Synthesis of soluble oligo- and polymeric pentacene-based materials

Article abstract of DOI:10.1016/j.tet.2008.09.041

Functionalization of pentacene at the 6- and 13-positions affords versatile building blocks for oligomer and polymer formation. Di- and trimeric materials are synthesized using unsymmetrical building block 18, while symmetrical diol monomer 17 allows for the synthesis of polymers. The materials reported herein are soluble in common organic solvents and air-stable. UV-vis and fluorescence spectroscopic properties have been investigated. Solid-state X-ray crystallography of building blocks 17 and 19 shows that these derivatives can π-stack with significant acene face-to-face interactions with spacing of less than 3.5 A?.

Full text of DOI:10.1016/j.tet.2008.09.041

Tetrahedron 64 (2008) 11449–11461
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Synthesis of soluble oligo- and polymeric pentacene-based materials
y
y
*
Dan Lehnherr, Robert McDonald , Michael J. Ferguson , Rik R. Tykwinski
Department of Chemistry, University of Alberta, Edmonton, AB T6G 2G2, Canada
a r t i c l e i n f o
a b s t r a c t
Article history:
Functionalization of pentacene at the 6- and 13-positions affords versatile building blocks for oligomer
and polymer formation. Di- and trimeric materials are synthesized using unsymmetrical building block
18, while symmetrical diol monomer 17 allows for the synthesis of polymers. The materials reported
herein are soluble in common organic solvents and air-stable. UV–vis and fluorescence spectroscopic
properties have been investigated. Solid-state X-ray crystallography of building blocks 17 and 19 shows
Received 3 September 2008
Accepted 12 September 2008
Available online 24 September 2008
Dedicated to Professor Reginald H. Mitchell
on the occasion of his 65th birthday
that these derivatives can
than 3.5 Å.
p
-stack with significant acene face-to-face interactions with spacing of less
Ó 2008 Elsevier Ltd. All rights reserved.
Keywords:
Pentacene
Oligomer
Polymer
p-Stacking
Organic semiconductor
Optoelectronic
Polycyclic aromatic hydrocarbon (PAH)
1. Introduction
used to enhance the semiconductive properties (i.e., charge-carrier
mobilities) and processability of pentacene-based materials.4
A
The last decade has seen an intensifying effort in the search for
organic materials for optoelectronic applications. Today, the field of
organic semiconducting materials is beginning to have a practical
impact and offers the potential to revolutionize applications in
photovoltaic cells, chemical sensors, thin film transistors (TFTs), as
well as nanowire-based transistors.1–3 Organic materials can pro-
vide numerous advantages over their inorganic counterparts, in-
cluding ease of tunability of the HOMO and LUMO levels through
chemical functionalization, compatibility with flexible substrates,
as well as lower manufacturing costs. The use of polycyclic aromatic
hydrocarbons (PAHs), such as pentacene, has dominated most
studies, but despite the substantial research on pentacene devices,
relatively few derivatives of pentacene are known.1,4 Historically,
efforts to enhance the performance of pentacene-based materials
have been through device fabrication techniques rather than syn-
thetic modifications of the pentacene skeleton. More recently,
however, synthetic derivatization of the pentacene core has been
natural progression in the development of organic advanced ma-
terials is the incorporation of functional chromophores into an
oligo- and/or polymeric framework with the goal of increasing the
solubility, as well as facilitating the formation of films and overall
processability. In spite of their potential, however, pentacene-based
oligomers and polymers remain rare.5
Unlike pentacene oligomers, anthracene oligomers are well
known,6–11 and they often show improved electronic properties as
a result of oligomerization.6–8,11 While the number of reports on
anthracene oligomers is too large to summarize herein, Figure 1
highlights several examples. Suzuki and co-workers reported
anthracene-based dimers 1–2 and trimers 3–4 in 2003, which were
synthesized by Suzuki cross-coupling.6 Functionalization with
solubilizing groups in dimer 2 and trimer 4 gave improved device
performance in field effect transistors (FETs) in comparison to
unfunctionalized oligomers 1 and 3. In fact, oligomers 2 and 4 had
FET mobilities of 0.13 and 0.18 cm2 Vꢀ1 sꢀ1, respectively. Conjugated
anthracene trimer 5 was reported in 2008 by Suranna and co-
workers and was synthesized by a Sonogashira cross-coupling to
afford materials suitable for FETs with a mobility as high as
* Corresponding author. Tel.: þ1 780 492 5307; fax: þ1 780 492 8231.
E-mail address: rik.tykwinski@ualberta.ca (R.R. Tykwinski).
0.055 cm2 Vꢀ1 sꢀ1 7
. Hybrid anthracene-thiophene oligomer 6 and
y
tetracene-thiophene oligomer 7 were reported by Frisbie and co-
X-ray Crystallography Laboratory, Department of Chemistry, University of
Alberta.
workers and synthesized via Stille cross-coupling.8 Oligomers 6 and
0040-4020/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2008.09.041

11450
D. Lehnherr et al. / Tetrahedron 64 (2008) 11449–11461
C10H21
R
R
1 R = H
2 R = C6H13
R
R
3 R = H
4 R = C6H13
C10H21
5
S
S
6
S
S
7
8
Figure 1. Selected examples of anthracene oligomers.
7 showed impressive performance in TFTs with hole mobilities as
high as 0.1 and 0.5 cm2 Vꢀ1 sꢀ1, respectively. Various conjugated
macrocyclic anthracenes have been studied by Toyota and co-
workers, including di-, tri-, and tetramers, as well as the corre-
sponding linear oligomers.9 And finally, even though cyclic
anthracene dimer 8 has been known for half a century,10 it was only
recently that Miao and co-workers studied its potential for appli-
cations in FETs.11 FETs were fabricated via thermal deposition of 8
(due to its insoluble nature) and hole mobilities of 0.05 cm2 Vꢀ1 sꢀ1
were measured.
Tokito and co-workers first reported pentacene polymers in
2001.12 The molecular structure of the random copolymers 9 was
a composition of functionalized fluorene with pentacene present as
either 1 or 10 mol % of the acene content (Fig. 2). The mixed acene
polymers were synthesized using Ni(0)-mediated Yamamoto-cou-
pling, and the materials were evaluated for their emissive properties.
Conjugated pentacene polymers 10 and 11 were reported by Bao and
Okamoto in 2007.13 In this case, the polymerization process was
conducted at the pro-cata positions14 of the pentacene moiety,
providing a mixture of the 2,9- and 2,10-isomeric linkages along the
polymer backbone. Recently, the synthesis of conjugated pentacene
dimers 12–14 has been reported by Tykwinski and Lehnherr, in
which dimer 14 exhibited photoconductive gain.15 There has been
one report of non-conjugated pentacene-based oligo- and polymeric
materials connected at the 6,13-positions (e.g., 15 and 16) commu-
nicated in 2007.5 A full account of this work is presented herein.
appear to be a combination of electronic and steric interactions that
contribute to provide bench-top stable materials, although the
exact nature of the electronic interaction has yet to be fully un-
derstood. Of the known pentacenes with 6,13-ethynylsilane sub-
stituents, triisopropylsilylethynyl is by far the most popular group
for substitution of the pentacene chromophore due to its ability to
afford stable, soluble pentacenes, which crystallize with significant
face-to-face interaction.4a–e These observations have been imple-
mented in the design of a pentacene monomer for assembly into
oligo- and polymers. Thus, building blocks 17 and 18 (Fig. 3) feature
pendent Si-groups that serve as an attachment point for two iso-
propyl groups to enhance solubility and stability, as well as
a ‘functional arm’ for polymerization. For the ‘functional arm’,
a primary alcohol was chosen to allow for the formation of oligo-
and polyesters via condensation reactions with either dicarboxylic
acids or bis-acid chlorides.
The synthesis of 17 and 18 was envisioned from 19 (Fig. 3),
which began by constructing the side groups in the form of
a functionalized silylacetylene that derived from 4-bromobenzyl
alcohol, 20. Although 20 is commercially available, in this case it
was made from 1,4-dibromobenzene (Scheme 1). Initial attempts to
form the mono-Grignard of 21 in either THF or ether and quenching
with paraformaldehyde gave unsatisfactory results (ca. 15% yield).
As a result, the formation of a suitable aryl nucleophile via lithium/
halogen exchange was attempted. Lithiation of 21 at ꢀ78 ꢁC or even
ꢀ40 ꢁC was problematic due to the low solubility of the starting
material. Success came by carrying out a mono-lithiation of 21 at
ꢀ15 ꢁC, even though not all of 21 was dissolved. Subsequent ad-
dition of paraformaldehyde to this solution gave 20 in a modest
yield of 65% after recrystallization from CH2Cl2/hexanes. Alcohol 20
was then protected as the tert-butyldimethylsilyl (TBS) ether to give
22 in 90% yield using an adaptation of the procedure reported by
Sessler and co-workers (CH2Cl2 or MeCN was used instead of
DMF).16 Purification of 22 could conveniently be obtained via
2. Synthesis of pentacene-based materials
2.1. Synthesis of pentacene building blocks
Anthony and co-workers4a–e have demonstrated that sub-
stitution of pentacene at the 6,13-positions with ethynylsilane
groups improves stability versus pristine pentacene. It would

D. Lehnherr et al. / Tetrahedron 64 (2008) 11449–11461
11451
Si
Octyl
Octyl
OR
n
RO
Si
m
9
10 R = Dodecyl
11 R = 2-ethylhexyl
n
R2
(R1)3Si
Si(R1)3
R2
12 R1= i-Pr, R2 = H
13 R1 = n-hexyl, R2 = H
14 R1 = i-Pr, R2 = SiMe3
O
O
Si
Si
HO
O
O H
n
m
15 n = 3; 16 n = 5
Figure 2. Pentacene oligo- and polymers.
reduced pressure distillation, thus avoiding the use of column
chromatography.
terminal alkyne product in 58% (based on 22) with no purification
necessary other than aqueous work up.
Two routes were then explored toward the formation of 23.
Initially, a one-pot sequence was developed, in which 22 was
subjected to lithium–halogen exchange with BuLi, and the product
was then added to a solution of i-Pr2SiCl2 in THF at ꢀ78 ꢁC. To this
solution was added TMS–C^C–Li, which afforded a w2:1 mixture
of 24 and 23 (as determined by 1H NMR spectroscopy) in 62%
overall yield after purification by reduced pressure distillation.
Since the desired product in this case was 23, the resulting mixture
was subjected to mild desilylation conditions to provide the
A significant improvement in the synthesis of 23 was sub-
sequently achieved using H–C^C–MgBr instead of TMS–C^C–Li
HO
Br
i) BuLi, –15 °C
40 min, Et2O
TBS-Cl
ii) paraformaldehyde
–15 °C to reflux
15 h, Et2O, 65%
rt, 16 h, CH2Cl2
90%
Br
21
Br
20
TBSO
TBSO
i) BuLi, –78 °C
30 min, THF
ii) i-Pr2SiCl2
–78 °C to rt
5.5 h, THF
TBSO
+
Si
Si
Si
Si
iii)
Li
TMS
OR2
R1O
rt, 16 h,THF
Br
22
TMS
24
H
23
(58% from 22)
17 R1 = R2 = H
18 R1 = TBS R2 = H
19 R1 = R2 = TBS
K2CO3, rt
1 h, wet MeOH
Figure 3. Pentacene building blocks 17–19.
Scheme 1. Synthesis of functionalized silylacteylene 23.

11452
D. Lehnherr et al. / Tetrahedron 64 (2008) 11449–11461
(Scheme 2). Thus, lithiated 22 was added to a solution of i-Pr2SiCl2
in THF at ꢀ78 ꢁC. The resulting solution was then allowed to warm
to room temperature and stir for 2 h. After again cooling the re-
action mixture to ꢀ78 ꢁC, H–C^C–MgBr was added and the
resulting solution stirred at ambient temperature overnight. This
process by either partial purification of the crude diol or by per-
forming an aqueous work up prior to the Sn(II)-mediated reduction
were ineffective. Lengthening the reaction time of the initial
alkynylation to 24 h did not improve the outcome. Likewise, in-
creasing the temperature from ambient to refluxing temperatures
did not yield better results either.
Given the challenges encountered during the attempted op-
timization of this two-step, one-pot procedure, this approach was
ultimately abandoned in favor of a stepwise sequence in which
the intermediate diol 25 was isolated and purified (Scheme 3).
Using this approach, it was quickly realized that the main prob-
lem was that 3 to 3.5 equiv of the lithium acetylide derived from
23 was not sufficient to form 25 in good yield; a significant
amount of monoaddition product 26 was observed (Fig. 4). This
issue was resolved by using 5.9 equiv (2.9 equiv per ketone
moiety) of the lithium acetylide. Further optimization in terms of
reaction times and temperatures resulted in the conditions
shown in Scheme 3.
TBSO
i) HexLi, –78 °C
30 min, THF
TBSO
ii) i-Pr2SiCl2
–78 °C to rt
2 h, THF
Si
iii)
H
MgBr
(1.7 equiv)
rt, 18 h, THF
85%
Br
22
H
23
Scheme 2. Optimized synthesis of 23.
Thus, under these optimized conditions a mixture of cis- and
trans-diol 25 (w5:95 ratio) was formed, from which the trans-diol
could be isolated in 88% yield by column chromatography. The cis-
diol 25 could also be isolated (which was indeed done for charac-
terization purposes), albeit the isolation process was tedious and
time consuming because it has an Rf very similar to that of the
major impurities/side products of the reaction. It is worth noting
that the excess 23 could be recovered almost quantitatively (>90%)
from this reaction during purification.
For the Sn(II)-mediated reduction of purified trans-diol 25, the
use of typical conditions4a in THF or in a mixture of THF/CH3CN
under reflux failed to provide satisfactory results both in terms of
yield and purity. Thus, milder conditions were sought, which in-
cluded reducing the temperature (room temperature). The success
of this step was solvent dependent, in terms of reaction rate, yield,
and purity. For example, reactions at room temperature in CH3CN
or acetone were complete in less than 30 min, but numerous
byproducts and low yields were encountered. Specifically, reactions
in acetone gave up to 68% yield, while those in CH3CN provided up
to 64% yield. The presence of byproducts, however, made purifi-
cation difficult and unsuitable for large-scale reactions. The re-
action was also attempted in MeOH at room temperature, as well as
sequence afforded directly 23 in 85% yield, circumventing the need
for a desilylation step. It is worth noting that this route also makes
use of less expensive reagents.
Both synthetic routes to 23 described above are scaleable to
tens-of-grams, and neither require column chromatography for any
step starting from 1,4-dibromobenzene. This is obviously quite
desirable when working on large-scale synthesis. In both synthetic
routes, however, the quality of the i-Pr2SiCl2 plays a crucial role in
the yield of this reaction. It is vital to use pure i-Pr2SiCl2 that has
been stored under a rigorously inert atmosphere to prevent its
degradation.
Initial attempts toward the synthesis of 19 employed a two-step,
one-pot method analogous to that reported by Anthony and co-
workers (Scheme 3).4a Thus, 23 was lithiated and added to 6,13-
pentacenequinone, followed by Sn(II)-mediated reduction. This
gave 19 in very low yields (ca. 5–10%), and the isolated product
consistently contained impurities. For example, 23 (3.7 equiv) was
reacted with BuLi (3.0 equiv) in THF at ꢀ78 ꢁC, added to 6,13-pent-
acenequinone (1 equiv) at 0 ꢁC, warmed to rt, and stirred for 2 h.
The reaction was quenched with water followed by the addition of
10% HCl saturated with SnCl2$2H2O to give 19 as an impure blue-
green solid after column chromatography. Attempts to improve the
i) BuLi (5.9 equiv)
HO
OH
–78 °C, 30 min, THF
Si
Si
23
ii) 6,13-pentacenequinone
(1.0 equiv), –15 °C to rt,
3 h, THF, 88% (trans-diol)
(excess)
TBSO
OTBS
25
(~95:5 trans:cis-diol)
SnCl2•2H2O (2.6 equiv)
rt, 6.5 h, THF, 83%
i) BuLi (ca. 3 equiv)
–78 °C, 30 min, THF
ii) 6,13-pentacenequinone
(1.0 equiv), rt, THF
iii) SnCl2•2H2O, H+
rt, THF, ca. 10%
Si
Si
TBSO
OTBS
19
Scheme 3. Functionalization of pentacene to give 19.

D. Lehnherr et al. / Tetrahedron 64 (2008) 11449–11461
11453
O
yield of dimer 34 (ca. 15%); the major product isolated was the
mono condensation product 37 (Fig. 5). Optimization was achieved
by (a) switching to a more hydrophobic solvent (CH2Cl2), (b) using
an excess of bis-acid chloride, (c) performing the reaction in
a minimal amount of solvent (to provide higher concentrations of
18), and (d) using only DMAP, rather than a mixture of Et3N and
DMAP. Ultimately, this led to an excellent yield for the dimerization
process (79%).
HO
Si
Dimer 34 was indeed readily soluble in a number of common
organic solvents. Thus, synthesis of dimers with gradually shorter
linkers was conducted, and dimers 27–34 with 1–8 methylene
groups were synthesized in excellent yield (79–97%, Scheme 7).
This allowed for a systematic study of the effects of tether length on
solubility, aggregation, and thin film formation. All of the dimers
were highly soluble in CH2Cl2, CHCl3, and THF, although those with
shorter linkers (n¼1 and 2) had slightly reduced solubility in
comparison to other dimers.
TBSO
Figure 4. Structure of monoaddition product 26 isolated during the formation of 25.
in 1:1 THF/CH3CN (both at room temperature and accompanied by
heating), but these also gave unsatisfactory results despite accel-
erating the rate of reaction.
Thus, best results were ultimately achieved by conducting the
Sn(II)-mediated reduction of trans-diol 25 in THF at room tem-
perature without HCl. The method gave protected monomer 19
in 83% yield as a blue solid. Over the course of attempts to
optimize this process, it became clear that when diol 25 was
exposed to SnCl2$2H2O in the presence of 10% HCl, loss of one
or both of the TBS group(s) occurred and more complex product
mixtures were produced. Thus, conditions that avoided the use
of aq HCl for the Sn(II)-mediated reduction of diol 25 were
employed and successfully prevented loss of the TBS protecting
group.
Pentacene trimers were then synthesized using desymme-
trized pentacene 18 to react with succinic or glutaric anhydride
affording 38 and 39 with
a pendent carboxylic acid group
(Scheme 6). Initially, work up to remove the excess anhydride
(along with DMAP) included washing with satd aq NaHCO3, fol-
lowed by 5% aq NaHCO3, and then satd aq NaCl. It was de-
termined, however, that basic conditions should be avoided to
circumvent the possible formation of the sodium carboxylate salt
during work up. The formation of 38 and 39 also typically gave
a trace of the corresponding dimer (ca. <5%) via an additional
coupling of the product under the reaction conditions. This
byproduct could be easily removed by size exclusion chroma-
tography to afford the pure carboxylic acids 38 and 39. Alterna-
tively, the byproduct could be separated after formation of trimer
35 or 36, at which point the difference in Rf values allowed
separation using column chromatography on silica gel.
A subsequent DCC mediated coupling of acid 38 or 39 with
monomer 17 afforded trimers 35 and 36. It was subsequently de-
termined that the synthesis of trimers could be simplified by
substituting EDC$HCl for DCC. This change in reagent greatly fa-
cilitated purification by virtue of forming water-soluble byprod-
ucts, and it also eliminated the need for product purification by size
exclusion chromatography. For example, EDC mediated coupling
gave trimer 36 in 94% yield. The EDC method was also explored for
the synthesis of one dimer (n¼2), and gave 28 in similar yield (90%)
as the bis-acid chloride method.
2.2. Desymmetrization via kinetic desilylation
Removal of the two TBS groups to unveil the diol functionality of
monomer 17 was accomplished in nearly quantitative yield using
dilute HCl in THF (Scheme 4). If the reaction time was limited to
45 min, monodeprotected 18 was isolated in 49% yield, along with
the recovery of 36% of unreacted 19 and 15% of diol 17. All three
compounds could be easily separated by column chromatography.
Alternatively, extending the reaction time to 1.5 h gave nearly equal
amounts of 17 (46%) and 18 (44%), along with 10% of unreacted 19.
With a series of dimers and trimers in hand, polymers 15 and 16
were targeted, and a number of polymerization conditions were
explored during optimization of their synthesis (Scheme 7). Ini-
tially, slow addition of a solution of the requisite bis-acid chloride
40 or 41 to a solution of 17 (w5ꢂ10ꢀ3 M) in the presence of DMAP
was tried, but this resulted in formation of a significant quantity of
cyclic oligomers, as determined by mass spectrometry analysis.
Reversing the order of addition (i.e., adding monomer 17 to a so-
lution of 40 or 41 in the presence of DMAP) led to the isolation of
primarily unreacted 17, seemingly due to quenching of the acid
chloride by adventitious water. Polymerization of 17 using the
corresponding bis-carboxylic acids and coupling agents such as
EDC$HCl in the presence of DMAP also gave significant amounts of
cyclic oligomers.
HCl, THF
rt, time
Si
Si
19
RO
OH
product yield (%)
19
18
17
time (h)
17 R = H; 18 R = TBS
0.75
1.5
36
10
–
49
44
–
15
46
98
6.0
Scheme 4. Desilylation to obtain building blocks 17 and 18.
2.3. Oligomerization process
Pentacene dimers 27–34 (Scheme 5) and trimers 35–36
(Scheme 6) were synthesized as model compounds to bridge the
gap between monomer 17 and polymers 15 and 16. The synthesis of
pentacene-based oligomers made use of unsymmetrical pentacene
building block 18. Initially, solubility was a concern and, as a result,
a pentacene dimer with a long alkyl tether linking the two ester
groups was targeted. The dimerization process was first attempted
in THF, with 1 equiv of 1,10-decanedioic acid chloride and 2 equiv of
18 in the presence of DMAP and Et3N. This gave an unsatisfactory
Ultimately, polymerization was best accomplished by pre-
paring a suspension of monomer 17 with DMAP in CH2Cl2
(w5ꢂ10ꢀ2 M) and quickly adding a solution of 40 or 41. Following
aqueous work up, purification of 15 and 16 could be effected
simply by recrystallization. MALDI mass spectrometry (MS)
analysis showed macromolecules of over m/z 17,000 were ach-
ieved (e.g., m¼20) for 15 and over m/z 15,000 (m¼17) for 16.
More specifically, this analysis showed a distribution of oligomers
from m¼2 to 20 and beyond for polymer 15, while the spectrum

11454
D. Lehnherr et al. / Tetrahedron 64 (2008) 11449–11461
18
O
O
DMAP
rt, 15 min, CH2Cl2
Cl
Cl
n
O
O
R
Si
Si
R
O
O
n
27 n = 1 80%
28 n = 2 97%
29 n = 3 95%
30 n = 4 87%
31 n = 5 93%
32 n = 6 96%
33 n = 7 86%
34 n = 8 79%
OTBS
R =
Si
Scheme 5. Synthesis of a homologous series of pentacene dimers.
O
O
O
OH
n
O
Si
O
n
O
17, DCC or
EDC•HCl, DMAP
(excess)
18
DMAP, rt, 2 h
CH2Cl2
rt, 3 h, CH2Cl2
R
38 n = 2 98%
39 n = 3 99%
O
O
OTBS
O
O
n
R =
Si
Si
Si
Si
R
R
35 n = 2 68% (via DCC)
O
O
36 n = 3 60% (via DCC)
Si
94% (via EDC)
O
O
n
Scheme 6. Synthesis of trimers 35 and 36.
for polymer 16 showed signals for m¼2 to 17 and beyond. In
addition to the generalized polymer structure shown in Scheme 7
for 15 and 16 (terminated with an acid and an alcohol), the
product mixtures also consisted of structures terminated with
two acids, two alcohols, and macrocyclic structures resulting from
intramolecular ester formation.17
3. UV–vis and fluorescence spectroscopy
UV–vis spectra of the new pentacene materials have been
measured in CH2Cl2. Figure 6 shows the absorption spectrum of
protected monomer 19, which is representative of all the de-
rivatives studied. A strong absorption is observed at lmax¼309 nm

D. Lehnherr et al. / Tetrahedron 64 (2008) 11449–11461
11455
O
O
8
Si
Si
O
OH
TBSO
37
Figure 5. Moncondensation side-product 37.
along with a series of low-energy absorptions, with the lowest
energy having a lmax at 645 nm. Compared to pristine pentacene,
which has lmax¼584 nm in ethanol18 and lmax¼576 nm in ben-
zene,19 this represents a red-shift of w65 nm.
Solvatochromism studies have been carried out for the more
diversely soluble pentacene 19 (Table 1) and small changes in the
position of lmax for the absorption bands are observed. Focusing
on the lowest energy lmax, one observes that the transition is
most red-shifted in CH2Cl2, CHCl3, and toluene, whereas the band
is most blue-shifted in hexanes, cyclohexane, and ethanol. In
Figure 6. UV–vis of 19 in CH2Cl2 representative of pentacene materials presented
herein. Inset: low-energy absorption region (right hand side trace), and fluorescence
using lexc¼309 nm (left hand side trace) normalized to unity.
red-shifted (<10 nm) in comparison with those measured in
solution.21
The emission of the pentacene materials has been investigated
in CH2Cl2 (Table 3). The fluorescence spectrum of 19 is represen-
tative, and shows a broad and relatively featureless emission with
lmax,em¼652 nm (lexc¼309 nm, Fig. 6). This represents a small
Stokes shift of only 7 nm and suggests minimal molecular rear-
rangement occurs upon photoexcitation of the molecule. A plot of
normalized fluorescence traces (lexc¼551 nm) for all these mate-
rials (Fig. 8) demonstrates that the overall emission profiles are
essentially identical.
Fluorescence quantum yields, FF, for all derivatives were mea-
sured using cresyl violet perchlorate as a standard,22 and the results
are summarized in Table 3. While FF seems to vary somewhat for
the three monomeric pentacenes 19, 17, and 18 (FF¼0.08–0.17), all
dimers, and both trimers show very similar FF values. Specifically
the dimers all tend to be around FF¼0.08–0.11, whereas the values
for the trimers, FF¼0.06, are about half that of the dimers.
Further analysis of FF data for dimers 27–34 suggests that the
length of the methylene tether between the two pentacene chro-
mophores might exert a small effect on fluorescence efficiency: the
longer the tether, the larger the FF (excluding 27). Ultimately,
however, this effect is on the same order of magnitude as the
expected error in the measurements (ca. 10%), so this correlation is
somewhat tenuous.
terms of
3
for the various bands as a function of solvent, minor
) in non-
changes are observed. Molar absorptivity coefficients (
3
polar solvents such as hexanes and cyclohexane were higher for
both the high-energy band as well as the low-energy transitions.
On the other hand, compared to the remaining solvents in the
table, toluene produces lower
sorption at 310 nm, without yielding lower
energy absorptions. No correlation was observed for either lmax or
as a function of the hydrogen-bonding capacity or the polarity of
the solvent.
The electronic structure of this basic pentacene unit found in,
3
values for the high-energy ab-
3
values for the low-
3
for example 19, is essentially unaffected by oligomer or polymer
formation. Comparing the UV–vis spectra of the homologous
series made up of monomer 19, dimer 29, trimer 36, and polymer
15 reveal that all molecules share nearly identical absorption
maxima (ꢃ1 nm) (Table 2 and Fig. 7). Molar absorptivity co-
efficients (
3) change monotonically as a function of oligomer
length (Table 2). In fact, a plot of
3
as a function of the number of
pentacene chromophores present in the molecule is essentially
linear. For example, this analysis has been made for monomers
17–19 in comparison to dimers 27–34 and trimers 35–36 at
309 nm and 645 nm, and both result in linear fits with R2¼0.991
and R2¼0.992, respectively.20 Thus, there appears to be no
intramolecular self-association of the pentacene moieties of these
oligomers in solution.
The absorption spectra of polymers 15 and 16 have been ex-
amined as thin films cast onto fused silica from CH2Cl2. In each case,
the low-energy absorptions between 475 and 675 nm are slightly
4. Aggregation
In dilute solutions, as used for UV–vis spectroscopy (w10ꢀ5 to
10ꢀ6 M), Beer’s Law is maintained for dimers 27–34. For example,
a plot of absorption at lmax¼645 nm as a function of concentration
O
O
Cl
Cl
n
O
O
40 n = 3, 41 n = 5
DMAP, rt, CH2Cl2
17
Si
Si
HO
O
O
H
n
m
15 n = 3 72%; 16 n = 5 85%
Scheme 7. Synthesis of polymers 15 and 16.

11456
D. Lehnherr et al. / Tetrahedron 64 (2008) 11449–11461
Table 1
Solvatochromism study of pentacene 19
Solvent
l/nm (3 )
/L molꢀ1 cmꢀ1
Hexanes
1-Butanol
269 (27,700)
270
306 (322,000)
307
327 (48,300)
328
438 (4840)
439
543 (6060)
546
585 (17,600)
588
637 (40,000)
639
a
Cyclohexane
Ethyl acetate
Acetone
270 (27,300)
269 (26,100)
db
307 (304,000)
306 (324,000)
db
328 (47,000)
328 (42,300)
327 (42,000)
328
439 (4500)
439 (4570)
439 (4430)
439
545 (5740)
548 (5320)
548 (5150)
549
586 (17,300)
589 (14,700)
590 (13,900)
592
639 (39,600)
640 (29,500)
640 (26,800)
641
a
Acetonitrile
269
306
Tetrahydrofuran
Toluene
Chloroform
270 (28,600)
db
308 (305,000)
310 (214,000)
310 (266,000)
309 (303,000)
329 (43,500)
330 (43,700)
330 (41,300)
330 (41,300)
439 (5120)
441 (4460)
440 (4340)
440 (4430)
549 (5730)
550 (4990)
551 (5120)
551 (5000)
592 (15,500)
594 (14,300)
593 (14,300)
594 (13,900)
643 (31,000)
645 (28,800)
645 (28,400)
645 (27,100)
271 (23,900)
271 (21,500)
Dichloromethane
Largest change
in lmax/nm
2
7200
25
4
110,000
34
3
7000
14
2
780
15
8
1070
18
9
3700
21
8
13,200
33
Largest change
in
3
/L molꢀ1 cmꢀ1
Largest percent
change in /%
3
a
Denotes solution of unknown concentration due to low solubility, solution was filtered to remove undissolved solids prior to measurement.
Peak could not be measured accurately due to UV–vis cut-off for the solvent.
b
Table 2
UV–vis data for pentacene materials in CH2Cl2
Compound
Number of pentacene units
lmax/nm (
3
/L molꢀ1 cmꢀ1
)
17, Monomer
18, Pseudo monomer
1
1
1
2
2
2
2
2
2
2
2
3
3
d
271 (22,600)
271 (24,400)
271 (21,500)
271 (45,600)
271 (45,800)
271 (46,500)
271 (46,700)
271 (46,400)
271 (44,800)
271 (41,800)
271 (40,200)
271 (66,900)
271 (62,400)
271
309 (285,000)
309 (279,000)
309 (303,000)
309 (536,000)
309 (508,000)
309 (547,000)
309 (523,000)
309 (540,000)
309 (514,000)
309 (531,000)
309 (512,000)
309 (802,000)
309 (803,000)
309
330 (40,200)
329 (40,200)
330 (41,300)
330 (78,500)
330 (73,400)
330 (79,800)
329 (75,800)
330 (78,800)
330 (73,000)
330 (74,900)
330 (72,100)
330 (113,000)
330 (111,000)
330
440 (4330)
440 (4240)
440 (4430)
440 (8310)
440 (7500)
440 (8410)
440 (8090)
440 (8340)
440 (7360)
440 (6940)
440 (6690)
440 (12,200)
440 (11,700)
440
551 (4940)
551 (4860)
551 (5000)
552 (9600)
551 (8660)
551 (9690)
550 (9190)
552 (9620)
552 (8570)
552 (8550)
552 (8240)
551 (13,700)
551 (13,400)
552
594 (13,400)
593 (13,400)
594 (13,900)
594 (26,200)
593 (24,400)
594 (26,600)
593 (25,200)
594 (26,300)
594 (24,300)
594 (24,900)
594 (24,000)
594 (37,700)
594 (37,000)
594
645 (25,900)
644 (25,800)
645 (27,100)
645 (50,400)
645 (47,100)
645 (51,100)
644 (48,500)
645 (50,600)
645 (47,100)
645 (48,800)
645 (47,000)
645 (72,400)
645 (71,400)
645
19, Protected monomer
27, Dimer n¼1
28, Dimer n¼2
29, Dimer n¼3
30, Dimer n¼4
31, Dimer n¼5
32, Dimer n¼6
33, Dimer n¼7
34, Dimer n¼8
35, Trimer n¼2
36, Trimer n¼3
15, Polymer n¼3
16, Polymer n¼5
d
271
309
330
440
552
594
645
for 28 (6.6ꢂ10ꢀ7 to 1.7ꢂ10ꢀ5 M) affords a linear best-fit line with
R-value of 0.9998. Dimers with shorter linker chains (27, 31, and
especially 28), however, aggregate in more concentrated solutions
(ca. >0.05 M) as determined by 1H NMR spectroscopy. In these
cases, multiple signals are observed where a single resonance is
expected (Fig. 9). For example, the singlet observed for the t-Bu
group of the TBS ether group is split into multiple signals. Upon
dilution of the NMR sample, these resonances coalesce into a single
Figure 7. Effect of the oligomerization process on the UV–vis spectra of pentacene
materials 19, 29, 36, and 15 (in CH2Cl2). The intensity of the absorption trace for
polymer 15 is plotted in arbitrary units.
Figure 8. Fluorescence spectra of selected pentacenes in CH2Cl2 measured with
lexc¼551 nm (intensity normalized to unity).

D. Lehnherr et al. / Tetrahedron 64 (2008) 11449–11461
11457
Table 3
Summary of FF of pentacene materials in CH2Cl2 measured using lexc¼551 nm
Table 4
Thermal stability of pentacene materials determined by TGA and DSC
Compound
FF
lmax,em/nm
Stokes shift/nm
Compound
TGA Td/ꢁC
DSC (decomposition)
Onset/ꢁC
Maximum/ꢁC
17, Monomer
0.14
0.17
0.08
0.09
0.08
0.08
0.09
0.09
0.10
0.11
0.11
0.06
0.06
653
652
652
652
652
652
652
652
652
652
652
652
652
6
8
7
7
7
7
8
7
7
7
7
7
7
18, Pseudo monomer
19, Protected monomer
27, Dimer n¼1
28, Dimer n¼2
29, Dimer n¼3
30, Dimer n¼4
31, Dimer n¼5
32, Dimer n¼6
33, Dimer n¼7
34, Dimer n¼8
35, Trimer n¼2
36, Trimer n¼3
17, Monomer
365
385
385
370
390
385
400
385
400
385
385
380
380
385
380
325
360
380
370
380
360
370
375
370
380
375
370
365
370
370
375
380
395
390
395
390
405
395
390
395
395
390
390
405
385
18, Pseudo monomer
19, Protected monomer
27, Dimer n¼1
28, Dimer n¼2
29, Dimer n¼3
30, Dimer n¼4
31, Dimer n¼5
32, Dimer n¼6
33, Dimer n¼7
34, Trimer n¼8
35, Trimer n¼2
36, Trimer n¼3
15, Polymer n¼3
16, Polymer n¼5
signal. Interestingly, no such similar aggregation behavior is ob-
served in either the trimers or the polymers.
Monomer 17 co-crystallizes with two molecules of MeOH (Fig. 10a).
Analysis of the solid-state packing for 17 reveals molecules
arranged in one-dimensional slipped-stacks (Fig. 12a),26 with an
interplanar distance between the acenes of 3.41 Å.27 Thus, in terms
of p-overlap and orientation in the solid-state, compound 17 is well
suited for intermolecular communication.
Attempts have been made to obtain solvent free crystals of 17,
but these efforts have been unsuccessful to date. Nevertheless, an
additional polymorph of 17 has been produced from a mixture of
CH2Cl2 and acetone.28 This results in a solid-state arrangement of
two structurally independent molecules of 17, namely molecules A
(Fig. 10b) and molecule B (Fig. 10c). These two molecules are linked
by hydrogen-bonding between their benzylic alcohol groups, ad-
ditionally molecule A hydrogen bonds to an acetone solvent mol-
ecule (Fig. 11). The polymorph of 17 co-crystallized with acetone is
also arranged in a one-dimensional slipped-stack configuration
(Fig. 12b), with an interplanar distance between the acenes of
3.39 Å (molecule A to molecule A) and 3.45 Å (molecule B to mol-
ecule B). Since the individual one-dimensional stacks are formed
5. Thermal analysis
Differential scanning calorimetry (DSC) and thermal gravi-
metric analysis (TGA) have been carried out on most of the
pentacene materials presented herein to assess thermal stability
(Table 4).23 Thermal stability as assessed by TGA shows no sig-
nificant weight loss (<5%) below 350 ꢁC in all cases. The oligo-
merization process increases the thermal stability slightly:
monomer 17 shows decomposition at 365 ꢁC, while all oligomers
decompose above this temperature. This is perhaps a result of
reducing the ratio of primary alcohols (nucleophilic) to pentacene
moieties. Further support for this premise is provided by the fact
that both 18 and 19, which have either one or both alcohol
moieties protected as the TBS ether, show higher thermal sta-
bility in comparison to monomer 17. DSC analysis shows no glass
transition temperature for polymers 15 and 16. Additionally, no
thermal event is observed in the DSC analysis below 350 ꢁC for
these materials, which could be associated with decomposition.
This contrasts the behavior of closely related monomeric penta-
cene materials that show decomposition well below this
temperature.4m,24
exclusively from either molecules A or molecules B, no p-stacking
interactions are found between molecules A and B in the solid-
state.
Single crystals of 19 have been grown at 4 ꢁC by allowing
a CH2Cl2 solution of 19 layered by a small amount of acetone to
evaporate slowly.29 Although disorder is apparent in the tert-
butyldimethylsilyloxybenzyl group of 19, the remainder of the
structure, including the bis-ethynylpentacene moiety, is uni-
formly arranged. Pentacene 19 reveals a two-dimensional slip-
ped-stacks arrangement similar to that of pentacene derivatives
exhibiting high hole mobility (Fig. 13).1–3,30 The interplanar dis-
tance between the acenes is 3.38 Å, which is slightly less than
those in the polymorphs of 17. These intermolecular distances in
both polymorphs of 17 and 19 between the acenes are compa-
rable to 6,13-bis(triisopropylsilylethynyl)pentacene, which has
spacing of 3.47 Å.4a
6. X-ray crystallography
Solid-state packing plays an important role for materials in
molecular electronics because electronic performance is often en-
hanced by strong electronic coupling mediated by strong in-
termolecular interactions between adjacent molecules.1–3 The
solid-state structure of the common building block, 17, has been
determined by X-ray analysis of crystals grown from THF/MeOH.25
7. Conclusions
In conclusion, we provide a full account of the first reported
synthesis of defined-length pentacene oligomers and representa-
tive polymeric derivatives, as well as a description of their physical
properties. All molecules exhibit good solubility and stability. The
synthetic strategy that has been developed is also amenable to
inclusion of other terminal groups on the monomer unit that en-
ables the synthesis of various other types of polymers and these
results will be presented in due course.
Figure 9. 1H NMR signal corresponding to t-Bu of TBS group in 27 at a concentration of
(a) w0.06 M (b) w0.002 M demonstrating the spectroscopic effects of aggregation in
CDCl3.

11458
D. Lehnherr et al. / Tetrahedron 64 (2008) 11449–11461
Figure 10. X-ray of 17 (a) polymorph grown from THF/MeOH. The polymorph of 17 grown from CH2Cl2/acetone showing the two structurally independent geometries, (b) molecule
A, and (c) molecule B.
8. Experimental section
8.1. General experimental
were distilled from CaH2. Anhydrous MgSO4 was used as the
drying agent after aqueous work up. Evaporation and concentra-
tion in vacuo were done at water-aspirator pressure. All reactions
were performed in standard, dry glassware under an inert at-
mosphere of nitrogen.
Reagents were purchased in reagent grade from commercial
suppliers and used without further purification. Preliminary
experimental procedures and spectroscopic data for compounds
15–20 and 22–39 have been previously reported.5 Detailed ex-
perimental procedures and spectroscopic data for compounds
15–20, 22–25, 27–36, 38–39, and spectroscopic data for 26 and 37
are provided as Supplementary data. 6,13-Pentacenequinone31
was recrystallized from N,N-dimethylformamide, washed with dry
THF, and dried under vacuum prior to use. THF and Et2O were
distilled from sodium/benzophenone ketyl; CH3CN and CH2Cl2
8.2. Synthesis of polymers 15 and 16
8.2.1. Synthesis of polymer 15
Compound 17 (0.105 g, 0.137 mmol), freshly recrystallized
from CH2Cl2/hexanes, was dissolved in dry CH2Cl2 (2.5 mL) and
DMAP (0.080 g, 0.65 mmol) was added. To this mixture under
stirring, a solution of 40 (0.27 M, 0.28 mL, 0.077 mmol), prepared
from 40 (0.46 g, 0.35 mL, 2.7 mmol) dissolved in dry CH2Cl2
(10 mL), was quickly added. After ca. 30 s, a second portion of
the solution of 40 (0.27 M, 0.25 mL, 0.069 mmol) was quickly
added. The reaction mixture was then allowed to stir for 3 h
before being diluted with CH2Cl2 (50 mL) and satd aq NaHCO3
(100 mL). The mixture was separated and the organic phase was
washed with 5% aq NaHCO3 (1ꢂ100 mL), satd aq NH4Cl
(2ꢂ150 mL), satd aq NaCl (150 mL), and dried (MgSO4). The
solvent was removed in vacuo. The residue was dissolved in
CH2Cl2 (2 mL) and precipitated by adding hexanes (40 mL) and
cooling to ꢀ78 ꢁC. After filtering, the resulting solid was washed
with hexanes (3ꢂ15 mL) and redissolved in CH2Cl2. This solution
was filtered through a fritted funnel to remove any undissolved
solids, concentrated in vacuo, and the resulting solid dried under
vacuum to afford polymer 15 (0.087 g, 72%) as a deep blue solid.
UV–vis (CH2Cl2) lmax: 271, 309, 330, 440, 552, 594, 645 nm. IR
(CDCl3, cast) 3042 (w), 3020 (w), 2943 (s), 2864 (s), 2136 (m),
1738 (s) cmꢀ1 1H NMR (500 MHz, CDCl3):
. d 9.35 (br s, 4H), 7.99–
7.94 (m, 4H), 7.88 (br d, J¼7.9 Hz, 4H), 7.46–7.38 (m, 8H), 5.17 (br
s, 4H), 2.47 (t, J¼7.4 Hz, 4H), 2.04 (quintet, J¼7.3 Hz, 2H), 1.61–
1.48 (m, 4H), 1.38 (br d, J¼7.3 Hz, 12H), 1.25 (d, J¼7.4 Hz, 12H).
13C NMR (125 MHz, CDCl3):
d 172.7, 137.1, 135.6, 133.4, 132.4,
130.7, 128.6, 127.5, 126.2, 118.2, 105.9, 105.3, 66.2, 33.3, 20.1, 18.2,
18.1, 12.0. TGA: Tdz385 ꢁC. DSC: decomposition, 370 ꢁC (onset)
and 405 ꢁC (peak).
Figure 11. Hydrogen-bonding interactions between adjacent molecule A (left) and
molecule B (right) of 17 and solvent acetone in crystals of 17 grown from CH2Cl2/
acetone.

D. Lehnherr et al. / Tetrahedron 64 (2008) 11449–11461
11459
Figure 12. Solid-state packing of 17 from crystals grown from (a) THF/MeOH (b) CH2Cl2/acetone (thermal ellipsoids for the carbon atoms have been omitted for clarity).
8.2.2. Synthesis of polymer 16
3045 (w), 2943 (s), 2890 (m), 2863 (m), 2136 (m), 1738 (s) cmꢀ1. 1H
NMR (500 MHz, CDCl3): 9.32 (br s, 4H), 7.96–7.90 (m, 4H), 7.88 (br
Compound 17 (0.205 g, 0.261 mmol), freshly recrystallized from
CH2Cl2/hexanes, was dissolved in dry CH2Cl2 (5 mL) and DMAP
(0.159 g, 0.130 mmol) was added. To this mixture under stirring,
a solution of 41 (0.56 M, 0.50 mL, 0.28 mmol), prepared from 41
(0.55 g, 0.46 mL, 2.8 mmol) dissolved in dry CH2Cl2 (5 mL), was
added dropwise over a period of 20 min. The reaction mixture was
then allowed to stir for 2 days before being diluted into CH2Cl2
(50 mL) and satd aq NaHCO3 (100 mL). The mixture was separated
and the organic phase was washed with 5% aq NaHCO3 (3ꢂ100 mL),
satd aq NaCl (150 mL), and dried (MgSO4). The solvent was re-
moved in vacuo. The residue was dissolved in CH2Cl2 (2 mL) and
precipitated by adding hexanes (40 mL) and cooling to ꢀ78 ꢁC.
After filtering, the resulting solid was washed with hexanes
(3ꢂ10 mL) and redissolved in CH2Cl2. This solution was filtered
through a fritted funnel to remove any undissolved solids, con-
centrated in vacuo, and the resulting solid dried under vacuum to
afford polymer 16 (0.202 g, 85%) as a deep blue solid. UV–vis
(CH2Cl2) lmax: 271, 309, 330, 440, 552, 594, 645 nm. IR (CDCl3, cast)
d
d, J¼7.9 Hz, 4H), 7.42–7.35 (m, 8H), 5.12 (br s, 4H), 2.35 (t, J¼7.5 Hz,
4H), 1.69–1.60 (m, 4H), 1.59–1.47 (m, 4H), 1.36 (br d, J¼7.4 Hz, 12H),
1.39–1.31 (2H, this signal overlaps a broad doublet at 1.36), 1.22 (d,
J¼7.4 Hz, 12H). 13C NMR (125 MHz, CDCl3):
d 173.3, 137.3, 135.5,
133.2, 132.4, 130.7, 128.6, 127.4, 126.2, 118.2, 105.8, 105.3, 66.0, 34.0,
28.5, 24.5, 18.2, 18.1, 12.0. TGA: Tdz380 ꢁC. DSC: decomposition,
370 ꢁC (onset) and 385 ꢁC (peak).
8.3. Synthesis of dimers 27–34
As a general procedure for the synthesis of pentacene-based
dimers, procedures for 28 are presented as examples. The other
syntheses are analogous and details are provided in Supplementary
data.
8.3.1. Synthesis of 28 using succinyl chloride
Compound 18 (0.075 g, 0.085 mmol) was dissolved in dry
CH2Cl2 (2 mL) and DMAP (0.0258 g, 0.211 mmol) was added. To this
mixture under stirring, a solution of succinyl chloride (0.17 M,
0.25 mL, 0.042 mmol), prepared from succinyl chloride (0.13 g,
0.094 mL, 0.85 mmol) dissolved in dry CH2Cl2 (5 mL), was added
dropwise over a period of 5 min. The reaction mixture was allowed
to stir at rt for 3 min before adding additional solution of succinyl
chloride (0.17 M, 0.25 mL 0.042 mmol) dropwise followed by dry
CH2Cl2 (5 mL). The reaction mixture was allowed to stir for 5 min
before being diluted with dry CH2Cl2 (20 mL) and 5% aq NaHCO3
(20 mL). The mixture was separated and the organic phase was
washed with satd aq NaCl (25 mL), dried (MgSO4), and the solvent
removed in vacuo. Column chromatography (silica gel, CH2Cl2)
afforded 28 (0.0757 g, 97%) as a deep blue solid.
8.3.2. Synthesis of dimer 28 using EDC$HCl
Compound 18 (0.156 g, 0.177 mmol), EDC$HCl (0.169 g,
0.855 mmol), and succinic acid (0.010 g, 0.085 mmol) were dis-
solved in dry CH2Cl2 (4 mL) and DMAP (0.108 g, 0.855 mmol) was
added. The reaction mixture was allowed to stir at rt for 8 h before
being poured into 5% aq NaHCO3 (100 mL), which was then diluted
with CH2Cl2 (50 mL). The mixture was separated and the organic
phase was washed with 5% aq NaHCO3 (100 mL), satd aq NaCl
(150 mL), dried (MgSO4), and the solvent removed in vacuo. Col-
umn chromatography (silica gel, CH2Cl2) afforded 28 (0.141 g, 90%)
as a deep blue solid. Rf¼0.65 (CH2Cl2). UV–vis (CH2Cl2) lmax
(3): 271
Figure 13. Solid-state arrangement of 19 (thermal ellipsoids for the carbon atoms have
been omitted for clarity).
(45,800), 309 (508,000), 330 (73,400), 440 (7500), 551 (8660), 593

11460
D. Lehnherr et al. / Tetrahedron 64 (2008) 11449–11461
(24,400), 645 (47,100) nm. Fluorescence (CH2Cl2): lexc¼551 nm,
lmax,em¼652 nm, FF¼0.08. IR (CDCl3, cast) 3048 (w), 2952 (s), 2863
observed). MALDI (CH2Cl2, DCTB) m/z 2721.1 ([MþH]þ, 100). TGA:
Tdz380 ꢁC. DSC: decomposition, 365 ꢁC (onset) and 390 ꢁC (peak).
(s), 2136 (m), 1740 (m) cmꢀ1. 1H NMR (500 MHz, CDCl3):
d 9.40 (s,
4H), 9.35 (s, 4H), 8.03–7.97 (m, 8H), 7.93 (d, J¼8.0 Hz, 4H), 7.92 (d,
J¼8.1 Hz, 4H), 7.47 (d, J¼8.2 Hz, 4H), 7.46 (d, J¼8.2 Hz, 4H), 7.45–
7.41 (m, 8H), 5.21 (s, 4H), 4.83 (s, 4H), 2.78 (s, 4H), 1.59 (septet,
J¼7.3 Hz, 4H),1.55 (septet, J¼7.2 Hz, 4H),1.42 (d, J¼7.3 Hz,12H),1.40
(d, J¼7.3 Hz, 12H), 1.29 (d, J¼7.3 Hz, 12H), 1.26 (d, J¼7.3 Hz, 12H),
8.4.3. Synthesis of acid 39
Compound 18 (0.2966 g, 0.3365 mmol) and glutaric anhydride
(0.150 g, 1.31 mmol) were dissolved in dry CH2Cl2 (7 mL) and DMAP
(0.158 g, 1.29 mmol) was added. The reaction mixture was allowed
to stir at rt for 2 h before being diluted with CH2Cl2 (70 mL) and
satd aq NaHCO3 (200 mL). The mixture was separated and the or-
ganic phase was washed with satd aq NaHCO3 (200 mL), 5% aq
NaHCO3 (2ꢂ200 mL), satd aq NaCl (250 mL), dried (MgSO4), and the
solvent removed in vacuo. This afforded 39 (0.3324 g, 99%) as
a deep blue solid. Size exclusion chromatography (BioRad Bio-
Beads S-X2, 200–400 mesh, CH2Cl2, gravity flow) could be
employed to remove traces of dimer 29 (ca. <5%) by collecting the
second blue band, which elutes off the column (the first band is
dimer 29). IR (CDCl3, cast) 3048 (w), 2952 (s), 2863 (s), 2136 (m),
0.98 (s, 18H), 0.14 (s, 12H). 13C NMR (125 MHz, CDCl3):
d 172.1, 142.9,
137.0,135.6,135.3, 133.5,132.44,132.41, 131.43, 130.74,130.71,128.7,
128.6, 127.5, 126.4, 126.22, 126.18, 125.5, 118.5, 118.1, 105.9, 105.8,
105.6, 105.2, 66.5, 64.9, 29.2, 26.0, 18.4, 18.25, 18.20, 18.17, 18.13,
12.12, 12.07, ꢀ5.3 (one signal not observed). ESI MS m/z 1866.9
([MþNa]þ, 100). TGA: Tdz390 ꢁC. DSC: decomposition, 380 ꢁC
(onset) and 395 ꢁC (peak).
8.4. Synthesis of trimers 35–36
1739 (s), 1710 (s) cmꢀ1 1H NMR (500 MHz, CDCl3):
. d 9.40 (s, 2H),
As a general procedure for the synthesis of pentacene-based
trimers, procedures for 36 (and its precursor compound 39) are
presented as examples. The other syntheses are analogous and
details are provided in Supplementary data.
9.38 (s, 2H), 8.02–7.97 (m, 4H), 7.94 (d, J¼8.2 Hz, 2H), 7.92 (d,
J¼7.7 Hz, 2H), 7.48–7.41 (m, 8H), 5.20 (s, 2H), 4.83 (s, 2H), 2.49 (t,
J¼7.4 Hz, 2H), 2.45 (t, J¼7.3 Hz, 2H), 2.01 (quintet, J¼7.3 Hz, 2H),
1.58 (septet, J¼7.3 Hz, 4H), 1.42 (d, J¼7.3 Hz, 12H), 1.29 (d, J¼7.3 Hz,
12H), 0.98 (s, 9H), 0.14 (s, 6H). gCOSY NMR (500 MHz, CDCl3):
8.4.1. Synthesis of trimer 36 using DCC
d
9.4048.02–7.97; 9.3848.02–7.97; 8.02–7.9749.40, 9.38;
Compounds 39 (0.184 g, 0.185 mmol) and 17 (0.0669 g,
0.0872 mmol) were dissolved in dry CH2Cl2 (5 mL), and DMAP
(0.0710 g, 0.581 mmol) and DCC (0.050 g, 0.24 mmol) were added.
The reaction mixture was allowed to stir at rt for 3 h before placing
the flask in the refrigerator (4 ꢁC) overnight to allow for most of the
dicyclohexylurea to precipitate. The reaction mixture was filtered
and the filtrate was diluted with CH2Cl2 (80 mL) and satd aq
NaHCO3 (200 mL). The mixture was separated and the organic
phase was washed with satd aq NaHCO3 (200 mL), 5% aq NaHCO3
(2ꢂ200 mL), satd aq NaCl (250 mL), dried (MgSO4), and the solvent
removed in vacuo. Column chromatography (silica gel, CH2Cl2)
afforded 36 with some residual dicyclohexylurea, which was later
removed using size exclusion chromatography (BioRad Bio-Beads
S-X2, 200–400 mesh, CH2Cl2, gravity flow) to afford 36 (0.142 g,
60%) as a deep blue solid.
7.9447.48–7.41; 7.9247.48–7.41; 7.48–7.4148.02–7.97, 7.94, 7.92,
5.20, 4.83; 5.2047.48–7.40; 4.8347.48–7.41; 2.4942.01;
2.4542.01; 2.0142.45, 2.01; 1.5841.42, 1.29; 1.4241.58;
1.2941.58. 13C NMR (125 MHz, CDCl3):
d 178.3 (br), 172.7, 142.9,
137.1, 135.6, 135.3, 133.5, 132.44, 132.42, 131.4, 130.74, 130.71, 128.7,
128.6, 127.5, 126.4, 126.23, 126.19, 125.5, 118.5, 118.1, 105.9, 105.8,
105.6, 105.2, 66.2, 64.9, 33.2, 29.7, 26.0, 19.9, 18.4, 18.25, 18.21, 18.15,
18.16, 12.11, 12.08, ꢀ5.3 (two signals not observed). ESI MS m/z
993.5 ([MꢀH]ꢀ, 100). ESI HRMS m/z calcd for C63H73O5Si3 ([MꢀH]ꢀ)
993.4771, found 993.4758.
Acknowledgements
This work has been generously supported by the University of
Alberta and the Natural Sciences and Engineering Research Council
of Canada (NSERC) through the Discovery Grant program. D.L.
thanks NSERC (PGS-D), the Alberta Ingenuity Fund, the University
of Alberta, and the Killam Trusts for scholarship support. We thank
Prof. Robert E. Campbell (University of Alberta) for helpful discus-
sions regarding fluorescence spectroscopy.
8.4.2. Synthesis of trimer 36 using EDC$HCl
Compound 39 (0.160 g, 0.161 mmol), 17 (0.054 g, 0.070 mmol)
and EDC$HCl (0.194 g, 1.01 mmol) were dissolved in dry CH2Cl2
(4 mL) and DMAP (0.148 g, 1.21 mmol) was added. The reaction
mixture was allowed to stir at rt for 14 h before being poured into
5% aq NaHCO3 (100 mL) and diluted with CH2Cl2 (50 mL). The
mixture was separated and the organic phase was washed with 5%
aq NaHCO3 (100 mL), satd aq NaCl (150 mL), dried (MgSO4), and the
solvent removed in vacuo. Column chromatography (silica gel,
CH2Cl2) afforded 36 (0.180 g, 94%) as a deep blue solid. Rf¼0.24
Supplementary data
Experimental procedures and spectroscopic data for com-
pounds 15–20 and 22–39, MS spectra for polymers, UV–vis spectra
of thin films of the polymers, and graph of molar absorptivity as
a function of number of pentacene units. Supplementary data as-
sociated with this article can be found in the online version, at
doi:10.1016/j.tet.2008.09.041.
(CH2Cl2). UV–vis (CH2Cl2) lmax (3): 271 (62,400), 309 (803,000), 330
(111,000), 440 (11,700), 551 (13,400), 594 (37,000), 645
(71,400) nm. Fluorescence (CH2Cl2): lexc¼551 nm, lmax,em¼652 nm,
FF¼0.06. IR (CDCl3, cast) 3048 (w), 2943 (s), 2863 (s), 2136 (m),
1739 (s) cmꢀ1. 1H NMR (500 MHz, CDCl3):
d
9.41 (s, 4H), 9.38 (2ꢂs,
References and notes
8H), 8.03–7.97 (m, 12H), 7.93 (m, 12H), 7.59–7.48 (m, 24H), 5.19 (s,
8H), 4.83 (s, 4H), 2.50 (t, J¼7.4 Hz, 8H), 2.07 (quintet, J¼7.3 Hz, 4H),
1.59 (septet, J¼7.4 Hz, 4H), 1.56 (septet, J¼7.3 Hz, 4H), 1.56 (septet,
J¼7.1 Hz, 4H), 1.43 (d, J¼7.2 Hz, 12H), 1.41 (d, J¼7.3 Hz, 12H), 1.41 (d,
J¼7.3 Hz, 12H), 1.30 (d, J¼7.4 Hz, 12H), 1.28 (d, J¼7.4 Hz, 12H), 1.27
(d, J¼7.3 Hz, 12H), 0.98 (s, 18H), 0.15 (s, 12H). 13C NMR (125 MHz,
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monoclinic space group P21/c (No. 14); rcalcd¼1.156 g cmꢀ3; a¼8.8223(6) Å,
b¼19.0409(13) Å, c¼14.6224(10) Å;
b
¼103.5400(16)ꢁ; V¼2388.1(3) Å3; Z¼2;
m
¼0.118 mmꢀ1
.
Final R1(F)¼0.0433 (6400 observations [F2oꢄ2
s
(Fo2)]);
5. A preliminary account of this work has appeared: Lehnherr, D.; Tykwinski, R. R.
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s
(Fo2)]; CCDC
654502. X-ray data have been deposited at the Cambridge Crystallographic
Data Centre, 12 Union Road, Cambridge CB21EZ, UK; fax: (þ44)122 333 6033.
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18. Goodings, E. P.; Mitchard, D. A.; Owen, G. J. Chem. Soc., Perkin Trans. 1 1972,
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27. The interplanar distance was obtained by defining a mean-square plane from the
22 pentacene carbons and measuring the distance to the adjacent acene plane.
28. X-ray crystallographic data for 17$acetone: C52H54O2Si2$(CH3)2CO, M¼825.21;
monoclinic space group P21/c (No. 14); rcalcd¼1.144 g cmꢀ3; a¼8.9392(9) Å,
b¼52.448(5) Å, c¼15.6959(16) Å;
b
¼102.5066(16)ꢁ; V¼7184.3(13) Å3; Z¼6;
¼0.
m
116 mmꢀ1. Final R1(F)¼0.0728 (11,705 observations [Fo2ꢄ2
s
(Fo2)]); wR2(F2)¼0.
1841 for 812 variables and 14,079 data with [Fo2ꢄꢀ3
s
(F2o)]; CCDC 697879.
29. X-ray crystallographic data for 19: C64H82O2Si4, M¼995.66; triclinic space
group P1 (No. 2); rcalcd¼1.111 g cmꢀ3
;
a¼7.3144(7) Å, b¼78.8444(8) Å,
ꢀ
c¼26.168(3) Å;
a
¼89.717(2)ꢁ,
b
¼183.279(2)ꢁ,
g
¼86.803(2)ꢁ; V¼1488.8(3) Å3;
Z¼1;
m
¼0.141 mmꢀ1
.
Final R1(F)¼0.0934 (3021 observations [Fo2ꢄ2
s
(Fo2)]);
wR2(F2)¼0.2937 for 397 variables, 49 restraints, and 5245 data with
[Fo2ꢄꢀ3
s
(Fo2)]; CCDC 697880. The t-BuMe2SiOCH2C6H4 side chain exhibited
disorder over two positions in which both components were 50% occu-
pied. The more poorly behaved component was restrained to have the
same geometry (distances and angles) as the better behaved one by use
of the SHELXL instruction SAME. Additionally, the t-butyl group of the
better behaved orientation was restrained to have the same Me–C bond
distances. Finally, equivalent atoms between the t-BuMe2Si groups were
constrained to have the same anisotropic displacement parameters.
30. (a) Sheraw, C. D.; Jakson, T. N.; Eaton, D. L.; Anthony, J. E. Adv. Mater. 2003, 15,
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T. N.; Anthony, J. E. J. Am. Chem. Soc. 2008, 130, 2706–2707.
31. Ried, W.; Antho¨fer, F. Angew. Chem. 1953, 65, 601.