Wide-Range Columnar and Lamellar Photoluminescent Liquid-Crystalline Lanthanide Complexes with Mesogenic 4-Pyridone Derivatives

Article abstract of DOI:10.1002/chem.201801781

A series of liquid crystals with various lanthanide ions (Eu^III, Sm^III, and Tb^III) was designed and prepared starting from the corresponding lanthanide nitrates and N-alkylated 4-pyridone derivatives bearing mesogenic 3,4,

Full text of DOI:10.1002/chem.201801781

A Journal of
Accepted Article
Title: Novel wide-range columnar and lamellar photoluminescent liquid
crystalline lanthanides complexes with mesogenic 4-pyridone
derivatives
Authors: Viorel Circu, Laura F Chiriac, Iuliana Pasuk, Mihai Secu, and
Marin Micutz
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Novel wide-range columnar and lamellar photoluminescent liquid
crystalline lanthanides complexes with mesogenic 4-pyridone
derivatives
Laura F. Chiriac,[a] Iuliana Pasuk,[b] Mihail Secu[b], Marin Micutz[c] and Viorel Cîrcu*[a]
Abstract: A series of novel liquid crystals with various lanthanide
ions (Eu(III), Sm(III) and Tb(III)) was designed and prepared starting
from corresponding lanthanide nitrates and N-alkylated 4-pyridones
derivatives with mesogenic 3,4,5-tris(alkyloxy)benzyl moieties (alkyl
= hexyl, octyl, decyl, dodecyl, tetradecyl or hexadecyl). These new
lanthanidomesogens were investigated for their mesogenic
properties by a combination of differential scanning calorimetry
(DSC), polarizing optical microscopy (POM) and temperature
dependent powder X-ray diffraction (XRD). Additionally, their thermal
stability was assessed by thermogravimetric analysis (TG). All these
complexes display an enantiotropic liquid crystalline behavior with
either a lamellar (SmA), in the case of shorter chains analogues (6
and 8 carbon atoms) or a hexagonal columnar (Colh) phase, for
complexes with higher number of carbon atoms (12, 14 and 16),
role and they are regarded as a promising class of light-emitting
liquid crystals with excellent emission properties such as the
narrow emission lines of high purity colors, the choice of tuning
the emission color by changing the lanthanide ion or the
possibility to create luminescent materials with polarized
emission in the liquid crystal (LC) state.[9-22] Due to their wide-
ranging applications, from telecommunications to life sciences,
development of novel lanthanide-based mesogens is much
needed both for fundamental research and applications.[23-26]
Among the f-block metals, the Eu(III) and Tb(III) complexes were
found to be the strongest emitters, red and green respectively,
while the Sm(III) compounds were less studied due to their
weaker luminescence intensity in comparison to Eu(III) or Tb(III)
compounds.[27, 28]
assigned on the basis of their characteristic texture and XRD studies. The design of advanced material based on lanthanide ion with
For complexes with an intermediate number of carbon atoms in the
side chains (10), both a lamellar phase at lower temperatures and a
Colh phase at higher temperatures were evidenced. In solid state, all
these complexes show characteristic emissions assigned to the
corresponding lanthanide ion. In addition, the luminescence decay
curves showed single exponential decays with the characteristic
times in the ms range (0.75-0.90 ms for Eu(III), 0.045-0.060 ms for
Sm(III) and 0.75-1.05 ms for Tb(III), respectively).
higher coordination number (usually 8 or 9) in a highly
anisotropic molecule presenting both mesomorphism and
emission properties, represents a challenge for the current
research. Furthermore, the fact that the trivalent lanthanide ions
absorb weakly the radiation due to their forbidden f→f transitions
poses additional challenges to the development of novel
lantanidomesogens with interesting photophysical properties.
Indeed, the low emission of the lanthanide ions can be improved
by using an organic ligand with a large absorption coefficient
that play the antenna role, by absorbing light and transfer
excitation energy to the emitting levels of the lanthanide
ion.[29,30] Thus, the choice of suitable ligands capable of
having the antenna properties while forming thermodynamically
stable complexes with lanthanide ions becomes essential in the
design of novel luminescent lanthanidomesogens. Preparation
of metallomesogens with highly coordination number might be
possible either through the separation of mesogenic units and
coordination units via long flexible alkyl spacers or by increasing
the number of long flexible alkyl chain attached to the
coordinating unit.[31, 32] The two mechanisms mentioned above
have led to the research and development of several classes of
lanthanidomegens starting from ligands such as: Schiff bases,
macrocyclic ligands (phtalocyanine, porphyrine, etc.), alkanoates,
β-diketonate, bis (benzimidazolyl) pyridine, etc.[1] A new class of
thermotropic lanthanidomesogens bearing mesogenic ligands
derived from 4-pyridone was reported recently.[31] Mesogenic
compounds based on the 4-pyridone core have been known
since long time ago; [33, 34] but, as far as we are concerned,
these compounds have not been utilized in the coordination of
metals to give metallomesogens despite the fact that they show
excellent coordination abilities, in particular to lanthanides.[35-
37] Importantly, the 2- or 4-pyridone derived compounds have
been previously employed as O-coordinated ligands in the study
of lanthanide complexes with interesting magnetic and optical
properties.[38-45] The group of mesogenic monodentate ligands
Introduction
In the last decades, introduction of d- or f-block metals in organic
liquid crystalline matrices to give metallomesogens was widely
studied for their interesting electronic, magnetic and optical
properties.[1-3] Liquid crystals are very attractive materials with
various interesting applications (manufacturing of LCDs,
molecular sensors and detectors, optical switches, etc.) [4-8] In
search for highly emissive metallomesogens, the lanthanide-
containing liquid crystals (lanthanidomesogens) play a central
[a]
L. F. Chiriac, Dr. V. Cîrcu
Department of Inorganic Chemistry
University of Bucharest
23 Dumbrava Rosie st., Bucharest, sector 2, 020484, Romania
E-mail:viorel.circu@chimie.unibuc.ro
Dr. I. Pasuk, Dr. M. Secu
National Institute of Materials Physics
P.O. Box MG-7, Magurele, 077125, Romania
Dr. M. Micutz
[b]
[c]
Department of Physical Chemistry
University of Bucharest
4-12 Elisabeta Blvd., Bucharest 030018, Romania
Supporting information (DSC traces, XRD patterns, luminescence
spectra and POM pictures) for this article is given via a link at the
end of the document.
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10.1002/chem.201801781
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developed by us possesses cyanobiphenyl groups attached to
the 4-pyridone coordinating unit via a flexible long alkyl spacer.
The new Eu(III) complexes show lamellar phases (SmA) and a
very high thermal stability while preserving the emission
properties in the LC phase at high temperature up to clearing
point. Their mesogenic properties depend on the spacer length;
while the transition temperatures were influenced by the number
of mesogenic group employed and the spacer length.[31]
In this paper, we would like to show that, by employing relatively
easy to design and synthesize non-chelating O-donor
monodentate ligands, it is possible to tune the liquid crystalline
properties of the lanthanidomesogens, ranging from lamellar, as
proven earlier, to long range lamellar and hexagonal columnar
phases in the present study, and, in the meantime, preserving
the emission properties of the lanthanide ions. The present work
reports on the synthesis of some new ligands, N-3,4,5-
tris(alkyloxy)benzyl-4-pyridone, and their new thermotropic
lanthanidomesogens containing the Eu(III), Sm (III) and Tb(III)
ions. In this case, the organic ligand has three long flexible alkyl
chains with variable number of carbon atoms (6 to 16) attached
to the coordinating unit, represented by the 4-pyridone unit.
Moreover, these new complexes have mesomorphic properties
bidentate nitrate ions complete the coordination geometry with
coordination number CN=9.[31]
H2n+1CnO
H2n+1CnO
H2n+1CnO
H2n+1CnO
H2n+1CnO
H2n+1CnO
HO
HO
ii
i
CH2OH
COOEt
HO
COOEt
H2n+1CnO
H2n+1CnO
H2n+1CnO
OCnH2n+1
H2n+1CnO
H2n+1CnO
iii
iv
CH2Br
CH2
N
PyO-n
O
v
OCnH2n+1
H2n+1CnO
CH2
N
H2n+1CnO
OCnH2n+1
OCnH2n+1
OCnH2n+1
H2n+1CnO
H2n+1CnO
H2n+1CnO
H2C
N
CH2
N
O
O
O
M
O
O
N
O
3
and
exhibit
either a lamellar or hexagonal
columnar
mesophase, depending on the alkyl chain length, over a large
temperature range, in some cases more than 200oC, as shown
n=6,8,10,12,14,16
[M(PyO-n)3(NO3)3]
Eu-n, Sm-n, Tb-n
by
differential scanning calorimetry (DSC). A detailed
Scheme 1. General reaction pathway for synthesis of N-alkyl-4-pyridones
ligands and their lanthanide complexes: i) CnH2n+1Br,K2CO3/MEK, reflux for 3
days, ii) LiAlH4/THF, iii) PBr3/CH2Cl2, iv) 4-hydroxypyridine, THF/H2O, NaOH
characterization of their thermal and self-organization properties
were investigated by powder X-ray diffraction (XRD) and
polarizing optical microscopy (POM) observations.
.
2N, TBABr, reflux 24h, 75oC, v) M(NO3)3 xH2O (x=5 or 6), EtOH, stirring 2h at
room temperature.
Results and Discussion
Thermal stability of the ligands and complexes
The results of the thermogravimetric analyses show that the
lanthanide complexes have a high thermal stability, their
decompositon started at temperatures around 300oC. For
exemplification, Figure 1 gives the TG curves for 4-pyridone
ligand PyO-10 and its lanthanide complexes Eu-10, Sm-10 and
Tb-10. The ligand PyO-10 is highly stable in the 25 – 350oC
range. At temperatures higher than 350oC, the thermogram
indicates a small mass loss, followed by the total decomposition
at a temperature higher than 370oC. In comparison, the
thermogravimetric analysis of the corresponding Ln(III)
complexes show that these materials are stable in the 0–300oC
region, while their decomposition started at temperatures higher
than 300oC. On the other hand, these TG curves do not contain
steps or other indication of mass loss in this region, therefore
the complexes do not contain small molecules (water or solvent).
All the lanthanide complexes undergo complete decomposition
up to 450oC.
Synthesis and characterization of ligands and complexes
The novel monodentate ligands having three alkoxy chains with
variable number of carbon atoms were prepared through the
reaction of 4-hydroxypyridine with 3,4,5-tris(alkyloxy)benzyl
bromide derivatives in THF, in the presence of NaOH and
tetrabutylammonium bromide (TBABr) as phase transfer catalyst
to give the 4-pyridone derivatives PyO-n, as the main product in
relatively high yield 65-70% (Scheme 1). [46, 47] All the new N-
1
alkylated-4-pyridone derivatives were fully characterized by H-
and 13C-NMR and IR spectroscopies as well as by elemental
analyses.
In the next step, the nitrate lanthanide (III) complexes were
obtained by reacting the 4-pyridone ligands with the
.
corresponding M(NO3)3 xH2O salt (Ln = Eu, Tb, Sm, x = 5 or 6)
in hot ethanol (Scheme 1). By mixing the two hot solutions, white
precipitates were obtained within minutes. The solids were
isolated by filtration and then recrystallized from a mixture of
dichloromethane/ethanol 1/1 v/v at -20oC.The TG and elemental
analyses indicated that there is no crystallization or coordination
water or solvent molecules. Based on the crystal structure of the
related Eu(III) complex with N-benzyl-4-pyridone ligand, the
structure of these complexes can be described in the following
way: three molecules of 4-pyridone ligand are coordinated to
lanthanide ion by means of the oxygen atom, and three
Liquid crystalline properties
The liquid crystalline properties were investigated by
a
combination of hot stage polarizing optical microscopy (POM)
and differential scanning calorimetry (DSC), followed by powder
X-rays diffraction (XRD) in the mesophase range to confirm the
nature of the LC phases. The transition temperatures for ligands
and their corresponding lanthanide (III) complexes are
presented in Table 1. Only selected ligands and their lanthanide
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(III) complexes were studied by variable-temperature powder X-
ray diffraction with the aim of elucidating the nature of the liquid
crystalline phase and getting insights regarding the effects of
delicate structural modifications onto the packing of molecules
within the LC phase and the mesophase temperature range.
a=33.3Å. By calculating the number of molecules per column
unit, N = Vcell/Vmol, where Vcell=Scolxh (assuming that h~4.5Å,
.
Scol=√3a2/2 and Vmol=Mw/(NA ); Mw is the molecular weight,  is
the density and it was approximated to 1 g.cm-3 and NA is the
Avogadro’ number), [1, 48] then it is fair to assume that the discs
are composed of three molecules of 4-pyridone ligands having in
total nine alkyl chains at the periphery. The aggregation of three
molecules of 4-pyridone derivative produces a rough disklike
arrangement of the three benzylic groups compatible with a
hexagonal columnar organization.
100
PyO-10
Eu-10
The melting temperatures of the complexes, measured during
the first heating run, do not depend much on the size of the
lanthanide(III) ion, their variation being more influenced by the
size of the side chains, ranging between ca. 53°C for
compounds with twelve carbon atoms Eu-12, Sm-12 and Tb-12,
and 110oC for compounds with six carbon atoms Eu-6, Sm-6
and Tb-6 (Figure 4).
80
60
40
20
0
Sm-10
Tb-10
The clearing temperatures of the three series of lanthanide
complexes (Table 1, measured during the first heating run)
again do not depend significantly on the size of the lanthanide
ion (Figure 4). The lowest clearing temperature was recorded for
Eu-10 and Tb-10 (189oC), while the highest clearing
temperature was measured for Sm-12 (238oC).
100
200
300
400
500
600
Temperature [oC]
Figure 1. TG curves for ligand PyO-10 and its lanthanide complexes Eu-10,
Sm-10 and Tb-10.
Further, the new lanthanidomesogens described in this study
show similar mesomorphic behaviors, no matter the lanthanide
ion employed. Interestingly, mesogenic compounds were
obtained for shorter chain length (6 or 8 carbon atoms), with
large temperature ranges for all three lanthanide ions employed.
The mesophase displayed by these materials is stable over a
large temperature range, with clearing temperatures higher than
190oC for all compounds with the exception of Tb-10. When the
comparison is made with the non-coordinated ligands, it is easily
seen that these transition temperatures are higher by more than
120oC. Additionally, besides the high stability of the LC phase
induced by the presence of the lanthanide ions, the mesophase
displayed by these complexes has an enantiotropic character.
It was found that both ligands and their corresponding
lanthanide(III) complexes show mesogenic properties. For
example, the 4-pyridone ligands, PyO-n display monotropic
hexagonal columnar LC phases with isotropic to mesophase
transitions in the 40 – 65oC temperature range. The heating run
of DSC curves recorded for these organic compounds show only
one transition assigned to a crystal-to-isotropic state transition.
On cooling from the isotropic state, two transitions were
observed, an isotropic to hexagonal columnar mesophase
transition followed by a crystallization step (Figure 2a). The only
exception seen was compound PyO-6, which, upon cooling, the
occurrence of a hexagonal columnar texture around 48oC was
not detected by DSC but was rather seen by POM observations.
On the other hand, compound PyO-8 show a slightly different
behavior; thus, on cooling the material from the isotropic liquid,
no crystallization was detected, and the compound remains in
the glassy state at ordinary temperature. The clearing
temperatures recorded for the 4-pyridone derivatives depend on
the number of carbon atoms of the terminal alkyl groups; a
general trend of increasing the clearing temperature was seen
on increasing the number of carbon atoms with the exception of
PyO-8 that clears at 65oC. The assignment of the hexagonal
columnar phases was done based both on the polarized
microscopy optical textures and XRD analysis. Typical textures
of a hexagonal columnar phase were developed from the
isotropic state (Figure 3a and Figure S5). Due to the monotropic
character of the LC phase, the XRD measurements were difficult
to perform for each product. Still an XRD diffractogram of the
compound PyO-10 was possible to be recorded at 40oC (Figure
S14a), right below the transition temperature from the isotropic
to columnar phase. Two diffraction Bragg peaks were observed
in the low-angle region, in the 1:2 ratio, giving an interplanar
distance of 28.85Å (d100) and a corresponding lattice parameter
Figure 2. DSC first heating–cooling cycle for PyO-14 (a) and second heating-
cooling cycles for Eu-6 (b), Tb-10 (c) and Sm-14 (d).
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Depending on the type of phases exhibited at room temperature,
these compounds could be divided into two groups: the
compounds with 4-pyridone derivatives with shorter terminal
alkyl groups (6, 8, 10 and 12 carbon atoms) show a glass
transition below 50oC after a previous heating run to isotropic
state (Figure 2b), while the compounds with longer alkyl groups
(14 and 16 carbon atoms) show a crystalline phase at room
temperature (Figure 2d).
For the first group of compounds, with the exception of
compounds with 4-pyridone ligands with ten carbon atoms, two
transitions were observed on the DSC traces recorded on the
second cooling steps: first transition correspond to an isotropic-
to-mesophase transformation and the second one assigned to a
glass transition. This behaviour was confirmed by POM when
the previously isolated LC phases freeze on further cooling
down to room temperature (Figures S10 and S11). For the
second group of complexes, with longer terminal alkyl chains,
n=14 and 16, the second transition was assigned to the
crystallization process at lower temperatures.
Table 1. Transition temperaturesa (in °C) and enthalpies (kJ.mol-1) for ligands
and their lanthanide (III) complexes.
Compound
PyO-6
Heating/cooling scans
1st: Cr 56 (17.3)Iso 48 Colh[c] 26 (14.2) Cr
[b]
PyO-8
1st: Colh 65 (0.7) Iso 65 (0.4) Colh
PyO-10
1st: Cr1 58 (45.7) Iso 40 (0.9) Colh 12 (5.2) Cr2
2nd:Cr2 25 (10.4) Cr3 42 (1.2) Cr1 58 (10.4) Iso 40 (1.2) Colh
12 (7.8) Cr2
PyO-12
1st: Cr1 71 (74.4) Iso 56 (1.4) Colh 13 (22.3) Cr2
2nd: Cr2 12 (13.3) Cr3 29 (25.0) Cr1 70 (80.3) Iso 56 (1.5) Colh
13 (19.0) Cr2
PyO-14
PyO-16
1st: Cr1 80 (79.7) Iso 58 (1.3) Colh 30 (47.4) Cr2
2nd:Cr2 37 (35.3) Cr1 79 (98.5) Iso 58 (1.6) Colh 30 (44.3) Cr2
1st: Cr1 85 (110.9) Iso 55 (0.9) Colh 46 (47.4) Cr2
2nd: Cr2 47 (29.8) Cr1 85 (110) Iso 55 (0.86) Colh 46 (55.1)
Cr2
All of the complexes display perfectly reproducible subsequent
heating-cooling cycles with the same onset temperatures
meaning that these complexes are stable during the heating on
the thermal range investigated. Homeotropic extended regions
were observed by POM for compounds with shorter alkyl chains
(6, 8) at the temperatures below the clearing points; by shearing,
poorly developed textures with low birefringence were seen for
these compounds (Figure S9), so that the microscopy
observations were not helpful enough for the phase
identification and the assignment was mainly done based on the
XRD measurements. The other complexes, having longer alkyl
side chains, give well-defined textures when cooling from the
isotropic state and several examples are presented in Figure 3.
The XRD analysis revealed different features of the LC phase
displayed by shorter chains complexes (n=6, 8 and 10) when
compared to their longer chains analogues (n=12, 14, 16). In the
XRD patterns of the lanthanide complexes with shorter alkyl
chains, a series of three or four equidistant sharp diffraction
Eu-6
1st: Cr1 52 (7.7) Cr2 105 (34.1) SmA 236 (5.9) Iso 236 (4.7)
SmA 40 g
2nd: g 40 SmA 236 (5.4) Iso 236 (4.9) SmA 40 g
Eu-8
1st: Cr 54 (18.2) SmA 193 (2.8) Iso 193 (1.9) SmA 50 g
2nd: g 50 SmA 193 (1.9) Iso 193 (1.9) SmA 50 g
Eu-10
1st: Cr 67 (6.2) SmA 115 (1.1) Colh 189 (1.4) Iso 189 (1.5)
Colh 115[c] SmA 38 g
2nd: g 38 SmA 115 (0.7) Colh 189 (1.0) Iso 189 (1.1) Colh 114
(0.3) SmA 38 g
Eu-12
Eu-14
Eu-16
1st: Cr 53 (79.0) Colh 238 (4.2) Iso 236 (3.7) Colh 50 g
2nd: g 50 Colh 238 (4.1) Iso 236 (3.6) Colh 50 g
1st: Cr 67 (220) Colh 217 (4.0) Iso 216 (3.4) Colh 22 (66.7) Cr
2nd:Cr 20 (66.4) Colh 218 (3.8) Iso 217 (3.4) Colh 22 (66.8) Cr
1st: Cr 83 (162.0) Colh 231 (2.3) Iso 230 (1.4) Colh 41 (101.0)
Cr
2nd: Cr 46 (94.0) Colh 233 (2.1) Iso 229 (1.0) Colh 40 (100.0)
Cr
peaks were observed in the low-angle region assigned to d001
,
d002, d003 and d004 reflections leading to a rather lamellar
structure of the mesophase (Figure 4a). An additional peak
around ~5Å found for Eu-8 suggests that there is a periodicity at
this distance, and it could be assigned to an interaction between
neighboring molecules arranged in a typical lamellar packing
model.[49-51] Several lanthanidomesogens were found to adopt
a lamello-columnar mesophase organization, having different
type of ligands such as substituted imidazo[4,5-f]-1,10-
phenanthroline with cyanobiphenyl mesogenic groups [32] or
Sm-6
1st: Cr1 52 (6.0) Cr2 107 (23.1) SmA 235 (3.0) Iso 235 (3.3)
SmA 45 g
2nd: g 45 SmA 235 (3.3) Iso 235 (3.9) SmA 45 g
Sm-8
1st:Cr 54 (9.9) SmA 195 (2.0) Iso 194 (2.2) SmA 46 g
2nd: g 46 SmA 195 (1.9) Iso 194 (2.1) SmA 46 g
Sm-10
1st: Cr 68 (21.1) SmA 130 (1.2) Colh 191 (1.0) Iso 190 (0.9)
Colh 130 (1.0) SmA 45 g
2nd: g 45 SmA 130 (1.2) Colh 192 (0.9) Iso 191 (1.3) Colh 130
(0.9) SmA 45 g
2,6-bis(1-ethyl-benzimidazol-2-yl)pyridine
lipophilic gallic acid derivatives.[52]
substituted
particular thermal
with
A
behavior was seen for compounds with ten carbon atoms in the
side alkyl chains. These compounds can be regarded as
bridging the behavior of shorter chains analogues with the one
of the longer chains compounds. In fact, their thermal properties
are the sum of those displayed by shorter chains complexes
(n=6 and 8) with the properties displayed by the longer chains
analogues (n=12,14,16).
Sm-12
Sm-14
1st: Cr 57 (65.0) Colh 238 (4.1) Iso 237 (3.6) Colh 50 g
2nd: g 50 Colh 238 (3.4) Iso 237 (3.4) Colh 50 g
1st: Cr1 48 (21.4) Cr2 90 (92.0) Colh 220 (2.9) Iso 220 (0.9)
Colh 23 (67.7) Cr1
2nd:Cr 23 (61.0) Colh 220 (2.3) Iso 220 (0.9) Colh 23 (60.0) Cr
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shows a strong fundamental reflection (100), followed by a
series of less intense, but sharp small angle peaks in the 1/3/2
ratio, assigned to (110) and (200) reflections (Figure 5b and c),
respectively, compatible to a 2D-hexagonal packing.
Sm-16
Tb-6
1st: Cr1 54 (5.0) Cr2 73 (156.0) Colh 221 (1.4) Iso 220 (1.5)
Colh 43 (108) Cr1
2nd: Cr 46 (112.0) Colh 221 (1.0) Iso 220 (2.7) Colh 43
(108.0) Cr
1st: Cr1 66 (4.7) Cr2 110 (50.8) SmA 228 (4.2) Iso 227 (4.4)
SmA 50 g
(a)
(b)
2nd: g 50 SmA 228 (4.6) Iso 227 (4.7) SmA 50 g
Tb-8
1st: Cr 65 (4.2) SmA 191[c] Iso 185[c] SmA 60 g
2nd: g 60 SmA191[c] Iso 185[c] SmA 60 g
Tb-10
1st: Cr1 36 (8.4) Cr2 82 (4.0) SmA 125 (1.0) Colh 189 (0.6) Iso
189 (1.1) Colh 125 (1.4) SmA 36 g
2nd: g 36 SmA 121 (1.0) Colh 189 (1.2) Iso 188 (1.3) Colh 123
(1.3) SmA 36 g
Tb-12
Tb-14
1st: Cr1 54 (175) Cr2 85 (8.8) Colh 235[c] Iso 231[c]Colh 50 g
2nd: g 50 Colh 235[c] Iso 231[c] Colh 50 g
(c)
(d)
1st: Cr1 51 (6.1) Cr2 95 (134.0) Colh 207 (3.0) Iso 206 (3.6)
Colh 23 (64.1) Cr1
2nd: Cr 21 (66.7) Colh 207 (3.3) Iso 206 (3.0) Colh 23 (62.2) Cr
Tb-16
1st: Cr 62 (171.0) Colh 209 (br) Iso 207 (2.1) Colh 41 (101.6)
Cr
2nd: Cr 47 (101.1) Colh 207 (0.9) Iso 207 (1.9) Colh 40 (98.3)
Cr
Figure 3. Pictures taken at the polarizing optical microscope showing the
optical textures of PyO-12 at 55oC (a), Eu-14 at 210oC (b), Sm-14 at 215oC(c),
Eu-10 at 164oC (d).
[a]g, br, Cr, SmA, Colh, Iso denotes glass transition, broad
transition, crystalline state, lamellar smectic A and hexagonal
columnar phases respectively; [b]glass transition or
crystallization could not be detected by DSC or POM;
[c]transition detected by POM.
Iso
Colh
Three transitions were detected in the DSC traces both during
the heating and cooling runs: a transition from the isotropic state
to a hexagonal columnar phase, a second transition from the
hexagonal columnar phase to a lamellar phase followed by a
third transition assigned to a glass transition near 50oC. All these
transitions were detected by POM as well, and an example of
textural changes for compound Tb-10 is presented in Figure 6.
The existence of two different LC phases for these compounds
having an intermediate number of carbon atoms in the side alkyl
chains was also confirmed by temperature dependent XRD
analysis. For compound Tb-10, the XRD pattern recorded at
145oC shows a series of three sharp diffraction peaks in the
1/3/2 ratio indicating a hexagonal columnar phase (Figure
S15a). On the other hand, the XRD pattern recorded at 95oC
contains a series of four equidistant sharp peaks in the low-
angle region, located at 2.84, 5.62, 8.42 and 11.14o, with
spacing in the 1/2/3/4 ratio assigned to the (001), (002), (003)
and (004) of a lamellar phase (Figure S15b).The hexagonal
columnar (Colh) mesophase of the compounds with longer alkyl
chains (n=12,14,16) were confirmed by XRD studies. The XRD
data of one compound for each lanthanide ion were selected as
representative examples to be discussed here. These XRD
results are summarized in Table 2 and in Tables S1-S4
(suplemmentary information). For complexes Eu-14 and Sm-16,
the X-rays diffraction pattern, recorded on cooling the
compounds from the isotropic state to liquid crystalline phase,
Tb-16
SmA
g
Cr
Tb-14
Tb-12
Tb-10
Tb-8
Tb-6
Sm-16
Sm-14
Sm-12
Sm-10
Sm-8
Sm-6
Eu-16
Eu-14
Eu-12
Eu-10
Eu-8
Eu-6
0
50
100
150
200
250
Temperature [oC]
Figure 4. Plot of phase-transition temperatures recorded during the second
cooling run for the lanthanide complexes.
These results are consistent with the POM studies, where typical
optical textures of a hexagonal columnar (Colh) phase were
observed for all the lanthanide complexes with higher number of
carbon atoms (n=12,14,16) (Figure 3).
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15000
Table 2. X-ray powder diffraction data for selected compounds.
(a)
d100
Compd
Meso-
phase
T/oC[a]
Indexatio
n
d-
d-
Lattice
spacing
obs. /Å
spacing
calc. /Å
parameter
a/ Å[b]
2000
PyO-10
PyO-14
Eu-14
Colh
Colh
Colh
40
100
28.47
14.35
4.5
28.47
14.23
32.91
37.25
40.02
5Å
d300
d200
200
d400
Broad[c]
0
5
10
15
20
25
30
55
100
32.22
16.17
4.5
32.22
16.11
2 [deg]
200
Broad[c]
85
100
110
34.62
20.25
17.31
4.6
34.62
20.01
17.31
8000
d100
200
(b)
Broad[c]
6000
Sm-14
Tb-14
Colh
Colh
130
130
100
200
33.69
16.72
4.99
4.5
33.69
16.85
38.88
38.66
1000
500
0
Broad[c]
d110
d200
100
200
broad
broad[c]
33.44
16.72
8.5
33.44
16.72
5
10
15
20
25
30
4.5
2 deg]
[a]On cooling from isotropic state. [b]The Colh lattice
parameter a=2<d100>/√3.[c]Broad peak assigned to
molten alkyl chains.
18000
17000
d100
(c)
3000
2000
1000
0
d110
d200
4.6Å
8Å
5
10
15
20
25
30
2 [deg]
Figure 5. X-ray powder diffractograms, recorded on cooling from the isotropic
state, for selected compounds: Eu-8 at 100oC (a); Eu-14 at 85oC (b) and Sm-
16 at 125oC (c).
Additionally, the XRD patterns of the mesomorphic lanthanide
complexes exhibit one broad signal with the maxima located
around 4.5 – 4.6Å assigned as usually to the lateral short-range
order of the molten alkyl chains. Interestingly, the XRD pattern of
Sm-16 contains an additional broad peak around 8Å which
presumably could be assigned to some additional correlations
within the columns at this distance (Figure 5c).
Figure 6. Pictures taken at the polarizing optical microscope showing the
change of optical textures of Tb-10 on cooling from the isotropic state.
Taking into account the distance and the breadth of this signal,
together with the absence of any signal around 3.5-3.6Å which is
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normally assigned to the stacking distance within the columns,
one presumably consider the minimum thickness of a single disc
of approximately h~8Å. Then, the associated volume of the unit
cell can be calculated with the relation: Vcell=Scolxh to give a
value of 11039 Å3. Based on a density value of 1 g.cm-3, the
calculations will give N~2.17, meaning that two molecules of
Sm-16 must occupy a column slice of 8 Å thickness.
Some interesting properties of these complexes can be seen by
further analysis of the X-ray data. A plot of the interplanar
distance versus the number of carbon atoms in the terminal
chain for Eu(III) and Sm(III) complexes is linear (Figure S13).
Similar correlation for analogue Tb(III) complexes could not be
made as only three complexes, Tb-8, Tb-10 and Tb-14 were
selected for XRD analysis. For the first series of complexes, Eu-
n, a highly linear relationship (R=0.999) was found, with a slope
of 0.934Å/CH2 and an intercept of 22.36Å. A close behavior was
found for the second series of complexes, Sm-n, (R=0.997) with
a slope of 0.733 Å/CH2 and an intercept of 23.41Å. It seems that
for extended chain length (n=12,14,16), due to packing
constraints, the molecules tend to self-assemble in a hexagonal
columnar fashion; while for shorter chains (six and eight carbon
atoms) a lamellar packing is preferred. Their analogues with ten
carbon atoms lie at the border between the two groups, as these
complexes display both hexagonal columnar phase at higher
temperatures and a lamellar phase at lower temperatures.
Figure 7. Emission spectra of lanthanide complexes: Eu-n (a); Sm-n (b) and
Tb-n (c) in solid state at room temperature. Within the insets are represented
the lanthanide-luminescence characteristic lifetime dependence on the
number of carbon atoms in the terminal alkyl chains, n; luminescence decay
curves have been recorded for the strongest luminescence and showed single
exponential decays.
Photoluminescence properties
All eighteen Eu(III), Sm(III) and Tb(III) complexes show strong
and characteristic emission spectra in the solid state with long-
lived (millisecond timescale) excited states. As displayed in
Figure 7a, the Eu(III) complexes Eu-n exhibit the strong and
characteristic emission spectrum of Eu(III) ions upon UV
excitation. The spectra contain characteristic sharp bands of the
Eu(III) ion in the 570–720 nm spectral range, which can be
assigned to 5D0→7FJ (J = 1, 2, 3, 4) transitions. For all
complexes, the spectrum is dominated by the 5D0→7F2 transition
at 616 nm, which gave a strong red luminescent output for these
compounds (Figure 8). The luminescence spectra recorded for
Sm(III) complexes Sm-n are represented in Figure 7b. These
spectra show the typical emission of the Sm(III) ion, which
(a)
(b)
4
6
displays four maxima assigned to the G5/2 → H5/2 at 564 nm,
4G5/2 → 6H7/2, at 599 nm, 4G5/2 → 6H9/2 at 644 nm, 4G5/2 → 6H11/2 at
4
707nm transitions, respectively. The G5/2
→
6H9/2 transition at
648 nm presents the highest relative emission intensity and this
transition is responsible for the orange luminescence of the
compounds (Figure 8b). Under UV light excitation, the Tb(III)
complexes Tb-n give the characteristic Tb(III) ion emissions, in
the 480 to 640 nm range, as shown in Figure 7c. These
emission bands were assigned to transitions from the excited
(c)
Figure 8. POM pictures of Eu-16 in hexagonal columnar mesophase at 55oC,
(a) and of Eu-16 in columnar hexagonal mesophase under UV light at 55oC
(b); pictures of Eu-14, Sm-14 and Tb-14 deposited on glass slides under UV
light (254 nm) (c).
7
5D4 state to the different J-levels of the lower FJ state: 5D4→7F6
at 491 nm, 5D4→7F5 at 544 nm, 5D4→7F4 at 588 nm and 5D4→7F3
at 621 nm, respectively. These spectra are dominated by the
5D4→7F5 transition, which gave a strong green luminescent
output for the Tb(III) compounds (Figure 8c).
Moreover, the europium(III), samarium(III) and terbium(III)
complexes have proven to be luminescent both in solid state at
room temperature and in LC phases. To study the thermal
stability of photoluminescence in the LC phase, the temperature
dependence of fluorescence intensity was measured between
room temperature and clearing temperature. One example of
variable temperature luminescence spectra recorded during the
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heating from 25oC up to 205oC is presented in Figure S16a for a
Sm(III) complex, Sm-12. It is obvious that the luminescence
intensity decreased monotonically along with molecular disorder
due to the increase of the temperature. The reverse process
takes place on cooling from the isotropic state, the increase of
the emission intensity due to the molecular reordering in the
mesophase and in the glassy state when the temperature
decreases down to room temperature (Figure S16b).
(0.75-0.90 ms for Eu(III), 0.045-0.060 ms for Sm(III) and 0.75-
1.05 ms for Tb(III), respectively). The photoluminescence
spectra have showed a clear influence of the number of carbon
atoms in the side alkyl chains attached to the bulky metal centre
on the optical properties: both a broadening effect was seen on
analyzing the shape of the emission peaks and a shortening of
the lifetimes with about 30% on going from the shorter chains
analogues with six carbon atoms, Eu-6, Sm-6 and Tb-6, to the
longest chains complexes with sixteen carbon atoms, Eu-16,
Sm-16 and Tb-16.
The photoluminescence spectra of all lanthanide complexes
show a clear influence of the number of carbon atoms (n) in the
side alkyl chains attached to the bulky metal centre on the
optical properties (Figure
7
a-c). It was observed an
inhomogeneous broadening of the luminescence peaks that
becomes more obvious as the number of carbon atoms (n)
increases; the effect is more evident for the Eu(III) complexes
(Figure 7a). The broadening effect was assigned to a certain
distorsion of the lanthanide ion nine-fold coordination polyhedron
but still close to its original C3 symmetry.[31] In addition, the
luminescence decay curves showed single exponential decays
with the characteristic times in the ms range (not shown) and a
shortening with about 30% (insets of the Figure 7 a-c)
associated with an increase of the nonradiative decay channels.
Therefore, it is presumable that as the alkyl chains length
increases they become less rigid and more flexible affecting
both the coordination polyhedron of the lanthanide ion and the
radiative decays rates.
Experimental Section
All the chemicals were used as supplied. C, H, N analyses were carried
out with a with an EuroEA 3300 instrument. IR spectra were recorded on
a Bruker spectrophotometer using an ATR device. 1H and 13C NMR
spectra were recorded on an Avance 300 Bruker spectrometer operating
1
at 300 MHz, using CDCl3 as solvent. H chemical shifts were referenced
to the solvent peak position, 7.26 ppm. Variable temperature emission
spectra in solid state were recorded with an OceanOptics QE65PRO
spectrometer attached to the microscope and using a Nikon Intensilight
excitation source. Photoluminescence (PL) spectra and PL lifetimes
measurements have been recorded at room temperature using a Jobin
Yvon Fluorolog spectrophotometer; the spectra were not corrected for
the spectral sensitivity. Thermogravimetric analysis was performed on a
TA Q50 TGA instrument using alumina crucibles and nitrogen as purging
gas. The heating rate employed was 10°C min-1 from room temperature
(approximately 25°C) to 550oC.
Conclusions
Synthesis of ligands PyO-n
These novel lanthanidomesogens with 4-pyridone ligands
having mesogenic 3,4,5-tris(alkyloxy)benzyl moieties are able to
form lamellar and columnar mesophases (SmA and Colh) on a
large temperarure ranges. It was found that the number of
carbon atoms contained in the side alkyl chains influence the
thermal behaviour of these liquid crystals. The DSC analysis and
POM observations revealed that all these complexes display an
enantiotropic liquid crystalline behavior with either a lamellar
phase (SmA), in the case of shorter chains analogues (six and
eigth carbon atoms) or a hexagonal columnar (Colh) phase, for
complexes with higher number of carbon atoms (twelve,
fourteen and sixteen). The phase assignment was made on the
basis of their characteristic texture and confirmed by
temperature variable XRD studies. As a particular case, the
complexes with an intermediate number of carbon atoms in the
side chains (ten), display a lamellar phase at lower temperatures
and a Colh phase at higher temperatures. On the other hand, the
clearing temperatures of the three series of lanthanide
complexes do not depend significantly on the size of the
lanthanide ion. The lowest clearing temperature was recorded
for Eu-10 and Tb-10 complexes (189oC), while the highest
clearing temperature was measured for Sm-12 (238oC). In solid
state, all these complexes show characteristic emissions
assigned to the corresponding lanthanide ion. Moreover, the
emission properties are preserved also in the LC phase. In
addition, the luminescence decay curves showed single
exponential decays with the characteristic times in the ms range
N-Alkyl-4-pyridone was produced through a slightly modified known
procedure. [46, 47] The PyO-n (n=6,8,10,12,14,16) ligands were
obtained by adding to
a
solution of 4-hydroxypyridine and
tetrabutylammonium bromide (phase transfer catalyst) in tetrahydrofuran
a 2N aqueous solution of sodium hydroxide. Water was added and the
mixture was stirred until the muddy solution became clear, then 3,4,5-
trialkoxybenzyl bromide was added and heated under reflux for 24 hours
(Scheme 1). After cooling, the solvent was evaporated and the product
was extracted with dichloromethane and water (1/1). The organic layer
was collected and the solvent was removed using a rotary evaporator.
The crude product was treated with ethyl acetate and the precipitate was
filtered and purified by column chromatography using dichloromethane
and methanol (95/5) as eluents. White products were obtained with yields
between 65-70%.
PyO-6: Anal. Calcd. For C30H47O4N: C%74.19, H%9.75, N%2.88.
1
Found: C%74.12, H%9.72, N%2.88. H NMR(CDCl3, 300 MHz): 7.38(d,
2H); 6.39(d, 2H); 6.32(d, 2H); 4.80(d, 2H); 3.81-3.85(m, 6H);1.6-1.78(m,
6H), 1.2-1.43(m,18H); 0.76-0.84(m, 9H). IR (ATR, cm-1): 2955, 2931,
2860, 1639, 1590, 1114, 856, 728.
PyO-8: Anal. Calcd. For C36H59O4N: C%75.88, H%10.44, N%2.46.
Found: C%75.86, H%10.45, N%2.46. 1H NMR(CDCl3, 300 MHz): 7.38(d,
2H); 6.42(d, 2H); 6.36(d, 2H); 4.82(d, 2H); 3.82-3.88(m, 6H);1.65-1.82(m,
6H), 1.2-1.46(m,30H); 0.9(m, 9H). IR (ATR, cm-1):2955, 2926, 2855,
1639, 1589,1114, 853, 723.
PyO-10: Anal. Calcd. For C42H71O4N: C%77.13, H%10.94, N%2.14.
Found: C%77.16, H%10.94, N%2.13. 1H NMR(CDCl3, 300 MHz): 7.4(d,
2H); 6.44(d, 2H); 6.31(d, 2H); 4.8(d, 2H); 3.85-3.95(m, 6H);1.68-1.8(m,
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6H), 1.26-1.48(m,42H); 0.87(m, 9H). IR (ATR, cm-1): 2956, 2922,
2852,1640, 1574, 1211, 847, 72.
and the residual solid was recrystallized from
a
mixture of
dichloromethane and diethyl ether. The white precipitates of Sm-n
complexes were filtered, washed with diethyl ether and dried in vacuo.
White products were obtained with yields in the 45-50% range.
PyO-12-structural characterization for this ligand was reported by us in a
previous study. [46]
Sm-6: Anal. Calcd. For C90H141SmN6O21: C%60.27, H%7.95, N%4.69.
Found: C%59.25, H%7.72, N%4.60. IR(ATR, cm-1): 2955, 2927, 2857,
1636, 1592,1441, 1248, 1112, 1032, 851, 786, 721, 467.
PyO-14: Anal. Calcd. For C54H95O4N: C%78.87, H%11.64, N%1.70.
Found: C%78.87, H%11.64, N%1.70. 1H NMR(CDCl3, 300 MHz): 7.37(d,
2H); 6.43(d, 2H); 6.28(d, 2H); 4.81(d, 2H); 3.82-3.88(m, 6H);1.64-1.82(m,
6H), 1.18-1.5(m,66H); 0.83(m, 9H). IR (ATR, cm-1): 2956, 2917, 2850,
1640, 1574,1116, 846, 721.
Sm-8: Anal. Calcd. For C108H177SmN6O21: C%63.40, H%8.72, N%4.11.
Found: C%63.56, H%8.64, N%3.39. IR (ATR, cm-1): 2956, 2926, 2855,
1636, 1591, 1442, 1244, 1114, 1023, 857, 787, 723, 468.
PyO-16: Anal. Calcd. For C60H107O4N: C%79.50, H%11.90, N%1.55.
Found: C%79.49, H%11.92, N%1.55. 1H NMR(CDCl3, 300 MHz): 7.4(d,
2H); 6.45(d, 2H); 6.36(d, 2H); 4.87(d, 2H); 3.84-4(m, 6H);1.7-1.9(m, 6H),
1.2-1.54(m,78H); 0.85(m, 9H). IR (ATR, cm-1): 2956, 2917, 2850, 1640,
1574,1116, 846, 721.
Sm-10: Anal. Calcd. For C126H213SmN6O21: C%65.84, H%9.34, N%3.66.
Found: C%65.16, H%9.15, N%3.54. IR (ATR, cm-1): 2955, 2924, 2853,
1635, 1590, 1442, 1246, 1115, 1031, 857, 787, 722, 468.
Sm-12: Anal. Calcd. For C144H249SmN6O21: C%67.80, H%9.84, N% 3.29.
Found: C%67.28, H%9.69, N% 3.33. IR (ATR, cm-1): 2956, 2922, 2852,
1635, 1590, 1441, 1245, 1116, 1030, 857, 787, 721, 468.
Synthesis of Eu(III) complexes, Eu-n
Eu(NO3)3x6H2O (1 mmole) in ethanol (5 ml) was added to the solution of
N-alkyl-4-pyridone (3 mmole) in ethanol (5ml) dropwise and with constant
stirring. The solution became turbid with the addition of the europium
nitrate. The reaction mixture was stirred at room temperature for two
hours. The white precipitate formed was filtered off, taken up in
dichloromethane and filtered over celite. Then the solvent was removed
by rotary evaporator and the residual complex was recrystallized from
dichloromethane and diethyl ether. The white precipitates of Eu-n were
filtered, washed with diethyl ether and dried in vacuo. White products
were obtained with yields about 50%.
Sm-14: Anal. Calcd. For C162H285SmN6O21: C%69.41, H%10.25, N%3.00.
Found: C%69.36, H%10.31, N%2.83. IR (ATR, cm-1): 2955, 2919, 2851,
1634, 1589, 1441, 1247, 1118, 1023, 856, 787, 721,468.
Sm-16: Anal. Calcd. For C180H321SmN6O21: C%70.75, H%10.59, N%2.75.
Found: C%69.26, H%10.31, N%2.68. IR (ATR, cm-1): 2956, 2918, 2850,
1635, 1589,1441,1243, 1117, 1023, 856, 787, 721, 467.
Synthesis of Tb(III) complexes, Tb-n
Eu-6: Anal. Calcd. For C90H141EuN6O21: C%60.22, H%8.65, N%4.68.
Found: C%59.41, H%8.22, N%4.52. IR (ATR, cm-1): 2955, 2932, 2859,
1636, 1591,1442, 1248, 1112, 1033, 852, 786, 737, 468.
Over a solution of N-alkyl-4-pyridone (3 mmole) in ethanol (5 ml) was
added dropwise a solution of Tb(NO3)3x5H2O (1 mmole) in ethanol (5 ml).
The reaction mixture was stirred at room temperature for two hours. The
white precipitate formed was filtered off, taken up in dichloromethane and
filtered over celite. The solvent was removed and the residual complex
was recristalized from dichloromethane and diethyl ether. The white
precipitates of Tb-n complexes were filtered and then washed with
diethyl ether and dried in vacuo. White products were obtained with
yields around 45%.
Eu-8: Anal. Calcd. For C108H177EuN6O21: C%63.35, H%8.71, N%4.10.
Found: C%62.83, H%8.62, N%4.04. IR (ATR, cm-1): 2955, 2924,
2854,1635, 1590, 1442,1245, 1115, 1031, 857, 817, 737, 468.
Eu-10: Anal. Calcd. For C126H213EuN6O21: C%65.80, H%9.33, N%3.65.
Found: C%65.65, H%9.42, N%3.51. IR (ATR, cm-1): 2955, 2924, 2854,
1635, 1590, 1442, 1245, 1115, 1031, 857, 817, 737, 468.
Tb-6: Anal. Calcd. For C90H141TbN6O21: C%60.84, H%8.62, N%4.66.
Found: C%59.92, H%8.14, N%4.53. IR (ATR, cm-1):2956, 2919, 2851,
1634, 1588,1441,1248, 1118, 1023, 856, 787, 721, 468.
Eu-12: Anal. Calcd. For C144H249EuN6O21: C%67.76, H% 9.83, N%3.29.
Found: C%67.19,H% 9.87, N%3.22. IR (ATR, cm-1): 2956, 2921, 2851,
1636, 1590, 1442, 1242, 1115, 1030, 856, 817, 721,467.
Tb-8: Anal. Calcd. For C108H177TbN6O21: C%63.14, H%8.68, N%4.09.
Found: C%63.36, H%8.51, N%3.84. IR (ATR, cm-1): 2956, 2925,
2854,1635, 1591, 1441, 1245, 1114, 1032, 858, 787, 723, 468.
Eu-14: Anal. Calcd. For C162H285EuN6O21: C%69.37, H%10.24, N%3.00.
Found: C%69.26 , H%10.32, N%2.76. IR (ATR, cm-1): 2955, 2919, 2851,
1636, 1590,1442, 1242, 1115,1030, 861, 817, 721,467.
Tb-10: Anal. Calcd. For C126H213TbN6O21: C%65.60, H%9.31, N%3.64.
Found: C%65.32, H%9.29, N%3.51. IR (ATR, cm-1): 2956, 2923, 2852,
1641, 1590, 1442, 1254, 1112, 1036, 847, 774, 718, 463.
Eu-16: Anal. Calcd. For C180H321EuN6O21: C%70.71, H%10.58, N%2.75.
Found: C%70.65, H%10.72, N%2.44. IR(ATR, cm-1): 2956, 2918, 2850,
1635, 1590, 1442, 1244, 1117, 1030, 857, 817, 721, 467.
Tb-12: Anal. Calcd. For C144H249TbN6O21: C%67.57, H%9.81, N%3.28.
Found: C%67.09, H%9.74, N%3.11. IR (ATR, cm-1): 2956, 2920,
2851,1635, 1590, 1441, 1243, 1115, 1021, 861, 790, 721, 466.
Synthesis of Sm(III) complexes, Sm-n
Over a solution of N-alkyl-4-pyridone (3 mmole) in ethanol (5 ml) was
added dropwise to a solution of Sm(NO3)3x6H2O (1 mmole) in ethanol (5
ml). The reaction mixture was stirred at room temperature for two hours.
The white precipitate formed was filtered off, taken up in dichloromethane
and filtered over celite. The solvent was removed by rotary evaporator
Tb-14: Anal. Calcd. For C162H285TbN6O21: C%69.20, H%10.22, N%2.99.
Found: C%69.10, H%10.41, N%2.70. IR (ATR, cm-1): 2955, 2930,
2859,1635, 1591, 1441, 1249, 1113, 1022, 855, 786, 741, 469.
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10.1002/chem.201801781
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Tb-16: Anal. Calcd. For C180H321TbN6O21: C%70.55, H%10.56, N%2.74.
Found: C%70.47, H%10.62, N%2.48. IR (ATR, cm-1): 2955, 2918, 2850,
1636, 1589, 1441, 1243, 1117, 1023, 857, 788, 721, 468.
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J.-C. G. Bünzli, Acc. Chem. Res. 2006, 39, 53-61.
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Keywords: lanthanidomesogens • liquid crystals • hexagonal
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