ACS Applied & Mat Interfaces 2015

Surface-Confined Heterometallic Triads on the Basis of Terpyridyl
Complexes and Design of Molecular Logic Gates
Prakash Chandra Mondal,*,†,‡,⊥
Vikram Singh,†
Yekkoni Lakshmanan Jeyachandran,§,#
and Michael Zharnikov*,§
†
Department of Chemistry, University of Delhi, Delhi 110007, India
‡
Department of Chemical Physics, Weizmann Institute of Science, Rehovot 76100, Israel
§
Applied Physical Chemistry, Heidelberg University, Heidelberg 69120, Germany
*S Supporting Information
ABSTRACT: Surface-confined heterometallic molecular triads (SURHMTs) were
fabricated on SiOx-based solid substrates using optically rich and redox-active Fe-, Os-,
and Ru-based terpyridyl complexes as metalloligands and Cu2+
ions as linkers. Optical and
electrochemical studies reveal efficient electronic intramolecular communication in these
assemblies. The UV−vis spectra of the triads exhibit a superposition of the metal-to-ligand
charge-transfer bands of individual complexes, providing a significant enlargement of the
optical window, useful for application. Similarly, cyclic voltammograms of SURHMT layers
show a variety of redox peaks corresponding to individual complexes as well as multi-redox
states at a low potential. Interaction of a representative SURHMT assembly with redox-
active NOBF4 was investigated and used as a basis for configuring molecular logic gates.
KEYWORDS: metallo-ligands, layer-by-layer, molecular triads, logic gates, redox states
■ INTRODUCTION
Since the advent of coordination chemistry, transition metal
complexes have attracted significant interest of both academic
and industrial communities, in view of their flexible molecular
design, versatile physical and chemical properties, and a variety
of potential applications. A particular advantage of these
complexes is a possibility to immobilize them on various solid
substrates as monolayers, oligomer wires, metallo-organic
assemblies, etc.1−6
Such assembles are a good alternative and
a useful addition to the analogous systems based on the
coordination of organic ligands by metal ions as well as to
increasingly popular metal−organic frameworks.7−9
These
assemblies, which can also be considered as surface-confined
inorganic−organic hybrid materials,10,11
likely offer a potential
route to different applications, including sensors, switches,
memory elements, logic integration, electrochromic films, and
photonic materials,12−16
to name a few.
Popular systems in this context are transition-metal-based
polypyridyl complexes, which became a focal point in diverse
research areas due to their tunable photophysical17,18
and
electrochemical properties19
as well as potential applications in
molecular electronics,20
catalysis,21
solar energy conversion,22,23
etc. Most of these applications rely on controlled immobiliza-
tion of the polypyridyl complexes on solid substrates. Such an
immobilization was indeed successively performed in form of
monolayers, molecular dyads, as well as homogeneous and
heterogeneous oligomer films, with individual polypyridyl
complexes serving as molecular building blocks.24−27
The
primary methodology to build the above systems is stepwise
coordination reaction of the individual complexes (metallo-
ligands) and inorganic coupling units. This methodology has
potential to create even more complex molecular assemblies of
novel architectures, with desired thickness, structure, orienta-
tion, and physicochemical properties.28−31
Along these lines, we report here the fabrication of surface-
confined heterometallic molecular triads (SURHMTs) on glass,
Si (100), and indiuim tin oxide (ITO)-coated glass substrates.
For this purpose we used well-known terpyridyl complexes32
(so-called metalloligands), specifically, Fe(pytpy)2·2PF6, Ru-
(pytpy)2·2PF6, and Os(pytpy)2·2PF6 (pytpy = 4′-(4-pyridyl)-
2,2′:6′,2″-terpyridyl) as the molecular building blocks (Scheme
1) and Cu2+
ions as the metallolinkers. These metalloligands
are abbreviated below as Fe-PT, Ru-PT, and Os-PT,
respectively. They were chosen due to their strong binding
affinity to the transition metal ions, tunable photophysical and
electrochemical properties (based on the well-separated optical
bands and redox peaks, respectively), as well as expected
electronic communication between individual M-PT units (M =
Fe, Ru, Os) within the derived assemblies.33
This communi-
cation is assumed to be mediated by conductive metallolinkers
(Cu2+
) and is superior to the case of organic linkers, which can
be potentially advantageous for molecular electronics applica-
tions. To the best of our knowledge, this is the first report on
heterogeneous molecular triads, where three different metallo-
Received: February 1, 2015
Accepted: April 8, 2015
Published: April 8, 2015
Research Article
www.acsami.org
© 2015 American Chemical Society 8677 DOI: 10.1021/acsami.5b00953
ACS Appl. Mater. Interfaces 2015, 7, 8677−8686

ligands of group VIII transition metal ions (Fe2+
, Ru2+
, and
Os2+
) were assembled together on a single platform and studied
in context of their optical and electrochemical properties.
Among other advantages, a combination of three different
metallo-organic units in one functional moiety increases the
parameter space, allowing to store more “bits of information”,
which has potential applications in the development of
molecular memory.16,34
■ EXPERIMENTAL SECTION
Synthesis and characterization data for the 4′-pyridyl terpyridyl and
the derived Fe-PT, Ru-PT, and Os-PT complexes are given in the
Supporting Information (Scheme S1 and Figure S1), along with a
detailed description of all fabrication steps for the preparation of the
triad films.
■ RESULTS AND DISCUSSION
General Design and Fabrication of Surface-Confined
Heterometallic Molecular Triads. The SURHMT assem-
blies were fabricated by stepwise coordination reactions of
metallolinkers and metalloligands with pyridine-terminated
template layers (Scheme 1; see the Supporting Information
for details) following the established strategy of our previous
publications.24−27
Initially, an iodine-terminated coupling layer
(CL) was formed, serving as substrate for the attachment of the
first M-PT unit (M = Fe2+
, Ru2+
, Os2+
). The resulting
monomolecular template layer was allowed to react with
Cu(NO3)2 in acetonitrile. Subsequently, the Cu-terminated
template layer was immersed in a solution containing a
dissimilar M-PT unit to fabricate a heterometallic molecular
dyad. Finally, reaction with Cu(NO3)2 and subsequent
coordination with a dissimilar M-PT unit were repeated once
more, resulting in a SURHMT. Combining different M-PT
units, different SURHMTs were fabricated.
Basic Characterization of the Intermediate Layers and
Molecular Triads. Formation of SURHMTs was monitored
by static water contact angle (CA) goniometry, atomic force
microscopy (AFM), spectroscopic ellipsometry, X-ray photo-
electron spectroscopy (XPS), and near-edge X-ray absorption
fine structure (NEXAFS) spectroscopy. Representative of all
possible combinations of the M-PT units within a triad, Fe-PT/
Cu/Ru-PT/Cu/Os-PT assembly was chosen, but also some
selected data for other triads are presented.
Contact Angle. The CA goniometry measurements on the
representative Fe-PT template layer showed a moderately
hydrophobic surface, associated with a free pyridine group, with
a CA of 76 ± 2° (Supporting Information, Figure S2a). In
contrast, Cu-terminated Fe-PT template layer (Fe-PT/Cu) had
a lower value of CA, namely, 58 ± 0.5° (Supporting
Information, Figure S2b), which agrees with the expected
hydrophilicity of the Cu2+
ions. For the final triad assemblies, a
CA of 74−83° was observed, similar to the Fe-PT template
layer, which is understandable in view of the same termination
by the pyridine moieties.
Atomic Force Microscopy. A tapping mode AFM image of
the representative Fe-PT template layer on Si showed a
relatively smooth film (see Figure S3a in the Supporting
Information). The root-mean-square roughness (Rrms) meas-
ured for this layer over a scan area of 500 × 500 nm2
was
estimated at 0.56 ± 0.03 nm. Such a low roughness value
Scheme 1. Schematic Representation of the Fabrication Procedure for SURHMTs by the Example of the Fe-PT/Cu/Ru-PT/Cu/
Os-PT Assemblya
a
(i) Immobilization of 3-iodo-n-propyl-trimethoxysilane on a SiOx substrate to form coupling layer (CL), (ii) quaternization of pendant pyridyl
group of M-PT (M = Fe2+
, Ru2+
, Os2+
) to form template layer (TL), (iii) coordination of Cu(NO3)2 with pyridine-terminated monomolecular
template layer to form Cu-terminated template layer, (iv) coordination of a different M-PT unit, (v) coordination of Cu2+
, and (vi) coordination of
the third M-PT unit to form the desired SURHMT.
ACS Applied Materials & Interfaces Research Article
DOI: 10.1021/acsami.5b00953
8678

suggests the formation of a uniform film without contamination
or physisorbed molecules. The Rrms value increased to 0.78 ±
0.08 nm and further to 0.99 ± 0.1 nm for the dyad (Fe-PT/Cu/
Ru-PT) and triad (Fe-PT/Cu/Ru-PT/Cu/Os-PT) layers,
respectively, suggesting a progressive increase of roughness in
the course of the successive coordination. The corresponding
AFM images are presented in the Supporting Information
(Figure S3b,c, respectively). In addition, variation of roughness
as a function of the deposition steps is given in the Supporting
Information (Figure S4).
Ellipsometry. For the representative Fe-PT/Cu/Ru-PT/Cu/
Os-PT assembly, ellipsometry-derived thicknesses of the Fe-PT
template layer and the successive dyad and triad films were
found to be ∼18.5, 25−32, and 38−42 Å. The observed gradual
increase in the thickness suggested an efficient attachment of
the individual building blocks. Comparison of the thickness
values with the theoretically optimized length of the M-PT
units implies that they are considerably tilted (30−40°) with
respect to the surface normal. Note that the individual pyridine
rings comprising the terpyridine moieties of a M-PT unit are
not coplanar but twisted with respect to one another.
X-ray Photoelectron Spectroscopy. Selected XP spectra,
following the successive fabrication of the representative Fe-
PT/Cu/Ru-PT/Cu/Os-PT triad layer, are presented in Figure
1; additional XPS data for Si 2p binding energy (BE) region can
be found in the Supporting Information (Figure S5). For CL, a
weak C 1s emission at a BE of ∼284.6 eV and I 4d doublet at a
BE of 51 eV (I 4d5/2) are observed, in agreement with the
expectations. For the Fe-PT template layer, characteristic Fe 2p
doublet at BEs of ∼711 eV (Fe 2p3/2) and 724.8 eV (Fe 2p1/2)
as well as Fe 3p doublet at a BE of ∼56 eV (Fe 3p3/2) are
perceptible, evidencing the coordination of Fe-PT to CL
(Figure 1a). For the dyad assembly, additional signals of Cu
(Cu 2p3/2 and Cu 2p1/2 at BEs of 932.4 and 952.2 eV,
respectively, Figure 1b) and Ru (Ru 3d5/2 at a BE of ∼281.0 eV,
Figure 1c) can be traced, manifesting the coordination of Ru-
PT to the Cu2+
linker. Accordingly, the C 1s intensity increases,
and the Fe-related signals get attenuated to some extent as
compared to the Fe-PT spectra. Finally, for the triad assembly,
a characteristic signal of Os is exhibited (Os 4f7/2 and Os 4f5/2
at BEs of 51.4 and 54.2 eV, respectively), indicating the
coordination of Os-PT (Figure 1d). Simultaneously, there is a
further progressive increase of the C 1s signal and a decrease of
the Fe-related signals as well as an increase of the Cu signal
(second Cu2+
linker) and decrease of the Ru 3d5/2 signal. In
general, the XP spectra in Figure 1 suggest successive
coordination of the Fe-PT, Ru-PT, and Os-PT units over the
Cu2+
linkers, in accordance with the expected architecture of
the Fe-PT/Cu/Ru-PT/Cu/Os-PT assembly.
Near-Edge X-Ray Absorption Fine Structure Spectroscopy.
The formation of the molecular triads on the SiOx substrates
was additionally monitored by NEXAFS spectroscopy. The
respective spectra of the Fe-PT/Cu/Ru-PT/Cu/Os-PT triad,
acquired at the C and N K-edges, are presented in Figure 2a,b,
respectively. These spectra were collected at so-called magic
angle of X-ray incidence (55°), to avoid any effects related to
molecular orientation.36
The C K-edge spectrum of CL exhibits
a weak, pre-edge π* resonance at ∼285.2 eV (0), along with a
comparable strong absorption edge and several low intense, π*-
like resonances at higher photon energies. The N K-edge
spectrum of CL shows no nitrogen signal as can be expected
Figure 1. Fe 2p (a), Cu 2p (b), C 1s/Ru 3d (c), and Os 4f (d) XP spectra of the coupling (CL), template (Fe-PT), dyad (Fe-PT/Cu/Ru-PT), and
triad layers for the Fe-PT/Cu/Ru-PT/Cu/Os-PT assembly on Si(100). The characteristic emissions are marked. All assignments are performed in
accordance with ref 35.
Figure 2. C (a) and N (b) K-edge NEXAFS spectra of the coupling
(CL), template (Fe-PT), template-Cu, dyad, dyad-Cu, and triad layers
for the Fe-PT/Cu/Ru-PT/Cu/Os-PT assembly on Si(100). The
spectra were acquired at an X-ray incidence angle of 55° (magic
angle). Some of the characteristic resonances are marked.
DOI: 10.1021/acsami.5b00953
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from the molecular composition (Scheme 1). In contrast, the
analogous spectra of all subsequent layers exhibit a strong π*
resonance at 399.6−399.8 eV (1), characteristic of the
pyridine37
and terpyridine38,39
moieties in the Fe-PT, dyad,
and triad assemblies. Interestingly, this resonance is accom-
panied by another one, at ∼405.7 eV (2), in the case of the Cu-
terminated layers. We believe that the latter feature
corresponds to the residuals of the NO3 groups still attached
to the terminal Cu2+
ions.40
The characteristic absorption
structure of pyridine37
and terpyridine moieties38
is also
observed in the C K-edge spectra, for all successive layers,
coordinated to CL. The major feature is a double, pre-edge π*-
like resonance at ∼284.9 and ∼285.7 eV (1), with these values
varying slightly through the series. Such a double peak is related
to the different C 1s core binding energies of the carbon atoms
in the different positions within the pyridine ring.41
Along with
the magic-angle NEXAFS spectra, those at X-ray incidence
angles of 20° and 90° were acquired to look for so-called linear
dichroism effects, namely, the dependence of the absorption
resonance intensity on the incidence angle of the linearly
polarized light.36
Such a dependence is usually observed for
highly oriented molecular ensembles as far as the characteristic,
average tilt angle of involved molecular orbitals differs
noticeably from the magic angle of 54.7°.36
In the given case,
for all layers within the Fe-PT/Cu/Ru-PT/Cu/Os-PT
assembly, no linear dichroism was observed (Supporting
Information, Figures S6 and S7). On one hand, this can
mean a lack of the orientational order, but, on the other hand, it
may correspond to an average tilt angle of ∼35° for the
pyridine rings (corresponding to ∼54.7° for the respective π*
orbitals). The latter explanation seems to be more realistic, in
view of the ellipsometry data (see above).
Optical Properties. UV−vis spectra of several representative
triads as well as the spectra of the corresponding template
(monolayer) and dyad layers are presented in Figure 3, along
with the “baseline” spectrum of the glass substrate. The spectra
of the Fe-PT, Ru-PT, and Os-PT monolayers, serving as the
references for the individual building blocks of the triads, are
given in Figure 3a,c,d, respectively. These spectra exhibit
characteristic metal-to-ligand charge-transfer (MLCT; dπ(M =
Fe, Ru, Os)-π*(4′-pytpy) process) bands at 586 nm (Fe-PT)
and 505 nm (Ru-PT) as well as at 505 and 694 nm (Os-PT).
For all four M-PT combinations presented in Figure 3, the
spectra of the individual building blocks superimpose on one
another, first upon the assembly of the dyads, and then upon
the formation of the triads. The relative intensities of the M-
PT-related MLCTs bands differ somehow for different
arrangements, but all these bands are clearly present in the
overall spectra of the triads, covering the entire spectral range
from 400 to 800 nm. The presence of such bands, associated
with the specific M-PT units, gives broad possibilities for the
design of versatile sensors and related logic gates as will be
discussed below. The analysis of the UV−vis data for a
representative Fe-PT/Cu/Ru-PT/Cu/Os-PT assembly gives
information about the surface coverage and the efficiency of
coordination in the triad layers. The surface coverage of the Fe-
PT template layer on glass, Γ, was estimated at 6.6 × 1013
metalloligands/cm2
(i.e., ∼1.51 nm2
/metalloligand), in good
agreement with our previous reports;26,27,42
this value was
calculated using the equation Γ = (NAAλ)/2ελ, where NA is the
Avogadro’s constant and Aλ and ελ are, respectively, the
absorbance and the isotropic molar extinction coefficient at
wavelength λ. Note that the surface coverage observed was
below an ultimate limit of ∼1 × 1014
metalloligands/cm2
(i.e.,
∼1 nm2
/metalloligand) given by the cross section of the M-PT
moieties, so that the interaction between individual triads was
limited, and an even closer packing was, in principle, possible.
The surface coverage of the dyad and triad layers was estimated
at 6.01 × 1013
metalloligands/cm2
(1.66 nm2
/metalloligands)
and 5.3 × 1013
metalloligands/cm2
(1.88 nm2
/metalloligands),
respectively. These values give the yield of the dyad and triad
assemblies of ∼91% and ∼80%, respectively, with respect to the
monolayer template. Analogous values were observed for other
triads, suggesting a rather high efficiency of the coordination
reactions involved.
Additional information can be obtained by a detailed analysis
of the UV−vis spectra. An important observation is red shifts of
the MLCT bands associated with the individual building blocks
(M-PT) as compared to the solution (acetonitrile) spectra (see
Figure S8 in the Supporting Information). These shifts are
most likely related to quaternization of the pyridine group and/
or cofacial orientation of the molecules within the purview of
exciton theory.25,43,44
They amount to 17−19 nm in the case of
the representative Fe-PT/Cu/Ru-PT/Cu/Os-PT film and are
accompanied by additional shifts of the MLCT bands in the
course of the coordination steps (Figures 3a). In particular, for
the Fe-PT/Cu/Os-PT/Cu/Ru-PT assembly (Figure 3b), the
position of the Fe-PT MLCT band changed from 586 nm for
the template layer to 597 and 590 nm for the dyad and triad
layers, respectively. This phenomenon is especially important
since it suggests an efficient intramolecular electronic
communication between the individual M-PT units within the
triads. Since Ru-PT is kinetically more inert and has a larger
highest occupied molecular orbital−lowest unoccupied molec-
ular orbital gap, it tends to withdraw the electron density from
Fe-PT and Os-PT, which should lead to a blue shift of the
MLCT band, exactly as observed in the UV−vis spectra in
Figure 3.
Further important parameters of the metallo-organic
assemblies are full width at half-maximum (FWHM) of the
Figure 3. UV−vis spectra of Fe-PT/Cu/Ru-PT/Cu/Os-PT (a), Fe-
PT/Cu/Os-PT/Cu/Ru-PT (b), Ru-PT/Cu/Os-PT/Cu/Fe-PT (c),
and Os-PT/Cu/Ru-PT/Cu/Fe-PT (d) layers on glass substrates.
Black, red, blue, and purple solid lines represent the spectra of the glass
substrate (“baseline”) and the template, dyad, and triad layers,
respectively. The characteristic MLTC bands are marked.
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MLCT bands and optical band gap (Eg). For the Fe-PT/Cu/
Ru-PT/Cu/Os-PT case, FWHM of the MLCT band of the Fe-
PT template layer was estimated at ∼58 nm, which is larger by
∼7 nm than the analogous value in acetonitrile solution. A
similar band broadening was observed for the Ru-PT and Os-
PT moieties. This effect can be tentatively explained by
intermolecular interactions in the densely packed films.44
Note
that FWHM of the MLCT band increased even further in the
dyad and triad layers, which can be considered as a further
evidence of the intramolecular communication between the
individual M-PT units within the triads.
Optical band gap (Eg) for the template (Fe-PT), dyad, and
triad layers on glass substrate was evaluated by extrapolating the
MLCT band and estimated at 1.84, 1.79, and 1.61 eV,
respectively, which is substantially lower than the values
obtained in acetonitrile.26
A progressive narrowing of the
optical band gap shows a potential of multi-M-PT assemblies
for design of metallo-organic materials.
Electrochemical Properties. Electrochemical properties of
the template, dyad, and triad layers on ITO-coated glass
substrates were investigated using cyclic voltammetry (CV,
Figure 4). The active surface area of the electrodes was kept at
∼2.1 cm2
. The template layers showed single-electron oxidation
and reduction processes at +1.18/1.14 (Fe-PT), +1.46/1.42
(Ru-PT), and +1.03/0.99 V (Os-PT) (vs Ag/AgCl),
respectively (see Figure S9 in the Supporting Information for
the solution data). Accordingly, the half-wave redox potentials
E1/2 for the Fe-PT, Ru-PT, and Os-PT template layers were
estimated at +1.16, +1.44, and +1.01 V, respectively. A linear
behavior (R2
= 0.98−0.99) of the peak current densities
(cathodic and anodic currents) as functions of the scan rate
shown in Figure 5 unequivocally suggests that the electro-
chemical process in the M-PT template layers is diffusionless.
In addition, the template layers showed excellent thermal
stability (up to 200 °C) and high redox stability (>1 × 103
“read”−“write” cycles; see Figures S10 and S11 in the
Supporting Information).
Significantly, there are shifts of the redox potentials upon the
triad assembly, which can be clearly seen by the example of the
Ru-PT/Cu/Fe-PT/Cu/Os-PT heterotriad. For the respective
Ru-PT/Cu/Fe-PT dyad layer, an oxidation peak at +1.53 V
assigned to the Ru2+/3+
center was anodic shifted by ∼70 mV as
compared to that in the template layer (Figure 4a). Upon the
successive assembly of Os-PT (triad), the oxidation potential
decreases further to +1.48 V. At the same time, a merging of
oxidation and reduction peaks associated with the Fe/Os2+/3+
redox processes was observed at +1.08/1.0 V, exhibiting a
cathodic shift of 100 mV in the triad layer (Ru-PT/Cu/Fe-PT/
Cu/Os-PT) as compared to the dyad layer (Ru-PT/Cu/Fe-
PT) (Figure 4a). These observations indicate the presence of
electronic communication between the metalloligands in the
triad assembly. Significantly, a similar conclusion was derived
on the basis of the UV−vis spectra where the MLCT band of
the Fe-PT showed a blue shift of 7 nm upon the coordination
Figure 4. CVs of Ru-PT, Ru-PT/Cu/Fe-PT, and Ru-PT/Cu/Fe-PT/Cu/Os-PT layers (a), Os-PT, Os-PT/Cu/Fe-PT, and Os-PT/Cu/Fe-PT/Cu/
Ru-PT layers (b), and Os-PT, Os-PT/Cu/Ru-PT, and Os-PT/Cu/Ru-PT/Cu/Fe-PT layers (c) on ITO-coated glass substrates. Red, blue, and
purple solid lines represent the cyclic voltammograms of the corresponding template, dyad, and triad layers, respectively. The cyclic voltammograms
were recorded at 200 mV s−1
. The characteristic redox processes (M2+/3+
, where M = Fe, Ru, and Os) are marked.
Figure 5. Linear behavior (R2
= 0.98−0.99) of the peak current densities (cathodic and anodic currents) as functions of the scan rates (ν) for the Fe-
PT (a), Ru-PT (b), and Os-PT (c) template layers on ITO-coated glass substrates.
DOI: 10.1021/acsami.5b00953
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of Ru-PT to the Fe-PT/Cu/Os-PT dyad. Further, relatively low
values (∼35−50 mV) in the peak-to-peak separation ΔEP,
between the anodic and cathodic processes, reveal a confine-
ment of the metalloligands on the conductive substrates.
Similar observations were made with the other heterotriads.
For example, the Os-PT/Cu/Fe-PT/Cu/Ru-PT layer exhibited
single-electron redox process at +1.43 V (Ru2+/3+
) and a
merging of oxidation/reduction features for Fe/Os2+/3+
at
+1.08 and +1.04 V as shown in Figure 4b. The oxidation feature
was anodic shifted by 70 mV as compared to the Os-PT
template. The E1/2 value for the Ru-center in the heterotriad
was estimated at +1.38 V, which corresponds to a noticeable
cathodic shift (by ∼60 mV) as compared to the Ru-PT layer
(vide supra). The decrease in the E1/2 value is an indication for
the transfer of electron density from the Fe-PT and Os-PT
units toward the Ru-center of the Ru-PT moiety in the triad
assembly. Also the Os-PT/Cu/Ru-PT/Cu/Fe-PT heterotriad
showed similar trend in its redox behavior (Figure 4c). In
particular, FWHM values of the oxidation peak, ΔEpa,1/2, in the
template, dyad, and triad layers were estimated in the range of
+120−210 mV, which deviate from an ideal Nernstian
electrochemical relation (ΔEP,1/2 = 90/n mV).45
This anomaly
could be related to the interaction between the redox sites and/
or their heterogeneity.42,46,47
In addition to the redox peaks
discussed above, there are also some features at lower potentials
(+0.5 to +0.6 V) that are most likely related to the formation of
a mixed-valence redox species.26
Interestingly, these features are
not prominent for the dyad layers but well-perceptible for the
triad assemblies.
Significantly, the distinct redox states (oxd/red) in the
heterometallic triads can be considered as equivalent to the
change of a bit of information (either “0” or “1”). Considering
both M-PT related and the mixed-valence redox processes, four
well-defined redox states can be achieved which are 000, 100,
110 (two-oxidized) and 111. The 000 memory setting is
associated with the nonoxidized state at a relatively lower
potential (less than 0.3 V); the 100 setting indicates one
oxidation state at +0.5 to +0.6 V; 110 corresponds to two
oxidation states at 1.2 V; and 111 is akin to three oxidized states
at relatively high potential (more than 1.42 V). Interestingly,
the oxidized redox states can be reconfigured in the original
states (M2+
) by applying the corresponding reduction potential.
The charge retention time of individual systems can be
modified by introducing a barrier between the metalloligand
and the conductive substrates.48
Note that after recording the voltammograms, the electrolyte
solution was cross-checked by UV−vis and CV measurements,
but no optical or redox signal related to the released
metalloligands was observed. These experiments clearly
indicate that there was no desorption of the M-PT units
during the CV experiments. In addition, the triad systems
showed good electrochemical stability under electrochemical
stress of +2 V, underlining a high quality of these assemblies.
Interaction of Molecular Triad with NO+
. The hetero-
metallic molecular triads were utilized for detection of small,
redox-active molecules. For instance, the redox-active NOBF4
induced selective oxidation of the metal centers (Fe/Os) in the
Os-PT/Cu/Ru-PT/Cu/Fe-PT layers on glass substrate, which
could be monitored by UV−vis spectroscopy in the visible
range. The triad film was exposed to a dry acetonitrile solution
containing 100 ppm of NOBF4. Subsequently, changes in the
UV−vis spectra were monitored as a function of time (Figure
6), tracing the effect of the NO+
ions forming upon the
dissolving of NOBF4. When the triad layer was immersed in the
NOBF4 solution for 30 s, the MLCT band at λmax = 577 nm
associated with the Fe-PT unit exhibited a bathochromic shift
by 19 nm, along with a simultaneous decrease in absorbance
(by ∼60%) as depicted in Figure 6a. These changes were
attributed to quaternization of the free pyridyl group by NO+
followed by oxidation of Fe2+
to Fe3+
.43
Additionally, the joint
1
MLCT transition at λmax = 508 nm was blue-shifted by 7 nm,
accompanied by a significant decrease in absorbance. Full
oxidation of the metal centers, M2+
to M3+
(M = Fe, Os) was
observed within 10 min as corroborated by diminishing MLCT
bands of Fe-PT and Os-PT (Figure 6). Interestingly, the Fe-
center got oxidized faster than the Os-center, even though the
former one has a higher oxidation potential than that of the
latter one. This observation can be tentatively explained by the
fact that the Fe center experienced a stronger effect of NO+
than the Os center, since the Fe-PT moiety constitutes the
topmost part of the triad layer. Notably, the Ru-center in the
triad layer did not get oxidized, as confirmed by the presence of
the MLCT band at 501 nm even after the NOBF4 exposure for
10 min. This can be expected since the oxidation potential of
the Ru center is higher than that of the Fe/Os centers (vide
supra). Probably, there is a transfer of electron density from the
Fe/Os centers toward the Ru center in the triad assembly. Note
that the time required for the oxidation was found somewhat
shorter than that for the individual M-PT (M = Fe, Os)
template layer(s). Importantly, the original spectrum was
recovered after the addition of H2O and Et3N to the solution,
as the former one reduces the metal centers (Fe/Os3+
), while
the latter one dequaternizes NO+
from the pyridyl group (see
Figure 6. (a) Ex situ UV−vis spectra of Os-PT/Cu/Ru-PT/Cu/Fe-PT triad layer before and after exposure to 100 ppm of NOBF4 (release of NO+
)
in dry acetonitrile for a certain time (marked at the curves) and (b) absorbance of the most prominent MLCT bands (λmax at 508, 577, and 690 nm)
as a function of the exposure time.
DOI: 10.1021/acsami.5b00953
8682

Supporting Information, Figure S12). On the basis of the above
experimental results, a plausible mechanism of the relevant
processes was proposed, as shown in Figure 7. Note that this
mechanism is additionally supported by the experiments with
the Fe-PT solution (in acetonitrile) as described in our
previous report.43
Development of Molecular Logic Gates. In general,
molecular assemblies that are capable of performing phys-
icochemical changes as reaction to external stimulus can serve
as switches or Boolean logic at the molecular level.13,49−54
A
particular useful building block for information processing and
computing devices at the nanoscale is a molecular logic gate.
Such a gate can be constructed on the basis of the heterotriads
such as Os-PT/Cu/Ru-PT/Cu/Fe-PT. As shown in previous
section, exposure of the triad layer to NOBF4 resulted in the
oxidation of the Os2+
centers in the assembly, accompanied by
diminishing the 3
MLCT band at 690 nm and blue shift of the
1
MLCT band from 508 to 501 nm. Upon addition of NO+
, the
MLCT band at 577 nm shows a red shift before the Fe2+
centers gets oxidized. The initial spectrum can, however, be
restored by addition of 5 μL of deionized H2O, corresponding
to the water-induced reduction of Fe3+
/Os3+
to Fe2+
/Os2+
state,
which is a well-established phenomenon.43,55
The respective
reversible changes of the optical properties can be used to build
logic gates at the molecular scale. To develop such devices, the
chemical inputs (NO+
, H2O, Et3N) and optical outputs
(absorbance at 577 nm and MLCT at 501 nm) were coded
with Boolean logic functions, which are 0 for the OFF state and
1 for the ON state.49
Chemical information processing with
NO+
, H2O, and Et3N inputs were considered as Input1, Input2,
and Input3, respectively, while absorbance at 577 nm was
considered as Output1. The present system conﬁgures a
combinatorial logic gate (Figure 8).
While monitoring the oxidation of Os2+
center in the
assembly, time is considered as one of the inputs, since the
metal center gets oxidize slowly upon NO+
stimuli. Assuming
Figure 7. Most plausible mechanism of oxidation/reduction and quaternization/dequaternization of Os-PT/Cu/Ru-PT/Cu/Fe-PT on glass
substrates; neutral form of triad (a), quaternized form of triad upon reaction with NO+
(b), and quaternized−oxidized form of the metal centers
(Os3+
, Fe3+
) in the triad (c). The reversibility can be achieved using H2O and Et3N as shown in Figure 12 in the Supporting Information.
Figure 8. Truth table (left) and schematic (right) for a Boolean logic
gate on the basis of the Os-PT/Cu/Ru-PT/Cu/Fe-PT heterotriads.
The gate has NO+
, H2O, and Et3N as inputs, namely, In1, In2, and In3,
respectively, while absorbance at 577 nm serves as output (out1).
DOI: 10.1021/acsami.5b00953
8683

NO+
, time (min), H2O, and Et3N as In1, In2, In3, and In4,
respectively, while monitoring the MLCT band at 501 nm as
output2 (Out2), the present chemical process allows us to
design an even more complex logic gate at the molecular scale.
The truth table and four input-based logic circuit diagram (a
combination of AND−NOR−AND gates) for this gate are
presented in Figure 9. Remarkably, the output A501 nm can be
readily RESET to A508 nm using an input (In3), while it can be
further SET using In1. Thus, the reversible switching behavior
(SET/RESET) can be implemented as feedback loop56
demonstrating a write−read−erase−read cycle (right panel).
Chemically driven four-input combinatorial logic gate using a
solid support is rare.
An important point is that we used only two of the three
available metal centers to design the logic gates described
above, based on their specific reaction to the chemical inputs.
Potentially, even more complex logic elements can be
developed as far as all metal centers of a heterotriad assembly
will be utilized. The chemical inputs can be provided by a
microfluidic system, allowing a miniaturization of the entire
device.
■ CONCLUSIONS
Using Fe, Ru, and Os terpyridyl complexes as metalloligands
and the Cu2+
ions as the metallolinkers, surface-confined
heterometallic molecular triads were fabricated on glass, Si
(100), and ITO-coated glass substrates by stepwise coordina-
tion reactions. Formation of SURHMTs was monitored and
confirmed by CA goniometry, AFM, spectroscopic ellipsom-
etry, XPS, and NEXAFS spectroscopy. Optical and electro-
chemical properties of the SURHMT layers were studied in
detail and compared to the behavior of the respective template
and dyad layers. These studies reveal efficient electronic
intramolecular communication in the SURHMT assemblies.
The UV−vis spectra of the triads exhibit a superposition of the
MLCT bands of individual complexes, providing a significant
extension of the optical window to entire visible region (400−
800 nm). Similarly, CVs of SURHMT layers show a
superposition of redox peaks corresponding to individual
complexes as well as distinct multiredox states at a low
potential.
The presence of dedicated MLCT bands and redox states
associated with the individual metal centers provides a basis for
different applications, including sensor fabrication, molecular
memory, and molecular logic. Along these lines, significant
changes in the optical spectrum of a representative SURHMT
layer upon the exposure to external chemical stimuli were
demonstrated, complemented by the restoration of the original
spectrum upon the effect of another stimulus. This behavior is
not only useful for an optical sensor but can also serve as a basis
for the design of complex molecular logic gates, as was
discussed by the representative examples.
Generally, supported heterometallic molecular assemblies
represent a versatile bottom-up strategy allowing design and
fabrication of application-specific systems for frontier areas of
nanotechnology. The SURHMT layers of the given study are
only a representative example in this context. Different
combinations of metalloligands and metallolinkers are possible,
depending on the requirements of a specific application.
■ ASSOCIATED CONTENT
*S Supporting Information
Synthetic detail and characterization data of the 4′-pyridyl
terpyridyl and the Fe-PT, Ru-PT, and Os-PT complexes; the
design and fabrication of SURHMTs; additional XPS and
NEXAFS data; UV−vis and CV solution data for the Fe-PT,
Ru-PT, and Os-PT complexes; 1
H NMR spectra; discussion of
activation of substrates, formation of coupling and template
layers, fabrication of heteromolecular triads, materials and
methods, thermal and electrochemical stability of template
layers; illustration of water contact angles; AFM topography
images; plot of roughness versus deposition; reversibility test.
This material is available free of charge via the Internet at
http://pubs.acs.org.
■ AUTHOR INFORMATION
Corresponding Authors
*E-mail: mondalpc@gmail.com. Phone: +972 54 797 8617.
(P.C.M)
*E-mail: Michael.Zharnikov@urz.uni-heidelberg.de. Phone:
+49 6221 544921. (M.Z)
Present Addresses
⊥
Department of Chemical Physics, Weizmann Institute of
Science, Rehovot 76100, Israel.
#
Department of Physics, Bharathiar University, Coimbatore
641046, India.
Notes
The authors declare no competing financial interest.
■ ACKNOWLEDGMENTS
This research was financially supported by the Dept. of Science
and Technology (SR/NM/NS-12/2010), Univ. of Delhi, New
Delhi, India, and the German Research Society (ZH 63/14-2).
P.C.M thanks CSIR, New Delhi, for senior research fellowship.
■ DEDICATION
A tribute to late Prof. T. Gupta, Dept. of Chemistry, Univ. of
Delhi, New Delhi, India.
Figure 9. (left) Truth table for the heterotriad based logic gate with all
possible variations according to 2n
rule, where n = 4. (right, upper)
Combinatorial logic circuits constructed using NO+
, time (min), H2O,
and Et3N as In1, In2, In3, and In4, while MLCT band at 501 nm as
output2 (Out2), respectively. (right, lower) Feedback loop represent-
ing a write−read−erase−read cycle upon chemical input on triad
assembly.
DOI: 10.1021/acsami.5b00953
8684

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