Stable and easily accessible functional dyes: Dihydrotetraazaanthracenes as versatile precursors for higher acenes

Article abstract of DOI:10.1002/chem.201500230

Abstract A series of new dihydrotetraazaanthracenes and one new dihydrotetraazatetracene as substances for applications in organoelectronic devices and as suitable building blocks for higher azaacenes was synthesised. The condensation of aromatic diamines with dichlorodicyanopyrazine led to these tricyclic/tetracyclic compounds. Syntheses of N-substituted phenylenediamines were developed to enable the introduction of multiple functional groups such as ester, amino, or nitro groups on the chromophoric system. Relationships between the structure and the spectroscopic properties could be derived from UV/Vis absorption and fluorescence spectroscopy, as well as by DFT and TD-DFT calculations of molecular and aggregate structures. The absorption spectra are dominated by π-π transitions of the single molecules, whereas aggregation needs to be taken into account to obtain reasonable agreement between theory and experiment in certain cases. Single-crystal X-ray analyses were carried out to examine the morphology and solid packing effects. Finally, a dihydrotetraazaanthracene was used as a building-block to create a mesoionic octaazapentacene. Dihydroazaacenes: A series of dihydrotetraazaanthracenes and one new dihydrotetraazatetracene as substances for applications in organoelectronic devices and as suitable building blocks for higher azaacenes was synthesised. Single-crystal X-ray analyses were carried out to examine the morphology and solid packing effects. Finally, a dihydrotetraazaanthracene was used as a building-block to create a mesoionic octaazapentacene.

Full text of DOI:10.1002/chem.201500230

DOI: 10.1002/chem.201500230
Full Paper
&
Sensitizers
Stable and Easily Accessible Functional Dyes:
Dihydrotetraazaanthracenes as Versatile Precursors for Higher
Acenes
Dominique Mario Gampe,[a] Martin Kaufmann,[a, b] Dçrthe Jakobi,[a] Torsten Sachse,[b]
Martin Presselt,[b] Rainer Beckert,*[a] and Helmar Gçrls[c]
Abstract: A series of new dihydrotetraazaanthracenes and
one new dihydrotetraazatetracene as substances for applica-
tions in organoelectronic devices and as suitable building
blocks for higher azaacenes was synthesised. The condensa-
tion of aromatic diamines with dichlorodicyanopyrazine led
to these tricyclic/tetracyclic compounds. Syntheses of N-sub-
stituted phenylenediamines were developed to enable the
introduction of multiple functional groups such as ester,
amino, or nitro groups on the chromophoric system. Rela-
tionships between the structure and the spectroscopic prop-
erties could be derived from UV/Vis absorption and fluores-
cence spectroscopy, as well as by DFT and TD-DFT calcula-
tions of molecular and aggregate structures. The absorption
spectra are dominated by p–p* transitions of the single mol-
ecules, whereas aggregation needs to be taken into account
to obtain reasonable agreement between theory and experi-
ment in certain cases. Single-crystal X-ray analyses were car-
ried out to examine the morphology and solid packing ef-
fects. Finally, a dihydrotetraazaanthracene was used as
a building-block to create a mesoionic octaazapentacene.
Introduction
observed. The introduction of nitrogen atoms, particularly,
leads to novel properties such as reduced HOMO-LUMO gaps,
and especially to dramatically increased chemical stabilities.[6]
Recent reviews[7] show that aza-substituted acenes are emerg-
ing in charge transport materials as well as in other functional
materials. Among the azaacenes, linear systems, for example,
1 “Pyrazinacenes”,[8] comprise by far the majority of derivatives.
However, the synthesis of higher azaacenes remains challeng-
ing because of 1) limited access to appropriate diamines and
the corresponding bis-electrophilic partners, 2) low yields of re-
actions with bis-electrophiles (classic conditions, partially Pd-
catalysed), and 3) poor solubility of the azaacenes because of
severe aggregation phenomena, which complicates characteri-
sation and processing.
Significant progress is being made in the development of or-
ganic electronic and optoelectronic devices.[1] However, many
of the recently developed material systems contain scarce or
expensive materials, such as redox systems containing rutheni-
um or fullerenes as acceptor materials, respectively.[2] Generally,
the use of expensive materials can negate the advantage of in-
expensive, large-scale production of organic electronic devi-
ces.[3] Hence, there is huge demand for replacing such systems
with stable organic dyes, for example, in solar cells.[4] Since the
successful stabilisation of pentacene, linear-condensed acenes
have found many applications as versatile organic semiconduc-
tors.[5] The replacement of CH by other atoms (mainly N or S)
results in heteroacenes in which a different reactivity can be
Most of the syntheses described use the condensation reac-
tion of vicinal diamines 1 with partners 2 possessing vicinal
leaving groups X, as depicted in Scheme 1, in which X=OH,[9]
X=halogen: nucleophilic heteroaromatic substitution[10] or Pd-
catalysis,[10c] X=CN,[11] diketones,[12] and quinone imines.[13] In
all of these cases, the rings A and B themselves can be hetero-
[a] Dipl.-Chem. D. M. Gampe, Dipl.-Chem. M. Kaufmann, D. Jakobi,
Prof. Dr. R. Beckert
Friedrich Schiller University Jena
Institute of Organic and Macromolecular Chemistry
Humboldtstraße 10, 07743 Jena (Germany)
E-mail: rainer.beckert@uni-jena.de
[b] Dipl.-Chem. M. Kaufmann, M. Sc. T. Sachse, Dr. M. Presselt
Friedrich Schiller University Jena
Institute of Physical Chemistry
Helmholtzweg 4, 07743 Jena (Germany)
[c] Dr. H. Gçrls
Friedrich Schiller University Jena
Institute of Inorganic and Analytical Chemistry
Humboldtstraße 8, 07743 Jena (Germany)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201500230.
Scheme 1. Retrosynthetic approach to azaacenes.
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cycles and members of bi- and polycyclic compounds. Cycload-
ditions of quinodimethides generated in situ[6d,7d] and oxidative
ortho-annulations,[14a] as well as special methods, for example,
for hexaazaanthracenes[14b] and hexaazaacridines,[14c] are rather
exceptional.
Results and Discussion
Syntheses
Diamines 10 can be obtained in four steps, starting with 4-
fluoro benzoic acid through nitration,[19a] followed by esterifica-
tion,[19b] nucleophilic fluoride substitution and reduction
(Scheme 3).[20] The vicinal diamines 12 were prepared starting
with 1,5-dichloro-2,4-dinitrobenzene by aminolyses to give
8.[21] Generally, the reduction of both nitro groups in 9 is possi-
ble; however, the tetramino derivatives of type 11 proved to
be very sensitive to oxidation and tended to decompose rapid-
ly. Naphthalene derivative 13 was synthesised by azo coupling
of commercially available 2-phenylamino-naphthalene with di-
azotated aniline and subsequent reduction with sodium di-
thionite.[22]
Tetraazaanthracenes were synthesised in 1988 by the con-
densation of phenylenediamines with 5,6-dichloro-2,3-dicyano-
pyrazine;[15] however, their first use as strong acceptors in the
form of n-type FET devices was reported in 2004.[16] Miao and
co-workers took advantage of these compounds as building
blocks for the production of highly electron-deficient hexaaza-
pentacenes and their dihydro precursors.[17]
In the present work, we focus on the synthesis and charac-
terisation of new derivatives of dihydrotetraazaanthracenes 16
and 17 (Scheme 2), because they are easily accessible, starting
from a functionalised diamine 10 and 12, and 5,6-dichloro-2,3-
dicyanopyrazine (14). We were particularly interested in the in-
troduction of functional substructures R into diamines that are
The model substance 15 was synthesised by cyclisation of
N-phenyl-o-phenylenediamine and 14 (Scheme 4). Triamines
12, diamines 10, and naphtha-
lene derivative 13 were cyclised
with 14 under the exclusion of
oxygen. In all cases, dihydrote-
traazaanthracenes 16 and 17
and the angularly fused dihydro-
tetraazatetracene 18 were isolat-
ed as the main products.
Scheme 2. Synthesised tetraaza-substituted dihydroanthracenes.
Nitro-substituted derivatives
17 and the tetracene 18 show
able to preserve the corresponding dihydroanthracene forms.
The substitution at the amino group causes a “frozen” dihydro
form and, consequently, prevents oxidation by air. There are
examples that such dihydro derivatives show much higher sta-
bilities[18] and intense fluorescence[7c,10a] compared with the
fully aromatic species. The simple introduction of substituents
enabled us to synthesise a multitude of different dihydroazaan-
thracenes. The reactions tolerate nearly all functionalities.
Therefore, it is possible to introduce electron-withdrawing and
electron-donating moieties and, furthermore, solubility-improv-
ing groups. Halogen atoms for further cross-coupling experi-
ments and alkyl chains possessing ester groups as precursors
for amphiphilic features can also be obtained. The nitro-substi-
tuted dihydrotetraazaanthracenes 17 are expected to allow
access to higher azaacenes by reduction and subsequent con-
densation reactions, whereas compounds 16 offer the possibili-
ty of modifications to the ester group. In addition, they might
serve as precursors for hexaazapentacenes by nucleophilic sub-
stitution of the cyano groups.[8,17] To study the properties of ex-
tended p-systems, derivative 15 and angular tetracene 18
were both integrated into the study. To investigate the interac-
tions between substituents and dihydroazaacene chromo-
phores, UV/Vis absorption and emission spectroscopy measure-
ments were performed, both of which were supplemented by
quantum chemical simulations of the spectral features and
line-fit analyses of the experimental spectra.
Scheme 3. Syntheses of vicinal diamines. Reagents and conditions:
a) 1. KNO3, H2SO4;[19a] 2. SOCl2, MeOH;[19b] 3. R-NH2: 36–99%; b) H2,Pd/C,
EtOH; c) EtOH, R-NH2: 40–97%; d) Raney-Nickel, 30% NH2NH2, EtOH; e) Ph-
NH2, NaNO2, EtOH/H2O;[22a] f) NaS2O4, DMF.[22b]
an intense red colour in both the solid state and in solution,
whereas derivatives of type 16 were obtained as orange crys-
tals. All derivatives were fully characterised by means of ele-
mental analysis (EA), MS and NMR experiments.
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Scheme 4. Syntheses of dihydrotetraazaanthracenes 15–17 and dihydrotetraazatetracene 18. Reagents and conditions: a) dioxane, 30 min reflux, 97%;
b) EtOH, reflux, 18–65%; c) dioxane, 4 h reflux, overnight 808C, 18–35%; d) dioxane, DIPEA, 4 h reflux, 68%.
Dihydrotetraazaanthracenes 16
like supramolecule. Despite having a similar structure, deriva-
tive 16d coordinates a solvent molecule at the cyclic NH
Single crystals of 16h and 16d that were suitable for X-ray
crystallographic analysis were grown by evaporating the sol-
vent from solutions in acetonitrile (16h) or acetone (16d). The
resulting crystal structures are illustrated in Figure 1. The 4-me-
thoxyphenyl substructure in derivative 16h is distorted with
an angle of 78.098 (C4-N2-C11-C12) out of plane. The bond
lengths C4ÀN2, N2ÀC3, C9ÀN3, and N3ÀC10 indicate the in-
volvement of the lone pair of N2 and N3 in the conjugated
system.
group, which efficiently prevents the formation of dimers and
allows the formation of the p–p-stacking framework.
All derivates of type 16 show a similar UV/Vis absorption be-
haviour. Typically, the spectra display a strong absorption at
approximately 267–274 nm and a less intense absorption at
389–393 nm, indicating that the substituents have only a small
electronic influence (Figure 3). The fluorescence spectra are
characterised by relatively broad, plateau-like emissions from
552 to 572 nm.
Contrary to expectations, p–p interactions of the neighbour-
ing stacks of 16h cannot be observed. As depicted in Figure 2,
two molecules form a point symmetric dimer through H-
bridges that connect to the next dimer due to the p–p interac-
tions of the 4-methoxyphenyl groups, thus generating a chain-
Quantum chemical calculations were performed to identify
the origins of the transitions in the visible spectral range utilis-
ing the range-corrected functional CAM-B3LYP together with
Ahlrichs-pvdz douple-z basis set and Grimmes dispersion cor-
rection (for technical details, see the Supporting Information),
which were shown to yield relia-
ble geometries and UV/Vis ab-
sorption spectra at reasonable
computational cost.[23–28] A com-
parison between theoretical and
experimental spectra is shown in
Figure 4 for 16e as a representa-
tive derivative. The theoretical
spectra are represented by
a stick spectrum that is addition-
ally broadened by simple Gaussi-
ans, with a full-width at half-
maximum of 0.2 eV and redshift-
ed by 0.8 eV. Results for those
derivatives not shown are
almost identical; thus, theory re-
produces the experimental ob-
servation that exchanging the
substituent R has little or no in-
Figure 1. View of 16h (left) and 16d (right, H-atoms are omitted for clarity).[35]
fluence on the main chromo-
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Figure 2. View of molecular packing 16h (left) and 16d (right).
Figure 4. a) Broadened and shifted theoretical (monomer, dimer, trimer:
dashed lines) and experimental (solid lines) UV/Vis absorption and fluores-
cence (right spectra, vertical dashed line in case of theory) spectra of deriva-
tive 16e. Charge difference densities are shown for the most prominent mo-
nomer transitions. b) Scheme illustrating the orbital contributions to the en-
ergetically lowest two transitions of the monomer compound.
charge transfer towards the cyano substituents in addition to
their p–p* character, as shown by the charge difference densi-
ties and canonical molecular orbitals in Figure 4a and b. It is
notable that none of the substances 16 show any contribution
around 450 nm, where the experimental spectrum exhibits
a weak and broad shoulder, in their monomer form according
to theory. However, progressing to higher aggregates, weak
signals emerge in that spectral region, as shown in the inset in
Figure 4a. Hence, theory supports the attribution of features
around 450 nm to aggregate effects. As examples, the aggre-
gate spectra of 16a, 16e and 16 f were calculated and are
shown in the Supporting Information.
Figure 3. Normalised absorption (line) and emission (points) spectra of se-
lected derivatives 16 in acetonitrile. lmax absorption/emission for 16a: 389/
562 nm; 16b: 389/552 nm, 16d: 391/567 nm, 16e: 391/572 nm, 16 f: 389/
565 nm, and 16j: 388/552 nm.
phore. This is further substantiated by inspection of the opti-
mised ground-state geometries, which were always character-
ised by a nearly perpendicular arrangement of the phenyl
groups R with respect to the main chromophore, thus, decou-
pling the two (not shown).
The experimental absorption maximum at approximately
400 nm, which shows a vibrational progression, as well as the
low-energy shoulder at approximately 500 nm are well repro-
duced by the TD-DFT calculations. The intense absorption at
approximately 400 nm is largely dominated by a p–p* transi-
tion from the HOMO to the LUMO+1 (contribution to the
total electronic transition: CI2 *100=87.6; the threshold for
plotting transition arrows in figures is set at CI2 *100=20;
arrow thicknesses in the transition schemes scale linearly with
CI2), whereas the low-energy transition at approximately
500 nm is from HOMO to LUMO. Both transitions show a weak
Dihydrotetraazatetracenes: Influence of an extended p-
system
We investigated the influence of an extended p-system or an
additional ester group at the chromophoric system on the
spectroscopic properties. The UV/Vis and fluorescence spectra
of the three comparable compounds are depicted in Figure 5.
As assumed, there is no significant difference between the ab-
sorption spectra of the tetraaza-substituted dihydroanthra-
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the S2 state to the low-energy S1 state is either forbidden or
occurs at a very low rate in 18. A more detailed analysis of
how the structural modifications from substance 15 to 18
change the excited state dynamics will be carried out in a sub-
sequent work.
The aggregation behaviour of such polycyclic aromatic com-
pounds is crucial for the electronic properties in the solid state
or thin film. Therefore, we studied the intermolecular interac-
tions by X-ray single-crystal analysis (see Figure 7). The crystals
were grown by slowly evaporating the solvents from solutions
of chloroform/ethanol (15) or acetonitrile (18).
Whereas 15 exhibits dimeric structures, 18 forms a step-like
p-stacking network. The distance between those steps is an
even 3.315 ꢁ because of an effective interaction, and the poly-
meric stacks of 18 are connected to each other because of the
solvent molecules. Every dihydrotetracene unit is linked by H-
bonds to an acetonitrile molecule, which itself interacts with
the neighbouring dihydrotetracene stack.
Figure 5. Comparison of the spectral data between the tetraaza-substituted
dihydroanthracenes 15, 16a and dihydrotetracene 18 in acetonitrile (absorp-
tion=line; emission=points). lmax absorption/emission for 15: 393/571 nm,
16a: 389/562 nm, and 18: 413/476 nm.
The dimeric structure in the case of 15 results in alternating
distances between the planes in the packing; depending on
the kind of interaction, they are 3.398 or 3.634 ꢁ. The dimers
are connected through H-bonds between the phenyl moieties
and the nitrogen of the pyrazine unit from the neighbouring
dihydroanthracene plane (3.634 ꢁ). The substructure on the
rear of each dimer is related through p-interactions to the next
(3.398 ꢁ).
cenes of type 15/16a and the dihydrotetracene derivative 18.
The enlarged p-system in 18, compared with those in 15 and
16a, means that all frontier molecular orbitals are spatially ex-
tended (Figure 6). Thus, the LUMO energy of 18 decreases
slightly, whereas the energies of HOMO and LUMO+1 are in-
creased slightly compared with those in 15. As a consequence,
the absorption bands are slightly shifted bathochromically, as
seen in the experimental absorption spectra. Furthermore, the
energy gap between the first two excited states increases due
to the extension of the p-system from 15 to 18.
In contrast to the rather small spectral differences between
the absorption spectra of 15, 16a and 18, a relatively strong
deviance was observed in their fluorescence spectra. The tricy-
clic derivatives exhibit a large shift between the absorption
maxima at approximately 390 nm and the fluorescence
maxima of approximately 180 nm. However, the onset of the
fluorescence spectra fit approximately to the low-energy ab-
sorptions at approximately 500 nm (Figure 6). If the low-energy
transition is the origin of the fluorescence, the reorganisation
energy would be expected to be rather small (energy of verti-
cal transition to the lowest excited state is approximated as
2.49 eV; the 0–0 fluorescence transition energy is estimated at
2.25 eV; thus, the excited state reorganisation energy is ap-
proximately 240 meV). The energy difference between the TD-
DFT-optimised S1 state and the S0 state at the same geometry
of an isolated molecule 15 of 2.17 eV (572 nm) approximately
match the energy of the experimental fluorescence spectrum
(ca. 2.18 eV, ca. 570 nm).
Nitro-substituted dihydrotetraazaanthracenes 17
Compared with derivatives of type 16, nitro-substituted dihy-
drotetraazaanthracenes 17 give rise to complex absorption
spectra. The strongest absorptions of 17a–d are located at l=
367–376 nm. Whereas alkyl-substituted derivatives 17c and
17d show a further absorption at lꢀ448 nm, those possessing
aromatic residues show only a shoulder. All derivatives in the
long-wavelength region exhibit two overlapping peaks be-
tween 504 and 550 nm. In the presence of a strong para-
donor substituent (dimethylamino, 17e), the spectrum differs
from those discussed previously, particularly in the spectral
range between 350 and 457 nm, where it features a single
broad absorption band that is centred at approximately
430 nm. The absorption bands of derivatives 17a–d between
350 and 457 nm appear to be slightly redshifted in the case of
17 f. In contrast to the other derivatives 17, lower energetic
absorption bands (cf. 475–600 nm) are unresolved for 17 f.
Given that all derivatives 17 contain a nitro group, mesomer-
ism is possible, in which the substituted nitrogen acts as an
electron donor. This results in a quinoid substructure, in which
the nitro group accepts the electron pair (see Scheme 5). Such
a structure may be the reason for the strong bathochromically
shifted absorbance compared with the other dihydroazaan-
thracenes described (15 and 16).
In contrast to the tricyclic derivatives, the fluorescence of
the tetracyclic analogue 18 reveals a much smaller shift be-
tween the absorption maximum at 413 nm and the fluores-
cence maximum at 476 nm of only 63 nm. Thus, the fluores-
cence appears to originate from the major p–p* excited state
at 413 nm, even if our TD-DFT calculations again predict a low-
energy absorption at 575 nm, but with very low intensity, and
a fluorescence at 641 nm. We assume that the transition from
To enable a reliable interpretation of the peaks found in the
absorption spectra of 17 (Figure 8), we performed line-fit anal-
yses of the experimental spectra to deconvolute the electronic
transitions from vibrational progression and we performed
quantum chemical calculations to unravel the origin of the
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Figure 6. a) Broadened and shifted theoretical (monomer, dimer, trimer: dashed lines) and experimental (solid lines) UV/Vis absorption and fluorescence (right
spectra, vertical dashed line in case of theory) spectra of derivatives 15 and 18. Charge different densities that result instantaneously with photoexcitation are
shown for the most prominent monomer transitions. b) Scheme illustrating the orbital contributions to the energetically lowest two transitions of 15 and 18.
electronic transitions. As examples, the results of these calcula-
tions are shown in Figure 9 for derivatives 17b, 17d and 17e.
In contrast to the spectra shown in Figure 4 and Figure 6, we
found three prominent peak structures around 525, 425 and
360 nm in Figure 9 to be best described by each using an ordi-
Scheme 5. Mesomeric structures of derivatives 17.
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nary vibrational progression[29] (for details see the Supporting
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Figure 7. Comparison between the packing of 15 (left) and 18 (right). Yellow lines show the interaction between atoms.[35]
that is, by using absorption parameters derived ab initio to-
gether with those obtained from line-fits of the experimental
spectra, particularly for the face-to-face conformer. The absorp-
tion features observed experimentally at 375, 425 and 525 nm
originate basically from HOMO to LUMO+2, LUMO+1 and
LUMO (in the latter case, the HOMO–LUMO excitation largely
dominates the total electronic transition: CI2 *100=85) transi-
tions, respectively. Whereas for the face-to-face conformer,
only the peak at approximately 525 nm is underestimated in
its relative oscillator strength by the TD-DFT calculations, both
the peaks at 525 and 425 nm are underestimated for the angu-
lar conformer. In the case of the alkyl-substituted derivatives
17c and 17d, the experimental absorption spectra are very
similar and we focus discussion of the particular features on
17d. As in the case of 17b, the simulated absorption spectrum
of the 17d face-to-face conformer (in the case of alkyl-substi-
tution labelled as cisoidal), fits the experimental spectrum
better than the simulated absorption spectrum of the angular
conformer, as shown in Figure 9. The energy spacing between
the three major electronic transitions is particularly large and
the long-wavelength absorption at approximately 525 nm is
significantly more intense in the case of the cisoidal conformer
compared with the angular conformer.
Figure 8. Normalised UV/Vis spectra of selected nitro-substituted dihydrote-
traazaanthracenes of type 17 in acetonitrile. lmax absorption for 17a:
372 nm, 17b: 376 nm, 17c: 370 nm, 17d: 367 nm, 17e: 413 nm, and 17 f:
382 nm.
Information). The vibrational progressions derived from the ex-
perimental spectra are applied to broaden the vertical pure
electronic transitions derived from the TD-DFT calculations, as
seen in Figure 9. In the TD-DFT calculations, we considered dif-
ferent conformers by means of a generalised rotamer search
algorithm[30,31] with subsequent quantum chemical optimisa-
tion. We found the central absorptions between 350 and
500 nm of the conformers of any particular derivative 17 to
differ significantly only if R contained a phenyl-ring, yielding
two distinct conformers in each case: the two residuals point-
ing away from each other, that is, angular, and a face-to-face
configuration of the rings. Alkyl chains turned out to have only
a very weak impact on the resulting spectra, although the
actual conformer geometries differed greatly.
In the case of the dimethyl-amino-phenylene-substituted de-
rivative 17e, the broad absorption at approximately 430 nm is
reproduced basically by a single electronic transition for the
angular conformer and by two superposed, similarly intense
transitions for the face-to-face conformer. The low-energy ab-
sorption between 500 and 550 nm is again predicted to be too
low for both conformers, whereas the face-to-face conformer
shows significantly higher oscillator strength for this low-
energy absorption.
In the case of 17 f, the semiempirical description of the ab-
sorption spectra presented above failed because of the lack of
visible vibrational progression in the spectra. Furthermore, the
theoretical spectra of 17 f could not reproduce the experimen-
tal findings at all and, instead, gave results that were qualita-
tively similar to those obtained with 17e. This might be due to
aggregation effects mediated by the spatially small residuals in
the solution and will be subject to further investigation. It
should be noted that, given the results shown in Figures 4 and
Considering that the experimental absorption spectra of
17a and 17b are very similar (cf. Figure 8), we focus on the
spectral features of our TD-DFT results regarding 17b in the
following. As shown in Figure 9, the experimental spectrum
could be reproduced well by using a semiempirical approach;
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Figure 9. a) Broadened and shifted theoretical (monomer, dashed line) and experimental (solid line) UV/Vis absorption spectra of derivatives 17b, 17d and
17e. Transition densities are shown for the most prominent monomer. b) Scheme illustrating the orbital contributions to the lowest three transitions of the
three substances.
6, agreement between theory and experiment is expected to
improve if aggregates of derivates 17 are investigated.
Contrary to other dihydrotetraazaanthracenes presented
here, none of the nitro derivatives 17 measured showed fluo-
rescence. The changes in the absorption and emission behav-
iour compared with derivatives 15, 16 and 18 can possibly be
attributed to the presence of the nitro group. This functional
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group is well-known for its ability to act as a quencher of fluo-
rescence in many cases.[32]
The similar absorption behaviour of derivatives 19 could be
explained by examining their X-ray structure. We succeeded in
obtaining single crystals from 19a and its X-ray crystal struc-
ture is depicted in Figure 11. The central tetranuclear unit is
almost ideally planar, but the phenyl residues are twisted sig-
nificantly, with torsion angles of 57.68 (C2-N1-C13-C14) and
68.28 (C4-N2-C19-C24) out of the plane. As a result of this
twisting, there is no interaction between the p-electrons of the
imidazodihydroanthracene unit and the phenyl moieties.
Therefore, the chromophoric system does not seem
to be enlarged (in 19a) compared with the aliphatic
substituted derivatives (19b, 19c), which explains
the almost identical absorption behaviour of the
three derivatives.
Imidazodihydroanthracenes
In a first attempt to cyclise the tetramines 11 synthesised in
situ with 14, the yellow-fluorescent imidazodihydroanthracene
derivatives 19 were isolated instead of the desired octaazapen-
tacenes (Scheme 6). Most likely, the reason for the formation of
A different approach to yield even larger azaacenes
than discussed in the previous section was enabled
by our development of derivatives 17. Thus, we suc-
ceeded in synthesising derivatives of octaazapenta-
cene of type 22d, as illustrated in Scheme 7. We
were recently successful in reducing the nitro group
Scheme 6. Tetracyclic imidazoles 19 as unexpected cyclisation products (30–58%).
derivatives 19 is the oxidising capacity of the cyclisation part-
ner 14. Together with small amounts of ethanol, which was
used as a solvent, acetic acid could be formed in the course of
a redox reaction. In a subsequent cyclisation reaction, this reac-
tive acid was able to easily transform one diamine part of 11
to yield the 2-methylimidazole derivatives of type 19.
These novel imidazole derivatives 19 were isolated in the
form of intense yellow-orange solids. Their solubilities increase,
as expected, from the phenyl derivative 19a through 19b (cy-
clohexyl) to the long-chained 19c (n-hexyl). The UV/Vis spectra
reveal that the chemical nature of the R group does not exert
any influence on the chromophoric system (Figure 10). All de-
rivatives show a strong absorption band at 305 nm and
a second broad band in the range around 410 nm. Larger dif-
ferences arise from the fluorescence spectra: whereas deriva-
tive 19a shows a substantial emission at l=529 nm, that of
19c is bathochromically shifted by 32 nm.
Figure 11. View of 19a showing the atom-numbering scheme.[35]
in 17d by means of Pd on activated charcoal and hydrogen
gas, and obtained a green fluorescent diamine of type 20d.
This proved to be very sensitive to atmospheric oxygen and
was, therefore, cyclised in situ with 14. In our initial studies, we
isolated derivative 22d in yields of approximately 10% as
1
a deep-blue solid with gold metallic luster. H NMR spectra and
MS data indicate the presence of the expected derivative of
octaazapentacene. It is noteworthy that the tetrahydro deriva-
tive 21d formed primarily was immediately oxidised to the
mesoionic azaacene (Scheme 7). It was nearly impossible to
isolate the intermediate substance 21d; the red solution very
rapidly becomes blue with red emission (Scheme 7). Further-
1
more, we could not detect NÀH signals in the H NMR spec-
trum. Additionally, the pentacene obtained shows similar spec-
troscopic properties to that of the mesoionic hexaazapenta-
cene developed by Fleischhauer et al.,[33] particularly with re-
spect to the broad-structured absorption behaviour in the
orange to red region of the visible spectrum. However, the ab-
sorption is, perhaps due to the electron-withdrawing cyano
residues, slightly redshifted by approximately 21 nm compared
Figure 10. Normalised absorption (line) and emission (points) spectra of
compounds 19 in acetonitrile. lmax absorption/emission for 19a: 408/
529 nm, 19b: 409/539 nm, and 19c: 409/561 nm.
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hence indicating slow relaxation
from higher excited states to the
S1 state in 18, none of the nitro
derivatives of type 17 are fluo-
rescent. An unexpected result
was obtained by cyclisation of
tetramines 10, synthesised in
situ, with 14. Instead of tetrahy-
drooctaazapentacenes 21, the
yellow-fluorescent derivatives of
imidazodihydroanthracenes 19
were isolated and fully character-
ised. All three groups of dihydro-
tetraazaacenes presented here
possess the preconditions to
serve as building-blocks for
higher
azaacenes
(see
Scheme 8). The nitro group of
derivates 17 can be reduced to
another vicinal diamine, which,
in subsequent reactions, can be
cyclised with a wide range of
bis-electrophilic partners (e.g.,
Scheme 7. Using the building block for synthesis of higher azaacenes; for example, derivative 17d. Photographs
of compound 22d dissolved in CH2Cl2 in daylight (left) and under irradiation with l=365 nm (right).
with the hexaazapentacene described by Fleischhauer. In addi-
tion, the small Stokes shift of the red emission points to low
energy loss after excitation, which is typical for rigid systems
such as higher acenes.[34] The positions of residues in diamine
20d mean that the pentacene has to be mirror symmetric, as
depicted in derivative 21d. Currently, we cannot provide any
information concerning the electronic state; however, experi-
mental data (ESR measurements) suggest a singlet and thus
mesoionic state for molecule 22d. This is consistent with the
Scheme 8. Dihydroazaanthracenes as suitable building blocks for higher
azaacenes.
findings of other groups for hexaazaanthracenes[14b] and hexaa-
zaacridines.[14c]
To our knowledge, the synthesised pyrazinacene 22d repre-
sents the zwitterionic pentacene with the highest number of N
atoms discovered to date.
with 14 to the mesoionic octaazapentacenes 22). Additionally,
all our dihydroazaacenes contain a vicinal dicyano substruc-
ture, which is suitable for further condensation reactions with
diamines. Previously, this twofold extrusion of hydrogen cya-
nide formed the basis for the synthesis of a series of edge-
sharing condensed oligopyrazine analogues.[8] By using both
routes, we were already successful in the synthesis of higher
azaacenes, which are redox switchable systems with interest-
ing properties such as high photostability, large molar extinc-
tion coefficient and fluorescent quantum yield. We will report
in a forthcoming paper on the use of derivatives 17 as starting
material for the construction of derivatives of octaazapenta-
cene and their redox behaviour, as well as on other unsymmet-
rical azaacenes, using 16, and their application as functional
dyes.
Conclusion
Our attempt to develop novel dihydrotetraazaanthracenes 16
and 17 as new materials for use in organic electronic devices
has been realised by cyclisation reactions of N-substituted o-
phenylenediamines with 14 in moderate yields. Analogously, 1-
amino-2-phenylaminonaphthalene was successfully trans-
formed into dihydrotetraazatetracene 18. The presence of moi-
eties such as bromine groups or ester substructures means
that derivatives 16 provide opportunities to introduce further
functional groups through cross-coupling or transesterification
reactions. All new derivatives feature intense absorptions in
the yellow-orange region of the visible spectrum. Donor/ac-
ceptor substituents at the peripheric phenyl moieties affect, in
the case of derivatives 16, their UV/Vis spectra only slightly.
Whereas dihydrotetraazaanthracenes 16 and dihydrotetraaza-
tetracene 18 show dramatically different shifted fluorescence,
Experimental Section
Further details of the synthetic procedures, quantum chemical cal-
culations, NMR spectra and single-crystal X-ray data can be found
in the Supporting Information.
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We thank Professor Todd Martꢂnez and Julia Preiß for scientific
exchange and support. Additionally, we thank Dr. Gꢃnther, G.
Sentis and H. Heinecke from HKI Jena for measuring the NMR
spectra, Mrs Schçnau and Mrs Heineck for the measurement of
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nancial support from the “Bundesministerium fꢃr Bildung und
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Published online on && &&, 0000
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FULL PAPER
&
Sensitizers
D. M. Gampe, M. Kaufmann, D. Jakobi,
T. Sachse, M. Presselt, R. Beckert,* H. Gçrls
&& – &&
Stable and Easily Accessible
Functional Dyes:
Dihydrotetraazaanthracenes as
Versatile Precursors for Higher Acenes
Dihydroazaacenes: A series of dihydro-
tetraazaanthracenes and one new dihy-
drotetraazatetracene as substances for
applications in organoelectronic devices
and as suitable building blocks for
higher azaacenes was synthesised.
Single-crystal X-ray analyses were car-
ried out to examine the morphology
and solid packing effects. Finally, a dihy-
drotetraazaanthracene was used as
a building-block to create a mesoionic
octaazapentacene.
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Chem. Eur. J. 2015, 21, 1 – 12
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ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!