A highly emissive distyrylthieno[3,2-b]thiophene based red luminescent organic single crystal: Aggregation induced emission, optical waveguide edge emission, and balanced ambipolar carrier transport

Article abstract of DOI:10.1016/j.orgel.2016.04.001

A highly emissive red luminescent single crystal which shows aggregation induced emission (AIE) property and optical waveguide edge emission based on small organic functional molecule, cyano-substituted 2,5-di((E)-styryl)thieno[3,2-b]thiophene (CNP2V2TT) has been prepared by the physical vapor transport (PVT) method. The fluorescence quantum efficiency of crystal is up to 37% and an emission peak maximum (λ_max) locates at 645 nm. Cystallographic data indicate that uniaxially oriented molecular packing with slipped face-to-face π-π stacking forms by the hydrogen bonding network among CNP2V2TT molecules. The single crystal FET devices were fabricated using Au and Ca as hole and electron injection electrodes, respectively. The molecular design, introducing cyano groups into molecular skeleton, effectively lower the LUMO level and achieve well-balanced ambipolar electron (0.13 cm² V^-1 s^-1) and hole (0.085 cm² V^-1 s^-1) mobilities.

Full text of DOI:10.1016/j.orgel.2016.04.001

Organic Electronics 34 (2016) 23e27
Contents lists available at ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
A highly emissive distyrylthieno[3,2-b]thiophene based red
luminescent organic single crystal: Aggregation induced emission,
optical waveguide edge emission, and balanced ambipolar carrier
transport
Shuai Mu a, Kazuaki Oniwa b, Tienan Jin b, Naoki Asao b, Masahiro Yamashita a,
,
*
Shinya Takaishi a
a Department of Chemistry, Graduate School of Science, Tohoku University, 6-3 Aramaki-Aza-Aoba, Aoba-ku, Sendai, Miyagi, 980-8578, Japan
b WPI-Advanced Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
A highly emissive red luminescent single crystal which shows aggregation induced emission (AIE)
property and optical waveguide edge emission based on small organic functional molecule, cyano-
substituted 2,5-di((E)-styryl)thieno[3,2-b]thiophene (CNP2V2TT) has been prepared by the physical va-
por transport (PVT) method. The fluorescence quantum efficiency of crystal is up to 37% and an emission
peak maximum (lmax) locates at 645 nm. Cystallographic data indicate that uniaxially oriented molecular
Received 30 January 2016
Received in revised form
19 March 2016
Accepted 1 April 2016
packing with slipped face-to-face
p-p stacking forms by the hydrogen bonding network among
CNP2V2TT molecules. The single crystal FET devices were fabricated using Au and Ca as hole and electron
injection electrodes, respectively. The molecular design, introducing cyano groups into molecular skel-
Keywords:
Red luminescence
Highly emissive crystal
Aggregation induced emission (AIE)
Optical waveguide edge emission
Cyano-substituted
eton, effectively lower the LUMO level and achieve well-balanced ambipolar electron (0.13 cm2 ꢀ1 sꢀ1
V )
and hole (0.085 cm2 Vꢀ1 ꢀ1) mobilities.
s
© 2016 Elsevier B.V. All rights reserved.
Balanced ambipolar carrier transport
1. Introduction
possess unusual high luminescence in the solid state but hardly
emissive in the solution state, which gives a promising strategy to
design highly emissive molecular solid for optoelectronic and
photonic applications.
Development of highly emissive materials in the solid state is a
crucial prerequisite for optoelectronic and photonic devices and has
become a hot topic of scientific research in the past decades [1e3].
In RGB (red, green, blue) luminescence, photoluminescence (PL)
quantum yield of red color is normally much lower than that of blue
and green colors. Red luminescence comes from the luminophor
with a narrow HOMO-LUMO energy gap, which is easy to result in
non-radiative recombination of excitons. On the other hand, sig-
nificant aggregation-caused quenching (ACQ) effect also renders
the red luminophor weakly emissive. An opposite effect named
aggregation induced emission (AIE) was found by Tang group [4].
AIE effect originates from the restriction of bond rotation in the
aggregation state, which can reduce the nonradiative loss and
result in the luminescence enhancement [5,6]. AIE molecules
Recently, the single crystal (SC) of linear
p-conjugated organic
molecule attracts much attention due to its distinct merits such as
few grain boundaries and defects which contribute to achieve good
charge transport performance [7,8]. In addition, some single crys-
tals based on linear p-conjugated organic molecule exhibit optical
waveguide edge emission, which is determined by the well-defined
transition dipole alignment in the crystal [9e12]. Unlike amor-
phous materials, organic single crystals exhibit various regular
intermolecular aggregation manners such as H-aggregate or J-
aggregate. It is well known that organic crystal exhibits distinctive
shift in absorption spectrum and emission spectrum depending on
the aggregation manner which characterizing transition dipole
alignment in the crystal [13e16]. Therefore, small change in ag-
gregation manner can induce large difference in optical properties
of organic single crystal, which gives a unique horizon to modulate
optical properties of organic single crystal by careful design of
* Corresponding author.
E-mail address: takaishi@mail.tains.tohoku.ac.jp (S. Takaishi).
http://dx.doi.org/10.1016/j.orgel.2016.04.001
1566-1199/© 2016 Elsevier B.V. All rights reserved.

24
S. Mu et al. / Organic Electronics 34 (2016) 23e27
molecular features. An effective route to modulate aggregation
manner is to introduce cyano groups into molecular skeleton.
Cyano groups can form CeH/N hydrogen bonding which can
effectively change the typical herringbone stacking motif of organic
single crystal. To date, some researches introduce cyano-
substitution into the vinylene unit or the terminal phenyls of mo-
lecular skeleton, giving rise to different aggregation manners, have
been reported [17e20]. To the best of our knowledge, highly
emissive red-emitting materials with confined emitting properties
for optoelectronic devices are very limited.
In this work, we report on synthesis, crystal structure and
physical properties in a novel molecular crystal of CNP2V2TT.
CNP2V2TT single crystal exhibits high quality red luminescence and
aggregation induced emission (AIE) property with a very high
photoluminescence quantum yield up to 37%. CNP2V2TT single
crystal also exhibits optical waveguide edge emission. By intro-
ducing cyano groups into the molecular skeleton, the strong
hydrogen bonding network forms and plays a crucial role in the
molecular packing and optical properties of CNP2V2TT crystal. On
the other hand, cyano group is a strong electron withdrawing
group, which can significantly lower LUMO level and improve the
electron injection performance. We fabricated single crystal field
effect transistor (SC-FET) devices using Au and Ca as hole and
electron injection electrodes and balanced ambipolar carrier
transport was achieved.
solvent. Anal. Calcd for C24H14N2S2: C, 73.07; H, 3.58. N, 7.10; Found:
C, 72.94; H 3.91, N 7.08.
2.2. Crystal growth
Thin platelet crystals of CNP2V2TT were grown by physical va-
por transport (PVT) method. Ultrapure argon was used as the car-
rier gas. TG/DTA measurement exhibits high heat stability of
CNP2V2TT crystal over 350 ꢁC (see Fig. S1 in the Supporting data).
To grow high quality crystals, different temperature zones were
used. The high temperature zone (330 ꢁC) was used as the sub-
limating region, while the low temperature zone (250 ꢁC) was used
for crystal growth.
2.3. Instrumental process
Optical absorption spectrum in diluted solution was measured
using Shimadzu UV-3100 spectrometer. We obtained absorption
spectrum of CNP2V2TT crystal by transforming diffuse reflection
spectrum of CNP2V2TT microcrystal doped BaSO4 film measured
on the Shimadzu UV-3100 spectrometer coupled with an inte-
grating sphere. In details, BaSO4 was used as 100% reflection stan-
dard, which was filled into the sample cell and compressed. Then
several milligrams of platelet CNP2V2TT crystals obtained by PVT
method were carefully ground with a suitable amount of BaSO4 in
order to make CNP2V2TT microcrystals homogeneously distributed
in BaSO4 and compressed into a thin film on the BaSO4 substrate. By
using integrating sphere, we measured diffuse reflection spectrum
of the sample and then transformed it into absorption spectrum.
Fluorescence spectra in the diluted state were measured on a JASCO
FP-8300 spectrophotometer. Fluorescence spectra in the solid state
and absolute quantum efficiencies were measured by a photon-
counting method using an integration sphere (C9920-2, Hama-
matsu Co., Japan). Single crystals were grown by a physical vapor
transport (PVT) method under argon gas atmosphere using a sep-
aration temperature controller (AMF-9P-III, ASAHI RIKA). Single-
crystal crystallographic data were collected on a Rigaku Saturn70
2. Experimental section
2.1. Synthesis
The synthetic route of CNP2V2TT molecule is shown as Scheme
1. To a solution of thieno[3, 2-b]thiophene 1 (0.56 g, 4 mmol) and in
dry Et2O 50 ml, n-butyl lithium in n-hexane (6 ml, 10 mmol, 1.6 M)
was added dropwise under an argon atmosphere at 0 ꢁC. The stir-
ring was continued for 2 h and subsequently DMF (~1 ml, 13 mmol)
was added and stirred for additional 1 day at ambient temperature.
The solution was quenched with diluted aqueous HCl under
vigorous stirring. The precipitate was filtered and dried to give a
yellow solid 2 (354 mg, 1.8 mmol, yield 45%), which was practically
pure and can be used to the next reaction. 1H NMR (500 MHz,
CCD diffractometer with graphite-monochromated Mo K
a radia-
tion (
l
¼ 0.71075 Å) produced using a VariMax micro-focus X-ray
rotating anode source at 93 K. PXRD measurements were per-
formed at room T in air on a BRUKER D2 PHASER. Elemental
analysis was measured on J-SCIENCE Lab JM-10 and YANAKOHNS-
ah/HSU-20 in Research and Analytical Center for Giant Molecules,
Tohoku University. The TGA analyses were performed on a SHI-
MADZU DTG-60H instrument at 10 ꢁC minꢀ1 under a nitrogen at-
mosphere. The FET transport characteristics were carried out inside
a N2-filled glove box by using an Agilent Technology B1500A
semiconductor parameter analyzer.
CDCl3) d8.01 (s, 2H, AreH), 10.05 (s, 2H, CHO); CNP2V2TT was
synthesized via Knoevenagel condensation. To a solution of com-
pound 2 (100 mg, 0.5 mmol) and benzylcyanide (113 mg, 1.1 mmol)
in 20 ml THF, a solvent of potassium tert-butoxide (1 mg/ml) in
20 ml tert-butoxide was added dropwise within 5 min, then 4 drops
(~0.2 ml) of tetra-n-butylammonium hydroxide (1 M in methanol)
were added. The mixture stirred at ambient temperature for 10 h.
The orange precipitate was collected by filtration and washed with
methanol. The material was further purified by three sublimations
to afford CNP2V2TT as a red solid (36 mg, 0.076 mmol, yield 18%).
CNP2V2TT exhibits poor solubility in commonly used organic
3. Result and discussion
3.1. Optical properties
Optical properties of CNP2V2TT were characterized in solution
and solid states, respectively (see Fig. 2). The absorption peak of
CNP2V2TT molecule in the solution locates at 423 nm. The ab-
sorption peak of CNP2V2TT microcrystals doped BaSO4 film ex-
hibits an obvious blue shift, which locates at 400 nm. CNP2V2TT
exhibits single molecule state in the diluted solution while H-
aggregate in the CNP2V2TT microcrystal doped BaSO4 film based on
single crystal structure. According to excitonic coupling in dimers,
in H-aggregate case, the lower one of two excited states to the
ground state is optically transition forbidden, which means more
excitation energy is demanded for the absorption of crystal against
single molecule. (see inset of Fig. 2). In order to show that the
Scheme 1. Synthetic route of CNP2V2TT.

S. Mu et al. / Organic Electronics 34 (2016) 23e27
25
differences in the absorption spectra in the crystalline state and
solution state is not due to the difference in the molecular
conformation but due to the difference in the aggregation manners,
we calculated UVeVis spectra of CNP2V2TT single molecule in the
optimized conformation (¼ diluted solution) and conformation in
the single crystal, respectively, by TD-DFT method (see Figs. S3 and
S4 in Supporting data). From calculated results, we can find
CNP2V2TT molecule exhibits almost same excitation energy in the
two conformations, indicating that the difference in the absorption
spectra mainly originates from the different aggregation manners
of CNP2V2TT molecule in the diluted solution (single molecule
state) and single crystal (H-aggregate). Emission spectrum of
CNP2V2TT in solution is accompanied with vibronic structure and
two clear emission peaks at 491 nm and 528 nm can be observed.
However, emission spectrum of CNP2V2TT crystal shows unstruc-
tured and emission peak locates at 645 nm. Compared with the
highest emission peak in the solution state (491 nm), emission peak
of CNP2V2TT crystal exhibits a strong bathochromic (red) shift over
0.6 eV against solution, which is obviously larger by comparison
with that in typical herringbone like aggregate. Clear red edge
emission can be observed from photoluminescent image of
CNP2V2TT crystal under 365 nm UV light (see Fig. 1(c)). In solution,
CNP2V2TT shows a very low PL quantum yield less than 1%. In
contrast, CNP2V2TT crystal exhibits strong aggregation-induced
emission (AIE) with a very high photoluminescence (PL) quantum
yield up to 37%. This photoluminescence enhancement is similar
with other twisted conjugated molecules, such as CNDPDSB [21],
CNDSB [22], which show very low luminescence in solution but
strongly enhanced emission in the solid state. In the diluted state,
CNP2V2TT molecule exhibits a twisted conformation (see Fig 1(a)).
In the single crystal, the tight intermolecular packing in the solid
state restricts the bond rotation and gives rise to nearly planar
molecular conformation (see Fig. 1(b)), which plays a crucial role to
the aggregation induced emission (AIE) behavior.
planar molecular conformation and. molecules stack into a slipped
column by efficient face to face interaction with a short inter-
p-p
plane distance of 3.47 Å (see Fig. 3(c)). As cyano groups are intro-
duced, strong N/HeC hydrogen bonds (red dotted lines) forms
between the cyano group and two hydrogen atoms from the
vinylene unit and the terminal phenyls of the neighboring mole-
cule, resulting in uniaxially oriented molecular packing. Fig. 3(a)
shows CNP2V2TTcrystal image under visible light. From the inset of
Figs.1(d) and 3(a), we can find crystal growth exhibits a tendency to
form rhomboid shape, which matches well with the perspective
view of molecular packing in the CNP2V2TTcrystal from the top ab-
plane (see Fig. 2(b)). The powder X-ray diffraction (PXRD) pattern of
the solid samples is shown in Fig. S2 of Supporting data.
Here we consider the origin of large Stokes shift and edge
emission from the structural feature of CNP2V2TTcrystal. The angle
between the molecular long axis of CNP2V2TTand ab-plane is 75.2ꢁ
(see Fig. 3(d)). Based on TD-DFT calculation, the transition dipole
moment of CNP2V2TT molecule is identified to be nearly parallel to
the long axis of the molecule [22e24]. Thus it is reasonable that the
angle
q between the transition dipole moment of CNP2V2TT
molecule and crystal ab-plane is ca. 75.2ꢁ, which indicates the
transition dipole moments are nearly perpendicular to the crystal
3.2. Structure analysis
For a better understanding of optical properties, single crystal
structure of CNP2V2TT was measured. CNP2V2TT crystal shows
triclinic lattice with P-1 space group. Cell parameters are:
a ¼ 3.832(5) Å, b ¼ 5.947(6) Å, c ¼ 20.569(19) Å,
a
¼ 76.44(5)ꢁ,
Fig. 2. Normalized absorption and emission spectra of CNP2V2TT in chloroform
(dashed line) and solid state (solid line). Inset: schematic diagram of excitonic coupling
in H-aggregate.
b
¼ 84.88(5)ꢁ,
g
¼ 84.96(4)ꢁ and Z ¼ 1. CNP2V2TT exhibits nearly
Fig. 1. Molecular conformation in the diluted solution calculated by the Gaussian 09 package [25] (a) and very weak luminescence from diluted chloroform under 365 nm UV lamp
(c). Molecular conformation in the single crystal (b) and clear edge emission from the single crystal (d).

26
S. Mu et al. / Organic Electronics 34 (2016) 23e27
Fig. 3. CNP2V2TT crystal under visible light (a). Crystal structures of CNP2V2TT on the ab-plane (b). Side view and the illustration of intermolecular
p-p stacking (c). Side view of
intermolecular interactions (red dotted line represents the hydrogen bonding; blue plane represents the ab-plane; yellow plane represents the molecular plane of CNP2V2TT) (d).
Inset: the illustration of transition dipole alignment and photons transport pathway in the crystal. (For interpretation of the references to color in this figure caption, the reader is
referred to the web version of this article.)
ab plane. Schematic representation of traveling direction of light is
illustrated in the inset of Fig. 3(d). Waveguide behavior in the
organic single crystal is determined by transition dipole alignment
in the crystal. Because traveling direction of light is perpendicular
to direction of transition dipole moment, by considering Snell's law
and typical refractive index of organic crystals (n ¼ 1.5), as a result
critical angle (qc) at the crystal/air interface is ca. 41.8ꢁ. Thus, this
properties of the CNP2V2TT crystal, we fabricated the single crystal
FET devices (SC-FETs). The SC-FETs employ the top contact/bottom
gate configuration (see Fig. 4(b)). For minimizing electron traps at
the interface between the gate electrode and organic semi-
conductor, the octadecyltrichlorosilane (OTS) buffer layer was
coated on the SiO2 surface by immersing the silicon wafer into the
toluene solution containing OTS for two days at room temperature.
A thin CNP2V2TT single crystal was laminated on the substrate by
electrostatic adhesion. Au and Ca were employed as hole and
electron injection electrodes with thickness of 50 nm. FET mea-
surements were conducted in the glovebox. As structure analysis,
molecular packing is along the a-axis. Thus it is reasonable to make
the conduction channel along the a-axis (between parallel orien-
tation to the short edge and perpendicular orientation to the long
edge of rhomboid-shape crystal, as shown in Fig. 4(c)). Comparing
work function of Au (5.1 eV) and Ca (2.87 eV) with HOMO/LUMO
level of CNP2V2TT calculated by the DFT method (see Fig. 4(a)), the
Schottky barrier for electron injection is slightly smaller than the
hole injection. Fig. 4(d) illustrates the transfer characteristics of
CNP2V2TT single crystal device at various source-drain voltages
(Vds). As predicted, typical V-shape transfer curves for ambipolar
transport clearly indicate in P-channel and N-channel measure-
ments, respectively. Output characteristics from same CNP2V2TT
single crystal device is shown in Fig. 4(e). Both electron and hole
transport are evident in the N-channel and P-channel measure-
ments, respectively. Balanced ambipolar performance with elec-
tron mobility me up to 0.13 cm2 Vꢀ1 sꢀ1 and hole mobility mh up to
crystal meets the criteria for the total reflection because
a > qc,
thereby limiting photons emitted from the edge of the crystal.
According to the simplified Kasha point-dipole model for excitonic
coupling in dimmers [14,16], it regards no excitonic splitting occurs
when
q
¼ 54.7ꢁ called ‘magic angle’, at which electrical dipole-
dipole interaction becomes zero. So it is traditionally regarded
that H-aggregate forms when
gate forms when
is smaller than 54.7ꢁ. In the CNP2V2TT crystal,
the angle
is much larger than 54.7ꢁ, which definitively induces
q
is larger than 54.7ꢁ while J-aggre-
q
q
efficient excitonic splitting, giving rise to H-type aggregate. Then
considering unstructured emission spectrum which originates
from the vibronic coupling between efficient intermolecular
p-p
stacking [16], we can conclude that unusual large bathochromic
(red) shift in emission from crystal against solution contributes to
achieve high quality red luminescence, which is due to some de-
gree of excimeric contributions although crystal exhibits some
degree of slipped stack motif [26,27].
3.3. SC-FET measurement
0.085 cm2 ꢀ1 sꢀ1 were achieved.
V
In organic semiconductors, charge carriers transport along
molecular
(electronic coupling) significantly impact charge transport of
organic semiconductors. H-aggregates with ideal -stack exhibit
p-orbitals. Thus overlap degree of molecular orbitals
4. Conclusion
p
efficient electronic coupling and thus it is reasonable that charge
transport properties can be effectively improved along the molec-
ular packing direction. For clarifying the charge transport
In conclusion, we have developed a high quality red lumines-
cent (lmax ¼ 645 nm) organic single crystal (CNP2V2TT), which
exhibits aggregation induced emission (AIE) property with a very

S. Mu et al. / Organic Electronics 34 (2016) 23e27
27
Fig. 4. DFT-calculated (B3LYP/6-311G) molecular orbitals and HOMO/LUMO levels (a). Schematic representation of the SC-FET (b). Optical photomicrographs of the SC-FET devices
(c). Transfer (d) and output (e) characteristics of the SC-FET (Vg from ꢀ10 to ꢀ120 V and from 10 to 120 V in steps of 10 V. The channel width (W): 141 m and channel length (L):
30 m).
m
m
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high PL quantum yield up to 37%. CNP2V2TT single crystal also
exhibits optical waveguide edge emission. Cystallographic data
indicates the existence of CeH/N hydrogen bonding change the
common herringbone stacking motif to an uniaxially oriented
molecular packing with slipped face to face intermolecular
p-p
stacking. On the other hand, introduction of cyano group into
molecular skeleton can effectively lower the LUMO level and
improve the electron injection behavior. Balanced ambipolar
charge mobilities with me up to 0.13 cm2 Vꢀ1 sꢀ1 and mh up to
0.085 cm2 Vꢀ1 sꢀ1 were achieved from the single crystal FET de-
vices using Au and Ca as hole and electron injection electrodes,
respectively. Our research demonstrates CNP2V2TT sing crystal is a
promising highly emissive red-emitting material with confined
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Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.orgel.2016.04.001.
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