9,10-Ter-anthrylene-ethynylene: A new molecular architecture for solution processed anthracene-based thin film transistors

Article abstract of DOI:10.1039/b714865h

This study deals with the properties of a new molecular architecture for organic thin film transistors obtained by introducing 10-decylanthr-9-yl-ethynyl moieties at the anthracene 9, 10 positions. The resulting structure, formally a ter-anthrylene-ethynylene (D3ANT), is stable and endowed with sufficient solubility to guarantee the formation of good quality films. The molecule was characterised by ¹H NMR, mass spectrometry, IR, UV-vis and photoluminescence both in solution and in the solid state. Cyclic voltammetry measurements, combined with solution UV-vis, permitted the evaluation of HOMO and LUMO energy levels. A series of D3ANT-based OTFT devices were built both in bottom and top contact configurations by a spin coating technique. Top contact OTFT devices exhibited the best semiconducting characteristics, showing an average mobility of 1.2 × 10^-2 cm² V^-1 s^-1 with on-off ratios higher then 10⁴, while the highest mobility obtained was 0.055 cm² V^-1 s^-1. AFM thin-film characterization showed evidence of a granular structure. High-resolution STM images of D3ANT monolayers adsorbed on Au(111) and HOPG reveal highly-ordered self-organized domains with molecules lying flat on the substrates, but, remarkably, no reflections were recorded by out-of plane GIXRD of spin coated films on SiO₂. The Royal Society of Chemistry 2008.

Full text of DOI:10.1039/b714865h

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
9,10-Ter-anthrylene-ethynylene: a new molecular architecture for solution
processed anthracene-based thin film transistors
Antonio Dell’Aquila,a Francesco Marinelli,b Junie Tey,c Peisi Keg,c Yeng-Ming Lam,c
Oleksiy L. Kapitanchuk,†c Piero Mastrorilli,a Cosimo F. Nobile,a Pinalysa Cosma,b
d
Alexandr Marchenko, Denis Fichou, Subodh G. Mhaisalkar, Gian Paolo Suranna and Luisa Torsi
d
c
a
b
*
*
Received 26th September 2007, Accepted 10th December 2007
First published as an Advance Article on the web 9th January 2008
DOI: 10.1039/b714865h
This study deals with the properties of a new molecular architecture for organic thin film transistors
obtained by introducing 10-decylanthr-9-yl-ethynyl moieties at the anthracene 9,10 positions. The
resulting structure, formally a ter-anthrylene-ethynylene (D3ANT), is stable and endowed with
sufficient solubility to guarantee the formation of good quality films. The molecule was characterised
by 1H NMR, mass spectrometry, IR, UV–vis and photoluminescence both in solution and in the solid
state. Cyclic voltammetry measurements, combined with solution UV–vis, permitted the evaluation
of HOMO and LUMO energy levels. A series of D3ANT-based OTFT devices were built both in
bottom and top contact configurations by a spin coating technique. Top contact OTFT devices
exhibited the best semiconducting characteristics, showing an average mobility of 1.2 ꢀ 10ꢁ2 cm2 Vꢁ1 sꢁ1
with on–off ratios higher then 104, while the highest mobility obtained was 0.055 cm2 Vꢁ1 sꢁ1. AFM
thin-film characterization showed evidence of a granular structure. High-resolution STM images of
D3ANT monolayers adsorbed on Au(111) and HOPG reveal highly-ordered self-organized domains
with molecules lying flat on the substrates, but, remarkably, no reflections were recorded by out-of
plane GIXRD of spin coated films on SiO2.
obtainment of oligo-2,6-(9,10-bisalkynylanthrylene)s exhibiting
a hole mobility of ꢂ3 ꢀ 10ꢁ3 cm2 Vꢁ1 sꢁ1 as solution deposited
thin films.8 The oligomerization of anthracenes through the
9,10 positions would be an effective alternative as a higher
delocalization can in principle be achieved, besides those linking
positions are the most reactive ones. To the best of our know-
ledge no example has been provided so far of 9,10-linked anthra-
cenes exhibiting field-effect modulation properties, as poly and
oligo-9,10-anthrylenes9 show a high dihedral angle between the
anthracene units, hampering the effective delocalization. A
viable way of easing the steric hindrance between anthracene
units, while mantaining conjugation between them in a poten-
tially planar structure, is to link them through an ethynylene
bond. This concept was exploited for the synthesis of ladderlike
p-conjugated poly and oligo(anthrylene-ethynylenes)10
Introduction
The huge research efforts directed towards the synthesis of
organic semiconductor materials for applications in organic
thin-film transistors (OTFTs),1 have considerably widened the
library of promising structures. The building blocks acknow-
ledged as the most promising are without doubt thiophenes
and acenes.2 Among the latter, pentacene, particularly as high
vacuum deposited thin films, shows hole field-effect mobilities
as high as 3 cm2 Vꢁ1 sꢁ1 and on–off ratios higher then 106.3
Recently, 6,13-functionalized soluble and stable pentacenes
have been prepared,4 exhibiting hole mobilities as high as 0.17
cm2 Vꢁ1 sꢁ1 for solution deposited thin films.5 All this calls for
a focussed synthetic effort aimed at investigating compounds
holding a fused aromatic system, particularly as soluble systems.
Lately, the oligomerization of anthracene, which has only
recently been recognized as appealing for transistor applica-
tions,6 has been adopted as a strategy to extend the p-system
beyond the single anthracene unit.7 The introduction of solubi-
lising chains in the anthracene 9,10 positions allowed the
A
decyl alkyl chain functionalised 9,10-ter-anthrylene-
ethynylene, D3ANT, is the first example of a monodispersed
architecture in which three neighbouring anthracenes are
chemically linked by two ethynylene bonds. The end groups
provide D3ANT with a solubility allowing its deposition by
wet techniques. In this study, we report D3ANT synthesis,
characterization and field effect properties, as well as its
morphological structure, studied by AFM and STM.
aDepartment of Water Engineering and of Chemistry, Polytechnic of Bari,
Campus Universitario, via Orabona 4- Bari, Italy. E-mail: surannag@
poliba.it; Fax: +39 080 5963611; Tel: +39 080 5963603
bDepartment of Chemistry, University of Bari, Campus Universitario, via
Orabona 4- Bari, Italy
Experimental
cSchool of Materials Science and Engineering, Nanyang Technological
University, Singapore
dCEA-Saclay, Organic Nanostructures and Semiconductors Lab CNRS-
ˆ
CEA-UPMC, Bat. 461, SPCSI, Gif-sur-Yvette, France
The reactants were purchased from Aldrich or Acros and used
without further purification; 9,10-dibromoanthracene11 1 and
[(trimethylsilyl)ethynyl]zinc12 were synthesized according to
literature procedures. All manipulations were carried out under
† On leave from Bogolyubov Institute for Theoretical Physics, Kiev,
Ukraine
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inert nitrogen atmosphere using standard Schlenk techniques
unless otherwise specified. All solvents used were carefully dried
and freshly distilled. Chromatography was performed using
Merckꢀ Kieselgel 60 (230–400 mesh) silica gel or Macherey-
Nagel polygoprepꢀ 60–50 C18 reversed phase silica gel. 1H
NMR and 13C{1H} NMR spectra were recorded on a Bruker
Avance 400 and are reported in ppm relative to tetramethyl-
silane. FT-IR spectra (in KBr or as neat films between NaCl
plates) were recorded on a Bruker Vector 22 spectrophotometer.
UV–vis spectra were measured on a Kontron Uvikon 942.
Fluorescence spectra were obtained on a Varian Cary Eclipse
spectrofluorimeter. GC-MS data (EI, 70 eV) were acquired on
a HP 6890 instrument equipped with a HP-5MS capillary
column (crosslinked 5% Ph Me siloxane) 30.0 m ꢀ 250 mm ꢀ
0.25 mm coupled with a HP 5973 mass spectrometer.
(LYP).15 The standard basis set 6-31G(d) was used in all calcula-
tions, which were performed by the Gaussian’03 program
package16 with an energy convergences parameter of 10ꢁ4 eV.
No correction was applied for the zero energy reference, thus
resulting in a shift in absolute values, which was cancelled in
the determination of the energy gap.
9-Bromo-10-decylanthracene 2
To a suspension of 9,10-dibromoanthracene 1 (6.00 g, 17.85
mmol) in dry di-n-butyl ether (250 ml), a 2.0 M phenyllithium
solution in di-n-butyl ether (19.63 mmol) was added at ambient
temperature. After 3h stirring, 1-bromodecane was added
(7.90 g, 35.70 mmol) and the resulting solution was kept under
stirring overnight. The reaction mixture was poured into a satu-
rated NH4Cl solution and then extracted several times with
diethyl ether. The collected organic phases were dried over
anhydrous Na2SO4 and the solvent was removed under reduced
pressure. The crude product was chromatographed on reversed
phase (RP-18) silica using acetonitrile as eluent affording 2 as
a pale yellow solid (4.26 g, 60 %): 1H NMR (CDCl3) d ¼ 8.60 (d,
J ¼ 8.83 Hz, 2H), 8.29 (d, J ¼ 8.53, 2H), 7.61–7.58 (m, 4H), 3.59
(t, J ¼ 8.22 Hz, 2H), 1.85–1.77 (m, 2H), 1.62–1.55 (m, 2H),
1.42–1.21 (m, 12H), 0.88 (t, J ¼ 6.70 Hz, 3H); 13C NMR
(CDCl3) d ¼ 136.27, 130.37, 130.32, 128.65, 126.63, 125.59,
124.80, 121.56, 31.88, 31.41, 30.32, 29.61, 29.32, 28.38, 22.67,
14.11; nmax (KBr pellet)/cmꢁ1 3076, 3039, 2914, 2848, 1510,
1457, 1368, 1327, 1255, 1096, 1023, 893, 795, 745, 646, 575; m/z
(EI, 70 eV) 396 (M+), 269 (M+ ꢁ C9H19), 189 (M+ ꢁ C9H19 ꢁ Br).
LC-MS analyses were performed by direct injection on an
Agilent HPLC system equipped with DAD and MS systems
(Agilent 1100 LC-MS SL series) using an atmospheric pressure
chemical ionization interface (APCI). APCI conditions: positive
ion mode, flow rate 0.5 mL minꢁ1, nitrogen as nebulising and
drying gas, nebuliser pressure 60 psi, vaporizer temperature
350 ꢃC, corona current 4.0 mA, drying-gas flow 5 L minꢁ1, drying
gas temperature 350 ꢃC, capillary voltage 4000 V. Mass
spectrometry data were acquired in the scan mode (mass range
m/z ¼ 200–3000).
Thermogravimetric analyses (TGA) were carried out under a
nitrogen flow with a Perkin Elmer Pyris 6 thermobalance at a
scanning rate of 10 C minꢁ1; differential scanning calorimetry
ꢃ
(DSC) analyses were carried out on a DSC-Q200 TA Instruments.
Cyclic voltammetry measurements were carried out under inert
nitrogen atmosphere with an Autolab potentiostat PGSTAT 10
using a three-electrode cell (glassy carbon as working electrode,
Ag/Ag+ as reference electrode, and a platinum counter electrode).
The scan rate was 200 mV sꢁ1 and the supporting electrolyte was
an n-Bu4NClO4 (0.1 M) solution in CH2Cl2. Furthermore,
HOMO and LUMO energy levels have been estimated from the
onset of the first oxidation and reduction.13
[(10-Decylanthracen-9-yl)ethynyl]trimethylsilane 3
To a three-necked round bottomed flask was added 1.590 g
(4.00 mmol) of 9-bromo-10-decylanthracene 2, 231 mg (0.2 mmol)
of Pd(PPh3)4, 30 mL of toluene and a solution of 4.4 mmol of
[(trimethylsilyl)ethynyl]zinc chloride in THF. The mixture was
kept under stirring overnight at 80 ꢃC. The reaction mixture
was then poured into 100 mL of 1N HCl and repeatedly
extracted with diethyl ether. The solvent was removed in vacuo
and the residue was chromatographed on silica gel using
Out-of-plane grazing angle X-ray data for thin films of D3ANT
were performed on a Bruker D8 Discover GADDS with a 2D
detector for fast acquisition to determine the crystallinity of films.
Blank silicon substrates were used for comparison. Scanning
tunnelling microscopy (STM) was performed at the n-tetradecane–
substrate interface at room temperature in the constant current
mode using a Pico-LE scanning probe microscope (Molecular
Imaging/Agilent Technology). STM tips were prepared from
a Pt–Ir (80 : 20) wire by mechanical cutting. Highly oriented
pyrolytic graphite (HOPG) substrates (Goodfellow) were freshly
cleaved using adhesive tapes prior to deposition. Au(111) thin
films (150 nm) were prepared by vacuum-deposition on cleaved
mica (Goodfellow) and then annealed using a butane flame
to achieve surface reconstruction. A small amount of D3ANT
was dissolved in n-tetradecane (n-C14H30, Aldrich, 99.99%,
ꢂ0.05 mg mlꢁ1) prior to droplet deposition onto the substrates.
The quantum-chemical calculations were applied to the
isolated D3ANT molecule being in neutral configuration. The
procedure of full geometry optimization and all electronic struc-
ture calculations were performed at the density functional theory
(DFT) level with the hybrid B3LYP of three parameter Becke
functional (B3)14 and the functional of Lee, Yang, and Parr
ꢃ
petroleum ether (40–60 C) as eluent to yield 1.327 g (80%) of
1
3 as a reddish oil: H NMR (CDCl3) d ¼ 8.63 (d, J ¼ 8.33 Hz,
2H), 8.28 (d, J ¼ 8.33 Hz, 2H), 7.60–7.52 (m, 4H), 3.60 (t, J ¼
8.2 Hz, 2H), 1.84–1.76 (m, 2H), 1.61–1.35 (m, 2H), 1.44–1.20
(m, 12H), 0.89 (t, J ¼ 6.8 Hz, 3H), 0.43 (s, 9H); 13C NMR
(CDCl3) d ¼ 137.61, 132.81, 129.11, 127.75, 126.20, 125.65,
124.88, 105.81, 102.14, 31.97, 31.55, 30.40, 29.72, 29.70, 29.64,
29.41, 28.43, 22.76, 14.21, 0.38; nmax (neat)/cmꢁ1 3084, 3064,
2964, 2923, 2853, 2135, 1945, 1816, 1705, 1621, 1561, 1522,
1467, 1440, 1410, 1378, 1249, 1153, 1077, 1040, 858, 797; m/z
(EI, 70 eV) 414 (M+), 287 (M+ ꢁ C9H19).
9-Decyl-10-ethynylanthracene 4
1 mL of a 1.0 M solution of anhydrous tetrabutylammonium
fluoride in THF was added to a solution of 3 (414.7 mg,
1 mmol, in 5 mL THF) kept in a two-necked flask protected
from light by an aluminium foil. The reaction mixture was stirred
at 0 ꢃC for 20 min. The mixture was then poured into water and
extracted with diethyl ether. The collected organic phases were
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dried over anhydrous Na2SO4 and the solvent was removed
in vacuo. The crude was eventually chromatographed on silica
gel using petroleum ether (40–60 ꢃC) as eluent to afford 332
Pd(PPh3)4 (26 mg, 2.25 ꢀ 10ꢁ2 mmol) and the mixture was stirred
at reflux for 1 day. The reaction mixture was then poured in
water and repeatedly extracted with chloroform. The organic
phases were concentrated in vacuo and the crude was purified
by flash chromatography on silica gel using chloroform as eluent
1
mg (97%) of 4 as a brown solid: H NMR (CDCl3) d ¼ 8.65
(d, J ¼ 8.04 Hz, 2H), 8.29 (d, J ¼ 8.77 Hz, 2H), 7.60–7.52 (m,
4H), 3.97 (s, 1H), 3.61 (t, J ¼ 8.2 Hz, 2H), 1.84–1.76 (m, 2H),
1.62–1.55 (m, 2H), 1.46–1.21 (m, 12H), 0.89 (m, 3H); 13C
NMR (CDCl3) d ¼ 137.99, 133.07, 129.09, 127.53, 126.33,
125.70, 124.93, 87.99, 31.96, 31.55, 30.42, 29.72, 29.69, 29.63,
29.40, 28.45, 22.75, 14.20; nmax (KBr pellet)/cmꢁ1 3285, 3273,
3084, 3062, 2951, 2920, 2851, 1620, 1497, 1469, 1377, 1261,
1100, 1020, 843, 792.
1
yielding 232 mg (60%) of D3ANT as a violet solid: H NMR
(C7D8, 80 ꢃC) d ¼ 9.19 (d, J ¼ 8.48 Hz, 4H), 9.13–9.11 (m,
4H), 8.29 (d, J ¼ 8.78, 4H), 7.50–7.45 (m, 8H), 7.41–7.37 (m,
4H), 3.59–3.56 (m, 4H), 1.90–1.83 (m, 4H), 1.58–1.51 (m, 4H),
1.43–1.24 (m, 24H), 0.90 (bt, J ¼ 6.72 Hz, 6H); nmax (KBr
pellet)/cmꢁ1 3079, 3038, 2948, 2919, 2848, 1619, 1525, 1463,
1438, 1326, 1401, 1256, 1028, 926, 798, 757, 721, 672, 650; m/z
(APCI) 859.5 ([M + H]+ requires 859.52).
Organic thin-film transistor (OTFT) devices were fabricated in
‘‘bottom’’ and ‘‘top’’ contact configurations. Top contact devices
were fabricated using a highly n-doped silicon wafer (resistivity:
20 Ohm cm) as the gate electrode on which 100 nm of dielectric
(SiO2) were thermally grown. All the Si/SiO2 substrates were
cleaned as follows: the wafers were first immersed in a piranha
solution for 8 min in order to remove traces of adsorbed organic
materials, rinsed with deionised water and eventually dried with
a nitrogen flow. They were then washed according to the SC1
and SC2 cleaning procedures. Films of D3ANT were deposited
by spin coating—from 0.05 wt% solutions in anhydrous CHCl3
at 2000 rpm for 30 s. Drain and source electrodes were fabricated
on the organic semiconducting layer by thermal evaporation
through a shadow mask at various channel width and length
(Table 1). Some of the films were subjected to an annealing
9,10-Bis[(10-decylanthracen-9-yl)ethynyl]anthracene D3ANT
To a three-necked flask was added 4 (308 mg, 0.9 mmol), 9,10-
dibromoanthracene 1 (151.2 mg 0.45 mmol), CuI (4.3 mg, 2.25
ꢀ 10ꢁ2 mmol), toluene (15 mL), triethylamine (6 mL), and
Table 1 Electrical properties of OFET based on the ter(anthrylene-
ethynylene) D3ANT
Device no. W/L
Vt/V
ꢁ10
Mobility/cm2 Vꢁ1 sꢁ1 Ion/Ioff
1a
2a
3a
4a
5a
1085/5
ꢁ7.3 1.4 ꢀ 10ꢁ3
4 ꢀ 103
3 ꢀ 103
7 ꢀ 104
2 ꢀ 104
2 ꢀ 104
1085/5
1070/2
980/20
980/20
1.2 ꢀ 10ꢁ3
ꢁ8.1 1.0 ꢀ 10ꢁ3
ꢁ7.6 1.6 ꢀ 10ꢁ3
ꢁ8.3 1.1 ꢀ 10ꢁ3
d
d
d
1.3 0.2 ꢀ 10ꢁ3
ꢃ
procedure consisting of a treatment at 100 C for 30 min under
6ab
7ab
8ab
9ab
10ab
126880/5
117640/10 ꢁ13
ꢁ13
2.5 ꢀ 10ꢁ3
2.3 ꢀ 10ꢁ3
2.0 ꢀ 10ꢁ3
3.0 ꢀ 10ꢁ3
2.8 ꢀ 10ꢁ3
2.5 0.4 ꢀ 10ꢁ3
5 ꢀ 105
6 ꢀ 104
6 ꢀ 104
2 ꢀ 104
6 ꢀ 104
a nitrogen atmosphere. The devices were measured with an
Agilent 4155 C semiconductor parameter analyzer kept in a glove
box at room temperature. The mobilities in saturation regimes
were extracted using the equation:
77600/50
56040/100 ꢁ11
56040/100 ꢁ10
ꢁ11
11bc
12bc
13bc
14bc
15bc
1000/100
500/150
2000/200
4000/200
2000/200
ꢁ10
1.2 ꢀ 10ꢁ2
4 ꢀ 105
1 ꢀ 103
2 ꢀ 103
5 ꢀ 103
1 ꢀ 103
Ids ¼ Cim(W/2L)(Vg ꢁ Vt)2 at Vds > Vg
ꢁ9.6 1.6 ꢀ 10ꢁ2
ꢁ11
ꢁ10
1.0 ꢀ 10ꢁ2
1.0 ꢀ 10ꢁ2
where Ids is the drain-source current, Ci is the capacitance per
unit area of the gate dielectric layer, Vg is the gate voltage and
Vt is the threshold voltage. Vt was extrapolated from the (Ids)1/2
vs. Vg plot. Fig. 1 shows the current–voltage characteristics
(Ids vs. Vds) for top contact devices (W/L ¼ 500/150) (mm/mm)
ꢁ9.0 1.0 ꢀ 10ꢁ2
1.2 0.3 ꢀ 10ꢁ2
a
b
c
‘‘Bottom contact’’ device. Annealed at 100 ꢃC for 30 min. ‘‘Top
contact’’ device. d Average values and standard deviations of mobilities.
Fig. 1 FET characteristics (top contact device): substrate deposited by spin coating from a 0.05 wt% solution. Output characteristics (left) and transfer
characteristics (right) at Vds ¼ ꢁ30 V.
788 | J. Mater. Chem., 2008, 18, 786–791
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Scheme 1 Synthesis of D3ANT.
with D3ANT film spin coated from 0.05 wt% chloroform
solution and annealed as previously described. Fig. 1 also shows
1/2
Ids and Ids vs. Vg plots at Vds ¼ ꢁ30 V.
Results and discussion
Fig. 2 Top: absorption spectra of D3ANT in CHCl3 (7.5 ꢀ 10ꢁ6 M) and
in the solid state (drop casting from 0.05 wt% chloroform solution).
Bottom: emission spectra of D3ANT in CHCl3 (ꢂ10ꢁ6 M, lex ¼ 504
nm) and in the solid state (drop casting from 0.05 wt% chloroform
solution, lex ¼ 532 nm).
The synthetic approach followed to obtain 9,10-bis[(10-decylan-
thracen-9-yl)ethynyl]anthracene D3ANT is shown in Scheme 1.
9,10-Dibromoanthracene 1 was mono-metallated with phenyl-
lithium and subsequently reacted with decyl bromide to afford
9-bromo-10-decylanthracene 2. The use of BuLi as the metalla-
ting agent was avoided due to the formation of butyl bromide
as byproduct. A Negishi coupling of 2 with [(trimethylsilyl)ethy-
nyl]zinc chloride yielded the trimethylsilyl-ethynyl derivative 3.
Deprotection of 3 with tetrabutylammonium fluoride gave
9-ethynyl-10-decylanthracene 4, which was subsequently reacted
with 1 in a Sonogashira coupling for the obtainment of D3ANT.
Product D3ANT was obtained in good yield (60%) as a purple-
red powder slightly soluble (1.0 mg mlꢁ1) at room temperature
in CHCl3. The proposed structure is fully supported by its
molecular mass (APCI-MS), as well as by the 1H-NMR spectrum
recorded in toluene at 80 ꢃC. The thermal properties of D3ANT
were evaluated by means of thermogravimetric analyses (TGA)
and differential scanning calorimetry (DSC). A good thermal
stability was revealed, the decomposition temperature being
375.9 ꢃC at 5% weight loss. For DSC, the sample was subjected
to a heating scan to 300 ꢃC, a cooling scan and a re-heating scan,
all carried out at 10 ꢃC minꢁ1 under inert atmosphere. The only
recorded thermal event in both heating scans was the D3ANT
fusion at 247.1 ꢃC with the same fusion enthalpy. No thermal
event ascribable to decomposition could be identified in the
explored temperature range.
CHCl3 (lex ¼ 504 nm) showed a band at 568 nm. In the solid
state, by exciting the sample at 532 or 603 nm, a red-shifted
emission band at 677 nm was recorded owing to the above
mentioned aggregate formation.
The electrochemical behavior of D3ANT was studied in
CH2Cl2 using Ag/Ag+ as the reference electrode (calibrated
with respect to the Fc/Fc+ couple). The cyclic voltammogram
showed two quasi-reversible oxidation peaks at 1.05 V and
1.20 V and an irreversible oxidation peak at 1.49 V. This beha-
vior is likely due to the sequential abstraction of three electrons
from the three anthracene moieties. Two irreversible reduction
peaks at ꢁ1.26 V and ꢁ1.53 V were also observed. The
HOMO and LUMO energy levels, estimated from the onset of
the first oxidation and reduction, are ꢁ5.23 eV and ꢁ3.15 eV
respectively, resulting in an energy gap of 2.1 eV, in reasonable
agreement with the optical gap in CHCl3 (2.2 eV) and the
HOMO–LUMO gap calculated at the DFT/B3LYP/6-31G(d)
level (2.3 eV).
OTFT devices were fabricated both in bottom and top contact
configurations. A thin film of D3ANT (10 nm) was deposited
onto the SiO2 (100nm)/n-doped Si substrate by spin coating
from a 0.05 wt% chloroform solution. Gold was evaporated as
the source–drain contact material. The results obtained on 15
representative devices are reported in Table 1. An average
mobility of 1.3 ꢀ 10ꢁ3 cm2 Vꢁ1 sꢁ1 with on–off ratios of most
The UV–vis spectrum of D3ANT recorded in CHCl3 (Fig. 2)
showed a structureless absorption band with a maximum at
504 nm. In the solid state, the spectrum exhibited two bands
with maxima at 535 nm and 603 nm, the latter one due to inter-
molecular interactions. The fluorescence spectrum of D3ANT in
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Fig. 3 AFM micrograph of D3ANT as deposited on Si/SiO2. 1 ꢀ 1 mm2.
devices higher than 104 were reached with a ‘‘bottom’’ contact
geometry. These figures were improved by annealing the sub-
strate film for 30 min at 100 ꢃC resulting in an average mobility
of 2.5 ꢀ 10ꢁ3 cm2 Vꢁ1 sꢁ1. A consistent mobility improvement
was achieved in a ‘‘top’’ contact geometry comprising a spin
coated and subsequently annealed film of D3ANT. The average
mobility reached the value of 1.2 ꢀ 10ꢁ2 cm2 Vꢁ1 sꢁ1, although
a highest mobility of 0.055 cm2 Vꢁ1 sꢁ1 was achieved in few cases.
Data reported in Fig. 1 are relevant to one of these devices.
An investigation of the D3ANT film surface morphology was
carried out by means of atomic force microscopy (AFM). The
micrograph, reported in figure 3, shows a good substrate cover-
age despite the moderate solubility. A granular morphology is
observed and the grain width of about 0.04 mm is clearly evident.
Preliminary structural characterization of D3ANT films
deposited on Si/SiO2 and Au(111) substrates was carried out
by means of out-of-plane grazing incidence X-ray diffraction
(GIXRD). A thin film was deposited by spin coating from
CHCl3 solution on both substrates and analysed. In both cases,
the diffractograms are featureless, thus indicating the absence of
long range structural order in the bulk active layer and in the
out-of-plane direction. Similarly to D3ANT, thin films of mole-
cules of the same family, i.e. possessing two (trimethoxyphenyl)
ethynyl substituents at the 9,10-positions of anthracene, are also
inherently disordered although they do not show any field-effect
modulation.17 It is reasonable to suppose that the introduction of
two 10-decylanthr-9-yl-ethynyl groups at the 9,10-positions of
anthracene extends the p-interactions allowing for improved
charge mobility independently of the structural arrangement of
the D3ANT systems in the active layer. However, GIXRD
data provide structural information on the bulk organization
of D3ANT films but cannot access the molecular arrangement
of the first monolayer in direct contact with the a-SiO2 surface
where the channel of the field-effect transistor presumably forms.
In order to shed more light on the two-dimensional self-
organization of D3ANT, we investigated monolayers on highly
organized pyrolitic graphite (HOPG) and Au(111) by scanning
tunnelling microscopy (STM) in a liquid environment of n-tetra-
decane.18 The STM study allows disclosing of the highly ordered
Fig. 4 Typical constant current STM images of self-organized mono-
layers of D3ANT adsorbed (a) at the n-tetradecane–HOPG interface
(16 ꢀ 16 nm2; Vt ¼ ꢁ333 mV; It ¼ 27 pA) and (b) at the n-tetra-
decane–Au(111) interface (16 ꢀ 16 nm2; Vt ¼ 62 mV, It ¼ 83 pA).
2D organization of the monolayers when adsorbed on conduc-
ting and atomically flat substrates When adsorbed on HOPG,
monolayers of D3ANT self-organize into highly-ordered mono-
domains extending over distances of 100–400 nm. Each domain
consists of a closed-packed arrangement of linear and parallel
rows of molecules (Fig. 4a). Adjacent rows are separated by a
distance of ꢂ1.3 nm (in dark contrast) corresponding to the decyl
chains in a slightly tilted position. The bright contrast areas
correspond to the p-conjugated cores of D3ANT with a length
of ꢂ2.0 nm, in good agreement with the calculated core length
(1.9 nm) and the profile of frontier wave functions which is
mainly defined by three anthracene sub-units. The 2D unit cell
parameters as measured by STM are a ¼ 3.1 nm; b ¼ 1.2 nm;
a ¼ 94ꢃ. Finally, the three anthracene sub-units of each indivi-
dual molecule are clearly visible on the high-resolution STM
image thus reflecting a nearly planar adsorption of the entire
molecule on HOPG, that is with no twist angle and full conjuga-
tion between the anthracene sub-units.
When adsorbed on reconstructed Au(111) (Fig. 4b), D3ANT
also forms self-organized domains whose 2D structure in linear
and parallel rows resembles that on HOPG. However, these
domains do not extend over distances as large as on HOPG.
790 | J. Mater. Chem., 2008, 18, 786–791
This journal is ª The Royal Society of Chemistry 2008

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5 M. M. Payne, S. R. Parkin, J. E. Anthony, C.-C. Kuo and
T. N. Jackson, J. Am. Chem. Soc., 2005, 127, 4986.
6 A. N. Aleshin, J. Y. Lee, S. W. Chu, J. S. Kim and Y. W. Park,
Appl. Phys. Lett., 2004, 84, 5383.
Another noticeable difference between both substrates is that the
distance between adjacent D3ANT rows on Au(111) (ꢂ0.3 nm) is
markedly smaller than that on HOPG and therefore cannot
accommodate decyl chains which are probably folded below or
above the anthracene sub-units. This results in a more densely
packed 2D structure on Au(111) as compared to HOPG. Again,
the individual anthracene sub-units of each molecule are clearly
visible.
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Conclusions
We conceived and synthesised a new solution-processable
ter(anthrylene-ethynylene) based architecture exhibiting a field-
effect mobility as high as 0.055 cm2 Vꢁ1 sꢁ1. This result is the
best one obtained so far for solution-processed anthracene based
OTFTs. The ter-anthrylene-ethynylene molecules self-organize
into well-ordered monolayers both on the HOPG and Au(111)
surfaces when deposited at the interface with an n-tetradecane
solution. However, GIXRD carried out on D3ANT films spin-
coated onto Si/SiO2 and Au(111) seem to indicate a disordered
nature of the thin film. Work is in progress to further investigate
the organic semiconductor–dielectric interface properties and
optimize the device performances by tuning of the molecular
structure.
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The authors gratefully acknowledge Dr Anup Purna Chandra
Lohani for carrying out the X-ray experiments, Dr Giuseppe
Ciccarella (University of Lecce) for the carrying out of the
ESI-MS experiment; Dr Paola Fini (University of Bari) for
carrying out DSC analysis. This work has been partially
supported by the FIRB project Micropolys (Italian MIUR), by
A*Star and Nanyang Technological University.
16 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria,
M. A. Robb, J. R. Cheeseman, J. A. Montgomery, Jr., T. Vreven,
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