Green and Rapid Hydrothermal Crystallization and Synthesis of Fully Conjugated Aromatic Compounds

Article abstract of DOI:10.1002/anie.201801277

Highly fused, fully conjugated aromatic compounds are interesting candidates for organic electronics. With higher crystallinity their electronic properties improve. It is shown here that the crystallization of three archetypes of such molecules—pentacenetetrone, indigo, and perinone—can be achieved hydrothermally. Given their molecular structure, this is a truly startling finding. In addition, it is demonstrated that perinone can also be synthesized in solely high-temperature water from the starting compounds naphthalene bisanhydride and o-phenylene diamine without the need for co-solvents or catalysts. The transformation can be drastically accelerated by the application of microwave irradiation. This is the first report on the hydrothermal generation of two fused heterocycles.

Full text of DOI:10.1002/anie.201801277

A Journal of the Gesellschaft Deutscher Chemiker
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Accepted Article
Title: Green and Rapid Hydrothermal Crystallization and Synthesis of
Fully Conjugated Aromatic Compounds
Authors: M. Josef Taublaender, Florian Glöcklhofer, Martina Marchetti-
Deschmann, and Miriam Margarethe Unterlass
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201801277
Angew. Chem. 10.1002/ange.201801277
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Green and Rapid Hydrothermal Crystallization and Synthesis of
Fully Conjugated Aromatic Compounds
M. Josef Taublaender,[a][b] Florian Glöcklhofer,[a] Martina Marchetti-Deschmann,[c] and Miriam M.
Unterlass∗[a][b]
Abstract: Highly fused, fully conjugated aromatic compounds are
interesting candidates for organic electronics. With higher crystallinity
their electronic properties improve. Here we show that the
These mixtures are of intense red color. The pure, separated
isomers show cleaner, individual colors: cis-perinone is of
blueish-red hue, while the industrially more important trans-
perinone has a brilliant orange-reddish shade.[4] To the best of our
knowledge, there is not a single direct synthetic approach that
allows for obtaining exclusively either of the two isomers. This is
due to the fact that they are able to co-crystallize: Indeed, both
isomers form a solid solution, which is structurally highly tolerant
as reflected by the fact that the two isomers can be combined in
a broad range of ratios by local, translational and positional
disorder.[5] As research on perinone was in the last decades
dominated by the development of isomer separation techniques
rather than syntheses, there is only a handful of synthetic
procedures towards perinone. These include (i) refluxing the
starting compounds in conc. acetic acid for several of hours;[6] (ii)
condensing the starting compounds in imidazole using Zn(OAc)2
as catalyst,[7] which is analogous to LANGHALS’ method for
generating perylene bisimides;[8] (iii) rapid condensation (15 min)
in H3PO4 at 190 °C;[9] and (iv) precondensation to α−amino-
imidazoles or α-carboxylic acid imides in H2O at reflux, followed
by solid-state condensation to perinone.[10] Clearly, all of the
these syntheses except the latter two-step route reported by
MAMADA et al. are far from being green approaches.[10]
With this contribution, we have set out to hydrothermally
prepare perinone using nothing but high-temperature water
(HTW) and the starting compounds in a stoichiometric ratio
prompted by our recent reports on the fully green hydrothermal
(HT) synthesis of perylene and naphthalene bisimides,[11] and
polyimides.[12–14] While others have used near-critical water (250
– 350 °C) for the preparation of benzimidazoles,[15] and
supercritical water (≈ 400 °C) for various benzazoles,[16] our HT
method only requires HTW (typically 180 – 250 °C). Consequently,
energy consumption is lower and required safety measures are
reduced. However, to date HT condensation has only been
reported for the formation of single heterocycles and non-cyclic
amides. [12–14,17] In the latter cases, the action of HTW has been
shown to generate superior crystallinity. Therefore, we were
intrigued to expand the scope of HT synthesis and crystallization
for the very first time towards fused heterocycles.
crystallization of three archetypes of such molecules
–
pentacenetetrone, indigo and perinone can be achieved
–
hydrothermally. Given their molecular structure, this is a truly startling
finding. In addition, we demonstrate that perinone can also be
synthesized in solely high-temperature water from the starting
compounds naphthalene bisanhydride and o-phenylene diamine
without the need for co-solvents or catalysts. The transformation can
be drastically accelerated by the application of microwave irradiation.
This is the first report on the hydrothermal generation of two fused
heterocycles.
Organic colorants are molecules that show strong color
when interacting with light. Aside from their oldest use, i.e.
coloring other substances, in recent years they have become of
interest for optoelectronic applications in e.g. organic solar cells,[1]
or as organic transistors.[2] One important class of organic
colorants are so-called carbonyl dyes, which are typically highly
resistant to heat, solvents and weathering.[3] Perinone, i.e.
naphthalene tetracarboxylic acid bisbenzimidazole, is such a
carbonyl dye. The name perinone is generally used for mixtures
of cis- and trans-isomers (Fig. 1).
Figure 1. HT synthesis of perinone: o-PDA and NBA undergo
cylcocondensation to a mixture of cis- and trans-perinone. The initially white-
yellow suspension is transformed into a red solid and an orange, translucent
supernatant.
[a]
M. Josef Taublaender, Dr. Florian Glöcklhofer, Dr. Miriam M.
Unterlass
In an initial experiment, we investigated the general
feasibility of hydrothermally generating perinone from the starting
compounds o-phenylene diamine (o-PDA) and naphthalene
bisanhydride (NBA), as shown in Fig. 1. Therefore, o-PDA and
NBA (2:1 molar ratio; at an equivalent concentration ceq = 0.01
mol/L) were suspended in deionized H2O and subjected to 200 °C
in a non-stirred batch autoclave (see SI for experimental details).
After a reaction time tR of 16 h, the autoclave contained two
distinct phases: a red, flocculent solid, sedimented at the bottom,
and an orange, translucent aqueous supernatant phase (see Fig.
1). The bottom phase, whose red color was already indicative for
the formation of perinone, was collected, dried and characterized
without further purification. Attenuated total reflectance Fourier
Institute of Applied Synthetic Chemistry
TU Wien
Getreidemarkt 9/163, 1060 Wien, Austria
E-mail: miriam.unterlass@tuwien.ac.at
M. Josef Taublaender, Dr. Miriam M. Unterlass
Institute of Materials Chemistry
TU Wien
Getreidemarkt 9/165, 1060 Wien, Austria
Prof. Martina Marchetti-Deschmann
Institute of Chemical Technologies and Analytics
TU Wien
[b]
[c]
Getreidemarkt 9/164, 1060 Wien, Austria
Supporting information for this article is given via a link at the end of
the document.
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transform infrared (ATR-FTIR) analysis (Fig. 2; SI for details) of
the crude product revealed the absence of modes characteristic
for the starting compounds. Instead, we found several intense
modes indicative for perinone.
desorption/ionization high resolution mass spectrometry (LDI
HRMS) measurements (see SI) allowed to identify the byproduct
as
naphthalene
tetracarboxylic
acid
monoanhydride
monobenzimidazole (NMM). Consequently, it was also possible
1
to calculate NMM’s amount in crude perinone. According to H-
NMR analysis NMM accounts for approximately 7 mol% of the
unpurified product mixture (see SI for calculation). From these
measurements the cis:trans ratio in crude as well as in
reprecipitated perinone was determined to be approximately 2:3
(see SI for details).
A further set of experiments revealed, that (i) a minimum of
tR = 12 h was necessary to yield maximum conversion of the
starting compounds, and (ii) that NMM’s formation could not be
avoided at any tested tR (up to 48 h). Moreover, it became evident,
that NMM is formed as an intermediate during the reaction and its
amount steadily decreases with tR until a certain threshold is
reached (after tR ≥ 12 h).
The HT method for the condensation of o-PDA and NBA
described so far clearly qualifies as green synthesis, since (i) no
co-solvents or catalysts are required, and (ii) H2O is not only the
sole reaction medium but also forms as sole condensation
byproduct. Nonetheless, we were interested in making the
transformation even greener and more energy-efficient. We
therefore turned our attention towards microwave (MW) assisted
HT condensation.[20,21] We conducted a series of experiments in
a stirred MW oven, where it was possible to exactly control
reaction temperature TR, heating time tH (time until TR is reached),
and tR (reaction time at TR). When we performed those
experiments at TR = 200 °C and tH = 10 min, we found, that already
after tR = 2 min the starting compounds had completely reacted to
form perinone. Evidently, not only efficient heating via MW
irradiation, but also the application of stirring is highly beneficial
for the reaction in order to proceed rapidly. Hence, by using this
MW-assisted set-up it was possible to decrease the total reaction
time at TR = 200 °C from 12 h to 12 min. Lowering TR to 180 °C
drastically increases tR, whereas at the highest tested TR of
250 °C it is not even necessary to keep this TR for a certain time.
This tremendous reduction of tR upon increasing TR can be
attributed – among other factors – to temperature-dependent,
significant changes in H2O’s physicochemical properties.[22] With
increasing TR the static dielectric constant decreases steadily,
which allows for better dissolving organic compounds that are
completely insoluble in H2O at rt. Furthermore, an elevation of TR
leads to an increasing ionic product of H2O until a maximum at
250 °C is reached. This phenomenon makes H2O itself a powerful
acid/base catalyst in the HT regime, which is indeed beneficial for
organic condensation reactions.
Figure 2. ATR-FTIR spectra of crude perinone (red) and starting compounds
(black).
The amount of obtained crude perinone corresponded to
approximately 90 % of the theoretical yield. In order to further
ascertain the formation of perinone, we performed 1H solution
nuclear magnetic resonance (NMR) analysis. 1H-NMR
measurements (Fig. 3; SI) revealed, that the crude perinone is
composed of a mixture of cis- and trans- isomers.[10,18] However,
the two multiplets labeled as A and B in Fig. 3 indicated the
presence of a small amount of byproduct. Attempts to remove this
byproduct via extraction were not successful. Therefore, we
turned our attention to selective precipitation - a technique
commonly used to purify or fractionate polydisperse polymers.[19]
Selective precipitation from trifluoroacetic acid (TFA) with H2O
finally allowed for purifying the target compound and isolating the
byproduct. The combined results of ATR-FTIR, 1H-NMR and laser
Interestingly, as long as full conversion of o-PDA and NBA
is achieved, the NMM-content as well as the cis:trans ratio (≈ 2:3)
stayed constant independent of the reaction conditions. The
typical amount of ≈ 7 mol% of NMM was always formed. Even
extending tR, increasing tH, varying the pH or lowering ceq had no
impact on the ratio of isomers as well as the byproduct content.
For the HT condensation of perylene and naphthalene
bisanhydride with monoamines to yield bisimides it had been
shown that the presence of a non-nucleophilic base such as
HÜNIG’s base had a positive effect (reduction of tR, increasing
yields) on the formation of some derivatives.[11] Unfortunately, for
the reaction of NBA with o-PDA the addition of HÜNIG’s base did
not suppress or reduce the formation of NMM. One could
furthermore speculate that NMM always forms due to a lack of o-
Figure 3. 1H-NMR spectrum of crude perinone and peak assignment for Ha, Hb,
Hc and Hd. The signals A and B can be attributed to the presence of the minor
byproduct NMM.
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PDA, which could be consumed by the well-known oxidative
polymerization of aromatic amines.[12] When we evaporated the
liquid, orange supernatant phase – that was always found after
the HT reaction – to dryness, a small amount of dark purple solid
was obtained. FTIR-ATR, 1H-NMR and 13C-NMR analysis yielded
spectra with a multitude of signals (SI), which are likely to
correspond to various products of the oxidative auto-
polymerization of o-PDA. In order to check if o-PDA’s oxidative
polymerization is responsible for NMM’s formation, we employed
a slight molar excess of o-PDA. However, this did not result in the
suppression of NMM formation. Clearly, in the absence of O2,
oxidative polymerization of o-PDA cannot occur. However,
performing a perinone synthesis after thoroughly degassing the
reaction mixture still led to the usual amount of NMM.
The for us unexpectedly high disorder in reprecipitated perinone
provided an excellent opportunity for attempting the very
challenging task of its HT crystallization. This task is ambitious,
because perinone is much more hydrophobic than its precursors
NBA and o-PDA. Hence, higher difficulties regarding perinone’s
dissolution must be expected. At 250 °C H2O’s polarity compares
to that of ethanol at rt,[23] which is certainly not apolar enough for
expecting it to be a good solvent for perinone.
For testing the feasibility of HT crystallization, perinone was
simply dispersed in H2O and heated to the HT regime. Fortunately,
PXRD patterns showed that the HT treatment tremendously
increased crystallinity (Fig. 4). Furthermore, scanning electron
microscopy (SEM) measurements clearly revealed, that the HT
treatment also improved the morphology (see SI): The
reprecipitated roundish particles (0.1 - 0.5 µm) with a rough,
random surface texture were transformed into agglomerates of
fine needles (1 - 2 µm in length). When commercially available
trans-perinone was subjected to HT conditions, the morphological
improvement was even more significant: beautiful needles of
approximately 5 - 10 µm in length with a rather narrow size
distribution featuring smooth crystal facets were obtained (Fig. 5).
The fact that the formation of NMM is not suppressed under
any of the tested conditions, including the addition of a
condensation promoter or the use of an excess o-PDA, points at
an incorporation of NMM into the crystal lattice of perinone as
alternative explanation. We suspect that the similar size and the
planarity of NMM as well as the electronic similarity to cis- and
trans-perinone facilitate NMM’s incorporation into perinone’s
crystal lattice through local, positional and translational disorder.
Once NMM is implemented, it can be considered as “entrapped”
and consequently it is not available for further condensation. As
for the curious fact that we always find ≈ 7 mol% of NMM in crude
perinone, we speculate that perinone crystals incorporate the
amount of NMM that the crystal lattice can maximally tolerate.
Therefore, although the HT condensation of o-PDA and
NBA was found to be highly robust and proceeds rapidly at TRs ≥
200 °C, selective precipitation from TFA was inevitable to remove
traces of NMM. Interestingly, powder X-ray diffraction (PXRD)
measurements revealed that this purification technique
significantly decreased crystallinity (Fig. 4). In fact, reprecipitation
with a non-solvent rapidly quenches the perinone solution,
thereby generating a strongly disordered liquid-crystalline (LC)
phase. When attempting to index the reflections to a rectangular
lattice all peaks fitted relatively well (see SI for detailed
information) to a columnar rectangular LC phase.
Figure 5. SEM micrographs of trans-perinone: before (A) and after (B) HT
treatment.
Hence, HT crystallization of perinone is indeed possible.
These drastic morphological changes can only arise from HT
dissolution of reprecipitated/commercially purchased perinone
and its subsequent crystallization. Considering H2O’s polarity at
250 °C, it is very surprising that perinone dissolves. We suspect
that the increased ionic product of HTW allows for protonating
perinone, which is indeed the mechanism for its dissolution in
some strong acids such as TFA. Additionally, the gain in lattice
energy upon passing from the LC to the fully crystalline state
could be a major driving force.
Thrilled by these findings, we decided to attempt the HT
crystallization of other, fully conjugated aromatic compounds, also
of interest for organic electronics and for which crystallinity
matters. Specifically, we chose indigo (2,2'-bis(2,3-dihydro-3-
oxoindolyliden)) and pentacenetetrone (pentacene-5,7,12,14-
tetrone).[24,25] In accordance with perinone, both of these
compounds are insoluble in H2O at rt due to their ability to strongly
π-stack.[26,27] For testing the feasibility of their HT crystallization,
we applied the optimized reaction conditions found for
crystallizing reprecipitated perinone. ATR-FTIR, 1H-NMR
measurements revealed that both substances are stable under
HT conditions (see SI). Furthermore, PXRD results indicated that
dissolution and crystallization of these compounds is indeed
possible under HT conditions, since reflections became sharper
Figure 4. PXRD patterns of crude, reprecipitated and crystallized perinone:
crude perinone (bottom) is highly crystalline, whereas the purified,
reprecipitated one (middle) shows a LC type of ordering. Subsequent HT
treatment leads to a highly crystalline product (top) again.
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and more pronounced (see SI). SEM analysis (Fig. 6; SI)
additionally showed a striking improvement of the morphology for
both compounds.
Keywords: crystalline materials • hydrothermal synthesis •
carbonyle dye • green chemistry • fused heterocycles
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In summary, we show with this contribution that perinone
can be synthesized hydrothermally solely from its starting
compounds in stoichiometric ratio. The synthesis is highly robust
and remains unaffected by various reaction parameters (TR, tR, tH,
ceq, pH, O2 presence) and does not require condensation
promotors or catalysts. The obtained product only contains minor
amounts of NMM-byproduct. Moreover, we demonstrate that one
can do both, crystallize intentionally amorphisized perinone, and
recrystallize commercially obtained perinone. Furthermore, we
could show that HT crystallization can also be successfully
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The authors acknowledge TU Wien, the Austrian Science Fund
(FWF) and the Christian Doppler Research Association (CDG) for
funding this project with grant no. PIR 10-N28. XRD
measurements were carried out at the X-ray Center of TU Wien
(XRC), and SEM was performed at the interfaculty electron
microscopy facility of TU Wien (USTEM). Furthermore, the
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The fully conjugated organic
compounds indigo, pentacenetetrone
and perinone can be crystallized, and
in the case of perinone also
synthesized, in nothing but high-
temperature water.
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