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A New Solution to an Old Problem: Synthesis of Unsubstituted Poly-(para-Phenylene) Ali Abdulkarim, Felix Hinkel, Daniel Jänsch, Jan Freudenberg, Florian Ernst Golling, and Klaus Müllen J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b10254 • Publication Date (Web): 30 Nov 2016 Downloaded from http://pubs.acs.org on November 30, 2016
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Journal of the American Chemical Society
A New Solution to an Old Problem: Synthesis of Unsubstituted Poly-(para-Phenylene) Ali Abdulkarim,a,b,d Felix Hinkel,b,c Daniel Jänsch,a,b Jan Freudenberg,a,b Florian E. Golling,a,b,d and Klaus Müllen*,d a
InnovationLab, Speyererstr. 4, 69115, Heidelberg, Germany
b
Organisch-Chemisches Institut, Ruprecht-Karls-Universität Heidelberg, 69120, Heidelberg, Germany
c
Centre for Advanced Materials, Ruprecht-Karls-Universität Heidelberg, 69120, Heidelberg, Germany
d
Max Planck Institute for Polymer Research, Ackermannweg 10, 5128 Mainz, Germany
KEYWORDS: poly-para-phenylene, organic electronics, precursor-route, film-processing, thermal aromatization
ABSTRACT: Unsubstituted and structurally well-defined poly-(para-phenylene) (PPP) is long-desired as an organic semiconductor prototype of conjugated polymers. To date, several attempts to synthesize unsubstituted, pristine and high molecular-weight PPP failed. Here, we solve this synthetic problem by a versatile precursor route. Suzukipolymerization of kinked disubstituted 1,4-dimethoxy-cyclohexadienylene monomers yielded a well-soluble, nonaromatic precursor polymer. Its solubility allowed processing by spin-coating into nanometer-thick films. Subsequent additive-free thermal treatment induced aromatization and led to exclusively para-connected, highly fluorescent PPP with a length of about 75 phenylene units.
As the prototype of a conjugated polymer, unsubstituted and structurally well-defined poly-(para-phenylene) (PPP) was the quest of various synthetic approaches. In neutral form, PPP is an insulator, whereas upon doping its conductivity increases up to 500 Ω-1cm-1.1 Its compressive strength (207 MPa), low density (1.210 g cm-3) as well as high stability towards temperature, oxygen and moisture render PPP a promising candidate for applications in organic electronics.2 In the past, several routes towards PPP have been developed. Since already short para-phenylene oligomers (≤ 6 phenylenes) are barely soluble in common organic solvents, all direct arylaryl couplings via 1,4-dihalobenzenes lead to regioselectively formed but very short oligomers (Scheme 1 a).3 The oxidative coupling of benzene results in a significant amount of ortho-connected and hence illdefined phenylene units.4 Longer PPP (n≈30), could be obtained by the introduction of solubilizing side-chains (alkyl/alkoxy groups) in ortho-position (Scheme 1b),5 unfortunately resulting in low charge carrier mobility and an undesired blue-shift of the emission bands due to deplanarization of the polymer backbone.6 Recently, several approaches for the surface-assisted synthesis of PPP have been published.7 Drawbacks of this approach were the requirement of the high Miller-index metal surfaces, the obtained low quantity of PPP, lack of follow-up processability as well as high experimental effort. A promising strategy towards unsubstituted PPP is the socalled precursor-route, by which Grubbs could successfully address the problem of regioselectivity
(Scheme 1 c).8 Although precursor-polymers for subsequent syn-elimination reactions were obtained, structurally perfect and unsubstituted PPP without impurities (e.g. acid residues) could not be achieved so far (Scheme 1).9
[3-7]
Scheme 1. Different synthetic approaches towards PPP.
We sought to overcome the above-mentioned obstacles of the synthesis of regioregular high molecular weight PPP by pursuing a novel precursor approach. Herein, we report the first facile synthesis of unsubstituted,
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structurally well-defined PPP. By utilizing kinked monomers containing disubstituted cyclohexadienylene moieties, good solubility is introduced to the resulting precursor polymer. Furthermore, this cyclohexadienylene is convertible to a phenylene unit by aromatization reactions. Hence, processing of thin films from solution, subsequent additive-free thermal aromatization to PPP layers and their optical characterization are enabled.
Scheme 2. Synthesis of monomers and (precursor) polymer. Conditions: i) n-BuLi, THF, -78 °C; ii) NaH, MeI, THF, 25 °C; iii) n-BuLi, isopropoxyboronic acid pinacol ester, THF, -78 °C; iv) Pd(PPh3)4, Cs2CO3 , THF/H2O; v)
