Tailor-made highly luminescent and ambipolar transporting organic mixed stacked charge-transfer crystals: An isometric donor-acceptor approach

Article abstract of DOI:10.1021/ja312197b

We have rationally designed a densely packed 1:1 donor-acceptor (D-A) cocrystal system comprising two isometric distyrylbenzene- and dicyanodistyrylbenzene-based molecules, forming regular one-dimensional mixed stacks. The crystal exhibits strongly red-shifted, bright photoluminescence originating from an intermolecular charge-transfer state. The peculiar electronic situation gives rise to high and ambipolar p-/n-type field-effect mobility up to 6.7 × 10^-3 and 6.7 × 10^-2 cm² V^-1 s^-1, respectively, as observed in single-crystalline OFETs prepared via solvent vapor annealing process. The unique combination of favorable electric and optical properties arising from an appropriate design concept of isometric D-A cocrystal has been demonstrated as a promising candidate for next generation (opto-)electronic materials.

Full text of DOI:10.1021/ja312197b

Article
pubs.acs.org/JACS
Tailor-Made Highly Luminescent and Ambipolar Transporting
Organic Mixed Stacked Charge-Transfer Crystals: An Isometric
Donor−Acceptor Approach
Sang Kyu Park,† Shinto Varghese,‡ Jong H. Kim,† Seong-Jun Yoon,† Oh Kyu Kwon,† Byeong-Kwan An,§
Johannes Gierschner,*,‡ and Soo Young Park*,†
†Center for Suprameolcular Optoelectronic Materials and WCU Hybrid Materials Program, Department of Materials Science and
Engineering, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 151-744, Korea
‡Madrid Institute for Advanced Studies, IMDEA Nanoscience, Calle Faraday 9, Ciudad Universitaria de Cantoblanco, 28049 Madrid,
Spain
§Department of Chemistry, The Catholic University of Korea, Bucheon-si, Geyonggi-do 420-753, Korea
S
* Supporting Information
ABSTRACT: We have rationally designed a densely packed 1:1
donor−acceptor (D−A) cocrystal system comprising two isometric
distyrylbenzene- and dicyanodistyrylbenzene-based molecules,
forming regular one-dimensional mixed stacks. The crystal exhibits
strongly red-shifted, bright photoluminescence originating from an
intermolecular charge-transfer state. The peculiar electronic
situation gives rise to high and ambipolar p-/n-type field-effect
mobility up to 6.7 × 10−3 and 6.7 × 10−2 cm2 V−1 s−1, respectively,
as observed in single-crystalline OFETs prepared via solvent vapor
annealing process. The unique combination of favorable electric and
optical properties arising from an appropriate design concept of
isometric D−A cocrystal has been demonstrated as a promising
candidate for next generation (opto-)electronic materials.
of such D−A cocrystal systems has rarely been achieved,12
1. INTRODUCTION
especially for ambipolar OFETs application.
Over several decades, much interest has been focused on
organic solid-state semiconducting materials, due to their
Organic CT complexes can exhibit two different types of
binary molecular stacking structures, being segregated or mixed
potential use in various (opto-)electronic device applications,
stacks. Most in-depth investigations so far have been focused on
such as organic light emitting diode (OLED),1 organic
the former cases due to their unconventional metallic
photovoltaic devices (OPV),2 and organic field-effect transistors
(super)conductivities.14 Very recently, mixed stacked D−A
(OFETs).3 Among them, ambipolar charge carrier transport in
organic semiconductors4 have raised much attention as
CT complexes have also been reexamined theoretically and
experimentally aiming at their promising features of ferroelec-
potential alternatives for complementary metal oxide semi-
tricity15 and ambipolar semiconductivity.11,16 The electronic
conductors (CMOS) in high-performing memory devices.5
properties of the CT structures are mainly driven by the FMO
However in the majority of the cases, organic semiconductors
show only unipolar charge transport mainly due to the
unbalanced electronic coupling of the frontier molecular
offsets of D and A as well as their specific molecular
arrangement. Thus, careful selection of D/A pair with
orbitals (FMOs).6 Therefore, several different approaches
appropriate FMOs is necessary to achieve efficient CT cocrystal
system. Up to now, only limited numbers of such ambipolar
have been explored to realize ambipolar organic materials,
such as bridged electron donor−acceptor (D−A) moieties,7 p-/
binary CT cocrystals were realized, all being based on
n-channel semiconductor blends or bilayers,8,9 and cocrystal-
bis(ethylenedithio)- (BEDT) or dibenzo- (DB) substituted
lization or coassembly of D and A molecules.10,11 In some
tetrathiafulvalene (TTF) as donor and (fluorinated) tetracya-
noquinodimethane (F2)TCNQ or organometallic and ionic
appropriate cases, such D−A structures form charge-transfer
counter parts as acceptor (Cu[N(CN)2]Br, I).11 Mixed stacked
(CT) complexes, opening a new prospect for realizing next
generation (opto-)electronic applications.12 To this end, many
supramolecular chemists have devoted their efforts to find
predictable D−A stacking structures with prominent CT
interaction from the extensive libraries of organic material
systems.13 In spite of such efforts, (opto-)electronic application
BEDT-TTF−F2TCNQ exhibits distinct ambipolar charge
transport only at low temperatures (2−60 K, metallic transport
Received: December 13, 2012
© XXXX American Chemical Society
A
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>10 K) with drain voltage-dependent mobilities of about 10−3
cm2 V−1 s−1 for holes and electrons (μh/μe: ∼1.5).11b Mixed
stacked DBTTF−TCNQ (prepared by cosublimation) ex-
hibited ambipolar behavior only when specific Fermi-level
tuned electrodes were used.11c,d On the other hand, organic
secondary bonding interactions. It should be noted that this
design concept of isometric D/A pairs is conceptually different
and advanced compared to the recently reported strategy of
“lock arm supramolecular ordering” (LASO),15 since the latter
requires integrating an additional supramolecular self-assembly
motif to the D and A units. Due to the remarkable structural
characteristics of DCS derivatives (i.e., “twist elasticity”),18a
single-crystalline OFETs (SC-OFETs) of the D−A cocrystal
could be easily prepared by solvent vapor annealing (SVA)
from solid-solution type spin-coated film.5b,22 An in-depth
structural, electrical, spectroscopic, and theoretical study
allowed for a full exploration of all relevant structure−property
relationships in this unique class of ambipolar CT cocrystal
systems.
superconducting materials, such as layer-structured κ-(BEDT-
11g
TTF)2Cu[N(CN)2]Br11e,f and α-(BEDT-TTF)2I3
(formed
by electrochemical process) crystals, exhibited negative pressure
induced phase transition (Mott insulating or charge ordered
states, respectively) when placed on SiO2/Si substrate at low
temperature, which resulted in mainly n-type field-effect
transport and also weak ambipolarity depending on surface
states of crystal samples. It has been reported that the
segregated stacked BEDT-TTF−TCNQ (formed by solution
casting) exhibited balanced ambipolar field-effect mobility
∼10−2 cm2 V−1 s−1, even at room temperature,11h while
metal-like behavior was observed in the electron accumulation
mode above 240 K. Very recently, remarkable and promising
electronic properties of mixed stacked D−A CT crystals were
theoretically predicted via quantum chemical calculations by
2. RESULTS AND DISCUSSIONS
2.1. Molecular Design. 4M-DSB (D) and CN-TFPA (A)
were synthesized and characterized according to the procedures
shown in the Experimental Section and Supporting Information
(SI). The substitution pattern of electron-donating (CH3) and
electron-withdrawing (CF3) groups was determined so that: (i)
the luminescence is bright and in the visible; (ii) the solubility
is sufficiently large by using the moderately small DSB
backbone with multiple D or A substituents; (iii) dense
packing is promoted by avoiding long or bulky substituents;
(iv) the structural differences of D and A are as small as
possible to favor mixed stacking; (v) the substituents of D (H,
CH3) and A (CN; CF3) are complementary to promote
alternating π-stacking and extend H-bonding; and (vi) the MO
offset of D and A is maximized. The latter was effectively
achieved by employing both CN and CF3 functionalities in A,
which significantly lowered the FMOs against those in D.
Indeed, density functional theory (DFT, for details see
Experimental Section) calculations suggest stabilization of A
by 1.6 eV (1.7 eV) for the highest(lowest) (un)occupied MOs,
HOMO (LUMO), see Table S1, in qualitative agreement with
our experimental results (see SI). Accordingly, the electron
affinity of A (EAcalc = 3.8 eV) is large enough to provide a
strong driving force for a direct CT from D (ionization
potential IPcalc = 5.2 eV) to A and to promote ambipolar charge
carrier transport when composing a D−A (1:1 stoichiometric)
mixed stacked CT cocrystal.
As a convenient method of realizing solid-state CT structures
from the D and A molecules, we prepared nanoparticle (NP)
suspensions from a 1:1 binary mixture solution of D and A in
tetrahydrofurane (THF) through reprecipitation into water
according to the method reported earlier by us (see
Experimental Section).23 Uniform and stable NP suspension
was obtained, whose absorption and photoluminescence (PL)
spectra were significantly different from those of the pure D or
A NP suspensions, as shown in Figure 2c. In contrast to the
rather small changes in the dilute solution system (Figure 2a),
spectral changes due to the CT interaction and stacking are
very prominent in the NP states. According to the strong CT
interaction, new red-shifted absorption bands of moderate
strength (533, 495 nm) coupled with intense orange PL (570
nm) strongly bathochromically shifted against those of D and A
were newly observed in the D−A NPs. Based on such
promising CT interaction in NPs, we grew high-quality bright
emissive, dark-orange D−A (1:1) binary single crystals with a
strongly elongated hexagonal shape, see Figure 3a.
Bred
́
as and co-workers,16 stressing the significance of super-
exchange along the stacking direction.
Unfortunately, however, ambipolar transporting CT crystals
other than TTF−TCNQ based systems (the latter normally
exhibit ambipolarity at low temperature) have rarely been
demonstrated. While various π-conjugated D−A CT supra-
molecular structures have been investigated aiming at the
(opto-)electronics device applications, only few of them
exhibited ambipolarity, but of rather limited performance.12c
Therefore, it would be of great impact to demonstrate high-
performance ambipolar CT cocrystals at this moment, even
better if combined with bright luminescence, which might pave
the way to advanced optoelectronic devices. Such alternative
concepts require however tailor-made and energetically fine-
tuned D/A pairs. In the past we and others have demonstrated
that dicyanodistyrylbenzene (DCS) represent a class of
molecules which are electronically significantly stabilized
compared to the parent distyrylbenzene counterparts
(DSB)17 and at the same time offer a versatile route to realize
multifunctional, color variable, and brightly luminescent single
crystals.18 Thus, a combination of DSB/DCS derivatives as D/
A pairs could be considered ideal a priori, especially since the
similar chemical structure and size of D and A (i.e., isometric
approach) should promote facile and regular cocrystalliza-
tion.19,20 We thus designed a single-crystalline binary molecular
cocrystal system comprising two isometric DSB- and DCS-
based donor (4M-DSB, Figure 1) and acceptor (CN-TFPA,
Figure 1. Chemical structures of 4M-DSB (D) and CN-TFPA (A).
Figure 1),21 with appropriately tuned FMOs of the D and A
molecules, to demonstrate unconventional bright emission
together with outstanding ambipolar charge transport. As will
be shown, the particular (opto-)electronic features of the newly
designed D−A system are generated by a unique combination
of the molecular electronic properties and the one-dimensional
(1D) densely packed supramolecular arrangement driven by
2.2. Solid State Structure. X-ray analysis of the D−A CT
cocrystal confirmed the 1:1 stoichiometry within a monoclinic
B
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densely packed planar D−A−D−A stacks, which is considered
favorable for both p-/n-type charge transport.
2.3. Electronic and Optical Properties. The consequen-
ces of the close π-stacked D−A arrangements on the electronic
and optical properties are conveniently and reliably elucidated
by DFT calculations, by using the experimental molecular
coordinates obtained from the X-ray structure analysis, see
Figure 4a. It was found that the HOMO and LUMO of the
stacks are totally localized on D and A moieties, respectively, as
implied by the large MO offset between the designed
molecules. Other MOs adjacent to the frontier orbitals,
however, showed some MO delocalization with rather strong
asymmetries.
The lowest electronic transition (S0→S1) arises mainly from
a HOMO→LUMO excitation (Table S2) and thus gives a
significant red-shift against the absorption of A (Figure 2b,c).
Due to the pronounced localized character of the FMOs, the
transition reveals a strong CT character as visualized by the
electron−hole wave function plot in Figure S6.19 Due to the
strong CT character of the electronic transition, the transition
dipole moment (TDM; with oscillator strength of 0.07) is
oriented fairly along the direction of the SDM (Figure 4a), i.e.,
in the b-direction of the crystal. The energy of the transition
(2.67 eV; 464 nm) agrees quite well with experiment, assuming
that the low-lying peaks at 532 nm (2.32 eV) and 494 nm (2.51
eV) of the experimental spectrum (Figure 2c) represent
apparent vibronics of S1, so that the vertical transition might
be around 2.5 eV. The observed band found at 428 nm (2.89
eV) should then correspond to S2, calculated at 3.19 eV (389
nm). The main absorption band (experimentally observed at
360 nm; 3.44 eV, see Figure 2c) is assigned to the S0→S3
transition, calculated at 3.53 eV (351 nm). This transition of
the D−A (1:1) CT cocrystal is described mainly by a
HOMO→LUMO+1 excitation (Table S2), thus essentially
generated by the frontier orbitals of D; hence, the energetic
position of S0→S3 as well as the orientation of the TDM agree
well with the S0→S1 transition of D, as indeed observed in
Figure 2c.
The bright emission (PL quantum yield ΦPL = 0.31) and the
rather well-structured PL spectrum of the D−A (1:1) CT
cocrystal (which becomes especially obvious at low temper-
atures, see Figure 2d) might be a surprise at a first glance, since
systems with significant CT character mostly show unstruc-
tured exciplex-like emissions of low intensity. We attribute the
high PL efficiency of the D−A (1:1) CT cocrystal to the non-
negligible oscillator strength of the S1 state, as discussed above,
and also to the low nonradiative decay often found in single
crystals,24 which is most likely due to the low trapping
probability of the originally formed CT exciton. With respect to
the well-resolved vibronics (which can be correlated to the
respective Raman modes, see Figure S7), it is to be reminded
that the decisive factor for vibronic coupling is the extent by
which the normal coordinates of a considered vibration
coincide with the geometrical change upon electronic de/
excitation. Hence, excimer- or exciplex-like PL spectra with
strongly red-shifted, unstructured features in binary CT
compounds are mainly obtained, when there is a significant
change in the intermolecular separation during electronic de/
excitation.25 This change is suggested to be small in the present
D−A (1:1) CT cocrystal case attributed to the localized nature
of the FMOs. Due to this strong localization, a pronounced
change in the intermolecular coordinate upon electronic de/
excitation is not expected; therefore, intermolecular modes
Figure 2. (a) Normalized UV−vis absorption and PL spectra of 4M-
DSB (D), CN-TFPA (A), and D + A mixture in THF solution (5 ×
10−6 M). (b) Calculated vertical electronic transition energies of D, A,
and a D−A (1:1) dimer (broadened by a Gaussian); for D−A,
transitions are also indicated as bars. (c) Normalized UV−vis
absorption and PL spectra of D, A, and D−A (1:1) NP suspensions
in THF:water (1:9, v:v). (d) PL spectra of the D−A (1:1) CT
cocrystal at rt and 77 K with minimized reabsorption.
system, space group P2(1)/c, see Figure 3. The crystal consists
of regular 1D mixed D−A−D−A stacks with intermolecular
distances of ∼3.36 Å (Figure 3d and S1) along the b-axis,
attributed to the significant CT and subsequent Coulombic
interaction between the D and A molecules, which automati-
cally fulfilled positional locking unlike the recently reported
LASO concept.15 The molecules are practically planar in the
D−A CT cocrystal (Figure S2), consistent with the “twist
elasticity behavior”18a of the β-cyano-functionalized A mole-
cules, which are further driven by intermolecular H-bonding
(Figure S3). For a D−A pair, the electronic offset of the FMOs
gives rise to a static dipole moment (SDM) of 1.22 D as
calculated at the DFT(CAM-B3LYP) level of theory. The SDM
of the D−A pair points in a 45° angle against the long axis of
the molecules, thus oriented exactly along the crystallographic
b-axis, as shown in Figure S4; therefore, in view of ensuring
electroneutrality in the crystalline state, 90° tilt between
neighboring D−A−D−A stacks is well rationalized (Figure
3b), of which the driving force is the enthalpy gain to promote
dense packing. This interstack arrangement is further assisted
by considerable intermolecular H-bonding interactions
(−N···HC−) along the c-axis, similar to the molecular stacking
features in the pure A crystal, see Figure S5.21 This hinge-like
arrangement of adjacent stacks minimizes the electronic
interactions between neighboring stacks and gives rise to
strongly anisotropic quasi-1D electronic characteristics of the
C
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Figure 3. (a) Optical microscope image of D−A (1:1) CT cocrystal under ambient light irradiation (insets: images under 45° rotating polarizer).
(b) Single-crystal XRD structure of the D−A (1:1) CT cocrystal; blue molecule: D, green molecule: A. (c) Counter pitch angle (90° − pitch angle),
and (d) counter roll angle (90° − roll angle) and π−π distance (interlayer distance) measured from single-crystal XRD results.
should have a minor influence in the vibronic structure, thus
mimicking single molecular vibronics, as is apparently the case
shown in Figure 2.
device operation stability, the SVA method allows for a precise
evaluation of the charge transporting characteristics in solution-
grown defect-free single crystals. The red emissive character-
istics of the SVA-grown crystals (Figure 5c) are inherent to the
D−A binary CT cocrystals, as described above, and exclude the
possibility of mixing separate D and A crystals.
2.4. Charge-Transport Properties. The exclusive local-
ization of the FMOs, as discussed above, is in fact an important
prerequisite for favorable ambipolar charge transport. To
estimate the potential of the binary CT cocrystal for electronics
applications, we have calculated the transfer integrals for both
hole and electron (th, te) along the stacking direction (i.e., b-
axis, D−A−D−A) based on single-crystal XRD analysis, as
shown in Figure 4b. The transfer integral values are known to
be strongly governed by the degree of orbital overlap and to
affect the effective mass of the charge carrier.26 Following the
method of Zhu et al,16 we calculated th and te from the MO
differences in D−A−D and A−D−A stacks (Figure 4b), giving
th = 0.034 eV and te = 0.039 eV. The calculated values point to
effective superexchange along the π-stacking direction,16
suggesting efficient and well-balanced ambipolar charge
mobility, which supports the strategy of our CT stack design
concept. Since the mobility scales with t2, the μe is suggested to
be 30% higher than μh within the frame of this rather simplistic
computational model.
Based on these promising results, we have carried out
electrical measurements for the bottom-gate and Au top-
contact SC-OFETs, see Figure 5. For this, we have employed
an efficient and simple solution process, SVA,5b,22 which we use
here to grow D−A (1:1) CT cocrystals based on a ternary
solid−solution type spin-coated film (composed of D, A and
poly(methylmethacrylate), PMMA, as insulating polymer
binding material, see Experimental Section). Here, by exposing
the spin-coated blended layer to organic solvent vapor (e.g.,
dichloromethane, DCM), high-quality D−A (1:1) mixed
stacked single crystals could be fabricated on the surface of a
PMMA film, as shown in Figure 5c. Owing to the well-
established advantages of introducing a polymer insulating
layer, such as surface trap-site coverage and improvement of
As shown in Figure 5a,b, transfer and output characteristics
of D−A (1:1) SC-OFET were measured in both hole-/
electron-enhancement mode at ambient temperature. The V-
shape transfer curves clearly indicate the transition from
ambipolar to unipolar transport with increasing VG in both
hole-/electron-enhancement mode (see Figure 5a). In addition,
diode-like behaviors are apparent at low VG regimes due to the
increased injection of the opposite charge carriers; however,
saturation characteristics for unipolar carrier at high VG regimes
are clearly seen in the output curves (see Figure 5b).27
Measurements along the long crystal axis gave hole and
electron mobilities up to 6.7 × 10−3 cm2 V−1 s−1 (avg. 4.3 ×
10−3 cm2 V−1 s−1) and 6.7 × 10−2 cm2 V−1 s−1 (avg. 5.4 × 10−2
cm2 V−1 s−1), respectively, for which the capacitance values of
SiO2/PMMA layer were separately measured and used, see
Figure S8. Although the experimental ambipolar character (μe/
μh ∼ 12.6) is somewhat less balanced compared to the
theoretical predictions (μe/μh ∼ 1.3), the overall good charge-
transport characteristics both for p/n-channel transport
strongly support our strategy to design ambipolar D−A CT
cocrystals which is ascribed to the superexchange nature along
the stacking direction. As one can expect, the currently
measured ambipolar mobility (μh up to 6.7 × 10−3 cm2 V−1
s−1 and μe up to 6.7 × 10−2 cm2 V−1 s−1) in this work is rather
moderate compared to the large n-channel mobility of pure A
crystal (5.5 × 10−1 cm2 V−1 s−1) in good agreement with the
calculated transfer integral values of D−A mixed stack (th =
0.034 eV and te = 0.039 eV) and pure A arrangement (te =
0.079 eV);21 however it is still good and well-balanced, due to
the fact that the isometric D/A pairs with rationally designed
D
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Figure 5. (a) Transfer and (b) output characteristics of the single-
crystal OFET, fabricated by SVA. ID, VD, VG, and L/W indicate drain
current, drain voltage, gate voltage and length-to-width ratio of the
active channel cocrystal. Inset: schematic drawing of the device
structure. (c) Optical microscope image of a single-crystal OFET (left:
under ambient light, right: under UV).
Figure 4. (a) MO diagram for the D−A (1:1) system as calculated by
DFT(CAM-B3LYP); the main CI contribution for the S1 ← S0
transition is indicated; the orientation of the respective TDM is
depicted on the left. (b) DFT calculated transfer integrals for electrons
(te) and holes (th) along the b-axis based on the energy-splitting
method.16
long crystal axis and thus explains the favorable, efficient, and
ambipolar charge transport in this direction.
substituents on them actually gave rise to the most tight D−A−
D−A packing, revealing high counter role and counter pitch
angles (89.4° and 54.4°, respectively) with close π-stacking
distance (3.36 Å), to ensure excellent ambipolar transporting
characteristics, see Figure 3c,d.
3. CONCLUSION
In conclusion, we have comprehensively investigated the
rational design of a mixed stack single-crystalline CT system
with efficient red luminescence (ΦPL = 31%) and ambipolar
quasi-1D charge-transport properties (μh up to 6.7 × 10−3 cm2
V−1 s−1 and μe up to 6.7 × 10−2 cm2 V−1 s−1). To this end, an
isometric D and A pair with appropriately tuned FMOs was
successfully designed and synthesized; DSB- and DCS-based D
and A molecules, respectively.
Our combined in-depth structural, electric, spectroscopic,
and computational analysis allowed for a full exploration of all
relevant structure−property relationships in this unique system,
which can be summarized as follows:
In order to prove whether the favored charge transport
direction (along the long crystal axis) indeed coincides with the
stacking direction (b-axis), we investigated the absolute
orientation of the molecules with respect to the crystal facets.
Combined out-of-plane XRD measurements (Figure S9a) and
height profile from atomic force microscopy (AFM) (Figure
S9b) evidence that the crystal facet coincides with the (1 0 0)
plane. To determine the orientation of the stacking direction
(b-axis) against the long crystal axis, we carried out polarized
PL experiments (see Experimental Section and Figure S10 for
the details) since the TDM of the emitting dipole is pointing
into the stacking direction (vide supra) and thus provides the
optimal tool to determine the absolute orientation of the b-axis.
According to the results in Figure 6, maximum PL intensity was
observed along the elongated crystal axis (further support came
from pronounced optical birefringence, see inset of Figure 3a).
Thus, the spectroscopic measurements indeed confirmed that
the molecular stack direction along the b-axis coincides with the
(1) The molecules could be self-assembled into a 1:1 D−A
mixed columnar structure driven by the tailor-made
electronic and geometric properties of D and A, i.e.,
isometric geometry, favorable Coulomb and H-bonding
interactions, and the requirements for electroneutrality
and enthalpic arguments.
(2) The lowest excited state in the D−A (1:1) CT cocrystal
is mainly ascribed by a HOMO→LUMO excitation,
E
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Figure 6. (a) measured PL intensity anisotropy of the D−A (1:1) CT cocrystal. (b) Schematic drawings of the molecular packing structure with
respect to the macroscopic crystal morphology; crystallographic axes are indicated.
spectrometer. PL spectra were measured using a Varian, Cary Eclipse
spectrophotometer. PL spectra on D−A (1:1) CT cocrystals were
recorded using an Anton SP2500 series spectrometer equipped with a
nitrogen-cooled CCD camera and with 300 line/mm grating; the
spectra were recorded with minimized self-absorption by proper
crystal adjustment. Polarized measurements were done with a Glan−
Thompson polarizer for the incident laser (405 nm); the analyzer was
varied in intervals of 10°. The ΦPL was measured in an integrating
sphere (Hamamatsu C9920−01). AFM measurements were carried
out in the Nanowizard scanning probe microscope (version 1.3) of
JPK instruments, and the images were obtained in the contact mode
using a soft cantilever.
Theoretical Quantum Chemical Calculation Methods.
Ground-state geometries and MOs of the D and A compounds were
calculated by DFT, imposing C2h symmetry. Vertical EAs and IPs were
done on the radical anions and cations based on the neutral geometry,
using DCM as a solvent as described by the polarizable continuum
model; adiabatic IPs and EAs were obtained on optimized geometries
of the radical ions. Excited states of the single molecular species were
calculated at the TD-DFT level of theory. For all single molecules the
B3LYP functional and 6-311G* basis set were employed as defined in
the Gaussian09 program package.28 D−A dimer and trimer
calculations were done by taking the intermolecular coordinates
from X-ray analysis, replacing the molecules by the DFT-optimized
ones. Single point (TD-)DFT calculations employed the Coulomb-
attenuated method (CAM) variant of the B3LYP functional29 as
implemented in Gaussian09 to correctly account for long-range
interactions. The orbital topologies were plotted with Molekel.30
Device Fabrication and Measurement. For substrates prepara-
tion, SiO2/Si substrates (p-doped 300 nm) were rinsed with acetone
and isopropyl alcohol, respectively, for 10 min in an ultrasonicator,
followed by 15 min UV (360 nm) O3 treatment. D, A compounds and
PMMA (average Mw ∼15 000) were dissolved in 1,2-dichloroethane
(0.5 wt % of PMMA, 0.1 wt % of A and D, 1:1). The solution was
spin-coated at 2500 rpm for 1 min. For the SVA process, 4 mL of
DCM were injected in 8 mL vial and covered with an as-casted film for
30 min to ensure full growth of the cocrystal. For the top-contact SC-
OFETs, 50 nm of Au were thermally deposited through a metal mask
in a vacuum chamber. The I−V characteristics of six individual devices
were measured using a Keithley 4200 SCS. All procedures were carried
out in a N2-filled glovebox.
where the HOMO and LUMO are entirely localized on
the D and A moiety, respectively. The distinct CT
character of the transition gives rise to a strong red-
shifted absorption band with moderately small oscillator
strength which allows for bright and highly polarized red
emission.
(3) Solution-processed single-crystalline OFETs show effi-
cient anisotropic and ambipolar charge carrier transport
behavior, with hole and electron mobilities up to 6.7 ×
10−3 and 6.7 × 10−2 cm2 V−1 s−1. The transport was
proven to occur along the direction of D−A−D−A
mixed stacks promoted by the densely packed, planar
molecules with localized FMOs, which follows a
superexchange mechanism to give rise to high p- and
n-channel behavior.
Summing up the remarkable properties of the investigated
system, our isometric D−A approach opens a new and versatile
route toward highly efficient ambipolar and at the same time
bright luminescent materials, which holds important implica-
tions for next-generation (opto-)electronic applications.
4. EXPERIMENTAL SECTION
Synthesis and Characterization. 1,4-bis(3,5-dimethylstyryl)-
benzene, 4M-DSB, was synthesized by a two-step procedure, as
detailed in the SI. (2Z,2′Z)-3,3′-(1,4-phenylene)bis(2-(3,5-bis-
(trifluoromethyl)phenyl)acrylonitrile), CN-TFPA, was synthesized by
Knoevenagel condensation, as previously reported.21
Sample Preparation. The D−A (1:1) NP suspension (5 × 10−6
mol L−1) was obtained by reprecipitation from THF:water (1:9, v:v).
For this, D and A were dissolved in THF in 1:1 molar ratio, then
distilled water was slowly injected as a poor solvent, left for 2 h before
the optical studies, see Figures S11 and S12. Pure D and A NP
suspensions were prepared accordingly. Single-crystalline bulk D−A
(1:1) CT cocrystals were prepared by solvent diffusion from a 1:1
mixture in DCM/methanol.
Structure Analysis. Single-crystal structure analysis was done with
a SMART-APEX II ULTRA (Bruker). Out-of-plane XRD measure-
ments were performed using PANalytical X’pert PRO system with a
Bragg−Brentano geometry using CuK radiation with a graphite
monochromator on the secondary side.
ASSOCIATED CONTENT
■
General Characterization. 1H NMR was recorded using a Bruker,
Avance-300 (300 MHz) in CDCl3 solution for all materials. 13C NMR
was recorded in CDCl3 on a Bruker Avance-500 (500 MHz).
Elemental analyses were conducted with a EA1110 (CE Instruments).
Mass spectra were measured using a JEOL, JMS-600W spectrometer.
UV−vis absorption spectra were done with a Shimazu UV-1650 PC
S
* Supporting Information
Synthesis and characterization, crystallographic information,
quantum chemical calculation methods and results, exper-
imental and calculated Raman spectra, capacitance value, AFM,
field emission SEM, experimental methods, crystallographic
F
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Journal of the American Chemical Society
Article
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data (CIF file, CCDC 905525). CCDC 905525 contains the
supplementary crystallographic data for this paper. This data
can be obtained free of charge via www.ccdc.cam.ac.uk/data_
request/cif or by emailing data_request@ccdc.cam.ac.uk or
contacting The Cambridge Crystallographic Data Centre, 12,
Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
parksy@snu.ac.kr; johannes.gierschner@imdea.org
(12) (a) Rao, K. V.; George, S. J. Chem.Eur. J. 2012, 18, 14286.
■
(b) Schmidt-Mende, L.; Fechtenkotter, A.; Mullen, K.; Moons, E.;
̈
̈
Friend, R. H.; MacKenzie, J. D. Science 2001, 293, 1119. (c) Percec, V.;
Glodde, M.; Bera, T. K.; Miura, Y.; Shiyanovskaya, I.; Singer, K. D.;
Balagurusamy, V. S. K.; Heiney, P. A.; Schnell, I.; Rapp, A.; Spiess, H.-
W.; Hudson, S. D.; Duan, H. Nature 2002, 419, 384. (d) Sagade, A. A.;
Rao, K. V.; Mogera, U.; George, S. J.; Datta, A.; Kulkarni, G. U. Adv.
Mater. 2013, 25, 559.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This research was supported by Basic Science Research
Program (CRI; RIAMI-AM0209(0417-20090011)) and
World Class University (WCU) project (R31-2008-000-
10075-0) through National Research Foundation of Korea
funded by the Ministry of Education, Science and Technology.
The work in Madrid was supported by the ‘Ministerio de
Economia y Competitividad’ (CTQ2011-27317, RyC-2007-
01559) and Comunidad de Madrid (S2009/MAT-1726,
S2009/PPQ-1533). We thank S. Casado, Madrid, for his
assistance with the AFM and R. Wannemacher, Madrid, for the
help with the low temperature measurements. S.V. thanks for
an Amarout grant of the EC (grant no. 229599).
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