Molecules under Pressure: The Case of [n]Cycloparaphenylenes

Abstract

High pressures in the 0–10 GPa range cause molecules to deform in unusual ways. A series of precisely defined carbon nanohoops consisting of n para-linked phenyl groups, [n]-cycloparaphenylenes ([n]CPPs, n = 7, 8, 9, 10, and 12) were studied in this pressure range using Raman spectroscopy and density functional theory (DFT) and compared with more rigid smaller 5- and [6]CPPs and with the longer carbon nanotubes. The presented analysis sheds light on the different responses to pressure depending on the nanohoop size. Surprisingly, the pressure coefficients, the rate of the Raman shifts as a function of pressure, change at a particular pressure which is characteristic of each [n]CPP. We identified this pressure as the beginning of ovalization of the nanohoops in analogy to carbon nanotubes. This pressure induced ovalization is reversible in the range of pressure studied for [n]CPPs with n = 7, 9, 10, and 12. In the case of [8]CPP, we find a metastable conformation at 8 GPa with significantly changed dihedral angles of adjacent phenyls. This high pressure molecular phase of [8]CPP provides an example for a new mechanism of irreversibility involving different conformations upon high pressure treatment. Modeling provided atomic level insights into the changes of conformations and the development of aromatic vs quinonoid structures as a function of pressure.