Semiconducting 2,6,9,10-Tetrakis(phenylethynyl)anthracene derivatives: Effect of substitution positions on molecular energies

Article abstract of DOI:10.1021/ol200299s

2,6-Bis((4-hexylphenyl)ethynyl)-9,10-bis(phenylethynyl)anthracene, 4, and 9,10-bis((4-hexylphenyl)ethynyl)-2,6-bis (phenyl ethynyl)anthracene, 5, have been synthesized to study their electronic and photophysical properties. It should be noted that the dif

Full text of DOI:10.1021/ol200299s

ORGANIC
LETTERS
2011
Vol. 13, No. 8
1948–1951
Semiconducting 2,6,9,10-
Tetrakis(phenylethynyl)anthracene
Derivatives: Effect of Substitution
Positions on Molecular Energies
Jung A Hur†, Suk Young Bae, Kyung Hwan Kim, Tae Wan Lee, Min Ju Cho, and
Dong Hoon Choi*
Department of Chemistry, College of Science, Research Institute for Natural Sciences,
Korea University, 5 Anam-dong, Sungbuk-gu, Seoul 136-701, Korea
dhchoi8803@korea.ac.kr
Received January 31, 2011
ABSTRACT
2,6-Bis((4-hexylphenyl)ethynyl)-9,10-bis(phenylethynyl)anthracene, 4, and 9,10-bis((4-hexylphenyl)ethynyl)-2,6-bis (phenyl ethynyl)anthracene, 5,
have been synthesized to study their electronic and photophysical properties. It should be noted that the difference between these compounds is
the substitution position of 1-ethynyl-4-hexylbenzene groups into an anthracene ring. In particular, substitution in the 9,10-positions of the
anthracene ring enhanced J-aggregated intermolecular interactions. Since 5 has a lower bandgap energy and more compact film morphology, it
exhibited higher hole mobility (∼0.27 cm2 V-1 s-1) in thin-film transistor devices.
Because of recent rapid progress in the development of
novel p-type organic semiconductors, derivatives of pen-
tacene, rubrene, anthracene, and thiophene have attracted
attention because of their applications in high-perfor-
mance organic thin-film transistors (OTFTs).1-5 Research
has focused on replacing vacuum-processable materials
with solution-processable materials to overcome severe
limitations in large-scale device fabrication due to the
complexity of the process.6-10 Two-dimensional π-
extended conjugated molecular structures have shown
effective charge-transport properties as films, satisfying
the requirements for soluble semiconducting materials.11,12
It is well recognized that electronic structure, molecular
structure, and morphology of thin films are important for
enhancing carrier mobility in TFT devices. With respect to
“electronic structure,” small bandgap energy, large band-
width, small ionization potential, and large π-conjugation
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r
10.1021/ol200299s
Published on Web 03/16/2011
2011 American Chemical Society

length are essential requirements for designing high-mobility
semiconducting molecules. To steer the bandgap energy
and molecular energy levels, such as the highest occupied
molecular orbital (HOMO) and the lowest unoccupied
molecular orbital (LUMO) energies, varying the conjuga-
tion length of a molecule or introducing donor/acceptors
into the π-conjugated molecular frames are often adopted
for designing new semiconducting materials, which is the
basis of “molecular structure”. Instead of changing the
entire molecular structure, it is preferable to use a method
capable of changing the molecular energies in a film only
by changing the position of alkyl substituents (e.g.,
solubilizing groups) while maintaining an identical π-
conjugated molecular skeleton. Even if the electronic
and photophysical properties of solutions of two dif-
ferent molecules are identical, their solid states can
exhibit different properties, which is the basis of “crys-
talline morphologhy.”
Scheme 1. Synthesis of 2,6,9,10-Tetrakis(phenylethynyl)
anthracene Derivatives
In this paper, we present one interesting π-conjugated
coreunit, 2,6,9,10-tetrakis(phenylethynyl)anthracene. Our
molecular design principle is to vary the substitution
positionsof 1-ethynyl-4-hexylbenzenegroupsinananthra-
cene ring. We tethered two 1-ethynyl-4-hexylbenzene
groups into 2,6-positions for 4 and into 9,10-positions for
5. We show that the hexyl side group enhances not only the
solubility but also the degree of self-organization (i.e.,
molecular ordering). The facile and high-yield synthesis of
two new p-type anthracene-based semiconducting mole-
cules is reported in this work. Scheme 1 illustrates the syn-
thetic routes for the two molecules.
New anthracene-containing π-conjugated molecules
have been synthesized using reduction and Sonogashira
coupling reactions. Ethynylbenzene was synthesized fol-
lowing the literature method.13 2,6-Dibromoanthracene-
9,10-dione was also synthesized according to an estab-
lished method.14,15 Addition of the dione compound to 2
molar equiv of 1-ethynyl-4-hexylbenzene in the presence of
bis(triphenylphosphine)palladium(II) dichloride and cop-
per iodide afforded 1. The reduction reaction using tin
chloride in an acidic medium gave the desired compound,
2.16 For preparation of 4, 2,6-bis((4-hexylphenyl)ethynyl)
anthracene-9,10-dione was also used to anchor the ethy-
nylbenzene through reduction, similar to the method for 2.
Sonogashira coupling reactions were performed, provid-
ing 5 in a 47% yield.
It should be noted that 2,6,9,10-tetrakis(phenylethynyl)
anthracene is not soluble in common organic solvents.
After tethering hexyl chains to phenyl rings, the molecules
showed good solubility (10 mg/1 mLofCHCl3 for 4 and 5).
The identity and purity of the synthetic materials were
confirmed by 1H NMR, 13C NMR, HRMS, and elemental
analysis (see the Supporting Information ).
Recently, we reported the facile synthesis of 5,50-((9,10-
bis((4-hexylphenyl)ethynyl)anthracene-2,6-diyl)bis-
(ethyne-2,1-diyl))bis(2-hexylthiophene), which exhibited
superior p-type semiconducting behavior evidenced by very
high carrier mobility in a TFT device.16 However, the
origin of strong intermolecular interactions in film form
was ambiguous. Therefore, the common phenylethynyl
groups in four positions were introduced into the anthra-
cene ring, and then,we varied two substitution positions of
1-ethynyl-4-hexylbenzene in the ring.
The thermal properties of the molecules were character-
ized by differential scanning calorimetry (DSC) and ther-
mogravimetric analysis (TGA). DSC measurements were
performed under nitrogen with the highest temperature
limited to below the decomposition temperature. Mole-
cules 4 and 5 exhibited distinct crystalline-isotropic transi-
tion temperatures of 183 and 223 °C and cold crystalli-
zation temperatures (Tms) of 154 and 127 °C, respectively
(see Figure 1S, Supporting Information). Comparison
showed that the presence of two 1-ethynyl-4-hexylbenzene
peripheral groups at the 9,10-positions of the anthracene
ring induced higher Tm, suggesting that a stronger inter-
molecular interaction exists for the denser molecular pack-
ing along the 9,10-directions in 5. TGA measurements
revealed that the molecules had almost identical, good
thermal stabilities (Td’s of 4 and 5 = 428 °C, see Figure 2S,
Supporting Information).
(10) Tang, M. L.; Reichardt, A. D.; Miyaki, N.; Stoltenberg, R. M.;
Bao, Z. J. Am. Chem. Soc. 2008, 130, 6064–6065.
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Li, H.; Hu, W. Chem. Mater. 2009, 21, 2840–2845.
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Choi, D. H. Chem. Lett. 2010, 39, 396–397. (b) Pisula, W.; Menon, A.;
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Commun. 2009, 35, 5290–5292.
We hypothesized that the two peripheral hexyl groups in
4 and 5 gave rise to high crystallinity due to the tight
Org. Lett., Vol. 13, No. 8, 2011
1949

Table 1. Measured and Calculated Parameters of Molecules 4 and 5
molecule Tm (°C) Td (°C) λmaxabsa (nm) λmaxPLa (nm) λmaxabsb (nm) λmaxPLb (nm) HOMOc (eV) LUMOd (eV) Egopte(eV)
4
5
183
223
429
427
430, 459, 486
430, 466, 486
506, 539
513, 543
448, 473, 510
468, 499, 540
525, 563
557, 592
-5.78
-5.73
-3.44
-3.52
2.34
2.21
a Measured in chloroform with a concentration of 10-5 M. b Films were spin-coated from chloroform solution. c The values were obtained from cyclic
voltammograms. *Sample: film on Pt electrode. d HOMO (eV) - Eg (eV). e The optical bandgaps were obtained from absorption spectra of film
opt
samples.
packing of molecules along the lateral directions and,
hence, may enhance carrier-transport properties in TFT
devices.
To study intermolecular interactions between the mole-
cules, absorption spectra of the samples in chloroform
(conc 1 ꢀ10-6 mol/L) and were obtained as thin films
(Figure 1 and Table 1).
spun film, while no annealing effect was observed in the
film spectra of 5. The emission bands of 5 appeared at
longer wavelengths than those of 4, which is consistent
with the results of absorption spectral analysis. For in-
stance, a nonaggregated emission band appeared at 557
nm, and the emission band at 592 nm can be assigned to the
J-aggregated characteristic band. In particular, we ac-
quired the emission spectra of single crystals, which dis-
played longer emission bands compared with those of
solutions and films. The fluorescence lifetimes obtained
from time-correlated single-photon-counting measurement
can support the above explanation (see Figures 6S and 7S,
Supporting Information). The emission band in 5 was
slightly longer than that in 4, indicating formation of more
highly ordered structures in the crystalline morphology.
Figure 1. UV-vis absorption spectra of solution (-), as-spun
film (--), and thermally annealed film (- -): (a) 4, (b) 5. Δλ(a)
=
3
17 nm, Δλ(b)= 54 nm.
In 4 and 5, we observed a significant bathochromic shift
in the absorption spectra upon film formation, indicating
the generation of strong intermolecular interactions. In
contrast to the solution phase spectra, the spectra of the
two molecules as spin-cast films were more structured.
Comparing the wavelength shift from the absorption
maxima in solution and annealed film spectra indicated
that molecules of 5 are subject to stronger intermolecular
interactions in the solid state.
Figure 2. Photoluminescence (PL) spectra of 4 (a) and 5 (b):
solution spectra (-); as-spun film (--); single crystals (- -);
3
thermally annealed film (- -). *Single crystals were grown
3 3
via solvent slow diffusion methods at 25 °C (THF/MeOH =
1:3 v/v).
Compared with 4 (Δλ(a) = 17 nm), 5 exhibited a larger
bathochromic shift, (Δλ(b) = 54 nm), implying that a
higher degree of J-aggregation exists in the films.17-19
The optical bandgap (Egopt) of 5 is 2.21eV, which is smaller
than that of 4 (Egopt = 2.34 eV). Switching the substitution
positions of 1-ethynyl-4-hexylbenzene from the 2,6- to
9,10-positions caused the bandgap to decrease in the solid
state, although the solution state gave identical bandgap
energies (Egopt = 2.44 eVz).
Importantly, no thermal annealing effect was observed
in film samples of the two molecules, implying that the
films were fully comprised of polycrystalline domains that
were preorganized after spin-coating.
To elucidate the HOMO and LUMO energy levels of
these molecules, their electrochemical properties were in-
vestigated by cyclic voltammetry. Cyclic voltammograms
(CV) were recorded on a film sample, and the potentials
were obtained relative to an internal ferrocene reference
(Fc/Fcþ). These CV scans in the range of -1.5 V to þ1.5 V
(vs Ag/AgCl) show quasi-reversible oxidation peaks. Un-
fortunately, the reduction behaviors were irreversible;
therefore, we were unable to accurately estimate their
HOMO and LUMO energies.
To determine the LUMO levels, we combined the
oxidation potential in CV with the optical energy bandgap
(Egopt) resulting from the absorption edge in the absorp-
tion spectrum. Voltammograms of 4 and 5 in film form
Figure 2 shows emission spectra of the two molecules
with identical skeletons under an excitation wavelength of
370 nm. For 4, the emission spectrum of the thermal
annealed film was more structured than that of the as-
show that their lowest oxidative waves commonly occur
onset
around 1.38 and 1.33 V (Eox
solutions). As shown in Table 1, the films have HOMO
= þ1.09 V for 4 and 5
1950
Org. Lett., Vol. 13, No. 8, 2011

levels of -5.78 eV for 4 and -5.73 eV for 5. In addition, 4
and 5 have LUMO energy levels of around -3.44 and
-3.52 eV. Results from CV analyses of films confirmed the
effect of substitution position on the molecular energy
level.
Bottom-gate top-contact OTFT devices were fabricated
under ambient conditions without thermal annealing, and
gold source and drain electrodes were thermally evapo-
rated using the conventional method.20 A 400-nm thin film
of the semiconductor was deposited by spin-coatinga1wt%
solution of the molecules in chloroform without thermal
annealing. The channel length was L = 100 μm, and the
channel width was W= 1500 μm. Devices I and II contained
semiconducting layers of 4 and 5, respectively.
than that of device I (μ = 0.17 cm2 V-1 s-1). Interesting
properties of the devices include very high on/off current
ratios (Ion/off = 106-107). In device II, Ion/off >107 in-
dicates that the device was well fabricated, having great
interfacial contact and limited leakage current. Note that
carrier mobilities larger than 0.1 cm2 V-1 s-1 have rarely
been achieved in devices of as-spun films of organic
semiconducting molecules. The significantly large mobility
of device II is mainly attributable to the high degree of micro-
structural order of grains as well as the two-dimensional
molecular arrangement within grains associated with the
molecular structure of two hexyl peripheral groups.
In conclusion, we successfully synthesized and charac-
terized new anthracene-based conjugated molecules that
are solution processable. Depending on the substitution
positions of 1-ethynyl-4-hexylbenzene, the molecular en-
ergy levels and bandgap energies were tuned. Molecule 5,
bearing 1-ethynyl-4-hexylbenzene subsitituents at the 9,10-
positions in the anthracene ring, showed a much smaller
bandgap energy in film form, although the two molecules
are identical in the solution phase. Remarkably, the device
bearing the as-spun film of 5 showed greater enhancement
of carrier mobility, indicating that the uniform lamella and
π-stacking of molecule 5 facilitates carrier transport more
effectively.
Figure 3. Transfer characteristics of two OTFT devices: (a)
device I, (b) device II. *Insulator: n-octyltrichlorosilane (OTS)-
treated SiO2 *Inset: atomic force microscopy (AFM) images of
the prestine film in TFT devices.
Our study unambiguously demonstrates that molecular
energy levels, including optical and photophysical proper-
ties of films, can be modulated only by changing the
substitution positions with alkyl chains, although the
central conjugated molecular skeletons are identical.
The OTFTs of the molecules exhibited typical p-channel
field-effect transistor (FET) characteristics (Figure 3). The
output characteristics showed very good saturation beha-
viors and clear saturation currents that were quadratic to
the gate bias. The mobilities were obtained from the source
(S)-drain (D) current-voltage curves (IDS vs VDS) in well-
resolved saturation regions. The mobility of device II (μ =
0.27 cm2 V-1 s-1; μmax = 0.30 cm2 V-1 s-1) was higher
Acknowledgment. This research was supported by
Korea Science and Engineering (KOSEF R012007000-
1128402008) and by the Priority Research Centers Pro-
gram through the National Research Foundation of
Korea (NRF) funded by the Ministry of Education,
Science and Technology (NRF20100020209).
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Supporting Information Available. Experimental pro-
cedures and full characterizations data. Fabrication of
TFT devices, results of DFT calculations, thermal ana-
lysis data, lifetime measurements, NMR spectra of 4 and
5 etc. This material is available free of charge via Internet
at http://pubs.acs.org.
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