The influence of side chains on the structures and properties of functionalized pentacenes

Article abstract of DOI:10.1039/b717082c

We investigated substituent-induced variations in microstructure and physical properties of a family of functionalized pentacenes, materials currently of intensive interest for making organic electronic devices such as thin film transistors, to shed light on the complex relationships between functionalization, film formation, stability, and microstructure. In this study, the pentacenes were modified with alkyl acetylene or alkylsilylethynyl groups with systematic variations in the alkyl chain length. With a proper side chain, this modification can effectively disrupt the herringbone packing seen in neat pentacene, promoting face-to-face arrangements between the acene rings and providing solubility in a variety of convenient solvents. Thin films can be readily formed by solution casting from THF, bromobenzene, toluene and other organic solvents. We have investigated the structure and properties of the functionalized pentacenes using UV-vis spectroscopy, hot stage optical microscopy, differential scanning calorimetry, transmission electron microscopy, X-ray and electron diffraction. The materials show regular variations in their thermal behavior, crystal packing and macroscopic properties as the chemistry of the side-group substituent changes.

Full text of DOI:10.1039/b717082c

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Volume 18 | Number 17 | 7 May 2008 | Pages 1937–2052
ISSN 0959-9428
HIGHLIGHT
PAPER
GregoryV. Hartland et al.
Jihua Chen et al.
Dark-field microscopy studies of single
metal nanoparticles: understanding the
factors that influence the linewidth of
the localized surface plasmon resonance
The influence of side chains on
the structures and properties of
functionalized pentacenes
0959-9428(2008)18:17;1-W

PAPER
www.rsc.org/materials | Journal of Materials Chemistry
The influence of side chains on the structures and properties of
functionalized pentacenes†‡
Jihua Chen,ab Sankar Subramanian,d Sean R. Parkin,d Maxime Siegler,d Kaitlin Gallup,b
b
Chelsea Haughn, David C. Martin
abc
d
*
*
and John E. Anthony
Received 5th November 2007, Accepted 15th January 2008
First published as an Advance Article on the web 12th February 2008
DOI: 10.1039/b717082c
We investigated substituent-induced variations in microstructure and physical properties of a family of
functionalized pentacenes, materials currently of intensive interest for making organic electronic
devices such as thin film transistors, to shed light on the complex relationships between
functionalization, film formation, stability, and microstructure. In this study, the pentacenes were
modified with alkyl acetylene or alkylsilylethynyl groups with systematic variations in the alkyl chain
length. With a proper side chain, this modification can effectively disrupt the herringbone packing seen
in neat pentacene, promoting face-to-face arrangements between the acene rings and providing
solubility in a variety of convenient solvents. Thin films can be readily formed by solution casting from
THF, bromobenzene, toluene and other organic solvents. We have investigated the structure and
properties of the functionalized pentacenes using UV-vis spectroscopy, hot stage optical microscopy,
differential scanning calorimetry, transmission electron microscopy, X-ray and electron diffraction.
The materials show regular variations in their thermal behavior, crystal packing and macroscopic
properties as the chemistry of the side-group substituent changes.
p–p interaction through alignment of the linear alkyl tails,
1. Introduction
thus improving mobility by often more than an order of magni-
tude; but at some critical length, it has been found that long-tail
side groups began to occupy the spaces between aromatic planes,
reducing the mobility of the films substantially.5,7,8,14–17
Molecular structure plays a central role in optimizing the perfor-
mance of organic electronic devices such as solar cells, thin film
transistors, and organic light emitting diodes,1–10 with either
organic small molecules or polymeric semiconductors.11 For
organic small molecules, extensive research has been done to
study the effects of conjugation length and side chain type on
device performance. In the case of oligoacenes in particular,
conjugation length was found to have a close relationship with
band gap (Eg), with a continuous decrease in gap observed as
the number of acene rings increased from benzene to penta-
cene.12 A similar trend was reported by Garnier et al. in oligo-
thiophenes,5,7,8 and Yanagi et al. in biphenyl oligothiophenes.13
In addition, Garnier et al. pioneered the influence of a-, u-,
and b-substituted alkyl side groups on the microstructure and
properties of sexi-thiophene.5,7,8 The effect of side chain on
charge carrier mobility has also been reported in a number of
conjugated oligomer series, such as oligothienylenevinylenes,14
and other oligothiophenes.15–17 In general, proper side chains
on the edge of the conjugated core units tended to improve the
Through the past decade, pentacene made steady progress in
the ‘‘mobility race’’ and has been studied extensively in various
applications such as field-effect transistors,18,19 circuits,20,21 and
paper-like electronic displays.22–24 The key to extracting
maximum performance from pentacene is to insure proper
morphology of the thin films of this material, and since it has
no appended alkyl groups to tune self-assembly, morphology is
typically controlled by modification of the surface of the
substrate,25–27 which requires additional process steps. Other
limitations with pentacene include poor solubility, which
restricts its application in solution processes. In addition, penta-
cene molecules are known to adopt a herringbone structure in
their polymorphs, limiting their p–p overlap.28
In this paper, we report the effect of systematic variation of
side chains on the crystal packing, thin film morphology, thermal
and optical properties of functionalized pentacenes. Functional
groups in this study were either alkylethynyl or alkyl(diisopro-
pylsilylethynyl) groups, with varying alkyl chain lengths. This
chemical modification can effectively disrupt the herringbone
packing of pentacene crystal, resulting in regular p–p stacking
and giving solubility in a variety of organic solvents.
aMacromolecular Science and Engineering Center, The University of
Michigan, Ann Arbor, Michigan, 48109, USA
bDepartment of Materials Science and Engineering, The University of
Michigan, Ann Arbor, Michigan, 48109, USA
cDepartment of Biomedical Engineering, The University of Michigan, Ann
Arbor, Michigan, 48109, USA. E-mail: milty@umich.edu
dDepartment of Chemistry, The University of Kentucky, Lexington,
Kentucky, 40506, USA. E-mail: anthony@uky.edu
2. Experiments
† CCDC reference numbers 671507–671513. For crystallographic data in
CIF or other electronic format see DOI: 10.1039/b717082c
‡ Electronic supplementary information (ESI) available: Detailed phase
transition information extracted from DSC; UV-vis molar extinction
coefficients; single crystal analysis. See DOI: 10.1039/b717082c
2.1 Synthetic details
General. Solvents (acetone, methylene chloride, hexanes) were
purchased from Fisher. Dry THF was either purchased from EM
This journal is ª The Royal Society of Chemistry 2008
J. Mater. Chem., 2008, 18, 1961–1969 | 1961

Science or distilled over sodium/benzophenone under N2
atmosphere. Commercial acetylenes were purchased from GFS
Chemicals. Silica gel 230–400 mesh was bought from Sorbent
Technologies. NMR spectra were measured on Varian (Gemini
200 MHz/Unity 400 MHz) spectrometers, chemical shifts
reported in ppm relative to CDCl3 as internal standard. Mass
spectroscopy was performed in EI mode at 70 eV or by MALDI
with TCNQ matrix on a JEOL (JMS–700 T) Mass Spectrometer.
22.88, 29.37, 29.52, 29.59, 32.11, 79.15, 105.8, 118.55, 125.91,
126.35, 128.93, 130.76, 132.33 ppm. MS (EI 70 eV) m/z 445
(25%), 429 (45%), 308 (100%).
1
6,13-Bis(dodecynyl)pentacene (12-P). Yield ¼ 0.3 g (61%). H
NMR (200 MHz, CDCl3): d 0.87 (t, J ¼ 3.3 Hz, 6 H), 1.20–
1.60 (m, 24 H), 1.77 (quintet, J ¼ 3.5 Hz, 4 H), 1.96 (quintet,
J ¼ 3.53 Hz, 4 H), 2.94 (t, J ¼ 3.4 Hz, 4 H), 7.39 (dd, J ¼ 5
Hz, J ¼ 3.3 Hz, 4 H), 8.02 (dd, J ¼ 1.5 Hz, J ¼ 3.3 Hz, 4 H),
9.20 (s, 4 H) ppm. 13C NMR (50 MHz, CDCl3): d 14.23, 20.81,
22.85, 29.37, 29.55, 29.87, 29.93, 32.09, 79.14, 105.79, 118.54,
125.91, 126.36, 128.94, 130.76, 132.33 ppm. MS (EI 70 eV) m/z
606 (20%, M+), 479 (20%, M+ ꢁ nonyl).
General procedure for the preparation of functionalized penta-
cenes. To an oven-dried 100 mL round-bottom flask containing
the desired 1-alkyne (4.6 mmol) dissolved in 25 mL of dry THF
(kept under a dry nitrogen environment) was added slowly
n-BuLi (2.5 mmol), and this mixture was stirred for 40 min at
room temperature. Then, 6,13-pentacenequinone (0.25 g, 0.8
mmol) was added quickly, and the mixture was stirred under
a blanket of dry nitrogen overnight. Stannous chloride dihydrate
(2 mmol) followed by 10% HCl (2 mL) were added to the
reaction mixture which was stirred for an additional 7 h, after
which it was extracted into hexanes, washed with saturated brine
solution, dried over anhydrous MgSO4 and concentrated. The
crude products were purified by silica chromatography (typically
using hexanes–methylene chloride (9 : 1 v : v)) and recrystallized
from hexanes to yield pure materials.
6,13-Bis(diisopropyl ethylsilylethynyl)pentacene (2-Si). Yield ¼
1
90%. H NMR (200 MHz, CDCl3): d 0.90–1.10 (m, 4 H), 1.26–
1.44 (m, 34 H), 7.43 (dd, J ¼ 1.6 Hz, J ¼ 3.4 Hz, 4 H), 8.00
(dd, J ¼ 1.6 Hz, J ¼ 3.3 Hz, 4 H), 9.31 (s, 4 H) ppm. 13C
NMR (50 MHz, CDCl3): d 2.55, 8.74, 12.18, 18.57, 18.86,
104.89, 107.51, 118.65, 126.32, 126.61, 128.99, 130.94, 132.64
ppm. MS (EI 70 eV) m/z 611 (56%, M+).
6,13-Bis(diisopropyl n-butylsilylethynyl)pentacene (4-Si). Yield
1
¼ 95%. H NMR (200 MHz, CDCl3): d 0.96–1.04 (m, 8 H),
1.29–1.46 (m, 30 H), 1.50–1.60 (m, 4 H), 1.70–1.80 (m, 4 H), 8.00
(m, 4 H), 7.43 (m, 4 H), 9.31 (s, 4 H) ppm. 13C NMR (50 MHz,
CDCl3): d 10.28, 12.29, 14.10, 18.57, 18.84, 27.05, 27.28 104.64,
107.62, 118.46, 126.18, 126.44, 128.80, 130.75, 132.44 ppm. MS
(EI 70 eV) m/z 667(65%, M+), 610(20%, M+ ꢁ butyl).
6,13-Bis(hexynyl)pentacene (6-P). Yield ¼ 0.28 g (80%), mp ¼
83 ꢀC (decomp.). 1H NMR (200 MHz, CDCl3): d 1.15 (t, J ¼ 3.6
Hz, 6 H), 1.74–1.99 (m, 8 H), 2.95 (t, J ¼ 3.4 Hz, 4 H), 7.39 (dd, J
¼ 1.5 Hz, J ¼ 3.3 Hz, 4 H), 8.01 (dd, J ¼ 1.5 Hz, J ¼ 3.2 Hz, 4 H),
9.18 (s, 4 H) ppm. 13C NMR (50 MHz, CDCl3): d 13.93, 20.46,
25.52, 31.43, 79.11, 105.69, 118.52, 125.92, 126.34, 128.94,
130.73, 132.32 ppm. MS (EI 70 eV) m/z 438 (100%, M+), 395
(35%, M+ ꢁ propyl).
6,13-Bis(diisopropyl sec-butylsilylethynyl)pentacene (4II-Si).
Yield ¼ 88%.1H NMR (200 MHz, CDCl3): d 1.2 (m, 8 H), 1.4
(m, 32 H), 1.58 (m, 4 H), 2.03–2.09 (m, 2 H), 7.44 (m, 4 H),
8.00 (m, 4 H), 9.32 (s, 4 H) ppm. 13C NMR (50 MHz, CDCl3):
d 11.89, 14.13, 14.95, 19.22, 19.15, 19.30, 19.54, 25.93, 104.94,
107.70, 118.58, 126.23, 126.53, 128.88, 130.85, 132.47 ppm. MS
(EI 70 eV) m/z 667 (68%, M+), 610 (20%, M+ ꢁ sec-butyl).
6,13-Bis(heptynyl)pentacene (7-P). Yield ¼ 0.28 g (75%), mp ¼
115 ꢀC. 1H NMR (200 MHz, CDCl3): d 1.06 (t, J ¼ 3.6 Hz, 6 H),
1.50–1.61 (m, 4 H), 1.7 (quintet, J ¼ 3.5 Hz, 4 H), 1.97 (quintet,
J ¼ 3.5 Hz, 4 H), 2.94 (t, J ¼ 3.4 Hz, 4 H), 7.39 (dd, J ¼ 1.4 Hz, J
¼ 3.4 Hz, 4 H), 8.01 (dd, J ¼ 1.6 Hz, J ¼ 3.2 Hz, 4 H), 9.20
(s, 4 H) ppm. 13C NMR (50 MHz, CDCl3): d 14.31, 20.76,
22.57, 29.03, 31.66, 79.13, 105.78, 118.54, 125.94, 126.35,
128.93, 130.76,132.34 ppm. MS (EI 70 eV) m/z 466 (100%,
M+), 409 (20%, M+ ꢁ butyl).
6,13-Bis(diisopropyl hexylsilylethynyl)pentacene (6-Si). Yield ¼
91%. 1H NMR (200 MHz, CDCl3): d 0.90 (m, 12 H), 1.35 (m, 38
H), 1.76 (m, 4 H), 7.45 (m, 4 H), 8.00 (m, 4 H), 9.30 (m, 4 H)
ppm. 13C NMR (50 MHz, CDCl3): d 10.68, 12.38, 14.38, 18.65,
18.92, 22.92, 25.11, 31.92, 33.90, 104.74, 107.71, 118.55,
126.22, 126.53, 128.89, 130.82, 132.50 ppm. MS (EI 70 eV) m/z
723 (68%, M+), 638 (10%, M+ ꢁ hexyl).
6,13-Bis(octynyl)pentacene (8-P). Yield ¼ 0.25 g (62%). 1H
NMR (200 MHz, CDCl3): d 0.99 (t, J ¼ 3.35 Hz, 6 H), 1.4–1.6
(m, 8 H), 1.77 (quintet, J ¼ 3.3 Hz, 4 H), 1.96 (quintet, J ¼
3.53 Hz, 4 H), 2.95 (t, J ¼ 3.4 Hz, 4 H), 7.39 (dd, J ¼ 1.5 Hz,
J ¼ 3.35 Hz, 4 H), 8.01 (dd, J ¼ 1.55 Hz, J ¼ 3.3 Hz, 4 H),
9.21 (s, 4 H) ppm. 13C NMR (50 MHz, CDCl3): d 14.30, 20.81,
22.92, 29.17, 29.33, 31.74, 79.13, 105.81, 115.59, 126, 126.42,
128.99, 130.82, 132.4 ppm. MS (EI 70 eV) m/z 494 (95%, M+).
6,13-Bis(diisopropyl octylsilylethynyl)pentacene (8-Si). Yield ¼
85%. 1H NMR (200 MHz, CDCl3): d 0.92(m, 10 H), 1.38 (m, 48
H), 1.77(m, 4 H), 7.43 (dd, J ¼ 1.5 Hz, J ¼ 3.3 Hz, 4 H), 8.00
(dd, J ¼ 1.6 Hz, J ¼ 3.3 Hz, 4 H), 9.30 (s, 4 H) ppm. 13C
NMR (50 MHz, CDCl3): d 10.71, 12.42, 14.29, 18.67, 18.94,
22.88, 25.19, 29.60, 29.70, 32.22, 34.26, 104.93, 107.85, 118.71,
126.36, 126.68, 129.05, 131.00, 132.69 ppm. MS (EI 70 eV) m/z
779 (68%, M+), 666 (15%, M+ ꢁ octyl).
1
6,13-Bis(decynyl)pentacene (10-P). Yield ¼ 0.25 g (56%). H
NMR (200 MHz, CDCl3): d 0.91 (t, J ¼ 3.3 Hz, 6 H), 1.25–1.6
(m, 16 H), 1.77 (quintet, J ¼ 3.55 Hz, 4 H), 1.96 (quintet, J ¼
3.53 Hz, 4 H), 2.94 (t, J ¼ 3.5 Hz, 4 H), 7.39 (dd, J ¼ 1.6 Hz,
J ¼ 3.4 Hz, 4 H), 8.02 (dd, J ¼ 1.7 Hz, J ¼ 3.3 Hz, 4 H), 9.20
(s, 4 H) ppm. 13C NMR (50 MHz, CDCl3): d 14.23, 20.81,
2.2 UV-vis spectroscopy
A Cary 50-Bio UV Visible Spectrometer was used to examine
THF solutions (30 ppm) of functionalized pentacenes.
1962 | J. Mater. Chem., 2008, 18, 1961–1969
This journal is ª The Royal Society of Chemistry 2008

Measurements were taken between wavelengths of 250 nm and
800 nm at medium speed.
2.3 Solution casting and thin film morphology
‘‘Uncovered’’ and ‘‘covered’’ solution casting of functionalized
pentacenes (from 0.1–0.2% wt. of THF, bromobenzene, or
toluene solution) was performed on clean glass slides (as received
from Fisher Scientific) or silicon wafers (cleaned as described
elsewhere29) inside four-inch-diameter Petri dishes, either with
or without cover on. A Spot RT Color 2.2.1 CCD camera
(from Diagnostic Instruments, Inc.), and a Nikon OptiPhot2-
POL optical microscope were used to take polarized optical
micrographs.
Fig. 1 The synthesis and yields of functionalized pentacenes. There were
two types of pentacenes studied in this present work. Alkyl acetylene
series included 6-P, 7-P, 8-P, 10-P, and 12-P, while alkylsilylethynyl series
had 2-Si, 3-Si, 4-Si, 4II-Si, 6-Si, and 8-Si.
Dilute THF solution droplets (0.1% wt.) of functionalized
pentacenes were applied onto amorphous carbon thin films
(20–50 nm) on copper grids (400 mesh, from Ted Pella), and
then dried in air. Bright-field TEM experiments were conducted
either on a Philips CM12 at 120 kV, or a JEOL 3011 at 300 kV.
stability properties of functionalized pentacene (Fig. 1). The
requisite ethynyllithium solutions were prepared by treatment
of the alkyne with n-BuLi in THF, followed by addition of
commercially available 6,13-pentacenequinone. After the
quinone had dissolved completely, the reaction was quenched
with 10% aqueous HCl / tin (II) chloride, which induced
deoxygenation and yielded the desired pentacene derivatives.
Even during the workup process, the alkylethynyl pentacenes
(n-P) proved to be poorly stable, with the decynyl and dodecynyl
(10-P and 12-P) derivatives particularly prone to decomposi-
tion—in fact, the decynyl derivative 10-P was barely stable
enough to obtain full characterization. The diisopropylsilyl
derivatives, in contrast, were significantly more stable, soluble
and easy to purify. The instability of the n-alkylethynyl penta-
cenes is likely due to the facile Diels–Alder reaction between
unhindered alkynes and the pentacene chromophore.30,31 Longer
side chains tended to hinder crystal formation and facilitate the
proper geometry conditions for Diels–Alder reaction. Thus 8-Si,
10-P, 8-P, and 7-P were found to be easier to degrade than their
shorter-side-chain counterparts.
2.4 Differential scanning calorimetry and hot-stage light
microscopy
A Perkin Elmer DSC6 with Pyris Thermal Analysis System
(Version 3.52) was used to examine the thermal properties ꢀof
powder samples (5–10 mg). Heating and cooling rates of 10 C
minꢁ1 were applied throughout this work. An isothermal holding
of 1 and 5 min was programmed at the lowest and highest
temperatures in the thermal cycles, respectively. Nitrogen gas
was flushing the system during the DSC experiments.
A Linkham TH 1500 hot stage with a TMS 91 controller was
combined with our polarized optical microscope (Nikon
OptiPhot2-POL) to achieve hot-stage optical microscopy
experiments.
2.5 Electron diffraction
Selected area electron diffraction experiments were conducted on
either a Philips CM12 at 120 kV, or a JEOL 3011 at 300 kV. Thin
thermally evaporated gold films were adopted as diffraction
standard. Au (111) planes (0.2355 nm) were generally used as
a calibration for d-spacing measurements of unknowns.
Table 1 summarizes the solubility properties of various
functionalized pentacenes as compared to triethylsilylethynyl
anthradithiophene (TES-ADT), a promising new candidate for
high-performance organic thin film transistors.32 TES-ADT
2.6 Molecular simulation
Table 1 The solubilities of functionalized pentacenes in hexane and
THF as compared to triethylsilylethynyl anthradithiophene (TES-ADT)a
Crystal structure visualization and comparison were made with
Cerius2 (Accelrys, Inc.) and Mercury 1.4.1 (Cambridge Crystal-
lographic Data Centre) platforms. Unit cell information of
different pentacene variations was obtained with single crystal
X-ray diffraction experiments. Data for the determination of
functionalized pentacene crystal structures were collected on
either a Nonius kappaCCD or a Bruker-Nonius X8 Proteum
instrument.
Hexane
THF
6-P
7-P
8-P
10-P
12-P
2-Si
<0.1%
ꢂ0.1%
<0.1%
ꢂ0.1%
1%
1%
0.5%
2.5%
2.5%
2.5%
5%
1%
3-Si
4-Si
4II-Si
6-Si
8-Si
0.25%
0.5%
1%
1%
2.5%
<0.1%
1%
2.5%
1%
2.5%
5%
1%
3. Results and discussion
3.1 Synthesis and molecular structure
TES ADT
The n-alkylethynyl (n-P) and diisopropyl alkylsilylethynyl (n-Si)
pentacene derivatives were prepared to study the effects of alkyl
group length on crystal structure as well as optical, thermal and
a
Two samples for each data point. The unit is mg solute per mL solvent.
J. Mater. Chem., 2008, 18, 1961–1969 | 1963
This journal is ª The Royal Society of Chemistry 2008

and most of the functionalized pentacenes examined in this work
had significantly higher solubility in THF than in hexane, except
that 4II-Si had roughly the same solubility in both solvents. To
describe and predict the extent of mixing interactions between
the solutes and solvents, solubility parameters33 offered a conve-
nient and easy means of quantitative evaluation. Solubility
parameter (d) is defined as the square root of the cohesive energy
density (CED).33 Since the solubility parameter of hexane is 14.9
(MPa)1/2 and that of THF is 18.6 (MPa)1/2, the solubility para-
meters of most functionalized pentacenes were estimated to be
around 18–19 (MPa)1/2, if one assumed that the Gibbs free energy
of mixing 6Gm ¼ 6Hm ꢁ T6Sm, the entropy of mixing 6Sm
was negligible, and the enthalpy of mixing 6Hm ¼ (d1
ꢁ
d2)24142V was a function of the difference in solubility parame-
ters between solute and solvent (d1, d2; subscript ‘‘1’’ stands for
solvent, ‘‘2’’ for solute), the volume fractions (41, 42) and the
volume of the mixture (V).34
Fig. 2 UV-vis spectra of the alkylsilylethynyl pentacene series in THF
solution.
Unlike unfunctionalized pentacene, in a good solvent such as
THF, alkylsilylethynyl and alkylethynyl pentacenes exhibited
rather high solubilities of 0.5–5% (solute mass in mg per solvent
volume in mL), which were sufficient for potentially low-cost and
large-area solution deposition processes. Alkylsilylethynyl
pentacenes showed substantially higher solubilities (1–5% in
THF, 0.25–2.5% in hexane) than those of their non-silicon-based
counterparts with similar chain lengths (0.5–2.5% in THF, <0.1–
1% in hexane), possibly because of the fractal shapes of side
chains. In addition, longer side chains tended to yield better
solubility for both alkylsilylethynyl and alkylethynyl pentacenes
in all solvents, although there were a few exceptions such as 2-Si
and 4-Si. Most of the functionalized pentacenes shown here had
superior solubility to that of TES-ADT because of their larger
side chains.
Fig. 3 UV-vis spectra of the alkyl acetylene pentacenes in THF solution.
The temperature dependent solubilities of 3-Si, also called 6,13-
bis(triisopropylsilylethynyl) (TIPS) pentacene, are presented in
Table 2. The solubilities of other functionalized pentacenes were
expected to undergo similar trends as the temperature increased.
This information is especially useful when elevated-temperature
deposition is necessary in order to fabricate films with larger
grains, or to speed up the deposition rate.
in THF all showed a strong 645 nm peak, which started at about
660 nm (1.88 eV), the energy of the optical gap. The almost
identical UV-vis absorption peaks of alkylsilylethynyl
pentacenes suggested that the silyl groups effectively isolate the
chromophore from the effect of variations in side chain length.
Calculated molar absorption coefficients at selected wave-
lengths are available as ESI.‡ The absorption intensity differ-
ences between functionalized pentacenes were partly due to the
slight variation in solution concentrations, although efforts
were made to keep the sample concentration around 30 mg L
(in THF) for all samples in a standard 10 mm cuvette.
3.2 UV-vis spectrocopy
The alkylsilylethynyl pentacenes demonstrated very similar
UV-vis spectra in the range of 350–700 nm (Fig. 2), while the
alkylethynyl series exhibited subtle changes in absorption
maxima depending on alkyl chain length. Notably, both 10-P
and 12-P decomposed rapidly in solution, yielding byproducts
with anthracene-like chromophores (evidenced by absorptions
at 380, 410 nm) (Fig. 3). Furthermore, the pentacene absorption
for 12-P appears slightly blue-shifted relative to the absorptions
for the other alkyl derivatives. The alkylsilylethynyl pentacenes
According to the crystal structure, the alkyne in 7-P is within
˚
3.5 A of one of the aromatic rings of an adjacent pentacene,
which is the right distance and geometry for a Diels–Alder
addition to take place in the solid state. For this reason, 7-P
will be less stable during storage than other pentacene derivatives
that have a crystal structure identified in this work.
3.3 Thin film morphology (optical and transmission electron
microscopy)
Table 2 The solubilities of TIPS pentacene (3-Si) in bromobenzene as
a function of temperaturea
Solution casting of functionalized pentacenes was performed on
clean glass slides inside four-inch-diameter Petri dishes, either
with or without cover on. This allowed rough control over the
solvent evaporation rate during the film formation process.
Optical micrographs of alkylsilylethynyl pentacene films are
shown in Fig. 4. Covered drying turned out to be not always
RT
1%
50 ꢀC
2.5%
105 ꢀC
5%
TIPS pentacene
a
Three samples for each data point. The unit is mg solute per mL solvent.
1964 | J. Mater. Chem., 2008, 18, 1961–1969
This journal is ª The Royal Society of Chemistry 2008

In practice, the alkylethynyl pentacenes degraded much faster
than their silicon-based counterparts both in solution and in the
solid state, as observed from either a color change (blue to
yellow) in solution, or birefringence loss in solution-cast films.
Thus, in general, higher evaporation rates were needed for the
alkylethynyl derivatives in order to generate crystalline films.
Uncovered drying usually yielded more uniform and non-
degraded films than covered drying in the case of alkylethynyl
pentacenes, but still, their chemical stability, film coverage and
crystalline domain sizes (Fig. 5) were poor when compared to
the alkylsilylethynyl series (Fig. 4).
Depending upon the vacuum deposition conditions such as
substrate temperature, chamber pressure, and growth rate, the
crystalline domain sizes of pentacene films were generally smaller
than 10 mm.35–38 The side-group modified pentacenes shown in
this work, however, can exhibit domain sizes up to millimeter
or centimeter dimensions, with relatively consistent morphology,
which are very desirable in order to fabricate large-area low-cost
organic electronic devices. Issues on uniform coverage, and
consistent crystal orientation will need to be taken into conside-
ration in order to further improve their performance and reduce
their performance variation.
Fig. 4 Optical micrographs of solution-cast films of alkylsilylethynyl
pentacenes. Films were cast from 0.1% wt. THF solutions.
advantageous, since the molecules were kept in solution (in air)
for long enough that they degraded and formed non-birefringent
amorphous-looking droplets.
Both bright-field TEM and optical microscopy data demon-
strated that crystalline domain sizes in 4-Si, 4II-Si, and 6-Si
were generally larger than those of 2-Si and 8-Si. In fact, unlike
other samples in Fig. 4, the 8-Si films were barely birefringent
under the polarized light microscope. Our available results
suggested that too long ($8) or too short (#2) a side group
impedes crystallization of these pentacene derivatives.
Optical images of solution-cast alkylethynyl pentacenes also
illustrated the effect of evaporation rate on thin film morphology
(as shown in uncovered and covered 12-P in Fig. 5). Uncovered
12-P films, although less uniform, exhibited significantly greater
crystal sizes than covered 12-P films. The films of 10-P assumed
non-birefringent droplet shapes, again supporting their quick
degradation after synthesis.
3.4 Thermal properties by differential scanning calorimetry and
hot-stage light microscopy
DSC of the two pentacene series showed systematic variations of
thermal properties depending upon the choice of side group, and
thermal stability of the silicon-based (alkylsilylethynyl) penta-
cenes were by far superior than the non-silicon-based (alkyle-
thynyl) derivatives (Fig. 6 and 7). A table of phase transition
temperatures and corresponding enthalpies is given in the ESI.‡
For the alkylethynyl series, degradation temperatures were so
low (around 120–150 ꢀC as indicated by the exothermic peaks
upon heating) that endothermic melting transitions could be
hardly seen except in the case of 12-P (Fig. 7). The melting and
degradation transitions in 12-P almost overlapped with each
other, so that the exothermic peak at 132 ꢀC closely followed
the endothermic transition at 128 ꢀC. In addition, increasing
length of the alkyl side groups from 6-P, 7-P, to 8-P seemed to
correlate well with the decreased degradation temperature. It
was expected that the longer linear side chains tended to decrease
X-Ray and electron diffraction experiments on these samples
generally agreed with birefringence observations in polarized
light microscopy.
Fig. 6 Differential scanning calorimetry (DSC) of the alkylsilylethynyl
pentacenes. The heating started at 30 ꢀC, and ended at 300 or 350 ꢀC
before cooling back to 40 ꢀC.
Fig. 5 Optical micrographs of solution-cast films of alkylethynyl
pentacenes. Films were cast from 0.1% wt. toluene solutions.
This journal is ª The Royal Society of Chemistry 2008
J. Mater. Chem., 2008, 18, 1961–1969 | 1965

stability. The thermal stabilities of alkylethynyl pentacenes were
reduced by more than 160 ꢀC, upon the addition of side groups.
In some of the alkylsilylethynyl pentacenes (3-Si, 4II-Si, and
2-Si), however, the differences were successfully minimized to
40–90 ꢀC. Further information on the chemical stability of
functionalized pentacenes upon heating could be obtained by
employing thermogravimetric analysis (TGA), which was not
pursued in this study.
3.5 Electron diffraction and crystallinity
Selected area electron diffraction patterns revealed that the
alkylsilylethynyl pentacenes generally exhibited higher crystallin-
ity and more single-crystal features than the alkylethynyl penta-
cenes. Using a 5 mm diffraction aperture, it was relatively easy to
obtain sharp single-crystal patterns with 6-Si, and 4-Si (Fig. 8),
and arcs with 2-Si or 8-Si (Fig. 9 and Fig. 10 left). For 3-Si,
a single-crystal [001] pattern was frequently observed as evidence
of the edge-on orientation of acene rings on the substrate.39
However, with n-alkylethynyl pentacenes, broad rings were
usually seen under the same conditions. A representative ring
pattern of 12-P is shown in Fig. 10 (right). In addition, silicon-
based pentacenes generally had distinctive facetting in
bright-field TEM images, whereas their n-alkylethynyl counter-
parts often adopted an irregular appearance. These results
Fig. 7 DSC of the alkyl acetylene pentacenes. The heating started at
30 ꢀC, and ended at 300 ꢀC before cooling back to 40 ꢀC.
the strength of the interaction between the aromatic portions of
these pentacenes, making crystalline packing more difficult and
thus reducing the thermal stability of the molecules. The degra-
ꢀ
ꢀ
dation point of 8-P (123 C) was found to be 19 C lower than
ꢀ
that of 6-P (142 C) because of this side chain length effect.
The addition of silicon-based substituents to the system
improved the thermal stability dramatically as in ꢀ2-Si, 3-Si,
ꢀ
ꢀ
and 4II-Si (melting points: 226 C, 263 C, and 220 C, respec-
tively), although too long a linear alkyl substituent could also
promote melting and for this reason the melting temperatures
of 8-Si, 6-Si, and 4-Si were rather low (68 ꢀC, 51 ꢀC, and
142 ꢀC) (Fig. 6). Furthermore, 3-Si, 4-Si, and 6-Si even demons-
trated one or more structure changes upon heating. In the case of
3-Si, also known as 6,13-bis(triisopropylsilylethnyl) pentacene
(TIPS pentacene), the sub-melting endothermic peak was caused
by a well-characterized solid-to-solid structural phase transition,
accompanied with characteristic cracking and facetting.39 For
6-Si, the three endothermic transitions below the melting point
suggested the possibility of thermotropic liquid crystalline
phases. Although hot-stage optical microscopy demonstrated
melting and recrystallization of 6-Si films upon heating and
cooling, clear evidence of the exact nature of mesophases present
has yet to be conclusively identified. In the cases of all alkylsilyl-
ethynyl pentacenes, as long as the temperature of DSC is not
taken above 170 ꢀC, their melting transitions are reversible
according to DSC experiments. The broad exothermic transition
Fig.
8 Representative electron micrograph and corresponding
diffraction pattern for 6-Si thin film. Sample prepared with THF solution
(0.1% wt.) on amorphous carbon substrate.
ꢀ
centered at 185 C for 8-Si, 6-Si and 4-Si likely corresponds to
Diels–Alder reaction between alkyne substituents and pentacene
chromophores in the melt30—for those compounds that melt
above this temperature, this decomposition occurs immediately
upon melting, due to the ability of molecules in the melt to adopt
the correct geometric orientation necessary for this reaction to
occur.
The melting temperature in the Si-based system correlates
strongly with the amount of p-overlap observed in the crystals
(also discussed below in section 3.6). 3-Si possesses the strongest
p-overlap, and exhibits the highest melting point. 2-Si and 4-Si
have significantly less p-overlap, and thus melt at significantly
lower temperatures; there is no p-overlap seen in the crystals
of 6-Si and 8-Si, leading to exceptionally low thermal stabilities
for these compounds.
Fig. 9 Representative electron micrograph and corresponding electron
diffraction pattern (aperture as shown in the TEM image) for 2-Si films.
The dotted ring in the diffraction pattern was the spacing for Au (111)
(0.2355 nm). Sample prepared with THF solution (0.1% wt.) on
amorphous carbon substrate.
Compared to unsubstituted pentacene (melting point
>300 ꢀC), all the functionalized pentacenes showed less thermal
1966 | J. Mater. Chem., 2008, 18, 1961–1969
This journal is ª The Royal Society of Chemistry 2008

Fig. 10 Representative electron diffraction patterns for 8-Si (left) and
12-P (right). The dotted rings in diffraction patterns were the spacing
for Au (111) (0.2355 nm). Sample prepared with THF solution (0.1%
wt.) on amorphous carbon substrate.
suggest that silyl substituents enhance the formation of crystal-
line films.
According to the electron pattern and fading observed in
electron diffraction experiments with the same imaging condi-
tions, a comparison of crystallinity could be made between
samples. The crystallinity sequence was determined to be 3-Si
> 6-Si, 4-Si, 4II-Si > 2-Si > 8-Si. This agreed well with the previ-
ous findings in the literature that an intermediate alkyl length
can promote crystal packing and p–p interactions.15,16,40 In the
case of alkylsilylethynyl pentacene, an alkyl chain size of 3–6
carbons was found to be optimal for maximizing crystallinity.
Because of the alkyl side chains on each acene unit, the aromatic
content in these functionalized pentacenes is substantially lower
than that of plain pentacene. The critical electron dose of beam
damage for 3-Si was experimentally determined to be 0.045 C
cmꢁ2 at 300 kV,34 which was several times lower than that of
pentacene (0.2 C cmꢁ2 at 100 kV).41 As the electron beam resistivity
of organic crystals is closely related to their thermal stability 42,43
and 3-Si had the best thermal stability (melting and degradation
temperature around 260 ꢀC) among the alkylsilylethynyl and
alkylethynyl pentacenes, the other functionalized pentacenes
examined in this work were expected to have much lower electron
critical doses and shorter lifetimes under the transmission electron
microscope than those of 3-Si and straight pentacene.
Fig. 11 Crystal packing of the functionalized pentacene unit cells. Top
row: 2-Si, 3-Si, and 4-Si; second row: 6-Si and 8-Si; third row: 6-P, 7-P,
and 8-P. In contrast to the regular p–p stacking in functionalized penta-
cenes studied in this work, unsubstituted pentacene has a ‘‘herringbone’’
arrangement between acenes.
pentacene chromophore begin to dominate the packing, and
p-stacking interactions are lost completely. Further increases
in alkyl chain length (8-Si, 8-P) only serve to more completely
insulate the pentacene unit from adjacent pentacene chromo-
phores, making them poor choices for applications in electronic
devices. Just as with the substituted oligothiophenes, in both the
alkylethynyl and alkylsilylethynyl pentacenes the optimum chain
length appears to lie between C2 and C5.
The available lattice parameters for the two series of function-
alized pentacenes are tabulated with their unit cell symmetry
information in Table 3, compared to two common polymorphs
of pentacene (Pentacene 1 is a common bulk phase28 and Penta-
cene 2 was a thin film phase45). There were no values available for
10-P and 12-P, possibly due to the fact that the long linear chains
in those molecules made crystal packing rather difficult and the
fact that 10-P degraded readily upon synthesis.
3.6 Crystal packing as a function of systematically varied side
chains
Although the general packing arrangement of pentacene mole-
cules in these two classes of compounds did not change in
a monotonic fashion with increasing length of alkyl chain, the
intermolecular interactions did follow a general trend across
both of these systems. First of all, in contrast to the ‘‘herring-
bone’’ arrangement of acene cores in neat pentacene crystals,
most of the functionalized pentacenes in this work adopted
a co-planar arrangement, with many exhibiting regular p–p
interactions (Fig. 11). In the cases with shorter alkyl chains
(ethyl, isopropyl, butyl) the pentacene units adopted strongly
3.7 Electrical properties
Thin-film transistors were constructed from several of the
derivatives reported in this work, although so far only 2-Si and
3-Si exhibited field effects. Since we did not apply self-assembled
monolayers to pre-treat the silicon dioxide substrate and did not
optimize the channel width/length based on the specific film
morphology, better device performances of these materials are
likely to be observed with more thorough electrical characteriza-
tion in the future.
˚
p-stacked arrangements, with contacts as close as 3.39 A (as
˚
compared with 6.27 A in unsubstituted pentacene) between car-
bon atoms in adjacent pentacene units (Fig. 11 shows interaction
between acene nearest neighbors in the solid state).44 As the
length of the aliphatic chain is increased beyond 4 carbon atoms
(e.g. 6-Si, 7-P), interactions between the long alkyl chain and the
As with the previously reported 8-P,46 most of the alkylethynyl
pentacenes (except 6-P) did not yield thin films of high enough
This journal is ª The Royal Society of Chemistry 2008
J. Mater. Chem., 2008, 18, 1961–1969 | 1967

Table 3 Crystal structures of the pentacene derivatives with systematically changed side chains, as compared to two pentacene polymorphs (Pentacene
1 is a common bulk phase: Holmes et al.;28 Pentacene 2 is a thin film phase: Drummy et al.45). The units of lattice parameters a, b, c are angstroms
Material
Crystal system
Space group
a
b
c
a
b
g
6-P
7-P
8-P
2-Si
3-Si
4-Si
6-Si
8-Si
Monoclinic
Monoclinic
Triclinic
Triclinic
Triclinic
Monoclinic
Orthorhombic
Triclinic
Triclinic
Orthorhombic
P21/C
P21/N
19.8513
12.3212
6.6368
10.6177
7.55
5.3006
5.142
8.3446
13.6171
7.73
22.324
15.4649
11.129
7.71
22.6989
20.803
13.7421
14.0829
16.76
90
90
105.5261
75.154
89.5
90
90
101.9194
76.75
90
93.5526
90
90
92.736
71.123
84
90
90
101.2905
84.52
90
101.1599
100.2523
73.915
78.7
110.146
90
93.8211
88.01
90
ꢀ
P1
ꢀ
P1
ꢀ
P1
P21/C
PBCA
8.884
10.797
10.3715
8.8577
6.28
5.9 ꢃ 0.2
27.5066
12.5975
14.44
30.0 ꢃ 0.8
ꢀ
P1
ꢀ
P1
N/A
Pentacene 1
Pentacene 2
7.4 ꢃ 0.2
quality for transistor studies. Of the alkylsilylethynyl derivatives,
3-Si has well known solution-processed transistor properties
comparable to, or even superior to, pentacene in some cases,
which is in part made possible by the strong two-dimensional
p-stacking observed in this material.29,34,39,47–52 In contrast,
neither 6-Si nor 8-Si exhibit any close contacts between aromatic
rings in the solid state, and the thin films of these two materials
were either not uniform enough, or suffering with degradation
on the silicon wafer (evidenced by the birefringence loss in
polarized light microscopy). All of 2-Si, 4-Si, and 4II-Si were
found to be chemically stable upon solution casting, forming
well-covered and textured thin films on silicon wafers. In the
future, it is expected that a reasonable compromise between
the thermal issues observed in 3-Si and high mobility may be
found by designing proper side chains to provide minimal
mechanical cracking, acceptable thermal stability, and decent
electrical properties for various device applications.
JEOL3011 was obtained from the support of NSF grant
#DMR-0315633.
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Acknowledgements
The authors would like to thank the Office of Naval Research
and the National Science Foundation for financial support
(DMR-0084304 and DMR-6518079). TEM studies were
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