Formation of stable neutral copper bis-dithiolene thin films by potentiostatic electrodeposition

Article abstract of DOI:10.1039/c1cc12344k

Thin films of neutral copper dithiolenes have been prepared by potentiostatic electrodeposition. This method allows the isolation of near infrared (NIR) active species, in a useable form, that are otherwise unobtainable by conventional chemical methods.

Full text of DOI:10.1039/c1cc12344k

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Formation of stable neutral copper bis-dithiolene thin films by
potentiostatic electrodepositionw
Simon Dalgleish,ab Kunio Awaga and Neil Robertson*a
b
Received 21st April 2011, Accepted 16th May 2011
DOI: 10.1039/c1cc12344k
9
Thin films of neutral copper dithiolenes have been prepared by
potentiostatic electrodeposition. This method allows the isolation of
near infrared (NIR) active species, in a useable form, that are
otherwise unobtainable by conventional chemical methods.
monoanion, contrary to other structurally analogous systems.
However, the complex was shown to be unstable in the few
solvents in which it was soluble (DMF and DMSO), and thus
a rigorous molecular characterization was prohibited. Formation
of the stable monoanionic salt of 1 could be achieved using
[TMA][OH] as a base in THF, followed by addition of
The preparation of thin films of NIR active materials is
an active area of research within materials chemistry, with
CuCl
2
ꢀ2H
2
O, and leaving the flask open to atmospheric
1
potential applications such as organic photodetectors,
2
electrochromic devices and broad spectrum photovoltaics.
oxidation, with pure [TMA][1] precipitating slowly from solution
upon cooling.
3
4
Neutral and anionic metal bis-dithiolenes of Ni, Pd, Pt and
5
neutral analogues of Au characteristically show strong
In contrast, for mi-5hdt both these synthetic routes led only
to isolation of the monoanionic salt of the complex.
The TMA salts of 1 and 2 showed only very weak absorption
absorption in this region and, coupled with numerous reports
6
of semiconductive behaviour, are therefore interesting targets
ꢁ1
in the NIR region at 1557 nm (6423 cm , e = 550) and 1486 nm
ꢁ1
for optoelectronic application. Isoelectronic with the neutral
Au and monoanionic Ni, Pd, Pt dithiolenes, neutral Cu
dithiolenes are expected to show very low energy NIR absorption
due to an intense p - p* transition predominantly within the
(6729 cm , e = 523) for [TMA][1] and [TMA][2], respectively
(Fig. S1, ESIw). By comparison with the redox series of the
nickel dithiolenes, the monoanionic copper dithiolenes are
7
isoelectronic with the dianionic nickel dithilolenes. The
7
ligand moieties, however, study into such systems has been
comparative weakness of the NIR absorptions, compared to
2
their monoanionic nickel analogues, can thus be understood,
limited due to their poor stability in solution, prohibiting their
isolation.z
since the orbital corresponding to the SOMO in the mono-
anionic nickel dithiolenes, is filled for the monoanionic copper
We here report the fabrication of thin films of two
7
novel neutral copper dithiolenes, [Cu(mi-5edt)
2
] (1) and
systems, and thus prohibits an analogous p - p* transition.
[
Cu(mi-5hdt) ] (2) (Fig. 1), by potentiostatic electrodeposition
2
The weak NIR absorption observed for [TMA][1] and
[TMA][2], is therefore likely due to d - d transitions on the
copper centre.
from their isolable monoanionic TMA salts and, by investigation
of their optical and redox properties, demonstrate that such
8
species are only obtainable by this direct fabrication method
Both complexes showed one strong absorption in the
UV/Vis region of the electromagnetic spectrum [418 nm
[
(mi-5edt) = 1-(N-methylindol-5-yl)-ethylene-1,2-dithiolate;
mi-5hdt) = 1-(N-methylindol-5-yl)-hex-1-ene-1,2-dithiolate;
ꢁ1
(
(23 923 cm , e = 11 220) for [TMA][1] and 393 nm
1
ꢁ
TMA = tetramethyl-ammonium].
(25 445 cm , e = 26 181) for [TMA][2]], and a second
absorption outside the solvent window for spectroscopic
investigation. The intensity and energy of the UV/Vis transition
suggests it to be a ligand centred transition. Over several days,
the spectra of [TMA][1] changed, losing the peak at 418 nm,
We have previously reported the synthesis of a stable ligand
2
precursor for (mi-5hdt). The route to the analogous
(
mi-5edt), as well as details of the synthesis of both mono-
anionic copper salts, is given in the ESI.w Deprotection of the
ligand precursors was first attempted using NaOMe/MeOH,
and subsequent addition of CuCl
2
ꢀ2H
2
O. For mi-5edt, this
method resulted in the immediate precipitation of, what was
suggested by elemental analysis to be neutral 1, and not the
a
School of Chemistry and EaSTCHEM Research School,
University of Edinburgh, West Mains Road, Edinburgh,
EH9 3JJ, UK. E-mail: neil.robertson@ed.ac.uk
Department of Chemistry and Research Center of Materials Science,
b
Nagoya University, Chikusa-ku, Nagoya 464-8602, Japan
w Electronic supplementary information (ESI) available. See DOI:
10.1039/c1cc12344k
Fig. 1 Structures of the copper bis-dithiolenes used in this study:
2 2
R = H : Cu(mi-5edt) (1), and R = Bu : Cu(mi-5hdt) (2).
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Chem. Commun., 2011, 47, 7089–7091 7089

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2
On scanning back to positive potentials, past E , no increase
ox
p
) was observed compared to the
in the oxidation current (I
clean electrode. This suggested that the deposited films were
comprised of the neutral species (1 or 2), and were molecular
in nature, due to their identical redox behaviour to the
monomer in solution. The formed neutral films could then
be quantitatively reduced, and thus redissolved, by scanning
2
past E .
When a similar experiment was performed in an identical
electrolyte solution of DMF, an irreversible oxidation process
Fig. 2 Electrochemical studies of [TMA][1] (a) and [TMA][2] (b).
3
was observed for E , however, no film was observed to form
with the resulting spectrum reminiscent of that of chemically
synthesised 1 upon dissolution/decomposition in DMF. This
suggests that [TMA][1] is slowly oxidised in solution, whereupon
it undergoes the same fate as observed for chemically synthesised
neutral 1 in solution. This might explain the difficulties
encountered for single crystal growth of [TMA][1], and isolation
of the monoanionic complex from the NaOMe/MeOH route.
Such a process is not observed for [TMA][2], suggesting that it
is more resistant to oxidation, possibly due to a more out-of-
plane indolyl group (due to greater steric hindrance from
the -Bu group), not raising the HOMO level as much as
2
on the electrode and, upon scanning back past E 1/2, no
increase in peak current was observed for the reduction or
oxidation process. This different behaviour is consistent with
the electrogeneration of a species that is unstable when
solublised in the DMF solvent system.
In order to further characterise the electrogenerated films of
1 and 2 from MeCN, films were grown on fluorine doped tin
oxide (FTO) coated glass electrodes, so that the electronic
absorption properties could be investigated. Continuous
amorphous films of 1 and 2 (observed by SEM and thin film
XRD, Fig. S5, ESIw) were grown under potentiostatic control,
by maintaining a constant potential (E = 0.5 V and 0.6 V for
the electrodeposition of 1 and 2, respectively), and recording
the charge passed with time (Fig. S3, ESIw). The deposited film
of 1 was shown to be more conductive than 2, due to the
passing of a greater current at the working electrode, however
both 1 and 2 continued to grow on the working electrode,
resulting in the formation of continuous films, that were gold
in colour, for both complexes. The UV/Vis/NIR absorption
spectra of both complexes showed pronounced differences
from their respective monoanions, investigated in solution
(Fig. 3a). Both complexes showed an intense absorption in
8
for [TMA][1].
The electrochemistry of both [TMA][1] and [TMA][2] was
investigated in a 0.1 M TBABF
4
electrolyte solution of MeCN
(
Fig. S2, ESIw). Both complexes showed one electrochemically
2
reversible redox process at E 1/2 = ꢁ0.59 V and ꢁ0.80 V for
[
TMA][1] and [TMA][2], respectively. This reversible process,
2ꢁ/1ꢁ
assigned to the [ML2]
redox couple, was observed to be
highly sensitive to the ligand environment, with the reduction
to the dianion occurring at less negative potentials for
[
TMA][1] than for [TMA][2]. This is likely due to the more
in-plane twist of the indolyl group in [TMA][1] being better
able to stabilise the development of a second negative charge,
through greater delocalisation onto the ligand.
ꢁ
1
the NIR range [B1270 nm (7849 cm for both 1 and 2)], as
well as several other absorptions across the visible region of
the spectra. This suggested that the formed films were indeed
neutral molecular copper bis-dithiolenes, with the oxidation
processes corresponding to the loss of an electron from the
dithiolene ligand, thus facilitating an intense p - p* transition,
as observed for the isoelectronic monoanionic nickel dithiolenes.
The films of neutral 1 and 2, deposited from MeCN, were
completely soluble in DMF, however, the UV/Vis spectra of
the dissolved films were shown to be very different from those
in the solid-state, losing all the absorption processes in the
visible and NIR regions of the spectra (Fig. 3a), resembling the
spectra for 1 in DMF, formed by chemical synthesis, which
was shown to decompose upon dissolution. It is therefore
apparent that electrodeposition provides a route to thin films
of neutral 1 and 2 that are otherwise unobtainable by solution
methods.
For [TMA][1], a second reduction process was observed
1
(
E
1/2 = ꢁ1.62), that was shown to be chemically reversible,
though not electrochemically reversible, as the associated
ox
oxidation current (I
p
) decreased at faster scan rates. This
2
+/1+
presumably corresponds to a Cu
transition, since the
ligand environment is already fully reduced. This process is
usually associated with a geometry change from (pseudo)
square-planar to tetrahedral, and thus is not expected to be
1
0
electrochemically reversible. The fact that an associated
oxidation process is observed, at low over-potential, might
suggest that the observed geometry change is slight, and that
the solution geometry of the monoanion may not be rigorously
1
1
square planar. On cooling the electrochemical cell to ꢁ40 1C,
the reduction process shifted outside the solvent window and,
as such, no associated oxidation process was observed. On
scanning to positive potentials, an irreversible process was
3
observed at E
= 0.42 V and 0.45 V for [TMA][1] and
In order to shed more light on the nature of the absorption
of the deposited complex, density functional theory (DFT)
calculations were performed on a modelled complex of 1,
using the same level of theory as has proven successful for
ox
1ꢁ/0
[
TMA][2], respectively, corresponding to a [ML2]
transition,
followed by several other irreversible oxidation processes. In
MeCN, this first oxidation process led to the deposition of a
film on the electrode surface for both [TMA][1] and [TMA][2].
For both complexes, by scanning the coated electrode back to
2
12–15
an analogous nickel complex ((U)B3PW91/6-31G*).
Removing symmetry constraints, and starting from an
16,17
2
red
negative potentials, past E , a greater reduction current (I
2
p
)
idealised tetrahedral geometry,
geometry optimisation
was observed, at the same potential as for E , compared to an
identical scan cycle for a clean electrode (Fig. 2).
showed the complex to converge upon an energy minimum
that can be best described as a squashed tetrahedron, with the
7
090 Chem. Commun., 2011, 47, 7089–7091
This journal is c The Royal Society of Chemistry 2011

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use of the resources provided by the EaStChem Research
Computing Facility (http://www.eastchem.ac.uk/rcf). This
facility is partially supported by the eDIKT initiative
(
http://www.edikt.org).
Notes and references
16
17
z The chemical and electrochemical preparation of a neutral
copper dithiolene has previously been reported. However, it is likely
that it is the TTF-functionalised ligands, used in both these systems,
that are oxidised, rather than the dithiolene core. It is noteworthy that
neither study analysed the solution properties of these species, likely
due to poor solubility.
Fig. 3 Experimental and calculated UV/Vis/NIR spectroscopy of an
electrodeposited film of neutral 1 on FTO; (a) Thin film spectra,
compared to the stable monoanionic complex [TMA][1] in MeCN, and
showing the decomposition of neutral 1 upon dissolution. Quotation
marks ‘‘x’’ denote that the species is no longer its identity upon
dissolution, due to an unknown decomposition process; (b) Thin film
spectrum of 1 compared to calculated spectra from TD-DFT.
1 M. C. Aragoni, M. Arca, M. Caironi, C. Denotti,
F. A. Devillanova, E. Grigotti, F. Isaia, F. Laschi, V. Lippolis,
D. Natali, L. Pala, M. Sampietro and P. Zanello, Chem. Commun.,
2
004, 1882.
two dithiolene rings distorted by 43.561 away from co-planarity.
Following geometry optimisation, information regarding the
nature of the electronic transitions for 1 was sought using
time-dependent density functional theory (TD-DFT). This
was done to model the electronic absorption spectrum, for
comparison with the electrodeposited thin films, and to probe
the origins of the individual absorptions. TD-DFT calculations
showed reasonable agreement with the experimentally determined
absorption spectrum (Fig. 3b), and showed the low energy
NIR absorption to be predominantly due to a HOMO -
LUMO transition (Table S2, ESIw). Since these orbitals
are almost exclusively located on the dithiolene SCCS cores
2 S. Dalgleish and N. Robertson, Chem. Commun., 2009, 5826;
S. Dalgleish and N. Robertson, Coord. Chem. Rev., 2010, 254,
1
549.
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`
re,
´
,
6
¨
2
(
Fig. S4, ESIw), this may explain the relatively small difference
in energy of the NIR absorption in the absorption spectra of
9
ꢁ1
both films (DlNIR E 31 cm ). The thin film spectrum was
generally well reproduced by TD-DFT, consistent with a film
formed of neutral copper dithiolene complexes, and that the
low energy NIR absorption was due to a p - p* transition
within the ligand moieties—analogous to the isoelectronic
7 R. K. Szilagyi, B. S. Lim, T. Glaser, R. H. Holm, B. Hedman,
K. O. Hodgson and E. I. Solomon, J. Am. Chem. Soc., 2003, 125,
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7
monoanionic nickel dithiolenes. It should be noted that
2
S. Dalgleish, H. Yoshikawa, M. M. Matsushita, K. Awaga and
N. Robertson, Chem. Sci., 2011, 2, 316.
´ `
S. Raba c¸ a, A. C. Cerdeira, A. I. S. Neves, S. I. G. Dias, C. Meziere
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copper dithiolenes have been shown to have hugely variable
coordination geometry around the metal centre, and that the
solution geometry (and thus the colour) is not always the same
8
9
18
and M. Almeida, Polyhedron, 2009, 28, 1069.
0 G. A. Razuvaev, V. K. Cherkasov and G. A. Abakumov,
as that in the solid state. Within this limitation, the consistency
between the calculated and experimental spectra is striking.
In summary, by exploiting the reduced solubility of the
neutral species, compared to the monoanionic species, thin
films of two neutral copper dithiolenes, highly active in the
NIR region, were prepared. We have previously reported on
the use of electrodeposition as a novel route to fabricate thin
films of nickel bis-dithiolenes, whose solubility is too low, and
melting point too high for effective processing by solution, or
1
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We thank the EPSRC and the Japanese Science and
Technology agency (JST) for funding. This work has made
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This journal is c The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 7089–7091 7091