Temperature-mediated polymorphism in molecular crystals: The impact on crystal packing and charge transport

Article abstract of DOI:10.1021/cm503439r

We report a novel synthesis to ultra high purity 7,14-bis((trimethylsilyl)ethynyl)dibenzo[b,def]-chrysene (TMS-DBC) and the use of this material in the growth of single crystals by solution and vapor deposition techniques. We observe that the substrate temperature has a dramatic impact on the crystal growth, producing two distinct polymorphs of TMS-DBC; low temperature (LT) fine red needles and high temperature (HT) large yellow platelets. Single crystal X-ray crystallography confirms packing structures where the LT crystals form a 1D slipped-stack structure, while the HT crystals adopt a 2D brickwork motif. These polymorphs also represent a rare example where both are extremely stable and do not interconvert to the other crystal structure upon solvent or thermal annealing. Single crystal organic field-effect transistors of the LT and HT crystals show that the HT 2D brickwork motif produces hole mobilities as high as 2.1 cm² V^-1 s^-1, while the mobility of the 1D structure is significantly lower, at 0.028 cm² V^-1 s^-1. Electronic-structure calculations indicate that the superior charge transport in the brickwork polymorph in comparison to the slipped-stack polymorph is due to the presence of an increased dimensionality of the charge migration pathways.

Full text of DOI:10.1021/cm503439r

Article
pubs.acs.org/cm
Temperature-Mediated Polymorphism in Molecular Crystals: The
Impact on Crystal Packing and Charge Transport
Loah A. Stevens,† Katelyn P. Goetz,† Alexandr Fonari,‡ Ying Shu,§ Rachel M. Williamson,∥
Jean-Luc Bredas,‡,⊥ Veaceslav Coropceanu,*,‡ Oana D. Jurchescu,*,† and Gavin E. Collis*,§
́
†Department of Physics, Wake Forest University, Winston-Salem, North Carolina 27109, United States
‡School of Chemistry and Biochemistry and Center for Organic Photonics and Electronics, Georgia Institute of Technology, Atlanta,
Georgia 30332-0400, United States
§CSIRO Manufacturing Flagship, Private Bag 10, Clayton South MDC, Victoria 3169, Australia
∥MX Beamlines, Australian Synchrotron, 800 Blackburn Road, Clayton, Victoria 3168, Australia
⊥Solar and Photovoltaics Engineering Research Center, Division of Physical Sciences and Engineering, King Abdullah University of
Science and Technology, Thuwal 23955-6900, Kingdom of Saudi Arabia
S
* Supporting Information
ABSTRACT: We report a novel synthesis to ultra high purity
7,14-bis((trimethylsilyl)ethynyl)dibenzo[b,def ]-chrysene
(TMS-DBC) and the use of this material in the growth of
single crystals by solution and vapor deposition techniques.
We observe that the substrate temperature has a dramatic
impact on the crystal growth, producing two distinct
polymorphs of TMS-DBC; low temperature (LT) fine red
needles and high temperature (HT) large yellow platelets.
Single crystal X-ray crystallography confirms packing structures
where the LT crystals form a 1D slipped-stack structure, while
the HT crystals adopt a 2D brickwork motif. These
polymorphs also represent a rare example where both are extremely stable and do not interconvert to the other crystal
structure upon solvent or thermal annealing. Single crystal organic field-effect transistors of the LT and HT crystals show that the
HT 2D brickwork motif produces hole mobilities as high as 2.1 cm2 V−1 s−1, while the mobility of the 1D structure is significantly
lower, at 0.028 cm2 V−1 s−1. Electronic-structure calculations indicate that the superior charge transport in the brickwork
polymorph in comparison to the slipped-stack polymorph is due to the presence of an increased dimensionality of the charge
migration pathways.
valuable insight into the complexity and correlations associated
with material structure, molecular packing, processing con-
ditions, and charge transport properties.17−20
1. INTRODUCTION
The use of small-molecule organic semiconductors in organic
field-effect transistors (OFETs) has increased significantly over
the past decade. Small molecules are easy to synthesize and
purify, readily accessible in large quantities, can be deposited by
solution and evaporation techniques, and have produced the
highest mobilities reported to date.1−11 The design of new
materials with even higher mobilities and long-term device
stability for practical applications is a continuing chal-
lenge.5,12,13 Crystal packing in small molecule semiconductors
plays an extremely important role in the electrical performance
of devices based on these materials. However, other factors that
are less well understood and not easily measured also
contribute, making designing new materials with desirable
properties an empirical process.14,15
These relationships are best studied using single crystal
organic field effect transistors (SCOFETs).21−29 Single-crystal
devices offer highly ordered systems with minimal grain
boundary defects, single polymorphs, a continuous crystal
lattice, and access to materials containing a minimum level of
impurities. Not surprisingly, mobility data acquired from single
crystals can be substantially higher than those from thin films;
for example pentacene SCOFET have produced mobilities as
high as 35 cm2 V−1 s−1,30 while thin-film OFET mobilities of 5
cm2 V−1 s−1 have been reported.4 SCOFETs offer a unique
platform to study the impact of polymorphism on charge
transport, where the effect of subtle changes in molecular
packing can be investigated without the complexity of the
Progress in the molecular design of semiconductor materials
by crystal engineering using in-silico methods is still in its
infancy in the field of organic electronics.16 In recent years, a
large number of publications on structure−property relation-
ships of small molecules in organic transistors have provided a
Received: September 17, 2014
Revised: December 15, 2014
Published: January 2, 2015
© 2015 American Chemical Society
112
DOI: 10.1021/cm503439r
Chem. Mater. 2015, 27, 112−118

Chemistry of Materials
Article
microstructure, which is always present in thin-film devices.
Such information is critical in material design and crystal
engineering. SCOFET data also allow easier comparison with
the results of electronic-structure calculations, to evaluate the
intrinsic charge-transport processes in these materials. Poly-
aromatic hydrocarbon (PAH) materials with a nonlinear
molecular structure have recently received much attention as,
unlike their linear oligoacene counterparts, they have been
shown to be much less reactive with other molecular entities,
such as fullerenes,31,32 and have produced high mobility in
OFETs.17,18,33 Here, we describe the synthesis and character-
ization of a high-purity PAH, 7,14-bis((trimethylsilyl)ethynyl)-
dibenzo[b,def ]-chrysene (TMS-DBC) 1. The results reveal the
first example of a single compound that exhibits two distinct
crystal structures (with 1D and 2D packing, respectively) and
very high crystal structure stability. The crystal packing is
dictated by the substrate temperature from which the crystals
are grown, and the extremely stable nature of the two
polymorphs does not allow interconversion between the two
different crystal lattice structures once formed within the
processing parameters investigated in this study. We evaluated
the intrinsic electronic properties of the two polymorphs using
SCOFETs and performed electronic-structure calculations in
order to understand the differences observed in the charge-
transport properties.
acetylide, followed by reduction with acidic tin oxide afforded,
after chromatography and recrystallization, high purity TMS-
DBC 1 (HPLC >99.3%) that agrees with the literature NMR
data for TMS-DBC 1 obtained previously from the commercial
DBC dye source.18 Although the minor isomer 6 presumably
also undergoes addition of TMS-acetylide, the five-membered
ring prevents cycloaromatization from occurring, and therefore
the minor isomer is easily removed during purification. TMS-
DBC 1 is thermally stable up to 315 °C (see SI Figure S1) and
shown to have a high hole mobility (1.17 cm2 V−1 s−1) when
fabricated into thin film bottom gate/top contact OFETs by
vacuum deposition.18
2.1. Polymorphism, Crystal Growth Methods, and
Single Crystal X-ray Crystallography. Polymorphism of
small molecule organic semiconductors has previously been
reported for systems such as pentacene,35−37 5,11-substituted
(hetero)pentacenes,38−40 oligothiophenes,41 tetrathiafulva-
lene,42 and diketopyrrolopyrroles.43 Polymorphism has also
been reported in bulk films of pentacene and oligiothiophene
derivatives, where the crystal packing of the first few
monolayers can be significantly different from the molecular
packing found in the bulk.35,44 To date, polymorphs derived
from the same molecular formula have produced different
crystal unit-cell structures that show minor variations in crystal
packing. These arise from small changes to the crystal
intermolecular contact distances and the degree of π−π
overlap. Polymorph formation is typically dictated by the
choice of solvent employed in crystal growth or film deposition,
postsolvent and thermal annealing of thin films, and more
recently, by the mechanical forces created during the processing
of materials into films (e.g., shearing and spin coating).6,7 Some
linear acenes are known to have many polymorphic structures
that are metastable under processing conditions or that can be
easily interconverted.36−39,45 In any of these cases, it is well
established that crystal polymorphs contribute differently to
charge transport within these crystal architectures, and
understanding and controlling these crystal transitions is critical
in maximizing device performance and long-term device
stability.
2. RESULTS AND DISCUSSION
High purity TMS-DBC 1 was accessed from a new synthetic
route shown in Scheme 1 (see the Supporting Information (SI)
Scheme 1. (a) Pd(PPh3)4, Toluene, 2M Na2CO3, Reflux; (b)
CF3SO3H, CH3SO3H, 140 °C; and (c) (i) −78°C, TMS-
Acetylene, n-BuLi, (ii) SnCl2·2H2O/Aqueous 1M HCl
Here, we find that two polymorphs can form in TMS-DBC 1,
which strongly differ in their electronic properties, and are not
interconvertible under the conditions tested in this study. We
have grown crystals by both solution and gas-phase methods,
and our results show that the resulting polymorphism type is
dictated by the substrate temperature at which crystal growth
occurs and is independent of the growth method. We refer to
the two structures as the low temperature (LT) and the high
temperature (HT) polymorphs, respectively (Figure 1).
Physical Vapor Transport (PVT)21,27,46 of TMS-DBC (sub-
limation temperature 240 °C) results in the formation of both
polymorphs in the crystallization tube. A temperature gradient
is present in the PVT tube, and we observe that TMS-DBC
crystal formation occurs in the temperature regions from ∼25
°C up to 170 °C (see SI Figure S2 for a detailed description of
the growth tube). Visual examination of the different regions of
the PVT tube clearly indicates the formation of two differently
colored and shaped crystals. In regions where the tube
temperature is ∼25−65 °C (the end of the tube), we observed
fine small red needles we refer to as the TMS-DBC red LT
polymorph, while from 130 °C to 170 °C, we observed large
yellow platelets, which we refer to as the TMS-DBC yellow HT
polymorph. The temperature region from 65 °C to 130 °C is a
transition region containing both red and yellow crystals. The
for full synthetic details). Suzuki coupling of boron ester 2 with
bis-triflate 334 gave the diester product 4 in high yield.
Subjection of diester 4 to extremely acidic conditions (TFSA/
MSA at 140 °C) facilitated intramolecular Friedel−Crafts
acylation to give two isomeric products, the 6,6-diketone 5 and
1
5,6-diketone 6 as an inseparable mixture. H NMR spectros-
copy confirmed the formation of two isomers with the major
compound being the desired dibenzochyrsene[b,def ]dione
(DBC) 5. Reaction of this isomeric mixture with TMS-
113
DOI: 10.1021/cm503439r
Chem. Mater. 2015, 27, 112−118

Chemistry of Materials
Article
Figure 1. (a) and (d) Crystal color, size, and shape of the LT red and HT yellow polymorphs of TMS-DBC. (b) and (c) Side and top views of the
crystal packing in the red LT polymorph. (e) and (f) Side and top views of the crystal packing in the yellow HT polymorph. The directions
corresponding to the largest calculated electronic couplings are indicated with arrows.
two observed polymorphs of TMS-DBC 1 crystallize in
significantly different triclinic cells with very different degrees
of cofacial packing. Unit cell parameters for the LT and HT
polymorphs are detailed in Table 1 and graphically represented
in SI Figures S3a and S4a, respectively. It is clear from the
crystal structures that the c-axis of the unit cell of the yellow
HT polymorph is significantly longer than that of the LT
polymorph, with the interactions between the alkyl groups
giving rise to a less closely packed structure and a greater gap
between neighboring π−π stacks.
LT polymorph (SI Figure S3b) the acetylene bond lies ∼2.36°
out of the aromatic plane, while in the HT polymorph it is
further distorted at ∼3.04° (SI Figure S4b). Finally, the most
obvious difference is the size and shape of the crystals. The HT
polymorph is obtained as large platelets, whereas the LT
polymorph occurs as small needles. Crystal growth by solution
methods was also investigated using drop cast (DC) and
solvent-assisted crystallization (SAC) techniques.47 Crystals
were grown from solutions of TMS-DBC 1 in xylenes,
acetonitrile, dimethyl sulfoxide, chlorobenzene, or mixtures of
these solvents on device substrates.
Table 1. Single-Crystal X-ray Crystallographic Data of TMS-
DBC 1 Polymorphs Measured at 100 K
Interestingly, we observe that in all cases, when the
crystallization occurs at or near room temperature, only red
crystals are obtained. When the substrate temperature is raised
over 65 °C, a mixture of red LT and yellow HT polymorphs is
observed. Unfortunately, rapid solvent evaporation precludes
crystal growth in the temperature range where the pure HT
polymorph is expected. Our observations on both PVT and
solution-grown crystals indicate that the crystallization temper-
ature plays a key role in determining which polymorph is
formed. Combined experimental and theoretical work is
currently in progress to elucidate the mechanism for this
polymorphism formation and to determine a complete
parameter space (e.g., pressure, solvent type, cooling rate, and
combinations of the above) that yield each of the structures.
With the crystal structures of the two polymorphs resolved,
interconversion between the polymorphs was investigated.
Attempts to transition the red LT polymorph to the yellow HT
polymorph by thermal annealing were unsuccessful. This can
also be observed from the feature-less DSC spectrum around
the temperature where the HT polymorph is expected to form
(SI Figure S1). In addition, extended heating (2 days) above 65
°C has also not resulted in polymorphism interconversion.
Likewise, room temperature solvent annealing for several hours
of the red LT and yellow HT polymorph also proved to induce
no change on the crystal structure. Polymorphism intercon-
version by solvent annealing at higher temperatures was
precluded by rapid evaporation of the solvent. We were also
not able to test PVT crystal growth by resublimation of the
crystals obtained in one run because of the low yield of our
crystal growth. We did note, however, that solution-grown LT
polymorph can be converted in a mixture of HT and LT
crystals upon subliming it using PVT.
red LT polymorph
yellow HT polymorph
a (Å)
6.4070 (13)
8.8220 (18)
12.726 (3)
72.82 (3)
6.1670 (12)
6.5710 (13)
18.077 (4)
92.65 (3)
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
V (Å3)
space group
76.84 (3)
90.94 (3)
83.84 (3)
113.17 (3)
672.3 (2)
668.6 (2)
triclinic, P-1
triclinic, P-1
Close examination of the red crystals by single-crystal X-ray
diffraction indicates these to be identical to the crystal
previously grown at room temperature from solution
methods.18 The red LT polymorph exhibits a 1D slipped
stacked packing (Figure 1b), where the intrastack distance
between aromatic carbon−carbon atoms (CC) is short at
∼3.353 Å, with a high degree of spatial intermolecular π−π
overlap (Figure 1c) between adjacent PAH units (i.e., 6 close
CC contacts per 3 DBC units).18 The crystal packing of the
newly discovered HT polymorph exhibits a 2D brickwork motif
with a slightly larger unit-cell volume compared to the LT
polymorph (Figure 1e). Even closer CC contacts of ∼3.336
Å are observed; however, this is compensated by a slightly
lower degree of π−π spatial overlap (Figure 1f) between
adjacent PAH rings (i.e., 4 close CC contacts per 3 DBC
units), presumably due to steric interactions of nearby TMS-
capping groups. The steric congestion brought by the TMS-
groups is confirmed by both the LT and HT polymorphs
showing distortion of the TMS−acetylene linkages, with these
bonds not being fully coplanar with the DBC PAH core. In the
114
DOI: 10.1021/cm503439r
Chem. Mater. 2015, 27, 112−118

Chemistry of Materials
Article
In contrast to other aromatic and heteroacene systems such
as TIPS-PEN45 and TES-ADT derivatives,38,39 where temper-
ature-induced transitions resulted in phase-interconversion, the
type of temperature-driven polymorphism discovered in TMS-
DBC appears to be distinctly different and rare, owing to its
reluctance to interconvert between polymorphs. Temperature-
induced formation of the 1D or 2D polymorphs is, to the best
of our knowledge, the first example where a significant and
irreversible change in crystal packing has been observed for
organic molecular crystals. From a device perspective, the
formation of a stable polymorph that can be determined by
regulation of the substrate temperature is highly advantageous
for the uniformity, reproducibility, long-term stability, and
operation of devices.
channel length L = 100 μm, and channel width W = 108 μm
yielded a mobility μ = 2.1 cm2V−1s−1. Variation of mobility data
for SCOFETs were observed. Such differences are typical and
originate from incomplete crystal lamination, anisotropy effects,
or morphology defects in the crystal studied,38 as shown in the
histogram presented in SI Figure S5a.
The red LT polymorph crystals gave devices with
significantly lower hole mobilities on the order of 10−5−10−3
cm2 V−1 s−1, the best reaching 10−2 cm2 V−1 s−1 (see SI Figure
S5b for the mobility distribution). The device presented in
Figure 2c for example has a mobility of μ = 0.028 cm2 V−1 s−1.
These values are significantly different from the ones measured
in the HT polymorph, where the highest mobility was in the
order 1 cm2 V−1 s−1, with the majority of the samples showing
mobilities of 10−1 cm2 V−1 s−1 (SI Figure S5a). Note that the
crystals were not aligned during device fabrication; however, by
measuring a large number of devices (>50) for each polymorph,
we have accessed all crystallographic directions, and we can
conclude that the HT polymorphs yield better hole transport.
The inferior electrical performance of the LT polymorph is
partially due to the fact that the majority of these crystals were
obtained by crystallization from solution, which typically
produces lower quality crystals (increased surface roughness,
lower purity) compared to the PVT, which yielded most of our
HT polymorph crystals. This is also supported by the higher
value of the subthreshold slope measured in the LT crystals
(e.g., 6.67 V/dec for the device in Figure 2c, compared to 1.04
V/dec for the transistor in Figure 2b). Nevertheless, these
effects cannot fully explain the large differences observed in the
device mobilities, and thus other factors will be discussed
below.
2.2. SCOFET Measurements. Crystals grown by the three
different methods were evaluated in bottom-gate, bottom-
contacts SCOFETs (Figure 2a). The substrates used consisted
2.3. Electronic Structure. In order to get a better
understanding of the intrinsic charge-transport properties of
the crystalline polymorphs, we investigated their electronic
couplings (transfer integrals, t) and related electronic band
structures using density functional theory (DFT) (see Figures
1b,e and 3). In the LT polymorph, the largest transfer integrals
Figure 2. (a) Schematic representation of the SCOFET device
structure. (b) Transfer curve for a crystal exhibiting the HT
polymorph. (c) Transfer curve for a crystal exhibiting the LT
polymorph.
of highly doped silicon for the gate electrode, 200 or 300 nm
thermally grown silicon dioxide for the gate electrode, and an
array of gold source and drain contacts. The PVT single crystals
were laminated by hand onto these substrates. The solution-
grown crystals were allowed to crystallize directly onto the
prefabricated testbeds for SAC and DC crystals.
Figure 3. DFT-B3LYP band structure for the relaxed geometry of LT
(left) and HT (right). The points of high symmetry in the first
Brillouin zone are Γ = (0, 0, 0), X = (0.5, 0, 0), Y = (0, 0.5, 0), Z = (0,
0, 0.5), V = (0.5, 0.5, 0), U = (0.5, 0, 0.5), T = (0, 0.5, 0.5) all in
crystallographic coordinates. The zero of energy corresponds to the
top of the valence band.
Figure 2b,c represent typical current−voltage characteristics
for the HT and LT polymorphs, respectively. On the right axis,
in blue, we show the drain current (ID) as a function of the
applied gate-to-source voltage (VGS) for a constant source-drain
voltage (VDS = −40 V). On the left axis, in black, we show the
square-root value of the ID, and the slope of this curve was used
in the determination of the field-effect mobility in the
saturation regime (SI Eq. S1). The device in Figure 2b, with
were found along the 1D slipped stack (Figure 1b), with the
corresponding values for holes and electrons estimated as 76
and 5.5 meV. In the framework of a one-dimensional tight-
binding model, the bandwidths correspond to four times the
transfer integrals, therefore such transfer integrals would result
in valence and conduction bandwidths of 0.3 and 0.02 eV,
115
DOI: 10.1021/cm503439r
Chem. Mater. 2015, 27, 112−118

Chemistry of Materials
Article
respectively. These values are in good agreement with those
obtained directly from the periodic boundary conditions
calculations (0.35 and 0.05 eV, respectively; see Figure 3). As
a result of a relatively large hole electronic coupling, the
effective hole mass along the stacking direction is small, 0.85 m0
(where m0 is the electron mass in vacuum). The other
components of the effective mass tensor are large (>3m0),
which suggests 1D character of the hole transport. The smallest
electron effective mass is calculated to be 4.04 m0 (SI Table
S2).
In the HT polymorph, as a result of a 2D brickwork packing
motif, there are two large transfer integrals (see Figure 1e), t1
and t2 (53 and 50 meV for holes, and 52 and 66 meV for
electrons). The valence and conduction bandwidths estimated
from band-structure calculations are about 0.40 and 0.44 eV.
For this polymorph, two small effective masses are found along
the stacking directions: 1.03 m0 and 2.54 m0 for holes and 1.34
m0 and 3.0 m0 for electrons, which is indicative of an intrinsic
2D transport. For the sake of comparison, we note that the two
smallest components of the hole effective mass in rubrene are
0.78 m0 and 1.95 m0, respectively.48
nonlocal coupling is also not likely.52,53 If the nonlocal coupling
is treated as a perturbation (band-like transport regime), then
the calculations based on the 1D Boltzmann theory (see SI)
indicate that the intrinsic hole mobility can be as large as 6 and
7 cm2 V−1 s−1 in the LT and HT polymorphs, respectively. We
note, however, that the applicability of the Boltzmann theory is
strictly justified only when σ/⟨t⟩ ≪1, therefore, transport
models that can be applied for intermediate values of σ/⟨t⟩ ∼1
need yet to be developed to obtain more accurate values of the
charge carrier mobilities in these and similar systems.
Overall, on the basis of the results of the electronic-structure
calculations, the LT and HT polymorphs are expected to
exhibit very good intrinsic charge-transport properties for holes
with one-dimensional and two-dimensional character, respec-
tively. Thus, the significant difference observed between the
SCOFET mobilities in the LT and HT polymorphs is believed
to be related to the lesser effect played by trap states and
defects on charge transport in systems with higher dimension-
ality.
3. CONCLUSIONS
We have described a novel synthesis to ultrahigh purity TMS-
DBC 1, the growth of polymorph single crystals by solution
and evaporation conditions, the evaluation of these crystals in
SCOFET devices, and the elucidation of the differences in the
intrinsic charge transport of these polymorphs by undertaking a
theoretical analysis of their electronic structure. Two
polymorphs (LT and HT) of TMS-DBC can be formed with
1D and 2D packing motifs, respectively, which can be
generated by controlling the temperature at which crystal
growth occurs. The polymorphs are visually markedly different,
as the LT crystals occur as fine red needles, while the HT
crystals are large yellow platelets. These polymorphs represent
a rare example of a situation where both polymorphs are stable
and do not interconvert to the other crystal structure upon
solvent or thermal annealing. SCOFET studies indicate that the
HT polymorph, with a 2D brickwork crystal packing motif,
exhibits hole mobilities as high as 2.1 cm2 V−1 s−1, while the
hole mobility of the LT polymorph with 1D slipped stacking
packing is considerably lower, 0.028 cm2 V−1 s−1. The
electronic-structure calculations point to a single large hole
electronic coupling in the LT polymorph and two large hole
electronic couplings in the HT polymorph. The higher mobility
of the HT polymorph is likely related to the better crystal
quality and the availability of alternative pathways for charge
carriers encountering defects or trap states. The current work
highlights further opportunities for functionalized PAH semi-
conductors systems for use in organic electronics applications,
as well as the need for further investigations of conduction in
single crystals with different dimensionalities of the crystal
packing.
In addition to electronic interactions, the charge-transport
properties also depend on electron−phonon interactions. In a
previous work, we have shown that the local electron-phonon
interaction (the modulation by vibrations of the site energy) in
TMS-DBC like systems29 leads to polaron binding energy for
holes of about 80 meV. This value is smaller than the half-width
of the valence bands in both LT and HT polymorphs implying
that formation of small molecular polarons in these systems is
not favorable. It is also important to note that the main
contribution to the polaron binding energy results from the
interaction with high-frequency molecular vibrations whose
excited levels are thermally inaccessible even at room
temperature, therefore it is expected that the main role of
local electron−phonon interactions is to moderately reduce
(renormalize) the electronic couplings.14
There is currently a growing consensus that a second
electron-vibration mechanism referred to as the nonlocal
coupling mechanism plays a dominant role in organic
semiconductors. This mechanism is due to the dependence of
the transfer integrals on the distances between adjacent
molecules and their relative orientations.14 The overall strength
of the nonlocal electron−phonon coupling at a given
temperature is given by the variance of the transfer integral
due to thermal fluctuation, σ (see SI).49−51 A combination of
molecular dynamics simulations and quantum-chemical calcu-
lations was used to compute the probability distribution of the
transfer integrals. From this distribution, we then estimated σ
and ⟨t⟩ (Table 2, SI Table S3), here ⟨···⟩ represents the average
over geometrical configurations resulting from thermal motion.
For the largest hole coupling, the calculations show that the
ratio σ/⟨t ⟩ is 0.3 for the HT polymorph and 0.4 for the LT
polymorph. This is smaller than unity in both polymorphs
implying that formation of self-localized polarons due to
ASSOCIATED CONTENT
* Supporting Information
■
S
Detailed synthetic experimental procedures; full character-
ization data of TMS-DBC; differential scanning calorimetry
(DSC) data; single crystal growth conditions; single crystal X-
ray data; and SCOFET device data. This material is available
free of charge via the Internet at http://pubs.acs.org.
Table 2. Calculated Average Values (⟨th⟩) and Standard
Deviation (σ) of the Transfer Integrals for Holes
⟨th⟩, meV
σh, meV
σh/⟨th⟩
HT t1
HT t2
LT
−77.3
−41.4
−86.8
26.0
32.5
33.2
0.3
0.8
0.4
AUTHOR INFORMATION
Corresponding Authors
*E-mail: coropceanu@gatech.edu.
■
116
DOI: 10.1021/cm503439r
Chem. Mater. 2015, 27, 112−118

Chemistry of Materials
Article
(21) Reese, C.; Bao, Z. Mater. Today 2007, 10 (3), 20−27.
*E-mail: jurchescu@wfu.edu.
*E-mail: Gavin.Collis@csiro.au.
Notes
(22) Hasegawa, T.; Takeya, J. Sci. Technol. Adv. Mater. 2009, 10,
024314.
(23) Vehoff, T.; Baumeier, B.; Troisi, A.; Andrienko, D. J. Am. Chem.
The authors declare no competing financial interest.
Soc. 2010, 132, 11702−11708.
(24) Yi, H. T.; Chen, Y.; Czelen, K.; Podzorov, V. Adv. Mater. 2011,
23, 5807−5811.
ACKNOWLEDGMENTS
■
(25) McGarry, K. A.; Xie, W.; Sutton, C.; Risko, C.; Wu, Y.; Young,
This work was supported by the Flexible Electronics Theme
and is part of the CSIRO Future Manufacturing Flagship. We
acknowledge financial support from the CSIRO Office of the
Chief Executive program for Y.S. and G.C. Work at WFU was
supported by the National Science Foundation, under Grant
ECCS 1254757 and GRFP DGE-0907738. The work at
Georgia Tech was supported in part by the National Science
Foundation under Award No. DMR-1105147. Data for X-ray
structure determination were collected on the MX2 beamline at
the Australian Synchrotron, Victoria, Australia.
́
V. G., Jr.; Bredas, J.-L.; Frisbie, C. D.; Douglas, C. J. Chem. Mater.
2013, 25 (11), 2254−2263.
(26) Mitsui, C.; Okamoto, T.; Matsui, H.; Yamagishi, M.; Matsushita,
T.; Soeda, J.; Miwa, K.; Sato, H.; Yamano, A.; Uemura, T.; Takeya, J.
Chem. Mater. 2013, 25, 3952−3956.
(27) Jurchescu, O. D.; Subramanian, S.; Kline, R. J.; Hudson, S. D.;
Anthony, J. E.; Jackson, T. N.; Gundlach, D. J. Chem. Mater. 2008, 20,
6733−6737.
(28) Gershenson, M. E.; Podzorov, V.; Morpurgo, A. F. Rev. Mod.
Phys. 2006, 78 (3), 973−989.
(29) Zhang, L.; Fonari, A.; Zhang, Y.; Zhao, G.; Coropceanu, V.; Hu,
W.; Parkin, S. R.; Bredas, J. L.; Briseno, A. L. Chem.Eur. J. 2013, 19,
17907−17916.
REFERENCES
(1) Murphy, A. R.; Frec
1096.
■
́
het, J. M. J. Chem. Rev. 2007, 107, 1066−
(30) Jurchescu, O. D.; Popinciuc, M.; van Wees, B. J.; Palstra, T. T.
M. Adv. Mater. 2007, 19, 688−692.
(2) Pron, A.; Gawrys, P.; Zagorska, M.; Djurado, D.; Demadrille, R.
Chem. Soc. Rev. 2010, 39, 2577−2632.
(3) Klauk, H. Chem. Soc. Rev. 2010, 39, 2643−2666.
(4) Wang, C.; Dong, H.; Hu, W.; Liu, Y.; Zhu, D. Chem. Rev. 2012,
112 (4), 2208−2267.
(5) Braga, D.; Horowitz, G. Adv. Mater. 2009, 21, 1−14.
(6) Yuan, Y.; Giri, G.; Ayzner, A. L.; Zoombelt, A. P.; Mannsfeld, S.
C. B.; Chen, J.; Nordlund, D.; Toney, M. F.; Huang, J.; Bao, Z. Nat.
Commun. 2014, 5, 3005.
(7) Giri, G.; Verploegen, E.; Mannsfeld, S. C. B.; Atahan-Evrenk, S.;
Kim, D. H.; Lee, S. Y.; Becerril, H. A.; Aspuru-Guzik, A.; Toney, M. F.;
Bao, Z. Nature 2011, 480 (22), 504−509.
(8) Minemawari, H.; Yamada, T.; Matsui, H.; Tsutsumi, J.; Haas, S.;
Chiba, R.; Kumai, R.; Hasegawa, T. Nature 2011, 475, 364−367.
(9) Sundar, V. C.; Zaumseil, J.; Podzorov, V.; Menard, E.; Willett, R.
L.; Someya, T.; Gershenson, M. E.; Rogers, J. A. Science 2004, 303,
1644−1646.
(10) Nakayama, K.; Hirose, Y.; Soeda, J.; Yoshizumi, M.; Uemura, T.;
Uno, M.; Li, W.; Kang, M. J.; Yamagishi, M.; Okada, Y.; Miyazaki, E.;
Nakazawa, Y.; Nakao, A.; Takimiya, K.; Takeya, J. Adv. Mater. 2011,
23, 1626−1629.
(11) Takeya, J.; Yamagishi, M.; Tominari, Y.; Hirahara, R.; Nakazawa,
Y.; Nishikawa, T.; Kawase, T.; Shimoda, T.; Ogawa, S. Appl. Phys. Lett.
2007, 90, 102120.
(31) Winzenberg, K. N.; Kemppinen, P.; Fanchini, G.; Bown, M.;
Collis, G. E.; Forsyth, C. M.; Hegedus, K.; Singh, T. B.; Watkins, S. E.
Chem. Mater. 2009, 21, 5701−5703.
(32) Cao, Y.; Liang, Y.; Zhang, L.; Osuna, S.; Hoyt, A.-L. M.; Briseno,
A. L.; Houk, K. N. J. Am. Chem. Soc. 2014, 136, 10743−10751.
(33) Zhang, L.; Fonari, A.; Liu, Y.; Hoyt, A.-L. M.; Lee, H.; Granger,
D.; Parkin, S.; Russell, T. P.; Anthony, J. E.; Bredas, J. L.; Coropceanu,
V.; Briseno, A. L. J. Am. Chem. Soc. 2014, 136 (26), 9248−9251.
(34) Takeuchi, M.; Tuihiji, T.; Nishimura, J. J. Org. Chem. 1993, 58,
7388−7392.
(35) Fritz, S. E.; Martin, S. M.; Frisbie, C. D.; Ward, M. D.; Toney,
M. F. J. Am. Chem. Soc. 2004, 126, 4084−4085.
(36) Mattheus, C. C.; Dros, A. B.; Baas, J.; Oostergetel, G. T.;
Meetsma, A.; de Boer, J. L.; Palstra, T. T. M. Synth. Met. 2003, 138,
475−481.
(37) Cheng, H.-L.; Lin, J.-W. Cryst. Growth Des. 2010, 10, 4501−
4508.
(38) Jurchescu, O. D.; Mourey, D. A.; Subramanian, S.; Parkin, S. R.;
Vogel, B. M.; Anthony, J. E.; Jackson, T. N.; Gundlach, D. J. Phys. Rev.
B 2009, 80, 085201.
(39) Yu, L.; Li, X.; Pavlica, E.; Koch, F. P. V.; Portale, G.; da Silva, I.;
Loth, M. A.; Anthony, J. E.; Smith, P.; Bratina, G.; Kjellander, B. K. C.;
Bastiaansen, C. W. M.; Broer, D. J.; Gelinck, G. H.; Stingelin, N. Chem.
Mater. 2013, 25, 1823−1828.
(40) Chen, J.; Shao, M.; Xiao, K.; Rondinone, A. J.; Loo, Y.-L.; Kent,
P. R. C.; Sumpter, B. G.; Li, D.; Keum, J. K.; Diemer, P. J.; Anthony, J.
E.; Jurchescu, O. D.; Huang, J. Nanoscale 2014, 6, 449−456.
(41) Pan, H.; Liu, P.; Li, Y.; Wu, Y.; Ong, B. S.; Zhu, S.; Xu, G. Adv.
Mater. 2007, 19, 3240−3243.
(12) Shirota, Y.; Kageyama, H. Chem. Rev. 2007, 107, 953−1010.
(13) Dodabalapur, A. Mater. Today 2006, 9 (4), 24−30.
(14) Coropceanu, V.; Cornil, J.; da Silva Filho, D.; Olivier, Y.; Silbey,
R.; Bredas, J.-L. Chem. Rev. 2007, 107, 926−952.
́
(15) Mas-Torrent, M.; Rovira, C. Chem. Rev. 2011, 111 (8), 4833−
4856.
(16) Sokolov, A. N.; Atahan-Evrenk, S. A.; Mondal, R.; Akkerman, H.
(42) Pfattner, R.; Mas-Torrent, M.; Bilotti, I.; Brillante, A.; Milita, S.;
Liscio, F.; Biscarini, F.; Marszalek, T.; Ulanski, J.; Nosal, A.; Gazicki-
Lipman, M.; Leufgen, M.; Schmidt, G.; Molenkamp, L. W.; Laukhin,
V.; Veciana, J.; Rovira, C. Adv. Mater. 2010, 22, 4198−4206.
(43) Salammal, S. T.; Balandier, J.-Y.; Arlin, J.-B.; Olivier, Y.; Lemaur,
V.; Wang, L.; Beljonne, D.; Cornil, J.; Kennedy, A. R.; Geerts, Y. H.;
Chattopadhyay, B. J. Phys. Chem. C 2014, 118, 657−669.
(44) Lercher, C.; Resel, R.; Balandier, J.-Y.; Niebel, C.; Geerts, Y. H.;
Sferrazza, M.; Gbabode, G. J. Cryst. Growth 2014, 386, 128−134.
(45) Giri, G.; Park, S.; Vosgueritchian, M.; Shulaker, M. M.; Bao, Z.
Adv. Mater. 2014, 26, 487−493.
́
B.; Sanchez-Carrera, R. S.; Granados-Focil, S.; Schrier, J.; Mannsfeld, S.
C. B.; Zoombelt, A. P.; Bao, Z.; Aspuru-Guzik, A. Nat. Commun. 2011,
2, 437.
(17) Burke, K. B.; Shu, Y.; Kemppinen, P.; Singh, B.; Bown, M.; Liaw,
I. I.; Williamson, R. M.; Thomsen, L.; Dastoor, P.; Belcher, W.;
Forsyth, C.; Winzenberg, K. N.; Collis, G. E. Cryst. Growth Des. 2012,
12, 725−731.
(18) Shu, Y.; Collis, G. E.; Dunn, C. J.; Kemppinen, P.; Winzenberg,
K. N.; Williamson, R. M.; Bilic, A.; Singh, T. B.; Bown, M.; McNeill, C.
R.; Thomsen, L. J. Mater. Chem. C 2013, 1, 6299−6307.
(19) Zhang, F.; Hu, Y.; Schuettfort, T.; Di, C.; Gao, X.; McNeill, C.
R.; Thomsen, L.; Mannsfeld, S. C. B.; Yuan, W.; Sirringhaus, H.; Zhu,
D. J. Am. Chem. Soc. 2012, 135, 2338−2349.
(20) Shu, Y.; Mikosch, A.; Winzenberg, K. N.; Kemppinen, P.;
Easton, C. D.; Bilic, A.; Forsyth, C. M.; Dunn, C. J.; Singh, T. B.;
Collis, G. E. J. Mater. Chem. C 2014, 2, 3895−3899.
(46) Laudise, R. A.; Kloc, C.; Simpkins, P. G.; Siegrist, T. J. Cryst.
Growth 1998, 187, 449−454.
(47) Goetz, K. P.; Li, Z.; Ward, J. W.; Bougher, C.; Rivnay, J.; Smith,
J.; Conrad, B. R.; Parkin, S. R.; Anthopoulos, T. D.; Salleo, A.;
Anthony, J. E.; Jurchescu, O. D. Adv. Mater. 2011, 23, 3698−3703.
117
DOI: 10.1021/cm503439r
Chem. Mater. 2015, 27, 112−118

Chemistry of Materials
Article
(48) Li, Y.; Coropceanu, V.; Bredas, J. L. J. Phys. Chem. Lett. 2012, 3,
3325−3329.
(49) Delgado, M. C. R.; Pigg, K. R.; da Silva Filho, D. A.; Gruhn, N.
E.; Sakamoto, Y.; Suzuki, T.; Osuna, R. M.; Casado, J.; Hernandez, V.;
Navarrete, J. T. L.; Martinelli, N. G.; Cornil, J.; Sanchez-Carrera, R. S.;
Coropceanu, V.; Bred
(50) Troisi, G.; Orlandi, J. J. Phys. Chem. A 2006, 110, 4065−4070.
(51) Yi, Y. P.; Coropceanu, V.; Bredas, J. L. J. Chem. Phys. 2012, 137,
164303−6.
́
as, J. L. J. Am. Chem. Soc. 2009, 131, 1502−1512.
́
(52) Fratini, S.; Ciuchi, S. Phys. Rev. Lett. 2009, 103, 266601.
(53) Li, Y.; Yi, Y. P.; Coropceanu, V.; Bredas, J. L. Phys. Rev. B 2012,
85, 245201.
118
DOI: 10.1021/cm503439r
Chem. Mater. 2015, 27, 112−118