Abstract
The synthesis of unsubstituted oligo‐para‐phenylenes (OPP) exceeding para‐hexaphenylene—in the literature often referred to as p‐sexiphenyl—has long remained elusive due to their insolubility. We report the first preparation of unsubstituted para‐nonaphenylenes (9PPs) by extending our precursor route to poly‐para‐phenylenes (PPP) to a discrete oligomer. Two geometric isomers of methoxylated syn‐ and anti‐cyclohexadienylenes were synthesized, from which 9PP was obtained via thermal aromatization in thin films. 9PP was characterized via optical, infrared and solid‐state 13C NMR spectroscopy as well as atomic force microscopy and mass spectrometry, and compared to polymeric analogues. Due to the lack of substitution, para‐nonaphenylene, irrespective of the precursor isomer employed, displays pronounced aggregation in the solid state. Intermolecular excitonic coupling leads to formation of H‐type aggregates, red‐shifting emission of the films to greenish. 9PP allows to study the structure–property relationship of para‐phenylene oligomers and polymers, especially since the optical properties of PPP depend on the molecular shape of the precursor.
Highlights
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-[a] Dr A
We report the first synthesis of unsubstituted para-nonaphenylene (9PP) via a precursor route. 9PP can be considered as a model compound to study the underlying structure–property relationship of para-phenylene oligomers and polymers, especially since the optical properties of PPP crucially depend on the precursor geometry[1,2,3] and the aromatization temperature
Basically any larger monodisperse oligophenylene is accessible by combining methoxylated anti-cyclohexadienylenes as masked 1,4-phenylene building blocks with the desired number of phenylene spacers and end-cappers, bridging the gap between solution-phase synthesis of the smaller oligophenylenes up to p-hexaphenylene and the phenylene polymers
Summary
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-. F. Bunz Organisch-Chemisches Institut Ruprecht-Karls-Universität Heidelberg Im Neuenheimer Feld 270, 69120 Heidelberg (Germany). Melzer Kirchhoff-Institut für Physik Ruprecht-Karls-Universität Heidelberg Im Neuenheimer Feld 227, 69120 Heidelberg (Germany). F. Bunz Centre for Advanced Materials Ruprecht-Karls-Universität Heidelberg Im Neuenheimer Feld 225, 69120 Heidelberg (Germany). For polymeric para-phenylenes, this issue has been resolved through the development of precursor routes: non-aromatic and non-planar precursor polymers containing „masked“ phenylenes (e.g. substituted cyclohexadienylenes and cyclohexenylenes) are, as a key step, aromatized, overcoming the problem of solubility and yielding high-molecular PPP. Precursor strategies towards OPPs can be divided into two categories (Scheme 1): Methods of solution-mediated aromatization (Scheme 1 a and b) suffer from necessary follow-up steps to isolate and characterize the insoluble products as well as diminished yields;[12,13] precursor routes to OPPs involving solely thermal solid-state aromatization processes are challenging for small-molecules.
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