Rectification in nanoscale devices based on an asymmetric five-coordinate iron(III) phenolate complex

Article abstract of DOI:10.1002/anie.201306765

One-way street: An asymmetric iron(III) phenolate complex with amphiphilic and redox properties was used in the formation of Langmuir-Blodgett (LB) films. These were used as a basis for nanoscale Au|LB film|Au devices that are capable of current rectifica

Full text of DOI:10.1002/anie.201306765

.
Angewandte
Communications
DOI: 10.1002/anie.201306765
Molecular Rectification
Rectification in Nanoscale Devices Based on an Asymmetric
Five-Coordinate Iron(III) Phenolate Complex**
Lanka D. Wickramasinghe, Meeghage Madusanka Perera, Li Li, Guangzhao Mao,
Zhixian Zhou, and Clꢀudio N. Verani*
Rectification consists of an asymmetric flow of electric
current. In a macroscopic electrical circuitry, rectifiers, such
as vacuum tubes or solid-state diodes, control the mobility of
current, enabling it to flow in one direction and preventing
reversibility. This directionality is fundamental to the con-
version of alternating into direct current. Molecular rectifi-
cation, which was proposed in the celebrated Aviram–Ratner
the formation of high-quality Langmuir–Blodgett (LB)
films.
[7]
Although it has been shown that self-assembled mono-
layers of polypyridine–cobalt(II) complexes in octahedral
[
8]
environments can act as single-electron transistors and
induce increased resistance (Coulomb blockade) at cryogenic
temperatures, the incorporation of transition-metal com-
plexes into electrode j molecule junctions has generally
employed symmetric molecules and has been rather slow in
development. Examples involve assemblies based on metal-
[
1]
ansatz, anticipates the feasibility of a current flow in one
direction that takes place in an electrode j molecule j elec-
trode junction. Central to this paradigm is the existence of
asymmetric molecules that incorporate electron-donor and
electron-acceptor moieties, [DA], with an excited state
[
9]
[10]
loporphyrins, terpyridine–ruthenium(II) complexes,
as
[11]
[12]
well as trivalent cobalt
and rhodium
azo-containing
+
ꢀ
[2]
[
D A ] of higher, but accessible, energy. Usually, donor
species in octahedral environments that are capable of the
symmetric conductance that is relevant for memory-switching
devices. An example of rectification based on an octahedral
bipyridine/acac–ruthenium(II) system has been reported
and acceptor are separated by a s- or p-bridge to decrease
[3]
electronic coupling, and if the requirements are fulfilled,
rectification occurs with contributions from Schottky, asym-
metric, and/or unimolecular mechanisms. Schottky rectifica-
tion is based on interfacial dipoles from electrode contact or
on covalent bonding between the molecule and the elec-
[
13]
(acac = acetylacetonate), but the effect of lowering global
symmetries around the metal center is yet to be tested.
Our group is engaged in an effort to integrate bioinspired
asymmetry principles into new molecular materials, with the
aim of developing redox-responsive metallosurfactants with
topologies that display unique structural, spectroscopic, and
surface patterning behavior. We recently reported on the
redox and electronic behavior of five-coordinate complexes
where the iron(III) ion is bound to low-symmetry, phenolate-
rich, [N O ] environments, and it was shown that geometric
[
4]
trode. Asymmetric and unimolecular mechanisms rely on
the use of frontier molecular orbitals of the molecule;
whereas the former relies on an asymmetric placement of
the HOMO or the LUMO in the electrode j molecule j elec-
trode assembly, the latter is based on small HOMO–LUMO
[
4a,5]
gaps that allow for through-molecule current flow.
Although experimental distinction between asymmetric and
unimolecular contributions can be ambiguous, there is con-
2
3
and electronic constraints determine the sequence by which
the metal and each of the phenolate substituents gets
[6]
sensus that electroactive molecules with local low symmetry
[
14]
III
1
constitute good candidates for this enterprise, and well-
documented cases of molecular rectification heavily rely on
oxidized.
Herein, a new iron(III) species [Fe L ] (1;
Scheme 1) is reported. This species shows marked local
asymmetries and is based on a newly synthesized amphiphilic
and redox-active [N O ] ligand. The complex takes advantage
2
3
of the presence of the phenylenediamino–metal and phenyl/
phenolate moieties that can act as electron-acceptors and
electron-donors, respectively. The viability of 1 as a precursor
[
*] L. D. Wickramasinghe, Prof. C. N. Verani
Department of Chemistry, Wayne State University
5101 Cass Ave, Detroit, MI 48202 (USA)
E-mail: cnverani@chem.wayne.edu
Homepage: http://chem.wayne.edu/veranigroup/
M. M. Perera, Prof. Z. Zhou
Department of Physics & Astronomy, Wayne State University
666 W. Hancock, Detroit, MI 48201 (USA)
L. Li, Prof. G. Mao
Department of Chemical Engineering & Materials Science
Wayne State University
5050 Antony Wayne Dr., Detroit, MI 48202 (USA)
[**] This work was funded by the National Science Foundation through
the grants CHE1012413 and CHE0718470 (C.N.V.), ECCS-1128297
(
(
Z.Z.), and CBET-0755654 and CBET-0619528 (G.M.). Dr. S. Shilov
Bruker) is acknowledged for comments on the IRRAS spectra.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201306755.
III
1
Scheme 1. The five-coordinate iron(III) complex [Fe L ].
1
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for LB films as well as its use in the fabrication of nanoscale
To build nanoscale devices based on LB films of 1, first the
rectifying devices are probed.
properties of its air–water interface have to be evaluated.
Therefore, isothermal compression associated with Brewster
angle microscopy (BAM) was used at 238C to assess the
average area per molecule, the collapse pressure of the
monolayers, and the film topology and homogeneity
1
The ligand H L was obtained by treatment of an
3
[18]
alkoxylated phenylenediamine, which was synthesized by
[
15]
a multistep method,
with 2,4-di-tert-butyl-6-(chlorome-
thyl)phenol in presence of triethylamine (Scheme 2). The
(Figure 1). A good amphiphilic distribution between the
1
Scheme 2. Multistep synthesis of H L : a) CH OCH CH Br, NaOH,
3
3
2
2
EtOH, reflux, 48 h; b) HOAc, HNO , CH Cl , RT, 72 h; c) Pd/C,
3
2
2
hydrazine, EtOH, reflux, 18 h; d) Et N, CH Cl , reflux, 72 h.
3
2
2
1
ligand H L was then treated with anhydrous FeCl3 in
3
presence of sodium methoxide in methanol under inert
conditions. Detailed descriptions of the synthetic procedures
Figure 1. Compression isotherm for complex 1, its first derivative
curve in inset), and selected BAM micrographs (right; see also the
Supporting Information, Figures S5 and S6).
III
1
are available in the Supporting Information. The [Fe L ] (1)
species was characterized by exact mass spectrometry (Sup-
porting Information, Figure S1), and IR and UV/Vis spectro-
scopic methods; the results are in excellent agreement with
the combustion-analysis results.
(
hydrophobic tert-butyl groups attached to the phenolate
moieties and the hydrophilic methoxyethane chains was
found for species 1. As a consequence, it starts to show
intermolecular interactions at the air–water interface at about
The limited information that was obtained by X-ray
analysis of the methoxy-substituted analogue (Figure S2)
supports the idea that 1 contains an iron(III) metal ion
surrounded by three phenolate oxygen atoms and two amine
nitrogen atoms of the ligand in a distorted trigonal-bipyr-
amidal geometry. This description is in excellent agreement
with experimental and calculated data obtained for species
2
ꢀ1
260 ꢀ molecule . Further compression leads to the forma-
tion of a homogeneous film at surface pressures of about 10–
ꢀ
1
20 mNm , as observed by BAM images. The formation of
Newton rings that are associated with film instability was
eventually observed. An apparent phase rearrangement was
[
16]
previously described by our group, where average FeꢀN
[17]
ꢀ1
and FeꢀO bond distances of 2.20 and 1.87 ꢀ were found.
Evidence from UV/Vis and IR spectroscopy suggests that
retains its amine character, rather than being converted into
observed between 25 and 35 mNm , which was indicated by
a change in the slope of the isotherm, as well as by the absence
of Newton rings. The microscopic homogeneity of the film
tracked by BAM seems to remain unaltered until ridge
1
i
the equivalent imine species 1 by ligand oxidation, as usually
observed.
[16a]
ꢀ1
This amine/imine conversion will only be
formation that is indicative of collapse appears at 63 mNm .
[
19]
observed at the air–water interface and in LB films.
According to the classic Ries mechanism, collapse involves
folding, bending, and breaking of the monolayer. The simplest
model for molecular arrangement at the air–water interface
suggests that the methoxyethane moiety is submerged in
water, whereas the hydrophobic iron–phenolate core remains
at the air subphase.
Complex 1 shows well-defined metal- and ligand-centered
redox processes in dichloromethane with TBAPF6 as the
supporting electrolyte (TBA = tetra-n-butylammonium; Fig-
ure S3 and Table S1). A cathodic process that appears at
+
approximately ꢀ1.49 V versus Fc /Fc (DE = 0.18 V, j I /I j
p
pa pc
III
II
=
0.81; Fc = ferrocene) is assigned to the Fe /Fe redox
Thus, the approximate area occupied by this core is
equivalent to a circle with a radius corresponding to the
distance between the outermost tert-butyl group of the ligand
and the metal ion. In known structures, this distance is
couple, whereas three consecutive phenolate/phenoxyl pro-
cesses are seen at 0.43 V (DE = 0.09 V), 0.69 V (DE =
p
p
+
0
.12 V), and 0.9 V versus Fc /Fc. The first two processes are
[
14,16b]
quasi-reversible, whereas the third is ill-defined. The elec-
tronic origin of these attributions has been investigated in
detail, along with the consequences of the lack of orbital
degeneracy, which is due to the low symmetry of these five-
approximately 7.9 ꢀ,
which results in an area of
2
approximately 200 ꢀ when optimal packing is assumed.
Thus, the critical areas should yield a tightly packed film with
optimized topology towards the end of the phase transition at
[14]
ꢀ1
coordinate species, to redox cycling.
35 mNm .
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LB films were deposited either as monolayers or multi-
layers onto glass substrates and studied by UV/Vis spectros-
shifting to lower wavenumbers, as associated with a highly
ordered and well-packed film. This relationship requires
[
20]
copy, IR reflection/absorption spectroscopy (IRRAS), and
by their contact angle, or they were deposited onto quartz and
mica and investigated by atomic force microscopy (AFM).
This characterization yielded pivotal understanding of the
chemical composition, orientation, and thickness of the
resulting films. The response of a single monolayer in UV/
Vis spectroscopy and IRRAS cannot be detected accurately,
and the best results were obtained for films with 25–50 layers
on glass. The general p–p* and ligand-to-metal charge
transfer (LMCT) features present in the solution spectrum
caution because the CH contribution is smaller than that
2
observed for the CH groups.
3
i
Each molecule of 1 contains 20 methyl groups (18
attached to the phenolates and two attached to the alkoxy
chain) and only 6CH groups (four in the alkoxy chains and
2
two in the methylaminophenolate moiety). Therefore, anal-
ysis of the film cannot be based on the intensity of the signals,
but must rather be based on the peak position. Indeed, the
ꢀ1
CH3 peaks shift from 2954 cm in the bulk sample to
ꢀ
1
2962 cm in the film (Figure S13). Considering the average
area at collapse reported above, this data corroborates the
notion of a well-packed film where the iron/phenolate moiety
points outwards. Although the intrinsically complex molec-
[
14,16a]
(
Figure S4 and Table S2) of 1 are clearly maintained,
but
III
obvious changes take place: the intensification of the N–Fe
LMCT band at 330 nm, the hypsochromic shift of the in- and
out-of-plane PhO –Fe band at 470 nm, and a new compo-
nent at approximately 400 nm (Figure S7). This new band is
a phenolate-to-azomethine charge transfer associated with
the amine/imine conversion of the ligand that takes place at
ꢀ
III
i
ular structure of 1 prevents us from proposing a tilt angle of
alignment, further information about the orientation of the
molecules can be gathered by the static contact angle of the
compound on glass, which was measured by a KSV CAM 200
goniometer at room temperature. Whereas a value of 7.458
was obtained for the glass substrate alone, the film yielded
a value of 85.308, which confirms its hydrophobic nature.
i
the air–water interface. The 1!1 conversion (Scheme S1)
was observed by UV/Vis spectroscopy (Figures S8 and S10)
and ESI mass spectrometry (Figures S9 and S11), and by
IRRAS on glass at an angle of 308 under s-polarized light. In
i
The morphology of the LB-monolayer of 1 deposited on
i
ꢀ1
ꢀ1
the IR spectrum of 1 , a new peak at 1583 cm that is
associated with the C=N group is seen (Figure 2b). As this
mica at surface pressures of 10, 25, 30, 33, and 40 mNm was
measured by AFM. At low surface pressures, the monolayer
shows large pinhole defects that become smaller and less
frequent as the surface pressure increases (Figure 3a,b;
ꢀ
1
Figure S14). At 33 mNm , the monolayer becomes smooth,
which is optimal for nanofabrication. At higher surface
pressures, the film becomes rougher owing to material
aggregation. In multilayer films, the surface roughness
increases with an increasing number of layers (Figure S15
and Table S3), and more particles tend to aggregate to the
surface of the film. The thickness of a single monolayer was
determined on quartz substrates containing one to fifteen
layers by blade-scratching the film and measuring the scratch
depth in the tapping mode (Figure 3c; Figure S16 and
Table S3). The resulting values yield a linear relationship
between the thickness and the number of layers, which
indicates homogeneous film deposition (Figure 3d). Each
layer is approximately 19 ꢁ 2 ꢀ thick and, based on available
crystallographic data for related complexes, the approximate
i
length of 1 reaches 15–17 ꢀ. With an estimated 12 ꢀ between
Figure 2. Comparison of the IR spectrum for 1 in KBr (a) and the
IRRAS spectra of a 50-layer 1 –LB film (b) and of moisture at 308 with
s-polarized light (c).
the catechol-like oxygen atoms and the bulky terminal tert-
i
[14,16]
butyl groups,
these results are in excellent agreement
with the notion of a true monolayer. Thus, taking into account
the shift in the pronounced CH peak observed by IRRAS,
3
the thickness of the monolayer determined by AFM, and the
hydrophobic nature of the film, it is possible to conclude that
the LB film of 1 is composed of well-packed molecules where
the alkoxy chains are in contact with the solid substrate and
the tert-butyl-rich iron/phenolate moiety points outwards at
the air–solid interface.
peak is absent in the bulk IR spectrum of 1 and is not
associated with adventitious moisture (Figure 2c), it further
validates the intraligand conversion. Other features include
the fingerprint region, where peaks that are due to aromatic
i
ꢀ
1
C=C stretchings (1610–1510 cm ) and angular -CH - and
2
ꢀ
1
[21]
-
CH deformations (1360–1470 cm ) appear (Figure S12).
Having gained valuable knowledge about the monolayers
at the air–water and air–solid interfaces, we focused on device
3
Relevant CH symmetric and antisymmetric stretching vibra-
2
ꢀ
1
i
tions were observed between 2850 to 2920 cm and a prom-
inent asymmetric CH feature appears at 2962 cm . In
previous studies,
fabrication. The Langmuir monolayer of 1 was transferred at
ꢀ
1
ꢀ1
33 mNm onto a precleaned gold-coated mica substrate to
3
[20,22]
a correlation was drawn between the
yield a defect-free film. After drying the monolayer for five
i
high intensity of the CH vibrations of alkyl chains and its
days under reduced pressure, an Au j 1 LB j Au device was
2
1
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i
Figure 4. I–V diagram for complex 1 (c) from 4 to ꢀ4 V.
These results further indicate that the rectifying behavior
of the current response is directly associated with the
i
presence of the LB film of 1 . Although the exact origin of
the rectifying behavior is not certain, unimolecular contribu-
tion can be assumed as viable, and a description of the
+
ꢀ
[4b,c,5b,6d,23]
possible [DA] and [D A ] states becomes relevant.
The redox behavior of 1 in dichloromethane indicates the
III
II
allowed oxidation states, as the Fe !Fe reduction was
III
IV
observed, but not the oxidation Fe !Fe oxidation. Con-
i
ꢀ
Figure 3. a,b) Surface morphology of the LB monolayers for 1 at
versely, the PhO !PhOC oxidation from phenolate to phe-
ꢀ1
ꢀ1
1
0 mNm (a) and at 33 mNm (b). c) 3D view of an AFM image of
noxyl was observed, whereas the formation of any reduced
anionic radical remains unseen for these ligand systems. Thus,
i
a 15-layer film. d) Layer thickness (nm) versus the number of layers; 1
2
(
&
). e) Optical micrograph of the device; image size=1240ꢀ930 mm .
ꢀ
III
it is viable to propose the involvement of Au j PhO ···Fe j Au
f) Device layout.
II
and Au j PhOC···Fe j Au states, where the LUMO of the
ansatz is metal-based, and the HOMO is ligand-centered.
Although further considerations will be necessary, DFT
built. The top gold electrode was deposited with an Effa-
Coater gold sputter by the shadow-masking method using
argon as the carrier. Three assemblies with an average of 16
devices each were fabricated, and the current–voltage (I–V)
characteristics were reproducibly measured in four to five
devices per assembly using a Keithley 4200 semiconductor
parameter analyzer and a Signatone S-1160 probe station at
ambient conditions. A few devices were short-circuited, which
[14]
calculations for similar systems
suggest that partially
occupied metal-based SOMOs might be energetically acces-
sible.
In conclusion, we have presented the synthesis and
thorough characterization of a new [N O ] ligand and its
2
3
pentacoordinate iron(III) complex. The balanced amphiphilic
i
nature of 1, along with imine conversion into 1 at the air–
[6d]
was likely to be due to monolayer defects.
An optical
water interface, was pivotal in optimizing the properties of
defect-free LB monolayers, as confirmed by spectroscopic
and microscopic methods. Furthermore, the asymmetric
nature of the complex seemed very useful in designing
suitable precursors with current-rectifying properties, as
indicated by the I–V characteristics of fabricated devices.
Asymmetric responses indicative of rectifying behavior were
micrograph of the fabricated devices and the device layout
are shown in Figure 3e and f. In each of these I–V measure-
ments, a higher current was observed in the third quadrant
than in the first quadrant. This asymmetric I–V that is
characteristic of a sharp negative response and a negligible
positive response is indicative of rectification behavior, as
shown in Figure 4.
i
observed for well-ordered LB monolayers of 1 that were
[
6d]
The rectification ratio RR (I at ꢀV /I at + V ) for the
sandwiched between two gold electrodes. Considering the
o
o
i
i
monolayer of 1 varies from 4.52 to 12 between ꢀ2 to + 2 V
and from 2.95 to 36.7 between ꢀ4 to + 4 V, respectively.
Reversing the drain and the source contacts led to a reversed
current response, thus demonstrating the retention of the
rectification behavior (Figure S17c). Upon repeating scans,
the current drops and the I–V behavior displays increased
symmetry (Figure S17e,f). This phenomenon has been
observed for organic materials and is tentatively explained
by the reorganization of dipole moments in the presence of
high electric fields to attain a stable monolayer by decreasing
molecular structure of 1 , along with the associated I–V
characteristics, it is suggested that the phenolate moiety and
the metal center play a role as electron-donors and electron-
acceptors in the generation of a directional flow of current.
This example of rectification based on an asymmetric
coordination complex paves the road towards the investiga-
tion of enhanced nanoscale rectifying devices in greater
detail. Our laboratories are currently engaged in the opti-
mization of molecular structures that contain new iron(III)
complexes with different donating and withdrawing substitu-
[
6d]
III
II
ꢀ
their energy.
ents to modulate the Fe !Fe and the PhO !PhOC redox
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couples, and with predominantly amphiphilic or hydrophobic
characters to yield defect-free LB films. The determination of
the extent by which redox potentials enhance rectification will
allow us to consider the magnitude of asymmetric versus
unimolecular contributions. Further results will be reported in
due course.
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