Block Copolymers for Directional Charge Transfer: Synthesis

Mar 24, 2015 - A series of styrenic triarylamines bearing electron-withdrawing or electron-donating substituents were synthesized and readily polymeri...
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Block Copolymers for Directional Charge Transfer: Synthesis, Characterization, and Electrochemical Properties of Redox-Active Triarylamines Robert Schroot,† Ulrich S. Schubert,*,†,‡ and Michael Jag̈ er*,†,‡ †

Laboratory of Organic and Macromolecular Chemistry (IOMC), Friedrich Schiller University Jena, Humboldtstraße 10, 07743 Jena, Germany ‡ Jena Center of Soft Matter (JCSM), Friedrich Schiller University Jena, Philosophenweg 7, 07743 Jena, Germany S Supporting Information *

ABSTRACT: A series of styrenic triarylamines bearing electron-withdrawing or electron-donating substituents were synthesized and readily polymerized by nitroxide-mediated polymerization (NMP). The utility of the homopolymers as macroinitiators for the preparation of defined block copolymers is demonstrated. All redox-active polymers were characterized in detail by NMR, MS, and SEC measurements; in addition, their electrochemical properties were studied. The homopolymers undergo reversible oxidation, whereby the redox potential is modulated by the substitution pattern. In the case of the copolymers, the sequential oxidation of the individual blocks is observed and corroborated by (spectro)electrochemical measurements. The ratio of transferred charges per redox-active block agreed with the stoichiometric composition, as readily quantified by the semi-integral analysis of the electrochemical data. In addition, redox-titration experiments revealed effective electron transfer between the redox-active polymers following the order of their redox potentials. These results demonstrate the potential to achieve directional charge transfer in hierarchically defined block copolymers.

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INTRODUCTION Triarylamines (TARA) are reversible electron donors and widely used as hole-conducting units in (supra)molecular triads,1 nanocomposites for light-induced charge separation,2 porous organic polymer materials,3 solar cells,4,5 or organic light-emitting diodes (OLEDs).6,7 Significant charge carrier mobilities of poly(triarylamine)s (pTARA) are reported ranging from 10−8 up to 10−5 cm2 V−1 s−1.8 The versatile application originates from their facile synthesis (e.g., by Hartwig−Buchwald coupling9,10 or modified Ullmann reactions11) and from modern controlled radical polymerization techniques (e.g., nitroxide-mediated polymerization (NMP)8,12−14 or reversible addition−fragmentation chain transfer (RAFT) polymerization2,15,16) that enable the synthesis of functional pTARA. Consequently, these methodologies allow the facile preparation of block copolymers and the postpolymerization modification of the end groups,17−19 which can be exploited to design and prepare defined redox-active architectures. Triarylamines exhibit two oxidation steps, in which the redox potential is easily tuned by the substituents of the aryl rings.20,21 The first redox step is generally reversible, if an inert substituent blocks the reactive para-position (R ≠ H). At higher potentials, a second oxidation process leads to the irreversible intramolecular formation of carbazoles upon release © XXXX American Chemical Society

of the two protons. Intermolecular coupling of two triarylamine units is reported for poly(triarylamine), which leads to the quasi-reversible formation of σ-dimers, which require harsh reducing condition for re-reduction.22 In addition, suitable peripheral substitution patterns may lead to self-organization, e.g. as nanowires,23−25 and can be utilized to tailor the thermal parameters, e.g., the glass transition temperature exploiting long alkyl chains. In this contribution, the preparation of functionalized styrenic triarylamines with tailored redox-potentials is presented, followed by their nitroxide-mediated polymerization to homo- and block copolymers and detailed optical and redoxchemical characterization. Novel block copolymers were designed and prepared by NMP to elucidate directional charge transport processes due to the inherent redox gradient between the blocks. All obtained polymers were characterized by nuclear magnetic resonance (NMR), mass spectrometry (MS), and size exclusion chromatography (SEC). The optical and redox chemical properties were detailed by UV−vis spectroscopy, spectroelectrochemistry, and cyclic voltammetry (CV). It is commonly observed for redox-active polymers that their cyclic Received: March 3, 2015 Revised: March 10, 2015

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DOI: 10.1021/acs.macromol.5b00449 Macromolecules XXXX, XXX, XXX−XXX

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35.1, 33.7, 22.5, 14.0. HR-ESI ([C28H33N]H+) m/z: calcd: 384.2686; found: 384.2671. Error: 3.8 ppm. Bis(4-trifluoromethylphenyl)-(4-vinylphenyl)amine (5). Method A. Compound 5 was prepared as 3 using 1-bromo-4-trifluoromethylbenzene (0.357 g, 1.59 mmol) and 6 (0.418 g, 1.59 mmol) in toluene (15 mL) (colorless oil, 0.040 g, 6%). Method B. Compound 5 was prepared as 3 using 1-iodo-4trifluoromethylbenzene (0.645 g, 2.37 mmol) and 6 (0.568 g, 2.16 mmol) in toluene (20 mL). The reaction mixture turned purple during the addition of the solvent. The desired product was isolated in traces (0.023 g, 3%). 1 H NMR (300 MHz, CDCl3) δ: 7.41 (d, J = 8.6 Hz, 4H, ArH), 7.30 (d, J = 8.6 Hz, 2H, ArH), 7.06 (d, J = 8.6 Hz, 4H, ArH), 7.00 (d, J = 8.6 Hz, 2H, ArH), 6.62 (dd, J = 17.6, 10.9 Hz, 1H, CHCH2), 5.63 (d, J = 17.6 Hz, 1H, CHCH2−trans), 5.17 (d, J = 10.9 Hz, 1H, CHCH2-cis). 13C NMR (100 MHz, CDCl3) δ: 149.9, 145.6, 135.8, 134.5, 128.3, 127.7, 126.7 (2×), 126.6 (2×), 125.9, 125.6, 125.3, 125.0, 124.6, 124.3, 123.1, 122.9, 120.2, 113.8. HR-ESI ([C22H15F6N]+) m/z: calcd: 407.1103; found: 407.1111. Error: 1.9 ppm. MS (MALDI-TOF, DCTB) m/z: 407.182 ([M]•+). (4-Trifluoromethylphenyl)-(4-vinylphenyl)amine (6). Method A. Compound 6 was prepared as 3 using 1-bromo-4-trifluoromethylbenzene (2.000 g, 8.90 mmol) in toluene (40 mL) (colorless oil, 0.418 g, 36%). Method B. Compound 6 was prepared as 3 using 1-iodo-4trifluoromethylbenzene (2.000 g, 7.40 mmol) in toluene (40 mL) (0.150 g, 16%). 1 H NMR (300 MHz, CDCl3) δ: 7.44−7.25 (m, 4H, ArH), 7.07− 6.93 (m, 4H, ArH), 6.61 (dd, J = 17.6, 10.9 Hz, 1H, CHCH2), 5.86 (s, 1H, NH), 5.59 (dd, J = 17.6, 0.8 Hz, 1H, CHCH2-trans), 5.11 (dd, J = 10.9, 0.8 Hz, 1H, CHCH2-cis). General Polymerization Procedure. A glass tube equipped with a septum and an external overhead flushing with nitrogen was used for the polymerizations. The reaction vessel was charged with monomer, initiator, and solvent, purged with nitrogen for 20 min, and placed in a preheated oil bath (120 °C). Samples were taken for NMR and SEC characterization. The purification is described for each polymer. Reaction time, used initiator, monomer concentration, and M/I ratio are given in Table 1 and the Supporting Information.

voltammogram deviates from the ideal reversible behavior (symmetric peak shape, small peak split). However, the mathematical transformation (vs t1/2) of the current−voltage data into its semi-differential/semi-integral form simplifies the quantitative analysis of the redox processes:26 The semi-integral form m(t) of the faradaic current i(t) is then independent of the scan rate and reaches a maximum plateau when the surface concentration of the analyte is zero; moreover, the obtained signal is then independent from diffusional processes.27 Consequently, the semi-differential shows a sharp symmetrical peak for the forward and the backward scan,28,29 which further serves to discern subtle differences that are usually hidden in the “shape” of the CV wave.30,31 Although this described transformation is generally not performed routinely, it is implemented in most electrochemical software packages or available by simple postprocessing of the ordinary CV data. More importantly, it not only provides the potential of a redox process (abscicca) but also contains valuable information on the current data (ordinate) to quantify the same process. In addition, the spectroelectrochemical analysis of the homopolymers yielded the characteristic absorption spectra of the oxidized pTARA units, which constitute the basis for the subsequent electron transfer studies of the block copolymers. First, the stepwise electrochemical oxidation of block copolymers was investigated to exclude any undesired interference of the blocks. In this regard, the semi-integral analysis is a convenient tool to determine the ratio of transferred electron per block and compare this value to the stoichiometric ratio of both redox units within the macromolecule. Finally, directional charge transfer between the blocks was investigated by UV−vis redox titration. This process was followed by the characteristic spectral changes that occur upon addition of a solution of a chemically oxidized homopolymer to a second solution containing a homopolymer with lower oxidation potential. The experiment mimics the directional electron transfer that also occurs after light-induced charge separation32 and, thus, addresses the general utility of pTARA-based block copolymers for light-driven energy conversion schemes.

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Table 1. Selected Characterization Data of Homopolymers and Copolymers Prepared by NMP

EXPERIMENTAL SECTION

Materials. 4-Methyl-N-p-tolyl-N-(4-vinylphenyl)aniline (2), 4methoxy-N-(4-methoxyphenyl)-N-(4-vinylphenyl)aniline (1), and 4fluoro-N-(4-fluorophenyl)-N-(4-vinylphenyl)aniline (4) were prepared as described in the literature.9,10 All monomers are sensitive to UV light, leading to irreversible reactions; thus all compounds were protected from light and stored in the fridge. Bis(4-(n-butyl)phenyl)-(4-vinylphenyl)amine (3). A flask was charged with 4-aminostyrene (4.000 g, 33.56 mmol), 1-bromo-4-nbutylbenzene (14.310 g, 66.72 mmol), sodium tert-butoxide (11.285 g, 133.71 mmol), 8,9-triisobutyl-2,5,8,9-tetraaza-1phosphabicyclo[3.3.3]undecane (0.095 g, 0.27 mmol), and bis(dibenzylideneacetone)palladium(0) (0.154 g, 0.27 mmol). After flushing with nitrogen, dry degassed toluene (500 mL) was added, and the reaction mixture was heated to 85 °C overnight. The reaction mixture was cooled to room temperature and filtered through Celite. Afterward, the filtrate was concentrated in vacuo and was purified by flash column chromatography (silica, hexane/dichloromethane 90/10) (colorless oil, 3.500 g, 27%). 1H NMR (400 MHz, CDCl3) δ: 7.25 (d, J = 7.5 Hz, 2H, ArH), 7.14−6.95 (m, 10H, ArH), 6.65 (dd, J = 17.6, 10.9 Hz, 1H, CHCH2), 5.61 (d, J = 17.6 Hz, 1H, CHCH2− trans), 5.12 (d, J = 10.9 Hz, 1H, CHCH2-cis), 2.56 (t, J = 7.7 Hz, 4H, 2 × CH2), 1.81−1.46 (m, 4H, 2 × CH2), 1.46−1.30 (m, 4H, 2 × CH2), 1.03−0.84 (m, 6H, 2 × CH3). 13C NMR (100 MHz, CDCl3) δ: 148.0, 145.3, 137.7, 136.4, 131.0, 129.2, 126.9, 124.5, 122.5, 111.6,

entry

polymera

Mnb [g/mol]

Đ

initiator

1 2 3 4 5 6 7 8 9 10 11

p111 p212 p250 p312 p411 p49 p56c p44-stat-p280 p49-b-p2170 p49-b-p1120 p212-b-p1144

3300 3500 10600 4200 3200 2600 3000 25000 38000 26000 39000

1.20 1.13 1.17 1.09 1.12 1.09 1.07 1.21 1.20 1.42 1.32

CMSt-TIPNO CMSt-TIPNO CMSt-TIPNO CMSt-TIPNO CMSt-TIPNO CMSt-TIPNO CMSt-TIPNO CMSt-TIPNO p49 p49 p212

a

Number of repeating units according to 1H NMR analysis. According to SEC analysis (chloroform/isopropylamine/triethylamine 94/2/4, polystyrene calibration). c Main fraction after preparative SEC (see text).

b

p111 was prepared according to the general procedure using 1 (0.500 g, 1.51 mmol), CMSt-TIPNO (0.028 g, 0.08 mmol), and anisole (0.6 mL). After the given time the reaction mixture was diluted with dichloromethane and precipitated in cold methanol. Unreacted monomer was removed by preparative SEC (Bio-Beads S-X1, dichloromethane). The polymer was obtained as a white powder after precipitation in methanol. Yield: 0.215 g. B

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Scheme 1. Schematic Representation of the Synthesis of Styrenic Triarylamines 1−5 by Hartwig−Buchwald Coupling and Preparation of Homo- and Copolymers by NMPa

Reagents and conditions: (i) Pd0, phosphine ligand, NaOtBu, toluene, N2, 85 °C, 16 h; (ii) anisole, initiator, monomer, 120 °C (see Supporting Information for details); (iii) anisole, monomer, 120 °C (see Supporting Information for details).

a

mL). After the given time the reaction mixture was diluted with dichloromethane and precipitated in cold methanol. Unreacted monomer was removed by preparative SEC (Bio-Beads S-X1, dichloromethane). The polymer was obtained as a white to light yellow powder after precipitation in methanol. Yield: 0.250 g. p49-b-p1120 was prepared according to the general procedure using 1 (1.000 g, 3.02 mmol), p49 (0.039 g, 0.02 mmol), and anisole (1.0 mL). After the given time the reaction mixture was diluted with dichloromethane and precipitated in cold methanol. Unreacted monomer was removed by preparative SEC (Bio-Beads S-X1, dichloromethane). The polymer was obtained as a white to light yellow powder after precipitation in methanol. Yield: 0.393 g. p49-b-p2170 was prepared according to the general procedure using 2 (1.000 g, 3.34 mmol), p49 (0.043 g, 0.02 mmol), and anisole (1.0 mL). After the given time the reaction mixture was diluted with dichloromethane and precipitated in methanol. Unreacted monomer was removed by preparative SEC (Bio-Beads S-X1, dichloromethane). The polymer was obtained as a white to light pink powder after precipitation in methanol. Yield: 0.405 g. p44-stat-p280 was prepared according to the general procedure using 2 (0.803 g, 2.68 mmol), 4 (0.041 g, 0.13 mmol), CMSt-TIPNO (0.005 g, 0.01 mmol), and anisole (1.6 mL). After the given time the reaction mixture was diluted with dichloromethane. Unreacted monomer was removed by preparative SEC (Bio-Beads S-X1, dichloromethane). The polymer was obtained as a white powder after precipitation in methanol. Yield: 0.327 g. Preparation of Oxidized Polymers for UV−Vis Experiments. p312ox. A solution of p312 (0.028 g, 0.01 mmol) was prepared in dichloromethane (1.0 mL). Subsequently an excess of a 1 M SbCl5 solution in dichloromethane (0.2 mL) was added with a syringe. The reaction mixture turned dark blue. Precipitation in pentane and methanol gave the oxidized product as dark blue powder. Yield: 0.030 g. Elem. Anal. Calcd for C23H32ClNO(C28H33NSbCl6)6(C28H33N)6: C, 67.77%; H, 6.80%; N, 2.87%; Cl, 14.4%. Found: C, 62.81%; H, 5.98%; N, 2.82%; Cl, 15.95%. p49-b-p1120ox. A solution of p49-b-p1120 (0.030 g, 0.09 mmol monomer units) was prepared in dichloromethane (1.0 mL). Then a substoichiometric amount of a 1 M SbCl5 solution in dichloromethane (0.025 mL) was added with a syringe. The reaction mixture turned dark blue. Precipitation in methanol gave the oxidized product as dark blue powder. Yield: 0.032 g. UV−Vis Titration Using p312ox. A stock solution of compound p312ox (1.454 mg) was prepared in dichloromethane (2.0 mL). The titer was determined by titration of ferrocene (0.1306 mg, 7 × 10−4

p212 was prepared according to the general procedure using 2 (0.500 g, 1.67 mmol), CMSt-TIPNO (0.031 g, 0.08 mmol), and anisole (0.6 mL). After the given time the reaction mixture was diluted with dichloromethane and precipitated in cold methanol. Unreacted monomer was removed by preparative SEC (Bio-Beads S-X1, dichloromethane). The polymer was obtained as a white powder after precipitation in methanol. Yield: 0.206 g. p250 was prepared according to the general procedure using 2 (1.800 g, 6.012 mmol), CMSt-TIPNO (0.023 g, 0.060 mmol), and anisole (2.0 mL). After the given time the reaction mixture was diluted with dichloromethane and precipitated in cold methanol. Unreacted monomer was removed by preparative SEC (Bio-Beads SX1, dichloromethane). The polymer was obtained as a white to light orange powder after precipitation in methanol. Yield: 0.492 g. p312 was prepared according to the general procedure using 3 (0.522 g, 1.36 mmol), CMSt-TIPNO (0.025 g, 0.07 mmol), and anisole (0.5 mL). After the given time the reaction mixture was diluted with dichloromethane and precipitated in cold methanol. Unreacted monomer was removed by preparative SEC (Bio-Beads S-X1, dichloromethane). The polymer was obtained as a white powder after precipitation in methanol. Yield: 0.222 g. p49 was prepared according to the general procedure using 4 (0.532 g, 1.73 mmol), CMSt-TIPNO (0.032 g, 0.09 mmol), and anisole (1.0 mL). After the given time the reaction mixture was diluted with dichloromethane and precipitated in cold methanol. Unreacted monomer was removed by preparative SEC (Bio-Beads S-X1, dichloromethane). The polymer was obtained as a white powder after precipitation in methanol. Yield: 0.176 g. p411 was prepared according to the general procedure using 4 (0.330 g, 1.07 mmol), CMSt-TIPNO (0.020 g, 0.05 mmol), and anisole (0.7 mL). After the given time the reaction mixture was diluted with dichloromethane, and unreacted monomer was removed by preparative SEC (Bio-Beads S-X1, dichloromethane). The polymer was obtained as a white powder after precipitation in methanol. Yield: 0.135 g. p56 was prepared according to the general procedure using 5 (0.040 g, 0.01 mmol), CMSt-TIPNO (0.002 g, 0.001 mmol), and anisole (0.25 mL). After the given time the reaction mixture was diluted with dichloromethane. Insoluble compounds were removed by filtration. Fractionation by preparative SEC (Bio-Beads S-X1, dichloromethane) and precipitation in methanol gave the polymer as a beige powder. Yield: 0.004 g. p212-b-p1144 was prepared according to the general procedure using 1 (1.000 g, 3.02 mmol), p212 (0.052 g, 0.02 mmol), and anisole (1 C

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Macromolecules mmol) in dichloromethane (2.0 mL): Defined volumes of titrant p312ox were added with an Eppendorf pipet, and the UV−vis spectra were recorded immediately. The titration end point was determined by analysis of the absorption at 741 nm, which is assigned to an excess of oxidant (amount of active units: 9.7 × 10−4 mmol/mg). Then a cuvette was charged with a solution of compound p111 (0.0078 mg, 2.36 × 10−5 mmol monomer units) in dichloromethane (2.0 mL). Defined volumes of a solution of compound p312ox (2.096 mg) in dichloromethane (10.0 mL) were added. The end point was determined by analysis of the adsorption at 744 and 690 nm. The end point was reached after the addition of 357 μL of oxidant.

the methodology is demonstrated by the two prepared batches of p4 (entries 5 and 6). Because of the small amount of available monomer 5, a lower concentration of the reagents was necessary. Hence, the lower reaction rate due to dilution required extended reaction times (44 h) to achieve a comparable monomer conversion. However, it also led expectedly to more pronounced side reactions, as confirmed by the higher dispersity of the crude product. Therefore, only the main fractions from the preparative SEC were combined to yield a batch of polymer p56 (entry 7) with a low Đ (1.07) due to fractionation. The homopolymers were further characterized by MALDI-ToF MS (see Supporting Information). In line with our previous more detailed study,10 fragmentation of the nitroxide was observed leading to multiple polymer series, depending on the exact conditions of ionization. More importantly, the distances between two peaks in any series reflects the molar mass of the respective repeating unit (see Supporting Information) and, therefor, complement the assignments on the basis of 1H NMR and SEC data. Preparation of Copolymers. Two homopolymers (p49 and p212) were utilized as macroinitiators for the preparation of the three possible combinations of the substituents (X = OMe, Me, and F) in the corresponding block copolymers (entries 9− 11). A statistical copolymer was also synthesized (entry 8) to serve later as a reference for the optical and electrochemical studies. The degree of polymerization was calculated from the 1 H NMR data: The total peak area in the aromatic region was compared to that of the methyl or methoxy groups, which provides the molar ratio of the repeating units. The degree of polymerization of the second block was subsequently calculated according to the known degree of polymerization of the macroinitiator. The copolymerization of the monomers 2 and 4 resulted in the formation of the statistical copolymer p44-statp2 80 , and the observed ratio of the repeating units corresponded to the initial feed. The preparation of the block copolymers was readily achieved from the macroinitiators p49 and p212, without the need to remove unreacted chains (“dead” macroinitiator). The SEC elugrams of the block copolymers p49-b-p2170, p49-b-p1120, and p212-b-p1144 (entries 9−11) after removal of the monomer are depicted in Figure 1 and demonstrated an almost quantitative initiation. In general, the dispersity of the block copolymers is larger (Đ = 1.20−1.42) compared to the homopolymers but parallels the observed increased dispersity for a higher degree of polymerization

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RESULTS AND DISCUSSION Monomer Synthesis. The palladium-catalyzed Hartwig− Buchwald coupling of 4-vinylaniline with 2 equiv of aryl halide allowed the synthesis of a series of triarlyamines (X = OMe (1), Me (2), F (4), Scheme 1).9,10 The synthetic versatility of this route on a multigram scale originates from the commercial availability of the reagents and the facile chromatographic purification. Two new monomers bearing n-butyl (3) and electronwithdrawing CF3 (5) substituents were synthesized and characterized. Monomer 3 was isolated in 27% yield, although the re-examination of the crude product by NMR analysis showed a conversion of the starting material >60%. The much lower isolated yield is attributed to losses during purification, in line with our previous finding of irreversible photoreactions occurring upon UV-light exposure, even under ambient light conditions.10 The general procedure was initially applied to the synthesis of 5, but instead of the bis-arylated product only the monoarylated 6 was isolated (36%). Hence, a second coupling step of intermediate 6 was performed to yield 5 (6%). The difficulties to achieve high conversion and the corresponding low yields prompted us to test the corresponding aryl iodide, which is generally more reactive in cross-coupling reactions than the bromide analogue. However, the opposite result was observed, and the yields decreased to 16% and 3%, respectively. The purple coloring of the reaction mixtures indicated the generation of elemental iodine as a side reaction. No further improvement of the reaction conditions was attempted, e.g., a variation of the ligand or base,33 because the obtained amounts were sufficient for subsequent polymerization and characterization. Preparation of Homopolymers. The monomers 1−5 were polymerized by NMP with a commercially available nitroxide initiator as reported,10 using anisole as solvent to enable high monomer concentrations (up to 3 M), and thereby, high reaction rates and short reaction times were achieved. The reaction mixtures were purified by removal of unreacted monomer, and the isolated polymers were analyzed by SEC, revealing a controlled character of the polymerization as judged from the low dispersity values Đ (Table 1). The degree of polymerization (DP) was determined from 1H NMR data based on the end-group analysis; i.e., the characteristic chloromethyl protons of the initiator at approximately 4.5 ppm. The synonyms of the prepared polymers are complemented by the number of repeating units (subscript): For example, p111 denotes an average of 11 repeating units of monomer 1. In line with the previously reported higher reactivity of monomer 1, the dispersity of polymer p111 (Đ = 1.20, entry 1) is slightly larger as for the other homopolymers with comparable degree of polymerization (DP = 6−12, Đ < 1.13). A higher degree of polymerization (DP = 50) also led to a slightly larger dispersity (entry 3), while the reproducibility of

Figure 1. SEC elugrams of block copolymers and macroinitiators (SEC, chloroform/isopropylamine/triethylamine 94/2/4). D

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emission maxima of the copolymers are generally shifted to lower wavelength compared to the dominating chromophore constituent (Table 2 and Supporting Information). In case of the two copolymers dominated by the methyl-substituted chromophore (Table 2, entries 6 and 7), the blue-shifted emission (22 nm vs p212, entry 2) accidently coincides with the emission of the minor fluoro-decorated chromophore (p49, entry 4). Note that any energy transfer process would expectedly occur in the opposite direction, i.e., populating the lower-energy emissive chromophore. The assignment of chainlength-dependent emission is further corroborated by inspection of the two methoxy-dominated copolymers (entries 8 and 9), which display a similar blue-shift (ca. 33 nm) with respect to homopolymer p111, irrespective of the second chromophore. Electrochemical Properties. The electrochemical properties were investigated by cyclic voltammetry (CV) and its semiintegral/semi-differential analysis. The influence of electrondonating and -withdrawing substituents on the oxidation potential versus ferrocene is summarized in Table 2. A quasireversible oxidation wave was detected for the homopolymers except for the trifluoromethyl-substituted p56 (Figure 3). This irreversible behavior was also found for monomer 5 and is attributed to the instability of the strongly electron-deficient radical cation. A plausible side reaction involves the loss of a proton and/or coupled additional redox processes at this high potential, which may lead to similar irreversible processes as reported upon applying high anodic potentials.22 As expected, the two different alkyl substituents exhibit identical redox potentials (entries 2 and 3), while the electron-donating substituent (OMe) leads to milder oxidation potentials (entry 1) and the electron-withdrawing substitution (F, CF3) to higher oxidation potentials (entries 4 and 5). These measured redox potentials (in dichloromethane) are cathodically shifted by approximately 10 mV in comparison to reported data in dimethylformamide (DMF).10 An additional prepeak is observed in the CV for the polymer p49, which is attributed to the irreversible cleavage of the nitroxide moiety. This assignment is supported by the irreversible oxidation of the NMP initiator at +0.78 V (see Supporting Information). The onset of degradation shifts to lesser positive values for the polymers, attributed to the stabilization of the cationic fragment by delocalization within the aromatic subunit (TARA vs Ph). The same behavior is found for the remaining homopolymers but overlaps with the oxidation process of the polymer. Instead, the semi-differential

(entry 2 vs 3). Because of their high molar mass, no meaningful mass spectra of the copolymers were obtained. Optical Properties. All polymers display a strong absorption band centered around 300 nm, almost independent of the substituents. The molar absorption coefficients (per repeating unit) range from 17 000 (p111) up to 24 500 L mol−1 cm−1 (p312) (Table 2 and Supporting Information). In stark Table 2. Selected Spectroscopic and Electrochemical Data entry

polymer

1 2 3 4 5 6 7 8

p111 p212 p312 p49 p56 p44-stat-p280 p49-b-p2170 p49-b-p1120

9

p212-b-p1144

a

E1/2 [V]

ΔE [mV]

λabsa [nm]

λema [nm]

ε [L mol−1 cm−1]

0.20 0.37 0.36 0.50 0.79b 0.40 0.39 0.19 0.49 0.19

80 97 84 61

300 303 306 297 n.d. 303 303 303

427 409 411 379 n.d. 377 377 393

17000 20000 24500 18500 n.d. 18000 22500 21000

303

394

19000

120 127 36 18 40

In dichloromethane. bPeak potential from square-wave voltammetry.

contrast, the emission energies of the short homopolymers are significantly influenced by the substituents (Figure 2, left). The methoxy substitution leads to emission around 427 nm, whereas a hypsochromic shift is found with increasing electron-withdrawing character of the substituents, i.e., for the alkyl substitution around 410 nm and for the fluoro substitution at 379 nm. In addition, isoabsorbing solutions of the methyldecorated homopolymers p212 and p250 revealed a hypsochromic emission shift (17 nm) between the short and long polymer chains (Figure 2, right). The origin of this observation is tentatively assigned to the different environment of the chromophores, which parallels the well-known solvatochromic effect of organic dyes. The proximal attachment of the chromophore to the polymer backbone suggests a compact polymer conformation, particularly present in the longer polymers and, thus, would lead to steric shielding (solvent exclusion) of the interior chromophore units. Hence, their less polar environment (dichloromethane vs the average “aromatic” polymer itself) would lead to less stabilization of the emissive state and consequently supports the observed blue-shifted emission. The same trend is found upon careful analysis of the emission spectra of the (high molar mass) copolymers: The

Figure 2. Normalized emission spectra of the homo- and copolymers in dichloromethane (left panel) and comparison of p212 and p250 showing a hypsochromic shift with increasing degree of polymerization shift in dichloromethane (right panel). E

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Figure 3. Electrochemical data (0.1 M TBAPF6, dichloromethane, 200 mV/s) of the homopolymers: cyclic voltammograms (left panel) and semidifferential plot for p212 and p312 illustrating a quasi-reversible oxidation of the TARA units and the irreversible cleavage of the nitroxide (right panel).

Figure 4. Electrochemical analysis of copoylmers (0.1 M TBAPF6, dichloromethane). Left panel: semi-differential of the prepared copolymers (scan rate ν = 200 mV/s); labeled vertical dashed lines indicate potentials of the corresponding homopolymers. Right panel: semi-integral of p49-b-p1120, recorded with different scan rates. Inset: semi-integral at ν = 50 mV/s with relative contributions of each redox-active unit.

analysis clearly shows the irreversible cleavage exemplified for polymers p212 and p312; i.e., the forward scan shows an electrochemical process that is absent in the backward scan (Figure 3, right). Furthermore, the irreversibility is also seen in the semi-integral as an offset between the forward and backward scan (Supporting Information). In contrast to the ideal reversible case involving only faradaic processes, the cyclic voltammograms of all polymers exhibit a substantial peak broadening and an unsymmetrical peak shape. This observation becomes clearer in the semi-differential representation and is therefore assigned to the kinetics of the oxidation processes. In line with previous reports,22,34,35 a second irreversible oxidation step for all polymers is observed upon applying high positive potentials, although to a smaller degree. The copolymers CVs’ are composed of the overlapping individual oxidation waves of the corresponding blocks, indicating that the differently substituted repeating units are oxidized independently. The semi-differential plot (Figure 4, left) shows also the broad unsymmetrical quasi-reversible peaks. For example, the two distinct redox steps of p49-b-p1120 are well-resolved and occur at the typical potentials for the homopolymers (OMe and F). The semi-integral representation shows a large influence of the scan rate (Figure 4, right), indicative of additional kinetic processes and/or adsorption.36,37 More importantly, at low scan rates (0.86 V vs Fc+/Fc0).21 Although the G

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Figure 7. Left panel: comparison of the spectroelectrochmical data of p111 and p312 with the spectrum at the titration end point. Right panel: Δabs spectra with different amounts of added oxidant p312ox showing the generation of p111ox.

Figure 8. Left panel: comparison of experimental and simulated spectra of p111ox and p312ox (top) at the end of the titration with residuals (bottom). Right panel: concentration change of the compounds p111ox and p312ox during the titration.

UV−vis spectrum of p312ox differs somewhat from the spectra obtained from spectroelectrochemistry (Supporting Information), the analysis of the titration with p111 is facilitated by monitoring only the spectral changes, which compensates for the electrochemical inactive side products. The spectral changes are depicted in Figure 7 (right). At the beginning, the rise of the characteristic absorption at 740 nm was found, which almost perfectly matches of spectroelectrochemical data of polymer p111ox (Figure 7, left). The addition of an excess of titrant led to the development of a shoulder in the spectrum, which can be reproduced by the superposition of the absorption spectra of p111ox and p312ox (Figure 8). The absorption coefficients of the titrant and the analyte were determined from the UV−vis and spectroelectrochemistry data and served to calculate the amount of each oxidized species at each step and, consequently, allowed to follow the course of the titration quantitatively. Interestingly, approximately 3 equiv were required for the quantitative oxidation of p111, which is tentatively assigned to inactive titrant due to overoxidation (vida supra). Alternatively, the use of a substoichiometric amount of SbCl5 led to the exclusive oxidation of the p1 block of p49-b-p1120, in line with the oxidation potential (not shown). The UV−vis experiments clearly reveal that an intermolecular charge transfer between the redox-active blocks occurs.

prepared by reinitiation and furnished the preparation of copolymers with a defined internal redox cascade between the blocks. All substances were fully characterized by 1H NMR, MS, and SEC measurements. The electrochemical properties of all homopolymers were investigated in detail by cyclic voltammetry and quantified by the semi-integral and semi-differential analysis in detail. Thereby, the effect of the substituents (OMe < Me = Bu < F < CF3) on the redox potentialsspanning a potential window from 0.20 to 0.50 Vwas clearly demonstrated, despite the broad peak observed in typical CV curves. In general, the oxidations are quasi-reversible except for nitroxide cleavage and irreversible side reactions for the highly electron-deficient trifluoromethyl-substituted triarylamine. In the case of the copolymers, the semi-integral analysis revealed the electrochemical stability of the redox-active polymer, while at high scan rates additional kinetic processes were noticed by strong deviations from the theoretical curve. More importantly, the comparable electrochemical activities of the blocks were confirmed by the relative contributions, in excellent agreement with the stoichiometry determined by 1H NMR. The spectral changes upon oxidation followed expectedly the successive oxidation of the individual blocks (as determined for the homopolymers), determined by the redox potential of the substituted TARA units. Finally, the UV−vis redox titration unambiguously demonstrated that the inherent driving force set by redox potential difference between the blocks is sufficient to promote directional charge transfer. The versatility of similar photoredox-active macromolecular architectures for lightinduced charge separation has been recently reviewed by Meyer et al.32,39

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CONCLUSIONS The use of a commercial NMP initiator enabled the preparation of new triarylamine-based monomers and their homopolymers (X = Bu, CF3). Novel block copolymers based on a series of substituted monomers (X1, X2 = OMe, Me, F) were readily H

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(18) Mansfeld, U.; Pietsch, C.; Hoogenboom, R.; Becer, C. R.; Schubert, U. S. Polym. Chem. 2010, 1 (10), 1560−1598. (19) Hadjichristidis, N.; Iatrou, H.; Pitsikalis, M.; Mays, J. Prog. Polym. Sci. 2006, 31 (12), 1068−1132. (20) Creason, S. C.; Wheeler, J.; Nelson, R. F. J. Org. Chem. 1972, 37 (26), 4440−4446. (21) Connelly, N. G.; Geiger, W. E. Chem. Rev. 1996, 96 (2), 877− 910. (22) Yurchenko, O.; Freytag, D.; zur Borg, L.; Zentel, R.; Heinze, J.; Ludwigs, S. J. Phys. Chem. B 2011, 116 (1), 30−39. (23) Moulin, E.; Niess, F.; Maaloum, M.; Buhler, E.; Nyrkova, I.; Giuseppone, N. Angew. Chem., Int. Ed. 2010, 49 (39), 6974−6978. (24) Faramarzi, V.; Niess, F.; Moulin, E.; Maaloum, M.; Dayen, J.-F.; Beaufrand, J.-B.; Zanettini, S.; Doudin, B.; Giuseppone, N. Nat. Chem. 2012, 4 (6), 485−490. (25) Nyrkova, I.; Moulin, E.; Armao, J. J.; Maaloum, M.; Heinrich, B.; Rawiso, M.; Niess, F.; Cid, J.-J.; Jouault, N.; Buhler, E.; Semenov, A. N.; Giuseppone, N. ACS Nano 2014, 8 (10), 10111−10124. (26) Oldham, K. B. J. Electroanal. Chem. 1976, 72 (3), 371−378. (27) Whitson, P. E.; VandenBorn, H. W.; Evans, D. H. Anal. Chem. 1973, 45 (8), 1298−1306. (28) Goto, M.; Ikenoya, K.; Kajihara, M.; Ishii, D. Anal. Chim. Acta 1978, 101 (1), 131−138. (29) Goto, M.; Kato, M.; Ishii, D. Anal. Chim. Acta 1981, 126 (0), 95−104. (30) Klička, R. J. Electroanal. Chem. 1998, 455 (1−2), 253−257. (31) Pedrosa, J. M.; Martín, M. T.; Ruiz, J. J.; Camacho, L. J. Electroanal. Chem. 2002, 523 (1−2), 160−168. (32) Alstrum-Acevedo, J. H.; Brennaman, M. K.; Meyer, T. J. Inorg. Chem. 2005, 44 (20), 6802−6827. (33) Surry, D. S.; Buchwald, S. L. Chem. Sci. 2011, 2 (1), 27−50. (34) Heinze, J.; Frontana-Uribe, B. A.; Ludwigs, S. Chem. Rev. 2010, 110 (8), 4724−4771. (35) Amthor, S. Redox Properties of Bis-Triarylamines and Ligand Properties of Thianthrenophane. PhD Thesis, Würzburg, 2005. (36) Bowling, R.; McCreery, R. L. Anal. Chem. 1988, 60 (6), 605− 608. (37) Leddy, J.; Bard, A. J. J. Electroanal. Chem. 1985, 189 (2), 203− 219. (38) Dapperheld, S.; Steckhan, E.; Brinkhaus, K.-H. G.; Esch, T. Chem. Ber. 1991, 124 (11), 2557−2567. (39) Morseth, Z. A.; Wang, L.; Puodziukynaite, E.; Leem, G.; Gilligan, A. T.; Meyer, T. J.; Schanze, K. S.; Reynolds, J. R.; Papanikolas, J. M. Acc. Chem. Res. 2015, 48 (3), 818−827. (40) Kübel, J.; Schroot, R.; Wächtler, M.; Schubert, U. S.; Dietzek, B.; Jäger, M. J. Phys. Chem. C 2015, 119 (9), 4742−4751.

In conclusion, the facile assembly of defect-free redox-active copolymers by nitroxide-mediated polymerization is described, which constitute an attractive platform to attach a photosensitizer leading to effective primary photoinduced charge separation.40 Hence, the inherent redox gradients among the blocks of the copolymer may be further exploited to diminish unproductive recombination by directional charge migration.

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ASSOCIATED CONTENT

S Supporting Information *

Additional instrumental details and analytical data of 1−6 and all polymers (NMR, MS, and SEC) provided for completion. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (U.S.S.). *E-mail [email protected] (M.J.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS M.J. thanks the Carl-Zeiss-Foundation and the Friedrich Schiller University Jena (“Nachwuchsförderung”) for financial support. We thank Sarah Crotty for MALDI-ToF analysis.

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REFERENCES

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