Tetrahedral zinc complexes with liquid crystalline and luminescent properties: Interplay between nonconventional molecular shapes and supramolecular mesomorphic order

Article abstract of DOI:10.1021/ja073639c

Novel metallomesogens with luminescent properties and liquid crystalline behavior at room temperature have been achieved by the preparation of zinc complexes with polycatenar pyrazole and bis-(pyrazolyl)methane ligands. Their molecular structures do not h

Full text of DOI:10.1021/ja073639c

Published on Web 08/22/2007
Tetrahedral Zinc Complexes with Liquid Crystalline and
Luminescent Properties: Interplay Between Nonconventional
Molecular Shapes and Supramolecular Mesomorphic Order
Emma Cavero,† Santiago Uriel,‡ Pilar Romero,† Jose´ Luis Serrano,† and
Raquel Gime´nez*,†
Contribution from the Departamento de Qu´ımica Orga´nica y Qu´ımica F´ısica, AÄ rea de Qu´ımica
Orga´nica, Facultad de Ciencias-Instituto de Ciencia de Materiales de Arago´n, UniVersidad de
Zaragoza-CSIC, 50009 Zaragoza, Spain, and Centro Polite´cnico Superior-Instituto de Ciencia
de Materiales de Arago´n, UniVersidad de Zaragoza-CSIC, 50015 Zaragoza, Spain
Received May 21, 2007; E-mail: rgimenez@unizar.es
Abstract: Novel metallomesogens with luminescent properties and liquid crystalline behavior at room
temperature have been achieved by the preparation of zinc complexes with polycatenar pyrazole and bis-
(pyrazolyl)methane ligands. Their molecular structures do not have a conventional shape in that they are
far from the typical rod-like and flat disc-like geometries of common liquid crystals. They consist of a
nonplanar nucleus due to the methylene spacer and/or the coordination to the tetrahedral center, as
confirmed by single crystal analysis of the cores. The different numbers and positions of side chains in the
pyrazole ligand enabled us to access lamellar and columnar mesophases and, of particular interest, to
obtain columnar arrangements at room temperature. Supramolecular models for the organization of the
molecules in the mesophases are proposed on the basis of the small-angle XRD diffractograms. The zinc
complexes display luminescence in the near UV-blue region with large Stokes shifts. An interplay between
non-conventional molecular shapes (due to the tetrahedral core) and the supramolecular mesomorphic
order (due to the ligand design) led to materials that interestingly embody two rather opposite properties,
a columnar self-organizational ability and luminescence with weak intermolecular interactions.
Introduction
approach to multifunctional materials on the basis that they
combine the self-organization and self-healing ability of liquid
The study of supramolecular organizations has evolved into
a significant topic for the nascent nanotechnologies of molecular
electronics and photonics, as it constitutes a starting point for
the bottom-up approach to functional materials.1-8 Metallo-
mesogens, or metal-containing liquid crystals,9-21 offer a viable
crystals14 (LCs) with the properties of metal complexes.22 In
particular, the combination of luminescence and liquid crystal-
linity is one possibility that is of relevance for applications in
OLEDs, information storage, sensors, and enhanced constrast
displays.23 Luminescent metallomesogens have been reported
with metals of the lanthanide group,24-29 Pd,30-32 Pt,33-35 Ni,36
† Facultad de Ciencias-Instituto de Ciencia de Materiales de Arago´n.
‡ Centro Polite´cnico Superior-Instituto de Ciencia de Materiales de
Arago´n.
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10.1021/ja073639c CCC: $37.00 © 2007 American Chemical Society

Luminescent Liquid Crystalline Zinc Complexes
A R T I C L E S
Au,37,38 Cu,39 Ag,40,41 and a few examples have concerned their
properties in the mesophase. An important example of the
interplay between self-organization and luminescence is the
recent report of a copper metallomesogen for security ink
applications.39
Scheme 1
Zinc complexes with nitrogen ligands display interesting
optical properties for LED devices and sensors,42-47 but the
tetrahedral geometry that the metal center usually tends to adopt
has proven to be detrimental for mesomorphism. For example,
in the extensively studied compounds with salicylaldiminate48
or diketonate ligands,16 complexes derived from Pd, Cu, VO,
Ni, Fe, or Mn are LCs but the zinc analogs are not,16,49 probably
due to the unfavorable geometry and the lack of additional inter-
actions between the metal centers.49 Indeed, all previous litera-
ture reports on zinc metallomesogens describe the zinc atom in
a different coordination geometry, for example, in a planar
coordination with porphyrin ligands50 and extended analogs, or
pentacoordinated with a trigonal-bipyramidal geometry with tris-
(2-aminoethyl)amine,51 dithiobenzoates,52 or tridentate pyri-
dines.53,54 Therefore, obtaining LC tetrahedral zinc complexes
has remained an elusive objective for long time.
and mononuclear complexes with Au,55,56 B,57,58 and Rh,59
respectively.
As a result of the versatility of the pyrazole ring, we recently
reported that tetrahedral zinc complexes exhibit LC behavior
with a flexible ligand design, and we prepared the first liquid
crystalline tetrahedral zinc complex by using a nonrigid ligand
derived from bis(pyrazolyl)ethane.60 Afterward, another report
on a tetrahedral zinc complex with columnar mesomorphism
appeared, and this example contained a nondiscoid dipyridyl
ligand.61
We have been investigating pyrazole-based metallomesogens
and found that 3,5-diarylpyrazoles are excellent building-blocks
for the induction of liquid crystalline behavior at room tem-
perature, as demonstrated by a variety of trinuclear, dinuclear,
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Encouraged by our previous results, we explored new Zn
complexes with the aim of obtaining LCs at room temperature
and luminescence properties both in solution and in the liquid
crystalline state. This objective has been achieved through the
preparation of several pyrazoles, Hpz 1-6, which have different
numbers (ranging from 2 to 6) and positions of side chains
(Scheme 1 and Chart 1), their bis(pyrazolyl)methane derivatives
Bpzm 7-12 and their respective Zn complexes of formulas
[ZnCl2(Hpz)2] 13-18 and [ZnCl2(Bpzm)] 19-24. All of the
compounds possess a nonplanar core due to the presence of
the methylene spacer and/or coordination to the tetrahedral
center. As a result, the molecules do not have a conventional
shape in that they are far from the typical rodlike and flat
disclike geometries of common LCs. This particular design may
have important implications in the optical properties. The
synthesis and characterization of the new ligands and complexes,
the crystalline structures of complexes 18 and 24, their supra-
molecular structures in the mesophase, and the optical prop-
erties are all discussed. The different numbers and positions of
side chains in the pyrazole ligand enabled us to access a variety
of mesophases and, in particular, to obtain columnar arrange-
ments with luminescent properties at room temperature. An
interplay between nonconventional molecular shapes (due to
the tetrahedral core) and the supramolecular mesomorphic order
(due to the ligand design) led to materials that combine a
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A R T I C L E S
Chart 1
Cavero et al.
columnar-ordered organization with weak intermolecular inter-
actions.
was performed on the methoxy-substituted pyrazole 6 and its
derivatives (12, 18, and 24) by H NMR spectroscopy.
1
The aromatic region of the 1H NMR spectrum of compound
6 displays a singlet at 6.87 ppm, which corresponds to the four
aromatic protons (Figure 1a, Hl, He), and another singlet for
the proton in position 4 of the pyrazole ring (Figure 1a, Hb).
After dimerization takes place to give Bpzm 12, the spectrum
display four singlets, with the signals at 7.04 and 7.31 ppm being
due to the aromatic protons located in positions 3 (Figure 1b,
He) and 5 (Figure 1b, Hl) of the pyrazole ring, respectively.
The signals were assigned by means of NOE experiments (see
Supporting Information). When compound 12 complexes with
Results and Discussion
Synthesis and Characterization in Solution. Polyalkoxy-
lated pyrazole ligands Hpz were prepared from 1,3-diketones
as reported previously.58 These compounds were dimerized to
the corresponding bis(pyrazolyl)methanes, Bpzm 7-12, by
reaction with dibromomethane using a solid-liquid phase
transfer catalysis method.62 The reaction proceeded in a step-
wise manner through a 1-(bromomethyl)pyrazole intermediate
that reacted with a second molecule of pyrazole. The zinc
complexes [ZnCl2(Hpz)2] and [ZnCl2(Bpzm)] were readily
prepared by reaction of the Hpz or Bpzm ligands with anhydrous
zinc chloride in a 2:1 or 1:1 molar ratio, respectively (Scheme
1).63 All complexes were isolated from the reaction media as
white solids or light-yellow greasy materials at room temper-
ature.
1H-Pyrazoles display 1,2-prototropy, that is, equilibrium of
the NH proton between the two nitrogens, which makes
positions 3 and 5 of the heterocycle equivalent on the NMR
time scale. Therefore, all pyrazoles, be they symmetrically or
unsymmetrically substituted, appear as a single compound.
However, when a Bpzm dimer is formed, the 3- and 5-positions
of the pyrazole ring become inequivalent and separate signals
are observed in the spectra. Similar spectra are also expected
for the zinc complexes due to the reduced symmetry. For the
full characterization of the new compounds, a careful analysis
1
zinc to give [ZnCl2(Bpzm)] 24, the H NMR spectrum shows
four singlets (Figure 1c). Similar NOE experiments were car-
ried out, and it was found that the signal at 7.00 ppm corre-
sponds to the aromatic protons at the 3 position, which is near
to the zinc atom, and these remain almost unchanged on com-
plexation (Figure 1c, He). However, the protons in the 5-posi-
tion, which are located far from the metal, are considerably
shielded upon coordination (Hl, from 7.31 ppm in Figure 1b to
6.32 ppm in Figure 1c). This unexpected effect is due to the
particular conformation of the molecule, as the two aromatic
rings in position 5 of the pyrazole rings are very close to one
another and are almost parallel, causing a shielding effect due
to the anisotropic ring current (see single-crystal X-ray discus-
sion).
On the other hand, the 1H NMR spectrum of complex [ZnCl2-
(Hpz)2] 18 only shows one signal for the aromatic protons
(Figure 1d, He, Hl), which is indicative of a high level of
symmetry in the compound. This suggests the existence of a
1,2-metallotropic equilibrium in solution, that is, a fast exchange
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Luminescent Liquid Crystalline Zinc Complexes
A R T I C L E S
1
Figure 1. Aromatic region of the H NMR for compound 6 and derivatives 12, 18, and 24 in CDCl3.
of the Zn atom between the two nitrogens, and this phenomenon
has been observed previously in other Rh and Ag complexes
with other pyrazole ligands.59,64-66 The equilibrium was con-
firmed by variable temperature 1H NMR experiments, in which
a lowering of the temperature gave rise to splitting of the signals
and provides evidence of a reduction in symmetry due to the
freezing of the metal interchange.
and one trans (Figure 2). Pyrazoles with small substituents, like
3-methylpyrazole, give rise to the three isomers as a nearly
statistical mixture,67,68 but in our case, Bpzms 8 and 10 were
obtained as a mixture of two regioisomers, one trans and one
cis in a 2/1 ratio. Only one of the two possible cis isomers could
be identified by COSY, NOESY, and selective 1D-ROESY
experiments, and this was identified as the one with fewer
alkoxy chains in the 5-position of the pyrazole (Figure 2, cis
1). Therefore, steric effects appear to be of importance in the
region close to the methylene bridge and influence the relative
proportions of the dimeric isomers obtained. Repeated purifica-
tion enabled the separation and identification of the trans isomer
of compound 8 in its pure form, which helped us to make a
full assignment of the NMR signals through COSY and NOE
experiments (see Supporting Information).
The proton in the 4-position of the pyrazole (Hb) is almost
unchanged upon dimerization (compound 12) and/or complex-
ation (compounds 18 and 24), whereas the signals for the protons
of the methylene bridge (Ha) are shifted downfield upon
complexation (from 6.20 ppm in 12 to 6.80 ppm in 24).
The above discussion applies to symmetrical decyloxy-
substituted Hpz (1, 3, and 5) and derivatives. However, more
complex spectra were obtained with unsymmetrically substituted
Hpz compounds 2 and 4 due to the presence of regioisomers.
In these cases, three possible Bpzms can be obtained, two cis
In the 1H NMR spectra of the [ZnCl2(Bpzm)] complexes 20
and 22, the signals are broad but the splitting remains unchanged
in comparison to that in Bpzm 8 and 10. Thus, with unsym-
metrically substituted ligands, two regioisomers can also be
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A R T I C L E S
Cavero et al.
Figure 2. Possible Bpzm isomers derived from unsymmetrically substituted Hpz 2.
Table 1. Crystal Data and Refinement for Compounds 18 and 24
compound
18
24
chem formula
C42H48Cl2N4O12Zn‚CH2Cl2
C43H48Cl2N4O12Zn‚3CH2Cl2
crystal size/ mm3
0.27 × 0.23 × 0.08
0.39 × 0.33 × 0.15
crystal system
Triclinic
Triclinic
space group
formula weight
P1h
1022.04
P1h
1203.92
T/K
173(2)
173(2)
θ range data collec /deg
a /Å
2.72-26.41
10.8967(4)
2.65-26.39
10.98830(3)
b /Å
14.5460(7)
13.2389(3)
c /Å
16.0768(9)
18.4296(5)
R /deg
â /deg
γ /deg
Z
80.091(4)
69.480(4)
74.809(4)
2
92.625(2)
99.990(2)
97.269(2)
2
F (000)
1060
2294.06(19)
1.48
0.834
18149
7673 (Rint ) 0.0548)
0/589
1238
2612.83(9)
1.53
0.943
17658
8843 (Rint ) 0.0223)
0/696
V /Å3
D
calc /gr cm-3
µ/mm-1
collected reflns
unique reflns
restraints/params
final R indices [I > 2σ(I)]
R indices (all data)
R1 ) 0.0394, wR2 ) 0.0756
R1 ) 0.0918, wR2 ) 0.0824
R1 ) 0.0305, wR2 ) 0.0724
R1 ) 0.0456, wR2 ) 0.0760
observed in the same 2/1 ratio. Selective 1D-ROESY experi-
ments confirmed that a conformational effect occurs on com-
plexation, and this leads to shielding of the aromatic protons at
the 5-position of the pyrazole ring, as observed with the methoxy
compound 24.
1H NMR spectra of the [ZnCl2(Hpz)2] complexes 14 and 16
display broad signals that are shifted upfield with respect to
those in the Hpz ligands, and splitting is not observed. These
results suggest the existence of a metallotropic equilibrium in
solution, as observed for the methoxy compound 18.
Single-Crystal Structure of Compounds 18 and 24. The
hexamethoxy-substituted complexes 18 and 24 were prepared
with the explicit intention of providing further insights into the
molecular structure of these new complexes. X-ray quality single
crystals were obtained for both compounds by vapor diffusion
of hexane into solutions of the compounds in dichloromethane.
Data collection and parameters are given in Table 1. ORTEP
drawings of the molecular structures with atom numbering
schemes and tables containing representative distances and
angles are included in the Supporting Information.
Compound 18 crystallizes with a molecule of dichlo-
romethane in the triclinic space group P1h, with two formula
units per unit cell. The zinc atom is surrounded by two chlorine
atoms and two nitrogen atoms in a distorted tetrahedral geometry
with a Cl-Zn-Cl angle of 115.7° and an N-Zn-N angle of
119.4°. The largest angular deviation is found for the N-Zn-N
angle, a situation in contrast to that found in the structure of
[ZnCl2(dmpz)]69 where the Cl-Zn-Cl angle is 119.2° and the
N-Zn-N angle is 103.5°. This difference could be due to the
structure of the bulky pyrazole ligand. The NH groups of the
two pyrazole rings point in opposite directions in a transoid
conformation (Figure 3a). The mean planes containing the two
pyrazole ligands form an angle of 124°, which gives the
compound an overall roof-shape (Figure 3b). The compound is
arranged into dimers by a double intermolecular H-bond
between the chloro ligand of one molecule and an NH from a
pyrazole ring of another molecule situated almost in the same
plane (Figure 3c). Both of the NH‚‚‚Cl distances are ∼2.27 Å.
In the dimeric structure, only one pyrazole ring of each molecule
is involved in an H-bond, being the pyrazole rings not involved
in the H-bonding in parallel planes. These dimers are stacked
along the a-axis (Figure 3d).
Compound 24 also crystallizes in the triclinic space group
P1h with two formula units per unit cell. The zinc atom has a
distorted tetrahedral coordination with Cl-Zn-Cl and N-Zn-N
angles of 114.4 and 95.6°, respectively. The N-Zn-N angle
measured for this compound is beyond the upper limit of the
values (71-92°) found for other complexes of bis(pyrazolyl)-
(69) Bouwman, E.; Driessen, W. L.; de Graaff, R. A. G.; Reedijk, J. Acta Cryst.
1984, C40, 1562-1563.
9
11612 J. AM. CHEM. SOC. VOL. 129, NO. 37, 2007

Luminescent Liquid Crystalline Zinc Complexes
A R T I C L E S
Figure 3. Compound 18. (a) Detail of the geometry around the metal center, (b) molecular structure, (c) detail of the double intermolecular H-bond, and
(d) packing along the a-axis (benzene rings and their substituents have been omitted in (a), (c), and (d) for clarity).
alkanes.63,70-72 The carbon atom of the methylene bridge is also
distorted from the tetrahedral geometry as the N-C-N angle
is 112.4°. The central CN4Zn metallacycle possess a distorted
boat conformation in which the carbon atom is the bow and
the zinc atom is the stern, situated at angles of 60° (carbon atom)
and 10° (zinc atom) with respect to the mean plane formed by
the four nitrogens (Figure 1a). The boat shape gives a roof-
shaped morphology to the whole molecule, being the fold angle
between the mean planes which contain the two pyrazole rings
of 132.5° (Figure 4b). The conformation of the metallacycle
also forces the aromatic rings situated in the 5-position of the
pyrazole rings to arrange closely, at a mean distance of 4 Å in
an almost parallel fashion (the mean planes form an angle of
∼11°). This conformation probably exists in solution and causes
the anisotropic shielding effect of the ring current observed in
the 1H NMR experiments for the aromatic protons in those rings.
On the other hand, the presence of the metal atom makes the
aromatic rings in the 3-position of the pyrazoles lie further from
each other. Molecular packing along the b-axis consists of tilted
roof-shaped complexes intercalated by solvent molecules (one
ordered and two disordered dichloromethane molecules) and
arranged with same boat geometry, which is all-up or all-down
(Figure 4c).
Thermal and Mesomorphic Properties. The liquid crystal-
line properties of the new series of compounds were investigated
using polarized light optical microscopy (POM) and differential
scanning calorimetry (DSC). Transition temperatures and their
associated enthalpy changes are gathered in Table 2.
Pyrazole ligands do not show mesomorphic behavior, except
for Hpz 1, which displays two smectic phases.73 In the Bpzm
series, the flexible spacer and the lack of hydrogen bonding
gives rise to a considerable lowering of the transition temper-
atures compared with the pyrazole ligands, and compounds 10
and 11, with 10 and 12 alkoxy chains, respectively, are liquid
crystals and both display a hexagonal columnar phase at room
temperature with typical pseudofocal-conic textures.
The zinc complexes [ZnCl2(Hpz)2] 13-17 display varied
mesomorphism with a gradual change from calamitic to
(70) Bovio, B.; Cingolani, A.; Bonati, F. Z. Anorg. Allg. Chem. 1992, 610, 151-
156.
(71) Pettinari, C.; Cingolani, A.; Bovio, B. Polyhedron 1996, 15, 115-126.
(72) Pettinari, C.; Marchetti, F.; Cingolani, A.; Leonesi, D.; Colapietro, M.;
Margadonna, S. Polyhedron 1998, 17, 4145-4154.
(73) Barbera´, J.; Cativiela, C.; Serrano, J. L.; Zurbano, M. M. Liq. Cryst. 1992,
11, 887-897.
9
J. AM. CHEM. SOC. VOL. 129, NO. 37, 2007 11613

A R T I C L E S
Cavero et al.
Figure 4. Compound 24. (a) Boat-shape of the metallacycle, (b) molecular structure, and (c) packing along the b-axis (benzene rings and their substituents
have been omitted in (a) and (c) for clarity).
columnar mesophases on increasing the number of aliphatic
chains, despite the tetrahedral geometry and the low aspect ratio
of the molecular structures. In particular, complex 13 (with only
4 decyloxy chains) is not a liquid crystal but, on increasing the
number of chains to 6 and 8 (compounds 14 and 15), the
appearance of lamellar mesomorphism was observed with SmA
and SmC mesophases. A further increase to 10 chains (com-
pound 16) led to the disappearance of mesomorphism, and this
proved to be the crossover point from lamellar to columnar
mesophases because compound 17, with 12 decyloxy chains,
exhibits columnar mesomorphism over a wide thermal range,
including room temperature. A marked increase in transition
temperatures and extended liquid crystal behavior is observed
in these complexes in comparison to the Bpzm series due to
the possibility of the ZnCl2 bridge to interact by additional inter-
or intramolecular interactions such as dipolar interactions or
H-bonding.69,74
The complexes [ZnCl2(Bpzm)] 19-23 display a SmA meso-
phase (compound 20) or a columnar mesophase when they bear
6 or more chains (compounds 21-23). Therefore, for these
structures columnar behavior is preferred to lamellar behavior.
Zinc complexation of the chelate Bpzm ligands gives a six-
membered ring and makes the molecular core more rigid. The
fixed structure in these complexes is associated with increases
in transition temperatures and stabilities due to a better core/
tail microsegregation leading to the arrangement of the mol-
ecules in columns.
SmA phases (Figure 5b). The SmC phases display an unchar-
acteristic grainy texture (Figure 5c).
Structural Characterization of the Mesophases. All of the
mesomorphic compounds were studied by XRD to confirm the
type of mesophase and to study their supramolecular organiza-
tions. The observed reflections, proposed indexing, and lattice
constants are included in Table 3.
The X-ray diffraction patterns obtained for the SmA phases
at high temperatures for complexes 14 and 20, both of which
bear 6 alkoxy chains, contained a diffuse halo corresponding
to a mean distance of 4.4 Å. This feature is characteristic of
the liquid-like order of the aliphatic chains in a liquid crystal
phase. A single sharp reflection was also observed at small
angles. The type-A smectic mesophase (SmA) was assigned by
textural studies and the small angle reflection was therefore
indexed as the first-order reflection of a lamellar mesophase
with measured layer spacings of 28.7 and 29.0 Å for compounds
14 and 20, respectively. These layer spacings are considerably
smaller than those measured for the SmA phase of the rod-like
pyrazole ligand 1 (34.7 Å)73 and also smaller than for a Zn
complex derived from bis(pyrazolyl)ethane (32 Å).60 The small
value is reasonable if we bear in mind the particular geometry
of these molecules as nonplanar and roof-shaped, an arrange-
ment that may leave more free volume to accommodate the
conformationally disordered alkyl chains within the layer.
The monotropic nature of the SmC phases of complexes 14
and 15 did not allow us to perform high-temperature experi-
ments. However, we were able to freeze the mesophase at room
In both series of Zn complexes, all of the columnar phases
gave pseudofocal-conic textures on cooling from the isotropic
liquid (Figure 5a). Fan-shaped textures were observed for the
(74) Libertini, E.; Yoon, K.; Parkin, G. Polyhedron 1993, 12, 2539-2542.
9
11614 J. AM. CHEM. SOC. VOL. 129, NO. 37, 2007

Luminescent Liquid Crystalline Zinc Complexes
A R T I C L E S
Table 2. Phase Behavior of 7-11, 13-17, and 19-23
Figure 5. Microphotographs of the textures observed by POM. (a)
Columnar phase for 21 at 106 °C. (b) SmA phase for 20 at 123 °C. (c)
SmC phase for 14 at 80 °C.
Table 3. X-Ray Diffraction Data for Liquid Crystalline Compounds
compd
phase
T/
°
C
d-spacing/Å
Miller index/hkl
lattice constant/Å
10
11
14
Colh
Colh
SmA
SmC
r.t.
r.t.
90.0
r.t.
24.5
25.0
28.7
32.2
14.8
10.8
32.9
16.5
11.0
24.1
14.0
29.0
33.6
23.9
24.4
14.1
24.2
14.0
100
100
001
001
002
003
001
002
003
100
110
001
001
100
100
110
100
110
a ) 28.2
a ) 28.9
d ) 28.7
d ) 32.2
15
SmC
r.t.
r.t.
d ) 32.9
17
20
Colh
a ) 27.8
SmA
116
r.t.
90
d ) 29.0
d ) 33.6
a ) 27.5
a ) 28.2
21
22
Colh
Colh
r.t.
23
Colh
r.t.
a ) 27.9
as in the crystal structure of 18, or lateral correlations, as in
other folded structures with orthopalladated imines described
previously.75
a Cr, Cr’ ) crystal, I ) isotropic liquid, Colh ) hexagonal columnar,
SmA ) smectic A, SmC ) smectic C. b DSC onset peaks. c DSC peak
maximum. d Enthalpy sum of two transitions. e POM data.
The layer spacing of the SmC phase is larger than that of the
SmA phase (see XRD data for compound 14). This difference
is due to the loss of conformational freedom at low temperatures,
which causes the more extended chains to overcome the effect
of tilting (Figure 6a,b).76 The same behavior occurs for the SmA
phase of compound 20, where there is a significant increase of
the layer spacing on cooling down to room temperature (Figure
6c,d).
With respect to the columnar phases, compounds 10, 11, and
21 display typical POM textures and enthalpy values of
hexagonal columnar phases, but their X-ray diffraction patterns
only show a sharp maximum in the low-angle region and a
diffuse halo characteristic of the liquid-like order between the
aliphatic chains at 4.4 Å. Although this pattern does not
unambiguously confirm the type of mesophase, it rules out other
columnar symmetries like rectangular or oblique phases, and it
temperature, and this gave rise to characteristic patterns of
smectic mesophases. These patterns showed three sharp maxima
in the low-angle region with a reciprocal spacing ratio of
1:2:3. The maxima can be assigned to the (001), (002), and
(003) reflections of a lamellar arrangement. The diffuse halo
due to the liquid-like arrangement of the aliphatic chains also
appeared in these patterns at 4.4 Å. The textural studies, as well
as the presence of a SmA phase at higher temperatures in
complex 14, led us to confirm the type-C smectic mesophase
(SmC) for the compound. Taking into account the molecular
structure for the core provided by the methoxy-substituted
complexes and from the powder XRD data, we can propose
models for the arrangement of these complexes in the SmA and
SmC mesophases, in which roof-shaped molecules arrange in
layers (Figure 6). From the X-ray diffraction patterns, it is
impossible to extract further information about the arrangement
of the molecules within the layers, either as dimeric associations,
(75) Espinet, P.; Pe´rez, J.; Marcos, M.; Ros, M. B.; Serrano, J. L.; Barbera, J.;
Levelut, A. M. Organometallics 1990, 9, 2028-2033.
(76) Baena, M. J.; Barbera, J.; Espinet, P.; Ezcurra, A.; Ros, M. B.; Serrano, J.
L. J. Am. Chem. Soc. 1994, 116, 1899-1906.
9
J. AM. CHEM. SOC. VOL. 129, NO. 37, 2007 11615

A R T I C L E S
Cavero et al.
Figure 6. Schematic illustration of the packing modes of the complexes 14 and 20 in the lamellar phases.
has also been found in other structures previously described as
hexagonal columnar phases.58,77-82 In contrast, complexes 17,
22, and 23 yielded diffractograms that are characteristic of
hexagonal columnar mesophases. Patterns due to these phases
contain a set of two sharp maxima in the low-angle region with
a reciprocal spacing ratio of 1:x3. These maxima can be
assigned to the (100) and (110) reflections from the two-
dimensional hexagonal lattice. Furthermore, in all three cases,
only a diffuse band at 4.4 Å, characteristic of the liquid-like
arrangement of the aliphatic chains, was visible in the large
angle region, suggesting a poor structural correlation between
molecules along the columns.
The supramolecular organization in the columnar mesophase
can be analyzed by estimating the number of molecules in a
columnar stratum. For this calculation, we take into account
the relationship between the columnar cross-section Scol, ob-
tained from the a parameter of the hexagonal lattice, and the
molecular volume Vm,83 calculated with the assumption that the
density of these compounds in the mesophase at room temper-
ature is ∼1 g cm-3. Such a value is acceptable and is commonly
used in LCs. From this analysis, we obtain that a single molecule
of Bpzm compound 11 occupies the cross-section of one column
with a mean stacking distance of 5.3 Å. In the case of the Zn
complexes 17 and 23, a single complex occupies the cross-
section of one column with a larger mean stacking distance,
ca. 6.1 Å. As said before, the maximum corresponding to the
stacking distance could not be observed in any case in the XRD
experiments performed in the mesophase due to the diffuseness
together with the failure to obtain oriented patterns, which
indicates that the intracolumn order extends only to very short
distances. The a parameters of the hexagonal lattice for the zinc
complexes are similar; therefore, we exclude the formation of
dimers for the [ZnCl2(Hpz)2] complex 17 that could be proposed
on the basis of the single-crystal structure of the analogue
complex 18. The lattice parameter is similar to that found for
other nonplanar columnar LCs described previously (e.g.,
pyrazabole derivatives),58 supporting the conclusion that one
single complex occupies the cross section of one column. To
ensure a circular spreading of the flexible chains and a loose
correlation in the stacking, we propose that the complexes in
one column should adopt conformations in which the pyrazole-
phenyl torsion angles are different from those observed in the
single-crystal structures. The fact that both types of zinc
complexes (17 and 23) have almost the same lattice parameters
despite their different core structures is not surprising if we take
into account that they both arrange with one molecule per
column stratum, possess similar molecular volumes, and the
molten disordered state of the alkyl chains may adopt different
conformations around the core. In conclusion, the model
proposed for the columnar mesophase involves the stacking of
one asymmetric bent-disc molecule in each column for the
[ZnCl2(Hpz)2] complex 17 or the stacking of one symmetric
bent-disc for [ZnCl2(Bpzm)] complexes 21-23 (Figure 7).
Optical Properties. We investigated the optical properties
of the Bpzm, [ZnCl2(Hpz)2], and [ZnCl2(Bpzm)] series by UV-
vis and luminescence spectroscopies in dilute chloroform
solutions (Table 4). The spectra of the Bpzm series display a
featureless band at around 265 nm, and this corresponds to a
(77) Carfagna, C.; Roviello, A.; Sirigu, A. Mol. Cryst. Liq. Cryst. 1985, 122,
151-160.
(78) Gramsbergen, E. F.; Hoving, H. J.; Dejeu, W. H.; Praefcke, K.; Kohne, B.
Liq. Cryst. 1986, 1, 397-400.
(79) Mertesdorf, C.; Ringsdorf, H. Liq. Cryst. 1989, 5, 1757-1772.
(80) Zheng, H. X.; Carroll, P. J.; Swager, T. M. Liq. Cryst. 1993, 14, 1421-
1429.
(81) Barbera, J.; Puig, L.; Serrano, J. L.; Sierra, T. Chem. Mater. 2004, 16,
3308-3317.
(82) Barbera, J.; Bardaji, M.; Jimenez, J.; Laguna, A.; Martinez, M. P.; Oriol,
L.; Serrano, J. L.; Zaragozano, I. J. Am. Chem. Soc. 2005, 127, 8994-
9002.
(83) Both parameters are related by the equation c Scol) Z Vm, where c is the
stacking periodiciy along the columnar axis and Z is the number of
molecules within a columnar stratum. For the hexagonal lattice Scol ) x3/2
a2. Molecular volume Vm ) M / (0.6023 d), where M is the molecular mass
and d is the density of the material.
9
11616 J. AM. CHEM. SOC. VOL. 129, NO. 37, 2007

Luminescent Liquid Crystalline Zinc Complexes
A R T I C L E S
Figure 7. Schematic illustration of the packing mode for 17 (left) and 23
(right) in the columnar phase.
Table 4. Absorption and Emission Data
a
chain
λ
(log
abs/nm
λem
/
Stokes shift/
1
b
number
compd
ꢀ)
nm
cm-
I*/IBpzm*
4
6
7
13
19
8
14
20
9
15
21
10
16
22
11
17
23
263 (4.87)
276 (4.58)
277 (4.79)
263 (4.82)
272 (4.77)
277 (4.64)
265 (4.77)
270 (4.73)
274 (4.59)
266 (4.79)
272 (4.77)
279 (4.61)
267 (4.80)
277 (4.74)
277 (4.62)
338
351
367
347
384
405
341
382
401
363
390
419
385
412
437
8440
7740
8850
1
6.5
5.2
1
10.8
18.7
1
8.4
12.8
1
2.6
9.1
1
9200
10700
11400
8410
8
10900
11600
10000
11100
12000
11500
11800
13200
10
12
3.7
7.5
a Excited at the absorption maxima. b I* ) Intensity of the emission band
corrected with the absorption value at λexc. IBpzm* ) I* of the Bpzm ligand
with the same number of chains.
π-π* transition. The zinc complexes show a broader and less
absorptive band than the Bpzm ligands, with a bathochromic
shift of 10-15 nm.
All compounds are luminescent and exhibit an emission band
in the near UV-blue region with large Stokes shifts (up to 13 000
cm-1). Excimer emission is excluded as excitation spectra were
similar to absorption spectra. In all series, the emission
maximum shifts to the red on increasing the number of alkoxy
chains. A general trend is also observed if we compare the
compounds derived from the same ligand (i.e., with the same
number of chains); the emission maxima increase on going from
the Bpzm series to the [ZnCl2(Hpz)2] and to the [ZnCl2(Bpzm)]
series. To estimate the relative emission intensities, we compared
the normalized intensities and in all cases found a significant
chelation-induced emission enhancement (Table 4). In this
respect, the [ZnCl2(Bpzm)] chelate complexes are more lumi-
nescent (by 5-19 times) than the corresponding Bpzm ligands.
In terms of efficiency, we calculated the quantum yield of the
compounds with 10 chains to be 0.45% for 10, 1.3% for 16,
Figure 8. Absorption and emission spectra in dichloromethane solution
(blue), absorption and emission in the liquid crystalline state at r.t. (black),
and excitation spectra (red) for compounds 11, 17, and 23.
and 5.3% for 22 in dichloromethane solutions with respect to
quinine sulfate (54.6% in a 1 N H2SO4 solution).84 These values
follow the same trend as the tabulated normalized emission
intensities.
The optical properties of the liquid crystalline compounds
11, 17, and 23, all with 12 decyloxy chains, at room temperature
were studied in thin films prepared by pressing the samples
between two quartz plates. We found that all three compounds
(84) Eaton, D. F. J. Photochem. Photobiol. B 1988, 2, 523-531.
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J. AM. CHEM. SOC. VOL. 129, NO. 37, 2007 11617

A R T I C L E S
Cavero et al.
are luminescent in the near UV-blue region and exhibit a similar
trend in emission and intensities to the solution experiments,
where 23 is the compound with the highest emission wavelength
(Figure 8). By comparing solution and liquid crystalline state
spectra, we observe that compound 17 displays the same
emission wavelength both in solution and in solid state, whereas
an unexpected blue-shift was observed for 11 (26 nm) and 23
(33 nm). A blue-shift of the fluorescence maximum has been
observed previously in the columnar mesophase of intrinsically
luminescent liquid crystals when studying the compounds in
different phases and has been associated with a decrease in the
degree of disorder along the column and a reduction in the
ability of the molecules to form self-quenching aggregates.85
On the other hand, the design of tetrahedral or globular
molecules has proved to be a good synthetic strategy to prevent
interchromophore contacts that lead to aggregation or excimer
formation, red-shift of the emission bands, and quenching.86 In
our case, the nonplanar shape of the compounds and the
supramolecular organization proposed within the column (see
previous section) prevent intracolumnar correlation of the
mesogenic cores and could explain why there is no red-shift of
the emission band with respect to solutions.
Because the Stokes shift is usually dependent on the structural
relaxation of the excited state, the large values found in solution
may result from a significant conformational change that occurs
upon excitation. Structural changes are sensitive to their
microenvironment; therefore, in the liquid crystalline state, the
possible different conformation from solution, viscosity, and
local order affect the internal rotations leading to a blue-shift
in the emission maxima. This effect would be more important
for compounds 11 and 23 than for 17, according to the spectral
changes observed. More comprehensive studies are needed to
evaluate the mechanisms of this behavior as the compounds
may act as intrinsic viscosity probes via internal rotations.
Therefore, these liquid crystalline tetrahedral zinc complexes
show attractive photophysical properties that make them inter-
esting subjects for future studies in their own right. Furthermore,
mesomorphic glassy films of these materials may provide an
approach to noncrystalline but organized films of pure com-
pounds with luminescent properties without close intermolecular
contacts. Such systems could be useful for the design of novel
functional assemblies.
Conclusions
We have developed a synthetic strategy for the preparation
of luminescent tetrahedral zinc complexes that show liquid
crystal phases at room temperature. The synthetic route involves
complexation of zinc(II) chloride to Hpz and Bpzm ligands.
These compounds exemplify the delicate balance that leads to
liquid crystals in which there is an interplay between an
unfavorable tetrahedral core with a low aspect ratio and
supramolecular lamellar or columnar order. Luminescent prop-
erties have been found in the near UV-blue region and these
are associated with large Stokes shifts. The special molecular
geometry is also responsible for the optical properties in the
mesophase. In this respect, the loose yet ordered columnar
packing of luminescent molecules gives rise to emissive pure
thin films without appreciable formation of aggregates. The
photophysical properties and the potential for optoelectronic
applications and UV sensors are open lines to be explored in
the near future.
Acknowledgment. We thank the Ministerio de Educacio´n y
Ciencia and FEDER, projects MAT2003-07806-C02-01, MAT-
2006-13571-CO2-01, the Ramo´n y Cajal (R.G.) and FPU (E.C.)
programs, and the Gobierno de Arago´n for financial support.
Supporting Information Available: Full synthetic and ana-
lytical details for all new compounds. 1D-ROESY spectra for
1
12. H NMR spectra for the characterization of isomers in 8
and 20. Variable temperature 1H NMR spectra for 18. ORTEP
molecular drawings with atom numbering scheme and repre-
sentative distances for 18 and 24. Crystallographic information
files (CIF) for 18 and 24. This material is available free of charge
via the Internet at http://pubs.acs.org.
(85) Levitsky, I. A.; Kishikawa, K.; Eichhorn, S. H.; Swager, T. M. J. Am.
Chem. Soc. 2000, 122, 2474-2479.
(86) Wang, S.; Oldham, W. J. J.; Hudack, R. A. J.; Bazan, G. C. J. Am. Chem.
Soc. 2000, 122, 5695-5709.
JA073639C
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11618 J. AM. CHEM. SOC. VOL. 129, NO. 37, 2007