A way to stable, highly emissive fluorubine dyes: Tuning the electronic properties of azaderivatives of pentacene by introducing substituted pyrazines

Article abstract of DOI:10.1002/chem.201103350

Pentacene and its derivatives are among the most important examples of π-electron-rich molecules used in organic field effect transistors. The replacement of CH groups by nitrogen atoms opens an elegant way to generate highly electron-deficient molecules, known as oligoazaacenes. We describe the synthesis and spectroscopic properties of two novel derivatives of this family, namely the zwitterionic and quinoidal conjugated forms of dihydro-5,6,7,12,13, 14-hexaazapentacene (fluorubine). We outline a powerful strategy to tune the electronic properties of these redox-active azaacenes by the selective introduction of substituted pyrazines. Their acidochromic and solvatochromic behaviour is investigated experimentally and interpreted with the help of theoretical calculations. The simple "exchange" of substituents or protonation is shown to significantly alter the spectroscopic and electronic properties of these remarkably stable π-systems. Their exceptional optical properties, such as high fluorescence quantum yields combined with a redox-active behaviour, make them promising candidates for sensor materials. Additional marked features in the solid state, such as herringbone packing in combination with short π-π distances, will open access to electronic materials.

Full text of DOI:10.1002/chem.201103350

FULL PAPER
DOI: 10.1002/chem.201103350
A Way to Stable, Highly Emissive Fluorubine Dyes: Tuning the Electronic
Properties of Azaderivatives of Pentacene by Introducing Substituted
Pyrazines
Jan Fleischhauer,*[a] Stefan Zahn,[b] Rainer Beckert,*[c] Ulrich-Walter Grummt,[d]
Eckhard Birckner,[d] and Helmar Gçrls[e]
One life ends another begins—In memoriam to Nelly Jahn 18.07.1997–29.09.2011—You will be forever in our hearts
Abstract: Pentacene and its derivatives
are among the most important exam-
ples of p-electron-rich molecules used
in organic field effect transistors. The
replacement of CH groups by nitrogen
atoms opens an elegant way to gener-
ate highly electron-deficient molecules,
known as oligoazaacenes. We describe
the synthesis and spectroscopic proper-
ties of two novel derivatives of this
family, namely the zwitterionic and qui-
noidal conjugated forms of dihydro-
5,6,7,12,13,14-hexaazapentacene (fluo-
rubine). We outline a powerful strategy
to tune the electronic properties of
these redox-active azaacenes by the se-
lective introduction of substituted pyra-
zines. Their acidochromic and solvato-
chromic behaviour is investigated ex-
perimentally and interpreted with the
help of theoretical calculations. The
simple “exchange” of substituents or
protonation is shown to significantly
alter the spectroscopic and electronic
properties of these remarkably stable
p-systems. Their exceptional optical
properties, such as high fluorescence
quantum
yields
combined
with
a redox-active behaviour, make them
promising candidates for sensor materi-
als. Additional marked features in the
solid state, such as herringbone packing
in combination with short p–p distan-
ces, will open access to electronic mate-
rials.
Keywords: azo compounds · densi-
ty functional calculations · dyes/pig-
ments · heterocycles · hexaazapen-
tacenes
Introduction
The field of molecular organic materials has gained signifi-
cant importance in recent decades. Organic materials have
been successfully applied as the active components in photo-
voltaic devices, molecular diagnostics, photodynamic therapy
(PDT), optical data storage, organic light-emitting diodes
(OLEDs) and organic field effect transistors (OFETs).[1]
Pentacene and its derivatives have been established as
benchmark p-electron-rich materials, and are particularly
important in OFETs as they display high hole mobility in
the solid state.[1a,2] Nitrogen substitution in the pentacene
scaffold has been proposed as a potentially fruitful route to
build up n-type molecules, and so oligoazaacenes have
become the subject of increased interest in theoretical
chemistry and material science.[2,3] More recently, excellent
reviews have been published covering historical aspects and
modern developments of azaacens.[2] Here, we demonstrate
how the molecular properties of compounds 1–5 can be
tuned by altering their nitrogen substitution pattern by
fixing the different hydroforms by non-hydrogen substitu-
ents, and further refining by protonation or alkylation.
[a] Dr. J. Fleischhauer
Ruprecht-Karls-University Heidelberg
Department of Chemistry
Im Neuenheimer Feld 270
69120 Heidelberg (Germany)
Fax : (+49)6221-548404
E-mail: j.fleischhauer@oci.uni-heidelberg.de
[b] Dr. S. Zahn
Monash University, School of Chemistry
Clayton, Victoria 3800 (Australia)
[c] Prof. Dr. R. Beckert
Friedrich Schiller-University Jena
Department of Organic and Macromolecular Chemistry
Humboldtstr. 10, 07743 Jena (Germany)
E-mail: Rainer.Beckert@uni-jena.de
[d] Prof. Dr. U.-W. Grummt, Dr. E. Birckner
Friedrich Schiller-University Jena
Department of Physical Chemistry
Lessingstr. 10, 07743 Jena (Germany)
[e] Dr. H. Gçrls
Friedrich Schiller-University Jena
Department of Inorganic Chemistry
Lessingstr. 8, 07743 Jena (Germany)
It has been proven that the replacement of two CH
groups with nitrogen atoms gives p-type materials, such as
5,14-dihydrodiazapentacene (5,14-DDP)[4a], whereas higher
substitution levels can give rise up to n-type conductors, as
exemplified by 5,7,12,14-tetraaza-6,13-pentacenequinone
Supporting information for this article (details of synthetic proce-
dures, DFT calculations including Cartesian coordinates of the calcu-
lated structures, X-ray structural analysis, CV and UV/VIS measure-
ments) is available on the WWW under http://dx.doi.org/10.1002/
chem.201103350.
Chem. Eur. J. 2012, 18, 4549 – 4557
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hexaazapentacenium ions 2 with variable substituents at po-
sitions 5, 7 and 13 (Scheme 1).[5g,j] In this paper, we will ad-
dress the synthesis and spectroscopic properties of two other
novel derivatives of the hexaazapentacene family, namely
the zwitterionic and quinoidal fluorubine derivatives 3 and 4
(Scheme 1, Scheme 2). To the best of our knowledge, these
(TAPQ).[4b] Additionally, TAPQ possesses high electron mo-
bility due to the combination of its quadruple CH···N/O hy-
drogen bonds and strong p–p-interactions.[4b] Theoretical
calculations dealing with modifications of the pentacene
scaffold predict that azaacenes containing higher levels of
azasubstitution will be promising n-type materials with suita-
ble charge mobility and electron affinities.[3b] Studies of
larger oligoazaacenes have also concluded that molecules
with separated dihydropyrazine moieties will be more stable
than derivatives with adjacent pyrazine rings.[3g] From this
point of view the fluorubine derivatives presented here fulfil
all the requirements to be stable, n-type organic–electronic
materials.
Fluorubine derivatives (dihydro-5,6,7,12,13,14-hexaaza-
pentacenes) are a class of azaacenes that contain three di-
rectly annulated pyrazine rings within the core of the penta-
cene scaffold. Historically, 6,13-dihydrohexaazapentacene
(1) (Scheme 1, R=H) was discovered as a condensation
Scheme 2. Synthesis of regioisomeric hexaazapentacenes 3a and 4a.
compounds have not yet been reported. We demonstrate
that the synthesis of 3a and 4a (Scheme 1, R=Ph) can be
achieved by condensation of N-phenyl-o-phenylenediamine
(8) with suitable electrophiles (Scheme 2), and we compare
the optoelectronic properties of these derivatives with
ꢀ
benchmark acenium salt 2 (R=Ph, R’=Me, X=BF4 ).
Results and Discussion
Synthesis and structural characterisation: It has been previ-
ously demonstrated that ipso substitution of both chlorine
atoms of 2,3-dichloro-5,6-dicyanopyrazine allows the forma-
tion of redox-active dodecaazahexacenes.[6] Additionally tet-
raaryl-substituted pyrazinopyrazines have been shown to
form highly functionalised octaazahexacenes as a result of
double ortho-annulation reaction.[6] Because pyrazine forms
the central core of hexaazapentacenes of type 3 and 4, tetra-
chloropyrazine (7) seemed to be a promising candidate for
their synthesis (Scheme 2). Thus, the fourfold substitution of
chlorine with two equivalents of N-phenyl-o-phenylenedia-
mine (8) should yield both regioisomeric hydroforms 3aꢁ
RED and 4aꢁRED that will be oxidised to give the desired
azaacenes. The synthesis of compound 7 has been described
as a high-yielding procedure, attained by the treatment of
2,5-dioxopiperazine (6) with chlorine and phosphorus penta-
chloride in POCl3 as the solvent.[7a] In our hands, all at-
tempts to realise this method failed and we only isolated
a fully chlorinated hexachloro-dihydropyrazine (see the Sup-
porting Information). Treating compound 6 with a mixture
Scheme 1. Structures and labelling of investigated dihydro-5,6,7,12,13,14-
hexaazapentacenes 1–5.
product of dichloroquinoxaline and diaminoquinoxaline in
1903 by Hinsberg and Schwantes,[5a] and several patents
claim its derivatives for applications as dyes.[5b–d] Besides the
established industrial procedures, there are few approaches
dealing with the synthesis of functionalised fluorubine deriv-
atives.[5e–i] Consequently, modular approaches to obtain se-
lectively functionalised, electron-deficient heterocycles are
still relatively rare and often demand novel synthetic con-
cepts.[3,6] We tackled this challenge and developed new strat-
egies, for instance for the synthesis of the highly fluorescent
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Stable, Highly Emissive Fluorubine Dyes
FULL PAPER
of PCl5/POCl3 allowed the formation and isolation of tetra-
chloropyrazine (7).[7b]
In order to assist chlorine displacement, collidine was
found to act as both a high-boiling-point solvent and non-
nucleophilic base. Initially, the 1:1 condensation product,
2,3-dichloro-5-phenyl-5,10-dihydropyrazinoACTHUNRTGNEUNG[2,3-b]quinoxa-
line (9), was observed as the main product when the reac-
tion was maintained at 120–1308C for 2–3 days; and could
be isolated in up to 50% yield through precipitation by
using dichloromethane/ethanol. Subsequent investigation
has shown that intermediates 7 and 9 can act as building
blocks for the syntheses of azaacenes 3a and 4a. Increasing
the reaction temperature and extending the reaction time
(1408C, 5 days) with compound 7, allowed us to isolate the
thermodynamically preferred isomer 4a as the main product
(20%). Unfortunately, the mesoionic derivative 3a was only
isolable in traces (3%). The yields of both isolated com-
pounds are relatively low, which seems to be a more general
drawback of this type of condensation reactions. Bunz et al.
have investigated the use of Hartwig–Buchwald protocols to
generate tetraazapentacenes in high yields.[8] An extension
of this chemistry may provide an attractive alternative to
the condensation reactions presented here. First attempts to
apply this strategy to systems of type 7 were not successful
and modified cyclisation procedures are currently being in-
vestigated in our laboratories.
The poor solubility of model compounds 3a and 4a hin-
ders their purification and spectroscopic characterisation.[9]
In spite of this, we confirmed the structures of 3a and 4a by
means of MS, HRMS and NMR measurements, and single
X-ray analysis of suitable crystals (see the Supporting Infor-
mation). From chloroform/methanol mixtures we were able
to obtain crystals of sufficient quality and confirmed the mo-
lecular structures of compounds 3a and 4a in the form of
their chloroform adducts (Figures 1 and 2). In case of 3a the
phenyl substituents are nearly perpendicular to the slightly
twisted hexaazapentacene plane (the torsion angle is ꢁ3.88)
and in excellent agreement with the results of DFT structure
optimisations (level of theory TZVP/PBE(RI); see also
Computational Details in the Supporting Information). Due
to strong p-interactions, the mesoionic derivative 3a forms
face-to-face stacked dimers (dꢁ3.387 ꢂ), which themselves
build highly symmetric molecular arrangements in the solid
state (Figure 1).
Figure 1. ORTEP plot of the molecular structure (top) and the solid-state
packing (bottom) of the mesoionic fluorubine 3a (50% probability ellip-
soids).[10] The co-crystallised chloroform molecules are omitted for clari-
ty.
The solid-state derivative 4a, which co-crystallises with two
molecules of chloroform, adopts a herringbone structure in
one dimension and a columnar face-to-face packing in the
second dimension—similar to rubrene—which is one of the
prerequisites for efficient multidimensional charge trans-
port.[1l] Thereby, the molecular distances are approximately
2.78 ꢂ for the edge-to-face separation and approximately
3.359 ꢂ between molecules that were stacked in parallel
(Figure 2). The p—p-stacking distances in dimers of 3a are
slightly larger than those obtained by Ariga et al. for aza-
pentacene derivative 5 (dꢁ3.197 ꢂ),[5h] but comparable to
those of TAPQ (dꢁ3.32 ꢂ).[4b]
ꢀ
ꢀ
Shortening of the bond lengths C7 N2 (1.326(3) ꢂ), C8
ꢀ
ꢀ
N4 (1.323(2) ꢂ), C16 N6 (1.327(3) ꢂ) as well as C15 N6
(1.330(2) ꢂ) points to partial charge localisation at the cen-
tral pyrazine core (Figure 1). On the other hand, compound
4a, though calculations predict it to be 31.3 kJmolꢀ1 more
stable than compound 3a, clearly shows the quinoidal struc-
Spectroscopic and electronical properties: In this section, we
will illustrate the influence of different pathways of conjuga-
tion of the azaacene scaffold on the spectroscopic and elec-
tronic properties. Hexaazapentacenes 3a and 4a exhibit re-
markable spectroscopic properties, which are significantly
influenced by the position of the phenyl substituents. Be-
cause the electronic structure of the fluorubinium ion 2a
ꢀ
ꢀ
ture by a shortening of the C7 N2 (1.310(5) ꢂ) and C8
ꢀ
N3a (1.308(4) ꢂ) bonds and a slight elongation of the C7
ꢀ
N3 (1.371(5) ꢂ) and C8 N1 (1.372(4) ꢂ) bonds, which is
also in excellent agreement with the calculated bond param-
eters of the azaacene framework (Figure 2). contrast to 3a
the scaffold of 4a is completely planar, whereby the phenyl
substituents are twisted out of the azaacene plane by 718.
ꢀ
(R=Ph, R’=Me, X=BF4 , Scheme 1)[5j] resembles the for-
mally methylated derivative of 3a, we use it as benchmark.
The photophysical properties of compounds 2a–4a are sum-
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4551

J. Fleischhauer, R. Beckert et al.
marised in Table 1. In dichloromethane, the purple solution
of the zwitterionic derivative 3a exhibits a broad structured
p!p* transition between l=500 and 610 nm with moderate
to high extinction coefficients, where the vibronic coupling
can be related to an in-plane skeletal mode of the aromatic
ring system (Figure 3 top, n˜ ꢁ1320 cmꢀ1). Typical for zwitter-
ionic compounds, compound 3a shows a pronounced nega-
tive solvatochromic behaviour (see the Supporting Informa-
tion). The peak absorption (lmaxabs) of compound 3a shifts
hypsochromically from l=630 to 588 nm by going from tol-
uene to methanol. By comparing to previous structure–prop-
erty-relationship elucidations of related mesoionic 5,7,12,14-
tetraazapentacenes, these spectra are hypsochromically
shifted about 160 nm, which is remarkable.[11a] As indicated
by the solvent-depended Stokes shift (DnSt), and further sup-
ported by DFT calculations (def-SV(P)/BP86), the chromo-
phore 3a exhibits a small change in its dipole moment (m)
going from its ground (7.90 D) to first exited state (5.80 D).
Decreasing the dipole moment during excitation results in
an Franck-Condom exited state that is formed in a more
strained surrounding solvent cage of oriented dipole mole-
cules. Thus increased solvent polarity energetically stabilises
the ground state more than the excited state and leads to
pronounced hypsochromically shifted absorption spectra.[12]
In case of the fluorubinium ion 2a a less-pronounced nega-
tive solvatochromic behaviour combined with small Stokes
shifts was observed,[5j] which can be attributed to a decreased
change of its dipole moment by going from the ground to
the first exited state.
As described here, the apparently simple “exchange” of
a phenyl substituent from position 7 to 12 significantly alters
the optical properties of the hexaazapentacenes 3a and 4a.
The yellow solution of the quinoidal derivative 4a exhibits
a well-structured p!p* transition between l=400 and
520 nm showing vibronic couplings (Figure 3 bottom, n˜
ꢁ1390 cmꢀ1) and high extinction coefficients. In contrast to
compounds 2a und 3a, the UV/VIS absorption of 4a shows
no solvatochromic behaviour as the dipole moments of the
ground and excited state of 4a are almost identical,
Figure 2. ORTEP plot of the solid-state structure (top) and the solid-
state packing (bottom) of the quinoidal fluorubine 4a (50% probability
ellipsoids).[10] The co-crystallised chloroform molecules are omitted for
clarity.
resulting in a negligible Stokes shift of 110 cmꢀ1.
Table 1. Photophysical data of selected hexaazapentacenes in DCM, 36% HCl and
acetic acid.
Within experimental error, in CH2Cl2 compound 4a
exhibits a very high fluorescence quantum yield
(ff), which is comparable with Rhodamine 6G and
the hexaazapentacenium salt 2a.[5j] The mesoionic
dye 3a exhibits a slightly smaller ff of 0.58 that is
orders of magnitude higher than that of related de-
rivatives.[11a]
abs
em
[a]
[c]
Solvent
lmax
lmax
DnSt
[cmꢀ1
ff[b] t[c]
[ns]
kf[c]
[nsꢀ1
knr
[nsꢀ1
e[a]
[mꢀ1 cmꢀ1
]
[nm] [nm]
G
]
E
]
U
]
G
2a
CH2Cl2
AcOH
HCl (36%) 540
CH2Cl2
AcOH
535
525
546
543
557
642
577
596
516
569
595
380
630
570
840
1720
370
110
670
380
0.95 4.32 0.22
0.95 4.47 0.21
0.17 1.0 0.17
0.58 7.20 0.081
0.72 4.14 0.17
0.31 2.16 0.14
0.012 80000
0.011 67700
[d]
2Ha
3a
3Ha
0.83
n.d.
609
525
0.058 48600
0.068 67700
The UV/VIS spectra acquired in glacial acetic
acid differ significantly from those obtained in
CH2Cl2, whereby the absorption of 3a is blue-shift-
ed by 84 nm. Furthermore, the absorption and emis-
sion transitions are more structured and have in-
creased extinction coefficients (e) as well as ff
values. The observed longest wavelength transition
does not match the expected relationship to the sol-
vent polarity, but the spectroscopic behaviour is
similar to that of 2a, pointing to a monoprotonated
3HHa HCl (36%) 583
4a
4Ha
0.32
n.d.
CH2Cl2
AcOH
513
548
1
3.27 0.31
<0.004 98800
0.086 57400
0.66 3.94 0.17
0.24 1.70 0.14
4HHa HCl (36%) 582
0.45
n.d.
[a] Determined for the longest wavelength transition. [b] The fluorescence quantum
yields ff (error ꢂ10%) were calculated according to the literature relative to rhoda-
mine 6G in ethanol used as a standard (ff =95%).[13] The absorbance at the excitation
wavelength was kept below 0.05 for the samples and the standard. [c] Details for pho-
tophysical determination of fluorescence life times (t), rates of radiative decay (kf)
and rates of non-radiative decay (knr) are listed in the Supporting Information. [d] Not
determined.
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Stable, Highly Emissive Fluorubine Dyes
FULL PAPER
relative to CH2Cl2, which indi-
cates the presence of the
double-protonated
species
3HHa. The relative quantum
yield ff of the species is further
decreased to 31%. In hydro-
chloric acid the positively
charged
hexaazapentacenium
ion 2a will be additionally pro-
tonated to furnish 2Ha. This
protonated derivative repre-
sents
a formally mixed ana-
logue to 3HHb, but is energeti-
cally less preferred as compared
to 3HHa (for details see the
Supporting Information). In
contrast to 3a, monoprotona-
tion of compound 4a in aque-
ous acetic acid leads to a broad-
ened and bathochromically
shifted absorption spectrum
combined with lower e and ff
values. Double protonation of
3a and 4a in hydrochloric acid
is supported by increased solu-
bility in aqueous media, and
a shift in colour to dark purple.
In case of the double-protonat-
ed azaacenes 3HHa and 4HHa
the UV/VIS spectra are almost
superimposable
and
show
a structured absorption at ap-
proximately 582 nm (Figure 3).
For both isomers, this observa-
tion can be explained by one of
Dꢃhnes colour rules, which
states that non-alternating poly-
methines exhibit particularly
bathochomic absorption.[14] Ob-
viously, successive protonation
of 3a and 4a leads to non-alter-
nating polymethine chromo-
Figure 3. Normalised absorption spectra of the longest wavelength transitions of the neutral (left), mono
(middle) and double-protonated (right) fluorubines 3a (top) and 4a (bottom) in their most stable tautomeric
forms as indicated by DFT calculations (TZVP/PBE(RI); c=CH2Cl2, a=AcOH, g=HCL (36%)).
species, such as those of 3Ha. This species was calculated to
be the most favoured single-protonated species (Figure 3). It
is worth noting this structural similarity, because 2a repre-
sents the formal alkylation product of 3a. By going from di-
chloromethane to acetic acid the absorption and emission
spectra of benchmark 2a, which is not protonated under
these conditions, are shifted hypsochromically and in agree-
ment with previously observed negative solvatochromism of
azaaceniumsalts of type 2.[5j] In comparison to 2a, the de-
creased fluorescence quantum yield of 3a can be attributed
to the protonated nitrogen atom that may open non-radia-
tive deactivation pathways. In hydrochloric acid, that is
under more acidic conditions, a more structured absorption
spectrum of compound 3a was observed, bathochromically
shifted compared to AcOH (Ac=acyl) though blue-shifted
phores. The fluorescence life times (t) of compounds 2a–4a
typically ranges from 2 to 4 ns, whereby we observed a mon-
oexponential decay and good correlation between the rates
of radiative deactivation (kf) and the Strickler–Berg equa-
tion (see the Supporting Information). In all investigated
solvents, the values of the radiative decay of compounds
2a–4a are in the same range, within the experimental error.
On the other hand t and ff decreases significantly by going
from CH2Cl2 or acetic acid to hydrochloric acid, whereby
the rates of non-radiative decay (knr) increase by a factor of
five, pointing to a strong solvent effect in case of hydrochlo-
ric acid.
Gas-phase DFT calculations were initially completed for
all the competing neutral and protonated species, with the
substituents labelled R in Scheme 1 fixed as hydrogen, by
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4553

J. Fleischhauer, R. Beckert et al.
using the Gaussian 03[15] program on the theoretical levels of
6-31+G/B3LYP and 6-31+G*/B3LYP. Starting from these
optimised structures the longest wavelength transitions were
estimated (see the Supporting Information). Because hydro-
gen substitution offers only a simplified means to compare
the different hydroforms, the phenyl-substituted derivatives
were additionally modelled. The calculated longest wave-
length transitions (lmaxabs) and oscillator strengths (f) of 3a
and 4a (and of their hypothetical reference structures, in
which the aromatic substituents have been replaced by hy-
drogen) are in acceptable agreement to the experimental re-
sults and are listed in the Supporting Information. The pho-
tophysical properties of compounds 3a and 4a, particularly
their high extinction coefficients and fluorescence quantum
yields, make these scaffolds ideal for designing new chromo-
phores for chemical biology or molecular diagnostics. In
case of 3a these properties go hand in hand with a pro-
nounced solvatochromic behaviour, making it a strong can-
didate for novel multi-parameter chromophores, which are
used to probe and correlate different parameters at the
same time, for example, location of participation and polari-
ty of the surrounding microenvironment or cell dynamics.[16]
structure is almost identical to those of the cationic fluoru-
bine derivative 2a, which explains their similar spectroscopic
behaviour. However, the energy differences between each
set of single-fold protonated isomers of 3a and 4a are rela-
tively small, so it is assumed that both compounds exist in
a dynamic equilibrium within their different protonated
forms (see the Supporting Information). The models also in-
dicate that double protonation of 3a and 4a occurs predom-
inantly at the peripheral pyrazine rings (B and D ring),
which may account for the similar optical properties of
3HHa and 4HHa.
The improved solubility mediated by protonation or alky-
lation is further evidence for the presence of strong p–p-in-
teractions, because introduction of charged nitrogen atoms
leads to increased repulsion forces between the molecules.
As mentioned above, with strong acids the hexaazapenta-
cenes 3a and 4a were protonated twice, resulting in water
solubility as well as changes to the optical properties. These
protonations are reversible, so that addition of base regener-
ates 3a and 4a. In case of azaacene 4a we were able to
obtain single crystals from hydrochloric acid, suitable for X-
ray structural analysis, which supports the results obtained
by DFT calculations (see the Supporting Information). Even
in the protonated state, the hexaazapentacene plane of
4HHa tends to form dimers through p–p interactions. It is
clearly observable that the quinoidal substructure is still pre-
Prototropism: As can be observed from the photophysical
data, the electronical properties of the dihydrohexaazapen-
tacenes are strongly dependent on the substitution pattern
of the central pyrazine core. This impressive feature is limit-
ed to non-hydrogen substituents at the attended pyrazine
moieties, leading to the fixing of benzoid (1), quinoidal (4)
and zwitterionic (3) structures. As a result of gas-phase cal-
culations with the TURBOMOLE[17] program suite employ-
ing the TZVP/PBE(RI) method, it was found that in com-
parison to 3a that was used as reference system (0 kJmolꢀ1),
ꢀ
served, with the double bond lengths of C7 N2 (1.321(2) ꢂ)
ꢀ
and C8 N3a (1.323(2) ꢂ), being slightly elongated com-
pared to their equivalents in the uncharged derivative 4a.
Electronical properties: Detailed structural analysis by using
the DFT approach on the level of TZVP/PBE(RI) offered
insights into the chromophore and particularly its charge de-
localisation and aromaticity (see Computational Details in
the Supporting Information). Figure 4 depicts the electro-
the
benzoid
conjugated
derivative
1a
(R=Ph,
ꢀ63.1 kJmolꢀ1) is energetically preferred followed by com-
pounds 4a (ꢀ31.1 kJmolꢀ1) and
5a (ꢀ29.4 kJmolꢀ1). It is
worthy to note, that up to now
there are only few simplified
theoretical studies that consider
the influence of this range of
conjugation pathways on the
electronic properties of related
hexaazaacene systems.[5l–m] The
various possible mono- and
double-protonated regioisomers
of compounds 3a and 4a were
subsequently investigated, and
the energetically most favoured
Figure 4. Electrostatic potentials of azaacenes 2a–4a as well as their mono- and double-protonated counter-
parts at the isodensity surface with 0.05e/bohr3 (TZVP/PBE(RI))
forms (3Ha, 4Ha, 3HHa,
4HHa)
are
included
in
Figure 3.
In case of derivative 3a the first protonation at the central
pyrazine moiety (C ring Scheme 1 and Figure 3) is energeti-
cally preferred, whereas the peripheral pyrazine moiety (B
ring) is the favoured initial protonation site of the quinoidal
hexaazapentacene 4a. In the case of 3Ha the optimised
static potential surfaces of the most important compounds.
In general, the non-substituted nitrogen atoms carry the ma-
jority of the electron density, whereas the positive charge is
localised at the hydrogen atoms. Successive protonation of
the azaacene core leads to increased charge at the carbon
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Chem. Eur. J. 2012, 18, 4549 – 4557

Stable, Highly Emissive Fluorubine Dyes
FULL PAPER
atoms of the central pyrazine ring. From there, it spreads to
the two other pyrazine moieties and finally to the annulated
benzene moieties. The Hirschfeld partial charge analysis[18]
underlines this finding and shows the similarity between
3Ha and 2a. An additional Wiberg bond order analysis[19]
and calculation of the nucleus-independent chemical shifts
(NICS)[20] by using benzene and pyrazine as references, con-
firmed that the pyrazine nitrogen substituents significantly
influence the electronic conjugation of the azaacenes. The
NICS values of the peripheral benzene moieties are similar
to the reference system and approach benzene when proton-
ated. Additionally, the NICS values of the benzene moieties
of the benzoid conjugated derivatives 1a and 5a (R=Ph, A
and E rings) are more benzene-like than those of 3a and
4a. It should be noted that the NICS values of the aryl-sub-
stituted pyrazine moieties differ significantly from those of
their unfunctionalised annulated analogues and the ideal
theoretical value of pyrazine itself. The same trend was ob-
served by inspection of the deviations of Wiberg bond order
analysis for which the aryl-functionalised rings show the
strongest discrepancies to the aromatic reference as well.
The corresponding frontier orbitals are fully delocalised
over the whole molecule. By inspecting the values of the
HOMO/LUMO orbitals it becomes clear that selective func-
tionalisation of the pyrazine core, that is, by altering the pe-
ripheral substituents or by subsequent protonation or alkyla-
tion, represents a powerful tool to tune the electronic prop-
erties of the hexaazapentacenes presented here. In this
study, auxochromic groups such as alkoxy substituents have
not been included. Such groups will alter the absorption and
emission wavelengths and give rise access to large regions of
the UV and visible spectrum.
In case of 3a and 4a the calculated energy levels of the
LUMO orbitals match very well to those obtained by cyclic
voltammetric (CV) measurements, although the energies of
the HOMO orbitals were systematically overestimated (see
the Supporting Information). Unfortunately, in case of the
charged derivatives (i.e., 2a, 3Ha, 3HHa, 4Ha and 4HHa)
the calculated values of the frontier orbitals are less satisfy-
ing and can be only regarded as rough estimation. Not that
surprisingly, it was also observed that the values of the cor-
responding orbitals strongly depend on the applied level of
theory. More accurate HOMO energy levels should be avail-
able by using Hartree–Fock (HF) calculations for which the
RIJ-COSX approximation was employed (see the Computa-
tional Details in the Supporting Information), but no signifi-
cant improvement was observed for our systems. Because
the malfunction of DFT for orbital energies is well known[21]
and solvation might also influence the orbital energy, we use
these calculations only to illustrate trends. There already
exist refined approaches such as AIMD calculations that
consider aspects such as solvation, but here we are limited
due to computational capacities. It is worth noting, that the
measured LUMO energy levels (electron affinities) of 3a
and 4a already reach the required values (ꢁ3 eV) to allow
efficient electron injection from metal electrodes, making
theses materials strong candidates for n-type OFET devi-
ces.[3b] On the other hand, the band-gap energies measured
by CV are in excellent agreement to those obtained by opti-
cal methods. Figure 5 summarises the results of DFT investi-
gations and is illustrating how in principle the energies of
frontier orbitals can be modified by the factors discussed
above.
Figure 5. Calculated energy of the frontier orbitals of neutral and charged
azaacenes 1a–5a, showing the dependence of the energies on the substi-
tution of the chromophore core (method: TZVP/PBE(RI)).
These chromophores can be considered as formal anti-ar-
omatic and stable 24 p-electron systems. The reduced (3aꢁ
RED/4aꢁRED, 26 p electrons) and oxidised (3aꢁOX/4aꢁ
OX, 22 p electrons) species represent formal 4n+2 aromatic
systems, though they are energetically disfavoured compared
to their formal 4n anti-aromatic counterparts 3a and 4a.
These findings are in agreement to those of related systems,
where it was found that the formal anti-aromatic oligoazaa-
cenes become energetically more favourable by expanding
the p-system.[2d,3e] We wish to note that clear evidence for
the leuco-dye form of compounds 3a and 4a is observable,
by both CV measurements and through chemical reduction.
Reducing the systems in aqueous THF solutions with
sodium dithionite leads to hypsochromically shifted absorp-
tion and emission bands of the leuco-dye forms (3aꢁRED
lmax =466, lem =546 nm), which are quickly re-oxidised
when exposed to air. Hexaazapentacenes 3a and 4a also ex-
hibit complex cyclic voltammograms with two irreversible
oxidation peaks and two quasi-reversible reduction peaks
(CV in CH2Cl2 on glassy carbon as counter and working
electrode and with glassy carbon//Ag/AgCl as the reference
electrode; see the Supporting Information) and similar com-
plex CV spectra have been previously observed for other di-
hydroderivatives of azaacenes.[5h] More detailed studies of
redox activity are beyond the scope of this paper, because
different prototropic and hydroforms may contribute in
a complex way[5h,j] and will be described in subsequent
work.
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4555

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Conclusion
We introduced tetrachloropyrazine as a building block for
the synthesis of oligoazaacenes and obtained two novel fluo-
rubine derivatives. These regioisomeric derivatives were
characterised by NMR spectroscopy, MS, X-ray structural
analysis and UV/VIS measurements as well as DFT meth-
ods. Strong packing effects were observed in the solid struc-
ture due to marked p–p-interactions, which were proven by
X-ray structural analysis and ab-initio calculations of model
compounds 3a and 4a. We discussed their exceptional opti-
cal properties, such as their high fluorescence quantum
yields, which are comparable to those of rhodamine 6G, as
well as their strong solvatochromic and acidochromic behav-
iour. We have demonstrated the tuning of the electronic
properties of azaacenes by selective introduction of substi-
tuted pyrazines. These chromophores are promising candi-
dates for functional materials, and particularly for electro-
optical devices, because of their pronounced p-interactions
and redox activity. In forthcoming publications we will
detail improved synthetic procedures that allow us to intro-
duce functionalities in a selective manner, and we will
report on their ability to act as n-type conducting materials
for OFET devices.
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Acknowledgements
We would like to thank E. Kielmann for measuring photophysical data,
DFG for founding of postdoctoral fellowship (FL720/1-1), and H. A. Col-
lins and J. K. Sprafke for language polishing.
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