Tetra-Aza-Pentacenes by means of a One-Pot Friedl?nder Synthesis

Article abstract of DOI:10.1002/chem.201805655

Tetra-aza-pentacenes are attractive n-type small molecules for optoelectronic device applications, yet their syntheses are often laborious. Disclosed here is a one-pot Friedl?nder synthesis of 1,7,8,14-tetraazapentacece (tAP) derivatives (linear and/or be

Full text of DOI:10.1002/chem.201805655

A Journal of
Accepted Article
Title: Tetra-Aza-Pentacenes via a One-Pot Friedländer Synthesis
Authors: Narcisse Ukwitegetse, Jonathan R. Sommer, Patrick J. G.
Saris, Ralf Haiges, Peter I. Djurovich, and Mark E. Thompson
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To be cited as: Chem. Eur. J. 10.1002/chem.201805655
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Tetra-Aza-Pentacenes via a One-Pot Friedländer Synthesis
Narcisse Ukwitegetse, Jonathan R. Sommer, Patrick J. G. Saris, Ralf Haiges, Peter I. Djurovich, and
Mark E. Thompson*
Abstract: Tetra-aza-pentacenes are attractive n-type small
molecules for optoelectronic device applications, yet their syntheses
are often laborious. Disclosed here is a one-pot Friedländer synthesis
of 1,7,8,14-tetraazapentacece (tAP) derivatives (linear and/or bent),
fully aromatized in-situ despite the absence of an exogenous oxidant.
The photophysics of linear tAPs resembles that of regular pentacene
while their crystal structures differ. A LUMO energy of -3.71 eV for
di-tert-butylanisole-substituted linear tAP is similar to that of the well-
known acceptor, C60.
energy (3.37 eV), dipole moment (μ = 0.00 D), and electron
reorganization energy (λ = 0.15 eV), that closely resemble values
found in C60. Our retrosynthetic analysis (Figure 1, A) of the target
tAP involves the construction of two naphthyridines by ring fusion
at
2,5-positions
of
cyclohexadione
with
substituted
amino-pyridines. Similar acid-catalyzed Friedländer syntheses of
di-aza-pentacenes starting from 1,4-cyclohexanedione and
2-aminobenzaldehyde derivatives have been reported to yield
linear dihydro-di-aza-pentacene adducts, which were then
oxidized to fully aromatized products.[8] Several reports claim that
o-amino-aryl-aldehydes and 1,4-cyclohexanedione exclusively
give products formed by fusion at the 2,3-positions of the dione
under basic conditions, Figure 1 (B).[9-12] In our hands, we observe
adducts resulting from the cyclohexanedione-ring fusion at the
2,5- and 2,3-positions (Scheme 1), which will be hereafter
referred to as “linear” (l) and “bent” (b) products, respectively.
Pentacene derivatives are promising candidates for the
development of high performance organic optoelectronic devices,
such as photovoltaics (OPVs) and organic field effect-transistors,
due to their strong optical absorptivities and high charge carrier
mobilities.[1, 2] In most cases, pentacene and its derivatives have
been used as p-type semiconductors in these devices; however,
n-type pentacene derivatives have been formed through
cyclopentannulation strategies or by the introduction of peripheral
electron withdrawing groups such as chlorine, fluorine,
trifluoromethyl, and nitrile substituents.[2, 3] Peripheral electron
withdrawing groups are effective in lowering HOMO and LUMO
energy levels, but increase the molecular volume, perturbing the
favorable bulk morphology of pentacene. Aza-substitution is an
alternative method of lowering the frontier orbital energy levels
that does not impact the molecular volume of pentacene.
Houk and Winkler used DFT calculations to identify a series
of aza-pentacenes comprised of pyridine moieties as potentially
promising n-type materials.[4] We have also carried out a
theoretical
study
on
a
combinatorial
library
of
tetra-aza-pentacenes.[5]
In our study, we determined that
Figure 1. (A) Retrosynthesis of tetra-aza-pentacenes comprised of pyridine
residues. (B) A reported example of a not-fully aromatized Friedländer product
from base-catalyzed conditions with ring fusion at the 2,3-positions of the
dione.[12]
incorporating four nitrogens into the pentacene framework lowers
the energy level of the LUMO to approximately that of C60, making
these aza-acenes attractive candidates as OPV acceptor
materials. Of the 135 possible substitution patterns for
tetra-aza-pentacene, only 3 have been reported, all containing
pyrazine
moieties
derived
from
condensations
of
7]
orthodiaminoaryls.[6,
Moreover, synthetic routes to prepare
aza-acenes with pyridine rather than pyrazine moieties remain
scarce. Herein we disclose a base-catalyzed Friedländer
synthesis that provides access to these promising materials,
previously thought to be inaccessible.[7]
This report focuses on 1,7,8,14-tetraazapentacene (tAP),
which is predicted to have several properties, i.e. LUMO
N. Ukwitegetse, P. J. G. Saris, Dr. J. R. Sommer, Dr. R. M. Haiges,
Dr. P. I. Djurovich, Prof. M. E. Thompson
Department of Chemistry
Scheme 1. One-pot Friedländer synthesis of tetra-aza-pentacenes. i) EtOH, aq.
University of Southern California
Los Angeles, CA 90089
2M NaOH (0.5 eq), N2 atm, 12-hour reflux.
[*]
Corresponding author’s e-mail: met@usc.edu
Supporting information for this article is given via a link at the end of
the document.
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The reaction between 1,4-cyclohexanedione and
2-amino-nicotinaldehyde (1a) in refluxing ethanol, promoted by
2M aqueous NaOH, yielded a dark purple suspension containing
6,13-dibenzylated tAP (5l) was isolated. The presence of two
atropisomers (syn and anti) of 5l was indicated by NMR
spectroscopy. A NOESY H NMR spectrum confirmed that the
1
both the linear (2l) and bent (2b) Friedländer products (Scheme 1). singlets of the benzylic methylene ( = 4.87 and 5.10 ppm) are not
Compound 2b is air-stable and relatively soluble in
geminal, and hence must belong to two atropisomers (See SI,
dichloromethane, which facilitates purification and
spectrum S11).
characterization. However, the air-sensitivity of 2l – like regular
pentacene[13] – made purification challenging, and no acceptable
NMR data could be recorded due to its poor solubility.
Nevertheless, we were able to obtain crystals of 2l suitable for
X-ray crystallography in low yields (<1%) by sublimation at 290 oC
and 10-6 torr.
The formation of linear Friedländer aza-pentacenes from
the same starting materials (1,4-cyclohexanedione and 1a) was
hinted at by Majewicz and Caluwe. They reported the formation
of unidentified “intractable and multi-colored products” when
[10]
Figure 2. Absorption (solid lines) and emission (dotted lines) spectra of linear
tAPs 2l, 3l, and 5l (left), and bent tAPs 2b and 3b (right) in DMSO.
KOH was used as a catalyst.
When piperidine was used in
similar syntheses, bent Friedländer products were exclusively
isolated, illustrating the importance of the catalyst choice.[11, 12,
14]The presumptive intermediate after the first Friedländer event is
The UV-visible absorption and emission spectra of the three
linear tAPs (2l, 3l, and 5l) in DMSO are shown in Figure 2 (left).
The compounds exhibit well-defined vibronic progressions
observed in other known tetra-aza-pentacenes[17] and acene
molecules in general.[18] The unsubstituted tAP (2l) and the
dibenzylated tAP (5l) show identical absorption and emission
maxima, (abs = 567 nm and em = 580 nm), whereas spectra for
3l are red-shifted (abs = 577 nm and em = 588 nm). The
difference indicates the absence of conjugation between the
benzyl groups and the aza-acene core as opposed to weak
conjugation between the DTBA groups and the parent tAP core.
Compared to the TIPS-bearing tetra-aza-pentacenes (pyrazine-
based) reported by Bunz, et al.[17] the absorption spectra of our
pyridine-based linear tAPs are hypsochromically shifted, implying
a wider band-gap in the present tAPs. Thus, changing the
substitution patterns of four nitrogens in the pentacene core can
significantly affect the absorption/emission properties of the
compounds. On the other hand, absorption and emission spectra
of the bent derivatives, 2b and 3b, are considerably blue-shifted
relative to the linear counterparts (Figure 2, right). According to
Clar’s rule,[19] the bent isomers have more aromatic π-sextets (i.e.
two versus one) which results in a wider HOMO – LUMO gap,
hence the observed hypsochromic shift. A greater red-shifting
observed from 2b to 3b is indicative of more pronounced
conjugation of the 3b tAP core with DTBA groups.
Cyclic voltammetry measurements of soluble tAPs (3b and
3l) were carried out under anaerobic conditions (see SI). The
linear isomer, 3l, exhibits an irreversible reduction wave with a
reduction peak at -0.95 V versus Fc/Fc+. This value is similar to
the first reduction potential for fullerene C60 (Ered = -0.97 V vs.
Fc/Fc+).[20] The 6,13-ethynylated tetraazapentacene reported by
Miao et. al., showed the first reduction at a half-wave potential of
-0.79 V vs. Fc/Fc+.[21] Using the conversion equation by
Sworakowski and co-workers,[22] the reduction potential of 3l
corresponds to ELUMO of -3.71 eV. Hence, from purely
hydrocarbon pentacene (Ered = -1.76 V, i.e. LUMO = -2.75 eV)[23]
to 3l, the LUMO was lowered by ca. 1 eV, emphasizing the effect
of incorporating inductively electron-withdrawing atoms in the
framework. The cyclic voltammogram of bent DTBA-tAP (3b)
exhibits two reversible reduction peaks at -1.65 V and -1.99 V
a
naphthyridocyclohexanone, which has inequivalent alpha
positions with respect to the ketone. Deprotonation of the more
acidic alpha proton leads to the bent isomer, while deprotonation
at the less acidic site leads to the linear isomer, meaning that the
bent isomer is the product of the more favored thermodynamic
intermediate enolate. In the previously reported conditions using
weak base (piperidine), a rapid acid-base equilibrium quickly
tautomerizes all of the intermediate to the thermodynamic enolate,
leading to exclusive formation of the bent isomer. Our conditions
employing strong base (NaOH) trap some of the less favored
kinetic enolate, leading to formation of the linear isomer, although
the bent isomer is still the major product.
The in-situ aromatization of the synthesized tAPs was
somewhat surprising due to the lack of an added oxidizing agent
in the reaction. Reactions were carried out in rigorously
oxygen-free environments, which eliminates the possibility of O2
as an oxidant. One plausible mechanism for the aromatization of
tAPs is deprotonation-hydride elimination. Reetz and Eibach
demonstrated a similar method to aromatize dihydro-acenes into
their corresponding acenes in good yields, with potassium
fencholate or n-butyl lithium as the catalyzing base.[15]
The poor solubility of the unsubstituted tAP (2l) hinders
purification, therefore we used a substituted nicotinaldehyde
precursor (1b) bearing 2,6-di-tert-butylanisole (DTBA) as a
solubilizing group. Subjecting 1b to 1,4-cyclohexadione in the
same Friedländer conditions leads to new linear (3l) and bent (3b)
tetra-aza-pentacene derivatives which are substantially more
soluble than 2l/2b. The DTBA groups did not show improvement
in the air-sensitivity of 3l (see Figure S2 for photo-oxidation
monitoring of 3l). Given molar absorption spectra of individual
pure components, as well as the UV-vis absorption spectrum of
the crude product mixture (see SI), we used a mixture analysis
technique described by Harris[16] to determine the 3l:3b product
ratio of 1:7. The reaction yields both the linear and bent
tetra-aza-pentacenes because 1,4-cyclohexanedione has only
hydrogen substituents on the alpha carbons. Therefore, to
promote
formation
of
the
linear
tAP,
2,5-dibenzylcyclohexane-1,4-dione (4) was used in place of the
unsubstituted cyclohexanedione, and as expected, only the linear
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(see SI). The more negative reduction potentials in the bent
isomer are consistent with Clar’s argument referred to above.
Oxidation waves were not observed for either 3b or 3l.
Single crystal structures for 2l, 3b and 5l are shown in
Figure 3 (Cambridge Crystallographic Data Center numbers
(CCDC) are 1866946, 1866947, and 1866945, respectively).
crystal packing structures (see SI for 3b and 5l packing
discussion). Neighboring molecules in 2l are organized into
ribbons with adjacent nitrogen atoms aligned opposite each other
at a distance close to the sum of van der Waals radii (NꞏꞏꞏN
spacing = 3.005(2) Å, Figure 3). The molecules in these ribbons
are arranged in a shallow staircase with a step height of 0.8 Å.
The series of slip-stacked ribbons form a sheet propagating in the
crystallographic ab plane. The molecules in these sheets are not
cofacially stacked alongside adjacent sheets, but rather are
disposed edge-to-face at an angle of ~53°. The unit cell contains
two sheets, related by a glide plane, that alternate along the
crystallographic c axis in a fashion reminiscent of herringbone
packing. Thus, 2l has cofacial pi-stacking in two dimensions, and
herringbone-like packing in the third. In contrast, the herringbone
arrangement in the crystal packing of pentacene (edge-to-face
angle = 52°) propagates in two dimensions, with collinear
end-to-end packing in the third.
ꢀ
Structures 3b and 5l belong to the triclinic space group P1, and
2l belongs to the monoclinic C2/c. Carbon–carbon bond lengths
in all three tAP cores fall in the range of 1.349(4) Å – 1.454(2) Å,
and follow a pattern of bond length alternation observed in
pentacene (short perimeter versus long internal C-C bonds).[24]
The carbon-nitrogen bonds are short, reminiscent of the bond
length pattern observed in acridine.[25] The structure core of 2l
and 5l are planar, whereas that of 3b is twisted with an offset
angle of 17.4o between planes of the two terminal naphthyridines.
Isoda and co-workers reported different packing behaviors
in the crystal of 5,6,13,14-tetraazapentacene due to induced
CHꞏꞏꞏN hydrogen bonds between molecules from neighboring
columns.[27] Similarly, Campbell et.al. proposed that packing in
multi-nitrogen-substituted pentacene structures should be
dominated by flattened herringbone or sheet-like arrangements
due to intermolecular CHꞏꞏꞏN interactions.[28] For the case of 2l,
however, CHꞏꞏꞏN hydrogen bonds are overshadowed by NꞏꞏꞏN
interactions.
In conclusion, a Friedländer reaction has been used to form
fully aromatized tetra-aza-pentacene products in the absence of
an exogenous oxidant. Peripheral substitution yields more
soluble materials and allows for the isolation and characterization
of pure
tetra-aza-pentacene molecules. Unsubstituted
1,4-cyclohexanedione permits the formation of both linear and
bent Friedländer products, whereas blocking its 2,5-positions
enforces exclusive formation of the linear isomer.
The
photophysical properties of the linear tAPs are similar to
pentacene, whereas their crystal structures differ. The
electrochemistry of dibenzylated tAP agrees well with
computational results, with reduction potentials comparable to
that of fullerene acceptors. Therefore, this class of tAPs are
promising as potential n-type semiconductors to contribute to the
fast-growing research on optoelectronic devices, such as OPVs
and/or OFETs, and the adopted synthetic strategy opens up
avenues to access other aza-substituted acenes of the same
family.
Experimental Section, Supporting Information
Synthetic procedures and characterization data (elemental analysis,
MALDI, NMR, electrochemistry, X-ray crystallography, and theoretical
calculations) as well as supplementary discussions can be found in the
supporting information.
Figure 3. X-ray single crystal structures of 2l (with its molecular packing), 5l,
and 3b. Nitrogen atoms are in blue; grey and green colors are used in crystal
packing of 2l for clearer view. Hydrogen atoms are omitted for clarity. Crystal
packing and discussion for 5l and 3b can be found in SI.
The arrangement of pentacene in the crystal lattice is well
studied and known for its edge-to-face herringbone packing
motif.[24, 26] The tetra-aza-pentacenes adopt substantially different
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10.1002/chem.201805655
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[9]
R. P. Thummel, e-EROS Encyclopedia of Reagents for Organic
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This work was financially supported by NanoFlex Power
Corporation
and
the
National
Science
Foundation
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Keywords: tetra-aza-pentacene • acenes • Friedländer reaction
[14]
[15]
[16]
• one-pot synthesis • n-type molecules
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Chemistry - A European Journal
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TOC
N. Ukwitegetse, P. J. G. Saris,
Dr. J. R. Sommer, Dr. R. M. Haiges,
Dr. P. I. Djurovich,
Prof. M. E. Thompson*
Page No. – Page No.
Tetra-Aza-Pentacenes via a One-Pot
Friedländer Synthesis
A
one-pot Friedländer synthesis catalyzed by strong base (NaOH) yields
1,7,8,14-tetraazapentacenes (tAPs), fully-aromatized in-situ, despite the absence of
an exogenous oxidizing agent. The LUMO level of linear tAPs is comparable to C60.
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