New Silver(I)-P4 Coordination Polymers Strongly Adsorb Congo Red to Yield Composites with Enhanced Photocurrent Responses

Article abstract of DOI:10.1002/ejic.202100228

Reaction of tetraphosphine (P4) ligand dppbpda with Ag(I) oxylates yielded new coordination polymers (CPs) of formula [(dppbpda)Ag₄(Ox)₄]_n (Ox=Bz (1), Sal (2), LA (3), Bz=benzoate, Sal=salicylate, LA=lactate). The solid-st

Full text of DOI:10.1002/ejic.202100228

Communications
doi.org/10.1002/ejic.202100228
New Silver(I)-P4 Coordination Polymers Strongly Adsorb
Congo Red to Yield Composites with Enhanced
Photocurrent Responses
Min Zhang,[a] Meng-Yao Feng,[a] Jia-Jun Yan,[a] Hai-Yan Li,[b] David James Young,[c]
Hong-Xi Li,[a] and Zhi-Gang Ren*[a]
cence sensors.[15,16] Many CPs absorb organic dyes and the
Reaction of tetraphosphine (P4) ligand dppbpda with Ag(I)
resulting
composites
exhibit
improved
photoelectric
oxylates yielded new coordination polymers (CPs) of formula
[(dppbpda)Ag4(Ox)4]n (Ox=Bz (1), Sal (2), LA (3), Bz=benzoate,
Sal=salicylate, LA=lactate). The solid-state structures of 1–3
consist of one dimensional (1D) chains constructed from
tetranuclear μ-η2,η2- connected Ag4(Ox)4 cluster cores and
dppbpda bridging ligands. Compounds 1–3 absorbed Congo
Red in two distinct steps, with maximum uptake quantities of
1438 (1), 1417 (2) and 1656 (3) mg/g, inversely correlating with
the size of the anions in the CPs. These CPs had band gaps in
the semiconducting region (Eg) 3.05–3.15 eV and all three
showed enhanced two-peak photocurrent responses during the
stepwise adsorption of CR, with maximum photocurrent
densities of 227 (1), 163 (2) and 386 (3) μAcmÀ 2 for composites
with optimized dye coverage.
responses.[17,18] For example, a one dimensional (1D) CP of Zn(II),
1,2,3-BTA (1,2,3-benzenetricarboxylate) and N,N’-di(3-pyridyl)-
succinamide showed superior Congo Red (CR) sorption up to
1129 mg/g, and the resulting composite exhibited a remarkably
enhanced photocurrent response.[17] Scientists usually aim to
maximize the uptake of dye and thereby to maximize photo-
current, but without specifically designing for this parameter.[19]
PPh2 containing bridging ligands with Ag(I) and Au(I)
cations have been used for the construction of photoelectro-
active materials.[20] We have previously focused on the design of
complexes derived from transition metals and multidentate
phosphine or PN hybrid ligands. For example, a tetraphosphine
(P4)
ligand
dpppda
(N,N,N’,N’-tetrakis
(diphenylphosphinomethyl)phenyl-4,4’-diamine) composed of
four À PPh2 groups and p-phenylenediamine spacer was com-
In recent years, coordination polymers (CPs) have been
designed with metal nodes and tightly-coordinated bridging
ligands to be photoelectric materials[1–3] and photocatalysts
with good stability and energy gap (Eg) values in the semi-
conductor region.[4] The photoelectric properties of these CPs
can be adjusted by changing the metal centers or bridging
ligands. Various types of materials with interesting structures
and good performance have been produced by employing
ligands containing multidentate coordination sites such as
carboxylate, pyridyl, Schiff base and other donors, to assembly
transition metal cations including Zn(II), Cd(II), Co(III), Ni(II),
Cu(I)/Cu(II) and Mn(II).[5–11] Coating semiconductive substrates
with organic dyes can be an effective method to improve the
photoelectric performances of these materials for applications
including dye-sensitized solar cells[12–14] and chemolumines-
bined with AgNO3 to yield
a
CP with formula
[Ag4(NO3)4(dpppda)]n, that catalyzed the photo-degradation of
nitro-aromatics.[21] Other CPs of similar composition were found
to contain the dpppda· + cationic radical and exhibited
enhanced photocurrent responses.[22] These results encouraged
us to design
a
P4 ligand dppbpda (N,N,N’,N’-tetrakis
(diphenylphosphinomethyl)biphenyl-4,4’-diamine) with a longer
p-diphenylenediamine spacer and to prepare CPs from the
reactions of dppbpda with Ag(I) oxylates. Herein, we report the
syntheses and structures of these CPs and their blending with
Congo Red (CR) to yield semiconductive composites with
enhanced photocurrent responses.
The air-sensitive ligand dppbpda was synthesized in 79%
yield by
a
typical Mannich reaction[23] of benzidine,
formaldehyde and HPPh2 (molar ratio 1:4:4) in toluene at
°
110 C for 12 h (Scheme 1). Reactions of dppbpda with AgBz,
AgSal and AgLA (molar ratios 1:4, Bz = benzoate, Sal =
salicylate, LA = lactate) in CH2Cl2 and MeCN or MeOH, followed
by Et2O diffusion gave single crystals of the three CPs
[(dppbpda)Ag4(Ox)4]n ·m(Sol) (1·CH2Cl2 ·Et2O, Ox = Bz, yield
68%, 2·2CH2Cl2, Ox = Sal, yield 75%, 3·2.5MeOH·MeCN, Ox =
LA, yield 55%). The guest solvent molecules in the crystals of
the CPs evaporated in air to give solventless 1–3, which were
stable to air and moisture and insoluble in water and organic
solvents. Their elemental analysis data, IR and NMR spectra
were consistent with their molecular structures (see Supporting
Information).
[a] M. Zhang, M.-Y. Feng, J.-J. Yan, H.-X. Li, Prof. Z.-G. Ren
College of Chemistry, Chemical Engineering and Materials Science
Soochow University
215123 Suzhou, Jiangsu, P. R. China
E-mail: renzhigang@suda.edu.cn
https://chemistry.suda.edu.cn/31/6f/c17986a405871/page.htm
[b] Prof. H.-Y. Li
Analysis and Testing Center
Soochow University
215123 Suzhou, Jiangsu, P. R. China
[c] Prof. D. J. Young
College of Engineering, Information Technology and Environment
Charles Darwin University
0909 Darwin, Northern Territory, Australia
Single-crystal X-ray diffraction (SCXRD) of 1·CH2Cl2 ·Et2O,
2·2CH2Cl2 and 3·2.5MeOH·MeCN indicated that they crystal-
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/ejic.202100228
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doi.org/10.1002/ejic.202100228
(840 mg/g).[27] Likewise, these CPs absorbed more CR than a CP
of Zn(II) ions and 1,2,3-BTA (1129 mg/g).[17] The adsorbed CR
couldn’t be removed from the composites by washing with
EtOH, and SEM images (Figure 2, Figure S2 and Figure S3)
indicated the rough surfaces were obviously smoother.
We further examined the absorption kinetics using aqueous
solutions containing 10 mg of CP and the maximum uptake
amounts of CR (1: 14.3 mg, BET surface 1.9 m2/g; 2: 14.1 mg,
BET surface 4.2 m2/g; 3: 16.5 mg, BET surface 3.0 m2/g). The
change in concentration (Figure 3b, Figure S4b and Figure S5b)
clearly showed a two-step absorption process with the kinetics
of each step obeying a zero-order rate equation (Table 1). For
all three CPs, step-1 was complete within 2.25–3.83 h, much
faster than step-2 (>100 h). The rate of step-1 followed the
order 3>1>2, which inversely correlates with the anion sizes
LA<Bz<Sal. We hypothesize that smaller anions permit closer
interaction of CR with the surface of the CPs. By comparison,
the rate constant for step 2 was similar in all three CPs. The
change in concentration of Na+ in the solutions after
absorption was measured by ICP and revealed only trace
amounts of this counterion, indicating absorption of the salt,
rather than an ion exchange. Evaporation of the solution gave
hardly any residue consistent with this proposal. It seems
Scheme 1. Syntheses of dppbpda and CPs 1–3.
lized in the monoclinic P21/c (1,2) and C2/c (3) space groups.
Their frameworks were quite similar (Figure 1 and Figure S1),
containing a Ag4(Ox)4 cluster core of two O atoms and two
eight-membered rings from two Ag atoms and two Ox anions.
This core is μ-η2,η2- ligated by the two À PPh2 groups from the
ends of two neighboring dppbpda molecules, while dppbpda
ligands bridge neighboring Ag4(Ox)4 cluster cores in a μ-η2,η2-
end-to-end coordination mode, thus forming a 1D Zigzag chain
extending along the b axis (1,2), or a wave-like chain extending
in the [1,0,1] direction (3). There were no noteworthy
intermolecular interactions between these chains.
Adding the fine ground powders of 1–3 (10 mg) into
aqueous solutions containing excess CR (0.04 M, 28 mg in
10 mL H2O) caused a decrease in the intensity of the color of
the solution and concomitant coloring of the powders. The
mixtures were stirred continuously in the dark for two weeks to
ensure saturation. The maximum CR uptake amounts were
calculated from the decrease of intensity at 497 nm of the
solutions to be 1438 (1), 1417 (2) and 1656 (3) mg/g, respec-
tively. These values are significantly larger than those of
FexCo3À xO4 nanoparticles (128.6mg/g),[24] ZnO@Ze composite
(300 mg/g),[25] Ce(III) doped UIO-66 nanocrystals (826 mg/g)[26]
or carbon hybridized montmorillonite nanosheets C/MMT
Figure 2. Photos and SEM images of compound 1 (left) and its CR composite
(right).
Figure 1. A section of the 1D chain in 1·CH2Cl2 ·Et2O. H atoms and solvent
molecules are omitted, and À Ph groups of dppbpda and Bz groups are
represented as hexagons for clarity. Atom color codes: Ag, turquoise; P, pink;
N, blue; O, red; C, gray.
Figure 3. (a) UV-Vis spectra of the CR solutions (0.687 mmol/L, 30 mL) after
treatment with 1 (10 mg) for different periods of time. (b) Time dependent
concentrations of the CR solutions calculated from UV-Vis absorbance.
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Table 1. Absorption of CR by 1–3.
Compound
1
2
3
Total cCR uptake (mmol/L)
Max. uptake capacity (mg/g)
0.687
1438
0.503
2.25
8.6
0.678
1417
0.531
3.83
3.7
0.792
1656
0.561
2.44
cCR after step 1 (mmol/L)
Step 1 elapsed time (h)
Step 1 uptake rate constant
10.0
(×10À 2 mmol·LÀ 1 ·hÀ 1
)
Step 1 uptake amount (mg/g)
Step 1 uptake percentage
Step 2 uptake rate constant
385
26.8%
0.32
307
21.7%
0.31
483
29.2%
0.31
(×10À 2 mmol·LÀ 1 ·hÀ 1
)
Step 2 uptake amount (mg/g)
Step 2 uptake percentage
Step 1 linear fit R2
1053
1109
1173
73.2%
0.98891
0.98884
78.3%
0.95949
0.97232
70.8%
0.97433
0.99438
Step 2 linear fit R2
reasonable that the Ox anions are tightly coordinated with the
Ag ions and unlikely to exchange with the larger CRÀ anion.
The band gap energies (Eg) of the CPs were determined by
the Kubelka-Munk method (Figure S6) to be 3.15 eV (1), 3.08 eV
(2) and 3.05 eV (3), which are in the semiconductivity range.
The photocurrent responses of 1–3 and their composites with
CR were measured using a three electrode cell in 0.1 M Na2SO4
aqueous solution under Xe light. The working electrodes were
prepared by coating the powders on indium tin oxide (ITO)
glasses, with SCE and Pt wire as reference and auxiliary
electrodes, respectively. The cyclic voltammetry (CV) curves for
1–3 were measured under Xe light irradiation and in the dark.
All three CPs showed strong reduction peaks for AgI!Ag0 at
À 0.6 V (vs SCE) and weak oxidation peaks for Ag0!AgI at 0.38 V
(vs SCE, Figure S7), indicating that these CPs may decompose to
Ag metal at lower potentials, but are stable at higher potentials
up to 1.1 V. Therefore, the bias potential was set at 0.75 V to
measure the anodic H2O!O2 photocurrents.
Figure 4. Photocurrent responses of 1 and its CR composites under Xe light
irradiation. (a) Photocurrents during 20s-time on/off light intervals. (b) Photo-
ocurrent intensities within 280–300s of the composite formation at different
CR uptake amounts and (c) an expansion of (b) in the range of 0–100 mg/g
CR uptake.
composites with two photocurrent peaks were designated
CP@CR-1 and CP@CR-2. The order of photocurrent was 3@CR-
2>1@CR-1>2@CR-2 inversely correlating with the size of the
oxylate ligands LA<Bz<Sal. We propose that CR anion binding
in 3@CR-2 is tighter because of the smaller LA anion, resulting
in more efficient transfer of photo-generated electrons to the
surface of 3, with a commensurately stronger photocurrent
response.
Raman spectroscopy was used to probe the interaction of
CR and the CPs. The Raman spectrum of CR changed
significantly when combined with CP1 in 1@CR-1 and 1@CR-s
(Figure 5). Upon coating on 1, the ph-N= stretching frequency
of CR[28] at 1157 cmÀ 1 remained unchanged, while the naphthyl
ring CÀ C stretching[29] at 1352 cmÀ 1 was blue-shifted to
1370 cmÀ 1, suggesting intermolecular π-π interaction with the
Upon repeated light irradiation with a shutter interval of
20s, CPs 1–3 and their CR composites all showed rapid, steady
and repeatable photocurrent responses (Figure 4a, Figure S8a
and Figure S9a). The composites showed interesting two-peak
responses related to the degree of CR uptake (Figure 4b,
Figure 4c, Figure S8b, Figure S8c, Figure S9b and Figure S9c).
The two peaks for the composites were 227/171 (1@CR),
140/163 (2@CR), 215/386 (3@CR) μAcmÀ 2, respectively. The
maximum photocurrents were 2.3, 3.3 and 4.5 times larger than
those of the pure CPs (97 (1), 49 (2) and 85 (3) μAcmÀ 2). The
two peaks all occur with composites prepared from the step-1
CR adsorption, even at different uptake amounts. Composites
prepared during step-2 CR absorption exhibited reduced photo-
currents that faded with increasing CR absorption. The photo-
currents of the dye saturated products (CP@CR-s) were
26 μAcmÀ 2 for 1@CR-s, 37 μAcmÀ 2 for 2@CR-s and 52 μAcmÀ 2
for 3@CR-s, all lower than the corresponding pure CPs. This
reduction in activity may be attributed to an outer CR layer
blocking the light from impinging on the photoactive centers,
and potentially blocking the transport of electrons.
This enhancement of photocurrent at lower dye content
encouraged us to explore the interaction of CR with the
substrate CPs in the early stage of step-1 adsorption. The
Figure 5. Raman spectra (900–1800 cmÀ 1) of 1, 1@CR-1, 1@CR-s and CR.
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CP surface. Likewise, the peak at 1248 cmÀ 1 attributed to the
Keywords: Adsorption · Congo Red · Coordination polymers ·
Photocurrent response · Silver · Tetraphosphine ligands
S=O stretching frequency[30] of 1@CR-1 was blue-shifted from its
original value of 1210 cmÀ 1 for pure CR. When the coverage of
CR on the composite was increased in 1@CR-s, the relative
intensity of this peak was reduced, presumably because the
À
À SO3 groups of the outer CR can’t interact with the surface of
1 directly. The same trends were observed in the Raman spectra
of 2, 3 and their composites (Figure S10). Therefore, we suppose
the two-step adsorption composes the first one which involve
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In summary, reactions of dppbpda with AgOx (Ox=Bz, Sal,
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Deposition Number(s) 2071252 (for 1·CH2Cl2 ·Et2O), 2071253
(for 2·2CH2Cl2), and 2071254 (for 3·2.5MeOH·MeCN) contain
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This work was financially supported by the National Natural
Science Foundation of China (Grant No. 21671144) and the
Priority Academic Program Development of Jiangsu Higher
Education Institutions. We are also grateful to Prof. Min-Min Xu of
Soochow University for his kind assistance with the FT-Raman
spectroscopy.
Conflict of Interest
Manuscript received: March 19, 2021
Revised manuscript received: April 28, 2021
Accepted manuscript online: April 30, 2021
The authors declare no conflict of interest.
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COMMUNICATIONS
Reactions of a tetraphosphine (P4)
ligand dppbpda with Ag(I) oxylates
resulted in three 1D CPs [(dppbpda)
Ag4(Ox)4]n, which exhibited excellent
two-step Congo Red (CR) sorption ca-
pacities up to 1438, 1417 and
M. Zhang, M.-Y. Feng, J.-J. Yan,
Prof. H.-Y. Li, Prof. D. J. Young, H.-X. Li,
Prof. Z.-G. Ren*
1 – 5
New Silver(I)-P4 Coordination
Polymers Strongly Adsorb Congo
Red to Yield Composites with
Enhanced Photocurrent Responses
1656 mg/g. Compared to pure CPs,
the composites CP@CR showed
enhanced two-peak photocurrent
responses, with optimized current
densities being 227, 163 and
386 μAcmÀ 2, respectively.