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Valence Tautomeric Thin-Films
Sérgio Mendes, Daniel Ruiz-Molina and Emilia Evangelio
Institut de Ciència dels Materials de Barcelona (CSIC),Esfera UAB, 08193, Cerdanyola, Catalonia, Spain
Abstract
The discovery of molecular materials exhibiting magnetic bistability, some of them even magnetization
relaxation phenomena characteristic of nanodomain particles, has attracted great deal of interest in the
last few years. The current work was conducted at Institut Català de Nanotecnologia (ICN) in the
NanoSFun group particularly in valence tautomeric compounds. Besides the bistability capacity, these
molecular materials are normally soluble in common solvents providing advantages in the processing for
potential application. Homogeneous, readily amenable to variations in their chemical and topological
structure. These complexes show high potential to be used as molecular magnets to store information data
as switches or thermochromic devices. One of the first descriptions of valence tautomerism, based on
quinone ligands, and an excellent example of charge distribution sensitivity, to temperature or pressure for
III
example, is exhibited by the cobalt bis(quinone) complex [Co (3,5-DTBCat)(3,5-DTBSQ)(bpy)]. However,
the Valence Tautomeric (VT) complex used to integrate a polymeric matrix in a thin film form was
III
[Co (Cat-N-SQ)(Cat-N-BQ]. This complex was incorporated in poly(bisphenol A carbonate) (PBC),
poly(methyl methacrylate) (PMMA) and poly(vinyl acetate) (PVA), in different concentrations (2%, 4% and
8% wt). The thin films were prepared by drop casting and were characterized by optical microscopy, UV–
Vis, X-ray, optical microscopy with polarized light and confocal microscopy. Only in this way by systematic
studies of the characterization we can understand the valence tautomeric material behaviour and how can
we manage, to control and switch their properties on the transition from a crystal to a surface.
Keywords: Valence tautomerism / Bistability / Thermochromism / Thin films
Introduction
There is currently active interest in the
development of molecular electronic devices that
can be used for optical and/or magnetic data
storage. Compounds of specific interest are
bistable molecular materials having two nearly
degenerated states with different optical and/or
i
magnetic properties [ ]. These complexes are
quite sensitive to the environment so an external
perturbation, such as temperature or irradiation,
may lead to an interconversion between the two
degenerated electronic states, settling down the
conceptual basis of a molecular device.
Examples of electronic labile complexes are
ii
iii
mixed-valence,[ ] spin-crossover [ ] and valence
iv
tautomeric complexes [ ]. The comprehension
and understanding of the characteristic features
for all these complexes have been developed in
parallel over the last decades, rising out a large
interest in the scientific community as
demonstrated by the large amount of examples
developed for each case.
Valence tautomeric (VT) metal complexes with
at least two redox-active centres, the metal ion
and an electro-active ligand, are characterized
by the existence of two electronic isomers
(valence tautomers) with different charge
distributions, and consequently, different optical,
v
electric and magnetic properties [ ]. The
interconversion between the different electronic
isomers is accomplished by a reversible
intramolecular electron transfer involving the
metal ion and the redox active ligand. One of the
first descriptions of valence tautomerism and an
excellent example of the above mentioned
charge distribution sensitivity is exhibited by the
III
cobalt
bis(quinone)
complex
[Co (3,51
DTBCat)(3,5-DTBSQ)(bpy)],[ ]
where
3,52DTBCat and 3,5-DTBSQ refer, respectively, to
2the catecholate (DTBCat ) and semiquinonate
(DTBSQ ) forms of 3,5-di-tert-butyl-o-quinone,
and bpy is 2,2’-bipyridine. In solution, the
equilibrium in Figure 1 can be shifted by
variations of temperature and monitored by
magnetic measurements and spectroscopic
techniques such as UV–VIS, NMR and EPR.
1
Figure 1: Valence Tautomerism of [CoIII(3,5-DTBCat)(3,5-DTBSQ)(bpy)].
Moreover, since valence tautomeric complexes
are electronically labile, they exhibit significant
vibronic
interactions
and
therefore
an
appreciable sensitivity to the environment. As a
consequence, intramolecular electron transfer
(IET) can be induced not only by temperature
variations but also by irradiation or pressure. For
instance, Hendrickson et al. have reported
results of the first picosecond time-resolved
optical experiments on valence tautomeric
vi
complexes in solution [ ] and in the solid state
vii viii ix
after illumination at low temperatures [ , , ].
Light-induced VT has also been observed in
polynuclear complexes. Examples of pressureinduced VT have also been reported although in
less extent than thermally-driven or light-induced
transitions, mainly due to experimental
x
difficulties [ ]. In these experiments, the increase
of the molecular size on passing from the low to
the high-spin isomer due to the population of
antibonding orbitals is used to favour the lowspin isomer after application of pressure as an
external stimulus. In summary, the interest for
studying this family of complexes is
considerable. First, they are unique model
systems which provide insight into the basic
factors affecting intramolecular electron transfer
in coordination complexes. And second, from an
applied perspective, the large changes in the
optical, structural, and magnetic properties that
often accompany the valence tautomeric
interconversion have potential applications in
bistable molecular materials and devices. By
choosing the appropriate solvent and by
controlling the temperature of the solution, the
tautomeric equilibrium of this family of
complexes can be modulated and one of the two
isomers can be made to be the dominant
species in solution. Moreover, there is no a
direct correlation between the Tc ,critical
temperature, values obtained experimentally for
each solvent and their corresponding dielectric
constant values. These complexes can
interconvert between the VT isomers also in the
solid state though with significant differences.
Indeed, whereas in solution the interconversion
takes place over a large temperature range, in a
crystalline matrix they can interconvert
cooperatively on a narrow temperature range.
III
interconversion in solution, the number of
examples exhibiting a valence-tautomeric
interconversion in the solid state is rather limited.
The spin state degeneracy is not expected to
vary on the different solvents under study.
However, the variations on the metal-ligand
bond lengths, i.e. the density of vibrational
states, are expected to be strongly dependent
on the surrounding matrix.
The VT complex, [Co (Cat-N-SQ)(Cat-N-BQ)]
(1), (see Figure 2)was selected to be
incorporated in a polymeric matrix, and using
temperature as external stimulus.The variations
found for the behaviour of these complexes in
solution and in solid state are not atypical. In
fact, it is important to emphasize that whereas
most of the valence tautomeric complexes thus
far reported exhibit a temperature-dependent
2
Figure 2: VT equilibrium of complex 1 induced by temperature.
The dispersion of the valence tautomeric
complex 1, shown in Figure 2, in a polymeric
matrix may lead to a dependence of the glass
transition temperature, Tg, of the polymer. The
increase of the structural disorder above Tg
contributes to a greater volume occupied by the
molecules involving volume entropy and
enthalpy decrease.
So, the best way to
determine if the Tg is relevant for the VT
phenomenon is test different polymers, with
different Tg values. In this work we used three
polymers as matrixes: the first one is the
poly(Bisphenol
A
carbonate)
(PBC),
a
thermoplastic and hydrophobic polymer, (see
Figure 3) and it has a Tg about 155ºC. The
second polymer is the poly(methyl methacrylate)
(PMMA), a transparent thermoplastic, which has
a Tg between 96-104ºC. The third polymer,
PVA, poly(vinyl acetate), is a rubbery synthetic
polymer, with the lower Tg value among the
three polymers used in this work (29ºC). This Tg
could influence the valence tautomerism (VT)
phenomenon and as temperature is an external
stimuli, the VT could be associated to a change
in colour, known as thermochromism. There are
previous examples of stimuli-sensitive polymers
[xii]
in thin films , though the observation of
thermochromic valence tautomeric complexes in
thin films is rare. This work is intended to
increase the understanding of this subject.
Figure 3: Repeat units of 1) PBC, 2) PVA and 3) PMMA.
2. Experimental Section
2.1. Materials and Sample Preparation. PBC,
PMMA and PVA were purchased from SigmAldrich. PBC (Mw = 64000 g/mol, PDI=1.45),
PMMA (Mw = 350000, PDI= 1.10, white solid)
and PVA (Mw ~100,000, PDI=1.18, transparent
solid). Complex 1 was obtained from the
synthetic procedure.
Before carrying out the optical, x-ray and
confocal experiments, each sample( for each
complex 1 concentration) was heated up to
140ºC(below the glass transition temperature,
Tg = 155ºC).
To characterize complex 1, UV–Vis was carried
out in DCM (this complex has high solubility in
this solvent). Complex 1 is a purple solid at room
temperature and PBC is a transparent solid.
Both materials were used without further
purification.
To prepare the thin films, in a first step, PBC
was dissolved in 10 mL dichloromethane (DCM)
with a concentration of 15 wt% (0,150g). In a
second step, complex 1 was dissolved with the
selected concentrations (2%, 4% and 8%) wt in
the PBC/DCM solution. After 10 minutes testing
period, the mixtures were cast onto a glass
substrate. During the drop casting the glass
plate was placed inside a closed chamber in
order to control the initial evaporation rate of the
solvent from the prepared film. After solvent
evaporation, the films where characterized
without being removed from the substrate.
Samples with relative large plane surfaces,
homogeneous and without pinholes are
required.
The thicknesses of the films are in the range
from 50 to 100 µm. While the samples with 2%
wt of complex 1 are light ciano/green, those of
higher concentrations (4% and 8% wt) are
increasingly purple. For the other polymers,
PMMA and PVA the procedure was the same
described above only with the following
differences: PMMA samples with 2% wt of
complex 1 are light brown and PVA samples
3
with 2% wt of complex 1 are light purple. PBC,
PMMA and PVA units are shown in the Figure 3.
Figure 4: (a) Absorption spectrum for complex 1 in solution during heating in the range 25 – 55ºC. (b) Colour change of
complex 1 in DCM.
2.2. Methods. Electronic absorption spectra
were recorded on a Perkin Elmer Lambda 35
spectrophotometer in the Laboratoire de Chimie
de la Coordination (LCC-CNRS) of Toulouse.
The instrument was equipped with a
thermostatic cell holder that can operate
between 180 and 370 K. Temperature stability
was better than ±5K. Spectra were collected
after the sample had been allowed to thermally
equilibrate at each temperature for 10 min. In
some specific experiments, Tc, the temperature
at which the isomer ratio is 1:1, was deduced
from the temperature at which the peaks of the
low-spin and the high-spin exhibit similar
intensities. The N2 was the carrier gas used in
the cooling and heating programmes.
X-ray powder diffraction data have been
recorded on an INEL Diffractometer (DebyeScherrer geometry and CPS120 curved
detector) at the Institut de Ciència dels Materials
de Barcelona (ICMAB-CSIC) which collects 120º
of the diffraction circle using the Kα radiation of
copper. The sample was introduced in the
diffractometer in 0.3 mm capillars.
Optical microscopy (Zeiss Observer.z1m) was
applied to investigate the structure of the
complex in the different polymeric matrixes. This
method is sensitive and the sample for the
optical microscopy measurement is placed in the
microscope plate and observed with different
magnification objectives (5, 10, 20, 50 and
100x).
Optical microscopy with polarized light (applied
to investigate the structure and the presence of
crystals) was carried out with an Optical
Microscope Leica DMRB.To see with more detail
the Stereoscopical LUPA Leica MZFLIII was
used to. The images where obtained with the
40x objective and the samples were covered
with oil before the measurement to enhance the
performance of the equipment.
Confocal microscopy technique used to
investigate the morphology and topology of the
crystal structures and the data was obtained
from the Confocal Leica Systems – Laser
Confocal Optical Microscope TCS SP2 AOBS
and Confocal Microscope Olympus. The
samples were analyzed with a 40 x objective.
Figure 5: (a) Non reversible change in the visible spectrum of 2% wt complex 1 PMMA thin film. The insert indicates the
appearance of complex 1 at -110 (brown) and 140 ºC (green). (b) Change in the colour of complex 1 at room
temperature from brown (LS) to green at 140ºC (HS) during 1 hour, embedded in poly(methyl methacrylate) matrix.
4
3. Results and Discussion
III
In solution, the valence tautomerism of ls-Co
(Cat-N-SQ)(Cat-N-BQ) is identified by a colour
change. At 25ºC the solution is purple and with
the heating up to 55ºC, the colour changes to
II
blue corresponding to hs-Co (Cat-N-SQ)2. At
room temperature, the UV-Vis spectra of
complex 1 show bands at 391 nm, 439 nm and
533 nm characteristic of the ls-Co(III) (S=1/2)
tautomer. An increase of the solution
temperature
to
55ºC,
promotes
an
intramolecular electron transfer from the ligand
to the metal ion. In consequence of this electron
transfer above 55ºC, the intensity of such bands
decreases and bands at 721 nm and 797 nm
characteristic of the hs-Co(II) tautomer increase
in intensity (Figure 4). In solution, two isosbestic
points at 590 and 856 nm are identified,
demonstrating that at least two species are
interconverting within the studied temperature
range. The different bands aren’t attributed to
any ligand-metal electronic transfer, but to
electronic transitions within the ligands,
modulated by the metal at large wave lengths.
Even though there isn’t any band that could be
associated to a ligand-metal electronic transfer.
In the Figure 4, the band at 390 nm that is
marked with a red symbol, is associated to the
radical in the Cat-N-SQ ligand of the tautomer lsIII
Co (Cat-N-SQ)(Cat-N-BQ), radical placed. This
band decreases as the temperature increases,
due to the tautomer conversion from ls-Co(III) to
[xiii]
hs-Co(II).
Girgis et al. predicted that the major bands
observed in the electronic spectra of complexes
1with general formula ML2, where L is Cat-N-BQ
2and/or Cat-N-SQ , are associated with
electronic transitions within the ligands though
modulated specially at higher wavelengths
depending on the nature of the metal ion. One of
them is the band centred around 390 nm,
attributed to the radical character of the Cat-N2SQ ligand. This fact can provide information
about the electronic distribution of the complex 1
at a specific temperature. Figure 5 shows the
absorbance of the 2% wt complex 1 in PMMA
III
initially (T=25ºC) in the form of Co LS state,
which is converted upon heating to 140ºC into
II
Co HS state, with well defined isosbestic points
at the same wavelength 590 and 856 nm that
appear in solution state (Figure 4). In addition,
this thin film with 2% wt complex 1 exhibits
thermochromism: with the increment of
temperature from -110 to 140ºC the brown
III
coloured ls-Co is converted into a light green
II
species (Co ). The thermochromism is well
demonstrated in the Figure 5.In sequence of
these results, we conclude that, firstly the
exhibition of the valence tautomerism,
III
associated to the conversion of the ls-Co into
II
the hs-Co with the influence of temperature,
accompanied by a colour change from brown to
light green. These properties are crucial for the
development of new devices like thermosensors
or memory devices.
Figure 6: (left) X-ray diffraction patterns of PMMA with different complex 1 concentrations. (right) X-ray
diffraction patterns of PBC with different complex 1 concentrations.
Referring to x-ray studies the thin films with
different concentrations of complex 1 were
incorporated in two distinct polymeric matrixes,
PBC and PMMA, were characterized to evaluate
if the increase of complex 1 concentrations has
any influence one the x-ray diffraction patterns
(Figure 6). Some analogies were done, after
looking to the Figure 6, first in PBC then in
PMMA.
5
Figure 7: X-ray diffraction patterns of 50% wt complex 1 in PMMA in comparison with simulated X-ray
powder diffraction pattern of complex 1 single crystal.
A 50% wt complex 1 in PMMA sample
was prepared to verify that the above X-ray data
correspond to complex 1 crystals and not to any
crystal impurity. The proof that the crystal
structures match up with complex 1 crystals is in
figure 7, were a association is made with the
crystallographic data of complex 1 single crystal.
Comparing the observed reflection peaks of
figure 6 with the standard single crystal complex
1, Figure 7, confirms that the recoded pattern
corresponds to some peaks, 8,42, 16,82 and
25,32º, from complex 1 single crystal data . The
calculated parameters for the three peaks are
[100], [200] and [300] (a,b and c are
coordinates) shows a preferential growth in the
film in the (h00) x plan direction. In all polymeric
matrixes the increase of the complex 1
concentration influence and increase the
intensity of the peaks. In addition, we find no
diffraction peaks for 2% wt complex 1, two peaks
for 4% wt and three peaks for 8% wt at similar 2
Theta
values.
Figure 8: (a)Confocal microscope image from the PMMA thin film at room temperature: projection xy. (b)Optical
microscope image with polarized light from the PMMA thin film at room temperature.
The presence of crystals in the thin films is
therefore confirmed not only by the optical
microscope images but also by the results from
X-ray data (Figures 6 and 7). To obtain
additional confirmation of the presence of
crystals, were used optical microscopy with
polarized light. In this technique, the sample is
illuminated with plane polarized light and its
rotation at 0º, 90º, 180º and 270º reveals the
presence of crystal structures. Thin PMMA films
without the complex 1 were first characterized,
to show that PMMA is an amorphous polymer.
The different samples were illuminated and the
polarizer rotated 0º and 270ºC to produce the
results shown in Figure 9.
The 2% wt complex 1 thin film in PMMA was
characterized at room temperature. Like was
said earlier, in the UV visible characterization, at
2% wt complex 1 there is a structure that seems
like a crystal that I named as “pre-crystal” (see
Figure 9). In this thin film this was the only
structure detected, like the optical microscopy
technique showed. The rest of the thin film there
wasn’t any structures, the presence of the precrystal is a tentative of crystal formation in this
case, and the complex 1 concentration isn’t
enough for the crystal growth.
6
Figure 9: Confocal microscope images from the pure PMMA and PMMA with 2% wt complex 1 thin film at room
temperature. Optical microscope images with polarized light from the PMMA thin film with 2% wt Complex 1 at room
temperature: light polarization (left) 0º and (right) 270º.
7
Conclusion
The valence tautomerism phenomenon induced by
temperature only occurs with two complex 1
concentrations, 2 and 4% wt, and only in PMMA. In
reality, for the 2% wt of complex 1 the increase of
temperature, from room temperature to 120ºC,
leads to the unambiguous observation of the
thermochromism effect, associated to the valence
tautomerism phenomenon. There’s an abrupt colour
change, from brown to light green, corresponding,
III
respectively, to the LS state [Co (Cat-N-SQ)(Cat-NII
BQ)] and the HS state [Co (Cat-N-SQ)2].
This is due to the fact that at the 2% wt of complex 1
in 15% wt PMMA the thin film behaves like a
“solution” due to segregation phase phenomenon,
rarefied of solid structures, the molecules are
disorganized and there’s an entropy gain.
The increase of crystal structures number and
complex 1 concentration in the polymeric matrix
blocks the valence tautomerism phenomenon.
The crystal structures present in the 15% wt PMMA
films are really complex 1 crystals that preferentially
grow in the plan direction (h00).
For complex 1 concentrations higher then 4% wt in
15% wt PMMA there’s no valence tautomeric
phenomenon associated, as the thin film starts to
behave like a solid.
On other hand, the valence tautomerism
phenomenon also occurs in PBC for 2% complex 1
however, less significant. One of the many
differences in the two polymeric matrixes, PMMA
(96-104ºC) and PBC (155ºC), that could determine
the valence tautomerism occurrence is the Tg.
Although, to reach to the conclusion that the Tg is
the main and determinant factor for the valence
tautomerism, SQUID, DSC or TGA measurements
should be done.
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