Organic semiconducting materials from sulfur-hetero benzo[k]fluoranthene derivatives: Synthesis, photophysical properties, and thin film transistor fabrication

Article abstract of DOI:10.1021/jo800606b

(Graph Presented) A new family of air-stable sulfur-hetero oligoarenes based on the benzo[k] fluoranthene unit has been facilely developed as the active materials for thin film organic field-effect transistors. The Diels-Alder reaction between cyclopentadienone 1 and 2,2′-(ethyne-1,2-diyl) bisthiophene followed by decarbonylation afforded fluoranthene derivative 2. After bromination and subsequent substitution through Suzuki coupling reaction, the FeCl₃-oxidative cyclization produced sulfur-hetero benzo[k]fluoranthene derivatives 8-12. In dilute chloroform solution, the absorption and emission behaviors of 2 and 4-7 showed characteristic features of the fluoranthene units, while their emission λ_max red-shifted with an increase of the effective conjugation length. The steady state absorption and emission spectra of these newly synthesized compounds were thoroughly investigated and discussed. Thin film organic field-effect transistors (OFETs) using 8-11 as active materials were fabricated in a top contact configuration. Substituents at the skeleton play an important role in the film morphologies, which lead to different mobilities, while the charge mobilities of 8-11 from OFETs were improved after thermal annealing of the thin films. A carrier mobility as high as 0.083 cm² V^-1 s^_1 and current on/off ratio of 10⁶ were achieved through vacuum-deposited film followed by the thermal annealing process from 11.

Full text of DOI:10.1021/jo800606b

Organic Semiconducting Materials from Sulfur-Hetero
Benzo[k]fluoranthene Derivatives: Synthesis, Photophysical
Properties, and Thin Film Transistor Fabrication
Qifan Yan,† Yan Zhou,† Ben-Bo Ni,† Yuguo Ma,*,† Jian Wang,*,‡ Jian Pei,*,† and
Yong Cao‡
Beijing National Laboratory for Molecular Sciences (BNLMS), Key Laboratories of Bioorganic Chemistry
and Molecular Engineering and of Polymer Chemistry and Physics of Ministry of Education, College of
Chemistry, Peking UniVersity, Beijing 100871, and Institute of Polymer Optoelectronic Materials and
DeVices, South China UniVersity of Technology, and Key Laboratory of Specially Functional Materials of
Ministry of Education, Guangzhou 510640, China
jianpei@pku.edu.cn
ReceiVed March 18, 2008
A new family of air-stable sulfur-hetero oligoarenes based on the benzo[k]fluoranthene unit has been
facilely developed as the active materials for thin film organic field-effect transistors. The Diels-Alder
reaction between cyclopentadienone 1 and 2,2′-(ethyne-1,2-diyl)bisthiophene followed by decarbonylation
afforded fluoranthene derivative 2. After bromination and subsequent substitution through Suzuki coupling
reaction, the FeCl3-oxidative cyclization produced sulfur-hetero benzo[k]fluoranthene derivatives 8-12.
In dilute chloroform solution, the absorption and emission behaviors of 2 and 4-7 showed characteristic
features of the fluoranthene units, while their emission λmax red-shifted with an increase of the effective
conjugation length. The steady state absorption and emission spectra of these newly synthesized compounds
were thoroughly investigated and discussed. Thin film organic field-effect transistors (OFETs) using 8-11
as active materials were fabricated in a “top contact” configuration. Substituents at the skeleton play an
important role in the film morphologies, which lead to different mobilities, while the charge mobilities
of 8-11 from OFETs were improved after thermal annealing of the thin films. A carrier mobility as high
as 0.083 cm2 V-1 s-1 and current on/off ratio of 106 were achieved through vacuum-deposited film followed
by the thermal annealing process from 11.
Introduction
substrate. Moreover, their optoelectronic properties can be tuned
through chemical engineering. Therefore, organic field-effect
transistors (OFETs) have been extensively employed in organic-
based electronic circuits to drive displays,5–8 to operate sensors,9,10
and so on.
Among various organic semiconductors for OFETs, pentacene
is regarded as a promising candidate owing to its high hole
mobility.11–13 However, pentacene is rather unstable, especially
under an electric field, and difficult to process due to its poor
Field-effect transistors (FETs) have been investigated inten-
sively since their discovery in the 1960s owing to their important
role in integrated circuits.1 In comparison with inorganic
semiconducting materials such as silicon, organic semiconduc-
tors can be easily processed with low cost and low temperature
processing techniques, such as spin-casting and inkjet printing,2–4
which enable large area electronics fabricated on a flexible
† Peking University.
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M. A.; Schmidt, M. A.; Bulovic´, V. AdV. Func. Mater. 2007, 17, 2722–2727.
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‡ South China University of Technology.
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5328 J. Org. Chem. 2008, 73, 5328–5339
10.1021/jo800606b CCC: $40.75 2008 American Chemical Society
Published on Web 06/25/2008

Sulfur-Hetero Benzo[k]fluoranthene DeriVatiVes
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J. Org. Chem. Vol. 73, No. 14, 2008 5329

Yan et al.
Moreover, discotic polycyclic aromatic hydrocarbons (PAHs)
such as hexabenzocoronenes (HBCs) and perylene derivatives
were also employed as active materials for OFETs.20,23,87–93
carrier transporting is usually maximized along the π-stacking
orientation.22,25,108 Fluoranthene and benzo[k]fluoranthene de-
rivatives have previously been employed to serve as the basic
skeleton to prepare a blue-light emitter for organic light-emitting
diodes (OLEDs).109 Such a π-extended planar core containing
sulfur atoms is employed to facilitate molecular self-association
driven by π-π stacking. Herein, we report the synthesis of a
family of novel hetero oligoarenes based on fluoranthene and
benzo[k]fluoranthene unit with good stability in air. We have
developed an expedient route to target molecules 8-11 in four-
step reaction sequences. Fused thiophene aromatic compounds
were developed as new organic semiconductors to avoid
sterically induced twist between proximal aromatic rings. Such
large planar conjugated molecules with flexible side chains
provide a variety of intra- and intermolecular interactions in
these thiophene-based materials, such as van der Waals interac-
tions, π-π stacking, and sulfur-sulfur interactions, to es-
sentially achieve high charge carrier mobilities.47,102,110,111 The
compounds reported herein combine the elements of oligoarenes
and peripheral sulfur atoms, which, we speculate, would be
beneficial to device performance in OFETs. All compounds
exhibit good thermal and oxidative stability in air. The thermal
annealing process was conducted directly after vacuum-deposit-
ing the material onto the cold, n-octadecyltrichlorosilane (OTS)
treated substrate. Crystalline films were obtained, and an increase
in the mobility of several orders of magnitude and on/off ratio
was achieved with such a technique. It is a promising, efficient
technique to fabricate OFETs. Thermal annealing was employed
to improve transistor performances, which is seldomly used for
crystalline small molecular materials. The thin film transistors
with carrier mobility of 0.083 cm2 V-1 s-1 are realized from
11.
Since both the molecular size and shape of oligoarenes play
very important roles in their electronic properties and assembly,
it is necessary to design and synthesize oligoarenes with diverse
structures for systematic investigation of their structure-property
relationships and specific applications.1,47,54,87,93–106 In our
previous contributions, we reported several π-extended, con-
densed, sulfur-rich benzothiophenes favoring π-π interaction.
Such oligoarenes can easily form bulk quantity individual
microwires through self-assembly. The organic transistors based
on these molecules with the mobility of 0.01 cm2 V-1 s-1 were
successfully achieved, demonstrating that such a processing
strategy for air-stable π-extended condensed benzothiophenes
with good performance is quite attractive.28,107 Theoretical
calculations and experimental results suggest that the charge
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Results and Discussion
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Synthesis and NMR Characterization. Scheme 1 illustrates
the synthetic approach to the designed sulfur-hetero benzo[k]-
fluoranthene derivatives 8-11. Cyclopentadienone 1, which is
usually in equilibrium with its dimer, was obtained as a light
yellow solid in good yield according to the literature.112 The
Diels-Alder reaction between cyclopentadienone 1 and 2,2′-
(ethyne-1,2-diyl)bisthiophene followed by decarbonylation in
refluxed 1,2,4-trimethylbenzene produced 2 in 69% yield. Prior
to the oxidative cyclization by FeCl3 to form the carbon-carbon
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5330 J. Org. Chem. Vol. 73, No. 14, 2008

Sulfur-Hetero Benzo[k]fluoranthene DeriVatiVes
SCHEME 1. Synthesis of Benzo[k]fluoranthene Derivatives 8-11 and Structure of Fluoranthene and Benzo[k]fluoranthene
1
SCHEME 2. Synthesis of Benzo[k]fluoranthene Model 12
downfield as shown in the H NMR spectrum of 8. A similar
1
downfield shift (ca. 0.9 ppm) of Ha was also observed in H
NMR spectra of the other three compounds. The data clearly
indicate the formation of the large polycyclic aromatic planar
π-system after the cyclization (see the Supporting Information).
All compounds were readily soluble in common organic
solvents, such as CH2Cl2, CHCl3, and THF.
Photophysical Properties in Solutions. Figure 1 shows the
absorption spectra of 2 and 4-7 in dilute chloroform (CHCl3)
solutions (1.0 × 10-5 M) in comparison with that of fluoran-
thene. The absorption spectrum of fluoranthene in dilute solution
exhibited two major bands with absorption λmax at about 289
and 360 nm.115 In comparison with fluoranthene, the absorption
spectrum of 2 in dilute CHCl3 solution also showed two major
bands with absorption λmax at about 287 and 383 nm. As shown
in Figure 1, 4-7 exhibited two major bands with absorption
phenylboronic acid or methoxyphenylboronic acids produced
4-7 in 85-92% yield, respectively. Optimizing the oxidative
cyclization by examining the amount of FeCl3 added as well as
the reaction temperature, the oxidative cyclization protocol
described in our previous contributions28,106,113,114 was followed
to construct the benzo[k]fluoranthene skeleton via thienyl-thienyl
carbon-carbon bond formation and afforded 8-11 in 79-96%
yield. Compound 12 as a model was also prepared through the
same oxidative cyclization reaction from 3 as shown in Scheme
λmax at about 300 and 380 nm, which showed almost the same
behaviors at the absorption band at 380 nm as that of 2.
Although the absorption bands of 4-7 at 380 nm were not
affected by the introduction of the phenyl ring, their absorption
bands at about 300 nm became very broad in comparison with
that of 2.
Figure 1 also shows the emission spectra of 2 and 4-7 in
dilute chloroform solutions (1.0 × 10-5 M). The emission
spectrum of 2 peaked at 467 nm, which slightly red-shifted
caused by thiophene ring substituents compared to those of
fluoranthene (443 and 463 nm).115 Compound 4 showed almost
the same emission behaviors as 6, and the emission spectra of
5 and 7 were approximately identical. In comparison with that
of 2, the emission λmax red-shifted about 33 nm to ca. 500 nm
for 5 and 7 and about 43 nm to ca. 510 nm for 4 and 6,
respectively. The emission λmax of 4 and 6 red-shifted about 10
nm compared to those of 5 and 7.
1
2. H NMR was employed to monitor oxidative cyclization
reactions. Two main transformations were found to be strong
evidences to support carbon-carbon bond formation: first, two
resonance signals of the protons in thiophene rings converted
into one single peak downfield; second, the resonance signals
of the R-CH2 of the alkyl chains became broader and shifted
downfield after oxidative cyclization reactions. The signals of
Hb exhibited almost the same chemical shift at about 6.75 ppm
1
for 4-7 and disappeared in the H NMR spectra of 8-11. In
1
the H NMR spectrum of 4, the signal assigned to Ha was at
chemical shift of 7.33 ppm as a duplet. After the cyclization,
such a signal converted to a singlet and moved ca. 0.94 ppm
(113) Pei, J.; Zhang, W.-Y.; Mao, J.; Zhou, X.-H. Tetrahedron Lett. 2006,
47, 1551–1554.
Figure 2 shows the absorption spectra of 8-12 in dilute
(114) Zhou, Y.; Liu, W.-J.; Zhang, W.; Cao, X.-Y.; Zhou, Q.-F.; Ma, Y.;
Pei, J. J. Org. Chem. 2006, 71, 6822–6828.
chloroform solutions (1.0 × 10-5 M), in which the benzo[k-
J. Org. Chem. Vol. 73, No. 14, 2008 5331

Yan et al.
FIGURE 1. Absorption and emission spectra of 2, 4-7, and fluoranthene in dilute CHCl3 solutions (1.0 × 10-5 M). The emission spectra were
recorded using the following excitation wavelengths: 274 nm for 2; 298 nm for 4; 300 nm for 5; 290 nm for 6; and 296 nm for 7.
FIGURE 2. Absorption and emission spectra of compounds 8-12 in dilute CHCl3 solutions (1.0 × 10-5 M). The emission spectra were recorded
using the following excitation wavelengths: 474 nm for 8; 492 nm for 9; 479 nm for 10; and 474 nm for 11.
FIGURE 3. Fluorescence spectra of 8 (a), 9 (b), 10 (c), and 11 (d) in chloroform solutions with different concentrations and in the solid state.
]fluoranthene unit was constructed by the C-C bond formation
between two thiophene units. Semiconductors 8-11 demon-
strated very similar absorption and emission behavior, due to
the same skeleton in their molecular structures. For 12, its
absorption spectra exhibited two major bands with absorption
λmax at about 341 and 436 nm, which dramatically red-shifted
in comparison with those of fluoranthene. Such a red-shift was
attributed to the larger polycyclic aromatic benzo[k]fluoranthene
π-planar system. Three vibronic bands were observed with
absorption λmax at 390 (25 641 cm-1), 411 (24 330 cm-1), and
436 nm (22 936 cm-1) for 12. The vibrational energy levels in
the excited state of 12 appeared to be equally spaced with a
wavenumber separation of about 1394 cm-1, which is expected
from the theoretical model of an anharmonic oscillator.116
(115) Amin, S.; Balanikas, G.; Huie, K.; Hussain, N.; Geddie, J. E.; Hecht,
S. S. J. Org. Chem. 1985, 50, 4642–4646.
(116) Turro, N. J. Modern Molecular Photochemistry 1991; University
Science: Mill Valley, CA, 1991; Chapter 4.
5332 J. Org. Chem. Vol. 73, No. 14, 2008

Sulfur-Hetero Benzo[k]fluoranthene DeriVatiVes
TABLE 1. Photophysical Properties of 2 and 4-11 in Dilute
Solutions and in Solid States in Comparison with 12 and
Fluoranthene
Therefore, it was likely that the absorption bands of 12 at 436,
411, and 390 nm were corresponding to 0-0, 0-1, and 0-2
transitions, respectively. Similarly, the absorption bands of 8-11
at about 450, 426, and 407 nm were corresponding to 0-0,
0-1, and 0-2 transitions, respectively. However, in comparison
with 12, both major absorption bands of 8-11 red-shifted about
20 nm owing to the increase of effective conjugation length
with the introduction of aromatic substituents. The absorption
spectra of 8 and 10 exhibited a slight red-shift compared to
those of 9 and 11. This was compatible with the effect of
methoxy substitution in the phenyl unit. The o-methoxy and
p-methoxy groups diminish the energy difference between
LUMO and HOMO more effectively and thus cause the red
shift of adsorption and emission spectra. However, the m-
methoxy group did not cause any difference in absorption and
emission spectra of 9 compared to those of 11 that has no
methoxy substituents, indicating little extension in effective
conjugated length.
As shown in Figure 2, the emission λmax of 8-11 red-shifted
about 20 nm in comparison with that of 12. For 12, the emission
λmax peaked at 448 nm (22 321 cm-1) with a vibronic peak at
470 nm. The wavenumber separation (615 cm-1) between
absorption at 436 nm and emission λmax at 448 nm was less
than that of vibronic absorption bands (1394 cm-1). According
to the theoretical model of an anharmonic oscillator,116 it might
indicate that the emission band of 12 at 448 nm was corre-
sponding to a 0-0 transition. Similarly, the emission bands of
8-11 at ca. 470 nm were corresponding to 0-0 transitions.
The emission λmax of 8 and 10 all red-shifted several nanometers
relative to those of 9 and 11, similar to what has been observed
in their absorption spectra. The emission features of 8 were the
same as those of 10, while the emission features of 9 were same
as those of 11. In comparison with 4-7, the emission λmax of
8-11 blue-shifted about 35-40 nm due to the formation of
the more rigid skeleton after the cyclization.117
We also investigated their emission spectra at various
concentrations and in thin films. As shown in Figure 3a, with
an increase of the concentration of 8 in chloroform, after
normalization, we found that the relative emission intensity of
the shoulder at longer wavelength was continuously enhanced
and finally dominated the emission behaviors, which might be
due to the formation of the aggregates or excimers at high
concentration. The exciton might be delocalized in the whole
assembly through π-π interaction and thus had a lower energy
state than one localized in a single molecule.118 The aggregation
was also evidenced by the excitation spectra, consistent with a
monomer-aggregate transformation when the concentration
increased from 1.0 × 10-5 M to 1.0 × 10-4 M. Similar results
were observed from the emission behaviors of 9-11.
Figure 3 also shows the emission spectra of precipitates 8-11
evaporated from chloroform solutions (1 mg/mL). The emission
λmax red-shifted relative to those of solutions of their corre-
sponding compounds. This was natural because the molecules
in solid states had tighter and closer intermolecular interaction,
which would lower the energy of the exciton. For 8-11, the
emission λmax in drop-cast films red-shifted in comparison with
those in the dilute solution, which was in accordance with larger
aggregates in the solid state. Table 1 summarizes their photo-
physical properties both in solutions and in solids.
λ
abs/nm
(log ε)
λ
abs/nm
(solid)
λ
PL/nm
λPL/nm λPL/nm
b
compds
(CHCl3)
(CHCl3) (evapor- (drop- ΦF
ation) cast)a
2
274 (4.45), 275, 330, 467
287 (4.40) 369, 386
366 (4.11),
/
486
/
383 (4.11)
4
298 (4.57), 300, 313, 513
313 (4.40) 372, 388
369 (4.15),
/
492
/
384 (4.15)
5
6
7
8
299 (4.73), 301, 370, 499
367 (4.23) 387
384 (4.20)
300 (4.76), 303, 372, 510
369 (4.26) 388
384 (4.23)
299 (4.77), 301, 371, 500
368 (4.25) 388
384 (4.23)
343 (4.95), 382, 453, 472
360 (4.96) 484
429 (4.52),
/
485
503
497
524
/
/
/
/
/
526
0.16
0.23
0.18
0.23
/
455 (4.58)
9
341 (4.85), 380, 444, 465
357 (4.88) 475
426 (4.41),
531
518
508
/
513
518
510
/
452 (4.48)
10
11
12
343 (4.75), 376, 448, 470
361 (4.79) 478
427 (4.32),
454 (4.36)
340 (4.88), 370, 442, 465
355 (4.92) 472
424 (4.41),
450 (4.51)
289 (3.62),
302 (3.63)
341 (3.86),
390 (3.07)
411 (3.27),
436 (3.38)
/
448
fluoranthene 289 (4.69),
324 (3.92)
/
443, 463
/
/
/
344 (4.03),
360 (4.04)
a From chloroform solution (10-4 M). b ΦF of compounds 8-11 were
measured in chloroform using 9,10-diphenylanthracene (in cylcohexane,
ΦF ) 1.00) as reference.
Thermal Properties. Thermal properties of semiconductors
8-11, which were crucial for thermal annealing and device
performance, were measured by differential scanning calorimetry
(DSC), and the curves from the heating (negative heat flow,
downward peaks)/cooling (positive heat flow, upward peaks)
cycle are shown in Figure 4 (corresponding data are compiled
in Table 2). Melting points were estimated from the onset of
the endothermic peaks (negative heat flow), and fusion enthal-
pies were calculated from the integration of the melting curves.
All these four compounds exhibit good thermal stability with
melting temperatures above 250 °C (decomposition temperatures
are approximately 400 °C under a nitrogen atmosphere measured
by thermal gravimetric analysis). Melting/crystallization pro-
cesses were reversible through several heating/cooling cycles,
while neither a glass state nor a liquid crystal (LC) phase
transition was observed from the tests. Theses results are
attributed to the short length of the flexible alkyl chains in
(117) Peaden, P. A.; Lee, M. L.; Hirata, Y.; Novotny, M. Anal. Chem. 1980,
52, 2268–2271.
(118) Schwab, A. D.; Smith, D. E.; Rich, C. S.; Young, E. R.; Smith, W. F.;
de Paula, J. C. J. Phys. Chem. B 2003, 107, 11339–11345.
J. Org. Chem. Vol. 73, No. 14, 2008 5333

Yan et al.
FIGURE 4. Differential scanning calorimetry curves of powder of compounds 8-11. Upward peaks indicate endothermic processes, while downward
peaks indicate exothermic processes. Scan rate: 10 deg/min.
TABLE 2. Thermal and Electrochemical Properties
layer, consequently affecting the threshold voltage of the OFET
devices.120 The HOMO energies for semiconductors 8-11 in
the solid state were estimated by conventional electrochemical
techniques: cyclic voltammetry (CV) of drop casting films of
compounds 8-11 performed in CH3CN (Figure 6), in which
n-Bu4NPF6 was used as the supporting electrolyte. All com-
pounds showed partially reversible oxidation waves yielding
radical cations, and HOMO levels of thin films were determined
using the onset positions of the oxidation peaks, which were
internally referenced to ferrocene’s HOMO level (Table 2).
Methoxy substituents at the outer benzene rings resulted in an
increase in the HOMO levels, as we observed in 8-10 compared
to 11. Methoxy groups at ortho- and para-positions of the
benzene rings can conjugate more effectively with the aromatic
cores than that at meta-positions, causing the HOMO levels of
8 and 10 to be higher than that of 9. These results are consistent
with UV-vis absorption spectra.
endothermic peak
∆Hfus
Eox HOMO
(V) (eV)
mp
(°C)
compds
onset (°C)
(kJ·mol-1
)
8
9
10
11
283
260
315
316
52
44
51
45
0.81 -5.69 285.5-286.5
0.93 -5.81 268-269
0.82 -5.70 >320
0.99 -5.87 >320
Thin Film Fabrication and Characteristics. Vacuum
Deposition and Thermal Annealing of the Thin Films.
Semiconductors 8-11 were vacuum-evaporated onto n-octa-
decyltrichlorosilane (OTS) treated Si/SiO2 substrate at 4 × 10-4
Pa at a rate of 0.1 nm s-1 to form thin films (ca. 50 nm thick)
as active layers in transistors. The enhancement of OFET device
performance could be realized via the two most common
methods: raising the temperature of substrate during small-
molecule active layer materials vacuum deposition and thermal
annealing after active layer materials solution deposition.
Thermal annealing was often employed after semiconductor film
was formed by spin casting a solution of polymer or small-
molecule materials onto substrates.36,55,62,74,121–124 This process
requires materials to have a glass state or liquid crystal phase
upon heating in general. Small molecules such as pentacene
that do not have flexible alkyl chains do not undergo such a
phase transition process. Compounds like pentacene and perylene
diimides are often vacuum-deposited onto heated substrates,
FIGURE 5. Molecular model of compound 11 optimized by the MM3
+ UFF method. Molecular dimensions: x, 1.69 nm; y, 1.54 nm.
compounds 8-11. According to the molecular model (Figure
5), the alkyl chains are on longer than rigid aromatic skeletons.
Therefore, the alkyl side chain might not provide enough
flexibility to form a liquid crystal phase.119 As neither a glass
state nor a LC phase transition was observed for semiconductors
8-11, thermal annealing experiments were conducted below
the melting temperatures.
Electrochemical Properties in the Solid State. In the case
of organic p-type semiconducting materials where charge
transport occurs predominantly by hopping through HOMOs,
the relative HOMO energetic positions are crucial in determi-
nation of the injection of holes from the electrodes to the active
(119) Etchegoin, P. Phy. ReV. E 1997, 56, 538–548.
5334 J. Org. Chem. Vol. 73, No. 14, 2008
(120) Zaumseil, J.; Sirringhaus, H. Chem. ReV. 2007, 107, 1296–1323.

Sulfur-Hetero Benzo[k]fluoranthene DeriVatiVes
FIGURE 6. Cyclic voltammograms of drop casting films of compounds 8-11. Scan rate, 100 or 150 mV s-1; working electrode, Pt disk; auxiliary
electrode, Pt wire; reference electrode, Ag/AgCl; supporting electrolyte NBu4PF6 (0.1 M, CH3CN). Fc/Fc+ was used as the internal reference (0.36
V vs SCE). HOMO was estimated vs vacuum level: EHOMO ) -4.88 - Eox.106
FIGURE 7. AFM images of pristine thin films of compounds 8-11: (a) 8, (b) 9, (c) 10, (d) 11.
without further treatment.12,106 In our system, we used cold
substrate (25 °C) to receive crystalline molecules in the gas
phase to form thin films during vacuum deposition and then
employed thermal annealing to align molecules on the OTS-
treated SiO2 surface. When molecules in the gas phase came in
contact with cold substrates, the molecules formed an amorphous
film consisting of microcrystalline grains (this was verified by
X-ray diffraction experiments and atomic force microscope
(AFM) images, which will be discussed in the next sections).
(123) Ong, B. S.; Wu, Y.; Liu, P.; Gardner, S. AdV. Mater. 2005, 17, 1141–
1144.
(124) McCulloch, I.; Heeney, M.; Bailey, C.; Genevicius, K.; MacDonald,
I.; Shkunov, M.; Sparrowe, D.; Tierney, S.; Wagner, R.; Zhang, W.; Chabinyc,
M. L.; Kline, R. J.; Mcgehee, M. D.; Toney, M. F. Nat. Mater. 2006, 5, 328–
333.
(121) Salleo, A.; Arias, A. C. AdV. Mater. 2007, 19, 3540–3543.
(122) Chang, P. C.; Lee, J.; Huang, D.; Subramanian, V.; Murphy, A. R.;
Fre´chet, J. M. J. Chem. Mater. 2004, 16, 4783–4789.
(125) Roduner, E. Chem. Soc. ReV. 2006, 35, 583–592.
J. Org. Chem. Vol. 73, No. 14, 2008 5335

Yan et al.
FIGURE 8. AFM images of thin films of compounds 8-11 after thermal annealing: (a) 8, (b) 9, (c) 10, (d) 11.
FIGURE 9. Optical microscopy images of thermal annealed films of 10. Scale bar: 20 µm. Yellow region: gold metal. Green region: semiconducting
materials.
Ultrathin films or crystals at nano size (50 nm) have lower
melting temperature than the bulk materials caused by surface
effect.125 If the crystal melts upon heating, small crystals will
combine together to form large crystals that have the small
surface area driven by surface free energy, which will often
cause “dewetting”.30 Taking these into consideration, prior to
gold metal electrode deposition, we conducted thermal annealing
of these semiconductor films at approximately 40 °C below their
bulk melting points under a nitrogen atmosphere for 20 min,
followed by slowly cooling the thin films to 25 °C. Such a
process is easier to carry out and more economic compared to
elevating the temperature of substrates during vacuum deposi-
tion. In other words, this method demands simpler instruments
because the vacuum system is less complicated if the substrate
does not need heating. Also, a smaller amount of semiconductor
sample is required because after a one-time vacuum deposition
different temperatures can be applied during the thermal
annealing process to achieve different device performance.
Thermal annealing of these crystalline films effectively con-
verted microcrystallines to larger connected crystalline grains
with π-conjugated cores aligned approximately perpendicular
to the dielectric surface (this was verified by X-ray diffraction
experiments and AFM images discussed below), which will
favor hole transport in OFET devices.126–128
Thin Film Morphology by AFM. Topographical images of
the films of semiconductors 8-11 newly vacuum-deposited and
after thermal annealing obtained from AFM experiments were
carefully investigated. Figures 7 and 8 shows AFM images of
the vacuum-deposited films of 8-11 on OTS-treated SiO2/Si
substrates. Thermal annealing at proper temperatures effectively
combined small crystalline grains into continuously connected
large crystals. All films showed one transformation in common
after thermal energy was provided under certain temperatures:
increased grain size and decreased boundaries. This transforma-
tion would lead to hole-mobility enhancement, because higher
defect concentration in grain boundaries of multicrystal thin
films usually leads to poor carrier transport.127,128
The images of pristine films of 8 and 9 in Figures 7a and 7b
indicated that these rough films consist of discontinuous islands
(126) Sun, Y.; Liu, Y.; Zhu, D. J. Mater. Chem. 2005, 15, 53–65.
44.(127) Bao, Z.; Lovinger, A. J.; Dodabalapur, A. AdV. Mater. 1997, 9, 42–
(128) Bao, Z.; Lovinger, A. J.; Brown, J. J. Am. Chem. Soc. 1998, 120, 207–
208.
5336 J. Org. Chem. Vol. 73, No. 14, 2008

Sulfur-Hetero Benzo[k]fluoranthene DeriVatiVes
FIGURE 10. X-ray diffraction pattern of vacuum-deposited films of compounds 8-11: blue lines in the upper halves, before thermal annealing;
red lines in the bottom halves, after thermal annealing. The diffraction peak at 2θ ) 28.44° with huge intensity is due to the SiO2/Si substrate.
TABLE 3. d-Spacing Calculated from 2θ Diffraction Peaks of Films of 8-11 and Performance Parameters of OFETs Based on These Films
pristine film
annealed film
µ
compds
D (nm)
(cm2 V-1 s-1
)
Vth (V)
Ion/Ioff
D (nm)
µ (cm2 V-1 s-1
)
Vth (V)
Ion/Ioff
8
9
10
11
1.16
N.D.a
1.60
1.46
2 × 10-5
1.7 × 10-4
7.7 × 10-3
-1.4
-9.8
-33
6 × 102
1 × 104
2 × 102
1 × 105
N.D.a
1.57
1.57
1.50
2.6 × 10-3
4.9 × 10-3
3.5 × 10-3
0.083
-5.9
-12.6
8.5
5 × 105
7 × 105
2 × 103
3 × 106
7 × 10-5
-11
-37
a Not determined.
with the size of ca. 0.1 µm. After annealing for 20 min, isolated
grains combined into larger crystalline grains connected to each
other with the size of ca. 0.5 µm. The longest crystal had the
length of ca. 1 µm and an aspect ratio of approximately 10, as
shown in Figures 8a and 8b, resulting in increased grain size
and decreased grain boundaries. The AFM images in Figures
7c and 7d of the pristine films of 10 and 11 obviously showed
one-dimensional nanostructures, which can be attributed to
stronger one-dimensional elongation trends of these two mol-
ecules’ crystals compared to 8 and 9. These one-dimensional
elongation trends, resulting from π-π stacking between aro-
matic cores, would lead to improved charge carrier mobility in
transistor devices, proved by theoretical calculations and
experimental results,25,47,129 (supported by the OFET charac-
terization of films before thermal annealing). After thermal
annealing at 270 °C for 20 min, the film of 11 was composed
of the largest crystalline grains among these four compounds
with the size of ca. 2.0 (maximum size 5 µm) close to each
other, which indicated that crystalline boundaries were ef-
fectively decreased and would enhance OFET performance
consequently.
which would be used as the TFT (thin film transistor) active
layer, exhibited a highly ordered one-dimensional nanostructure
with length approaching the magnitude of micrometer indicated
by the scale bar. Notably, it is very rare that this kind of ordered
one-dimensional nanostructure can be achieved through simple
vacuum deposition of the materials onto bare OTS-treated SiO2/
Si substrate, without introducing materials such as para-
hexaphenyl88 or an advanced film fabrication technique such
as zone casting.23 As optical microscope pictures taken from a
larger area of the same region for AFM are shown in Figure 9,
the ordered nanostructure of 10 associates into macroscale
channels with a length of ca. 100 µm. However, unfortunately,
the nanostructures were grown heading random directions within
the substrate plane. As shown by the picture on the right side,
we concluded both of the one-dimensional structures in the
direction nearly parallel to the gold electrode edge along with
one-dimensional channels that intersected would countercheck
charge transporting in OFET and lead to a hole mobility lower
than expected.
Thin Film X-ray Diffraction. X-ray diffraction experiments
have been carried out to acquire insight into the microstructure
of the vacuum deposited thin films of the semiconductors 8-11.
We carefully compared the diffraction patterns of the films
before and after thermal annealing to understand the change of
the microstructures, which would affect charge transport proper-
ties of organic thin films in transistor devices. X-ray diffraction
scans are utilized to obtain out-of-plane d-spacings in the films,
The most interesting images were obtained from the films of
10, shown in Figure 7c and 8c. Thermal annealed films of 10,
(129) Miao, Q.; Lefenfeld, M.; Nguyen, T. Q.; Siegrist, T.; Kloc, C.; Nuckolls,
C. AdV. Mater. 2005, 17, 407–412.
(130) Pal, M.; Kundu, N. G. J. Chem. Soc., Perkin Trans. 1 1996, 5, 449–
452.
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Yan et al.
FIGURE 11. Electrical characterization of OFETs based on 8-11 after thermal annealing. (a), (c), (e), (g): output curves of 8-11 taken at different
gate voltages, respectively. (b), (d), (f), (h): transfer curves of 8-11 at constant VD ) -60 V, respectively.
which allow us to determine the molecular orientations relative
to the substrate surface. As shown in the upper halves in Figure
10, it was quite difficult to find any sharp diffraction peaks in
the pristine films, which indicated the crystalline quality of these
polycrystalline organic thin films was very poor. Only first-
order diffraction peaks around 2θ ) 5 o with very low intensity
were observed in pristine films of 8, 10, and 11. But d-spacings
of 0.347 nm calculated from diffraction small peaks at 2θ )
25.7 ° (arrow pointed), which were typical distances of π-π
interactions, showed up in the XRD patterns of pristine films
of 9 and 11. The observation of these diffraction peaks could
be due to the molecules lying on the substrates in different
orientations in pristine films. After thermal annealing, the quality
of crystalline of these films improved remarkably, which was
confirmed by the increased intensity of diffraction peaks. Except
8, annealed films of 9-11 exhibit several peaks for the
predominating phases, which have d-spacings of nearly 1.6 nm
(calculated d-spacing are summarized in Table 3). These
d-spacings, which were assignable to (00h) reflections,45 were
close to the molecular dimensions obtained from the molecular
model, and we could judge that the aromatic cores of the
molecules are aligned approximately perpendicular to the SiO2/
Si surface in the predominating phase.4 Molecule 8, due to its
torsional strain introduced by the ortho-methoxy group, might
not stack so effectively as other isomers, resulting in a less-
ordered crystalline structure.
Field-Effect Transistor Characterization. Organic field-
effect transistor devices using 8-11 as the active layers were
fabricated in a top-contact configuration to analyze the materials’
chargetransportcharacteristicsviaevaluationofthecurrent-voltage
response. Three performance parameters were extracted from
the FET I-V response curves: the charge carrier mobility (µ),
5338 J. Org. Chem. Vol. 73, No. 14, 2008

Sulfur-Hetero Benzo[k]fluoranthene DeriVatiVes
current on-off ratio (Ion/Ioff), and threshold voltage (Vth). Both
newly deposited films and thermal annealed films of 8-11 were
employed as active layers in FET devices after gold metal was
vacuum-deposited as an electrode. All OFET devices were tested
under ambient condition several times, and their performance
remained persistent over two weeks (stored in air), indicating
the high stability of these transistors based on 8-11. In this
contribution, we proved that thermal annealing is an efficient
method to improve the transistor device performance based on
crystalline small molecules without the glass state or LC phase:
both hole mobility and on/off current ratio in annealed films
were several orders of magnitude larger than that in newly
deposited films. (Performance data are summarized in Table 3,
and output and transfer curves of annealing films of 8-11 are
depicted in Figure 11.) These device performance improvements
are expected as results of increased grain size and higher
crystalline quality of active layers, which were verified by AFM
and XRD, respectively. The threshold voltages of the transistor
device based on annealed films of 8-11 are -5.9, -12.6, 8.5,
and -37 V, respectively. Vth values of semiconductor films of
8-11 are in good agreement with CV results: the higher the
HOMO levels are, the easier the molecules are oxidized and
the easier the holes are injected from electrodes. The trends of
mobility of semiconductor films of 8-11 are in the sequence
11 > 9, 10 > 8, which is consistent with AFM and XRD results:
the higher the quality of crystalline is, the easier holes are
transported. Some conclusions regarding structure-property
relationships could be drawn from the transistor performance
parameters and data gathered from AFM and XRD experiments.
First, methoxy groups at ortho-positions, which would lead to
a larger torsion angle between the outer benzene ring and rigid
core compared to that at meta- and para-positions, caused less-
ordered crystalline structure on the substrate, disfavoring charge
carrier transporting. Second, decreased quality of crystalline
caused by methoxy substitution would lower the mobilities of
transistors. As a result of these two reasons, the best performance
was realized from 11. Hole mobility in annealed films of 11
could reach as high as 0.083 cm2 V-1 s-1, along with the current
on/off ratio of 106, without further device optimization. These
results indicate that our novel fluoranthene-based compounds
are promising candidates for organic field-effect transistor active
layer materials.
that such a conjugated skeleton is very rigid. OFETs based on
thin films of 8-11 have been fabricated. As shown by Figures
8c and 9, long-range one-dimensional nanostructures achieved
previously only by means of inducing layers or zone-casting
can now be realized via chemical engineering. Substituents at
different positions on the same conjugated aromatic core in
8-11 affect the HOMO levels, quality of crystalline, and film
morphologies, consequently influencing transistor performance.
More specifically, the structure-property relationships could
be deduced from our experimental result. Decreasing crystalline
degree caused by methoxy groups would lower the charge carrier
mobility in FET devices. The thermal annealing process is an
efficient way to decrease the grain boundary and increase the
crystalline degree. The performance of thin film transistors has
been remarkably improved by means of thermal annealing
developed by our group, which provides a new possibility in
fabricating OFET devices.
Experimental
Fabrications of Organic Films and OFETs. Organic field-effect
transistors based on fluoranthene derivatives 8-11 were fabricated
in a “top contact” configuration. A heavily doped, n-type Si wafer
was used as the gate electrode and substrate. A thermally grown
SiO2 layer (ca. 300 nm thick) acted as a gate insulator with a unit
capacitance of 11 nF cm-2. Prior to the deposition of semiconduc-
tors, the Si/SiO2 substrate was immersed in 10 mg/mL n-
octadecyltrichlorosilane (OTS) in toluene at 60 °C for 20 min.
Semiconductors 8-11 were vacuum-evaporated onto Si/SiO2
substrate at 4 × 10-4 Pa at a rate of 0.1 nm s-1 to form thin films
(ca. 50 nm thick) as active layers. Source and drain electrodes (ca.
100 nm thick) were prepared by deposition of Au with a shadow
mask. The defined channel width (W) and channel length (L) were
100 µm and 10 mm, respectively. The annealing procedure was
done before the deposition of Au under nitrogen. Thin films of
semiconductors 8-11 were thermally annealed at 240, 235, 230,
and 270 °C for 20 min, respectively.
Characterization of Thin Film OFETs. The thin films of
semiconductors were imaged with the tapping mode of AFM. The
organic transistors were tested under ambient condition at room
temperature. Field-effect mobilities (µFET) were extracted from the
saturation region of Id using the equation Id ) (WCi/2L)µFET(Vg-
Vth)2.
Acknowledgment. This research was financially supported
by the Major State Basic Research Development Program (No.
2006CB921602 and 2007CB808000) from the Minister of
Science and Technology and by the National Natural Science
Foundation of China.
Conclusions
In summary, we have developed a facile approach to a new
family of air-stable sulfur-hetero oligoarenes containing the
benzo[k]fluoranthene unit through the reaction sequence of the
Diels-Alder reaction, decarbonylation, and FeCl3-oxidative
cyclization. In comparison with their corresponding precursors
4-7 in dilute solutions, the absorption λmax of 8-11 red-shifts
after the formation of a larger polycyclic aromatic skeleton;
however, their emission λmax blue-shifts, which demonstrates
Supporting Information Available: Synthesis details and
1H and 13C NMR spectra of all new compounds 2-12 (PDF).
This material is available free of charge via the Internet at
http://pubs.acs.org.
JO800606B
J. Org. Chem. Vol. 73, No. 14, 2008 5339