Conformation of p-terphenyl under hydrostatic pressure

2002

Changes in the conformations of conjugated molecules affect the optical and electronic properties significantly. Hydrostatic pressure has been used to probe the conformations of biphenyl and deuterated biphenyl at liquid-helium temperatures. Infrared (IR) spectra of these materials have been taken up to a pressure of 2 GPa. A disappearance of certain IR absorption peaks has been found to occur between 0.07 and 0.45 GPa, due to the phase transition from a twisted to a planar conformation.

The Journal of Physical Chemistry A, 2001

Hydrostatic pressure is used to modulate the intermolecular interactions in the conjugated oligophenyl, parahexaphenyl. These interactions affect the structural properties and also cause changes in the molecular geometry that directly alter the electronic properties. We use Raman spectroscopy to investigate the nature of the structural changes. Our Raman studies in the temperature range of 12 K to 300 K, under pressures up to 70 kbar, indicate that the potential energy of two neighboring phenyl rings as a function of the torsional angle is "W"-shaped. The libration of the phenyl rings between the two minima of the "W"-shaped potential can be modulated by either promoting the molecule to a higher energy state (activation energy of 0.045 eV) by raising the temperature or by decreasing the intermolecular separation, which makes the potential more "U"shaped. Both these situations make the molecule seem more planar. We infer the shape of the potential from the relative intensity of the interring CsC stretch Raman mode at 1280 cm-1 to the CsH bending mode at 1220 cm-1 (I 1280 /I 1220). These results are interpreted within the framework of ab initio electronic and vibrational spectra calculations of a biphenyl molecule. We have also conducted X-ray studies to check the sample purity.

Synthetic Metals, 2001

We investigate the in¯uence of intermolecular interactions on the optical and structural properties of oligophenyls in solid ®lms of polycrystalline nature. To this end, we have performed X-ray powder diffraction (XRD) experiments in a pressure range up to 2 kbar yielding the lattice constants of biphenyl (2P) as a function of pressure. The optimized geometry of the 2P molecule Ð including the interring torsion angle Ð is then determined using three-dimensional (3D) band structure calculations within density functional theory (DFT). We ®nd a red shift upon increasing pressure in the computed optical absorption spectra which is due to conformational changes of the 2P molecule. A planarization accompanied by a reduction of the inter-ring bond length increases the conjugation, thereby reducing the band gap. Pressure dependent Raman measurements on 2P, terphenyl, quaterphenyl and sexiphenyl support the calculated conformational changes. Moreover, experimental optical absorption spectra on 2P ®lms under hydrostatic pressure were measured in a pressure range up to 3 kbar. The observed red-shift in the absorption quantitatively agrees with the computed values. #

2001

The different conformations of molecular compounds play important roles in biochemistry and organic solid-state technology. Hydrostatic pressure has been applied to para-quaterphenyl to probe its molecular structure at liquid-helium temperatures. The molecules transform from a twisted to a planar conformation at a critical pressure of 0.9 GPa. This conformational change results in the abrupt disappearance of five infrared-absorption peaks.

Synthetic Metals, 2001

The goal of this combined experimental and computational study is to investigate the structural conformation of oligo(para-phenylenes) in the crystalline phase, in particular the planarity of this type of molecules. To this end we have performed Raman experiments on paraterphenyl and para-quaterphenyl in a pressure range from 0 to 70 kbar and at temperatures from 10 to 300 K. The positions and the relative intensities of the C±C interring stretch mode at 1280 cm À1 and the C±H in-plane bend mode at 1220 cm À1 have been tracked. We ®nd that upon increasing temperature at ambient pressure the intensity ratio I 1280 /I 1220 drops rapidly at temperatures that coincide with the crystallographic phase transition for the investigated materials. At ambient temperature also, this intensity ratio drops rapidly upon increasing pressure up to about 15 kbar. In the computational part, the Raman frequencies and activities of isolated 3P and 4P molecules were calculated within restricted Hartree±Fock formalism with the interring tilt angles varying from 0 to 908. These calculations con®rm that the I 1280 /I 1220 intensity ratio can be related to the planarity of the molecules. Three-dimensional bandstructure calculations within density functional theory were applied to determine phonon frequencies and estimate Raman activities for the polymer poly(paraphenylene). These simulations show that the same conclusions hold for crystalline environment. # 2001 Published by Elsevier Science B.V.

Physical Chemistry Chemical Physics, 2013

In this work we theoretically investigated the characteristics of the structure of biphenyl at zero temperature. The calculations were carried out with density functional theory using periodic boundary conditions. Semiempirical van der Waals (vdW) corrections were applied. We focused on the phenylphenyl dihedral angle and its shift with increasing pressure. We furthermore investigated the bond lengths of different bonds during the compression. The experimental transition pressure of a phase transition could be reproduced with satisfactory accuracy.

Chemical Physics Letters, 1993

The hydrostatic pressure-induced shift of the zero-phonon line of single pentacene molecules in a pterphenyl host crystal was studied at cryogenic temperatures. Five molecules in the inhomogeneously broadened &t-S, absorption band of the O1 site (592.32 nm) were selected. In the low-pressure regime (App6 600 hPa), all molecules showed a linear and reversible frequency shift to the red, varied from -0.74 to -1.0 MHz/hPa.

The Journal of Chemical Physics, 1991

The n-pentane molecule has the basic conformations, TT, TG, and GG. It is important for understanding the conformational behavior of chain molecules. We have studied the effect of pressure on the conformations of n-pentane in the condensed phase by using high pressure infrared spectroscopy with a diamond anvil cell up to 28.9 kbar. Compression of liquid n-pentane increases the concentration of the more globular conformers, while the TT conformer only exists as a high pressure solid. The volume changes for TT to TG, TG to GG, and TT to GG forms are −1.0±0.2, −1.1±0.3, and −2.1±0.3 cm3/mol, at 20 °C, respectively. The conformational volume changes in liquid n-alkanes are discussed in terms of the solvent-excluded volume.

The Journal of Physical Chemistry B, 2014

Chemistry: A European Journal, 2017

We find evidence for the surprising formation of polymeric phases under high pressure for conjugated nanohoop molecules. This paper represents one of the unique cases where the molecular-level effects of pressure in crystalline organic solids is addressed and provides a suitable general approach based on vibrational Raman spectroscopy combining experiments and computations. In particular we studied the structural and supramolecular chemistry of the cyclic conjugated nanohoop molecule, [5]cyclo-para-phenylene under high pressures of 0 to 10 GPa. The theoretical modeling for periodic crystals predict good agreements with Raman spectra in the molecular phase. In addition we have discovered two stable polymeric phases that arise in the simulation. The critical pressures in the simulation are too high, but the formation of polymeric phases at high pressures provides a natural explanation for the observed irreversibility of the Raman spectra upon pressure release between 6 and 7 GPa. The geometric parameters show a deformation toward quinonoid structures at high pressures accompanied by other deformations of the nanohoops. The quinonoidization of the benzene rings is linked to the systematic change of the bond length alternation as a function of pressure, providing a qualitative interpretation of the observed spectral shifts in the molecular phase.