Raman spectroscopic characterization of a thiophene-based active material for resistive organic nonvolatile memories

Article abstract of DOI:10.1002/jrs.2449

A combined theoretical and experimental Raman study is presented on a diphenyl bithiophene molecule known as a good candidate for the development of organic nonvolatile memory devices. Spectroscopic markers suitable to distinguish the different stable conformers of the molecule have been predicted and detected. The combined analysis of theoretical and experimental Raman spectra recorded in solution indicates that at room temperature a dynamical equilibrium, characterized by interconversion between the two more stable conformers (namely trans and cis), takes place and that the more populated species is the cis form. Referring to the solid phase instead, Raman spectra of single-crystal samples show the presence of the only trans conformer, as confirmed by X-ray measurements. Finally, Raman spectra of thin films, as those used for the memory device, were collected; samples just deposited from solution and after few hours from the deposition were analyzed. Following the evolution of selective spectroscopic Raman markers, an isomerization process from the abundant cis (as-deposited) to the totally trans (after few hours) conformer in the solid phase was detected. These results open the way to the identification of the molecular isomers present in the tthin film of the memory cell and finally of the active molecular species involved in the switching mechanism of the operating device.

Full text of DOI:10.1002/jrs.2449

Research Article
Received: 17 March 2009
Accepted: 21 June 2009
Published online in Wiley Interscience: 6 October 2009
(www.interscience.wiley.com) DOI 10.1002/jrs.2449
Raman spectroscopic characterization of a
thiophene-based active material for resistive
organic nonvolatile memories
Daniele Fazzi,a Eleonora Valeria Canesi,a Chiara Bertarelli,a
Chiara Castiglioni,a∗ Fabrizia Negrib and Giuseppe Zerbia
A combined theoretical and experimental Raman study is presented on a diphenyl bithiophene molecule known as a good
candidate for the development of organic nonvolatile memory devices. Spectroscopic markers suitable to distinguish the
different stable conformers of the molecule have been predicted and detected. The combined analysis of theoretical and
experimental Raman spectra recorded in solution indicates that at room temperature a dynamical equilibrium, characterized
by interconversion between the two more stable conformers (namely trans and cis), takes place and that the more populated
species is the cis form. Referring to the solid phase instead, Raman spectra of single-crystal samples show the presence of the
only trans conformer, as confirmed by X-ray measurements. Finally, Raman spectra of thin films, as those used for the memory
device, were collected; samples just deposited from solution and after few hours from the deposition were analyzed. Following
the evolution of selective spectroscopic Raman markers, an isomerization process from the abundant cis (as-deposited) to the
totally trans (after few hours) conformer in the solid phase was detected. These results open the way to the identification of
the molecular isomers present in the thin film of the memory cell and finally of the active molecular species involved in the
c
switching mechanism of the operating device. Copyright ꢀ 2009 John Wiley & Sons, Ltd.
Supporting information may be found in the online version of this article.
Keywords: diphenyl bithiophene; Raman markers; DFT; isomerization process; resistive organic memory
Introduction
Raman spectroscopy (and in general vibrational spectroscopy)
plays a key role in the study of molecular structure and struc-
ture–property relationships. It contributes to the material’s char-
acterization at the level of single molecule, but it is also sensitive to
the supra molecular organization. Moreover, it provides relevant
information for the understanding of how electronic processes
in organic materials are governed and modulated by structural
and vibrational properties.[15] Vibrational spectroscopy has been
also used for the in situ characterization of organic electronic
devices,[16,17] confirming its relevant contribution to the under-
standing of the molecular physics beyond the device operation
involving electronic mechanisms, such as charge injection from
the electrode to the organic film and charge transport across the
device.[18]
Recent and emerging applications in organic electronics are
represented by molecular switches and organic memories.[1,2] An
innovative field is that of resistive nonvolatile organic memories
(NVM), where, by applying a suitable voltage (threshold bias, Vth)
to the device electrodes, a change in the electrical conductivity
of the organic material is obtained, going from a low conductive
state (OFF state of the memory cell) to a higher one (ON state).
By cycling the bias between proper threshold values, the organic
material can be written on and erased many times working as a
random access memory (RAM) or can be written on once and read
many times (WORM memory).[3]
This property, in which two states with different conductivity
triggered by a proper electric field are present, is called electrical
bistability, and several experimental studies have been carried out
to define the key parameters (at the molecular level) governing
the phenomenon and the device performances.[4–6] Variations
in conductivity have been explained in terms of structural
rearrangements[7] (conformational changes) or as induced by
redox processes.[8–11]
In this work, we carried out a joint experimental and theoretical
Raman study to investigate the more stable conformers and
their Raman markers of a molecule, which belongs to the class of
diphenylbithiophenes(DPBTs).DPBTs,synthesizedastheso-called
∗
Correspondence to: Chiara Castiglioni, Dipartimento di Chimica, Materiali
Since the electronic properties of organic materials (i.e. charge
mobility or energy transport) are strongly sensitive both to the
single-molecular structure (intra-molecular properties) and to the
materialmorphology(inter-molecularproperties)inthesolidstate,
the understanding of such features, in terms of (1) molecular
structures, (2) stable conformers and their stabilization energies,
and (3) occurrence of crystalline phases and crystal packing[12–14]
is of particular relevance.
e Ingegneria Chimica ‘G. Natta’, Politecnico di Milano, P.zza Leonardo da
Vinci 32, 20133 Milano, Italy, and INSTM, UdR Politecnico di Milano, Italy.
E-mail: chiara.castiglioni@polimi.it
a
Dipartimento di Chimica, Materiali e Ingegneria Chimica ‘G. Natta’, Politecnico
di Milano, 20133 Milano, Italy, and INSTM, UdR Politecnico di Milano, Italy
`
b
Dipartimento di Chimica ‘G. Ciamician’, Universita di Bologna, 40126 Bologna,
Italy, and INSTM, UdR Bologna, Italy
c
J. Raman Spectrosc. 2010, 41, 406–413
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Raman characterization of resistive organic nonvolatile memories
OMe
t-Bu
t-Bu
OMe
t-Bu
t-Bu
t-Bu
MeO
t-Bu
HO
OH Pd(PPh3)4
NaHCO3
B
NBS
Br
+
S
2
Br
S
S
1
3
4
OMe
t-Bu
t-Bu
1) LiBu
2) CuCl2
OMe
t-Bu
S
4
S
t-Bu
4, Ni(dppp)Cl2
Mg
t-Bu
MgBr
S
t-Bu
MeO
Z-OCH3
Figure 1. Synthesis of Z-OCH3 compound.
‘linear’ (L) or ‘Z’ shape isomers,[19] are good candidates for resistive
memory applications.[5,6] A recent theoretical investigation[20]
suggested a possible mechanism for their electrical bistability
behavior. In Ref. [20] we carried out a complete conformational
study, finding the more stable conformers of DPBT molecules
(namely trans and cis for both L and Z isomers). For the Z isomer,
thetheoreticalresultsmatchedfavorablythestructuralparameters
obtained by X-ray measurements carried out on single-crystal
samples. From density functional theory (DFT) simulations, the
energy difference between trans and cis conformers turned out to
be negligible, and the energy barrier for cis–trans interconversion
was predicted to be very small (a few kcal/mol).
Finally, we describe some preliminary outcomes of the Raman
study on Z–OCH3 thin films, which are strictly related to the active
layer in the memory devices.
Methods
Computational details
Z-OCH3 structures, vibrational wavenumbers, and intensities are
obtained by using the three-parameter exchange correlation
functional B3LYP[21] with the split-valence Pople basis set 6-
31G ,
∗∗ [22] by using the Gaussian03 code.[23] The choice of B3LYP/6-
The vibrational study here reported refers to the Z structural
isomer with the OCH3 functional group on the phenyl moieties
(hereafter labeled as Z-OCH3, see Fig. 1), and has the purpose of
finding spectroscopic markers that can be used as fingerprint to
detect the existence in the sample of different stable conformers.
The paper is organized as follows: first we outline the synthetic
procedureemployedtoprepareZ-OCH3 molecule.Then,webriefly
summarize the results of the conformational analysis reported in
Ref. [20] and we analyze the simulated Raman spectra (in vacuo)
of the more stable conformers of Z-OCH3 in order (1) to define
their spectroscopic differences and (2) to find suitable spectral
regions for the identification of those spectroscopic markers
useful for the identification of trans, cis, or any other Z-OCH3
stable conformer.
Once the markers have been identified, a direct comparison
between the theoretical results and the experimental Raman
spectra of DPBT collected in different solvents allows us (1) to
study the effect of different environments on the Raman features
of Z-OCH3 DPBT and (2) to estimate, by using a simple fit model,
the relative populations of the two major conformers.
31G∗∗ level of theory is related to the fact that (1) a good match
between experimental X-ray and theoretical structural parameters
(intermsofbondlengthsandangles)hasbeenalreadyobtained[20]
and (2) by using a proper scaling factor applied to the vibrational
wavenumbers[24] good agreement between experimental and
theoretical vibrational spectra is reached.
A fitting procedure that maximizes the overlap between
theoretical and experimental Raman spectra has been devised
to obtain an estimate of the relative populations of the two more
stable trans and cis Z-OCH3 conformers. The fitting procedure is
based on a weighted sum of the calculated Raman spectra of
the cis and the trans conformers, where bands originating by a
single vibrational transition are described by a Lorentzian function
characterized by its peak wavenumber and area, which are given
by theoretical wavenumber νi and intensity Ii, respectively.
Once a suitable wavelength range is selected, the theoretical
spectrum (S) is built as reported in Eqn (1):
ꢀ
Strans/cis(ν) =
IiLi(ν)
(1)
i
We then analyze the Raman spectra of a Z-OCH3 single crystal
which has been fully characterized in terms of molecular structure
as shown by a previous X-ray analysis.[20]
in Eqn (1) Ii is the Raman intensity of the ith normal mode obtained
from quantum chemical simulations and Li is a Lorentzian function
c
J. Raman Spectrosc. 2010, 41, 406–413
Copyright ꢀ 2009 John Wiley & Sons, Ltd.
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D. Fazzi et al.
defined as
synthetic route is reported). The synthesis consists in the
Suzuki coupling of 1-methoxy-2,6-di-tert-butyl-4-bromo benzene
(1) with 3-thiophen boronic acid (2) to give half a molecule.
The resulting intermediate (3) is brominated in 2-position with
N-bromosuccinimide: the reaction was conducted in different
solvents, such as carbon tetrachloride or a mixture of acetic
acid : chloroform = 1 : 1, with comparable yield but a difference in
the reaction time resulted, the latter being much faster than the
former.[25]
Li(ν) = (ꢀ/2π)/[(ν − νi·t)2 + (ꢀ/2)2]
(2)
where ꢀ is the full width at half-maximum (FWHM), νi is the
vibrational wavenumber of the ith normal mode and t is the
wavenumber scaling factor. In order to mimic the Raman spectra
in solution, where probably both trans and cis conformers are
present, the analogous theoretical Raman spectra is obtained by
a linear combination of the two Strans(ν) and Scis(ν) spectra:
Two different coupling strategies were followed to yield
the targeted molecule: intermediate (4) was lithiated with
buthyllithium and subsequently coupled with CuCl219, or the
correspondent Grignard reagent was reached from (4) followed
by a Kumada coupling, catalyzed by Ni(dppp)Cl2. Following
this second route, an enhancement of the yield was obtained,
going from 33% to 67%. These syntheses were preferred to the
Suzuki–Miyaura coupling recently reported[26] because of the
difficulty to obtain 3,5-di-tert-butyl-4-hydroxyphenylboronic acid
as a pure product in high yields.
S(ν) = ctrans·Strans(ν) + ccis·Scis(ν)
(3)
where ctrans and ccis are the relative conformer populations. The
theoretical Raman spectra S(ν) is then fitted with the experimental
one by following a minimization mean-square errors procedure.
The fitted parameters are the relative population ctrans and ccis, the
wavenumber scaling factor t, and the Lorentzian’s width ꢀ.
Experimental
See the Supporting Information for experimental procedures
and 1H-NMR spectra.
Procedures for the synthesis of the Z-OCH3 and the intermediates
are reported in the Supporting Information. Unless otherwise
specified, all reagents, catalysts, and reagent-grade solvents were
commercial (Sigma Aldrich, Panreac), while anhydrous diethyl
ether was dried over a Na–K alloy. All the molecules have been
characterized with 1H-NMR spectroscopy.
All solvents used for the spectroscopic analysis were purchased
by Sigma Aldrich or J.T. Baker and used without further treatments.
A solution concentration around 10−1 M was chosen, thus
providing a good signal-to-noise spectral ratio and avoiding
aggregation phenomena.
Crystals of Z-OCH3 were obtained by slow evaporation from
solution.
Thin films were obtained on glass substrates using both
casting and spin-coating (1500 rpm) techniques, from chloroform
solutions with a concentration of 60 mg/ml.
FT-Raman spectra of samples in solution or in crystal powder
were recorded with a Nicolet FT-Raman Nexus NXR 9650
instrument, which employs a Nd/YAG laser with excitation at
λ = 1064 nm. For each spectrum, 512 scans were acquired with
2 cm−1 resolutionand800mWoflaserpower. Nospectralchanges
inducedbythelaserwereobservedduringthespectraacquisitions.
The Raman wavenumber range considered is between 100 and
3700 cm−1. Intensity was corrected with white-light internal
correction. Both liquid and solid samples were sealed in NMR
tubes suitable for the Raman spectrometer. For thin-film samples,
Raman spectra were recorded with a Horiba Jobin Yvon Labram
HR800 instrument using the 514.5 nm line of an argon ion laser for
excitation and equipped with a microscope that can be focussed
on specific areas of the sample with a resolution of the order of
micrometers, thus allowing distinguishing the different regions of
the film. The laser power was some tens of milliwatts. In order to
evaluate the possible effect of the laser excitation on the spectral
features of the material, the same setup was used also for some
of the crystalline samples analyzed with FT-Raman. No significant
differences in terms of wavenumber shifts were detected.
Theoretical results
Because of the bulky tert butyl (tBu) substituents and of the
presence of relatively flexible exocyclic CC bonds, Z-OCH3
is characterized by a complex and conformationally flexible
nonplanar structure. As reported in Ref. [20], from a detailed
conformational analysis carried out on Z-OCH3 it turns out that,
among the many possible stable conformers, only two can be
selected on the basis of their energy, namely the trans and
the cis forms (Fig. 2), both showing a perfect C2 point group
symmetry. From DFT calculations (in vacuo), the trans Z-OCH3
species is more stable than the cis by 0.37 kcal/mol. Also, DFT
simulations carried out mimicking the solution phase (by using
the polarizable continuum model (PCM)[27] as implemented in
the Gaussian03 code), give the trans form as the more stable
(ꢁE = 0.4–0.5 kcal/mol). However, we should note that an error
of the order of 1 kcal/mol in the computed relative energies of
the two conformers is within the accuracy of the level of the
theory (B3LYP/6-31G∗∗) adopted in these calculations. By varying
the dihedral angle (α) between the two thiophene rings, it is
possible to transform the trans conformer to the cis; accordingly,
we calculated the potential energy surface (PES) scan by varying α
and optimizing the other internal molecular degrees of freedom
at each step. The calculated energy barrier between the two
conformers turned out to be ≈5 kcal/mol. From these results we
concluded that, at room temperature and in solution, the trans–cis
isomerization process should easily occur for Z-OCH3. However,
this conclusion has not been supported yet by any systematic
experimental study. To further clarify this point, we simulated the
Raman spectra of both trans and cis Z-OCH3 conformers in order to
study their vibrational properties and to identify suitable spectral
regionsinwhichthetwoformsmightshowselectivespectroscopic
markers. This will be particularly useful for their characterization
and recognition both in solution and in the solid phase.
In order to point out the relevant spectroscopic markers in
the simulated Raman spectra, we consider here two regions,
namely spectral region I: 1700–1350 cm−1 and spectral region II:
650–400 cm−1. In Fig. 3 we report the unscaled theoretical trans
and cis Z–OCH3 Raman spectra in this two regions.
Results and Discussion
Synthesis
The molecule involved in this study is 3,3ꢁ-bis(3,5-di-tert-butyl-4-
methoxyphenyl)-2,2ꢁ-bithiophene (Z-OCH3, Fig. 1, in which the
From the spectral region I (Fig. 3a) the following observations
can be made: (1) the trans form shows higher Raman intensity
c
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J. Raman Spectrosc. 2010, 41, 406–413

Raman characterization of resistive organic nonvolatile memories
a CC bond sequence along the whole molecular backbone (thus
involving CC bonds of the bithiophene and phenyl units) and a
CC bond sequence confined to the bithiophene unit. Accordingly,
normal vibrations are either delocalized along the π-conjugation
path on the molecular backbone (modes B–C–D, bonds sequence
1–11 see Fig.4) or confined to the bithiophene unit (mode E,
bonds sequence 12–13–7–6–5–14–15, see Fig. 4). The strong
Raman activity of these vibrational transitions and, in particular,
the relevance of vibrations involving CC stretchings belonging to
a chain of conjugated thiophene rings (as for band E) have been
already observed and described for thiophene-based molecular
systems[31,32] andgenerallyforotherπ-conjugatedmolecules[33,34]
in the framework of the effective conjugation coordinate (ECC)
theory.[35,36]
In Fig. 4, the computed values of the derivative of the
polarizability tensor’s trace with respect to the CC bond stretching
(∂<α>/∂RCC; <α> = Tr [α]) are reported for the two different π
conjugation paths, as mentioned above. The sign of ∂<α>/∂RCC
oscillates from positive to negative values, with the only exception
of the next-to-last bond along the sequence. This behavior is
responsible of the large ∂α/∂Q value (and hence of the Raman
intensity) shown by normal modes, which can be described as a
collective out-of-phase CC stretching, namely the -vibrations
(Fig. 4). Moreover, it is interesting to note that all the ∂<α>/∂RCC
values are slightly larger in the trans conformer than in the cis, a
fact which justifies the larger intensity of the trans Raman pattern
with respect to the cis, in the spectral region I.
Figure 2. Molecular sketch of the trans and cis conformer of Z-OCH3 with
the indication of the main dihedral angles involved in the conformational
study. Optimized molecular structures as obtained by B3LYP/6-31G∗∗
simulations. For a detailed analysis of the geometry see Ref. [20].
than the cis; (2) the intensity ratios of the three Raman bands, from
1612 to 1566 cm−1 (bands B, C, D, Fig. 3a), are different in the two
conformers; and (3) a slight blue shift of 5 cm−1 is observed for the
cis band at 1575 cm−1 (D) with respect to the corresponding band
of the trans form.
For region II (650–400 cm−1, Fig. 3b), the theoretical Raman
spectra of trans and cis conformers show relevant differences in
their pattern. Bands a (νa = 500 cm−1) and b (νb = 600 cm−1
)
The detailed normal mode analysis (reported in the Supporting
Information) reveals that, for both the trans and cis species, the
are stronger for the trans conformer than for the cis. (The Raman
intensities of the II spectral region are 2 orders of magnitude
lower than those of the I region (1700–1450 cm−1)). Band a at
500 cm−1 (A species, Fig. S4) is highly sensitive to the different
molecular structures of the Z-OCH3; the analysis of the computed
normal mode eigenvectors shows that the associated vibrational
displacements involve a torsional motion around the C–C bond
linking the two thiophene rings. Interestingly, this 500 cm−1 mode
has larger Raman intensity for the trans than the cis form: its
relative intensity can be proposed as a Raman marker to probe the
occurrenceoftrans,cis,orbothconformers.Asafurtherproofofthe
correctness of our assignment, we have simulated also the Raman
spectra of a molecular moiety corresponding to one half Z-OCH3,
namely the intermediate 3 in Fig. 1, thus excluding the degree of
freedom related to the variation of the dihedral angle between
thiophene rings. Indeed, in the predicted Raman spectrum of this
compound, the a band localized at 500 cm−1 disappears.
Raman active vibrations in this spectral range correspond to C
C
and C–C bond stretching, which are either localized onto the
two phenyl groups or delocalized along the whole molecular
backbone, following the π-electrons delocalization path.
Accordingly, band A (νA = 1640 cm−1) corresponds to two acci-
dentally degenerate transitions for both trans and cis conformers,
consisting of a symmetric (A species referring to the C2 point
group) and an antisymmetric (B species) vibration localized onto
the phenyl moieties; as sketched in the Supporting Information
(Fig. S1), this vibration can be described as an oscillation of the
phenyl rings between an aromatic and a more quinoid structure.
Its intensity is not surprising since the normal mode localized
on the phenyl rings bears a parentage with the Raman active ν8
(Wilson notation) normal mode of benzene or even with the G
band of fused polycyclic aromatic hydrocarbons and ultimately of
graphite.[28,29]
Band b at 600 cm−1 consists of two degenerate modes, one
corresponding to the A species (high Raman intensity) and one to
the B species (low Raman intensity) related to out-of-plane modes
localized on the phenyl-thiophene moiety, as reported in Fig. S5.
In the 550 cm−1 region, broad features for both trans and cis
are observed showing different intensity patterns; the normal
mode analysis shows that the vibrations involved are related to
out-of-plane vibrations localized on the phenyl and/or on the
tert-butyl moieties differently coupled in the normal modes of the
two conformers.
From these theoretical results we can conclude that: (1) There
are many differences, in terms of bands intensity and wavenumber
shifts, in the Raman spectra of trans and cis Z-OCH3 conformers;
(2) While in the region I there is a broad change of the spectral
pattern and it is difficult to find simple and well-defined makers for
The normal mode of A species has a higher Raman intensity than
that of the B species by a factor of 1.5. Bands B (νB = 1612 cm−1, A
species), C (νC = 1593 cm−1, A species), and D (νD = 1566 cm−1
,
B species), whose eigenvectors are reported in Supporting
Information (Fig. S2), are related to alternating CC stretching
and shrinking along the π-conjugation path involving the whole
molecular backbone, while band E (νE = 1500 cm−1, A species)
is the in-phase CC stretching and shrinking more localized on
the bithiophene unit (see Fig. S3 in the Supporting Information).
These bands (B–E) can be referred to the well known -modes,
consisting of alternated stretching and shrinking of adjacent
CC bonds belonging to a sequence of carbon atoms bearing
conjugated π electrons.[30] In the molecule under study, there
are different possible conjugation paths for π electrons, namely
c
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D. Fazzi et al.
Figure 3. Simulated Raman spectra: (a) I spectral region, 1700–1350 cm−1 and (b) II spectral region, 650–400 cm−1 of both Z-OCH3 trans (solid line) and
cis (dotted line) conformers (unscaled B3LYP/6-31G∗∗ wavenumber).
Figure 4. Z-OCH3 molecular structure with the two conjugation paths: 1–11 and 12–13–7–6–5–14–15. Upper panel: theoretical values of the derivative
of the polarizability tensor’s trace with respect to the CC bond stretching (left) and the corresponding
first conjugation path (right). Bottom panel: the same results displayed along the bithiophene moiety (12-13-7-6-5-14-15), and the
–mode (band C, νC = 1593 cm−1) along the
–mode localized
onto the thiophene rings (band E, νE = 1500 cm−1). Red bars for trans conformer and blue bar for cis.
c
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Raman characterization of resistive organic nonvolatile memories
Figure 5. Comparison between experimental (top) Raman spectra (chloroform solution, dotted line) and simulated (bottom) spectra of Z-OCH3, trans
(solid line) and cis (dotted line) conformers. (a) I spectral region, 1700–1350 cm−1 and (b) II spectral region, 650–400 cm−1. Theoretical frequencies are
scaled in the I region by a factor of 0.9755 and are unscaled in the II region.
a conformational diagnosis, the spectral region II (650–400 cm−1
)
dichloromethane, tetrahydrofuran (THF), diethyl ether, and n-
hexane, which cover a quite wide range of values of dielectric
constants and dipole moments (as reported in Table 1 of the
Supporting Information).
is the most suitable region for the detection (through the analysis
of the experimental spectra) of the different conformers in real
samples and possibly for a semiquantitative determination of their
relative populations by means of fitting procedures (see below).
Spectroscopic markers in this region turn out to be the bands a
(νa = 500 cm−1), b (νb = 600 cm−1), and the intensity pattern
Despite the different properties of the solvents, the differences
observed in the conformer population ratio after the fitting
procedure of Raman spectra are quite small and confirm the
4 : 1 cis to trans ratio for each case, thus excluding a strong
correlation between the stability of the two molecular structures
and the chemical-physical properties of the environment. We
can conclude that the dielectric constant and the permanent
dipole moment of the solvents have negligible effect on the
stability of the conformers for the molecule Z-OCH3 and that the
cis form is always the dominant species in solution (Fig. S7). On
the other hand, in accordance with the theoretical simulations,
the values of permanent dipole moment (µtrans = 0.7768 D,
µcis = 0.6659 D) are not very different; this fact allows us to
dismiss the hypothesis of the existence of remarkably different
dipole–dipole interactions between the two conformers and the
polar solvents used. Moreover, the Raman spectra simulated with
the presence of solvents such as chloroform by using the PCM
approach show no relevant wavenumber shifts both in the I and II
region considered here.
A further support to these results has been obtained by
considering the possible effect on the Raman features of Z-
OCH3 of a bad solvent: by adding portionwise methanol to
a chloroform solution of Z-OCH3, no significant differences in
the Raman markers were observed until the two solvents were
around 1 : 1 vol; at the saturation limit, a Z-OCH3 precipitate as a
white powder was obtained. The spectrum of this powder (see
Supporting Information, Fig. S8) shows the typical markers of the
trans isomer, in accordance with the experimental results reported
in the next section.
around 550 cm−1
.
Based on the conclusions of our theoretical analysis, the
experimental data will be analyzed in the wavenumber range
identified through the modeling, namely regions I and II.
Experimental Raman spectra in solution
The two reference regions of the FT-Raman spectrum of a solution
of Z-OCH3 in chloroform are reported in Fig. 5 and compared with
the simulations in which all frequencies of the region I are scaled
by a suitable factor (0.9755). In the spectra of solutions, the signals
of the solvent have been subtracted.
The comparison between the experimental Raman spectra and
the theoretical results does not show evidence of any dominant
species between the two conformers, thus indicating that both
forms can occur in solution at room temperature. Both the band
pattern around 1500 cm−1 and the broad experimental intensity
pattern around 550 cm−1 well match with those of the simulated
cis form, but the observation of weaker Raman bands around 600
and 500 cm−1 (red arrows in Fig. 5b) suggests the existence of also
the trans species.
The different relative intensities of the features in the region
between 480 and 600 cm−1 make possible a study on the relative
conformerspopulation:thefittingproceduregivesacis : transratio
of ca 4 : 1 (Fig. S6 of the Supporting Information).
This ratio suggests that the cis form is more stable than the
trans one, at least in solution. Considering a classical Boltzmann
distribution, acis : transratioofca4 : 1atroomtemperatureimplies
an energy difference of 0.8 kcal/mol in favour of the cis species,
consistent with the theoretical results previously discussed.
In order to explore whether the relative conformers popu-
lation and stabilization energies estimated are influenced by
the environment, Raman spectra of Z-OCH3 in different sol-
vents were analyzed. The solvents considered were acetone,
Experimental Raman spectra in the solid state: single crystals
and thin films
Single crystals were obtained through slow evaporation from
concentratedsolutionsofZ-OCH3 indiethylether.X-rayanalysis[20]
indicates that the crystalline structure of the molecule is the trans
form. The result is confirmed by Raman spectroscopy; the trans
c
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D. Fazzi et al.
Figure 6. Comparison between experimental (single crystal obtained from diethyl ether solution, upper broken line) and simulated (bottom) spectra of
Z-OCH3, trans (solid line) and cis (dotted line) conformers. (a) I spectral region, 1700–1350 cm−1 and (b) II spectral region, 650–400 cm−1. Theoretical
frequencies are scaled in the I region by a factor of 0.9755 and are unscaled in the II region. Arrows indicate the band assignment and intensity trend for
cis and trans conformer.
Figure 7. Comparison between experimental Raman spectra of deposited (casting) Z-OCH3 thin film from chloroform solution (upper dotted line
‘transparent’) and after 24 h of exposure in air (upper broken line, ‘opaque’). (a) I spectral region, 1700–1350 cm−1 and (b) II spectral region,
650–400 cm−1. Theoretical simulated (bottom) Raman spectra of Z-OCH3, trans (solid line) and cis (dotted line) conformers, are also reported for direct
comparison.
isomer is indeed immediately recognized by considering, in the
spectral region II, the sharp peak near 500 cm−1, the well-defined
weak band at 600 cm−1, and the pattern of three bands near
550 cm−1 (Fig. 6b). Accordingly, for the spectral region I, we notice
theredshiftoftheDbandnear1525–1515 cm−1,whichisassigned
to the trans conformer. Moreover, the intensity of the 1375 cm−1
band (Fig. 6a) leads to the hypothesis that the cis form, if present,
is not the dominant one. Crystals obtained from THF and ethyl
acetate provide the same Raman features, thus indicating that
negligible effects result from changing the starting solution.
The same spectroscopic study was applied to thin film samples,
representative of the organic layer in nonvolatile memories based
on Z-OCH3[5] in order to highlight which active species are present
in the device and thus involved during the electrical process; this
is certainly a key point to determine the molecular origin of the
switching mechanism.
allows a better signal-to-noise ratio in the Raman spectra. For
both the cast and spin-coated films, a progressive transition
to an opaque film, which started from nucleation sites usually
located near the film boundary, was observed during time. At
room conditions, this phenomenon starts a few hours after the
deposition and takes some days to complete, depending on the
uniformity and the extension of the film.
Forthesesamples,theRamanspectraoftransparentandopaque
regions of the film were obtained with a Raman spectrometer
equipped with a microscope that allowed focusing the laser line
(in the visible) on specific points of the sample with a resolution
of the order of micrometers. Interestingly, the transparent areas
of the film provide the same features of the solution spectra
reported previously (abundance of cis conformer, see Fig. 7b).
In contrast, the opaque regions show the appearance of the
bands a around 500 cm−1 and b at 600 cm−1, typical of the trans
conformer (Fig. 7b). These findings lead to the hypothesis that a
cis to trans isomerization occurs in the solid state, starting from a
Thin films were cast from a chloroform solution to yield thicker
filmswithrespecttospin-coatedlayers(somemicrons vshundreds
of nanometres); this difference of around one order of magnitude
c
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Copyright ꢀ 2009 John Wiley & Sons, Ltd.
J. Raman Spectrosc. 2010, 41, 406–413

Raman characterization of resistive organic nonvolatile memories
more abundant cis form in the film as deposited and reaching a
new population dominated by the trans species after few hours.
Atthisstageofthework,theRamananalysisgivesthisindication:
in the solid state (film), the system tends to form a crystalline phase
as indicated by the observation of the specific markers of the trans
conformer in opaque regions. On the other hand, the remarkably
different Raman features of the film observed immediately after
deposition, together with its transparent appearance, suggest
that in the starting film the material morphology corresponds to a
mainly amorphous molecular arrangement.
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One structural isomer, namely Z-OCH3, displaying two different
conformers of similar stability, has been considered. From a direct
comparison between experimental and simulated Raman spectra,
it is apparent that unambiguous spectroscopic markers related to
the presence of the trans or of the cis Z–OCH3 conformer can be
identified and rationalized in two specific spectral regions. Raman
spectra of Z-OCH3 in solution show that trans and cis conformers
arebothpresentindynamicequilibrium. Afittingprocedurebased
on predicted spectra indicates that in solution the relative popula-
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Acknowledgement
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This work has been partly supported by grants from the
Italian Ministry of Education, University and Research through
FIRB projects ‘Molecular compounds and hybrid nanostructured
materials with resonant and nonresonant optical properties for
photonic devices’ (RBNE033KMA) and from MIUR grant ex 60%.
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J. Raman Spectrosc. 2010, 41, 406–413
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