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.

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

“…As in the compression of phase II of benzene [45], and in line with former results, the GA 1g mode of [6]LPP linearly upshifts [16,50]. The pressure at which the [n]CPPs change the slope and, thus, the cycle, becomes oval is higher the more strained and rigid the cycle is, in agreement with the change in slope at around 5 GPa in [6]CPP and below 1 GPa in [12]CPP [40]. In the case of the non-conjugated cycloparaphenylene H 4 [6]CPP, the C-C stretching mode from the aromatic units of H 4 [6]CPP linearly upshift to approximately 8 GPa, where there is a change in slope.…”

“…It has been reported that compression of [12]CPP induces the deformation of the cycle towards a more oval configuration, creating shorter and longer diameters [7,40]. The onset of ovalisation is below 1 GPa [40]; however, the FTIR spectra presented in Figure 5c show that compression within the oval regime does not seem to affect the sp 2 framework as the C-H stretching modes broaden and upshift while maintaining their band shapes. After pressure is released, the spectrum is identical to that of the pristine sample.…”

“…It has already been established that there is a common Raman response that reveals structural ovalisation of [n]CPP. The deformation of the cycle is revealed by a change of the rate of frequency shift as a function of pressure [40]. The most intense Raman bands appear in the 1560–1600 cm −1 range, having originated from symmetric vibrations arising from collective C-C stretching modes of the benzenoid rings along the transversal direction.…”

“…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.…”

“…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. [n]CPPs undergo ovalisation with compression at lower pressures with increasing sizes [40]. However, it is still unknown how the optical properties of [n]CPPs respond to deformation of the cycle.…”