Ruthenium acetate cluster amphiphiles and their langmuir-blodgett films for electrochromic switching devices

Article abstract of DOI:10.1002/ejic.201301442

We synthesized a long-chain phenylazopyridine ligand and used it to obtain three new amphiphilic trinuclear ruthenium acetate clusters. The amphiphilicity and anisometric form of the long-chain ligand allowed the complexes to self-assemble at the liquid-a

Full text of DOI:10.1002/ejic.201301442

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DOI:10.1002/ejic.201301442
Ruthenium Acetate Cluster Amphiphiles and Their
Langmuir–Blodgett Films for Electrochromic Switching
Devices
Karine P. Naidek,[a] Denize M. Hoffmeister,[a] Jaqueline Pazinato,[a]
Eduard Westphal,[b] Hugo Gallardo,[b] Marcelo Nakamura,[c]
Koiti Araki,[c] Henrique E. Toma,[c] and Herbert Winnischofer*[a]
Keywords: Ruthenium / Cluster compounds / Amphiphiles / Langmuir–Blodgett films / Electrochemistry
We synthesized a long-chain phenylazopyridine ligand and
used it to obtain three new amphiphilic trinuclear ruthenium
acetate clusters. The amphiphilicity and anisometric form of
the long-chain ligand allowed the complexes to self-
assemble at the liquid–air interface and enabled their deposi-
tion by the Langmuir–Blodgett technique. We obtained the
linear complex [Ru3O(C2H3O2)6(py)2L]+ [L = 4-(4-dodecyl-
oxyphenylazo)pyridine, py = pyridine], which was more
stable than the triangular [Ru3O(C2H3O2)6L3]+ and more suit-
able for application in electrochromic switching devices, as
attested by its fast switching and optical changes in the near-
IR region. Indeed, our electrochromic device remained stable
for hundreds of switching cycles.
in electrochromic switching devices in the near-IR. Optical
attenuators that work in the near-IR are technologically im-
portant for the fabrication of new data storage and optical
telecommunication devices.[5] Toma et al.[6] reported the
first example of an electrochromic switching film based on
triruthenium clusters; these authors described how they im-
mobilized the ruthenium clusters on a Prussian blue ana-
logue. In another paper, Toma et al.[7] demonstrated a very
stable electrochromic device based on triruthenium clusters
adsorbed on nanocrystalline titanium dioxide films. Re-
cently, Matsuse et al.[8] presented a new device based on
triruthenium cluster metallopolymers that displays three-
color electrochromism associated with the [Ru3O-
(C2H3O2)6]2+, [Ru3O(C2H3O2)6]+, and [Ru3O(C2H3O2)6]0
species. The metallopolymer film exhibited an optical
change response time of 5 s for the [Ru3O(C2H3O2)6]+/0
couple (monitored at 923 nm) and 80% conversion.
In the aforementioned studies, the ancillary ligands of
the triruthenium complexes play a special role in the immo-
bilization of the complex onto a polymeric film or in the
direct attachment of the complex to the electrode surface.
Here, we used the long-chain-derived phenylazopyridine li-
gand (Scheme 1) to obtain amphiphilic triruthenium clus-
ters.
Introduction
Trinuclear ruthenium acetate clusters bearing a core with
the general formula [Ru3O(C2H3O2)6]n and D3h symmetry
have long been studied.[1] In these clusters, the metal centers
strongly interact through μ-oxo and carboxylate bridges,
which results in a fully delocalized electronic structure and
provides versatile molecular models for the investigation of
electron transfer.[2] The charge delocalization in these tri-
ruthenium clusters gives rise to rich electrochemical behav-
ior: up to five reversible monoelectronic redox processes
separated by 1 V occur in electrolyte solutions in aceto-
nitrile.[3] This behavior has led scientists to incorporate tri-
ruthenium clusters as films in electrodes[4] to render them
electroactive. In addition, the monoelectronic redox pro-
cesses centered at the [Ru3O] core culminate in drastic chro-
matic changes, especially in the near-IR regions. As spectral
changes in the 700 and 900 nm regions accompany the most
accessible redox pair [Ru3O(C2H3O2)6]+/0 in the 0.0–0.2 V
[vs. standard hydrogen electrode (SHE)] range, researchers
have explored the application of triruthenium cluster films
[a] Department of Chemistry, Universidade Federal do Paraná,
Curitiba, Paraná 81531-980, Brazil
E-mail: hwin@ufpr.br
http://www.quimica.ufpr.br/site/
Mahato et al.[9] recently used a similar strategy to deposit
films of ruthenium polypyridine complexes. We synthesized
and fully characterized the 4-(4-dodecyloxyphenylazo)pyr-
idine (L) ligand and the amphiphilic triruthenium clusters.
We explored the self organization of the clusters at a liquid–
air interface to prepare films by the Langmuir–Blodgett
(LB) technique. We then investigated the resulting 15-layer
[b] Department of Chemistry, Universidade Federal de Santa
Catarina,
Florianópolis, Santa Catarina 88040-900, Brazil
[c] Department of Inorganic Chemistry, Chemistry Institute
Universidade de São Paulo,
São Paulo, São Paulo 05508-000, Brazil
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201301442.
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exhibits an intense redox pair with E1/2 = –0.90 V and we
detected additional peaks typical of adsorbed species after
some scans over this redox process. These peaks probably
correspond to species originating from cleavage of the inves-
tigated complex. In addition, the cathodic–anodic peak sep-
aration for process I is 0.150 V, whereas the separation is
ca. 0.08 V for processes II and III.
Scheme 1. Synthetic route to the 4-(4-dodecyloxyphenylazo)pyr-
idine (L) ligand and the amphiphilic triruthenium clusters 1a, 1b,
and 2. (i) HBF4 (48 wt.-% in H2O), NaNO2; (ii) phenol, NaOH,
H2O; (iii) C12H25Br, K2CO3, butanone; (iv) [Ru3O(C2H3O2)6-
(py)2CH3OH]PF6 (or [Ru3O(C2H3O2)6(pic)2CH3OH]PF6), CH2Cl2;
and (v) [Ru3O(C2H3O2)6(CH3OH)3]C2H3O2, CH2Cl2 (see Exp.
Sect. for details).
electroactive film as a near-IR electrochromic switching de-
vice. We expected that the hydrophobic character of the
long-chain ligand should protect the ruthenium center from
parallel reactions and deactivation[10] and that the π-ac-
ceptor character of the phenylazopyridine group should sta-
bilize the metal–ligand bond.
Figure 1. Electrochemical characterization of 1a. (A) Cyclic vol-
tammetry of 1a in a 0.1 m tetrabutylammonium perchlorate (TBA-
ClO4) dichloromethane solution, v = 100 mVs–1. (B–D) Spectroe-
lectrochemistry of 1a showing the spectral changes related to redox
processes I–III. The arrows indicate the changes related to re-
duction processes in I and II and to oxidation process in III.
Results and Discussion
Figure 1 (B–D) illustrates the spectroelectrochemical
We successfully synthesized the amphiphilic phenylazo- changes that occur during the redox processes I, II, and III
pyridine ligand L and complexes 1a, 1b, and 2. The green depicted in Figure 1 (A). The spectrum of 1a (Figure 1, C)
color of 1 and 2 distinguished them from the deep yellow displays an intense band at 383 nm, assigned to L, as well
color of the L solution and aided their separation by the as the intracluster charge-transfer (ICCT)[11] band at
silica column chromatography. The complexes are soluble 700 nm. The 383 nm band stems from the n–π*[13] or π–
in dichloromethane and chloroform, in contrast with the π*[14] transition of the whole conjugate system involving
[Ru3O(C2H3O2)6(py)2(CH3OH)]+ {or [Ru3O(C2H3O2)6- both aromatic rings and the azo group.
(pic)2(CH3OH)]+ (py = pyridine, pic = 4-picoline)} and
[Ru3O(C2H3O2)6(CH3OH)3]+ precursors.
We designate the broad band relative to the phen(π*)ǟazo-
(π) charge-transfer transition as n–π* to distinguish it from
In all our experiments, complexes 1a and 1b gave similar a narrow π–π* band related to the localized transition
results. We conducted the electrochemical assays with an in the N-heterocyclic rings with maximum below 250 nm.
electrolyte solution in dichloromethane instead of aceto- The characteristic triruthenium metal-to-ligand charge-
nitrile or dimethylformamide (DMF). Figure 1A shows the transfer (MLCT) band is covered by the n–π* band. During
cyclic voltammogram of 1a. This complex undergoes three redox process II (Figure 1, C), the n–π* band diminishes
redox processes separated by ca. 1 V in the analyzed poten- and shifts to 372 nm, the band at 700 nm disappears, and
tial window. The half-wave potentials (E1/2) appear at 1.30, the [Ru3O(C2H3O2)6]0 ICCT band appears at 950 nm. The
0.16, and –0.84 V and are ascribed to the redox couples reduction of the cluster promotes a hypsochromic shift in
[Ru3O(C2H3O2)6]2+/+
, , and [Ru3O- the n–π* band, which is consistent with increased electron
[Ru3O(C2H3O2)6]+/0
(C2H3O2)6]0/–1, respectively. Despite the stronger π-acceptor density in the π* orbital and evidences participation of the
character of the ligand L as compared with pyridine, the aromatic portion of the ligand L bound to the ruthenium
experimental E1/2 value obtained for the [Ru3O(C2H3O2)6- ion. Redox process I (Figure 1, B) decreases the n–π* band
(py)2L]+/0 couple coincides with the E1/2 value of the [Ru3O- and slightly alters the bands at 606 and 925 nm. The n–π*
(C2H3O2)6(py)3]+/0 couple.[11] The [Ru3O(C2H3O2)6L3]+/0 becomes less intense probably owing to reduction of the
couple (from 2) is the only couple to have E1/2 shifted by ligand L; however, the other minor changes are consistent
+0.2 V relative to the [Ru3O(C2H3O2)6(py)3]+/0 couple; with reduction of the cluster. As the redox wave for process
therefore, the ligand L stabilizes the lower oxidation state I has similar intensity to those of processes II and III (Fig-
of the ruthenium cluster. The process at –0.84 V should also ure 1, A), indicating that they are monoelectronic processes,
be associated with reduction of L; in this case, L is reduced the reaction for redox process I should involve an orbital
at ca. +0.28 V more positive potentials than free L.[12] We delocalized over the cluster and L. The n–π* band does not
assumed that the ligand L is reduced at –0.84 V, because 2 disappear completely: the applied potentials are close to the
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lower limit of the potential window, so the solvent probably point to oxidation of the cluster.[6,7,15] The presence of
also undergoes reduction, and this hinders reduction of the isosbestic points in the spectra of Figure 1 (B–D) indicates
complex to some extent. Redox process III (Figure 1, D) that simple equilibrium reactions occur during redox pro-
reveals slight changes in the n–π* and near-IR bands that cesses I–III.
The Π–A isotherm of the free ligand L (Figure 2, A) re-
veals three defined regions that correspond to the typical
pattern of a surfactant. The molecular area A is 66 Å2,
which is three times larger than that of arachidic acid.[16]
The large value of A indicates that the L molecules are tilted
with respect to the surface. The coordination of the ruth-
enium cluster to L modifies the shape of the Π–A isotherm,
as shown in Figure 2 (B) for 1b; complex 1a exhibits similar
behavior. We found an increase in the surface pressure to
ca. 600 Å2, and the molecular area increases to A = 204 Å2.
This value is comparable to the molecular area of an am-
phiphilic ruthenium polypyridine complex.[9]
Although 1b behaves as an amphiphile, hydrophobic li-
gands surround the charge at the triruthenium cluster. This
should hinder orientation of the molecules at the liquid–air
interface, which accounts for the large molecular area and
explains why intermolecular interactions occur at 600 Å2.
Nevertheless, the monolayer film formed at the liquid–air
interface is very stable and it collapses at more than 30
mNm–1, which is close to the value obtained for L (Fig-
ure 2, A and B).
In contrast to 1a and 1b with linear symmetry, complex
2 has triangular symmetry. Hence, the surface pressure al-
ters the Π–A isotherm to ca. 800 Å2. The symmetry of 2 is
the reason for this change at the beginning of the liquid
region: the molecules probably acquire flat positions at the
liquid–air interface, and the three organic chains point to-
ward the neighboring molecules. The phase transition
modifies the slope of the curve at ca. 340 Å2: the flat mole-
cules rearrange themselves as a monolayer of standing mo-
lecules. The condensed region explains the rise in the curve
below ca. 250 Å2. The measured molecular area is 253 Å2,
which is close to the molecular area obtained for 1b; a col-
lapse occurs at 35 mNm–1. On the basis of the Π–A results,
we decided to deposit films at a surface pressure of 25
mNm–1.
We used AFM to analyze the deposited multilayer films.
The L film (Figure 3, A) resembles piles of layers. The step
distance between these layers varies from 50 to 60 Å, which
is consistent with the size of a 4-(4-dodecyloxyphenylazo)
pyridine bilayer in a head-to-head (or tail-to-tail) confor-
mation and is as expected for classic Langmuir–Blodgett
film deposition.[17] Films of 1b and 2 (Figure 3, B and C,
respectively) display regularly distributed round structures
with base diameters of ca. 105 and ca. 170 nm, respectively.
Figure 2. Π–A curves obtained for (A) L, (B) 1b, and (C) 2; bar
speed: 100 cm2 min–1.
Figure 3. AFM images of 18-layer films of (A) L, (B) 1b, and (C) 2.
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Hence, 1b and 2 form aggregates rather than the densely allel) polarization clearly demonstrates these changes: all of
packed layers verified for free L. The molecular areas of the IR bands are more intense. The IR band at 1253 cm–1
both complexes probably create vacancies. Along successive is the one that intensifies the most and is assigned to phenyl
depositions, the cluster molecules fill these vacancies; this ether asymmetric stretching. As the L molecules arrange
culminates in a multilayer film that consists of regularly dis- as densely packed layers (Figure 4, E), the azophenyl ether
tributed aggregates instead of organized layers of the am- groups are oriented closely parallel to the normal plane and
phiphilic molecules. Scheme 2 illustrates this case of film vibrate collectively. Consequently, p polarization intensifies
deposition. Nevertheless, the films containing the com- this stretching mode. As mentioned above, the molecular
plexes should preserve open structures that create channels area value indicates that the L molecules are tilted with re-
and allow for species to diffuse during the electrochemical spect to the surface. Nevertheless, the polarized IR spectra
processes.
indicate that the phenyl ether groups are oriented closer to
the normal plane than to the surface.
Films of 1 and 2 do not pack like the L film. The spectra
of the former films (such as their p-polarized spectra) do
not evidence relative intensification of any IR bands (Fig-
ures 5 and S13). As in the case of the L film, films of 1 and
2 exhibit intense IR bands in the p-polarized spectra and
low signal-to-noise ratios in the s (perpendicular) polarized
spectra. Despite intrinsic optical limitations,[19] the amount
of material deposited as a multilayer film should be enough
for detection even under s polarization. These observations
suggest an orientation closer to parallel for molecules of 1
and 2 with respect to the normal plane in the films. We
investigated the triruthenium cluster LB films by cyclic vol-
tammetry. Films of 1b exhibit a redox pair related to the
process [Ru3O(C2H3O2)6(py)3]+/0 (C+/C0) at 0.08 V (vs.
Scheme 2. Proposed mechanism for the deposition of films of 1a
or 1b. The large molecular area of the complex creates cavities that
other molecules can fill; head-to-head and tail-to-tail stacking also
occurs. As a consequence, a regular layer of molecules is missed in
the deposition, and the amount of deposited molecules is larger
than expected.
To further characterize the films and minimize the con- SHE; Figure 6, A). The voltammetric response of the 1b (or
tribution from the substrate, we conducted infrared spec- 1a) film is quite stable and remains unaltered for hundreds
troscopy at a grazing angle of 80°. The use of polarized of potential scan cycles. In contrast, the film of 2 exhibits
radiation provides information about the orientation of the broad current waves during the first scan. Although their
species with respect to the substrate.[18] Figure 4, B–D com- shape changes during successive cycles, the current response
pares the IR spectra of the L film with the IR spectrum of stabilizes after 250 cycles and more defined current peaks
the L powder (Figure 4A).
arise (Figure S14). This conditioning procedure should
The spectrum of the L film recorded in the absence of prompt diffusion of ions from the electrolyte solution into
polarized radiation (Figure 4, C) displays the same bands the electroactive film. The initial broad current probably
as the spectrum of the L powder, but the band intensities results from the hydrophobic and closed structure of the
are different. The spectrum of the L film recorded at p (par- film of 2, which makes ion exchange with the solution diffi-
Figure 4. FTIR spectra of L: (A) diffuse reflectance of the powder and (B–D) specular reflectance of the film at 80° grazing angle. The
film spectra were recorded with (B) p and (D) s polarizations. Spectrum (C) corresponds to data recorded without polarization. The
bands in (A) face downward and the bands in (B–D) face upward for better visualization and comparison. (E) Scheme of the densely
packed layer arrangement in the L LB film.
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cult. Conversely, as the film of 1b affords a more stable of different thicknesses, from monolayer (single deposition)
electrochemical response, it is probable that it has an open up to 18 layers (depositions). The current peak intensities
structure, which allows ions to easily diffuse into the mate- increase linearly with the scan rate (Figure 6, B), which is
rial.
characteristic of an electron-transfer-limited process. The
current versus scan rate plot tends to deviate from linearity
and approaches a diffusion-limited process for thicker films
with 50 and 100 layers (Figure S15). Up to 18 layers, the
current peak intensities also increase linearly with the
number of layers. We obtained an unexpected result when
we analyzed the peak potential values shown in Figure 6
(A): the anodic, and more drastically, the cathodic peak po-
tentials shift to positive values as the number of layers in-
crease. As a result, the half-wave potentials (E1/2) shift to
positive values as the films become thicker. In Figure 6 (C),
we plot the mean values of E1/2 for different films as a func-
tion of the number of layers. Figure 6 (C) also illustrates the
close relationship between the shifts in E1/2 and the anodic–
cathodic peak potential separations (ΔE) as the thickness
increases. The anodic–cathodic peak potential separation is
a kinetic factor related to resistivity, that is, to electrons
hopping between the triruthenium centers in the film.
As the number of layers increases, the charge diffusion
thickness increases, which results in a larger ΔE value. How-
ever, E1/2 is a thermodynamic factor and its shift relates to
electronic perturbation in the triruthenium cluster inside
the material. We believe that the electrode influences this
phenomenon by augmenting the electron density on the
surface-immobilized triruthenium clusters. This effect is
more evident in thin films, in which most of the triruthen-
Figure 5. FTIR spectra of 1b: (A) diffuse reflectance of the powder
and (B–D) specular reflectance at 80° grazing angle. Film spectra
were recorded with (B) p and (D) s polarizations. Spectrum (C)
corresponds to data recorded without polarization. The bands in
(A) face downward and the bands in (B–D) face upward for better
visualization and comparison.
This structure most likely stems from the symmetry of
1b compared with that of 2; the linear arrangement of 1b
exposes the electroactive ruthenium cluster sites more. Fig-
ure 6 (A) presents the cyclic voltammograms of films of 1b
Figure 6. Electrochemical characterization of films of 1b. (A) Cyclic voltammograms (CV) of films of 1b of different thicknesses. Scan
rate = 10 mVs–1; 0.5 m aqueous KNO3 solution. (B) Current vs. scan rate relationships obtained from the CV experiments. (C) Mean
values of E1/2(᭹) and ΔE (᭺) obtained from the CV data as a function of the number of layers. (D) Electrochemical surface concentration
(Γ) as a function of the expected surface coverage considering the LB results.
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Figure 7. Spectral variation measured at 950 nm for a 15-layer film of 1b as the bias potential switches between –0.20 and 0.20 V.
ium centers remain close to the electrode interface; in thick for a common spectrophotometer to detect the alterations
films, most of the redox centers are immersed in the film. in the near-IR absorbance. We mounted the electrochemical
Therefore, triruthenium clusters in films with few layers do system by placing the 1b-modified ITO electrode in a
not constitute isolated species, but they are under the influ- quartz cuvette cell of 1 cm path length containing aqueous
ence of the electron density of the electrode. The E1/2 value electrolyte solution. The plot of the optical change at
in Figure 6 (C) approaches the E1/2 value observed for 1a 950 nm (related to the ICCT band of the [Ru3O-
in solution (E1/2 = 0.16 V; Figure 1). The plot of the triru- (C2H3O2)6]0 species) as we switched the bias potential be-
thenium cluster surface concentration (Γ) versus the surface tween –0.20 and 0.2 V is shown in Figure 7. We expanded
coverage is shown in Figure 6 (D). We calculated Γ from the portion of the graph relative to the first minute to illus-
the area below the Faradaic peak in the cyclic voltammog- trate the fast spectral variation. The resolution of the ab-
rams; we estimated the surface coverage by considering the sorption measurement is 0.5 s, and the switching process
molecular area obtained from the Π–A isotherms. For films takes 1 s to convert Ͼ90% of the absorbance, which is much
of up to 18 layers, Γ rises linearly with the surface coverage, faster than analogous films of polymeric complexes re-
but the slope is larger than expected. For the 18-layer film, ported in the literature.[8] In addition, the film is quite
the Γ value (3.4ϫ10–9 molcm–2) is more than twice the ex- stable: the absorbance loss is negligible over 30 min and
pected surface coverage (1.5ϫ10–9 molcm–2). We attribute hundreds of cycles. This outcome is attributed to the hydro-
this result to the way that the 1b molecules are packed in phobic character of the phenylazopyridine ligand, which
the film. As discussed above, as the layers have large molec- prevents lixiviation of the film into the aqueous solution
ular area, they contain vacancies that other molecules can and strongly binds to the triruthenium cluster.
fill (Scheme 2). Consequently, a larger amount of molecules
can be deposited and this results in the higher slope ob-
served in Figure 6 (D). Films thicker than 50 layers afford
slightly lower Γ values than the theoretical 45° slope; the
Conclusions
We synthesized amphiphilic triruthenium acetate clusters
and prepared their LB films. The hydrophobic character of
the phenylazopyridine ligand allows them to self-assemble
at the liquid–air interface and renders the LB films stable
against lixiviation. The film of the linear complex 1 (a or b)
is electrochemically more stable and, thus, more suitable for
use in electrochromic switching devices. The half-wave po-
tential of films with less than 20 layers fluctuates, so we
concluded that the electrode affects these films. The elec-
trochromic switching device experiences fast near-IR op-
tical changes with a response time in the scale of seconds.
The device is quite stable over hundreds of cycles, and the
maximum absorptivity decreases only very slightly.
100-layer film furnishes ca. 25% of the expected value. We
also measured the visible absorbance of these films de-
posited on transparent fluorine-doped tin oxide (FTO) elec-
trodes. We noticed that the absorption intensity actually in-
creases for the 50- and 100-layer films (Figure S16). Hence,
the lower Γ values observed in Figure 6 (D) are probably
cause by a decrease in the number of electroactive sites in
these thick films. In thicker films, some of the triruthenium
sites immersed in the film do not participate in the hopping
mechanism of charge percolation and are, thus, inactive.
Notably, ΔE (Figure 6, C) for the 100-layer film is smaller
than ΔE for the 18- and 50-layer films.
The studied complexes undergo a redox process that is
accompanied by intense optical changes in the near-IR re-
gion, as shown in Figure 1 (C). We explored this property
of the LB films to build a near-IR electrochromic switching
device with potential applications in information storage
and data transmission. To this end and on the basis of the
trend observed in Figure 6C, we decided to investigate a 15-
layer film of 1b film deposited on a transparent indium tin
oxide (ITO) electrode. This thin film exhibits stable and fast
Experimental Section
Materials and Chemicals: All reagents and solvents were of analyti-
cal grade; they were purchased from commercial sources (Merck,
Sigma–Aldrich, Fluka, and Acros) and used as receive. TLC was
performed on Si60-F254 (Merck) silica gel. Purifications were per-
formed by recrystallization with commercial grade solvents and by
electrochemical response, but the film remains thick enough column chromatography on 60–200 mesh 60A silica gel (Acros).
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The dichloromethane used in the electrochemical experiments was
treated with anhydrous calcium chloride and distilled with calcium
hydride before use. For the LB experiments, ultrapure water was
obtained in a Millipore® Milli-Q system (R = 18.3 MΩcm, 20 °C)
and used immediately.
5.00 (m, Hacetate), 5.57 (m, Hβ-py), 6.04 (m, Hβ-L), 6.31 (m, Hγ-
py), 6.85 (d, Ar-H), 7.61 (d, Ar-H) ppm. 1H NMR (400 MHz,
25 °C,CDCl3, TMS): 1b: δ = 0.25 (m, Hα-L), 0.86 (t, CH3), 1.19–
1.45 (m, CH2), 1.76 (m, CH2CH2O), 2.71 (s, CH3 pic), 3.98 (t,
OCH2), 4.52, 4.84 (m, Hacetate), 5.82 (m, Hβ-py), 6.86 (d, Ar-H),
7.64 (d, Ar-H) ppm. IR (KBr): νmax = 2925, 2853, 1595, 1500, 1427,
˜
4-(4-Hydroxyphenylazo)pyridine (B): (i) Diazotization: A mixture of
4-aminopyridine (A; 4.00 g, 42.5 mmol) in tetrafluoroboric acid
(35 mL, 48 wt.-% in H2O) was cooled to –5 °C with an ice/NaCl/
1258, 1139, 1030, 843, 682 cm–1. MS: m/z = 1199.10 (1a) [M]+,
1228.17 (1b) [M]+.
acetone bath. Next, solid NaNO2 (3.08 g, 44.6 mmol) was added Complex 2: To synthesize 2, a procedure similar to that used to
in small portions over 45 min with intense stirring. After NaNO2
addition was complete, the cold bath was maintained for another
hour. The suspension was then filtered through a Büchner funnel
to yield the diazonium salt. H NMR (200 MHz, [D6]DMSO): δ =
7.26 (d, J = 7.2 Hz, 2 H, Py-H), 8.57 (d, J = 7.2 Hz, 2 H, Py-H)
obtain 1a and 1b was used. In a 125 mL reaction vessel, [Ru3O-
(C2H3O2)6(CH3OH)3](C2H3O2) (100 mg, 0.121 mmol) was dis-
solved in dichloromethane (20 mL). Then, L (220 mg, 0.6 mmol)
dissolved in dichloromethane (50 mL) was added to the system,
which was kept under reflux for 24 h in the dark. The mixture was
rotary evaporated to dryness. The product was purified by silica
column chromatography with hexane/acetone (9:1) as eluent, to re-
move impurities and excess L, followed by dichloromethane. This
procedure furnished 2 (75 mg, 32%). 1H NMR (200 MHz, 30 °C,
CDCl3, TMS): δ = 0.88 (m, Hα-L, CH3), 1.26–1.57(m, CH2), 3.98
(m, OCH2), 4.13 (m, Hacetate), 5.57 (Hβ-L), 7.12 (d, Ar-H), 7.33 (d,
1
ppm.
(ii) Azo Coupling: To an aqueous solution (300 mL) containing
phenol (4.00 g, 42.5 mmol) and NaOH (4.08 g, 102 mmol) at room
temperature, the freshly prepared diazonium salt was added in
small portions to give a deep red solution. After complete addition,
the mixture was stirred for a further 30 min. The pH was adjusted
to 7. The resulting precipitate was collected by filtration and
washed with water. Recrystallization in DMF/H2O afforded 6.68 g
(79%) of a red solid; m.p. 253.8–256.0 °C (ref. 265 °C).[20] 1H NMR
(400 MHz, [D6]DMSO): δ = 6.96 (d, J = 6.8 Hz, 2 H, Ar-H), 7.66
(d, J = 4.5 Hz, 2 H, Py-H),7.86 (d, J = 6.8 Hz, 2 H, Ar-H), 8.75
Ar-H) ppm. IR (KBr): ν
= 2918, 2849, 1560, 1421, 1258, 1022,
˜
max
1143, 844, 682 cm–1. MS: m/z = 1776.57 [M]+.
LB Film Deposition: Surface pressure–molecular area (Π–A) iso-
therms and LB film depositions were conducted in a Nima Tech-
nology 311D Trough system at 20 °C. A dichloromethane solution
(d, J = 4.5 Hz, 2 H, Py-H), 10.59 (s,1 H, OH) ppm. 13C NMR of the investigated compound (0.5 mgmL–1) was spread onto an
(100.6 MHz, [D6]DMSO): δ = 116.46, 116.86, 126.47, 145.91,
ultrapure water subphase with the aid of a microsyringe. The Π–A
curves were obtained at several bar speeds. The film depositions
were accomplished at 25 mNm–1 constant surface pressure and at
100 cm2 min–1 and 10 mmmin–1 bar and dip speeds, respectively.
FTO, ITO, gold, or silicon plates were used as substrates.
152.01, 157.48, 162.99 ppm. IR (KBr): ν
= 3453, 3038, 2565,
˜
max
1583, 1472, 1400, 1294, 1260, 1229, 1173, 1137, 1006, 837 cm–1.
MS: m/z = 200.08 [M]+.
4-(4-Dodecyloxyphenylazo)pyridine (L): A mixture of B (1.55 g,
7.79 mmol), 1-bromododecane (2.3 mL, 9.6 mmol), K2CO3 (2.15 g,
15.6 mmol), and butanone (30 mL) was added to a round-bot-
tomed flask and stirred under reflux for 15 h. After this period, the
suspension was filtered and washed with hot butanone. The solvent
was removed under reduced pressure, and the product was purified
by silica column chromatography with hexane/ethyl acetate (8:2) as
Apparatus and Measurements for B and L: Infrared spectra were
recorded with a Perkin–Elmer 283 spectrometer with samples as
KBr pellets. 1H and 13C NMR spectra were recorded with a Varian
Mercury Plus spectrometer operating at 400 and 100.6 MHz,
respectively; the 1H NMR spectrum of the diazonium intermediate
was recorded with a Bruker AC-200F spectrometer operating at
eluent to give 2.40 g (84%) of a red solid; m.p. 70.1–71.2 °C (ref. 200 MHz. Melting points were determined with an Olympus BX50
69.2–71.0 °C).[21] 1H NMR (400 MHz, CDCl3): δ = 0.87 (t, J =
microscope equipped with a Mettler Toledo FP-82 hot stage. High-
6.8 Hz, 3 H, CH3), 1.20–1.40 (m, 16 H, CH2), 1.47 (m, 2 H, CH2), resolution mass spectra were recorded with a Bruker micrOTOF-
1.82 (m, 2 H, CH2CH2O), 4.05 (t, J = 6.5 Hz, 2 H, CH2O), 7.02 Q II APPI mass spectrometer.
(d, J = 9.0 Hz, 2 H, Ar-H), 7.72 (d, J = 5.2 Hz, 2 H, Py-H), 7.96
Apparatus and Measurements for 1 and 2: A Bruker Vertex 70 spec-
(d, J = 9.0 Hz, 2 H, Ar-H), 8.78 (br, 2 H, Py-H) ppm. 13C NMR
trophotometer was used to acquire FTIR spectra in the 4000 to
(100.6 MHz, CDCl3): δ = 14.37, 22.93, 26.21, 29.34, 29.59, 29.79,
400 cm–1 range. The spectra were obtained with powder samples in
29.82, 29.87, 29.89, 31.25, 68.77, 115.14, 116.74, 126.03, 146.92,
KBr pellets; for the diffuse reflectance mode (DRIFT), the com-
150.52, 158.12, 163.40 ppm. IR (KBr): ν
= 3026, 2918, 2847,
˜
max
pounds were embedded in KBr by using a Pike Technologies Easi-
Diff accessory with 4 cm–1 resolution. The FTIR spectra of the LB
films (18-layer samples) deposited on gold substrates were recorded
with the A518/Q specular reflectance unit at 80° with the aid of a
grazing angle accessory with 8 cm–1 resolution. The polarized spec-
tra were collected at 0 and 90° by using an F-350 mid-infrared
(MIR) polarizer, KRS-5 (TlBr/TlI) optical crystals, and an A-110
rotatable holder. UV/Vis spectra were obtained from dichlorometh-
ane solutions placed in quartz cuvettes by using a Shimaduzu UV
2401 PC or Agilent 8453 spectrophotometer. A Bruker Daltonics
1602, 1582, 1498, 1469, 1404, 1317, 1252, 1142, 1003, 845 cm–1.
MS: m/z = 368.26 [M]+.
Complexes 1a and 1b: In a reaction vessel, the previously prepared
[11]
[Ru3O(C2H3O2)6(py)2(CH3OH)]PF6
or [Ru3O(C2H3O2)6(pic)2-
(CH3OH)]PF6 (40 μmol, 40 mg) was dissolved in dichloromethane
(17 mL). Next, 4-(4-dodecyloxyphenylazo)pyridine (L; 56 mg,
136 μmol) dissolved in dichloromethane (30 mL) was added to the
system, which was kept under reflux for 24 h in the dark. NH4PF6
(20 mg, 123 μmol) in ethanol (10 mL) was then added, and the sys-
tem was kept under reflux for 1 h. Then, the mixture was rotary MICROTOF-Q IITM mass spectrometer was used to record the
evaporated to dryness. Impurities and excess L were removed by
silica column chromatography with hexane/acetone (9:1) followed
by dichloromethane as eluents. This procedure furnished 1a (22 mg,
41%) and 1b (24 mg, 46%). 1H NMR: 1a: (200 MHz, 30 °C,
CDCl3, TMS): δ = 0.54 (m, Hα-py), 0.69(m, Hα-L), 0.86 (t, CH3),
1.19–1.45 (m, CH2), 1.76 (m, CH2CH2O), 3.97 (t, OCH2), 4.89,
ESI mass spectra of the compounds at a concentration of
10–2 mgmL–1 in dichloromethane. A Bruker DPX 200 spectrometer
1
operating at 4.7 T was used to acquire the H NMR (200.13 MHz)
spectra of the compounds; the signals were assigned relative to that
of tetramethylsilane (TMS) at 303 K. AFM images were recorded
from LB films (18-layer samples) deposited on silicon plates in a
Eur. J. Inorg. Chem. 2014, 1150–1157
1156
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.eurjic.org
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Received: November 11, 2013
Published Online: January 30, 2014
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© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim