Article pubs.acs.org/JPCC
High-Pressure Raman Study of [2.2]Paracyclophane Wei Li,† Zhilei Sui,‡ Huarong Liu,† Zengming Zhang,‡ and Hewen Liu*,† †
CAS Key Laboratory of Soft Matter Chemistry, Department of Polymer Science and Engineering, University of Science and Technology of China, Jinzhai Road 96, Hefei 230026, China ‡ The Center of Physical Experiments, University of Science and Technology of China, Hefei 230026, China ABSTRACT: The high-pressure structural properties of PCP were investigated by using Raman scattering techniques in a diamond anvil cell up to 10 GPa at room temperature. Compression caused a relative flat xylyl fragment and twisted ethylene bridges in PCP. Raman analysis showed that more peaks related to vibrational modes of aliphatic CH2 emerged due to the transition to a low symmetric phase after compression pressure reached 3.9 GPa. Other blue-shifting peaks showed discontinuity in the frequency− pressure curves at pressure 3.9 GPa because of molecular relaxation by phase transition. Compression strengthened the conjugation of xylyl fragments. When releasing pressure, the normal vibrational modes could be recovered with a hysteresis of ∼1.3 GPa. The normal phase of PCP could be fully recovered under ambient conditions.
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INTRODUCTION The effects of pressure in molecular materials have attracted long-term interests in diverse fields, including materials science and even biophysics, and so on.1−3 It is known that thermodynamic states and properties of materials are dependent on pressure and temperature. Pressure responsive materials have found important applications. A promising example is the high-pressure induced phase change materials because of their application in next-generation optical data storage devices and partially because very large compressive pressure up to 5 to 6 GPa is momentarily generated in recorded molten bits.4−8 Moreover, the advances in high-pressure techniques make it possible to exploit the full power of high pressure as an ideal means to tune electronic, magnetic, structural, and vibrational properties. Raman spectroscopy is one of the most informative tools for studying material properties including materials phase crystal structure under high pressure.9−14 Pressure-induced Raman shifts of embedded microcrystals can also be used as a nanogauge for laser-induced shock wave studies or monitoring inner stress in composites. Many materials including carbonaceous materials, minerals, and even amino acid crystals have been investigated under high pressure.3,10−12 [2.2]Paracyclophane (PCP) is an attractive aromatic compound whose two benzene rings are fixed with π-stacked systems.15,16 Such π-stacked systems were relevant to singlemolecule electronics, molecular wires, molecular rectifiers, and so on.17−19 Paracyclophanes have also led to a new wave of research into molecular nonlinear optical phenomena in donor−acceptor complexes, hydrogen storage materials, and so on.20−22 PCP was incorporated as a repeating unit into the conjugated polymer main chains in an effort to search for materials with novel optical and electrochemical properties.23−26 Theoretical calculation of the Raman spectrum of PCP has been reported. Walden et al. carried out geometry optimizations and vibrational analysis of both the D2 and D2h conformers of PCP using the hybrid Hartree−Fock/density © 2014 American Chemical Society
functional method B3LYP and 4-31G(d) basis set and presented complete assignment of all 90 normal harmonic vibrations of [2.2]paracyclophane.27 Henseler et al. optimized PCP at the MP2/6-31G(d) level and investigated the influence of the molecular twist on the vibrational spectra. 28 Experimental Raman spectra have been obtained for the entire series of [2n]-cyclophanes, and several band assignments have been proposed.29 Solid-state Raman investigation of mono/ dihydroxy substituted PCP was also recently reported.30 For the purpose of those applications of PCP, pressure is an essential parameter to be considered. To our surprise, the effects of high pressure on PCP have not been reported anywhere. In this work, we study the effects of high pressure on PCP with high-pressure Raman spectroscopy.
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EXPERIMENTAL SECTION Materials. PCP was purchased from Aladdin Industrial Corporation, Shanghai, whose crystalline structure was checked to be a tetragonal system (P42/mnm) with a = b = 7.781, c = 9.290, in accordance with that reported by Hope et al.31 There are two PCP molecules per unit cell. Acquisition of Raman Spectra at High Pressure. A diamond anvil cell (DAC) was used to produce high pressure. Samples were packed into a hole in diameter of 0.2 mm in a stainless-steel gasket. The ruby fluorescence method was used to monitor the pressure. The pressure was calculated by applying the well-known pressure shift of Ruby luminescence R1 line.32,33 Silicone oil was used as pressure-transmitting medium. The maximum pressure achieved in the experiments was ∼10 GPa. A confocal microscope Raman spectrometer system (equipped with Princeton Instruments Acton SP2750 monochromator and Princeton Instruments Pixis 100-BR Received: May 5, 2014 Revised: June 23, 2014 Published: July 2, 2014 16028
dx.doi.org/10.1021/jp504390u | J. Phys. Chem. C 2014, 118, 16028−16034
The Journal of Physical Chemistry C
Article
Table 1. Calculated Modes for [2.2]Paracyclophane MSa
Gaussianb
scaledc
exptld
normal mode descriptione
1593, 1588 1586 1450, 1447 1432, 1433 1180 1178 945, 952, 959, 960 888, 890 830, 829 782 630 561, 558 455, 459 237 150
1659 1620 1528 1505 1225 1195 964 903 853 792 652 580 463 235 157
1598 1560 1471 1449 1180 1150 928 870 821 763 628 559 463 235 157
1599 1589 1454 1430 1186 1175 955 900 837 789 635 566 460 240 155
benzene ring stretching (ip) benzene ring stretching (op) CH2 scissoring (op) CH2 scissoring (ip) Cr-Cb stretching + in-plane CH scissoring bending Cr-Cb stretching + in-plane CH bending CH twisting + CH2 twisting ring breathing mode I CH wagging (op) ring breathing mode II in-plane benzene ring bending (ip) out-of-plane benzene ring bending (op) in-plane benzene ring bending (ip) ring breathing mode III translation of each p-xylyl fragment in opposite directions
a
Calculated from PCP hexagonal crystals with CASTEP module of Materials Studio. bCalculated from a PCP molecule with Gaussian03. cGaussian calculated frequencies >550 cm−1 are scaled by 0.963, and those
