Article pubs.acs.org/JPCB
High-Pressure Reactivity of Triptycene Probed by Raman Spectroscopy Paramita Ray,†,⊥ Jennifer L. Gray,⊥ John V. Badding,†,‡,§,⊥ and Angela D. Lueking*,∥ †
Department of Chemistry, ‡Department of Physics, §Department of Materials Science and Engineering, ⊥Materials Research Institute, ∥Department of Energy & Mineral Engineering, Department of Chemical Engineering, and EMS Energy Institute, Pennsylvania State University, University Park, Pennsylvania 16802, United States S Supporting Information *
ABSTRACT: The high-pressure reactivity of caged olefinic carbons and polyatomic aromatic hydrocarbons (PAHs) are of interest because of their ability to produce unique C−H networks with varying geometries and bonding environments. Here, we have selected triptycene to explore the creation of pores via high-pressure polymerization. Triptycene has internal free volume on a molecular scale that arises due to its paddle wheel-like structure, formed via fusion of three benzene rings via sp3-hybridized bridgehead carbon sites. At 25 GPa and 298 K, triptycene polymerizes to yield an amorphous hydrogenated carbon, with FTIR indicating an sp3 C−H content of approximately 40%. Vibrational spectroscopy conclusively demonstrates that triptycene polymerizes via cycloaddition reactions at the aromatic sites via a ring opening mechanism. The bridgehead carbons remain intact after polymerization, indicating the rigid backbone of the triptycene precursor is retained in the polymer, as well as molecular-level (∼1−3 Å) internal free volume. High resolution transmission electron microscopy, combined with dark field imaging, indicates the presence of ∼10 nm voids in the polymer, which we attribute to either polymeric clustering or a hierarchical tertiary porous network. Creation of a polymerized network that retains internal voids via high-pressure polymerization is attributed to the presence and retention of the bridgehead carbons.
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INTRODUCTION Extreme pressures may induce reactivity in otherwise unreactive systems and lead to unique reaction pathways and products. Thus, the use of high pressure to tune the chemical and physical properties of small molecules has been of interest for a long time.1−8 Examples include linear acenes, unsaturated hydrocarbons, and the C60 cage structure.9−13 In-situ vibrational spectroscopy in a diamond anvil cell (DAC)14,15 provides a means to probe these unique reaction pathways, including, but not limited to, the molecular arrangement and rearrangement in the crystal structure. An example of particular interest is the pressure-induced rehybridization of small aromatics and PAHs (polyromatic hydrocarbons) to form high hardness sp3 carbon materials.16−18 In certain cases, pressure-induced polymerization may better control the topochemistry of the end product, relative to comparable plasma deposition methods. Gradients in pressure, such as those induced under nonhydrostatic (without a pressure transmitting medium) conditions in a DAC, tend to increase reactivity, as observed in nonhydrostatic19 and shock20 compression of anthracene, as well as nonhydrostatic compression of benzoic acid.21 This increased reactivity under nonhydrostatic conditions as compared to hydrostatic conditions (in the presence of pressure transmission medium) has been attributed to the plastic deformation and formation of dislocations within the © 2016 American Chemical Society
crystal. In more rare cases, unusual and unexpected products may result: for example, a sp3 bonded crystalline onedimensional carbon nanothread was formed via high-pressure polymerization, followed by slow decompression, of benzene.22 Here, we explore the high-pressure reactivity of triptycene,23 which is composed of three benzene rings fused into a Yshaped paddle wheel (Figure 1), leading to internal free volume with fixed geometry, and high rigidity due to a high energy barrier for twisting and deformation. The rigidity and nonplanarity of the [2.2.2] bridgeheads are important structural models for the study of arene−arene interactions and the formation of molecular rotors.24,25 Cross-linking of triptycene
Figure 1. Structure of triptycene. Received: May 20, 2016 Revised: September 26, 2016 Published: September 27, 2016 11035
DOI: 10.1021/acs.jpcb.6b05120 J. Phys. Chem. B 2016, 120, 11035−11042
Article
The Journal of Physical Chemistry B
Figure 2. In-situ visible Raman spectra with 633 nm excitation of triptycene pressurized in a DAC. Spectra are normalized with respect to the diamond mode at 1332 cm−1. (a) Polymerization is observed at 25.73 GPa, as evidenced by a strong photoluminescence and loss of triptycene features; low resolution of the full spectra is included to show the appearance of the broad G peak at 1650.9 cm−1 (see arrow). (b) The lower wavenumber region of the spectra shows splitting and broadening of the vibrational modes as pressure is increased. In both cases, the ambient pressure measurement is within the diamond cell without the top diamond; low pressure denotes placement within the DAC with top diamond, without any measurable pressure changes by the Ruby fluorescence method.
in the sample in order to perform in situ pressure measurements by monitoring the R1 ruby fluorescence band shift.28 Additional experiments (reported in the Supporting Information) were performed under hydrostatic conditions, using 23 000 psi hydrogen gas as a pressure transmitting medium, within a gas loading setup described elsewhere.29 The high pressure of 23 000 psi was required to ensure penetration of the H2 into the 160 μm DAC sample chamber. 2.2. Characterization. In-situ Raman spectra were collected in a DAC using 633 and 514 nm laser Renishaw Invia spectrometer. The Raman spectrum (from 1000 to 2000 cm−1) was fit to a Lorentzian profile with a cubic background subtraction. The slit width for the Raman studies was 50 μm and since the fwhm of the UV Raman peaks were in the range of 57.7 to 200.9 cm−1, a Lorentzian profile provided the best fit. Ex-situ FT-IR spectra were collected with a Hyperion 3000 FTIR microscope in transmission mode with 400 scans from 4000−400 cm−1. Ex situ UV Raman of polymerized triptycene was collected using a 0.2 mW power, 244 nm laser. No sample damage was observed while using a laser power of 0.2 mW or below. The UV Raman of pristine triptycene could not be obtained due to sample damage as a result of rapid photopolymerization of triptycene to a semibulvallene type structure reported previously.30 Transmission electron microscopy (TEM) samples were prepared by transferring the solid sample from the diamond cell gasket hole onto a lacey carbon supported TEM grid by a needle. Imaging and analysis was then done on regions of the sample over vacuum and not over the carbon support. TEM images were taken using a JEOL 2010 TEM with a LaB6 thermionic source, operated at an accelerating voltage of 200 kV. No beam damage was observed at this accelerating voltage. HAADF-STEM images and electron energy lossspectroscopy
led to a two-dimensional structure that evolves into hollow spheres,26 suggesting potential as building blocks for porous materials in supramolecular chemistry. Similarly, photochemical polymerization of triptycene led to the formation of a (nonbenzo) norcaradiene type structure.27 Due to its internal free volume and three bladed rigid geometry, we selected triptycene to explore pressure-induced polymerization, anticipating inefficient packing might produce voids to retain high-pressure trapped gases. Here, we report high-pressure polymerization of triptycene led to the formation of a fluorescent polymer-like hydrogenated amorphous carbon (PLHC) with 40% sp3 C−H content. Uniaxial (nonhydrostatic) stress induces reactions at the aromatic sites, but the bridgehead carbons remain intact after high-pressure treatments. Bridgehead carbons are nonreactive at high-pressure, as an increase in the activation volume is highly unfavorable at high pressures. We propose a reaction pathway, that is corroborated with visible and UV Raman spectroscopy, FTIR, EELS, TEM, and HAADF-STEM. High-pressure polymerization (at 25 GPa and 298 K) of triptycene, with retention of bridgehead carbons, thus serves as a candidate to form a porous polymer network with potential applications in gas trapping.
2. EXPERIMENTAL SECTION 2.1. Methods and Materials. Triptycene was purchased from Sigma-Aldrich and was used without further purification. The triptycene was loaded into a steel gasket which was pre indented to a thickness of 35 μm and a 160 μm hole acted as the sample chamber. The primary results discussed in this article were performed with nonhydrostatic compression (i.e., in the absence of a fluid to uniformly transmit the applied pressure), at room temperature in a Mao-Bell type DAC with a 400 μm culet using type-II diamonds. A ruby chip was inserted 11036
DOI: 10.1021/acs.jpcb.6b05120 J. Phys. Chem. B 2016, 120, 11035−11042
Article
The Journal of Physical Chemistry B (EELS) data were collected on a JEOL 2010F field emission TEM, also at 200 keV, using a GatanEnfina 1000 spectrometer with a 1.1 eV energy resolution. Due to the lack of suitable standard sp2 carbon references, the sp3 content could not be quantified by EELS. Graphite could not be used as a 100% sp2 carbon reference because of its anisotropic distribution of sp2 sites. Moreover quantification is only possible if both the reference and the sample have the same medium range order.31 Positron annihilation lifetime spectroscopy (PALS) was used to monitor pore size and relative porosity of the triptycene precursor.32 A standard PALS spectrometer with an evacuated sample chamber was used for the triptycene powder.
benzene had an 80% sp3 content,22 the polymerized triptycene has a higher sp2 content (∼40% sp3, as determined by FTIR, see below), consistent with a smaller band gap. Further information can be ascertained by the low wavenumber modes (Figure 2b). Assignment of these modes, summarized in Table 1, was based upon a previous study.36 Table 1. Raman Active Vibration Modes of Triptycene with Assignments from Ref 36a
3. RESULTS AND DISCUSSION 3.1. Pressure-Induced Polymerization of Triptycene. Triptycene (Figure 1) belongs to the D3h symmetry group with each aromatic ring experiencing local C2v symmetry.33 Visible Raman spectra (Figure 2a) of triptycene33 at ambient pressure have a CC double bond stretching mode at 1599.7 cm−1 and a CC breathing mode at 1324.3 cm−1 (see also Supporting Information, Figure S3). Once placed in the DAC for in situ Raman studies, the diamond signal at 1332 cm−1 masks the CC breathing mode. As pressure is increased within the DAC under nonhydrostatic conditions, the mode at 1599.7 cm−1 broadens and blue shifts (Figure 2). With pressurization, the intensity of the 798.93 cm−1 (ν22) mode in triptycene increases relative to the 803.97 cm−1 (ν21) mode. At 10 GPa these modes increase in intensity and combine to one broad peak, suggesting the formation of dimers, likely intermediates in the polymerization process. With pressurization, significant photoluminescence (PL) arises in the triptycene spectra by 17.3 GPa (Figure 2) yet the triptycene modes are retained. Significant broadening of the triptycene modes suggests formation of an excimer (i.e., excited dimer).34 After pressure release from 17.3 GPa, the PL disappears and the triptycene spectra is fully recovered (data not shown), consistent with the formation of an excimer without polymerization. At 25.73 GPa, the PL background is further increased, and the vibrational modes of the triptycene molecule disappear under visible excitation. A very broad peak centered at 1650.9 cm−1 (Figure 2a, arrow) is suggestive of the G peak of amorphous hydrogenated carbon,35 a supposition that is discussed further in the next section. After depressurization from 25.73 GPa, the triptycene modes are not recovered, confirming triptycene has irreversibly polymerized. The recovered polymer is orange (Supporting Information, Figure S4), a marked change from the off-white color of the triptycene precursor. Notably, when hydrogen gas was used as a pressuretransmitting medium for hydrostatic compression, triptycene did not polymerize up to 34 GPa at room temperature and was recovered after compression (Supporting Information S1). This demonstrates that a pressure gradient induces reactivity, as seen in previous studies19−21 and attributed to induced dislocations and defects in the crystal.19 Relative to benzene,22 triptycene polymerizes at a slightly higher pressure (25.7 versus 23 GPa). This can be attributed to inefficient packing of the triptycene molecules due to the paddle wheel structure and internal free volume. The darker color of the polymerized triptycene suggests the band gap to be smaller than that of white polymerized benzene,22 and the orange color is attributed to the closure of the HOMO− LUMO band gap at high pressure. Whereas polymerized
a
experimental Raman frequency (cm−1)
symmetry
mode
350.9 361.7 497.3 626.1 646.8 798.9 804.0
E′ A1′ A1′ E′ E″ E′ A1′
ν6 ν7 ν11 ν14 ν15 ν22 ν21
Note the modes at 1050 and 1200 cm−1 were not assigned.
Splitting and broadening of the low frequency vibrational modes with pressurization (Figure 2b) suggests that intermolecular interactions increase with pressurization. At 3.35 GPa, changes in the slope of the pressure-frequency curves (dotted line, Figure 3) demonstrate a second order phase
Figure 3. Low wavenumber vibrational modes (from Figure 2b) shift as triptycene is pressurized. Assignments are summarized in Table 1. The vertical dashed line shows the phase transformation at 3.35 GPa.
transition occurs prior to polymerization.37,38 The change in slope is particularly pronounced for the 1026 and 1208 cm−1 modes. As high pressure is expected to favor transitions to a high density phase, this is likely a phase transition from the orthorhombic crystal structure of triptycene39 to a high density phase. The intensity of the 798.93 cm−1 (ν22) increases with respect to 803.97 cm−1 (ν21) mode and at 10 GPa both combine to form a broad peak. This may be explained due to the formation of intermediate species or dimers. Although the skeletal modes below 400 cm−1 could possibly shed light on the relative motion of the benzene rings,36 these modes were not studied in this work as they became weak at high pressure. 3.2. Polymerized Triptycene is an Amorphous Hydrogenated Carbon. The recovered polymerized triptycene exhibited a strong PL, masking all features in visible Raman spectra at all excitations studied (i.e., 488, 514, 633, and 754 11037
DOI: 10.1021/acs.jpcb.6b05120 J. Phys. Chem. B 2016, 120, 11035−11042
Article
The Journal of Physical Chemistry B
whereas the G peak is associated with sp2 bonded carbon in rings and chains.41 The presence of sp2 rings and chains shifts the G peak position relative to graphite (1582 cm−1). The low fwhm of the G peak (57.73 cm−1) is in the range (
