Formation of a trithia[5]helicene in an unexpected photoreaction of a methyl-substituted bis(dithienylethenyl)thiophene through a double sequence of 6π-electrocyclization/aromatization (dehydrogenation/demethylation)

Article abstract of DOI:10.1016/j.jphotochem.2015.10.012

Photochemical reactions of 2,5-bis[2,2-di(thien-2-yl)ethenyl]thiophene (1a) and its tetramethyl derivative 1b, under direct photoexcitation, and photoinduced electron-transfer conditions, were studied. Our initial prediction was that 1b would undergo a photoreaction as part of a reversible photochromic system, while a reaction of its non-methyl substituted analog 1a would undergo a typical double sequence of 6π-electrocyclization (6π-EC)/aromatization (AR) (double dehydrogenation, [sbnd]2H) to give 7,11-di(thien-2-yl)trithia[5]helicene (2a). In fact, we observed that photoirradiation of 1b leads to formation of the 7,11-bis(3-methylthien-2-yl)trithia[5]helicene (2b). In this process, 2b is formed via a double sequence of 6π-EC/AR (dehydrogenation/demethylation, [sbnd]H/[sbnd]Me). Moreover, the yield of formation of 2b is much higher than that of 2a, which reacts through a double sequence of 6π-EC/AR ([sbnd]2H). Thus, this photochemically-induced paradoxical cascade reaction based on the newly uncovered reactivity serves as an efficient method to construct the trithia[5]helicene framework.

Full text of DOI:10.1016/j.jphotochem.2015.10.012

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JPC 10032 No. of Pages 8
Journal of Photochemistry and Photobiology A: Chemistry xxx (2015) xxx–xxx
Contents lists available at ScienceDirect
Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Formation of a trithia[5]helicene in an unexpected photoreaction of a
methyl-substituted bis(dithienylethenyl)thiophene through a double
sequence of 6p-electrocyclization/aromatization (dehydrogenation/
demethylation)
Atsushi Yamamoto , Yasunori Matsui , Eisuke Ohtaa,b, Takuya Ogaki , Hiroyasu Sato ,
a
a,b
a
c
d
d
a
a,b,
Taniyuki Furuyama , Nagao Kobayashi , Kazuhiko Mizuno , Hiroshi Ikeda
*
a
Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture University, 1-1 Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531, Japan
The Research Institute for Molecular Electronic Devices (RIMED), Osaka Prefecture University, 1-1 Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531, Japan
Rigaku Corporation, 3-9-12 Matsubara-cho, Akishima, Tokyo 196-8666, Japan
b
c
d
Department of Chemistry, Graduate School of Science, Tohoku University, 6-3 Aramaki-azaaoba, Aoba-ku, Sendai, Miyagi 980-8578, Japan
A R T I C L E I N F O
A B S T R A C T
Article history:
Photochemical reactions of 2,5-bis[2,2-di(thien-2-yl)ethenyl]thiophene (1a) and its tetramethyl
derivative 1b, under direct photoexcitation, and photoinduced electron-transfer conditions, were
studied. Our initial prediction was that 1b would undergo a photoreaction as part of a reversible
photochromic system, while a reaction of its non-methyl substituted analog 1a would undergo a typical
Received 24 August 2015
Received in revised form 13 October 2015
Accepted 15 October 2015
Available online xxx
double sequence of 6
p
-electrocyclization (6
p
-EC)/aromatization (AR) (double dehydrogenation, ÀÀ 2H)
to give 7,11-di(thien-2-yl)trithia[5]helicene (2a). In fact, we observed that photoirradiation of 1b leads to
formation of the 7,11-bis(3-methylthien-2-yl)trithia[5]helicene (2b). In this process, 2b is formed via a
Keywords:
Helicene
double sequence of 6
p
-EC/AR (dehydrogenation/demethylation, ÀÀ H/ ÀÀ Me). Moreover, the yield of
Electrocyclization
Aromatization
Demethylation
Photoinduced electron transfer
Thiophene
formation of 2b is much higher than that of 2a, which reacts through a double sequence of 6
p-EC/AR
( ÀÀ 2H). Thus, this photochemically-induced paradoxical cascade reaction based on the newly uncovered
reactivity serves as an efficient method to construct the trithia[5]helicene framework.
ã 2015 Elsevier B.V. All rights reserved.
1. Introduction
Photo- and SET-induced 6p-EC reactions of 1,3,5-hexatrienes
that are followed by AR [3] rank with C ÀÀ H activation [4], and
oxidative [5] and reductive coupling reactions [6] as a useful
method for the synthesis of important polycyclic aromatic
compounds. This situation is highlighted by the fact that aromatic
fused thiophenes serve as key substances in organic electronic
devices such as organic field-effect transistors (OFETs) [7] and
organic photovoltaic systems [8]. We previously described the
preparation of tetrathienonaphthalene [9] (TTN), the parent
structural and electronic framework that makes it a good
charge-carrier transport material. In that effort, TTN was generated
through a photoinduced reaction of tetra(thien-3-yl) ethene (TTE),
6
p
-Electrocyclization (6p-EC) reactions of 1,3,5-hexatrienes
promoted by photoirradiation or single electron-transfer (SET)
oxidation are of great importance in organic chemistry. As
exemplified by the diarylethene [1] (DAE, Scheme 1), a 6p-EC
process can serve as the basis of photochromic systems.
Specifically, alkyl groups such as Me groups at the terminal
carbons of the 1,3,5-hexatriene play an important role in
preventing aromatization (AR) and/or 1,2-dyotropic rearrange-
ments from the formed 1,3-cyclohexadiene product [2]. Thus,
strategy that takes into account the effects of substituents is an
important component of designing applications of 6
of 1,3,5-hexatrienes.
p
-EC reactions
taking place by a double sequence of 6p-EC/AR (double
dehydrogenation, ÀÀ 2H). In addition, angularly-fused and, espe-
cially, curved polycyclic aromatic compounds such as helicenes
[10,11], which are intriguing next generation functionalized
materials, have been prepared by using this type of photochemical
or SET process.
In the study described below, we explored direct photoexcita-
tion and photoinduced electron-transfer (PET) reactions of 2,5-bis
*
Corresponding author at: Department of Applied Chemistry, Graduate School of
Engineering, Osaka Prefecture University, 1-1 Gakuen-cho, Naka-ku, Sakai, Osaka
99-8531, Japan. Fax: +81 72 254 9289.
5
E-mail address: ikeda@chem.osakafu-u.ac.jp (H. Ikeda).
http://dx.doi.org/10.1016/j.jphotochem.2015.10.012
010-6030/ã 2015 Elsevier B.V. All rights reserved.
1
Please cite this article in press as: A. Yamamoto, et al., Formation of a trithia[5]helicene in an unexpected photoreaction of a methyl-substituted
bis(dithienylethenyl)thiophene through a double sequence of 6 -electrocyclization/aromatization (dehydrogenation/demethylation), J.
Photochem. Photobiol. A: Chem. (2015), http://dx.doi.org/10.1016/j.jphotochem.2015.10.012
p

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2
A. Yamamoto et al. / Journal of Photochemistry and Photobiology A: Chemistry xxx (2015) xxx–xxx
2
.2. General procedure of photoreactions
Photoreactions were carried out using irradiation from a Xe
lamp (150 W) loaded into JASCO FP-6300, an USHIO UXL-2003D-O
Xe arc lamp (2 kW) with DSB-2001A power supply through a
TOSHIBA Y-44 glass filter, and an Eikosha EHB-W-300 high-
pressure Hg lamp (300 W) through a Pyrex filter.
2
.2.1. 2,5-Bis[2,2-di(thien-2-yl)ethenyl]thiophene (1a)
To a dry diethyl ether (20 mL) solution containing t-BuOK
(
2.24 g, 20 mmol) was added a diethyl ether (50 mL) solution
containing di(thien-2-yl) ketone (5a, 1.95 g, 10 mmol) and 2,5-bis
diethylphosphonomethyl) thiophene (6, 1.93 g, 5.0 mmol) under
Ar at room temperature. After stirring for 40 h, the reaction was
quenched with sat. aq. NH Cl. The solution was extracted with
EtOAc (50 mL Â 3), and the extracts were washed with water and
brine, and dried over anhydrous Na SO . Concentration of the
(
4
Scheme 1. 6
p-EC-based photochromism of DAE (top) and a photoinduced double
sequence of 6
p
-EC/AR ( ÀÀ 2H) reaction of TTE (bottom).
2
4
organic layer under reduced pressure gave a residue that was
subjected to column chromatography [silica gel, n-hexane/
benzene = 10/1 (v/v)] to give orange oil. Recrystallization of this
oil from CHCl
3
–EtOH gave 1a (923 mg, 2.0 mmol, y. 40%) as orange
ꢀ
1
solid. mp 130.0–131.0 C; H NMR (300 MHz, CDCl ) dppm 6.74 (dd,
3
J = 3.6, 1.2 Hz, 2H), 6.76 (s, 2H), 6.92 (dd, J = 3.6, 1.2 Hz, 2H), 6.93 (dd,
J = 5.1, 3.6 Hz, 2H), 7.08 (dd, J = 5.1, 3.6 Hz, 2H), 7.17 (s, 2H), 7.18 (dd,
13
J = 5.1, 1.2 Hz, 2H), 7.47 (dd, J = 5.1, 1.2 Hz, 2H); C NMR (75 MHz,
CDCl ppm 122.9 (2C), 125.1 (2C), 126.0 (2C), 126.6 (2C), 127.8 (2C
2C), 127.9 (2C), 128.9 (2C), 129.5 (2C), 138.2 (2C), 142.1 (2C), 147.4
3
) d
+
À1
(
2C); IR (KBr)
n
/cm 3095, 3047, 2991,1654,1529,1419,1241,1030,
8
83, 855, 790, 706, 614, 538; HR-mass [FAB, 3-nitrobenzyl alcohol
+
(
NBA)] m/z calcd. for [C24
16
H S
5
] = 463.9856, Found 463.9829; anal.
calcd. for C24H
16
S
5
, H: 3.47, C: 62.03, S: 34.5, Found H: 3.48, C:
61.55.
Scheme 2. A photoinduced double sequence of 6
reaction of 1a or 1b.
p
-EC/AR ( ÀÀ 2H or ÀÀ H/ ÀÀ Me)
2.2.2. 2,5-Bis[2,2-bis(3-methylthien-2-yl) ethenyl]thiophene (1b)
To a dry diethyl ether (20 mL) solution of t-BuOK (1.36 g,
1
(
3
2 mmol) was added a diethyl ether (30 mL) solution containing bis
3-methylthien-2-yl) ketone (5b, 1.33 g, 6.0 mmol) and 6 (1.16 g,
.0 mmol) under Ar at room temperature. After stirring for 60 h, the
reaction was quenched with sat. aq. NH Cl. The solution was
extracted with EtOAc (50 mL Â 3), and the extracts were washed
with water and brine, and dried over anhydrous Na SO
[2,2-di(thien-2-yl) ethenyl]thiophene (1a, Scheme 2) and its
tetramethylderivative 1b, which contains Me groups on the
terminal carbons of the two 1,3,5-hexatriene moieties. At the
outset of this effort, we expected that 1a under both conditions
4
2
4
.
would undergo a double sequence of 6
give the 7,11-di(thien-2-yl)trithia[5]helicene (2a). In contrast, we
anticipated that 1b would participate in a photochromic system
p
-EC/AR ( ÀÀ 2H) reaction to
Concentration of the organic layer under reduced pressure gave
a residue that was subjected to column chromatography [silica gel,
n-hexane/benzene = 5/1 (v/v)] to give orange oil. Recrystallization
involving 3b, the product formed by a 6
in the study show that, in fact, upon photoirradiation both 1a and
b undergo a double 6 -EC/AR sequence to produce 2a and 7,11-
p-EC. Observations made
of this oil from CHCl
as orange needles. mp 113.0–115.0 C; H NMR (300 MHz, CDCl
3
–EtOH gave 1b (625 mg, 1.2 mmol, yield 40%)
ꢀ
1
3
)
1
p
d
ppm 1.99 (s, 6H), 2.09 (s, 6H), 6.78 (d, J = 5.1 Hz, 2H), 6.82 (s, 2H),
bis(3-methylthien-2-yl)trithia[5]helicene (2b), respectively. In the
unexpected reaction of 1b (and intermediate 4b, Scheme 2),
unusual dehydrogenation/demethylation ( ÀÀ H/ ÀÀ Me) processes
6
.82 (d, J = 5.1 Hz, 2H), 7.06 (s, 2H), 7.08 (d, J = 5.1 Hz, 2H), 7.33 (d,
13
J = 5.1 Hz, 2H); C NMR (75 MHz, CDCl
3
) dppm 14.2 (2C), 15.3 (2C),
123.5 (2C), 125.5 (2C), 125.9 (2C), 126.3 (2C + 2C), 129.0 (2C), 130.3
occur following the 6p-EC steps to give 4b (and then 2b).
(
(
1
2C), 132.0 (2C), 134.4 (2C), 136.8 (2C), 140.2 (2C), 142.0 (2C); IR
Moreover, the yield of 2b formed by sequence of the 6
p
-EC/AR
À1
KBr)
n
/cm 2911, 1685, 1654, 1560, 1529, 1433, 1225, 1112, 1056,
(
ÀÀ H/ ÀÀ Me) is much higher than that of 2a generated by the
+
004, 931, 890, 839; LR-mass (FAB, NBA) m/z 520 (M ); anal. calcd.
classical 6
p
-EC/AR ( ÀÀ 2H). As a result, this investigation shows
-EC/AR ( ÀÀ H/ ÀÀ Me) sequence can be employed to
construct trithia[5]helicene frameworks in an efficient manner.
for C28 , H: 4.64, C: 64.57, S: 30.78, Found H: 4.56, C: 64.21.
24 5
H S
that the 6
p
2
.2.3. 7,11-Di(thien-2-yl)trithia[5]helicene (2a) and 7,11-bis(3-
methylthien-2-yl)trithia[5]helicene (2b)
A CDCl or C (0.5 mL) solution containing 1 (1a, 2.32 mg,
mol; 1b, 2.60 mg, 5.0 mol) and p-chloranil (p-CA), o-
chloranil (o-CA), or anthraquinone (AQ) (2.46 mg, 10 mol for p-
CA and o-CA; 2.10 mg, 10 mol for AQ) as an electron or hydrogen/
2. Materials and methods
3
6 6
D
5
.0
m
m
2.1. Measurement of magnetic circular dichroism (MCD) spectra [12]
m
m
MCD spectra were recorded on a JASCO J-725 spectrodichrom-
proton acceptor (Acc) in a Pyrex NMR tube was irradiated with a 2-
kW Xe lamp through a Y-44 cutoff filter or a 300-W high-pressure
eter equipped with a JASCO electromagnet capable of producing
magnetic parallel and antiparallel fields of up to 1.03 T with both.
The magnitudes are expressed in terms of molar ellipticity per tesla
Hg lamp through a Pyrex filter at room temperature. The
1
conversion of 1 and yield of 2 were determined by using
H
À1
À1 À1
(
[
u
]
M
/deg M cm
T
).
NMR analysis with 1,3,5-trioxane as an internal standard. A similar
Please cite this article in press as: A. Yamamoto, et al., Formation of a trithia[5]helicene in an unexpected photoreaction of a methyl-substituted
bis(dithienylethenyl)thiophene through a double sequence of 6 -electrocyclization/aromatization (dehydrogenation/demethylation), J.
Photochem. Photobiol. A: Chem. (2015), http://dx.doi.org/10.1016/j.jphotochem.2015.10.012
p

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JPC 10032 No. of Pages 8
A. Yamamoto et al. / Journal of Photochemistry and Photobiology A: Chemistry xxx (2015) xxx–xxx
3
Table 1
Photoreactions of 1a.
photoreaction (0.50 mmol scale) followed by preparative GPC
CHCl ) and recrystallization from toluene–n-hexane gave 2a and
b. Compound 2a; yellow needles; mp 203.5–205.0 C; H NMR
300 MHz, CDCl ppm 7.26 (dd, J = 5.4, 3.5 Hz, 2H), 7.48 (dd, J = 5.4,
.9 Hz, 2H), 7.74 (dd, J = 3.5, 0.9 Hz, 2H), 7.78 (d, J = 5.7 Hz, 2H), 8.06
s, 2H), 8.42 (d, J = 5.7 Hz, 2H); 13C NMR (75 MHz, CDCl
ppm 118.4
(
2
(
0
(
(
(
(
(
3
ꢀ
1
3
) d
3
) d
2C), 125.0 (2C), 125.9 (2C), 126.2 (2C), 126.8 (2C), 127.9 (2C), 128.4
2C), 129.5 (2C), 135.2 (2C), 137.1 (2C), 137.9 (2C), 142.0 (2C); IR
Entry
lEX
nm
Acc
Sol
Time
h
Con.a
%
Yielda (C.Y.b)
%
À1
KBr)
n
/cm 3110, 3076, 1565, 1420, 1357, 1157, 896, 640; HR-mass
+
1
2
3
4
5
6
7
8
9
>410c
–
CDCl3
10
25
10
12
12
10
10
12
12
25
71
48
52
33
84
49
66
49
0 (0)
FAB, NBA) m/z Calcd. for [C24
H
12
S
5
] = 459.9543, Found 459.9598.
ꢀ
1
>410c
Compound 2b; pale yellow plates; mp 254.0–255.0 C; H NMR
300 MHz, CDCl ppm 2.28 (s, 6H), 7.05 (d, J = 5.1 Hz, 2H), 7.39 (d,
J = 5.1 Hz, 2H), 7.74 (d, J = 5.7 Hz, 2H), 7.87 (s, 2H), 8.42 (d, J = 5.7 Hz,
–
CDCl
CDCl
CDCl
C D
6 6
CDCl
C D
6 6
CDCl
6 6
C D
3
3
3
20 (28)
5 (10)
14 (27)
9 (27)
4 (5)
13 (27)
17 (26)
6 (12)
d
>313
–
(
3
) d
>313d
p-CA
p-CA
o-CA
o-CA
AQ
AQ
>313d
13
2
H); C NMR (75 MHz, CDCl
3
)
dppm 15.3 (2C), 121.0 (2C), 125.1
d
>313
3
>313d
>313d
>313d
(
1
2C + 2C), 127.4 (2C), 128.4 (2C), 130.0 (2C), 130.7 (2C), 134.8 (2C),
À1
35.1 (2C), 136.1 (2C), 137.6 (2C), 140.4 (2C); IR (KBr)
n/cm 3083,
3
2
915, 1571, 1447, 1336, 1149, 833, 650; HR-mass (FAB, NBA) m/z
+
Calcd. for [C26
H
16
S
5
] = 487.9856, Found 487.9828; anal. calcd. for
a
Determined by using 1H NMR analysis with 1,3,5-trioxane as an internal
C
26
H
16
S
5
, H: 3.30, C: 63.90, S: 32.80, Found H: 3.45, C: 64.46.
. Results
.1. Synthesis of 1
,5-Bis[2,2-di(thien-2-yl)ethenyl]thiophenes 1a and 1b were
standard.
b
C.Y. stands for conversion yield, representing (Yield/Con.) Â 100.
c
A 2-kW Xe lamp with a Y-44 cutoff filter.
3
d
A 300-W high-pressure Hg lamp with a Pyrex cutoff filter.
3
produce 2a in low yields (Entries 4–9). Finally, an attempt to
prepare 2a by using SET-oxidation reaction of 1a with anhydrous
2
FeCl
complex mixture of CDCl
Photoreaction of 1b, promoted by direct irradiation (a 150-W Xe
lamp equipped with a monochromator, EX = 410–450 nm), was
3
in CH
2
Cl
2
, was unsuccessful and resulted in formation of a
prepared by using the corresponding thiophenes 7a and 7b as
starting materials (Scheme 3). The ketone 5a was generated 2a
3
-insoluble substances.
(
93% yield) by a sequence involving lithiation of 7a followed by
l
treatment with N,N-dimethylcarbamoyl chloride. Subjecting 2-
bromo-3-methylthiophene (8b) [13], prepared by a NBS-promoted
bromination of 7b, to similar reaction conditions produced ketone
monitored by using UV–vis spectroscopy. A CH
2
Cl
2
solution of 1b
À5
(
(
2.5 Â10 M) displayed an intense absorption band at 430 nm
Fig. 1a, red). During photoirradiation, the intensity of this band
5
b (64%). 2,5-Bis(chloromethyl)thiophene (9), formed by reaction
of 7a [14] with 1,3,5-trioxane and hydrochloric acid (48%), was
subjected to Michaelis–Arbuzov reaction with P(OEt) to generate
[15] (94%). Finally, Horner–Wadsworth–Emmons reactions of 6
with 5a and 5b were used to produce [16] 1a and 1b, respectively
both 40% yield).
decreases and a new band at 330–370 nm concurrently rises
isosbestic point 380 nm). However, 330–370-nm irradiation (the
50-W Xe lamp equipped with the monochromator) of the
(
1
3
6
resulting solution for 1 h did not lead to recovery of the 430-nm
band (Fig. 1b, blue), indicating that irradiation of 1b does not
promote formation of a photochromic system with high revers-
ibility.
(
3.2. Photoreactions of 1
3
Photoreactions (Xe lamp, lEX > 410 nm) of CDCl solutions of 1b
does not give rise to formation of identifiable products (Table 2,
Entry 1). However, photoirradiation (high-pressure Hg lamp,
Direct photoirradiation of an aerated CDCl
3
solution of 1a, using
EX > 410 nm), for 25 h
-EC/AR ( ÀÀ 2H) reaction to form
a (Table 1, Entry 2, con. 71%, yield 20%). Interestingly, 2a was not
generated after 10-h irradiation under the same conditions (Entry
), indicating that the reaction is relatively inefficient. A similar
photoreaction of 1a in CDCl by using a 300-W high-pressure Hg
lamp ( EX = 313, 365, 400, and 436 nm) also leads to formation of
a (Entry 3, con. 48%, yield 5%) [17]. In contrast, any photoreactions
of 1a does not take place under these conditions when C is
employed as solvent. The results of additional studies revealed that
PET reactions of 1a using p-CA, o-CA, or AQ as Acc in CDCl or C
a 2-kW Xe lamp and a Y-44 cutoff filter (
promotes a double sequence of 6
l
lEX = 313, 365, 400, and 436 nm) leads to formation of 2b as the
p
sole identified product (Entry 2, con. 100%, yield 36%). This
substance arises by elimination of two hydrogen atoms and two
Me groups at the terminal carbons of the two 1,3,5-hexatriene
2
1
3
l
2
6 6
D
3
6 6
D
Scheme 3. Preparation of 1a and 1b. Reagents and conditions: (i) n-BuLi, N,N-
Fig. 1. Time-dependent changes of UV–vis absorption spectra of direct photoexci-
ꢀ
ꢀ
À5
dimethylcarbamoyl chloride in THF, À78 C; (ii) NBS in THF, 0 C; (iii) n-BuLi, N,N-
tation reaction of 1b (2.5 Â10 M) in aerated CH
2 2
Cl upon (a) 410–450-nm and
then (b) 330–370-nm irradiation. (For interpretation of the references to colour in
ꢀ
dimethylcarbamoyl chloride in diethyl ether, À78 C; (iv) 1,3,5-trioxane and conc.
ꢀ
aq. HCl, rt; (v) P(OEt)
3
, 150 C; (vi) t-BuOK, 5a/5b in diethyl ether, rt.
the text, the reader is referred to the web version of this article.).
Please cite this article in press as: A. Yamamoto, et al., Formation of a trithia[5]helicene in an unexpected photoreaction of a methyl-substituted
bis(dithienylethenyl)thiophene through a double sequence of 6 -electrocyclization/aromatization (dehydrogenation/demethylation), J.
Photochem. Photobiol. A: Chem. (2015), http://dx.doi.org/10.1016/j.jphotochem.2015.10.012
p

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JPC 10032 No. of Pages 8
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A. Yamamoto et al. / Journal of Photochemistry and Photobiology A: Chemistry xxx (2015) xxx–xxx
Table 2
Photoreactions of 1b.
at 2.28 ppm [18] arises as the reaction proceeds and then
disappears when the photolysate is purged with Ar gas. Moreover,
we demonstrated that formation of p-CAH takes place in the PET
reactions in the presence of p-CA (Entries 3 and 4). Finally, 1b also
reacts with anhydrous FeCl , but the process does not yield 2b.
2
3
3
.3. Crystal packing structure of 2b
Entry
l
nm
EX
Acc
Sol
Time
h
Con.a
%
Yielda (C.Y.b)
%
X-Ray crystallographic analysis of a single crystal of 2b [19],
c
1
2
3
4
5
6
7
8
>410
–
CDCl
CDCl
CDCl
3
3
3
10
12
12
15
10
12
12
12
37
100
97
100
100
100
88
0 (0)
obtained by recrystallization from toluene–n-hexane, revealed
that this substance possesses a slightly helical crystalline state
geometry (Fig. 3a) with a disordered thienyl group. The dihedral
>313d
–
36 (36)
42 (43)
60 (60)
22 (22)
44 (44)
24 (27)
10 (14)
>313d
p-CA
p-CA
o-CA
o-CA
AQ
d
>313
C
6
D
6
>313d
angle
u
between terminal thieno moieties, including S1 and S2 in
CDCl
C D
6 6
CDCl
3
3
ꢀ
d
trithia[5]helicene framework of 2b, is 33.9 . In addition, analysis of
a packing diagram (Fig. 3b) shows that the crystal consists of equal
amounts of P-2b (red) and M-2b (blue) isomers, which comprise a
locked dyad with a large degree of herring-bone like overlap. On
>313
>313d
>313d
AQ
C
6
D
6
74
a
Determined by using 1H NMR analysis with 1,3,5-trioxane as an internal
standard.
the other hand, the dihedral angle of the parent trithia[5]helicene
b
ꢀ
C.Y. stands for conversion yield, representing (Yield/Con.) Â 100.
(u
= 36.6 ) [20], is slightly larger than that of 2b, and the crystal
c
A 2-kW Xe lamp with a Y-44 cutoff filter.
packing structure consists of racemic dyads ordered in a face-to-
face manner.
d
A 300-W high-pressure Hg lamp with a Pyrex cutoff filter.
1
moieties in 1b. The structure of 2b was confirmed by using H and
3
.4. Physical Properties of 2
13
C NMR spectroscopy and FAB-mass spectrometry. Remarkably,
the 6 -EC/AR photoreaction of 1b was found to be more efficient
than that of 1a, in spite of the presence of Me groups, which should
cause steric hindrance that could suppress 6 -EC reactions of the
,3,5-hexatriene moieties. In contrast to the observation that 2b is
not generated by direct photoirradiation of a C solution of 1b,
the corresponding PET reaction of this substance with p-CA in C
generates 2b in the highest yield among all reactions investigated
Entry 4).
In addition, the results of 1H NMR monitoring of time-
dependent changes occurring during the photoreaction of 1b in
CDCl , showed that a singlet signal arises at 3.0 ppm, which is
assigned to MeCl (Fig. 2a). The time-dependent changes taking
p
Studies were carried out to elucidate the photophysical
properties of trithia[5]helicenes 2a and 2b. UV–vis absorption
spectra of 2a and 2b in CH Cl have maxima at AB = 381 and
64 nm (Table 3 and Fig. 4, bottom, red), respectively. Photoexci-
tation of 2a and 2b provides fluorescence with respective maxima
at FL = 429 and 417 nm [17]. Both AB and FL values of these
p
2
2
l
1
3
6 6
D
6 6
D
l
l
l
photoproducts occur at shorter wavelengths than those of the
corresponding precursors 1a and 1b. In addition, fluorescence
quantum yields of 2a and 2b are larger than those of their
precursors, a likely consequence of increased structural rigidity.
Cyclic voltammograms of 2a and 2b contain anodic peaks with
irreversible oxidation waves at EAP = +1.41 and +1.47 V vs SCE in
(
3
1
place in the H NMR spectrum of a C
6
D
6
solution of 1b and p-CA in
solution in the
2 2
CH Cl , respectively [17]. The EAP values are higher than those of
the PET reaction are similar to those of a CDCl
3
direct photoirradiation reaction (Entry 2). However, generation of
the 3.0-ppm signal (MeCl) does not occur (Fig. 2b). Rather, a signal
Fig. 3. (a) ORTEP plots of X-ray crystallographic data of 2b showing 50%-probability
thermal ellipsoids (T = 296 K). (b) The crystal packing structure of 2b. Red and blue
molecules indicate P- and M-2b, respectively. (For interpretation of the references
to colour in this figure legend, the reader is referred to the web version of this
article.).
Fig. 2. Time-dependent changes of 1H NMR spectra observed during (a) direct
photoexcitation reaction for 1b in CDCl
3
(Table 2, Entry 2) and (b) PET reaction for
1
b in C with p-CA (Table 2, Entry 4).
6 6
D
Please cite this article in press as: A. Yamamoto, et al., Formation of a trithia[5]helicene in an unexpected photoreaction of a methyl-substituted
bis(dithienylethenyl)thiophene through a double sequence of 6 -electrocyclization/aromatization (dehydrogenation/demethylation), J.
Photochem. Photobiol. A: Chem. (2015), http://dx.doi.org/10.1016/j.jphotochem.2015.10.012
p

G Model
JPC 10032 No. of Pages 8
A. Yamamoto et al. / Journal of Photochemistry and Photobiology A: Chemistry xxx (2015) xxx–xxx
5
Table 3
minus (363 nm) at ca. 400 nm, which corresponds to the presence
of two electronic transitions. The fact that the MCD signals are
relatively intense indicates that the low-energy transition is a
UV–vis absorption and fluorescence maxima (
l
AB and
l
FL), fluorescence quantum
Cl
yields (
F
FL) and anodic peak potentials (EAP) of 1 and 2 in aerated CH
2
2
.
a
b
c
Compd.
l
nm
AB
l
FL
F
FL
E
AP
trithia[5]helicene
p–p* transition, because signal intensities in
nm
V vs SCE
MCD depend on changes in angular momentum between the
ground and excited states. A clear plus to minus Faraday B terms,
which is associated with the absorption band at 267 nm, also
appear at around 270 nm. The MCD spectrum of 2b contains
similar features, indicating that Me groups in this substance have
only marginal effects on the electronic structure of the trithia[5]
helicene unit.
1a
1b
2a
2b
294, 429
293, 428
267, 381
259, 364
529
527
429
417
–d
+1.08, +1.55
+1.05, +1.24
+1.41
0.01
0.18
0.08
+1.47
a
EX = 430 nm for 1a and 1b, 380 nm for 2a, and 360 nm for 2b. 2.5 Â10À5 M.
l
l
b
À5
EX = 429, 381, and 364 nm for 1b, 2a, and 2b, respectively. 2.5 Â10 M.
Determined by using an integrating sphere system.
c
+
–
À1
4 4
Electrolyte: n-Bu N BF (0.1 M), scan rate: 0.1 V s , reference electrode: SCE,
working and counter electrodes: Pt.
4. Discussion
d
Not available.
4
.1. Mechanism for formation of 2
The photoreaction of 1 leading to 2 is initiated by excitation of
1
either 1 or p-CA (Scheme 4). In the former case, 1* is generated,
and then undergoes 6
p-EC reaction to form 3. Thus, this is a direct
photoexcitation reaction. Hydrogen abstraction from 3 by p-CA
ꢁ
ꢁ
ꢁ
takes place to give p-CAH and 10 . In the case of 10a (R = H),
another hydrogen-abstraction reaction occurs to form 4a and p-
ꢁ
CAH
(
2
in the final step of the first AR ( ÀÀ 2H). In the case of 10b
ꢁ
R = Me), 4b is generated by elimination of Me , which then
undergoes chlorine abstraction from CDCl
sequence of 6
examples [22,23] have been reported previously of AR reactions
occurring through elimination of a Me group. Finally, 4 is
transformed to 2 through a similar mechanistic sequence. In the
3
to form MeCl [a
-EC/AR ( ÀÀ H/ ÀÀ Me) reaction]. Note that only two
p
3
cases of the reaction of 10 without p-CA (or other Acc) in CDCl , the
dissolved molecular oxygen most likely works as a hydrogen
1
acceptor instead of p-CA. Alternatively, 1* could also undergo SET
to p-CA (PET1,
pair, [1 /p-CA ] (especially in CDCl
D
G
PET1ꢂ –1.2 eV, Table 4) to generate a radical–ion
ꢁ
+
ꢁ–
3
), which probably has a
singlet multiplicity. Thus, here is a PET reaction (i.e. 1-sensitized
ꢁ
+
Fig. 4. MCD (top) and UV–vis absorption spectra (bottom, red) in aerated CH
2
Cl
2
PET reaction) triggered by excitation of 1. 5
p
-EC reaction of 1
ꢁ+
and electronic transitions (bottom, blue) calculated with TD-B3LYP/6-31G(d,p) of 2a
then forms 3 , in which a proton is abstracted ( ÀÀ H in 6 -EC) by p-
p
À5
À5
(
a) and 2b (b). MCD spectra; [2a] = 7.6 Â10 M, [2b] = 6.1 Â10 M. UV–vis
ꢁ–
ꢁ
CA to give 10 .
À5
absorption spectra; [2a or 2b] = 2.5 Â10 M.
In the latter case, where the reaction is initiated by p-CA
excitation followed by intersystem crossing to form its triplet state,
another PET reaction (i.e. p-CA-sensitized PET reaction, PET2) is
their precursors owing to the presence of lower energy HOMOs in
contrast to those of 1a and 1b.
3
triggered. PET from 1 to p-CA* (
D
G
PET2 ꢂ –0.6 eV) also gives the
ꢁ
+
ꢁ–
radical-ion pair, [1 /p-CA ], which has a triplet multiplicity. The
observation that the yields of 2b in the PET reactions in C are
higher than those in CDCl (Table 2) is likely as a result of the fact
MCD spectroscopy was utilized to gain insight [21] into the
6 6
D
electronic structures of the
Analysis of the MCD spectrum of 2a in aerated CH
p
-conjugated molecules 2a and 2b.
Cl [Fig. 4 (top)]
3
d–
2
2
d+
that exciplex [1b /p-CA ]* is more resistant to decomposition
shows that the sign of the signal changes from plus (392 nm) to
Scheme 4. Plausible mechanisms for formation of 2 through a double sequence of 6
p
-EC/AR ( ÀÀ 2H or ÀÀ H/ ÀÀ Me) reaction triggered by excitation of 1 or p-CA.
Please cite this article in press as: A. Yamamoto, et al., Formation of a trithia[5]helicene in an unexpected photoreaction of a methyl-substituted
bis(dithienylethenyl)thiophene through a double sequence of 6 -electrocyclization/aromatization (dehydrogenation/demethylation), J.
Photochem. Photobiol. A: Chem. (2015), http://dx.doi.org/10.1016/j.jphotochem.2015.10.012
p

G Model
JPC 10032 No. of Pages 8
6
A. Yamamoto et al. / Journal of Photochemistry and Photobiology A: Chemistry xxx (2015) xxx–xxx
Table 4
Observations in this study show that photoreaction of 1b is
more efficient than that of 1a, in spite of the fact that the Me groups
in 1b are anticipated to provide steric hindrance in the 6p-EC
T
Cathodic peak potentials (ECP), triplet excitation energies (E ) of Acc, and changes in
Gibbs free energy associated with electron transfer from 1* to Acc (
from 1 to Acc* (
1
D
G
PET1), and
3
DG
PET2) in CH
2 2
Cl .
processes. The differences in the reactivities of 1a and 1b in the
direct photoirradiation reactions can be explained by considering
differences in the LUMO orbital coefficients on the 1,3,5-
hexatriene-terminal carbons in the s-trans,s-cis conformers
(Fig. 5). In the LUMO of 1a, the two thienyl groups that constitute
the photolabile 1,3,5-hexatriene moiety are oriented nearly
perpendicular. As a result, the orbital coefficient at C8 of 1a is
Acc
E
CP(Acc) (Acc)
E
T
D
eV
G
PET1
D
eV
G
PET2
b,c
a
V vs SCE
eV
1ad
1be
1ad
1be
p-CA
o-CA
AQ
À0.12
À0.01
À0.99
2.00f
À1.19
À1.30
À0.32
À1.22
À1.33
À0.35
À0.57
À1.30
À0.41
À0.60
À1.33
À0.44
f
2.62
2.71g
a
D
G
PET1 = EAP(1) À ECP(Acc) À E
S
(1) +
v. See Ref. [24].
small and, as a consequence, 6
other hand, the orbital coefficient at C8 in 1b is larger than that in
a, suggesting that 6 -EC should take place more efficiently in this
p-EC proceeds inefficiently. On the
b
c
d
e
f
v
D
= 0.23 eV. See Ref. [25].
G
PET2 = EAP(1) – ECP(Acc) – E
T
(Acc) +
v.
1
p
EAP(1a) = +1.08 V vs SCE, E
S
(1a) = 2.62 eV [17].
(1b) = 2.62 eV [17].
case. Note that the distribution of orbital coefficients at C8 of the
LUMOs of double-s-cis conformers of 1a and 1b resemble those of s-
trans,s-cis conformers.
EAP(1b) = +1.05 V vs SCE, E
S
Obtained by using phosphorescence wavelength of p-CA (620 nm) or o-CA
(
474 nm) in n-BuCl at 77 K [17].
g
In methylcyclohexane/isopentane = 5 (v/v). See Ref. [26].
ꢀ
Additionally, since the dissociation energy (DH ) of t-Bu ÀÀ Me
À1
bond (87.5 kcal mol ) is smaller than that of the t-Bu ÀÀ H bond
ꢁ
+
ꢁ–
ꢁ+
than [1b /p-CA ] or a free ion, 1b . After all, three mechanistic
routes, direct irradiation, PET1 and PET2 can be involved in this
process.
ꢀ
À1
ꢁ
ꢁ
(
DH = 96.5 kcal mol ) [28], elimination of Me is more likely to
ꢁ
occur than that of H . Thus, AR ÀÀ H/ ÀÀ Me) from 10b should take
ꢁ
place more readily than AR ( ÀÀ 2H) from 10a . Therefore, it appears
that for introducing Me groups at the 1,3,5-hexatriene-terminal
carbons, an electronic advantage that outweighs steric disadvan-
4
.2. DFT calculations providing a plausible explanation for differences
in the efficiencies of photoreactions of 1a and 1b
tages exists to promote efficient 6p-EC.
To gain an understanding of the differences in the efficiencies of
photoreactions of 1a and 1b, knowledge about conformational
preferences associated with rotation about their single bonds is
required. From a consideration of geometric requirements, the “
double-s-cis” and “s-trans,s-cis” conformers of 1 can undergo 6p-EC
reactions while the “double-s-trans” conformer should be unable to
4
.3. DFT calculations providing information about differences in
electronic structures of 2a and 2b
DFT calculations [B3LYP/6-31+G(d,p)] on 2a and 2b were
employed to gain insight into their electronic structures. In an
optimized geometry, the thienyl groups in 2a exist in a twisted
trithia[5]helicene unit with a dihedral angles,
groups and trithia[5]helicene framework (Fig. 3) of 42.69 and
participate in this process (Table 5). The fact that these conformers
v
, between thienyl
1
cannot be discerned by using H NMR spectroscopy, suggests that
ꢀ
they are in rapid equilibrium under conditions employed. As a
result, we have utilized DFT calculations [27] [B3LYP/6-31+G(d,p)]
to estimate their relative energies (Erel) and populations. The
results show that the s-trans,s-cis conformer of 1a is of the lowest
ꢀ
41.61 . This distortion from planarity is caused by steric repulsion
between thienyl groups and the trithia[5]helicene framework. The
v
owing to the presence of Me groups. Thus, thienyl groups of 2b do
not have significant orbital coefficients in the HOMO–1, LUMO, and
LUMO+1 (Fig. 6). The energy of the LUMO of 2b is higher than that
of 2a, while energies of the HOMO–1 of 2a and 2b are nearly the
ꢀ
ꢀ
are larger (69.88 and 62.86 ) in the optimized structure of 2b
À1
energy (Erel = 0 kcal mol ) and that it comprises 53% of the
conformer mixture at 298 K, according to Boltzmann distribution.
À1
Moreover, the double-s-trans (Erel = 0.18 kcal mol ) and double-s-
À1
cis (Erel = 1.19 kcal mol ) conformers of 1a comprise 40 and 7%,
same. The HOMO–LUMO energy gap (DEH–L) in 2b is calculated to
respectively, of the mixture. For 1b, s-trans,s-cis conformer is the
be 3.76 eV [= –1.92 eV – (–5.68 eV)], a value that is larger than that
of 2a (3.49 eV). These differences in electronic structures are
reflected in the UV–vis absorption and fluorescence spectra [17] of
À1
most stable (Erel = 0 kcal mol , 67%) and the double-s-trans (Erel
0
=
À1
À1
.69 kcal mol , 21%) and double-s-cis (Erel = 1.00 kcal mol , 12%)
conformers are of higher energy. Therefore, the 6 -EC photolabile
p
2
a and 2b (Table 3 and Fig. 4).
conformers of 1a and 1b exist predominantly at ambient
temperature.
4
.4. Analysis of MCD spectra
Analysis of the MCD spectrum of 2a suggests that the 390-nm
Table 5
À1
absorption peak is comprised of two bands. The plus-to-minus sign
transition pattern seen for ascending energy indicates that
Relative energies (Erel/kcal mol ) of conformers of 1a and 1b calculated using
B3LYP/6-31+G(d,p) at 298 K.a
DE
H
[
[
= E
= EL+1 – E
H
– EH–1 = –5.55 eV – (–5.99 eV) = 0.44 eV] is smaller than
DE
L
L
= 0.96 eV] [21]. This finding is supported by the results of
frontier orbital calculations on 2a and 2b. The difference between
the MOs of 2a and 2b is likely a consequence of the large dihedral
1
1
a
1.19
7%)
1.00
12%)
0
0.18
(40%)
0.69
(
(53%)
0
(67%)
b
(
(21%)
a
The numbers in parentheses are the populations based on Boltzmann
Fig. 5. The LUMOs of (a) 1a and (b) 1b in s-trans,s-cis conformer and estimated by
distribution.
using DFT method at B3LYP/6-31+G(d,p) level of theory.
Please cite this article in press as: A. Yamamoto, et al., Formation of a trithia[5]helicene in an unexpected photoreaction of a methyl-substituted
bis(dithienylethenyl)thiophene through a double sequence of 6 -electrocyclization/aromatization (dehydrogenation/demethylation), J.
Photochem. Photobiol. A: Chem. (2015), http://dx.doi.org/10.1016/j.jphotochem.2015.10.012
p

G Model
JPC 10032 No. of Pages 8
A. Yamamoto et al. / Journal of Photochemistry and Photobiology A: Chemistry xxx (2015) xxx–xxx
7
Scheme 5. AR ( ÀÀ 2H, ÀÀ H/ ÀÀ Me, or ÀÀ 2Me) Reactions of the 1,3-cyclohexadiene
moieties formed by 6 -EC reactions.
p
hexatrienes. Thus, an exploration of the photoreactions of 1,3,5-
hexatrienes is a goal of our studies in this area.
Acknowledgements
Fig. 6. The HOMOÀ1, HOMO, LUMO, and LUMO+1 energies (abbreviated as HÀ1, H,
L, and L+1, respectively) of 2 calculated using B3LYP/6-31+G(d,p).
A.Y. and Y.M. gratefully acknowledge financial support (Nos. 25-
04 and 23-307) provided by the Sasakawa Scientific Research
3
Grant from the Japan Science Society, respectively. E.O. acknowl-
edges financial support in the form of a Grant-in-Aid for Young
Scientist (B) (No. 24750044) and the Challenging Exploratory
Research (No. 26620034) from the Ministry of Education, Culture,
Sports, Science and Technology (MEXT), Japan. N.K. gratefully
acknowledges financial support in the form of a Grant-in-Aid for
Scientific Research on Priority Areas “Stimuli-responsive Chemical
Species” (Nos. 25109502 and 15H00910). K.M. gratefully acknowl-
edges financial support in the form of a Grant-in-Aid for the
Scientific Research (C) (Nos. 23550058 and 26410049) and
Cooperation for Innovative Technology and Advanced Research
in Evolutional Area (Wakayama) program from the MEXT of Japan.
H.I. gratefully acknowledges financial support in the form of a
Grant-in-Aid for Scientific Research on Priority Areas ‘New
Frontiers in Photochromism’ (Nos. 20044027 and 21021025 in
ꢀ
ꢀ
angles (
v= 69.88 and 62.86 ) between the trithia[5]helicene unit
and thienyl groups in 2b.
4
.5. Crystal structure
Some helicenes serve as organic semiconductors [29]. However,
the presence of large cavities between the parent trithia[5]helicene
units of adjacent molecules in the crystal structure of 2b, caused by
steric hindrance, might prevent its use as an OFET, which requires
high carrier mobility. On the other hand, if 2a and 2b is able to be
amorphous, they might be useful as semiconductors, which do not
require high mobility, e.g., hole transporting materials for organic
light-emitting diode and donors of thin-layer photovoltaics.
the Area No. 471), Innovative Areas ‘
3108718 in the Area No. 2007) and ‘Stimuli- responsive Chemical
Species’ (No. 24109009), the Scientific Research (B) (Nos.
0044027 and 23350023), and the Challenging Exploratory
p-Space’ (Nos. 21108520 and
5
. Conclusion
2
In the study described above, we demonstrated that photo-
2
reactions of bis(dithienylethenyl) thiophenes 1a and 1b produce
the corresponding trithia[5]helicenes 2a and 2b by double
Research (Nos. 21655016 and 24655037) from the MEXT, Japan.
sequence of 6
p
-EC/AR ( ÀÀ 2H or ÀÀ H/ ÀÀ Me) reactions of their
Appendix A. Supplementary data
two 1,3,5-hexatriene moieties. Surprisingly, the efficiency of the
photoreaction of the tetramethyl derivative 1b is much higher than
that of 1a in spite of the presence of Me groups in the former that
could have sterically-interfered with 6p-EC/AR reaction. Two
reasons exist for this paradoxical cascade reaction, one is
associated with Me group effects on the distribution of the LUMO
Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/j.
jphotochem.2015.10.012.
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more favored. Finally, the results show that PET reaction of 1b in
the presence of p-CA is the ideal method for synthesis of 2b.
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Please cite this article in press as: A. Yamamoto, et al., Formation of a trithia[5]helicene in an unexpected photoreaction of a methyl-substituted
bis(dithienylethenyl)thiophene through a double sequence of 6 -electrocyclization/aromatization (dehydrogenation/demethylation), J.
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Please cite this article in press as: A. Yamamoto, et al., Formation of a trithia[5]helicene in an unexpected photoreaction of a methyl-substituted
bis(dithienylethenyl)thiophene through a double sequence of 6 -electrocyclization/aromatization (dehydrogenation/demethylation), J.
Photochem. Photobiol. A: Chem. (2015), http://dx.doi.org/10.1016/j.jphotochem.2015.10.012
p