Unexpected photooxidation of H-bonded tetracene

Article abstract of DOI:10.1021/ol800620s

In our attempts of tuning the molecular packing of tetracene with H-bonding, we found that tetracenediamide was much more vulnerable to photooxidation than tetracene in crystals despite their similar sensitivity to photooxidation In solution. Unexpectedly photooxidation of tetracenediamide in solution and in crystals show different regioselectivities. To explain the regioselectivity, a mechanism Involving H-bonding is proposed. This study Indicates that molecular packing in solid state can play an important role In solid-gas reactions.

Full text of DOI:10.1021/ol800620s

ORGANIC
LETTERS
2008
Vol. 10, No. 10
2007-2010
Unexpected Photooxidation of H-Bonded
Tetracene
Zhixiong Liang,† Wei Zhao,† Shenglong Wang,‡ Qin Tang,† Sheung-Chuen Lam,†
and Qian Miao*,†
Department of Chemistry, the Chinese UniVersity of Hong Kong, Shatin, New
Territories, Hong Kong, China, and Department of Chemistry, New York UniVersity,
New York
miaoqian@cuhk.edu.hk
Received March 17, 2008
ABSTRACT
In our attempts of tuning the molecular packing of tetracene with H-bonding, we found that tetracenediamide was much more vulnerable to
photooxidation than tetracene in crystals despite their similar sensitivity to photooxidation in solution. Unexpectedly photooxidation of
tetracenediamide in solution and in crystals show different regioselectivities. To explain the regioselectivity, a mechanism involving H-bonding
is proposed. This study indicates that molecular packing in solid state can play an important role in solid-gas reactions.
Organic field effect transistors (OFETs) offer promising
applications in large-area, flexible, and low-cost electronics.1
However, degradation of device performance when operated
in the presence of oxygen and moisture remains a critical
challenge for the development of OFETs.2 One key factor
that leads to the device performance degradation of OFETs
is the oxidation of organic semiconductor molecules in
ambient air. Pentacene and tetracene and their derivatives
are an important family of organic semiconductors applied
in OFETs3 but suffer from the atmospheric instability toward
photooxidation in air.4,5> One widely accepted mechanism
for such photooxidation is that the photoexcited acene
molecule sensitizes the formation of singlet oxygen,5,6 which
then reacts with ground-state acene to generate endoperox-
ide.7–9 In our attempts to modulate the molecular packing
of tetracene with H-bonding, we found that yellow crystals
of tetracenediamide (structure shown in Scheme 1) underwent
photooxidation with the color faded and the crystallinity not
destructed when they were exposed to ambient air and light.
Our unexpected findings are that molecular packing in solid
state plays an important role in photooxidation of tetracene-
diamide, showing effects not only on the reaction rate but
also on the product regioselectivity (shown in Scheme 1).
† The Chinese University of Hong Kong.
(5) Maliakal, A.; Raghavachari, K.; Katz, H.; Chandross, E.; Siegrist,
T. Chem. Mater. 2004, 16, 4980–4986.
‡ New York University.
(1) (a) Dimitrakopoulos, C. D.; Malenfant, P. R. L. AdV. Mater. 2002,
14, 99–117. (b) Dodabalapur, A. Mater. Today 2006, 9, 24–30. (c) Jang, J.
Mater. Today 2006, 9, 46–52.
(6) (a) Aubry, J.-M.; Pierlot, C.; Rigaudy, J.; Schmidt, R. Acc. Chem.
Res. 2003, 36, 668–675. (b) Greer, A. Acc. Chem. Res. 2003, 39, 797–804.
(7) Ono, K.; Totani, H.; Hiei, T.; Yoshino, A.; Saito, K.; Eguchi, K.;
Tomura, M.; Nishida, J.; Yamashita, Y. Tetrahedron 2007, 63, 9699–9704.
(8) (a) Chan, S. H.; Lee, H. K.; Wang, Y. M.; Fu, N. Y.; Chen, X. M.;
Cai, Z. W.; Wong, H. N. C. Chem. Commun. 2005, 66–68. (b) Zhou, X.;
Kitamura, M.; Shen, B.; Nakajima, K.; Takahashi, T. Chem. Lett. 2004,
33, 410–411.
(2) (a) Facchetti, A. Mater. Today 2007, 10, 28–37. (b) Murphy, A. R.;
Fre´chet, J. M. J. Chem. ReV. 2007, 107, 1066–1096. (c) Anthony, J. E Chem.
ReV. 2006, 106 (12), 5028–5048.
(3) (a) Bendikov, M.; Wudl, F.; Perepichka, D. F. Chem. ReV. 2004,
104 (11), 4891–4945. (b) References 2b and 2c.
(4) Yamada, M.; Ikemoto, I.; Kuroda, H. Bull. Chem. Soc. Jpn. 1988,
(9) Bjarneson, D. W.; Petersen, N. O. J. Photochem. Photobiol. A: Chem.
1992, 63, 327–335.
61, 1057–1062
.
10.1021/ol800620s CCC: $40.75
Published on Web 04/24/2008
2008 American Chemical Society

Scheme 1
.
Photooxidation of Tetracene and Its Derivatives
Scheme 2. Synthesis of Tetracenediamide
contrast, the crystals were stable in ambient air but in the
1
dark. As indicated by H NMR and HRMS, the resulting
colorless crystals only contained peroxide 2a (shown in
Figure 1), indicating that the photooxidation of tetracene-
The study below details synthesis and unexpected photo-
oxidation of tetracenediamide.
This work was initiated by our attempts to better
understand the relationship between molecular packing and
electrical properties, which is another key question in the
research of OTFTs.10 In our study on tuning molecular
packing of tetracene, we designed N,N′-dihexyl-5,12-
tetracenedicarboxamide, which equips tetracene backbone
with amide groups for formation of H-bonds11 and alkyl
groups for better solubility. Due to lack of direct ways to
functionalize tetracene at C5 and C12,12 dihydrotetracene13
4 was used as shown in Scheme 2 so that the two internal
rings of tetracene backbone could be differentiated.
Treatment of 4 with n-BuLi at the presence of TMEDA
yielded the dianion of tetracene, which was quenched with
CO2 to yield dicarboxylic acid 5. Diamide 6 happened to
be inert to attempted dehydrogenation by DDQ or Pd/C.
The dehydrogenation step was finally achieved by heating
6 with CuO.14
Figure 1. Photooxidation of tetracenediamide in crystals monitored
by microscopy: (a) viewed with normal light; (b) viewed with
polarized light between two crossed polarizer.
diamide was complete. The photooxidation of tetracenedia-
mide in crystals was monitored by microscopy with the
crystals being exposed to ambient air and the light from the
halogen lamp of microscope. As shown in Figure 1a, no
change on the crystal shape is observed during the progress
of photooxidation. The polarized light micrographs for
crystals of tetracenediamide shown in Figure 1b were viewed
between crossed polarizers during the progress of photooxi-
dation and clearly display birefringent crystals of the
photooxidation product. These facts indicate that the pho-
tooxidation occurs in molecular crystals of tetracenediamide
without destruction of crystallinity and suggest that only
small motions of atoms are involved during photooxidation.
In contrast, crystals of tetracene showed no observable
changes after being exposed to the ambient air and the same
light for 24 h.
The yellow crystals of tetracenediamide turned colorless
after being exposed to ambient air and light for 24 h. In
(10) (a) Coropceanu, V.; Cornil, J.; da Silva Filho, D. A.; Olivier, Y.;
Silbey, R.; Bredas, J.-L. Chem. ReV. 2007, 107 (4), 926–952. (b) Refer-
ence 2.
(11) (a) Bushey, M. L.; Hwang, A.; Stephens, P. W.; Nuckolls, C. J. Am.
Chem. Soc. 2001, 123, 8157–8158. (b) Lightfoot, M. P.; Mair, F. S.;
Pritchard, R. G.; Warren, J. E. Chem. Commun. 1999, 1945–1946. (c) Yang,
E.; Fan, J.; Geib, S. J.; Stoner, T. C.; Hopkins, M. D.; Hamilton, A. D.
J. Chem. Soc. Chem. Commun. 1995, 1251.
Thin films of tetracenediamide that were deposited by
thermal evaporation behaved as semiconductors in OFETs
initially but turned insulating after being exposed to air and
ambient light for several hours with the yellow films turning
colorless. The device degradation is apparently due to
photooxidation of tetracenediamde.
(12) Direct bromination of tetracene yields 5,11-dibromotetracene or a
mixture of isomers, see: (a) Avlasevich, Y.; Mu¨llen, K. Chem. Commun.
2006, 4440–4442. (b) Kyushin, S.; Ishikita, Y.; Matsumoto, H.; Horiuchi,
H.; Hiratsuka, H. Chem. Lett. 2006, 35, 64–65.
(13) Dihydrotetracene was previously synthesized by a different method,
see: (a) Luo, J.; Hart, H J. Org. Chem. 1987, 52 (22), 4833–4836. For
example of using LiAlH4/AlCl3 to reduce quinone to dihydro-arene, see:
(b) Shahlai, K.; Hart, H. J. Org. Chem. 1991, 56, 6905–6912.
2008
Org. Lett., Vol. 10, No. 10, 2008

Photooxidation of tetracenediamide and tetracene in solu-
tion was investigated. As shown in Figure 2a, the solutions
level of lowest unoccupied molecular orbital (LUMO) of
pentacene. Such stabilizing effect of amide substituent
appears to be very small in tetracenediamide not only due
to the weak electron-withdrawing nature of amide but also
because the unfavorable interactions between amide groups
and H atoms of the adjacent rings prevent effective conjuga-
tion between amide and tetracene core.
To answer the first question, the molecular packing of
tetracenediamide has to be considered. Unfortunately, crystals
of tetracenediamide grown from solutions were not suitable
for X-ray crystallography. On the other hand, the crystal
structure of its smaller analogue, N,N’-dihexyl-9,10-an-
thracenediamide (7), is solved and shown in Figure 3. Crystal
Figure 2. (a) UV-vis absorption of 0.05 mM solutions of
tetracenediamide and tetracene in CHCl3; (b) the absorbance as a
function of time for the solutions of tetracenediamide and tetracene
(0.05 mM in CHCl3) exposed to ambient light.
Figure 3. Molecular and crystal structure of N,N′-dihexyl-9,10-
anthracenediamide with amide groups highlighted as balls and sticks
and the other parts displayed as fine sticks.
of tetracene and tetracenediamide in CHCl3 have similar
UV-vis absorption spectra showing the longest wavelength
absorption at 475 and 488 nm, respectively, which decreases
with the progress of photooxidation because conjugation of
tetracene backbone is broken in the products. Therefore the
decrease in the absorbance of tetracene at 475 nm and that
of tetracenediamide at 488 nm as a function of time was
used to monitor the progress of photooxidation. As shown
in Figure 2b, tetracene and tetracenediamide in solutions have
similar stability to photooxidation. To characterize the
photooxidation products, solutions of the two compounds at
higher concentrations were open to air and subject to
irradiation with a 500W tungsten lamp, and the two major
products 2a and 2b were isolated by column chromatography
in a yield of 25 and 49%, respectively, as shown in Scheme
1. The structures of 2a and 2b were determined by comparing
structure of 7 clearly shows H-bonds between the amide
groups, which are out of the aromatic plane with NsH atoms
on the opposite sides. The closest H-to-C and C-to-C
distances between anthracene backbones are 2.99 Å and 3.56
Å respectively indicating a loose packing of anthracene
backbones in comparison with the herringbone packing of
unsubstituted anthracene, which has the closet intermolecular
H-to-C and C-to-C distances of 2.53 and 3.51 Å, respec-
tively.15 We assume that tetracenediamide has a similar
packing motif in solid state as its anthracene analogue with
H-bonds between amide groups and a loose packing of
tetracene backbones. Such assumption is supported by the
UV-vis absorption of tetracenediamide in thin films. The
longest wavelength absorbance of tetracenediamide (shown
in Figure S-1 in Supporting Information), essentially does
not change on going from solution to a thin film while that
of tetracene shifts to the red by approximately 45 nm on
going from solution (475 nm) to a thin film (520 nm). The
red shift of tetracene can be attributed to electronic delo-
calization due to aromatic interactions16 whereas the absence
of red shift in tetracenediamide suggests weak interactions
between aromatic backbones due to loose packing. The high
reactivity of tetracenediamide to photooxidation can thus be
1
their H NMR spectra with that of 1 (the photooxidation
product of tetracene).9
The above studies raised two questions: why tetracene-
diamide is much more vulnerable to photooxidation than
tetracene in the crystal state although the two compounds
have similar stability in solution and why photooxidation of
tetracenediamide shows different regioselectivity in solution
and in crystals. The similar stabilities of tetracene and
tetracenediamide in solutions reflect the chemical properties
of isolated molecules because no association was observed
at the concentration used in those experiments. It has been
reported that ethynyl5 and pentafluorophenyl7 substituents
at C6 and C13 can increase the stability of pentacene toward
photooxdiation in solution because they lower the energy
(14) For example of dehydrogenation by CuO, see: Satchell, M. P.;
Stacey, B. E. J. Chem. Soc. 1971, 468.
(15) Mason, R. Acta Crystallogr. 1964, 17, 547.
(16) Lim, S.-H.; Bjorklund, T. G.; Spano, F. C.; Bardeen, C. J. Phys.
ReV. Lett. 2004, 92, 107402.
Org. Lett., Vol. 10, No. 10, 2008
2009

attributed to its loose molecular packing, which allows
diffusion of O2 into the crystal lattice. On the other hand,
the condensed packing of tetracene prevents O2 diffusion.4,5
To explain the regioselectivities for photooxidation of
tetracenediamide in crystals and in solution, we propose a
mechanism involving H-bonds between photosentisized
singlet O2 and tetracenediamide.17 As might be expected,
we found two locally stable geometries with almost equal
energy for tetracenediamide in density funcitonal theory
(DFT) optimization with B3LYP/6-31G(d) method.18 As
shown in Figure 4a, one geometry has the N-H atoms of
conformation exclusively in crystals as its anthracene
analogue (7) does (shown in Figure 3). Thus photooxidation
of the anti conformer in crystals yields 2a as the only product.
The proposed mechanism is supported by several
experimental facts. Photooxidation of tetracenediester
(shown in Scheme 1), an analogue of tetracenediamide
that can not form H-bonds, occurred in solution yielding
peroxide 3 as the major product in an isolated yield of
88%. Meanwhile solids of tetracenediester showed no
observable changes after being exposed to the ambient
air and light for 24 h. The second evidence comes from
photooxidation of tetracenediamide in solution at the
presence of DMSO, a H-bond acceptor that can compete
with O2. Upon adding 40 µL of DMSO-d6 to a 10 mM
solution of tetracenediamide in 0.5 mL of CDCl3, the
photooxidation product ratio 2a:2b changed from 1:2.7
1
to 1.7:1 as monitored by H NMR. The two experiments
above suggest that H-bonds play an important role in the
photooxidation of tetracenediamide in solution. Moreover
the energy minmized model of 2b displayed in Figure 4a
shows N-H atoms of both amide groups at the same side
forming H-bonds to bridge oxygen atoms. In contrast,
intramolecular H-bonds are not observed in energy
Figure 4. (a) DFT optimization models of tetracenediamide (syn
and anti) and peroxdide 2b with hexyl chains simply shown as a
carbon atom for clear view; (b) proposed H-bonds between syn
tetracenediamide and O2.
1
minimized model of 2a. In fact H NMR spectra of 2a
and 2b at the same concentration (10 mM) in CDCl3 show
chemical shifts of N-H at 5.97 ppm and 6.40 ppm
respectively. Moreover, adding 100 mM of DMSO-d6 to
these solutions leads to downfield shift of N-H of 2a to
6.60 ppm while causing almost no change on chemical
shift of N-H of 2b (6.43 ppm). These NMR studies
indicate that N-H and bridge O of 2b form intramolecular
H-bonds, which are the trace of intermolecular H-bonds
between syn tetracenediamide and O2.
In summary, the unexpected photooxidation of tetracene-
diamide highlights the important role of molecular packing
in solid-gas reactions. This study suggests that selective
oxidation of organic substrates by photosentisized singlet O2
can be achieved in crystals, where molecular conformations
are fixed.
amide groups on the same side of the tetracene core (syn)
and the other has the N-H atoms on opposite sides (anti).
We propose that the syn conformer of tetracendiamide can
bind O2 with a pair of H-bonds as shown in Figure 4b in a
similar manner as 1,3-benzenediamde binds halide anions.19
Such cooperative H-bonds guide O2 to access the substituted
ring resulting peroxide 2b. In contrast, the anti conformer
has amide N-H atoms on the opposite sides so that such
pair of H-bonds to O2 can not form. As a result, the anti
conformer has oxygen added to the unsubstituted ring, which
should be electron-richer and less sterically hindered, yielding
peroxide 2a. Because the two conformers of tetracenediamide
can convert to each other during the reaction in solution,
the conformer that reacts faster should yield the major
product. Because the syn conformer reacts faster with
assistance of the H-bonds between O2 and itself, 2b is the
major product when photooxidation occurs in solution. On
the other hand, tetracenediamide is assumed to adopt the anti
Acknowledgment. We acknowledge the finacial support
from Center of Novel Functional Molecules, CUHK and
RGC Research Grant Direct Allocation. We thank Prof. T-W.
Chan (CUHK) for help on MS.
(17) To the best of our knowledge, amide binging O2 via H-bonds was
not reported. For an example of hydroxyl group binding singlet O2 via
H-bonds, see: Adam, W.; Peters, E. M.; Peters, K.; Prein, M.; von Schnering,
H. G. J. Am. Chem. Soc. 1995, 117, 6686–6690.
Supporting Information Available: Experimental details
of photooxidation and synthesis, crystallographic information
file for 7. This material is available free of charge via the
Internet at http://pubs.acs.org.
(18) DFT optimization was performed using: Frisch, M. J.; Gaussian
03, revision D.01; Gaussian, Inc.: Wallingford, CT, 2004.
(19) Kavallieratos, K.; Bertao, C. M.; Crabtree, R. H. J. Org. Chem.
1999, 64, 1675–1683.
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