Small Change, Big Impact: The Shape of Precursor Polymers Governs

Article Cite This: Macromolecules 2019, 52, 4458−4463

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Small Change, Big Impact: The Shape of Precursor Polymers Governs Poly‑p‑phenylene Synthesis Ali Abdulkarim,†,‡ Karl-Philipp Strunk,§,# Rainer Bäuerle,†,§ Sebastian Beck,†,§ Hanna Makowska,∥ Tomasz Marszalek,∥,⊥ Annemarie Pucci,†,§,# Christian Melzer,†,§,# Daniel Jänsch,†,‡ Jan Freudenberg,†,‡ Uwe H. F. Bunz,‡,# and Klaus Müllen*,⊥ †

InnovationLab, Speyerer Str. 4, 69115 Heidelberg, Germany Organisch-Chemisches Institut, Ruprecht-Karls-Universität Heidelberg, Im Neuenheimer Feld 270, 69120 Heidelberg, Germany § Kirchhoff-Institut für Physik, Ruprecht-Karls-Universität Heidelberg, Im Neuenheimer Feld 227, 69120 Heidelberg, Germany ∥ Department of Molecular Physics, Faculty of Chemistry, Lodz University of Technology, Zeromskiego 116, 90-924 Lodz, Poland ⊥ Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany # Centre for Advanced Materials, Ruprecht-Karls-Universität Heidelberg, Im Neuenheimer Feld 225, 69120 Heidelberg, Germany

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‡

S Supporting Information *

ABSTRACT: The synthesis of unsubstituted, structurally perfect poly(para-phenylene) (PPP) has remained elusive for many decades. By modifying our previously reported precursor route towards PPP, we were able to simplify and optimize the precursor polymer synthesis and yields, the thermal conversion process to PPP, and the resulting material properties. We describe the synthesis of unprecedented anti-dialkoxycyclohexadienylenes, polymerized via Suzuki coupling to yield linear PPP precursor polymers. Changing the geometry and overall shape of the precursor viz upon going from syn- to anti-configuration of the monomer has two important consequences: (i) formation of the precursor polymer becomes more selective since cyclization of the monomer is no longer possible and (ii) the precursor polymer adopts a “stretched” geometry and becomes more similar to the rigid-rod of PPP, impacting the aromatization process and material properties. Films of the precursor polymers are thermally aromatized via dealkoxylation to yield structurally perfect and highly ordered, insoluble PPP. Long-range ordering within the thin films, not observed for its syn-analog, is induced as evidenced by atomic force microscopy, X-ray scattering, and IR and UV−vis/photoluminescence spectroscopy. The aromatization temperature, now feasible for fabrication of plastic devices, is significantly lowered from previously reported 300 °C to below 250 °C. The kinetics of the aromatization process were monitored via time-dependent IR measurements at different annealing temperatures, showing much faster quantitative aromatization for thin layers.

A

Scheme 1. PPP Syntheses via Thermal Elimination of a Precursor Polymer Containing Dialkoxycyclohexadienylenes in Syn- (Previous Work, syn1) and Anti-Configuration (This Work)

polymer most commonly referred to as the prototype of a conjugated semiconductor, unsubstituted and structurally well-defined poly(para-phenylene) (PPP), has remained a synthetic challenge. Its rigid-rodlike nature is responsible for its insolubility, thus complicating synthesis and processability.1−9 Prior to our work,2 PPP was obtained either by direct methods,3,4 resulting in oligomeric materials, or via precursor protocols.5 Those products were ill-defined as they contained meta-phenylene defects or acid contaminants.6−10 However, the mechanical strength, high thermal and chemical stability, electrical conductivity upon doping, and electroluminescence make PPP a promising candidate for applications in organic electronics and have raised high expectations for structurally perfect (without meta-defects or impurities), unsubstituted PPP.11−13 Although surface-assisted syntheses furnish PPP of higher quality, the minuscule quantities obtained prohibit further use.14−16 On the journey toward pristine PPP, our group has recently reported a new solution for this old problem by the synthesis of a cyclohexadienylene-based precursor polymer (Scheme 1).2 However, this approach leads to low polymer quantities, as it © 2019 American Chemical Society

requires a challenging precursor purification to remove macrocyclic byproducts of different sizes and structures (e.g., catenanes).17 Received: April 17, 2019 Revised: May 24, 2019 Published: June 14, 2019 4458

DOI: 10.1021/acs.macromol.9b00792 Macromolecules 2019, 52, 4458−4463

Article

Macromolecules Scheme 2. Synthesis of syn- and anti-4a,b/5a,ba

Conditions: (i) sodium hydride, (4-bromophenyl)lithium, tetrahydrofuran (THF), −78 °C, 1 h, (ii) (4-bromophenyl)lithium, THF, −78 °C, 1 h, (iii) MeI or n-BuI, NaH, THF/DMF, 0 °C to room temperature, 16 h (iv) n-BuLi, (i-PrO)B(Pin), THF, −78 °C, 1 h.

a

Scheme 3. Suzuki Polymerization and Aromatization Strategy by Thermal Annealing of Thin Filmsa

Conditions: (i) Pd(PPh3)4, Cs2CO3, THF/H2O (10:1), 60 °C, 16 h, PhBPin 1 h, PhBr 1 h, (ii) for anti-1b: thin film, 250−300 °C, 5−30 min.

a

Our synthesis (Scheme 2) started from ketone 2b (/2a), which itself could be synthesized by different routes (for details, see the Supporting Information (SI)).17 In contrast to the deprotonated alcohol 2a, which, upon subsequent arylation with monolithiated 1,4-dibromobenzene, furnishes the synadduct stereoselectively due to electrostatic shielding of one hemisphere by the emerging alcoholate (cf. Supporting Information, electrostatic model),20−24 2b yielded a mixture of the syn-isomer and anti-3 in a ratio of 3.5:1 (syn/anti). After isolation of anti-3 by column chromatography, also possible via crystallization since the anti-product crystallizes very effectively and the oily syn-isomer remains in solution, subsequent alkylation employing sodium hydride and alkyl iodide furnished methoxylated anti-4a (99%) or butoxylated anti-4b (34%). Both dihalides were converted to bisboronic esters anti-5a and anti-5b by double lithiation and reaction with isopropoxyboronic acid pinacol ester ((i-PrO)B(Pin)) in yields ranging from 59 to 68%.

The key step of the PPP synthesis is the additive-free thermal aromatization of thin films of a well-soluble and thus processable precursor polymer, offering opportunities for its practical application (cf. Scheme 1). The syn-configuration of the cyclohexadienylenes most likely results in random coiled polymer conformations in solution, preserved also in the solid state.18 Thus, the polymer backbone needs to undergo an additional geometrical reorganization from non-“extended” chains to linear rigid-rods upon aromatization in the solid state, which is reflected in the high reaction temperatures (up to 300 °C) and long aromatization times (40 min) to achieve full conversion.2 The postulated geometrical change upon aromatization also hinders the alignment of the PPP chains, as the obtained PPP thin films are amorphous. Thus, the geometry of the precursor polymer is critical for purification, aromatization, and morphology.19 To resolve the aforementioned issues, we designed the precursor polymer by replacing syn- with anti-cyclohexadienylene moieties. 4459


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resulting from head-to-head and head-to-tail connectivities (see the SI). For a detailed analysis of the aromatization process, Fourier transform infrared (IR) spectroscopy of spin-coated thin films was applied. This technique has previously been employed to quantify the conversion process of syn-1.2,19 Transmission IR spectra of anti-1b, annealed stepwise at 200, 250, and 300 °C, illustrate the gradual transition from the precursor to PPPanti (Figure 1b for 250 °C and SI), with the product spectra being similar to that of PPPsyn and DFT-calculated spectra.2,19 The strongest absorption bands assigned to calculated vibrational modes and fitted models for the dielectric functions of anti-1b and PPPanti are supplied in the Supporting Information (see Section 6). The measured IR spectra leave no doubt that aromatization at 300 °C leads to complete conversion after only 5 min, whereas at 250 °C, 2 h are necessary (conversion exceeds 90% at 1 h), and although aromatization does proceed at 200 °C, transformation is inefficient as it does not exceed 80% even after 10 h (Figure S14). In comparison to the precursor polymer with syn-configuration, the conversion process for anti-1b is independent of the layer thickness, and anti-1b reaches full conversion at 250 °C in films (see the SI). The aromatization temperature has thus been lowered significantly (by up to 50 °C) supporting the TGA/DSC results obtained from bulk analysis (vide supra). For thin layers, quantitative aromatization is much faster and more benign, thus enabling the use of PPP for the fabrication of plastic devices.28 This improved conversion process (aromatization kinetics, lowering Tarom, and independency of the layer thickness) is due to a smaller geometrical reorganization during aromatization of the “streched” form of anti-1b to PPP. This is also reflected in absorbance and emission of the asgenerated PPPanti. Although precursor polymers anti-1b and syn-1 show similar absorption maxima at 271 and 269 nm before thermal treatment (Figure 2), after aromatization at 300 °C for 10 min (to ensure comparability), PPPanti revealed an absorption maximum at 360 nm and PPPsyn at 350 nm. We attribute this red shift to better intra- and interchain interactions of the phenylene units, a phenomenon observed also for other polyconjugated π-systems with different degrees of order such as poly-p-phenylenevinylene or polythiophenes,29−35 most likely caused by less torsion in the para-phenylene backbone formed from anti-1b. This is plausible since the precursor polymer adopts a similar rigid geometry in comparison to PPP due to anti-connectivity in contrast to the more coiled syn-1. The latter has to undergo more reorganizational changes in geometry upon aromatization and results in more pronounced torsion of the polymer backbone. The PL spectra further deviate, since PPPanti displays narrower emission bands and transitions at higher energies (λem = 426 and 451 nm) are reduced. The main emission bands at λem = 485 and 520 nm of PPPanti appear unaltered compared to PPPsyn (Figure 2 and see SI), whereas the PL quantum yield is almost doubled from 18% for PPPsyn to 34% for PPPanti. The difference in the optical properties of PPPanti to PPPsyn is a direct result of different polymer arrangements in thin films, which are further investigated by atomic force microscopy (AFM) in combination with angle-dependent IR spectroscopy and compared to those of PPPsyn. Atomic force micrographs of three polymer-coated Sisubstrates were recorded after thermal treatment at different temperatures for 10 min. At 200 °C, the film remains

Selective AA/BB-polymerization under Suzuki conditions of anti-4a with anti-5a leads to the insoluble, methoxylated polymer anti-1a in 94% yield (Scheme 3), which is a consequence of the linear monomers, since cyclizations are now suppressed. The polymer’s insolubility in common organic solvents is due to the linear and rigid nature in comparison to the coiled and thus more soluble syn-analog.2 Unfortunately, the insolubility of anti-1a prohibited further analysis (except thermogravimetry and differential scanning calorimetry (TGA/DSC) of the as-obtained powder, vide infra) and processing (film formation) prior to aromatization. Regioirregular replacement of one methoxy group with an nbutoxy substituent was sufficient to endow anti-1b, obtained in a yield of 91%, with good solubility in chloroform. Its molecular weight distribution was estimated via analytical gel permeation chromatography (GPC) vs polystyrene in chloroform (Table S1) with an average molecular weight (Mw) of 10.3 kg mol−1 and a dispersity (Đ) of 2.4, expected for stepgrowth polymerization. After aromatization, 13 (mMn) or 31 (mMw) repeating units correspond to 40 and 93 phenylene moieties, respectively (see SI). Further optimization of the polymerization conditions, e.g., variation of temperature, scale, or even reaction type (e.g., Stille polymerization) might even lead to higher molecular weights. However, compared to our previously reported PPP (PPPsyn), both dispersity (Đsyn = 2.1) and chain length (mMn[syn] = 18 and nMw[syn] = 39) remain comparable.2,25−27 The dealkoxylation of the precursor polymers anti-1a and anti-1b was studied via TGA/DSC analyses in the bulk (Figure 1). In comparison to syn-1 (Tarom = 275−300 °C),2 the aromatization temperatures by TGA/DSC were significantly lowered by ∼50−75 to 220 °C and 222 °C for anti-1a and anti-1b (SI). While anti-1a displayed a sharp exothermic peak in the DSC, anti-1b gave rise to a broad signal ranging from 181 to 260 °C, presumably due to its regioirregularity,

Figure 1. (a) DFT-calculated relative transmission spectrum of a precursor anti-1b layer on silicon. (b) Experimental relative transmission spectra of 55 nm precursor anti-1b on silicon as cast and for increasing annealing times at 250 °C and a relative transmission spectrum of 34 nm PPPsyn prepared from syn-1 annealed at 300 °C for 60 min on silicon (purple). (c) DFT-calculated relative transmission spectrum of a PPP layer on silicon. 4460


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Figure 2. Optical spectra of anti-1b and its conversion into PPPanti (left) and of syn-1 and its thermal aromatization into PPPsyn (right) on a quartz substrate.

Figure 3. Atomic force micrographs of PPPanti obtained after annealing of anti-1b at 200 °C for 10 min (a), 250 °C for 10 min (b), and 300 °C for 10 min (c). GIWAXS patterns of anti-1b: (d) before heating, (e) annealing at 200−250 °C (2 h), and (f) at 200−250 to 300 °C (2 h).

orientation for the anti-precursor as well as for the as-formed PPPanti (for details, see the SI). Both polymer films, before and after conversion, are not isotropic but rather exhibit a preferred face-on molecular orientation, which differs from previously published isotropic PPPsyn and syn-1. Grazing incidence wide-angle X-ray scattering (GIWAXS, see Figure 3d−f and SI) reveals a significant increase in order after thermal annealing as well as a face-on orientation of the polymer strands. This is in agreement with the IR and AFM studies. The low intensity of the reflection at qz = 1.62 Å−1 (corresponding to 390 pm), assigned to the π-stacking of PPPanti, is attributed to the ordered domains (∼500 nm) being randomly oriented over the thin film. Note that no reflections were observed for the syn-analogous PPPsyn. Thus, changing the geometry of the precursor polymer is essential to an improved morphology. In conclusion, the correct choice of the overall shape of a precursor polymer is vital for massively improved material

amorphous at that microscale resolution (Figure 3, left; see Supporting Information for AFM of nonannealed anti-1b). Further heating at 250 °C (Figure 3, middle) leads to ordered domains, which are attributed to aggregates of partly aromatized rigid-rodlike PPP chains, since the TGA/DSC and IR studies show that aromatization proceeds at this temperature, but remains incomplete at this timescale. Finally, after annealing at 300 °C (Figure 3, right), complete conversion into PPP occurs and large domains of highly ordered PPP chains are formed. Expulsion of small molecules from thin films is known to sometimes decrease their integrity. However, substitution of one methoxy group with a single butoxy group, compared to our previous work, does not negatively affect film integrity or uniformity as evidenced by atomic force micrographs and polarization micrographs (SI). Contrastingly, AFM images of aromatized syn-1 are completely devoid of structure and, in some cases, holes are observed.19 Angle-dependent IR spectra reveal a preferred molecular 4461


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Musterbildung in der materiellen Welt” of the University Heidelberg for Funding.

properties of the desired target polymer, demonstrated in this work by design of a precursor route toward unsubstituted and pristine PPP. A simple change of the geometry of the cyclohexadienylene monomers from syn- to anti-configuration increases the selectivity of polymerization and improves the aromatization kinetics and morphology of the target polymer. A similar rigid-rodlike nature of the precursor polymer anti-1b with the product PPPanti minimizes reorganization during aromatization and increases its tendency to align. This PPPanti shows better intra- and interchain interaction, a feature highly desirable for improved charge carrier mobilities.36 Aromatization temperatures were lowered below 250 °C, ensuring compatibility to adjacent organic layers and substrates in multilayer devices for OE. We will study the use of PPP for organic lasing, as it solely consists of sp2-hybridized carbon atoms and should display excellent photostability besides its demonstrated thermostability. We will also study the lateral fusion of aligned single aromatized PPPanti chains from our optimized precursor into graphene sheets, since PPPsyn was not suitable for this objective. Furthermore, this optimized precursor route, in which a well processable polymeric precursor leads to a structurally perfect target material, may also be considered as a general access to insoluble and thus inprocessable aromatic polymers and will be adapted for further aromatic π-systems in the future.

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Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the German Federal Ministry of Education and Research (BMBF) for financial support within the Interphase project (FKZ 13N13657) and the POESIE project (FKZ 13N13695). K.-P.S. thanks the German Academic Foundation for the generous Ph.D. scholarship. K.-P.S. and C.M. thank the Field of Focus 2: “Struktur- und Musterbildung in der materiellen Welt” of the University Heidelberg for Funding. We thank Dr. Frank Rominger for single-crystal X-ray analysis and Carsten Placke for preparative assistance.

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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.9b00792. General information, synthetic procedures, formation of syn- and anti-isomers, gel permeation chromatography (GPC) analysis, film forming and annealing protocol, thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) data for the precursor polymer, atomic force micrographs of anti-1b and PPPanti, grazing incidence wide-angle X-ray scattering (GIWAXS) data, polarization micrographs, absorption and fluorescence spectra, crystal structures of anti-3, and NMR spectra (PDF) Crystallographic data (CIF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Sebastian Beck: 0000-0003-2194-6842 Annemarie Pucci: 0000-0002-9038-4110 Uwe H. F. Bunz: 0000-0002-9369-5387 Klaus Müllen: 0000-0001-6630-8786 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding

The authors acknowledge financial support from the Federal Ministry for Education and Research (BMBF) through Grant FKZ: 13N13695 and FKZ 13N13657.K.-P.S. was funded through a Ph.D. scholarship of the German Academic Foundation and the Field of Focus 2: “Struktur- und 4462


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Small Change, Big Impact: The Shape of Precursor Polymers Governs

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