One-pot synthesis of stable NIR tetracene diimides via double cross-coupling

Article abstract of DOI:10.1021/ja207630a

Tetracene tetracarboxylic diimides have been synthesized based on direct double ring extension of electron-deficient naphthalene diimides involving metallacyclopentadienes. Atomic structure and electronic transitions responsible for their NIR absorption spectra are investigated with quantum-chemical calculations. In light of their unique structure and admirable photophysical and electronic properties, this new molecular skeleton is promising candidate for n-type semiconductors.

Full text of DOI:10.1021/ja207630a

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One-Pot Synthesis of Stable NIR Tetracene Diimides via Double
Cross-Coupling
||
Wan Yue,†,‡ Jing Gao,†,‡ Yan Li,*,† Wei Jiang,† Simone Di Motta,§, Fabrizia Negri,*,§,|| and Zhaohui Wang*,†
†Beijing National Laboratory for Molecular Science, Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of
Science, Beijing 100190, China
‡Graduate School of the Chinese Academy of Sciences, Beijing 100190, China
§Dipartimento di Chimica ‘G. Ciamician’, Universitꢀa di Bologna, Via F. Selmi, 2, 40126 Bologna, Italy
INSTM, UdR Bologna, Italy
S Supporting Information
b
stable and NIR tetracene derivatives functionalized by tetracar-
ABSTRACT: Tetracene tetracarboxylic diimides have been
synthesized based on direct double ring extension of electron-
deficient naphthalene diimides involving metallacyclopen-
tadienes. Atomic structure and electronic transitions re-
sponsible for their NIR absorption spectra are investigated
with quantum-chemical calculations. In light of their unique
structure and admirable photophysical and electronic prop-
erties, this new molecular skeleton is promising candidate
for n-type semiconductors.
boxylic diimides substituents in one pot under mild conditions.
Inspired by fused aromatic rings via intermolecular coupling of
metallacyclopentadiene with dihalo aromatics,7 we envisage that
double cross-coupling of metallacyclopentadiene with electron-
deficient tetrabrominated naphthalene tetracarboxylic diimides
will provide the desired tetracene diimides. Accordingly, 2,3,6,7-
tetrabromo-1,4,5,8-naphthalene tetracarboxylic diimides 1 (TBNDIs)
are prepared according to a known procedure.8 The coupling
reaction of compound 1 and zirconacyclopentadiene 2 proceeds
well in THF in the presence of copper chloride at 50 °C
(Scheme 1) to give desired tetracene diimides (TDIs) derivative
5 in good yield.
he design and synthesis of acenes have attracted consider-
Encouraged by the success of such a facile double aromatic
annulation, we used a range of zirconacyclopentadienes to check
the scope of our procedure. By this method, different substitu-
ents can be conveniently introduced into the extended aromatic
cores. Interestingly, the coupling of TBNDIs with zirconacyclo-
pentadiene 3 or 4 prepared from 3,9-dodecadiyne or 1,2-di(hept-
2-yn-1-yl)benzene provides octahydrotetracene 6 or tetrahydro-
tetracene 7 also in a moderate yield, which is expected to be
further converted to higher acenes tetracarboxylic diimides. Nota-
bly, the introduction of semiperfluoroalkyl chain at the imides
position of the TBNDIs also proceeded well. Incorporation of
this semiperfluoroalkyl chain is known to have a strong influence
not only on their self-assembled structures, but also on their
transporting properties.9
Palladium-catalyzed double cross-coupling of 9-stannafluor-
ene 9 with 1,2-dihaloarenes is known to serve as a new entry to
aromatic annulation and provide a variety of triply annulated
benzene derivatives.10 As we expected, the cross-coupling reac-
tions of 9-stannafluorene 9 with TBNDI 1 in the presence of
[Pd(P(t-Bu)3)2] and CsF in THF at 70 °C proceed well to afford
tetrabenzotetracene diimides 11 (Scheme 2). Meanwhile, when
we choose 1,1-dimethyl-2,3,4,5-tetraphenylstannole11 as cou-
pling reagent under the same conditions, surprisingly, we get
the product 10, which has extreme steric hindrance due to the
eight phenyl groups in a moderate yield.
T
able interests owing to their potential applications as p-type
organic semiconductors in organic electronics, such as field-effect
transistors, organic light-emitting diodes, and solar cells.1 Typi-
cally, tetracene and pentacene derivatives have been the subject
of intense study because of the unique electronic properties
associated with their π-bond topology.2 One efficient approach
to optimize the performance of acenes is to integrate functional
groups into the parent molecules to improve their stability and
processability.3 However, the systematic change of effective func-
tional groups on acenes poses a great challenge since there are
very few synthetic methodologies that can serve this purpose.
Compared with aryl, alkynyl-subsitituted, and other function-
alized groups, introduction of strong electron-withdrawing sub-
stituent to acenes is believed to be very crucial to invert the majority
carrier sign to achieve efficient electron transport.4 Perylene
tetracarboxylic diimides (PDIs) and naphthalene tetracarboxylic
diimides (NDIs), which combine p-type perylene or naphthalene
aromatic cores with strong electron-withdrawing tetracarboxylic
diimides substituents, were used as promising building blocks for
most extensively investigated n-type semiconductors.5 Thus, a
similar strategy is expected to create new series of acene tetra-
carboxylic diimides as new n-type building blocks.
Aromatic annulation by double cross-coupling reactions of
organometallic reagents and electron-rich acenes with dihalides
function groups is an efficient method for region- and stereo-
specific formation of CꢀC bonds, which would provide a straight-
forward and promising method for extension of polycyclic aromatic
molecules.6 However, the efficient synthetic approach based on
electro-deficient acenes for big π-delocalized functional mol-
ecules are rarely reported. Herein, we present a new family of
To determine the molecular structure of TDIs derivatives,
crystal of 5c suitable for single-crystal X-ray structure analysis was
Received: August 12, 2011
Published: October 17, 2011
r
2011 American Chemical Society
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dx.doi.org/10.1021/ja207630a J. Am. Chem. Soc. 2011, 133, 18054–18057
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Scheme 1. The Synthetic Route towards TDIsa
Figure 1. Molecular structure of 5c, the X-ray structure along the
crystallographic b axis (right), the X-ray structure along the crystal-
lographic c axis (left).
a Conditions: TBNDIs, zirconacyclopentadiene, CuCl, THF, 50°C, argon.
The yields of compounds: 5a (55%), 6a (48%), 5b (52%), 7 (48%), 5c
(44%), and 6b (35%).
Scheme 2. Palladium-Catalyzed Annulation of TBNDI with
Stannane Reagentsa
Figure 2. UVꢀvis-NIR absorption spectra of 5a (black), 10 (red), and
11 (blue) in chloroform.
The absorption spectra of the representative TDIs in chloro-
form are shown in Figure 2, showing broad absorption which
covers the whole visible and NIR region, rendering them as “full-
absorption dyes’’ and therefore potential objects in solar cells.
Compared with their all-carbon parent, tetracene (474 nm),13
those TDIs display broad and significantly red-shifted spectra
and have higher molar extinction coefficient in the visble-NIR
region. The remarkable difference between TDIs and tetracene
in their absorption peaks and intensities reflect the substantial
electronic effect of the attachment of strong electron-withdraw-
ing diimides groups. Furthermore, in contrast with NDI (380 nm),14
which show absorption only in the UV region, the fusion of two
additional aromatic rings also leads to much more red shift in
absorption as a reflection of a larger extensive conjugation over
the electronic system (see below).
a Conditions: TBNDIs, 8 or 9, [Pd(P(t-Bu)3)2], CsF, THF, 70 °C,
argon. The yields of compounds: 10 (32%) and 11 (42%).
obtained by slow evaporation of a solution of dichloromethane
and methanol at room temperature.12 As can be seen from
Figure 1, the molecule crystallizes in the monoclinic Cc space
group, the crystal structure reveals that the molecule has crystal-
lographically imposed inversion symmetry. Owing to the steric
encumbrance effect between oxygen atoms and neighboring
group, the tetracene core is found to be markedly nonplanar as
well as the two imides rings with dihedral angles of 30ꢀ32°. The
heliciform structure revealed by the crystal structure corresponds
indeed to the lowest energy computed structure (B3LYP/6-31G*)
of 5 and 11 (see Figures S2ꢀS3 and Table S1).
The absorption spectra of 11 lead to a remarkable change.
The purple-red solution of compound 11 exhibits a major
absorption band at 584 nm with a blue shift of 176 nm relative
to 10 (λmax = 740 nm), as a reflection of the interruption of
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Table 1. Opto-Electronic Properties of TDIs and Energy
Levels of Representative Compounds
λ1 [nm]a E1r [V]b LUMO [eV]c HOMO (eV)d Eg [eV]e
NDIf
5a
380
756
780
774
737
584
ꢀ0.63
ꢀ0.40
ꢀ0.40
ꢀ0.29
ꢀ0.35
ꢀ0.32
ꢀ3.90
ꢀ4.06
ꢀ4.06
ꢀ4.17
ꢀ4.12
ꢀ4.14
ꢀ7.02
ꢀ5.54
ꢀ5.50
ꢀ5.50
ꢀ5.68
ꢀ6.13
3.12
1.48
1.44
1.43
1.56
1.99
6a
5c
10
11
a λ1 as peak of the visible-NIR regions b V vs Ag/AgCl. Half wave redox
potential (in V vs Ag/AgCl) measured in CH2Cl2 with a scan rate of
0.1 V/s. c Calculated by measuring the onset potential of the first
reduction wave. d Estimated from LUMO levels and Eg. e Obtained from
f
the edge of the absorption spectra. NDI: N,N0-di (n-octyl)-naphtha-
lene-1,2:6,7-tetracarboxylic bisimide.
Figure 3. Energies and shapes of B3LYP/6-31G* frontier orbitals
(HOMO and LUMO) of tetracene, NDI, and a planar model of 5.
which could be related to the increasing conjugation length, and
the LUMO fall into ꢀ4.0 to ꢀ4.2 eV indicating that all of them
could be served as candidates for air-stable n-type semiconduc-
tors. Notably, the value of the gaps are comparable to the
HOMOꢀLUMO gap of heptacene,15 which means that the
introduction of electron-withdrawing tetracarboxylic diimides
can reduce the gaps more efficiently than the fusion of rings.
All the TDIs derivatives show good solubility in common
organic solvents such as dichloromethane, chloroform, toluene,
and tetrahydrofuran. Their structures are unambiguously identi-
tetrabenzo-units to the effective conjugation along the cores
of the TDIs.
An analysis in terms of the orbital nature of the TD-B3LYP/6-
31G* computed electronic transitions of 5 and other TDI
derivatives (see Tables S1ꢀS3 and Figures S4ꢀS5 for a compar-
ison between computed and observed spectra) sheds more light
on the nature of the lowest energy transitions (and specifically
the NIR transitions of TDIs). The computed excitation energies
show that the lower energy portion of the absorption spectra of
5 (and other TDI derivatives) and 11 is dominated by two low
lying transitions, the first of which is weaker than the second one,
considerably more intense (see also Tables S1ꢀS3). The lowest
energy and weaker transition (responsible for the NIR absorp-
tion of TDIs) corresponds always to the HOMO f LUMO
excitation.
The HOMO orbital of all the TDI derivatives is dominated by
the tetracenic HOMO nature, while the LUMO orbital shows the
typical character of diimide derivatives (see Figure 3, Figure S6
and Table S2). Therefore, the low energy transition of TDI
derivatives is a new transition reflecting the peculiar frontier
orbital nature. While for 5 the S0 f S1 transition is well separated
in energy from the next, higher intensity transition, the two
excitations are almost degenerate for 11 (see Table S1). Inter-
estingly, the second (more intense) transition of TDIs is also
dominated by a π f π* electronic excitation from an occupied
orbital of tetracenic nature to the LUMO (see Figure S7 and
Table S3).
1
fied by H NMR, 13C NMR spectroscopy, and MALDI-TOF.
They exhibited very good stability on exposure to light and air
condition, whereas the tetracene is air and light sensitive materials,
that is, the introduction of strong electron-withdrawing imides
groups into the tetracene improved their photo-oxidative resis-
tances significantly and make them to be stable n-type semi-
conductors and NIR dyes (Supporting Information).16
In conclusion, a new family of stable low band gap tetracene
tetracarboxylic diimides derivatives has been designed and syn-
thesized for the first time from readily available TBNDIs. In light
of their unique structure and admirable photophysical and
electro-optical properties, this new molecular skeleton is promis-
ing candidate for air stable n-type semiconductors and solar cell
materials. We believe that this straightforward route, based on
direct double ring extension involving organometallic reagents,
opens a new way to access elusive stable and higher acenes
tetracarboxylic diimides in the near future.
’ ASSOCIATED CONTENT
The redox properties of these TDIs were studied by cyclic
voltammetry in dichloromethane (in V vs Ag/AgCl). The half-
wave reduction potentials of the representative compounds are
ꢀ0.63, ꢀ1.11 V for NDI; ꢀ0.40, ꢀ0.73 V for 5a; and ꢀ0.32,
ꢀ0.64 V for 11. The first reduction potentials of these TDIs are
much less negative than those of parent NDI (Table 1), thus,
revealing the extremely strong electron-accepting abilities, and
the first reduction potential of 5c is less negative by about 0.1 V
than the other alkyl substituted TDIs, which is probably due to
the introduction of semiperfluoroalkyl chains.
S
Supporting Information. Experimental details and char-
b
acterization data of all new compounds, computational details, opti-
mized structures of 5 and 11, computed absorption spectra, room-
temperature UVꢀvis-NIR spectral measurements. This material is
available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Authors
wangzhaohui@iccas.ac.cn; fabrizia.negri@unibo.it; yanli@iccas.ac.cn
The optical band gaps and LUMO energy levels of these
compounds were calculated based on UVꢀvis-NIR absorption
data and the onset potential of the first reduction wave. It is worth
mentioning that the band gap of these TDIs are much smaller
than the gap of NDI (λmax = 380 nm, optical gap = 3.12 eV),
’ ACKNOWLEDGMENT
For financial support of this research, we thank the National
Natural Science Foundation of China (91027043), 973 Program
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dx.doi.org/10.1021/ja207630a |J. Am. Chem. Soc. 2011, 133, 18054–18057

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(2011CB932301), NSFC-DFG joint project TRR61, Solvay, the
Chinese Academy of Sciences, and Italian PRIN 2008, project
JKBBK4.
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