Broadband near-IR absorbing Au-dithiolene complexes bearing redox-active oligothiophene ligands

Article abstract of DOI:10.1039/c8dt03915a

A series of three homoleptic, monoanionic gold dithiolene complexes of oligothiophene ligands which coordinate via a central thiophene-3,4-dithiolate chelate are presented. The oligomer chains are three, five and seven thiophenes long and the complexes display hybrid optoelectronic properties featuring characteristics of both the oligothiophene chains and the delocalised metal dithiolene centre. The properties of the complexes have been characterised using a variety of spectroscopic and electrochemical methods complemented by computational studies. Solid state spectroelectrochemistry has revealed that upon oxidation these complexes display intense and broad absorption across the visible spectrum. In attempting to produce nickel analogues of these materials a single crystal of a photo-oxidised nickel dithiolene complex has also been isolated.

Full text of DOI:10.1039/c8dt03915a

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Broadband near-IR absorbing Au-dithiolene
complexes bearing redox-active oligothiophene
Cite this: Dalton Trans., 2019, 48, 107
ligands†
Iain A. Wright, *‡a Claire Wilson, §b Simon J. Coles b and
Peter J. Skabara *§a
A series of three homoleptic, monoanionic gold dithiolene complexes of oligothiophene ligands which
coordinate via a central thiophene-3,4-dithiolate chelate are presented. The oligomer chains are three,
five and seven thiophenes long and the complexes display hybrid optoelectronic properties featuring
characteristics of both the oligothiophene chains and the delocalised metal dithiolene centre. The pro-
perties of the complexes have been characterised using a variety of spectroscopic and electrochemical
methods complemented by computational studies. Solid state spectroelectrochemistry has revealed that
upon oxidation these complexes display intense and broad absorption across the visible spectrum. In
attempting to produce nickel analogues of these materials a single crystal of a photo-oxidised nickel
dithiolene complex has also been isolated.
Received 28th September 2018,
Accepted 15th November 2018
DOI: 10.1039/c8dt03915a
rsc.li/dalton
Conjugated polymers21–24 and oligomers25–27 of thiophene
are archetypical organic p-type semiconductors that have been
Introduction
Binding two or more electronically delocalised substructures widely studied for use in optoelectronic devices. Combining
together by fusion across a shared aromatic bond presents an the rich optoelectronic properties of the metal-containing non-
alternative and powerful method by which to facilitate elec- innocent dithiolene core with the straightforward processing
tronic communication between redox-active sub-units.
and easily tunable electronic properties of a conjugated
Bis(dithiolene) complexes have a non-innocent [C4S4M] polymer or oligomer presents an appealing approach in the
chelate and have fascinated coordination and materials che- search for new functional materials. A number of polymerisa-
mists alike for over 50 years.1–5 These complexes are often ble complexes have been realised and subjected to
intensely coloured with a characteristic near-IR (NIR) absorp- electropolymerisation28–34 or electrodeposition35,36 to give low
tion6,7 and display numerous reversible electron transfer pro- band gap metallopolymers, while discrete thiophene-contain-
cesses.8 They are known to exhibit a range of fascinating and ing dithiolene ligands have recently provided some novel
useful material properties including ferromagnetism,9 metallic photochromic complexes.37
conductivity and superconductivity,9–12 ambipolar charge
transport,13–15 non-linear optical activity16,17 and catalytic
water splitting.18–20
aWestCHEM, Department of Pure and Applied Chemistry, University of Strathclyde,
Glasgow G1 1XL, UK. E-mail: i.a.wright@lboro.ac.uk, peter.skabara@glasgow.ac.uk
bUK National Crystallography Service. School of Chemistry, University of
Southampton, Southampton SO17 1BJ, UK
†Electronic supplementary information (ESI) available: General experimental
and synthetic and crystallography details and spectroscopic data. CCDC
1861791. For ESI and crystallographic data in CIF or other electronic format see
Of particular relevance to this work are the studies by Belo,
DOI: 10.1039/c8dt03915a
Almeida and co-workers on complexes of thiophene-dithiolate
‡Present address: Department of Chemistry, Loughborough University,
ligands38–42 which have given rise to a number of complexes
Loughborough LE11 3TU, UK. E-mail: i.a.wright@lboro.ac.uk.
and salts thereof with interesting structural, electronic and
magnetic properties.43–52 The ligands thiophene-2,3-dithiolate
§Present address: WestCHEM, School of Chemistry, University of Glasgow,
Glasgow G12 8QQ, UK. Email: peter.skabara@glasgow.ac.uk.
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(23TDT) and thiophene-3,4-dithiolate (34TDT) consist of an
aromatic, electron-rich thiophene ring, fused to the electroni-
cally delocalised, typically anionic, metal dithiolene chelate. In
both 23TDT and 34TDT further functionalisation of the thio-
phene moiety can be used to modify and tune the materials’
electronic or mechanical properties. Mono-53 and di-alkyl54
substituted derivatives of 23TDT have been prepared with a
dialkyl substituted complex behaving as a single molecule con-
ductor, while robust synthetic routes have been established for
the synthesis of semiconducting π-extended 23TDT ligands
bearing aromatic rather than alkyl substituents.55,56
Our interest in this area involves extending the conjugation
length of the 34TDT ligand. Metallopolymers obtained by the
electropolymerisation of complexes of the symmetrical 34TDT
ligand 2,5-di(thien-2-yl)thiophene-3,4-dithiolate displayed
broad absorption which is desirable for harvesting solar
energy.33 An analogue of this ligand was synthesised with
methyl groups capping the terminal α-positions of the oli-
gothiophene chains to inhibit polymerisation; this material
provided electrochemically stable complexes.34
Our family of ligands has grown to include methyl end-
capped terthiophene (3T), quinquethiophene (5T) and sep-
tithiophene (7T) chains which we have previously exploited as
precursors in the synthesis of new tetrathiafulvalenes57–59 and
spirocyclic germanium complexes60,61 for applications in
organic field effect transistor (OFET) and organic solar cell
(OSC) devices, respectively. These ligands also bear hexyl
groups to improve solubility. Here, we continue these studies
by reporting the synthesis of monoanionic homoleptic Au
complexes using 3T, 5T and 7T as 34TDT ligands.
The complexes have been characterised by electrochemical
and spectroscopic methods and the optoelectronic properties
of the neutral complexes have been probed using electronic
absorption spectroelectrochemistry. Alongside this we
attempted the synthesis of a complementary Ni complex which
proved to be poorly stable, although we did obtain a single
crystal of the complex [Ni(3T)2]2− in a partially oxidised state,
the structure of which is also presented.
Scheme 1 Synthesis of ligand precursors 12–14 and the Au complexes
thereof.
CH2Cl2/AcOH. Finally, the 34TDT core was obtained by Paal–
Knorr cyclisation using P4S10 and NaHCO3 in 1,4-dioxane to
produce the ring-fused oligothiophenes 12–14.
Results and discussion
Synthesis
The final complexes were obtained (Scheme 1) by treating a
THF solution of the desired ligand precursor 12–14 with
freshly made NaOMe prior to addition of K[AuCl4] followed
immediately by a tetra(alkyl)ammonium bromide to provide the
complexes in moderate to high yields. Purification was achieved
by recrystallisation and verified by elemental analysis. Although
the elemental analysis results for the largest complex [Au(7T)2]−
showed some deviation from the predicted values, the results of
optical and vibrational spectroscopy and its electrochemical
properties are consistent with the other members of the series
therefore we will include it within our discussion.
All of the ligand precursors and complexes were synthesised by
the same process (Scheme 1). Full experimental details can be
found in our earlier publications57,58,60,62 and the ESI.†
Hexafluorophosphate salt 1 was treated with NaHS·H2O in a
3 : 1 mixture of DMF/AcOH to produce the 1,3-dithiol-2-thione
2.63 Compound 2 was dissolved in tetrahydrofuran and treated
twice, sequentially, at −78 °C with LDA followed by the corres-
ponding 2-thienyl aldehyde 3–5. The temperature had to be
carefully controlled throughout this process. This provided
intermediate 1,4-diols which were then rapidly oxidised by
MnO2 in CH2Cl2 to give the desired 1,4-diketone intermediates
Electronic absorption spectroscopy
6–8. Transchalcogenation of the 1,3-dithiol-2-thiones to 1,3- The absorption spectra of the complexes (Fig. 1 and Table 1)
dithiol-2-ones 9–11 was achieved with Hg(OAc)2 in a mixture of were recorded in CH2Cl2 solution and are dominated by
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A characteristic low energy absorption which stretched
towards the NIR region is also observed for each of the com-
plexes and is associated with d → d transitions over the
large Au atom (Fig. 1, inset).64–66 Increasing the ligand size
from [Au(3T)2]− to [Au(5T)2]− results in a notable red-shift of
this band however it does not shift further when the chain
length increases to [Au(7T)2]−. The fact that this band is
shifting to longer wavelengths as the oligothiophene chain
increases in length serves to indicate that there is indeed
some electronic communication with the delocalised dithio-
lene core.
Combined, these results indicate that while the Au dithio-
lene core itself does have interaction with the delocalised
π-system of the ligands, it can facilitate only limited electronic
communication between the π-systems of the two ligands, the
extent of which decreases with increasing chain length.
Fig. 1 Electronic absorption spectra for complexes [Au(3T)2]−, [Au
(5T)2]− and [Au(7T)2]− with low energy d–d transitions highlighted (inset).
Vibrational spectroscopy
Fourier transform infra-red (FTIR) spectroscopy of the ligands
and complexes was performed using KBr pellets (Fig. 2 and
Table S1†). There is good complementarity in the fingerprint
regions of both the ligands and the complexes.
intense bands in the visible region associated with π → π* tran-
sitions over the oligothiophene chains. As may be expected,
the π → π* transitions become red-shifted as the conjugation
length of the oligothiophene ligand increases. For [Au(3T)2]−
this band has a λmax at 350 nm whereas complexes [Au(5T)2]−
and [Au(7T)2]− have λmax values of 441 and 481 nm respect-
ively. These are typical values for oligothiophenes of these
chain lengths and are only slightly red-shifted in comparison
with the ligands 3T–7T.
Common features include two high energy absorptions at
∼255 nm and ∼300 nm which are significantly more intense in
complex [Au(5T)2]−. [Au(7T)2]− displays substructure featuring
aspects of both of the smaller analogues, including a peak at
∼345 nm which coincides with the lowest energy π → π* band
for [Au(3T)2]−; the peak maximum for [Au(5T)2]− correlates
closely with a shoulder in the spectrum of [Au(7T)2]− indicat-
ing that some spectral characteristics of the smaller chains are
maintained as the chain length increases.
The magnitude of the difference in extinction coefficients
of the π → π* transitions between the free ligand and the
complex decreases systematically as the chain length increases.
It more than doubles in the case of 3T but increases by only a
third in the case of 7T. Calculation of the energy of these π →
π* transitions using the onset wavelengths reveals that they
have only decreased by 0.04–0.07 eV with respect to their pre-
cursor ligands.
The ligands 3T–7T display peaks at ∼1650 and ∼1700 cm−1
due to the central carbonyl of the thieno-[3,4,d]-1,3-dithiol-2-
one moiety. This correlates reasonably well with reported
values, albeit at slightly higher wavenumber, and can be
assigned confidently due to the absence of this peak in the
FTIR spectra of the complexes.67,68 Aromatic overtones of the
thiophene rings dominate the spectrum from 1650–1400 cm−1
(ref. 69) although a peak in this region between ∼1560 cm−1 is
only observed in the metal complexes (hidden in [Au(7T)2]−)
while a weak to medium intensity peak at ∼1377 cm−1 arises
from deformations of the terminal –CH3 groups.69,70 A sharp
peak at ∼1285 cm−1 is only present in the complexes. By
comparing with Au complexes of 1,2-benzenedithiolate the
absence of any intense bands around ∼1085 cm−1 indicates
that the ligands have no significant radical character.64,66 The
remaining notable characteristics are bands of moderate to
strong intensity between 850–785 cm−1 assigned to aromatic
C–H stretches of the remaining β-hydrogens on the thiophene
backbone.69,70
Cyclic voltammetry
Cyclic voltammetry was used to examine the electrochemical
properties of the complexes (Fig. 3 and Table 2). All of the
Table 1 Peak maxima, extinction coefficients and optical π → π* and d → d energy gaps for ligands 3T, 5T and 7T and complexes [Au(3T)2]−, [Au
(5T)2]− and [Au(7T)2]
λmax/nm
εmax × 103/mol−1 L cm−1
π → π* energy gap/eV
d → d energy gap/eV
3T
5T
7T
271(sh), 345
262, 302, 435
260(sh), 339, 467
254, 310, 349, 543
258, 299, 362(sh), 441, 616
251, 307, 342, 438(sh), 481, 611
19.7
45.3
61.2
42.6
78.6
83.2
3.03
2.44
2.29
2.99
2.37
2.24
—
—
—
1.64
1.42
1.27
[Au(3T)2]−
[Au(5T)2]−
[Au(7T)2]−
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Fig. 2 FTIR spectra for ligand precursors 3T, 5T and 7T and complexes
[Au(3T)2]−, [Au(5T)2]− and [Au(7T)2]−.
Fig. 3 CV scans for the three gold complexes [Au(3T)2]−, [Au(5T)2]− and
[Au(7T)2]− showing oxidation (upper) and reduction (lower) events.
complexes are significantly more easily oxidised than their
ligand precursors57,58 and demonstrate a number of reversible
or quasi-reversible processes. The complexity of the waveform
increases and the reversibility decreases with increasing chain
length.
Table 2 Electrochemical data for [Au(3T)2]−, [Au(5T)2]− and [Au(7T)2]−
[Au(3T)2]−
[Au(5T)2]−
[Au(7T)2]−
The oxidative behaviour of aromatic Au dithiolene com-
plexes is predominantly ligand based while the reductive
behaviour is more metal centered.64,66 which can allow for
great control to be exerted over the complexes’ oxidation
potentials.65
E1ox/V
E2ox/V
E3ox/V
E4ox/V
+0.61/0.57
+0.69/0.64
+1.17/1.15
—
+0.43/0.38
+0.53/0.47
+0.88/0.80
+1.12/1.04
−1.04/0.95q
—
+0.31/0.28
+0.83/0.63
+0.99/0.95
+1.04irr
E1red/V
−1.30/1.19q
—
−1.00irr
−1.53irr
1.04
E2red/V
HOMO–LUMO gap/eVa
1.68
1.19
In this series of compounds all major oxidative processes
shift to lower potential as the oligothiophene chain lengthens,
due to the increase in the number of π-electrons and extent of
delocalisation, which result in stronger electron donating pro-
perties. Using the first oxidation potential as an example, the
half-wave potential decreases steadily: E1/2 of +0.59 V for [Au
a Electrochemical HOMO–LUMO gap calculated as the difference
between the onset of the first oxidation and reduction waves. qQuasi-
reversible peak. irrIrreversible peak.
(3T)2]−, +0.40 V for [Au(5T)2]− and +0.29 V for [Au(7T)2]−. On peaks and agree well with the d → d transition energies calcu-
the other hand, reduction processes shift to a more positive lated from the absorption spectra.
potential in a similar fashion. The trend in reduction poten-
tials coupled with the low onset of oxidation in all cases
further indicates that some electronic coupling between the
Some specific comments about each complex follow:
[Au(3T)2]−
This complex displays greatly improved electrochemical
ligand π-system and the metal dithiolene core does indeed reversibility when compared to the analogous non-capped
occur.
complex which only had a single irreversible oxidation at +0.57
Electrochemical HOMO–LUMO gaps were calculated from V.34 It has closely overlapping first and second oxidations with
the difference in the onsets of the first oxidation and reduction peak profiles and separations that indicate fairly good reversi-
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bility. In keeping with our previous observations we have
assigned this to sequential radical cation formation on each
terthiophene, which is localised primarily over the ligands. A
third quasi-reversible oxidation occurs at E1/2 of +1.16 V, of
which the narrow profile of the reverse peak may indicate
some adsorption of the triply charged complex to the electrode
surface. The complex undergoes a single quasi-reversible
reduction at −1.26 V.
[Au(5T)2]−
In complex [Au(5T)2]−, the first oxidation peaks are similar
to those observed for complex [Au(3T)2]−. The next region of
the voltammogram shows no clearly defined peaks although
there is evidence for a further oxidation occurring at approxi-
mately +0.83 V followed by a fourth, more easily discernible
quasi-reversible peak centred at E1/2 = +1.08 V. This sequence
of events is likely due to dication formation on each of the
quinquethiophene chains. The complex undergoes a single
quasi-reversible reduction at E1/2 = −1.00 V and an irreversible
reduction at −1.48 V.
[Au(7T)2]−
Complex [Au(7T)2]− displays a voltammogram with a shape
that is largely complementary to that of [Au(5T)2]−. A first
reversible oxidation is centered at +0.28 V with another oxi-
dation at +0.97 V. There appears to be at least one quasi-revers-
ible oxidation in the broad waveform between these two oxi-
dations. An irreversible shoulder can be identified at +1.04 V
prior to the final quasi-reversible formation of a second dica-
tion at +1.27 V. The complex undergoes two irreversible
reductions at −1.00 and −1.53 V. The magnitude of the current
response of these reductions is very large and no reversibility
is apparent.
Computational studies
DFT methods were used to gain a further insight into the elec-
tronic properties of [Au(3T)2]− and [Au(5T)2]−. The orbital con-
tours for [Au(3T)2]− and [Au(5T)2]− are shown in Fig. 4 and 5
respectively. As it was desirable to see how the hexyl chains
influence the geometry adopted by the ligands upon complexa-
tion they were included in the calculation. Similar obser-
vations can be made for both complexes:
(1) In both cases steric hindrance results in significant
twisting of the terminal thiophene rings of the oligothiophene
chains
(2) The frontier orbitals of the HOMO manifold are deloca-
lised over both the oligothiophene chains and metal dithio-
lene core. In [Au(5T)2]− it is not until the HOMO−4 that purely
oligothiophene based contours begin to be observed while in
[Au(3T)2]− even the HOMO−5 contains some influence from
the coordinating sulfurs.
(3) The LUMO is localised almost exclusively over the gold-
dithiolene centre for both complexes and is close in energy to
the higher HOMO orbitals which correlates well with the low
energy d → d band observed in the absorption spectra.
(4) The LUMO+1 and higher lie at significantly more posi-
tive energies than the LUMO and are delocalised over the con-
jugated π systems of the ligands with small contributions from
Fig. 4 Frontier orbital profiles and energies for the [Au(3T)2]− anion.
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between the HOMO, the HOMO−1 and the HOMO−2 are rela-
tively small, it appears that the longer conjugation length of
the 5T ligand leads to it having an increased influence on the
frontier HOMOs than the less extended 3T ligand.
Paying particular attention to the mixed oligothiophene/
dithiolene nature of the HOMO orbitals, these results clearly
indicate the presence of constructive electronic interplay
between the ligands and the gold-dithiolene core. Overall, the
calculated electronic structure of the gold TDT core agrees
fairly well with existing studies on both gold dithiolenes64,65,71
and other metal TDT complexes.56
Spectroelectrochemistry
Neutral Au 34TDT complexes have not as yet been isolated.
Upon oxidation of the monoanionic complexes with iodine
they yield poorly defined and insoluble polycrystalline precipi-
tates which show some paramagnetic character.39,42 UV-vis
spectroelectrochemical (SEC) measurements present one route
by which to begin to observe their spectral properties. SEC
allows changes in a molecule’s spectroscopic properties upon
oxidation or reduction to be monitored.
Thin films of the complexes were deposited on ITO slides
by drop casting. The slides were then suspended in an electro-
lytic solution and used as the working electrode of a standard
three-electrode electrochemical cell held within a UV/vis
spectrometer. A potential was applied to the film and its
absorption spectrum measured at intervals of +0.10 V in a
step-wise fashion. Upon changing potential the film was given
approximately 60 seconds to equilibrate prior to recording the
spectrum. The results of these experiments are presented in
Fig. 6. Spectroelectrochemical studies on thin films of the
ligands 5T and 7T can be found in our previous publi-
cations.57,58 The smallest ligand 3T is soluble in the electrolyte
solution therefore thin film studies were not possible.
Prior to oxidation, the thin film absorption spectra of all of
the complexes are dominated by ligand based π → π* tran-
sitions with the weak d → d transition of the Au dithiolene
core also evident. Upon oxidation a large increase in absorp-
tion across the entire visible spectrum is observed, featuring
characteristics of both the conjugated ligand and the deloca-
lised Au dithiolene centre. This is a somewhat gradual process
for [Au(3T)2]− and [Au(7T)2]− but is very sharp for [Au(5T)2]−
and remains remarkably stable at higher potentials.
The solid state SEC of bis(terthiophene) Au dithiolene
complex [Au(3T)2]− is shown in Fig. 6a. Upon increasing the
potential from 0.00 V to +0.40 V the intensity of the d → d
absorption drops slightly then increases rapidly as at least one
electron is lost with strong peaks emerging at 515 and 734 nm.
The wavelength of the first of these peaks is due to the terthio-
phene based radical cation while the second will occur over
the now neutral [AuC4S4]0 dithiolene centre. The intensity of
Fig. 5 Frontier orbital profiles and energies for the [Au(5T)2]− anion.
the coordinating dithiolene sulfur atoms. This indicates that the absorbance across the spectrum begins to stabilise at
these orbitals make a much more substantial contribution to potentials >+0.70 V.
the shorter wavelength π → π* transitions.
The SEC plot for [Au(5T)2]− (Fig. 6b) shows that upon oxi-
(5) The HOMO in [Au(3T)2]− is quite similar to the dation at +0.30 V a large and sudden spectral response occurs:
HOMO−2 of [Au(5T)2]−. Noting that the energy differences the π → π* absorption drops slightly in intensity and is red-
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shifted from 429 to 459 nm and this is typical behaviour
observed in conjugated thiophene oligomers and polymers.72
Concomitantly, the intensity of absorption across the rest of
the spectrum is raised greatly with peaks at 602 nm and
1036 nm pushing deep into the NIR. The very limited change
in the spectrum at higher potentials may indicate that it has
become insulating.
The complex [Au(7T)2]− displays characteristics similar to
those of the 5T analogue. At potentials above +0.30 V the gene-
ration of broad bands with peaks at 679 and >1100 nm respect-
ively (Fig. 6c).
X-Ray crystallography
As previously mentioned, we attempted to synthesise nickel
analogues of the gold complexes by using NiCl2·6H2O in place
of K(AuCl)4 but found them poorly stable and therefore
difficult to isolate. However we were able to isolate a small
amount of the terthiophene nickel complex by diffusion of
cyclohexane into a tetrahydrofuran solution and were sur-
prised by our findings.
A single crystal of (Et4N)2[Ni(3T)2] grew as an orange plate.
X-ray crystallography revealed that the complex displayed
varying amounts of oxidation of the coordinating sulfur
atoms. We can only attribute this partial oxidation to the
complexes having reacted with dissolved molecular oxygen.
Similar spontaneous aerobic oxidation of dithiolenes has
only been reported on a small number of occasions.73–75 This
behaviour also leads us to postulate that oxidation of our Ni
complexes could be an underlying factor of their poor stabi-
lity. The asymmetric unit of the nickel complex contains two
half complexes with the Ni lying on an inversion centre and
two tetraethylammonium cations (Fig. 7). The coordinated S
atoms are partially oxidised and have two oxygen sites mod-
elled for each S which are refined with a common occupancy
for each pair (for Ni(1) 0.19(3) and 0.50(2), for Ni(1a) 0.24(2)
and 0.42(2)).
The terthiophene chains do not adopt the all-anti confor-
mation typical of short chain oligothiophenes, presumably
due to steric hindrance between the hexyl chains and possible
Fig. 6 Spectroelectrochemistry plots for the oxidation of thin films of
(a) [Au(3T)2]−, (b) [Au(5T)2]−, and (c) [Au(7T)2]−.
Fig. 7 X-ray molecular structure determined for the partially oxidised
[Ni(3T)2]2− dianion.
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intermolecular forces between S(5)⋯O(3) (3.050(2)). The tor-
sional angles between the terminal thiophenes are 72(2)° and
73(2)° for C3–C4–C5–C6 and C2a–C1a–C10a–C11a respect-
ively and 150(2)° and 148(2) for C2–C1–C10–C11 and Ca–C4a–
C5a–C6a respectively. The root mean square deviation from
planarity for the central dithiolene rings, Ni–S2–C3–C2–S3
Notes and references
1 G. N. Schrauzer and V. Mayweg, J. Am. Chem. Soc., 1962, 84,
3221.
2 H. B. Gray, R. Williams, I. Bernal and E. Billig, J. Am. Chem.
Soc., 1962, 84, 3596–3597.
and Ni1a–S2a–C3a–C2a–S3a are 0.067
respectively.
Å
and 0.054
Å
3 A. Davison, N. Edelstein, R. H. Holm and A. H. Maki, Inorg.
Chem., 1963, 2, 1227–1232.
4 J. A. McCleverty, in Progress in Inorganic Chemistry,
ed. F. A. Cotton, John Wiley & Sons, Inc., Hoboken, NJ,
USA, 1968, vol. 10, pp. 49–221.
5 R. Eisenberg and H. B. Gray, Inorg. Chem., 2011, 50, 9741–
9751.
Conclusions
In conclusion, six new homoleptic bis(thiophene-3,4-dithio-
late) complexes featuring nickel or gold metal centres bound
between end-capped oligothiophene ligands with chain
lengths of three, five and seven thiophenes were synthesised.
The Ni complexes proved to be poorly stable due to facile auto-
oxidation as confirmed by the isolation of a crystal of a par-
tially oxidised complex, the molecular structure of which was
confirmed by X-ray crystallography.
The monoanionic formally Au(III) complexes show hybrid
optoelectronic properties resulting from interplay between the
dithiolene core and the oligothiophene ligands including low
oxidation potentials and a low energy metal centred absorp-
tion band, the position of which is sensitive to the oligothio-
phene chain length. This hybrid electronic behaviour has been
confirmed using spectroscopic and electrochemical studies
and corroborated with computational analysis of the two
smaller complexes.
Spectroelectrochemical results for thin films of the Au(III)
complexes in the solid state have shown that in all cases oxi-
dation, ostensibly to the neutral Au(IV) complex, results in a
large and sudden increase in the intensity of absorption across
the entire visible window and into the NIR, which persists at
higher potentials. Although their isolation remains challen-
ging, the interesting optoelectronic and magnetic properties of
neutral gold 34TDT complexes with extended conjugation
means that they remain intriguing targets for further develop-
ment. In particular, their broad and strong absorption across
the visible and NIR regions of the spectrum make them excel-
lent candidate materials for broadband light sensing and har-
vesting applications.76
6 M. L. Kirk, R. L. McNaughton and M. E. Helton, in
Dithiolene Chemistry. Synthesis, Properties, and Applications.
Progress in Inorganic Chemistry, ed. E. I. Stiefel and
K. D. Karlin, John Wiley & Sons, Inc., Hoboken, NJ, USA,
2004, vol. 52, pp. 111–212.
7 S. D. Cummings and R. Eisenberg, in Dithiolene Chemistry:
Synthesis, Properties, and Applications. Progress in Inorganic
Chemistry, ed. E. I. Stiefel and K. D. Karlin, John Wiley &
Sons, Inc., Hoboken, NJ, USA, 2004, vol. 52, pp. 315–367.
8 K. Wang, in Dithiolene Chemistry: Synthesis, Properties, and
Applications. Progress in Inorganic Chemistry, ed. E. I. Stiefel
and K. D. Karlin, John Wiley & Sons, Inc., Hoboken, NJ,
USA, 2004, vol. 52, pp. 267–314.
9 N. Robertson and L. Cronin, Coord. Chem. Rev., 2002, 227,
93–127.
10 R. Kato, Chem. Rev., 2004, 104, 5319–5346.
11 M. Bousseau, L. Valade, J. P. Legros, P. Cassoux,
M. Garbauskas and L. V. Interrante, J. Am. Chem. Soc.,
1986, 108, 1908–1916.
12 D. G. Branzea, F. Pop, P. Auban-Senzier, R. Clérac,
P. Alemany, E. Canadell and N. Avarvari, J. Am. Chem. Soc.,
2016, 138, 6838–6851.
13 T. D. Anthopoulos, G. C. Anyfantis, G. C. Papavassiliou and
D. M. De Leeuw, Appl. Phys. Lett., 2007, 90, 122105.
14 T. D. Anthopoulos, S. Setayesh, E. Smits, M. Cölle,
E. Cantatore, B. De Boer, P. W. M. Blom and D. M. De
Leeuw, Adv. Mater., 2006, 18, 1900–1904.
15 S. Dalgleish, J. G. Labram, Z. Li, J. Wang, C. R. McNeill,
T. D. Anthopoulos, N. C. Greenham and N. Robertson,
J. Mater. Chem., 2011, 21, 15422–15430.
16 P. Deplano, L. Pilia, D. Espa, M. L. Mercuri and A. Serpe,
Coord. Chem. Rev., 2010, 254, 1434–1447.
Conflicts of interest
17 D. Espa, L. Pilia, C. Makedonas, L. Marchiò, M. L. Mercuri,
A. Serpe, A. Barsella, A. Fort, C. A. Mitsopoulou and
P. Deplano, Inorg. Chem., 2014, 53, 1170–1183.
18 A. Zarkadoulas, E. Koutsouri and C. A. Mitsopoulou, Coord.
Chem. Rev., 2012, 256, 2424–2434.
There are no conflicts to declare.
19 W. R. McNamara, Z. Han, C.-J. Yin, W. W. Brennessel,
P. L. Holland and R. Eisenberg, Proc. Natl. Acad. Sci. U. S.
A., 2012, 109, 15594–15599.
Acknowledgements
PJS and IAW acknowledge the EPSRC for funding. IAW
acknowledges Loughborough University and the Royal Society 20 W. R. McNamara, Z. Han, P. J. Alperin, W. W. Brennessel,
of Chemistry Research Fund (Grant No. RF18-5353) for
funding.
P. L. Holland and R. Eisenberg, J. Am. Chem. Soc., 2011,
133, 15368–15371.
114 | Dalton Trans., 2019, 48, 107–116
This journal is © The Royal Society of Chemistry 2019

View Article Online
Dalton Transactions
Paper
21 J. Roncali, Chem. Rev., 1992, 92, 711–738.
22 T. P. Kaloni, P. K. Giesbrecht, G. Schreckenbach and
M. S. Freund, Chem. Mater., 2017, 29, 10248–10283.
44 D. Belo, M. J. Figueira, J. P. M. Nunes, I. C. Santos,
L. C. Pereira, V. Gama, M. Almeida and C. Rovira, J. Mater.
Chem., 2006, 16, 2746–2756.
23 A. L. Kanibolotsky, N. J. Findlay and P. J. Skabara, Beilstein 45 J. P. M. Nunes, M. J. Figueira, D. Belo, I. C. Santos,
J. Org. Chem., 2015, 11, 1749–1766.
B. Ribeiro, E. B. Lopes, R. T. Henriques, J. Vidal-Gancedo,
J. Veciana, C. Rovira and M. Almeida, Chem. – Eur. J., 2007,
13, 9841–9849.
24 T. M. S. K. Pathiranage, D. S. Dissanayake, C. N. Niermann,
Y. Ren, M. C. Biewer and M. C. Stefan, J. Polym. Sci., Part A:
Polym. Chem., 2017, 55, 3327–3346.
25 A. Mishra, C. Q. Ma and P. Bäuerle, Chem. Rev., 2009, 109,
1141–1176.
46 D. Belo, J. Mendonça, I. C. Santos, L. C. J. Pereira,
M. Almeida, J. J. Novoa, C. Rovira, J. Veciana and V. Gama,
Eur. J. Inorg. Chem., 2008, 5327–5337.
26 L. Zhang, N. S. Colella, B. P. Cherniawski, 47 D. Belo, L. C. Jesus Pereira, M. Almeida, C. Rovira,
S. C. B. Mannsfeld and A. L. Briseno, ACS Appl. Mater.
Interfaces, 2014, 6, 5327–5343.
27 T. Otsubo, Y. Aso and K. Takimiya, J. Mater. Chem., 2002,
12, 2565–2575.
J. Veciana and V. Gama, Dalton Trans., 2009, 4176–4180.
48 A. I. S. Neves, J. C. Dias, B. J. C. Vieira, I. C. Santos,
M. B. Castelo Branco, L. C. J. Pereira, J. C. Waerenborgh,
M. Almeida, D. Belo and V. da Gama, CrystEngComm, 2009,
11, 2160–2168.
28 C. L. Kean and P. G. Pickup, Chem. Commun., 2001, 815–816.
29 C. L. Kean, D. O. Miller and P. G. Pickup, J. Mater. Chem., 49 A. I. S. Neves, I. C. Santos, L. C. J. Pereira, C. Rovira, E. Ruiz,
2002, 12, 2949–2956. D. Belo and M. Almeida, Eur. J. Inorg. Chem., 2011, 4807–4815.
30 T. Anjos, S. J. Roberts-Bleming, A. Charlton, N. Robertson, 50 D. Belo, A. I. S. Neves, L. C. J. Pereira and M. Almeida, Phys.
A. R. Mount, S. J. Coles, M. B. Hursthouse, M. Kalaji and
P. J. Murphy, J. Mater. Chem., 2008, 18, 475–483.
31 S. Dalgleish, C. A. Morrison, D. S. Middlemiss,
A. R. Mount, A. Collins, L. Pilia, A. Serpe, M. L. Mercuri,
Status Solidi, 2012, 9, 1199–1201.
51 E. Laukhina, V. Lebedev, V. Laukhin, A. P. del Pino,
E. B. Lopes, A. I. S. Neves, D. Belo, M. Almeida, J. Veciana
and C. Rovira, Org. Electron., 2012, 13, 894–898.
S. J. Roberts-Bleming, A. Charlton, P. Deplano, P. J. Murphy 52 R. Silva, B. Vieira, M. Andrade, I. Santos, S. Rabaça,
and N. Robertson, J. Mater. Chem., 2009, 19, 6194–6200.
32 S. Dalgleish and N. Robertson, Coord. Chem. Rev., 2010,
254, 1549–1558.
33 C. Pozo-Gonzalo, R. Berridge, P. J. Skabara, E. Cerrada,
M. Laguna, S. J. Coles and M. B. Hursthouse, Chem.
Commun., 2002, 2408–2409.
E. Lopes, J. Coutinho, L. Pereira, M. Almeida and D. Belo,
Molecules, 2018, 23, 424.
53 A. I. S. Neves, I. C. Santos, J. T. Coutinho, L. C. J. Pereira,
R. T. Henriques, E. B. Lopes, H. Alves, M. Almeida and
D. Belo, Eur. J. Inorg. Chem., 2014, 2014, 3989–3999.
54 T. Higashino, O. Jeannin, T. Kawamoto, D. Lorcy, T. Mori
and M. Fourmigué, Inorg. Chem., 2015, 54, 9908–9913.
34 P. J. Skabara, C. Pozo-Gonzalo, N. Lardiés Miazza,
M. Laguna, E. Cerrada, A. Luquin, B. González, S. J. Coles, 55 C. M. Amb and S. C. Rasmussen, Synth. Met., 2009, 159,
M. B. Hursthouse, R. W. Harrington and W. Clegg, Dalton
Trans., 2008, 3070–3079.
35 S. Dalgleish, H. Yoshikawa, M. M. Matsushita, K. Awaga
and N. Robertson, Chem. Sci., 2011, 2, 316–320.
36 E. Allwright, G. Silber, J. Crain, M. M. Matsushita, K. Awaga
and N. Robertson, Dalton Trans., 2016, 45, 9363–9368.
2390–2393.
56 C. M. Amb, C. L. Heth, S. J. Evenson, K. I. Pokhodnya and
S. C. Rasmussen, Inorg. Chem., 2016, 55, 10978–10989.
57 I. A. Wright, P. J. Skabara, J. C. Forgie, A. L. Kanibolotsky,
B. González, S. J. Coles, S. Gambino and I. D. W. Samuel,
J. Mater. Chem., 2011, 21, 1462–1469.
37 J. Wang, L.-X. Shi, J.-Y. Wang, J.-X. Chen, S.-H. Liu and 58 I. A. Wright, N. J. Findlay, S. Arumugam, A. R. Inigo,
Z.-N. Chen, Dalton Trans., 2017, 46, 2023–2029.
A. L. Kanibolotsky, P. Zassowski, W. Domagala and
38 D. Belo, H. Alves, E. B. Lopes, V. Gama, R. T. Henriques,
P. J. Skabara, J. Mater. Chem. C, 2014, 2, 2674–2683.
M. T. Duarte, M. Almeida, A. Perez-Benitez, C. Rovira and 59 R. G. D. Taylor, J. Cameron, I. A. Wright, N. Thomson,
J. Veciana, Synth. Met., 2001, 120, 699–702.
O. Avramchenko, A. L. Kanibolotsky, A. R. Inigo, T. Tuttle
39 D. Belo, H. Alves, E. B. Lopes, M. T. Duarte, V. Gama,
and P. J. Skabara, Beilstein J. Org. Chem., 2015, 11, 1148–1154.
R. Teives Henriques, M. Almeida, A. Pérez-Benítez, 60 I. A. Wright, A. L. Kanibolotsky, J. Cameron, T. Tuttle,
C. Rovira and J. Veciana, Chem. – Eur. J., 2001, 7, 511–519.
40 D. Belo, M. J. Figueira, J. Mendonça, I. C. Santos,
M. Almeida, R. T. Henriques, M. T. Duarte, C. Rovira and
J. Veciana, Eur. J. Inorg. Chem., 2005, 3337–3345.
41 A. I. S. Neves, I. C. Santos, D. Belo and M. Almeida,
CrystEngComm, 2009, 11, 1046–1053.
P. J. Skabara, S. J. Coles, C. T. Howells, S. A. J. Thomson,
S. Gambino and I. D. W. Samuel, Angew. Chem., Int. Ed.,
2012, 51, 4562–4567.
61 S. Arumugam, I. A. Wright, A. R. Inigo, S. Gambino,
C. T. Howells, A. L. Kanibolotsky, P. J. Skabara and
I. D. W. Samuel, J. Mater. Chem. C, 2014, 2, 34–39.
42 D. Belo and M. Almeida, Coord. Chem. Rev., 2010, 254, 62 P. J. Skabara, I. M. Serebryakov, D. M. Roberts,
1479–1492.
I. F. Perepichka, S. J. Coles and M. B. Hursthouse, J. Org.
Chem., 1999, 64, 6418–6424.
43 D. Belo, H. Alves, S. Rabaça, L. C. Pereira, M. T. Duarte,
V. Gama, R. T. Henriques, M. Almeida, E. Ribera, C. Rovira 63 F. S. Guziec, J. M. Russo, F. F. Torres, G. C. Long and
and J. Veciana, Eur. J. Inorg. Chem., 2001, 2001, 3127–3133.
M. R. Tellez, J. Chem. Soc., Perkin Trans. 1, 1989, 1068–1070.
This journal is © The Royal Society of Chemistry 2019
Dalton Trans., 2019, 48, 107–116 | 115

View Article Online
Paper
Dalton Transactions
64 K. Ray, T. Weyhermüller, F. Neese and K. Wieghardt, Inorg. 71 S. Kokatam, K. Ray, J. Pap, E. Bill, W. E. Geiger,
Chem., 2005, 44, 5345–5360.
R. J. LeSuer, P. H. Rieger, T. Weyhermüller, F. Neese and
65 N. C. Schiodt, P. Sommer-Larsen, T. Bjornholm,
K. Wieghardt, Inorg. Chem., 2007, 46, 1100–1111.
M. F. Nielsen, J. Larsen and K. Bechgaard, Inorg. Chem., 72 R. Berridge, S. P. Wright, P. J. Skabara, A. Dyer, T. Steckler,
1995, 34, 3688–3694.
A. A. Argun, J. R. Reynolds, R. W. Harrington and W. Clegg,
J. Mater. Chem., 2007, 17, 225–231.
73 G. N. Schrauzer, C. Zhang and R. Chadha, Inorg. Chem.,
1990, 29, 4104–4107.
74 R. Bolligarla and S. K. Das, Eur. J. Inorg. Chem., 2012, 2012,
2933–2939.
75 E. J. Uzelac and S. C. Rasmussen, Eur. J. Inorg. Chem., 2017,
2017, 3878–3883.
66 K. Ray, T. Weyhermüller, A. Goossens, M. W. J. Crajé and
K. Wieghardt, Inorg. Chem., 2003, 42, 4082–4087.
67 L. Y. Chiang, P. Shu, D. Holt and D. Cowan, J. Org. Chem.,
1983, 48, 4713–4717.
68 X. Chen, N. R. de Tacconi and R. L. Elsenbaumer, J. Org.
Chem., 2009, 74, 9188–9190.
69 D. Williams and I. Fleming, Spectroscopic Methods in
Organic Chemistry, McGraw-Hill Publishing Company, 76 D. Cortizo-Lacalle, C. T. Howells, S. Gambino, F. Vilela,
London, 5th edn, 1995.
70 C. Wang, J. L. Schindler, C. R. Kannewurf and
M. G. Kanatzidis, Chem. Mater., 1995, 7, 58–68.
Z. Vobecka, N. J. Findlay, A. R. Inigo, S. A. J. Thomson,
P. J. Skabara and I. D. W. Samuel, J. Mater. Chem., 2012, 22,
14119–14126.
116 | Dalton Trans., 2019, 48, 107–116
This journal is © The Royal Society of Chemistry 2019