Synthesis, characterization, and transistor applications of new linear small molecules: Naphthyl-ethynyl-anthracene-based small molecules containing different alkyl end group

Article abstract of DOI:10.1016/j.dyepig.2016.04.017

A novel small molecule consisting of triple bond-bridged naphthalene-anthracene core was synthesized with either a pentyl or decyl end group. The linear and planar shape of naphthalene-anthracene core allowed herringbone packing pattern with enhanced conjugation length and close molecular packing structure. The film morphology of naphthalene-anthracene based small molecules was also affected by the pentyl and decyl groups. The pentyl substituted molecule exhibited more unified and ordered packing patterns than did the decyl substituted example, which showed at least two polymorphs. The p-type semiconducting properties of the two new molecules were characterized by organic field-effect transistor measurements. In annealed devices at 80 °C, the field-effect mobility of pentyl analogue was 0.045 cm²/(V·s), which was about 3 times higher than the decyl substituted compound.

Full text of DOI:10.1016/j.dyepig.2016.04.017

Dyes and Pigments 131 (2016) 349e355
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Synthesis, characterization, and transistor applications of new linear
small molecules: Naphthyl-ethynyl-anthracene-based small
molecules containing different alkyl end group
,
,
, ***
Hyeng Gun Song c 1, Yebyeol Kim b 1, So-Min Park c, Tae Kyu An d, Soon-Ki Kwon c
,
Chan Eon Park b **, Yun-Hi Kim a
,
, *
a Department of Chemistry and RIGET, Gyeongsang National University, Jin-ju, 660-701, Republic of Korea
b POSTECH Organic Electronics Laboratory, Polymer Research Institute, Department of Chemical Engineering, Pohang University of Science and Technology,
Pohang, 790-784, Republic of Korea
c Department of Materials Engineering and Convergence Technology, Gyeongsang National University, Jinju, Gyeongnam 660-701, Republic of Korea
d Department of Polymer Science & Engineering, Korea National University of Transportation, Chungju, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel small molecule consisting of triple bond-bridged naphthalene-anthracene core was synthesized
with either a pentyl or decyl end group. The linear and planar shape of naphthalene-anthracene core
allowed herringbone packing pattern with enhanced conjugation length and close molecular packing
structure. The film morphology of naphthalene-anthracene based small molecules was also affected by
the pentyl and decyl groups. The pentyl substituted molecule exhibited more unified and ordered
packing patterns than did the decyl substituted example, which showed at least two polymorphs. The p-
type semiconducting properties of the two new molecules were characterized by organic field-effect
transistor measurements. In annealed devices at 80 ꢀC, the field-effect mobility of pentyl analogue
was 0.045 cm2/(V∙s), which was about 3 times higher than the decyl substituted compound.
© 2016 Elsevier Ltd. All rights reserved.
Received 3 January 2016
Received in revised form
4 April 2016
Accepted 6 April 2016
Available online 7 April 2016
Keywords:
Organic field-effect transistor (OFET)
2D-grazing incidence X-ray diffraction
(GIXD)
Herringbone
Alkyl end group length
Structureeproperty relationship
1. Introduction
The most energetically-stable conformation of small molecule is
determined by van der Waals (VDW) dispersion and electrostatic
In small molecules that contain aromatic rings, the effect of
crystal structure on charge carrier transport has been studied by
experimental [1,2] and theoretical [3] methods. In the solid state,
charge carrier transport in small molecules is attributed to the
packing patterns, crystal size, and inter-crystal connectivity of the
film morphology [4e11]. Among these factors, the packing patterns
of the small molecule should be subject to their interaction types.
The small molecules usually have three energetically-attractive
interaction types: parallel, T-shaped, and slipped-parallel [12].
interaction [13] caused by their geometrical and chemical structure
of their conjugated core [14e16] and substituents [16e19].
Linear-shaped small molecules generally adopt a herringbone-
type packing with T-shaped interactions because these in-
teractions are stabilized by a number of short intermolecular C$$$C
contacts and weak acid-base contacts between CeH bonds (weak
hydrogen-bonds; soft acids) and
p-systems (electron rich; soft
bases) [12,13]. Additionally, the substituted alkyl group of linear-
shaped small molecules can affect the film morphology by
affecting VDW interaction and the steric hindrance between adja-
cent molecules [20] and also their solubility in a processing solvent.
In previous reports, our group has tried to reveal how the mo-
lecular structure of linear-shaped small molecules influences their
film morphology and charge carrier transport of the corresponding
OFETs [21e25]. Modification of the conjugated core of several series
of linear-shaped small molecules can yield molecular structures
which could fabricate favorable film morphology to charge carrier
* Corresponding author. Department of Chemistry and ERI, Gyeongsang National
University, Jin-ju, 660-701, Republic of Korea.
** Corresponding author.
*** Corresponding author.
E-mail addresses: skwon@gnu.ac.kr (S.-K. Kwon), cep@postech.ac.kr (C.E. Park),
ykim@gnu.ac.kr (Y.-H. Kim).
1
Hyeng Gun Song and Yebyeol Kim are equally contributed as first authors.
http://dx.doi.org/10.1016/j.dyepig.2016.04.017
0143-7208/© 2016 Elsevier Ltd. All rights reserved.

350
H.G. Song et al. / Dyes and Pigments 131 (2016) 349e355
transport: a large fused or triple-bond-bridged conjugated core
with planar geometrical structure produces improved intra-
2.1.6. 2-Bromo-6-pentylnaphthalene (5)
Compound 5 was synthesized according to the literature [28].
molecular
p-conjugated system as well as intermolecular in-
Yield: 0.6 g, 64%. 1H NMR (CDCl3, 300 MHz):
d 7.97 (s,1H), 7.70e7.50
teractions [26]. Moreover the shape and length of alkyl group have
a strong influence on molecular ordering, particularly during for-
mation of films by solution processing [1,2].
(m, 4H), 7.38e7.35 (m, 1H), 2.78e2.74 (m, 2H), 1.34e1.27 (m, 6H),
0.87e0.86 (t, J ¼ 3 Hz, 3H).
Previous work in our group has investigated the small molecules
that have a triple-bond-bridged phenylene-anthracene conjugated
core (phenylethynylanthracene; PEA) which exhibited good charge
carrier transport. For the PEA core, a pentyl end group was found to
be most suitable alkyl end group for improving both solubility and
molecular arrangement [27]. However, it is still necessary to discuss
the packing patterns of the molecular assembly. Moreover, the
further fine-tuning opportunity is allowed to extend conjugation
length of PEA by simply substituting a larger fused conjugated core
than that of the PEA core.
In this paper, we present two linear-shaped small molecules
that contain a triple bond-bridged naphthalene-anthracene con-
jugated core (NEA) which has longer conjugation length than that
of PEA. Therefore, the fine-tuned NEA-based small molecules are
expected to provide improved carrier transport through their
2.1.7. 2-Bromo-6-decylnaphthalene (6)
Compound 6 was synthesized according to the literature [28].
Yield: 0.5 g, 52%. 1H NMR (CDCl3, 300 MHz):
d 8.04e8.02 (s, 1H),
7.70e7.58 (m, 4H), 7.38e7.35 (m, 1H), 2.84e2.82 (m, 2H),
01.31e1.27 (m, 16H), 0.87e0.86 86 (t, J ¼ 3 Hz, 3H).
2.1.8. 2-((6-Pentylnaphthalen-2-yl)ethynyl)anthracene (PNEA)
2-Bromo-6-pentylnaphthalene (1.0 g, 3.6 mmol), Pd(PPh3)2Cl2
(1.0 mg, 2 mmol), copper(I) iodide (2.0 g, 10.8 mmol), 20 mL trie-
thylamine were dissolved in 20 mL toluene. 2-ethynyl-anthracene
(0.8 g, 3.9 mmol) was added at room temperature. Mixture was
refluxed 24 h at 90 ꢀC. After cooled to room temperature, reaction
mixture poured into water and then extracted with methylene
chloride. The organic layer was separated and dried over MgSO4,
and then the solvent was removed by using a rotary evaporator. The
residue purified by column chromatography on silica gel with
methylene chloride/hexane (1:5) to obtain the product. Yield: 0.8 g,
56%. mp: 212 ꢀC, IR(KBr) cmꢁ1, 3057-3021 (aromatic CeH), 2950-
2843 (aliphatic CeH), 2301-2194 (triple bond). 1H NMR (CDCl3,
extended intramolecular p-conjugated system as well as increased
intermolecular interaction in comparison to the PEA-based small
molecules. The NEA core was substituted with two end groups of
differing alkyl length: pentyl (2-(p-pentylnaphthylethynyl)anthra-
cene; PNEA) and decyl (2-(p-decylnaphthylethynyl)anthracene;
DNEA). We will discuss the film morphology including packing
patterns of NEA-based small molecules, and the relationship be-
tween morphology and OFET properties depending on alkyl length.
300 MHz):
d 8.43 (s, 2H), 8.20 (s, 1H), 8.10 (s, 1H), 8.05e8.00 (m,
3H), 7.80e7.78 (d, J ¼ 6 Hz, 2H), 7.65e7.63 (m, 3H), 7.50e7.41 (m,
2H), 7.38 (m, 1H), 2.82e2.78 (m, 2H), 1.77e1.71 (m, 2H), 1.41e1.37
(m, 4H), 0.95e0.91 (m, 3H). 13C NMR (CDCl3, 500 MHz):
d 141.680,
133.115,132.185,132.128,131.844,131.556,131.347,131.152,130.698,
128.465, 128.379, 128.312, 128.280, 128.234, 128.211, 127.692,
127.584, 126.334, 126.269, 126.258, 125.823, 125.758, 120.167,
119.661, 91.017, 90.144, 36.163, 31.556, 30.986, 22.577, 14.036.
2. Experimental section
2.1. Synthesis method
C31H26 Cal. C, 93.42; H, 6.584; Found: C. 93.94; H, 6.54 HRMS Cal.
398.2035, Found: 398.2033.
2.1.1. Materials
2-Bromoanthracene, trimethylsilyl acetylene, copper(I) iodide,
tetrabutylammonium fluoride were purchased from Aldrich.
Pd(PPh3)Cl2 catalyst purchased from Strem.
2.1.9. 2-((6-Decylnaphthalen-2-yl)ethynyl)anthracene (DNEA)
DNEA was prepared using the same method as described for
compound PNEA using 2-bromo-6-nonylnaphthalene (1.0 g,
2.8 mmol), copper(I) iodide (1.6 g, 8.4 mmol), 2-ethynyl-anthracene
(0.6 g, 3.1 mmol), 20 mL triethylamine. Yield: 0.8 g, 62%. mp: 202 ꢀC,
IR(KBr) cmꢁ1, 3058-3026 (aromatic CeH), 2939-2840 (aliphatic
2.1.2. Anthracen-2-ylethynyl-trimethyl-silane (1)
Compound 1 was synthesized according to the literature [27].
Yield: 25.28 g, 79%. 1H NMR (CDCl3, 300 MHz):
d: 8.38 (s, 2H), 8.19
CeH), 2316-2202 (triple bond). 1H NMR (CDCl3, 300 MHz):
d 8.43 (s,
(s, 1H), 8.03e7.99 (m, 2H), 7.95e7.92 (d, J ¼ 9 Hz, 1H), 7.52e7.45 (m,
2H), 8.28 (s, 1H), 8.10 (s, 1H), 8.04e7.99 (m, 3H), 7.80e7.77 (d,
J ¼ 9 Hz, 2H), 7.64e7.61 (m, 3H), 7.52e7.50 (m, 2H), 7.40 (d, 1H),
2.82e2.77 (m, 2H), 1.75e1.70 (m, 2H), 1.36e1.12 (m, 14H),
3H), 0.33 (s, 9H).
0.92e0.87 (m, 3H). 13C NMR (CDCl3, 500 MHz):
d 141.700, 133.114,
2.1.3. 2-Ethynyl-anthracene (2)
132.184, 132.126, 131.850, 131.550, 131.355, 131.150, 130.698,
128.464, 128.320, 128.287, 128.243, 128.134, 127.780, 127.697,
127.587, 126.341, 126.270, 125.831, 125.765, 123.691, 120.164,
119.649, 91.019, 90.141, 36.206, 31.918, 31.326, 29.639, 29.610,
29.549, 29.382, 29.348, 22.701, 14.131. C36H36 Cal. C, 92.26; H, 7.74;
Found: C. 92.08; H, 7.76 HRMS Cal. 468.2817, Found: 468.2818.
Compound 2 was synthesized according to the literature [27].
d 8.41 (s, 2H), 8.22 (s,
1H), 8.03e7.95 (m, 3H), 7.53e7.47 (m, 3H), 3.22 (s, 1H).
Yield: 2.5 g, 85%. 1H NMR (CDCl3, 300 MHz):
2.1.4. 1-(6-Bromonaphthalen-2-yl)pentan-1-one (3)
Compound 3 was synthesized according to the literature [28].
Yield: 14.2 g, 60%. 1H NMR (CDCl3, 300 MHz):
d 8.46 (s, 1H),
2.2. Measurements
8.10e8.07 (d, J ¼ 9 Hz, 2H), 7.87e7.84 (t, J ¼ 4.5 Hz, 2H), 7.66e7.63
(d, J ¼ 9 Hz, 1H), 3.10e3.09 (t, J ¼ 1.5 Hz, 2H), 1.82e1.80 (m, 2H),
1.61e1.52 (m, 2H), 0.89e0.87 (t, J ¼ 3 Hz, 3H).
1H NMR spectra were recorded using a Bruker Advance-300
spectrometer. The thermal analyses were performed on a TA TGA
2100 thermogravimetric analyzer under N2 atmosphere at a rate of
10 ꢀC/min. Differential scanning calorimeter (DSC) was conducted
under N2 atmosphere using a TA instrument 2100 DSC. The sample
was heated at 10 ꢀC/min from 30 ꢀC to 300 ꢀC. UVevis absorption
spectra were measured using a UV-1650PC spectrophotometer.
Cyclic voltammetry (CV) was performed on an EG and G Parc model
273 Å potentiostat/galvanostat system with a three-electrode cell in
2.1.5. 1-(6-Bromonaphthalen-2-yl)decan-1-one (4)
Compound 4 was synthesized according to the literature [28].
Yield: 11.0 g, 57%. 1H NMR (CDCl3, 300 MHz):
d 8.45 (s, 1H),
8.09e8.06 (d, J ¼ 9 Hz, 2H), 7.87e7.81 (t, J ¼ 9 Hz, 2H), 7.66e7.63 (d,
J ¼ 9 Hz, 1H), 3.10e3.09 (t, J ¼ 1.5 Hz, 2H), 1.80e1.78 (m, 2H),
1.58e1.29 (m, 12H), 0.89e0.87 (t, J ¼ 3 Hz, 3H).

H.G. Song et al. / Dyes and Pigments 131 (2016) 349e355
351
a solution of 0.1 M tetrabutylammonium perchlorate (Bu4NClO4) in
acetonitrile at a scan rate of 50 mV/s. The small molecule films were
coated on a square carbon electrode by dipping the electrode into
the corresponding solvents, then dried in N2 atmosphere. A Pt wire
was used as the counter electrode, and an Ag/AgNO3 (0.1 M) elec-
trode was used as the reference electrode.
2D-grazing incidence X-ray diffraction (2D-GIXD) experiment
was performed using synchrotron radiation on the 9A beam line
with an energy of 11.02 keV and a sample-to-detector distance of
224 mm at the Pohang synchrotron Laboratory, Pohang, Korea.
Topography of the films was investigated using an Atomic Force
Microscope (AFM: Dimension 3100 þ Nanoscope V 7.0, VEECO) in
tapping mode using Si tips. The NEA small molecule films for 2D-
GIXD and AFM analysis, were prepared by spin-coating from a
0.5 wt% solution in chloroform which was used as the processing
solvent onto octyltrichlorosilane (OTS)-modified silicon wafer, then
stored in a vacuum chamber overnight.
To measure of OFET characters based on the NEA small mole-
cules, they were spin-coated onto SiO2/nþþ Si substrate (0.5 wt% in
chloroform, 2000 rpm). A heavily Arsenide doped n-type silicon
substrate was used as a gate electrode. A silicon dioxide of 300 nm
thickness was thermally grown as a gate dielectric. To increase
hydrophobicity and reduce hydroxyl groups on the surface, the
substrate was treated with OTS before deposition of small mole-
cules. Gold source and drain electrodes were thermally evaporated
coupling reaction (Scheme 1). These small molecules were purified
by column chromatography and recrystallization. Their chemical
structures were verified using 1H NMR, 13C NMR and Mass spec-
troscopy (Fig. 1).
Although PNEA and DNEA were soluble in chloroform which
was used as processing solvent in this report, the solubility of PNEA
and DNEA was less than that of small molecules having PEA core
with pentyl (PPEA) or decyl (DPEA) alkyl end group [27], which
have phenyl-anthracene cores (The molecular structures are veri-
fied in Fig. S1). PPEA and DPEA were well dissolved in a high
concentration of more than 10 mg in chloroform (1 mL) at 25 ꢀC,
but PNEA and DPEA were supersaturated in an amount >8 mg and
>5 mg, respectively in the same condition.
The thermal properties of PNEA and DNEA films were charac-
terized by thermogravimetric analysis (TGA) and DSC (Fig. 2). Both
synthesized small molecules exhibited good thermal stabilities,
with high decomposition temperature (at which a compound loses
5% of its weight): PNEA: 368 ꢀC; DNEA: 427 ꢀC. The thermally stable
behavior of both small molecules facilitated further processing
steps to deposit upper layers such as the passivation layer and
emission layers, which are required when fabricating electronic
circuits. The DSC thermogram contained one melting endothermic
peak on the heating curve at 215 ꢀC for PNEA and at 196 ꢀC for
DPEA. The higher melting points and narrower peak shape of PNEA
(215 ꢀC) indicates the stronger intermolecular interaction and
narrow size distribution than those of DNEA (196 ꢀC).
using a shadow mask. The channel width was 1000
mm and channel
length was 100 m. Both PNEA and DNEA devices were thermally
m
As compared with PEA-based small molecules, the higher
melting points of PNEA (215 ꢀC) and DNEA (196 ꢀC) than those of
previously reported small molecules PPEA (169 ꢀC) and DPEA
(153 ꢀC) [27], indicate that the NEA core induces stronger interac-
tion between adjacent molecules than does the PEA core.
These findings are in good agreement with the UVevis spectra
(Fig. 3a). The band gaps of PNEA (2.72 eV) and DNEA (2.72 eV) were
calculated from band edge of the UVevis spectra, and are narrower
than those of PPEA (2.82 eV) and DPEA (2.83 eV). The narrow band
gaps of NEA-based small molecules (PNEA and DNEA) suggest that
they have longer conjugation length and increased intermolecular
interaction in the crystal structure than do PEA-based small mol-
ecules (PPEA and DPEA). The electrochemical behaviors of PNEA
annealed at 80 ꢀC under N2 atmosphere. The OFET measurement
was performed using a Keithley 2400/236 source/measure units in
ambient atmosphere in a dark room. The OFET parameters were
extracted in the saturation regime with drain voltage ¼ e 80 V. The
OFET parameters were extracted in the saturation regime from the
slope of the source-drain current.
3. Results and discussion
3.1. Synthesis and characterization
PNEA and DNEA were synthesized using the Sonogashira
Scheme 1. Synthesis scheme of PNEA and DNEA.

352
H.G. Song et al. / Dyes and Pigments 131 (2016) 349e355
Fig. 1. NEA-based small molecules confirmed by 1H NMR.
and DNEA were observed by using CV to define the oxidation po-
tential, which was used to calculate the highest occupied molecular
orbital (HOMO) energy level (Fig. 3b). The onset of oxidation po-
tential was 1.32 V for PNEA and 1.29 V for DNEA; these values
correspond to HOMOs of 5.75 and 5.72 eV, respectively. Unfortu-
nately, the lowest unoccupied molecular orbital (LUMO) energies
could not be accurately estimated because reduction behavior was
not observed by CV. Therefore, the LUMO levels were calculated
indirectly by using oxidation potentials from the CV and optical
band gaps from the UVevis spectra (Table 1). PNEA and DNEA are
due to the same NEA conjugated core.
3.2. Morphological studies
To observe the crystalline structure of NEA-based small mole-
cules, their 2D-GIXD patterns were identified (Fig. 4aed). Both
PNEA and DNEA were thermally annealed at 80 ꢀC to improve
molecular ordering by providing the thermal energy for molecular
rearrangement and to decrease the amount of solvent residue
[29,30]. In a direction to the surface normal, the 2D-GIXD diffrac-
tions clarified after thermal annealing, but those of DNEA still
exhibited at least two polymorphic molecular packing structures
which could be clarified by the extracted out-of-plane profiles for
annealed sample (Fig. 4e). DNEA spectrum exhibited a predomi-
nant peak having a 37.9 Å d-spacing of (00k) with a weak and broad
peak having a 10.4 Å d-spacing of (00k)* (marked by a light blue (in
Fig. 3. Photochemical properties for compounds by Normalized UVevis absorption
spectra as solid films (a) and CV (b).
Fig. 2. Thermograms for PNEA and DNEA by TGA (a) and DSC (b).

H.G. Song et al. / Dyes and Pigments 131 (2016) 349e355
353
Table 1
Measured and calculated parameters of the NEA-based small molecules.
Td [ꢀC]a
Tm [ꢀC]a
Band gap [eV]b
HOMO [eV]c
LUMO [eV]d
PNEA
DNEA
368
427
215
200
2.72
2.72
ꢁ5.75
ꢁ5.72
ꢁ3.03
ꢁ3.00
a
Obtained from DSC measurements.
Obtained at the onset of the UVevis absorption spectroscopy.
Calculated from the CV using the empirical equation: HOMO (eV) ¼ 4.43 þ Eonset
b
c
.
d
calculated with optical band gapb and HOMO level.c
web version) arrow in Fig. 4e). This result is probably indicating
that the long decyl group could not be sufficiently stretched or
regularly arranged during spin coating and thermal annealing. In
the direction parallel to the substrate, the stronger diffraction
patterns of PNEA indicate a more regular structure than that of
DNEA.
In the direction normal to the surface, the d-spacing was ob-
tained from (001) peaks for each film (Fig. 4e). The proposed
configuration of PNEA and DNEA which was obtained by the d-
spacing in direction of out-of-plane was provided in Fig. 5. PNEA
had a shorter d-spacing (26.3 Å, qz ¼ 0.24 Åꢁ1) than that of DNEA
(37.9 Å, qz ¼ 0.17 Åꢁ1) because the alkyl end group was shorter in
PNEA than in DNEA. The crystalline structure of the NEA-based
molecules could be affected by the van der Waals interaction be-
tween the end groups as well as electrostatic interaction between
the conjugation cores [27].
Fig. 5. Estimated configuration of PNEA and DNEA in direction of out-of-plane. Grey:
NEA skeleton; orange: pentyl group; red: decyl group. (For interpretation of the ref-
erences to color in this figure legend, the reader is referred to the web version of this
article.)
adjacent molecules in the same unit cell of a herringbone pattern
[3]. Although PNEA and DNEA show similar in-plane d-spacing, the
peak intensity of PNEA is more pronounced than that of DNEA
(Fig. 4f). Collectively, the long alkyl chain of DNEA disturbed the
The diffraction peaks in the parallel direction (Fig. 4f) corre-
sponded with the (11L), (02L), and (12L) diffractions of a herring-
bone packing pattern [19,31,32]. The absence of (01L) and (10L)
diffractions indicates an antiparallel configuration of the two
Fig. 4. 2D-GIXD analysis of PNEA and DNEA; 2D-GIXD images (aed) and extracted out-of- and in-plane profile for annealed sample (e: out-of-plane, f: in-plane).

354
H.G. Song et al. / Dyes and Pigments 131 (2016) 349e355
Fig. 6. AFM images of PNEA (a) and DNEA (b). The root-mean-square roughness (Rq) was calculated with NanoScope Analysis program. The below plots show the cross-sectional
height profiles which are marked with blue line in the AFM images. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this
article.)
molecular ordering due to its steric hindrance, whereas the well-
ordered packing structure of PNEA was observed resulting the
enhanced OFET performances.
small molecules decreased solubility in the processing solvent and
increased the heterogeneity of the morphology during film depo-
sition, compared to those of PEAs. In other words, PNEA which has
X-ray diffraction analysis provides specific information about
the molecular packing in crystal grains. However, analysis of
microscale morphology and inter-grain connectivity is also
important because the carriers travel several tens of micrometers
through grain boundaries. As shown in AFM images (Fig. 6), DNEA
produced rougher film morphology with smaller crystals than that
of PNEA. This result is due to the poor molecular ordering of DNEA,
as revealed in corresponding 2D-GIXD analyses. These findings
indicate that the pentyl group is more morphologically suitable to
enhance charge carrier transport for the NEA core than is the decyl
group.
a short pentyl group, showed lower
lower solubility caused by enlarged conjugated core, whereas
DNEA, which has a long decyl group exhibited higher than DPEA
because the long alkyl group could complement the solubility to
form more ordered morphology during solution processing for film
fabrication.
m than did PPEA because of the
m
3.3. OFET properties
The NEA-based small molecules showed typical p-type char-
acter (Fig. 7); the OFET parameters such as field-effect mobility (m)
with average value (mAVG) from six individual devices and max
value (mMAX), on/off current ratio, and threshold voltage (VTH) were
summarized in Table 2. The thermal annealing process can
marginally increase the average mAVG of NEA-based small molecules
from 0.036 0.003 to 0.041 0.003 cm2/(V∙s) for PNEA and from
0.011 0.002 to 0.015 0.001 cm2/(V∙s) for DNEA. The higher mAVG
of PNEA compared with DNEA is reasonable considering the 2D-
GIXD and AFM analysis. The long decyl group of DNEA disrupts
charge carrier transport in the direction parallel to the substrate
because the bulkiness of the group limits the molecular ordering; it
also disrupts charge carrier transport in the vertical direction (from
the source to the channel and from the channel to the drain) due to
increased insulation of the alkyl region.
By comparing the mobility of PEA-based small molecules having
pentyl and decyl end groups, PNEA had lower
m
than that of PPEA
(0.55 cm2/(V∙s)), but DNEA had higher
m than that of DPEA
(0.0014 cm2/(V∙s)). This result is due to the balance of the conju-
gated core and the alkyl end group. Although the NEA-based small
molecules that have a naphthalene core allow a longer intra-
molecular
p-conjugation system than do the PEA-based small
Fig. 7. Out-put characteristics for annealing samples at the 80 ꢀC. The gate voltage (VG)
is changed from 0 to ꢁ80 V with an interval of ꢁ20 V (a, b). Transfer characteristics at a
fixed drain voltage of ꢁ80 V (c, d).
molecules, the excessively strong intermolecular interaction
(described by DSC thermograms and UVevis spectra) of NEA-based

H.G. Song et al. / Dyes and Pigments 131 (2016) 349e355
355
Table 2
OFET parameters of PNEA and DNEA.
PNEA
DNEA
mMAX (cm2/(Vs))
mAVG (cm2/(Vs))
On/off current ratio
VTH (V)
mMAX (cm2/(Vs))
mAVG (cm2/(Vs))
On/off current ratio
VTH (V)
ꢁ12
Pristine
Ann 80
0.040
0.045
0.036 0.003
0.041 0.003
6 ꢂ 104
ꢁ29
ꢁ25
0.014
0.017
0.011 0.002
0.015 0.001
2 ꢂ 104
1 ꢂ 104
9 ꢂ 104
0.4
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4. Conclusion
NEA-based small molecules were combined with pentyl (PNEA)
and decyl (DNEA) end groups. NEA with a larger fused conjugated
core was expected to improve charge carrier transport with an
extended intramolecular
compared to those of PEA conjugated cores that were investigated
previously. Although PNEA and DNEA showed strong intermolec-
ular interaction with extended
observed by DSC and UVevis spectra, the excessive strong inter-
molecular interaction rather reduced the solubility in the pro-
cessing solvent. As a result, PNEA exhibited reduced
that of PPEA. Among NEA-based small molecules, PNEA promoted
good molecular ordering and had higher
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m compared to
m than DNEA, which has
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Acknowledgment
[19] Kang MJ, Yamamoto T, Shinamura S, Miyazaki E, Takimiya K. Unique three-
This work was supported by the Ministry of Science, ICT and
Future Planning (NRF-2014R1A2A1A05004993) and
(2015R1A2A10055620).
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a new
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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.dyepig.2016.04.017.
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