Dinuclear rhenium complexes as redox-active pendants in a novel electrodeposited polycyclopentadithiophene material

Article abstract of DOI:10.1021/ic501840p

The novel [Re₂(μ-H)(μ-OOC-CPDT)(CO)₆(μ-3-Me-pydz)] complex (1; OOC-CPDT = 4H-cyclopenta[2,1-b:3,4-b′]dithiophene-4-carboxylate, 3-Me-pydz = 3-methylpyridazine) has been prepared and characterized by single-crystal X-ray diffraction, density functional theory (DFT), and time-dependent DFT computations, UV-vis absorption and emission spectroscopy, and cyclic voltammetry (CV). The measured properties indicate the lack of electronic communication in the ground state between the CPDT and the rhenium diazine moieties. Oxidative electropolymerization of 1, achieved by repeated potential cycling (-0.4 to +1.0 V vs Fc⁺/Fc, in acetonitrile) with different supporting electrolytes, on different electrodes, afforded an electroactive and stable metallopolymer (poly-1). In situ measurements of the mass of the growing film (on a gold electrode, with an electrochemical quartz microbalance) confirmed the regularity of the polymerization process. The polymer exhibits two reversible oxidation peaks of the thiophene chain and a broad irreversible reduction peak (-1.4 V, quite close to that observed for the reduction of monomer 1), associated with a remarkably delayed sharp return peak, of comparable associated charge, appearing in close proximity (+0.3 V) to the first oxidation peak of the neutral polythiophene chain. This charge-trapping effect can be observed upon repeated cycles of p and n doping, and the negative charge is maintained even if the charged electrode is removed from the solution for many hours. Electrochemical impedance spectroscopy showed that the main CV oxidation peak corresponds to facile charge transfer, combined with very fast diffusion of both electrons and ions within the polymer. In summary, poly-1 provides a new example of a metallopolymer, in which the conductive properties of the π-conjugated system are added to the redox behavior of the pendant-isolated complexes. (Figure Presented).

Full text of DOI:10.1021/ic501840p

Article
pubs.acs.org/IC
Dinuclear Rhenium Complexes as Redox-Active Pendants in a Novel
Electrodeposited Polycyclopentadithiophene Material
†
†
,†,‡
,†,‡
†,§
Elsa Quartapelle Procopio, Valentina Bonometti, Monica Panigati,* Pierluigi Mercandelli,*
,†,‡,§
⊥
†,‡
Patrizia R. Mussini,*
Tiziana Benincori, Giuseppe D’Alfonso, and Francesco Sannicolo
̀
†
Dipartimento di Chimica, Universita
̀
degli Studi di Milano, via Golgi 19, 20133 Milano, Italy
di Ricerca di Milano del Consorzio INSTM, via Golgi 19, 20133 Milano, Italy
Centro Interdisciplinare Materiali e Interfacce Nanostrutturati, via Celoria 16, 20133 Milano, Italy
degli Studi dell’Insubria, via Valleggio 11, 22100 Como, Italy
‡
Unita
̀
§
⊥Dipartimento di Scienza e Alta Tecnologia, Universita
S Supporting Information
̀
*
ABSTRACT: The novel [Re (μ-H)(μ-OOC-CPDT)(CO) (μ-3-Me-pydz)]
2
6
complex (1; OOC-CPDT = 4H-cyclopenta[2,1-b:3,4-b′]dithiophene-4-
carboxylate, 3-Me-pydz = 3-methylpyridazine) has been prepared and
characterized by single-crystal X-ray diffraction, density functional theory
(
DFT), and time-dependent DFT computations, UV−vis absorption and
emission spectroscopy, and cyclic voltammetry (CV). The measured
properties indicate the lack of electronic communication in the ground state
between the CPDT and the rhenium diazine moieties. Oxidative electro-
polymerization of 1, achieved by repeated potential cycling (−0.4 to +1.0 V vs
+
Fc /Fc, in acetonitrile) with different supporting electrolytes, on different
electrodes, afforded an electroactive and stable metallopolymer (poly-1). In
situ measurements of the mass of the growing film (on a gold electrode, with
an electrochemical quartz microbalance) confirmed the regularity of the
polymerization process. The polymer exhibits two reversible oxidation peaks of the thiophene chain and a broad irreversible
reduction peak (−1.4 V, quite close to that observed for the reduction of monomer 1), associated with a remarkably delayed
sharp return peak, of comparable associated charge, appearing in close proximity (+0.3 V) to the first oxidation peak of the
neutral polythiophene chain. This charge-trapping effect can be observed upon repeated cycles of p and n doping, and the
negative charge is maintained even if the charged electrode is removed from the solution for many hours. Electrochemical
impedance spectroscopy showed that the main CV oxidation peak corresponds to facile charge transfer, combined with very fast
diffusion of both electrons and ions within the polymer. In summary, poly-1 provides a new example of a metallopolymer, in
which the conductive properties of the π-conjugated system are added to the redox behavior of the pendant-isolated complexes.
INTRODUCTION
steric constraints of the metal complexes can orient the π-
conjugated materials into specific solid-state arrangements,
affecting their conductivity properties.
■
Many efforts have been devoted to incorporating metal centers
in conjugated polymers, pointing toward expanding function1-
Many synthetic routes to metallopolymers have been
alities and performances in many fields, such as sensing,
2
3
4
developed. The electrochemical polymerization of monomers
catalysis, photovoltaics, electroluminescence, information
10
5
6
containing transition-metal complexes has been used for
functionalized fluorene or vinylene monomers or for monomers
containing nitrogen heterocycles, while only a few examples
concerned thiophenes functionalized with substituents capable
of anchoring transition metals (usually polypyridines, as ligands
for ruthenium, iron, platinum, and, in very few cases,
storage, and electrochromism. To tune the properties of the
metallopolymers, different metal centers have been used, from
transition to main-group metals to lanthanides. Moreover,
different metal−polymer linkages have been exploited, ranging
from strong covalent bonds to more labile coordination
7
8
interactions, resulting in “static” or “dynamic” binding modes.
11,7e,g
rhenium).
The metal centers have been either incorporated into the
conducting skeleton, with strong coupling between the metal
orbitals and those of the π-conjugated system, or bonded as
We report here the characterization of a new metallopolymer
obtained by electropolymerization of the dinuclear
7e,9
hydridocarbonylrhenium(I) complex [Re (μ-H)(μ-OOC-
pendants of the polymer chain.
In this arrangement, the
2
CPDT)(CO) (μ-3-Me-pydz)] (1, Chart 1), where 3-Me-pydz
metal complexes are not involved in the p conductivity of the
backbone but can make available a parallel conduction pathway,
through an “outer-sphere” charge-transfer mechanism, provided
6
Received: July 29, 2014
7
e,9
they have redox properties.
Moreover, the geometric and
©
XXXX American Chemical Society
A
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16
Chart 1. Chemical Structure of 1
[Re (μ -H) (CO) ] with 2 equiv of 3-Me-pydz. Dihydrido
4 3 4 12
complexes like 2 can be easily functionalized, owing to their
high reactivity toward acid molecules. This reactivity was
exploited to connect a COOH-functionalized CPDT molecule
to the dinuclear rhenium moiety (Scheme 1). The treatment of
2
with a slight excess of CPDT-COOH (toluene, reflux, 32 h)
resulted in H evolution and coordination of the CPDT-
containing fragment through the carboxylate anion. The H
2
1
NMR spectrum showed, besides the pyridazine signals, one
hydride resonance (δ −6.80 ppm) and the expected five
resonances of the CPDT moiety due to the C symmetry of 1.
1
Crystals of 1 suitable for X-ray analysis were obtained by the
slow diffusion of n-hexane (−25 °C) into a saturated CH Cl
2
2
solution.
indicates 3-methylpyridazine and CPDT the 4H-cyclopenta-
Solid-State Structure. The structure of the complex has
been determined by single-crystal X-ray analysis. Figure 1
[
2,1-b;3,4-b′]dithiophene-4-yl group. The CPDT system was
chosen for its very high polymerogenic ability, independent of
the bulkiness of the substituents in position 4, and for the
12
generally high stability of the resulting polymers. Moreover,
CPDT-based materials display a wide range of interesting
physical properties, including high electrical conductivity, low
band gap, and extended π conjugation. In addition, it is possible
to functionalize these materials because a wide range of groups
can be bonded in the 4 position, allowing one to tune the
properties of the material. In 1995, Zotti et al. reported the
preparation and electropolymerization of cyclopentadithio-
13
phenes carrying redox centers, such as ferrocene groups.
Examples are also known of CPDT polymers containing a
ruthenium(II) phenanthroline complex hanging on a CPDT
backbone.
Figure 1. Frontal and lateral views of 1 with a partial labeling scheme.
Only the major component of the disordered 3-methylpyridazine
ligand is depicted. Displacement ellipsoids are drawn at the 50%
probability level. Hydrogen atoms are given arbitrary radii. Color code:
C, gray; H, white; N, blue; O, red; Re, teal; S, yellow.
1
4
The dinuclear rhenium complex anchored to the CPDT
backbone belongs to a family of neutral carbonylrhenium(I)
complexes with the general formula [Re (μ-X) (CO) (μ-1,2-
2
2
6
15
16
diazine)], where X can be halogen, hydride, or alkoxide or
thiophenoxide. Some of them have recently gained interest
for their intense yellow/green emission, occurring from triplet
1
7
shows frontal and lateral views of the molecule. In this
dinuclear species, each rhenium atom attains a distorted
octahedral coordination and bears three terminal carbonyl
ligands in a facial arrangement, one of the nitrogen atoms of the
bridging pyridazine ligand, a bridging hydrido ligand, and one
of the two oxygen atoms of the bridging 4H-cyclopenta[2,1-
b:3,4-b′]dithiophene-4-carboxylato ligand. Because of the
presence of the methyl substituent on the pyridazine, the
molecule is overall asymmetrical. This asymmetry is particularly
evident in the skewed conformation adopted by the CPDT-
carboxylato ligand, as measured by the H(1)···C(8)−C(34)−
H(34) torsional angle of 145.0° (in a symmetrical structure, it
should be 0° or 180°). This conformation maximizes the
intramolecular distance between the pyridazine and cyclo-
pentadithiophene rings. However, the main structural motif
observed in the crystal is the π−π stacking between the
pyridazine and cyclopentadithiophene rings of two adjacent
molecules (see Figure S1 in the Supporting Information, SI).
Computational Study. The new rhenium complex 1 was
studied by means of density functional theory (DFT) and time-
dependent DFT (TD-DFT) computations. Two optimized
structures have been found, differing in the rotation of the
cyclopentadithiophene substituent around the C(8)−C(34)
bond. One of them is similar to the conformer observed in the
solid state, having an H(1)···C(8)−C(34)−H(34) torsional
3
metal-to-ligand charge-transfer ( MLCT) states, showing the
modulation effect of the diazine substituents on energy,
1
8
lifetimes, and quantum yields of the emission. These
properties allowed their successful use as phosphorescent
19,20
dopants in organic light-emitting devices.
A distinctive
feature of this class of complexes is the relatively low-lying
lowest unoccupied molecular orbital (LUMO) level, localized
on the bridging diazine ligand. Here this feature has been
exploited to obtain, by electroplymerization of 1, a material that
joins the conductive properties of a thiophene polymer to the
easy reducibility of the pendant complex.
RESULTS AND DISCUSSION
Synthesis and Characterization of the Monomer 1.
The starting material was the electronically unsaturated (32
■
valence electrons) dinuclear hydridocarbonyl complex [Re (μ-
2
H) (CO) (μ-3-Me-pydz)] (2, Scheme 1), obtained by reaction
2
6
of the unsaturated tetranuclear hydridocarbonyl cluster
Scheme 1. Synthesis of Monomer 1
−1
angle of 161.4°; the other one (more stable by 6.9 kJ mol )
shows a closer arrangement of the pyridazine and cyclo-
pentadithiophene rings and has a torsional angle of −12.8° (see
Figure S2 in the SI). The torsional barrier for their
−1
interconversion is computed to be as low as 8.0 kJ mol ,
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Figure 2. Partial MO diagram for complex 1. Orbitals are colored according to their main contribution: rhenium “t ” orbitals (dark green),
2g
cyclopentadithiophene π orbitals (orange), pyridazine π* orbitals (light blue), rhenium “e ” orbitals (light green), and cyclopentadithiophene π*
g
orbitals (light orange). Arrows correspond to electronic transitions involved in the absorption spectra (computed TD-DFT wavelengths are given).
In particular, values on the left and on the right refer to the cyclopentadithiophene π−π* and MLLCT absorptions, respectively. Some relevant MOs
are depicted (isovalue = 0.04 au).
suggesting that free rotation occurs at room temperature. This
can be explained by the strongly asymmetrical arrangement of
the carbonyl ligands that, because of the different size of the
bridging hydrido and carboxylato ligands, leave one side of the
complex almost unhindered (see Figure 1).
Orbital energies and absorption wavelengths computed for
the two rotamers are qualitatively similar, and only the ones for
the more stable conformation are discussed (data can be found
in Tables S1 and S2 in the SI). A partial orbital energy diagram
is depicted in Figure 2, along with some relevant molecular
orbital (MO) plots.
The highest occupied molecular orbital (HOMO) is the
highest-lying π orbital of the cyclopentadithiophene group.
Figure 3. Black lines: absorption and emission spectra of 1 in a
Lower in energy lie six orbitals corresponding to the “t ” set of
2
g
deaerated dichloromethane solution at room temperature. Red line:
absorption spectrum of the free CPDT-COOH ligand under identical
conditions.
the two rhenium atoms in a pseudooctahedral environment. In
particular, three of them show a strong rhenium μ-carboxylato
π* character, in close analogy with the derivatives previously
1
5,17
described.
As is typical for these species, LUMO and
transfer character of such transitions is supported by their
strong solvent dependence: indeed, as was already reported for
LUMO+1 are the two lowest-lying π* orbitals of the
coordinated pyridazine. LUMO+2 is the lowest-lying π* orbital
of the cyclopentadithiophene group, while the following four
18
the related dirhenium μ-diazine complexes, a blue shift is
observed upon increasing solvent polarity (see Figure S3 in the
SI).
MOs are the “e ” set of the two rhenium atoms (showing in
g
addition a large CO π* character).
The position of the two absorptions of the CPDT-COOH
2
1
The absorption spectrum down to 270 nm was simulated by
computing the lowest 30 singlet excitation energies. Selected
transitions are depicted in Figure 2. The absorptions starting
ligand, arising from π−π* transitions of the thiophene system,
did not show significant changes upon complexation, indicating
that the CPDT π system does not significantly interact with the
from the rhenium “t ” orbitals can be described as metal−
2
g
Re -pydz orbitals. The change of the relative intensities can be
2
1
ligand-to-ligand charge-transfer ( MLLCT) transitions, taking
into account the significant contribution of the bridging
ancillary ligands to the metal-centered HOMO set. The three
more intense transitions (from HOMO−n to LUMO) are
listed in Figure 2 and Table S2 in the SI.
attributed to a partial overlap between the highest-energy band
of the ligand and the d−d transitions of the Re (μ-X) (CO)
2
2
6
1
7,18
moiety.
Upon excitation at 450 nm, complex 1 exhibits weak
photoluminescence, centered at 607 nm, attributed to the
3
Photophysical Characterization of 1. The absorption
spectra of complex 1 and CPDT-COOH ligand are shown in
Figure 3. On the basis of DFT computations, the lowest-energy
absorption of the complex, at 413 nm, can be assigned to the
emission from MLCT excited states (Figure 3), typical of this
18
class of complexes. The photoluminescence quantum yields
of 1 (Φ = 0.05%) are lower than those of related [Re (μ-H)(μ-
2
OR)(CO) (μ-3-Me-pydz)] complexes (Φ = 0.55% and 0.07%
6
1
22
convolution of multiple MLLCT transitions. The charge-
for R = Ph and Me, respectively). This might be due to the
C
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+
lower stiffness of the Re (μ-H)(μ-X) scaffold, with X is bridging
through two donor atoms rather than one (Re−O−C−O−Re
instead of Re−O−Re).
reduction peak of 1, located at −1.45 V (Fc /Fc), is nearly
coincident with that of 1′, both in position and in shape. In
agreement with many related dirhenium pyridazine com-
2
18
Electrochemical Characterization of 1. Cyclic voltam-
metry (CV) was employed for the characterization of 1, for its
anodic polymerization, and for the characterization of the
resulting polymer. The normalized CV pattern of 1, recorded
on a glassy carbon (GC) electrode, in MeCN with 0.1 M
TBAPF (TBA = NBu ) as the supporting electrolyte, is
plexes, the reduction of 1 and 1′ is monoelectronic and
reversible, both chemically (symmetrical return peaks,
independent of the scan direction) and electrochemically
(facile electron transfer, which is indicated by the 57 mV
half-peak widths and the almost-zero E vs log ν slopes). This
p
6
4
agrees with the very fast formation of a stable radical anion.
Therefore, the CV features of 1 confirm that the HOMO is
localized on the CPDT unit and the LUMO essentially consists
of the π* orbitals of the pyridazine ligand, in agreement with
the DFT computations. Moreover, the values of the oxidation
and reduction potentials, very similar to those observed in free
CPDT-COOH and in the model complex 1′, confirm the lack
of communication between the pyridazine ring and the CPDT
moiety, consistent with the indications of UV−vis absorption
data (see the previous section).
reported in Figure 4. For comparison, the CV patterns of the
Electropolymerization. Regular oxidative electropolyme-
rization of monomer 1 was achieved by repeated potential
+
cycles between −0.4 and +1.0 V vs Fc /Fc, in MeCN, with
different supporting electrolytes, on different electrodes [GC,
platinum, gold, and indium-doped tin oxide (ITO)]. Sequential
cyclic voltammograms of 1 (Figure 5a) gave evidence that the
polymer films remain highly electroactive during the polymer-
ization process: oxidative and reductive peak currents regularly
increased with no apparent threshold (very regular growth was
observed even up to 60 cycles; see Figure S4 in the SI).
The sequential cyclic voltammograms of free CPDT-COOH,
recorded under similar conditions (Figure 5b), showed that the
two monomers have a comparable polymerization ability.
Therefore, the binding of the CPDT moiety to the metal
complex does not significantly hamper its oxidative coupling, in
spite of the bulkiness of the organometallic fragment.
The two polymerization patterns are similar, which is
reasonable, considering that polymerization involves the same
α positions available in the CPDT unit in both cases. In
particular, both show two large, reversible peaks, the first one
smaller than the second one, at 0.2 V s− scan rate. Such
reversible peaks are particularly well-defined and symmetrical in
the case of 1, pointing to an easy solid-state charging/
uncharging process.
Figure 4. Normalized CV patterns, recorded on GC, at 0.2 V s−1 in
+
MeCN with a 0.1 M TBAPF medium (starting from −0.4 V vs Fc /
6
Fc), for complex 1 together with reference compounds 1′ and CPDT-
COOH.
corresponding free ligand CPDT-COOH and of the related
rhenium complex [Re (μ-H)(μ-OOC-Ph)(CO) (μ-3-Me-
2
6
2
2
pydz)] (1′), which contains one hydride and one benzoate
anions as ancillary ligands, are also reported. The peak and
onset potentials, together with the related HOMO and LUMO
energies, calculated as previously described, are collected in
Table 1. It is worthwhile to notice that the HOMO−LUMO
energy gap calculated from the electrochemical experiment
23
1
(
2.16 eV, max, or 1.93 eV, onset) is much narrower than the
energy gap calculated from the lowest-energy UV−vis
absorption peak (3.01 eV, max). This is consistent with the
fact that the electronic absorption arises from a MLLCT
1
transition, which does not involve the HOMO level (π-CPDT;
The stability of the poly-1 films was tested by transferring the
carefully rinsed polymer-modified electrodes into a monomer-
free solution and by performing repeated oxidative cycles
around the first two oxidation peaks, as shown in Figure S5 in
the SI. The polymers exhibited remarkable stability, and their
CV characteristics remained almost unchanged even upon
prolonged cycling.
see the previous section).
The first oxidation peak of 1 at 0.71 V vs Fc /Fc (chemically
+
irreversible) can be assigned, by comparison with CPDT-
COOH, to the formation of a radical cation delocalized over
24
the cyclopentadithiophene moiety. The second oxidation
+
peak at +1.02 V vs Fc /Fc corresponds to oxidation of the
binuclear rhenium core, by comparison with the behavior of the
These general features were observed in all operating
model complex 1′ (see Table 1). On the other hand, the first
conditions, albeit small differences in the polymer CV patterns
+
Table 1. Oxidation and Reduction Peak and Onset Potentials, Referring to the Fc /Fc Couple, and Related HOMO and LUMO
Values, for the Monomer 1 Together with the Reference Molecules 1′ and CPDT-COOH
energy levels and gaps/eV
reduction peak/V
+
+
(
Fc /Fc)
oxidation peak(s)/V (Fc /Fc)
onset
max
max
onset
onset
0.52
max
0.67
onset
max
LUMO
HOMO
Eg
LUMO
HOMO
Eg
CPDT-COOH
−3.08
−1.52
−1.45
−2.88
−1.42
−1.34
−1.92
−3.38
−3.46
−5.32
−5.66
−5.39
3.40
2.28
1.93
−1.72
−3.28
−3.35
−5.47
−5.77
−5.51
3.75
2.49
2.16
1
1
′
0.86
0.97
1.02
0.59
0.71
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Figure 5. Oxidative electropolymerization of (a) monomer 1 (0.5 mM, MeCN, 0.1 M TBAPF ) performed on a GC electrode by potential cycling
6
+
−1
between −0.4 and 1.0 V vs Fc /Fc (0.2 V s ) and of (b) CPDT-COOH (0.5 mM), performed under similar conditions, with potential cycling
between −0.66 and 0.94 V. The Roman numerals indicate the number of potential cycles.
were observed, changing the supporting electrolyte, the
electrode substrate, and/or the width of the potential range
for the polymerization cycles (examples are provided in Figures
S4, S6, and S7 in the SI); in particular, a narrower potential
range resulted in a slower film growth because of the reduced
interval time and the lower conversion of monomer within this
potential window.
toward more negative values, revealing a broad irreversible
reduction peak at −1.4/−1.5 V (vs Fc /Fc, with the onset
+
being at −1.1/−1.2 V; Figure 7). The potential value, quite
close to that observed for the reduction of monomer 1,
indicates that the negative charge is injected into the π* orbitals
27
of the diazine units.
The oxidative CV pattern of poly-1, similar to that of other
5
,25,26
cyclopentadithiophene polymers,
is characterized by the
two reversible thiophene oxidation peaks observed in the
electropolymerization process.
The regularity of the polymerization can be finely
appreciated by measuring in situ the weight mass of the
growing film on gold electrodes with an electrochemical quartz
microbalance (EQCM; Figure 6). The weight of the forming
Figure 7. CV traces showing the charge-trapping phenomenon,
−1
recorded at 0.2 V s in a monomer-free solution, on a film obtained
from a 60-cycle electropolymerization of monomer 1 on GC, in a
MeCN and 0.1 M TBABF medium. For comparison, the gray curve
4
provides a typical stability cycle of the same film in a monomer-free
solution, limiting the potential scan to the reversible oxidation region.
The reduction peak is associated with a remarkably delayed
sharp return peak, of comparable area (i.e., associated charge),
appearing in close proximity to the first oxidation peak of the
neutral polythiophene chain. This behavior has been observed
in all of the experiments involving poly-1, using different
supporting electrolytes, and on all of the tested electrode
surfaces, and it is consistent with the presence of a solid-state
charge-trapping phenomenon, implying that the negative
charge, injected at a given cathodic potential, is recovered at
Figure 6. EQCM monitoring (as mass variation per active surface area,
cycles I−VI) of the electropolymerization of monomer 1 on gold (0.25
−1
mM in MeCN and 0.1 M TBAPF at 0.2 V s ).
6
28
much higher potentials in the reverse scan.
polymer (monitored from the decrease of the frequency of the
quartz oscillator supporting the gold electrode; see the lower
section of Figure S8a in the SI) increased linearly with time
Charge trapping, a typical property of electroactive materials
bearing electron-rich and -poor moieties, can be explained by
invoking different processes, depending on the molecular
structures and the working conditions involved. Indeed, the
higher than expected reoxidation potential can be ascribed to a
(
Figure S8b in the SI), with a periodic fine structure
corresponding to the ingress/egress of ions and solvent within
each cycle.
29
strong stabilization of the reduced species or to some
30
Charge Trapping. After the poly-1 stability was tested at
conformational changes of the film components (concurrent
positive potentials, the potential scan range was widened
with the ingress or egress of ions and solvent molecules),
E
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resulting in a more stable supramolecular structure. Such
changes could result in kinetic barriers to the efficient reuptake
of the ejected species (or to the egress of the entered species)
upon reoxidation, which can be overcome only by providing a
significant energy supplement. On the other hand, charges
created upon reduction can become trapped in isolated
microdomains in the insulating film; therefore, they can be
too immobile or too dilute to allow charge transfer to the
electrode. These charges can be released only when the film is
scanned positive enough to begin oxidation of the backbone,
becoming conductive and thus able to mediate electron
released upon oxidation) even if the negatively charged
electrode is removed from the solution and stored for many
hours (for instance, overnight) at room temperature.
3.8. Electrochemical Impedance Spectroscopy (EIS).
The film was also studied in a monomer-free solution by
comparing its electrochemical impedance at different potentials
as representative of different possible states. Figure 9 provides a
synopsis of the relevant Nyquist, Bode modulus, and Bode
phase diagrams.
3
1,32
transfer.
In our case, the assumption of a supramolecular reorganiza-
tion is supported by the observation that the amount of charge
associated with the reduction and the charge-trapping process
regularly decreases with increasing scan rates, pointing to a
somehow slow solid-state process occurring at the first
29
reduction peak, concurrently with charge transfer (Figure 8).
Figure 9. EIS features of the conducting film obtained by 20-cycle
electropolymerization of 0.25 mM monomer 1 on a GC electrode, in a
−1
MeCN and 0.1 M TBAPF medium, at 0.2 V s : (a and b) Nyquist
6
phase diagrams of the imaginary versus real components of the
impedance Z, focusing on medium and high frequencies, respectively;
(c) Bode modulus phase diagram; (d) Bode phase diagram.
+
At −0.79 V vs Fc /Fc, poly-1 is in its neutral state (see
Figure 8), far away from both its oxidation and reduction
potentials. Accordingly, it exhibits purely capacitive behavior
(
corresponding to a nearly vertical line from high to low
frequencies in the Nyquist phase diagram and a nearly 90°
asymptote in the Bode phase diagram).
At sufficiently positive potentials to allow charge transfer
from the polymer to the electrode, changes are observed in the
shapes of the curves, in both the Nyquist and Bode diagrams,
which can be rationalized in terms of the nature of the rate-
determining steps at the different frequencies, following the
Figure 8. CV cycles including the charge-trapping phenomenon,
recorded at different scan rates on a film obtained from a 60-cycle
electropolymerization of monomer 1 on GC, in MeCN and 0.1 M
33
TBABF medium, starting from −0.4 V and scanning the cathodic
4
treatment described by Musiani (summarized in the SI).
region before.
At more positive potentials than −0.79 V, the capacitive
straight line is replaced in the Nyquist phase diagram at high
frequencies (frequency decreases from left to right in the
horizontal axis) by half-cycles of decreasing diameter, pointing
to electron transfer as rds and decreasing charge-transfer
resistance upon increasing potential. Thus, it can be observed
that some electron transfer takes place already at −0.29 V,
although it is kinetically very hindered and possibly
corresponds to only a few reaction sites per surface unit (R
≈ 8000 Ω; Figure 9a). Then, shifting to −0.02 V, electron
transfer becomes easier, the half-circle diameter remarkably
decreases (R ≈ 2300 Ω; Figure 9a), and a small portion of the
In any case, this effect appears to be quite interesting, in
many terms. At sufficiently low scan rates, the associated charge
is high, almost comparable to the charge generated in polymer
oxidation. Moreover, the charges associated with the forward
and backward charge-trapping peaks are comparable, pointing
to the fact that the entire negative charge is firmly trapped until
the first oxidation peak is attained. Furthermore, the charge-
trapping process is reversible upon repeated cycling of p and n
doping, showing that the polymer network is undamaged by
this process. Finally, a negative charge is maintained (and
F
dx.doi.org/10.1021/ic501840p | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
3
4
Warburg straight line, with 45° slope, can be perceived at the
lowest frequencies. This implies the diffusion of charges (both
electrons and ions) within the polymer to be kinetically
determining. At 0.27 V, the charge-transfer resistance is
dramatically lower (R ≈ 100 Ω; Figure 9b); furthermore, the
contiguous “diffusive” Warburg section is practically unnoticed,
pointing to high conductivity (electronic and ionic) within the
polymer. As a consequence, after the small RC semicircle, the
system in the “charge saturation” state turns into a capacitive
response with no apparent intermediate diffusive regimen.
Finally, at 0.51 V the charge-transfer resistance is nearly
unperceivable and the system soon reaches the capacitive
behavior.
shifts to longer wavelengths, with respect to the corresponding
band in monomer 1, and this red shift increases with the
number of electropolymerization cycles (from 496 nm for 10
polymerization cycles, as in Figure 10, to 545 nm for 40 cycles),
21
as a consequence of the π-conjugation extension. This
absorption maximum is at lower wavelength than that of the
13
bulk polymer (600 nm), suggesting a rather low degree of
polymerization, as was previously observed for CPDT-COOH
21
itself, or other electropolymerized CPDT monomers carrying
35
redox pendant groups.
Upon a stepwise increase in the electrode potential up to
.36 V, the absorption band of the polythiophene chain
0
progressively disappears and a broad band emerges at 830 nm,
36
1
attributable to the radical cation. The MLLCT band at 410
nm is still perceivable, in agreement with the electrochemical
data, indicating that the metal core is not involved in backbone
oxidation.
This indicates that the main CV oxidation peak corresponds
to extremely facile charge transfer combined with very fast
diffusion of both electrons and ions within the polymer. The
good conductivity (both electronic and ionic) of the polymer
was also revealed by the presence of the oxidation peak of
monomer 1, at about 0.71 V, in the CV traces recorded during
the electropolymerization of 1 (see Figure 5a).
The decreasing charge-transfer resistance is also well
recognized in the Bode modulus phase diagram (Figure 9c),
in terms of horizontal plateau, providing the R values (with
respect to the niveau associated with the electrolyte solution
resistance); it gets lower and shifts to higher frequencies with
increasing potential. It can also be recognized in the Bode
phase diagram (Figure 9d), in terms of peaks, again shifting to
higher frequencies with increasing potential.
A further increase of the potential to 0.81 V results in the
progressive growth of new absorption bands at 690, 900
(
shoulder), and 1050 nm. The formation of a bipolaronic
species should afford only one new absorption, at lower energy
than the radical cation. Therefore, the presence of different
absorptions, one of which is at higher energy, could be a hint of
π dimerization of the polarons, which affords spinless
bipolaronic species, stabilized with respect the two separated
37
polarons. According to the selection rules for the π dimer,
three absorption bands (often overlapped) are expected, two of
them at higher energy than the corresponding polaronic
36
band.
Spectroelectrochemistry. The electrooptical properties of
the films deposited onto ITO-coated glass electrodes in a
MeCN solution (0.1 M TBAPF ) have been investigated. At
When the potential is settled at negative values (down to
1.4 V vs Fc /Fc, corresponding to the first reduction
+
−
6
potential), no changes in the UV−vis spectrum of poly-1 are
observed, in agreement with the absence of electronic
communication between the thiophene polymer backbone
and pyridazine moieties.
+
−
0.39 V (vs Fc /Fc), the film is neutral. Accordingly, its UV−
CONCLUSIONS
■
A new dinuclear rhenium complex containing a cyclo-
pentadithiophene moiety has been synthesized and fully
characterized. At the molecular level, the spectroscopic and
electrochemical properties of the complex result from the
addition of the properties of two separate building blocks,
namely, the CPDT-COOH and organometallic fragments, in
agreement with the lack of electronic communication in the
ground state between the two fragments, indicated by the DFT
computations.
In spite of the steric hindrance of the dinuclear scaffold of the
dirhenium complex, electropolymerization of the CPDT moiety
does occur, on different electrodes, although with a low
polymerization degree. The metallopolymer is constituted by a
poly-CPDT conjugated chain, regularly decorated by pendant
dirhenium diazine complexes (one complex for every CPDT
group). It provides an example of a metallopolymer, in which
the conductive properties of the π-conjugated system are added
to the redox behavior of the pendant-isolated complexes. Easy
charge injection and diffusion (both electrons and ions) within
the polymer have been confirmed by EIS measurements. The
good stability of the conductive polymer film on different
electrodes, from GC to ITO to platinum and gold, has been
indicated by its reproducible behavior upon repeated potential
cycles.
Figure 10. Spectroelectrochemistry of a film (10 polymerization
cycles) of poly-1 on ITO-coated glass (MeCN and 0.1 M TBAPF ) at
6
+
applied potentials between −0.39 and 0.81 V (vs Fc /Fc).
vis spectrum (Figure 10) shows a (partially overlapped)
1
MLLCT transition at 410 nm and a π−π* transition of the
polythiophene backbone at 496 nm. The first band lies at the
same energy as that of the band observed for monomer 1
(
Figure 3) because the diazine ligands are not involved in the
polymerization process and are not conjugated to the polymer
In addition, poly-1 exhibits not only the high p-doping
stability typical of poly-CPDT but also a high n-doping stability
backbone. On the contrary, the band due a π−π* transition
G
dx.doi.org/10.1021/ic501840p | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Scheme 2. Synthetic Approach to CPDT-COOH
because of the presence of the electron-poor rhenium diazine
groups, which are the site of reduction. The negative charge is
removed at much higher potential than the reduction (ca. 1.7 V
more positive), in concomitance with the onset of the p-doping
process. Such a charge-trapping process can be observed upon
repeated potential cycles, indicating that the polymer network
is undamaged by the process. Notably, the negative charge is
preserved even if the charged electrode is removed from the
solution and stored overnight at room temperature. Possible
applications for rewritable memory devices can be envisaged.
Moreover, immobilization of the complex on a conductive film
on an electrode surface will provide a favorable arrangement for
testing in heterogeneous conditions the properties of
dirhenium diazine complexes already ascertained in the
homogeneous phase, such as the recently reported activity in
1′ was prepared from 2 and benzoic acid, using the same procedure
2
2
as that described for 1. Anal. Calcd for Re C H O N : C, 28.57; H,
2
18 12
8
2
1
2
9
.60; N, 3.70. Found: C, 28.77; H, 1.46; N, 3.43. IR (νCO, toluene):
−1 1
040m, 2020s, 1940s, 1913s cm . H NMR (400 MHz, CD Cl ): δ
2
2
.29 (d, 1H, Ha), 7.90 (d, 1H, Hb), 7.69 (dd, 1H, Hc), 7.93 (dd, 2H,
H , phenyl), 7.38 (t, 2H, H , phenyl), 7.49 (t, 1H, H , phenyl), 3.06
o
m
p
(
s, 3H, Me), −6.68 (s, 1H, hydride). GC (M = [Re (μ-H)(μ-OOC-
2
+
Ph)(CO) (μ-3-Me-pydz)], relative intensity): m/z 759 ([M] , 24),
6
+
+
758 ([M − H] , 23), 730 ([M − H − CO] , 100).
Electrochemistry. Complex 1 (together with reference com-
pounds CPDT-COOH and complex 1′) was characterized by CV at
−1
potential scan rates ranging from 0.005 to 10 V s , in 0.25 or 0.5 mM
MeCN solutions, deaerated by N2 purging, with different 0.1 M
supporting electrolytes: TBABF and TBAPF (TBA = n-Bu N). The
4
6
4
ohmic potential drop was compensated for by the positive feedback
technique. The experiments were carried out using an AUTOLAB
PGSTAT 12 potentiostat of EcoChemie (Utrecht, The Netherlands)
run by a PC with the GPES 4.9 software of the same manufacturer.
38
the electrocatalytic reduction of carbon dioxide.
The working electrode was either (i) a GC disk embedded in Teflon
2
(
Amel, 0.071 cm ), (ii) a gold disk embedded in Teflon (Amel, 0.0314
EXPERIMENTAL SECTION
■
2
2
cm ), (iii) a platinum foil (1.44 cm ) for bulk depositions, or (iv) an
Materials and Instrumentation. All of the reactions were
performed under dinitrogen using the Schlenk technique and solvents
deoxygenated and dried by standard methods. 3-Methylpyridazine was
used as received from Aldrich. [Re (μ-H) (CO) (μ-3-Me-pydz)]
−1
ITO-coated glass slide (Aldrich, sheet resistance 9−12 Ω sq ). The
counter electrode was a platinum wire. The operating reference
electrode was an aqueous saturated calomel electrode of a redox
potential of −0.39 V in our MeCN working solutions versus that of the
2
2
6
16
(
(
2), and 4H-cyclopenta[2,1-b:3,4-b′]dithiophene-4-carboxylic acid
+
Fc /Fc couple (the intersolvental redox potential reference currently
39
CPDT-COOH, Scheme 2) were prepared according to literature
41
recommended by IUPAC). The optimized polishing procedure for
1
methods. H NMR spectra were recorded on a Bruker DRX300 or
DRX400 spectrometer. IR spectra were acquired on a Bruker Vector22
FT instrument. Electronic absorption spectra were recorded on an
Agilent 8543 spectrophotometer, at room temperature. Mass
spectrometry by secondary-ion ionization were recorded using a VG
AUTOSPEC M246 magnetic spectrometer using 3-nitrobenzyl alcohol
as the matrix.
the working disk electrodes consisted of surface treatment with a
synthetic diamond powder of 1 mm diameter from Aldrich on a DP-
Nap wet cloth from Struers.
Electrochemical polymerization of 1 by repeated CV cycling in the
potential range of the first anodic peak between −0.4 and 1.0 at a 0.2 V
−1
s
potential scan rate resulted in regular deposition of conducting
films on the working electrodes. The CV stability of the films was
tested by repeated cycling in the same potential range at different scan
rates, in a monomer-free solution; finally, the potential range was
progressively extended negatively in search of the first reduction
potential and possible related charge-trapping phenomena.
The polymer growth was also monitored by electrochemical
piezoelectric microgravimetry using an EQCM 5710 electrochemical
quartz crystal microbalance (Institute of Physical Chemistry of the
Polish Academy of Science) connected with the Autolab PGSTAT and
run by EQCM 5710-S2 software, depositing the polymer on gold-
coated AT-cut quartz crystal resonators (5 MHz).
Synthesis of CPDT-COOH. CPDT-COOH was obtained by
benzoinic condensation of 3-thiophenecarboxyaldehyde, mediated by
thiamine chlorohydrate in ethanol, at pH 9 following a modified
40
literature procedure. Control of the pH has a crucial role in the
reaction because thiamine chlorohydrate is inactive at lower or higher
values. It is worth noting that the pH decreases constantly during the
reaction; therefore, continual KOH addition is necessary. The resulting
3
4
,3′-thienoin 3 was oxidized with copper sulfate to give the 3,3′-thienyl
, which, in the presence of KOH, underwent benzylic transposition to
give the hydroxyl acid 5. CDPT-COOH resulted then from
intramolecular Friedels-Crafts-type ring closure, promoted by AlCl3
in benzene.
The mass variation was calculated according to Sauerbrey’s
42
equation:
Synthesis of Complexes 1 and 1′. A mixture of 2 (39.8 mg,
0
.0625 mmol) and CPDT-COOH (16.9 mg, 0.0750 mmol, 1.2 equiv)
2
−
2f Δm
0
was dissolved in freshly distilled toluene (4 mL). The solution was
refluxed for 32 h and then evaporated to dryness. Caution! This
Δf =
A μ ρ
q q
reaction generates H and should be carefully kept under nitrogen. The
2
addition of n-hexane to the reaction mixture afforded 1 as a
spectroscopically pure solid (isolated yield 40.21 mg, 0.0468 mmol,
where Δf = frequency change, Δm = mass variation, A = active
2
piezogravimetric area = 0.1963 cm , ρ = quartz density = 2.648 g
q
−
3
11
−2
7
5%). Anal. Calcd for Re C H O N S : C, 29.44; H, 1.41; N, 3.27.
cm , and μ = shear modulus of quartz = 2.947 × 10 dyn cm ,
2
21 12
8
2
2
q
Found: C, 29.45; H, 1.33; N, 3.50. IR (νCO, toluene): 2041m, 2021s,
1
under the assumption that the film density and shear modulus are the
same as those of the quartz (an assumption reasonable for thin films
with no compressive stress of mass <1% quartz crystal mass).
The film deposited on a GC electrode (20 cycles, MeCN with 0.1
−
1 1
940s, 1917s cm . H NMR (400 MHz, CD Cl ): δ 9.11 (d, 1H, Ha),
2 2
7.81 (d, 1H, Hb), 7.60 (dd, 1H, Hc), 7.19 (dd, 2H, Hα, thiophene
ring), 7.10 (d, 1H, Hβ, thiophene), 7.07 (d, 1H, Hβ, thiophene), 4.52
(
s, 1H), 2.92 (s, 3H, Me), −6.80 (s, 1H, hydride). GC (M = [Re (μ-
M TBAPF ) was also characterized in a monomer-free solution by EIS,
using the above-described working cell and an AUTOLAB PGSTAT
302N (Metrohm Autolab) potentiostat equipped with a FRA2 module
2
6
H)(μ-OOC-CPDT)(CO) (μ-3-Me-pydz)], relative intensity): m/z
8
6
+
+
+
59 ([M] , 38), 858 ([M − H] , 36), 830 ([M − H − CO] , 100).
H
dx.doi.org/10.1021/ic501840p | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
(
EcoChemie, Utrecht, The Netherlands), run by a PC with NOVA
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by means of DFT calculations. The parameter-free hybrid functional
44
PBE0 was employed along with the standard valence double-ζ
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3
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46
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(10) Heinze, J.; Frontana-Uribe, B. A.; Ludwigs, S. Chem. Rev. 2010,
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X-ray crystallographic data of complex 1 in CIF format and
details on the electrochemical, spectroscopic, crystallographic,
and computational characterization. This material is available
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AUTHOR INFORMATION
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■
*
13) Zotti, G.; Zecchin, S.; Schiavon, G.; Berlin, A.; Pagani, G.;
Corresponding Authors
E-mail: monica.panigati@unimi.it. Phone: +390250314352.
Fax: +390250314405.
E-mail: pierluigi.mercandelli@unimi.it. Phone:
390250314447. Fax: +390250314454.
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*
+
*
Mercandelli, P.; Sironi, A.; Mussini, P.; D’Alfonso, L. Inorg. Chem.
2008, 47, 4243−4255.
E-mail: patrizia.mussini@unimi.it. Phone: +390250314211.
Fax: +390250314405.
(16) Panigati, M.; Donghi, D.; D’Alfonso, G.; Mercandelli, P.; Sironi,
A.; D’Alfonso, L. Inorg. Chem. 2006, 45, 10909−10921.
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643.
19) Mauro, M.; Quartapelle Procopio, E.; Sun, Y.; Chien, C.-H.;
The authors declare no competing financial interest.
2
(
ACKNOWLEDGMENTS
■
1
(
M.P., P.M., and G.D. thank the Italian Ministero dell’Istruzione,
Universita
009PRAM8L and PRIN-2012A4Z2RY).
̀
e Ricerca, for financial support (Grants PRIN-
Donghi, D.; Panigati, M.; Mercandelli, P.; Mussini, P.; D’Alfonso, G.;
De Cola, L. Adv. Funct. Mater. 2009, 19, 2607−2614.
2
I
dx.doi.org/10.1021/ic501840p | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
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dx.doi.org/10.1021/ic501840p | Inorg. Chem. XXXX, XXX, XXX−XXX