Silver and gold luminescent metallomesogens based on pyrazole ligands

Article abstract of DOI:10.1039/b808487d

A series of ionic bis(pyrazole)-silver(i) and -gold(i) complexes [ML ₂][A] (M = Ag, Au; A = BF₄^-, PF ₆^-, NO₃^-), prepared by coordination of the mesomorphic L = Hpz^2R(n) or non-mesomorphic L = Hpz ^R(n) pyrazole ligands (Hpz^2R(n) = 3,5-bis(4- alkyloxyphenyl)pyrazole; Hpz^R(n) = 3-(4-alkyloxyphenyl)pyrazole), has been studied. The complexes exhibit enantiotropic behaviour, showing smectic A (SmA) mesophases. The choice of the ligands allows the achievement of 'H' or 'U' molecular shapes, which appear to be responsible for the attainment of liquid crystal mesophases, these not being dependent on the coordinating or non-coordinating nature of the A counteranions. The new complexes are photoluminescent both in the solid state and in solution at room temperature. In addition, the luminescent behaviour of selected compounds as a function of the temperature indicates that the luminescence is maintained in the mesophase.

Full text of DOI:10.1039/b808487d

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PAPER
www.rsc.org/dalton | Dalton Transactions
Silver and gold luminescent metallomesogens based on pyrazole ligands†
Mar´ıa Jose´ Mayoral,a Paloma Ovejero,a Jose´ Antonio Campo,a Jose´ Vicente Heras,a Elena Pinilla,a,b
Mar´ıa Rosario Torres,b Carlos Lodeiro*c and Mercedes Cano*a
Received 20th May 2008, Accepted 12th September 2008
First published as an Advance Article on the web 3rd November 2008
DOI: 10.1039/b808487d
-
-
A series of ionic bis(pyrazole)-silver(I) and -gold(I) complexes [ML2][A] (M = Ag, Au; A = BF4 , PF6 ,
NO3 ), prepared by coordination of the mesomorphic L = Hpz2R(n) or non-mesomorphic L = HpzR(n)
-
pyrazole ligands (Hpz2R(n) = 3,5-bis(4-alkyloxyphenyl)pyrazole; HpzR(n) = 3-(4-alkyloxyphenyl)-
pyrazole), has been studied. The complexes exhibit enantiotropic behaviour, showing smectic A (SmA)
mesophases. The choice of the ligands allows the achievement of ‘H’ or ‘U’ molecular shapes, which
appear to be responsible for the attainment of liquid crystal mesophases, these not being dependent on
the coordinating or non-coordinating nature of the A counteranions. The new complexes are
photoluminescent both in the solid state and in solution at room temperature. In addition, the
luminescent behaviour of selected compounds as a function of the temperature indicates that the
luminescence is maintained in the mesophase.
showing luminescence in the solid state and/or in solu-
Introduction
tion are limited to a few compounds which contain gold,9,10
silver,11 palladium,12 platinum13 or nickel14 centres. In addi-
tion, compounds exhibiting luminescence in the mesophase are
quite rare and include those showing mesomorphism at room
temperature.2,15–18 However, it is remarkable to note that in the last
few years Espinet et al. have reported several stable mesomorphic
tetrafluorophenyl gold(I) isocyanide complexes which represent
the first examples of luminescent metallomesogens displaying
strong luminescence in the mesophase, the solid state and in
solution.17
Ordered molecular materials are increasingly used in active
electronic and photonic devices and the supramolecular structures
of liquid crystals can be tailored to improve such devices in order to
make their performances conform to the requirements for practical
applications.1 New technologies require the use of devices based on
physical properties as light emission or charge transport (displays,
solar cells, data treatment and storage, image components), and
in this context emitting liquid crystals are useful materials for
displays applications.2,3
In particular, the interest in liquid crystal materials as a support
of the modern technology of LCDs demands an increase in their
research which is specially evident in the cost-saving color LCDs
displays. Those devices present several limitations such as low
brightness and energy efficiency due to the use of polarizers and
color filters which transform a great part of the incident light into
thermal energy4 and one way to overcome this shortcoming can
be the use of luminescent liquid crystals.
Although a large number of luminescent liquid crystal or-
ganic compounds have been reported, metallomesogens (metal-
containing liquid crystals) exhibiting luminescence are scarcely
described, and most of them involve lanthanide derivatives.4–8
However, luminescent transition metal-based metallomesogens
On designing luminescent mesophases, the metallomesogens
offer an interesting opportunity. They combine the liquid crystal
properties with those characteristic of the metal ions that they
contain, therefore typical photoluminescent behaviour of selected
metal complexes can be used as a support for extending those
properties to related metallomesogens.
In this context, looking for new luminescent liquid crystal
materials, appropriately designed gold(I) and silver(I) complexes
can be considered as a good choice. Gold and silver complexes
frequently possess luminescence, this behaviour being usually,
but not necessarily, associated with the presence of metal–metal
interactions.9 Then, it is possible to suggest that in the liquid
crystal state the supramolecular ordering of the mesophases
could contribute to overtake metal–metal interactions, and by this
favouring the luminescence properties.
Previous works from our lab have been driven to estab-
lish the mesomorphism of metal complexes based on pyrazole
ligands containing long-chained alkyloxyphenyl R substituents
(R = C6H4OCnH2n + 1). We have proved that, independent of the
mesomorphic nature of the 3,5-disubstituted pyrazoles Hpz2R(n)
or the non-mesomorphism of the 3-substituted ligands HpzR(n)
(Hpz2R(n) = 3,5-bis(4-alkyloxyphenyl)pyrazole; HpzR(n) = 3-(4-
alkyloxyphenyl)pyrazole), both were able to induce mesomor-
phism upon coordination to several metal fragments.19,20 So the
coordination of the non-mesomorphic HpzR(12) ligand to the AuCl
fragment gave rise to the complex [AuCl(HpzR(12))] which, in
aDepartamento de Qu´ımica Inorga´nica I, Facultad de Ciencias Qu´ımicas,
Universidad Complutense, E-28040, Madrid, Spain. E-mail: mmcano@
quim.ucm.es
bLaboratorio de Difraccio´n de Rayos-X, Facultad de Ciencias Qu´ımicas,
Universidad Complutense, E-28040, Madrid, Spain
cREQUIMTE, Departamento de Qu´ımica, Faculdade de Cieˆncias e Tecnolo-
gia, Universidade Nova de Lisboa, 2829-516 Campus da Caparica, Monte
De Caparica, Portugal. E-mail: lodeiro@dq.fct.unl.pt
† Electronic supplementary information (ESI) available: Synthesis and
characterization of the new compounds, including a table containing the
analytical data (Table S1); absorption and normalized fluorescence of
compounds 1, 5, 13 and 15 (Fig. S1). CCDC reference numbers 688083.
For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/b808487d
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Table 1 Classification of the compounds including the numbering
[Ag(Hpz2R(n))2][A] [Ag(HpzR(n))2][A]
addition to the liquid crystal behaviour, exhibited luminescent
properties.9
On the other hand, we have also recently reported the crys-
tal structure of the related luminescent compounds containing
the disubstituted pyrazole with shorter alkyl chains (n = 4),
[Ag(Hpz2R(4))2][BF4] and [Au(Hpz2R(4))2][O3S–C6H4–p-CH3], both
exhibiting an unconventional ‘H’ shape (Fig. 1 and 2a),21 which has
been found to be consistent with the supramolecular ordering of
their mesophases.22,23 Then we considered that related compounds
with monosubstituted pyrazoles HpzR(n) should be expected to
form ‘U’ shaped molecules, which have also been proved to be
useful for liquid crystal materials (Fig. 2c).24,25
Number
n
A
Type I
Number
n
A
Type III
-
-
-
-
-
-
-
-
1
2
3
4
12 BF4
14 BF4
16 BF4
18 BF4
IBF4
19
20
21
22
12 BF4
14 BF4
16 BF4
18 BF4
IIIBF4
-
-
-
-
-
-
-
-
-
5
6
7
8
12 PF6
14 PF6
16 PF6
18 PF6
IPF6
23
24
25
26
27
4
PF6
IIIPF6
12 PF6
14 PF6
16 PF6
18 PF6
-
-
-
-
-
-
-
-
9
12 NO3
14 NO3
16 NO3
18 NO3
INO3
28
29
30
31
12 NO3
14 NO3
16 NO3
18 NO3
IIINO3
10
11
12
-
32
4
PO2F2
[Au(Hpz2R(n))2][A]
[Au(HpzR(n))2][A]
Type II Number
Number
n
A
n
A
Type IV
-
-
13
14
12 BF4
14 BF4
IIBF4
IIPF6
IINO3
33
34
12 BF4
14 BF4
IVBF4
-
-
-
-
-
-
15
16
12 PF6
14 PF6
35
36
12 PF6
14 PF6
IVPF6
IVNO3
Fig. 1 Molecular structure of [Ag(Hpz2R(4))2][BF4] exhibiting an uncon-
ventional ‘H’ shape.21
-
-
-
-
17
18
12 NO3
14 NO3
37
38
12 NO3
14 NO3
Following those precedents, in this work we are involved in the
synthesis and study of new complexes of the types [M(HpzR(n))2][A]
and [M(Hpz2R(n))2][A] (R = C6H4OCnH2n + 1, M = Ag, Au; A =
the mesophases (Fig. 2). Different counteranions were considered
to check their influence on the liquid crystal properties. Silver
and gold(I) centres were also selected as support for luminescence
properties. Table 1 depicts the compounds described in this work,
including the type and the numbering.
-
-
-
BF4 , PF6 , NO3 ) containing mono- and disubstituted pyrazole
ligands as well as different counteranions. The selection of the
ligands was directed to attain potential ‘H’ or ‘U’ molecular
shapes as building blocks to achieve the required ordering on
Fig. 2 Schematic representation of the ‘H’ and ‘U’ molecular shape ((a) and (c)) and their potential ordering on the mesophases ((b) and (d)) for
[M(Hpz2R(n))2][A] and [M(HpzR(n))2][A] complexes, respectively.
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Yvon SPEX Fluorolog 3.22 spectrofluorimeter equipped with
a ThermoNeslab RTE7 bath. The linearity of the fluorescence
emission vs. concentration was checked in the concentration
range used (10-4–10-6 M). A correction for the absorbed light
was performed when necessary. All spectrofluorimetric titrations
were performed as follows: the stock solutions of the ligands
(ca. 10-3 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).
Experimental
Materials and physical measurements
All commercial reagents were used as supplied. The 3-(4-alkyl-
oxyphenyl)-1H-pyrazoles (HpzR(n)) and 3,5-bis(4-alkyloxyphenyl)-
1H-pyrazoles (Hpz2R(n)) (R = C6H4OCnH2n + 1; n = 4, 12, 14,
16, 18) and the starting Au complex [AuCl(tht)] (tht = tetrahy-
drothiophene) were prepared by procedures described in previous
works.19,20,26 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 cm-1 region: vs (very strong), s (strong), m (medium), w (weak).
1H NMR spectra were performed at room temperature on
a Bruker AC200 or on a Bruker DPX-300 spectrophotometers
(NMR Service of Complutense University) from solutions in
CDCl3. Chemical shifts d are listed relative to Me4Si using the
signal of the deuterated solvent as reference (7.26 ppm), and
coupling constants J are in hertz. Multiplicities are indicated as s
(singlet), d (doublet), t (triplet), m (multiplet), br (broad signal).
Preparation of the complexes of the types I and III
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 tetrahydrofurane was added AgA in a 2 : 1 molecular ratio
under dinitrogen atmosphere (Scheme 1). The mixture was stirred
for 24 h in absence of light, and then filtered through Celite. The
clear filtrate was removed in vacuo and the residue was dissolved
in dichloromethane. Addition of hexane gave rise to a colorless
solid, which was filtered off, washed with hexane and dried
in vacuo. Spectroscopic and analytical data are given as ESI.†
1
The H chemical shifts and coupling constants are accurate to
0.01 ppm and 0.3 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 polarized light.
Preparation of the complexes of the types II and IV
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 min-1.
The X-ray diffractograms at variable temperature were recorded
on a Panalytical X’Pert PRO MPD diffractometer in a q–q
configuration equipped with a Anton Paar HTK1200 heating stage
(X-Ray Diffraction Service of Complutense University).
Absorption spectra were recorded on a Shimadzu UV-2501PC
spectrophotometer and fluorescence emission on a Horiba-Jobin-
To a mixture of AgA and [AuCl(tht)] in dry tetrahydrofu-
rane was added a solution of the corresponding 3,5-bis(4-
alkyloxyphenyl)-1H-pyrazole (Hpz2R(n)) or 3-(4-alkyloxyphenyl)-
1H-pyrazole (HpzR(n)) in a 1 : 1:2 molecular ratio under dinitrogen
atmosphere (Scheme 1). The mixture was stirred for 24 h in
absence of light, and then filtered through Celite. The clear
filtrate was removed in vacuo and the residue was dissolved in
dichloromethane. Addition of hexane gave rise to a colorless solid,
which was filtered off, washed with hexane and dried in vacuo.
Spectroscopic and analytical data are given as ESI.†
Scheme 1
6914 | Dalton Trans., 2008, 6912–6924
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Table 2 Crystal and refinement data for [Ag(HpzR(4))2][PO2F2] (32)
The supplementary crystallographic data have been passed to
the Cambridge Crystallographic Data Centre (CCDC deposition
number 688083). For crystallographic data in CIF or other
electronic format see DOI: 10.1039/b808487d
32
Empirical formula
Formula weight
Crystal system
Space group
[C26H32N4Ag][PO2F2]
641.40
Monoclinic
C2/c
25.047(4)
7.936(1)
16.167(3)
117.768(3)
2840.0(8)
4
Results and discussion
˚
a (A)
˚
b (A)
Synthetic studies and structural characterization
˚
c (A)
b (◦)
V (A )
Z
The silver(I) compounds [Ag(Hpz2R(n))2][A] (type I) and
[Ag(HpzR(n))2][A] (type III) were prepared by reaction of the
pyrazole ligands and the corresponding silver salts AgA according
to Scheme 1. The related gold(I) complexes [Au(Hpz2R(n))2][A] (type
II) and [Au(HpzR(n))2][A] (type IV) were obtained by reaction
of [AuCl(tht)] with the selected pyrazole and addition of the
corresponding silver salt AgA (Scheme 1). Table 1 shows the
classification of compounds including the numbering.
3
˚
T (K)
296(2)
1312
1.500
0.817
F(000)
rcalc. (g cm-3)
m (mm-1)
Scan technique
Data collected
q range (◦)
w and j
(-29,-5, -19) to (29,9,16)
1.84–25.00
10553
Reflections collected
Independent reflections
Completeness to maximum q
Data/restraints/parameters
Observed reflections [I > 2s(I)]
Ra
All Ag(I) and Au(I) derivatives were isolated as white solids and
they were fully characterized by elemental analysis and IR and
NMR spectroscopies (see Experimental section and ESI†). The
Au derivatives containing the longest chains (n = 16, 18) were
always isolated as mixtures from which we were unable to obtain
the pure complexes.
2511 (Rint = 0.1584)
100%
2511/3/160
1001
0.0652
b
RwF
0.2017
ꢀ
ꢀ
ꢀ
ꢀ
1
a
2
1/2
[|Fo| - |Fc|]/ |Fo|. b
{
[w(Fo - Fc2)2]/ [w(Fo2)2]}
.
The H NMR spectra in CDCl3 solution at room temperature
show in all cases signals attributed to the pyrazole ligands. Silver
compounds I containing 3,5-disubstituted pyrazoles exhibit one
set of signals each type of protons, indicating the equivalence of
the two pyrazoles as well as of their substituents. By contrast,
the spectra of the related gold compounds II show two signals
of the Ho, Hm and OCH2 protons of the pyrazole substituents
and a single signal of the H4 pyrazolic proton, according with
the unequivalence of the substituents each of the two equivalent
pyrazoles.
The above results can be explained in terms of the presence
of a metallotropic equilibrium for the silver derivatives, similar
to that previously found for related complexes.21,28 The absence
of NH-pyrazolic signals of these silver compounds, in contrast to
those observed for the gold derivatives in the region of 12–14 ppm,
agrees with the metallotropic equilibrium above suggested.
The complexes containing 3-substituted pyrazoles (III, IV) show
well-defined signals each proton, indicating the equivalence of the
two pyrazole ligands.
X-Ray structure determination of [Ag(HpzR(4))2][PO2F2] (32)
Yellow prismatic single crystals of compound [Ag(HpzR(4))2]-
[PO2F2] suitable for X-ray diffraction experiments were success-
fully grown by slow diffusion of hexane into a dichloromethane
solution of the parent compound [Ag(HpzR(4))2][PF6] (23). Data
collection was carried out at room temperature on a Bruker
Smart CCD diffractometer, with graphite-monochromated Mo-
˚
Ka radiation (l = 0. 71073 A), operating at 50 kV and 30 mA. The
data were collected over a hemisphere of the reciprocal space by
combination of the three exposure sets. Each frame exposure time
was of 10 s, covering 0.3◦ in w. The first 100 frames were recollected
at the end of the data collection to monitor crystal decay. No
appreciable drop in the intensities of standard reflections was
observed. The cell parameters were determined and refined by a
least-squares fit of all reflections. A summary of the fundamental
crystal data and refinement data is given in Table 2. The value of
Rint for independent reflections is slightly high. The quality of the
crystal, along with its needle-type morphology, have not allowed
to get lower Rint. In spite, other agreement parameters are adequate
to give a good convergence.
Finally it is interesting to note the influence of the counteranion
on the chemical shifts. So, it can be observed that, in general terms,
the resonances of the compounds carrying the most coordinative
-
counteranion (NO3 ) appear slightly more downfield shifted than
those of the compounds containing counteranions with low
-
-
coordinative ability (BF4 , PF6 ).
The structure was solved by direct methods and conventional
Fourier techniques. The refinement was done by full-matrix
least-squares on F2.27 Anisotropic parameters were used in the
last cycles of refinement for all non-hydrogen atoms except for
C13, C14 and C15, which have been refined isotropically with
geometrical restraints and a variable common carbon–carbon
distance. Hydrogen atom bonded to N2 atom has been located in
a Fourier synthesis, included and refined as riding on the bonded
atom. The remaining hydrogen atoms were included in calculated
positions and refined as riding on their respective carbon atoms.
Final R (Rw) values were 0.065 (0.2017).
The infrared spectra in the solid state show the characteristic
bands of pyrazole ligands as well as those of the corresponding
=
=
counteranions. In particular the (n(C N) + n(C C)) and n(NH)
absorption bands from the ligands appear at ca. 1600 cm-1 and
in the 3200–3400 cm-1 range, respectively. On the other hand, the
presence of the counteranions is also established from the strong–
medium broad bands at ca. 850 cm-1 n(PF), 1380 cm-1 n(NO)
-
-
and 1030 cm-1 n(BF) from the counteranions PF6 , NO3 and
BF4 , respectively.29 These bands exhibit slight splitting and/or
-
weak shoulders which can suggest the presence of coordinative
or hydrogen bonding interactions (M ◊ ◊ ◊ X, X ◊ ◊ ◊ H–N; X = F
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of 177.8(5)◦ agrees with the typical linear coordination around the
silver centre.
◦
˚
Table 3 Selected lengths (A) and angles ( ) for 32
Ag–N1
N1–N2
N2–C3
C3–C4
C4–C5
N1–C5
P–O2
2.089(7)
1.350(9)
1.36(1)
1.38(1)
1.35(1)
1.33(1)
1.420(6)
1.510(5)
1.00
N1–Ag–N1¢
N2–N1–Ag
C5–N1–Ag
C5–N1–N2
N1–N2–C3
N2–C3–C4
C3–C4–C5
O2–P–F
177.8(5)
122.0(6)
134.3(7)
103.6(7)
112.2(7)
105.2(7)
106.0(8)
108.4(4)
142.4
The pyrazole ligands exhibit a cis orientation of the NH groups,
as deduced by the torsion angle t defined by the four nitrogen
atoms of 19(1)◦. From this disposition, the PO2F2 counteranion
-
is bonded through a moderate hydrogen-bond between each
O2 atom with each NH group of the cationic unit (Table 3),
giving rise to an almost planar N2–N1–Ag–N1¢–N2¢–O2¢–P–O2
(symmetry operation: -x, y, -z + 3/2) metallocycle with the
P–F
N2–H2
N2 ◊ ◊ ◊ O2
O2 ◊ ◊ ◊ H2
C3–N2–H2
N1–N2–H2
N2–H2 ◊ ◊ ◊ O2
2.702(9)
1.72
105.2
166.8
˚
maximum deviation of 0.32(1) A observed in the O2 atoms. The
cis-molecular cation adopts a special ‘U’ or bowl-like molecular
shape (Fig. 3) in which the pyrazole planes are twisted at an angle
of 19.6(6)◦, and the benzene plane with its own pyrazole plane
forms an angle of 9.8(7)◦. The alkyl chains are almost in the same
plane as the pyrazole rings, with the two arms pointing out in the
same direction.
Symmetry operation (¢): -x, y, -z + 3/2.
or O) in agreement with the lowering of the symmetry of the
free counteranions. Structural data of the hydrogen-bonding
interactions N–H ◊ ◊ ◊ O observed in 32, as will be described later,
support the above suggestion.
The cationic entities [Ag(HpzR(4))2]+ are packed in interdigitated
bilayers in which neighbouring cations are oriented with their
alkyl chains alternated (Fig. 4a). In turns, each cation connects
with neighbouring cations in a column along the b axis through
weak coordinative Ag ◊ ◊ ◊ F (x, y - 1, z; -x, y–1, -z + 3/2)
Crystal structure of [Ag(HpzR(4))2][PO2F2] (32)
Repeatedly we have tried to grow crystals of the different types of
compounds described in this work for X-ray purposes, but all at-
tempts were unsuccessfully. Then we prepared several homologous
derivatives containing shorter chains, which are usually proved
to crystallize. So the ionic silver derivative [Ag(HpzR(4))2][PO2F2]
bearing the butoxyphenyl group as substituent at the 3-position of
the pyrazole, and PO2F2- as counteranion was obtained. The coun-
˚
interactions of 3.242(6) A involving the fluorine atoms of the
-
neighbouring PO2F2 counteranion (Fig. 4b). No short contacts
between columns are observed.
-
teranion was generated through a hydrolysis process of the PF6
anion during the crystallization of the starting [Ag(HpzR(4))2][PF6]
(23) (Scheme 1), this feature being frequently observed in some
examples described in the literature.30–32 The molecular structure
of [Ag(HpzR(4))2][PO2F2] (32) was determined by single X-ray
diffraction methods (Fig. 3). The data collection and refinement
parameters are detailed in the Experimental section (Table 2) and
Table 3 lists selected distances and angles.
Fig. 4 (a) Packing of the cationic entities of 32 in the bc plane.
(b) Molecular chain along the b axis.
Mesomorphic behaviour
The thermal behaviour of all synthesized compounds was estab-
lished by using both polarized-light optical microscopy (POM)
and differential scanning calorimetry (DSC) techniques. The
mesophases were also characterized by small-angle X-ray diffrac-
tion (XRD).
All complexes of the series I and II containing Hpz2R(n) present
enantiotropic mesomorphism (Table 4), showing smectic SmA
mesophases which were characterized by their focal-conic fan-
shaped textures (Fig. 5).
Fig. 3 ORTEP plot of [Ag(HpzR(4))2][PO2F2] (32) with 35% probability.
Hydrogen atoms, except H2, have been omitted for clarity. Symmetry
operation (-x, y, -z + 3/2).
By contrast the complexes of the classes III and IV containing
the non-mesomorphic HpzR(n) ligands are not liquid crystal except
-
The compound crystallizes in the monoclinic system, space
[Ag(HpzR(12))2][PF6] (24) and [Au(HpzR(12))2][A] (A = PF6 (35),
-
group C2/c. The asymmetric unit consists of the half of the
NO3 (37)) (Table 4).
-
cationic unit [Ag(HpzR(4))2]+ and of the counteranion PO2F2 , with
DSC thermograms of the complexes I (IBF4, IPF6 and INO3), with
the exception of that of 5, exhibit a similar pattern which is now
described for [Ag(Hpz2R(18))2][BF4] (4).
the silver and phosphorus atoms located in an axis. The cationic
part is a two-coordinated silver complex in which the silver centre
has two strongly bonded pyrazoles. The Ag–N distance of 2.089(7)
The first heating cycle shows two endothermic peaks. The
◦
˚
A is similar to those observed in related compounds containing
first one at 70.0 C (DH = 36.0 kJ mol-1) corresponds to the
heterocyclic ligands.24,33–35 The N1–Ag–N1¢(-x, y, -z + 3/2) angle
transition of a crystal phase to the mesophase and the second
6916 | Dalton Trans., 2008, 6912–6924
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Table 4 Phase behaviour of new compounds 1–18, 24, 35 and 37
determined by optical microscopy and DSC
Complex
T/◦C (DH/kJ mol-1)
n
Cr→SmA
SmA→IL
IBF4
12
14
16
18
12
14
16
18
12
14
16
18
12
14
12
14
12
14
12
12
12
1
64 (31.7)
69 (32.4)
69 (38.2)
70 (36.0)
144 (3.1)
138 (8.8)
132 (8.8)
126 (3.8)
2
3
4
IPF6
5a
6
77 (36.3)
79 (13.6)
60 (12.3)
40d
125 (7.8)
125 (2.5)
104 (3.1)
140 (4.7)
132 (1.2)
121 (2.9)
124 (2.7)
127b,d
7
8
INO3
9
10
11
12
13
14
15
16
17
18
24
35
37
40d
51 (6.6)
68 (11.4)
56d
IIBF4
IIPF6
IINO3
61d
138b,d
83d
125b,d
85d
127b,d
75d
130b,d
99d
134b,d
IIIPF6
IVPF6
IVNO3
47 (27.1)
—
—
89 (4.8)
135c,d
130c,d
a Complex 5 exhibits the following phase transitions:
93 29.4
116 6.5
154 3.8
(
)
(
)
(
)
Cr ææææÆ SmC æææÆ SmA æææÆIL
b At the beginning of the clearing process. c Monotropic behaviour (35
exhibits SmA mesophase at 81 ◦C and 37 exhibits SmA mesophase at
120 ◦C). d Temperatures determined by POM.
Fig. 5 Microphotographs of the SmA mesophases observed by POM of
(a) [Ag(Hpz2R(14))2][PF6] (6) at 111 ◦C, and (b) [Au(Hpz2R(12))2][PF6] (15) at
103 ◦C.
◦
peak at 126.0 C with a lower enthalpy value of 3.8 kJ mol-1
corresponds to the transition of the mesophase to isotropic liquid.
On cooling, from the isotropic liquid, the first exothermic peak
◦
at 115.0 C (DH = -3.5 kJ mol-1) is due to the formation of a
mesophase transition and SmA to SmC mesophase transition,
respectively. A third peak at 66 ◦C (DH = -11.6 kJ mol-1) agrees
with the crystallisation process.
SmA mesophase in agreement with the typical fan-shaped texture
◦
optically observed. A second exothermic peak at 64.0 C (DH =
-15.0 kJ mol-1) is related to the transition of the mesophase to the
solid phase. In the remaining complexes this latter process was not
always detected from their corresponding DSC thermograms due
to the limitant lowest temperature of ca. 50 ◦C of the equipment
used. So, complexes 9 and 10 could be observed as solids at ca.
40 ◦C by optical microscopy.
The gold compounds II and IV with disubstituted and mono-
substituted pyrazole ligands exhibit a similar mesomorphism to
that of silver derivatives but also they show two particular features.
The first one deals with a higher thermal instability evidenced by
a partial decomposition at temperature close to the clearing. The
second feature is related to the rate of the transition processes.
In particular on heating the clearing process is very slow and the
total phase transformation from mesophase to isotropic liquid
is only reached after increasing the temperature to a value that
depends of the counteranions. On cooling the mesophase is slowly
formed from the isotropic liquid and in all cases it is maintained
below 50 ◦C with no evidence of crystallization. Indeed the
viscosity suggests that these systems have formed a smectic glass.
In agreement with the above results the processes were not detected
by DSC.
Complex 5 exhibits four endothermic transitions in the first
◦
heating cycle. The first one at 76.4 C (DH = 12.2 kJ mol-1)
corresponds to the transition of a crystal phase to another crystal
phase, only observed by DSC. The second peak at 93.0 ◦C (DH =
29.4 kJ mol-1) is assigned to the transition of the crystal phase to
a mesophase identified as the smectic C (SmC) mesophase by its
sanded-schlieren-type texture as well as small-angle XRD studies
(Table 5). A fan-shaped texture of a smectic A (SmA) mesophase
is clearly observed at 116 ◦C, this temperature being related to
the third peak of the DSC thermogram (DH = 6.5 kJ mol-1), and
associated to a mesophase–mesophase transition. The presence
of this SmA phase at higher temperatures allows us to confirm
the ab◦ove assignment of the SmC mesophase. The fourth peak at
154.0 C with a small enthalpy value (3.8 kJ mol-1) corresponds
to the transition of mesophase to isotropic liquid. On cooling two
Table 4 contains the results of thermal behaviour of compounds
of classes I–IV obtained by DSC and/or POM.
The influence of the counteranion on the thermal behaviour
was clearly observed for all metallomesogenic derivatives I and II
and several trends can be pointed out. In general terms, for each
chain length the variation of the melting temperatures is in the
exothermic peaks at 154 C (DH = -7.9 kJ mol-1) and 80 ◦C
order PF6 > BF4 > NO3 . However the trend on the clearing
◦
-
-
-
-
-
(DH = -1.4 kJ mol-1) are related to the isotropic liquid to a SmA
temperatures variation is partially reversed, BF4 > PF6 . Taking
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Table 5 X-Ray diffraction data for liquid crystalline complexes
behaviour has been described for gold(I) and silver(I) isocyanide
ionic complexes and hexafluorophosphate and tetrafluoroborate
pyridine silver derivatives.37,38
Complex T/◦C
Position/2q d-spacing/A Miller indices (hkl)
˚
On the other hand, the inverse variation of the clearing tendency
of BF4- and PF6- derivatives could be considered in agreement with
the melting of the anion–cation arrangement in the mesophase. So
the bulkiest anion should introduce higher difficulties to allow the
cation–cation ordering of the fluid phases giving rise to the lowest
clearing points.
1
2
3
4
5
115
115
76
2.5
5.0
7.5
34.8
17.5
11.7
4.5
(001)
(002)
(003)
a
19.9
2.4
4.9
7.3
37.0
18.2
12.1
4.1
(001)
(002)
(003)
Thermal and thermodynamics effects of increasing the alkyloxy
chain length are also studied. So for complexes of the series IBF4
(1–4) and INO3 (9–12) (Table 4), the temperatures of the phase
transition (Cr → M) increase upon increasing the chain length,
a
21.0
2.1
4.1
6.1
42.5
21.6
14.5
4.3
(001)
(002)
(003)
being the same behaviour suggested for all Au derivatives II (IIBF4
,
a
20.6
IIPF6, IINO3; 13–18) on the basis of their optical data (Table 4). This
behaviour is parallel to that observed on [Ag(R-imidazol)][NO3]
and attributed to the melting of alkyl chains.24 Therefore it is
possible to suggest that, mainly in silver compounds I, strong van
der Waals interactions between chains should govern the variation
of the melting process, being less important the potential lowering
on the core–core interactions produced by the increased motion
of longer chains.
80
2.0
3.9
5.8
44.9
22.7
15.2
4.5
(001)
(002)
(003)
a
19.5
110
130
2.2
4.4
6.6
19.8
2.3
4.6
39.7
19.8
13.3
4.2
37.5
18.9
12.4
4.4
(001)
(002)
(003)
a
The inverse variation on the melting temperature observed
(001)
(002)
-
in IPF6 (5–8) can be attributed to the bulky size of the PF6
7.1
20.0
(003)
counteranion, which by increasing the alkyloxy chain length
should produce a less favourable packing of the solid reducing
the melting temperatures.
On the other hand, the clearing temperatures of the silver
complexes I are reduced with the increase of the chain length,
which is the inverse of the trend of their counterparts Au
compounds II.
a
6
109
100
90
2.4
4.8
7.0
36.5
18.4
12.3
4.3
(001)
(002)
(003)
a
20.2
9
2.6
5.0
7.6
34.5
17.5
11.7
4.4
(001)
(002)
From the above results it is deduced that the silver compounds I
(003)
with BF4- and NO3- as counteranions present a wider temperature
a
20.1
-
range of mesophase than those with PF6 following the order
-
-
-
13
15
35
37
2.7
5.3
7.9
33.0
16.7
11.2
4.1
(001)
(002)
NO3 > BF4 > PF6 . However for the gold derivatives II, the
increase in the length of the chain (from 12 to 14 carbon atoms)
affects in the same way the melting and clearing temperatures and
as a consequence the mesophase range follows the general trend
(003)
a
21.4
-
-
-
70
2.5
4.8
6.9
35.9
18.4
12.7
4.2
(001)
(002)
in the order BF4 > PF6 > NO3 . The above different results in
the mesomorphic behaviour of silver and gold derivatives can be
understood on the basis of the presence of covalent coordinative
interactions from the O or F atoms of the counteranions with
the metal centres, which are more common and stronger for
silver(I) than gold(I) derivatives. Evidences of those kinds of
interactions have been found in several pyrazolyl derivatives
previously described by us.39 Then it is not surprising that
related metal–counteranion interactions can be expected in our
compounds being responsible for the variable stabilization of the
mesophases experimentally observed.
(003)
a
21.3
70b
50b
2.1
4.2
5.5
42.0
21.0
13.8
4.5
(001)
(002)
(003)
a
20.0
2.6
4.2
5.0
34.4
17.1
11.1
4.1
(001)
(002)
(003)
a
20.0
Mesomorphism of complexes III and IV (Table 4) is restricted to
those bearing 12 carbon atoms on the alkyl chain (24, 35 and 37).
Complex 24 shows lower melting and clearing temperatures than
those of its counterpart with disubstituted pyrazole ligands and
the gold derivatives 35 and 37 exhibit monotropic behaviour. The
absence of liquid crystal properties of the counterparts containing
longer alkyl chains (n > 12) can be explained as a consequence
of the decrease of the interpenetration of the chains of the ‘U’
or ‘bowl’ shaped molecules, and therefore to a decrease in the
anisotropy, which renders weaker core–core interactions.
a Halo of the molten alkoxy chains. b On cooling.
-
into account the volume of the spherical counteranions PF6
>
-
BF4 ,36 it is possible to suggest that the molecular arrangement of
the solid should be more effective for those derivatives containing
-
the bulkiest PF6 counteranion, so generating the highest melting
-
points as has been found experimentally. By contrast, the flat NO3
group should be less effective than the BF4 and PF6 and as a
consequence it should produce the lowest melting points. Similar
-
-
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Selected compounds of the different families were subjected to
temperature-dependent power X-ray diffraction (XRD) measure-
ments. The XRD pattern shows in all cases three sharp reflections
in a 1 : 2 : 3 ratio which were indicative of a lamellar structure and
they are indexed as the (0 0 1), (0 0 2) and (0 0 3) reflections,
than that of SmC phase, this feature can be due to the loss of
conformational freedom at low temperatures, which causes the
more extended chains to overcome the effect of tilting.16,40
The homologous complexes exhibit similar XRD results (Ta-
˚
˚
˚
˚
˚
ble 5) showing layer spacing of 34.8 A (1), 37.0 A (2), 42.5 A (3),
˚
˚
˚
˚
˚
respectively. In addition a broad halo at ca. 4.5 A corresponds to
44.9 A (4), 37.5 A (5), 36.5 A (6), 34.5 A (9), 33.0 A (13) and 35.9 A
(15) in the SmA mesophase.
the liquid-like order of the molten alkylic chains. The results are
summarized in Table 5 and Fig. 6 shows the diffraction patterns
of compounds 3 and 13 selected as representative examples.
At this point of the discussion and in order to establish potential
solid–mesophase structure relationships, we think that it could be
interesting to consider the X-ray structure of [Ag(Hpz2R(4))2][BF4]
(Fig. 1), previously reported by us,21 as a potential model for
compounds I and II. On the basis of a similar ‘H’–shaped
molecular structure for the complexes IBF4 (Table 5) with chains
of n = 12, 14, 16, 18 carbon atoms, we can estimate a molecular
length L of the cationic entity in a full-extended configuration of
˚
ca. 43, 48, 54 and 59 A, respectively. Because the d-spacings of the
˚
smectic layer (35–45 A, Table 5) are shorter than L, we can deduce
that in the layer the molecules should present interdigitation
(Scheme 2), analogously to that established on the crystalline
structure of [Ag(Hpz2R(4))2][BF4] (Fig. 7).21
Scheme 2 Schematic representation of the packing modes of the com-
plexes of the types I and II in the SmA mesophases.
Fig. 6 X-Ray diffraction pattern of (a) [Ag(Hpz2R(16))2][BF4] (3) at 76 ◦C
on heating, and (b) [Au(Hpz2R(12))2][BF4] (13) at 90 ◦C on heating.
The XRD diffractogram of 5 requires a special comment. As
measured at 110 ◦C, it shows three peaks in the small-angle region
from the (0 0 1), (0 0 2) and (0 0 3) reflections in agreement with
˚
a lamellar structure with a layer distance of 39.7 A. In addition, a
˚
broad halo at ca. 4.2 A is observed in the wide-angle region, which
Fig. 7 View of the molecular distribution in a layer of the compound
[Ag(Hpz2R(4))2][BF4].21
is characteristic of the diffraction of the side chains in the liquid-
like order. The X-ray diffractogram recorded at 130 C shows a
◦
similar pattern with three sharp small-angle peaks indexed in a
lamellar mesophase with the Miller indices of (0 0 1), (0 0 2) and
Photophysical studies
˚
(0 0 3) and a layer distance of 37.5 A. XRD analysis allows to
identify these two mesophases as smectic phases being assigned
as SmC and SmA mesophases by textural studies. It is important
to note that the layer spacing of the SmA mesophase is smaller
The silver(I) and gold(I) compounds based on the 3,5-disubstituted
pyrazole ligands (Hpz2R(n)) 1, 5, 9 (type I) and 13, 15, 17
(type II) with n = 12 were considered for photoluminescence
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Table 6 Optical data for compounds 1, 5, 9, 13, 15 and 17 in chloroform solution at room temperature
a
Compound
l
max Abs (nm)
e (L mol-1 cm-1)
Conc./10-6
M
lemis (nm) (lexc = 274 nm)
Dl (nm)
Hpz2R(12)
267
272
272
272
280
276
278
55063
71672
80126
75065
77062
79566
68453
3.24
2.84
2.51
4.45
2.08
3.30
4.90
362b
374
380
371
380
371
373
95
102
108
99
100
95
1
5
9
13
15
17
95
a Stocks shift in chloroform, b lexc = 261 nm.
Table 7 Optical data for compounds 1, 5, 9, 13, 15 and 17 as solid samples at room temperature
Compound
lmax Exc (nm)
lemis (nm) (lexc = 274 nm)
lemis (nm) (lexc = 350 nm)
Colora
Color (lexc = 366 nm UV lamp)
1
272
272
272
280
276
278
360
362
354
359
391
356
416
410
Not emissive
512
438
White
White
White
White
White
White
Orange
Orange
Orange
Purple
Orange
Pale orange
5
9
13
15
17
Not emissive
a Solid powder samples.
studies. Related to their counterparts, these compounds exhibit
the largest mesophase range of each counteranion, allowing
a better determination of the influence of the anionic group
on the luminescence properties. On the other hand our pre-
liminary studies of complexes containing pyrazole-type ligands
revealed that photophysical characteristics remained constant
despite variations in the chain length of the ligands.9,41 From
these considerations, the photophysical properties of the selected
complexes [M(Hpz2R(12))2][A], (M = Ag, Au; A = BF4 , PF6 , NO3 )
were analyzed. All of those derivatives, including the free ligand,
are luminescent at room temperature both in solution and in solid
state (Tables 6 and 7). The complexes displaying liquid crystal
behaviour were also proved to be luminescent at the mesophase
by variable temperature luminescence studies.
Solution studies. The UV-Vis absorption spectra of dilute
solutions of all the complexes in CHCl3 exhibit a broad absorption
band in the region of 240–320 nm with the maximum centered
at ca. 280 nm (Table 6). In analogy to the related compounds
previously published,9 as well as to the free ligand (Table 6),
that absorption was associated with the p–p* electronic transition
of the pyrazole groups. The metal complexation of the Hpz2R(12)
ligand induces a slight red-shift in all cases, but no significant
differences attributed to the influence of the counteranions are
observed (Table 6, Fig. 8 and ESI†).
-
-
-
The emission spectra of complexes in fresh chloroform solutions
show a broad band between 320–480 nm which is bathochromi-
cally shifted related to that of the free ligand and attributed
to a ligand-centered transition (Table 6). Again, no appreciable
Fig. 8 Absorption and normalized fluorescence emission spectra of compounds 9 and 17 in fresh chloroform solution at concentrations 4.45 ¥ 10-6
M
(9) and 4.90 ¥ 10-6 M (17) (lexc = 274 nm; room temperature).
6920 | Dalton Trans., 2008, 6912–6924
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Fig. 9 Normalized fluorescence emission spectra of compounds 1, 5, 9, 13, 15 and 17 in solid state (lexc = 274 nm; room temperature).
Fig. 10 Normalized fluorescence emission spectra of compounds 1, 5, 9, 13, 15 and 17 in solid state (lexc = 350 nm; room temperature).
-
-
-
spectral differences ascribed to the counteranions are observed
(Fig. 8 and ESI†).
complexes, and BF4 ~ PF6 > NO3 in the gold derivatives. This
feature suggests that the presence of coordinative interactions
between the metal centre and the counteranions, favoured by the
Solid studies. The photophysical properties in the solid state
were also examined in order to establish the influence of the crystal
packing on the luminescence behaviour.
-
NO3 group, contributes to modify the luminescence properties.
On the other hand the broadness of the luminescence bands
appears to reflect the highly distorted excited states.
The emission spectra of silver (1, 5, 9) and gold (13, 15, 17)
complexes exhibit a similar pattern consistent with two broad
bands in the range of ca. 300–800 nm when they are excited
with l = 274 nm (Table 7). The two bands were observed for
all derivatives at ca. 300–400 nm and ca. 600–800 nm, respectively.
The first one exhibiting the highest intensity is again attributed to
a ligand-centered transition and the latter band of low intensity is
assigned to a metal–ligand charge transfer (MLCT)42–46 (Fig. 9).‡
By changing the counteranion, the intensity of the emission
Compound 13 exhibits a third band between 420–600 nm.
A related emission was not clearly observed in the remaining
compounds, because it should be occluded by the broad and
intense band at 300–400 nm. In order to elucidate that option,
a new excitation at 350 nm was used. As expected, the emission
appears in the range of ca. 400–600 nm as a broad band (Fig. 10),
which could be assigned to a metal-centered emission.
By comparing the above results with those from the solution
spectra, it is evident that both silver and gold derivatives exhibit a
slight bathochromic shift of the emission maxima, which is almost
unaffected by the change of the metal.
-
-
-
bands decreases in the order PF6 > BF4 > NO3 in the silver
‡ The band at ca. 600–800 nm observed in all the complexes could be
assigned to a triplet state (centered in the pyrazole unit or in the metal–
ligand fragment). In order to shed further light on this point the life time of
this band, in solid complexes 5 and 15 excited at 274 nm, has been studied
using a Perkin-Elmer LS45 spectrofluorimeter in phosphorescence mode.
Unfortunately, life times obtained gave values below 50 ms; these values
are in our lamp life time region and this result prevents any assignment.
However, the solid state spectrum of the free ligand in the same conditions
does not present the band above mentioned (600–800 nm) suggesting to
us its assignment to a metal–ligand charge transfer (MLCT).
Variable-temperature studies. The luminescent behaviour of
compounds 1, 5, 9 (type I) and 13, 15, 17 (type II) was studied at
different temperatures from the solid state to the isotropic liquid,
in order to prove the luminescence at the liquid crystal state.
The emission spectra were recorded in the 300–800 nm range,
using a fiber optic system connected to the Horiba-Jobin Yvon-
Spex Fluorolog 3.22 spectrofluorometer. The solid samples were
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heated over a hotplate with an external temperature control
provided with a digital thermo par.
the increased non-radiative deactivation, a residual emission is
generally maintained. Indeed, in our compounds, as usual, a
remained residual emission is observed at higher temperatures
than the clearing. From the above results it is also deduced
that the mesophase aggregation does not quench the emission.
In fact we can observe that, when the samples overtake the
melting temperature generating the mesophase (Table 4), the
luminescence is maintained in all cases although with less intensity,
following the mentioned variation of the intensity with the tempe-
rature.
Fig. 11 displays the emission spectra of the 1, 5, 9 and
13, 15, 17 complexes as a function of the temperature. They
were recorded from room temperature until temperatures slightly
higher than their respective clearing temperatures. As a general
trend it is observed that the intensity of the luminescence
decreases upon temperature increases and at the clearing tem-
perature the emission almost disappears. In spite of the emis-
sion quenching of the isotropic liquids, as a consequence of
Fig. 11 Normalized fluorescence emission spectra of compounds 1, 5, 9, 13, 15 and 17 in solid state as a function of the temperature (lexc = 274 nm).
The spectra and the temperature list follow the same top to bottom order. The smectic and IL phases are given in all cases.
6922 | Dalton Trans., 2008, 6912–6924
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On the other hand, the overall pattern of the spectra is
maintained along all of the temperatures used. Therefore the
emission maxima in the solid and in the liquid crystal state are the
same (the latter with less intensity) suggesting that the structural
changes and viscosity of the mesophase do not affect to the excited
state.
The investigated silver and gold complexes I and II constitute
a new contribution to the scarce examples of luminescent metal-
lomesogens exhibiting luminescence in the mesophase as well as in
the solid state and in solution. New related compounds based on
these pyrazole ligands will be investigated looking for luminescent
mesophases at lower temperatures.
Acknowledgements
We thank the Ministerio de Educacio´n y Ciencia of Spain, project
CTQ2006-13344/BQU, and the Comunidad de Madrid (Spain)
and Universidad Complutense de Madrid (Spain) project CCG07-
UCM/PPQ-2844 for the financial support. We are also indebted to
Fundac¸ao para a Cieˆncia e a Tecnologia/FEDER (Portugal/EU)
by project PTDC/QUI/66250/2006 FCT-FEDER.
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Conclusions
Mesomorphic molecular materials can be used for liquid crystal
or OLEDs technologies. To these purposes stability over an ap-
propriate temperature range and emission in the visible spectrum
are required to perform as liquid crystal OLED materials.
The ionic silver and gold complexes, presented in this work,
of the type I [Ag(Hpz2R(n))2][A] and II [Au(Hpz2R(n))2][A] based on
disubstituted pyrazole ligands are new metallomesogens exhibit-
ing lamellar phases over a broad of temperature range. The non-
conventional ‘H’ molecular shape of these derivatives allows to
achieve SmA mesophases, which could be related to the layer-like
structural packing observed in the solid.
By contrast the complexes III and IV, [M(HpzR(n))2][A], bearing
the monosubstituted pyrazole ligands, are not liquid crystals with
the exception of those with n = 12. Rationalizing this different
behaviour is not a simple matter and an explanation of those
results is supported by the following considerations. The ‘U’
shaped two-chained molecules of III and IV are arranged in the
solid in a columnar fashion (viewed in Fig. 4), which is different
to that of the layer-like structure found in the four-chained ‘H’
shaped molecules of the related compound [Ag(Hpz2R(4))2][BF4].21
Those structural features could be reflected in the difficulty to
achieve the required supramolecular ordering of the lamellar
mesophases for compounds III and IV. Alternatively, although
a layer-like ordering of the ‘U’ molecules on the mesophase could
also be considered, as that depicted in Fig. 2d, the increase of
the length chain above of n = 12 should produce a decrease
on the interdigitation giving rise to a higher random motion of
the chains. The flexibility of the alkyl chains tends to disrupt the
core–core interactions, this fact being responsible for the lack of
mesomorphism.
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