Selective Ag(I) binding, H2S sensing, and white-light emission from an easy-to-make porous conjugated polymer

Article abstract of DOI:10.1021/ja411067a

Separating silver (Ag⁺) from lead (Pb²⁺) is one of the many merits of the porous polymer framework reported here. The selective metal binding stems from the well-defined chelating unit of N-heterocycles, which consists of a triazine (C₃N₃) ring bonded to three 3,5-dimethylpyrazole moieties. Such a rigid and open triad also serves as the distinct building unit in the fully conjugated 3D polymer scaffold. Because of its strong fluorescence and porosity (e.g., BET surface area: 355 m ²/g), and because of the various types of metal species that can be readily taken up, this versatile framework is especially fit for functionalization. For example, with AgNO₃ loaded, the framework solid exhibits a brown color in response to water solutions of H₂S, even at the dilution of 5.0 μM (0.17 ppm); whereas cysteine and other biologically relevant thiols do not cause notable change in color. In another example, tunable white-light emission was produced when an Ir(III) complex was doped (e.g., about 0.02% of the polymer weight) onto the framework. Mechanistically, the bound Ir(III) centers become highly emissive in the orange-red region, complementing the broad, bluish emission from the polymer host to result in the overall white-light quality: the color attributes of the emission are therefore easily tunable by the Ir(III) dopant concentration. With this exemplary study, we intend to highlight metal uptake as an effective approach to modify and enrich the properties of porous polymer frameworks and to stimulate interest in further examining metal-polymer interactions in the context of sensing, separation, catalyzes, and other applications.

Full text of DOI:10.1021/ja411067a

Article
pubs.acs.org/JACS
Selective Ag(I) Binding, H2S Sensing, and White-Light Emission from
an Easy-to-Make Porous Conjugated Polymer
Jie Liu,† Ka-Kit Yee,† Kenneth Kam-Wing Lo,† Kenneth Yin Zhang,†,§ Wai-Pong To,‡ Chi-Ming Che,‡
and Zhengtao Xu*,†
†Department of Biology and Chemistry, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon, Hong Kong, China
‡Department of Chemistry and HKU-CAS Joint Laboratory on New Materials, The University of Hong Kong, Pokfulam Road, Hong
Kong, China
S
* Supporting Information
ABSTRACT: Separating silver (Ag+) from lead (Pb2+) is one of the many merits
of the porous polymer framework reported here. The selective metal binding
stems from the well-defined chelating unit of N-heterocycles, which consists of a
triazine (C3N3) ring bonded to three 3,5-dimethylpyrazole moieties. Such a rigid
and open triad also serves as the distinct building unit in the fully conjugated 3D
polymer scaffold. Because of its strong fluorescence and porosity (e.g., BET
surface area: 355 m2/g), and because of the various types of metal species that can
be readily taken up, this versatile framework is especially fit for functionalization.
For example, with AgNO3 loaded, the framework solid exhibits a brown color in
response to water solutions of H2S, even at the dilution of 5.0 μM (0.17 ppm);
whereas cysteine and other biologically relevant thiols do not cause notable
change in color. In another example, tunable white-light emission was produced when an Ir(III) complex was doped (e.g., about
0.02% of the polymer weight) onto the framework. Mechanistically, the bound Ir(III) centers become highly emissive in the
orange-red region, complementing the broad, bluish emission from the polymer host to result in the overall white-light quality:
the color attributes of the emission are therefore easily tunable by the Ir(III) dopant concentration. With this exemplary study,
we intend to highlight metal uptake as an effective approach to modify and enrich the properties of porous polymer frameworks
and to stimulate interest in further examining metal−polymer interactions in the context of sensing, separation, catalyzes, and
other applications.
including N-heterocycle units,5c,d,8 poly(thienylene arylene),5e
INTRODUCTION
■
Troger’s base,9 Lin’s binaphthol motif,10 and isocyanurate.5b
̈
Porous polymer frameworks (PPFs) as a growing class of
The importance of such groups is obvious. For example, they
advanced materials have been intensely studied owing to the
can be directly utilized as heterogeneous acid/base catalysts.5f,11
salient advantages of stability and functionality.1 The stability
Moreover, the donor groups often facilitate the uptake of metal
arises from the strong constituent covalent bonds that are often
species and thereby potentially enable within these novel
deployed in spatially confined and rigid configurations.
Remarkable examples of this sort are seen in the covalent
porous mediums a much wider range of catalytic, photo-
triazine-based frameworks (CTFs)2 and the diamond-like
chemical, and photophysical processes. Among the numerous
networks based on tetraphenylmethane building blocks:3 both
systems withstand higher temperatures (e.g., 400 °C) and
strong acid/base treatments, with stability of the former (i.e.,
CTFs) further highlighted in a recently reported application for
electrochemical energy storage device, in which the CTF
efforts to integrate donor units and to upload metal species into
PPFs, the major intent is often focused on catalytic applications.
By comparison, little has been reported on the use of PPFs for
selective binding and separation of metal ions.
Here we report a PPF that, besides being able to selectively
bind and separate Ag(I) ions from base metal ions (e.g., Pb2+,
system continues to hold up after thousands of electrochemical
Cd2+, and so on; in aqueous solutions), offers multiple
cycles.4 The functional versatility is rooted in the diverse
advantages. For example, the majority of Ag(I) can be readily
modifications provided by organic chemistry and is further
extricated from the polymer matrix, thus enabling the recycling
buttressed by the extraordinary stability of these covalent
of the polymer host. The remainder (e.g., about 20%) of the
framework materials. Presently, the major thrust of the field is
shifting from framework syntheses/pore characterization (e.g.,
via gas adsorption measurements) to the functionalization of
Ag(I) species in the polymer host, on the other hand, can be
used to detect and differentiate H2S from other biologically
the networks, in order to drive forward catalyzes,1b,c,5
relevant mercaptan molecules. Interestingly also, the conjugated
separation,6 and other applications.7
One effective approach of functionalization is to integrate
Lewis base donor groups into the framework structure,
Received: October 30, 2013
Published: January 24, 2014
© 2014 American Chemical Society
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polymer framework features distinct photoluminescent proper-
ties that can be easily tuned to generate white-light emission
amid a wide range of other wavelength profiles. Let us start with
the synthesis and design of this polymer framework.
The results of elemental analyses (e.g., of C, H, and N; EDX
indicates no Cl remaining in the solid product of TMBPT,
Figure S1) are consistent with the formation of the TMBPT
framework (see SI). Thermogravimetric analysis (TGA, Figure
S2) on the TMBPT powder reveals a stable weight region up to
300 °C, indicative of the nonvolatile, polymeric nature of the
product; while the X-ray powder diffraction pattern exhibits
three very broad peaks, suggesting a rather low degree of
crystalline order of the TMBPT solid (Figure S3). The
formation of the TPT core in the TMBPT polymer framework
is also demonstrated by the synthesis of the triad molecule M1
(Scheme 2) when excess TMBP (relative to CC) was used
RESULTS AND DISCUSSION
■
Convenient Synthesis. Both monomer building blocks are
easy to obtain: 2,4,6-trichloro-1,3,5-triazine (i.e., cyanuric
chloride, CC) is an industrial material, and 3,3′,5,5′-
tetramethyl-4,4′-bipyrazol (TMBP, see Scheme 1) is available
Scheme 1. Synthetic Scheme for the TMBPT Polymer
a
Framework
Scheme 2. Molecule M1
under similar reaction conditions (see Scheme S1). Also
revealing is a comparison of the IR and Raman spectra of the
TMBP reactant, the intermediate product M1, and TMBPT
polymer (Figure 1). Most notably, the M1 triad and the
TMBPT shares a number of distinct features that are absent in
the TMBP reactant, including: the peaks at 745, 812, 1095, and
1129 cm−1 in the IR spectra; the strong peak 990 cm−1 and the
overall resemblance of the Raman spectra. Such similarities
point to the common structural features between the M1
molecule and the TMBPT polymer and are associated with the
TPT/triazine core (as highlighted in Scheme 1). For example,
the strong Raman peak at 990 cm−1 arises from the symmetrical
ring deformation (C, N out of phase) of the triazine core, and
the medium IR peaks at 1095 and 1129 cm−1 can be ascribed to
the stretch of the C−N bonds off the triazine ring. The efficient
reactions observed here are also in line with that of similar
reactions between CC and amine molecules13 as well as the
near quantitative reaction between CC and the model
compound 3,5-dimethylpyrazole (DMP, see Scheme S2 and
Figure S4). In addition, the solid-state 13C NMR measurement
of the TMBPT sample also verifies the efficiency of the
polymerization reaction. As seen in Figure S5, the aromatic
region of the solid-state 13C NMR spectrum of TMBPT is
dominated by four distinct peaks (164.52, 152.63, 142.45, and
115.00 ppm) corresponding to the carbon signals of the
polymer backbone, whereas a small peak found at higher field
(106.66 ppm) can be assigned to the dangling, monoreacted
TMBP units (as TMBP was added in slight excess relative to
CC in the reaction).
a
Highlighted at lower right is a TPT unit.
commercially or from an inexpensive preparation using 2,4-
pentanedione and hydrazine.12 The assembly of the polymer
framework simply involves refluxing a mixture of THF
(tetrahydrofuran), CC, TMBP, together with the mild base
N,N′-diisopropylethylamine (DIPEA) as a promoter/catalyst
(see Scheme 1). The mild reaction requires no strong acid/base
reagents and is especially compatible with industrial production.
The metal-free procedure also obviates the toxicity and other
detrimental effects of metal residues that often interfere with
applications in medical and optical/electronic technologies.
The resultant TMBPT (the last T standing for triazine)
polymer framework features the tritopic node of the triazine
and its three dimethylpyrazole neighbors (i.e., the TPT unit as
highlighted in Scheme 1). The relative rigidity of the TPT unit,
together with the large torsional angle enforced by the methyl
groups, is meant to promote the formation of a 3D open
framework that contains significant void. The conjunction of
triazine core and the pyrazole groups also creates distinct
chelating motifs of the nitrogen donors for potentially effective
binding of metal ions. In principle, two types of orientations are
possible for the three pyrazole moieties in the TPT unit: one
features three equivalent bidentate motifs (i.e., the lower left
one in Scheme 1); the other is less symmetrical and contains
the mono-, bi- and tridentate modes (the lower right one in
Scheme 1). No effort was made to assess the ratio of the two
configurations, but in either case, the chelating character is
prominent.
Gas Sorption. CO2 sorption experiments at 273 K
(pressure range: from 8 × 10−3 to 780 mmHg) on the
TMBPT sample (previously activated by Soxhlet extraction in
methanol) revealed highly reproducible typical type-I gas
adsorption isotherms (CO2 gas, 273 K, Figure 2) with a
Brunauer−Emmett−Teller (BET) surface area of 355 m2/g.
The validity of the BET model is also demonstrated by the
linear fit of the data, as seen in the inset of Figure 2, and by the
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Figure 1. The IR (A) and Raman (B) spectra of TMBP, the M1 molecule, and the TMBPT polymer. The stars mark selected peaks seen in both M1
and TMBPT.
access to the mesopores. The bottleneck effect is readily seen in
one of the adsorption/desorption isotherms we obtained,
wherein the distinct H2-type hysteresis loop corresponds to the
delayed desorption caused by the blocking of the exit pathways
(Figure S7).14 The specific surface area (230 m2/g) as well as
the pore size/volume data (e.g., Figure S8) thus derived may
therefore involve larger degree of uncertainty than systems of
more regular porous features.
Aside from the gas sorption behaviors, porosity often
expresses itself rather differently in other situations, e.g., in a
solvent where swelling of the pores becomes significant and
helps to open up the micropores and the bottlenecks to the
mesopores. The importance of this effect has begun to be
recognized even in metal−organic frameworks with well-
Figure 2. CO2 sorption isotherm at 273 K for an activated TMBPT
sample (126 mg, activated by evacuating at 120 °C for 12 h). Inset: the
BET isotherm.
ordered, crystalline structures.15 As will be seen in the
metalation processes described below, this effect is also
especially pronounced for the flexible, organic TMBPT system
featuring wide pore size distribution. On the other hand, typical
gas sorption (e.g., BET curves) indicates open and accessible
pores in vacuum, thus demonstrating the structural persistence
and stability of the framework with regards to solvent/guest
loss. Such so-called permanent porosity, however, does not
always guarantee advantages, especially in solution-based
applications. After all, many “permanent” pores (e.g., in
MOFs) are readily degraded by moisture or other common
reagents.
Effective Metal Uptake. In a typical run, the activated
TMBPT solid (300 mg, equivalent to about 0.83 mmol of the
TPT unit in Scheme 1) was placed in an aqueous solution
containing excess AgNO3 (495 mg, 2.91 mmol in 33 mL of
water). After the mixture was stirred for 48 h at room
temperature in the dark, the resultant solid (which remained
yellowish) was separated by filtration and washed repeatedly by
water (e.g., 10 × 10 mL, to remove residual unbound Ag+ salts).
CHN elemental analyses on the resultant solid sample found
[C (38.73%), H (4.82%), N (23.98%)], while ICP indicated Ag
content to be 15.6%. These elemental data can be fitted by the
formula (C18H18N9)·(AgNO3)0.80·(H2O)3.5 [calcd C (38.65%),
H (4.50%), N (24.54%), Ag (15.43%)]. This formula indicates
an ∼0.8:1 AgNO3/TPT molar ratio, and the Ag(I) uptake is
thus found to be substantial, further demonstrating the open
and porous nature of the bulk sample under the conditions.
Uptake of other metal species is also possible. For example,
PdCl2 can be loaded to reach a 2.2:1 Pd/TPT ratio (see SI for
details), a ratio that compares favorably with the pioneering
system of Hatn-based (hexaazatrinaphthylene) network poly-
mer reported by McKeown and co-workers.8a The effective Pd
positive, reasonable C constant (C = 88), which is associated
with the heats of adsorption (first layer) and condensation
(multilayer). Monte Carlo analysis on pore size distribution and
pore volume (Figure S6) of the CO2 adsorption isotherms (273
K) indicated an average pore width of 0.48 nm and a modest
micropore volume of 0.149 cm3/g. The small pore size thus
uncovered points to the importance of the micropore and
ultramicropores in the structure, which could complicate
porosity studies in certain cases (e.g., due to significant
diffusion barriers).
One such case is the N2 sorption experiments (e.g., at the
much lower temperature of 77 K). Herein, adsorption at the
micropore domain (i.e., at very low relative pressure) was not
observed, indicating difficulty in accessing the very narrow
micropores. Such a marked difference from the typical
adsorption observed of CO2 is often observed of microporous
polymeric materials and has been well rationalized. First, CO2
has a much higher saturation pressure P0 (i.e., smaller relative
pressure P/P0) at 273 K, making it possible to investigate the
microporosity at this higher temperature, where stronger
thermal motion of the framework provides for faster diffusion
and easier access of the pore regions. Also, the diffusion of CO2
into micropores is facilitated by the following: CO2 has a
quadrupole moment and is more polarizable, thereby
interacting more effectively with host material; CO2 (0.33
nm) has a slightly smaller kinetic diameter than N2 (0.364 nm).
The micropores and ultramicropores in this system also
complicate the characterization of the mesoporous property
(i.e., at higher relative pressures) based on N2 sorption (at 77
K), because they tend to create the “bottlenecks” restricting
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loading in the TMBPT matrix shall facilitate future studies on
the TMBPT polymer for potential applications in heteroge-
neous catalysis.
The Ag(I) and Pd(II)-loaded samples thus prepared become
much weaker in photoluminescence, whereas the as-made
sample features an intense, broad peak that comes off as whitish
with a slight blue quality (Figure 3). The response of the
Selective Ag(I) Uptake. Silver as a precious metal often
naturally occurs in mineral forms together with ores of copper,
nickel, zinc, and lead, and separation of Ag(I) from these base
metal species is of great industrial importance.16 Tests of
selective Ag(I) binding are therefore especially meaningful for
further exploring the applicability of TMBPT polymer system
in separation science. For this, we first dissolve in water AgNO3
and the following: Cu(NO3)2·3H2O, Zn(NO3)2·6H2O, Co-
(NO3)2·2H2O, Cd(NO3)2·4H2O, Ni(NO3)2·6H2O, and Pb-
(NO3)2. The concentration of each of the metal ions was set at
about 50 ppm (see Table S2). An as-made TMBPT sample (50
mg, about 0.14 mmol of the TPT unit) was placed into 50 mL
of the above mixture solution, which contains 2.6 mg (0.024
mmol) of Ag(I) and a total of 0.21 mmol of metal ions. In this
test, the TPT units are set to decidedly outnumber the Ag(I)
ions, so as to give other metal ions ample opportunity to access
the TPT sites and to fully unveil the selectivity for the
individual metal ions. The mixture was then stirred in the dark,
and the solution of the mixture was sampled at different times
(1, 3, 8, 19, 26, 44 h) for monitoring the concentrations of the
metal species present in the solution.
Figure 3. Room-temperature solid-state emission spectra: (a) activated
TMBPT polymer, (b) AgNO3-loaded TMBPT, and (c) PdCl2-loaded
TMBPT powders. The excitation wavelength λex was 360 nm.
The remarkable selectivity for Ag(I) is readily seen in the
plot of Figure 4 and the data in Table S2. Within the first 8 h,
emission properties to metal guests is potentially useful for
tuning the photoluminescent properties (see below for the
white-light emission) and for monitoring the presence of metal
species. As expected, CO2 sorption (273 K, Figure S9) reveals a
smaller specific surface area of 136 m2/g for the Ag(I)-loaded
TMBPT sample (cf. 355 m2/g before Ag upload). Also the
sorption isotherms feature a significant H2-type hysteresis,
which, as discussed above, suggests aggravated bottleneck
effects due to the loaded AgNO3 further blocking the passage of
the gas molecules.
The uploaded Ag species can be retrieved, and the TMBPT
matrix can reused for additional cycles of metal uptake. For
example, tetramethylthiourea (TMTU, Scheme 3), with its
Scheme 3. TMTU
Figure 4. A plot of the concentrations (ppm) of the individual metal
ions in a mixture solution at different hours after being mixed with
TMBPT.
distinct sulfur donor, was found to be effective in stripping the
Ag(I) species off the TMBPT matrix. Such stripping can be
achieved by stirring the AgNO3-loaded TMBPT solid (300 mg,
prepared as mentioned above) in a water solution of TMTU
(300 mg in 30 mL water) at 50 °C for 16 h in the dark. After
the resultant solid was washed with water (about 200 mL) and
THF (5 × 10 mL) to remove the residual TMTU, the Ag
content was found by ICP to have dropped from 15.6% to
4.8%. Thus, the majority (e.g., 70%) of the loaded Ag(I) species
can be extracted by the TMTU agent, which also serve to
demonstrate the penetrable nature of the host matrix (e.g., with
regard to TMTU as the guest). The cycle of upload and
stripping can be repeated without significant compromise on
the uptake and retrieval capabilities (see Table S1). For
example, after the AgNO3 was uploaded in the third round, the
BET isotherm features remain little changed (surface area: 120
m2/g, Figure S10), as compared with the AgNO3-loaded
sample in the first round.
Ag(I) ion rapidly decreases in concentration, indicating efficient
uptake by the TMBPT matrix, whereas concentrations of all
other ions remain little changed. Specifically, by 8 h, over 90%
of the Ag(I) ions in solution has been taken up by the TMBPT
solid; later on at 44 h, the Ag(I) concentration was further
reduced to 0.35 ppm (i.e., <1% of the original 52 ppm). Among
the other ions, only Cu(II) exhibits significant decrease in
concentration, but only after 8 h, and it remains above 35 ppm
even at the 44 h (cf. the original being 51 ppm). The majority
of the Cu(II) ions (about 70%) thus remain in solution; they
are not taken up by TMBPT under the conditions. Of
particular interest is Pb(II), because PbCl2, like AgCl, is
insoluble in water, which makes it impractical to separate the
two by means of halide precipitation. The distinct selectivity of
TMBPT for Ag(I) thus points to a convenient protocol for its
separation from Pb(II). Generally speaking, the strong selective
binding of Ag(I) is possibly driven by the higher (less negative)
Gibbs free energy of hydration for silver ions than other metal
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ions16 as well as by the kinetics/liability of the coordination
properties of Ag(I).
a
Scheme 4. Ir2(pq)4Cl2
Detection of H2S. The metal-laden, porous TMBPT
polymer framework offers advantages for potential sensing
applications. First, the porosity and large surface area of the
solid-state matrix allow analytes to diffuse throughout the bulk
and to fully access the anchored metal sites. Also, the anchoring
of the metal centers (i.e., via the nitrogen coordination donors)
is only moderate in strength, so that they remain potentially
reactive/responsive upon being accessed by the guest analyte.
The anchoring, on the other hand, is robust enough to prevent
leaching and aggregation and thus to provide for better stability,
e.g., the AgNO3-loaded TMBPT is stable to water and does not
turn black even after hours of exposure to light (unlike many
light-sensitive silver salts).
As a preliminary test to exploit the nice balance of reactivity
and stability in the solid state, we demonstrate here the highly
sensitive colorimetric detection of H2S in water. Most notably,
even at rather low loading of Ag(I) (e.g., using a depleted
sample after extraction by TMTU; with the AgNO3/TPT ratio
being about 1/5), a light but distinct brown coloring quickly
developed even when the H2S concentration was as low as 5.0
μM (0.17 ppm), as shown in Figure 5. At the higher
a
Bonds not drawn to scale.
(as being bound to the nitrogen donors on the TMBPT
polymer host), one could potentially generate white-light
emission, a property that is of great importance for the lighting
industry.19 Indeed, simply by stirring the TMBPT powder (e.g.,
18.0 mg) and a dilute solution of Ir2(pq)4Cl2 (e.g., 7.5 ppm in
CH2Cl2, 1.0 mL, the Ir content being 0.02% of the polymer
weight) for 15 min, one can effectively dope the Ir(III) complex
into the TMBPT polymer host, and distinct white-light
emissions were observed of the resultant solid sample (labeled
as TMBPT-Ir-a). The white emission appears homogeneous
throughout the powder sample (see the photo insets of Figure
6) and remains unchanged even after being repeatedly washed
by CH2Cl2, indicating the robust and stable binding of complex
Ir2(pq)4Cl2 onto the TMBPT host.
The emission spectra of the pristine TMBPT polymer are
shown in Figure 6. Overall, a longer excitation wavelength (λex)
tends to red-shift the emission maximum, e.g., with λex being
320, 360, and 375 nm, the emission peaks at 455, 465, and 485
Figure 5. Photo images of the setup for the detection of H2S (left) and
of the AgNO3-loaded TMBPT powder after being stirred for 4 h at 50
°C in: (1) 5 μM H2S; (2) 10 μM H2S; (3) 50 μM H2S; (4) 100 μM
H2S; (5) 5 mM glutathione; (6) 5 mM cysteine; (7) 5 mM NaSCN;
(8) 5 mM Na2SO3; (9) 5 mM Na2S2O3; (10) Kpi buffer; (11) DI
water; (12) 5 mM H2O2 solution. The scale bar is 0.80 mm.
concentrations of 10, 50, and 100 μM, the coloring becomes
darker in a monotonic fashion, indicating the potential for
colorimetric analysis. Moreover, the color change thus triggered
is highly specific for H2S. For example, biologically relevant
thiols, such as glutathione and cysteine, and other reactive
sulfur species (e.g., thiocyanate, sulfite, and thiosulfate, see
Figure 5) do not cause any notable color change to the AgNO3-
loaded TMBPT powder even at the high concentration of 5
mM (i.e., over 1000 times the detection limit of H2S). In
particular, the water stability of the Ag-TMBPT solid is
especially suited for the topical application of monitoring H2S
in physiological settings and in industrial waste/pollution
control.17
Tuning for White-Light Emission. As previously demon-
strated by Lo and others,18 Ir2(pq)4Cl2 (Scheme 4), which is
nonemissive per se, can be rendered strongly emissive in the
orange-red region, when the Cl atoms are displaced by
nitrogen-based donor molecules. One implication here is
obvious: by complementing the broad, bluish emission of the
TMBPT polymer the orange-red features of the Ir(III) centers
Figure 6. Room-temperature emission spectra of a pristine TMBPT
powder (a) and the TMBPT-Ir-a powder (b). The excitation
wavelengths for 1−3 are 320, 360, and 375 nm, respectively. The
scale bar is 0.60 cm.
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nm, respectively, with the corresponding CIE20 coordinates of
(0.18, 0.21), (0.19, 0.25), and (0.21, 0.31); all within the bluish
region of the chromaticity graph (see Figure 7). Notice also
AUTHOR INFORMATION
Corresponding Author
zhengtao@cityu.edu.hk
■
Present Address
§Key Laboratory for Organic Electronics & Information
Displays and Institute of Advanced Materials, Nanjing
University of Posts and Telecommunications, Nanjing
210023, China
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work is supported by City University of Hong Kong
(projects 7002723 and 7008095). Z.X. acknowledges the
Alexander von Humboldt Foundation for a Humboldt Research
Fellowship for Experienced Researchers (host: Prof. M.
Antonietti). We thank Prof. Cheng Wang and Ms. Huimin
Ding of Wuhan University (College of Chemistry and
Molecular Sciences) for conducting the 13C solid-state NMR
study on TMBPT.
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Figure 7. CIE-1931 chromaticity diagram and the positions for
emissions of prisitne TMBPT [irradiated by 320 (a), 360 (b), and 375
nm (c)] and TMBPT-Ir-a [irradiated by 320 (a′), 360 (b′), and 375
(c′) nm] as marked by the stars and the crosses, respectively.
that the emission intensity is the greatest with λex = 360 nm.
For the doped polymer TMBPT-Ir-a, the emission maxima
becomes less dependent on the excitation wavelengths: when
excited at 320, 360, and 375 nm, respectively, the emission
from the polymer host peaks at 445, 458, and 465 nm, whereas
the new emission feature, arising from the Ir(III) moiety,
remains at 570 nm for all three cases, and the corresponding
CIE coordinates are now modified to be (0.24, 0.25), (0.25,
0.26) and (0.32, 0.34); all within the white-light region (see
Figure 7) in the chromaticity graph. In general, the CIE
coordinate as well as other aspects of the color quality of the
emission can be conveniently tuned over a broad range by
adjusting the concentration of the Ir(III) dopant in the polymer
host. For example, when the Ir(III) content was raised to 0.04%
(as in sample TMBPT-Ir-b), the emission from the Ir(III)
centers becomes more significant, resulting in a stronger
orange-red quality in the overall emission feature (e.g., see the
top spectrum in panel 3B of Figure S11). Measurements on the
powder samples of TMBPT, TMBPT-Ir-a, and TMBPT-Ir-b,
using the integration sphere setup,19u,21 indicate a fluorescence
quantum efficiency on the order of 5−6%. Notice though that
the integration sphere method tends to underestimate the
actual value because of reabsorption of the emitted light by the
sample.21c,22 Overall, the quantum efficiencies found here are
similar to a rare-earth-doped MOF reported by Chen and co-
workers,19d while comparing favorably with a white-emitting
nanocrystal sample of CdSe.23
ASSOCIATED CONTENT
* Supporting Information
■
S
Additional experimental procedures, X-ray powder diffraction
pattern of TMBPT, sorption isotherms and related graphs,
detailed procedures for quantum yield measurement. This
material is available free of charge via the Internet at http://
pubs.acs.org.
2823
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Journal of the American Chemical Society
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