Template-Assisted Benzannulation Route to Pentacene and Tetracene Derivatives and its Application to Construct Amphiphilic Acenes That Self-Assemble into Helical Wires

Article abstract of DOI:10.1002/chem.201703084

Pentacene is one of the most versatile organic semiconductors. New synthetic strategies to construct the pentacene skeleton are imperative to produce pentacene derivatives with appropriate solubility, stability, and optoelectronic properties for various applications. This paper describes a template-directed approach to pentacene derivatives. In the retrosynthesis, the acene skeleton is viewed as a laddered double strand polyene instead of the more intuitive linearly fused hexagons. Based on this vision, the template strand of polyene is constructed with Wittig olefination, whereas the second strand is accomplished with Knoevenagel condensation to produce pentacene and tetracene derivatives. The synthetic scheme is flexible enough to generate an array of acene derivatives with substitution patterns that were hitherto difficult to access. Amphiphilic pentacene and tetracene derivatives were also synthesized by the template strategy. One pentacene based amphiphilic rod-coil molecule undergoes self-assembly to form helical wire structures that were visualized with TEM.

Full text of DOI:10.1002/chem.201703084

A Journal of
Accepted Article
Title: Template-Assisted Benzannulation Route to Pentacene
and Tetracene Derivatives and its Application to Construct
Amphiphilic Acenes that Self-Assemble into Helical Wires
Authors: Chih-hsiu Lin, Bikash Pal, Chun-Hsiung Chang, and Cian-Zhe
Zeng
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.201703084
Link to VoR: http://dx.doi.org/10.1002/chem.201703084
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Template-Assisted Benzannulation Route to Pentacene and
Tetracene Derivatives and its Application to Construct Amphiphilic
Acenes That Self-Assemble into Helical Wires
Bikash Pal, Chun-Hsiung Chang, Cian-Jhe Zeng, and Chih-Hsiu Lin*
Abstract: Pentacene is one of the most versatile organic
semiconductors. New synthetic strategies to construct the pentacene
skeleton are imperative to produce pentacene derivatives with
appropriate solubility, stability, and optoelectronic properties for
various applications. This paper describes a template-directed
approach to pentacene derivatives. In the retrosynthesis, the acene
skeleton is viewed as a laddered double strand polyene instead of the
more intuitive linearly fused hexagons. Based on this vision, the
template strand of polyene is constructed with Wittig olefination,
whereas the second strand is accomplished with Knoevenagel
condensation to produce pentacene and tetracene derivatives. The
synthetic scheme is flexible enough to generate an array of acene
derivatives with substitution patterns that are hitherto difficult to
access. Amphiphilic pentacene and tetracene derivatives were also
synthesized by the template strategy. One pentacene based
amphiphilic rod-coil molecule undergoes self-assembly to form helical
wire structures that were visualized with TEM.
acene structures. The template-directed multiple benzannulation
approach to tetracene and pentacene structures described in this
article is an attempt towards this direction.
This intuition to adopt the stepwise benzannulation approach is
the result of viewing the acene skeleton as a linear array of fused
benzene rings. However, acene can also be viewed as double
strand polyenes. In fact, computational studies have long
recognized that the electronic structures of high acene derivatives
resemble those of double strand ladder polyenes.9 Taking a hint
from such different perspective, we propose an alternative
retrosynthetic approach to acene skeleton perceiving acene as a
double strand laddered polyene instead of the linear array of
fused benzene rings. The most prominent path to construct
double strand systems, exemplified by DNA synthesis in
biological systems, is to pre-form one strand as the template to
assemble the second strand. To employ this principle to acene
synthesis, the template polyene formed first must carry the
building blocks for the second strand. The second strand can
then be assembled with the first strand as its template (scheme
1). The benzannulation and precursor routes are merged in this
template approach since the acene skeletons are accomplished
only in the last step from non-acene precursors via multiple
benzannulation.
Introduction
In the pursuit of organic electronic material,1 the linearly fused
pentacene skeleton has been proven one of the most privileged
structural motifs.2 Pentacene derivatives have shown great
promise as the semiconducting components in many devices.
Parallel to this emerging endeavor, the synthetic effort towards
organic conjugated systems in general also experienced a recent
renaissance. Innovative methodologies are invented and refined
to deliver target compounds in practical scale for physical studies
or device fabrications.3 In the present study, we wish to report a
conceptually novel synthetic strategy towards pentacene and
tetracene skeletons via template-directed benzannulation.
Regioselective installation of various functional groups onto the
acene skeletons was achieved with the new methodology.
On surveying the literature, retrosynthetic plans towards acene
homologues rely heavily on stepwise benzannulation strategies.
Notable examples include the Diels-Alder approaches,4 the
double aldol condensation approach,5 the zirconocene mediated
cyclization,6 and the iterative elongation tactic based on Wittig-
Knoevenagel benzannulation.7 An alternative approach is the
“precursor route” where the acene skeleton is established only in
the last step from a precursor molecule via photochemical or
thermolytic extrusion reactions.8 Despite their wide applications,
neither protocol is particularly effective towards multiply
substituted acene derivatives. An innovated synthetic strategy
that combines the merits of benzannulation and precursor
approaches could potentially open access to a wider scope of
Acene retrosynthesis based on
repetitive benzannulation strategy
Acene retrosynthesis based on
template directed strategy
Scheme 1: Two distinct acene synthesis strategies based on repetitive
benzannulation and template-directed construction of double strand polyene.
(*) Chih-Hsiu Lin,
Institute of Chemistry, Academia Sinica, Academia Road, Sec.
2, No. 128, Taipei, Taiwan, Republic of China
E-mail: chemopera@gate.sinica.edu.tw
The Supporting Information is available free of charge via the
Internet at SI file.
Results and Discussion
A model study was carried out with the simple 2,3-anthracene
diester 5 as the target. Due to their efficiency in olefin synthesis,
Wittig-Horner reaction and Knoevenagel condensation are
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CH3COOH/H2O/CF3COOH (2:2:1:1), r.t., 24 h; (g) CH2Cl2, M. S. (4 Å), DBU, r.t.,
chosen to construct the two strands of polyene in the two stages.
Since both reactions require aldehyde as their electrophilic
components, the aldehyde groups for constructing the second
strand must be protected until the first strand is accomplished.
The fulfillment of this template synthesis concept is depicted in
Scheme 2 in which the template strand (including its Wittig
synthons) are depicted in red, the second strand in green, and the
two sets of orthogonal protecting groups in blue. According to the
analysis, the starting compound should be a semi-protected
phthalaldehyde (as 1,3-dioxane acetal) as 1.10 The elongation
unit 2 carries an imine based Wittig-Horner reagent11 to construct
the first strand and a masked aldehyde (protected as diethyl
acetal) as the building block of the second strand. (This
compound was synthesized via formylation of diethyl (3,3-
diethoxypropyl)phosphonate followed by imine formation.)12
Wittig-Horner reaction between 2 and 1 furnished the imine
intermediate which was immediately hydrolyzed to aldehyde 3 in
oxalic acid, whose acidity is weak enough to preserve the acetal
groups intact. The unmasked aldehyde then underwent another
Wittig reaction (diethyl maleate/PEt3, according to the modification
reported by McCombie and Townsend)13 to give 4. The diene
moiety in 4 is the template strand. The acetal protecting groups
were hydrolyzed at this stage. The two liberated aldehyde groups
then underwent double Knoevenagel cyclization (DBU) to
accomplish the anthracene skeleton.
30 min., 83 % (over two steps).
After the template concept is verified, tetracene and pentacene
derivatives in principle can be synthesized with a few extra
elongation cycles. However, for this purpose, to carry out
bidirectional template-directed benzannulation is far more step-
economical. The realization of this proposal is demonstrated in
Scheme 3a. 2,5-Bis(1,3-dioxan-2-yl)terephthalaldehyde (8) is
employed as the starting compound in place of the monoaldehyde
5 in scheme 2. This compound was synthesized from known 2,5-
dibromoterephthalaldehyde14 in three steps. The two aldehyde
groups were first protected as 1,3-dioxne acetal. The bromides
were then converted to aldehydes with Heck coupling and
ozonolysis. The subsequent steps are identical with those in
Scheme 2. After a double Wittig-Horner olefination was carried
out with the elongation unit 2, the imine groups were hydrolyzed
to give dialdehyde 9. The second round of Wittig reaction (diethyl
maleate and dibutyl fumarate/PEt3) was then performed to
accomplish the template strand diene in 10a and 10b respectively.
The yield to construct the template strand over three steps (four
Wittig reactions overall) is 63 %. At this stage, all four acetal
protecting groups were hydrolyzed. The intermediate tetra-
aldehyde undergoes spontaneous cyclization to give anthracene
intermediates 11a and 11b. A final DBU catalyzed Knoevenagel
condensation effect the tetraethyl and tetrabutyl pentacene
2,3,9,10-tetracarboxylate 12a and 12b.
The extraordinary
When compared to other known benzannulation strategies, the
new template-directed synthesis is not the most step-economical.
However, this preliminary model study reveals several
advantages of the template strategy. First, several steps are
operationally simple protection-deprotection reactions that
proceed with nearly quantitative yields. Secondly, all aldehyde
groups for the annulation are introduced with the right redox state.
No cumbersome functional group transformation reactions are
required. The good yield of the last two steps (83 percent for two
deprotection and two cyclization steps) indicates this
methodology should be applicable to synthesize longer acene
derivatives.
efficiency of the template strategy is validated with the
respectable 60 % yield in the final four-fold deprotection and four-
fold cyclization steps.
With the template-directed synthesis of pentacene 2,3,9,10-
tetraester accomplished, pentacene with other electron-
withdrawing substitutions (nitrile, imide, sulfone) at these
positions can also be considered. According to our previous
experiences, N-alkyl pentacene 2,3,9,10-bisimide derivatives are
too insoluble for purification and full characterization.15 Yet, with
the greatly enhanced synthetic efficiency, more complex
solubilizing groups like 3,4,5-tris(dodecyloxyphenyl)16 can now be
incorporated to overcome the solubility problem. In Scheme 3b,
the pentacene diimide 15 was synthesized from 9 with N-3,4,5-
tris(dodecyloxyphenyl)maleimide replacing diethyl maleate in
Scheme 3a in the Wittig reaction. With both template diene
moieties in place, compound 13 underwent deprotection to
liberate four aldehyde groups to furnish the pentacene skeleton.
As in Scheme 3a, anthracene intermediate 14 was obtained right
after the hydrolysis due to spontaneous Knoevenagel
condensation. A second Knoevenagel condensation (catalyzed
by DBU) furnishes the pentacene diimide 15. The overall yield for
the template-directed quadruple cyclization is 53 %.
N
EtO
OEt
OEt
a), b)
OEt
OEt
2
EtO
OEt
EtO
OEt
CHO
COOEt
COOEt
CHO
O
O
O
c) d)
e)
O
O
O
1
3
4
CHO
COOEt
COOEt
COOEt
COOEt
f)
g)
CHO
4' (
not ioslated)
5
Wittig synthon to construct the template strand
Knoevenagel synthon to construct the second strand
Orthogonal protecting groups to differentiate the propagation of strands
Scheme 2: Synthesis of diethyl 2,3-anthracene diester via template strategy-
the model reaction. Reagents and conditions: (a) (i) LDA, THF, -78oC, 1 h; (ii)
ethyl formate, -78oC, 1 h, 76 % (over two steps); (b) MeOH, cyclohexylamine,
0oC to r.t., 3 h, 96 %; (c) (i) NaH, 2, THF, 0oC, 1 h, (ii) reflux, 2h; (d) CH2Cl2/THF
(4/1), oxalic acid, 0oC to r.t., 2 h, 74 % (over two steps); (e) (i) toluene, diethyl
maleate, PEt3, 0oC to r.t., 30 min., (ii) 3, reflux, 25 h, 80 %; (f) CH2Cl2/
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EtO
OEt
O
O
Br
O
OHC
Br
O
COOC4H9
O
O
O
O
Br
c)
Br
CHO a)
b)
COOC4H9
O
O
7
9
6
O
O
b), c)
a)
OEt
EtO
OEt
CHO
EtO
OEt
COOR
EtO
CHO
O
16'
CHO
O
O
O
OHC
OHC
COOC4H9
COOC4H9
O
COOR
NC
NC
COOC4H9
COOC4H9
O
OHC
O
d), e)
f)
O
O
O
OEt
8
OEt
d)
O
O
EtO
CHO
EtO
16
17
10a, 10b
9
ROOC
COOR
CHO
COOR
Scheme 4a: Synthesis of unsymmetrically substituted 2,3 diester-9,10 dinitrile
pentacene. Reagents and conditions: (a) (i) toluene, dibutyl fumarate (1.4 eq.),
PEt3, r.t., 1 h, (ii) 9, reflux, 36 h, 78 %; (b) CH2Cl2/CH3COOH/H2O/CF3COOH
(2:2:1:1), r.t., 24 h; (c) CH2Cl2, M. S. (4 Å), DBU, r.t., 30 min., 62 % (over two
steps); (d) CH2Cl2, PEt3, fumaronitrile, 0oC, 3 h, 31 %.
COOR
OHC
OHC
ROOC
ROOC
COOR
COOR
g)
CHO
CHO
OHC
11a, 11b
ROOC
COOR
not isolated
ROOC
ROOC
To further simplify the procedure and improve its efficiency, two
consecutive Wittig reaction can be carried out on 9 using different
electron deficient alkenes (dialkyl fumarate, fumaronitrile, and
tris(dodecyloxyphenyl) maleimide) to produce 18’ and 19’
(Scheme 4b). The subsequent template-directed cyclization were
conducted accordingly to give the pentacene imide diester 18,
and pentacene imide dinitrile 19 in practical yields (40-50%) and
scale (100-200 mg).
COOR
COOR
10a, 11a, 12a R = Et
10b, 11b, 12b R = n-Bu
h)
12a, 12b
Scheme 3a: Synthesis of 2,3,9,10-pentacene tetraester via bidirectional
template strategy. Reagents and conditions: (a) toluene, PTSA,
HOCH2CH2CH2OH, reflux, 5 h, 96 %; (b) DMF/Et3N (1/1) Pd(OAc)2, tri(o-
tolyl)phosphine, styrene, 90oC, 16 h, 72 %; (c) CH2Cl2/MeOH (5/1), pyridine,
O3/O2, -78oC to r.t,. 1 h, Me2S, r.t., 12 h, 85 %; (d) (i) NaH, 2, THF, 0oC, 1 h, (ii)
reflux, 2h; (e) CH2Cl2/THF (4/1), oxalic acid, 0oC to r.t., 2 h, 76 % (over two
steps); (f) (i) toluene, diethyl maleate or dibutyl fumarate, PEt3, 0oC to r.t., 1 h,
(ii) 9, reflux, 36-48 h, 84 % or 83 %; (g) CH2Cl2/CH3COOH/H2O/ CF3COOH
(2:2:1:1), r.t., 24 h; (h) CH2Cl2, M. S. (4 Å), DBU, r.t., 30 min., 58 % or 60 % (over
two steps).
EtO
OEt
O
O
N
O
O
R2
9
O
O
c)
a), b)
OEt
EtO
R1
18', 19'
R1
R1
OC12H25
OC12H25
OC12H25
R1
O
O
R1 OHC
R1
EtO
OEt
N
R2
N
R2
O
d)
CHO
O
N
O
O
O
18'', 19''
O
R
18, 19
9
O
O
b)
a)
OEt
18 R1 = COOC12H25
19 R1 = CN
R2
=
EtO
O
13
N
O
R
O
O
O
O
O
OHC
R
N
N R
R
N
N R
Scheme 4b: Simplified synthesis of unsymmetrically substituted pentacene.
c)
CHO
Reagents and conditions: (a) (i) toluene, N-3,4,5-tris(dodecyloxyphenyl)
maleimide, PEt3, 0oC, 1 h, (ii) 9, reflux, 36 h, 78 %; (b) (i) toluene, didodecyl
fumarate or fumaronitrile, PEt3, r.t. or 0oC, 1 h, (ii) 9’, reflux, 36-48 h, 64 % or
54 %; (c) CH2Cl2/CH3COOH/H2O/CF3COOH (2:2:1:1), r.t., 24 h; (d) CH2Cl2, M.
S. (4 Å), DBU, r.t., 30 min., 54 % or 42 % (over two steps).
O
O
O
14
15
OC12H25
OC12H25
OC12H25
R =
Scheme 3b: Synthesis of soluble pentacene 2,3,9,10 bisimide. Reagents and
conditions: (a) (i) toluene, N-3,4,5-tris(dodecyloxyphenyl) maleimide, PEt3, 0oC,
1 h, (ii) 9, reflux, 48 h, 76 %; (b) CH2Cl2/CH3COOH/H2O/CF3COOH (2:2:1:1), r.t.,
24 h; (c) CH2Cl2, M. S. (4 Å), DBU, r.t., 30 min., 53 % (over two steps).
Furthermore, we can take advantage of the C2h symmetry of 9 to
synthesize pentacene derivatives with the same symmetric
property. With dodecyl-3-cyano-2-(triphenylphosphoranylidene)
propanoate employed in the Wittig reaction (Scheme 4c), the
template strategy opened access to pentacene 2,9-diester 3,10-
dinitrile (20), a new class of pentacene derivatives with C2h
symmetry. The yield for the template directed four-fold cyclization
is near 50 %. It is difficult to envision a selective synthesis of
pentacene derivatives with this substitution pattern by any other
methodologies hitherto available.
Pentacene derivatives with unsymmetrical substituents at
terminal rings are quite rare in literature.17 The present synthetic
strategy can easily be adopted to make this class of elusive
compounds. In Scheme 4a, 9 was employed in Wittig reactions
with 1.3 equivalents of dibutyl fumarate and triethyl phosphine to
produce 16’. After deprotection of acetal groups and DBU
catalyzed
dicarboxylate 16 was produced. This compound then underwent
further elongation (fumaronitrile/PEt3/ DBU) to furnish
pentacene diester dinitirle 17 in 31 % yield.
cyclization,
dibutyl-8,9-diformyltetracene-2,3-
a
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EtO
OEt
EtO
OEt
COOC12H25
CN
R1
O
O
R1
O
O
21
O
O
O
O
a), b)or b), c), or a), c)
9
OEt
a)
R2
R2
EtO
24',25'.26'
20'
COOC12H25
d), e)
CN
24' R1 = CN, R2 = COOC12H25
25' R1 = COOC12H25
NC
NC
COOC12H25
COOC12H25
b), c)
O
N
OC12H25
NC
COOC12H25
CN
24
R2
=
O
OC12H25
OC12H25
C12H25
O
O
COOC12H25
COOC12H25
C12H25OOC
O
C12H25
O
N
O
R1 = CN,
26'
20
C12H25
O
N
OC12H25
25
R2
=
Scheme 4c: Synthesis of pentacene 2,9-diester 3,10-dinitrile with C2h symmetry.
Reagents and conditions: (a) toluene, dodecyl-3-cyano-2-(triphenylphospho-
ranylidene) propanoate,, reflux, 48 h, 73 %; (b) CH2Cl2/CH3COOH/H2O/
CF3COOH (2:2:1:1), r.t., 24 h; (c) CH2Cl2, M. S. (4 Å), DBU, r.t., 30 min., 46 %
(over two steps).
O
N
OC12H25
OC12H25
C12H25
O
O
CN
CN
O
C12H25
O
C12H25
O
26
Scheme 5b: Synthesis of unsymmetrically substituted tetracene. Reagents and
condition: (a) (i) toluene, fumaronitrile, PEt3, 0oC, 1 h, then reacts with the
appropriate aldehyde, reflux, 36 h,; (b) toluene, didodecyl fumarate, PEt3, 0oC,
1 h, then reacts with the appropriate aldehyde, reflux, 36 h (c) toluene,
maleimide, PEt3, 0oC, 1 h, then reacts with the appropriate aldehyde, reflux, 36
h, 64%, 69%, and 58% (over two steps) (d) CH2Cl2/CH3COOH/H2O/CF3COOH
(2:2:1:1), r.t., 24 h; (e) CH2Cl2, M. S. (4 Å), DBU, r.t., 30 min., 77 % , 62 % and
59 % (over two steps).
To expand the scope of this template-directed annulation,
tetracene derivatives are the most obvious targets.
The
bidirectional approach in Scheme 3a and 3b cannot be directly
adopted, because the tetracene does not possess a central ring
to launch the elongation. However, the desymmetrization of 8 can
be accomplished by using one equivalent of 2 as the limiting
reagent (Scheme 5a). The following steps (hydrolysis of imine,
Wittig olefination, hydrolysis of acetals, Knoevenagel cyclization)
are identical as pentacene synthesis in Scheme 3a and 4a.
Tetraester (86 %) and bisimide (61 %) tetracene derivatives (22
and 23) were produced accordingly in practical yields.
EtO
OEt
CN
O
COOC12H25
O
NC
COOC12H25
CN
O
EtO
OEt
21
EtO
OEt
CHO
a)
b), c)
O
C12H25OOC
O
R
C12H25OOC
CHO
O
O
O
27
O
R
27'
CN
O
OHC
O
O
O
O
O
b)
OHC
a)
Scheme 5c: Synthesis of tetracene 2,8-diester 3,9-dinitrile with C2h symmetry
Reagents and conditions: (a) toluene, dodecyl-3-cyano-2-(triphenylphospho-
ranylidene) propanoate,, reflux, 48 h, 75 %; (b) CH2Cl2/CH3COOH/H2O
/CF3COOH (2:2:1:1), r.t., 24 h; (c) CH2Cl2, M. S. (4 Å), DBU, r.t., 30 min., 51 %
(over two steps).
O
8
R
21
R
22',23'
c), d)
22 R = COOC12H25
OC12H25
OC12H25
OC12H25
O
R
R
R
23 R =
N
R
In addition to creating novel-systems through chemical
O
22, 23
synthesis, another important objective in the current exploration
of organic electronic materials is to assemble flat aromatic
systems into 3-D supramolecular architectures with well-defined
shapes and sizes. It is generally acknowledged that the
intermolecular packing orders of chromophores can have
profound influences on the charge transporting and energy
transfer properties of the materials, sometimes overturn the
structural effects of individual chromophores.18 Perylene diimide
based self-assembly systems are now commonly employed in
sensors and photovoltaic devices.19 Self-assembly studies of
pentacene scaffold mostly focus on crystal engineering of 6,13-
disubstituted derivatives.5f, 20 For terminal substituted acenes,
coil-rod anthracene and tetracene derivatives were reported to
form self-assembled organic gelators which display energy
transfer properties.21 Since the template strategy opened the
synthetic access to unsymmetrically substituted pentacene and
tetracene derivatives, we perceived the opportunity to construct
amphiphilic acene derivatives as the building blocks for new types
of acene based self-assembly structures. The target molecules
for such investigation are composed of rigid acene moieties
substituted with hydrophilic groups at one end of the structure and
hydrophobic groups on the other. To accommodate the template-
Scheme 5a: Tetracene synthesis via bidirectional template strategy. Reagents
and conditions: (a) (i) NaH, 2 (1 eq.), THF, 0oC, 1 h, (ii) reflux, 2h; (iii)
CH2Cl2/THF (4/1), oxalic acid, 0oC to r.t., 2 h, 71 %; (b) (i) toluene, didodecyl
fumarate or maleimide, PEt3, r.t. or 0oC, 1 h, (ii) 21, reflux, 36-48 h, 74 % or 71
%; (c) CH2Cl2/CH3COOH/H2O/CF3COOH (2:2:1:1), r.t., 24 h; (d) CH2Cl2, M. S.
(4 Å), DBU, r.t., 30 min., 86 % or 61 % (over two steps).
Tetracene diester dinitrile 24, diester imide 25, dinitirle imide 26,
and C2h symmetric 27 tetracene derivatives were also generate in
good yields (51-77% in the triple cyclization steps) with the
template-directed strategy (Scheme 5b and 5c).
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directed synthetic protocol, oligo ethylene glycol esters and N-
dodecyl imide are chosen as the hydrophilic and hydrophobic
moieties respectively. In an attempt to fine-tune the volume ratio
among aromatic rod, hydrophobic coil, and hydrophilic coil,
diethylene glycol, triethylene glycol, and tetraethylene glycol are
50 % THF/water solution (10-4~10-5 M). Slow evaporation of
solvents allows the molecules to undergo self-assembly under
mild condition. The molecular assemblies thus formed were then
visualized with TEM and AFM (Figure 1). In such investigation,
compound 29b, 29c, 30b, and 30c do not exhibit any visible
incorporated into these systems.
Three 2,3,8,9-tetracene
organized structure under the microscope.
However, two
tetrakis(oligo ethylene glycol)ester (28a, 28b, and 28c) were first
synthesized (Scheme 6a) to test whether the oligo ethylene glycol
chain is compatible with the multiple benzannulation reactions.
Satisfactory yields were recorded for all three model compounds
(80-83%).
compounds with diethylene glycol chains (29a and 30a) formed
notable features under the same condition. As shown the Figure
1, the amphiphilic tetracene 29a forms network structures
composed of fibrous assembly of various diameters ranging from
30-60 nm. The result from amphiphilic pentacene derivative 30a
is more fascinating. The compound assembles into wire like
nanostructures with more uniform diameter near 20 nm. From the
TEM image, these nanowire assemblies are clearly helical.
Furthermore, these wires bundled up to form aggregates under
the assembly condition. Since the dimension of compound 30a is
about 4 nm in its extended conformation, it is quite probable that
the observed wire and fiber structures are composed of folded
EtO
OEt
O
OR
COOR
COOR
O
O
OR
O
O
RO
O
O
21
O
b), c)
OR
a)
molecular
bilayer
stacks as in Aida’s amphiphilic
COOR
COOR
28a' R= (CH2CH2O)2CH3
28a R= (CH2CH2O)2CH3
28b
hexabenzocoronene assembly.22 However, since it is impossible
to preclude pentacene derivatives undergo decomposition during
the assembly process, it must be cautioned that such molecular
assembly might contain some decomposition products of 30a.
R= (CH2CH2O)3CH3
28c R= (CH2CH2O)4CH3
28b'
R= (CH2CH2O)3CH3
R= (CH2CH2O)4CH3
28c'
Scheme 6a: Synthesis of water soluble tetracene derivatives. Reagents and
conditions: (a) (i) toluene, bis(di, tri, or tetra-ethylene glycol methyl ether)
fumarate, PEt3, r.t., 1 h (ii) 21, reflux, 48 h, 95 %, 89 %, and 86 %; (b) CH2Cl2/
CH3COOH/H2O/CF3COOH (2:2:1:1), r.t., 24 h, 84 %, 82 %, and 80 %; (c) CH2Cl2,
M. S. (4 Å), DBU, r.t., 30 min., 83 % , 80 %, and 81 %.
Next, amphiphilic tetracene (29a, 29b, and 29c) and pentacene
(30a, 30b, and 30c) derivatives were synthesized according to the
procedures in Scheme 6b and 6c. The yields for the template-
directed multiple cyclization steps are comparable with those
observed previously (Scheme 4b, 5b).
O
O
O
O
O
O
O
n-C12H25
N
EtO
OEt
O
O
O
COOR
COOR
O
O
O
O
OR
O
n-C12H25
N
O
O
21
O
c), d)
a), b)
OR
O
29a R= (CH2CH2O)2CH3
N
29b
R= (CH2CH2O)3CH3
R= (CH2CH2O)4CH3
O
nC12H25
29c
29a' R= (CH2CH2O)2CH3
29b'
29c'
R= (CH2CH2O)3CH3
R= (CH2CH2O)4CH3
Scheme 6b: Synthesis of amphiphilic tetracene derivatives via bidirectional
template strategy. Reagents and conditions: (a) (i) toluene, bis(di, tri, or tetra-
ethylene glycol methyl ether) fumarate, PEt3, r.t., 1 h (ii) 21, reflux, 48 h; (b) (i)
toluene, N-dodecyl maleimide, PEt3, 0oC, 1 h; (ii) 21’, reflux, 48 h, 65 %, 63 %,
and 64 %; (b) CH2Cl2/CH3COOH /H2O/CF3COOH (2:2:1:1), r.t., 24 h, 87 %, 85
%, and 84 %; (c) CH2Cl2, M. S. (4 Å), DBU, r.t., 30 min., 85 % , 83 %, and 84 %.
EtO
OEt
O
O
O
O
O
O
O
N
O
O
OR
O
O
nC12H25
O
C12H5n
N
n-C12H25
N
O
O
9
O
OEt
O
O
a), b)
O
O
c), d)
OR
EtO
O
30a
R= (CH2CH2O)2CH3
30b R= (CH2CH2O)3CH3
30c R= (CH2CH2O)4CH3
COOR
COOR
30a' R= (CH2CH2O)2CH3
30b' R= (CH2CH2O)3CH3
Figure 1: TEM image of amphiphilic tetracene and pentacene self-assemble
(helical assemblies of 30a are highlighted in yellow boxes)
30c'
R= (CH2CH2O)4CH3
Scheme 6c: Synthesis of amphiphilic pentacene derivatives via bidirectional
template strategy. Reagents and conditions: (a) (i) toluene, bis(di, tri, or tetra-
ethylene glycol methyl ether) fumarate, PEt3, r.t., 1 h (ii) 9, reflux, 48 h; (b) (i)
toluene, N-dodecyl maleimide, PEt3, 0oC, 1 h; (ii) 9’, reflux, 48 h, 59 %, 55 %,
and 53 %; (c) CH2Cl2/CH3COOH /H2O/CF3COOH (2:2:1:1), r.t., 24 h, 87 %, 86
%, and 82 %; (d) CH2Cl2, M. S. (4 Å), DBU, r.t., 30 min., 64 % , 62 %, and 61 %.
Conclusions
In conclusion, we have developed a novel synthetic scheme to
construct the acene skeletons. The new strategy adopts a distinct
retrosynthetic approach by treating the acene structure as a
double strand polyene instead of a linearly fused array of benzene
Preliminary self-assembly studies were carried out with these
amphiphilic compounds. The compounds were dissolved in 45-
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10.1002/chem.201703084
Chemistry - A European Journal
FULL PAPER
rings. The key conceptual renovation is to pre-form the first
strand of diene and use it as the template to direct the synthesis
of the second diene strand. In practice, the first strand of diene
was constructed with Wittig-Horner reaction while the second
strand is accomplished with Knoevenagel condensation.
Combining the template approach with the bidirectional growth
strategy, we successfully conducted three-fold and four-fold
cyclization reactions to furnish tetracene and pentacene
derivatives in moderate to good yields (20-50 % over 7 steps from
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readily available compounds).
Unsymmetrically substituted
pentacene and tetracene derivatives with ester, nitrile, and imide
substituents are also accessible through this strategy. Moreover,
the capacity of the template strategy was showcased in the
synthesis of amphiphilic acene derivative with oligo ethylene
glycol ester and alkyl imide substituents at both ends of the
molecules. One of these amphiphilic pentacene derivatives 30a
forms helical wire assemblies in aqueous/THF medium. This
strategy should prove particularly handy when screening the
physical properties of a series of compounds. It should also be
applicable to the synthesis of higher acene and related polycyclic
aromatic systems.
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Acknowledgements
This work was supported by Academia Sinica and National Science
Council of Taiwan.
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Keywords: Pentacene. Tetracene. Template Synthesis.
Annulenes. Self-assembly
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