A stable heptacene derivative substituted with electron-deficient trifluoromethylphenyl and triisopropylsilylethynyl groups

Article abstract of DOI:10.1021/ol101158y

(Equation Presented). A heptacene derivative 1 substituted with four electron-deficient trifluoromethylphenyl and two triisopropylsilylethynyl (TIPSE) groups was prepared by a new synthetic strategy. Photo-oxidative resistance studies showed that this new

Full text of DOI:10.1021/ol101158y

ORGANIC
LETTERS
2010
Vol. 12, No. 15
3360-3363
A Stable Heptacene Derivative
Substituted With Electron-Deficient
Trifluoromethylphenyl and
Triisopropylsilylethynyl Groups
Hemi Qu and Chunyan Chi*
Department of Chemistry, National UniVersity of Singapore, 3 Science DriVe 3,
Singapore 117543
chmcc@nus.edu.sg
Received May 20, 2010
ABSTRACT
A heptacene derivative 1 substituted with four electron-deficient trifluoromethylphenyl and two triisopropylsilylethynyl (TIPSE) groups was
prepared by a new synthetic strategy. Photo-oxidative resistance studies showed that this newly developed heptacene compound persisted
47 h in solution under ambient light and air conditions, and it represents the most stable heptacene derivative reported to date.
Acenes, linear fused aromatic hydrocarbons, have been the
subject of extensive studies owing to their potential applica-
tions in organic electronics.1 While the chemistry of short
acenes (n e 5) is well explored, our knowledge of longer
acenes (n > 5) is still limited due to the presence of challenges
in synthesis. According to Clar’s sextet rule, as the length
of acene increases, the stability significantly decreases,
causing difficulties in synthesis. Furthermore, the intrinsically
poor solubility would make those species even more
intractable. Thus, in early syntheses of higher acenes, with
no exception, all reports aroused controversies.2 Only
recently did Necker and co-workers demonstrate a successful
photochemical route for the synthesis of unsubstituted
hexacene (n ) 6) and heptacene (n ) 7) in a polymer
matrix.3
Whereas acenes (n > 5) without any substituent are
unstable under ambient conditions and insoluble in a variety
of organic solvents, chemical modifications to introduce
substituents have been developed to prepare their stable and
soluble derivatives. Anthony and co-workers first reported
the synthesis and characterization of silylethynyl derivatives
of hexacene and heptacene in 2005.4 Later, Wudl and co-
workers reported a further functionalized silylethynyl hep-
(1) (a) Clar, E. Polycyclic Hydrocarbons; Academic Press: London, New
York, 1964; Vols. 1 and 2. (b) Ronald, G. H. Polycyclic Aromatic
Hydrocarbons; Wiley, VCH: New York, 1997. (c) Anthony, J. E. Chem.
ReV. 2006, 106, 5028. (d) Bendikov, M.; Wudl, F.; Perepichka, D. F. Chem.
ReV. 2004, 104, 4891. (e) Anthony, J. E. Angew. Chem., Int. Ed. 2008, 47,
452.
(3) (a) Mondal, R.; Shah, B. K.; Neckers, D. C. J. Am. Chem. Soc. 2006,
128, 9612. (b) Holger, F. B.; Mondal, R.; Necker, D. C. Chem. Commun.
2007, 5209. (c) Mondal, R.; Tonshoff, C.; Khon, D.; Neckers, D. C.;
Bettinger, H. F. J. Am. Chem. Soc. 2009, 131, 14281.
(4) (a) Payne, M. M.; Parkin, S. R.; Anthony, J. E. J. Am. Chem. Soc.
2005, 127, 8028. (b) Purushothaman, B.; Parkin, S. R.; Anthony, J. E. Org.
Lett. 2010, 12, 2060.
(2) (a) Clar, E. Chem. Ber. 1942, 75B, 1330. (b) Bailey, W. J.; Liao,
C. W. J. Am. Chem. Soc. 1955, 77, 992.
10.1021/ol101158y  2010 American Chemical Society
Published on Web 07/14/2010

tacene derivative with additional phenyl rings at the 5, 9,
14, and 18 positions.5 Miller and co-workers then demon-
strated that arylthio substituents can enhance photo-oxidative
resistance, and thus, arylthio derivatives of acenes (n ) 7,
9) were successfully synthesized and fully characterized.6 It
should be noted that a strategy of “center-to-edge” was used
both in Wudl’s and Miller’s work; i.e., the precursors were
constructed by a Diels-Alder reaction of a central dienophile
(or diene) with two edge diene (or dienophile) components.
Aside from reports by these three groups, no other derivatives
of higher acene were reported elsewhere, despite theoretical
calculations that predicted an array of interesting properties
for higher acenes.7
In parallel to these works, we have been working on
alternative approaches to prepare soluble and stable higher
acenes by using different synthetic methods and new
solubilizing and stabilizing substituents. Based on previous
studies on the higher acenes and also peri-fused oligoacenes
(e.g., bisanthene),8 we noticed that steric and electronic
effects were important factors in determining the photo-
oxidative stability and solubility of largely extended π-con-
jugated systems. (1) Steric effect: bulky substitution, such
as the triisopropylsilylethynyl groups, can prevent the
oxidative addition of singlet oxygen to the acene core.4,9
Meanwhile, the solubility can be also improved as a result
of diminished intermolecular interactions. (2) Electronic
effect: to form stable acene derivatives, HOMO levels need
to be reduced and thus electron-withdrawing groups should
be attached.10,11 Both trifluoromethylphenyl and the triiso-
propylsilylethynyl (TIPSE) groups are electron-withdrawing
groups and expected to enhance the stability of the electron-
rich acene core. Herein, we demonstrate that a new heptacene
derivative 1 bearing four trifluoromethylphenyl groups at the
5, 9, 14, and 18 positions and two TIPSE groups at the 7
and 16 positions (Scheme 1) was prepared by a new synthetic
approach, and it showed significant improvement in its
stability against photo-oxidation under different irradiation
conditions.
3-methoxyphthalide 3 quantitatively.13 Addition of 2 equiv
of 4-trifluoromethylphenyl Grignard reagent at 0 °C to a
solution of 3-methoxyphthalide 3 in THF, followed by acidic
workup,furnishedisobenzofuran4in56%yield.14 Diels-Alder
reaction between isobenzofuran 4 and dimethyl maleate gave
epoxide 5, which was subsequently treated with p-toluene-
sulfonic acid (TsOH) in refluxing toluene to remove one
molecule of water. The resulting dehydrated compound 6
was reduced by LiAlH4 to give (1,4-bis(4-(trifluoromethyl)-
phenyl)naphthalene-2,3-diyl)dimethanol 7. The Swern oxida-
tion of diol 7 did not give the expected dialdehyde, which
was planned to be used for the preparation of the heptacene
quinone 10 by base-mediated condensation with 1,4-cyclo-
hexanedione. Alternatively, only one alcohol was oxidized
into aldehyde, which then condensed with the neighboring
alcohol to give the compound 4,9-bis(4-(trifluoromethyl)phe-
nyl)-1,3-dihydronaphtho[2,3-c]furan-1-ol 8 in 55% yield.
Fortunately, this type of compound (e.g., 8) was established
as a convenient precursor for isonaphthofurans.15 Thus,
treatment of compound 8 with acetic acid produced the
isonaphthofuran intermediate, which then underwent a 2-fold
Diels-Alder reaction with benzoquinone to give the dual
cycloaddition product 9 in 90% yield. The resulting com-
pound 9 was then treated with TsOH in refluxed toluene with
a Dean-Stark apparatus to give the desired heptacene
quinone 10 in 50% yield.
Finally, nucleophilic addition of organometallic reagents
to the heptacene quinone 10 in toluene followed by reduction
of the as-formed diol 11 afforded the desired product. For
the formation of diol 11, use of TIPSE lithium reagent
prepared by lithiation of triisopropylsilyl acetylene (TIPSA)
with n-BuLi was not successful, probably due to side
reactions with the trifluoromethyl groups (-CF3). The
synthesis of diol 11 required the use of TIPSE Grignard
reagent prepared by treatment of TIPSA with i-PrMgCl
solution to minimize formation of side products, as previ-
ously reported by Ong et al.10b and Tykwinski et al.11
Interestingly, we found that low polar solvent was also
required, and only one TIPSE group was introduced if THF
was used instead of toluene as solvent for the preparation of
this diol 11. Diol 11 was then reduced using excess
SnCl2·2H2O in toluene to obtain our target compound 1 as a
yellow green solid.11 To further improve solubility, we also
attempted to replace TIPSE groups with a long alkynyl chain
such as -Ct CC12H25. It was found that the reduction of
the corresponding diol was not successful because steric
hindrance imposed by the alkyl chain (-C12H25) was not
sufficient to stabilize heptacene.
In our synthetic strategy shown in Scheme 1, a heptacene
quinone 10 was first prepared by an “edge-to-center”
approach, a method generally used in constructing pentacene
derivatives.12 The synthesis began with 2-carboxybenzalde-
hyde 2, which was refluxed in dry methanol to give
(5) Chun, D.; Cheng, Y.; Wudl, F. Angew. Chem., Int. Ed. 2008, 47,
8380.
(6) (a) Kaur, I.; Stein, N. N.; Kopreski, R. P.; Miller, G. P. J. Am. Chem.
Soc. 2009, 131, 3424. (b) Kaur, I.; Jazdzyk, M. N.; Stein, N.; Prusevich,
P.; Miller, G. P. J. Am. Chem. Soc. 2010, 132, 1261.
(7) (a) Kivelson, S.; Chapman, O. L. Phys. ReV. B. 1983, 28, 7236. (b)
Bendikov, M.; Duong, H. M.; Starkey, K.; Houk, K. N.; Carter, E. A.;
Wudl, F. J. Am. Chem. Soc. 2004, 126, 7416.
The heptacene derivative 1 was slightly soluble in
chloroform, dichloromethane, and tetrachloroethane. The
solubility could be improved in an aromatic solvent such as
(8) (a) Yao, J.; Wu, J.; Chi, C.; Loh, K. P. Chem.sEur. J. 2009, 15,
9299. (b) Zhang, K.; Huang, K.; Li, J.; Luo, J.; Chi, C.; Wu, J. Org. Lett.
2009, 11, 4854. (c) Li, J.; Zhang, K.; Zhang, X.; Huang, K.; Chi, C.; Wu,
J. J. Org. Chem. 2010, 75, 856.
(9) Kaur, I.; Jia, W.; Kopreski, R. P.; Selvarasah, S.; Dokmeci, M. R.;
Pramanik, C.; McGruer, N. E.; Miller, G. P. J. Am. Chem. Soc. 2008, 130,
(12) (a) Rainbolt, J. E.; Miller, G. P. J. Org. Chem. 2007, 72, 3020. (b)
Martin, N.; Behnisch, R.; Hanack, M. J. Org. Chem. 1989, 54, 2563. (c)
Ried, W.; Antho¨fer, F. Angew. Chem. 1954, 66, 604.
(13) Cresp, T. M.; Giles, R. G. F.; Sargent, M. V.; Brown, C.; Smith,
D. O’N. J. Chem. Soc., Perkin Trans. 1 1974, 2435.
(14) Benderradji, F.; Nechab, M.; Einhorn, C.; Einhorn, J. Synlett 2006,
13, 2035.
16274
.
(10) (a) Maliakal, A.; Raghavachari, K.; Katz, H.; Chandross, E.; Siegrist,
T. Chem. Mater. 2004, 16, 4980. (b) Li, Y.; Wu, Y.; Liu, P.; Prostran, Z.;
Gardner, S.; Ong, B. S. Chem. Mater. 2006, 19, 418. (c) Jiao, C.; Huang,
K.; Luo, J.; Zhang, K.; Chi, C.; Wu, J. Org. Lett. 2009, 11, 4508
.
(11) Lehnherr, D.; McDonald, R.; Tykwinski, R. R. Org. Lett. 2008,
(15) Smith, J. G.; Dibble, P. W.; Sandborn, R. E. J. Org. Chem. 1986,
51, 3762.
10, 4163
.
Org. Lett., Vol. 12, No. 15, 2010
3361

Scheme 1. Synthetic Route of Heptacene Derivative 1
toluene (ca. 1 mg/10 mL at 30 °C). The solution of
compound 1 is stable enough for further characterizations.
The 1H NMR signals obtained for compound 1 in
CDCl2CDCl2 (see the Supporting Information) are sharp and
can be assigned to the desired structure, indicating that this
functionalized heptacene has a closed-shell electronic
structure.4a The MALDI-TOF mass spectrum of compound
1 shows one molecular ion peak with m/z at 1314.482 with
well-resolved isotope distribution (Figure S10b in the Sup-
porting Information), and there are no additional peaks with
m/z plus 16 or 32, indicating that this compound is stable
and there are no oxygen addition products. The UV-vis-NIR
absorption spectrum for 1 (Figure 1a) is consistent with a
highlyconjugatedstructurepossessingasmallHOMO-LUMO
gap. Compound 1 shows intense absorption bands at
300-400 nm and weak absorption bands in the longer
wavelength. The longest wavelength absorption for 1 in
toluene solution is centered at 870 nm. On the basis of the
onset of this absorption, the optical HOMO-LUMO gap for
1 was determined to be 1.35 eV. Compound 1 in toluene
also shows weak fluorescence with emission maximum at
553 nm when excited at 353 nm (Figure S1, Supporting
Information). The electrochemical property of compound 1
was studied by cyclic voltammetry in chlorobenzene. As
shown in Figure 1b, two reversible reduction waves with
half-wave potentials at -1.33 and -1.64 V (vs Fc+/Fc) and
one reversible oxidation wave with half-wave potential at
0.25 V were observed, indicating that the heptacene unit can
be reversibly reduced into radical anions and dianions and
oxidized into radical cation. The electron-withdrawing TIPSE
and the 4-trifluoromethylphenyl groups are believed to further
stabilize the reduced species. A HOMO energy level of
-4.93 eV and a LUMO energy level of -3.61 eV were
estimated on the basis of the onset potential of the first
oxidation and the first reduction wave, respectively.16 An
energy gap of 1.32 eV was then obtained which is in
agreement with the optical band gap. It is worth noting that
the absence of trifluoromethyl subsitutent could lead to an
upshift of about 0.1 eV for both HOMO and LUMO levels
as reported in Wudl’s heptacene derivative, which has a
HOMO at -4.8 eV and LUMO at -3.5 eV.5
We then investigated photostability of compound 1 in
toluene under different irradiation conditions. The photoin-
duced degradation was quantified by monitoring the decrease
of vibronic absorption in the near-IR region (780-890 nm)
as a function of elapsed photolysis time (Figure 2). The
photostability was studied with both deoxygenated solution
and solution exposed to ambient air atmosphere. Upon
irradiation with white light (100 W), UV light (4 W), and
ambient light, the deoxygenated solutions gradually decom-
posed with a decrease of the optical intensity at the NIR
band and appearance of new absorption band in the shorter
wavelength (ca. 306 and 500 nm) (Figure 2a). In contrast to
the unstable parent heptacene, compound 1 is still detectable
after 66 h in deoxygenated toluene under ambient light
irradiation. The half-lives (t1/2) of around 1950, 200, and 100
min were estimated under ambient light, white light (100
W), and UV lamp (4 W), respectively, by plotting the
absorption intensity at 870 nm with the irradiation time
(Figure 2b). When the solution of 1 in toluene was exposed
to air and ambient light irradiation, its presence was
detectable after 47 h (Figure S2, Supporting Information).
Compared with Wudl’s heptacene derivative, which can
survive 41 h under the same conditions,5 our molecule
exhibited further improvement in its stability, and this can
be explained by the introduction of an electron-withdrawing
(16) Chi, C.; Wegner, G. Macromol. Rapid Commun. 2005, 26, 1532.
Org. Lett., Vol. 12, No. 15, 2010
3362

Figure 2. (a) Change of UV-vis-NIR absorption spectra of a
deoxygenated solution of 1 when exposed to UV light (4 W)
irradiation. (b) Change of the optical density at 870 nm of
deoxygenated toluene solution of 1 (3.7 × 10-5 M) as a function
of irradiation time in ambient light, white light (100 W), and UV
light (4 W).
Figure 1. (a) UV-vis-NIR absorption spectrum of 1 in toluene;
(b) cyclic voltammogram of 1 in hot chlorobenzene (70 °C) under
nitrogen atmosphere with 0.1 M Bu4NPF6 as supporting electrolyte,
AgCl/Ag as reference electrode, Au disk as working electrode, Pt
wire as counter electrode, and scan rate at 50 mV/s. Fc+/Fc was
used as internal reference.
of the higher acenes. Compound 1 represents the most stable
heptacene derivative reported to date. Our synthetic meth-
odology may also lead to opportunities to prepare stable
higher acenes with electron-deficient substituents in the
future.
trifluoromethyl group onto the phenyl rings. During irradia-
tion, in all cases, the color of compound 1 in solution changed
from yellow green to light red and the FAB mass spectrum
of the solution after irradiation revealed a small peak with
m/z at 1314.5 [M+] along with two intense peaks at (M+ +
Acknowledgment. This work is supported by National
University of Singapore under MOE AcRF FRC Grant No.
R-143-000-412-112.
1
32) and (M+ + 16) corresponding to a O2 adduct of 1 and
its fragment, respectively. This kind of color change and the
appearance of UV absorption around 500 nm, in combination
with the mass spectrum, indicated that tetracene-containing
species were presumably formed during the photo-oxidative
process.5
In conclusion, we have prepared the stable and soluble
heptacene derivative 1 utilizing an “edge-to-center” approach.
Attachment of electron-withdrawing groups such as 4-trif-
luoromethylphenyl groups further improved the photostability
Supporting Information Available: Experimental pro-
cedures, characterization data of all new compounds, fluo-
rescence spectrum, and photo-oxidative resistance studies.
This material is available free of charge via the Internet at
http://pubs.acs.org.
OL101158Y
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3363