Synthesis and charge transport studies of stable, soluble hexacenes

Article abstract of DOI:10.1039/c2cc33919f

Acenes larger than pentacene are predicted to possess enticing electronic properties, but are insoluble and prone to rapid decomposition. Utilizing a combination of functionalization strategies, we present stable, solution-processable hexacenes and an evaluation of their hole and electron transport properties.

Full text of DOI:10.1039/c2cc33919f

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Synthesis and charge transport studies of stable, soluble hexacenes
Balaji Purushothaman,a,b Sean R. Parkin,a Mark J. Kendrick,c Daniel David,d Jeremy W. Ward,d Liyang
Yu,e Natalie Stingelin,e Oana D. Jurchescu,d Oksana Ostroverkhovac and John E. Anthony*a
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/c0xx00000x
Acenes larger than pentacene are predicted to possess
enticing electronic properties, but are insoluble and prone to
rapid
decomposition. Utilizing
a
combination
of
10 functionalization strategies, we present stable, solution-
processable hexacenes and an evaluation of their hole and
electron transport properties.
Recent interest in larger acenes is founded on pentacene's
excellent device performance in
a variety of electronic
15 applications,1 as well as theoretical predictions of enticing
properties for fused aromatic systems with more than five rings.2
This promise has generated a number of new functionalization
strategies designed to yield reasonably stable versions of
molecules such as hexacene and heptacene.3 Although a number
20 of quinoidal4 or heteroaromatic5 acenes with more than five fused
rings have been studied in electronic devices, to-date soluble
versions of hexacenes and larger acenes have not been prepared
in
a
sufficiently stable form for film deposition and
characterization of transport properties. Our recent study of the
25 predominant decomposition pathways for silylethyne-substituted
hexacenes provided guidelines for the preparation of suitably
stable hexacenes,6 and our earlier studies on pentacenes7 and
anthradithiophenes8 further showed that partial fluorination
yielded significant improvements in solution stability. Combining
30 these functionalization and fluorination strategies, we report here
the synthesis of tricyclohexylsilylethynyl-substituted derivatives
of hexacene (1), tetrafluorohexacene (2) and octafluorohexacene
(3) (Fig. 1), and our initial study of the charge transport
properties of solution-deposited films of these hexacenes.
Figure 1: For hexacene derivatives 1 – 3, (a) structures, (b) energy levels
(pleas see supporting information for redox levels) and (c) decomposition
rate of hexacene 3 in solution measured in air under laboratory lighting,
55 with measurements taken every hour. The control trace consists of a
similar sample kept in the dark. Inset: integrated absorption spectra (640
nm < λ < 824 nm) as a function of exposure time for 1 – 3 (lines
connecting data points added for clarity).
35
Silylethyne-substituted hexacenes are prepared from the
a Department of Chemistry, University of Kentucky, Lexington, KY 40506-
0055, USA. Fax: 01 859 323 1069; Tel: 01 859 257 8844; E-mail:
Anthony@uky.edu
40 b Current address: School of Chemistry, Bio21 Institute, The University of
Melbourne, Parkville, VIC 3010, Australia
corresponding hexacenequinones, and the requisite fluorinated
60 quinones were formed by condensation of tetrafluoronaphthalene
2,3-dicarboxaldehyde with either 1,4-dihydroxyanthracene or
tetrafluoro 1,4-dihydroxyanthracene, as described in the
supporting information. Hexacenes 1 – 3 are green crystalline
solids that did not decompose appreciably after several months'
65 storage in the solid state in the dark (Fig. S1). Solutions of the
hexacenes were characterized by absorption and emission
spectroscopy, as well as by electrochemistry (Fig. 1). The
disappearance of the long-wavelength absorptions upon exposure
c Department of Physics, Oregon State University, Corvallis, OR 97331,
USA.
d Department of Physics, Wake Forest University, Winston-Salem, NC
45 27109, USA.
e Department of Materials and Centre for Plastic Electronics, Imperial
College London, London, SW7 2AZ, United Kingdom
† Electronic Supplementary Information (ESI) available: Synthesis,
experimental procedures, device fabrication and characterization details.
50 See DOI: 10.1039/c0xx00000x
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to bright laboratory lighting in air was used to gain initial insight 35 the difference between the two. While films of all three
into the solution stability of these materials (Figures 1c, S2 and
S3), and as expected the fluorine substituents led to a significant
increase in solution half-life. Electrochemical analysis of 1 – 3,
performed by cyclic voltammetry and differential pulse
hexacenes exhibited photoresponse (Fig. 2a), compounds 2 and 3
were considerably more photoconductive compared to 1. In all
films at higher voltage, the voltage (V) dependence of the dark
currents (Id) could be described as Id ~ Vα with α ≈ 2. This
5
voltammetry, provided estimates of the HOMO and LUMO 40 suggests the space-charge limited current (SCLC) regime,
energies9 as presented in Fig. 1b. Crystallographic analysis of
hexacene 1 was reported previously, and the structure consists of
enabled by efficient hole injection from Au electrodes (as
expected from the estimated HOMO energies of 1 – 3). From the
SCLC measurements, we estimated effective charge carrier
mobility (µ) values, which in our planar electrode experimental
a
strongly 1-dimensional π-stacked array with significant
10 distortion of the hexacene backbone due to crystal packing
effects.6 Hexacenes 2 and 3 also formed crystals suitable for 45 geometry depend upon the film thickness (see SI for details).12 In
structural analysis. Unfortunately, disorder in the structure of 2
was extensive, and only a rough assessment of crystal packing
order could be obtained; the molecules appear to adopt a two-
15 dimensional π-stacked arrangement similar to functionalized
the limiting case of infinitely thin film (infinite half-space), the
mobility values yielded 1.7x10-4 cm2/Vs (3.8x10-4 cm2/Vs) for 1,
8.4x10-3 cm2/Vs (1.9x10-2 cm2/Vs) for 2 and 6.9x10-4 cm2/Vs
(1.6x10-3 cm2/Vs) for 3 (Fig. 2b). These values should be
pentacenes.10 There was significantly less disorder in the 50 regarded as lower bounds of charge carrier mobility, as the SCLC
structure of 3, although it did incorporate recrystallization solvent
into the structure, and the hexacene chromophores were again
found to adopt a highly layered, strongly π-stacked arrangement
20 as shown in Fig. S4. Thus, similar to prior-reported pentacene
materials, both 2 and 3 are expected to show reasonable charge
transport properties.11
trap-free limit was not achieved in our measurements.
Figure 3. Field-effect transistors fabricated from hexacene 2. (a)
Schematic representation of the FET geometry. (b) Top-view cross-
55 polarized optical image of a device. (c) Evolution of the drain current ID
with the gate voltage VGS for the same device.
The higher mobility and good stability of 2 led us to explore its
use in bottom-gate field-effect transistors (FETs), which were
fabricated on SiO2 gate dielectric and chemically modified Au
60 contacts (Fig. 3a). Heavily doped Si was used as gate electrode
and 300 nm of thermally-grown SiO2 at its surface served as gate
dielectric. Ti/Au source and drain contacts were deposited by e-
beam evaporation, and were treated with pentafluorobenzene
thiol to enhance the crystallization of the fluorinated
65 semiconductor.13 The organic semiconductor was drop-cast from
a 0.2 wt% chlorobenzene solution, and the solvent was allowed to
evaporate slowly in a covered petri dish in the dark for up to three
days.14 Figure 3b shows an optical micrograph of the crystalline
films obtained by this process – the large, leaf-shaped crystals
70 appear single, and span the channel of the transistor, with crystal
dimensions ranging from a few µm to hundreds of µm. The
resulting FETs were measured under ambient conditions, and
over 100 devices were sampled to yield hole mobility ranging
from 0.01 to 0.10 cm2/Vs. The variability arises from different
75 crystal quality along with the natural anisotropy present in these
π-stacked systems.15 Mobilities were calculated in the saturation
regime (drain-to-source voltage VDS = -40V), and Fig. 3c shows
the transfer characteristics (log (ID) right axis and √ID left axis
versus gate-to-source voltage VGS) for the device shown in Fig.
80 3b, demonstrating a mobility of 0.1 cm2/Vs and current on/off
25 Figure 2. (a) Dark and photocurrents for 1 – 3 as a function of applied
voltage. (b) Dark current density of 1 – 3 as a function of voltage squared,
along with the linear fits from which SCLC effective charge carrier
mobilities were calculated. Inset: The low voltage and high voltage
current trace for 3, showing the transition from Ohmic behavior.
30
For the initial electronic characterization of compounds 1 – 3,
drop-cast films were prepared on glass substrates with
interdigitated Au electrode pairs. Current was measured as a
function of voltage, both in the dark and under 765 nm
continuous wave illumination, and photocurrent was calculated as
2 | Journal Name, [year], [vol], 00–00
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by the National Science Foundation (NSF, DMR-1035257) and
the Office of Naval Research (N00014-11-1-0329). JWW and
45 ODJ thank the National Science Foundation (ECCS-1102275),
MJK and OO thank the NSF CAREER program (DMR-
0748671), and LY and NS acknowledge the Dutch Polymer
Institute (LATFE programme) for financial support.
Notes and references
50 ‡ Crystallographic data for 3: T = 90.0(2) K, C73H80F8Si2, M =
1165.55, triclinic space group P 1, a = 9.5223(1) Å, b = 10.2081(1) Å, c =
18.3986 (3) Å, α = 89.2381(6), β = 80.7371(6), γ = 63.7452(6), V =
1579.62 Å3, Z = 1, reflections collected/obs 9479/14355, R1 (all data) =
0.1076, R1(obs) = 0.0635
55
1
2
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binary blend of 3 and HDPE, 1:1 weight ratio. (Channel length L = 10
µm; channel width W = 10 mm)
3
5
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of fluorinated pentacene derivatives16 suggested that octafluoro
hexacene 3 might be suitable for electron transport studies. Films
4
5
For example, Q. Miao, M. Lefenfeld, T.-Q. Nguyen, T. Siegrist, C.
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solutions of 3 onto pre-formed device substrates (Au electrodes)
in an inert atmosphere glove box – devices were measured in the
same glove box. While initial measurements on neat films of 3
were promising, poor film formation from solution, perhaps
15 complicated by the incorporation of solvent in the crystals, led to
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molecules with insulating polymers has recently been shown an
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20 high-density polyethylene (HDPE) in a 1:1 weight ratio were thus
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transfer characteristics are shown in Fig. 4. Thus, octafluoro
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25 hexacene
3 behaves as an ambipolar semiconductor with
impressively balanced hole and electron mobilities.
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sufficiently soluble and sufficiently stable for studies in solution-
cast electronic devices. The fluorinated derivatives adopt π-
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2 formed large, plate-like
crystals on transistor device substrates and yielded hole mobility
as high as 0.1 cm2 / Vs. Octafluoro derivative 3 required blending
with an insulating polymer to give acceptable thin-film
35 architectures, but rewarded this added complexity with ambipolar
transistors exhibiting well-balanced hole and electron mobility.
These promising results based on the first stable, soluble
hexacene derivatives suggest that further tuning of the
trialkylsilyl substituents to optimize crystal packing and enhance
40 film morphology will yield materials with desirable electronic
properties.
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Synthesis of acene semiconductors (JEA, BP) was supported
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