Small Change, Big Impact: The Shape of Precursor Polymers Governs Poly-p-phenylene Synthesis

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.

Beyond p‐Hexaphenylenes: Synthesis of Unsubstituted p‐Nonaphenylene by a Precursor Protocol

“…Since the same emission spectra were obtained, regardless of the geometry of the precursor, we conclude that the different optical properties of the polymers (see Supporting Information, Figure S7), which emit at 485, 520 and 560 nm for PPP anti and 426, 450, 485 and 560 nm for PPP syn , respectively, are a consequence of their differing morphologies and solid‐state order. Furthermore, the estimated timescales of the aromatization process for both isomers are comparable to each other and with that of PPP anti (5 min) and much faster than that of PPP syn (40 min) [2, 3] …”

“…The easily evaporable methoxy‐masked para ‐terphenyl makes the use of a sealing‐tube indispensable, in which the leaving‐groups (formally methoxy radicals) are trapped in the vessel and might cause additional side‐reactions. In contrast, no rearrangements were observed for our precursor polymer thin films (Scheme 1 d and e), in which dialkoxylated cyclohexadienylene moieties were thermally aromatized under nitrogen atmosphere and ambient pressure [1–3] . We concluded that aromatization was far more selective in thin films: the precursor polymers PPP syn,pre and PPP anti,pre did not melt in contrast to methoxy‐masked para ‐terphenyl (cf.…”

“…The respective temperatures T arom amounted to 260 °C for 9PP syn,pre and 250 °C 9PP anti,pre , as displayed by sharp exothermic peaks in the DSC profiles accompanied by a mass loss in the thermograms. T arom of the two isomers only differed by 10 °C (250 °C vs. 260 °C), while those of their polymeric counterparts ( PPP syn vs. PPP anti ) deviated more from each other (Δ T arom =55 °C; PPP anti =222 °C vs. PPP syn =277 °C) [1–3] . Both compounds 9PP anti,pre and 9PP syn,pre did neither sublime nor melt in the relevant temperature regime up to 300 °C and thus fulfilled the above‐mentioned criteria for solid‐state aromatization.…”

“…T arom of the two isomerso nly differed by 10 8C( 250 8Cv s. 260 8C), while those of their polymeric counterparts (PPP syn vs. PPP anti ) deviated more from each other (DT arom = 55 8C; PPP anti = 222 8C vs. PPP syn = 277 8C). [1][2][3] Both compounds 9PP anti,pre and 9PP syn,pre did neither sublimen or melt in the relevant temperature regime up to 300 8Ca nd thus fulfilled the above-mentioned criteria for solid-state aromatization. Smallerm ethoxy-masked OPPs prepared in our group, such as cyclohexadienylenebased heptaphenylenep recursors, or different geometrici somers of nonaphenylene precursors (see Figure S8 for structures, Supporting Information), had too low melting points or sublimed during annealing.…”

“…The structural motif of para ‐phenylenes has experienced renewed interest in the last two decades, scientifically spurred by the synthesis of cyclo‐ para ‐phenylenes (CPPs), as well as the development of precursor routes yielding pristine, defect‐free poly‐ para ‐phenylene [1–3] . The lasting enthusiasm for materials with para ‐phenylene moieties recently led to the development of curved and topologically new para ‐phenylene derivatives, for example, catenated CPPs, [4, 5] CPP knots, [4, 5] CPP‐PPP hybrids, [6] [2.2]paracyclophane‐containing cyclic OPPs, [7] or spirobifluorene‐OPP ladder‐type hybrids [8] .…”