Novel neutral lanthanide complexes of 5-aryl-2,2′-bipyridine-6′-carboxylic acids with improved photophysical properties

Article abstract of DOI:10.1016/j.poly.2016.07.025

By means of “1,2,4-triazine” strategy new 2,2′-bipyridine-type lanthanide ligands, i.e. 5-aryl-2,2′-bipyridine-6′-carboxylic acids and its cycloalkane-annulated derivatives, i.e. 6-(4-aryl-6,7-dihydro-5H-cyclopenta[c]pyridin-1-yl)picolinic acids, have bee

Full text of DOI:10.1016/j.poly.2016.07.025

Polyhedron 118 (2016) 30–36
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Polyhedron
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Novel neutral lanthanide complexes of 5-aryl-2,20-bipyridine-
60-carboxylic acids with improved photophysical properties
a
Dmitry S. Kopchuk a,b, , Alexey P. Krinochkin , Dmitry N. Kozhevnikov a,b, Pavel A. Slepukhin a,b
⇑
a Ural Federal University, 19, Mira St., 620002 Yekaterinburg, Russian Federation
b Postovsky Institute of Organic Synthesis of RAS (Ural Division), 22/20, S. Kovalevskoy/Akademicheskaya St., 620990 Yekaterinburg, Russian Federation
a r t i c l e i n f o
a b s t r a c t
By means of ‘‘1,2,4-triazine” strategy new 2,20-bipyridine-type lanthanide ligands, i.e. 5-aryl-2,20-bipyr-
idine-60-carboxylic acids and its cycloalkane-annulated derivatives, i.e. 6-(4-aryl-6,7-dihydro-5H-cyclo-
penta[c]pyridin-1-yl)picolinic acids, have been synthesized, and their luminescent neutral lanthanide
complexes have been prepared. Due to the presence of an annulated cyclopentene fragment the last
ligand lanthanide complexes exhibited high solubility in non-polar solvents. In comparison with early
published chelates the herein reported complexes exhibit improved photophysical properties in non-
polar solvents namely more pronounced bathochromic shift in absorption spectra and higher europium
luminescence quantum yields. The structures of two europium complexes have been confirmed based on
X-ray diffraction analysis.
Article history:
Received 17 May 2016
Accepted 20 July 2016
Available online 30 July 2016
Keywords:
5-Aryl-2,20-bipyridine-60-carboxylic acids
Luminescence
Lanthanide complexes
aza-Diels–Alder reaction
1,2,4-Triazine
Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction
to one of the pyridine rings (the general formula of these ligands is
shown in Fig. 1, structure A) [8]. Such a possibility has been
The creation of electroluminescent materials [1] is one of the
important uses of neutral luminescent lanthanide complexes. In
order to provide the high luminescence intensity of the resulted
lanthanide complex the first coordination sphere of the lanthanide
cation needs to be fully saturated [2]. In addition, these complexes
should be stable, able to sublime in vacuum or to be soluble in non-
polar solvents for the aim of doping semiconductive polymers.
Diketonate-based lanthanide complexes [3] are the most com-
monly used for the creation of electroluminescent materials. How-
ever, the complexes of such ligands have a number of drawbacks;
in particular, they may suffer from a poor photostability upon UV
irradiation [4]. Also in this aspect such ligands as 2,20-bipyridine-
6-carboxylic acid derivatives [5,6] as well as their aza-analogs [7]
should be mentioned. Their neutral lanthanide complexes [Ln
(L)3] (L = 2,20-bipyridine-6-carboxylate) with high quantum effi-
ciency of a lanthanide luminescence are potentially useful as com-
ponents for OLEDs. However, according to the literature, in most
cases such complexes have low solubility in organic solvents,
which complicates their practical use. Recently, we have demon-
strated the possibility for the significant increasing of the solubility
of these chelates by means of the cyclopentene moiety annulation
achieved through the use of so-called ‘‘1,2,4-triazine” synthetic
approach towards the oligopyridine ligands [9], and the cycloalk-
ane ring annulation to the pyridine ligand can be easily carried
out by means of aza-Diels–Alder reaction with 1-morpholinocy-
clopentene as a dienophile. Thus obtained neutral europium com-
plexes exhibited the luminescence lifetime values up to 1.1 ms and
the quantum yield of luminescence of europium cation up to 11%,
which can be considered as a moderate result.
It should be noted that these complexes demonstrated absorp-
tion maxima in short wavelengths, i.e. in a range of 295–309 nm,
which may be attributed to the possible hampering of the conjuga-
tion between the aromatic substituent (b-position) and the bipyr-
idine moiety due to the steric hindrances caused by the presence of
the carboxyl group in an a-position of pyridine ring. As a result the
aromatic substituent is not coplanar towards 2,20-bipyridine moi-
ety. We have assumed that by the transfer of the carboxyl group
from one pyridine ring to another within the 2,20-bypyridine moi-
ety will possibly increase the conjugation and thus improves the
photophysical properties of the resulting complexes. In this work,
we are pleased to report the approach towards 5-aryl-2,20-bipyri-
dine-60-carboxylic acids as a new ligands for luminescent neutral
lanthanide complexes and results of our studies of the photophys-
ical properties for these complexes.
⇑
Corresponding author at: Ural Federal University, 19, Mira St., 620002
Yekaterinburg, Russian Federation. Fax: +7 343 374 0458.
E-mail address: dkopchuk@mail.ru (D.S. Kopchuk).
http://dx.doi.org/10.1016/j.poly.2016.07.025
0277-5387/Ó 2016 Elsevier Ltd. All rights reserved.

D.S. Kopchuk et al. / Polyhedron 118 (2016) 30–36
31
fractions was removed under reduced pressure. The residue was
recrystallized from ethanol.
R
R
2.4. General method for synthesis of acids 6, 7
N
The corresponding ester 3 or 4 (1.5 mmol) and NaOH (0.12 g,
3 mmol) were suspended in ethanol (60 ml) and the resulting mix-
ture was refluxed for 30 min and then kept at room temperature
for 3 h. Ethanol (40 ml) was removed under reduced pressure,
hydrochloric acid (5 N) was added to adjust pH = 2. The precipitate
was filtered off, washed with ethanol and water, dried in vacuum.
O
N
N
OH
N
B
A
HO
O
Fig. 1. The structure of the previously described ligands for neutral lanthanide
complexes (A) and the structure of the ligands reported in this paper (B).
2.5. General method for synthesis of lanthanide complexes
2. Experimental
Corresponding acid 6 or 7 (0.35 mmol) was suspended in
methanol (40 ml), sodium hydroxide (14 mg, 0.35 mmol) was
added and the resulting mixture was refluxed until clear solution
was obtained. Then mixture was cooled to room temperature
and chloride of corresponding lanthanide (0.117 mmol) was added.
Mixture was kept at room temperature for 2 h. Solvent was
removed under reduced pressure; water (20 ml) was added to
the residue. The precipitate was filtered off, washed with water,
dried in vacuum and solved in mixture methanol–DCM
(1:1, 30 ml). Unsolved part was filtered off, solvents were removed
under reduced pressure. The product was dried in vacuum.
All reagents were purchased from commercial sources and used
without further purification. NMR spectra were recorded on a Bru-
ker Avance-400 spectrometer, 298 K, digital resolution 0.01 ppm,
using TMS as internal standard for 1H and 13C NMR or CFCl3 for 19
F
NMR. Infrared spectra were measured on a Bruker Alpha FTIR spec-
trometer with an ATR accessory (ZnSe). UV–Vis spectra were
recorded on Lambda 45 spectrophotometer (Perkin Elmer). Lumi-
nescence spectra were recorded on a Cary Eclipse spectrofluorom-
eter (Varian). Mass spectra were recorded on a MicrOTOF-Q II mass
spectrometer (Bruker Daltonics) with electrospray ionization. Ele-
mental analysis were performed on a PE 2400 II CHN-analyzer
(Perkin Elmer). Hydrazones of isonitrosoacetophenones 1 [12],
6-metoxycarbonylpyridine-2-carbaldehyde [13], 3-(6-metoxycar-
bonyl-2-pyridyl)-6-(4-methoxyphenyl)-1,2,4-triazine 2c [11] were
synthesized as described in literature.
3. Results and discussion
It is obvious that for achievement of the proposed synthetic goal
the same ‘1,2,4-triazine’ synthetic methodology can be used. In
order to improve the solubility of the resulting lanthanide
complexes the introduction of fused cyclopentene fragment into
bipyridine ligand has been proposed. The choice of 5-aryl-2,20-
bipyridine-60-carboxylic acids as target ligands (Fig. 1, structure
B) has been made based on their synthetic accessibility. Apart from
that the introduction of an additional aromatic substituent in the
b-position of the adjacent pyridine ring will not cause significant
steric hindrances for the coordination of the lanthanide cation.
The key precursors for the synthesis of the target ligands are 6-
aryl-3-(2-pyridyl)-1,2,4-triazines substituted at the 6-position of
the pyridine ring with any functional group which can be further
transformed into carboxylic group. Based on the analysis of pub-
lished data on possible methods for the synthesis of 6-aryl-3-(6-
R-2-pyridyl)-1,2,4-triazines, for instance substituted at the posi-
tion 60 by hydroxymethyl [10], methyl [11], or ester [11] group
we have selected 60-ester-substituted 1,2,4-triazines in a view of
the lower complexity of the synthesis (Scheme 1, compound C).
For the synthesis of 1,2,4-triazine precursors C of target ligands
B two methods are possible (Scheme 1). In particular, according to
our previously reported method [10] in first path cyclization
2.1. General method for the synthesis of triazines 2
The corresponding hydrazone 1 (15 mmol) was solved in etha-
nol (30 ml) and solution of the 6-metoxycarbonylpyridine-2-car-
baldehyde (2.48 g, 15 mmol) in ethanol (25 ml) was added. The
resulting mixture was kept at room temperature for 12 h. The pre-
cipitate was filtered off, washed with ethanol and dried. Then the
obtained intermediate was suspended in acetic acid (50 ml) and
mixture was heated to reflux two times. Solvent was removed
under reduced pressure. Ethanol (30 ml) was added to the residue;
the resulting crystals of 2 were filtered off, washed with ethanol
and dried. The crude triazines were used directly in the next step
without addition purification.
2.2. General method for synthesis of bipyridines 3b,d
The corresponding triazine 2 (5 mmol) was suspended in
o-xylene (25 ml), 2,5-norbornadiene (0.46 ml, 4.5 mmol) was
added and the resulting mixture was refluxed for 19 h with addi-
tion of 2,5-norbornadiene (0.46 ml, 4.5 mmol) every 7 h. Solvent
was removed under reduced pressure. The product was isolated
by column chromatography (chloroform, Rf = 0.25). Solvent from
containing product fractions was removed under reduced pressure.
The residue was recrystallized from ethanol.
between isonitrosoacetophenones hydrazones
D
[12] and
6-methoxycarbonylpyridine-2-carbaldehyde E can be used. The
6-methoxycarbonylpyridine-2-carbaldehyde E can be prepared as
described early [13]. In the second approach the reaction of 2-
bromoacetophenone G with two equivalents of hydrazide of
6-methoxycarbonylpyridine-2-carboxylic acid F [14] can be used
according to a procedure described by Saraswathi and Srinivasan
in 1971 [15]. Obviously, the first approach seems to be more effi-
cient, since it does not involve the significant consumption of less
synthetically accessible mono-hydrazide F.
For the synthesis of 1,2,4-triazines C we have followed our pre-
viously reported method for the preparation of 3-(6-methoxycar-
bonylpyridin-2-yl)-6-(3-nitrophenyl)-1,2,4-triazine [11]. Thus,
according to the described procedure 1,2,4-triazine precursors 2a,
b,d,e (Scheme 2) have been obtained starting from hydrazones
2.3. General method for synthesis of bipyridines 4
Corresponding triazine 2 (3 mmol) was suspended in o-xylene
(35 ml), 1-morpholinocyclopentene (0.96 ml, 6 mmol) was added
and the resulting mixture was refluxed for 2 h. Then addition por-
tion of 1-morpholinocyclopentene (0.48 ml, 3 mmol) was added
and mixture was refluxed for 1 h. Solvent was removed under
reduced pressure, the product was isolated by column chromatog-
raphy (chloroform, Rf = 0.25). Solvent from containing product

32
D.S. Kopchuk et al. / Polyhedron 118 (2016) 30–36
Scheme 1. Retrosynthetic analysis for the synthesis of necessary ligands.
Scheme 2. Synthesis of neutral lanthanide complexes of ligands 6 and 7. Reagents and conditions: (i) 6-methoxycarbonylpyridin-2-carbaldehyde, 10 h, 20 °C, then AcOH,
118 °C, 5 min; (ii) 6-methoxycarbonylpyridin-2-carboxylic acid hydrazide (2 equiv.), AcONa, ethanol–AcOH (4:1), 50 °C, 15 h; (iii) 2,5-norbornadiene, o-xylene, 143 °C, 19 h;
(iv) 1-morpholinocyclopentene, o-xylene, 143 °C, 3 h; (v) NaOH, ethanol, 78 °C, 1 h, then HCl, 20 °C; (vi) NaOH (1 equiv.), methanol, 65 °C, 5 min, then EuCl3*6H2O
(0.33 equiv.), 20 °C.
1a,b,d,e. The resulting 1,2,4-triazines were pure enough and they
were immediately subjected to an aza-Diels–Alder reaction with
1-morpholinocyclopentene or 2,5-norbornadiene. The resulted
2,20-bipyridines 3 and 4 can be isolated very easily in pure form
by column chromatography. It is noteworthy to mention that in
case of 1,2,4-triazine 2c this approach failed. Therefore, for the syn-
thesis of 1,2,4-triazine 2c the second approach was successfully
used [11] starting from 2-bromo-40-methoxyacetophenone 5c.
Subsequent alkaline hydrolysis of esters 3 and 4 afforded the free
acids 6 and 7.
aromatic substituent and the bipyridine core since the carboxyl
group does not impede the coplanar arrangement of the aromatic
substituent towards bipyridine moiety. However, in the case of
complexes of cyclopentene-annelated bipyridine acids Ln*7 values
of the bathochromic shift of the absorption maxima are smaller.
Europium cation luminescence spectra of complexes Euꢀ6 and
Eu*7 contain narrow emission lines at 592, 616 and 695 nm, which
are common for the europium complexes (Figs. 2 and 3).
The europium cation luminescence quantum yields of com-
plexes Euꢀ6 and Eu*7 (up to 28%) and europium luminescence life-
time (up to 1.5 ms) are generally higher of those for earlier
published complexes [8] (Table 1). As it was expected based on
the previously obtained data, the complexes Euꢀ6 showed very
low solubility in non-polar solvents, and this makes them less pre-
ferred from the viewpoint of practical use. With regards to the
complexes Eu*7, the annulation of cyclopentene fragment resulted
in the significant increase in their solubility (up to 10 g/L in
CH2Cl2). However, the annulation of aliphatic cycle resulted in a
lowering of europium cation luminescence quantum yield for the
complexes Eu*7 compared to Euꢀ6 while maintaining the same
characteristics of the emission spectra (Table 1, Fig. 3). Apart from,
the luminescence lifetime and quantum yield for the complexes
Euꢀ6 and Eu*7 are lower than that of the same properties
As a final step 2,20-bipyridine carboxylic acids 6 and 7 were sub-
jected to react with LnCl3*6H2O in the presence of a base to afford
Euꢀ6 and Ln*7 complexes, whose metal-to-ligand ratio is the same
as in the complexes published earlier by us [8]. According to the
analytical data each complex contains one lanthanide cation coor-
dinated to three ligand molecules (in a carboxylate form), i.e. the
resulting complexes are neutral.
Table 1 contains a photophysical data for the complexes Euꢀ6
and Ln*7. Compared to the earlier published complexes [8] the
absorption maxima for these complexes are shifted towards longer
wavelengths (313–330 nm), thus indicating the lower values of the
energy of
tions is a consequence of the improved conjugation between the
p– p–
p⁄-transitions. The reducing of energy of p⁄-transi-

D.S. Kopchuk et al. / Polyhedron 118 (2016) 30–36
33
Table 1
Photophysical properties of lanthanide complexes of ligands 6 and 7.
a
Ligand structure
Ar
Complex
Absorption, kmax, nm (
e
, 10ꢁ3 Mꢁ1 cmꢁ1
)
Ub
s
c, ms
Ph
Tol
4-MeOC6H4
4-BrC6H4
4-FC6H4
4-FC6H4
Eu*7a
Eu*7b
Eu*7c
Eu*7e
Eu*7d
Tb*7d
313 (50.7)
322 (40.4)
321 (44.4)
320 (32.7)
275 (28.9), 318 (45.6)
257 (18.4), 279 (19.3), 321 (34.7)
0.095
0.013
0.12
0.28
0.15
–
1.5
1.5
1.3
1.1
1.5
Ar
N
COOH
COOH
N
N
Ph
Tol
4-FC6H4
Euꢀ6a
Euꢀ6b
Euꢀ6d
280, 328*
282, 330*
280, 324*
0.26
0.15
0.23
0.9
1.1
1.2
Ar
N
a
b
c
In a solution of CH2Cl2 at room temperature.
Europium cation luminescence quantum yield in CH2Cl2 solution were determined using optically matching solutions of [Ru(bpy)3]Cl2 [16].
Europium cation luminescence lifetime.
*
Due to the low solubility the extinction coefficient could not be measured.
500
0.4
400
0.3
300
0.2
200
0.1
100
0.0
250
0
300
350
400
580
600
620
640
660
680
700
720
Wavelength, нм
Wavelength, nm
a)
b)
Fig. 2. The absorption (a) and europium cation luminescence (b) spectra of europium complexes Euꢀ6 (Euꢀ6a – black lines; Euꢀ6b – red lines; Euꢀ6d – blue lines) in CH2Cl2
solution at room temperature. (Color online).
0.5
0.4
0.3
0.2
0.1
0.0
500
450
400
350
300
250
200
150
100
50
0
580
600
620
640
660
680
700
250
300
350
400
Wavelength, nm
Wavelength, nm
a)
b)
Fig. 3. The absorption (a) and europium cation luminescence (b) spectra of europium complexes Eu*7 (Eu*7a – black lines; Eu*7b – red lines; Eu*7c – blue lines;
Eu*7d – green lines; Eu*7e – dark yellow lines) in CH2Cl2 solution at room temperature. (Color online.)
previously reported for some of known lanthanide complexes, for
instance reported in a source [6] luminescence lifetime up to
2.81 ms and the quantum yield of up to 85%.
In addition, for a number of complexes Eu*7 besides the
europium cation luminescence (lifetime up to 1.5 ms) the own
fluorescence of aryl-2,20-bipyridine moiety with a much more
shorter wavelength emission maximum (350–450 nm) is observed.
The comparison of the intensity of fluorescence (due to the emis-
sion from S1 state localized on the ligand) and europium cation
emission demonstrates the influence of the nature of aryl

34
D.S. Kopchuk et al. / Polyhedron 118 (2016) 30–36
substituents on the efficiency of the ligand-to-metal energy trans-
fer. The maximum share of the fluorescence in the total lumines-
cence quantum yield was observed for the complex Eu*7c
bearing 4-methoxyphenyl-2,20-bipyridine moiety. Thus, introduc-
tion of the electron-donating substituents into the ligand core
complicates the energy transfer. Apparently, this is due to the sig-
nificant reduction of the energies of S1 and T1 levels of the ligand
in respect to the 5D0 state of the chelated europium cation, which
makes energy transition between T1 and 5D0 states to be less
favorable. The introducing of halogen atoms (fluorine or bromine,
complexes Eu*7d and Eu*7e) into the aromatic substituent of the
2,20-bipyridine ligand resulted in the most effective energy trans-
fer: the almost complete quenching of fluorescence with the
increasing of the europium cation luminescence quantum yield
were observed.
of terbium cation [8] which is probably due to the less pronounced
conjugation between 2,20-bipyridine moiety and the aromatic sub-
stituent and, consequently, higher excited state energy of the
ligand.
Besides the europium cation luminescence in solutions the sim-
ilar emission in crystalline form of the EuIII complexes is possible.
Except the absence of short-wavelength bands of fluorescence
these spectra do not differ from the ones recorded in a solution.
In a temperature range of 77–300 K no temperature dependence
of phosphorescence spectra is also observed. In particular, highly
resolved europium cation luminescence spectra were recorded in
organic solvents glasses, i.e. a mixture of dichloromethane and
methyltetrahydrofuran (MTHF-DCM) at the temperature of 77 K.
Fig. 4 shows one of such europium cation emission spectra
recorded for the complex Eu*7d.
Unfortunately, no terbium cation luminescence was observed
for the complex Tb*7d. Apparently, an increase of the degree of
conjugation of the ligand reduces its energy of the excited states,
which greatly complicates the energy transfer to the higher ener-
getic 5D4 level of terbium cation, and, thus, the sensitisation of
its luminescence. In the europium complexes the ligand-to-metal
energy transfer takes place on the 5D0 level, which is much lower
in energy, therefore the sensitisation of europium cation lumines-
cence is observed.
The structures of the complexes were confirmed by elemental
analysis, mass spectrometry (ES-MS), and IR spectroscopy. The
obtaining data correlate with ones for the previously described
by us complexes of 5-aryl-2,20-bipyridine-6-carboxylic acids [8].
For the more detailed study the X-ray analysis has been performed
for the complexes Eu*7b and Eu*7d.
According to the X-ray data (Figs. 5 and 6), in both cases the
central metal atom coordinates with three anions of bipyridine car-
boxylic acid, i.e. EuIII coordination number in these complexes is 9.
These data clearly confirm the absence of solvent molecules in the
coordination sphere of the Eu cation.
It should be noted that in case of using 5-aryl-2,20-bipyridine-6-
carboxylic acid as a ligand, we observed a very faint luminescence
In both cases the aromatic substituent is significantly twisted
out of the plane of the associated pyridine ring (the values of torsion
angles are in the range of 36.29–47.95°). The bond lengths between
the europium atom and the pyridine nitrogen atom are in the range
of 2.52–2.72 Å, with oxygen atoms in the range of 2.3–2.4 Å.
The neutral complex Eu*7d is crystallized as solvate with
methanol (1:3). The molecules of the MeOH are fixed in the crystal
packing by intermolecular H-bonds with O-atoms of the carboxylic
moiety of the ligands (distances are in the range 1.97–2.07 Å, see
Fig. S1, Supporting information). In the crystal packing there are
the hydrogen bonds between fluorine atoms and hydrogens of
cyclopentene moiety, distances between them are 2.5 Å (Fig. S2,
Supporting information). Ligand molecules are packed in stacks
(Fig. 5b), which is explained by the presence of intra- and inter-
molecular p–p-interactions. This is supported by an almost copla-
nar arrangement of the pyridine rings of the interacting ligands of
complex (Fig. 5a, the value of the angle between the planes of rings
is 26°), and the short distance between these rings, i.e. 2.9–4.0 Å.
Moderate intermolecular
p–p-interactions are supported in the
same way: in this case the distance is 3.7 Å. As a result, there is a
significant deviation from the coplanar arrangement of two out
Fig. 4. Europium cation luminescence emission spectrum of the Eu*7d complex
recorded in a frozen glass of MTHF-DCM at 77 K.
Fig. 5. (a) Molecular structure of the complex Eu*7d (the dimming indicates the close contact between pyridine rings) and (b) a stacking in Eu*7d complex crystals according
to X-ray diffraction (methanol molecules and hydrogen atoms are omitted for clarity).

D.S. Kopchuk et al. / Polyhedron 118 (2016) 30–36
35
Fig. 6. (a) The molecular structure of the complex Eu*7b and (b) the arrangement of the molecules of the complex Eu*7b in the crystal according to X-ray data (solvent
molecules and hydrogen atoms are omitted for clarity).
Fig. 7. The coordination polyhedra of the complexes Eu*7d (a) and Eu*7b (b) according to the X-ray data.
of three of bipyridine fragments of europium complex (the values
of torsion angles are 12° and 27°), which is non-typical for the
bipyridine chelates. It is worthy to mention that the similar effect
has been described previously for neutral lanthanide complexes of
ligands bearing the extended aromatic system such as 6-benzimi-
dazolylpyridine-2-carboxylic acids [17] or 2-benzimidazolyl-8-
hydroxyquinolines [18].
self-organization of europium complexes molecules in the crys-
talline phase. It should be noted that the presence of more efficient
intermolecular p–p-interaction can facilitate energy transfer from
the conjugated polymer to dye, which is important for creating a
device based [19].
The complex Eu*7b is crystallized as (aqueous) solvated with
methanol, in which, as well as for compound Eu*7d, MeOH is fixed
in the crystal packing by intermolecular H-bonds with O-atoms of
the carboxylic moiety of the ligands. The molecules of the complex
Eu*7b are ordered in a crystal in one-dimensional structure due to
4. Conclusions
In summary, based on previously published results [8] we opti-
mized the synthetic approach to 5-aryl-2,20-bipyridine-6(60)-car-
boxylic acids as ligands for lanthanide cations. In general, the
proposed synthetic approach allows to vary the structure of the
ligand and, therefore, to tune the desired properties of the complex
by the variation of three factors: the nature of the aromatic sub-
stituent, the position of the carboxyl group and the presence of
annulated aliphatic carbocycle.
It was found that the novel ligands are not suitable for sensiti-
sation of terbium cation luminescence in contrast to europium. The
europium complexes based on the ligands having
cyclopentene fragment, are more promising in view of their better
solubility in non-polar solvents (up to 10 g/L in CH2Cl2). Europium
complexes of 6-(4-aryl-6,7-dihydro-5H-cyclopenta[c]pyridin-1-yl)
picolinic acids, reported in a current manuscript, are more interest-
ing in comparison with chelates of 4-aryl-1-(2-pyridyl)-6,7-dihy-
dro-5H-cyclopenta[c]pyridine-3-carboxylic acids reported earlier
intermolecular
p–p-interactions between the aromatic sub-
stituents (this is confirmed by their coplanar arrangement, as well
as the closest distance between them of 3.36 Å), and the pyridine
rings of the ligands (the distance between them is 3.4–4.0 Å)
(Fig. 6b). In this case the intramolecular
not observed.
p–p-interactions were
For the both complexes the europium cation with three bipyri-
dyl ligands forms 6 five-membered chelated cycles (Fig. 7), with
formation of the distorted trigonal antiprismatic coordination
polyhedron.
a fused
Thus, according to the obtained XRD results, the introduction of
the aromatic substituents in the 50-position of 2,20-bipyridine-6-
carboxylic acids increases the intra- and intermolecular
p–p-inter-
actions between the ligand molecules and thereby controls the

36
D.S. Kopchuk et al. / Polyhedron 118 (2016) 30–36
by us [8] due to their longer absorption wavelengths maxima, i.e.
more less energy for their excitation, as well as their higher euro-
pium cation luminescence efficiency. It should be mentioned that
introducing of halogen atoms, such as fluorine or bromine, in the
aromatic substituent of the 2,20-bipyridine core resulted in the
almost complete quenching of the ligand fluorescence. Despite of
the lower efficiency of europium cation luminescence of new com-
plexes (quantum yield up to 28% and luminescence lifetime up to
1.5 ms) compare to the described earlier chelates, the substantially
higher solubility in non-polar solvents of first ones will determine
the practical interest to them.
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033; or e-mail: deposit@ccdc.cam.ac.uk.
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pounds and complexes, the detailed description of 1H, 13C and
19F NMR spectra for all new compounds, the pictures of 1H, 13C
and 19F NMR spectra for all new compounds, the IR data for the
new complexes and additional figures) associated with this article
can be found, in the online version, at http://dx.doi.org/10.1016/j.
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