Photocyclization and photooxidation of 3-styrylthiophene

Article abstract of DOI:10.1016/S0040-4039(00)00097-6

The photocyclization, dye-sensitized photooxidation and auto- photooxidation of 3-styrylthiophene have been examined, cis-3-Styrylthiophene undergoes photochemical cis-trans isomerization and cyclization to dihydronaphtho-[1,2-b]thiophene. The quantum efficiency for photocyclization in nonpolar solvents is greater than that in polar solvents. Dye-sensitized photooxidation of 3-styrylthiophene gives benzaldehyde and 3- thiophenecarboxaldehyde. This oxidation proceeds via a superoxide radical anion pathway rather than singlet oxygen pathway. In the presence of oxygen photoirradiation of 3-styrylthiophene solution results in photocyclization, oxidation and dimerization. The mechanism of the latter two reactions was described in terms of formation of a charge transfer complex between oxygen and the substrate. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4039(00)00097-6

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 41 (2000) 1951–1954
Photocyclization and photooxidation of 3-styrylthiophene
Kai Song,a Li-Zhu Wu,b,∗ Chun-He Yang b and Chen-Ho Tung a
aInstitute of Photographic Chemistry, the Chinese Academy of Sciences, Beijing 100101, China
bCenter for Molecular Science, Institute of Chemistry, the Chinese Academy of Sciences, Beijing 100080, China
Received 6 October 1999; accepted 11 January 2000
Abstract
The photocyclization, dye-sensitized photooxidation and auto-photooxidation of 3-styrylthiophene have
been examined. cis-3-Styrylthiophene undergoes photochemical cis–trans isomerization and cyclization
to dihydronaphtho-[1,2-b]thiophene. The quantum efficiency for photocyclization in nonpolar solvents is
greater than that in polar solvents. Dye-sensitized photooxidation of 3-styrylthiophene gives benzaldehyde
and 3-thiophenecarboxaldehyde. This oxidation proceeds via a superoxide radical anion pathway rather than
singlet oxygen pathway. In the presence of oxygen photoirradiation of 3-styrylthiophene solution results in
photocyclization, oxidation and dimerization. The mechanism of the latter two reactions was described in terms of
formation of a charge transfer complex between oxygen and the substrate. © 2000 Elsevier Science Ltd. All rights
reserved.
Thiophene and its derivatives have been shown to be the best monomers for the production of desirable
conductive polymers in terms of high conductivity, processibility and ease of manufacture.1–3 Although
the electrochemical oxidation of a wide variety of thiophene derivatives, leading to the formation of
conductive polymers, has been extensively investigated, the photochemical oxidation of these compounds
has rarely been reported.4–6 It is known that polythiophenes are susceptible to photodegradation in the
presence of oxygen. Thus, the study of photochemical reactions of polythiophenes and their monomers
is critical for understanding the behavior and for the manufacture of high quality conductive polymers.
Here we report the photocyclization and photooxidation of 3-styrylthiophene in the presence and absence
of a sensitizer.
3-Styrylthiophene exists as two isomers: trans-3-styrylthiophene (1) and cis-3-styrylthiophene (2).7
While 1 undergoes only trans–cis isomerization, 2 may undergo a photochemical ring closure reaction to
form dihydronaphtho-[1,2-b]thiophene (3) in addition to the cis–trans isomerization (Scheme 1). In the
present study, we used a mixture of 1 and 2 as starting materials. Photoirradiation of starting materials
(1×10−2 M, degassed by purging with nitrogen) in organic solvents was carried out in a quartz reactor
with a 450 W Hanovia medium pressure Hg arc lamp at room temperature. The products were isolated
1
by column chromatography on silica and identified from their H NMR and MS spectra. It was found
∗
Corresponding author.
0040-4039/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)00097-6
tetl 16370

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that 3 was oxidized to naphtho-[1,2-b]thiophene (4, Scheme 1) by air during the isolation.8 The yield of
this product was 100% based on the consumption of the starting materials 1 and 2, suggesting that any
unidentified product must be minor. The efficiency of the ring closure reaction from 2 to 3 is dependent
on the solvent. In acetonitrile, photoirradiation of a 1×10−2 M solution of 2 for 20 min leads to a 30%
conversion of 2 to 3. Under identical conditions, photoirradiation of 3 in cyclohexane for the same period
results in a 40% conversion. By reference to the photochemical reaction of cis-stilbene,9 we attributed
the high efficiency of the photocyclization of 2 in a nonpolar solvent to the small rate constant of the
cis–trans isomerization compared with that in a polar solvent.
Scheme 1.
Photosensitized oxidation of 1 and 2 was carried out by using several sensitizers in both polar
and nonpolar solvents. A combination of Corning 0–52 and 7–60 glass filters were used to isolate
the λ>350 nm light, which was only absorbed by the sensitizer. Photoirradiation of oxygen-saturated
solutions of the mixtures of 1 and 2 in acetonitrile containing 9,10-dicyanoanthracene (DCA) gave 3-
thiophenecarboxaldehyde (5) and benzaldehyde (6) (Scheme 2). The products were isolated by column
chromatography and analyzed by GC. The ratio of 5 to 6 was 1:1, and the yield was 100% based
on the consumption of the starting material. Generally, after 3 h of irradiation the conversion of the
starting material was near 100%. There are two well established types of dye-sensitized photooxidation
of alkenes: the energy transfer pathway and the electron transfer pathway.10 It has been established that
DCA can act both as a singlet oxygen sensitizer and as an electron transfer sensitizer.11 To clarify the
mechanism of the DCA-photosensitized oxidation of 1 and 2 we examined the fluorescence quenching.
The fluorescence of DCA in acetonitrile was quenched by 1 and 2 following the linear Stern–Volmer
relationship and the quenching rate constant (kq) was 9.0×109 M/S (assuming the fluorescence lifetime
of DCA in acetonitrile in the absence of quencher to be 13.4 ns12), close to the diffusion-controlled
rate constant. The energy of the singlet excited state of DCA (ca. 66 kcal/mol13) is lower than that of
1 and 2 (ca. 96 kcal/mol, as determined from the absorption spectra), suggesting that singlet–singlet
energy transfer from the excited DCA to 1 and 2 is impossible. Thus, we examined the possibility of
electron transfer from 1 and 2 to the singlet excited state of DCA as the cause of the fluorescence
quenching. The free energy change (∆G) involved in a photostimulated electron transfer process can
be estimated by Eq. (1),14 where ∆E00 is the singlet excited state energy of DCA. Eox (D) and Ered (A)
are the oxidation potential of 1 and 2 (1.48 V vs SCE15) and the reduction potential of DCA (–0.89 V vs
SCE13), respectively, determined in acetonitrile solution.
∆G = Eox (D)–Ered (A)–∆E00–e2/εRcc–e2/2 (1/r + 1/r ) (1–1/ε)
(1)
+
−
Scheme 2.

1953
e2/εRcc is the Coulombic interaction (Rcc=r++r , ε=dielectric constant of the medium) in the ion pair
−
state. The last term in Eq. (1) is the Born Correction to the solvation energy. Assuming both donor and
acceptor are spherical, we set r+ and r to equal 3.8 Å and 4.8 Å, respectively. The calculation according
−
to Eq. (1) reveals that electron transfer from 1 or 2 to the DCA singlet excited state in acetonitrile is
exothermic by 12.7 kcal/mol. Thus, the fluorescence quenching indeed can occur via the electron transfer
process, and the photosensitized oxidation may proceed via the superoxide radical anion pathway.
The proposal of the electron transfer mediated mechanism for the photosensitized oxidation of 1 and
2 was further supported by an N,N-dimethylaniline (DMA) quenching experiment. Because DMA has
a lower oxidation potential16 than 1 and 2, electron transfer from DMA to the excited state of DCA
should be more favorable. Thus, one might expect that DMA would intercept the photooxidation of 1
and 2 sensitized by DCA. Experimentation revealed that this is indeed the case. Addition of DMA to
the oxygen-saturated solution of 1, 2 and DCA in acetonitrile led to efficient quenching of the oxidation
reaction.
We also studied the photosensitized oxidation of 1 and 2 in cyclohexane by using tetraphenylporphyrin
(TPP) as the sensitizer. TPP is a typical sensitizer for the production of singlet oxygen, particularly
in nonpolar solvents.17 Photoirradiation of an oxygen-saturated solution of a mixture of 1 and 2 in
cyclohexane containing TPP with visible light for 10 h resulted in no reaction of 1 and 2. All the above
observations suggest that in the dye-sensitized oxidation of 1 and 2 the reaction proceeds via an electron
transfer pathway rather than an energy transfer pathway.
Compounds 1 and 2 can also undergo auto-photooxidation in the absence of the sensitizer. Photoir-
radiation of an oxygen-saturated solution of 1 and 2 in acetonitrile with 254 nm light resulted in the
ring closure product 4 as well as the oxidation products 5 and 6 (Scheme 3). The product distribution is
given at the bottom of Scheme 3. Material balance was greater than 90%. Photoirradiation of oxygen-
saturated solutions of 1 and 2 in cyclohexane gave a dimer of 4 (7, Scheme 3) as a precipitate in addition
to 4, 5 and 6. The dimer 7 was identified by its 1H NMR and MS spectra.18 It is possible that the triplet
excited state of 1 and/or 2 (generated by intersystem crossing from the singlet excited state) interacts with
oxygen to produce singlet oxygen. However, the singlet oxygen does not react with 1 or 2, Holdcroft and
his co-workers reported that molecular oxygen forms reversible charge transfer complexes (CTC) with
oligo-and poly(3-alkylthiophenes).4 We proposed that 1 and 2 also might form a CTC with oxygen. The
complex possesses character of the radical pair formed from superoxide anion radical and the thiophene
cation radical, and the products 5 and 6 were produced from the radical pair as in the case of the electron
transfer pathway in the dye-sensitized oxidation. The dimer 7 might be formed from the radical cation of
4.
Scheme 3.
In summary, 3-styrylthiophene may undergo photocyclization to dihydronaphtho-[1,2-b]thiophene in
the absence of oxygen. In the presence of oxygen photoirradiation of this compound gave the oxidation
products, benzaldehyde, 3-thiophenecarboxaldehyde, and the cyclization product 4. In DCA sensitized

1954
oxidation the reaction proceeds via an electron transfer pathway. We believe that this study may provide
the basis for improvement of the photostability of polythiophene conductive polymers.
Acknowledgements
Financial support from the National Science Foundation of China (No. 29992530) and the Bureau for
Basic Research, Chinese Academy of Sciences, is gratefully acknowledged.
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