Mechanochemistry in [6]Cycloparaphenylene: A Combined Raman Spectroscopy and Density Functional Theory Study

Abstract

Raman spectroscopy under high pressures up to 10 GPa and density functional computations up to 30 GPa are combined to obtain insights into the behavior of a prototypical nanohoop conjugated molecule, [6]cycloparaphenylene ([6]CPP). Upon increasing pressure, the nanohoop undergoes deformations, first reversible ovalization and then at even higher pressures aggregates are formed. This irreversible aggregation is caused by the formation of new intermolecular σ-bonds. Frequencies and derivatives of the Raman frequency shifts as a function of pressure are well reproduced by the computations. The frequency behavior is tied to changes in aromatic/quinonoid character of the nanohoop. The modeling at moderate high pressures reveals the deformation of the [6]CPP molecules into oval-like and peanut-like shapes. Surprisingly the pressure derivatives of the observed Raman mode shifts undergo a sudden change around a pressure value that is common to all Raman modes, indicating an underlying geometrical change extended over the whole molecule that is interpreted by the computational modeling. Simulations predict that under even larger deformations caused by higher pressures, oligomerization reactions would be triggered. Our simulations demonstrate that these transformations would occur regardless of the solvent, however pressures at which they happen are influenced by solvent molecules encapsulated in the interior of the [6]CPP.

Linear, Non-Conjugated Cyclic and Conjugated Cyclic Paraphenylene under Pressure

“…The compression of tubular [6]CPP above 6 GPa causes a permanent deformation of the cycle that could lead to the formation of intermolecular covalent linkages [39]. Figure 5d shows that at 10 GPa, the C-H stretching modes of the aromatic phenyl units upshift and broaden but their overall profile persists.…”

“…Many different aspects of the ambient pressure chemistry of [n]CPPs have been already addressed in different laboratories [36]. In previous work, we explored high-pressure structural and mechanical properties of [n]CPPs [38,39,40]. The two smallest and most highly strained members of the series, [5]CPP and [6]CPP, polymerised at pressures between 5 and 6 GPa, whereas larger [n]CPPs are softer and remain in a monomeric state up to 10 GPa.…”

Measuring structural inhomogeneity of a helical conjugated polymer at high pressure and temperature

“…The forthcoming work includes more comprehensive thermal studies against pressure clarifying the nature of observed thermal transition as well as molecular dynamics simulations incorporating both pressure and temperature. In the future, we will also screen avenues for high pressure polymer chemistry as is already emerging elsewhere …”

Molecules under Pressure: The Case of [n]Cycloparaphenylenes

“…The trend is that the pressure cycling shows more reversibility for larger[n]CPPs compared to the least reversible small CPPs. Irreversibility of[5]-and[6]CPP comes from the formation of new -bonds leading to new phases with aggregation and polymerization of the CPP molecules that start to occur around 6 GPa 21,22. The Raman spectra of[n]CPPs shown inFigures S14 and S18indicate that the spectra are irreversible after application of pressure for n=5, 6, and 8 while the spectra of[7]CPP,[9]CPP,[10]CPP, and[12]CPP are almost completely reversible in the pressure range studied.…”

“…Based on their elastic response to pressure,[n]CPPs can be qualitatively divided into two categories: rigid and soft. The group of rigid[n]CPPs includes [5]CPP 21 , [6]CPP22 , and[7]CPP. These…”