Synthesis and characterization of a luminescence metallosupramolecular hyperbranched polymer

Article abstract of DOI:10.1039/c3cc41069b

A luminescence supramolecular hyperbranched polymer is reversibly fabricated by successively adding acid and base to a solution of an AB ₂ platinum(ii) monomer in dichloromethane on the basis of acid-base controllable host-guest recognition of dibenzo[24]crown-8 moieties with dialkylammonium ion centers. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c3cc41069b

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Synthesis and characterization of a luminescence
metallosupramolecular hyperbranched polymer†
Cite this: Chem. Commun., 2013,
49, 3333
Bingran Yu, Shuwen Guo, Lipeng He and Weifeng Bu*
Received 7th February 2013,
Accepted 5th March 2013
DOI: 10.1039/c3cc41069b
www.rsc.org/chemcomm
A luminescence supramolecular hyperbranched polymer is reversibly
fabricated by successively adding acid and base to a solution of an AB2
platinum(II) monomer in dichloromethane on the basis of acid–base
controllable host–guest recognition of dibenzo[24]crown-8 moieties
with dialkylammonium ion centers.
Hyperbranched polymers synthesized by covalent polymerization
are of great current interest because of their highly branched
nanoarchitectures and low melt and solution viscosities in compar-
ison with their linear analogues.1 The combination of hyper-
branched polymers with metal complexes leads to the formation
of functional polymer hybrids with interesting redox,2a,b catalytic2c,d
and luminescence properties.2e On the other hand, the occurrence
of noncovalent recognition in the solutions of organic AB2 or other
monomers facilitates the fabrication of supramolecular hyper-
branched polymers (SHPs).3 They are usually believed to be highly
Fig. 1 (Top) The synthetic routes of 1 and 2. (Bottom) Schematic representation
of the reversible formation of the SHP, TFA-1.
dynamic and responsive to external stimuli.3e,f In this context, the of trifluoroacetic acid (TFA) to a solution of 1 in CH2Cl2 and
noncovalent fabrication of functional SHPs by using metal complex thus protonation of the amine groups, the SHP of TFA-1 is
monomers is important for potential applications in the fields of fabricated on the basis of the host–guest recognition between
chemical sensing, catalysis, and optoelectronic devices. However, DB24C8 moieties and dialkylammonium ion centers (Fig. 1).
such assemblies are rarely explored.
The target AB2 monomer 1 and the model compound 2 were
Square-planar platinum(II) complexes have received great atten- synthesized according to the routes depicted in Fig. 1. The
tion in the past few decades due to their intriguing photochemical platinum(II) complex 4 was achieved by stirring a mixture of 3
and photophysical properties.4 Among them, the platinum(II) (ref. 6a) and Pt(DMSO)2Cl26b in CH2Cl2 at room temperature. The
complexes of [Pt(diimine)(CRCR)2] show strong luminescence complexes 2 and 5 were synthesized by the chloride–arylacetylide
in their fluid solutions at room temperature.4a,d–k Here, we exchange reactions of 4 under CuI-catalyzed conditions. The
synthesized a luminescence [Pt(diimine)(CRCR)2] monomer 1 imine 6 was obtained by treating benzylamine with 5, which was
with a dibenzo[24]crown-8 (DB24C8) ring and two secondary further reduced to form 1. They were characterized in detail
amine centers (Fig. 1). It is well known that a DB24C8 ring can using 1H NMR and high resolution electrospray ionization mass
+
host a dialkylammonium ion center (–CH2NH2 CH2–) on the spectrometry (ESI-MS) and elemental analyses.
basis of a cooperative combination of [N+–Hꢀ ꢀ ꢀO] and [C–Hꢀ ꢀ ꢀO]
The solution of 1 in CD2Cl2 was treated with a slight excess of
1
hydrogen bonds and p–p stacking interactions.5 Upon addition TFA (2.2 equiv.), which was then subjected to H NMR measure-
ments. The resulting 1H NMR spectrum showed significant changes
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 City,
Gansu Province, China. E-mail: buwf@lzu.edu.cn; Fax: +86 931 8912582
† Electronic supplementary information (ESI) available: Experimental details and
supplementary figures. CCDC 923693 and 923694. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c3cc41069b
in comparison with that of 1 (Fig. 2a and b). With reference to
previous 1H NMR studies on the host–guest recognition of DB24C8
derivatives with dialkylammonium ion groups,5 a broad peak at
d = 4.5 ppm was clearly assigned to the benzylic methylene
protons adjacent to the NH2 centres hosted by the DB24C8
moieties (Hac + Hbc), whereas the broad signal at d = 3.8–3.9 ppm
+
c
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After adding 2.2 equivalents of TFA to this solution, two modes
were observed at Dhs of 60 ꢁ 20 nm and 8330 ꢁ 416 nm
(Fig. S1d, ESI†). The former revealed the SHP formation of
TFA-1, whereas the latter corresponded to multi-aggregate
associates of the SHP. Similar results were also observed in
the other supramolecular polymers.3b,f When the same solution
was treated with 2.4 equivalents of P1-t-Bu, Dh returned back to
1.2 ꢁ 0.2 nm (Fig. S1e, ESI†), demonstrating once again
the reversible formation of TFA-1. The size and morphology
of TFA-1 were further examined by transmission electron
microscopy (TEM). Fig. S1f and g (ESI†), respectively, show
typical images of the aggregates obtained by dropcasting the
dichloromethane solutions of TFA-1 at the concentrations of
1 ꢂ 10ꢃ3 mol Lꢃ1 and 5 ꢂ 10ꢃ5 mol Lꢃ1 onto carbon-coated
copper grids. The former image shows aggregates with a
diameter of 20–220 nm, whereas in the latter case, the diameter
of aggregates ranged from 15 to 80 nm. The increase in the size
of TFA-1 with the concentration was consistent with the viscosity
measurement listed above and the picture of other supramole-
Fig. 2 1H NMR spectra (600 MHz, in CD2Cl2, 1.0 ꢂ 10ꢃ3 mol Lꢃ1) of 1 (a), TFA-1
produced by adding 2.2 equivalents of TFA into the solution of 1 (b), 1 obtained
by treating 2.4 equivalents of P1-t-Bu to TFA-1 (c).
cular polymers.2,7 Therefore, four lines of experimental evidence,
1
was assigned to the methylene protons adjacent to the uncom- such as the H NMR spectrum, viscosity, DLS, and TEM, were
plexed ammoniums. The methylene proton Ha on the crown consistent with the formation of the SHP of TFA-1.
ether was shifted upfield from d = 4.5 to 4.4 ppm together with
Both 1 and TFA-1 in CH2Cl2 (5 ꢂ 10ꢃ5 mol Lꢃ1) exhibited a
the significant broadening. Furthermore, the resonances of the 5d(Pt) - p*(phenanthroline) metal-to-ligand charge transfer
phenanthroline core in TFA-1 were broadened to such an extent (MLCT) transition at around 410 nm (Fig. 3a). The difference is
that they were hardly detected. This phenomenon was assigned that the absorption band between 415 and 500 nm in TFA-1
to the presence of strong p–p stacking interactions in TFA-1. showed a significant blue shift in comparison with that in 1,
These signal shifts indicated that the crown ether moieties were which could be basically recovered by further treating 2.4
threaded by the dialkylammonium ions. The resonance broad- equivalents of P1-t-Bu with the solution of TFA-1 (Fig. 3a). Upon
ening was a typical feature of polymers because of the long excitation at 410 nm, the dichloromethane solution of 1
relaxation times. Therefore, the SHP of TFA-1 was formed upon (Fig. 3b) exhibited a strong 3MLCT luminescence band at a
treating 1 with 2.2 equivalents of TFA (Fig. 1). The broad
lmax of 565 nm. With the addition of 2.2 equivalents of TFA and
resonances of both Hf and Hac + Hbc + Ha were basically thus the formation of TFA-1, the emission band was blue
independent of others (Fig. 2b). Therefore, the percentage shifted to 548 nm together with a notable decrease in its
recognition (p) and polymerization degree (n) could be intensity (Fig. 3b). The further treatment of this solution with
determined according to the following equation: p = n/(n + 1) =
A(Hac + Hbc + Ha)/[2A(Hf)], where A(Hf) and A(Hac + Hbc + Ha)
are the average integrals of Hf and Hac + Hbc + Ha, respectively.
The values of p and n were estimated to be 99.0 ꢁ 0.2% and
99 ꢁ 15 at a concentration of 1.0 ꢂ 10ꢃ3 mol Lꢃ1. The
corresponding molecular weight was then calculated to be
1.4 ꢁ 0.2 ꢂ 105 g molꢃ1. Upon the successive addition of
2.4 equivalents of N-tert-butyl-N0,N0,N00,N00,N0 0 0,N00 0-hexamethyl-
phosphorimidic triamide (P1-t-Bu) in the same solution, the
chemical shifts of the 1H NMR signals in the regenerated
spectrum were almost consistent with those in the original one
(Fig. 2c), indicating that the formation of TFA-1 was reversible.
Further evidence for the formation of the SHP resulted from
the viscosity measurement at 18 1C. The reduced viscosity of the
solution of TFA-1 in CH2Cl2 was remarkably enhanced relative
to that of 1 and increased with the increasing concentration
(Fig. S1a and b, ESI†). This result was consistent with the
Fig. 3 Absorption (a) and emission (b) spectral changes of 1 (5 ꢂ 10ꢃ5 mol Lꢃ1
,
SHP formation of TFA-1 and thus the increase in the size of
TFA-1 with the increasing concentration. The dynamic light
scattering (DLS) measurement of the solution of 1 in CH2Cl2
(1 ꢂ 10ꢃ3 mol Lꢃ1) provided a hydrodynamic diameter (Dh) of
1.3 ꢁ 0.2 nm (Fig. S1c, ESI†), consistent with its molecular size.
in CH2Cl2), TFA-1 obtained by adding 2.2 equivalents of TFA into the solution of 1,
and 1 obtained by treating 2.4 equivalents of P1-t-Bu to TFA-1. Absorption (c) and
ꢃ
emission (d) spectral changes of 2 (5 ꢂ 10ꢃ5 mol Lꢃ1, in CH2Cl2), [2ꢀ7]+PF6
obtained by adding equimolar 7 into the solution of 2, and 2 obtained by
treating 2.4 equivalents of P1-t-Bu with [2ꢀ7]+PF6
.
ꢃ
c
3334 Chem. Commun., 2013, 49, 3333--3335
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2.4 equivalents of P1-t-Bu resulted in a remarkable lumines-
This work was supported by the NSFC (51173073, 20901034,
cence enhancement centered at 555 nm, where the intensity and 20931003), the Program for New Century Excellent Talents
was unexpectedly higher than the original level of 1 (Fig. 3b). in University (NCET-10-0462), the Specialized Research Fund for
To clarify the mechanism of this unusual luminescence the Doctoral Program of Higher Education (20100211110023),
behavior, we further studied binding behaviour of the model the Fundamental Research Funds for the Central Universities
compound 2 with dibenzylammonium hexafluorophosphate (7ꢃ). (lzujbky-2012-k14, lzujbky-2011-29 and lzujbky-2010-162), the
Quantitative formation of the host–guest complex of [2ꢀ7]+PF6
Gansu NSF (1107RJZA214) and the Open Project of State Key
1
was evidenced by the H NMR spectra (Fig. S2, ESI†) and high Laboratory of Supramolecular Structure and Materials of Jilin
resolution ESI-MS (Fig. S3, ESI†). This pseudorotaxane can be University (sklssm201314).
unthreaded by the addition of 1.2 equivalents of P1-t-Bu. In this
acid–base cycle, significant decrease and increase in the lumines-
cence intensities were also observed (Fig. 3d), whereas the absorp-
Notes and references
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(Fig. 3c). The changes in the luminescence intensities were
consistent with those of 1 and TFA-1. However, no shift was
observed for the emission band in the model compounds
(Fig. 3d), which was in sharp contrast to the situation of 1 and
TFA-1 mentioned above. Therefore, the notable blue shifts of both
the absorption and emission bands in TFA-1 were attributed to
the presence of strong p–p stacking interactions, as revealed by its
1H NMR spectrum. The decrease in the luminescence intensities
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enhancements were also detected (Fig. S4, ESI†). Therefore, the
unexpected luminescence enhancement could be due to the
presence of the generated TFAꢀP1-t-Bu salt in the solutions of 1
and 2 after an acid–base cycle.
Crystals of both 8ꢀCH2Cl2 and 8 were, respectively, grown by
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The former structure revealed a weak C–Hꢀ ꢀ ꢀp(CRC) interaction
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bis(acetylide) moiety (Fig. S5 and Table S1, ESI†), whereas such a
weak interaction did not exist in the latter case (Fig. S6 and Table
S1, ESI†). The solution of 2 obtained after an acid–base cycle was
further analysed by ESI-MS and a new peak appeared at m/2z =
517.1167 due to [2ꢀCH2Cl2 + 2H]2+ with a calculated value
of 517.1216 (Fig. S7, ESI†). We therefore inferred that the weak
C–Hꢀ ꢀ ꢀp(CRC) interactions occurred between the proton of the
CH2Cl2 molecule and the bis(acetylide) moiety of both 1 and 2 in
the solutions containing TFAꢀP1-t-Bu or other organic salts,
leading to their unexpected luminescence enhancements.
In summary, we have fabricated a luminescence SHP of
TFA-1 by adding a slight excess of TFA to a solution of 1 in
CH2Cl2 on the basis of the host–guest recognition between
DB24C8 moieties and dialkylammonium ion centers. The
resulting TFA-1 is depolymerised by adding a slight excess of
P1-t-Bu. Such acid–base controlled fabrication of the SHP
induces controllable changes of the luminescence band and
its intensity in the solution. This switchable supramolecular
polymer may set a stage for developing smart materials and
nanomachinery on the basis of functional metal complexes.
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c
This journal is The Royal Society of Chemistry 2013
Chem. Commun., 2013, 49, 3333--3335 3335