Design, synthesis, and study of main chain poly(N-heterocyclic carbene) complexes: Applications in electrochromic devices

Article abstract of DOI:10.1021/ja104051x

A series of poly(N-heterocyclic carbene) complexes in which the carbene functionalities are orthogonally connected to the main chains of the respective polymers have been synthesized via oxidative electropolymerization of various bis(bithiophene)-substituted monomers with appended transition metal or main group entities (M) Ir, Au, Ag, or S). The polymers were characterized using a range of electrochemical, X-ray photoelectron spectroscopy, UV-vis spectroscopy, profilometry, and four-point probe conductivity measurements. Most of the polymers exhibited an intense absorbance wave at 700 nm under oxidative conditions which was attributable to the formation of polarons along the main chains. The iridium-containing thin film poly(8) was found to possess a significant NIR absorbance at 1100 nm in which the metal moiety effectively functioned as an electron sink. Electrochemical analyses of the polymer thin films revealed that they exhibited highly reversible electrochromic phenomena.

Full text of DOI:10.1021/ja104051x

Published on Web 07/07/2010
Design, Synthesis, and Study of Main Chain
Poly(N-Heterocyclic Carbene) Complexes: Applications in
Electrochromic Devices
Adam B. Powell, Christopher W. Bielawski, and Alan H. Cowley*
Department of Chemistry and Biochemistry and Center for Nano and Molecular Science and
Technology, The UniVersity of Texas at Austin, 1 UniVersity Station,
A5300, Austin, Texas 78712-0165
Received May 12, 2010; E-mail: cowley@mail.utexas.edu
Abstract: A series of poly(N-heterocyclic carbene) complexes in which the carbene functionalities are
orthogonally connected to the main chains of the respective polymers have been synthesized via oxidative
electropolymerization of various bis(bithiophene)-substituted monomers with appended transition metal or
main group entities (M ) Ir, Au, Ag, or S). The polymers were characterized using a range of electrochemical,
X-ray photoelectron spectroscopy, UV-vis spectroscopy, profilometry, and four-point probe conductivity
measurements. Most of the polymers exhibited an intense absorbance wave at 700 nm under oxidative
conditions which was attributable to the formation of polarons along the main chains. The iridium-containing
thin film poly(8) was found to possess a significant NIR absorbance at 1100 nm in which the metal moiety
effectively functioned as an electron sink. Electrochemical analyses of the polymer thin films revealed that
they exhibited highly reversible electrochromic phenomena.
Introduction
Electroconducting polymers (ECPs) have become an increas-
ingly active area of research following the work of Shirakawa
and co-workers in 1977 on the conducting properties of
polyacetylene.1 Organic ECPs, in particular, have been studied
extensively and have found utility in many photovoltaic,2
sensing,3 and medicinal applications,4 among others. Such
materials are generally polymerized electrochemically since this
methodology offers a number of advantages which include
synthetic ease and the ability to deposit the polymer directly
onto an electrode or conducting substrate of choice.5 Thiophene
is commonly used as a monomer for accessing ECPs due to its
Figure 1. Examples of polymers synthesized via electropolymerization.
high functional group tolerance; however, ethylenedioxythio-
phene (EDOT) and bithiophene-based monomers are also often
employed because they can be electrochemically polymerized
at relatively low potentials.6
(1) Chiang, C. K.; Fincher, C. R.; Park, Y. W.; Heeger, A. J.; Shirakawa,
H.; Louis, E. J.; Gau, S. C.; MacDiarmid, A. G. Phys. ReV. Lett. 1977,
39, 1098–1101.
The incorporation of transition metal and main group atoms
into ECPs has been shown to afford significant increases in
conductivity, thus greatly expanding the utilities of these
materials.7 For example, the polymers shown in Figure 1 have
been used for the support of elements which span the periodic
table.8 Of the various electropolymerizable scaffolds depicted,
the most widely employed are those derived from the N,N′-
ethylenebis(salicylimine) (Salen) ligand framework, mainly
because such ligands can be prepared via simple condensation
of ethylene diamine and salicylaldehyde. However, a notable
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2007, 6, 521–527. (b) Knapton, D.; Rowan, S. J.; Weder, C.
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10184 J. AM. CHEM. SOC. 2010, 132, 10184–10194
10.1021/ja104051x  2010 American Chemical Society

Main Chain Poly(N-Heterocyclic Carbene) Complexes
A R T I C L E S
Scheme 1. Synthesis of Gold NHC Complex 4
disadvantage of Salen and related ligands is that the attachment
of electroactive, polymerizable substituents, such as thiophene
or bithiophene, is challenging due to the coupling chemistry
required.9 Moreover, the rigid geometry of the resulting Salen
ligand constrains the coordinated metal to a tight, four-coordi-
nate environment which can complicate further derivatization.
Given the widespread use of R-diimines for the support of a
significant variety of p-, d-, and f-block functionalities, we were
prompted to develop a new electropolymerizable ligand class
via the attachment of flanking thiophene substituents to an
R-diimine. Such a system was envisioned to create a more open
site for metal coordination and also to avoid the use of the
aforementioned coupling reactions. Although our initially
synthesized diimine ligand, 1,2(bis)-thiophene diazadiene met
these requirements, it failed to undergo electropolymerization
under reasonable conditions.10,11 To overcome this limitation,
the thiophene moieties in this substrate were replaced with
bithiophenes, and the ligand was incorporated into an N-
heterocyclic carbene (NHC) scaffold.
Scheme 2. Electrochemical Polymerization of Bithiophene-
Substituted NHC-Metal Complexes 3-8
NHCs have attracted considerable attention over the last two
decades due to their ability to stabilize many transition metal
and main group complexes under a wide range of conditions.12
Furthermore, polymeric materials have been shown to possess
superior stabilities and enhanced electrocatalytic kinetics when
compared with their monomeric analogues.13-16 Although many
examples are known in which NHCs have been incorporated
into the backbones of polymeric materials,16 no examples existed
until very recently of polymers in which the NHCs were
positioned orthogonally with respect to the main chains and thus
available for ligation to other functionalities.17 As summarized
in Scheme 1, the requisite monomer for accessing such main-
chain poly(NHC)s was synthesized by the cyclization of diimine
1 to form the corresponding imidazolium chloride 2. Subsequent
metalation of 2 with Ag2O (to form 3) followed by transmetalla-
tation with tetrahydrothiophene (tht)AuCl (tht ) tetrahydrothiophene)
afforded 4, which was subsequently electropolymerized and
characterized using standard techniques.17
Inspired by a recent account which demonstrated the impact
of different coordinated metals on the electronic behavior of
ECPs,18 we have undertaken a comprehensive investigation of
the influence of the ligated metal centers on the electronic
properties of the aforementioned main-chain poly(NHC) com-
plexes. More specifically, the methodology detailed for the gold
NHC polymer was expanded to include polymers containing
other transition metal and main group moieties (see Scheme
2), which were characterized using a suite of electrochemical,
X-ray photoelectron and UV-vis spectroscopy, profilometry,
and four-point probe conductivity measurements.
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Trans. 2009, 14, 2521–2527.
(11) Interestingly, a recent publication revealed a similar outcome for
electropolymerizable phosphine-substituted monomers, see: Velau-
thamurty, K.; Higgins, S. J.; Rajapakse, R. M. G.; Bacsa, J.; van
Zalinge, H.; Nichols, R. J.; Haiss, W. J. Mater. Chem. 2009, 19 (13),
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6), 596–609.
During the course of these studies, a noticeable color change
was observed when the gold NHC complex 4 was electropo-
lymerized on an indium tin oxide (ITO) slide. It was surmised
that this change originated from the development of polarons
along the polymer main chains. In general, polymers such as
poly(3,4-ethylenedioxythiophene) (PEDOT) are well-known to
exhibit such electrochromic characteristics, but examples of
organometallic-based ECPs are relatively rare.18 Electro-
(13) Winter, A.; Friebe, C.; Chiper, M.; Hager, M. D.; Schubert, U. S. J.
Polym. Sci., Part A 2009, 47, 4083–4098.
(14) Leung, A. C. W.; Chong, J. H.; MacLachlan, M. J. Macromol. Symp.
2003, 196, 229–234.
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Chem. Commun. 2006, 16, 1727–1729. (c) Boydston, A. J.; Rice, J. D.;
Sanderson, M. D.; Dykhno, O. L.; Bielawski, C. W. Organometallics
2006, 25, 6087–6098. (d) Boydston, A. J.; Bielawski, C. W. Dalton
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9
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A R T I C L E S
Powell et al.
using Starna quartz cells. A three-electrode cell was constructed in
which each polymer was deposited onto a ITO-coated glass and
used as the working electrode. The counter- and pseudoreference
electrodes consisted of tungsten and silver wires, respectively, using
0.1 M TBAPF6 in CH2Cl2 solution. An Eco Chemie Autolab
PGSTAT30 equipped with GPES software was used to adjust the
potential of the three-electrode cell in the UV-vis spectrophotom-
eter. The polymers that were deposited onto the ITO electrodes
were analyzed using UV-vis-NIR spectroscopy (1600 nm-300
nm) while increasing the voltage in stepwise intervals of 0.10 V,
from 0.00 to 1.50 V. The stability of each film was determined by
continuous cycling at 0.10 V/s until the electrochromic signal
observed at approximately 700 nm decreased to 50% of its initial
absorbance.
X-ray Photoelectron Spectroscopy. All XPS spectra were
recorded on a Kratos Axis Ultra X-ray photoelectron spectrometer
utilizing a monochromated Al KR X-ray source (hν ) 1486.5 eV),
hybrid optics (employing a magnetic and electrostatic lens simul-
taneously), and a multichannel plate and delay line detector coupled
to a hemispherical analyzer. The photoelectron takeoff angle of 0°
(measured from the surface normal) was used in all experiments.
All spectra were recorded using an aperture slot of 300 × 700 µm,
and the high resolution spectra were collected with a pass energy
of 20 eV. The pressure in the analysis chamber was typically held
at 2 × 10-9 Torr during data acquisition. Kratos XPS analysis
software was used to determine the stoichiometries of the samples
from the corrected peak areas by applying the appropriate sensitivity
factors for each element of interest. All reported ratios represent
the average of two independent sample acquisitions, and all binding
energies are reported with respect to the carbon 1s signal at 286.1
eV.
X-ray Crystallography. All crystals were grown in the absence
of light. Crystals suitable for data collection were covered with
hydrocarbon oil and mounted on thin nylon loops. The X-ray
diffraction data were collected at 153 K on a Rigaku AFC-12
diffractometer equipped with a Saturn 724 and CCD area detector,
an Oxford Cryostream low-temperature device, and a graphite-
monochromated Mo KR radiation source (λ ) 0.71073 Å).
Corrections were applied for Lorentz and polarization effects. The
structures were solved by direct methods and refined by full-matrix
least-squares cycles on F2.25 All non-hydrogen atoms were refined
with anisotropic thermal parameters, and all hydrogen atoms were
placed in fixed, calculated positions using a riding model (C-H,
0.96 Å).
Profilometry and Conductivity. Profilometry measurements for
3-8 were performed on a Dektak 6 M Stylus Profilometer in a
clean-room facility. The average thicknesses of the polymers were
based on an average of 12 data acquisitions on two different
samples. The samples were prepared by electropolymerizaton of
the polymers on ITO slides using cyclic voltammetry (50 cycles)
between -0.2 V and +1.7 V.
chromism is a useful property that has found commercial
applications in color-changing windows and mirrors,19 polymer
light-emitting diodes,20 and near-infrared (NIR) devices.21
Moreover, there is a current demand for organic materials which
exhibit excellent color efficiencies and fast switching capabili-
ties.22 On a different level, the study of electrochromic materials
is also useful for analysis of the electronic structures of p-doped
conjugated polymers in both the oxidized (cationic) and reduced
(neutral) states.23 The metallopolymer scaffolds shown in
Scheme 2 are well suited for use in electrochromic devices since
they: (1) should enable coordination of a range of transition
metals and main group elements, each of which may display
different characteristics, and (2) feature orthogonally positioned
carbene groups in direct electronic contact with the polymer-
izable scaffold, which may be further derivatized to tailor the
electronic properties exhibited by the materials in which they
are contained.
Experimental Section
General Procedures. All solvents and reagents were used as
received from commercial sources unless stated otherwise. The
synthesis of the bithiophene-substituted diimine 1 and imidazolium
chloride 2 are reported elsewhere.17 Methylene chloride was
distilled over calcium hydride prior to use. NMR spectra were
recorded at 298 K on a Varian Inova instrument (1H NMR, 499.40
MHz; 13C NMR, 125.57 MHz) using residual solvent or tetra-
methylsilane (TMS) as an internal reference unless stated otherwise.
Low-resolution chemical ionization mass spectral data (MS-CI)
were collected on a Thermo Scientific TSQ Quantum GC mass
spectrometer. High-resolution CI-MS spectra were recorded on a
magnetic sector Waters Autospec Ultima instrument or on an
IonSpec 9.4T FT mass spectrometer equipped with an ESI source,
and data are reported as m/z (relative intensity). Infrared spectra
were recorded in the solid state (KBr) using a Nicolet Avatar 260
FT-IR spectrometer. The spectroelectrochemical and absorbance
measurements were recorded on a Varian Cary 5000 UV-vis-NIR
spectrophotometer using Starna quartz fluorometer cells with a path
length of 10 mm for 3 and 5-8 and a 0.1 mm path length for
poly(3)-poly(8).
Electrochemistry. Electrochemical syntheses and studies were
performed in a glovebox under a nitrogen atmosphere using a CH
Instruments Electrochemical Workstation (series 700B). All elec-
trochemical experiments were carried out in a three-electrode cell
consisting of a platinum button working electrode, a tungsten
counter-electrode, and a silver wire pseudoreference electrode. A
0.1 M solution of [(n-Bu)4N+][PF6-] (TBAPF6) was used as the
supporting electrolyte. The TBAPF6 was purified via recrystalli-
zation three times from hot ethanol before being dried for 3 days
at >100 °C under a dynamic vacuum. Electrosyntheses of the films
onto a platinum button were performed from 1 × 10-3 M monomer
solutions by continuous cycling between -0.20 and 1.50 V (upper
limit of 1.70 V for 6) at V ) 100 mV s-1. Analogous continuous
cycling was performed between -0.20 V and +1.70 V for
deposition of the polymers onto either stainless steel or ITO slides.
The resulting films were then washed with copious amounts of
CH2Cl2 and stored in the absence of light. All potentials listed in
the manuscript and Supporting Information were referenced to SCE
by shifting Fc*0/Fc*+ to -0.057 V.24
Bithiophene-Substituted Silver NHC Complex 3. The imida-
zolium chloride 217 (0.115 mmol, 0.080 g) and silver(I) oxide (0.069
mmol, 0.016 g) were added to a round-bottomed flask containing
dry CH2Cl2 (15 mL), and the flask was covered with aluminum
foil. The reaction mixture was stirred at ambient temperature for
48 h in the absence of light, after which it was filtered through
Celite. The solvent was removed under reduced pressure to afford
the desired product 3 as a yellow solid (95% yield). Single crystals
suitable for X-ray data collection were obtained by slow vapor
diffusion of hexanes into a solution of 3 in THF (1:3.00 THF:
hexanes v/v) over a period of five days at ambient temperature. 1H
NMR (499.40 MHz, CD2Cl2, TMS): δ 7.28 (d, 4H, J ) 8.0 Hz),
7.22 (d, 4H, J ) 8.0 Hz) 7.15 (dd, 2H, J ) 5.0, 1.0 Hz), 7.00 (dd,
2H, J ) 3.5, 1.0 Hz), 6.90-6.88 (m, 4H), 6.69 (d, 2H, J ) 3.5
Spectroelectrochemistry. Spectroelectrochemical data were
recorded on a Varian Cary 5000 UV-vis-NIR spectrophotometer
(20) Mortimer, R. G. Chem. Soc. ReV. 1997, 26, 147–156.
(21) (a) Sonmez, H.; Meng, H.; Wudl, F. Chem. Mater. 2003, 15, 4923–
4929. (b) Meng, H.; Tucker, S.; Chaffins, Y.; Chen, R.; Helgeson, R.;
Dunn, B.; Wudl, F. AdV. Mater. 2003, 15, 146–149.
(22) Kraft, A.; Grimsdale, A. C.; Holmes, A. B. Angew. Chem., Int. Ed.
1998, 37, 403–428.
(24) Noviadri, I.; Brown, K. N.; Fleming, D. S.; Gulyas, P. T.; Lay, P. A.;
Masters, A. F.; Phillips, L. J. Phys. Chem. B 1999, 103, 6713–6722.
(25) Sheldrick, G. M. SHELL-PC, version 5.03; Siemens Analytical X-ray
Instruments, Inc.: Madison, WI, 1994.
(23) Pang, Y.; Li, X.; Ding, H.; Shi, G.; Jin, L. Electrochim. Acta 2007,
52, 6172–6177.
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Main Chain Poly(N-Heterocyclic Carbene) Complexes
A R T I C L E S
Hz), 2.60 (t, 4H, J ) 7.5 Hz), 1.55 (quint, 4H, J ) 7.0 Hz), 1.29
(sextet, 4H, J ) 7.5 Hz), 0.85 (t, 6H, J ) 7.0 Hz). 13C{1H} NMR
(125.57 MHz, CD2Cl2): δ 145.38, 141.22, 136.40, 136.22, 132.15,
129.81, 128.32, 127.50, 127.43, 126.01, 125.73, 124.81, 123.80,
35.39, 33.60, 22.73, 14.06. The carbenoid carbon was detected when
CDCl3 was used as the solvent: 13C{1H} NMR (150.83 MHz,
CDCl3): δ 181.21(br). LRMS (CI+ m/z): 804 (100% M + H+).
HRMS (CI+, CH4): calcd for C39H36N2S4107AgCl, 802.0504; found,
802.0501. Calcd for C39H36N2S4109AgCl, 804.0493; found, 804.0497.
The bis-ligated silver cation was also detected. HRMS (ESI): calcd
for C78H72N4S8107Ag, 1427.25681; found, 1427.25317. IR (cm-1):
3060, 2954, 2926, 2856, 1628 (C-Carom), 1510 (C-Carom), 1388,
1229, 1048, 1019, 838, 800, 696. λmax ) 325 nm. ε ) 4.9 × 104
chloro(1,5-cyclooctadiene)iridium(I) dimer (0.075 mmol, 0.051 g)
were added to a round-bottomed flask containing dry CH2Cl2 (15
mL), and the reaction vessel was covered with aluminum foil. The
reaction mixture was stirred at ambient temperature for 72 h, after
which it was filtered through Celite and the solvent was removed
under reduced pressure to afford the desired product 7 as a brown
powder (95% yield). 1H NMR (499.40 MHz, CD2Cl2, TMS): δ 7.57
(d, 4H, J ) 8.0 Hz), 7.16 (d, 4H, J ) 8.5 Hz), 7.12 (dd, 2H, J )
5.0, 1.0 Hz), 6.98 (dd, 2H, J ) 3.5, 1.0 Hz), 6.87 (m, 4H), 6.71 (d,
2H, J ) 3.5 Hz), 4.13 (m, 2H), 2.59 (t, 4H, J ) 7.0), 2.50 (m, 2H),
1.56 (quint, 6H, J ) 7.5 Hz), 1.27 (sextet, 8H, J ) 7.5 Hz), 1.11
(m, 2H), 0.83 (t, 6H, J ) 7.5 Hz). 13C{1H} NMR (125.57 MHz,
CD2Cl2, TMS): δ 182.62, 142.87, 139.38, 135.54, 134.86, 130.83,
128.04, 127.41, 127.11, 125.94, 125.90, 124.31, 123.47, 122.51,
82.14, 50.75, 34.50, 32.75, 32.06, 28.09, 21.35, 12.94. LRMS (CI+
m/z): 996 (100% M+ + H+), 959 (55% M - Cl). HRMS (CI+,
CH4): calcd for C47H48N2ClS4193Ir, 996.2024; found, 996.2018. IR
(cm-1): 3067, 2954, 2924, 2858, 1509 (C-Carom), 1420 (C-Carom),
1375, 1327, 1262, 1019, 837, 799, 689. λmax ) 324 nm. ε ) 4.2 ×
M-1 cm-1
.
Bithiophene-Substituted Silver NHC Complex 5. The imida-
zolium chloride 217 (0.057 mmol, 0.040 g) and silver(I) acetate
(0.143 mmol, 0.024 g) were added to a round-bottomed flask
containing 3 Å sieves and dry CH2Cl2 (15 mL). The reaction vessel
was then covered with aluminum foil, and the reaction mixture was
stirred at ambient temperature for 48 h in the absence of light. The
reaction mixture was then filtered through Celite, and the solvent
was removed under reduced pressure to afford the desired product
5 as a yellow solid (96% yield). Single crystals suitable for X-ray
data collection were obtained after one week by layering a THF
104 M-1 cm-1
.
Bithiophene-Substituted Iridium NHC Complex 8. Complex
7 was added to a round-bottomed flask containing dry CH2Cl2 (5
mL). The reaction vessel was then fitted with a rubber septum, and
the solution was degassed. The resulting solution was stirred under
an atmosphere of carbon monoxide for 12 h, after which it was
filtered through Celite and the solvent was removed under reduced
pressure to afford the desired product 8 as a brown powder (98%
yield). 1H NMR (499.40 MHz, CD2Cl2, TMS): δ 7.36 (d, 4H, J )
8.5 Hz), 7.20 (d, 4H, J ) 8.5 Hz), 7.12 (dd, 2H, J ) 5.0, 1.0 Hz),
6.97 (dd, 2H, J ) 4.0, 1.5 Hz), 6.86 (m, 4H), 6.65 (d, 2H, J ) 4.0
Hz), 2.61 (t, 4H, J ) 5.0 Hz), 1.57 (quint, 4H, J ) 8.0 Hz), 1.28
(sextet, 4H, J ) 8.0 Hz), 0.85 (t, 6H, J ) 7.5 Hz). 13C{1H} NMR
(125.57 MHz, CD2Cl2, TMS): δ 180.33, 174.44, 167.45, 144.14,
140.07, 135.28, 134.00, 130.94, 128.12, 127.98, 127.16, 126.84,
124.69, 124.55, 123.64, 122.62, 34.51, 32.39, 21.55, 12.97. LRMS
(CI+ m/z): 944 (20% M+), 909 (100% M - Cl). HRMS (CI+, CH4):
calcd for C41H36N2ClO2S4193Ir, 944.0978; found, 944.0974. IR
(cm-1): 2953, 2926, 2856, 2058 (C)O), 1974 (C)O), 1658, 1510
(C-Carom), 1398 (C-Carom), 1334, 1261, 1202, 1020. λmax ) 326
1
solution of 5 with hexanes at -40 °C (1:3 THF:hexanes v/v). H
NMR (400.27 MHz, CDCl3, TMS): δ 7.29 - 7.20 (m, 8H), 7.16 (d,
2H, J ) 5.2 Hz) 7.00 (d, 2H, J ) 4.0 Hz), 6.91-6.88 (m, 4H),
6.68 (d, 2H, J ) 3.2 Hz), 2.61 (t, 4H, J ) 7.6 Hz), 1.81 (s, 3H),
1.56 (quint, 4H, J ) 8.4 Hz), 1.30 (sextet, 4H, J ) 7.2 Hz), 0.86
(t, 6H, J ) 7.2 Hz). 13C{1H} NMR (125.57 MHz, CD2Cl2): δ
175.86, 144.06, 140.07, 135.30, 135.20, 131.01, 128.63, 127.19,
126.40, 126.27, 124.98, 124.58, 123.67, 122.66, 34.45, 32.48, 21.59,
21.47, 12.94. As in the cases of similar complexes, the carbenoid
carbon could not be detected by 13C NMR spectroscopy.26 LRMS
(CI+ m/z): 661 (100% M+ + H+ - AgC2H3O2), 828 (20% M +
H+). The bis-ligated silver cation was also detected. HRMS (ESI):
calcd for C78H72N4S8107Ag, 1427.25681; found, 1427.25771. IR
(cm-1): 3067, 2954, 2927, 2858, 1578 (C-Carom), 1510 (C-Carom),
1382, 1327, 1018, 838, 804, 696. λmax ) 325 nm. ε ) 3.0 × 104
M-1 cm-1
.
nm. ε ) 1.8 × 104 M-1 cm-1
.
Bithiophene-Substituted Thione 6. The imidazolium chloride
217 (0.143 mmol, 0.100 g), elemental sulfur (0.179 mmol, 0.006
g) and K2CO3 were added to a Schlenk flask. Degassed MeOH (25
mL) was then added Via cannula to the reaction mixture, which
was stirred subsequently at ambient temperature for 24 h. The
product was then extracted into CH2Cl2 (50 mL) and washed with
water (3 × 25 mL). The aqueous layer was back-extracted and
organic layers combined. The organic layer was then dried with
MgSO4, filtered and concentrated to afford the desired product 6
as an orange solid (98% yield). Single crystals suitable for X-ray
data collection were obtained after three days by the slow vapor
diffusion of hexanes into a solution of 6 in THF (1: 2.5 THF:
hexanes v/v) at ambient temperature. 1H NMR (499.40 MHz,
CD2Cl2, TMS): δ 7.24 (d, 4H, J ) 8.5 Hz), 7.21 (d, 4H, J ) 8.5
Hz) 7.12 (dd, 2H, J ) 5.5, 1.0 Hz), 6.96 (dd, 2H, J ) 3.5, 1.0 Hz),
6.96 (dd, 2H, J ) 5.0, 1.5 Hz), 6.83 (d, 2H, J ) 3.5 Hz), 6.62 (d,
2H, J ) 4.0 Hz), 2.58 (t, 4H, J ) 7.5 Hz), 1.55 (quint, 4H, J )
Hz), 1.27 (sextet, 4H, J ) 7.5 Hz), 0.84 (t, 6H, J ) 7.0 Hz). 13C{1H}
NMR (125.57 MHz, CD2Cl2): δ 167.11, 144.67, 140.27, 136.63,
134.71, 131.50, 129.45, 129.17, 128.24, 127.14, 125.42, 124.52,
123.58, 123.36, 35.35, 33.66, 22.70, 14.07. LRMS (CI- m/z): 692
(100% M+). HRMS (CI+, CH4): calcd for C39H36N2S5, 693.15548;
found, 693.15543. IR (cm-1): 3067, 2956, 2928, 2857, 1510
(C-Carom), 1385 (C-Carom), 1344, 1229, 1048, 1019, 838, 800, 696.
Results and Discussion
Monomer Synthesis and Characterization. The monomers 3
and 5-8 were synthesized as summarized in Scheme 3;
monomer 4 was prepared as previously described.17 The
precursor bithiophene-substituted diimine 1 was synthesized by
sodium cyanide-catalyzed coupling of the requisite aldimine in
DMF solution. Cyclization of 1 with paraformaldedyde afforded
the anticipated imidazolium chloride 2 in 87% yield. The
1
imidazolium proton was identified by H NMR spectroscopy
as a singlet at δ ) 10.41 ppm in CD2Cl2 solution. Access to
silver NHC 3, which has been previously utilized as a carbene
transfer reagent,17 was obtained by metalation of the requisite
imidazolium chloride with Ag2O. As reported for other silver
NHC complexes,27 an equilibrium was observed in solution
between 3 and its dimeric analogue (see Scheme 4). As a
consequence, the carbene nucleus of 3 could not be detected
by 13C NMR spectroscopy in CD2Cl2 solution. However, it was
observed as a broad singlet at δ ) 181 ppm when the NMR
spectrum was recorded in CDCl3. In turn, this broad singlet
resolved into two sets of doublets (in CDCl3) at -50 °C, due
to coupling of the 13C nucleus to 107Ag and 109Ag with coupling
constants of approximately 260 Hz. Related examples of this
phenomenon have been reported previously for both silver
λmax ) 324 nm. ε ) 4.3 × 104 M-1 cm-1
.
Bithiophene-Substituted Iridium NHC Complex 7. The
bithiophene-substituted silver NHC 3 (0.137 mmol, 0.110 g) and
(26) Ogle, J. W.; Zhang, J.; Reibenspies, J. H.; Abboud, K. A.; Miller,
(27) Newman, C. P.; Clarkson, G. J.; Rourke, J. P. J. Organomet. Chem.
2007, 692, 4962–4968.
S. A. Org. Lett. 2008, 10, 3677–3680.
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Powell et al.
Scheme 3. Syntheses of Monomers 3 and 5-8
Scheme 4. Equilibrium Between 3 and a Dimeric Analogue
phosphine and silver carbene complexes.28 Silver NHC 3 was
also detected by double-sector high-resolution mass spectrom-
etry, and the presence of a cationic complex consisting of the
aforementioned complex featuring a Ag atom bound to two
NHC ligands was evident from ESI spectrometry.
As shown in Figure 2 (left), a single-crystal X-ray diffraction
experiment revealed the monomeric structure of 3. Interestingly,
the packing diagram exhibited a 1D array of metal atoms
connected by short Ag-Ag contacts (Figure 2, right). The
presence of these argentophilic interactions in silver carbene
structures has been shown to be dependent upon the extent of
steric crowding proximal to the metal center. For example,
phenyl-substituted silver carbenes analogous to 3 exhibit
comparable 1-D arrays of Ag-Ag contacts;26 however, silver
carbene analogues with bulky mesityl or diisopropylphenyl
groups fail to do so in the solid state.29 The distance between
the carbene nucleus and the Ag atoms of 3 was measured to be
2.087(3) Å, a value that is significantly longer than the analogous
carbon-metal 1.950(8) Å distance previously observed17 for the
gold NHC 4 as anticipated on the basis of relativistic effects.30
Attention was next directed toward the synthesis of the
relatively bulky silver NHC 5 in order to discern whether the
differences in solid state structure would affect the stabilities
of the resulting polymers (see below). Complex 5 was synthe-
sized in nearly quantitative yield by metalation of the precursor
imidazolium chloride 2 with silver(I) acetate in CH2Cl2 solution.
(28) (a) Chen, F.; Oh, S. W.; Wasylishen, R. E. Can. J. Chem. 2009, 87,
1090–1101. (b) Ramnial, T.; Abernethy, C. D.; Spicer, M. D.;
McKenzie, I. D.; Gay, I. D.; Clyburne, J. A. C. Inorg. Chem. 2003,
42, 1391–1393.
(29) Paas, M.; Wibbeling, B.; Fro¨hlich, R.; Hahn, F. E. Eur. J. Inorg. Chem.
2006, 1, 158–162.
(30) Bayler, A.; Schier, A.; Bowmaker, G. A.; Schmidbaur, H. J. Am. Chem.
Soc. 1996, 118, 7006–7007.
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A R T I C L E S
Figure 2. (left) POV-Ray view of 3 showing 50% probability thermal ellipsoids and selected atom labels. (right) Packing diagram for 3. Hydrogen atoms
have been omitted for clarity.
Figure 4. POV-Ray view of 6 showing 50% probability thermal ellipsoids
and selected atom labels.
of the chloride ligand.32 The iridium NHC complex 8 was
Figure 3. (left) POV-Ray view of 5 showing 50% probability thermal
ellipsoids and selected atom labels. (right) Packing diagram for 5. Hydrogen
synthesized by exposure of 7 to an atmosphere of carbon
atoms have been omitted for clarity.
monoxide at ambient temperature for 12 h. Although solutions
of 8 were unstable to prolonged storage as previously reported
1H NMR spectroscopic analysis revealed broad resonances for
the phenyl groups, which suggested that this complex may exist
in equilibrium with its dimeric counterpart, as observed in the
case of 3. Indeed, analysis by electrospray ionization (ESI) mass
spectrometry revealed the presence of the diligated silver cation
as shown in Scheme 4. Decomposition of 5 in solution was
observed over the course of a few days, but slowed at lower
temperatures. In contrast, 5 was found to be remarkably robust
for other iridium NHC carbonyl complexes,33 two IR absorptions
were detected at νCO ) 2058 and 1974 cm-1 corresponding to
the two chemically distinct CO ligands.34 The carbene 13C NMR
signal for 8 was detected at δ ) 180 ppm (CD2Cl2) as a sharp
singlet which is a similar value to that observed for 7 (δ ) 183
ppm; CD2Cl2).
In parallel with the aformentioned metalation reactions, efforts
were also directed toward the synthesis of a monomer that
contained a main group element in lieu of a transition metal.
in the solid-state and was stored for several months under
atmospheric conditions without noticeable decomposition. X-ray
As shown in Scheme 3, thione 6 was prepared via treatment of
crystallographic analysis revealed that 5 was isolated as the
imidazolium chloride 2 with a mixture of S8 and K2CO3 in
methanol. The 13C NMR resonance assigned to the thione carbon
was observed at δ ) 167 ppm (CD2Cl2), and the expected
composition was confirmed by ESI mass spectrometry. Analysis
of 6 via single-crystal X-ray diffraction provided additional
support for the structure of this molecule (see Figure 4).35
monomeric silver carbene (Figure 3, left), replete with dimeric
argentophilic interactions (Figure 3, right),31 presumably due
to the increased steric demands of the acetate ligand (in
comparison with the chloride ligand in 3).
Bithiophene-substituted iridium NHC complex 7 was syn-
thesized by transmetalation of 3 with [Ir(cod)Cl]2 (cod ) 1,5-
cyclooctadiene) in CH2Cl2 solution. In comparison with those
of the starting material, the ortho-phenyl protons of 7 were
shifted significantly downfield (δ ) 7.27 to 7.57 ppm) as the
reaction proceeded toward product. Furthermore, the vinyl
protons of the cod ligand were split into two separate resonances,
one at δ 2.50 (assigned to the olefin cis to the NHC) and the
other at δ 4.15 (assigned to the olefin trans to the NHC) due to
the stronger donating character of the NHC compared with that
Electropolymerization Studies. In general, the aforementioned
complexes were independently electropolymerized in a three-
electrode cell using a platinum button as the working electrode.
The polymers synthesized from these reactions were obtained
(32) Chang, Y. H.; Fu, C. F.; Liu, Y. H.; Peng, S. M.; Chen, J. T.; Liu,
S. T. Dalton Trans. 2009, 861–867.
(33) Ogle, J. W.; Miller, S. A. Chem. Commun. 2009, 38, 5728–5730.
(34) Bittermann, A.; Herdtweck, E.; Ha¨rter, P.; Herrmann, W. A. Orga-
nometallics 2009, 28, 6963–6968.
(35) (a) Türkmen, H.; S¸ahin, O.; Büyükgüngör, O.; Çetinkaya, B. J. Inorg.
Chem. 2006, 23, 4915–4921. (b) Burford, N.; Phillips, A. D.; Spinney,
H. A.; Robertson, K. N.; Cameron, T. S.; McDonald, R. Inorg. Chem.
2003, 42, 4949–4954.
(31) Lee, C. K.; Vasam, C. S.; Huang, T. W.; Wang, H. M. J.; Yang, R. Y.;
Lee, C. S.; Lin, I. J. B. Organometallics 2006, 25, 3768–3775.
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Powell et al.
as thin films and then characterized using a suite of electro-
chemical,36 XPS,37 UV-vis spectroscopy, profilometry, and
four-point probe conductivity measurements.
The first monomer studied, the bithiophene-substituted di-
imine 1, was successfully electropolymerized by continuous
electrochemical cycling between -0.2 and +1.7 V. Although
the current from the generated thin film of poly(1) was found
to increase linearly with respect to the scan rate as expected,38
the respective cyclic voltammogram of 1 revealed a reduction
process near 0.0 V (see Supporting Information). For this reason,
1 was deemed unsuitable or coordinating various transition
metals and forming metallopolymers. As described above, 1 was
cyclized to form the imidazolium salt 2, which was envisioned
as a stable NHC precursor. Unfortunately, attempts to elec-
tropolymerize 2 were unsuccessful,39 a likely consequence of
the high oxidation potential of the monomer.
In contrast, the silver NHC complex 317 was successfully
electropolymerized under the aforementioned conditions. As
shown in Figure 5 (top), the oxidation and reduction waves
overlapped and the current measured increased in a linear
fashion over 10 cycles. Cyclic voltammetry experiments con-
ducted on a thin film of poly(3) revealed that, as expected, the
current generated was directly proportional to the applied scan
rate (see Figure 5, bottom).
To further characterize poly(3), its respective monomer was
also electropolymerized onto stainless steel. Two distinct phases
resulted, both of which were analyzed by XPS to determine
their chemical compositions. One phase, which was visibly white
in color, featured of a 1:1 molar ratio of silver to chlorine atoms.
The lack of sulfur or nitrogen signals suggested to us that the
bithiophene-substituted NHC moiety was absent. By inference,
silver chloride had formed and was presumed to originate from
-
the electrochemical oxidation of the AgCl2 anion which is
typically present in solutions of silver NHC complexes.40
Analysis of the other phase, which was visibly yellow in color,
revealed a S:Ag ratio of 3.70 and an N:Ag ratio of 1.83 (Table
1), values which were in agreement with the anticipated
canonical structure of poly(3).41 Moreover, when poly(3) was
Figure 5. (top) Overlay of continuous cyclic voltammograms of silver NHC
3 over time. Inset: plot of current maxima vs number of cycles. (bottom)
Cyclic voltammogram of thin film poly(3) at different scan rates. Inset:
plot of current vs scan rate.
Table 1. Selected Properties of the Thin Films Prepared
deposited onto an ITO slide, the yellow film exhibited a λmax
)
a
b
poly.
MLn
λred (nm) λox (nm) stabilityc (cycles) thicknessd (nm)
399 nm, a value that was red-shifted by 74 nm with respect to
that measured for its monomer and consistent with the formation
of an electronically delocalized polymeric structure.
poly(3) AgCl
poly(4) AuCl
poly(5) AgOAc
399
392
395
367
333
374
558
348
1173
126
294
750
702
677
687
704
692e
35
12
20
30
50
Although poly(3) was successfully synthesized, subsequent
efforts focused on the silver NHC acetate complex 5, a monomer
whose electropolymerization characteristics were predicted to
not entail competitive or deleterious processes.42 Using the
method described above, electropolymerization of 5 afforded a
thin film that was measured to be 1173 nm thick by profilometry.
As shown in Figure 6 (top), a series of cyclic voltammetry
experiments revealed that the oxidation current observed at 0.8
poly(6)
S
poly(7) Ir(cod)Cl
poly(8) Ir(CO)2Cl
a Absorbance maxima at 0.0 V. b Absorbance maxima for the signal
attributed to polaron formation. c Stability was defined as the number of
cycles (listed) observed until half of the absorbance peak measured near
700 nm was depleted. d Determined by profilometry. e An absorption at
1092 nm was observed and attributed to bipolaron formation.
(36) Scan rates for poly(3)-poly(8) above 100 mV/s resulted in distortions
of the electrochemical profiles due to kinetic limitations on the
counterion diffusion in and out of the polymer matrix.
(37) XPS analysis of the monomers was unsuccessful due to their inability
to form thin films.
(38) The diimine thin film poly(1) could not be deposited on either stainless
steel or ITO glass for further characterization.
(39) The cyclization succeeded in eliminating the reduction of the diimine
moiety, but thin films of poly(2) were not detected on the electrode
in monomer-free solutions.
(40) de Fre´mont, P.; Scott, N. M.; Stevens, E. D.; Ramnial, T.; Lightbody,
O. C.; MacDonald, C. L. B.; Clyburne, J. A. C.; Abernethy, C. D.;
Nolan, S. P. Organometallics 2005, 24, 6301–6309.
(41) Hesse, R.; Streubel, P.; Szargan, R. Surf. Interface Anal. 2005, 37,
589–607.
V increased along with the anticipated potential shifts following
each subsequent cycle. The oxidation and reduction waves
assigned to the polymer were clearly delineated from those of
the monomer. Moreover, the current generated by oxidizing or
reducing poly(5) was directly related to the scan rate up to 100
mV/s (Figure 6, bottom).
XPS examination of a thin film of poly(5) revealed that the
ratios of the Ag, N, and S atoms were in accord with the
(42) For a recent review see: Hindi, K. M.; Panzner, M. J.; Cannon, C. L.;
Youngs, W. J. Chem. ReV. 2009, 109, 3859–3884.
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Main Chain Poly(N-Heterocyclic Carbene) Complexes
A R T I C L E S
Figure 6. (top) Overlay of continuous cyclic voltammograms of silver NHC
5 over time. Inset: plot of current maxima vs number of cycles. (bottom)
Cyclic voltammogram of thin film poly(5) at different scan rates. Inset:
plot of current vs scan rate.
Figure 7. (top) Overlay of continuous cyclic voltammograms of thione 6
over time. Inset: plot of current maxima vs number of cycles. (bottom)
Cyclic voltammogram of poly(6) at different scan rates. Inset: plot of current
vs scan rate.
anticipated structure of this material. As predicted,43 the silver
3d core electrons of poly(5) was measured to possess a binding
energy of 368.6 eV, which is significantly higher than that
measured for poly(3) (367.4 eV). The UV-vis spectrum of
poly(5) also exhibited a strong bathochromic shift of 69 nm
when compared to that measured for its respective monomer.
In agreement with the results described above, this observation
is consistent with the formation of an electronically delocalized
polymeric structure.
Electropolymerization of the thione 6 was accomplished in
a similar fashion to the analogous electrochemical experiments
described above. As shown in Figure 7 (top), the maximum
oxidation current was observed at 1.15 V, which is a higher
potential than those observed for the other polymers analyzed
thus far. In the first three electrochemical cycles, an oxidation
was also detected near 0.9 V. Previous reports have suggested
that this feature represents the oxidative attachment of the thione
sulfur onto the platinum button44 and that the accompanying
reduction wave (-0.1 V) reflects detachment. After the first
three cycles, however, poly(6) was observed to coat the surface
of the electrode, which in turn prevented any further thione
attachment.45 As shown in Figure 7 (bottom), the current
generated by poly(6) was also found to increase linearly with
the scan rate up to 100 mV/s.
XPS analysis of poly(6) on a stainless steel substrate revealed
a S:N ratio of 2.52, which is in close agreement with the
expected value of 2.50. This observation provided additional
evidence that 6 did not decompose during polymerization and
that the anticipated structure of the polymer had formed.
Examination of the UV-vis spectrum of this material revealed
a λmax ) 367 nm, a value which is bathochromically shifted by
43 nm in comparison with that of the corresponding monomer.
The bithiophene-substituted iridium NHC 7 was electropo-
lymerized next, and the plot of current versus potential is
displayed in Figure 8 (top). In accord with related systems, a
quasi-reversible oxidation of the metal center was measured at
a potential of 0.75 V and the corresponding reduction peak was
measured at 0.60 V.46 Both of these electrochemical features
decreased rapidly in intensity with repeated cycling and were
no longer present after five cycles. Since a similar observation
was made for the thin film poly(6), it was concluded that coating
(43) Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. Handbook of
X-ray Photoelectron Spectroscopy, 2nd ed.; Perkin-Elmer: Eden Prarie,
1992.
(44) Bates, J. R.; Kathirgamanathan, P.; Miles, R. W. Thin Solid Films
1997, 299, 18–24.
(45) To support this argument, the thione analogue of the Kuhn carbene
was synthesized, analyzed by cyclic voltammetry, and shown to
possess similar electrochemical features as described in the literature
for other thione compounds. The electrochemistry of the Kuhn thione
is shown in the Supporting Information.
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A R T I C L E S
Powell et al.
Figure 8. (top) Overlay of continuous cyclic voltammograms of iridium
NHC 7 over time. Inset: plot of current maxima vs number of cycles.
(bottom) Cyclic voltammogram of thin film poly(7) at different scan rates.
Inset: plot of current vs scan rate.
Figure 9. (top) Overlay of continuous cyclic voltammograms of iridium
NHC 8 over time. Inset: plot of current maxima vs number of cycles.
(bottom) Cyclic voltammogram of thin film poly(8) at different scan rates.
Inset: plot of current vs scan rate.
the electropolymerization of iridium NHC 8 (Figure 9, top),
metal oxidation was observed to increase from 0.75 to 1.20 V.
As a result, and in contrast to 7, a linear relationship between
the increase in polymer current and the number of electrochemi-
cal cycles was observed. Poly(8) also exhibited a λmax ) 374
nm, a value that is bathochromically shifted by 48 nm compared
to its monomer, which is a significantly larger shift than that
measured upon polymerization of iridium NHC 7. The elec-
trodeposited thin film poly(8) was therefore expected to be
comprised of relatively high molecular weight polymers (and
not oligomers). Electrochemical cycling at different scan rates
revealed a significant cathodic shift of the reduction wave of
poly(8) and a broadening of its oxidation feature above scan
rates of 25 mV/s (Figure 9, bottom). Distortions at low scan
rates are consistent with a lower degree of ion mobility for
poly(8), particularly when compared with poly(3)-poly(7).
Electrochromic Studies. It is well established that electro-
chemical oxidation of polythiophene-type systems increases the
degree of planarity along the main chain. As a result, an
absorption feature typically appears near 700 nm and has been
assigned to polaron-induced π f π* transitions that occur along
the backbone of the polymer.23 Further oxidation of these
materials gives rise to new absorption features in the NIR region
that have been attributed to π f π* bipolaron excitations.48 In
the case of the polymers described herein, metal-ligated imi-
of the electrode by poly(7) was responsible for this phenomenon.
A linear relationship between current increase and number of
consecutive cycles was not evident due to the concurrent
oxidation of the coordinated iridium metal. Moreover, the metal
oxidation process could not be detected by electrochemical
cycling of poly(7) (Figure 8, bottom) in monomer-free solution,
thus suggesting that the metal may not be present in the
polymeric thin film. Related iridium-containing polymers,
however, have been shown to lack a metal-centered oxidation
process.47 Hence, to gain additional insight into the composition
of poly(7), the material was analyzed by XPS which revealed
that the iridium, sulfur, and nitrogen atoms were present in the
expected ratios. Surprisingly, the λmax of the polymer was found
to be only slightly red-shifted (9 nm) with respect to its
monomer, which suggested to us that the film consisted
primarily of oligomers of 7.
To minimize the concurrent oxidation of the metal center
during electropolymerization, the cod ligand of 7 was displaced
with carbon monoxide since the latter is expected to anodically
shift the oxidation potential of iridium NHC complexes. During
(46) Tennyson, A. G.; Rosen, E. L.; Collins, M. S.; Lynch, V. M.;
Bielawski, C. W. Inorg. Chem. 2009, 48, 6924–6933.
(47) Deng, Y.; Liu, S. J.; Fan, Q. L.; Fang, C.; Zhu, R.; Pu, K. Y.; Yuwen,
L. H.; Wang, L. H.; Huang, W. Synth. Met. 2007, 157, 813–822.
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Main Chain Poly(N-Heterocyclic Carbene) Complexes
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Scheme 5. Proposed Mechanism of Polaron and Bipolaron Formation For Thin Films Poly(4)-Poly(8) in TBAPF6/CH2Cl2 Solution under
Oxidative Conditions
dazolylidene moieties are infused into the main chain and
connected through quarterthiophene linkages. It was envisioned
that polaron and bipolaron excitations would likewise occur at
the oligothiophene linkages of poly(4)-poly(8); hence, these
polymers were subsequently analyzed for their abilities to exhibit
electrochromic characteristics.
activity was observed, it is unlikely that bipolarons are being
formed in the aforementioned materials. Nevertheless, in all
cases studied, the polaron absorbance near 700 nm shifted
hypsochromically above potentials of 1.2-1.3 V, which is likely
due to overoxidation (with concomitant disruption of planarity
along the main chain).50
Spectroelectrochemical measurements were performed on
poly(4)-poly(8) by increasing the potential from 0.0 to 1.5 V
in 0.1 V increments and in conjunction with the acquisition of
UV-vis-NIR spectral data. An absorption feature near 700
nm was observed upon oxidation of a thin film of poly(4), which
was found to be stable for 35 cycles. The material underwent a
drastic color change from yellow (reduced state) to dark blue
(oxidized state) during the oxidation process. Although similar
results were obtained for poly(5) (Figure 10), this material was
found to be stable for only 12 cycles, which is in accord with
the labile nature of 5 in solution (vide supra). To underscore
the importance of a coordinated transition metal, a thin film of
poly(6) was also examined and found to exhibit diminished
electrochromic behavior in comparison with thin films of poly(4)
and poly(5). This observation suggested to us that the absorption
features observed near 700 nm were enhanced by the presence
of the coordinated transition metal.
Poly(7), which contains a coordinated iridium metal, exhibited
a weak UV-vis absorption near 700 nm under oxidative
conditions. In contrast, a thin film of poly(8) exhibited significant
absorption at 692 and 1092 nm at 1.2 V (see Figure 11). The
absorbance at 1092 nm was assigned to the formation of
bipolarons along the quarterthiophene chains of poly(8) (see
Scheme 5). In this respect, poly(8) is unique in that its CO
ligands may facilitate the formation of polarons and bipolarons
On the basis of the data presented, combined with the well-
known electrochromic properties of oligothiophenes, a proposed
mechanism for the observed electrochromic activity is illustrated
in Scheme 5. At high potentials, the quarterthiophene moieties
undergo oxidation to generate polarons which causes the
thiophenes to planarize. In turn, this results in the appearance
of the new absorbances near 700 nm.49 Since no significant NIR
Figure 10. UV-vis profiles for silver NHC thin film poly(5) at different
potentials (see color coded legend). The film was used as the working
electrode of a three-electrode cell assembled in a quartz cuvette.
(48) For a recent review see: Beaujuge, P. M.; Reynolds, J. R. Chem. ReV.
2010, 110, 268–320.
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A R T I C L E S
Powell et al.
moieties have been synthesized and studied. Many of these
complexes were successfully polymerized under oxidative
conditions to afford stable thin films, which were characterized
using a range of electrochemical, XPS, and UV-vis-NIR
spectroscopic techniques and found to be consistent with their
canonical structures. A unique feature of these materials is that
the ligated main group or transition metals are orthogonally
positioned with respect to the polymer main chain. In addition,
most of the new polymers prepared were found to exhibit
electrochromic characteristics that were dependent on the
incorporated metal. Collectively, the results presented herein
substantiate the pivotal role that NHCs and their ligated
transition metals play in optimizing and enhancing the electronic
properties exhibited by NHC-based polymeric materials.
Acknowledgment. We are grateful to the Robert A. Welch
Foundation (Grant Nos. F-0003 and F-1621) for support of this
work. We also thank Dr. Vince Lynch, Brent Norris, and Maria
Cabezas for their assistance and the National Science Foundation
(Grant No. 0618242) for the X-ray photoelectron spectrometer used
in this work.
Figure 11. UV-vis-NIR profiles for iridium NHC thin film poly(8) at
different potentials. The film was used as the working electrode of a three-
electrode cell assembled in a quartz cuvette.
through the stabilization of these species (Figure 11). Regardless,
thin films of poly(8) were found to be relatively robust as
evidenced by the high degree of electrochromic reversibility
observed (>50 cycles).
1
Supporting Information Available: H and 13C NMR, elec-
trochemical studies of the Kuhn thione, diimine 1, imidazolium
chloride 2, XPS, UV-vis, spectroelectrochemistry, and X-ray
crystallographic data. This material is available free of charge
via the Internet at http://pubs.acs.org.
Conclusions
In summary, high-yielding syntheses of various silver, gold,
iridium, and sulfur-substituted NHCs with bis(bithiophene)
JA104051X
(49) (a) Skotheim, T. A.; Reynolds, J. R. Handbook of Conducting
Polymers; 3rd ed.; CRC Press: Boca Raton, 2007. (b) Farchioni, R.;
Grosso, G. Organic Electric Materials; Springer-Verlag: Berlin, 2001.
(50) Tehrani, P.; Robinson, N. D.; Kugler, T.; Romonen, T.; Hennerdal,
L. O.; Ha¨ll, J.; Malstro¨m, A.; Leenders, L.; Berggren, M. Smart Mater.
Struct. 2005, 14, N21–N25.
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