Synthesis of cyano-substituted diaryltetracenes from tetraaryl[3]cumulenes

Article abstract of DOI:10.1002/anie.201402299

A versatile, two-step synthesis of highly substituted, cyano-functionalized diaryltetracenes has been developed, starting from easily accessible tetraaryl[3]cumulenes. This unprecedented transformation is initiated by [2+2] cycloaddition of tetracyanoethy

Full text of DOI:10.1002/anie.201402299

Angewandte
Chemie
DOI: 10.1002/anie.201402299
Tetracenes
Synthesis of Cyano-Substituted Diaryltetracenes from
Tetraaryl[3]cumulenes**
Przemyslaw Gawel, Cagatay Dengiz, Aaron D. Finke, Nils Trapp, Corinne Boudon, Jean-
Paul Gisselbrecht, and FranÅois Diederich*
Dedicated to Professor Armin de Meijere on the occasion of his 75th birthday
Abstract: A versatile, two-step synthesis of highly substituted,
cyano-functionalized diaryltetracenes has been developed,
starting from easily accessible tetraaryl[3]cumulenes. This
unprecedented transformation is initiated by [2+2] cycloaddi-
tion of tetracyanoethylene (TCNE) to the proacetylenic central
double bond of the cumulenes to give an intermediate
zwitterion, which after an electrocyclization cascade and
dehydrogenation yields 5,5,11,11-tetracyano-5,11-dihydrote-
tracenes in a one-pot procedure. A subsequent copper-assisted
decyanation/aromatization provided the target 5,11-dicyano-
6,12-diaryltetracene derivatives. All of the postulated structures
were confirmed by X-ray crystallography. The new chromo-
phores are thermally highly stable and feature promising
fluorescence properties for potential use in optoelectronic
devices. They are selective chemosensors for CuI ions, which
coordinate to one of the CN substituents and form a 1:1
complex with an association constant of Ka = 1.5 ꢀ 105 LmolÀ1
at 298 K.
1.366 ꢀ),[7] and in polarized push–pull-substituted [3]cumu-
=
lenes the central C C bond approaches the length of an
[8]
ꢀ
acetylenic C C triple bond (down to 1.20–1.21 ꢀ). By
applying the [2+2] cycloaddition–retroelectrocyclization
(CA-RE) reaction between electron-rich alkynes and elec-
tron-deficient alkenes,[9] we recently observed proacetylenic
reactivity in a push–pull-substituted [3]cumulene.[10] It reacted
with tetracyanoethylene (TCNE) via the CA-RE cascade to
give a stable zwitterionic product. With this in mind, we
became interested in exploring the potential proacetylenic
=
reactivity of the longer central C C bond of non-polarized
tetraaryl[3]cumulenes with TCNE.
Herein, we report a one-pot synthesis of 5,5,11,11-
tetracyano-5,11-dihydrotetracenes 2a–e, which we propose
is initiated by the CA-RE reaction of tetraaryl[3]cumulenes
1a–e with TCNE, and their subsequent Cu-promoted thermal
oxidation to 5,11-dicyanotetracenes 3a–c,e (Scheme 1). We
describe the structures and remarkable optoelectronic prop-
erties of these new chromophores and their use as a selective
chemosensor for CuI ions.
I
n the early 1920s, Brand reported the first synthesis of
tetraarylbuta-1,2,3-trienes ([3]cumulenes).[1] Since then,
[3]cumulenes have attracted increasing attention, and a vari-
ety of procedures have been developed for their synthesis.[2]
However, only a limited amount of research has been done to
investigate the chemistry of [3]cumulenes themselves.[3,4]
[3]Cumulenes show the largest bond-length alternation
The synthesis of functionalized [3]cumulenes 1a–e fol-
lowed a well-established procedure,[2,11] and gave the desired
compounds in multigram quantities in four steps, starting
(BLA)[5] in the [n]cumulene series.[6] The central C C bond of
=
[3]cumulenes (average bond length 1.236 ꢀ) is much shorter
=
than the two terminal C C bonds (average bond length
[*] P. Gawel, C. Dengiz, Dr. A. D. Finke, Dr. N. Trapp,
Prof. Dr. F. Diederich
Laboratorium fꢀr Organische Chemie, ETH Zꢀrich
Vladimir-Prelog-Weg 3, HCI, 8093 Zꢀrich (Switzerland)
E-mail: diederich@org.chem.ethz.ch
Prof. Dr. C. Boudon, Dr. J.-P. Gisselbrecht
Laboratoire d’Electrochimie et de Chimie Physique du Corps Solide,
Institut de Chimie-UMR 7177, C.N.R.S., Universitꢁ de Strasbourg
4, rue Blaise Pascal, 67000 Strasbourg (France)
[**] This work was supported by the Swiss National Science Foundation
and the ERC Advanced Grant No. 246637 (“OPTELOMAC”). We
thank Dr. Ori Gidron for help with the determination of fluorescence
quantum yields, Dr. David Schweinfurth with the complexation
studies, and Dr. Bruno Bernet for NMR interpretation. X-ray services
were provided by the Small Molecule Crystallography Center of ETH
Zꢀrich (http://www.smocc.ethz.ch).
Scheme 1. Synthesis of 5,11-dihydrotetracenes 2a–e and tetracenes
3a–c,e. In the first step, the second set of conditions is used only for
1e. TCNE: tetracyanoethylene; TCE: 1,1,2,2-tetrachloroethane.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201402299.
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from para-substituted benzophenones (Supporting Informa-
tion, Section S2). X-ray crystal structures of 1b–e showed no
influence of the substituents on the butatriene moiety with the
temperatures (3108C), all other 5,11-dihydrotetracenes
decompose to a mixture of products (including small amounts
of target tetracene derivatives 3, see below) at around 3008C.
X-ray crystallography (Section S3) revealed that the four
fused rings of 2b and 2c are coplanar, while there is
a significant puckering in the dihydrotetracene core of 2a,
with two nitriles in a pseudoaxial and the others in a pseu-
doequatorial position (Figure 1). Compounds 2a–e are fluo-
rescent, both in solution (CH2Cl2) and in the solid state
(Section S5). Compounds 2a and 2b exhibit low fluorescence
quantum yields (FF = 0.10 in CH2Cl2), whereas compounds
2c–e display higher fluorescence quantum yields (FF = 0.22
for 2c in CH2Cl2; for details, see Section S5).
=
length of the central C C bond varying between 1.24 and
=
1.25 ꢀ and of the two terminal C C bonds between 1.33 and
1.35 ꢀ (for details on the butatrienes, see the Supporting
Information, Sections S3, S4, S7, S8).
Upon heating 1a–d in acetonitrile with TCNE, 5,11-
dihydrotetracenes 2a–d were obtained in good yields (39–
67%, Scheme 1). The polar solvent and the elevated temper-
ature (908C) seem to be essential for the reaction.[12]
[3]Cumulene 1e required higher temperatures (1408C) and
a
different solvent (1,1,2,2-tetrachloroethane, TCE) to
undergo this reaction with TCNE.
We propose a cascade mechanism for the unprecedented
transformation to the dihydrotetracenes, initiated by formal
We subsequently transformed the 5,11-dihydrotetracenes
into the corresponding tetracenes by formal elimination of
cyanogen ((CN)2). Tetracenes feature extraordinary elec-
tronic properties and find application in organic devices, such
as field effect transistors (FETs), organic light-emitting
diodes (OLEDs), and solar
=
[2+2] cycloaddition of TCNE to the central C C bond of the
tetraaryl[3]cumulenes (Scheme 2).[9,10] Retroelectrocycliza-
cells.[14] Heating a finely ground
mixture of 2a–c,e with an excess of
copper powder[15] to 2808C yielded
tetracene derivatives 3a–c,e in fair
to
good
yields
(18–80%,
Scheme 1). In the case of 2d, the
reaction conditions led to the
decomposition of starting material.
Other attempts towards oxidative
or reductive eliminations failed.[16]
5,11-Dicyano-6,12-diaryltetra-
cenes 3a–c,e are thermally stable
solids, sublimable at 2308C
(10À6 mbar), and are intensely
pink colored with yellow to
orange glow in solution and in the
solid state. They are structurally
similar to rubrene (5,6,11,12-tetra-
phenyltetracene) and also have
Scheme 2. Proposed mechanism for the transformation of tetraaryl[3]cumulenes with TCNE to give
5,11-dihydrotetracenes.
tion of the formed cyclobutene ring generates the zwitterionic
species A with the negative and positive charges stabilized by
the dicyanovinyl and diphenylmethyl groups, respectively.
The requirement for stabilization of the highly polar,
zwitterionic intermediates explains the favorable influence
of the polar solvent acetonitrile on the reaction. A disrotatory
6p-eÀ electrocyclization provides intermediate B, followed by
charge neutralization to give C.[13] The crystal structure of the
previously reported zwitterion formed by a push–pull buta-
triene shows the right geometry for the proposed disrotatory
ring closure.[10]
Intermediate C undergoes a second disrotatory 6p-eÀ
Figure 1. Left: Crystal structure of 2b. Right: crystal structure of 2a.
T=100 K. Ellipsoids are set at 50% probability; hydrogen atoms are
omitted for clarity.
electrocyclization to give 4a,5,10a,11-tetrahydrotetracene D.
We presume that this is possibly the rate-determining step and
that the high temperatures are required to enable the right
cyclization geometry. A formal 1,8-elimination of hydrogen,
driven by gain in benzenoid aromaticity, finishes the reaction
cascade.
a similar appearance.[17] X-ray crystal structures were deter-
mined for all obtained compounds. The cyano groups are in
plane with the tetracene core, and the phenyl rings at the 6-
and 12-positions are nearly orthogonal to it. Repulsion from
the orthogonal phenyl rings causes substantial bending of the
5,11-Dihydrotetracenes 2a–e are pale- to deep-yellow
solids. Except for 2d, which gives a black tar at elevated
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(Figure 3). In comparison to 5,11-dihydrotetracenes 2a–e,
tetracenes 3a–c,e show reversed substituent dependence on
fluorescence efficiency (see Section S5). While unsubstituted
3a displays the highest fluorescence quantum yield (FF =
0.44), it is almost half as muchfor methoxy-substituted 3c
(FF = 0.24). The fluorescence quantum yield of 3a is in
between the values for tetracene (FF = 0.17)[19] and rubrene
(FF = 0.61).[20] Tetracenes 3a–c,e show rather constant elec-
trochemical gaps (2.27 V for 3c to 2.30 V for 3a), which are in
full agreement with the spectroscopically determined gaps
(for full electrochemistry data, see Sections S4 and S7).
Prominent electronic and spectroscopic properties, along
with the unique molecular environment of the cyano groups
in tetracenes 3a–c,e, inspired us to explore their use as
fluorescent chemosensors for ions with the potential to
coordinate to this group.[21] We exposed compounds 3a–c,e
to different metal triflates (Li+, Mg2+, Ba2+, Ni2+, Cu2+, Cu+,
Ag+, Zn2+, Hg2+, Sn2+, In3+) in CH2Cl2 and found them to be
highly selective fluorescent molecular sensors for Cu+ and
Ag+ (see Section S6). These soft ions interact well with the
soft nitrogen atoms in cyano groups.[22] Their selectivity may
be controlled by the size of the metal cation, as space is
limited by the close proximity of the adjacent phenyl ring.
Moreover these ions are known to favorably interact with p-
surfaces by undergoing cation–p interactions, while the soft
aromatic p-surface is not capable of sufficiently “solvating”
the harder ions, such as alkali and alkaline earth metal ions.[23]
The chromophoric response to the complexation of Ag+
and Cu+ ions is substantial and allows for the easy detection of
these ions both by absorption and emission spectroscopy in
CH2Cl2 solution (Figure 4). A change in color occurs upon
complexation, which is readily monitored in daylight (Fig-
ure S47). With all four cyanotetracenes, Cu+ ions produce
bathochromic shifts of the lowest-energy absorption of
approximately 25 nm and Ag+ ions shifts of approximately
15 nm. Treatment of the tetracenes with these ions leads to
lowered fluorescence emission intensity and bathochromic
shifts of the emission maxima. We quantified the binding of
Cu+ ions (as the more readily soluble [Cu(MeCN)4]BF4 salt)
by evaluating absorption and emission titrations (Figure 4),
assuring the 1:1 stoichiometry both by the appearance of
distinct isosbestic points in the absorption spectra and by Job
plot analysis (Figure S59). Non-linear least-square curve
fitting of the absorption titration data from multiple runs
Figure 2. Left: Crystal structure (left) and crystal packing pattern
(right) of 3a. T=100 K. Thermal ellipsoids at 50% probability.
nitrile groups, with a deviation of up to 108 (Figure 2). The
crystal packing is dominated by p–p stacking interactions.
Packing patterns of 3a,c,e are similar, with single molecules
oriented on parallel planes in a regular manner. In the case of
3b, the packing is less regular, where the molecules are
located on different nonparallel planes (Figure S24). The UV/
Vis spectra of compounds 3a–c,e are characteristic for acenes,
with a very strong band around 300 nm and vibronically
coupled bands with significant fine structure at lower energy
(Figure 3). The lowest energy absorption is not a charge
transfer band, but rather associated with the p–p* transition
in the tetracene moiety, as evidenced by time-dependent DFT
calculations (see Section S8).[18] The HOMO and LUMO are
located on the tetracene core, with no polarization along the
molecules. Tetracenes 3a–c,e are strong fluorophores
afforded
an
association
constant
Ka = 130600 Æ
20000 LmolÀ1. This value was confirmed by the evaluation
of the fluorescence titration, with excitation at the isosbestic
point at 513 nm, which yielded Ka = 170000 Æ 22000 LmolÀ1.
Fluorescent copper ion sensing is a rapidly developing field
owing to the importance of these ions in human metabo-
lism.[24]
In summary, we reported an unprecedented access to
substituted tetracenes in two steps, starting from [3]cumu-
lenes. Their reaction with TCNE in the first step to give
5,5,11,11-tetracyano-5,11-dihydrotetracenes most probably
involves the reactivity of the proacetylenic, central cumulenic
bond in a CA-RE reaction. Formal cyanogen elimination in
the second step provided functionalized tetracenes with
a rubrene-like substitution pattern as thermally stable,
Figure 3. UV/Vis absorption spectra (top) and normalized photolumi-
nescence spectra of 3a–c,e (bottom) in CH2Cl2 at 298 K. The table
gives the fluorescence quantum yields FF (for details, see the Support-
ing Information, Section S5).
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Figure 4. Molecular sensing of Cu+ ions (as [Cu(MeCN)4]BF4) by
absorption (top, [3b]=10À5 m) and emission (bottom, [3b]=10À6 m)
spectroscopy. On the legends on the right side are given the
equivalents of Cu+. T=298 K.
highly fluorescent molecular materials with potential for
application in optoelectronic devices.[25] We have now shown
that these new chromophores, with their unique structural
feature of coaligned neighboring phenyl and cyano substitu-
ents on the tetracene ring, have potential as selective
molecular sensors of soft ions such as Cu+ and Ag+. Addi-
tional synthetic modifications for chemosensor applications,
also in aqueous solution, are planned.
[13] In case of identical atom positions, B and C are of course
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Received: February 11, 2014
Published online: March 24, 2014
[15] A similar cyanogen elimination from tetracyanoquinodime-
thanes on Cu(100) surfaces was recently reported: C. Urban,
Y. Wang, J. Rodriguez-Fernandez, R. Garcia, M. A. Herranz, M.
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Keywords: [2+2] cycloaddition · [3]cumulenes · chemosensors ·
electrocyclization · tetracenes
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