Electrochemical deposition of highly-conducting metal dithiolene films

Article abstract of DOI:10.1039/c6dt01166g

Electrochemical deposition has been used to prepare a thin film of neutral 4′,4-(3-alkyl)-thiophene-5′,5-hydogen-nickel and copper dithiolenes (Ni-C2, Cu-C2). The application of molecular electrodeposition provides a means to solution process molecular semiconductors of poor solubility, which results from the strong intermolecular interaction required for charge transport. Both Ni-C2 and Cu-C2 form continuous thin films that show intense NIR absorptions, extending to 1800 nm and 2000 nm respectively giving evidence for the strong intermolecular interactions in the solid state. Both films are highly conducting and temperature dependence of resistance gave an activation energy of 0.42 eV and 0.072 eV respectively, with the near-metallic behaviour of Cu-C2 attributed to the additional presence of an unpaired electron.

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ARTICLE TYPE
Electrochemical Deposition of Highly-conducting Metal Dithiolene Films
a
a
b
c
c
Emily Allwright, Georg Silber, Jason Crain, Michio M. Matsushita, Kunio Awaga and Neil
Robertson *
a
Received (in XXX, XXX) XthXXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
Electrochemical deposition has been used to prepare a thin film of neutral 4',4ꢀ(3ꢀalkyl)ꢀthiopheneꢀ5',5ꢀ
hydogenꢀnickel and copper dithiolenes (NiꢀC2, CuꢀC2). The application of molecular electrodeposition
provides a means to solution process molecular semiconductors of poor solubility, which results from the
strong intermolecular interaction required for charge transport. Both NiꢀC2 and CuꢀC2 form continuous
thin films that show intense NIR absorptions, extending to 1800 nm and 2000 nm respectively giving
evidence for the strong intermolecular interactions in the solid state. Both films are highly conducting and
temperature dependence of resistance gave an activation energy of 0.42 eV and 0.072 eV respectively,
with the nearꢀmetallic behaviour of CuꢀC2 attributed to the additional presence of an unpaired electron.
1
0
The electrodeposition of metal dithiolene molecules, based on
Introduction
[
Ni(bꢀ3ted) ], was initially investigated by Dalgleish et al.
2
1
2
2
3
3
4
4
5
5
0
5
0
5
0
5
0
Metal bisꢀ1,2ꢀdithiolene complexes, comprising two unsaturated 55 However, the film conductivity was hindered by their packing,
22
bidentate ligands, each bound to a central atom through two1
owing to twisting of the peripheral thienyl groups. Further work
sulfur donor atoms, have been investigated since the 1960s.
gave a film of neutral [Cu(miꢀ5hdt) ], demonstrating unique
2
2
While their magnetic properties have been of interest, it is their
stabilisation of a small, neutral, NIRꢀabsorbing Cuꢀdithiolene
23
optical and electronic properties in particular that have made
complex in the solid state, contrasting with anionic Cuꢀ
2,3,4,5
24,25,26
them attractive candidates for study for the last 50 years.
60 dithiolenes which are non NIR absorbing.
Many neutral metal dithiolene complexes show strong
absorptions in the near infrared region (NIR) and have been6
successfully employed in Qꢀswitching lasers, photodetectors,
In this work, we have designed and studied new Ni and Cu
dithiolene complexes, aimed at achieving highlyꢀconducting
singleꢀcomponent molecular films. The novel 4',4ꢀ(3ꢀalkyl)ꢀ
65 thiopheneꢀ5',5ꢀhydogenꢀnickel and copper dithiolenes were
prepared in their soluble, monoanionic form. By including
sterically undemanding Hꢀatoms on the dithiolene, the ligands
were designed to enable an extended, largely planar structure to
occur in the solid state, such that strong intermolecular
7
semiconductors for fieldꢀeffect transistors, molecular metals and
8,9,10,11
superconductors.
The unsaturated, nonꢀinnocent ligands give rise to a square planar
core geometry with metals such as Ni, Pd or Pt, and reversible
12
redox processes. These properties, tunable by changing the
metal, extending the ligand conjugation or varying the side 70 interactions could be achieved to give high film conductivity, as
2
7,28,29
chains, make dithiolene complexes ideal for charge transport if
observed in similar αꢀquarterthiophene analogues.
the packing of the molecule results in strong intermolecular
interaction. One very intruiging area of dithiolene complex
research has involved the synthesis of singleꢀcomponent
Electrochemical deposition of the neutral molecules through
oxidation of the soluble anions was then used to form the films.
Due to the very strong intermolecular interactions that result, the
2
molecular metals through strong intermolecular interactions that 75 neutral complexes in the films are completely insoluble.
1
3,14,15,16
close the bandgap in the solid.
A serious challenge exists
in this field however, since the necessary strong intermolecular
interactions typically result in dithiolene complexes having
extremelyꢀlow solubility which prohibits processing or device
incorporation. Furthermore, this situation can be viewed as
symptomatic of molecular conducting materials in general, where
the desirable strong intermolecular interactions are always
balanced against the need to maintain solubility for solution
Results and Discussion
The complexes were sythesised through a precursor route tailored
to yield the correct functionality of the back bone,
8,30,31
via the αꢀ
halo carbonyl and its subsequent reaction with a xanthateꢀester,
followed by the acid catalysed ring closure to produce the 1,3ꢀ
dithioleneꢀ2ꢀone ligand precursor. This can then be conveniently
converted to the dianion in situ and reacted with the nickel
dichloride salt to give the mono anionic complex (Fig. 1). The
identity and purity of the sythesised complexes were shown by
80
1
7,18,19,20
processing, or volatility for vapour processing.
We have previously presented electrochemical molecular
deposition as a suitable method for the formation of films of
insoluble molecules onto conductive substrates. The method 85 NMR, MS and CHN (ESI, Fig. S1†). A series of Ni complexes
involves disolving the more soluble anionic form of a molecule,
which is then electrochemically oxidised into the insoluble
neutral form which deposits onto the conducting substrate.
with varying alkyl chains was synthesised (Fig. 1) and for
comparison of the electrodeposited products, the Cu analogue
with ethyl chain was also synthesised.
21,22,23
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DOI: 10.1039/C6DT01166G
O
S
S
S
R
R
Cl
S
-
S
R
S
S
S
M
S
S
R
S
O
S
Figure 1: Synthesis of anionic dithiolene complexes as
tetrabutylammonium salts for M = Ni, R = Ethyl, Butyl, Hexyl, Octyl,
Decyl, Dodecyl, labelled as TBAꢀNiꢀCX (X = 2, 4, 6, 8, 10, 12
respectively); and as tetramethylammonium salt for M = Cu, R = Ethyl,
labelled as TMAꢀCuꢀC2.
5
Figure 2: Experimental spectrum of TBAꢀNiꢀC2 in DCM and calculated
transitions for monoanionic and neutral NiꢀC1 calculated at B3PW91/6ꢀ
31+G(d) using DCM within the PCM
Electronic Absorption of TBA-Ni-CX compounds
45
The absorption spectrum for TBAꢀNiꢀCX (TBA
tetrabutylammonium) is shown in Figure 2, with all compounds
showing essentially identical spectra with two major peaks at
=
In contrast to the anionic Ni complexes, TMAꢀCuꢀC2 showed no
low energy absorption (Figure S2†), which is as expected since
the additional electron prevents the NIR HOMO → SOMO
transition observed in the Ni analogue.
10
15
20
25
30
35
40
3
19 nm and 982 nm and molar absorption coeffcients of 31700
ꢀ
1
-1
-1
-1
M cm and 10500 M cm , respectively. Extension of the alkyl
chain had negligible impact on the absorbance spectra as any
increase in electron inductive effect is not significant and the
thienyl pendant groups are already electron rich. The small band
in the visible region is responsible for the dark red appearance of
the salt. Significant redꢀshifts compared with lessꢀconjugated
5
0
EPR
TBAꢀNiꢀCX were investigated in the solid state by room
temperature powder EPR spectroscopy (Fig S3, Table S3).
2
8
55 Except for TBAꢀNiꢀC2, which showed a rhombic type signal with
g = 2.123, g = 2.004 and g = 1.997, all compounds displayed
analogues are the result of the pendant thiophene rings giving
more delocalised frontier orbitals and a decrease in the energy
gap, in keeping with increased delocalisation, enabled by only
containing two pendant thienyl groups, allowing for greater
planarity.
1
2
3
axial type signals with gǁ and g┴ lying at around 2.1 and 2.03
respectively. This corresponds to results reported for similar
61
nickel centres where Ni (I = 3/2) enriched samples were used to
show that 30ꢀ40% of the spin density resides in the metal d
6
6
7
0
5
0
3
2,35,36
orbitals while the rest is spread over the ligand system,
also
DFT and TDꢀDFT calculations were carried out on the neutral
and monoanionic form of NiꢀC1 using B3PW91/6ꢀ31+G(d)
incorporating dichloromethane (DCM) as the solvent and the TDꢀ
DFT results are included in Figure 2. Reasonable qualitative
agreement with the experimentally observed spectrum is
observed, as although the energy of the NIR transition was
34
backed up by electronic structure calculations. While the axial
symmetry is expected for the square planar centres, the slight
distortion giving rise to the rhombic type signal for compound
TBAꢀNiꢀC2 is not unusual and has been reported before for other
36,37
nickel dithiolene salts.
ꢀ
1
underestimated by 1300 cm , the peak at 546 nm was clearly
reproduced. Similar calculations on the neutral species showed
the NIR transition to be only slightly higher in energy and
without the peak in the visible region. This was expected as the
NIR absorption is due to the HOMO → SOMO/LUMO (for
anion/neutral) transition of the π electron, while the visible
transition was primarily due to the SOMO→ LUMO+3 transition.
The former is expected to be stronger in the neutral form while
Electrochemistry
TBAꢀNiꢀCX exhibited completely reversible redox processes in
DCM at around ꢀ0.3 V and ꢀ1.1 V corresponding to the changes
from the neutral to monoanionic then dianionic form. Additional,
irreversible oxidations close to the solvent window were also
observed. The energy of the LUMO of the neutral complex was
estimated from the first reduction (Table 1) and a representative
75 voltammogram is shown in Figure 3. The small changes observed
between the compounds lie within the experimental error and in
combination with the spectroscopic data suggest that the
compounds have very similar electronic properties.
3
2
the latter should not occur. This is a well documented effect for
nickel dithiolenes and electrochemical reduction of the
3
2,33,34
complexes has been used to switch the NIR absorption.
+
80
Table 1: NiꢀCX and CuꢀC2 redox potentials against Fc/Fc . E1/2 for
reversible potentials and peak potential for irreversible processes*, plus
extracted LUMO energy values.38
Compound
NiꢀC2
E
red2 [V]
E
red1 [V]
E
ox [V ]
LUMO [eV]
ꢀ4.42
ꢀ1.13
ꢀ1.14
ꢀ1.15
ꢀ1.12
ꢀ1.13
ꢀ1.14
ꢀ1.37*
ꢀ0.30
ꢀ0.32
ꢀ0.33
ꢀ0.32
ꢀ0.30
ꢀ0.31
ꢀ0.32
0.83*
0.83*
0.82*
0.81*
0.83*
0.83*
NiꢀC4
ꢀ4.41
NiꢀC6
ꢀ4.39
NiꢀC8
ꢀ4.40
NiꢀC10
NiꢀC12
CuꢀC2
ꢀ4.42
ꢀ4.41
ꢀ4.40
2
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ꢀ2
ꢀ2
Figure 4: Deposition of 2.40x10 Ccm of (a) CuꢀC2 at 0.96 V and (b)
45
NiꢀC2 at 0.86 V
2
The charge per cm of the deposition of the films of CuꢀC2 and
NiꢀC2 onto FTO continues to rise steadily with time suggesting
that both dithiolene films are conductive and transport the current
to the surface of the formed film.
Figure 3: CV voltammograms of TBAꢀNiꢀC2 in DCM at different scan
rates, using 0.3 M [TBA][BF ] as supporting electrolyte
4
50
5
The potential of Ered1 and thus the calculated LUMO energy are
very similar to those observed for the phenyl substituted
analogues, consistent with the small contributions of the pendant
The initial current is very large due to the formation of the double
layer and the initial adsorption of any dithiolene monoanions
present at the surface of the FTO substrate. It then decreases
rapidly until it levels off, decreasing very slowly which indicates
that the formation of the film is diffusionꢀdependent (Figure S4†).
This is probably due to the formation of a continuous layer of
neutral complex on the substrate surface, since the deposition of
molecules onto alreadyꢀdeposited molecules is more energetically
favourable than the nucleation of molecules on the substrate
surface.
3
9,40
groups to the LUMO.
For CuꢀC2, the 1ꢀ/0 oxidation,
5
5
important for the subsequent electrodeposition experiments, is
similar to that of the Ni complexes. The Eox value of 0.82 V for
the Ni complexes is very similar to that observed for the phenyl
substituted analogues, and slighlty smaller than the previously
reported tetrakis thienyl substituted analogue, suggesting that the
increased planarity of the thienyl ring may allow for more
delocalisation of the additional charges, while avoiding the
electron induction effects of the additional, twisted thienyl
moieties. The values, however, remain well above that of the
1
0
60
1
5
Electronic Absorption of Ni-C2 films
more delocalised [Ni(dmit) ] (dmit = 2ꢀthioxoꢀ1,3ꢀdithioleꢀ4,5ꢀ
2
4
1
dithiol)
.
2
0
In contrast to DCM, when carrying out electrochemistry in
acetonitrile solution, film formation was observed if a potential
was applied sufficient to form the neutral compound, resulting in
CV characteristics similar to those observed in previous studies
2
2,23
2
3
3
4
5
0
5
0
of metal dithiolene electrodeposition.
This contrasts with CV
in the DCM solution in which no electrodeposition of the neutral
species was observed. We note also that no electropolymerisation
was observed, following the irreversible oxidation, attributed to
the terminal alkyl groups as expected, and evidenced by reꢀ
dissolution of the molecular film upon reꢀreduction of the
molecules.
4
2
ꢀ
1
2
65
Figure 5: Electronic Absorption of NiꢀC2 films: (a) 1.54x10 Ccm , (b)
ꢀ2 ꢀ2 ꢀ2 ꢀ2 ꢀ2 ꢀ2
6
.38x10 Ccm , (c) 3.85x10 Ccm , (d) 2.40x10 Ccm and (e) TBAꢀ
NiꢀC2 in DCM for comparison
Electrodeposition of Ni-C2 and Cu-C2
Films of NiꢀC2 and CuꢀC2 were electrodeposited onto FTO
(fluorineꢀdoped tin oxide) conducting glass and interdigitated
electrode substrates to investigate the optical and charge transport
properties of both films. Several potentials above Eox=0.31 V for
the CuꢀC2 and Eox=0.34 V for the NiꢀC2 were investigated for the
electrodeposition. Uniform films occurred at 0.96 V for CuꢀC2
and at 0.86 V for NiꢀC2. Typical depositions of CuꢀC2 at 0.96 V,
Table 2: Peak positions of the NiꢀC2 films shown in Figure 5
ꢀ
2
Film (Ccm )
Peak Positions (nm)
ꢀ2
2.40x10 (a)
305
305
319
434
316
758
757
786
1051
1050
1091
1095
980
1455
1464
1556
1668
ꢀ2
3.85x10 (b)
ꢀ2
6
1
.38x10 (c)
ꢀ1
.54x10 (d)
(
a), and NiꢀC2 at 0.86 V, (b), in 0.1M TBABF MeCN electrolyte
4
Solvent (e)
551
are shown in Figure 4.
70
The electronic absorption of the thinnest film on FTO has peaks
at 305 nm, 758 nm, 1051 nm and 1455 nm (Fig. 5 (a)). The new
band at low energy implies strong intermolecular interactions,
most likely by πꢀπ stacking. Apart from the broad peak at around
3
00 nm, all the other peaks have redꢀshifted and become broader,
2
75
also attributed to πꢀπ stacking. Films with a low charge per cm ,
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ꢀ
2
ꢀ2
less than 5.00x10 Ccm , have similar peak positions, with the
exception of the last peak at around 1400 nm, whereas for the
thicker films the peaks start to further redꢀshift with increasing
thickness, probably due to an increase in the extensive
intermolecular interactions.
5
Electronic Absorption of Cu-C2 films
Figure 8: SEM images of NiꢀC2
The continuous films of both NiꢀC2 and CuꢀC2 contain features
of between 50 nm and 500 nm in size. SEM images of the
interdigitated electrodes were recorded and the results are shown
in Figures S7 and S8†. The SEM images of the interdigitated
areas show that both dithiolenes deposit as a combination of a
thin film and fibres. The CuꢀC2 has deposited as an even layer
with small needleꢀshaped fibres crossing the electrodes. The Niꢀ
C2 has also deposited as a thin film plus needleꢀshaped fibres, but
in this case both types of deposition also seem to be in
competition as there is no thin film for about 20 ꢁm where the
fibres have formed.
4
4
5
5
0
5
0
5
ꢀ
2
ꢀ2
1
0
Figure 6: Electronic absorption of CuꢀC2 films: (a) 5.68x10 Ccm , (b)
ꢀ2 ꢀ2 ꢀ2 ꢀ2 ꢀ3 ꢀ2
4
.35x10 Ccm , (c) 2.38x10 Ccm , (d) 5.80x10 Ccm , (e)
ꢀ3 ꢀ2 ꢀ3 ꢀ2
2
.94x10 Ccm and (f) 2.30x10 Ccm
The films show similar absorptions to the parent TMAꢀCuꢀC2 in
solution, between 300 nm and 600 nm, although redꢀshifted in a
similar way to the NiꢀC2 species above. However, there are also
three new absorptions at 830 nm, 1170 nm and 1628 nm for
Resistance measurements
The resistance of the electrodeposited films of CuꢀC2 and NiꢀC2
was measured as a function of temperature upon cooling (Figure
9, Figure S9†). The activation energy of the complexes can be
calculated from the temperatureꢀdependent resistance using the
following equation,
15
20
25
30
2
thicker films deposited via higher charge passed per cm , with
corresponding weak absorptions in the thinner films. The
presence of these two NIR peaks and one MIR (midꢀIR) peak
confirms that the neutral CuꢀC2 has been formed, as this complex
is now isoelectronic with the NIR absorbing monoanionic TBAꢀ
NiꢀC2 complex. The peaks are also very broad covering the range
of 800 nm to 2000 nm compared to the previously
electrodeposited neutral Cu(miꢀ5hdt) , which had a peak from
800 nm to 1500 nm. This can be attributed to the greater
planarity of CuꢀC2 with only one substitutent on each ligand.
ꢈꢉ 1
ꢀ
ꢁꢂꢃꢄ ꢅ ꢀꢁꢂꢃ ꢄ ꢇ
ꢆ
ꢊ ꢋ
where σ is the conductivity, σ
0
is the preꢀexponential factor, E is
a
2
2
3
60 the activation energy, R is the gas constant and T is the
temperature in K. This is illustrated by a plot of ln(σ) against 1/T
where the gradient of the plot is ꢀE /R.
a
Scanning Electron Microscopy
SEM imaging of several films of CuꢀC2 and NiꢀC2 was
performed and representative images of the surface of the films
are shown in Figures 7 and 8 and Figures S5 and S6†.
Figure 9: Plot of Log(σ) against 1/T of (a) CuꢀC2 and (b) NiꢀC2
65
The activation energies of CuꢀC2 and NiꢀC2 were calculated to
be 0.072 eV and 0.42 eV respectively. Both measurements
suggest that the films contain strongly πꢀstacked molecules,
which result in significant intermolecular interaction, showing a
narrow gap between the occupied and unoccupied levels. The
activation energy of the NiꢀC2 film broadly agrees with the
electronic absorption measurements (Fig. 5), where the
absorption onset around 1800 nm corresponds to a gap of around
Figure 7: SEM image of CuꢀC2
35
70
0
.7 eV and, assuming similar behaviour to a band semiconductor,
4
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the activation energy equals the bandgap divided by two. The Cuꢀ
C2 activation energy is significantly smaller than that of NiꢀC2
and does not correspond to a similar transition in the electronic
absorption measurements. For CuꢀC2 (Fig. 5) the absorption
onset around 2000 nm corresponds to a separation of 0.6 eV. This
suggests that the extra unpaired electron in the radical CuꢀC2 is
playing a role, giving an exceptionally low activation energy. The
resulting, almostꢀmetallic behaviour, is reminiscent of the
isoelectronic Auꢀbisꢀdithiolene complexes that show metallic
8. U. T. MuellerꢀWesterhoff, B. Vance, and D. I. Yoon,
Tetrahedron,1991, 47, 909ꢀ 932
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We have achieved highlyꢀconducting thin films of neutral metalꢀ
bisꢀdithiolene complexes, which in the case of M = Cu shows
1
2
2
5
0
5
nearly metallic properties. This adds distinctive new examples to 80 16. E. Fujiwara, A. Kobayashi, H. Fujiwara, H. Kobayashi, Inorg.
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1
7. W. E. Buschmann, S. C. Paulson, C. M. Wynn, M. A. Girtu, A. J.
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semiconductors, our extension of the electronic delocalisation to
increase intermolecular interaction also reduced the solublity of
the neutral complexes and precluded solution processing. To
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9
9
5
0
5
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We thank EPSRC for studentship support. The work was
supported by the Fonds National de la Recherche, Luxembourg,
902480. This work was partially supported by a GrantꢀinꢀAid for
Scientific Research from the Ministry of Education, Culture,
Sports, Science, and Technology (MEXT). Funds were also
provided by the JSPS CoreꢀtoꢀCore Program, A. Advanced
Research Networks and the Leverhulme Trust to support the UKꢀ
Japan collaboration.
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