9, 103.4, 80.7, 46.7, 40.0, 39.6,
30.4, 28.5, 20.5, 14.0 ppm.
1
typical H NMR, DOSY, and Ubbelohde viscometry meth-
ods, which provide new light on the ring–chain polymeri-
zation mechanism. We also used the photoreaction to study
the UPy dimer dissociation in DMSO/CHCl3. Moreover, we
fabricated a mixed polymer film with a fluorescent dye non-
covalently endcapping the linear photochromic assemblies
through quadruple hydrogen bonds. The film fluorescence
could be switched by UV/Vis light, presenting a fluorescent
switch with nondestructive readout ability for data storage
and high-resolution imaging technology. Further investiga-
tions of the supramolecular polymer based on ureido-pyri-
midinone and its application as smart materials are in prog-
ress.
Compound F1: Trifluoroacetic acid (15 mL) was gradually added to a so-
lution of F3 (1.42 g, 3.45 mmol) in CH2Cl2 (15 mL). The reaction mixture
was stirred for 10 h. The solvent was evaporated in vacuo to afford a
brown oil, which was dissolved in dry CHCl3 (40 mL) and neutralized
with N-methylmorpholine (NMM) to pH 7–8. Then imidazolide 3 (1.09 g,
3.61 mmol) was added to the solution under nitrogen and the reaction
mixture was stirred for 12 h. Then dry CHCl3 (50 mL) was added and the
organic layer was washed with 1n HCl (40 mL), saturated NaHCO3
(40 mL), and brine (40 mL). After drying with Na2SO4 the organic layer
was reduced to about 5 mL by evaporation in vacuo. The concentrated
solution was slowly added to MeOH (30 mL) under vigorous stirring,
which resulted in a yellow precipitate. The precipitate was filtered off
and washed thoroughly with MeOH to obtain a yellow solid (0.98 g,
Experimental Section
General: All reactions were performed in air unless noted otherwise. The
commercially available reagents and solvents were either employed as
purchased or dried according to procedures described in the literature.
All yields are given as isolated yields. NMR spectra were recorded with a
Bruker DPX 300 MHz spectrometer with tetramethylsilane (TMS) as in-
ternal standard and solvent signals as internal references. CDCl3 was
used as received. IR spectra were recorded on Bruker Vector 22 as KBr
pellets. UV/Vis spectra were obtained with Perkin–Elmer Lambda 25
and Shimadzu UV-2401 spectrometers. Liquid-phase and solid-phase
photoluminescence spectra (LEM and SEM) were recorded on Lambda
55 and Aminco Bowman Series 2 luminescence spectrometers. UV and
visible irradiations were carried out with a CHF-XM500W power system
(China) using a suitable band-pass filter. Matrix-assisted laser desorption/
ionization time-of-flight (MALDI-TOF) mass spectra were recorded in
positive-ion mode using an Autoflex III (Bruker Daltonics, Germany)
time-of-flight mass spectrometer. Low-resolution electrospray ionization
mass spectra (LR-ESI-MS) were obtained on a Finnigan Mat TSQ 7000
instrument. High-resolution electrospray ionization mass spectra (HR-
ESI-MS) were recorded on an Agilent 6210 TOF LCMS instrument
equipped with an electrospray ionization (ESI) probe operating in the
positive ion mode with direct infusion. DOSY experiments were per-
formed with a Bruker DPX 500 MHz spectrometer. Viscosity measure-
ments were carried out with Ubbelohde micro-viscometers (Shanghai
Liangjing Glass Instrument Factory, 0.40 mm and 0.71 mm inner diame-
ter) at 258C in chloroform.
1
55%). H NMR (300 MHz, CDCl3, 248C): d=13.12 (brs, 1H), 11.89 (brs,
1H), 10.71 (brs, 1H), 8.55 (d, J=7.2 Hz, 1H), 8.44 (d, J=8.3 Hz, 2H),
7.51 (t, J=7.7 Hz, 1H), 6.67 (d, J=8.4 Hz, 1H), 5.94 (s, 1H), 4.15 (t, J=
7.4 Hz, 2H), 3.96 (brs, 1H), 3.75 (brs, 2H), 3.61 (brs, 2H), 2.46–2.35 (m,
1H), 1.82–1.55 (m, 6H), 1.50–1.18 (m, 6H), 0.98–0.86 (m, 9H) ppm;
13C NMR (75 MHz, CDCl3, 248C): d=173.2, 164.7, 164.1, 157.1, 156.3,
154.6, 149.8, 134.5, 130.8, 129.9, 127.1, 124.1, 123.0, 120.6, 110.2, 106.3,
103.8, 45.4, 44.0, 39.9, 38.8, 33.0, 30.3, 29.4, 26.7, 22.5, 20.4, 13.9,
11.8 ppm; LRMS (ESI): m/z (%): 547.25 (100) [M+H]+, (28) 569.33
[M+Na]+; HRMS (ESI): m/z: calcd for C30H38N6NaO4 [M+Na]+:
569.2852; found: 569.2856, error 0.7 ppm; m/z: calcd for C60H76N12NaO8
[2M+Na]+: 1115.5807; found: 1115.5813, error 0.5 ppm.
Acknowledgements
We gratefully thank the financial support of the National Natural Science
Foundation of China (Grant No. 20932004 and 21072093), the National
Basic Research Program of China (Grant No. 2007CB925103 and
2011CB808600), the Program for New Century Excellent Talents in Uni-
versity (Grant No. NCET-07-0425) and The Doctoral Fund of the Minis-
try of Education of China (Grant No. 20090091110017). We are grateful
to Dr. Chen Lin from Nanjing University for helpful discussions.
Compound 1: 1,2-Bis[2-methyl-5-[p-(3-aminopropoxy)phenyl]-3-thienyl]-
cyclopentene (2; 0.90 g, 1.53 mmol) and imidazolide (3; 1.07 g,
3.52 mmol) were dissolved in dry CHCl3 (20 mL) and this solution was
stirred overnight under nitrogen. Dry CHCl3 (50 mL) was added to the
reaction mixture and the organic layer was washed with 1n HCl (20 mL),
saturated NaHCO3 (20 mL), and brine (20 mL). The organic layer was
dried over Na2SO4 and the solvent was evaporated to dryness. Purifica-
tion of the crude product by flash column chromatography (silica,
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a white powder
(1.18 g, 75%). 1H NMR (300 MHz, [D6]DMSO, 248C): d=11.43 (brs,
2H), 9.63 (brs, 2H), 7.60 (brs, 2H), 7.43 (d, J=8.7 Hz, 4H), 7.10 (s, 2H),
10722
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Chem. Eur. J. 2011, 17, 10716 – 10723

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Received: March 5, 2011
Published online: August 11, 2011
Chem. Eur. J. 2011, 17, 10716 – 10723
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
10723