Triketonate difluoroboron complexes. Substitution-dependent liquid crystal and photophysical properties

Article abstract of DOI:10.1016/j.dyepig.2015.10.029

Novel alkyloxyphenyl-substituted β,δ-triketonate difluoroboron complexes have been efficiently synthesised and thoroughly characterised. Significant features such as liquid crystal behaviour and solid and solution fluorescence are observed in all cases. The mesomorphism was determined by the presence of one, two or three alkyl chains in each substituent aromatic group of the triketonate ligand. So, smectic C mesophases were found for compounds carrying two lateral chains, each of them located at one of each phenyl substituent, while the presence of four or six chains at the whole molecule gave rise to smectic A or discotic lamellar and hexagonal columnar mesophases, respectively. Fluorescence in the solid state and in solution is again proved to be dependent on the ligand substitution as well as to be maintained in the mesophase. These multifunctional materials also present luminescent sensor activity towards Hg²⁺ and Cu²⁺.

Full text of DOI:10.1016/j.dyepig.2015.10.029

Accepted Manuscript
Triketonate difluoroboron complexes. Substitution-dependent liquid crystal and
photophysical properties
Ignacio Sánchez, Adrián Fernández-Lodeiro, Elisabete Oliveira, José Antonio Campo,
M. Rosario Torres, Mercedes Cano, Carlos Lodeiro
PII:
S0143-7208(15)00412-X
10.1016/j.dyepig.2015.10.029
DYPI 4976
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 18 August 2015
Revised Date: 21 October 2015
Accepted Date: 25 October 2015
Please cite this article as: Sánchez I, Fernández-Lodeiro A, Oliveira E, Campo JA, Rosario Torres M,
Cano M, Lodeiro C, Triketonate difluoroboron complexes. Substitution-dependent liquid crystal and
photophysical properties, Dyes and Pigments (2015), doi: 10.1016/j.dyepig.2015.10.029.
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ACCEPTED MANUSCRIPT
Revised version
Triketonate difluoroboron complexes. Substitution-dependent liquid
crystal and photophysical properties
a
b,c
b,c
a
Ignacio Sánchez, Adrián Fernández-Lodeiro, b,cElisabete Oliveira, José Antonio Campo, M. Rosario
d
a
Torres, Mercedes Cano,* and Carlos Lodeiro*
a
Department of Inorganic Chemistry I, Faculty of Chemistry, Universidad Complutense de Madrid, 28040 Madrid,
Spain. E-mail: mmcano@ucm.es; Fax: +34 91 3944352; Tel: +34 91 3944340.
b
BIOSCOPE Group, UCIBIO-REQUIMTE, Chemistry Department, Faculty of Science and Technology, University
NOVA of Lisbon, Caparica, 2829-516, Portugal. E-mail: cle@fct.unl.pt; Fax: +351 2129 48550; Tel: +351 2129
1
1402.
c
PROTEOMASS Scientific Society, Rua dos Inventores, Madam Parque, Caparica Campus, Portugal.
d
X-Ray Diffraction Laboratory, Faculty of Chemistry, Universidad Complutense de Madrid, 28040 Madrid, Spain.
Abstract
Novel alkoxyphenyl-substituted β,δ-triketonate difluoroboron complexes have been efficiently
synthesised and thoroughly characterised. Significant features such as liquid crystal behaviour and solid
and solution fluorescence are observed in all cases. The mesomorphism was determined by the presence
of one, two or three alkyl chains in each substituent aromatic group of the triketonate ligand. So, smectic
C mesophases were found for compounds carrying two lateral chains, each of them located at one of each
phenyl substituent, while the presence of four or six chains at the whole molecule gave rise to smectic A
or discotic lamellar and hexagonal columnar mesophases, respectively. Fluorescence in the solid state and
in solution is again proved to be dependent on the ligand substitution as well as to be maintained in the
2
+
mesophase. These multifunctional materials also present luminescent sensor activity towards Hg and
2
+
Cu .
Keywords: Difluoro(triketonate)boron compounds; liquid crystals; luminescence; bifunctional
materials; molecular design
1

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1
. Introduction
High-efficiency fluorophores as are the conventional difluoroboron dipyrromethenes
(
[
BODIPYs) have been proposed to be used in the field of materials and biological optical imaging
1-3]. However, this kind of compound presents self-quenching related with their small Stokes
shifts, so preventing them to be applied as electroluminescent materials in organic light-emitting
diodes (OLEDs) [4]. By contrast, the highly emissive β-diketonate-difluoroboron complexes [5-9]
have received considerable attention as a potential alternative to emitters in OLED devices [6-9].
In this context, in previous work we have established that the coordination of 1,3-
unsymmetrically long-chained alkyloxyphenyl-substituted β-diketonates to the BF fragment give
2
rise to liquid crystal materials which exhibit bent-core mesophases in the range of 120-145 ºC, in
addition to an intense fluorescence in the green region [7,8]. However, in designing luminescent
liquid crystal materials for technological applications, the supramolecular order of columnar
liquid crystals into 2D lattices [10] increases their suitability to these purposes [11]. So, when
luminescent columnar liquid crystals are used in the fabrication of OLEDs they can behave as the
emissive layer [12]. On this basis we were able to achieve luminescent boron derivatives
exhibiting hexagonal columnar mesophases by a strategical control of the ligand established by
the employment of polycatenar unsymmetrical β-diketones [9].
Following these results on β-diketonate-difluoroboron complexes as luminescent columnar
liquid crystal materials [6-9], we are now involved in extending the study to new boron
derivatives by using mono and polycatenar alkyloxyphenyl- substituted β,δ-triketonate ligands
towards the BF group. This type of polyketonate ligands have been reported to be useful in
2
achieving complexes with strong intermolecular associations and molecular superstructures as
well as to attain homo and heteronuclear bimetallic assemblies, thus leading to multifunctional
mesogenic materials with potential magneto and electro-optical applications [13,14].
Currently, our main objective is centred on using β,δ-triketone ligands in which effects like the
presence of a largest molecular core, the increasing of the molecular polarisation and the
expansion of the π conjugation will be determining factors in order to achieve mesomorphic
complexes with improved liquid crystalline properties. In addition, polycatenar systems, which
would potentially decrease the phase transition temperatures, have also been strategically used.
On the other hand, luminescent liquid crystals having entities able to react to external stimuli are
2

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interesting for sensor applications [15,16]. In this context, the BF -β,δ-triketonate complexes offer
2
new opportunities as chemosensors towards external metal centres by considering the presence of
an uncoordinated oxygen atom as a potential coordinative site. Thus, evaluating the changes on
the photophysical properties could be exploited for signalling purposes [17].
In this work, four families of complexes (Ia-Id) have been prepared. They have been classified
according to the number and position of the alkyl chains in the alkyloxyphenyl groups at the 1-
and 5-position of the 1,3,5-pentanotrione core (Table 1). For comparative purposes, all the free
ligands (IIa
-IId) (Table 1), some of which had already been previously described [13,18], are
also included along the study.
2
. Experimental Section
2
.1. Materials and physical measurements
All commercial reagents were used as supplied. Hydroxybenzoates,
n-alkyl bromides,
tetrafluoroboric acid diethyl etherate, sodium hydride, quinine sulphate, acridine yellow, and the
metal salts, NaBF ·xH O, Ca(OOCCF ) ·xH O, Zn(BF ) ·xH O, Pb(BF ) ·xH O,
Cd(OOCCF ) ·xH O, Ni(NO ) ·xH O, Co(NO ) ·xH O, Cu(BF ) ·xH O, Hg(NO ) ·xH O,
4
2
3 2
2
4 2
2
4 2
2
3
2
2
3 2
2
3 2
2
4 2
2
3 2
2
Al(NO ) ·xH O and Cr(NO ) ·xH O were purchased from Stream Chemicals, Sigma-Aldrich or
3
3
2
3 3
2
Solchemar.
Elemental analyses for carbon and hydrogen were carried out by the Microanalytical Service
of Complutense University (validated range: %C 0.50-94.7, %H 0.50-7.6 and %N 0.50-23.0). IR
spectra were recorded on a FTIR Thermo Nicolet 200 spectrophotometer with samples as KBr
-
1
pellets in the 4000-400 cm region: vs (very strong), s (strong), m (medium), w (weak), sh
1
13
19
(
shoulder). Solution H-, C- and F-NMR spectra were performed at room temperature on a
Bruker DPX-300 spectrophotometer (NMR Service of Complutense University) from solutions in
CDCl . Chemical shifts are listed relative to Me Si using the signal of the deuterated solvent as
δ
3
4
1
13
19
reference (7.26 and 77.0 ppm for H and C, respectively), and to trifluorotoluene for F.
Coupling constants are in hertz. Multiplicities are indicated as s (singlet), d (doublet), t (triplet),
dd (doublet of doublets), m (multiplet). The H chemical shifts and the C and F ones are
J
1
13
19
1
3
accurate to ±0.01 and ±0.1 ppm, respectively, and coupling constants to ±0.3 Hz. Solid state C-
NMR spectra of compound 24 have been obtained on a Bruker WB 400 spectrometer (Faculty of
3

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Sciences of the National University of Distance Education) at 300 K using a 4 mm DVT probe
head. Samples were carefully packed in a 4 mm diameter cylindrical zirconia rotor with Kel-F
end-caps. The mass spectrum of compound 42 was registered on a Bruker Ultraflex
MALDI/TOF-TOF spectrometer using dichloromethane as the solvent and dithranol as the matrix.
Phase studies were carried out by optical microscopy using an Olympus BX50 microscope
equipped with a Linkam THMS 600 heating stage. The phase transition temperatures were
determined from the DSC thermograms by using a Perkin Elmer Pyris 1 differential scanning
calorimeter with the sample (1 – 4 mg) sealed hermetically in aluminium pans and a heating or
-
1
cooling rate of 5 – 10 Kmin . X-ray diffractograms at variable temperature were recorded on a
Panalytical X’Pert PRO MPD diffractometer in a configuration equipped with an Anton Paar
θ-θ
HTK1200 heating stage (X-Ray Diffraction Service of Complutense University).
UV-Vis absorption spectra were recorded with a Jasco V-650 spectrophotometer equipped
with a Julabo thermostat and the fluorescence emission by a Horiba JY Scientific Fluoromax-4
spectrofluorimeter. The linearity of the fluorescence emission vs. concentration was checked in
-
5
-6
the concentration range used (10 – 10 M). A correction for the absorbed light was performed in
the case of absorbance (A) higher than 0.2 at the excitation wavelength. The photophysical
-3
characterisations were performed using stock solutions of the compounds (ca. 10 M), prepared
by dissolving the appropriate amount of the compounds 10 11 15 19 20 24 26 31 32 36
and 41 in dichloromethane in a 10 mL volumetric flask. The stock solutions were then diluted
3,
5
,
,
,
,
,
,
,
,
,
,
,
4
0
-5
-6
until 10 – 10 M. Titrations of
3, 10, 11, 24, 31 and 32 were carried out by the addition of
+
2+
2+
2+
2+
2+
3+
microliter amounts of standard solutions of the metal ions, Na , Ca , Zn , Cd , Cu , Hg , Al
3
+
and Cr , in dry acetonitrile. All measurements were performed at 295 K. Luminescence quantum
yields were measured using solution of quinine sulphate in sulphuric acid (0.1 M) [ΦF = 0.54] for
ligands and acridine yellow in ethanol [ΦF = 0.47] for complexes, as a standard [19]. The values
were corrected for different refraction indexes of solvents. Fluorescence spectra of solid samples
were recorded using a fibre optic system connected to the Horiba J-Y Fluoromax-4
spectrofluorimeter exciting at appropriate
λ (nm) of the solid compounds.
4

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2
.2. Synthetic procedures
Ethyl 4-alkyloxybenzoates, ethyl 3,4-dialkyloxybenzoates, methyl 3,5-dialkyloxybenzoates
and methyl 3,4,5-trialkyloxybenzoates were prepared by a Williamson alkylation of the
corresponding commercially available hidroxybenzoate derivatives [13,20].
2
.2.1. Ligands II (21-41)
β,δ-triketones were synthesised by adapting literature methods and fully characterised
according to the previous results when corresponding [13,18]. All compounds were characterised
1
by IR and H-NMR spectroscopies and elemental analyses. In addition, compound 24 was also
1
3
characterised by C-NMR as a representative example. A procedure for the synthesis and
characterisation of a selected compound is given as follows, and the complete characterisation of
all the derivatives is deposited in the ESI†.
2
.2.1.1. 1,5-Bis-(4’-octyloxyphenyl)-pentane-1,3,5-trione (24).
To a solution of acetone (0.52 g, 9 mmol) and sodium hydride (60 % mineral oil suspension,
.16 g, 54 mmol) in 1,2-dimethoxyethane (100 mL), ethyl 4-octyloxybenzoate (5.00 g, 18 mmol)
2
was added under N at room temperature. The mixture was then refluxed for 24 h, and the
2
resulting dark brown suspension was cooled to room temperature and poured into ice (ca. 200
mL) where the excess NaH was quenched. A small amount of concentrated HCl was added to
neutralise the triketone. The product was extracted with chloroform (3
distilled water (3 50 mL) and dried with anhydrous MgSO . The solvent was removed and the
solid residue was recrystallised from tetrahydrofurane/methanol to give 6.75 g of a yellow powder
72 % yield).
Due to the presence of tautomers, H- and C-NMR signals are listed below and a full
× 50 mL), washed with
×
4
(
1
13
assignation is shown in Figs. S1 and S3†.
Mp 103 ºC. Elemental analysis: Found: C, 75.2; H, 8.5%. C H O ·0.25H O requires C, 75.2;
3
3
46
5
2
-
1
H, 8.9%. νmax/cm : 3084w
593vs (C=O)enol, 1567m
CH ), 1.29-1.44 (20H, m, CH ), 1.80 (4H, m,
ν
(O-H), 2932s, 2871s
ν
(C-H), 1688w
ν
(C=O)keto, 1604s
ν
(C=C)Ar,
3
1
ν
ν
(C=C)enol
.
δH (300 MHz; CDCl ; Me Si)/ppm: 0.88 (6H, t,
J
6.6,
3
4
3
3
J
6.6, CH ), 4.02 (4H, t,
J
6.6, OCH ), 4.26 (s,
2
3
2
2
3
3
COCH CO), 5.91 (s) and 6.23 (s) (CCHCO), 6.90 (d,
J
8.9) and 6.92 (d,
J
8.9) (4H, H ), 7.80
2
m
5

ACCEPTED MANUSCRIPT
3
3
3
3
(
d,
J 8.9), 7.82 (d, J 8.9), 7.92 (d, J 8.9) and 8.02 (d, J 8.9) (4H, H ), 14.87 (s) and 16.08 (s)
o
(
OH). δC (75.48 MHz; CDCl ; Me Si)/ppm: 14.5 (CH ), 23.1, 26.4, 29.4, 29.5, 29.7, 32.2 (CH ),
3
4
3
2
5
1
0.8 (C-2), 68.7 (OCH ), 95.8, 96.4 (C-4), 114.5, 114,8 (C ), 126.3, 126.6 (C ), 129.6, 131.7 (C ),
2 m i o
62.5, 163.4 (C ), 164.1 (C-5), 173.7 (C-1, C-5), 183.0 (C-5), 189.8, 192.9 (C-3), 193.6 (C-1). δC
p
(
100.73 MHz; solid; Me Si)/ppm: 14.5 (CH ), 23.7, 27.0, 29.5, 31.2, 31.7, 33.8 (CH ), 68.3
4 3 2
(
OCH ), 94.9, 95.3 (C-4), 95.0, 97.6 (C-2) 110.6, 112.6 (C ), 116.2, 118.6 (C ), 126.6, 129.0 (C ),
2 m i o
1
60.5, 161.7 (C ), 162.7 (C-5), 167.8 (C-3), 173.1 (C-1, C-5), 192.8, (C-1), 193.6 (C-3).
p
2
.2.2. Boron derivatives I (1-20)
9
The boron derivatives were prepared by adapting a procedure previously reported by us. All
1
19
compounds were characterised by IR and H- and F-NMR spectroscopies and elemental
1
3
analyses. Compound
3 was also characterised by C-NMR as a representative example. A general
procedure for the synthesis and characterisation of a selected compound is given as follows, and
the complete characterisation of all the complexes is deposited in the ESI†.
2
.2.2.1. Difluoride(1,5-bis-(4’octyloxyphenyl)-pentano-1,3-dionate-5-one)boron
(3)
To a solution of 1,5-bis-(4’-octyloxyphenyl)-pentane-1,3,5-trione (0.523 g, 1 mmol; 24) in
dichloromethane (50 mL), sodium hydride (60 % mineral suspension, 0.042 g, 1.05 mmol) was
added. The solution was left to stir for 30 minutes and then tetrafluoroboric acid diethyl ether
complex (0.2 mL) were added. After 48 hours of stirring at room temperature, the suspension
formed was filtered through a Celite pad and the clear dark green solution was concentrated to
obtain a viscous residue, which was purified by silica chromatography using ethyl acetate as the
eluent. The solvent was evaporated and the oily product was recrystallised from hexane/methanol
to give the title compound as an orange powder (0.37 g, 65 % yield).
1
19
13
Due to the presence of tautomers, H-, F- and C-NMR signals are listed below and a full
assignation is shown in Figs. S2 and S4†.
Mp 128 ºC. Elemental analysis: Found: C, 69.6; H, 7.8%. C H BF O requires C, 69.5; H,
3
3
45
2
5
-
1
7
.9%. νmax/cm : 2923m, 2853m
(C=C)enol, 1383m (B-O), 1040s
CH ), 1.29-1.46 (20H, m, CH ), 1.81 (4H, m,
ν
(C-H), 1608s
ν
(C=C) , 1595sh, 1543vs
ν
(C=O), 1506vs
Ar
3
ν
ν
ν
(B-F). δH (300 MHz; CDCl ; Me Si)/ppm: 0.89 (6H, t,
J
6.6,
3
4
3
3
J
6.6, CH ), 4.01-4.08 (4H, m,
J
6.6, OCH ), 4.15
3
2
2
2
6

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3
3
(
s, COCH CO), 5.92 (s), 6.25 (s) and 6.64 (s) (CCHCO), 6.91 (d,
J
8.9) and 6.96 (d,
J
8.9) (4H,
2
3
3
3
H ), 7.83 (d,
J
8.9), 7.97 (d,
J
8.9) and 8.04 (d,
143.1, 139.7. δC (75.48 MHz; CDCl ; Me Si)/ppm: 14.1 (CH ),
2.7, 26.0, 29.1, 29.2, 29.3, 31.8 (CH ), 52.3 (C-4), 68.5 (OCH ), 92.6, 94.4 (C-2), 97.1 (C-4),
J 8.9) (4H, H ), 11.40 (s, OH). δF (282.40 MHz;
m
o
CDCl ; trifluorotoluene)/ppm:
−
−
3
3
4
3
2
1
2
2
14.7, 114,8, 115.2 (C ), 124.1, 124.5, 125.6 (C ), 129.3, 130.5, 131.3 (C ), 163.5, 164.2, 166.0
m
i
o
(
C ), 173.7, 179.9 (C-3), 190.7 (C-5), 192.5, 193.2 (C-1), 208.9 (C-5).
p
2
.2.3. Bis(1,5-bis-(4’-octyloxyphenyl)-pentane-1,3,5-trionate)dicopper(II) (42)
The binuclear copper(II) complex containing the ligand 24 was synthesised by a similar
procedure to that reported for some of its homologues [13], and characterised by elemental
analysis, mass spectrometry and IR spectroscopy.
To a yellow solution of of 1,5-bis-(4’-octyloxyphenyl)-pentane-1,3,5-trione (0.53 g, 1 mmol;
2
4
) in hot chloroform (20 mL) was added a solution of copper(II) acetate monohydrate (0.22 g,
1
.1 mmol) in hot methanol (25 mL). The solution turned green and was refluxed for 3 hours after
which it was cooled down to room temperature. The product was filtered off, washed with
methanol and recrystallised from a 1:1 mixture of tetrahydrofuran and methanol to yield a dark
green powder (0.53 g, 90% yield).
Mp 270 ºC. Elemental analysis: Found: C, 67.4; H, 7.3%. Cu C H O requires C, 67.8; H,
2
66 88 10
+
-1
7
.6%. MS 1068 (M ). νmax/cm : 2919vs, 2853vs ν(C-H), 1606s ν(C=C), 1528vs ν(C=O).
2
.3. X-Ray data collection and structure refinement
Data collection for compound 21 was carried out at room temperature on a Bruker Smart CCD
diffractometer using graphite-monochromated Mo-K radiation ( = 0.71073 Å) operating at 50
kV and 35 mA. The data were collected over a hemisphere of the reciprocal space by combination
of three exposure sets, each exposure of 20 s and covered 0.3º in . The first 100 frames were
α
λ
ω
recollected at the end of the data collection to monitor crystal decay, and no appreciable decay
was observed. A summary of the fundamental crystal and refinement data is given in Table 2.
The structure was solved by direct methods and refined by full-matrix least-square procedures
2
on
F
[21]. All non-hydrogen atoms were refined anisotropically. All hydrogen atoms were
included in calculated positions and refined riding on the respective carbon atoms, with some
7

ACCEPTED MANUSCRIPT
exceptions. The hydrogen atoms bonded to oxygen atoms (O1 and O3) were located in a Fourier
synthesis and refined riding on the respective bonded atoms.
CCDC 1061994 contains the supplementary crystallographic data for this paper. These data
can be obtained free of charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
3
. Results and discussion
3
.1. Synthetic procedures
All the new complexes
I as well as their free triketone ligands II have been synthesised
according to procedures summarised in Scheme 1.
The new triketone ligands II 21 22 29 33 37, 41; Table 1) have been prepared according to
(
,
,
,
-
the method reported by Serrette et al [13]. The procedure involves a Claisen condensation
between one acetone molecule and two equivalents of the corresponding alkyloxybenzoate III
(
Scheme 1) in the presence of an excess of sodium hydride to give intense yellow powders, which
crystallise with a variable number of water molecules. The remaining ligands (23-28, 30-32, 38-
; Table 1), which had been previously described [13,18], were again prepared by the same
4
0
procedure indicated in the literature. Because their full characterisation had not been reported it
has been now made and the results will be included along with those of the new related species in
order to achieve a systematic study.
The precursor alkyloxybenzoates III were synthesised through a Williamson alkylation
reaction from the corresponding commercially available hydroxybenzoate derivatives and the
appropriate n-alkyl bromides. These were carried out in the presence of catalytic amounts of
potassium iodide and an excess of potassium carbonate as base, in refluxing acetone for 24-96
hours depending on the number of alkyl chains [13,20].
All the BF -triketonate complexes have been obtained by a similar procedure to that used for
2
the related BF -diketonate derivatives [9]. The procedure involves the reaction of the
2
corresponding triketone ligand with an excess of tetrafluoroboric acid diethyl ether adduct in the
presence of sodium hydride at room temperature. The new complexes were purified by silica
chromatography using ethyl acetate (Ia) or dichloromethane (Ib
, Ic, Id) as a eluent.
Recrystallisation from hexane/methanol or chloroform/methanol, respectively, yields stable
8

ACCEPTED MANUSCRIPT
orange solids whose yellow-green solutions in common organic solvents are visually highly
luminescent.
All complexes
I have been characterised by C, H elemental analysis. In all cases, the one to
one relationship BF fragment/ligand was found, in agreement to the proposed formulation.
2
Likewise, the IR and NMR results suggest a mononuclear nature of the compounds, as discussed
below.
On the other hand, in order to explore the potential sensorial ability of selected complexes and
their corresponding ligands, several metal titrations followed by absorption and emission
2
+
2+
measurements were performed with Hg and Cu additions. For additional information and
comparative purposes, a Cu(II) complex containing 24 as a ligand was also prepared and
characterised. It was synthesised according to a similar procedure described for related triketonate
R(8)
R(8)
complexes [13], and characterised as the binuclear [Cu (O OO )2] complex (Scheme 2). The
2
analytical data, the mass spectrum, and the absence of IR bands associated to non-coordinated
keto groups in the ligand support the binuclear formulation proposed.
3
.2. Structural characterisation
R(n)x
R(n)x
3
1
.2.1. Solution studies of triketone ligands [HO
OO
H] II (x = 1, n = 1, 4, 6, 8, 10, 12,
4, 16, 18; x = 2, 3, n = 12, 14, 16, 18). Keto-enol equilibrium
1
The CDCl solution H-NMR spectra at room temperature of all the β,δ-triketones (21
-
41
)
3
exhibit a similar pattern showing well defined signals from the triketone core and the
alkyloxyphenyl substituents. According to a tautomeric equilibrium established in these type of
polyketone systems [13,14,22,23], three different keto-enol tautomeric forms can be expected
(
Fig. 1). In fact, the spectra show two characteristic signals at
δ
14.72 – 16.24 ppm, two signals at
, –CC CO and –
δ
5.91 – 6.23 ppm and one signal at δ ca. 4.26 ppm, assigned for –CO
H
H
COCH2CO–, respectively (Table 3). The pattern of peaks observed corresponds to the presence of
three tautomeric keto-enol forms as depicted in Fig 1. Those are established as the
undistinguishable monoenol forms exchanged together by a rapid process (II1), and two different
dienol forms (II2 and II3) that differ on the position of the keto group within the core, thus giving
rise to different chemical shifts of the two dienol tautomers. From the intensity ratio it can be
established that the dienol forms are present in a percentage of ca. 90%, being the monoenol
tautomers the minor ones. The lack of a signal at ca. 3.90 ppm indicates the complete absence of
9

ACCEPTED MANUSCRIPT
the triketo form. The splitting of the aromatic peaks into three signals is also remarkable.
Complete assignation of the chemical shifts is included in Fig S1†.
On the other hand it is important to note the observed changes in the spectra of compounds
after several months of their synthesis, the new spectra indicating the exclusive presence of the II3
tautomer in the solution. This fact suggests the greater stability of this form, which was the same
obtained by crystallisation from solutions of 21 (see section below).
1
3
C-NMR spectra in CDCl solution support the above results. The presence of the monoenol
3
and dienol forms in the freshly prepared compounds is established from the characteristic shifts of
the carbonyl signals of the different tautomers as well as by the splitting of the aromatic signals.
Again, after several months from the synthesis of the compounds, their spectra indicate the
presence of a sole tautomer.
1
3
On the other hand, in the solid state, the C-NMR spectra exhibited slightly different spectral
features. So, only signals from the two dienol forms (II2 and II3) are observed initially, thus
indicating the absence of the monoenol tautomer (II1). Further NMR investigation several months
after preparation shows that the original mixture evolved to the exclusive presence of the tautomer
II3, this fact again confirming that this must be the most stable tautomer among all the possible
ones.
1
3
Table 4 indicates the characteristic signals observed in the C-NMR study in CDCl solution
3
and in the solid state for a recently prepared sample of compound 24 selected as a representative
example. A detail of the chemical shifts is included in Fig S3†.
IR spectra of all compounds were recorded in KBr at room temperature and, in all cases
confirm the exclusive presence of dienol forms in the solid state, as indicated by the absorption
-
1
bands at ca. 3100, 1680 and 1570 cm from the ν(OH), ν(CO) and ν(CC) vibrations, respectively,
-
1
of the enol forms. The lack of a band at ca. 1720 cm involves the absence of keto groups from
the triketo and monoenol tautomers [24]. On the other hand, from the low frequency value of the
ν
(OH) hydroxyl stretching, strong hydrogen interactions are suggested, as it is expected in dienol
forms.
.2.2. Crystal structure of compound [HO OO H] 21
In order to clarify the tautomeric form in the solid state as well as the structural influence of
the triketone ligands for coordinative purposes, we were involved in determining the crystalline
R(1)
R(1)
3
1
0

ACCEPTED MANUSCRIPT
structure of these types of compounds. However, all attempts to grow crystals of sufficient quality
for single-crystal X-ray analysis were unsuccessful. Then, we synthesised and crystallised a
related derivative containing 4-methoxyphenyl substituents at the 1- and 5-position of the
pentanotrione core, which, as expected, should crystallise more easily than compounds containing
R(1)
R(1)
long-chained moieties. Thus, crystals of [HO OO H] were obtained by slow evaporation of a
dilute chloroform solution of the mentioned compound and characterised by single crystal X-ray
diffraction.
R(1)
R(1)
The molecular structure of [HO OO H] 21 is depicted in Fig. 2. Table 5 lists selected bond
distances and angles. The compound crystallises in the orthorhombic space group
four molecules per unit cell.
P2 2 2 , with
1 1 1
The molecular structure corresponds to the dienol symmetric form II3 described in the
tautomeric mixture in solution. This tautomer exhibits two enol groups and a keto group at the
terminal and central carbon atoms of the ligand backbone, respectively, as deduced from the C –
C and C – O distances and the bond angles (Table 5). The presence of non-symmetrical hydrogen
bonds between the enol and keto groups with the hydrogen atoms located closer to the oxygen
atoms at the terminal positions (Table 6) is also an indicator of this dienol tautomer.
The C – C bond distances in the benzene rings and also in the triketone core are consistent
with a delocalised
π system. The shape of the molecule is only slightly deviated from planarity.
The central backbone is almost planar, with the two planes described by the O3C5C4C3O2 and
O2C3C2C1O1 atoms forming a dihedral angle of 5.7(1)º. At the same time, the phenyl groups are
deviated from each of these planes 8.2(1) and 3.2(1)º, respectively.
The molecular packing can be described as corrugated layers in which molecules are packed in
a
zigzag disposition. The layers are formed through non-conventional C – O intermolecular
interactions between the C12 and O4 atoms of neighbouring molecules, with a distance of 3.45(1)
Å (symmetry operation;
between layers (Fig. 3).
−x
+ ½ + 1,
−y
+ 1,
z
+ ½), but no significant interactions are found
R(n)x
R(n)x
3
.2.3. Difluoroboron complexes [BF (HO
OO
)] I (x = 1, n = 4, 6, 8, 10, 12, 14, 16, 18; x
2
=
2, 3, n = 12, 14, 16, 18)
1
19
13
H, F and C-NMR spectra of all difluoroboron-β,δ-triketonate complexes
been recorded in CDCl solution at room temperature.
I (1-20) have
3
1
1

ACCEPTED MANUSCRIPT
The presence of an uncoordinated free oxygen atom not bonded to the BF fragment in the
2
triketonate backbone leads to the two posible keto-enol tautomers, as depicted in Fig. 4. This
result was found for Ia and Ic complexes, as it was deduced by the splitting of the triketonate core
1
13
and aromatic signals observed in the H and C-NMR spectra, as well as by the presence of two
1
9
fluorine signal peaks in the F-NMR measurements. By contrast, spectra of Ib and Id compounds
indicate the presence of the keto tautomer exclusively.
1
For Ia and Ic compounds, exhibing the keto-enol equilibrium, three H-NMR methyne signals
in the range of
δ 5.92 – 6.64 ppm, one methylene signal at 4.15 ppm and one hydroxyl signal at
1
1.40 ppm are consistent with the presence of both enol and keto tautomers (Fig. 4). A strong
coordination to the BF fragment is deduced from the shift of the methyne central protons signals
2
of the triketonate core to lower fields related to the free neutral β,δ-triketones. In addition, the
large shift to upper fields of the hydroxyl peak suggests that this group should be involved in a
strong hydrogen interaction with the neighbouring fluorine atoms in the enol form.
1
9
Two fluorine signals in the F-NMR spectra related with the two tautomers are observed in a
range of 139 to 143 ppm. Each of them is duplicated and consists in two sharp lines with an
intensity ratio of 1:4, in agreement to the natural abundance of the two boron isotopes, B and
−
−
1
0
1
1
B.
The C-NMR spectra show carbonyl, methyne and methylene carbon signals in their
1
3
characteristic regions, being low-field shifted with respect to those of the neutral ligands by effect
of the coordination.
Table 7 recovers the most characteristic NMR signals of
complete assignation is shown in figures S2 and S4†.
3 as a representative example. A
In the keto-enol mixture of Ia and Ic, the enol tautomer was found in the highest percentage as
1
deduced from the intensity ratio of the H-NMR methyne/methylene signals (60:40). By contrast,
derivatives of Ib and Id types showed a single peak due to the keto form, this fact can be related
with the enhanced steric effect produced by the presence of two or three adjacent chains in the
phenyl substituents. Therefore, as the keto form is less restrained to planarity, the equilibrium
should be shifted to this tautomer, allowing a better accommodation of the voluminous groups.
IR spectra of the complexes in the solid state exhibit characteristic bands of enolate groups at
-
1
ca. 1540 and 1500 cm associated to the ν(CO) and ν(CC) stretching absorptions. These bands are
1
2

ACCEPTED MANUSCRIPT
noticeably bathochromically shifted with respect to those of the neutral ligand, so indicating the
1
strong coordinative interaction with the BF fragment, in agreement with the H-NMR results.
2
-1
The carbonyl band at ca. 1595 cm from the neutral enol group along with the absence of a keto
-1
band at ca. 1720 cm indicate that the enol form is the unique tautomer in the solid state for all
-
1
the boron derivatives. Bands at ca. 1380 and 1030 cm from the ν(BO) and ν(BF) stretching
vibrations complete the main absorptions of the complexes [24].
3
.3. Thermal behaviour
Thermal properties of the free triketone compounds II as well as those of their corresponding
boron complexes were studied by polarised optical microscopy (POM), differential scanning
I
calorimetry (DSC) and X-ray diffraction at variable temperature, the latter technique used only in
representative complexes. Tables 8 and 9 list the phase transition temperatures and their
corresponding enthalpies established by DSC.
R(n)x
R(n)x
3
.3.1. Triketone derivatives [HO
OO
H] II (x = 1, n = 4, 6, 8, 10, 12, 14, 16, 18; x = 2, 3,
n = 12, 14, 16, 18)
None of the new triketone ligands II exhibited liquid crystalline behaviour in agreement to the
absence of mesomorphism of the related derivatives previously reported [13,18]. However, for the
latter the thermal results had not been explicitly mentioned and so, we were interested to compile
the melting temperatures of all the ligands used in this work. Phase transition temperatures on
heating of compounds II are depicted in Table 8.
POM observations show direct melting from the solid to the isotropic liquid at a temperature
range depending on the degree of alkyloxy substitution at the aromatic groups. On the other hand,
several solid phases are observed prior to the melting.
Variations in the melting temperatures were analysed as depending on the number, position
and length of the alkyl chains attached to the phenyl substituents (Fig 5). Polycatenar ligands IIc
-
IId show their melting transition at lower temperatures that those of the monocatenar ligands IIa
,
this fact suggesting a less effective packing in the solid state for the former ones. In addition, the
results reveal a quite different behaviour for each family of compounds, suggesting that a slight
change in the molecular structure translates in to a large modification of their properties.
1
3

ACCEPTED MANUSCRIPT
For compounds of families IIa, with monocatenar aryl groups, and IIc, carrying two chains at
the meta positions of the aryl group, the melting temperatures do not seem to be notably modified
by increasing the alkyl chain length, so suggesting that interpenetration between chains is not
favoured, as an increase in the melting point by increasing the alkyl length should be expected. By
contrast, the opposite behaviour was found for compounds of series IIb carriers of adjacent chains
at each aromatic substituent group, at difference of the alternating positions they occupy in
compounds IIc. In this series IIb a decrease of the melting temperatures by increasing the alkyl
chain length was observed. Therefore, we propose a higher packing efficiency potentially due to
lower mobility of the chains, which should be restricted in relation to that of the compounds IIa
and IIc. The decrease of the melting temperatures by increasing the chain length could be
explained on the basis of less effective alkyl interpenetration when longer chains are present.
Finally, opposite results were observed for the most substituted compounds IId which should
exhibit the lowest melting temperatures. However, the melting occurs at higher temperatures by
growing the alkyl length, thus indicating that the alkyl chain interdigitation effect is balanced by
the increment of the Van der Waals forces. In addition, lower molecular mobility would lead to a
more effective packing in the solid, thus leading to higher values of the melting temperatures in
comparison to those of family IIc
.
R(n)x
R(n)x
3
1
.3.2. Difluoroboron triketonate complexes [BF (HO
OO
)] I (x = 1, n = 4, 6, 8, 10, 12,
2
4, 16, 18; x = 2, 3, n = 12, 14, 16, 18)
All the complexes of families Ib
,
Ic and Id as well as compounds 7 and 8 from Ia exhibit
enantiotropic liquid crystal behaviour as revealed by POM observations and DSC thermograms.
The thermal properties of all the new adducts are collected in Table 9. In Figs. 6a, 7a, 8a and 9a
the stability range of the mesophases is depicted for a better quality of discussion.
It is interesting to remark that each family of compounds exhibit different mesophases, this
fact being related to their corresponding molecular geometry. Thus, a smectogenic behaviour has
been found for compounds Ia, containing monocatenar alkyoxyphenyl-substituted triketonate
ligands, and Ib, carriers of two adjacent alkyl chains at each aromatic ring of the ligand, while the
discotic nature of the derivatives Ic, with two separate alkyl chains, and Id, with three chains in
each aromatic group, leads to discotic lamellar and columnar mesophases, respectively.
1
4

ACCEPTED MANUSCRIPT
For compounds of family Ia, only those with long-tailed substituents
7
,
8
(
n
= 16, 18)
< 14) were
exhibited liquid crystal behaviour, while those derivatives with shorter alkyl chains (
n
not mesomorphic, showing crystalline polymorphism and a direct melt to the isotropic liquid (Fig.
a). Compounds and exhibit enantiotropic SmC mesophases, as deduced from the POM
6
7
8
observed characteristic Schlieren texture [25] (Fig. 6b). Their DSC thermograms show, on
heating, an endothermic peak related to a solid-solid phase transition followed by two overlapped
peaks from the solid-mesophase and mesophase-isotropic liquid transitions, in agreement with the
POM observations. On cooling, the mesophase was undoubtedly identified by the characteristic
Schlieren texture, previous to the crystallisation, but in the DSC traces only a wide peak
corresponding to both overlapped transitions was observed. The mesophase ranged between 114 –
1
22 ºC. The melting temperatures were the same for both 7 and 8 derivatives, but 8 exhibits a
higher clearing temperature, so indicating a slight expansion of the mesophase range by
increasing the alkyl length.
As an interesting feature it can be mentioned that, contrarily to the results reported by us on
monocatenar symmetrically alkoxyphenyl-substituted β-diketonate containing difluoroboron
complexes, which were not mesomorphic [6], the new long-tail monocatenar aryl-substituted
triketonate derivatives behave as liquid crystals. This fact can be explained on the basis of the
requirement of an appropriate width-to-length molecular ratio, which seems to be favoured with
the longest chains and the most extensive core of the triketones, besides of their increased and
extensive conjugation.
Complexes belonging to series Ib (9-12) also present a smectogenic mesomorphic behaviour.
The melting transition was easily detected by POM on heating by the appearance of a phase with
intense birefringence and high mobility, which happens in the range of 27 – 65 ºC depending on
the alkyl length. The presence of focal-conic domains and oily strikes on the borders of the
preparation agrees with its assignation as a SmA mesophase [25] (Fig. 7b). By increasing the
temperature, the mesophase solidifies in all cases between 54 and 75 ºC giving rise to the break-
up of the mesophase texture as well as a loss of the mobility. After the solidification, a melting to
the isotropic liquid occurs. DSC thermograms agree with this behaviour showing an exothermic
peak followed up by an endothermic peak related to the melting of the solid phase. On cooling
from the isotropic liquid, the mesophases were poorly detected by POM prior to the crystallisation
1
5

ACCEPTED MANUSCRIPT
and, according to that, in the DSC thermograms only a peak related to crystallisation could be
observed, both facts indicating that the mesophase formation involves a very slow process. As
depicted in Fig. 7a, the increase of the alkyl chains length leads to higher values of the phase
transition temperatures, being the stability range of about 10 ºC for all complexes with the
exception of complex with 12 carbon atoms at the alkyl chains, which, due to its noticeable low
value of the melting temperature the mesophase shows a stability range of 27 ºC before the
crystallisation.
Concerning compounds of family Ic, all of them 13-16 exhibit a similar liquid crystal
-
1
behaviour. Observation by POM at the typical rate of 10 Kmin failed to identify the mesophase
structure since only a viscous appearance was observed on heating before the isotropic liquid was
-1
formed. However, further experiments using a rate of 1 Kmin allowed us to observe the
appearance of a birefringent fibrous texture which was maintained until the isotropisation of the
sample. On cooling from the isotropic liquid, the compounds were allowed to cool slowly at 1
-
1
Kmin in steps of 5 ºC and the temperature maintained in each step for 3 hours until appearance
of the mesophase, thus allowing us to observe the formation of a stable broken fan-like texture
(
Fig. 8b) characteristic of discotic lamellar structures (Col ) [26,27]. Finally, after 12 hours at
L
room temperature, crystallisation of the sample was shown.
-
1
DSC thermograms made up at 10 Kmin only show a wide endothermic peak on heating
corresponding to both melting and clearing overlapped transitions. Due to the low phase transition
temperatures and the slow cooling rate needed to detect the mesophase on cooling, no further
DSC experiences were able to be attempted in order to confirm the behaviour exhibited by POM.
Bar diagram of Fig. 8a shows the stability range of the Col mesophase as a function of the
L
alkyl length. It is remarkable that, the mesophases appear at temperatures of 50 – 60 ºC and are
maintained until 61 – 77 ºC. As usual, the Van der Waals forces are enhanced by increasing the
alkyl length leading to an increment of the melting and clearing temperatures. The stability ranges
increase following the same tendency, thus the widest range being found for compound 16 with
n
=
18 carbon atoms at the alkyl chains.
The POM observations of compounds Id
(
17
-
20) indicate enantiotropic mesomorphism. On
heating from the solid, a birefringent phase is formed, with the exception of the compound
bearing the shortest chains 17 = 12) that exhibits this phase at room temperature. On cooling
(
n
1
6

ACCEPTED MANUSCRIPT
from the isotropic phase, a dendritic texture with linear birefringent defects could be observed,
subsequently giving rise to a mosaic texture with pseudo-focal conic regions, which is typical of
the hexagonal columnar mesophases [26,28,29] (Fig. 9b). DSC thermograms confirm this
behaviour, showing the corresponding endothermic peaks on heating as well as the exothermic
ones related to the reverse phase transitions. As depicted in the bar diagram (Fig. 9a), both
melting and clearing temperatures are increased by growing the alkyl length, as was observed for
the analogue Ia-Ic complexes. Also the stability range tends to decrease, the best results being
achieved for the shorter-chain derivative of the family which exhibits the Col mesophase at room
h
temperature and with a range of ca. 35 ºC.
3
.3.3. Comparative results from thermal behaviour of Ia-Id complexes
By comparing the mesomorphism of the different families of boron-complexes, several
conclusions can be established.
Firstly, by comparing the results of the new complexes to those previously reported on related
β-diketonate complexes [6-9], it has been established that the presence of the larger molecular
rigid core, involving an expansion of the π conjugation and the molecular polarisation has been a
useful tool to induce mesomorphism in the difluoroboron complexes. Therefore, the efficiency of
the β,δ-triketonate ligand core to achieve mesomorphic complexes has been proved.
On the other hand, it should be mentioned that in the β-diketonate systems, the molecular
asymmetry related to the length at the chains at both substituents, and/or the presence of a
different number of alkyl chains at both phenyl rings was a requirement in order to achieve
mesomorphic complexes [7-9]. However, this requirement was not necessary when using β,δ-
triketonate ligands towards the BF group, in which liquid crystal behaviour was always achieved,
2
even at room temperature.
We have also been able to modulate the type of mesomorphism by the controlled design of the
ligands, the molecular geometry being the main feature that governs the type of mesophase. That
is imposed by the number and position of the alkyl chains at the aromatic substituents at the 1-
and 5-positions of the triketone core. Thus, compounds bearing one chain or two adjacent chains
in each phenyl group display smectogenic behaviour, in agreement to the rod-like molecular
shape. By contrast, disk-shaped complexes are achieved when two alternate or three alkyl chains
1
7

ACCEPTED MANUSCRIPT
are attached to the aromatic substituents. As a consequence, the complexes exhibit lamellar
discotic or hexagonal columnar mesophases, respectively.
It is also of interest to compare the phase transition temperatures of the complexes of the four
families in order to establish which of them present the best liquid crystal properties. So, while the
monocatenar derivatives Ia show the higher melting and clearing temperatures, and the narrower
stability ranges of the mesophase, opposite results were found for complexes Ib-Id. Thus, it
seems that the use of polycatenar systems is a determinant factor to achieve lower transition
temperatures, thus giving rise to more suitable materials for technological applications.
Among the polycatenar compounds Ib-Id, it is noticeable that in each family, the melting and
clearing temperatures were higher for those with longer flexible chains, determining that the
presence of shorter chains appears as a requirement to achieve mesogens at nearly room
temperature.
On the other hand, the melting temperatures are in a similar range for compounds Ib and Id
,
while those of complexes Ic are slightly higher. In addition, since the clearing temperatures are
much higher for members of family Id and the mesophases of Ib suffer crystallisation upon
heating, the range of the mesophase is extended about 10 ºC, 15 ºC and 30 ºC for complexes Ib
,
Ic and Id, respectively. Therefore, it seems that the discotic six alkyl chains peripherally-
decorated complexes present the best liquid crystalline properties.
3
.3.4. Temperature-dependent powder X-ray diffraction studie
Variable-temperature powder X-ray diffraction experiments were performed on selected
mesomorphic compounds from Ia Id to confirm the structure of the mesophases observed by
s
-
POM. The results are summarised in Table 10.
Fig. 10 depicts the diffractogram of compound
7
, selected from Ia, at 115 ºC on heating,
temperature at which the mesophase have been proved to be present. Two sharp peaks in a 1 : 1/2
ratio indexed as the (001) and (002) reflections are indicative of a lamellar structure. In addition, a
broad halo in the medium-angle region at ca. 4.5 Å corresponds to the liquid-like order of the
molten alkyl chains. This pattern appears in both heating and cooling processes, indicating the
enantiotropic behaviour of the compounds. From the above result as well as the observed
Schlieren texture, a SmC structure was assigned for all the mesophases of Ia compounds.
1
8

ACCEPTED MANUSCRIPT
The identification of the smectic mesophase of compounds belonging to family Ib has been
harder since the mesophase subsequently crystallises upon heating. This feature prevented us
obtaining diffractograms free of peaks from a solid phase. In particular, Fig. 11 shows the
diffractogram of compound 12 at 68 ºC, temperature where the mesophase could be observed
although overlapped with a crystalline phase which starts to be formed. Four sharp peaks with a
d
-spacing ratio of 1 : 1/2 : 1/3 : 1/4 are associated to the (001), (002), (003) and (004) reflections,
thus confirming its smectogenic behaviour previously established for this family of derivatives.
The ColL mesophase of compounds Ic was undoubtedly confirmed from the diffraction
patterns of compounds 15 and 16 recorded at temperatures of mesophase existence. In both cases,
the diffractograms show, both the heating and cooling, two or three peaks from the (001), (002)
and (003) reflections with a d-spacing ratio of 1 : 1/2 : 1/3 of a lamellar phase. In addition, a
diffuse band indicating the liquid-like order of the molten chains was localised around 4.5 Å.
Finally, the Col nature of the mesophase of compounds Id could be extrapolated from the
h
diffraction patterns of compounds 19 and 20 recorded at temperatures at which the liquid crystal
state had been reached (Fig. 12). These exhibit a strong fundamental maximum in the low angle
region from the (100) reflection, followed up by a set of weak peaks with a
d
-spacing ratio of 1 :
1
/√3 : 1/√4 : 1/√7, corresponding to the (110), (200) and (210) reflections of a hexagonal
columnar phase. In addition, a broad band from the (001) reflection with a d-spacing value of 3.4
Å is consistent with a π-π stacking between mesogenic units of each column. The broad diffuse
band at ca. 4.5 Å is again related to the fluid order of the alkyl chains in the mesophase.
3
.4. Photophysical studies
The photophysical studies of the free ligands II and their BF complexes
I have been carried
2
out in such a way that factors related to the number, position and length of the alkyl chains were
analysed to establish their influence on the luminescent properties. Thus, the luminescent
behaviour of ligands 24 and 26
(
IIa), 31 and 32
(
IIb), 36
(
IIc), 40 and 41
(
IId) as well as their
Id) has been fully
counterpart complexes
studied.
3
and
5
(
Ia), 10 and 11
(
Ib), 15
(
Ic) and 19 and 20 (
1
9

ACCEPTED MANUSCRIPT
3
3
.4.1. Photophysical characterisation
The absorption, emission and excitation spectra of the
40 41) and their complexes 10 11 15 19
β
,δ-triketone ligands II
(24, 26, 31, 32,
6
,
,
I
(
3
,
5
,
,
,
,
, 20) were measured in dichloromethane
-5
-6
solutions (ca. 10 – 10 M) at 295 K. The emission spectra in the solid state of all compounds
were also recorded. The photophysical data obtained are gathered in Table 11.
The
characteristic of the electronic
maximum in the difluoro(triketonate)boron complexes
β
,δ-triketone derivatives II show absorption bands centred at ca. 391-409 nm, which are
* transition of chelating -ketonate ligands [5-9,30], while the
is centred at ca. 433-474 nm.
π
-
π
β
I
Regarding the emission, it was observed that complexation of the free ligands to the BF2
fragment led to a general red spectral shift, the spectra showing in all cases a broad band with an
unresolved vibronic substructure at ca. 418-470 nm for II and at ca. 483-525 nm for
On the other hand, no significant spectral variations were produced by the increasing of the
carbon atoms in the lateral-side chains in complexes Ia) and 10 11 Ib) as well as in the
ligands 24 26 IIa) and 40 41 IId).
Concerning the ligands, the increase of the number of alkyl chains from two to four or six in
26 IIa), 31 32 IIb) and 36 IIc), and 40 41 IId), respectively, practically does not affect
I.
3
,
5
(
,
(
,
(
,
(
2
4
,
(
,
(
(
,
(
the ground state of the molecules. By contrast, in the excited state the position of the maximum
appears to be dependent on the number of alkyl chains at the aryl substituents, as visible red shifts
from 442 nm to ca. 450 nm and to ca. 470 nm of the emission bands are detected in the above
compounds (24, 26; 36; 40, 41). This fact can be due to the strong electron-donating effect of the
alkyl groups produced by the increase in the number of alkyloxy chains. The degree of
substitution lowers the energy gap and changes the electronic distribution. This aspect had already
been observed in our previous difluoroboron alkyloxyphenyl-β-diketonate systems [6-9].
Related to the boron complexes, the increment of the number of alkyl chains from two in
Ia) to four in 15 Ic) again does not produce significant variations while the presence of six
chains in 19 and 20 Id) led to a red shift in the absorption and emission bands, this feature being
3 and
5
(
(
(
explained on the basis of the higher electron donation towards the BF group.
2
On the other hand, the position of the alkyl groups in the compounds containing four chains
(
31
, 32 (IIb), 36 (IIc), 10, 11 (Ib), 15 (Ic)) is also a factor that modifies the spectral behaviour.
Derivatives of Ib and IIb types, in which both chains at each substituent are adjacent, show
2
0

ACCEPTED MANUSCRIPT
absorption bands at higher energy compared to those of Ic and IIc types with separated chains
(
Table 11), concluding that the electron donor effect of the alkyl groups is somehow enhanced
when the two groups are in meta relationship one to each other.
The relative fluorescence quantum yield was calculated using quinine sulphate and acridine
yellow as a standard for ligands and complexes, respectively. All the studied ligands II show low
fluorescence quantum yields (
complexes , the highest values being observed for
.67-0.71). Surprisingly, the position of the chains seems to be a more influent factor to decrease
φ) at ca. 0.003-0.05. However those increased in the boron
I
3
,
5
(
Ia = 0.34-0.36) and 10 11 (Ib; φ =
;
φ
,
0
the quantum yields than the number of alkyl chains attached to the aromatic groups, as it was
expected. As the quantum yield is decreased in complexes Ic and Id, we suggest that the presence
of a flexible group at the 5-position of the phenyl ring tends to decrease the fluorescence
efficiency of the molecule. This fact can be probably attributed to a higher flexibility in the
excited state which would increase the non-radiative decay processes [6].
Regarding the emission in the solid state, a broad band peaked at ca. 487-520 nm and at ca
.
5
43-600 nm is observed for the ,δ-triketone derivatives II and boron complexes , respectively.
β
I
All emission bands observed in the solid state spectra are bathochromically shifted with respect to
the ones measured from the spectra in solution. As a representative example, Fig. 13 depicts the
full photophysical characterisation of
3 (Ia) as well as that of its precursor β,δ-triketone 24.
4
.2. Sensorial ability towards metal ions
In order to explore the potential sensorial ability of the β,δ-triketone derivatives II and that of
their boron complexes
I
, several metal titrations followed by absorption and emission
+
measurements were performed in dichloromethane solution with the increasing addition of Na ,
2
+
2+
2+
2+
2+
2+
2+
2+
3+
3+
Ca , Zn , Cd , Pb , Cu , Ni , Co , Hg , Al and Cr in dry acetonitrile. For that purpose,
the metal titrations were carried out with the ligands 24 IIa) and 32 IIb), and their boron
complexes Ia) and 11 Ib), as representative examples. Significant spectral changes were
observed with the addition of Cu and Hg while no spectral interaction was detected with Na ,
(
(
3
(
(
2
+
2+
+
2
+
2+
2+
2+
3+
3+
2+
2+
Ca , Zn , Cd , Pb , Al and Cr metal ions. Only, in the presence of Ni and Co , a
quenching of about 5-10% was observed.
2
1

ACCEPTED MANUSCRIPT
The most significant spectral variations in the absorption and the emission were observed in
2
+
compound 24 by addition of Cu metal ion. In the absorption spectra a decrease at 403 nm
followed by an increase in the absorbance at ca. 345 nm was observed. In the emission spectra, a
pronounced quenching in emission intensity at 442 nm is verified, as depicted in Fig. 14.
2
+
The ligand 24 and the complex
3 were the only species that interact with Hg metal ion,
whereas their absorption and emission spectra are depicted in Fig. 15. In compound 24, a decrease
in the absorbance at 403 nm is observed, as well as a quenching in the emission intensity at 442
nm. Regarding complex 3, a decrease at 465 nm and an increase at 375 nm are noticed in the
absorption spectra. Similarly, a quenching in the emission intensity is detected at 490 nm.
Regarding compound 32 and its complex 11, no spectral changes were observed upon the
2
+
2+
addition of Cu and Hg metal ions. This fact could be related to the presence of one more
alkyloxy chain nearby to the other.
In order to postulate the stoichiometry of the metal complexes formed in solution, the stability
2
+
2+
2+
constants for compounds 24/Cu , 24/Hg and
3/Hg were determined using the HypSpec
R(8)
R(8)
program [31]. The results suggest the formation of type L M complexes (L = O OO ) with a
2
2
2
+
2
:2 (ligand to metal) stoichiometry for the interaction of
β
,δ-triketone 24 with the metal ions Cu
2
+
2+
2+
and Hg as indicated by the values of log
Concerning the boron complex
with a stoichiometry of two boron complexes for one Hg metal ion from the obtained value of
β (Cu ) = 21.51 ± 0.04 and logβ (Hg ) = 18.44 ± 0.01.
R(8)
R(8)
3
, a L’ M type interaction was postulated (L’ = BF (O OO )),
2 2
2
+
2
+
log
β
(Hg ) = 11.94 ± 0.02.
Compound 24 is able to detect the minimal amount of 1.33 µM and 1.00 µM by absorption,
2
+
2+
and 0.26 µM and 0.23 µM by emission of Cu and Hg metal ions, respectively. Regarding
2
+
complex 3, the minimal amount of 0.35 µM and 0.11 µM of Hg metal ion was determined.
2
+
In the case of the Cu -to-ligand interaction, the 2:2 stoichiometry was confirmed by the
spectroscopic characterisation of the solid copper(II) complex of 24, specifically synthesised
according to a similar procedure described for the homologous polycatenar-containing triketonate
R(8)
R(8)
complexes [13] and characterised as a binuclear complex of the formula [Cu (O OO ) ] 42
2
2
(
see experimental section), and for which absorption and emission spectra were measured in THF
solution (Fig. 16). The copper complex shows an absorption band at 422 nm, with an extinction
4
-1
-1
molar coefficient of 6.9 × 10 M Lcm . As expected, a total absence of emission was observed.
2
2

ACCEPTED MANUSCRIPT
Unfortunately, the complex was not soluble in acetonitrile, chloroform or dichloromethane,
preventing further studies.
4
. Conclusions
Novel series of difluoroboron alkyloxyphenyl-β,δ-triketonate complexes containing peripheral
substituted alkyl chains have been prepared and characterised as highly luminescent liquid crystal
materials, exhibiting chemosensor ability.
The new complexes, exhibit a keto-enol tautomeric equilibrium, which was depending on the
peripheral substitution. Thermal studies reveal that, in contrast to the absence of mesomorphism
of the related β-diketonate complexes, all the new β,δ-triketonate derivatives behave as liquid
crystal materials. The larger molecular core, the expansion of the π conjugation and the increased
polarisation were proved as efficient factors to induce mesomorphism.
The smectic or columnar mesomorphism of the complexes was related to the molecular
geometry, which was determined by the number and position of the peripheral flexible chains at
the ligand, six-chained decorated complexes achieving the wider stability ranges of the
mesophases, which even occur at room temperature.
The fluorescent emission was also found to be dependent on the substitution, showing a red shift
by increasing the number of alkyl chains attached to the aromatic substituents. Likewise, the
higher fluorescence efficiency was observed for compounds having restricted mobility in the
2
+
2+
excited state. Chemosensor ability of ligands and complexes towards Cu and Hg metal ions
has been established.
In summary, modulation of the luminescent and liquid crystal properties of new boron
complexes can be achieved by controlling the molecular peripheral substitution, so giving rise to
polyfunctional soft materials which might offer new opportunities of research in technological
fields.
Acknowledgements
We thank Dr. Rosa Claramunt and Pilar Cornago, of National University of Distance Education
(
Spain) for their collaboration on the solid state NMR experiments. We are grateful to the
2
3

ACCEPTED MANUSCRIPT
Ministerio de Economía y Competitividad (Spain), project CTQ2011-25172. I. S. thanks the
Ministerio de Educación (Spain) for the predoctoral scholarship from the FPU program. E.
Oliveira acknowledges the post-doctoral grant from Fundação para a Ciência e a Tecnologia
(
(
(
Portugal) SFRH/BPD/72557/2010. A. F. Lodeiro thanks Fundação para a Ciência e a Tecnologia
Portugal) for the PhD grant number SFRH/BD/52528/2014. C. L. thanks to LAQV/REQUIMTE
UID/QUI/50006/2013), UCIBIO/REQUIMTE (UID/Multi/04378/2013) and PROTEOMASS
Scientific Society (Portugal) for funding.
Electronic Supplementary Information
Electronic Supplementary Information (ESI) available: Full characterisation of the boron
compounds, the corresponding triketones, and Figs. S1-S4 showing the full NMR assignation.
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[
(
[
(
[
2
8

ACCEPTED MANUSCRIPT
Table 1
[BF (HOR(n)2OOR(n)2)]
2
[
BF
2
(HOR(n)OOR(n))]
n
no.
n
no.
n
no.
9
F
F
F
F
4
1
2
3
4
12
14
16
18
5
6
7
8
12
B
B
O
O
OH
O
O
OH
6
8
14 10
16 11
H
H
2n+1
2n+1
C
C
n
n
O
O
OC
OC
n
n
H
H
2n+1
H
2n+1
C
n
O
OC
n
H
2n+1 10
2n+1 18 12
Ia
Ib
[BF
2
(HOR'(n)2OOR'(n)2)]
n
no.
[BF
2
(HOR(n)3OOR(n)3)]
n
no.
F
F
F
F
B
B
O
O
OH
12 13
O
O
OH
12 17
14 18
16 19
H
2n+1
C
n
O
OC
n
H
2n+1
H
2n+1
2n+1
C
n
O
O
OC
n
H
H
2n+1
2n+1
14
14
15
16
H
C
n
OC
n
OC
n
H
2n+1
OC
n
H
2n+1
OC
n
H
2n+1
OC
n
H
2n+1
18
16
18 20
Ic
Id
n
no.
n
no.
n
no.
HOR(n)OOR(n)H]
[HOR(n)2OOR(n)2H]
[
a
1
4
6
8
21
22
12 30
14 31
16 32
18 33
OH
O
OH
OH
O
OH
a
a
12 26
14 27
16 28
H2n+1C O
n
OC H
n
2n+1
a
a
a
a ,b
23
H2n+1CnO
OCnH2n+1
a
H
2n+1
C
n
O
n
OC H
2n+1
24
IIa
a
IIb
10 25
18 29
n
no.
n
no.
[
HOR'(n)2OOR'(n)2H]
[HOR(n)3OOR(n)3H]
OH
O
OH
OH
O
OH
b
1
2
4
6
34
35
36
12 38
14 39
16 40
H
2n+1
C
n
O
OC
n
H
2n+1
H
2n+1
2n+1
C
n
O
OC
OC
n
H
2n+1
b
b
1
1
H
C
n
O
n
H
2n+1
OC
n
H
2n+1
OC
n
H
2n+1
18 37
OC
n
H
2n+1
OC
n
H
2n+1
18 41
IIc
IId
a
b
Previusly reported in ref. [18]. Previously reported in ref. [13].
2
9