A tetraalkylated pyrene building block for the synthesis of pyrene-fused azaacenes with enhanced solubility

Article abstract of DOI:10.1039/c0cc02249g

The synthesis and characterisation of a soluble pyrene-fused tetraazaoctacene derivative has been achieved by developing a key pyrene-based building block with four solubilising groups.

Full text of DOI:10.1039/c0cc02249g

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A tetraalkylated pyrene building block for the synthesis of pyrene-fused
azaacenes with enhanced solubilitywz
Niksa Kulisic, Sandeep More and Aurelio Mateo-Alonso*
ab
ab
ab
Received 30th June 2010, Accepted 12th October 2010
DOI: 10.1039/c0cc02249g
2
1
The synthesis and characterisation of a soluble pyrene-fused
tetraazaoctacene derivative has been achieved by developing a
key pyrene-based building block with four solubilising groups.
insoluble, which hampers their deposition through solution-
based techniques. Even if the introduction of alkyl groups on
24
pyrene has been reported, the current solubilisation strategies
are based either on the use of soluble terminal building blocks
or on the postfunctionalisation of the azaacene core with
Semiconducting organic molecules and polymers are expected
to open up possibilities for new electronic products that will
not compete with the existing technology, but instead
will create a new market that could reach the size of silicon.
n-Acenes are polyaromatic hydrocarbons composed of linearly
fused benzene rings (where n refers to the number fused rings),
which are among the most promising organic semiconductors.
Although higher acenes (n > 5) are known to be unstable, a
lot of effort has been dedicated to study their properties.
Several stabilisation strategies have allowed the preparation
1
1,13,14,16,22,23
solubilising groups.
Ideally, for facilitating the
preparation and characterisation of higher pyrene-fused
azaacenes, it would be desirable to use pyrene building blocks
displaying suitable solubilising groups that yield directly
soluble azaacenes during the cyclocondensation step, without
the need of functionalisation at a later stage.
With all this in mind, we have developed a convenient
synthetic route (Scheme 1) for preparing 1,3,6,8-tetraoctyl-
4,5,9,10-tetraketopyrene (9). By using this building block, we
have conveniently synthesised a tetraazaoctacene derivative
(10) with four n-octyl solubilising chains in the pyrene core
that displays enhanced solubility in neutral organic solvents.
In a first attempt to synthesise soluble azaacenes, we focused
on the synthesis of tetraazaoctacene derivatives with two
solubilizing groups on the pyrene core (Scheme 1). tert-Butyl
groups can be introduced in azaacenes by using 2,7-di-tert-
butyl-4,5,9,10-tetraketopyrene (2), which is currently the most
commonly used building block since it can be synthesised in
1
–5
of higher acenes (5 r n r 9), which in some cases are stable
for several days. The optoelectronic properties observed on
the larger acenes have motivated scientists to explore other
members of the acene family.
Azaacenes have re-emerged as an alternative platform to
acenes since in some cases they can be more stable and easier
to synthesise. Azaacenes show the same degree of aromaticity
than acenes but structurally present N instead of C atoms in
6
some positions of the aromatic framework. By expanding
2
4
laterally the conjugation of the linear aromatic backbone with
additional aromatic rings, sextets of p-electrons are localised
on such external rings, which results in the enhancement of the
stability. This stabilisation strategy has allowed scientists
only two steps. Accordingly, pyrene was functionalised with
two tert-butyl groups through Friedel–Crafts alkylation to
give 2,7-di-tert-butylpyrene (1). Subsequently, 1 was oxidised
with NaIO in the presence of RuCl catalyst to tetraketone 2.
4 3
7
,8,10–23
to prepare not only higher pyrene-fused azaacenes
7
The reaction of 2 with 2,3-diaminonaphthalene (3) in a
degassed pyridine solution yielded the desired pyrene-fused
tetraazaoctacene 4 that was collected in pure form by filtration.
Azaacene 4 is sparingly soluble in neutral organic solvents. The
best solvents for solubilising 4 are strong acids, among which,
sulfuric and trifluoroacetic acid (TFA) are the most accessible.
The presence of the tert-butyl groups enhanced the solubility in
TFA and no dilution was necessary to obtain a resolved NMR
spectrum, in contrast to previous observations on an analogous
–9
(
6 r n r 16) but also polyazaacenes
to study their
length/property characteristics. One of the common features
of pyrene-fused azaacenes relies on their synthesis, which
proceeds through the cyclocondensation of 1,2-diamines and
diketones to fuse different polyaromatic residues through
pyrazine rings. A key building block in the preparation of
2
4
such oligoazaacenes is 4,5,9,10-tetraketopyrene (the substitution
numbering of pyrene is shown on Scheme 1). However the
azaacenes that evolve from 4,5,9,10-tetraketopyrene are highly
2
1
pyrene-fused tetraazaoctacene with no solubilizing groups.
The structure of 4 was indeed confirmed by NMR (TFA-d1,
see ESIz) and MS.
a
Freiburg Institute for Advanced Studies (FRIAS), School of Soft
To overcome the lack of solubility of tert-butyl functionalised
azaacenes in neutral solvents, more efficient solubilising
groups must be incorporated in the pyrene core. A route to
introduce n-alkyl groups on positions 2,7 of 4,5,9,10-tetra-
Matter Research, Albert-Ludwigs-Universita¨t Freiburg,
Albertstrasse 19, D-79104 Freiburg i. Brsg., Germany.
E-mail: amateo@frias.uni-freiburg.de
Institut fu¨r Organische Chemie und Biochemie (IOCB),
Albert-Ludwigs-Universita¨t Freiburg, Albertstrasse 21,
D-79104 Freiburg i. Brsg., Germany
b
24
ketopyrene derivatives has been reported. Nevertheless, this
route is not straightforward as it proceeds through several
reduction and oxidation steps that we found difficult to
reproduce. Our alternative approach is conveniently based
w This article is part of the ‘Emerging Investigators’ themed issue for
ChemComm.
z Electronic supplementary information (ESI) available: Full experi-
mental procedures and characterisation of compounds 4, 7, 8, 9, and
2
on the reaction of pyrene with Br to give 1,3,6,8-tetrabromo-
1
0. Structure of 7 in CIF format. CCDC 783204. For ESI and
crystallographic data in CIF or other electronic format see DOI:
0.1039/c0cc02249g
2
pyrene 5. This reaction is straightforward and 5 can be
5
1
obtained in a multigram scale without the need of other
5
14 Chem. Commun., 2011, 47, 514–516
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Scheme 1 Synthetic route for the preparation of tetraazaoctacenes 4 and 10. (a) tBuCl, AlCl
5 1C, 24 h, 36%; (c) pyridine, reflux, 3 days, 32%; (d) Br , PhNO , reflux, 4 h; (e) CuI, Pd(PPh
f) H , Pd/C (10%), 1 atm, rt, overnight, 89%; (g) NaIO , RuCl , CH Cl , CH CN, water, 45 1C, 3 days, 20%; (h) AcOH, rt overnight, then reflux
days, 17%.
3
, rt, 3 h; (b) NaIO
4
, RuCl
3 2 2 3
, CH Cl , CH CN, water,
i
3
2
2
3 2 2 2
) Cl , Pr NH, THF, 80 1C, overnight, 71%;
(
2
4
3
2
2
3
3
purification than filtration, as 5 is insoluble in organic solvents.
Tetrabromopyrene 5 was functionalised with 1-octyne (6)
under Sonogashira conditions. Eventhough a wide variety of
alkynes functionalised with alkyl chains are commercially
available, 6 was chosen because of the high solubilising power
of octyl chains and also because of its reasonable price. The
Sonogashira C–C coupling proceeded smoothly by using CuI
and evaporating the solvent. However it should be noted that
when the reaction times were exceeded 4,5-dihydropyrene
derivatives were detected by NMR.
The resulting tetraoctylpyrene 8 was oxidised to 1,3,6,8-
tetraoctyl-4,5,9,10-tetraketopyrene (9) by oxidation with NaIO4
catalysed by RuCl . Tetraketone 9 was obtained in pure form
3
as an orange solid after chromatography. Reaction of 2,3-
diaminonaphthalene (3) with tetraketone 9 in a degassed and
refluxing solution of AcOH yielded, after chromatography, the
desired pyrene-fused tetraazaoctacene 10 displaying four n-octyl
solubilising groups. Compound 10 presents enhanced solubility
3 2 2
and Pd(PPh ) Cl at 80 1C. Filtration and chromatography
yielded compound 7 as a pure material, which was characterised
by NMR and MS. Crystals suitable for X-ray diffraction
analysis were obtained by slow evaporation of a solution of
petrol ether (40–60 1C), which confirmed the structure (Fig. 1).
Pyrene 7 presents already a very high solubility in the most
common organic solvents. However the acetylene groups may
interfere in the oxidation of pyrene to tetraketopyrene.
For this reason, the acetylene residues of 7 were reduced
to methylenes by catalytic hydrogenation to yield 1,3,6,8-
tetraoctylpyrene (8). The reaction also proceeded smoothly
and 8 was obtained in pure form by filtrating off the catalyst
Fig. 2 Top left: absorption (thick line) and emission (thin line)
spectra of 10. Bottom left: cyclic voltammogram of 10 in 0.1 M
TBAP/ODCB. Right: experimental HOMO–LUMO gaps and LUMO
level. Calculated shapes of HOMO and LUMO levels.
Fig. 1 X-Ray structure of compound 7.
This journal is c The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 514–516 515

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Table 1 Selected photophysical and electrochemical data of 10
a
a
em
b
c
d
1/2
d
e
c
l
onset
l
E
gap(opt)
E
gap(calcd)
E
E
onset
E
LUMO(CV)
E
LUMO(calcd)
4
88 nm
478 nm
2.5 eV
2.5 eV
ꢀ1.37 V
Estimated from absorption onset. Calculated B3LYP/6-31G* using Spartan 08. Measured from 0.1 M ODCB/TBAP
ꢀ1.13 V
ꢀ3.2 eV
ꢀ2.7 eV
a
b
c
d
Measured in CHCl
3
.
e
vs. Ag wire, potential window (+1.1 to ꢀ2.0 V). Estimated from EONSET according to ELUMO = ꢀ4.8 ꢀ e (EONSET ꢀ E1/2 Fc) eV where E1/2 Fc
.48 V.
=
0
ꢀ
1
in organic solvents (c20 mg mL in CHCl ), which facilitated
the Verband der Chemischen Industrie (SK 185/13). We thank Dr
3
the confirmation of its structure by NMR (see ESIz).
¨
M. Keller, Dr J. Worth and Dr J. Geier (U. Freiburg) for
assistance on NMR, MS and X-ray characterisation, respectively.
Taking advantage of the enhanced solubility of 10,
its electronic properties were investigated by steady-state
UV-Vis and fluorescence spectroscopy (Fig. 2). In non-polar
Notes and references
3
solvents such as toluene and CHCl , well-resolved absorption
1
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features of 10 along the UV and the visible were observed,
with no sign of aggregation upon dilution. The optical
HOMO–LUMO gap of 10 (2.5 eV) was calculated from the
2
2
8
absorption spectrum onset of a CHCl
3
solution (Table 1). The
3 I. Kaur, N. N. Stein, R. P. Kopreski and G. P. Miller, J. Am.
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emission spectra of 10 were obtained by excitation with UV or
visible light. The spectra showed a peak centered at 478 nm
with a shoulder at 454 nm. Excitation spectra mirrored the
features observed on emission and Stokes shifts of 93 nm were
observed (see ESIz).
4
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voltammetry in a degassed solution of 1,2-dichlorobenzene
and tetrabutyl ammonium perchlorate (TBAP/ODCB)
6
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vacuo (2.5 eV, B3LYP/6-31G*) is in good agreement with the
measured optical bandgaps (Table 1). The overestimation by
1
1
1
1
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3
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1
,3,6,8-tetraoctyl-4,5,9,10-tetraketopyrene, a new and key
1
1
2
2
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building block for the synthesis of soluble pyrene-fused
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ꢀ
1
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the door for preparing pyrene fused azaacenes with enhanced
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This work was carried out with support of the Freiburg Institute
for Advanced Studies (FRIAS Junior Research Fellowship) and
2
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5
16 Chem. Commun., 2011, 47, 514–516
This journal is c The Royal Society of Chemistry 2011