Pressure-Induced Conformation Transition of o-Phenylene Solvated in Bulk Hydrocarbons

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.

Dimensional Evolution of Polyphenylenes: Expanding in All Directions

“…These molecules were termed “foldamers” because they demonstrated the ability to form helical structures as illustrated in Figure , where the extent of folding was dependent on the type and quantity of functional groups, solvent, and temperature. For example, when there were no polar groups on the oligomers, they did not exhibit helical formation, but upon introducing species such as triflate, cyano, or nitro groups, it was possible to quantitatively form the folded structures. − An interesting aspect of this research was the conformational influence on the photophysical properties of the oligophenylenes, where a hypsochromic shift in the absorption (230–250 nm) and fluorescence (320–340 nm) was observed when going from unoriented to helical assembly due to direct charge transfer though the π–π stacking direction.…”