Copper(i) coordination polymers of 2,2′-dipyridylamine derivatives: Syntheses, structures, and luminescence

Article abstract of DOI:10.1039/c2dt12032a

Reactions of CuX (X = Br^-, I^- or CN^-) with various types of 2,2′-dipyridylamine (dpa) derivatives have been performed via a hydrothermal-solvothermal method and the products have been structurally characterized by X-ray crystallography. Four ligands with different coordination motifs were employed in the reactions, including angular N,N,N′,N′-tetra(2-pyridyl)-2,6-pyridinediamine (tppda); linear N,N,N′,N′-tetra(2-pyridyl)-1,4-phenylenediamine (tppa) and N,N,N′,N′-tetra(2-pyridyl)biphenyl-4,4′-diamine (tpbpa); and star-shaped tris-[4-(2,2′-dipyridylamino)-phenyl]amine (tdpa), which yielded eight copper(i) complexes exhibiting different stoichiometries of Cu-dpa and variable coordination modes of dpa. The compound [Cu₂(tppda) (μ-I)₂]_n (1) forms a one dimensional (1D) coordination polymer exclusively through double μ₂-I bridges, which arranges to two dimensional (2D) metal-organic frameworks (MOFs) via the face-to-face π...π stacking interactions from pyridyl rings. The compound [Cu ₆(tppa)(μ₃-Br)₆]_n (2) forms a 2D network linked through multiple μ₃-Br bridges. The compound [Cu₂(tppa)(μ-CN)₂]_n (3) is also a 2D MOF containing 1D (CuCN)_n chains. The compounds [Cu(tpbpa)Br]_n (4) and [Cu₄(tpbpa)₂(μ-I)₄]_n (5) display two different 1D assemblies: a zig-zag chain for 4 and a linear structure for 5. The compound [Cu₄(tpbpa)(μ-CN)₄] _n (6) shows a pseudo-4,8² topological net, while the compound [Cu₈(tpbpa)(μ-CN)₈]_n· 2nH₂O (7) exhibits a three-dimensional (3D) framework containing a ...PM... double helical structure, although both of them contain (CuCN)_n chains. The compound [Cu₂(tdpa)(μ-I) ₂]_n (8) is a zig-zag chain based on the star-shaped molecule tpda, in which one of three dpa-arms is free of coordination to metal ions. All complexes exhibit luminescence in the solid state.

Full text of DOI:10.1039/c2dt12032a

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PAPER
Copper(I) coordination polymers of 2,2′-dipyridylamine derivatives:
syntheses, structures, and luminescence†
Jia Ni,a Kai-Ju Wei,*b Yuanzeng Min,b Yaowen Chen,a Shunze Zhan,c Dan Lic and Yangzhong Liu*b
Received 26th October 2011, Accepted 13th February 2012
DOI: 10.1039/c2dt12032a
Reactions of CuX (X = Br−, I− or CN−) with various types of 2,2′-dipyridylamine (dpa) derivatives have
been performed via a hydrothermal–solvothermal method and the products have been structurally
characterized by X-ray crystallography. Four ligands with different coordination motifs were employed in
the reactions, including angular N,N,N′,N′-tetra(2-pyridyl)-2,6-pyridinediamine (tppda); linear N,N,N′,N′-
tetra(2-pyridyl)-1,4-phenylenediamine (tppa) and N,N,N′,N′-tetra(2-pyridyl)biphenyl-4,4′-diamine
(tpbpa); and star-shaped tris-[4-(2,2′-dipyridylamino)-phenyl]amine (tdpa), which yielded eight copper(I)
complexes exhibiting different stoichiometries of Cu–dpa and variable coordination modes of dpa. The
compound [Cu2(tppda)(μ-I)2]n (1) forms a one dimensional (1D) coordination polymer exclusively
through double μ2-I bridges, which arranges to two dimensional (2D) metal–organic frameworks (MOFs)
via the face-to-face π⋯π stacking interactions from pyridyl rings. The compound [Cu6(tppa)(μ3-Br)6]n
(2) forms a 2D network linked through multiple μ3-Br bridges. The compound [Cu2(tppa)(μ-CN)2]n (3)
is also a 2D MOF containing 1D (CuCN)n chains. The compounds [Cu(tpbpa)Br]n (4) and
[Cu4(tpbpa)2(μ-I)4]n (5) display two different 1D assemblies: a zig-zag chain for 4 and a linear structure
for 5. The compound [Cu4(tpbpa)(μ-CN)4]n (6) shows a pseudo-4,82 topological net, while the
compound [Cu8(tpbpa)(μ-CN)8]n·2nH2O (7) exhibits a three-dimensional (3D) framework containing a
⋯PM⋯ double helical structure, although both of them contain (CuCN)n chains. The compound
[Cu2(tdpa)(μ-I)2]n (8) is a zig-zag chain based on the star-shaped molecule tpda, in which one of three
dpa-arms is free of coordination to metal ions. All complexes exhibit luminescence in the solid state.
derivatives have also been widely investigated as potential mol-
Introduction
ecular wires and magnetic materials.10,11 However, the use of Cu
The coordination chemistry of 2,2′-dipyridylamine (dpa) and its
derivatives has received considerable attention since dpa is a
high-performance group combining molecular flexibility and
strong chelating ability for transition metal ions.1–6 The mono-,
di-, tri-, tetra- and even hexa-derivatives of dpa have been
reported, in which the secondary nitrogen of each dpa is directly
bound to the aryl core. These ligands have been widely
employed for the synthesis of novel functional materials in metal
complexes and supramolecular compounds.2–6 Pd(II) and Pt(II)
complexes of such dpa derivatives have been investigated as
potential anticancer agents due to their structural similarity to
cisplatin ions.7 Ag(I), Zn(II), Cd(II) and Hg(II) complexes of dpa
derivatives have been investigated as potential luminescent and
molecular recognition materials.8,9 Linear polynuclear Mn(II),
Cu(II), Co(II), Ni(II), Cr(II) and Ru(II) complexes of dpa
(I) in this range of metal–dpa complexes and supramolecular
materials has been rarely reported,12 partly due to the coordi-
nation difficulty of this d10 metal ion to dpa under regular con-
ditions, although its tendency to form strong complexes with
nitrogen donor ligands under solvo- or hydro-thermal conditions
has been well documented.13 Furthermore, the copper centers in
these materials are often associated with supramolecular inter-
actions (including Cu–Cu and Cu–π interactions) and some of
them can give rise to unusual electronic and photophysical
properties.14
On the other hand, luminescent complexes have recently
attracted much research interest because of their potential appli-
cations in optoelectronic devices and as fluorescent sensors and
probes.8,15 By choosing appropriate metal ions and organic
building blocks, it is possible to control and manipulate the
structures and the photonic properties of metal complexes2–4,8
because the assembly and structural morphology of these coordi-
nation complexes are highly dependent on the conformations
and sizes of the ligands. Therefore, fine tuning of the lumines-
cence properties can be achieved by modifying the subunits in
ligands.16
aCenter Lab, Shantou University, Shantou, 515063, P. R. China
bDepartment of Chemistry, University of Science and Technology of
China, Hefei 230026, P. R. China. E-mail: kjwei@ustc.edu.cn,
liuyz@ustc.edu.cn; Fax: +86-551-3600874
cDepartment of Chemistry, Shantou University, Shantou, 515063,
P. R. China
Encouraged by these findings and the further exploration of
2,2′-dipyridylamine derivatives as linkers to achieve novel
†CCDC 279821, 773756–773762. For crystallographic data in CIF or
other electronic format see DOI: 10.1039/c2dt12032a
5280 | Dalton Trans., 2012, 41, 5280–5293
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Scheme 1 Schematic representation of the structures of ligands tppda, tppa and tpbpa.
Scheme 2 Preparation of the ligand tdpa.
extended luminescent structures, we have designed and syn-
thesized a series of 2,2′-dipyridylamine compounds (Schemes 1
and 2) with various configurations and then investigated their
reactions with Cu(I) salts. These ligands include angular N,N,N′,
N′-tetra(2-pyridyl)-2,6-pyridinediamine (tppda), linear N,N,N′,
N′-tetra(2-pyridyl)-1,4-phenylenediamine (tppa) and N,N,N′,N′-
tetra(2-pyridyl)biphenyl-4,4′-diamine (tpbpa), and the star-
shaped molecule tris-[4-(2,2′-dipyridylamino)phenyl]amine
(tdpa). The reactions were carried out using a hydrothermal–sol-
vothermal method since it is advantageous to prepare low-solubi-
lity crystalline organic–inorganic hybrid materials.17 The desired
products were obtained by controlling the hydrothermal–sol-
vothermal conditions, such as reaction time, temperature, pH
value, solvent system and molar ratio of reactants.18–22 Herein,
we investigated a series of dpa complexes for systematic studies
on structural variations and photoluminescence of Cu(I)
complexes. These compounds exhibit interesting and unusual
structural features, and also show effective luminescent
properties. They are [Cu2(tppda)(μ-I)2]n (1), [Cu6(tppa)
(μ3-Br)6]n (2), [Cu2(tppa)(μ-CN)2]n (3), [Cu(tpbpa)Br]n (4),
FTIR spectrophotometer. Mass spectra were obtained from a
MALDI-TOF MS spectrometer. Elemental analyses were carried
out on an Elementar Vario EL-III analyzer. Photoluminescence
analyses were performed on a Perkin-Elmer LS 55 luminescence
spectrometer at room temperature.
N,N,N′,N′-Tetra(2-pyridyl)-2,6-pyridinediamine (tppda) was
prepared according to our procedure described.9a N,N,N′,N′-Tetra
(2-pyridyl)-1,4-phenylenediamine (tppa) and N,N,N′,N′-tetra(2-
pyridyl)-biphenyl-4,4′-diamine (tpbpa) were prepared using the
Ullmann method according to the procedure described by Wang
et al.23
Synthesis of tris-(4-iodophenyl)amine (tipa)
Tris-(4-iodophenyl)amine was synthesized according to the pro-
cedures reported earlier.24 1H NMR (300 MHz, CDCl3, ppm): δ
7.55–7.52 (d, 6H), 6.82–6.79 (d, 6H).
Synthesis of tris-[4-(2,2′-dipyridylamino)phenyl]amine (tdpa)
[Cu4(tpbpa)2(μ-I)4]n
[Cu8(tpbpa)(μ-CN)8]n · 2nH2O (7), and [Cu2(tdpa)(μ-I)2]n (8),
respectively.
(5),
[Cu4(tpbpa)(μ-CN)4]n
(6),
A mixture of 2,2′-dipyridylamine (1.54 g, 9 mmol), tris-(4-iodo-
phenyl)amine (1.25 g, 2 mmol), anhydrous K2CO3 (2.1 g,
15 mmol), Cu powder (0.081 g, 1.2 mmol) and 0.1 ml of 18-
crown-6 in anhydrous xylene (40 mL) was refluxed for 120 h
under dry nitrogen. The reaction mixture was cooled and the
solvent was removed by vacuum distillation. The residue was
cooled to ambient temperature and then dissolved into dichloro-
methane (200 mL) and water (50 mL). The aqueous phase was
discarded and the organic phase was washed with distilled water
to neutral pH and then dried with MgSO4. The product was
purified by a chromatographic column (the chromatographic
support used was SiO2) with ethyl acetate as the eluent. A white
solid was obtained. Yield: 0.9 g, 60%. 1H NMR (300 MHz,
CDCl3, ppm): δ 8.37 (6H, Py), 7.61–7.58 (6H, Py), 7.19 (6H,
Py), 7.10 (6H, Py), 6.99–6.91 (16H, Ph). MS m/z 752 (M+).
Experimental procedures
Materials and physical measurements
Copper(I) bromide, copper(I) iodide and copper(I) cyanide were
purchased from Acros. Other starting chemicals and solvents
used during the synthesis were of reagent grade and were
obtained from commercial sources. All reagents and solvents
were dried and purified by conventional methods. The FT-IR
samples were prepared using KBr pellets, and the spectra were
recorded in the 400–4000 cm−1 region on a Nicolet Avatar 360
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Synthesis of [Cu2(tppda)(μ-I)2]n (1)
8 mL water according to the same procedure described in the
synthesis of complex 1. Yield: 0.0230 g, 27%. IR (cm−1): 2140
(vs), 1590(s), 1565(s), 1496(m), 1474(vs), 1462(vs), 1435(s),
1324(s), 1294(m), 1278(m), 776(s), 748(m). Calc. for
C18H12Cu2N5.5: C, 50.00; H, 2.797; N, 17.81. Found: C, 49.88;
H, 2.789; N, 17.82%.
A mixture of CuI (0.0571 g, 0.3 mmol), tppda (0.0417 g,
0.1 mmol) and 8 mL water was sealed in a Teflon-lined reactor,
heated at 180 °C for 3 days, and slowly cooled to room tempera-
ture; the resulting mixture was washed with water and ethanol
and dried in air to give yellow crystals. Yield: 0.0240 g, 36%. IR
(cm−1): 1662(vs), 1610(vs), 1542(m), 1499(m), 1452(m), 1195
(w), 1029(w), 1001(w), 840(w), 762(w). Calc. for
C25H19Cu2I2N7: C, 37.61; H, 2.399; N, 12.28. Found: C, 37.70;
H, 2.392; N, 12.36%.
Synthesis of [Cu8(tpbpa)(μ-CN)8]n·2nH2O (7)
Yellow crystals of complex 7 were obtained from a mixture of
CuCN (0.0720 g, 0.8 mmol), and tpbpa (0.0493 g, 0.1 mmol),
1 mL water and 7 mL acetonitrile according to the same pro-
cedure described in the synthesis of complex 1. Yield: 0.0159 g,
15%. IR (cm−1): 2140(vs), 1591(s), 1565(s), 1496(m), 1474(vs),
1461(vs), 1435(s), 1324(s), 1294(m), 1278(m), 776(s), 748(m).
Calc. for C10H7Cu2N3.5O0.5: C, 38.59; H, 2.267; N, 15.75.
Found: C, 38.46; H, 2.250; N, 15.67%.
Synthesis of [Cu6(tppa)(μ3-Br)6]n (2)
Yellow crystals of complex 2 were obtained from a mixture of
CuBr (0.0861 g, 0.6 mmol), tppa (0.0417 g, 0.1 mmol) and
8 mL water according to the similar procedure described in the
synthesis of complex 1 (160 °C heating temperature was used
instead of 180 °C). Yield: 0.0409 g, 32%. IR (cm−1): 1602(s),
1560(m), 1499(s), 1474(s), 1428(s), 1384(m), 1331(m), 1266
(m), 787(m), 770(m). Calc. for C13H10Br3Cu3N3: C, 24.45; H,
1.578; N, 6.580. Found: C, 24.39; H, 1.581; N, 6.574%.
Synthesis of [Cu2(tdpa)(μ-I)2]n (8)
Yellow crystals of complex 8 were obtained from a mixture of
CuI (0.0572 g, 0.3 mmol), and tdpa (0.0752 g, 0.1 mmol),
4 mL water and 3 mL DMF according to the same procedure
described in the synthesis of complex 1. Yield: 0.0161 g, 12%.
IR (cm−1): 1586(vs), 1565(vs), 1504(vs), 1466(vs), 1427(vs),
1314(s), 1268(vs), 776(s), 736(s). Calc. for C48H36Cu2I2N10: C,
50.85; H, 3.201; N, 12.35. Found: C, 50.72; H, 3.199; N,
12.27%.
Synthesis of [Cu2(tppa)(μ-CN)2]n (3)
Yellow crystals of complex 3 were obtained from a mixture of
CuCN (0.0359 g, 0.4 mmol), tppa (0.0417 g, 0.1 mmol) and
8 mL water according to the similar procedure described in the
synthesis of complex 2. Yield: 0.0179 g, 30%. IR (cm−1): 2106
(vs), 1587(vs), 1573(s), 1501(m), 1478(s), 1459(s), 1427(vs),
1325(s), 766(s), 661(m). Calc. for C14H10CuN4: C, 56.46; H,
3.385; N, 18.81. Found: C, 56.32; H, 3.390; N, 18.79%.
X-ray crystallography
X-ray diffraction data were collected on a Bruker-AXS SMART
CCD area detector diffractometer at 293 K using ω rotation
scans with a scan width of 0.3° and Mo-Kα radiation (λ =
0.71073 Å). The structures were solved by direct methods and
refined with full-matrix least-squares technique using
SHELXTL.25 Anisotropic thermal parameters were applied to all
non hydrogen atoms. All of the hydrogen atoms in these struc-
tures are located from the differential electron density map and
constrained to the ideal positions in the refinement procedure.
The crystallographic calculations were conducted using the
SHELXL-97 programs. Crystal data and experimental details for
all crystals are given in Tables 1 and 2.
Synthesis of [Cu(tpbpa)Br]n (4)
Yellow crystals of complex 4 were obtained from a mixture of
CuBr (0.0574 g, 0.4 mmol), tpbpa (0.0493 g, 0.1 mmol) and
8 mL water according to the similar procedure described in the
synthesis of complex 2. Yield: 0.0191 g, 30%. IR (cm−1): 1586
(s), 1567(m), 1494(m), 1475(m), 1457(vs), 1428(s), 1325(m),
175(m), 1155(w), 767(m), 746(m). Calc. for C32H24BrCuN6: C,
60.43; H, 3.803; N, 13.21. Found: C, 60.31; H, 3.795; N,
13.31%.
Synthesis of [Cu4(tpbpa)2(μ-I)4]n (5)
Results and discussion
Yellow crystals of complex 5 were obtained from a mixture of
CuI (0.0571 g, 0.3 mmol), tpbpa (0.0493 g, 0.1 mmol) and
8 mL water according to the same procedure described in the
synthesis of complex 1. Yield: 0.0271 g, 31%. IR (cm−1): 1583
(s), 1561(m), 1473(m), 1460(vs), 1448(vs), 1427(s), 1323(m),
1303(m), 1266(m), 1165(w), 772(m), 762(m). Calc. for
C32H24Cu2I2N6: C, 44.00; H, 2.769; N, 9.621. Found: C, 43.87;
H, 2.775; N, 9.603%.
Syntheses and characterization
The ligands tppda, tppa and tpbpa were synthesized by using
Ullmann condensation methods as reported previously.9a,23 The
molecular star tdpa was synthesized using the procedure in
Scheme 2, in which the key starting material, tris-(4-iodophenyl)
amine, was synthesized using the procedure in the literature.26
The Ullmann condensation reaction of tris-(4-iodophenyl)amine
with 2,2′-dipyridylamine was performed with a base (K2CO3)
and catalysts (Cu, 18-crown-6), and the product tris-[4-(2,2′-
dipyridylamino)phenyl]amine (tdpa) was obtained in a medium
yield (60%). Compounds tppda, tppa, tpbpa and tdpa are
Synthesis of [Cu4(tpbpa)(μ-CN)4]n (6)
Yellow crystals of complex 6 were obtained from a mixture of
CuCN (0.0360 g, 0.4 mmol), tpbpa (0.0493 g, 0.1 mmol) and
5282 | Dalton Trans., 2012, 41, 5280–5293
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Table 1 Crystallographic data for compounds 1–4
Compound
1
2
3
4
CCDC
Formula
Formula weight
773756
C25H19Cu2I2N7
798.35
773757
C13H10Br3Cu3N3
638.59
773758
C14H10CuN4
297.80
773759
C32H24BrCuN6
636.02
Crystal system
Monoclinic
0.15*0.15*0.14
P2(1)/n
Monoclinic
0.20*0.13*0.12
P2(1)/c
Monoclinic
0.36*0.32*0.08
P2(1)/c
Monoclinic
0.21*0.20*0.15
P2(1)/c
Crystal size/mm3
Space group
a/Å
b/Å
c/Å
α/°
8.6328(11)
22.983(3)
14.1392(17)
90.00
12.3130(9)
17.5191(12)
7.3772(5)
90.00
9.1973(18
15.376(3)
9.1789(18)
90.00
14.6291(13)
11.2293(10)
17.3085(15)
90.00
β/°
91.257(2)
90.00
2804.7(6)
1.891
104.2390(10)
90.00
1542.47(19)
2.750
111.737(3)
90.00
1205.7(4)
1.641
107.402(2)
90.00
2713.2(4)
1.557
γ/°
V/Å3
Dc/Mg m−3
Z
4
4
4
4
F(000)
1528
3.745
1204
11.865
604
1.800
1288
2.312
μ/mm−1
Reflections collected
Reflections unique
R(int)
Data/restraints/params
Final R indices [I > 2σ(I)]
15 066
4932
0.0291
4932/0/325
R1 = 0.0388
wR2 = 0.0852
R1 = 0.0521
wR2 = 0.0924
1.029
7232
2697
0.033
2697/0/199
R1 = 0.0399
wR2 = 0.1028
R1 = 0.0513
wR2 = 0.1119
1.034
5200
2119
0.0269
2119/0/172
R1 = 0.0353
wR2 = 0.0884
R1 = 0.0451
wR2 = 0.0958
1.047
11 767
4771
0.0280
4771/0/361
R1 = 0.0446
wR2 = 0.1095
R1 = 0.0677
wR2 = 0.1232
1.045
R indices (all data)
GOF on F2−
Table 2 Crystallographic data for compounds 5–8
Compound
5
6
7
8
CCDC
Formula
Formula weight
279821
C32H24Cu2I2N6
873.45
773760
C18H12Cu2N5.5
432.41
773761
C10H7Cu2N3.5O0.5
311.27
773762
C24H18CuIN5
566.87
Crystal system
Monoclinic
0.18*0.15*0.04
P2(1)/n
Orthorhombic
0.34*0.32*0.12
Pbcn
Orthorhombic
0.18*0.15*0.13
Pnnn
Monoclinic
0.26*0.18*0.11
C2/c
Crystal size/mm3
Space group
a/Å
b/Å
c/Å
α/°
18.6210(14)
10.0925(8)
18.7696(14)
90.00
12.7525(18)
18.711(3)
14.771(2)
90.00
7.413(4)
8.577(4)
37.051(16)
90.00
9.7052(8)
15.6248(12)
30.199(2)
90.00
β/°
117.6270(10)
90.00
3125.2(4)
1.856
90.00
90.00
3524.5(8)
1.630
8
90.00
90.00
2355.8(19)
1.755
8
95.2010(10)
90.00
4560.5(6)
1.651
γ/°
V/Å3
Dc/Mg m−3
Z
4
8
F(000)
1688
3.369
1732
2.425
1228
3.588
2232
2.332
μ/mm−1
Reflections collected
Reflections unique
R(int)
Data/restraints/params
Final R indices [I > 2σ(I)]
21 972
5485
0.0593
5485/0/379
R1 = 0.0582
wR2 = 0.1230
R1 = 0.0938
wR2 = 0.1393
1.028
16 824
3113
0.0397
3113/0/229
R1 = 0.0448
wR2 = 0.1263
R1 = 0.0647
wR2 = 0.1426
1.037
5964
2026
0.0427
2026/0/151
R1 = 0.0739
wR2 = 0.1618
R1 = 0.0940
wR2 = 0.1727
1.013
17 020
4007
0.0594
4007/0/282
R1 = 0.0445
wR2 = 0.0940
R1 = 0.0642
wR2 = 0.0985
1.037
R indices (all data)
GOF on F2−
fairly soluble in most common polar solvents, such as MeOH,
CH2Cl2, CHCl3 and EtOAc. The compound tdpa was fully
characterized by 1H NMR and MS spectroscopy.
The Cu(I) complexes 1–6 were obtained by reaction of Cu(I)
salts with tppda, tppa and tpbpa ligands in H2O using the
hydrothermal method. Compounds 7 and 8 were obtained in the
solvent mixtures CH3CN–H2O and DMF–H2O, respectively. In
this dpa–Cu(I) system, the hydrothermal–solvothermal method is
chosen because the solubilities of these Cu(I) salts are relatively
poor. All compounds 1–8 were examined by single-crystal X-ray
diffraction analysis (Tables 1 and 2). As shown in Fig. 1–13,
none of the ligands tppda, tppa, tpbpa and tdpa is planar;
instead, they adopt various arrangements based on flexible che-
lating modes displayed by dpa in 1–8.
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The complex [Cu2(tppda)(μ-I)2]n (1): from 1D chain to 2D
network
result, each copper cation serves as a tetrahedral coordination site
where each iodide anion is coordinated to two copper cations,
which show double μ2-I bridging mode, similar to the compound
[Cu(dpa)I]2 early reported.12 The steric demands of the ligand
and bridging modes of the anion are likely responsible for the
binding of only two of the four available pyridyl rings. Similar
bonding modes have been also observed in other M(II)–tppda
complexes (M = Zn, Cd and Hg).9a
The solid-state structure of compound 1 is shown in Fig. 1. The
angular ligand tppda bridges two Cu(I) ions with typical Cu–N
bond lengths (Table 3),13,27 through the four pyridyl rings of two
pairs of dpa groups, in a opposite-facing fashion. The iodide
anion functions not only as a ligand to coordinate a copper(I) ion
(Cu–I: 2.60–2.65 Å), but also as a bridge to link two neighbor-
ing Cu ions (Cu–Cu is 2.574 Å and the van der Waals radii for
copper is 1.40 Å28), forming a 1D coordination polymer. As a
Compared with other metal ions with d10 electronic configur-
ation (Ag+, Zn2+, Cd2+ and Hg2+), Cu(I) was considered to facili-
tate the formation of polyhalide bridging.13,27 This phenomenon
is also observed in compounds 2–8 in this work. These 1D
chains arrange in the bc plane to form 2D metal–organic frame-
works through double face-to-face π⋯π stacking interactions
between the lateral pyridyl rings of dpa groups with centroid–
centroid distances of 3.49 and 3.99 Å (Fig. 2). Evidently, this
complex is a good example suitable for the study of MOFs con-
structed via non-covalent interactions.
2D networks of the complex [Cu6(tppa)(μ3-Br)6]n, (2),
containing broad (CuBr)n chains
Fig. 1 View of 1D “zig-zag” chain of 1. (The dihedral angle is 6.71°
and the centroid-to-centroid distance is 4.30 Å between N(5) pyridyl
ring and N(1) pyridyl ring in tppda.) Hydrogen atoms have been
omitted for clarity in this and following figures. Symmetry code: #1 =
1/2 + x, 1/2 − y, −1/2 + z.
Single-crystal X-ray structure analysis showed that compound 2
is a 2D inorganic–organic CuBr coordination polymer. The
structure of 2 can be divided into two parts: tppa units and
(CuBr)n chains. As shown in Fig. 3, each tppa molecule lies
about an inversion center and coordinates to four copper ions
Fig. 2 2D metal–organic frameworks of 1 stacked together via face-to-
face π–π stacking interactions (the distance between N(7) pyridyl rings
is 3.99 Å; the distance between N(3) pyridyl rings is 3.49 Å). Symmetry
codes: #2 = 1 − x, 1 − y, 2 − z; #3 = 1 − x, −y, 2 − z.
Fig. 4 2D networks of complex 2 as seen along the crystallographic bc
plane.
Fig. 3 (a) The coordination environments of copper atoms in 2; (b) 1D broad (CuBr)n chains of 2 (symmetry codes: #1 = x, 2.5 − y, 1/2 + z; #2 =
x, y, 1 + z).
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Fig. 5 (a) 2D metal–organic frameworks of complex 3; (b) 63 topology of complex 3.
coordination metal with a Br4 environment. Interestingly, each
bromine anion in 2 is coordinated to three copper cations,
showing a less common μ3-Br bridging mode.30
In the complex 2, each tppa unit connects two broad (CuBr)n
chains through the Cu–N coordination to construct the 2D net-
works (Fig. 4). Polynuclear complexes of Cu(I) are of great inter-
est due to their remarkable photo-physical and catalytic
properties.31 In various d10 metal-clusters, the attractive inter-
actions between closed d10 shells have received special attention,
since the arrangement of metal centers and, in particular, the
number of metal atoms deeply affect the photo-physical beha-
viors of the resulting polynuclear systems.32 Therefore, the con-
trolled formation of small clusters or low-dimension structures of
the metal with defined nuclearities and the study of their struc-
ture–function correlations are of great importance in designing
functional materials.
Complex [Cu2(tppa)(μ-CN)2]n (3), 2D MOFs containing zig-zag
(CuCN)n chains
Fig. 6 (a) 1D zig-zag chain structure of complex 4; (b) 2D supramole-
cular networks of 4 formed through Br⋯H hydrogen-bonding (Br
(1)#1⋯H(18): 2.80 Å, ∠Br(1)#1⋯H(18)–C(18): 165°; Br(1)#1⋯H(22):
3.03 Å, ∠Br(1)#1⋯H(22)–C(22): 167°. Symmetry codes: #1 = x, 1/2 −
y, −1/2 + z; #2 = x, −1/2 − y, 1/2 + z).
Copper(I) cyanide is well known to form a diverse array of
MOFs with polydentate bridging ligands (L).33 Known
(CuCN)nL stoichiometries cover half-integer values, n = 1, 1.5,
2, 2.5, 3, 3.5, and even 4.34 The more copper-rich phases often
show evidence of partially or fully independent CuCN chains
within their structures. As part of an extensive study of lumines-
cent MOFs based on CuCN, a copper-rich network is syn-
thesized which is based on the tetradentate ligand tppa.
Hydrothermal synthesis using a mixture of CuCN–tppa
(molar ratio 4 : 1) produced yellow crystals of 3 suitable for
X-ray diffraction analysis. The structure of 3 contains one inde-
pendent Cu ion and one cyano unit, all of which are C/N site dis-
ordered. Since it is composed of only independent sub-lattices,
these sub-lattices form a simple 63 planar network of stoichi-
ometry [(CuCN)2(tppa)] which results from tppa-crosslinking of
parallel (CuCN)n chains with tppa located on an inversion
center. All six Cu atoms in every lattice are four-coordinated,
nevertheless all the cyano–Cu–cyano angles are >132°. The
sheets, which run parallel to the crystallographic bc plane, are
composed of tiled hexagonal [Cu6(CN)4(tppa)2] units. This
arrangement generates a very large, rippled 2D network consist-
ing of [Cu6(CN)4(tppa)2] rings (Fig. 5).
through nitrogen atoms of four pyridyl groups in two pairs of
twisted dpa groups (Cu–N 2.028(5)–2.044(5) Å). Both Cu(1)
and Cu(3) cations serve as tetrahedral coordination sites, but
they display different coordination environments: NBr3 for Cu
(1) and NCBr2 for Cu(3), respectively. That is, the C(13) atom
bonds to the Cu(3) ion (2.422(6) Å). This structure shows that
the benzene ring coordinates to two Cu(3) ions with an η1-
bonding mode. This η1-type interaction between the Cu(I) ion
with a phenyl ring has already been observed in some com-
plexes,29 although it is less common than the η2-type interaction
in which metal ions interact with one side of the phenyl ring.
The other part of the structure is the broad (CuBr)n chains,
which are composed of three zig-zag (CuBr)n units. Three single
chains are joined through Cu–Br (2.41–2.83 Å) and Cu–Cu (Cu
(1)–Cu(2): 2.86 Å) interactions to form a triple (CuBr)n chain
(Fig. 3b). Obviously, the Cu(2) atom is also a tetrahedral
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Fig. 7 1D chain structure of complex 5 (symmetry codes: #1 = 2 − x, −y, 1 − z; #2 = 1 − x, −y, −1 − z).
Fig. 8 2D metal–organic frameworks of complex 6 (symmetry code:
#1 = 2 − x, y, 1/2 − z).
Fig. 9 4.82 topological nets of complex 6 are stacked in a ⋯ABAB⋯
fashion, neglecting the biphenyl group of tpbpa.
The 2 : 1 CuCN–tppa complex forms hexagonal 63 sheets,
which are induced via face-to-face π⋯π stacking interactions
from the N(1) and N(3) pyridyl rings (dihedral angle: 12.62°,
centroid-to-centroid distance: 3.694 Å). Further, these sheets are
crosslinked by π⋯π stacking from N(3) pyridyl rings between
two adjacent sheets (the distance is 3.618 Å between planes) to
produce a 3D network. Threading of (CuCN)n chains through 63
topological networks has been previously observed for
(CuCN)3-(Pyz), (CuCN)n(Bpy) and (CuCN)20(Pip)7 (Pyz = pyra-
zine; Bpy = 4,4′-bipyridine, n = 3.5, 4; Pip = piperazine).35
The arrangement around the Cu(I) cation in 4 is similar to those
of other Cu(II) salts with dpa appended triazine derivatives.36
The shortest Cu⋯Cu distance is 16.322 Å, which is the size of
the ligand tpbpa. Further, the neighboring chains link each other
via multiple intermolecular C–H⋯Br H-bonding interactions to
construct a 2D supramolecular network.
The anion I− was used in the reaction with tpbpa in order to
compare with the Br− anion in the construction of the coordi-
nation structure. Yellowish crystals of [Cu4(tpbpa)2(μ-I)4]n (5)
were isolated. In contrast to 4, there are two independent Cu(I)
centers in 5. The Cu(1) cation is coordinated by a nitrogen atom
of a dpa group (Cu–N(3): 2.02 Å) and two iodide ions (Cu–I:
2.56–2.63 Å) to form a NI2 trigonal coordination sphere. The N
(1) atom coordinates to the Cu(1) cation weakly with a bond
length of 2.352 Å. The iodide anion also functions as a bridging
unit to link two equivalent Cu(1) atoms, and the distance of Cu–
Cu (2.53 Å) corresponds to that in 1. The coordination sphere
around the Cu(2) center is composed of one pyridyl nitrogen
atom from the dpa group (Cu–N: 1.999(5) Å) and the other nitro-
gen atom of dpa is free. Analogously, two iodide ions act also as
a double μ2-I bridge and links two Cu(2) units (Cu–I:
2.56–2.58 Å) with a Cu–Cu distance of 2.548 Å. Each Cu(2)
cation in 5 forms a trigonal-planar coordination environment as
the sum of the bond angles around Cu(2) is about 360°. Thus,
each tpbpa in turn links two Cu2I2 double cores to generate an
infinite chain (Fig. 7).
1D zig-zag chain [Cu(tpbpa)Br]n (4) and linear chain
[Cu4(tpbpa)2(μ-I)4]n (5)
In principle, the longer the spacers used, the larger dimensions
of assemblies obtained. Actually, it is very hard to expect the
same type of structure with an elongation in one of the dimen-
sions, in particular for flexible ligands (vide infra 2 and 4; 1
and 5).
To investigate assembly properties with a larger-sized linker,
we used tpbpa to react with CuBr according to the method for
2. However, a different 1D zig-zag polymer 4 was obtained.
Single-crystal analysis reveals that there is only one independent
crystallographic Cu(I) center in 4. As shown in Fig. 6, the
coordination geometry around the Cu(I) ion is trigonal-planar
with an N2Br-donor environment (Cu–N(1): 1.978(3), Cu–N(5):
1.994(3), Cu–Br: 2.457 Å). The two coordination pyridyl rings
with two free are organized in a manner that effectively closes
one side of the half-sphere, thus making a cup-shaped bottom.
It can be concluded from the architectures of the two
complexes (4 and 5) that the counterions (Br− vs. I−) play a key
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Fig. 10 (a) Double-helical structures of complex 7 along the a axis showing biphenyl group bridging interactions; (b) the view of structure units of 7
down the bc plane; (c) double helical structure of DNA molecules showing H-bond bridging interactions; (d) network structure of 7 (symmetry code:
#1 = 2 − x, 1 − y, 1 − z).
Fig. 11 A racemic structure schematic of complex 7.
role in determining the polymeric structures. Anion-dependent
extended structures have often been observed earlier.9 The differ-
ent van der Waals radii of two ions are probably the cause of
such different architectures (1.85 and 1.98 Å for Br and I,
respectively28). It is difficult to predict the final assembly of
flexible ligands, and the comparisons of 1 and 5, 2 and 4 are
actually these cases.
2D MOFs of the complex [Cu4(tpbpa)(μ-CN)4]n (6) and 3D
architecture of the complex [Cu8(tpbpa)(μ-CN)8]n·2nH2O (7)
containing a rare double helical structure
As part of an extensive study of luminescent MOFs based on
CuCN, we have further explored the copper-rich networks based
on the tpbpa molecule. Solvothermal synthesis using a 4 : 1
CuCN–tpbpa mixture produced yellow crystals of 6 suitable for
Fig. 12 MOFs of complex 7, showing 1D porous channel along the c
axis. Water guest molecules are omitted for clarity.
X-ray diffraction analysis.
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Fig. 13 The 1D zig-zag chain of complex 8 (symmetry code: #1 = 1 − x, −y, 1 − z).
Table 3 Relevant structural parameters (dihedral angles (Θ) between Py rings of dpa groups) in 1–8
Cu(I) coordination environment
Cu(dpa)I2
Cu–Ndpa# (Å)
Θ (°)
Cu⋯Cu (Å)
Cu–I (Å)
∠Cu–I–Cu (°)
1
2
2.085(4)#1
2.086(4)
2.089(4)#1
2.094(4)
56.62
58.35
2.5738(9)
2.60
2.65
58.30
59.36
CuNPyBr3
CuNPyCphBr2
2.028(5)
2.044(5)#2
67.70
2.8567(15)#3
3.245
3
4
Cu(dpa)(CN)2
Cu(NPy)2Br
2.102(3)
1.978(3)
2.160(3)
12.62
52.28
60.06
47.76
70.75
61.78
75.57
69.06
29.75
48.89a
1.994(3)#4
5
6
CuNPy 2
I
1.999(5)
2.173(4)
2.017(5)
2.5480(17)#5
2.5348(18)#6
2.56
58.42#6
59.54#5
2.58#5
CuNPy(CN)2
2.168(4)#7
7
8
CuNPy(CN)2
Cu(dpa)I2
2.104(7)
2.188(5)
2.024(5)
2.5790(14)#8
2.56#8
2.63
59.53#8
a Free dpa group of tdpa in compound 8. Symmetry codes: #1 = x + 1/2, −y + 1/2, z − 1/2; #2 = −x + 2, −y + 2, −z; #3 = x, −y + 5/2, z + 1/2; #4 =
x, −y + 1/2, z−1/2; #5 = −x + 1, −y, −z − 1; #6 = −x + 2, −y, −z + 1; #7 = −x + 2, y − 1, −z + 1/2; #8 = −x + 1, −y, −z + 1).
The structure of 6 contains two independent crystallographic
Cu centers and two cyano units, all of which are also C/N site
disordered. In contrast to CuCN–tppa in 3, four pyridyl nitro-
gens of two dpa groups in tpbpa coordinate to four Cu atoms
(Cu–N: 2.168(4)–2.173(4) Å) and each cyano unit bridges two
Cu ions, which accomplish a trigonal coordination environment.
Because of the presence of two flexible centers, ligand tpbpa
can coordinate to the metal ion suitably to meet the geometric
requirements of copper. Each tpbpa located on a two-fold axis
links four Cu(I) ions and each copper ion links two cyano units
to generate an infinite 2D network, which contains two different
metallacycles A and B without considering the biphenyl group
of tpbpa (Fig. 8). In A, two dpa units from two tpbpa
ligands and two cyano units connect four Cu(I) ions to form an
18-membered ring {Cu4(CN)2(dpa)2} with dimensions of 4.86 ×
5.32 Å for Cu⋯Cu. In B, two dpa groups and six cyano
units connect eight Cu(I) atoms to form a 30-membered ring
{Cu8(CN)6(dpa)2}. Accordingly, each 18-membered ring is sur-
rounded by four 30-membered rings, and each 30-membered
ring approaches four 18-membered rings and four 30-membered
rings. As a result, the 2D network of 6 can be regarded as a 4.82
topology, which is stacked in a ⋯ABAB⋯ pattern, in which the
biphenyl core is ignored and the dpa serves as a bidentate ligand
(Fig. 9).
interpenetrating structure of complex 6 is rare, which results
from the filling of biphenyl groups in each 30-membered ring.
All Cu atoms in lattices A and B are three-coordinate, neverthe-
less all the cyano–Cu–cyano angles are >149°. The (CuCN)n
chains are nearly linear along the tpbpa units, but they buckle at
the crosslinks.
Increasing the CuCN –tpbpa ratio to 8 : 1 in the hydrothermal
synthesis yielded yellow crystals of 7. Similarly to 6, the struc-
ture of 7 contains two independent Cu ions and two cyano units,
all of which are also C/N site disordered. Four pyridyl nitrogens
of tpbpa coordinate to four equivalent Cu(1) atoms (Cu–N:
2.104(7) Å), and both Cu(1) and Cu(2) bridge two cyano units.
As a result, a double helix assembly is formed along the a axis
(Fig. 10). The helix paths can be easily traced through the Cu–
cyano–Cu–cyano bonds anticlockwise around the two-fold
screw axis in the helix. Furthermore, two extended helix chains
enwind around each other to form a double-helical structure,
which is similar to the well-known structure of the biological
molecule DNA. However, the covalent bond of the biphenyl unit
in 7 is in the place of the interstrand hydrogen bond of DNA.
This assembly translates throughout the crystal into formation of
not only right-handed helices but also left-handed helices
through an auto-resolution process.
The space group of compound 7 is Pnnn, which has a centro-
symmetric structure. Therefore, equal amounts of right-helix and
left-helix structures are present in 7, giving a racemic structure.
Generally, interpenetrating 4.82 topological networks are
frequently observed in coordination polymers.24,37 The non-
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Scheme 3 Four typical possible coordination modes of di-dpa derived ligands in Cu(I) compounds.
Generally, most racemic helix arrangements have syndiotactic
chirality or atactic arrangements, that is random chirality.38
However, complex 7 exhibits heterotactic chirality, because the
type P and M helices adopt a layered arrangement in the c axis
(Fig. 11), which is a rare arrangement because the monomers
need to identify the helicity of the final product in the process of
polymer growth.39
To the best of our knowledge, there is no precedent of
(CuCN)8L (L = ligand) observed with a stoichiometry of eight.
These sheets are cross-linked by tpbpa-bridging cyano groups,
producing a 3D network. In contrast to 6, the threaded (CuCN)n
chains in this structure consist solely of 2D Cu centers but lack
ligands. It is interesting that the 3D network contains 1D open
∼14.2 Å. Although a few triangle-shaped ligands have been
reported as successful linkers for extended cyclic structures,8
examples of this class of supramolecular structures in 8 remain
relatively scarce.
It is worth noting that the compounds 1, 5 and 8 exhibit
various polymeric patterns, but all of them have the same
Cu2(μ-I)2 dimetal units as “nodes”. Two dpa groups of ligands
coordinate to these di-nuclear “nodes”, showing similar Cu–N
bond lengths (ranging from 1.999 to 2.188 Å). In these
[Cu2(μ-I)2(dpa)2] systems, the structural parameters (bond dis-
tances and angles; see Table 3) are similar: Cu–I bond lengths
ranging from 2.56 to 2.65 Å; Cu–Cu distances ranging from
2.5348 to 2.5790 Å; and the angle of Cu–I–Cu ranging from
58.30 to 59.53°. That is, the [Cu2(μ-I)2(dpa)2] structure is steady
in the CuI–dpa coordination system. We can not obtain
[Cu2(μ-X)2(dpa)2] structures (X = Cl, Br or CN), although we
have performed some relative experiments. It is proved again
that the [Cu2(μ-X)2(dpa)2] assembly is special for the CuI–dpa
system.
channels along the
c axis direction. Calculations using
PLATON40 showed that the open channels constitute about
8.63% (203.4 Å3) of the crystal volume and are filled with guest
water molecules (Fig. 12).
1D zig-zag chain of the complex [Cu2(tdpa)(μ-I)2]n (8)
In principle, large cavities in supramolecular structures based on
coordination compounds can be achieved by employing large
organic linkers with well-defined shape and coordination geome-
try.41 Large starburst or star-shaped ligands are attractive candi-
dates for this purpose because they form interesting and
uncommon extended structures such as hexagonal or triangular
macrocycles.8,42 Thus, we designed and synthesized the triangle-
shaped ligand tris-[4-(2,2′-dipyridylamino)phenyl]amine (tdpa)
and investigated its reaction with Cu(I) salts. However, only a 1D
coordination polymer 8 was obtained in crystal.
In 8, two Ph-dpa “arms” of the tdpa ligands located on a two-
fold axis are linked together by two Cu(I) ions in an alternating
up and down pattern to form a zig-zag chain (Fig. 13). Similarly
to 1, the iodide anion functions not only as a ligand to coordinate
a Cu(I) ion (Cu–I: 2.56–2.63 Å) but also as a bridge to connect
two neighboring Cu ions (Cu–Cu: 2.579 Å), forming a
[Cu2(μ-I)2] dimetal unit on an inversion center in a double μ-I
bridging mode. Interestingly, the third dpa-arm of tdpa is free.
The nearest Cu⋯Cu distance between the two arms of tdpa is
Structural properties
Scheme 3 exhibits four typical coordination modes of di-dpa
ligands in Cu(I) complexes 1–8. There are four defined confor-
mations (I, II, III, IV) and numerous intermediate conformations
between them, which suggests that di-dpa derived ligands can
generate various polymeric patterns to coordinate to metal ions.
The conformation of ligands in metal complexes may depend on
the solvents, counteranions and even reagent ratios. In other
words, the topologies of metal complexes can be controlled by
reaction conditions.
In 1D chain 1 and 2D metal–organic framework 3, two dpa
groups coordinate to two Cu(I) ions in a centro-symmetric
fashion, which results in a trans-conformation with the bonding
of two pairs of NPy to metal ions (Scheme 3a). Although both
1D chains 4 and 5 have free NPy atoms, two pairs of dpa groups
still adopt a trans-conformation (Scheme 3a). In 2D MOFs 2, 6
and 3D MOFs 7, four NPy atoms of the ligands coordinate to
four independent Cu(I) ions, but two pyridyl rings of dpa groups
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adopt different distortions: inward- and outward-type
(Scheme 3c for 2; Scheme 3d for 6 and 7). Evidently, it results
from the flexibility and free rotation of the dpa group. Similarly
to the coordination mode of 1 and 3, two pyridyl nitrogen atoms
of the dpa group coordinate to one Cu(I) ion in 8, but the
two coordinated-“arms” nearly adopt
a cis-conformation
(Scheme 3b) because of the flexibility and free rotation of the tri-
phenylamine core.
Evidently, all of 1, 5 and 8 contain the same [Cu2I2] core,
although they show different sizes of the spacers of the ligands.
When the flexible centers of the spacers that link dpas (pyridyl
for tppda, diphenyl for tpbpa and triphenylamine for tdpa)
increase, the rigidity of the ligands decrease. As a result, the dpa
units of the ligands potentially change from trans- to cis-confor-
mation to coordinate copper ions. In comparison to the tppa
ligand in 3, the ligand tpbpa, which contains the biphenyl core
in 6 and 7, is a relatively longer spacer. Thus, the steric effect is
small, and the dpas of tpbpa are prone to adopting outward-type
modes to bond metal ions. However, in comparison with 4, the
formation of the (CuBr)n cluster leads to a twisted inward-type
structure, although the core of tppa is rigid in 2.
Fig. 14 Emission spectra in the solid state of the free ligand tppda and
its copper(I) compound 1 at room temperature.
In this series of Cu–dpa complexes, the transformation of
dihedral angles between pyridyl rings of dpa groups is consistent
with the change of corresponding structures (Table 3) in order to
meet the coordinated geometric requirements of copper ion. In 1
and 3, two pairs of dpa units adopt a similar trans-conformation,
but they show different dihedral angles between pyridyl rings,
which result from the different conformation of ligands (angular
for tppda; linear for tppa). The dihedral angles are similar or
identical (56.62 and 58.35° for 1; 12.62° for 3). Although both
complexes 4 and 5 show trans-conformations, only one pyridyl
nitrogen atom of each dpa coordinates to a metal ion and the
other is free. As a result, the dihedral angles between pyridyl
rings of dpas are bigger than that of 3, although tpbpa is also
the linear ligand. In 2, 6 and 7, four pyridyl nitrogen atoms of
two pairs of dpas coordinate to four different copper ions, the
angles between pyridyl rings are larger and irregular, probably
due to the steric hindrance from the larger building block. In 8,
two nitrogen atoms of dpas again coordinate to a metal center,
so the angles between pyridyl rings are smaller than those of
complexes 2, 6 and 7. Obviously, all of those versatilities of
complexes 1–8 result from the flexibilities of ligands, since the
flexible “spacers” (–dpa) carry out flexible “assemblies” to con-
struct structural versatility.
Fig. 15 Emission spectra in the solid state of the free ligand tppa and
its copper(I) compounds 2 and 3 at room temperature.
Herein, the luminescent properties of a series of new 2,2′-dipyri-
dylamine derivative ligands and its copper(I) complexes 1–8 are
investigated.
All of the free ligands tppda, tppa, tpbpa, tdpa and their
compounds 1–8 exhibit photoluminescence in the solid state at
room temperature. The emission spectra of tppda and compound
1 are shown in Fig. 14. The free ligand tppda exhibits strong
emission bands in the 375–450 nm region (λmax = 397 nm) upon
excitation with λex = 276 nm, which is due to the π → π* tran-
sition. When tppda coordinates to Cu ions, a significant red shift
of the emission peak is observed (λmax = 519 nm). This emission
may originate from the halide-to-ligand charge transfer
(XLCT),43 because the coordination environment and the λmax
Luminescent properties and spectra analysis
For potential applications as luminescent materials, the lumines-
cent properties of the metal–organic coordination polymers have
been investigated. Due to the ability of inorganic–organic
coordination polymers to affect the emission wavelengths of
organic materials, syntheses have employed the smart design of
conjugated organic ligands and judicious choice of proper metal
centers in order to obtain new types of luminescent materials,
especially for d10 metal systems. We have explored the lumines-
cent properties of N,N,N′,N′-tetra(2-pyridyl)-2,6-pyridinediamine
(tppda) and its Zn(II), Cd(II), Hg(II) coordination polymers.9 The
results indicated that emission colors of organic spacers tppda
were remarkably affected by metal coordination compounds.
of the emission of this complex are similar to those of the
em
max
dimeric complex [Cu2I2(py)4] (py = pyridine) (λ
= 517 nm at
294 K).43a
Upon excitation at λex = 272 nm, ligand tppa and its com-
plexes 2 and 3 show strong emissions at 480 nm, 569 nm and
573 nm, respectively (Fig. 15). Free ligand tpbpa exhibits strong
emission bands in 450–600 nm region (λmax = 517 nm) upon
excitation of λex = 276 nm. When tpbpa coordinates to the Cu(I)
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Fig. 16 Emission spectra in the solid state of the free ligand tpbpa and
its copper(I) compounds 4–7 at room temperature.
Fig. 18 UV-vis spectra of tdpa at ambient temperature in CH2Cl2,
THF, CH3CN and DMF, respectively.
8 compared with the complexes 1 and 5, in which the ligand
over-twists to break the conjugation when it coordinates to the
Cu(I) ion, although the anion I− is present in all of three com-
plexes. Thus the photoluminescence mechanism of 8 is different
from 1 and 5, and the position of the red shift of 8 is only
67 nm, which is less than those of 1 and 5.
Originating from different charge-transfer mechanisms, the
red shifts of the emission bands of complexes 1–8 in comparison
with their corresponding free ligands indicate that the coordi-
nation of N-donor ligands to Cu(I) centers effectively increases
the ligands conformational rigidity and therefore results in a
decrease in the loss of energy by radiationless decay.45
In addition, we have carried out further investigation on the
fluorescence properties of the star-shape ligand tdpa in different
solvents. About 20 nm blue shift of the λmax of tdpa is observed
in THF relative to CH2Cl2 or DMF, showing that the emission
energy is dependent on the solvent (Fig. 18).46
Fig. 17 Emission spectra in the solid state of the free ligand tdpa and
its CuI compound 8 at room temperature.
CuI completely quenches the emission of the ligand tdpa in
solution, as demonstrated by the disappearance of luminescence
of tdpa in the presence of CuI in CH2Cl2–CH3CN solution. The
luminescence titration was performed in order to determine the
amount of CuI required to completely quench the luminescence
of tdpa. Results in Fig. 19 show that the molar equivalent of
CuI (CuI in CH3CN) causes an ∼60% decrease of emission
intensity of tdpa (5.0 × 10−6 M in CH2Cl2). The addition of
three equivalents of CuI nearly completely quenches the emis-
sion of the ligand. The binding of CuI to the dipyridylamino
units in tdpa brings the Cu center close to the chromophore,
which is clearly responsible for the observed luminescence
quenching.6
ion, different red shifts of emission peaks are observed. The λmax
of 4–7 are 595 nm (4), 589 nm (5), 595 nm (6) and 571 nm (7)
(λex = 276 nm), respectively (Fig. 16). All of these emission
bands are broad.
The emissions of all complexes 2–7 are likely to originate
from the metal-to-ligand charge transfer (MLCT) for their broad
emission spectra.44 In this system, the dpa-substituted ligands are
a series of large π-systems with a lower energy of the π*-orbital,
which is more prone to MLCT. The red shifts of the emission
peaks are probably due to the stabilized excitation states after
ligand–metal coordination, which lowers the overall energy of
the system.
The emission spectrum of ligand tdpa consists of a main band
(λmax = 423 nm) and a shoulder peak (λmax = 517 nm), possibly
due to the two chromophores NPh3 and NPy2 in tdpa. Whereas
for complex 8, only one broad emission band is observed (λmax
= 490 nm) (Fig. 17) under excitation at λex = 360 nm, which is
probably caused by the ligand-to-ligand charge transfer (LLCT).
The ligand tdpa adopts a different conformation in the complex
Conclusions
A series of linked dipyridylamine derivatives of Cu(I) in the
solid state has been synthesized by hydrothermal–solvothermal
reactions under different conditions. Analyzed by X-ray crystal-
lography, eight new (CuX)n-based (X = Br, I or CN) coordi-
nation polymers were successfully obtained incorporating three
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Fig. 19 Emission spectra of tdpa (5.0 × 10−6 M in CH2Cl2) with various ratios of tdpa versus CuI (5.0 × 10−6 M in CH3CN).
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Cu2I2 cluster units, one (CuBr)n chain, two (CuCN)n chains, and
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Acknowledgements
This work was supported by the National Natural Science
Foundation of China (Nos. 20873135, 50903081 and
60971075), the China Postdoctoral Science Foundation (No.
20100480683), the Natural Science Foundation of Jiangsu Pro-
vince (No. BK2008579) Science and Technology Planning
Project of Shantou City (2008-64), the Specialized Research
Fund for the Doctoral Program of Higher Education (SRFDP
20103402110020) and Science Funds for Young Scholar of
Shantou University (YR08007).
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