Luminescent liquid crystal materials based on unsymmetrical boron difluoride β-diketonate adducts

Article abstract of DOI:10.1039/c0nj00503g

A strategic design based on the use of asymmetrically 1,3-alkyloxyphenyl substituted β-diketonate ligands towards the BF₂ group allowed us to find unsymmetrical compounds that, in addition to a high photoluminescent quantum yield, possess a liquid crystal behaviour, which is related to the presence of the banana-shaped liquid crystal mesophases B₁. The Royal Society of Chemistry and the Centre National de la Recherche Scientifique.

Full text of DOI:10.1039/c0nj00503g

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PAPER
www.rsc.org/njc | New Journal of Chemistry
Luminescent liquid crystal materials based on unsymmetrical boron
difluoride b-diketonate adductsw
a
a
a
a
´
Ignacio Sanchez, Marıa Jose Mayoral, Paloma Ovejero, Jose Antonio Campo,
Jose Vicente Heras, Mercedes Cano* and Carlos Lodeiro
´
´
´
a
a
b
´
Received (in Victoria, Australia) 30th June 2010, Accepted 24th August 2010
DOI: 10.1039/c0nj00503g
A strategic design based on the use of asymmetrically 1,3-alkyloxyphenyl substituted b-diketonate
ligands towards the BF2 group allowed us to find unsymmetrical compounds that, in addition to
a high photoluminescent quantum yield, possess a liquid crystal behaviour, which is related to the
presence of the banana-shaped liquid crystal mesophases B1.
New studies have also been recently reported to provide
Introduction
evidence of the lack of excimer formation being their low
tendency to self-aggregation an interesting result for developing
applications based on highly concentrated solutions of the dye
or in the solid phase (OLEDs, fluorescent sensors, solid laser
dyes, etc.).11
In recent years a wide range of emissive materials have been
reported for use in electroluminescent devices due to their
possible applications in modern technologies such as molecular
recognition, fluorescence probes and microscopy imaging,
electroluminescent materials, electro-optical sensors, lighting
and organic light-emitting diodes (OLEDs), among many
others.1–5
However, our previous results on the symmetrically
substituted compounds confirmed that, although all of them
were highly luminescent materials, none of them exhibited
liquid crystal behaviour.10,11
As an extension of these materials, the luminescent liquid
crystals are a class of materials which are especially interesting
because the molecular self-organization can be exploited to
improve device performance in order to achieve linear polarized
electroluminescence.6 Other interest in these materials proceeds
from their carrier high charge mobility as well as their ability
to develop defect-free layers.7
At this point we have to take into account that in designing
thermotropic liquid crystals, anisometric rod-like or disk-like
molecules exhibit the common prerequisite to perform this
behaviour. However, liquid crystals based on a molecular
banana shape with a bent-core12 are not extensively reported,
especially for those based on non-organic molecules.13 This
kind of material is growing in importance since the discovery
of polar switching and their potential application in display
technology.14
Therefore achieving liquid crystal and luminescence
properties combined in the same material led to intrinsically
luminescent mesogens, which constitute a realistic challenge in
flat-panel technology.8
In the previous paper we also reported the crystal structure
of the compound [BF2(OO2R(4,4))] which can be considered as
having a molecular banana shape with a bent-core (Fig. 1)
related to that observed in several organic compounds.
On this basis we decided to explore the thermal behaviour
of unsymmetrical adducts of the type [BF2(OO2R(12,n))]
(OO2R(12,n) = 1-(4-n-dodecyloxyphenyl)-3-(4-n-alkyloxyphenyl)-
propane-1,3-dionate), related to those symmetrical ones
previously described by us.10,11 These were built by the
simple introduction of different alkyl chain lengths in the 1,3
On the other hand, the research of photophysical and
photochemical properties of boron difluoride b-diketonate
derivatives permanently increases because they exhibit high
emission quantum yields in the visible range, being potentially
used as components of light emitting devices. As an interesting
consideration, it is observed that the intensity of luminescence
of those compounds is dependent on their geometry and
electronic structure.9
In a previous paper we described the preparation and a
preliminary description of the photoluminescent behaviour of
a family of related boron derivatives containing b-diketonates
with alkyloxyphenyl substituents of the type [BF2(OO2R(n,n))]
(OO2R(n,n) = 1,3-di(4-n-alkyloxyphenyl)propane-1,3-dionate).10
a
´
Departamento de Quımica Inorganica I,
Facultad de Ciencias Quımicas, Universidad Complutens de Madrid,
´
´
28040, Madrid, Spain. E-mail: mmcano@quim.ucm.es;
Fax: +34 91394 4352; Tel: +34 91394 4340
b Grupo BIOSCOPE, Departamento de Quı´mica-Fı´sica,
Facultade de Ciencias, Campus de Ourense, Universidade de Vigo,
32004, Ourense, Spain. E-mail: clodeiro@uvigo.es;
Fax: +34 98838 7001; Tel: +34 98836 8894
w Electronic supplementary information (ESI) available: Table SI1
containing yields and the analytical data. See DOI: 10.1039/
c0nj00503g
Fig. 1 ORTEP plot of the symmetrical adduct [BF2(OO2R(4,4))].10
c
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The IR spectra in CH2Cl2 solution shows two bands at 1711
and 1683 cmꢀ1 corresponding to the n(CO) absorption in
agreement with the presence of the keto and enol forms
respectively. On the other hand the IR spectra in KBr show
three bands at 1676, 1607 and 3450 cmꢀ1 assigned to the
n(CO), n(CQC)Ø and n(OH) absorptions confirming the sole
presence of the enol form in the solid state.
The 1H-NMR spectra of the compounds of the type II (4–6)
in CDCl3 at room temperature show the characteristic
signals of the ligand b-diketonate bearing alkyloxyphenyl
substituents.
The proton of the central C(2)H group of the diketonate
core appears as a singlet at ca. 7.0 ppm consistent with the
coordination of the ligand as enolate form. The spectra also
show two doublets at ca. 8.1 and 7.0 ppm for the ortho and
meta aromatic protons of the phenyl ring respectively.
The IR spectra in solid state display the characteristic bands
of the carbonyl groups of the b-diketonate ligands at ca.
1562 cmꢀ1 bathochromically shifted with respect to the free
b-diketone ligand (ca. 1676 cmꢀ1). The lack of the n(OH)
absorption bands (from the enol tautomer of the b-diketone)
and the presence of the n(BO) band at ca. 1376 cmꢀ1 demon-
strate the coordination of the carbonyl groups of the ligand in
the b-diketonate form to the boron atom.
Scheme 1
substituents of the b-diketonate ligands. We consider that the
asymmetry of those bent-core molecules should play an
important role in the steric packing of the molecular self-
organization at the mesophase.
In this work we report the mesomorphic behaviour and
luminescent properties of the unsymmetrical bent-core
compounds of the type [BF2(OO2R(12,n))] (type II)
(Scheme 1b) as well as the liquid crystal behaviour of the
precursors b-diketones ligand (Scheme 1a; type I). All
the compounds were designed with two unsymmetrical arms
at the 1, 3 positions of the b-diketonate ligand. In particular
different alkyl chain with 14, 16 or 18 carbon atoms at one of
these positions (1 or 3) were used to elongate one of the arms,
while the other arm at the other position is maintained with
the same alkyl length with 12 carbon atoms.
Thermal studies
The thermal behaviour of the starting b-diketones (1–3) was
studied by polarized-light optical microscopy (POM) and
differential scanning calorimetry (DSC) (Table 1). The three
compounds exhibit enantiotropic mesomorphism showing a
SmC mesophase established on the basis of their characteristic
schlieren texture observed by POM (Fig. 3).
Results and discussion
Synthesis and characterisation
The DSC thermograms show three or four endothermic
peaks on heating (Table 1). The first peaks (one or two
depending on the diketone) are assigned to solid–solid phase
transitions. The last two peaks were related to the melting
and clearing processes respectively, these results being
consistent with the polymorphic nature of this type of
compounds.15 No significant differences in the mesophases
range (ca. 7 1C) of compounds 1–3 can be established. By
contrast, the melting and clearing points increase with the
length of the alkylic chain (that is, with the increase of the
asymmetry from 1 to 3).
The b-diketone ligands (1–3) and the boron difluoride adducts
(4–6) were prepared according to Scheme 1 and unambiguously
characterized by elemental analysis, IR and 1H-NMR
spectroscopies.
The CDCl3 solution 1H-NMR spectra of the ligands 1–3
show well defined signals for the b-diketone core and the
alkyloxyphenyl substituents. The presence of a singlet at ca.
6.7 ppm corresponding to the C(2)H group of the b-diketone
core, a signal at ca. 14.8 ppm from the OH group and a singlet
at ca. 4.5 ppm confirms the existence in solution of keto and
enol tautomers. From the 1H-NMR data it is deduced that the
enol is the major form but the NMR time scale do not allow to
distinguish between the two possible enol forms a and b
(Fig. 2).
Temperature-dependent powder X-ray diffraction (XRD)
studies for the b-diketones (1–3) gave rise to characteristic
patterns of smectic phases in agreement with the SmC
mesophases assigned by POM (Table 2). The XRD patterns
show two or three sharp maxima in the low-angle region with
a reciprocal spacing ratio of 1 : 2 : 3 corresponding to the
(0 0 1), (0 0 2) and (0 0 3) reflections of a lamellar mesophase.
The diffuse halo due to the liquid-like arrangement of the
aliphatic chains also appears at ca. 4.5 A.
In the aromatic region duplicated doublets at ca. 8.0, 7.9
and 7.0, 6.9 ppm corresponding to the ortho and meta protons
of the phenyl rings of the two tautomeric keto and enol forms
are observed.
The indubitable evidence of mesomorphic behaviour of
b-diketones 1–3 was in contrast with the suggested no liquid
crystal nature reported in the literature16 for the compound
namely as our b-diketone 1. However the results obtained for
all compounds of the type I allow us to establish the absolute
identity of those as liquid crystal materials.
Fig. 2 Keto-enol equilibrium.
c
2938 New J. Chem., 2010, 34, 2937–2942 This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2010

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Table 1 Phase behaviour of compounds 1–6 determined by DSC
points. So, we observe the presence of crystal, liquid crystal
and isotropic liquid phases partially overlapping, this feature
makes it difficult to completely identify the mesophase.
However by a careful and precise control of the temperature
(each 0.1 1C) we were able to observe of the formation of the
mesophase without the presence of crystals or the quick
appearance of the isotropic liquid (140.2, 139.8 and 139.6 1C
for 4, 5 and 6 respectively).
n
Transition
T/1C
DH/kJmolꢀ1
14
1
2
Cr - Cr0
Cr0 - SmC
SmC - I
I - SmC
SmC - Cr
Cr0- Cr0
Cr - Cr0 0
Cr0 0 - SmC
SmC - I
I - SmC
SmC - Cr
Cr0- Cr0
Cr - Cr0 0
Cr0 0 - SmC
SmC - I
I - SmC
SmC - Cr0
Cr0 - Cr0 0
Cr - Cr0
Cr0 - B1
B1 - I
72
80
87
81
72
71
81
97
103
91
75
76
9.9
26.5
1.1
ꢀ1.0
ꢀ31.7
10.1
24.3
29.7
1.5
ꢀ1.8
ꢀ25.3
6.6
18.9
28.1
3.2
ꢀ4.4
ꢀ24.8
ꢀ15.3
18.6
26.3
16
18
In this way the mesophase for the three compounds, was
characterized as a mesophase B1 which seems to exhibit a fluid
smectic structure in agreement with the literature data.17
The assignment is supported by the mosaic texture and the
dendritic growth17–19 (Fig. 4) observed by POM as well as by
XRD studies which will be later discussed. The establishment
of that bent-core liquid crystal phase agrees with that expected
for the designed bent-core adducts [BF2(OO2R(12,n))].
3
83
100
114
113
80
74
135
141a
The DSC thermograms of compounds 4–6 exhibit similar
features. In all cases the DSC thermograms show on heating
two endothermic peaks. The first one in the 125–135 1C range
is related to a solid-solid phase transition in agreement with
X-ray results. The second peak at higher temperatures
(ca. 140 1C) is assigned to an overlapping of the melting and
clearing processes above mentioned.
14
16
18
4
5
6
I - B1
138a
ꢀ27.2
B1 - Cr0
Cr0 - Cr
Cr - Cr0
Cr0 - B1
B1 - I
96
125
139a
ꢀ15.6
26.6
23.0
In order to establish a complete identification of the
mesophase and following the above mentioned temperature
requirements to be detected, the X-ray diffractograms were
registered in different stage at temperatures close to the
clearing process being maintained the temperature each stage
for several minutes. Several registers were taken for each stage
at sequential times. In particular, for compound 6 diffracto-
grams were taken for each stage at 138 and 141 1C on heating
and at 141 and 138 1C on cooling. Only on cooling from the
isotropic liquid (150 1C) until the appearance of the solid
phase (138 1C) we were able to obtain a diffractogram at
141 1C which allow us to establish the presence of a B1 phase.
The two reflection peaks, in the small angle region, indexed as
(1 0 1) and (0 0 2), indicate the rectangular columnar arrangement
(Table 2).
I - B1
136a
ꢀ21.4
B1 - Cr0
Cr0 - Cr
Cr - Cr0
Cr0 - B1
B1 - I
87
128
139a
ꢀ19.0
27.4
26.1
I - B1
B1 - Cr0
Cr0 - Cr
136a
84
ꢀ26.2
ꢀ23.6
a
Overlapped processes.
Once again it was remarkable that in the previous paper
above mentioned16 the authors described the liquid crystal
behaviour of a compound considered to be like 4 of our
work. However neither the mesomorphic behaviour nor the
luminescent properties described for that compound respond
to our current data. The absolute characterization of our
compounds as well as the regular mesomorphic behaviour
found for those allow us undoubtedly to establish their
identity. From the best of our knowledge the unsymmetrical
compounds [BF2(OO2R(12,n))] described in this work can be
considered between the first materials of these type which
exhibit both liquid crystal and luminescence properties.
Luminescence studies
Fig. 3 Texture observed by POM of SmC phase for 2 at 100 1C on
heating (the white bar indicates 100 mm).
Luminescence studies were carried out both in dichloro-
methane solution and in the solid state for compounds of
the type II (4–6)
Thermal behaviour of the boron difluoride adducts 4–6
(type II, Table 1) exhibits similar features. The range of the
existence of the mesophase found for those adducts is very
narrow and the melting temperatures are close to the clearing
Solution studies. The UV-Vis absorption spectra of dilute
10ꢀ6 M) in dichloromethane of all
solutions (4.45
ꢁ
compounds exhibit two absorption bands, the strongest one
c
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Table 2 X-Ray diffraction data for mesomorphic compounds 1–3 (type I) and 6 (type II)
T/1C
Phase
SmC
Position (1)/2y
d-spacing/A
Miller indices (h k l)
Lattice parameters/A
1
2
88
3.3
6.7
19.0
3.0
6.1
9.1
19.8
2.7
5.5
8.3
19.7
2.4
3.5
26.5
13.2
4.7
29.0
14.5
9.7
0 0 1
0 0 2
c = 26.4
a
90
SmC
SmC
B1
0 0 1
0 0 2
0 0 3
c = 29.0
c = 32.0
a
4.5
3
105
141
32.2
16.0
10.6
4.5
36.5
3.5
0 0 1
0 0 2
0 0 3
a
6
1 0 1
0 0 2
a = 52.3
c = 51.0
a
Halo of the molten alkyloxy chains.
Table 3 Absorption (labsmax) and emission (lemmax) maxima in nm,
molar absorption coefficients (e) in Lmolꢀ1cmꢀ1 and fluorescence
quantum yields (FF) of the compounds of the type II in aerated
4.45 ꢁ 10ꢀ6 M in dichloromethane solution and in the solid state at
room temperature
max
max
max
labs
lem
lem
c
n
solutiona
e/104
solutionb
FF
solidd
4
5
6
14
16
18
414
411
414
4.3
6.8
5.6
440
437
440
0.95
0.95
0.95
484
486
493
a
b
Estimated error: ꢂ1 nm. lex = 414 nm; estimated error: ꢂ1 nm.
c
d
l
ex = 414 nm; estimated error: ꢂ5%.
lex = 414 nm; estimated error:
ꢂ1 nm.
Fig. 4 Textures observed by POM of B1 mesophase for 4 at 140.2 1C
on cooling (the white bar indicates 100 mm).
Fig. 5 UV-vis and normalized excitation and fluorescence spectra of
adduct 4 in dichloromethane solution and in the solid state at room
temperature.
centered at ca. 414 nm (Table 3, Fig. 5). This absorption is
characteristic of the electronic p–p* transition of chelating
b-diketonate ligands.20
blue-violet range, makes this compound to be a good
candidate as fluorescent biological probes.
The fluorescence spectra display an unresolved vibronic
substructure in the blue region with a maximum at ca.
440 nm with an excellent photoluminescence relative quantum
yield (F = 0.95, estimated error of ꢂ5%) using a solution of
acridine yellow as standard (F = 0.47).21 The emissive nature
of these compounds was markedly higher than that reported
for several boron difluoride arylsubstituted-b-diketonates.9,11
The high quantum yield close to the unity, in addition to the
observed absorption in the yellow region and emission in the
Solid state studies. The emission spectra of the adducts 4–6
exhibit a similar pattern consistent with a broad band peaked
at ca. 490 nm, bathochromically shifted with respect to the
solution emission (Table 3; Fig. 5).
Variable-temperature studies. In order to prove the
luminescent behaviour of compounds in the liquid crystal
c
2940 New J. Chem., 2010, 34, 2937–2942 This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2010

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s (singlet), d (doublet), t (triplet), m (multiplet). The 1H
chemical shifts and coupling constants are accurate to
ꢂ0.01 ppm and ꢂ0.3 Hz, respectively.
Phase studies were carried out by optical microscopy using
an Olympus BX50 microscope equipped with a Linkam
THMS 600 heating stage. The temperatures were assigned
on the basis of optic observations with polarised light.
Measurements of the transition temperatures were made
using a Perkin Elmer Pyris 1 differential scanning calorimeter
with the sample (1–4 mg) sealed hermetically in aluminium
pans and with a heating or cooling rate of 5–10 K minꢀ1
.
The X-ray diffractograms at variable temperature were
recorded on a Panalytical X’Pert PRO MPD diffractometer
in a y–y configuration equipped with a Anton Paar HTK1200
heating stage (X-ray Diffraction Service of Complutense
University).
Absorption spectra were recorded on a JASCO 650 UV
spectrophotometer and fluorescence emission on a Horiba-
Jobin-Yvon SPEX Fluoromax 4 spectrofluorimeter equipped
with a JULABO 32 bath. The linearity of the fluorescence
emission vs. concentration was checked in the concentration
range used (10ꢀ4–10ꢀ6 M). A correction for the absorbed light
was performed when necessary. All spectrofluorimetric studies
were performed as follows: the stock solutions of the ligands
(ca. 10ꢀ3 M) were prepared by dissolving an appropriate
amount of the ligand in a 50 mL volumetric flask and diluting
to the mark with dichloromethane HPLC or UVA-sol grades.
The relative fluorescence quantum yields were determinate by
comparison with a standard solution of acridine yellow (AY)
in absolute ethanol (F = 0.47).21 In order to avoid the inner
filter effects, in all the experiments the absorbance of both
compounds was kept below 0.1. The excitation wavelength
chosen was the wavelength where both absorption spectra
cross it. The emission spectra were performed using the same
excitation wavelength for both compounds, and the area
of both emission spectra was calculated using the Origin
Program. The equation used was:
Fig. 6 Fluorescence spectra of compound 4 in solid state as a
function of temperature.
state, the emission spectra of the adducts 4–6 were studied at
different temperatures from the solid state to the isotropic
liquid. The fluorescence spectra were also registered on cooling
from the isotropic liquid until the solidification temperature
(Fig. 6) all of them exhibiting a similar pattern.
In general terms the behaviour of the emission band on
heating shows that the intensity of the fluorescence is
maintained until 127 1C and it decreased on increasing the
temperature, almost disappearing at ca. 145 1C. The pattern of
the band is maintained in all temperature range used so much
on heating and on cooling (Fig. 6). The above results allow us
to establish that the liquid crystal state does not quench the
emission.
On cooling, the emission is reversible and increases the
intensity when the temperature decreases being the total
fluorescence emission rescued until ca. 40%. It is noticeable
that the emission maximum is maintained in the heating/
cooling cycle indicating that the excited state is not modified.
I
n2
f ¼ f ꢁ
ꢁ
Ist n2st
Experimental
Materials and physical measurements
where f = unknown fluorescence quantum yield, fst
=
standard fluorescence quantum yield, n = unknown solvent
refraction index, nst = standard solvent refraction index, I and
Ist = integrated emission spectra.
All commercial reagents were used as supplied. The synthetic
route
to
unsymmetrical
alkyloxyphenyl-disubstituted
b-diketones (type I) was carried out following similar procedures
to that used for the related symmetrical derivatives.10,11,15
Commercial solvents were dried prior to use.
Fluorescence spectra of solid samples were recorded on the
spectrofluorimeter exciting the solid compounds at appropriate
l (nm) using a fiber-optics devices connected to the spectro-
fluorimeter. The emission spectra as a function of the
temperature were recorded in the 300–800 nm range, using a
fiber optic system connected to the spectrofluorimeter and the
solid samples were heated over a hotplate with an external
temperature control provided with a XS intrument digital
thermo par.
Elemental analyses for carbon, hydrogen and nitrogen were
carried out by the Microanalytical Service of Complutense
University. IR spectra were recorded on a FTIR Thermo
Nicolet 200 spectrophotometer with samples as KBr pellets
in the 4000–400 cmꢀ1 region: vs (very strong), s (strong), m
(medium), w (weak).
1H-NMR spectra were performed at room temperature on a
Bruker DPX-300 spectrophotometer (NMR Service of
Complutense University) from solutions in CDCl3. Chemical
shifts d are listed relative to Me4Si using the signal of the
deuterated solvent as reference (7.26 ppm), and coupling
constants J are in hertz. Multiplicities are indicated as
Preparation of the compounds of the type I: [HOO2R(12,n)] (1–3)
The compounds were obtained by Claysen synthesis. To a
solution of 0.04 mol of 4-n-dodecyloxyacetophenone and
0.1 mol of NaH (60 wt% dispersion in mineral oil) dissolved
c
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in dimethoxyethane (300 mL) was added 0.06 mol of the
corresponding ethyl 4-n-alkyloxybenzoate. The solution was
refluxed for 24 h to give a suspension of sodium salts that was
left to cool to room temperature, and then a hydrochloric acid
solution (spec. grav. 1.19) was added to obtain a brown
precipitate. The mixture was stirred for 24 h at room
temperature. The brown precipitate was filtered off and
washed repeatedly with hexane and dried in vacuo.
for the financial support. C.L. thanks Xunta de Galicia by the
Isidro Parga Pondal research contract.
All the team thanks to the program ‘‘Joint Portuguese-
Spanish Project 2009’’ for bilateral agreement number
69/09(Portugal)/HP 2008-0066 (Spain).
Notes and references
1 B. M. Krasovitskii, B. M. Bolotin and V. G. Vopian, Organic
Luminescent Materials, VCH, Weinheim, Germany, 1988.
2 Highly Efficient OLEDs with Phosphorescent Materials, ed.,
H. Yersin, Wiley, New York, 2007.
Preparation of the compounds of the type II: [BF2(OO2R(12,n))]
(4–6)
3 Luminescent Materials and Applications, ed., A. Kitai, Wiley,
New York, 2008.
To a solution in dichloromethane (20 mL) of the corresponding
b-diketone (0.20 mmol) or sodium b-diketonate (prepared
in situ from the b-diketone and 0.22 mmol of NaH 60%
dispersion in mineral oil), 0.60 mmol of HBF4ꢃOEt2 was
added. The mixture was stirred at room temperature for
24 h. The deep-yellow solid formed was filtered off and
purified by chromatography using dichloromethane as eluent.
Spectroscopic IR and 1H-NMR data are given for 2 and 5 as
selected representative examples of each type of compounds.
Data for the other homologues are essentially identical. Yields
and elemental analysis are collected in supporting information
(ESIw).
4 S. R. Rotman, Wide-Gap Luminescent Materials: Theory and
Applications, ed., Springer, Berlin-Heidelberg, Germany, 1996.
5 Luminescence Biotechnology: Instruments and Applications, ed.,
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Raton, Fla., 2001.
6 M. P. Aldred, A. J. Eastwood, S. M. Kelly, P. Vlachos,
A. E. A. Contoret, S. R. Farrar, B. Mansoor, M. O’Neill and
W. C. Tsoi, Chem. Mater., 2004, 16, 4928.
7 D. Adam, P. Schuhmacher, J. Simmerer, L. Haussling,
K. Siemensmeyer, K. H. Etzbachi, H. Ringsdorf and D. Haarer,
Nature, 1994, 371, 141; H. Tokuhisa, M. Era and T. Tsutsui, Adv.
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M. P. Aldred, M. O’Neill and S. M. Kelly, Chem. Commun.,
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[HOO2R(12,16))] (2). nmax(KBr)/cmꢀ1 3450w (OH), 1676s
(CQO enol form) and 1607s (CQC)Ø. nmax(CH2Cl2)/cmꢀ1
1711m (CQO keto form), 1683w (CQO enol form) and
1604m (CQC)Ø. dH (300 MHz; CDCl3; Me4Si) 0.88 (t, 3J
6.8, CH3), 1.26 (m, CH2), 1.80 (m, CH2), 4.02 (t, 3J 6.4,
OCH2), 4.52 (s, C(2)H2 keto form), 6.73 (s, C(2)H enol form),
8 S. M. Kelly, Flat Panel Displays: Advanced Organic Materials, ed.,
J. A. Connor, RSC Materials Monographs, Royal Society of
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9 E. Cogne-Laage, J. F. Allemand, O. Ruel, J.-B. Baudin,
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3
3
3
0
6.92 (d, J 8.9, Hm), 6.97 (d, J 8.5, Hm ) 7.94 (d, J 8.5, Ho ),
0
3
8.03 (d, J 8.9, Ho), 14.84 (s, OH).
10 M. J. Mayoral, M. Cano, J. A. Campo, J. V. Heras, E. Pinilla and
M. R. Torres, Inorg. Chem. Commun., 2004, 7, 974.
[BF2(OO2R(12,16))] (5). nmax(KBr)/cmꢀ1 1562vs (CQO),
1376m (BO) and 1038s (BF). dH (300 MHz; CDCl3; Me4Si)
0.88 (6 H, t, J 6.4, CH3), 1.26 (44 H, m, CH2), 1.83 (4 H, m,
11 M. J. Mayoral, P. Ovejero, M. Cano and G. Orellana, Dalton
Trans., 2010, accepted under review, DT-ART-06-2010-000588.
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3
CH2), 4.07 (4 H, t, 3J 6.4, OCH2), 7.00 (s, C(2)H), 7.00 (4 H, d,
3
3J 9.0, Hm), 8.11 (4 H, d, J 9.0, Ho).
Conclusions
In previous studies we reported that compounds
[BF2(OO2R(n,n))] with a banana-molecular shape and identical
alkyloxyphenyl substituents possess a strong luminescence in
the solid state and in solution, but they were not liquid crystal
materials. The strategic introduction of asymmetry in this type
of compounds by using substituents of different chain length in
the rigid BF2-diketonate core appears to provide a lateral
dipole so favouring the mesomorphic properties. This kind
of compound opens the study of new boron difluoride
b-diketonates unsymmetrically substituted looking for excellent
photoluminescent materials with liquid crystal properties.
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We thank the Secretarıa de Estado de Investigacion (Direccion
General de Investigacion) del Ministerio de Ciencia e Innovacion
of Spain, project CTQ2009-11880, and Universidad
Complutense de Madrid (Spain), project UCM2008-910300,
c
2942 New J. Chem., 2010, 34, 2937–2942 This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2010