Pressure-Induced Conformation Transition of o‑Phenylene Solvated in Bulk Hydrocarbons Massimo Riello, † Giovanni Doni, † Sorin V. Filip, ‡ Martin Gold, ‡ and Alessandro De Vita* ,† † Physics Department, King’s College London, Strand, London WC2R 2NS, United Kingdom ‡ BP Formulated Products Technology, Pangbourne, United Kingdom *S Supporting Information ABSTRACT: The conformational behavior of o-phenylene 8-mers and 10-mers solvated in a series of linear alkane solvents by means of classical molecular dynamics and first- principles calculations was studied. Irrespective of the solvent used, we find that at ambient pressure the molecule sits in the well-defined close-helical arrangement previously observed in light polar solvents. However, for pressures greater than 50 atm, and for tetradecane or larger solvent molecules, our simulations predict that o-phenylene undergoes a conformational transition to an uncoiled, extended geometry with a 35% longer head-to- tail distance and a much larger overlap between its lateral aromatic ring groups. The free energy barrier for the transition was studied as a function of pressure and temperature for both solute molecules in butane and hexadecane. Gas-phase density functional theory- based nudged elastic band calculations on 8-mer and 10-mer o-phenylene were used to estimate how the pressure-induced transition energy barrier changes with solute length. Our results indicate that a sufficiently large solvent molecule size is the key factor enabling a configuration transition upon pressure changes and that longer solute molecules associate with higher conformation transition energy barriers. This suggests the possibility of designing systems in which a solute molecule can be selectively “activated” by a controlled conformation transition achieved at a predefined set of pressure and temperature conditions. ■ INTRODUCTION o-Phenylenes are a recently developed class of polymers in which every aryl unit is linked to the previous one following o- polymerization. 1,2 This addition mechanism yields considerable steric crowding along the molecular backbone, resulting in a twisted (coiled) configuration (Figure 1(a)). 3,4 This peculiar arrangement is perhaps the reason why o-phenylene polymers have historically attracted little interest, along with increased difficulty in synthesizing the polymers via standard aryl−aryl addition and with using them in thin film electronics because of poor π conjugation between the monomeric units (this is not the case for the p isomer, which has been used extensively as a single-molecule wire). 5 Recently, however, interest in this class of optically active, chiral polymers has increased, especially because of their DNA- like structure, 6,7 which has led to the formulation of reproducible synthesis protocols. 8 This is of considerable interest because the inherent chirality of helical geometries ultimately allows for the possibility of finely controlling optical properties 9 and building new polymeric precursors for asymmetric catalysis. 10 In particular, newly synthesized helical o-phenylenes may in turn enable chiroptical activity, provide a molecular scaffold to help separate enantiomers, and be used in enantioselective synthesis. o-Phenylenes are also regarded to be promising graphene nanoribbon precursors. For the reasons listed above, control over the conformational behavior of o-phenylenes is being actively pursued. In a recent, significant step forward, the conformational flexibility of o- phenylenes was tuned via chemical substitution to promote and stabilize its folded conformation in solution. 11,12 Interestingly, the conformational changes of the molecules were found to display a certain degree of stimuli responsiveness, meaning that the folded, coiled conformer could be “locked” via special Received: September 23, 2014 Revised: November 6, 2014 Published: November 7, 2014 Figure 1. DFT-optimized structures of (a) coiled and (b) extended conformers of an o-phenylene 8-mer (hydrogen atoms not represented). Article pubs.acs.org/JPCB © 2014 American Chemical Society 13689 dx.doi.org/10.1021/jp5096272 | J. Phys. Chem. B 2014, 118, 13689−13696