Heteroleptic copper switches

Article abstract of DOI:10.1021/ja103937v

Heteroleptic copper compounds have been designed and synthesized on solid supports. Chemical redox agents were used to change the oxidation state of the SiO₂-immobilized heteroleptic copper compounds from Cu(I) to Cu(II) and then back to Cu(I). Optical spectroscopy of a dimethyl sulfoxide suspension demonstrated the reversibility of the Cu(I)/Cu(II) SiO₂-immobilized compounds by monitoring the metal-to-ligand charge transfer peak at about 450 nm. Electron paramagnetic resonance spectroscopy was used to monitor the isomerization of Cu(I) tetrahedral to Cu(II) square planar. This conformational change corresponds to a 90° rotation of one ligand with respect to the other. Conductive atomic force microscopy and macroscopic gold electrodes were used to study the electrical properties of a p⁺ Si-immobilized heteroleptic copper compound where switching between the Cu(I)/Cu(II) states occurred at -0.8 and +2.3 V.

Full text of DOI:10.1021/ja103937v

Published on Web 10/22/2010
Heteroleptic Copper Switches
Sanaz Kabehie,†,‡ Mei Xue,‡,§ Adam Z. Stieg,‡,| Monty Liong,†,‡ Kang L. Wang,§,|
and Jeffrey I. Zink*,†,‡
Department of Chemistry and Biochemistry, California NanoSystems Institute, and DeVice Research
Laboratory, Department of Electrical Engineering, UniVersity of California, Los Angeles,
California 90095, United States, and WPI Center for Materials Nanoarchitectonics (MANA),
National Institute for Materials Science, Tsukuba, Japan
Received May 14, 2010; E-mail: zink@chem.ucla.edu
Abstract: Heteroleptic copper compounds have been designed and synthesized on solid supports. Chemical
redox agents were used to change the oxidation state of the SiO2-immobilized heteroleptic copper
compounds from Cu(I) to Cu(II) and then back to Cu(I). Optical spectroscopy of a dimethyl sulfoxide
suspension demonstrated the reversibility of the Cu(I)/Cu(II) SiO2-immobilized compounds by monitoring
the metal-to-ligand charge transfer peak at about 450 nm. Electron paramagnetic resonance spectroscopy
was used to monitor the isomerization of Cu(I) tetrahedral to Cu(II) square planar. This conformational
change corresponds to a 90° rotation of one ligand with respect to the other. Conductive atomic force
microscopy and macroscopic gold electrodes were used to study the electrical properties of a p+
Si-immobilized heteroleptic copper compound where switching between the Cu(I)/Cu(II) states occurred at
-0.8 and +2.3 V.
Introduction
transition metals offer useful features due to the accessibility
of multiple oxidation states (low power consumption). Changes
State variables represent a class of essential components in
the emerging field of nanoelectronic devices1-6 that serve as
the physical representations of information used in memory and
logic applications.7 Important properties include switching speed,
on/off ratio, maximum spatial density, limiting scaling factor,
and on/off modulation method. Boolean logic remains the most
commonly used information processing methodology in current
technology and is based on two discrete states, represented by
1 or 0. A critical first step toward the fabrication of functional
nanodevices capable of carrying out Boolean computation is
the realization of a physical unit within the device that resides
in two stable, distinguishable states. Subsequent integration of
this nanostructure into the process of Si-based fabrication
technology represents the crucial second step.
in transition metals include coordination number, color, and
geometry.8 Of specific interest here, metal complexes of
copper9-18 and nickel19 are known to exhibit oxidation-state-
dependent geometries. These distinct molecular conformations
have potential to be employed as a state variable where the
oxidation state of the metal center is used to drive conforma-
tional motion such as intramolecular rotation. For example, Cu(I)
bis-1,10-phenanthroline compounds are tetrahedral, while Cu(II)
analogues are square planar (Scheme 1). Similarly, the unique
carbons in Ni(III) bis-dicarbollide cages adopt a transoid
geometry with respect to one another, while Ni(IV) analogues
(8) Cotton, F. A. AdVanced Inorganic Chemistry, 6th ed.; Wiley: New
York, 1999.
Rotational conformational states of transition-metal-containing
compounds constitute state variables for which information
storage relies on physical changes of the molecule. In particular,
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Attenkofer, K.; Meyer, G. J.; Coppens, P. J. Am. Chem. Soc. 2003,
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† Department of Chemistry and Biochemistry, University of California,
Los Angeles.
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‡ California NanoSystems Institute, University of California, Los Angeles.
§ Department of Electrical Engineering, University of California, Los
Angeles.
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| WPI Center for Materials Nanoarchitectonics.
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J. AM. CHEM. SOC. 2010, 132, 15987–15996 15987

A R T I C L E S
Kabehie et al.
Scheme 1. Cu(I) (Tetrahedral) and Cu(II) (Square Planar)
Bis-1,10-phenanthroline
Scheme 2. Individual Components of the SiO2-Immobilized
Heteroleptic Copper Compounds
nanoscale logic and memory devices.25,26 This system is
particularly attractive due to its relatively low energetic activa-
tion barrier (-0.8 and +2.3 V) and potential for high spatial
density. Switching between discrete conformation states occurs
on the picosecond time scale,9,25 and the molecules can be
readily integrated into a silicon or silica solid-state platform.25,26
A modular synthetic approach (Scheme 2) enables versatility
in the design of size, shape, and function within the nanoar-
chitecture of these functionally engineered materials.
adopt a cisoid geometry.20 Such isomerizations can thus be used
as a state variable based on a redox-induced intra-rotational
conformational change. Integration of nickel bis-dicarbollides
onto a solid silicon support and solid-state switching of these
systems have yet to be reported. The most highly developed
nanoelectrical applications involve organic rotaxanes in a cross
bar architecture.21-24
Results and Discussion
Heteroleptic copper compounds in solution undergo fast
ligand exchange reactions between copper and bidentate
ligands.36,37 Although pure homoleptic copper compounds have
been isolated, pure heteroleptic copper compounds in the solid
state are less common.38 The first reported mixed-ligand copper
systems, HETPHEN (heteroleptic bis(phenanthroline))39,40 were
synthesized via thermodynamic control. An approach used in
the past employs bulky aryl substituents at the 2,9- and 4,7-
bis(imine) coordination sites, where steric bulk and electronic
effects selectively control the metal complexation equilibrium
through π-π stacking interactions.41 In the reported solution
studies, steric and electronic factors governing the selective
HETPHEN principles were observed by 1H NMR and ESI-MS
titrations.42 Although strategic for locked positions,40,42-52 these
Silicon-immobilized heteroleptic copper compounds provide
an attractive option as potential physical state variables due to
their discrete rotational mechanical movement and robust
integration with the Si surface. This one-electron redox-induced
conformational change is reversible and occurs via the +1 and
+2 oxidation states of copper, which isomerizes from tetrahedral
to square planar geometries, respectively (Scheme 1). The redox
potential of the copper switch constitutes the minimum energy
required for information storage i.e., one electron per molecule
at the voltage at which the redox event occurs. Because
information storage is based on the nanoscopic movement of
atomic nuclei about the copper center, the switching speed is
on the order of the relevant rotational vibrational mode of the
molecule.9,25 Molecular integration onto a solid support neces-
sitates a heteroleptic design (a metal center with two different
ligands) whereby one ligand serves to immobilize the compound
while the second ligand is able to move freely. In this way, a
high-density layer of molecular switches can be immobilized
onto a bottom electrode (i.e., p+-doped Si) and sandwiched by
a top electrode (i.e., macroscopic gold electrodes or a conductive
AFM (cAFM) tip) to achieve state variable functionality.25,26
In this work, the design and synthesis of a family of
Si-immobilized heteroleptic copper compounds is presented. The
systems shown undergo reversible, intramolecular conforma-
tional motion upon a one-electron oxidation or reduction and
represent a physical state variable toward applications in
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Heteroleptic Copper Switches
A R T I C L E S
× 106, K3 ) 105),54 bisphosphine ligands also form favorable
bonds to copper.55
Multiple linker and rotator components that use these types
of ligands were designed and synthesized to allow for versatility
in the fabrication of nanodevices (Scheme 2). For example, the
designs include a short (∼1 nm) rotator that can be used for
memory in intramolecular vertical conductance measurements25
and long (∼2 or 3 nm) rigid rotators with long π-conjugated
arms that can undergo large-amplitude motion and function as
molecular machines. Along similar lines, immobilized linkers
(Scheme 3) with an sp3 center (iS1) or a monosubstituted 1,10-
phenanthroline ligand (iS2) allow for flexibility of linker rotation
if and when a rotator is prohibited from motion due to the
deposition of a top gold electrode via shadow mask evaporation
of Si-immobilized heteroleptic copper compounds.26 In other
words, the relative 90° rotational motion of the linker and
rotator, with respect to each other, will cause an overall
isomerization of the copper compound. S1 is particularly
attractive due to its rigidity in standing up on the surface.
Additionally, its phosphorus-containing ligands facilitate char-
acterization via solid-state 31P NMR. S2 is useful because it is
not easily oxidized chemically when switching between the
Cu(I) and Cu(II) SiO2-immobilized systems. S3, a disubstituted
1,10-phenanthroline, has linkers in the 4,7 positions that allow
for a more stationary linker that will not rotate about an sp3
center (a characteristic that is desirable for intermolecular
horizontal conductance measurements but not as important for
intramolecular conductance measurements).
Figure 1. Schematic diagram of the copper switch components. (1a) iL is
a bidentate bifunctional linker (triangle) that is covalently immobilized on
a solid support, a copper ion is the metal center (circle) that is coordinated
to both the linker and the rotator, and L is the bidentate rigid rotator
(rectangle). Two states are shown, (1a, upper left) Cu(I) tetrahedral and
(1a, upper right) Cu(II) square planar. In the Cu(I) system, the ligands are
perpendicular to one another while in the Cu(II) system the ligands are
parallel to one another. (1b) The surface-outward sequential synthesis
involves: (1) the solid support, (2) the immobilization of the linker, (3) the
addition of the copper center, and (4) the addition of the rotator.
interactions prohibit the isomerization of Cu(I) tetrahedral to
Cu(II) square planar geometries. For the purpose of using
heteroleptic copper compounds as a state variable, at least one
ligand needs to be able to rotate freely.
The strategy to synthesize heteroleptic copper compounds
with nonhindered ligands that can have a large-amplitude
rotational conformational change involves a surface-outward
sequential synthesis.26 The design of heteroleptic copper
compounds presented here involves four subunits: (1) a support
to serve as a solid-state platform, (2) a bifunctional chelating
ligand that is covalently immobilized on the support and can
bind to a copper center through bidentate coordination interac-
tions (linker), (3) a Cu(I) or Cu(II) metal center, and (4) a
bidentate rigid ligand (rotator). This method begins with an
immobilized ligand (iL, linker) on a solid support, with the
subsequent addition of copper (Cu+), followed by the chelation
of a second ligand (L, rotator), shown by eq 1 and illustrated
by Figure 1.
1. Homoleptic Copper Compounds. Homoleptic copper com-
pounds were prepared as model compounds for the solid-state
heteroleptic analogues. The homoleptic copper compound,
copper bis-3,8-di(ethynyltrityl)-1,10-phenanthroline (Cu(RM)2+),
is easily synthesized (Figure S14a, Supporting Information).
Despite the bulky ethynyltrityl groups present on the 3,8
positions of RM, large-amplitude intra-rotational motion of the
bisRM ligands with respect to one another occurs upon oxidation
of Cu(I) to Cu(II). This is further supported by the lack of
+
+
iL + Cu+ + L f [CuiLL]+
(1)
luminescence in Cu(RM)2 and(Cu(RL)2 as compared to the
highly luminescent 2,9-disubstituted aryl groups in homoleptic
copper compounds. The luminescence in copper(I) compounds
is attributed to the lack of isomerization in Cu(I) to Cu(II)
compounds caused by steric bulk.12-14,42,56,57 In order for the
phenanthroline ligands to isomerize with the most efficient
conformational change, the long, rigid π-conjugated substituents
(e.g., present in ligands RM and RL) need to be on the 3,8 (not
the 2,9) positions of 1,10-phenanthroline.
Two classes of bidentate ligands were used as the linker (iL):
substituted 1,10-phenanthrolines and bisphosphines. Modified
1,10-phenanthroline ligands are an important class of chelating
agents53 and are particularly attractive due to their rigidity, high
affinity for metal centers, and ease of modification. Although
diimines have one of the highest binding affinities for a copper
metal center (e.g., 1,10-phenanthroline: K1 ) 7 × 108, K2 ) 4
The reduction potentials obtained via cyclic voltammetry for
2+/+
2+/+
(44) Halper, S. R.; Malachowski, M. R.; Delaney, H. M.; Cohen, S. M.
Inorg. Chem. 2004, 43, 1242–1249.
Cu(RM)2
and Cu(RL)2
compared to copper bis-1,10-
phenanthroline ((Cu(phen)2)2+/+) and copper bis-2,9-dimethyl-
(45) Listorti, A.; Accorsi, G.; Rio, Y.; Armaroli, N.; Moudam, O.; Gegout,
A.; Delavaux-Nicot, B.; Holler, M.; Nierengarten, J.-F. O. Inorg. Chem.
2008, 47, 6254–6261.
1,10-phenanthroline ((Cu(RS)2)2+/+) are shown in Table 1.34,57,58
2+/+
2+/+
The relatively high Cu(RM)2
and Cu(RL)2
couples
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compared to the Cu(phen))22+/+ couple can be attributed to the
additional electron-donating groups provided by the trityl
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A R T I C L E S
Kabehie et al.
Scheme 3. Surface-Outward Sequential Synthesis for Selected Heteroleptic Copper Compounds
Table 1. Redox Potentials for Cuprous Phenanthroline
evaporation or a Pt/Ir AFM tip) is used to complete the device
assembly.25,26
Derivativesa
compound
CuII/I (V)
Among the linkers that have been immobilized on a silica
support, iS127 (Scheme 3a) is attractive due to its short length
and phosphorus-containing ligands that have a high isotopic ratio
for NMR and hence allow for the easy detection of surface-
modified silica particles via solid-state NMR studies. Phosphorus-
31 solid-state NMR spectroscopy is a useful tool due to the
phosphorus atom’s great natural abundance (100%), large
chemical shift dispersion with 1/2 spin nucleus, and the improve-
ment in the quality of spectra with magic angle spinning (MAS)
and cross-polarization (CP) techniques.59 On the other hand,
iS228 (Scheme 3b) is attractive due to its robustness against
chemical redox agents and the fact that its absorption spectrum
does not interfere with the RM πfπ* region (Figures S12 and
S13, Supporting Information). Both iS1 and iS2 have flexibility
in rotating when placed in a vertical device geometry due to
the sp3 center in iS1 and the monosubstituted 1,10-phenanthro-
line with an sp3 center in iS2.25 iS3 was designed as a
disubstituted 1,10-phenanthroline linker that allows full 90°
rotation of the upper ligand, since this linker (the bottom ligand)
is rigidly immobilized at two points of the molecule. This
rigidity is desirable for horizontal conductance measurements
in logic applications where alignment is critical.
3. Divergent Synthesis. S1 was immobilized on silica particles
to yield the immobilized linker iS1 (Scheme 3a). The 13C CP/
MAS solid-state NMR spectrum for iS1 (Figure S4a, Supporting
Information) shows two types of carbon peaks associated with
iS1. The aromatic region is attributed to the phenyl groups bound
to the phosphine. Two peaks appear in the aliphatic region at
59.46 and 16.36 ppm and are attributed to unbound ethoxy60
groups from iS1, from the methylene and methyl carbon atoms
of the ethoxy group, respectively. The appearance of these two
residual peaks is known because trialkoxysilane reagents do not
exclusively form three siloxane bonds upon condensation
reaction with silica surface silanol groups: there always exists
a distribution of silane species bound by one, two, and three
siloxane bonds.60 The 31P CP/MAS solid-state NMR spectrum
for iS1 shows two peaks at 31.38 and -28.30 ppm. The
rotational spin of the sample was 10 kHz, and corresponding
+
CuI(phen)2
0.54
0.84
0.92
0.99
+
CuI(RM)2
+
CuI(RL)2+
CuI(RS)2
a Measured at room temperature in 0.1 M TBAH/CH2Cl2 vs SCE
with ferrocene as an internal standard.
acetylene and 4-ethynylbiphenyl groups from RM and RL,
resepectively.57
2. Heteroleptic Copper Compounds. A total of six com-
pounds (three linkers and three rotators), shown in Scheme 2,
can be used in a modular synthetic approach to synthesize nine
combinations of surface-immobilized heteroleptic Cu(I) com-
pounds (or 18 total compounds when Cu(II) analogues are
included). Each component allows for the versatility of device
fabrication and serves useful purposes in the flexibility of the
design. Different combinations of linkers and rotators form
assemblies with criterion for through-molecule conductance
(intramolecular) or horizontal conductance (intermolecular)
device geometries. In other words, through-molecule conduc-
tance refers to intramolecular conductance from the top ligand
through the metal center to the linker, while horizontal
conductance refers to conductance from the π-conjugated top
ligand to neighboring π-conjugated top ligands.
Silica nanoparticles were chosen as a model solid support
because (1) their surface properties mimic the native oxide layer
of silicon, (2) linkers can be covalently attached with high
surface area, and (3) their high dispersibility in dimethyl
sulfoxide is extremely practical for optical measurements using
UV-vis absorption spectroscopy. Solid-state NMR, electron
paramagnetic resonance (EPR), and UV-vis absorption studies
cannot be carried out using p+ Si wafers. The commercially
available Merck silica is useful for solid-state NMR and EPR
characterization, and silica nanoparticles dispersed in liquids
are favorable for UV-vis absorption spectroscopy characteriza-
tion. Silicon is also a favorable support for the linkers shown
in Scheme 2 because alkoxysilanes form strong anchors to the
native oxide layer on silicon. Additionally, p+-doped silicon
wafers are useful for conductance measurements when a top
electrode (macroscopic gold electrodes via shadow mask
(59) Blu¨mel, J. Inorg. Chem. 1994, 33, 5050–5056.
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A R T I C L E S
Figure 2. 31P CP/MAS solid-state NMR of iS1 and iS1Cu(I)RS.
spinning sidebands are observed. The peak at 31.38 ppm is from
the formation of a phosphine-oxide with the surface (Figure
S4b).59,61 This is confirmed by addition of hydrogen peroxide
to iS1, where it is observed that the peak at -28.30 ppm has
been oxidized (Figure S4c) to the peak at 31.38 ppm.
agents cannot be used to demonstrate the reversible two states
of the Cu(I) and Cu(II) analogues in the presence of iS1.
The rotators are relatively short (RS, ∼1 nm), medium (RM,
∼2 nm), and long (RL, ∼3 nm) in size. RS is commercially
available, while RM and RL were synthesized by the Sono-
gashira coupling of 3,8-dibromo-1,10-phenanthroline to trity-
lacetylene and 4-ethynylbiphenyl, respectively.
S2 was also immobilized on silica, and the 13C CP/MAS solid-
state NMR spectrum of iS2 (Scheme 3b) shows carbon atom
peaks in the carbonyl, aromatic, and aliphatic regions (Figure
S5a, Supporting Information). Similar to iS1, the 13C CP/MAS
solid-state NMR of iS2 contains residual alkoxide signals at
59.22 and 16.96 ppm from unbound ethoxy groups. In addition,
a methanol wash of iS2 results in a methoxide peak that appears
at 50.46 ppm. 29Si CP/MAS solid-state NMR confirms the
covalent bonding that forms between S2 and the silica particles
(Figure S5b). The peaks corresponding to the various siloxane
Qm and organosiloxane Tn species can be identified clearly.62
The Tn bands [Tn ) RSi(OSi)nOH3-n, n ) 1-3] are resonances
of the Si to which the linkers are bonded, whereas the Qm bands
[Qm ) Si(OSi)m(OH)4-m, m ) 2-4] correspond to bulk and
surface Si to which only oxygen atoms are bonded.
The immobilized ligand, iS3, was produced by the im-
mobilization of S3 on the solid silica support. The 13C CP/MAS
solid-state NMR spectrum of iS3 confirms the presence of
aromatic and aliphatic carbon atoms (Figure S6a, Supporting
Information) in addition to the residual ethoxide signals. The
29Si CP/MAS solid-state NMR spectrum shows the T and Q
regions of the silicon atoms. The T region shows the binding
of one, two, and three ethoxy groups to the solid silica support
(Figure S6b).
4. Properties of iS1Cu(I)RS. To illustrate the properties of
compounds prepared by divergent syntheses and described in
the previous section, iS1Cu(I)RS is an example of a compound
where the first linker and shortest rotator are examined. Addition
of Cu(I) and RS to iS1 yields the compound iS1Cu(I)RS. The
31P CP/MAS solid-state NMR spectrum for iS1Cu(I)RS shows
the shift of the iS1 31P CP/MAS solid-state NMR signal from
-28.30 to -12.15 ppm. This confirms the complexation of Cu(I)
to iS1 (Figure 2).55 The signal in the 13C CP/MAS solid-state
NMR spectrum of the aromatic carbon atoms from the phenan-
throline portion of RS overlaps with the aromatic carbon atom
signals from the phenyl groups on the phosphorus atoms in iS1.
However, the methyl carbons on the 2,9 positions of the
phenanthroline ligand of RS are distinctly observed in the 13C
CP/MAS solid-state NMR spectrum of iS1Cu(I)RS and appear
at 23.81 ppm. Although this signal is broad enough to interfere
with the methylene carbon atom in iS1, a comparison of the
13C CP/MAS spectra of iS1 and iS1Cu(I)RS shows the distinc-
tion when RS is added to the compound (Figure S9b, Supporting
Information).
Overall, although the number of scans of the iS1Cu(I)RS 13
C
and 31P CP/MAS solid-state NMR spectra is greater than that
of the iS1 13C and 31P CP/MAS solid-state NMR spectra, the
signal-to-noise for iS1Cu(I)RS is significantly worse. This is a
consequence of the presence of a small amount of Cu(II) on
the silica surface. The paramagnetic nature of Cu(II) vs Cu(I)
significantly decreases the S/N ratio of the spectra (Figure S9a).
EPR was used to confirm the presence of Cu(II) on the silica
support (Figure S9c). Additionally, UV-vis absorption spectra
of independently synthesized iS1Cu(I)RS and iS1Cu(II)RS show
that a metal-to-ligand charge-transfer (MLCT) band is present
for the Cu(I) species, indicative of the tetrahedral geometry.
However, no MLCT band is present for the iS1Cu(II)RS,
implying a different geometry (i.e., square planar) from the Cu(I)
form (Figure 3).
Following the divergent synthetic scheme, subsequent at-
tachment of the metal center and rotator is carried out. In a
typical synthesis, copper(I) tetrakis(acetonitrile)hexafluo-
rophosphate was chosen as the starting oxidation state for the
copper metal center because of its high solubility in organic
solvents such as acetonitrile and methylene chloride. In the case
of iS1, starting independently with both Cu(I) and Cu(II) was
necessary since the phosphine atoms in iS1 oxidize in the
presence of oxidizing agents.59,61 In other words, chemical redox
(61) Sommer, J.; Yang, Y.; Rambow, D.; Blu¨mel, J. Inorg. Chem. 2004,
43, 7561–7563.
(62) Peng, C.; Zhang, H.; Yu, J.; Meng, Q.; Fu, L.; Li, H.; Sun, L.; Guo,
X. J. Phys. Chem. B 2005, 109, 15278–15287.
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Figure 3. An amount of 2.5 mL of the particle suspension (100 µg/mL) in DMSO of independently synthesized iS1Cu(I)RS and iS1Cu(II)RS. The Cu(I)
form has a MLCT around 450 nm, but the Cu(II) form does not. The red curve represents the reduced state, while the blue curve represents the oxidized
state.
5. Properties of iS2Cu(I)RS and iS2Cu(I)RM. To illustrate
the properties of compounds prepared by divergent synthesis,
iS2Cu(I)RS and iS2Cu(I)RM are examples of compounds
examined where the second linker is used with either the shortest
rotator or a medium-length rotator, respectively. The addition
of Cu(I) and RS to iS2 produces iS2Cu(I)RS (Scheme 3b).
Unlike iS1Cu(I)RS, the methyl groups from the 2,9 position of
the phenanthroline ring of RS are not clearly observed in the
13C CP/MAS solid-state NMR spectrum of iS2Cu(I)RS because
the signal interferes with the aliphatic carbons of iS2 (Figure
S10a,b, Supporting Information). Therefore, the subsequent
addition of Cu(I) and RM to produce iS2RM was used to
distinguish between the carbon atom signals of iS2 and RM.
The 13C CP/MAS solid-state NMR spectrum of RM shows
additional carbon signals that result from the tritylacetylene
groups of the phenanthroline ring present on the 3,8 positions
of RM. The acetylene group contributes to carbon signals at
99.16 and 81.73 ppm, while the trityl carbon results in a carbon
atom signal at 56.38 ppm (Figure S7b, Supporting Information).
Comparison of the 13C CP/MAS solid-state NMR spectra of
iS2 and iS2Cu(I)RM shows the addition of RM to iS2 and Cu(I)
(Figure S11c, Supporting Information). Like iS1Cu(I)RS, the
poor S/N ratio in the solid-state NMR spectra of iS2Cu(I)RS
and iS2Cu(I)RM shows the presence of paramagnetic Cu(II).
This presence is confirmed via EPR (Figures S10c and S11d).
It is noteworthy that, over time, the slow oxidation of Cu(I) to
Cu(II) occurs. Therefore, the addition of a reducing agent, such
as ascorbic acid, to the immobilized Cu(I) compounds returns
air-oxidized Cu(II) centers back to Cu(I).
The absorption spectra after adding ascorbic acid shows that
any remaining Cu(II) was reduced to Cu(I), and hence an
increase of the MLCT band at ∼450 nm was observed (Figure
S11e). UV-vis absorption spectroscopy was also used to
distinguish between iS2Cu(I)RS and iS2Cu(I)RM immobilized
on silica nanoparticles and dispersed in dimethyl sulfoxide
(DMSO) (Figure S11f). iS2Cu(I)RS and iS2Cu(I)RM can be
distinguished by the additional shoulder at ∼336 nm in the
iS2Cu(I)RM spectrum. This peak is attributed to the RM ligand
of iS2Cu(I)RM. The UV-vis absorption of iS2Cu(I)RS and
iS2Cu(I)RM is compared to that of their Cu(II) analogues that were
formed by the oxidizing agent, hydrogen peroxide (Figure 4).
6. Heteroleptic Copper Switches. The switching properties
of the SiO2- and p+ Si-immobilized heteroleptic copper com-
pounds were studied. The switching characteristics of the SiO2-
immobilized systems were explored via chemical redox agents,
UV-vis absorption spectroscopy, and EPR. The p+ Si-im-
mobilized heteroleptic copper switches were studied by two
methods: (1) macroscopic gold electrodes25 and (2) a cAFM
tip.26
7. Heteroleptic Copper Switches on Silica Nanoparticles
Using Chemical Redox Agents. Tetravalent copper compounds
are known to exhibit different geometries based on the charge
of the copper metal center and the absence of sterically hindering
substituents on the 2,9 positions of 1,10-phenanthroline. More
specifically, copper(I) bis-bidentate compounds are tetrahedral,
while the copper(II) analogues are generally square planar.10,63
Hence, up to a 90° rotation of the ligands, with respect to one
another, can be achieved via oxidation and reduction of the
copper metal center.
Absorption spectra of iS2Cu(I)RS, iS2Cu(I)RM, iS3Cu(I)RS,
and iS3Cu(I)RM were measured by suspending the modified
silica nanoparticles in DMSO. The MLCT band in all four
samples appears at ∼450 nm. Upon the addition of hydrogen
peroxide and then washing of the particles, all four samples
were separately oxidized to Cu(II) compounds. The presence
of Cu(II) was observed by the disappearance of the MLCT.
These samples were subsequently reduced back to Cu(I) by the
addition of ascorbic acid followed by a wash. The MLCT band
reappeared upon the reduction of Cu(II) to Cu(I) (Figures S10d,
S11e, S12, S13, Supporting Information).
EPR characterization is a powerful tool used to determine
the structure of paramagnetic compounds, including but not
limited to Cu(II) compounds.64,65 Additional data that support
the isomerization of Cu(I) tetrahedral to Cu(II) square planar
are extracted from EPR data. To begin with, the bis(1,10-
phenanthroline) Cu(I) compounds are known to be tetrahedral66
(Figure S14a, Supporting Information), while Cu(II) analogues
are square planar.63 Based on the MLCT band at ∼450 nm for
the heteroleptic copper compounds discussed above (iS1Cu(II)RS,
iS2Cu(II)RS, and iS2Cu(II)RM) and the crystal structure from
Figure S14a, the SiO2-immobilized heteroleptic copper com-
pounds are considered to be tetrahedrally shaped. Upon oxida-
tion of the Cu(I) center, isomerization of Cu(II) square planar
takes place, and this structural change is confirmed on the basis
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Figure 4. An amount of 2.5 mL of the particle suspension (100 µg/mL) in DMSO of (a) iS2Cu(I)RS and (b) iS2Cu(I)RM, where the red-curves represent
the reduced state. Disappearance of the MLCT band of Cu(I) (∼450 nm) is observed upon use of an oxidizing agent, H2O2, where the blue curves represent
the oxidized state.
Figure 5. An amount of 2.5 mL of the particle suspension (100 µg/mL) in DMSO of (a) iS3Cu(I)RS and (b) iS3Cu(I)RM, where the red curves represent
the reduced state. Disappearance of MLCT band of Cu(I) (∼ 450 nm) is observed upon use of an oxidizing agent, H2O2, where the blue curves represent
the oxidized state. The absorption spectrum of iS3 interferes with the πfπ* transition of RM; hence, iS3Cu(I)RS and iS3Cu(I)RM absorption spectra cannot
be distinguished.
of the pattern of the EPR signals shown in Figures S9c, S10c,
and S11d (Supporting Information).64,65
Ti/gold electrodes and localized cAFM, respectively.1 Discrete
switching events are commonly associated with the observation
of a distinct increase in conductance above specific threshold
voltages in both the positive and negative directions, a persistent
ON state current following a switching event, and/or a hysteresis
within the acquired I-V curve. As seen in Figure 6, I-V
characteristics of devices whose molecular layer consisted of
solely grafted linkers (iS1), Cu-ligated linkers (iS1Cu), and the
fully formed heteroleptic Cu switch (iS1CuRS) were investigated
in order to assess the source of observed solution-phase
switching events. Beginning with a device residing in the OFF
state, denoted by a lack of measured electrical current, bias
voltage sweeps were applied to both iS1 and iS1Cu and
resulted in the I-V curves seen in Figure 6d,e. As expected,
neither molecular device revealed any of the effects indicating
8. Current-Voltage Spectroscopy of iS1, iS1Cu, and
iS1CuRS on p+ Si Using Two Methods. Characterization of
switching events in electrical devices can be readily carried out
through I-V spectroscopy. Electrical properties of the im-
mobilized molecular monolayers represented in steps 2-4 of
Figure 1b were examined in a sandwich-type device architecture
at both the macro- and nanoscale through the use of macroscopic
(63) Amournjarusiri, K.; Hathaway, B. J. Acta Crystallogr., Sect. C 1991,
47, 1383–1385.
(64) Zink, J. I.; Drago, R. S. J. Am. Chem. Soc. 1972, 94, 4550–4554.
(65) Hyde, J. S.; Pasenkiewicz-Gierula, M.; Basosi, R.; Froncisz, W.;
Antholine, W. E. J. Magn. Reson. 1989, 82, 63–75.
(66) Pallenberg, A. J.; Marschner, T. M.; Barnhart, D. M. Polyhedron 1997,
16, 2711–2719.
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Figure 6. Schematic diagram of the p+ Si-immobilized heteroleptic copper switch components (a-c) and corresponding I-V curves acquired on each
component layer in using both macroscopic Ti/Au (black) and Pt-Ir cAFM (red) electrodes (d-f), where molecular switching events are solely evident in
the fully formed compound (f).
a molecular switching event described above. The lack of a
ligated rigid rotator in these devices serves to eliminate any
capability for intramolecular rotational motion. I-V curves
acquired on iS1CuRS demonstrated that application of a
positive bias >2.2 V produced a drastic increase in conduc-
tivity that persisted upon sweeping the bias toward a negative
potential,as seen in Figure 6f. Application of a bias greater
than -1 V caused the ON state to be switched OFF and
generated a corresponding, abrupt decrease in conductivity
that persisted until the positive threshold voltage was applied.
In combination, these results indicate that observed switching
effects in fully formed molecular devices are the result of
conformational effects and do not stem from either the ligand
subunits themselves or the interfaces between the ligands and
the metal electrodes.
and cAFM,26 make these materials promising candidates for
future memory and logic applications.
Experimental Section
1. Linkers. [Bis(diphenylphosphino)methyl]triethoxysilane (S1).27
A solution of bis(diphenylphosphino)methane (1.02 g, 2.60 mmol)
in 15 mL of toluene was treated with 1.62 mL (2.60 mmol) of 1.6
M n-BuLi in hexane and allowed to stir under an N2 atmosphere at
room temperature. After 6 h, 0.511 mL (2.60 mmol) of chlorotri-
ethoxysilane was added dropwise under rapid stirring. After
complete addition, the reaction mixture was stirred for an additional
30 min. Toluene was removed by vacuum evaporation, and the
reaction mixture was dissolved in pentane. Centrifugation was used
to separate the lithium salt, and the supernatant was dried in vacuo
to remove the pentane to yield 67% of product (0.824 g, 1.51
mmol). 1H NMR (500 MHz, C6D6): δ 7.99-7.36 (m, 8 H),
7.09-6.89 (m, 12 H), 3.95 (q, 6H), 2.79 (s, 1H), 1.22 (t, 9H). 31P
NMR (202 MHz, C6D6): δ -21.12.
5-N,N-Di(amidopropyltriethoxysilane)-1,10-phenanthroline (S2).28
A total of 0.200 g (1.02 mmol) of 5-amino-1,10-phenanthroline
was dissolved in 30 mL of CHCl3 and filtered into a reaction flask.
The solution was stirred while 1.6 mL (∼ 6.5 mmol) of 3-(tri-
ethoxysilyl)propyl isocyanate was added. The chloroform was
evaporated at atmospheric pressure, and the resulting mixture was
heated at 80 °C, under N2, overnight. The reaction mixture was
allowed to reach room temperature and was then placed in an ice
bath. Cold hexanes were added to precipitate an off-white powder.
Centrifugation, decanting of the supernatant, and then dissolution
of the solid in methanol were carried out. The solution was gravity
filtered, the methanol was then removed by rotary evaporation, and
the final product was dried under vacuum overnight to yield 0.274
Summary
The design and synthesis of a library of silica-immobilized
heteroleptic copper compounds was reported. The method used
to compose this library of heteroleptic compounds involved
surface-outward sequential synthesis. A modular synthetic
approach of three individual components comprised this class
of functional materials and included a (1) bidentate linker
immobilized on silica, (2) a Cu(I) or Cu(II) metal center, and
(3) a rigid and bidentate rotator. Solid-state NMR, EPR, and
UV-vis absorption spectroscopies were used to characterize
these silica-immobilized heteroleptic copper compounds. Hy-
drogen peroxide and ascorbic acid were used as chemical redox
agents to change the oxidation state of the heteroleptic copper
compounds from Cu(I) to Cu(II) and then back to Cu(I). Optical
spectroscopy of a DMSO suspension demonstrated the revers-
ibility of the Cu(I)/Cu(II)-immobilized compounds by monitor-
ing the disappearance and appearance of the MLCT band at
450 nm. EPR spectroscopy confirmed the structure of the SiO2-
immobilized Cu(II) species as square planar. The electrical
properties of these heteroleptic copper compounds immobilized
on p+ Si, as determined using macroscopic gold electrodes25
1
g, 0.397 mmol (39%). H NMR (500 MHz, CDCl3): δ 9.22 (m,
2H), 8.29 (m, 2H), 7.89 (s, 1H), 7.71 (m, 2H), 3.70 (q, 12H), 3.23
(m, 4H), 2.10 (br, 2H), 1.60 (m, 4H), 1.13 (t, 18H), 0.53 (m, 4H).
4,7-Di(3-(triethoxysilyl)propylthio)-1,10-phenanthroline (S3).29
4,7-Dichloro-1,10-phenanthroline (200 mg, 0.803 mmol) was stirred
with 20 equiv of 3-mercaptopropyltriethoxysilane (4.13 mL,16.1
mmol) and 40 equiv of K2CO3 (4.44 g, 32.1 mmol, dried in oven
overnight) in anhydrous THF under N2 for 36 h at 65 °C. The
mixture was cooled and centrifuged, and the THF from the
supernatant was removed under vacuum to give a green oil.
Dissolution of the oil in pentane followed by cooling to -115 °C
precipitated the final product in a pure form (72% yield, 378 mg,
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A R T I C L E S
0.578 mmol). 1H NMR (500 MHz, CDCl3): δ 9.56 (d, J ) 4.7 Hz,
2H), 8.55 (s, 2H), 8.12 (d, J ) 4.3 Hz, 2H), 3.82 (q, J ) 7.0 Hz,
12 H), 2.70 (t, J ) 7.0 Hz, 4H), 1.81 (m, 4H), 1.23 (t, J ) 7.0 Hz,
18 H), 0.73 (t, J ) 8.3 Hz, 4H). MS (ESI): calcd for
C30H49N2O6S2Si2 [M + H]+ 653.26, found 653.24.
2. Immobilization of Linkers. iS1, iS2, and iS3. A total of
0.3 g of silica was slurried in about 50 mL of toluene, and excess
S1 (3 × 10-4 mol dissolved in 4 mL of toluene), S2 (3 × 10-4 mol
dissolved in 4 mL of CHCl3), or S3 (3 × 10-4 mol dissolved in 4
mL of THF) was added. The reaction mixture was stirred for 18 h,
under N2, at ambient temperature. The reaction mixture was
centrifuged, and the supernatant was removed. The modified silica
particles were washed three times with toluene and dried overnight
under vacuum.
modified silica particles were washed three times and dried
overnight under vacuum.
5. Homoleptic Cu Species for Crystallography and Electro-
chemistry. [Cu(I) Bis(3,8-di(ethynyltrityl)-1,10-phenanthroline)]PF6
([Cu(I)bisRM]PF6). [Cu(I)bisRM]PF6 was prepared on the basis
of a previously reported procedure.34 Two equivalents of RM (7.22
mg, 1 × 10-5 mol) was dissolved in 5 mL of CH2Cl2, and the
resultant solution was deaerated with argon. To this solution was
added, with stirring, 1.9 mg (5 × 10-6 mol) of Cu(CH3CN)4(PF6)
in 3 mL of CH3CN under an argon atmosphere. An orange-red color
was immediately observed, and the reaction mixture was allowed
to stir at ambient temperature for 1 h. Next, 35 mL of argon-
saturated diethyl ether was added to the reaction mixture. After
3 h, an additional 20 mL of argon-saturated diethyl ether was added
to the reaction mixture and allowed to stir overnight under an argon
atmosphere. The formed precipitate was filtered onto a glass frit
and redissolved in a minimum amount of CH2Cl2. Slow evaporation
of diethyl ether produced air-stable crystals of the product in ∼88%
yield. MS (MALDI) calcd for C108H72CuN4 1488.51, found 1487.98.
[Cu(I) Bis(3,8-di(ethynylbiphenyl)-1,10-phenanthroline)]PF6 ([Cu(I)-
bisRL]PF6). Two equivalents of RL (5.33 mg, 1 × 10-5 mol) was
dissolved in 5 mL of tetrachloroethane, and the resultant solution
was deaerated with argon. To this solution was added, with stirring,
1.9 mg (5 × 10-6 mol) of Cu(CH3CN)4(PF6) in 3 mL of CH3CN
under an argon atmosphere. An orange-yellow color was im-
mediately observed, and the reaction mixture was allowed to stir
at ambient temperature for 1 h. Next, 35 mL of argon-saturated
diethyl ether was added to the reaction mixture and allowed to stir
overnight under an argon atmosphere. The formed precipitate was
filtered onto a glass frit. 1H NMR (500 MHz, CD2Cl2): δ 7.89-6.79
(m, 46H).
3. Rotators. 3,8-Di(ethynyltrityl)-1,10-phenanthroline (RM).
RM was prepared on the basis of modified procedures.30,31
A
mixture of 3,8-dibromo-1,10-phenanthroline32 (0.54 g, 1.60 mmol,
1 equiv), 3,3,3-triphenylpropyne33 (2.58 g, 9.60 mmol, 6 equiv),
dichlorobis(triphenylphosphine)palladium(II) (0.071 g, 100 µmol),
copper iodide (7.6 mg, 0.400 mmol), and anhydrous triethylamine
(3.4 mL) was suspended in anhydrous benzene (20 mL). After the
mixture was refluxed for 4 d under Ar, the solvent was evaporated.
The black residue was dissolved in dichloromethane (150 mL),
washed with 2% KCN solution (100 mL) and water (100 mL), and
dried over MgSO4. The residue was purified by column chroma-
tography by first dry-loading the crude product with silica, flushing
out 3,3,3-triphenylphosphine (hexanes, Rf ) 1), and then gradually
increasing the polarity of the elute (9:1, 6:1, 3:1, 1:1 hexanes/EtOAc,
respectively, and then EtOAc, Rf ) 0.16). The product was
recrystallized from EtOAc/hexanes. Yield: 0.890 g (1.25 mmol,
78%) of a colorless solid, mp 269 °C. IR (neat): νmax 3086, 3058,
6. UV-Vis Absorption Measurements. Silica nanoparticles
were synthesized35 then functionalized by the copper compound
(described in Supporting Information). A total of 2.5 mL of the
particle suspension (100 µg/mL) in DMSO was used. Either
ascorbic acid or hydrogen peroxide solution was added into the
suspension to reduce or oxidize the copper compounds bound on
the particle surface, respectively. After the addition of the redox
reagent, the particles were washed with and resuspended in DMSO
to measure absorbance.
1
3024, 2218, 1597, 1539, 1490, 1446, 1421 cm-1. H NMR (300
MHz, CDCl3): δ 9.24 (d, J ) 2.0 Hz, 2H), 8.36 (d, J ) 2.0 Hz,
2H), 7.77 (s, 2H), 7.37-7.29 (m, 30H). 13C (125 MHz), CP/MAS:
δ 153, 146, 143, 129, 120, 99, 82, 56. MS (ESI): calcd for C54H37N2
[M + H]+ 713.29, found 713.28.
3,8-Di(4-ethynylbiphenyl)-1,10-phenanthroline (RL). RL was
prepared on the basis of modified procedures.30,31 A mixture of
3,8-dibromo-1,10-phenanthroline (0.54 g, 1.60 mmol, 1 equiv),
4-ethynylbiphenyl (0.627 g, 3.52 mmol, 2.2 equiv), dichlorobis-
(triphenylphosphine)palladium(II) (0.056 g, 0.08 mmol, ∼10%),
copper iodide (0.3 g, 0.16 mmol), and anhydrous triethylamine (10
mL) was suspended in anhydrous benzene (20 mL). After the
mixture was refluxed for 1 d under Ar, the solvent was evaporated.
The black residue was dissolved in dichloromethane (150 mL),
washed with 2% KCN solution (100 mL) and water (100 mL), and
dried over MgSO4. The residue was purified by column chroma-
tography by first dry-loading the crude product with silica, flushing
out 4-ethynylbiphenyl (SiO2, hexanes, Rf ) 1), and then gradually
increasing the polarity of the elute (9:1, 6:1, 3:1, 1:1 hexanes/EtOAc,
respectively, EtOAc, and then CHCl3, Rf (10% MeOH in EtOAc,
0.1% NEt3) ) 0.5). The product was recrystallized from 1,1,2,2-
tetrachloroethane with slow diffusion of ether. Yield: 0.605 g (1.14
mmol, 62%) of a pale yellow solid, mp 246-248 °C. IR (neat):
7. Assembly of p+ Si-Immobilized Heteroleptic Cu Compounds.
A p+ Si wafer was first washed with a piranha solution (1 part
30% H2O2 and 3 parts H2SO4) and then an aqua regia solution (1:3
HNO3:HCl). The wafer was then immersed into a 1 × 10-5
M
solution of iS1 (in toluene), iS2 (CHCl3/toluene), or iS3 (THF/
toluene) inside a test tube. The test tube was equipped with a stir
bar and sealed under an N2 atmosphere. The wafer remained in
this stirred solution at ambient temperature, under N2, for ap-
proximately 18 h. After excess iS1, iS2,or iS3 was washed away
with toluene, acetonitrile, or THF, respectively, the modified wafer
containing immobilized linkers was subsequently immersed into a
test tube containing a 1 × 10-5 M solution of propylaminotriethox-
ysilane (PTS), equipped with a stir bar, under an N2 atmosphere,
at room temperature for an additional 18 h. The wafer was washed
with toluene to remove excess PTS, placed in a test tube containing
a 1 × 10-5 M solution of Cu(I)(CH3CN)4PF6 in CH3CN, under an
N2 atmosphere, and then stirred at ambient temperature for 3 h. A
concentration of 1 × 10-5 M dmp in CH3CN was added to the
reaction test tube, and the immersed wafer was allowed to stir under
ambient conditions and under an N2 atmosphere for an additional
2 h. The wafer was washed thoroughly with CH3CN, dried with a
flow of N2 gas, and then placed in a vacuum desiccator overnight.
8. Electrical Characterization. Electrical properties of the p+
Si-immobilized molecular layers were investigated through
current-voltage (I-V) spectroscopy in ambient conditions at both
the micro- and nanoscale. First, microscale devices were fabricated
by deposition of a Ti/gold film through a shadow mask onto the
surface of the molecular layer to form a top electrode. Electrical
contact to the device was made using a probe station. Bias voltage
1
νmax 3406, 2920, 2208, 1595, 1522, 1485, 1419 cm-1. H NMR
(500 MHz, (CD3)2SO): δ 9.28 (d, J ) 1.9 Hz, 2H), 8.81 (d, J )
1.9 Hz, 2H), 8.11 (s, 2H), 7.85-7.76 (m, 18H). MS (ESI): calcd
for C40H25N2 [M + H]+ 533.20, found 533.20.
4. Assembly of SiO2-Immobilized Heteroleptic Cu Compounds.
In a typical synthesis, 0.2 g of iS1, iS2, or iS3 is slurried in about
30 mL of toluene (see Supporting Information). A concentration
of 2 × 10-4 mol of [Cu(I)(CH3CN)4]PF6 in 4 mL of CH3CN was
added. The reaction mixture was allowed to stir for 2 h, under N2,
at ambient temperature. A concentration of 2 × 10-4 mol of a RS,
RM, or RL in 4 mL of CH3CN, CH2Cl2, or tetrachloroethane,
respectively (see Supporting Information), was added to the reaction
mixture and was allowed to stir for an additional 3 h. The reaction
mixture was centrifuged, and the supernatant was removed. The
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A R T I C L E S
Kabehie et al.
sweeps were applied to the top gold electrode with respect to the
grounded p+ Si substrate and analyzed by using a Keithley 4200
semiconductor characterization system. In order to average the
nonuniformity of the molecular monolayer, sweep delay and hold
times were optimized and resulted in individual measurements
requiring 10 min to complete, depending on the measured current
level. Second, local nanoscale I-V spectroscopic analyses were
carried out via cAFM using the Nanoscope V Dimension Icon
instrument (Veeco Instruments). Pt-Ir-coated silicon cantilevers
(ContPt, NanoWorld) with calibrated spring constants between 0.12
and 0.15 N/m, first longitudinal resonance frequencies between 11.5
and 13 kHz, and nominal tip radii of <25 nm were employed in
contact mode under a minimum constant applied load in order to
reduce sample perturbation during the measurement. Bias voltage
sweeps were applied to the sample with respect to a virtually
grounded cAFM probe tip at a rate of 0.25 V/s over the range of
interest. Each point within the bias sweep was sampled five times
and averaged to enhance the S/N ratio. Presented I-V curves are
composed of a single sweep. While small fluctuations attributed to
nonuniformity in the active molecular layer were observed between
individual devices, I-V characteristics were stable with repeated
positive and negative bias sweeps.
Acknowledgment. The authors gratefully acknowledge financial
support by the SRC-DARPA Focus Research Program (FCRP)
through the Center on Functional Engineered Nano Architechtonics
(FENA) and the National Science Foundation through grant
CHE0809384. We thank Dr. Bob Taylor, Dr. Bob Schwartz, Dr.
Christopher Henry, Dr. Khanhlinh Nguyen, and Dr. Edward
Plummer for their helpful discussions and Dr. Yanli Zhao for his
help with images. Characterization of this material is supported by
shared instrumentation funded by the National Science Foundation
and National Center for Research Resources under equipment grant
numbers DMR9975975 and S10RR024605, respectively.
Supporting Information Available: Spectral characterization
data. This material is free of charge via the Internet at http://
pubs.acs.org.
JA103937V
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15996 J. AM. CHEM. SOC. VOL. 132, NO. 45, 2010