Abstract
Despite a vast body of the literature devoted to the use of phenylene polymers in the fabrication of graphene nanoribbons, the study of the physical properties of these precursors still poses open questions whose answers will certainly contribute to the design of more efficient/precise synthesis protocols. Particularly, persistence length measurements combined with size exclusion chromatography techniques assign both semiflexible to semirigid structures depending on the molecular weight of the precursor (NaritaNat. Chem.2014, 6, 126−13224451588). Peculiarly, these results suggest an apparent structural change upon increasing the length of the polymers. To address this puzzle, we use single-chain models to study the stiffness of polyphenylene precursors in a theta-like solvent as a function of chain composition and monomer sequence. Steric effects are isolated by considering random walk chains with segment length distributions and the position of monomers determined by the nature of the arene substitution along the backbone. Moreover, two homopolymer limiting cases are defined, that is, meta and para sequences, by associating two types of monomers to each possible substitution pattern. We consider, within these two limiting cases, chains with different compositions and monomer sequences. We compute persistence lengths, mean square end-to-end distances, and gyration and hydrodynamic radii. We find that distinct values of the persistence length for apparently the same chain chemistry are the result of different mixing ratios and the arrangement along the chain of the two positional isomers of the same monomer. Finally, we discuss the relation between two-dimensional density of the number of crossings and the length of polyphenylene segments as they would occur upon strong chain adsorption onto a substrate.
Highlights
Novel bottom-up techniques for the fabrication of graphene nanoribbons (GNRs) guarantee control of the produced structures with atomistic precision and pave the way for designing GNRs with well-defined optoelectronic properties.2 As a matter of fact, long GNRs with various widths and edge structures have been obtained using solution-mediated synthesis.1 the tendency of long GNRs to aggregate, as evidenced by their poor solubility, prevents an accurate physical characterization and visualization and makes their deposition difficult
Solution-mediated synthesis allows the production of a unique GNR structure from polyphenylene precursors
We found that chain composition and monomer sequence affect the stiffness of a chain
Summary
Novel bottom-up techniques for the fabrication of graphene nanoribbons (GNRs) guarantee control of the produced structures with atomistic precision and pave the way for designing GNRs with well-defined optoelectronic properties. As a matter of fact, long GNRs with various widths and edge structures have been obtained using solution-mediated synthesis. the tendency of long GNRs to aggregate, as evidenced by their poor solubility, prevents an accurate physical characterization and visualization and makes their deposition difficult. The bond lengths, lbm, p, correspond to the distances between two successive phenyl rings (m-poly) and every other ring (p-poly) along the backbone This means that experimental monomers are represented by two beads in the meta-substitution pattern and by one bead in the para case. The position of this bead is related to the internal rotation angle, φ, which turns out to be the torsion (dihedral) angle between two phenyl rings, and is selected with a probability proportional to the Boltzmann factor exp( − E(φ)/kBT) This potential energy, E(φ), is the torsional potential between the two rings forming the backbone of the polymers. This is a consequence of both the meta or para position of the connecting bond between monomers and the asymmetrical arrangement of the bulky groups with respect to each other upon the dihedral rotation in each. All results discussed here are averaged over 104 independent configurations
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