Elsevier

Chemical Physics Letters

Volume 533, 23 April 2012, Pages 30-34
Chemical Physics Letters

Temperature dependent vibrational dynamics in crystalline para-terphenyl under high pressure

https://doi.org/10.1016/j.cplett.2012.03.001Get rights and content

Abstract

This study examines the effect of pressure-induced conformation changes on low-temperature vibrational dynamics in crystalline para-terphenyl (pTP). The temperature dependence of homogeneously broadened infrared bands under simultaneous high-pressure and low-temperature conditions is reported. High-pressure induces a conformation change in pTP from a twisted structure at ambient pressure to a planar structure at high pressure (1.35 GPa). The characteristic temperature dependent vibrational dynamics for in-plane versus out-of-plane hydrogen bending modes are correlated with the planarization of the pTP molecule at high-pressure.

Highlights

► High-resolution homogeneously broadened IR bands under simultaneous high-pressure and low-temperature. ► High-pressure measurements examined the effect of conformation changes on vibrational dynamics. ► Increased planarity of p-terphenyl under pressure decreased couplings of out-of-plane vibrations. ► Increased intramolecular interactions at high-pressure yield increased couplings of in-plane vibrations.

Introduction

Vibrational dynamics of molecular solids have been investigated extensively using time and frequency domain techniques. High resolution Fourier transform infrared spectroscopy (HR-FTIR) allows for the simultaneous investigation of a large number of vibrons in a homogeneously broadened crystal. Previous high-resolution FTIR experiments have focused on relatively small solid state molecular systems with a limited number of isolated vibrons (e.g. S8 [1] and CO2 [2]). In vibrationally sparse systems temperature dependent vibrational dephasing can be used to determine vibrational relaxation/dephasing pathways and the corresponding anharmonic (e.g. cubic and quartic) mode coupling parameters. Similar studies of larger systems (i.e. naphthalene [3]), with more congested vibrational spectra, have also yielded an understanding of the dominant relaxation processes in more complex molecular crystals [3]. The extension of anharmonic perturbation analysis to more complex systems is more limited, but can still provide useful upper bounds on anharmonic couplings [4] and the range of dominant frequencies involved in the vibrational dynamics [4].

Vibrational dynamics at ambient pressure provides insight into vibrational relaxation and dephasing pathways governed by the intermolecular potential associated with the low temperature crystal. Systematic investigations of the effect of molecular conformation, crystal symmetry, and/or intermolecular distances on vibrational dynamics are difficult given the constraints of the crystal structure. However, high-pressure has the ability to tune intermolecular interactions and systematically alter these properties. The major difficulties associated with high-resolution high-pressure IR measurements is the limited signal transmission through the high-pressure cell and the need to minimize spectral broadening associated with pressure gradients, phase transitions, and sample contraction. Because of these challenges, high-resolution FTIR studies under high pressure conditions have been limited to small molecule systems [5], and obtaining a homogeneously broadened spectra under simultaneous high-pressure and low-temperature conditions has been elusive. High-resolution coherent Raman measurements under high pressure have been less restricted by sample aperture constraints, but these investigations typically focus on individual isolated vibrational bands [6], [7].

Para-terphenyl (pTP) is an interesting molecular crystal system that exhibits several polymorphs as a function of temperature [8], [9] and pressure [10], [11] (see Figure 1). The intramolecular double-well potential for rotation of the central phenyl ring of pTP is correlated with an order–disorder crystalline phase transition that is tunable via temperature and pressure [10], [12], [13]. Structural and dynamic characteristics of the pTP polymorphs have been characterized by experimental and computational studies under variable temperature and pressure conditions [8], [9], [10].

The interplay between molecular conformation and dynamics and the crystallographic structure of pTP make it an appealing system for investigating temperature and pressure effects on vibrational dynamics. The low-temperature (<193 K) triclinic phase of pTP has four distinct crystallographic sites [8], [14], characterized by unique molecular torsion angles at each site (Figure 1a). High pressure acts to planarize the pTP molecule [10], [13] (Figure 1b), resulting in a different low-temperature crystallographic phase above 0.55 GPa [12], [15], [16]. The present study focuses on a comparison of temperature dependent vibrational dynamics in two different crystal forms composed of the same pTP molecular species. A prerequisite for this study requires obtaining homogeneously broadened FTIR spectra under variable temperature and pressure conditions. The vibrational dynamics in different crystal forms with characteristic molecular conformations and orientations are used to analyze intra- and inter-molecular contributions to the vibrational dynamics in crystalline pTP.

Section snippets

Experimental

Thin sublimation crystals of pTP were prepared and loaded into a high pressure diamond anvil cell (DAC) as previously described [15]. The sublimation flake was loaded into the DAC such that the gasket would cut the crystal so that it completely filled the gasket aperture. Once the crystal was cut, nitrogen was loaded as a hydrostatic pressure mediating fluid. IR spectra obtain using a Bruker HR-120 FTIR at a resolution of 0.01 cm−1. The high-pressure DAC was thermally mounted to a closed cycle

High-resolution spectra under variable low-temperature and high-pressure

Temperature and pressure dependent high-resolution FTIR spectra of crystalline pTP are illustrated in Figure 3. Previous lower resolution IR studies of pTP revealed spectral differences associated with changes in molecular structure [11], and crystal symmetry [15]. The present high-resolution study focuses on resolving the temperature dependent homogeneous linewidths associated with these spectral features. Figure 3a and b show temperature dependent spectra under ambient pressure, while Figure 3

Conclusions

The effect of pressure-induced conformational changes on low-temperature infrared linewidths in crystalline para-terphenyl (pTP) was determined. Annealing of very thin sublimation crystals in a hydrostatic pressure medium was successful at reducing inhomogeneous broadening (associated with polymorphic phase transitions as well as thermal and pressure-induced contraction), permitting resolution of homogeneously broadened IR bands under simultaneous high-pressure and low-temperature conditions.

Acknowledgments

B.S. would like to thank the Eberly Science Foundation for their generous support. E.L.C. acknowledges support by the ACS Petroleum Research Fund (#37400-AC5), the National Science Foundation (CHE-0612957), and the AFOSR (F496209810475) for the FTIR equipment grant.

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