Europium complex of 5-(4-dodecyloxyphenyl)2,2’-bipyridine-6’-carboxylic acid

Article abstract of DOI:10.1016/j.mencom.2017.07.026

Based on ‘1,2,4-triazine’ methodology, a new lanthanide(III) ligand, 5-(4-dodecyloxyphenyl)-2,2’-bipyridine-6’-carboxylic acid, was synthesized, and its luminescent europium(III) complex was prepared. The solubility in organic solvents for both the ligand

Full text of DOI:10.1016/j.mencom.2017.07.026

Mendeleev
Communications
Mendeleev Commun., 2017, 27, 394–396
Europium complex of 5-(4-dodecyloxyphenyl)-
2,2'-bipyridine-6'-carboxylic acid
Dmitry S. Kopchuk,*a,b Alexey P. Krinochkin, Grigory A. Kim and Dmitry N. Kozhevnikova,b
a
a,b
a
Ural Federal University, 620002 Ekaterinburg, Russian Federation
I. Ya. Postovsky Institute of Organic Synthesis, Ural Branch of the Russian Academy of Sciences,
b
620990 Ekaterinburg, Russian Federation. E-mail: dkopchuk@mail.ru
DOI: 10.1016/j.mencom.2017.07.025
C12H25O
Based on ‘1,2,4-triazine’ methodology, a new lanthanide(iii)
ligand, 5-(4-dodecyloxyphenyl)-2,2'-bipyridine-6'-carboxylic
acid, was synthesized, and its luminescent europium(iii)
complex was prepared. The solubility in organic solvents
for both the ligand and its europium complex was provided
by the presence of a long aliphatic dodecyl moiety. The
photophysical properties of the europium(iii) complex were
examined, the fluorescence quantum yield and the europium
cation luminescence quantium yield were determined.
BnO
O
N
N
N
O
N
O
N
O
N
OMe
N
N
Eu
O
O
O
N
N
OC12H25
OC12H25
1
5
Luminescent lanthanide complexes are of significant interest for
creation of phosphorescent labels for various bioimaging applica­
was improved using another common approach, namely, by intro­
duction of an aryl substituent bearing long alkyl chain fragment
into one of pyridine moieties of the 2,2'­bipyridine system.
To obtain the target ligand, the 1,2,4­triazine­based methodo­
1
tions, including the highly selective molecular probing of chiral
2
molecules, enantioselective recognition of c­type cytochromes
3
16
17,18
and vitamin B in aqueous solutions etc., for anion sensing
logy was used. Taking into account published data
and retro­
1
2
4
5
applications, and as electroluminescent materials for OLEDs.
In the latter case, to achieve a high emission intensity, it is
necessary to exclude water molecules from the first coordination
sphere of the lanthanide cation to prevent the possible quenching
synthetic considerations (see Online Supplementary Materials),
the target ligand was prepared starting from 4­hydroxyaceto­
phenone 1 (Scheme 1). For the protection of the aromatic
4­hydroxy group, benzylation was carried out to yield compound
iii
6
19
of the Ln ion emission via interaction with O–H oscillators.
2. Keto oxime 3 was obtained by further nitrosation under
2
0
The other necessary prerequisite is either high solubility of such
complexes in organic solvents or their high volatility in the gas
phase for the purposes of chemical doping or vacuum­deposition
on the surface of semi­conductive polymers.
the slightly modified conditions, viz., in benzene, particularly,
due to low solubility of compound 2 in ethanol (a common
2
1
solvent for the nitrosation of acetophenones ). The subsequent
reaction of compound 3 with hydrazine hydrate was performed
upon heating (cf. ref. 22). Then, according to a reported proce­
iii
According to a literature, the b­diketonate Ln complexes are
7
12
commonly used for OLED creation. However, these complexes
dure, the reaction between hydrazone 4 and 6­(methoxy­
carbonyl)pyridine­2­carbaldehyde followed by aromatization
afforded the 1,2,4­triazine 5. The aza­Diels–Alder reaction
between crude compound 5 and 2,5­norbornadiene in boiling
o­xylene furnished the 2,2'­bipyridine 6, which was isolated
analytically pure after recrystallization from acetonitrile. Finally,
debenzylation of 6 with ammonium formate over Pd/C, alkylation
of phenol moiety of 7 with dodecyl benzenesulfonate in DMF,
and the alkaline hydrolysis of ester 8 yielded the desired ligand 9.
The europium complex Eu·9 was prepared as described
2
3
have a number of disadvantages, in particular, their photostability
8
12
upon UV irradiation is very poor. On the other hand, some
9
,10
complexes of 2,2'­bipyridine­6­carboxylic acids
or their aza­
1
1
analogues are potentially useful as components for OLEDs
since they form neutral complexes [Ln(L) ] (L = 2,2'­bipyridine­
3
6
­carboxylate) with high quantum efficiency of a lanthanide
24
luminescence. For some of these complexes the absence of water
molecules in the first coordination sphere of the lanthanide cation
was confirmed by X­ray diffraction analysis.1
2,13
However, their
1
2
major drawback is poor solubility in common organic solvents.
recently. Since terbium complexes of 5­aryl­2,2'­bipyridine­
6(6')­carboxylic acids were reported to exhibit extremely low
iii
Recently, we have demonstrated that solubility of such Ln com­
1
2,14
plexes could be improved via the direct annulation of cyclopentene
moiety into one of the pyridine moieties of the 2,2'­bipyridine
luminescence intensity,
the synthesis of Tb·9 complex was
considered to be inappropriate.
1
2,14
system.
For their preparation, the 1,2,4­triazine inverse­
The stoichiometry of the Eu·9 complex has been determined
by elemental analysis and mass spectrometry, which is sufficient
according to the previous reports. Unfortunately, we failed
to grow single crystals of Eu·9 complex for X­ray diffraction
measurements. Meantime, the data obtained for complex Eu·9
electron­demand aza­Diels–Alder reaction was used. The thus
obtained neutral europium complexes of 5­aryl­2,2'­bipyridine­
1
0
6
1
(6')­carboxylic acids were soluble in dichloromethane in up to
0 g dm , while the luminescence lifetime of the europium(iii)
–
3
1
2
complex was as high as 1.5 ms, and the luminescence quantum
yield was up to 28%. In this paper, we report on the preparation
of other soluble europium complexes of 5­aryl­2,2'­bipyridines
correlate with those for similar chelates lacking long aliphatic
chains, whose structures were confirmed by X­ray studies.
In fact, the obtained complex Eu·9 demonstrated the enhanced
solubility in non­polar solvents (e.g., CH Cl ), as compared to
(
i.e. 5­aryl­2,2'­bipyridine­6'­carboxylic acid). The solubility
2
2
©
2017 Mendeleev Communications. Published by ELSEVIER B.V.
–
394 –
on behalf of the N. D. Zelinsky Institute of Organic Chemistry of the
Russian Academy of Sciences.

Mendeleev Commun., 2017, 27, 394–396
Me
O
Me
3
25
(a)
i
ii, iii
2
.0 230
HO
BnO
BnO
O
N
1
2
1.5
1.0
2
84 (sh.)
NH2
O
iv
BnO
NOH
NOH
0.5
3
4
O
N
CO2Me
0.0
CO2Me
2
25 250 275 300 325 350 375 400 425 450
N
N
N
N
N
l/nm
BnO
v
800
700
600
617
(b)
5
7
D
4
® F
2
5
CO2R2
FEu = 17.7%
t
Eu = 0.76 ms
5
4
3
00
00
00
vi
x
1
5
7
R O
4 1
D ® F
N
591
5
D
® F
695
7
4
4
1
2
6
R = Bn, R = Me
R = H, R = Me
R = C12H25, R = Me
R = C12H25, R = H
200
100
0
vii
viii
ix
5
7
1
2
D
® F
650
7
8
9
5
7
4
3
D
4
® F
0
1
2
1
2
5
75
600 625
650
l/nm
675
700
725
C12H25O
4
88
6
5
4
3
2
0
0
0
0
0
(c)
403
Fbipy = 47.5%
O
N
O
N
O
N
N
10
0
Eu
3
50 400 450 500 550 600 650 700
O
O
O
l/nm
N
N
OC12H25
Figure 1 (a) Absorption, (b) europium cation luminescence and (c) fluore­
scence spectra of complex Eu·9 in CH Cl solution at room temperature (the
OC12H25
2
2
excitation wavelength is 325 nm).
Eu·9
Scheme 1 Reagents and conditions: i, PhCH Br, K CO , 80°C, 12 h;
quantum yield was 47.5%). In other words, the introduction of
an electron­donating alkoxy group into the aromatic substituent
of the bipyridine ligand causes the significant lowering of the
2
2
3
i
ii, EtONa, then Pr ONO, benzene, 20°C, ~18 h; iii, AcOH, H O, 20°C, 30 min;
2
iv, NH NH ·H O, EtOH, 70°C, 3 h, then H O, 20°C; v, EtOH, 20ºC, 10 h,
2
2
2
2
then AcOH, 118°C, 5 min; vi, 2,5­norbornadiene, o­xylene, 143°C, 19 h;
energies of excited states of the chromophore, which may hamper
vii, HC(O)NH , Pd/C, THF–MeOH (1:1), reflux, 50 min; viii, PhSO OC H ,
4
2
12 25
5
the energy transfer from the ligand T level to the D level of
K CO , DMF, 100°C, 10 h; ix, NaOH, EtOH, 65°C, 30 min, then 20°C, 3 h,
1
0
2
3
6
then HCl, 20°C; x, NaOH, MeOH, 65°C, 5 min, then EuCl ·6H O, 20°C.
the europium cation. Structurally close 5­(4­methoxyphenyl)­
,2'­bipyridine exhibited very intense fluorescence in acetonitrile
3
2
2
described complexes lacking long alkyl chains.12 Absorption,
europium cation luminescence and fluorescence spectra of
complex Eu·9 and its photophysical characteristics are shown
solution (the fluorescence quantum yield was as high as 89%).
However, the residual fluorescence of 5­aryl­2,2'­bipyridine ligands
in such complexes was completely eliminated in solid state. In
this case only luminescence of europium cation was observed and
no residual fluorescence was detected. Thus, such a drawback of
the Eu·9 complex as the residual fluorescence could be eliminated
under certain conditions.
In summary, a convenient synthesis of 5­(4­alkoxyphenyl)­
2,2'­bipyridine­6'­carboxylic acids using the modified ‘1,2,4­tri­
azine’ methodology has been accomplished. The introduction
of long alkyl chain in the aromatic substituent is crucial for
improving the solubility in organic solvents of the resulting
2,2'­bipyridine ligands, as well as their neutral lanthanide(iii)
complexes. For neutral europium(iii) complex of 5­(4­dodecyl­
oxyphenyl)­2,2'­bipyridine­6'­carboxylic acid, the europium cation
luminescence quantum yield and lifetime value were close to
those reported previously for similar europium(iii) complexes.1
21
†
in Figure 1.
1
2
The photophysical studies of Eu·9 revealed a significantly
higher quantum yield of Eu3 luminescence (17.7%) compared
with those for a number of complexes (15–26%),1 while the
emission maxima positions were the same. The luminescence
lifetime value of Eu·9 (0.76 ms) was a little lower than those
+
2
(
typically 0.9–1.2 ms12,14) for the similar lanthanide complexes
5
7
of the ligands lacking long alkyl chains. The D ® F emission
band at 617 nm is dominating over the D ® F band (591 nm)
0
2
5
7
0
1
that correlates well with the published emission spectra of similar
9
,12–14
chelates.
Meanwhile, the Eu·9 complex demonstrated a
highly intensive residual fluorescence of arylbipyridine chromo­
phore corresponding to its p–p*­transitions (the fluorescence
2,14
†
Fluorescence quantum yield (Fbipy) in CH Cl solution was determined
2
2
25
This work was supported by the Russian Science Foundation
using quinine bisulfate in 0.1 n solution of H SO . Europium cation
2
4
(
grant no. 15­13­10033). D. S. Kopchuk acknowledges the support
luminescence quantum yield (FEu) in CH Cl solution was established
2
2
26
from the Council for Grants of the President of the Russian
Federation (grant no. MK­644.2017.3).
using optically matching solution of [Ru(bpy) ]Cl . The excitation wave­
3
2
length was 325 nm.
–
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Mendeleev Commun., 2017, 27, 394–396
13 N. M. Shavaleev, F. Gumy, R. Scopelliti and J.­C. G. Bünzli, Inorg.
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Received: 8th November 2016; Com. 16/5092
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