Pressure-Induced Diversity of π‑Stacking Motifs and AmorphousPolymerization in PyrroleWenbo Li, Defang Duan, Xiaoli Huang, Xilian Jin, Xue Yang, Shourui Li, Shuqing Jiang, Yanping Huang,Fangfei Li, Qiliang Cui, Qiang Zhou, Bingbing Liu, and Tian Cui*

State Key Lab of Superhard Materials, College of Physics, Jilin University, Changchun 130012, P. R. China

*S Supporting Information

ABSTRACT: The behavior of pyrrole under high pressure has been investigated by insitu high-pressure synchrotron X-ray diffraction (XRD) and Raman scattering up to 34GPa. A solid-to-solid transition at about 6.2 GPa with a large collapse of volume (∼40%)from Pnma to P21/c has been found after a liquid-to-solid transition at 0.6 GPa, which iscaused by the molecular rotational repacking of π-stacking. This new phase P21/c plays acentral role in the following pressure-induced polymerization due to the formation of aclosed dimer, a very important precursor. The threshold of C···C distance with sterichindrance in dimer is about 1.62 Å at ∼10.2 GPa. After this steric hindrance isovercome, a crystal−amorphous transformation starts at ∼14.3 GPa. When completelyreleased from 34 GPa, the recovered solid product with single bond is identified by insitu Raman measurement.

■ INTRODUCTION

“Sometimes amorphous forms of a material can haveadvantages over crystalline forms” is reported on ScienceDaily.1 Over the past few decades, the preparation of entirelynew high-density amorphous materials, polymerized byaromatic compounds under high pressure, has been extensivelyresearched,2 not simply because of its great potential appliedvalue3 but also because the transition mechanisms are certainlyof interest to high-pressure physical chemists who seek tounderstand the effect of pressure on polymerization andamorphization of molecular solids in fundamental science. In asolution, heterogeneous catalysts or enzyme can be used tofavor particular orientations of some reactants. In the solidstate, since a crystal lattice offers a greater degree of geometricalconstraint, high pressure is a good tool for controlling thebehavior of transitions.4 For instance, pressure induces inbenzene several structural changes, forming an amorphoushydrogenated carbon material (a-C:H), as a prototype forsynthesis of carbon-based high-density materials.5−9 Thismaterial is characterized by high mechanical strength, whichimplies the presence of a larger sp3 C concentration withpossible applications in optics. Similar pressure-inducedtransitions in furane,10 thiophene,11 pyridine,12 etc. have alsobeen found by Raman and IR spectroscopy experiments. Sincethese transformations take place in crystals, the onset oftransformation of molecular arrangement in the lattice and therelative distances among the molecules would surely reflectfundamental information for understanding of the polymer-ization path at the microscopic level. In this framework, severalstudies attempted to rationalize the transitions in aromaticcompounds, including a variety of computer simulation inorganic crystals.3,13 However, the relationship between the

precursor and polymerization for the simple aromaticcompound remains not fully clear, because the importantdispersion force about aromatics under high pressure is ignoredin theoretical computations. Additionally, previous under-standing of the transformation in solid-state polymerization ismostly based on qualitative interpretation of experimentalobservations. Thus, quantitative information, especially directobservation from in situ high-pressure synchrotron X-raydiffraction (XRD) under pressure, is extremely necessary andimportant.In this work, we focus our attention on pyrrole (C4H5N), an

important aromatic compound, because it is a fundamentalconstituent of petroleum. Up to now, research on pyrrole underhigh pressure has appeared only rarely. At ambient conditions,pyrrole exists in liquid form. At 103 K, pyrrole crystallizes intophase N with space group Pnma.14,15 In this paper, we presentthe first direct observations of pyrrole by means of in situ high-pressure synchrotron XRD and Raman scattering spectra up to∼34 GPa. A solid-to-solid transition is uncovered at about 6.2GPa with a huge collapse of volume (∼40%) from Pnma toP21/c, caused by the molecular rotational repacking of π-stacking. More, the new phase P21/c plays a central role for thefollowing pressure-induced amorphous polymerization due tothe formation of a closed dimer, a very important precursor. Athreshold of C···C distance resulting from repulsive force ofsteric hindrance in the pyrrole dimer is found to be about 1.62Å at 10.2 GPa by experimental Rietveld refinement. After thepotential steric hindrance is overcome, a crystal−amorphous

Received: December 10, 2013Revised: May 14, 2014Published: May 19, 2014

Article

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transformation starts at 14.3 GPa. As the pressure is releasedfrom 34 GPa, the single-bonded polymeric solid product isobtained.

■ EXPERIMENTAL SECTION

Pyrrole (98%) is purchased from Sigma-Aldrich as a clear liquidand loaded along with a ruby chip in a 100 μm diameter hole ofT301 stainless steel gasket that is preindented to a thickness of48 μm in a diamond anvil cell (DAC). The pressure isdetermined by measuring the ruby R1 fluorescence line.16

The Raman spectra are measured by an ActonSpctraPro500ispectrograph with a liquid-nitrogen-cooled CCD detector(Princeton Instruments, 1340 × 100). A solid-state, diode-pumped, frequency-doubled Nd:vanadate laser (λ = 532 nm) isused as the exciting laser. Raman spectra are collected in a backscattering geometry with a 1800 gr/mm holographic grating,and the slit width is selected as 80 μm, corresponding to aresolution of ∼2 cm−1. The sample image can be collectedthrough an achromatic lens and then focused onto a CCDdetector for visual monitoring during experiments.In situ angle-dispersive XRD experiments are carried out at

the Cornell High Energy Synchrotron Source using mono-chromatic radiation (λ = 0.4859 Å). The sample−detectordistance and geometric parameters are calibrated by means of aCeO2 standard. In light of the light atoms (C, H, and N) inpyrrole, we take 600 s as the average exposure time for eachspectrum to maintain sufficient intensity. The recorded two-dimensional data are analyzed to yield plots of intensity versus2θ with Fit2D software.17 Further analysis of XRD pattern isundertaken by the Reflex Module combined in the MaterialsStudio 5.5 program (Accelrys Inc.) to obtain the accuratesymmetries of phases, lattice parameters, and molecularpacking.18,19

Ab initio calculations are performed with the pseudopotentialplane-wave method based on density functional theoryimplemented in the CASTEP code.20 The generalized gradientapproximation (GGA) with the Perdew−Burke−Ernzerhof(PBE) exchange-correlation functional is used in the calculationof the crystal vibration spectrum. The norm-conservingpseudopotentials are employed with a plane-wave cutoff energyof 1000 eV.

■ RESULTS AND DISCUSSION

At 0.6 GPa, the liquid sample crystallizes into solid crystal inthe sample chamber of DAC. The refinement of the XRDpattern collected at 1.3 GPa has been carried out based on thereported low temperature structure with Pnma symmetry.15,16

From Figure 1a, the herringbone N−H···π interactions stronglyinfluence the crystal packing. The refined result identifies thesymmetry of the solid phase as space group Pnma with thesame structure at low temperature, which is shown in Figure 1b.The refined lattice constants are a = 7.185 (4) Å, b = 10.21 (0)Å, c = 4.912 (2) Å, with unit cell volume V = 360.36 (7) Å3.Typical XRD patterns under pressures are presented in

Figure 2a. Diffraction peaks shift toward higher angles withincreasing pressure, which is generally due to the contraction ofcorresponding d-spacings. The diffraction pattern is easilyindexed by using orthorhombic symmetry at 1.3 GPa. In Figure2a, both peak position and relative intensity of the XRD patternchange during the phase transition. First, with pressureincrease, the intensities of (020) and (011) peaks at 1.3 GPadecrease and even vanish at 4.6 GPa. Then, up to 6.2 GPa,

additional new peaks emerge at 5.9° and 6.5°. More, therelative intensity of the (101) peak at 1.3 GPa increases withpressure below 6 GPa, while the (210) peak weakens. Uponcompression to 4.6 GPa, the relative intensities of the XRDpattern change obviously. All these obvious phenomena suggestthat the phase transition takes place at 4.6 GPa, then completesat 6.2 GPa. Besides, Figure 2b presents the anisotropiccompressibility of pyrrole. The detailed fitting compressibilityof every main peak is listed in Table 1. A discontinuity of theslope of d-spacing versus pressure is visible in the range of 4.6−6.2 GPa, which is consistent with the pressure range of thephase transition. In addition, from Table 1, the slope of peak(011) in phase Pnma is the largest one (−0.092), followed bythat of peak (101) (−0.072). It is noteworthy that the direction

Figure 1. (a) The orthorhombic structure of pyrrole crystal at 1.3 GPawith herringbone stacking and the N−H···π hydrogen-bondeddenoted by dashed lines. (b) The refinement of Pnma phase withrespect to the synchrotron XRD pattern collected at 1.3 GPa.

Figure 2. (a) Typical synchrotron XRD patterns of pyrrole withincreasing pressure up to 34 GPa at room temperature (incidentwavelength λ = 0.4859 Å). The diffraction peaks of the Pnma phase areindexed by Miller indexes (hkl). The new weak peaks are marked byasterisks. (b) Variation of the d-spacings of main peaks under highpressure. The dashed line is used to divide the two-phase field.

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(101) closes to the N−H···π direction. The faster gradient indirection (101) makes the β-angle enlarge in the pressure-induced phase transition, which lowers the crystallinesymmetry.Above 6.2 GPa, no structural changes are observed in the

XRD profiles up to 10.2 GPa. At 14.3 GPa, the XRD pattern,with emerging weak peaks at 6.5° and 7.9° combined with theprominent diffraction peaks, characterizes the coexistencewhich will be discussed later. At about 28 GPa, a broad peakis left in the XRD pattern, which suggests that the samplecompletely transforms into the amorphous state. The largerange (14.3−28.1 GPa) of the crystal−amorphous trans-formation under high pressures seems to be a gradual evolutionprocess. We have performed two sets of the compression−decompression experimental cycle on pyrrole by achievingdifferent maximum pressures (23 and 34 GPa). Our findingsare that, during decompression to ambient pressure from 23GPa, part of the sample polymerizes into the solid stateirreversibly, shown in Figure 3a. Next, during decompression to

ambient conditions from 34 GPa, the whole sample completelypolymerizes into a semitransparent solid product in the samplechamber, shown in Figure 3b. Therefore, the start ofirreversible amorphous polymerization should take placebelow 23 GPa.The X-ray results provide strong evidence for a pressure-

induced phase transition in pyrrole. However, the crystalstructure of the new phase is unknown. Based on our XRD

patterns from 6.2 to 10.2 GPa, using the Reflex Module in theMaterials Studio program (Accelrys Inc.), all of the XRDpatterns are best indexed with monoclinic symmetry containingfour molecules. After comparison of the simulated XRDpatterns by the possible monoclinic structures with ourexperimental XRD, the refinement results suggest thesymmetry with the space group P21/c. Then four moleculesC2H4N are added into the monoclinic crystal. By means ofRietveld refinement, the most likely structure of the high-pressure phase is space group P21/c with closed dimer. Thediffraction patterns of Pnma and P21/c structures seem to besimilar in the phase transition on compression. The Rietveldrefinement result utilizing Pnma structure presents obviousdifferences between simulative and experimental XRD patternsin Figure S1 in the Supporting Information. However, utilizingP21/c structure to fit is better with the small values Rp = 6.98%and Rwp = 13.39%. In the new packing structure of pyrrole,shown in Figure 4a, molecular layers arrange along an inclinedplane with an angle of a axis about 80°. The basic pattern of thenew phase structure (Z = 4) consists of two pairs of moleculesin the unit cell. Each pair is composed of parallel and displacedmolecules as dimer in Figure 4b. Along the b axis, the adjacentmolecules arrange in an antiparallel way, and they alternate withantiparallel ups and downs, as shown in Figure 4c,d.In Figure 5, the XRD patterns could be well refined with

P21/c symmetry from 6.2 to 10.2 GPa. The refined latticeconstants at 6.2 GPa are a = 3.87 (5) Å, b = 9.28 (2) Å, c = 4.84(8) Å, and β = 101.7 (8)°, with unit cell volume V = 170.62 (0)Å3. The values of Rp and Rwp suggest the acceptability of theproposed structure. The atomic coordinates in new phase P21/cat 6.2 GPa are shown in Table 2. The lattice parameters ofPnma and P21/c phases as a function of pressure are shown inFigure 6a. It is observed that all lattice parameters in two phasesdecrease monotonically with increasing pressure except angle βand the lattice parameter b at 10.2 GPa. Figure 6b shows thepressure dependence of the volume changes obtained fromexperimental data for the two phases. The experimentalpressure−volume data are fitted by the third-order Birch−Murnaghan (BM) equation of state (EOS),21

= −

× + ′ − −

− −

−

⎡⎣⎢⎢⎛⎝⎜

⎞⎠⎟

⎛⎝⎜

⎞⎠⎟

⎤⎦⎥⎥

⎧⎨⎪⎩⎪

⎡⎣⎢⎢⎛⎝⎜

⎞⎠⎟

⎤⎦⎥⎥⎫⎬⎪⎭⎪

PB V

VVV

BVV

32

134

( 4) 1

0

0

7/3

0

5/3

00

2/3

where V0 is the volume per formula unit, V is the volume performula unit at pressure P given in GPa, B0 is the isothermalbulk modulus, and B0′ is the first pressure derivative of the bulkmodulus. In our experimental data, B0′ is set as 4. Actually, thepredominant cohesive factors in phase Pnma of pyrrole arehydrogen-bond (N−H···π) and van der Waals interactions(electrostatic interactions, induction force, and dispersionforce) that lead to a large proportion of empty space left inthe crystal. During upward pressure runs, the increased energyof intermolecular interactions could rotate molecules andrearrange π-stacking. Nevertheless, molecules in crystals have atendency to achieve closer packing by the application of highpressure.22 As such, a large remarkable collapse (∼40%) ofvolume takes place from herringbone configuration to closeddimers packing as shown in Figure 6b and Figure 7.

Table 1. Detailed Fitting Compressibility of the Main XRDPeaks

(hkl) compressibility in Pnma (hkl) compressibility in P21/c

(020) −0.039 (020) (−0.020)(011) −0.092 (011) (−0.034)(101) −0.072 (100) (−0.014)(111) −0.063 (110) (−0.012)(200) −0.069 (021) (−0.021)(210) −0.062 (111 ) (−0.016)(121) −0.053 (120) (−0.016)(220) −0.051 (121 ) (−0.017)(201) −0.052 (031) (−0.014)(211) −0.049 (111) (−0.014)(031) −0.048

Figure 3. (a) Sample is decompressed to ambient conditions from 23GPa, with part of the mixture liquid going out and the other part leftsolid. (b) The solid product is obtained after 34 GPa is completelyreleased.

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For understanding the transition from crystal to amorphousphase, the new phase P21/c is believed to be the precursorbefore amorphous polymerization. Above 6.2 GPa, the XRDpatterns on compression show that the diffraction peak (020)of phase P21/c almost barely shifts from 8.7 to 10.2 GPa(actually, the peak (020) (along the b axis) shifts slightly to lowangle, with the lattice parameter b enlarging slightly at themicroscopic level). This is primarily because the existence ofrepulsive force resulting from steric hindrance makes it hard todecrease the distance between intermolecular atoms, either inclosed dimer of pyrrole or in adjacent molecules (antiparallel

arrangement with each other). Therefore, the thresholddistance in dimer of pyrrole remains almost constant at 1.62

Figure 4. Phase P21/c of pyrrole is viewed along different plane directions: (a) molecular layers arrange along an inclined plane; (b) the unit cellconsists of two pairs of closed dimers; (c, d) the adjacent molecules along the b axis arrange in an antiparallel way and alternate with antiparallel upsand downs. The blue, black, and white spheres represent nitrogen, carbon, and hydrogen atoms, respectively.

Figure 5. Rietveld refinement of the P21/c phase with respect to thesynchrotron XRD patterns from 6.2 to 14.3 GPa. The P21/c phasefitting is good for the diffraction pattern shown. The closest C···Cdistance in the dimer with increasing pressure is shown on the rightside.

Table 2. Atomic Coordinates in New Phase P21/c of Pyrroleat 6.2 GPa (Å)

label x y z

C1 1.233(0) 8.730(1) 0.642(0)C2 0.731(4) 8.336(3) −0.545(3)C3 1.233(0) 6.534(4) 0.642(0)C4 0.731(4) 6.928(1) −0.545(3)N1 1.827(9) 7.632(2) 2.183(6)H1 1.430(7) 9.630(1) 1.124(0)H2 0.452(2) 8.930(6) −1.206(5)H3 1.430(7) 5.634(3) 1.124(0)H4 0.452(2) 6.333(8) −1.206(5)H5 1.827(9) 7.632(2) 2.183(6)

Figure 6. (a) The lattice parameters of pyrrole at different pressures.(b) Pressure−volume data of Pnma and P21/c phases at 300 K. Solidspheres represent experiment data, and solid black lines are third-orderBirch−Murnaghan equation of state fitting data.

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Å from 8.7 to 10.2 GPa. When the pressure reaches 14.3 GPa,the C···C distance in the pyrrole dimer decreases to 1.53 Å,which is in the range of C−C single-bond distance (commonC−C is 1.54 Å).23 Thus, it is identified that steric hindrance atintermolecular separation must be overcome before forming aC−C single bond in the dimer.24 This process is very similar tothe explanation of benzene dimerization, proposed by Ciabiniet al. They have shown that the amorphous polymerizationstarts with the formation of a σ-bond in two close moleculeschanging into zwitterion species (intermediate), after apotential energy barrier is overcome. Therefore, in our XRDpattern at 14.3 GPa, the emerging two new weak peaks mightbe related to the pressure-induced polymerization species withC−C single bond and represent the onset of the crystal−amorphous transformation. Above 14.3 GPa, there exists a largepressure range of coexistence during the crystal−amorphoustransformation until the sample completely transforms intoamorphous phase at about 28 GPa.To understand the structural variation, the Raman spectrum

is also a vital tool combined with the XRD pattern. In Figure 8,

our Raman spectrum of pyrrole collected at 0.6 GPa is depictedas the starting point, which is in good agreement with ourresults of the theoretical calculation of the Raman spectrumbased on density functional theory (DFT). The list of observedcrystal vibrational modes along with their frequencies andassignments is given in Table 3. Below 200 cm−1 is assigned tothe lattice vibration region; from 500 to 1350 cm−1 are mainly

C−H and N−H bending vibration regions; from 1450 to 1600cm−1 is the ring frame CC vibration region; above 3000 cm−1

are the C−H and N−H stretching vibration regions.Typical Raman spectra of pyrrole upon compression are

shown in Figures 9−11. All the Raman modes show a blue shiftas a function of pressure in general (except N−H stretchingvibrational modes), which can be attributed to the increasingstrength of interactions between adjacent molecules or atomswith the reduction of mutual distances.25−28 The shift rates ofmodes exhibit a clear decreasing trend with pressure increase,indicating smaller compressibility of pyrrole in higher pressureregions, owing to a denser molecular packing uponcompression.The evolution of lattice modes in pyrrole can give essential

information on symmetry change. There exist five calculatedRaman modes above 75 cm−1 in the lattice vibration region at0.9 GPa, as shown in Table 3. In Figure 9a, we label the latticemodes using peak profile fitting legibly to reveal the variationtrends and the emergence of new lattice modes with increasingpressure. At about 2.6 GPa, a new lattice vibration modemarked with an asterisk emerges at 136 cm−1, and the relativeintensity of this peak increases with increasing pressure. Upomfurther compression to 6.5 GPa, another new lattice vibrationmode split from the original peak (205 cm−1) emerges at 213cm−1 and can be distinguished clearly with increasing pressure.The two new emerging lattice vibration modes are ascribed tothe reduction of crystalline symmetry in the process of phasetransition (Pnma to P21/c).

29 Thus, the changes of latticevibration modes suggest that the phase transition takes place at2.6 GPa, then completes at 6.5 GPa. The pressure range ofphase transition is consistent with our XRD results.

Figure 7. Comparison of motifs between herringbone and closeddimer stacking in phases Pnma and P21/c respectively. The pyrrolemolecules with bokeh effect are in the background.

Figure 8. Raman spectra of pyrrole: the red line is obtained by densityfunctional theory (DFT) calculation, and the green line is collected byRaman spectral measurements at 0.6 GPa.

Table 3. Tentative Assignments and Frequencies (cm−1) ofObserved Raman Vibration Modes of Pyrrole inComparison with Theoretical Calculation Results

expt(cm−1)

cryst calcn(cm−1) description

78 83 lattice vibration modes99 97130 104141 107157 124578 582 C−H out-of-plane wag604 619 C−H and N−H out-of-plane wag724 694 C−H out-of-plane wag829 820 C−H, N−H in-plane bend862 837 C−H in-plane bend868 852 C−H in-plane bend881 868 C−H in-plane bend1047 1005 C−H in-plane rocking1053 1035 C−H and N−H in-plane rocking1079 1066 C−H in-plane rocking1139 1119 ring breathing vibration and C−H in-plane

bend1147 1138 C−H and N−H in-plane rocking1248 1268 C−H and N−H in-plane rocking1380 1374 ring frame CC vibration and C−H bend1471 1453 ring frame CC vibration and N−H bend3108 3151 C−H stretch3117 3160 C−H stretch3136 3179 C−H stretch3359 3318 N−H stretch3374 3340 N−H stretch

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Simultaneously, Figure 9b shows the nonlinear Raman shiftdependent on pressure in the lattice mode region and anobvious discontinuous range from 2.6 to 6.5 GPa.Figure 10a shows the typical Raman spectra ranging from

550 to 950 cm−1 with pressure increase up to 32 GPa. Thereare two significant modes to note. The internal modes markedwith asterisks (828 and 893 cm−1) appear in the range of theC−H and N−H bending vibrational region at 8.9 and 6.5 GPa,respectively. Actually, after comparing with the calculatedvibration spectrum in this region, the two new bands can beexplained by the different pressure-induced Raman shift value

of the two adjacent modes at 0.9 GPa. From Figure 10b, adiscontinuity also exists in the range 2.6−6.5 GPa, consistentwith the proposed phase transition. Compared with the latticemodes, the internal modes show smaller shift rates. This isbecause noncovalent interactions are weak in essence and showmuch higher compressibility than covalent bonds. Thisphenomenon appears to be common among supramolecularsystems.30,31

Figure 11a shows the variations of modes in the range of1000−3400 cm−1 with increasing pressure. Above 15.7 GPa, theweakening of peaks in this range is particularly obvious.Especially, the intensities of modes in the ring frame CCvibration region (1370−1850 cm−1) decrease and thencompletely vanish at 26.8 GPa, which indicates a gradualprocess with the loss of ring CC character to polymerize.The C−H and N−H stretching modes are respectively assignedin the range of 3100−3200 cm−1 and 3200−3400 cm−1 at 0.9GPa. In Figure 11b, the N−H stretching modes exhibit a redshift obviously, which is in accordance with general rules that anincrease of pressure decreases the N−H stretching frequencies

Figure 9. (a) Typical Raman spectra of pyrrole in lattice mode regionupon compression−decompression cycle. The first peak is the systempeak of the instrument. (b) Raman shifts of pyrrole with increasingpressures in lattice mode region. Shadow region suggests that thephase transition takes place at 2.6 GPa and then completes at 6.5 GPa.

Figure 10. (a) Raman spectra of pyrrole in the region of 550−950 cm−1 at typical pressures upon compression−decompression cycle. (b) Ramanshifts of pyrrole on compression.

Figure 11. (a) Raman spectra of pyrrole in the region 1000−3400cm−1 at typical pressures upon compression−decompression cycle. (b)Raman shifts of pyrrole on compression.

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of weak strength N−H···π bond.32 In addition, the CambridgeStructural Database (CSD) structural search shows that the N−H···π bond always has close correlation with centrosymmetry inthe process of molecular self-assembly. That is why the newcrystal phase can keep the centrosymmetry after phasetransition. In brief, the internal modes do not seem toexperience large changes below 10 GPa. This suggests that thephase transition of pyrrole does not involve a large conforma-tional change of the constituent molecules.Above 26.8 GPa, it is obvious that all of the Raman vibration

modes vanish, which indicates that pyrrole completelytransforms into the amorphous state. In fact, similar amorphouspolymerization has been reported in several linear33−37 andcyclic9,38,39 monomers. In the case of benzene, Pruzan et al. firstreported the onset of intermolecular coupling between adjacentmolecules at high pressures.5 As a result, an acyclic system isproduced, and the transition is found to involve ring-openingpolymerization.40 Our Raman spectrum of the recoveredproduct from 34 GPa characterizes the amorphous state. Thebroad and diffuse band centered at 878 cm−1 is easily assignedto the C−H bending vibrational mode in Figure 10a. The otherband 1183 cm−1 (Figure 11a) is also in single-bondedvibrational region. The double bond (CC) is unseen duringthe decompression, signifying the product probably with singlebonds as the reported benzene released from high pressure.

■ CONCLUSIONS

In conclusion, our present synchrotron XRD and Ramanspectra have given crucial evidence of structural changes. Asolid-to-solid transition has been found at about 6.2 GPa, with alarge collapse of volume (∼40%) from Pnma to P21/c phase,after a liquid-to-solid transition at 0.6 GPa. It is primarilycaused by the molecular rotational repacking of π-stacking fromherringbone motif to closed dimers. The new phase P21/c playsa central role for the following pressure-induced polymerizationdue to the formation of closed dimers. The threshold of C···Cdistance with steric hindrance in the dimer is about 1.62 Å at∼10.2 GPa. After this steric hindrance is overcome, the closestC···C distance in pyrrole dimer decreases to ∼1.53 Å,representing the formation of a C−C single bond at 14.3GPa, and simultaneously the crystal−amorphous transforma-tion starts. After release from 34 GPa, solid amorphous productwith a C−C single bond is identified by in situ Ramanmeasurement. These results provide us with new ideas indesigning new materials by high-pressure technique, andcorresponding experimental and theoretical results need to beverified by further study. Furthermore, the new phase P21/cwith huge volume shrinkage might be more favorable fordesigning hydrogen-storage materials.41

■ ASSOCIATED CONTENT

*S Supporting InformationRietveld refinements of the diffraction patterns collected oncompression from 6.2 to 10.2 GPa. This material is availablefree of charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATION

Corresponding Author*E-mail: cuitian@jlu.edu.cn. Tel/fax: +86-431-85168825.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

The authors are grateful to Zhongwu Wang et al., staff scientistsin B2 station of Cornell High Energy Synchrotron Source, fortheir kind-hearted help during the experimental research. Thediffraction data were obtained using the angle dispersive X-raydiffraction technique and synchrotron radiation from theCornell High Energy Synchrotron Source (CHESS). Thiswork was also performed at 4W2 HP-Station, BeijingSynchrotron Radiation Facility (BSRF). We express our sincerethanks to Yanchun Li and Xiaodong Li staff scientists. Thiswork was supported by the National Basic Research Program ofChina (No. 2011CB808200), Program for Changjiang Scholarsand Innovative Research Team in University (No. IRT1132),National Natural Science Foundation of China (Nos.51032001, 11074090, 10979001, 51025206, 11274137,11004074, 11204100), and National Fund for FosteringTalents of Basic Science (No. J1103202).

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