Dicyanovinyl-substituted indolo[3,2-b] indole derivatives: Low-band-gap π-conjugated molecules for a single-component ambipolar organic field-effect transistor

Article abstract of DOI:10.1039/c6tc02777f

A series of low-band-gap π-conjugated molecules comprising N,N′-dihexylindolo[3,2-b]indole as an electron donor (D) and dicyanovinyl as an electron acceptor (A) with A-π-D-π-A architecture have been designed and synthesized to fabricate a single-component

Full text of DOI:10.1039/c6tc02777f

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ARTICLE
Dicyanovinyl-Substituted Indolo[3,2-b]indole Derivatives: Low
Band-Gap π-Conjugated Molecules for Single-Component
Ambipolar Organic Field-Effect Transistor
Received 00th January 20xx,
Accepted 00th January 20xx
Illhun Cho,a Sang Kyu Park,a Boseok Kang,b Jong Won Chung,a Jin Hong Kim,a Won Sik Yoon,a Kilwon
Cho,b and Soo Young Parka*
DOI: 10.1039/x0xx00000x
www.rsc.org/
A series of low band-gap π-conjugated molecules comprising N,N-dihexyl-indolo[3,2-b]indole as an electron donor (D) and
dicyanovinyl as an electron acceptor (A) with A-π-D-π-A architecture, are designed and synthesized to realize single-
component ambipolar organic field-effect transistor (OFET). Molecules with different π-bridging units (none, thiphene,
bithiophene) are synthesized and characterized to explore the structure-property correlation. Through cooperative effects
of intramolecular charge transfer (ICT) interaction and conjugation extension, band-gap of newly synthesized molecules is
reduced down to 1.41 eV in solution state. Among others, 2H2TIDID-DCV (with thiophene π-spacer) exhibits highly
balanced ambipolar charge transport with hole and electron mobilities of 0.08 cm2 V-1 s-1 and 0.09 cm2 V-1 s-1, respectively,
from vacuum-deposited OFET device. The spin-coated OFET devices using OD2TIDID-DCV, for which hexyl side chains of
2H2TIDID-DCV are replaced by 2-octyldodecyl, also exhibits ambipolar charge transporting nature (mobility of 9.67 Ɛ 10-2
cm2 V-1 s-1 for hole, and 3.43 Ɛ 10-3 cm2 V-1 s-1 for electron). Both 2H2TIDID-DCV and OD2TIDID-DCV exhibit favorable thin-
film morphology for charge transporting channel formation, and structure analyses of those films reveal the same
molecular
packing
characteristics
of
three-dimensional
lamellar
π-stacking
structure.
and lowest unoccupied molecular orbital (LUMO) of the
organic semiconductor relative to the work function of
Introduction
symmetric electrode is most essential for the favorable bipolar
8-11
In this regard, wide ranges of low band-gap
Research interests in the ambipolar organic field-effect
transistors (OFETs) have been significantly grown because of
their potential application to the complementary circuits like
inverters without complicated multi-step deposition process.1-2
Especially, single-component ambipolar OFETs simply
comprised of a single-layer active channel with symmetric
source-drain electrodes, have attracted much attention owing
to their expediency in practical device fabrication.2-4
Theoretically, many of organic semiconductors are known to
transport both types of charge carriers in their solid-state, if
and only if charge carriers are efficiently injected.2,5 However,
majority of reported organic semiconductors exhibited
unipolar characteristics, i.e., either hole- or electron-transport
only depending on the work function of the electrodes, mainly
due to the restriction of high injection barrier for one of the
carriers.6-8 Therefore, among various crucial issues in realizing
single-component ambipolar OFETs, balanced energy level
alignment of the highest occupied molecular orbital (HOMO)
injection.2,
organic materials have been extensively investigated, and
some of π-conjugated structures, such as TIPS-pentacenes,12-14
diketopyrrolopyrroles,15-18
indigos,19
isoindigos,20
and
quinoinal oligothiophenes,21-23 have been reported as
promising backbone structures for designing high performance
ambipolar semiconductor, through appropriate molecular
structure engineering.24 In spite of such extensive materials
research on ambipolar OSCs, molecular structural effect on the
operating mechanism of ambipolar OFETs has seldom been
explored. In this regard, exploring structure-property
correlation based on the comprehensive photophysical,
electrochemical, and quantum-chemical calculation studies is
essential in developing high performance organic
semiconductors.
Very recently, we have synthesized indolo[3,2-b]indole (IDID)
derivatives as promising p-type organic semiconductors with
remarkable hole mobility of 0.97 cm2 V-1 s-1 and versatile
processability.25 Besides apparent advantages of pyrrole-fused
structure such as facile solubility control and structural
derivatization, IDID core is characterized by stronger electron
donating nature than those of other reported pyrrole-fused
heteroarene cores (e.g., carbazole and indolocarbazole), owing
to the larger proportion of five-membered pyrrole units in the
backbone. To examine the possible bipolar carrier injection
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and transport, we herein designed and synthesized
intramolecular charge transfer (ICT)-type IDID derivatives
(IDID-DCV derivatives), 2HIDID-DCV, 2H2TIDID-DCV, and
2H4TIDID-DCV, which comprise the IDID as an electron donor
(D) and dicyanovinyl (DCV) as an electron acceptor (A) with A-
π-D-π-A type architecture. Three different π-linking units
(none, thiphene, bithiophene) were incorporated to explore
different degrees of ICT interaction and π-conjugation, which
enabled both molecular structure-property correlation and
precise control of electronic characteristics. To elaborate in-
depth analysis of structure-property correlation, we also
prepared p-type reference molecules, 2HIDID, 2H2TIDID, and
2H4TIDID, with which comprehensive studies were conducted
in terms of their theoretical, photophysical, electrochemical,
morphological, electrical, and structural characteristics. Among
others, 2H2TIDID-DCV (with thiophene π-linker) exhibited
dramatically reduced energy band-gap (1.62 eV in solution
state) as well as excellent film morphology, which afforded
highly balanced ambipolar charge transport with hole and
electron mobilities of 0.08 cm2 V-1 s-1 and 0.09 cm2 V-1 s-1,
respectively, in the vacuum-deposited (VD) OFET devices.
Aiming at
a solution processable IDID-DCV derivative,
OD2TIDID-DCV was also synthesized for which hexyl side
chains of 2H2TIDID-DCV was replaced by 2-octyldodecyl group.
The spin-coated OFET device of OD2TIDID-DCV also exhibited
Scheme 1 a) Synthesis route for IDID-DCV derivatives and their chemical structure ((I)
bis(pinacolato)diborone, Pd2(dppf)2, KOAc, anhydrous dimethylformamide, (II) n-BuLi,
tetrahydrofuran, (III) Pd(PPh3)4, 2N K2CO3, tetrahydrofuran, (IV) Al2O3, methylene
chloride); b) Chemical structure of IDID reference derivatives.
ambipolar charge transport behavior (mobility of 9.67
cm2 V-1 s-1 for hole, and 3.43 10-3 cm2 V-1 s-1 for electron). To
Ɛ
10-2
Ɛ
the best of our knowledge, this is the first report of single-
component ambipolar OFET based on pyrrole-fused
heteroacene family even including the widely studied
indolocarbazole derivatives.
For the comprehensive understanding of newly synthesized
IDID-DCV derivatives in terms of their photophysical
properties, we also prepared three IDID reference materials,
2HIDID, 2H2TIDID, and 2H4TIDID, which have exactly the same
molecular structures with those of 2HIDID-DCV, 2H2TIDID-
DCV, and 2H4TIDID-DCV but without acceptor unit (DCV),
respectively (see Scheme 1, 2H2TIDID and 2H4TIDID were
reported earlier,25 but 2HIDID was newly synthesized in this
work. Detailed synthetic procedure and characterization of
2HIDID is described in Electronic Supplementary Information).
The electronic characteristics of IDID-DCV derivatives were
evaluated through the molecular orbital calculation, UV-vis
spectroscopy, and cyclic voltammetry (CV) measurement.
Ground-state optimized geometries and electronic structures
of IDID derivatives were calculated within the density
functional theory (DFT) B3LYP method by using Gaussian 09
package with basis set of 6-31G d p. According to the
calculated frontier orbital energies shown in Fig. 1, it is noted
that the effect of different π-linking units (none, thiophene,
bithiophene) in IDID-DCV derivatives is quite different from
that in p-type IDID reference derivatives. Because IDID unit
intrinsically holds very low ionization potential, π-conjugation
extension with thiophene unit mostly resulted in the
stabilization of LUMO energies without any HOMO variation in
case of the p-type reference derivatives. While both the
HOMO and LUMO energies of a given IDID-DCV derivative
were strongly stabilized compared to those of respective IDID
reference derivative, the actual degree of stabilization
Result and Discussion
To understand the effect of the ICT interaction and molecular
structure alteration on electronic characteristics of IDID
derivatives, and also to realize ambipolar OSC for practical
application, we rationally designed molecular structure as
follows. Using the DCV unit as a strong electronic acceptor, we
constructed A-π-D-π-A type molecular architecture not only
for manifesting strong ICT interaction but also for enabling
efficient electron injection through lowering LUMO energy
26,27
level,
for which their electronic and intermolecular
interaction characteristics were delicately tuned by altering
the π-linker (none, thiophene, bithiophene). Based on this
idea, we have designed a series of three IDID-DCV derivatives,
2HIDID-DCV, 2H2TIDID-DCV, and 2H4TIDID-DCV (Scheme 1).
Synthesis of 2,7-dibromo-N,N-dihexyl-IDID (compound
25
1
) was
according to our previous report;
from which IDID-DCV
derivatives were successfully synthesized through lithium-
exchange formylation, palladium-catalyzed borylation, Suzuki-
Miyaura cross-coupling, and Knoevenagel-condensation
reaction. All the three target compounds were carefully
characterized by 1H NMR, 13C NMR, elemental analysis, and
mass analysis. Detailed synthetic procedures are described in
Scheme 1 and Experimental Section.
2 | J. Name., 2012, 00, 1-3
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Fig. 1 Plot of calculated energy levels for IDID-DCV derivatives and IDID reference derivatives, and their molecular orbital contour (DFT/B3LYP-6-31G d p, the alkyl chains were
replaced by methyl in calculation).
increased in the order of different π-linking units (none >
thiophene > bithiophene) as seen in Fig. 1. This can be
rationally correlated with the increasing strength of ICT
characteristics with decreasing D-A distance.
For the empirical exploration of electronic transitions, UV-vis
absorption spectra of IDID derivatives were measured at a
concentration of 1
Ɛ
10-5 M in tetrahydrofuran (THF) solutions
and also for spin-coated films (see Fig. 2, detailed data listed in
Table 1). In solution states, all the IDID-DCV derivatives
exhibited broad absorption bands consisting of three
absorption maxima. Interestingly, the location of lowest
energy transition maxima corresponding to the ICT transition
were virtually the same for all of them (544 nm for 2HIDID-
DCV, and 555 nm for 2H2TIDID-DCV and 2H4TIDID-DCV), in
spite of their different π-linker length from each other. This
feature can be comprehended by comparing their absorption
spectra with those of the reference derivatives. As shown in
Fig. 2c, IDID-DCV derivatives show different degree of
bathochromically shifted absorption spectra (∆ λ maxs, see
Table 1) which increases with decreasing D-A distance. The
apparently similar ICT absorption wavelengths among IDID-
DCV derivatives can thus be rationalized by compromised ICT
strength and conjugation length effects. Strength of ICT
interaction between IDID and DCV is getting weaker for the
molecules with larger number of thiophene π-spacer, which at
the same time is compensated by increased π-conjugation
length to give comparable absorption λ maximum. In the solid
states samples of IDID-DCV derivatives, further bathochromic
shifts of 90-100 nm with distinct vibronic features are
observed as shown in Fig. 2b and summarized in Table 1.
To investigate electrochemical properties of IDID-DCV
derivatives, cyclic voltammetry (CV) measurements were made
both in solution states and solid (film) states. The HOMO,
LUMO, and electrochemical band-gap energies which were
extracted from the first oxidation onset and the first reduction
onset of CV curves are listed in Table 1. As shown in Fig. 3, IDID-
Fig. 2 Absorption spectra of IDID-DCV derivatives for solution state (a) and film state
(b), and each solution state absorption spectra with correspondent reference (c).
DCV derivatives exhibited quasi-reversible multistage
electrochemical oxidation and reduction behaviors in THF
solvent. Consistent with the DFT calculation results in Fig. 1,
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Table 1 Optical and electrochemical properties of IDID-DCV derivatives
Compound
Abs. λ max
[sol./film/∆ λ max, nm] a)
544/636/89
∆ λ maxs
[nm] b)
179
HOMO
[sol./film, eV]c)
-5.34/-5.42
-5.08/-5.09
-4.90/-4.99
LUMO
Band-gap
[sol./film, eV]d)
1.81/1.71
[sol./film, eV]c)
-3.51/-3.71
-3.46/-3.86
-3.49/-3.74
2HIDID-DCV
2H2TIDID-DCV
2H4TIDID-DCV
555/657/103
151
1.62/1.23
555/641/89
118
1.41/1.25
a) λ max is extracted from lowest spin-allowed transition maximum, ∆ λ max presents λ max difference between solution state and film state, solution absorption spectra
were measured in THF (concentration of 1Ɛ10-5M), and film absorption spectra were measured with spin-coated sample (1500 RPM/60s, 0.3 wt% in chloroform); b) ∆ λ
c)
maxs presents Abs. λ
difference between IDID-DCV derivatives and correspondent IDID reference in solution state; HOMO and LUMO energy levels were
max
determined by cyclic voltammetry. Solution sample was prepared 3 Ɛ 10-3 M in THF and film sample was prepared on ITO patterned glass by drop-casting; d) band-gap
= Voxidation - Vreduction
.
Based on the photophysical and electrochemical properties
(i.e., satisfactorily low band-gap with suitable HOMO and
LUMO energy in the solid state for efficient bipolar injection
from symmetric source-drain electrodes), two of the IDID-DCV
derivatives, 2H2TIDID-DCV and 2H4TIDID-DCV, were selected
for the evaluation of OFET characteristics. Before the OFET
device fabrication, however, we have examined their VD thin-
film surface topography as a function of substrate temperature
(Tsub) alteration. As shown in Fig. S1, 2H2TIDID-DCV films
showed dense and compact grain features suitable for OFET
fabrication, whilst 2H4TIDID-DCV VD films showed coarse and
unfilled topological characteristics unsuitable for device
evaluation. Therefore, Charge transporting characteristics of
2H2TIDID-DCV was evaluated for its ambipolar mobility in the
geometry of bottom-gate top-contact (BGTC) VD OFET devices.
2H2TIDID-DCV was thermally evaporated and deposited on
octadecyltrichlorosilane (ODTS) treated SiO2/Si substrates at
different Tsub from room temperature (RT) to 140 °C to
optimize the device performance. As previous reported,
2H2TIDID OFET devices exhibited typical p-type OFET
behavior.(25) In contrast, 2H2TIDID-DCV OFET devices showed
ambipolar OFET behavior exhibiting typical V-shaped transfer
curves and unique output curves with superlinear regime as
seen in Fig. 4a to f. With the Tsub increase, hole and electron
mobilities were increased simultaneously (see Fig. S2 and
Table 2).
Fig. 3 Cyclic voltammograms of IDID-DCV derivatives in solution state (a) and film state
(b) (Inset: ferrocene).
In particular, the electron mobility was much more
dramatically enhanced by altering the deposition temperature
10-5 cm2
10-3
LUMO energies of IDID-DCV derivatives were strongly
stabilized and located at similar level (around -3.5 eV) as seen
in Fig. 3 and Table 1. Meanwhile, HOMO energies were
gradually destabilized with increasing π-spacer length; i.e., -
5.34, -5.08, and -4.90 eV. Consequently, we could achieve
quite small band-gap (i.e., down to 1.41 eV) using
compensated ICT interaction between IDID and DCV. In case of
film states, HOMO and LUMO energies of IDID-DCV derivatives
were even further stabilized with higher degree of stabilization
for LUMO energies than HOMO energies to give smaller band-
gap energies. Interestingly, largest solid state LUMO
stabilization to give the smallest band-gap energy of 1.23 eV
was observed for 2H2TIDID-DCV (see Table 1), which attests
the most strong and efficient electronic interaction in the
tightly packed solid state suggesting its excellent OFET mobility
(vide infra).
(nearly four orders of magnitude increase, from 1.5
V-1 s-1 to 9.2 10-2) than that of hole mobility (from 1.8
cm2 V-1 s-1 to 8.2 10-2), and the devices which were
Ɛ
Ɛ
Ɛ
Ɛ
deposited at high Tsub (> 70 °C), showed well balanced hole and
electron mobilities (hole mobility to electron mobility ratio is
0.63 at Tsub of 140 °C). Such different tendency of electron and
hole mobility modulation is most probably due to the higher
sensitivity for the trap site of electron charge carrier than that
of hole charge carrier.28 The mobility increment could be
attributed to the reduced density of trap sites originating from
increased grain size, as was rationalized from surface
topography study using atomic force microscopy (AFM)
measurement. As shown in Fig 5a to f, surface topography of
2H2TIDID-DCV film was transformed from uniform film (root-
mean-square roughness value of 0.7 nm) to highly ordered
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Fig. 4 OFET devices characteristics. (a to f) Representative OFET device characteristics of 2H2TIDID-DCV ((a and c) transfer curves of Tsub 120 °C device, (b and d) output curves Tsub
120 °C device, and (e and f) temperature dependent mobility for p- and n-channel operation, respectively. (white stars represent average value)); (g and j) Spin-coated OFET device
characteristics of OD2TIDID-DCV (annealing temperature is 150 °C, (g and i) transfer curves and (h and j) output curves for p- and n-channel operation, respectively).
synthetic procedures and characterizations are described in
Electronic Supplementary Information). Spin-coated OFET
devices of OD2TIDID-DCV were fabricated and characterized
for the same OFET structure and geometry as the VD ones, but
with the semiconductor material (OD2TIDID-DCV) deposited by
spin-coating method. As shown in Fig. 4g to j, optimized spin-
coated OFET device (post annealing temperature of 150 °C)
exhibited ambipolar charge transport behavior with hole and
electron mobilities of 9.6
Ɛ Ɛ
10-2 cm2 V-1 s-1 and 3.4 10-3 cm2
V-1 s-1, respectively. With respect to the charge carrier mobility
values, solution processed OD2TIDID-DCV devices exhibited
slightly higher hole mobility but relatively lower electron
mobility values than those of 2H2TIDID-DCV. Such different
mobility values are due to the somewhat different π-π
distance, vide infra, and rather smaller grain size compared to
that of optimized VD device (see Fig. 5g). Anyway,
experimentally evidenced ambipolarity in the 2H2TIDID-DCV
and OD2TIDID-DCV devices suggest that 2TIDID-DCV is a
promising π-conjugate backbone for the single-component
ambipolar OFET.
To investigate the molecular packing structure in the organic
semiconductor samples, out-of-plane X-ray diffraction (XRD),
powder XRD, and 2D grazing incidence X-ray diffraction (2D-
GIXD) analyses were carried out. Fig. 6a shows out-of-plane
XRD patterns of 2H2TIDID-DCV thin films. While low Tsub (RT
and 50 °C) samples exhibited quite dim diffraction peaks,
samples which were deposited at high Tsub (over 70 °C)
exhibited sharp and strong diffraction peaks up to the third
(300) order, at 2 theta degrees of 8.37°, 16.78°, and 25.28°;
this could be interpreted as lamellar diffraction with a d-
spacing of 10.58 Å. From the powder XRD, (Fig. 6b) identical
lamellar diffraction peaks with those of out-of-plane XRD could
be observed at almost same 2 theta degrees (8.36°, 16.67°,
and 25.16°), and moreover, we could recognize diffraction
peak at wide angle region (24.9°) which might be interpreted
as π-π distance with a d-spacing of 3.57 Å (inset of Fig. 6b). The
Fig. 5 AFM surface topologies (height images, 5 μm Ɛ 5 μm) of 2H2TIDID-DCV VD thin
films under different Tsub ((a) RT; (b) 50 °C; (c) 70 °C; (d) 100°C; (e) 120 °C; (f) 140 °C),
and OD2TIDID-DCV spin-coated film ((g), annealed at 150 °C).
crystalline films with terrace-structured grains along with
increasing Tsub; structured grains were developed from Tsub of
70 °C, and micron-sized grains were observed from Tsub of 120
°C. By correlating charge carrier mobility and surface
topography, it was found that the terrace-structured and
enlarged grains are responsible for the favorable
intermolecular packing structure for both hole and electron
charge carrier transport (vide infra).
To demonstrate a solution processable IDID-DCV derivative,
we additionally synthesized OD2TIDID-DCV, for which linear
hexyl side chains in 2H2TIDID-DCV were replaced by branched
2-octyldodecyl group (see Scheme 1 and Scheme S2, detailed
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Table 2 OFET device characteristics
Compound
Tsub
p-channel operation
μh, max/average Vth
n-channel operation
p/n ratio
[°C]a)
Ion/Ioff
μe, max/average
[cm2 V-1 s-1]b)
1.47 Ɛ 10-5/
8.79 Ɛ 10-6
2.32 Ɛ 10-4/
1.23 Ɛ 10-4
8.07 Ɛ 10-3/
5.39 Ɛ 10-3
4.46 Ɛ 10-2/
2.09 Ɛ 10-2
8.13 Ɛ 10-2/
5.45 Ɛ 10-2
9.24 Ɛ 10-2/
6.21 Ɛ 10-2
-
Vth
[V]c)
Ion/Ioff
[cm2 V-1 s-1]b)
1.85 Ɛ 10-3/
1.43 Ɛ 10-3
2.30 Ɛ 10-2/
1.39 Ɛ 10-2
2.76 Ɛ 10-2/
1.69 Ɛ 10-2
8.28 Ɛ 10-2/
4.30 Ɛ 10-2
6.86 Ɛ 10-2/
3.30 Ɛ 10-2
5.89 Ɛ 10-2/
4.41 Ɛ 10-2
3.20 Ɛ 10-2/
3.00 Ɛ 10-2
9.67 Ɛ 10-2/
5.28 Ɛ 10-2
[V]c)
2H2TIDID-DCV
RT
50
-42
3 Ɛ 104
1 Ɛ 105
4 Ɛ 104
1 Ɛ 103
4 Ɛ 103
1 Ɛ 104
1 Ɛ 104
4 Ɛ 104
60 (±5)
2 Ɛ 101
3 Ɛ 102
3 Ɛ 103
5 Ɛ 102
9 Ɛ 102
3 Ɛ 103
-
125.85
99.14
3.42
1.86
0.84
0.63
-
(±8)
-22
100 (±10)
67 (±18)
61 (±10)
61 (±13)
77 (±9)
-
(±12)
-9
70
(±9)
-7
100
120
140
70
(±3)
-13
(±4)
-15
(±3)
-11
2H2TIDIDe)
(±2)
-24
OD2TIDID-DCV
150d)
3.42 Ɛ 10-3/
1.25 Ɛ 10-3
50 (±11)
1 Ɛ 102
28.27
(±9)
a) Substrate temperature in vacuum deposition; b) The mobilities were extracted from the saturation regimes; c) Average threshold voltages. The values in parentheses
represent one standard deviation; d) Post annealing temperature of spin-coated device (°C); e) from reference 25.
Fig. 6 (a) Out-of-plane XRD pattern of 2H2TIDID-DCV VD films under different Tsub; (b) Powder XRD pattern of 2H2TIDID-DCV powder; (c) 2D-GIXD diffraction pattern image of
2H2TIDID-DCV VD films (Tsub 140°C); (d and e) Horizontal and vertical line profile of (c), respectively; (f) 2D-GIXD diffraction pattern image of OD2TIDID-DCV film; (g and h)
Horizontal and vertical line profile of (e), respectively; (i) Calculated molecular size of 2H2TIDID-DCV; (j) Schematic diagram of 2H2TIDID-DCV packing structure.
2D-GIXD result of 2H2TIDID-DCV thin film could further correlation between XRDs, DFT calculation, and AFM surface
support our interpretation; we observed diffraction peaks topographic results shown in Fig. 6, we could conclude that
corresponding to lamellar and π-π d-spacing at qz of 0.60 Å-1 2H2TIDID-DCV (and OD2TIDID-DCV) molecules are oriented on
(10.47 Å) and qxy of 1.76 Å-1(3.57 Å), respectively. Moreover, the substrate with edge-on manner, and their π-planes are
the 2D-GIXD patterns of OD2TIDID-DCV spin-coated film also stacked up toward surface parallel direction. On the other
exhibited similar result with that of 2H2TIDID-DCV; i.e., hand, van der Waals interactions of N-aliphatic chains enable
semiconductor molecules are oriented exactly same manner 2H2TIDID-DCV and OD2TIDID-DCV to construct well-organized
with that of 2H2TIDID-DCV VD film but with different values of three-dimensional lamellar structure with lamellar spacing of
d-spacing (the lamellar spacing and π-π spacing values are 17.9 10.58 Å and 17.9 Å, respectively (see Fig. 6j).
Å and 3.64 Å, respectively). Through the comprehensive
6 | J. Name., 2012, 00, 1-3
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DOI: 10.1039/C6TC02777F
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ARTICLE
4.64 (t, J = 7.5Hz, 4H), 2.00 (q, J = 7.5Hz, 4H), 1.43 (q, J = 7.5Hz,
4H), 1.35-1.24 (m, 8H), 0.83 (t, J = 7.5Hz, 6H). 13C-NMR
(500MHz, Tetrahydrofuran-d8, δ): 161.30, 142.50, 131.37,
127.80, 121.68, 120.41, 117.81, 115.66, 115.08, 115.04, 79.72,
46.40, 32.63, 31.28, 27.77, 23.54, 14.43. HRMS (FAB, m/z)
Calcd. for C34H34N6: 526.28; found: 526.2851. Elem. Anal.
Calcd. for C34H34N6: C 77.54, H 6.51, N 15.96; found: C 77.4905,
H 6.5662, N 15.8965.
Conclusions
We have designed and successfully synthesized A-π-D-π-A
type IDID-DCV derivatives with different π-spacer. It was found
that the compensated ICT interaction between IDID and DCV
of 2TIDID-DCV derivatives (with thiophene π-spacer) and their
efficient electronic interaction in the three-dimensional
lamellar π-stacking structure gave rise to the dramatically
reduced energy band-gap as well as excellent film morphology.
As a consequence, the VD OFET device using 2H2TIDID-DCV
exhibited highly balanced ambipolar charge transport with
hole and electron mobilities of 0.08 cm2 V-1 s-1 and 0.09 cm2 V-1
s-1, respectively, and the spin-coated OFET devices using
OD2TIDID-DCV, also exhibited ambipolar charge transport
Synthesis of 5,10-dihexyl-2,7-bis(4,4,5,5-tetramethyl-1,3,2-
dioxaborolan-2-yl)-5,10-dihydroindolo[3,2-b]indole (3): A 100
mL round-bottom-flask, equipped with a magnetic stirrer bar
and reflux condenser, was baked under reduced pressure and
backfilled with Ar for three times. Compound
1 (1.00 g, 1.878
mmol), bis(pinacolato)diboron (1.06 g, 4.133 mmol), [1,1′-
Bis(diphenylphosphino)ferrocene]-dichloropalladium(II) (345
mg, 0.413 mmol), potassium acetate (1.10 g, 11.271 mmol),
and anhydrous DMF (35mL) were added, then gently refluxed
for 24 hours. After reaction finished, reaction mixture was
quenched with brine (300 mL), and extracted with DCM. The
combined organic phase was dried with MgSO4 and
concentrated under reduced pressure. The crude product was
purified by column chromatography (EA/n-hex.; 1:9, v/v) to
behavior (mobility of 9.67
10-3 cm2 V-1 s-1 for electron).
Ɛ
10-2 cm2 V-1 s-1 for hole, and 3.43
Ɛ
Experimental section
Synthesis
The final products were synthesized according to the synthetic
procedure shown in Scheme 1. 2,7-dibromo-5,10-dihexyl-5,10-
dihydroinolo[3,2-b]indole (2-,7-dibromo-N,N-dihexyl-IDID) was
synthesized according to our previous report.[25] Unless stated
otherwise, all reagents were purchased at Sigma Aldrich, TCI,
and Alfa Aeasar.
afford
3
as yellow solid (350 mg, 41%). 1H-NMR (300 MHz,
CDCl3, δ): 7.93 (s, 2H), 7.84 (d, J = 7.86Hz, 2H), 7.62 (d, J = 7.86
Hz, 2H), 4.55 (t, J = 6.93 Hz, 4H), 1.98 (q, J = 6.57 Hz, 4H), 1.39
(s, 24H), 1.29 (m, 12H), 0.86 (t, J = 6.72 Hz, 6H)
Synthesis
of
5,5'-(5,10-dihexyl-5,10-dihydroindolo[3,2-
): (205
Synthesis of 5,10-dihexyl-5,10-dihydroindolo[3,2-b]indole-
b]indole-2,7-diyl)bis(thiophene-2-carbaldehyde)
(
4
3
2,7-dicarbaldehyde
(
2
):
A
100 mL round-bottom-flask,
mg, 0.223 mmol), 5-bromothiophene-2-carbaldehyde (131.3
mg, 0.469 mmol), tetrakis(triphenylhosphine)palladium(0)
(37.38 mg, 0.032 mmol), 2N K2CO3 aqueous solution (10 mL),
and THF (20 mL) were added into a 100 mL round-bottom-flask,
and the reaction vessel evacuated and backfilled with Ar. After
then, reaction mixture was gently refluxed for 12 hours. After
reaction finished, reaction mixture was quenched with 1N HCl
aqueous solution (300 mL), and extracted with DCM. The
combined organic phase was dried with MgSO4 and
concentrated under reduced pressure. The crude product was
purified by flash column chromatography (EA/n-hex.; 1:2, v/v)
equipped with a magnetic stirrer bar, was baked under
reduced pressure and backfilled with Ar for three times. A
solution of compound
1 (200 mg, 0.375 mmol) in anhydrous
tetrahydrofuran (THF) in baked reaction vessel was cooled
down to -17 °C, afterwards, n-butyllithium (n-BuLi, 1.6 M
solution, 0.36 mL, 0.902 mmol) was added slowly. One hour
later, dimethylformamide (DMF, 69 μL, 0.902 mmol) was
added to a reaction mixture, and then, reaction vessel was
warmed to room temperature. After reaction finished,
reaction mixture was quenched with distilled water (200 mL),
and organic compounds were extracted with methylene
chloride (DCM). Combined organic phase was separated and
concentrated. Crude product was purified by column
and recrystallization (EA) to afford 4 as red solid (190 mg, 94%)
1H-NMR (300 MHz, DMSO-d6, δ): 9.91 (s, 2H), 8.13 (s, 2H), 8.07
(d, J = 4.05 Hz, 2H), 8.00 (d, J = 8.37 Hz, 2H), 7.78 (d, J = 3.96
Hz, 2H), 7.58 (d, J = 8.40 Hz, 2H), 4.68 (t, J = 6.48 Hz, 4H), 1.88
(q, J = 6.57 Hz, 4H), 1.23 (m, 12H), 0.81 (t, J = 6.57 Hz, 6H)
chromatography (ethyl acetate (EA)/n-hexane (n-hex.); 1:19,
1
as yellow solid. (90 mg, 55.3 %) H-NMR (300
v/v) to afford
2
MHz, CDCl3, δ): 10.14 (s, 2H), 8.04 (s, 2H), 7.98 (d, J = 8.22 Hz,
2H), 7.74 (d, J = 8.19 Hz, 2H), 4.58 (t, J = 7.14 Hz, 4H), 2.00 (m, J
= 7.41 Hz, 4H), 1.44-1.25 (m, 12H), 0.87 (t, J = 7.02 Hz, 6H)
Synthesis of 2H2TIDID-DCV
by the same synthetic procedure as that for 2HIDID-DCV by
using compound (166 mg, 0.279 mmol), malononitrile (40.6
: 2H2TIDID-DCV was synthesized
4
Synthesis of 2HIDID-DCV: A mixed solution of
2 (200 mg,
mg, 0.615 mmol), Al2O3 (0.228 g, 2.535 mmol), and DCM (50
mL). The crude product was purified by flash column
chromatography (THF) and subsequently by recrystallization
(EA) to afford 2H2TIDID-DCV as dark purple solid (0.189 g,
0.462 mmol), malononitrile (67.9 mg, 1.017 mmol), Al2O3 (377
mg, 3.698 mmol), and DCM (50 mL) was vigorously stirred
during 2 hours at room temperature. After reaction finished,
reaction mixture was filtered through celite plug to remove
residual Al2O3, and filtrate was concentrated under reduced
pressure. The crude product was purified by flash column
chromatography (THF) and subsequently by recrystallization
(EA) to afford 2HIDID-DCV as dark purple solid (220 mg,
90.3%). 1H-NMR (500MHz, Tetrahydrofuran-d8, δ): 8.30 (s,
2H), 8.24 (s, 2H), 8.13 (d, J = 8.5Hz, 2H), 7.86 (d, J = 8.5Hz, 2H),
1
98%) H-NMR (500MHz, Tetrahydrofuran-d8, δ): 8.23 (s, 2H),
8.01 (d, J = 1.5Hz, 2H), 7.98 (d, J = 8.5Hz, 2H), 7.86 (d, J = 4Hz,
2H), 7.73 (d, J = 4Hz, 2H), 7.61 (dd, J = 8.5Hz, 1.5Hz, 2H), 4.66
(t, J = 7Hz, 4H), 2.00 (q, J = 7.5Hz, 4H), 1.45 (q, J = 7.5Hz, 4H),
1.38-1.25 (m, 8H), 0.85 (t, J = 7.5Hz, 6H). 13C-NMR (500MHz,
Tetrahydrofuran-d8, δ):158.93, 151.96, 142.73, 141.77,
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Journal Name
125.19, 119.90, 118.46, 116.19, 115.45, 114.78, 109.18, Nanoscope III multimode SPM with tapping mode using TREST
45.98,32.70, 31.35, 27.73, 23.57, 14.46. HRMS (FAB, m/z) cantilever. Out-of-plane X-ray diffraction and powder X-ray
Calcd. for C42H38N6S2: 690.26; found: 690.2563. Elem. Anal. diffraction measurement was performed using a Bruker D8-
Calcd. for C42H38N6S2: C 73.01, H 5.54, N 12.16, S 9.28; found: C Advance X-ray diffractometer. Two-dimensional grazing
73.0165, H 5.5255, N 12.1216, S 9.2924..
Synthesis of 5',5'''-(5,10-dihexyl-5,10-dihydroindolo[3,2- 3C beam-line of Pohang Acceleration Laboratory. Density
b]indole-2,7-diyl)bis([2,2'-bithiophene]-5-carbaldehyde) ( ): 5 functional theory (DFT) calculations were performed using
was synthesized by the same synthetic procedure used for Gaussian 09 package B3LYP method with the basis set of 6-31G
with compound (170 mg, 0.271 mmole), 5’-bromo-[2,2’- d p. The I-V characteristics of all devices were measured using
incidence X-ray diffraction measurement was performed at the
5
4
3
bithiophene]-5-carbaldehyde (135 mg, 0.569 mmol), Keithley 4200 SCS.
tetrakis(triphenylhosphine)palladium(0) (32 mg, 0.027 mmol),
2N K2CO3 aqueous solution (10 mL), and THF (20 mL). The OFET device fabrication and measurement
crude product was purified by column chromatography
To prepare the substrates, SiO2/Si substrates (p-doped 300
(CHCl3/EA/n-hex; 3:1:6, v/v) and subsequently by
recrystallization (EA) to afford compound as red solid (162
nm) were rinsed with acetone and isopropyl alcohol,
sequentially, for 10 minutes in an ultrasonicator, followed by
15 minutes UV (360 nm) O3 treatment. For the thermally
evaporated vacuum deposited OFET device fabrication,
octadecyltrichlorosilane (ODTS) was treated as self-assembled
monolayer (SAM) on prepared substrate to reduce charge trap
sites as well as to domain enlargements. ODTS was treated in
vapor phase in a vacuum oven; then the substrates were
passed into nitrogen filled glove box. 30 nm thick of organic
semiconductor active layers were thermally deposited with
deposition rate of 0.1 - 0.2 Å s-1 and different substrate
temperature (Tsub: room temperature, 50, 70, 100, 120, and
4
1
mg, 78%). H-NMR (300 MHz, Tetrahydrofuran-d8, δ): 9.83 (s,
2H), 7.90 (d, J = 8.28 Hz, 2H), 7.83 (s, 2H), 7.79 (d, J = 3.9 Hz,
2H), 7.48-7.46 (m, 6H), 7.39 (d, J = 3.9 Hz, 2H), 4.61 (t, J = 6.9
Hz, 4H), 1.98 (q, J = 7.47 Hz, 4H), 1.46-1.25 (m, 12H), 0.49 (t, J =
7.08 Hz, 6H)
Synthesis of 2H4TIDID-DCV
by the same synthetic procedure used for 2HIDID-DCV using
compound (162 mg, 0.213 mmol), malononitrile (43 mg,
: 2H4TIDID-DCV was synthesized
5
0.640 mmol), Al2O3 (0.228 g, 1.921 mmol), and DCM (60 mL).
The crude product was purified by flash column
chromatography (THF) and subsequently by recrystallization
(THF) to afford 2H4TIDID-DCV as black solid (0.110 g, 60%): 1H-
NMR (500MHz, Tetrahydrofuran-d8, δ): 8.14 (s, 2H), 7.90 (d, J
= 8.5Hz, 2H), 7.84 (s, 2H), 7.78 (d, J = 4Hz, 2H), 7.56 (d, J =
3.5Hz, 2H), 7.49 (m, 4H), 7.44 (d, J = 4Hz, 2H), 4.61(t, J = 7Hz,
4H), 2.03 (m, 4H), 1.48 (q, J = 7Hz, 4H), 1.39-1.27 (m, 8H) 0.85
(t, J = 7Hz, 6H). HRMS (FAB, m/z) Calcd. for C50H42N6S4: 854.24;
found: 854.2363. Elem. Anal. Calcd. for C50H42N6S4: C 70.23, H
4.95, N 9.83, S 15.0; found: C 69.8566, H 5.0406, N 9.8086, S
15.0143. (We could not obtain 13C NMR date due to
insufficient solubility of 2H4TIDID-DCV for common NMR
solvents such as CDCl3 or Tetrahydrofuran-d8.)
140 °C), under a vacuum of 7
Ɛ
10-7 Torr. As source and drain
electrode, 50 nm thick of gold (Au) was thermally deposited
with deposition rate of 0.2 - 0.3 Å s-1. For the spin-coated OFET
device fabrication, same substrate preparation procedure with
VD device was performed. OD2TIDID-DCV were dissolved in
chloroform (0.3 - 0.4 weight %), and spin-coated at 1500-3000
rpm for 1 minute in nitrogen filled glove box; then the
substrates were annealed with different temperature from
room temperature to 150 °C). Source and drain electrode were
thermally deposited same condition with VD device
fabrication. The I-V characteristics of all the OFETs were
measured in a nitrogen-filled glove box, using a Keithley 4200
SCS instrument connected to a probe station. All OFET
characteristics were obtained from the transfer curve in the
Characterization
Chemical structures of newly synthesized materials were fully saturated regime. For the charge carrier mobility calculations,
identified by 1H NMR, 13C NMR, (Bruker. Advanced-300 and we checked the channel width and length of the individual
Advanced-500), GC-Mass (JEOL, JMS-700) and elemental devices using an optical microscope.
analysis (CE Instrument, EA1110). UV-vis spectra were
recorded on a SHIMADZU UV-1650PC. HOMO and LUMO
Acknowledgements
energy levels of organic materials were obtained from the
cyclic voltammetry (CV) measurements. CV measurements
were performed using a 273A (Princeton Applied Research)
This research was supported by the National Research
Foundation of Korea (NRF) through a grant funded by the
Korean government (MSIP; No. 2009-0081571[RIAM0417-
20150013]).
with
a one-compartment platinum working electrode, a
platinum wire counter-electrode, and a quasi Ag+/Ag electrode
as a reference electrode. Measurement was performed in a 0.5
mM tetrahydrofuran solution with 0.1M tetrabutylammonium
tetrafluoroborate (TBATFB) as a supporting electrolyte, and
acetonitrile with 0.1 M TBATFB for film as a supporting
electrolyte, at a scan rate of 50 mV/s. Each oxidation and
reduction potential was calibrated using ferrocene as a
reference. Film surface topologies were obtained by a Bruker
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Journal of Materials Chemistry C
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DOI: 10.1039/C6TC02777F
Graphical and textual abstract
DicyanovinylꢀSubstituted Indolo[3,2ꢀb]indole Derivatives: Low bandꢀGap πꢀconjugated
Molecules for SingleꢀComponent Ambipolar Organic FieldꢀEffect Transistor
Illhun Cho,a Sang Kyu Park,a Boseok Kang,b Jong Won Chung,a Jin Hong Kim,a Won Sik Yoon,a Kilwon Cho,b
and Soo Young Parka*
A series of indolo[3,2ꢀb]indoleꢀdicyanovinyl derivatives are prepared to realize ambipolar
organic semiconductor. Through cooperative effects of intramolecular charge transfer
interaction and conjugation extension, low bandꢀgap πꢀconjugated molecules are successfully
prepared. Among others, 2H2TIDIDꢀDCV and OD2TIDIDꢀDCV exhibit ambipolar charge
transport behavior (hole/electron mobilities of 0.08/0.09 and 0.09/0.003 cm2 Vꢀ1 sꢀ1,
respectively) with same molecular packing motif of 3D lamellar πꢀstacking structure.