Silver pyrazole complexes with tunable liquid crystals and luminescent properties

Article abstract of DOI:10.1039/b9nj00753a

New silver ionic complexes based on pyrazole ligands of the type [Ag(Hpz^2R(n))₂][A] (Hpz^2R(n) = 3,5-bis(4-alkyloxyphenyl)pyrazole; R = C₆H₄OC _nH_2n+1; [A] = CH₃-p-C₆H ₄SO₃^- (PTS), CF₃SO₃ ^- (OTf)) (type I) are liquid crystal materials exhibiting photoluminescence in the solid state, in solution and in the mesophase. By way of contrast, the related counterparts [Ag(Hpz^R(n))₂][A] (Hpz^R(n) = 3-(4-alkyloxyphenyl)pyrazole; R = C₆H ₄OC_nH_2n+1; [A] = CH₃-p-C ₆H₄SO₃^- (PTS), CF₃SO ₃^- (OTf)) (type II), containing monosubstituted pyrazole ligands, are not mesomorphic but also behave as luminescent materials. The crystalline structures of 5 and 10, as representative examples of classes I and II, have been solved. Molecular "H" or "Z" shapes, depending on the cis or trans orientation of the NH groups of the pyrazole ligands, were found, respectively. The first type gave rise to a layer-planar network, while a corrugated layer was found for complex 10, these features being related to the mesomorphic behaviour of both kinds of compounds. The mesomorphic and luminescent properties of complexes I and II were compared with those of related compounds containing the less sterically-demanding counteranions PF ₆^-, NO₃^- and BF₄ ^-, described in our previous work. In all cases, the effect of the counteranions, as well as the substitution on the pyrazole groups, were analyzed for their corresponding properties. The Royal Society of Chemistry and the Centre National de la Recherche Scientifique.

Full text of DOI:10.1039/b9nj00753a

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PAPER
www.rsc.org/njc | New Journal of Chemistry
Silver pyrazole complexes with tunable liquid crystals and luminescent
propertiesw
a
a
a
Marıa Jose Mayoral, Paloma Ovejero, Jose Antonio Campo,
´
´ ´
a
b
c
´
Jose Vicente Heras, Marıa Rosario Torres, Carlos Lodeiro* and
´
Mercedes Cano*a
Received (in Gainesville, FL, USA) 11th December 2009, Accepted 11th June 2010
DOI: 10.1039/b9nj00753a
New silver ionic complexes based on pyrazole ligands of the type [Ag(Hpz2R(n))2][A]
ꢀ
(Hpz2R(n) = 3,5-bis(4-alkyloxyphenyl)pyrazole; R = C6H4OCnH2n+1; [A] = CH3-p-C6H4SO3
ꢀ
(PTS), CF3SO3 (OTf)) (type I) are liquid crystal materials exhibiting photoluminescence
in the solid state, in solution and in the mesophase. By way of contrast, the related
counterparts [Ag(HpzR(n))2][A] (HpzR(n) = 3-(4-alkyloxyphenyl)pyrazole; R = C6H4OCnH2n+1
;
ꢀ
ꢀ
[A] = CH3-p-C6H4SO3 (PTS), CF3SO3 (OTf)) (type II), containing monosubstituted pyrazole
ligands, are not mesomorphic but also behave as luminescent materials. The crystalline structures
of 5 and 10, as representative examples of classes I and II, have been solved. Molecular ‘‘H’’ or
‘‘Z’’ shapes, depending on the cis or trans orientation of the NH groups of the pyrazole ligands,
were found, respectively. The first type gave rise to a layer-planar network, while a corrugated
layer was found for complex 10, these features being related to the mesomorphic behaviour of
both kinds of compounds. The mesomorphic and luminescent properties of complexes I and II
were compared with thoseꢀof related compounds containing the less sterically-demanding
counteranions PF6ꢀ, NO3 and BF4ꢀ, described in our previous work. In all cases, the effect of
the counteranions, as well as the substitution on the pyrazole groups, were analyzed for their
corresponding properties.
and color filters, which transform a great part of the incident
Introduction
light into thermal energy.4 One way to overcome this short-
coming could be the use of luminescent liquid crystals.
On designing luminescent mesophases, metallomesogens offer
an interesting opportunity. They combine liquid crystal properties
with the characteristics of the metal ions they contain; therefore,
the typical photoluminescent behaviour of selected metal com-
plexes can be used as a support for extending those properties to
related metallomesogens.5
The design of molecular materials, in which light emission
or charge-transport properties are combined with a supra-
molecular organisation, could be a good strategy in order to
attain the required improvements demanded by electro-
optic devices for practical applications (displays, solar cells,
data treatment and storage date, image components).1 In this
context, luminescent liquid crystals could be used as multi-
functional materials on the basis that they combine lumines-
cence with the ordering of fluid phases.2,3
In particular, looking for new luminescent liquid crystal
materials, appropriately designed gold(I) and silver(I) com-
plexes can be considered good choices. Gold and silver com-
plexes frequently possess luminescence, this behaviour being
usually, but not necessarily, associated with the presence of
metal–metal interactions.6 Then, it is possible to suggest that
in the liquid crystal state, the supramolecular ordering of
mesophases could contribute to overtake metal–metal inter-
actions, and in this way favour the luminescence properties.
Since the report of the first luminescent metallomesogens of
gold(I),7 there have been growing developments in this type of
molecular material based specially on lanthanide ions,4,5,8–12
and to a lesser extent transition metals such as Pd,13 Ni,14
Pt,15,16 Zn,17,18 Cu,19 Ag20 and Au.21
The interest in liquid crystal materials as a support for the
modern technology of LCDs demands an increase of their
research, which is especially evident in cost-saving color LCD
displays. These devices present several limitations, such as low
brightness and energy efficiency, due to the use of polarisers
a
´
Departamento de Quımica Inorganica I,
Facultad de Ciencias Quımicas, Universidad Complutense,
´
´
E-28040 Madrid, Spain. E-mail: mmcano@quim.ucm.es
´
Laboratorio de Difraccion de Rayos-X,
Facultad de Ciencias Quımicas, Universidad Complutense,
b
c
´
E-28040 Madrid, Spain
´
´
Grupo BIOSCOPE, Departamento de Quımica-Fısica,
Facultade de Ciencias, Campus de Ourense, Universidade de Vigo,
32004 Ourense, Spain. E-mail: clodeiro@uvigo.es
Very recently, we have reported a series of ionic silver(I) and
gold(I) mesomorphic complexes based on pyrazole type ligands
containing long-chained alkyloxyphenyl R-substituents
w Electronic supplementary information (ESI) available: Selected lengths,
angles and hydrogen-bond geometries for 5 and 10 (Tables S1–S4),
together with analytical data of the compounds (Table S5). In addition,
Fig. S1 shows the ORTEP plot of the asymmetric unit of 10. CCDC
deposition numbers 756790 (5) and 756791 (10). For crystallographic
data in CIF or other electronic format see DOI: 10.1039/b9nj00753a
ꢀ
ꢀ
(R = C6H4OCnH2n+1), and PF6ꢀ, NO3 and BF4 counter-
anions, which exhibit an intense photoluminescent behaviour
in the solid state, in solution, as well as in the mesophase.22
c
2766 New J. Chem., 2010, 34, 2766–2776 This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2010

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We have proved that, independent of the nature of mesomorphic
Hpz2R(n) (Hpz2R(n) = 3,5-bis(4-alkyloxyphenyl)pyrazole) or
non-mesomorphic HpzR(n) (HpzR(n) = 3-(4-alkyloxyphenyl)-
pyrazole) ligands, they are able to induce mesomorphism
upon coordination to Ag(I) and Au(I) fragments. The non-
conventional ‘‘H’’ molecular shape and supramolecular order-
ing found in these complexes in the solid state are related to
liquid crystalline behaviour.
reaction of the pyrazole ligands Hpz2R(n) or HpzR(n) with
the corresponding silver salt AgA in a 2 : 1 molar ratio
(Scheme 1, Table 1). All compounds were isolated as white
solids, and characterised by elemental analysis (C, H, N) and
1
spectroscopic techniques (IR and H NMR) (see the Experi-
mental section). Matrix-assisted laser desorption/ionization
time-of-flight mass spectrometry (MALDI-TOF-MS) was also
used for complexes 1, 7 and 11.
In order to modify and hopefully improve the liquid crystal
and luminescence properties of this kind of compound, in this
work, we have extended our studies to ionic pyrazole-Ag(I)
complexes by using coordinative andꢀ sterically-demandinꢀg
counteranions such as CH3-p-C6H4SO3 (PTS) and CF3SO3
(OTf). In these systems, the influence of the counteranions on
the structural characteristics, as well as the photoluminescence
and liquid crystal properties, is studied and compared with
those of the related counterparts above-mentioned.22
The 1H NMR spectra of all the compounds in a CDCl3
solution at room temperature exhibited characteristic signals
of the alkyloxyphenyl (C6H4OCnH2n+1)-substituted pyrazole
ligands (Fig. 1). The presence of only one set of signals of each
type of proton is in agreement with the equivalence of the two
pyrazoles, as well as their substituents, this feature being
explained in terms of a metallotropic equilibrium similar to
that previously found for related complexes.23,24 On the other
hand, the absence of the NH-pyrazolic signal suggests the
presence of strong H-bond interactions. The resonances of
the alkyl protons of the C6H4OCnH2n+1 substituents appear in
the expected regions, not being influenced by the chain length.
The IR spectra in the solid state show, among other bands,
the characteristic bands of the pyrazole ligands and those of
the corresponding counteranions. As a general pattern, an
absorption band is observed at ca. 1600 cmꢀ1, associated with
n(CQN) and n(CQC) vibrations, and a sharp n(NH) band
Results and discussion
Synthesis and characterisation
The Ag(I) complꢀexes of the type I [Ag(Hpz2R(n))2][A] ([A] =
ꢀ
CH3-p-C6H4SO3 (PTS) (1–4), CF3SO3 (OTf) (5–9)) and
ꢀ
type II [Ag(HpzR(n))2ꢀ][A] ([A] = CH3-p-C6H4SO3 (PTS)
at ca. 3100–3300 cmꢀ1 25
The values of the n(NH) bands
.
agree with those observed in several complexes containing
(10–14) and CF3SO3 (OTf) (15–18)) were prepared by
Fig. 1 The aromatic region of the 1H NMR spectra of complexes
(a) [Ag(Hpz2R(12))2][CH3-p-C6H4SO3] (1), (b) [Ag(Hpz2R(12))2][CF3SO3]
(6), (c) [Ag(HpzR(12))2][CH3-p-C6H4SO3] (11) and (d) [Ag(HpzR(12))2]-
[CF3SO3] (15) in CDCl3.
Scheme 1 Schematic synthesis.
Table 1 Classification of the complexes, including their numbering
[Ag(Hpz2R(n))2][A]
[Ag(HpzR(n))2][A]
Number
n
A
Number
n
A
Type I
Type II
ꢀ
10
11
12
13
14
1
CH3-p-C6H4SO3ꢀ
CH3-p-C6H4SO3ꢀ
CH3-p-C6H4SO3ꢀ
CH3-p-C6H4SO3ꢀ
CH3-p-C6H4SO3
ꢀ
1
2
3
4
5
6
7
8
9
12
14
16
18
1
12
14
16
18
CH3-p-C6H4SO3ꢀ
CH3-p-C6H4SO3ꢀ
CH3-p-C6H4SO3ꢀ
CH3-p-C6H4SO3
IPTS
12
14
16
18
IIPTS
ꢀ
CF3SO3ꢀ
IOTf
IIOTf
ꢀ
CF3SO3ꢀ
CF3SO3ꢀ
CF3SO3ꢀ
CF3SO3
15
16
17
18
12
14
16
18
CF3SO3ꢀ
CF3SO3ꢀ
CF3SO3ꢀ
CF3SO3
c
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2010 New J. Chem., 2010, 34, 2766–2776 2767

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intermolecular NH-bond interactions,26 this result supporting
The molecular structure of [Ag(Hpz2R(1))2][OTf] (5) is
depicted in Fig. 2, and Table S1w lists select bond distances
and angles. The compound crystallises in the triclinic system,
space group P(ꢀ1). Because of minor disorder problems, the
oxygen and fluorine atoms of the OTf counteranion were only
refined isotropically (see Experimental section), this issue
being previously observed in compounds containing this
moiety.27
1
our previous H NMR deductions.
The presence of the counteranion OTf in compounds 5–9
and 15–18, and PTS in 1–4 and 10–14 is clearly confirmed by
the two bands at 1250 cmꢀ1, nas(SO3), and 1030 cmꢀ1, ns(SO3),
for the former, and at 1175 cmꢀ1, nas(SO3), and 1015 cmꢀ1
ns(SO3), for the latter.25
,
MALDI-TOF-MS spectra of complexes 1, 7 and 11 show
the peaks corresponding to the fragments [Ag(Hpz2R(12))]+,
[Ag(Hpz2R(14))]+ and [Ag(HpzR(12))]+, in agreement with a
1 : 1 metal/ligand stoichiometric ratio at m/z 695.4, 751.4 and
435.2, respectively, as the most intense signal. A peak corres-
ponding to the [AgA + Na]+ fragment is also clearly dis-
tinguished. Unfortunately, the molecular ion peak [M + A]+
(A = counteranion) could not be observed, thereby indicating
that these fragments are not stable under MALDI conditions,
in which the counteranion and one pyrazole ligand are lost
under the applied laser irradiation. However, these facts
suggest the high stability of the 1 : 1 species in all of the
complexes studied.
Crystal structures of complexes [Ag(Hpz2R(1))2][OTf] (5) and
[Ag(HpzR(1))2][PTS] (10)
All attempts to grow adequate crystals of the compounds
with long-chained ligands for X-ray diffraction studies were
unsuccessful. Therefore, we tried to obtain crystals of related
compounds containing shorter chain ligands. In particular we
were able to grow suitable crystals of [Ag(Hpz2R(1))2][OTf] (5)
and [Ag(HpzR(1))2][PTS] (10) by slow vapor diffusion of
hexane into a dichloromethane solution and then solving their
crystalline structures (Table 2).
Fig.
2
ORTEP plot of [Ag(Hpz2R(1))2][CF3SO3] (5) with 35%
probability. Hydrogen atoms, except for H2 and H4, have been
omitted for clarity.
Table 2 Crystal and refinement data for [Ag(Hpz2R(1))2][OTf] (5) and [Ag(HpzR(1))2][PTS] (10)
5
10
Empirical formula
Formula weight
[C34H32N4O4Ag][CF3SO3]
817.58
[C20H20N4O2Ag][CH3-p-C6H4SO3]
627.46
Crystal system
Space group
Space group number
a/A
Triclinic
P(ꢀ1)
Monoclinic
P21/c
14
10.223(1)
2
7.8645(4)
b/A
c/A
13.3014(7)
16.7157(9)
21.903(1)
12.946(1)
a (1)
b (1)
86.066(1)
88.383(1)
—
109.930(2)
g (1)
87.054(1)
1741.7(2)
—
2725.1(3)
V/A3
Z
T/K
2
296(2)
4
296(2)
F(000)
rc/g cmꢀ3
832
1.559
0.709
o and j
1280
1.529
0.860
o and j
m/mmꢀ1
Scan technique
Data collected
(ꢀ9, ꢀ15, ꢀ19) to (9, 15, 19)
(ꢀ12, ꢀ25, ꢀ15) to (11, 20, 15)
y range (1)
1.22 to 25.00
17121
1.86 to 25.57
15883
Reflections collected
Independent reflections
Completeness to maximum y (%)
Data/restraints/parameters
Observed reflections [I > 2Is(I)]
Ra
6106 (Rint = 0.0248)
99.6
6106/6/430
4633
0.0795
4613 (Rint = 0.0529)
90.2
4613/0/349
2667
0.0448
b
RwF
P
0.2641
0.1499
P
P
P
a
b
2
[|Fo| ꢀ |Fc|]/ |Fo|.
{
[w(Fo ꢀ Fc2)2]/ [w(Fo2)2]}1/2
.
c
2768 New J. Chem., 2010, 34, 2766–2776 This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2010

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The cationic unit shows a non-conventional ‘‘H’’ shape
defined by two disubstituted pyrazole rings, bonded to the
metal centre through the pyrazolic nitrogen. The coordination
environment around the silver atom is almost linear with an
N–Ag–N angle of 173.8(2)1 and Ag–N1 and Ag–N3 distances
of 2.127(5) A and 2.117(5) A, respectively, values similar to
those observed in related compounds containing heterocyclic
ligands.28
two molecules orthogonally arranged (Fig. S1w), with two
independent silver atoms, each lying on inversion centres.
The silver atom of each molecule is bonded to the two
pyrazolic nitrogens in a linear coordination with Ag1–N1 and
Ag2–N3 distances of 2.095(5) and 2.119(5) A, respectively, and
an N–Ag–N angle of 1801.
The cationic unit presents a high planarity, as deduced by
the dihedral angles between the phenyl rings and its own
pyrazole plane (dihedral angles 4.3(2) and 3.8(2)1). The alkyl
chains are almost in the same plane as the pyrazole rings, with
the two arms pointing out in opposite directions.
The phenyl rings from the two pyrazole substituents form
dihedral angles of 29.6(2) and 0.9(2)1, and 4.6(2) and 37.5(2)1
with their corresponding pyrazole planes, and are twisted
30.2(2) and 33.0(2)1 between them. The complex exhibits a
cis-orientation of the NH groups of both pyrazole ligands, as
deduced by the torsion angle t of 2.2(1)1, defined by the four
nitrogen atoms (N1, N2, N3 and N4) of the two pyrazole
ligands. The triflate counteranion is bonded through strong
hydrogen bonds with the NH groups of both heterocycles,
giving rise to an eight-membered metallocycle defined by the
atoms O6–N2–N1–Ag–N3–N4–O7–S1 (Table S2w).
On a supramolecular level, the bispirazolyl-silver cationic
units, bonded to their corresponding counteranion, are
packed in parallel layers in the (1 1 1) direction. The cationic
units adopt a face-to-face disposition, bonded through non-
conventional hydrogen bonds of the type C–Hꢁ ꢁ ꢁO of ca.
3.33 A, so generating a 1D network along the b axis (Fig. 3a).
Besides, the 1D chains connect with neighbouring chains
through new weak C–Hꢁ ꢁ ꢁO interactions, producing double
chains that are arranged through pꢁ ꢁ ꢁp interactions of ca.
3.64 A between phenyl and pyrazole rings (Fig. 3b).
As a relevant difference to the structural features observed
in 5, it is noted that the two cationic entities exhibit a trans-NH
group arrangement of the two pyrazole ligands, with a torsion
angle, t, of 1801. They are bonded to their corresponding OTf
counteranion through strong and bifurcated hydrogen bonds
involving the N2 and N4 atoms of the NH pyrazolic rings, and
the S1, O3 and S1A, O5A atoms of the two PTS counteranions
(Table S4w), so generating a special ‘‘Z’’ molecular shape, also
called a chair shape (Fig. 4).
On the other hand, the cationic entities [Ag(HpzR(1))2]+ are
also H-bonded to their corresponding counteranion, giving
As the same time, the double chains are bonded through
non-conventional C–Hꢁ ꢁ ꢁO hydrogen bonds of 3.45 A in a 2D
network defined by layers which lie parallel to the (1 1 1)
direction (Fig. 3c). Finally, new weak C–Hꢁ ꢁ ꢁO interactions
between layers can be considered as responsible for producing
a 3D network (Table S2w).
The molecular structure of [Ag(HpzR(1))2][PTS] (10) is
depicted in Fig. 4, and Table S3w lists selected bond distances
and angles. The compound crystallises in the monoclinic
system, space group P21/c. The asymmetric unit consists of
Fig.
4
ORTEP plot of [Ag(HpzR(1))2][CH3-p-C6H4SO3] (10)
with 30% probability. Hydrogen atoms, except for H2, and one entity
of the asymmetric unit have been omitted for clarity (symmetry
operations see Table S4w).
Fig. 3 The crystal structure of 5: (a) molecular chain along the b axis,
(b) double chain and (c) layered 2D network parallel to the (1 1 1)
direction.
Fig. 5 The crystal structure of 10: (a) columns, (b) schematic
representation of the A and B molecules in the asymmetric unit along
(1 0 ꢀ1) direction, and (c) a corrugated layer.
c
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2010 New J. Chem., 2010, 34, 2766–2776 2769

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rise to columns that exhibit Agꢁ ꢁ ꢁAg distances of 6.74 A
(Fig. 5a,b). Interactions of the type pꢁ ꢁ ꢁp between phenyl
groups of neighbouring columns of 3.6–3.8 A along the
(1 0 ꢀ1) direction produce a corrugated layer (Fig. 5c). New
C–Hꢁ ꢁ ꢁO interactions between layers can also be considered to
be responsible for a 3D network.
Mesomorphic behaviour
The thermal behaviour of all the new complexes was deter-
mined using polarised light optical microscopy (POM) and
differential scanning calorimetry (DSC) techniques. The nature
of the mesophases was confirmed by using X-ray diffraction
(XRD) at various temperatures. Table 3 summarises the
thermal data.
The compounds of type II (11–18) were not mesomorphic,
simply melting to the isotropic state.
By way of contrast, the complexes of type IPTS (1–4)
[Ag(Hpz2R(n))2][PTS] displayed mesomorphic enantiotropic
behaviour. On heating, SmA mesophases, characterised by
their typical fan-shaped texture, were observed at ca. 100 1C
(Fig. 6a). In addition to these mesophases, on cooling, SmC
mesophases exhibiting sanded or broken-focal conic textures
were also observed (Fig. 6b).
However the liquid crystal behaviour of the homologous
counterparts containing OTf (IOTf) as the counteranion was
restricted to those derivatives with n = 12 (6) and n = 14 (7),
which again exhibited enantiotropic SmA mesophases (Fig. 6c).
The DSC thermograms of complexes 1–4 show on the
first heating three endothermic peaks and on cooling three
exothermic peaks assigned as follows.
Fig. 6 Microphotographs of the textures on cooling observed by
POM of (a) SmA phase for [Ag(Hpz2R(18))2][CH3-p-C6H4SO3] (4) at
112 1C, (b) SmC phase for [Ag(Hpz2R(18))2][CH3-p-C6H4SO3] (4)
at 82 1C, and (c) SmA phase for [Ag(Hpz2R(14))2][CF3SO3] (7) at
109 1C.
For complexes 2 and 3, with n = 14 and n = 16, respec-
tively, the first two peaks from the solid–solid and solid–SmA
mesophase transitions are followed by a third one with a lower
enthalpy value, corresponding to the SmA–isotropic liquid
transition. On cooling from the isotropic liquid, the first
observed peak is due to the formation of an SmA mesophase
and the second is related to a new mesophase identified as
SmC. The last peak agrees with the crystallisation process. The
calculated enthalpy values of all the processes are consistent
with this assignment (Table 3).
Complex 4 shows an analogous thermogram on heating;
however, on cooling, the SmA–SmC transition observed by
POM at ca. 85 1C was not detected. A related feature is
observed for compound 1, whose DSC thermogram on cooling
shows only a peak corresponding to the formation of an SmA
mesophase from the isotropic liquid, the transition from SmA
to SmC mesophase being only detected by POM. In addition,
Table 3 Phase behaviour of complexes 1–4, 6 and 7 determined by POM and DSC
T/1C (DH/kJ molꢀ1
)
Cr
$
Cr0
$
SmC
$
SmA
$
I
n
Cycle
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
12
1
2
3
4
6
7
Heating
Cooling
Heating
Cooling
Heating
Cooling
Heating
Cooling
Heating
Cooling
Heating
Cooling
61 (37.2)
65a
81 (11.1)
70a
140 (3.0)
139 (ꢀ5.2)
b
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
14
16
18
12
14
51 (25.8)
53 (ꢀ16.4)
95 (26.2)
85 (ꢀ15.8)
71 (95.5)
61 (ꢀ11.2)
88 (20.5)
77 (ꢀ4.9)
105 (19.8)
100 (ꢀ5.1)
104 (8.3)
85a
145 (6.6)
144 (ꢀ6.4)
125 (6.0)
120 (ꢀ6.0)
127 (3.3)
126 (ꢀ3.9)
155 (7.0)
84 (30.8)
55 (ꢀ15.0)
60 (ꢀ11.5)
153 (ꢀ7.3)
97 (31.8)
150 (7.6)
149 (ꢀ4.0)
a
b
Temperatures determined by POM. crystal smectic glassy.
c
2770 New J. Chem., 2010, 34, 2766–2776 This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2010

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at 65 1C, the mobility of the SmC mesophase disappears,
although the optical texture is maintained, this fact suggesting
the formation of a crystal smectic glassy.
favourable packing produced by the spherical and bulky
ꢀ 22
counteranion PF6
.
On the other hand, within a given series of complexes, the
melting point of the mesomorphic homologues increases with
the chain length, e.g. between 81 and 105 1C for IPTS and
84 and 97 1C for IOTf, the same way that it has also been found
for IBF between 64 and 70 1C, and INO between 40 and
DSC thermograms for compounds 6 and 7 exhibit slight
differences. On heating, they show two endothermic peaks,
corresponding to the Cr–SmA and SmA–I transitions, and on
cooling, two peaks related to the expected formation of the
mesophase and crystallisation processes are observed.
4
3
68 1C.22 By way of contrast for IPF (whose melting point is, in
6
The clear variations between the thermal behaviour of the
IPTS and IOTf complexes evidence the influence of the counter-
anion on the mesomorphic properties, as already proposed in
our previous work.22 Therefore, at this point of the discussion,
a further comparison of the thermal results of the com-
plexes containing both the bulky (PTS, OTf) and small
(PF6ꢀ, NO3ꢀ, BF4ꢀ) counteranions is considered interesting
in order to shed light on the knowledge of the best structure–
properties relationship (Scheme 2). In particular, the melting
temperatures of the OTf derivatives are higher than those of
their homologues with PTS, this fact suggesting stronger
cation–anion interactions for the forꢀmer. However, in the
general terms, higher than those of the homologues IBF and
4
INO due to the best intermolecular packing),22 the increasing
of the length chain causes a lowering of the melting point.
Ignoring the behaviour of IPF , the increasing of this tempera-
3
6
ture with chain length could be explained by van der Waals
interactions between chains, which should be more important
than intermolecular core–core interactions.
The clearing point of the SmA phases decreases slightly in
all cases with the chain length, independent of the counter-
anion, giving rise to a lower stability range of the SmA
mesophase for the complexes with the longest chains.
All the above observations suggest that the melting transi-
tion may be well controlled by some of the intermolecular
interactions and ionic forces discussed above, and that these
can exert a greater effect than that of chain length alone.
ꢀ
ꢀ
,
related compounds containing PF6
NO3 and BF4
,
those with the bulkiest anion, PF6ꢀ, exhibit the higher melting
temperatures, these results being associated to the most
Scheme 2 Molecular shapes and mesomorphic behaviour of complexes [Ag(Hpz2R(n))2][A] (I) and [Ag(HpzR(n))2][A] (II): (a) and (b) ref. 22;
(c) and (d) current work.
c
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The compounds of type II with monosubstituted ligands
(11–18) displayed non-mesomorphic behaviour analogous to
that of the related compounds IIBF , IIPF and IINO , with
diffraction patterns of 2, 4 and 7, selected as representative
examples.
On cooling, the XRD diffractograms of compounds of type
IPTS confirmed the presence of two mesophases. So, for
example, for complex 2 at 120 1C, the small angle peaks at
2.5 and 4.81, and the two new ones at 2.4 and 4.61 at 70 1C with
a reciprocal space ratio of 1 : 2, indexed to the (0 0 1) and
(0 0 2) reflections, are typical of a layered arrangement. The
variation of the lamellar periodicity d at 70 1C with respect
to that of the corresponding diffractogram at 120 1C
(Table 4) evidences a SmA–SmC transition, which had also
been registered by DSC and observed by POM. It is important
to note that the layer spacing of the SmA mesophase is smaller
than that of the SmC phase, this feature being attributed to the
loss of conformational freedom at low temperature, which
causes the more extended chains to overcome the effect of
tilting.18,22,30
4
6
3
exception of the mesomorphism of IIPF , bearing 12 carbon
6
atoms on the alkyl chains (Scheme 2b,d).22 These complexes
exhibit bowl-shaped molecules, and the absence of liquid
crystal properties is explained as a consequence of the decreasing
interpenetration of the chains on the observed columnar
arrangement followed by a decreasing anisotropy and there-
fore weaker core–core interactions.22
In the actual complexes IIPTS and IIOTf, the ‘‘Z’’ molecular
shape appears to be responsible for the absence of liquid
crystalline properties. A similar behaviour was reported by
Lee et al.29 when considering the non-mesomorphic nature of
cationic silver bis-benzimidazole complexes associated with
their chair molecular shape. Selected complexes of the families
IPTS and IOTf were subjected to temperature-dependent
powder X-ray diffraction (XRD) measurements. The XRD
patterns show in all cases two or three sharp peaks in the small
angle region in a 1 : 1/2 : 1/3 ratio of the (0 0 1), (0 0 2) and
(0 0 3) reflections, corresponding to a lamellar structure, and a
broad halo at ca. 4.5 A owing to the liquid-like order
of the molten alkyl chains. This diffraction pattern is in
agreement with the lamellar structure of the SmA mesophase
assigned by POM. Table 4 summarises the results of the X-ray
Photophysical studies
Photoluminescence studies of selected complexes of type I
(1, 7) and type II (11, 15) were carried out. All selected
compounds, 1, 7, 11 and 15, are luminescent at room tempera-
ture, both in the solid state and in solution. Mesomorphic
complexes 1 and 7 were also luminescent in the liquid crystal
Table 4 X-Ray diffraction data of mesomorphic complexes
n
Complex
T/1C
Mesophase
SmA
Position (1)/2y
d-spacing/A
Miller indices (hkl)
14
2
135
2.4
4.8
7.0
19.3
2.5
4.8
19.8
2.4
4.6
19.2
2.3
4.6
18.9
2.3
18.7
2.1
19.4
2.4
4.8
7.1
36.2
18.5
12.0
4.6
36.3
18.5
4.5
37.0
19.0
4.6
38.2
19.3
4.7
39.1
4.7
42.7
4.6
36.5
18.3
12.3
4.3
(001)
(002)
(003)
a
120b
70b
SmA
SmC
SmA
(001)
(002)
a
(001)
(002)
a
18
4
7
125
(001)
(002)
a
120b
80b
SmA
SmC
SmA
(001)
a
(001)
a
14
100
(001)
(002)
(003)
a
20.1
a
b
Halo of the molten alkoxy chains. On cooling.
Table 5 Optical data for compounds 1, 7, 11 and 15 in chloroform solution at room temperature
Compound
lmax abs./nm
e/L molꢀ1cmꢀ1
[Conc.]/10ꢀ6
M
lemis/nm (lexc = 274 nm)
Dl/nma
Hpz2R(12)
1
267
270
270
260
260
265
55 063
57 500
38 900
62 231
97 960
66 300
3.24
1.00
1.00
2.25
1.00
1.00
362b
380
393
362b
320
327
95
110
123
101
60
7
HpzR(12)
11
15
62
a
b
Stokes shift in chloroform. lexc = 261 nm.
c
2772 New J. Chem., 2010, 34, 2766–2776 This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2010

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Table 6 Optical data for compounds 1, 7, 11 and 15 as solid samples at room temperature
Color (lexc = 366 nm)
UV lamp
Compound
lmax exc./nm
lemis/nm
lemis/nm (lexc = 360 nm)
Colora
1
7
11
15
265
265
265
265
352
386
394
328
372
398
455
450
White
White
White
White
Orange
Orange
Orange
Pale orange
a
Solid powder samples.
state, which has been proved by variable-temperature lumines-
cence studies.
the isotropic liquid until the solidification temperature (Fig. 7).
In this way, we could observe the fluorescence emission as a
function of temperature, as well as the aggregation state of the
compounds.
Solution studies. Absorption UV-vis spectra were measured
in CHCl3 solutions (ca. 10ꢀ6 M). In all cases, a broad
absorption band in the region of 240–320 nm was observed,
with the maximum centered at ca. 270 nm for compounds 1
and 7, and at ca. 265 nm for 11 and 15. This band, associated
with a p–p* electronic transition of the pyrazole group,6 is
slightly bathochromically-shifted with respect to the free
ligand but it is not modified by the different counteranions
(Table 5).
On heating, the behaviour of the emission band is similar
for both compounds, showing that the intensity of the fluores-
cence decreases upon the temperature increasing, and it almost
disappears at the clearing temperature. The pattern of the
band is maintained across the temperature range used on
heating and on cooling (Fig. 7). The emission maximum is
maintained in both the solid state and the liquid crystal state,
which is indicative that neither the structural changes nor the
mesophase viscosity affect the excited state.
Compounds I exhibit a broad fluorescence band in the near-
UV blue region. The band, attributed to a ligand-centered
transition (LCT), was red shifted to respect to that of the free
ligand (lemis = 362 nm), this shift depending on the counter-
anion nature (Table 5). So, for complexes 1 with PTS and 7
with OTf, it was observed at 380 nm (Stokes shift 110 nm) and
393 nm (Stokes shift 123 nm). By way of contrast, the emission
band of complexes 11 and 15 was blue-shifted, decreasing the
Stokes shift ca. 40 nm with respect to the free ligand (Table 5).
Solid studies. The emission spectra of complexes 1 and 7
exhibit a broad band from 300 to 500 nm (lexc = 265 nm),
attributed to a ligand-centered transition (Table 6). When an
excitation of 360 nm was used, a second lower intensity emission
band, corresponding to a metal-centered charge transfer
(MLCT), was registered.22 Analogous results were obtained
for complexes of type II (11 and 15).
The fluorescence emission of compounds in the solid
state corresponding to an LCT transition is blue-shifted with
respect to that observed in solution, the same way previously
22
observed for complexes IBF , IPF and INO .
3
In contrast, for
4
6
complexes IIPTS and IIOTf (11 and 15), this band is red-shifted,
the emission value being dependent on the counteranion
nature.
The differences in the luminescence behaviour of complexes
of type I and II in the solid state can be related to their
different molecular and structural features. Therefore, it is
suggested that the luminescent properties of complexes can be
modulated by modifying either the nature of the counteranion
or the substitution on the pyrazole ligand, which is in fact
responsible for the ‘‘H’’ (type I) or ‘‘Z’’ (type II) molecular
shapes found for these complexes.
Variable temperature studies. In order to prove the lumines-
cent behaviour of the compounds in the liquid crystal state, the
emission spectra of 1 and 7 were studied at different tempera-
tures from the solid state to the isotropic liquid. In both cases,
the fluorescence spectra were also registered on cooling from
Fig. 7 Fluorescence spectra of compounds (a) 1 and (b) 7 in the solid
state as a function of temperature (lexc = 274 nm). The spectra and
the temperature list follow the same top-to-bottom order (on cooling)
and bottom-to-top order on heating.
c
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shifts d are listed relative to Me4Si using the signal of the
deuterated solvent as reference (7.26 ppm), and coupling
constants J are in Hertz. Multiplicities are indicated as s
(singlet), d (doublet), t (triplet), m (multiplet), br s (broad
signal). The 1H chemical shifts and coupling constants are
accurate to ꢃ0.01 ppm and ꢃ0.3 Hz, respectively.
Phase studies were carried out by optical microscopy using
an Olympus BX50 microscope equipped with a Linkam
THMS 600 heating stage. The temperatures were assigned
on the basis of optic observations with polarised light.
Measurements of the transition temperatures were made
using a Perkin-Elmer Pyris 1 differential scanning calorimeter
with the sample (1–4 mg) sealed hermetically in aluminium
pans and with a heating or cooling rate of 5–10 K minꢀ1
.
The X-ray diffractograms at variable temperature were
recorded on a Panalytical X’Pert PRO MPD diffractometer
in a y-y configuration equipped with a Anton Paar HTK1200
heating stage (X-Ray Diffraction Service of Complutense
University).
Fig. 8 Fluorescence spectra of compound 11 in the solid state as a
function of temperature (lexc = 274 nm). The spectra and the
temperature list follow the same top-to-bottom order (on cooling)
and bottom-to-top order on heating.
Absorption spectra were recorded on a Shimadzu UV-2501PC
spectrophotometer and fluorescence emissions were measured
on a Horiba-Jobin-Yvon SPEX Fluorolog 3.22 spectro-
fluorimeter equipped with a ThermoNeslab RTE7 bath. The
linearity of the fluorescence emission vs. concentration was
checked in the concentration range used (10ꢀ4–10ꢀ6 M). A
correction for absorbed light was performed when necessary.
All spectrofluorimetric studies were performed as follows:
stock solutions of the ligands (ca. 10ꢀ3 M) were prepared by
dissolving an appropriate amount of the ligand in a 50 mL
volumetric flask and diluting to the mark with chloroform
HPLC or UVA-sol grades. Fluorescence spectra of solid
samples were recorded on the spectrofluorimeter exciting the
solid compounds at appropriated l (nm) using a fiber-optics
devices connected to the spectrofluorimeter. The emission
spectra as a function of the temperature were recorded in
the 300–800 nm range using a fiber optic system connected to
the spectrofluorimeter, and the solid samples were heated over
a hot plate with an external temperature control provided by a
digital thermo par.
The above results allow us to establish that the liquid crystal
state does not quench the emission.
On cooling, the emission tends to be recovered and increases
in intensity, although the total fluorescence emission is not
rescued. As an explanation of this result, it is possible to
suggest that the crystalline solid obtained after the crystalli-
sation process from the mesophase is different to that of the
initial solid before the first heating, which is in agreement
with the slight shift of the emission maximum from the
starting to final solid that is clearly observed in complex
[Ag(Hpz2R(14))2][OTf] (7) (Fig. 7b).
For comparative purposes, the variable temperature studies
of non-mesomorphic compound [Ag(HpzR(12))2][PTS] (11)
were also performed (Fig. 8). The most appreciable difference
with respect to the results obtained from the above-mentioned
mesomorphic complexes 1 and 7 (Fig. 7) is related to the shift
of the emission maximum to a lower wavelength when the
temperature is increased, indicating that the excited state is
slightly modified.
The MALDI-TOF-TOF-MS spectra of complexes 1, 7 and
11 were acquired on an Applied Biosystems 4700 proteomics
analyser using an N2 laser at 337 nm in the University of Vigo,
BIOSCOPE Group (Applied Biosystems, Foster City, CA,
USA). MALDI mass spectra were acquired in positive ion
reflectron mode, with an accelerating voltage of 20 kV, an ion
extraction delay of 80 ns, a grid voltage of 15 kV, and a guide
wire voltage of 40 V. The MALDI mass spectrum for each
sample was based on an average of 600 laser shots. All the data
were processed using the Data Explorer software package
(Applied Biosystems). All compounds were spotted on the
MALDI target plate (2 mg mLꢀ1) without any MALDI-matrix,
dissolving the compounds in acetonitrile or chloroform.
Experimental
Materials and physical measurements
All commercial reagents were used as supplied. The 3-
(4-alkyloxyphenyl)-1H-pyrazoles (HpzR(n)) and 3,5-bis(4-alkyl-
oxyphenyl)-1H-pyrazoles (Hpz2R(n)) (R = C6H4OCnH2n+1
;
n = 1, 12, 14, 16, 18) were prepared by procedures described
in previous work.31,32 Commercial solvents were dried prior
to use.
Elemental analyses for carbon, hydrogen and nitrogen were
carried out by the Microanalytical Service of Complutense
University. IR spectra were recorded on a FTIR Thermo
Nicolet 200 spectrophotometer with samples as KBr pellets
in the 4000–400 cmꢀ1 region: vs (very strong), s (strong), m
(medium), w (weak).
1H NMR spectra were performed at room temperature
on a Bruker DPX-300 spectrophotometer (NMR Service of
Complutense University) from solutions in CDCl3. Chemical
Crystallographic structure determination
Data collection for both compounds 5 and 10 was carried out
at room temperature on a Bruker Smart CCD diffracto-
meter using graphite-monochromated Mo-Ka radiation
(l = 0.71073 A), operating at 50 kV and 25 mA. Data were
c
2774 New J. Chem., 2010, 34, 2766–2776 This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2010

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3
C(4)H), 6.88 (4 H, d, JH,H 8.8, Hm), 7.21 (2 H, d, JH,H 8.0,
3
collected over a hemisphere of the reciprocal space by combi-
nation of three exposure sets. Each exposure of 20 s covered
0.31 in o.
The cell parameters were determined and refined by a least-
squares fit of all reflections.
3
Hm PTS), 7.63 (2 H, d, JH,H 2.3, C(5)H), 7.63 (4 H, d, JH,H
3
3
8.8, Ho), 7.89 (2 H, d, JH,H 8.0, Ho PTS)
[Ag(HpzR(12))2][OTf] (15). nmax(KBr)/cmꢀ1 3125w (NH),
1613m (CQN) + (CQC), 1250vs and 1028s (SO3). dH
(300 MHz; CDCl3; Me4Si) 0.88 (6 H, t, JH,H 6.5, CH3),
3
The first 50 frames were recollected at the end of the data
collection to monitor crystal decay, and no appreciable decay
was observed.
3
1.27 (36 H, m, CH2), 1.80 (4 H, m, CH2), 3.97 (4 H, t, JH,H
3
6.3, OCH2), 6.59 (2 H, br s, C(4)H), 6.93 (4 H, d, JH,H
A summary of the fundamental crystal and refinement
data is given in Table 2. For 5, a semi-empirical absorption
correction was applied.
8.0, Hm), 7.58 (4 H, d, 3JH,H 8.0, Ho), 7.68 (2 H, br s, C(5)H).
Conclusions
The structures were solved by direct methods and conven-
tional Fourier techniques and refined by applying full-matrix
least-squares on F2,33 with anisotropic thermal parameters for
the non-hydrogen atoms, with some exceptions. For 5, due to
a non-resolvable positional disorder found in the OTf group,
the O and F atoms were refined isotropically with restrained
S–O and C–F distances.
Mesomorphism of ionic silver complexes of type
I
[Ag(Hpz2R(n))2][A], containing disubstituted pyrazole ligands,
has been established on the basis of the adequation of the ‘‘H’’
molecular shape of their cationic entities to achieve the supra-
molecular ordering in the liquid crystal state. The mesophases
are controlled by inter-chain interactions over the core–core
interactions, their stability being favoured by the presence of
bulky and coordinating counteranions like PTS and OTf.
The absence of mesomorphic properties of the related
monosubstituted pyrazole compounds (type II) was also
rationalized in terms of the unfavourable ‘‘Z’’ or ‘‘U’’ molecular
shape of their cationic unities to achieve the ordering of
mesophases.
The hydrogen atoms were included at their calculated
positions determined by molecular geometry and refined
riding on the corresponding carbon atoms, except for the H2
and H4 bonded to N2 and N4 atoms, which were located from
the Fourier map included and were refined riding on the
corresponding nitrogen atoms for 5 and not refined for 10.
All complexes of types I and II exhibit luminescence in the
solid state and in solution at room temperature, this behaviour
being independent of substitution in the pyrazole ring. By way
of contrast, the presence of di- or monosubstituted pyrazole
ligands was reflected both in red or blue fluorescence shifts
with respect to that of their corresponding free ligand in the
order OTf > PTS, and in the fluorescence range (370–393 and
320–364 nm) for I and II, respectively.
Preparation of the complexes of the types I and II
To a solution of the corresponding 3,5-bis(4-alkyloxyphenyl)-
1H-pyrazole (Hpz2R(n)) or 3-(4-alkyloxyphenyl)-1H-pyrazole
(HpzR(n)) in dry tetrahydrofuran was added AgA in a 2 : 1
molecular ratio under a dinitrogen atmosphere (Scheme 1).
The mixture was stirred for 24 h in the absence of light, and
then filtered through Celites. The clear filtrate was removed
in vacuo and the residue was dissolved in dichloromethane.
The addition of hexane gave rise to a colorless solid, which was
filtered off, washed with hexane and dried in vacuo.
Fluorescence was also maintained in the liquid crystalline
state for complexes I.
In summary, we have established that the luminescent and
liquid crystalline properties of [Ag(L)2][A] (L = Hpz2R(n) or
HpzR(n)) can be modulated by controlling both the counter-
anion nature and substitution on the pyrazole ligand.
Spectroscopic IR and 1H NMR data are given for 1, 6
(type I) and 11, 15 (type II) as selected representative examples
of each type of compound. Data for the other homologues are
essentially identical. Yields and analytical data are collected in
Table S5 as ESI.w
Acknowledgements
[Ag(Hpz2R(12))2][PTS] (1). nmax(KBr)/cmꢀ1 3230m (NH),
1615s (CQN) + (CQC), 1175vs and 1035s (SO3). dH
(300 MHz; CDCl3; Me4Si) 0.88 (12 H, t, JH,H 6.3, CH3),
We thank the Secretarı
General de Investigacio
Innovacio
´
a de Estado de Investigacio
´
n (Direccio
´
n) del Ministerio de Ciencia
n
e
3
´
´
n of Spain, project CTQ2006-13344/BQU, and
1.27 (72 H, m, CH2), 1.78 (8 H, m, CH2), 2.39 (3 H, s, CH3
PTS), 3.91 (8 H, t, 3JH,H 6.3, OCH2), 6.67 (2 H, s, C(4)H), 6.76
(8 H, d, 3JH,H 8.3, Hm), 7.24 (2 H, d, 3JH,H 8.0, Hm PTS), 7.64
Universidad Complutense de Madrid (Spain), project UCM2008-
910300, for financial support.
3
3
C. L. is grateful to InOu-Universidade de Vigo at Ourense
Campus, Galicia (Spain) for financial support through project
2009K914122P64702 and thanks INCITE, Xunta de Galicia
(Spain) for the Isidro Parga Pondal Research program.
Masters student Sabina Rodriguez and Dr Laura Rodriguez
are kindly acknowledged for helping with some initial fluores-
cence measurements.
(8 H, d, JH,H 8.3, Ho), 7.96 (2 H, d, JH,H 8.0, Ho PTS).
[Ag(Hpz2R(12))2][OTf] (6). nmax(KBr)/cmꢀ1 3231m (NH),
1616m (CQN) + (CQC), 1254vs and 1030s (SO3). dH
3
(300 MHz; CDCl3; Me4Si) 0.88 (12 H, t, JH,H 6.4, CH3),
3
1.27 (72 H, m, CH2), 1.81 (8 H, m, CH2), 3.97 (8 H, t, JH,H
6.5, OCH2), 6.75 (2 H, s, C(4)H), 7.00 (8 H, d, 3JH,H 8.7, Hm),
3
7.71 (8 H, d, JH,H 8.7, Ho).
[Ag(HpzR(12))2][PTS] (11). nmax(KBr)/cmꢀ1 3150w (NH),
1613m (CQN) + (CQC), 1178vs and 1037s (SO3). dH
(300 MHz; CDCl3; Me4Si) 0.88 (6 H, t, JH,H 6.7, CH3),
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