A Four-Step Synthesis of Substituted 5,11-Dicyano-6,12-diaryltetracenes with Enhanced Stability and High Fluorescence Emission

Article abstract of DOI:10.1002/chem.201703903

A four-step synthesis of substituted 5,11-dicyano-6,12-diaryltetracenes was developed, starting from readily available para-substituted benzophenones. The key step of this straightforward route is the complex cascade reaction between tetraaryl[3]cumulenes

Full text of DOI:10.1002/chem.201703903

DOI: 10.1002/chem.201703903
Full Paper
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Polycyclic Aromatic Hydrocarbons |Hot Paper|
A Four-Step Synthesis of Substituted 5,11-Dicyano-6,12-
diaryltetracenes with Enhanced Stability and High Fluorescence
Emission
Nicolas Kerisit,[a] Przemyslaw Gawel,[a] Brian Levandowski,[b] Yun-Fang Yang,[b] Vꢀctor Garcꢀa-
Lꢁpez,[a] Nils Trapp,[a] Laurent Ruhlmann,[c] Corinne Boudon,[c] Kendall N. Houk,*[b] and
FranÅois Diederich*[a]
Abstract: A four-step synthesis of substituted 5,11-dicyano-
6,12-diaryltetracenes was developed, starting from readily
available para-substituted benzophenones. The key step of
this straightforward route is the complex cascade reaction
between tetraaryl[3]cumulenes and tetracyanoethene (TCNE)
resulting in 5,5,11,11-tetracyano-5,11-dihydrotetracenes. The
mechanism of this transformation was reinvestigated by
means of theoretical calculations. The target tetracenes were
obtained by a newly developed decyanation/aromatization
reaction catalyzed by CuI or CuII complexes in solution, con-
ditions compatible with a broad range of functional groups.
A computational mechanistic study sheds light on this trans-
formation. Structures of all tetracene derivatives were con-
firmed by X-ray crystallography. The presented dicyanotetra-
cene derivatives exhibit outstanding optoelectronic proper-
ties and enhanced photostability, significantly surpassing the
reference rubrene (5,6,11,12-tetraphenyltetracene).
er than the two neighboring C=C bonds,[1] providing a unique
proacetylenic reactivity. In the case of push–pull-substituted
[3]cumulenes, the BLA is even stronger with the central C=C
bond reaching 1.21–1.20 ꢂ,[5] a bond length characteristic for
CꢀC bonds of alkynes. Highly polarized [3]cumulenes were
shown to exhibit a proacetylenic reactivity with tetracyano-
ethene (TCNE), and stable zwitterionic products were obtained
as a result of a [2+2] cycloaddition–retroelectrocyclization
(CA–RE) reaction characteristic for acetylenes.[5c,d] In a remark-
able finding, tetraaryl[3]cumulenes reacted with TCNE to yield
5,5,11,11-tetracyano-5,11-dihydrotetracenes, which could be
thermally transformed into 5,11-dicyano-6,12-diaryltetracenes.[6]
Acenes feature a unique set of properties[7] and are highly
desirable molecules, used for the fabrication of organic field-
effect transistors (OFETs)[8] and organic light-emitting diodes
(OLEDs).[9] Recently, acenes have proven to be key players in
the field of singlet exciton fission (SF), the process in which
two charge carriers are formed upon absorption of one
photon[10] and which enables a significant increase in efficiency
of photovoltaics.[11] We recently evaluated the ability of 5,11-di-
cyano-6,12-diphenyltetracene derivatives to undergo very fast
and efficient SF.[12,13] The presence of cyano groups on the tet-
racene scaffold enhances the stability of the molecules, in ad-
dition of having a favorable effect on electronic energy
levels.[14] As a result, SF in cyanotetracenes is exoergic, which is
in contrast to tetracene[15] or rubrene (5,6,11,12-tetraphenyl-
tetracene),[16] for which SF is endoergic and must be thermally
activated or needs an entropic contribution.[17] High triplet ex-
citon yields up to 200% (quantitative) were achieved. It is
therefore important to further explore the chemical space of
Introduction
Recent advances in organic synthetic methodology enabled
the preparation of very long [n]cumulenes (up to n=9) on the
way towards carbyne.[1,2] In parallel, the investigation of cumu-
lene reactivity has been intensely pursued.[3] The interest in cu-
mulenes lies in their unique structure. Among the [n]cumulene
series, [3]cumulenes possess the largest bond length alterna-
tion (BLA),[4] that is, the central C=C bond is significantly short-
[a] Dr. N. Kerisit, Dr. P. Gawel, Dr. V. Garcꢀa-Lꢁpez, Dr. N. Trapp,
Prof. Dr. F. Diederich
Laboratorium fꢂr Organische Chemie, ETH Zꢂrich
Vladimir-Prelog-Weg 3, CH-8093 Zꢂrich (Switzerland)
E-mail: diederich@org.chem.ethz.ch
[b] B. Levandowski, Dr. Y.-F. Yang, Prof. Dr. K. N. Houk
Department of Chemistry and Biochemistry
University of California Los Angeles
California 90095-1569 (United States)
E-mail: houk@chem.ucla.edu
[c] Prof. Dr. L. Ruhlmann, Prof. Dr. C. Boudon
Laboratoire d’Electrochimie et de Chimie Physique du Corps Solide
Institut de Chimie-UMR 7177, C.N.R.S., Universitꢃ de Strasbourg
4 rue Blaise Pascal, 67081 Strasbourg Cedex (France)
Supporting information and the ORCID identification number(s) for the au-
thor(s) of this article can be found under https://doi.org/10.1002/
chem.201703903. It contains the materials and methods, experimental de-
tails and data for new compounds as well as copies of H, 13C, and 19F NMR
spectra, full data set of UV/Vis absorption spectroscopy, fluorescence spec-
troscopy, cyclic voltammetry and rotating disc voltammetry results, X-ray
crystallographic data and CIF files for 1b-f, 3h, and 4d,h,i, full data set for
the photostability study, computational details for 2e,g-i, 4d,e,g-i, as well
as full data set for the computational study of the dihydrotetracene forma-
tion and the copper-mediated decyanation/aromatization reaction.
1
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5,11-dicyano-6,12-diaryltetracene derivatives in order to fully
evidence their interesting features.
Except for compound 2g, which is soluble in Et2O and even in
n-pentane, [3]cumulenes precipitated during the transforma-
tion and could be conveniently separated from the reaction
medium by filtration, without any further purification needed.
The previously reported synthesis of tetraaryl[3]cumulenes
starting from benzophenones involved four steps[6] compared
to the two-step protocol described here.
Herein, we report an overall four-step synthesis of cyano-
substituted diaryltetracenes starting from easily accessible
para-substituted benzophenones. The final tetracenes were
prepared from 5,5,11,11-tetracyano-5,11-dihydrotetracenes
using a newly-developed methodology, which consists of a de-
cyanation/aromatization reaction catalyzed by CuI or CuII com-
plexes in solution. This novel transformation is investigated by
theoretical calculations that evidenced two consecutive single-
electron transfers from CuI complexes to the substrate. The op-
toelectronic properties as well as the photostability of the tet-
racene derivatives were investigated and show highly promis-
ing features for their potential application in functional devi-
ces.
5,11-Dihydrotetracenes 3a–i were obtained in the reaction
of tetraaryl[3]cumulenes with TCNE in acetonitrile at 90 or
1708C, either using classical heating or microwave (mw) irradia-
tion (Table 1). The new derivatives 3h and 3i were obtained in
28% and 53% yields, respectively.
5,11-Dihydrotetracenes were transformed into the corre-
sponding tetracenes by means of a newly-developed decyana-
tion/aromatization reaction carried out in solution. This is in
contrast to our previous reports for which heating of a finely
ground mixture of copper powder and dihydrotetracenes
3a–c,e–g neat at 2808C resulted in tetracene derivatives 4a–
c,e–g in fair to good yields.[6,13] However, this methodology is
rather harsh and leaves little room for condition tuning. Addi-
tionally, in the case of thiomethyl derivative 4d, the expected
tetracene could not be obtained under these conditions.
We were eager to investigate if this transformation could be
carried out under milder conditions, that is, in solution and at
a lower temperature. Due to the novelty of the starting
5,5,11,11-tetracyano-5,11-dihydrotetracene scaffold, finding
such conditions was not a trivial task. Given that elemental
copper promotes the transformation of dihydrotetracenes, we
focused our investigation on copper complexes. Dihydrotetra-
cene 3g was treated with CuI complexed with N,N’-dimethyl-
ethylenediamine (DMEDA) in toluene, producing tetracene 4g
in 68% yield (Table 2, entry 1). Changing the ligand or the sol-
vent gave lower yields or no conversion (Table 2, entries 2–4).
Results and Discussion
Synthesis
Tetraaryl[3]cumulenes are the key intermediates in the synthet-
ic route leading to cyanotetracenes and were conveniently
prepared in 2 steps from para-substituted benzophenones.
1,1,4,4-Tetraarylbut-2-yne-1,4-diols 1a–i were obtained in the
reaction of benzophenone derivatives with either commercially
available lithium acetylide ethylenediamine complex (Table 1,
method A),[18] or dilithium acetylide,[19] generated in situ by
adding trichloroethylene to a solution of nBuLi (Table 1, meth-
od B). The structures of 1b–f were confirmed by single-crystal
X-ray diffraction (see Supporting Information, Section S8).
Tetraarylbut-2-yne-1,4-diols 1a–i were subsequently trans-
formed into [3]cumulenes 2a–i in a reductive elimination pro-
cess induced by tin(II) chloride in an acidic Et2O (Table 1).[1,20]
Table 1. Synthesis of tetraarylbut-2-yne-1,4-diols 1a–i, [3]cumulenes 2a–i and dihydrotetracenes 3a–i.
R
Product
Method[a,b]
Yield of 1 [%][c]
Product
Yield of 2 [%][c]
Product
T [8C]
Yield of 3 [%][c]
H
Me
MeO
MeS
F
Cl
Me3Si
Ph
1a
1b
1c
1d
1e
1 f
1g
1h
1i
B
B
A
A
A
B
A
A
A
76
64
59
64
71
57
59
61
65
2a
2b
2c
2d
2e
2 f
2g
2h
2i
83
85
89
96
83
68
81
79
78
3a
3b
3c
3d
3e
3 f
3g
3h
3i
90
90
90
90
90
67[d]
48[d]
60[d]
39[d]
51[e]
25[d,f]
63[e]
28
90
170
170[g]
170[g]
p-Me3Si-C6H4
53
[a] Reaction conditions for method B: nBuLi (24 mmol), THF/Et2O, ꢁ808C, trichloroethylene (8 mmol), 258C; ꢁ808C, benzophenone derivative (8 mmol),
258C. [b] Both methods A and B were employed to prepare 1a–i (see Experimental Section as well as Supporting Information, Sections S3 and S4): only
the highest-yielding method is given here. [c] Isolated yields. [d] See reference [6]. [e] See reference [13]. [f] Reaction was carried out in 1,1,2,2-tetrachloro-
ethane at 1408C for 48 h. [g] Reaction was carried out in a microwave oven.
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Different copper sources were evaluated: [Cu(MeCN)4PF6] gave
moderate yield (43%, Table 2, entry 5 and 6), whereas CuCl2
(79% yield, Table 2, entry 7) and CuBr2 (87% yield, Table 2,
entry 8) were, to our delight, high-yielding in the first attempt.
Thus, maintaining CuBr2, DMEDA, and toluene as parameters
of the reaction, we investigated the influence of the catalytic
loading. Using an excess of CuBr2 (200 mol%) neither im-
proved nor hindered the reaction (68% yield, Table 2, entry 9).
Decreasing the catalytic loading from 80 mol% to 20 mol% im-
proved the yield from 66% to 87% (Table 2, entries 10–12),
and the reaction was still high yielding with only 10 mol% of
catalyst (74% yield, Table 2, entry 13). Switching from classical
heating (1118C) to microwave irradiation at higher temperature
(2008C) drastically reduced the reaction time from almost 30 h
to 4 h, without affecting the yield (Table 2, entry 14 vs. entry 8).
The best yield (97%) was achieved with 20 mol% CuBr2 with
an excess of DMEDA upon microwave irradiation at 2008C for
4 h (Table 2, entry 15). When using 40 mol% of DMEDA only,
the conversion was not complete; 4g was obtained in 32%
yield only, and 50% of the starting material (3g) were recov-
ered, showing thus the importance of using an excess of the
amine (Table 2, entry 16).
decomposition took place under such conditions. Using 2,2’-bi-
pyridyl also gave no conversion of 3g (Table 2, entry 18), and
N,N,N’,N’-tetramethylethylenediamine (TMEDA),
a chelating
amine slightly bulkier than DMEDA, gave 4g in only 51% yield
(table 2, entry 19). We presume that the aryl groups in the 6-
and 12-positions sterically hinder the cyano groups in the 5-
and 11-positions of 3g, leaving limited space for copper coor-
dination. Hence, bulky ligands prevent the copper complex to
reach the cyano groups, thus preventing the reaction from
taking place.
We were able to synthesize eight additional tetracene deriv-
atives adapting the conditions optimized for the preparation
of 3g. Surprisingly, 4c and 4i could only be prepared using a
microwave oven at 2008C and were both obtained in 89%
yield (Table 3, entry 3 and 8). All other derivatives afforded
lower yields and difficult purifications, or were even completely
decomposed, when using microwave irradiation. The latter,
even though time-saving, proved to be suitable only for dihy-
drotetracenes bearing bulkier substituents, that is, 3c,g–i. Nev-
ertheless, switching back to classical heating at 1118C enabled
the preparation of 4a,b,d–h (Table 3, entries 1, 2, 4–7). When a
catalytic amount of CuBr2 was used to prepare the thiomethyl
derivative 4d, only a partial conversion of the starting material
3d was observed, even after several days. Therefore, a large
excess of CuBr2 (250 mol%) and an extensive period of heating
(7 days), were necessary to observe a full conversion of 3d
into 4d. To our delight, starting material and product are
We observed that a limited bulkiness of the ligand was cru-
cial for the reaction to proceed. Dihydrotetracene 3g was
treated with CuI in toluene in the presence of 1,10-phenan-
throline, and the mixture was heated to reflux for 15 h
(Table 2, entry 17). Absolutely no conversion or any noticeable
Table 2. Synthesis of tetracene 4g and optimization of the conditions.[a]
Entry
Scale
Catalyst
CuI
[mol%]
Additive
DMEDA
[mol%]
Solvent
toluene
pyridine
pyridine
(CH2Cl)2
toluene/DMF
toluene/DMF
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
T [8C]
t [h]
Yield [%][b]
1
2
3
4
5
6
7
8
65 mmol
65 mmol
65 mmol
65 mmol
65 mmol
65 mmol
65 mmol
65 mmol
65 mmol
65 mmol
65 mmol
130 mmol
65 mmol
65 mmol
130 mmol
65 mmol
65 mmol
65 mmol
65 mmol
40
40
40
40
40
40
50
40
200
80
60
20
10
40
20
20
40
40
20
400
–
400
200
–
400
400
400
400
400
400
400
400
400
200
40
111
116
116
85
111
111
111
111
111
16
14
8
68
n.r.[c]
38
n.r.[c]
n.r.[c]
43
79
87
68
66
CuI
CuI
CuI
–
DMEDA
PPh3
18
22
48
40
27
30
17
46
4
[Cu(MeCN)4PF6]
[Cu(MeCN)4PF6]
CuCl2
–
DMEDA
DMEDA
DMEDA
DMEDA
DMEDA
DMEDA
DMEDA
DMEDA
DMEDA
DMEDA
DMEDA
1,10-phenanthroline
2,2’-bipyridyl
TMEDA
CuBr2
CuBr2
CuBr2
CuBr2
CuBr2
CuBr2
CuBr2
CuBr2
CuBr2
CuI
CuBr2
CuBr2
9
10
11
12
13
14
15
16
17
18
19
111
111
85
87
74
87
200 (mw)
111
200 (mw)
200 (mw)
200 (mw)
111
39
4
4
97
4
32[d]
n.r.[c]
n.r.[c]
51
80
400
200
15
30
4
111
200 (mw)
[a] Reactions were carried out under an argon atmosphere. [b] Isolated yields. [c] n.r.: No reaction observed, neither any decomposition. [d] 50% of 3g
were recovered.
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H2 elimination yielding the expected dihydrotetracene deriva-
tive. This hydrogen shift had not been considered in the earlier
mechanistic proposal. The density functional calculations pre-
dict high transition-state energies for the hydrogen shift and
the final H2 elimination step, even though both of these are
thermally allowed six-electron processes. The two reactions re-
quire significant distortion of a six-membered ring to allow
overlap between H and C or H and H, respectively, leading to
the high activation energies.[21]
Table 3. Synthesis of tetracenes 4a–i using the CuBr2·DMEDA complex in
toluene.[a]
For the decyanation/aromatization reaction, we speculated
that CuII precursors are reduced to CuI by the amine ligand in
situ, with the ligand most likely being oxidized to an imine, as
reported by others (Scheme 1).[22] The single-electron transfer
Entry Product
R
T [8C]
Reaction time Yield [%][b]
1
2
3
4
5
6
7
8
4a
4b
4c
4d
4e
4 f
4h
4i
H
111
111
40 h
29 h
14
73
89
89[c]
71
43
55
89
Me
MeO
MeS
F
Cl
Ph
200 (mw) 4 h
111
111
111
111
7 d
30 min
30 min
27 h
p-Me3Si-C6H4 200 (mw) 4 h
[a] Reactions were carried out under an argon atmosphere. [b] Isolated
yields. [c] Conditions: 3d (75 mmol), CuBr2 (250 mol%), DMEDA
(600 mol%), toluene (2.2 mL), reflux, 7 d.
Scheme 1. Proposed mechanism for CuI-mediated decyanation/aromatiza-
stable even in such harsh conditions and 4d was obtained in
89% yield (Table 3, entry 4). We suspect that the copper cata-
lyst binds to the MeS groups, competing thus with the decya-
nation reaction. This would give a reasonable explanation why
such a large excess of CuBr2 is required to achieve the full con-
version.
tion reaction involving two single-electron transfers (SET).
(SET) from the CuI complex to the substrate 3a forms the CuII
complex that is coordinated by the cyanide ion and also forms
the radical intermediate Int. Subsequently, a second SET occurs
from another CuI complex to the radical intermediate and
leads to the final tetracene derivative 4a. A second reduction
of the catalyst by the amine regenerates the CuI complex, and
we do not exclude the possibility of (DMEDA)CuCN to be pres-
ent in solution as well. The second reduction of the catalyst by
the amine is consistent with our experimental data, that is,
when only 40 mol% of DMEDA were used, the reaction was
not complete and tetracene 4g was obtained in 32% yield
only (See Table 2, entry 16). Therefore, an equal amount of
amine in respect to the catalyst, as well as one equivalent of
amine with respect to the dihydrotetracene, seems to be the
minimum required in order for the CuII species to be reduced
and for the reaction to proceed.
Compared to the previous (and only) methodology using
metallic copper at 2808C,[6,13] we now have established milder
conditions that could easily be modulated. Regarding the
known compounds 4b,c,e–g, the improvement was ranging
from moderate (4b: 73% vs. 68%[6] yield previously) to quite
significant (4c: 89% vs. 60%[6] yield previously). Only 4a was
low-yielding under the new conditions (14% vs. 80%[6] yield
previously). Furthermore, we could successfully obtain the thi-
omethyl derivative (4d) that could not be prepared using ele-
mental copper. Additionally, two new tetracene derivatives
(4h,i) were synthesized as well.
Mechanistic investigation
The free-energy profile for the two decyanation steps is
shown in Figure 1. The first decyanation requires a barrier of
29.7 kcalmolꢁ1 that leads to the radical intermediate Int. The
second decyanation step has a slightly lower barrier (27.2 kcal
molꢁ1) than the first decyanation step, and leads to the expect-
ed compound 4a. This whole transformation is exergonic by
4.7 kcalmolꢁ1. Transition structures TS1 and TS2 for the two
decyanation steps are shown in Figure 2. In both transition
states, the CuꢁC bond is partly formed and the CꢁC bond is
partly cleaved.
The mechanism of formation of dihydrotetracenes as well as
the decyanation/aromatization reaction leading to tetracene
derivatives were both subjected to computational mechanistic
analysis (see Supporting Information, Section S15 for full data
set).
For the formation of dihydrotetracenes, a mechanistic pro-
posal had already been reported.[6] Here, calculations confirm
that the reaction starts with a stepwise [2+2] cycloaddition be-
tween the proacetylenic central C=C bond of tetraaryl[3]cumu-
lene and TCNE via a zwitterionic intermediate leading to a
[2+2] cyclobutane adduct (Figure S62). Several subsequent
carbon–carbon bond cleavages and formations by two sequen-
tial electrocyclizations are necessary to build up the tetracyclic
carbon scaffold. A final 1,5-hydrogen shift occurs prior to the
X-ray crystal structures
X-ray crystal structures were obtained for alkynes 1b–f, dihy-
drotetracene 3h, and all new 5,11-dicyano-6,12-diaryltetra-
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The closest interplanar distances between tetracene cores are
3.40 ꢂ, between the two terminal rings. A second interaction
with the terminal ring of the acene core of a third molecule
shows an interplanar distance of 3.54 ꢂ. Similar close contacts
between two pairs of tetracenes was also observed in the case
of the unsubstituted derivative 4a, for which the closest inter-
planar p–p interactions were 3.40 and 4.09 ꢂ.[6] The strong in-
teraction between acene cores in these p-stacked arrays of 4e
shows similarities to rubrene[23] (Figures S13 and S14). Such a
characteristic of solid-state structure is important for charge
mobility in devices.[24]
Optoelectronic properties
Figure 1. Gibbs free-energy profile for the CuI-mediated decyanation/aroma-
tization reaction.
The UV/Vis absorption spectra of 4d,e,g–i in CH2Cl2 exhibit the
characteristic optical features of tetracenes.[6] They display a
very intense band between l=270 and 370 nm with a large
molar extinction coefficient
e ranging from 100000 to
200000mꢁ1 cmꢁ1 and vibronically coupled bands with fine
structure in the visible region, with e values ranging from 4000
to 23000mꢁ1 cmꢁ1 (Figure 4). The lowest energy bands of 4d,
4h, and 4i are slightly red shifted (ca. 20–30 nm), as compared
to 4e and 4g, and this difference is even larger for the maxi-
mum absorption band (ca. 30–50 nm), showing a noticeable
influence of the substituents on the absorption properties.
Figure 2. Decyanation transition structures TS1 and TS2. The distances are
shown in ꢂngstrçms.
cenes 4d,h,i (see Supporting Information, Section S8 for full
data set). It is worth mentioning that the achiral 1,1,4,4-tetra-
kis(4-methylphenyl)but-2-yne-1,4-diol 1b crystallizes in the
chiral space group P212121.
The fluorinated derivative 4e[13] displays the most remark-
able packing pattern among all dicyanodiaryltetracenes 4a–i
(Figure 3). Molecules are slip-stacked on top of each other
along the long axis of the acene core, in a bricklayer fashion.
Figure 4. Absorption spectra of tetracenes 4d,e,g— recorded in CH2Cl2 at
258C. Inset at the top right-hand corner shows a magnification of the visible
region.
The 5,11-dicyano-6,12-diphenyltetracenes 4a–i described
here are deep-red powders that are highly luminescent both in
solution and in the solid state. Solutions of these tetracenes
exhibit a yellow to red luminescence under a 366 nm UV lamp
(Figure S32). Absolute fluorescence quantum yields of deriva-
tives[25] 4a–i and rubrene were measured in CH2Cl2 and range
from 0.48 to 0.70 (Figure 5 and Table 4). It is noteworthy that
the absolute fluorescence quantum yields of tetracenes 4e
(FF =0.70), 4 f (FF =0.64), 4g (FF =0.66) and 4h (FF =0.64)
surpass rubrene (FF =0.61), and also tetracene (FF (toluene)=
0.17, measured relative to anthracene in ethanol[26]). The
narrow Stokes shift obtained for 4a–i (0.10–0.20 eV) reflects
the efficiency of the radiative emission process occurring on
such rigid scaffolds. Emission spectra of 4a–i and rubrene are
all Gaussian curves overlapping with a second, probably vi-
Figure 3. Packing pattern (top) and interplanar distances between acene
cores (bottom) in crystals of tetrafluoro-substituted tetracene 4e.
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Table 5. Summary of calculated, electrochemical, and optical energy
gaps for 4a–i.
[a]
[b]
[c]
Compd
R
Egap,calcd [eV]
Egap,elec [eV]
Egap,opt [eV]
4a
4b
4c
4d
4e
4 f
4g
4h
4i
H
Me
MeO
MeS
F
Cl
Me3Si
Ph
2.01
2.42
2.41
2.35
2.42
2.06
2.41
2.31
2.29
2.35
2.33
2.28
2.14
2.36
2.25
2.32
2.20
2.20
2.33
2.30
2.26
2.18
2.33
2.31
2.28
2.22
2.21
p-Me3Si-C6H4
Figure 5. Normalized emission spectra of tetracenes 4a–i and rubrene re-
corded in CH2Cl2 at 258C. For experimental details, see Section S11 in the
Supporting Information.
[a] TD-DFT calculations were performed at the CAM-B3LYP/6-31G(d) level
of theory with CH2Cl2 solvation (PCM). For compounds 4a–c,f, see refer-
ence [6]. [b] Electrochemical data obtained from RDV at a scan rate of
0.02 Vsꢁ1 in CH2Cl2 containing 0.1m nBu4NPF6 on a glassy carbon working
electrode. The following approximation were used: EHOMO =ꢁ(5.1+E[ox
vs.
[33]
Table 4. Summary of optical data for tetracenes 4a–i and rubrene.
Fc+/Fc]), ELUMO =ꢁ(5.1+E[red vs. Fc+/Fc]) and Egap =ELUMOꢁEHOMO
.
Electrochemi-
cal data for 4a–c,f were adapted from reference [6]. [c] lmax defined as
the wavelength measured at the maximum of the longest-absorption
[a]
Compd
R
lmax
lexc
lem
Stokes
Abs. FF
[nm]
[nm]
[nm]
Shift [eV]
band Egap =1240/lmax
.
4a
4b
4c
4d
4e
4 f
4g
4h
H
Me
MeO
MeS
F
Cl
Me3Si
Ph
532
540
548
568
532
537
543
559
561
527
498
504
509
529
495
501
507
520
520
490
559
575
600
616
556
564
572
593
597
554
0.11
0.14
0.20
0.17
0.10
0.11
0.12
0.13
0.13
0.11
0.55
0.56
0.48
0.59
0.70
0.64
0.66
0.64
0.55
0.61[b]
which displays the most red-shifted longest-wavelength ab-
sorption maximum of this series of compounds (lmax =568 nm,
2.18 eV).
4i
rubrene
p-Me3Si-C6H4
–
Stability towards photooxidation
[a] Abs. FF: Absolute fluorescence quantum yield.[25] See also reference [6]
for 4a–c,f for which lower values were measured with the comparative
method.[27] [b] Similar FF values for rubrene were reported in different
solvents: FF (toluene)=0.61;[28] FF (p-xylene)=0.56.[29]
Tetracenes are thermally stable compounds. The main flaw of
acenes in general, in terms of stability is photooxidation by
[4+2] cycloaddition reaction with photosensitized molecular
singlet oxygen, for which the acene derivative can act as the
photosensitizer. Tetracene, rubrene, pentacene, and their deriv-
atives have been reported to undergo photooxidation both in
solution and in the solid state.[34] Endoperoxides resulting from
such an oxidation reaction present little interest for materials,
since their p-system is disrupted and the intrinsic properties of
the acene core are lost (Scheme 2).
bronic band toward the near-infrared region. The emission
spectra of rubrene and the fluoro derivative 4 f are almost
overlapped with lem of 554 and 556 nm, respectively (Table 4).
Finally, the thiomethyl derivative 4d gave the most red-shifted
emission with lem =616 nm.
The electronic properties of tetracenes 4d,e,g–i were investi-
gated in detail by cyclic voltammetry (CV), rotating disc vol-
tammetry (RDV), and DFT calculations (for full data set, see
Supporting Information, Sections S13 and S14). The HOMO–
LUMO gaps measured by RDV are in good agreement with the
values obtained from the absorption spectra and range from
2.1 to 2.4 eV (Table 5). Calculations of the electronic transitions
of tetracenes were performed by means of TD-DFT at the
CAM-B3LYP/6-31G(d) level of theory using the software pack-
age Gaussian 09,[30] and solvation in CH2Cl2 was applied using
the polarizable continuum model (PCM).[31] The computed
lowest energy transitions S0!S1 differ by 0.1 to 0.3 eV from
the experimental optical energy gaps, which is reasonable con-
sidering the reported error on TD-DFT benchmarks.[32] The
HOMO and LUMO are located mostly on the tetracene core,
and the HOMO!LUMO transition corresponds to the lowest-
energy absorption band (Figure S61). The narrowest HOMO–
LUMO gap[33] was observed in the thiomethyl derivative 4d
(RDV: 2.14 eV) that is in agreement with its UV/Vis absorption,
Scheme 2. Photooxidation reaction of tetracene in the presence of singlet
oxygen.
In order to evaluate the stability of the 5,11-dicyano-6,12-di-
aryltetracene scaffold, solutions of tetracene, rubrene, and 4a–
i were subjected to UV light (254 nm/ 4ꢄ4 W) for several con-
secutive runs ranging from 0.25 to 2.0 min (Figure 6). Similar
stability studies have already been reported on other substitut-
ed tetracenes and higher acenes.[14d,34c,35] Irradiation experi-
ments were carried out in air-equilibrated CH2Cl2, and the
decay in tetracene absorption was followed by UV/Vis absorp-
tion spectroscopy. This was possible because the absorption
spectra of tetracenes differ significantly from their decomposi-
tion product (see Supporting Information, Section S12). The
data for tetracene, rubrene, and 4a–i were fitted to a pseudo-
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Conclusion
We report a straightforward 4-step route to 5,11-dicyano-6,12-
diaryltetracenes starting from easily accessible para-substituted
benzophenones. The catalytic decyanation/aromatization reac-
tion of 5,5,11,11-tetracyano-5,11-dihydrotetracenes in the final
step of the synthetic route is achieved for the first time in solu-
tion. The developed conditions are compatible with a broad
range of chemical functions and allowed the preparation of
nine highly functionalized tetracenes. The mechanisms of for-
mation of the dihydrotetracenes and the final tetracenes were
investigated by theoretical calculations. They confirm the spe-
cific proacetylenic reactivity of the central bond of tetraaryl[3]-
cumulene with TCNE as the first step of the reaction cascade
to the dihydrotetracenes. The computational study of the de-
cyanation/aromatization reaction revealed that the reaction
proceeds through two consecutive single-electron transfers in-
volving copper(I) complexes as well as a radical dihydrotetra-
cene intermediate. The properties of the 5,11-dicyano-6,12-di-
aryltetracenes were studied by means of UV/Vis absorption
and fluorescence spectroscopies, electrochemistry, and TD-DFT
calculations. They exhibit high photoluminescence quantum
yields in solution, which in several cases are superior to ru-
brene. The photostability of the 5,11-dicyano-6,12-diaryltetra-
cene scaffold was evaluated in solution and showed a signifi-
cant improvement towards oxidation, as compared to tetra-
cene or rubrene. The structures of all tetracenes were con-
firmed by single-crystal X-ray crystallography and the crystal
packings showed very close intermolecular contacts between
the p-systems of stacked molecules. Our recent investigations
showed their potential as singlet fission dyes for photovoltaics,
with highly promising results already obtained for some of the
5,11-dicyano-6,12-diaryltetracene derivatives.[12,13] With their
optoelectronic properties and high stability, paired with a
rapid, general synthetic access, 5,11-dicyano-6,12-diaryltetra-
cenes hold much interest in the context of advanced materials
science applications.
Figure 6. Photostability study of tetracene, rubrene, and 4a–i carried out in
CH2Cl2 at 258C under air, using a 254 nm UV-lamp (4ꢄ4 W). The y axis fea-
tures normalized DAbs, which is defined as the normalized difference of ab-
sorption between the absorption maximum at the longest absorption wave-
length and the absorption value after a given time at the same wavelength.
first-order rate kinetic model, which is consistent with what
has been observed with other acenes (Figure S46 and
Table S2).[36]
The parent tetracene was fully decomposed after 0.75 min
(45 s), with a half-life of 0.16 min (ca. 10 s), followed by rubrene
that was decomposed after 1.25 min (t1/2 =0.21 min) (Figure 6).
Derivatives 4a–i exhibited a remarkable robustness when mea-
sured under the same conditions, with half-lives ranging from
1.12 to 6.43 min, which represents a stability enhancement
that is 5 to 31 times greater than rubrene. MeO-substituted di-
cyanotetracene 4c, the least stable of this series, required
5 min to be fully decomposed, whereas fluoro-substituted 4e
and chloro-substituted 4 f were the most stable and required
24 and 28 min, respectively to be fully transformed.
Such a general increase in stability is mostly attributed to
the presence of the two cyano groups directly connected to
the tetracene core. The functionalization of acenes with
strongly electron-withdrawing groups (EWGs), such as nitriles,
is a known strategy to significantly improve stability, in addi-
tion of tuning electronic properties.[14,37]
Attempts to grow single-crystals of endoperoxides of 4a–i
or other possible decomposition products were unsuccessful.
Decomposition of 4c in an air-equilibrated solution was fol-
1
Experimental Section
lowed by H NMR spectroscopy. A complex mixture of prod-
ucts was observed that could not be characterized and result-
ed from either decomposition of the endoperoxide after exten-
sive periods of time in solution or the parallel formation of by-
products (Figures S44–S45).
Crystallographic data
CCDC 1517263 (1b), 1517264 (1c), 1517265 (1d), 1517266 (1e),
1517268 (1 f), 1517270 (3h), 1517272 (4d), 1517273 (4h) and
1517271 (4i) contain the supplementary crystallographic data for
this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre.
Within the 5,11-dicyano-6,12-diaryltetracene scaffold, the
nature of the substituents also has a significant influence on
the stability toward photooxidation. A tentative fitting of the
HOMO and LUMO energy levels versus the half-lives of tetra-
cenes (Figure S47) shows that the gain in stability follows the
trend of the decreasing energies of these orbitals. This obser-
vation is relevant for the further improvement of tetracenes,
which could benefit from additional EWGs directly connected
to the tetracene core.[37] The enhanced stability, compared to
tetracene or rubrene, when subjected in solution to UV light in
the presence of oxygen presumably indicates an even greater
stability in the solid state, which is desirable for optoelectronic
devices.
General procedure for the preparation of 1a–i from lithium
acetylide ethylenediamine (Method A)[18]
Lithium acetylide ethylenediamine (15 mmol) was added to a solu-
tion of benzophenone derivative (10 mmol) in THF (1m/benzophe-
none derivative). The solution was heated to reflux for 30 h, al-
lowed to cool down to RT, and diluted with EtOAc and a saturated
NH4Cl solution. The layers were separated, and the aqueous layer
was extracted with EtOAc (3x). The combined organic layers were
washed with brine once, dried over MgSO4, and evaporated. FC
(SiO2; cyclohexane/EtOAc) gave the desired product.
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followed by MeCN (19 mL). The vessel was septum-sealed and
heated to 1708C for 4 h in the microwave. The mixture was al-
lowed to reach RT. The precipitate was collected with a fritted
funnel, dissolved in dichloromethane, and filtered. After evapora-
tion, the residue was suspended in boiling MeOH and supernatant
was removed with a Pasteur pipette, affording 3i (119 mg, 53%) as
a yellow-green solid. Rf =0.43 (SiO2; cyclohexane/EtOAc 95:5); m.p.
General procedure for the preparation of 1a–i from tri-
chloroethylene (Method B)
A solution of trichloroethylene (8 mmol) in THF/Et2O 1:1 (v/v, 1.5m
with respect to trichloroethylene) was slowly added to a solution
of nBuLi (24 mmol of a 1.6m solution in n-hexane) in THF/Et2O 1:1
(v/v, 1.5m with respect to trichloroethylene) at ꢁ808C under N2.[19]
The solution became cloudy after 5 min and was stirred at RT for
2 h, cooled to ꢁ808C, and treated with the benzophenone deriva-
tive (8 mmol). The resulting mixture was stirred for 15 min at
ꢁ808C and for 24 h at RT and diluted with EtOAc and a saturated
NH4Cl solution. The layers were separated and the aqueous layer
was extracted with EtOAc (3ꢄ). The combined organic layers were
washed with brine once, dried over MgSO4, and evaporated. FC
(SiO2; cyclohexane/EtOAc) afforded the desired product.
1
3548C (decomp); H NMR (400 MHz, CDCl3, 258C): d=0.30 (s, 18H;
2 Me3Si), 0.34 (s, 18H; 2 Me3Si), 7.17 (d, J=8.3 Hz, 2H; H-C(1,7)),
7.57 (brd, J=8.3 Hz, 4H; 4 HAr), 7.60 (brd, J=8.2 Hz, 4H; 4 HAr)
overlapping with 7.61 (brd, J=8.3 Hz, 4H; 4 HAr) overlapping with
7.63 (brdd, J=8.3, 1.8 Hz, 2H; H-C(2,8)), 7.68 (brd, J=8.2 Hz, 4H; 4
HAr), 7.74 (brd, J=8.2 Hz, 4H; 4 HAr), 7.86 (brd, J=8.4 Hz, 4H; 4
HAr), 8.05 ppm (d, J=1.7 Hz, 2H; H-C(4,10)); 13C NMR (101 MHz,
CDCl3, 258C): d=ꢁ1.05 (2 Me3Si), ꢁ0.94 (2 Me3Si), 40.62 (C(5,11)),
113.13 (4 CꢀN), 119.36 (C(5a,11a)), 125.66 (C(4,10)), 126.47 (4 CH),
126.74 (4 CH), 126.83 (2 C), 127.89 (4 CH), 129.33 (C(2,8)), 129.97
(C(1,7)), 131.12 (2 C), 131.38 (4 CH), 132.06 (2 C), 134.12 (4 CH),
134.29 (4 CH), 138.88 (2 C), 140.35 (2 C), 140.59 (2 C), 141.59 (2 C),
142.50 (2 C), 143.45 (2 C), 144.15 ppm (2 C); IR (ATR): v˜ =3068 (w),
3015 (w), 2953 (w), 2897 (w), 1599 (w), 1537 (w), 1513 (w), 1488
(w), 1407 (w), 1387 (w), 1313 (w), 1246 (m), 1112 (m), 1059 (w),
1004 (w), 898 (w), 837 (s), 812 (s), 755 (m), 720 (w), 710 (w), 693
(w), 636 (w), 615 (w), 606 cmꢁ1 (w); UV/Vis (CH2Cl2): lmax (e)=380
General procedure for the preparation of [3]cumulenes 2a–
i[20]
A suspension of SnCl2·2H2O (1.1 equiv) in Et2O (0.1m/1,1,4,4-tetra-
phenylbut-2-yne-1,4-diol derivative) under N2 was treated with 2m
HCl in Et2O (2.2 equiv). The mixture was treated with 2 (1 equiv)
and stirred at RT. After completion of the reaction according to
TLC analysis, the suspension was filtered over a fritted funnel. The
solid was washed with a minimum of Et2O, affording the desired
product.
(30900), 269 nm (100700mꢁ1 cmꢁ1); HR-MALDI-MS: m/z (%):
1075.4391 (13), 1074.4355 (15) [M]+ (calcd for C70H66N4Si4
:
+
1074.4359), 1049.4356 (84), 1048.4327 (100) [MꢁCN]+ (calcd for
C69H66N3Si4+: 1048.4328), 1023.4328 (9), 1022.4295 (10) [Mꢁ2 CN]+
(calcd for C68H66N2Si4+: 1022.4298).
6,12-Di[(1,1’-biphenyl)-4-yl]-3,9-diphenyltetracene-5,5,11,11-
tetracarbonitrile (3h)
[3]Cumulene 2h (100 mg, 0.15 mmol) and tetracyanoethylene
(38 mg, 0.30 mmol) were placed in a microwave vessel under Ar
followed by MeCN (20 mL). The vessel was septum-sealed and
heated to 1708C for 4 h in the microwave. The mixture was al-
lowed to reach RT. The precipitate was collected with a fritted
funnel, dissolved with dichloromethane, and filtered. After evapo-
ration, the residue was suspended in boiling MeCN and superna-
tant removed with a Pasteur pipette. This operation was repeated
with boiling MeOH, affording 3h (33 mg, 28%) as a yellow solid.
Rf =0.53 (SiO2; cyclohexane/EtOAc 8:2); m.p. 3198C (decomp);
1H NMR (400 MHz, CDCl3, 258C): d=7.17 (d, J=8.24 Hz, 2H; H-
C(1,7)), 7.38–7.47 (m, 8H; 8 HAr), 7.52 (brt, J=7.6 Hz, 4H; 4 HAr),
7.57–7.62 (m, 10H; H-C(2,8) and 8 HAr), 7.75 (brdd, J=8.3, 1.4 Hz,
4H; 4 HAr), 7.86 (brd, J=8.3 Hz, 4H, 4 HAr), 8.05 ppm (d, J=1.7 Hz,
2H; H-C(4,10)); 13C NMR (101 MHz, CDCl3, 258C): d=40.63 (C(5,11)),
113.15 (4 CꢀN), 119.34 (C(5a,11a)), 125.67 (C(4,10)), 126.83 (2 C),
127.23 (4 CH), 127.45 (4 CH), 127.91 (4 CH), 128.13 (2 CH), 128.92 (2
CH), 129.10 (4 CH), 129.31 (4 CH), 129.37 (C(2,8)), 129.97 (C(1,7)),
131.06 (2 C), 131.38 (4 CH), 131.99 (2 C), 138.59 (2 C), 140.24 (2 C),
142.50 (2 C), 143.53 (2 C), 144.21 ppm (2 C); IR (ATR): v˜ =3031 (w),
2923 (w), 2847 (w), 1804 (w), 1739 (w), 1675 (w), 1605 (w), 1550
(w), 1512 (w), 1484 (m), 1447 (w), 1395 (w), 1306 (w), 1276 (w),
1181 (w), 1156 (w), 1141 (w), 1111 (w), 1075 (w), 1024 (w), 1008 (w),
972 (w), 915 (w), 893 (w), 841 (m), 767 (s), 753 (s), 737 (m), 695 (s),
674 (w), 610 (w), 624 cmꢁ1 (w); UV/Vis (CH2Cl2): lmax (e)=375
General procedure for the preparation of tetracenes 4a–i
A microwave vessel under argon was charged with dihydrotetra-
cene derivative (1 equiv), CuBr2 (20 mol%), toluene (dihydrotetra-
cene derivative/0.07m), N,N’-dimethylethylenediamine (200 mol%)
and was then septum-sealed. The mixture was either heated to
2008C in the microwave for 4 h or heated to reflux for a given
amount of time. The mixture was cooled down to RT, and diluted
with CH2Cl2 and water. The layers were separated. The organic
layer was washed with water, brine, dried over Na2SO4, and evapo-
rated. The residue was suspended in hot MeOH, and supernatant
was removed with a Pasteur pipette. This operation was repeated
with MeOH once more, and then with MeCN twice, affording the
tetracene derivative.
Acknowledgements
This work was supported by a grant from the Swiss National
Science Foundation SNF 200020_159802. The authors thank Dr.
Bruno Bernet for his assistance in the spectral evaluations and
Michael Solar for his help with collecting X-ray data. X-ray serv-
ices were provided by the Small Molecule Crystallography
Center of ETH Zurich (http://www.smocc.ethz.ch).
(25000), 264 nm (79300mꢁ1 cmꢁ1); HR-MALDI-MS: m/z (%):
761.2777 (61), 760.2745 (100) [MꢁCN]+ (calcd for C57H34N3
:
+
Conflict of interest
760.2747).
The authors declare no conflict of interest.
6,12-Bis[4’-(trimethylsilyl)(1,1’-biphenyl)-4-yl]-3,9-bis[4-(tri-
methylsilyl)phenyl]tetracene-5,5,11,11-tetracarbonitrile (3i)
Keywords: decyanation · photostability · polycyclic aromatic
hydrocarbons · tetraaryl[3]cumulenes · tetracenes
[3]Cumulene 2i (200 mg, 0.21 mmol) and tetracyanoethylene
(54 mg, 0.42 mmol) were placed in a microwave vessel under Ar
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Manuscript received: August 20, 2017
Accepted manuscript online: && &&, 0000
Version of record online: && &&, 0000
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ÝÝ These are not the final page numbers!

Full Paper
FULL PAPER
&
Polycyclic Aromatic Hydrocarbons
N. Kerisit, P. Gawel, B. Levandowski,
Y.-F. Yang, V. Garcꢀa-Lꢁpez, N. Trapp,
L. Ruhlmann, C. Boudon, K. N. Houk,*
F. Diederich*
From tetra steps to tetracenes: The
presented substituted 5,11-dicyano-
6,12-diaryltetracenes are synthesized
from 5,5,11,11-tetracyano-5,11-dihydro-
tetracenes, using a newly developed de-
cyanation/aromatization reaction cata-
lyzed by CuI or CuII complexes in solu-
tion. The mechanism of this transforma-
tion is explored by theoretical calcula-
tions. The obtained tetracene deriva-
tives exhibit outstanding optoelectronic
properties and enhanced photostability.
&& – &&
A Four-Step Synthesis of Substituted
5,11-Dicyano-6,12-diaryltetracenes
with Enhanced Stability and High
Fluorescence Emission
Chem. Eur. J. 2017, 23, 1 – 11
www.chemeurj.org
11
ꢃ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
These are not the final page numbers! ÞÞ