Mendeleev Commun., 2017, 27, 459–461
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(a)
(b)
(c)
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2b
2b
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Figure 1 Luminescence (a), (b) emission (excitation at l = 320 nm) and (c) excitation spectra (emission at l = 618 nm) for the europium-based coordination
polymers 2a and 2b.
The target polydentate ligand 1 was obtained in 85% yield† as
reveal significant broadening of the signals due to the effect of
europium ion. The determined composition of the complexes
evidences the water molecules coordination to the metal ion or
hydrogen bonding with the carboxylic groups.
white powder with poor solubility in most of the organic solvents
1
excepting polar ones (DMSO, DMF). Its H NMR spectrum
shows well resolved signals of aromatic protons only at elevated
temperature (55°C) due to slow intramolecular rotation of
aromatic fragments.
The nature of water molecules bonding in the complexes can
be elucidated by measurements of their luminescence.§ In the lan-
thanide coordination complexes, the luminescence of the metal
ion can be efficiently quenched by water molecules coordination.
Luminescence spectra of the complexes under study showed
typical europium emission lines in the red region corresponding
to transitions 5D0 ® 7FJ (J = 0, 1, 2, 3, 4) (Figure 1). The main
europium luminescence band (5D0 ® 7F2) was splitted into three
components for both complexes: 614 (the most intense), 619 and
623.5 nm. The spectra differed only in intensity (2a >> 2b) and
the ratios of these resolved components. The luminescence
lifetimes, the internal quantum efficiency and the ratio between
the integral intensities of the band located at 614–624 and that
at 595 nm (9.4 for 2a and 8.7 for 2b) did not differ markedly.6,9
Luminescence kinetics for both compounds deviated from
monoexponential decay is typical of coordination compounds in
solutions. The experimental data showed that at room temperature
luminescence kinetics consist of two components: fast (before
1 ms) and slow (after 1.5 ms) (Figure 2). We approximated the
experimental decays with two exponents. This luminescence
decay behaviour corroborates with the theory of energy migration
in heterogeneous environment of solid solutions of organic
compounds. The initial part of the decay kinetics was analyzed
within existing triplet-triplet annihilation models.10 The best
The europium complexes 2 with ligand 1 were prepared
by the reaction of the latter with either europium chloride or
nitrate salts (see Scheme 1).‡ Complexes 2 are light grey com-
pounds with extremely low solubility even in polar solvents
1
(DMSO or DMF). For this reason, their H NMR spectra in
DMSO-d6 solutions were recorded at 60°C.
The coordination of the europium atom seems to occur both
on bipyridine moiety and carboxylic group of the ligand. The
strong upfield shift of the CH-protons of benzoic acid moieties
observed for both complexes 2 testifies to the coordination
of carboxylic group to the Eu atom. Moreover, the spectra
†
4,4'-Bis(4-carboxyphenyl)-N,N'-diethyl-N,N'-diphenyl-2,2'-bipyridyl-
6,6-dicarboxamide 1. 4,4'-Dibromo-N,N'-diethyl-N,N'-diphenyl-
2,2'-bipyridyl-6,6-dicarboxamide (7.0 g, 11.5 mmol), 4-carboxyphenyl-
boronic acid (6.0 g, 36 mmol), K2CO3 (30 g), PdCl2(PPh3)2 (400 mg,
5 mol%) and Bu4NBr (1.6 g, 40 mol%) were refluxed in EtOH (80 ml)/
H2O (30 ml) mixture for 4 h. After that, the reaction mixture was poured
into water (200 ml) acidified to pH 2 with HCl and filtered. The solvent
was removed and the dark residue was dissolved in 400 ml of CH2Cl2–
MeOH (2:1) mixture. The resulting solution was passed through celite
to give 10.65 g of crude product. The brown powder was dissolved under
reflux in 400 ml of PriOH–DCM–MeOH (1:2:1) mixture. After cooling,
the sludge-like precipitate was filtered off and discarded, and the volume
of the filtrate was reduced by boiling at 78°C to 60 ml. Its cooling to
room temperature gave 4.42 g of pure product. The standing overnight
afforded additional crop (2.35 g) of pure 1. Yield 6.77 g (85%). 1H NMR
(600.1 MHz, DMSO-d6, 55°C) d: 1.21 (t, 3H, J 7.02 Hz), 3.98 (q, 2H,
J 7.03 Hz), 7.02 (t, 1H, J 7.11 Hz), 7.15 (t, 2H, J 7.34 Hz), 7.18–7.31
(m, 3H), 7.85 (d, 2H, J 7.89 Hz), 7.97 (br.s, 1H), 8.19 (d, 2H, J 7.61 Hz).
13C NMR (100.6 MHz, DMSO-d6, 55°C) d: 166.57, 166.39, 154.19,
153.40, 147.77, 142.52, 140.34, 131.70, 129.86, 128.49, 127.24, 126.93,
126.14, 121.84, 118.13, 44.30, 12.38. MS (MALDI-TOF), m/z: 729
[M+K]+, 691 [M+H]+.
40
(a)
y = 29.006e–1.138x
30
R2 = 0.979
20
y = 6.7523e–0.465x
R2 = 0.7927
10
0
0
1
2
3
4
5
t/ms
‡
Complexes of 4,4'-bis(4-carboxyphenyl)-N,N'-diethyl-N,N'-diphenyl
1250
1000
750
500
250
0
y = 1229e–1.459x
R2 = 0.9997
(b)
2,2'-bipyridyl-6,6-dicarboxamide with Eu 2. Ligand 1 (0.691 g, 1 mmol)
was suspended in refluxing acetonitrile (120 ml). The corresponding
europium salt hydrate EuCl3 ·6H2O or Eu(NO3)3 ·6H2O (1mmol) was
carefully added to the boiling suspension. The reaction mixture was
refluxed for 6 h. The solid ligand dissolved during the reflux and a new
precipitate formed. After cooling to room temperature, the pale brownish
solid was filtered off, washed with hot acetonitrile and air dried.
For 2a: yield 0.70 g (70%). 1H NMR (400 MHz, DMSO-d6, 60°C) d:
1.56 (t, 3H, J 7.34 Hz), 3.96 (q, 2H, J 6.36 Hz), 6.83 (d, 2H, J 8.07 Hz),
7.01 (t, 1H, J 6.97 Hz), 7.14 (t, 2H, J 5.87 Hz), 7.18–7.32 (m, 3H), 7.79
(d, 4H, J 8.19 Hz), 8.04 (s, 1H), 8.06 (s, 1H). Found (%): C, 50.12; H, 3.60;
N, 5.75. Calc. for C42H40Cl3EuN4O9 (%): C, 50.29; H, 4.02; N, 5.59.
For 2b: yield 0.827 g (84%). 1H NMR (400 MHz, DMSO-d6, 60°C) d:
1.12 (t, 3H, J 6.60 Hz), 3.89 (q, 2H, J 6.48 Hz), 6.92 (t, 1H, J 5.99 Hz),
6.96–7.08 (m, 3H) 7.08–7.25 (m, 3H), 7.38 (br.s, 2H), 7.66 (br.s,
1H), 7.78 (br.s, 1H). Found (%): C, 51.30; H, 3.58; N, 8.63. Calc. for
C42H36EuN6O13 (%): C, 51.23; H, 3.68; N, 8.53.
y = 854.93e–1.262x
R2 = 0.9966
1
2
3
4
5
0
t/ms
Figure 2 Luminescence kinetics in millisecond range for the europium-
based complexes (a) 2a and (b) 2b. Excitation wavelength 320 nm.
§
The photoluminescence emission and excitation spectra were collected
in the solid state at room temperature using a Hitachi 7000 luminescence
spectrometer with a spectral slitwidth of 1 nm.
– 460 –