PH-Controlled reversible formation of a supramolecular hyperbranched polymer showing fluorescence switching

Article abstract of DOI:10.1002/chem.201204315

A π-conjugated AB₂ monomer 1 with a dibenzo[24]crown-8 (DB24C8) ring and two secondary amine centres has been synthesised. Treatment of a solution of 1 in dichloromethane with trifluoroacetic acid (TFA) leads to protonation of the amine groups,

Full text of DOI:10.1002/chem.201204315

DOI: 10.1002/chem.201204315
pH-Controlled Reversible Formation of a Supramolecular Hyperbranched
Polymer Showing Fluorescence Switching
Bingran Yu,[a] Baoyan Wang,[a] Shuwen Guo,[a] Qian Zhang,[a] Xiaorui Zheng,[a]
Haitao Lei,[a] Weisheng Liu,[a] Weifeng Bu,*[a] Yun Zhang,[b] and Xin Chen[b]
Abstract: A p-conjugated AB2 mono-
mer with dibenzo[24]crown-8
ing interactions between the conjugat-
ed cores are evident in this polymer.
The supramolecular hyperbranched
polymer (SHP) can be completely de-
polymerised by adding a slight excess
tetrabutylammonium fluoride, or tetra-
butylammonium acetate. The acid–
1
a
G
ACHTUNGTRENNUNG
(DB24C8) ring and two secondary
amine centres has been synthesised.
Treatment of a solution of 1 in di-
chloromethane with trifluoroacetic acid
(TFA) leads to protonation of the
amine groups, and then the DB24C8
rings are threaded by the dialkylammo-
nium ion centres of other monomer
molecules, leading to the formation of
a supramolecular hyperbranched poly-
mer, TFA-1. Rather strong p–p stack-
base-controlled process induces a re-
versible change in the fluorescence in-
tensities of the solutions due to the
controllable presence of the p–p stack-
ing interactions between the conjugat-
ed cores. This dynamic behaviour is sig-
nificant with respect to “smart” supra-
molecular polymer materials.
of
hexa
N-tert-butyl-N’,N’,N’’,N’’,N’’’,N’’’-
AHCTUNGTRENGNmUN ethylphosphorimidic triamide,
Keywords: acid–base reaction · flu-
orescence switch · molecular recog-
nition · supramolecular chemistry ·
p–p stacking
Introduction
assemblies are usually considered to have a dynamic nature.
However, rare examples show reversible polymerisation and
depolymerisation in response to external stimuli.[2c] Despite
progress in this area, SHPs built up of star-shaped p-conju-
gated monomers, which might hold great promise for appli-
cations in optoelectronic devices, have not hitherto been
demonstrated.
Hyperbranched polymers adopt highly branched and globu-
lar architectures, are highly soluble in appropriate solvents,
and have low melt and solution viscosities in comparison
with their linear analogues.[1] They are usually synthesised in
high yields by one-pot covalent polymerisation of ABm
mono
mers (mꢀ2). Alternatively, noncovalent connections
The crown ether moiety of dibenzo[24]crown-8 (DB24C8)
can host a dialkylammonium ion centre (-CH2NH2+CH2-)
with a molar ratio of 1:1 driven by a cooperative combina-
of ABm or other monomers can result in the formation of
supramolecular hyperbranched polymers (SHPs).[2] The driv-
ing force behind such assembly processes is commonly
based on host–guest recognition, and examples of this have
involved crown ethers with viologen moieties,[2a] [60]fuller-
ene with concave aromatic electron donors,[2b] and b-cyclo-
dextrin with azophenyl[2c] and adamantyl[2d,e] groups. Such
tion of [N+ H···O] and [C H···O] hydrogen bonds and p–p
stacking interactions.[3,4] This recognition motif has a high
binding constant in CH2Cl2.[3c] Upon the addition of excess
base to the solution, deprotonation of the ammonium recog-
nition site occurs, and thus the pseudorotaxane complex dis-
sociates. Subsequent addition of acid can completely restore
the inclusion complex.[3d–g] Such acid–base-controllable
switching occurs very rapidly. Alternatively, such a pseudo-
ꢁ
ꢁ
AHCTUNGTREGrNNNU otaxane can be unthreaded by the addition of tetrabutylam-
[a] B. Yu, Dr. B. Wang, S. Guo, Q. Zhang, X. Zheng, H. Lei,
Prof. Dr. W. Liu, Prof. Dr. W. Bu
monium chloride because of the formation of a strong ion
pair of dialkylammonium chloride, whereas the pseudoro-
taxane structure is regenerated upon addition of tributylam-
monium hexafluorophosphate.[5] We speculated that by com-
bining these features we might achieve a reversible fabrica-
tion of SHPs on the basis of the above acid–base or ion-pair-
ing reactions, which would be accompanied by a controllable
fluorescence switch.
To this end, we synthesised a p-conjugated AB2 monomer
1 with a DB24C8 ring and two secondary amine centres
(Figure 1 and Scheme 1). The reasons for selecting 1 were
threefold. 1) On treating a solution of 1 in dichloromethane
Key Laboratory of Nonferrous Metals Chemistry and
Resources Utilization of Gansu Province
State Key Laboratory of Applied Organic Chemistry and
College of Chemistry and Chemical Engineering
Lanzhou University, Lanzhou, Gansu, 730000 (P.R. China)
Fax : (+86)9318912582
E-mail: buwf@lzu.edu.cn
[b] Y. Zhang, Dr. X. Chen
National Laboratory for Infrared Physics
Shanghai Institute of Technical Physics
Chinese Academy of Sciences, Shanghai, 200083 (P.R. China)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201204315.
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FULL PAPER
triangular structure of
1 is
highly fluorescent[6] and thus
the reversible formation of
TFA-1 is accompanied by a re-
versibility in intensity of both
the monomeric and excimeric
emissions.
Results and Discussion
The AB2 monomer 1 and the
model compounds
2 and 3
(Figure 1) were synthesised ac-
cording to the routes depicted
in Scheme 1, and Schemes S1
and S2 in the Supporting Infor-
mation, respectively. The inter-
mediate 8 was synthesised by
three Sonogashira cross-cou-
pling reactions starting from
1,3,5-tris(4’-iodophenyl)ben-
zene (4);[7] other reactants such
as 4-formylphenylacetylene[8]
and 7[9] were synthesised as de-
scribed previously. The imine 9
Figure 1. Top: Chemical structures of the AB2 monomer 1 and model molecules 2 and 3. Bottom: Schematic
representation of the reversible formation of the SHP, TFA-1.
with trifluoroacetic acid (TFA), strong host–guest interac-
tions between the dialkylammonium ion centres and
DB24C8 groups resulted in formation of the SHP TFA-1,
which could be subsequently depolymerised by adding
was obtained by treating 8 with benzylamine. Reduction of
the imine with sodium borohydride generated the final
product 1. The model molecules 2 and 3 were synthesised in
a similar manner. They were characterised by their high-res-
olution electrospray ionisation mass spectra (ESI-MS) and
1H and 13C NMR spectra. The supramolecular polymerisa-
tion of AB2 monomer 1 to the SHP TFA-1 was monitored
by H NMR spectroscopy, matrix-assisted laser desorption/
ionisation time-of-flight mass spectrometry (MALDI-TOF-
MS), solution viscosity, dynamic light scattering (DLS),
excess
N-tert-butyl-N’,N’,N’’,N’’,N’’’,N’’’-hexamethylphos-
phorimidic triamide (P1-tBu), tetrabutylammonium fluoride
(TBAF), or tetrabutylammonium acetate (TBAOAc).
2) The large distance between the secondary dialkylammoni-
um and DB24C8 groups excludes the possibility of intramo-
lecular recognition in the protonated monomer. 3) The rigid
1
Scheme 1. The synthetic route to AB2 monomer 1 (TMS=trimethylsilyl).
Chem. Eur. J. 2013, 19, 4922 – 4930
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W. Bu et al.
The broad resonances of both
Hd and Hac+Hbc were essential-
ly independent of the others,
as shown in Figure 2b. There-
fore, the percentage recogni-
tion (p) and polymerisation
degree (n) could be deter-
mined according to Equa-
tion (1) in which A(Hd) and
AACTHNUTRGNEUGN(Hac+Hbc) are the average in-
tegrals of Hd and Hac+Hbc, re-
spectively. The values of p and
n were estimated to be 98.8ꢂ
0.2% and 82ꢂ12, respectively,
at
a concentration of 1.0ꢁ
10ꢁ3 molLꢁ1 and 98.5ꢂ0.2%
and 66ꢂ10, respectively, at
5.0ꢁ10ꢁ4 molLꢁ1. They corre-
sponded to molecular weights
of (1.2ꢂ0.2)ꢁ105 and (9.5ꢂ
1.4)ꢁ104 gmolꢁ1, respectively.
According to the integrals of
the broad peaks at d=4.5
(linear units), 4.7 (terminal
units), and 4.9 ppm (dendritic
units), the degrees of branch-
Figure 2. 1H NMR spectra (600 MHz, in CD2Cl2, 5.0ꢁ10ꢁ4 molLꢁ1) of a) 1, b) TFA-1 produced by adding
2.2 equivalents of TFA to the solution of 1, c) 1 obtained by treating TFA-1 with 2.4 equivalents of P1-tBu, and
d) 1H22+F22ꢁ obtained by treating TFA-1 with 2.4 equivalents of TBAF.
transmission electron microscopy (TEM), and scanning
force microscopy (SFM).
ing (DB) were estimated to be 64.0ꢂ6.4% at a concentra-
tion of 1.0ꢁ10ꢁ3 molLꢁ1 and 66.6ꢂ6.6% at 5.0ꢁ
10ꢁ4 molLꢁ1, respectively, on the basis of Equation (2).[1c]
1
The H NMR resonance signals of 1 were fully assigned
1
on the basis of H–1H COSY spectra recorded from a solu-
tion in CD2Cl2. Upon addition of a slight excess of TFA
(2.2 equiv), significant changes of the 1H NMR spectrum
were observed, as shown in Figure 2a,b. With reference to
previous 1H NMR studies on host–guest recognition of
DB24C8 derivatives with dialkylammonium groups,[3,4] the
three broad peaks at d=4.5, 4.7, and 4.9 ppm could be un-
equivocally assigned to the benzylic methylene protons adja-
n
AðHacþHbcÞ
4AðHdÞ
ð1Þ
ð2Þ
p ¼
¼
ðnþ1Þ
½ðno: of dendritic unitsÞ þ ðno: of terminal unitsÞꢃ
ðtotal no: of unitsÞ
DB ¼
+
cent to the NH2 centres hosted by the DB24C8 moieties
(Hac+Hbc), whereas the broad signal at d=3.8–3.9 ppm
could be assigned to the methylene protons adjacent to the
uncomplexed ammonium groups. The characteristic
1H NMR signals of the crown ether between d=3.2 and
4.3 ppm were much broader and more complicated than the
corresponding peaks before the acid treatment, resembling
those observed in supramolecular “daisy chains”.[3d] Further-
more, the resonances of the conjugated core were broad-
ened and significantly upfield shifted relative to those of 1.
These shifts and the signal broadening indicated that the the
crown ether moieties were threaded by the dialkylammoni-
um ions, confirming the formation of the SHP TFA-1 upon
treatment of 1 with 2.2 equivalents of TFA. It is noteworthy
that the resonances of the conjugated core were shifted up-
field, which revealed rather strong p–p stacking interactions
in TFA-1.[3f] Indeed, it was this strong p–p stacking interac-
tion between the conjugated cores that promoted the forma-
tion of TFA-1, as discussed below. As a result, the resonance
signals of the conjugated core were significantly broadened.
A further decrease in the concentration would have led to
a large error in the calculated values of p and n because of
1
the weaker H NMR signals. Upon addition of 2.4 equiva-
lents of P1-tBu to the same solution, the chemical shifts of
the 1H NMR signals in the spectrum obtained (Figure 2c)
were essentially consistent with those in the original spec-
trum, indicating that the formation of TFA-1 seemed to be
nearly reversible. The difference was that the new spectrum
displayed sharper and better resolved signals. The resonan-
ces of the methylene protons Ha and Hb were changed from
singlets at d=3.81 and 3.83 ppm to doublets at d=3.81
(2J(H,H)=7.2 Hz) and 3.83 ppm (2J
ACTHUNTGRNENUG ACHTUNTGERN(NUGN H,H)=7.2 Hz). The
signal of the aromatic proton Hd was clearly observed at d=
6.87 ppm with a coupling constant of J=8.4 Hz. These clear-
er resonance signals after an acid–base switching cycle could
be attributed to a weakened p–p stacking interaction of 1 in
the solution containing the generated TFA·P1-tBu salt,
which was further supported by fluorescence spectroscopic
studies (see below).
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pH-Controlled Formation of a Fluorescent Hyperbranched Polymer
FULL PAPER
The MALDI-TOF mass spectrum of TFA-1, utilising di-
tranol as a matrix, further supported the formation of oligo-
meric species in the gas phase (Figure S1 in the Supporting
Information). Peaks attributable to monomeric species ap-
was treated with 2.4 equivalents of P1-tBu, Dh reverted to
1.2ꢂ0.2 nm (Figure 3e), demonstrating once again the re-
versible formation of TFA-1. The size and morphology of
TFA-1 were further examined by TEM and SFM. Solutions
of TFA-1 were drop-cast onto carbon-coated copper grids
for TEM observation. To obtain clear images, the cast films
of TFA-1 were stained with RuO4 vapour. Typical images
showed that spherical aggregates coexisted with crescent-
shaped aggregates (Figure 4a,b). Their respective diameters
peared
at
m/z
1238.1
([1+Na]+)
and
1329.1
([1+2H+CF3COO]+). Peaks at m/z 2252.3 and 2342.8 were
tentatively attributed to the tetramers [14+3H+CF3COO-
4PhCH2-2C6H5CH2NH]2+ and [14+3H+CF3COO-C6H5-
2C6H5CH2NH]2+, respectively, whereas the peak at m/z
2432.1 was assigned to the dimer [12+3H+2CF3COO-
3PhCH2NH-C6H5CH2NHCH2]+. Further evidence for the
formation of TFA-1 was provided by viscosity measure-
ments made at 188C. The reduced viscosity of solutions of 1
in dichloromethane remained almost constant with increas-
ing concentration (Figure 3a). Upon treating such a solution
Figure 4. TEM images of TFA-1 as drop-cast onto carbon-coated copper
grids at concentrations of a,b) 1ꢁ10ꢁ3 molLꢁ1 and c) 8ꢁ10ꢁ6 molLꢁ1
.
d) SFM image of TFA-1 as drop-cast onto a silicon substrate at a concen-
tration of 8ꢁ10ꢁ6 molLꢁ1
.
were estimated to be 115ꢂ15 and 800ꢂ50 nm at a concen-
tration of 1ꢁ10ꢁ3 molLꢁ1, smaller than the values obtained
from the DLS measurement. This was most probably due to
the swelling behaviour of the supramolecular aggregates in
solution. On decreasing the concentration to 8ꢁ
10ꢁ6 molLꢁ1, both TEM (Figure 4c) and SFM images (Fig-
ure 4d) revealed that TFA-1 formed spherical aggregates
with diameters in the range 18–65 nm. The decreasing
degree of polymerisation and size of TFA-1 with decreasing
concentration was consistent with the aforementioned vis-
cosity measurements and previous findings for other SHPs[2]
and supramolecular polymers.[10] Even when the concentra-
tion was decreased to the order of 10ꢁ6 molLꢁ1, TFA-1 still
formed spherical aggregates with a diameter of several tens
of nanometers. The MALDI-TOF MS, viscosity, and DLS
results, as well as the microscopy images, were consistent
Figure 3. Reduced viscosity as a function of the concentration of a) 1 and
b) TFA-1 in dichloromethane at 188C. DLS plots of c) 1 (in CH2Cl2, 1.0ꢁ
10ꢁ3 molLꢁ1), d) TFA-1 produced by adding 2.2 equivalents of TFA into
a solution of 1, e) 1 obtained by treating TFA-1 with 2.4 equivalents of
2ꢁ
P1-tBu, and f) 1H22+F2 obtained by treating TFA-1 with 2.4 equivalents
of TBAF.
with 2.2 equivalents of TFA, the reduced viscosity was re-
markably enhanced and increased with increasing concen-
tration (Figure 3b). This sharp contrast was consistent with
formation of the SHP of TFA-1 and thus an increase in the
size of TFA-1 with the concentration. A DLS plot of a solu-
tion of 1 in dichloromethane (1ꢁ10ꢁ3 molLꢁ1) showed a
peak at a hydrodynamic diameter (Dh) of 1.3ꢂ0.2 nm (Fig-
ure 3c), which was consistent with its molecular size. After
adding 2.2 equivalents of TFA to this solution, two modes
were observed at Dh =324ꢂ16 nm and 8885ꢂ444 nm (Fig-
ure 3d). The former revealed formation of the SHP of TFA-
1, whereas the latter corresponded to multi-aggregate asso-
ciates of the SHP. Similar results were obtained for the su-
pramolecular polymer prepared from [60]fullerene with con-
cave aromatic electron donors.[2b] When the same solution
1
with the formation of TFA-1 as revealed by the H NMR
spectra.
To further support the formation of TFA-1, we studied
the binding of the model compounds 2 and 3 under acidic
conditions. Upon adding a slight excess of TFA (1.1 equiv)
to a mixed solution containing 2 and 3 in a molar ratio of
1:1 (total concentration: 1.0ꢁ10ꢁ3 molLꢁ1), two distinct
broad peaks appeared at d=4.6 and 4.7 ppm in the
1H NMR spectrum, indicative of formation of the host–guest
complex [2·3·H]+TFAꢁ (Figure S2 in the Supporting Infor-
mation). The resonances of both the conjugated core and
crown ether in [2·3·H]+TFAꢁ were similarly broadened and
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W. Bu et al.
shifted as those for TFA-1, albeit to a much lesser extent
(Figure 2b and Figure S2b in the Supporting Information).
This difference may be attributed to the formation of TFA-1
being favoured by much stronger p–p stacking interactions
between the conjugated cores. The signal at d=7.78 ppm
due to the central aromatic protons was not overlapped with
others, and could therefore be used as an internal reference
to determine the yield of [2·3·H]+TFAꢁ. The ratio of the in-
tegral of the signal at d=7.78 ppm to that of the signals at
d=4.6 and 4.7 ppm was estimated to be about 1:1, which
was consistent with the corresponding value in the original
spectrum. This indicated quantitative formation of [2·3·H]+
TFAꢁ. After adding 1.2 equivalents of P1-tBu to the same
strong fluorescence band at lmax =393 nm together with a
shoulder near 480 nm. Upon dropwise addition of TFA, the
fluorescence band at 393 nm showed a stepwise decrease in
intensity, which was accompanied by a slight increase in in-
tensity of the shoulder band near 480 nm. These changes
reached their greatest extents at a TFA/1 molar ratio of
around 2 (Figure 5b). Of note was that the fluorescence
band at 393 nm for 1 was shifted toward lower energy, ap-
pearing at 400 nm for TFA-1, indicative of a slight modifica-
tion of the electronic conformation by [N+ H···O] and [C
H···O] hydrogen bonds. The resulting solution of TFA-1 was
further titrated with P1-tBu. The titration data showed that
the fluorescence intensities of the bands at both 393 and
480 nm reverted to the original levels for 1 at a P1-tBu/TFA-
1 molar ratio of 2 (Figure S4 in the Supporting Information
and Figure 5c). However, no change was observed in the
UV/Vis absorption spectrum. Taking these features together,
the bands at 393 and 400 nm were assigned to the monomer-
ic emissions of 1 and TFA-1, respectively, whereas the band
at 480 nm was assigned to an excimeric emission. The fluo-
rescence enhancement of the excimeric emission at 480 nm
could be attributed to the much stronger p–p stacking inter-
action in TFA-1 compared to that in 1, as revealed by their
1H NMR spectra. In sharp contrast, the fluorescence intensi-
ty of [2·3·H]+TFAꢁ was largely unaffected by the host–guest
recognition (Figure S5 in the Supporting Information),
ꢁ
ꢁ
1
solution, the H NMR spectrum obtained (Figure S2c in the
Supporting Information) was essentially consistent with the
original one, indicating that the formation of the model
complex [2·3·H]+TFAꢁ was pH-controlled and completely
and quantitatively reversible. A similar situation was evident
at a total concentration of 5.0ꢁ10ꢁ4 molLꢁ1. The formation
of [2·3·H]+TFAꢁ was further confirmed by the ESI mass
spectrum, which featured a new peak at m/z 1073.7187 due
to [2·3·H]+ (m/z calculated 1073.7164 (Figure S3 in the Sup-
porting Information)).
With these results in mind, we proceeded to investigate
how the fluorescence behaviour of the solutions was influ-
enced by the reversible formation of the SHP and thus the
controllable presence of p–p stacking interactions between
the conjugated cores. The electronic absorption spectra of
both 1 and TFA-1 showed a p–p* band centred at 315 nm.
Upon excitation at this wavelength, a solution of 1 in di-
1
which was consistent with the much narrower H NMR sig-
nals and thus a much weaker p–p stacking interaction in
[2·3·H]+TFAꢁ compared to that in TFA-1. Figure 5c illus-
trates a reversible intensity change of the fluorescence band
at 393 nm upon successive additions of TFA and P1-tBu,
albeit with partial loss of the
chloromethane (8ꢁ10ꢁ6 molLꢁ1, Figure 5a) exhibited
a
signal in the first cycles. The
fluorescence intensities after
repeated treatment with P1-tBu
were quite consistent with the
original intensity of 1. TFA-1
showed a stepwise increase in
its
fluorescence
intensity,
which reached
a
maximum
after seven acid/base cycles.
Further alternate additions of
acid and base to the solution
yielded reversible cycles of flu-
orescence changes with almost
no loss of the signal intensity.
The TFAꢁ ion can form a
strong ion pair with the diben-
zylammonium cation, so much
so that even host–guest recog-
nition between DB24C8 and
dibenzylammonium is inhibited
in solvents of low dielectric
constant.[4h,i] However, quanti-
tative recognition was ob-
served in solutions of both
TFA-1 and [2·3·H]+TFAꢁ in
Figure 5. a,b) Fluorescence spectral changes of 1 (8ꢁ10ꢁ6 molLꢁ1, dichloromethane) upon titration with TFA
(TFA/1=0, 0.5, 1, 1.5, 2.0, 2.5, 3.0) at both the monomeric and excimeric emission bands. Fluorescence intensi-
ty changes of 1 at 393 nm in c) dichloromethane and d) dichloromethane containing 50 equivalents of TFA·P1-
tBu upon alternate additions of TFA and P1-tBu for ten repeating cycles.
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pH-Controlled Formation of a Fluorescent Hyperbranched Polymer
FULL PAPER
dichloromethane, which may be attributed to a cooperative
combination of strong p–p stacking interactions between
the p-conjugated molecules, as revealed by their broadened
was found to increase from 7.75ꢂ0.39% to 46.23ꢂ2.31%.
Basically, the fluorescence lifetimes reverted to their origi-
nal values for 1 after treating a solution of TFA-1 in di-
chloromethane with 2.4 equivalents of P1-tBu. A solution of
1 in dichloromethane containing 50 equivalents of TFA·P1-
tBu showed a fluorescence decay similar to that of 1. With
the formation of TFA-1 by adding 2.2 equivalents of TFA to
this solution, the relative contributions of the shorter- and
longer-lifetime components were estimated to be 74.78ꢂ
3.74% and 25.22ꢂ1.26%, respectively, and were again re-
stored to their original values for 1 after the addition of a
slight excess of P1-tBu. These reversible fluorescence decays
were fully consistent with the intensity changes of the emis-
sion bands at both 393 and 480 nm. On the basis of these
data, the lifetimes of 1.10ꢂ0.11 and 5.23ꢂ0.52 ns could be
unequivocally assigned to the monomeric and excimeric
emissions, respectively.
and upfield-shifted 1H NMR signals, together with [N+
ꢁ
ꢁ
H···O] and [C H···O] hydrogen bonds. A high concentration
of TFAꢁ can completely suppress the host–guest recognition
of dibenzylammonium with DB24C8 in media of low polari-
ty.[4i] However, in the present case, such recognition still oc-
curred even after ten acid/base cycles, which may have been
due to a rather high binding constant of TFA with P1-tBu
(pKb =26.5).[3f,11] The partial loss of the fluorescence signals
(Figure 5c) may be attributed to the presence of traces of
free TFAꢁ in an ion-paired solution of TFA·P1-tBu produced
by the acid–base switching process. To further prove this
effect, a solution of 1 in dichloromethane (8ꢁ10ꢁ6 molLꢁ1)
containing 50 equivalents of TFA·P1-tBu was prepared,
which showed a fluorescence intensity consistent with that
of 1 at the same concentration. After adding 2.2 equivalents
of TFA, the fluorescence intensity at 393 nm decreased, be-
coming similar to that of TFA-1 after it had been subjected
to seven acid/base cycles (Figure 5d). With successive addi-
tions of P1-tBu and TFA to this solution, the increases and
decreases in the emission at 393 nm were reversible over at
least ten cycles with almost no loss of the signal intensity
(Figure 5d). This phenomenon might be attributable to the
almost constant concentration of TFAꢁ originating from the
ion-pairing effect and activity coefficient in a solution with
such a high ionic strength as TFA·P1-tBu.[4i]
Next, we investigated the fluorescence responsiveness of
TFA-1 towards various anions by means of fluorimetric ti-
trations. The fluorescence intensity at 393 nm increased
upon dropwise addition of TBAF and reached a maximum
value at a TBAF/TFA-1 molar ratio of 2:1 (Figure 6a),
which was fully consistent with that of 1 in dichloromethane
(8ꢁ10ꢁ6 molLꢁ1). The anionic responsiveness was further
1
confirmed by the H NMR spectrum (Figure 2d) and DLS
measurement (Figure 3 f). Upon addition of 2.4 equivalents
1
of TBAF to the solution of TFA-1, the resulting H NMR
spectrum was completely consistent with that of 1 (Fig-
ure 2d). A similar result was obtained upon addition of
TBAOAc to a solution of TFA-1 (Figure S8 in the Support-
ing Information). These results indicated that the additions
Due to the different excitation properties of 1 and TFA-1,
we measured the lifetimes of both the monomeric and exci-
meric bands at 393 and 480 nm, respectively, upon excitation
at 315 nm. The fluorescence decay profiles were well-fitted
by double-exponential curves, with the lifetimes being essen-
tially independent of the emission wavelengths of 393 nm
(Table 1 and Figure S6 in the Supporting Information) or
480 nm (Table S1 and Figure S7 in the Supporting Informa-
tion). When 2.2 equivalents of TFA were added to a solution
of 1 to form TFA-1, the faster component of t=1.10ꢂ
0.11 ns showed a decrease in its relative weighting from
92.25ꢂ4.61% to 53.77ꢂ2.67%, whereas the relative contri-
bution of the longer-lifetime component of 5.23ꢂ0.52 ns
of TBAF and TBAOAc led to the formation of the strong
2ꢁ
ion pairs 1H22+F2 and 1H2
C
led to dissociation of the dialkylammonium cations from the
DB24C8 groups. When various other anions, namely, Clꢁ,
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
Brꢁ, Iꢁ, BF4 , ClO4 , H2PO4 , HSO4 , NO3 , and PF6 , were
added to a solution of TFA-1, however, no significant fluo-
rescence enhancement was detected (Figure 6b). The results
showed that Fꢁ and OAcꢁ have much stronger ion-pairing
interactions with dialkylammonium cations than the other
anions.
Although the additions of 2.4 equivalents of the afore-
mentioned anions to a solution of TFA-1 did not produce a
significant change of the fluorescence intensities, upon sub-
sequent addition of 2.4 equivalents of TBAF to the solution,
the fluorescence intensity was essentially consistent with
that of 1 (Figure 6c). However, in such a competitive experi-
ment with TBAOAc, the fluorescence intensity was only
partially recovered (Figure S9a in the Supporting Informa-
tion). This contrasting result revealed that the binding of
TFA-1 with fluoride was highly selective. However, upon
successive additions of TFA and TBAF or TFA and
TBAOAc, the fluorescence signals were completely lost
after five TFA/TBAF or three TFA/TBAOAc cycles, which
may be attributed to rather strong ion-pairing effects of Fꢁ
or OAcꢁ and TFAꢁ with the dialkylammonium cations (Fig-
ure 6d and Figure S9b in the Supporting Information). This
Table 1. Luminescence lifetimes[a] (t1 and t2) for 1 and TFA-1.
Sample
t1 [ns]
RW1 [%][b]
t2 [ns]
RW2 [%][b]
1
1.10ꢂ0.11
1.39ꢂ0.14
1.05ꢂ0.10
1.06ꢂ0.11
1.10ꢂ0.11
0.99ꢂ0.10
92.25ꢂ4.61
53.77ꢂ2.67
94.12ꢂ4.71
91.39ꢂ4.57
74.78ꢂ3.74
94.12ꢂ4.71
5.23ꢂ0.52
5.97ꢂ0.60
4.92ꢂ0.49
4.42ꢂ0.44
4.93ꢂ0.49
4.83ꢂ0.48
7.75ꢂ0.39
46.23ꢂ2.31
5.88ꢂ0.29
8.61ꢂ0.43
25.22ꢂ1.26
5.88ꢂ0.29
TFA-1[c]
1[d]
1[e]
TFA-1[f]
1[g]
[a] 8ꢁ10ꢁ6 molLꢁ1 solutions in CH2Cl2 monitored at 393 nm upon excita-
tion at 315 nm. [b] Relative weighting (RW) of components in double-ex-
ponential fits. [c] Produced by adding 2.2 equivalents of TFA to a solu-
tion of 1. [d] Obtained by treating TFA-1 with 2.4 equivalents of P1-tBu.
[e] 8ꢁ10ꢁ6 molLꢁ1 solution in CH2Cl2 together with 50 equivalents of
TFA·P1-tBu. [f] Produced by adding 2.2 equivalents of TFA to the solu-
tion in e. [g] Obtained by adding 2.4 equivalents of P1-tBu to the solution
in f.
Chem. Eur. J. 2013, 19, 4922 – 4930
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www.chemeurj.org
4927

W. Bu et al.
optoelectronic devices.[12] In
this context, TFA-1 can be re-
garded as a p-conjugated SHP
and a dynamic material with
controllable p–p stacking inter-
actions between the conjugated
cores and thus controllable
photophysical properties.
Experimental Section
General considerations: All reaction
operations were performed under an
anhydrous Ar atmosphere. Anhy-
drous toluene and tetrahydrofuran
(THF) were distilled over Na and
benzophenone. Dichloromethane was
dried over CaH2. All chemicals were
used as received without any further
treatment. 1H and 13C NMR spectra
were recorded on Varian 600 MHz or
Bruker 400 MHz spectrometers with
tetramethylsilane (TMS) as an inter-
nal standard. Melting points were de-
Figure 6. a) Fluorescence spectral changes of TFA-1 produced by adding 2.2 equivalents of TFA to a solution
of 1 (8ꢁ10ꢁ6 molLꢁ1, dichloromethane) upon titration with TBAF (TBAF/1=0, 0.5, 1, 1.5, 2.0, 2.5, 3.0). b) Flu-
orescence response (IꢁIo) of TFA-1 upon addition of 2.2 equivalents of various anion salts. c) The selectivity
of TFA-1 for Fꢁ in the presence of other anions and d) with alternate additions of TBAF and TFA for five re-
peating cycles.
termined on
a Kofler apparatus.
High-resolution ESI mass spectra
were obtained on a Bruker APEX II
FT-MS mass spectrometer. MALDI-
TOF-MS were recorded on an auto-
flex III smartbeam mass spectrometer (Bruker Daltonics). DLS measure-
ments were performed on a Brookhaven BI-200SM spectrometer. TEM
situation is markedly different to that controlled by TFA/P1-
tBu cycles (Figure 5c,d), whereby a reversible change in the
fluorescence intensity was clearly observed, albeit with some
loss of the signal within the first cycles.
AHCTUNGTRENNUNG
images were acquired with an FEI Tecnai F30 microscope operating at
300 kV. SFM measurements were performed on a commercial Multimode
AFM apparatus (Nanoscope IIIa, Veeco Instruments, Santa Barbara) op-
erated in tapping mode. UV/Vis absorption spectra were recorded on a
Shimadzu 2550 spectrophotometer. Luminescence measurements were
made on a Hitachi F-7000 spectrofluorimeter with a xenon lamp as the
excitation source. Fluorescence lifetimes in solution were measured by
using a commercially available time-correlated single-photon counting in-
strument (Edinburgh Instruments, model FL920 CDT) excited with a
nanosecond flash lamp. The lifetime data were deconvoluted from the in-
strumental response and fitted to double-exponential equations.
Conclusion
We have fabricated a supramolecular hyperbranched poly-
mer (SHP), denoted as TFA-1, by adding a slight excess of
TFA to a solution of the AB2 monomer 1 in dichloro-
Synthesis and characterisation
ACHTUNGTRENNUNG
Compound 5: 1,3,5-Tris(para-iodophenyl)benzene[7] (4; 0.68 g, 1.0 mmol),
4-formylphenylacetylene[8] (0.13 g, 1.0 mmol), [Pd
ACHTNUGTRNEUNG(PPh3)4] (0.58 g,
ACHTUNGTRENNUNG
0.05 mmol), CuI (19.0 mg, 0.10 mmol), iPr2NH (0.20 g, 2.0 mmol), and
THF (20 mL) were added to a Schlenk tube under an argon atmosphere
and the resulting mixture was stirred for 48 h at 508C. The crude product
was obtained by removing the solvent under reduced pressure, and was
then further purified by column chromatography on silica gel (CH2Cl2/
petroleum ether, 2:1). Compound 5 was isolated as a yellow solid (89 mg,
13% yield). M.p. 119–1218C; 1H NMR (400 MHz, CDCl3, Me4Si): d=
stacking interactions between the conjugated cores. The re-
sulting TFA-1 was depolymerised upon the addition of a
slight excess of P1-tBu or Fꢁ, whereupon the host–guest rec-
ognition was lost as a result of deprotonation or a stronger
ion-pairing effect involving the ammonium recognition sites.
Such acid–base- or ion-pair-controlled fabrication of the
SHP induces a reversible change of the fluorescence intensi-
ty of the solution due to the controllable presence of p–p
stacking interactions between the conjugated cores. Upon
successive additions of acid and base to a solution of 1, re-
versible changes in the fluorescence intensity were observed
over at least ten cycles, after a partial signal loss within the
first cycles. The synthesis and characterisation of hyper-
branched p-conjugated polymers has attracted much atten-
tion recently because of their intriguing physical properties
and thus solution-processable applications in a variety of
7.45 (d, J
N
ACHTUNGTRENNUNG(H,H)=
1.6 Hz, 2H), 7.84 (d, J
N
ACHTUNGTRENNUNG
2H), 10.05 ppm (s, 2H); 13C NMR (100 MHz, CDCl3, Me4Si): d=89.6,
93.2, 93.6, 122.0, 125.2, 127.3, 128.9, 129.1, 129.4, 129.6, 132.1, 132.4,
135.5, 138.0, 140.2, 141.2, 141.6, 141.69, 191.4 ppm; MS (ESI): m/z calcd
for [C42H25IO2+H]+: 689.0977; found: 689.0989.
Compound 6: Compound 5 (0.69 g, 1.0 mmol), trimethylsilylacetylene
(0.15 g, 1.5 mmol), [PdACTHNUTRGNEUNG(PPh3)4] (0.58 g, 0.05 mmol), CuI (19.0 mg,
0.10 mmol), iPr2NH (0.20 g, 2.0 mmol), and THF (20 mL) were added to
a Schlenk tube under an argon atmosphere and the resulting mixture was
stirred for 48 h at 508C. The crude product was obtained by removing
the solvent under reduced pressure, and was then further purified by
column chromatography on silica gel (CH2Cl2/petroleum ether, 2:1). A
4928
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ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 4922 – 4930

pH-Controlled Formation of a Fluorescent Hyperbranched Polymer
FULL PAPER
yellow solid was isolated, which was redissolved in a mixture of EtOH
(15 mL) and CH2Cl2 (15 mL). K2CO3 (1.0 g) was added and the mixture
was stirred at room temperature for 3 h. It was then filtered and the sol-
vent was evaporated under reduced pressure. Compound 6 was obtained
versities (lzujbky-2012-k14, lzujbky-2011–29, and lzujbky-2010–162), the
Gansu Natural Scientific Foundation (1107RJZA214), and the Open
Project of State Key Laboratory of Supramolecular Structure and Mate-
rials of Jilin University (sklssm201214).
as
(400 MHz, CDCl3, Me4Si): d=3.18 (s, 1H), 7.62–7.74 (m, 16H), 7.80–7.82
(m, 3H), 7.89 (d, J
(H,H)=8.0 Hz, 4H), 10.04 ppm (s, 2H); 13C NMR
a
yellow solid (522 mg, 89% yield). M.p. 167–1688C; 1H NMR
ACHTUNGTRENNUNG
(100 MHz, CDCl3, Me4Si): d=78.2, 83.4, 89.6, 93.2, 121.6, 121.9, 125.2,
125.3, 127.2, 127.3, 129.5, 129.6, 132.1, 132.4, 132.7, 135.5, 141.0, 141.2,
141.6, 141.7, 191.4 ppm; MS (ESI): m/z calcd for [C44H26O2+H]+:
587.2011; found: 587.2022.
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Compound 9: Compounds
1.5 mmol), [Pd(PPh3)4] (0.58 g, 0.05 mmol), CuI (19.0 mg, 0.10 mmol),
6
(0.59 g, 1.0 mmol) and 7[9] (0.86 g,
ACHTUNGTRENNUNG
Et3N (0.20 g, 2.0 mmol), and toluene (20 mL) were added to a Schlenk
tube under an argon atmosphere and the resulting mixture was stirred
for 72 h at 1108C. The crude product was obtained by removing the sol-
vent under reduced pressure, and was further purified by column chro-
matography on silica gel (CH2Cl2/MeOH, 20:1). The intermediate 8 was
isolated as a brown solid (692 mg, 67%). MS (ESI): m/z calcd for
[C68H56O10+Na]+: 1055.3766; found 1055.3813. The intermediate
8
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(0.51 g, 0.5 mmol), benzylamine (0.11 g, 1.0 mmol), and toluene (10 mL)
were added to a Schlenk tube under an argon atmosphere and the result-
ing mixture was stirred for 24 h at 1108C. The solvent was removed
under reduced pressure. The crude product was recrystallised from di-
chloromethane and methanol. Compound 9 was isolated as a yellow solid
(581 mg, 96% yield). M.p. 83–878C; 1H NMR (400 MHz, CDCl3, Me4Si):
d=3.83 (s, 8H), 3.91–3.92 (m, 8H), 4.13–4.15 (m, 8H), 4.83 (s, 4H), 6.81
(d, J
N
ACHTUNGTREN(NUNG H,H)=1.2 Hz,
1H), 7.12 (d, JACHTUNGTRENNUNG
16H), 7.76–7.78 (m, 7H), 8.37 ppm (s, 2H); 13C NMR (150 MHz, CDCl3,
Me4Si): d=65.2, 69.5, 69.6, 69.9, 70.0, 71.4, 71.5, 88.0, 90.3, 90.6, 91.3,
113.5, 114.2, 185.8, 116.8, 121.5, 122.5, 123.0, 125.1, 125.2, 125.5, 125.6,
127.1, 127.2, 127.3, 127.4, 128.1, 128.1, 128.3, 128.4, 128.5, 128.6, 128.6,
128.7, 131.9, 132.1, 132.3, 135.9, 139.2, 140.3, 140.9, 141.7, 141.9,
161.2 ppm; MS (ESI): m/z calcd for [C82H70N2O8+H]+: 1211.5210; found:
1211.5321.
Compound 1: Compound 9 (0.12 g, 0.1 mmol), NaBH4 (0.19 g, 0.5 mmol),
and THF (10 mL) were added to a Schlenk tube under an argon atmos-
phere and the resulting mixture was stirred for 24 h at 608C. The crude
product was obtained by removing the solvent under reduced pressure.
Compound 1 was isolated as a yellow solid by recrystallising the crude
product from dichloromethane and methanol (112 mg, 92% yield). M.p.
90–938C; 1H NMR (400 MHz, CDCl3, Me4Si): d=3.81 (s, 4H), 3.83 (s,
4H), 3.84 (s, 8H), 3.91–3.93 (m, 8H), 4.14–4.17 (m, 8H), 6.82
(d, J
ACHTUNGTRENNUNG(H,H)=8.4 Hz, 2H), 6.86–6.89 (m, 4H), 7.05 (s, 1H), 7.13 (d,
JACHTUNGTRENNUNG(H,H)=8.4 Hz, 2H), 7.24–7.38 (m, 13H), 7.52–7.54 (m, 4H), 7.63–7.70
(m, 12H), 7.77–7.79 ppm (m, 4H); 13C NMR (150 MHz, CDCl3, Me4Si):
d=53.0, 53.2, 69.5, 69.6, 69.9, 70.0, 71.4, 71.5, 88.0, 89.2, 90.5, 90.6, 113.5,
114.2, 115.9, 116.8, 121.6, 121.9, 122.9, 123.0, 125.2, 125.5, 126.9, 127.0,
127.1, 127.2, 127.3, 127.4, 128.1, 128.2, 128.3, 128.4, 128.5, 128.6, 128.7,
131.8, 132.1, 132.2, 132.3, 140.2, 140.6, 140.9, 141.8, 141.8, 148.6, 149.0,
149.6 ppm; 1H NMR (600 MHz, CD2Cl2): d=3.74 (s, 8H), 3.78 (s, 4H),
3.80 (s, 4H), 3.83 (s, 4H), 3.85 (s, 4H), 4.09 (s, 4H), 4.12 (s, 4H), 6.84 (d,
J
N
ACHTUNGTREN(NUNG H,H)=8.4 Hz,
ACHTUNGTRENNUNG
J
N
N
ACHTUNGTRENNUNG
8.4 Hz, 2H), 7.64 (d, JACHTUNGTRENNUNG(H,H)=7.8 Hz, 4H), 7.71–7.77 (m, 6H), 7.81 (s,
1H), 7.84 ppm (s, 2H); MS (ESI): m/z calcd for [C82H74N2O8+H]+:
1215.5518; found: 1215.5530.
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This work is supported by the National Natural Scientific Foundation of
China (20901034, 20931003, and 51173073), the Program for New Centu-
ry Excellent Talents in University (NCET-10–0462), the Specialized Re-
search Fund for the Doctoral Program of Higher Education
(20100211110023), the Fundamental Research Funds for the Central Uni-
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Received: December 4, 2012
Published online: February 18, 2013
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Chem. Eur. J. 2013, 19, 4922 – 4930