Polymers Composed of Alternating Anthracene and Pyridine

Article pubs.acs.org/Macromolecules

Polymers Composed of Alternating Anthracene and Pyridine Containing Units by Radical Ring-Opening Polymerization: Controlled Synthesis, Optical Properties, and Metal Complexes Kazuhiro Nakabayashi, Shota Inoue, Yohei Abiko, and Hideharu Mori* Department of Polymer Science and Engineering, Graduate School of Science and Engineering, Yamagata University, 4-3-16, Jonan, Yonezawa 992-8510, Japan S Supporting Information *

ABSTRACT: Novel polymers composed of alternating anthracene and pyridine containing units were synthesized by radical ring-opening polymerization of pyridine-containing cyclic monomers via reversible addition− fragmentation chain transfer (RAFT) process. The ring-opening polymerization proceeded predominantly to afford well-defined polymers (Mw/Mn ∼ 1.30), which exhibited the characteristic absorption and fluorescence due to the anthracene unit. On the other hand, fluorescence quenching was observed in chloroform and acidic aqueous solutions. The quenching was explained by charge transfer from the excited anthracene units to the coordinated or protonated pyridine units. Additionally, fluorescence quenching independent of external ambient conditions was conducted by quaternization. In the thin film state, the obtained polymers exhibited fluorescence due to the formation of excimers. The formation of complexes between the anthracene-containing polymers and metal species (Zn, Cu, and Co) led to the change in the optical properties compared to those of pristine polymers. These results demonstrate that the unique optical properties could be developed under various conditions due to the interactions between the anthracene and pyridine units in the well-defined polymers.

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INTRODUCTION Anthracene-containing polymers have attracted significant research interest, due to their promising features as fluorescent labels, photon harvesters, and optoelectronic applications (e.g., organic light emitting diodes, organic field-effect transistors, photovoltaics).1−8 Among them, conjugated polymers containing anthracene units in the main chain can be readily modified to optimize their optoelectronic properties by adjusting the polymer structure (e.g., the elongation of conjugated bonding, incorporation of substituent groups into the anthracene unit, and so on), potentially bringing in the advantages for the optoelectronic applications.5−10 In addition to the large variety of anthracene-based conjugated polymers, nonconjugated polymers with anthracene units in the main chain linked at the 9,10-positions with flexible alkyl chains, such as poly(trimethyleneanthrylene)s,11,12 poly(octamethyleneanthrylene)s,13 and poly(9,10oxymethyleneanthrylene)s,14 have been reported as well. Furthermore, the anthracene-based nonconjugated polymers afforded paramagnetism by doping with iodine and enhanced electrical conductivity.14 Various polyamides,15,16 polyesters,17,18 polyethers,17 and polyurethanes16 containing anthracene units in the main chain have been synthesized so far. On the other hand, the control of their molecular weights, polydispersity, composition, and polymer structure is difficult because these polymers are synthesized by stepwise polymerization. In our previous work, we demonstrated the synthesis of well-defined nonconjugated polymers containing anthracene © XXXX American Chemical Society

units in the main chain from 10-methylene-9,10-dihydroanthryl-9-spirophenyl-cyclopropane (MDS) by radical ring-opening polymerization via reversible addition−fragmentation chain transfer (RAFT) process.19−22 The driving force of this process is the release of ring strain in the cyclopropane ring, which results in the formation of a stable aromatic ring.23,24 This reaction was able to yield nonconjugated polymers containing anthracene units that alternate with styrene (poly(MDS)s) with narrow polydispersity ( 2.1) were obtained in all cases. The SEC profiles of poly(MDS4P)s were sharp and monomodal; this result supports the progress of the controlled polymerization (Figure 1). The comparison of the conventional radical polymerization and RAFT polymerization of MDS4P reveals that the addition of CTA leads to the decreases in the molecular weight and polydispersity. This is consistent with the general tendency that higher CTA/initiator ratios (lower concentration of the initiator) may afford better overall control of the polymerization, due to a decrease in the number of radicals available for unfavorable side reactions. Given the obtained polydispersities, the optimal RAFT conditions for MDS4P is likely to be [M]/[CTA]/[AIBN] = 500/10/1 in the presence of CTA 2 (Run 4 in Table 2, Mw/Mn = 1.30).

Polymerization was carried out at 80 °C in DMF for 20 h ([M] = 0.335 mol/L). b[M]/[CTA]/[AIBN] = 100/0/1. c[M]/[CTA]/ [AIBN] = 100/2/1. dCalculated by 1H NMR in CDCl3. eYield of diethyl ether-insoluble part. fThe theoretical molecular weight (Mth) = (MW of MDS4P) × [MDS4P]/[CTA] × yield + (MW of CTA). g Measured by SEC using polystyrene standards in DMF (0.01 M LiBr). a

149.5, 134.5, 131.5, 129.7, 126.7, 126.2, 124.8, 123.6, 123.2, 48.8, 33.0. Mn = 3100 (Mw/Mn = 1.30). Formation of Metal Complex. Typical procedure is as follows (run 4 in Table 3). To the ethanol/chloroform solution (10 mL, 3/1 vol %) of poly(MDS4P) (0.03 g, 0.1 mmol) was added the ethanol/ chloroform solution (1.25 mL, 3/1 vol %) of CuCl2 (3 mg, 0.025 mmol) under nitrogen atmosphere, and the reaction was carried out at 60 °C for 2 h. Then the precipitated metal complex was collected with filtration (0.02 g, 68%). Instrumentation. 1H and 13C NMR spectra were recorded with a JEOL JNM-ECX400. The UV−vis and fluorescence spectra were recorded on a JASCO V-630BIO UV−vis and JASCO FP-6100 spectrofluorophotometers, respectively. FT-IR spectra were measured on a Horiba FT-720 spectrometer. The number-average molecular weight (Mn) and molecular weight distribution (Mw/Mn) were estimated by size-exclusion chromatography (SEC) using a Tosoh HPLC HLC-8220 system equipped with refractive index and ultraviolet detectors at 40 °C. The column set was as follows: four consecutive hydrophilic vinyl polymer-based gel columns [TSK-GELs (bead size, exclusion limited molecular weight): α-M (13 mm, >1 × 107), α-4000 (10 μm, 4 × 105), α-3000 (7 μm, 9 × 104), α-2500 (7 μm, 5 × 103), 30 cm each] and a guard column [TSK-guard column α, 4.0 cm]. The system was operated at the flow rate of 1.0 mL/min using DMF containing 10 mM LiBr as the eluent. Polystyrene standards were employed for calibration. Inductive-coupled plasma mass (ICP-MS) spectrometry measurements were performed on a Perkin-Elmer ELAN DRC II spectrometer. Nebulizer gas flow, ICP RF power, lens voltage, pulse stage voltage, dwell time, sweeps, readings per replicate, and flow rate were respectively as follows; 0.91−1.01 mL/min, 1.1 kW, 7.4 V, 900 V, 60 ns, 3 times, 3 times, and 0.96 mL/ min. Dynamic light scattering (DLS) was performed using a Zetasizer Nano (Sysmex) with a He−Ne laser. Thermal analysis was performed

Table 2. RAFT Polymerization of MDS4P and MDS2P using CTA 2a run

monomer

[M]/[CTA]/[AIBN]

convn (%)b

yield (%)c

Mthd

Mne

Mw/Mne

1 2 3 4 5 6 7 8 9 10 11 12

MDS4P

100/0/1 100/2/1 250/5/1 500/10/1 100/5/1 200/10/1 100/0/1 100/2/1 250/5/1 500/10/1 100/5/1 200/10/1

61 72 56 46 74 48 86 96 93 93 96 93

37 42 19 31 42 25 15 50 53 48 50 27

− 6600 3100 7100 4300 3100 − 7700 8100 7400 3200 1900

7200 3000 1700 3100 1500 1300 8000 5800 6700 5100 2900 2100

2.17 1.49 1.53 1.30 1.43 1.34 1.70 1.51 1.39 1.39 1.55 1.55

MDS2P

Polymerization was carried out using CTA 2 at 80 °C in DMF for 20 h ([M] = 0.335 mol/L). bCalculated by 1H NMR in CDCl3. cYield of diethyl ether-insoluble part. dThe theoretical molecular weight (Mth) = (MW of monomer) × [monomer]/[CTA] × yield + (MW of CTA). eMeasured by SEC using polystyrene standards in DMF (0.01 M LiBr). a

C

dx.doi.org/10.1021/ma400525m | Macromolecules XXXX, XXX, XXX−XXX

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Table 3. Formation of Complexes with Metal Species run

polymer

M

[Py]/[M]a

temp (°C)

yield (%)c

M content (wt %)d

Zn

1/5 4/1 1/5 4/1 1/5 4/1

r.t. (5 hb) 60 (2 hb) r.t. (5 hb) 60 (2 hb) r.t. (5 hb) 60 (2 hb)

31 83 31 68 32 77

6.4 5.7 6.1 4.1 1.0 0.1

1/5 4/1

r.t. (5 hb) 60 (2 hb)

21 21

4.2 5.5

poly(MDS4P) 1 2 3 4 5 6

Cu Co poly(MDS2P)

7 8

Cu

a [Py]/[M] = Molar ratio of [pyridine in the repeating unit]/[MCl2]. bReaction time. cYield of precipitated products. dDetermined by ICP measurement.

the poly(MDS4P) are functionalized with the dithioester end groups, which can be used as a macro-CTA for further chain extension reactions. The controlled polymerization was also achieved with MDS2P; the number-average molecular weight and polydispersity of poly(MDS2P) under the optimal condition ([M]/ [CTA]/[AIBN] = 250/5/1) were estimated to be 6700 and 1.39, respectively. The number-average molecular weights of poly(MDS2P)s were, compared to poly(MDS4P)s, more comparable to the theoretical molecular weight, which should resulted in the higher yield of poly(MDS2P)s than those of poly(MDS4P)s. Yet, the yields of both polymers were relatively low in all cases, which was probably due to unfavorable side reactions such as dimerization. In a previous work, we reported reduced yield in the free radical polymerization of the nonsubstituted MDS monomer,19 and thus, the same phenomena probably occurred with the pyridine-containing MDS monomers. Nevertheless, the molecular weight of poly(MDS2P) was controlled by the monomer/CTA molar ratio with maintaining relatively low polydispersity. As shown in Figure S3b (Supporting Information), a shift in the SEC trace toward a higher molecular weight regions with a unimodal peak was seen with increasing [M]0/[CTA]0 ratio. These results suggested that the polymers having relatively narrow molecular weight distributions and predetermined molecular weights were obtained by the ring-opening RAFT polymerizations of the pyridine-containing cyclic monomers, MDS4P and MDS2P. The obtained poly(MDS4P)s and poly(MDS2P)s exhibited good solubility in common organic solvents such as DMSO, DMF, THF, toluene, chloroform, and dichloromethane. The structure of the resulting polymers was characterized by 1H and 13 C NMR spectroscopies (Figure 2). In the 1H NMR spectrum of poly(MDS4P), the peaks corresponding to the aromatic rings and the polymer backbone are observed at 8.3−6.6 and 4.2−3.6 ppm, respectively. The peaks at 153.4−123.2 ppm correspond to those of the aromatic rings, and furthermore, two peaks at 48.8 and 33.0 ppm in the 13C NMR spectrum are clearly assigned to the two different aliphatic carbons in the polymer backbone. These results demonstrate that the desired ring-opening polymerization of cyclic monomers predominantly occurred under the optimal RAFT conditions to yield well-defined polymers composed of alternating anthracene and pyridine containing units. Optical Properties. The solvents- and pH-dependence of the optical properties of the poly(MDS4P) and poly(MDS2P) solutions were investigated. In Figure 3A, the absorption peaks at ca. 330−430 nm corresponding to the anthracene units are

Figure 1. SEC profiles of poly(MDS4P)s. See Table 2 for detailed polymerization conditions.

When the polymerization of MDS4P was conducted at different [M]0/[CTA]0 ratios (20 and 50) and a constant AIBN/CTA molar ratio (1/10), the number-average molecular weights of the poly(MDS4P)s increased with the [M]0/[CTA]0 ratio, and the molecular weight distributions remained narrow, indicating the feasibility of controlling the molecular weights. The unimodal SEC trace of the resulting poly(MDS4P) was shifted to the higher molecular weight region with increasing [M]0/[CTA]0 ratio, as shown in Figure S3a (see Supporting Information). Note that the experimental molecular weights determined by SEC are apparent values, due to the difference in hydrodynamic volume between poly(MDS4P) and the linear polystyrene standards used for SEC calibration. Previously, we reported that the experimental molecular weights determined by SEC were slightly lower than the theoretical values calculated from the monomer/CTA molar ratio and the polymer yield in the cases of the RAFT polymerization of the nonsubstituted MDS monomer.19 The same tendency was observed in the RAFT polymerization of the pyridinecontaining MDS monomer. Nevertheless, these results suggest that the molecular weights of poly(MDS4P)s can be adjusted by the monomer-to-CTA ratio, and the polymers having relatively low polydispersities can be obtained by RAFT polymerization of MDS4P under suitable conditions. For the further investigation, we carried out the chain extension experiment by the polymerization of styrene as a second monomer using poly(MDS4P) as a macro-CTA. As can be seen in Figure S4 (Supporting Information), a shift in the SEC trace toward higher molecular range was observed after the chain extension. These results suggest that most of the chain ends of D


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proton donor to form 1:1 complexes with pyridine rings.39,40 Thus, the fluorescence quenching resulted from charge transfer from the excited anthracene units to the pyridine units coordinated with chloroform. In a previous study, the incorporation of the pyridine rings had influence on the absorption and fluorescence behaviors of conjugated block copolymers having pyridine units.41,42 In contrast, the pyridine rings in the polymer side chains did not induce a clear effect on the optical properties of nonconjugated poly(MDS4P) and poly(MDS2P). Then the pH-dependence of the optical properties was also investigated (Figure 4). The spectrum of poly(MDS4P) solution (THF/water = 1/1 vol %) exhibits the absorption peaks at ca. 330−430 nm under pH = 2, 7, and 10; these peaks correspond to the anthracene units. The absorption intensity is drastically enhanced with increase in acidity, which is caused by the protonation of the pyridine units.43 The absorption intensity of poly(MDS2P) in pH = 2 is stronger than that in pH = 10. However, the pH-dependence of the absorption behavior is stronger in poly(MDS4P) than in poly(MDS2P). This difference suggests that the structure of the molecules may be a major factor affecting the pH-dependence of the absorption behavior (In fact, the absorption spectrum of poly(2-vinylpyridine) has been reported to be not significantly affected by protonation43). The fluorescence quenching of poly(MDS4P) and poly(MDS2P) under acidic conditions (pH = 2) can be due to an electron-transfer from the excited anthracene to the protonated pyridine units.44,45 The charge transfer mechanism based on the donor−acceptor system induced under specific conditions (i.e., use of chloroform as

Figure 2. 1H (top) and 13C (bottom) NMR spectra of poly(MDS4P) in CDCl3. The residual solvent peaks are marked.

clearly observed in the spectra of all poly(MDS4P) solutions (THF, DMF, DMSO, and chloroform). In the fluorescence spectra (Figure 3C), distinct fluorescence peaks due to the anthracene units are observed expect for in the chloroform solution, which was excited at 385 nm. The same trend was detected in the poly(MDS2P) solution (Figure 3, parts B and D), i.e., fluorescence quenching occurred in the chloroform solution. It have been reported that chloroform acted as a

Figure 3. Absorption and fluorescence of poly(MDS4P) (A and C) and poly(MDS2P) (B and D) in different solvents. Solution concentrations are 3.4 × 10−5 and 1.7 × 10−6 anthracene unit mol/L for the absorption and fluorescence measurement, respectively. The excited wavelength is 385 nm. E


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Figure 4. pH-dependence of absorption and fluorescence of poly(MDS4P) (A and C) and poly(MDS2P) (B and D) in THF/H2O (1/1 vol %). Solution concentrations are 3.4 × 10−5 and 1.7 × 10−6 anthracene unit mol/L for the absorption and fluorescence measurement, respectively. The excited wavelength is 365 nm.

solvent and acidic conditions) led to fluorescence quenching. Therefore, the well-defined structure should be attributed to the efficient charge transfer mechanism. So far, we have considered weak cationic polyelectrolytes, i.e., anthracene-containing polymers that possess a functional charge based on the pH of the medium. In the next stage, we converted the pyridine units to the quaternized ones, which rendered the pyridine component permanently charged over the entire pH range.46,47 The quaternization of poly(MDS4P) with iodomethane was then investigated (Scheme 3). Using an

amphiphilic polymer after the quaternization. The solution property of the quaternized poly(MDS4P) was characterized using dynamic light scattering (DLS). As can be seen in Figure S5a (Supporting Information), the broad peak with the maximum peak at around 60 nm was seen in DMSO (1 mg/ mL), suggesting that the formation of the amphiphilic polymer composed of alternating hydrophobic anthracene unit and quaternized pyridine unit leads to the segregation on the nanoscale. In contrast to the DMSO solution of pristine poly(MDS4P), the quaternized poly(MDS4P) exhibited fluorescence quenching in DMSO, which resulted from the charge transfer between the excited anthracene and cationic pyridine units. Note that the absorption and emission behaviors were measured by using extremely diluted solutions. Thus, even though the metal complexes and quaternized polymers can form the aggregates, such effect can be negligible under extremely diluted conditions. Chemical functionalization such as quaternization of pyridine units resulted in optical behaviors that were independent of the external ambient conditions (see Table S1, Figures S6 and S7 in details, Supporting Information). The absorption and fluorescence properties of the poly(MDS4P) thin film were investigated (Figure 5). Compared to the spectrum of the THF solution (Figure 3), the absorption spectrum of the poly(MDS4P) thin film exhibits no significant difference whereas the fluorescence spectrum is largely redshifted. The observed fluorescence peak corresponds to emissions due to excimers of anthracene units,48,49 which indicates that the formation of excimers derived from πstacking interaction could occur in nonconjugated anthracene-

Scheme 3. Quaternization of Poly(MDS4P) with Iodomethane

excess amount of iodomethane for poly(MDS4P) (>3 equiv), the quaternization was completely proceeded to yield the cationic poly(MDS4P). The quaternized poly(MDS4P) was soluble only in DMSO, DMF, and insoluble in THF, toluene, chloroform, and dichloromethane, methanol and water. In contrast, the pristine poly(MDS4P) was soluble in DMSO, DMF, THF, toluene, chloroform, and dichloromethane. The difference in the solubility between the pristine poly(MDS4P) and the quaternized one suggested the formation of the F


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Co in poly(MDS4P) (wt %) were estimated to be 5.7−6.4, 4.1−6.1, and 0.1−1.0 wt % by ICP measurement, respectively. These amounts depended on much rather the [Py]/[M] ratios than the reaction temperature and time. The resulting poly(MDS4P)−metal complexes also exhibited various colors, depending on the nature of the metal and reaction conditions (Figure S8, Supporting Information). A DLS measurement of the poly(MDS4P)-Co complex in DMSO showed extremely broad peak (Figure S5b, Supporting Information), suggesting the existence of aggregation. Poly(MDS2P)-Cu (metal contents: 4.2−5.5 wt %) showed FT-IR spectral changes whereas coordination with Zn and Co did not result in spectral changes (Figure S9, Supporting Information), indicating that the coordination sites (i.e., the position of the nitrogen atom in the backbone) significantly affected the progress of the coordination reactions. This may be attributed to the lower polarity and weaker polarization of the 2-vinylpyridine component than 4-vinylpyridine unit, leading to weaker coordination with metal species. The difference in the solubility between the poly(MDS4P)−metal and poly(MDS2P)−metal complexes was also observed, which may due to the difference in the degree of the coordination and coordination states. As shown in Table S2 (Supporting Information), poly(MDS2P)−Cu complexes were soluble in DMSO and DMF, independent of the [Py]/[M] ratio and reaction conditions. In contrast, only poly(MDS4P)−Zn and poly(MDS4P)−Co complexes prepared at [Py]/[M] ratio = 1/ 5 were soluble in DMSO and DMF in the cases of the poly(MDS4P)−metal complexes. The absorption and fluorescence behaviors of poly(MDS4P)−Zn, and poly(MDS4P)−Co are presented in Figure 7 (Note that the optical properties of poly(MDS4P)-Cu could not be measured because of the poor solubility). The absorption bands due to the anthracene units are clearly observed in the spectra of poly(MDS4P)-Zn and poly(MDS4P)-Co as well as that of pristine poly(MDS4P). On the other hand, coordination with metal species led to a decrease in the fluorescence intensity (Figure 7B). The fluorescence quenching of the complex between pyridinecontaining molecules and Cu, which was due to the excitation energy transfer from the molecules to the metal d-orbital and/ or the molecules to metal charge transfer,50,51 was previously reported. The similar phenomenon may also occur with poly(MDS4P)−Zn and poly(MDS4P)−Co, resulting in fluorescence quenching. As for poly(MDS2P)−Cu, the fluorescence was almost quenched as described in previous work (see Figure S10 in Supporting Information). The thermal stability of the complexes under nitrogen atmosphere was investigated by TG analysis. As can be seen in Figure 8, rapid decomposition was observed in pristine poly(MDS4P) (the residual weight at 400 °C was only 30%), whereas the complexes showed gradual decomposition (the residual weights at 400 °C was 75−60%). Thus, the coordination with metals contributed to the improvement of thermal stability at high temperature.

Figure 5. Absorption and fluorescence of poly(MDS4P) thin film. The excited wavelength is 365 nm.

containing polymers as well as in rigid conjugated anthracenecontaining polymers. Additionally, well-defined structure obtained by radical ring-opening polymerization under RAFT conditions may contribute to the formation of excimers. Formation of Metal Complexes. The coordination of metal species (Zn, Cu, and Co) with the pyridine rings in poly(MDS4P) and poly(MDS2P) has been investigated (Scheme 4). The reaction was carried out under two different Scheme 4. Coordination Reaction with Zn, Cu, and Co Species

conditions by adjusting the amount of metal salts, reaction temperature, and reaction time. In the spectrum of poly(MDS4P) after coordination, the characteristic peak corresponding to the pyridine rings (1620 cm−1) is shifted to 1615 cm−1 (Figure 6). Furthermore, the solubility of the obtained complexes drastically changed compared to pristine poly(MDS4P) (Table S2, Supporting Information), which should result from the formation of the complexes (poly(MDS4P)− Zn, poly(MDS4P)−Cu, and poly(MDS4P)−Co). As can be seen in Table 3, the amounts of the coordinated Zn, Cu, and

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CONCLUSIONS Novel nonconjugated polymers composed of alternating anthracene and pyridine containing units were successfully synthesized by radical ring-opening polymerization via RAFT process. The controlled radical ring-opening polymerization predominantly proceeded under suitable RAFT conditions to yield the desired polymers with low polydispersities (∼1.30).

Figure 6. FT-IR spectra of poly(MDS4P) before and after the coordination reaction with metal species at 60 °C: (a) poly(MDS4P), (b) poly(MDS4P)−Zn, (c) poly(MDS4P)−Cu, and (d) poly(MDS4P)−Co. G


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unique optical properties. These properties arise from the enhancement of the donor−acceptor system under various solution conditions. These polymers have a huge potential to be applicable for stimuli-responsive sensor materials.

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ASSOCIATED CONTENT

S Supporting Information *

Synthetic procedures and 1H NMR spectra of A2PC and MDS2P, SEC traces of the polymers obtained at different [M]0/[CTA]0 ratios, results of chain extension experiment, figures and tables summarizing the quaternization and coordination reaction, photos of the complexes made, and IR and fluorescence spectra. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*(H.M.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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Figure 7. Absorption (A) and fluorescence (B) spectra of poly(MDS4P), poly(MDS4P)−Zn (run 1 in Table 3), and poly(MDS4P)−Co (run 5 in Table 3) in DMSO. Solution concentrations are 3.4 × 10−5 and 1.7 × 10−6 anthracene unit mol/L for the absorption and fluorescence measurement, respectively. The excited wavelength is 385 nm.

Figure 8. TG profiles of poly(MDS4P), poly(MDS4P)-Zn (run 1 in Table 3), and poly(MDS4P)-Cu (run 3 in Table 3), and poly(MDS4P)-Co (run 5 in Table 3) under a nitrogen atmosphere.

The interaction between the excited anthracene and coordinated or protonated pyridine units under specific conditions (chloroform and acidic solution) led to fluorescence quenching. Furthermore, the quaternization of pyridine rings and complexes between polymers and metal species (Zn, Cu, and Co) induced fluorescence quenching that was independent of external ambient conditions. In the thin film state, the obtained polymers exhibited fluorescence due to the formation of excimers. This is the first report on the synthesis of well-defined nonconjugated polymers with a repeating structure with units containing alternating anthracene and pyridine groups, and H


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