Designing cyclometalated ruthenium(II) complexes for anodic electropolymerization

Article abstract of DOI:10.1002/chem.201301439

The anodic electropolymerization of thiophene-functionalized cyclometalated ruthenium(II) complexes is shown for the first time. Oxidative decomposition reactions can be overcome by modification of the involved redox potentials through the introduction of electron-withdrawing substituents, namely nitro groups, at the cyclometalating phenyl ring. The generated functionalized ruthenium(II) complexes allow the electrochemical preparation of thin polymer films, which show a broad UV/Vis absorption as well as reversible redox switchability. The presented complexes are promising candidates for future photovoltaic applications based on photo-redox-active films. Copyright

Full text of DOI:10.1002/chem.201301439

DOI: 10.1002/chem.201301439
Full Paper
&
Electropolymerization
Designing Cyclometalated Ruthenium(II) Complexes for Anodic
Electropolymerization
Christian Friebe,[a, b] Benjamin Schulze,[a, b] Helmar Gçrls,[c] Michael Jꢀger,[a, b] and
Ulrich S. Schubert*[a, b]
Abstract: The anodic electropolymerization of thiophene-
functionalized cyclometalated ruthenium(II) complexes is
shown for the first time. Oxidative decomposition reactions
can be overcome by modification of the involved redox po-
tentials through the introduction of electron-withdrawing
substituents, namely nitro groups, at the cyclometalating
phenyl ring. The generated functionalized ruthenium(II)
complexes allow the electrochemical preparation of thin
polymer films, which show a broad UV/Vis absorption as
well as reversible redox switchability. The presented com-
plexes are promising candidates for future photovoltaic ap-
plications based on photo-redox-active films.
(CuAAC),[8] allowing the facile assembly of functionalized li-
gands. The complexes showed prolonged excited-state life-
times, redox stability, and suitability for application in dye-sen-
sitized solar cells.[4a,9]
Introduction
The ruthenium(II)-polypyridyl motif represents a highly favora-
ble building block with regard to applications as photosensitiz-
er units (e.g., in solar cells, light-driven catalysis, and water
splitting).[1] In particular, the incorporation of strong electron-
donating ligands enables the formation of ruthenium(II) com-
plexes providing long excited-state lifetimes and broad absorp-
tion features, both of which are crucial for an efficient photo-
sensitizer dye.[2] Thereby, ruthenium(II) complexes that contain
thiocyanate ligands have been applied very successfully.[3]
However, the monodentate thiocyanates cause a lowered com-
plex stability and impede further functionalization with regard
to dye optimization. Hence, alternative polydentate ligands
that feature strong electron donors, namely anionic carbon[4]
and nitrogen[5] atoms as well as classical[6] and mesoionic[7] N-
heterocyclic carbenes, were designed for sensitizer applica-
tions. In this context, we recently presented a series of com-
plexes based on a cyclometalating, tridentate ligand possess-
ing 1,2,3-triazole moieties, which were introduced through
copper(I)-catalyzed azide–alkyne 1,3-dipolar cycloaddition
Most photosensitizer applications require the processing of
the dye in thin films to allow light absorption as well as effi-
cient charge transfer to the affiliated reaction site. Thereby, an
instrumentally simple technique for the formation of defined
layers is electropolymerization, i.e., the formation of insoluble
polymers through the coupling of electrochemically generated
monomer radicals on an electrode surface.[10] In this way, differ-
ent ruthenium(II) complexes of polypyridyl-type ligands have
already been used successfully to form polymeric coatings.[11]
Recently, cyclometalated systems were also polymerized
through electrochemical reduction,[12] and, very recently,
through an anodic approach.[13] However, successful oxidative
electropolymerization of cyclometalated ruthenium(II) com-
plexes was reported only rarely in the literature, although the
anodic polymerization allows the usage of aromatic electropo-
lymerizable units such as thiophene, 3,4-ethylenedioxythio-
phene (EDOT), and pyrrole. These enable the assembly of met-
allopolymers[14] that feature p-conjugated spacer units, provid-
ing additional chromophores and potentially enabling an intra-
molecular electron transfer after photo-oxidation of the com-
plex.[15] This, in turn, leads to more efficient UV/Vis absorption
and extended charge separation, respectively. A potential chal-
lenge is the high reactivity of the electron-rich central phenyl
ring of the cyclometalating ligand, which possesses a high spin
density, possibly leading to electrochemical coupling.[16] Never-
theless, redox stability could be shown in UV/Vis/NIR spectroe-
lectrochemical experiments, at least for the first oxidized
state.[4a] Still, the electron-rich aromatic moiety may give rise to
side reactions or decomposition under the highly positive po-
tentials required for the electropolymerization.
[a] Dr. C. Friebe,+ B. Schulze,+ Dr. M. Jꢀger, Prof. Dr. U. S. Schubert
Laboratory of Organic and Macromolecular Chemistry (IOMC)
Friedrich Schiller University Jena
Humboldtstr. 10, 07743 Jena (Germany)
E-mail: ulrich.schubert@uni-jena.de
[b] Dr. C. Friebe,+ B. Schulze,+ Dr. M. Jꢀger, Prof. Dr. U. S. Schubert
Jena Center for Soft Matter (JCSM)
Friedrich Schiller University Jena
Philosophenweg 7, 07743 Jena (Germany)
[c] Dr. H. Gçrls
Laboratory of Inorganic and Analytical Chemistry
Friedrich Schiller University Jena
Humboldtstr. 8, 07743 Jena (Germany)
[+] These authors contributed equally to this work.
In this work, we present the preparation and electrochemical
polymerization of ruthenium(II) complexes based on 1,2,3-tria-
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201301439.
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zole-containing cyclometalating ligands. Two structural motifs
are introduced, varying in the presence and absence of methyl
groups at the linking phenyl rings, which prevent and allow,
respectively, a coplanarization of the phenylthienyl and the
central, metal-coordinating triazole moieties, and thus, may
affect the extent of p conjugation between thiophene and tria-
zole. However, electronic coupling through the triazole is not
expected.[17] The synthesized monomer complexes were fully
characterized, with the prepared polymer films studied
through cyclic voltammetry, UV/Vis absorption spectroscopy,
and UV/Vis/NIR spectroelectrochemistry. Furthermore, support-
ing computational calculations based on density functional
theory (DFT) were performed to gain further insight into the
electrochemical behavior.
phene on an azide-functionalized bromobenzene first, afforded
the desired 2-(4-azidophenyl)thiophene building blocks in rea-
sonable yields. Notably, the Suzuki cross-coupling tolerated the
presence of aryl azides despite their known tendency to form
phosphazides and phosphimines with free or coordinated
phosphines of the palladium catalyst.[20] The subsequent
CuAAC afforded the thiophene-containing N^CH^N ligands in
good yields. In contrast to the initially attempted cross-cou-
pling method, the CuAAC greatly simplifies the purification be-
cause the educt and byproducts are removed readily, which is
particularly important in the case of low product solubility, for
example, for HTph (Scheme 1). For HTphMe, the solubility is
much higher owing to the steric interactions between the tria-
zoles and the ortho methyl groups of the outer phenyl rings,
which enforce a twisting out of plane and thereby preclude p
stacking. As a result, single crystals suitable for X-ray diffraction
could be grown through vapor diffusion of diethyl ether into
a concentrated dichloromethane solution (see Figure S42, Sup-
porting Information). The cyclometalation was achieved in fair
yields by using [RuII(tpy)(CH3CN)3][PF6]2 (tpy=2,2’:6’,2’’-terpyri-
dine) as the precursor.[4a]
Results and Discussion
Synthesis and electrochemical behavior of thiophene-
equipped complexes
As a first survey of the ability of the cyclometalated rutheni-
um(II) complexes to undergo anodic electropolymerization,
two complexes possessing 4-(2-thienyl)phenyl moieties at the
triazole rings were synthesized (Scheme 1) and studied
through electrochemical means.
Subsequently, the monomer complexes were characterized
electrochemically; the obtained cyclic voltammograms (CVs)
and differential pulse voltammograms (DPVs) are depicted in
the Supporting Information (Figures S45 and S46), and the first
redox potential values are given in Table 1. The CVs of RuTph
and RuTphMe reveal a reversible first oxidation process with
a half-wave potential of 0.10 V, which is assigned to the de-
population of a mixed ruthenium- and cyclometalating-ligand-
based orbital on the basis of previous computational investiga-
tions on related systems.[4a] In the region around 1.1 to 1.4 V,
further oxidation processes appear, including thienyl radical
cation formation, which is crucial for the electropolymerization.
Thereby, the thienyl-based signals possess a significantly larger
peak current than the first redox processes. This is attributed
to a lack of electronic coupling, because the 1,2,3-triazole is
known to interrupt p conjugation,[17] leading to simultaneous
oxidation of both thiophene moieties at the same redox po-
To allow a rapid and modular access to the thiophene-func-
tionalized 1,3-bis(1,2,3-triazol-4-yl)benzene (N^CH^N) ligands,
we rely on a click-derived triazole-based framework. It has
been demonstrated that the involved triazole units can be
used as analogues for pyridine donors.[18] Initial attempts to
couple 2-thienylboronic acid directly to the corresponding
bromo-functionalized N^CH^N ligand framework through
Suzuki cross-coupling were not successful (sluggish reaction,
intractable reaction mixtures). Likewise, attempted cyclometa-
lation using a bromo-functionalized N^CH^N ligand to install
the thiophene after complexation was precluded by partial de-
bromination resulting in an inseparable mixture of com-
plexes.[19] The alternative approach, that is, installing the thio-
Scheme 1. Synthesis of the thiophene-containing cyclometalated ruthenium(II) complexes: a) Pd(PPh3)4, K2CO3, DMF, 508C, 12 h, 40%; b) CuSO4·5H2O, NaAsc.,
CH2Cl2/EtOH/H2O (1:2:1), 508C, 12 h, 70%; c) EtOH/toluene (1:1) or DMF, 140 to 1608C, 30 to 120 min, 50%. Ru was prepared previously in an analogous syn-
thesis.[4a]
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of Lewis acids (e.g., BF3·OEt2, borate esters)[21] or weak bases
(e.g., water, 2,6-di-tert-butylpyridine),[10] did not result in suc-
cessful electropolymerization. BF3·OEt2 in acetonitrile even
caused the displacement of the cyclometalating ligand, leading
to the undesired recovery of the [RuII(tpy)(CH3CN)3]2+ precursor
species (see Figures S43 and S49, Supporting Information).
We assumed that the decomposition is caused by an inher-
ent electrochemical process of the complexes. This assumption
is supported by comparison of the cyclic voltammograms of
the thienyl-equipped system with those of its parent complex
devoid of thiophene units (Ru, Figure 2). The latter system re-
veals an additional, irreversible redox process, which is in the
same potential range as the oxidation of the thienyl moeities.
Thus, oxidation of the thienyl units would not be possible
without inducing an irreversible oxidation reaction of the cen-
tral complex moiety.
Table 1. Electrochemical characteristics of the monomer complexes
(10ꢀ4 m in CH2Cl2 with 0.1m Bu4NPF6).
Complex
E
1/2 [V] (ipa/ipc, DEp [mV])[a]
+1!+2
+1! 0
RuTph
0.10 (1.05, 65)
0.10 (1.0, 74)
0.29 (1.0, 67)
0.31 (1.0, 72)
ꢀ2.12 (irrev.)[b]
ꢀ2.17 (irrev.)[b]
ꢀ1.87 (1.0, 75)
ꢀ1.88 (1.05, 78)
RuTphMe
RuTphNO2
RuTphMeNO2
[a] Potentials vs. Fc+/Fc. [b] Peak potential of the cathodic wave.
tential. The first, ligand-based reduction appears at ꢀ2.04 V for
RuTph and ꢀ2.06 V for RuTphMe, and is irreversible in both
cases.
The electropolymerization experiments were performed po-
tentiodynamically in different solvents using 0.1m Bu4NPF6 as
the electrolyte. The first studies were executed in acetonitrile.
With the inclusion of the higher oxidation processes at 1.2 to
1.4 V in the potential cycling, both RuTph and RuTphMe show
a rapid decrease in all redox signals, indicating decomposition
of the complexes (see Figure S47, Supporting Information). Be-
cause changing the potential range or scan rate did not lead
to successful polymerization, the solvent was changed to di-
chloromethane, which has a lower nucleophilicity than acetoni-
trile, possibly leading to a diminished rate of side reactions[10]
(see Figure S48 and Figure 1 as an example for RuTphMe). For
RuTph, the cyclovoltammetric development is divided into
two phases: During the first eleven cycles, an increase in the
original RuIII/RuII-based redox signal occurs, indicating the for-
mation of the desired polymer, which is, however, accompa-
nied by additional signals arising at around ꢀ0.1 and 0.9 V,
suggesting the formation of byproducts. After the eleventh
cycle, the redox signals in the region between ꢀ0.4 and 0.4 V
start to decrease, which is probably because of the electro-
chemical decomposition of the formed compounds. For
RuTphMe, likewise, additional redox processes appear at
around ꢀ0.2 V, but already during the third cycle, the signal
decrease begins. In addition, further studies involving the use
Synthesis and electrochemical behavior of redox-modified
nitro-complexes
To overcome the problem of electrochemical decomposition,
we attempted to shift the respective redox potential beyond
the thiophene-related one. Therefore, a nitro group was intro-
duced at the 4-position of the central phenyl ring of the cyclo-
metalating ligand to increase the redox potentials as observa-
ble for the respective thiophene-free parent complex (RuNO2,
Figure 2).
Figure 2. Cyclic voltammograms depicting the oxidation processes of
RuTphMe and RuTphMeNO2 in comparison with the parent Ru and RuNO2
(10ꢀ4 m in CH3CN with 0.1m Bu4NPF6).
The nitro group was attached directly on the cyclometalat-
ing phenyl ring using Cu(NO3)2 (Scheme 2). Here, the common
Menke conditions[4a,22] had to be attenuated to prevent the ni-
tration of the thiophene moiety.[23] In fact, even if a high
excess of Cu(NO3)2 is used in dichloromethane/methanol as
a solvent mixture, only the nitration on the cyclometalating
phenyl ring is observed, which underlines the high reactivity in
the position para to the carbanion and, conversely, the ease of
manipulating the carbanion donation in cyclometalated com-
Figure 1. CV development during electropolymerization attempt for
RuTphMe in CH2Cl2 (10ꢀ4 m with 0.1m Bu4NPF6).
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ring of RuTph by using CuBr2 in dichloromethane/methanol,[25]
with the aim of blocking the reactive para position of the cen-
tral phenyl ring, but maintaining the oxidation potential at
a reasonably constant value. However, as for the nonbrominat-
ed species, the electropolymerization control experiments re-
sulted in nondefined electrochemical processes (see Fig-
ure S50, Supporting Information), thus ruling out potential side
reactions that are related to the reactive position para to the
carbanion.
Electrochemical characterization of RuTphMeNO2 and
RuTphNO2 showed that both the first oxidation and first re-
duction potentials are shifted anodically by about 200 mV (see
Table 1) because of the electron-withdrawing influence of the
nitro group. As for their nitro-free counterparts, the first, rever-
sible anodic signal is assigned to oxidation of the rutheniu-
m(II)/cyclometalating-ligand moiety.[4a] Importantly, as intend-
ed, the second oxidation is achieved more easily than the irre-
versible oxidation of the parent RuNO2 complex (see Figure 2).
Hence, this redox process is assigned to the thienyl-based oxi-
dation, which should, in turn, enable electrochemical polymeri-
zation without decomposition. In contrast to the preceding
complexes, the first reduction is reversible, and a further pro-
cess occurs at ꢀ2.10 V.
Scheme 2. Synthesis of the nitro-functionalized thiophene-containing cyclo-
metalated ruthenium(II) complexes: a) Cu(NO3)2, CH2Cl2/MeOH (2:1 to 3:1),
RT, 96 h, 67 to 85%. RuNO2 was prepared previously in an analogous synthe-
sis.[4a]
plexes. The pure complexes were obtained after counterion ex-
change to hexafluorophosphate and crystallization through
vapor diffusion of diethyl ether into a concentrated DMF solu-
tion. The desired nitro-functionalization was proven unambigu-
ously by single-crystal X-ray diffraction in case of RuTphMeNO2
(Figure 3).[24] The bond lengths and bite angles as well as the
dihedral angles of the nitro group relative to the central
In addition, DFT calculations were executed to examine the
energies and spin-density distributions of the singly and
doubly oxidized states of the four complexes (see Figure 4 and
Figures S68 to S77 in the Supporting Information). First, the as-
signment of the first oxidation to a metal- and cyclometalat-
ing-ligand-based process is confirmed for all the complexes.
The second oxidation process may formally lead to a singlet or
triplet state, depending on whether the removed electron has
Figure 3. ORTEP plot[24] of RuTphMeNO2: thermal ellipsoids drawn at 50%
probability level; solvent molecules, counterion, and hydrogen atoms omit-
ted for clarity. Disorder of a thiophene omitted for clarity. Selected bond
lengths (ꢂ) and angles (8): RuꢀC1, 1.981(4); RuꢀN1, 2.085(4); RuꢀN2,
2.032(3); RuꢀN3, 2.075(4); RuꢀN4, 2.082(3); RuꢀN5, 2.052(3); N4-Ru-N5,
155.56(13); N1-Ru-N3, 155.89(14).
phenyl ring (528) and of the mesityl ring relative to the triazole
(62–728) are comparable to the previously reported crystallo-
graphic data.[4a] Accordingly, the electron-withdrawing charac-
ter of the nitro group is attenuated, and the p conjugation be-
tween the complex and the N-substituents of the triazole is
broken. Note also that the p conjugation into the triazoles for
RuTph is only weak as indicated by DFT calculations (vide in-
fra), which is in line with the literature reports.[17] On the other
hand, the small torsion between thiophene and mesityl allows
extended conjugation, which may give rise to an additional
chromophore after the electropolymerization (vide infra). In ad-
dition, a bromo function was introduced to the central phenyl
Figure 4. Spin density of singly oxidized (top) and doubly oxidized triplet
(bottom) states of RuTphNO2 (dark and light regions indicate excess of
alpha and beta spin, respectively; iso value 0.002).
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alpha or beta spin, respectively. The singlet (closed shell) con-
figuration affords a metal–cyclometalating-ligand-based oxida-
tion, which requires a potential that is shifted anodically by
1.33 to 1.55 V in comparison with the first oxidation process.
Whereas the triplet configuration of the parent complexes Ru
and RuNO2 displays a small stabilization relative to the singlet
configuration, the thiophene-containing complexes reveal the
localization of the second spin on one phenylthienyl unit, caus-
ing a potential difference of only 0.80 to 1.15 V compared with
the first oxidation. Notably, the computed second oxidation
should be regarded as an upper limit, considering the artificial
stabilization of extended p-systems,[26] the effect of the sur-
rounding charges,[27] and the challenges involved in treating
spin–spin interactions accurately,[28] which become particularly
important in strongly coupled open-shell systems.[29] The intro-
duction of the nitro substituent leads to an anodic shift of the
calculated metal–cyclometalating-ligand-based redox poten-
tials by approximately 0.25 V, whereas the thiophene-based ox-
idation remains almost unchanged. Hence, the calculations
support the observed electrochemical behavior of the systems
upon nitration, but cannot definitely support the electrochemi-
cal decomposition of the thiophene-equipped complexes
through an irreversible oxidation of the central complex frag-
ment. However, the observed deviations between experimental
data and calculations for the model complex Ru, namely the
difference for the second oxidation, must also be taken into
account for the thiophene-functionalized complexes.
Figure 6. CVs of RuTphMeNO2 films showing the first oxidation process at
different scan rates. Inset: Relationship between peak currents and applied
scan rate. (Film on glassy-carbon disk electrode in CH2Cl2 with 0.1m
Bu4NPF6.)
voltammetry (depicted in Figure 6) and UV/Vis spectroscopy
(vide infra) was possible. The CV shows a reversible first oxida-
tion with a half-wave potential of 0.28 V, which is slightly shift-
ed cathodically compared with the dissolved monomer com-
plex. Furthermore, the peak current grows linearly with in-
creasing scan rate up to 500 mVsꢀ1, indicating the formation
of conductive films with only weakly diffusion-controlled
charge migration.[10,31]
The electropolymerization of RuTphMeNO2 in dichlorome-
thane was studied, and the respective cyclic voltammograms
are shown in Figure 5. The peak current of the first oxidation
of the complex increases during the first five to ten cycles, as
In a similar manner, anodic polymerization was also attempt-
ed for the nonmethylated congener RuTphNO2. Notably, only
relatively low concentrations (around 50 mgmLꢀ1 or 5ꢃ10ꢀ5 m)
could be applied because of the poor solubility of the com-
plex. However, in contrast to its methylated counterpart,
a linear increase with a steady slope of the monitored peak
current was observed within the 30 cycles conducted
(Figure 7),[32] and a surface coverage of G=2ꢃ10ꢀ9 molcmꢀ2
was obtained. Additional signals appeared at 0.45 and ꢀ0.05 V,
which are tentatively assigned to radicals that were not incor-
porated into the polymer. Comparable features had already
been observed in former electropolymerization studies.[19] The
Figure 5. CV and peak-current development during the electropolymeriza-
tion of RuTphMeNO2 in CH2Cl2 (10ꢀ4 m with 0.1m Bu4NPF6).
expected for a successful electropolymerization. However, the
slope is comparatively low, indicating a low polymerization
rate, and decreases afterwards, reaching a plateau at the twen-
tieth cycle.[30] Hence, the obtained films are very thin (the ap-
parent surface coverage was determined to be only G=1ꢃ
10ꢀ9 molcmꢀ2). Nevertheless, their characterization by cyclic
Figure 7. CV and peak-current development during the electropolymeriza-
tion of RuTphNO2 in CH2Cl2 (10ꢀ5 m with 0.1m Bu4NPF6).
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CVs of the obtained thin polymer films displayed a reversible
redox signal at 0.25 V, which is slightly shifted cathodically
with respect to both its corresponding monomer and the
RuTphMeNO2 polymer film. As for the latter, the peak-current/
scan-rate relationship shows linear behavior up to 500 mVsꢀ1
(Figure 8). Notably, the accompanying signals at 0.45 and
ꢀ0.05 V are not present in the film CVs, supporting their as-
signment to species that were not incorporated.
Figure 9. CV and peak-current development during the co-electropolymeri-
zation of RuTphMeNO2 with EDOT in CH2Cl2 (10ꢀ4 m with 0.1m Bu4NPF6)
using a molar ratio of 1:1.
of the RuIII/RuII-based peak-current increase is still reduced
after 15 cycles, and the further current development parallels
the pure PEDOT-based one. This indicates that only PEDOT is
formed from that cycle on, and that the ruthenium(II) complex
is no longer included. Eventually, an increase in the EDOT
molar ratio to 1:1 leads to significantly improved polymeri-
zation. The RuIII/RuII-related current increases linearly at least
up to the fortieth cycle with a larger slope than the PEDOT-re-
lated current, indicating that the ruthenium(II) moiety is still in-
corporated into the generated copolymer. A similar behavior
was observed for an excess of EDOT, namely for a rutheniu-
m(II)-complex/EDOT ratio of 1:5, with a higher current for the
PEDOT-related background, as expected. In this way, copoly-
mer films containing the cyclometalated ruthenium(II) complex
were prepared and characterized. Cyclic voltammetry revealed
a reversible oxidation process for both the 1:1 and 1:5 copoly-
mers at around 0.27 V accompanied by a broad, undefined
redox current assigned to the electrochemical doping of
PEDOT chains. In both cases, the peak-current/scan-rate rela-
tionship is linear up to 500 mVsꢀ1 (Figures 10 and S56), and
apparent surface coverages of 5ꢃ10ꢀ9 and 8ꢃ10ꢀ9 molcmꢀ2
(with respect to the complex moieties) were determined for
the 1:1 and 1:5 ratios, respectively.
Figure 8. CVs of RuTphNO2 films showing the first oxidation process at dif-
ferent scan rates. Inset: Relationship between peak currents and applied
scan rate. (Film on glassy-carbon disk electrode in CH2Cl2 with 0.1m
Bu4NPF6.)
In addition, electropolymerization studies on the nitro-func-
tionalized complexes using higher vertex potentials were per-
formed, which resulted in a nondefined reaction process, as al-
ready observed for the non-nitro species (Figures S52 and S53,
Supporting Information). Thus, the enhanced electropolymeri-
zation ability upon nitro-functionalization can indeed be attrib-
uted to shifted redox potentials, but not to a blocking of reac-
tive sites.
Copolymerization experiments
For further enhancement of the electropolymerization perfor-
mance of the RuTphMeNO2 complex, copolymerization with
3,4-ethylenedioxythiophene (EDOT) as a co-monomer was at-
tempted. The ruthenium(II) complexes are diluted within the
resulting copolymers, thus undesirable side reactions between
the metal complexes are expected to be diminished. Hence,
different complex/EDOT ratios were used to identify the EDOT
content that leads to improved electropolymerization. The re-
sulting CVs during the potentiodynamic anodic polymerization
experiments are shown for a molar ratio of 1:1 in Figure 9 (see
Figure S54 for ratios of 5:1, 2:1, and 1:5). With a 5:1 ratio, no
improvement in the polymerization process is noticeable at all.
As for the pure complex, the peak-current development shows
an interruption around the fifteenth cycle. However, an addi-
tional plateau arises between ꢀ0.2 and 0.6 V, which is assigned
to the formation of poly(3,4-ethylenedioxythiophene) (PEDOT)
chains (see Figure S58 for comparison). For a molar ratio of
2:1, enhanced PEDOT generation is observable, but the slope
As for its methylated analogue, copolymers of RuTphNO2
and EDOT were prepared electrochemically with molar ratios
of 1:1 and 1:5 (see Figure S55, Supporting Information). For the
equimolar ratio, only marginal differences, namely a small cur-
rent plateau between ꢀ0.2 and 0.5 V, occur in comparison
with the homopolymerization. This behavior can probably be
attributed to the very low concentration (below 10ꢀ4 m) of
EDOT, which is required because of the low complex solubility.
In contrast, a fivefold EDOT excess leads to the distinct forma-
tion of PEDOT moieties, as indicated by the development of
a broad current plateau. As for RuTphMeNO2, the peak current
corresponding to the RuIII/RuII redox couple increases faster
than the subjacent PEDOT-related current, showing that both
the ruthenium(II) complex and EDOT are copolymerized. Subse-
quent electrochemical characterization confirmed these find-
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phenyl spacer moieties causes a hypsochromic shift as well as
a decreasing extinction coefficient. This is probably because of
diminished p conjugation within the triazole-phenyl fragment
owing to steric hindrance by the methyl groups, precluding
coplanarization. Emission measurements revealed room-tem-
perature photoluminescence at around 740 nm for the nitro-
free RuTph and RuTphMe, whereas the emission maxima for
the nitro-substituted species are, as expected, blueshifted by
about 900 to 1000 cmꢀ1.
Comparison of the UV/Vis absorption spectrum of the
RuTphMeNO2 polymer film with that of the monomer complex
shows only a negligible redshift of the MLCT maximum of
110 cmꢀ1 from 520 nm for the dropcasted monomer film to
523 nm for the polymer, accompanied by a broadening and
a loss of structural features for the MLCT band (Figure 11, top).
Additionally, an intense peak arises at 341 nm, which is as-
signed to pp* transitions that are located on the bis(phenylth-
Figure 10. CVs of electropolymerized copolymer films from RuTphMeNO2
and EDOT showing the first oxidation process at different scan rates and re-
lationship between peak currents and applied scan rate (films on glassy-
carbon disk electrode in CH2Cl2 with 0.1m Bu4NPF6) for a molar ratio of 1:1.
ings. The cyclic voltammogram of the 1:1 polymer (Figure S57,
Supporting Information) resembles in principle the homopoly-
mer with only small deviations, whereas the films from the
higher EDOT ratio show significant PEDOT influence, that is,
a broad, underlying current plateau. Analysis of the RuIII/RuII-re-
lated current revealed an apparent surface coverage of approx-
imately 2ꢃ10ꢀ9 molcmꢀ2 in both cases.
UV/Vis spectroscopy and UV/Vis/NIR spectroelectrochemis-
try
The UV/Vis absorption and emission features of the monomer
complexes are depicted in Table 2 and the Supporting Informa-
tion (Figure S59). The absorption spectra exhibit a set of bands
Table 2. UV/Vis spectroscopic characteristics of the monomer complexes
(10ꢀ6 m in CH2Cl2).
Complex
lAbs [nm] (e [103 mꢀ1 cmꢀ1])[a]
lEm [nm]
RuTph
685s (0.7), 586s (5.0), 533 (9.5), 491 (11.0),
388 (43.8)
733
RuTphMe
689s (0.8), 590s (5.5), 536 (10.5), 491 (12.8),
372 (32.1)
645s (0.7), 570s (4.0), 507 (11.8), 479 (13.7),
389 (36.3)
642s (0.7), 560s (5.6), 515 (14.3), 482 (16.5),
361 (26.1)
743
689
690
RuTphNO2
RuTphMeNO2
Figure 11. UV/Vis absorption spectra of electropolymerized films (&) from
RuTphMeNO2 (top) and RuTphNO2 (bottom) in comparison with the drop-
casted monomer (~) (films on ITO-coated glass) and the dissolved mono-
mer (*) (10ꢀ6 m in CH2Cl2).
[a] s: shoulder.
between 450 and 700 nm, which are assigned to metal-to-
ligand and metal/ligand-to-ligand charge-transfer (MLCT and
MLLCT, respectively) transitions.[4a] The introduction of nitro
groups causes a blueshift of these bands owing to the elec-
tron-withdrawing nature of the nitro group, which gives rise to
a stabilization of the HOMOs, located on the cyclometalating
ligand and the metal.[4a] An additional band can be found at
around 400 nm. Here, the introduction of methyl groups at the
ienyl) moiety,[33] which is present in the polymer but not in the
monomer, and thus confirms the coupling of the monomer
complexes. Unfortunately, spectroelectrochemical investiga-
tions could not be executed because the obtained films were
too thin to give an observable absorption signal within the
used setup.
For the polymer film from RuTphNO2, the UV/Vis absorption
results exhibit a prominent, MLCT-based band at 531 nm,
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slightly redshifted by 220 cmꢀ1 with respect to the monomer
film (525 nm) (Figure 11, bottom) and by 290 cmꢀ1 compared
with the methylated analogue, suggesting a higher degree of
conjugation in the nonmethylated polymer. Likewise, the addi-
tional band that is present in the UV region, assigned to bi-
s(phenylthienyl) units[33] formed through the polymerization, is
redshifted by 590 cmꢀ1 to 348 nm. Furthermore, UV/Vis/NIR
spectroelectrochemical studies on the polymer film were per-
formed and an example is shown in Figure 12. The spectral
trochemical studies of the copolymer films showed a combina-
tion of Ru and PEDOT characteristics (see Figures S63 to S66,
Supporting Information): The disappearance of the MLCT band
between 400 and 600 nm, and the rise of a broad, intense
band in the NIR region, respectively. In the long-wavelength
visible-light region, the behavior of the metal complex, that is,
the formation of a new LMCT absorption band, is dominant.
Notably, for the 1:1 copolymer, the PEDOT-based NIR absorp-
tion is blueshifted with respect to the 1:5 copolymer, indicat-
ing the presence of shorter oligo-EDOT chains, which possess
a smaller conjugated p system, whereas the 1:5 system exhib-
its an NIR absorption maximum similar to the pure PEDOT ref-
erence, indicating that the maximum conjugation length is al-
ready achieved. The application of a re-reducing potential led
to the recovery of the initial spectrum in both cases, and simi-
larly, monitoring of the UV/Vis transmission while repeatedly
changing between the oxidizing and re-reducing potentials
showed a reversible redox switchability for at least 30 cycles.
The UV/Vis absorption spectrum of the RuTphNO2 1:1 copo-
lymer (see Figure S61, Supporting Information) is basically the
same as that of the homopolymer. In contrast, the film of the
1:5 copolymer shows an enhanced absorption in the NIR
region, characteristic of PEDOT. Remarkably, in contrast to its
methylated counterpart, a notable redshift of 1300 cmꢀ1 be-
tween the homopolymer and the 1:5 copolymer appears for
the absorption band around 520 nm, due to an overlap with
the PEDOT-based absorption (see Figure S67, Supporting Infor-
mation). UV/Vis/NIR spectroelectrochemical studies on the co-
polymer films revealed the typical spectral changes during oxi-
dation. For the 1:1 copolymer, as soon as the oxidation of the
metal center begins, the MLCT absorption band at around
500 nm vanishes, whereas a very broad, weak absorption
arises beyond 600 nm. In contrast to the homopolymer (vide
supra), the latter spans the region up to 1600 nm, and is attrib-
uted to the incorporated oligo-EDOT chains. However, their in-
fluence is significantly smaller than for the 1:1 copolymer of
the methylated analog. An isosbestic point at 590 nm, which is
not present for the other copolymer studies, supports the
presence of only one electro-optically determinant species,
namely the ruthenium(II) complex, whereas the other copoly-
mers show significant features of both metal complex and
PEDOT. In the case of the 1:5 copolymer, both the metal com-
plex moiety and PEDOT chains determine the spectra; a de-
crease in the complex MLCT absorption between 400 and
600 nm is accompanied by the emergence of a strong NIR ab-
sorption peaking at 1350 nm. Re-reduction of the 1:1 and 1:5
copolymer films recovered the initial UV/Vis/NIR absorption
spectra, demonstrating the redox stability of the systems. How-
ever, for the 1:5 copolymer, repetitive switching of the redox
state over 30 cycles showed a diminishing of the maximum ab-
sorption change to 95% of the initial value, which could not
be observed for the other systems.
Figure 12. Change in the UV/Vis absorption spectrum of an electropolymer-
ized film of RuTphNO2 during the oxidation and re-reduction (*) process.
(Note that the underlying absorbance between 800 an 1100 nm (*) is attrib-
uted to the ITO substrate.) Inset: Change of transmission at 510 nm over 30
cycles of switching between initial and oxidized state. (Film on ITO-coated
glass in CH2Cl2 with 0.1m Bu4NPF6.)
changes during the oxidation process with a half-wave poten-
tial of 0.25 V resemble in principle the characteristic features
observed for the present cyclometalated ruthenium(II) complex
moiety, namely a bleaching of the MLCT absorption and the
rise of a broad and weak band between 700 and 900 nm,
which is assigned to ligand-to-metal charge-transfer (LMCT)
transitions.[4a] Repeated switching between the initial and oxi-
dized state turned out to be reversible for at least the 30
cycles that were run, proving the redox stability of the pre-
pared polymer film, and revealed switching times (defined as
the time necessary to undergo 95% of the full transmission
change[34]) of 1.8 s.
The UV/Vis absorption spectra of the 1:1 and 1:5 copolymer
films of RuTphMeNO2 show absorption maxima at 523 nm and
341 nm, accompanied by a broad band in the NIR region (see
Figure S60, Supporting Information). With increasing EDOT
ratio, the relative intensity of the latter rises; this is attributed
to the growing content of PEDOT moieties, which exhibit
a strong NIR absorption (see Figure S62 for comparison). Si-
multaneously, the band at 341 nm, which is related to the bi-
s(phenylthienyl) moieties, decreases with respect to the MLCT
absorption because the bis(thienyl) bridges are replaced by
oligo-EDOT blocks for the copolymers. UV/Vis/NIR spectroelec-
Conclusion
Oxidative electrochemical polymerization can be applied suc-
cessfully to incorporate electron-rich ruthenium(II) complexes
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Full Paper
of cyclometalating, 1,2,3-triazole-based polypyridyl-type li-
gands equipped with electropolymerizable thiophene moieties
into polymeric thin-film coatings. The use of nonfunctionalized
thiophene-containing complexes leads to decomposition reac-
tions during the electropolymerization process, which are at-
tributed to an irreversible second oxidation of the electron-rich
central complex fragment. The subsequent selective introduc-
tion of a nitro group at the phenyl ring causes an anodic shift
of the decomposition-related redox potential, but not of the
thiophene-assigned one. Hence, the maximum potential
during the potentiodynamic polymerization can be chosen
such that the generation of thienyl radicals, which are crucial
for the formation of the polymer chains, is possible, without
degradation of the complex fragment. The problem of unde-
sired side reactions is expected to be a general issue for the
oxidative electropolymerization of cyclometalated complexes,
so this approach is believed to be a generally applicable strat-
egy for the processing of cyclometalated ruthenium(II)-poly-
pyridyl systems. Alternatively, the incorporation of 3,4-ethyle-
nedioxythiophene (EDOT) moieties instead of the thiophene
groups into the ruthenium(II) complex represents a possible
future approach for an improved electropolymerization, be-
cause the oxidation potential required for radical formation
would be lowered. Besides modifications of the metal com-
plexes themselves, an additional comonomer can be used to
enhance the polymerization performance further. Consequent-
ly, EDOT was utilized to form copolymers with different mono-
mer ratios.
Employment, and Technology (TMWAT) for financial support.
C.F. and B.S. thank the Fonds der Chemischen Industrie for
Ph.D. scholarships. M.J. is grateful for financial support of the
Carl-Zeiss-Stiftung. We also thank Prof. Dr. Anna Ignaszak for
discussions and Peter Schulze for synthetic support.
Keywords: click
chemistry
·
cyclometalation
·
electropolymerization · photosensitizers · ruthenium
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Received: April 16, 2013
Revised: October 27, 2013
Published online on January 23, 2014
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