Conjugated coordination polymers based on 8-hydroxyquinoline ligands: Impact of polyhedral oligomeric silsesquioxanes on solubility and luminescence

Article abstract of DOI:10.1039/c1jm11389e

A novel bis(8-hydroxyquinoline) zinc-based conjugated coordination polymer with polyhedral oligomeric silsesquioxanes (POSSs) in the side chains has been synthesized. In contrast to the traditional coordination polymers, this new coordination polymer has

Full text of DOI:10.1039/c1jm11389e

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PAPER
Conjugated coordination polymers based on 8-hydroxyquinoline ligands:
impact of polyhedral oligomeric silsesquioxanes on solubility and
luminescence†
*
Fanfan Du, Hu Wang, Yinyin Bao, Bin Liu, Haiting Zheng and Ruke Bai
Received 3rd April 2011, Accepted 4th May 2011
DOI: 10.1039/c1jm11389e
A novel bis(8-hydroxyquinoline) zinc-based conjugated coordination polymer with polyhedral
oligomeric silsesquioxanes (POSSs) in the side chains has been synthesized. In contrast to the
traditional coordination polymers, this new coordination polymer has good solubility in common
organic solvents and can be characterized and processed in solution. The thermal stability of the
polymer is also enhanced by POSSs and the fluorescence is not quenched by intermolecular aggregation
in the solid state. This fluorescence enhancement is due to the significant effect of the bulky POSS units.
Nanoparticles is obtained by a facile self-assembly method of the polymer materials and the
nanoparticles show strong fluorescence at 545 nm.
demanding groups into the conjugated polymers has attracted
Introduction
much attention recently, because it can improve solubility and
simultaneously maintain solid state luminescence. For instance,
iptycene units are particularly useful in the design of advanced
materials because of their three-dimensional, noncompliant
structures.14 Iptycenes can provide steric blocking, which can
prevent strong interactions between polymeric chromophores
that have a strong tendency to form nonemissive exciplex
complexes.
In recent years, metal–organic coordination polymers have
received increasing interest due to their attractive electronic,
catalytic, optic, and magnetic properties.1–4 In comparison with
common polymers, coordination polymers are easy to synthesize
and possess much greater design flexibility due to their abundant
and tunable synthons, e.g. variation in oxidation state, size, and
number of center metal atoms or ions, ligand environment, and
geometry.5 Conjugated coordination polymers whose structures
consist of conjugated ligands interconnected by metallic nodes
have been extensively studied because of their numerous appli-
cations including chemical sensors, electroluminescence devices,
electrocatalysis, batteries, smart windows, and memory
devices.6–11 Unfortunately, most of the conjugated coordination
polymers suffer from poor solubility that limits the ability to
characterize the materials in solution and to process them for
applications.7 To resolve this issue, significant efforts have been
focused on the synthesis of ligands with pendant organic groups,
such as long or bulky alkyl substituents, even with polymer
chains. The introduction of a flexible polymer chain into ligands
is efficient for enhancing the solubility, however this approach
usually yields much increased molecular weight, and the molec-
ular weight of the ligands is difficult to control due to uncon-
trolled polymerization.12,13 The incorporation of sterically
8-Hydroxyquinoline (8-HQ), as a very useful ligand, has often
been incorporated into conjugated coordination polymers. These
polymers may find potential applications in OLED devices,
optical sensors, and dye-sensitized solar cells due to their
exceptional optical properties and thermal stability.5,15–18
However, these polymers usually possess very poor process-
ability due to lack of thermoplasticity and solubility in common
solvents.19,20 Moreover, it was reported that the coordination
polymers often exhibit strong fluorescence in solution, but
undergo fluorescence quenching in the solid state because of
molecular aggregation.17 These disadvantages have remarkably
limited the applications of the coordination polymers in optical
and electronic devices.
Recently, polyhedral oligomeric silsesquioxanes (POSSs)21
have attracted a great deal of attention in the materials field
because of their unique nanoscale cage-shaped structures and
good solubility in organic solvents. Some of the novel hybrid
polymers with promising properties such as thermal and flam-
mability resistance or special surface properties have been
prepared by incorporating POSS into polymers.22–32 In addition,
POSS has also been successfully used in the enhancement of
solubility, luminescence efficiencies, and colour purities of the
conjugated polymers by suppressing undesired excimer
CAS Key Laboratory of Soft Matter Chemistry, Department of Polymer
Science and Engineering, University of Science and Technology of China,
Hefei, P. R. China 230026. E-mail: bairk@ustc.edu.cn; Fax: +0086-551-
3631760; Tel: +0086-551-3600722
† Electronic supplementary information (ESI) available: Synthetic
procedures, 1H NMR spectra, TGA curve of 2b, PL spectra, TEM
image. See DOI: 10.1039/c1jm11389e
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formation and intermolecular aggregation.22,23 However, there
has not been any paper reporting the conjugated metal coordi-
nation polymers with POSS moieties.
Temperature-dependent fluorescence spectra were acquired at
ꢁ
a heating rate of 0.2 C minꢀ1
.
In this paper, we report the synthesis and properties of a 8-HQ
conjugated coordination polymer with pendant POSS moieties.
The new coordination polymer materials have good solubility in
common low-boiling point solvents and simultaneously maintain
their luminescence in the solid state. Nanoparticles of the poly-
mer materials can be obtained by a facile self-assembly method
and the nanoparticles show strong fluorescence at 545 nm.
Transmission electron microscopy (TEM). TEM observations
were conducted on a JEM100-SX transmission electron micro-
scope at an acceleration voltage of 200 kV. The sample for TEM
observations was prepared by placing 10 mL of micellar solution
on copper grids coated with thin films of Formvar and carbon
successively.
Dynamic laser light scattering (DLS). A commercial spec-
trometer (ALV/DLS/SLS-5022F) equipped with a multi-tau
digital time correlator (ALV5000) and a cylindrical 22 mW
UNIPHASE He–Ne laser (l0 ¼ 632 nm) as the light source was
employed for DLS measurements. Scattered light was collected
at a fixed angle of 90ꢁ for duration of ꢂ10 min.
Experimental
Characterizations
Nuclear magnetic resonance spectroscopy (NMR). H and 13C
1
NMR spectra were recorded on a Bruker AVANCE II spec-
1
trometer (resonance frequency of 400 MHz for H) operated in
Material synthesis
the Fourier transform mode. CDCl3 and TMS were used as the
solvent and internal standard, respectively.
All reagents and solvents were purchased commercially and used
without further purification. Aminopropylheptakis (isobutyl)
POSS (POSS-NH2, 97%) was obtained from Hybrid Plastics.
5-Formyl-8-hydroxyquinoline (denoted as FHQ) was synthesized
from 8-hydroxyquinoline according to the reported procedure
(ESI†).33 The mainsynthesisprocedurewasillustratedinScheme1.
Fourier transform infrared spectroscopy (FT-IR). FT-IR
spectra were recorded as KBr pellets with a Bruker VECTOR-22
IR spectrometer. The spectra were collected at 64 scans with
a spectral resolution of 4 cmꢀ1
.
Elemental analyses. Elemental analyses were performed on
N-Propylheptakis(isobutyl)
POSS-3,5-dinitrobenzamide
a VARIO ELIII C, H, and N analyzer.
(PDNB). Modification of the procedures in the literature was
used.34 POSS-NH2 (3.5 g, 4 mmol), 3,5-dinitrobenzoic acid
(DNBA, 1.3 g, 6 mmol), dry tetrahydrofuran (THF) (30 ml), and
N,N0-dicyclohexylcarbodiimide (DCC, 1.0 g, 5 mmol) were
added into a flask and the mixture was stired for 24 h. After the
reaction, the precipitate was filtered off, and the filtrate was
rotary concentrated. The precipitate was obtained by dropping
the solution into acetonitrile, and the precipitation cycle was
repeated twice more. PDNB was collected by filtration and dried
in vacuo at 40 ꢁC for 24 h with a yield of 93%. 1H NMR dH (400
MHz; CDCl3; Me4Si): d ¼ 9.17 (1H, ArH), 8.93 (2H, ArH), 6.29
(1H, NH), 3.53 (2H, NHCH2), 1.85 (7H, SiCH2CH), 1.66 (2H,
Solubility measurement. A solubility experiment was per-
formed by gravimetric analysis of the evaporated solvent of
saturated solution.
Gel permeation chromatography (GPC). Molecular weights
and molecular weight distributions were determined by GPC
equipped with a Waters 1515 pump, a Waters 2414 RI detector,
and Waters UV/RI detectors (set at 30 ꢁC). It used a series of
three linear Styragel columns HR3, HR4, and HR6 at an oven
temperature of 45 ꢁC. The eluent was DMF at a flow rate of 1.0
ml minꢀ1. A series of low-polydispersity polystyrene (PS) stan-
dards were employed for the GPC calibration.
Thermogravimetric analysis (TGA) and differential thermal
analysis (DTA). TGA and DTA were carried out on a SDT Q600
V20.9 Build 20 instrument at a heating rate of 10 ꢁC minꢀ1 in air
ꢁ
from room temperature to 600 C.
Ultraviolet-visible spectroscopy (UV-vis). UV-Vis spectra of
solutions were recorded with a Pgeneral UV-Vis TU-1901
spectrometer.
Photoluminescence (PL) spectroscopy. Emission spectra of
solutions were measured using a RF-5301PC spectrometer. The
quantum yields in solution were determined twice. Quinine
bisulfate in 0.05 M H2SO4 solution was used as the standard. The
measurements were averaged. Solid-state emission spectra were
measured in powders by JY Fluorolog-3-Tou, equipped with
solid state accessories. The PL quantum yields in the solid state
were determined as absolute values in an integrating sphere.
Scheme 1 Synthetic procedure of the ligands and polymers.
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SiCH2CH2), 0.95 (42H, CH(CH3)2), 0.59 (16H, SiCH2). 13C
NMR dC (100 MHz; CDCl3; Me4Si): d ¼ 162.7, 148.8, 138.4,
127.2, 121.1, 43.2, 25.9–22.6, 9.7. Anal. Calcd for
C38H73N3O17Si8: C 42.74%; H 6.84%; N 3.94%. Found: C
42.76%; H 6.87%; N 3.92%.
solution was washed by dilute muriatic acid and dilute sodium
hydroxide three times, respectively. The organic solution was
dried with sodium sulfate and subsequently passed through
a neutral alumina column to move the salts. DDNB was
obtained by rotary evaporation and drying in vacuo at 40 ꢁC.
Yield ¼ 95%; 1H NMR dH (400 MHz; CDCl3; Me4Si): d ¼ 9.16
(1H, ArH), 8.94 (2H, ArH), 6.34 (1H, NH), 3.54 (2H, NHCH2),
1.68 (2H, NCH2CH2), 1.41–1.26 (18H, NCH2CH2(CH2)9), 0.88
(3H, CH3). 13C NMR dC (100 MHz; CDCl3; Me4Si): d ¼ 162.6,
148.5, 138.4, 127.3, 121.0, 43.1, 32.0, 29.6, 27.0, 22.7, 14.7. Anal.
Calcd for C19H29N3O5: C 60.14%; H 7.70%; N 11.07%. Found: C
60.13%; H 7.73%; N 11.06%.
N-Propylheptakis(isobutyl) POSS-3,5-diaminobenzamide
(PDAB). PDNB was reduced with ammonium formate catalyzed
by Pd/C.35 PDNB (3.2 g, 3 mmol), ammonium formate (1.9 g, 30
mmol), 5% palladium on carbon (0.1 g), and ethanol (30 ml) were
added into a flask and the mixture was refluxed for 6 h. After the
reaction was completed, the precipitate was filtered out, and the
filtrate was rotary evaporated. The solid was dissolved in 5 ml
THF and precipitation in 50 ml water. PDAB was collected by
filtration and dried in vacuo at 50 ꢁC for 48 h with a yield of 95%.
1H NMR dH (400 MHz; CDCl3; Me4Si): d ¼ 6.44 (2H, ArH), 6.11
(1H, ArH), 5.94 (1H, C(]O)NH), 3.69 (4H, NH2), 3.39 (2H,
NHCH2), 1.85 (7H, SiCH2CH), 1.66 (2H, SiCH2CH2), 0.95
(42H, CH(CH3)2), 0.59 (16H, SiCH2). 13C NMR dC (100 MHz;
CDCl3; Me4Si): d ¼ 168.4, 148.0, 137.5, 104.3, 104.1, 42.4, 25.8–
22.6, 9.7. Calcd for C38H77N3O13Si8: C 45.28%; H 7.65%; N
4.17%. Found: C 42.19%; H 7.76%; N 4.14%.
N-Dodecyl-3,5-diaminobenzamide (DDAB). DDAB was
prepared similar to PDAB with a yield of 91%. 1H NMR dH (400
MHz; CDCl3; Me4Si): d ¼ 6.45 (2H, ArH), 6.12 (1H, ArH), 5.97
(1H, NH), 3.70 (4H, NH2), 3.40 (2H, NHCH2), 1.56 (2H,
NCH2CH2), 1.32–1.25 (18H, NCH2CH2(CH2)9), 0.88 (3H,
CH3). 13C NMR dC (100 MHz; CDCl3; Me4Si): d ¼ 168.1, 148.2,
137.4, 104.3, 104.4, 40.0, 31.9, 29.7, 29.6, 29.4, 27.0, 22.7, 14.9.
Anal. Calcd for C19H33N3O1: C 71.43%; H 10.41%; N 13.15%.
Found: C 71.33%; H 10.50%; N 13.11%.
3,5-Bis(8-HQ Schiff base)-N-propyl heptakis(isobutyl) POSS
benzamide (1a). PDAB (2.2 g, 2.2 mmol), FHQ (0.76 g, 4.4
mmol), and 100 ml ethanol were refluxed for 30 h under nitrogen.
After the reaction, the solution was concentrated to obtain
a yellow product 1a. The product was then filtered off and dried
at 30 ꢁC for 24 h under vacuum. Yield ¼ 82%; 1H NMR dH (400
MHz; CDCl3; Me4Si): d ¼ 10.14 (2H, N]CH), 9.68 (2H, 4-H of
8-HQ), 8.85 (2H, 2-H of 8-HQ), 8.01 (2H, 6-H of 8-HQ), 7.65
(2H, 3-H of 8-HQ), 7.29 (2H, 7-H of 8-HQ), 6.44 (2H, ArH), 6.12
(1H, ArH), 5.96 (1H, C(]O)NH), 3.39 (2H, NHCH2), 1.85 (7H,
SiCH2CH), 1.66 (2H, SiCH2CH2), 0.95 (42H, CH(CH3)2), 0.59
(16H, SiCH2). 13C NMR dC (100 MHz; CDCl3; Me4Si): d ¼
167.2, 161.7, 158.0, 155.7, 154.0, 148.9, 140.6, 138.5, 134.9,
127.9–124.8, 116.7, 116.0, 42.4, 25.9–22.6, 9.7. Mn(GPC) ¼
1,400; Mw/Mn ¼ 1.09; Anal. Calcd for C58H87N5O15Si8: C
52.85%; H 6.60%; N 5.32%. Found: C 52.77%; H 6.64%; N
5.19%.
3,5-Bis(8-HQ Schiff base)-N-dodeyl benzamide (1b). 1b was
1
synthesized similar to 1a with a yield of 77%. H NMR dH (400
MHz; CDCl3; Me4Si): d ¼ 10.14 (2H, N]CH), 9.67 (2H, 4-H of
8-HQ), 8.85 (2H, 2-H of 8-HQ), 8.00 (2H, 6-H of 8-HQ), 7.65
(2H, 3-H of 8-HQ), 7.30 (2H, 7-H of 8-HQ), 6.44 (2H, ArH), 6.11
(1H, ArH), 5.99 (1H, C(]O)NH), 3.40 (2H, NHCH2), 1.56 (2H,
NCH2CH2), 1.32–1.25 (18H, NCH2CH2(CH2)9), 0.88 (3H,
CH3). 13C NMR dC (100 MHz; CDCl3; Me4Si): d ¼ 167.1, 161.5,
158.3, 156.1, 154.1, 148.9, 140.7, 138.3, 135.1, 127.9, 127.1, 126.7,
124.8, 116.5, 116.0, 40.4, 31.9, 29.7, 29.6, 29.4, 27.0, 22.7, 14.9.
Anal. Calcd for C39H43N5O3: C 82.98%; H 7.62%; N 12.41%.
Found: C 82.73%; H 7.66%; N 12.37%.
Preparation of the coordination polymer 2b. 2b was synthesized
similar to 2a with
a yield of 87%. Anal. Calcd for
(C39H41N5O3Zn): C 74.64%; H 6.54%; N 11.16%. Found:
C74.57%; H 6.61%; N 11.07%.
Preparation of the coordination polymer 2a. The zinc-based
coordination polymer 2a was prepared by adding an equivalent
amount of ZnCl2 into a solution of 1a in DMF under vigorous
stirring, and the solution turned from yellow to red color
immediately. The reaction mixture was poured into methanol
and a precipitate was formed. The red precipitate was washed
with absolute methanol and dried in vacuo at 40 ꢁC. Yield ¼ 80%;
Results and discussion
Ligand 1a was obtained as yellow powder from the reaction of
FHQ and amine derivatives bearing a POSS moiety, according to
the literature.5,36 The molecular structure was verified using
NMR spectroscopy, as shown in the ESI†. The coordination
polymer 2a was synthesized using an equimolar ratio of the
ligands and ZnCl2 in DMF via a standard approach.19 UV-Vis
absorption spectra were performed to track the polymerization.
As shown in Fig. 1, a new absorption peak appears at 385 nm
after the polymerization, which is attributed to the p–p* tran-
sition of the metal complex, indicating the complexation of the
ligands to Zn2+. The FT-IR spectra for 1a and 2a are shown in
Fig. 2. For comparison’s sake, the FT-IR spectrum of FHQ was
also measured. The characteristic stretching vibration band of
C]O in FHQ is clearly observed at 1663 cmꢀ1. The reaction of
FHQ and PDAB can be confirmed by the following three bands:
the sharp band of C]N bond (Schiff’s base) at 1639 cmꢀ1 with
Mn(GPC)
¼
220 000; Mw/Mn
¼
1.34; Anal. Calcd for
(C58H85N5O15Si8Zn)n: C 50.43%; H 6.16%; N 5.07%. Found: C
49.70%; H 6.34%; N 5.01%.
N-Dodecyl-3,5-dinitrobenzamide (DDNB). Dodecylamine
(0.74 g, 4 mmol) and triethylamine (0.6 ml, 4.2 mmol) were added
to 30 ml dry THF (30 ml). Then 3,5-dinitrobenzoyl chloride (0.92
g, 4 mmol) in 20 ml THF was added dropwise at 0 ꢁC. After the
addition, the mixture was warmed to room temperature and
stirred overnight. After having been filtered, THF was rotary
evaporated and the solid was dissolved in CH2Cl2. The CH2Cl2
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solubility of 2a in ethanol was measured as 17 mg mlꢀ1. Mean-
while, we also synthesized another bis(8-HQ) ligand 1b which
bears dodecyl instead of POSS. However, unlike ligand 1a, its
zinc complex 2b shows poor solubility in common low-boiling-
point solvents such as ethanol and THF, and it is only slightly
soluble in DMF. These results obviously indicate that POSS is an
efficient building block for the solubility enhancement of the
coordination polymers due to the bulky and unique nanoscale
cage-shaped structure with eight isobutyl groups.
The thermal stability of the coordination polymers was eval-
uated by TGA and DTA in air with a heating rate of 10 ꢁC minꢀ1
.
The onset of the thermal degradation was determined to be 205
ꢁC and the Td was about 213 ꢁC for 2b (Fig. S16 in the ESI†). In
the case of 2a, we observed an increase of 35 ꢁC in the value of Td
as compared to 2b, and the onset of the thermal degradation was
determined as 214 ꢁC (Fig. 3). Moreover, the ceramic yield of 2a
is approximately 18% higher than theory (19%), suggesting
Fig. 1 UV-Vis absorption spectra of 1a and 2a in ethanol with 4.0 ꢃ
10ꢀ5 M (repeat units).
ꢁ
incomplete decomposition of the polymer at 600 C. This result
indicates that the grafted bulky POSS can enhance the thermal
stability and reduce the flammability of the coordination poly-
mer, as expected in most cases.23
The fluorescent properties of 2a and 2b have been studied. In
dilute DMF solution, both 2a and 2b show similarly high emis-
sion bands with maxima around 500 nm under the same condi-
tions (Fig. 4). The quantum yields of 2a and 2b in DMF are
determined to be 0.43 and 0.41, respectively. However, the
fluorescence of 2b is strongly quenched by intermolecular
aggregation in the solid state as shown in Fig. 5. By contrast with
2b, 2a shows a well resolved band at 613 nm, indicating only
weakly interacting p-stacks of chromophores in the solid state.
Polymer 2a shows a much higher quantum yield (0.26) in the
solid state as compared to polymer 2b (0.02). This enhancement
of solid-state luminescence efficiencies has been widely reported
for conjugated polymers.22,23 The reason may attribute to the
bulky POSS groups of 2a on the side chains, as optically inactive
spacers,39 which reduce the packing density of the chromophores
in the solid state. To the best of our knowledge, this is the first
example of the introduction of POSS units into the conjugated
coordination polymers to prepare solid state fluorescent
materials.
Fig. 2 FT-IR spectra (KBr) of FHQ, 1a and 2a.
Motivated by the attractive potentials of fluorescent polymer
nanoparticles particularly for the nanosized optoelectronic
devices applications, researchers have engaged in developing
a red-shift relative to the C]O absorption in FHQ;37 a group of
bands at 2800–3000 cmꢀ1 of C–H vibration stretching on the
eight pendant organic arms of POSS, and the Si–O–C stretching
band of the cubic cores of POSS located at 1111 cmꢀ1. The FT-IR
spectra further demonstrate that ligand 1a has been successfully
synthesized. The FT-IR spectrum of the coordination polymer
2a is also shown in Fig. 2. In comparison to 1a, the strong C]N
band (aromatic ring) in 2a shifts to 1659 cmꢀ1 19
and a new
,
band appears at 1334 cmꢀ1, associated with C–O vibrations of
C–O–Zn,38 indicating the complexation of the ligands to Zn2+.
The molecular weights and molecular weight distributions of
polymer 2a were 220 000 and 1.34, as determined by GPC in
DMF.
Compared to 3,5-bis(8-HQ Schiff base)–benzoic acid reported
by Shi’s group,5 which showed poor solubility in common
solvents, both 1a and 2a are readily soluble in most polar organic
solvents, such as ethanol, THF, and DMF. Further, the
Fig. 3 TGA and DTA curves of 2a measured in air.
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Fig. 6 Concentration dependence of the emission of 2a in CHCl3 excited
at 385 nm: concentration of repeat units: C1 ¼ 2.5 ꢃ 10ꢀ6 M, C2 ¼ 5.0 ꢃ
10ꢀ6 M, C3 ¼ 7.5 ꢃ 10ꢀ6 M, C4 ¼ 1.0 ꢃ 10ꢀ5 M, C5 ¼ 2.5 ꢃ 10ꢀ5 M, C6 ¼
5.0 ꢃ 10ꢀ5 M, C7 ¼ 7.5 ꢃ 10ꢀ5 M, C8 ¼ 1.0 ꢃ 10ꢀ4 M, C9 ¼ 2.0 ꢃ 10ꢀ4 M.
Fig. 4 Fluorescence spectra of 2a and 2b in dilute DMF solution excited
at 385 nm.
wavelength in the PL spectra. We notice that with the increase of
concentration, the new emission band shifts to a higher wave-
length. The interesting concentration-dependent emissions can
be attributed to the superposition of fluorescence of the unimers
and the nanoparticles. The nanoparticles show strong fluores-
cence at 545 nm, which is strongly blue-shifted as compared to
the solid-state emission. This indicates that the packing density
of the chromophores in the nanoparticles is much lower than that
in the solid state.
The formation of the nanoparticles was also investigated with
DLS at different concentrations. In Fig. 7, a bimodal (two peaks)
distribution can be observed at lower concentrations, indicating
the unimers and the nanoparticles coexist in the solution. The
hRhi of the unimer is about 3 nm, tracked by DLS. The size of the
nanoparticles does not show obvious augmentation at higher
concentrations, which suggests that there is only one kind of
micellar aggregate.42 Beside the bimodal distribution, the
changes of their relative intensities are also in excellent agreement
Fig. 5 Fluorescence spectra of 2a and 2b in the solid state.
a new class of strongly fluorescent organic materials, i.e.,
aggregation-induced emission conjugated polymer, for prepara-
tion of fluorescent polymer nanoparticles,40,41 because of the
factor that the fluorescence of polymers is generally decreased or
quenched in their aggregation state. Herein, the new coordina-
tion polymer 2a has good solubility in common organic solvents
and can be processed in solution. We found that 2a could form
a micellar solution spontaneously with green fluorescence when it
was dispersed in CHCl3 by sonication, but 2b kept undissolved in
CHCl3 even vigorously sonicated for a long time. The TEM
image in Fig. S18 (ESI†) clearly reveals that spherical nano-
particles of 2a are formed in CHCl3.
The self-assembly behavior of 2a in CHCl3 and the fluorescent
properties of the nanoparticles have been studied. The critical
micelle concentration (CMC) of 2a in CHCl3 was measured by
fluorescence spectrophotometry. As shown in Fig. 6, the CMC is
determined to be about 5 mM (repeat units). In dilute CHCl3
solution, 2a displays maximum emission at 485 nm with 385 nm
excitation, and the 15 nm blue-shift from DMF solution
is attributed to the lower polarity of CHCl3. When the
concentrations exceed the CMC, nanoparticles are formed as
demonstrated from the appearance of a new peak at a higher
Fig. 7 Hydrodynamic radius distributions obtained for CHCl3 solution
of 2a at different concentrations. C1 ¼ 2.5 ꢃ 10ꢀ5 M, C2 ¼ 7.5 ꢃ 10ꢀ5 M,
C3 ¼ 2.0 ꢃ 10ꢀ4 M.
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Fig. 8 Temperature-dependent fluorescence spectra obtained for 10ꢀ4
(repeat units) CHCl3 solution of 2a.
M
with the PL spectra. The disappearance of the unimers at 10ꢀ4
M
indicates a completed self-assembly of 2a.
To test the stability of the fluorescent nanoparticles, temper-
ature-dependent fluorescence measurements of the nanoparticles
dispersed in CHCl3 (10ꢀ4 M repeat units) were carried out below
60 ꢁC, and the results are shown in Fig. 8. It can be seen that the
emission maxima was resistant towards the change of tempera-
ture. It is worthy to point out that strong fluorescence of the
nanoparticles can also be observed without the self-quenching of
aggregation because of the incorporation of POSS moieties.
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In summary, this work reports a new kind of soluble bis(8-
hydroxyquinoline) zinc-based conjugated coordination polymer.
The experimental results demonstrate that incorporation of POSS
moieties not only effectively improves the solubility of the bis(8-
hydroxyquinoline) coordination polymers, but also prevents the
fluorescence quenching effect caused by intermolecular aggrega-
tion. Both the solubility enhancement and the prevention of the
fluorescence self-quenching will promote the application of 8-
hydroxyquinoline-based conjugated coordination polymer as
electrooptic material. Moreover, this research provides a facile
approach for the design and preparation of soluble conjugated
metal coordination polymers as advanced materials.
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Acknowledgement
Financial support from the Ministry of Science and Technology
of China (NO. 2007CB936401) is gratefully acknowledged.
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10864 | J. Mater. Chem., 2011, 21, 10859–10864
This journal is ª The Royal Society of Chemistry 2011