Nitroxide-mediated polymerization of styrenic triarylamines and chain-end functionalization with a ruthenium complex: Toward tailored photoredox-active architectures

Article abstract of DOI:10.1021/ma302631f

The preparation of redox-active polymers and the chain-end functionalization with one ruthenium complex was investigated in detail. A series of substituted monomers, i.e., styrenic triarylamines bearing methyl, fluoro, or methoxy substituents, were prepared by a one-pot Hartwig-Buchwald coupling. The nitroxide-mediated polymerization (NMP) was studied by variation of the functional initiators, the monomer-to-initiator ratios, and the solvent. The kinetic analysis of the prototypical methyl-substituted triarylamine shows the controlled polymerization up to 75% conversion, but a considerable decrease of the polymerization rate was observed during the course of the reaction. Both chain-end functionalities of the purified oligomers were subsequently utilized, i.e., the nitroxide to serve as a macroinitiator for an additional NMP step and the chloromethyl group to introduce one ruthenium complex at the chain terminus. The products were analyzed in detail by size-exclusion chromatography, NMR spectroscopy, and mass spectrometry. The optical and electrochemical properties of the prepared poly(triarylamine)s show the application potential as charge transport materials in conjunction with the photoactive ruthenium complex.

Full text of DOI:10.1021/ma302631f

Article
pubs.acs.org/Macromolecules
Nitroxide-Mediated Polymerization of Styrenic Triarylamines and
Chain-End Functionalization with a Ruthenium Complex: Toward
Tailored Photoredox-Active Architectures
Robert Schroot, Christian Friebe, Esra Altuntas, Sarah Crotty, Michael Jager,* and Ulrich S. Schubert*
̈
Laboratory of Organic and Macromolecular Chemistry (IOMC), Friedrich-Schiller-University Jena, Humboldtstr. 10, 07743 Jena,
Germany, and Jena Center for Soft Matter (JCSM), Philosophenweg 7 07743, Jena, Germany
S
* Supporting Information
ABSTRACT: The preparation of redox-active polymers and the chain-
end functionalization with one ruthenium complex was investigated in
detail. A series of substituted monomers, i.e., styrenic triarylamines
bearing methyl, fluoro, or methoxy substituents, were prepared by a
one-pot Hartwig−Buchwald coupling. The nitroxide-mediated poly-
merization (NMP) was studied by variation of the functional initiators,
the monomer-to-initiator ratios, and the solvent. The kinetic analysis of
the prototypical methyl-substituted triarylamine shows the controlled
polymerization up to 75% conversion, but a considerable decrease of
the polymerization rate was observed during the course of the reaction.
Both chain-end functionalities of the purified oligomers were subsequently utilized, i.e., the nitroxide to serve as a macroinitiator
for an additional NMP step and the chloromethyl group to introduce one ruthenium complex at the chain terminus. The
products were analyzed in detail by size-exclusion chromatography, NMR spectroscopy, and mass spectrometry. The optical and
electrochemical properties of the prepared poly(triarylamine)s show the application potential as charge transport materials in
conjunction with the photoactive ruthenium complex.
appropriate.] In addition, the desired monomers can be readily
polymerized in a controlled fashion by reversible addition−
fragmentation chain-transfer (RAFT) polymerization13,15,16 or
nitroxide-mediated polymerization (NMP).11,12,17 The latter
technique offers the advantage to directly access telechelic
polymers18 by means of unimolecular radical initiators, but it
generally requires higher reaction temperatures and the use of
particularly reactive nitroxides,3 e.g., 2,2,5-trimethyl-4-phenyl-3-
azahexane-3-nitroxide (TIPNO) and N-(2-methylpropyl)-N-
(1-diethylphosphono-2,2-dimethylpropyl)-N-oxyl (SG1). If the
complementary initiator fragment contains a functional group,
its modification enables the defined introduction of functional
components at the chain terminus. A few NMP initiators are
commercially available to fulfill this task, e.g., SG1 equipped
with methacrylic acid (MAA), which can be modified prior to
polymerization,19 or TIPNO attached to chloromethylstyrene
(CMSt), which is readily exploited by nucleophilic substitution.
In this contribution, the assembly of a redoxactive polymer
and the subsequent attachment of a terminal photoactive unit
(dye) are presented (Scheme 1). Within this architecture the
dye carries out the charge separation and the injection into the
adjacent redoxactive chain. Ruthenium polypyridyl-type com-
plexes are attractive photoactive units due to their favorable
combination of absorption of visible light, formation of long-
INTRODUCTION
■
The development of materials that harness light energy to
generate a redox-chemical potential is the key to a modern
sustainable energy supply. In general, the working principle
involves the absorption of light to create an excited state
(exciton), followed by charge separation, and ultimately the
accumulation of redox equivalents. However, the efficient
conversion of the transient excitation energy requires a well-
tuned interplay of the elementary steps to compete with any
unproductive back-reactions. In this regard, modern controlled
radical polymerization techniques provide powerful method-
ologies to design and prepare such photo- and redox-active
materials,1−3 as shown by the rapid progress of organic
photovoltaics4−7 or (hybrid) storage systems.8,9 In particular,
the possibility to connect multiple (active) units provides the
basis to mimic charge transport and accumulation processes
also on a molecular level. In this regard, many (small) organic
molecules can serve as redox-active components in electron-
transfer reactions.10 Noteworthy, the resulting transfer steps
can be very efficient if such units are held close to each other,
e.g., by the backbone to restrict the conformational freedom.
For example, para-substituted styrenic triarylamines are
attractive monomer candidates for electron-donor polymers
due to their inherent good conducting properties11−13 and
redox stability14 as well as the possibility to tune the redox
potentials on a molecular level by peripheral substituents. [In
this article, the term “polymer” is used consistently for clarity,
although the term “oligomer” may be occasionally more
Received: December 25, 2012
Revised: January 31, 2013
© XXXX American Chemical Society
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Scheme 1. General Assembly Strategy of the Telechelic Redox-Active Macromolecules via Polymerization, Chain Extension, and
End-Group Modification with a Photoredox-Active Dye
eluting with hexanes/dichloromethane) to yield a light-yellow solid
(7.679 g, 76%). H NMR (300 MHz, CDCl3): δ 7.28 (d, J = 8.2 Hz,
lived excited states, suitable redox potentials, chemical stability
in the different oxidation states, and facile functionalization via
the ligand scaffold.20,21 In addition to the extensive fundamental
studies of electron-transfer processes in molecular assem-
blies,10,22,23 there is a continuing interest to utilize this class of
compounds in macromolecules with light-harvesting and redox-
active units.24 An instructive review by Meyer et al. describes
the development of ruthenium-based polymeric architectures,
including their in-depth photophysical analysis and molecular
modeling.25 Noteworthy, an efficient charge separation is
achieved by placing the redox-active units in close proximity.
However, the quest of subsequent efficient vectorial charge
transfer and accumulation of the redox equivalents is
challenging due to trapping and recombination of charges
within the statistical copolymer architecture. The placement of
the photoredox-active unit in between a donor chain and an
acceptor chain would, in principle, allow for unidirectional
migration of the separated charges. The success of this design
relies on the precise chain-end modification of the redox-active
polymers. Hence, the design and synthetic approach of the
target architecture are based on the following criteria: (1)
Triarylamines-containing monomers are selected to ensure
quantitative functionalization in the latter polymer, while the
attached vinyl group ensures minimal spatial separation of the
redox-active polymer with maximal through-space interac-
tion.26,27 (2) A novel RuII(dqp)2-based (dqp is 2,6-di-
(quinolin-8-yl)pyridine) complex serves as the photoactive
unit, which typically displays excited-state lifetimes in the
microsecond time scale at room temperature.28−30 (3) The
nitroxide-mediated polymerization using functional initiators
allows the direct preparation of functional telechelic polymers,
which can be further utilized to reinitiate NMP or to introduce
the ruthenium complex. In the first part of this work, the
polymerization kinetics are investigated by SEC, NMR
spectroscopy, and complemented by mass spectrometry. The
second part describes the isolation and characterization of a
ruthenium-decorated poly(triarylamine), including a brief
discussion of the ground state UV−vis absorption behavior
and electrochemical features.
1
2H), 7.08 (apparent d of AA′BB′, J = 8.4 Hz, 4H), 7.02 (apparent d of
AA′BB′, J = 8.4 Hz, 4H), 7.00 (d, J = 8.2 Hz, 2H), 6.67 (dd, J = 17.6,
10.9 Hz, 1H), 5.63 (d, J = 17.6 Hz, 1H), 5.15 (d, J = 10.9 Hz, 1H),
2.34 (s, 6H). 13C NMR (75 MHz, CDCl3): δ 148.0, 145.3, 136.5,
132.7, 131.1, 130.0, 127.0, 124.8, 122.5, 111.7, 20.9. EI-MS m/z (M+)
calcd for C22H21N 299; found 299.
4-Fluoro-N-(4-fluorophenyl)-N-(4-vinylphenyl)aniline (2). Using
4-vinylaniline (0.298 g, 2.5 mmol, 1 equiv), in toluene (50 mL).
Yield 65%. 1H NMR (400 MHz, CDCl3): δ 7.31 (apparent d of
AA′BB′, J = 8.6 Hz, 2H), 7.12−6.94 (m, 10 H), 6.68 (dd, J = 17.6, 10.9
Hz, 1H), 5.66 (d, J = 17.6, 1H), 5.18 (d, J = 10.9 Hz, 1H). 13C NMR
(100 MHz, CDCl3): δ 160.3, 157.9, 147.7, 143.8 (2×), 136.2, 131.8,
127.3, 126.2 (2×), 122.5, 116.4, 116.2, 112.3, Anal. Calcd for
C20H15NF2: C, 78.16; H, 4.92; N, 4.56. Found: C, 77.55; H, 5.21; N,
4.64. EI-MS m/z (M+) calcd for C20H15NF2 307; found 307.
4-Methoxy-N-(4-methoxyphenyl)-N-(4-vinylphenyl)aniline (3).
Using 4-vinylaniline (0.298 g, 2.5 mmol, 1 equiv), in toluene (50
1
mL). Yield 33%. H NMR (300 MHz, CDCl3): δ 7.26 (apparent d of
AA′BB′, J = 8.6 Hz, 2H), 7.08 (apparent d of AA′BB′, J = 8.9 Hz, 4H),
6.92 (apparent d of AA′BB′, J = 8.9 Hz, 2H), 6.86 (apparent d of
AA′BB′, J = 8.6 Hz, 4H), 6.66 (dd, J = 17.6, 10.8 Hz, 1H), 5.61 (dd, J
= 17.6, 1.0 Hz, 1H), 5.12 (dd, J = 10.8, 1.0 Hz, 1H), 3.82 (s, 6H). 13C
NMR (75 MHz, CDCl3): δ 156.0, 148.6, 141.0, 136.5, 130.1, 127.0,
126.6, 120.6, 114.8, 111.2, 55.6. EI-MS m/z (M+) calcd for
C22H21NO2 331; found 331.
[Ru(dqp)(dqpOH)][PF6]2 (4). A flask was charged with [Ru(dqp)-
(MeCN)3][PF6]2 (0.513 g, 0.484 mmol), 2,6-di(quinolin-8-yl)pyridin-
4-ol (0.170 g, 0.487 mmol), and ethylene glycol (20 mL). The reaction
mixture was heated to 120 °C under N2 for 16 h. The reaction mixture
was allowed to cool to room temperature, followed by dropwise
addition into an aqueous solution of ammonium hexafluorophosphate.
The solids were filtered off, washed with a slight amount of water, and
redissolved in acetonitrile. The crude product was purified by column
chromatography (silica, eluent acetonitrile/water/potassium nitrate
40:4:1), the streaking red band was collected, and the excess of solvent
was removed under reduced pressure. Aqueous ammonium hexa-
fluorophosphate was added, and the suspension extracted three times
with dichloromethane. The solids were filtered off, washed with little
water and diethyl ether, and dried under reduced pressure to yield a
first crop (0.251 g). The combined organic layers were washed with
water, and excess of solvent was removed under reduced pressure. The
product was recrystallized from acetonitrile by vapor diffusion of
diethyl ether to yield a second crop (0.097 g). Total yield (0.348 g,
EXPERIMENTAL SECTION
■
1
67%). H NMR (250 MHz, CD3CN): δ 8.13 (t, J = 8.0 Hz, 1H),
Materials. [Ru(dqp)(MeCN)3][PF6]2 and 2,6-di(quinolin-8-yl)-
pyridin-4-ol were prepared as in the literature.30
8.12−7.99 (m, 8H), 7.85 (d, J = 8.1 Hz, 2H), 7.73−7.62 (m, 8H), 7.43
(t, J = 7.8 Hz, 2H), 7.42 (d, J = 7.8 Hz, 2H), 7.37 (s, 2H), 7.07 (dd, J =
8.3, 5.2 Hz, 2H), 7.03 (dd, J = 8.3, 5.2 Hz, 2H). 13C NMR (CD3CN,
63 MHz): δ 167.1, 159.6, 158.4, 157.9, 147.7, 147.6, 138.8, 138.5,
138.3, 133.9, 133.7, 133.0, 132.8, 131.5, 128.9, 127.8, 127.8, 127.7,
123.0, 122.8, 117.1. ESI-MS m/z ([M-H-2PF6]+) calcd for
C46H29N6ORu 783.1453; found 783.1441.
Polymerization. General Procedure for Small-Scale Polymer-
ization Using MAA-SG1 (“Blocbuilder”). A stock solution of the
initiator (18.3 mg) was prepared using toluene/anisole (9:1, 2.5 mL).
A microwave vial was charged with the monomer and stock solution,
sealed, and purged for 30 min with N2, which was bubbled through a
4-Methyl-N-p-tolyl-N-(4-vinylphenyl)aniline (1). A flask was
charged with 1-bromo-4-methylbenzene (11.411 g, 66.72 mmol), 4-
vinylaniline (4.001 g, 33.56 mmol), sodium 2-methylpropan-2-olate
(11.285 g, 133.71 mmol), 2,8,9-triisobutyl-2,5,8,9-tetraaza-1-
phosphabicyclo[3.3.3]undecane (0.095 g, 0.27 mmol), bis-
(dibenzylideneacetone)palladium(0) (0.154 g, 0.27 mmol), and dry
toluene (300 mL). The reaction mixture was purged with N2 for 5 min
and heated to 85 °C for 10 h. The reaction mixture was allowed to
cool to room temperature, filtered, and rinsed with dichloromethane,
and the excess of solvent was removed under reduced pressure. The
crude product was purified by flash column chromatography (silica,
B
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mixture of toluene/anisole (9:1) before. The reaction mixture was
heated to 115 °C for 13 h. The reaction mixture was allowed to cool to
room temperature, diluted with toluene, and analyzed by SEC and
NMR. The crude product was isolated by precipitation into ice-cold
methanol (50 mL), filtered, washed with cold methanol and dried
under reduced pressure. Remaining monomer was removed by size-
exclusion chromatography on Biobeads to yield an off-white solid.
P1 (Table 1, entry 1) was prepared according to the general
procedure using 4-methyl-N-p-tolyl-N-(4-vinylphenyl)aniline (1)
(0.228 g, 0.762 mmol) and 0.38 mL of the stock solution of MAA-
SG1 (2.78 mg, 7.29 μmol). Yield: 0.180 g.
P2 (Table 1, entry 2) was prepared according to the general
procedure using 4-fluoro-N-(4-fluorophenyl)-N-(4-vinylphenyl)aniline
(2) (0.240 g, 0.781 mmol) and 0.39 mL of the stock solution of MAA-
SG1 (2.85 mg, 7.47 μmol). Yield: 0.150 g.
P3 (Table 1, entry 3) was prepared according to the general
procedure using 4-methoxy-N-(4-methoxyphenyl)-N-(4-vinylphenyl)-
aniline (3) (0.228 g, 0.688 mmol) and 0.34 mL of the stock solution of
MAA-SG1 (0.251 mg, 0.658 μmol). Yield: 0.038 g.
P4 (Table 1, entry 4) was prepared according to the general
procedure using styrene (0.121 g, 1.162 mmol) and 0.58 mL of the
stock solution of MAA-SG1 (4.246 mg, 0.011 mmol). Product was
analyzed but not isolated.
P5 (Table 1, entry 5). A microwave vial was charged with 4-methyl-
N-p-tolyl-N-(4-vinylphenyl)aniline (1) (0.500 g, 1.67 mmol) and
MAA-SG1 (0.064 g, 0.167 mmol) and sealed with a septum. Dry N,N-
dimethylformamide (1.7 mL) was added via a syringe, and the mixture
was purged for 1 h with N2. The reaction mixture was heated to 115
°C, and samples were taken for SEC and NMR analysis. The polymer
was precipitated into cold methanol, redissolved in dichloromethane,
and further purified by size-exclusion chromatography on Biobeads
(SX-3) for final NMR analysis.
General Procedure with CMSt-TIPNO. The reaction vessel, a
sealable glass tube (1 mL) equipped with external overhead flushing
with protective gas (see Supporting Information), was charged with 4-
methyl-N-p-tolyl-N-(4-vinylphenyl)aniline (1), N-(tert-butyl)-O-(1-(4-
(chloromethyl)phenyl)ethyl)-N-(2-methyl-1-phenylpropyl)-
hydroxylamine (CMSt-TIPNO), and solvent. The reaction vessel was
sealed, purged with N2 for 20 min, and immersed in a preheated oil
bath (120 °C). Samples were taken at given times for SEC and NMR
analysis.
P6 (Table 1, entry 6) was prepared according to the general
procedure using 4-methyl-N-p-tolyl-N-(4-vinylphenyl)aniline (1)
(0.500 g, 1.67 mmol), CMSt-TIPNO (0.031 g, 0.083 mmol), and
N,N-dimethylformamide (1.7 mL). The remaining reaction mixture
was directly used for the postmodification with 4 (read below).
P7 (Table 1, entry 7) was prepared according to the general
procedure using 4-methyl-N-p-tolyl-N-(4-vinylphenyl)aniline (1)
(1.818 g, 6.07 mmol), CMSt-TIPNO (0.023 g, 0.061 mmol), and
anisole (2.0 mL). The polymer was precipitated into cold methanol,
redissolved in dichloromethane, and further purified by size-exclusion
chromatography on Biobeads (SX-1). Reprecipitation of the polymer
into methanol gave a white powder. Yield: 0.492 g.
increasing the polarity of the eluent (acetonitrile/water/potassium
nitrate 40:4:1). The crude product was further purified by size-
exclusion chromatography on Biobeads (SX-3) to yield Ru−P6 as a
reddish solid (0.040 g).
RESULTS AND DISCUSSION
■
Monomer Synthesis. The palladium-catalyzed Hartwig−
Buchwald coupling of p-aminostyrene with 2 equiv of a para-
substituted bromobenzene gave the substituted styrenic
triarylamines (1−3) (Scheme 2).31,32 The convenient isolation
of the product by column chromatography facilitates the
straightforward up-scaling, e.g., the preparation of 1 on a
multigram scale (10 g) with 76% yield. The NMR spectra (see
Supporting Information) are composed of the partially
overlapping signals of the aromatic protons (7.3−6.8 ppm)
and the well-resolved vinylic group at approximately 6.7, 5.7,
and 5.2 ppm. The methyl groups appear as singlets at 2.34 ppm
(1) and 3.82 ppm (3). However, the isolated yield of 3 was
lower, and a significant photoinstability of 3 was noticed in
solution under ambient conditions. When a sample is exposed
to sun light, the quantitative conversion to a new species with
the double molar mass is observed. The structural assignment
of the photoproduct is provided in the Supporting Information
1
and supported by NMR analysis. The H NMR spectrum
shows the typical chemical shifts and the coupling constant (J =
16.0 Hz) of a trans-alkene group adjacent to a CH group (see
Supporting Information). In addition, an aliphatic methyl group
appears as a doublet (J = 7.0 Hz) with a pronounced high-field
shift (1.5 ppm). It is well-known that Lewis acids (e.g., protons
or transition metals) promote the dimerization of styrenes, but
the formation of excimers or the presence of oxygen also lead
to similar photoreactions, as observed in zeolites.33 However,
the conversion of the methyl- and fluoro-substituted triaryl-
amines required exposure to UV light for an extended time.
The photochemical behavior of the monomers suggests that the
dimerization via a radical cation is likely: no cyclic [2 + 2]
photoproducts or polymeric side products were found, and the
reactivity increases (F < Me ≪ OMe) with decreasing
oxidation potentials (see Electrochemistry).
Polymerization with Blocbuilder. The nitroxide-mediated
polymerization of 1−3 was first investigated using the MAA-
SG1 initiator. The reactions were performed applying typical
conditions of NMP (Scheme 2),3 i.e., a monomer/initiator ratio
of 100:1 in 2 M solutions in toluene/anisole (9:1) at 115 °C
(Table 1, entries 1−4) for 13 h. The SEC analysis of P1−P3
confirmed the successful polymerization, although the polymers
display a broad range of molar masses and significantly larger
PDI values than expected. In particular, the electron-rich
monomer 3 revealed the loss of control during polymerization.
For comparison, the polymerization of styrene under identical
conditions was investigated (entry 4). The analysis of P4
showed similar conversion (66% by 1H NMR), but a somewhat
larger molar mass (Mn = 8470 g/mol) and PDI value (1.26)
than anticipated. The nonideal behavior of these initial
experiments are attributed to the small scale (0.2 mL), e.g.,
by evaporation of solvent or termination reactions by residual
oxygen. Hence, the real radical concentrations would differ
significantly from the initial values and thereby affect the
apparent rate of conversion, the obtained molar masses, and
PDI values. However, the absolute values of the SEC analysis
should also be taken with care due to the unknown
hydrodynamic volume of the polymers. In order to gain a
detailed insight into the course of the reaction in the early stage,
Reinitiation of P6 was performed according to the general
procedure using 4-methyl-N-p-tolyl-N-(4-vinylphenyl)aniline (1)
(0.740 g, 2.46 mmol), P6 (0.050 g, 2.45 μmol assuming Mn
=
20 400 g/mol from poly(styrene) calibration of SEC), and anisole (2.5
mL). The final reaction solution was added dropwise to cold methanol
and analyzed by SEC.
In situ modification of P6. The reaction mixture of crude P6 (read
above) was allowed to cool to room temperature, followed by addition
of 4 (0.098 g, 0.091 mmol), potassium carbonate (0.012 g, 0.087
mmol), and N,N-dimethylformamide (2 mL). The solution was
purged for 20 min and heated to 60 °C for 4 days afterward. The
reaction mixture was poured into aqueous ammonium hexafluor-
ophosphate (100 mL); the formed precipitate was filtered, rinsed with
water, and redissolved in dichloromethane. The dark red solution was
washed with water and dried over sodium sulfate. The crude product
was purified by column chromatography on silica eluting with
dichloromethane/methanol (9:1). Unreacted 4 was recovered by
C
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a
Scheme 2. Synthesis and Polymerization of Triarylamines 1−3
a
Reagents and conditions: (i) 2 equiv of para-substituted bromobenzene, Pd0, phosphine ligand, NaOtBu, toluene, N2, 85 °C, 10 h; (ii) see Table 1.
Table 1. Polymerization of 1−3 at 115 °C
a
b
polymer
monomer
initiator
M/I
100
solvent
conc [M]
2
time [h]
13
conv [%]
Mn [g/mol]
PDI
c
c
c
c
c
c
c
c
P1
P2
P3
P4
P5
1
MAA-SG1
toluene/anisole (9:1)
66
>99
76
8060 (27)
26200 (85)
231000 (697)
8470 (81)
1.51
1.27
1.96
1.26
1.19
1.24
1.28
1.13
1.17
c
c
c
c
c
c
d
d
2
3
styrene
66
1
MAA-
SG1
10
DMF
1
1.2
2
74
1700 (5.7)
2230 (5.4)
2600 (8.6)
81
3
86
d
d
P6
P7
1
1
CMSt-TIPNO
20
60
DMF
1
3
22
73
3840 (9.5)
CMSt-TIPNO
anisole
20
77
10550 (35)
a
b
c
Determined from 1H NMR data. Apparent molar mass by PS calibration; the degree of polymerization is given in parentheses. SEC in DMAc +
d
0.21% LiCl with PS calibration. SEC in CHCl3, isopropanol, triethylamine (94:2:4) with PS calibration.
we followed the conversion of 1 with a lower monomer/
initiator ratio (10:1). In view of a subsequent in situ
postpolymerization modification, the solvent was changed
(DMF) to promote the solubility of a suitable RuII complex
(see below). Samples of P5 were taken hourly and analyzed by
the characteristic signals of the initiator, i.e., the CMSt and the
TIPNO moiety, can be used to analyze the reaction mixtures.
However, the 1H NMR spectrum of the initiator is complicated
due to diastereomers (see Figure 2, t = 0 min), which can be
identified by 2D data in agreement with the literature data (see
Supporting Information).34 The two CH2 signals of the
diastereomeric chloromethyl groups appear as singlets at 4.58
and 4.61 ppm. The TIPNO fragment displays two sets in the
aliphatic region, with a pronounced high-field shift of one CH3
group (0.22 ppm). Upon incorporation into the polymer (P6),
both fragments exhibit a significant line broadening and a small
but distinct high-field shift (0.1−0.2 ppm), which is attributed
to the different magnetic environment of the triarylamine unit
(Figure 2). The initiation stage can be estimated by the changes
of the characteristic signals of the end groups, but the exact and
reliable determination of the initiator efficiency suffers from the
low intensity, overlapping signals, and the formation of
diastereomers. However, the consumption of the initiator
occurs within ∼75 min, followed by the formation of transient
oligomeric species, as judged from the sharp signals around
4.50 ppm. At later times, the broad signals of the polymer
evolve around 4.40 ppm. Noteworthy, the remaining minor
contribution of the initiator suggests incomplete initiation.
The SEC analysis of P6 provides a more detailed insight into
the polymerization (Figure 2). During the initial stage (<2 h),
the transient oligomeric species (10.5−11.0 min) are resolved
due to the sufficiently large molar mass difference. The
separation between each species agrees well with the theoretical
value (see Supporting Information), although a strict
comparison would require a precise calibration. At later
times, the polymer chains grow and the curves shift to shorter
elution volumes. In line with previous experimental results, a
1
SEC and H NMR spectroscopy (Figure 1). The controlled
polymerization is reflected by low PDI values (<1.3), but the
fast conversion precluded a more detailed kinetic analysis. The
small shoulder (t = 22.3 min) is attributed to side reactions
before the first sampling because this contribution remains
unchanged in latter samples. At this stage, the somewhat larger
PDI values at later times may also arise from chain−chain
coupling reactions. The polymerization was stopped, and the
product purified by preparative SEC. The 1H NMR spectrum of
P5 shows the backbone signals of the polymer (1.4−2.4 ppm)
and the characteristic resonances of the SG1 end group. In
comparison to the initiator, the typical signal broadening within
polymers is observed, whereas the different magnetic environ-
ment results in a high-field shift of the end group’s signals. In
addition, the new chiral centers of the backbone lead to
diastereomers, which cause a more complex set of signals of the
SG1 group. In this regard, the phosphorus-decoupled spectrum
can be finally utilized to unambiguously assign the different P-
containing species (see Figure 1, at 3.2−3.4 ppm).
Polymerization with a TIPNO-Based Initiator. The
polymerization of 1 was investigated next using CMSt-
TIPNO as initiator in DMF to assist the in situ modification
with a ruthenium complex (see below) and in anisole for
enhanced solubility. The course of the reaction was followed by
1
SEC and H NMR spectroscopy (Figure 2). The conversion
1
was determined by H NMR from the disappearance of the
vinyl group with respect to the aromatic protons. In addition,
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1
Figure 1. (top) Normalized SEC traces of P5 at given times and H
NMR spectra (CDCl3, 400 MHz, aliphatic region): (3rd from bottom)
MAA-SG1 with P-decoupling, (2nd from bottom) P5 with P-
decoupling, and (bottom) without P-decoupling, including proton
assignment of the SG1 moiety with typical regions (in brackets) and
effect of P-decoupling (vertical lines) of the PCH group (around 3.3
ppm).
Figure 2. Polymerization of 1 in DMF: (top) normalized SEC traces
(UV detection at 342 nm) of P6 after given times, and (bottom) H
NMR spectra (300 MHz, CDCl3, aliphatic region) of crude P6 after
given times; the changes of the CMSt group (4.55 ppm) and TIPNO
(0.18 ppm) are indicated by arrows.
1
chain-length-dependent effects, the penultimate effect, etc.35
However, a more detailed kinetic analysis, i.e., the determi-
nation of the individual rate constants of initiation, propagation
or termination steps, is beyond the scope of this study. More
importantly, the slower propagation at later times seems to
compensate for the relatively long initiation period, as judged
from the decrease of the apparent PDI values. Hence, the faster
growth of late-initiated chains narrows the polymers’
polydispersity during the reaction. Finally, termination
reactions seem to play only a minor role, as judged from the
low PDI value at maximum conversion (up to 75%). A similar
behavior was found for P7 in anisole with a 3-fold M/I ratio,
which gave a polymer with ∼3-fold molar mass (Figure 3).
Mass Spectrometry. The polymerization products were
further investigated by mass spectrometry (Figure 4). A cutoff
of ions with molar masses below 780 m/z was applied to
increase the signal intensity in the high molar mass region. The
MALDI spectrum shows a major distribution with the
characteristic molar mass of the repeating unit (300 m/z),
reaching up to 7000 m/z. However, the ion mass differs from
the proposed dormant species, which could originate from the
known decomposition of nitroxides.38 The fragmentation of
TIPNO-based polymers is reported to depend on the exact
ionization conditions (matrix, additives, laser intensity, etc.) to
give both N−C scission products, the related H-abstraction
products,39 and/or the main chain scission products,40 in
particular at higher laser intensities.41 However, none of such
low-molar-mass species is observed throughout the polymer-
ization. The amount seems invariant because no changes are
observed for the latter samples, whereas the relative
contribution decreases for the first samples due to the
normalization of the UV signal. These findings are in line
with the results of the NMR analysis. The kinetic analysis of P6
shows a linear increase of the molar mass with low PDI values
(<1.2) (Figure 3). The second important feature of the
polymerization is the steady conversion of monomer over time.
The linear relationship between ln(M0/Mt) and time is
predicted in the ideal case of (1) a fast initiation vs propagation
rate, (2) the absence of terminating reactions, and (3)
propagation rates independent of the chain length. However,
if these assumptions are not met, a diverse kinetic behavior is
observed instead, as discussed in a recent review on the kinetics
of NMP.35 The experimental data of P6 show a pronounced
deviation from the ideal behavior, i.e., a significant decrease of
the propagation rate during the course of the reaction. The
common explanation of such a behavior is provided by the
persistent radical effect (PRE),36 arising from termination
reactions of the polymer radicals and the buildup of free
nitroxide. Analytical solutions of the rate laws were derived to
account for the PRE, i.e., plotting the data vs t2/3 instead.36,37
Although a linear relationship can be reasoned up to 2 h, later
samples show a slower growth of the chains. This finding
suggests further rate-retarding processes, which are ascribed to
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Figure 5. Normalized SEC-UV traces of the reinitiation study of P6:
macroinitiator (P6), reaction product (P6 + P6′), and deconvolved
trace of the chain-extended polymer (P6′).
resulting ions contain also the initiator moiety. In case of the
P6, the characteristic isotope pattern of CMSt with one or two
repeating units is identified (see Figure 4 and Supporting
Information).
The ESI-MS data of the polymer show a strong
discrimination of high molar masses in comparison to the
MALDI technique (Figure 4). The main distributions are singly
charged and display the typical mass difference of the repeating
unit, but none of the species belong to the intact polymer chain.
However, the thermally labile nitroxide may cleave off during
the ESI process to form secondary products. A more detailed
analysis of the exact mechanism of ionization and fragmenta-
tion is beyond the scope of this study and subject of a
forthcoming publication applying complementary state-of-the-
art MS techniques.43 However, the controlled nature of the
polymerization and the NMR analysis indicates the presence of
the nitroxide, whereas the MS and MS/MS data support only
the presence of the CMSt group.
Figure 3. Kinetic analysis of P6 (crosses) using M/I 20:1, 1 M in
DMF, and P7 (circles) using M/I 60:1, 3 M in anisole. Mn, Mw, and
PDI values were determined by SEC (CHCl3, isopropanol, triethyl-
amine 94:2:4), conversion determined from 1H NMR data. See
Supporting Information for linear plot of ln(M0/Mt) vs time.
species can be correlated with the experimental data. A
plausible explanation for this unusual behavior is the absorption
of the laser light (355 nm) by the triarylamine chromophore,
which may initiate photoreactions that compete with the
conventional desorption process.42 The MS/MS fragmentation
of the ions leads primarily to cleavage of the backbone and
formation of fragments with low molar masses, which are
assigned to one or two triarylamine units with up to three
methylene groups (see Supporting Information), respectively. If
the site of fragmentation is close to the chain terminus, the
Reinitiation. The polymer P6 was tested to reinitiate the
polymerization of 1 using the general procedure. A monomer-
to-initiator ratio of 1000:1 was chosen to compensate for the
low propagation rates. After purification, the SEC analysis
Figure 4. Mass spectrometry data of P6: MALDI-ToF (top left, matrix: dithranol) and ESI-ToF (bottom left), and expansion (right) of a
representative fragment ion from the MALDI-ToF spectrum (a), isotope simulation (b), and the proposed structure.
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shows a bimodal distribution, which can be assigned to the
unreacted macroinitiator (P6) and the chain-extended polymer
(P6′) (Figure 5). The efficiency of the reinitiation can be
estimated from the UV absorption data plotted vs the molar
mass based on the following simplifications: (1) the degree of
polymerization is proportional to the apparent molar mass
using PS calibration, and (2) the number of repeating units is
proportional to the absorbance. Hence, the ratio of
incorporated monomer units in the P6′ vs macroinitiator is
ca. 95:5, as derived from the deconvolved peak areas (Figure 5).
The controlled growth of the polymer is an indirect argument
for the presence of the nitroxide group in P6, in line with the
SEC and NMR data (see above).
extent of possible side reactions of the CMSt group during
isolation. Noteworthy, the concurrent growth of the chains is
negligible due to the substantially lower reaction temperature.
The product was readily purified by column chromatography
on silica because the high polarity of the cationic ruthenium
complex enables the removal of excess of nonfunctionalized
polymer and the recovery of 4. A final preparative size-exclusion
chromatography step was performed to separate any remaining
nonfunctionalized polymer. The purified product was analyzed
by SEC, 2D NMR spectroscopy, and mass spectrometry.
The 3D SEC plot of Ru−P6 is shown in Figure 6. The UV
trace is composed of the very strong absorption from the
triarylamine units below 350 nm and the typical MLCT
transition of the ruthenium fragment around 500 nm.
Interestingly, the elution volume is slightly larger than for the
nonfunctionalized precursor polymer (+0.1 mL, see Supporting
Information). The apparent smaller hydrodynamic volume can
by reasoned by a more compact conformation of Ru−P6
compared to P6 or by retention due to surface interactions of
the charged species. However, the size-exclusion effect is
evident from the comparison with the precursor complex 4,
which elutes significantly later (+0.8 mL, see Supporting
Postpolymerization Modification with Ruthenium.
The defined introduction of a ruthenium complex was
investigated via a nucleophilic substitution reaction of the
CMSt group of P6 (Scheme 3). The hydroxyl-functionalized
a
Scheme 3. Postpolymerization Modification of P6
1
Information). The H NMR spectrum of the purified sample
shows the strong signals of the triarylamine units between 6.3
and 7.0 ppm and the well-resolved peaks of the ruthenium
fragment above 7.0 ppm. The methylene protons of the linker
unit can be unambiguously identified by the phase-sensitive
C−H correlation technique (Figure 7). However, the cross-
peaks of the TIPNO group in the aliphatic region overlap with
the strong backbone signals and preclude a definite assignment.
The comparison of the aromatic signals of the ruthenium and
triarylamine moieties gives a numerical ratio of 1:14 for Ru−
P6, in good agreement with the precursor polymer.
a
Reagents and conditions: (i) K2CO3, DMF, 60 °C, N2.
ruthenium complex (4) was readily prepared by ligand
substitution reaction of [Ru(dqp)(MeCN)3]2+ in analogy to a
literature procedure.30 The modification was performed in situ
at 60 °C in the crude polymer solution in order to minimize the
The MALDI-ToF measurements required a cutoff to detect
any higher molar mass components (Figure 8). The major
series of the singly charged polymer shows the broad isotope
Figure 6. 3D SEC data (wavelength vs elution time, DMAc + 0.08% NH4PF6) of Ru−P6 with projections: (top) UV trace at 19.1 mL; (right)
chromatogram at 515 nm in au.
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Figure 7. Expanded HSQC spectrum of Ru−P6 with assignment of characteristic cross-peaks, assigned to the Ru fragment (dotted), the
poly(triarylamine) (solid), and the CH2−group of the linker (dashed).
pattern of ruthenium, which can be assigned to the formula
investigated by cyclic voltammetry (CV) and differential pulse
voltammetry (DPV). The potentials were referenced vs the
ferrocene/ferrocenium couple (Table 2). First, the three
substituted polymers show an oxidation process formally
assigned to the oxidation of the triarylamine moiety. The
electronic influence of the substituents (F, Me, OMe) is
reflected by a shift in potential of the redox couple. The
oxidation of P1 occurs around +0.41 V, whereas the electron-
withdrawing fluoro substituents of P2 cause a shift to higher
potential (+0.58 V), while the electron-releasing methoxy
substituents of P3 enable the oxidation already at lower
potential (+0.26 V). The prototypical [Ru(dqp)2]2+ complex is
oxidized at +0.63 V under similar conditions. With these data in
hand, it is possible to calculate the thermodynamic driving force
of electron transfer from the homopolymer to a RuIII center,
which is generated after oxidative quenching of the excited
state. The calculated values increase in the order P2 < P1 < P3
with driving forces of −0.05, −0.22, and −0.37 eV, respectively.
The broad waves of the homopolymers further indicate
additional processes beyond diffusion and the mere electrode
surface kinetics. For example, the hypothetical oxidation of one
triarylamine unit will affect the nearby units, leading to higher
potentials required for their subsequent oxidation. In addition,
further phenomena may contribute to the electrochemical
response of the polymers, e.g., conformational changes of the
polymer backbone to accommodate the charges and the
associated counterions, aggregation phenomena upon (multi-
ple) oxidation, or charge migration among the redox-active
units on the time scale of the experiments. Homopolymer P1
was also investigated in dichloromethane, which caused an
appreciable cathodic shift by ∼60 mV. A similar behavior was
found for the Ru−P6, which maintained the individual features
of the components. First, the ligand-based reduction is found at
−1.73 V, whereas the intense but broad wave around +0.37 V
can be attributed to the oxidation of the triarylamine units.
Upon re-reduction, the RuIII/II redox process becomes visible,
followed by the reversible re-reduction of the polymer. Because
of overlapping peaks, the potentials were extracted from the
DPV data and agree very well the data of the individual
components. The oxidation potential of the ruthenium center
[Ru−P6 + PF6 −H+] without the TIPNO fragment. The MS/
−
MS investigation showed the dominant C−O cleavage (783
m/z): the major signal without using the cutoff, the elimination
of HPF6 (−146 m/z), and two nonidentified ruthenium-
containing species (see the Supporting Information). However,
a more detailed analysis was precluded by the low signal
intensity, which may arise from the challenging desorption of
the doubly charged ruthenium species and competitive
absorption of the laser light, which explains also the
pronounced C−O scission. Noteworthy, a higher laser power
only leads to more pronounced fragmentation (see above).
Hence, the softer ESI-ToF technique may provide additional
structural information on the ionic species. The spectrum of
Ru−P6 consists of several high molar mass species (Figure 8),
in which the main series can be assigned to doubly and triply
charged [Ru−P6−H+] with fitting isotope pattern (Figure 8).
However, no intact Ru−P6 with the attached TIPNO group
was detected. Furthermore, two minor unidentified species
were found with +15 and +31 m/z relative to the major series.
The MS/MS analysis of the major series provides additional
support for the proposed structure (see the Supporting
Information): First, the ruthenium-containing fragments can
be easily identified by their characteristic isotope pattern. The
most intense signal originates from the cleavage of the complex
(783 m/z), whereas the fragmentation along the polymer
backbone gives significantly less intense signals. The organic
fragment (583 m/z) can be assigned to the allyl cation bearing
two triarylamine units, which can be explained by main chain
scission. In addition, a characteristic series of organic fragments
containing the styrene unit with multiple repeating units are
observed. This fragmentation behavior can be reasoned by
assuming the concurrent scission along the backbone upon
cleavage of the ruthenium moiety. In conclusion, the MS and
MS/MS data support the presented structure of Ru−P6, but no
indication of the nitroxide moiety was found. However, thermal
cleavage of the labile nitroxide group during the ESI process
cannot be ruled out.
Electrochemical and Optical Properties. The redox
properties of the homopolymers P1−P3 and Ru−P6 were
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Table 2. UV−Vis Absorption Data and Electrochemical Data vs Fc/Fc+
a
compound
[Ru(dqp)2]2+
λabs [nm]
Ered [V] (dqp/dqp•−
)
Eox [V] (TARA•+/TARA)
Eox [V] (RuIII/RuII)
ΔG [eV] vs RuIII/RuII
b
n.d.
−1.73
+0.63
0
b
P1
302
+0.41
+0.58
+0.26
−0.22
−0.05
−0.37
−0.28
b
P2
294
b
P3
299
c
d
P1
n.d.
+0.35 (0.22)
c
d
e
e
e
Ru−P6
302, 502
−1.73 (0.10)
d
+0.37
+0.63
−0.26
a
b
c
e
In DMAc. In DMF. In dichloromethane. Peak split (ΔEp) in V. From DPV data.
Figure 8. Mass spectrometry data of Ru−P6: MALDI-ToF (top left, matrix: dithranol), ESI-ToF (bottom left), and expansion (right) of a
▼
representative specimen ( ) from the ESI-ToF spectrum (a), isotope simulation (b), and the proposed structure.
CONCLUSION
■
A redox-active architecture was synthesized by polymerization
and subsequent modification with a photoactive ruthenium
complex. Three styrenic triarylamine monomers were readily
prepared by a one-pot procedure, including electron-with-
drawing and -releasing substituents (1−3). Next, the nitroxide-
mediated polymerization was investigated by SEC, NMR
spectroscopy, and mass spectrometry. A kinetic analysis was
performed for 1 to elucidate the scope of this approach toward
telechelic redox-active architectures. Under typical NMP
conditions, the initiation is completed within the first hour,
and the polymer chain grows in a controlled fashion up to 75%
conversion with low PDI values (<1.2). Noteworthy, the
apparent polymerization rate decreases considerably during the
reaction, beyond the anticipated influence of the persistent
radical effect (t2/3 plot). However, this effect compensates for
the long initiation period. Both end groups of the telechelic
redox-active polymer were successfully utilized: (a) to reinitiate
the polymerization and (b) in the postmodification with a
ruthenium complex via nucleophilic substitution. The homo-
polymers (P1−P3) and the Ru−P6 were further investigated
by UV−vis absorption spectroscopy and electrochemistry. The
absorption spectra of the polymers are composed of the strong
transitions in the UV region (<370 nm) of the triarylamine
subunits and the typical MLCT of the ruthenium complex
(∼500 nm). The oxidation of the homopolymers occurs at less
Figure 9. UV−vis absorption spectra of P1−P3 and Ru−P6 (in
DMAc).
matches the value of the model complex in DMF. Hence, all
homopolymers can act as an electron donor for the RuIII center
with adjustable driving force for the charge-injection step.
The UV−vis absorption spectra (Figure 9) of the
homopolymers show a strong transition in the UV region
centered around 300 nm. In case of P2 and P3, a shoulder at
higher wavelength is observed up to 380 nm. The Ru−P6
displays the preserved optical features of the individual
components: the triarylamine units dominate in the UV region,
whereas the typical MLCT transition of the ruthenium complex
is observed in the visible region (ca. 500 nm).
2+
oxidizing potentials with respect to the Ru(dqp)2 and obeys
the electronic influence of the substituents. The related Ru−P6
shows a similar behavior with a driving force of electron transfer
of ∼260 mV. Hence, all homopolymers can act as electron
donors to the RuIII center (driving force of 0.05−0.22 eV). In
I
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(12) Barea, E. M.; Garcia-Belmonte, G.; Sommer, M.; Huttner, S.;
̈
conclusion, the NMP of styrenic triarylamines represents a
versatile route to assemble well-defined redox-active polymers,
due to the following aspects: (a) the facile synthesis and
isolation of the redox-tuned monomers, (b) the availability of
functional initiators to yield telechelic linear polymers, (c) the
successful reinitiation to access future block copolymers, and
(d) the possibility to introduce a photoredox-active unit via
postpolymerization manipulation. Hence, more sophisticated
architectures with well-defined structure can be efficiently
prepared, e.g., by connecting the complementary redox chain to
the ruthenium center or by construction of redox-active
segments utilizing block copolymers. In addition, the in-depth
characterization of the photophysical and redox processes is
expected to provide the ground to design the next generations
of energy conversion systems.
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ASSOCIATED CONTENT
* Supporting Information
■
S
Additional instrumental details and analytical data of 1−4, P6,
and Ru−P6 (NMR, MS, MS/MS, and SEC) provided for
completion. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: michael.jager.iomc@uni-jena.de (M.J.); ulrich.
schubert@uni-jena.de (U.S.S.).
■
(24) Happ, B.; Winter, A.; Hager, M. D.; Schubert, U. S. Chem. Soc.
Rev. 2012, 41, 2222.
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Rev. 2005, 249, 457.
Author Contributions
(26) Morisaki, Y.; Chujo, Y. In Conjugated Polymer Synthesis; Wiley-
VCH Verlag GmbH & Co. KGaA: Berlin, 2010; p 133.
(27) Morisaki, Y.; Ueno, S.; Saeki, A.; Asano, A.; Seki, S.; Chujo, Y.
Chem.Eur. J. 2012, 18, 4216.
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
Notes
̈
(28) Abrahamsson, M.; Jager, M.; Osterman, T.; Eriksson, L.;
̈
The authors declare no competing financial interest.
Persson, P.; Becker, H.-C.; Johansson, O.; Hammarstrom, L. J. Am.
̈
Chem. Soc. 2006, 128, 12616.
̈
ACKNOWLEDGMENTS
(29) Abrahamsson, M.; Jager, M.; Kumar, R. J.; Osterman, T.;
Persson, P.; Becker, H.-C.; Johansson, O.; Hammarstrom, L. J. Am.
̈
■
̈
M.J. is grateful for financial support of the Carl-Zeiss-Stiftung.
We also thank Dr. Grit Festag for assistance with SEC
measurements and Dr. Christine Weber for valuable discussion
during the preparation of this manuscript.
Chem. Soc. 2008, 130, 15533.
(30) Jager, M.; Kumar, R. J.; Gorls, H.; Bergquist, J.; Johansson, O.
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ABBREVIATIONS
DMAc, N,N-dimethylacetamide; DMF, N,N-dimethylforma-
mide.
■
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