The Power of Packing: Metallization of an Organic Semiconductor

Article abstract of DOI:10.1021/jacs.6b12814

Benzoquino-bis-1,2,3-dithiazole 5 is a closed shell, antiaromatic 16π-electron zwitterion with a small HOMO-LUMO gap. Its crystal structure consists of planar ribbon-like molecular arrays packed into offset layers to generate a "brick-wall" motif with str

Full text of DOI:10.1021/jacs.6b12814

Communication
pubs.acs.org/JACS
The Power of Packing: Metallization of an Organic Semiconductor
Aaron Mailman,† Alicea A. Leitch,† Wenjun Yong,‡ Eden Steven,⊥ Stephen M. Winter,†
Robert C. M. Claridge,† Abdeljalil Assoud,† John S. Tse,§ Serge Desgreniers,∇ Richard A. Secco,‡
and Richard T. Oakley*,†
†Department of Chemistry, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada
‡Department of Earth Sciences, University of Western Ontario, London, Ontario N6A 5B7, Canada
⊥Department of Physics, Florida State University, Tallahassee, Florida 32310, United States
§Department of Physics, University of Saskatchewan, Saskatoon, Saskatchewan S7N 5E2, Canada
∇Department of Physics, University of Ottawa, Ottawa, Ontario K1N 6N5, Canada
S
* Supporting Information
Further reduction in the HOMO−LUMO gap can be made by
ABSTRACT: Benzoquino-bis-1,2,3-dithiazole 5 is a
closed shell, antiaromatic 16π-electron zwitterion with a
small HOMO−LUMO gap. Its crystal structure consists of
planar ribbon-like molecular arrays packed into offset
layers to generate a “brick-wall” motif with strong 2D
interlayer electronic interactions. The spread of the valence
and conduction bands, coupled with the narrow HOMO−
LUMO gap, affords a small band gap semiconductor with
σRT = 1 × 10−3 S cm−1 and Eact = 0.14 eV for transport
within the brick-wall arrays. Closure of the band gap to
form an all-organic molecular metal with σRT > 101 S cm−1
can be achieved by the application of pressure to 8 GPa.
incorporating a pyridone or benzoquinone bridge, as in the 16π-
electron bisdithiazoles 4 and 5. The former is known to possess a
very small HOMO−LUMO gap with ΔEST near 0.15 eV,7 but its
crystal structure allows for very little band spreading, and it shows
no measurable conductivity. By contrast, the quinone-bridged
zwitterion 5, whose preparation and transport properties we
describe here, possesses a crystal structure with the much sought
after “brick-wall” packing pattern recognized as effective for the
development of strong 2D electronic interactions in organic
semiconductor devices.8 As it happens, the zwitterion is
isostructural with the 17π-electron oxobenzene-bridged radical
6,9 which displays high conductivity at ambient pressure (σRT
∼
10−2 S cm−1), metallizes at 3 GPa and forms a Fermi liquid state at
6 GPa.10 In the case of 5, the small HOMO−LUMO gap,
combined with the spreading of the valence and conduction
n the absence of an electric field, chemical doping, or
Iphotoexcitation most closed shell organic molecular solids are
insulators.1 The HOMO−LUMO gaps are too large, and
intermolecular interactions too small to generate spreading of
the valence and/or conduction bands and allow closure of the
band gap Eg. A variety of strategies to reduce the HOMO−
LUMO separation and improve charge transport have been
considered.2 Notable targets have included formally antiaromatic
(4nπ) systems such as quinonemonoimines 1, hexaazaanthra-
cenes 2 and bisdithiazoles 3 (Chart 1) in which a charge-
separated zwitterionic singlet state competes with a triplet or
singlet biradical.3−5 These and related materials have been
explored extensively, as the HOMO−LUMO gap, and
consequent singlet/triplet splitting ΔEST, can be fine-tuned by
variations in substituents and heteroatoms. However, reduction
of the band gap to the point where charge carriers can be easily
generated by thermal excitation has remained a challenge.6
bands, affords a small band gap semiconductor with σRT ∼ 10−3
S
cm−1 at ambient pressure. Moreover, the application of physical
pressure leads to closure of the band gap and formation of a single
component organic metal with σRT > 101 S cm−1 at 8 GPa.
Scheme 1
The preparation of 5 (Scheme 1) begins with 2,6-diamino-1,4-
dihydroxybenzene 7 (as its bishydrochloride salt), which is
prepared by nitration and reduction of commercially available
1,4-diacetoxybenzene.11 Subsequent condensation of 7 with
sulfur monochloride yields the chloride salt of the protonated
oxobenzene-bridged bisdithiazolium cation [5]H+, which may be
Chart 1
Received: December 13, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/jacs.6b12814
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Journal of the American Chemical Society
Communication
converted to the more soluble triflate salt by metathesis with
trimethylsilyl triflate. Deprotonation of this material with Proton-
Sponge yields the zwitterion, which sublimes at 240 °C/10−4
Torr to afford air-stable, purple-black crystalline blocks.
compared with a calculated (TD-DFT) estimate of 837 nm.
While 5 is closed shell and EPR silent, the open-shell 2B1 radical
cation [5]+ can be generated in situ by the comproportionation of
the doubly oxidized salt [5][AlCl4]2 with neutral 5 in liquid SO2.
The resulting EPR spectrum (Figure 2) displays the expected
five-line pattern (g = 2.01125), but with a substantially smaller aN
value (0.063 mT) than typically seen in radicals like 6, an
observation suggestive of a semiquinone radical cation
formulation.12 In the case of [5]+, spin density (Figure S1) is
localized on the basal oxygen; the calculated aN values are
consistent with those observed experimentally.
Figure 1. ORTEP drawings of 5 and [5]H+ (in its OTf− salt) with atom
numbering, with selected metrics in Å. For averaged values, numbers in
parentheses are the greater of the difference and the standard deviation.
The internal structural metrics of 5, obtained by single crystal
X-ray diffraction (Figure 1), are broadly consistent with the
valence bond formulation 5A. The marked shortening of the C−
O2 bond that accompanies deprotonation of the [5]H+ cation (in
the triflate salt) and the resulting similarity in the C−O1 and C−
O2 distances suggests little resonance with the internal salt
formulation 5B. The charge density distribution and low
molecular dipole moment obtained from DFT calculations at
the B3LYP/6-311G(d,p) level are consistent with this
interpretation (Figure S11).
Figure 3. (a) Unit cell of 5, showing sheet-like packing in the bc plane;
the shaded layer is offset by a/2. (b) 2D brick-wall π-stacking of
molecules in the ab plane, with four equivalent interlayer hopping
interactions tπ.
As noted above, zwitterion 5 carries appeal from a solid state
perspective because of its structural similarity to radical 6; the two
compounds constitute an isostructural “matched pair” in the
space group Cmc21. In each case, there are four molecules in the
unit cell (Figure 3), located on crystallographic planes at x = 0, 1/
2, and within these coplanar layers neighboring molecules along
the z-direction are related by 2-fold screw axes. Although there
are small, expected differences in the internal metrics of the two
compounds (Table S2), the coplanar sheet-like molecular arrays
are essentially identical; the network of close (inside van der
Waals separation)13 intermolecular S···N′ and S···O′ contacts
found in 5 maps perfectly onto the S···N′ and S···O′ (and S···F′)
contacts found in 6 (Figure S2).9 Adjacent layers of radicals along
the x-direction are related by b-glides at x = 1/4 and 3/4, which
produces a classic “brick-wall” packing pattern in the ab plane,
with interlayer separation of a/2, or 3.164 (2) Å for 5 and
3.151(1) Å for 6.9 The number and closeness of the
intermolecular contacts occasioned by this 2D architecture, in
particular the four equivalent π-type hopping interactions tπ, has a
profound impact on the transport properties of both compounds.
Despite the absence of unpaired spins present in radical 6,
zwitterion 5 shows remarkably high conductivity for a metal-free
single component system. Variable temperature (VT) 4-probe
single crystal measurements taken parallel (∥) and perpendicular
(⊥) to the ab plane (Figure 4a) indicate significant anisotropy,
with enhanced performance within the brick-wall arrays, resulting
Figure 2. (a) Frontier Kohn−Sham π-orbital manifold (not to scale) of
the zwitterionic 1A1 state of 5, (b) DFT spin distribution of the 3B2 triplet
state, (c) 1B2 open shell singlet state of 5 and the 1A1 to 1B2 excitation in
the near IR spectrum and (d) EPR spectrum of the 2B1 radical cation 5+
recorded in SO2(l).
The DFT calculations reveal a frontier π-orbital manifold
(Figure 2) for the zwitterionic 1A1 state of 5 consisting of a closely
spaced b1 HOMO and a2 LUMO, with an additional low-lying b1
LUMO+1 that is of importance in understanding the solid state
electronic features (vide infra). In the 3B2 triplet state, which lies
slightly above the zwitterionic singlet, with ΔEST = 0.15 eV, as
found for 4, spin density is partitioned between the two dithiazole
rings. The corresponding open-shell 1B2 singlet lies significantly
higher in energy, and photoexcitation to this state gives rise to an
intense near IR band at 732 nm (in DMSO), which may be
B
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4 × 4 Monkhorst−Pack k-point mesh. The resulting band
structure (Figure 6) can be understood in terms of the three
frontier orbitals in Figure 2. In the first Brillouin zone, the valence
band is composed of two crystal orbitals (COs) arising from the
b1 HOMO, whereas the conduction band comprises four COs
arising from heavily hybridized (in-phase and out-of-phase)
combinations of the a2 LUMO and b1 LUMO+1.17
Figure 4. (a) Single crystal conductivity σ of 5 as function of temperature
measured ∥ and ⊥ to the ab plane, with corresponding Eact values. (b)
Pressure dependence of pressed pellet conductivity σRT and thermal
activation energy Eact of 5, measured using a multianvil press.
in σRT (∥) = 1.0 × 10−3 S cm−1 and Eact (∥) = 0.14 eV, with σRT
(∥):σRT (⊥) ∼ 50:1. This behavior may be compared to that of
the donor−acceptor conductor bisthiadiazolo p-quino-bisdi-
thiole (BTQBT).14 To explore the response of both σRT and
Figure 6. DFT calculated band structure of 5 at (a) 0 GPa and (b) 7.4
GPa. COs in the valence band (in blue) derive from the molecular
HOMO, whereas the conduction band (in green) is composed of
hybridized COs based on the LUMO and LUMO+1 (in Figure 2). The
Fermi level is shown in red.
Eact of 5 to applied pressure, we performed high pressure (HP) 4-
probe conductivity measurements over the range P = 0−12 GPa
on a pressed pellet sample using a multianvil press. The results
(Figure 4b) indicate a rapid rise in σRT with increasing pressure to
a non-plateau value near 102 S cm−1 at P = 12 GPa. At the same
time, the value of Eact (measured over the range T = 25−150 °C)
steadily drops, reaching zero near 8 GPa.
It is evident that at 0 GPa there is already significant dispersion
in both the valence (0.9 eV) and conduction bands (1.0 eV), to
afford an indirect band gap Eg near 0.2 eV (Figure 6a), which is
significantly smaller than that calculated for BTQBT (∼1.1 eV)18
but close to that estimated from the VT conductivity measure-
ments (Eg ∼ 2 Eact (∥) = 0.28 eV). The slight discrepancy may be
related to the tendency of DFT methods to underestimate
correlation effects.19 As expected, band dispersion is greatest
along the reciprocal space directions Γ → X and Γ → Y, which
correspond in real space to interactions within the brick-wall
arrays. By comparison, interactions between neighboring walls
(Γ→ Z) are small. With applied pressure, dispersion in all
directions increases, including Γ → Z, as neighboring walls are
compressed together. By 7.4 GPa, overall dispersion of the
valence band is near 1.6 eV, whereas the width of the conduction
band is over 2.0 eV. As a result, there is significant overlap of the
two bands, suggestive of formation of a metallic state.
Pressure induced metallization of single component, closed
shell organic (metal-free) materials by band gap closure has only
rarely been observed, and pressures >30 GPa are usually
required.20 The presence of sulfur,21 selenium22 and tellurium23
heteroatoms can facilitate intermolecular interactions, but even
then metallic or metallike behavior requires pressures approach-
ing or in excess of 20 GPa. In this light, the onset of a metallic state
in 5 at 8 GPa by closure of the band gap without molecular
deformation24 is exceptional. The phenomenon results, we
postulate, from the combination of (i) the small HOMO−
LUMO gap in the molecular building block and (ii) the highly 2D
solid state electronic structure afforded by its brick-wall packing
pattern. Although this architecture is known to improve the
performance of organic FET materials,8 electronic interactions
therein are typically cloaked by steric congestion and the actual
band gap remains large. By contrast, the total absence of steric
crowding in 5 allows for enhanced intrawall and (with pressure)
interwall interactions, so that dispersion of the valence and
conduction bands is substantial, even at ambient pressure. Given
the presence of this chemical pressure, very little physical pressure
is required to close the residual band gap.
To explore the electronic origins of the remarkably high value
of σRT for 5 observed at ambient pressure, and also its rapid
pressure induced transformation to a metal, we have explored its
band electronic structure as a function of pressure. As a first step,
HP crystallographic studies were performed using synchrotron
radiation and DAC techniques. Powder diffraction data were
collected at room temperature as a function of increasing pressure
up to 12 GPa, with helium as the pressure transmitting medium.
The data sets so obtained were indexed and the structures solved
in DASH and refined in GSAS using a molecular model derived
from the ambient pressure crystal structure. In the final Rietveld
refinement only the unit cell parameters were optimized. As
shown in Figure 5, which illustrates variations in the unit cell
dimensions of 5 as a function of pressure, there is no evidence for
a structural phase change. All three axes contract uniformly, the
most significant response being parallel to the a-axis, as found
previously for 6, an observation that reflects the relative ease of
compression of the layered π-stacks.
Figure 5. Contraction in unit cell dimensions of 5 with applied pressure.
Using the unit cell parameters and atomic coordinates
provided by the HP structural work, we performed DFT band
structure calculations using the Quantum Espresso15 package
with ultrasoft PBE pseudopotentials,16 a plane-wave cutoff of 25
Ry and a 250 Ry integration mesh. The SCF calculations were
based on crystallographic atomic coordinates and employed a 4 ×
C
DOI: 10.1021/jacs.6b12814
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Journal of the American Chemical Society
Communication
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Hansen, N. H.; Musiol, T.; Kalblein, D.; Pflaum, J.; Tao, X.; Brill, J.;
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/jacs.6b12814.
■
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Experimental details and data (PDF)
Crystal data for [5][HOTf] (CIF)
Crystal data for 5 (CIF)
High pressure crystal data for 5 (CIF)
AUTHOR INFORMATION
Corresponding Author
*oakley@uwaterloo.ca
ORCID
Richard T. Oakley: 0000-0002-7185-2580
Notes
The authors declare no competing financial interest.
■
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ACKNOWLEDGMENTS
■
We thank the NSERC of Canada and the U.S. NSF (Grant DMR
1309146) for financial aid, the NSERC for a postgraduate
scholarship to S.M.W., the Government of Canada for a Tier I
Canada Research Chair to J.S.T., and the Canada Foundation for
Innovation for funding to R.A.S. for a 3000 ton multianvil press.
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DOI: 10.1021/jacs.6b12814
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