Double-core nanothread formation froma-furilvia
a pressure-induced planarization pathway†
Samuel G. Dunning, ‡*aAnirudh Hari, aLi Zhu, bBo Chen, cd
George D. Cody, aSebastiano Romi, aDongzhou Zhang e
and Timothy A. Strobel *a
The packing and geometry of compressed small molecule precursors largely dictate the kinetically
controlled formation processes of carbon nanothread materials. Structural ordering and chemical
homogeneity of nanothread products may deteriorate through competing reaction pathways, and
molecular phase transitions can disrupt precursor stacking geometries. Here, we report the formation of
well-ordered, double-core nanothreads from compresseda-furilviaa unique polymorphic transition
pathway that serves to facilitate a pressure-induced reaction. At∼1.6 GPa,a-furil transforms to the
photoactivetrans-planar conformation, which was previously theorized but not observed. Crystalline
packing of thetrans-planar structure provides closely overlapping molecular stacks that result in
topochemical-like Diels–Alder cycloaddition reactions between furan rings upon further compression.
The controlled reaction pathways on both sides of the molecule produce two linked“cores”of
chemically homogenous nanothreads, and successive nucleophilic addition reactions crosslink a large
fraction of the diketone bridges between monomers.
Introduction
Diamond nanothreads represent a novel class of one-
dimensional sp 3carbon-rich nanomaterials, formed from the
compression of small molecules to high (GPa) pressures. 1–15 The
initial discovery of crystalline nanothreads from benzene1,2
suggested synthetic design principles for controlled high-
pressure reactions, in contrast to previous work that showed
the formation of disordered products.
16–23 While crystalline one-
dimensional nanothreads are in stark contrast to the previous
paradigm of disorder, this crystallinity is typically the result of
the macromolecular packing of individual chemically inhomo-
geneous threads.
1–3,14,24–26 That is, atomically precisenanothreads with full three-dimensional long-range order still
represent a major synthetic challenge, and most threads show
a range of local chemical environments due to competing
reaction pathways.
27,28
The initial discovery of nanothreads in 2015 sparked new
interest in high-pressure solid-state organic reactions and
nanothreads with a diverse range of chemical functionalities
and structural motifs have since been produced.
3–15 Several
strategies to improve thread homogeneity and 3D order have
been employed, and different approaches can signicantly
constrain reaction paths to produce highly ordered mate-
rials.
5,6,9 Solution phase reactions 29,30 and theoretical studies 31
have shown that certain reaction pathways, such as [4 + 2] Diels–
Alder cycloaddition reactions, are accelerated by increasing
pressure due to their negative activation volumes. Such reac-
tivity can also be translated to solid-state reactions, and
accordingly, precursors containingve-membered ring systems
(e.g., furan,
4–6 thiophene 7) have been shown to promote the
formation of nanothreads with greatly increased chemical
homogeneity, largely due to the incorporation of heteroatoms
that constrain reactivity along a single [4 + 2] Diels–Alder
cycloaddition pathway.
4–6,32,33 While this reaction pathway may
seem unorthodox in the context of traditional organic chem-
istry, recent theoretical studies have shown that small mole-
cules such as furan can act as both diene and dienophile at
extreme pressures.
33 Indeed, therst“perfect”threads were
synthesized from furans 4–6,32 and pyridazine 9(a 6-membered
diazine ring system). In addition, reduced aromaticity caused by
aEarth and Planets Laboratory, Carnegie Institution for Science, Washington, District
of Columbia 20015, USA. E-mail: [email protected]; tstrobel@
carnegiescience.edu
bPhysics Department, Rutgers University-Newark, 101 Warren Street, Newark, NJ
07102, USA
cDonostia International Physics Center, Paseo Manuel de Lardizabal, 4, 20018
Donostia-San Sebastian, Spain
dIKERBASQUE, Basque Foundation for Science, Plaza Euskadi 5, 48009 Bilbao, SpaineHawaii Institute of Geophysics and Planetology, School of Ocean and Earth Science
and Technology, University of Hawaii at Manoa, Honolulu, HI 96822, USA
†Electronic supplementary information (ESI) available. CCDC 2343198–2343200.
For ESI and crystallographic data in CIF or other electronic format see DOI:
https://doi.org/10.1039/d4sc07412b
‡Current address: Division of Sciences and Mathematics, University of the
District of Columbia, 4200 Connecticut Avenue NW, Washington, District of
Columbia 20008, USA. E-mail: [email protected]
Cite this:Chem. Sci.,2025,16,4144
All publication charges for this article
have been paid for by the Royal Society
of Chemistry
Received 1st November 2024
Accepted 2nd January 2025
DOI: 10.1039/d4sc07412b
rsc.li/chemical-science
4144 |Chem. Sci.,2025,16,4144–4151 © 2025 The Author(s). Published by the Royal Society of Chemistry
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heteroatom incorporation can contribute to lower reaction
pressures for nanothread formation,e.g.,10–13 GPa 4–6,9,32 for
furan/pyridazinevs.∼20 GPa for benzene. 1,34 Interestingly, the
formation of“perfect”nanothreadsviasequential [4 + 2]
cycloaddition reactions from carbonyl-substituted furans (e.g.,
2,5-furandicarboxylic acid)
5,6 suggest that high-pressure reac-
tions may be a route to increase the reactivity of typically
electron-poor dienes. It is also important to note that the most
chemically homogeneous threads have been synthesized from
planar systems which crystallize with appropriatep-stacking for
topochemical-like nanothread formation, allowing nano-
threads to be formed under rapid, quasi-hydrostatic compres-
sion conditions in a diamond anvil cell. Precursors that
crystallize without a signicant degree ofp-stacking overlap
(e.g., furan, benzene) may undergo non-topochemical nano-
thread formation induced by anisotropic stress, typically under
slow (GPa hour
−1) uniaxial compression. This is important to
keep in mind as we move towards synthetic strategies that
incorporate precursors with increasing chemical diversity, in
which functional groups will impactp-stacking geometries.
Recent research has investigated nanothread formation in
larger molecular precursors with different functionalities and
increased structural complexity.
5,6One interesting class of more
complex nanothreads are the so-called“double-core”
nanothreads.
11–14,35 First reported by Romi and coworkers,
double-core nanothreads are formed from the compression of
molecules containing linked aromatic rings (e.g., stilbene,
14
azobenzene 11,12 ), resulting in the formation of two 1D nano-
thread polymers connected together by a bridging motif.
Double/multi-core nanothreads provide an opportunity to
extend nanothread systems to pseudo-2D structures with an
expanded range of chemical properties. For example, double-
core nanothreads produced from diphenylacetylene represent
tunable low-bandgap polyacetylene-like polymers, protected on
the sides by sp
3nanothread cores. 13,35 To date, double-core
threads have only been produced from precursors containing
six-membered aromatic ring systems, and the resulting nano-
thread products contain a large degree of disorder and coexist
with an amorphous sp
2–sp 3carbon matrix due to the presence
of multiple competing reaction pathways (cf.benzene). This
disorder has made precise structural determination a signi-
cant challenge and has led to debate over thenal structure of
the high-pressure reaction products for these systems. For
example, recent publications on the pressure-induced poly-
merization of azobenzene have proposed both double-core
nanothread,
11,24 and pseudo-two dimensional nanoribbon 36
structures for the recovered polymeric materials.
One factor that has hindered the formation of larger,
chemically homogeneous systems is their molecular geometry.
Planar molecules are regularly used as precursors for nano-
thread formation as their molecular geometries at ambient
pressure oen already occupy the smallest volume, and will
therefore be favored under high-pressure conditions. While
high-pressure phase changes may result in alteration of the
overall molecular packing, such phase changes are unlikely to
result in signicant structural rearrangements of planar mole-
cules. Therefore, the crystal structures of these systemscollected at ambient may be used to predict high-pressure
reaction pathways. While this approach has been successful,
it has naturally excluded studies of non-planar systems.
Nevertheless, many bridged systems are non-planar, yet possess
aromatic rings that are well positioned for nanothread forma-
tion and structural motifs that would be interesting to incor-
porate into nanothread materials. We are therefore interested
in investigating the high-pressure behavior of a non-planar
system that also incorporatesve-membered ring systems.
Such a precursor would allow us to test if the design principles
that have allowed for the formation of“perfect”nanothreads
from small molecule precursors can also be applied to more
chemically and structurally complex precursors in order to
extend reaction control to larger systems with diverse future
applications.
Herein, we report the high-pressure behavior of the 1,2-
diketone systema-furil. Compression ofa-furil leads to the
formation of high-pressuretrans-planar phase with excellentp-
stacking overlap, which had previously been theorized but not
validated experimentally. Additional compression results in the
formation of an extended, double-core nanothread polymer
through sequential [4 + 2] Diels–Alder cycloaddition reactions
between overlapping furan rings.
Results and discussion
In situcharacterization of high-pressure phase changes ina-
furil
a-Furil (2,2
0-furil, C 10H6O4) is a potential candidate for the
formation of double-core nanothreads with improved order and
homogeneity due to the presence of overlappingve-membered
furan rings. These furan rings, linked by a 1,2-diketone bridge,
show parallel-displacedp-stacking and short intermolecular
C/C distances (i.e., <4 Å), compatible with previous reports of<br />
temperature
37and pressure-induced 3–6,9,10,32 [4 + 2] Diels–Alder
cycloaddition reactions. We hypothesize that a cascade Diels–
Alder polymerization reaction would proceed along the stacking
axis with high selectivity
3to produce a double-core nanothread
consisting of two connected degree-4 threads, each forming in
thesynconformation (i.e., all oxygen atoms aligned along the
same side). Double-core threads linked by a 1,2-diketone bridge
could enable photoreactions such as those observed for struc-
turally analogous systems such as benzil,
38 phenan-
threnequinone, 38retenequinone, 38diacetyl, 39and pyruvic acid, 40
which have all been found to react with alkenes, alkynes and
water in the presence of sunlight. Nevertheless, unlike the
majority of nanothread-forming precursors,a-furil is non-
planar; the furan ring planes of furil are canted by∼48° from
one another and the molecule exists in a buckled conformation
such that simultaneous nanothread formation for both ring
stacks would induce signicant bridge strain. Furthermore,
intermolecular ketone interactions could potentially impact
reactivity of the rings.
To investigate reactivity under high-pressure conditions,
powder samples ofa-furil were loaded into diamond anvil cells
(DACs), and the structure and behavior was monitored within
situX-ray diffraction (XRD) and vibrational spectroscopy. PXRD
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patterns of startinga-furil at 1 atm readily index to the previ-
ously reported orthorhombicFdd2 phase. 41,42 Upon compres-
sion, several new diffraction peaks emerge, indicating
a pressure-induced phase transition (Fig. 1A and S1†). Single-
crystal diffraction ofa-furil collected between 0.5 and 4.4 GPa
was used to solve the structure of this high-pressure phase
(Fig. 1B and Table S1†). At 0.5 GPa,a-furil remains in the
ambient-pressureFdd2 phase, albeit with a 6.7% reduction in
cell volume. Upon further compression to 1.6 GPa,a-furil pla-
narizes, resulting in a transition to the monoclinic space group
P2
1/nwith cell parametersa=14.586(5) Å,b=7.0535(14) Å,c=
3.4512(6) Å,b=93.132(6)° (see ESI†). This new cell is in good
agreement with the analogous thiophene structure,a-thenil,
which crystallizes in thetrans-planar conformation, occupying
aP2
1/nlattice (a=14.73,b=7.27,c=4.75,b=112.41°) albeit
with minimal stacking overlap between aromatic rings. 43,44
Planar 1,2-diketone systems are generally uncommon due to
steric interactions between carbonyl oxygens and attached ring
systems.
41However, the dihedral angle between rings ina-furil
at ambient conditions is notably larger than analogous
complexes (138.4°vs.114.9° in benzil
45) indicating that the
smaller ring size may reduce steric clashes and therefore the
barrier to planarization. Indeed, our theoretical calculations
indicate a minimal (+0.2 kcal mol
−1) energy difference between
the ambient-pressure andtrans-planar phases ofa-furil. This is
consistent with previous theoretical studies that indicate that
a free molecule ofa-furil with a dihedral angle of 180° is very
close in energy to the global minimum (156.1°).
46It is important
to note that the isolated molecular global minimum does not
account for stabilization effects caused by crystal packing,
which result in the smaller, crystallographically observed
dihedral angle. The reduced volume of the high-pressure phase
Fig. 1 (A) Comparison of experimental XRD data collected at 0, 1.9, and 4.2 GPa (black lines) and phase profiles from Le Bail refinements at the
same pressures calculated using phase I (blue dash) and phase II (red dash) structures ofa-furil (l=0.4340 Å); (B) top: structures of phase I and
phase IIa-furil taken from crystal structure data collected at 0 GPa and 4.4 GPa respectively. The orientation of the leftmost furan ring is kept
constant between structures to highlight the twisting of the 1,2-diketone bridge between phases. Black lines indicate the closest contact
intermolecular C/C distances corresponding to a [4 + 2] cycloaddition pathway. Red lines represent the closest contact intermolecular C/O
distances. Bottom: View of the same structures along the 1,2-diketone bridge, highlighting the torsion angle between the four carbons in the
bridge motif; (C) comparison of the FTIR spectra fora-furil collected at 0, 2.0, and 4.2 GPa, between 600–1800 and 2900–3300 cm
−1. The
region between 1800 and 2900 cm −1with diamond absorption has been omitted for clarity. Highlighted are vibrational modes corresponding
with phase I (blue dots, decreasing in intensity as a function of pressure) and phase II (red asterisks, increasing in intensity as a function of
pressure); (D) comparison of the Raman/photoemission spectra for phase I (black) and phase II (red)a-furil (excitation: 532 nm). Spectra have
been normalized based on collection time. The inset shows photomicrographs of ana-furil crystal (∼50×50mm) across the transition with
laser-induced emission.
4146
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is a result of improved packing efficiency of thetrans-planar
structure. Conrmation of the existence of thetrans-planar
phase ofa-furil validates previous studies into the photophysics
of 1,2-dicarbonyl systems that theorized the existence of a short-
livedtrans-planar excited state fora-furil, from which photo-
emission occurs.
46–48 A second structure ofa-furil collected at
4.4 GPa (see ESI†) indicates that phase II persists upon further
compression. Again, the furan rings are well placed to undergo
a pressure-induced [4 + 2] Diels–Alder cycloaddition reaction,
with an average intermolecular C/C distance of 2.98 Å (see
Fig. 1B).
The low-pressure FTIR spectra ofa-furil are in good agree-
ment with previously reported studies.
49 However, above
1.1 GPa, several peaks disappear and new peaks emerge, atca.
665, 874, 1167, 1234, 1287, 1552, 1629, 3131, and 3186 cm
−1
(Fig. 1C and S2†), caused by the change to thetrans-planar
phase. Above 8.6 GPa, a new set of peaks begin to emerge atca.
879, 1022, and 1141 cm
−1, increasing in intensity as a function
of pressure. These new peaks are generally consistent with
changes seen upon the onset of nanothread formation in the
case of 2,5-furandicarboxylic acid (cf.1163, 899 cm
−1) which
polymerizes to form asyn-furan nanothread wherein all the
furan atoms have the same orientation. At pressures above
15 GPa peaks corresponding to C]O stretches above 1600 cm
−1
begin to dramatically decrease in intensity, complimented by
a corresponding increase in the new peak atca.1022 cm −1,
which is generally consistent with the formation of sp 3C–O
bonds. While this may indicate reaction of the carbonyl groups
at pressure, it is important to note that furan nanothread
formation will also result in the formation of sp
3C–O–C bonds.
Raman studies show behavior that is consistent with FTIR,
and at 1.5 GPa several new vibrational modes appear atca.206,
616, 656, 1405, and 1562 cm
−1 (Fig. S3†). The Raman spectra
also show thata-furil exhibits a mechanochromic response to
pressure. At ambient pressure a broad emission can be seen at
ca.623 nm, which may be related to trace impurities which can
be seen in the
1H NMR of thea-furil starting material (see ESI†).
Above 3.3 GPa a second emission centered atca.541 nm
emerges, becoming more intense with increasing pressure and
overwhelming Raman scattering above 4 GPa (Fig. 1D). Previous
studies into the photoemission behavior ofa-furil in solution
have assigned emission in this region to the phosphorescence
bands from the triplet state of a theorizedtrans-planar
intermediate.
46–48 However, thistrans-planar phase has not been
directly observed due to its transient nature. The vibrational
changes observed in both Raman and FTIR studies were found
to be reversible up to at least 4.6 GPa (see Fig. S2†), strongly
indicating that they are related to the polymorphic molecular
phase change and not the onset of an irreversible chemical
reaction.
Powder X-ray diffraction patterns and IR spectra ofa-furil
collected above 1.9 GPa are in good agreement with the new
planar phase. Interestingly, while single crystals ofa-furil
compressed under quasi-hydrostatic conditions show complete
conversion to thetrans-planar phase above 1.9 GPa, powder
samples compressed without a pressure transmitting medium
(i.e., anisotropic compression) show coexistence of both phasesat similar pressures (see Fig. 1A). Further compression ofa-furil
results in increased conversion to thetrans-planar phase and
a uniform shiof Bragg peaks to higher angles, consistent with
pressure-induced contraction of the unit cell. Above 8.5 GPa,
signicant changes are observed in the diffraction pattern with
the emergence of new peaks atd=6.30, 5.08, 4.00 and 3.37 Å
(see Fig. S1†). These new reections grow in intensity with
increasing pressure and no further changes are noted upon
additional compression to 25.2 GPa. These new reections
remain upon decompression to ambient pressure, suggesting
that these new peaks are not due to another polymorphic
pressure-induced phase change, and indicate thata-furil has
undergone a pressure-induced chemical reaction.In situIR data
collected aer the phase change are consistent with the changes
observed in XRD. The intensity of the additional vibrational
modes that emerge above 8.6 GPa increases upon further
compression. At the same time the intensity of the peaks cor-
responding to phase I and IIa-furil decreases signicantly up to
19.6 GPa. Additionally, the large photoemission intensity
observed aer the transition to thetrans-planar phase broadens
and shis to lower energy above 7 GPa, shiing toca.634 nm at
18 GPa. This broad emission remainsca.634 nm upon
decompression to ambient pressure (see Fig. S3†), and is
consistent with the formation of other“perfect”nanothreads
such as pyridazine-derived threads, which also shows broad
uorescence atca.630 nm.
Characterization of nanothread polymer
Given the relatively low reaction onset pressure, synthesis was
readily scaled using a Paris–Edinburgh (PE) press at∼20 GPa to
produce bulk samples for advanced analysis. XRD and FTIR of
recovered PE press and DAC samples are in good agreement
(Fig. S4†
), with the PE press sample exhibiting clearly dened
diffraction out todz2.13 Å. Unlike molecular furan which is
a yellow powder, the recovered sample is a deep red color. The
recovery of a highly colored sample is consistent with other
nanothread systems, such as pyridazine,
9pyridine, 25,26 and 2,5-
furandicarboxylic acid 5,6 which all possess an intense orange
color. While the exact cause of this color change is likely to be
system dependent, it is commonly believed to be due to the
presence of defects along the thread backbone comparable to
diamond, which can become highly colored by the inclusion of
minor defects.
Comparison of solid-state
13C NMR collected ona-furil, and
the washed product (Fig. 2A) agrees with the formation of
a nanothread material rich in sp
3carbon. Two new peaks can be
seen atca.50 and 83 ppm, which are consistent with previous
solid-state NMR studies carried out onsyn-2,5-furandicarboxylic
acid nanothreads (cf., 58 and 89 ppm).
5However, the peak
widths in the recovered sample are notably larger than those
seen in comparablesyn-furan nanothreads, strongly indicating
that the presence of a broader distribution of sp
3C environ-
ments. Accordingly, we can only generally assign the peak atca.
50 ppm to sp
3C–C bonds, and the peak atca.83 ppm to sp 3C–O
bonds. Interestingly, the recovered product shows no signi-
cant intensity associated with C]O functionality atca.178 ppm
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in the molecular phase, which strongly indicates reaction of the
1,2-diketone bridge during polymerization. It is also important
to note that the recovered sample exhibits signicantly different
relaxation dynamics compared to the starting material, result-
ing in a signicant decrease in the observed signal-to-noise
ratio under the same collection times, and it is possible that
a small number of sp
2C]O environments remain, but are not
detected. Indeed, some weak intensity above the baseline can be
seen up to approximately 200 ppm. While carbon environments
in this region would be consistent witha,b-diketones attached
to saturated, sp
3hybridized carbon, 50,51 it is not possible to carry
out meaningful interpretation of this data due to poor signal
intensity. Although cross-polarization magic-angle spinning
(CPMAS) NMR is not quantitative (see ESI†), the presence ofsmaller peaks at 148 ppm and 113 ppm potentially indicates
retention of some amount of residual sp
2carbon aer washing. 1H NMR studies collected on the same sample (Fig. 2B) conrm
the presence of residual sp 2carbon environments. The resulting
spectrum is described by two peaks atca.3.6 and 7.3 ppm,
which we assign to sp
3and sp 2C–H bonds, respectively.
Comparison of the area under each peak indicates a 2 : 1 ratio of
sp
3:sp 2C–H bonds, consistent with the formation of a nano-
thread containing residual sp 2carbon defects and/or a disor-
dered sp 2material. It is important to note that while the 1H
spectrum can be used to estimate the relative proportion of
hydrogen environments within the sample, the broad nature of
the spectrum limits the certainty of quantication.
The sp
2carbon peaks in the 13C NMR spectrum of the
recovered nanothread are in good agreement with the peaks
assigned to the carbons in the a and c positions of the furan ring
in unreacteda-furil, furthest away from the diketone bridge
(Fig. 2A(a) and (c),ca.148 and 112 ppm at ambient respectively),
potentially indicating retention of this bond. Indeed, the FTIR
spectrum of the washed nanothread also shows evidence for the
retention of sp
2C]C vibrations atca.1452 cm −1 and sp 2C–H
stretching vibrations atca.3150 cm −1, although their
Fig. 2 (A) Solid-state 13C CPMAS NMR spectra ofa-furil (black,
bottom) and the recovered nanothread (red, top) between 300 and
−100 ppm. Inset: Assignment of
13C NMR peaks in phase Ia-furil. Only
chemically unique carbons are shown. No additional peaks were
observed outside of the spectral range shown; (B) solid-state
1H NMR
spectrum of the same recovered nanothread (solid red line) between
50 and−50 ppm. Dashed black and red lines indicate thefits corre-
sponding to sp
2and sp 3C–H bonds respectively; (C) FTIR spectra of
phase Ia-furil (black) and the nanothread recovered to ambient
pressure (red). The diamond absorption region between 1800 and
2900 cm
−1 has been omitted for clarity. Inset: Optical photomicro-
graphs ofa-furil before (yellow) and after (red) compression, sample
width is∼1 mm.
Fig. 3 X-ray diffraction pattern (l=1.541 Å) ofa-furil nanothreads
synthesized in a Paris–Edinburgh press and recovered to 1 atm (black
points) with Rietveld refinement using nanothread structure C (red
line). Black dots indicate peaks corresponding to unidentified phases.
Inset, top: A segment of refined nanothread structure C and associated
structural formula. Inset, bottom: X-ray diffraction pattern ofa-furil
nanothreads between 8 and 19°. Green tick marks indicate the relative
positions of the three low-angle reflections for the refined nanothread
polymer. Purple pattern and tick marks indicate the relative positions
of the three low-angle reflections for an expanded lattice of phase IIa-
furil.
4148
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intensities are signicantly reduced (Fig. 2C). While the shortest
intermolecular O/C separation distance between a carbonyl
oxygen and a furan ring in phase IIa-furil is longer than the
closest contact [4 + 2] cycloaddition pathway (3.15vs.2.98 Å at
4.4 GPa, respectively), it is still possible that some reactivity
occurs between ketone groups and the closest C]C double
bond in the furan ring. Such a reaction would therefore prevent
[4 + 2] cycloaddition and would result in the formation of sp
2
C]C defects at the position furthest away from the diketone
bridge.
The IR spectrum of the recovered PE press sample, washed to
remove any residual starting phase, conrms the formation of
a new material rich in sp
3carbon (Fig. 2C). The emergence of
new sp 3C–H stretches atca.2968 and 3000 cm −1, clearly indi-
cates the formation of a sp 3-C rich material consistent with
nanothread formation. Additionally, the presence of new peaks
atca.1730 and 1770 cm
−1 are consistent witha,b-diketones
attached to sp 3-hybridized carbons (cf., 1690–1769 cm −1 in
diacetyl 52) and analogous carbonyl-functionalized nanothreads
(1742 cm−1 in 2,5-furandicarboxylic acid nanothreads 5). Peaks
at 775, 852, and 877 cm −1 are also consistent with ring vibra-
tions ofsyn-furan nanothreads (cf., 765, 851, and 885 cm −1 in
syn-2,5-furandicarboxylic acid nanothreads 5). The formation of
a double-core nanothread product, wherein the furan rings are
arranged in thesynconformation (i.e., all oxygen atoms
arranged on the same side of the nanothread core), is consistent
with the orientation of the starting molecular structure.
However, the presence of the carbonyl peak atca.1656 cm
−1
(albeit with notably decreased intensity) suggests partial reten-
tion of the diketone bridge as seen in the molecular phase.
Additionally, the relatively broad absorbance background indi-
cates the presence of a disordered component coexisting with
the resulting nanothread.
In order to determine if the carbonyl peak at 1656 cm
−1 is
due to the presence of residual trapped molecular precursor or
polymerization defects, we carried out a combined thermogra-
vimetric analysis/differential scanning calorimetry study on the
same sample used for FTIR. While moleculara-furil exhibits an
endothermic mass loss beginning at approximately 153 °C,
consistent with sublimation, the recovered nanothread shows
no mass loss in the same region (Fig. S5†). Instead, the recov-
ered nanothread does not begin to exhibit any mass loss until
the onset of an exothermic decomposition process at approxi-
mately 364 °C, indicating the formation of an extended, poly-
meric system.
To understand the potential side reactions which may result
in loss of ketone functionality, it is necessary to reexamine the
structure of phase IIa-furil. In this planar phase the diketone
bridge is indeed well positioned for potential reaction with only
a 2.81 Å intermolecular O/C separation between carbonyl
groups at 4.4 GPa, a shorter spacing than any of the intermo-
lecular C/C separation distances observed in phase IIa-furil
(average=2.98 Å). Indeed, studies in analogous systems have
shown that carbonyl groups can readily act as dienes in Diels–
Alder reactions,
53,54 and can react with 5-membered ring
systemsviaalternative reaction pathways such as electrophilic
aromatic substitution.
55 To investigate possible reactionpathways, including possible diketone reactions, we carried out
rst-principles molecular dynamics (MD) simulations on phase
IIa-furil (see Fig. S6†) at 50 GPa and 500 K (note that more
extreme PT conditions were necessary to compensate for rare-
event reaction observations on a computationally limited time
scale). Aer 150 fs, a spontaneous [4 + 2] cycloaddition reaction
occurs between furan rings, before the onset of a second reac-
tion between ketones aer 450 fs.
Based on thesendings we optimized the packed crystal
structures of four potential close-contact nanothread products
for direct comparison to experimental data (see Table S2†).
These structures represent the nanothread formed from [4 + 2]
Diels Alder cycloaddition reactions between furan rings with (A)
no reaction between ketones, (B) partial reaction between
ketones (i.e., only one set of ketones react), (C) complete reac-
tion between ketones, and (D) complete reaction between
ketones with alternative diagonal bonding due to the close
O/C contacts of 2.96 Å between ketone groups at 4.4 GPa.
Initial comparison of the energies for the packed structures
relative to structure A indicates that structure D is signicantly
less stable (+1.52 kcal mol
−1) than the other candidates and can
likely be discounted as a product.
All of the considered DFT-optimized structural models
exhibit calculated X-ray diffraction patterns that reproduce
most of the experimentally observed features. Rietveld rene-
ment of the recovered PE press sample indicates that the fully
reacted structure (model C) provides the best description of the
experimental data among the four basic models (Fig. 3,
Tables S3 and S4†). While models A and B are poorer matches to
the XRD of the recovered sample, some degree of partial reac-
tivity must be present in the recovered structure, based on the
experimental evidence for residual sp
2C–H and C]O groups in
the corresponding FTIR and NMR spectra. The actual polymer
product clearly exhibits more complexity than the four base
models, however, major structural aspects are well reproduced,
and the prohibitively large number of all possible furan ring/
diketone defect structures were not examined. Although the
major features of the XRD of the recovered nanothreads are
described by the simplied nanothread models, XRD patterns
also contain small peaks at low angle that are not described by
any of the model structures. Preliminary analysis shows that
these peaks can be described by an expanded version of the
monoclinic cell associated with phase IIa-furil, and the impu-
rity structure might be related to trapped molecular precursor
or insoluble crystallizedn-mer fragments. The intensity of these
impurity peaks can be reduced, but not removed entirely,
through washing the sample with dichloromethane.
Comparison of calculated and experimental IR spectra
provides some additional insights into the local structure of the
recovered product (Fig. S7†). All model structures contain asyn-
furan nanothread ring system and can therefore each generally
describe the peaks seen between 700 and 1000 cm
−1. While
structure C is consistent with NMR studies (i.e., no signicant
evidence of carbonyl species) this structure alone cannot
account for the peaks at 1656, 1730 and 1770 cm
−1 seen in the
FTIR spectrum of the recovered nanothread. Structure A is
calculated to exhibit a single C]O vibration atca.1654 cm
−1,in
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excellent agreement with the experimentally observed peak at
ca.1656 cm −1. Structure B is also calculated to exhibit two,
higher energy C]O vibrations in reasonable agreement with
the experimentally observed vibrations atca.1730 and
1770 cm
−1.
Based on these studies, we can conclude that the structure of
the resulting nanothread can largely be described by structure
C, as twosyn-furan nanothread cores containing a polymerized
bridge. However, it is also evident from FTIR and NMR studies
that the 1,2-diketone bridge is not fully reacted and the recov-
ered nanothread contains some degree of unreacted carbonyl
defects which can be described by the bridging motifs shown in
structures A and B. This behavior is consistent with previous
studies into double-core nanothreads that have shown uncer-
tainty in the extent of reaction of bridging groups.
11,14,35,36,56
Conclusions
In summary,a-furil undergoes pressure-induced planarization
to the previously hypothesizedtrans-planar phase above 1 GPa,
before undergoing a topochemical-like polymerization above
8.5 GPa. The discovery of thetrans-planar phase validates
previous photochemical studies of 1,2-dicarbonyl systems that
theorized the existence of atrans-planar excited state from
which photoemission occurs. Upon further compression,a-furil
polymerizes to form a double-core nanothread material with
a high degree of crystalline order. This work also emphasizes
the importance of polymorphic phase transitions on the high-
pressure behavior of small organic molecules and subsequent
chemical reactions. The application of pressure may enable
topochemical-like reaction pathways within complex molecular
systemsviastructural rearrangement that might otherwise be
overlooked based on initial crystalline geometries.
Data availability
The data supporting this article have been included as part of
the ESI.†Additional experimental details, plots of diffraction
and spectroscopic data, as well as computational models are
provided in PDF form. Crystal structures ofa-furil at 0.5, 1.6,
and 4.4 GPa are provided in cif form. CCDC 2343198–2343200
contain the supplementary crystallographic data for this paper.
These data can be obtained free of chargevia
www.ccdc.cam.ac.uk/data_request/cif, or by emailing
[email protected], or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
Author contributions
SGD, AH and TAS designed the research project and conducted
experimental studies. BC and LZ carried out all theoretical
calculations. GDC performed all NMR measurements. SR and
DZ assisted with X-ray diffraction studies and structure solu-
tions. All authors contributed to the writing of this manuscript.
Conflicts of interest
There are no conicts to declare.
Acknowledgements
Portions of this work were performed at GeoSoilEnviroCARS
(The University of Chicago, Sector 13), Advanced Photon Source
(APS), Argonne National Laboratory. GeoSoilEnviroCARS is
supported by the National Science Foundation–Earth Sciences
(EAR–1634415). This research used resources of the Advanced
Photon Source; a U.S. Department of Energy (DOE) Office of
Science User Facility operated for the DOE Office of Science by
Argonne National Laboratory under Contract No. DE-AC02-
06CH11357. The authors acknowledge funding support from
The Arnold and Mabel Beckman Foundation (Arnold O. Beck-
man Postdoctoral Fellowship in Chemical Sciences Program).
TS acknowledges support from NSF-DMR under award No.
2226699. BC acknowledgesnancial support from MCIN/AEI/
10.13039/501100011033/FEDER, UE (Project PID2021-
123573NA-I00).
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