From byproducts to efficient fluorophores: A route to the synthesis of fluorubines

Article abstract of DOI:10.1002/chem.200901961

The reaction of bis(imidoyl) chlorides of oxalic acid with monoalkyl hydrazines leads to substituted ?²-l,2diazetines, which are versatile building blocks for ring-transformation reactions. One remarkable product originating from side reactions featured by a strong orange/red fluorescence was confirmed as a novel fluorubine derivative. In continuing our studies to substituted oligoazaacenes, we developed several synthetic entries to build up novel fluorubine derivatives, in which particularly aminobridged bis(quinoxaline)s are the key products for cationic hexaazapentacenes. We would like to discuss the possible formation pathways of these fluorubine derivatives, which exhibit interesting photophysical and chemical properties. The structures of all new derivatives were confirmed by common analytical methods (NMR spectroscopy, CV, U V/Vis, mass spectrometry, elemental analysis, and X-ray structural analysis) and will be discussed on selected examples in more detail.

Full text of DOI:10.1002/chem.200901961

FULL PAPER
DOI: 10.1002/chem.200901961
From Byproducts to Efficient Fluorophores:
A Route to the Synthesis of Fluorubines
Jan Fleischhauer,[b] Rainer Beckert,*[a] Yvonne Jꢀttke,[a] David Hornig,[a]
Wolfgang Gꢀnther,[a] Eckhard Birckner,[c] Ulrich-W. Grummt,[c] and Helmar Gçrls[d]
Abstract: The reaction of bis
(imidoyl)
novel fluorubine derivatives, in which
particularly aminobridged bis(quinoxa-
line)s are the key products for cationic
hexaazapentacenes. We would like to
discuss the possible formation path-
ways of these fluorubine derivatives,
which exhibit interesting photophysical
and chemical properties. The structures
of all new derivatives were confirmed
by common analytical methods (NMR
spectroscopy, CV, UV/Vis, mass spec-
trometry, elemental analysis, and X-ray
structural analysis) and will be dis-
cussed on selected examples in more
detail.
chlorides of oxalic acid with monoalkyl
hydrazines leads to substituted D2-1,2-
diazetines, which are versatile building
blocks for ring-transformation reac-
tions. One remarkable product origi-
nating from side reactions featured by
a strong orange/red fluorescence was
confirmed as a novel fluorubine deriva-
tive. In continuing our studies to substi-
tuted oligoazaacenes, we developed
several synthetic entries to build up
Keywords: cyclization reactions
·
dyes/pigments · fluorescence · fluo-
rubines · pentacenes
Introduction
to tetraazafulvalenes and vinylogous derivatives,[1] which
themselves provide the preconditions for the construction of
novel functional dyes.[2] In 2002 we reported on the synthesis
of D2-1,2-diazetines 2 by a simple cycloacylation reaction of
monosubstituted hydrazines with 1 (Scheme 1).[3] Due to
their ring strain, these four-ring systems proved to be valua-
ble starting materials for transformation reactions to yield
several, in a wide range of variable substitutable, five- and
six-membered heterocycles.[4] Surprisingly, cationic deriva-
tives of fluorubines (dihydro-5,6,7,12,13,14-hexaazapenta-
cenes) 3 were identified as byproducts in some cases. These
The easily accessible bisACTHUNRGTNE(NUG imidoyl) chlorides of oxalic acid 1
were developed as C2-building blocks by our group. Due to
their well-tuned selectivity, they are versatile educts for cyc-
lization reactions with different types of binucleophiles. One
specific feature is their marked tendency to undergo cascade
reactions. For example, their multistep one-pot reactions
with amidinium salts form the basis for the synthetic entry
[a] Prof. Dr. R. Beckert, Y. Jꢀttke, D. Hornig, W. Gꢀnther
Institute of Organic and Macromolecular Chemistry
Humboldtstr. 10, 07743 Jena (Germany)
Fax : (+49)3641-804-210
E-mail: Rainer.Beckert@uni-jena.de
[b] Dr. J. Fleischhauer
FB 18, Nature science, Chemistry of Mesoscopic Systems
Heinrich-Plett-Str. 40, 34132 Kassel (Germany)
[c] Dr. E. Birckner, Prof. Dr. U.-W. Grummt
Institute of Physical Chemistry, Lessingstr. 10
07743 Jena (Germany)
[d] Dr. H. Gçrls
Institute of Inorganic Chemistry, Lessingstr. 8
07743 Jena (Germany)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200901961.
Scheme 1. From byproduct to novel highly functionalized fluorubines 3.
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derivatives and especially their solutions are characterized
by strong orange/red fluorescence with high fluorescence
quantum yields. Additionally, they exhibit a remarkable sta-
bility against oxidative damage. For example, a sample of 3a
in ortho-dichlorobenzene did not bleach when exposed to
daylight for three years—only loss of the solvent was ob-
served—which is really remarkable for organic dyes. Such
well-defined highly functionalized fluorubines similar to 3
are not mentioned in the chemical literature up to date.
Since the first recommendation of the “classical fluorubines”
by Hinsberg in 1903, several papers that deal with syntheses
and applications of the 6,13-dihydroform of fluorubines 4
can be found.[5]
On the other hand, during the past years the immense
progress in the field of applied organic materials, for exam-
ple, conjugated polymers, organic light-emitting diodes, and
organic semiconductors (OFETꢂs), shows the necessity to
develop novel well-tuned materials. In comparison to the
wide range of available p-type single molecules and poly-
meric-organic materials, such as pentacene and thiophene-
based materials, there only exist few examples that deal
with n-type devices.[6] One possibility to overcome this defi-
cit can be achieved by the introduction of electron-accepting
groups, such as fluorine or cyanosubstituents.[7] A second
possibility to get an access to electron-deficient molecules is
based on the successive replacement of CH moieties by ni-
trogen. In 2007, Winkler showed in a theoretical work that
in this manner the HOMO/LUMO energies, which repre-
sent an important feature for organic devices, of linear fused
azaacenes could be effectively influenced.[8] So there exist
several molecular examples, such as the diphenylanthra-
as chloroform. The isolation and characterization of this
photochemical-induced “byproduct” by means of NMR
spectroscopy and MS led us to the assumption that a dimeri-
zation of 2 by ortho-annulation of the aromatic rings and
formal loss of methylhydrazine has occurred. The crucial in-
1
dication was the H NMR spectrum of 3a in which a singlet
at d=6.49 ppm in the region of the aromatic protons was
clearly detectable. In addition to these experimental results,
we have found that a quaternization of one nitrogen atom,
which leads to the cationic charge of derivative 3a, has oc-
curred. We confirmed these findings by HRMS, which
showed that the estimated mass of m/z: 509.2448 u for the
cationic core ([C33H29N6]+) of 3a is in agreement with the
measured mass of m/z: 509.2461 u. After anion exchange
into hexafluorophosphate, single-crystal X-ray analysis de-
livered the unambiguous structural assignment of the dihy-
dro-5,6,7,12,13,14-hexaazapentacenium salt 3a (Figure 1) in
which the ortho-annulation of N1/N5 at positions C1/C14
took place. In addition, one molecule of dioxane was incor-
porated into the molecular unit cell.[10]
azolines,
5H,7H-
and
5H,12H-dihydroqinoxalino-
ACHTUNGTRENNUNG
handled as potent n-type as well as p-type organic semicon-
ductors.[9] Stimulated by the challenge to develop novel,
stable electron-deficient materials, such as 3, and by the lack
of their synthesis, we have developed several independent
syntheses for linear-fused hexaazapentacenes. In this paper,
we are going to present one of our novel synthetic possibili-
ties to achieve aza-based polynuclear acenes. In addition, we
will discuss the structures, their photophysical and redox
chemical characteristics, and their possible formation mech-
anisms in more detail.
Figure 1. Solid-state structure and bond lengths of the cationic fluorubine
derivative 3a (Scheme 1; Ar=4-Me-C6H4, R=Me, X=PF6À).[10]
As depicted in Figure 1, the nitrogen atoms N1, N3, and
N5 at the positions 5, 7, and 13 of the fluorubine core are
substituted with a remarkable selectivity. In view of the
structure-determining bond lengths of 3a, almost alternating
distances between the neighboring atoms were measured.
This result points out the efficient delocalization of the con-
jugated system. The cationically charged derivatives 3 repre-
sent the alkylation products (at position 13) of the mesoion-
ic derivatives 5 (Figure 2), which themselves are not men-
tioned in the chemical literature and represents the diaza-
homologues of 5H,7H-5,7-diphenyl-dihydroquinoxalino-
Results and Discussion
Due to its characteristic AA’XX’-pattern in proton reso-
nance spectroscopy, the p-tolyl group (Scheme 1; Ar=4-Me-
C6H4) is one of our most used substructures for the synthesis
of novel heterocycles based on 1. Decomposition of a corre-
sponding substituted D2-1,2-diazetine (2a) furnished a pot-
pourri of oligo- as well as polymeric derivatives originating
from isonitriles. To our surprise, a substance showing a
strong orange fluorescence on TLC was detected. This side
reaction occurred in approximately 1% and was always
present in daylight and by use of halogenated solvents, such
AHCTUNGTREG[NNNU 2,3b]phenazines. From this point of view, they can be re-
garded as zwitterionic “double-barreled” cyanine dyes, in
which the negative strain is formally alkylated. The slightly
shortened bond lengths between C7/N2 (1.279 ꢃ) and C8/
N4 (1.307 ꢃ), which indicate a partial double-bond charac-
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A Novel Route for the Synthesis of Fluorubines
FULL PAPER
This result is in good agreement with the band gap of
2.08 eV, which we have determined by electrochemical
measurements (Eegc). We found in initial cyclic voltammetry
experiments and more clearly in differential pulse polarog-
raphy measurements in CH2Cl2 one oxidation peak (1.65 V)
and two reduction potentials (À0.43, À1.0 V). Taking the as-
sumption that the oxidation peak of ferrocene/ferrrocenium
1
in CH2Cl2 in our measurements is situated at E = =0.71 V
2
and that the energy level of this system below vacuum is
À4.80 eV, we estimate that the HOMO-edge (ionization po-
tential) of 3a is situated at À5.74 eV and the LUMO-edge
(electron affinity) at À3.66 eV.
Stimulated by theses interesting features, we tried to im-
prove this synthesis of azaacenes 3. However, all further at-
tempts to develop an efficient and practicable protocol,
Figure 2. Packing effects of the crystal lattice of the fluorubine derivative
3a (Scheme 1: Ar=4-Me-C6H4, R=Me, X=PF6À).[10]
ter, as well as the slightly elon- Table 1. Optical data of selected compounds, the full data are attached in the Supporting Information.
abs [c]
max
em [c]
max
opt [e]
No R1[a]
R[b]
Ar[b]
l
l
Stokes shift
ff[d]
[%]
E
g, 10%
gated bond lengths of C7/N3
(1.388 ꢃ) and C8/N3 (1.379 ꢃ)
support this assumption. On the
other hand, the positive valence
band is predominantly located
between the nitrogen atoms N1
and N5. The molecular plane of
[nm]
[nm]
[cmÀ1
]
[eV]
3a Me
3b Me
3c Me
3d Me
Me
nBu
tBu
4-Me-C6H4
4-nBu-C6H4
4-tBu-C6H4
546
546
546
531
563
569
569
546
553
740
740
517
92
90
93
94
2.18
2.16
2.16
2.25
CO2Et 4-CO2Et-
C6H4
naphthyl
C6H5
C6H5
C6H5
3e Me
–
587
528
530
529
615
546
552
551
775
624
752
755
67
100
98
1.99
2.25
2.24
2.24
the fluorubine core is nearly 3f Me
H
H
H
3g nPr
planar and the methyl subsitu-
3h 4-Me-
ent at N3 is slightly twisted out
C6H4
97
of it by approximately 108. In
[a] See Scheme 3. [b] See Scheme 1. [c] Measured in CHCl3. [d] ff in CHCl3 vs. rhodamine 6G in EtOH.
addition, herein the p-tolyl sub-
stituents at positions N1 and N5
show an orthogonal arrange-
opt
[e] Optical band gap (E
) estimated by the 10% method.
g, 10%
ment. Due to p–p interactions between the fluorubine mole-
cules, the formation of molecular-stacked dimers (d
ꢀ3.41 ꢃ) was observed in the crystal lattice. These dimers
themselves are connected by two hexafluorophosphate
anions to build up “molecular wires”, which, in principle,
could have good preconditions for charge-transport process-
es. Additionally, packing elements of the herring bone con-
formation can be recognized in the crystal lattice of the
“face-to-face” packed dimeres of 3a.
based on the polychromatic irradiation of 2a were not suffi-
cient. Thus, irradiation of D2-1,2-diazetines 2b–e in tetra-
chloromethane with daylight for approximately four weeks
produced in traces the fluorubine derivatives 3b–d. The de-
composition of the naphthalene-substituted 1,2-diazetine 2e
was studied too and the formation of a double benzo-annu-
lated derivative of fluorubine 3e was observed (Scheme 2).
In comparison to derivative 3a, the absorption maximum of
this hexaazapentacene has a bathochromic shift of about
40 nm (CHCl3: lmax =587 nm).
The optical absorption maximum of fluorubine 3a in
chloroform lies at 546 nm combined with a small Stokes
shift of 553 cmÀ1 and a high fluorescence quantum yield (ff)
of 92%. Additionally, we observed negative solvatochromic
behavior from its UV/Vis spectra. Thus increasing the polar-
ity of the solvent leads to hypsochromically shifted absorp-
tion maxima (CH2Cl2: lmax =553, CHCl3: 546, THF: 540,
methanol: 537 nm). We estimated the optical band gap
Furthermore, we choose the D2-1,2-diazetine 2a as model
compound and irradiated it with monochromatic light at its
absorption maximum (lmax =365 nm) with different irradia-
tion intervals and times and in several different solvents.
(Egopt) of this molecule by two methods, namely the so-
opt
g, 10%
called 10% method (E
; of the absorption maxima at the
lower-energy site), which was introduced by Hçrhold and
the commonly used tangential through the turning point
method (E ).[11] Both methods delivered nearly similar re-
opt
g, T
sults, but we would like to mention that the 10% method is
still easier in its handling. However, as listed in Table 1 we
found an optical band gap of approximated 2.19 eV for 3a.
Scheme 2. Formation of the naphthalene-substituted derivative 3e (Ar=
1-naphthyl, X=ClÀ).
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R. Beckert et al.
Unfortunately, in view of the selective formation of 3a,
these experiments have failed up to now. As mentioned
above, 2a is unstable towards irradiation in chloroform. In
tetrachloromethane and less efficiently in chloroform, a
transient with a broad absorption peak at 490 nm was ob-
served, with the solute irreversibly decomposed. These find-
ings point towards an electron transfer to the solvent as the
primary process.
Without going into details here, we can safely conclude
from this series of long-term irradiation experiments that
more than one photon of different energy is required to gen-
erate the final product. In our belief, the mechanism of this
cascade reaction proceeds in three steps. First, an ortho-an-
nulation process forms the quinoxaline subunit. This cycliza-
tion sequence most likely represents the initial step and will
be discussed here. Subsequently, the cleavage of the four-
membered diazetine under formation of a reactive inter-
mediate might take place. Finally, a dimerization involving
the extrusion of a MeNN fragment yields the fluorubines.
To understand these results, we used DFT calculations on
the B3LYP/6-31g(d) level[12] to investigate the reaction coor-
dinate of the ring annulation reaction of a hypothetical
phenyl-substituted 1,2-diazetine 2 f (Scheme 1; R=H, Ar=
C6H5), in which the reaction takes place between the aryla-
spect to cleavage. Hydrogen-shift reactions that lead to
much more stable isomers are predicted to show significant-
ly higher barriers. (oxidation would produce a radical that
À
should be stabilized by cleavage of the N N bond.) The rad-
ical cation shows a minimum which allows further reactions
to compete with ring cleavage with a calculated barrier of
18.0 kJmolÀ1.
The theoretical and experimental findings point against a
second possible reaction mechanism, similar to those ortho-
annulation reactions reported by Sakurai,[13] which we first
assumed as a possible pathway for our reactions. In these
photochemical-supported reactions, a base, such as triethyla-
mine, formed an ion pair with the substrate. In contrast to
them, a mechanism that involves the ring cleavage of a cat-
ionic intermediate seems to be more plausible and is cur-
rently one of our further research fields. Some very recent
experimental findings have indicated that the key step
seems to be the formation of aminyl radical cations that are
capable of cyclizing intramolecularly. We will discuss the
mechanism of these remarkable ortho-annulation reactions
in more detail in a forthcoming paper.
Hence, at this point we focused our work on step-by-step
synthesis of the novel fluorubine derivatives. The most
common way to deliver the “classical fluorubines” 4 can be
achieved by the Hinsberg protocol,[5a] in which 2,3-dichloro-
and 2,3-diaminoquinoxalines are melted together. Several
authors have described syntheses of similar 6,13-dihydro-
5,6,7,12,13,14-hexaaza-pentacenes 4, which are used up to
date as functional dyes, especially for inkjet printers and
synthetic fibres.[5b,k] In 2009, a paper that deals with the syn-
thesis and self-assembly of 5,14-dihydro-5,6,7,12,13,14-hex-
aazapentacenes was been published by Hill and Ariga.[5l] To
the best of our knowledge, structurally well-defined 5,7-di-
hydroderivatives, such as derivatives 3 and 5 (Figure 4) are
unknown to date.
À
mine nitrogen C NHAr and the ortho carbon atom position
of the imine-substituted aryl moiety C=NAr (p6-electrocyclic
reaction). The most stable form with respect to syn/anti iso-
merism and tautomerism (2 f-A; Figure 3) was used. This
Figure 3. Investigated isomers and relative energies of the three energeti-
cally preferred prototropes of hypothetical 1,2-diazetine 2 f based on the
theoretical level of B3LYP/6-31g(d).[12]
tautomeric form is 4.8 kJmolÀ1 more stable than the most
stable syn/anti isomer with the mobile proton residing on
one of the exocyclic nitrogen atoms (2 f-B). Rotating the
phenylamino group leads to a further rotamer sterically suit-
able for an electrocyclization (2 f-C), which is another
3.7 kJmolÀ1 more unstable. Due to the low energetic barri-
ers, the coexistence of such isomers was confirmed by NMR
spectroscopic experiments. Obviously, these isomers rapidly
interconvert on the time scale of NMR spectroscopy, thus
we were unable to assign a particular isomer as predominat-
ing in solution. The essential conclusions derived from these
calculations are: the cyclized product of the radical anion
does not possess a minimum and, if photochemically gener-
ated as an intermediate, should undergo an extremely rapid
ring cleavage. The cyclized product of the neutral molecule
exhibits a shallow minimum of about 7.6 kJmolÀ1 with re-
Figure 4. Selected isomeric fluorubines 3–5.
For our synthetic purpose of building up the fluorubine
derivatives 3, we choose the cyclization reaction of commer-
cially available N-phenyl-phenylenediamine (6) with oxalic
acid diethylester to yield the N-phenyl substituted quinoxa-
line-2,3-dione (7). In a second step, it was converted by
means of thionyl chloride into 3-chloro-quinoxaline-2-one
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A Novel Route for the Synthesis of Fluorubines
FULL PAPER
(8). The subsequent aminolysis reaction of 8 with selected
amines yielded the crystalline 3-aminoderivatives 9a–e in
very good yields (80–100%, Scheme 3). 3-Aminoalkyl/aryl-
bridged bis(quinoxaline)s 10a,b were obtained in yields of
90 and 60%. Within the use of a similar protocol to gener-
ate the aryl amine-bridged bis(quinoxaline)s 10c–e, all at-
tempts failed without the use of the Pd0/2,2’bis(diphenyl-
phosphino)-1,1’-binaphthyl (BINAP)-system under Hart-
wid–Buchwald conditions. Otherwise, the use of bases, such
as alkali alkoxides, led to a straight reaction to the corre-
sponding alkoxy derivatives[14g,h] exemplified by the conver-
sion of 8a with NaOMe in THF/MeOH to yield the meth-
oxy-derivative 11a (Scheme 3). Similarly, we employed a
modified protocol, in which at first KH is used as a base, fol-
lowed by cross-coupling delivering the best results for the
aromatic-substituted derivatives of 10. Elemental analysis
and NMR spectroscopic/MS data confirmed the presence of
1:1 acylation products. We succeeded in obtaining single
crystals of bis(quinoxaline)s 10a (R1 =Me) and of derivative
10d (R1 =4-Me2N-C6H4) and confirmed our postulated
structures by X-ray structural analysis.[10]
The solid-state structure of derivative 10a is depicted in
Figure 5. Conspicuously, both quinoxaline moieties are twist-
ed within a dihedral angle of approx. 358 and show a cis ar-
Scheme 3. General syntheses of amino-substituted quinoxalines 9a–e,
amino-bridged bis(quinoxaline)s 10a–e (9/10a: R1 =Me, 9/10b: R1 =nPr,
9/10c: R1 =4-Me-C6H4, 9/10d: R1 =4-Me2N-C6H4, 9/10e: R1 =4-tBu-
Me2SiO-C6H4), and fluorubines of type 3. a) (EtCO2)2, 1608C; b) SOCl2,
toluene, 1108C; c) R1NH2, THF, RT to 658C; d) NaOMe, THF, MeOH,
RT; e) 1) KH, THF; 2) 8 (R1 =Ar), BINAP/[Pd
ACHTNUGTRNEUNG(PPh3)4] (10 mol%);
f) Pathway A: 1) PCl5, CH2Cl2, RT; 2) NH3, THF, RT; 3) NH4Cl, DMF,
1608C; Pathway B: Tf2O, CH2Cl2, RT; 2) NH3, CH2Cl2, RT; 3) NH4Cl,
mesitylene, 1608C.
substituted quinoxaline-2-ones of type 9 were already de-
scribed in the literature before.[14] Due to their physiological
activity, for example, as inhibitors of glycogen-phosphoryla-
se,[14c] different syntheses have been developed in the past
decades and a wide range of patents have claimed their fea-
tures.[14f–j] In a further step, the amino derivatives 9a–e were
acylated with 8 to yield the desired symmetrical amino-
bridged bis(quinoxaline)s 10a–e. Surprisingly, these bis(qui-
noxaline)s of type 10, which can be regarded as tertiary aryl/
heteroaryl amines, are also not mentioned in the literature
to date. There only exists a few data dealing with the synthe-
ses and properties of similar but unsubstituted 2-aminoderi-
vatives and 3-alkyl- and 3-aminoalkyl substituted bis(qui-
noxaline)s.[15] The chemical nature of the base was one of
the decisive factors in the transformation of 9 into 10. All
attempts to use Huenigꢂs base or nBuLi in THF and also the
reaction in DMF/K2CO3 at 1508C did not furnish the de-
sired amino-bridged bis(quinoxaline)s 10. However, sodium
and potassium hydride proved to be suitable for such acyla-
tion processes. To achieve a homogenous reaction, we fa-
vored potassium hydride, which formed a more soluble
amide deriving from 9. Upon subsequent addition of the
chloro-derivative 8 to the potassium salt of 9 the alkylamin-
Figure 5. Solid-state structure of the aminomethyl-bridged bis(quinoxa-
line) 10a.[10]
rangement in view to each other. Due to the formation of a
À
À
partial amidinium resonance between N1 C1 N2 and be-
À
À
À
tween N1 C15 N4, the bond lengths of N1 C1 (1.389 ꢃ)
À
and N1 C15 (1.390 ꢃ) are slightly shortened in comparison
À
À
to typical C N single bonds, such as N1 C29 (1.465 ꢃ).
By starting from 10 we developed two practicable proce-
dures to yield the target molecules 3. On the one hand, we
used a three-step protocol in which at first a solution of 10
in dichloromethane was treated with phosphorous penta-
chloride. Therefore, the formation of the bisACTHUNRTGNEUNG(iminium) salts
10-A can be assumed (Scheme 4). By using toluene as a sol-
vent in this reaction step, the formation of a yellow precipi-
tate was detected, thus supporting our assumption. After re-
moving and changing the solvent into THF, dried ammonia
was passed through the reaction flask and the formation of
a sluggish mixture was observed containing NH4Cl and the
possible bis
ACHTUNGTREN(NUNG imino)-derivatives 10-B. Due to the scrupulous
sensitivity of these intermediates against moisture, whereby
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R. Beckert et al.
droxyphenoles into the well-known rhodamines occurs in
yields between 5 and 35% respectively.[17]
À
After anion exchange (TfÀ/ClÀ into BF4 ) of derivative
3 f, we were able to achieve single crystals. The result of the
single-crystal X-ray analysis and some bond parameters are
depicted in Figure 6. In general, the bond lengths and struc-
Scheme 4. Postulated mechanism for the activation and cyclization se-
quence of bis(quinoxaline)s 10 by formation of fluorubines of type 3.
a) PCl5, CH2Cl2, RT, 12 h; b) Tf2O, CH2Cl2, RT, 12 h; c) NH3, THF, RT,
1 h; d) NH3, CH2Cl2, RT, 1 h; e) NH4Cl, DMF, 1608C, 24 h.
Figure 6. Solid-state structure of the cationic fluorubine derivative 3 f
(Scheme 2; R1 =Me, X=BF4À).[10]
hydrolysis to the starting materials 10 takes place immedi-
ately, no attempts for their isolation were made. In a final
step, the solvent was changed once again into DMF, NH4Cl
was added, and the mixture was heated in a sealed tube at
1608C for 24 h. Under these conditions, most likely the in-
tramolecular formation of a cyclic aminal 10-B takes place,
which finally, upon protonation and elimination of ammo-
nia, formed the cationic fluorubine derivatives 3. In a
second protocol, we activated the cyclic amide substructure
with trifluoromethanesulfonic anhydride (Tf2O) in dichloro-
tural parameters are very similar to that of 3a (see above),
only the molecular plane of the fluorubine core is more
twisted than it. Additionally, we observed remarkable pack-
ing effects in the crystal lattice of 3 f. Due to p–p-interac-
tions between the fluorubine cations of 3 f, there also exists
molecular stacked dimers (dꢀ3.37 ꢃ), which themselves are
connected by two molecules of tetrafluoroborate. As conse-
quence of this charge interaction between the fluorubine
core on the one hand and the corresponding anion on the
other hand, regular molecular architectures are built up (see
the Supporting Information).
methane,[16] which also leads to bis
ACTHNUTRGNE(UNG iminium) salts 10-D orig-
inating from oxygen sulfonation by formation of esters of
trifluoromethanesulfonic acid (Scheme 4). These esters were
converted directly into their imino homologues without
changing the solvent. After removing the solvent, addition
of NH4Cl, and heating in mesitylene, the desired fluorubines
3 were furnished in comparable yields. We would like to
note that this way is a bit more practicable, because it is not
recommended to change the solvents before the final cycli-
zation step. Furthermore, experiments with several high-
boiling solvents in the final step showed that mesitylene
leads to the formation of fewer byproducts, thus simplifying
the workup.
Unfortunately, our selected arylamine derivatives 10d–e
decomposed during the ring-closure procedure. Mainly, we
reasoned these findings as due to their functional groups
1
1
À
À
(10d: R =4-Me2N C6H4, 10e: R =4-tBuMe2SiO C6H4),
which are not stable enough under the investigated reaction
conditions giving us undefined mixtures. From this point of
view, there is currently a lack in the tolerance of certain
functional groups (silylether, amino groups), which show
themselves a high affinity to the Lewis acids Tf2O or PCl5.
However, in general, it should be possible to introduce func-
tionalized aryl moieties at position 13 (Figure 4), which is
still one of our further research topics.
We used the bis(quinoxaline)s 10a–e, mentioned in
Scheme 3, as starting materials for the cyclization sequence.
In the case of the aliphatic bridged derivatives 10a,b and
the aryl derivative 10c, we produced the highly functional-
ized 5,7-dihydro-5,6,7,12,13,14-hexaazapentacenium salts
3 f–h in a one-pot synthesis in yields of approximately 20–
40%. These currently achieved overall yields by conversion
of 10 into 3 are not sufficient, but can be compared to those
of similar systems. For instance, the thermal cyclization reac-
tion of 4-bromo-benzaldehyde with substituted 2-amino-hy-
According to the extension of the conjugated p-system in
the order 9–10–3, a subsequent redshift in the absorption
and emission spectra was observed. Exemplified by the
methylamine derivative 9a (Figure 7), the absorption maxi-
mum lies at 331 nm showing a Stokeꢂs shift of approximatley
2800 cmÀ1. In comparison, herein a significant Stokeꢂs shift
of 5086 cmÀ1 of bis(quinoxaline) derivative 10a (lmax
=
385 nm, ff ꢀ0.1) indicates a remarkable change of geome-
tries in its ground and exited state. The final cyclization re-
action of 10a to 3 f is attended with a bathochromic shift of
12804
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ꢁ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2009, 15, 12799 – 12806

A Novel Route for the Synthesis of Fluorubines
FULL PAPER
Acknowledgements
We would like to thank D. Berg and W. Poppitz for measuring the mass
spectra, G. Sentis and B. Friedrich for measuring the NMR and the UV/
Vis spectra, M. Kuse for CV measurements and A. Jacobi for irradiation
experiments.
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140 nm in its UV/Vis spectrum (3 f: lmax =528 nm) and addi-
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g, 10%
the gap energy (E
) decreases from approximately 3.45
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Conclusion
Based on the structure of a highly fluorescent decomposi-
tion product of D2-1,2-diazetine 2a, a novel route for the
synthesis of functionalized fluorophores was developed. In
this paper, the syntheses and the spectroscopic features of
novel amino-bridged bis(quinoxaline)s 10 and their ring-clo-
sure products “fluorubines” 3 were described. The commer-
cially available phenylenediamine 7 proved to be a suitable
starting material. The new synthesized derivatives of hexaa-
zapentacenes 3, which are stable crystalline compounds, are
featured by a strong fluorescence between 500–600 nm and
high fluorescence quantum yields. It is noteworthy that the
cationic fluorubines represent redox systems, which could
make them powerful candidates for organic n-type devices.
In a forthcoming paper, we will report on their use as start-
ing materials for the construction of other functional dyes,
which can be used as molecular sensors. Additionally, we
will discuss in more detail the remarkable formation reac-
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tions and measurements can be found in the Supporting Information.
Chem. Eur. J. 2009, 15, 12799 – 12806
ꢁ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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Received: July 15, 2009
Published online: October 21, 2009
12806
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Chem. Eur. J. 2009, 15, 12799 – 12806