Tunable blue-emitting fluorophores-benzo[1,2-b:4,3-b′]dithiophene and trithia[5]helicene end-capped with electron-rich or electron-deficient aryl substituents

Article abstract of DOI:10.1016/j.tet.2007.12.029

Light-emitting fluorophores 1-10b based on aryl substituted benzo[1,2-b:4,3-b′]dithiophenes (BDT) and trithia[5]helicenes (T5H) have been synthesized using various combinations of Suzuki coupling, the Wittig, or McMurry reaction, and subsequent photocycli

Full text of DOI:10.1016/j.tet.2007.12.029

Available online at www.sciencedirect.com
Tetrahedron 64 (2008) 2251e2258
www.elsevier.com/locate/tet
Tunable blue-emitting fluorophoresdbenzo[1,2-b:4,3-b0]dithiophene
and trithia[5]helicene end-capped with electron-rich or
electron-deficient aryl substituents
Ying Hu a, Brigitte Wex b, Marc W. Perkovic c, Douglas C. Neckers a,
*
a Center for Photochemical Sciences, Bowling Green State University, Bowling Green, OH 43403, United States
b Division of Natural Sciences, Lebanese American University, Byblos, Lebanon
c Department of Chemistry, Western Michigan University, Kalamazoo, MI 49008, United States
Received 12 October 2007; received in revised form 27 November 2007; accepted 4 December 2007
Available online 15 December 2007
It is with sorrow that we dedicate this paper to the memory of Professor Perkovic
Abstract
Light-emitting fluorophores 1e10b based on aryl substituted benzo[1,2-b:4,3-b0]dithiophenes (BDT) and trithia[5]helicenes (T5H) have
been synthesized using various combinations of Suzuki coupling, the Wittig, or McMurry reaction, and subsequent photocyclization of the di-
thienylethenes thus obtained. Photophysical properties of the helical compounds end-capped with different electron-rich and electron-deficient
aryl moieties thus resulting were evaluated. Photocyclization of a dithienylethene derivative 10a was investigated, and the X-ray crystal structure
of dinaphthyl-substituted BDT (4) was obtained. With this series of compounds 1e10b, we demonstrate that the optical properties of all of the
new compounds, and by extension many conjugated materials, can be tuned over the entire blue range (400e480 nm).
Ó 2007 Elsevier Ltd. All rights reserved.
1. Introduction
design and the optimization of functional organic helical con-
jugated materials.
Organic semiconducting molecules particularly for applica-
tions as organic light-emitting diodes (OLED), organic field-
effect transistors (OFET), in nonlinear optics (NLO), and as
photovoltaic cells have attracted considerable attention. Non-
linear polycyclic compounds used as NLO materials have
been studied both theoretically and experimentally.1e3 They
serve as model compounds for understanding the chemical
and physical properties of related materials as well as for
the investigation of structureeproperty correlations among
similar electronic materials. Our particular interest was in
probing structural features to affect useful functions and prop-
erties using materials comprised of aryl substituted heteroheli-
cenes in that these compounds are essential toward the rational
The helix is a common structure in nature, and its intrinsic
3-D architecture also offers much interest to synthetic chem-
ists because of the potential interactions between overlapping
layers, and the presence of a powerful inherent chirality.4e6 In
biology, important life secrets lie in helical DNA, and many
proteins adopt a helical structure. In chemistry, since Newman
synthesized and resolved hexahelicene,7,8 Martin (Brussels)9
and Wynberg (Groningen)10 developed numerous helicenes
of all types, including carbohelicenes, double helicenes, and
heterohelicenes. Replacing benzenes with thiophenes has ben-
eficial effects on the nonlinear optical properties of previously
known achiral conjugated helicene structures, due to the
unique 2-position of thiophene and the electron-rich sulfur
atoms. In principle, these features make functionalization of
helicene systems possible, thus opening the way for obtaining
modified helicenes targeted for specific uses, and for tuning
and monitoring properties.11
* Corresponding author.
E-mail address: neckers@photo.bgsu.edu (D.C. Neckers).
0040-4020/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2007.12.029

2252
Y. Hu et al. / Tetrahedron 64 (2008) 2251e2258
R1
R2
applied, the synthesis of 2 was achieved in w80%, Scheme 2.
2-Arylthiophenes are susceptible to rearrangement on irradia-
tion.20 Photolysis of 2-(a-naphthyl)thiophene in dilute solu-
tion, for example, yields 3-(a-naphthyl)thiophene as the only
product.10,21,22 Photolysis of 3-(a-naphthyl)thiophene in dilute
solution led to slow decomposition, and no migration of the
naphthalene ring was observed.10,21,22 In contrast, 2- and 3-ar-
ylbenzo[b]thiophenes show no significant migration of the aryl
group or decomposition upon photolysis.23,24 Our materials,
2-arylbenzo[1,2-b:4,3-b0]dithiophenes, showed no evidence
of group migration as verified by gas chromatography,
NMR, and UV absorption spectroscopies, or X-ray crystallo-
graphy. In this case, conjugation might stabilize carbons 2 and
3 of the thiophene moiety toward rearrangement by rigidifying
the structure and delocalizing the electrons.
S
S
1. R1 = 4-methoxyphenyl, R2 = H
2. R1 = α−naphthyl, R2 = H
3. R1 = 4-cyanophenyl, R2 = H
4. R1 = R2 = α−naphthyl
5. R1 = 4-acetylphenyl, R2 = H
6. R1 = 4-methoxyphenyl, R2 = (E)-2-thienylethenyl
R3
R4
S
S
S
7. R3 = 4-methoxyphenyl, R4 = H
8. R3 = 4-methoxyphenyl, R4 = 4-cyanophenyl
9. R3 = 4-methoxyphenyl, R4 = 4-acetylphenyl
(a)
(b)
Chart 1. Molecular structures of BDT and T5H derivatives 1e9.
O
Br
R1
S
S
R1
S
Many alkyl thia[n]helicenes, n<9, have been reported,11e15
but aryl thia[n]helicenes have never been prepared. We herein
report the first synthesis of helicenes end-capped with elec-
tron-rich and electron-deficient aryl moieties (Chart 1). Tun-
able blue-emitting p-conjugated helical compounds based on
benzo[1,2-b:4,3-b0]dithiophene (BDT) and trithia[5]helicene
(T5H) were synthesized, and their structureefunction rela-
tionships were investigated.16 Both BDT and T5H emit in
the UV region at 333 nm, 365 and 379 nm, respectively.
Therefore, tunable blue-emitting compounds can be obtained
by carefully choosing the number of aromatic substituents
and end-capped moieties. The parent BDT itself is planar.
Aryl substituents increase steric crowding on BDT molecule
and thereby induce helicity. We attribute the good photo and
thermal stability in 1e5 and 7e9 to their non-planarity and
higher percentage of benzene character.17,18
(c)
(d)
R2
R1
R1
10a. R1 = α−naphthyl
S
S
S
S
10b. R1 = R2 = α−naphthyl
(e)
(e)
R2
S
R1
S
2. R1 = α−naphthyl, R2 = H
4. R1 = R2 = α−naphthyl
Scheme 1. Reagents and conditions: (a) R1B(OH)2, toluene, (C4H9)4NOH,
Pd(PPh3)4, reflux, overnight. (b) n-BuLi, DMF, ꢀ78 ꢁC, 40 min. (c)
Diethyl(2-thienylmethyl)phosphonate, NaH, DME, 0 ꢁC for 30 min, reflux
3 h. (d) TiCl4, Zn, THF, ꢀ18 ꢁC for 1 h, reflux 4 h. (e) I2, propylene oxide,
hv, 1 day.
2. Results and discussion
Photocyclization of 10a, the precursor of 2, was carried out
in C6D6 in an NMR tube. The solution was irradiated under
350 nm light and proton NMR spectra were recorded every
10 min. Selected spectra are shown in Figure 1. During irradi-
ation, 10a, i.e., (E)-1-{2-[5-(1-naphthyl)thienyl]}-2-(2-thienyl)-
ethene, was photochemically isomerized to the Z-isomer,
i.e., (Z)-1-{2-[5-(1-naphthyl)thienyl]}-2-(2-thienyl)ethene. This
Z-isomer then underwent photocyclization and oxidation to
form the desired product 2.
At t¼0 min, only the starting 10a existed. The protons of
the ethene group appeared around 7.11 ppm as a singlet. The
formation of the Z-isomer was confirmed by appearance and
disappearance of peaks in the 6.35e6.45 ppm region of the
spectra. At t¼570 min, the signal of the proton at the 2-posi-
tion of thiophene changed to a doublet at 7.08 ppm. Peaks
above 9.50 ppm were assigned to aldehydic oxidation products
of the double bond. Chart 2 shows a plot of the change in the
percentage of starting material (10a), intermediates, product
(2), and by-products. At the beginning of the reaction, 10a
photoisomerized to the Z-isomer and immediately formed
a 1:1 photostationery state of both isomers. This 1:1 ratio of
2.1. Synthesis
Photocyclization of stilbene-like olefins followed by in situ
oxidation using iodine is a crucial step in the synthesis of
higher numbered helicenes.13,19 This reaction requires dilute
solutions and is typically carried out in toluene. Application
to higher helicenes and heterohelicenes is limited due to their
poor solubility, the small scale of the synthesis, and the fact
that the products act as filters for incident radiation.12
Suzuki coupling is an efficient method for introducing aryl
groups. In this method, a palladium catalyst is used to cross-
couple an organoboronic acid with an aryl halide. For our tar-
get compounds 1e9, there are theoretically two approaches,
i.e., Suzuki coupling followed by Wittig/McMurry reaction
or Wittig/McMurry reaction followed by Suzuki coupling.
Compounds 2 and 4 were synthesized by the former approach
in overall yield of 27e47%, Scheme 1. Decomposition prod-
ucts were detected by NMR spectroscopy during photochemi-
cal cyclization (step e, Scheme 1). When the latter approach was

Y. Hu et al. / Tetrahedron 64 (2008) 2251e2258
2253
R2
I
R1
R1
(a)
(b)
S
S
S
S
S
S
1. R1 = 4-methoxyphenyl
2. R1 = α−naphthyl
3. R1 = 4-cyanophenyl
5. R1 = 4-acetylphenyl
11. R1 = 4-methoxyphenyl,
2 = CHO
(c)
R
6. R1 = 4-methoxyphenyl,
R
2 = (E)-2-thienylethenyl
(d)
R1
R3
R1
S
I
R1
S
S
S
S
S
(a)
(e)
S
S
S
8. R1 = 4-methoxyphenyl,
12. R1 = 4-methoxyphenyl
7. R1 = 4-methoxyphenyl
R3 = 4-cyanophenyl
9. R1 = 4-methoxyphenyl,
R3 = 4-acetylphenyl
Scheme 2. (a) DME, K2CO3 (2 M), Pd(PPh3)4, R3B(OH)2, reflux, overnight. (b) n-BuLi, DMF, ꢀ78 ꢁC, 40 min. (c) Diethyl(2-thienylmethyl)phosphonate, NaH,
DME, 0 ꢁC for 30 min, reflux 3 h. (d) I2, propylene oxide, hv, 1 day. (e) n-BuLi, I2, DME, ꢀ78 ꢁC, 1 h.
isomers was observed until the end of the photocyclization
reaction.
In order to avoid solubility problems, 7e9 were synthesized
100
80
60
40
20
0
by a combination of both approaches (Scheme 2). Our at-
tempts to purify 12 have continually failed due to similarities
in polarity of 7 and 12. The Suzuki cross-coupling of crude 12
with the corresponding boronic acid afforded 8 and 9, respec-
tively. For both products, 8 and 9, bright yellow solids were
recrystallized from dichloromethane. The yields for 8 and 9
over these two steps starting from 7 were 69e73%. Notewor-
thy, the synthesis of 8 starting from 3, did not yield the desired
product. The synthesis was attempted in an approach similar to
that outlined in Scheme 2. In this case, however, the formyl-
ation of 3 did not furnish the desired aldehyde (110 with R1¼
4-cyanophenyl, R2¼CHO). This finding is attributed to the de-
activation of the formylation site (i.e., R2, Scheme 2) of BDT by
the electron-deficient cyanophenyl group. As anticipated, the
starting material 10a
Z-isomer of 10a
product 2
by-product
0
100
200
300 400
Time (min)
500
600
700
Chart 2. Photocyclization data profile of 10a.
electron-releasing nature of the methoxyphenyl group stabilized
the reaction intermediates and afforded 11, and hence 7, 12, and
8 in satisfactory yields.
2.2. Crystal structure
Slow recrystallization of 4 from benzene resulted in wedge-
like crystals. Figures 2 and 3 show the molecular structure of
425 and the packing obtained by single-crystal X-ray diffrac-
tion. Although both naphthalene and BDT are planar, the steric
bulk of the naphthyl arms causes a twisting in 4. It has a C2
molecular symmetry with the twofold axis bisecting the cen-
tral ring of the molecules.
The angle between the two naphthalene rings is 114ꢁ. Mole-
cules in the crystal are well oriented and exist as a racemic
mixture. Pi-stacking is observed between neighboring naph-
thalene rings and between neighboring BDT rings. The 57ꢁ
Figure 1. 1H NMR profile of photocyclization of 10a.

2254
Y. Hu et al. / Tetrahedron 64 (2008) 2251e2258
Figure 2. X-ray crystal structure of 4.
Figure 4. Crystals of 4 under UV irradiation.
conjugation further in the ground state as these structures are
non-planar. The electron-rich methoxyphenyl group helps to
lower the ground state energy slightly more than the electron-
deficient cyano- and acetylphenyl groups do.
A fine emission-color tuning of 1e10b in the region of
380e480 nm is observed in the fluorescence spectra as shown
in Figure 6. Comparing the emission maxima of 1 (379 nm), 3
(413 nm), and 5 (430 nm) with those of parent BDT (333 nm)
shows that aryl substituents introduce a red-shift of emission.
The electron-rich aryl group, methoxyphenyl, introduces
a red-shift of 46 nm, while the electron-deficient aryl groups,
cyanophenyl and acetylphenyl, introduce red-shifts of more
than 80 nm in dichloromethane. Likewise, the emission max-
ima of 7 (409 nm), 8 (469 nm), and 9 (477 nm) show the
same trend when compared with T5H (363 nm). The fluores-
cence quantum yields of 1 and 7 are 0.55 and 0.12, respec-
tively. In comparison, the fluorescence quantum yield of
BDT is 0.08 and that of T5H is 0.02.13 This implies that the
Figure 3. Packing pattern of 4.
angle may be the reason for the absence of interaction between
naphthalene and BDT. Figure 4 depicts crystals of 4 under UV
light.
2.3. Absorption and emission spectra
Table 1
Photophysical properties of 1e10 in CH2Cl2
Table 1 summarizes absorption and emission properties of
compounds 1e10b recorded at room temperature. Normally,
p-conjugated systems exhibit strong pep* electronic absorp-
tion in the UVevis region. Compounds 1e10b fluoresce under
UV light, and are mostly blue emitters. For comparison, the
absorption and emission spectra of parent BDT and T5H are
shown in Figure 5.
Compounds 1e6 showed similar absorption spectra be-
cause they have the same heterocyclic backbone. Compounds
7e9 also show similar absorption spectra. Compounds with
one or two naphthyl substituents, 2 and 4, exhibited absorption
maxima at 307 and 333 nm, respectively. Phenyl-substituted
compounds 1, 3, and 5 absorb at 333, 346, and 350 nm, respec-
tively. These observations suggest that a chain extension at the
2- and 7-position of BDT does not improve the p-orbital
a
lmax (nm)
c
lF,max (nm)
d
Compound
3b (ꢂ104 Mꢀ1 cmꢀ1
3.0
)
FF
1
333
307
379
405
413
423
430
445
409
469
477
453
475
0.55
0.17
0.22
0.25
0.12
0.28
0.12
0.09
0.07
0.25
0.19
2
2.4
3.2
3
346
333
4
2.7
3.2
5
350
376
6
4.8
7
295, 357
323, 383
324, 380
370
3.5, 4.1
4.9, 4.3
5.4, 4.9
3.8
8
9
10a
10b
a
388
4.2
lmax¼absorption maximum.
b
c
d
3¼molar decadic absorption coefficient.
lF,max¼fluorescence maximum.
FF¼fluorescence quantum yield.

Y. Hu et al. / Tetrahedron 64 (2008) 2251e2258
2255
3 in hexane
1.0
0.8
0.6
0.4
0.2
0.0
Absorption of BDT
3 in dichloromethane
3 in acetonitrile
8 in hexane
8 in dichloromethane
8 in acetonitrile
Emission of BDT
Absorption of T5H
Emission of T5H
1.0
0.8
0.6
0.4
0.2
0.0
25
0
3
00
350
400
Wavelength (nm)
450
500
550
350
400
450
500
Wavelength (nm)
550
600
650
700
Figure 5. Normalized absorption and emission spectra of BDT and T5H in
CH2Cl2.
Figure 7. Normalized fluorescence spectra of compounds 3 and 8 in different
solvents.
electron-rich aryl moiety affects a smaller red-shift in fluores-
cence while dramatically increasing the fluorescence quantum
yields. Fluorescence quantum yields of compounds 1 (0.64), 3
(0.56), and 7 (0.07) were also measured in the solid-state. The
solid-state fluorescence spectra are provided in Supplementary
data.
Interestingly, the emission pattern and quantum yields of
fluorescence of 1 and 7 show no significant solvatochromism.
However, compounds with electron-deficient cyanophenyl
substituents (3 and 8) exhibit a large solvatochromic effect,
Figure 7 and Table 2. This observation hints of a polar fluoro-
phore.26 In the polar solvents, dichloromethane and acetoni-
trile, a shift in fluorescence to a longer wavelength and
a loss of the structured fluorescence were observed, Figure 7.
Polar solvents stabilize a more polarized excited state, such as
an internal charge-transfer (ICT) state, more effectively, which
may explain the observed pattern difference and red-shift of
the emission maximum. This suggests that the excited states
Table 2
Fluorescence quantum yields of 1, 3, 7, and 8 in different solvents
Compound
FF
Hexane
Dichloromethane
Acetonitrile
1
3
7
8
0.55
0.16
0.12
0.10
0.55
0.22
0.12
0.09
0.55
0.41
0.12
0.05
of compounds with the electron-deficient cyanophenyl moie-
ties (3 and 8) might be more polar than their ground states.
3. Conclusions
Two series of blue-emitting aryl substituted helical com-
pounds were successfully synthesized. Compound 2 was pre-
pared by two approaches. The photocyclization of 10a was
1
carried out in an NMR tube and followed by H NMR spec-
1
troscopy. Compound 10a immediately formed an equilibrium
1:1 photostationery state with the Z-isomer. Single-crystal
X-ray crystallographic analysis carried out on 4 shows that
the molecules exist as a racemic mixture in the crystal.
From the packing pattern, interaction between aryl substitu-
ents was observed and their contribution to the self-assembly
was confirmed. The photophysical properties of all the new
compounds were studied. By carefully choosing different
electron-deficient and electron-rich aryl substituents, the emis-
sion of BDT and T5H parent compounds can be well tuned in
the blue region of about 400e480 nm. Solvent polarity has
a stronger effect on the fluorescence of compounds with
electron-deficient groups than on the fluorescence of those
with electron-rich groups. The substantial changes in fluores-
cence spectra and quantum yields of 3 and 8 imply that their
excited states might form ICT or even charge-separated diradi-
cal states. A detailed photophysical study is currently
underway.
1.0
2
3
4
0.8
5
6
0.6
7
8
9
0.4
10a
10b
0.2
0.0
400
450
500
Wavelength (nm)
550
600
650
700
Figure 6. Normalized fluorescence spectra of compounds 1e10b in CH2Cl2
solution at room temperature.

2256
Y. Hu et al. / Tetrahedron 64 (2008) 2251e2258
4. Experimental section
4.1. General
4.2.2. 2-(4-Methoxyphenyl)-7-[(E)-2-(2-thienyl)ethenyl]-
benzo[1,2-b;4,3-b0]dithiophene (6)
Silica gel column chromatography with hexane/dichloro-
methane 1:1 as eluent produced pure 6, yield: 71%, mp
1
Solvents were dried and freshly distilled following usual
protocols prior to use. Reagents were used as received from
˚
commercial suppliers. Standard grade silica gel (60 A, 32e
63 mm) and silica gel plates (200 mm) were purchased from
Sorbent Technologies. Reactions that required anhydrous
conditions were carried out under argon in oven-dried
glassware.
Mass spectra were recorded on a Shimadzu GCMS-
QP5050A instrument equipped with a direct probe (ionization
70 eV). Melting points are uncorrected. A Bruker spectrome-
220e222 ꢁC. H NMR (300 MHz, CDCl3): d 3.88 (s, 3H),
6.97 (m, 1H), 7.00 (m, 1H), 7.03 (m, 1H), 7.12 (m, 1H), 7.18
(d, 2H), 7.23 (m, 1H), 7.60 (s, 1H), 7.65 (m, 1H), 7.68e7.74
(m, 4H); 13C NMR (75 MHz, CDCl3): d 55.4, 114.5, 116.3,
118.1, 118.9, 121.1, 121.9, 123.7, 124.9, 126.7, 127.1, 127.7,
127.8, 134.9, 135.5, 135.7, 135.9, 142.1, 142.7, 144.9, 159.9;
HRMS (EI): m/z [Mþ] calcd for C23H16OS3, 404.0363; found,
404.0368.
4.2.3. (E)-1,2-Bis[5-(1-naphthyl)thiophen-2-yl]ethene (10b)
To a stirred solution of 5-(a-naphthyl)thiophene-2-carb-
aldehyde (2.8 g, 10 mmol) in THF (100 mL) was added
titanium(IV) chloride (3 mL, 3 mmol) over a period of 30 min
at ꢀ18 ꢁC (dry ice with ethanol/water¼1:4). After stirring at
this temperature for 30 min, zinc powder (4 g, 60 mmol) was
added in small portions over a period of an additional
30 min. The mixture was stirred at ꢀ18 ꢁC for 30 min,
warmed to room temperature, and refluxed for 4 h. The reac-
tion was quenched by the addition of ice water (50 mL), and
the resulting solid was collected by filtration and dried. The
solid was dissolved in dichloromethane (70 mL) and the insol-
uble inorganic material was removed by filtration. The filtrate
was evaporated and the residue was recrystallized from cyclo-
hexane to give orange solid 10b,31 yield: 60%, mp 156e
1
ter (working frequency 300.0 MHz for H and 75.0 MHz for
13C) was used to record the NMR spectra with CDCl3 and
C6D6 as solvents. Chemical shifts relative to TMS at 0.00
ppm are reported in parts per million (ppm) on the d-scale
1
for H NMR. For 13C NMR, chemical shifts were reported
in the scale relative to CDCl3 at 77.00 ppm and C6D6 at
128.00 ppm. Absorption and fluorescence spectra were re-
corded on a Shimadzu UV-2401 spectrophotometer and a Fluo-
rolog-3 spectrometer, respectively. All measurements were
carried out at room temperature.
2-(a-Naphthyl)thiophene, 5-(a-naphthyl)thiophene-2-carb-
aldehyde, benzo[1,2-b;4,3-b0]dithiophene, and 2-iodobenzo-
[1,2-b;4,3-b0]dithiophene were synthesized following the
literature procedures.27e30
1
158 ꢁC. H NMR (300 MHz, CDCl3): d 7.13e7.16 (m, 6H),
7.48e7.55 (m, 6H), 7.60 (d, 2H), 7.85e7.92 (m, 4H), 8.28e
8.31 (m, 2H); 13C NMR (75 MHz, CDCl3): d 121.4, 125.3,
125.7, 126.1, 126.5, 126.6, 127.9, 128.1, 128.4, 128.5, 131.7,
132.3, 133.9, 140.8, 142.8; HRMS (EI): m/z [Mþ] calcd for
C30H20S2, 444.1006; found, 444.1006.
4.2. General procedure for Wittig reaction
To a stirred, ice-cooled solution of diethyl(2-thienylme-
thyl)phosphonate (2.3 g, 10 mmol) in dimethoxyethane (DME)
(40 mL) was added sodium hydride (60% dispersion, 0.52 g,
12 mmol). The mixture was stirred for 30 min in an ice bath
and then a solution of 5-(a-naphthyl)thiophene-2-carbaldehyde
(2.4 g, 10 mmol) in DME (10 mL) was added dropwise over
a period of 30 min. The mixture was slowly warmed and re-
fluxed for 3 h. After quenching the reaction by the addition of
ice water (50 mL), the mixture was extracted with ether. The ex-
tract was washed with water, dried over anhydrous Na2SO4, and
evaporated. The residue was recrystallized from cyclohexane or
purified by silica gel column chromatography.
4.3. General procedure for photocyclization
Compounds 10a, 10b, or 6 (0.5 mmol) were dissolved in
toluene (300 mL) and exposed to 300 nm light from a UV
lamp in a Rayonet reactor for 1 day. I2 (0.25 g, 0.1 mmol)
and propylene oxide (3 mL) were added to the solution and
air was bubbled through. The solvent was evaporated and the
residue recrystallized from hexane or purified by silica gel col-
umn chromatography.
4.3.1. 2-(a-Naphthyl)benzo[1,2-b;4,3-b0]dithiophene (2)
Yellow solid, yield: 64%, mp 91e93 ꢁC. 1H NMR
(300 MHz, CDCl3): d 7.50e7.57 (m, 4H), 7.67e7.71 (m, 2H),
7.82e7.83 (m, 3H), 7.89e7.93 (m, 2H), 8.31e8.34 (m, 1H);
13C NMR (75 MHz, CDCl3): d 118.3, 118.7, 121.9, 122.2,
125.2, 125.8, 126.2, 126.6, 126.7, 128.4, 128.6, 128.9, 131.9,
132.4, 133.9, 134.6, 135.1, 136.7, 136.9, 142.5; HRMS (EI):
m/z [Mþ] calcd for C20H12S2, 316.0381; found, 316.0382.
4.2.1. (E)-1-{2-[5-(1-Naphthyl)thienyl]}-2-(2-thienyl)ethene
(10a)
Recrystallization from cyclohexane gave pure yellow solid,
yield: 42%, mp 95e96 ꢁC. 1H NMR (300 MHz, CDCl3): d 7.0
(m, 1H), 7.06 (d, 1H), 7.09 (s, 2H), 7.11 (d, 1H), 7.14 (d, 1H),
7.20 (d, 1H), 7.47e7.54 (m, 3H), 7.59 (d, 1H), 7.85e7.91 (m,
2H), 8.28e8.31 (m, 1H); 13C NMR (75 MHz, CDCl3):
d 121.4, 121.5, 124.4, 125.3, 125.7, 126.0, 126.1, 126.5, 126.6,
127.7, 128.0, 128.1, 128.4, 128.5, 131.7, 132.4, 133.9, 140.8,
142.5, 142.7; HRMS (EI): m/z [M]þ calcd for C20H14S2,
318.0537; found, 318.0536.
4.3.2. 2,7-Di-(a-naphthyl)benzo[1,2-b;4,3-b0]dithiophene (4)
Yellow solid, yield: 78%, mp 148e150 ꢁC. 1H NMR
(300 MHz, CDCl3): d 7.51e7.57 (m, 6H), 7.71 (d, 2H), 7.86

Y. Hu et al. / Tetrahedron 64 (2008) 2251e2258
2257
(s, 2H), 7.88 (s, 2H), 7.90e7.95 (m, 4H), 8.34e8.37 (m, 2H);
13C NMR (75 MHz, CDCl3): d 118.4, 122.2, 1253, 125.8,
126.2, 126.7, 128.4, 128.6, 129.0, 131.9, 132.5, 133.9, 135.1,
137.2, 142.7; HRMS (EI): m/z [M]þ calcd for C30H18S2,
442.0850; found, 442.0843.
4.4.2. 2-(a-Naphthyl)benzo[1,2-b;4,3-b0]dithiophene (2)
Elution with hexane/dichloromethane (10:1) gave a yellow
solid in 89%.
4.4.3. 2-(4-Cyanophenyl)benzo[1,2-b;4,3-b0]dithiophene (3)
Elution with hexane/dichloromethane (1:1) gave yellow
solid, yield: 83%, mp 181e183 ꢁC. 1H NMR (300 MHz,
CDCl3): d 7.62 (d, 1H), 7.71e7.75 (m, 3H), 7.78 (d, 1H), 7.85
(m, 2H), 7.88 (m, 1H), 8.03 (s, 1H); 13C NMR (75 MHz,
CDCl3): d 29.7, 111.4, 118.4, 119.8, 120.0, 121.7, 126.7, 127.2,
132.8, 134.8, 136.9, 138.7, 141.9; HRMS (EI): m/z [M]þ calcd
for C17H9NS2, 291.0176; found, 291.0175.
4.3.3. 2-(4-Methoxyphenyl)trithia[5]helicene (7)
Silica gel column chromatography with hexane/dichloro-
methane (1:2), yield: 80%, mp 152e153 ꢁC. 1H NMR
(300 MHz, CDCl3): d 3.88 (s, 3H), 7.02 (d, 2H), 7.73e7.76
(m, 3H), 7.81 (d, 1H), 7.87 (d, 1H), 7.93 (d, 1H), 8.02 (d,
1H), 8.41e8.43 (m, 2H); 13C NMR (75 MHz, CDCl3):
d 55.4, 114.6, 118.6, 119.0, 119.2, 120.7, 121.0, 124.4, 126.2,
127.1, 127.8, 130.4, 134.7, 135.8, 136.9, 137.0, 137.4, 138.1,
144.3; HRMS (EI): m/z [M]þ calcd for C23H14OS3, 402.0207;
found, 402.0208.
4.4.4. 2-(4-Acetylphenyl)benzo[1,2-b;4,3-b0]dithiophene (5)
Elution with hexane/dichloromethane (1:1) gave yellow
solid, yield: 78%, mp 175e176 ꢁC. 1H NMR (300 MHz,
CDCl3): d 2.65 (s, 3H), 7.61 (d, 1H), 7.75 (d, 1H), 7.78 (d,
1H), 7.83 (s, 1H), 7.85 (m, 1H), 7.88 (m, 1H), 8.02 (m, 1H),
8.04e8.05 (m, 3H); 13C NMR (75 MHz, CDCl3): d 197.3,
143.0, 138.8, 137.0, 136.8, 136.4, 135.5, 134.8, 129.2, 127.0,
126.3, 121.8, 119.6, 119.2, 118.5, 26.6; HRMS (EI): m/z [M]þ
calcd for C18H12OS2, 308.0330; found, 308.0332.
4.3.4. 2-Iodo-11-(4-methoxyphenyl)trithia[5]helicene (12)
A 2.5 M solution of n-BuLi in hexane (0.44 mL, 1.1 mmol)
was added dropwise under stirring to a solution of 2-(4-me-
thoxyphenyl)trithia[5]helicene 7 (0.4 g, 1 mmol) in 20 mL of
dry THF at ꢀ78 ꢁC. The solution was stirred for 10 min at
ꢀ78 ꢁC and for 15 min at room temperature. The resulting
light yellow solution was cooled at ꢀ78 ꢁC, and a solution
of I2 (0.27 g, 1.05 mmol) in 5 mL of dry THF was added drop-
wise. After 2 h, the reaction was quenched with aqueous
Na2SO3 (5%, 20 mL) and warmed to room temperature. The
THF was removed under reduced pressure. The yellow mate-
rial that precipitated was filtered and washed with water. The
crude compound was dried under vacuum and used without
further purification.
4.4.5. 2-(4-Cyanophenyl)-10-(4-methoxyphenyl)trithia[5]-
helicene (8)
Bright yellow solid was precipitated from dichloromethane.
The yield starting from 7 was 73%; mp 295e296 ꢁC. 1H NMR
(300 MHz, CDCl3): d 3.90 (s, 3H), 6.98 (d, 2H), 7.70e7.75 (m,
4H), 7.83 (d, 1H), 7.88e7.97 (m, 5H), 8.46 (s, 1H), 8.79 (s,
1H); HRMS (EI): m/z [M]þ calcd for C30H17ONS3, 503.0472;
found, 503.0467.
4.4.6. 2-(4-Acetylphenyl)-10-(4-methoxyphenyl)trithia[5]-
helicene (9)
4.4. General procedure for Suzuki reaction
Bright yellow solid was precipitated from dichloromethane.
The yield starting from 7 was 69%; mp 289e291 ꢁC. 1H NMR
(300 MHz, CDCl3): d 2.66 (s, 3H), 3.89 (s, 3H), 6.98 (d, 2H),
7.76 (d, 2H), 7.83 (d, 1H), 7.88 (d, 1H), 7.93e7.98 (m, 4H),
8.02e8.05 (d, 2H), 8.51 (s, 1H), 8.80 (s, 1H); HRMS (EI):
m/z [M]þ calcd for C31H20O2S3, 520.0625; found, 520.0629.
Under argon, a mixture of 2-iodobenzo[1,2-b;4,3-b0]-
dithiophene (1 mmol, 316 mg), aryl boronic acid (1.05 mmol),
and Pd(PPh3)4 (0.03 mmol, 3.0%) in a mixed solvent of DME
(3 mL) and aqueous K2CO3 (2 M, 1 mL) was refluxed for
overnight. Then, water (5 mL) was added. After separation,
the organic phase was washed twice with water (2ꢂ5 mL)
and the collected aqueous phases were extracted twice with
5 mL dichloromethane. The organic phases were collected,
dried over Na2SO4, and the solvent was removed under re-
duced pressure to give a residue that was subjected to silica
gel chromatography to obtain the pure products.
4.4.7. 7-(4-Methoxyphenyl)benzo[1,2-b;4,3-b0]dithiophene-
2-carbaldehyde (11)
A 2.5 M solution of n-BuLi in hexane (0.44 mL, 1.1 mmol,
1.1 equiv) was added dropwise under stirring to a solution of
2-(4-methoxyphenyl)benzo[1,2-b;4,3-b0]dithiophene 1 (296 mg,
1 mmol), in dry THF (25 mL) at ꢀ78 ꢁC. The solution was
stirred for 5 min at ꢀ78 ꢁC and 15 min at room temperature.
The resulting blue solution was cooled at ꢀ78 ꢁC and treated
with dry DMF (0.1 mL, 1.3 mmol). After 2 h at ꢀ78 ꢁC, the
solution was warmed to room temperature, and quenched
with a saturated aqueous solution of NH4Cl (5 mL). The
THF was removed under reduced pressure, the crude material
was taken up with dichloromethane (40 mL), and washed with
a saturated aqueous solution of NH4Cl until pH 5 (2ꢂ10 mL).
The organic phases were dried over Na2SO4, the solvent was
removed under reduced pressure, and the crude material was
4.4.1. 2-(4-Methoxyphenyl)benzo[1,2-b;4,3-b0]dithiophene (1)
Elution with hexane/dichloromethane (1:1) gave yellow
solid, yield: 77%, mp 168e169 ꢁC. 1H NMR (300 MHz,
CDCl3): d 3.87 (s, 3H), 6.97 (m, 1H), 7.00 (m, 1H), 7.65 (d,
1H), 7.72e7.70 (m, 3H), 7.76 (d, 2H), 7.80 (s, 1H); 13C
NMR (75 MHz, CDCl3): d 55.4, 114.4, 116.4, 118.4, 118.5,
121.9, 126.4, 127.2, 127.8, 134.4, 135.7, 135.8, 136.7, 144.7,
159.8; HRMS (EI): m/z [M]þ calcd for C17H12OS2, 296.0330;
found, 296.0338.

2258
Y. Hu et al. / Tetrahedron 64 (2008) 2251e2258
7. Newman, M. S.; Lednicer, D. J. Am. Chem. Soc. 1956, 78, 4765.
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purified by flash column chromatography on silica gel with
hexane/dichloromethane (1:1) as eluent to provide yellow
solid, 96% yield, mp 198e200 ꢁC. 1H NMR (300 MHz,
CDCl3): d 3.88 (s, 3H), 6.99 (m, 1H), 7.01 (m, 1H), 7.70 (m,
1H), 7.73 (m, 1H), 7.77 (d, 1H), 7.85 (s, 1H), 7.91 (d, 1H),
8.39 (s, 1H), 10.17 (s, 1H); 13C NMR (300 MHz, CDCl3):
d 55.5, 114.6, 116.1, 118.6, 122.7, 126.6, 127.9, 131.8,
133.5, 136.3, 137.3, 140.8, 143.0, 146.8, 184.2; HRMS (EI):
m/z [M]þ calcd for C18H12O2S2, 324.0279; found, 324.0282.
11. Licandro, E.; Rigamonti, C.; Ticozzelli, M. T.; Monteforte, M.; Baldoli,
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Acknowledgements
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Chem. Commun. 2000, 1139.
Y.H. thanks the McMaster Endowment for providing a fel-
lowship. B.W. thanks LAU for a Travel Grant (URC-t2007-49)
and the McMaster Endowment for a 2007 McMaster Summer
Fellowship. We thank Dr. Thomas H. Kinstle, Dr. Jeanette A.
Krause, and Dr. Bipin K. Shah for helpful discussions. The
work was supported by ONR00014-06-1-0948. Contribution
no. 654 from the Center for Photochemical Sciences.
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Supplementary data
23. Kitamura, T.; Morizane, K.; Taniguchi, H.; Fujiwara, Y. Tetrahedron Lett.
1997, 38, 5157.
Thin film fluorescence of 1, 3, and 7, NMR, and IR spectra
of compounds 1e10b were provided. This material is avail-
able free of charge via the Internet. Supplementary data asso-
ciated with this article can be found, in the online version, at
doi:10.1016/j.tet.2007.12.029.
24. Kitamura, T.; Zhang, B.; Fujiwara, Y. Tetrahedron Lett. 2002, 43,
2239.
25. Crystallographic data (excluding structure factors) for the structure in this
paper have been deposited with the Cambridge Crystallographic Data
Centre as supplementary publication nos. CCDC 667991. Copies of the
data can be obtained, free of charge, on application to CCDC, 12 Union
Road, Cambridge CB2 1EZ, UK (fax: þ44 (0) 1223 336033 or e-mail:
deposit@ccdc.cam.ac.Uk).
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