Ladder-Type Polymers and Ladder-Type Polyelectrolytes with On

Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

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Ladder-Type Polymers and Ladder-Type Polyelectrolytes with OnChain Dibenz[a,h]anthracene Chromophores Florian Trilling, Michelle-Kathrin Ausländer, and Ullrich Scherf* Macromolecular Chemistry Group and Institute for Polymer Technology, Bergische Universität Wuppertal, Gaußstraße 20, 42119 Wuppertal, Germany

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S Supporting Information *

ABSTRACT: The alternating copolymerization of fluorene and diethynyl benzene building blocks and postpolymerization electrophilic cyclization result in the generation of well-defined ladder poly(dibenz[a,h]anthracene)s (PLDBAs). PLDBAs combine the specific optical properties of the aromatic building blocks with well-resolved, vibronically structured photoluminescence spectra and a small Stokes shift (5 nm), as typical characteristics of ladder polymers, with an excellent solubility in organic solvents. Success of the ladderization step was confirmed by NMR- and IR-spectroscopies. The presence of syn- and anti-isomeric repeat units along the polymer backbone was proven by an investigation of corresponding model compounds. In addition, this concept was used for the synthesis of the corresponding ladder polyelectrolytes, bearing cationic imidazolium groups in their sidechains. The polyelectrolytes show very similar optical and electronical properties if compared to their noncharged counterparts but are now soluble in protic, polar solvents as methanol.

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INTRODUCTION Since synthesis of the first soluble, planarized conjugated ladder polymer (cLP), namely, ladder poly(para-phenylene) (LPPP), by Scherf and Müllen in 1991,1 there has been a growing interest in the synthesis and characterization of cLPs until today.2−8 Characterized by their rigid and planar structure, inhibiting torsion along the polymer backbone, cLPs show unique optical and electronical properties and excellent intrachain charge mobilities.9−14 In combination with their outstanding chemical and thermal stability, cLPs are promising candidates for many optoelectronic applications and were already tested as active materials in organic lasers, organic light emitting diodes, and organic field effect transistors.5,15−20 However, generation of novel cLPs is still challenging. Any synthesis strategy needs to fulfill two important requirements: on the one hand, the ring closure procedure has to occur in a quantitative yield to prevent structural defects along the polymer backbone.21 On the other hand, the cLPs and all intermediates need to provide an adequate solubility in common organic solvents, often hampered by the strong π−π interactions.22,23 During the last decades, several potential synthesis strategies were developed. The most widely used © XXXX American Chemical Society

methods consist of multistep approaches: a pre-organized polymeric precursor with functional groups attached to the polymer backbone is build-up in a first coupling step and annulated in quantitative yields in one (or more) polymer analogous reaction(s). Ladder polymers as LPPP or poly(thienylene-alt-phenylene) were built-up from polyketone precursor polymers, which were reduced with alkyllithium reagents and cyclized by Lewis-acid mediated, intramolecular Friedel−Crafts-analogous alkylations.7,11,21 Another example for the formation of ladder polymers in kinetically controlled annulation reactions is an electrophilic cyclization of alkynyl sidegroups, first reported by Goldfinger and Swager in 1994.24 Their attempt based on synthesis of diethynyl-functionalized precursor polymers and their polymer analogous annulation using electrophiles as acids or iodonium salts. In addition to the resulting graphenic-nanoribbon (GNR)-like ladder polymers, they also investigated the scope of this method by synthesis of a variety of annulated small molecules as model Received: February 26, 2019 Revised: March 29, 2019

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DOI: 10.1021/acs.macromol.9b00396 Macromolecules XXXX, XXX, XXX−XXX

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Scheme 1. Synthesis Scheme toward Ladder Polymer PLDBA: (i) K2CO3, Pd(PPh3)4, Water, THF, 80 °C, 72 h; (ii) TFA, Dichloromethane (DCM), RT, 3 h

Figure 1. Normalized absorption (solid lines) and PL spectra (dotted lines) of PELOD-F8 and PLDBA in solution (chloroform, left) and as thin film (right).

of five-membered rings between the isolated dibenz[a,h]anthracene (DBA) building blocks should improve the photooxidative stability of the polymer backbones through an increased highest occupied molecular orbital (HOMO)/lowest unoccupied molecular orbital (LUMO) band gap and, thereby, also influence the optical properties. Moreover, the electrophilic cyclization procedure is also tested as a tool for the preparation of the corresponding, cationic ladder polyelectrolytes.

compounds.25 Nowadays, the solution-based synthesis of cLPs was much stimulated and inspired by the availability of new, surface-assisted coupling schemes, see extensive work by Mü llen, Fasel and co-workers,26−28 as well as of new postpolymerization methods for ladderization.29 Moreover, one-handed helical ladder polymers have been made in an electrophilic cyclization procedure by Swager and co-workers.30 The optical spectra of GNRs and other ladder polymers with a two-dimensional π-framework significantly differ from those of typical one-dimensional, cLPs like LPPP.24,31,32 In this light, the primary goal of this study is the synthesis and characterization of ladder-type poly(dibenz[a,h]anthracene)s (PLDBA) in an electrophilic cyclization reaction. Introduction

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RESULTS AND DISCUSSION Scheme 1 depicts the synthesis for ladder polymers containing the DBA unit. The corresponding precursor polymer PELODF8 was synthesized by Suzuki coupling between the B


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Figure 2. 1H NMR-spectra of PELOD-F8 (top) and PLDBA (bottom) in C2D2Cl4.

lesser intense, red-shifted shoulder at 368 nm. After cyclization, PLDBA shows significantly bathochromic-shifted absorption features, indicating the extension of the conjugated system due to the more rigid, planarized structure of the polymer backbone. The UV/vis spectrum shows a sharpened absorption edge. The absorption maximum is red-shifted by 83 nm up to 419 nm if compared to the single-stranded precursor PELOD-F8. Moreover, PLDBA displays an absorption shoulder at 436 nm, the so-called α-band of polyaromatic hydrocarbons as previously reported for DBA-derived oligomers as well as for GNRs.9,34 In the PL spectra of PLDBA, the emission maximum is also shifted to higher wavelengths upon cyclization [PLDBA in solution (chloroform): λmax = 441 nm, (λexc = 380 nm)] if compared to PELOD-F8 [in solution (chloroform): λmax = 414 nm, (λexc = 320 nm)]. Moreover, PLDBA exhibits a well-resolved emission spectrum with narrow PL bands and vibronic progressions at 470 and 507 nm, which can be assigned to the 0−1 and 0−2 transitions as described for other ladder polymers as LPPP.1,35 Furthermore, the ladderization leads to a strong decrease of the Stokes shift from 78 nm (PELOD-F8) down to 5 nm (PLDBA, calculated from the low energy absorption shoulder), which is almost similar to that of LPPP (ΔλStokes = 4 nm).15 In the thin film (Figure 1 right), the absorption spectra of PLDBA appear almost unaffected, indicating the absence of aggregation phenomena, whereas the PL-spectra extended into the vis region and show an additional maximum peaking at 520 nm. This maximum is probably caused by the presence of the socalled keto-defects along the polymer backbone and changes the solid state emission color from blue (in solution) to yellowish in the thin film. The spectral differences of ketodefect-containing cLPs in solution and the solid state have

dibrominated diethynyl building block (ELOD-Br) and the diboronic ester-substituted fluorene derivative F8-BPin. ELODBr was synthesized according to a modified literature procedure (branched octyldodecyl instead of linear dodecyl sidechains):24,33 starting from 4-iodophenol, ELOD-Br was generated in an excellent overall yield of 71% over four steps (see Supporting Information). Suzuki cross-coupling of ELODBr and F8-BPin was performed under optimized reaction conditions with a comparably high catalyst loading of 10 mol % [catalyst: tetrakis(triphenylphosphine)palladium(0)]. Attempts with lower catalyst loadings resulted in defect formation as indicated by occurrence of an absorption shoulder around 500 nm with a color change of the products from yellow to brownish (in solution and in the solid state). Subsequently, those precursor polymers could not be successfully transferred into the corresponding ladder polymers with satisfying precision. However, by application of the optimized reaction conditions (10 mol % Pd(PPh3)4) PELODF8 could be synthesized as a slightly yellow solid in 72% yield with a number average molecular weight (Mn) of 10.0 kDa. The following intramolecular, electrophilic cyclization of PELOD-F8 with an excess of trifluoroacetic acid (TFA, 350 equiv) led to the formation of the corresponding ladderized PLDBA in a nearly quantitative yield. During the reaction, the color of the solution turned from slightly yellow to deep green, caused by partial protonation of the aromatic system.25 After quenching with diisopropylamine, the solution is colored deep yellow and shows a strong blue photoluminescence (PL). Optical spectra of PELOD-F8 and PLDBA are shown in Figure 1. The absorption spectra of PELOD-F8 in solution (Figure 1, left) show a single maximum at 336 nm accompanied by a C


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Figure 3. Possible isomer formation, occurring during the formation of PLDBA.

Scheme 2. Syntheses of Ladderized Model Compounds anti-DFA and syn-DFA: (i) K2CO3, Pd(PPh3)4, Water, THF, 80 °C, 16 h; (ii) TFA, DCM, RT, 3 h

Comparing the 1H NMR-spectra (see Figure 2) of the polymers, the protons of the fluorene moieties and the protons of the central benzene units of the diethynyl-building block (PELOD-F8) only show one multiplet signal (black, top) at a chemical shift of 7.96−7.66 ppm. The remaining aromatic signals at 7.34 and 6.84 ppm correspond to the protons of the outer benzene rings of the diethynyl units in meta- (gray, top) and ortho-position (brown, top) to the alkoxy sidechains. After formation of the aromatic DBA moieties, the aromatic signals are strongly downfield-shifted. The central protons of the DBA unit (black, bottom) can be observed at 9.27 ppm, caused by the strong deshielding effect of the DBA moiety. Moreover, the signals of the protons in ortho- (blue) and meta-position (green) to the methylene bridge of the fluorene unit are shifted to 8.90 and 8.43 ppm. The signals are perfectly fitting to those of literature-known DBA-oligomers.9 The remaining aromatic signals are also downfield-shifted. Surprisingly, several lower intensity signals can be observed in the area from 9.0 to 7.0 ppm. Those signals may result from the formation of different isomeric repeat units during the polymer analogous reaction step (see Figure 3). Hereby, the ring closure is possible into the 3/6- or 1/8-positions of the fluorene cores with the sterically more crowded 1/8-positions as the minority

been related to an interchain exited state energy transfer to defect centers.36 The spectral changes during ladderization indicate that the planarization of the π-system distinctly affects the electronic properties of the polymers. Due to the red shift of the optical spectra, the optical band gap of PLDBA (3.06 eV) is decreased by 0.33 eV in comparison to PELOD-F8 (3.39 eV). While the position of the HOMO levels is nearly unaffected (PLDBA: −5.64 eV vs PELOD-F8: −5.67 eV), the formation of the extended aromatic DBA units lowers the LUMO levels from −2.28 (PELOD-F8) to −2.58 eV (PLDBA). Success of the polymer analogous reaction was also monitored by IR- and NMR-spectroscopic studies. In addition to aromatic and aliphatic vibrations, PELOD-F8 shows a weak vibration band 2210 cm−1 that can be assigned to the stretching vibrations of the carbon−carbon triple bonds. As expected, PLDBA shows no IR vibration bands in this area, caused by the full consumption of the ethynyl groups upon ladderization. The complete conversion is also demonstrated by 13C{H}-NMR-spectroscopy. For PELOD-F8, two acetylenic resonances at 94.7 and 88.5 ppm can be observed; they fully disappear after cyclization into PLDBA. D


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Figure 4. 1H NMR-spectra of anti-DFA (top), PLDBA (middle), and syn-DFA (bottom) in C2D2Cl4.

target, since an attack of the fluorene carbon atoms in paraposition (3/6-position) relative to the 9-methylene bridge should be sterically favored. To investigate the degree of isomer formation, a model compound (ELOD-2F8) was synthesized in a Suzuki-type cross coupling of ELOD-Br with two equivalents of a monoborylated fluorene derivative (F8-mBPin) (Scheme 2). In the cross coupling toward the model compounds, identical reaction conditions were applied as already described for the polymer formation. The precursor ELOD-2F8 was isolated and subsequently treated with TFA for cyclization. After purification and isomer separation by column chromatography, two different products were isolated in a 2:1 molar ratio (total yield: 95%). High-resolution mass spectrometry of the products yielded identical masses for the two model compounds with m/z = 1649.3. Next, the chemical structure of both isomers could be identified by NMR-spectroscopy: the majority isomer as anti-difluoreno[a,h]anthracene (anti-DFA), with both linkages to the fluorenes cores in para-position (3position), and the minority isomer as syn-DFA, with one paraand one ortho-linkage toward the fluorene cores (3- or 1position). Due to its high symmetry, anti-DFA shows a lowpeak-number 1H NMR-spectrum (see Figure 4, top), whose

signals well compare to the higher intensity signals of the polymer PLDBA [central DBA proton: PLDBA: 9.27 ppm (black, middle) versus anti-DFA: 9.25 ppm (black, top)]. In contrast, syn-DFA shows a more complicated 1H NMRspectrum: the configuration of this isomer was derived from the signals of the fluorene hydrogens in para- (gray, bottom) and meta-position (brown, bottom) relative to the methylenebridge (9-position). For these hydrogens, the spectrum shows two doublets at 8.08 and 7.96 ppm of very similar intensity. The 1H NMR-spectrum of syn-DFA also relates to the minority signals of the PLDBA spectrum most probably caused by the presence of “syn”-configurated, isomeric repeat units of the polymer backbones. The intensities of those signals imply that ca. every sixth linkage with the fluorene cores is ortho-positioned, thus leading to an isomeric mixture of repeat units in the polymer backbone. Conjugated Ladder Polyelectrolytes. The electrophilic cyclization method described here can be also exploited for the formation of ladder polymers with ionic sidechains, the socalled conjugated ladder polyelectrolytes (cLPEs), by annexing a final postpolymerization step. Those were generated by the introduction of ω-halogenated alkylene sidechains into one or E


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Macromolecules Scheme 3. Syntheses of cLPEs PLDBA2I and PLDBA4I: (i) 1-Methyl-1H-imidazole, THF, 80 °C, 16 h

Figure 5. Normalized absorption (solid lines) and PL spectra (dotted lines) of PELOD-F6Br, PLDBA2Br, and PLDBA2I (left) and of PELHBr-F6Br, PLDBA4Br, and PLDBA4I (right) in solution (PELOD-F6Br, PLDBA2Br, PELHBr-F6Br, and PLDBA4Br in chloroform; PLDBA2I in an 1:1 mixture of methanol and chloroform; PLDBA4I in methanol).

both of the building blocks, followed by a final polymeranalogous quaternization with N-nucleophiles. (see Scheme 3). For details of the synthetic pathway, see the Supporting Information, in short: copolymerization of ELOD-Br with a diboronic ester-substituted fluorene building block bearing 6bromohexyl sidechains (F6Br-BPin) produces the precursor polymer PELOD-F6Br in 40% yield with a comparably high Mn of 21.5 kDa. Another precursor polymer was synthesized from a di(bromoalkyl)-substituted diethynyl monomer, also bearing 6-bromohexyl sidechains (ELHBr-Br), synthesized in an overall yield of 26% over four steps. The corresponding precursor

polymer PELHBr-F6Br, with four 6-bromohexyl groups per repeat unit, was isolated with a molecular weight Mn of 13.0 kDa. Subsequently, both polymers were transferred into ladder polymers by reaction with TFA using the conditions already described for PLDBA synthesis. The ladderized polymers PLDBA2Br and PLDBA4Br were isolated in nearly quantitative yield and show very similar optical and electronical properties if compared to PLDBA (for IR- and NMR-spectroscopy data see the Supporting Information). Finally, PLDBA2Br and PLDBA4Br were treated with an excess of 1-methyl-1H-imidazole (Scheme 3) in tetrahydrofuran F


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(THF) at 80 °C for the generation of the corresponding imidazolium salts. Formation of the ionic polymers is accompanied by distinct solubility changes and precipitation of the polymer from THF solution during the reaction. PLDBA2I shows a poor solubility both in polar and unpolar solvents, but could be easily solubilized in 1:1 mixtures of chloroform and methanol. In contrast, the higher density of charged groups in PLDBA4I (four instead of two per repeat unit) strongly increases the solubility in polar solvents as methanol. After dialysis (cutoff molecular weight of the membrane: 3.5 kDa) against a suited solvent/solvent mixture, the cLPEs were obtained in nearly quantitative yields for the quaternization step. Apart from the solubility differences, PLDBA2I and PLDBA4I show almost identical electronic and optical properties (see Figure 5), without significant differences to those of the precursors PLDBA2Br and PLDBA4Br. The cLPEs exhibit a combination of typical ladder polymer (low Stokes shift, strong fluorescence, well-resolved vibronic progressions in absorption and PL) and polyelectrolyte properties (solubility in polar solvents as methanol). Again, the full conversion of the precursor polymers after postpolymerization cyclization and quaternization was crosschecked by IR and NMR spectroscopy. The differences between the non-ionic PLDBA4Br and the quaternized PLDBA4I are now discussed as an example for the cLPe synthesis in general. PLDBA4Br shows, in addition to the already discussed signals of the aromatic DBA units, two characteristic resonances of the methylene groups in α-position to the bromo functionalities of the sidechains (3.50 and 3.24 ppm) in the 1H NMR-spectrum (see Supporting Information). After the polymer-analogous substitution with 1-methyl-1Himidazole, those signals are downfield-shifted to form a multiplet at 3.81−3.69 ppm, as a consequence of the stronger deshielding effect of the imidazolium groups. The introduction of the imidazolium groups is also indicated by the appearance of an additional aliphatic singlet at 4.01 ppm, which can be assigned to the N-methyl group of the imidazolium unit. PLDBA4I shows a broad band at 3375 cm−1 in the IR spectrum, resulting from the hydrogen stretching vibrations of the heterocyclic substituent (see Supporting Information).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.9b00396. Experimental details and analytics including materials and instruments, synthesis of the diethynyllinkers, polymer synthesis, literature, and spectral data (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Florian Trilling: 0000-0001-6096-7895 Notes

The authors declare no competing financial interest.

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REFERENCES

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CONCLUSIONS In summary, we have successfully synthesized novel ladder polymers containing DBA main chain units using an electrophilic postpolymerization cyclization procedure. The resulting ladder polymer PLDBA shows a sharp absorption edge, well-resolved vibrationally structured PL features, and a small Stokes shift of ca. 5 nm, as typical features of cLPs. PLDBA is formed as an isomeric mixture of repeat units (3/6or 1/8-substitution of the fluorene units), as demonstrated by synthesis and characterization of two model compounds synDFA and anti-DFA. Finally, we also applied the electrophilic cyclization for the formation of ionic ladder polymers with imidazolium groups in their sidechains, as the so-called cLPEs. The cationic cLPEs PLDBA2I and PLDBA4I, with two or four imidazolium side groups per repeat unit, were obtained with molecular weights of >20 kDa, whereby the derivative with four ionic groups per repeat unit is soluble in polar, protic solvents as methanol. The electronic and optical properties of the polymers were almost unaffected by the conversion into the cLPEs. The solubility in methanol opens possibilities for an orthogonal solvent-based solution processing of multilayer assemblies. G


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Ladder-Type Polymers and Ladder-Type Polyelectrolytes with On

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