Liquid crystal polymorphism of boron difluoride β-diketonates

Article abstract of DOI:10.1134/S1070363210110150

Boron difluoride 2-alkyl-β-diketonates possessing liquid crystal polymorphism and luminescence properties were first obtained. Influence of the ligand structure on the liquid crystal properties of the boron difluoride complexes was demonstrated. Pleiades Publishing, Ltd., 2010.

Full text of DOI:10.1134/S1070363210110150

ISSN 1070-3632, Russian Journal of General Chemistry, 2010, Vol. 80, No. 11, pp. 2317–2322. © Pleiades Publishing, Ltd., 2010.
Original Russian Text © O.A. Turanova, G.G. Garifzyanova, A.N. Turanov, 2010, published in Zhurnal Obshchei Khimii, 2010, Vol. 80, No. 11, pp. 1854–
1859.
Liquid Crystal Polymorphism
of Boron Difluoride β-Diketonates
O. A. Turanovaa,b, G. G. Garifzyanovac, and A. N. Turanova,b
a Zavoiskii Kazan Physico-Technical Institute, Kazan Research Centre,
Russian Academy of Sciences, Kazan, Tatarstan, Russia
b Kazan State Energy University,
ul. Krasnosel’skaya 51, Kazan, Tatarstan, 420066 Russia
e-mail: lightur@mail.ru
c Kazan State Technological University, Kazan, Tatarstan, Russia
Received December 21, 2009
Abstract―Boron difluoride 2-alkyl-β-diketonates possessing liquid crystal polymorphism and luminescence
properties were first obtained. Influence of the ligand structure on the liquid crystal properties of the boron
difluoride complexes was demonstrated.
DOI: 10.1134/S1070363210110150
In the recent years an interest increased to the study
of new light-emitting media, the organic light-emitting
diodes (OLED), owing to a possibility of using them
for the creation of thin and flexible displays [1–2].
New systems containing lanthanides [3], platinum [4]
and iridium [5] were obtained and studied. Much
attention attracted fluorescent boron complexes [6–8].
Incorporation of electron-donor triphenylamine into
the boron complex was shown to lead to the formation
of bipolar substance which was used as a source of
yellow color for producing an effective monolayer
electroluminescence device [9]. β-Diketonate com-
plexes of boron difluoride show strong luminescence
both in solutions and in solid state [10–11]. The
quantum yield values of such complexes were shown
to depend on the structure of β-diketonate ligands [12–14].
there are the prospects to impart desired supra-
molecular organization to the substance through its
mesogenic properties that would be retained in the
solid state.
In this work we report on the attempts to obtain the
liquid-crystalline boron difluoride β-diketonate com-
plexes using α-substituted β-diketones as the ligands.
The β-diketones containing long alkyl substituents
in the α-position were synthesized by two different
pathways. Their phase transfer temperatures and yields
are given in the table.
Compounds Ia and IIa were obtained by long (72 h)
refluxing the starting materials in chloroform along the
procedure [15]. The products were purified by column
chromatography eluting with dichloromethane.
We have reported [8] on the synthesis of the first
mesogenic boron difluoride complex using a non-
mesogenic β-diketone. Besides, the obtained coor-
dination compound showed luminescence properties,
therewith the emission intensity of the mesogenic
sample was of the same order of magnitude as in the
case of its non-liquid-crystalline analog. The presence
of mesogenic properties extends essentially the field of
potential application of these compounds. Due to the
mesomorphism the coordination compounds acquire
new collective properties combined with the high
system sensitivity to external influence. In addition,
R1
C
R1
C
O
O
O
O
R2
H
+ R2I
H2C
C
−HI
C
C
R3
R3
Ia, IIa
The ligand IIIa was prepared by heating 2-bromo-
dibenzoylmethane with sodium dodecyloxybenzoate in
ethanol.
2317

2318
TURANOVA et al.
O
R1
C
R1
C
O
O
O
H
+
O
O
C
C12H25O
C12H25O
C
C
−NaBr
Br
C
ONa
O
C
H
C
R3
R3
IIIа
The compounds obtained were identified by elemental
analysis and 1Н NMR spectra. All the synthesized 2-sub-
stituted β-diketones show no liqud crystal properties.
As mentioned above, all the β-diketonates used as
ligands did not possess liquid crystal properties. We
expected the appearance of such properties due to the
strengthening of intramolecular interactions in the
complexes. The absence of mesogenic properties in
compound Ib can be explained by the weak intra-
molecular interaction in comparison with complexes
IIb and IIIb containing aromatic rings that are
involved in the π–π-interaction.
2-Substituted boron difluoride diketonates were
prepared by the reaction of the corresponding β-
diketonate with boron trifluoride etherate:
R1
R1
C
O
O
C
O
O
H
F
F
R2
C
Owing to the presence of the long alkyl chains in
the molecule it was impossible to grow single crystals.
Therefore for the more detailed examination of the
relationship between the molecule geometry and the
presence of liquid crystal properties we calculated
geometric parameters of compounds IIb and IIIb by
B3LYP/6-31G(d) method (Figs. 1 and 2, respectively).
As a result, we found the structures with minimum
formation enthalpy. For occurrence of the liquid
crystal state the anisotropy of the electronic
polarizability should be inherent to the molecule. In
the first approximation the polarizability anisotropy is
determined by the geometric anisotropy (the ratio of
the molecule length L to its width D) [16]. The
calculated geometric anisotropy of the complexes is
1.22 (IIb) and 2.15 (IIIb). The increased geometric
anisotropy of the molecule IIIb makes it possible to
appear in this compound of mesogenic properties.
B
+ BF3 Et2O
C
R2
−HF
C
C
R3
R3
The products precipitated at cooling the reaction
mixture to –40ºС with liquid nitrogen. After crys-
tallization of all the compounds formed, the reaction
mixture was left to warm up, therewith gaseous
hydrogen fluoride liberated. The complexes obtained
were purified by recrystallization from diethyl ether.
These coordination compounds were characterized by
1
the elemental analysis, Н, 11В NMR and UV spectra.
The formation of coordination compounds is
1
confirmed by the absence in the Н NMR spectra of a
signal at 5–5.5 ppm of the proton in the diketone α-
position. The phase transfer temperatures and yields
are given in the table.
Phase transition temperatures and yields of ligands Ia–IIIa
and complexes Ib–IIIb
The absence of liquid crystal properties in complex
IIb can be explained by the low geometric anisotropy
value and by the insufficient intramolecular π-electron
interaction between benzene rings (Fig. 1).
Comp.
no.
Yield,
%
R1
R2
R3
mp, °С
CH3 C9H19
CH3
Ph
53
72
34
70
65
50
0
48
Iа
It was reported earlier that the presence of ester
bridge group increases flexibility and lability of some
part of the molecule that affects the mesophase
existence, type, and temperature range [17–20]. The
introduction of such a group into the molecular struc-
ture of boron difluoride β-diketonate leads not only to
appearance of the liquid crystal properties, but also to
polymorphism, that is, to the presence of nematic and
smectic mesophases. The observed textures (shlieren
and radial textures for nematic and smectic meso-
Ph
Ph
C9H19
IIа
IIIа
Ib
C12H25OC6H4COO Ph
86
CH3 C9H19
CH3
Ph
>300
195
Ph
Ph
C9H19
IIb
IIIb
C12H25OC6H4COO Ph
89 (Сr→SА),
102 (SА→N),
124 (N→I)
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 80 No. 11 2010

LIQUID CRYSTAL POLYMORPHISM
2319
H
C
H
C
H
C
C
H
F
C
H
H
C
C
H
H
H
H
H
C
H
O
H
H
C
C
H
C
C
H
F
B
C
C
C
C
H
H
H
C
H
H
H
H
C
H
O
H
C
C
C
H
H
C
C
C
H
H
Fig. 1. Molecular model of compound IIb.
H
H
H
C
C
C
H
H
C
C
H
H
H
H
H
H
C
H
C H
H
H
C
H
H
C
H
C
C
C
H
C
H
C
C
C C
H
C
H
H
H
C
C
H
O
C
C
H
H
H
H
C
O
F
C
C
C
H
H
H
C
H
C
O
H
B
F
C
O
C
H
H
C
C
C
H
H
C
H
Fig. 2. Molecular model of compound IIIb.
phases, respectively) are shown in Figs. 3 and 4,
respectively.
evidences molecular coplanarity of the latter: benzene
ring of one molecule is located under the chelate ring
of the adjacent molecule that proves an effective
interaction between the π-electrons and thus promotes
the appearance of intense luminescence and formation
of liquid crystal properties due to the increase in
geometric anisotropy of the ligand molecule.
Mesogenic properties were determined by means of
polarization polythermal microscopy: the mesophase
structure was revealed by observing a typical texture in
the image obtained from a thin layer of the mesomor-
phous substance in the polarized light of the microscope.
The comparison of the data of X-ray analysis of the
boron acetylacetonate and dibenzoylmethanate [14, 21]
In the UV spectra of the complexes dissolved in
chloroform there are very strong absorption bands in
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 80 No. 11 2010

2320
TURANOVA et al.
Fig. 3. Texture of smectic mesophase of compound IIIb at
101°С.
Fig. 4. Texture of nematic mesophase of compound IIIb at
119°С.
the range of 287–363 nm due to the π–π*-transfer in
the chelate ring. For compounds IIb and IIIb also
weak absorption bands are noted in the region of
longer waves (385–400 nm), that is, the presence of
aromatic substituents in the ligands leads to
strengthening the formed chelate cycle. It is known
[22] that the luminescence of boron difluoride β-
diketonates is caused by the absorption in the long-
wave range, which is attributed to a forbidden
transition in the В–О fragment.
Thus, the boron difluoride 2-substituted β-di-
ketonates combining luminescence properties and
liquid crystal polymorphism were obtained.
EXPERIMENTAL
All reagents and solvents were preliminary purified
by recrystallization or distillation to coincidence of
their constants with the literature data. Textures and
phase transition temperatures were determined on a
Boetius polarization microscope equipped with
temperature block. The error in the temperature
measurement was ±0.1°С. The IR spectra were
registered on a Bruker-JFS66V/S spectrometer from
Figure 5 shows normalized luminescence spectra at
room temperature of the polycrystalline samples of the
complex compounds: (1) Ib, λex 370 nm, (2) IIb, λex
330 nm, and (3) IIIb, λex 420 nm. The luminescence
quantum yields in the obtained compounds are
apparently of same order as that in boron difluoride β-
diketonates reported earlier [8].
1
mulls in mineral oil. The Н and 11В NMR spectra
were recorded on an Avance 400 spectrometer, solvent
CDCl3, concentration ~10–3 mol l–1, internal reference
TМС for 1Н and boron trifluoride etherate for 11В. The
UV spectra were taken on a Lambda 35 instrument
from solutions in CHCl3 at the concentration 5×10–5–
2×10–4 mol l–1. The luminescence spectra of polycrys-
talline samples were obtained at the irradiation of
compounds at room temperature with a DK xenon
lamp with EL-1000 using MDR-23 monochromator (in
the registration canal), the measured spectra were not
corrected for the apparatus function. For luminescence
registration a FEU-106 instrument was used operating
in the photon counting mode.
I, rel. units
1.00
0.75
2
1
0.50
0.25
3
Sodium acetylacetonate. A mixture of 97.30 mmol
of the freshly distilled acetylacetone and 97.30 mmol
of sodium metal was refluxed in anhydrous toluene.
The white precipitate formed was filtered off, washed
with toluene, and dried in a vacuum. Yield 72%, mp >
250°С. Found, %: С 49.04; Н 5.79. С5Н7О2Na.
Calculated, %: С 49.18; Н 5.70.
0.00
400
500
600
700
λ, nm
Fig. 5. Normalized luminescence spectra of polycrystalline
samples of (1) boron 2-nonyl-1,3-pentadionate difluoride
Ib, (2) boron 2-nonyl-1,3-dibenzoylmethanate difluoride
IIb, and (3) boron 2-(4-dodecyloxyphenyl)-1,3-dibenzoyl-
methanate difluoride IIIb at room temperature.
Sodium dibenzoylmethanate was obtained similarly.
Yield 90%, mp > 300°С. Found, %: С 73.10; Н 4.31.
С15Н11О2Na. Calculated, %: С 73.17; Н 4.50.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 80 No. 11 2010

LIQUID CRYSTAL POLYMORPHISM
2321
1
2-Nonyl-1,3-pentanedione (Iа). A mixture of
3.19 mmol of nonyl iodide and 3.99 mmol of sodium
acetylacetonate in methyl ethyl ketone was refluxed
for 72 h. Then the solvent was removed and to the
mixture water was added. The precipitate was
extracted with diethyl ether (4×25 ml). Organic layer
was concentrated. The crude product was subjected to
chromatography on silica gel L100/400 eluting with
dichloromethane. Pure product was obtained as white
needle crystals after removing the solvent and cooling.
and washed with ethanol. Yield 34.22% (0.23 g). Н
NMR spectrum (СDCl3), δ, ppm: 0.86 t (3Н, СН3),
1.27–1.81 m (20Н, СН2), 4.02 t (2Н, ОСН2), 5.54 s
(1Н, СНСО), 6.93 d (2Н, Ph), 7.50 t (2Н, Ph), 7.62 m
(4Н, Ph), 7.97 d (4Н, Ph), 8.08 d (2Н, Ph). Found, %:
С 77.23; Н 7.61. С34Н40О5. Calculated, %: С 77.24;
Н 7.62.
Boron 2-nonyl-1,3-pentadionate difluoride (Ib).
To 15.07 mmol (3.40 g) of 2-nonyl-1,3-pentadione was
added dropwise at stirring 15.07 mmol (2.08 ml) of
boron trifluoride etherate. When the self-heating
stopped, 10 ml of diethyl ether was added, and the
mixture was cooled to –40ºС. The white precipitate
was filtered off, washed with cold (–40ºС) diethyl
ether, and recrystallized from ether. Yield 70% (2.79 g).
1Н NMR spectrum (СDCl3), δ, ppm: 0.86–1.06 m
(9Н, СН3), 1.24 m (14Н, СН2), 1.62 t (2Н, СН–СН2).
Found, %: С 61.61; H 9.13. С14Н25О2BF2. Calculated,
%: С 61.36; Н 9.13.
1
Yield 89%. Н NMR spectrum (СDCl3), δ, ppm:
0.86–0.99 m (9Н, СН3), 1.24 m (14Н, СН2), 1.56 m
(2Н, СН–СН2), 5.01 t (1Н, СНСО). Found, %: С
74.21; Н 11.54. С14Н26О2. Calculated, %: С 74.29; Н
11.58.
2-Nonyl-1,3-dibenzoylmethane (IIа) was ob-
tained similarly. Yield 78%. 1Н NMR spectrum
(СDCl3), δ, ppm: 0.86 t (3Н, СН3), 1.25 m (14Н,
СН2), 1.55 m (2Н, СН–СН2), 5.17 t (1Н, СН), 7.45 t
(2H, Ph), 7.56 m (4H, Ph), 7.96 m (4Н, Ph). Found,
%: С 82.08; Н 8.62. С24Н30О2. Calculated, %: С
82.24; Н 8.63.
Boron 2-nonyl-1,3-dibenzoylmethanate difluo-
ride (IIb) was obtained similarly. Yield 65% (0.25 g).
1Н NMR spectrum (СDCl3), δ, ppm: 0.86 t (3Н,
СН3), 1.25 m (14Н, СН2), 1.56 t (2Н, СН–СН2), 7.57
t (2H, Ph), 7.70 m (4H, Ph), 8.17 m (4Н, Ph). 11B
NMR spectrum (СDCl3), δB, ppm: 1.34. Found, %: С
72.70; Н 7.28. С24Н29О2BF2. Calculated, %: С 72.40;
Н 7.29.
Sodium dodecyloxybenzoate. A mixture of
3.20 mmol (1.00 g) of dodecyloxybenzoic acid and
3.20 mmol (0.12 g) of sodium hydroxide in ethanol
with a low water content was heated on a water bath
for 10 min. After cooling, a white precipitate formed
was filtered off and washed with ethanol. Yield
98.2% (1.02 g), mp > 300ºС. Found, %: С 69.51; Н
8.81. С19Н29О3Nа. Calculated, %: С 69.49; Н 8.90.
Boron
2-(4-dodecyloxyphenyl)-1,3-dibenzoyl-
methanate difluoride (IIIb). A mixture of 0.29 mmol
(0.15 g) of dodecyloxybenzoyloxy-1,3-dibenzoyl-
methane and 0.45 mmol (0.07 ml) of BF3 etherate in
toluene was heated at stirring with a magnetic stirrer.
The solution color changed as follows: colorless–
yellow–brown–green. The product was precipitated by
adding cold (–40ºС) diethyl ether and kept in liquid
nitrogen. The yellow precipitate was filtered off and
2-Bromo-1,3-dibenzoylmethane. A mixture of
2.23 mmol (0.50 g) of dibenzoylmethane and 2.23 mmol
(396.94 mg) of N-bromosuccinimide in CCl4 was
refluxed for 15 min. The precipitated succinimide
was filtered off and washed with a hot carbon
tetrachloride. Combined filtrates were concentrated
and dissolved in petroleum ether. After cooling, a
white precipitate formed was filtered off and washed
with cold ССl4. Yield 34.22% (0.23 g), mp 176ºС. 1Н
NMR spectrum (СDCl3), δ, ppm: 6.55 s (1Н, СНBr),
7.47–7.98 m (10Н, Ph). Found, %: С 66.88; Н 7.05;
Br 18.33. С24Н30О2Br. Calculated, %: С 66.99; Н
6.98; Br 18.59.
1
dried in a vacuum. Yield 50% (0.39 g). Н NMR
spectrum (СDCl3) δ, ppm: 0.87 t (3Н, СН3), 1.21–
1.84 m (20Н, СН2), 4.02 t (2Н, ОСН2), 6.94 d (2Н,
Ph), 7.51 t (2Н, Ph), 7.62 m (4Н, Ph), 7.98 d (4Н,
Ph), 8.09 d (2Н, Ph). Found, %: С 70.92; Н 6.69.
С34Н39О5BF2. Calculated, %: С 70.96; Н 6.78.
2-(4-Dodecyloxybenzoyloxy)-1,3-dibenzoylme-
thane (IIIа). A mixture of 0.76 mmol (238.06 mg) of
sodium dodecyloxybenzoate and 0.76 mmol (231.25 mg)
of bromine-substituted dibenzoylmethane were dis-
solved in aqueous ethanol, refluxed for 7 h, and
cooled. The white precipitate formed was filtered off
ACKNOWLEDGMENTS
Authors are grateful to S.I. Nikitin and O.I. Gnez-
dilov for registration of NMR and luminescence
spectra.
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TURANOVA et al.
Tulachev, V.Ya., and Shkadarevich, A.P., Zh. Prikl.
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