Silver coordination complexes as room-temperature multifunctional materials

Article abstract of DOI:10.1002/chem.200600408

A series of bischelate ionic silver complexes [Ag(L*)₂][X] was prepared by complexation of a newly synthesized 2,2′-bipyridine containing chiral alkoxy chains in the 4,4′ positions. The appropriate choice of the construction motifs allows the preparation of new materials in which several functionalities can be introduced.Indeed, when the anion X ^- is a triflate or a dodecylsulfate group, the right combination of intermolecular interactions promotes the production of liquid crystalline mesophases. Therefore, the presence of coordinating anions, which drives the supramolecular assembly, is essential to generate, at the same time, room-temperature columnar hexagonal mesomorphism, the columnar helical supramolecular structure, and excimeric emission.

Full text of DOI:10.1002/chem.200600408

FULL PAPER
DOI: 10.1002/chem.200600408
6738
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2006, 12, 6738 – 6747

FULL PAPER
Silver Coordination Complexes as Room-Temperature Multifunctional
Materials
[
a]
[a]
[a]
[a]
Daniela Pucci,* Giovanna Barberio, Anna Bellusci, Alessandra Crispini,
[
c]
[b]
[a]
[a]
Bertrand Donnio, Loris Giorgini, Mauro Ghedini, Massimo La Deda, and
Elisabeta Ildyko Szerb
[
a]
ꢀ
Abstract: A series of bischelate ionic
Indeed, when the anion X is a triflate
crystalline mesophases. Therefore, the
presence of coordinating anions, which
drives the supramolecular assembly, is
essential to generate, at the same time,
room-temperature columnar hexagonal
mesomorphism, the columnar helical
supramolecular structure, and excimer-
ic emission.
silver complexes [Ag(L*) ][X] was pre-
or a dodecylsulfate group, the right
combination of intermolecular interac-
tions promotes the production of liquid
2
pared by complexation of a newly syn-
thesized
2,2’-bipyridine containing
chiral alkoxy chains in the 4,4’ posi-
tions. The appropriate choice of the
construction motifs allows the prepara-
tion of new materials in which several
functionalities can be introduced.
Keywords: chirality · helical colum-
nar mesophases · ionic metallo-
mesogens · luminescence · silver
[8–11]
Introduction
larly attractive for ferroelectric switching.
Moreover, the
introduction of a further functionality such as a metallic re-
active center, could be a good strategy for obtaining helical
superstructures in columnar mesophases.
Liquid crystals are attractive as new materials in practical
device applications because of their luminescence and
[12]
[1–3]
charge-transport properties.
Among the rich variety of
Supramolecular assembly plays a key role among the pos-
sible approaches to functionalize soft materials such as
[4]
structural mesomorphic species, ionic and/or discotic sys-
tems are promising candidates for new applications in opto-
[13]
liquid crystals. We have recently reported on the design of
metal-containing mesogens, the molecular shapes of which
differ from the archetypal discogenic molecules. In these
systems, the molecular organization in the mesophase is
mainly driven by intermolecular attractive interactions that
can induce liquid-crystalline properties regardless of the co-
ordination geometry of the metal ion and the number of
[
5–6]
electronics.
within discoid mesogens may induce chiral supramolecular
The incorporation of a stereogenic center
[7]
columnar organizations, making these molecules particu-
[14–16]
flexible chains.
[
a] Prof. D. Pucci, Dr. G. Barberio, Dr. A. Bellusci, Prof. A. Crispini,
Prof. M. Ghedini, Dr. M. La Deda, Dr. E. I. Szerb
Centro di Eccellenza CEMIF.CAL-LASCAMM
CR-INSTM Unità della Calabria, Dipartimento di Chimica Universi-
tà della Calabria (Italy)
With this in mind, we aim now to extend this strategy, to
develop new chiral helical superstructures in columnar mes-
ophases, through a synthetic pathway based on several con-
struction motifs ranging from metal coordination, p–p inter-
actions, ionic self-assembly, stereogenic centers, and the co-
ordinating ability of counterions.
E-mail: d.pucci@unical.it
[
b] Dr. L. Giorgini
Unità INSTM of Bologna, Dipartimento di Chimica Industriale e dei
Materiali, University of Bologna
Bologna (Italy)
Relevant results are described herein, in which we report
the synthesis and characterization of four new ionic bische-
ꢀ
[
c] Dr. B. Donnio
late silver complexes [Ag(L*) ][X] (X = BF , tetrafluoro-
2
4
Institut de Physique et Chimie des MatØriaux de Strasbourg - Groupe
des MatØriaux Organiques
UMR 7504(CNRS-UniversitØ Louis Pasteur), 23 rue du Loess
BP43, 67.034 Strasbourg cedex 2 (France)
ꢀ
ꢀ
borate, 1; PF , hexafluorophosphate, 2; CF SO , trifluoro-
6
3
3
ꢀ
3
methanesulfonate (triflate), OTf, 3; and C H OSO , dode-
1
2
25
cylsulfate, DOS, 4), based on a 2,2’-bipyridine ligand (L*)
containing the S-(ꢀ)-b-citronellyl group as substituent in the
4,4’ positions. Optical microscopy, DSC, and powder X-ray
Supporting information for this article is available on the WWW
under http://www.chemeurj.org/ or from the author.
Chem. Eur. J. 2006, 12, 6738 – 6747
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
6739

D. Pucci et al.
diffraction analysis have been used to characterize the
liquid-crystalline behavior of the new silver derivatives.
the ionic silver complexes 1–4, in good yields and of analyti-
cal purity.
Since interesting luminescence properties have been report-
The ionic nature of complexes 1–4 was confirmed by
using IR spectroscopy to identify the characteristic bands of
the corresponding counterions. The H NMR spectra of both
n
ed for achiral [Ag(L ) ]X complexes, at room temperature,
2
[15]
1
both in solution and in the solid state, the photophysical
properties of complexes 1–4 were also investigated by using
UV/Vis spectroscopy. Moreover, to evaluate the possibility
of the appearance of any helical organization in the meso-
phase the optical activity was investigated by using circular
dichroism (CD) spectroscopy.
the free ligand and the silver complexes are very similar, but
we observed a significant increase in the chemical shifts
3
(D=0.1 ppm) associated with the bipyridine protons (H ,
3
’
H ), which are within the shielding region exerted by the
ring of the complementary bipyridine.
Mesomorphism: The thermal behavior of all synthesized
species was characterized by using both polarized optical
microscopy (POM) and differential scanning calorimetry
(DSC), while the nature of the mesophase was confirmed by
using temperature-dependent powder X-ray diffraction
(XRD). All thermal data are summarized in Table 1.
The thermal behavior of the
Results and Discussion
Synthesis: As shown in Scheme 1, the chiral ligand L* was
obtained through a three-step route, following the proce-
dure previously reported for nonchiral analogous ligands.
[14]
[
Ag(L*) ][X] complexes was
2
found to be strongly dependent
on the nature of the counterion.
The tetrafluoroborate (1) and
the hexafluorophosphate (2) sil-
ver(I) complexes lack meso-
morphism: At room tempera-
ture, complex 1 is an oil, where-
as complex 2 is a partially crys-
talline solid that directly dis-
solves on heating without
showing any sign of monotropic
mesophase formation on cool-
ing.
Scheme 1. Preparation of the L* ligand.
However, the triflate and the
dodecyl sulfate derivatives (3
In particular, 4,4’-dimethyl-2,2’-bipyridine was oxidized to
give the corresponding dicarboxylic acid I, which, once
transformed into the more reactive diacid chloride II ana-
and 4, respectively) are liquid
crystals already at room temperature, and exhibit large mes-
ophase stability across a wide range of temperatures. For
both complexes textures characteristic of a hexagonal col-
umnar mesophase were observed with polarized light micro-
scopy. The triflate derivative 3 exhibits a focal conic texture
resulting from the formation of large cylindrical domains
along with homeotropic zones, whereas the dodecyl sulfate
[17]
logue, reacted with the chiral alcohol S-(ꢀ)-b-citronellol
(
S-3,7-dimethyl-6-octen-1-ol).
Complexation of the L* ligand with several AgX salt solu-
tions, at a 2:1 ratio, under a nitrogen atmosphere in di-
chloromethane, under reflux, for 24h (Scheme 2), afforded
derivative
4
shows mosaic and fan-shaped textures
(
Figure 1). DSC measurements over repeated heating–cool-
ing cycles demonstrated that the two complexes are highly
thermally stable.
Temperature-dependent powder XRD patterns confirmed
that both compounds 3 and 4 exhibit a hexagonal columnar
mesophase between room temperature and the temperature
of the corresponding isotropic liquid. At room temperature,
both X-ray diffraction patterns, as shown in Figure 2,
showed a set of small-angle reflections, with a sharp and in-
tense single peak (fundamental reflex) alongside three or
four signals of lower intensity. These reflections were per-
fectly indexed in a hexagonal-planar symmetry with the
Miller indices (10), (11), (20), (21), and (30) (Figure 2). The
symmetry of the mesophase was unaffected by the counter-
2
Scheme 2. Synthesis of silver complexes, [Ag(L*) ][X], 1–4.
6740
www.chemeurj.org
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2006, 12, 6738 – 6747

Silver Coordination Complexes
FULL PAPER
Table 1. Thermal behavior of L* and [Ag(L*)
complexes 3 and 4.
2
][X] complexes and X-ray characterization of the mesomorphic salts was observed in the solid
state, in which the effect of a
[
a]
Compound
Transition
temperatures[8C]
d
meas [Š]
I
Index
d
calcd [Š]
Mesophase
parameters
measured at
Mesophase
parameters
measured at
308C
“
pincer”, such as a coordinating
counterion, induces metal–
metal distances sufficiently
short to be considered as an ar-
gentophilic interaction (3.05–
3.74Š). These values are con-
sistent with the stacking perio-
dicity h0 observed for com-
plexes 3 and 4. Thus, in the bis-
chelate DOS and OTf deriva-
tives, the counterions force the
molecules into a strongly dis-
torted environment, with the
Ag ions in a geometry closer to
the square-planar,
ment with the formation of
strong stacking intra- and inter-
dimers.
3
08C
hk
L*
1
2
oil
oil
Cr
Col
X
60.6 I
91.0 (3.55 ) I
[
b]
3
3
h
21.5
vs
m
m
m
w
br
br
br
vs
m
m
m
br
br
br
10
11
20
21
30
h’
21.5
12.4
10.75
8.1
V
V
mol =1810 Š
1
1
8
7
7
4
3
2.45
a=24.8 Š
2
0.85
.15
.1
.0
.7
S=535 Š
h=3.4Š
7.15
h
h
ch
.55
0
I
[
b]
3
4
Col
h
56.8 (2.05 ) I
24.2
10
11
20
21
h’
24.2
14.0
12.1
9.15
mol =2110 Š
[20]
1
1
9
7
4
3
4.0
2.1
.15
.0
.7
.50
a=28.0 Š
in agree-
2
S=680 Š
h=3.1 Š
h
h
ch
Dilatometry
experiments
0
were performed on complex 3
to investigate the structural or-
d (h +k +hk) ], where Nhk is the number of hk reflections; I is the intensity of ganization of the columns
[
a] Col
h X
: Hexagonal columnar phase. Cr : Partially crystalline solid. I: Isotropic liquid. dmeas and dcalcd are the
measured and calculated diffraction spacing, respectively; dcalcd is deduced from the following mathematical ex-
pression: <d10 >=1/Nhk
the sharp reflections (vs: very strong, m: medium, w: weak), br stands for broad scattering; hk are the indices
of the reflections corresponding to hexagonal 2D symmetry; a is the lattice parameter (a=2<d10 >/ 3), S is
2
2
1/2
A
C
H
T
R
E
U
N
G
[Æhk
hk
A
H
R
U
G
within the mesophase.
p
From a geometrical point of
view, columnar structures are
the lattice area (S=a”<d10 >). The molecular volume was measured for compound 3 and estimated for com-
pound 4 according to: Vmol(4)~Vmol(3)+VDOSꢀVOTf (see text); h
dicities determined by X-ray diffraction corresponding to the molecular-stacking distance (h
order of the molten chains (hch), and double periodicity h’=2h
; h is the intracolumnar repeating distance, de- ters: the columnar cross section
duced directly from the measured molecular volume and the columnar cross-section according to h=Vmol/S.
0
, hch, and h’ are the various short-range perio-
), the liquid-like characterized by two parame-
0
0
(
S) and the stacking periodicity
ꢀ
1
[
b] DH [kJmol ].
(
h ) along the molecular-disc
0
[21]
normal.
The knowledge of
these two structural parameters
permits the understanding of a specific molecular packing
inside the columns and the proposal of models of molecular
ion, remaining the same for both OTf and DOS derivatives.
Only a slight expansion of the hexagonal cell was observed
with the most voluminous DOS anion (a=24.8 Š for com-
pound 3 versus a=28.0 Š for compound 4), increasing the
columnar cross-section of the disc. The magnified views in
Figure 2 show that the diffraction patterns of compounds 3
and 4 exhibit broad scattering in the wide-angle region. In
[22]
organizations.
The intracolumnar repeating periodicity
along the columnar axis, h, the columnar cross section, S,
and the molecular volume (Vmol) are linked analytically
through the relation h·S=N·Vmol in which N is the number
[21]
of molecules within the fraction of the column. While S
addition to the typical halo at approximately 4.7 Š (h ),
(and h ) are obtained directly from the X-ray diffraction
ch
0
characteristic of the diffraction of the side chains in their
liquid-like order, a scattering diffuse band at about 3.5 Š
data (see Table 1), N is chosen to equal unity as the mole-
cule is disc-like. The molecular volume (Vmol) is determined
experimentally by dilatometry, whereas h is deduced from
both XRD and dilatometry measurements. The comparison
(
h0), corresponding to the molecular-stacking distance along
the disc normal within the column, and another weakly in-
tense halo at 7.0 Š (h’) were observed (vide infra).
of h and h is very informative about the various possible or-
0
These data are in agreement with the presence, in both
cases, of discrete dimers in which the silver atoms of each
molecule are gripped by two triflate (or sulfate) groups in a
double-bridged fashion, giving rise to a supramolecular ar-
ganisations of the disc-like molecules within the column as
shown in Figure 4.
Prior to the analysis, the variation of the molecular
volume of complex 3 as a function of temperature was
monitored. The experiment was carried out, with approxi-
mately one gram of compound, between room temperature
rangement favorable to
Figure 3).
The so-formed dimers interact with each other along the
a
liquid-crystalline assembly
(
and about 1158C, and thus included the Col -to-I phase
h
columns as indicated by the presence of the halo h’=2h cen-
tered at 7 Š in the powder XRD spectra.
transition (Figure 5).
The temperature dependence of the molecular volume is
quasilinear over the entire range of experimental tempera-
tures despite the change from the mesophase to the isotrop-
A similar coordination network observed in bis(phenylth-
[18]
[19]
io)propane-
and 4-methoxystyrylpyridinatosilver(I)
Chem. Eur. J. 2006, 12, 6738 – 6747
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
6741

D. Pucci et al.
Figure 1. Optical texture (magnification 40”) of the hexagonal columnar
mesophase of compound: a) [Ag(L*) [OTf] (3) as observed on cooling
at 838C; b) [Ag(L*) [DOS] (4) as observed on cooling at 318C.
2
]
ACHTREUNG
2
]
ACHTREUNG
ic liquid. From this experiment, the volume of compound 3
3
varies as Vmol =1772.7+1.341T (in Š , and T in 8C). The
slope is in fairly good agreement with the volume variation
[23]
of the methylene groups in liquid alkanes, an indication
that the volume of the rigid part is almost unchanged with
temperature in the mesophase. Thus at 308C, the volume of
3
complex 3 is equal to about 1810 Š . The good reproducibil-
ity of the experiment was verified over several heating–cool-
ing cycles (see Supporting Information).
The volume of complex 4 was not measured experimen-
tally but could be estimated from that of complex 3. Indeed,
Vmol can be written as a sum of elementary volumes associat-
ed with the different segments of the molecule according to
Figure 2. Powder X-ray diffraction patterns of the columnar hexagonal
phase at room temperature for complexes 3 (a) and 4 (b).
Vmol =VCore +V , whereby V
and VCh are the volumes of
Core
Ch
the rigid core and of the aliphatic chains, respectively. The
only difference between the volumes of complexes 3 and 4
is the nature of the counterion, and thus this sum of volumes
can be re-written according to Vmol(4)~Vmol(3)+VDOSꢀV
OTf
where VDOS =VSO4 +11VCH2 +VCH3, VOTf =VSO3 +VCF3, VCH2
=
2
6.5616+0.02023T (volume of one methylene group),
2
VCH3 =53.8221+0.03492T+0.0004258T (volume of the
methyl
2
group),
VCF3 =68.903+0.41998Tꢀ0.0004258T
(
volume of one fluoromethyl group). The difference in
volume between the sulfate and sulfonate heads is deduced
from the difference between the volume of sulfuric acid and
Figure 3. Sketch of model molecules of complex 3 stacked in a discrete
dimer and gripped by two triflate ions.
3
sulfurous acid, D=VSO4ꢀV =ꢀ43.81 Š . The expression of
SO3
Vmol(4) can be simplified as Vmol(4)~V (3)ꢀD+11V
+
mol
CH2
3
VCH3ꢀVCF3. At 308C, Vmol(4)=2110 Š . From the V
of
It can be seen that the stacking distance h is larger, for
mol
0
complexes 3 and 4 it is possible to calculate the h values of
both cases, than the intracolumnar repeat unit h, particularly
in the case of complex 4, indicating either strong undula-
3.4and 3.1 Š, respectively (Table 1).
6742
www.chemeurj.org
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2006, 12, 6738 – 6747

Silver Coordination Complexes
FULL PAPER
Table 2. UV/visible absorption maxima of the ligand and solutions of
complexes 1–4 in CH Cl .
2 2
ꢀ
1
ꢀ1
Compound
Abs, l[nm] (e,
A
C
H
T
R
E
U
N
G
[m cm
(13900)
(28800)
(26080)
(21800)
]
L*
1
2
241(12400), 300
A
H
R
U
G
ACHTREUNG
240(sh), 308
240(sh), 310
240(sh), 311
ACHTREUNG
ACHTREUNG
3
ACHTREUNG
4
240(sh), 310 (24200)
However, the analysis of UV/Vis emission spectra showed
that the substituents in the 4,4’ positions play a crucial role
in the deactivation path of the excited state. Indeed, where-
n
Figure 4. Representation of three types of stacking of discs within the
as the silver(I) achiral complexes [Ag(L ) ]X were lumines-
2
column and the correspondence between the different parameters h, h
0
,
cent in solution, showing a maximum at about 350 nm at-
S, and S (obtained from X-ray diffraction and dilatometry) allowing for
0
[15]
tributable to a ligand-centered transition, no emission was
detected for complexes 1–4, in solution, or for their respec-
tive ligand.
their discrimination; y is the angle between the columnar axis and the
molecular disc normal: Left: The discs are slightly tilted with respect to
the columnar axis; middle: The disc normal and columnar axis are per-
fectly collinear: right: The discs are tilted and arranged in such a way
that they precess around the columnar axis (helical packing).
The emission properties have been investigated also in
pure solid samples at room temperature. Interestingly, while
complexes 1 and 2 are not luminescent, complexes 3 and 4,
which are mesomorphic at room temperature, are lumines-
cent, with the lifetime of the excited state lasting less than
3
0 ms for both complexes (Figure 6 and Table 3).
Figure 5. Temperature dependence of the molecular volume of complex
3
.
[24]
[25]
tions of the columnar cores
or helical twisting,
about
the columnar axis (lateral expansion combined with tilting
of the disc, and thus a larger apparent surface area). This
result supports the hypothesis of the formation of helical
columns (Figure 4, right).
Figure 6. Emission (on the right; lex =330 nm) and excitation (on the left;
em =460 nm) spectra of compound 3 (neat compound at room tempera-
ture).
l
UV/Vis and CD spectra: The UV/Vis absorption and emis-
Table 3. Luminescence data for complexes 3 and 4 in the mesophase, at
room temperature.
sion spectra of [Ag(L*) ][X] were obtained to investigate
2
the role of the chiral centers introduced into the peripheral
tails.
The absorption UV/Vis spectra of dilute solutions of com-
pounds 1–4 in dichloromethane exhibited a broad absorp-
tion band in the region 250–400 nm, with the maximum cen-
tered at around 310 nm and a shoulder in the range 340–
Compound
Ex, l[nm]
Em, l[nm]
3
4
335
335
460
486
3
80 nm (Table 2).
The most important features in the photoluminescence
emission and excitation spectra of complexes 3 and 4 are
the large Stokes shift and the structureless emission band.
The observed large Stokes-shifted emission could be attrib-
uted to phosphorescence, but the lifetimes exclude a triplet
emission. Large Stokes shifts are also indicative of highly
In analogy to the related achiral compounds previously in-
[15]
vestigated, the absorptions at high energy are associated
with the p–p* electronic transition of the single aromatic
chromophores, while the weak n–p* transitions are related
to the carboxylic groups.
Chem. Eur. J. 2006, 12, 6738 – 6747
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
6743

D. Pucci et al.
distorted excited states, as reflected in the broadness of the
luminescence bands.
Cotton effects, and intensity were observed by changing the
counterion. It is evident that in dilute solutions (concentra-
ꢀ
4
These observations proved that the structureless emission
tion of the complexes approximately 3.0”10 m) the small
circular dichroic behavior of complexes 3 and 4 is attributa-
ble to the presence of intrinsic dissymmetry of the S-(ꢀ)-b-
citronellyl unit within the side chains, which induces a chiral
perturbation of the electronic transitions.
[26]
arises from an excimer-like organization, only present in
the mesomorphic arrangement, since no emission was de-
tected in very concentrated solutions of complexes 3 and 4.
Indeed, in the mesomorphic state the supramolecular ar-
rangement proposed for compounds 3 and 4 can allow the
single molecule to be separated by a stacking distance of
about 3.5 Š, the typical value found for solid-state excimers
and/or exciplexes related to the presence of metal–metal in-
Thin films were prepared by spin coating the solutions of
compounds 3 and 4 onto fused-silica slides. The thickness of
the films was regulated to give UV/Vis spectra with maxi-
mum absorbance between 0.7–1.0 units, to enable the direct
correlation between the properties in the dilute solutions
with those exhibited in the mesophase. UV/Vis and CD ex-
periments were performed on these thin solid films (area ex-
[27–29]
teractions.
The presence of four stereogenic centers on each mole-
*
cule of the [Ag(L ) ]X complexes can confer a formal opti-
2
2
cal activity derived from the helical organization in the mes-
ophase rather than just the response of the molecular chiral-
amined 0.5 cm ), at room temperature (mesophase) and at
1008C (isotropic phase). Figure 8 illustrates the CD elliptici-
ty and UV/Vis absorption spectra of both complexes in the
mesophase, after isotropization and in dilute dichlorome-
thane solutions.
The CD spectra of the isotropic melt and solution of both
complexes exhibited similar Cotton effects of low intensity,
while the CD spectra (of both complexes) in the mesomor-
phic state were very different to those of the complexes in
dilute solutions (where the complex molecules are isolated
from each other) and in the isotropic state.
[
12]
ity. Thus, circular dichroism (CD) experiments were car-
ried out to confirm this possibility.
The CD spectra of solutions of the OTf and DOS deriva-
tives in dichloromethane displayed similar features: A weak
positive dichroic absorption with maxima strictly related to
their UV/Vis absorption maximum (Figure 7 and Table 4).
No particular differences between absorption wavelengths,
The CD spectrum of the thin film of compound 3 at room
temperature revealed an intense broad circular dichroism in
the 280–400 nm spectral region, characterized by two nega-
tive CD peaks with maxima at 321 and 330 nm and a weak
positive dichroic band at 300 nm, (Figure 8a). No significant
CD bands were visible in the dilute solution and the isotrop-
ic sample of compound 3. The CD spectrum of compound 4
(
Figure 8b) in its mesomorphic state is quite similar to that
of compound 3 but a positive broad and weak dichroic
signal is visible, centered at around 375 nm, which could be
due to the weak n–p* transition of carboxylic groups as ob-
served in the solid-state UV/Vis spectrum.
In the CD spectra of compounds 3 and 4, the overlapping
of several CD signals, which is correlated with electronic
transitions (see UV/Vis transitions present in solution and
described in the previous section) in the 250–400 nm spec-
trum region, may be responsible for a slight shift in the
Cotton-effect bands with respect to the corresponding UV/
Vis maximum absorption, preventing the full identification
of every CD peak.
In addition, an exciton splitting of low intensity for com-
plex 3, and of higher intensity for complex 4, is present as
indicated by two bands of opposite signs with a crossover
point at the same wavelength of the relative UV/Vis elec-
tronic transition, nevertheless we observe a partial overlap-
ping with other dichroic signals correlated with several elec-
tronic transitions.
However, since the presence of a long alkyl chain in the
DOS counterion of complex 4 slightly modifies the colum-
nar cross-section of the disc with respect to complex 3, as
evidenced by the X-ray diffraction measurements, their CD
spectra are similar but not perfectly superimposable.
Figure 7. UV/Vis (bottom) and CD (top) spectra of solutions of the com-
plexes 3 (b) and 4 (c) in CH Cl .
2 2
Table 4. CD spectra of solutions of complexes 3 and 4 in CH
2 2
Cl at
2
58C.
Sample
CD band
[
a]
[b]
l
max
De
1
3
4
300
310
+0.27
+0.32
[
a] Wavelength [nm] of maximum dichroic absorption. [b] De expressed
ꢀ
1
ꢀ1
in m cm
.
6744
www.chemeurj.org
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2006, 12, 6738 – 6747

Silver Coordination Complexes
FULL PAPER
the whole columns as a result of the four stereogenic centers
in each molecule.
Stability of the helical arrangement was confirmed as the
CD spectra obtained on anisotropic films stored for six
months at room temperature were comparable to those orig-
inally obtained, indicating that the conformational arrange-
ment in the liquid-crystalline state is stable over time and
does not undergo further structural changes.
Conclusion
We have synthesized a new, chiral non-mesomorphic 2,2’-bi-
pyridine, L*, and the corresponding bischelate ionic silver(I)
complexes [Ag(L*) ][X]. Mesomorphism is observed only
2
for those derivatives containing coordinating counterions
such as OTf and DOS, compounds 3 and 4, respectively.
Therefore the coordinating ability of these counterions
can induce a supramolecular arrangement in which two bis-
chelate cations are connected by two triflate (or sulfate)
groups, in a double-bridged fashion, through Ag–O interac-
tions, giving rise to discrete dimers or infinite polymers
(
Figure 3). The resulting supramolecular arrangement will
favour a liquid-crystalline assembly that would otherwise be
prevented by the molecular shape of the [Ag(L*) ][X] com-
2
plexes.
X-ray diffraction investigations on complexes 3 and 4 con-
firmed the formation of the room-temperature hexagonal
columnar phases initially observed by POM analysis. More-
over, volume measurements in the mesomorphic state indi-
cate helical twisting about the columnar axis and support
the hypothesis of the formation of helical columns. To con-
firm these results CD experiments were performed in solu-
tion and on the liquid crystalline films of compounds 3 and
4
. From these data we can confirm that, in these systems,
chirality is induced from the single-molecule level to a su-
pramolecular columnar liquid-crystalline organization.
Therefore, the supramolecular arrangement achieved gives
rise to a helical structure formed by the entire molecular
columns in the mesophase.
Finally, the photophysical characterization of compounds
3
and 4 has pointed out that these complexes are lumines-
Figure 8. UV (lower) and CD (upper) spectra of 3 (a) and 4 (b) under
cent, at room temperature, in their mesomorphic state, and
this behavior is very promising for applications as light-emit-
ting diodes. In fact, very few metallomesogens that are lumi-
nescent in the mesophase have been reported up to now
(lanthanide-based columnar and gold calamitic meso-
2 2
different conditions: Diluted solution in CH Cl (d), thin film in the
mesophase at 258C (c), and in the isotropic phase at 1008C (b).
Since the appearance of supramolecular chirality in col-
umnar structures can be obtained by different mecha-
[31–32]
gens)
but none of them show mesomorphism and lumi-
[
7,25]
nisms
such as the molecular shape and modes of aggregation,
analogous CD experiments were performed on the achiral
other than the presence of stereogenic centers,
nescence at room temperature.
[30]
Moreover the blue luminescence exhibited in the meso-
phase by compounds 3 and 4 is not usual and can be ascri-
bed to the existence of excimers arising from argentophilic
interactions that are intradimeric within the columns.
The synthetic strategy based on several construction
motifs has been useful to synthesize materials the properties
of which can be properly and finely modulated through the
specific intermolecular interactions. Indeed, in this class of
n
[
Ag(L ) ][X] complexes in their mesomorphic state, to clari-
2
fy the nature of the conformational chiral aggregation in the
columnar mesophase in these systems. In these cases, no cir-
cular dichroism was observed. Therefore the optical activity
manifested by complexes 3 and 4 in the mesophase reflects
the formation of a supramolecular helical arrangement of
Chem. Eur. J. 2006, 12, 6738 – 6747
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
6745

D. Pucci et al.
complexes, we have several construction motifs with a varie-
ty of possible molecular and intermolecular organizations.
Both bipyridines and the silver(I) ion are versatile systems
that can lead to different structures depending on coordina-
tion modes, solvent, reaction conditions, and nature of
corded as the sample in the thin-film state was rotated through successive
6
08 increments around the light source. The temperature was maintained
[
33–36]
constant by using a plate holder.
The UV/Vis and CD spectra did
not change significantly so averaged spectra were used for the analysis.
Spectrofluorimetric grade dichloromethane (Acros Organics) was used
for investigating the photophysical properties of the complexes in solu-
tion at room temperature. Neat compounds were examined on a quartz
window. A Perkin–Elmer Lambda 900 spectrophotometer was employed
to measure the absorption spectra, while the corrected emission spectra,
all confirmed by excitation spectra, were recorded with a Perkin–Elmer
LS 50B spectrofluorimeter, equipped with a Hamamatsu R928 photomul-
tiplier tube.
anions. In the case of the [Ag(L*) ][X] compounds, the tet-
2
racoordination is achieved through bischelation of two bi-
pyridine units around the silver center. The network forma-
tion is then controlled by the choice of the counterion that
is the first step towards a supramolecular organization es-
sential to generate, at the same time, room-temperature col-
umnar hexagonal mesomorphism, columnar helical supra-
molecular structure, and excimeric emission.
Dicitronellyl 2,2’-bipyridyl-4,4’-dicarboxylate (L*): A mixture of 4,4’-di-
carboxy-2,2’-bipyridine, I, (0.50 g, 0.96 mmol) and thionyl chloride
(
19 mL) were refluxed under nitrogen until a clear yellow solution was
obtained. Excess thionyl chloride was removed and the residue was dried
under vacuum for 2 h. Acid chloride II was suspended in toluene
(
4
20 mL) and treated with a slight excess of S-(ꢀ)-b-citronellol (0.90 mL,
.91 mmol). The mixture was heated under reflux for 3 h. The solvent
Experimental Section
was evaporated before the addition of chloroform (40 mL) and the mix-
ture was washed with a solution of saturated sodium hydrogen carbonate
General information: All reagents were used as received from the re-
spective suppliers: 4,4’-dimethyl-2,2’-bipyridine, silver tetrafluoroborate,
silver hexafluorophosphate, silver trifluoromethanesulfonate, silver ni-
trate, sodium dodecylsulfate from Aldrich, and S-(ꢀ)-b-citronellol from
Fluka. Likewise the solvents were used as received from commercial
sources without further purification and were dried by using standard
methods wherever necessary. The synthesis of 4,4’-dicarboxy-2,2’-bipyri-
(
1
40 mL). The aqueous layer was then rigorously mixed in chloroform (3”
5 mL). The organic extract was washed with water (100 mL), dried over
anhydrous sodium sulfate, filtered, and evaporated to dryness. The pure
product was obtained by chromatography by using the solvent mixture
dichloromethane/diethyl ether (9:1) as elution medium. Evaporation of
ꢀ1
the solvent gave a yellowish oil in a 78% yield. IR (KBr): n˜ =1730 cm
1
3,3’
3
(
3
C=O); H NMR (300 MHz, CDCl ): d=8.94(s, 2H; H ), 8.87 (d, J=
[
17]
dine (I) was performed as previously reported. Thin films were pre-
pared by spin coating solutions of samples in CH Cl (2% w/w) onto
clean fused-silica substrates. Films were then dried under vacuum at
58C overnight to eliminate any residual solvent. Film thickness was
regulated to allow for maximum UV/Vis spectral absorbance between
.7–1.2 units, depending on the conditions, to enable the direct correla-
6,6’
3
4
5,5’
4
(
1
.7 Hz, 2H; H ), 7.90 (dd, J=4.7 Hz and J=1.7 Hz, 2H; H ), 5.10
2
2
b,b’
h,h’
c,c’
e,e’
m, 2H; H ), 4.43 (m, 4H; H ), 2.01 (m, 4H; H ), 1.87 (m, 2H; H ),
a,a’
d,d’,g,g’
3
.64(m, 12H; H ), 1.33 (m, 8H; H
), 0.99 ppm (d, J=6.0 Hz, 6H;
2
f,f’
H ); C32
44 2 4
H N O (520.71).
General procedure for the synthesis of silver complexes [Ag(L*)
2
][X]: A
0
solution of dicitronellyl 2,2’-bipyridyl-4,4’-dicarboxylate (0.365 mmol) in
dichloromethane (10 mL) was added to a stirred suspension of silver(I)
salt (0.183 mmol) in dichloromethane (10 mL) and the mixture was
heated under reflux for 24h. The solvent was evaporated and the residue
recrystallized from methanol.
tion between the properties of the complexes in thin films and in dilute
solutions. As sample thicknesses ranged between 100–200 nm the extinc-
tion coefficient and ellipticity values could be considered negligible for
the spectra of bulk samples.
1
H NMR solution spectra were acquired on a Bruker Avance DRX-300
(
(
(
2,2’-bipyridyl-4,4’-bis-citronellylcarboxylate)silver(i)tetrafluoroborate,
1): Yellow oil. Yield 47%. IR (KBr): n˜ =1730.6 (C=O), 1066.9 cm
6,6’
3 6
spectrometer in CDCl and [D ]DMSO, with TMS as internal standard.
ꢀ
1
IR spectra were obtained on a Spectrum One FTIR Perkin–Elmer spec-
trophotometer by using the KBr disc method. Elemental analyses were
carried out with a Perkin–Elmer 2400 analyser. The textures of the meso-
phases were studied with a Zeiss Axioscope polarising microscope equip-
ped with a Linkam CO 600 heating stage. The transition temperatures
and enthalpies were measured by using a Perkin–Elmer DSC 6 Differen-
1
3
4 3
BF ); H NMR (300 MHz, CDCl ): d=8.93 (d, J=5.1 Hz, 4H; H ),
3
,3’
3
5,5’
3
8
H
H
.82 (s, 4H; H ), 8.12 (d, J=5.1 Hz, 4H; H ), 5.11 (t, J=7.0 Hz, 4H;
b,b’
3
h,h’
c,c’
), 4.49 (t, J=6.2 Hz, 8H; H ), 2.02 (m, 8H; H ), 1.89 (m, 4H;
e,e’
a,a’
d,d’,g,g’
3
), 1.64(m, 2 4H ; H
.0 Hz, 12H; H ); AgBC64
), 1.37 (m, 16H; H
(1236.10).
), 1.01 ppm (d, J=
f,f’
6
F
4
H
88
N
4
O
8
ꢀ
1
(2,2’-bipyridyl-4,4’-bis-citronellylcarboxylate)silver(I)hexafluorophos-
phate, (2): Yellow solid. Yield 64%. Mp 60.6 8C. IR (KBr): n˜ =1730.8
(
tial Scanning Calorimeter with a heating and cooling rate of 108Cmin
.
The apparatus was calibrated with indium. At least two heating–cooling
cycles were performed on each sample. The powder X-ray diffraction
patterns were obtained by using a Bruker AXS General Area Detector
Diffraction System (D8 Discover with GADDS) with CuKa radiation;
The highly sensitive area detector was placed at a distance of 20 cm from
ꢀ
1
1
3
C=O), 832.1 cm (PF
6
); H NMR (300 MHz, CDCl
3
): d=8.88 (d, J=
6
,6’
3,3’
3
5,5’
4
.7 Hz, 4H; H ), 8.82 (s, 4H; H ), 8.13 (d, J=5.1 Hz, 4H; H ), 5.11
b,b’
3
h,h’
c,c’
(
m, 4H; H ), 4.50 (t, J=6.2 Hz, 8H; H ), 2.03 (m, 8H; H ), 1.90 (m,
e,e’
a,a’
d,d’,g,g’
3
4
6
H; H ), 1.65 (m, 24H; H ), 1.37 (m, 16H; H
), 1.01 ppm (d, J=
f,f’
.4Hz, 12H; H ); elemental analysis calcd (%) for AgC64F H N O P
6 88 4 8
D
the sample and at an angle 2q of 128. A CalCTec (Italy) variable heating
ꢀ
1
(1294.26): C 59.39, H 6.85, N 4.33; found: C 59.23, H 6.76, N 4.31.
stage was used to heat the samples at a rate of 5.08C min to the appro-
priate temperature. Measurements were performed by charging samples
in Lindemann capillary tubes with inner diameters of 0.5 mm.
(2,2’-bipyridyl-4,4’-bis-citronellylcarboxylate)silver(i)trifluoromethanesul-
fonate, (3): Yellow wax. Yield 73%. Thermotropic behavior shown in
ꢀ
1
Table 1. IR (KBr): n˜ =1730.5 (C=O), 1285.8 (OTf), 1254.9 cm (OTf);
A Perkin–Elmer Lambda 19 UV/Vis spectrophotometer was used to
1
3
6,6’
3
H NMR (300 MHz, CDCl ): d=8.93 (d, J=5.1 Hz, 4H; H ), 8.78 (s,
record absorption spectra on solutions in CH
2 2
Cl at 258C in the 700–
3
,3’
3
5,5’
b,b’
3
4
H; H ), 8.08 (d, J=4.9 Hz, 4H; H ), 5.11 (m, 4H; H ), 4.49 (t, J=
2
40 nm spectral region by using cell path lengths of 0.1 cm. Concentra-
h,h’
c,c’
e,e’
ꢀ
4
ꢀ4
ꢀ1
6.2 Hz, 8H; H ), 2.01 (m, 8H; H ), 1.89 (m, 4H; H ), 1.66 (m, 24H;
tions of 3.0”10 and 3.7”10 molL of compounds 3 and 4, respective-
ly, were used. These compounds are fairly stable in dichloromethane, as
demonstrated by the constancy of their absorption spectra over a week.
CD spectra were recorded at 258C at a scanning speed of 50 nmmin on
a Jasco 810 A dichrograph, by using the same path lengths and solution
concentrations as for UV/Vis measurements. De values, expressed as
a,a’
d,d’,g,g’
3
f,f’
H
), 1.35 (m, 16H; H
mental analysis calcd (%) for AgC65
6.83, N 4.32; found: C 59.52, H 6.83, N 4.34.
), 1.01 ppm (d, J=6.4Hz, 12H; H ); ele-
3 88 4
F H N O
11S (1298.36): C 60.13, H
ꢀ
1
(2,2’-bipyridyl-4,4’-bis-citronellylcarboxylate)silver(i)dodecylsulfate, (4):
Yellow wax. Yield 37%. Thermotropic behavior shown in Table 1. IR
(KBr): n˜ =1730.1 (C=O), 1255.9 (DOS), 1220 cm (DOS); H NMR
6
(300 MHz, [D ]DMSO): d=8.95 (d, J=4.9 Hz, 4H; H ), 8.83 (s, 4H;
H
H
ꢀ
1
ꢀ1
ꢀ1
1
L mol cm , were calculated by using the following equation: De=[V]/
2
ꢀ1
3
6,6’
3
300 where the molar ellipticity [V] is expressed in deg cm dmol . To
3
,3’
3
5,5’
b,b’
avoid linear effects (linear dichroism and linear birefringence) due to ori-
entation anisotropy in the ordered systems, several CD spectra were re-
), 7.94(d, J=4.9 Hz, 4H; H ), 5.10 (m, 4H; H ), 4.43 (m, 8H;
h,h’
3
c,c’
), 3.69 (t, J=6.6 Hz, 2H; CH
2
of DOS), 1.98 (m, 8H; H ), 1.81 (m,
6746
www.chemeurj.org
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2006, 12, 6738 – 6747

Silver Coordination Complexes
FULL PAPER
e,e’
a,a’
d,d’,g,g’
4
(
H; H ), 1.66 (m, 26H; H , CH
2
of DOS), 1.40 (m, 16H; H
), 1.25
[20] E. C. Constable, C. E. Housecroft, B. M. Kariuki, M. Neuburger,
C. B. Smith, Aust. J. Chem. 2003, 56, 653–655.
3
f,f’
3
m, 18H; CH
2
of DOS), 0.96 (d, J=6.4Hz, 12H; H ), 0.85 ppm (t, J=
6
.8 Hz, 3H; CH of DOS); elemental analysis calcd (%) for
2
[21] D. Guillon, Struct. Bonding (Berlin) 1999, 95, 41–82.
AgC76
H
113
N
4
O
12S (1414.68): C 64.53, H 8.05, N 3.96; found: C 64.97, H
[22] a) F. Morale, R. W. Date, D. Guillon, D. W. Bruce, R. L. Finn, C.
Wilson, A. J. Blake, M. Schrçder, B. Donnio, Chem. Eur. J. 2003, 9,
8
.09, N 4.16.
2
484–2501; b) B. Donnio, B. Heinrich, H. Allouchi, J. Kain, S.
Diele, D. Guillon, D. W. Bruce, J. Am. Chem. Soc. 2004, 126,
5258—15268.
1
[
[
23] A. K. Doolittle, J. Appl. Phys. 1951, 22, 1471–1475.
24] B. Donnio, B. Heinrich, T. Gulik-Krzywicki, H. Delacroix, D. Guil-
lon, D. W. Bruce, Chem. Mater. 1997, 9, 2951–2965.
Acknowledgements
This work was partly supported by grants from the Italian Ministero del-
lꢁIstruzione, dellꢁUniversità e della Ricerca (MIUR) through the INSTM-
FIRB RBNE01P4JF, MIUR PRIN2004 and Centro di Eccellenza CE-
MIF.CAL. B.D. and E.I.S. gratefully thank Dr. Benoît Heinrich for his
help in the dilatometry experiment and fruitful discussions.
[
[
25] J. L. Serrano, T. Sierra, Chem. Eur. J. 2000, 6, 759–766.
26] a) T. H. Lowry, K. Schuller-Richardson in Mechanism and Theory in
Organic Chemistry, Harper & Row, New York, 1981; b) N. J. Turro
in Modern Molecular Photochemistry, Benjamin/Cummings, Menlo
Park, CA, 1978; c) A. A. Lamola in Energy Transfer and Organic
Photochemistry (Eds.: A. A. Lamola, N. J. Turro), Wiley-Inter-
science, New York, 1969 d) M. Gordon, W. R. Ware in The Exciplex,
[
1] R. J. Bushby, O. R. Lozman, Curr. Opin. Colloid Interface Sci. 2002,
, 343–354.
(
Eds.: M. Gordon, W. R. Ware), Academic Press, New York, 1975;
7
e) J. Kopecky in Organic Photochemistry: AVisual Approach , VCH,
New York, 1991; f) J. Michl, V. Bonacic-Koutecky in Electronic As-
pects of Organic Photochemistry, Wiley, New York, 1990, pp. 274–
[
[
[
[
2] M. OꢁNeill, S. Kelly, M. Adv. Mater. 2003, 15, 1135–1146.
3] J. Hanna, Opto-Electron. Rev. 2005, 13, 259–267.
4] K. Binnemans, Chem. Rev. 2005, 105, 4148–4204.
5] R. J. Bushby, O. R. Lozman, Curr. Opin. Sol. State Mater. Sci. 2002,
2
86.
[
27] a) S. A. Clodfelter, T. M. Doede, B. A. Brennan, J. K. Nagle, D. P.
6
, 569–578.
Bender, W. A. Turner, P. M. LaPunzina, J. Am. Chem. Soc. 1994,
[
[
6] S. Kumar, Chem. Soc. Rev. 2006, 35, 83–109.
7] H.-S. Kitzerow, C. Bahr in Chirality in Liquid Crystals, Springer,
New York, 2001.
1
1
16, 11379–11386; b) J. K. Nagle, B. A. Brennan, J. Am. Chem. Soc.
988, 110, 5931–5932.
[
[
28] K. R. Kylie, P. C. Ford, J. Am. Chem. Soc. 1989, 111, 5005–5006.
29] a) M. A. Omary, H. H. Patterson, Inorg. Chem. 1998, 37, 1060–
[
[
8] J. MalthÞte, J. Jacques, N. H. Tinh, C. Destrade, Nature 1982, 298,
4
6–48.
1
1
066; b) M. A. Omary, H. H. Patterson, J. Am. Chem. Soc. 1998,
20, 7696–7705.
9] H. Bock, W. Helfrich, Liq. Cryst. 1992, 12, 697–703.
[
[
10] G. Scherowsky, X. H. Chen, J. Mater. Chem. 1995, 5, 417–421.
11] G. Heppke, D. Krüerke, M. Müller, H. Bock, Ferroelectrics 1996,
[
30] a) R. A. Reddy, C. Tschierske, J. Mater. Chem. 2006, 16, 907–961;
b) H. M. Keizer, R. P. Sijbesma, Chem. Soc. Rev. 2005, 34, 226–234.
31] S. Suàrez, D. Imbert, F. Gumy, C. Piguet, J.-C. G. Bunzli, Chem.
Mater. 2004, 16, 3257–3266.
32] R. Bayon, S. Coco, P. Espinet, Chem. Eur. J. 2005, 11, 1079–1085.
33] J. Schellmann, H. P. Jensen, Chem. Rev. 1987, 87, 1359–1367.
34] A. Hans-Werner, S. Dꢂhne, A. Ouart, C. Spitz, J. Phys. Chem. B
1
79, 203–209.
[
[
[
12] J. L. Serrano, T. Sierra, Coord. Chem. Rev. 2003, 242, 73–85.
13] T. Kato, N. Mizoshita, K. Kishimoto, Angew. Chem. 2006, 118, 4 4 –
[
[
[
7
4; Angew. Chem. Int. Ed. 2006, 45, 38–68.
[
[
[
14] D. Pucci, G. Barberio, A. Crispini, O. Francescangeli, M. Ghedini,
Mol. Cryst. Liq. Cryst. 2003, 395, 155–165.
15] D. Pucci, G. Barberio, A. Bellusci, A. Crispini, M. La Deda, M.
Ghedini, E. Szerb, Eur. J. Inorg. Chem. 2005, 2457–2463.
16] G. Barberio, A. Bellusci, A. Crispini, M. Ghedini, A. Golemme, P.
Prus, D. Pucci, Eur. J. Inorg. Chem. 2005, 181–188.
2
000, 104, 8664–8669.
[
[
35] L. Angiolini, R. Bozio, L. Giorgini, D. Pedron, G. Turco, A. Daurꢃ,
Chem. Eur. J. 2002, 8, 4241–4247.
36] A. Rodger, B. Norden in Circular Dichroism and Linear Dichroism,
Oxford University Press, Oxford, 1997.
[
[
17] D. W. Bruce, K. E. Rowe, Liq. Cryst. 1995, 18, 161–163.
18] M. O. Awaleh, A. Badia, F. Brisse, Inorg. Chem. 2005, 44, 7833–
7
845.
[
19] H. Adams, N. A. Bailey, D. W. Bruce, S. C. Davis, D. A. Dunmur,
P. D. Hempstead, S. A. Hudson, S. Thorpe, J. Mater. Chem. 1992, 2,
Received: March 23, 2006
3
95–400.
Published online: July 21, 2006
Chem. Eur. J. 2006, 12, 6738 – 6747
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
6747