Cascade cyclization to produce a series of fused, aromatic molecules

Article abstract of DOI:10.1021/ol100656d

Figure presented An efficient synthesis of fully aromatic, fused-ring, monoaza-acenes through a nucleophile initiated cascading cyclization is illustrated. Photophysical properties of the resulting molecules are reported.

Full text of DOI:10.1021/ol100656d

ORGANIC
LETTERS
2010
Vol. 12, No. 9
2146-2148
Cascade Cyclization To Produce a
Series of Fused, Aromatic Molecules
William J. Behof, Dongchuan Wang, Weijun Niu, and Christopher B. Gorman*
Department of Chemistry, North Carolina State UniVersity, Box 8204,
Raleigh, North Carolina 27695-8204
cbgorman@ncsu.edu
Received March 19, 2010
ABSTRACT
An efficient synthesis of fully aromatic, fused-ring, monoaza-acenes through a nucleophile initiated cascading cyclization is illustrated.
Photophysical properties of the resulting molecules are reported.
Fused, aromatic molecules are among the most attractive
candidates for materials in organic thin layer devices. They
typically form close contacts in the solid state as a result of
their generally planar geometries. This behavior maximizes
the electronic coupling between adjacent units. There are,
however, few methods for the synthesis of fused, aromatic
molecules that can be generalized to prepare molecules of
various lengths. Since an increase in length generally makes
these molecules easier to both oxidize and reduce, new
methods for the synthesis of long fused aromatic molecules
are desirable.
Here, we illustrate an efficient synthesis of fused-ring,
monoaza-acenes through a nucleophile initiated cascading
cyclization of nitrile groups followed by spontaneous CH to
NH tautomerization to give fully aromatic, fused hetero-
cycles. The first step of this process is based on nucleophilic
attack on the electrophilic cyano carbon. This is the basis
for many approaches in heterocycle synthesis,4 and is
reminiscent of the first step in the pyrolysis of poly(acry-
lonitrile) to form carbon fibers.5
Monoaza-acene oligomers containing 2-5 fused rings
were synthesized by adapting a previously reported cycliza-
tion to produce amino pyridines.1h,6 The precursor, cyano-
Nitrogen-containing oligomers have several potential ap-
plications. Unfused oligo-pyridines are widely useful as
building blocks for biological applications, chelators, and
metallo-organic conducting materials.1 Molecules containing
fused diaza-acene units have been prepared and extensively
studied as candidates in thin film organic devices.2 Fused
monoaza-acenes are multipoint, hydrogen bond acceptors.3
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10.1021/ol100656d  2010 American Chemical Society
Published on Web 04/08/2010

containing oligomers (1a-c) were synthesized (see the
Supporting Information) through both nucleophilic aromatic
substitution and palladium mediated pathways. The reaction
of 1a-c with n-BuLi (Scheme 1) afforded cyclized deriva-
tives 2a-c in acceptable to excellent yields. In each case
the benzylic hydrogens tautomerized to nitrogen to form a
completely aromatic heterocycle. To our knowledge, this is
the first example of a cascading cyclization reaction to form
fused, completely aromatic molecules. Note that only the
terminal cyano group is attacked by the nucleophile. Products
of the attack at internal cyano groups were not observed.
Furthermore, only one regioisomer was observed in each case
where the starting oligomer was not symmetric.
Figure 1. Absorbance and emission spectra of 2a (black), 2b (red),
and 2c (blue) taken in CH3CN solvent.
Scheme 1
Note that the pendant phenyl group has a large effect on the
quantum yield of these molecules. This behavior is observed
when the quantum yields of anthracene and phenyl an-
thracene are compared.12 In that case, the pendant phenyl
group affects the relative energies of the first excited singlet
and triplet states, reducing the rate of intersystem crossing
substantially and thus increasing the quantum yield for
fluorescence. A similar mechanism may be in operation here.
Packing, intermolecular distance, and self-assembly are
extremely important properties in organic conducting materi-
als and can be determined through the crystal structures.
Analysis of the crystal structures for molecules 2b (Figure
2) and 2c (Figure 3) indicates two structural features in these
molecules that influence organization. First, all form mul-
tipoint hydrogen bonding interactions via the intermolecular
interaction of the nitrogen as the hydrogen bond acceptor
and N-H as the hydrogen bond donor. This type of
organizational motif has been illustrated previously.3b,13
Molecule 2b has two nonbonded N-N distances of 3.05 Å
and molecule 2c has an average N-N distance of 3.04 Å.
These values are comparable to those found for N-N
distances in nucleic acids.14
The synthesis of oligonitriles (e.g., molecules with a
-CRdN- repeating unit) has been accomplished by using
several approaches including stepwise construction7 and ring-
opening methods.8 Open-chained examples assume a helical
conformation.9 Saturated heterocycles have been prepared
via nucleophile-initiated attack of one cyano group on
another.10 This latter approach is reminiscent of the first step
shown here.
Photophysical behaviors of cyclized oligomers 2a, 2b, and
2c are shown in Figure 1. Molecules 2a and 2b have similar
absorption spectra indicating that a fused and pendant phenyl
group have a similar electronic effect here. This trend has
been observed in other types of fused and conjugated
systems.11 As expected molecule 2c shows a characteristic
red-shift due to extended conjugation as compared to 2a and
2b. All molecules fluoresce with quantum yields (φF) of 0.63,
0.16, and 0.58 for molecules 2a, 2b, and 2c respectively.
Second, molecule 2b forms tubular 2D close-packed layers
uncharacteristic of most tetracene derivatives.15 Here, hy-
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intuitive18 that longer, fused, aromatic molecules should face-
to-face stack more readily. This nicely illustrates the
minimum size at which this packing motif is found in this
series of molecules.
Figure 2. Hydrogen bonding interactions (top) and packing (bottom)
of molecule 2b.
drogen bonding appears to dominate over the face-to-face
packing found in hydrocarbons. Thus, at most two molecules
stack face-to-face with an average 4.0 ( 0.1 Å distance
between them. The longer molecule 2c forms relatively flat,
although slipped, 2D close-stacked layers with an average
3.8 ( 0.2 Å distance between molecules in the stacking
direction. The slipped, 2D packing motif is typical of
substituted pentacenes,15d,16 and is the packing motif that
offers the highest performance in thin film devices.17 It is
Figure 3. Hydrogen bonding interactions (top) and packing (bottom)
of molecule 2c.
Acknowledgment. This work was supported by the
Department of Energy (Grants DE-FG02-05ER46238 and
DOE No. 08NT0001925). Mass spectra were obtained at the
NCSU Department of Chemistry Mass Spectrometry Facility.
Funding was obtained from the North Carolina Biotechnol-
ogy Center, and the NCSU Department of Chemistry.
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Supporting Information Available: Experimental pro-
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material is available free of charge via the Internet at
http://pubs.acs.org.
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